SOAP SKIM TANK

Alkaline pulping of resinous wood species results in the saponification of fats, resins, and
neutral oils present in the wood. Removal of these tall oil soaps has an economic benefit
to the pulp mill. In addition to the commercial value of the soap itself, removal of soap
improves evaporator performance by reducing foaming and condensate contamination as
well as reducing tube side deposits in the concentrated effects. It also permits
concentration to higher black liquor solids for more efficient recovery boiler operation.

As the solubility of soap in black liquor decreases with increasing liquor solids
concentration, the general practice is to divert liquor from the multiple effect evaporators
to a retention tank (soap skim tank). There, the lower density soap particles can separate
from the liquor before the liquor is returned to the evaporator train.

Selecting the location of the skim tank in the multiple effect evaporator train for optimum
soap removal should be determined by the mill's technical department as it will vary with
wood species being pulped and the individual evaporator configuration.

To reduce heat losses and prevent flashing with resulting thermal currents which could
effect proper operation of the skim tank, the liquor at the point selected must have a
temperature below 100 degrees C. The liquor solids will normally be between 24 and 32 percent.
Liquor samples meeting the above criteria can be collected from evaporator effects and
allowed to separate.

Once the optimum skim tank location has been selected, the skim tank can be designed.
The main criteria for sizing soap skim tanks is the rate of rise of the soap particle in the
liquor. This varies with soap particle size and black liquor viscosity as well as the density
differential between soap and liquor.

As soap particles are not uniform, a compromise must be made in sizing the skim tank
or the skim tank would become uneconomical for the incremental increase in soap
collection. Based on numerous trials on pine liquors in the southern United States, a rise
rate of 0.5 to 0.7 percent tall oil based on black liquor solids is used. This translates to
2.29 sq. ft. of skim tank area per US gpm or 3.375 m per liter/sec of black liquor flow.

The liquor depth does not enter into the calculation, but for convenience we have found
that a liquor depth of 3 meters plus an additional 0.6 meter height for a soap pad provides
adequate space for liquor distribution and level control.

In order for the above philosophy to be implemented, conditions approximating plug flow
of liquor must be achieved. Short circuiting from liquor inlet to outlet will result in local
liquor velocities exceeding the soap rise rate with loss of efficiency.

To achieve the equivalent of plug flow in large diameter skim tanks, baffles are provided
to direct the liquor in a horizontal laminar flow, thus effectively eliminating problems of
distribution over a large area.

Maintaining steady state conditions in the multiple effect evaporators is another
requirement of soap skim tank design.

If the skim tank is serving only one set of evaporators, a relatively simple standpipe
arrangement with a variable overflow weir has proven to be the most reliable. The
setting of the overflow weir determines the thickness of the soap pad floating on the black
liquor. Care must be taken to adequately size the standpipe and weir length so that
changes in black liquor flow do not result in measurable changes in pressure drop. For
example, a one-inch change in weir height will result in a four-inch change in the height
of the soap pad. In a 50 ft. diameter skim tank, this would result in a change in liquor
retention of approximately 1224 U.S. gallons and four times that quantity of soap.

If more than one line of evaporators is feeding the same soap skim tank, individual flow
meters should be provided on the supply and the return liquor lines to permit balancing
of the system.

Soap Skim Tank & Soap Skimmer
SS-1011-B