Stripping Section Operating Line (SOL)

Analysis of the stripping section is not really required, as SOL can be automatically fixed once ROL and q-line are drawn.

The SOL is the last line to draw, by connecting xB on the diagonal to the point of intersection between the ROL and q-line. With ROL fixed, when the q-line changes the SOL changes as well.

The Figure below shows 3 different SOL that correspond to 3 different q-lines, for a given ROL (i.e. fixed R and xD).

Analysis of Stripping Section

Analysis of SOL is presented below using the Figure below which shows the stripping section of a distillation column. The reboiled vapour in equilibrium with bottoms liquid leaving the column.

Material balance : IN = OUT

Assuming constant molal overflow

L'm = L'm+1 = .... = L' = constant

V'm = V'm+1 = ..... = V' = constant

Material balance:

Overall : L' = V' + B'

MVC : L' xm = V' ym+1 + B xB

Substituting, and re-arrange in the form y = f(x), we obtain

Again, dropping the subscripts "m+1" and "m" we have:

Substituting V' = L' - B: we have the stripping operating line (SOL)

This is straight line with slope ( L' / L' - B)
and intercept ( B xB / L' - B )

In addition, when x = xB , y = xB, i.e. the operating line passed through ( xB, xB ) on the 45o diagonal line.

Using the equilibrium diagram, the stripping section operating line can be drawn and the number of theoretical stages in the stripping section can be done in the same manner.

Reminder: The last "stage" on the graphical construction represents the reboiler.