Page 100 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 89

3. Determine (Xi/Xh)2, for tray No. 2 (second from
top), for each component, using the x values for the
(8- 176)
reflux as the initial xi (n + 1).
4. Total this column to yield Z(xi/xh). This equals 1/Xh.
Z,i = 1.0 (Including xh) 5. Determine xi for each component by:

(8- 180)

The compositions of each component are obtained This is liquid composition on tray.
from the (Xi/Xh)n ratio. 6. Continue down column using the composition calcu-
The tray temperature is obtained from: lated for tray above to substitute in Equation 8-176 to
1 obtain the (Xi/Xh) for the tray below.
Kh =- (8- 177)
Z aixi '7. Test to determine if a is varying to any great extent by
calculating aixi for a test tray. Zajxi = Deter-
is evaluated at the column pressure by use of suitable mine temperature and evaluate corresponding val-
K charts. ues. Use new ai if significantly different.
Stripping section: reference component is heavy key, 8. Continue step-wise calculations until the ratio of light
Xhr yh. to heavy key on a tray equals (or nearly so) that ratio
in the liquid portion of the feed. This is then consid-
(8- 178) ered the feed tra~7.
9. If there are components in the feed and bottoms
which do not appear in the overhead product, they
(Yi/)'h)m = 1.0 (including yb)
must gradually be introduced into the calculations.
The estimated position above the feed tray to start
The composition of each component on a tray is introducing these components is determined by:
obtained from (Yi/Yh)m.
The tray temperature is obtained from:
(8- 181)

where XF~ = mol fraction of a component in feed that does
at the column pressure using K charts for the heavy key or not appear in overhead
reference component. x, = small arbimy mol fraction in the liquid p"
plates above the feed plate
Procedure: p" = number of plates above the feed where introduc-
tion of components should begin
A. Rectzfjing Section
B. Stripping Section:
1. Determine material balance around column, includ-
ing reflux L, distillate product D, bottoms product B. 1. Determine bubble point temperature of bottoms and
(a) With total condenser, the reflux composition is composition of vapor, yBi, up from liquid. Calculate
equal to the condensed distillate product compo- relative volatility of light to heavy component at this
sition. temperature.
(b) With a partial condenser, the product D is a 2. From these calculate vapor compositions, using Equa-
vapor, so a dew point must be run on its compo- tion 8-178 calculate the ratio (yi/yh) for the first tray
sition to obtain the liquid reflux composition. at the bottom.
2. Determine top tray temperature for use in relative 3. Total 2 (yi/yh) to obtain l/yh
volatility calculations by running a dew point on the 4. Calculate yi for tray one
overhead vapor. For total condenser its composition
is same as distillate product. For a partial condenser, (8 - 182)
run a dew point on the column overhead vapor com-
position as determined by a material balance around
the partial condenser, reflux, and product. zyi = 1.0