Poison deposition

Complete characterization of poisoned catalysts, of course, requires much more than chemical analysis. For example, the interaction of poisons with catalyst constituents and with each other has been studied by X-ray diffraction and by electron microscopy, the morphology of the poison deposits by optical methods, the distribution within the catalyst pellets and washcoats by the microprobe, and the distribution of poison on the surface of the active metals by Auger spectroscopy. 

In heterogeneous catalysis, chemisorption of reagents is a preliminary step of the surface reaction. Thus every discussion about catalytic modification (activity, selectivity, and lifetime of the catalyst), induced by poison deposition, has to be carried out in parallel with a study of the poison... 

Pore mouth deactivation by poisoning or fouling is more likely when precursor molecules are large and the pores are narrow and long. These factors make the Thiele modulus for poison deposition, h, large. In such instances the poison precursor molecules will reside in the vicinity of the pore mouth longer and be more likely to lie down there. 

Conversions and production levels both increased with increases in the pellet deactivation time (pore-mouth) or time constant for poison deposition (uniform). 

One of the difficulties to study this catalyst is the possible influence of the poisons deposited on the active phases during the test and originated from the gasohne or motor oil components such as Si, Ca, P, Zn, S. .. [11,12]. In particular, the TPR study may become totally erroneous if additional reducible compounds are present. To take into account this influence and to evidence an eventual aging gradient along the axis of the monolith, three samples were selected after the test, at the inlet, in the middle and at the outlet of the monohth. 

ESCA and Auger ° may be employed to provide surface-sensitive detection of the deposited poisons. ESCA has been employed in a limited number of studies (e.g., the study of SCR catalysts O). The sensitivity of these last techniques is relatively less than the elemental analyses. However these are more surface sensitive. This also means that only poisons deposited on the particle exterior (and not within the interior of the pore network) will, in general, be detected. 

Poison Deposition on the Catalyst. XRF analysis of spent catalyst units revealed that only a small percentage of the lead and phosphorus burned in the engine was retained by the catalyst. This agrees with the concept of particulate formation upon combustion in the engine with resultant low retention on the catalyst. Most of the lead and phosphorus species analyzed on the catalyst probably are present as particulates, and toxic species on the catalyst constitute only a very small portion of the... 

We also identified a first order relation for poison deposition on the catalyst. Therefore, accumulation of poison on any element of catalyst is directly proportional to the local concentration of poison in the gas phase, and the poisoning process may be diffusionally controlled. 

The geometric configurations of the catalysts may be ranked by their effectiveness factors, but a better measure is their CO conversion efficiency under identical inlet concentrations, space velocities, and temperatures. Catalyst deterioration caused by poison deposition and thermal damage should also be considered. These factors should be quantified in order to ascertain the optimum thickness of catalytic layers. 

A similar model that specifically considers the poison deposition in a catalyst pellet was presented by Olson [5] and Carberry and Gorring [6], Here the poison is assumed to deposit in the catalyst as a moving boundary of a poisoned shell surrounding an unpoisoned core, as in an adsorption situation. These types of models are also often used for noncatalytic heterogeneous reactions, which was discussed in detail in Chapter 4. The pseudo-steady-state assumption is made that the boundary moves rather slowly compared to the poison diffusion or reaction rates. Then, steady-state diffusion results can be used for the shell, and the total mass transfer resistance consists of the usual external interfacial, pore diffusion, and boundary chemical reaction steps in series. 

The mathematical statement of the rate of poison deposition is as follows ... 

Further extensions of these catalyst poisoning models to complex reactions have been made by Sada and Wen [9]. The poison deposition was described as in Eq. 

S. 1 For shell progressive poisoning, the shrinking core model of Chapter 4 was utilized to derive the time rate of change td poison deposition. Eq. S.2c-6 complete the steps leading to this result. 

The amount of poison deposited is given as a function of the dimensionless process time by Fig. 5.2a -l. Also, the deactivation function for given poison levels is in Fig. 5.2.C-2. Combine these in a figure for the deactivation function as a function of dimensionless time for the shell progressive mechanism. 

This is an implicit equation for Cpj)/Cpj, which is represented in Fig. 5.2.3-1. These results can be used to predict the poison deposition as a function of time and the physiochemical parameters. 

Sada and Wen [1967] extended these catalyst poisoning models to sets of simultaneous reactions. The poison deposition was described as in (5.2.3-7), but for very rapid poisoning, Da . The results were expressed in terms of the dimensionless position of the poison boundary, = tc I R. The profiles are described by... 

Arrhenius nuinbers for CO oxidation and selective poison deposition respectively, Ecq/ To E VRTq diffusivity ratio, dimensionless axial coordinate, z/L... 

Alkali metals The majority of aU alkali metals report to char so not a big problem, high ash feed, incomplete solids separation Catalyst poisoning, deposition of solids in combustion, erosion and corrosion, slag formation, damage to turbines... 

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