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To ensure the proper operation of steam generation systems,it is essential that the purity of the boiler feed water is high.
High quality feed water will reduce the use of boiler
chemicals because of less frequent blowdown requirements
(reducing blowdown frequency by as much as a factor of ten).
Lower blowdown frequency lowers fuel costs. Furthermore, scale
build-up is reduced because there are less impurities in the boiler
feed water to foul heat transfer surfaces. The lower concentration
of impurities also reduces corrosion rates in the boiler. When a
boiler is used to run a steam turbine, blade erosion is less likely to
occur because of the high quality of steam generated.
The use of reverse osmosis (RO) in feed water purification
systems can cuts chemical costs by reducing the frequency of ionexchange regeneration. A complete system, which includes RO
and ion exchange, typically produces a more cost effective system
when compared to systems that do not use RO. However,
conventional RO systems require pretreatment units, which
obviously adds to the total cost of the system.
Silicate and colloidal deposits decrease boiler efficiency and
also cause premature failure of turbines. Ultrafiltration (UF) can
remove more than 99% of colloidal silica, as well as precipitatediron and aluminum. The reduction in particulate matter,
suspended solids and total organic carbon (TOC) also enhance
turbine and boiler efficiency. However, the use of conventional
membrane technologies can suffer from substantial membrane
fouling problems.
VSEP Membrane System
Technological advances in membrane filtration systems have
created an opportunity for industrial boiler and steam power
plants to effectively treat boiler feed water streams. The
Vibratory Shear Enhanced Processing technology or VSEP
®
makes it possible to filter influent water, without the fouling
problems exhibited by conventional membrane systems. The
VSEP membrane system can significantly reduce total dissolved
solids (TDS), total suspended solids (TSS), total organic carbon
(TOC), colour bodies, silica and hardness from influent streams
originating from city water, well water, and river/surface waters,
and therefore minimize treatment cost.
The VSEP treatment system can use UF, nanofiltration
(NF) or RO membrane modules to generate a permeate
stream that meets the quality criteris for boiler feed water,
e.g. concentrations of suspended and dissolved solids, silica
and hardness. After final polishing with ion exchange
resins, the clear permeate can be used as boiler feed water.
In summary, the VSEP treatment system can be used to
treat boiler feed water with only minor final polishing by
ion exchange resins. VSEP will eliminate pretreatment
requirements and can substantially reduce chemical usage
for regeneration, and therefore reduce disposal of spent
regenerant and the associated costs.
Process Conditions
The influent water to a power plant can come from a variety
of sources, including groundwater well (aquifer), surface
water or a municipality, so it has to be treated to meet thequality requirements for boiler feed water.
For boiler feed water treatment, depending on its
requirements, a number of processes can be used,
including chemical treatment/lime softening, dual media
28 June 2003 ISSN 0015-1882/03 © 2003 Elsevier Ltd. All rights reserved
Feed Water Treatment forIndustrial Boilers & Power Plants
In steam generation systems, ensuring the quality of the boiler feed water is essential for successful unit operation. Power plants traditionally
employ a multi-step process to remove impurities from the influent water.
Belwo, New Logic Research Inc, USA, explains why its VSEP technology,fitted with NF or RO membrane modules, is a commercially viable
alternative to exisitng boiler feed water treatment processes.
featurearticle
City water
VSEPtreatmentsystem
Chemical injection(opional)
Concentrateto disposal
Ion exchangepolishing units
Boilers
Posttreatment filter
Supply pump
Storage tank withsterile vent filter
Figure 1: A single VSEP membrane system can replacea multi-stage pretreatment for boiler feed water.
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filtration, carbon
adsorption,
conventional RO
membranes and final
ion exchange resin
polishing. However,
significant waste is
generated from these
unit operations,
including spent carbon
and spent regenerating
chemicals from the ion
exchange resins.
To avoid scale in the
boilers, power plants
normally utilize a
multi-step process to
remove the hardness of
the incoming water, i.e.chemical treatment or
ion exchange, as well as
other less advanced
methods, such as multi-
media filtration. These
multi-step processes can be replaced by a single pass VSEP
system, which will purify the influent in one step.
A process schematic for the treatment of boiler feed water
using a VSEP treatment system is presented in Figure 1. This
system can use either NF or RO membrane filtration system to
replace the entire pretreatment step that would be usually
required, i.e. lime or cation softening, dual media filters, carbonadsorbers, and conventional RO units. City water can be treated
in a single step VSEP treatment system followed by a final ion
exchange polishing unit. The treated boiler feed water can then
be fed to the boiler via a supply pump.
Boiler Feed Water Treatment
Figure 2 is a process block flow diagram, showing the integration
of VSEP systens into a boiler feed water treatment process at an
industrial manufacturing facility. The city water is preheated in
the Stage I heat exchanger, and then further heated with a
steam-heated exchanger to a temperature of 42 °C. The higher
feed temperature improves permeate flux through the VSEP
treatment unit and provides the energy drive for the first
exchanger. The preheated water is introduced into the
equalization tank, where sulfuric acid can be added for pH
adjustment. Other chemicals, such as alum and polymer, can
also be injected at this point to help coagulation. The
equalization tank effluent is then transferred to the feed storage
tank where it is fed to the VSEP units at a rate of 473 litres/min.
Two industrial scale VSEP units process the preheated city water.
VSEP generates a permeate stream of approximately
380 litres/min, which is sent to the heat recovery exchanger and
then stored in the treated water tank prior to final polishing by
ion exchange. A concentrated waste stream is generated at a flowrate of 95 litres/min, which is routed to the holding ponds and
then the sewer.
Two industrial VSEP treatment units are used. The raw city
water has a concentration of 160 to 240 mg/l of hardness,
10 mg/l of silica, and 510 µS of conductivity. The permeate
concentration is reduced to <15 mg/l of hardness, <1 mg/ l of
silica, and ~10 µS of conductivity, all well below the designed
water quality limits.
Conclusions
Using a NF or RO module in the VSEP system is a
commercially viable option for the treatment of boiler feed
water at industrial boilers and power plants. Nearly 80% of the
influent water is recovered as treated water suitable for final
polishing at a boiler facility, while less than 20% is disposed as
concentrate.
Membrane selection is based on material compatibility, flux
rates (capacity) and concentration requirements (hardness, silica,
TDS, conductivity). In this example, the hardness reduction is
over 90% and TDS and conductivity are also reduced by a
similar amount.
New Logic Research has successfully installed VSEP
separation technology into many industrial processes. The
industrial boilers and power plants, as well as the cogeneration
industries’ effort to meet system performance criteria and
environmental regulations can be helped by combining
membrane filtration with VSEP technology.
The development towards applications for industrial boilers
and power plants, along with the availability of new membrane
materials and VSEP technology, make it possible to treat the
more difficult streams with technically and economically
successful results.
Contact:
New Logic Research Inc,1295 Sixty Seventh Street,
Emeryville, CA 94608, USA.Tel: +1 510 655 7305,Fax: +1 510 655 7307;
E-mail: [email protected];Website: www.vsep.com
Filtration+Separation June 2003 29
City water
Heat exchanger(stage I)
Heat
exchanger(stage II)
Equalizationtank
Concentratetank
Concentrate todisposal
Steam condensateto drain
Treatedwater tank
Treated
water pump
Feedpump
Treated water to final polishing
Steam H2SO
4
Feedtank
Two industrialscale VSEPs
PLC
featurearticle
Figure 2: Schematic showing how VSEP integrates into a boiler feed watertreatment process.
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