A new approach to the modeling of deactivation in the conversion of methanol on zeolite catalysts

The deactivation of a zeolite catalyst in the conversion of methanol to hydrocarbons is described as a reduction of the effective amount of catalyst with time on stream. With the assumptions that the conversion of methanol is a first-order reaction, and that the loss of active catalyst is proportional to the conversion, an expression for the conversion with time on stream is obtained, which describes the experimental data well. This expression contains the rate constant, that characterizes the activity, and a deactivation coefficient that describes the deactivation behavior as parameters. It is shown that active catalysts show a more sudden decrease in conversion, and that the deactivation rate determines the time at which the decrease in conversion is observed. If the initial conversion is close to 100%, the lifetime to 50% conversion does not depend on the activity, and the deactivation coefficient is directly derived from the experimental data, by dividing the measured lifetime to 50% conversion by the applied contact time. The lifetime to all other conversion levels is dependent on both deactivation and activity, which implies that a catalyst lifetime to breakthrough of methanol does not scale with the deactivation rate. Likewise, it is shown that the conversion capacity is a good characterization of the deactivation, and this can be readily calculated as the product of the space velocity of methanol (WHSV) and the lifetime to 50% conversion. The amount of converted methanol at other conversion levels depends on the deactivation, the activity, and applied contact time (space velocity), and is therefore less appropriate to use as a characterization of the deactivation behavior.