Poisoning bulk-phase concentrations

The thermodynamics of nickel sulphide phases [372] indicates that the formation of a bulk phase sulphide (NisS2) at temperatures around 500-700 C requires a H2S/H2 concentration ratio in the order of 10. This ratio is about 100—1000 times above what would normally cause poisoning at those temperatures. 

Since the rate of poisoning in the fixed bed reactor is a slow process the intraparticle concentration of P, Cp, may be taken as equal to the concentration of P in the bulk gas phase. 

Mechanisms of Inhibition. A number of diffusion experiments were run in which various concentrations of N,N -diphenyloxamide (< 0.1 wt % or 3 X 10 3 mol/kg) were loaded in the polyethylene films. The great majority of runs showed essentially no effect of the additive on the diffusion rate, nor were any unusual surface phases reproducibly noted (see Tables I and III for typical results). Previous work (2,5) has indicated that the inhibition effect of a deactivator may be caused by both surface and homogeneous scavenging effects. On the basis of the present results we conclude that the major effect of the deactivator involves surface-interface reactions rather than bulk scavenging mechanisms. The former may consist of poisoning of active surface sites on the Cu20/Cu film (13) and/or conversion of an interfacial copper carboxylate layer to a relatively inert phase of insoluble copper complex (5). Work is in progress to separate these mechanisms further. 

The reference time, i f and concentration, Cp. f, are chosen for a specific application (e.g., in a flow reactor, the mean residence time and feed concentration, respectively). Equation 5.2.C-6 now permits a solution for the amount of poison, /Cpia, to be obtained as a function of the bulk concentration, Cp, and the physicochemical parameters. In a packed bed tubular reactor, Cp varies along the longitudinal direction, and so Eq. 5.2.C-6 would then be a partial differential equation coupled to the flowing fluid phase mass balance equation—these applications will be considered in Part Two—Chapter 11. 

To summarize, Eq. 5.2.C-13 gives a theoretical expression for the ratio of rate with to that without poisoning in terms of the reaction physicochemical parameters and the amount of poison C., >/Cn ). The amount of poison, in turn, is found from Eq. 5.2.C-6 with the poisoning physicochemical parameters and the fluid phase bulk concentration, Cp, at a point in the reactor. It is the only such complete case at the present time, since all other treatments require at least some empirical formulas. 

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