Catalytic Cracking in Fixed- and Moving-Bed Processes

Catalytic Cracking in Fixed- and Moving-Bed Processes D. B. ARDERN and J. C. DART, Houdry Process Corp., Philadelphia, Pa.

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1951 | doi: 10.1021/ba-1951-0005.ch003

R. C. LASSIAT, Sun Oil Co., Philadelphia, Pa.

The application of cracking catalysts to the production of high octane motor gasoline from petroleum distillates was initially investigated by Eugene J. Houdry. The results of those studies led to the development of the fixed-bed process and later to the Thermofor, Houdriflow, and modified Thermofor air lift moving -bed processes. Effects of operating variables and characteristics and performance of catalysts upon yields and quality of products have been fully investigated. The important process and engineering phases of fixed- and moving-bed processes and information on the cracking reaction, properties of catalytic products, and characteristics of catalysts are presented in this paper.

T h e application of catalysis to the production of motor fuel b y cracking of less volatile petroleum oils was first investigated i n France by Eugene J . Houdry i n the period 1927 to 1930. The results from these investigations clearly established the superiority of catalytically cracked gasoline over that made by the thermal processes; the economic possibilities were also indicated. I t had been previously recognized that certain activated clays, of the type used for decolorization, catalyzed to some degree the decomposition of heavy oils into products of lower boiling range. Concurrently with the production of lighter hydrocarbons, products of condensation were formed and retained on the catalyst i n the form of a coke deposit. This deposit impaired the catalyst activity and gave decreased yields of the lower boiling range product. E a r l y i n the course of his studies, Houdry discovered that a high level of catalyst activity could be maintained if the catalyst was regenerated frequently b y burning the coke deposit at controlled temperatures before its concentration on the catalyst had attained a level that would substantially impair the cracking reaction. A s a result, a process was conceived in which oil i n the vapor phase was passed through a bed of catalyst until the catalyst deposit had reached a predetermined amount. The oil charge was then discontinued, and the catalyst deposit was burned b y circulation of oxygen-bearing gas. These two operations succeeded each other i n relatively rapid alternation. B y maintaining high catalyst activity i n this manner, a motor fuel high i n antiknock quality and low i n sulfur and gum-forming components could be produced, irrespective of the character of the oil charged. Ultimate gasoline yields were comparable to or better than those realized by the thermal processes. The total recovery of liquid products was relatively high and the residual liquid i n the boiling range above motor gasoline was a clean, low boiling fuel irrespective of the boiling range of the charge stock. When heavy distillates were processed, this last phenomenon meant an actual upgrading of a large 13

In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

ADVANCES IN CHEMISTRY SERIES


14

portion of the charge stock i n addition to the production of high quality gasoline. A l l these factors had an important bearing on the better utilization of petroleum crudes and, therefore, on the economy of the refining industry. Late i n 1930, H o u d r y was brought to the United States b y the Vacuum O i l C o . , subsequently the Socony-Vacuum O i l Co. H i s activities were transferred to the Sun O i l C o . i n 1933, at which time the Houdry Process Corp. was organized. Socony rejoined the development i n 1935. W i t h the financial and technical help of those two oil companies, extensive development work on the catalytic cracking process was carried out on a laboratory a n d semiplant scale. T h i s included the study of catalysts and the process variables, as well as the development of new engineering concepts which led to the first commercial application of this process i n 1936. The first commercial plant was a H o u d r y fixed-bed three-case unit charging light gas oil at a capacity of 2000 barrels per day. A large commercial plant charging heavy oil with a capacity of 10,000 barrels per day was completed i n 1937. A t the start of W o r l d W a r I I , sixteen full scale Houdry fixed-bed units were operating or under construction. Altogether 29 fixed-bed units having a charging capacity of 375,000 barrels per day were erected. A number of process limitations and a relatively high initial investment cost, which were inherent i n the fixed-bed process, added impetus to the development of the movingbed process. Thermofor catalytic cracking was a result of this development. During the late war years and immediately thereafter, 34 catalytic cracking units of the Thermofor design having a capacity of 375,000 barrels per day were built. T h e catalyst i n the Thermofor units flows i n a continuous stream alternately through a reactor and a regenerator. Mechanical elevators are used to move the catalyst from the bottom of one vessel to the top of the other. Investment cost and capacity limitations of the elevators, together with other engineering and process features, led to the development i n the past 4 years of the latest concept i n moving-bed catalytic cracking. This new development, utilizing the original catalytic cracking principles, employs pneumatic lifts to transport the catalyst between process vessels. B o t h the Houdriflow catalytic cracking process (5), licensed b y the H o u d r y Process Corp., and the basically similar Thermofor air lift process, licensed b y the Socony-Vacuum O i l Co., are of this type.

Cracking Reaction The cracking reaction i n all catalytic cracking processes is affected b y the following factors (2): catalyst type and inherent activity; charge stock characteristics and m i d boiling point; space rate, usually measured i n terms of liquid o i l volume per volume of catalyst per hour; ratio of catalyst to o i l , the amount of catalyst i n the reaction zone per unit of oil reacted, which i n the fixed-bed process becomes the ratio of reciprocal space rate to time on stream, and i n the moving-bed process is the ratio of catalyst rate to oil rate; temperature; and oil partial pressure. I n present catalytic cracking processes the production of gasoline is accompanied b y the formation of substantial amounts of coke, as well as the production of hydrogen and light hydrocarbons. A s the control of the combustion of coke was the main problem facing the commercial development, the effects of variables are conveniently expressed in terms of coke formation i n the following discussion. Effect of Catalyst Activity. I n the p r e l i m i n a r y investigations of cracking catalysts, i t was found that each type of catalyst h a d a so-called inherent a c t i v i t y which could be adjusted during the process of manufacture. T h e activity index (A.I.) is the yield of gasoline produced from a specific charging stock under standard conditions of pressure, temperature, space rate, and time on stream ( i ) . T h e activity of a catalyst is correlated with its chemical properties (13) and, for a given type of catalyst, i t is somewhat related to its physical properties. The inherent activity will decrease i n operation, owing mostly to exposure to certain vapors such as steam. Catalyst activity is a n independent variable. Replacing activity b y other operating variables to maintain In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

ARDERN, DART, AND LASSIAT—CATALYTIC CRACKING

15


constant conversion results i n lower gasoline yield. Figure 1 shows the yield of gasoline as a function of the coke yield for catalysts of different activity index (1). Effect of Charge Stock Characteristics and Mid-Boiling Point. T h e chemical composition of the charge stock has a substantial effect o n the y i e l d of gasoline a t a given coke production. A s a rule, the higher the naphthene concentration i n the charge the higher is the y i e l d of gasoline.

9 *

I

1

2ol

I

I

0

I

2 3 Y 5 6 COKE YIELD, % OF CHARGE

I

1

1 7

1

1

8

9

Figure 1. Effect of Catalyst Activity on Gasoline Yield Once-through cracking light East Texas gas oil

Figures 2 and 3 show the product distribution versus coke yield for a gas oil from mixed-base crude, and yields of gasoline versus coke for different types of charge stock. The effect of the mid-boiling point on product distribution at constant temperature and coke yield is shown i n Figure 4. A n increase i n mid-boiling point favors the produc tion of unsaturates i n the gasoline (8) and the lighter products. Stocks of higher mid-boiling point are more readily decomposed and necessitate the use of milder conditions for a given rate of coke production. T h i s is attained i n the fixed-bed units b y the use of lower oil partial pressures and catalyst activities, and i n the moving-bed units b y higher space rates. Effect of Space Rate and Ratio of Catalyst to Oil. Space rate, R, affects coke formation as an exponential function of the form Coke (weight % of charge) = Κ (R)

x

where Κ is a constant. function of the form

Ratio of catalyst to oil affects coke formation as an exponential

Coke (weight % of charge) = Ki The exponents vary somewhat with charge stocks. F o r East Texas light gas oil the values 6f χ and y are —0.4 and 0.6, respectively. W h e n other conditions are constant, any combination of these two variables which results i n a given yield of coke will also result i n given yields of gasoline, gas, and residual gas oil (20). Therefore, space rate and ratio of catalyst to oil are interchangeable variables. Effect of Temperature. Figure 5 illustrates the effect of temperature on product distribution. A n increase i n temperature decreases yields of gasoline and increases gas yields. I n addition, the yield of butylènes i n the C cut increases with increased temperature and the octane number of the gasoline produced is higher, 4


16


Effect of Pressure. Pressure is a n independent variable. A n increase i n pressure favors the production of coke. I t also favors production of gasolines of low olefin content, which is necessary i n the production of a v i a t i o n gasoline. Effect of Recycle Operation. C a t a l y t i c a l l y cracked gas oils are as a rule not so favorable charge stocks as the v i r g i n materials. T h e ratio of hydrogen to carbon i n the cracked gas oil is less than i n the virgin charge. A s the extent of cracking increases, the gas oil product becomes more refractory and aromatic. I f a small heavy fraction is removed from the cycle oil, the production of gasoline from a given amount of virgin feed can be increased at constant coke yield b y recycling the catalytic gas oil. T h e extent of recycling is limited by economic considerations. CATALYTIC OAS OIL


70 60

Ν

50 40 111

i

MOTOR GASOLINE

< 30 u 50

I Ο >

40 30 GAS

2

3

4 5 6 7 8 9 COKE YIELD, WEIGHT % OF CHARGE

Figure 2.

10

II

Product Distribution vs. Coke Yield

Once-through cracking gas oil distillatefrommixed-base crude

Figure 6 illustrates the comparison between once-through and recycle cracking i n fixed-bed operation. A n illustration of moving-bed recycling is also shown. These two curves are not to be compared, as both charge stock and catalyst differ i n the two examples. For practical use, combination charts (12, 20) have been prepared which show the effect of the above process variables on product distribution and also the interrelation ships of the yields of the various products. Although the reaction i n fixed- and moving-bed units is essentially the same, some deviations i n results occur. These are due to the following facts: I n the fixed-bed unit, commercial reactor design results i n a wide variation of tem perature during the oil cycle. I n the fixed-bed unit, the character of the catalyst varies during the cycle time, owing to the variation of temperature mentioned above and to the progressive accumulation of coke. I n the moving-bed unit, the oil is always exposed to catalyst of the same average coke concentration. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

17



Properties of Catalytic Products Catalytic cracking enjoys the distinction of being capable of producing gasoline of high quality from almost any crude oil. Generally, the F - l or Research octane rating of a 10-pound R V P catalytic unleaded gasoline is in the range of 86 to 92 octane number. T h e F-2 or A S T M motor octane number falls in the range of 78 to 82. Catalytic gasolines show an excellent response to tetraethyllead. Catalytic gasolines are generally low in gum-forming constituents and i n corrosive sulfur compounds, so that the requirements for chemical treating are minimized. A n important feature of catalytic gasolines is the uniformly good octane throughout the boiling range of the gasoline (4). The reason for the good octanes obtained with catalytic gasolines is substantiated by chemical analysis. The front end of the catalytic gasoline is predominantly isoparaffins, which contribute markedly to the high octane performance obtained. Table I shows the composition of the hexane cuts from cata lytic, thermal, and straight-run gasolines (3). T y p i c a l gasolines have been analyzed for paraffin, olefin, naphthene, and aromatic contents in narrow boiling cuts as illustrated i n Figure 7. The large concentration of higher boiling aromatics accounts for the good octanes shown by catalytic gasolines of high end point. This fact allows much latitude in cutting gasoline end point best to fit the refiner's interest without affecting gasoline quality as measured by octane number. 60

50 Ο oc

30 Κ = ( HOLAL BOILING P0INT-°R) SPECIFIC GRAVITY

20! 3 4 5 COKE YIELD, WEIGHT % OF CHARGE

Figure 3.

6

Effect of Type of Charge on Gasoline Yield Once-through cracking of distillate gat oils

W i t h naphthenic charge stocks, the higher boiling fraction of the gasoline may show even higher octane than the average gasoline octane because of the high aromatic content in this boiling range. Overlap between end point of the gasoline and initial boiling point of the virgin charge stock will cause a loss in octane number because of the inclusion of low octane uncracked naphtha i n the higher boiling fraction of the gasoline product. The virgin heavy naphtha, which is much more refractory than the gas oil, is substan-* tiaily unaffected by passing over the cracking catalyst at normal cracking conditions, The final test of a good gasoline is the road performance. A t low engine speed, gasolines respond i n accordance with F - l or Research octane numbers; at higher engine speeds, the gasoline performance follows the F-2 or motor octane number. A single example of one of many tests comparing catalytic, straight-run, and thermal gasolines is In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

18


Ό oc

C

H

FREE MOTOR 6AS0LINΕ, VOL. %


Χ

Q -I UJ

1 ISOBUTANE, VOL.*

— 4 •C

3

1

l~AND LIGHTER,

1 r——_

BUTYLENES, WT.*

550

600

Figure 4.

650 700 750 MID-BOILING POINT OF CHARGE, °F.

800

850

Product Distribution vs. Mid-Boiling Point of Charge

Constant temperature, constant coke yield, 5 weight % of charge

shown i n Figure 8 (4). T h e catalytic fuel shows road test octane ratings above the maximum required i n this particular test. T h e availability of high octane catalytic gasoline has permitted automotive engineers to develop higher compression engines to put more power and economy into today's cars. The lighter hydrocarbons and fixed gas from the catalytic cracking reactions are principally i n the C to C boiling range. The hydrogen production is about 0.1 weight % of the charge, and the total methane, ethane, and ethylene gas amounts to between 1 and 2 weight % of the charge at 5 0 % conversion. Distribution between saturated and unsaturated hydrocarbons i n the C and C4 cuts is affected b y a number of variables, including temperature, o i l partial pressure, boiling range and type of charge stock, and type of catalyst. H i g h cracking temperature, higher boiling range charge, and low oil partial pressure a l l favor unsaturation. Natural clay catalyst will give more unsaturation and less isobutane than synthetic silica-alumina catalysts, other factors being equal. 3

4

3

Table I.

Composition of Hexane Cut of Houdry Catalytic, Thermal, and Straight-Run Gasolines (Per cent by volume)

Paraffins from Ce cut π-Hexane 2- Methylpentane 3- Methylpentane 2,3-Dimethylbutane 2,2-Dimethylbutane Total isoparaffins

Houdry Catalytic Gasoline

Thermal Gasoline

9 48 27 13 3 91

63 18 16 3 .._ 37

East Texas Straight-Run Gasoline


51 32 16 1 49


19

Butanes produced are predominantly isobutane (2-methylpropane), the proportion ranging from 80 to 90 % of the total C saturates. The cracked liquid products boiling above gasoline have been utilized i n Diesel, distillate, and residual fuels, i n addition to catalytic recycling and thermal cracking for additional gasoline yield (4, 6, 7,15, 21). The catalytic Diesel fuel has a lower cetane number than the corresponding component of the virgin charge because of the changes i n chemical composition effected i n the cracking operation. However, i n mild cracking conditions, the drop may amount to only a few numbers i n Diesel index. Nos. 1 and 2 distillate fuels cut from catalytic gas oils are lower i n both gravity and aniline point than corresponding virgin stock fuels. B u t the catalytic fuels have been used satisfactorily i n atomizing pressure-type oil burners.


4

Figure 5,

Effect of Temperature on Product Distribution Fixed-bed operation


20


Catalytic Aviation Gasoline


Prior to the development of the catalytic cracking process, aviation gasoline was produced b y adding tetraethyllead to blends of commercial iso-octane (2,2,4-trimethyipentane) and selected straight-run petroleum fractions. The commercial development of catalytic cracking made available additional supplies of blending stocks having the necessary requirements of volatility, stability, and antiknock value. A t the same time, by-product isobutane and butylènes provided charging stocks for the newly developed alkylation processes.

A t the outbreak of World W a r I I a synthetic-type cracking catalyst (22) was made available and the quality of the gasoline produced was further improved. The production of this aviation fuel added greatly to the over-all output of finished aviation gasoline (18). E v e n so, the availability of the components of commercial iso-octanes and alkylates was limiting. A s far back as 1938, i t had been proved commercially that repassing or treating the primary aviation gasoline over the cracking catalyst resulted i n a product of lower olefin content, higher aromatic content, and improved response to tetraethyllead, decreasing sharply the proportion of alkylate necessary i n the blend. This process was fully utilized i n commercial units early i n 1942 and played an i m portant part in satisfying the ever-increasing requirements of our Armed Forces. Usually, a debutanized motor gasoline was charged to the treating unit at relatively severe conditions to crack the heavier fractions and increase the yield and quality of aviation base stock. T y p i c a l results are given i n Table I I . In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

21

ARDERN, DART, AND LASSIAT—CATALYTIC CRACKING 100 90

50

10


50 100

1 100

-

t

1

f "^

ι

1

50

«*00°F.

100

Figure 7.

-

Ί

1

I

I

T~

200 300 BOILING POINT

«WO°F.

AROMATIC

0

C

10

1

OLEFIN

5l0

1

200 300 BOILING POINT

^

NAPHTHENE

A. Houdry gasoline from light gas oil from mixed-base crude B. Thermal gasoline from light gat oil from mixed-base crude C. Houdry gasoline from heavy gas oil from mixed-base crude

PARAFFIN

VOL. % CUT IN GASOLINE | I

1

50

100

I

ι

200

ι

ι

Composition of Gasolines vs. Boiling Point

ι

300

^

ι HOO°F.

BOILING POINT

Catalyst Types and Properties. T h e properties of the catalysts used w i t h the H o u d r y fixed-bed process, and w i t h a l l true catalytic cracking processes, comprise the major factor not only i n promoting the desired reactions, but also i n the design and economy of construction and operation of the plant. The cracking process requires that the catalyst take part i n two alternating reac tions: (1) the decomposition of heavy to predominantly lighter hydrocarbon products, which is an endothermic reaction; and (2) the exothermic oxidation of the nonvolatile hydrocarbons retained on the catalyst during the cracking operation. This complete cycle of operations demands of the catalyst unusual chemical and physical properties, so that i t not only promotes the desired reactions when first applied, but also remains effec tive for a long time. The requirement of chemical stability will be appreciated when i t is considered that the catalyst is exposed during the cracking step to hydrocarbon vapors, sulfur and nitro gen compounds, and water vapor. During the regeneration step i t is exposed to sulfur dioxide, carbon monoxide, carbon dioxide, water vapor, and possibly nitrogen compounds in addition to air. Table II.

Aviation Gasoline from Catalytic Cracking and Treating

Chemical analysis, vol. % Paraffins Olefins Naphthenes Aromatics F-3 octane rating (4 ml. T E L ) 7-lb. R V P alkylate in blend required for 100 octane rating, vol. %

Gasoline from Light Gas Oil After treating Before treating

Gasoline from Heavy Gas Oil After treating Before treating

57.7 8.1 15. ô 18.7 94

55.3 3.3 8.1 33.3 98

40.7 29.6 11.3 18.4 91

54.2 2.7 8.1 35.0 98

46

22

56

22


22


Physical stability is required of the catalyst i n view of high temperatures reached during the regeneration step and because it is subjected to considerable mechanical strain from external sources, such as temperature fluctuations and impact loading. The earliest catalyst developed for commercial use was produced from naturally occur ring bentonitic-type clays. Such clays are carefully selected and further refined and acti vated b y chemical means to bring out their latent cracking characteristics. A typical analysis of this type of commercial catalyst follows: Wt. %

Wt. % 67.1 15.6 0.5 2.0 1.8

SiOa AbOa

Total alkali as NasO Fe 0 CaO


2

3

3.8 0.001 0.001 8.8

MgO CuO NiO Ignition loss

The inherent variability of the raw mineral, particularly with respect to minor con stituents which i n certain cases were known to have major effects on the cracking reac tion, led to the development b y the H o u d r y Process Corp. of a synthetic silica-alumina catalyst of controlled chemical composition and more stable catalytic properties. F u l l scale manufacture of synthetic catalyst was started i n 1939. The superiority of synthetic catalyst over the natural clay type for the production of aviation gasoline from a yield and octane standpoint is shown i n the following com parisons: One-Pass Aviation Gasoline Gas oil from Naphtha from coastal crude mixed-base crude Synthetic Synthetic Clay Clay

Charge stock Catalyst Yield, vol. % Octane, A F E -1C + 4 ml. T E L

38 96.2

34 93.9

34 95.2

32 93.

Two-Pass Aviation Gasoline Gas oil from California crude Synthetic Clay 32 94.9

37 98.9

The synthetic catalyst also yields a n aviation fuel of considerably lower olefin con tent which, i n effect, reduces gum-forming tendencies and improves storage stability. F o r use i n commercial plants, the natural-type catalyst is produced as pellets a p proximately 4 m m . i n diameter and length. T h e synthetic-type catalyst finds general 90

90

Ε 85 Ul

ζ

!

ui oc ui 80

a

oc 75

Catalytic Cracking in Fixed- and Moving-Bed Processes

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