Title of Invention

A METHOD FOR RECOVERING SULFURIC ACID FROM BY-PRODUCTS OF 2-HYDROXY-4 METHYLTHIO BUTYRICACID (MHA)

Abstract

The invention relates to a method for recovering sulfuric acid from sulfurous by-products which arise in processes for preparing 2-hydroxy-4-methylthiobutyric acid (MHA) , wherein the sulfuric acid accumulates in a grade in which it can be directly re-employed in such processes For this purpose the combustion gas containing S02 is passed through a sulfuric H202 solution in which the S02 is oxidised to form sulfuric acid.

Full Text

-2- The invention relates to a method for recovering sulfuric acid from by-products containing sulfate which arise in processes for preparing 2-hydroxy-4-methylthiobutync acid (MHA) , wherein the sulfuric acid accumulates in a grade in which it can be directly re-employed in such processes. MHA is the hydroxy analogue of the essential amine acid methionine in racemic form and can be employed as an additive in feedstuffs, in particular for the rearing of poultry, but also in many other fields, above all in the form of its aqueous concentrates. The synthesis pathway that is used exclusively in technical applications starts out from methylmercaptopropior.aldehyde (MMP) which by addition of HCN is converted into the corresponding cyanohydrin (MMP-CH) which is then firstly catalysed with sulfuric solution to form MHA amide and in a further step is hydrolysed to form the hydroxy acid MHA, the sulfuric acid being converted into ammonium hydrogensulfate and, optionally, ammonium sulfate. Starting from MHA hydrolysate, which consequently contains, in addition to corresponding portions of water, ammonium hydrogensulfate/sulfate, there are various methods for -3- isolating MHA, which are described in summary form in German published application DE-OS 19 524 054 and in DE-PS 4 428 608. These methods involve either solvent extraction or precipitation steps or a combination of both for the purpose of separating MHA from the salt that has been formed at the same time. With each of these processes, in addition to the product MHA a portion of ammonium hydrogensulfate/sulfate corresponding to the quantity of sulfuric acid previously introduced is produced in the form of a solution or in a more or less pure solid grade. Since production of MHA is of the order of about 20,000 to 300,000 tpa per plant, quantities of sulfate salt also accumulate in quantities of approximately the same magnitude, which hitherto had to be disposed of in expensive manner or dumped. However, in particular the dumping of such gigantic quantities of salt is unjustifiable from the ecological point of view and is also not economically sensible as a result of further increasing charges. With a view to utilising the waste salts it is therefore particularly desirable to recover sulfuric acid from the sulfates and to return said sulfuric acid into the MHA process. A quite similar composite method is already known from the technical process for preparing methyl methacrylate (MMA), as described, for example, in US Patent No. 3,549,320. In this case acetone cyanohydrin is firstly converted with sulfuric acid into methacrylamide sulfate and subsequently with methanol into methyl methacrylate and ammonium bisulfate After separation of the MMA the residue containing sulfuric acid and ammonium bisulfate is transformed into sulfuric acid in a cracking contact plant. This method, which is known from the state of the art, is proposed in US Patent No. 5,498,790 with a view to the utilisation of waste salt in the MHA process. In this connection the raffmate solution arising from the MHA 970068 FU -4- extraction, principally containing ammonium bisulfate or ammonium sulfate, water and small quantities of organic byproducts, is firstly subjected to combustion in a cracking furnace together with a fuel at about 900 to 1,200 ° C to form S02, N2, 02, C02 and H20: 2 NH4HSO4 + 1/2 02 -> N2 +2 S02 + 5 E20 After cooling to 35 - 45 ° C with a view to condensation of portions of water and subsequent reheating together with oxygen, the hot combustion gas containing S02 is conducted into a contact furnace and the S02 is oxidised there on the V205 contact substance at temperatures of at least 420 ° C to form SO3: S02 + 1/2 02 -> S03. However, the economic implementation of this method is tied to minimum concentrations of S02. The contact gas containing sulfur trioxide is absorbed in oleum in accordance with a long-established procedure and, as a result of subsequent dilution with water, sulfuric acid is produced having a concentration of about 65 wt-% that is needed for the MHA process. The disadvantages of this method which is described in USPS 5,498,790 consist in the elaborate and complicated process technology for the contact furnace, wherein the stream of cracking gas containing S02 has to be conducted with oxygen over several contact trays and, after intermediate absorption of the SO3 which has already formed, conversion of the remaining sulfur dioxide is brought about on the last tray by intermediate coding and reheating to the reaction temperature. 970068 FU -5- Further disadvantages are the high reaction temperature of over 420 ° C, which in addition is attained by reheating the S02 combustion gas which has cooled down to 35 - 45 ° C, and also the non-quantitative conversion of the S02 portion in the course of catalytic oxidation in the contact furnace, which results in a residual S02 content in the waste gas leaving the contact furnace. This portion has to be lowered to the permitted legal limits by means of a suitable after treatment. The method described in US-PS 5,498,790 preferably operates with a minimum concentration of 70 wt-% of sulfate salt in the input stream of the combustion furnace, since a lower concentration results in an additional demand for fuel for evaporation of the high proportion of water and in higher inert-gas portions consisting of C02, N2 etc. In this case the S02 concentration falls so considerably that the contact furnace can no longer be operated effectively. In particular, solutions containing ammonium as described in US-PS 4,912,257, the maximum sulfate concentration of which is clearly below 70 wt-%, can be introduced in the form of a suspension only after further concentration. In operation this leads to greater difficulties such as blockages, for example, and is consequently not very practicable (cf. US Pat No. 5,498,790, bottom of col. 14 and top of col 15). Further disadvantages consist in the use of a heavy-metal catalyst that is tied to sufficiently high concentrations of S02, and the risk resulting therefrom of contamination of the product with catalyst residues and also with NOx which can be formed as a result of catalytic oxidation at the stated reaction temperatures. In the course of returning the sulfuric acid into the MHA process these contaminants may find their way into the MHA end product which is employed as feedstuff additive. By their nature they are undesirable therein. 970068 FU -6- The object of the invention is to make available a simpler method for recovering sulfuric acid from by-products containing sulfate that arise from the preparation of MHA, with the aid of which method, under conditions that are as mild as possible and without contamination by heavy metals and N0X, a grade of sulfuric acid is obtained1that can be directly re-employed in the hydrolysis stage of a process for the preparation of MHA. The method is furthermore to be capable of being employed irrespective of the S02 concentration of the combustion gases and, after the production of sulfuric acid, is to yield a waste gas that can be emitted into the atmosphere without further aftertreatment. The invention provides a method for recovering sulfuric acid from solutions or solids containing sulfate chat accumulate in the course of the sulfuric hydrolysis of MMP cyanohydrin, wherein the sulfates are converted in a combustion furnace into S02, said method being characterised in that the gas mixture containing S02 is passed through an aqueous sulfuric solution of H202 and is converted into sulfuric acid in accordance with the following overall equation: S02 + H202 -> H2S04 In this process it is possible to produce sulfuric acid in a concentration and in a grade that can be directly employed for the preparation of MHA by hydrolysis of MMP cyanohydrin. No detectable NOx contaminant can be found in the sulfuric acid prepared in accordance with the invention, so this sulfuric acid can be employed for the hydrolysis step without misgivings. According to the invention the procedure is preferably such that the combustion gas containing S02 is passed at a temperature between 0 and 100 ° C, preferably between 10 and 80 ° C, in particular 20 to 60 ° C, through a packed column in which an acidic aqueous solution of hydrogen 970068 FU -7- peroxide is located which is circulated. Hydrogen peroxide is preferably employed having a concentration between 10 and 90 wt-%, preferably between 20 and 80 wt-%, in particular between 30 and 60 wt-%, with the proviso that consumed hydrogen peroxide corresponding to the quantities of S02 in the desired or necessary concentration is made up by further metering. As is a matter of course to a person skilled in the art, the gas containing S02 is simultaneously cooled in this device and/or prior to entering this device. In a conventional manner of working a simple packed column is employed for the purpose of absorbing the sulfur dioxide. Said packed column is generally operated in the form of a cooled absorption circuit However, the at least two-stage absorption in one of the absorption columns that are known for these purposes is to be regarded as the preferred working form. The procedure is advantageously such that the combustion gas containing S02 is introduced into the column in the vicinity of the sump. The column packing is optionally circulated repeatedly. In a preferred working form the recycled liquid is brought into contact with the gas containing S02 until such time as practically no H202 is present any longer in the solution of the circuit. The desired H2S04 concentration is adjusted via the H202 concentration of the hydrogen-peroxide solution employed. This is between 10 and 90 wt-%, in particular between 20 and 80 wt-%, most preferably between 30 and 60 wt-%. According to the invention, working preferably takes place in such a way that a max. sulfuric concentration of 50 to 78 wt-% is attained. 970068 FU -8- In an advantageous working form the sulfuric acid that has been produced in this way is continuously discharged from the optionally multi-stage reactor. Depending on the S02 concentration in the combustion gas, differing quantities of H2S04 per unit of time are produced for a given volumetric flow of the combustion gas. In one technical working form it is advisable to employ a two-stage apparatus that consists of 2 absorption circuits (cf. Figure 2), the 1st absorption circuit being operated with a molar deficit of H2O2 with respect to the SO2 portion in the combustion gas introduced therein and the 2nd absorption circuit being operated with an excess of H2O2. The partly oxidised gas stream discharged from the 1st circuit is supplied to the 2nd circuit and the SO2 residue is converted therein. To this end the 2nd circuit is charged with fresh H2O2 in a molar excess relative to the SO2 portion that is fed in there. The sulfuric acid with residual H2O2 content that is located in the 2nd circuit is continuously discharged with a view to supplying the 1st circuit and therein the remaining H202 from the 2nd circuit is caused to react completely with the crude gas. H2O2-free sulfuric acid having the appropriately adjusted concentration is continuously obtained from the 1st circuit as recycling product. An insignificant proportion of H2SO3 which may possibly be contained in the sulfuric acid produced in this way, arising from unreacted SO2, is not critical for the intended purpose of hydrolysis of MMP cyanohydrin to form MHA, because no side reactions of any kind can be caused thereby. In particularly elegant manner it is possible for such regeneration of H2SO4 and recycling to be achieved within an appropriate cyclic process in which the sulfuric acid can be returned into the stage for sulfuric hydrolysis of MMP-CH to form MHA amide (cf. Figure 3) . 970068 FU -9- Furthermore, with a view to producing SO2 in the combustion furnace it is readily possible to make use of additional sulfur sources besides the sulfates from an MHA process On the one hand, elemental sulfur can be directly subjected to combustion, which is sensible in particular in the case of increased demand for sulfuric acid. On the other hand, however, sulfates from other processes can also be used concomitantly. To be considered in this connection are, in particular, wash solutions containing NH4HSO4 or (NH4)S04, arising for example from a hydrocyanic acid process. In such a process the residual NH3 in washers is washed out with aqueous sulfuric acid in the course of the production of HCN from CH4 and NH3. The solutions containing sulfate that are produced in this way can be directly utilised concomitantly in a combustion furnace, irrespective of their concentration. In the subsequent H2O2-oxidation stage sulfuric acid is again produced therefrom which can be returned into the HCN process. Since HCN is consumed for the purpose of producing MMP cyanohydrin and since, by reason of the fact that transfers of hydrocyanic acid are only possible to a limited extent for reasons of safety, as a general rule an appropriate HCN plant has to be situated in the immediate vicinity of an MHA plant, here too a direct composite process is possible. The latter is exceptionally advantageous from economic and ecological points of view and also from the point of view of modern safety engineering. Strong-smelling sulfurous waste waters or waste gases such as typically accumulate, in addition to sulfate, in an MHA process but which may also originate from other plants, preferably at the same location, can also be utilised concomitantly in the combustion furnace. Here, in contrast to the catalytic atmospheric oxidation of combustion gas containing SO2 at > 420 ° C which is described in US-PS 5,498,790, the amount of gas ballast produced at the same time by virtue of increased inert 970068 FU -10- portions or excess consumption of fuel in the course of combustion of dilute solutions is totally irrelevant. In the H2O2 oxidation, used in accordance with the invention, of such a combustion gas, arbitrarily low SO2 concentrations can also be transformed quantitatively into sulfuric acid with hydrogen peroxide, as is known from corresponding methods for waste-gas desulfurisation. In contrast with the method according to US-PS 5,498,790 there is no minimum requirement for the sulfate or sulfur concentration in the input stream to the combustion furnace. The combustion gas containing S02 that has cooled down to The claimed method is distinguished in particular by the fact that it avoids waste streams and the sulfuric acid employed for the purpose of hydrolysis can be completely recovered. Even defective charges containing MHA can be introduced as fuel into the combustion furnace The sulfur contained therein is converted into sulfur dioxide which, in accordance with the invention, is converted with H202 into sulfuric acid. 970068 FU -11- The following preparative Examples serve to clarify the subject of the invention further: Analytical methods of determination and definitions The contents of MHA monomer and of MHA amide were determined quantitatively in the process solutions by HPLC by comparison with an external standard (pure substance) The content of total MHA (MHAtot) = MHA monomer + MHA (dimers + oligomers) + MHA amide (optionally) was [sic] by titrimetric determination of the thioether function with KBr/KBrO3 standard solution and was expressed as the sum of the corresponding MHA monomer equivalents in [wt-%] or [g] or [mole] or [mole-%]. The content of MHA dimers + MHA oligomers (DIM + OLD was ascertained by calculating the difference of total MHA and MHA monomer + MHA amide (optionally) and was expressed as the sum of the corresponding MHA monomer equivalents in [wt-%] or [g] or [mole] or [mole-%]. The content of sulfuric acid was determined by alkalimetric titration, the H2O2 content was determined by a Merck rapid test. The water content was determined by titration according to Karl-Fischer, the nitrate, sulfite, sulfate and ammonium contents were determined by ion chromatography according to standard methods. 970068 FU -12- Description of method with reference to Figure 1 The schematic structure of the apparatus used for Example 1 is shown in Figure l. Said apparatus substantially consists of the following appliances. Storage tank for hydrogen peroxide B 1 Receiving tank for sulfuric acid formed B 2 Metering pump for hydrogen peroxide P 1 Circulating pump P 2 Glass absorption column with packings K 1 Heat exchanger for cooling W 1 The gas mixture, which corresponded in its composition to the crude gas typically resulting from the combustion, was passed at a temperature of 20 - 25 ° C into the lower part of an absorption column K 1 filled with packings. The aqueous sulfuric acid which was formed and which was recycled via the head of the column with the aid of the circulating pump P 2 served as absorption solution. The energy arising in the course of the reaction was captured by the heat exchanger W 1, and in this way the temperature in the circuit was limited to max. 3 ° C above the gas-inlet temperature. The necessary hydrogen peroxide for the reaction (50 wt-% H2O2) was pumped into the circuit from the storage tank B 1 with the aid of the metering pump P 1. The excess of H2O2 in the absorption liquid amounted to max. 10 mole-%, relative to the stream of SO2 introduced. The sulfuric acid formed was let off into the receiving tank B 2 in a side stream. 970068 FU -13- Example 1 The absorption column was filled with just so much water that the cooling and absorption circuit could be operated safely. 100 1/h of a gas mixture having the composition shown below were introduced into the column. The composition corresponds to that of a typical combustion gas from a combustion furnace for producing SO2 from sulfate-containing raff mate arising from an MHA process: 82 vol-% (3 33 mole/h) N2 7 vol-% (0 28 mole/h) C02 5 vol-% (0 20 mole/h} S02 6 vol-% (0 24 mole/h) 02 Addition of the 50 wt-% H202 (13.6g/h, 0.2 mole/h) was effected with low initial feed in a stoichiometric ratio. The gas-inlet temperature amounted to 2 0 ° C. The temperature in the cooling circuit did not rise above 23 ° C. After an interval of several hours the H2SO4 content in the sulfuric acid flowing off was determined. The H2SO4 concentration had risen to 74.7 wt-%. No loss of hydrogen peroxide could be detected. On average 2 6 g/h (0.2 mole/h) of a max. 75-wt-% H2SO4 were obtained in the receiving tank B2. The sulfuric acid produced had the following composition: Content of H2S04 74.7 wt-% Content of H20 24 85 wt-% Content of SO32- 1,570 mg/kg Content of H202 Content of NO3- 970068 FU -14- Example 2 306 g of 150-% hydrogen peroxide {4.5 mole) were submitted in the absorption apparatus and pumped in a circuit. At a max. temperature of 23 °C the gas mixture was introduced for several hours in a manner analogous to Example 1. 596 g of a sulfuric acid resulted having a content of 74.0 wt-% (4.5 mole). No loss of hydrogen peroxide could be detected. Example 3 236 g of 74.7 wt-% H2S04 (1.8 mole) from Example 1 were submitted in a reaction tank equipped with stirrer, internal thermometer and reflux condenser. Within 30 mm. 403 g (3.0 mole) of 97.7 wt-% MMP cyanohydrin were metered into the agitated reactor. The reaction temperature was maintained at 50 ° C during the inflow and the subsequent secondary-reaction time of 30 min. When the secondary reaction was over, the reaction mixture was diluted with 540 g water and immediately transferred into a 2-1 Büchi pressure reactor equipped with stirrer and internal temperature-measuring device. The reaction solution had a content of 22.8 wt-% MHA amide and 15.3 wt-% MHA and, subject to stirring, was heated up to 120 ° C and stirred for a further 3 hours at this temperature. After cooling to room temperature, 1,176 g MHA hydrolysate were isolated having the following composition: 0.33 wt-% ( 0.9 % of theor ) MHA amide 36.26 wt-% (94.6 % of theor.) MHA 1.71 wt-% (about 4 5 % of theor ) MHA dim+oli 38.30 wt-% (100 % of theor.) MHAtot 256 g (0.663 mole MHA) of the MHA hydrolysate were extracted with 2-times 100 ml methyl tert. butyl ether, the organic phases were combined and concentrated by 970068 FU -15- evaporation in a water-jet vacuum. The MHA high concentrate (100 g) primarily produced was diluted with 12 g water and analysed: MHAtot 87.7 wt-% 100 mole-% MHA monomer 77.2 wt-% 88.0 mole-% MHA -dim+oli 10.5 wt-% 12.0 mole-% H20 12.0 wt-% SO42- 0.2 wt-% S032- NO3- -16- We Claim: 1. Method for recovering sulfuric acid from sulfate salts accumulating in solution or in solid form in the course of sulfuric hydrolysis of methylmercaptopropionaldehyde (MMP) cyanohydrln, wherein the sulfates are converted into S02 in a combustion furnace, characterized in that the gas mixture containing SO2 is passed through an aqueous sulfuric solution containing H2O2 and the SO2 is oxidized into sulfuric acid at a temperature between 0 and 100°C. 2. Method as claimed in claim 1, wherein the concentration of the hydrogen peroxide employed is between 10 and 90 wt-%, preferably between 20 and 80 wt-%, most preferably between 30 and 60 wt-% 3 Method as claimed In claim 2, wherein the concentrations of the hydrogen peroxide and of the S02 are so chosen that a 50 to 78-%, preferably a 60 to 75-%, aqueous sulfuric acid is obtained 4. Method as claimed in claim 1, wherein in the course of the oxidation of SO2 the temperature is preferably between 10 and 80°C, most preferably between 20 and 60°C. 5 Method as claimed in claim 1, wherein the oxidation of S02 with H2O2 is optionally carried out in a one-stage, two-stage or multi-stage apparatus. 6. Method as claimed in claim 1, wherein in addition to the sulfates from an 2-hydroxy-4-methyithiobutyric acid (MHA) process, sulfate portions from other sources and/or sulfurous waste gases or waste waters from an 2-hydroxy-4-methylthiobutync acid (MHA) process or other processes and/or organic waste from an 2-hydroxy-4-methyithlobutyric acid (MHA) process or other processes are supplied to the combustion furnace Dated this 3rd day of December 1998. The invention relates to a method for recovering sulfuric acid from sulfurous by-products which arise in processes for preparing 2-hydroxy-4-methylthiobutyric acid (MHA) , wherein the sulfuric acid accumulates in a grade in which it can be directly re-employed in such processes For this purpose the combustion gas containing S02 is passed through a sulfuric H202 solution in which the S02 is oxidised to form sulfuric acid.

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