D . B . ARDERN, R . H . NEW TON,
AN DG. L. BARCUS
H o u d r y P ro cess C o r p o r a tio n , M a r c u s H o o k , P a.T o o b ta in b a s ic d a ta o n t h e c h e m is tr y o f c a ta ly tic crack in g , fo u r n a r r o w -b o ilin g c u ts o f C oa sta l cru d e o il, h a v in g A .S .T .M . m id b o ilin g p o in ts o f 307°, 406®, 543°, an d 687® F ., w ere crack ed over H ou d ry s y n th e tic silic a -a lu m in a c a ta ly s t a t c o n d itio n s o f te m p e ra tu r e (650-900° F .) a n d sp a ce v e lo c ity (0 .2 -2 .0 ) c h o se n t o o b ta in v a ria tio n in th e sev erity o f c ra ck in g . D e p e n d en t variab les (pressu re a n d c a ta ly s t a c tiv ity ) w ere c o n tr o lle d to give c a ta ly s t d ep o sits e q u iv a le n t t o th o s e o b ta in e d in p la n t o p era tio n . T h e c o m p o s itio n o f t h e cracked p ro d u cts w as fo u n d to vary reg u la rly w ith t h e severity o f cra ck in g . T h e greater th e se v e rity , th e g rea ter w a s t h e c o n c e n tr a tio n o f a r o m a tic s in th e crack ed p r o d u ct. T h ere w as in d ic a tio n t h a t th e m a x i
m u m c o n c e n tr a tio n o f crack ed n a p h th e n e s w ill occu r a t m o d e r a te cra ck in g sev erity . T h e s u m m a tio n o f m o les o f rin g c o m p o u n d s decreased as sev erity in crea sed , in d ic a tin g
a d e s tr u c tio n o f r in g c o m p o u n d s a n d n o n e t in c r e a se in rin g c o m p o u n d s fo r m ed fr o m c h a in m a te r ia l. T h e i n crea se in a r o m a tic s w a s a lw a y s le s s t h a n t h e d e c re a se in n a p h th e n e s . O n ly a b o u t 40% o f t h e n a p h t h e n e s d e stro y ed ap p eared a s a r o m a tic s. H ow ever, t h e p r o d u c tio n o f h y d ro g en w a s m u c h le s s t h a n w o u ld co rr esp o n d t o s im p le d e h y d r o g e n a tio n o f n a p h th e n e s t o a r o m a tic s . A t c o n d itio n s o f rea so n a b ly c o n s t a n t se v e rity , t h e t o t a l p ro d u c tio n o f o lefin s in c re a se d w ith in c r e a s in g b o ilin g r a n g e , w h ile to ta l iso p a ra ffin p r o d u c tio n d ec re a se d . B o th tr en d s w ere m o s t e v id en t in t h e lig h te r -b o ilin g c o m p o u n d s . T h e ra tio o f iso to n o r m a l s a tu r a te d h y d ro c a r b o n s d id n o t a p pear to vary a p p rec ia b ly w it h c h a r g in g s to c k o r c ra c k in g c o n d itio n s . I n t h e lo w e r -b o ilin g a r o m a tic s p r o d u ce d , th e w e ig h t r a tio o f b e n z e n e :to lu e n e :x y le n e a v era g ed 1 : 6 : 1 6 , a lth o u g h i t varied so m e w h a t w it h c o n d it io n s .
W
IT H th e w idespread application of cataly tic cracking to th e production of high-octane m o to r fuel, th ere is a growing in te re st in th e chem istry of th e cracking process. A previous p ap er (J) gave th e chem ical com position of gasolines from two cata ly tic cracking operations a n d com pared them w ith several th erm al an d n a tu ra l gasolines. O ther recent investigators (3) have stu d ied th e reactions occurring w hen pure compounds are passed over cracking catalysts.
B ecause C oastal crude oils provide a satisfactory charging stock for c ataly tic cracking due to th e high yield of gasoline in propor
tio n to coke deposited on th e catalyst, a n d also due to th e high qu ality of th e m otor fuel produced, a series of narrow-boiling cu ts from th is crude were cracked a t several different conditions.
T he conditions chosen were those comm only used in th e H oudry fixed bed cracking process (2) an d are those in use commercially a t th e p resen t tim e. R easonably com plete analyses of th e various charge stocks an d products were obtained. Because of th e rath er large num ber of com pounds involved, it is difficult to analyze the results to indicate individual reactions; however, th e over-all results, including th e in teractio n of th e products and secondary reactions, are of considerable interest. T he purpose of this paper is to present th e d a ta from these experim ents an d to show th a t th e com position of th e cracked p roduct m ay be easily controlled over a wide range by control of th e cracking variables.
APPARATUS AND PR O C ED U RE
L ab o rato ry p ilo t-p lan t equipm ent was used. F o r all runs, ex
cep t th e low-conversion ru n a t 650° F ., th e cracking u n it was a sm all case having a capacity of 6.8 liters of catalyst. T he cata
ly st bed w as 36 inches deep in a 1-inch an n u lar space obtained by arran g em en t of concentric tubes. F o r control of th e tem perature in th e c ata ly st bed, a liquid comm ercial h eat-treatin g salt was circulated over th e inner an d o u ter surfaces confining th e catalyst bed. T he tem p eratu res m easured by pencil therm ocouples lo
cated in th e c ataly st bed were q u ite uniform th ro u g h o u t th e case.
M axim um tem p eratu res during th e regeneration periods were som etim es 300° F . above th e controlled sa lt tem perature, b u t th e c ata ly st bed was alw ays retu rn ed to th e desired ru n tem pera
tu re before oil was charged to th e cataly st.
T h e ru n a t 650° F . was m ade in a p ilo t p lan t u n it having a capacity of 20 liters of catalyst. T he in tern al design of th a t u n it
duplicated th e configuration of in le t a n d o u tle t tu b es a n d m u lti
finned sa lt cooling tubes, w hich are sta n d a rd in com m ercial H oudry fixed-bed cataly tic cases.
S tan d ard lab o rato ry auxiliary eq u ip m en t was used w ith b o th units, including charge pum p, rotom eters, vaporizer, an d pro d u ct coolers an d condensers. T h e c ata ly sts were sy n th e tic silica- alum ina pellet catalysts. T h ey w ere o b tain ed d irectly from stan d ard commercial production of H o u d ry sy n th e tic cracking catalyst. C ataly sts of different activ ities w ere em ployed as one of th e dependent variables to a id in ob tain in g desired severity of cracking as described below.
A cyclic operation was used for th e experim ents in th e 6 .8- liter case. E ach cycle was m ade as follows: 5-m inute evacuation to 1 inch of m ercury absolute pressure; 10-m in u te charging oil a t desired space ra te an d pressure; 5-m inute purge w ith steam an d evacuation; 20-m inute regeneration w ith a ir a t 3 cubic feet per m inute an d 40 pounds per square inch gage. I n each rim a few cycles were m ade d uring w hich a d ju s tm e n t for p ro p er cata
ly st deposit was m ade. T h en a n u m b er of cycles w ere ru n in continuous operation, a n d sam ples a n d d a ta w ere obtained.
In norm al p lan t practice, cracking conditions a re controlled to obtain a c ataly st deposit of a b o u t 5.0% by w eight of the charge a t a space ra te of 1.0 or 2.5% a t a space ra te of 2.0. Con
trol of th e cataly st deposit is o b tain ed by selection of th e cata
ly st of proper activ ity a n d by ad ju stin g th e o p eratin g pressure.
T he lower-boiling refracto ry stocks require a c a ta ly s t of rela
tively high activ ity a n d higher operating pressures. As th e boil
ing range of th e stock increases, less a ctiv e c ata ly sts are used, th e case operating pressure is reduced, an d steam m ay b e ad d ed to reduce fu rth er th e p a rtia l pressure of th e stock in th e case.
On this basis b o th c ata ly s t a ctiv ity a n d o p eratin g pressure were varied in these experim ents. T he m axim um case working pressure of 60 pounds per square inch proved to be a lim iting fac
to r w hich resulted in c ata ly st deposit figures for cracking th e 307 0 F . m idboiling-point c u t w hich were 6 to 36% u n d er th e average of o th er cuts a t corresponding conditions. I t w as n o t necessary to employ steam to p re v en t excessive c ata ly st deposits in a n y of these runs.
T he low conversion ru n m ade a t 650° F . was com posed of a single 90-m inute period w hen oil was charged to th e c ata ly s t bed an d gas an d liquid p roducts were recovered, followed b y a regen
eration period controlled to o b tain essentially to ta l com bustion of th e c ata ly s t deposit.
Sam ples of b o th gas a n d liquid p ro d u cts were distilled in stan d a rd P odbielniak colum ns, an d all lig h t p roducts th ro u eh n pentane were determ ined. T h e liquid p ro d u ct was distilled a k n in equipm ent used regularly for m aking an aly tical fraction«
th ro u g h th e gasoline range. T h is eq u ip m en t consisted of a 3000
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 547 EX PER IM EN TA L DATA
F ig u res 1 an d 2
ml. flask, electrically heated and connected to a one-inch-diam eter Bruun column having forty bubble plates which was operated a t a 15 to 1 reflux ratio. T he analytical fractions were exam ined for certain physical an d chemical properties on w hich th e paraffin, cracking work done upon th e charge stock under th e imposed cracking conditions. T he m easure increasing severity of cracking. V ariation in severity was obtained b y fixing conditions of these were converted to w eight per cent using the densities of th e compounds. T h e analyses for hexanes an d heavier were converted from th e liquid volume percentages, determ ined an aly ti
cally, to weight percentages by applying th e w ith th e density calculated for th e fraction using th e determ ined volume of each ty p e of organic compounds and th e corresponding densities for those compounds a t th e m idboiling po in t of the fraction. Above 392 0 F. th e w eight per cent data are less firm, possibly as a result of errors in tro duced by th e presence of bicyclic compounds which were not identified separately in th e analyses.
E F F E C T O F VARIABLES
TableIII.Yields(inWeightPerCentofCharge) fromCrackingCoastai, Crude
Inn«, 1945
W
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 549 N ap h th en es boiling below th e initial boiling p oint of th e chargelikewise show a v ariatio n w ith severity, as p lo tted in Figure 2.
T he d a ta indicate t h a t a m axim um concentration of lower-boiling naphthenes w ould be found a t a low severity. T he highest naph- thene yield was obtained when operating a t 650° F ., b u t tem pera
tu re was n o t trea ted as an independent variable an d these d a ta do n o t suffice to evaluate th e effect of tem p eratu re alone. On the hypothesis th a t th e high-naphthene yield is a function of severity, it should be possible to o b tain ab o u t th e sam e production of naphthenes a t higher tem p eratu res if pressure and space ra te are a d ju sted to assure low severity of cracking.
Bo i l i n g Ra n g e o f Ch a r g e. W hen cracking under condi
tions of co n stan t tem p eratu re, space rate, an d cataly st deposit, the boiling range of th e charge stock has a direct bearing upon th e production of lower-boiling isoparaffins an d olefins (Figure 3).
While th e variatio n s are pronounced in th e C 4 range, in th e C t and heavier analytical fractions th e boiling range has little or no effect. F o r th e d a ta in F igure 3 th e severity varied from 34.3 to 42.2 weight % C 6 an d lighter.
R IN G COMPOUND BALANCES
D ata on arom atics an d naphthenes, obtained when cracking the 406° F. m idboiling-point charge, were converted to a molal basis by m eans of a correlation of m olecular w eight vs. boiling point from th e d a ta of W ard an d K u rtz an d the determ ined mid
boiling points of th e analytical fractions. T he to ta l moles of ring compounds in th e pro d u ct decreased w ith increasing severity of cracking as shown in Figure 4. Likewise, th e am ount of residual ring compounds, w hich includes unreacted and polymer com
pounds, decreased rapidly w ith increasing severity.
The ring balance for th e m ost severe cracking operation follows (basis, moles per 1000 grams of charge):
4.70 moles charge 1 naphthenes I 0.96 mole charge [ arom atics J
f0.54 mole residual naphthenes "I ,
\0.53 mole residual arom atics J fO.48 mole lighter naphthenes
\1.84 moles lighter arom atics 4.16 moles charged naph-1
thenes destroyed I 0.43 mole charged aro-f m atics destroyed J 4.59 moles charged rings
destroyed
1
0.48 mole lighter naphthenes produced 1.84 moles lighter aro
m atics produced 2.32 moles lighter rings
produced
Similar d a ta can be tab u la te d for th e other conditions on cu t 2 also. In general, only ab o u t 50% of th e to ta l ring compounds destroyed appeared as ring com pounds produced. There ap peared to be no gain in ring compounds owing to th eir form ation from chain m aterial. T he gain in arom atics was always less th an the loss of naphthenes. Only 35-40% of th e naphthenes de
stroyed appeared as lower-boiling arom atics a t th e condition of greatest severity, an d th a t proportion was less in less severe oper
ation. The am ount of hydrogen determ ined in th e cracking products was m uch less th a n w ould correspond to simple dehy- .drogenation of naphthenes to arom atics.
F ig u re 3. Is o p a ra ffin a n d O lefin Y ield s vs. B o ilin g P o in t o f C h a rg e
d en t from th e large naphthene concentration present. T he b o t
tom s fraction was prim arily m aterial boiling above the end point of th e charge stock an d was composed principally of arom atics an d olefin polymer.
F ig u re 4. V a ria tio n in R in g B a l
a n c e w ith S ev erity o f C ra ck in g
C O M PO SITION O F PRODUCTS
I n Figure 5 th e percentage PO N A d istribution is shown for each analytical fraction obtained, in cracking th e 406° F . m id- boiling-point cu t a t th e four conditions used. I t is evident th a t th e wide range of severity did n o t affect radically th e characteris
tic distribution of compounds w ithin an y analytical fraction.
However, th e trends of arom atic an d naphthene production pointed out above are ap p aren t. Olefins are m ost evident in th e run a t greatest severity, b u t th is is probably due principally to th e 900° F. tem perature. B eyond th e 325° F . m idpoint frac
tion th e overlap into th e boiling range of th e charge stock is
evi-E xcept for th e runs m ade w ith th e 307° F . midboiling-point charge, th e distribution of th e lower-boiling arom atics was nearly constant throughout th e experiments. Benzene production averaged 2.5% of th e C9 and lighter arom atics, an d toluene pro
duction averaged 14.5%. N either severity nor charge h ad any effect upon th e relative concentration of those compounds. The ratio of xylenes to Cs arom atics was 1 to 1 w ith th e 406° m id- boiling charge and averaged 2 to 3 using th e 687° midboiling cut.
T he ratio showed slight increase w ith increasing severity. T he average weight ratio benzene ¡toluene :xylene was 1:6:16, b u t it varied appreciably in individual runs. E xam ination by super
530 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T S !
si. ai,
6CRACKING AT 900» F.. 1.0 SPACE RATE CRACKING AT > 4 0 *E . 1.0 SPACE «ATE
0, c* c c4 c* 8 g J g s g f I
M io BOILWO PT.OF FRACTION ANALYTICAL FRACTION
c , o £ cï a , c 9 g p | g g g ç |
M ia BOtLINS PT.OF FRACTION a n a l y t ic a lfr a c tio n
F ig u re 5. C o m p o sitio n s o f A n a ly tic a l F ra ctio n s from C racking th e 406° F . M id b o ilin g -P o in t C u t
fractio n atio n of tw o of th e xylene fractions of th e products indi
c ated a b o u t 85% m eta an d p ara isomers, an d did n ot indicate any appreciable a m o u n t of ethylbenzene. T he lowest-boiling-range charge stock, w hich contained xylenes an d Cs arom atics, gave a w eight ra tio of benzene to toluene of ab o u t 1 to 3 in th e cracked p roduct, w hich is a t variance w ith th e 1 to 6 ratio obtained w ith th e higher-boiling charges.
T h e d istrib u tio n of paraffins betw een iso and norm al com
pounds d id n o t v a ry appreciably w ith aharging stock or operating conditions, in so far as th e paraffins were identical. The average w eight ra tio s of iso to to ta l paraffins were:
Isobutane to to tal butanes 0.807 Isopentane to total pentanes 0.935 Isohexane to to ta l hexanes 0.973
S everal of th e lower-boiling an aly tical fractions were distilled in a sm all b a tc h u n it designed for a high degree of fractionation.
T h e hexane fractio n w as found to be principally 2-m ethylpentane, w ith som e 3 -m ethylpentane a n d a sm all am o u n t of 2,3-dime th y 1- b u tan e . T h ere was no indication of neopentane (2,2-dim
ethyl-p roethyl-pane) or neohexane (2,2-dim ethy - b u tan e). A m o n g t h e l o w e r - b o i l i n g n ap h th en es no cyclopentane was foun , b u t m ethylcyclopentane an d cyclo- hexane w ere present.
CO NCLUSIO N S
T h e com position of cracked p ro d u cts w as found to v a ry regularly w ith th e severity of cracking. A lthough cracking tem p eratu re was n o t chosen as a n in
d ependent variable, it was v aried along w ith th e o th er conditions. I n th is w ay th e relativ e contents of n ap h th e n e s (cyclopara-ffins) an d aro m atics w ere v aried over a wide range. T h e g re ater th e severity of cracking conditions (i.e., th e g reater th e production of m aterial boiling lower th a n th e hexanes), th e g reater was th e concentration of arom a- tics in th e cracked d istillate. I t is in
d icated t h a t m axim um p ro d u ctio n of cracked n ap h th en es will occur a t m oder
a te cracking severity.
As th e boiling range of th e charging stock increases, th e im posed cracking conditions are less severe, since th e higher-boiling m ate ria l is less refracto ry . I n order to p re v en t “ overcracking” an d th e d estru ctio n of desirable p roducts, a lower p a rtia l pressure is im posed on th e oil, e ith e r b y lowering to ta l pressure o r b y intro d u cin g steam . Also c a ta ly sts of su itab le a c tiv ity m u st be used. In th is w ay th e carbon deposited on th e cataly st is controlled a n d lim ited to th e desired figure of a b o u t 5 .0 % b y w eight a t 1.0 space ra te or 2 .5 % a t 2.0 space ra te . W hen cracking conditions a re ad ju sted a s described above, it is found t h a t th e p ro d u ctio n of lig h t olefins in
creases w ith increase in boiling range.
C orrespondingly, th e p ro d u ctio n of lig h t isoparaffins decreases w ith increase in boiling range of th e charge stock.
F o r th e over-all cracking reaction, th ere is no evidence of th e fo rm atio n of ring com pounds from chain* m aterial.
T he to ta l a m o u n t of moles of ring w as found to decrease as crack
ing severity increased, in d icatin g a d estru ctio n of rings. T h e in
crease in arom atics was alw ays less th a n th e decrease in naphthenes. W h atev er th e m echanism of conversion of naphthenes to arom atics, th e prod u ctio n of hydrogen is far less th a n w ould correspond to sim ple dehydrogenation of n a p h th en es to arom atics, an d only approxim ately 40% of th e n a p h th e n e s de
stroyed ap p ear as arom atics.
In th e lower-boiling arom atics (Cs an d lighter) produced, th e concentration of benzene was alw ays low, averaging 2 .5 % of th e arom atics. T h e toluene production was fairly co n stan t a t a b o u t 14.5%. T h e ratio of xylenes to C9 arom atics was 1 to 1, using a 406° F . m idboiling-point charge stock, an d decreased w ith th e heavier charge cuts. Also th is ratio was highest a t th e m o st severe cracking conditions. O n th e average, th e w eight ra tio benzene: toluene ¡xylene was ab o u t 1:6:16, although it v aried som ew hat w ith conditions.
T he ra tio of iso to norm al s a tu ra te d hydrocarbons did n o t a p p e ar to v a ry appreciably w ith charging sto ck or operating condi
tions. As in previous w ork, n eopentane (2,2-dim ethylpr0p an e)
_______
June, 1945 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y SSI and neohexane (2,2-d im ethylbutane) were su b stan tially absent,
as was oyclopentane.
ACKNOW LEDGM ENT
T he au th o rs acknow ledge th e assistance of Jo h n Snyder, who perform ed th e special fractionation work, an d th e research staff of the H ou d ry L aboratories, C ataly tic D evelopm ent C orporation, where all w ork in th is investigation was perform ed.
LITERATURE CITED
(1) Bates, Kurtz, Rose, and Mills, In d. En g. Ch e m., 34, 147 (1942).
(2) Houdry, Burt, Pew, and Peters, Oil Gas J., 37, 40 (1938); Peter-kin, Bates, and Broom, Refiner, 18, 504 (1939).
(3) Thomas, Div. of Petroleum Chem., A.C.S., April, 1944; Block and Thomas, Ibid.; Thomas, Hoekstra, and Pinkstone, Ibid.
(4) Ward and Kurtz, In d. En o. Ch e m., An a l. Ed., 10, 559 (193S).
Pr e s e n t e daa part of the Symposium on Catalysis in the Petroleum Indus
try before the Division of Petroleum Chemistry a t the 108th Meeting of the Am e r i c a n Ch e m i c a l So c i e t yin New York, N. Y.