Auxiliary reboiler

Two distillation columns are heat-inicgraled as shown in the sketch below. The first column has an auxiliary condenser to take any excess vapor that the second column does not need. The second column has an auxiliary reboiler that provides additional heat if required. 

Figure 3.6a shows how costs for a new distillation system vary with the reflux ratio. It expresses the capital cost as an annual cost. This can be achieved hy dividing the capital cost by the expected payout period. A discounted cash flow (DCF) analysis is used for estimating this payout period. The capital cost shonld include the costs of auxiliaries (reboiler, condenser, vacuum equipment, pumps, piping in many cases, costs of vent systems, coolant, and heating medium handling equipment are also affected). The operating costs should include reboiler... 

The control performance of the heat-integrated reactor-column system shown in Fig.. 5.9 deteriorates as the auxiliary rehoiler provides less and less heat to the column. The reason is that uncontrolled variations in the steam pressure of the waste heat boiler affect the heat supplied to the column. When these variations are of the same order of magnitude as the total heat supplied by the auxiliary reboiler, the latter cannot compensate properly for the variations. Part of the prob-... 

Figure 5.9 Reactor/column heat integration with auxiliary reboiler in parallel.

This has implications in two different cases. For columns with auxiliary reboilers, we should obtain better control by using heat input to the base reboiler (operating at the highest temperature) compared with heat input to the other reboilers. We should also expect better control for columns with large temperature differences between the top and bottom than for columns with small temperature differences. 

The feed split is approximately 50/50. but slightly more feed goes to the low-pressure column if the separation is easier at lower pressure. The system as pictured runs neat. i.e., all the heat available from condensing the vapor from the high-pressure column is used to reboil the low-pressure column. In some systems auxiliary reboilers and/or condensers are used to balance the heat loads both at steady state and dynamically. 

This subject is discussed in detail in Chap. 5, along with the use of auxiliary reboilers and condensers as well as the idea of using total heat input controllers. 

Column pressure drop is controlled on the first column by manipulating steam flow to the auxiliary reboiler. 

A high-column-pressure override controller pinches steam to both the evaporator and the auxiliary reboiler. 

Next we smdy the case where the two columns are fuUy heat integrated and no auxiliary reboiler or condenser is used. The only energy input is the heat-transfer duty in the... 

Figure 6.21 Flowsheet equations with auxiliary reboiler.

Figure 6.23 Control structuie with auxiliary reboiler and ratio.

An alternative control structure would be to use a Total Q controller. In this scheme the heat transfer in the condenser/reboiler and the heat input in the auxiliary reboiler are added together and become the process variable signal to a Total Q controller. The setpoint of this controller is the output signal of the TCI temperature controller. The output signal of the Total Q controller goes to the control valve in the steam line feeding the auxiliary reboiler. 

The energy requirement in the low-pressure column is 14.79 MW, so the auxiliary reboiler must provide 14.79-5.86 = 8.93 MW (32GJ/h, as shown in the TCI faceplate in Fig. 6.30). Flowsheet equations similar to those described in the THF-water system are needed in this system. The pressure-compensated temperature measurement uses the temperature (411.5 K) and pressure (10.354 atm) on Stage 53. 

These results show that there is no control penalty for going to the partial heat-integrated systems when auxiliary reboilers or condensers are added. This result is to be expected because there is no loss of control degrees of freedom. 

In this section we compare the steady-state design and the dynamic control of heat-integrated extractive and pressure-swing processes. The same numbers of trays used in the base-case designs are used in both systems. The systems have not been reoptimized for heat integration. Only partial heat integration is considered in which an auxiliary reboiler is used. 

The total heat transfer in the base of the extractive column is the sum of the heat removed in the condenser of the methanol column Qrq and the heat added in the auxiliary reboiler Qavx in the extractive column. 

The two temperature controllers are toned using the methods discussed previously in Section 11.3. Tuning parameters are given in Table 11.2. Remember that the temperature controller in the extractive column is manipulating the energy input Qaux from the auxiliary reboiler (2.10 MW or 7.6 GJ/h as given in the TCI faceplate in Fig. 11.13). 

These are important for startups and shutdowns, changes in production rate, changes in fe ccanposition, and changes in product specifications. Generally speaking, however, auxiliary reboilers and condensers should be avoided, if... 

In another version a following column in the train supplies heat to a preceding column as shown in Figure 8.4. In this particular case, the first column gets only part of its heat from the second coltomn the remainder comes from an auxiliary reboiler. Interactions between the two columns may be severe. Again, for the cases studied, we have found it advantageous to let pressure find its own level in the second column, that is, the one supplying heat. 

An interesting practical problem here is how to adjust the auxiliary reboiler on the first column. After examining some complex heat-balance schemes, we decided that the simplest approach was to use column AP. Vapor flow to the first column frcan the condenser—reboiler will not be constant, but the AP control will provide a rapid method of ensuring constant boilup. The AP control, in turn, may have its set point adjusted by a composition controller for the lower section of the first coltomn. 

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