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'3 PRETREATMENT OF AN ION

EXCHANGE DEMINERALIZER WITH REVERSE OSMOSIS

Presented at the High Purity Water Conference and Exposition

Philadelphia, PA April 2-4, 1990

Albert L. McAfee Billy Nowlin TU Electric Company 400 N. Olive L.B. 81 DaUas, TX 75201

Scott Beardsky FilmTec Corporation 7200 Ohms Lane Minneapolis, MN 55435

, FORM#609-00010 --

PRETREATMENT OF AN ION EXCHANGE BEMINERAWZER 'REVERSE~lWOSfS

Steam electric stations require large quantities of high purity water for the production of electricity. Water treatment plants are built and operated along with steam electric power plants to supply demineralized water for boiler feedwater. Many plants are located in areas with poor raw water quality, thus making treatment requirementsquite rigorous.

In 1973, TU Electric Company added a 540 MW generating unit at the Permian Basin Steam Electric Station. A 180 gpm water plant was also installed to supply the plant's makeup. This paper discusses the early operating experience of the water plant, the decision to reduce makeup costs and improve reliability by adding a reverse osmosis unit, and the impact of the reverse osmosis unit on water production.

OYEBVIEW

The Permian Basin Steam Electric Station is a two (2) unit gas/oil fired electric generating station located in Monahans, Teras. Unit 5 is R 115 MW cyciing unit w i t h boiler operating conditions of 1500 p i g and steam conditions of 1005 OFfiOOS OF. Unit 6 is a 540 MW cycling unit with boiler operating conditions of 2600 psig and steam conditions of 1005 OF/1005 OF. Located approximately 1/2 mile from the s team generating plants are five GE Frame 7 gas turbines.

EARLY OPERATING EXPERIENCE

Unit 6 of the station w a s installed in 1973. A Permuitt water treatment plant consisting of a sand filter, a clearwell, a RO unit and a demineralizer was installed along with the steam generator. The configuration of the water plant w a s unusual in that the demineralizer train had a capacity of 180 gpm, yet the RO unit could produce only 26 gpm of permeate. The RO unit w a s designed to treat only a sidestream (approximately 14.4%) of the demineralizer influent. Raw water for treatment was purchased from the Colorado River Municipal Water District (CRMWD). Water received at the plant w a s a mixture of surface water and ground water. Problems were experienced with the RO unit from the beginning. It should be noted tha t RO technology was in its infancy when this unit w a s designed and installed. Pretreatment for the system consisted of graded sand filtration, 5 micron filtration, and sulfuric acid dosing to a pH of 5.5. An antiscalant was not included in the design. The RO experienced feed pump and membrane fouling problems constantly. A few days of uninterrupted service would be followed by an unscheduled outage. RO operation became very t ime consuming and expemive. Since only a small portion of the DI influent was RO permeate, it was more economical t o bypass the RO unit. This decision was reached within a year after system startup..

Demineralizer feedwater was difficult to treat with or without the aid of the sidestream RO. The feedwater w a s high in sodium, silica, and dissolved solids. Table A contains the water analyses used to design the demineralizer train. To meet the required effluent quality, large ion exchange vessels were used. Regenerant dosages were also high with cation dosages of 8 Ibs. of sulfuric acid per cubic foot and anion regenerant dosages Of 15 Ibs. of caustic soda per cubic foot. Table B contains the design data for the demineralizer.

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Demineralizer effluent quality typically met design conditions. There were occasions when the DI train experienced silica overruns. Overruns were the result of fluc~uations in the iRfluent wager quaiity. and a gradual bss of e ~ n g base ex- capacity of the instailed Type II anion resin. To prevent Silica O V e m r t s the cation-anion throughput capacity had be reduced periodically in proportion to the loss of strong base capacity. When strong base capacity got too low, resin replacement w a s the only aiternative. During this operating period, anion resin replacement was at a frequency of 2 to 3 years.

While effluent. water quality w a s usually acceptable, laboratory technician time and costs associated w i t h . demineralizer operation was excessive. Demineralizer performance w a s unsatisfactory due to the following reasons:

1) Anion resin replacements were too frequent due to a loss of strong base capacity.

2) Regenerations required high dosages of acid and caustic.

3) Throughput capacity was too low thus requiring frequent regenerations.

- A convenient way to evaluate 8 demineralizer’s efficiency is to compare the amount of solids removed by an exchanger to the amount of acid or caustic used for regeneration. If the amount of solids removed are 3040% of the amount of regenerant used, the exchanger is considered to performing efficiently. The efficiency rating for Permian Basin’s DI train was: cation exchanger 29% and anion exchanger 19.3%. Below is the formula for the efficiency calculation:

Total Kilograins Removed by the Exchanqerl’l Pounds of Regenerant Used by the Exchanger = % Efficiency

In 1979, the plant stopped using CRMWD water for demineralizer influent, and began using well water located on plant property. The plant’s wel l water was not as difficult to treat as the purchased CRMWD water. The dissolved solids concentration w a s 500 mgll. The sodium and silica concentrations were 60 mg/l and 30 mg/l respectively. The change in raw water source allowed the anion regenerant dosage to be reduced to 12 pounds of caustic per cubic foot. Throughput capacity was increased and the cost of producing DI water was decreased.

The new source of water improved demineralizer performance and efficiency, however it was far from ideal. The plant still experienced unpredictable silica overruns and a 2 to 3 year replacement frequency from the anion resin. The cost for producing DI water during this period was $7.82/1000 gallons.

IMPROVEMENTS EVALUATED

Further improvements were desirable in our water plant. We desired to reduce technician time required for regenerations and to reduce water production costs. Consideration was given for a decarbonater retrofit downstream of t h e cation exchanger to remove CO2. Consideration was also given for a ful l flow reverse osmosis unit. We evaluated both options and decided that a RO system would be the most cost effective option. h decarbonator could have been installed for approximately $65,000. A decarbonator addition would increase anion run length by 50%. A RO system, on the other hand, could be installed for approximately $250,000. The RO could increase both cation and anion run lengths by a factor of 20. Our evaluation indicated tha t the RO was t h e best alternative and could reach a pay back period within 4 years.

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BID SPECIFICATION

In 1984, bids were requested from several water treatment equipment manufactweft for a 100 gpm RO. A 100 gpm system was specified although the demineralizer capacity is 180 gpm. Based upon an annuai production of 20,000,000 gallons, the everage station makeup rate was 38 gpm. It was felt that a 100 gpm system could meet demand during all periods except emergency situations. During emergency situations, the RO could be bypassed and raw water fed directly to the demineralizer at 180 gpm. By specifying a RO unit with a lower flow rate, system costs were reduced by approximately $1,000 per gpm of rated capacity. Also specified was FILMTEC* thin film composite membranes. Up to this time our company was not familiar with the FilmTec Corporation. Our RO membrane experience at other stations had been with cellulose acetate membranes and with Dupont B-9 membranes. FnmTec worked with our .technical personnel and performed computer projections of the plant's well water. The projections indicated that dissolved solids as well 85 Siiicti and bicarbonates could be rejected at greater than 95%. Experience with cellulose acetate membranes showed only 85% rejection for silica and bicarbonates. Since the plant% well water w a s high in silica and bicarbonates, the FILMTEC membranes were specified due to its superior rejection capabilities.

RO XNSTALLATION AND OPERATION

The RO system was installed at the station in April 1985. This was a turnkey project with both construction and installation performed by the successful bidder. Figure 3 shows the details of the RO system.

The initial operation of the RO system was excellent. Data taken several weeks after startup showed greater than 98.5% salt rejection. Performance from t h e system exceeded projections and expectations. The RO feedwater had a conductivity of 710 microS/cm and the permeate had a conductivity of less than 10 microS/cm. The RO installation has a dramatic effect on demineralizer performance. The low dissolved solids content of the demineraiizer feedwater allows a change from a Type I1 anion resin to a Type I. Type I anion resins have a lower exchange capacity, but they do not lose strong base capacity as readily as 'Type 11 anion resins. Demineralizer production has increased from an average of 165,000 gallons per regeneration to 10,000,000 gallons per regeneration. Regenerations previously performed at a rate of 2 to 3 per week are now performed at a rate of 3 per year. Silica overruns are now nonexistent. These positive benefits have allowed time technicians would normally spend for regenerations to be used for other duties. The cost for producing DI water has been reduced from $7.80/1000 gallons to $1.54/1000 gallons. Table E contains a comparsion for water cost before and after the RO installation.

SYSTEM MAINTENANCE

Operation of the RO system h a s been relatively trouble free. Maintenance requirements are cartridge filter replacement once per month, instrumentation

' calibration and upkeep, and occasional pump maintenance. The membranes have been cleaned four times over a period of five years. All of the cleanings were based upon differential pressure drop increase across the pressure vessels or visual inspection of the product piping for microbiological fouling.

The first cleaning was performed 15 months after startup. The suspected foulant w a s silica. The RO bank was cleaned wi th an alkaline solution followed by an acid ammonium bifluoride solution. Analysis of the spent cleaning solutions after cleaning indicated that silica was not the foulant. The actual foulant was a mixture of CaC03 and microorganisms. After rinsing and placing the system back in service, the Original normalized values were obtained.

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* Tradanark of FhTcc Corporation

The next three cleanings were performed specifically for biological fading. Alkaline solutions followed by an antimicrobical cleaner were used for the cieanings. Following the second cleaning, the last pressure vessei of the 3 x 2 x 1 amay did not return te star.tup conditions. Following the third cleaning, it was observed tha t the RO system had not been programmed to flush with permeate when the system was taken out of service. The system was reprogrammed t o provide a permeate flush when the system was shut down. After the fourth cleaning, the membranes in the last stage pressure vessei were replaced. This set of membranes had attained a service life of 49 months. After replacement of the last stage membranes, the original normalized performance values were attained.

System experience indicates that the last stage membrane replacement would not have been necessary had a system flush been included in the original program. The system is now. treated weekly with a non oxidizing biocide to control biofoulinq. The biocide' is a solution of potassium dimethyldithiocarbamate fed at a concentration of 20 mg/l for one hour.

Overall the performance of the RO system has been favorable. The norm- flux had decreased during membrane fouling episodes but would return to near design conditions after cleaning. Flux is now at the original normalized flux since new membranes have been installed in the last stage pressure vessel. Figure 4 shows a graph of flux vs. time. Normalized salt rejection has been maintained at greater than 98.5%. Normalized salt rejection vs. time is shown on figure 5. The membranes in the first two arrays have now been in service for 5 years and are performing 85 good as new membranes.

SUMMARY

The RO installation at the Permian Basin Steam Electric Station has reduced the cost of DI water from %7.80/1000 gallons to $1.54/1000 gallons. Regenerations of the demineralizer is now performed at a frequency of 3 times per year. The orighal RO membranes are still in use except for the last stage membranes which attained a service life of 49 months prior t o replacement. Replacement of demineralizer resins have not been necessary since startup of t he RO system five year ago. Specifying a 100 gpm RO system was a good decision. The makeup rate of 100 gpm h a s been able to meet station demand.

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REFERENCES

McCormack, Austin, "Capacity, Efficiency - Keys To Demineralizer Performance1*, Power Magazine, August 1989. - Whippie, S. S., Ebach, E. A., and Beardsiey, S. S., 'The Economics of Reverse Osmosis and Ion Exchange", presented at the Ultrapure Water Conference and Exposition, Philadelphia, Pa., April 1987.

McGarvey, F. X., Hauser, E. W., Back, B., Stellitano, J., 'Thermal Degredation of Strongly Basic Anion Exchange Resins in Caustic Regenerants", presented at the International Water Conference, Pittsburg, Pa., November 1987.

Ca Mg Na HCOQ Cl so4 Si02 Total Solids

TABLE A DEMINERALIZER

DESIGN WATEK ANALYSIS

Raw Water

203 144 525 115 534 163

50 1000

Anion Effluent

0.1 18

Mixed Bed Effluent

- 0.01

1

All constituents expressed as mg/l CaC03.

TABLE B DEMINERALIZER DESIGN

DATA

Diameter, Inches Height, ft. - in. Ion Exchange Type

Bed Volume, cu-ft- Bed Depth, inches Capacity Kg/cu.ft. Regenerant Dosage, lb./cu.f t. Flow Rate, gpm Total Gallons

Anion Cation - - 120 108 12-0 14-0 Permuitt Q 10 0 Permui tt S-20 0

620

16.9 a7

392 67

14.7

n .\ 15 (NaOH)

146,500 180

197,350

Mixed Bed

48 10-0 Permuitt €jllS/. Permuitt S-1 00 39/36

36

- 180

1,000,000

TABLE C REVERSE OSMOSIS SYSTEM

DESIGN CONDITIONS

Feed Flow Product Flow Feed Pressure Number of Pressure Vessels Number of Elements/Vessei Element Model A m y Feed pH Salt Rejection @ 100 gpm

TABLE D RO/DI SYSTEM PROFILE

WATER ANALYSIS

Constituent

Ca Mg Na K Fe Sr C1

HCO3 so4

si02

PH Conductivity

Raw Water - 47 15 60

4 <0.01

6 1 105 190

28

0.88

7.8 705

RO Permeate

0.11 0.04 1.0

< O S <o.or <0.01 0.9 1.3 4.9 Q*45

7.5 10.7

133 gpm 100 gpm 250 psi

6 4

3 x 2 ~ 1

95% ( m i d

FILMTEC BW30-8040

7.0 - 7.6

% Rejection

99.8 99.7 98.3 - -

>98. 98.5

97.4 98.4

98.8

- 98.5

Mixed Bed Effluent

<o.of. < O . O l (0.01 <0.01

< O . O l <0.01 (0.01

<0.002

-

-

6 to a 0.1

Al l constituents e x c e p t for pH and conductivity are expressed as mg/I as shown. pH is expressed as standard units and conductivity is expressed as micro S/cm @ 25OC.

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Water Power Chemicals Resin Replacement Membrane Replacement Filter Replacement

Total Cost $/lo00 gallom

TABLE E COST COMPARISON OF

DI WATER PRODUCTION

Before RO 1984

0.77 0.02 6.19 0.84 -

7.82

After RO 1986

0.013 0.16 0.207 0.60 0.526 0.035

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