Hydrothermal deactivation, catalysts

For hydrothermal deactivation, catalyst samples were kept at 300"C in a nitrogen stream containing steam for 16 hours and CO oxidation activity was measured. 

Hydrothermal deactivation (catalyst), 170-171 Hydrothermal reaction, 4 Hydrotreated vacuum resid, 49... 

The activity of catalyst degrades with time. The loss of activity is primarily due to impurities in the FCC feed, such as nickel, vanadium, and sodium, and to thermal and hydrothermal deactivation mechanisms. To maintain the desired activity, fresh catalyst is continually added to the unit. Fresh catalyst is stored in a fresh catalyst hopper and, in most units, is added automatically to the regenerator via a catalyst loader. 

Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

Catalyst residence time in the stripper is determined by catalyst circulation rate and the amount of catalyst in the stripper. This amount usually corresponds to the quantity of the catalyst from the centerline of a normal bed level to the centerline of the lower steam distributor. A higher catalyst residence time, though it increases hydrothermal deactivation of the catalyst, will improve stripping efficiency. 

The regenerator design, either single-stage or two-stage, should provide uniform catalyst regeneration, increase flexibility for processing a variety of feedstocks, and minimize thermal and hydrothermal deactivation of the catalyst. 

Kiss, G., KJiewer, C. E., DeMartin, G. J., Culross, C. C., and Baumgartner, J. E. 2003. Hydrothermal deactivation of silica-supported cobalt catalysts in Fischer-Tropsch synthesis. J. Catal. 217 127-40. 

Catalyst hydrothermal deactivation was carried out in two different equipments a lOOg capacity fixed bed steamer was used for the advanced cracking evaluation (ACE) unit tests and a 5 kg capacity fluidized bed steamer was used for the other testing protocols. Steaming conditions in the two cases were the same 788°C for 5 hours under 100% steam flow. Although conditions were similar, higher pressure buildup in the fixed bed steamer led to lower surface area retentions. 

The catalyst used in this study corresponds to a fresh commercial catalyst used in one FCC unit of ECOPETROL S.A. This solid is hydrothermal deactivated at the laboratory in cycles of oxidation-reduction (air-mixture N2/Propylene) at different temperatures, different times of deactivation, with and without metals (V and Ni), and different steam partial pressures. Spent catalysts (with coke) are obtained by using microactivity test unit (MAT) with different feedstocks, which are described in Table 10.1. 

The catalysts with metals are previously impregnated with solutions of vanadyl and nickel naphtenates based on the Mitchell method [4], Before hydrothermal deactivation the samples were calcined in air at 600°C. The activity was performed in the conventional MAT test using 5 grams of catalyst, ratio cat/oil 5, stripping time 35 seconds, and reaction temperature 515°C. Elemental analyses to determine the total amount of carbon in the spent catalysts were done by the combustion method using a LECO analyzer. 

Hydrothermal Deactivation of Catalyst Impregnated with Different Levels of Metal... 

ECC catalyst is subject to hydrothermal deactivation. This occurs when the A1 atom in the zeolitic cage is removed in the presence of water vapor and temperature. The result is a loss of activity and unit conversion. The effect of temperature on this process is nonlinear. The deactivation rate increases exponentially with temperature. Units that experience high afterburn have attributed high rates of catalyst deactivation on the higher dilute phase temperatures. This phenomenon is more apparent on units with high combustion air superficial velocities. The high velocity not only increases afterburn, but also increases catalyst entrainment to the cyclones and dilute area. COP is used to decrease afterburn and minimize catalyst deactivation. 

This loop is, however, affected by the availability of the reactant oxygen, which in surplus destroys the precursor VPO. Further, oxygen is positively needed to activate and re-oxidize the VxOy sites but leads also to more water formation that in turn hydrothermally deactivates the active mass. Likewise, water is needed to separate, via hydrolysis, the vanadium phosphate into VxOy and mobile phosphate. The multiplicity of the feedback loops is at first puzzling but explains the apparent stable steady state that can be reached with a catalyst undergoing so many chemical and microstructural transformations the multiplicity of controls prevents one single factor becoming dominant and thus potentially destabilizing the whole process. 

Laboratory steam deactivations represent a significant compromise in the effort to simulate equilibrium catalyst. Since hydrothermal deactivation of FCC catalysts is not rapid in commercial practice, deactivation of the fresh catalyst in the laboratory requires accelerated techniques. The associated temperatures and steam partial pressures are often in substantial excess of those encountered in commercial units. In some instances, the effect of contaminant metals is measured by an independent test not affiliated with steam deactivation. In subsequent yields testing, interactions between different modes of deactivation may be overlooked. Finally, single mode deactivation procedures can not reproduce the complex profile of ages and levels of deactivation present in equilibrium catalyst. 

E - Severe Hydrothermal Conditions - Catalyst deactivation/ stability problems - High Activity and Stability 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. 

Figure 1 shows the effect of steam during hydrothermal deactivation for a 1990 s state of-art medium REs03 zeolite catalyst, containing also an active matrix contribution. 

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 [14t 15], more sophisticated dynamic equations will be needed to describe this behaviour also including the effects of the catalyst age distribution [15,16,17]. 

Reversible deactivation is affecting hydrothermal deactivation, via the deterioration of the coke selectivity of the catalyst and hence higher regenerator temperatures. This can continue until the regenerator reaches a new equilibrium, because of the drop in catalyst activity. 

Irreversible deactivation can have a similar effect on the hydrothermal deactivation by deteriorating coke selectivity (for instance for nickel poisoning). The hydrothermal deactivation on its turn will now also have an effect on the catalyst poisons, as for instance on the mobility of vanadium and on the deactivation of vanadium and nickel as dehydrogenation catalysts [2]. 

Short-cuts, even in the case of hydrothermal deactivation, can lead to critical errors in the performance ranking of FCC catalysts. 

Rajagopalan et al [5] brought experimental evidence that fresh catalyst is diffusionally limited when cracking West Texas heavy gas oil at 773°K. But, it is not clear whether this limitation remains after the steaming, which simulates the hydrothermal deactivation of fresh catalyst to the equilibrium catalyst. Moreover, since the result of activity in M.A.T. is the average performance of a decaying catalyst, it is impossible to determine whether the effectiveness factor of uncoked catalyst is less than 1. 

Cu-MFI catalysts have been the most extensively studied. They are very active and selective towards nitrogen under certain experimental conditions, but subjected to hydrothermal deactivation... 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

Pore size. The pore size distribution of the catalyst matrix plays a key role in the catalytic performance of the catalyst. An optimum pore size distribution usually helps in a balanced distribution of smaller and larger pores, and depends on feedstock type and cracking conditions. The pore size distribution of the matrix changes when another component is added e.g. by adding 35-40% kaolin to a silica-alumina gel, a pore structure with a significant amount of micropores can be obtained. Figure 27.9 Pore volume. Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

Hydrothermal stability is a critically important property of the constituents of FCC catalysts accordingly prototype and reference materials were subjected to a hydrothermal deactivation treatment at a temperature of 788°C, 4 and 8 hours and 100% steam. Results are summarized in Table 3. Synthetic silico-aluminate (MX-0994) and alumina/kaolin (MM-0894) base matrices, which are used in the preparation of commercialFCC catalysts designed to crack heavy feedstocks, were used as references. 

The loss in conversion is also partly caused by lower "effective catalyst activity" in the riser as a result of increased coke blockage of the catalyst pores with coke and higher vanadium and hydrothermal deactivation of the catalyst. The negative effects of resid processing on FCC yields can be reduced by adjusting the FCC process conditions (lower feed preheat, increased catalyst make-up, increased steam dispersion and stripping) and by the use of FCC catalyst formulations more suitable to such applications. 

Properties of cerium-zeolite catalysts and the lost of area after hydrothermal deactivation by steaming. 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

Regarding the composition of diesel exhaust gases (containing amongst others water and SO2), developing a stable, zeolite based diesel exhaust deNOx catalyst is a challenging task. Zeolites can show dealumination under hydrothermal conditions accompanied by a loss of active material furthermore SO2 can also cause deactivation. Many authors already have reported on the hydrothermal stability of zeolite SCR catalysts [e.g. 7-9] and also some papers exist on the stabilization with respect to hydrothermal deactivation of zeolite SCR catalysts by the choice of proper cations [10-13]. A small number of articles describes the influence of SO2 on zeolite SCR catalysts [14-17]. The current paper gives the results of measurements on both the short term hydrothermal stability and the influence of SO2 on CeNa-MOR and CeH-ZSM-5 zeolite catalysts. 

Deactivation Mechanisms of Cu/Zeolite SCR Catalysts 5.3.1 Hydrothermal Deactivation... 

Hydrothermal deactivation. Caused by exposing catalyst to a steam atmosphere at high (1,300+°F) temperature. Since most of the water vapor in... 

Inject water in the regenerator torch oil nozzles. Of course, this accelerates hydrothermal deactivation of the catalyst. 

Remove oil of catalyst Increased regenerator air Higher regenerator temperature Loss product yield Hydrothermal deactivation Alter steam rate to test Change riser temperature with steam Check CO/CO2 ratio... 

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