THE CATALYST DEACTIVATION PROBLEM

Chapter 7 briefly dealy with the catalyst deactivation problem and presents elementary information on the use of heterogeneous models for the optimization of reactors experiencing catalyst deactivation. 

APPENDIX C APPLICATION OF THE COLLOCATION METHOD TO THE CATALYST DEACTIVATION PROBLEM 449... 

APPENDIX D MAXIMUM PRINCIPLE FOR THE HETEROGENEOUS MODEL. (THE CATALYST DEACTIVATION PROBLEM, CHAPTER 7) 451... 

Application of the Collocation Method to the Catalyst Deactivation Problem... 

Maximum Principle for the Heterogeneous Model. (The catalyst deactivation problem, chapter 7)... 

Reaction (13.2) is highly undesired because SO, reacts with water present in the flue gas in large excess and with ammonia to form sulfuric acid and ammonium sulfate salts. The ammonium sulfate salts deposit and accumulate on the catalyst if the temperature is not high enough, leading to catalyst deactivation, and on the cold equipment downstream of the catalytic reactor, causing corrosion and pressure drop problems. The catalyst deactivation by deposition of ammonium sulfate salts can be reversed upon heating. 

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,. 

In the second generation catalysts, deactivation problems have led to chromium being substituted by other metals that play various roles. The newer patents report rather complicated catalyst compositions, e.g. in 1989 United Catalysts described an optimum catalyst with composition, 78.6% Fe203, 9.5% K2O, 5.0% Ce02, 2.5% M0O3, 2.2% CaO, 2.2% MgO and 1000-5000 ppm Cr. To prepare a catalyst of this type, a slurry of the components is extruded as pellets which are then dried, and subsequently calcined at high temperature, usually 800-1000°C. 

The coke deposition on a catalyst under operation of the latter Is a typical reason for the catalyst deactivation. This process also can be considered as a manifestation of nonselectivity in the conversion of various organic compounds. Hence, the practically important problem is to find the conditions of the coking prevention. As an example, let us identify the conditions of no coke deposition on catalysts during the "dry" methane reforming described by a stepwise transformation as follows ... 

The main problem in case of thermocatalytic cracking of polymers is the activity loss of catalysts therefore first-order kinetics is applicable only with some simplifications in thermocatalytic cases. On the other hand there is a relation modelling the fluid catalytic cracking taking into consideration the catalyst deactivation in refineries [31] ... 

Commercial catalyst are available for the production of methanol and other liquid fuels from synthetic gases. The main problem is the catalyst deactivation due to chemical poisoning from chlorine and sulphur. 

The main problem is that all the catalyst deactivate in 1.0-8.0 hours. The deactivation is mainly caused by coke deposition- Ansimultaneous thermal and catalytic cracking of the tars present in the exit gas-... 

A typical dual catalyst system studied in our lab consisted of a commercial, Cu-based methanol synthesis catalyst and y-alumina. While all the merits of this process were verified, the study showed that both catalysts suffered rapid deactivation under LPDME conditions. The problem seems common to many other dual catalyst systems containing different dehydration catalysts. This paper focuses on our investigation of the cause and the mechanism of the catalyst deactivation in this dual catalyst system. 

Nevertheless, efforts to understand, treat, and model sintering/thermal-deactivation phenomena are easily justified. Indeed, catalyst deactivation problems greatly influence priorities in research, development, design, and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate, drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover, it may be possible under selected circumstances to reverse the sintering process through redispersion. 

Aromatics alkylation over ZSM-5 was found to be remarkably free of catalyst deactivation problems even in the absence of hydrogen or a hydrogenation component on the catalyst. Process studies indicate that this is... 

The adsorption of acids is assumed to be responsible for the catalyst deactivation observed at pH 2 which blocks further conversion of glyceric acid. This problem is alleviated simply by neutralising the acids. Thus, at pH 4, 5 and 6, conversion proceeds smoothly and there is an increase in the initial rate of reaction as the pH is increased. The anomalously high rate at pH 2 is assumed to be due to the initial rapid adsorption of glyceric acid on the platinum metal which increases the reaction rate at first since it brings the substrate into the immediate proximity of the bismuth. 

The heteroatom reactants do not themselves deactivate the catalyst the catalyst deactivates due to coke formation from hydrocarbons and metal deposition from the mineral in coal. The commercial HT process cannot be easily carried out on a bench scale, because of materials handling and pressure problems however, the process is carried out on a demonstration scale at the Advanced Coal Liquefaction Research and Development Facility at Wilsonville, AL. Small portions of the catalyst are removed at various deactivation levels, quantified as the weight of product per weight of catalyBt, with the metals and coke deposits on the removed solid being characterized. 

The deactivation of zeolite in the acylation of anisole has recently been studied [38,39], Both teams agree that the main deactivation is reversible and attributable to non-desorption of the product, acetylanisole, from the catalyst. This problem can be circumvented by modifying the ratio of reagents. Another deactivation which is not reversible is consecutive reaction of the product with the reagents or the product to form a heavy product that remains on the catalyst. Those products will also block the accessibility of the reactants to the micropores of the catalyst. This problem which is second order to the first deactivation causes slow deactivation of the zeolite. The zeolite can be regenerated by calcination with air at high temperature. A third potential deactivation that has been identified is dealu-mination of the catalyst. It has been shown that when the reaction is performed under harsh conditions (for a long time) acetic acid can extract small quantities of aluminum from the zeolite. 

The energy input requirements for TCD are significantly less than that of steam methane reforming (37.8 and 63.3 kJ/mol H2, respectively). Due to the absence of oxidants (e.g., H2O and/or O2), no carbon oxides are formed in the reaction. The choice of an efficient and durable methane decomposition catalyst is vital for the development of a TCD process. Two major problems associated with existing catalysts relate to their rapid deactivation (due to carbon deposition) and coproduction of large amounts of CO2 during the catalyst regeneration step. The successful development of efficient and stable carbon-based catalysts for a methane decomposition process can solve both the catalyst deactivation and CO2 emission problems. 

The intrinsic complexity of three phase systems creates some difficulties in the scale-up and in the prediction of performances of three phase reactors. But this complexity is also often a serious advantage, as the simultaneous occurrence of three phases offers such a large number of design possibilities that almost all technical and chemical problems (heat removal, temperature control, selectivity of the catalyst, deactivation, reactants ratio etc..,) can be solved by a proper choice of the equipment and of the operating conditions. For example, countercurrent flow of gas and liquid can be used to overcome thermodynamic limitations and solvent effects can be used to improve selectivity and resistance to poisoning of the catalyst. 

Solid acid catalysts such as clays and zeolites are also utilized for phenol acylation however, these processes suffer from catalyst deactivation problems and lack C-selectivity. In the acylation of phenol with acetic anhydride, HZSM-5 zeolite shows a very high ort/io-selectivity (48% o-HAP yield, <1% p-HAP yield), although phenyl acetate is isolated in only approximately 20% yield [115]. The SAR value has a remarkable influence on the selectivity of the process when the reaction is carried out in the presence of HZSM-5(30), HZSM-5(150), and HZSM-5(280) zeolites, the o-HAP yields are 42,40, and 15%, respectively, whereas the O-acylation is noticeably increased. These results mean that C-acylation requires higher Brpnsted acidity and that lower acidity leads to phenyl acetate formation. It must be noted that the reaction performed with an amorphous aluminosilicate acid catalyst gives mostly phenyl acetate without isomer selectivity. These results suggest that the C-acylation of phenol occurs in the channels of zeolites and not on the external surface. 

Additional theoretical investigations of the intraparticle deactivation problem, which unfortunately we cannot treat in detail here, have been reported by Luss and co-workers (28,29) on the modification of selectivity upon poisoning, and by Hegedus (30) on the combined influence of interphase and intraparticle gradients on deactivation It is of interest that deactivation in certain instances can actually have beneficial results on selectivity and in the long run the problem may be to achieve the best balance between diminished activity and enhanced selectivity such results are reminiscent of those pertaining to deactivation of bifunctional catalysts (19,20) ... 

Desalting is a water-washing operation performed at the production field and at the refinery site for additional cmde oil cleanup. If the petroleum from the separators contains water and dirt, water washing can remove much of the water-soluble minerals and entrained soflds. If these cmde oil contaminants are not removed, they can cause operating problems duting refinery processiag, such as equipment plugging and corrosion as well as catalyst deactivation. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

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