21.1 DEVICE TYPE
Venturi type scrubbers have been used for decades for the direct contact evaporation
of water from process liquids such as pulp mill black liquor. These devices were
typically used in stages wherein the black liquor was gradually increased in concen-
tration so as to avoid foaming issues and to provide adequate control over the solids.
The Venturi design had advantages since the gas path is essentially open and thus
reduces the chance of plugging.
The design has also been applied to waste incinerators wherein a zero or near-zero
liquid discharge is required. Venturi scrubbers are used since they can recirculate at
a higher solids content than most spray or tray type devices.
Of course, any wet scrubber that uses the direct contact of hot, dry gas with water
will experience evaporation. As mentioned in Chapter 1, “Air Pollution Control 101,”
the proper application of a wet scrubber to a hot, dry gas source will result in that gas
stream becoming saturated with water vapor. The source of that vapor is the evapo-
ration of the water from the liquid being recycled. Fluidized bed type contactors can
also be used as evaporators; however, the peak solids content that can be reached is
typically less than that of a Venturi scrubber.
21.2 TYPICAL APPLICATIONS
Direct contact concentrators have been used to increase the solids content of landll
leachate, to produce water from natural gas recovery operations (such as in “frack-
ing”), and to a lesser extent for process liquid stream concentration. The device is
used effectively wherein direct contact of the liquid with hot gases is acceptable or
where surface type evaporators plug with solids on the heat exchange surfaces.
In recent years at the time of this writing, interest has increased in evaporating
excess water from liquid streams, particularly from those liquid streams relating to
gas recovery and/or leachate treatment. Evaporating excess water reduces the vol-
ume and cost of trucking or pumping the liquid streams away for treatment off-site.
If the treatment is on-site, removing much of the excess water can reduce the size
and cost of the on-site treatment equipment. Sometimes, the evaporated water can be
recovered through condensation if the process to which the system is attached is in
need of make-up water.