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E x p e r im e n t a l S e c t i o n

R e a c t o r U n i t . A schematic flow diagram of thereactor unit is given in Figure 1. The unit consists ofthree once-through isotherma l reactors in parallel,designed for simultaneous testing of different catalysts(or feedstocks). The thr ee reactors a re operated inde-pendently, which means that a set of three differentexperimenta l conditions can be investigated at t he sam etime. The reactors are heated in thr ee separat e bathsof circulating molten salt, assuring very high rates ofheat t ra nsfer and good tempera tur e control as reportedby Van Tr impont et al. (1988). Each r eactor tu be hasan int erna l diamet er of 19 mm and a n available lengthof appr oximately 750 mm an d is equipped with a centr altube for a moveable thermocouple, leaving about 190cm 3 of reactor volume.

The catalyst bed was diluted by a low-surface, chloride-free alumina wi th the same part icle shape and size(1/16 in. extru dates) as th e cata lyst itself. The degree ofdilution was varied in three different zones of eachreactor, with the highest dilution at the reactor inlet,in order to minimize the t emperature drop caused bythe r apid, endotherm ic naph then e dehydrogenat ion. The

temperatur e drop at th e reactor inlet , measured withthe axial ther mocouple, was less th an 5 C.The reactor effluent was analyzed by on-line GC

ana lysis prior to condensa tion. Ea ch reactor line wasequipped with a HP 5890 GC with a flame ionizationdetector (FID), inter faced with a PC for da ta ha ndlingand storage. The method of ana lysis, based on HPsPONA ana lysis, included all import an t hydr ocar bons u pto C11. Heavier component s than th is were only presen tin t race amounts and were n ot analyzed. Researchocta ne n um bers (RON) were calculat ed from GC analy-sis based on an ada pted version of the met hod presentedby Anderson et al . (1972). The h ydrogen yield wascalculated from GC analysis as the hydrogen balanceover the reactor.

The cri t ical operating variables (temperatures andpressu res) were monitored continuously by an in-house-programmed PC which would shut the uni t downaut omat ically if certa in sa fety limits were exceeded.

T e s t P r o c e d u r e s . The t ests were performed with35 g of catalyst in each reactor, an d t he catalyst wasoxychlorinated, reduced, and sulfided in the reactorsprior to testing. Oxychlorination was carried out inorder to ensure uniform chloride content as well as a

highly dispersed metal function on th e cata lyst. Oxy-chlorin at ion wa s carr ied out in flowing air wit h a givenra tio of H 2O a nd HCl at 500 C, before rejuvenationof the met al fun ction in dry air. The cata lyst was th enreduced in H 2 at 10 bar , with temperatur e ram ping of+20 C/h from 400 to 480 C and k ept a t 480 C for 2 h.Finally the catalyst was presulfided with 0.08% H 2S inH 2 at 425 C unt il break thr ough of H2S. After flushingwith pu re H 2, naphth a was introduced at 400 C, andthe reactor temperature was slowly raised to 480 C.

The tests were carried out at WHSV ) 2.03, a molarratio H 2/hydrocarbon ) 4.34, and pressures of 12, 16,20, and 25 bar. For each pressure level the reactionwas examined at t hr ee different t empera tur es, 480, 495,and 510 C, and each temperatur e was kept constan tfor 30-50 h. Pr ior to this the cata lyst was lined outat ei ther 16 or 25 bar at 480 C for about 125 h tostabilize th e fresh cata lyst.

By car ry i ng out a num ber of repea t ed runs , t hestan dar d deviations for r eformate a nd h ydrogen yieldswere determined as 0.25 and 0.02 wt %, respectively,and 0.25 units for RON. In order to achieve this, adeta iled calibra tion of th e GC system s ha d to be carr iedout, and th e reactor ther mocouples, the hydrogen massflow controllers, an d naphth a feed pumps were th or-oughly calibrated between each test run.

C a t a ly s t a n d F e e d . The tests wer e car ried out witha commercial, balanced Pt-Re (0.3 wt % Pt a nd 0.3 wt% Re) reformin g catalyst, supplied, and used in the form

F i g u r e 1 . Flow diagram for each reactor of the pilot reactor unit (MFC ) mass flow contr oller; PC ) pressur e contr oller).

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of 1/16 in . ext rudates. A hydrot reated, s t raight-runnaph tha from a North Sea crude was used as feedstockin the test program. The compostion as determined byGC analysis i s given in Table 1. The hydrot reatednapht ha cont a ined l ess t han 0 .5 ppm su lfur . Thenap hth a was dried over a 4- molecular sieve an d storedunder an inert gas (Ar) prior to use. Using Karl-Fischer analysis, the water content of dried naphthawas measured t o 5-8 wt ppm. In order to compensa tethe effect of the rema ining water on the chloride contentof the catalyst, 0.8 wt ppm chloride as 1,1,2-trichloro-ethan e was added to the na phth a. The chloride contentof the catalyst at different positions in the reactor was

determined after each test run, to check the water/ chloride balance during t esting.H 2 (99.995%, Norsk Hydr o), sup plied from gas cylin-

ders, was passed over a deoxo catalyst (BASF R3-11)at 70 C and a 4- molelular s ieve to r emove tr aces ofoxygen and water. The deoxo catalyst as well as themolecular sieve were regenera ted before each t est r un.

R e s u l t s a n d D i s c u s s i o n

The conversion and selectivity in catalytic reformingar e functions of thermodynamic an d k inetic factors forthe great n umber of rea ctions tak ing place. The desiredreactions are the following (Gates et al., 1979):

(a) Dehydrocyclization of paraffins into aromatics.(b) Isomerization of alkylcyclopentanes into cyclohex-anes.

(c) Dehydrogenation of cyclohexanes into aromatics.(d) Isomerizat ion of linear par affins in to isopar affins.The dehydrocyclization and dehydrogenation reac-

tions are desired also becaus e th ey produce hydrogen.Both r eactions are favored by low reaction pressu re an dhigh temperature.

The most importa nt side reactions a re a s follows:(a) Hydrocracking of naphthenes and paraffins.(b) Hydrodealkylation of aromatics.(c) Alkylation and tr ans alkylation of aromatics.(d) Coke forma tion.

Hydrocracking a nd hydrodealkylation are mostlyundesired reactions because they lower the reformateand hydrogen yields, while coke formation deactivatesthe catalyst an d redu ces th e cycle length, i.e., the timebetween cata lyst regenera tion. In directly, coke forma -tion a lso influences th e reforma te a nd h ydrogen yieldsbecause the loss in catalyst activity is normally com-pensated for by higher reaction temperatures whichfavor hydrocracking reactions (Little, 1985).

O c t a n e N u m b e r a n d R e f o r m a t e Yi e l d . While thedehydrogenation of cyclohexanes to aromatics is selec-tive and nearly complete, the reactions involving paraf-fins an d a lkylcyclopenta nes are the most critical onesfor obtain ing good reform at e yield dur ing high-severityreforming (Hughes et a l., 1988). High octan e num bers

can only be achieved th rough extensive par affin dehy-drocyclization, as the octane number obtained fromdehydrogenation of naphthenes and from isomerizationof par affins is well below th e desired values (Ster ba a ndHa ensel, 1976). Pa ra ffin dehydrocyclization is relativelyslow and comparable to paraffin hydrocracking undernorma l reform er conditions (Sterba a nd Ha ensel, 1976).However, under high-severity reforming paraffin dehy-

drocyclization to aromatics can approach thermody-namic equilibrium values. Furthermore, hydrocrackingof par affins contr ibutes to the h igh octane nu mbers byconcentrating the aromatics in the reformate.

RON depends highly on the concentration of aromat-ics in the reforma te. As shown in Figure 2, there is alinear correlation between RON and the total concen-tration of aromatics in the reformate, regardless ofreaction pressure a nd temperatu re. For a given feed-stock, variations in the yields of other components(para ffinic or na pht hen ic) are th erefore of minor impor-tance for the research octane number at high-severityreforming. RON is prim ar ily a function of th e yield ofar omat ics an d th e yield of reforma te (C5+), as t hose two

values combined define the concentration of aromaticsin th e reformat e. As RON is so closely l inked t o theconcentra tion of aromat ics in the reforma te, changingthe operating variables will not influence the contentof aromatics in the reformate at a given RON.

A simple m odel for calculat ion of RON d irectly fromthe aromat ics concentrat ion has been proposed byMcCoy (1975). Based on a linear r elationsh ip betweenthe octan e num ber an d the concentra tion of aromatics,RON can be expressed as:

The values obtained by linear regression were A )

0.4822 an d B ) 64.976 with a correlation coefficient r) 0.9970 over t he wh ole severity r an ge of 95-105 RON.However, it has been demonstra ted tha t this meth od

is not universal, as the coefficients A and B depend onthe feedstock (Petr off et a l., 1988). Various modelsbased on a l inear octane contribut ion of differentcomponents determined by gas chromatography havebeen proposed (Jenkins et al., 1968; Walsh and Mor-timer , 1971; Anderson et a l., 1972; Dorbon et al., 1990).RON values calculated from GC analysis by an adaptedversion of the method presented by Anderson et al .(1972) corresponded to test engine RON values for thedebutanized reformate samples (25 samples) with asta nda rd deviat ion of 0.55 RON un its, which is close tothe accuracy of the engine measurements.

T a b l e 1 . N a p h t h a C o m p o s i t i o n ( w t %) A s D e t e r m i n e d b yG C A n a l y s i s

paraffins cyclopentanes cyclohexanes aromatics total

C5 0.2 3.2 4.9 1.4 0.2C6 6.3 5.1 8.7 6.4 15.8C7 11.2 6.2 1.5 8.3 31.4C8 14.2 1.2 6.8 2.7 30.2C9 8.0 0.1 0.4 18.7C10 3.2 3.7C11 0.1 0.1

tota l 43.2 15.7 22.0 19.2 100.1

F i g u r e 2 . RON as a function of t he total concentrat ion of aromatics in the reformate at different reaction conditions (dif-ferent t emperatures a nd pressures).

RON ) A (% ar omat ics) + B

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In par ts a a nd b of Figure 3, RON and th e reforma teyield are shown as a function of temper atu re a t 12, 16,20, an d 25 bar press ur e. Both RON and reform ate yieldar e favored by low pres sur e. As a consequ ence, a lowertemp era tu re is needed to obta in a given RON at a lowerreaction pressu re, and t he reforma te yield increases asa d irect resu lt of low pressure as w ell as of lowtempera tur e. The effect of pressur e on reformate yieldincreases with the temperature, while the effect ofpressure on RON is almost temperature-independent.

For a given severity in t he r an ge of 99-102 RON, th etemperature can be lowered by 8-9 C when going from25 to 20 bar and by 4-5 C when going from 16 to 12bar (Figure 3a). As can be seen from Figure 3b, atem pera tu re r eduction of 9.0 C gives a yield benefit of2.7 wt % at 20 bar , in addit ion t o a yield increa se of 1.5wt % when reducing the pressure from 25 to 20 bar atconsta nt tempera tur e (498.0 C, corresponding to 99.0RON at 25 bar). The yield benefit when r educing thepressure from 16 to 12 bar at a constan t temperat ure(484.3 C, corresponding to 99.0 RON at 16 bar) is 1.0wt %, while a tem pera tu re r eduction of 4.7 C at 12 ba rgives an add itional 1.0 wt % yield benefit. At 102 RONthe gain in reforma te yield by reducing the pr essure is

even greater tha n at 99 RON. This is a r esult of thesteeper decline in reforma te yield with t empera tur e athigh pr essur e (Figur e 3b), so th at t he differen ce in yieldincreases with the severity.

The incremental values, calculated as weight percentreforma te yield increase per bar reduction in pr essure,ar e given as a fun ction of RON in Figure 4. The benefitin weight percent reformate yield per bar is higher inthe h igh-pressure range t han in th e lower, regardlessof severity. Furt hermore, the yield benefit per barincreases substantially with increased severity.

A ro m a t i c C o m p o n e n t s . The ar omatics did accountfor 62-82 wt % of the reformate, depending on thereaction conditions. Toluene was by far the singledominat ing component , a ccounting for 23-31 wt % of

the reformate, while benzene accounted for 7-12 wt %.In F igur e 5 the yields of the m ost import ant aromat icsare shown as a function of temperature at pressuresbetween 12 and 25 bar.

The yields of benzene a nd t oluen e respond to changesin the temperatu re in a similar way: As expected theyields increase with increasing t emperature becausedehydrocyclization an d deh ydrogenat ion reactions ar efavoured by high temperat ure. Furt hermore, benzeneand toluene formation is favored by lower pressure. Onthe contr ar y, the yield of ethylbenzene is reduced withincreasing temperature, probably as an effect of moreside-chain hydrodealkylation. This reaction is favoredby high pressure and gives benzene and ethane asproducts (Van Broekhoven et al., 1990). The xylenesar e less susceptible to h igh-severity hydrodealkylationthan ethylbenzene, and both p- and m -xylene increasewith increased temperature for all reaction pressuresexcept a t 25 bar, wher e th e yields level off. The yieldofo-xylene, however, rea ches a ma ximum a t 495 C an ddecreases at higher temperatur es. p-Xylene d iffers fromthe t wo other xylenes by a much lower pr essure depen-dency. The heavier aroma tics, lumped as C9+, diminish

with increasing tempera tur e for all the pressu res stud-ied but are m ore pronounced the h igher the pressure.The C9+ lump consists of alkylated benzenes, and thestrong pressure dependency as with ethylbenzene isprobably a result of higher dealkylation tendency forlarger alkyl groups on benzene. For C9+ aromat ics, th isresu lts in a net loss to lighter ar omatics such as ben zenean d toluene with increasing severity. As RON is closelylinked to the total concentration of aromatics in thereforma te (Figure 2), chan ging th e operating var iableswill only affect the distribution between the differentar omatic component s, while the total conten t of aromat-ics in t he r eformate is defined by the RON va lue.

The conten t of benzene in th e reforma te is shown as

function of RON for different pressures in Figure 6. Itis clearly seen tha t at h igh severity th e benzene contentdepends on the reaction pressure. At severities aboveca. 99 RON the content of benzene increased withincreasing pressure as a result of increased benzene(and toluene) form ation from hea vier aromat ics. At 102RON the content of benzene in the reformate differedby 1.0 wt % (10%) when comparing the results at 12and 25 bar pressure. At high severity the benzenecontent of the reformate will therefore be significantlyreduced when reformer pressur e is lowered. However,at severit ies below 99 RON the opposite effect wasobserved.

i-Paraffins. Isomerizations ofn-paraffins to i-paraf-fins, and particularly multibranched i-para ffins, are

F i g u r e 3 . RON (a) and reformate yield (b) as a function of reaction temperature at different reaction pressures.

F i g u r e 4 . Increase in the yield of reformate per bar pressurereduction in the intervals of 20-25 and 12-16 bar.


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desired reactions that contribute to the increase inoctane n umbers during naph tha reforming. It is com-mon to assume that the isomerization reactions arerapid enough to closely approach thermodynamic equi-librium at normal reforming conditions (Gates et al.,1979). However, the reactivity of the paraffins de-creases as their carbon number decreases (Van Trim-pont et al., 1988). Fu rt her more, multibr an ched isomersare secondary products formed via single-branchedisomers (Van Trimpont et al., 1988), and equilibriumis largely atta ined between norm al para ffins and t heirsingle-branched isomers (Kmak and Stuckey, 1973).Since th e i-paraffins crack much easier th an t he corr e-

sponding n-paraffins, i t is not obvious that the i- ton -ratios for all paraffins are defined by their thermo-dynam ic equilibrium a t high-severity reform ing. As th epar affin isomerization rea ctions ar e slightly exoth ermic,the equilibrium ratio between i- and n-paraffins dimin-ishes sl ightly with increasing reaction temperature.Furthermore, the isomerization equilibrium is indepen-dent of the reaction pressure.

Figure 7 shows the experimenta l i/n-ratios for C4, C5,C6, and C7 para ffins at different pr essures an d temper-atures. The i/n -ratios for C6 and C7 increase from 480to 495 C, signifying th at chemical equ ilibrium is notatta ined between i- and n -paraffins for hexanes and

F i g u r e 5 . Yield of different a romat ics as a function of reaction tem perat ure a t different reaction pressur es: (a) benzene, (b) toluene, (c)ethylbenzene, (d) p-xylene, (e) m -xylene, (f) o-xylene, and (g) C9+ aromatics.


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heptanes at t hese severit ies. At higher temperatures(510 C) the i/n -rat ios for C 6 and C7 par affins appr oachther modynamic values only at th e lowest pressu res, 12and 16 bar. At h igher pressures an increase in thetempera tur e above 495 C results in a net consum ptionofi-heptanes, exceeding t he consum ption ofn -heptanedue to hydrocra cking. The same was observed for thehexan es as well, alth ough to a lesser extent. For C6 andC7 paraffins the i/n -ratios at high severity seem to be

governed by th e consum ption ofi- and n -paraffins byhydrocra cking. The isomerization rea ctions a re obvi-ously not fast enough at those reaction conditions tocompet e with t he hydr ocra cking reactions, in par ticularat h igh pressures.

On the contr ary, the i/n-ratios of the pentanes dimin-ish with increasing temperature independent of thepressur e, as is expected for th e isomerization r eactionat th ermodynamic equilibrium. The measur ed value at480 C (Kexp ) 1.8) corresponds very well with thecalculated equilibrium consta nt (Kt h ) 1.89, with datafrom Stull et al . , 1969) but falls more rapidly withincreasing temperatur e tha n expected. This could becaused by either extensive cracking of i-pentane orformation ofn-pentane from cra cking. As can be seenfrom F igur e 2, the var iations in the degree of para ffinisomerization with the operating conditions did notinfluence RON significantly.

H y d r o g e n . For a given feedstock the yield of hydro-gen is determined by t he balance between h ydrogen-producing an d hydrogen-consu ming reactions. Dehy-drogenation and dehydrocyclization are the most im-porta nt hydrogen-producing reactions, while hydro-cracking a nd hydrogenolysis, both undesired reactionswhich lower th e reform at e yield, are hydr ogen consu m-ing.

Figur e 8 shows the yield of hydr ogen a s a fun ction ofRON at different pressures. Hydrogen production isfavored by low pressures, and more so the higher the

severity. At 102 RON, the yield of hydrogen increasedby 0.35 wt % (16%) when the pressure was lowered from16 to 12 bar a nd by 0.60 wt % (50%) when th e pressu rewas lowered from 25 to 20 bar. The same pressurereductions at 99 RON gave lower hydrogen yield ben-efits, 0.25 and 0.52 wt %, respectively.

At 12 and 16 bar the hydrogen yield increases withincreasing severity. At higher pressur es the hydrogen-consuming reactions are favored and the hydrogenyields diminish with increasing severity. The severityat which the hydrogen-consuming reactions balance thehydrogen-producing reactions decreases with increasingpressur e. At 25 bar th e maximum hydrogen yield wasattained at a severity below 98 RON, while at 12 barth e maximum h ydrogen yield was above 104 RON. The

effect of severity on hydrogen yields therefore dependson the operating conditions: At reformer pressuresbelow 16 bar the hydrogen yields decrease when theseverity i s reduced, while at higher pressures thehydrogen yields may even increase when lowering th eseverity.

C o n c l u s i o n s

The study shows that a reduction of the reactionpressure resul ts in higher reformate and hydrogenyields. This effect is more pronounced at high-severityoperation an d in the high-pressure ran ge. Maximum

Figu re 6. Concent rat ion of benzene in th e reforma te as a functionof RON at different reaction pressures.

Figu re 7. i/n-ratios for different paraffins as a function of reactiontemperature at d ifferen t reaction pressures: (a) bu tanes, (b)

pentanes, (c) hexanes, and (d) heptanes.


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hydrogen yields were observed at lower severity thehigher th e pressure.

A linear correlation between RON and the content ofaromatics in the reformate was found: Independent ofthe reaction conditions, a given concentration of aro-matics in the reformate is needed to achieve a certainRON value. However, th e relat ive amount of thedifferent ar omatic component s depends on th e opera tingconditions. Less h ydrodealkylation to benzene a nd

toluene results in a shift toward xylenes and heavieraromatics a t lower r eaction pressure. Therefore, thebenzene content of the reformat e decreases when thepressur e is lowered a t high-severity operat ion.

Var iations in t he degree of par affin isomerization a sa result of changing reaction conditions did not influenceRON significan tly at th ose severities. The i/n -rat ios ofC6 a n d C7 paraffins approach chemical equilibriumvalues only at low rea ction pres sur es and h igh severity.At low severity chemical equilibrium apparently was notrea ched for t hose isomer ization rea ctions, while at h ighpressure and high severity the branched isomers werehydrocracked to give lower than equilibrium i- t on-ratios.

A c k n o w l e d g m e n t

The authors are grateful to Statoil for supporting thiswork and for the permission to publish the results.

L i t e r a t u r e C i t e d

Anderson, P. C.; Sha rkey, J . M.; Walsh, R. P . Calculation of theResearch Octane Number of Motor Gasolines from Gas Chro-matographic Data and a New Approach to Motor GasolineQuality Contr ol. J. Inst. Pet. 1972, 58 (560), 83.

Bournonville, J. P .; Franck, J . P. H ydrogen an d Cat alytic Reform-ing. In Hyd rog en Ef f ects i n C a t al ysi s. Fu n d a m en t al s a n d Practical Applications; P aa l, Z., Menon, P. G., Eds.; MarcelDekker, Inc.: New York, 1988; Vol. 31, p 653.

Davis, B. C. Adventures in Clean Air Act Amendments Imple-mentation. Hyd rocarbon Process. 1992, 71 (5), 91.

Dorbon, M.; Durand, J. P.; Boscher, Y. On-line Octane-numberAnalyser for Reforming Unit Effluent s. Prin ciple of the Analyserand Test of Prototype. Anal. Chim. Acta 1990, 238 , 149.

Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of CatalyticProcesses; McGraw-Hill Book Co.: New York, 1979; p 184.

Hughes, T. R.; J acobson, R. L.; Tamm , P. W. Cat alytic Processes

for Octane E nha ncement by Increasing the Aromatics Contentin Gasoline. In Catalysis 1987; Ward, J. W., Ed.; Elsevier:Amsterdam, The Netherlands, 1988, p 317.

Jenkins, G. I.; McTaggart, N. G.; Watkin, B. L. H. GLC for On-Stream Octane Number Rating of Stabilized Catalytic Refor-mates. In Gas Chromatography 1968; Har bourn , C. L. A., Ed.;Instit ute of Petroleum, London, 1968, p 185.

Kmak, W. S.; Stuckey A. N., Jr. Powerforming Process Studieswith a Kinetic Simulation Model. (Paper No. 56a). AIChENational Meeting, New Orleans, March 14, 1973.

Little, D. M. Catalytic Reforming; Penn Well Publishing Co.:Tulsa, OK, 1985; p 55.

McCoy, R. D. Catalytic Reformer Product Octane Measurementvia Total Aromatics. I S A T r a n s. 1975, 14 , 161.

Petroff, N.; Boscher, Y.; Durand, J. P. Determination automatiquede l ' indice doctane et de la composition des reformats parchromatographie en phase gazeuse. Rev. Inst. Fr. Pet. 1988, 43

(2), 259.Sie, S . T.; Blauwhoff, P . M. M. Laboratory Equipment andProcedures for E valuat ion of Catalysts in Cat alytic Reforming.Catal. Today 1991, 11 , 103.

Sterba , M. J.; Haensel, V. Cata lytic Reforming. Ind. Eng. Chem.,Prod. Res. Dev. 1976, 15 (1), 2.

Stull, D. R.; Westrum, E . F.; Sinke, G. C. The Chemical Thermo-dynam ics of Organic Compoun ds; John Wiley & Sons, Inc.: NewYork, 1969.

Unzelman , G. H. Reformulat ed Gasolines will Challenge ProductQuality Maintena nce. Oi l & Ga s J. 1990 , 88 (15), 43.

Van Broekhoven, E. H.; Bahlen, F.; Hallie, H. The Reduction ofBenzene in Reformate. AIChE Spring Meeting, Orlando, FL,March 18-20, 1990.

Van Trimpont, P. A.; Marin, G. B.; Froment, G. F. Reforming ofC7 Hydrocarbons on a Sulfided Commercial Pt/Al2O3 Catalyst.

Ind. Eng. Chem. Res. 1988, 27, 51.

Walsh, R. P.; Mortim er, J . V. New Way to Test P roduct Quality. Hydrocarbon Process. 1971 , 50 (2), 153.

Received for review October 6, 1994Revised m anu script received May 19, 1995

Accepted September 12, 1995X

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X Abstract published in Advance ACS Abstracts, November15, 1995.

F i g u r e 8 . Yield of hydrogen as a function of RON at differentreaction pressures.


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