FOUL CONDENSATE STRIPPING


In recent years the in-plant treatment of waste condensate has become an accepted
method for the removal of odorous gases and BOD (Biological Oxygen Demand). This
treatment has been necessitated for environmental reasons as the pollution control
regulations have become increasingly stringent. A. H. Lundberg Inc. has developed
steam stripping systems designed for the removal of malodorous gases or both the gases
and BOD.

The condensate streams of concern include the liquor condensates of the multiple effect
evaporator, the overflow of the blow heat accumulator and the underflow from the
turpentine decanter. Each of these streams are contaminated by organic sulfides (TRS-
Total Reduced Sulfur) and volatile organic compounds that contribute to the BOD load.

The TRS gases are a result of the kraft pulping reactions. Among the TRS compounds
are hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide. These
are noxious gases with a very low threshold of odor detectability, as shown in Table I.
Thus, before any of these streams can be used in the mill or discharged to waste water
treatment, the TRS compounds must be removed at a high efficiency.



TABLE I: THRESHOLD OF ODOR LIMITS IN AIR
PPB BY VOLUME
    Hydrogen Sulfide    H2S     5
    Methyl Mercaptan    CH3SH     5
    Dimethyl Suifide     CH3)2S 50
    Dimethyl Disulfide (CH3)2S 100

The sources of BOD are primarily methanol and turpentine with ethanol and acetic acid
also contributing to the load. Turpentine is a wood extractive and its concentration is
dependent upon the species of wood being pulped. Likewise, since methanol is formed
by a pulping reaction with lignin, its concentration is also dependent upon the wood being
pulped. Hardwoods, for example alder, may potentially yield twice as much methanol as
softwoods, such as red cedar or Douglas fir. The BOD equivalents of methanol,
turpentine and acetic acid in pounds of oxygen per pound of material are 1.0,1.08, and
0.65 respectively.

Economically, the collection and stripping of all the foul condensate streams cannot be
justified. However, if the condensate streams are judiciously chosen, it may be possible
to isolate 80% of the methanol and a larger fraction of the TRS and turpentine in only
40% of the total condensate. Table II shows a typical condensate methanol content and
the amount of condensate expected for a kraft mill. The condensate from the second
through the fifth effect of evaporation are relatively clean and therefore may be reused
within the mill without treatment. The other streams mentioned in Table II should at least
be treated for the removal of TRS if not methanol.


TABLE II: CONTAMINATES IN CONDENSATE
        Condensate    Methanol
        (Gal/ATD Pulp)    (Ib/ATD Pulp)

Accumulator    270 8.0
Turpentine Decanter 60 3.0
Evaporator Effects    990     1.7
2-5
Evaporator Effect 6    610    6.8
Surface Condenser


The benefits of removing each of these contaminants is a part of the decision for the type
of system to be installed. First, methanol or BOD removal will be of economic benefit by
reducing the size of biological treatment plant required. This is of importance both in
capital cost of a new plant and in reducing the requirement from an overloaded system.
Second, odor removal results in clean condensate for reuse (lower fresh water
requirements) or lower odor at the secondary aeration plant. Depending upon the wood
being pulped, and the resultant turpentine concentration in the condensate, recovery of
turpentine may also be attractive. It must be emphasized that the steam used for
stripping condensate is not lost heat, but is recovered as hot stripped condensate for use
in the pulp washers or as hot process water recovered in the reflux condenser, etc.

Three basic types of stripping systems are available:
1.    Steam stripping for TRS removal.

2.    Steam stripping for TRS removal and turpentine recovery.

3.    Methanol distillation.

It should be noted that there is no precise division of these systems since in stripping
TRS, some methanol may be removed; while in methanol distillation, TRS and turpentine
removal is quite efficient. Since each mill has different requirements, it would be unwise
to say that one of these processes is superior. However, a mill with an adequately sized
biological treatment plant or the ability to reuse the stripped condensate within the mill
may choose the first alternative. A mill pulping a highly resinous wood such as pine may
choose the second process, while a mill requiring a significant BOD as well as odor
reduction may decide the last option is most desirable.

The stripping steam required is dependent upon the process chosen as well as the
contaminate removal efficiency. A usage of 5% steam, based upon the weight of the foul
condensate, will achieve approximately a 95% TRS removal and at the same time, an
80% removal of turpentine. To achieve a 90% of both TRS and turpentine, a steam
usage of approximately 20%, based upon the condensate flow, is required.

Drawing DL-003920 shows a typical process flow diagram for a methanol distillation
system. The equipment as indicated in this drawing produces a liquid methanol distillate
product along with a noncondensible gas. The liquid methanol water mixture is stored
for later incineration while the stripped condensate is either reused within the mill or
discharged to waste treatment.

The basic theory behind methanol distillation is the removal of contaminates (TRS and
methanol) from the condensate counter-currently by an immiscible fluid, in this case,
steam. The steam enters the column at the bottom and as it passes upward through the
stripping section (3a) it removes the contaminates from the condensate. The steam is
further enriched in the rectifier section (3b).

The foul condensate is cleaned of solids such as fibers and shives, etc., in the fiber filter
(9) prior to the heat exchanger. The condensate is preheated in the stripper exchanger
(4) by the stripped condensate. This is done to prevent flashing of the stripped
condensate in the process pumps and to reduce the amount of steam required to supply
sensible heat to the foul condensate. The amount of condensate preheat is limited only
by a reasonable approach temperature to the stripper column, in the range of 15 to 20 degrees F.

The overhead vapors are partially condensed in the reflux condenser (5), the reflux either
being pumped (10) or gravity fed back to the top of the rectifier section. The
uncondensed vapors, in the amount required to remove the methanol, enter the
secondary cooler with a major portion of the water and methanol being condensed. The
remaining vapors are primarily the noncondensible gases, TRS and some turpentine,
which are incinerated. The methanol solution may be stored for later incineration. By
reducing the condensation, the methanol can be transferred as a gas to the incinerator.

Foul Condensate Stripping System
DL-3920