Single Pellets

When a carrier is impregnated with a solution, where the catalyst deposits will depend on the rate of diffusion and the rate of adsorption on the carrier. Many studies have been made of Pt deposition from chloroplatinic acid (HgPtClg) with a variety of acids and salts as coim-pregnants. HCl results in uniform deposition of Pt. Citric or oxalic acid drive the Pt to the interior. HF coimpregnant produces an egg white profile. Photographs show such varied distributions in a single pellet. 

C. Cavalca, G. Larsen, C.G. Vayenas, and G. Haller, Electrochemical Modification of CH3OH oxidation selectivity and activity on a Pt single-pellet catalytic reactor, /. Phys. Chem. 97, 6115-6119(1993). 

I.V. Yentekakis, and S. Bebelis, Study of the NEMCA effect in a single-pellet catalytic reactor, J. Catal. 137, 278-283 (1992). 

Figure 8.57. Effect of catalyst potential on the rates of formation of C2H6, C2H4) HzCO, CH3OH and CH3CHO during CO hydrogenation on Pd/YSZ. The rate of CH4 formation is of the order 10 9 mol/s and is only weakly affected by UWr Single pellet design P=12.5 bar, T=350°C. pH2/pco= -8, flowrate 85 cm3 STP/min.5 59...

Last but not least, one should check the inertness of the auxiliary electrodes in single-pellet arrangements, both under open and closed circuit conditions and, also, via the closure of the carbon balance, the appearance of coke deposition. This is especially important in systems with a variety of products (e.g. selective oxidations), where the exact value of selectivity towards specific products is of key interest. This in turn points out the importance of the use of a good analytical system and of its careful calibration. 

The minute sample sizes allowed in SFE-SFC analysis (typically 0.5 mg cf. the approximate weight of 30 mg for a single pellet), which is several orders of magnitude smaller than the sample weights used in GC, HPLC or IR analysis (5-10g), allows us to perform additive dispersion studies on a pellet-to-pellet basis [106]. 

A single pellet of resin is exposed to a flow of solution and the temperature is maintained constant. The take-up of exchanged ion is followed automatically and the following results are obtained ... 

Further examples are furnished by the spectra of Figs. 10 and 11. A single pellet of virgin catalyst ( 7 x 10mm) was placed in a cell (Fig. 2) and degassed at room temperature, and spectrum S2 was recorded (the main spectral features are the strong absorptions of the kieselguhr support, but some sulfate absorptions can also be discerned). The catalyst was then exposed to 90 torr of SO2 at room temperature and spectrum was recorded with SO2 in the cell, when new features appeared. 

There have been many studies of the effect of boundary films on mass and heat transfer to single pellets and in packed beds, including the work of Ranz and Marshall 27 and Dwivedi and Upadhey(28). Other theories of mass and heat transfer are discussed in Volume 1, Chapter 10, although only the steady-state film-theory is considered here. It is assumed that the difference in concentration and temperature between the bulk fluid and the external surface of a pellet is confined to a narrow laminar boundary-layer in which the possibility of accumulation of adsorbate or of heat is neglected. 

When used as part of a commercial operation with gas or liquid mixtures, the single pellets discussed in the context of rate processes are consolidated in the form of packed beds. Usually the beds are stationary and the feed is switched to a second bed when the first becomes saturated. Whilst there are applications for moving-beds, as discussed later, only fixed-bed equipment will be considered, here as this is the most widely used type. 

A single pellet of alumina is exposed to a flow of humid air at a constant temperature. The increase in mass of the pellet is followed automatically, yielding the following results ... 

The optimal distribution of silver catalyst in a-Al203 pellets is investigated experimentally for the ethylene epoxidation reaction network, using a novel single-pellet reactor. Previous theoretical work suggests that a Dirac-delta type distribution of the catalyst is optimal. This distribution is approximated in practice by a step-distribution of narrow width. The effect of the location and width of the active layer on the conversion of ethylene and the selectivity to ethylene oxide, for various ethylene feed concentrations and reaction temperatures, is discussed. The results clearly demonstrate that for optimum selectivity, the silver catalyst should be placed in a thin layer at the external surface of the pellet. 

Although this reaction network has been studied extensively, its mechanism is still under debate (10). In this study, a single-pellet reactor was used, and the pellet was prepared mechanically by pressing the active catalyst layer between two alumina layers. In this way a step-type catalyst pellet was produced, which approximated a Dirac-type catalyst distribution. 

Figure 1. Single-pellet reactor. (Reproduced with permission from ref. 9. Copyright 1992 AIChE.)...

A natural extension of the investigations of single-pellet micro-imaging is to explore the effect of pore structure on the spatial distribution of coke deposition... 

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