Other Transient Responses

The basic dynamic model was modified to simulate two other disturbances to the reactive distillation system. Firstly, catalyst ageing was simulated by re-extending the model to incorporate reaction kinetics and decreasing the amount of active catalyst present according to a simple ramp function. Secondly, changing ambient temperature was simulated by modifying the reflux rate according to a pattern that might result if an air cooler operated at its duty limit with a control configuration that manipulated the reflux drum level with the condenser duty.

7.4.2.1 Catalyst Ageing

Amberlyst 15™ is susceptible to both short term poisoning and long term deactivation. A time frame of 15 hours was assumed here for the amount of active catalyst to linearly decrease to 10% of the initial catalyst load, mcat (equivalent to 1% deactivation every 10 minutes), to simulate severe short term poisoning. The initial catalyst load was estimated based on the dimensions of the pilot scale column.

The full reaction rate equation, proposed for conditions where the ethanol concentration is below 4.0mol% (Jensen, 1996), was used to provide the required kinetic information. However, the ethanol adsorption equilibrium constant (KA) was assumed to be constant over the reaction zone temperatures to reduce the total number of equations to be solved. The additional equations, given below, replace equation 7.5 in the basic dynamic model (equations 7.1-7.47).

ai But

ETBE

ETBE

krate=7.418*10"exp\-

Figure 7.4 shows the column response. Until the amount of active catalyst falls to around 20% of the starting load, the reductions in conversion and product purity are only slight and would probably go undetected in an industrial environment. However, after that point the effects become more exaggerated and would necessitate a unit shutdown after only a few more hours if the poisoning was allowed to continue unabated. Longer term catalyst ageing (deactivation) would probably follow a similar path on a different time scale.

Figure 7.4 shows the column response. Until the amount of active catalyst falls to around 20% of the starting load, the reductions in conversion and product purity are only slight and would probably go undetected in an industrial environment. However, after that point the effects become more exaggerated and would necessitate a unit shutdown after only a few more hours if the poisoning was allowed to continue unabated. Longer term catalyst ageing (deactivation) would probably follow a similar path on a different time scale.

Catalyst Ageing

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