Pillow Plate Steel Exchanger In Black Liquor Evaporation During Paper Pulp
In recent years, with the maturity of mechanical vapor recompression (MVR) technology. This technology has been widely used in wastewater treatment (including chemical pulp black liquor, APMP & BCTMP and other chemical mechanical pulp wastewater), food industry, seawater desalination and other fields.
Black Liquor rom the brown stock washers is typically at 13-18%TS. Most of this water content must be evaporated to produce a material with high enough solids to support effective combustion in the recovery boiler, typically between 65% and 80%TS.
During evaporation to this level of solids, various volatile components (sulfur compounds, methanol, etc.) are released from the liquor and must be separated from the condensate to allow reuse in the fiberline and recausticizing. From this point of view, the evaporation plant actually serves as a"water factory"within the mill. Black liquor also contains a substantial fraction of inorganic compounds which, during the evaporation process, reach their solubility limit and can deposit as scale on the evaporator heat transfer surfaces greatly limiting the operating capacity of the evaporation plant and of the entire recovery island.
Evaporation of weak black liquor to separate water and create a combustible product-strong black liquor
The three main processes that occur in the evaporator are:
Black liquor+heat to Strong Black liquor + Water + Steam
Condensate + Steam to Clean + Dirty condensate + NCG
Black liquor to Black liquor + Soap (only softwood)
Multi-effect evaporation steam consumption
(t steam/ t water)
MULTIPLE EFFECT EVAPORATORS
Black liquor evaporators use indirect contact with condensing steam (or vapors)on one side of a heat transfer surface to evaporate water from the black liquor circulating on the other side. As with all indirect heat exchangers, the overall heat transfer between the condensing steam and the black liquor is governed by the basic relationship:
In this equation, the heat flow Q represents the amount of heat transfer that can be accomplished. As shown, this heat flow is dependent on the overall heat transfer coefficient U, the amount of heat transfer area A available to condense the incoming vapors and the temperature differential AT between the condensing vapors and the black liquor within the evaporator. Any reduction in U, A or AT will result directly in a reduction of Q. Evaporation can only take place when heat is added (in the form of condensing steam) to a surface area and a AT is established across that surface area.
The heat transfer coefficientU represents the rate at which heat transfer is accomplished across the given surface under the operating conditions of the evaporator.It is a complex number which varies with the material and quality of the heating surface, the temperature, concentration and viscosity of the black liquor and the level of turbulence within the liquor. The heat transfer coefficient is adversely affected by scale and deposits on the heat transfer surface,by low turbulence and also by the finish and thickness of the heat transfer surface.
Multiple effect evaporators (MEEs)are always used in black liquor service. The term muliple effectcomes from the multiple effective use of energy to perform the evaporation task. In such configuration,live steam is condensed only in the first effect evaporator, generating vapors that are then sent to condense in a second effect where additional evaporation takes place. The process can then be repeated until reaching the last effect evaporator where generated vapors are condensed in a condenser using cooling water.
Steam generation in the Kraft mill is a significant operating expense and every effort must be made to conserve its use. The evaporation plant is by far the major consumer of that steam for the removal of water from the weak black liquor.Economic operation of the evaporator is therefore predicated upon the multiple effective use of the heat available from the steam and therefore on the number ofeffects in the MEE.
Application of MVR Technology:
When the concentration of evaporated liquid is raised and its boiling point is raised within a certain limit, for example, when the concentration of evaporated black liquid is below 20%, the boiling point is raised no more than 1.5 ℃.
It is used in the areas or enterprises where the electricity price is cheap and the heat energy is expensive.
Application of technology in black liquor evaporation of pulp
Taking the evaporation of wheat straw pulp black liquor as an example, the combination evaporation process of MVR technology and multi effect evaporation was compared with the multi effect evaporation process. The details are as follows.
2.1 main parameters and evaporation scheme
The results show that the flow rate of dilute black liquor is 128.6t/h, the temperature of dilute black liquor is 70 ℃, the concentration of dilute black liquor is 10%, the concentration of concentrated black liquor is 45%, and the evaporation water is 100t / h. The multi effect evaporation process is 5-effect evaporation station, and the combined evaporation process is MVR pre concentration + 5-effect evaporation station.
Our production Capability