half century of condensate polishing

HALF A CENTURY OF CONDENSATE POLISHING

by Eli Salem & Terrance LaTerra, Graver Water Systems, Inc.

Introduction The use of in place regenerated mixed bed demineralizers for condensate polishing originated in the 1950’s. By the end of the decade, Graver Water had patented a method of transferring the resins to an external regeneration system and designed the first system for Louisiana Power & Light’s Little Gypsy Station. This was the first high pressure (2400 psig) drum boiler to employ 100% deep bed type condensate polishing. Over the next 40 years, many innovations to the external regeneration concept were introduced including Graver’s Seprex and SepraEight (see figure 1) processes. In addition, a radically different concept, Powdex, was invented by Graver Water in 1962. This process utilized finely ground ion exchange resin in a disposable precoat format and eliminated the necessity of chemical regeneration altogether. This technology evolved into today’s state of the art Powdex process employing advanced precoating techniques, Air Surge backwash and an enhanced flow distribution tube. Figure 2 is a schematic diagram of a Powdex system with an enhanced flow distribution tube, Air Surge type backwash, and advanced precoat system. During the service mode, figure 3a, condensate enters the service vessel at the center of the bottom head and is directed to the chamber above the tube sheet. At this point, some of the condensate enters the filter chamber below the distribution tube while the balance enters

above the distribution tube. This split insures that the condensate is distributed equally to the top and bottom of the filter elements and avoids localized high velocities which could disrupt the precoat. Treated condensate passes through the filter elements and proceeds through the tube sheet to the Powdex vessel outlet. Likewise, figure 3b depicts the precoat cycle in which the direction of flow is the same as the service mode. During the backwash cycle, the vessel is partially drained (figure 3c). Then Air Surge is applied using a short burst of air introduced to the plenum to displace the water between the tube sheet and the tube sheet downcomers (figure 3d). This process is repeated several additional times at different water levels in the filter chamber as implied in figures 3e & 3f. This technique insures the filter elements are restored to their low initial pressure drop with minimal water consumption. Both the SepraEight external regeneration type systems and the Powdex precoat type systems are applied to meet the condensate polishing requirements of today’s high pressure boilers. As a world leader in condensate polishing technology with hundreds of installations including both external regeneration and precoat type systems, this paper will discuss Graver’s views on condensate polishing and the relative advantages and limitations of these two technologies.

BACKWASHINLET DRAIN DRAIN

RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN

CATION TRANSFER LINE

ANIONTRANSFER

LINE

Separation & AnionRegeneration Vessel

Cation RegenerationVessel

Mix & Hold Vessel

Fig. 1a - Receive Exhausted Resin Charge


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1c - Hydraulic Classification of

Exhausted Resin

NaOH ACID


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1e - Regenerate resins with Acid & Base


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN

AIRINLET


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1b - Transfer Regenerated Resin

to Service Vessel


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1d - Transfer Cation Resin to

Cation Regeneration Vessel

RINSE RINSE


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1f - Settle & Rinse Resins

Figure 1 - SepraEight Regeneration Process


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1g - Transfer Cation Resin to Mix

& Hold Vessel

BACKWASHINLET DRAIN AIR

RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1i - Air Mix & Backflush Resin

Transfer Lines


RESINOUTLET

RESININLET

XE

XT

XE

XT

DRAIN


ANIONTRANSFER

LINE



Mix & Hold Vessel

Figure 1h - Transfer Anion Resin to Mix

& Hold Vessel

Color Key:

- Mixed Resin

- Anion Resin

- Cation Resin

- Liquid Stream

Figure 1j - Color Key

Figure 1 - SepraEight Regeneration Process

Purpose

Condensate polishing is installed to control corrosion transport and condenser in-leakage in order to maintain cycle purity and efficiency. As presented by Black and Veatch1 at an EPRI conference in 1997, the key benefits, in order of importance, are: • improved availability • turbine efficiency loss prevention

• faster start ups • reduced chemical cleaning. For a 360 Mwe plant the annual savings were reported to exceed $500,000 after subtracting the annual fixed charges on capital and operating cost.1

FEED IN

DISTRIBUTIONTUBE

ELEMENTS

SERVICEVESSEL

DRAIN

SUMP

PRECOAT INLET

HOLD PUMP

VENTFEEDOUT

AIR WATERININ

SUMP

SUMP

SUMP

WATERIN SLURRY

TANK

AUXILIARYTANK

PRECOATRECYCLE

PUMP PRECOATINJECTION

PUMP

Figure 2 - Powdex Process with Advanced Precoat System

For those systems with copper bearing alloys, the need for condensate polishing is even more pronounced. Without condensate polishing, it is not unusual for a 400 MW unit to lose 10 % of its generating capacity in six months due to turbine deposits.2

For co-generation facilities operating above 1000 psig, it is strongly recommended that condensate polishing be installed to provide very low boiler feedwater iron and copper and provide high purity steam.3,3b Most end users interested in condensate polishing must choose between precoat filter demineralizers and externally regenerated deep bed (mixed bed) type polishers. Each have their own benefits and disadvantages

and selection is based on system design and operating parameters as well as environmental requirements. Those conditions that favor deep beds include: • use of high TDS cooling water such as

sea water or high TDS cooling towers • the requirement to operate long term

with small condenser leaks • the inability to control excessive air in-

leakage. If deep beds are selected, SepraEight can consistently provide the highest degree of regeneration and ion exchange resin utilization.4

VentDistribution

Tube

Precoat Element

Inlet Outlet

Figure 3a - Powdex Service Mode

VentDistribution

Tube

Precoat Element

Inlet Outlet

Air Inlet Air Inlet

Figure 3c - Drain prior to Air Surge

Vent

DistributionTube

Precoat Element

Inlet Outlet

Air Inlet Air Inlet

Figure 3e - Drain prior to Air Surge

VentDistribution

Tube

Precoat Element

Inlet Outlet

Figure 3b - Powdex Precoat Mode

VentDistribution

Tube

Precoat Element

Inlet Outlet

Air Inlet Air Inlet

Figure 3d - Air Surge

Vent

DistributionTube

Precoat Element

Inlet Outlet

Air Inlet Air Inlet

Figure 3f - Air Surge

Fig. 3 - Powdex Service, Precoat, and Air Surge Backwash Modes

At times, the difficulty in producing satisfactory results with deep bed condensate polishers, as reported in China4b, would lead one to believe that the simplicity of Powdex would overcome many of the difficulties encountered. Those conditions that favor Powdex include: • low TDS cooling water • titanium condensers • limited operator availability and training

(regeneration of deep beds is a complex regeneration sequence and involves the use of strong acids and base)

• need to avoid handling and neutralizing large quantities of acid and base

• frequent start ups and restarts (precoat filters provide superior crud removal and offer the most cost efficient means for controlling corrosion transport)

• need for rapid start up • need to minimize pressure drop

requirements for condensate polishing • operating policy of orderly shut down

and repair in the event of a condenser leak

• need to minimize costs including equipment, installation, and operating costs.

• Limited space availability Powdex precoats can perform these functions since they are composed of highly regenerated, finely divided ion exchange resins which provide enhanced kinetics for dissolved solids removal and excellent filtration properties. There is no need to worry about the degree of regeneration as with deep bed polishers.5 Powdex precoat systems can remove over 90 % of all metals from the condensate. Graver has been a leader introducing innovative improvements to condensate

polishing. Many of these improvements, such as the distribution tube6,7, advanced precoating system6,7 (constant slurry concentration), body feed6, Air Surge backwash6,8 and positive element installation hardware (SealFast™)6, have been incorporated into recent Powdex systems. Data will be presented on some of these plants as well as older installations. The key benefits of precoat condensate polishing include: • Reduced start-up and restart time • Improved cycle chemistry • Clean-up of cycle before firing • Reduced make up requirements. • Allows for orderly shut down for repairs • Allows for operation during minor in-

leakage • Minimizes any deposition within boiler

or turbine • Improved availability Crud Removal Contaminant levels during start up are high due to construction materials and corrosion products associated with new equipment. Under these conditions, new units may require chemical cleaning during the first scheduled outage because of inadequate crud removal. The bulk of any corrosion transport takes place during start up and restarts. This is partly due to the ingress of oxygen into the cycle. The Powdex process combines superior filtration with ion exchange by using finely ground, highly regenerated resins precoated onto specially designed elements. The precoated elements are contained in a system which is designed for uniform precoating using an enhanced flow distribution tube and element cleaning using Air Surge. In many

newer systems, advanced precoating is supplied to provide for a more uniform precoat. There are many precoat materials available. Those consisting solely of ion exchange material and those containing ion exchange material and fibers for improved filtration capacity. A discussion on the various precoat materials available is a topic of a separate discussion. In contrast, deep bed condensate polishers offer limited crud removal efficiency relative to Powdex. The removal efficiency for deep beds is typically 90% removal of black iron oxides (magnetite - Fe3O4) and 50% removal of yellow (Fe2O3) and red iron oxides. With Powdex systems the removal efficiency for all oxides is typically >95%. Figure 4 compares the Millipore stains, before and after condensate polishing, obtained with a deep bed system and with Powdex. An additional benefit of Powdex is its ability to hold on to the retained crud during flow surges. With deep beds, crud may be released into the condensate during a flow surge. It should be noted that heater drains that cascade to the deaerator and not the hot well are not treated with condensate polishing and could be a source of contaminants in the cycle. Figure 5 provides a convenient chart to quickly estimate the concentration of iron oxides in condensate. Silica and Dissolved Solids Removal Control of silica and dissolved solids within the boiler and turbine manufacture’s limits is required to operate the system efficiently. Condensate polishing is an effective means to achieve the desired conditions for a rapid startup or restart. One advantage of Powdex over deep beds is its ability to remove colloidal silica. Since Powdex resin is composed of highly regenerated, finely divided ion exchange resins, it can

effectively remove silica and other ionic contaminants to very low levels. Depending on the operating pH, removal efficiencies of up to 99% can be achieved. To maximize the utilization of anion resin, it is important that air in-leakage be controlled. Air contains about 400 ppm of carbon dioxide that can react with amines when it enters the condenser to form a salt. The salt can then only be removed with ion exchange. Another advantage of Powdex is the ability to vary the precoat dosage and composition to help effect the desired result (i.e., use hydrogen form resin to lower pH). Economic Considerations Table 1 compares the capital equipment and operating costs for both full flow mixed beds and Powdex. The lower cost for Powdex is due primarily to the fact that the system is skid assembled and has no chemicals, minimizing installation cost. The capital and operating costs are only rough approximations based on US data and should be adjusted for local conditions. Case Histories Typical Once Through Boilers Some of the installations were discussed earlier.9 Texas Utilities operates twelve supercritical, once through steam generators (8,125 MWe installed capacity). All the stations use Powdex for condensate polishing. The units operate with once through cooling using fresh water and typically have stainless steel condensers. In 1991 Texas Utilities’ Monticello station was retrofitted with the distribution tube and an advanced precoat system. These improvements resulted in more uniform

Mn. Cond. Pump MB Polisher Out Powdex Inlet Powdex Outlet

Figure 4a - Millipore Samples before and after Deep Beds (mixed beds) and Powdex


Figure 4b - Millipore Samples before and after Deep Beds (mixed beds) and Powdex


Figure 4c - Millipore Samples before and after Deep Beds (mixed beds) and Powdex

Figure 4 - Three Sets of Millipore Filter Stains*

*Data from Allegheny Power Fort Martin Station which has Deep Bed and Powdex condensate polishers

Fe3O4

Fe2O3

Fe2O3 +

Fe3O4 (1:1)

Figure 5 - Iron Determination by Membrane Filters

The color and density of the stains on this chart are approximately the same as those produced when one liter of water containing the indicating concentration of suspended Iron in parts per billion (Fe) is passed through white filter membrane having a pore size of 0.45 microns.

TABLE 1 - ECONOMICS Plant Design Basis: Plant Capacity 600 MW Coal Fired Type Boiler Supercritical with Oxygenated Treatment Condensate pH 8.5 M/B Resin Ratio 1.5/1 C/A Design Pressure 40 Bar Design Data Mixed Bed Powdex Number of Service Vessels Three x 50% Three x 50% Number of Regen Vessels Three One Precoat System Velocity Through S/V 50 gpm/ft2 (120 m/hr) 3.5 gpm/ft2 (8.4 m/hr) Capital Costs Estimated Equipment Cost $2,000,000 $1,200,000 Estimated Installation Cost $1,000,000 $ 600,000 Total Capital Costs $3,000,000 $1,800,000 Annual Operating costs Total Regens/Precoats per Yr 24 30 Acid Consumption 10 metric tons N/A Acid Cost @ $0.2/kg HCl $2000 N/A Base Consumption 10 metric tons N/A Base Cost @ $0.30/kg $3000 N/A D/M Water Consumption 4000 m3 1000 m3 Cost of D/M Water @ $.5/m3 $2000 $500 Cost of Precoat Material N/A $32,000 Resin Replacement @ 20%/yr $15,000 N/A Maximum Pressure Drop 50 psig (3.3 bar) 30 psig (2 bar) Pressure Drop Penalty @ $.03/kwh $20,000 $0 Total Annual Operating Cost $42,000 $40,500 Net Present Value (Capital + Operating Costs over 20 yrs @ 8%)

$3,412,362 $2,197,635

TABLE 2 - TYPICAL OPERATING DATA

Plant Velocity Precoat Effluent Quality

Type gpm/ft2 type Cond. Na+ Silica Cl- Fe End Pt. (m/hr) µmho/cm ppb ppb ppb ppb days

Drum 3.5 (8.4) Ecodex < 0.3 < 1 < 5 < 0.2 35-45 SC* 2.9 (7.0) 1C+1A < 0.09 < 0.5 < 5 < 1 50** SC* 3.5 (8.4) Ecodex < 0.08 < 0.5 < 5 < 1 < 0.1 60**

Drum 3.3 (8.0) Ecodex < 0.3 < 1 < 5 DP SC* 3.0 (7.2) 1C+1A < 0.1 < 1 < 5 < 1 90**

* Supercritical with oxygenated treatment ** The end point termination is administrative.

precoat and flow distribution which produced a greater than two fold improvement in precoat ulilization.7 Typical systems currently operate with oxygenated treatment which allows for a relatively low pH in the condensate (8.0-8.5) and lower levels of crud. Air in-leakage is controlled at <0.5 scfm/100 MW. Typically the run lengths on the Powdex units are in excess of 50 days. The normal precoat dosage is 0.2 lb/ft2. Recent typical data from several US utilities is provided in Table 2. Typical Drum Boilers Data on San Miguel Electric Cooperative (drum boiler) was first reported at the 44th International Water Conference.10 The rated capacity is 410 MW at 2400 psig. Internal treatment consists of coordinated phosphate during start up and all volatile treatment at full load. Cooling towers are used and operate with well water.

The system was retrofitted with Powdex (with Air Surge) condensate polishing in 1982. This decision was made because start ups were taking longer than expected due to the time required to attain proper boiler water quality as well as the large volume of blowdown required. Due to the low pressure drop across the Powdex units, the existing condensate pumps were found capable of handling the additional head without any modification. In addition, the space required and installation cost were far less than a deep bed system. Upon installation, improved boiler water chemistry was realized as well as a drastic reduction in start up time. Start up times were reduced by 11 to 15 hours while consistently maintaining the desired silica levels in the cycle. The precoat filters at that time were only used for restarts. Air in-leakage at that time was a problem and contributed about 150 ppb of carbon dioxide to the condensate. This ingress of air limited the run length of the Powdex units to about 10 days.

A typical restart, after less than a 24 hour shut down, requires no more than two precoats to reach full power. Each precoat uses 9 boxes of Ecodex P303N costing about $700. When the cost savings (faster start up, greater availability, etc.) were applied, the pay back period was less than one year. More recent data indicates that San Miguel has solved the air in leakage problem and now uses the Powdex units continuously to polish the condensate. Most drum boilers operate with stainless steel or copper alloy condensers. Air in leakage is controlled to < 1 scfm/100 MW. Typical precoat run lengths are between 30 to 45 days and are terminated due to pressure drop. The precoat dosage is normally 0.2 lb/ft2. The pH of the condensate is typically about 8.8-9.2. Effluent quality is typically <0.3 µmho, <1 ppb Na, <5 ppb silica and < 0.2 ppb crud. Recent Powdex Installations Huaneng Power International - Fuzhou Power Station, Peoples Republic of China includes two 66” diameter Powdex units for each of two 350 MW units, complete with Air Surge, body feed, advanced precoat, Seal Fast elements and an enhanced flow distribution tube. Each vessel is sized for full flow. Unit 3 started up during the summer of 1999. End points of 60 ppb silica and 0.3 µmho were initially established due the short runs encountered during the plant startup phase. At last report, run length had reached one week using 14 bags of Ecodex P-202 and alternative precoat materials were being considered. International Generating Company - Rocksavage Power Company, England - This is a combined cycle, 750 MW power

station, operating at 1650 psi with drum boilers and cooling towers. The conductivity of the cooling tower water is about 2,500 µmho/cm. The condensate contains 0.6 to 1 ppm ammonia with a pH of 9.3 - 9.5. The condensate polishing system consists of two 48” diameter Powdex units. Each vessel is sized for 50% of flow. The plant started up in the summer of 1998. Currently, during normal operation, Powdex run lengths are about one month and shorter during start up conditions. They experience about 20 restarts/year. Precoat end points are usually due to pressure drop. Typical effluent quality values are a cation conductivity of 0.12 µmho/cm, and silica concentration of 3 ppb. The precoat dosage is normally 9 bags/vessel. Annual precoat consumption is about 12 precoats for each vessel/year. Conclusion The extensive use of condensate polishing world wide is testimony to the usefulness of this technology. While deep bed systems offer extended condenser leak protection, Powdex can be a cost effective alternative where local operating conditions permit. References 1. B.A. Larkin, et al., “Condensate Polishing Cost Benefit Analysis”, EPRI 5th International Cycle Chemistry Conf., 1997 2. D. Daniels, “Turbine Deposits Rob Megawatts, But You Can Catch a Thief”, Power, p. 83, March/April 1999 3. M. Wisdom, H. Weed, “Condensate Considerations in the Development of High Pressure Co-Generation Facilities”, Power-Gen International, December 1997 3b. K. Sheilds, R. B. Dooley, et al, “Copper Transport in Fossil Plants”, EPRI 5th International Cycle Chemistry Conf., 1997 4. M. O’Brien, E. Salem, “Deep Bed Condensate Polishing Separation

Technology Requirements for Operation in the Amine Cycle”, EPRI Condensate Polishing Workshop, Scottsdale, AZ, 1991 4b. Z. Chengxin, C. Long, “The Special Problems Might be Encountered with the Operation of Condensate Polishing Systems in China”, International Water Conf., 1998 5. J. Duff, J. Levendusky, “Powdex a New Approach in Condensate Purification”, American Power Conference, 1962 6. E. Salem, M. O’Brien, “Design Improvements - Powdered Resin Equipment”, BWR Owner’s Group, Condensate Committee Meeting, New Orleans, La., 1991 7. L. Morris and M. O’Brien, “DistributionTube & Advanced Precoat

System Plant Trials at TU Electric Monticello”, EPRI Conference on Filtration of Particulates in LWR Systems, King of Prussia, Pa, 9/91 8. L. Nolan, “Evaluation of Filter / Demineralizer Precoat Materials on Monticello Nuclear Power Station Condensate System”, International Water Conference, 1979 9. P. Gross, J. Longo, M. Wadlington, “150 Million Megawatts Hours of Condensate Polishing”, International Water Conf., 1981 10. E. Lange, D. Johnstone, P. Gross, “San Miguel: A Case for Drum Boiler Condensate Polishing Retrofit”, International Water Conference, 1983

About the Authors

Eli Salem VP of Technology Education BChE 1957 - CCNY MChE 1967 - Polytechnic Institute of Brooklyn MBA finance 1977 - NYU Employment - Joined Graver Water in 1957 and has held various positions leading to VP of Science and Technology. Recognized as an expert in ion exchange and water treatment. Authored and presented numerous papers and holder of 21 patents in the field of ion exchange and water treatment. E-mail: [email protected] Terrance LaTerra VP of Sales & Marketing Education BS Engineering 1969 - Boston University MBA 1994 - The Wharton School, U. of Penn. Employment - Joined Graver Water in 1971 and has held various positions leading to VP of Sales & Marketing. Authored and presented numerous papers and holder of 3 patents in the field of ion exchange and water treatment. Has traveled to the Peoples Republic of China since 1985 and has made several presentations on Condensate Polishing in the 1990’s. E-mail: [email protected]

half century of condensate polishing

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