Chemical Engineering September 2014 - 49

Distilled water
to process
Distilled water
to process
Feed from
wastewater
tank
Evaporator
Evaporator
concentrate
tank
Filter press
or other sludge
thickener
;
Solids
to disposal
Figure 4. Shown here is an evaporator-crystallizer schematic
for wastewater treatment. This process, or variations
thereof, allow the plant to operate with zero liquid discharge
microbiological treatment is chlorine
dioxide (ClO2). This product
must be generated on-site and is
more expensive than bleach, but
it does not react with ammonia
and is not consumed by standard
organics
Even with these technologies in
place, the wastewater produced
during LNG production still needs
proper treatment.
Wastewater. Cooling-tower blowdown
is often the largest wastewaCrystallizer
ter
stream at heavy
industrial facilities,
and at LNG plants,
petroleum refineries,
petrochemical
plants and similar
facilities. At many
plants, the blowdown
is released to
a receiving body of
water, provided the
discharge meets the
plant's National Pollutant Discharge
Elimination System (NPDES) guidelines.
However, this option is becoming
more limited for many process
operators [8].
Excluding the cooling tower, a generic
water balance at an LNG facility
with power generation might
closely resemble Figure 3. The unit
operations are not shown in elaborate
detail in Figure 3, but the objective
is to provide a system overview.
The process shown is similar
to that of a typical natural-gasfired,
combined-cycle plant, with a
few exceptions. The large bulk of
the makeup water goes toward production
of steam for process use and
as a source of demineralized feed
to the process. The aqueous amine
used for acid-gas scrubbing needs
regular replenishment. As seen in
this particular design, plant and
process drains are treated in an oilwater
separator (OWS) prior to further
treatment. The OWS stream
combines with boiler blowdown and
demineralizer waste in the wastewater
tank, from which the liquid is
forwarded for further processing.
One possibility for wastewater
treatment is to remove non-volatile
impurities as solids and recycle the
distillate, as shown in Figure 4.
Evaporator-crystallizers are commonly
used in the chemical process
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Chemical Engineering September 2014

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