Page 332 - Air Pollution Control Engineering

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Condensation 311

2.2. Prevention of VOC Emission from Condensers
In most control applications, the emission stream contains large quantities of noncon-
densible gases and small quantities of condensible compounds. Design and operation
must limit emissions of VOCs from discharged condensate (i.e., secondary emissions).
Subcooling of the condensate may be required. Uncondensable air contaminants in the
gas stream must be either dissolved in the condensate or vented to other control equip-
ment. Gas streams at Superfund sites usually contain a variety of contaminants, and the
recovered stream may fail purity specifications and be unsalable. Such streams must be
disposed of by incineration or other methods. Another consideration is the moisture con-
tent of the gas stream. Any water condensing with the organic vapors dilutes the solvent
stream. Finally, condenser off-gas not meeting emission standards will require further
treatment, usually with activated carbon. Disposal problems and high power costs are
some of the disadvantages associated with condensation.

2.3. Proper Maintenance
Proper maintenance of a condenser system is essential to maintaining performance.
Scale buildup over time fouls condenser systems. This significantly increases fluid
pressure drop or decreases heat transfer, resulting in higher fluid outlet temperature
and decreased efficiency. Adequate control of hazardous air pollutants (HAPs) requires
continuous monitoring of the emission stream outlet temperature. Cleaning must be
performed without delay because scale buildup becomes much harder to remove over
time (1,11,16).
2.4. Condenser System Design Variables
The required condensation temperature represents the key design variable for con-
denser systems. As stated previously, a condenser’s removal efficiency greatly depends on
the concentration and nature of emission stream components. For example, compounds
with high boiling points (i.e., low volatility) condense more readily compared to those
with low boiling points. Assume, as a conservative starting point, that condensation will
be considered as a HAP emission control technique for VOCs with boiling points above
100ºF. Therefore, the concentration and nature of an emission stream are also important
design variables.
The temperature necessary to achieve a given removal efficiency (or outlet concen-
tration) depends on the vapor pressure of the HAP in question at the vapor–liquid
equilibrium. The removal efficiency for a given HAP can be determined from data on
its vapor pressure–temperature relationship. Vapor pressure–temperature data for typical
VOCs appear graphically in Cox charts (see Fig. 4). Coolant selection depends on the
required condensation temperature. All aforementioned parameters must be considered
for a proper condenser system design. See Table 1 for a summary of practical limits for
coolant selection.
3. ENGINEERING DESIGN

3.1. General Design Information
This section describes a shell-and-tube heat exchanger with the hot fluid (emission
stream) on the shell side and the cold fluid (coolant) on the tube side. Condensate forms