Page 23 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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CHEMICAL ENGINEERING
have to be made to hold several days, or weeks, storage to smooth out fluctuations and
interruptions in supply. Even when the materials come from an adjacent plant some
provision is usually made to hold a few hours, or even days, supply to decouple the
processes. The storage required will depend on the nature of the raw materials, the method
of delivery, and what assurance can be placed on the continuity of supply. If materials are
delivered by ship (tanker or bulk carrier) several weeks stocks may be necessary; whereas
if they are received by road or rail, in smaller lots, less storage will be needed.
Stage 2. Feed preparation
Some purification, and preparation, of the raw materials will usually be necessary before
they are sufficiently pure, or in the right form, to be fed to the reaction stage. For example,
acetylene generated by the carbide process contains arsenical and sulphur compounds, and
other impurities, which must be removed by scrubbing with concentrated sulphuric acid
(or other processes) before it is sufficiently pure for reaction with hydrochloric acid to
produce dichloroethane. Liquid feeds will need to be vaporised before being fed to gas-
phase reactors, and solids may need crushing, grinding and screening.

Stage 3. Reactor
The reaction stage is the heart of a chemical manufacturing process. In the reactor the
raw materials are brought together under conditions that promote the production of the
desired product; invariably, by-products and unwanted compounds (impurities) will also
be formed.

Stage 4. Product separation
In this first stage after the reactor the products and by-products are separated from any
unreacted material. If in sufficient quantity, the unreacted material will be recycled to
the reactor. They may be returned directly to the reactor, or to the feed purification and
preparation stage. The by-products may also be separated from the products at this stage.

Stage 5. Purification
Before sale, the main product will usually need purification to meet the product specifi-
cation. If produced in economic quantities, the by-products may also be purified for sale.

Stage 6. Product storage

Some inventory of finished product must be held to match production with sales. Provision
for product packaging and transport will also be needed, depending on the nature of the
product. Liquids will normally be dispatched in drums and in bulk tankers (road, rail and
sea), solids in sacks, cartons or bales.
The stock held will depend on the nature of the product and the market.

Ancillary processes
In addition to the main process stages shown in Figure 1.3, provision will have to be
made for the supply of the services (utilities) needed; such as, process water, cooling