Page 338 - Distillation theory

P1: JPJ/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c08 CB644-Petlyuk-v1 June 11, 2004 20:20

312 Synthesis of Separation Flowsheets

To choose the entrainer among a number of alternative entrainers, it is nec-
essary to carry out comparative estimation of expenditures on separation. For
preliminary estimation at extractive distillation, the value of minimum flow rate
of entrainer can be used and, at the sequence in Fig. 8.22b, the length of possible
product composition segment at the side of the concentration triangle can be used.
For more precise estimation, it is necessary to calculate the summary vapor flow
in the columns in the mode of minimum reflux at several values of excess factor
at the flow rate of entrainer.

8.7. Petroleum Mixtures

8.7.1. Peculiarities of Petroleum as Raw Material for Separation

A number of significant peculiarities distinguishes petroleum from other kinds of
raw materials:

1. The largest world volume of production. It attaches especially great impor-
tance to the solution of the task of optimal designing of petroleum refining
units.
2. Continuity of properties of components. A large number of components
leads to the fact that dependence of properties of components on their
normal bubble temperature is continuous. Therefore, in practice, while
designing one deals not with the true components but with pseudocom-
ponents (i.e., with groups of components boiling away in a set interval of
temperatures), and the quality of the products is characterized not by their
purity but by their refinery inspection properties.
3. Wide intervals of bubble temperatures of components from negative tem-
◦
peratures for light hydrocarbon gases up to over 800 C for heavy fractions
of mazut and tar.
4. Thermolability–thermal decomposition of components at high tempera-
◦
tures (360 ÷ 400C). Thermolability limits separability of petroleum and
the possibility of recovery of the most valuable components. Therefore,
the main task of synthesis of petroleum mixture separation flowsheets is
the creation of such flowsheets that allow recovery of a maximum number
of valuable components at a permissible temperature (i.e., without thermal
decomposition of components).

8.7.2. Methods of Petroleum Separability Increase

To increase recovery of valuable (light) components, it is necessary to increase the
amount of the heat brought in without exceeding the set maximum permissible
temperature. There are several methods to achieve this goal:
1. It is necessary to bring in heat, not in the reboiler of the column, but
in its feeding, bringing in the bottom a light stripping agent (most often,
steam serves as one). At such a method of creation, vapor flow maximum