Page 216 - Separation process engineering

' Loop in stripping section stepping off stages with equilibrium
and operating eqs.
Do
If yeq < yint Then
Cells(i + 7, 3).Value = "Reflux rate too low"
Exit Do
End If
y = a6 * x ^ 6 + a5 * x ^ 5 + a4 * x ^ 4 + a3 * x ^ 3 + a2 * x
^ 2 + a1 * x + a0

' Record values of stage number, x and y values on spreadsheet.
Cells(i + 7, 1).Value = i
Cells(i + 7, 2).Value = x
Cells(i + 7, 3).Value = y
i = i + 1
x = (y / LbaroverVbar) + (LbaroverVbar - 1) * xb /
LbaroverVbar
Loop While i < feedstage
' Calculations in enriching section.
Do While y < xd
If yeq < yint Then
Cells(i + 7, 5).Value = "Reflux rate too low"
Exit Do

End If
y = a6 * x ^ 6 + a5 * x ^ 5 + a4 * x ^ 4 + a3 * x ^ 3 + a2 *
x ^ 2 + a1 * x + a0
' Record values of stage number, x, and y on spreadsheet
Cells(i + 7, 1).Value = i
Cells(i + 7, 2).Value = x
Cells(i + 7, 3).Value = y
i = i + 1
x = (y / LoverV) - (1 - LoverV) * xd / LoverV
If x < 0 Then
Cells(i + 7, 4).Value = "Feed stage too low"
Exit Do
End If
Loop
End Sub

By varying the feed stage location, the optimum location (stage 20) can easily be found. The minimum
external reflux ratio can be determined quickly by trial and error (see Problem 4.H2). The program can
also be coded to find either of these values automatically (see Problems 4.H1 and 4.H3).
Obviously this problem can be solved graphically. The problem can also be solved with a hand
calculation on a spreadsheet without using VBA by writing the equilibrium and operating equations in cell

form and then repeating them as needed. The advantages of a programmed spreadsheet or other software
tools (e.g., Binous, 2008) are a huge number of trials can be done by changing just one number (e.g., z or
L/D), different systems can easily be studied by using different values for the VLE coefficients, and one
does not have to manually step off 25 stages. For a single use graphical or hand-calculation with a