Pilot Plant METHYL MERCAPTAN FROM METHYL CHLORIDE


C. B. SCOTT, W. S. DORSEY, and H. C. HUFFMAN. Research Division, Union Oil Co. of California, Brea, Calif. CONSIDERABLE attention has been focusedĀ ...
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Methyl Mercaptan from Methyl Chloride C.

B. SCOTT, W. S. DORSEY,

AND

H. C. HUFFMAN

Research Division, Union O i l Co. of California, Brea, Calif.

c

OSSIDERABLE attention has been focused recently upon methyl mercaptan (methanethiol) as an intermediate for jet fuel additives, fungicides, and methionine. Because future demands for methyl mercaptan probably will exceed the supply recoverable from petroleum refining, numerous routes of synthesis have been under investigation. The chemical literature describes many methods of preparing mercaptans, including reactions of the appropriate alcohols with hydrogen sulfide ( 8 ) , dialkyl sulfates with alkali hydrosulfides ( 7 , 8 ) ,and alkyl halides withalkali hydrosulfides (1,3-6, 10). For methyl mercaptan, each route described in the literature had an important disadvantage. Reaction of methanol with hydrogen sulfide over thoria at 380" C. (716" F.) gave poor yields of mercaptan and objectionable amounts of by-products, primarily formaldehyde. The reaction of dimethyl sulfate with sodium hydrosulfide was straightforward, but yields of mercaptan above 50 mole % were difficult to attain. The expense and high toxicity of dimethyl sulfate are important factors in ruling out this route as a commercial process. Since the authors' work wae completed the Pan American Refining Corp. has reported (2) a successful synthesis of methyl mercaptan from methanol and hydrogen sulfide. Reaction of methyl chloride with sodium hydrosulflde requires excess hydrogen sulfide

The reaction of methyl chloride with sodium hydrosulfide appears obvious and simple. This method has been applied frequently to higher alkyl chlorides, but no successful attempt with methyl chloride is described in the literature. The basic reaction is : CHsCl

+ NaSH

-

CHaSH

+ NaCl

(1)

Possible side reactions are:

+ NaSH CH3SNa + CHIC1

CH3SH

2NaSH

F;?

Na2S

+ H2S CHaSCH, + SaCl

CH,SNa +

+ H2S

(3)

(4)

+ NaSS CH3SCHs + 2NaCl 2CHaSH + oxidizing agent CH3SSCH3 HS- + HzO F? HzS + HOCHaCl + HO- * CHsOH + C12CH3Cl

(2 1

-.f

4

(5)

(6)

(7)

(8)

Consideration of these reactions indicates that the following requirements should be met: 1. Good contact of reactants to promote rapid reaction. As methyl chloride is not appreciably soluble in aqueous sodium hydrosulfide, a solvent or excellent stirring must be used.

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2.

Pressure equipment to provide sufficient concentrations of

mf'khg:i ~~~$~~~~$!$'' suppress side reactions,

Equilibria in Reactions 2, 4, and 7 emphasize the need for hydrogen sulfide, as both sodium methyl mercaptide and hydroxide ion are very reactive toward methyl chloride. Semipitot continuous reactor was designed -

The aemipilot unit which was found to meet these requirements is shown schematically in Figure 1. Designed to produce methyl mercaptan from aqueous sodium hydrosulfide, methyl chloride, and hydrogen sulfide, the unit consisted of three feed lineis, a mixer, a reactor immersed in a constant temperature bath, a phase separator, a dryer, and a condenser for gaseous products. Both feed pumps were of the proportioning type with stainless steel bodies and check valves. Sodium hydrosulfide ( 5 N ) was pumped from a glass reservoir. More concentrated hydrosulfide was unsuitable because of precipitation of sodium chloride in the seat of the back-pressure control valve. Methyl chloride was pumped as a liquid by raising the pressure in the methyl chloride reservoir to 125 pounds per square inch gage with nitrogen. Additional methyl chloride was charged into the reservoir from the storage cylinder as required. The overpressure of nitrogen was necessary to prevent vapor locking of the methyl chloride pump. Hydrogen sulfide was admitted as a gas. Except for the glass drying tower with connections of rubber tubing, all parts of the unit which came in contact with reactants or products were constructed of Type 304 or 316 stainless steel. Piping was 1/4-inch stainless pipe and the mixer was a standard pipe cross. Two reactors were employed: a coiled tube 80 feet long and inch in inside diameter, and a coiled tube 10.75 feet inch in inside diameter. Best mixing of reactants was long and realized in the S/le-inch reactor. Effluent from the reactor was reduced t o atmospheric pressure in the phase separator, gaseous products were sent overhead to the drying and condensing apparatus, and liquid aqueous products were retained in the separator for periodic withdrawal. A vessel 2 inches in diameter and 4 feet long with piping as shown in Figure 1 served as an efficient phase separator. By steam-tracing the phase separator with atmospheric steam it was possible to achieve essentially complete separation of organic and aqueous products. The organic products were dried in a glass tube, 2 X 18 inches, packed with calcium chloride and closed with large rubber stoppers containing glass nipples to accommodate rubber tubing connectors. Amber gum rubber tubing proved most satisfactory. By using a glass dryer it was possible to observe the exhaustion of the calcium chloride and prevent solid hydrates from plugging the

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol 47, No. 5

PILOT PLANT condensation system, which consisted of a coil immersed in a dry ice-acetone mixture. Residence or contact times in the reactor were determined by mixing a small quantity of radioactive cesium chloride with the sodium hydrosulfide and following the passage of this material with a Geiger counter. With the reactor inch in inside diameter, the cesium chloride left the reactor in a small, well defined

VENT

tray Oldershaw columns gave 90.5 mole % pure mercaptan. The methyl chloride and hydrogen sulfide were suitable for recycling without further purification. I n the reactor I/.z inch in inside diameter the efficiency and amounts of by-products were essentially the same as with the reactor '//le inch in inside diameter; however, the conversion per pass fell from 87 to 33 mole %. Phase study showed nature of reaction mixture

VENT

t

PHASE SEPARATOR

c

L

t

NaSH

U CH3CI

PUMP

PUMP

Figure 1.

CONSTANT TEMP. BATH

Semipilot p l a n t d i a g r a m of methyl mercaptan unit

packet. With the reactor L/z inch in inside diameter, having the same volume, the radioactivity was dispersed t o such an extent that detection became extremely difficult, suggesting laminar or slug-type flow in the large reactor and turbulent or emulsion-type flow in the small reactor. Conversion of methyl chloride to methyl mercaptan was satisfactory

At 70" C. ( 1 5 8 O F.) and 175 pounds per square inch gage and with contact times of 11 minutes in the a/,,-inch-diameter reactor, conversions of methyl chloride to methyl mercaptan of 87 mole % per pass and efficiencies over 98 mole % were obtained. Main by-products were dimethyl sulfide and dimethyl disulfide. A slight excess of hydrogen sulfide suppressed byproducts to less than 4 weight % of the total organic sulfur compounds. No evidence of methanol could be found, indicating no hydrolysis or reaction of hydroxide ion with methyl chloride. Typical effluent gas from the phase separator was comprised of 57 mole % methyl mercaptan, 28 mole % hydrogen sulfide, 14 mole % methyl chloride, 0.9 mole % dimethyl sulfide, and 0.1 mole yo dimethyl disulfide. Low-temperature distillation in 30-

May 1955

AOUEOUS PRODUCTS

I n attempting to determine the nature of the reaction mixture, a brief phase study was made of the system: methyl chloride, methyl mercaptan, hydrogen sulfide, and 5N sodium hydrosulfide. The three phases that were present-organic, aqueous, and gaseous-formed an emulsion with only mild agitation, Although both reactors had the same volume, the higher velocity in the small reactor of 7.3 feet per minute compared with 1 foot per minute for the large reactor, presumably caused sufficient turbulence to form an emulsion conducive to a good reaction rate. The alternative possibility that slug flow existed in the reactor of smaller bore does not appear to be in accordance with the higher observed yields. Acknowledgment

The authors gratefully acknowledge the counsel and assistance of W. D. Schaeffer, Leroy Nyquist, L. C. Smith, and K. W. Fort throughout the project and particularly during the lengthy production runs.. literature cited (1) ' Beanblossom. J. E.. and Kimball. R. H. ( t o Hooker Electrochemical Co.), U. S.Patent 2,404,425 (July 23, 1946). (2) Chem. Eng. News, 33, 833 (1955). (3) Clark, L. H., and Deibel, C . W. (to Sharples Solvents Corp.). Ibid.,2,147,400 (Feb. 14, 1939). (4) Ellis, L. M., and Reid, E. E.. J . Am. Chem. Soc.. 54, 1674 (1932). ( 5 ) Fore, D., and Bost, R. W., Ibid., 59, 2557 (1937). (6) Genelin, S.,2.physik. chem. Interricht, 43, 80 (1930). (7) Kiprianov, A. I., Suitnikov, Z. R., and Suich, E. D., J . Gen. Chem. (U.S.S.R.), 6, 576 (1936). (8) Klason, P., Ber., 20, 3407 (1887). (9) Kramer, R. L., and Reid, E. E., J . Am. Chem. SOC.,43, 880 (10)

(1921). Wirth, W. V. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,395,240 (Feb. 19, 1946).

RECEIVED for review February 25, 1955. ACCEPTED April 7, 1955. Presented before the Division of Petroleum Chemistry a t the 127th MeetSOCI~TY Cincinnati, , Ohio. ing of the A h f E R I C A N CHEMICAL

INDUSTRIAL AND ENGINEERING CHEMISTRY

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