Coke Formation on Metal Surfaces - American Chemical Society

15 Process of Coke Formation in Delayed Coking C . A . A U D E H and T . Y . Y A N

Downloaded by EMORY UNIV on June 23, 2014 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch015

Mobil Research and Development Corporation, Central Research Division, Princeton, N J 08540

The process of coke formation in a delayed coker has been studied. Rates of reaction and s e l e c t i v i t i e s have been determined from which an overall sequence of coke development i s suggested. Kinetic, tracer and quenching studies have shown that feed entering the coker at the incipient coking temperature undergoes a two stage thermal decomposition to gas and liquid products and coke. In the f i r s t stage, thermal cracking takes place producing gas and liquid products at a fast rate R , with simultaneous condensation reactions which result in the formation of non-volatile semicoke. The volatiles leave the coking zone rapidly. Steam injected with the feed f a c i l i t a t e s the removal of v o l a t i l e s to minimize secondary cracking. Hot feed continues to enter the drum and pushes the semicoke upward with l i t t l e or no mixing. Meanwhile, in the second stage, the semicoke continues to undergo pyrolysis, leading to more gas, l i q u i d and coke, at a slower rate f

At 915 °P, in the f i r s t stage the reaction rate i s 10.9 times that of the second stage (R /R »10.9) and the liquid yield i s higher. The product s e l e c t i v i t i e s for gas, l i q u i d and coke were 4, 57 and 39 and 7, 21 and 72% wt. for the fast and slow stages respectively. f

s

In connection with thermal and catalytic processes such as coking, pyrolysis and catalytic cracking f o r the conversion of petroleum fractions, there i s considerable interest in the mechanism of the transformation of various 0097-615 6/82/0202-0295 $06.00/0 © 1982 American Chemical Society

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


296

COKE

FORMATION

organic materials into coke. In particular, Singer (1) studied the i n i t i a l steps associated with the coking of a series of pure organic compounds. Whittaker e t . a l . , ( 2) concentrated on the coking of residual o i l s and studied the nature of coke precursors and their p l a s t i c i t y . Others (3,4) have studied other aspects, with particular emphasis on kinetics, and have developed complex mathematical models to describe the coking process (4). A l l these studies agree that coking i s a thermal process"in which feed i s decomposed to generate free radicals and light products. Some free radicals undergo condensation/polymerization reactions which result in the formation of coke. Published kinetic data were generally obtained in batch reactors (1-5). The data obtained were observed to f i t f i r s t order kinetics notwithstanding the complexity of the feeds studied and the constant change in the nature of the product-forming intermediates. It has been shown (3,5) that batch coking has a definite induction period and that the usually observed f i r s t order coking behavior of complex feeds i s only apparent. Also i t was determined that the rates observed could f i t third order kinetics for decomposition and fourth order for polymerization/ condensation (5). Clearly, kinetics of batch coking are only approximations. Delayed coking i s a well developed commercial process (6), and operates on a semi-continuous basis. Feed, usually vacuum residue, mixed with steam, i s continuously pumped through tubular heaters in which i t i s heated to i t s incipient coking temperature. At this temperature the feed i s injected into an insulated drum where coking takes place. The vapors produced in the drum during coking are continuously removed and fractionated. The fractions usually include coker naphtha and light and heavy coker gas o i l s . As a drum f i l l s up, feed i s switched to another drum. Meanwhile, the f u l l drum i s steam stripped, cooled and the coke d r i l l e d out. Whereas feed i s continuously supplied to the drum, the coke i s recovered intermittently. In considering the nature of delayed coking in a continuous process i t i s clear that no specific amount of feed i s under consideration i f only part of the coking period i s studied. Also, feed i s continuously transformed during the coking process. Volatiles, with the help of steam, leave the coking zone continuously as soon as they are formed and pass through layers of material that are at various stages of the coking process. It would appear that a yet more involved model would be needed to describe the coking process in the continuous mode than in the case of batch coking. An aspect of coking which has received l i t t l e or no attention in the published literature (7) concerns the gross


15.

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Coking

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physical changes involved in delayed coking. Such information could be helpful in commercial unit design and in the interpretation of commercial unit performance. Some information i s , however, available about the apparent densities of the vapor phase within a commercial coke drum (8). Apparent density i s a measure of the relative position of the solid phase in a coke drum, usually such information i s used as an aid in determining optimum operating levels of a specific coke drum. We have studied the process of coke formation in a laboratory-scale continuous coker from which the overall sequence of coke development could be described. Experimental Figure 1 depicts the 100 ml, 7/8" ID steel coker drum used in these experiments. The coker was heated in a furnace, f i t t e d with three independently controlled zones, and was equipped with gas and liquid measurement and recovery systems. Coke recovery in the form of plugs was carried out by the use of a hydraulic ram or lathe operating on an aluminum rod with the same diameter as the preheater plug. The feed used was a 1050 °F+ residue from a commercially operated unit and was processed at typical commercial conditions, 915°F and 40 psig. In addition, operation at lower temperatures and pressures as well as higher temperatures were explored in these studies. In a l l the coking experiments, steam, 2% by wt. of the feed, was introduced with feed in the preheating zone. For tracer experiments, where two residues with different metals content were used, feed to the coker was interrupted to allow for the feed change. Metals p r o f i l e in the coke was obtained by taking sections from the coke plug and determining the metals concentration in each section. The metals, nickel and vanadium, were determined by atomic absorption. In the quenching experiments, the usual coking procedure was i n i t i a l l y followed. However, after the required amount of feed was pumped into the drum, the experiment was stopped by quickly removing the drum from the furnace and immersing i t in a mixture of ice and water. The quenched contents were carefully removed from the drum, inspected and sectioned. Each section was then extracted in a Soxhlet extractor with toluene, u n t i l the toluene extract became colorless. Results and Discussion Formation of Coke.

Tracer and quenching studies were


COKE FORMATION


298

carried out to follow the progress of feed conversion from the point of entry into the coking drum. In the tracer studies two residues with different metals concentrations were used. I n i t i a l l y i t was determined that coke prepared from each of the residues had a s i g n i f i c a n t l y different concentration of metals. Thus by taking contiguous sections from the coke plug and determining the metals concentration, the metals p r o f i l e of the coke plug recovered from the coker could be determined. Since coke produced from each of the feeds has a known metals concentration, i t i s possible to determine from which feed the coke was derived and thus assess the flow pattern of each residue. Figure 2 depicts the concentration p r o f i l e of the metals in the coke. It i s apparent that the coke formation i s representative of plug flow through the coker. The top portion of the coke has a metals concentration of 425 ppm V and 125 ppm Ni, and the bottom portion 855 ppm V and 300 ppm Ni. These metals concentrations correspond to those of coke derived from the f i r s t residue followed by coke from the second residue. This observation shows that feed entering the drum does not penetrate and mix with the material already in the coke drum. The carbonizing mass continues i t s reaction process with l i t t l e or no co-mingling of the materials at the various intermediate stages that, ultimately lead to coke formation. Early (9) and recent (2) reports have shown that coking of residues involves a sequence of phase changes with different p l a s t i c i t y characteristics. Whittaker (2) found that, depending on the feed, the carbonizing mass retains p l a s t i c i t y up to 95% of the residence time at the coking temperature. Quenching experiments, Table I, show that the carbonization progresses continuously from the entrance of the feed and that the carbonizing mass would have the p l a s t i c i t y required for flow at the coking temperature. Formation of Products. The rate of conversion of feed into products was determined in terms of the rate of formation of v o l a t i l e s , gas and l i q u i d products, and by difference the non-volatiles. The gaseous product includes C -C hydrocarbons and the liquid C -iooo°F. Gas rates were obtained from the composition and volume of the gas generated during each experiment. Similarly, the rate of l i q u i d product was obtained by determining the weight of liquid product condensed at room temperature for each coking experiment. It was found that, at the temperatures studied, volatiles formed at a constant rate as long as feed i s introduced into the coker; upon stopping the feed, however, the rate drops (Figure 3). Similarly the rate behavior of gas formation was also observed (Figure 4). 1

4

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• VOLATILE PRODUCTS 900 800


700 600 500 400 300 200 100 2

•

4 6 INCHES

8 10

FEED Figure 1.

Experimental coker drum and typical temperature profile.

125 ppm Ni 4 2 5 ppm V in coke

Top

Low Metals Feed (42 ppm Ni, 126 ppm V) 2 8 % coke

High Metals Feed (130 ppm Ni, 4 2 0 ppm V) 4 6 % coke Bottom

3 0 0 ppm Ni 8 5 5 ppm V in coke

Figure 2. Consecutive coking of two residues with different metal content. Key: low metals, 90 cm ; and high metals, 50 cm . Conditions: temperature, 915°F; pressure, atmospheric; and steam, 2% by weight. 3

3


COKE FORMATION

300

Table I


Toluene Solubles Determined for a Quenched Coking Reaction in a Continuous Coker

Temperature : Pressure : Steam: Operation time, min:

a. Coke Plug

915 ° F atmospheric 2% by weight 320

Approximate % of Position Total in Drum, in. "Coke"

Section 1

0-1/4

Section 2

Toluene Soluble % wt.

Approximate "Residence" Time, min.

4

35

3-12

1/4-1 1/4

24

5

12-80

Section 3

1 1/4-4 1/2

56

Coke Formation on Metal Surfaces - American Chemical Society

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