RECOVERY OF ALKANOL IN CHLOROPRENE PRODUCTION

Abstract: In the extraction of an alcohol from an organic phase resulting from the dehydrochlorination of 3,4-dichlorobutene to give chloroprene in a two-phase mixture of water and an alcohol, undesirable emulsification is prevented by the addition of sufficient acid to reduce the pH to below 5.

Description: Recovery of alkanol in chloroprene production 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 3,4-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 of 40 to 70°C is disclosed in German patent specification 2 707 073. At some stage in the operation of the process it will be desirable to separate the alcohol from other constituents of the organic phase produced in the dehydrochlorination process. Thus it may be necessary to take a bleed from the organic phase and to recover alcohol from this phase. Our copending applications (Case 4723/Case 4727) and (Case 4728) disclose methods for recovering the alcohol by extraction with water. However it is sometimes found that on carrying out the extraction step a highly undesirable formation of emulsion takes place. It has now been found that the emulsion formation is associated with alkaline conditions during the extraction step. According to the present invention the process for the recovery of alkanol contained in an organic phase resulting from the dehydro- chlorination of 3,4-dichlorobutene to give chloroprene is a reaction medium which is a two phase liquid mixture of water and an alcohol in the presence of a alkali is characterised OMPI by subjecting the organic phase to an extraction step with water and separating an aqueous phase containing the alcohol from the organic phase, acid being added to the extraction step in a quantity sufficient to reduce the pH of the aqueous phase to below 5. 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 and unsubstituted alkanols for example those containing 3 to 8 carbon atoms in the molecule. An example of a suitable alkoxyalkanol is 2-butoxyethanol. Preferred unsubstituted alkanols are those containing 3 to 5 carbon atoms e.g. propanol-1 . The ability to form a separate liquid phase will depend on the nature of the alcohol, the relative quantities of alcohol and water, and the concentration of dichlorobutenes 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 containin 227 wt/wt NaOH at a volume ratio of alcohol to aqueous .phas . . - corresponding to that to be used in the dehydrochlorination process, forms a separate phase, which phase contains at. least 0.1% wt/w '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 a fast reaction rate. . -.-.■-: - . - ' Thus when various alcohols were shaken with a saturated briae solution containing 22% wt/wt NaOH at a volume ratio -of. alcohol to aqueous phase 3:1, the values obtained for the.naOH 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-1) -_ . 0.6% wt/wt 2-butoxyethanol 3.6% wt/wt It will be seen that butanol-2 is not a preferred solvent, QMPI ' while n-butanol and 2-butoxyethanol are preferred solvents. It is particularly preferred to use 2-butoxyethanol. The dehydrochlorination step is usually carried out with alkaline dehydrochlorination agents and traces of these may remain in the organic phase recovered from the dehydrochlorination reaction and give rise to subsequent problems during extraction with water. The use of alkaline dehydrochlorination agents does not inevitably lead to emulsification problems however, particularly if the alkali concentration in the organic phase recovered from the dehydrochlori¬ nation step is low. It is believed that problems with emulsification; are particularly likely if the organic phase has been heated to temperatures greater than 100°C, particularly 105°C, in the presence of alkali before the extraction step is carried out. Heating at such temperatures may occur during the dehydrochlorination step particularly if chloroprene is distilled off from the reaction mixture which is formed. Such a heating in the presence of alkali may also take place if the dehydrochlorination step is carried out at temperatures below 100 α C e.g. at 40°C to 70°C and a liquid organic phase containing chloroprene is removed from the dehydrochlorination reaction and is subsequently distilled to separate the chloroprene from the other constituents of the organic phase prior to extraction with water. Alkali may be present during this distillation because of alkali carried over from an alkaline dehydrochlorination agent. However alkali may also-be added during the distillation to protect the distillation column. gainst corrosion. The present invention is particularly suitable for recovering alkanol- from a bleed stream taken from the organic phase to prevent • the'.concentration of high boiling material in the organic<-phase becoming excessive. The acid is most conveniently added to the extraction.step by introducing it into the water used to carry out the extraction step. Alternatively the acid may be added to the mixture during the extraction step. ' . -.-. Examples of suitable acids are strong mineral acids. such as HCL, H3PO4 as well as organic acids e.g. acetic. The nature and quantity OMPI of acid added must be sufficient to decrease the pH of the aqueous extract to below 5, preferably to below 2. Addition of acid may take place continuously so as to maintain the pH below 5 or may be carried out discontinuously, addition only taking place when a problem of emulsion formation arises. The process of the present invention may be combined with the process of our copending application (Case 4723/2727) which describes a process for the dehydrochlorination of 3,4-dichlorobutene- to give chloroprene in a two phase liquid mixture of water and an alkanol in the presence of an alkali in which a bleed is taken from the organic phase and is subjected to extraction with water, the volume ratio of water and organic phase brought into contact at the first extraction step is not less than 15:1 or if the bleed is substantially free drom chloroprene, not less than 6:1. In addition the process of the invention may be combined with copending application (Case 4728) which discloses a process for the dehydrochlorination of 3,4-dichlorobutene-1 to give chloropren in a reaction medium which is a two phase liquid mixture of water and an alkanol in the presence of an alkali in whcih a bleed is taken fro the organic phase and is extracted with water, the temperature at which the extraction takes place being not more than 32°C, preferably not more than 20°C. The accompanying drawings are diagrammatic representations of apparatus of apparatus suitable for carrying out the dehydrochlorinati of 3,4-dichlorobutene-l to give chloroprene to which the process of the present application may be applied. Figure 1 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. Figure 2 is a diagrammatic representation of apparatus suitable for carrying out the process of the invention when the dehydrochlori¬ nation is carried out at temperatures such that chloroprene is retaine OMPI 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 operation of the embodiment shown in Figure 1 will now be described in more detail. A mixture of nitrogen and nitric oxide, sodium hydroxide solution, and 3,4-dichlorobutene-l are introduced into reactor 1, provided with stirrer 2, through pipes 3, 4, and 5 respectively. The reactor contains a mixture of water and alkanol (e.g. 2-butoxyethanol) . The reactor 1 is heated by heating means (not shown) to maintain the contents at a temperature such that chloroprene will distill off from the reaction mixture as it is formed. (Examples of suitable temperatures are those in in the range SO to 105°C). Vapour passes through pipe 6 into distillation column 7 and condensate returns through pipe 8. Chloroprene is removed overhead at 9. The reactor 1 is provided with a stand pipe 10 through which the liquid contents of reactor 1 overflow into decanter 11, where they separate into lower aqueous phase containing dissolved NaCl, and an upper organic phase. The aqueous phase is discarded through pipe 12. The organic phase consisting of unreacted dichloro- butenes, chloroprene, and high boiling impurities is recycled through pipe 13 to the reactor. A bleed is taken through pipe 14 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 through 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 through pipe 19. The upper aqueous phase containing the extracted alkanol is returned through pipe 20 to the reactor 1. In Figure 2 those items which are identified by the same numbers as those in Figure 1 perform the same function as in Figure 1. 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 50°C to 7o°C). All the chloro- prene therefore remains in the organic phase from decanter 11 which passes through pipe 130 to distillation column 71 from which chloroprene OMPI is recovered overhead at 91. The product from the base of the column is returned through pipe 131 and mixed with the 3,4-dichloro- butene-1 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 1, the aqueous extract containing alkanol is returned to the reactor through pipe 20, the organic phase being discarded through pipe 19. In the arrangement shown in Figure 2 it is particularly desirabl to feed aqueous alkali solution to the lower portion of distillation column 70 to protect it against corrosion and to convert some of the dichlorobutenes in the bleed to chloroprene. However it is believed that the risk of emulsion formation is increased when this addition of aqueous alkali takes place and that therefore the additio of acid in accordance with the invention is particularly advantageous. The invention will now be illustrated by the following Examples. In all these Examples the 3,4-dichlorobutene-l was continuously dehydrochlorinated with NaOH in a mixture of water and butoxyethanol at a temperature (90 to 105°C) such that the chloroprene was recovere overhead from a distillation column connected to the reactor. The liquid reaction mixture was fed to a decanter and the upper organic phase recycled to the reactor. A bleed was taken from this organic phase and was distilled to remove substantially all of the residual chloroprene. This was the bleed subjected to extraction. In these Examples, Example A is a comparative Example not according to the invention. The arrangement used was thus substantially that illustrat by Figure 1. Examples 3 and 4 differ from the arrangement shown in Figure 1 in having two stages of mixing and separation instead of only one. In the Examples all the extractions were carried out at ambient temperature. Example A -- --— The butoxyethanol in organic bleed containing by weight 66% butoxyethanol, 6.5% dichlorobutenes, 12% high boilers and 0.3% chloro¬ prene and previously heated in the presence of alkali, could not be extracted with 10, 12 or 15 bolumes of fresh water because of formatio of a stable emulsion. The pH of the emulsion was about 6. • - υs- E O PI Example 1 A sample of the bleed used in Example A was washed with 12 volumes of water containing _ca 0.4% by weight of hydrogen chloride and no emulsification occurred. The aqueous phase contained _ca 83% 5 of the butoxyethanol in the system and had a pH of 1.8. Example 2 Another sample of the same organic bleed as in Example A was extracted with 12 volumes of water containing 0.01% by weight of hydrogen chloride and no emulsion was produced. The separated aqueous 10 phase pH was about 4.5. Example 3 Organic bleed previously heated in the presence of alkali and containing by weight 66.4% butoxyethanol, 5.5% dichlorobutenes, 11.8% high boilers and 0.5% chloroprene was extracted continuously by 8 15 volumes of water containing 0.1% by weight acetic acid in a two stage counter-current system. The final aqueous extract contained 97.7% of the butoxyethanol feed and had a pH of 3.8. Example 4 Organic bleed previously heated in the presence of alkali and 20 containing by weight 67% butoxyethanol, 8.3% dichlorobutenes, 11.8% high boilers and 0.7% chloroprene was extracted continuously by 8 volumes of water containing 0.1% by weight of ortho phosphoric acid in a two stage counter-current system. The final aqueous extract contained 97.5% of the butoxyethanol feed and the pH was 4.2. 25

Claims: Claims: 1. A process for the recovery of alcohol contained in an organic phase resulting from the dehydrochlorination of 3,4-dichlorobutene to give chloroprene in a reaction medium which is a two phase liquid mixture of water and an alcohol in the presence of alkali is characterized in that the organic phase is subjected to an extraction step with water and an aqueous phase containing the alcohol is separated from the organic phase; and an acid is added to the extraction step in a quantity sufficient to reduce the pH of the aqueous phase to below 5. 2. A process according to Claim 1 wherein the alcohol is an alkoxyalkanol or unsubstituted alkanol containing 3 to 8 carbon atoms in the molecule. 3. The process according to Claim 2 wherein the alcohol is butoxyethanol. 4. The process according to Claim 2 wherein the alcohol is n-butanol. 5. A process according to any one of the preceding Claims wherein the organic phase has been heated to a temperature greater than 100°C in the presence of alkali before the extraction step is carried out. 6. A process according to any one of the preceding Claims wherein the organic phase to which the extraction step is applied is a bleed stream taken from the total organic phase resulting from the- dehydrochlorination. 7. A process according to any one of the preceding Claims wherein the acid is a strong mineral acid. OMPI 8. A process according to any one of Claims 1 to 6 wherein the acid is an organic acid. 9. A process according to Claim 8 wherein the acid is acetic acid. 10. A process according to Claim 9 when the quantity of acid is sufficient to decrease the pH to below 2. OMPI _ . WIPO «

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