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DESALINATION ELSEVIER

Desalination 117 (1998) 311-321

The Hollow Fiber CartridgeTM Operational experience with an improved reverse osmosis device/technology G. Reibersdorf&, C.P. Shields h*, J.W.

Stranzb

~DuPont Permasep ~, Sagewerkstrasse 3, D-83395 Freilassing, Germany bDuPont Permasep~, Glasgow Site, Wilmington, DE 19898, USA TeL: + 1 002) 451-9309; Fax." + 1 (302) 451-9686 RecOver 20 July 1998

Abstract Since the introduction &the Hollow Fiber (HF) CartridgeTM at the IDA World Congressin Madrid in October 1997, DuPont Permasep® Products commercialized a breakthrough in reverse osmosis (RO) device technology and advanced new membrane products for desalination. The new HF CartridgeT M product line has capabilities far beyond the current products. The sum of all improvements further increases the HF device capacity and compactness, and provides new capabilities to operate RO systems more effectively at very high recoveries in a singlestage array with low pressure drop. This paper presents results of field experience on various applications and the results obtained.

Keywords: Desalination; Reverse osmosis; Membranes; Water treatment; New RO technology; I-IF CartridgeTM

1. Introduction When introduced a year ago, the new HF Cartridge~' RO products represented a breakthrough in device engineering and the "repackaging" o f proven, aramid HF membranes.

*Corresponding author.

The new products were designed to bring many new benefits to end users in seawater desalting as well as in brackish and other lowsalinity RO applications. This paper reviews the new RO device and technology, and presents results o f field experience with a number o f commercial plants and test units operating on sea and low-salinity feedwaters.

Presented at the Conferenceon Membranesin Drinkingand IndustrialWaterProduction,Amsterdam, September21-24. 1998, International Water Services Association,European DesalinationSocietyand American Water Works Association 0011-9164/98/$ - See front matter lC~! 998 ElsevierScience B.V. All rightsrosorved. PII S0011-9164(98)00125-8

312

G. Reibersdorferet al. / Desalination 117 (1998) 311-321

2.

The HF Cartridge"` features and benefits

devi~

design,

Following is a brief review of the design innovations and features of the new HF Cartridge" device and the benefits it brings to designers and operators of RO systems.

2.1. Improved performance through "doubleended" design RO science predicts performance improvements across a given RO membrane by increasing net driving force (NDF) in a new RO device. The increased performance would apply in all applications. The new HF Cartridge" device increases NDF by withdrawing permeate from both ends of the fiber membrane/bundle (i.e., "doubleending"). Traditional HF devices withdraw from only one end. The double-ended design provides three significant performance improvements* with current HF membranes, as illustrated by computer modeling and the following comparisons to traditional single ended HF membranes/bundles having the same dimensions:

2.1.1. Increased permeate flow The comparative increase in permeate flow for a double-ended device (vs. traditional singleended) can range from 24% with seawater feeds to 52% with brackish water [1]. Double-ending significantly increases membrane flux because pressure drop in the bore of the fiber is significantly reduced, thereby increasing NDF. * An extensive discussion of the science and calculations underlying the increased NDF and the resulting performante improvements of a double-ended design can be found in the IDA paper introducing the new technology [ll.

2.1.2. Improved rejection)

permeate

quality

(salt

The comparative improvement in permeate quality (salt rejection) can range from a 19% reduction in permeate salt concentration for a seawater application to a 34% reduction for brackish water [1]. The higher permeate flow dilutes a given amount of .salt, resulting in a lower permeate salt concentration (higher salt rejection).

2.1.3. Better internal flow uniformity A subtle, yet very important benefit of the double-ended device is that it produces a more uniform flux along the fiber length because the NDF is more uniform. This produces a more uniform recovery along the length of fibers inside the membrane bundle, resulting in important RO system design and operational benefits. The magnitude of improvements finally realized (for each application area) depends on the final HF Cartridge" design chosen, considering the trade-off between the actual amount of fiber packed into a practical device versus the operational characteristics desired (i.e., lower pressure drop, increased resistance to fouling, etc.).

2.2. HF Cartridge improvements

TM

device--additional design

The overall objective for the reengineered HF Cartridge"` was to develop a practical new HI: device that would produce higher flow per pressure vessel (unit volume) and lower overall RO system p~meate costs, while further improving operational characteristics. Additional device design improvements also contribute significantly to this objective and are summarized below:

G. Reibersdorfer et aL / Desalination 117 (1998) 311- 321

SUPPORT BLOCK -~,\

/~

END HOUSING

/--,C..EN-rER PRODUCT

313

END HOUSING ~

U-CUP BRINE SEAL

4,,-

FEED BYPASS PERMEATE

FEED PERMEATE

,'-- BRINE

" // ~FEED

\/ BUSHING

.......

L - - F E E D TUBE ANNULUS

--

BRINE BUSHING

Fig. 1. The HF Caruidge". iJ

'

Fig. 2. MultipleHF Cartridge assembly. TM

2. 2.1 Innovative design for multiple assemblies The breakthrough [1] to enable highercapacity, multiple-cartridge assemblies is a unique mechanical design which allows permeate to be collected in a center product water tube. This innovative design allows multiple HF Cartridges to be interconnected easily in a single pressure vessel. The new design was patented in 1995 [2] for HF devices. The HF Cartridge design and water flow pattern is illustrated in Fig. 1. Details o f this unique single-cartridge construction are described in the introductory paper [1 ]. The ability to join the center product water tube o f one HF Cartridge tc another enables multiple assemblies to be made o f two, three or more cartridges in a single pressure vessel, as illustrated in Fig. 2. TM

TM

TM

Fig~3.shows the flow pattern for three Cartridges m a single pressure vessel and illustrates in more detail how they are joined together with interconnectors. 2. 2. 2. SeiJ:regulating feed bypass A special feature o f the HF Cartridge design is a self-regulating feed "bypass" hole located in the brine bushing (Figs. 1 and 3). The feed bypass hole allows a portion of the feedwater of one cartridge to bypass its bundle directly to the next cartridge. This reduces flow though the first in a series o f cartridges and avoids a high flow resistance, which would occur if 100% o f the feedwater passed radially through every cartridge in series. By reducing flow through the fiber bundle, pressure drop across it is lowered, and a higher net driving TM

314

G. Reibersdorfer et aL / Desalination 117 (1998) 311-321 141:: ( " A I ~ T I ~ I I'1~. I:: ~

TM

:lINE

FEED

qMEATE

PERMEATE <

Fig. 3. Assemblyof threeHF Cartridges

TM

force is provided to the downstream cartridges. The feed bypass also has self-regulating characteristics, whereby pressure drops, and flows automatically and beneficially adjusts during operation [1]. The self-regulating feed bypass feature is very important and allows: • More cartridges installed in a single vessel, • Longer time between membrane cleanings, and • Lower overall pressure loss across the cartridge assembly. 2.2. 3. Reengmeered bundle cross-section

Another important design feature of the HF Cartridge is a reengineered cross-section to optimize inside-to-outside radial crossflow through the fiber bundle. The HF Cartridge has a smaller bundle OD and a larger bundle ID than the traditional HF bundle, part of the tradeoff to achieve the overall performance objectives. Pressure drop through the HF CartridgeTM is significantly lower than traditional designs because flow velocity through the inner portion of the bundle, where the most pressure drop occurs, has been reduced by 20%, and the radial flow path through the fiber bundle has been reduced 22% [1]. This feature, combined with other improvements obtained with the HF CarTM

TM

tddge (i.e., more uniform recovery along the length of the fiber, bypass hole, etc.), is expected to further improve fouling resistance and reduce energy consumption. TM

2, 3. Commercial H F Cartridge T M products

The first in a series of new commercial products, using current HF membranes, is described here. The initial focus was on the development and commercialization of two seawater models and one brackish water cartridge in a nominal 8½" diameter by 40" length for a wide range of RO plant sizes. For seawater desalination, the two new cartridge models are: . SW-M-8540, designed for operation on medim-salinity seawater at maximum pressure up to 1,015 psig (70 bar). * SW-H-8540, designed for operation on highsalinity seawater at maximum pressure up to 1,200 psig (83 bar). For brackish water and other low-salinity applications, the first low-pressure cartridge model is: * BW-L-8540, designed for operation on typical brackish water, at a nominal 225 psig (l 5 bar) pressure.

G. Reibersdorfer et al. /Desalination 117 (1998) 311-321

315

Tablel Permasep®HF Cartridge" products and performance

Products Seawater For medium salinity/medium-pressure applications (Model SW-M-8540, on feed of 32,000 mg/1NaCI) For high salinity/highpressure applications (Model SW-H-8540, on feed of 35,000 mg/l NaCI) Brackish For typical brackish/lowpressure applications (Model BW-L-8540, on feed of 1,500 mg/l NaC1)

Applied pressure, psig

Recovery,% GPD

Cartridge performance Flow Salt rejection, m3/d %

1,000 1,000

35 20*

10,400 11,500

39.4 43.5

99.45 99.56

1,000

1O*

12,000

45.4

99.60

1,000 1,200

35 35

8,000 11,400

30.3 43.1

99.60 99.73

1,200 1,200

20* 10*

12,500 13,000

47.3 49.2

99.78 99.80

225 225

75 20*

11,000 12,400

41.6 46.9

97.30 98.90

225

1O*

12,500

47.3

99.00

*Projectedperformancefor comparisonto spiral-woundelements. Performance o f the three initial commercial products is shown in Table 1 for a range o f applied pressures and conversions. Higher recovery operation is possible, but naturally depends on application conditions. (The other lower recoveries are shown for comparison to spiral dements.) The three initial HF Cartridge TM products are pictured in Fig. 4. Pressure vessels are currently available for assemblies o f one, two or three HF Cartridges TM. Flow is approximately doubled or tripled when two or three cartridges are used in a single vessel versus one cartridge. Salt rejection remains about the same with one, two or three cartridges.

2.4. Translating HF Cartridge" design improvements into RO ~3,stem benefits The new HF Cartridge TM technology provides a breakthrough in multiple-cartridge assemblies and embodies many design improvements for RO systems while retaining the benefits o f current HF membranes.

2. 4.1. High recovery per cartridge efficient system designs/lower costs A single cartridge can be operated up to about 45% recovery in seawater and about 70% recovery in brackish water, depending on conditions. Multiple-cartridge assemblies (for large-

316

G. R ei bersdorfer et al. / Desalination 117 (1998) 311-321

cellent, long-term performance and low membrane replacement costs.

3. Field experience

Fig. 4. HF Cartridge TM products.

scale plants) can be operated up to about 60% recovery in seawater and 90% recovery in brackish water. Because of these high recovery capabilities, most RO system designs using HF Cartridges will be single-staged arrays. This important capability simplifies system design, reduces pressure losses and energy consumption, and improves system compactness, all of which lead to reduced capital and operating costs. TM

2. 4. 2. Device compactness--plant compactness/ lower costs

The first prototype HF Cartridge" was produced in September 1995. Many thousands of hours of in-house testing were performed to fully evaluate the feasibility of this concept, improve design and test durability of the unit and its individual components before commercialization in October 1997. Field experience with the new HF Cartridge products is being gained rapidly in both seawater and lower salinity applications, ineluding brackish. The following five cases summarize field experience to date with HF CartridgeT M in applications where sufficient data were available to evaluate RO system performance over time. TM

3.1. Seawater~.~aribbean

This commercial seawater plant operates at an existing B-10 plant location in the Caribbean. It comprises 12 model SW-H-8540 HF Cartridge in four pressure vessels (each with three cartridges) in a single-staged array. The seawater is drawn t~om wells. It has a normal composition in the 36,000-37,000 mg/l range, except for a high level of organics. Pretreatment simply consists of 5~t cartridge filters. The plant is rated at 270 m3/d (71,000 US GPD), under normalized conditions, and provides potable water to a hotel. The plant started up in February 1998 and ran for more than 2,000 hours through early June. Operating conditions and performance are monitored weekly. The plant operates at a feed pressure of 69-76 bars (1,000-1,100 psig), recovery of about 50% and temperature of 27-28°C. TM

I-IF Cartridges are the most compact (i.e., GPD per ft3 volume) of all RO devices. They have approximately twice the flow per unit volume as a similar capacity spiral-wound assembly. This reduces vessel support racks, building size and capital cost. TM

2. 4. 3. Robust construction membrane performance/lower costs

long-term

The HF Cartridge (like its forenlnners, traditional HF permeators) h ~ a very robust construction. The HF Cmialdge is capable of withstanding >80 psig pressure drop in a pressure vessel. This, together with durable Aramid HF membranes, contributes strongly towards exTM

TM

G. Reibersdorfer et al. / Desafination 117 (1998) 311-321 3. 2. Seawater--Atlantic Ocean (USA)

Permeate Flow 50O 40O 3OO 20O

100

0

I

I

OPERATING PERIOD--MONTHS

Permeate Quality

I

800

0

317

This commercial seawater plant operates at one of the world's largest shrimp farms located in Florida. It contains 12 model SW-H-8510 HF CartridgeTM in six pressure vessels (each with two cartridges in a single-staged array. The plant was first built in 1986 with another type of RO membrane, but was upgraded with new membranes in early 1998. The objectives of the upgrade were to provide higher capacity, 284 m3/d (75,000 US GPD),* in a very limited space, to operate at higher recovery, and to provide a better product water quality for blending to make the controlled salinity required to raise shrimp. A photo of the membrane system is shown in Fig. 6.

NORMALIZED PERMEAIEll)~i t ........... R~TF~PERMI=~TETOS

11 OPERATING PERIOD--MONTHS

Fig. 5. HF Cartridge" performance:Seawater-Caribbean (ModelSW-H-8540).

Fig. 6. Compactseawatersystem,usingmodelSW-H-8549 I-IFCartridges"m "slide-portentry"pressurevessels.

Performance of the plant is summarized in Fig. 5, showing normalized data plots of permeate flow and quality versus time. The unit has met flow requirements and delivered an outstanding peimeate quality, averaging about 200 mg/1, much better than the potable water standard. The HF CartridgesTM have experienced no operating problems and, despite the high level of organics, have not required cleaning. Plant performance met expectations.

Atlantic Ocean seawater of about 35,00036,000 mgh TDS is withdrawn from seawells on the farm property. The only pretreatment is a 51a cartridge filter. Operating dam are taken daily. The HF Cartridge system started up in February 1998 and has operated for about 2,500 hours. Operating conditions are---feed pressure: 67-75 bars (970-1,090 psig); recovery: about 45%; and feed temperature: 22-24°C. TM

*Normalizedto operatingconditions.

G. Reibersdorfer et al. / Desalination 117 (1998) 311-321

318

Permeate Flow

the available space and for better pei-meate quality at higher recovery.

5OO

3.3. Low salinity--USA (Site ,4)

4O0

3OO

2OO

100

0

......

L,

2~

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OPERATING PERIOD--MONTHS

This commercial installation uses 4 model BW-L-8540 HF CartridgesTM in an assembly of eight pressure vessels (each with three cartridges) in a single-staged array. The RO unit is rated at 855 m3/d (225,000 US GPD)* and is part of a larger water treatment system, including an ion exchange softener. A photo of the RO rack showing the HF Cartridge vessels and control panel is shown in Fig. 8. TM

Permeate Quality 1,00( 80( 60(

i..................... 40(

0 ~

<

~

OPERATING PERIOD--MONTHS

Fig. 7. HF Cartridge performance: Seawater--Atlantic Ocean (ModelSW-H-8540). TM

Plant performance is indicated in Fig. 7, showing normalized permeate flow and quality through the operating period to date. Permeate flow has leveled o f f t o about 310 m3/d (81,850 US GPD), above rated plant capacity*, and permeate quality has averaged less than about 250 mg/l, well below the 500 mg/l quality needed for diluting and controlling seawater concentration. Pressure drop across the pressure vessels is between 0.3 and 0.7 bars (5 and 10 psig). No operating problems have been experienced. The upgraded plant has exceeded the owner's expectations for more plant capacity in

Fig. 8. Expandable,low-pressuresystem,using modelBWL-8540 HF Cartridges in a three-cartridgeassembly. TM

The combination soflener/RO system is intended to "'polish" a low-salinity, municipal "tap" water for an industrial process (and other) use. The new treatment system replaced an older activated carbon system that had difficulties providing a consistent process water quality. The new system started in January 1998 and has provided purified water on demand to a manufacturing process for a period of six months. Water system operating parameters are *Normalizedto operatingconditions.

G. Reibersdorfer et aL / Desalination 117 (1998) 311-321

monitored about once a week on average. The high-alkalinity, softened RO feedwater has variable composition and feed conductivity ranging fIom about 80-140 BS/cm (40-65 rag/1 TDS), depending on the combination of wells used at the source. The RO plant has operated between the following ranges: feed pressure, 12-16 bars (175-235 psig); recovery, 80-85%; pI-I, 6-7; and temperature, 10-20°C. RO plant performance is indicated in the normalized data plots of Fig. 9. The plant has reliably produced the permeate flow demanded by the manufacturing process and has consistently supplied the desired permeate quality of conductivity 2-6 BS/cm (1-3 mg/1 TDS). Pressure drop across the three-cartridge assemblies is in the 1.0--1.4 bar (15-20 psig) range. No operating problems have been experienced. The combined water system has met its dual objecfives of: delivering a more consistent water quality and providing the expected benefits to the manufacturing process.

319

Permeate Flow

1,20 1,00 .............

~

8O

~

40

..........

I

OPERATING PERIOD--MONTHS

Permeate Quality 10 1.0

..',1

iL !........ \

,..,

~'....,. !

..... ~

'

X

/i~

~,.",

..........-'''" 10 ,0 ~0

3.4. Low salinity--USA (Site B) This test used two model BW-L-8540 HF Cartridge in a single pressure vessel. The test operated at an industrial plant site in the US, which provided softened well water to the RO unit. RO permeate is fed to an existing ion exchanger to produce demineralized process water. The test started in June 1997 and has run for nearly a year. The high-alkalinity, softened feedwater had a conductivity ranging fxom 150250 BS/cm (TDS about 135-215 mg/l). Operating conditions have been: feed pressure, 14.515.5 bars (210-225 psig); recovery about 70%; feed pH,7-8; and feed temperature, 11-15°C. Test unit performance showing normalized data plots of permeate flow and quality, is graphed in Fig. 10. The normalized data show

~.C 0 =

TM

OPERATING P E R I O D - - M O N T H S

Fig. 9. HF CartridgeT M performance: low salinity--USA (Site A) (Model BW-L-8540).

that permeate flow has been constant at about 67 m3/d (17,700 US GPD) and above the flow rating.* Permeate conductivity has been relatively constant at about 5 ~S/cm (about 3 mg/1 TDS). Pressure drop across the pressure vessel has been a constant 0.7 bar (10 psig). No operating problems have been experienced. Performance of the I-IF Cartridges has met test expectations. TM

*Normalized to operating conditions.

320

G. Reibersdorfer et al. /Desalination 117 (1998) 311-321

Permeate Flow

NORMAI.I~J:'D FLOW

Fig. 11. One of three compacttrains, using Model BW-L8540 HF CarU'idges"in a three-cartridgeassembly.

OPERATING PERIOD--MONTHS

Permeate Quality

~.' ,"~~r...........,....

250 :~ 200

b~

~..r~ d.~-

~ d'

~

~'"

k"

150

..................~?~o%,~%~A~ .... u~,,.~__

8 50

t I

0 '

~

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OPERATING PERIOD~MONTItS

Fig. 10. HF Cartridge performance:low salinity--USA (Site B) (ModelBW-L-8540). TM

3.5. RO Permeate--Caribbean

This commercial plant contains a total of 189 model BW-L-8540 HF Cartridges" installed in three separate trains. Each train has 21 pressure vessels, containing three cartridges per vessel, in a single-staged array. The plant is used to polish permeate from a companion B-10 seawater desalination plant to below 300 pS/cm. Because of the excellent permeate quality from the B-10 portion of the plant, only two of the three polishing trains are used. One train is shown in Fig. 11.

The plant started up in April 1997 and has performed exceptionally well. Operating conditions for the BW-L-8540 trains are varied to maintain overall permeate quality leaving the plant at 2?5-300 lxS/cm and to maximize total permeate production. The number of seawater trains on-line also affects the polisher operating conditions. Feed to these polishing units ranges from about 400--500 ~tS/cm (225-300 mgh TDS). Feed pressure varies from 9.3-13 bar (135-190 psig), while recovery ranges from 83-89% and temperature varies between 27 and 30°C. Comparin~ data taken at start-up and 13 months later* at comparable conditions shows that performance of the BW-L-8540 system has not changed. Each train is producing a permeate flow of about 1,960 m3/d (517,400 US GPD) with a p~m~eate quality of about 21 btS/crn (11 mg/1 TDS). Pressure drop across the pressure vessels remained constant at 0.7-0.8 bar (10-12 psi). Other than three leaky end-plate seals at start up, there have been no operating problems, and the owner is pleased with the polisher system performance. *Continuousdatanot availablefor graphing.

G. Reibersdorfer et al. / Desalination 117 (1998) 311-321

321

4. Summary and conclusions

Acknowledgements

The field experience o f five RO installations using the innovative, new DuPont HF Cartridge TM products was reviewed. Detailed normalized data were presented showing successful operation o f two commercial seawater applications using Permasep ® HF CmetridgerTM model SW-H-8540 and three low-salinity applications using model BW-L-8540. Field experience with the new HF Cartridges TM in the above operating systems h ~ thus far met expectations and has confirmed improvements sought in redesigning traditional HF pei-meators for both seawater desalination and purification o f lower-salinity feedwatefs.

The authors wish to thank g . o . Myers, Jr., and plant owners and operating personnel for arrangements made to provide data on the HF Cartridge TM RO systems.

References [1] T.J. Eekman, C.P. Shields, J W. Strantz, et al., Proe., IDA World Congress on Desalination and Water Reuse, Madrid, Spain, October 1997, vol. HI, pp. 441457. [2] T.J. Eckman,US Patent No. 5,470,569, Hollow Fiber CartridgeTM, November28, 1995.