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Wet and Dry Scrubbing 215

Table 7
Annual Cost Factors for Absorbers Systems
Cost item Factor
Direct Cost
Utilities
Electricity $0.059/kWh
3
Solvent (water) $0.20/10 gal
Operating Labor
Operator labor $12.96/h
Supervisor 15% of operator labor
Maintenance
Maintenance labor $14.96/h
Materials 100% of maintenance labor
Indirect Costs
Overhead 0.60 (Operating labor and maintenance)
Administrative 2% of TCC
Property taxes 1% of TCC
Insurance 1% of TCC
Capital recovery a 0.1628 (TCC)
a The capital recovery cost is estimated as i (1 + i) /[ (1 + i) − 1],
n
n
where i is interest (10%) and n is equipment life (10 yr).
Source: Data from refs. 9 and 12.

The USC is the unit solvent cost. The costs of various solvents are given in Table 7.
As of January 1990, the solvent cost of water, on average, was $0.20 per 1000 gal in the
United States (1 US gal = 3.785 L).

2.5. Venturi Wet Scrubber Design
2.5.1. General Design Considerations
Venturi scrubbers provide excellent removal efficiency for particulate matter of 0.5
to 5 µm in diameter (see Fig. 1c). A general design criterion of Venturi scrubbers is that
for any given pressure drop across a scrubber, the longer the constriction or “throat,” the
higher the removal efficiency. The throat cannot be made so long as to have frictional
loses become significant, however. Suppliers of Venturi scrubbers also provide for vari-
able throat sizes as a control mechanism of the scrubber. Changing the throat size will
result in an adjustment of gas velocity, which affects pressure drop as well as efficiency.
In a typical Venturi scrubber, liquid, normally water, is introduced upstream of the
Venturi or throat. As the water flows down the convergent sides of the throat, the sudden
acceleration of gas velocity in the throat atomizes the water. This is referred to in industry
as the wetted approach to Venturi scrubber design. As so implied, a nonwetted method
is also possible. In this design, the water (or other liquid) is injected directly into the
throat. The nonwetted scheme is used if the gaseous stream being scrubbed is near its
saturation point. The nonwetted method requires that very clean water be used to avoid
plugging of the injection nozzles. The wetted scheme is used if the gas being treated is
hot, as this means that some amount of water must be evaporated (18).