^^he feed system for a column should function as a filter for incoming disturbances in feed flow rate, feed composition, and sometimes feed enthalpy. For minimum energy consumption operation, it should also send the feed to the proper feed tray. And for startup/shutdown of the column being fed, it may serve to receive recycled column product streams. In the following discussion, we will assume that (1) process material-balance control is in the direction of flow, (2) feed-tank level control is of the averaging type, and (3) there is good mixing in the tank.
Figure 5.1 shows three commonly encountered feed flow schemes. When the upstream pressure is higher than the column pressure, then only a letdown valve is required, as shown by Figure 5.1 A. If column pressure is greater than upstream pressure, then a pump is required, as shown by Figure 5. IB. If, however, upstream or downstream pressures can vary significantly, then a cascade level control/liquid flow control system such as that of Figure 5.1C is required. The flow signal should be linear—one should use a linear flowmeter or a square-root extractor with an orifice and AP transmitter. This (Figure 5.1C) is really the preferred overall design; it provides the most protection and offers the operator the maximum flexibility.
For small feed rates, it may be considerably cheaper to use a positive-displacement pump (piston type). Control may be by stroke adjustment on a constant-speed pump as shown by Figure 5.2A or by adjustment of a variablespeed drive of a pump with a fixed stroke as shown by Figure 5.2B. If the minimum stroke rate is at least three times the reciprocal of the feed-tray holdup time, no pulsation damper is required.
Some care should be taken in locating the feed control valve if flashing can occur. Hydraulic problems have been experienced in columns where the feed
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