Deactivation and Testing of Hydrocarbon-Processing Catalysts

Chapter 10

Catalyst Deactivation in Fluid Catalytic Cracking A Review of Mechanisms and Testing Methods

Downloaded by UNIV OF PITTSBURGH on September 29, 2013 | http://pubs.acs.org Publication Date: June 6, 1996 | doi: 10.1021/bk-1996-0634.ch010

Paul O'Connor, E. Brevoord, A. C. Pouwels, and H. N. J. Wijngaards Akzo Nobel Catalysts, Nieuwendammerkade 1-3, P.O. Box 37650, 1030 BE Amsterdam, Netherlands

The consequences of deactivation on FCC catalyst activity and selectivity are reviewed and possible relations between the various deactivation phenomena are qualitatively indicated. A few cases of FCC catalyst deactivation are highlighted, specifically addressing the question how to simulate the deactivation phenomena properly.

Mechanisms of FCC catalyst deactivation. Deactivation of FCC catalysts does not only yield a drop in activity, but usually also a change in selectivity. Basically, three types of phenomena should be considered when studying the changes in catalyst activity and selectivity: Catalyst Aging: How does the catalyst change its behaviour in time. Catalyst Poisoning: How do external poisons affect catalyst behaviour in time. Catalyst Fouling: How does formation of coke and/or metal deposits affect catalyst behaviour. One can also distinguish between reversible and irreversible forms of deactivation as illustrated in Table 1. Hydrothermal Deactivation. With amorphous silica-alumina catalysts [5, 6], the primary mode of aging involves steam-induced loss of surface area by the growth of the ultimate gel particles, resulting also in loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process.

0097-6156/96/0634-0147$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.


148

DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

The introduction of zeolites in cracking catalysts combined with various non-zeolite matrix types (a.o. higher stability silica-alumina types) certainly complicates the picture of FCC hydrothermal deactivation. Letzsch et al [7] have shown that like amorphous catalysts the zeolite is more strongly deactivated hydrothermally than purely thermally. The first 10 to 25% of steam has the greatest influence[4]. The zeolite unit cell size reduction, which should give an indication of the zeolite activity loss by dealurnination [8] is not very sensitive to steam partial pressure, with the exception that some steam is necessary for cell size shrinkage. Chester et al [6] indicate that the relative contributions of zeolite deactivation (e.g. loss of crystallinity) and matrix deactivation (e.g. loss of porosity) in different temperature ranges can be significantly different. They therefore conclude that increasing temperature as a means of increasing catalyst steam deactivation severity can give misleading estimates of overall catalyst stability. This has also been confirmed with "today's" FCC catalysts [10]. As the relative contribution of zeolite and matrix activity will have an impact on catalyst selectivity, we can conclude that the foregoing is also valid for catalyst selectivity. Deactivation by Poisons and deposits. Basic and polar molecules e.g. nitrogen compounds which are readily adsorbed on to the catalyst acidic sites, lead to an instantaneous, but temporary deactivation [1,2]. Also polycyclic aromatics and other organic and non-strippable molecules which lead to coke formation are considered reversible (regenerable) catalyst poisons [11,12]. Irreversible catalyst poisons (or deposits) can even influence the catalyst during the first passage through the reactor, but are not (easily) removed during the stripping and/or regeneration stages. Examples are the heavy metals in feed as vanadium and nickel and other poisons such as alkali components, iron and copper. If we assume that the poisoning effect will increase with the concentration of poisons on the catalyst [13,14,15], we can model this effect by for instance assuming steady state addition and removal of catalyst, see for example Leuenberger [14]. The catalyst poisoning effect will then be proportional to the ratio between catalyst replacement and feed rate. Unfortunately, the metal level on FCC catalysts is hardly ever in equilibrium and as catalyst deactivation by vanadium does not take place in isolation, but combined with and influenced by hydrothermal deactivation [14, 15], more sophisticated dynamic equations[4] will be needed to describe this behaviour by including the effects of the catalyst age distribution [15, 16, 17].



10. O'CONNOR ET AL.

Catalyst Deactivation in FCC

149

In principle the poisoning effect of vanadium on the FCC catalyst can be partially reversed [9], this type of regeneration however does not usually take place in conventional FCC operations. With the deposits of catalyst poisons as coke and heavy metals, fouling and pore mouth plugging phenomena can be observed [18, 19]. Fouling can result in bigger differences in selectivity of various catalysts, because of changes in pore architecture [2, 10]. The catalysts which are relatively less accessible for large hydrocarbons will be more sensitive to pore mouth blocking and plugging [18]. Khouw et al [20] report that catalysts contaminated to high vanadium levels are still capable of converting light feeds, but not heavier feeds (see Table2) Apparently catalyst poisons can block access for the larger hydrocarbon molecules to the most accessible sites. The foregoing can be explained with a simple supply and demand model of cracking [4,10]: if a larger fraction of the sites are more accessible, the detrimental effect of poisons on the resid cracking selectivity will be less as both the poisons and the large molecules compete for the most accessible sites. How to simulate a low metals catalysts deactivation. Assuming that the metals and other poisons on catalyst are low, we may expect that traditional catalyst steaming will be sufficient to simulate catalyst deactivation. Key worth et al [16] recommend making a composite of several steamings in order to address the age distribution of equilibrium catalyst in a commercial unit. Beyerlein et al [17, 21] critically question the possibility of improving catalyst aging procedures, which rely only on steam treatment at constant temperature for varying times. We find [10, 22] that the decay behaviour of zeolite catalysts by steaming differs significantly from the activity and selectivity results after cyclic deactivation without metals. One of the typical features of the FCC operation is the continuous regeneration of the catalyst which is being circulated. The average catalyst goes undergoes 10.000 to 50.000 regeneration cycles. As described by Gerritsen et al [23] in a Cyclic Deactivation procedure the catalyst is deactivated by means of several reaction and regeneration (coke burning) cycles. This is essential for the realistic deposition and aging of the metals. Strangely enough our data consistently shows that even without metals, the catalyst seems to deactivate differently by Cyclic Deactivation compared to steaming.



150


Table 1.

Forms of Deactivation [1, 2, 3, 4]

Deactivation

Reversible

Catalyst Aging Catalyst Poisoning Catalyst Fouling

Coke, N, S, O (Polars) Coke deposits

Table 2.

Irreversible

Hydrothermal Na, V, Ni, etc. Metal deposits

Vanadium contamination has higher effect on conversion of residue feed

Feedstock Activity Loss in wt% Conversion per 1000 ppm V From [20] Own Data

VGO, CCR < < 1 wt% RESID, CCR = 3-4 wt%

1 3

0.7 1.8

CCR: %wt Conradson Carbon Residue in feed.


10. O'CONNOR ET AL.

151

Catalyst Deactivation in FCC

An example is given in the next Table: Table 3.

Ranking changes dependent on deactivation conditions

Method

ST

CD-I

CD-2

Conversion, %wt Coke, %wt C -olefmicity

67.7 2.1 0.64

72.5 2.5 0.60

68.5 3.4 0.61

Catalvst B Conversion ,%wt Coke, %wt C -olefinicity

67.0 2.2 0.69

72.7 2.6 0.60

69.2 3.9 0.57


Catalvst A

4

4

ST CD-I CD-2

: :

Steaming 5 hours at 788 °C, 100% steam Cyclic deactivation, 50 hrs, no metals Cyclic deactivation, 50 hrs, 1000 ppm Ni, 1000 ppm V For CD 1 and 2 regeneration temperature 788 C, 50% Steam

This example shows that a significant change in ranking is obtained with respect to C -olefinicity (total C = olefins / total C ) and hence hydrogen transfer activity of the catalyst. A possible explanation for this is that while dealumination in a commercial unit is fast, migration of Non-Framework Alumina (NFA) from within the zeolite structure will be a function of temperature and steam partial pressure [24]. In traditional high temperature steaming methods NFA will migrate quickly, while under commercial conditions we do not encounter these conditions. Here the Cyclic Deactivation method approaches the commercial conditions much more closely than traditional steaming methods. The presence of coke and coke burning in the regenerator stage may also have an effect on the mobility and aging of the non-framework alumina species. This has also been demonstrated to be the case for vanadium [1, 25]. 4

4

4

How to simulate a catalyst poisoned by metals. The literature on FCC catalyst deactivation by vanadium and nickel is extensive [1, 2, 10, 13, 14, 26, 27]. Basically nickel and vanadium influence the catalyst via four main reactions:


152


Mechanism

Metal Potencv

Destruction or neutralization of active catalyst sites

(V > Ni)

Dehydrogenation reactions leading to coke and gas formation

(Ni > V)

Oxidation promotion, a higher C0 /CO ratio in the regenerator [28]

(Ni > V)

Pore mouth blockage

(Ni > V)


2

The following table gives a rough impression of the relative poisoning power and dehydrogenation activity of some fresh compounds based on several literature sources available [10, 26, 27 , 29, 30, 31, 32]. Table 4.

Indications for Fresh Poisoning Power and Dehydrogenation Activity

Relative Relative Relative Activity Loss*' Activity Loss*) H Production** per ppm weight per ppm moles per ppm weight

>

2

V Ni Fe Cu

1.0 0.1 0.1 0.1

1.0 0.1 0.1 0.1

0.3 1.0 0.3 0.4

Na K Mg Ca Ba

0.9 0.9 0.5 0.5 0.1

2.0 1.2 1.0 0.6

Deactivation and Testing of Hydrocarbon-Processing Catalysts

Recommend Documents