As shown elsewhere,1 the size and location of tanks and the concept of overall material-balance control used can have a great influence on plant investment and process control. If the design engineer uses the concept of control in the direction opposite to flow, tanks may be smaller and plant fixed investment and working capital can be lower than if control in the direction of flow is used. The meaning of these expressions is illustrated, for simple tanks with level controllers, in Figures 1.1 and 1.2.
For the last tank in a series (final product storage), the demand flow is always the shipments to a customer. As shown by Figures 1.3 and 1.4, the choice in control strategy is between adjusting the flow into the last tank (control in direction opposite to flow) or adjusting flow into the first process step (control in direction of flow). In the first case, we can easily use simple automatic controls. In the second case, it is more common to have an operator make the adjustment.
When more than one tank is involved, other advantages of control in the direction opposite to flow are (1) less difficulty with stability problems, and (2) reduced internal turndown requirements. "Turndown," as used here, is the ratio of maximum required flow rate to minimum required flow rate. In this instance the meaning is that, in response to a given change in demand flow, the required change in the manipulated flows will be smaller in one case than in the other.
Once the basic concept of material-balance control has been selected for a process, one must apply the same concept to all process steps. It is for this reason that the first step in designing column controls is to determine the material-balance control arrangement. Control in the direction of flow is the most commonly used concept (although the least desirable), and a frequently encountered arrangement is shown on Figure 1.5. Here level in the condensate receiver (also commonly called reflux drum or accumulator) sets the top product, or distillate flow, while the level in the base of the column sets the bottom product flow; in other columns base level sets steam or other heat-transfer media to the reboiler, in which case the condensate receiver level sets top product flow.
Generally speaking the direction of material-balance control is determined by the demand stream. In recycle systems we may find some material-balance controls in the direction of flow while others are in the direction opposite to flow.25
Material-balance control, in the direction opposite to flow, can lead to many interesting level-control and flow-ratio options. These are discussed in detail in Chapter 6.
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