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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.

Feed: Cold liquid

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.

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The following Figures showed 2 other feed conditions: case (b) for saturated liquid (left) and case (c) for vapour-liquid mixture (right):

Feed: Saturated liquidFeed: Vapour-liquid mixture

And the following Figures showed 2 other feed conditions: case (d) for saturated vapour (left) and case (e) for superheated vapour (right):

Feed: Saturated vapourFeed: Superheated vapour

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Subcooled feed or superheated feed can be controlled (see Figure below) by preheating (left) or desuperheating (right) the feed prior to column entry:

Feed condition control

A superheated bottom feed can be cooled by injecting a quench stream as shown in the Figure below.

Quenching bottom feed

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