RECOVERY OF ALKANOL IN THE PRODUCTION OF CHLOROPRENE

Abstract: In the dehydrochlorination of 3,4-dichlorobutene-1 to give chloroprene in a two-phase mixture of water and an alcohol, a bleed stream may be taken from the organic phase to prevent high boiling impurities accumulating. Problems in recovering the alcohol from the bleed are overcome by extracting the bleed with water at a volume ratio of water and organic phase brought into contact at the first extraction step of not less than 15:1, or if the organic phase is substantially free of chloroprene, not less than 6:1.

Description: RECOVERY OF ALKANOL IN THE PRODUCTION OF CHIOROPRENE The present invention relates to the production of chloroprene from 3,4-dichlorobutene-l in a two phase mixture of water and alcohol and to the recovery of alcohol from other organic materials present in organic phases from the dehydrochlorination reaction. The production of chloroprene by a dehydrochlorination of 34-dichlorobutene-l with alkali in a two phase mixture of water and an alcohol at elevated temperatures is disclosed in United States patent specification 3 079 446. The chloroprene is distilled off from the reaction mixture as it is formed. The dehydrochlorination of 3,4-dichlorobutene-l to give chloroprene at temperatures in the range 40 to 700C is disclosed in German patent specification 2 707 073. In the course of the dehydrochlorination there is an accumulation of impurities in the organic phase. The above mentioned German specification states that it may be desirable to discard some of the organic phase and to replace it by fresh alcohol to prevent an accumulation of undesirable higher boiling materials in the system. In a continuous process this is conveniently carried out by taking a bleed stream from the organic phase once it has settled in a separator and before recycle to the reactor. This bleed may be taken from an organic phase which contains substantially no chloroprene, which may be the case if chloroprene is distilled off as it is formed or if a chloroprene removal step is carried out on the organic phase outside the dehydrochlorination reactor before the bleen is taken. Alternative the bleed may be taken from organic phase from which chloroprene has not been removed. In either case it is desirable for economic reasons to recover the alcohol from the organic bleed. One possible method which might suggest itself for recovering the alcohol from the organic bleed. is extraction with water. Although the quantity of material removed by solvent extraction step is related to the total quantity of solvent used it is well known that the most efficient way of carrying out a solvent extraction step so as to remove the greatest volume of material for a given volume of solvent is to carry out maximum possible number of extractions with the given volume of solvent. Theoretically if one volume of liquid is to be subjected to extraction with a total of four volumes of solvent more material will be extracted by four separate extractions with one volume each than by a single extraction with four volumes of solvent. Extraction of the bleed with low volume ratios of water to organic phase (e.g. 1:1 to 3:1) gives only low recoveries of the alkanol. However on carrying out further extractions with additional quantities of water it was found either that separation of the resulting phases could not be obtained or separation was too slow for use as a commercial process (i.e. separator residence times of over 2 hours would be required). It would therefore appear that solvent extraction is not a suitable technique for removing the alkanol from the organic phase and that other techniques such as distillation should be employed. We have now found the recovery of alkanol from the organic bleed can be carried out provided that the volume ratio of water to organic phase brought into contact in the first extraction step exceeds a certain minimum value. According to a first aspect the present invention a process for the dehydrochlorination of 3,4-dichlorobutene-l to give chloroprene in a reaction medium which is a two phase liquid mixture of water and an alcohol in the present of alkali in which a bleed is taken from the organic phase is characterised in that the bleed is subjected to extraction with water and the volume ratio of water and organic phase brought into contact at the first extraction step is not less than 15:1. The process of the present invention (in that aspect in which the organic bleed is not substantially free of chloroprene) may be applied to organic bleed streams produced in dehydrochlorination reactions carried out at temperatures such that the chloroprene is not distilled from the dehydrochlorination reactor but is retained in the liquid removed from the reactor, which is separated by decantation into an aqueous and an organic phase the chloroprene being subsequently recovered from the organic phase by distillation. The bleed may then be taken from the organic phase before the distillation step. This is a less preferred process. The process of this first aspect of the present invention is most preferably applied to organic bleed streams produced during dehydrochlorination at temperatures such that chloroprene is distilled from the reaction mixture as it is formed. There will be an equilibrium between the chloroprene in the liquid and vapour phases and it is usually found necessary to operate the dehydrochlorination process so as to maintain quite high chloroprene concentrations in the reaction mixture. The process of the present invention is particularly applicable to such chloroprene-containing bleed streams. The volume ratios in which the water and organic phase are brought into contact in the first extraction step must be at least 15:1 and is preferably at least 16:1, when the organic phase is not substantially free of chloroprene. The applicant has found that if the bleed is substantially free of chloroprene the water to organic phase volume ratio can be substantially reduced. This enables a greater quantity of organic phase to be treated in apparatus of given size and reduces the quantity of water introduced into the process on recycle of the aqueous phase to the reactor. Any water introduced into the total dehydrochlorination process has finally to be removed as an aqueous effluent. It is desirable to reduce the volume of such effluents. According to a second aspect of the present invention a process for the dehydrochlorination of 3,4-dichlorobutene-l to give chloroprene in a reaction medium which is a two phase liquid mixture of water and an alcohol in the presence of alkali in which a bleed is taken from The organic phase is characterised in that the bleed is substantially free of chloroprene when it is 'extracted with water at a volume ratio of water and organic phase brought into contact at the first extraction step is not less than 6:1. - - When using the smaller volume ratios of water to organic phase possible when the bleed is substantially free of chloroprene, the bleed substantially free of chloroprene may be obtained by carrying out the dehydrochlorination in which a way that the chloroprene is distilled off as it is formed. A bleed is taken from the organic phase from the dehydrochlorination redaction, and is subjected to a distillation to remove chloroprene. Alternatively the dehydrochlorination reaction product may be removed from the reactor in which it is formed and the aqueous phase separated from the organic phase. The organic phase may then be distilled to remove chloroprene and the bleed may be taken from the organic phase after the chloroprene has distilled off. In this second aspect of the invention the organic phase must not contain any substantial quantity of chloroprene. The presence of minor amounts of chloroprene insufficient to adversely affect the extraction can be tolerated. It is preferred however that the chloroprene concentration in the organic phase subjected to extraction does not exceed 3% wt/wt, more preferably does not exceed 2% wt/wt and preferably does not exceed 0.5% wt/wt. When the organic phase is substantially free of chloroprene, the volume ratio in which the water and organic phase are brought into contact in the first extraction step must be at least 6:1 and is preferably at least 8:1. The dehydrochlorination of 3,4-dichlorobutene-l to give chloroprene in a reaction medium which is a two phase liquid mixture of water and an alkanol is well known in the art. The reaction medium must consist of two liquid phases and the alcohol employed must be such as to give with the water two liquid phases under the reaction conditions used. Examples of suitable alcohols are higher alkoxyalkanols e.g. butoxyethanol, and higher alkanols e.g. those alkoxyalkanols and alkanols containing 3 to 8 carbon atoms. An example of suitable alkoxyethanol is butoxyethanol. Example of suitable unsubstituted alkanols are those having 3 to 5 carbon atoms, eg propanol-l, propanol-2, butanol-l. The ability to form a separate liquid phase will depend on the nature of the alcohol, the relative quantities¯¯of alcohol and water, nd the concentration of dichiorobutenes and chloroprene in the organic phase and of alkali metal hydroxide and alkali metal chloride in the aqueous phase, but for any given reaction mixture a suitable alcohol can be selected by simple test. Thus it is preferred to use an alcohol which, when shaken with a saturated brine solution containing 22% wt/wt NaOH at a volume ratio of alcohol to aqueous phase corresponding to that to be used in the dehydrochlorination process, forms a separate phase, which phase contains at least 0.1% wt/wt NaOH. An example of a volume ratio of alcohol to aqueous phase at which the test may be carried out is 3:1. It is particularly preferred to use alcohols which give a separate phase containing at least 0.5% wt/wt NaOH in order to obtain fast reaction rates. Thus when various alcohols were shaken with a saturated brine solution containing 22% wt/wt NaOH at a volume ratio of alcohol to aqueous phase 3:1, the values obtained for the NaOR content of the alcohol phase (at room temperature) were: Alcohol NaOH Content sec-butanol (butanol-2) 0.05% wt/wt iso-butanol (trimethyl methanol) 0.19% wt/wt n-butanol (butanol-l) 0.6% wt/wt 2-butoxyethanol 3.6% wt/wt It will be seen that butanol-2 is not a preferred solvent, while n-butanol and 2-butoxyethanol are preferred solvents. It is particularly preferred to use 2-butoxyethanol. The quantity of organic phase taken as a bleed varies with the purity of the feed and with the reaction conditions. Typically the bleed is in the range 2 to 10% of the organic phase present. The extraction step is a step of bringing into contact and then separating the organic and aqueous phases. This may be carried out by any of the well known methods. A suitable method is by mixing the water and the organic phase together in a stirred vessel or a static in-line mixer and then feeding the mixture to a decanter where the organic phase separates from the aqueous phase under gravity. Depending on the degree of recovery of alkanol required it may be necessary to use only one extraction step if higher recoveries of alkanol are necessary additional extraction steps may be carried out. No restriction is required on water: organic ratio for second extraction as the problem of achieving satisfactory extraction is overcome by the use of the large volume of water in the first extraction. The invention will now be illustrated by reference to the accompanying drawings which are diagrammatic representations of apparatus suitable for carrying out the present invention. Figure 1 is a diagrammatic representation of an apparatus suitable for carrying out a process according to the presnet invention in which the organic phase subjected to extraction is not substantially free from chloroprene. Figure 2 is a diagrammatic representation of apparatus suitable for carrying out the process of the invention when dehydrochlorination is carried out under conditions such that chloroprene is distilled from the reaction mixture as it is formed and a portion of the organic phase removed from the reactor is subject to a separate distillation to remove substantially all the chloroprene before feeding to the extraction s tep. Figure 3 is a diagrammatic representation of apparatus suitable for carrying out the process of the invention when the dehydrochlorination is carried out at temperatures such that chloroprene is retained in the liquid withdrawn from the reactor, and the chloroprene is subsequently recovered by distillation from an organic phase separated from the reaction mixture. The apparatus shown in Figure 1 comprises a dehydrochlorination reactor equipped with an agitator 2 and heating means (not shown). A mixture of nitrogen and nitric oxide, aqueous sodium hydroxide solution, and liquid 3,4-dichlorobutene-l may be fed into the reactor 1 through pipes 3, 4 and 5 respectively. Vapour may be removed from the reactor through pipe 6 to distillation column 7 and liquid condensate returned through line 8. Distillate may be removed from the head of the column by pipe 9. The reactor 1 is provided with an internal stand pip 10 through which liquid in the reactor 1 can overflow when the liquid level in the reactor reaches the top of the pipe. Pipe 10 is connected to a decanter 11 in whcih a lower phase is discarded through pipe 12 and the upper phase is returned through the pipe 13 to reactor 1. A bleed can however be taken through pipe 14 to stirred tank 15 to which water can be added through pipe 16. The tank is connected to a decanter 17 by pipe 18. Pipe 19 allows the lower phase to be discarded while the upper phase is recycled to reactor 1 through pipe 20. In operation 3,4-dichlorobutene-l, aqueous sodium hydroxide and nitrogen/nitric oxide are fed to the reactor 1, which is charged initially with a water/alcohol mixture which is maintained at a sufficiently high temperature to allow the cloroprene formed to distill off from the reaction mixture. (Examples of suitable temperatures are those in the range 800C to 1050C). Vapour passes into column 7 and chloroprene is recovered overhead through 9. Although chloroprene is distilled directly from the reaction mixture there will be an equilibrium concentration of chloroprene in the liquid organic phase which (depending on temperature and pressure in the reactor, and the alcohol used) may be from 3 to 20% by weight of the organic phase. The organic and aqueous phases are separated in decanter 11 and the aqueous phase containing NaCL produced from the NaOH initially fed is discarded through pipe 12. The organic phase containing unreacted 3,4-dichlorobutene-l, alkanol some chloroprene and high boiling impurities formed during the reaction is returned to the reactor through pipe 13. To prevent the concentration of high boiling impurities increasing to undesirable levels a bleed is taken through pipe 14. By extraction with water fed through pipe 16 followed by separation in decanter 17, an upper aqueous phase containing the alkanol is obtained which is returned to the reactor. The lower organic phase is discarded through pipe 19. To compensate for the slight loss of alkanol, fresh alkanol may be added directly to the reactor or at any point in line-20. The operation of the reactor 1 distillation column 7 and decanter 12 is the same in-Figures¯l-and¯2 and the description given for Figure 1 applies also to Figure 2 as far as the step of taking a bleed through pipe 14. The bleed taken through pipe 14 is not, as in the embodiment of Figure 1 fed directly to an extraction step but is fed instead to a distillation column 70 in which the residual chloroprene is distilled overhead at 90. The material from the base of the distillation column is passed throgh line 140 to a stirred vessel 15 to which water is introduced through pipe 16. The resulting mixture passes through pipe 18 to decanter 17 from which a lower organic phase is separated and discarded. The upper aqueous phase containing the extracted alcohol is returned through pipe 20 to the reactor 1. In Figure 3 those items which are identified by the same numbers as those in Figure 2 perform the same function as in Figure 2. However the dehydrochlorination reactor is operated at a temperature such that the chloroprene produced does not boil off from the reaction mixture (Examples of suitable temperatures are 500C to 70"G). All the chloroprene therefore remains in the organic phase from decanter 11 which passes through pipe 130 to distillation column 71 from which chloroprene is recovered overhead at 91. The product from the base of the column is returned through pipe 131 and mixed with the 3,4-dichlorobutene-l feed to the reactor (although of course it can be added separately). A bleed is taken through pipe 14 and subjected to extraction with water as in the apparatus disclosed in Figure 2 and the aqueous extract containing alkanol is required to the reactor through pipe 20. In both Figure 2 and 3, as in Figure 1 the organic phase from decanter 17 is discarded through pipe 19. The invention will now be illustrated by reference to the following Examples. Example 1 A bleed stream was taken from the decanted organic phase produced in accordance with the description of Figure 1 using 2butoxyethanol as the alcohol. A sample (94.4g/lOOml) of the bleed stream-wa-s- subjected- to a-batchsextraction with water (1700ml) at ambient temperature, the water: organic phase volume ratio thus being 17:1. The sample contained 15.lg chloroprene and 56.6g of 2-butoxyethanol. The aqueous phase recovered from the extraction step contained 52.7g of 2-butoxyethanol and 3.3g of chloroprene. Only trace amounts of the impurities originally present in the sample passed into the aqueous phase, the remainder being removed with the organic raffinate which had separated as a heavy organic phase and which was discarded. Example 2 This Example shows a continuous extraction with water as shown in Figure 1. The organic bleed, of the same composition as in Example 1 and water were fed to a stirred 1 litre glass vessel at a volume ratio of 1:17. The mixture passed to a separator from which the organic phase was withdrawn from the base and discarded. The aqueous phase was recovered as an upper layer. In a period in which 215g of organic bleed were fed to the mixer, 68.7g of organic raffinate containing 10.8% of 2-butoxyethanol were removed from the separator. The recovery of alcohol in the aqueous phase was therefore 94% by weight. Example 3 An experiment was carried out using the apparatus of Figure 2. The bleed taken from the organic phase was subjected to distillation to remove substantially all the chloroprene present in the bleed. The residue from the distillation which contained 69.6% wt of 2-butoxy-ethanol and an insignificant amount of chloroprene (0.15% wt) was fed to the stirred vessel (corresponding to 15 to Figure 1) at a rate of 45 ml/hour (52.9 g/hour) together with water at 611 ml/hour. This corresponds to a water: organic phase volume ratio of 11.3:1. The resulting mixture was then passed to a decanter from which the aqueous phase was removed overhead and returned to the dehydrochlorination reaction. The organic residue was removed from the base of the decanter at a rate of 16.25g/hour. The alkanol concentration i the organic residue was 15.2% wt and the alkanol recovery in the aqueous extract was--therefore--937° wt. - The- extraction was carried out with the bleed cooled to ambient temperature. Example 4 Organic bleed containing 67.2% butoxyethanol, 7.7% dichlorobutenes, 12.9% high boilers and 0.3% chloroprene obtained as in Example 3 was continuously extracted in a mixer/decanter system with 8 volumes of water at 170C and 86% of the alkanol was recovered into the aqueous phase. The organic raffinate containing 24.6% butoxy-ethanol, 21.9% dichlorobutenes and 43.2% high boilers was re-extracted continuously in a mixer/decanter system with 10 volumes of water at 16 C and 85.2% of the butoxyethanol was recovered into the aqueous phase. Combining the two aqueous extracts gave a total alkanol recovery of 97.9% at an overall ratio of aqueous extractant to organic bleed of 12:1. This showed a 5 points percent increase in recovery compared with a single extraction 12:1. In this Example all percentages are by weight. The process of the present invention may be combined with the process of our copending application (Case 4731) in which emulsion formation during the extaction step is prevented by the addition of sufficient acid to the extraction step to bring the pH of the aqueous phase below 5. The process of the present convention may also be combined with the process of our copending application (Case 4728) in which the extraction with water is carried out a a temperature of not more than 329C.

Claims: Claims: 1. A process for the dehydrochlorination of 3,4-dichlorobutene-l to give chloroprene in a reaction medium which is a two phase liquid mixture of water and an alcohol in the present of alkali in which a bleed is taken from the organic phase characterised in that the bleed is subjected to extraction with water and the volume ratio of water and organic phase brought into contact at the first extraction step is not less than' 15:1. 2. A process according to claim 1 wherein the bleed is taken from the organic phase recovered from a dehydrochlorination reaction in which chloroprene is distilled from the reaction mixture as it is formed. 3. A process according to Claims 1 or 2 wherein the volume ratio at which the water and organic phase are brought into contact in the first extraction step is at least 16:1. 4. A process according to Claim 1 characterised in that the organic phase subjected to extraction with water is substantially free from chloroprene and that the volume ratio of water and organic phase brought into contact at the first extraction step is not less than 6:1 instead of not less than 15:1. 5. A process according to Claim 4 wherein the organic phase substantially free of chloroprene is obtained by distilling off chloroprene from the dehydrochlorination reaction as it is formed, taking a bleed from the organic phase from dehydrochlorination reaction and subjecting the bleed to a distillation to remove chloroprene. 6. A process according to Claim 4 wherein the organic phase substantially free from chloroprene is obtained by removing the dehydrochlorination reaction product from the reactor in which it is formed spearating the organic phase from the -aqueous- phase, distilling the organic phase to remove chloroprene and taking a bleed from the organic phase after the chloroprene has been distilled off. 7. A process according to any one of claims 4 to 6 wherein the organic phase subjected to extraction contains not more than 3% wt/wt of chloroprene. 8. A process according to Claim 7 wherein the organic phase contains not more: than 0.5% wt/wt chloroprene. 9. A process according to any one of Claims 4, 5 or 6 wherein the volume ratio of water and organic phase brought into contact in the first extraction step is at least 8:1. 10. A process according to any one of the preceding claims wherein the quantity of organic phase taken as a bleed is in the range to 10% of the total organic phase. 11. The process according to any one of the previous claims wherein the extraction step is carried out by mixing the water and organic phase together in a stirred vessel or in a static mixer and the feeding the resulting mixture to a decanter. 12. A process according to any one of the previous claims wherein the alcohol is 2-butoxyethanol or n-butanol.

Inventors: HALL A
Applicants: BP CHEM INT LTD, HALL A, BP CHEM LTD
Application year: 1980
Main classification: C07C17
IPC codes: C07C17, C07C21, C07C29, C07C67