Evaluation of Coke Selectivity of Fluid Catalytic Cracking Catalysts

Chapter 25

Evaluation of Coke Selectivity of Fluid Catalytic Cracking Catalysts E. Brevoord, A. C. Pouwels, F. P. P. Olthof, H. N. J. Wijngaards, and Paul O'Connor

Downloaded by UNIV LAVAL on July 11, 2014 | http://pubs.acs.org Publication Date: June 6, 1996 | doi: 10.1021/bk-1996-0634.ch025

Akzo Nobel Catalysts, Nieuwendammerkade 1-3, P.O. Box 37650, 1030 BE Amsterdam, Netherlands

In resid cracking the high feed metals and Conradson Carbon Residue (CCR) require careful consideration when assessing both catalyst design and performance evaluation. This paper addresses the issues of the latter with respect to coke, delta coke and catalyst deactivation. Overall, evaluation of catalysts on resid feedstocks requires sophisticated and well integrated catalyst deactivation, catalyst stripping and cracking systems. It is important to determine not only the coke yield, but each of its components (Catalytic coke, contaminant coke, CCR coke and stripper (soft) coke). This paper provides details on how each of the components of the coke yield may be experimentally determined using catalyst metallation by cyclic deactivation, catalyst strippability measurements and modified catalytic cracking techniques. In fixed-bed catalytic cracking tests the proper decreasing delta coke response as catalyst-to-oil is increased is possible if a constant catalyst load and a constant feed injection rate are maintained. As CCR increases above 4 wt%, however, fixed-bed cracking methods are suspect because the mass balance drops significantly and the cracking performance can be measured better using other techniques (e.g.s., circulating pilot plants or fluidized-bed reactors).

Introduction: Resid cracking and delta coke. Since the early 80's the number of FCCU's processing resids has increased dramatically. New units are being built, designed for processing 100% atmospheric resid with a conradson carbon residue content up to 10%. Many units

0097-6156/96/0634-0340$15.00/0 © 1996 American Chemical Society

In Deactivation and Testing of Hydrocarbon-Processing Catalysts; O'Connor, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

25. BREVOORD ET AL.

Coke Selectivity of FCC Catalysts

341

are revamped to allow processing heavy feed. One of the major modifications is the implementation of a catalyst cooler, allowing a higher coke yield. When processing resid, coke formation is often a major limit (1). Problems associated with processing heavy resid feeds are (1,2):


1. Poor atomization and vaporization of high boiling components. 2. More irreversible deactivation (hydrothermal, metals), if the feed is not hydrotreated. 3. A quick reversible deactivation in the riser through the adsorption and deposition of polars and coke, which are burnt off in the regenerator. The types of coke formed in Resid FCC can be classified based on the length of time needed for their formation. Conradson carbon residue (CCR) coke will form nearly instantly at the inlet of the reactor and is therefore also called "entrance coke". The second type of coke is formed by the adsorption of highly aromatic and basic materials on even weakly acidic surfaces; this process also occurs rapidly. A part of the adsorbed polars can be stripped off, but still causes reversible deactivation. Finally, reaction or catalytic and dehydrogenation coke will form which are the slowest coke forming processes. Consequently, the relative importance of non reaction coke will increase in resid operations with a short contact time riser. In order to correctly evaluate the coke selectivity of a catalyst, it is necessary to distinguish between the types of coke made. In what follows we will also distinguish between hard and soft coke, whereby the hard coke is the coke after a long period of ideal stripping and the soft coke the difference between total and hard coke. In summary, coke consists of several components (3): 1. 2. 3. 4. 5.

Feed conradson carbon residue (CCR) coke. Non strippable additive coke, being adsorbed aromatics and polars (AC). Reaction or catalytic coke (RC). Dehydrogenation or contaminant coke, caused by metals (DHC). Soft coke (SC), being: - adsorbed hydrocarbons - trapped hydrocarbons caused by pore mouth blocking - hydrocarbons entrained in interstitial spaces

In this paper we tackle the issue on how to test and evaluate resid catalysts in respect to their coke make, consisting of: Coke=

CCR

+

AC

+

RC

+

DHC

+

SC

How to determine reaction or catalytic coke. For light feedstocks and low metals operations, determining the coke selectivity by making use of yields from a Micro Activity Test (MAT) is generally the preferred route. Several modifications of the test and test procedures, for


342

DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

instance the Micro Simulation Test (MST), have been proposed for improved relevancy of the test results (4, 5,6). In MAT/MST the conversion is usually varied by changing the cat/oil ratio, which is realized by varying either:


1. The amount of catalyst in the reactor and injecting a constant amount of feed or: 2. The amount of feed, increasing the feed flow at a constant feed injection time and catalyst content Contrary to commercial performance, this results in an increase in delta coke versus cat/oil ratio. Catalytic coke yield (% on feed) is expected to be proportional with cat/oil ratio. Consequently that delta coke should remain constant or drop: Delta coke

= =

coke yield / cat/oil ratio = (CCR coke+ catalytic coke) / cat/oil ratio (a+b* cat/oil ratio) / cat/oil ratio = a / cat/oil ratio + b (a, b are constants)

Moreover, often catalyst ranking reverses with changing cat/oil ratio, especially if yields are evaluated at constant coke yield. This phenomena can be explained with the increasing pressure in the MST reactor, when the catalyst bed height is increased, as illustrated in figure 1. Pressure affects bimolecular reaction rates and consequently higher reactor pressures result in more hydrogen transfer and coke formation. Figure 2 shows that a good correlation exists between coke make and hydrogen transfer, a high LPG olefinicity being a sign of low hydrogen transfer. 80

-20

I

I

I

I

1

0

10

20

30

40

Run time, sec ^ . C T O = 2.5 _*.CTO= 3.5 4a-CTO= 4.5 Figure 1.

Impact cat. bed height on pressure in MST


25. BREVOORD ET AL.

343



To simulate a commercial unit, it would be better to have the same pressure profile for different cat/oil ratios. In table 1 and figure 3 is shown that delta coke drops slightly with increasing cat/oil ratio, if variable feed injection is applied, keeping the catalyst bed height and feed flow rate constant. To change cat/oil the amount of feed and feed injection time is varied. The consequence of variable feed injection is that the liquid contact time varies with cat/oil ratio. This can be circumvented by injecting a fixed amount of feed in fixed time, and varying the amount of catalyst as is done traditionally, and keeping the catalyst bed height constant by adding inerts. This is a good method to obtain a constant pressure, but is a rather laborious testing method as quantities of catalyst and inerts must be weighted in accurately. Table 1.

MST results using different methods to increase cat/ oil ratio

Case

Lower bed height Base

Cat/oil (t/t)

3.5 — >

4.5

3.5

Cat. bed (gram)

3.5

4.5

4.5

Feed pumping time (sec)

15

15

19

Conversion (wt%)

69.6

74.0

72.3

Coke yield (wt%)

3.39

4.85

3.82

Delta coke (wt%)

0.97 — >

1.08

Const, bed height

4-5%) in a fluidized bed system or in a riser unit. One of our pilot riser units (PRU) is a modified Arco FCC design. When processing resid feedstocks, problems were encountered with coke formation in the reactor riser. Consequently the feed injection system was modified, allowing feed CCRs of 10% or more. To get a better insight of what is happening in the bottom of the riser, the vaporization time was calculated in a similar way as described by Buchanan (11). We conclude that the time to vaporize the feed depends mainly on the oil droplet size after atomization. For a droplet size of 100 micrometers, less than 10 milliseconds is required to vaporize 75% of the feed, being sufficient for our purposes. Though the catalyst return temperature does not seem of great importance for the vaporization time, it does of course affect the percentage of feed which is vaporized and thus the percentage of liquid cracking that is taking place. Test results have shown so far that coke make is very sensitive to dehydrogenation activity, when processing high CCR feed, using catalysts with high nickel and vanadium. In figure 5 is shown how the coke make relates to dehydrogenation activity. As CCR coke deposits immediately after feed introduction and no catalytic cracking is required for CCR coke deposition, we can distinguish between CCR coke and other types of coke by extrapolating the coke yield to cat/oil ratio = 0 (12) (figure 6). As expected the CCR coke mostly depends on the feed CCR . From figure 7 it can be calculated that more than 100% of feed CCR is converted to coke. This was caused by too much backmixing and too low temperature of the catalyst/feed mixing zone. By improving the design of the feed inlet, the percentage of CCR converted to coke dropped to 80-85%, being much closer to commercial practice.

How to determine soft delta coke The hydrocarbons which are entrained or adsorbed by the catalyst and are not stripped off before the catalyst enters the regenerator, will clearly contribute to the total delta coke. Fast and effective stripping will therefore be essential in


348

DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

7.5 > c o o


0

s

00

7

-

6.5

-

6

-

5.5

-

5

-

CO

Q) >» 0) o

o

4.5

2.2

2.4

2.6 2.8 3 3.2 CTO at w hich conv = 6 8 % , t/t Figure 4.

3.4

Dehydrogenation coke

0.4 H2 yield (wt%) Figure 5.

Dehydrogenation activity in PRU


3.6


25.

BREVOORD ET AL.

T3

CD *>»

14

r

12

-

10

-

8

-

6

I

4

-

349


Evaluation of Coke Selectivity of Fluid Catalytic Cracking Catalysts

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