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Figure 13.12 (Continued ).

peak errors for the temperature and composition control are roughly the same, except that offset-free product composition can be obtained (Figs. 13.6d and 13.12d). Thus, composition control may be an attractive alternative for the BuAc system. For the AmAc system, the peak errors are almost the same for both temperature control and composition control structures. However, the steady-state offset can be eliminated using one composition loop. This is another likely system to consider for composition control (Figs. 13.6e and 13.12e). However, asymmetrical responses can also be seen for the AmAc system.

For feed ratio disturbances, the responses of the EtAc, IPAc and BuAc systems are symmetrical with acceptable speeds of response (Fig. 13.13). Again, oscillatory behaviors are observed for the MeAc system (Fig. 13.13a) and limited operability can be seen for the AmAc system (can handle only +2% feed ratio changes) as shown in Figure 13.13e. Table 13.3 also gives the dynamic performance for one-temperature, one-composition control for all five systems.

In summary, one-temperature, one-composition control is recommended for the BuAc system, in which steady-state error can be eliminated without sacrificing closed-loop dynamics. For the AmAc system, we observed limited operability (Fig. 13.13e) when composition control is applied. For the MeAc system, composition control leads to a large peak error and oscillatory response. Therefore, a tradeoff has to be made between the off-spec acetate composition and poor control performance. An alternative is to use a temperature/composition cascade to solve this problem (e.g., CS3 in Figs. 12.58 and 12.84). As for the EtAc and IPAc systems, composition control provides little improvement in terms of the steady-state error.

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Figure 13.13 Composition control responses for +5% feed ratio changes for (a) MeAc, (b) EtAc, (c) IPAc, and (d) BuAc systems; and (e) +2% changes for AmAc system.

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Figure 13.13 Composition control responses for +5% feed ratio changes for (a) MeAc, (b) EtAc, (c) IPAc, and (d) BuAc systems; and (e) +2% changes for AmAc system.

Time (h)

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