Page 101 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 79

Development of a Pollution Control System
Developing a pollution control system involves an engineering evaluation of
several factors which encompass a complete system. These include investigation
of the pollution source, determining the properties of the pollution emissions,
design ofthe collection and transfer systems, selection of the control device, and
dispersion of the exhaust to meet applicable regulations.
A key responsibility of the design engineer is to investigate the pollutants
and the total volume dispersed. It is axiomatic that the size of equipment is
directly related to the volume being treated and thus equipment costs can be
reduced by decreasing the exhaust volume. Similarly, stages of treatment are
related to the quantity of pollutants that must be removed. Any process change
that favorably alters the concentrations will result in savings. Additionally,
consideration should be given to changing raw materials used and even process
operations if a significant reduction in pollution source can be attained. The
extent to which source correction is justified depends on the cost of the
proposed treatment plant.
For example, the characteristics of equipment for air pollution control, as
specified in Table 8, often limit the temperature and humidity of inlet streams
to these devices. Three methods generally considered for cooling gases below
500°F are dilution with cool air, quenching with a water spray, and the use of
cooling columns. Each approach has advantages and disadvantages. The method
selected will be dependent on cost and limitation imposed by the control device.
Selection of the most appropriate control device requires consideration of
the pollutant being handled and the features of the control device. Often, poor
system performance can be attributed to the selection of a control device that is
not suited to the -pollutant characteristics. An understanding of the equipment
operating principles will enable the design engineer to avoid this problem.

Air Pollution Abatement
The most recent changes in the U.S. Clean Air Act Amendments have changed
the regulatory ground rules so that almost any air-pollutant-emitting new facility
or modification is subject to the provisions of the law. For most situations, a
New Source Review (NSR) application will have to be filed before construction
is allowed. Source categories covered at this time include petroleum refineries,
sulfur recovery plants, carbon black plants, fuel conversion plants, chemical
process plants, fossil-fuel boilers (greater than 250 MM Btu/h heat input), and
petroleum storage and transfer facilities (greater than 300,000-barrel capacity).
To obtain a construction permit, a new or modified source governed by the
Clean Air Act must meet certain requirements. These include a demonstration
that “best available control technology” (BACT) is to be used at the source. In
addition, an air quality review must demonstrate that the source will not cause