Condensation of Acetylene by Molten Salts

Condensation of Acetylene b-y J

Molten Salts PHILIP C. JOHNSON' .4YD SHERLOCK SWdNiV. JR.

Vnicersity of Illinois, Urbar-Lu, Ill.

A wetted-wall column with molten salt and gas in direct contact passing countercurrent to each other proved successful in controlling the highly exothermic reactions involved in the condensation of acetylene to aromatic hydro- carbon liquids. Numerous salt systems, all of them halides, were investigated over a temperature range of 530" to 625" C. Both pure acetylene and acetylene-ethylene mixtures were studied. Molten systems containing zinc chloride were found to catalyze the reaction as well as to

STUDY of the polymerization of acetylene was undertaken A because of a pressing need for benzene in the war effort The following pertinent conclusions concerning this reaction were reached from a comprehensive survey of the literature.

The most suitable reaction temperature lies between GOO- 700' C., the slow rate of reaction belop 600' C. setting the lower limit, and the spontaneous decomposition of acetylene to carbon and hydrogen with rapid propagation of P flame setting the upper limit. A series of condensation plus hydrogenation reactions takes place between acetylene and hydrogen a t temperatures up to 350' C. (5, 6). However, the products are principally of a non- aromatic nature, so that the results obtained in this temperature range are not comparable to those obtained a t higher temperatures.

The pnncipal reactions involved are of an extremely exothermic nature.

3C2H2(g) + C G H B ( ~ )

ATIux--~Mo c = - 1 q O O O g -cal. /$.-mole CBH6

3C,H,(g) + 6C + 3EIz

1H6W--700~ C. = - 162,000 g -tal /3 g -moles C,B.

It is believed tha t this high heat of reartion is the cause of most of the difficulties encountered in the work carried out so far. Local overheating in the catalyst bed certainly occurs. Lewes (12) actually measured a localized overheating of about' 200" C. in passing acetylene through a silica tube a t 800" C. This local overlieating has two serious effects. It promotes the spontaneous decomposition of acetylene to carbon and hydrogen and fa- vors the formation of the higher polymers. Once started, this decomposition is difficult to control because the heat liberated raises the surrounding gas to decomposition temperature, and, as expressed by several investigators (2, g l ) , the light fluffy carbon which is the main product of decomposit,ion actually catalyzes the decomposition of more acetylene. BerI and Hofmann (2) were, however, able to get a 98.8% yield of liquids by removing this fluffy carbon with water vapor as fast as i t was formed. The water vapor did not affect the layer of graphitic carbon serving as catalyst in their experiments.

The free energy change of the reaction 3C1H2 --j CaHs(g) is very negative (18) and, although it decreases n-ith a rise in

Present address, Carbide and Carhon Chemicals Corporation, South Charleston, W, Va.

J

s e n e as media for temperature control. Distillation of the products obtained under varying conditions yielded nearly identical fractions in every case. These fractionq, furthermore, were similar to those obtained by previous investigators using higher reaction temperatures and carbon catalysts. The results indicate that, in certain cases, fused salt systems could replace to advantage the heat ex-

change tubes commonly found in commercial catalyst beds.

temperature, the equilibrium lies 99.5% in favor of benzene at 1000°C. Equilibrium is not, therefore, a factor in the choice of operating conditions.

The only catalyst found so far for the high temperature polymerization is carbon. It is of particular significance that, in all experiments, heavy layers of carbon are deposited on the so- called catalysts. That it is this deposit of carbon which is the true catalyst has been suggested by both Tiede and Jenisch (20) and Iiovache and Tricot ( I O ) . Tiede and Jenisch, using several different contact material3 with a very carefully controlled temperature of 610" C., obtained oil yields which were identical in amount and gave identical fractions upon distillation. Identical yields were also obtained at a carefully controlled temperature of 600" C. with another series of contact materials. However, similar preliminary experiments in rshich the control was 600" C. * 10% showed erratic results for this same series of materials. Furthermore, these results were not reproducible. This lack of agreement, which was due to temperature alone, could easily ex- plain the differences in apparently similar experiments reported in the literature by different investigators. The actual contact material was probably the same in every case-an adherent type of carbon covering the tube packing.

K i th these facts in mind, the authors considered that a molten salt bath, because of several unique properties, would be particularly desirable as a medium for carrying out this reaction.

FEATURES OF AIOLTEN SALT BATH

l loltcn salts have a high specific heat which affords excellent temperature control. Although data on specific heats of molten salts are not plentiful, an average value of 0.3 g.-cal./gram/" C'. is a good approximation. With a density of 2 grams/cc. the heat capacity of 1 cc. of molten salt is equivalent to about 3000 cc. of acetylene at GOO" C.

The composition of a salt mixture can be varied over a wide range by adding various amounts of fresh salt to an existing mixture. This eliminates the necessity of repeating a tedious catalyst preparation for each of a series of catalysts being tested. Here the catalyst, preparation is not an a r t where physical struc- ture plays an important role, since all catalysts are prepared by mixing quantities of anhydrous salts and melting them t o a liquid bath.

The molten salt can be circulated through a column and any fluffy carbon which has been formed will be continually washed from it. I n this manner the decomposition of acetylene catalyzed by this t,gpc of carbon can be kept at a minimum. Carbon re-

This bath is always reproducible.

990

October, 1946 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY 99 1

moval will also prevent the column from becoming blocked, as is the case with stationary catalvst, beds.

The catalyqt d l be the salt rather than the carbon-covered packing material, as in stationary catalyst heds. The carbon \\-aslied from the column rises to the surface af the salt reservoir. By pumping carbon-free salt from the bottom of the reservoir, the column is a lx iys n e t with pure molten salt.

APPARATUS

;i ,series of preliminary experiments to determine possible ac- tir-it>- of various salt tctms was curied out i n a stoppercd 38 X 300 nim. test tube equipped with inlet and exit gas tuhes. The test tube v,-m iriinic~racd in a ralliant coil furnace and was half full of molten salt (ahour 200 gram>). Acetylene was admitted through a glass inlet tube to the bottom of the bath. The rcitc- t ion products were passed through ice and dry ice-isopropanol traps to remove the condensabies. Snrnplea of the noncondeiis- able gases n-ere analyzed for unreaeted acetylene, unsaturated and saturated hydrocarbons, and hydrogen.

From the preliminary studies it x a s decided definitely that the time of cuntact necessary for appreciable con Riderably exceed that attainable by allowing a gas bubhle t o rise through a layer of liquid. Consequently, a column n-as devised in which the gas arid liquid salt puaed coiiutercurreiit to each other (Figure 1). In this systeni time of contact %\-as e a d y controlled, since i t is a function of rate of gas input and volume of the column. The eolunin was made by putring tn-enty-five pairs of indentations into opposite sides of a 25-mm, Pyrcx tube, the indentations covering a 50-em. section of the tube. The faces of the indentations were designed so that all surfaces could be mashed continually by the flowing salt t o prevent dcposition of carbon.

The molten salt \vas circulated by a bellows pump. The salt was prevented from entering the bellows by an air bulb placed in the line between the bellows and the salt reservoir. On the buction stroke the air was drawn from the bulb into the bellon-s x-liile the salt entered the bulb. On the compression stroke the air was forced from the bellows back into the bulb and the salt, in turn, n-as forced up into the lines leading to the top of the column. The valve system, also shown in Figure 1, was made of 6-nim. Pyrex rod enlarged to a 9-mm. ball on one end. The valves fitted into a seat made by shrinking 14-mm. tubing to approximately 6 mm. Both the hall surface of the valve and the seat were ground with carborundum dust and oil to give a good seal. Very little difficulty \vas encountered once the pump was in operation but it ~ i i s often difficult to start.

The products leaving the top of the column turned, upon cooling, to a cloud of small liquid drops in an excess of noncondensable gases. The drops were so small that they were impossible to remove in a 'condenser, in traps, or even by stripping through an oil bath. .4 combination water-cooled condenser and Cottrell precipitator \\-as, therefore, set up. The design of the precipitator n-as similar to that used by Billman and Cash (5). In addition to the condenser-precipitator, the exit gases passed through ice and dry ice-isopropanol traps.

The pulsation of the salt bath caused by the pump, and the slight increase in pressure drop through the column as the run continued, made it necessary to use a flow system which would give a constant rate regardless of the changes in downstream pressure. Such a flow was obtained by using nozzles with an upstream pressure so great tha t the linear velocity of the gas through the throat of the nozzle reached the acoustic velocity while the pressure in the throat still exceeded the dov,mstream pressure. L-nder such conditions the flow rate of the gas is a function of upstream pressure and is entirely independent of the downstream pressure so long as this pressure does not exceed tha t in the t'hroat of the nozzle (19). .4 series of nozzles was made by collapsing 8-mm. tubing and then calibrating. By using up-

W E L L

SPHERICAL

SECTlON SHOWING INDENTATIONS

22 CM.

Figure 1. Reaction Column

stream pressures of 30 to 60 pounds gage, constant flow mteswerr maintained regardless of the changes in pressure in the reaction system.

PROCEDURE

Figure 2 shows the complete arrangement of apparatus as used in actual operation. A new column and salt pump were used for each experiment. The purification train consisted of 5.25y0 sodium hypochlorite solution, 50y0 sodium hydroxide solution, 75% by weight sulfuric acid, and a U-tube containing calcium chloride. The salt reservoir was a 64 X 300 mm. test tube. Radiant coil furnaces surrounded both the column and the salt reservoir. The section between the two furnaces was heated by means of a loosely wrapped coil of asbestos-covered resistance wire.

The gas leaving the dry ice trap passed through a three-way stopcock which permitted it to exhaust to an aspirator or to en- ter a sample buret. The total exhaust flow rate was measured a t hourly intervals by means of this buret, and the sample was transferied to an Orsat apparatus for analysis.

First the column volume was determined by filling it with water and measuring the amount coming from the reaction zone on draining. The column was dried and placed in the furnace. The salt reservoir was lowered and filled Kith salt. Both column and salt were heated to approximately 525' C. During the heating period the flow rate was calculated, based on contact time, column volume, and reaction temperature. The proper nozzle was selected and the gage pressure adjusted so that the desired flow

Experiments were carried out in the following manner:

992 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 10

of acetylene, determined by actual measurements, !T-a? obtained. The reservoir was then raised around the column base until it touched the gas entrance tube. The resistance coil was wounJ around the exposed section and brought UP to temperature. The salt Pump started and adjusted to give its maximum flow of aPPoximateb' 250 CC. of salt Per minute. The column was flushed with natural gas and then acetylene admitted to the colurn~i. During Ihe next 20 minutes the column and salt Kere brought to the desired experiment temperature. At' the end of this period (liquid was usually just beginning to drop into the ice bath), the time clock was started. During the next 10 hours the flow rate of the exit gas vas taken every hour and the sample of gas obtained was analyzed.

At the conclusion of the run the salt Pump and furnaces 4 w e shut off, the acetvlene was disconnected, and the apparatus was flushed with natural gas. The inlet acetylene rate was checked again, and an arithmetical mean of the two values m s used to compute the total f l o ~ to tile column. The liquid product was sealed in a samplc bottle for later distillation.

DISCUSSIOS OF RESULTS

The of the preliminary experiments using the 3-incll layer of molten salt in a test t,ube are given ill Table 1, Althoug)l contact. bet%-een salt and gas was admittedly poor, enough contact resulted to give small amounts of liquid condensation product when salts possessing catalyt,ic activity xvere used. All salt bath conlpositions in Table I are given in mole per cent unless other- \vise stated and, in cases TThere the coIlstitution diagram is knolvn, t,he reference is given. The melting points are those at \ylijch a single liquid phase exists having the samp compo;ition as the solid salts making up the bath. -411 chemicals us!:tl were of reagent grade.

Studies covering the low melting salts and their binary mixtures shoned that 0111s zinc chloride appears to have any catalytic activity for the forlnatiorl of liquid products from acetylene. H ~ , ~ ~ ~ , ~ ~ , in the pure state this salt catalyzed the dccompositiol, to carbon and hydrogen, ~~~~~~i i,.~lo used zinc chloride on c~larcoal and pumice to catalyze this Same reaction, ohservetl considerable decomposition. All salts except sodium chloride were unsatisfactory as diluents. With SOYG sodium chloride its activity for the decomposition of acetylene, although depressed, DISTILLATION AND ANALYSIS OF PRODUCTS

Hourly analyses of the waste gases were made during the course of an experiment. The samples were analyzed for acetylene and unsaturated hydrocarbons. The acetylene absorbent was a solution of 20 grams of mercuric cyanide in 100 cc. of 2 sodium hydroxide. This solution absorbs acetylene without affecting ethylene and other unsaturated hydrocarbons ( I ) . The unsaturated hydrocarbons, mainly ethylene, were absorbed in luniiny sulfuric acid. The remaining gases consisted almost entirely of hydrogen and saturated hydrocarbons. All waste gases remaining after the acetylene was removed were designated as inerts. Periodic analyses of the acetylene and ethylene used showed t h t each contained less than 1% of inert gas. In all calculations, therefore, these gases were assumed to be 1 0 0 ~ G pure.

The liquid product was fractionated in a column of small liquid holdup. The column vas made by wrap- ping a 75-cm. length of a/le-inch rod with a spiral of L:s-inch copper tubing. The individual turns of tubing were wound on '/4-inch centers along the entire length of the rod. Thc rod and spiral were then forced into a 14-mm. Pyrex tube. The column was jacketed with a 19-mm. Pyrex tube which was wound with resistance wire. The reflux rate was controlled by the poxer input to this coil. All distillations were carried out a t atmos- pheric pressure which was approximately 750 mm. Be- fore distillation the specific gravity of the product xas determined by means of a Westphal balance.

0 n

was still great. However, the melting point of baths with great,er amounts of sodium chloride were too high for use. Ternary systems \yere next investigated in order to reduce the concentration of zinc chloride still further. The first bat,h, a eutectic of sodium chloride, potassium chloride, and zinc chloride, gave great promis. and \vas used during most of the subsequent work.

The two experiments in Table I show tha t this bath possessed considerable catalytic activity. However, the contact !vas poor and eventually the tube became blocked with carbon. -1 wetted wall column was used, therefore, to improve contact and to remove carbon as it mas formed.

Table I1 presents the results using pure acetylene gas with various salts in the column. The definitions of the terms used are

Figure 2. Complete Apparatus Assembly

October, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 993

TABLE I. RESULTS OF PRELIMINARY EXPERIMENTS WITH ACE:YLEKE AS INLET Gaa

Salt Bath d l I s 8 1 5 . l lBr r19% NaBr

50% .UCIs-50% S a C l

58.9% . i l C 1 ~ - ' 4 1 , 1 ~ S a c 1

ZnClz

ZnClz

ZnCh

(binary eutectic)

70 wt. % ZnClz-30 wt. % FeCll (briars eutectic)

50% ZnClz-50(7o CulClz

cusc12

5fl% ZnClz-50% NaCl

PbCli

5% ZnC12-95% PbClz

50% ZnC12-50% PbClz

90% ZnCls-lO% PbClz

12.5% NaC1-59.0% IiCl -28.5% ZnClz (ternary eutectic

12.570 h-aC1-59.0% KCl -28.5% ZnCl2 iternarv eutectcc)

Expt. NO.

2 1

23

21

4

5

6

3

7

8

10

12

10

18

19

24

25

Bath

C. 191 90

250

hJ.P.,

124

318

318

318

214

350

422

350

501

490

375

290

402

402

Temp. of Expt . ,

210-250 10*250

250-500

13&150

400-450

510

600

c.

250-400

500

500

5'40

550

600

500

550

550

600

Dura- tion of Expt.,

Hr. 1

3

1

1

1

1

0 . 2 5

. . .

3 . 5

0 . 5

1 . 3

1 . 5

4 . 0

3 . 5

5 . 0

Waste Gas CtHh GHz H I satd. hyd;oc&hons

CzHi

CiHz

CzHz, Hz, satd. hydrocarbons


CzH2, Hz, satd. hydrocarbons

CZHZ. Hz, satd. hydrocarbons

CZHZ, Hz, satd. hydrocarbons

CZHZ, Hs. satd. hydrocarbons

CzHz, Hz, satd hydrocarbons

CzHa, H2, satd. hydrocarbons


CzHz Hz satd hyd;ocLrhon$


CzHz. Ha, satd. hydrocarbons

CZHI, HI, satd. hydrocarbons

Liquid Formation

None None

Kone

Sone

None

Heavy cloud

4 g.

Trace

1 . 5 g.

None

of drops

1 1 . l g .

Trace

Trace

Trace

Small

7 . 0 g . amounts ,

l l . 6 g .

Carbon Formation

None Excessive amounts

None

Kone

Excessive amounts

Excessive hut less than a t 450' C.

Excessive amounts

Trace

Excessive amounts

Excessive amounts

Considerable amounts

Trace

Trace

Trace

Excessive amounts



Comment No activity Extremely active for de-

compn. of CzH2, even at 90 activity, low vapor pressure of AlClr, even at 550' C.

No activity, high vapor pressure of AlCls

Carbon blocked reaction tuhe: bath viscous

Bath appears more EO- tive for liquid formation a t higher temp.

Bath active for formation of liquid, too active in decompn. of acetylene

Bath unstable, gave off FeCla or FeCL fumes

Bath too active in decompn. of acetylene

Carbon blocked appars- tus immediately, too EC- tive

2 ~ 7 ~ conversion to liquid, hath definitely active for liquid forniation

S o t active, ossible diluent for ~n81z

Kot active, free metal in bottom of bath

Kot active, free metal in hottom of bath

Too active in decompn. of acetylene

23.2% c o n v e r s i o n t o liquid

24.7% c o n v e r s i o n t o liquid

1000 c,.

given at the end of the article. different baths will be discussed separately.

The results obtained with the

SODIUM CHLORIDE-POTASSIUM CHLORIDE-ZINC CHLORIDE SYSTEM

The eutectic bath of 12.5% sodium chIoride-59.0% potassium chloride-28.5yo zinc chloride (8) showed the most favorable catalytic activity of all systems investigated. The bath melts at 402' C. to a water-white, nonviscous liquid which can be pumped with ease. I t s specific gravity varies from 1.95 a t 500" C. to 1.85 a t 600" C. The bath is stable to the atmosphere up to 625" C., but above this temperature fumes of zinc chloride appear.

The first seven experiments in Table I1 n-ere carried out with this bath. Let us consider first the conversion to liquid product obtained in a single pass. The data are summarized in Figure 3, where curves are given for both the 10- and 15-serond contact. For the 10-second contact the conversion increases from 12% at 550" C. to 54% at 625' C.; with 15-second contact a 48% conversion a t 550" C. is increased only 7% at 600" C. The slight increase in an already high conversion by a more drastic condition may be explained by Figure 4, which shows the concentration of acetylene in the exit gases for these experiments. For the 15- second contact the acetylene in the waste gas fell only from 74 to 677,, although the temperature was increased from 550" to 600" C. Apparently the diluent effects of the 30% inert gases after 50% conversion, plus the additional diluent effect of the vaporized reaction product, are sufficient t o retard further condensation. This decreased reaction at lower acetylene concentra- t'ions eliminates the possibility of successful application of this catalyst to gases containing only small amounts of acetylene.

The mean value for the yield of liquid product for experiments 28-33 was found to be 68%. Any divergence from this value shows n o consistent correlation with either contact time or temperature. .4pparently 68% of the acetylene disappearing ap-

peared as liquid product. Combustion analyses of the inert waste gas showed that a n additional 207, of the reacted acetylene was converted to hydrogen and saturated hydrocarbons. The remaining 12% was lost, probably by solution in the salt bath, with subsequent liberation t o the atmosphere.

A summary of the data for experiments 26-33 indicates tha t the yields of products are independent of ,operating conditiona and that , with a contact time of 15 seconds, the conversons are affected only slightly by changes of temperature within the range investigated.

E F F E C T O F ZINC CHLORIDE COXCENTRATIOX

The possibility of changing results by varying the concentration of zinc chloride has been investigated. For this work a binary eutectic of 58y0 lithium chloride-42% potassium chloride having a melting point of 364" C. was used. T o this bath were added varying amounts of zinc chloride. The ternary systems formed a single liquid phase at the temperatures of the experiments. The results are presented in Table I1 under experiments 36-39.

The results with these baths were not particularly successful. The lOy0 zinc chloride bath exhibited low activity a t 550" C.- 19.1% conversion compared to 46.7% conversion under the same conditions using the ternary eutectic. The bath with 2.5% zinc chloride gave off strong fumes of zinc chloride, and gave low yields of liquid product and excessive amounts of carbon and fixed gases. This bath was too active for the decomposition of acetylene to carbon and hydrogen.

The unfavorable results obtained with these two baths indicate tha t other factors besides the zinc chloride content are important in the bath composition. I n the sodium chloride-potassium chloride-zinc chloride eutectic, the sodium chloride and potassium chloride are not serving merely as diluents. The favorable catalytic activity of this bath may be due to actual compounds of these inorganic salts.


50. Y 9

40. J

0

Z 30. 0 II: W t 20.

a\"

v

10-

Mean Time of Inlet Gas Expt . Wt. of Wt . of Concn. % CzHp yo Con- FlC Yield Cc. Inerts/ G . Liquid1

E r p t . Temp. Contact, Kate, Duration, Liquid, Carbon, C?H? in Disap- version t o of c c . iC?lI? c;. s o . c. Sec. Cc. , Sec. Hr. G. G. Waste, '5 pearing Liquid l iqu id Di-npprunrigj Curbon

Catalyst, 1 2 . 5 % NaCl-5Ll.070 KCl-28.5% ZnClr

26 530 8 85 5 6.5 1 2 5 3; 9 1 5 08 7 32 0 12 8 40 1" 0 0270 8 4

32 350 1 3 0 3 0 10 0 58 3 8 2 i 4 60 0 0 16.5 7 1

30 575 10 0 4 4G 10 0 4-1 3 4 . 1 93 5 33 G 23 8 66 z 0 1 2 8 10 8 GOO 10.0 4 29 10 0 fi6 2 11 .i 83 0 2 8 . G 27 0 63 0 0 . 1 4 i i 8

31 5i5 15.0 3 1-1 9 " 5 63 2 5 9 6 8 . 2 7 1 5 5 2 0 G9.i 0. 1.57 10 i 33 600 l j . 0 2 8 3 8.25 5 3 4 5 7 6 0 . 3 7 6 5 5 4 . 8 7 1 6 0 136 9 i

29 2s 625 10 0 4 63 10 2 5 105 6 8 8 0 3 9 ,.,. p i; ; 68 8 0 l t j l 12 0

Catalyst, Binary Eutectic of 5S% LiCl-I?C, KC1 with ZnClz Added

36 550 15 0 3 2 2 5 0 1 2 9 1 . 0 85 2 3 . 2 19 1 7 2 5 0 14,; I ? 9 37 600 15.0 3 01 5 0 33 6 2 4 7 3 . 4 73 .8 5 3 . 3 7 2 . 5 0 133 !4 0

Catalybr, Binary Eutectic of 58CA LiCl-4'2% KC1 with 2 5 5 ZnClz Added

38 550 1. i .n 3 1 : ,5 0 19 4 4 . 9 7 : 8 5 7 . 0 2 9 . 2 51 3 0 216 1 0 39 600 15.0 3 03 1 5 10 .0 b

Catalyst, 24.0% SaCl-43.070 KC1-33.0r7, CdClz

34 550 15.0 3,50 5 .0 8 9 1 . 0 9 6 . 0 2 i . 2 1 2 . 2 1 8 . 4 0 . 12.5 8 U 35 600 15 .0 3 . 1 8 1 , 5 c 7 . 1 0 . 8 8 1 . 8 6 3 . 3 3 5 . 6 56 4 0.129 8 9

a The conversion and yield of liquid product are too low in this experiment because no Cottrell precipirator was used and a large amount of t h e liquiil e*-

f The tube hecame hlocked with carbon after 1 . 5 hours and the erperiinent was ended. ea ed in the form of a cloud of small droplets.

One of the furnaces burned out after 1.5 hours of opeiation, and the experiment was ended.

The Cottrell precipitator was used in all subsequent experiments.

SODIUM CHLORIDE-POTASSIUM CHLORIDE-CAD3TIU31 CHLORIDE SYSTEM

Since zinc chloride exhibited such favorable catalytic activity, it n-as believed that other elements in Group I1 of the periodic system might be active as well. Therefore, a bath of somewhat aimilar composition to the zinc chloride ternary eutectic, but in which the zinc chloride w ~ s replaced by cadmium chloride, was used. This bath, a ternary eutectic of 24.073 sodium chloride- 43y0 potassium chIoride-33,0% cadmium chloride (a), has a melting point of 386" C. The molten bath was a clear, nonviscous liquid xvith specific gravities of 2.32 a t 500" C. and 2.2-1 at 600" C. The molten salt did not wet the walls of the column but acted like water flowing down an oily glass tuhe.

Two experiments '17-ere carried out x i th 15-second contact a t 650" and 600" C. The results are given in Table I1 and plotted in Figure 3. The conversions and yields of liquid product were much lower than those obtained using tlie ternary zinc chloride eutectic under similar conditions. The bath had little if any catalytic activity. AIost of the product probably resulted from contact hetwcrn the gas and carbon dcposited on the column walls. The reaction wa3 probably catalyzed by carbon n-ith the added feature of close temperature control by means of molten salt

ACETYLENE DILUTED WITH ETHYLENE

The possibility that the ternary catalyst 12.5n0 sodium chloride--59.0% potassium chloride-28.5% zinc chloride would act 011

ethylene or mixtures of acetylene plus et liyleiie was consitlcrcd next.

.4n experiment (not shown here) was carried out with pure ethylene with 30-second contact a t GOO" C. =iitc,r 30 minutes of operation the exit gas shoxed 7.5T0 inerts, but no liquid product or carbon appeared. The bath vas not active enough to condense pure ethylene.

Experiments 41 and 42 viere carried out x i th IZ-mond contact time and a gas containing 25% ethylene and 75y0 acetylene. The conversions of 18% and 41 % arc below those using pure acct- ylene, but the main point of interest is that slightly o v e r . 2 0 5 of the ethylene entering the column disappeared. The ratio of acetylene to ethylene disappearing varies considerably x i th temperature; i t is 4.3 to 1 a t 550" C. and 10.3 t o 1 a t 600" C. These experiments indicate tha t the rate of condensatiorl of acet-

The results are presented in Table 111.

ylene n-ith itself increases more rapidly n it11 temperatiire than the rate of reaction of acetylene with ethylene.

60 1

5 50 575 600 625

TEMPERATURE, OC.

Figure 3. Conversion of .icetyleue to Liquid on a Single Pass (aboce) and Amount of Acetylene in Taste Gnseh

(below) 3 12.5% NaCl-59.070 KC1-28.5% ZnCln and 10-second contact 0 12.5% NaC1-59.070 KC1-28.570 ZnCli and 15-second contact

24.070 NaC1-43.0~0 KCl-33.0vo CdCli and 15-second contact

October, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 995 -

TABLE 111. RESULTS USING ACETYLESE-ETHYLENE ~ I IXTCRES IS RE.ICTIOS CoLunm (Catalyst, 12.5% SaC1-59.0% KC1-28.5% ZnClr) hIolee

CtHi Disap-

Inlet con- Gas Rates, 2:- wt. of Wt. of in Waste C2H2 C2H4 Conver- Yield CZ%) Liquid/ CZH4

Cc. Inerts/ cc. pearing/

Expt. Temp., Gas, % tact Cc./Sec. tion, Liquid, carbon, Gas, "0 Dis;ip- Disap- sion to of Disip- G. Disap- S O . 0 C . C*H* C ~ I I ~ ~ e c . ' C~H, CJG Hr. G. G. C ~ H * c~?& pearing pearing Liquid Liquid pearing Carbon pearing

Time of Inlet %lean Conen. ~ 7 0 % % % ( c ~ H ~ + G. M O ~ S

41 550 2 ; 25 15.0 2.41 0.78 5 . 0 12 .0 1 . 3 7 0 . 5 2 5 . 4 28.7 2 0 . 5 1 7 . 6 6 6 3 0.118 9 . 2 4 . 3 42 600 , a 2a 1 5 . 0 2 . 2 8 0 . 6 9 6 . 0 31.4 3 . 3 4 4 . 2 3 6 5 ( 1 . 9 23 .2 4 1 . 4 6 9 . 0 0 1 5 5 9 . 5 10.3 43 GO0 50 50 15.0 1.52 1 .52 12 .0 44 .1 -1.3 28 .4 6 1 . 3 62 0 1 7 . 7 2: 8 71 .5 0.173 1 0 . 3 3 . 5

Because of the low conversion n-ith 76% acetylene at 550" C., a singlca i,xprriment with a 50% gas was carried out a t 600" C. and l5-second contact time. The conversion fell from 41%, obtained with the 75% gas, to 28% for the 50% gas. Onlg 18% of the ethylene was used compared to 627, of the acetylene, a 3.5 to 1 ratio of acetylene to ethylene. It might be expected that the character of the product in these experiments would dif- fer from that of previous products because of the reaction of appreciable quantities of ethylene. Kozlov and Fedoseev (11) showed that butadiene was formed when acetylene and ethylene were allowed to pass over certain catalysts, including zinc chloride.

DISTILLATION OF PRODUCTS

,411 liquid products were fractionated in the column described and the results are given in Table IV. About 1% of phenyl-& naphthylamine was added to each sample before distillation to prevent possible polymerization of styrene or other similar hydrocarbons. The first drop appeared in the receiver a t a vapor tem- pernture of 40" C. The first fraction was taken off to a final vapor temperature of 90" C. Upon redistillation of some of this material, a fraction representing over 90% of the charge came over between 80-81 C. This fraction had a refractive index of 1.5000 a t 20' C. This compares favorably with the literature refractive index of 1.5014 a t 20" C. given for benzene; therefore the first fraction in these distillations t i l l be called the benzene fraction in the folloving discussion. Only about 1057, of the charge distilled over in the interval 90-150" C.; this fraction probably coiisisted of toluene and xylenes. The fraction from 150-225O C . started to distill over as a liquid; then considerable quasititicbs of a nhite solid, and finally more liquid, appeared.

The solid was identified as naphthalene by its odor and its melting point of 80-81 ' C. It composed roughly half of this fraction. Slight evolution of a noncondensable gas as the vapor tcmpera- ture approached 225' C. gave evidence of cracking of the residue. This residue was fluid a t the still temperature when the distillation stopped, but it set to 8 firm tar upon cooling. Further iden- tification of the individual compounds in the fractions was not attempted. An indication of the types of compounds present may be found in a series of papers by Xeyer (14-17).

The product of experiment, 26, in which no Cottrell precipitator was used, showed a small benzene fraction. This was t o be expected and these results are not included in the following discussion.

Considering experiments 28-33, the specific gravity of the product increased both with temperature and time of contact. A similar increase is observed in the amounts of the tar fraction. I n both cases, the increase is more noticeable with the 10-second contact time. To counter this increase in tar a corresponding decrease in the benzene and naphthalene fractions occurs. Al- though these trends are present, the variation in any fractions over the entire range of contact time and temperature is negligible for all practical purposes. It may therefore be stated that, in addition to the yield, the composition of the liquid is also constant throughout. The fractionation is roughly

To 90' C. 50- 1 50' oC. 150-225 C. Residue

40% 1 0 5 20% 30%

The product obtained with the 10 and 25% zinc chloride systems separated into fractions similar in size t o those of the ot,her

TABLE IV. DISTILLATION ANALYSIS OF LIQTID PRODUCT (Pressure, 750 rnrn.; crude gravities a t 20')

Wt. yo in Boiling Range: Weight Conditions Liquid Specific

Expt. c- Product, Gravity, T o 90- 125- 130- Residue, Loss, K O , c2h2 c:h c. Sec. Grams Crude 90' C. 125' C. 150' C. 225' C. ICt. $3 Kt. % Comment

Catalyst, 12.57, XaCl-5Q.0% KC1-28.5% ZnClx

26 100 . . 530 8 85 28.2 0.952 3 0 . 8 1 .8 1 0 . 3 19 .8 31 .6 5 . 7 No Cottrell precipitator 30 100 , . 575 1 0 0 3 8 . 6 0 .959 4 2 . 5 3 . 1 3 . 1 2 2 . 8 22.0 6 . 5 . . . . . . . . . . . . . . 28 103 , , GOO 10 0 5 7 . 5 0 .964 3 8 . 6 3 . 5 4 .7 2 1 . 2 29 .6 2 . 4 . . . . . . . . . . . . . . . 29 100 , . 625 10 .0 99 .1 0 9 7 4 3 8 . 2 6 . 9 4 . 0 1 7 . 4 3 3 . 4 0 , l . . . . . . . . . . . . . . . 32 100 , , ?2o 15 .0 5 1 . 7 0 9 7 3 35.2 5 . 8 2 . 3 20.5 3 1 . 8 1 . 4 . . . . . . . . . . . . . . . . . 31 100 , , a , a 15.0 55.5 0 . 9 7 9 38 .0 5 . 4 2 9 2 1 . 2 3 2 . 2 0 . 3 . . . . . . . . . . . . . . . . 33 100 , , GOO 15.0 4 8 . 7 0 . 5 8 0 3 6 . 6 5 . 8 3 . 3 1 5 . 9 33 .4 1 . 0 . . . . . . . . . . . . . . .

Catalyst, Binary Eutectic of 58% LiC1-42% IiCl with l o g Z n C h Added

38.37 100 . . . 550-600 15 40 .1 0.566 39.4 6 . 0 2 . 7 19.7 27.4 4 . 8 . . . . . . . .

Catalyst, Binary Eutectic of 58% LiCl-42% KCl with 2 5 7 , ZnCls Added

38 ,39 100 ,. 550-600 15 2 4 . 4 0.555 36 .0 4 . 5 7 . 0 2 0 . 8 . . . . Residue lo3t

Catalyst, 24% NaC1-43% I<C1-337, CdClz

34 ,37 100 , . 550-600 15 11.0 0.940 52 .7% boiled below 83' C. 36 .4 10.9 Sample too small

Catalyst, 12.57, SaC1-59.O% RC1-28.57, ZnClt

41,42 7 5 25 550-600 15 38.8 0.942 32.0 1 1 . 6 4 . 6 2 0 . 9 28 .6 2 . 3 10% came over below 50' C. 43 50 50 600 15 38.2 0 .914 31.6 14 .1 4 . 2 2 0 . 4 25 .2 4 . 2 L O 5 came over below 50' C.


zinc chloride baths. ilpparently changes in zinc chloride concentration affect only the conversion obtained on a single pass.

The 11-gram sample obtained from the cadmium chloride catalyst was too small to fractionate successfully. About 50% came over below 83 O C., but a t this time the still was nearly empty so the distillation was discontinued. The yield of benzene obtained with this catalyst was considerably higher than that obtained with catalysts containing zinc chloride, but the analysis is not reliable because of the small size of the sample.

The 0.942 and 0.914 specified gravities of the product from the dilute gas experiments lead one to expect a difference in the distillation results. The,benzene fraction decreased about 10% and this decrease was balanced by a corresponding increase in the 90- 125” C. fraction. The residue also decreases, only slightly for the gas containing 75% acetylene, but 5 7 , for the gas containing 50YO acetylene. Thus dilution with ethylene increases the lower benzene homolog fraction at the expense of the benzene and tar fractions.

The results of these distillations indicate that, under the conditions of these experiments, the type of product changes very little. The fractions correspond fairly well to results obtained by other investigators who used carbon in packed tubes as the catalyst. However, i t is rather difficult to make a comparison because nearly all of the investigators used differcnt temperature ranges for their distillation fractions. The catalytic conditions of these experiments do not preferentially favor the formation of any particular fraction, but merely lower the temperature of operation.

CONCLUSIONS

The condensation of acet,ylene t o hydrocarbons of higher molecu- lar weight can be catalyzed by mixtures of molten salts. The only true catalyst found in this investigation was zinc chloride. In the pure molten state this salt was too active and caused excessive decomposition of acetylene. Numerous binary and ternary systems of fused halides, of which zinc chloride was one component, were studied. A molten bath of 12.5Yo sodium cMoride-59.0% potassium chloride-28.5% zinc chloride proved to be the best catalyst for the condensation of acetylene.

With this catalyst a 68% yield of liquid hydrocarbons was obtained. The yield was independent of the experimental conditions within the range studied. The conversion to liquid in a single pass increased with higher temperatures and longer periods of contact to a maximum value. The composition of the liquid product, as determined by distillation, was practically independent of the conditions.

Pure ethylene is unaffected by the 12.57, sodium chloride- 69.0% potassium chloride-28.5% zinc chloride catalyst, but mixtures of ethylene and acetylene undergo mutual condensation. The conversion in a single pass decreases with increasing concentration of ethylene, but the ratio of ethylene to acetylene disappearing increases. An analysis of the liquid products shows tha t the condensations involving ethylene result in a larger fraction of low boiling products.

The yield and type of products obtained by the use of molten salt catalysts appear to be similar to those obtained by previous investigators who used carbon as the active catalyst. However, the molten salt catalyst has several advantages over the fixed bed oatalysts of the previous investigators. The temperature of operation is lower, -the yield of carbon is lo^, the temperature control is excellent, and the catalyst is easily prepared and re- covered. No regeneration is necessary because all the carbon rises t o the top of the bath and can be skimmed off. It would also be possible to recover the heat of reaction as high pressure steam by the use of proper heat-exchange equipment. From these results i t appears that molten salt systems might serve as catalysts for reactions occurring above 300” C., particularly in caaes where temperature control is difficult because of large heats of reaction. This technique is particularly suitable where the

reactants and products are all volatile at the reaction temperatures, such as is the case with many organic reactions.

DEFINITIONS

CONVERSIOS TO LIQUID. The weight of liquid product divided by the total weight of acetylene passed through the column.

YIELD OF LIQVID. The weight of liquid product divided by the veight of acetylene disappearing during an experiment.

GAS DISAPPEARISG. The weight of acetylene entering the column minus weight of acetylene leaving in the waste.

WASTE Gas. All noncondensable gases leaving the dry ice- isopropanol trap. These consist of unreactcd acetylene, hydrogen, saturated hydrocarbons, and small amounts of olcfins.

IXERT Gas. T h a t portion of the waste gas remaining after the acetylene has been removed.

TIME OF CONTACT. The volume of the column dividcd by the entering gas flow rate corrected t o the temperature and pressure in the column. KO account is taken of the volume of the salt film on the column walls or the decrease in gas volume due to rc- action.

All gas volumes and gas rates are given as cubic centimeters a t 0 ” C. and 760 mm. The total Tolumes of waste components were obt,ained by graphical integration of the results from hourly analyses and rates.

The weight of carbon removed from the top of the salt reservoir a t the conclusion of an experiment. This carbon was washed with water until no chloride ion could be de- tected with silver nitrate. I t was then washed Kith acetone t o remove soluble hydrocarbons, then dried at 110 O C.

G.AS VOLUMES.

WEIGHT OF CARBON.

The above definitions must be modified slightly when mixtures of acetylene plus ethylene are used. I n such experiments the term “acetylene” must be replaced by “acetylene plus ethylene”.

ACKNOWLEDG.MENT

The authors wish t o thank W. E. Winsche and W. 11. Langdon for their many helpful suggestions on apparatus design given during the course of this investigation. They also appreciate the cooperation of the Office of Production Research and Develop ment of the War Production Board, under whose sponsorship the research was undertaken.

LITERATURE CITED

“Allen’s Commercial Organic Analysis”, Vol. 4, London, J. and

Berl, E., and Hofmann, K., 2. angew Chem., 44, 259 (1931). Billman, J. H., and Cash, R. V., J . Chem. Education, 18, 261

Bull. acad. sci. U.R.S.S., Classe sei. chim., 1939, 391-406. Fischer, F., and Peters, K., Brennstofl-Chem., 12, 286-93 (1981) Fischer, F., Peters; K., and Koch, H., IDid., 10, 383-5 (1929). Herrrnann, G., 2. anorg. Chem., 71, 257 (1911). International Critical Tables, Vol. IT’, New York, SIcGraw-

Kendall, J., Crittenden, E. D., Miller, H. K., J . Am. Chem. SOC.,

Korache, .1., and Tricot, E., Chiinie 6: industrie, 13, 361-72,

A. Churchill, Ltd., 1925.

(1941).

Hill Book Co., Inc., 1928.

45, 963 (1923).

53747 (1925). Rozlov, K., and Fedoseev, P., Sintet. Kauchuk 3, No. 5 , 36

Lewes, V. R., Proc. Roy. SOC. (London), 57, 455 (1806). Lozovoi. A. V.. J . Gen. Chem. (U.S.S.R.), 1, i17 (1931).

(1934); il’utl. Petroleum il’ews, 37, No. 10, R224 (1946).

lleyer, R., Ber., 45, 1609-33 (1912). hfeyer R., and Fricke, H., Ibid. , 47, 2766-74 (1914). hfeyer, R., and Meyer, W., Ibid. , 51, 1571-87 (1918). hIeyer R., and Taneen. A., Ibid. , 46, 3183-99 (1913). Parks, G. S., and Huffman, H. M., ”Free Energy of Some Or-

ganic Compounds”, New York, Chem. Catalog Co., Inc., 1932.

Perry, J. H., Chemical Engineers’ Handbook, 2nd ed., S e w York, McGraw-Hill Book Go., Inc., 1941.

Tiede, E., and Jenisch, W., Brennstos-Chem., 2, 5 (1921). Zelinsky, N. D., Compt. rend., 177, 882 (1923).

Documents

Condensation of Acetylene by Molten Salts