Effects of catalyst deactivation and

Combined Effects of Catalyst Deactivation and Intrapartide Diffusion on the Dynamics of a Fixed Reactor J. J, Yu and J. B, Butt... 

COMBINED EFFECTS OF CATALYST DEACTIVATION AND INTRAPARTICLE DIFFUSION ON THE DYNAMICS OF A FIXED REACTOR... 

A maximum phenol conversion of 65% was reached, due to the fact that the consumption of benzoic acid was higher than that of phenol. Indeed, despite the 1/1 load ratio, the selectivity to those products the formation of which required two moles of benzoic acid per mole of phenol, made the conversion of benzoic acid approach the total one more quickly than phenol. A non-negligible effect of catalyst deactivation was present in fact, when the catalyst was separated from the reaction mixture by filtration, and was then re-loaded without any regeneration treatment, together with fresh reactants, a conversion of 52% was obtained after 2.5 h reaction time, lower than that one obtained with the fresh catalyst, i.e., 59% (Figure 1). The extraction, by means of CH2CI2, of those compounds that remained trapped inside the zeolite pores, evidenced that the latter were mainly constituted of phenol, benzoic acid and of reaction products, with very low amount of heavier compounds, possible precursors of coke formation. 

This study describes the results of processing both conventional solvent refined coal extract (SRC-I) and short contact time coal extract. Both coal extracts have been run at several space velocities, temperatures, and total reactor pressures for comparative purposes. The effect of catalyst deactivation has also been considered. The short residence time coal extract was run in both a deashed and non-deashed mode of operation. 

The Oscillatory Behavior. We have so far focused our attention on three questions (a) Does an oscillatory regime exist for this reaction system (b) What is the effect of hydrocarbon impurities and (c) What is the effect of catalyst deactivation on the dynamic behavior ... 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

Rates of deactivation of Ni and Ni bimetallic catalysts as a result of poisoning by 10-ppm H2S during methanation were investigated in a series of studies by Bartholomew and co-workers (23, 113, 161, 194). Effects of catalyst composition and geometry, gas composition and reaction temperature on the rate of deactivation were considered. Deactivation rates were found to be relatively insensitive to temperature and quite sensitive to gas and catalyst composition (194). In fact, the rates of deactivation were 2-3 times more rapid in a H2-rich mixture (H2 /CO = 99), compared to a normal synthesis (H2/CO = 3-4) mixture. 

In the mixed-phase CD reaction system, propylene concentration in the liquid phase is kept extremely low (<0.1 wt%) due to the higher volatility of propylene to benzene. This minimizes propylene oligomerization, the primary cause of catalyst deactivation and results in catalyst run lengths of 3 to 6 years. The vapor-liquid equilibrium effect provides propylene dilution unachievable in fixed-bed systems, even with expensive reactor pumparound and/or benzene recycle arrangements. 

Blocking the pore mouth and reducing the diffiisivities of the xylenes does not change this overall picture for toluene methylation, but enhances the p- selectivity [258]. As a negative side effect the catalysts deactivate and this has to be balanced with higher reaction temperatures. The higher reaction temperatures are required to open new reaction channels (dealkylation, transalkylation, disproportionation) to drain products fi om the pores as the longer residence times lead to polymethylated products that are unable to leave the zeolite pores and would eventually block all acid sites [258]. 

The modeling of real industrial reactors is usually the most difficult step in process simulation. It is usually easy to construct a model that gives a reasonable prediction of the yield of main product, but the simulator library models are not sophisticated enough to fully capture all the details of hydraulics, mixing, mass transfer, catalyst and enzyme inhibition, cell metabolism, and other effects that often play a critical role in determining the reactor outlet composition, energy consumption, rate of catalyst deactivation, and other important design parameters. 

A fundamental kinetic freunework is developed for deactivation by site coverage, coke grovth and blockage in pores and networks of pores. Diffusional limitations are also accounted for The methodology of kinetic studies of catalyst deactivation by coke formation is discussed by means of a number of practical examples. Finally, the effect of catalyst deactivation on the behavior of reactors is illustrated. 

In this review the intrinsic kinetic aspects are dealt with in the first place The progressive coverage of active sites of the catalyst, which affects its activity and the process selectivity, is cast in a mechanistic form. These kinetic aspects are then studied in combination with the influence of the catalyst morphology, first at the pore level, then at the particle level, seen as a network of pores. Next, growth of coke, leading eventually to pore blcx kage and diffusional limitations are introduced The practical application of the models in kinetic studies is given particular attention. Finally, the effect of catalyst deactivation on the behavior of the reactor is discussed. 

The model developed has been used to study the effect of catalyst pretreatment and operation temperatures upon the activation/deactivation processes during isopropyl alcohol (IPA) dehydrogenation on a Cu/Si02 catalyst. 

In the presence of both SO and HjO, the NO conversion of CuNZA decreases to a steady state of about 30% within 2 h. This activity drop is similar to that observed with only SOj in the feed gas stream. Therefore, the apparent loss of the catalytic activity over the CuNZA catalyst is mainly due to the presence of SO2. No synergistic effect of catalyst deactivation by both SO2 and H2O is observed for either catdyst type. Although the simultaneous effect of SO2 and H2O on the performance of was not... 

However, in this case the relationships obtained were only valid during the first 40 minutes of reaction since they were determined from the initial conversion values, although at 550°C the difference between the calculated and experimentally determined values was only 2.5%. A further model could be derived where the effects of catalyst stability and deactivation were also considered in order to define the best selection of variables in the catalyst preparation. 

H. Chang and B. Crynes, "Effect of Catalyst Pore and Pellet Sizes on Deactivation in SRC Oil Hydrotreating", AIChE Journal. 1986,22-2, 224-232. 

Deactivation. - The effects of catalyst deactivation in VOC abatement have been reviewed extensively by Spivey and Butt [15], and will not, therefore, be discussed in depth here. The causes of deactivation include high temperatures and the presence of chlorinated, fluorinated and/or sulfated substances, including the reactant VOCs and their partially oxidised products, water or other catalyst poisons, such as metals, in the feed gas. It should be noted, however, that the presence of water vapour in the effluent stream may be beneficial for some catalysts, particularly in terms of their specificity towards completely oxidised products this is discussed later in this review. 

WHSV = 1.0, 100-400 °C, degree of exchange has no marked effect on the extent of catalyst deactivation, and zeolite-Y is not suitable for synthesizing long straight chain hydrocarbons... 

The analysis of the effects of catalyst deactivation on CSTR performance is straightforward and there is really not too much to write about however, this can be of considerable importance in the design of slurry reactors, which will be discussed in Chapter 8. We can start with the familiar relationship for a first-order reaction given in equation (4-68)... 

The effects of catalyst decay and transport were qualitatively similar to those discussed for the simulation of la-aft lignin. Intrinsic kinetics resulted in the highest yields of monomeric products, and the presence of both deactivation and internal transport limitations resulted in the lowest. 

Our intention in this chapter is limited, however formulate approaches to the design of two main classes of catalytic reactors, fixed and fluidized bed briefly describe selected procedures along with a few numerical (or methodological) examples to illustrate their use and outline a procedure for incorporating the effects of catalyst deactivation in reactor design and operation. 

For an economic evaluation of the FT slurry process it is timely to carryout studies oncatalyst life-time and deactivation. This should be accompanied by the development of strategies how to minimize the detrimental effects of catalyst deactivation. Another important point concerns the check of competitive regeneration techniques of spent catalysts. 

Froment and Bischoff (1961, 1962) examined the effect of catalyst decay and reactor performance when coke is produced from both products and reactants. TTiey showed a Voorhies type law holds over certain operating ranges and defined a deactivation function as the fraction of active sites remaining active on the catalyst. They related this function, to the coke content, Cc, by the following two empirical relationships which are equivalent at low coke concentrations ... 

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