COKING METHODS AND PRODUCTS
I
B. V. MOLSTEDT and J. F. MOSER, Jr. Esso Research Laboratories, Louisiana Division, Esso Standard Oil Co., Baton Rouge, La.
Fluid Coking Development:
A Mechanically Fluidized Reactor This pilot plant is remarkably reliable for commercial scale-up
IN
FLumuw TECHNIQUE^ for the petroleum industry, the Fluid Coking process, developed by the Esso Research and Engineering Co., is one of the most recent developments (2-5). This is a true continuous proces for convertinp residual and related stocks to end prod ucts, consisting predominantly of easoline ". gas oil, and coke. Needed h e a t i supplied by the small, heated fluidize; coke particles produced in the process the coking reaction and cake-reheatinj are conducted in fluidized beds. The basic elements of the fluid coke are the reactor, or caking vessel, ani the burner, or heater v e d . Residua feed stocks are sprayed as a liquid di rectly into the coking reactor. Here thi liquid feed is distributed as a thin, oi film on the hot, fluidized coke partidm As the oil in this liquid film cracks, it i vaporized and quickly removed fron the coking zone, thus avoiding unde sirable secondary reactions. After steam stripping, dry coke is re moved from the coking reactor and con veyed by conventional fluid solid tech niques to the heater 4.Here, portion of the coke produced in the re actor is burned directly with air to rais the temperature of the coke bed. Thi reheated coke is then circulated to th coking reactor to supply the heat for th, process. Any excess coke produced i withdrawn from the heater vessel quenched, and conveyed to storage tank in more or less conventional fluid solid equipment. Thus, the Fluid Coking process i truly continuous. Only a single reacto and single heater vessel are required Also, because the heat required can b easily supplied internally by the fluid ized solids, it is not necessary to prehea the &dual feed stock in furnaces In typical operation, the reactor, o coking vessel, is operated a t abou
900' to 1000° F. and the burner vessel at approximately l l O O o to 1175' F. In most cases, v e n d top p r e m e a are maintained in the order of 5 to 10 pounds per square inch gage. The s is r---xly flexible *-a vir-
-"--_~
tually any feed which can be handled and pumped can he coked directly in the reactor. I n developing the Fluid Coking p r o w , one of the principal problems encountered wa-*-Iuplicatinp proposed
--
.
VOL. 50, NO. 1 .
~
JANUARY 19U ,
21
commercial process conditions in smallscale pilot equipment. Unlike most fluidized solids processes, the reaction is predominantly a liquid-phase reaction ; hence, the feed itself contributes little gas or vapor to maintain adequate fluidizing velocities near the bottom of the reactor. As a result, it was necessary to add appreciable quantities of extraneous diluent, normally steam, to fluidize adequately the coke particles. However, because of the small size of the pilot equipment, the extraneous diluent required was many times greater than that anticipated for the commercial units, For example, in a conventional 2-inch fluidized reactor, approximately 50 to 100 weight % of diluent on feed would be required to maintain adequate fluidizing velocities. On the other hand, projected commercial designs, because of much higher bed heights, required only about 5 to 10 weight %. Hence, reactor conditions in the pilot plant would be far different from those anticipated in the commercial plants. To overcome this difficulty, a stirred reactor was developed in which mechanical agitation was substituted for fluidizing gas to obtain the necessary fluidization in the solids bed. This stirred reactor was used principally to study the coking reaction, and the pilot unit was operated as a one-vessel system. Many problems were associated with the design of such equipment. First and most basic was the development of a suitable stirring device which \vould simulate, if not duplicate, the solids flow patterns in conventional fluidized solids units. Of the many types of agitation devices tested and observed in glass equipment, a centrifugal stirrer was chosen to give a type of fluidization most similar to commercial operation in large-scale equipment, The final design selected consists of four flat, vertical blades mounted horizontally in a row and attached to a central shaft. Each blade is pitched at an angle from the periphery of rotation so as to force solids into it against the centrifugal force. Also, horizontal curved prongs or blades are attached to the upper part of the shaft in such a fashion as to drive the solids inward at the upper part of the bed. Clearances between the stirrer and the walls and bottom of the vessel are ‘ / 8 inch or less. The solids flow pattern actually obtained is downward in the center and upward at the wall of the vessel. Of course, this is the reverse of that obtained in normal gas fluidization, but it represents a similar degree of solids back mixing. I t was not considered necessary to duplicate the actual pattern of gas fluidization. In addition, there is a tendency for a vortex to form in the center of the bed. With this stirrer,
22
I
~ _ _ _ _ _ _
c c 123
Zd’
S
Figure 1. density
a2 3.:
o j
43’
PPM
Effect of stirrer speed on bed
adequate solids back mixing could be obtained at superficial gas velocities of about 0.05 feet per second and a t stirring speeds in the order of 200 to 500 r.p.m. Because there is a natural tendency for the gas to be squeezed in toward the center of the bed, gas injected into the bed was entered near the periphery of the stirrer. Other important performance criteria of this stirring mechanism were high heat transfer coefficients and minimum solids attrition and entrainment. Heat transfer from the reactor wall into the stirred bed was good with coefficients of about 100 to 200 B.t.u./hr./sq. ft. F. during normal fluidization conditions. This is equivalent to the best obtained in gas fluidization. Also, at stirrer speeds of about 200 to 500 r.p.m., solids bed densities are comparable to those obtained a t 30 to 50 times the vapor velocity in normal fluidized beds. Thus, stirrer speed replaces fluidizing gas velocity in determining solids bed density (Figure 1). Attrition of solids is not a major problem if stirrer speeds do not exceed 500 r.p.m. Because of low fluidizing gas velocities, entrainment of solids is negligible. Although the stirring device is the heart of the pilot plant, it can only succeed if other components of the reactor system are satisfactory. The integrated design of the final pilot plant as set up
Table I.
In the stirring device, pitched blades force solids against centrifugal force
S i I RRER
The stirred bed circulates in this manner
for Fluid Coking is illustrated. The entire reactor is immersed in a fluidizedsolids heating bath, a technique for heating small-scale pilot plants which
Fluid Coking Yields
(Vacuum residua) Crude Source South La. Tia Juaiia Feed inspections Gravity, OAPI Conradson carbon, wt. % ’
B/D
c3, wt. %
12
Ultimate Yields on Coker Feedc Stirred 100 Btirred 100 reactor B/D reactor B/D 11
4 3.5 vol. % 24 25 C6-430’ F., V O ~ .% 430’ F. 4- gas oil, vol. 7% 52 51 20 21 Coke, wt. 7 0 (gross) Yields for 100-B/D plant are actual; yields for c4,
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1
2.4 30.0
7.4 21.5
10.6 19.2 100
Elk Basin
12 4.5 22 50 23
11 3.5 22 51 24
14 5.5 21 36 36
stirred reactor are correlated.
Stirred reactor 14 5 23 36 34
COKING METHODS A N D PRODUCTS
Table 11. Fluid Coking Yields (Atmosphere residua) Crude Source Casabe plus Mixed Coastal Aramco Feed inspections Gravity, OAPI Conradson carbon, wt. Sulfur, wt. % yo distilled at 1000° F.
15.1 9 1.2
%
55
Ultimate yields on coker feed C8, wt. % Ca, vol. % cs-430° F., V O ~ % . 430-1015° F., V O ~ .% Coke, wt. % (gross)
5.5 1.5 13 75 11.5
12.5 13 3.1 40
4.7 19 7.8 35
6.5 2 16.5 69.5 13.5
8 2.5 20.5 61 17.5
stocks, ranging from 19 to 30 weight % of Conradson carbon, the agreement is excellent and well within the limits of experimental error. Thus, it is concluded that results obtained in the stirred reactor are reliable and can be used to explore process variables to obtain more optimum operating conditions. A large number of feed stocks have been tested in the stirred reactor, ranging from atmospheric and vacuum virgin residua to asphalts and highly cracked thermal tars (Tables I1 and 111). This range of feed stocks well illustrates the versatility of the Fluid Coking process.
has been described previously ( 7 ) . The stirrer is driven by an electric motor acting through a pulley mechanism. This arrangement makes it convenient to remove the reactor to change the solids charge without removing the motor, Length and diameter of the stirrer shaft were carefully chosen to minimize whipping motion, because a reasonably close tolerance is desired between the stirrer and the reactor walls to minimize dead areas. Entrained solids are retained by a conventional filter which consists of a perforated cylinder wrapped loosely with a glass-wool cloth. Provisions are made for the stirrer shaft to extend through the center line of this filter. The residual feed is injected into the periphery of the mechanically fluidized bed along with a small amount of fluidizing steam. Results obtained in the mechanically fluidized, stirred reactor agree remarkably well with fluid coking data obtained in conventionally fluidized pilot plants. Table I shows a comparison between the actual recycle coking data obtained in a large-scale 100 barrel-perday pilot plant (2) and the correlated results obtained in the small-scale stirred reactor. O n three residual feed
Table 111.
Zaca
Conclusions
A stirred reactor pilot plant has been developed which substitutes mechanical agitation for fluidizing gas to obtain a pseudo fluidized solids bed. The stirred reactor eliminates the necessity for adding extraneous diluent to a smallscale, fluidized solids pilot plant to maintain adequate fluidizing velocity. As a result, projected commercial unit process conditions can be more closely simulated in the pilot plant. Such a stirred reactor was invaluable in developing the Fluid Coking process,
Fluid Coking Yields for Los Angeles Basin Residual Stocks Residuum . Virgin Visbreaker pitch tar
Feed inspections Gravity, OAPI Conradson carbon, wt. % Sulfur, wt. % % distilled at 1000° F.
17 2.0 25
Ultimate yields on coker feed Ca, wt. % c4, POI. % cS43O0 F., VOl. % 430-1015° F.,vol. % Coke, wt. % (gross)
8 2.5 16.5 61.5 20.5
6.7
-0.2 33 2.3 0
-3.5 41 2.1 5
10
11.5
3 17.5 44.5 36
3 14.5 32.5 48.5
-
II"l:lzlM
FOP sa,*
"e. SIiH
Integration of the stirred reactor with the final pilot plant
and data obtained in the small pilot plant agree remarkably well with the results obtained in a large-scale 100barrels-per-day pilot plant.
Acknowledgment The authors wish to acknowledge the valuable contributions of Ivan Mayer, Esso Research and Engineering Co., in developing a mechanically fluidized reactor.
literature Cited
(I) Adams, C. E., Gernand, M. O., Kimberlin, C. N., Jr., IND. ENC. CHEM. 46, 2458 (1954). ( 2 ) Johnson, F. B.? Wood, R. E., Petroleum Rejner 33, 156 (1954). (3) Martin, H. Z.! Barr, F. T., Krebs, R. W., OilGus J . 166,169-71 (1954). (4) Othmer, D. F., "Fluidization in Practice," p. 139, Reinhold, New York, 1956. ( 5 ) Voorhies, A,, Jr., Martin, H. Z., PTOG. Am. Petrol. Znst. Sect. III 33, 39-46 (1953). RECEIVED for review June 26, 1957 ACCF.PTED October 7, 1957 Division of Gas and Fuel Chemistry, Symposium on Coking Methods and Products, 131st Meeting, ACS, Miami, Fla., April 1957. VOL. 50, NO. 1
JANUARY 1958
23
