For a given feed composition and enthalpy, there is an optimum feed-tray location that permits making the specified separation with the least energy consumption. It is also the tray that will permit maximum feed rate without causing the column to flood. As shown in Figure 5.6, a column should generally be equipped with a number of alternative feed trays to handle changes in operating conditions from those assumed for design.
The magnitude of the energy savings to be realized by changing feed tray location can be very significant in some systems (10-20 percent reduction in heat input), but in other columns the effects can be small. Each system must be examined to determine the strategy and the incentives for controlling to an optimum feed-tray location. Sometimes unexpected results occur. Luyben5 has shown that the optimum feed-tray location in some columns rises higher in the column as the feed becomes fighter (increase in more volatile component concentration), while in other columns exactly the opposite is true.
Many columns have more than one feed. Each feed stream should be introduced onto its optimum feed tray if energy consumption is to be minimized. These feeds should not be mixed together and introduced onto the same feed tray if their compositions differ.
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