PROCESS FOR PURIFYING ALKANESULFONIC ACIDS

Abstract

The invention relates to a process for purifying alkanesulfonic acids which comprises the steps of: (a) distilling a melt comprising crude alkanesulfonic acid (1) to completely or partly remove low boilers, wherein the low boilers are drawn off at the top of a distillation column (3) or of a one-stage evaporation apparatus and a material stream (7) comprising alkanesulfonic acid, high boilers and residual low boilers is withdrawn at the bottom of the distillation column (3) or of the one-stage evaporation apparatus, (b) sending the stream (7) comprising alkanesulfonic acid, high boilers and residual low boilers into a melt crystallization (9) as the starting melt to form crystals of the alkanesulfonic acid, of hydrates of the alkanesulfonic acid or of a mixture of both suspended in mother liquor, (c) performing a solid-liquid separation to remove the crystals from the mother liquor, (d) optionally washing the crystals to remove mother liquor adhering to the crystals.

Claims

1. A process for purifying an alkanesulfonic acid, the process comprising: (a) distilling a melt comprising crude alkanesulfonic acid to completely or partly remove low boilers, wherein the low boilers are drawn off at a top of a distillation column or a one-stage evaporation apparatus and a material stream comprising alkanesulfonic acid, high boilers and residual low boilers is withdrawn at a bottom of the distillation column or the one-stage evaporation apparatus, (b) sending the material stream into a melt crystallization as a starting melt to form crystals of the alkanesulfonic acid, hydrates of the alkanesulfonic acid or a mixture of both of the alkanesulfonic acid and the hydrates of the alkanesulfonic acid suspended in mother liquor, (c) performing a solid-liquid separation to remove the crystals from the mother liquor, and (d) optionally washing the crystals to remove mother liquor adhering to the crystals.

2. The process according to claim 1, wherein the mother liquor after removal of the crystals and/or the mother liquor generated is/are at least partly recycled into the melt crystallization or into the distilling.

3. The process according to claim 1, wherein the mother liquor after removal of the crystals and/or the mother liquor generated is/are at least partly sent into a high boilers removal to remove high boilers from the mother liquor.

4. The process according to claim 3, wherein the mother liquor after removal of the high boilers is recycled into the distilling.

5. The process according to claim 3, wherein the mother liquor after removal of the high boilers is partially condensed and a condensed portion of the mother liquor is recycled into the distilling and an uncondensed portion of the mother liquor is discharged from the process as low boilers.

6. The process according to claim 1, wherein the alkanesulfonic acid is methanesulfonic acid.

7. The process according to claim 1, wherein the low boilers comprise water or sulfur trioxide.

8. The process according to claim 1, wherein the material stream sent to the melt crystallization comprises in addition to water or sulfur trioxide not more than 6 wt % of further impurities based on a total mass of the material stream.

9. The process according to claim 1, wherein the melt crystallization is carried out at a temperature of from 15 C. to 19 C.

10. The process according to claim 1, wherein the material stream comprises at least 76 mol % of alkanesulfonic acid based on a total amount of alkanesulfonic acid and water in the material stream.

11. The process according to claim 1, wherein the material stream comprises at least 87 mol % of alkanesulfonic acid based on a total amount of alkanesulfonic acid and sulfur trioxide in the material stream.

12. The process according to claim 10, wherein the melt crystallization is carried out at a temperature of from 10 C. to 19 C.

13. The process according to claim 1, wherein the material stream comprises 31 to 75 mol % of alkanesulfonic acid based on a total amount of alkanesulfonic acid and water in the material stream.

14. The process according to claim 13, wherein the melt crystallization is carried out at a temperature of from 15 C. to 12 C.

15. The process according to claim 1, wherein the optionally washing comprises washing the crystals with molten crystallizate.

16. The process according to claim 15, wherein the molten crystallizate for washing the crystals has a temperature of from 0.1 C. to 15 C. above a solidification temperature of an alkanesulfonic-acid-containing crystallizate.

17. The process according to claim 4, wherein the material stream withdrawn from the distilling is cooled before being sent to the melt crystallization and the mother liquor which is recycled into the distilling is heated.

18. The process according to claim 17, wherein heat from the material stream which is to be cooled is transferred to the mother liquor which is to be heated.

19. The process according to claim 1, wherein the melt crystallization is a suspension crystallization or a layer crystallization.

20. An alkanesulfonic acid produced by the process according to claim 1.

21. An alkanesulfonic acid hydrate produced by the process according to claim 1.

22. The process according to claim 11, wherein the melt crystallization is carried out at a temperature of from 10 C. to 19 C.

Description

[0086] Exemplary embodiments of the invention are shown in the figures and are more particularly described in the description which follows.

[0087] FIG. 1 shows an overview representation of a process for alkanesulfonic acid production.

[0088] FIG. 2 shows a process flow diagram of the process according to the invention.

[0089] An overview diagram of a process for alkanesulfonic acid production is depicted in FIG. 1.

[0090] A first process step 2 is supplied with starting materials 4 for the production of alkanesulfonic acid. These starting materials 4 depend on the production process. In the first process step 2 crude alkanesulfonic acid 1 is produced. Arranged downstream of the first process step 2 is a low boilers removal 3 in which low boilers are removed from the crude alkanesulfonic acid. A material stream 5 comprising low boilers and a material stream 7 comprising alkanesulfonic acid, high boilers and residual low boilers are withdrawn from the low boilers removal 3. In the low boilers removal 3 an alkanesulfonic acid concentration suitable for crystallization is established in the material stream 7 comprising alkanesulfonic acid, high boilers and residual low boilers.

[0091] The material stream 7 comprising alkanesulfonic acid, high boilers and residual low boilers is sent to a melt crystallization 9 as the starting melt. In the melt crystallization 9 the starting melt is part crystallized and a suspension 12 of mother liquor and a crystalline solid phase of alkanesulfonic acid is generated which is sent to a solid-liquid separation 14. In the solid-liquid separation 14 the mother liquor 10 is largely separated from the crystalline solid phase. The solid phase is sent to a washing means 11 in which a washing liquid 16 is used to remove residual mother liquor adhering to the crystals and the product 13 is obtained. The washing liquid is preferably recycled into the melt crystallization or the low boilers removal.

[0092] The mother liquor 10 removed in the solid-liquid separation 14 is preferably recycled into the process for purifying alkanesulfonic acids. Possible suitable recycling procedures for the mother liquor 10 are shown by way of example. Thus, for example, in a crystallization cycle 18 mother liquor 10 may be completely or partly recycled into the melt crystallization 9. As an alternative or in addition mother liquor 10 may be completely or partly recycled into the low boilers removal 3 in a distillation cycle 20. It is also possible to remove a high boilers purge stream 26 in a high boilers cycle 22 in a high boilers removal 19 and to recycle the stream thus worked up by removal of the high boilers into the low boilers removal 3 or the melt crystallization 9.

[0093] FIG. 2 shows a process flow diagram of the process according to the invention.

[0094] A melt 1 which comprises crude alkanesulfonic acid and originates from a process for producing alkanesulfonic acid is fed into a low boilers removal 3 for purification. In the low boilers removal 3 low boilers are removed from the crude alkanesulfonic acid. The low boilers removal 3 may be carried out using a distillation column for example. A material stream 5 comprising low boilers is obtained at the top of this distillation column employed for low boilers removal 3. The material stream 5 comprising low boilers is removed from the process and worked up separately or sent for disposal. A material stream 7 comprising alkanesulfonic acid, high boilers and residual low boilers is withdrawn at the bottom of the distillation column employed for low boilers removal 3. Said stream is sent to a melt crystallization 9 as the starting melt. As described hereinabove the melt crystallization 9 may be, for example, a suspension crystallization or a layer crystallization and the melt crystallization may be carried out in any desired crystallizer suitable for the particular crystallization process.

[0095] A material stream comprising alkanesulfonic acid crystals and alkanesulfonic-acid-depleted mother liquor is withdrawn from the melt crystallization 9. This material stream is sent to a solid-liquid separation and a washing means 11 in which the alkanesulfonic acid crystals are removed and washed to remove the mother liquor and impurities adhering thereto. To this end it is possible, for example, to run the alkanesulfonic acid crystals suspended in mother liquor in countercurrent to a washing liquid, for example molten crystallizate. The washing liquid washes off the mother liquor and impurities from the crystals. After separation of the crystals from the washing liquid via a solid-liquid separation method a pure product 13 is obtained and withdrawn from the process. If desired, the alkanesulfonic acid obtained may subsequently be diluted to a desired concentration by addition of water.

[0096] The washing liquid comprising mother liquor and impurities is likewise withdrawn from the apparatus for solid-liquid separation and washing means 11. Since this material stream still comprises a large proportion of alkanesulfonic acid, said stream is preferably not removed from the process.

[0097] It is thus possible, for example, to recycle the washing liquid and alkanesulfonic-acid-depleted material stream withdrawn from the washing means 11 into the melt crystallization 9 as recycle stream 15. This may comprise recycling either the entire stream or merely a substream.

[0098] To counter an impermissible level of accumulation of low boilers in this recycle stream, the washing liquid and the alkanesulfonic-acid-depleted stream withdrawn from the washing means may be recycled into the low boilers removal 3 as recycle stream 17. This is advisable particularly when the melt crystallization 9 affords pure alkanesulfonic acid rather than the monohydrate of alkanesulfonic acid as product. By contrast, in the crystallization of the monohydrate a suitable low boilers concentration may alternatively be established by adjusting the bottoms temperature of the distillation column employed for low boilers removal 3. It will be appreciated that another option comprises recycling a substream of the material stream comprising the washing liquid and alkanesulfonic-acid-depleted mother liquor into the melt crystallization 9 as recycle stream 15 and a further substream into the low boilers removal 3.

[0099] To remove high boilers from the washing liquid and alkanesulfonic-acid-depleted material stream withdrawn from the washing means, at least some of the mother liquor, the so-called high boilers purge 24, is sent to an evaporator 19 or otherwise utilized. In the evaporator 19 alkanesulfonic acid and low boilers are evaporated and drawn off as vapors 21. The unevaporated portions are withdrawn from the evaporator in liquid form as worked up high boilers purge stream 26 and sent for further use or disposal.

[0100] The vapors 21 withdrawn from the evaporator 19 may be recycled into the process for purifying alkanesulfonic acids or may be sent to another application, for example the production of 100 wt % MSA. In the embodiment depicted here the vapors 21 flow into a partial condenser 23. In the partial condenser 23 the alkanesulfonic acid present in the vapors condenses and is recycled into the low boilers removal 3 as condensate 25. Without partial condensation the vapors can only with difficulty be passed directly into the low boilers removal 3 in gaseous form because the evaporator 19 and the distillation column employed for low boilers removal 3 are typically operated at different pressure levels. An alternative recycling option for the condensed vapors is use, after cooling, as washing liquid 16 for the washing means 11.

[0101] The uncondensed portion comprising low boilers is withdrawn from the partial condenser 23 in gaseous form and drawn off from the process together with the low-boilers-comprising material stream 5 from the distillation column 3.

[0102] When only some of the material stream which comprises washing liquid and depleted mother liquor and is withdrawn from the washing means is fed into the evaporator 19, the remainder may be recycled either into the melt crystallization 9 as recycle stream 15 or into the distillation column 3 as recycle stream 17 or else partly into the melt crystallization as recycle stream 15 and partly into the distillation column 3 as recycle stream 17.

[0103] For reasons of energy economy it is advisable to use the smallest possible amount of high boilers purge 24. The lower limit for reducing the amount of high boilers purge 24 is the amount at which just sufficient amounts of high boilers are still discharged from the cycle to keep the concentration of said high boilers below the concentration above which they impede crystallization.

Examples

[0104] Crystallization of Methanesulfonic Acid and Methanesulfonic Acid Hydrate

[0105] Starting melts 1 to 4 as per table 1 comprising methanesulfonic acid and water as well as defined impurities, for example sulfuric acid, nitric acid, methyl methanesulfonate and chlorine (as total chlorine), were charged at atmospheric pressure and room temperature into a jacketed stirred vessel of 1 l in volume and 150 mm in diameter comprising a close-clearance helical stirrer. The starting melts were then cooled to the respective final temperature specified in table 1 at a cooling rate of 1 K/h.

[0106] Crystals were formed during cooling which were kept in suspension by stirring at a rotational speed of 180 min.sup.1. Pure methanesulfonic acid was crystallized from starting melt 1 and pure methanesulfonic acid hydrate was crystallized from each of starting melts 2 to 4. The obtained crystals in the suspension were removed on a pressure filter at the respective end temperature and washed with a 70% methanesulfonic acid-water solution to partially remove the mother liquor adhering to the crystals. An amount of washing liquid identical to the amount of crystallizate was chosen.

[0107] The proportion of water in the washed crystals and in the mother liquorapart from in the crystals and in the mother liquor from starting melt 4was determined by Karl-Fischer titration. The proportions of sulfuric acid and nitric acid in the washed crystals and in the mother liquor were captured by ion chromatography (IC). The proportion of methyl methanesulfonate in the washed crystals and in the mother liquor was determined by gas chromatography (GC). The proportion of total chlorine in the washed crystals and in the mother liquor was determined by Coulometry.

[0108] The portions of water and impurities are summarized in tables 2 and 3. Table 4 reports the distribution coefficients of the impurities. The distribution coefficient of a component is the proportion of the component in the crystals divided by the proportion of the component in the mother liquor. Distribution coefficients smaller than 1 show that the proportions of impurity component in the crystals are smaller than the proportion in the mother liquor, i.e. the impurity is depleted by crystallization.

[0109] The results reported in table 4 show that, proceeding from the exemplary starting melts 1 to 4, impurities can be removed from methanesulfonic acid and methanesulfonic acid hydrate by crystallization.

[0110] It is apparent that depending on purity requirements a further depletion of the impurities in the crystallization is achieved by crystallizing the crystallizate not only once but repeatedly and/or washing it in cocurrent or in countercurrent.

TABLE-US-00001 TABLE 1 Compositions and amounts of employed starting melts 1-4 and the respective associated end temperature in the crystallization starting melt 1 2 3 4 total amount [g] 1250 1135 1390 1080 methanesulfonic acid 96.02 81.59 79.55 83.32 [wt %] water [wt %] 1.96 15.51 15.21 15.68 sulfuric acid [wt %] 1.97 1.90 4.70 nitric acid [wt %] 1 0.54 methyl methanesulfonate 500 [ppm] total chlorine [wt %] not determined end temperature [ C.] 4 6 4 4

TABLE-US-00002 TABLE 2 Water and impurities in the mother liquor starting melt 1 2 3 4 water [wt %] 3.35 16.08 15.4 not determined sulfuric acid [wt %] 3.21 3.97 8.8 nitric acid [wt %] 1.96 0.89 methyl methanesulfonate 1490 [ppm] total chlorine [wt %] 0.5

TABLE-US-00003 TABLE 3 Water and impurities in the washed crystals starting melt 1 2 3 4 water [wt %] 1.24 17.76 17.72 not determined sulfuric acid [wt %] 0.48 0.47 0.85 nitric acid [wt %] 0.229 0.062 methyl methanesulfonate 185 [ppm] total chlorine [wt %] 0.034

TABLE-US-00004 TABLE 4 Distribution coefficients of the impurities starting melt 1 2 3 4 sulfuric acid 0.150 0.118 0.096 distribution coefficient nitric acid 0.117 0.069 distribution coefficient methyl methanesulfonate 0.124 distribution coefficient total chlorine 0.068 distribution coefficient

LIST OF REFERENCE NUMERALS

[0111] 1 crude alkanesulfonic acid [0112] 2 first process step [0113] 3 low boilers removal [0114] 4 starting materials [0115] 5 material stream comprising low boilers [0116] 7 material stream comprising alkanesulfonic acid, high boilers and residual low boilers [0117] 9 melt crystallization [0118] 10 mother liquor [0119] 11 washing means [0120] 12 suspension [0121] 13 product [0122] 14 solid-liquid separation [0123] 15 recycle stream into melt crystallization [0124] 16 washing liquid [0125] 17 recycle stream into distillation column [0126] 18 crystallization cycle [0127] 19 evaporator, high boilers removal [0128] 20 distillation cycle [0129] 21 vapors [0130] 22 high boilers cycle [0131] 23 partial condenser [0132] 24 high boilers purge [0133] 25 condensate [0134] 26 worked up high boilers purge stream