Chemical Engineering May 2012 - 55

All photos: SINOPEC
FIGURE 4. Installation of alternate technologies can have a large effect on water savings.
Examples include replacing water cooling with air cooling in a hydrocracking
system (top left); a once-through water cooling can be replaced by a closed recirculating
cooling system (top right); a closed recirculating cooling system routed though a cooling
tower (bottom)
Water losses
Cooling tower
makeup water
Cooling tower
Recycle
Main
processes
Fresh water
Boiler
feedwater
Boiler
Boiler
makeup water
Deep treatment
Regeneration
Auxiliary
processes
Cooling tower blowdown
Boiler blowdown
Fresh water
Wastewater
treatment
Environmentally
acceptable
WWTP
effluent
plant (WWTP)
Closed-loop
circulating
system
Mild treatment
Regeneration
Reuse
FIGURE 5. Water savings opportunities expand through additional treatment of environmentally
acceptable WWTP effluent. The additional treatment allows WWTP effluent
to be used as cooling-tower makeup water (mild treatment), or as boiler makeup water
(deep treatment)
leakage and loss, and others
* Water reused (tons), in the following
categories: steam condensate
reused, amount of wastewater treatment
plant (WWTP) effluent reused
after further treatment
* Corrosion monitoring, including testtube
corrosion rate in mm/yr and adhesion
rate in mg/(cm2) per month.
With this monthly accounting available,
financial and personnel resouces
can be directed at areas most ripe for
improvement, while the best-performing
areas are replicated.
Process changes
Effective process changes can maximize
water reuse and minimize
wastewater generation in existing
chemical plants [3]. And intelligently
integrating the use of materials
and energy through process
integration can optimize water and
energy savings. The following items
are examples of process changes
implemented as a result of the project
team's efforts that had a positive
impact on water savings.
Hot-material feed and heat integration.
Hot product streams from
upstream process units are sent to
downstream process units directly as
feeds, without entering intermediate
storage tanks or undergoing water
cooling or feed preheating [6].
For example, in one refinery,
products from the crude distillation
unit (CDU) include naphtha,
diesel, vacuum wax oil and vacuum
residue. The gas oils to the fluid
catalytic cracking (FCC) unit come
from atmospheric and vacuum distillation,
delayed coking and solvent
de-asphalting units. The liquefied
gas product from the FCC goes to
the gas fractionation unit, while the
gasoline and diesel products from
the FCC unit are sent to the hydrotreating
unit. The delayed-coking
unit processes the residue from
the CDU, and sends its gasoline and
diesel products to the hydrotreating
unit. The solvent de-asphalting unit
processes the residue from the CDU,
and sends its product oil to the FCC
and fertilizer plant. By raising
the outlet temperatures of several
product streams that serve as feed
streams to downstream processing
units, uncessary water-cooling
or feed-preheating can be avoided,
thus lowering energy costs and recirculating
cooling-water flowrates.
Raising storage temperatures.
Another example of a successful
process change is raising the storage
temperature of selected products.
Temperatures must still be below
their safe-storage temperatures, but
raising storage temperatures helps
avoid the need for water cooling.
Recovery of low-temperature heat
sources. Recovering low-temperature
heat sources can also realize waterCHEMICAL
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Chemical Engineering May 2012

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