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23 4The Plasma Induced Rea cti on of Hydrogen Su l f i d e wi th Hydrocarbons

F. J . Va s to l a a nd W. 0. S t a c yDepar tment of Fue l Sc ie nce

The P e n n s y l v a n i a S t a t e U n i v e r s i t y , U n i v e r s i t y P a r k , P a .

I n t r o d u c t i o n IT h e i n t e r a c t i o n of s u l f u r w i t h h y dr o ca rb o ns u nd er a r a nge of e xpe r im e n ta l

con di t io ns has been in ve s t ig a t ed by va r io us workers . Knight and co-workers1 re -a c t e d e x c i t e d s u l f u r at o ms , p ro du ce d by ' i n s i t u ' p h o t o l y s i s of COS a t 25OoC, witha range o f pa r a f f in i c hyd r oc a r bons a nd f ound as pr im a r y p r oduc t s on ly t h e c o r re -ponding m e rc a p ta ns . B ryce and H insnelwood2 s t ud ie d th e r e a c t i on o f s u l f u r va porwi th pa r a f i nn ic hyd r oc a r bons i n t he t e m pe r a t u r e r ange 320C t o 349C and obser vedt h a t t h e p r im a ry p r o d u c t s w er e hy d ro g en s u l f i d e a nd u n s a t u r a t e d h y dr o ca rb o ns .S tudy o f t h e r e a c t i on o f s u l f u r and m etha ne 3-7 ove r a c a t a l y s t a t 500C t o 7OO0Chas shown t h e p r o d u ct s t o b e c ar bo n d i s u l f i d e and hy dro ge n s u l f i d e a c c o rd i n g t ot h e r e a c t i o n

CH4 + 2S2 -+ CS + 2H2S. (1)Thomas an d S t r i c k l a n d - C o n s t a b l e 8 s t u d i e d t h e i n t e r a c t i o n o f s u l f u r a nd h y d ro ca rb o nsa t te mp er at ur es 1200.OK t o 1500'K a nd i n t h e a b s e nc e o f a c a t a l y s t o b s er v e d no c ar bo nd i s u l f i d e f o rm a ti o n . W it h a c a t a l y s t , h o wev er, t h e r e a c t i o n p ro ce ed ed y i e l d i n gc a rb o n d i s u l f i d e , h y d ro g en s u l f i d e , and hy dr og en ; r e a c t i o n s s i m i l a r t o (1) f o rm e tha ne , a bove 1200K, be ing in c r e a s i ng ly supe r se ded by t hose s imi la r t o ( 2 ) :

CH4 + S + CS2 + 2H2*2Reported now a r e s o m e r e s u l t s o f a s t u d y o f t h e r e a c t i o n s o f e x c i t e d at o mi c

s p e c i e s , g e n e r a t e d by t h e d i s s o c i a t i o n o f h yd ro gen s u l f i d e i n a plasma j e t , wi thmethane and neopentane.

9This s t udy was sug ges ted by an e a r l i e r i n ve s t ig a t io n o f Va s to l a and co -wor ke rsi n t o t h e r e a c t i o n b e tw ee n c ar bo n a nd t h e p r o d u c t s o f water vapor microwave d i sch arge ,where i t w a s obser ve d t ha t t h e p r e se nc e o f c a r bon downs tr ea m in h i b i t e d t he r ecom-b i n a t i o n , t o e i t h e r o xy gen or water , o f t h e a c t i v e a t om i c s p e ci e s . I n s t e a d , t h ea c t i v e o xy ge n r ea c t e d p r e f e r e n t i a l l y w i th c ar bo n.

Exper imenta l P rocedureThe p la sm a r e a c to r i s shown i n F igu re 1. The r e a c t o r c o n s i s t s o f a 1 0 x 0 .75

i n c h f u s ed q u a r t z t u be . Th e g a s t o be b ro u gh t t o t h e pl as ma s t a t e i s in t roducedt a n g e n t i a l l y u p st r ea m fr om t h e o u t p u t c o i l o f a 1 . 2 KW , 120 MHz RF in du c ti o n he at e r.Ihe s p i r a l i n g moti on i m pa rt ed by t h e t a n g e n t i a l i n p u t i n c r e a s e s t h e d w e l l ti me oft h e g a s i n t h e p la sm a zo ne . T he h y dr o ca rb o n g a s e s t o b e r e a c t e d w i t h t h e pl as mad i s s o c i a t i o n p r o d u ct s a r e i n t r o d u c e d down stream f rom t h e p lasma f lame .the hyd r oc a r bon f e e d t u b e t h e d i s t a n c e be t we en t h e p l as ma f l am e a nd t h e p o i n t o fh y dr oc ar bo n i n t r o d u c t i o n c an b e v a r i e d .

By moving

TO i n i t i a t e t h e p l a s m a d i s c h a r g e a r g on i s pa sse d th r ough the reactor (0.5 C Uf t / h r , 1 Atm) and a g r a p h i t e r od i s p l ac e d i n t h e i n d u ct i o n f i e l d .g r a p h i t e r o d s t a r t s t h e d i s c h a r g e i t i s removed and t h e gas fee d is s w i t c h e d t o t h ed e s i r e d H2S-Ar mix ture . Mass s p e c t r o m e t r i c g a s a n a l y s e s were made be fo re and a f t e rt h e r e a c t i on .

A f t e r t h e he at ed

1

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, 2 3 5. . . . . . 1. a. . . . I > ..: . , . . I , . . * . . .. . .,. . . (.. .

i ?..

'LI0 .

H

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236R e su l t s a nd D i sc uss ion

When hydrogen s u l f i d e i s i n t r od uc e d i n t o a n a r gon pl asma c onde ns ing e l e m e n ta ls u l f u r p r od uc es a dense fo g dovns t ream f rom t h e p lasma f lame. A s t h e amount ofnydr oc a r bon tha t i s i n t r o du c e d i n t o t h i s r e g io n i s i n c r ea s e d t h e e l e m en t a l s u l f u rp r o d u c t i on d e c r e as e s t o a p o i n t w he re i t c a n no lon ge r be obse r ve d . W ith l a r g ee x c e s s e s o f h y dr oc ar bo n t h e c o l o r o f t h e plasma f l a m e c ha nge s f r om b l ue t o o r a nged ue t o b ac k d i f f u s i o n o f t h e h y d ro c a rb o n .

T a b l e 1 g i v es t h e r e a c t a n t a nd p ro d u ct c o n c e n t r a t i o n s f o r r e a c t i o n s o f d i s -s o c i a t e d l iydrogen s u l f i d e w i t h methane and neopentane . With th e excep t ion of runnumber 3 t h e o n l y g a s eo u s p r o d u c t s d e t e c t e d were hydr oge n su l f ide , un r e a c t e d hydr o -c a r b o n , c a r b on d i s u l f i d e , a nd hy dr og en .

Ta b le 1Reac t ant and Produc t Ana lyses

R e a c ta n t Mix tu r e P r oduc t sComposition Mole % Composition Mole %

RUXINo. R e a c ta n t s HC H 2 S h K HC H2S H2 CS2 C2H2 AK

1 H2S, CH4 6 .4 17 .6 76 .5 2 .2 4 .5 18 . 5 3.8 0 70 .12 I, 6.4 17 .6 76.5 4 . 3 6.7 13.4 1 . 6 0 71.13 11 25 .5 9.9 64.7 13.0 1 .3 21.9 3.5 3.0 57.8

5 11 1 5 . 3 14 .9 70.2 13.4 4.8 12.9 2.8 0 70.04 H2S, C5Hl2 15 . 3 1 4 . 9 70.2 11.0 0 .7 18 . 1 3.9 0 69.7

The f o l lowin g s to i c h io m e t r y would be e x p ec t ed f o r t h e r e a c t i o n of methane:2H2S + CH4 -+ CS2 + 4H2. (3)

I n r u n num be r 3 t h e a c e t y l e n e was p ro du ce d by t h e b ac k d i f f u s i o n o f m e th an e i n t o t h ep l a sm a r e g i o n .

The s t o i c h i o m e t r y o r t h e n e o pe n t an e r e a c t i o n w ould b elOH2S + C5H12 -f 5cs2 + l6HzS. (4 )

The r e a c t i o n s o f n e o p e n ta n e were per formed wi th a l a r g e e xc e s s o f hyd roc ar bon w i tht h e ho pe o f d e t e c t i n g s u l f u r i n s e r t i o n c ompounds O K r e a c t ion in t e r m e d ia t e s . How-ever , hydr oge n a nd c a r bon d i su l f ide were t h e o n l y ga s e ou s s p e c i e s p ro du ce d. T h i si n d i c a t e s t h a t o n ce t h e ne o pe n ta n e reac ts w i t h o n e s u l f u r atom i t s r e s i s t a n ce t of u r t h e r r e a c t i on i s lowered .

Ta b le 2 shows t h e v a r i a t i o n of r e a c t a n t de c om pos i t i on w i t h c ha nge s o f hyd roge ns u l f i d e t o h yd ro ca rb on f l o w r a t i o an d p la sm a- hy dr oc ar bo n i n j e c t i o n d i s t a n c e . FromT a b l e 2 i t can b e s e e n t h a t t h e e x t e n t o f h y dr og en s u l f i d e de c o mp o s i ti o n i s afu nc t i on of th e amount of hydrocarbon p re se nt downst ream f rom t h e p lasma f lame andt h e d i s t a n c e fr om t h e f l a me t h a t t h e hy d ro c ar b on i s i n t r oduc e d . t h e h i gh e s t decom-P o s i t i o n , 9 5 p e r c e n t , o c c u r r i n g i n r u n number 4 w i t h a n a p p r o x i m a t e l y ten f o l d e x c e s s Of neopentane in t r od uc ed 1 / 4 i nc h f rom th e f l am e . The f i gu r e s f o r t he de com pos it i on Of h y d r o g en s u l f i d e t e n d t o b e m i s l e a d i n g , t h e h y d r oc a r bo n a d d i t i o n d o e s n o t c h an ge 1t h e de g re e of d i s s o c i a t i o n of t h e h yd ro ge n s u l f i d e b u t r a t h e r it r e a c t s w it h t h ea t o m i c s u l f u r m i n im i z in g f o r m a t io n o f h yd ro g en s u l f i d e by t h e re c o mb i n at i o n o f t h e I

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237Ta b le 2

Expe rim e n ta l Va r i a b le s a nd Xe ac t ion C onve r sion E f f i c i e n c i e sHC i n l e t

Flow d i s t an ce Convers ion of decomposedRun r a t e f ro m R e a ct a n t r e a c t a n t s t o :-O. Rea c ta nts ( t o t a l ) HZS/HC plasma decomposit ion CS? H?-H2S HC H2S I H C & S + HCc . f . h . i n c h e s % % -% x %1 H2S, CH4 0. 14 2.75 11 4 72 6 3 6 5 *lo0 %lo02 I, 0 .14 2.75 1 59 28 33 1.100 %lo03 , I 0 . 3 3 0.39 114 8 5 4 3 93 36 8 14 HzS, CgH12 0 .25 1 . 0 3 1/ 4 95 28 55. 19 465 I , 0 .25 1 .03 1 68 12 56 30 61*c u f t / h r .a t o mi c h yd ro ge n and s u l f u r . T he d a t a o n e x t e n t o f c o n v e r s i o n t o h yd ro ge n a n dc ar bo n d i s u l f i d e i n T a b l e 2 show t h a t i n r u n s 1 and 2 e s s e n t i a l l y a l l o f t h ecarbon from the decomposed methane i s c o nv e r te d i n t o c a rb on d i s u l f i d e a nd t h ehydrogen i n to molecula r hydrogen . I n run number 3 th e a c e t y l e n e p r od u ce d a c c o u n t sf o r t h e miss ing ca rbon and hydrogen .t h e n eo p en t an e t h a t w a s decomposed w a s c o n v e r t e d i n t o a s o l i d p r od u c t.In runs number 4 and 5 a l a r g e f r a c t i o n o f

T a b l e 2 a l s o s hows t h e p e r c e n t a g e o f t h e s u l f u r , r e s u l t i n g f rom t h e decom-p o s i t i o n of t h e h yd ro gen s u l f i d e , t h a t i s c on ve rt ed i n t o c a rb on d i s u l f i d e ; th er e m a inde r o f t h e s u l f u r a p p ea r s as e l em e nt a l s u l f u r ( r un s 1-3) o r c a n b e i n co r -p o r a t e d w i t h t h e ca rb on -h yd ro ge n p o l y me r i c s o l i d t h a t i s p ro du ce d i n t h e r u n sus ng ne ope n ta ne.

T he se r e s u l t s i n d i c a t e t h a t h yd ro gen s u l f i d e ca n b e , co m p le t el y d i s s o c i a t e d byan e l e c t r i c d i s c h a r g e a n d t h e e x c i t e d a t o m ic hy dr og en a n d s u l f u r g e n e r a t e d c an b er e a c t e d w i t h h y dr o c ar b o ns t o p r od u ce m o l e c u l a r hy d ro g en a nd c a r b o n d i s u l f i d e .Under c o n d i t i o n s wh ic h mi n im i ze b ac k d i f f u s i o n o f t h e hy d ro c ar b on i n t o t h e e l e c t r i cd i s c h a r g e s u b s t a n t i a l l y c o mp l et e c o n v e r s io n o f t h e h yd ro ge n s u l f i d e an d h yd ro ca rb ont o h yd ro ge n an d c ar b o n d i s u l f i d e ca n b e e f f e c t e d .

Refe rences1.2.3.4.5.6 .7.8.9.

Knight , A . R . , S t r a u s z , 0. P. and Gunning, H. E . , J . A m. Chem. Soc. 85, 2349,Bryce , W. A. and Hinshelwood, C . , J . Chem. Soc. 4, 3379, (1949) .Nabor, G. W. and Smith, J. M . , Ind. Eng. Chem. 45, 1272, (1953) .Tha c ke r , C. M. and M i l l e r , E . , Ind. Fng. Chem. 36, 182 , ( 1944) .F o l k i n s , H . O . , Mille r , E. and Hennig, H . , Ind. Eng. Chem. 2, 2201, (1950).F i s h e r , R. A. and Smith, J. M., Ind. Eng. Chem. 42, 704, (1950) .Forney , R . C. and Smith, J . H., Ind. Eng. Chem. 43, 1841, (1951) .Thomas, W. J. a nd S t r i c k l a nd - C ons ta b le , R. F . , Trans . Fa rad . Soc . 53 , 972, (1957)V a s t o l a , F . J., Walker, P. L . , J r . and Wightman, J. T . , Carbon 1,11, (1963).

(1963) .