Chinese pillow plate jacket exchanger for falling film black liquor evaporator
Black Liquor
Black liquor is the waste liquor from the kraft pulping process after pulping is completed. It contains most of the original cooking inorganic elements and the degraded, dissolved wood substance. The latter includes acetic acid, formic acid, saccharinic acids, numerous other carboxylic acids (all as the sodium salts), dissolved hemicelluloses (especially xylans), methanol, and hundreds of other components. It is an extremely complex mixture. About 7 tons of black liquor at 15% solids (about 10% organic chemicals and 5% inorganic chemicals with a total solid is produced per ton of pulp. The black liquor must be concentrated to as high a solids content as possible before being burned in the recovery boiler to maximize the heat recovery. The viscosity rises rapidly with concentration above 50%, with softwood black liquors being more viscous than hardwood black liquors. Black liquor is usually fired at 65%–70% solids content.
EVAPORATOR EQUIPMENT
There are two basic types of evaporator equipment in service today for black liquor evaporation:
Rising Film Evaporators
Also referred to as a Long Tube Vertical (LTV) evaporator, this design has dominated the Industry for decades and remains a common sight in older mill operations.
Falling Film (FF) Evaporators
This evaporator design relies either on tubes or plates as heat transfer surfaces. Liquor is processed on the inside of the s intubular units but on the outside of the heat transfer surface in plate designs.
Comparison
| Name | Pillow plate series | Shell and tube heat exchanger | Detachable plate heat exchanger | Spiral plate heat exchanger |
| Operating temperature range | <800℃ | <800℃ | <170℃ | <350℃ |
| Maximum pressure | <60 bar | <200 bar | <32 bar | <25 bar |
| Heat transfer coefficient to water[W/m2·℃] | 3500 | 2700 | 5600 | 2000 |
| Application of air and water heat exchange | fit | fit | not fit | Partial fit |
| Immersion in tank or water | fit | Partial fit | not fit | not fit |
| Welding of tank and reactor | Applicable | not applicable | not applicable | not applicable |
| Install into the existing reactor and other equipment | Flexible application | Partial applicable | not applicable | not applicable |
| All welded construction | Applicable | Applicable | not applicable | not applicable |
| Heavily contaminated liquids and other applications | Applicable | Applicable | Partial Applicable | Applicable |
| Weight per unit area | low | high | low | high |
| Falling film, condenser and evaporator | fit | fit | Partial fit | Partial fit |
Black liquor evaporation is an essential part of the chemical recovery process as it significantly concentrates the dry solid matter in black liquor so that the liquor can be effectively combusted in recovery boiler. The water removed from the liquor can be segregated and processed so that it can be reused in the mill – reducing fresh water requirements, effluents, and saving money.
Evaporators and concentrators, with their lamella heating surfaces, are adept at processing the spent cooking liquors or mill effluents using the minimum amount of energy while maximizing product dry solids, plant availability, and condensate quality. The lamella heating surfaces are inherently non-scaling. This allows the use of various heat sources inside the lamella, allowing the mill to select the most energy-efficient process connection.
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