THE ANATOMY OF A CHEMICAL MANUFACTURING

PROCESS

The basic components of a typical chemical process are shown in Figure 1.3, in which

each block represents a stage in the overall process for producing a product from the raw

materials. Figure 1.3 represents a generalised process; not all the stages will be needed for

any particular process, and the complexity of each stage will depend on the nature of the

process. Chemical engineering design is concerned with the selection and arrangement

of the stages, and the selection, specification and design of the equipment required to

perform the stage functions.

Stage 1. Raw material storage

Unless the raw materials (also called essential materials, or feed stocks) are supplied

as intermediate products (intermediates) from a neighbouring plant, some provision will 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.