lique fac t ion o f coal

8/13/2019 Lique Fac t Ion o f Coal

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Lique fac t ion o f Coa l

4 .1 In t roduc t ionToday, the i s sue o f energy secur i ty i s e s sen t i a l fo r su rv iva l , i . e . , fo r the sus ta inab le deve lop-m e n t o f t h e g lo b a l c o m m u n i t y. T h e w o r l d p o p u l a t io n i s e x p e c t e d t o r e a c h 7 .0 b i ll i o n b y2 0 1 0 , a n d o v e r 5 5 % o f th i s n u m b e r w i l l b e t a k e n u p b y th e p o p u l a t i o n o f A s i a. B e c a u s e t h ed e m a n d f o r p e t r o l e u m a n d n a t u r a l g a s i n th e i n d u s t r i a li z e d n a t i o n s c o n t i n u e s t o r is e , w o r l dp e t r o l e u m s u p p l ie s a r e a n t ic i p a t e d t o b e c o m e e i t h e r u n r e l i a b le o r i n a d e q u a t e i n t h e n e a r f u -tu re . The loca l d i s t r ibu t ion o f pe t ro leum in the wor ld has resu l t ed in severa l c r i ses and sup-p l y i n t e r r u p t io n s , e s p e c i a l l y s i n c e 1 9 7 3 w h e n t h e O rg a n i z a t i o n o f P e t r o l e u m E x p o r t i n gC o u n t r i e s ( O P E C ) f i r s t a c h i e v e d s u f f ic i e n t p o w e r t o h a v e a m a j o r i m p a c t o n w o r l d o i l m a r-ke t s (Wi l son , 1980) . N ow ada ys , the re is no doub t tha t coa l i s a va lua b le resource and m aybe the ma jor fue l o f the 21s t cen tu ry. Coa l can be cons ide red to be one o f the mos t a t t r ac -t iv e a l te r n a t iv e s o u r c e s o f p e t r o l e u m o i l f o r t h e fo l l o w i n g t w o r e a s o n s . 1 ) T h e a m o u n t o fc o a l d e p o s i t s e s t i m a t e d w o r l d w i d e i s t e n t i m e s l a rg e r t h a n t h a t f o r t h e o t h e r c a r b o n a c e o u sr e s o u r ce s . 2 ) C o a l r e s o u r c e s a r e l o c a t e d m o r e w i d e l y t h r o u g h o u t t h e w o r l d t h a n a re o il r e -se rves . The re fore , coa l wi l l be mo re wid e ly ava i l ab le than c rude o i l in the fu tu re . I t i s nec -e s s a r y t o d e v e l o p n e w p r o c e s s e s f o r t r a n s f o r m a t i o n o f s o l i d c o a l i n t o c l e a n l i q u i d f r o m t h ev iewpoin t o f energy secur i ty in the fu tu re .

4 . 1 .1 C o a l L i q u e f a c t i o n

Many sc ien t i s t s and eng ineers be l i eve tha t in the fu tu re coa l l iquefac t ion wi l l be requ i red tom e e t t h e d e m a n d f o r l i q u id t r a n s p o r ta t i o n f u e ls . C o a l l i q u e f a c t io n m e a n s t h e c o n v e r s i o n o fso l id coa l to fue l l iqu ids (Whi teh urs t e t a l ., 1980) . Coa l com po sed m os t ly o f ca rbo n andh y d r o g e n a n d h a s l o w e r h y d r o g e n c o n t e n t t h a n p e t r o l e u m . T h e r e f o r e , t h e p r o c e s s o f c o a l

l i q u e fa c t i o n p r o d u c e s l i q u id c o m p o u n d s c o n t a i n i n g h y d r o g e n a t le v e l s o f a p p r o x i m a t e l y 1 0to 15% by weigh t (Wu and S to rch , 1968) . Ty p ica l comp os i t ions fo r coa l s , l iqu id fue l s , ands o m e h y d r o c a r b o n s a r e g iv e n i n Ta b l e 4 . 1 .

Table 4.1 Com positionof T ypical Fuel Oils and Hydrocarbons

Element, wt% (moisture and ash-free)Fuel C a r b o n H y d r o g e n O x y g e n S u l f u r N i t ro g e n

Typica l crud e oil 86.0 11.0 0.7 1.5 0.5Fuel oil 86.0 13.4 0 .2 0.3 0.1Gaso line 85.0 15.0 0.0 0.1 0.0Bituminous coa l 78.0 5.7 11.6 3.3 1.0Subbituminous coa l 71.9 6.1 20.2 0.6 1.0Benzene 92.3 7.7 0.0 0.0 0.0Naphthalene 93.7 6.3 0.0 0.0 0.0


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182 4 Liquefactionof Coal

There are many technologies for coal l iquefact ion: indirect l iquefact ion, ref inement ofcoa l t a r ob ta ined in carboniza t ion a t 630 -770 K, f lash hydropyro lys i s in the h igh pressureof hydrogen , ex t rac t ion wi th so lvent and/or c r i t i ca l gas , and d i rec t l iquefac t ion by hy-

droge nolysis in the present of catalyst , solvent and high pressure of hydrog en. Reg ardingthe gas i f ica t ion of coa l involv ing f lash hydropyro lys i s and carboniza t ion i s descr ibed inchapter 3 . The cha pter empha sizes on the direct l iquefact ion.

In indirect l iquefact ion, coal is gasif ied at 1300 K or over in the presence of s team andoxyge n to produce a synthes is gas conta in ing mos t ly carbon monox ide and hydrogen . Thissynthesis gas (syngas) , af ter being cleaned of impuri t ies and adjusted to the desired H2/COrat io ( i f required) , is converted to l iquid fuels in the presence of catalysts . A unique featureof the indirect l iquefact ion is the abi l i ty to produce a broad array o f sulfur and ni t rogen freeproducts inc lud ing motor fue ls , methanol , oxygenates (oc tane enhancers ) , and chemica lswith the use of different com binat ions of catalysts and process condit ions. The conv ersion

of syngas to mo tor fue ls is known as F ischer-Tropsch (F-T) synthes i s . Com merc ia l ind i rec tl iquefact ion plants in operat ion s ince 1955 have included coal based plants in South Africaand the U.S. , and natural gas base d plants in South Africa, New Ze aland and M alaysia . Inal l the plants , the syngas is conv erted in gas phase reactors . Bec ause o f the high exothermassociated with the react ions, i t has long been known that a l iquid phase reactor could offercost and operabi l i ty advantages over gas phase reactors due to i ts superior heat t ransfer ca-pabi l it ies . Earl ier effor ts in develo ping a liquid phase F-T reactor af ter W orld W ar II weresuspended in the la te 1950s because of the avai labi l i ty of cheap petroleum crude (Poutsma,1980). Intere st in this area was reviv ed in late 1970s with the rise in pet role um cru de price.Scop ing economics s t ud i e s suppor t ed by t he US Depa r tmen t o f Ene rgy (DOE) and t he

Electr ic Power Research Inst i tute (EPRI) indicated that the capabi l i ty of a l iquid phase re-actor to process a low H2/CO rat io syngas from advanced coal gasif ier could offer s ignif i -cant cost advantag es ov er gas phase reac tors (Gray et a l. , 1980, Bro wn et a l., 1982). In co-operat ion w ith industr ia l organizat ions, DO E in 1981 began to support a research and de-ve lopment program to advance the l iqu id phase reac tor t echnology for coa l -based syngasconv ersion be yon d that of the la te 1950s. The init ia l focus of this prog ram has been on thel iqu id phase reac tor technology deve lopm ent for methanol and F-T synthes is . The de ta il sof the prog ram hav e been reviewed (Shen et a l ., 1996). Liquid phase methan ol develop-ment was success fu l ly comple ted a t the proof -of -concept (POC) sca le in 1989 , and ad-v a n c e d t o c o m m e r c i a l d e m o n s t r a t i o n i n 1 9 93 u n d e r t h e s u p p o r t o f D O E C l e a n C o a lTec hnolog y program. Deve lopme nt of l iqu id phase reac tor t echnologies for F-T synthesi sand for syngas convers ion to oxygenates and chemica ls have been under way a t the POCunit.

Direct coal l iquefact ion involves the conve rsion o f sol id coal to l iquids w ithout the pro-duct ion of synthesis gas , a mixture of carbon monoxide and hydrogen, as an intermediatestep. Direct coal l iquefact ion should be the most energet ical ly eff ic ient metho d of produc-ing l iquids and this metho d m akes i t possible to obtain the highest oi l yield. Althoug h thedevelopment of coal l iquefact ion wil l depend on both economics and the rel iabi l i ty of pe-troleum and natural gas suppl ies from the Middle East and other main export ing areas , i t isimportant to develop coal l iquefact ion technology to insure new sources of energy in the fu-ture.

The d i r ec t l i que fac t i on o f coa l ha s been s tud i ed ex t ens ive ly. M os t o f t he cu r r en tprocesses for the l iquefac t ion of coa l have been deve loped f rom ear ly works (Berg ius andBil l ivi l ler, 1918) and have some co mm on features . In mo st of the conve rsion processes , asshow n in Fig. 4 .1 which is a f low diag ram o f the coal l iquefact ion proces s , coal is transport-


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4.1 Introduction 183

Coal

Slurrypreparation

Hydrogen

Liquefaction

f

~ Distillation

Light oil Hydrogenation

r " ~ Liquidproducts

M._._ Dist illa tion Heavy oil

Fig. 4.1 A flow diagram of the coal liquefaction process.

ed to a coal s lurry preparat ion process , in which the coal is dr ied and ground, then mixedwith a hyd roge n-do nor solvent and/or a coal-derive d recycle oi l to form coal s lurry. The nthe s lurry is pumped into the l iquefact ion process , in which the coal is l iquefied by hydro-gen at a high temperature and pressure in the absence/presence of a catalyst in one and/orsevera l h igh-pressure reac tors . The l iquefac t ion prod uct is separa ted in the d i s ti l l a tionprocess . The he avy oi l ( residue) and a port ion of l ight gas oil is hyd roge nize d in the sol-vent hydrogena t ion process and t ransfer red to the hydro gen-do nor so lvent.

4 .1 .2 M echan i sm of Coa l L iquefac t ion

I t has become a lmos t ax iomat ic to formula te coa l l iquefac t ion as a f ree rad ica l p rocess .The concept has i ts or igins in the contr ibut ion of Curran et a l . (1967) who pointed out thesignif icance of the relat ionship between the extent of the conversion of the intractable coalmolecules to so luble products and the amount of hydrogen t ransfer red to the l iqu id coa lproducts . The y prop osed a f ive-step react ion [Eqs. (4 .1)-(4.5)] sequen ce focused on thehom olyses of carbon-carbon bonds in the coa l molecules . In th is reac t ion sequence , theradicals produced in the ini t ia l react ion, Ri , react with other coal molecules or with hydro-gen-a tom donor-so lvent molecules , DH2, to form o ther rad ica ls . A var ie ty of recombina-t ion react ions terminate the chain react ions.

C oa l ---> 2Ri ~ (4.1 )

Ri" + DH2 ~ R i l l q - D H - (4.2)

Ri ~ -Jr- C oa l- -)R il l -I-- Rj . (4.3)

Ri 9 -}- D H "-- -)R i l l q - D (4.4)

R i ~ -4- R j . ---->Rill -+-Argj (4.5)

This view is supported by the general observat ion that f ree radical react ions control thepyro lysis chem istry of mo st organic substances. Gen eral kinet ic features of coal l iquefac-t ion have also been used to support this view (Neavel , 1982). A detai led considerat ion ofthe chemical s t ructure of coal and i ts react ion products a lso s t rongly suggests that f ree-radi-ca l reac t ions cont ro l coal chemis t ry. The a romat ic and hydroarom at ic un i ts found in coa ltars and l iquids and presumed to be dominant s t ructures in coal i tself are known to be veryreact ive toward free radicals. M oreove r, resona nce-stab i l ized radicals der ived from these


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s t ruc tures a re formed and reac t read i ly a t coa l decompos i t ion tempera tures ( -350 ~Methyl and hydroxy subst i tuents serve to increase the overal l f ree radical react ivi ty of themolec ules to which they are at tached. The present analysis accepts the importanc e of f ree

radicals in coal chemistry and at tempts to develop a more detai led and unif ied view of theirs t ructures and propert ies .

In many respects , l iquefact ion is c losely related to pyrolysis . The y share an ident icalini t ia l s tep - the thermal generat ion of radicals f rom the coal by way of homolyt ic bondscission. In pyrolysis , these radicals are ei ther capped by an internal ly t ransferred hydrogenor they combine with carbon to form mater ial of heavie r molecular weight (char). Thesetwo even ts a lso occur in l iquefaction, a long with t ransfer of hydrog en to the radicals f rom ahydro gen source. The net effect is that l iquefact ion produces greater amounts of liquid andgaseous products than convent ional pyrolysis , but a t the expense of addi t ional hydrogenconsum ption. Liquids from hydrol iquefact ion are substant ial ly depleted of heteroatoms as

com pared with ei ther the parent coal or pyrolysis l iquids. A wide range of different tech-niques is used to make l iquids from coal , even though they al l share the thermal conversionstep. These m ethods differ in whether the hydrog en is provided from an organic donor orf rom mo lecular hydrog en , e i ther ca ta ly t ica l ly or non-ca ta ly t ica l ly. The y a l so d i ffe r inwhe ther a solvent, and w hat kind of solvent , is used. Thus, a s tudy of the physical proper-t ies of solut ions of coal macromolecules in various solvents as wel l as the col loidal natureof the solutions wou ld be helpful . An understand ing of the phase behavior at high tempera-tures and under high hydrogen pressures would help to elucidate the l iquefact ion process .Catalysis in l iquefact ion has rec eived m uch at tention, a l though thus far the use of such cata-lysts as cobal t -molybdenum has not a l tered process temperature or pressure requirements

(Johnson, 1978). Res earch should be ca rde d out to develop catalysts that wil l posi t ively af-fect the initial coal conversio n. It is relative ly easy to affect the course of reactions after theprima ry products are out of the coal part ic le . How ever, by this t ime the product dis t r ibut ionmay already have been determined. I f a catalyst that could inf luence the product dis t ribu-t ion of the pr imary products as they are formed could be found, ent i rely different types andquantities o f prod ucts m ight result. It wo uld be less impo rtant, but stil l valuab le, to deter-mine whether the use of catalysts in coal l iquefact ion improves the qual i ty of the l iquidproduct . Com parison s of the heteroatom content , a l iphat ic/aromatic ratios , viscosi ty andcompatibi l i ty with petroleum l iquids of catalyzed and noncatalyzed coal l iquids would bevaluable in determining the best disposi tion o f var ious coal l iquid fract ions. I t would alsobe valuable to: (1) understand the relationship between coal structure and its reactivity; (2)establ ish the ul t imate disposi t ions of e lements; (3) bet ter understand the kinet ics and mech-anism of the reaction involved in the catalytic effect, hydrogen transfer, etc.; and (4) devel-op data which l ink coal character is t ics to process condit ions and product type, qual i ty andultima tely, utilization. In this chapter, the adv ance s in the areas me ntion ed abov e are de-scribed.

4.1.3 Hy drogen Transfer Reaction in Coal LiquefactionThe hydrogen-transfer react ions that occur during coal l iquefact ion react ions are essent ialfor the conversion o f intractable coal molec ules into l iquids and soluble products . Vir tual lyall the practical processes for coal l iquefaction, such as the solvent-refined Coal II process(Schm id and Jackson, 1981), the Exxon donor-solve nt process (Furlong et al ., 1976), the in-tegrated two-stage l iquefact ion process (Whitehurst e t a l . , 1980, Neuworth and Moroni ,1981), and the Chevron coal l iquefaction process (Rosenthal et al . , 1982), use a portion ofthe l iquid coal products as a solvent for the dissolut ion react ion. In the recent ly developed


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4.2 Coa l Structure and Reactivity 185

Chevron coa l l iquefac t ion process , the l iquefac t ion reac t ion i s ca rded ou t in two separa te ,but c losely coupled, reactors . A s lurry of the coal in a port ion of the coal l iquid (recy cleoi l ) is introduced into the f i rs t -s tage reactor and the product of this phase of the react ion is

then fed in to the second-s tage reac tor. The la rge coa l mo lecu les a re deco mp osed and , inpart , d issolved in the f i rs t reactor and the ini t ia l product is ref ined catalyt ical ly in the sec-ond reactor to yield the coal l iquefact ion products which include a f ract ion sui table for useas the so lven t fo r the reac tion . The c onvers ion reac t ions requi re no t on ly the addi t ion o fhydrogen bu t a l so the red is t r ibu t ion of the hydroge n a tom a l ready presen t in the coa l mo le-cu les . Thus , hyd rogen t rans fe r reac t ions occur be tween the coa l molecu les and the com po-nen t s o f t he r e ac t i on so lven t and be tween t he coa l mo lecu l e s and t he added hyd ro ge n .Hydrogen t rans fe r reac t ions a l so take p lace be tween the l iqu id coa l p roduc ts , the so lven tmolecu les , and gaseous hydrog en . M any o f these reac t ions , par t i cu la r ly those tha t occur inthe ini t ia l s tages of the coal l iquefact ion process , are qui te rapid even in the absence of cata-

lys ts . Indeed , som e coa ls a re such good hydro gen a tom donors tha t they on ly need to behea ted in a f lu id medium to cause ex tens ive degrada t ion of the ca rbon ske le ton wi th an a t -tendant red i s tr ibu t ion of the hydrog en a toms (N eave l , 1982) . M ore of ten , how ever so lven tstha t are good hyd roge n-a tom donors a re used in coa l l iquefac t ion reac t ions to p rov ide a f lu -id medium for the produc ts as wel l as to p rov ide a convenien t , mobi le source of hydrogenfor the deco mp os ing coa l molecu les . In add it ion , these so lven t molecu les enab le the t rans -fe r o f hydrogen a toms be tween the a r ray of hydrogen donors in the so l id , l iqu id , and gasphases and the reac t ive coa l molecu les . The reac t ion pa thw ays im por tan t fo r the trans fe r o fhydrogen a toms dur ing coa l l iquefac t ion have been s tud ied in tens ive ly in the pas t few yearsto es tab l i sh a more secure bas i s fo r the deve lopment o f e ff ic ien t methods of coa l l iquefac-

t ion us ing the ava i lab le hydrogen a toms in the coa l molecu les as wel l as the hydrogena toms in dono r-so lven t molecu les and added hydrogen . The recen t work on th i s ma t te r i salso discussed in this chapter.

4.2 Co al Structure and Reactivi ty

4.2.1 Stages of Coal LiquefactionPr ior to l iquefac tion , coa l is o f ten washed to rem ove inorganic minera l s and dr ied . Thisprocess somet imes changes the s t ruc ture and assemblage of coa l macromolecu les , whichprofoun dly in f luences the reac t iv i ty o f coa l (Moc hida and S akanish i , 1994). In the pre -heater, coal with or without catalysts is rapidly heated to react ion temperature in the pres-ence of so lven t and pressur ized hydrogen ex tens ive decarboxyla t ion , fo rmat ion of ca rbon-a tes , and dehyd ra t ion take p lace in the prehea te r (Neave l , 1976). Coa l i s be l ieved to besubs tan t ia l ly d i sso lved in the prehea te r a t th is s tage . Rapid hea t ing of up to severa l hundreddegrees per m inute i s be l ieved to be very essen t ia l in ob ta in ing h igh o i l y ie lds and prevent -ing re t rogress ive reac t ions , wh ich ma y take p lace a t the sam e t ime . Ca ta lys t s a re no t ex-pec ted to be e ffec t ive in the prehea te r s tage due to insuff ic ien t con tac t t ime . Hy drog endonor so lven ts p lay an impor tan t ro le in suppress ing the re t rogress ive reac t ions a t th i ss tage , and i t i s impor tan t tha t the capac i ty o f the donor no t be exceeded in the prehea te r.The amount o f hydrogen consumed f rom so lven t has been cons idered to be re la t ive to thehea t ing ra te . S low hea t ing ra tes a l low mo re so lven t dehydrog ena t ion (Derbysh i re et a l .,1986b) . I t i s wel l know n tha t the v i scos i ty increases very rap id ly wi th b i tuminou s coa lstha t a re d i sso lved in the so lven t rap id ly in the prehea te r. This so met im es causes p rob lem sof s lu r ry t ranspor ta tion in nar row prehea te r tubes . The preh ea ted coa l s lu r ry (essen tia l lyl iquef ied) i s sen t to the reac tor, where thermal and ca ta ly t ic c rack ing , hydrogena t ion and


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hydro cracking take p lace . These reac t ions occur ra ther slowly because fewer reac t ivebond s are involved in this stage, which prod uces dis t i l la te range small molecules . In theearl ier Bergius process, the react ion at this s tage was perform ed u nder v ery high pressure at

high temperature with disposable catalysts of low act ivi ty and was completed in a s inglestep. Current l iquefact ion processes ut i l ize two or three s tages under mo re moderate condi-t ions. Hy drog en donor solvents also assis t in mo derat ing the condit ions required. Thus,the primary products in the f i rs t s tage, together with the used solvent , are further hydroc-racked and/or hydro refined products as well as rehyd rogena ted solvent . Various types offeeds, distil lates, nondistillable liquids free of minerals, the catalyst, preasphaltenes and un-reacted coal of the f i rs t s tage or whole products , including the catalyst and minerals , arecharged to the second stage, depend ing on the liquid/sol id separat ion procedure u t i lized andthe durability of the catalyst in this stage. Stage d heating is som etim es utilized in the firsts tage where the react ion temperature of each reactor is control led separately to obtain the

bes t o i l y i e ld w i th min imum fo rma t ion o f hyd roca rbon gases and avo idance o f cok ing(Burg ess and Scho bert, 1991, Da vis et al., 1989). The oil is furthe r refined in the follow ingstages. Such a process sch em e practiced at present is called mu ltistage liquefaction. A se-r ies of react ion temperatures is expected to improve select ively specif ic react ions at differ-ent temp eratures in a ser ies of consecutive react ions. Higher degrees of desulfurizat ion anddenitrogenat ion, longer catalyst l i fe , less s ludge formation, and higher yields of dis t i l la teare reportedly obtained by the mult is tage processing and ref ining of petroleum products(M och ida et al., 1988a; 1 990a). The function of the catalysts in the various liquefactionstages are described in the following sections.

4 . 2 .2 C o a l D i s s o l u t i o n , D e p o l y m e r i z a t i o n , a n d R e t r o g r e s s i v e R e a c t i o n s

The l iquefact ion of coal is the conversion of an ensemble of macromolecules as describedabove into smaller hydro carbon molecu les that are dis ti l lable . Shinn (1984) has describedthe changes in representative molecular structures of intermediates in the three steps of liq-uefaction as show n in Fig. 4.2a -c. The first step in the liquefac tion o f solid coal is the for-mation of l iquid phase. Sm all molecules of the coal fuse above 350 ~ to form a l iquidphase together w ith solvent ( i f present) ; some m acrom olecules ma y be dissolved in this l iq-uid phase (fusion and dissolut ion mechanism s). Other mo lecules undergo thermal f ission attheir weakest bonds, such as methylene and benzylether bonds, producing fragmented radi-cals (Whitehurstet e t a l ., 1980). W hen the radicals are capped with hydrog en from the sol-vent or the catalyst , they form smaller molecules that are soluble in the solvent or evenfusible by themselves (f i rs t mechanism) increasing the quanti ty of l iquid phase (Poutsma,1990). This pyro lys is cont inues whi le the reac t ive bonds and s tab i l iz ing hydrogen areavai lable. Atom ic or mo lecular hydrog en avai lable in the reactor system can hydrogenatereact ive s ites on the aromatic r ings. W hen the ipso-posi t ion of the s trong aryl-aryl bond inthe aromatic r ing is hydrogenated, the bond becomes weakened and bond cleavage becomespossible via the f i rs t mechanism of depolymerizat ion and faci le s tabi l izat ion (second mech-anism ) (McM illen et al., 1987; Kam iya et al., 1988; M och ida et al., 1988b). V ery reactivehydrogen may at tack the aryl-aryl bond direct ly, leading to i ts breakage ( third mechanism)(Mo chida et al ., 1990b). Aro matic r ings are very s table unless they are hydrog enated tonaphthenic t ings, which may be thermally or catalyt ical ly cracked to open the r ing (fourthmec hanism ) (Ma lhotra and M cMillen, 1990). Unless the fragm ented radicals are s tabi l ized,they recombine or react with other molecules , form ing thermally s table bonds. Repet i t ionof recombinat ion react ions produces large molecules that have resis tance toward depoly-merizat ion. Co king takes place when such large molecules remain at e levated temperatures


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4.2 C oal Structure and Reactiv i ty 187

OH HO O O OH

H3C HO"~0 ( ~ ~ C H 3 ~"~"~-'~ 0 ~ . OHH O ' V N - ~ ~ ~ [ ~ )H ~ . -. ~ 'S ' ~~ O - 0 ~ . , ~

H O ,~ j [~J - ~ O I ~ ~ ~ ? HO O I O No ~ o ~ ~.H,~ ~ . '~ o

a . f ~ o ~ . j s ~ 2.~ ~ e ~ ~ ~OOH~ x - .. ,' m c ~ t T _ o ~ , N v y o

H,C~~ ~_~~ ~ ( H o ~ H, OHH O ~ ~ OOH "~ k ~ ~ " N ~ O ~ L . ~

O C

O HO

N HO O /H O ~ NNN~ OHOH

(a)

(b)

S

Fig. 4.2 The Shinn m odel of a bi tuminou s coal s tructure[From Shinn, J .H. , Fuel , 63, 1190 , 1191 , 1194 (1984)]

H20 H20

20

3H8

2 H20H20

CH

H 3C ~~ ; ~ ~ ' ~ CH3 l / - O ~ ~ Y ~ O ~ ~ ~ Ok H3C~c~HH - ~ ~ % ~ H O ~ I ~ o H s ~ C ~ H 3 ~H20

\ . ,o = ~ . . ~ L o ~ O - l / . .~o't~,It'CH; H2SH20

fo r long t ime pe r iods , fo r example , i n the loca t ions o f low f low ra t e , such as nea r r eac to rwa l l s , i n bends o f t r ans fe r l i nes , o r on ca ta lys t su r faces ( r e t rogress ive mechan i sm) (Ba teand Har r ison , 1992 ) . W hen rad ica ls a re t r apped in the cage o f coa l ma crom olecu les , such


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188 4 Liquefaction of Coal

Table 4.2 Half-life Estimated for Carbon-Carbon Bond Homo lysis in Some RepresentativeComp ounds at 400 ~ in Tetralina

Compoundb Half-life (min)

1,2-Diphenylethane, C6HsCH2-CH2C6H52,3-D iphenylb utane, C6HsC(CH3)H-C(CH3)HC6H52,3-Diphenyl-2, 3-dimethylbutane, C6HsC(CH3)2-C(CH3)2C6H59-(1-P henyleth yl) anthracene, 9-CI4H9CH2-CH2C6H59-Be nzyl-9,1 0-dihy droph enanthr ene, (9,10-C14HI1)-9-CH2C6H5Bitetrayl, (1-CIoHI )-(1-Cl0Hll)Ben zyl p heny l ether, C6HsCH2-OC6H5Ben zyl p heny l thioether, C6HsCH2-SC6H5

168020

0.25

141

0.71

a The values were selected from the com pilation provided by Stein (1981).bThe carbon-carbon, carbon-o xygen, or carbon-sulfur bond cleaved in the hem olytic reaction is

indicated in the structural reprasentation.[Reproduced with p ermission from Stein,S. E., ACS Symposium Ser,No. 169.New Approaches

in Coal Chemistry,7, 104 (1981)]

retrogressive react ions come to act and are accelerated as the radicals f requent ly encountereach other. The cage hinders l iberat ion of radicals and the part ic ipat ion of donors . Hence,the dissolut ion of depolymerized coal molecules to break the cage is very important and ef-fective in the liquefac tion proce ss (Saka ta et al . , 1990). Strong dissoc iative properties ofthe solvent are important in minimizing macromolecular interact ions of coal components orcoal-derived products .

4 .2 .3 F r e e R a d i c a l s in C o a l L i q u e f a c t i o n

It is general ly accepted that f ree radicals are key react ive intermediates in thermal coalchem istry. How ever, because of the chem ical complexi ty of coal , s t ructural and kinet icpropert ies of these radicals cannot be direct ly obtained from experiments on coal i tself .Therefore, the work that is based on resul ts of wel l control led "model" experiments andpredict ive theory has been ca rde d out (ste in 1981). This work extends a previous analysisof coal chem istry (Stein, 1981) in which basic pred ict ive theory was f i rs t appl ied to coal-re-lated molecules and react ions and then used to analyze selected resul ts of model compoundexperime nts . The present analysis uses these predict ive m ethods and results a long with rel-evant experimental data to divide coal-derived free radicals into classes according to theirreact ivi ty and to examine the probable behavior of each of these radical c lasses in coal con-version chem istry. The pr imary focus of this work is on react ions below 500 ~ To mak ethis analysis t ractable , the fol lowing working assumptions are made:1) Coal is compo sed of random ly oriented, subst i tuted, hyd roarom atic clusters t ied togetherby short covalent l inkages (biphen yl , methylen e, e ther (W hitehurst e t a l. , 1980)2) Free radical react ions account for a l l covalent bond breaking and forming processes andmost types of hydrogen t ransfer3) All var iet ies of e lemen tary free radical react ions o f importance in coal conversion are al-ready known (O'Neil l and Benson, 1973)4) Steric and diffusional effects do not influence reaction kinetics.

In the past decade, many contr ibut ions have been presented to support the general reac-t ion scheme as shown in Eqs. (4 .1)-(4.5) . The f i rs t react ion in the sequence involving thehomoly t ic c leavage of carbon-carbon bonds in the coa l molecules i s the c r i t i ca l s tep .Studies of the kinet ics of the decomposi t ion of a var iety of compounds with relat ively weakcarbon-carbon bonds have shown that the rates of decomposi t ion of these substances bywell-known free radical pathways are comparable with the rates of decomposi t ion of coal


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4.2 Coa l Structure and Reactivity 189

mo lecules . This feature is wel l i llust ra ted by the data presented in Table 4 .2 (Stein, 1981) ,One of the bes t lines o f ev idence for the involvem ent o f f ree rad ica l s in these processes

s t ems f rom t he s t udy o f t he r ap id py ro ly s i s o f coa l i n t he cav i t y o f EPR spec t rome t e r s

(Petrakis and Gran dy, 1981; Spre cher and Retc ofsky , 1983) . In one qui te per t inen t s tudy.Sprecher and Retcofsky inves t iga ted the thermal decompos i t ion or a b i tuminous coa l sus -pende d in s i li ca (Sprecher and Retcofsky, 1983) . The co ncent ra t ion of rad ica l s increasedby a fac tor o f 4 in about 5 min a t 470 ~ The addi t ion of an equa l weigh t o f 9 ,10-d ihy-drophena nthrene to the reac t ion m ix ture inh ib i ted the format ion of rad ica l s, w hereas the ad-d i t ion of an equiva len t amount o f phenanthrene had no e ffec t on the rad ica l concent ra t ion .In addi t ion, i t was found that the radical concentrat ion increased by an addi t ional factor of 2wh en the vo la t i le p roduc ts fo rm ed in the pyro lys i s o f th i s coa l were a l lowed to escape f romthe reac t ion vesse l. These resu l t s s trongly suppor t the idea tha t coa l molecu les de com poseby hom olyt ic react ions to yield t ransient react ive radicals . In addi t ion, the resul ts are com -

pa t ib le wi th the v iew tha t hydrogen-a tom-abs t rac t ion reac t ions occur rap id ly wi th e ffec t ivedonor mo lecu les such as 9 ,10-d ihydrophena nthrene . As i l lus t ra ted in reac t ions (4 .2 ) and

Rj,-+- ~ ~ RjH + ~ (4.6)

(4 .6) , a new coa l mo lecu le i s p roduced toge ther wi th a mo bi le rad ica l. These reac t wi thother coal molecules as i l lustra ted in Eqs. (4 .3) and (4.7) to yield new coal molecules and ad i ffe ren t se r ies o f coa l rad ica ls resu l t ing f ro m the abs t rac t ion of hydrog en a toms ra ther than

f rom the homolyses o f ca rbon-carbon bonds .R i 9 + volatile products ---->Rill n- R3" (4.7)

The four th reac t ion in the sequence descr ibes the behavior o f donor-so lven t molecu les ,such as t e t ra l in and 9 ,10-d ihydrophenanthrene , tha t can read i ly be ox id ized to a romat iccom pounds . The abs t r ac ti on r eac t ions o f t he d ihyd ronap h tha l enes [Eq. (4 .9 ) ] a re morerap id than the abs t rac t ion react ions of te t ral in . The rem oval o f hydrogen a toms f rom the in -te rmedia te rad ica l s by reac t ions wi th o ther coa l rad ica l s a re a l so presumably very rap idprocesses . Som e rad ica l s der ived f rom the coa l and f rom the so lven t engag e in dehyd ro-gena t ion react ions. Th e f if th react ion in the basic seq uen ce i l lustra tes the not ion that hyd ro-gen a toms a re t rans fe r red among coa l molecu les to y ie ld bo th hydrogen- r ich and hydrogen-poor com poun ds dur ing the coa l l iquefac t ion process (Neave l , 1982). These processes a re

~ + Ri9 ---> ~ --t--Rin (4.8)

@ --t- Ri9 ----) @ 9 -F- RiH (4.9)

@ o + Ri9 ~ ~ -+ Rill (4.10)

complemented by another se r ies o f reac t ions which a re in i t i a ted by the abs t rac t ion of hy-drogen a toms f rom the coa l molecu les to p rov ide another uns tab le se r ies o f rad ica l s [Eq .(4 .3) ]. These reac t ions a re ce r ta in to be imp or tan t in coa l s wi th abund ant hydro arom at ic


10/87


structures because the benzyl ic carbon-hydrogen bonds are relat ively weak and the act iva-t ion energies for the t ransfer of hydrogen atoms from these posi t ions to other coal radicalsa re mode s t . W hi le some of the rad ica l s formed in benzyl ic hyd roge n-a tom abs t rac tion

eventual ly undergo aromatizat ion, other radicals formed as out l ined in Eq. (4.3) decomposeby the very well-known zr-sciss ion process (Sweet ing and Wilshire , 1962; Coll ins et a l . ,1979; Poutsma and Dyer, 1982; King and Stock, 1984) to yield a new radical and a highlyreact ive alkene as i l lustrated for 1 ,3-diphenylprop ane in Eq. (4.11) . The fragm entat ion re-act ions decrease the molecular weight of the coal molecules and the react ive products prop-agate the l iquefact ion react ion.

C6HsCHC H2CH2C 6H5 -'-) C6HsC H29 + C6HsCH - CH2 (4.11)

While most discussions have focused at tent ion on the dominant f ree radical processesthat take place during the thermal dec om posi t ion of coal molecules du ring coal l iquefac-

t ion, i t is apparent that per icycl ic react ions can make a major contr ibut ion to the degrada-t ion react ions of the com plex mo lecules un der appropriate condit ions. The re are a var ietyof plausible per icycl ic react ions that must be considered in the development of an adequatetheory for the nonca talyt ic therm al react ions of coal molecules . M oreov er, processes of thiskind are cer tain to be more important in the more severe react ions of coal molecules at tem-pera tures in excess of 500 ~ The reac t ions inc lude the un imolecular t ransfer o f hydrogenf rom one hydroca rbon to another hydrocarbon . Doer ing and Rosentha l (1967) provided theclassic example of the react ion when they showed that the Z isomer, ra ther than the E iso-mer, of 1 ,2-dimethylcyclohexane was obtained preferent ial ly (6% yield) during the decom-pos i t ion of the d ihydronaphtha lene . How ever, few o ther au thent ic ex amples of the reac t ion

have been reported presumably because free radical chain react ions occur competi t ively ob-scuring the nonc hain pericycl ic react ions (Flem ing and W ildsmith, 1970). For example,s tudies of the hydrogen-transfer chemistry of 1 ,2- and 1,4-dihydronaphthalene and other di-hydroarom at ic com pound s une quivoca l ly ind icate tha t pericyc l ic processes a re no t involvedin the product-forming s tages of the react ions with E-st i lbene, phenanthrene, and tetracene(King and Stock, 1981) and Heesing and Mullers (1980) demonstrated that the hydrogen-transfer react ions which take place during the disproport ionat ion of 1 ,2-dihydronaphthaleneat 300 ~ do not occu r in a pairwise fashion or s tereo specif ical ly.

H H3C H3C H

H H3C U3C H

(4.12)

Unim olecular dehydroge na t ion i s a lmos t cer ta in ly a more impor tan t p rocess . Evidencefo r t h i s r eac t i on wh ich l eads d i r ec t l y t o a roma t i c compounds i s much more abundan t(Derb yshire et a l. , 1982). The se processe s m ay be regarde d as terminat ion react ions in coall iquefact ion.

~ @ + H2 (4.13)

The not ion that per icycl ic react ions are important for carbon-carbon bond-cleavage re-ac t ions remains a mat te r o f cont roversy. Vi rk and h is co-workers have advanced the v iewthat retroene react ions and related processes are important in the decomposi t ion react ions of


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4.3 C ataly sis n Coal Liquefaction 191

s imple hydroca rbons a t the th reshold tempera ture of l iquefac t ion , 400 ~ (Vi rk , 1979).Tes t s o f th i s p roposa l by the s tudy of the ra tes and products o f decompos i t ion of 1 ,2-d ipheny le thane , 1 ,3 -d ipheny lp ropane , and 1 ,4 -d ipheny lbu t ane r evea l t ha t t he r e t roene

process is insignif icant for the formation of the react ion produ cts (Stein, 1981; Poutsm a andDye r, 1982; Hun g and Stock, 1982). The s i tuat ion is wel l i llustrated by the pathwa y fol-lowed in the decompos i t ion of l abe led 1 ,4-d iphenylbutane . The rad ica l cha in decompos i -t ion react ion predicts that unlabeled toluene [Eqs. (4 .14)-(4.16)] be formed, whereas the

C6HsCH2CD2CD2CH2C6H5 q- Ro --4 RD + C6HsC H2C DC D2C H2C 6H5 (4.14)

C6HsCHzCDCD2CH2C6H5 ~ C6HsCH 2CD=CD 2-1- 9 (4.15)

9CH2C6H5 + Tetra lin --~ C6H sCH 3 + 1-Tetraly l radica l (4.16)

retroene process requires the form ation of toluene-2 -d [Eqs. (4 .17)-(4.18)] . No m ore than2% toluene-2-d is obtained during the dec om posi t ion react ion in te tralin at 400 ~ Thus,react ions Eqs. (4 .14)-(4.16) appea r to be the pr incipal toluen e-form ing react ions. W hile i tseems clear that re troene processes are not responsible for the product-forming react ions, i ti s q u i t e p o s s i b l e t h a t s u c h p e r i c y c l i c p r o c e s s e s m a y b e r e s p o n s i b l e i n d i r e c t l y f o r t h e

C6HsCH2CD2CD2CH2C6H5

[~CH2

HD

cvI :

HD

+ C6HsCHzCD = CD2 (4.17)

severalsteps > To luen e-2-d (4.18)

Th i s i dea r equ i r e s cons ide ra t i on because t hen i t ia t ion of f ree rad ica l cha in reac t ions .re t roene reac t ions often produce very uns tab le in te rmedia tes . To exam ine the concept, werecent ly s tud ied the decom pos i t ion o f 9 - [3- (perdeuter iophenyl )propyl -3 ,3-d2] -phenanthrene( S t o c k , 1 9 8 5) . G r o u p - a d d i t i v i t y c o n s i d e r a t i o n s s u g g e s t t h a t t h e r e t r o e n e r e a c t i o n is

CH2

-+- C6DsCD ----CH2 (4.19)

CH2CH2CD2C6D5

_._)

endothermic by no more than 38 kca l mol -1 . How ever, the products formed in th is reac-t ion are qui te unstable under the condit ions of coal l iquefact ion and may ini t ia te f ree radicalcha in reac t ions wi th qu i te low effec t ive ac t iva t ion energ ies . Therm al chem ica l ana lyses(Benson , 1976) sugges t tha t the molecule- induced homolys i s shown in Eq . (4 .20) i s en-dotherm ic by only 21 kca l mol-1 . M ore impor tan t , the abs t rac t ion of the benzy l ic and

CH2 CH2 ~

+ C6 Hs CH --C H2 ~ -q- C6H5CHCH3 (4.20)


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a l ly l ic hydrogen a tom in the 9-methylphenanthrene i somer i s a very fac i le p rocess [Eq .(4.21)] which leads to a new radical capable of ini t ia t ing the decomposi t ion of the or iginalalkylphen anthrene b y a convent ional react ion sequence. Considerat ions of this kind sup-

port the view that cer tain low-energy pericycl ic react ions may lead to the ini t ia t ion of f reeradical chain react ions. In this sense, such react ions ma y inf luence the rates of the thermaldecomposi t ion react ions of coal molecules .

C H 2 C H 2 ~

~ +Ri" ~ ~ -'l-Rill (4.21)

Other per icycl ic react ions such as the decarbonylat ion of phenolic aromatic compounds[Eq. (4.22)] and the fragmentation of tetralin [Eq. (4.23)] apparently are too slow to be im-po r t an t unde r t he conven t iona l cond i t i ons u sed i n l i que fac t i on r eac t i ons (Cypre s andBettens, 1974; Berman et al . , 1980).

~ OPhenol --+ -+ CO + Cyc iopentad iene (4.22)~ CH2Te tralin --+ + C H 2 = C H 2 (4.23)

CH2

4.3 Catalysis in Coal LiquefactionThe coal l iquefact ion proceed s e ven in the absence of the catalyst . There are essential ly nouse of ca ta lys t s in the processes deve loped in USA such as SRC, SR C-I I , EX X ON DO NO RSOLVENT (EDS), in which the i ron species present in ash of coal and hydrogen-donor sol-vent were ut i l ized to enhance the l iquefact ion of coal (Suzuki and Ikenag a, 1998). On theother hand, the use of catalysts is considered to enhance the oi l yield in coal l iquefact ionand has been spot l igh ted in a lo t o f s tud ies , espec ia l ly in Japan . In fac t , the NED OLprocess , which the catalyst and hydrogen-donor solvent was used, had showed higher oi lyield than any present processes .

The m ost convent ional ca talyt ic mater ial is i ron sulf ide in various types. I ron-sulfurcatalyst systems have been successful ly employed for direct hydrogenat ion during coal l iq-uefact ion on a com me rcial scale (Wu and Storch, 1968). Now aday s, they are preferred be-cause they are s imple to use and becau se of economic reasons. I ron-sulfur catalyst systemshave been widely invest igated by several authors (Montano and Granoff , 1980; Montano etal. , 1981; Cypres et al . , 1981; Baldwin and Vinciguerra, 1983; Lambert, 1982; Stenberg etal. , 1983; M ukh erje e and Mitra, 1984; Trew hella, 1987). It was sugg ested that the highestconversion of coal to l iquid products w as associated with a pyrrhot ite , wh ich had the largestnum ber o f vacancies . Mo reover, both H2S and pyrrhot ite appeare d to play a s ignif icant rolein the coal l iquefact ion process (Montano and Granoff , 1980; Montano et a l . , 1981; Suzukiand Ikenag a, 1998). In most s tudies , it was recognized that the act ive form o f i ron-sulfurcatalysts was pyrrhot i te . There is an al ternat ive interpretation that the catalyst was actual lyH2S produ ced from the reduct ion of pyri te (Lambe rt , 1982; Mu kherjee and Mitra , 1984).

Pyri te , pyrrhot i te , and various nonstoichiometr ic sulf ides are known, and pyrrot i te is


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4.3 C atalysis n Coal Liquefaction 193

postulated as the act ive form. I ts precursors are red mud, residue o f bauxi te af ter the sepa-rat ion of a lumina, i ron ores of var ious sources, synthet ic and natural pyri te , f ine i ron part i -cles , i ron dust f rom converters , i ron sulfate , i ron hydroxide, e tc . (Montan et a l . , 1981;

Suzuki e t a l. , 1989). I t is a lso imp ortant to elucidate the catalyt ic roles played by i ron-based catalysts and sulfur, which are added during coal l iquefact ion.

The next mos t wide ly used mater ia l s a re Co-Mo and Ni-Mo su l f ides , which have beenwidely used in petroleu m ref ineries . Th ey are usual ly supported on alumina of designedpore s t ructures in which the pore diameter is usual ly larger than that for convent ional petro-leum residue (Stephens et al ., 1985; Derbyshire , 1989).

A third type o f mater ial is the chloride o f t ransi tion metals , such as ZnC12 and SnC14(M obley and Bel l , 1980; M izum oto et al ., 1985). This group of catalysts works in moltenstate in contrast to the sol id s tate of the previous two groups. The c orrosive nature and in-s tabi li ty ma y exclude their pract ical applicat ion. No detai ls are review ed here. Ru has been

used as an addi t ive to Co-Mo and Ni-Mo (Hirschon et a l . , 1987) to improve their hydro-genat ion and deni trogena t ion act ivit ies . Hy drog en sulf ide in the react ion atmosp here hasbeen reported to accelerate l iquefact ion direct ly, in addi t ion to control l ing the extent of sul-t id ing of i ron, Ni -Mo , and Co-M o ca ta lys t s (O gaw a e t a l. , 1984; Hi rschon and Laine ,1984) . Recent ly, ca rbon b lack was repor ted to ca ta lyze coa l liquefac t ion (Farcas iu andSmith, 1990, 1991); this ma y ini tia te radical react ions o f bond breakage .

4 .3 . 1 P r e p a r a t i o n o f C a t a l y s t s

Sol id l iquefact ion catalysts have been prepared by three procedures .1) Fine Powde r Catalysts . Mo st i ron catalysts are used in pow dered form. Since their par-

t ic le s ize s trongly inf luences their act ivi ty, f ine powde rs are preferred. Natural produc ts areground extensively. M agne t i te for the mag net ic tape is nee dle - l ike crystal of which the di-ame ter is less than 1 /2m. Rece nt ly, ul t ra-f ine pow ders (nanom eter to tens-of-n anom etersize scale) of i ron oxides and sulf ides have been prepared by means of vapor-phase hydrol-ys i s o f vo la t i l e compounds in a hydrogen-oxygen f la rhe to produce nanometer-s ized i ronoxides (aerosol) (Bacaud et a l . , 1993; Lacroix et a l . , 1989); rapid thermal decomposi t ion ofso lu tes (RTDS) , such as Fe(NO3)3 so lu t ions (Matson e t a l . , 1993) ; l aser pyro lys i s o fFe(CO)5 and C2H4 to produce i ron carbides fol lowed by in s i tu sulf idat ion (Hager et a l . ,1993); precipi ta t ion/crystal l izat ion seq uence fro m the sulfated and oxyh ydrox ides o f i ron(Pradhan et a l . , 1993); and a chemical reduct ion or an exchange/replacement react ion ofiron salts solubil ized in inverse micel les of react ion m edia (M art ino et a l ., 1993). Finerpowd ers of the i ron sulf ide are expected to be expensive as wel l as active. The co st /perfor-mance should be careful ly evaluated.2) Suppor ted Cata lys t s . Co-M o and Ni-M o su l f ides a re usua l ly suppor ted on a lumina .Select ion of the specif ic a lumina is convent ional ly s tudied on the basis of the pore s ize dis-t r ibution and acidic character is t ics . The support ing proce dure a nd the amou nt of supportedsulf ides are very inf luent ial in catalyst activi ty. Alternat ive supports to alumina are the fo-cus of current research. Ti tania and carbon have recent ly been exam ined as supports fori ron and Ni-M o sulf ides (Moc hida et a l ., 1984; Der bysh ire et a l ., 1986a) . Bifunct ional andstrong interact ive roles of the support should be emphasized in addi t ion to physical proper-t ies (Ehrburge r et a l ., 1976; Showa, 1982). The s earch for addi t ives such as phosp hate andsulfate , which have been ut i l ized for commercial Co-Mo and Ni-Mo base catalysts , has alsobeen r ece iv ing much r ecen t a t t en t i on (Lewis and Kydd , 1991 ; Mora l e s e t a l . , 1984 ;DeCanio et al . , 1991).3) Highly Dispersed C atalysts on Coal . Sulf ide catalysts have been dispersed direct ly on


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the coal surface. V ery high dispersion on the catalyst may al low direct interact ions be-tween the catalyst and sol id coal . The f i rs t appl icat ion of this approach ut i l ized moltenchloride as the s tar t ing material . Later, oi l - and water-solub le i ron precursors w ere impreg-

nated or ion exchange d o nto the coal surface throug h the interaction w ith oxyge n funct ionalgroups (Cugini et a l . , 1991; Hirschon and Wilson, 1991a; Curt is and Pel legrino, 1989;Sna pe et al., 1989; Pradha n et al., 1991). Rece ntly, hig hly dispersed, hig hly active, or high-ly funct ional catalysts have been extensively invest igated to reduce the amount of catalystrequired for recovery and regenerat ion (Suzuki et a l . , 1984, 1987, 1991; Holloway andNelson, 1977; Takemura et a l . , 1989; Curt is and Cahela, 1989; Ryan and Stacey, 1984;M endez -V ivar et a l. , 1990). Very f ine part icles of i ron sulf ide are very prom ising catalystsbecause of lower cost and moderate act ivi ty. Presulf iding t reatments for act ivat ion, ion ex-change, and dispersed impregnat ion of catalysts or catalyst precursors are combined to en-hance the catalyt ic act ivi ty and reduce the amount of catalyst required (Naumann et a l . ,

1982; Y oko yam a et a l ., 1983). The use of highly dispersed catalysts from soluble salts ofm olyb den um is another ap proach to the reduct ion of catalyst amoun t because of their excel-len t ac t iv i ty despi te the i r h igher pr ice . Recent ly, meta l carbony l com-p ound s , such asFe(CO)5, Ru3(CO)12, and Mo(CO)6 have been inves t iga ted as meta l c lus te r ca ta lys t s .P repa ra t i on invo lved the i r depos i t i on and decompos i t i on on ca t a ly s t suppor t su r f aces(Parad han et al., 1991; Bu rgess and Schobert, 1991; Co w ans et al., 1987). It has been re-ported recent ly that highly dispersed catalyst on coal grains can accelerate the l iquefact ionof the coal grains without supporting catalysts (Cugini et al. , 1991; Pradhan et al. , 1991).The f ine po wde rs of the catalyst are indicated to be m obile du ring the l iquefact ion, suggest-ing no importance of direct interaction. Recov erable catalysts also offer a prom ising way to

econo mize the cost of l iquefact ion catalysts (Joseph, 1991; Pelofsky, 1979). Do w designeda process that ut il ized f ine powd ers of MoS2 that were reported to be recoverable by hydro-clone; however, specif ic detai ls have not been published (Wh itehurst , 1980).

4 . 3 .2 F e - b a s e d C a t a l y s t s

It was reported that all Fe-based catalysts (FeOOH, Fe203, pyrite, FeSO4, etc.) show similaryields of l iquid when 0.5% Fe was added. On the other hand, FeOO H catalyst that was dis-t r ibuted on the surface of coal as f ine particles via impreg nat ion showed higher yield of liq-uid than other catalysts, as shown in Fig. 4.3 (Cug ini et al., 1994). The sintering of theFeOOH nanometer part icle catalyst s t i l l occurred in the sulf idat ion and resul ted in a de-crease of surface area even though its initial value wasca . 138 m2 /g, resulting in low er cat-alytic activity. This indica tes that the high er the distribu tion of the sulfides is, the higherthe catalytic activity.

I t was proposed that natural pyri te was act ivated via the grinding in NEDOL processbecause o f the lower act ivi ty of natural pyrite . The relat ionship between part icle s ize ofpyri te and the yield of l iquid in the l iquefact ion of Wandoan coal is shown in Fig. 4 .4(Hiran o et al., 1997). The yield of oil significantly incre ased w hen the average particle sizewas u nder 50 ~tm, and the yield increased from 40% for part icle s ize of 100/~m , i .e. , un-ground pyr i te , to 58% for par t ic le s ize of 0 .86/ . tm, which i s over the objec t ive of theNE DO L m ethod. By XPS analysis , it was found that there isca . 10% of S O ~ - on the sur-face of natural pyrite. In contrast, mos t of the surface of ground py rite in air is present inthe form of SO 2- . Thus, the grinding in the oil phase or the addition of sulfur after the oxi-dation will restore the catalytic activity of the pyrite (Hirano et al. , 1997).

The preparat ion of nanometer part icle ~,-gei t i te (FeOOH) was developed by Kaneko etal. (1995). In the l iquefact ion of Yallourn coal , the yield of oi l was only 40- 43 % in the


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4.3 Catalysis in Coal Liquefac tion 195

100

7 5 -

50

~.~D

25

I I I I I I I

= = ~ z0 0 + nap htha lene > te t ra lin , ind ica t ing the reverse orde r of the hydr oge n donat ingqual i t ies o f these solvents . The 3H conc entrat ions in coal products fol low the same order in

the presence of the catalyst . I t suggests that , in toluene solvent , the l iquefact ion proceedswith hydrogen spi l lovers from hydrogen adsorbed on the catalyst , but , in naphthalene sol-vent , coal is l iquefied by hydrogen donat ion from tetral in converted from naphthalene.

In the absence of a catalyst , 3H concentrat ions in heavier coal products are larger thanthat in l ighter coal products . This agrees with Skow ronski 's resul ts (1984). The reason forthis is not c lear but i t seems that the heavier products have much more mobile hydrogena toms such as in OH or NH, w hich exchange wi th hydrogen molecules even wi thout a ca ta -lyst.

From the da ta above , the am ount of hydrog en t ransferred by addi t ion or exchange reac-t ions am ong gase ous hydroge n, solvent and coal products can be calculated. The calculatedresul ts are shown schematical ly in Fig. 4 .22, in which the sol id and dot ted arrows indicatethe d irec t ions of hydroge n addi t ion and e xchange among shown phases , respec tive ly.

The data presented in Fig. 4 .22 i l lustrate that the catalyst s l ight ly decreases the hydro-gen addi t ion from solvent to coal but i t considerably increases hydrogen addi t ion from gasphase to coal products . The la t ter increased w ith react ion t ime. The increase in hydrog enconsum pt ion i s assume d to be equa l to the am ount used for the hydrocrac king of the coa ll iquids, becau se the yields of the l iquefact ion, which are calculated from the am ounts of un-conv erted coal , are almos t the same in both cases w ith and without the catalyst . W hen coaland/or heavy coal l iquids are present , however, the hydrogenat ion of solvents by gaseoushydrogen is rather s l ight even in the presence of the catalyst compared with the case of hy-droge nat ion of solvents in the absence of coal. In other words, the hyd roge n exchange be-tween gaseous hydrogen and so lvents i s suppressed s ince so lvents cannot eas i ly be ad-sorbed on the catalyst in the presence o f the heavier coal products . The hyd roge n exchangebetween gas phase and coal products , on the contrary, proceeds considerably even in theabsenc e of catalyst . This resul t me ans that there is mu ch t ransferable hyd roge n in the coal


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4.3 Catalysis in Coal Liquefactio n 213

Gas phase Gas phase Gas phase

Without 0.01 /,/'~1.22 0"20j~,,'~1.20 0"24 ///9', ,' 1.32catalyst r 0.01 yr yr '0.36. . . . . . . ~ . 0 .40

Coal = Solvent Coal = -~ Solvent C oa l . . . . . . . . ~ Solvent0.88 0.76 0.04

Gas phase Gas phase Gas phaseWith 9' ~ 9'catalyst 0 "60 /. '1 00 08 0 ', ~ 0. 04 0.88~,,,~.60 0.84~',~0.04


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30i

~ q

~,~6 2 0

~ 10x


~ ------- - - - -D-

O 1 2 3 4 5Reaction time (h)

3 0 1

o o 20

~ ~ 1020 ~ ~

.o .~

(b)0

60 ~r

0

4o ~, . o

~

20 ~0

0 0 0

0 1 2 3 4 5Reaction time (h)

Fig. 4.23 I-] Gas, X Naph tha, A Ligth oil, O HS, V HIS-BS, V BIS-T HFS , 9 TH FIS.Changes in product distributions with reaction time for Datong coal liquefaction at 400 ~a) without, and (b) with a catalyst.[Reproduced with permission from Kabe. T. et el.,Fue l P roce s . Techno l . ,25, 48, Elsevier(1990)]

How ever, the ca ta lys t chang ed the produc t d i s t r ibu t ion by acce le ra t ing the hydrocrack ing ofpreasp hal tene s a t f i rs t and then hydro gen ate asphal tenes to oi l . Th ese resul ts suggest thatthe catalyst was not able to hydrogenate the coal direct ly but that the heavier products in

l iqu ids a re p redom inant ly adsorbed on the ca ta lys t sur face and reac t wi th hydrogen . Thesame resu l t s were observed for Ta ihe iyo and Wandoan coa ls (Kabe e t a l . , 1986a , 1987a) .Besson e t a l . (1986) repor ted more de ta i led s tud ies on the e ffec t s o f Ni -Mo/A1203 andFeeO3 catalysts on coal l iquefa ct ion at 400 ~ in te t ra lin solvent and reach ed a s imilar con-clusion, i .e . , that the catalysts had l i t t le effect on the conversion of the coal into THF-solu-b le p roduc ts bu t increased the amo unt o f aspha l tenes ( to luene so lub le p roduc ts ) .

The hydrogen d i s t r ibu t ions among gas phase , so lven t and coa l p roduc ts a re shown inFig. 4 .24. W ithou t a catalyst , the hyd rogen dis t r ibut ion does not chang e with react ion time.W hen a c a ta ly s t was added , hy d rogen and coa l p roduc t s i nc r ea sed and t ha t o f gas d e -c r ea sed . I n o rde r to c la r i f y the amou n t o f hyd rogen t r an s f e r r ed t o coa l p roduc t s , t heamou nts o f hydrogen t rans fe r red f rom the gas phase and so lven t to coal p roduc ts were p lo t -ted with elapse of t ime in Fig. 4 .25. W ithout the catalyst , the amou nts of hydrog en ini t ia l lytransferred to 100 g of coal were 0.21 g from the gas phase and 0.42 g from the solvent , re-spect ively. The hy drog en t ransfer f rom solvent to coal was the m ain react ion in the ini t ia ls tage. Th e amo unt of hyd roge n t ransferred gradu al ly increase d and reache d 1.1 and 1.0 gfrom the gas phase and the solve nt a t 5 h, respect ively . W ith the catalyst , the amou nts ofhydrogen t rans fe r red to coa l p roduc ts were 0 .73 g f rom the gas phase and 0 .28 g f rom thesolvent a t 0 h. This indicates that hyd rogen t ransfer f rom gase ous h ydro gen to coal prod-ucts was rapid, even at the init ia l stage of the react ion w hen the catalyst was used. Theamount o f hydrogen t rans fe r red f rom gaseous hydrogen to coa l p roduc ts increased to 2 .2 gat 5 h , however, that f rom the solvent did not increase so much and was only 0.69 g at 5 h .S ince naphtha lene prod uced by hyd rogen t rans fe r from te t ra lin to coa l was re -hydrogena tedto te t ra l in on the catalyst , the amount of hydrogen t ransferred from the solvent apparent lydid not increase. Total am oun ts of hyd rogen t ransferred to coal were 2.1 and 2.9 g withou tand with the catalyst , respec t ively. Fig. 4 .24 also shows t r i t ium dis t r ibut ions am ong gas


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4.3 C atalysis in Coal Liquefaction 215

p h a s e , s o l v e n t a n d c o a l p r o d u c t s . W i t h o u t t h e c a t a l y s t , t h e t r i t iu m d i s t r i b u t i o n s h o w s t h a tt r i t iu m t r a n s f e r s t o c o a l p r o d u c t s i n t h e i n i ti a l s t a g e , t h e n t r i t i u m i n s o l v e n t i n c r e a s e s . T h i ss u g g e s t s t h a t , a s a f i rs t st e p , g as p h a s e t r i t i u m t r a n s f e r s t o c o a l f o l l o w e d b y t h e e x c h a n g e

b e t w e e n c o a l a n d s o lv e n t . W h e n t h e c a t a l y s t w a s a d d e d , t r i t iu m w a s t r a n s f e r r e d to c o a lm o r e r a p i d l y a t t h e in i ti a l s ta g e o f t h e r e a c t i o n . T r i t i u m w a s a l s o t r a n s f e r r e d t o s o l v e n t a tt h e i n it i al s t a g e , i n c o n t r a s t t o t h e c a s e w i t h o u t t h e ca t a l y s t . A s s h o w n i n F i g . 4 . 2 5 , h y d r o -

80

60=.o

. I D

~ 40. , . .~

20

Conversion (%) Conversion (%)

32.2 44.9 63.4 67.2 53.9 66.8 75.0 80.4I I I I i l I I I

L(b)

- ~ -

a '

- j_ /

_ _ ' I I I I I h i I I I I I

0 1 2 3 4 5 0 1 2 3 4 5Reaction time (h) Reaction time (h)

A

.o. . a

"~ 4c ~

. ~

a A

Fig. 4.24 Changes in balance of hydrogen and tritium in Datong coal liquefaction at 400 ~(a) without and (b) with a catalyst.Hy drog en distribution: D gas, A solven t, C) coal; Tritium distribution: 9 gas, A solven t,9 coal. [Reproduced with perm ission from Kabe. T. et el.,Fuel Process. Technol. 25, 49(1990)]

2o

c..)e ~ 0

. , . .~

~ D

o ( v

~ _ _ A _ _ _ _ . _ - - ~ - - - - - - - ~ -

0 1 I I I I I

0 1 2 3 4 5

Reaction time (h)

Fig. 4.25 Changes in amounts o f hydrogen addition w ith reaction time in Datong coal liquefaction at 400 ~H added to coal from gas phese: C) without a catalyst, @ with a catalyst;H add ed through tetralin: A without a catalyst, A with a catalyst.[Reproduced w ith permission from K abe. T. et el.,Fuel Process. Technol. 25, 49, Elsevier (1990)]


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O

20

.o

10

(a)

300 ~ 300min440 ~ 180min |

230 ~ 120min 9

0 20 40 60 80 100Conversion (%)

o

x.~ 20

o

~ 10-o

E

o

( b )

440 ~ (0-'- 300 min)

I I I

20 40 60 80 100Conversion (%)

Fig. 4.26 9 Residu al coal, V P reasphaltene, V Asp haltene, O Oil, F-I Solve ntChange in tri tium concentrations with conversion of D atong coal l iquefaction at 400 ~(a) Without, and (b) w ith a catalyst[Reproduced with permission from Kabe. T. et el.,Fuel Process. Technol. 25, 50,Elsevier (1990)]

gen was t ransferred to coal f ro m tetral in to produce naphthalen e at the init ia l s tage, and tr i-t ium was t ransferred to the solvent through hydrogenat ion of naphthalene on the catalyst .From Fig. 4 .24 (b) , af ter 5 h, the t r i t ium dis tr ibut ion among gas phase, solvent and coalproduc ts approached the hyd rog en d is t r ibu t ion . This ind ica tes tha t hydro gen exchangereaches the equi l ibr ium among gas phase, solvent and coal products a t 5 h when the catalystis added. Fig. 4 .26 show s the chan ge in t r i tium concentrat ions of each l iquefied product

and the solvent with conversion. The ex perime ntal points in Fig. 4 .26 corresp ond to thosein Fig. 4 .23 (400 ~ 0- 5 h) , unless otherwise noted. The points with special notat ion inFig. 4 .26 show the resul ts a t different temperatures . The two horizontal dot ted l ines in Fig.4.26 indicate the expected mean equi l ibr ium tr i t ium concentrat ions for coal , represent ingthe radioact ivi ty in one gram of coal a t the equi l ibr ium in the hydrogen exchange react ion.If the equi l ibr ium among coal , solvent and gas phase ( lower l ine) , and between coal and gasphase only (upper l ine) , were establ ished in the hydrogen exchange react ion, the equi l ibr i -um tr i t ium conce ntrat ions for coal w ould be 5 X 103 and 19 X 103 dpm /g-coal , respect ively.The expec ted concent ra t ion for the so lvent under equi l ib r ium condi t ions among the th reecomponents in the system is shown by an arrow on the f ight-hand s ide of the graphs (10 X103 dpm /g tetral in) . W ithout the catalyst, t ri t ium conc entrat ions in coal products increasedwith the conv ersion of coal. Since t r i t ium is t ransferred from the gas phase to the solventvia coal in the absence of the catalyst , the t r i t ium concentrat ion in the solvent began to in-crease af ter about 30 wt% of coal was converted. On the other hand, w hen the catalyst wasadded , a l a rge number of hydrogen in coa l components exchanged wi th gaseous hydrogenat the ini tia l s tage of react ion. Hy drog en exchang e reached equ i l ibr ium am ong gas phase,solvent and coal products a t the f inal s tage of the react ion consis tent with the resul ts givenin Fig. 4.24.

Figure 4.27 shows the product dis t r ibut ion as a funct ion of temperature at a react iont ime of 2 h without the catalyst . In the tempe rature range 300 -44 0 ~ the conve rsion ofcoal increases with increasing temperature. The hyd rocra cking to l ighter products has l it tlechance of occur r ing be low 300 ~ F ig . 4 .28 shows the amounts of hydrogen transfer redand exchan ged at 300, 350 and 400 ~ in the absence o f catalyst. The sol id and dot ted ar-rows ind ica te the d i rec t ions of hydrogen t ransfer and exchange among shown phases , re -spect ively. Nu m bers with the arrows indicate the amoun ts (g) per coal (100 g) of the t rans-


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4.3 C atalysis in Coal Liquefaction 217

f e rred o r exch anged hyd rogen fo r 2 h (Kab e e t a l ., 19 90c) . S ince the d i r ec t hyd rogen ex -change be tween gas phase and so lven t ha rd ly occur red in the absence o f coa l and ca ta lys t ,i t was ignored in F ig . 4 .28 .

Th e am oun t s o f hy drogen t r ans fer red and exchan ged inc reased wi th r i se in t empera tu re .The amoun t o f hydrogen t r ans fe r red f rom the gas phase to coa l doub led wi th a r i se f rom30 0 to 350 ~ Ho we ver, t he am oun t o f hyd rogen t rans fe r red f rom so lven t to coa l r emark-ab ly inc reased wi th a r i se f rom 350 to 400 ~ Th ese r e su lt s ind ica te tha t gase ous hyd rogenis the main hy drog en donor in coal l iquefact ion a t 35 0 ~ and that the capabi l i ty of tet ra linas a hyd rogen donor appea red a t 40 0 ~ S ince hyd rogen in t e tr a lin t r ans fe r s to coa l morerap id ly than gas eous h ydrog en at 40 0 ~ on ly a s l igh t am oun t o f hy drog en tr ans fe r redf rom gas ph ase to coa l wou ld inc rease wi th a r i se f rom 3 50 to 40 0 ~

F igure 4 .29 shows the r e l a t ionsh ip be tween the t r i t i um concen t ra t ions o f the coa l p rod-uc t s and the r eac tion t ime at 30 0 ~ In a l l cases a t 30 0 ~ in the unca ta lyzed exp er imen t s ,

50 100

4000

030

20

lO

0 I200 3 00 400

8000

n~

060

, d

e~O.,..~

4o9

.,...,.o

20 m

Tem perature (~

9 Residue , 9 Preaspha ltene, A Asph altene, O Oil

Fig. 4.27 Effect of temperature on the fractional weigh ts of products. [Reprod uced with perm issionfrom Kabe. T. et el.,F ue l Pro cess . Technol . , 25, 51, Elsevier (1990)]

Reaction temperature

300 ~ 350 ~ 400 ~

Gas phase Gas phase Gas phase

0 . 2 5 ~ 0 ~ 0 .5 0 g / ~ , " " 0 .6 2 g / ", . 0 8 g / , , " 0 . 5 8 g / ~ , , " 1 .8 6

~p '(" 0.12 g ~ Jcp'!" 0.34 g ~ ~p'( 1.48 g p-

Coal 4 Solvent Coal ~ Solvent Coal0.18 g 0.27 g 0.65 g Solvent

Fig. 4.28 Schem e of hydrogen transfer and exchange (g/100 g coal) between three phases in the absence of cata-lyst.


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t h e tr i t i u m c o n c e n t r a t i o n s o f r e s i d u e s a n d p r e a s p h a l t e n e s w e r e h i g h e r t h a n a s p h a l t e n e s a n do i ls . U n l i k e t h e r e s u l t s o b t a i n e d a t 4 0 0 ~ t h e c o n c e n t r a t i o n o f t r i t i u m i n t h e p r o d u c t s i n-c r e a s e d w i t h i n c r e a s i n g r e a c t i o n t i m e u n t i l 4 h . A f t e r t ha t , t h e p r o d u c t s a p p e a r e d t o b e s at u -

r a t e d w i t h tr i t iu m . H o w e v e r , t h e t r i t i u m c o n c e n t r a t i o n s o f t e t r a li n w e r e v e r y l o w, w h i c h in -d i c a te s t h a t th e h y d r o g e n e x c h a n g e r e a c t i o n o f t e tr a li n r e q u i r es h i g h e r a c t i v a t i o n e n e r g y.Ta b l e 4 . 5 s h o w s t h e t r i t i u m c o n c e n t r a t i o n s o f t h e p r o d u c t s t o g e t h e r w i t h th e t r it i u m a n d m a -

4

o

2

8QL)

V

O

~7

O

I I 1

0 2 4 6Reaction time (h)

9 Residue, V THF S-B IS, V BS-H IS, O HS, [] Tetralin

Fig. 4.29 Tritium conc entrations in coal products and tetralin in unca talyzed reaction of Dato ng coal at 300 ~[Reproduced with permission from Kabe. T. et el.,Fuel Process . Technol . ,25, 52, Elsevier (1990)]

Table 4.5 Tritium Concentrations and M aterial B alances in Datong Coal Liquefaction a at300 ~ for 6h

Tritium Tritium Am ount of Recove redconcentration balance produ ct ratio

(dpm/g) (%) (g) (%)

Coal product - - 7.47 24.60Residue 2767 6.65 21.94 85.30Preasp haltene 3443 0.31 0.83 3.23Asp haltene 2144 0.26 1.12 4.35Oil 1770 0.09 0.45 1.75Light oil 5256 0.09 0.15 0.80Naphtha 5993 0.07 0.10 0.39Gas 0 0.00 0.01 0.04

Solv ent m 2.68 71.61Tetralin 210 2.23 70.31 93.18Nap hthalene 4116 0.44 0.96 1.34Me thylinda n 210 0.00 0.10 0.14

Deca lin 210 0.01 0.24 0.34Gas 89.83

alnitial conditions: C oal 25.05 g, Tetralin 75.01 g, and H2 1.25 g. [Reproduced w ithpermission from Kabe. T. et el.,Fuel Process . Technol . ,25, 53, Elsevier (1990)]


39/87

4.4 HydrogenTransfer Reaction in Coal Liquefaction 219

ter ial balances in Dato ng coal l iquefact ion at 300 ~ and a react ion t ime of 6 h. Fro m theseda ta and the hydroge n conten t o f the res idue (4 .6 wt%) , the ra t io of exchangeab le h ydroge nin the res idue hydrog en can be ca lcu la ted and the exchange d hy drogen a t 6 h (approximate-

ly equi l ib r ium va lue) amounts to 7 .8 a tom% of hydrog en in the res idue . This va lue maysugges t the ra t io of the ac t ive hydroge n such as in the form of -O H and -NH to to ta l hydro-gen in coal . This shows which hydro gen in coal is excha nged. Since oil , asphal tene s and apar t o f p reaspha l tenes a re d i sso lved in the so lvent , these a re expec ted to undergo morerap id hydrog en exchange than the inso luble carbonaceous m ateria ls . How ever, the resu l tsare different. This may sugge st that there is mo re mobile hydro gen in residue. Further, a t300 ~ on ly the ac t ive hydrogen in coa l p roducts is exchanged by hydrogen . Al though ade ta iled ana lys i s o f phenol ic OH group of Datong coa l has no t been done , a com parableana ly s i s o f b i t um inous coa l s ha s been r epo r t ed (P es t ryako c , 1986; M aek aw a , 1975 ).Bi tuminou s coa ls which have a chemica l com pos i t ion of C: 75-85 % and H: 5 .0-5 .4 wt%

con ta in 6 -12 a tom% of pheno l i c OH hyd roge n fo r t o ta l hyd rogen . Ko ta n igaw a e t a l.(1979) conc luded tha t the exchange reac t ion be tween deuter ium gas and phenol took p lacerap id ly a t 350 ~ wi th ZnO-Fe203 ca ta lys t . OH hyd rogen of po lycond ensed a romat ic phe-nol ics is able to exchang e at lower temp eratures .

4.4 Hydrogen Transfer Reaction in Coal Liquefaction

4.4.1 IntroductionSince the reac t ions involved in coa l l iquefac t ion inc lude hydrocracking and hydrogena t ionby donor so lvents and molecular hydrogen , a number of a t tempts have been made to e luc i -

da te the m echan ism of hydrogen t ransfer occur r ing dur ing coa l l iquefac t ion in the presenceof so lvents . For th is purpose , the use of var ious m odel com pounds re la ted to coa l is veryeffec t ive because the reac t ions wi th those compounds a re much s impler than coa l l iquefac-t ion and i t i s muc h eas ie r to trace the i r behavior of hydrogen . Am ong these m odel com -pounds , t e t ra l in is one of the mos t s imple , in te res t ing and conve nien t model com pound s be-cause i t is cheap and easier to obtain, has an aromatic r ing and a naphthene r ing in i ts s t ruc-ture and can serve as a hydrogen donor so lvent . The h ydroge n t ransfer f rom te tra l in to coa lmolecules , and from gas phase to te tral in especial ly has been extensively s tudied to under-s tand the me cha nism of coal liquefact ion (C ronau er et al ., 1978, 1979; Bil lmers et a l ., 1986;Sko wro nski e t a l. , 1984).

The mechanisms of hydrogen t ransfer f rom a donor so lvent such as te t ra l in to var iouscoa l s t ruc tures and the i r subsequent f ragments remain la rge ly unspec if ied , no twi ths tandingthe effor ts in this area over the years . M uch d iscussion on do nor solvent in coal l iquefac-t ion i s based on the presumpt ion tha t the pr inc ipa l m echan ism involves thermal sc i ss ion ofweak bonds , fo l lowed by capping of the resu l t ing f ree rad ica l s by hydrogen a tom abs t rac-t ion f rom do nor so lvent (Kuhlma nn e t a l. , 1985), even thoug h a num ber of researchers havepointed o ut a l ternat ive possibi l it ies (Franz and C ama ioni , 198 l a ; Stein, 1982). I t has alsobeen sugges ted tha t hydr ide t ransfer (Brower, 1977) and concer ted H2 t ransfer p rocesses(Virk, 1979) play importa nt roles in coal liquefact ion. In addi t ion, direct t ransfer of hydr o-gen atom from solvent der ived radicals to subst i tuted posi t ions in aromatic t ings as prel imi-nary s teps in depolym er iza t ion of coa l s t ruc tures has a l so been sugg es ted to be impor tan t incoal l iquefact ion (M cM illen et a l ., 1987).

In the e luc ida t ion of the hydro gen t ransfer, hydro gen exch ange i s o ther form of hydro-gen t r ans f e r and i s a l so ex t ens ive ly s t ud i ed t o unde r s t and t he mechan i sms o f va r iousprocesses (Benjamin e t a l . , 1982b, 1983; Davis and Garne t t , 1975; Gamet t and Kenyon,


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41/87

4.4 Hydrogen Transfer Reaction in Coal Liquefaction 221

H e r e , t h e l i q u e f a c t i o n b e h a v i o r o f W a n d o a n c o a l u n d e r a 3 H - l a b e l e d h y d r o g e n a t m o s -p h e r e w a s i n v e s t i g a t e d t o c l a r if y t h e r o le o f g a s e o u s h y d r o g e n ; t h e e f f e ct o f s o l v e n t w a sa l s o d e t e r m i n e d u s i n g u n l a b e l e d s o l v e n t s s u c h a s t e t r a l i n , n a p h t h a l e n e a n d d e c a l i n ( K a b e e t

a l . , 1987a) .To d e t e r m i n e t h e h y d r o g e n t r a n s f e r p a t h f r o m t h e g a s p h a s e , l i q u e f a c t i o n e x p e r i m e n t s

w e r e c a r r i e d o u t u s i n g 3 H - l a b e l e d h y d r o g e n g a s i n u n l a b e l l e d t e t r a li n , n a p h t h a l e n e a n d d e -c a l in . T h e d i s t r i b u t io n s o f p r o d u c t s a n d t r it i u m a r e s h o w n i n R u n s 1 -8 in Ta b l e s 4 .6 a n d4 . 7. I n th e s e t a b le s , t he d e c a l in f r a c t i o n c o n t a i n s d e c a l i n , 1 - m e t h y l i n d a n a n d b u t y l b e n z e n e .T h e d e c a l i n i s v e r i f i e d t o b e a n i m p u r i t y c o n t a i n e d i n t e t r a l i n , a n d t h e o t h e r t w o s u b s t a n c e ss e e m t o b e c o n v e r t e d f r o m t e t ra l i n in t h e c a s e o f t e t r a li n s o l v e n t .

I n t et r a li n s o l v e n t , th e c a t a l y s t d o e s n o t e n h a n c e l i q u e f a c t i o n y i e l d s a s c a l c u l a t e d f r o mt h e a m o u n t o f r e s i d u e , b u t i t i n c r e a s e s t h e c o n s u m p t i o n o f g a s e o u s h y d r o g e n a n d t h e h y d r o -c r a c k i n g o f a s p h a l t e n e ( R u n s 1 a n d 2 ). T h e c a t a l y s t d o e s n o t a f f e c t t h e f o r m a t i o n o f d e-

c a l in , b u t i t r e d u c e s n a p h t h a l e n e f o r m a t i o n d u r i n g c o a l li q u e f a c t i o n . T h e d i s t r i b u t i o n a n dc o n c e n t r a t i o n s o f 3H in d i c a t e t h a t th e r a t e o f 3H tr a n s f e r f r o m g a s p h a s e t o c o a l p r o d u c t s i nt h e p r e s e n c e o f a c a t a l y s t i s h i g h e r t h a n t h a t i n i ts a b s e n c e ( Ta b l e 4 . 7 , R u n s 1 a n d 2 ) .T h e r e f o r e , i n t e t r a l i n s o l v e n t , t h e c a t a l y s t p r o m o t e d h y d r o g e n t r a n s f e r f r o m t h e g a s p h a s e t o

Table 4.6 Product Distribution for Wan doan Coal Liquefaction

Run No. 1 2 3 4 5 6 7a 8aSolve nt Tetralin Naphthalene D ecalin TetralinCatalyst -- + -- + -- + -- +

Products (wt%)Residue 26.6 25.0 70.3 30.8 51.4 33.5Preasphaltene 22.7 22.4 12.0 13.7 12.6 12.1Asp haltene 27.2 21.2 3.5 24.5 13.7 21.7Oil 12.9 17.1 6.6 22.0 11.4 18.9Lig ht oil 8.7 12.1 5.9 5.9 6.7 10.8Nap htha 1.0 1.1 0.7 1.9 3.3 1.9Gas 1.0 1.0 1.0 1.2 0.8 1.0

Solvent (wt%)Nap hthalene 13.8 7.9 99.4 93.8 3.4 0.9Tetralin 84.7 90.6 0.1 5.7 5.5 2.6Decalin b 1.4 1.4 0.5 0.5 91.1 96.6

0.499.2

0.4

2.585.611.9

a W ithout the coal; bDecalin fraction contains decalin, 1-m ethylindan, and butylbenzen e[Reproduced with permission fro m K abe. T. et el.,Fuel, 66, 1 327, Elsevie r (1987)]

Table 4.7 Tritium Distribution for W andoa n Coal Liquefaction

Run No. 1 2 3 4 5Tritium (%)

Residue 2.4 7.2 11.3 9.2 3.7 14.9Preasphaltene 1.9 6.2 1.3 3.7 0.8 5.7Asphaltene 2.0 5.6 0.4 5.6 1.0 8.2Oil 0.9 5.1 0.6 5.8 0.8 8.3Lig ht oil 0.9 1.3 0.9 3.2 1.1 2.6Nap htha 0.3 1.3 0.3 2.7 1.6 2.5 mNap hthalene 1.0 1.7 4.4 21.9 0.1 0.1 0.0Tetralin 7.1 22.9 0.2 10.2 0.2 0.2 0.0Decalin b 0.3 0.4 0.2 0.4 2.6 6.5 0.0Sum in solvent

fraction 8.4 25.0 4.8 32.5 2.9 6.8 0.0Gas phase 83.4 48.3 80.4 37.2 88.2 75.6 100.0

n

m

15.366.8

1.0

83.116.9

a W ithout the coal; bDecalin fraction contains decalin, 1-rnethylindan, and butylbenzene.[Reproduced with permission from Ka be. T. et el.,Fuel, 66, 1 327, Elsevier (1987)]


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coa l p roduc ts . In naphtha lene so lven t , the degree of liquefac t ion w as obviou s ly low wi th-out catalyst (Run 3) . Ho we ver, l iquefac t ion proceed s at a substant ia l ra te in the presence ofthe ca ta lys t (Run 4) . This sugg es t s tha t gaseous hy drogen i s used for liquefac t ion in naph-

thalene solvent in the presence of a catalyst .On the o ther hand , when deca l in i s used as the so lven t in the presence of a ca ta lys t

(Run 6) , the amo unt , o f res idue i s a lmos t the sam e as fo r naphtha lene so lven t bu t the prod-ucts are l ighter. W ithou t the catalyst (Run 5) , liquefact ion in decal in procee ds m ore exten-s ive ly than in naphtha lene . The am ounts o f t e tra l in and naphtha lene der ived f rom deca l inshow tha t l iquefac t ion proceeds to a cons iderab le ex ten t accompanying the hydrogen dona-t ion from decal in in the absence of a catalyst .

The las t co lumn s (Run 8) o f Tables 4 .6 and 4 .7 show the d i st r ibu t ions ob ta ined exper i -m enta l ly without the coal and coal products . Th e experim ental 3H dis t r ibut ion, 83% in theso lven t and 17% in the gas phase , agreed wi th the ca lcu la ted va lues based on the assump-

t ion of a comple te sc rambl ing of hydrogen a toms of the so lven t and molecu la r hydrogen .These r e su l ts show tha t if coa l o r co a l p roduc t s a re no t p r e sen t, t h e hyd rog ena t i o n o fte t ra l in to deca lin and the hydrogen exchan ge be tw een so lven t and molecu la r hydrogen wi l lp roceed rap id ly in the presence of a ca ta lys t . On the o ther hand , no hydrogen a t ion of so l-ven t o r hydrogen exchang e occu rs in the absence of ca ta lys t (Run 7) .

F igure 4 .30 shows3H conc entrat ions in l iquefied produc ts and in the solvent . I t showsthat 3H concentrat ions in coal products produced in te t ra l in and decal in solvents are lowerthan those in naphth alene solvent in the absence of a catalyst . Bu t in the presenc e of a cata-lys t , the amount o f3H from the gas phase incorporated into coal products is largest in de-cal in solvent excep t for l ight oi l. I t show s that decal in is a goo d hyd rogen d ono r without a

ca ta lys t , bu t molecu la r hydrogen i s a be t te r hydrog en donor than deca l in whe n a ca ta lys t i spresent . W ith a catalyst , naph thalene h as a s imilar act ion to te t ra l in . In Fig. 4 .30, a fa i r lylow con cent ra t ion of 3H in each so lven t shows tha t the hydroge n ex change be tween so lven tand mo lecu lar hydrog en is small in the prese nce of coal and coal products . On the otherhand, the 3H concentrat ion in te t ra l in converted from naphthalene is high because te t ra l inmo lecu l e s a r e f o rmed by hyd rogena t i on o f naph tha l ene u s ing mo lecu l a r hyd rogen . Th eva lue of the 3H concent ra t ion of t e t ra l in conver ted f rom naphtha lene so lven t was equa l tothe va lue ca lcu la ted un der the assumpt ion tha t four hydrogen a toms f rom the gas phase a readded to one naphtha lene mo lecu le . The 3H concent ra t ion of deca l in frac t ion conver tedf rom te t ra l in and tha t o f naphtha lene conver ted f rom te t ra l in were the same as the3H con-centration in te t ra lin so lvent i tself, in the prese nce of the catalyst .

Compar ing the unca ta lyzed exper iments in the th ree so lven ts shown in Table 4 .6 , thedeg ree o f l i que f ac t i on , de t e rmined f rom the amoun t o f r e s i due , dec r ea se s i n t he o r de rte t ra l in > deca l in > naphtha lene . This o rder conform s to tha t o f the hydrog en do na t ingpo we r of the solvents . Ho we ver, the order is te t ra lin > naph thalene -- decal in in the pres-ence of the catalyst . This shows that the hyd roge n donat ing cycle , nap hthalen e ~ te t ra lin---) nap hthalen e in naphtha lene solvent , is as effect ive for coal l iquefact ion as hy drog enat ionin decal in solvent in the presence of a catalyst .

Table 4 .7 shows tha t the amounts o f3H in coa l p roduc ts in unca ta lyzed exper iments(Runs 1 and 5) are a lm ost the same in both te t ra l in and decal in solvents . In Ru n 3 in naph-thalene without catalyst , the3H content in the residue is higher than that in te t ra lin or in de-cal in solvent . In the absence of a catalyst , Fig. 4 .30 shows that the amo unt of3H incorpo-rated into the products increa ses in the order oil < asphal tenes < preasp hal tenes < residue,i r respec tive of the so lven t used . These resu l t s agreed wi th those ob ta ined by S kowron sk ie ta l . (1984) in a coa l -deu te r ium gas sys tem. The am ount o f3H t r ans fe r f rom g aseous hydro-


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4.4 Hydrogen Transfer Reaction in Coal Liquefaction 223

0Residue P A A O LO

;LN T D

25

20 _.9.

~ 15 ~ ~ Z

_ _

10

5

I IFig. 4.30 Effects of solvents and catalyst on tri t ium counts o f products for Wandoan coal l iquefaction at 400 ~

for 30 min. PA: Preasphaltene; A: Asphaltene; O: Oil; LO: Light oil; N: Naphthalene; T: Tetralin; D:Decalin; 1-Methyl-indan and butylbenzene[Reproduced w ith permission from Kabe. T. et el .,Fuel, 66, 1327, Elsevier (1987)]

gen to coal products increases su bstant ial ly in the presence o f a catalyst . Fig. 4 .30 also

shows that 3H concentrat ion of coal products is a lmost the same as that in te t ral in and naph-thalene solvents in the presence of a catalyst , and i t seems to show that , in naphthalene sol-vent , a fair ly large part of the hydrol iquefact ion was conducted by te tral in which was pro-duced from naphthalene. On the other hand,3H concent ra t ions in coa l p roducts p roduced indecal in solvent are higher than those in other solvents in the presence of a catalyst . This in-d ica tes tha t d i rec t hydrogena t ion of coa l by gaseous hydrogen and the hydrogen exchangebetween hydrog en molecules and coa l componen ts a re enhanced in deca l in solvent. Thisalso suggests that i t is energet ical ly more favorable for l iquefied products to react with hy-drogen d issoc ia ted on the ca ta lys t than to be hydroge na ted by the deca l in i t se l f. From theseresul ts i t is concluded that naphthalene behaves as a hydrogen carr ier f rom the gas phase tocoal by being h ydro gena ted to te tral in with the help o f the catalyst .

Here the rou te of hydrogen incorpora t ion f rom so lvents and gaseous hydrogen to coa lproducts is d i scussed . Hyd rogen incorpora t ion dur ing coa l l iquefac tion involves two reac-tions, i . e . , hydrog en addi t ion to coa l p roducts and hydrogen exch ange am ong the coa l p rod-ucts , the solvent and the gas phase. To clar i fy the correlat ion of these react i

lique fac t ion o f coal

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