FOAM AND SOAP ...A DISCUSSION

In kraft pulping, foam frequently becomes a process problem. When resinous woods are
pulped, foam is created by the presence of resin and fatty acids. Broken foam containing
these compounds is usually referred to as soap. Soap becomes tall oil when acidified.

Foam is usually found to be a problem in brown stock washing. When vacuum drum
filters are used for washing, the drawing of air through the pulp mat and down the washer
drop leg usually generates foam. When this liquor is reused in washing, the air entrained
can hinder the pickup of the pulp on the wire. Most frequently defoamers are added to
the stock to aid in breaking the foam bubbles.

In some cases, the presence of foam in the first filtrate tank will create a back pressure
which reduces the washer capacity. This backpressure can be relieved by placing a
Foam Breaker on the filtrate tank or by directing the foam to a "foam tower." Many older
mills have a "foam tower" which is a tank of 10 to 12 foot diameter by 40 to 50 foot tall.
One or more Foam Breakers are usually mounted on the tank top to break foam as the
excess air is released to atmosphere.

The most important thing to be practiced in any foam breaking system is the removal of
the broken foam from the liquor at the earliest opportunity. Light foam broken in a Foam
Breaker may still require additional processing of foam concentration to increase the
density sufficiently that it becomes a soap rather than a foam.

Practical removal of soap usually is accomplished in the evaporator train as the liquor is
concentrated to the 23 to 28 percent solids range. This solids concentration conveniently
occurs at a point when the evaporators are at atmospheric pressure. Thus, it is possible
to divert the liquor to a soap skimming tank where the soap is permitted to float on the
surface of the liquor for removal.

Often when Southern pine is being pulped, the soap concentration in the liquor is so great
that some separation will occur in the filtrate tanks, in the weak liquor storage tanks and
in the strong liquor storage tanks. Frequently provisions are made to remove surface
soap from storage tanks either by utilizing a floating soap skimmer or by providing draw
down pipe nozzles at several elevations near the top of the tank. When these nozzles
are utilized, the liquor level is permitted to rise so that soap can be discharged.

Pulp mill experience has also shown that when black liquors generated from pulping
resinous woods are mixed with black liquors from pulping hardwood, foam problems
diminish.

During our extensive experience in black liquor oxidation, A. H. Lundberg, Inc., developed
two centrifugal machines for the breaking of the foam and concentrating it to heavy soap
consistency. These machines are now known as the Lundberg Foam Breaker for light
foam, and the Soap Concentrator for heavier foams. The Foam Breaker is designed to
reduce light foam with a density of as low as 0.1 lb/ft3 to 3 to 5 lb/ft3. The concentrator
will further reduce the air content of these foams to obtain a soap with a density of 7 to
8 lb/ft3. The development of these machines as an integral part of black liquor oxidation
was required since the successful air oxidation of black liquor resulted in foam generation.

PROBLEM:    Foam in first filtrate tank.

When pressure builds up in the first filtrate tank due to foam accumulation, the pressure
can be relieved through a Lundberg Foam Breaker. The foambreaker is installed on the
side of the filtrate tank at a level that will be a foot or two above the liquor level. The
internals of the filtrate tank frequently require modification so that the incoming liquor and
air from the washer are discharged in a seal pot that will facilitate separation of the air
from the liquor. The liquor to be used in pulp dilution is withdrawn from the bottom of the
tank. The excess liquid destined to the evaporators is overflowed into a funnel and
discharge pipe which establishes liquor level in the tank. The foam layer is established
on the top of the liquor and above the discharge funnel. It is above this liquor level that
the foam nozzle leading to the Foam Breaker is located. Broken foam from the Foam
Breaker can be flushed with a side stream of overflow liquor from the transfer pump on
the weak liquor storage.

PROBLEM:    Soap accumulation on weak liquor in storage.

When soap separates from weak liquor in storage, a layer of soap will accumulate on top
the liquor. In extreme cases, weak liquor tanks have become filled with soap and a false
liquor inventory is indicated. As liquor inventory is depleted, soap can be pumped to the
evaporators. This usually results in fouled evaporators that require washout.

To control this problem, the soap layer can be withdrawn either through a floating soap
skimmer, or through drawdown pipe nozzles, as mentioned above. In cases where
severe accumulations are encountered, we recommend the use of two or more weak
liquor storage tanks which is normally maintained at a constant liquor level. As soap is
accumulated, it will overflow into a Lundberg soap concentrator. The concentrated soap
is then pumped to the soap handling or soap storage system.

Liquor enters the first tank near the top to facilitate soap separation. A second inlet at
the bottom of the tank is provided when the liquor inventory in the first storage tank is
required. Lundberg engineers are available to assist in design of a weak liquor system
with soap removal capabilities.

PROBLEM:    Removal of soap in the evaporators.

Most common method of soap recovery is the skimming of liquor at 23 to 28 percent
solids. This is accomplished at an intermediate stage of evaporation. The sizing of a
soap skimming tank is dependent upon the rate of rise of soap in liquor. Typically the
rate of rise for soap is assumed to be 3.5 feet per hour when actual data is not provided.
The skim tank should also be baffled to elongate the flow of liquor through the tank and
to minimize short-circuiting. A liquor depth of about 10 feet is recommended, with a two
foot soap layer accumulation on the liquor surface. The tank baffles extend into the mid-
level of the soap.

Supported by a truss system across the tank top is the skim rake and drive. The skim
rake moves the soap to a launderer. Removed soap flows into a Lundberg soap
concentrator for removal of entrained air. The deaerated soap then enters a small tank
where some liquor is drained and soap accumulates for transfer to soap storage.

A special overflow weir is recommended for use in maintaining a constant liquor level in
the skim tank.

PROBLEM:    Liquor is entrained in soap.

Foam is air bubbles surrounded by a film of fatty or resin acid
soap in the liquor system. When the bubbles are broken, some liquor remains with the soap.
The handling of foam and soap requires first the removal of air from the foam, and
second the removal of liquor from soap. Soap collected by flotation usually contains 12 to 15
percent liquor. Upon acidification, the alkali content of the black liquor must be
neutralized prior to saponification of the soap. Removal of the excess liquor prior to
acidification reduces the acid required.

In order to remove liquor from soap, Lundberg has developed the liquor/soap separator
tank and rake system. In this system, a tank dedicated to liquor/soap separation is fitted
with a rake system. The rake system consists of fingers which provide a path for the
liquor to flow down ward through the soap into the bottom of the tank where a liquor level
accumulates. Periodically the liquor is pumped to storage. At the top of the tank, a skim
arm pushes the soap to a launderer and then it is transferred with a soap pump to soap
storage.

A. H. Lundberg, Inc. process engineers are available to work with mills on solving foam
and soap handling problems. Please provide us with a complete description of your
problems and existing equipment with your inquiry.