Page 89 - Separation process engineering

T drum and V/F at the same time), or should we converge sequentially? Both techniques will work, but
sequential convergence tends to be more stable. If we use sequential convergence, then a third question is:
Which variable should we converge on first, V/F or T drum ? To answer this question we need to consider

the chemical system we are separating. If the mixture is wide-boiling, that is, if the dew point and bubble
point are far apart (say more than 80 to 100°C), then a small change in T drum cannot have much effect on
V/F. In this case we wish to converge on V/F first. Then when T drum is changed, we will be close to the

correct answer for V/F. For a significant separation in a flash system, the volatilities must be very
different, so this is the typical situation for flash distillation. The narrow-boiling procedure is shown in
Figure 6-1 for distillation.
The procedure for wide-boiling feeds is shown in Figure 2-13. Note that the energy balance is used last.

This is standard procedure since accurate values of x and y are available to calculate enthalpies for the
i
i
energy balance.
Figure 2-13. Flowsheet for wide-boiling feed