Page 343 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.9 Design illustration e batch distillation 345

(ii) To find R min , draw operating line from the point (x Di , x Di ) through the point (x Bi ; y )on

Bi
equilibrium curve to locate the point ðy int ; 0Þ on the ordinate. Calculate R min ¼ðx D =y int Þ 1.
(iii) Select R in the range 1.5 R min < R < 10 R min . Larger the R, higher is the vapour boil-up rate and
more is the column diameter and an economic balance may be struck.
(iv) Estimate x Davg , V and Q from Eqs. 11.50e11.52 by replacing F with B i . The expressions are

x Bi x B ðB=B i Þ
x Davg ¼ ; V ¼ DðR þ 1Þ
1 ðB=B i Þ
x
Z B
B i dx B Q
ln ¼ ; ¼ð1 B i = FÞðR þ 1Þ
B x Di x Bi Fl F
x Bi
(b) Operation with constant distillate composition (by continuously increasing R)
(i) Initially R is minimum corresponding to liquid composition x Bi in still. This is found by trial e
drawing operating lines through (x Di x Di ) intersecting the equilibrium curve and meeting the
ordinate at (y int , 0). On each operating line N stages are constructed starting from (x Di x Di ) and
ending in still composition of x B . The operating line corresponding to still composition of x Bi is
chosen and the corresponding y int value is read off the ordinate scale. Calculate
R ¼ðx D =y int Þ 1
(ii) Find B by integrating Eq. 11.47 after replacing F with B i
x
Z B
B dx B
ln ¼
B i x D x Bi
x F
(v) Use Eq. 11.53 through 11.55 to find B, D, Q, and V after replacing F with B i .
The expressions are
x D x Bi x Bi x B
B ¼ B i ; D ¼ B i
x D x B x D x B
Q Z x B R þ 1
dx B
2
¼ðx D x Bi Þ
Fl F
x Bi ðx D x Bi Þ
and
Z
x B
dx B
V ¼ B i ðx D x Bi Þ
2 R
x Bi
R þ 1
ðx D x Bi Þ 1
11.9 Design illustration e batch distillation
Few drums containing ortho-xylene contaminated with toluene (25% mol/mol toluene, 75% mol/mol O-
xylene) are available. As much as possible amount of toluene is to be recovered from this material.
Decide on process to recover the toluene. Minimum acceptable purity of toluene-rich phase produced is
40% but this fetches a low selling value. The price improves substantially if the purity is above 55 mole%.