Poisoning sensitivity coefficient

The minimum concentration required to eliminate the catalytic activity is one possible measure of the sensitivity of a catalyst to a poison. Sensitivity to poisoning is most properly defined by the amount, np, of the poison adsorbed on a unit amount of catalysts which causes a given fractional decrease of the catalytic activity (a = (a0 - ap)/a0), where aQ and ap are the activities in the absence or presence of poison, respectively) ot/rtp is a measure of the sensitivity to poisoning. One may also use the ratio of a to the concentration cp of the poison in the feed, namely a/cp, but this is less precise, as this depends on the adsorption coefficient of the poison. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

There is actually a whole spectrum of possible phenomena, between very strong irreversible poisoning and normal competition between molecules for a given site (this competition leading to a decrease of activity). Inhibitors, as defined above, correspond to a behaviour intermediate between those just mentioned. The sensitivity to inhibitors can thus be expressed either as in the case of true poisons, or as in the case of competition. In the latter formulation, for example, the value or relative value, of the adsorption coefficient could characterize the inhibitor. 

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