Open Loop Control

The most basic distillation control system considers only the column inventory and relies on the process operator to counteract disturbances to the process by adjusting (when required) one or both of the manipulated variables which are not being used for inventory control. The effectiveness of this approach depends on the variable pairings (i.e. the control configuration).

It is convenient to adopt a nomenclature to concisely describe the variable pairings or control configuration. The most widely accepted method of describing control configurations employs two letter designations that correspond to the variables which are not used for inventory control. For example, the LV configuration uses the distillate product draw rate (D), the bottoms rate (B) and the condenser duty (Qc) to control the reflux accumulator level, the reboiler sump level and the column pressure, respectively.

The requirement for dynamic responsiveness in the inventory control loops (as described in Section 9.2) restricts the number of acceptable control configurations. Effectively, one of the distillate rate and reflux rate and one of the bottoms rate and reboiler duty must be used for inventory control. Thus, only four pairs of manipulated variables are suitable: LV, DV, LB and DB. Of these, the DB configuration cannot be used in open-loop as the two manipulated variables are not independent at steady state because of the column material balance. Ratios and other linear combinations of the manipulated variables can also be considered so that many more control configurations are made available. The (L/D)V configuration (also known as Ryskamp's scheme: Ryskamp, 1980) and the (L/D)(V/B) or double ratio configuration are prominent among the ratio control schemes. Ratios involving the feed rate (F) are also possible, for example, the (D/F)V configuration which effectively incorporates steady state feed-forward control into the control structure.

Every control structure has the capability to reject (or attenuate) certain disturbances without requiring the intervention of the process operator. For example, in ideal binary distillation where the constant molar overflow assumption applies, constant vapour and liquid loadings (i.e. constant reflux rate and reboiler duty) presupposes constant fractionation. Therefore, if the feed rate increases, the product compositions will change only very slightly if no corrective action is taken. Similarly, if the column material balance is fixed via one of the product draw rates, at least one of the product compositions will remain close to its initial value following small changes in the feed composition without any change in the manipulated variables.

The open-loop disturbance rejection of several control structures were determined for the 10 stage ETBE hybrid reactive distillation column described in Chapter 3 using the SpeedUp™ dynamic simulation model. The results from this study are shown in Table 9.1 and indicate that only the double ratio configuration provided effective rejection of feed rate disturbances and that only the (L/D)V configuration was effective against feed composition disturbances. This is not surprising considering the narrow range of acceptable operating conditions predicted in Chapter 4. In practice, none of the control structures that were examined could be used successfully in open-loop operation of the ETBE column without regular intervention from the process operator. This suggests that some form of closed-loop composition control will always be required.

Table 9.1 - Oncn Loop Gains lor feed Rale and Feed Composition Disturbances

Feed Rate Disturbance

Feed Composition Disturbance

LV

-3.6°C / % feed rate

-0.45°C / % excess

DV

-3.8°C / % feed rate

+0.50°C / % excess

LB

+2.6°C / % feed rate

-6.7°C / % excess

(L/D)V

-3.4°C / % feed rate

-0.01°C / % excess

(L/D)(V/B)

+0.01°C/% feed rate

-3.2°C/% excess

(D/F)V

-0.4°C / % feed rate

+0.50°C/% excess

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