Solids concentration in the black liquor for the purpose of firing the thick liquor

Chemical Recovery Processes

In a recovery boiler. Depending on the process used, a final solids concentration

in thick liquor from kraft pulping up to 85% is achievable. Most commonly, kraft

thick liquor concentrations are in the range of 65% to 80% dry solids. Thick

liquors from sulfite pulping reach 50–65% dry solids.

Either steam or electrical power can be used to provide the energy to evaporate

Water from the black liquor. As there are usually sufficient quantities of low-pressure

Steam available in a pulp mill, the most economic solution in almost all cases

Is multi-effect evaporation with steam as the energy source. The use of mechanical

Vapor recompression (an electrical power-consuming process) is economically

Restricted to the evaporation of liquors with a low boiling point rise. Vapor recompression

Is therefore viable mainly for the pre-thickening of kraft black liquors at low

Dry solids concentrations, or for the evaporation of liquors from sulfite pulping.

Evaporators

Today’s evaporators are mainly of the falling film type, with plates or tubes as

Heating elements. For applications involving high-viscosity liquor, or liquor with a

Strong tendency to scaling, forced circulation evaporators are also employed. Evaporators

Are usually constructed from stainless steel.

A schematic diagram of a falling film evaporator equipped with plate (lamella)

Heating elements is shown in Fig. 9.4. Thin liquor is fed to the suction side of the

Circulation liquor pump, which lifts the black liquor up to the liquor distribution.

The liquor distribution ensures uniform wetting of the heating element surfaces.

As the liquor flows downwards on the hot surface by gravity, the water is evaporated

And the concentration of the liquor increases. The concentrated liquor is collected

In the sump at the bottom of the evaporator. The generated vapor escapes

From between the lamellas to the outer sections of the evaporator body, and then

Proceeds to the droplet separator, where the entrained liquor droplets are

Retained.

The steam, which drives the evaporation on the liquor side, is condensed inside

The lamellas. Steam may actually be fresh steam or vapor coming from elsewhere,

For example, another evaporator. In the latter case, the vapor often contains gases

Which are not condensable under the given conditions, such as methanol and

Reduced sulfur compounds from kraft black liquor or sulfur dioxide from spent

Sulfite liquor. If not removed, noncondensable gases (NCG) accumulate on the

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