Page 246 - Distillation theory

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220 Trajectories of the Finite Columns and Their Design Calculation

different reflux numbers). Conceptual design calculation can precede simulating
of the process. In this case, the found values of reflux numbers, trays numbers,
profiles of liquid and vapor flows, and temperature and component concentrations
at the trays of the column are used as initial ones for the realization of simulating
calculation by means of known methods at a small number of iterations. At the
other approach, the conceptual design calculation used without the simulating
calculation.
In complex columns and distillation complexes, geometric conditions of joining
of section trajectories are similar to those for simple columns.
Therefore, the algorithm of design calculation of simple columns is a basis
for the algorithms of design calculation of any complex distillation columns and
distillation complexes.
In connection with it, we examine in detail conditions of the joining of sec-
tion trajectories and the algorithms of design calculation of simple columns at
various splits, and then on this basis we examine these questions for complex
columns.

7.2. Distillation Trajectories of Finite Columns: Possible Compositions
in Feed Cross Section

7.2.1. Location of Section Trajectories

The main difference between distillation trajectories of finite columns and those
of infinite columns consists of the fact that finite columns trajectories do not pass
2
2
1
1
through stationary points S , S , S , S ... From this, it follows that, in particular,
r s r s
absolutely sharp separation at which sections trajectories should pass through
1
1
points S and S is not feasible in finite columns. Also from this, it follows that
r s
the parameter (L/V) r for finite columns cannot have any value at which section
2
2
trajectories should pass through points S and S .
s
r
Section trajectories at quasisharp and nonsharp separation and at reflux bigger
than minimum are examined below. At quasisharp separation, each product of
the column contains, besides the product components itself, small amounts of
impurity components, mostly of the key nonproduct component. The purpose of
separation is to obtain in each product a prescribed set of product components at
a prescribed summary concentration of impurity components.
For example, for the split 1, 2,... k : k + 1, k + 2 ... n, components 1, 2 . . . k are
product ones for the top product and impurity ones for the bottom product and
vice versa for components k + 1, k + 2 ... n; key components are k and k + 1.
Along with that, the heavy key component (k + 1) is a main impurity one in the
top product and the light key component k is a main impurity one in the bottom
product. The remaining impurity components are contained in the products in
small amounts.
If a component is distributed, it is product one for both products of the col-
umn. For example, for the split 1, 2 ... k, k + 1: k + 1, k + 2,... n components
1, 2 ... k, k + 1 are product ones for the top product and components k + 2,... n
are impurity ones, components k + 1, k + 2 ... n are product ones for the bottom