Page 357 - Chemical process engineering design and economics

336 Chapter 6

Table 6.28 Continued

6. Calculate the relative volatility, a f, of the light and heavy key components at the top tray
and at the bottom of the column, from Equations 6.27.18 and 6.27.22 (i = LK and i = HK).
7. Calculate the column geometric-average relative volatility, (a() avg, (feed, distillate, and
bottom product) of the light and heavy key components from Equation 6.27.19 (i = LK and
i = HK).
8. Calculate the minimum reflux ratio, RM_ from the Underwood equations (Equations
6.27.3 and 6.27.4).
9. Calculate the optimum reflux ratio, RQ, from the Van Winkle and Todd correlation,
(Equations 6.27.5 to 6.27.7).
10. Calculate the minimum number of equilibrium stages, N M, from the Fenske equation,
Equation 6.27.2.
11. Calculate the number of equilibrium stages, N c, from the Gilliland correlation, (Equa-
tions 6.27.8 to 6.27.11).
12. Locate the feed point from the Kirkbride equation, Equations (6.27.12 and 6.27.13).

13. Calculate the column diameter, D, using the procedure outlined in Table 6.24.
14. Calculate the mole-fraction average of the relative volatility, ttj, and feed viscosity, Uj,
at the average of the top tray and bottom temperature. Use Equations 6.27.20, 6.27.24, and
6.27.25.
15. Calculate the column overall efficiency, E o, from Equation 6.27.21.

16. Calculate the length, L s, at the bottom of the column required for surge capacity from
Equation 6.27.16.

17. Calculate the column height, Z, from Equation 6.26.15 for a tray column or from Equa-
tion 6.27.17 and 6.27.26 for a packed column.

The height of a tray fractionator is equal to the number of trays times the
tray spacing plus additional height above the top tray and below the bottom tray.
These additional sections are needed for removal of liquid entrained in the vapor
from the top tray and to provide surge capacity for the bottom product. Table 6.25
lists the tray spacing as a function of pressure. Because tray spacing influences the
height of a column, it should be kept as small as possible. Tray spacing may be
influenced by maintenance considerations. There should be sufficient space be-
tween the trays to facilitate inspection and repairs, but occasionally, other consid-

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