Absorption systems design involves selecting a solvent and the design of the absorber.
Solubility is the most important consideration in the selection of a solvent for absorption. The higher the solubility, the lower the amount of solvent required to remove a given amount of pollutants. The solvent should also be relatively nonvolatile to prevent an excessive carryover in the gas effluent. Other favorable properties include low flammability and viscosity, high chemical stability, acceptable corrosiveness, and low toxicity and pollution potential. The final selection criterion is an economic comparison with other control technologies.
Any gas-liquid contactors that promote the mass transfer across the phase boundary can be used in absorption operations. The most popular devices are spray, packed, and tray columns as well as venturi scrubbers. For gas pollution control, the combination of high gas flowrate and low pollutant concentration suggests that the absorber should exhibit a low pressure drop. The mass-transfer efficiency of the absorber determines the height of the column, but it is not as important a consideration as the pressure drop. Clearly, spray and packed columns are the best devices to satisfy the preceding criteria. Spray columns are used where fouling and low pressure drops are encountered. The design of a spray column is straightforward and is detailed in other publications (Kohl 1987). The packed column is the device used most often.
Two types of packings are used for absorption: random packing and structured packing. Table 5.20.1 shows a list of packing suppliers. Structured packing tends to be proprietary, and the design procedure is normally obtained from suppliers. Random packings (e.g., Pall rings and Intalox saddles) can be purchased from several suppliers, and the system design procedure is published. Table 5.20.2 shows the properties of typical random packings.
As a general rule, randomly packed columns have the following characteristics (Strigle 1987):
A pressure drop between 2 and 5 cm H2O/m of packed depth
Air velocity between 1.7 and 2.4 m/sec for modern, high-
capacity plastic packings An inlet concentration of pollutants below 0.5% by volume
A superficial liquid rate in the range of 1.35 to 5.5 L/m2 • sec
Figure 5.20.1 is a schematic diagram of a typical packed column installation.
In designing a packed column, the design engineer must consider the solvent rate, column diameter, and column height.
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