Although this book deals primarily with distillation control in design projects, it is pertinent to consider briefly the controls of typical, existing columns, the opportunities for their improvement, and how to troubleshoot them when necessary. Frequently encountered problems include unstable or ineffective controls, off-specification product or products, and flooding or dumping. In addition, it is fairly common practice to use excessive boilup and reflux to make sure of meeting or exceeding product specifications. This not only wastes energy; it also reduces column capacity. To provide a perspective on energy savings, one may note that 100 lbm/hr of steam is worth $3200 per year (basis $4.00/1000 pounds, 8000 hours per year). To save this amount of steam would probably be only a modest accomplishment for most columns.
If composition control of each product stream is desired (and this is usually
* In some companies columns are designed by the probabilistic methods recommended by Fractionation Research, Incorporated.
the case), the most obvious deficiency of most existing columns is the lack of appropriate composition measurements. Most commonly temperature in the upper or lower section of the column (or both) is used in lieu of true composition measurements. Frequently composition control is attempted at only one end of the column, and sometimes at neither end.
Another shortcoming frequently observed is the use of fixed flow controls for steam, reflux, or product drawofF. Any such unaided flow control should be regarded with suspicion. With rare exceptions flow-control set points must be changed to accomplish either composition control or material balance control.
In view of the preceding comments about problem areas and likely opportunities for improvement of composition control and reduction in energy consumption, the following guidelines are suggested:
1. Make sure that column material-balance controls are properly designed and tuned, and that hardware, especially level and flow transmitters and control valves, is in good working condition. If PI level controllers are"used, follow the tuning procedures of Chapter 16; auto overrides or nonlinear controllers should be used. It is usually desirable to cascade level control to flow control, in which case flow measurement should be linear.
2. Provide averaging level control of column feed. Column feed rate should also be held between maximum and minimum limits.
3. Provide a linear flow measurement for steam flow control. Also provide temperature and pressure compensation.
4. If condenser controls are a problem, review the control schemes in Chapter 3.
5. At this point with flows established, smoothed, and in some cases limited, it will probablv be possible to see some improvement in composition control, at least for part of the time. For further improvement provide steam-to-feed ratio control, internal reflux-to-feed or distillate-to-feed ratio control, high AP override on steam to protect against flooding, and a minimum steam flow limiter to protect against dumping.
6. If composition is measured and is cascade controlled via reflux or boilup, or both, ratio controls should be replaced by impulse feedforward compensation (see Chapter 12) if feed flow turndown is greater than 2:1.
7. For pressurized or vacuum columns, make sure an adequate scheme is provided for maintaining an inert balance.
8. If temperature by itself is not an adequate measure of composition, consider one of the schemes in Chapter 10 for using temperature, pressure, and, in some cases, flow measurements to deduce compositions.
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