Title of Invention	"METHOD OF OXIDATION REACTION FLUE GAS TREATMENT AND ENERGY RECOVERY"
Abstract	A system in which the amount of contents of oxidation reaction flue gas emitted from the process of aromatic dicarboxylic acid production is reduced and in which the pressure held by the flue gas is recovered as power energy at relatively low temperature range (150° → 0°C) ; and a system unified with the production process in which the thus recovered energy is used as power for compression of air for oxidation reaction and in which flue gas of low pressure (0.1 Kg/cm2G) is used as a supply gas for processing, such as drying or transportation, of produced dicarboxylic acid powder. Oxidation reaction flue gas is first treated at 40°C or below by the use of a high-pressure absorbing tower according to acetic acid/water two-stage washing method (1), and thereafter the pressure held by the flue gas is recovered as power energy by means of a two-stage expansion turbine with the use of steam (5 Kg/cm2G steam) generated at the time of reaction as heating source (2). As a result, the recovered energy can be used as rotating power, and the dew point of low-pressure flue gas becomes 0°C or its vicinity or higher, thereby attaining conversion to gas and energy recycled in the production process.

Title of Invention

"METHOD OF OXIDATION REACTION FLUE GAS TREATMENT AND ENERGY RECOVERY"

Abstract

A system in which the amount of contents of oxidation reaction flue gas emitted from the process of aromatic dicarboxylic acid production is reduced and in which the pressure held by the flue gas is recovered as power energy at relatively low temperature range (150° → 0°C) ; and a system unified with the production process in which the thus recovered energy is used as power for compression of air for oxidation reaction and in which flue gas of low pressure (0.1 Kg/cm2G) is used as a supply gas for processing, such as drying or transportation, of produced dicarboxylic acid powder. Oxidation reaction flue gas is first treated at 40°C or below by the use of a high-pressure absorbing tower according to acetic acid/water two-stage washing method (1), and thereafter the pressure held by the flue gas is recovered as power energy by means of a two-stage expansion turbine with the use of steam (5 Kg/cm2G steam) generated at the time of reaction as heating source (2). As a result, the recovered energy can be used as rotating power, and the dew point of low-pressure flue gas becomes 0°C or its vicinity or higher, thereby attaining conversion to gas and energy recycled in the production process.

Full Text	DESCRIPTION METHOD OF OXIDATION REACTION FLUE GAS TREATMENT AND ENERGY RECOVERY TECHNICAL FIELD [0001] The present invention relates to technology concerning resource saving for treating the heat and the oxidation reaction waste gas respectively generated in large quantities in the industrial manufacture of aromatic dicarboxylic acid and utilizing the recovered energy. Detailedly, the present invention relates to an oxidation reaction waste gas treatment system that recovers pressure energy held in the following gas by an expander as power energy after the high-pressure oxidation reaction waste gas generated by liquid phase oxidation in acetic acid solvent with oxygen-containing gas applied to aromatic dialkyl hydrocarbon such as para-xylene as a material under the existence of a catalyst is treated in high-pressure absorbing towers by acetic acid and water at the respective temperature which is controlled. BACKGROUND ART [0002] It has been industrially performed on a large scale as the applications of aromatic dicarboxylic acid are extended and grow to manufacture aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon such as para-xylene as a material using acetic acid for a solvent under the existence of a catalyst containing cobalt, manganese and bromine by high-pressure gas containing molecular oxygen. In the said manufacturing method, oxidation reaction waste gas which is discharged the oxygen content from an oxidation reactor, and which is accompanied by vapor of acetic acid and water in the solvent generated by the oxidation so as to remove heat of reaction generated in large quantities is discharged from the reactor in large quantities. Therefore, after the gas containing the vapor is cooled to remove the heat and the accompanied condensable components are condensed, the condensate is separated and is returned to the oxidation reactor, and the oxidation reaction waste gas is discharged from the reactor. [0003] As the oxidation reaction waste gas contains saturated steam of acetic acid and water at a separation temperature, the oxidation reaction waste gas is discharged from the said manufacturing unit after the contained components containing acetic acid as the solvent are industrially further reduced by passing the oxidation reaction waste gas in a high-pressure absorbing tower by scrubbing with water (refer to Patent Publication 1: Japanese Examined Patent Application Publication No.S39-8818). Because of the increase of need for the subsequent environmental problem and resource-saving problem, a method of contacting oxidation reaction waste gas emitted from the high-pressure absorbing tower, that is, the said gas emitted from the manufacturing unit to adsorbent such as activated carbon, further removing organic components contained in the waste gas and recovering them is proposed. However, as the method for adsorbing in activated carbon and others has problems that the desorption and the recovery are not easy and the ability of adsorption is reduced by moisture contained in the gas, the improvement and the device of adsorbent are proposed (refer to Patent Publication 2: Japanese Patent Application Laid-Open Publication No.H02-32040 and Patent Publication 3: Japanese Patent Application Laid-Open Publication No. H04-74153) . Besides, as disclosed in Patent Publication 4 (Japanese Patent Application Laid-Open Publication N0.HO8-268953), a method for contacting the waste gas to an oxidation catalyst, oxidizing the contained organic components and making them non-noxious is also proposed. [0004] In the meantime, as the said treated waste gas is discharged in accompanied by the pressure in oxidation, a method for converting the pressure energy which the effluent gas has to power by passing the waste gas in an expansion turbine and recovering the power is proposed. Incidentally, in Patent Publication 5 (Japanese Patent Application Laid-Open Publication No.S55- 99517), Patent Publication 6 (Japanese Patent Application Laid-Open Publication NO.S56- 72221) and Patent Publication 7 (Japanese Patent Application Laid-Open Publication N0.HO8-155265), a method for converting pressure which waste gas has to power energy by passing the waste gas in an expansion turbine and recovering the power energy after the temperature of the waste gas is raised and the waste gas is made non-noxious by catalytic oxidation (combustion) by a catalyst or directly by combustion of the waste gas kept in a high-pressure state is also proposed. However, in the method, as methyl bromide which is one of by-products of oxidation is contained a little amount in the waste flue gas, there also occurs a problem that the pressure and the temperature (to be a temperature higher than a dew point) of the gas in the turbine and others are required to be controlled because the methyl bromide may be converted to a corrosive bromine compound by combustion and corrosion may occur when the combusted gas is conducted into the expansion turbine (refer to Patent Publication 8: Japanese Patent Application Laid-Open Publication No.2000- 189753 and Patent Publication 9: Japanese Patent Application Laid-Open Publication No.2001-515576). [0005] As described above, in the method for removing and recovering residual contained components in the oxidation reaction waste gas generated in the aromatic dicarboxylic acid manufacturing system, the special adsorbent and technique are required and in addition, no process of the pressure of the waste gas is described. Besides, as to the method of combusting the waste gas in a high-pressure state, it is proposed to conduct the high-temperature and high-pressure gas into the expansion turbine and to recover heat and pressure energy as power, however, a measure to prevent corrosion in high-pressure combustion facilities and the expansion turbine is required. Therefore, in the related art, an efficient waste gas treatment system unified with the aromatic dicarboxylic acid manufacturing process is not configured. [0006] Patent Publication 1: Japanese Examined Patent Application Publication No.S39-8818 Patent Publication 2: Japanese Patent Application Laid-Open Publication No.H02-32040 Patent Publication 3: Japanese Patent Application Laid-Open Publication No.H04-74153 Patent Publication 4: Japanese Patent Application Laid-Open Publication NO.H08-268953 Patent Publication 5: Japanese Patent Application Laid-Open Publication No.S55-99517 Patent Publication 6: Japanese Patent Application Laid-Open Publication NO.S56-72221 Patent Publication 7: Japanese Patent Application Laid-Open Publication NO.H08-155265 Patent Publication 8: Japanese Patent Application Laid-Open Publication No.2000-189753 Patent Publication 9: Japanese Patent Application Laid-Open Publication No.2001-515576 Patent Publication 10: Japanese Patent Application Laid-Open Publication No.S53-84933 Patent Publication 11: Japanese Patent Application Laid-Open Publication No.S54-100310 Patent Publication 12: Japanese Patent Application Laid-Open Publication No.H06-304700 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0007] In the above-mentioned situation, these inventors examined overall in relation to the degree of a fall of the temperature in the expansion turbine and a dew point of the waste gas in addition to components contained in the oxidation reaction waste gas discharged from its production process and their composition with it as objects (1) to reduce and recover the residual contained components in the waste gas in one step in the manufacturing system, (2) to recover pressure which the waste gas held as power energy, (3) to control a temperature lowered in recovering as the power energy to be a temperature at which components contained in the waste gas, particularly moisture did not reach a dew point and not to adopt special materials against the corrosion, (4) therefore, to enable using steam generated in the manufacturing unit or the surplus steam and (5) to use low-pressure effluent gas (approximately 0.1 kg/cm2G) acquired after the recovery of the power energy for inert gas used for drying, mixing and transporting aromatic dicarboxylic acid powder generated in the manufacturing system. [0008] First, the oxidation reaction waste gas discharged from the manufacturing unit of aromatic dicarboxylic acid, that is, from the high-pressure absorbing tower generally has composition shown in Table 1 and it is said that the contained organic components are acetic acid, methyl acetate, benzene, toluene, para-xylene and methyl bromide. It is known that for organic components contained in oxidation reaction waste gas from a manufacturing system of terephthalic acid using para-xylene for a material, methyl bromide of 25 ppm, methyl acetate of 900 ppm, para-xylene of 80 ppm, benzene of 9 ppm and toluene of 4 ppm are contained (refer to a fourth page of Patent Publication 3 and Embodiment 1), and it is known that a most contained organic component in the organic composition is methyl acetate and a second most contained organic component is para-xylene as the material. In the data, although acetic acid as a solvent is not described, it is considered that acetic acid of quantity equivalent to the quantity of methyl acetate is contained. For example, in the composition shown in Table 1 of oxidation reaction waste gas as the objects for recovering pressure energy, organic components of 0.2 mol% are contained as described in Patent Publication 9, the organic components are methyl acetate, acetic acid, hydrocarbon as a material and methyl bromide, and it is thought that acetic acid is contained. [0009] [Table 1] Composition of oxidation reaction waste gas (Japanese Patent Application Laid-Open Publication No. 2001-515576) (Table Removed) Note: methyl bromide is contained in organic components by approximately 50 ppm [0010] However, in the method for recovering the pressure energy of oxidation reaction waste gas as power as described above, after the waste gas is combusted (at 300 °C or a higher temperature under a catalyst, at 800°C or a higher temperature when no catalyst exists) and organic components in the waste gas are made non-noxious, a combustion process by adding auxiliary fuel and conducting air for combustion according to circumstances is added so as to combust the waste gas containing few combustible components (CO: 0.45 mol%, organic components: 0.2 mol%) as shown in Table 1 in a step for conducting the acquired high-temperature and high-pressure waste gas into the expansion turbine and recovering it as power energy. [0011] Therefore, it is described in [0046] on a sixth page of Japanese Patent Application Laid-Open Publication No. 2000-189753 that the temperature and the pressure of the waste gas after energy is recovered by the expansion turbine are 320°C higher than a dew point and 2 atm. and the high dew point (110°C) is avoided. As described on fifth to ninth lines on page 16 of Japanese Patent Application Laid-Open Publication No. 2001-515576, as to a temperature higher than a dew point, the temperature and the pressure of gas at an outlet of an expander are 140 to 200°C and 1.2 bar (absolute pressure) and it similarly comes into problem that the dew point is high. That is, in these methods, as a result of generating a corrosive bromine compound and combusting it by adding auxiliary fuel, moisture of 72.7 mol% ([0045] of the sixth page of Patent Publication 8) is contained in the flue gas after combustion and as a result of raising the dew point, a problem occurs. [0012] As described above, they occur a vicious circulation that the dew point in the expansion turbine is raised by adding the auxiliary fuel and combusting the gas as the result so as to acquire a high temperature required for making the contained substance which originally exists by small quantity in the oxidation reaction waste gas non-noxious and recovering power and the operation in a high-temperature range is required as a result of being required so that the temperature of the gas after the recovery of energy is higher to be approximately 14°C or higher. As a result, the waste gas treatment is made more complex technique (combustion under high pressure, the measure against corrosion, the recovery of heat and others). [0013] Then, these inventors found that the whole temperature range required for the recovery of power could be lowered by further reducing contained components which were originally contained by small quantity in oxidation reaction waste gas and lowering a dew point of the waste gas and examined the configuration of a power recovery system using steam in a temperature range generated in the manufacture process as an object. MEANS OF SOLVING THE PROBLEMS [0014] A moisture content (0.5 mol%) to be a problem as a component of dew condensation in oxidation reaction waste gas (effluent gas at an outlet of a high-pressure absorbing tower) discharged from an aromatic dicarboxylic acid manufacturing system is far less than a moisture content (72.7 mol%) after the combustion, and it is found that the much moisture content after the combustion results from saturated steam by washing water in the high-pressure absorbing tower and organic components can be greatly reduced up to 1/3 to 1/10, compared with those in the high-pressure absorbing tower by water as shown in test results in Embodiments 1 to 4 by adding an acetic acid washing process in a high-pressure absorbing tower and controlling a washing temperature so as to remove other contained organic components of 0.2 mol%. [0015] Then, these inventors represented a moisture content in oxidation reaction waste gas resulting from saturated steam in the high-pressure absorbing tower and causing dew condensation as an absorbing tower temperature, represented the temperature of dew condensation depending upon relation between pressure and temperature in a phenomenon of adiabatic expansion in an expansion turbine for recovering power as a high-pressure absorbing tower temperature, and examined a limit of the absorbing tower temperature in the expansion turbine. These inventors examined a method for recovering pressure energy which high-pressure oxidation reaction waste gas held as power energy without combusting the high-pressure oxidation reaction waste gas. Simultaneously, these inventors limited conditions for generating oxidation reaction waste gas to the similar conditions in the industrial manufacture of aromatic dicarboxylic acid and set it to an object to configure a method according to the present invention as the most efficient power recovery system applied to a production process of aromatic dicarboxylic acid. [0016] A condition of reaction in an oxidation reactor in the industrial manufacture of aromatic dicarboxylic acid has been currently condensed to a temperature range of 185 to 205°C in consideration of economy. Therefore, the pressure of oxidation reaction waste gas is condensed to a range of 12.5 to 16.5 kg/cm2G though the pressure slightly varies depending upon the water content and materials in the reaction solvent and the oxidation reaction waste gas discharged from the manufacturing unit, that is, oxidation reaction waste gas discharged from the high-pressure absorbing tower has pressure normally in a range of 12.0 to 16.0 kg/cm2G. Therefore, the effluent gas from the absorbing tower contains moisture normally in a range of 0.44 to 1 mol% (at the absorbing tower temperature of 40 to 50°C under the pressure of 12 to 16 kg/cm2G) as saturated moisture and it is known from Table 6 and Embodiments 1 to 4 that a moisture content can be greatly reduced by lowering the temperature in the absorbing tower. It can be expected that the fall of the temperature in the absorbing tower has a large effect on lowering a dew point in the expansion turbine. Simultaneously, as shown by results of tests described later, content of organic components can be greatly reduced by combining the high-pressure absorbing tower by acetic acid and a problem that a dew point in the expansion turbine causes corrosion may be also solved. [0017] Accordingly, it is known that the washing and the absorption of components contained in oxidation reaction flue gas conducted into the expansion turbine to recover pressure energy by combining a series of two-stage high-pressure absorbing towers by acetic acid and water and the lowering of the absorption temperature more securely makes a dew point of the waste gas lowered, the emission of the organic components can be expected to be greatly reduced and the method is desirable for a process by which the loss of acetic acid as a solvent and material not reacted yet such as para-xylene can be saved. The reduction of the discharge of methyl acetate contributes to the retrenchment of the loss of acetic acid as a solvent by inhibiting the loss in combustion of acetic acid by recovering and returning to an oxidation step or by recovering as acetic acid by hydrolysis as proposed in Patent Publication 10 (Japanese Patent Application Laid-Open Publication No.S53-84933) and Patent Publication 11 (Japanese Patent Application Laid-Open Publication NO.S54-100310). [0018] Next, when power is recovered from high-pressure waste gas of the absorbing tower decreased the absorption temperature, the decreased temperature need to be compensated together with the fall of the temperature caused by adiabatic expansion. In the method in the related art, a high temperature is created by combustion, however, in the method according to the present invention, an energy saving system that limits to the utilization of steam to an extent of steam (normally 5 kg/cm2G, 158°C) generated in a steam generator annexed to an upper part of the oxidation reactor is set. Therefore, conditions of the waste gas at an inlet of the expansion turbine are set in a range of 12 to 16 kg/cm2G and 140 to 150°C. The outlet temperature when gas in such a state (12 to 16 kg/cm2G, 140 to 150°C) is conducted into the first- stage expansion turbine as material gas for recovering power and gas of 0.1 kg/cm2G is acquired is -85 to -70°C. In the meantime, the temperature in the high-pressure absorbing tower by washing water and a dew point (the starting temperature of dew condensation) of the effluent gas of 0.1 kg/cm2G are as follows. [0019] [Table 2] Dew point of effluent gas of 0.1 kg/cm2G (Table Removed) Note: pressure in high-pressure absorbing tower: 12 to 16 kg/cm2G [0020] It is known that in a case of waste gas (12 to 16 kg/cm2G, 140 to 150°C) , the temperature of the gas at an outlet of one-stage expansion turbine is far below a dew point and there is no executability in one-stage expansion turbine. Therefore, that is the reason why a high temperature is required for the introducing gas to compensate a fall of the temperature in one-stage expansion turbine. Conditions of the expansion turbine on which the outlet gas temperature is higher than the dew point (pressure: 0.1 kg/cm2G in Table 2) even under the above- mentioned conditions of the waste gas are a key to solve a problem of the power recovery system. A dew point under the pressure of 0.1 kg/cm2G after washing by water of 40°C or below in the high-pressure absorbing tower of the waste material gas (12 to 16 kg/cm2G, temperature: absorbing tower temperature) is in the vicinity of a freezing point, and a power recovery method by which the temperature of gas at the outlet of the expansion turbine is the freezing point (0°C) or higher is expected. Effluent gas of the outlet gas temperature which is the freezing point or higher can be also utilized for inert gas for processing the aromatic dicarboxylic acid powder processed at an ordinary temperature or a higher temperature as it is without any treatment. As a result, these inventors found that the outlet gas of temperature which was the freezing point or higher could be acquired by the two-stage expansion turbines under the following relational expression by conducting the oxidation reaction waste gas of the temperature which was 140 to 150°C and the pressure of which was 12 to 16 kg/cm2G into the expansion turbine as material gas, recovering power and analyzing about the condition (temperature that did not reach the dew point) where a dew of the gas at the pressure which fell to 0.1 kg/cm2G was not formed. [0021] First, relation between pressure and temperature in adiabatic expansion of gas is calculated in the following expression and a moisture content to be a dew point is calculated based upon saturated vapor pressure of water at temperature of a high-pressure absorbing tower. (T2/T1) = (P2/P1)(1)  = Cp/Cv =1.4 In this case, Tl, P1: temperature and pressure on side of inlet T2, P2: temperature and pressure on side of outlet (Temperature and pressure are converted to absolute notation) : ratio of specific heat at constant pressure Cp to specific heat at constant volume Cv (Air or diatomic gas is 1.4) Oxidation reaction waste gas (gas at the outlet of the high-pressure absorbing tower) having the pressure of 12 to 16 kg/cm2G and discharged from the aromatic dicarboxylic acid manufacturing system is heated with steam of 5 kg/cm2G and the oxidation reaction waste gas having the pressure of 12 to 16 kg/cm2G and the temperature of 140 to 150°C is conducted into the expansion turbine as material gas for recovering power. After power (Wl) is recovered from the gas conducted into the first-stage expansion turbine as rotating energy by adiabatic expansion, the gas is discharged emitted from the outlet of the first stage at a temperature of (T2)l and at pressure of (P2)l. Next, the gas is heated with steam of 5 kg/cm2G to be gas of the temperature of 140 to 150°C and the pressure of (P1)2 and is conducted into the second-stage turbine. First-stage outlet pressure (P2)l and second-stage inlet pressure (P1)2 are equal and are called intermediate pressure. [0022] After power (W2) is recovered from the gas conducted into the second stage by adiabatic expansion, second-stage outlet gas is discharged from the second-stage turbine at a temperature of (T2)2 and at the pressure (P2)2 of 0.1 kg/cm2G. In a process which the temperature and the pressure of the waste gas fall by adiabatic expansion in the relation of the above-described expression, a moisture content that reaches the dew point is calculated using the temperature in the high-pressure absorbing tower for an index as a limit of high-pressure absorbing tower temperature that reaches the dew point on the respective turbine conditions. That is, the flue gas treated at the high-pressure absorbing tower of the limit temperature or lower temperature in the absorbing tower is an index that does not reach the dew point (that forms no dew) on the respective turbine conditions. Conversely, the gas treated at the limit temperature or higher temperature is an index in which dew condensation occurs in the turbine. [0023] As maximum power is recovered in the two-stage expansion turbines when power recovered in the first stage and power recovered in the second stage are equal (W2/W1 = 1), the first-stage outlet pressure (P2)l (or the second-stage inlet pressure (Pl)2) in which the waste gas is expanded from the inlet pressure (12 to 16 kg/cm2G) to 0.1 kg/cm2G by the two-stage expansion turbine and the respective recovered power is equal is calculated as optimum intermediate pressure and is shown by a full line in Fig. 2 (Recovered power ratio W2/W1=1). The following table shows the calculated results of the temperature and the pressure at the inlet and at the outlet of the two-stage expansion turbines at that time. The table simultaneously shows moisture content to be a dew point at the outlets of the respective turbines as the limit of the temperature of the high-pressure absorbing tower (Note: temperature 140°C at which dew condensation may be more easily formed in the temperature range of the gas conducted into the expansion turbine of 140 to 150°C is shown). [0024] [Table 3] Two-stage expansion turbines when ratio W2/W1 of recovered power is 1 (Heated temperature: 140°C) (Table Removed) [0025] Table 3 shows that though the outlet temperature of the first-stage turbine and the second-stage turbine at this time is equal, a condition of dew condensation is more severe in the first-stage turbine and a limit of high-pressure absorbing tower temperature at which dew condensation occurs is 28 to 39°C as to the temperature in the high-pressure absorbing towers containing moisture to be a dew point at the temperature and the pressure at the respective outlets, while on a condition in the second-stage turbine, no dew condensation occurs in a moisture content in which the temperature in the high-pressure absorbing tower is 50°C. [0026] Accordingly, Table 3 shows that when the temperature in the high-pressure absorbing tower is 25°C, there is no risk of dew condensation on the above-mentioned conditions of reaction in recovering power by the two-stage expansion turbines. Besides, Table 3 shows that there is a safe range for dew condensation at the high-pressure absorbing tower temperature of 30°C (15.1 kg/cm2G or below) or 35°C (13 kg/cm2G or below) depend on the waste gas pressure. Relation between temperature and pressure when the ratio W2/W1 of recovered power in the expansion turbines is 1.2 and 1.4 is calculated and each high-pressure absorbing tower temperature limit in the respective turbines is also shown below. Intermediate pressure at the ratio of recovered power is shown by a dotted and a fine line in Fig. 2. As these results indicate, as the ratio of recovered power increases (intermediate pressure increases), the severe condition of dew condensation in the first-stage turbine is relieved and the condition in the second-stage turbine is turned severe. [0027] That is, when the ratio W2/W1 of recovered power is 1.2, the limit of high-pressure absorbing tower temperature is a similar temperature (37 to 50°C) in both the first-stage turbine and the second-stage turbine and a risk of dew condensation is substantially balanced. It is known that when the temperature in the high-pressure absorbing tower is 35°C, both of the first-stage and the second-stage expansion turbines are in a safe range free of a risk of dew condensation. Besides, it is known that when the temperature in the high-pressure absorbing tower is 40°C and the pressure of waste gas is substantially 15 kg/cm2G (14.8 kg/cm2G) or below, there is a safe range for dew condensation. [0028] [Table 4] High-pressure absorbing tower temperature limit when ratio W2/W1 of recovered power is 1.2 (Heated temperature:14 0°C) (Table Removed) [0029] When the ratio W2/W1 of recovered power is 1.4, the emission temperature of the first-stage turbine rises and a risk of dew condensation is greatly reduced, however, the gas discharge temperature in the second-stage turbine decreases and the temperature limit of the high-pressure absorbing tower is turned severe. However, it is known that when the temperature in the high-pressure absorbing tower is 25°C, a risk of dew condensation is completely avoided at the pressure of waste gas of 15.6 kg/cm2G or below, that is, in the expansion turbine substantially according to the above-mentioned conditions of reaction waste gas. It is simultaneously known that when the temperature in the high-pressure absorbing tower is 30°C, there is no risk of dew condensation in the second-stage turbine up to the pressure of waste gas of 14 kg/cm2G or below. When the ratio (W2/W1) of recovered power exceeds 1.4, a fall of the temperature of waste gas proceeds on the side of the second-stage turbine and the condition of dew condensation is turned further severe. Therefore, the temperature in the absorbing tower is required to be further lowered from 25°C, a cooling load is industrially applied, and a countermeasure for temperature is required to utilize waste gas. In the meantime, from a viewpoint of the recovery of power, when the ratio W2/W1 of recovered power is 1.4, recovered power decreases by approximately 0.7%, compared with the case that the ratio W2/W1 of recovered power is 1, however, it is known that the quantity of the decrease does not come into the problem as the loss of power. [0030] [Table 5] High-pressure absorbing tower temperature limit when ratio W2/W1 of recovered power is 1.4 (Heated temperature: 140°C) (Table Removed) [0031] Accordingly, it was found that high-pressure oxidation reaction waste gas discharged from the industrial manufacturing system of aromatic dicarboxylic acid by the currently executed oxidation of aromatic dialkyl hydrocarbon such as para-xylene was conducted into the two-stage expansion turbines and power could be recovered in the balanced range of 1 to 1.4 as the ratio (W2/W1) of recovered power in the first-stage turbine and in the second-stage turbine by only heating with steam (5 kg/cm2G) generated in the oxidation reaction by selecting the temperature in the high-pressure absorbing tower installed in the manufacturing system out of 40°C or below, desirable 35°C or below, preferable 30°C or below and the most desirable 25°C or below for the pressure of the oxidation reaction waste gas. It was also found that when the turbine the ratio (W2/W1) of recovered power on the conditions (12 to 16 kg/cm2G, 140 to 150°C) of the oxidation reaction waste gas of which was designed to be 1.2 was used, a risk of dew condensation was avoided in the overall range of turbine inlet conditions by setting a washing temperature in the high-pressure absorbing tower to 35°C and substantially maximum power could be recovered. Similarly, the high-pressure absorbing tower temperature is required to be set to 25°C for the expansion turbines in the ratio (W2/W1) of recovered power of which is 1.2 and 1.4 to correspond to turbine inlet gas conditions. [0032] Power recovered in the expansion turbine is calculated using an expression of power W executed in adiabatic expansion derived from an equation of an adiabatic state (PV=K= constant). W=K{(V2)(1 - -(Vl)(1 - )}/(l-7) V1, V2: gas volume on sides of inlet and outlet 7=Cp/Cv: ratio of specific heat at constant pressure Cp to special heat at constant volume Cv (Air or diatomic gas is 1.4) PV=nRT: Convert V1, V2 in a right relational expression [0033] The treatment of oxidation reaction waste gas generated by oxidation for the manufacture of aromatic dicarboxylic acid industrially produced on a large scale is substantially performed in the above-mentioned range of conditions. The economical and efficient power recovery system can be configured by annexing the high-pressure absorbing tower by acetic acid to the high-pressure absorbing tower (by washing water) also provided in the related art, lowering the treatment temperature in the those towers and afterward, using the two-stage expansion turbines that recover power at two stages using steam (5 kg/cm2G) generated in the said manufacturing unit for a heating source without requiring facilities required in front of and at the back of the expansion turbine in high-pressure combustion performed in the related art and the heart recovery at a high temperature. Fig. 1 shows a schematic flow of treatment in the recovery system. [0034] That is, there is configured the system where high-pressure oxidation reaction waste gas discharged from the industrial manufacture of aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon as a material using acetic acid for a solvent under the existence of the catalyst containing cobalt, manganese and bromine at the temperature range of 185 to 205°C by oxygen-containing gas is conducted into the two-stage expansion turbines after l)the said waste gas is treated at 40°C or below in the high-pressure absorbing towers by two-stage washing liquid of acetic acid and then water and components contained in the waste gas are reduced, 2)the gas conducted into the first stage and the second stage is respectively heated with steam (5 kg/cm2G) at the temperature of 140°C or higher with the heat and the pressure energy of the conducted gas are recovered as power and the low-pressure gas of 0.1 kg/cm2G or more can be acquired. At that time, the turbine designed so that the ratio of power recovered by the second-stage turbine to power recovered by the first-stage turbine of the two-stage expansion turbines is 1 to 1.4 is required to be used, and the treatment temperature in the high-pressure absorbing tower is required to be set to desirable 35°C or below, preferable 30°C or below and the most desirable 25°C or below depending upon the ratio of recovered power and the pressure of the oxidation reaction waste gas. It is also known that as a result, the acquired waste gas can be used for gas for drying, mixing and transporting the above-formed aromatic dicarboxylic acid powder. EFFECTS OF THE INVENTION [0035] For the effect of the method according to the present invention, it directly contributes to the reduction of the facility cost and the recovery cost and the reduction of the manufacturing cost of aromatic dicarboxylic acid which is the body that when the pressure energy of the gas used in oxidation is recovered as power, power recovery facilities requiring facilities does not require the facilities for high-pressure combustion and high-temperature heat recovery facilities in the related art, the power can be recovered at a relatively low temperature (140 to 150°C) by only the additional installation of the high-pressure absorbing tower and lowering the treatment temperature in the absorbing towers and the recovery system is unified with the dicarboxylic acid production process which is the body. Besides, the worthy aromatic dicarboxylic acid production process gentle with environment in which emission matters and waste heat of the manufacturing system are reduced is configured. [0036] The concrete effect of individuals is as follows. 1) Effective components to be materials contained in the waste gas combusted or discarded in the past are reduced to 1/3 to 1/10 and are recovered by only the additional installation of the high-pressure absorbing tower and the management of the temperature in the absorbing towers. 2) Power can be recovered at the relatively low-temperature range (150°C → 0°C) by adopting the two-stage expansion turbines. 3) Therefore, steam generated in the oxidation in the body can be used for a heating source of gas for recovering power. 4) Recovered power can be utilized for power for compressing air required in the oxidation. 5) The effluent gas after the recovery of power can be used for inert gas for processing dicarboxylic acid generated powder without any treatment. BEST MODE FOR CARRYING OUT THE INVENTION [0037] It is the temperature and the pressure (Fig. 1 [2]) at an outlet of a high-pressure absorbing tower (2) of material gas the power of which is to be recovered that are important so as to embody the present invention. As ordinary-temperature water (ion exchange water and others containing few impurities) was used in the past, the waste gas was discharged at varying temperatures normally in the vicinity of 50°C from the absorbing tower, however, an area free of dew condensation is secured in an expansion turbine by cooling the wash water and positively controlling so that the outlet gas temperature of the high-pressure absorbing tower is 40°C or below, desirably 35°C or below, preferably 30°C or below and most desirably 25°C or below. In the high-pressure absorbing tower, contained components were absorbed and reduced by only washing by water in the past, however, it is proved that contained organic substances such as methyl acetate and para-xylene which cannot be reduced by only washing by water can be greatly reduced so that an amount which is normally 0.2 mol% of organic substances is approximately 0.05 mol% and is further approximately 0.02 mol% or below by additionally installing a high-pressure absorbing tower by acetic acid (Embodiments 1 to 4 described later). Therefore, it is desirable that the temperature of washing and absorption in the added high-pressure absorbing tower by acetic acid is 40°C or below similarly to the temperature in the absorbing tower by water and it is preferable that the temperature is further 35°C or below. Therefore, control and management over temperature and pressure (Fig. 1 [1]) at an outlet of a high-pressure absorbing tower (1) are required for reducing the cost of raw materials and for measures to prevent the corrosion by dew condensation, coolers (7, 9 in Fig. 1) for cooling washing liquid to the respective high-pressure absorbing towers are installed, the coolers respectively control the temperature of the gas by cooling the washing liquid. The temperature of gas at the outlet of the absorbing tower becomes a temperature substantially close to the temperature of the washing liquid though the temperature of gas at the outlet depends upon the temperature of the gas at an inlet, however, it is desirable that as to the control and management, the temperature of gas at the outlet is managed. As the temperature of gas at the outlet of the high-pressure absorbing tower by water is directly related to a dew point in the turbine, it is managed in relation to the pressure of oxidation reaction waste gas as described above, however, it is desirable that the temperature of gas in the high-pressure absorbing tower by acetic acid is 35°C or below in consideration of the results of tests in the embodiments described later. In view of the effect (particularly, methyl acetate) of greatly reducing organic components by the additional installation of the high-pressure absorbing tower by acetic acid, the temperature of gas is sufficient if only it is 35°C or below, however, from a viewpoint of the management of a dew point of the waste gas, the same temperature as the temperature in the high-pressure absorbing tower by water or the temperature below it is desirable. [0038] Next, for a heating source of oxidation reaction waste gas when the waste gas is conducted into the expansion turbine, it is desirable as a heat source for recovering power in a low-temperature range (150 → approximately 0°C) according to the present invention and as a loop of energy to use steam of 5 kg/cm2G (5 in Fig. 1) generated in oxidation as described above, however, the heat source is not limited to the said generated steam. Waste steam of approximately 5 kg/cm2G that exists in a manufacturing system can be used and incidentally, spent waste steam in a refiner annexed to the manufacturing system can be effectively utilized. [0039] However, as to relation between conditions in the expansion turbine and a risky range of dew condensation, a risky range by the adiabatic expansion at the heated lowest temperature 140°C of the conducted waste gas is calculated, and the temperature and the pressure ([3], [5] in Fig. 1) of the gas at inlets of respective expansion turbines are important for temperature management. The rise exceeding 140°C of the temperature of the said conducted gas leads to a safer range from dew condensation and has no problem. While the expansion turbines designed on predetermined conditions are operated under the management of the temperature and the pressure, no problem occurs, however, when an abnormality occurs on the side of the aromatic dicarboxylic acid manufacturing system, outlet conditions ([4],[6] in Fig. 1) from the respective expansion turbines are important to prevent dew condensation. [0040] However, as known from Embodiment 5 described later, a fall of the temperature due to adiabatic expansion does not reach the calculated temperature in the actual operation of the expansion turbines. Concretely, the temperature merely falls to a temperature equivalent to approximately a little more than eighty % of calculated temperature fall width. Therefore, on this test condition, the turbine outlet temperature is higher than the calculated temperature by approximately 20 to 25°C and as the turbine outlet temperature enters in the safer range of dew condensation, the temperature in recovering power according to the method by the present invention can be managed in the sufficiently safe range. Next, rotational energy recovered by the expansion turbines is directly transmitted to a electric generator and can be recovered as electric energy, however, the said manufacturing unit requires compressed air for oxidation, it is useful for the reduction of an energy cost to transmit the rotational energy to a shaft for rotating an electric motor or a steam turbine for driving an air compressor (10, 11, 12, 13 in Fig. 1) and to directly use the rotational energy for the rotational power, and the system is very desirable as a system in which the energy of the manufacturing unit is looped. [0041] The effluent gas which is treated and recovered in the above-mentioned process holds a moderate temperature and pressure, the dew point falls (a dew point under 0.1 kg/cm2G in the table: 0 to -15°C) owing to the great reduction of contained condensed substances, and the effluent gas can be used for inert gas for drying, transporting and mixing the above-formed dicarboxylic acid powder and for sealing the system without requiring any treatment as it is. For example, the effluent gas is desirable gas for the replenishment of gas for transporting crude aromatic dicarboxylic acid powder acquired in oxidation. [0042] For material gas for purifying the crude aromatic dicarboxylic acid by a method such as a purifying process by hydrogenation and transporting the purified refined dicarboxylic acid powder to be a final product of the aromatic dicarboxylic acid manufacturing unit, when organic components (0.01 to 0.05 mol%) extremely minutely contained are to come into problem, the contained components are further removed by passing the material gas in an adsorption tower using activated alumina and others for adsorbent and can be used, however, the used quantity and a life of the absorbent are more advantageous than those of gas by a well-known method (refer to Patent Publication 12: Japanese Patent Application Laid-Open Publication No.H06-304700) and the cost is reduced. Embodiments [0043] Referring to concrete embodiments, the method according to the invention will be described further in detail below, however, the present invention is not limited to them. [0044] Embodiments 1 to 4 and Comparative example 1 Liquid phase oxidation is applied to para-xylene as a material at the temperature of 19 5°C under the pressure of approximately 14.3 kg/cm2G using acetic acid for a solvent under the existence of cobalt, manganese and bromine by blowing air, a part of oxidation reaction waste gas is branched from a pipe for the oxidation reaction effluent gas discharged from a gas-liquid separator located in front of a high-pressure absorbing tower and annexed to an oxidation reactor of a plant for manufacturing terephthalic acid and is continuously extracted, the part of the oxidation reaction effluent gas is conducted into high-pressure absorbing tower test equipment shown in Fig. 3, and a test of the effect of organic components contained in the effluent gas according to washing conditions in a high-pressure absorbing tower is made. The high-pressure absorbing tower test equipment shown in Fig. 3 includes a high-pressure absorbing tower 1 and a high-pressure absorbing tower 2, in the high-pressure absorbing tower 1, the said high-pressure effluent gas branched from a lower part is washed by acetic acid injected from an upper part by a volumetric pump, and the effluent gas which the contained components are reduced is discharged from the upper part. As for the composition of components contained in the effluent gas, the material gas conducted in from the lower part is branched from the upper part if necessary, is sampled, and the contained organic components such as acetic acid, methyl acetate and para-xylene are analyzed. The temperature of the gas is measured and managed by a thermometer installed at an outlet of the absorbing tower. [0045] The high-pressure absorbing tower is a 4" Sch 80 SUS pipe having a diameter of 97. 1 mm and is configured by a packed column 1.2 m high (a packed part: 1.0 m) as an upper packing part into which a 1/4 Raschig ring is packed and a lower liquid phase bubble column 0.7 m high (a liquid level: 0.5 m). A cooler is attached to control the temperature of acetic acid when washing acetic acid is injected. Acetic acid used for washing is discharged to a washed liquid recovery pot via a siphon type communicating pipe, keeping a liquid level in the bubble column and the acetic acid is intermittently extracted from the recovery pot at suitable time. A high-pressure absorbing tower 2 is a system provided with the same structure as the high-pressure absorbing tower 1, gas emitted from the high-pressure absorbing tower 1 is conducted from a lower part of the high-pressure absorbing tower 2, water for washing is injected from an upper part, and the similar operation and the similar test method to those of the high-pressure absorbing tower 1 are executed. After the effluent gas emitted from the upper part of the high-pressure absorbing tower 2 is once stored in a mist separation pot, the pressure in the mist separation pot is measured, the effluent gas quantity is controlled by a valve at the back of the pot, and the emitted gas quantity is measured by a gas meter. [0046] Table 6 shows a result of a test executed using the above-mentioned test equipment, a saturated water content at the temperature and pressure shown in Table 6 is measured, and is shown in Table 6. A test of washing and absorption by only water performed as a comparative example 1 is made by bypassing the high-pressure absorbing tower 1 and directly conducting the gas branched from the terephthalic acid manufacturing unit into the lower part of the high-pressure absorbing tower 2. Conditions in this test are as follows. Operating pressure : 13.9 to 14.1 kg/cm2G Gas flow rate : 120 Nm3/hr. Acetic acid injected quantity: 7 kg/hr. Water injected quantity : 3.5 kg/hr. The temperature of acetic acid and water in the high-pressure absorbing towers 1, 2 is equalized. As the result shown in Table 6, the total quantity of acetic acid, methyl acetate and material not reacted yet (para-xylene) is approximately 1400 ppm (0.14 mol%) according to washing by only water in the high-pressure absorbing tower, the contained organic components (acetic acid, methyl acetate, material not reacted yet (para-xylene) are greatly reduced by washing by acetic acid and water and lowering the temperature of the washing liquid, and it is proved that the effect of washing organic contained substances by acetic acid is great. The saturated water content is also greatly reduced by a fall of the temperature in the absorbing tower. An acetic acid content has a tendency to slightly increase by washing by acetic acid of 40°C, however, an effect on the acetic acid content by washing by acetic acid is removed by lowering the temperature(35°C or below). [0047] [Table 6] High-pressure absorbing tower test result (Table Removed) [0048] Embodiment 5 Condensable components in gas discharged from an oxidation reactor of a terephthalic acid manufacturing unit (50,000Ton/Y) are condensed via a steam generator 2 and a condenser 3 as shown in a flow in Fig. 1, the oxidation reaction waste gas after condensate is separated in a gas-liquid separator 4 is washed by acetic acid and water, and power is recovered as follows using the high-pressure waste gas treated in a high-pressure absorbing tower (1) 6 (the 12-shelf tower 1000 f10300H) installed to absorb and reduce components contained in the waste gas and a high-pressure absorbing tower (2) 8 (the 16-stage tray tower l000f10500H). Terephthalic acid manufactured by the manufacturing unit is produced at a rate of approximately 5.4 ton/hr., air required for the production is compressed up to 19.5 kg/cm2G by a compressor 10 (four-stage compression) and is supplied at a rate of approximately 13,300 Nm3/hr., and oxidation is regularly performed. Washing liquid is cooled so that both outlet gas temperatures of the high-pressure absorbing towers are 35°C and is injected. As for the quantity, acetic acid is approximately 2.3 ton/hr. and water is approximately 0.5 ton/hr. Power is recovered using two-stage expansion turbines 12, 13 designed on a design basis shown in Table 8 and pressure energy is recovered using steam of 5 kg/cm2G generated in a steam generator 5 annexed to the oxidation reactor for a heating source. The recovered energy is supplied to an electric motor by coupling a shaft for rotating the turbine to the electric motor 11 of an air compressor 10. Therefore, the recovered quantity of electric power is calculated based upon the retrenched quantity of current which the electric motor requires. [0049] [Table 7] Outlet gas pressure of high-pressure absorbing tower and quantity of contained components (Table Removed) [0050] The above-mentioned treatment of oxidation reaction waste gas and the recovery of energy have been satisfactorily performed for approximately half a year since the start without trouble in a rotor of the turbine. Table 7 and Table 8 show representative results of the quality of treated gas during operation and the operational conditions of the expansion turbines. Table 2 shows that a content of organic components is approximately 300 ppm (0.03 mol%) and a dew point of the effluent gas (0.1 kg/cm2G) is -5°C. Table 8 tells that the expansion turbine is provided with substantially designed performance and power can be recovered at a low-temperature range of 30 to 150°C using the two-stage expansion turbines. Table 8 also tells that recovered low-pressure gas can be recovered at a higher temperature than the dew point (-5°C) of the oxidation reaction waste gas. However, as to the temperature at an inlet and at the outlet of each turbine, the width of the fall of the temperature in actual operation is approximately 82.4% for the width of calculated values and operation in a safer range for the dew point is actually performed. Next, Table 9 shows the calculated quantity of electric power retrenched by the expansion turbines in the air compressor. Table 9 tells that power of expected quantity calculated in Table 8 could be substantially recovered. The rotor and a casing of the expansion turbine are made of a stainless casting (equivalent to SCS14), the expansion turbine is disassembled after the operation is stopped and is checked for corrosion, and it was verified that the expansion turbines had no problem. [0051] [Table 8] Operating conditions of power recovery turbine (Table Removed) Note: recovered power: estimated value in view of operating conditions Outlet temperature in ( ): calculated value in adiabatic expansion expression Fall rate: temperature fall rate in actual operation for calculated value width between inlet temperature and outlet temperature of each turbine [0052] [Table 9] Power recovered quantity by expansion turbine (Table Removed) Note: power: value acquired from characteristic curve based upon measured current BRIEF DESCRIPTION OF THE DRAWINGS [0053] [Fig. 1] Fig. 1 is a system diagram showing a flow of the treatment of oxidation reaction waste gas in an aromatic dicarboxylic acid production process by a method according to the present invention and a flow in a pressure energy recovery system. [Fig. 2] Fig. 2 shows relation between turbine inlet pressure and intermediate pressure (outlet pressure in a first stage = inlet pressure in a second stage) in two-stage expansion turbines using the ratio of recovered power for parameters (Wl: a calculated value of power recovered in the first stage, W2: a calculated value of power recovered in the second stage). [Fig. 3] Fig. 3 is a schematic diagram showing test equipment that makes an absorption test by injecting washing liquid (acetic acid and water) into a high-pressure absorbing tower. EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS [0054] 1 Oxidation reactor, 2 Steam generator, 3 Condenser, 4 Gas-liquid separator, 5 Steam storage tank, 6 High-pressure absorbing tower (1), 7 Acetic acid cooler, 8 High-pressure absorbing tower (2), 9 Water cooler, 10 Air compressor, 11 Electric motor, 12 First-stage expander, 13 Second-stage expander, 14 Heater (1), 15 Heater (2), 16 Condensed water storage tank. CLAIMS 1. A method for treating oxidation reaction waste gas in a production process of aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon as a material using acetic acid for a solvent under the existence of a metallic catalyst containing cobalt and manganese and bromine as the promoter at a temperature range of 185 to 205°C in an oxidation reactor by oxygen-containing gas, comprising the steps of: cooling the oxidation reaction waste gas discharged from the above-mentioned oxidation reactor and separating subsequent to condensing the condensable components from the high-pressure oxidation reaction waste gas; scrubbing the said resulting waste gas at 40°C or below in high-pressure absorbing towers by washing liquid on two-stage of acetic acid and subsequently water and reducing the contained components in it; and subsequently, conducting the said treated oxidation reaction waste gas into two-stage expansion turbines after heating the said gas respectively conducted into the first-stage and the second stage of the said expansion turbine to be 140°C or a higher temperature with steam (approximately 5 kg/cm2G) and recovering the heat and the pressure energy of the conducted gas as power. 2. The method for treating oxidation reaction waste gas according to Claim 1, wherein as to the treatment temperature in the high-pressure absorbing towers, it is desirable that the temperature of the gas at an outlet of the high-pressure absorbing tower is 35°C or below, it is more desirable that the temperature of the gas is 30°C or below, and it is the most desirable that the temperature of the gas is 25°C or below; and the components contained in the oxidation reaction waste gas are reduced. 3. The method of treating oxidation reaction waste gas according to Claim 1 or 2, wherein the steam (approximately 5 kg/cm2G) from a steam generator annexed with the oxidation reactor is used for the steam used in Claim 1 or 2. 4. The method for treating oxidation reaction waste gas according to any of Claims 1 to 3, wherein the two-stage expansion turbines where a ratio of power recovered from the second-stage turbine to power recovered from the first-stage turbine is designed to be between 1 and 1.4 are used. 5. The method for treating oxidation reaction waste gas according to any of Claims 1 to 4, wherein the low-pressure gas (0.1 kg/cm2G) acquired in any of Claims 1 to 4 is used for material gas for drying, transporting and mixing generated aromatic dicarboxylic acid powder without any treatment.

Full Text

DESCRIPTION METHOD OF OXIDATION REACTION FLUE GAS TREATMENT AND ENERGY
RECOVERY
TECHNICAL FIELD [0001]
The present invention relates to technology concerning resource saving for treating the heat and the oxidation reaction waste gas respectively generated in large quantities in the industrial manufacture of aromatic dicarboxylic acid and utilizing the recovered energy. Detailedly, the present invention relates to an oxidation reaction waste gas treatment system that recovers pressure energy held in the following gas by an expander as power energy after the high-pressure oxidation reaction waste gas generated by liquid phase oxidation in acetic acid solvent with oxygen-containing gas applied to aromatic dialkyl hydrocarbon such as para-xylene as a material under the existence of a catalyst is treated in high-pressure absorbing towers by acetic acid and water at the respective temperature which is controlled. BACKGROUND ART [0002]
It has been industrially performed on a large scale as the applications of aromatic dicarboxylic acid are
extended and grow to manufacture aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon such as para-xylene as a material using acetic acid for a solvent under the existence of a catalyst containing cobalt, manganese and bromine by high-pressure gas containing molecular oxygen.
In the said manufacturing method, oxidation reaction waste gas which is discharged the oxygen content from an oxidation reactor, and which is accompanied by vapor of acetic acid and water in the solvent generated by the oxidation so as to remove heat of reaction generated in large quantities is discharged from the reactor in large quantities. Therefore, after the gas containing the vapor is cooled to remove the heat and the accompanied condensable components are condensed, the condensate is separated and is returned to the oxidation reactor, and the oxidation reaction waste gas is discharged from the reactor. [0003]
As the oxidation reaction waste gas contains saturated steam of acetic acid and water at a separation temperature, the oxidation reaction waste gas is discharged from the said manufacturing unit after the contained components containing acetic acid as the solvent are industrially further reduced by passing the oxidation
reaction waste gas in a high-pressure absorbing tower by scrubbing with water (refer to Patent Publication 1: Japanese Examined Patent Application Publication No.S39-8818).
Because of the increase of need for the subsequent environmental problem and resource-saving problem, a method of contacting oxidation reaction waste gas emitted from the high-pressure absorbing tower, that is, the said gas emitted from the manufacturing unit to adsorbent such as activated carbon, further removing organic components contained in the waste gas and recovering them is proposed. However, as the method for adsorbing in activated carbon and others has problems that the desorption and the recovery are not easy and the ability of adsorption is reduced by moisture contained in the gas, the improvement and the device of adsorbent are proposed (refer to Patent Publication 2: Japanese Patent Application Laid-Open Publication No.H02-32040 and Patent Publication 3: Japanese Patent Application Laid-Open Publication No. H04-74153) .
Besides, as disclosed in Patent Publication 4 (Japanese Patent Application Laid-Open Publication N0.HO8-268953), a method for contacting the waste gas to an oxidation catalyst, oxidizing the contained organic components and making them non-noxious is also proposed.
[0004]
In the meantime, as the said treated waste gas is discharged in accompanied by the pressure in oxidation, a method for converting the pressure energy which the effluent gas has to power by passing the waste gas in an expansion turbine and recovering the power is proposed. Incidentally, in Patent Publication 5 (Japanese Patent Application Laid-Open Publication No.S55- 99517), Patent Publication 6 (Japanese Patent Application Laid-Open Publication NO.S56- 72221) and Patent Publication 7 (Japanese Patent Application Laid-Open Publication N0.HO8-155265), a method for converting pressure which waste gas has to power energy by passing the waste gas in an expansion turbine and recovering the power energy after the temperature of the waste gas is raised and the waste gas is made non-noxious by catalytic oxidation (combustion) by a catalyst or directly by combustion of the waste gas kept in a high-pressure state is also proposed. However, in the method, as methyl bromide which is one of by-products of oxidation is contained a little amount in the waste flue gas, there also occurs a problem that the pressure and the temperature (to be a temperature higher than a dew point) of the gas in the turbine and others are required to be controlled because the methyl bromide may be converted to a corrosive bromine compound
by combustion and corrosion may occur when the combusted gas is conducted into the expansion turbine (refer to Patent Publication 8: Japanese Patent Application Laid-Open Publication No.2000- 189753 and Patent Publication 9: Japanese Patent Application Laid-Open Publication No.2001-515576). [0005]
As described above, in the method for removing and recovering residual contained components in the oxidation reaction waste gas generated in the aromatic dicarboxylic acid manufacturing system, the special adsorbent and technique are required and in addition, no process of the pressure of the waste gas is described.
Besides, as to the method of combusting the waste gas in a high-pressure state, it is proposed to conduct the high-temperature and high-pressure gas into the expansion turbine and to recover heat and pressure energy as power, however, a measure to prevent corrosion in high-pressure combustion facilities and the expansion turbine is required.
Therefore, in the related art, an efficient waste gas treatment system unified with the aromatic dicarboxylic acid manufacturing process is not configured. [0006]
Patent Publication 1: Japanese Examined Patent
Application Publication No.S39-8818
Patent Publication 2: Japanese Patent Application Laid-Open Publication No.H02-32040
Patent Publication 3: Japanese Patent Application Laid-Open Publication No.H04-74153
Patent Publication 4: Japanese Patent Application Laid-Open Publication NO.H08-268953
Patent Publication 5: Japanese Patent Application Laid-Open Publication No.S55-99517
Patent Publication 6: Japanese Patent Application Laid-Open Publication NO.S56-72221
Patent Publication 7: Japanese Patent Application Laid-Open Publication NO.H08-155265
Patent Publication 8: Japanese Patent Application Laid-Open Publication No.2000-189753
Patent Publication 9: Japanese Patent Application Laid-Open Publication No.2001-515576
Patent Publication 10: Japanese Patent Application Laid-Open Publication No.S53-84933
Patent Publication 11: Japanese Patent Application Laid-Open Publication No.S54-100310
Patent Publication 12: Japanese Patent Application Laid-Open Publication No.H06-304700
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION [0007]
In the above-mentioned situation, these inventors examined overall in relation to the degree of a fall of the temperature in the expansion turbine and a dew point of the waste gas in addition to components contained in the oxidation reaction waste gas discharged from its production process and their composition with it as objects (1) to reduce and recover the residual contained components in the waste gas in one step in the manufacturing system, (2) to recover pressure which the waste gas held as power energy, (3) to control a temperature lowered in recovering as the power energy to be a temperature at which components contained in the waste gas, particularly moisture did not reach a dew point and not to adopt special materials against the corrosion, (4) therefore, to enable using steam generated in the manufacturing unit or the surplus steam and (5) to use low-pressure effluent gas (approximately 0.1 kg/cm2G) acquired after the recovery of the power energy for inert gas used for drying, mixing and transporting aromatic dicarboxylic acid powder generated in the manufacturing system. [0008]
First, the oxidation reaction waste gas discharged
from the manufacturing unit of aromatic dicarboxylic acid, that is, from the high-pressure absorbing tower generally has composition shown in Table 1 and it is said that the contained organic components are acetic acid, methyl acetate, benzene, toluene, para-xylene and methyl bromide.
It is known that for organic components contained in oxidation reaction waste gas from a manufacturing system of terephthalic acid using para-xylene for a material, methyl bromide of 25 ppm, methyl acetate of 900 ppm, para-xylene of 80 ppm, benzene of 9 ppm and toluene of 4 ppm are contained (refer to a fourth page of Patent Publication 3 and Embodiment 1), and it is known that a most contained organic component in the organic composition is methyl acetate and a second most contained organic component is para-xylene as the material.
In the data, although acetic acid as a solvent is not described, it is considered that acetic acid of quantity equivalent to the quantity of methyl acetate is contained.
For example, in the composition shown in Table 1 of oxidation reaction waste gas as the objects for recovering pressure energy, organic components of 0.2 mol% are contained as described in Patent Publication 9, the organic components are methyl acetate, acetic acid, hydrocarbon as a material and methyl bromide, and it is
thought that acetic acid is contained. [0009]
[Table 1] Composition of oxidation reaction waste gas (Japanese Patent Application Laid-Open Publication No. 2001-515576)

(Table Removed)
Note: methyl bromide is contained in organic components by approximately 50 ppm
[0010]
However, in the method for recovering the pressure energy of oxidation reaction waste gas as power as described above, after the waste gas is combusted (at 300 °C or a higher temperature under a catalyst, at 800°C or a higher temperature when no catalyst exists) and organic components in the waste gas are made non-noxious, a combustion process by adding auxiliary fuel and conducting air for combustion according to circumstances is added so as to combust the waste gas containing few combustible components (CO: 0.45 mol%, organic components: 0.2 mol%) as shown in Table 1 in a step for conducting the acquired high-temperature and high-pressure waste gas into the
expansion turbine and recovering it as power energy. [0011]
Therefore, it is described in [0046] on a sixth page of Japanese Patent Application Laid-Open Publication No. 2000-189753 that the temperature and the pressure of the waste gas after energy is recovered by the expansion turbine are 320°C higher than a dew point and 2 atm. and the high dew point (110°C) is avoided.
As described on fifth to ninth lines on page 16 of Japanese Patent Application Laid-Open Publication No. 2001-515576, as to a temperature higher than a dew point, the temperature and the pressure of gas at an outlet of an expander are 140 to 200°C and 1.2 bar (absolute pressure) and it similarly comes into problem that the dew point is high.
That is, in these methods, as a result of generating a corrosive bromine compound and combusting it by adding auxiliary fuel, moisture of 72.7 mol% ([0045] of the sixth page of Patent Publication 8) is contained in the flue gas after combustion and as a result of raising the dew point, a problem occurs. [0012]
As described above, they occur a vicious circulation that the dew point in the expansion turbine is raised by adding the auxiliary fuel and combusting the gas as the
result so as to acquire a high temperature required for making the contained substance which originally exists by small quantity in the oxidation reaction waste gas non-noxious and recovering power and the operation in a high-temperature range is required as a result of being required so that the temperature of the gas after the
recovery of energy is higher to be approximately 14°C or higher. As a result, the waste gas treatment is made more complex technique (combustion under high pressure, the measure against corrosion, the recovery of heat and others). [0013]
Then, these inventors found that the whole temperature range required for the recovery of power could be lowered by further reducing contained components which were originally contained by small quantity in oxidation reaction waste gas and lowering a dew point of the waste gas and examined the configuration of a power recovery system using steam in a temperature range generated in the manufacture process as an object.
MEANS OF SOLVING THE PROBLEMS [0014]
A moisture content (0.5 mol%) to be a problem as a component of dew condensation in oxidation reaction waste
gas (effluent gas at an outlet of a high-pressure absorbing tower) discharged from an aromatic dicarboxylic acid manufacturing system is far less than a moisture content (72.7 mol%) after the combustion, and it is found that the much moisture content after the combustion results from saturated steam by washing water in the high-pressure absorbing tower and organic components can be greatly reduced up to 1/3 to 1/10, compared with those in the high-pressure absorbing tower by water as shown in test results in Embodiments 1 to 4 by adding an acetic acid washing process in a high-pressure absorbing tower and controlling a washing temperature so as to remove other contained organic components of 0.2 mol%. [0015]
Then, these inventors represented a moisture content in oxidation reaction waste gas resulting from saturated steam in the high-pressure absorbing tower and causing dew condensation as an absorbing tower temperature, represented the temperature of dew condensation depending upon relation between pressure and temperature in a phenomenon of adiabatic expansion in an expansion turbine for recovering power as a high-pressure absorbing tower temperature, and examined a limit of the absorbing tower temperature in the expansion turbine. These inventors examined a method for recovering pressure energy which
high-pressure oxidation reaction waste gas held as power energy without combusting the high-pressure oxidation reaction waste gas.
Simultaneously, these inventors limited conditions for generating oxidation reaction waste gas to the similar conditions in the industrial manufacture of aromatic dicarboxylic acid and set it to an object to configure a method according to the present invention as the most efficient power recovery system applied to a production process of aromatic dicarboxylic acid. [0016]
A condition of reaction in an oxidation reactor in the industrial manufacture of aromatic dicarboxylic acid has been currently condensed to a temperature range of 185
to 205°C in consideration of economy. Therefore, the pressure of oxidation reaction waste gas is condensed to a range of 12.5 to 16.5 kg/cm2G though the pressure slightly varies depending upon the water content and materials in the reaction solvent and the oxidation reaction waste gas discharged from the manufacturing unit, that is, oxidation reaction waste gas discharged from the high-pressure absorbing tower has pressure normally in a range of 12.0 to 16.0 kg/cm2G.
Therefore, the effluent gas from the absorbing tower contains moisture normally in a range of 0.44 to 1 mol%
(at the absorbing tower temperature of 40 to 50°C under the pressure of 12 to 16 kg/cm2G) as saturated moisture and it is known from Table 6 and Embodiments 1 to 4 that a moisture content can be greatly reduced by lowering the temperature in the absorbing tower.
It can be expected that the fall of the temperature in the absorbing tower has a large effect on lowering a dew point in the expansion turbine. Simultaneously, as shown by results of tests described later, content of organic components can be greatly reduced by combining the high-pressure absorbing tower by acetic acid and a problem that a dew point in the expansion turbine causes corrosion may be also solved. [0017]
Accordingly, it is known that the washing and the absorption of components contained in oxidation reaction flue gas conducted into the expansion turbine to recover pressure energy by combining a series of two-stage high-pressure absorbing towers by acetic acid and water and the lowering of the absorption temperature more securely makes a dew point of the waste gas lowered, the emission of the organic components can be expected to be greatly reduced and the method is desirable for a process by which the loss of acetic acid as a solvent and material not reacted yet such as para-xylene can be saved.
The reduction of the discharge of methyl acetate contributes to the retrenchment of the loss of acetic acid as a solvent by inhibiting the loss in combustion of acetic acid by recovering and returning to an oxidation step or by recovering as acetic acid by hydrolysis as proposed in Patent Publication 10 (Japanese Patent Application Laid-Open Publication No.S53-84933) and Patent Publication 11 (Japanese Patent Application Laid-Open Publication NO.S54-100310). [0018]
Next, when power is recovered from high-pressure waste gas of the absorbing tower decreased the absorption temperature, the decreased temperature need to be compensated together with the fall of the temperature caused by adiabatic expansion. In the method in the related art, a high temperature is created by combustion, however, in the method according to the present invention, an energy saving system that limits to the utilization of steam to an extent of steam (normally 5 kg/cm2G, 158°C) generated in a steam generator annexed to an upper part of the oxidation reactor is set. Therefore, conditions of the waste gas at an inlet of the expansion turbine are set in a range of 12 to 16 kg/cm2G and 140 to 150°C.
The outlet temperature when gas in such a state (12 to 16 kg/cm2G, 140 to 150°C) is conducted into the first-

stage expansion turbine as material gas for recovering
power and gas of 0.1 kg/cm2G is acquired is -85 to -70°C.
In the meantime, the temperature in the high-pressure absorbing tower by washing water and a dew point (the starting temperature of dew condensation) of the effluent gas of 0.1 kg/cm2G are as follows. [0019] [Table 2] Dew point of effluent gas of 0.1 kg/cm2G

(Table Removed)
Note: pressure in high-pressure absorbing tower: 12 to 16
kg/cm2G
[0020]
It is known that in a case of waste gas (12 to 16 kg/cm2G, 140 to 150°C) , the temperature of the gas at an outlet of one-stage expansion turbine is far below a dew point and there is no executability in one-stage expansion turbine.
Therefore, that is the reason why a high temperature is required for the introducing gas to compensate a fall of the temperature in one-stage expansion turbine.
Conditions of the expansion turbine on which the outlet gas temperature is higher than the dew point (pressure: 0.1 kg/cm2G in Table 2) even under the above-
mentioned conditions of the waste gas are a key to solve a problem of the power recovery system.
A dew point under the pressure of 0.1 kg/cm2G after
washing by water of 40°C or below in the high-pressure absorbing tower of the waste material gas (12 to 16 kg/cm2G, temperature: absorbing tower temperature) is in the vicinity of a freezing point, and a power recovery method by which the temperature of gas at the outlet of the expansion turbine is the freezing point (0°C) or higher is expected.
Effluent gas of the outlet gas temperature which is the freezing point or higher can be also utilized for inert gas for processing the aromatic dicarboxylic acid powder processed at an ordinary temperature or a higher temperature as it is without any treatment.
As a result, these inventors found that the outlet gas of temperature which was the freezing point or higher could be acquired by the two-stage expansion turbines under the following relational expression by conducting the oxidation reaction waste gas of the temperature which was 140 to 150°C and the pressure of which was 12 to 16 kg/cm2G into the expansion turbine as material gas, recovering power and analyzing about the condition (temperature that did not reach the dew point) where a dew of the gas at the pressure which fell to 0.1 kg/cm2G was
not formed. [0021]
First, relation between pressure and temperature in adiabatic expansion of gas is calculated in the following expression and a moisture content to be a dew point is calculated based upon saturated vapor pressure of water at temperature of a high-pressure absorbing tower.
(T2/T1) = (P2/P1)(1)
 = Cp/Cv =1.4
In this case,
Tl, P1: temperature and pressure on side of inlet
T2, P2: temperature and pressure on side of outlet
(Temperature and pressure are converted to absolute notation)
: ratio of specific heat at constant pressure Cp to specific heat at constant volume Cv
(Air or diatomic gas is 1.4)
Oxidation reaction waste gas (gas at the outlet of the high-pressure absorbing tower) having the pressure of 12 to 16 kg/cm2G and discharged from the aromatic dicarboxylic acid manufacturing system is heated with steam of 5 kg/cm2G and the oxidation reaction waste gas having the pressure of 12 to 16 kg/cm2G and the temperature of 140 to 150°C is conducted into the expansion turbine as material gas for recovering power.
After power (Wl) is recovered from the gas conducted into the first-stage expansion turbine as rotating energy by adiabatic expansion, the gas is discharged emitted from the outlet of the first stage at a temperature of (T2)l and at pressure of (P2)l. Next, the gas is heated with steam of 5 kg/cm2G to be gas of the temperature of 140 to 150°C and the pressure of (P1)2 and is conducted into the second-stage turbine. First-stage outlet pressure (P2)l and second-stage inlet pressure (P1)2 are equal and are called intermediate pressure. [0022]
After power (W2) is recovered from the gas conducted into the second stage by adiabatic expansion, second-stage outlet gas is discharged from the second-stage turbine at a temperature of (T2)2 and at the pressure (P2)2 of 0.1 kg/cm2G. In a process which the temperature and the pressure of the waste gas fall by adiabatic expansion in the relation of the above-described expression, a moisture content that reaches the dew point is calculated using the temperature in the high-pressure absorbing tower for an index as a limit of high-pressure absorbing tower temperature that reaches the dew point on the respective turbine conditions.
That is, the flue gas treated at the high-pressure absorbing tower of the limit temperature or lower
temperature in the absorbing tower is an index that does not reach the dew point (that forms no dew) on the respective turbine conditions. Conversely, the gas treated at the limit temperature or higher temperature is an index in which dew condensation occurs in the turbine. [0023]
As maximum power is recovered in the two-stage expansion turbines when power recovered in the first stage and power recovered in the second stage are equal (W2/W1 = 1), the first-stage outlet pressure (P2)l (or the second-stage inlet pressure (Pl)2) in which the waste gas is expanded from the inlet pressure (12 to 16 kg/cm2G) to 0.1 kg/cm2G by the two-stage expansion turbine and the respective recovered power is equal is calculated as optimum intermediate pressure and is shown by a full line in Fig. 2 (Recovered power ratio W2/W1=1). The following table shows the calculated results of the temperature and the pressure at the inlet and at the outlet of the two-stage expansion turbines at that time. The table simultaneously shows moisture content to be a dew point at the outlets of the respective turbines as the limit of the temperature of the high-pressure absorbing tower (Note: temperature 140°C at which dew condensation may be more easily formed in the temperature range of the gas conducted into the expansion turbine of 140 to 150°C is
shown).
[0024]
[Table 3] Two-stage expansion turbines when ratio W2/W1 of
recovered power is 1 (Heated temperature: 140°C)

(Table Removed)
[0025]
Table 3 shows that though the outlet temperature of the first-stage turbine and the second-stage turbine at this time is equal, a condition of dew condensation is more severe in the first-stage turbine and a limit of high-pressure absorbing tower temperature at which dew condensation occurs is 28 to 39°C as to the temperature in the high-pressure absorbing towers containing moisture to be a dew point at the temperature and the pressure at the
respective outlets, while on a condition in the second-stage turbine, no dew condensation occurs in a moisture content in which the temperature in the high-pressure absorbing tower is 50°C. [0026]
Accordingly, Table 3 shows that when the temperature
in the high-pressure absorbing tower is 25°C, there is no risk of dew condensation on the above-mentioned conditions of reaction in recovering power by the two-stage expansion turbines. Besides, Table 3 shows that there is a safe range for dew condensation at the high-pressure absorbing tower temperature of 30°C (15.1 kg/cm2G or below) or 35°C (13 kg/cm2G or below) depend on the waste gas pressure. Relation between temperature and pressure when the ratio W2/W1 of recovered power in the expansion turbines is 1.2 and 1.4 is calculated and each high-pressure absorbing tower temperature limit in the respective turbines is also shown below. Intermediate pressure at the ratio of recovered power is shown by a dotted and a fine line in Fig. 2.
As these results indicate, as the ratio of recovered power increases (intermediate pressure increases), the severe condition of dew condensation in the first-stage turbine is relieved and the condition in the second-stage turbine is turned severe.
[0027]
That is, when the ratio W2/W1 of recovered power is 1.2, the limit of high-pressure absorbing tower temperature is a similar temperature (37 to 50°C) in both the first-stage turbine and the second-stage turbine and a risk of dew condensation is substantially balanced. It is known that when the temperature in the high-pressure absorbing tower is 35°C, both of the first-stage and the second-stage expansion turbines are in a safe range free of a risk of dew condensation. Besides, it is known that when the temperature in the high-pressure absorbing tower is 40°C and the pressure of waste gas is substantially 15 kg/cm2G (14.8 kg/cm2G) or below, there is a safe range for dew condensation. [0028]
[Table 4] High-pressure absorbing tower temperature limit when ratio W2/W1 of recovered power is 1.2 (Heated
temperature:14 0°C)

(Table Removed)
[0029]
When the ratio W2/W1 of recovered power is 1.4, the emission temperature of the first-stage turbine rises and a risk of dew condensation is greatly reduced, however, the gas discharge temperature in the second-stage turbine decreases and the temperature limit of the high-pressure absorbing tower is turned severe. However, it is known that when the temperature in the high-pressure absorbing tower is 25°C, a risk of dew condensation is completely avoided at the pressure of waste gas of 15.6 kg/cm2G or below, that is, in the expansion turbine substantially according to the above-mentioned conditions of reaction waste gas. It is simultaneously known that when the temperature in the high-pressure absorbing tower is 30°C, there is no risk of dew condensation in the second-stage turbine up to the pressure of waste gas of 14 kg/cm2G or below.
When the ratio (W2/W1) of recovered power exceeds 1.4, a fall of the temperature of waste gas proceeds on the side of the second-stage turbine and the condition of dew condensation is turned further severe. Therefore, the temperature in the absorbing tower is required to be
further lowered from 25°C, a cooling load is industrially applied, and a countermeasure for temperature is required to utilize waste gas.
In the meantime, from a viewpoint of the recovery of power, when the ratio W2/W1 of recovered power is 1.4, recovered power decreases by approximately 0.7%, compared with the case that the ratio W2/W1 of recovered power is 1, however, it is known that the quantity of the decrease does not come into the problem as the loss of power. [0030]
[Table 5] High-pressure absorbing tower temperature limit when ratio W2/W1 of recovered power is 1.4 (Heated temperature: 140°C)
(Table Removed)
[0031]
Accordingly, it was found that high-pressure oxidation reaction waste gas discharged from the industrial manufacturing system of aromatic dicarboxylic acid by the currently executed oxidation of aromatic dialkyl hydrocarbon such as para-xylene was conducted into the two-stage expansion turbines and power could be recovered in the balanced range of 1 to 1.4 as the ratio (W2/W1) of recovered power in the first-stage turbine and in the second-stage turbine by only heating with steam (5 kg/cm2G) generated in the oxidation reaction by selecting the temperature in the high-pressure absorbing tower installed in the manufacturing system out of 40°C or below, desirable 35°C or below, preferable 30°C or below and the most desirable 25°C or below for the pressure of the oxidation reaction waste gas.
It was also found that when the turbine the ratio (W2/W1) of recovered power on the conditions (12 to 16 kg/cm2G, 140 to 150°C) of the oxidation reaction waste gas of which was designed to be 1.2 was used, a risk of dew condensation was avoided in the overall range of turbine inlet conditions by setting a washing temperature in the
high-pressure absorbing tower to 35°C and substantially maximum power could be recovered.
Similarly, the high-pressure absorbing tower temperature is required to be set to 25°C for the
expansion turbines in the ratio (W2/W1) of recovered power of which is 1.2 and 1.4 to correspond to turbine inlet gas conditions. [0032]
Power recovered in the expansion turbine is calculated using an expression of power W executed in adiabatic expansion derived from an equation of an adiabatic state (PV=K= constant). W=K{(V2)(1 - -(Vl)(1 - )}/(l-7) V1, V2: gas volume on sides of inlet and outlet 7=Cp/Cv: ratio of specific heat at constant pressure Cp to special heat at constant volume Cv (Air or diatomic gas is 1.4)
PV=nRT: Convert V1, V2 in a right relational expression [0033]
The treatment of oxidation reaction waste gas generated by oxidation for the manufacture of aromatic dicarboxylic acid industrially produced on a large scale is substantially performed in the above-mentioned range of conditions. The economical and efficient power recovery system can be configured by annexing the high-pressure absorbing tower by acetic acid to the high-pressure absorbing tower (by washing water) also provided in the related art, lowering the treatment temperature in the
those towers and afterward, using the two-stage expansion turbines that recover power at two stages using steam (5 kg/cm2G) generated in the said manufacturing unit for a heating source without requiring facilities required in front of and at the back of the expansion turbine in high-pressure combustion performed in the related art and the heart recovery at a high temperature. Fig. 1 shows a schematic flow of treatment in the recovery system. [0034]
That is, there is configured the system where high-pressure oxidation reaction waste gas discharged from the industrial manufacture of aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon as a material using acetic acid for a solvent under the existence of the catalyst containing cobalt, manganese and bromine at the temperature range of 185 to 205°C by oxygen-containing gas is conducted into the two-stage expansion turbines after l)the said waste gas is
treated at 40°C or below in the high-pressure absorbing towers by two-stage washing liquid of acetic acid and then water and components contained in the waste gas are reduced, 2)the gas conducted into the first stage and the second stage is respectively heated with steam (5 kg/cm2G) at the temperature of 140°C or higher with the heat and the pressure energy of the conducted gas are recovered as
power and the low-pressure gas of 0.1 kg/cm2G or more can be acquired.
At that time, the turbine designed so that the ratio of power recovered by the second-stage turbine to power recovered by the first-stage turbine of the two-stage expansion turbines is 1 to 1.4 is required to be used, and the treatment temperature in the high-pressure absorbing tower is required to be set to desirable 35°C or below, preferable 30°C or below and the most desirable 25°C or below depending upon the ratio of recovered power and the pressure of the oxidation reaction waste gas.
It is also known that as a result, the acquired waste gas can be used for gas for drying, mixing and transporting the above-formed aromatic dicarboxylic acid powder.
EFFECTS OF THE INVENTION [0035]
For the effect of the method according to the present invention, it directly contributes to the reduction of the facility cost and the recovery cost and the reduction of the manufacturing cost of aromatic dicarboxylic acid which is the body that when the pressure energy of the gas used in oxidation is recovered as power, power recovery facilities requiring facilities does not
require the facilities for high-pressure combustion and high-temperature heat recovery facilities in the related art, the power can be recovered at a relatively low temperature (140 to 150°C) by only the additional installation of the high-pressure absorbing tower and lowering the treatment temperature in the absorbing towers and the recovery system is unified with the dicarboxylic acid production process which is the body. Besides, the worthy aromatic dicarboxylic acid production process gentle with environment in which emission matters and waste heat of the manufacturing system are reduced is configured. [0036]
The concrete effect of individuals is as follows.
1) Effective components to be materials contained in the waste gas combusted or discarded in the past are reduced to 1/3 to 1/10 and are recovered by only the additional installation of the high-pressure absorbing tower and the management of the temperature in the absorbing towers.
2) Power can be recovered at the relatively low-temperature range (150°C → 0°C) by adopting the two-stage expansion turbines.
3) Therefore, steam generated in the oxidation in the body can be used for a heating source of gas for
recovering power.
4) Recovered power can be utilized for power for compressing air required in the oxidation.
5) The effluent gas after the recovery of power can be used for inert gas for processing dicarboxylic acid generated powder without any treatment.
BEST MODE FOR CARRYING OUT THE INVENTION [0037]
It is the temperature and the pressure (Fig. 1 [2]) at an outlet of a high-pressure absorbing tower (2) of material gas the power of which is to be recovered that are important so as to embody the present invention.
As ordinary-temperature water (ion exchange water and others containing few impurities) was used in the past, the waste gas was discharged at varying temperatures
normally in the vicinity of 50°C from the absorbing tower, however, an area free of dew condensation is secured in an expansion turbine by cooling the wash water and positively controlling so that the outlet gas temperature of the high-pressure absorbing tower is 40°C or below, desirably 35°C or below, preferably 30°C or below and most desirably 25°C or below.
In the high-pressure absorbing tower, contained components were absorbed and reduced by only washing by
water in the past, however, it is proved that contained organic substances such as methyl acetate and para-xylene which cannot be reduced by only washing by water can be greatly reduced so that an amount which is normally 0.2 mol% of organic substances is approximately 0.05 mol% and is further approximately 0.02 mol% or below by additionally installing a high-pressure absorbing tower by acetic acid (Embodiments 1 to 4 described later).
Therefore, it is desirable that the temperature of washing and absorption in the added high-pressure
absorbing tower by acetic acid is 40°C or below similarly to the temperature in the absorbing tower by water and it
is preferable that the temperature is further 35°C or below.
Therefore, control and management over temperature and pressure (Fig. 1 [1]) at an outlet of a high-pressure absorbing tower (1) are required for reducing the cost of raw materials and for measures to prevent the corrosion by dew condensation, coolers (7, 9 in Fig. 1) for cooling washing liquid to the respective high-pressure absorbing towers are installed, the coolers respectively control the temperature of the gas by cooling the washing liquid.
The temperature of gas at the outlet of the absorbing tower becomes a temperature substantially close to the temperature of the washing liquid though the

temperature of gas at the outlet depends upon the temperature of the gas at an inlet, however, it is desirable that as to the control and management, the temperature of gas at the outlet is managed.
As the temperature of gas at the outlet of the high-pressure absorbing tower by water is directly related to a dew point in the turbine, it is managed in relation to the pressure of oxidation reaction waste gas as described above, however, it is desirable that the temperature of gas in the high-pressure absorbing tower by acetic acid is 35°C or below in consideration of the results of tests in the embodiments described later. In view of the effect (particularly, methyl acetate) of greatly reducing organic components by the additional installation of the high-pressure absorbing tower by acetic acid, the temperature of gas is sufficient if only it is 35°C or below, however, from a viewpoint of the management of a dew point of the waste gas, the same temperature as the temperature in the high-pressure absorbing tower by water or the temperature below it is desirable. [0038]
Next, for a heating source of oxidation reaction waste gas when the waste gas is conducted into the expansion turbine, it is desirable as a heat source for recovering power in a low-temperature range (150 →
approximately 0°C) according to the present invention and as a loop of energy to use steam of 5 kg/cm2G (5 in Fig. 1) generated in oxidation as described above, however, the heat source is not limited to the said generated steam. Waste steam of approximately 5 kg/cm2G that exists in a manufacturing system can be used and incidentally, spent waste steam in a refiner annexed to the manufacturing system can be effectively utilized. [0039]
However, as to relation between conditions in the expansion turbine and a risky range of dew condensation, a risky range by the adiabatic expansion at the heated
lowest temperature 140°C of the conducted waste gas is calculated, and the temperature and the pressure ([3], [5] in Fig. 1) of the gas at inlets of respective expansion turbines are important for temperature management. The
rise exceeding 140°C of the temperature of the said conducted gas leads to a safer range from dew condensation and has no problem.
While the expansion turbines designed on predetermined conditions are operated under the management of the temperature and the pressure, no problem occurs, however, when an abnormality occurs on the side of the aromatic dicarboxylic acid manufacturing system, outlet conditions ([4],[6] in Fig. 1) from the respective
expansion turbines are important to prevent dew
condensation.
[0040]
However, as known from Embodiment 5 described later, a fall of the temperature due to adiabatic expansion does not reach the calculated temperature in the actual operation of the expansion turbines. Concretely, the temperature merely falls to a temperature equivalent to approximately a little more than eighty % of calculated temperature fall width. Therefore, on this test condition, the turbine outlet temperature is higher than the calculated temperature by approximately 20 to 25°C and as the turbine outlet temperature enters in the safer range of dew condensation, the temperature in recovering power according to the method by the present invention can be managed in the sufficiently safe range.
Next, rotational energy recovered by the expansion turbines is directly transmitted to a electric generator and can be recovered as electric energy, however, the said manufacturing unit requires compressed air for oxidation, it is useful for the reduction of an energy cost to transmit the rotational energy to a shaft for rotating an electric motor or a steam turbine for driving an air compressor (10, 11, 12, 13 in Fig. 1) and to directly use the rotational energy for the rotational power, and the
system is very desirable as a system in which the energy
of the manufacturing unit is looped.
[0041]
The effluent gas which is treated and recovered in the above-mentioned process holds a moderate temperature and pressure, the dew point falls (a dew point under 0.1
kg/cm2G in the table: 0 to -15°C) owing to the great reduction of contained condensed substances, and the effluent gas can be used for inert gas for drying, transporting and mixing the above-formed dicarboxylic acid powder and for sealing the system without requiring any treatment as it is. For example, the effluent gas is desirable gas for the replenishment of gas for transporting crude aromatic dicarboxylic acid powder acquired in oxidation. [0042]
For material gas for purifying the crude aromatic dicarboxylic acid by a method such as a purifying process by hydrogenation and transporting the purified refined dicarboxylic acid powder to be a final product of the aromatic dicarboxylic acid manufacturing unit, when organic components (0.01 to 0.05 mol%) extremely minutely contained are to come into problem, the contained components are further removed by passing the material gas in an adsorption tower using activated alumina and others
for adsorbent and can be used, however, the used quantity and a life of the absorbent are more advantageous than those of gas by a well-known method (refer to Patent Publication 12: Japanese Patent Application Laid-Open Publication No.H06-304700) and the cost is reduced. Embodiments [0043]
Referring to concrete embodiments, the method according to the invention will be described further in detail below, however, the present invention is not limited to them. [0044] Embodiments 1 to 4 and Comparative example 1
Liquid phase oxidation is applied to para-xylene as
a material at the temperature of 19 5°C under the pressure of approximately 14.3 kg/cm2G using acetic acid for a solvent under the existence of cobalt, manganese and bromine by blowing air, a part of oxidation reaction waste gas is branched from a pipe for the oxidation reaction effluent gas discharged from a gas-liquid separator located in front of a high-pressure absorbing tower and annexed to an oxidation reactor of a plant for manufacturing terephthalic acid and is continuously extracted, the part of the oxidation reaction effluent gas is conducted into high-pressure absorbing tower test
equipment shown in Fig. 3, and a test of the effect of organic components contained in the effluent gas according to washing conditions in a high-pressure absorbing tower is made.
The high-pressure absorbing tower test equipment shown in Fig. 3 includes a high-pressure absorbing tower 1 and a high-pressure absorbing tower 2, in the high-pressure absorbing tower 1, the said high-pressure effluent gas branched from a lower part is washed by acetic acid injected from an upper part by a volumetric pump, and the effluent gas which the contained components are reduced is discharged from the upper part. As for the composition of components contained in the effluent gas, the material gas conducted in from the lower part is branched from the upper part if necessary, is sampled, and the contained organic components such as acetic acid, methyl acetate and para-xylene are analyzed. The temperature of the gas is measured and managed by a thermometer installed at an outlet of the absorbing tower. [0045]
The high-pressure absorbing tower is a 4" Sch 80 SUS pipe having a diameter of 97. 1 mm and is configured by a packed column 1.2 m high (a packed part: 1.0 m) as an upper packing part into which a 1/4 Raschig ring is packed and a lower liquid phase bubble column 0.7 m high (a
liquid level: 0.5 m). A cooler is attached to control the temperature of acetic acid when washing acetic acid is injected. Acetic acid used for washing is discharged to a washed liquid recovery pot via a siphon type communicating pipe, keeping a liquid level in the bubble column and the acetic acid is intermittently extracted from the recovery pot at suitable time.
A high-pressure absorbing tower 2 is a system provided with the same structure as the high-pressure absorbing tower 1, gas emitted from the high-pressure absorbing tower 1 is conducted from a lower part of the high-pressure absorbing tower 2, water for washing is injected from an upper part, and the similar operation and the similar test method to those of the high-pressure absorbing tower 1 are executed.
After the effluent gas emitted from the upper part of the high-pressure absorbing tower 2 is once stored in a mist separation pot, the pressure in the mist separation pot is measured, the effluent gas quantity is controlled by a valve at the back of the pot, and the emitted gas quantity is measured by a gas meter. [0046]
Table 6 shows a result of a test executed using the above-mentioned test equipment, a saturated water content at the temperature and pressure shown in Table 6 is
measured, and is shown in Table 6. A test of washing and absorption by only water performed as a comparative example 1 is made by bypassing the high-pressure absorbing tower 1 and directly conducting the gas branched from the terephthalic acid manufacturing unit into the lower part of the high-pressure absorbing tower 2.
Conditions in this test are as follows.

Operating pressure : 13.9 to 14.1 kg/cm2G
Gas flow rate : 120 Nm3/hr.
Acetic acid injected quantity: 7 kg/hr.
Water injected quantity : 3.5 kg/hr. The temperature of acetic acid and water in the high-pressure absorbing towers 1, 2 is equalized.
As the result shown in Table 6, the total quantity of acetic acid, methyl acetate and material not reacted yet (para-xylene) is approximately 1400 ppm (0.14 mol%) according to washing by only water in the high-pressure absorbing tower, the contained organic components (acetic acid, methyl acetate, material not reacted yet (para-xylene) are greatly reduced by washing by acetic acid and water and lowering the temperature of the washing liquid, and it is proved that the effect of washing organic contained substances by acetic acid is great. The saturated water content is also greatly reduced by a fall
of the temperature in the absorbing tower.
An acetic acid content has a tendency to slightly increase by washing by acetic acid of 40°C, however, an effect on the acetic acid content by washing by acetic acid is removed by lowering the temperature(35°C or below). [0047]
[Table 6] High-pressure absorbing tower test result

(Table Removed)
[0048] Embodiment 5
Condensable components in gas discharged from an
oxidation reactor of a terephthalic acid manufacturing unit (50,000Ton/Y) are condensed via a steam generator 2 and a condenser 3 as shown in a flow in Fig. 1, the oxidation reaction waste gas after condensate is separated in a gas-liquid separator 4 is washed by acetic acid and water, and power is recovered as follows using the high-pressure waste gas treated in a high-pressure absorbing tower (1) 6 (the 12-shelf tower 1000 f*10300H) installed to absorb and reduce components contained in the waste gas and a high-pressure absorbing tower (2) 8 (the 16-stage tray tower l000f*10500H).
Terephthalic acid manufactured by the manufacturing unit is produced at a rate of approximately 5.4 ton/hr., air required for the production is compressed up to 19.5 kg/cm2G by a compressor 10 (four-stage compression) and is supplied at a rate of approximately 13,300 Nm3/hr., and oxidation is regularly performed.
Washing liquid is cooled so that both outlet gas temperatures of the high-pressure absorbing towers are 35°C and is injected. As for the quantity, acetic acid is approximately 2.3 ton/hr. and water is approximately 0.5 ton/hr.
Power is recovered using two-stage expansion turbines 12, 13 designed on a design basis shown in Table 8 and pressure energy is recovered using steam of 5
kg/cm2G generated in a steam generator 5 annexed to the oxidation reactor for a heating source. The recovered energy is supplied to an electric motor by coupling a shaft for rotating the turbine to the electric motor 11 of an air compressor 10. Therefore, the recovered quantity of electric power is calculated based upon the retrenched quantity of current which the electric motor requires. [0049] [Table 7] Outlet gas pressure of high-pressure absorbing
tower and quantity of contained components

(Table Removed)
[0050]
The above-mentioned treatment of oxidation reaction waste gas and the recovery of energy have been satisfactorily performed for approximately half a year since the start without trouble in a rotor of the turbine. Table 7 and Table 8 show representative results of the quality of treated gas during operation and the operational conditions of the expansion turbines.
Table 2 shows that a content of organic components is approximately 300 ppm (0.03 mol%) and a dew point of the effluent gas (0.1 kg/cm2G) is -5°C.
Table 8 tells that the expansion turbine is provided with substantially designed performance and power can be
recovered at a low-temperature range of 30 to 150°C using the two-stage expansion turbines. Table 8 also tells that recovered low-pressure gas can be recovered at a higher temperature than the dew point (-5°C) of the oxidation reaction waste gas.
However, as to the temperature at an inlet and at the outlet of each turbine, the width of the fall of the temperature in actual operation is approximately 82.4% for the width of calculated values and operation in a safer
range for the dew point is actually performed.
Next, Table 9 shows the calculated quantity of electric power retrenched by the expansion turbines in the air compressor. Table 9 tells that power of expected quantity calculated in Table 8 could be substantially recovered.
The rotor and a casing of the expansion turbine are made of a stainless casting (equivalent to SCS14), the expansion turbine is disassembled after the operation is stopped and is checked for corrosion, and it was verified that the expansion turbines had no problem. [0051]
[Table 8] Operating conditions of power recovery turbine

(Table Removed)
Note: recovered power: estimated value in view of
operating conditions
Outlet temperature in ( ): calculated value in adiabatic expansion expression
Fall rate: temperature fall rate in actual operation for calculated value width between inlet temperature and outlet temperature of each turbine
[0052]
[Table 9] Power recovered quantity by expansion turbine

(Table Removed)
Note: power: value acquired from characteristic curve
based upon measured current
BRIEF DESCRIPTION OF THE DRAWINGS
[0053]
[Fig. 1] Fig. 1 is a system diagram showing a flow of the treatment of oxidation reaction waste gas in an aromatic dicarboxylic acid production process by a method according to the present invention and a flow in a pressure energy recovery system.
[Fig. 2] Fig. 2 shows relation between turbine inlet pressure and intermediate pressure (outlet pressure in a first stage = inlet pressure in a second stage) in two-stage expansion turbines using the ratio of recovered power for parameters (Wl: a calculated value of power recovered in the first stage, W2: a calculated value of power recovered in the second stage). [Fig. 3] Fig. 3 is a schematic diagram showing test equipment that makes an absorption test by injecting washing liquid (acetic acid and water) into a high-pressure absorbing tower.
EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS [0054]
1 Oxidation reactor, 2 Steam generator, 3
Condenser, 4 Gas-liquid separator, 5 Steam storage
tank, 6 High-pressure absorbing tower (1), 7
Acetic acid cooler, 8 High-pressure absorbing tower
(2), 9 Water cooler, 10 Air compressor, 11
Electric motor, 12 First-stage expander, 13
Second-stage expander, 14 Heater (1), 15 Heater
(2), 16 Condensed water storage tank.

CLAIMS
1. A method for treating oxidation reaction waste gas in a production process of aromatic dicarboxylic acid by applying liquid phase oxidation to aromatic dialkyl hydrocarbon as a material using acetic acid for a solvent under the existence of a metallic catalyst containing cobalt and manganese and bromine as the promoter at a temperature range of 185 to 205°C in an oxidation reactor by oxygen-containing gas, comprising the steps of:
cooling the oxidation reaction waste gas discharged from the above-mentioned oxidation reactor and separating subsequent to condensing the condensable components from the high-pressure oxidation reaction waste gas;
scrubbing the said resulting waste gas at 40°C or below in high-pressure absorbing towers by washing liquid on two-stage of acetic acid and subsequently water and reducing the contained components in it; and
subsequently, conducting the said treated oxidation reaction waste gas into two-stage expansion turbines after heating the said gas respectively conducted into the first-stage and the second stage of the said expansion turbine to be 140°C or a higher temperature with steam (approximately 5 kg/cm2G) and recovering the heat and the pressure energy of the conducted gas as power.
2. The method for treating oxidation reaction waste gas according to Claim 1,
wherein as to the treatment temperature in the high-pressure absorbing towers, it is desirable that the temperature of the gas at an outlet of the high-pressure absorbing tower is 35°C or below, it is more desirable that the temperature of the gas is 30°C or below, and it is the most desirable that the temperature of the gas is
25°C or below; and
the components contained in the oxidation reaction waste gas are reduced.
3. The method of treating oxidation reaction waste
gas according to Claim 1 or 2,
wherein the steam (approximately 5 kg/cm2G) from a steam generator annexed with the oxidation reactor is used for the steam used in Claim 1 or 2.
4. The method for treating oxidation reaction waste
gas according to any of Claims 1 to 3,
wherein the two-stage expansion turbines where a ratio of power recovered from the second-stage turbine to power recovered from the first-stage turbine is designed to be between 1 and 1.4 are used.
5. The method for treating oxidation reaction waste gas according to any of Claims 1 to 4,
wherein the low-pressure gas (0.1 kg/cm2G) acquired in any of Claims 1 to 4 is used for material gas for drying, transporting and mixing generated aromatic dicarboxylic acid powder without any treatment.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=ahwTtLnDiieGd/GPW8e43w==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

279186

Indian Patent Application Number

5334/DELNP/2009

PG Journal Number

03/2017

Publication Date

20-Jan-2017

Grant Date

13-Jan-2017

Date of Filing

19-Aug-2009

Name of Patentee

HITACHI PLANT TECHNOLOGIES, LTD.

Applicant Address

5-2, HIGASHI-IKEBUKURO 4-CHOME, TOSHIMA-KU, TOKYO 170-8466, JAPAN.

Inventors:

#	Inventor's Name	Inventor's Address
1	HARA NORIAKI	C/O HITACHI PLANT TECHNOLOGIES LTD., OF 5-2, HIGASHI-IKEBUKURO 4-CHOME, TOSHIMA-KU, TOKYO 170-8466, JAPAN.
2	ITO TOSHINOBU	C/O HITACHI PLANT TECHNOLOGIES LTD., OF 5-2, HIGASHI-IKEBUKURO 4-CHOME, TOSHIMA-KU, TOKYO 170-8466, JAPAN.
3	YAMAZAKI HATSUTARO	92-1-402, TAKANOSHIMIZU-CHO, SAKYO-KU, KYOTO-SHI, KYOTO 606-8102, JAPAN.

PCT International Classification Number

C07C 51/265

PCT International Application Number

PCT/JP2007/053785

PCT International Filing date

2007-02-28

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	NA	1900-01-01	IB