Chemical Engineering May 2012 - 56

TABLE 2. WATER QUALITIES AFTER ULTRAFILTRATION TREATMENT
Cover Story
pH
savings. For example, the overhead
of the FCC-gas fractionation unit can
be used as a heat source to heat the
recirculating desalted water, which is
then used as the heat source for the
low-temperature depropanizer and
propylene purification units.
Heat integration. Another technique
for water savings is to integrate the
heat from low-temperature streams,
such as steam condensate and desalted
water. For example, in Sinopec's
largest system, which recovers 1,120
ton/h of steam condensate, the project
team implemented a change whereby
the condensate is cooled to its purification
temperature of 50°C by heating
up the desalted water and other lowtemperature
streams.
Reuse of clean effluent. Water can
be saved by reusing the clean effluent
from steam stripping as makeup
water streams flowing to the desalter,
the quench tower, the washing column,
the hydrotreating unit and others. The
clean effluent from steam stripping of
sour water is a quality water source
that can replace industrial water
(freshwater) for several process units.
Clean effluent is also a good source
of injection water for hydrotreating
columns. Figure 3 illustrates this application,
in which the qualities of the
clean effluent are acceptable when
compared to those of industrial water
and injection water. Throughout our
project, the team used the same approach
as shown in Figure 3 to present
our water-reuse proposal, in which
we compared the available flowrates
and qualities of both the water source
and water demands for each reuse proposal.
The engineering and economic
evaluation specialists on our project
team helped the facilities estimate the
investment cost and payback period
for each water-reuse proposal.
THE FOUR " R " S
Water saving initiatives involve a
careful focus on regeneration, recycling,
reuse and replacement.
Replacement refers to a change
from one system within a process to
an alternative that allows water savings
to be realized. An example can be
found in Figure 4, which illustrates an
air cooler that replaces a water cooler
in a hydrocracking system. Figure 5
CODcr(mg/L)
Oil, mg/L
NH3-N, mg/L
Turbidity, NTU
2-, mg/L
SO4
Hardness,
CaCO3, mg/L
Alkalinity, mg/L
Conductivity, μS/
cm
Tianjin Jinan Wuhan Maoming Cooling tower
makeup water
7.9
51
0.75
0.2
0.03
7.2
38
8.4
55
0.80 0.86
0.86 2.9
0.23 0.12
190 230 202
150 65
88
170 50
360
~6.0-9.0
52
1.5
5.6
0.2
74.2
62.2
39.7
2,640 2,270 1,150 972
~6.5-9.0
≤60
≤2
≤10
≤10
≤300
~50-300
~50-300
≤1,200
TABLE 3. WATER QUALITIES AFTER REVERSE OSMOSIS TREATMENT
Tianjin,
RO
pH
Oil, mg/L
NH3-N, mg/L
Turbidity, NTU
Hardness, CaCO3
mg/L
Cl-, mg/L
Iron, mg/L
Alkalinity, mg/L
effluent
5.75
0.41
N.D.
N.D
N.D
11.2
N.D.
5.5
Conductivity, μS/cm 65
Tianjin cooling
tower
makeup water
7.93
--0.43
94
53
0.13
117
576
also
shows the once-through water
cooling that an old chemical subsidiary
was practicing for years, taking
advantage of the water from the
Yangtze River. Finally Figure 4 shows
a closed recirculating cooling system,
which also illustrates the definition
of cycle of concentration of a cooling
tower. By replacing the once-through
water cooling with a closed recirculating
cooling system throughout this
chemical subsidiary, its yearly freshwater
consumption dropped from 1.4
to 0.49 million tons, representing a
65% reduction [7].
WWTP effluent reuse,
regeneration and recycling
In refining and chemical plants, cooling-tower
makeup water and boiler
makeup water represent two of the
significant freshwater users with specific
water-quality constraints. Significant
water savings can result through
additional mild treatment of the environmentally
acceptable wastewater
treatment plant (WWTP) effluent
as cooling tower makeup water, or
through further deep treatment as
boiler makeup water (desalted water)
(Figure 5).
Mild treatment. One approach to additional
mild treatment of WWTP effluent
is aimed at using it as coolingtower
makeup water. Mild treatment of
38 CHEMICAL ENGINEERING WWW.CHE.COM MAY 2012
Jinan,
RO effluent
6.33
0.66
N.D
N.D
N.D
6.1
0.15
5.5
50
180
450
295
Choose
water
treatment chemicals to
control odor, corrosion, scaling
and microganism
growth
Bench-scale static
laboratory tests
Onsite dynamic plant tests
Onsite tests for 2 to 3 months,
gradually increasing the percentage
of treated WWTP effluent being used
Jinan cooling
tower makeup
water
7.6
<0.02
38.3
Reuse of 100% treated
WWTP effluent
FIGURE 6. Implementation of a mild
treatment scheme for WWTP effluent can
allow its use as cooling-tower makeup
water
the WWTP effluent will make its water
qualities comparable to those acceptable
as cooling-tower makeup water.
Table 1 shows a typical comparison.
Our project applied the technology
developed by the Water Treatment
Center of Sinopec's Research Institute
of Petroleum Processing (RIPP) in Beijing
for mild treatment of the WWTP
effluent. The technology combines
physical, biological, and chemical
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Chemical Engineering May 2012

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