The continuous improvement in the closing of pulp and paper mill process circuits has
resulted in the reduced needs of hot water in most mills. The recovery of low level heat
by the production of hot water can therefore result in an excess of hot water, which the
mill must dispose of without any benefit.
A Pre-Evaporator overcomes this problem by utilizing the hot foul condensate as the heat
source for pre-evaporation of pulp mill spent liquor. A typical source for the foul
condensate is the digester blow heat recovery system. The net result of pre-evaporation
using mill waste heat is the reduction of live steam required in the multiple effect
evaporators and/or the increased through-put in the multiple effect units. While the pre-
evaporator utilizes the waste heat, it does not rid the mill of it, thus this heat is still
present, but at a lower level.
An efficiently designed blow heat system will produce hot condensate at approximately
205 degrees F. This high temperature permits a two-stage pre-evaporation unit which allows a
heat economy of more than one, and thus a more efficient use of the heat present. The
cooled foul condensate is returned to the bottom of the accumulator, sufficiently cooled
to permit total condensation of the blow steam. The amount of condensate recycled is
determined by the amount of heat incoming with the blow steam.
Hot water flows from the blow heat accumulator tank through filters to remove fiber
bundles prior to flashing in the first stage flash tank. The vapor flashed is fed to the
heater of the first effect. Frequently, the weak liquor feed temperature is above the
saturation temperature in the first effect vapor head, resulting in liquor flashing. The
vapor formed by the liquor flashing and boiling from the supplied heat source (vapor from the flash
tank) after leaving the vapor head enters the heater of the second effect.
Additional heat to the second effect is supplied by flashing the condensate from the first
effect heater. The vapor formed by the flashing and boiling of the liquor in the second
effect is condensed in a surface condenser. Foul condensate is pumped from the second
effect heater to the hot well.
The concentrated weak liquor exits the second effect and is returned to the weak liquor storage
tank. Returning it to this tank provides operating flexibility. The exiting weak liquor is at a lower
temperature than the entrance liquor due to the two-stage liquor flash that occurs in the
evaporator. By recycling the product liquor through internal heaters in the two bodies, it is
possible to increase the temperature of the weak liquor returning to the storage tank. The vacuum
in the surface condenser is maintained by controlling the cooling water exit temperature from the
surface condenser. Non-condensible gases are removed by a steam jet ejector or mechanical
vacuum pump.
The operator has the choice of controlling solids increase at varying heat inputs by letting
the liquor flow rate float (by ratio control) and adjusting throughput by inventory shift, or
maintain flow dictated by liquor production and letting solids increase float. If the installed
capacity of the evaporator is to be used at all times, the operator may control liquor flow
and solids increase under varying waste heat loads by adding make-up steam on
differential flow control to E-1 heater. Either rising film or falling film evaporator design
may be used.
The multi-effect evaporator will operate under the design conditions or at reduced rates
equally well. No attention or control changes are required by the operator. The co-current
arrangement with complete series feed will control itself within the rates attainable.
The cooled foul condensate is returned to the bottom of the blow heat accumulator tank
from the flash tank.
Pre-Evaporation with Flash Steam Diagram
SO-1038-B