Page 304 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

306 Chapter 11 Distillation

The annualised total cost is a function of ðR=R min Þ. The curve of annualised total cost versus
ðR=R min Þ starts from infinity at minimum reflux ðR=R min ¼ 1Þ and passes through a minimum (at
R ¼ R opt ) for specified composition of feed, distillate and bottom product. Usually ðR opt =R min Þ lie
between 1.1 and 1.5, with the lower value applying to a difficult separation and the higher value to an
easy separation. However, optimal reflux ratio is not sharply defined in most cases and for greater
operational flexibility, columns are mostly designed for reflux ratios slightly higher than the optimum.
In an existing plant, a decrease in reflux ratio can save only the utility cost and a reflux sufficient to
obtain the desired product yield and purity should be used. Usually the total annual cost is dominated
by the cost of reboiler energy input except at close to minimum reflux condition. Also, in reality, often
the credit for additional separation overshadows the utility savings. In such cases of existing columns,
the operating ðR=R min Þ used is as high as possible without causing flooding or entrainment in the
column. Thus, the optimum reflux for an column depends upon the product values and the desired
degree of separation.

11.4.3 Multicomponent distillation
There are two approaches for multicomponent distillation design:
(i) Rigorous approach: This considers multicomponent thermodynamics and mass transfer along
with heat transfer and involves tray-to-tray/stagewise heat and mass balances.
(ii) Short cut approach based on similarities with binary system: This starts with identifying a pair of
key components whose separation represents the separation target of the distillation process
being designed.
Prior to a discussion of the aforementioned approaches, it is necessary to get acquainted with a few
definitions relevant to multicomponent distillation.
Definitions
Volatility is the tendency of a component to vaporise. At a temperature T, it is the ratio of the pure
sat
sat

component vapour pressure (p ) to the total system pressure (P), i.e., a i ¼p ðTÞ P
i i
Relative volatility (a i;j ) of component i with respect to component j is the quantitative comparison
of the volatilities of two components. In case of an ideal system,
sat sat
i j (11.10)
a i;j ¼ p ðTÞ=p ðTÞ
sat
sat
where p ðTÞ and p ðTÞ are the pure component vapour pressures at temperature T.
i j
K i
ðy i =x i Þ
In case of a non-ideal system; a i;j ¼ ¼
y j =x j K j
where K i ¼ y i =x i is the distribution coefficient of component i. The VLE relationship in terms of a i;j is
a i;j x i
(11.11)
y i ¼
1 þ a i;j 1 x i
Since a i;j is a function of temperature, which is different at the top and the bottom of the column,
their geometric mean is used for designing the column.
1=2
avg Bottom Top
a i;j ¼ a i;j a i;j (11.12)