Evaporation plants which deliver high-end thick liquor concentrations usually

Have mixing of recovery boiler ash and chemical make-up to an intermediate

Liquor before the concentrator. The suspended solids then act as crystallization

Seeds for salts precipitating in the concentrator, thus making heating surfaces less

Susceptible to fouling. Thick liquors of high dry solids concentrations require a

pressurized tank for storage at temperatures of 125–150 °C.

Chemical Recovery Processes

Thick liquor:

75%

Effect 2: 42%

Effect 3: 29%

Effect 4: 22%

Feed: 15%

Effect 5: 18%

10% 30% 50% 70% 90%

Water, tons per ton dry solids

Dry solids concentration, wt.-%

Fig. 9.6 Water in black liquor as a function of the dry solids

Concentration. _, calculated dry solids concentrations in a

five-effect evaporation plant with 15% dry solids in weak

liquor feed and 75% dry solids in thick liquor.

Increasing the dry solids concentration brings a number of considerable advantages

For subsequent firing in the recovery boiler, including more stable furnace

Conditions, higher boiler capacity, and better steam economy.

Vapor Recompression

The concept of mechanical vapor recompression is based on a process where evaporation

Is driven by electrical power. In general, vapor coming from the liquor

Side of an evaporator body is compressed and recycled back to the steam side of

The same body for condensation. The principle is shown schematically in Fig. 9.7.

The vapors from all bodies are collected and fed to a fan-type, centrifugal compressor.

The compressed vapors then return, at an elevated temperature, to the different

Bodies and condense at the steam sides, by that evaporating new water on the

Liquor sides.

The liquor is pumped from body to body. In contrast to multiple-effect plants,

Where the flow rates of condensate from all effects are similar (at the same surface

Area), vapor recompression plants have the highest condensate flow rate from the

Thin liquor stage and the lowest flow rate from the thick liquor stage. This is

Caused by the reduced driving temperature difference due to the increasing boiling

Point rise at higher dry solids concentrations. In some applications, it is useful

To install a second fan in series to the first one. The second fan (shown in dotted

Style in Fig. 9.7) is dedicated to supplying the higher-concentration bodies with

Vapor of more elevated temperature, thus considerably improving their performance.

Recovery

THIN LIQUOR

THICK LIQUOR

CONDENSATE

COMPRESSOR

Fig. 9.7 Principle of vapor recompression evaporation

Demonstrated on a system with three evaporator bodies.

Дата добавления: 2015-10-21