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Distillation Column’s q-line

The Engineering Concepts
Distillation column q line

Distillation column q line

Distillation Column’s q-Line

The Feed Line (q-line)

The parameter q is the mole fraction of liquid in the feed and the slope of the q-line is q / (q – 1).

If the feed composition is such that it does not coincide with the intersection of the operating lines, this means that the feed is not a saturated liquid. The condition of the feed can be deduced by the slope of the feed line or q-line.

The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.

Depending on the state of the feed, the feed lines will have different slopes. For example,

q = 0 (saturated vapour)

q = 1 (saturated liquid)

0 < q < 1 (mix of liquid and vapour)

q > 1 (subcooled liquid)

q < 0 (superheated vapour)

The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.

Depending on the state of the feed, the feed lines will have different slopes. For example,

q = 0 (saturated vapour)

q = 1 (saturated liquid)

0 < q < 1 (mix of liquid and vapour)

q > 1 (subcooled liquid)

q < 0 (superheated vapour)

The q-lines for the various feed conditions are shown in the diagram.

Using Operating Lines and the Feed Line in McCabe-Thiele Design.

If we have information about the condition of the feed mixture, then we can construct the q-line and use it in the McCabe-Thiele design. However, excluding the equilibrium line, only two other pairs of lines can be used in the McCabe-Thiele procedure.

These are:

1) feed-line and rectification section operating line

2) feed-line and stripping section operating line

3) stripping and rectification operating lines

This is because these pairs of lines determine the third.

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