Page 341 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.8 Batch distillation 343

t
d
Z
ðBSx Bi l i Þdt (11.63)
Q ¼
dt
0
A rigorous evaluation of Q should make use of Eq. 11.63 and for quicker estimate Eq. 11.52
or Eq. 11.55 may be used depending upon the mode of operation selected.
(11.64)
Q 2 ¼ BC pB ðT bubB T B Þþ mC p ðT bubB T B Þ
where T B is the final temperature of the bottoms product.

Q 2 3
The average cooling water requirement; m CW ¼ m =hr (11.65)
t 3 C pCW DT cooling
In the above equation t 3 ; C pCW and DT cooling are the still cooling time duration, specific heat of
cooling water and temperature range of cooling water, respectively.
Heat transfer coefficient in each stage of operation is different. Exchanger design must be capable
of handling each of these functions.
One may also like to operate by selecting a fixed Q still based on the still heater heat load capacity/
rating. This is particularly true when electric reboilers with fixed wattage are being used. In such case
the reflux ratio needs to be suitably altered as distillation proceeds. The heating could be by steam
pipes or electrical heating elements which need to remain submerged till the end of the batch. In case
of electrical heating, extra heating elements are included. Say if a bench scale set up requires a total
power of 12 kW that can be served by using 2 kW elements, instead of using six elements, an addi-
tional two elements are usually provided. This requires selection of vessel dimensions such that the
liquid holdup at the end of the batch is sufficient to keep all the eight elements completely submerged
with a minimum of 80 mm liquid level above and below the heater section.
The condenser duty (Q C ) is estimated based on the vapour leaving the top of the column and is
given by

Z t
dD
ðh V h L Þdt (11.66)
Q C ¼
dt
0
where h V and h L are the enthalpy values of D at its dew point and bubble point temperature,
respectively.

11.8.3 Design steps
The design steps and calculation procedure are outlined as follows:
1) Decide the operating pressure (P op ) e Distillation pressure is decided with the same
considerations as design of rectification columns that are detailed in Section 11.4.1.
2) Generate the T-x-y data and x-y plot or find a from vapour pressure data obtained from Antoine
equation or available experimental data.
3) Note the bubble point at x F and dew point at x D . Find y corresponding to x F .

F