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Feed Preheat Control
Feed preheat is usually practised for heat recovery or to attain the desired vapour and liquid traffic above and below the feed tray. The objective of the preheat control system is to supply the column with a feed of consistent specific enthalpy. With a single-phase feed, this becomes a constant feed temperature control; with a partially vapourised feed, a constant fractional vapourisation is required.
As an example, consider case (a) as shown in the Figure below whereby the feed is a cold liquid. In this case, all the liquid feed will go to the stripping section. In addition, because the feed is cold, it will also condense some of the rising vapour.
As a result, the amount of liquid
flow in the stripping section is much
larger than the liquid flow in the rectifying
section. The vapour flow in the rectifying
section is lower than the vapour flow
in the stripping section because of the condensation
into the liquid.
The following Figures showed 2 other feed conditions: case (b) for saturated
liquid (left)
and case (c) for vapour-liquid mixture (right):
And the following Figures showed 2 other feed conditions:
case (d) for saturated vapour (left)
and case (e) for superheated vapour (right):
Subcooled feed or superheated feed can be controlled (see Figure below) by preheating (left) or desuperheating (right) the feed prior to column entry:
A superheated bottom feed can be cooled by injecting a quench stream as shown in the Figure below.
