Aging catalyst

The Chevron process was used in two U.S. plants, although it is no longer used. Cycle lengths tanged from 6—30 d, depending on catalyst age and OX content of the feed. Operating conditions were temperature of 370—470°C and space velocity of about 0.5/h. Addition of 5 wt % steam reduced disproportionation losses. 

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138). 

Al Ti in the range of 0.9—1.0 appeared optimum for i7j -l,4-polyisoprene yield (20). Other factors such as catalyst preparation temperature, influence of the R group in the alkyl aluminum compound (R Al), and catalyst aging have been extensively studied (16,17). Another variable studied was the effect of... 

In other instances, reaction kinetic data provide an insight into the rate-controlling steps but not the reaction mechanism see, for example, Hougen and Watson s analysis of the kinetics of the hydrogenation of mixed isooctenes (16). Analysis of kinetic data can, however, yield a convenient analytical insight into the relative catalyst activities, and the effects of such factors as catalyst age, temperature, and feed-gas impurities on the catalyst. 

Modeling. There is as yet no rapid simulated laboratory aging test for catalysts that is recognized as a good predictor of catalyst aging in the vehicle. 

Fig. 25. Experimental and predicted mid-bed temperature. Vehicle Galaxie, 289 cu. in. engine with thermactor. Converter radial, 392 cu. in. Catalyst aged type F (12,500...

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

As a result, selectivity is not optimal and excessive hydrocracking results (10). Catalyst aging is also excessive. 

In this chapter a two a selectivity model is proposed that is based on the premise that the total product distribution from an Fe-low-temperature Fischer-Tropsch (LIFT) process is a combination of two separate product spectrums that are produced on two different surfaces of the catalyst. A carbide surface is proposed for the production of hydrocarbons (including n- and iso-paraffins and internal olefins), and an oxide surface is proposed for the production of light hydrocarbons (including n-paraffins, 1-olefins, and oxygenates) and the water-gas shift (WGS) reaction. This model was tested against a number of Fe-catalyzed FT runs with full selectivity data available and with catalyst age up to 1,000 h. In all cases the experimental observations could be justified in terms of the model proposed. 

There are also changes that occur while the catalyst ages. For instance, it was observed that the methane and C02 selectivities of the Fe-LTFT synthesis increase concomitantly with increasing catalyst age, as illustrated in Figure 13.12. From... 

FIGURE 13.12 Change in CH4 and C02 selectivities as the Fe-LTFT catalyst ages. 

Janse van Vuuren, M. J., Huyser, J., Kupi, G., and Grobler, T. 2008. Understanding Fe-LTFT selectivity changes with catalyst age. Prepr. Pap.-Am. Chem. Soc. Div. Petrol. Chem. 53 129-30. 

Figure 1.19 AES data from a Ru/Al203 catalyst aged in a reaction (CO+H2) mixture containing trace amounts of H2S [148], Spectra are shown for the sample before (a) and after (b) sputtering with an Ar+ beam for 2 min. The difference between the two spectra indicates the presence of S on the surface but not the subsurface of the poisoned catalyst. (Reproduced with permission from Elsevier.)...

Tail gas cleanup is required because a well-designed Claus plant with three catalytic stages and fresh catalyst will recover only 95-97% of its feed sulfur (8), which is not generally sufficient to meet current emission standards. In addition, feed impurities and catalyst aging will reduce overall recovery in some plants to about 92% just before catalyst changeout. Therefore, tail-gas cleanup is required. Tail-gas treating processes are generally classified as follows ... 

As discussed above, the potential octane boost which can be achieved from ZSM-5 addition is a function of five parameters the regenerator temperature and steam partial pressure (which determine the activity maintenance) the base and ZSM-5 catalyst makeup rates (which determine the catalyst age) and the base gasoline octane. The sensitivity of the model to these parameters is discussed below. 

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