Gas Emission Control - Mathematics and Statistics for the Environment - page 481

16.6 CONDENSATION
Condensation is a process by which volatile gases are removed from the contaminant stream and
changed into a liquid. That is, it is a process that reduces a gas or vapor to a liquid. Condensers con-
dense vapors to a liquid phase by either increasing system pressure without a change in temperature
or by decreasing the system temperature to its saturation temperature without a pressure change.
The common approach is to reduce the temperature of the gas stream, because increasing the pres-
sure of a gas is very expensive (USEPA, 1981, p. 6-1). Condensation is affected by the composition
of the contaminant gas stream. When different gases are present in the streams that condense under
different conditions, condensation is hindered.
In air pollution control, a condenser can be used in two ways: either for pretreatment to reduce the
load problems with other air pollution control equipment or for effectively controlling contaminants
in the form of gases and vapors. There are two basic types of condensation equipment—contact
and surface condensers. In a contact condenser (which resembles a simple spray scrubber), spraying
liquid directly on the vapor stream (see Figure 16.17) cools the vapor. The cooled vapor condenses,
and the water and condensate mixture are removed, treated, and disposed of.
A surface condenser is normally a shell-and-tube heat exchanger (see Figure 16.18). It uses a
cooling medium of air or water where the vapor to be condensed is separated from the cooling
medium by a metal wall. Coolant flows through the tubes, while the vapor is passed over the tubes,
condenses on the outside of the tubes, and drains off to storage (USEPA, 1971).
In general, condensers are simple, relatively inexpensive devices that normally use water or air
to cool and condense a vapor stream. Condensers are used in a wide range of industrial applica-
tions, including petroleum refining, petrochemical manufacturing, basic chemical manufacturing,
dry cleaning, and degreasing.
Clean gas out
Water in
Dirty gas in
Water and condensate out
FIGURE 16.17 Contact condenser. (From USEPA, Control Techniques for Gases and Particulates , U.S.
Environmental Protection Agency, Washington, DC, 1971.)