Title of Invention

A METHOD OF PRODUCING BLACK LIQUOR FROM A PULPING PROCESS

Abstract

The invention relates to a method of producing black liquor from a pulping process for evaporation and combustion processes, comprising the steps of treating the black liquor in a reactor vessel having a vapour space a temperature of 150-350°C, characterisd in that the black liquor is introduced into the vapour space of the reactor vessel, which vapour space is located above the liquid level, and the liquor is brought into direct contact with steam when the liquor and steam flow counter-crrently in order to heat the liquor to said temperature, the difference between the steam saturation and said temperature being 0-20°C, and the liquor is maintained in the vessel at said temperature for a period of time.

Full Text

The invention relates to a method of producing black liquor from a pulping process in a reactor vessel at a temperature of 150-350C. When evaporating black liquor to a high (60-75 %) dry-solids content, the viscosity of the liquor increases rapidly. At the same time its evaporability decreases remarkably as the black liquor sticks to the heat surfaces thus impairing the heat transfer from the heat surfaces to the black liquor, which results in a decrease in the efficiency of the evaporator. It may also result in overheating of the heat transfer surfaces. The viscosity of the nonwood black liquors is significantly higher than that of the wood black liquor. In the digestion of nonwood fibers, such as bagasse, bamboo, reed, wheat or rice straw, the conditions are milder than in the digestion of wood fibers, e.g. the cooking temperature is usually below 160C, whereby the majority of the polysaccharides does not degrade, which is at least one reason for a higher viscosity. Due to the high viscosity, the evaporability of the black liquor is poor and the dry substance content of only about 40 % is achieved in the evaporation plant. It is, however, advantageous to evaporate the liquor so as to have an as high dry-solids content as possible for further treatment. The effective combustion value of the black liquor is then higher than that of a more dilute liquor and the volume of the steam generated by the recovery boiler is respectively greater. Canadian Patent 1,288,203 discloses a method of decreasing the viscosity of black liquor in which the black liquor is exposed to a temperature above the cooking temperature for a period of time in order to reduce the size of long polymers in the liquor and thus reducing the visicosity. The treatment temperature required is typically 180-190 °C. By pressure heating the black liquor the viiscosity thereof decreases so that the liquor becomes easily pumpable. The decrease of viscosity is irreversible. Thus the liquor may be evaporated to a considerably higher dry substance content than is possible without the heat treatment. Thej transfer of liquor having a high dry substance content to a recovery boiler is possible, too. On the other hand, the described pressure heating can also be used to remove selectively sulpliur from sulphur-containing waste liquors, e.g. blaclc liquors of kraft pulping processes. During the treatment sulphur is removed as a gas containing dimethyl sulphide and nethyl mercaptan, as described in US Patents 5,277,759 and 5,:26,433. Canadian Patent 2,059,2 64 discloses ci system for performing the heat treatment. The thermal system includes n+1 heat exchangers in series and n flash tanks in series.. The system is interposed between two subsequent effects of a multiple effect evaporator for black liquor. The liqaor is discharged from one of the evaporator effects and pumped through the series of heat exchangers into a reactor vessel. In the (n+l)th heat exchanger the liquor is brought up to the reactor vessel temperature by live steam from an external source. After the reactor vessel, the liquor is flash cooled through the series of flash tanks and returned to the evaporator train. Each of the flash tanks is connected to one of the first n heat exchangers. The flash vapour is thus used to accomplish the bulk of the heating up tc thereactor vessel temperature. In the (n+l)th heat exchanger the black liqucr can be heated using live steam indirectly or directly to the heat treatment temperature. Normally the mills have three kinds of live steam available: high, medium ani low pressure steam. The respective typical pressure ranges are 50-8 0, 10-15 and 2-5 bar(g). With high pressure steam the required final heat treatment temperature, e.g. 13C-190°C, can be easily reached. However, the use of high pressure steam means high investment and operation costs. As; the saturation temperature of medium pressure steam is normally only around 190°C its availability is not good as that of high pressure steam. At the highest heat treatment temperatures the availability of high pressure steam may also be; limited. It should be noted that the temperature can have a strong effect on the black liquor viscosity within oven a short temperature range. In the range of 180-190°C a few degrees of temperature may be of great significance. This means that a heat treatment system should be arranged so that the pressure of heating steam, such as medium pressure steam, may be utilized as efficiently as possible to obtain a good heat treatment effect. So far the heat treatment units (indirect or direct) are based on a concept that the whole system is kept full of liquid (liquor) . The pressure is adjusted with a valve after the reactor. The desired reactor temperature is maintained by means of a stean ccntrol valve. In the indirect system the liquor pressure is kept with a sufficient margin above the boiling point of the liquor. The steam pressure does not impose any constreints to liquor pressure as they are not in direct contact with each other. The maximum liquor temperature is lower than the saturation temperature of live steam. This difference, temperature approach, is specific for the live steam heat exchanger used depending on its effectiveness. The live steam heater is susceptible for fouling on the liquor side. Viher it happens, the difference, the temperature approach, inc;reases. All this means that the pressure level of the medium pressure steam may not be sufficient to guaremtee a good heat treatment effect. The direct steam usage is carried out by a ievice in which steam is compelled to be mixed with the liquor. At this point the pressure of the liquor has to te adjusted below the steam pressure. There may be some pressure variation in the steam line, which require a considerable margin. On the other hand the pressure at the highest point must be kept above the boiling temperature of the liquor. So the liquor pressure control requires attention in order to maintain an efficient operation. Also with this system the maximum liquor temperature does not reach the potential of the steam. As the whole volume of the unit is kept full of liquid, harmful hydraulic shocks are expected. Depending on the system there may arise vibration with pipe lines or devices. It is an object of the invention is to provide such a system, in which the pressure of heating steam can be as completely as possible utilized for heating the black liquor to a desired heat treatment temperature. The above mentioned object of the invention is realized by a method, the main characteristic feature of which is that the black liquor is introduced into the vapour space of the reactor vessel and brought into direct contact with steam in order to heat the liquor to said temperatun, whereafter the liquor is maintained in the vessel at said temperature for a period of time. According to the invention the black liquor is heated in direct heat exchange with steam in the vapour space of the reactor vessel. The black liquor is fed to the vessel from the top thereof. When flowing or falling downward encounters steam which has been led to the vessel at a lower level than the liquor. The invention allows the use of heating steam having the saturation temperature close to the desired heat treatment temperature. The difference between the saturation temperature and the heat treatment temperature can be 0-20°C, even 0-10C, and good heat treatment effect is still achieved. Accordingly the present invention provides a method of producing black liquor from a pulping process for evaporation and combustion processes, comprising the steps of treating the black liquor in a reactor vessel having a vapour space at a temperature of 150-350°C, characterised in that the black liquor is introduced into the vapour space of the reactor vessel, which vapour space is located above the liquid level, and the liquor is brought into direct contact with steam when the liquor and steam flow counter-currently in order to heat the liquor to said temperature, the difference between the steam saturation and said temperature being 0-20C, and the liquor is maintained in the vessel at said temperature for a period of time. The invention is described in more detail in the following with reference to the accompanying drawings, in which FIG. 1 shows schematically a system for carrying out the process according to the invention, and FIG. 2 a and b show the upper part of a reactor vessel, where a contact zone for black liquor and heating steam is arranged. According to FIG. 1 weak black liquor from the digester house of a pulp mill or black liquor from one of the evaporation effects of the multiple effect evaporator system of the mill is transferred through a line 10 to a system comprising n heat exchangers HEXI-HEXIV, n flash tanks FTI -FTIV and a reactor vessel 20 including a vertical shell 21. The black liquor can be heat treated prior to evaporation, during or after evaporation. Usually the heat treatment is carried out between the last two evaporation effects. The retention time in the reactor vessel is typically 1-60 min, preferably 5-30 min. The heat exchangers HEXI-HEXIV are connected in series through lines 12, 14 and 16 and they operate at successive increasing temperatures. The black liquor is successively heated by vapour in the heat exchangers. All heat exchangers are heated by vapours generated in the flash tanks FTI -FTIV by successive expansion of the heat treated black. The flash tanks operate at successive decreasing temperatures and pressures. The liquor is heated indirectly in the first two heat exchangers, HEXI and HEXII. At higher temperatures there may appear fouling of heat transfer surfaces on the liquor side. Therefore the black liquor is heated directly in the heat exchangers, HEXIII and HEXIV. It is;, of course, possible to use indirect heat exchangers if fculing is not a problem. The black liquor from the last heat exchanger HEXIV in the series is transferred to the reactor vessel through a line 18. Heating steam is fed to the vessel 20 through a line 22. In the upper part of the vessel there is a vapiour space 24 where the black liquor is brought into direct contact with steam above the liquid level 26 in order to heat the black liquor to a desired temperature in the range of 150-350°C. Vent vapours from the reactor vessel and the heat exchangers HEXI-HEXIV are discharged through a line 42 to a gas purification system of the mill. The vent: vapour from the reactor 20 can be led through some of the heat exchangers HEXI-HEXIV in order to maximize the heat recovery. After a suitable retention time (typically 1-60 min) in the reactor vessel 2 0 the black liquor is transferired through a line 28 to the last of the flash tanks, FTI7. The flash tanks are operatively connected in series through lines 30, 32, and 34, and the black liquor flows through ihem counter-currently to the flow of black liquor through the heat exchangers HEXI-HEXIV. Each flash tank includes a vapour outlet which, through a line 54, 56, 58 or (50, is connected to a corresponding one of the heat exchangers according to the rule: the first flash tank FTI is connected to the first heat exchanger HEXI, the second flash tank FTII to the second heat exchanger HEXII and ultimately the nth flash tank to the nth heat exchanger. Thus, all the heat exchangers are heated by black liquor vapours. The vapours condense in the indirect heat exchangers; and the condensate is passed to the next upstream heat exchanger in relation to the black liquor flow at a lower pressure. The black liquor which has been thermally treated is then returned through a line 36 to the multiple effect evaporator system to be further concentrated or a combustion device. The reactor vessel 20 has a level controller 46 thereon coupled to a level control valve 48. The retention time of the liquor is adjusted by means of this control. The vessel has also a temperature controller 50, but the: ccmtroller can be left out, if wanted. If the maximum liquor temperature is tried to roach as it is normally in the case with medium pressure steam, the steam control valve (if it exists) is kept totally open. Then essentially the same pressure exists in th In a situation when the steam pressure for seme reason drops and the reactor pressure exceeds it a back pressure valve 52 closes. If the steam pressure stays longer at the lower level, it may take some time (exothermic reaction) before the reactor pressure is levelled down to steam pressure. After this the back pressure valve opens and the reactor pressure is adapted according to the new situation. In this way the system to a great extent controls itself. The factors controlling the heat treatment effect are the reactor temperature and the retention time. In some, situations there may be a desire to limit the effect. In this case if we do not have a temperature control system, the retention time (liquid volume) can ba used for this. In a system with the whole volume full of liquor the possibility does not exist. The system of the invention is not especially susceptible to liquid shocks. They are damped because of the relatively large vapour volume acting as a buffer againsit the hydraulic shocks. In general it can be stated that the system is easy to be operated and controlled. FIG. 2 a shows in detail an embodiment for the contact zone for liquor and steam in the vapour space of the reactor vessel. There are mounted inclined baffle plates 38, or trays, one above the other. Black liquor is introduced into the top of the vessel 20, from which it flows from plate to plate down the upper part of the vessel. Heating steam in the line 22 is fed below the plates 38, which are disposed so as to leave space in between so that the steam can flow up toward the top of the vessel and is discharged through the line 42. Then the steam comes into a counter-current contact with the black liquor which is thus heated up to the heat treatment temperature. The baffle plates are connected to the shell 20. At least one of the edges of the plate is provided with a vertical lip 40 over which the liquor falls to the next plate during which very intimate contact between the counter-current liquor and steam takes place. FIG. 2b discloses another embodiment of th It is clear that the invention is not limited -to the plate systems of FIG. 2. The contact system for liquor and steam may be any system known in the art in which systein efficient contact between liquid and steam can be brov.ght. about. The system of the present invention providos at least the following advantages. The pressure of heating steam is fully exploited, whereby steam having a saturation temperature close to the heat treatment temperature can be used to Jaeat black liquor. Of course, steam having a saturation temperature substantially higher than the heat treatment temperature can also be utilized i the claimed method. The apparatus for realizing the invention is simple and inexpensive, and easy to use and adjust. It is also easy to dimension due to the low design pressurs. It is very applicable at small plants. Vibrations in the heat treatment apparatus are also eliminated. The present invention provides a method of heat treating black liquor in an efficient way. Variations of the method are possible within the inventive scope defined in the accompanying claims. For example, the origin of black liquor is not limited to what was described in connection with the invention. WE CLAIM: 1. A method of producing black liquor from a pulping process for evaporation and combustion processes, comprising the steps of treating the black liquor in a reactor vessel having a vapour space at a temperature of 150-350°C, characterised in that the black liquor is introduced into the vapour space of the reactor vessel, which vapour space is located above the liquid level, and the liquor is brought into direct contact with steam when the liquor and steam flow counter-currently in order to heat the liquor to said temperature, the difference between the steam saturation and said temperature being 0-20C, and the liquor is maintained in the vessel at said temperature for a period of time. 2. The method as claimed in claim 1, wherein the liquor is preheated in n heat exchangers, and at least the last of them, prior to feeding the black liquor to the vessel, is a direct heat exchanger. 3. The method as claimed in claim 2, wherein the heat treated black liquor is discharged from the reactor vessel to n flash tanks arranged in series from first to last, wherein each of said flash tanks, the heat treated liquor is partially vaporized; and vapour formed in each of said flash tanks is utilized to heat the liquor in the heat exchangers, wherein the direction of flow of the heat treated liquor is counter to the direction of flow of the liquor in the heat exchangers, and wherein the vapour from the first of the series connected flash tanks supplies heat to the last direct heat exchanger, and the vapour from the last of the series connected flash tanks supplies heat to the first heat exchanger. 4. The method as claimed in claim 3, wherein the energy of the steam vented from the reactor vessel is recovered in the heat exchangers. 5. A method of producing black liquor from a pulping process, substantially as herein described with reference to the accompanying drawings.

Documents:

0142-mas-96 others.pdf

0142-mas-96 abstract.pdf

0142-mas-96 claims.pdf

0142-mas-96 correspondence others.pdf

0142-mas-96 correspondence po.pdf

0142-mas-96 description (complete).pdf

0142-mas-96 drawing.pdf

0142-mas-96 form-1.pdf

0142-mas-96 form-13.pdf

0142-mas-96 form-26.pdf

0142-mas-96 form-4.pdf

0142-mas-96 form-6.pdf

0142-mas-96 petition.pdf