Page 155 - Separation process engineering

When stepping off stages from the top down, the fractional number of stages can be calculated as (see
Figure 4-8B and 4-8C),

(4-23)

where the distances are measured horizontally on the diagram. The fraction has no physical meaning
because we build either five or six stages; however, the fraction is somewhat useful when the stage
efficiency is < 1.

Now that we have seen how to do the stage-by-stage calculations on a McCabe-Thiele diagram, let us
consider how to start with the design problem given in Figure 3-8 and Tables 3-1 and 3-2. The known
variables are F, z, q, x , x , L /D, p, saturated liquid reflux, and we use the optimum feed location. Since
D
B
0
the reflux is a saturated liquid, there will be no change in the liquid or vapor flow rates on stage 1 and L 0
= L and V = V . This allows us to calculate the internal reflux ratio, L/V, from the external reflux ratio,
1
1
2
L /D, which is specified.
0

(4-24)

With L/V and x known, the top operating line is fully specified and can be plotted.
D
Since the boilup ratio, /B, was not specified, we cannot directly calculate / , which is the slope of the
bottom operating line. Instead, we need to utilize the condition of the feed to determine flow rates in the
stripping section. The same procedure used with the Lewis method can be used here. The feed quality, q,