Time hrs

Figure 7.4 - Transient Response to Catalyst Ageing Ambient Temperature Rise

The reflux rate was varied in a sinusoidal pattern over a period of six hours (with a minimum of 80% of the initial value after three hours, corresponding to the daily maximum temperature) to simulate an ambient temperature rise in the morning followed by a temperature fall in the afternoon. The initial and final conditions are equivalent to the steady state described by Table 7.1. The reboiler duty was constant throughout the simulation.

This scenario corresponds to a process where the reflux drum level is controlled by manipulating the condenser duty, and the condenser is directly and strongly affected by ambient conditions (e.g. an air cooler operating at its maximum speed). In practice, the reflux drum would essentially run dry and the reflux flow control valve would saturate during periods of high ambient temperature, causing the reflux rate to fall as the ambient temperature rises. The simulation output reflects the changes to operation that would occur if the reboiler duty was not manipulated to compensate for the fall in reflux rate.

The column response to the change in reflux is shown in Figure 7.5. The changes are dramatic and clearly show the need to constantly monitor and adjust the reboiler duty to maintain acceptable operating conditions following a loss of condenser duty. The hiccup in the response that occurs between 80 and 90 minutes after the start of the simulation appears to be a result of the column readjusting itself to the relatively fast changes that occurred. The reaction conditions are temporarily stabilised due to interactions between the changing composition and reaction equilibrium. The shape of the response was not affected by rounding errors in the simulation. The response is not symmetrical due to the lags and dynamics present in the system.

Figure 7.5 - Transient Response to Variation in Ambient Temperature

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Time (min)

Figure 7.5 - Transient Response to Variation in Ambient Temperature

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