Reference Manual

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Chapter 11
Black Liquor Evaporator & Concentrator
The evaporator/concentrator system serves as a
bridge between the pulp mill and powerhouse.
This is the first step in reclaiming spent cooking
chemicals. The evaporator receives weak black
liquor from the pulp washers (or continuous
digester) and concentrates the solution by
evaporating a large portion of the water content.
The concentrated black liquor is then sent to the
powerhouse for combustion in the recovery boiler.
A portion of the water content must be removed to
maintain safe and efficient combustion in the
recovery boiler. Although various methods exist
for this process, the primary purpose of
economical evaporation of water is a common
goal.
Multiple-Effect Evaporator
The most common method, referred to as a
multiple-effect evaporator, uses a series of
evaporator bodies to remove water from weak
black liquor. These evaporator bodies typically
receive weak black liquor at 12 - 15% solids
concentration and evaporate a portion of the water
to raise the solids concentration to 50 - 60%.
“Solids” refers to the organic wood constituents
and inorganic cooking chemicals. The product is
referred to as heavy or strong black liquor.
The primary advantage of multiple evaporator
bodies is steam economy. A series of evaporator
bodies makes it possible to remove 4 - 6 pounds
of water per pound of motive steam used. This is
accomplished by connecting the bodies in series
so the vapor generated from one evaporator
becomes the steam supply for the next evaporator
in the series. Effects are numbered in order of the
steam flow. Weak black liquor starts at the last
effect, thus moving in a counter-current flow to
steam. Although the number of effects may vary,
six, or a sextuple-effect, is most common. Capital
cost usually offsets any increase in steam
economy if more effects are used.
A typical sextuple-effect evaporator set is shown
in figure 11-1. Each individual evaporator consists
of a heating element and a vapor head. Each
heating element consists of a tube bundle with
upper and lower tube sheets. Liquor flows on the
inside of the tubes and steam on the outside of the
tubes. Some designs use plate-type heating
elements instead of tubes. In either design, the
transfer of heat causes the liquor to boil and the
vapor that forms is carried to the next effect to
continue the evaporation process. Condensate
formed by condensing steam vapor is also
removed.
Motive steam is fed to the first effect and weak
black liquor feed split between the 5th and 6th
effects. The black liquor increases in solids
content and boiling temperature as it progresses
toward the first effect. In order for boiling to take
place, the pressure on the liquor side of the tubes
must be less than the pressure on the steam side.
Thus, the pressure must be different in each effect
and decrease from a high in the first effect of 30 -
40 psig to a low in the sixth effect of 20 - 25
inches of Mercury (Hg) vacuum. Maintaining this
partial vacuum is accomplished by piping vapors
from the sixth effect to a condenser and removing
non- condensible gases with an ejector or vacuum
pump.
Evaporator Types
As mentioned earlier, the most common type of
evaporator is the multiple-effect type. However,
many variations of this concept are used. The
largest installed base of evaporators is the rising
film or long tube vertical (LTV) type (see figure
11-1). This was the prevalent design until the late
1970s. In this design, the liquor enters a cavity