Patent Publication Number: US-3876714-A

Title: Chloroprene removal from pyrolyzed 1,2-dichlorethane

Description:
United States Patent Coppens Apr. 8, 1975 [54] gs g g gg fi g igigggfigr FOREIGN PATENTS OR APPLICATIONS L 720.894 3/1969 Belgium 260/656 [75] inventor: glllgllizlrlnme Coppens, Brussels, OTHER PUBLICATIONS Vinyl and Diene Monomers Leonard, part 3 (i971), [73] Asslgnce: Solvay &amp; Cle, Brussels. Belgium Tp 1180 v v pp |24l l245 [22] Filed: Feb. 9, l97l Primary Examiner-Bernard Helfin [2H Appl&#39; &#34;3,985 Assisranl Eruminer-A. Siege] Rdated U Application D Attorney, Agent, or Firm-Spencer &amp; Kaye [63] Continuutiun-in-part of Ser. No. 760.847. Sept. 19.  
 I968. abandoned. [57] ABSTRACT In a process for treating the raw gaseous products of [30] Foreign Application Priority Data the pyrolysis of 1,2-dichlorethane to form vinyl chlo- Feb. 20. I970 Belgium x5450 3 Procedure for 6limimiliflg Chloroprene involving the steps of cooling the raw gaseous product com- 521 u.s. Cl 260/656 R ing from the Py y rsarrtion under Pressure above 51 lm. Cl. C076 21/02 atmospheric to Obtain a gaseous Phase and a liquid [58 Field 6: Search 260/656 R P and treating the liquid Phase with Chlorine in an amount of between 0.0] and 5 percent by weight 5 References Cited with respect to the l,2-dichlorethane not converted UNITED STATES PATENTS during the pyrolysis. After such treatment. the resulting mixture is separated into individual components. 3 l25.607 3/1964 Keatmg et al 260/656 3.142.709 7/1964 Gause et ul 260/656 10 (llalms. 2 Drawing Flgures 1,2-C H Cl 5 IFVICE PYROLYSIS WEN OONEENSER Clz PATENTEU 81975 3,876,714  
 SHEET 1 0f 2 2 3 2 1. 2 I 5 STORAGE DEWCE pvaofv sls OVEN CONDENSER v SEPAR ION SEPARATION COLU COLUM Hcl/\C2H3u A /1 12c H Cl 1 2 I. 2  
 QIYFIF i/Jl/M/J INVENTOR. Guillaume Coppens ATTORNEYS.  
 CHLOROPRENE REMOVAL FROM PYROLYZED l,2-DlCHLORETHANE CROSS REFERENCE TO RELATED APPLICATION The subject application is a continuation-in-part of application Ser. No. 760,847, filed Sept. 19, 1968 now abandoned refiled Apr. 28, I971 as Ser. No. 138,364, now U.S. Pat. No. 3,801,660.  
 BACKGROUND OF THE INVENTION The present invention relates to a process for treating the raw gaseous product coming from the pyrolysis of l,2-dichlorethane into vinyl chloride.  
  It is known, during the pyrolysis of l,2-dichlorethane to form vinyl chloride and hydrogen chloride, to con dense the raw product coming from the pyrolysis in order to obtain a gaseous phase and a liquid phase which one allows to stand for at least 2 hours in order to eliminate the butadiene therefrom. Such a process is disclosed in Belgian Pat. No. 720,894, filed Sept. 16, 1968. Then the hydrogen chloride and the vinyl chloride are separated from the l,2-dichlorethane which was not converted during the pyrolysis.  
  The 1,2dichlorethane, which still contains certain impurities, such as chloroprene, is then purified before being recycled to the pyrolysis.  
  The purification is effected by distillation in two successive columns in such a manner as to eliminate fractions having a boiling point below 83C and fractions having a boiling point greater than 83C. The pressure of the chloroprene then proves to be very troublesome. In effect, when the distillation intended to separate from the l,2-dichlorethane the fractions having a low boiling point is carried out, the concentration of chloroprene at the top of the column is increased, and this chloroprene can polymerize after its concentration reaches 5 percent by weight. The polymer formed provokes the blocking of conduits and condensers which necessitates frequent stoppages and cleanings.  
  In order to avoid this inconvenience, it has already been proposed to subject the l,2-dichlorethane being recycled, before distillation, to a treatment with catalytic quantities of anhydrous aluminum chloride. Such a procedure is proposed, for example, in Belgian Pat. No. 7l9,865 issued on Aug. 23, I968. This operation is effective but requires supplemental steps which are difficult to carry out on an industrial scale. Thus, it is necessary to dry the l,2-dichlorethane containing the fractions having a low boiling point by agitation with phosphoric anhydride and then to heat the anhydrous product obtained in the presence of aluminum chloride at l30-l50C under pressure for 24-48 hours.  
 SUMMARY OF THE INVENTION It is a primary object ofthe present invention to avoid the above drawbacks and difficulties.  
  A further object of the invention is to simplify the purification of l,2-dichlorethane.  
  A further object of the invention is to provide an improved method for removing chloroprene from the 1,2- dichlorethane which was not converted during the pyrolysis operation.  
  The present invention is based essentially on applicants discovery of a procedure for eliminating the chloroprene accompanying the nonconverted l,2- dichlorethane leaving the pyrolysis region without introducing any supplemental steps, such as have been heretofore proposed.  
  The invention is thus directed to a process for the treatment of the raw gaseous product derived from the pyrolysis of l,2-dichlorethane into vinyl chloride, which process includes the following steps: cooling the gaseous product (issuing from the pyrolysis) under a pressure greater than atmospheric pressure in order to obtain a gaseous phase and a liquid phase, treating this liquid phase with chlorine in an amount of 0.0l-5 percent by weight with respect to the l,2-dichlorethane which was not converted during the pyrolysis, and sep arating the mixture obtained into its various constituents.  
  Surprisingly, applicant has discovered that the liquid phase obtained after treatment with the chlorine contained only negligible quantities of chloroprene. Simultaneously, applicant noted that the concentration of butadiene in the liquid phase treated by the chlorine had also been greatly diminished.  
  More specifically, the present invention is directed to a process for the treatment of the raw gaseous product coming from the pyrolysis of the l,2-dichlorethane and containing principally vinyl chloride, in a mixture with hydrogen chloride, unconverted l,2-dichlorethane, and small quantities of chloroprene, the process being for the purpose of obtaining pure vinyl chloride and including the following steps: cooling of the product coming from the pyrolysis of the l,2-dichlorethane to a temperature between 0 and 300 C by quenching with liquid l,2-dichlorethane at a pressure greater than atmospheric pressure and generally between I and 30 kg/cm condensing the cooling mixture obtained in the cooling step at a pressure generally between I and 30 ltg/cm and a temperature between 0 and C so as to obtain a liquid phase principally containing 1.2- dichlorethane, vinyl chloride, a small quantity of hydrogen chloride and small quantities of chloroprene, and a gaseous phase principally containing hydrogen chloride and vinyl chloride; treating the liquid phase obtained by the condensing step with chlorine in an amount of 0.0 l-S percent by weight with respect to the l,2-dichlorethane which was not converted by the pyrolysis in such a manner as to obtain a liquid mixture practically free of chloroprene; separating the liquid mixture obtained during the treating step by distillation in a manner to obtain polymerizable vinyl chloride; and separating the gaseous phase obtained during the condensing step by distillation, this separation being carried out either independently of the liquid mixture separation step or simultaneously with the distillation associated with the liquid mixture separation step.  
  When the gaseous product coming from the pyrolysis of the l,2-dichlorethane contains also a small amount of butadiene, the latter is divided during the condensation between the gaseous phase and the liquid phase obtained and it is then advantageous to add a supple mental step of washing the gaseous phase obtained during the condensing step with a part of the liquid mixture obtained during the treating step in a manner to obtain a gaseous phase which is practically free of butadiene, the washing liquid being then returned to the treating step where it is treated with chlorine.  
  As a practical matter, the raw product coming from the pyrolysis is condensed in two steps, according to the process described in Belgian Pat. No. 720,894 the gaseous product coming from the pyrolysis is first of all cooled, by being brought into contact with liquid l,2- dichlorethane, under a pressure of 1-30 kg/cm and preferably 4-20 kg/cm to a temperature of between and 300C and preferably l00200C; and the gase ous mixture thus obtained is condensed, under a pressure of between l30 kg/cm and preferably 4-20 kg/cm to a temperature of between 0 and 100C and preferably between 20 and 80 in order to obtain a gaseous phase and a liquid phase.  
  The liquid phase obtained mainly contains 1,2- dichlorethane, vinyl chloride, hydrogen chloride and impurities, such as chloroprene and butadiene. The gaseous phase contains hydrogen chloride, vinyl chloride, and a fraction of the light impurities present in the liquid phase, such as butadienev Chlorine is introduced into the liquid phase thus ob tained and this liquid phase remains in a reservoir with out it being necessary to add thereto any reactant whatsoever. Obviously, however, one can introduce into the reservoir a chlorination catalyst such as ferric chloride or any other known chlorination catalyst, as disclosed in the text by P. H. Groggins, Unit Processes in Organic Synthesis, McGraw-Hill, 1958, page 265.  
  The quantities of chlorine utilized are between 0.01 and 5 percent by weight with respect to the 1,2- dichlorethane which was not converted during the pyrolysis and the quantity of chlorine is preferably between 0.02 and 1 percent by weight with respect to that l,2 dichlorethane.  
  Advantageously, one part of the liquid phase treated with chlorine can be utilized to wash in a countercurrent flow the gaseous phase obtained during the condensation stage in order to eliminate the residual impurities. The liquid product coming from the washing column is then treated again with chlorine, preferably in the same reservoir as that in which the liquid phase coming from the condensation step is treated. Of course, use could also be made of a separate reservoir without departing from the scope of the invention.  
  The constituents of the liquid phase treated with chlorine and including essentially hydrogen chloride, vinyl chloride and unconverted 1,2-dichl0rethane, can then be separated in any manner known per se. It is possible, for example, to distill the hydrogen chloride and the vinyl chloride in two successive distillation columns. The hydrogen chloride can be distilled at a temperature of between 80 and +C and at a pressure of between 1 and 30 kglcm The vinyl chloride can be distilled at a temperature of between 14and +60C and a pressure of between 1 and 10 kg/cm&#34;.  
  The dichlorethane obtained is then purified in its turn by distillation in two successive columns permitting elimination of the fractions having a boiling point below 83C and the fractions having a boiling point above 83C. The distillation column which permits the elimination of fractions whose boiling point at atmospheric pressure is below 83C operates at a temperature of between 40 and 120C and at a pressure of be tween 0.5 and 3 kg/cm&#34;. The distillation column which permits the elimination of fractions whose boiling point at atmospheric pressure is greater than 83C operates at a temperature between 60 and 130C and at a pressure of between 0.5 and 3 kg/cm It is noted, in applying the process according to the invention, that the liquid phase treated with chlorine no longer contains practically any chloroprene. Consequently, no polymerization, and hence no blocking, is any longer observed in the column where the vinyl chloride is distilled or in the column where the 1,2-  
 dichlorethane is separated from fractions having a boiling point below 83C. In addition, it is noted that there is no longer any formation of tars in the boiler of the column where the l,2-dichlorethane is separated from the fractions having a boiling point greater than 83C. These tars are bothersome because they accumulate in the boiler of this column where they provoke clogging. Finally, the vinyl chloride obtained now contains only small traces of butadiene and as a result is suitable for polymerization without undergoing any further purification.  
 BRIEF DESCRlPTlON OF THE DRAWINGS FIG. 1 is a block diagram ofa first embodiment of apparatus for carrying out the method according to the invention.  
  FIG. 2 is a view similar to that of FIG. 1 showing another embodiment of apparatus for carrying out the method according to the invention.  
 DESCRIPTION OF THE PREFERRED EMBODMENTS In the arrangement of FIG. 1, a pyrolysis oven 1 is maintained at a temperature of 500C and under a pressure of the order of l l kg/cm The oven, which may be furnished with a packing of inert material having a silica base, is supplied with pure 1,2- dichlorethane. The product from this oven, which com prises, mainly, e.g., 5 to 45 percent by volume of vinyl chloride, 5 to 45 percent by volume of hydrogen chloride, 10 to percent by volume of nonconverted 1,2- dichlorethane, up to 3 percent by volume of chloroprene and up to 0.2 percent by volume of butadiene, passes to a condenser 2 where it is quenched to bring its temperature to around 150C, and then cooled to about 40C, to obtain a liquid and a gaseous phase.  
  The gaseous phase which is withdrawn via conduit 3 contains from 5 to 45 percent by volume of vinyl chloride, from 55 to percent by volume of hydrogen chloride and from 0 to 0.2 percent by volume of butadiene and is fed into column 4. There is no change in butadiene content in conduit 3 and butadiene goes unchanged into the final vinyl chloride.  
  The liquid phase flows via conduit 5 from the bottom of condenser 2 under a pressure of the order of 10 kg/cm and passes into a storage device 7. This liquid phase almost entirely comprises 1,2-dichlorethane, vinyl chloride, a small amount, e.g. from 1 to 20 mole percent, of hydrogen chloride, 0 to 0.2 mole percent of butadiene, up to 4 mole percent of chloroprene, and.  
 some other impurities. The storage device 7 may be a suitable vessel provided with an inlet and outlet so as to avoid back-mixing of the liquid; this vessel may be made, for example, of steel, alloys, cladded steel or enamelled steel. Via the conduit 6 there is simultaneously introduced into the storage device 7 about 0.015 percent by weight of chlorine with respect to the 1,2-dichlorethane. The liquid phase which remains in the storage device 7 contains essentially 1,2- dichlorethane, vinyl chloride and a little hydrogen chloride.  
  The butadiene present in the product entering unit 2 is divided equally between conduits 3 and 5 while the chloroprene is mainly collected in conduit 5. Surprisingly, however, the liquid phase in conduit 8, which phase is maintained in the presence of chlorine, is virtually free from chloroprene and butadiene; it contains l,2-dichlorethane, 20-80 mole percent; hydrogen chloride, 1-20 mole percent; vinyl chloride, l-50 mole percent.  
  Hydrogen chloride is removed from the top of column 4 at around -30C under a pressure of 10 kg/cm while a liquid phase, removed via line 9 from a heater maintained at 90C, is directed to column 10. Vinyl chloride is separated in column 10 at 40C under a pressure of kg/cm and carried off via the head of column 10, while the base of the column, maintained at l50C, contains virtually only 1,2-dichlorethane and heavy products consisting mainly of chlorinated C products. The components at the base of the column are conveyed by line 11 into the column 12 where the light impurities having a boiling point below 83C are separated, in the head of the column, and removed via line 13, while the 1,2-dichlorethane containing only heavy products is conveyed via line 14 into column 15. In column 15, the pure l,2-dichlorethane in a condition to be recycled is removed from the head of that column and conveyed via line 17, while at the base of the column the heavy products are collected and carried off via line 16. The columns 4, l0, l2 and I5 are well known to those skilled in the art and may be provided with any kind of trays or packing. Columns and trays for 4, l0, l2 and IS may be made ofa suitable material, such as steel, stainless steel and also Ni-alloys, e.g. inconel, hastelloy and monel.  
  It has been noted that as a result of the introduction of chlorine into storage device 7, there is no longer practically any chloroprene in column 12 and no polymerization is any longer noted in the head of this column, which is connected to line 13. In addition, the boiler of the column 15 no longer contains any troublesome tarry residue. Finally, the process permits the production of polymerizable vinyl chloride starting from pyrolysis products containing at the most l0 parts per million of butadiene with respect to the vinyl chloride.  
  When the initial butadiene concentration, at the output of the pyrolysis, is greater than ten parts per million with respect to the vinyl chloride and when it is desired to obtain a vinyl chloride which is free of butadiene, it is advantageous to utilize an apparatus of the type illustrated in FIG. 2. This apparatus is identical with that of FIG. I with the exception that it also includes a countercurrent washing column 3&#39; connected to receive the material passing through line 3 and to wash this material in a countercurrent flow with a liquid phase taken from the conduit 8. The liquid passing through column 3&#39; is returned to the input of storage device 7 where it is again brought into contact with chlorine. As a result of this arrangement, a reduction is effected in the quantity of butadiene present in the gaseous phase conveyed via line 3&#34; to the column 4.  
  It should of course be appreciated that the specific embodiments shown in FIGS. 1 and 2 represent only examples of apparatus which can be employed for carrying out the process of the present invention and that other forms of apparatus can be employed. For example, means could be provided for subjecting the gaseous phase leaving the head of the washing column, or tower, 3&#39; to another countercurrent washing by another liquid fraction taken from conduit 8, etc.  
  There will now be presented below a specific nonlimiting example of the process according to the invention demonstrating the surprising and advantageous results of this process.  
  This example of a process according to the invention is based on the actual operation of an apparatus having the form illustrated in FIG. 1.  
  THe pyrolysis oven 1 is maintained at around 500C under a pressure of the order of l 1 kg/cm This oven is supplied with pure l,2-dichlorethane. The products exiting from this oven and which contain essentially vinyl chloride, hydrogen chloride and nonconverted l,2-dichlorethane (50 percent mole) pass to condenser 2 where they undergo first of all a quenching which brings their, temperature to about l50C. In a second stage, the gaseous mixture thus obtained is cooled to 40C under a pressure of the order of 10 kg/cm in order to produce a liquid phase and a gaseous phase. The gaseous phase, which exits via line 3 and which contains vinyl chloride, hydrogen chloride and a small quantity of butadiene, is conveyed to column 4. The liquid phase removed via line 5 from the bottom of condenser 2 has the following composition:  
 Components 1 by weight vinyl chloride 29.4 l 2-dichlorethane 60.] l,l-dichlorethane 0.02 HCI l0.l chloroprene 0.09 trichlorethylene 0.27 butadiene 0.00027 l.2,4-trichlorobutane 0.024 l,2-dichlorobutanc 0.0 l 8 l l,2-trichlorethane 0.006 remaining components 0.0l2  
 Components 7( by weight vinyl chloride 29.4 I ,2-dichlorethane 60.] l l -dichlorethane 0.02 HCI l0.l chloroprene 0.0l trichlorethylene 0.27 butadiene 0.00003 1 ,2.4-trichlorobutane 0.024 I ,2-dichlorobutane 0.0 l 8 l l ,Z-trichlorethane 0.006 remaining components 0162 It is noted from a comparison of the above tables that the concentrations of chloroprene and butadiene in the product leaving storage device 7 are greatly reduced.  
  In the head of column 4, hydrogen chloride is separated at a temperature of around +30C and under a pressure of 10 kg/cm, while from the boiler of that column, maintained at C, there is removed, via line 9, a liquid phase which is conveyed to column 10.  
  In the head of column 10, vinyl chloride is separated at 40C and under a pressure of 5 kg/cm while the base of the column, maintained at C, supplies essentially l,2-dichlorethane and chlorinated C products, the material provided by the base of the column being delivered to line 11.  
 The heavy fraction removed at the base of column 10 vaporisation of the latter. The vapors formed are condensed in a heat exchanger and part of the collected liquid is recycled to the quenching device.  
 it will be understood that the above description of the is Q y y line 11 into Column 12 in head of 5 present invention is susceptible to various modificawillcii here are P 1 at Q&#34;? and finder i j tions, changes and adaptations, and the same are inp f Pre55ureg p having a boiling tended to be comprehended within the meaning and point lower than 83C, while the boiler of this column, range f equivmems f the appended chin-ls maintained at 83C, furnlshes essentially 1,2- I Claim: dlchlc&#39;l&#39;eil&#39;gane and Products having boiling P i 1. In a process for the purification of vinyl chloride Flbove 83 BeFauSe of the P &#39;Q of Orme obtained by pyrolyzing l,2-dichlorethane to form a raw mto siozrage device 7, no polymerization is noted in colgaseous product Containing vinyl chloride, hydrogen fg l&#39;z d M eth e t 1 1 hea y p d chloride, chloroprene,butadiene and unconverted 1,2-  
  e or an Con ammg y 0 dichlorethane and cooling the raw gaseous product to gg f fi zg g fgi z g tggggg zl gg ii gl 2: form a gaseous phase and a liquid condensate phase,  
  said liquid condensate phase containing 1.2- and under atmos henc ressure. The boiler of column which is at a z p of 100C furnishes essen dichlorethane, vinyl chloride, hydrogen chloride, chlob t d&#39; th tlally chlorinated C products. Also as the result of the iizi z f j f gszg ig ggg fi mz g ggf z ig g introduction of chlorine into storage device 7, practip g the raw gaseous product to form a liquid mixture, the cally no tarry residues are found to appear in the boiler of column 15. amount of chlorine being from 0.01 to 5 percent by The dichlorethane thus obtained at the head of colb Y i umn l5, and carried off via line 17, can be recycled to Chlorethaile m the hqulil i p remollnig.hydm the input of the pyrolysis oven 1 without any other adgen.chlo.nde from l hquld mixture dlsnnmg the ditiona] &#34;eatmem liquid mlxture resulting from said removing step to ob- When a portion of treated effluent from storage detalzn jf i i f d t vice 7 is used for washing noncondensed gases in washprocess 0r tea i 8 l gaseous pro ing column 3&#39;, from about 5 to about 80, and preferaf l from l pyrolysls, 9 l&#39;2&#39;dlchl0rethane to My about percem by weight of the treated gmuent 30 tam vmyl chloride, compnsmg the steps of: coollng the is generally recycled for this purpose. The wash liquid gaseous product commg from pyrolifsls under a from column 3&#39; is either separately treated with chlopressure ,than atmosphel&#39;lc to Obtam i F &#39;I rine in storage device 7 or is treated therein in admixphase i a Phase; &#34;eating the resumng hquld ture with fresh condensate from condenser 2. There is phase chlofme &#34;K amount of between? and no limit to the respective proportions of raw condensPercem by we&#39;ght respect to the &#39;l of sate (from condenser 2) and wash liquid effluent (from fhchlorethane not Gonvened by h pyrolyslsr separat&#39; column being tmated at any particular time in OF mg hydrogen chlorlde from the mixture resulting from age device 7. said treating step and distilling the liquid mixture rewith reference to FIGS 1 and 2 Ordinary Operating sulting from said separating step to obtain polymerizranges for the several units are: 40 able vlnl Chloride- Appalrotus Temperature Pressure Dwell Time Range (Tl Range lhourl (kg/cm quick cooling (2) (J to 300 to 30 condensing l3) 0 to 100 l to 30 \HJShillg (3&#39;) it] to +ltll) l to 3U l) to 0.5 storage (7) Z(I to +|UU l to 30 U.()| to ll) column (4) head 8(] to +ll] i to 30 boiler fill to I70 0.] to ll] column I Hi! head l-l to +60 l to \U boiler R0 to ZUU 0.1 to 6 column I ll) head 40 to lZtl }(J.5 to 3 boiler m :50 0.| m to column 151 head to 130 }0.5 to 3 boiler lo 150 0.2 m 20 By q as Used in the iflsiam i5 meam 3. A process as defined in claim 2 wherein said step id fin Applicable methods of rapid coolmg are of cooling is carried out as to&#39;obtain a gaseous phase well known to those iramlllflr with lhls T p 60 and a liquid phase at a temperature of between 0 and ferred manner is to mix the hot gases emeggmg fro m C d comprising the further step of washing the the Pyrolysis Oven at a lemp&#39;iraml&#39;e of 400 to C gaseous phase produced by said cooling step with part With a liquid having a temperature of f 300 C f of the liquid mixture produced by said treating step, consisting mainly of l,2diChl0r t l&#39;la In y device and subsequently returning the liquid employed in said which brings in close contact liquid and gas such as a 65 washing step to be subjected anew to said treating step.  
 cooling tower or an ejector. The hot gases are cooled to the temperature of the liquid (from 0 to 300C) by 4. A process as defined in claim 2 wherein the quantity of chlorine utilized in said treating step is between 0.02 and 1 percent by weight with respect to the amount of ll-dichlorethane which was not converted by the pyrolysis and which is subjected to said treating step.  
  5. A process for treating the raw gaseous product of the pyrolysis of l,2-dichlorethane, which product contains principally vinyl chloride in mixture with hydrogen chloride, non-converted l,2-dichlorethane and small quantities of chloroprene. the process being for the purpose of eliminating the chloroprene and obtaining pure vinyl chloride and comprising the steps of:  
 a. cooling the product of the pyrolysis of l,2-  
 dichlorethane to a temperature of between and 300C by quenching it with liquid [.2- dichlorethane under a pressure greater than atmospheric;  
 bi condensing the mixture obtained by said cooling step under a pressure of between 0 and 30 kg/cm and at a temperature of between 0 and 100C for obtaining a liquid phase and a gaseous phase, the liquid phase principally containing 1,2- dichlorethane, vinyl chloride, a small quantity of hydrogen chloride and a small quantity of chloroprene, and the gaseous phase principally containing hydrogen chloride and vinyl chloride;  
 c. treating the liquid phase obtained by said condensing step with chlorine in an amount of 0.01 to 5 percent by weight with respect to the quantity of 1,2-dichlorethane which was not converted by the pyrolysis and which is contained in the liquid phase for producing a liquid mixture substantially free of chloroprene;  
 d. separating hydrogen chloride from the mixture resulting from said treating step;  
 e. separating the liquid mixture obtained by step (d) by distillation to obtain polymerizable vinyl chloride; and f. separating the gaseous phase obtained by said condensing step, by distillation, for obtaining polymerizable vinyl chloride.  
  6. A process as defined in claim 5 wherein said step of cooling is carried out under a pressure of between I and 30 kg/cm? 7. A process as defined in claim 5 wherein said two separating steps are carried out together.  
  8. A process as defined in claim 5 wherein each of the liquid phase and gaseous phase produced by said con densing step additionally contains a small quantity of butadiene, and said treating step produces a liquid mixture which is also substantially free of butadiene; and comprising the further step of washing the gaseous phase produced by said condensing step with a portion of the liquid mixture produced by said treating step for obtaining a gaseous phase substantially free of butadiene, and subsequently again subjecting said portion of the liquid mixture to said treating step.  
  9. A method as defined in claim 5 wherein the quantity of chlorine employed in said treating step is between 0.02 and 1 percent by weight with respect to the quantity of l.2-dichlorethane not converted by the pyrolysis and subjected to said treating step.  
  10. A process according to claim 1 which comprises maintaining the chlorine in admixture with the liquid condensate for a period sufficient to remove all but at most a negligible quantity of chloroprene from said liquid condensate.