REFINERY COKE-1 MAJOR GROWTH IN COKE PRODUCTION TAKES PLACE

Edward J. Swain
Bechtel Corp.
Houston

U.S. petroleum coke production has increased 64% during the 10-year period from 1980 to 1990.

This dramatic rise makes it timely to discuss the history and future of U.S. coking capacity, production, and processing.

A following article will cover the properties and uses of the various grades of petroleum coke, as well as pricing and market trends.

WORLDWIDE CAPACITY

Several publications, including Oil & Gas Journal, report 24 countries with petroleum coking capacity. The reported world capacity is 96,756 short tons/sd. Five countries account for almost 90% of coking capacity, the U.S. having the largest capacity (Table 1).

The U.S. Department of Energy (DOE) reports that there were 187 operating refineries in the U.S., with 15 million b/cd crude oil capacity, as of Jan. 1, 1990. Of these refineries, 55 have coking facilities.

Although these 55 represent only 28% of U.S. refineries, they contain 52% (or some 8 million b/cd) of the available crude capacity. These 55 refineries are mostly medium-to-large, complex facilities.

U.S. refineries utilize three types of coke-processing technology: delayed, fluid, and flexi. The majority of the units are of the delayed coking type (Table 2).

U.S coking capacity grouped by Petroleum Administration for Defense (PAD) district is presented in Table 3. PAD district (PADD) No. 3 contains the largest number of refineries with coking capacity (20), and the next largest group (15) is sited in PADD No. 5.

The ten petroleum refining companies leading in coke capacity are listed in Table 4. Mobil Oil Corp., with a delayed coking unit in each of its five refineries, has the largest coking capacity.

Delayed coking is open-art technology. Although the process is offered by several licensors, no major differences exist between the different technologies. Most of the differences are related to the licensor's design and commercial experience.

One exception is the Conoco Inc. technology, which involves a specific scheme for increasing liquid yield relative to the other coking technologies. The Conoco scheme can result in lower coke production.

Fluid and flexi-coking technologies are offered by Exxon Research & Development Co. A licensing fee is paid for the use of these technologies. Fluid and flexi-cokers in operation throughout the U.S. are listed in Table 5.

U.S. COKE PRODUCTION

Coke production increased during the last decade although crude runs dropped about 1.8 million b/cd in the early 1980s. By 1990, crude oil runs had returned to near the 1980 level. The 10-year history of coke production is presented in Table 6 and illustrated in Fig. 1.

In a recent Arthur D. Little Report, "Motor Fuels in a Clean Fuels Environment," coke production was projected to increase by 16% over the next 10 years, whereas the increase in crude oil runs was projected at only 7.7% during the same period.

Coke production by PAD district from 1985 to 1990 is shown in Table 7. Because the refineries in PADD 3 represent about 50% of U.S. coker capacity, it is reasonable to expect that those refineries would be leaders in coke production.

The heavy fuel oil market is limited in PADD 3. Therefore, refineries in this district have installed coking units to aid in reducing the production of heavy fuel oil.

Coke production in PADD 3 has increased about 32% in the past 5 years alone.

As the expansion of existing coking units and the addition of new coking units are being carried out by PADD 3 refineries, coke production in the district should continue to increase faster than national growth.

The 1989 coke production quantity-66,005 short tons/cd-from U.S. petroleum refining operations represented about 91% of coke capacity. During 1989, the coke production factors within the PAD districts were estimated to be: PADD 1, 58.8%; PADD 2, 91.5%; PADD 3, 99.3%; PADD 4, 85.9%; PADD 5, 85.9%.

These production factors are calculated by dividing coke yield by coker capacity. They are influenced by crude oil quality (Table 8).

The 5-year history of crude oil quality presented in Table 8 and a recent in-house review of the 9-year history of crude oil quality, indicate a decrease of about 0.21 API gravity per year for crude processed in U.S. refineries.

If this trend continues, coupled with the installation of additional downstream processing units to upgrade the bottom-of-the-barrel fraction into transportation fuels, the resulting coke production should approach the values projected in the Arthur D. Little report.

Coke production from refineries, expressed in short tons per thousand barrels crude oil input, has increased 65% over the past 10 years (Table 9 and Fig. 2). There appears to be a leveling out of coke yields over the past several years.

However, as coking units in PADD 2 and PADD 3 expand and heavier crude oils are processed, the coke yields should start increasing again.

COKE CLASSIFICATION

Petroleum coke is produced from delayed cokers as either sponge coke or needle coke, and from fluid cokers in a small-particle-size form called fluid coke. Most U.S. coking capacity is devoted to the production of sponge coke; by far the most important coke product. When calcined, sponge coke is sold as a premium-grade coke used primarily in the manufacture of graphite electrodes and shaped products.

Approximately 65% of the delayed coke produced in the U.S. is used as fuel. The remaining 35% is calcined and converted into anodes and shaped products.

COKE CALCINATION

Calcination is a high-temperature pyrolysis treatment of green coke, the major objective of which is to produce a coke having properties suitable for a particular end use. In the calcination cycle, moisture and volatile matter are removed and the carbonization and aromatization processes which began in the coker are completed.

Dehydrogenation and dealkylation reactions continue, and large aromatic structures fuse into highly organized shapes with definite crystalline characteristics.

In normal calcining operations, the physical and chemical properties of green coke undergo considerable changes. These changes are affected by the following variables: calciner atmosphere, residence time, temperature gradient, and particle size. The particle size of green coke charged to a calciner is normally less than 4 in., with a minimum of fines.

However, there are some calciners that charge coke having a particle size of less than 3 in. Porosity, density, resistivity, chemical composition, and granulemetry are some of the properties of the coke which may be affected during calcining operations.

The 1985 U.S. petroleum coke calcining capacity was reported to be approximately 6.5 million tons/year, of which about 6.1 million tons/year, or almost 95%, used rotary kilns. Worldwide, about 8892% of calcining operations use rotary kilns. A full listing is not readily available because industry does not release this information.

Coke calcining technology is available through at least three companies: Conoco, Kennedy Van Saun Corp. (KVS), and GKT Gesemashaft Fur Kohle-Technologie MBH (GKT).