Page 96 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 85

Actual Number of Trays (Nact), (S, - l)/Eo (for columns with reboilers) (8-1 69)
=
From the theoretical trays at operating reflux the actual For systems with wide variation in relative volatility, the
trays for installation are determined: suggestion of Cicalese, et al. [9] is often used to evaluate
the theoretical total equilibrium stages in the rectifylng
and stripping sections:
The reboiler is considered 100% efficient, and likewise
any partial condenser, if used. Therefore the value No rep-
resents the thoretical trays or stages in the column proper, (8 - 170)
excluding the reboiler and partial condenser. E,, repre
sents the overall tray efficiency for the system based upon (XI / hh )F
actual test data of the same or similar systems, or from the s, = log (XI /Xh )B (8- 171)
plot of Figure 8-29, giving operating information prefer- log a (average below feed)
ence (if reliable).
where Sr = number theoretical trays/plates in rectifylng section
Feed Tray Location
S, = number theoretical trays/plates in stripping section
The approximate location can be determined by the
ratio of the total number of theoretical stages above and Maas [lo81 presents a useful analysis for selecting the
below the feed plate from the Fenske total reflux relation: feed tray in a multicomponent column. For accuracy it
involves the use of a tray-by-tray computer calculation.
Kirkbride’s [174] method for estimating the ratio of
theoretical trays above and below the feed tray allows esti-
mation of the feed tray location:
The relation is solved for SJS,. The results are not
exact, because the feed tray composition is very seldom
the same as the feed; which is the assumption in this rela- (8- 172)
tion. Actually, the feed point or correct location for the
feed may be off by two or three theoretical trays. This will
vary with the system. It does mean, however, that when this where Nn = number of trays above feed tray
approach is used for feed plate location, alternate feed N, = number of trays below feed tray
nozzles should be installed on the column to allow for D = mols per hour of overhead product
experimental location of the best feed point. These extra W = mols per hour of bottoms product
nozzles are usually placed on alternate trays (or more)
both above and below the calculated location. A minimum Subscripts
of three alternate nozzles should be available. h = heavy key
When the feed point is located by a match from tray-by- 1 = light key
tray calculation, the correct point can be established with F = feed
greater confidence, but still alternate nozzles are suggest- UT = bottoms
ed since even these detailed calculations can be off to a D = overhead
certain extent.
The actual number of trays in the rectifying section Estimating Multicomponent Recoveries
(N,J, can be determined by:
Yaws et al. [141] present a useful technique for estimat-
ing overhead and bottoms recoveries with a very good
comparison with tray-to-tray computer calculations. The
procedure suggested uses an example from the reference
with permission:
Solve for S,, because SM and S,/S, are known.
Obtain S, by difference. 1. Plot relative volatility (q) and % desired recovery for
LK and HK. Draw a straight line through these two
(N,,,), = S,/Eo (for total condenser; if partial condenser use points. The non-key component points will also be on
(S, - 1)/Eo this straight line.