Reverse Osmosis and Ultrafiltration - American Chemical Society

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Downloaded by STANFORD UNIV GREEN LIBR on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch008

Design Features of the Polyether Composite (PEC)-1000 Spiral-Wound Membrane Element T. ITOH, M. KURIHARA, N. KANAMARU, and T. TONOMURA Technical Development Department, Toray Industries, Inc., 3-Chome, Sonoyama, Otsu, Shiga 520, Japan The PEC-1000 membrane is a composite type of membrane consisting of (1) crosslinked polyether salt-rejecting barrier layer, (2) polysulfone supporting layer and (3) polyester fabric ((2) & (3) are both reinforced by crosslinked polyether resin). So it displays strong physical structure and zero m-value (no-compaction). As for the structural material of the RO element, extensive hydrodynamic study under high pressure exposure has been done specially for the selection of the permeate carrier. The above features, and the special modular design providing the spiral pattern for brine flow are the essential factors which assure the high temperature and pressure stability of the membrane, and uniform flow velocity over the membrane surface. The latter feature permits the use of feed waters with relatively high fouling index, and easy cleaning of the membrane surface even after heavy fouling. These advantages are actually shown in many commercial RO plants in the world. The Special Feature of the PEC-1000 Membrane Toray*s composite membrane, designated as PEC-1000 (PEC is derived from polyether ^Composite), has been developed from many years of research, development, testing and it represents the latest in membrane technology CUjO* PEC-1000 is specially designed for single-stage seawater desalination, and we took into consideration both its performance and mechanical structure. Single-stage seawater desalination permits low TDS permeate from high temperature-high salinity seawater feed at a high recovery rate of feedwater. To meet the challenge of dependability and durability in basic seawater membrane performance, Toray has built into its design the highest salt rejection, permeate flow rate, and durability for high temperature and high pressure operation. This design feature is not found in other single-stage, partial singlestage or two-stage membranes. 0097-6156/85/0281-0099$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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REVERSE OSMOSIS AND ULTRAFILTRATION


Mechanical Structure and Performance of PEC-1000 Membrane The PEC-1000 membrane shown in Figure 1 is made of a composite type structure (3) consisting of 1) a protective membrane layer (0.2ii) 2) a thin 300 A salt barrier layer (being thicker than that in other composite membranes) 3) a porous supporting membrane made of polysulfone, and 4) polyester fabric as the base structure. The ultrathin salt barrier layer, which determines membrane performance, is constructed from a crosslinked polyether, which yields 1) high salt rejection 2) high permeate flow rate 3) wide pH range operation and stability 4) high temperature durability, and 5) dependability And it should be noticed that the resin of ultrathin layer also reinforces both the polysulfone supporting layer and the polyester fabric to give them more elasticity. As a result, the Young's Modulus of the polysulfone layer is increased for example, to 16.7 kg/mm2 from 10.3 kg/mm2 which is the common value for the general polysulfone substrate. However, as for the polyester fabric, the effect of the reinforcement is not so obvious. Such unique membrane substrate provides the Toray composite membrane with 6) high pressure durability 7) durability for handling and service The PEC-1000 membrane exhibits excellent high salt rejection characteristic, an average of 99.85%, and high permeate flow rate of 0.4 m 3 /m 2 .day at 56 kg/cm2 (800 psi), for 3.5% NaCJl or seawater fed at 25°C (77°F) as shown in Figure 2 0_,3>)» The membrane also has the durability for high pressure operation (70 kg/cm2-1000 psi) with high feed-water temperature. Many experimental data indicate there is no effect of compaction on membrane performance and therefore the m-value (flux decline slope) for the PEC-1000 membrane is zero. Element Configuration and Its Flow Pattern in PEC-1000 Spiral Wound Membrane Element PEC-1000 membrane elements are different in configuration (4^_5) from other spiral-wound elements. One main design advantage is that the feedwater flows tangentially into the element along the cylindrical surface, flows spirally around towards the centre (permeate gathering) pipe in the element and exits the element from a circular opening located near the centre pipe at the downstream end of the element. PEC-1000 membrane element configuration is shown in Figure 3. In the case of other spiral-wound elements, water flows into the element from the feed end of the element, flows parallel with the center pipe of the element, and exits the element at the downstream end. The improved configuration of the PEC-1000 membrane element posseses the following advantages over other type of elements (ji) • 1) The uniform distribution and high flow velocity of the feedwater over the membrane surface minimizes concentration polarization. Hence, the membrane performance does not



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PEC-1000 Spiral- Wound Membrane Element

Figure 1. Electronmicrograph of the cross-section and sectional drawing of a PEC-1000 composite membrane. (Reproduced with permission from Ref. 3. Copyright 1983 Elsevier Science.)

Permeate Flow Rate ( m 3 / m 2 d )

Figure 2. Diagrammatic comparison of desalination performance of different commercially available membranes for seawater (3.5% seawater, 56 kg/cm^, 25 °C. (Reproduced with permission from Ref. 3. Copyright 1983 Elsevier Science.) In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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decline even at a high recovery rate which results in low flow rate of brine, as illustrated in Figure 4. In the spiral type elements, the axial deformation of the flow path sometimes occurs and forms a few small ridges in rolling-up the membrane leaves. This results in nonuniform velocity distribution and decrease in element performance in other type of elements. Therefore the PEC-1000 membrane element could maintain the uniform flow velocity distribution and the element performance even if such deformation occurred, 2) There is no dead space when the element is inserted in the pressure vessel. As a result, the following advantages are obtained, a. Difficulty in bacteria growth. This is because the feedwater does not stagnate in the space between the outside surface of the element and the inside surface of the vessel, b. Rapid replacement of the feedwater after cleaning and disinfection of the module, c. Rapid approach to steady permeate flow rate after startup of the plant, 3) There is no trouble based on telescopic deformation. When a large amount of the feedwater flows into a spiral-wound element, or when pressure drop is increased by fouling, other types of elements deform easily, and a telescopic deformation occurs. On the other hand, ends-sealed PEC-1000 membrane element resists greatly to the telescopic deformation; the telescopic deformation of the element does not occur even at a pressure drop of 3 kg/cm2 per one element. From the advantages mentioned above, Toray PEC-1000 composite membrane module makes it possible to produce pure water at lower cost than other types of modules. Data of long-term field tests and demonstration plants suggest that PEC-1000 RO process is now well established for the single-stage seawater desalination (6t^j**.?)• Structural Material of the PEC-1000 Membrane Element Basic Quality of the Structural Material, PEC-1000 membrane element has been developed for single-stage desalination of seawater, especially for high salinity and high temperature seawater in the Middle East. This means that PEC-1000 membrane element would be used at very severe operating conditions such as 50-70 kg/cm2 operating pressure, and 30-40°C feedwater temperature. Therefore the structural material must have the required durability against pressure and temperature. So we had paid much attention to select the material of each part of the element and examined its toughness and durability. In the following section, we describe how we made the selection of permeate spacer, which has the most important influence on element performance. Permeate Spacer of PEC-1000 Membrane Element. Permeate spacer is installed to maintain the permeate flow by supporting the membrane against the operating pressure. We can



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Figure 3. PEC-1000 element configuration. (Reproduced with permission from Ref. 4. Copyright 1983 Elsevier Science.)

Figure 4. Comparison of the performance between PEC-1000 and another type element.


REVERSE OSMOSIS A N D ULTRAFILTRATION

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express the above function in more specific terms as shown in Table I. These characteristics are naturally needed in any RO element. But in the case of the PEC-1000 membrane element, they are needed even more.


Table I.

Basic Characteristics Needed for Permeate Spacer

1, 2,

High pressure durability Low fluid resistance

3,

No dissolution of material into the permeate water Thinner material in thickness to gain more membrane area per unit volume

4,

± ^^\

rigid material proper configuration to prevent the membrane from being deformed

As the permeate spacer of RO element, some kinds of tricot knit fabrics made rigid in some way, are commonly used. Figure 5 shows an example of tricot knit fabric. The specification of fabric is given in terms of wale No,, the number of ribs per inch, and the course No,, the number of cross points per inch. The groove between ribs is used as permeate water channel, so wale No, has much relevance to flow resistance as shown later. As for the way to make tricot rigid, resin treatment commonly using melamine resin is applied in many cases. However, conventional melamine resin type tricot has the following two problems. One is less durability against pressure, particularly in the case of high operating pressures over 50 kg/cm2, and the other is the dissolution of materials, mainly melamine resin, into the permeate water during long-term operation, by oxidative degradation. We had started the study of the selection of the permeate spacer from this point of view. We discussed this problem with fabric and textile researchers in our company, Toray Ind, Inc. is a maker of synthetic fibers and textiles originally, so we have much potential and experience in this field. We studied about 1) the material and the size of tricot yarn, 2) the number of filament of each yarn, 3) fabric structure, 4) temperature of heat treatment, and so on • We resolved the problem of the melamine type tricot by applying an improved tricot knit fabric, the so called mixed or conjugate yarn type, as shown in Figure 6. It is composed of at least one composite yarn, each yarn having the first and the second components of high polymers, different in a specific characteristic, such as melting point, solubility, and degree of swelling (10), The first yarn component is fused, dissolved into a specific solvent or swelled with a specific solvent to adhere to the second yarn component, and thus, the first yarn component of each yarn is bound



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Course: the number of cross points per inch. Flow direction of permeate water. Wale:

Figure 5.

the number of ribs per inch.

An example of tricot knit fabric.

Figure 6. Sectional view of a tricot knit fabric composed of mixed yarns made rigid by heat treatment.


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directly to that yarn and/or the other yarns so that the cloth is made rigid. The resultant cloth is rigid enough to resist a high fluid pressure of the feed solution, so that a smooth flow of the permeate water is ensured. And also there is no problem of dissolution in this improved type. The experimental data for comparing some of the improved type to melamine type is shown in Table II. The flow resistance coefficient H, the so called H-value, means pressure drop per unit flow rate and is given by Equation 1 below.

where W, L, dp, Q are the channel width [m], channel length [m], pressure drop [atm] and flow rate [m3/day] respectively. And m H given by Equation 2 means the increase of H-value with increase in operating time, therefore smaller mu means more durability.

where t, H Q , H represent operating time, initial H-value, H-value after t has elapsed, respectively. One can see the effect of our improvement from Table II. On the other hand, we also made a hydrodynamical study on the proper configuration of permeate spacer to make fluid flow resistance less, and reduce spacer thickness at the same time. As the first step, we adopted the plain model of the section configuration shown in Figure 7. If we suppose the membrane is a homogeneous film, its deformation is given by balancing the static load working on the membrane. The schematic cross section of the membrane under operating pressure is shown in Figure 8, where two basic equations are given. From the balance of the force acting vertically.

From the relation between the stress of the membrane and its elongation,

where a, E, AL are the stress [kg/m 2 ], the Young's Modulus [kg/m 2 ], elongation [m] of the membrane respectively. Considering that

where t is thickness of the membrane [m], Equation 5 turns into the form of Equation 7 as shown below.


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Table II.

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Flow Resistance Characteristics of Some Kinds of Tricot Knit Fabric R.g.d Cloths

" 0 W Re8i8'ance Characteristics

Reduction in flow rate after

1Q()

houf8


H (atm.day/m3) iTlH

Ex. 1 Polyester conjugate yarns

Heating treatmeent at 2 4 0 c

0.78 3.82

0.03 0.15

0 (negligible)

Ex.2 Polyester mixed yarns


1.41

0.04

1.1

Ex.3 Polyester mixed yarns


1.23

0.02

0 (negligible)

3.56

0.19

Control Polyester yarns with melamine

u H

WdP = 1TQ-

H: flow resist, coefficient (atm.day/m3) W: channel width (m) L: channel length (m) dP: pressure drop (atm) Q: flow rate (m3/day)

„_ m H

=

13.0

log Ht/Ho log t

. .. t: t , m e

,. „ r% (hour)

Figure 7, The plain model of the section configuration of the permeate spacer.



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The values of 9 and T are given by satisfying Equation 3 and Equation 7 simultaneously. In this way, we can calculate the elongation of the membrane and thus, the depth of the bending of the membrane. Hence, we can fix the effective flow channel area of permeate water and the channel resistance H. The calculated H-value of a certain configuration is shown in Figure 9. In this case, membrane is PEC-1000, operating pressure is 70 kg/cm2, and groove depth is 0.15 mm. H-value is indicated with the parameter, wale No. This result shows, naturally, that the smaller channel width gives the bigger H-value. The bigger channel width means the smaller H-value but its effect is decreased by the deformation of the membrane. In the case of too big a channel width, H-value could be even increased, when we must consider the limitation in making tricot knit fabric. As for the rib width, the smaller is the better, but about 0.2 mm is the minimum we can reach. The dotted line in Figure 9 means the rib width is 0.2 mm, so the tricot we can make is in the upper area of the dotted line. Another example of calculated H-value is given in Figure 10. The difference from the previous one is channel depth, 0.13 mm in this case. This result shows that only 0.02 mm smaller depth increases H-value greatly. Such study brought us to the specific design about the most proper configuration which makes the H-value the least. Those are summarized as follows: 1) the bigger channel depth 2) channel width should be in the range from 0.2 mm to 0.3 mm 3) wale number; about 50 is preferable. We made a lot of samples of tricot whose configurations were similar to the ideal one. We measured the H-value of those samples while comparing them with each other. Figure 11 shows the experimental data. H-value is plotted versus the thickness of tricot. So in this figure, left and lower area means good configuration. It is obvious that our improved tricot has a better configuration than conventional ones. Though the calculated H-value is slightly different from the measured one because of the simplification, experimental data are enough to make us ensure the correctness of our study. We studied the permeate spacer design to get better performance of the element, mainly by the two ways described above. By applying these results of the development, PEC-1000 membrane elements exhibit excellent high salt rejection at an average of 99.7% and a high permeate flow rate at 56 kg/cm2 (800 psi) using 3.5% NaCU or seawater, as feed at 25°C (77°F) and 13.5% water recovery. Plant Performance of the PEC-1000 Membrane Element As stated above, PEC-1000 membrane and element have the characteristics required for single-stage seawater desalination. That means high durability against pressure and temperature, and of course high salt rejection and water flux from the membrane. The reasons for such characteristics are the following: A. high performance of salt barrier layer, B. very strong physical structure of the membrane, C. improved configuration of the element, D. proper selection of the structural material.



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Figure 8. The deformation of the membrane under operating pressure. Key: P, operating pressure (kg/m 2 ); T, tension of the membrane per unit width (kg/m); 8, angle of deformation (rad.); I, a half of the channel width (m); and m, a half of the rib width (m).

Figure 9. Examples of the calculated H-value (1). Wale 30 means 30 channels per inch of the tricot width. Upper area of the dotted line means that the rib width is bigger than 0.2 mm. Membrane: PEC-1000. Operating pressure: 70 (kg/cm2).



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Figure 10. Examples of the calculated H-value (2^ .Wale 30 means 30 channels per inch of the tricot width. Upper area of the dotted line means that the rib width is bigger than 0.2 mm. Membrane: PEC-1000. Operating pressure: 70 (kg/cm2).

Figure 11. H-value of improved tricot knit fabric. Key: 1, zone of H-value of commonly used tricot: 2, zone of H-value of improved tricot; and A, •, O, •, •,; actual experimental data of improved tricot.


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These characteristics are actually shown in many commercial RO

plants (£,Z.,£,2). Literature Cited 1. 2. 3.


4. 5. 6.

7. 8.

9. 10.

Kurihara, M.; Kanamaru, N.; Harumiya, N.; Yoshimura, K.; Hagiwara, S. Desalination 1980, 32, 13. Kurihara, M.; Watanabe, T.; Inoue, T. U.S. Patent 4366062. Chen, J.Y.; Kurihara, M.; Pusch, W. Desalination 1983, 46, 379. Bairinji, R.; Tanaka, T.; Kurihara, M.; Kanamaru, N.; Tonomura, T. Desalination 1983, 46, 57. Kanamaru, N.; Fujino, H. U.S. Patent 3933646, 1976. Kurihara, M.; Nakagawa, Y.; Takeuchi, H.; Kanamaru, N.; Tonomura, T. Proc. 10th Annual Conferen. Trade Fair of WS1A., 1982. Kurihara, M.; Nakagawa, Y.; Takeuchi, H.; Kanamaru, N.; Tonomura, T. Desalination 1983, 46, 101. Doelle, R.A.; Kallenberg, K.H.; Heyden, W. Poster Presentation No. 29 at First World Congress on Desalination and Water Reuse. Florence Italy, May 26 (1983). Kunisada, Y.; Okada, K.; Sonoda, T.; Setogawa, S.; Ishiwatari, T. Zosui Gijitsu 1983, 9, No. 4, 13. Bairinji, R.; Tanaka, T.; Kawabata, T. G.B. Patent 2000694.

RECEIVED February 22, 1985


Reverse Osmosis and Ultrafiltration - American Chemical Society

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