Document VJN5geR8w7Mj29kjJbYQpYwdg

V CE Refresher * Vinyl Chloride Processes 01 - MODERN f CHEMICAL M ' TECHNOLOGY 1; , : ! -- - ! '] .: ,[ . PART 13 Conversion of acetylene by hydrochlorination to vinyl chloride or that of ethylene by vapor-phase or liquid-phase chlorination to dichloroethane, followed by pyrolysis to vinyl chloride, are fundamental reactions for rts manufacture by different processes or their combinations. LYLE F ALBRIGHT, Purdue Univertity Large-scale production of vinyl chloride in the U.S, reflects important proce-s developments of the last few years. To understand this growth, we will consider in this article the c. rrent information for these developments. In later articles of this series, we will continue the discussion on vinyl chloride production and also review that for polyvinyl chloride. Several combinations of chemical reactions are used in commercial units ir. -.rder to obtain the best over-all economics. Reactants for these operations Include: (a) acetylene ana/or ethylene, and (b) hydrogen chlo ride and/or ch.o. ine. Vinyl chloride is now produced for as low as /lb,, or even less in some of the newer processes. Relatively small plants for vinyl chloride probably cannot compete economically with newer and larger units. It is thought to be only a matter of time before smaller plants are shut down or enlarged. The trend in the U.S., in particular, has been to large units, some of which have capacities greater than 300 million lb./yr.*- Announcement has just been made by B. F. Goodrich Co. that its plant of over 300 million lb./yr. will be expanded to 1 billion lb./yr. by 1968. Decreased cost and new developments in polymeriza tion of vinyl chloride are expected to reduce signifi cantly the production cost of polyvinyl chloride polymer's. General Chemistry of the Reaction Vinyl chloride is produced from acetylene and hydro gen chloride in the presence of a catalyst as follows: CH -CH + HCt -- C1I- CHC1 (1) When ethylene is the starting hydrocarbon, it is To meet the author, see Chem. Bug., Feb. 13. 1SS7, p. 1<4. chlorinated or oxychlorinated to produce 1,2-dichloro ethane by the following catalytic reactions: CH, - CH, + Cl, -- CH,C1--CH-C1 v-') CH, - CH, + 2HC1 + JO, -- CH..C1 - CH/'l + H,0 SI Reaction (2), the chlorination reactic''.. is practiced commercially in both the vapor phase and the id phase. Each method will be discussed in this artio * Pyrolysis of dichloroethane produces both v:,,yi chloride and hydrogen chloride: heat CHtd-CHgCl--------* CHg-CHQ -h Ha (-4) Hydrogen chloride from Reaction (4) can be used as the reactant in Reaction (1) or (3), All of the above four reactions occur with relatively few side reactions, i.e., they have high yields. Attempts have been made to produce vinyl chloride from ethylene by substitutive chlorination of ethylene as follows: CH, - CH, + CU -- CH, - CHCI -r HC1 (S) However, yields for such a reaction have been too low for commercial adaptation. Some industrial repre sentatives believe that such a process may yet be developed. Production of vinyl chloride from acetylene [Ren.- tion (1)] has been the principal method until very recently. The flowsheet for such a plant is shown in Fig. 1, and it will be considered in detail later in this article. Other commercial methods of producing vinyl chlo ride involve two or more of the above reactions. One example involves the chlorination of ethylene [Reaction (2) ] as a first step, followed by a pyrolysis step [Reac tion (4)]. About half of the chlorine ends up in the vinyl chloride, and about half produces hydrogen chloride. Companies that have a Urge captive use for Chenicul Engineering--March 27,1967 CBY 1110160 123 RSV0032965 v CE REFRESHER . . HYDROCHLORINATION of pore acetylene Is one commercial route for vinyl chloride productior--Fig. 1 hydrogen chloride may find this two-step method of making vinyl chloride attractive. A balanced process that uses essentially equal moles of relatively pure ethylene and acetylene has been used by several companies. In this process, there Is a rcmbinarion of Reactions (2), (4) and (1). All (or essentially all) of the chlorine eventually ends up in vinyl chloride because the hydrogen chloride from Reaction (4) is used in Reaction (1). Relatively new and somewhat similar balanced processes have been developed by the Societe Beige de L'Azote (S.B,A,)."-S and by Kureha Chemical Industry Co., Ltd. of Japan,13 ** In these processes, a hydro carbon feedstream that contains essentially equal moles of acetylene and ethylene (produced by pyrolysis of naphtha) is reacted first with hydrogen chloride to remove essentially all of the acetylene. The remainder of the hydrocarbon stream that contains ethylene is now reacted with chlorine. The resulting dichloroethane is separated and then pyrolyzed. Thia method is relatively unique compared to several previous bal anced processes in that no attempt is made to use pure ethylene or acetylene. In other words, the acetylene and ethylene in the effluent stream from the naphtha pyrolysis do not have to separated. Such a separation step Is relatively expensive. Several processes have recently been developed in which one of the chemical steps is oxychlorination of ethylene. The first step in such a process is the chlori nation of ethylene. The resulting dichloroethane is then pyrolyzed to produce vinyl chloride and hydrogen chloride. The hydrogen chloride is then used for oxychlorination. Hydrochlorination of Acetylene Fig, 1 is the flowsheet for a commercial process for producing vinyl chloride from relatively pure acetylene. Care must be taken in handling pure acety lene because It can detonate under some conditions. General safety procedures include:1-1,1 1. Avoidance of alloys or solders containing copper, silver or mercury. 2. Preference for relatively small-diameter piping 124 BY 1110161 March 27,1947--Chemical Enginesring RSV0032966 >> or tubing:. Sizes up to C in. I.D. have been used at relatively low pressures with suitable flame or flash arresters Installed in the line. 3. Compression by means of centrifugal, liquid- sealed compressors has been successful. When hydrocarbon streams (such as the effluent stream from a naphtha cracker) contain only a small fraction of acetylene, less stringent safety precautions are necessary. Vinyl chloride must also be handled with care. In 1964, a large plant for the polymerization of vinyl chloride was almost completely destroyed as a result of a leak in which vinyl chloride escaped and formed a mixture with air and detonated.8 Production of Acetylene Up to several years ago. production of vinyl chloride accounted for 30% of all acetylene manufactured in this country. Hence the general economics of acetylene manufacture are important relative to the future of vinyl chloride production. Based on predictions for the next five years, it Is unlikely that new acetylene plants will be built in the U.S. unless a substantially cheaper acetylene is made.10 Within the last few years, new acetylene plants have been built using hydrocarbons as the feed stock12* l0-1T-10- rather than the older processes using calcium carbide. Acetylene from these hydrocarbon processes is thought to be considerably cheaper than that from carbide processes. Claims, based on research information, have been made that acetylene can be produced from 3 to Sd/lb.2* However, at least two companies using hydrocarbon processes hsve experi enced considerable difficulties and expenses in reaching design capacities and yields. Present cost may, in many if not all units, be as high as 7 to Bd/lb. In 1963, about 53% of the acetylene produced In this country was still by the carbide process, and the re mainder from various hydrocarbon processes. Mechanism of Hydrochlorination Mercuric chloride is the usual catalyst for the hydrochlorination reaction. It is thought to react with acetylene to form the intermediate compound, trana-2chlorovinylmercuric chloride:--20 ah CH CH + HgClj --- u (6) H ^gCl This compound is attacked at the C-Hg bond by hydrogen chloride to form vinyl chloride and to re generate the mercuric chloride. As indicated by Reac tion (6), acetylene is chemisorbed by the catalyst Vinyl chloride and possibly also hydrogen chloride are also strongly adsorbed on the catalyst30 The con trolling chemical step is probably the surface reaction between adsorbed reactants. At startup, hydrogen chloride should enter the catalyst bed first, and then the acetylene flow should be started slowly.30 Mercuric chloride is, as a rule, Impregnated on activated-carbon pellets. Transfer of the acetylene and hydrogen chloride to the catalyst surface, and of vinyl chloride away from the surface, are necessary steps in the over-all process. The surface available in the carbon pellets affects the resistance to transfer of the materials to and from the catalyst surface. One or more of these transfer steps is partly controlling in terms of the kinetics of the over-all reaction. Feed Purification A commercial process for the production of 30 million lb./yr. of vinyl chloride will now be described. Fig, 1 indicates the flowsheets Acetylene at about 1.5 atm. absolute pressure is first compressed to 2 atm. with a Nash compressor that uses water as the sealing medium. Acetylene is then purified by passing it through an entrainment separator that contains refrigerated water. The temperature of the acetylene is lowered to 15 C., and considerable water is con densed and removed from the gas. A rotary, positivedisplacement meter measures the gas to within 0,7%. Next the acetylene is scrubbed in two columns with sulfuric acid to complete the drying. The first scrubber usps relatively dilute acid, and is 2-ft.-dia, by 24 ft. high. The steel tower is lined with acid brick. The second tower uses a more concentrated acid, and is 1.67-ft.-dia. by 22 ft. high. Each column is packed with 2-in. Roschig rings. The acid in each column is recycled many times until its strength has dropped sufficiently. It is then replaced with fresh, strong acid. An external heat exchanger maintains the temperature of the recycle acid ax 20 to 30 C. [This part of the process is not shown in Fig. 1.] Acetylene from the second scrubber is then passed through an activated-carbon filter to remove traces of catalyst poisons such as sulfides. The filter is 3-ft.-dia, by 7 ft high. A second filter is provided as a standby for use after the first filter has been depleted. The purifying section for HC1 eliminates undesired impurities such as water, chlorine or chlorinated or ganic materials that may be present from previous chlorinations. Refrigerated condensers are sometimes used. The HC1 is then compressed to about 2.25 atm. absolute by using a reciprocating compressor. The HC1 at 70 C. is then passed through a separator tank to remove any entrained liquid. An automatic ratio controller maintains the desired molar ratio of acety lene to HC1. A ratio of about 1:1 is normal but slight variations are sometimes maintained. The HC1 and acetylene streams are mixed in a mix tank. Reactors Are Shell-and-Tube Exchangers Five reactors are used, but only two are shown in Fig. 1. The five reactors are arranged: two in parallel, t Detail** on this particular plant were b\ (Wneral Tire end Rubber Co. *11 a *euilnar fjKw**re<l by AIChE at Akron. Ohio on Oct. 16. 1964. R. R. Matllko and r. R. Sayre were responsible for presentation of the materiel on vinyl chrorlde synthesis at this seminar. Chemical Engineering--March 27,1947 CBY 1110162 | j f f j >. [ j | 1 i ; RSV0032967 cs refresher . . . r followed by two more in parallel, and then the last infrared on-stream analyzer monitors composition of one. Each reactor is essentially a shell-and-tube heat the HC1 in the exit gases. Kinetic information con exchanger that contains 200 tubes, each 2-in. 1.0. cerning the hydrochlorination reaction has been pre by 16 ft. long. The steel tubes are packed with acti- sented by Wesselhoft and others.510 Their predicted vated-carbu s pellets impregnated to a weight of about rates of reaction are less than those in the commercial 10% mercuric chloride. The reaction is highly exo unit. The differences are probably due to variations thermic (about 24,500 cal./gm.-mole of vinyl chloride in catalyst activity. formed at reaction conditions). A heat-transfer fluid on the shellside maintains the desired temperatures. Stripping Section The temperature used in the reactor varies from about 90 to 140 C and depends on age and condition Exit gases from the reactor are fed directly into of the catalyst. Lower temperatures are used with the bottom of the water scrubber that is packed with fresh catalyst. At higher temperatures, mercuric chlo 2-in. Raschig rings and that is built of brick-lined ride begins to sublime. There is evidence that the steel. The cooling water (absorbent) is recirculated mercuric chloride sublimes from the hotter inlet of several times countercurrent to the gases, but this the reactor (where most of the reaction occurs), and feature is not shown in Fig. 1. The exit solution from then recondenses near the cooler outlet. As a result, the scrubber contains from 3 to 10% HC1. As the the locations where the highest rates of reaction occur concentration increases, a higher percentage of HC1 vary as the catalyst is exhausted.5* The suggestion is carried over to the caustic scrubber, also packed has been made that the life of the catalyst can be with Rosehip rings. The gases leave the scrubber increased if the direction of flow to each reactor is essentially free of HQ and flow through a suction occasionally reversed, but it is not known if such a knock-out tank to remove entrained liquids. The gases technique is used commercially. In such a case, the are next compressed to about 7 atm. absolute. mercuric chloride would tend to ''move" from one end of the reactor tube to the other, and vice versa. Main Recovery Section taining the catalyst at temperatures less than 120 C. gives little or no sublimation loss.1 At such tempera The gases at 7 atm. are partially condensed by nor tures, the amount of vinyl chloride produced per pound mal cooling water and then cold brine. This pressure of catalyst consumed is much greater than it is at was chosen in order to obtain the desired amount of higher temperatures. condensation with normal cooling water. Most of the As indicated above, temperatures of the catalyst vinyl chloride, water and chlorinated hydrocarbon by in the tubes vary vertically. Temperatures also vary products condense. A decanter separates the water radially because of the cooling fluid on the shellside layer from the organic layer that is fed to a stripping of the tubes. The temperature and rate at which the column. This column contains 20 bubble-cap trays and cooling fluid is pumped through the shellside of the is 27 ft. high. The bottom of the column is maintained reactor affects the temperature gradients in the re at 46 C., using hot water in the reboiler of the stripper. actor. Reliable information on catalyst life has not Hot water is used instead of steam in order to obtain been published, but each batch of catalyst is estimated lower surface temperatures and hence minimize de to last about half p year, assuming catalyst poisons composition of vinyl chloride or of a relatively unstable are minimized in the feeds treams. byproduct. The bottom product of the stripper is crude Reactor pressure is maintained at 1.5 to 1,6 atm. vinyl chloride, which after cooling with brine is fed I absolute. (Some processors perhaps operate at lower to a storage tank. The top gaseous product of the i pressures.-'0) Higher pressures are not used for at stripper is combined with the exit gas stream from the least two reasons: knock-out tank. Mild steel is used for construction of a. Increased dangers involved with higher-pressure the stripper section. acetylene. b. Increased rates of reaction cause increased prob lems in local hot spots, and hence affect the tempera ture control of the catalyst. Oil is used as the cooling fluid for the process described here, but presumably other heat-transfer liquids would work satisfactorily. Several years ago, a heat exchanger using water cooled the oil, but now an air-cooled heat exchanger is used. At startup, the oil is heated with steam in order to raise the catalyst to reaction temperatures. About 98 to 99% conversions of each-reactant occur in the reactor. Essentially all of the product is vinyl chloride, but trace amounts of trichloroethylene, dichloroethylcne, and aldehydes are also present. In addi tion, a small amount of unreacted acetylene and HC! are contained in the product stream. A continuous Acetylene Recovery The uncondensed materials from the brine cooler are primarily acetylene, vinyl chloride and inert gases. These are fed into the bottom of the absorber. Tri chloroethylene, a byproduct of the process, is used as the solvent In the absorber and stripper. The absorber operates at about 4 atm, absolute and 30 C., and the stripper at about 90 C. (in the reboiler) and 2.25 atm. Both columns are packed with 1-In. saddles. The vent gases from the absorber contain 90% inerts, and the remainder is primarily acetylene. The gas stream from the stripper is cooled with brine to condense most of the trichloroethylene. Un condensed gas contains 40% vinyl chloride plus 20% acetylene, which represents nearly all of the un reacted 126 CBY 1110163 March 27, 1967--Chemical Engineering RSV0032968 ^acetylene. These gases, recycled to the reactors, also coutnin small amounts of chlorinated hydrocarbons, primarily trichloroethylene. As additional trichloro- . ethylene (and other chlorinated byproducts) are re* covered, they are fed to the heavy-end storage tank. Refining Section Chlorinated organic materials and aldehydes arc removed from the vinyl chloride in a distillation col umn. This column contains 30 bubble-cap trays and is 48 ft. high. It is operated at 4.0 to 4.5 atm. absolute so that the relatively pure vinyl chloride overhead stream can be condensed at 35 C. with normal cooling water. Feed to this column enter* at the twentieth tray from the bottom. The top trays and that portion of the equipment contacting vinyl chloride are built from stainless steel in order to minimize iron contamination of the vinyl chloride. Hot water is used as the heating lluid in the reboiler of this column In order to min imize surface temperatures in the roboiler, and hence minimize decomposition of vinyl chloride. The bottom product from the column is discarded. For every 2,000 lb. (82 lb.-moles) of vinyl chloride produced, the following are leqvired: 1,183 lb. (32.5 lb.-moles) HC1, and 877 lb. (33.7 lb.-moles) of acet ylene. Heavy ends produced are 23 lb. (0.18 lb.-mole if they ore assumed to be pure trichloroethylene) Other Hydrochlorination Processes Kalr7' has recently described an unspecified European plant, which produces about 5 million lb./yr. of vinyl chloride. The hydrochlorination reactor contains about 1.000 tubes, each 3-cm. dia. and 3 m. long, and each is packed with catalyst. Calculations indicate that the rate of production for a given bulk volume of the cat alyst is only about one-quarter to one-third that in the General Tire process. A hydrochlorination reaction is also part of several balanced processes. In the S.B.A. or Kureha processes, * the hydrocarbon feedstream is relatively dilute in re gard to acetylene--9.1% was reported for the Kureha process.11 In this latter process, the partial pressure of acetylene was less than 0.6 atm. Hence the total pressure in the reactor was less than 6.6 atm. Tem peratures for the reactor were specified as 120 to 180 C. The Kureha process is writ instrumented and is com puter controlled.80 Acetylene content of the feedstream to the hydrochlorination unit is continuously measured by a stream analyzer.11 Flow of hydrogen chloride is metered in order to provide slightly less than the stoi chiometric amount. The excess acetylene is specified as less than 1% according to one reference,90 but is somewhat greater according to another.11 Yields of product are 95 to 98% based on the acetylene in the feedstream, and more than 99% based on hydrogen chloride. The design for the hydrochlorination reactor of the Kureha process is similar to that previously dis cussed.11 Temperature control with a dilute acetylene feedstream is considerably easier than with essentially pure acetylene. As a result, at least one of the follow ing modifications seems possible: 1. Larger diameter tubes for the packed catalyst bed. 2. Higher gas temperatures since diluents in the gas stream will minimize local hot spots on the solid catalyst Gas temperatures will in general be more uniform throughout the catalyst bed. Somewhat longer reactor tubes and lower space ve locities may be required to obtain essentially complete reaction with the more-dilute acetylene streams. The catalyst used in the Kureha process11 Is a "mer curic chloride catalyst," o.r.:l in probably similar to those used with pure acetylc..e. Although specific de tails about the cathirst are not reported, the amount of mercuric chloride added and the type of support used may be somewhat different. Reactions with eth ylene, carbon oxides, hydrogen and other gases in the feedstream (from the naphtha crackeri are "hardly observed." Although fewer details of the reactor de sign are available for the S.B.A. process, presumably it is similar to that of the Kureha process. Vapor-Phase Chlorination of Ethylene Ethylene is chlorinated in the vapor phase by a freeradical chain mechanism at temperatures from about 90 to 130 C. The initiating step of the over-all reaction is probably the rate-controlling one, and is: Cl, ?=t2Cl- (free radical) (7) Propagation steps are: - CH--CH, + Cl---* CHCI --ClI, (in < lauical) (8) CH.C1-CH, + CU-- CH,C1--CHjOl -i- Cl- (9) The free-radical chains terminate when two free radicals combine, or when free radicals collide and react at the reactor wall. The walls of the reactor and/or the catalyst such as iron (probably an iron chloride formed in sitv) or calcium chloride have a pronounced effect on the over all reaction.''-'1**4-87 These materials are involved in the initiation step [Reaction (7)j, and may be consid ered as catalysts. Surface temperatures in the reactor are therefore important. Semenov90 and others have indicated that the walls of the reactor frequently act both to initiate and to terminate the free-radical chains. Further re search is needed, however, to characterize the role of the reactor. Preliminary results of such a project now in progress at Purdue University clearly indicate that the role of the reactor walls is often highly compli cated and significant. In many cases, failure to develop simple kinetic equations for the chlorination reaction is not surprising, although several past investigators did not seem to understand this fact. In addition to type of surface, the amount of surface is also important The role of the surface is compli cated especially if it is porous or rough; hence the controlling factors may be both mass transfer and heat transfer to it. Rust and Vaughn-4 found that the rate of chlorination is directly proportional to the chlorine concentration, and to the square of the ethylene con- Chemical EaBlMtriae--March 27,1967 ...... CBY 111016** 127 t4 i || l1 \1 RSV0032969 CE REFRESHER . . . centration In the gas phase. However, the kinetics of vide sufficient surface to generate eatalyst for the the reaction were increased by increased surface. reaction. Such a tinned surface would also provide A pressure of about 7 to 10 atm. is probably used good heat transfer, and promote turbulence in the to obtain almost complete condensation and recovery fluids. The flns may be coated in some eases with cal of dlchloroethane--using water as coolant. Tempera cium chloride or lead chloride.14 As a result, reaction ture control may be a problem since the reaction is rates as high, or higher, than 0.3 Ib.-tnole/(hr.) (cu.ft.) highly exothermic (about 3D kilocal,/g.-mole of prod of dlchloroethane seem probable with temperatures of uct). A large excess of cue reactant or an inert gas is about 125 C. generally used for this reason. If only relatively pure Selectivity to the desired product of at least 93%, ethylene and chlorine react, excess ethylene would be based on ethylene, is possible; but tbe selectlvities used since excess chlorine would promote undesired decrease with increased temperature.17 The reactors side reactions and the formation of over-chlorinated are cooled with water, and pressure must be sufficiently products. low to allow complete vaporization of all products and Mixing the chlorine and ethylene is an important reactants The pressures depend on the amount of step. Equipment for combining the two gas streams inerts in the feedstream. should be designed to provide quick and intimate mix Exhaust gases from the chlorination reactors are ing so as to minimize local concentration gradients of cooled in two or more heat exchangers in series to the reactants. The temperature of each stream would about --20 C. in order to condense dichloroethane. also have to be carefully controlled; otherwise a run Uncondensed gases are contacted in two scrubbers; away, or even explosive, mixture might occur during first with water, and then with caustic, to remove small the mixing step. When excess ethylene is used, it must amounts of unreocted chlorine. The remaining gases be recovered and recycled for economical operation. are probably used as fuel. When the hydrocarbon stream contains ethylene in Tbe condensed product is stored In a tank, and the rather low concentrations as in the SJ3.A. process,4- liquid is primarily 1,2-dichloroethane plus small the '.emperature can no doubt be controlled without amounts of polychlorinnted materials formed by addi using large, excess amounts of ethylene. There are tive and substitutive reactions of chlorine. Ethyl chlo already adequate Inert gases present in the feedstream. ride is sometimes present. Dichloropropenes or di- A slight excess of ethylene is normally provided to chloropropnnes will be present if propylene or propane minimize the amount of unreacted chlorine in the were present in the feedstream*. Generally special process. attempts are made to eliminate or minimize C* hydro carbons in the feed because their chlorinated products Commercial Vapor-Phase Chlorination are difficult to separate by distillation from 1,2-dichlo roethane. Tbe combined condensed product from the Fig. 2 is the flowsheet for the vapor-phase chlorina chlorinators is fractionated to produce essentially pure tion of ethylene in a dilute hydrocarbon stream such 1,2-dichloroethane that is later cracked to produce - as the S.B.A. process. Liquid chlorine from a storage vinyl chloride and HC1. tank is pumped to a heat exchanger where it Is vapor ized and heated to perhaps 80 to DO C. The hydrocarbon Liquid-Phase Chlorination of Ethylene feed stream is also adjusted to a similar temperature in a heat exchanger. The two streams are combined Liquid-phase chlorinntions occur at essentially am at the inlet of the reactor. bient temperatures, from about 20 C. or higher, up to In one case, these reactors were apparently heat perhaps 70 C. In the Kureha process, temperatures exchangers, and the catalyst was packed inside the from about 50 to 70 C. arc indicated.11 but temperature tubes. Steel fins inside the tubes may be used to pro control is not critical.*" Pressures in the latter process 12S CBY 1110165 Morch 27, 19*7--Oiemieel Engineering &SV0032970 * If,4 1 'l to ntm., and the chlorine and ethylene * feddtllaavffeamr s are bubbled through the liquid that la niaiifly 1,2-ciichJoroethane. The mechanism for the liquid-phase chlorination re daction is less well understood than that for the vapor phase. Iii the polar liquid solution, the reaction is thought to be partly polar (or ionic) in nature. Chlo rine atoms presumably add one at a time to produce a half-chlorinated intermediate such as: m, - cn. or CHtC3-CH,+ This intermediate reacts with Cl" or Cl- to form the desired product. There is some evidence that part of the chlorination reactions are free radical in nature. Catalyst for liquid-phase reactions are metal chlo rides such as iron chloride. They are dissolved in small quantities in liquid 1,2-dichloroethane. These metal chlorides complex with the ethylene and likely provide chlorine ions for the reaction. The reactor walls often serve as additional catalytic sites for the reaction. The actual chlorination is thought to occur in the liquid phase. Hence ethylene and chlorine must be transferred from the gas pha*e and dissolved in the liquid. The following analysis is then applicable to each reactant, specified here aa 7?.1 Xrt transfer T J* hVt mu'tMml f Accumulation"1 [of A* from rail I -- 1 or H in the I -r 1 >f R in I to liquid pinin'J Lliquid plkiw J L liquid phase J For a differential volume dV of the reactor: (10) l*,.i((V-c,,)]<;r - k[f(Cu,c.,1.,1]fir + Jwi' oi) Eq, (11) can be integrated over the entire volume V. At steady-state conditions in the reactor, the accumula tion term is zero. The chlorination reactions can be assumed to be irreversible at reaction conditions; hence only the forward reaction has. to be considered. In Eq. (11), the exact form of the kinetic equation is unknown, but it is proportional to some function of the concentrations of various materials in the liquid phase. Eq. (11), or its integrated form, is most useful for indicating the effect of various operating variables on both the transfer of components between phases and the kinetics of the reaction* in the liquid phase. 1. Temperature affects the forward rate constant A; for the chlorination reaction, and indirectly the dis solved concentrations in the liquid of each reactant C* plus any other compound such as the product Cwfcrr. The equilibrium concentration at reaction conditions of the reactant Ct<* also varies with temperature. The surface tension and viscosity of the liquid change with temperature, and as a result the mass-transfer eoeflicient from the gas to the liquid l\A also changes. The iuterfaclal area A likely varies significantly with tem perature as the gas bubbles upward through the liquid, or as mechanical agitation i* applied. t Similar snslyre* hai e Iwn In.lje r,,r .ir,.m.uH- nitration* and for tl>e nil,} Union of ImOnK-tnr h>- f'Urm. Hup., Apr. 35, ID**. pp. iss-172: July 4. tact. pr> lo-liti. 2. Agitation or the type of bubbling affects h,A, and hence indirectly C. 3. Total pressure of the system and composition of the reuctant gases affect C*.* 4. Composition of the liquid phase affects factors that control mass-transfer resistances. High dissolved ratios of ethylene to chlorine are desired In order to minimize formation of byproducts, which sire mainly trichloro and other polychlorinated ethanes.--14 A better understanding of the reaction would be possible if the solubilities of the reactants in the liquid phase were known. In most cases, the actual solubility Ck is probably much lower than the equilibrium solubility Ck*--i.e., mass-transfer resistaiiccs are partially controlling. Commercial Liquid-Phase Chlorination Liquid-phase chlorinations are used in the processes of Shell Development Co.1"' and Kureha Chemical In- dustry Co.11 There may be significant differences in the liquid-phase processes but the following features probably apply to all. The product (1.2-diehloroethanc) is used as the solvent in which ethylene and chlorine are dissolved. A catalyst such as a small amount of ferric chloride is dissolved in the solvent, and rapid chlorination occurs in the liquid phase. In the Kureha process, about 0.06 to 0.0S moles of chlorine are introduced per mole of ethylene. The ethylene is chlorinated rapidly at 4 to 5 atm, and at 50 to 70 C. At such conditions, about 90% of the chlorine reacts to form the desired product whereas 05 to 98% of the ethylene reacts. Side reactions are negligible. In processes using essentially pure ethylene, a fairly large excess of ethylene is used in some vases, and unreacted ethylene is recovered and recycled. The conversions in the Kureha process are affected to some extent by the hyuroesrbon-gas stream used, which is the product stream from a cracking furnace containing several component- * -ch a* hydrogen, car bon monoxide, carbon dioxiac. tr.Khi ne and others. AU of these components are considered to be inert relative to the chlorination reaction. These inert gases act as stripping agents as they pass upward through the reactor. Similar conversions can be obtained, however, in those processes in which relatively pure ethylene is the hydrocarbon feed. Other factors affecting the reaction are: X. Methods of contacting the liquid phase with gas eous ethylene and chlorine. 2. Agitation at or near the entrance points for the ethylene and chlorine would help to minimize high local concentrations of reactants if a pool of liquid solvent were used. Bubbling the feed gases into the solvent would cause some mixing of it, but would not necessaiily provide adequate shear at the entrance points. 3. Recirculation, either internal or external, of the gases may be necessary in some cases. Nair21 describes a commercial chlorination reactor that is 1-m. dia. and G m. high. This reactor is packed Chemical Ensinteriiig--March 27,19(7 CBY 1110166 129 j ; j j I j j | j j ; 1 j V { ; ' } RSV0032971 CE REFRESHER w^th iron ceramic rings. Ethylene is introduced into the capital and operating costs for the liquid-phase the bottom of the reactor and bubbles upward through process are probably relatively low. the liquid and around the Raschig rings. Three exit lines are provided near the top of the reactor. The Acknowledgment lower of these is used to recirculate liquid (mainly 1,2-dichloroethane) through an external heat ex changer. Normal cooling water Is used in this ex changer to maintain recycle liquid at essentially 40 C. Chlorine gas is introduced into the recycle liquid just The author is especially appreciative that General Tire and Rubber Co. released information on their vinyl chloride plant for use in the preparation of this article. before it re-enters the reactor. The second of the exit lines at the top of the reactor is for crude dichloro- References ethane product and the top line ia for exhaust gases that may include any Inert gases introduced into the It Badische Anil In ttnd Soda Fabrlk A.Q.. wVinyl Chloride from Acetylene and Hy<ir*ustan Chloride.'* British Patent ?i*?,773 (Mar 3 3, system. 1, Bahr. K. nnd 7.tn'er. H. The Interaction ot Chlorine on Ethylene, X At,gur is, 333 (1930). The iron packing in (he reactor serves two purposes. 3. Bhatnagar, H. K Selection of a Process for Manufacture of EthlleiM Bichloride. Chivu Ape /niltir. Sept 1906. p. .*21- First, it maximizes the heat- and mass-transfer sur 4. Braconler. P, F Manufacture of vinyl Chloride Starting with Naphtha. Ox\Ken and Chlorine. Cheni. Age Jttdut. Juno faces between the gas and liquid. Second, it acts to produce iron chlorides that dissolve to some extent in 13*53.0. 473. 5 Bmconler, P F, How 80 A. Make* Vinyl Chloride, Hydro carbon Procc*. Nov I9t4, ji 140. the liquid, and are catalysts for the reaction. Produc $ Burke. 1> P and Miller. R. L.. Oxychkwlnation. Cfcnsi, Week, AUK. 25, i*U4, pp **3-11*. tion capacity of this reactor was not specified, but it is 7 Aret>lene Tr*imlMihm Syatems, Chem Sep. Sept S. I960, pp. 13M34. probably a small unit. A high local concentration of 8. FVe Plant Goo-* the Limit tor Safe Operation. Chcm. g > Sent 13. 1*413, pp IH4*U<. chlorine in contact with the product is probably un 9 Bold Stroke at Calvert City. Chcm Week. Auer. 29, 1944. pp 141-108. desired. The method for introducing chlorine to this reactor does not seem to be the ideal one, 10 Chopev, X, P.. Cheap Hydrocarbons Put Pressure on A.c.eBtky/liernve, CV Actu, 0Flulgp .( rFeCUb .5S8, 419r6w6V,j pp. S441*-S48 11. Conn, J. B., Kitl.ikow*ky, G B, and Smith. E. A. Hants Apparently some chlorination reactors contain pools of Organic Reactions----Vlj AudIHition of* "Halogen to O`lefins, JAOA. HU. 5744 <1938). of liquid solvent through which ethylene and chlorine 12. Gladlech, H,, How Fuel* Make Acetyleoe by DC Are, Uttdrocarbon firm tea ..lime p. 159. are bubbled. A mechanical agitator is probably used 13. Goml, S. Japan** New Vinyl Chloride Process, Hydro carbon t*roccitn . Nov. Pfil, p 160. to decrease mass-transfer resistances. Back-mixing might be a problem in such a reactor. To minimize back-mixing and to obtain higher conversions, two or 14. Ornll. H P. A.. Henme, Om Runt. F. F. and Vaughan, W. E, Chlorination of Olefin* and QleAn-Parafiln Mixture* at Moderate Temperature* Indined Substitution. Ind ng, Chrm 91, 1589 (1!39>. 15. Flowsheet*. J/Mroewrbon Process., Nov 1963, pp. 239-540: more liquid-phase reactors could be provided in series. Counterflow of the liquid relative to the gas is a possi Xov. l*irJi, pp 16 Knmptner, H K. Krause. W R and Sch liken, H. P.. High* Temperntnre Crocking. Chrm. Bug,. Frh S3, IPCfi, pp 93-nS 17. Kiunntner. H K,, Krause. W. R nnd SchUken. H P., bility, and it could be achieved using a tower column that is packed, or contains plates. Liquid-phase chlorination has been used for proc Acetylene from Naphtha Pyrolysis, Chrm, Bng,, Feb. 58, 1 ?GG. pp. 80-82 13. KnhlrmtiWi E., Traduction of Vinyl Chloride," French Patent 1.13H.124 (June 23. 1987), 14 Miller. S A. `Acetylene* its Properties, Manufacture, and esses in which ethylene enters in both dilute and con Uses,1* Vo la 1 nnd 2, Aindeudc j>resal No* York. 2963 and 3 966a 20 Nnlr. K. 8, Commercial Manufncturo of Vinyl Chloride centrated streams. The liquid-phase process would by Low-PreeKure Synthesis, Chrm. Ape ladtri, Jan 1463, p. 80. 81. Nnlr, K. 9- Notes on the Manufacture of Commercially appear to have several advantages as compared to gas- Important Product*. Chun, Ape lndw> May 1966. p, 385 25 Nexmeyanov. A. N , Qu.iai-CniripteK Orgftnometnlllc Com phase ones: pounds. null, ncail ari USSR. p. *29, Clxume *d chem. <101'). 53, Othmer. D, F., Make 3 to 5-cent Acetylene. J/pdroenrboa a. Better use of the heat of chlorination. Reactant /races* , M*r i p. 14 3. 1 '! streams can be introduced into the reactor relatively cold. A pilot-plant investigation indicated that heat 54. Rust, F. F. ami Vaughan, W E, The Hlgh-Tempornture Chlorination of Olehn Hydrocarbon*. J. Ora, t'ftcm., 3, 472 (1940). 25 Semenov, X, N, MS**me Problems hi Chemical Kinetic* and of reaction can be used to vaporize dichloroethane,* Reactivity.'* Vol. 1. pp 511-257, English translation by M, Boutinrt. Princeton University Press, 1958 With this technique, little or no iron chloride catalyst is carried over, and purification steps for the product are simplified. 20 Shilov. S. A nnd Rmlrnov*2atnkov, I V., ftterooehemlstrv and Mechanism of the Formntton and Decomposition of cl*- nnd trans-chlorovinyi Chloride. Dopox'idi Aknrf, XAttk Ukr. /? g /?* 1951. p 87 27 Societe Beige de L'Asote, Trace** for the Preparation of b. . Better temperature control of reactor. Large ex Vhtvl Chloride,** British Patent 4*14,79! (Apr. 4. 1944). 58. fttohnugh, 11 n,, Vet\lenc: How, Where, Vho-Futnre. cess of liquid, good agitation of the liquid, and excel Ilvdrorurhon Process,. Aug. 1866, p. 125 59. Wnshlmi. 1C. nnd A^kom, M, Computer Control of a i lent heat transfer between gas and liquid should be Vinyl Chloride Plant Provide* Procoss Optimization, Ckcm Eu. Oct. 24. JMfi, pn. 123-138. Nov. 21, 1966, np, 121*15e sufficient to maintain isothermal conditions in the reactor. 36 VVesselhoft, R. P,, Wonda^ J. >f. nnd Amlth, J M , Vinyl Chloride from Arrtrlen* nnd Hydrogen Chloride: CatalyllcRate Studies. AlChR, J , *, 301 (1959). c. Increased safety because chlorine and ethylene are not premixed. Key Concepts for This Article d. Large excesses of ethylene are not needed nor is an inert gas in order to maintain adequate tempera ture control. This advantage would be particularly valuable when essentially pure ethylene and chlorine are used as reactants. Based on rather sketchy literature descriptions, the 'eactor for a liquid-phase process is simple in design and operation. Corrosion is not a problem if precau tions are made to exclude water from the system.8 Both Active (6) Pnmihf (4) lnpiit/Feedateek(l) Guti*ut/Prilset (5) Reviewing ProE-csiwi** Acetylene* Vinyl chloride* Vinrhylol tlUc* nUthhl\ol*r*tmne** Ktlnlene dJcUioride* HynilarotieoUn*lorl- llvd,innbe\iMrtriocuhslo* ride, ClhrhtalUin* V,tjor iihsm'* Liquid pltttse" (Words In bold are m)e Indlcntor*: numbers correspond to EJCAlChK system cxrpt for Hole 8 mndldcntlon. /Vsterlsks m.irlc key concept* *ugKesiril fot Indexing. Other* are added to Improve rending us mi .ibstnut. Inrirxing In described In Chvm. /.`irry.. Oct |1, 19*5, )i. ik": nr \o may order Key Concept reprint, &0& uilnc Header Service Postcard.) 130 CBY 1110167 March 27,1967--Chemical Engineering RSV0032972