POWER September 2012 - 24

The crystallizer is fed with WWRO reject water from the
ZLD crystallization feed tank (not shown in Figure 4). Acid is
dosed inline prior to the inlet of a forced draft decarbonator
to convert scaling bicarbonates to CO2
and water in order to
prevent scaling in the downstream equipment. The feedwater
is then pumped to the feed heat exchangers. The heat exchangers
preheat the feedwater by exchanging heat with the
distillate streams from the crystallizer FCHX. The preheated
feed streams are then fed directly into the crystallizer flash
tank.
The superheated brine flashes as it discharges into the
flash tank, which is operating at a slightly lower pressure
than the brine side of the FCHX. The evaporation of
water from the slurry supersaturates the brine slurry, and
salts crystallize from the brine slurry to release this supersaturation.
The
crystallizer concentrates the feedwater stream into
heavy slurry of crystals using forced circulation evaporation
technology. The design of the system is such that it overcomes
the saturation limitations of a conventional evaporator:
low-solubility scaling salts, such as calcium sulfate and
calcium carbonate, and high-solubility salts, such as sodium
chloride, sodium sulfate, and sodium nitrate.
A crystallizer recirculation pump is provided to recirculate
the slurry at a high velocity through the FCHX tubes to suspend
solids, avoid scale formation, and maintain high heat
transfer coefficients. The flash tank is provided with sufficient
residence time to allow the crystals to grow and reach
super-saturation.
Flashed vapor is disengaged from the recirculating slurry
in the flash tank. The vapor flows through an internal mist
eliminator to remove suspended droplets from the evaporated
vapor. The mist eliminator is intermittently washed with
distillate to prevent the buildup of solids. Solids buildup will
reduce the efficiency of the droplet removal and result in an
increased distillate total dissolved solids concentration. A
foam separator is used to remove liquid slugs that can dam5.
Concentrating the dissolved solids. The crystallizer, using
carefully chosen chemical additives, captures the dissolved solids
in the wastewater into brine slurry that can be processed by a filter
press for easy disposal. Courtesy: Aquatech International Corp.
age the crystallizer vapor compressor. The vapor flows from
the foam separator to the suction of the compressor.
The FCHX is heated by condensing steam/water vapor on
the outside of tubes. The vapor generated by evaporation
from the brine slurry provides the steam required. To utilize
this steam, a motor-driven mechanical vapor compressor is
used.
The density of the crystallizer recirculating slurry is monitored
and controlled. On high density, the filter press automated
valves open and slurry is fed to the filter press, where
a dry salt cake is formed. The salt cake is then manually
removed from the plates and falls into a dumpster.
Test Results
The water treatment and ZLD systems underwent 24-hour and
12-hour tests, respectively. Two tests focused on the performance
of each of the two UF/RO water treatment trains,
designed as 2 x 100% systems, each followed by a 12-hour
test of one of the two crystallizers. The tests were conducted
in December 2010 using canal water (see Table 1). The
performance criteria were that effluent to the mixed-bed
polishers (rented equipment) from each RO system could be
maintained at 60 gpm under full automatic control and the
quality of the water produced must meet the criteria shown
in Table 2. Each equipment train was tested at full-load conditions.
Test results are also shown in Table 2.
In summary, the tests demonstrated that effluent to the
rental mixed-bed polishers from each RO system could be
maintained at rated flow under full automatic control (essentially
hands-off operation for the 24-hour period). The
Table 1. Raw water quality analysis. Source: Aquatech International
Corp.
Water quality parameter
Minimum design water temperature
Maximum design water temperature
Turbidity
Color
pH
Total suspended solids
Total dissolved solids
Hardness as CaCO3
Calcium
Magnesium
Sodium
Potassium
Total alkalinity
Bicarbonate
Sulfate
Chloride
Nitrate
Fluoride
Iron
Silica
Average concentration
50F
86F
3.8 NTU
10
8.3
35 mg/l
94 mg/l
62 mg/l
20 mg/l
2.9 mg/l
8.0 mg/l
1.2 mg/l
78 mg/l
78 mg/l
7.4 mg/l
5.4 mg/l
1.2 mg/l
<0.1 mg/l
<0.1 mg/l
8.8 mg/l
Notes: µg/l = micrograms per liter (ppb), mg/l = milligrams per liter (ppm), NTU =
nepholometric turbidity units.
24
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POWER September 2012

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