Reference Manual

11−5
today’s concentrator, it was often referred as a
direct contact evaporator. The two most widely
used types, the cyclone and cascade evaporators,
utilized hot flue gas exiting the recovery boiler to
further concentrate the black liquor. However, the
direct contact of flue gas and black liquor strips
sulfur compounds from the liquor which results in
air pollution and sulfur loss. Most mills have
eliminated direct contact evaporators in favor of
more modern indirectly heated concentration
equipment.
D Forced Circulation Concentrators
To overcome the limitations of direct contact
evaporators, indirectly heated concentrator bodies
are used for final liquor concentration. This type of
installation typically involves one or more
additional effects ahead of the first effect in the
multiple-effect evaporator set. The first effect is
fed with a separate supply of steam and
concentrates the product liquor from the first effect
of the multiple-effect evaporators.
The concentrator effects are generally used in a
switching arrangement such that one effect is
concentrating high solids black liquor while the
other effect is concentrating lower solids liquor.
This type of arrangement uses the lower solids
liquor to wash deposits left from concentrating
high solids liquor.
D Falling Film Crystallizers
One of the most recent concentrator designs is
referred to as a falling film crystallizing
concentrator. As mentioned earlier, salt crystals
tend to form once black liquor becomes saturated
at about 55% solids. In typical evaporators these
crystals tend to stick to heat exchange surfaces
and prohibit heat transfer. Crystallizers are
designed to control crystal formation such that
newly formed crystals will bond to crystals
contained in the recirculating liquor rather than the
heating surfaces. This allows for extremely high
solids concentration (up to 80%) with reduced risk
of fouling. The FFC concentrator can be used for
all effects in an evaporator system where
crystallization will occur.
D Other Designs
A couple of other designs may also be used as
concentrators. One type is a single concentrator
body with two or three separate sections. In this
design one section(s) is washed with weak liquor
from the evaporators while the other section(s) are
used to achieve final liquor concentration. A
second type combines preheat, falling, and rising
film sections in one body. These units typically use
forced circulation and are complicated to operate.
Although figures 11-1 and 11-2 indicate many of
the critical control valves, other general service
control valves are required for the successful
operation of an evaporator/concentrator set. Many
of the valves are on mill supply water, instrument
air, plant air, or low pressure steam heating lines.
Control Valve Selection
Black liquor is a thick media which can be erosive,
corrosive, or cause scaling problems in valves.
Valves must be able to perform in both control and
tight-shutoff applications, with special
consideration give to valves where black liquor is
the thickest; typically before entering and exiting
the concentrator.
In applications where black liquor has lower solids
content, the Control-Disk butterfly valve may be
used. However, in most cases, the Vee-Ball
segmented ball valve is the primary valve of
choice.