Properties of Cokes Produced in the Flexicoking ... - ACS Publications

3 Properties of Cokes Produced in the Flexicoking

Downloaded by GEORGETOWN UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch003

Process W. J. METRAILER Exxon Research and Development Laboratories, Exxon Co., U.S.A., P.O. Box 2226, Baton Rouge, La. 70821 R.C.ROYLE Exxon Research and Engineering Co., P.O. Box 51, Linden, N.J. 07036 G.C.LAHN Exxon Research and Engineering Co., P.O. Box 101, Florham Park, N.J. 07932 FLEXICOKING i s a process designed to convert high boiling petroleum fractions (residua) into more valuable light hydrocarbons, low sulfur fuel gas and coke. The coke produced i n the FLEXICOKING process i s quite different from cokes produced by conventional Fluid Coking or Delayed Coking. In any petroleum coking process essentially all of the non-volatiles (metals) and much of the sulfur i n the residuum feed ends up i n the coke product. In Delayed Coking, the coke product i s accumulated i n a drum and then mechanically removed. Therefore, the quality of the coke i s primarily related to the quality of the residuum feed. In Fluid Coking, typically about twenty-five percent of the coke product is burned to supply process heat. This burning concentrates the metals somewhat but does not reduce the sulfur content of the coke product. Gasification of the coke i n FLEXICOKING produces a substantial concentration of metals i n the coke product with a high coke gasification operation. Gasification of the coke also affects significant desulfurization of the residual coke. The unique system employed i n the FLEXICOKING process permits operation to give substantial coke desulfurization even at reduced gasification levels. Thus coke gasification can be minimized when there i s an attractive outlet for the low sulfur petroleum coke. This paper w i l l give a brief description of the FLEXICOKING process and then discuss 1. the mechanism of FLEXICOKE formation and desulfurization, 2. the characteristics of the coke, and 3. potential uses for the coke product. Process Description The FLEXICOKING process has been described i n detail i n other publications (1, 2). A brief description of the process, shown i n Figure 1, w i l l help to explain the unique mechanism of coke formation and how the mechanism of the process affects coke properties. FLEXICOKING i s a continuous coking-gasification 38 process carried out i n three vessels containing fluidized solids (coke) which are interconnected to permit transfer of solids from

In Petroleum Derived Carbons; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


3. METRAiLER ET AL.

Flexicoking Process

39

one vessel to another. In the reactor, steam i s injected into the bottom of the reactor to fluidize the solids and coke i s circulated to and from the heater to maintain the reactor at 925-1000°F. Residuum feed i s sprayed into the fluidized bed where i t i s cracked to form light hydrocarbons that pass overhead and are recovered i n conventional fractionating equipment. The coke formed i s deposited on the external surface of the "seed" coke i n the f l u i d bed. The deposited coke causes the particle size of the seed coke to increase and, as i n Fluid Coking (3), internal a t t r i t i o n i s supplied i n the system to maintain a fluidizable particle size range. The coke which i s generated i n the reactor passes through the heater where i t i s p a r t i a l l y devolatilized and then i s circulated to the gasifier. Here the coke i s gasified to the extent desired with a i r and/or oxygen and steam. In Fluid Coking, only sufficient coke i s burned to provide heat for the process. In FLEXICOKING, additional coke is burned to provide heat needed for gasification. The amount of coke gasified can be varied depending on the needs of the refiner. Hot gases from the gasifier pass overhead and into the bottom of the heater where they provide fluidization gas for this vessel. The heater recovers most of the heat from the gases and provides control of the temperature i n the f l u i d solids system. Mechanism of Flexicoke Formation and Desulfurization A f l u i d coke particle i s formed by repeated deposition of a thin film of fresh coke on "seed" coke particles as they pass to and from the reactor and gasifier. This i s illustrated i n Figure 2. Seed coke i s circulated through the reactor/heater system where made coke CM i s deposited on the seed to build up a film of new coke. The thickness of this film w i l l va^y somewhat with feedstock and operating conditions but typically the new coke deposit C^j i s about 5 microns thick. The reactor/ heater coke represented as R-l i s transferred to the gasifier where part of the coke i s gasified with steam/air mixture. The amount of coke gasified i s designated as Cq. The net product coke (Cp = C M - C G ) remains on the i n i t i a l seed as a thin deposit and this material R-2 i s returned to the reactor heater where additional coke i s deposited. Since the "seed" coke particles are internally generated when the system reaches equilibrium, the composition of the coke i n the system w i l l be that of the net coke product Cp. FLEXICOKING operations have been carried out i n both a 2 B/D p i l o t plant and a 750 B/D Prototype unit. Analysis of the circulating coke from the FLEXIC0KER prototype during 60 days of operation with a residuum containing 28% Conradson carbon and 4.6 wt. % sulfur are shown i n Figure 3. The i n i t i a l "seed" coke (from a commercial Fluid Coker) contained 4.3 wt. % sulfur. As the run progressed, the coke i n the system i s replaced by FLEXICOKER coke product and the sulfur content of the coke lined


PETROLEUM DERIVED CARBONS

SCRUBBER FUEL GAS *TO SULFUR RECOVERY


HYDROCARBON PRODUCTS

VAC. RESID. -*»! BED COKE

STEAM

AIR AND STEAM Figure 1.

Flexicoking designflowplan

?6

REACTOR . HEATER

l

o

,

R-2

G A S I F I Ε R T—*Cp« C -C M

R-1 COKE

Figure 2.

6

R-2COKE

Flexicoke growth mechanism



3.

MÉTRA π,ER ET AL.

Flexicoking Process

41

out at about 2 wt. %. Normal Fluid Coking of this residuum would produce a coke containing about 6.0 wt. % sulfur. The 66% coke desulfurization obtained would not be predicted based on previous experience i n hydrodesulfurization of cokes. It i s known that coke can be hydrodesulfurized (£); however, most of the studies reported have been carried out at relatively high hydro gen partial pressures. The hydrogen partial pressure i n the gasifier-heater system during this FLEXICOKER operation was only about 5 psia. The fact that coke desulfurization can be affected at these mild conditions i s attributed to the thin film like nature of the new coke described above. Electron probe analyses of a cross-section of a coke particle obtained from operations i n the 2 B/D FLEXICOKER p i l o t plant (shown i n Figure 4) confirm that the desulfurization i s occurring on the layers of the new coke. This operation started with a "seed" coke containing 5.8 wt. % sulfur. At the time the sample shown was taken, the unit had not operated long enough to dis place a l l of this "seed" coke from the system. The core of the coke particle had undergone l i t t l e or no desulfurization. How ever, the new shell of FLEXICOKE had been desulfurized to about 1.2 wt. % sulfur. The residuum feed employed i n this operation contained 4.6 wt. % sulfur and would have produced a coke containing about 8.0 wt. % sulfur i n normal Fluid Coking. Thus, the equivalent of 85% coke desulfurization was obtained i n this experimental p i l o t plant FLEXICOKING operation. Characteristics of Flexicoke Two types of coke can be obtained from a FLEXICOKER (see Figure 1). The fine coke carried over from the heater can be removed with a scrubber and recovered as a product. The metals in the coke tend to concentrate i n this stream. If a larger quantity of coke i s desired, coke product can be withdrawn from one of the beds. The Prototype FLEXICOKER operation discussed earlier was carried out at about 95% coke gasification. Here the fine coke leaving the system with the gas from the heater made up a major portion of the coke product. The operation i n the 2 B/D pilot plant discussed earlier was carried out at about 60% coke gasification. In this type of operation most of the coke product would be withdrawn from one of the fluidized coke beds. Analyses of the fine coke from two high gasification runs and bed coke from a low gasification operation are shown i n Table I. The fine coke from high gasification represents about 2 wt. % of the residuum feed, however, this may be 50 to 100 T/D of useful coke product from a typical size FLEXICOKER. The metals are concentrated i n this product. In the run with West Texas residuum, the vanadium i n the product i s 1.17 wt. % which represents a seventy fold concentration of the vanadium i n the feed. In the other high coke gasification run with Boscan




5.0

ι ιιIιιιι

ιιι ι ι ιι ιMIιι ι ι I''ι I

llllll

I .0 • • ' ' ' • ' 0 10 20 30 40 RUN LENGTH, DAYS 1

1

1

1

1

1

1

1

1

1

1

I I I I

50

60

Figure 3. Flexicoker prototype circulating coke sulfur level

8 SULFUR 6 IN COKE, WT. % 4

SEED COKE CORE 5.8% FLEXICOKE

FLEXICOKE] SH^LL

2 0

120 60 0 60 120 DISTANCE FROM CENTER OF PARTICLE, MICRONS

Figure 4. Electron probe analyses of coke particle



3.

METRAiLER ET AL.

43

Flexicoking Process

(Venezuelan) residuum feed, a fine coke product containing 15 wt. % vanadium was obtained. Metal contents of this level should be of interest for metals recovery (5). The sulfur content of the fines stream has been slightly higher than the circulating coke, however, i t s t i l l has undergone sufficient desulfurization to reduce the sulfur level to less than 3 wt. %. The fine particle size of the coke should permit use without grinding where fine coke i s required. Coke produced during the p i l o t plant FLEXICOKING operation at 60% coke gasification i s also compared i n Table I to coke that would be produced by conventional Fluid Coking of the same material. The density and the particle size of the two cokes would be about the same. The sulfur content of the coke from FLEXICOKING i s indicated to be less than 2.0 wt. %. As shown earlier i n Figure 4, the coke product from this particular operation contained only 1.2 wt. % sulfur. In addition to the lower sulfur content, the coke from FLEXICOKING has a surface area of 100 m^/gm compared to less than 10 m/gm for Fluid Coke, and the metals content of FLEXICOKE i s higher reflecting the higher consumption of coke by gasification. Combustion characteristics are important i n many processes employing coke. Combustion profiles have been obtained on coke from the low gasification operation. This i s a test which has been proposed by some furnace manufacturers (6). In the test a sample of coke i s placed on a continuous weighing device and heated at a constant rate while exposed to a i r . In Figure 5 the results from the FLEXICOKE sample are compared to data on a low v o l a t i l e bituminous coal and an anthracite coal. A l l tests were on pulverized material. The coke i s s l i g h t l y easier to burn than anthracite but more d i f f i c u l t than the low v o l a t i l e bituminous coal. Thus, FLEXICOKE should perform satisfactorily in furnaces designed to handle anthracite coal. Other combustion comparisons show FLEXICOKE to be similar to Fluid Coke which i s currently being used for power generation. 2

Potential Uses FLEXICOKING at reduced coke gasification could produce over a thousand tons per day of coke product i n a typical plant. The operation can be moved i n this direction i f an attractive market for the additional coke i s available. In addition to use of this coke as fuel, i t could be used i n blast furnaces, foundries, and other ferrous metallurgical processes, i n the manufacture of Portland cement, and to make carbon electrodes for use i n aluminum production. The f a i r l y low sulfur content of FLEXICOKE could make i t a useful material i n ferrous metallurgical processes. Petroleum coke i s reportedly already being used to make metallurgical grade coke for use i n foundries (7). In order to be used as part of the burden to a blast furnace, coke of a fluidizable


44


TABLE I

COKE PRODUCT INSPECTIONS WEST TEXAS

BOSCAN

SULFUR, WT. %

4.6

5.2

4.6

VANADIUM, PPM

160

1150

104

RESIDUUM FEED

-FLEXICOKING

TYPE OF OPERATION


HEAVY ARABIAN

COKE CONSUMED, WT. %

95

95

COKE PRODUCT STREAM

FINE COKE

FINE COKE

90

120

FLUID COKING^ ) 1

60

25 BED COKE

COKE INSPECTIONS DENSITY, LBS/CU. FT.

(2)

SURFACE AREA, M /G 2

98

95

100

Properties of Cokes Produced in the Flexicoking ... - ACS Publications

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