Page 125 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 101

customer even during a plant difficulty or unforeseen shutdown. An additional
need for adequate storage is often encountered when it is necessary to meet
seasonal demands from steady production.
Bulk storage of liquids is generally handled by closed spherical or cylindri-
cal tanks to prevent the escape of volatiles and minimize contamination. Since
safety is an important consideration in storage-tank design, the American
Petroleum Institute? and the National Fire Protection Association* publish
rules for safe design and operation. Floating roof tanks are used to conserve
valuable products with vapor pressures which are below atmospheric pressure at
the storage temperature. Liquids with vapor pressures above atmospheric must
be stored in vapor-tight tanks capable of withstanding internal pressure. If
flammable liquids are stored in vented tanks, flame arresters must be installed
in all openings except connections made below the liquid level.
Gases are stored at atmospheric pressure in wet- or dry-seal gas holders.
The wet-gas holder maintains a liquid seal of water or oil between the top
movable inside tank and the stationary outside tank. In the dry-seal holder the
seal between the two tanks is made by means of a flexible rubber or plastic
curtain. Recent developments in bulk natural gas or gas-product storage show
that pumping the gas into underground strata is the cheapest method available.
High-pressure gas is stored in spherical or horizontal cylindrical pressure
vessels.
Solid products and raw materials are either stored in weather-tight tanks
with sloping floors or in outdoor bins and mounds. Solid products are often
packed directly in bags, sacks, or drums.

MATERIALS HANDLING
Materials-handling equipment is logically divided into continuous and batch
types, and into classes for the handling of liquids, solids, and gases. Liquids and
gases are handled by means of pumps and blowers; in pipes, flumes, and ducts;
and in containers such as drums, cylinders, and tank cars. Solids may be
handled by conveyors, bucket elevators, chutes, lift trucks, and pneumatic
systems. The selection of materials-handling equipment depends upon the cost
and the work to be done. Factors that‘must be considered in selecting such
equipment include:

1. Chemical and physical nature of material being handled
2. Type and distance of movement of material
3. Quantity of material moved per unit time
4. Nature of feed and discharge from materials-handling equipment
5. Continuous or intermittent nature of materials handling

tAmerican Petroleum Institute, 50 W. 50th St., New York, NY.
SNational Fire Protection Association, 60 Batterymarch St., Boston, MA.