Page 47 - Distillation theory

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2.1 Purpose and Process Essence of Distillation 21

Q Q
a) con b) con
L D D
N N N L N
x D x D
j+1
F, z F f V, y j L ,x
L , x F j i,j j+1 i,j+1
F
V , y V , y L , x
F F j-1 i,j-1 j i,j
1
V 1

B
Q
Reb
x
B
Figure 2.1. (a) A distillation column with condenser, reboiler, and
reflux capacity; (b) control volume (dotted line) for obtaining material
balance equations for the top section.

2.1.1. Description of Distillation Process

To begin with, let’s consider a schematic diagram of a distillation column with a
condenser and a reboiler (Fig. 2.1a).
The column feed, in the common case, is vapor–liquid mixture of flow rate F
and with concentrations of components z iF , where i is the component number, the
vapor feed of flow rate V F with concentrations of components y iF , and the liquid
feed of flow rate L F with composition x iF .
The reboiler serves to make a vapor flow, which goes upward along the column
length; the condenser serves to make a liquid flow, which flows down from tray
to tray from top to bottom. On tray j, the liquid flow from tray j + 1 meets the
vapor flow from tray j − 1. These flows are not equilibrium and, therefore, a mass
exchange takes place on the tray; a part of the lightest components converts from
liquid into the vapor phase, and a part of the heaviest components (of higher
boiling temperatures) converts from vapor into the liquid phase.
Although on real trays complete equilibrium between liquid and vapor is never
reached, in the world practice a model of a theoretical tray (Sorel, 1893) for which
thisequilibriumisachieved(1,1÷2realtrayscorrespondtoonetheoreticaltray)is
accepted. Due to the mass exchange between liquid and vapor, the composition on
the trays varies along the column length – on the overhead trays, there is a high con-
centration of light components and, on the bottom trays, there is a high concentra-
tion of heavy components. Thus, a separation of the initial mixture occurs. The va-
por rising from the column is condensed in the condenser. One part of the formed
liquid is removed as an overhead product (distillate is the amount D of composi-
tion x iD ), and the other part comes back into the column (reflux in the amount L N
of the same composition x iD ). Such a condenser, which condenses all vapor from
the column, is called a total condenser. Sometimes a partial condenser is applied,