Catalyst deactivation irreversible poisoning

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Moffat and Clark 84> found that a Langmuir-Hinshelwood model applied to a heterogeneous surface can be used to describe both the general kinetics and the rate-temperature maxima reported by Banks and Bailey (Fig. 2) for olefin disproportionation on cobalt molybdate-alumina catalyst. They conclude that the rate-temperature maximum was caused by the reversible deactivation of sites superimposed on the irreversible poisoning of sites. 

This value was verified in a continuous laboratory reactor used to study the catalyst deactivation in long time kinetic runs[2]. On the basis of experimental observations, we recognized that the palladium catalyst is subjected to both reversible and irreversible poisoning. Water beiing responsible for reversible poisoning of the catalyst. Thus, we suggested, the following mechanism ... 

It is well known that metal catalysts are poisoned by compounds of Group VB and VIB elements (ref. 90). The precise effect of a given poison however, may vary from system to system. In addition, catalyst deactivation may also result from the adsorption of a product of the reaction onto the active surface. The poisoning effect may be reversible, and in some cases catalytic activity can be restored by eliminating the source of poison. On the other hand, when poisoning occurs by an irreversible process, regeneration of the catalyst may not be possible, and so it may have to be discarded. 

In Older to explain the diverse deactivation behavior of the catalyst in processing these three types of feedstocks, we formulated a simplified dewaxing model which assumes that the dewaxing reaction can be described as an irreversible reaction (ix., cracking of waxy paraffinic molecules) coupled with a order catalyst deactivation reaction. The deactivation reaction is assumed to be conceniratioti independent while the fractional catalytic activity at any time Is a function of a number of variables including number of catalytic sites and concentration of poisons in the feedstock. 

All heavy crude oil residues have heavy metals such as Ni, V or Fe in their structure. These metals are bonded as organometalic compounds. At high temperatures and for hydrogenation reactions, these compounds are cracked and heavy metals are deposited on the catalyst surface. These metals can also react with hydrogen sulfur from the gas phase to form metal sulfides. The deposition of sulfides of iron, vanadium or nickel leads to irreversible poisoning of the catalyst. This is the difference between catalyst deactivation by metals and deactivation by coke the former leads to an irreversible loss of the catalyst activity. 

Catalyst deactivation can result from poisoning [2], from fouling [31 or ft on sintering [4]- Catalyst poisons may or may not be removable, depending on the nature of the poison. One common foulant, coke deposited on catalysts, can be removed to regenerate the catalyst. Sintering, on the other hand, is largely Irreversible and necessitates replacement of the catalyst. 

Reversible/Irreversible Poisoning- Further definition, however, is needed to clarify the related concepts of reversible and irreversible deactivation, and poisons versus simple competitive adsoiption. Deactivation, which may involve the loss of the catalyst s conversion ability, activity or selectivity, over time is often irreversible. If irreversible, continued in situ operation of the catalyst in the absence of the deactivating agent does not restore the catalyst to its original activity. 

Sulfur from SO2 can poison noble metal catalysts by its strong bonding with the metal, forming the metal sulfide and even penetrating into the bulk metal l 3. When alumina is used as the catalyst support, irreversible deactivation can result from the formation of Al2(S04)3 with concurrent substantial reduction in surface area and pore volume " . Similar activity loss with decreased surface area and pore volume accompanying sulfur accumulation in the catalyst can result from the formation and deposition of sulfates of ammonia, particularly at lower operating temperatures, but these effects can usually be reversed by heating 2,47... 

Exposure to reactant mixtures containing HMDS caused a decreased in catalytic activity. The magnitude of the decrease in catalytic activity was dependent on the HMDS concentration, and deactivation was much larger for methane than for butane (Fig.4). For the purpose of recovery, the deactivated catalyst was purged with the mixture of hydrocarbon. It was found that the catalysts were almost irreversibly poisoned for methane oxidation, but much of the activity was recovered for butane oxidation, (Fig. 4). 

Anode contamination by fuel impurities such as traces of H2S or NH3 are more irreversible than CO poisoning. Catalyst deactivation by H2S can be partially relieved by bringing the anode to oxidizing potentials and subsequent operation under hot and humid conditions [43]. 

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