Poisons Order

When the Levenspiel model was used, the poison orders were significantly different. In fact, while poison order corresponding to acetic acid was 1.3, the poison order for 3,5-DMP was close to 1. As consequence of this, the comparison of the values of the intrinsic deactivation constant, kj, is not straightforward. On the other hand, it is important to note that the Levenspiel model only allows to describe the initial deactivation behavior due to the activity decay curves reach a pseudo-steady state. On the contrary, all experimental data,... 

Experiments involving the use of dimethyl sulphate should be carried out by students only under immediate supervision. Not only is the vapour of dimethyl sulphate highly poisonousy but the cold liquid itself is absorbed easily through the skin, with toxic results individual susceptibility to ditnethyl sulphate poisoning varies and may be very high. If the sulphate is splashed on to the hands, wash immediately with plenty of concentrated ammonia solution in order to hydrolyse the methyl sulphate before it can be absorbed through the skin (see p. 528). 

The palladium may be recovered by heating the spent catalyst to redness in order to remove organic impurities this treatment may reduce some of the barium sulphate to barium sulphide, which acts as a catalytic poison. The palladium is then dissolved out with aqua regia and the solution evaporated the residue is dissolved in hot water and hydrochloric acid to form palladium chloride. 

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

Catalysis by Metal Sulfides. Metal sulfides such as M0S2, WS2, and many others catalyze numerous reactions that are catalyzed by metals (98). The metal sulfides are typically several orders of magnitude less active than the metals, but they have the unique advantage of not being poisoned by sulfur compounds. They are thus good catalysts for appHcations with sulfur-containing feeds, including many fossil fuels. 

Most refinery/petrochemical processes produce ethylene that contains trace amounts of acetylene, which is difficult to remove even with cryogenic distillation. Frequently it is necessary to lower the acetylene concentration from several hundreds ppm to < 10 ppm in order to avoid poisoning catalysts used in subsequent ethylene consuming processes, such as polymeri2ation to polyethylene. This can be accompHshed with catalytic hydrogenation according to the equation. 

To find the effectiveness under poisoned conditions, this form of the modulus is substituted into the appropriate relation for effec tiveness. For first-order reaction in slab geometry, for instance,... 

The three preceding equations may be solved simultaneously by the shooting method. A result for a first-order reaction is shown in Fig. 23-20, together with the case of uniform poisoning. 

FIG. 23-20 Poisoning of a first-order reaction (a ) uniform poisoning, (h) pore month poisoning. 

Interest in the ZGB model arises due to its rieh and eomplex irreversible eritieal behavior. In faet, in two dimensions and for the asymptotie regime (t — cxd), the system reaehes a stationary state whose nature solely depends on the parameter 7 - For Fa < Fia = 0.3874 (Fa > F2A = 0.5250) the surfaee beeomes irreversibly poisoned by B (A) speeies, while for Fia < Fa < F2A a steady state with sustained produetion of AB is observed. Fig. 2 shows plots of the rate of AB produetion (J ab) and the surfaee eoverage with A ( a) and B ( s) speeies versus Fa. So, just at Fja and F2A the ZGB model exhibits IPTs between the reaetive regime and poisoned states, whieh are of seeond and first order, respeetively. Experimental evidenee of a first-order transition-like behavior has been reported for the eatalytie oxidation of earbon monoxide on Pt(210) and Pt(lll) [19], as shown, e.g., in Fig. 3. 

Negleeting CO desorption, as in the standard ZGB model, the CO-poi-soned state is irreversible sinee there is no possibility of removing CO from the surfaee. So, CO desorption has to be eonsidered in order to avoid the fully CO-poisoned state. The adsorption and desorption of X then drives the system from a state with high eoneentration of adsorbed CO to the reaetive state and baek. This proeess ean be understood with the aid of Fig. 8. At low X eoverage only the reaetive state is stable. Inereasing X eoverage eauses site bloeking and eonsequently the adsorption of both CO and O2 is redueed. 

The phase diagram of the MM model is quite simple for I"a< — 1/2 (7a > 7]a) the catalyst becomes poisoned by A (B) species, respectively. Thus one has a first-order IPT where 7ia = 1/2 is a trivial critical point given by the stoichiometry of the reaction. In contrast to the ZGB model. 

The MM model with one species desorption (say B) has also been studied [90]. Due to desorption, the B-poisoned state is no longer observed and the system undergoes a second-order IPT between a reactive regime and an A-poisoned state. The behavior of the MM model with one species desorption is similar to another variant of the MM model which incorporates the Eley-Rideal mechanism [57]. 

J. W. Evans, M. S. Miesch. Catalytic reaction kinetics near a first-order poisoning transition. Surf Sci 245 401-410, 1991. 

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