Catalyst time-on-stream

Catalyst° Time on Stream, hrs Inlet S/G Inlet Hot Spot Outlet... 

Fig. 7.4 Comparison of Mordenite (250°C) vs. Sulfated Zirconia (200°C) based Isomerization Catalysts Time on stream...

Catalyst Time on stream h % w/w in liquid di- tri- tetra- Conversion % w/w... 

Usually, these catalysts are characterized in terms of their efficiency in some test reaction, usually CO oxidation, under arbitrary conditions (O2 and CO concentrations, humidity, flow rates, and catalyst time on stream) that would be difficult to reproduce in another laboratory. Characterization on any length scale, from molecular to morphological, is extremely difficult. In many cases, it is uncertain whether all the reactants and by-products have been removed from the catalyst surface. It is reported that chlorides poison the active sites. This raises the question of a role for gold ions, and these, rather than Au(0), are reported to be the active sites in the water-gas shift reaction. ... 

Kinetic measurements were made at 623 - 773K using a circulatory flow installation. Reactions were studied in the fixed bed catalyst. Time - on- stream was varied within the range 1,5 - 10s at a reactant ratio of C2H6 HCI 0 2=1 1 - -3,3, 1 1,4. Air was used as a source of oxygen. 

Figure 3 Benzene conversion as function of catalyst time on stream at different temperatures...

An increase of nitrous oxide feed concentration led to an increase inreaction rate and benzene conversion. On the other hand benzene conversion decreased with increasing benzene feed concentration (Table 2). Phenol yield basically changed in the same way as benzene conversion in all cases. Upon variation of the feed concentrations, the highest phenol production was obtained at 26% nitrous oxide and 12.5% benzene at T=400 C, W/F=92 gmin/mol and a catalyst time on stream of 40 minutes [5-7]. 

Table2 Benzene conversion as a function of nitrous oxide and benzene partial pressures (temperature=400 "C, W/F= 92 g min/mol,catalyst time-on-stream-40 minutes)...

The conversion of chloromethane over ZSM-5 to gasoline-range hydrocarbons occurred under conditions comparable to those for the conversion of methanol. The reaction was typically conducted at constant temperature, whereas conversions and product distributions were determined as functions of space velocity or catalyst time-on-stream. The mass-selective detector allowed identification of most of the components in the liquid samples. Generally, the products contain ten carbons or less, and a large fraction of the products are aromatic. The ZSM-5 catalyst was stable under extended exposure to chloromethanes. Figure 2 illustrates the catalyst activity after nearly 700 hours of exposure to chloromethane, during which time the catalyst had been oxidatively regenerated to remove coke that had been deposited on the catalyst. 

To further understand the role of strong acid sites in the aromatization activity of the catalysts, time-on-stream (TOS) activity studies were carried out over these three catalysts, and the deactivation patterns of acid sites were studied. It can be seen from fig.4 that, the as-synthesized catalysts T and WT show a constant aromatization activity, where as the hydrothermally treated sample (HT) shows rapid deactivation with in 12 hrs. The highest initial activity in aromatization with the steep fall within 12 hrs TOS observed in case of the catalyst HT, can be understood by considering the high turn over number of super acid sites created during hydrothermal treatment, and their propensity to rapid deactivation (8). 

Catalyst Time on stream (h) T( K) Activity xlO3 /mol IT1 (g catal.y1... 

The heterogeneous catalysts used in the past had some drawbacks such as incomplete conversion in most cases, low selectivity because of consecutive aldol condensation to form preferably trimers and low service time [i.e. catalyst time on stream (TOS)]. 

However, it should be noted that, in the open scientific literature, there is a lack of information regarding the catalyst time on stream and the effect of some impurities on catalyst performance, such as higher hydrocarbons or sulfur compounds still contained in the methane stream. Therefore, future investigation should be realized for filling these lacks. 

One of the earliest and most important studies on the kinetics of coking was published by Voorhies (1945). The simple rate law developed fi om his work has played a vital role in the design of most commercial FCC units. The amount of coke deposited on both natural and synthetic clay catalysts at various temperatures and with a wide variety of charge stocks was studied by Voorhies. He showed that coke formation was primarily a function of catalyst to oil contact time or catalyst time-on-stream, t ... 

Levenspiel (1972) and Wojciechowski (1968) developed similar expressions but based on the catalyst time-on-stream as the main variable. This approach also used by Gustafson (1972) and Weekman (1969) includes a decreasing exponential decay function. This so-called exponential law is a particular case of a more general power law function which has been used in various forms by various researchers to fit their particular experimental data (Wojciechowski, 1974 Pachovsky et al., 1973 Newson, 1975 Mann et al., 1986 Mann and Thomson, 1987 Tan and Fuller, 1970 Forzatti et al., 1984 Fuentes, 1985 El-Kady and Mann, 1982a, 1982b). 

A deactivation function is needed to account for the catalyst activity decay due to coke deposition on the catalyst. The various forms of this function were discussed in Chapter 2 where can be a function of catalyst time-on-stream or more appropriately as a function of coke content on the catalyst. The kinetic constant, k, is the overall gas oil cracking rate constant which is the sum of ki and kj from the 3-lump scheme. Substituting this value for rA in equation (3.1) results in ... 

S Symbol for aromatic substituent groups in the 10-lump model t Catalyst time on stream (s)... 

Time-on-stream behaviour of 25wt%VPO/Ti02 (0.7 P/V) catalyst Time-on-stream behavioirr of 25wt%... 

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