Coke Formation in Reactor Vapor Lines of Fluid Catalytic Cracking Units

Chapter 9

Coke Formation in Reactor Vapor Lines of Fluid Catalytic Cracking Units A Review with Some New Insights E.Brevoord1and J . R. Wilcox2,3

Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0571.ch009

1

Akzo Chemicals bv, Stationsplein 4, P.O. Box 247, 3800 AE Amersfoort, Netherlands Akzo Chemicals, Houston, TX 2

Coke formation in the Fluid catalytic cracking unit reactor vapour line is still a problem of big concern to some refineries. The commercial experience is summarized and the coking process has been simulated in a pilot riser. Catalyst and feed quality effects have been investigated. It is concluded that coke formation is caused by thermal cracking at high temperatures at the bottom of the riser, resulting in the formation of di-olefins. Di-olefins are very reactive and form with other large convertable components, probably cyclic molecules with an extremely high molecular weight, which, if not converted, condense downstream of the riser. The influence of unit design, unit conditions, feedquality and catalyst formulation is discussed. Recommendations are given to prevent coking. Coking in the reactor vapour line is not a new subject. In the seventies and the eighties several units suffered from coking in the vapour line, resulting in a considerable pressure drop, and causing severe fouling in downstream equipment. By changing unit design and catalyst formulation, usually the coking rates could be reduced to acceptable levels. With the introduction of new reactor/riser designs, aiming for a reduction of thermal cracking, the problems seem to arise again; So far several theories exist to explain the reaction mechanism: 1. Large molecules in the feed do not evaporate under reactor conditions and leave theriserunconverted. They form a fine mist and condense in dead zones and/or the vapour line (1). 2. High hydrogen transfer results in the formation of aromatics, which can polymerize and form coke (2). 3. Other types of large molecules are formed which are not converted in the riser and as a result, settle in the vapourline, where they dehydrogenate and form solid coke. 3Current address: Refining Process Services, 1708 Pittsburgh Street, Suite 1, Cheswick, PA 15024 0097-6156/94/0571-0110$08.00/0 © 1994 American Chemical Society

In Fluid Catalytic Cracking III; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Coke Formation in Reactor Vapor Lines111

It remains an interesting question which catalyst design (matrix and zeolite activity, pore size distribution) can reduce the coking rates. A high zeolite or matrix activity may result in conversion of large polymers, a large macropore volume may allow the catalyst to absorb the coke precursors. To determine which of the three mechanisms mentioned above is the correct one and how coking can be prevented, the commercial experience is summarized. A pilotriserprogram was performed, during which coking could be simulated and catalyst and feed quality effects investigated.


Commercial experience The coking problem is clearly associated with the introduction of the modem allriser concept dating from the mid-seventies. At comparable feedstock qualities, reactor temperatures and conversions, units started to face coking problems only after the modification from a bedreactor to a reactor riser. Units however, still operating on an older type low density, high porosity type zeolite catalyst, seemed to suffer less than units using the improved high density, zeolite containing grades (2). Most units were able to cope with the problem, by taking the following actions (1): * improved insulation of the vapour line * prevent deadzones in the reactor dome * higher superficial velocities in the vapour line (35-45 m/s) During the eighties some changes in operating strategy and unit design took place. To reduce fuel oil production, the feedstock became heavier; the feed end point increased and more metals were introduced. In a few cases coking rates were related with the amount of resid processed and some refineries claimed that only problems are noticed when high nickel-feeds are processed. The latter would indicate that dehydrogenation activity plays a role in the coking process. However, the influence of the feed endpoint on coking rates is not very clear. Units processing clean feeds with an endpoint below 530°C, are known to have coking problems as well. Consequently it is unlikely that coking is only caused by insufficient conversion of large molecules, which do not vaporize in the reactor riser (mechanism 1 in the introduction). To be able to process resids, often the reactorriserswere modified. Especially the feed introduction had a lot of attention. Improved feed dispersion and feed/catalyst mixing resulted in more selective cracking. It appeared that a considerable reduction in coking rates could be established as well. In 1984 it was thought that a reduced H-transfer activity would help to reduce coking, as the formation of polyaromatics via Η-transfer was a proposed route for coke formation (2). The leadphase down followed by an increase in octane requirements resulted in a high demand for low R E ^ octane catalysts, having a low Η-transfer activity. So far however, no relation between hydrogen transfer activity and coking rates have been found, making it unlikely that coking takes place via the formation of aromatics (mechanism 2 of introduction).


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Recently short contact time risers and closed cyclone systems became popular, the latter reducing the residence time in the dilute phase. A reduction of thermal cracking and delta coke allowed the refiner to operate at higher severities. Though many modified units run without any problems, some cases are known where coking started after the modification. To investigate the coking phenomena in more detail, at one refinery several actions were taken while the coking rate was monitored:


* all unit conditions were varied, especially the operating severity. Only a high catalyst activity and high cat/oil ratio seemed to have beneficial effects, (variations in RXT may have been too small to see a significant effect) * as it was thought that coking is caused by certain large molecules, a switch to a high matrix catalyst was made to convert them. Though bottoms yield reduced with 4% at constant unit conditions, coking rates remained high. * A major reduction in coking rates was established after increasing the dispersion steam rate. The effect of feed dispersion is illustrated in figure 1. During the first period in this graph, feed quality remained relatively constant. The coking rates (monthly averages) were calculated by measuring the weight of the vapour line. The values were checked using a radioactive probe. Initially the dispersion steam rate was doubled resulting in a considerable reduction in coking rates. After a turnaround pressure drop over the feednozzles and steamrate could be increased furthermore. An improved feed/catalyst mixing results in a better temperature distribution at the bottom of the riser. Temperatures exceeding 560°C are prevented and as a result thermal cracking is reduced. A clear relation was found between the coking rates and the degree of catalytic cracking (C /LPG-ratio), enhanced by a higher catalyst activity, cat/oil ratio and improved feed dispersion (figure 2). The C4/LPG ratio can be seen as a measure of catalytic cracking. A value of 0.5 wt/wt means that thermal cracking dominates, 0.7 wt/wt means mainly catalytic cracking (3). Finally the problem was solved by a further improvement in feed introduction, while also the feed quality changed considerably (last 3 data points in figure 1). As a result a further increase in catalytic cracking could be achieved. We may conclude that feed introduction and the degree of catalytic cracking play an essential role. 4

Pilot plant simulation A pilot plant study was performed to investigate the coking phenomena. Three feeds were used, two being very paraffinic and easy to convert, one being aromatic. As it was assumed that coking is enhanced if more thermal cracking takes place or if an unit operates at a low severity, steamed fresh catalyst was diluted with inert steamed equilibrium catalyst, to achieve low catalyst activities and low



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Coke Formation in Reactor Vapor Lines 113

BREVOORD & WILCOX

F i g u r e 1. C o k i n g r a t e s feed d i s p e r s i o n .

d r o p p e d as a r e s u l t

of

better

MORE CATALYTIC CRACKING: LESS COKING

Ole

I

0.57

Ô58

I

0.59

06

I

0.61

Ô62

I

0.63

Ô64

I

0.è6

0.65

C4/(C3+C4) RATIO (wt/wt)

Figure 2. Coking rates are high thermal cracking takes place.

i f a l a r g e degree o f


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FLUID CATALYTIC CRACKING III

conversion. Several catalyst grades were used with a variety of matrix activity and pore size distributions. It appeared that also in the pilot riser (figure 3) coking took place in the transfer line. This only occurred when operating at low conversion levels and when using the waxy feed. At high conversions, the coke precursers were apparently converted.


Coking resulted in blockages in the vapour line and consequently in an unsteady catalyst circulation. As a measure of the coking tendency the stability of the operation could be used, being monitored using the temperature of the catalyst liftline at the cooler outlet. A high matrix activity did not seem to have a substantial effect, but a catalyst with a very open structure and high accessibility appeared to eliminate the formation of coke, even at low conversion levels (

Coke Formation in Reactor Vapor Lines of Fluid Catalytic Cracking Units

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