Page 113 - Applied Process Design For Chemical And Petrochemical Plants Volume II

1 02 Applied Process Design for Chemical and Petrochemical Plants

), Contaminated air out, The topic of control of distillation columns has been
-- MY discussed by many authorities with a wide variety of expe-
rience [ 117-120,2371, and is too specialized to be covered
Liquid (water) in, 4- in this text.
Contaminated
L, xo
Nomenclature for Part 1: Distillation Process
Performance
- Packing or Trays
Contacting Section
Numbers indicate relative A, B thru K = Constants developed in original article
locations of either
theoretical trays (stages), a, b, c = Correlation constants (distillation recoveries
or number of transfer unlts 11411)
a = Activity of component
ai = Activity of component, i
av or avg = Average
B, C, D = Vinal coefficients, Equation &11
B = Bottoms product or waste, lb mols/hr, also = W
Bb = Mols of component, b, used as reference for
volatility, after a given time of distillation
Bb, = Mols of component, b, used as reference for
volatility, at start of distillation
7 (water) out,
Liquid
Decontaminated
Bi = Mols of component, i, after a given time of
distillation
Figure 8-55. Schematic stripping tower using air to strip organics Bi, = Mols of component, i, at start of distillation
from water solution. Adapted and used by permission, Li, K. Y. and BT~
= Total mols of liquid in bottoms of still at time, T1
Hsiao, K. J., Chern. Eng., V. 98, No. 7 (1991) p. 114. BT,, = Total mols liquid (not including any steam) in
bottom of still at start time To (batch charge)
b = y intercept of operating line; or constant at fixed
pressure for Winn's relative volatility
bi = Mols of component, i, in bottoms
C = No. components present, phase rule; or no. com-
ponents, or constant
file of the operating fluids on each tray or through the C,i = Factor in Colburn Minimum Reflux method,
packing of a packed column. pinch conditions, stripping
Other troubleshooting techniques can include comput- C,i = Factor in Colburn Minimum Reflux method,
er modeling, checking the reliability of instrumentation, pinch conditions, rectifying
measuring quality of product streams with varying reflux C, = Specific heat, Btu/lb (OF)
rates, measuring column tray temperatures at close inter- D = Mols of distillate or overhead product, lb
mols/hr; or batch distillation, mols
vals, stabilizing the feed rate, bottoms withdrawal and di = Mols component, i, in distillate
overhead condensing rates. Surprising results can be E =Vaporization efficiency of steam distillation
obtained, including: EG = Overall column efficiency
E, = Overall tray efficiency
E& = EOG = Murphree point efficiency, fraction
1. Trays may have damage to caps, valves, distributors, Ew0 = Murphree plate/tray efficiency, = EM
sieve holes, or packing for packed towers. F = Degrees of Freedom, phase rule; or, charge to
2. The contacting devices of (1) above may actually be batch still, mols
missing, i.e., blown off one or more trays, so all that is F = Feed rate to tower, lb mols/hr; or, mols of feed,
existing is a "rain-deck" tray with no liquid-vapor con- (batch distillation) entering flash zone/time all
components except noncondensable gases
tacting. Fm = Factor for contribution of other feed flow to min-
3. Crud, polymer, gunk and other processing residues, imum reflux
plus maintenance tools, rags, or overalls may be plug- FL = Mols of liquid feed
ging or corroding the liquid flow paths. FV = Mols of vapor feed
4. Entrainment. F, = F + V, = mols feed plus mols of noncondensable
gases
5. Weeping of trays, or flooding of packing or trays. FR = FSR,k = Factor for contribution of sidestream, k,
6. Foaming limitations. flow to minimum reflux
7. Unusual feed conditions, unexpected or uncon- FSR = Factor for contribution of sidestream flow to min-
imum reflux
trolled. Fw = Factor for contribution of feed, j, flow to mini-
8. Many other situations, almost too odd to imagine. mum reflux
References on this topic include 159-166, 182, 238. f = Fugacity at a specific condition