Conservation

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For distillation, conservation means designing and operating a column so that it makes the specified separation with the least amount of energy per pound of feed. We have a number of techniques to accomplish this:

1. Automatic control of composition of product streams. Operators commonly overreflux conventional columns with a single top product and a single bottom product. Extra heat is used to ensure the meeting or exceeding of specified product purities.

Geyer and Kline1 give, as an example, a 70-tray column separating a mixture with a relative volatility of 1.4 and with specifications of 98 percent low boilers overhead and 99.6 percent high boilers in the base. If the operator adds enough boilup and reflux to increase overhead purity to 99 percent and base purity to 99.7 percent, an increase of 8 percent in energy consumption results.

2. Feed provided at the proper feed tray. It can be shown that this results in a lower energy requirement per pound of feed than would feeding on any other tray. As feed composition or enthalpy deviates from design values, the optimum feed-tray location also changes.

3. Column operation at minimum pressure.2 Lower pressure usually means higher relative volatility. Therefore, the necessary separations can be accomplished with lower boilup/feed and reflux/feed ratios. Condenser capacity may be limited, however, and the column may flood at lower boilup rates than it would when operating at higher pressures.

4. Use of lowest pressure steam available.1 In many plants excess low-pressure steam is available that otherwise would be vented to the atmosphere. This steam is usually cheaper than high-pressure steam. Where reboiler AT might be too small if the steam were throttled, one may use a partially flooded reboiler (see Chapters 4 and 15) and throttle condensate. Since low-pressure steam is seldom available at constant pressure or steam quality, pressure and temperature compensation of flow measurements is highly desirable if steam is throttled instead of condensate.

5. Use of steam condensate receivers. In many plants steam traps require considerable maintenance and have significant leakage. The use of steam condensate receivers instead of traps reduces maintenance and steam losses.

6. Possible use of mechanical vacuum pumps. For vacuum columns there is some opinion1 that mechanical vacuum pumps offer energy savings over steam jets. The difference, however, is usually small.

7. Dry distillation. For columns now using live steam, it is sometimes economical to switch to steam-heated reboilers.

8. Insulation. Older columns, designed before the energy crunch, can often benefit from new, increased insulation.

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