Page 36 - Separation process principles 2

4 Chapter 1 Separation Processes

Use the concepts of key components and separation power to measure the degree of separation between two key
components.
Make a selection of feasible separation operations based on factors involving the feed, products, property
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differences among chemical components, and characteristics of different separation operations.

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1.1 INDUSTRIAL CHEMICAL PROCESSES change pressure), mixing or dividing of streams or batches
of material, solids agglomeration, size reduction of solids,
The chemical industry manufactures products that differ in and separation of solids by size.
chemical content from process feeds, which can be (1) natu-
The key operations for the separation of chemical mixtures
rally occurring raw materials, (2) plant or animal matter,
into new mixtures and/or essentially pure components are
(3) chemical intermediates, (4) chemicals of commerce, or
of central importance. Most of the equipment in the average
(5) waste products. Especially common are oil refineries [I], chemical plant is there to purify raw materials, intermediates,
which, as indicated in Figure 1.1, produce a variety of useful
and products by the separation techniques described briefly in
products. The relative amounts of these products produced
this chapter and discussed in detail in subsequent chapters.
from, say, 150,000 bbllday of crude oil depend on the con-
Block-JEow diagrams are used to represent chemical
stituents of the crude oil and the types of refinery processes.
processes. They indicate, by square or rectangular blocks,
Processes include distillation to separate crude oil into vari-
chemical reaction and separation steps and, by connecting
ous boiling-point fractions or cuts, alkylation to combine
lines, the major process streams that flow from one process-
small hydrocarbon molecules into larger molecules, catalytic
ing step to another. Considerably more detail is shown in
reforming to change the structure of medium-size hydro- process-JEow diagrams, which also include auxiliary opera-
carbon molecules, fluid catalytic cracking to break apart large
tions and utilize symbols that depict more realistically the
hydrocarbon molecules, hydrocracking to break apart even
type of equipment employed. The block-flow diagram of a
larger molecules, and other processes to convert the crude-oil
continuous process for manufacturing hydrogen chloride gas
residue to coke and lighter fractions. from evaporated chlorine and electrolytic hydrogen [2] is
A chemical process is conducted in either a batchwise,
shown in Figure 1.2. The heart of the process is a chemical
continuous, or semicontinuous manner. The operations may
reactor, where the high-temperature gas-phase combustion
be classified as key operations, which are unique to chemical
reaction, H2 + C12 -+ 2HC1, occurs. The only auxiliary
engineering because they involve changes in chemical com-
equipment required consists of pumps and compressors to
position, or auxiliary operations, which are necessary to the
deliver feeds to the reactor and product to storage, and a heat
success of the key operations but may be designed by me-
exchanger to cool the product. For this process, no separa-
chanical engineers as well because the auxiliary operations
tion operations are necessary because complete conversion
do not involve changes in chemical composition. The key
of chlorine occurs in the reactor. A slight excess of hydrogen
operations are (1) chemical reactions and (2) separation of
is used, and the product, consisting of 99% HCI and small
chemical mixtures. The auxiliary operations include phase
amounts of H2, N2, H20, CO, and C02, requires no purifica-
separation, heat addition or removal (to change temperature
or phase condition), shaft-work addition or removal (to tion. Such simple commercial processes that require no sep-
aration of chemical species are very rare.
Some industrial chemical processes involve no chemical
Clean fuel aas reactions, but only operations for separating chemicals and
phases, together with auxiliary equipment. A block-flow dia-
gram for such a process is shown in Figure l .3, where wet
Sulfur
natural gas is continuously separated into six light-paraffin
Motor gasoline
t
Diesel fuel L 99% HCI
Crude oil Jet fuel 7
Oil
7 *
150.000 bbllday refinen/ Lubricants
-L-
Waxes L
Water-jacketed
L
I I combustion chamber
Fuel oils
I' Coke *
-
Chlorine vapor
Figure 1.1 Refinery for converting crude oil into a variety of
marketable products. Figure 1.2 Synthetic process for anhydrous HCl production.