Distillation columns are designed using VLE data for the mixtures to be separated. The vapour-liquid equilibrium characteristics (indicated by the shape of the equilibrium curve) of the mixture will determine the number of stages, and hence the number of trays, required for the separation. This is illustrated clearly by applying the McCabe-Thiele method to design a binary column.

McCABE-THIELE DESIGN METHOD

The McCabe-Thiele approach is a graphical one, and uses the VLE plot to

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determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow and this implies that:

molal heats of vaporisation of the components are roughly the same

heat effects (heats of solution, heat losses to and from column, etc.) are negligible

for every mole of vapour condensed, 1 mole of liquid is vaporised

The design procedure is simple. Given the VLE diagram of the binary mixture, operating lines are drawn first.

Operating lines define the mass balance relationships between the liquid and vapour phases in the column.

There is one operating line for the bottom (stripping) section of the column, and on for the top (rectification or enriching) section of the column.

Use of the constant molar overflow assumption also ensures the the operating lines are straight lines.

The operating line for the rectification section is constructed as follows. First the desired top product composition is located on the VLE diagram, and a vertical line produced until it intersects the diagonal line that splits the VLE plot in half. A line with slope R/(R+1) is then drawn from this instersection point as shown in the diagram below.

 

R is the ratio of reflux flow (L) to distillate flow (D) and is called the reflux ratio and is a measure of how much of the material going up the top of the column is returned back to the column as reflux.

Operating Line for the Stripping Section

The operating line for the stripping section is constructed in a similar manner. However, the starting point is the desired bottom product composition. A vertical line is drawn from this point to the diagonal line, and a line of slope L_s/V_s is drawn as illustrated in the diagram below.

L_s is the liquid rate down the stripping section of the column, while V_s is the vapour rate up the stripping section of the column. Thus the slope of the operating line for the stripping section is a ratio between the liquid and vapour flows in that part of the column.

Equilibrium and Operating Lines

The McCabe-Thiele method assumes that the liquid on a tray and the vapour above it are in equilibrium. How this is related to the VLE plot and the operating lines is depicted graphically in the diagram on the right.

A magnified section of the operating line for the stripping section is shown in relation to the corresponding n'th  stage in the column. L's are the liquid flows while V's are the vapour flows. x and y denote liquid and vapour compositions and the subscripts denote the origin of the flows or compositions. That is 'n-1' will mean from the stage below stage 'n' while 'n+1' will mean from the stage above stage 'n'. The liquid in stage 'n' and the vapour above it are in equilibrium, therefore, x_n and y_n lie on the equilibrium line. Since the vapour is carried to the tray above without changing composition, this is depicted as a horizontal line on the VLE plot. Its intersection with the  operating line will give the composition of the liquid on tray 'n+1' as the operating line defines the material balance on the trays. The composition of the vapour above the 'n+1' tray is obtained from the intersection of the vertical line from this point to the equilibrium line.

Number of Stages and Trays

Doing the graphical construction repeatedly will give rise to a number of 'corner' sections, and each section will be equivalent to a stage of the distillation. This is the basis of sizing distillation columns using the McCabe-Thiele graphical design methodology as shown in the following example.

Given the operating lines for both stripping and rectification sections, the graphical construction described above was applied. This particular example shows that 7 theoretical stages are required to achieve the desired separation. The required number of trays (as opposed to stages)  is one less than the number of stages since the graphical construction includes the contribution of the reboiler in carrying out the separation.

The actual number of trays required is given by the formula:

(number of theoretical trays)/(tray efficiency)

Typical values for tray efficiency ranges from 0.5 to 0.7 and depends on a number of factors, such as the type of trays being used, and internal liquid and vapour flow conditions. Sometimes, additional trays are added (up to 10%) to accomodate the possibility that the column may be under-designed.

The Feed Line (q-line)

The diagram above also shows that the binary feed should be introduced at the 4'th stage. However, 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-lines for the various feed conditions are shown in the diagram on the left.

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:

feed-line and rectification section operating line

feed-line and stripping section operating line

stripping and rectification operating lines

This is because these pairs of lines determine the third.

 

OVERALL COLUMN DESIGN

Determining the number of stages required for the desired degree of separation and the location of the feed tray is merely the first steps in producing an overall distillation column design. Other things that need to be considered are tray spacings; column diameter; internal configurations; heating and cooling duties. All of these can lead to conflicting design parameters. Thus, distillation column design is often an iterative procedure. If the conflicts are not resolved at the design stage, then the column will not perform well in practice. The next set of notes will discuss the factors that can affect distillation column performance.