Control of Volatile Organic Compounds Emissions from Manufacturing

are immiscible with the coolant. The direct contact between the vapor
and the coolant limits the application of contact condensers slnce the
spent coolant can present a secondary emission source or a wastewater
treatment probl em,34 unless it is economical ly feasible to separate the
two in a subsequent process.
Surface, or indirect, condensers are usually common shell-and-tube
heat exchangers. The coolant usually flows through the tubes and the
vapor condenses on the outside of the tubes. In some cases, however, it
may be preferable to condense the vapor inside the tubes. The condensate
forms a film on the cool tube and drains to st0ra~e.35 The shel 1-and-tube
condenser is the optimum configuration from the standpoint of mechanical
integrity, range of allowable design pressures and temperatures, and
versatility in type of service. She1 1 -and-tube condensers may be designed
to safely handle pressures ranging from full vacuum to approximately
41.5 MPa (6,000 psig), and for temperatures in the cryogenic range up to
approximately 1,lOO°C (2,000~~) ,36 Surface condensers usual ly requi re
more auxiliary equipment for operation (such as a cooling tower or a
refrigeration system) but offer the advantage of recovering valuable VOC
without contaminating the coolant, thereby mi nimizi ng waste di sposal
problems. The successively more volatile material returned from the
condenser to the distil lation column is termed "reflux," or overhead
product. The heavier compounds removed at the bottom are often called
column "bottoms. "37
The major pieces of equipment used in a typical refrigerated surface
condenser system are shown in Figure 3-6.38 Refrigeration is often
required to reduce the gas phase temperature sufficiently to achieve low
outlet VOC concentrations. This type of system includes dehumidification
equipment (1 ), a shel 1-and-tube heat exchanger (Z), a refrigeration
unit (3), recovery tank (4), and operating pumps (5). Heat transfer
within a shel 1 -and-tube condenser occurs through several materi a1 1 ayers,
including the condensate film, combined dirt and scale, the tube wall,
and the coolant film. The choice of coolant used depends on the saturation
temperature of the VOC stream. Chilled water can be used to cool down
to 4OC (40°F), brines to -34°C (-30°F), and chlorofluorocarbons below
-34°C (-30"~).39 Temperatures as low as -62°C (-80°F) may be necessary
to condense some VOC streams.34