Control of Volatile Organic Compounds Emissions from Manufacturing

I gases
may consist of a single condensable component or any number of
gaseous components which may or may not all be condensable or miscible
with one another. Example gas streams found in the polystyrene industry
may consist of a single condensable component (styrelne); a mixture of
condensable and noncondensable components (styrene and air); a mixture
of condensable, but immiscible, components (styrene and steam); or a
mixture of condensable, but immiscible, components with a noncondensable
component (styrene, steam, and ai r).
Condensers are designed and sized using the pri nciples of thermodynamics.
At a fixed pressure, a pure component will condense isothermally at the
saturation or equilibrium temperature, yielding a pure liquid conden-
sate. A vapor mixture, however, does not have a single condensate
temperature. As the temperature drops, condensation progresses, and the
composition, temperature, enthalpy, and fl owrate of both the remaining
vapor and the condensate will change. These change:; can be calculated
from thermodynamics data, if it is assumed that the vapor and liquid
condensate are in equi 1 ibrium. Variations in composition and temperature
will affect most of the physical and transport properties which must be
used in condenser design calculations. When these properties change,
the calculations governing the heat transfer process are adjusted to
accom~mdate these changes.
In a two-component vapor stream with one noncondensable component,
condensation occurs when the partial pressure of the condensable component
is equal to the component's vapor pressure. To separate the condensate
from the gas at fixed pressure, the temperature of the vapor mixture
must be reduced. The liquid will begin to appear when the vapor pressure
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of the condensable component becomes equal to its partial pressure, the
Mdew point." Condensation continues as the temperature is further
reduced.
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The presence of a noncondensable componenl: interferes with the
condensation process, because a layer of noncondensable on the condensate
acts as a heat transfer barrier.
Two types of condensers are employed: contact and surface, Contact,
or direct, condensers cause the hot gas to mingle intimately with the
cooli ng medium. Contact condensers usual ly operate by sprayi ng a cool
liquid directly into the gas stream. Contact condensers also may behave
as scrubbers since they sometimes collect noncondensable vapors which