2308
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE VIII.
IGNITION
Cup Temp. a t Flesh,
Sample0 Natural rubber
1.
0
3.
Butadiene-acrylonitrile Neoprene
4.
a
c.
415
GR-S
2.
CHSRACTERISTICS O F
437
430 410
SMOKE
Sample Temp.
at Flash, 0
c.
270 342 380 315
Burning Characteristics after Flash Continuous a t sample Continuous a t sample Continuous a t sample Vapor extinguished pilot which could not he relighted
Compositions in Table I.
top, and a small pilot flanie was kept burning from the jet during the rapid heating process. The temperatures mere read a t intervals of one minute, and the temperature a t which the vapor flashed was noted. The samples were of the same size and shape as those used in the smoke density determinations (Table 3'111). I n all cases the initial flash at the pilot occurred exactly at the beginning of the rapid rise in temperature of the sample. T h e flame descended t o the natural rubber, GR-S, and butadieneacrylonitrile samples which continued to buiii. With neoprene, flashes occurred a t the pilot but did not ignite the vapors or the sample. The flashes increased in frequency and intensity until the pilot Iight was extinguished. The interval between the flash and the extinction of the pilot light above the neoprene stock varied in different trials. A lighted taper n a s now lowered into the tube and was extinguished before reaching the pilot. The gas a t the mouth of the chimney burned brieflv and then the flame went out. The taper was reintroduced as rapidly as possible for 7 minutes after the first ignition, but continuous burning was never attained. SUMMARY O F RESULTS
All of the elastomer compositions tested from natural rubber, GR-8, butadiene-acrylonitrile copolymer, and neoprene under60 a n exothermic decomposition when rapidly heated. Increase in the heating rate enhances the abruptness and magnitude of the exothermic rise. Long heating a t 190-200" C. eliminates exothermic action, The rapid action also takes place in a n atmosphere of nitrogen. Various compounding agents may modify this behavior, but they do not eliminate it. The exothermic action is therefore characteristic of the elastomers in the stocks. The only known structural characteristic common to all these
elastomers is t,he residual double bond. The exothermic art,lvit,y is therefore ascribed primarily to this source. This int'erpretation is further supported by the work of Midgley and Henne, which showed t h a t the C--C bond in the 1-3 position t,o the residual double bond in crepe rubber is most readily cleaved. It is rendered still more probable by the evolution of products from the gum stocks tested, which require active hydrogen atoms for their formation. Very little hydrogen chloride is evolved before rapid action begins, and the rat'e of evolution increases sharply a t the peak of exothermic act,ion. Similarly, the huhdiene-acrylonitrile copolymer yields hydrogen cyanide, arid natural rubber yields saturated gaseous hydrocarbons. The evolut'ion of smoke does not necessarily parallel the exothermic action. Smoke densities sufficient t o cause 50% extinction are always obtained before the beginning of the rapid rise i n temperature. Complete extinction of the light in the apparatuF: used occurs at a point between the beginning and the peak of the exothermic action. The smoke just above the sample of some stocks may clear completely during the exothermic action. Stocks containing zinc and magnesium oxides give colored smokes of a greatly increased order of density during the exothermic action. Phosgene cannot be detected in the smoke from neoprene stocks. The outside hcating temperatures at which the smoke and volatile gases flash are very similar for the four elastomers in the gum stpcks. The initial flash occurs a t the beginning of the exothermic rise. The smoke from neoprene extinguishes the pilot light while the other samples are ignited and continue to burn. ACKNOWLEDGMEST
Grateful acknodcdgment is made to Jackson Laboratory, &z Company, Inc., for aid in this work and for criticisms by members of its staff.
E. I. d u Pont de Ncmours
LITERATURE CITED
(1) Carothers, Williams, Collins, and Kirby, J . Am. Chem. SOC.,53,
4207 (1931). (2) Midgley and Henne, Ibid., 51, 1215 (1929); 53, 203 (1931). (3) Olsen, Ferguson, Sabett,a, and Scheflan, ISD. ENG.CHEM.,ANAL. ED.,3, 189 (1931). (4) Prentiss, "Chemicals in War," p. 17 (1937). (5) Prettyman, IND. ENG.CHEM.,34, 1294 (1943). (6) Rose and Simonsen, J . Chem. Soc., 1944, 101-3. (7) Yant, Olsen, Storch, LittlefieId, and Rcheflan. ISD. ENG.CBEX., ANAL.ED.,8, 20 (1936). RECEIVED September 27, 1947.
Presented before the Division of Rubber Chemistry a t the 112th Meeting of the . ~ M E R I C A NCHmiIc.41, SOCIITY,New York. X. Y.
v
d
VOl. 40, No. 12
d
v 0'
rc WALTEN A. SCHULZE, J. P. LYON, AND G. H. SHORT Phillips Petroleum Company, Bartlesville, Okla.
ITERATURE references (1, W, 4, 7 ' , 9) t o the synthesis of mercaptans (thiols) indicate thc use of a rather wide variety of raw materials and reaction conditions. From the standpoint of commercial scale process economics, however, b y far the most attractive synthesis method involves the direct addition of hydrogen sulfide t o olefinic hydrocarbons. Both patent and literature references t o the olefin-hydrogen sulfide reaction are found after about 1925. A number of these (b, 6, 8) pertain t o the formation of aliphatic mercaptans of low molecular weight in low concentration through the reaction of a relatively small portion of the light olefins and hydrogen sulfide over certain solid contact catalysts. These references, however, do not indicate process features or economics approaching a commercially feasible synthesis of mercaptans of high molecular weight.
DEVELOPiMENT PROGRAM
Early experimental n-ork in the authors' laboratory had est& lishcd the activity of certain catalysts for the reaction C,€L
+ H2S i CJL,.iSH
Conditions were knovn, for example, under which mercaptans could be converted t o olefins and hydrogen sulfide, and this process (5,10, 1 1 ) had been in commercial application for 8 number of years Low pressures, temperatures of 600' t o 800 O F., and catalysts with cracking or depolymerizing activity promote this reaction. It n-as found possible t o employ some of the same types of catalysts, with suitable modification, under the reverse conditions to promote the addition of hydrogen sulfide t o olefins to produce mercaptans. This addition reaction is favored
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2309
I t was predicted t h a t the by high pressures and temaddition reaction would be peratures well below those A synthesis process is described for the production of promoted by superatmosa t which mercaptans are demercaptans (thiols) of high molecular weight. This pheric pressures at least up t o composed. process incorporates the direct addition of hydrogen the range required for mainPreliminary experiments sulfide to olefin hydrocarbons of petroleum origin in the taining the reactants in prehad shown the favorable presence of catalysts. Research studies i n laboratory and dominantly liquid phase durcharacteristics of a silicabench scale apparatus established engineering design data ing contact with the solid alumina gel type catalyst for for the use of solid catalysts-e.g., silica-alumina gel-in catalysts. Accordingly, exthe condensation reaction. the synthesis of mercaptans i n the range of 12 to 16 carbon perimental tests were made at This catalyst is prepared by atoms. Supplementary experimental studies were depressures in the range of 500 forming silica gel and activatvoted to a number of process variables, including the type to 1500 pounds per square ing the partially dried gel of hydrocarbon feedstoclrs, the effect of impurities in the inch gage. Liquid space vewith a minor percentage of hydrogen sulfide, and the comparative activity of a numlocities in the range of l t o 10 alumina under closely conber of solid and liquid catalysts. Plant construction and liquid feed volumes per voltrolled and specific conditions operating procedures reflect the characteristic properties ume of catalyst per hour were (1%). The finished catalyst of hydrogen sulfide and mercaptans. Process design feaused. ordinarily contains about 1 to tures for the dehydration and liquefaction of relatively 5 weight 70of alumina. With these process condipure hydrogen sulfide, and the vacuum fractionation of tions selected, the practical Process studies concerning high boiling mercaptans are reviewed and properties of degree of olefin conversion the commercial synthesis of some of the products are summarized. per pass was primarily dealkyl mercaptans were initipendent on the nature of the ated in March 1943, as a olefin feedstock, although the result of efforts by the Rubheat release accompanying ber Reserve Corporation t o the exothermic reaction placed a n upper limit on conversion in contract for a supply of the dodecyl mercaptan (dodecancthiol) plant scale equipment. Various means for controlling the temutilized in butadiene-styrene copolymerization. The process deperature rise in the catalyst bed a t high conversions were invelopment program occupied approximately nine months. Comvestigated and for plant design purposes experimental d a t a were mercial plant construction was started in September 1943 and comobtained using a light paraffin hydrocarbon heat carrier and a permercial mercaptan production began in February 1944, or slightly pass conversion of olefin within the range of about 20 to 50%. less than one year after process development work was started. As the hydrogen sulfide-olefin reaction is capable of producing Studies of the hydrogen sulfide-olefin reaction were first dialkyl sulfides as well as mercaptans, feed compositions were carried out in bench scale equipment of relatively simple design. adjusted t o result in highly selective mercaptan formation. Feed materials were blended according to the desired molar The adjustment comprised the use of feed mixtures containing proportions in pressure cylinders and the solution of hydrogen a hydrogen sulfide-olefin molar ratio greater than unity and prefsulfide in hydrocarbon liquid was displaced by nitrogen gas t o the erably at least 1.5 t o 1. The use of a n excess of hydrogen reaction vessel. When solid contact catalysts were employed, the sulfide was found desirable also in providing additional quantities catalyst case consisted of a short length of high-pressure steel pipe with caps drilled for inlet and outlet connections. This case of heat carrier material. was immersed in a n electrically heated oil bath and preheating of Hydrocarbon feed stocks were obtained from plant streams in the feed was accomplished by a n inlet coil also immersed in the oil nearly all cases. Pilot plant fractionating columns were used in bath. The catalyst case effluent was passed through a pressuretrimming the plant stream samples t o the boiling range correreducing valve, condenser, and gas separator and liquid products were recovered.in the separator. sponding t o the desired olefin molecular weight. For the synThe conversion was measured by titration of the mercaptan thesis of dodecyl mercaptans, this olefin feedstock boiling range content of effluent liquid samples and catalyst temperatures were was approximately 330' to 390" F. The hydrogen sulfide for adjusted t o maintain a desired conversion level. T h e total liquid experimental laboratory tests was purchased in cylinders from the products from test runs were fractionated in small packed laboratory columns t o separate unconverted hydrocarbons from the Barium Reduction Company, Charleston, W. Va. This was a mercaptan products. high-purity product with minor amounts of impurities such a s water and traces of carbon dioxide. En the course of the bench scale studies a number of process variables related t o plant design and feed stock preparation were studied with silica-alumina gel type catalysts. I n addition, studies of other catalysts were made t o the extent permitted b y the relatively brief over-all time period prior to plant operation. The major portion of the experimental program necessarily pertained t o the selcction of specifications for the plant feed stocks and catalyst, although much supplementary information was obtained regarding the synthesis reaction and generalized process engineering. REACTION VARIABLES
The initial bench scale test confirmed the high activity of silicaalumina gel catalyst in promoting the reaction of hydrogen sulfide with a Clz olefin fraction at temperatures within the range (roughly from about 100' t o about 500 F.) considered practical €or theoreticaI reasons. It was expected t h a t temperatures above this range would introduce process complications resulting from isomerization, polymerization, and/or depolymerization reactions involving the olefinic hydrocarbons and, furthermore, would be in the region of incipient mercaptan decomposition.
EXPERIMENTAL RESULTS WITH SOLID CONTACT CATALYSTS
I n the tables are shown the d a t a from typical bench Beale tests which were made t o study reaction variables and to establish process conditions for the synthesis of various alkyl mercaptans. The length of the test period and the conditions employed varied according t o the particular factor being investigated. Because the activity of solid catalysts changes with use, the general test procedure which was developed involved starting up a t the minimum temperature a t which the desired conversion could be obtained, and subsequently raising the catalyst temperature t o maintain conversion. The upper temperature limit was reached when conversion could no longer be maintained or when the extent of side reactions indicated t h a t the economic life period of the catalyst had been exceeded. Procedures for catalyst regeneration were developed, b u t the initial life of the silica-alumina gel type catalyst was too long t o justify the expense of plant regeneration facilities. Extensive tests were carried out on the reaction of Clz olefins with hydrogen sulfide to study the variables of olefin feedstock
2310
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 12
FEEDSTOCK 1MPURITIES
TABLE1.
REACTIONO F TWELVECARBON
OLEFINS
WITH
HYDROGEr;
I n the course of pilot plant studies it wa? noted that erratic results were sometimes encountered Per Pass Yields during initial stages of a test run which indicated Light ClZ Olefin Average Olefin, hfercaptan, Mercaptans, catalyst poisoning. This led to a study of the Boiling Feed Conversion wt. 70 of wt. 70 of Range, Temperature, per Pass, olefin olefin effect of varioua impurities in both hydrocarbon Olefin Source F. F. wt. 7 0 charge charge and hydrogen sulfide feedstocks. Various conTriisobutylene con328-356 220 28.5 17.8 19.0 centrate taminants and impurities were found to suppress CaH6-CaHs polymer 330-380 230 49 7 61.9 5.9 the activity of silica-alumina catalyst, presumPropslcne polymer 330-390 230 54.7 55,s 4.5 Gray towera polymer 330-390 230 18.4 19.3 2.1 ably by adsorption during periods of low tema Liquid polymer from vapor phase clay treatment of gasoline. perature operation. iZmong the materials a hich impaired the activity of silica-alumina catalyst at lorn teniperatures n ere water, carbon dioxide. T.4BLE 11. C O K D E X s 4 T I O X O F HYDROGEN S U L F I D E WITH OLEFINS O F oxygenated organic compounds, and arsine. Arsine VARIOUS k1OLECULAR WEIGHTS OVER SILICA-ALU31INh CATALYST was found in commercial tylinder hydrogen -_ Olefin Feedstock Rango of Ultimate Boiling Conversion Olefin sulfide in trace quantities, but was not a facNo. range, Temperature, Conversion, Mercaptan tor in the process when using hydrogen sulfide carbons O F. Source F. W t . 70 Product t ecovered from hydrocarbon gases. Oxygenated 4 15-25 Isobutylene 80-2;10 100 teri-Butyl 5 80-120 Refinery amylenes 80-250 90 tetit-Amyl oiganic compounds nere encountered only in 6 148-162 Refinery hexenes 200-300 60 tert-Hexyl CaHs-C4Hn 8 205-250 160-250 85 tert-Octvl pilot plant and laboratory work and were usual11 polymer attributable to oxidation of olefins during stor10 290-320 Pentene dimer 200-300 80 tert-Decyl 14 420-460 Heptene dimer 200-300 80 teit-Tctradecvl age in small quantities and in frequently opened 1G 430-500 Octene dimer 200-300 75 tert-Hexadecyl containers or to residual traces of solvents used t o clean and dry apparatus. Water and carbon dioxide were more effectivtsource and structure. Some typical experimental results are poisons when present together than when present separately shown in Table I. The tests listed employed silica-alumina Because it was necessary to eliminate water from the plant catalyst and hydrogen sulfide-olefin molar ratios of 1.5 t o 1. reaction system to suppress corrosion, it vi as assumed that Operating pressure was 1000 pounds per square inch gage and dry feedstocks mould be available. I n the absence of uater, it a liquid space velocity of 2.0 was maintained in the catalyst case. n a s found t h a t hydrogen sulfide containing LID t o about 10 T h e values li,ted in Table I for olefin conversion per pass and mole yo of carbon dioxide could be used without serious poibonfor yields of the deaired long-chain mercaptans indicate some of ing effects when initial reaction temperatures were in the rangc of the variations in olefin reactivitj The first stock nas a narron130' to 200' F. cut fraction containing mostly isobutylene (2-methylpropane) Light hydrocarbon impurities in the hydrogen sulfide m r e trimer. Although the conversion was kept relatively low, the found to be inert as far as the addition reaction was condepolymerization was severe, as indicated by the high yield of cerned. K h e n allowance u as made lor the reduced hydi ogen sulfide purity by adjusting feed compositions, it mas feasible to light mercaptans and the low yield of dodecyl mercaptans. When the C12 olefin was prepared from propylene-isobutylene polymer, use high percentages of propane diluent as a heat carrier in the hydrogen sulfide stream. The presence of methane arid ethane the yield of dodecyl mercaptans wa3 much higher, even at a higher temperature and per-pass convcraion. Results rvith the in the hydrogen sulfide stream, hon ever, was objectionablc h r n the standpoint of hydrogen sulfide losses due to the hsndling of propylene polymer stock appeared very satisfactory. However, noncondensablc gases in the plant compression system. the C12 olefin fraction from Gray T o ~ ~ polymers er had a low reactivity factor under the conditions employed. Another factor affecting the comparative reactivity of olefin CATALYST STUDIES stocks from different sources was the variation of reaction rate One phase of bench scale studies concerned the comparative with olefin structure. Earlier experimental tests had indicated testing of a number of solid catalysts to evaluate their actibitv for that in mixtures of various olefin types the tertiary olefins reacted the hydrogen sulfide-olehn 1eaction. With silica-alulnlna gel with hydrogen sulfide at a much faster rate. The result of this established as a standard of comparison, tcst conditions 1%CI'P difference in reaction rate was that under conditions for partial approximately the same as those emploj ed 11ith standard silicaolefin conversion a selective conversion of the tertiary olefins alumina catalyst in the synthesis of Ci, inelcaptans. These occurred, and the mercaptan products often contained 90% conditions included the use of temperatures in the range of 150 or more of tertiary mercaptans. Inasmuch as prospective plant to 450" F., pressure of 1000 pounds per square inch gage, liquid operation embodied controlled per-pass conversion with recycle space velocity of 1.O, and hydrogen sulfide-olefin molar ratio o f of unconverted olefins, it was apparent that tertiary mercaptans 1.5. The olefin stock for all the comparative tests was a fraction would be formed selectively as long as tertiary olefins werr of propylene-isobutylene polymer with a boiling range of about present in the ole& feedstocks. Changing process conditioiis Each catalyst vas tested b y operation for 300" to 380" F. to promote the addition of hydrogen sulfide to other types of successive 4-hour periods at 50 F. temperaturc intervals over the olefins was effective only in the substantial absence of tertiary range of 150" to 300" F., inclusive. I n the caw of tho higlili' olefins. active silica-alumina gel, tempcratures were not advanced beT h e hydrogen sulfide-olefin reaction was carried out with yond 200 ' F. bemuse of the fact t h a t conversion wa3 in the range many other olefinic hydrocarbons of varying source, structure, of 80 to 90% a t the minimum temperatures. and molecular weight. Most of the test work on other than CIZ The solid catalysts tested and the test results are listed iii olefins was done subsequent t o a period of commercial operation Table 111. Of the catalysts employed the activated alumina, in the production of CU mercaptans and was for the purpose bauxite, silica, and silica gel were commercial products of grades of evaIuating new raw material sources and producing new usually specified for catalyst preparatlon. The thoria-alumina mercaptans for evaluation research. catalyst was a coprecipitated gel coinposition containing a major I n Table I1 are listed a number of the olefinic hydrocarbon proportion of alumina. The alumina-base catalysts 5%erc prcstocks examined. SULFIDEOYER SILICA ALUMINACATALYST
O
INDUSTRIAL AND ENG INEERING CHEMISTRY
December 1948
TABLE 111. COMPARISON OF ACTIVITY OF SOLID CATALYSTS FOR REACTION OF HYDROGEN SULFIDEWITH TWELVE-CARBON OLEFINSTOCK
Wt. % lMeroaptan Sulfur in Effluent ZOOo 250° F. F. 12.6 ... 2.1 1.1 1.4 1.0 0.8 0.7 0.7 0.8 0.6 1.6 0.7 0.8 1.1 1.3 1.2 0.6 0.8 0.3 0.3 0.4 0.8 0.6 0.6 0.9 1.1 0.9 0.9 0.5 0.3 1.2 0.6 0.4 0.3 1.6 2.0 0.4 Assuming no formation of lighter mercaptans.
Catalyst Silica-alumina gel Activated alumina Bauxite Silica gel Diatomaceous earth (silica) Nickel sulfide-alumina Nickel oxide-alumina Thoria-alumina pel
150°
F. 13.9 0.5 0.9 0.8
L
Calculated5 Av. Yield per paJs of
c,s
3.1
Mercaptan, Wt. % 87.0 11.1
3.0 0.7 1.4 2.9 1.2 1.8 1.0 1.3 0.9 2.4 0.5 2.6
10.0 4.9 5.4 9.7 9.1 5.6 4.1 5.6 6.1 4.2 2.8 10.0
300' F.
...
231 1
Exploratory bench scale tests were also carried out with a number of liquid catalysts including anhydrous hydrogen fluoride, aqueous hydrofluoric acid, and boron trifluoride and its various complexes such as the (IS)hydrate and the phosphoric acid complex. These tests were made in a small turbomixer reactor on feed mixtures comprising olefin, hydrocarbon diluent, and hydrogen sulfide. The hydrocarbon diluent employed aided adjusb ment of the hydrogen sulfide-olefin molar ratio at moderate pressures to values in the range of about 2:l to about 8:1 t o obtain mercaptans in preference to dialkyl sulfides. The results of a few of the experimental tests, summarized in Table IV, indicate the extent of the olefin conversion obtainable with these liquid catalysts. Higher yields of the mercaptans of molecular weight corresponding to olefin feed may be obtained by adjustment of reaction conditions and by utilizing olefin stocks less susceptible to depolymerization. PLANT DESIGN AND CONSTRUCTION
pared by impregnating alumina pellets with about 10 to 20 weight yoof the various metal salts from aqueous solutions. Conventional methods were used t o convert the adsorbed metal salts to the oxide or sulfide. All the catalysts were dried prior to testing to produce maximum activity. It will be noted from t h e calculated mercaptan yield values of Table I11 that only the standard silica-alumina gel catalyst is outstanding in activity for the reaction. This catalyst produced initial conversions above 80% per pass (based on weight of olefins charged) at the minimum test temperatures. The superiority is even greater than indicated by these calculated yields because substantially all of the mercaptans produced by the catalysts having very low activity (such as silica gel) were light mercaptans resulting from depolymerization of the olefin feedstock. The activity of none of the alumina-base catalysts promoted by metal oxides or sulfides, according to the specific methods employed, excecded the values obtained with activated alumina alone as catalyst. I n some instances it appeared t h a t the principal effect of the promoter was to suppress the depolymerizing action of the alumina without greatly affecting the rate of addition of hydrogen sulfide t o the long-chain olefins. Bauxite was approximately equivalent to activated alumina. As it appeared that the optimum temperature range had not been reached with either bauxite or activated alumina, tests were continued with these catalysts at 50" F. intervals from 325' t o 425" F. Conversions based on the total mercaptan yields increased uniformly, but maximum yields of dodecyl mercaptans were noted at about 375" F. with bauxite and at about 325" F. with activated alumina. Thcse maximum mercaptan yield values were approximately 25 weight % (based on olefin charged) for each catalyst. I n the case of the alumina catalyst, increased mercaptan production above about 325' F. was due almost entirely to olefin depolymerization and formation of low boiling mercaptans.
Process design work for a commercial scale unit to produce tertiary dodecyl mercaptans proceeded concurrently with t h e latter portion of the pilot plant program. The basic process design for the catalytic condensation reaction was not greatly different from the pattern defined by early laboratory tests. However, various other plant operations were studied on a sniall scale prior to designing the large scale equipment.. The commercial scale plant process flow and operation are illustrated by the schematic flow diagram of Figure 1. Fresh hydrogen sulfide gas is taken into the compression system together with the recycle hydrogen sulfide stream from the highpressure stripper. The combined gas streams are passed through a bauxite dehydrator (not shown) t o the two-stage compression system. High stage gas passes through a scrubber t o remove entrained liquid and then through a condenser and to the liquid hydrogen sulfide surge tank. Feed t o the catalyst case comprises fresh olefin recycle olefin and hydrogen sulfide. The hydrogen sulfide-olein feed stream passes through a steam preheater and then through one of t h e catalyst cases (in alternating service) containing a 4 X 7 foot bed of granular catalyst. The catalyst case effluent stream passes through a pressure-reducing valve and a stream heater into the high-pressure stripper. I n the high-pressure stripper a nearly complete separation of unreacted hydrogen sulfide is made, and the overhead vapors from the stripper are recycled t o the compression system. Liquid products from the bottom of the highpressure stripper then flow to a lower pressure stripping column for further removal of low-boiling components. The low-pressure stripper reboiler serves also as preheater for the liquid feed t o the subsequent fractionation step. The first fractionation step effects the separation of unreacted olefin stock from mercaptan products at a n absolute pressure of about 5 mm. of mercury. The overhead liquid from this tower returns t o the recycle olefin storage tank. The kettle product from the recycle olefin tower is ordinarily specification grade mercaptan when charging a CI?olefin stock. However, a second vacuum fractionation is carried out in the mercaptan tower at absolute pressure of about 3 mm. of mercury t o obtain the mercaptan as a n overhead product. Kettle residues in this second distillation step are negligible. The overhead product from the mercaptan tower passes to finished product storage tanks. The process features, aside from the catalytic reaction step which received particular attcntion, included:
TABLEIV. HYDROGEN SULFIDE ADDITIONTO OLEFINS IN PRESENCE OF HYDROGEN FLUORIDE AND BORON FLUORIDE COMPLEXCATALYSTS HzS/ Olefin Mole Temp., Catalyst Olefina Ratio O F. HF (anhydrous) Diisobutylene 3.3 90 80% H F Triisobutylene 3.5 90 BFa-Hz0 Triisobutylene 2.8 90 BFs-HsP04 Triisobutylene 2.2 90 BFa-H3PO4 Triisobutylene 6.5 40 alO% solution of olefin in n-pentane.
Dehydration of hydrogen sulfide and compression and liquefaction of this raw material. Fractional distillation of high boiling mercaptans which are inherently unstable at high temperaturei. Selection of materials of construction. Inspection, analysis, and storage of the finished products. ~
Yield of CorreW t 41 sponding Oldfin" Mercaptan, CopWt. % version of Olefin 100 70
68 76 64
72
37 41 61 72
The plant site was selected in the Borger, Texas, area where hydrogen sulfide is available from the desulfurization of gas streams of high sulfur content. I n this particular operation, a crude hydrogen sulfide concentrate is obtained from the amine reactifier still of a Girbotol gas
INDUSTRIAL AND ENGINEERING CHEMISTRY
2312 c
RECYCLE
Vel. 40, No. 12
OLEFIN
3 -STAGE
4-STAGE STEAM EVACTOR
MERCABTA N
I
TO LOADING DOCK
Figure 1.
I-.
Source
tert-Hexadocyl
5
Hexene dimer
Heptane dimer Octene dimer
LONG-CHSIN TERTIARY ALKYLMERCAPTANS
nlercaptan Distillation Range, Rolling
tert-Tetradecyl tert-Hexadscyl tert-Dodecyl tert-Tetradecyl tert-Hexadecyl
inch gage. Plant compression in t,vm stages effects liquefaction at pressures in the range of 350 to 400 pounds per square inch gage. The vapor pressure of the condensed liquid a t ordinary plant cooling water temperatures varies with the purity of the hydrogen sulfide. By constantly venting noncondensed gases from the liquid hydrogen sulfide surge tank the concentrations of carbon dioxide, methane, and ethane are maintained a t about the values of t,he incoming fresh hydrogen sulfide gas &ream. A standard gas engine-driven compressor with two power and two compression cylinders is used. The cornpression cylinders have some special fittings such as Ni-Resist liners, stainless st,eel valve assemblies, and Inconel garter springs for t,he inet'allic packing. lnterstage cooling and the condensation of hydrogen sulfide are done in conventional shell-and-tube exchangers fit,t,ed with steel tubes. The catalyst cascs are fabricated from alloy steel. Other plant vessels, piping, and tankage are of common steel with adequate corrosion allorrance. I n general, most of the selected materials of construction have given satisfactory service. Fatigue corrosion due to hydrogen sulfide has been encountered in some cases. This type of corrosion which results in cracking and fracture occurs with metal parts (including many alloy steel parts) which are subjected to tension and/or compression and flexing. Examples are pump plungers, springs of all types, gate valve seats and wedges, ball
P H Y S I C A L A43D CHEMIC.41, FROPERTIES O F
Olefin Feedstock
Product tert-Dodecyl tert-Tetradecyl
ii
Schematic Flow Sheet of Mercaptan Plant
treat>ingunit as a vapor stream s a h r a t e d with n-ater at atmospheric temperature and a pressure of 5 to 10 pounds per square inch gage. The crude concentrate is cooled, scrubbed to remove condensed water, and dehydrated by passage through a bed of bauxite desiccant to produce a dew point of -30' to -40' F. at 50 pounds per square inch gage. The degree of dehydration is controlled by regeneration of the bauxite a t suitable intervals. With dew points in t,Ksrange, corrosion of metal equipment is suppressed and hydrate formation in subsequent compression steps is eliminated. This Iatter factor is important, since the hydrate formed at, high pressures is a rather high melting solid capable of plugging lines and eschangers in the compression system. Preliminary experimental testing of solid desiccants for hydrogen sulfide indicated t,hat bauxite was satisfactory with regard t>o dew point lowering, water capacity, and service life. However, it was found necessary to use bauxite of lorn iron content in order to carry out uncomplicated regeneration of the desiccant. This was based on the discovery that bauxite of high iron content tends t o release relatively large amounts of sulfur dioxide and elemental sulfur during regeneration with hot inert g a m at temperatures above about 350" F. This difficult?; has not been encountered with bauxite of low iron content-Le., belox about' 3 to 5% I.'ezOa. The dry hydrogen sulfide gas stream is delivered to the mercaptan plant by pipe line a t pressures of 20 to 50 pounds per square
TABT,E
MERCAPTAN
FINISHED
c
range, F. 320-380 400-450 450-500 400-450 450-500 372-405 415-460 460-5 00
IJ3P-'35VQ
condensed a t 5 mm. Erg 172-207 218-252 252-305 221-235 302-327 196-222 240-280 260-310
Specially purified for research purposes from a commercial produot.
F.
condensed *Bp-Q5% a t 760 mm. Hg 420-462 484-320 520-590 488-502 590-615
452-485
51O-ZG0
540-610
Sp. Gravity, 60' F./60° F. 0.8713 0 .a v o 0 5830
0.8821 0.8890 0.8945 0,9046
0.9121
Mercaptan Sulfur, Wt. yo 15.9 11.9 10.3 14.1
12.4
15.6 12.2
11,o
Average Molecular Weight 194 230 241) 226 2.55 203 234 2 50
Apparent RIercaptan Puilty, 7c
97.0 85.0 80.0 99.25 99.1"
99.3 8'3.3 85.1
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
thrust bearings, and the Bourdon tubes of pressure gages and instruments. Fracture of common steel relief valve springs was troublesome until aluminum-plated springs were installed. Inconel springs are apparently resistant to this type of corrosion and have given good service. The failure of Bourdon tubes is reduced by the use of molybdenum-stabilized 18-8 stainless steel tubes. Recording pressure controllers are fitted with tantalum diaphragms between the transmitting and recording systems. The vacuum fractionators are of novel design and relatively high efficiency. The design is based on requirements for (1) high vacuum operation with minimum internal pressure drop, ( 2 ) minimum residence time for heating the mercaptan-containing liquids, and (3) heat exchange under positive pressure. The recycle olefin column, for example, is a 5 X 65 foot tower containing 30 feet of 1.5-inch Berl saddle packing. The internal pressure drop in the column is in the range of 5 t o 7 mm. of mercury, and column surveys during operation have indicated a fractionating efficiency of 7 t o 8 theoretical plates when a pressure of 5 mm. of mercury is maintained at the top of the column. External exchangers are provided for heating the kettle liquid and cooling the overhead condensate, and are located on the discharge lines of the respective circulating pumps. Residence time in the reboiler is very short, and flash vaporization of the heated liquid within the column results in a kettle temperature considerably below that a t the reboiler outlet. The overhead vapors are condensed in an internal condensing section bv direct exchange with a portion of the cooled overhead condensate; the remainder of the cooled condensate 1s withdrawn t o product storage. Reflux liquid is withdrawn ahead of the cooler and returned t o the column just below the condmsing section. MERCAPTAN PRODUCTS
The tertiary dodecyl mercaptan which was the primary objective of the work was produced from a 330 to 390 O F. fraction of propylene-isobutylene polymer. This product, as would be expected from the nature of the feed olefin, is a mixture of mercaptans having 11 to 13 carbon atoms with a n average molecular weight corresponding to about 12 carbons. The compounds are of branched chain structure and the location of the mercaptan group is not known except in so far as the location of the olefinic linkage in the polym-er molecule can be deduced. The characteristic reactions of the mercaptan mixture indicate t h a t the mercaptan groups are almost entirely attached to tertiary carbons, indicating t h a t the addition of hydrogen sulfide probably occurs according to Markovnikov's rule. The high yield of tertiary mercaptans from a n olefin stock of this nature is also in agreement with the theoretical considerations regarding the location of the double bond in hydrocarbon polymer molecules and the known tendency of the double bond to migrate toward the center of the molecule under polymerization conditions. It is also further confirmation of the more rapid rate of reaction of hydrogen sulfide with the tertiary olefins under the conditions employed. Because the high molecular weight mercaptan products are mixtures of isomers of a particular narrow molecular weight range, inspection tests and analyses for mercaptan purity are based on boiling range, average mean molecular weight, and mercaptan sulfur content. The latter is the most convenient analytical method, as other sulfur compounds are absent and the mercaptan sulfur determination is a rapid and accurate volumetric or amperometric analysis. The boiling range of the high boiling mercaptans is determined in a standardized distillation apparatus a t a constant pressure of 5 mm. of mercury. Average molecular weight is determined by the cryoscopic method in which the freezing point depression of benzene is measured. Impurities present in the mercaptan products are hydrocarbons which have been incompletely separated in the distillation steps or which, in some cases, occur in the mercaptan boiling range as a
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result of further polymerization reactions undergone by t h e original olefin feed stock. I n general, separation of hydrocarbon impurities can be relatively complete within the limitations of t h e plant fractionation equipment since the boiling point increment due to the introduction of the mercaptan group is of the order of 60 O t o 80' F. The tertiary alkyl mercaptans are stable at temperatures below about 300" F. Oxidation is relatively slow, and metals are not appreciably attacked under usual storage conditions by the compounds of higher molecular weight. Product storage tanks of mild steel are used and are closed to the atmosphere and blanketed with a n inert gas. Retain samples have been observed over a period of 3 years without indications of deterioration, as the mercaptan sulfur content, distillation range, and color have remained constant. Commercial scale production of the high molecular weight tertiary mercaptans has yielded a number of products identified as tertiary dodecyl, tetradecyl, and hexadecyl mercaptans. Typical properties of these products are listed in Table V. The properties listed in Table V indicate the gradation of physical characteristics and purity between the commerical CIZ,C I ~ , and Cre products. The plant distillation equipment permits both light and heavy end separations on the CIS and C14 carbon mercaptans, and these products can be produced in very high purity. The hexadecyl mercaptan product, however, is obtained in the plant as a n undistilled kettle product, so t h a t commercial purities are somewhat lower. Two of the products listed were obtained by chemical purification of commercial tetradecyl and hexadecyl mercaptan mixtures t o separate associated hydrocarbons. These long-chain mercaptans are mobile liquids of moderate viscosities a t ordinary temperatures, and colors ranging from water white t o light straw, depending on the extent of purification to remove hydrocarbon impurities. They have distinctive and persistent odors which, however, are not objectionable like the odors of the lower molecular weight mercaptans. Higher purity and higher molecular weight are accompanied by a decrease in odor intensity. They are soluble in hydrocarbon solvents, the lower alcohols, and many common organic solvents, and are substantially insoluble in water. -4number of typical reactions have been studied. These tertiary mercaptans as a class undergo metal salt formation, oxidation t o disulfides, and other known reactions of the sulfhydryl group but with gradations of reactivity attributable t o the molecular weight and the tertiary mercaptan characteristics. LITERATURE CITED (1) Arndt, F , Milde, E., and Eckert, G., Ber., 44, 2236 (1911). (2) Braun, J. von, N d . , 42, 4568 (1909). (3) Buell, A. E., and Schulze, W. A. (to Phillips Petroleum Co.), U. S.Patent 2,016,271 (Oct. 8, 1935). (4) Claus, A., Ber., 5, 659 (1872).
( 5 ) Duffey, M. R., Snow, R. D., andKeyes, D. B., IND.ENG. CHEM., 2 6 . 9 1 (1934).
Johansen: E.-'M. (to Gray Processes Corp.), U. S. Patents 1,836,170 and 1,836,171 (Dee. 15, 1931). (7) Kramer, R. L., and Reid, E. E.. J. -4m. Chem. SOC.,43, 881 (6)
(1921).
Nisson, P. S.,and Mandelbaum, M. It. (to Gray Processes Corp.), U. S. Patent 1,836,183 (Dec. 15, 1931). (9) Roemer, H., Rer., 6 , 7 8 4 (1873). (10) Schulze, . '%1 A. (to Phillips Petroleum e o . ) , U S. Patent 2,151,721 (March 28, 1939). (8)
(11) I b i d . , 2,162,319 (June 13, 1939). (12) Ibid., 2,392.554 (Jan. 8, 1946), 2,392,555 (Jan. 8, 1946), and 2,426,646 (Sept. 2, 1847). '. and Crouch W. W.( t o Phillips Petroleum Co.), (13) Schulze, 1%A., I b i d . , 2,426,647 and 2,426,648 (Sept. 2 1947). RECEIVEDSeptember 23, 1947. Presented before the Division of IndustrisI and Engineering Chemistry a t the 112th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y .
