new high-performance spacers in electro-dialysis reversal (EDR) systems

Transcription

1 Water Technologies & Solutions technical paper new high-performance spacers in electro-dialysis reversal (EDR) systems Authors: Antonia von Gottberg, Product Manager, Ionics, Incorporated This paper was originally published in Proceedings of 1998 AWWA Anual Conference, June 21-25, 1998, Dallas Texas. Reprinted with the permission of the American Water Works Association. Note: SUEZ purchased Ionics in abstract A new high-performance spacer, which promotes greater turbulent flow in an electrodialysis stack, has been developed and is in operation. The maximum demineralization that can be obtained in a single hydraulic stage has been increased in comparison to one hydraulic stage of conventional stacks. These high-performance spacers are incorporated into a new membrane stack that has 38% more usable membrane area than a conventional stack. The increased available membrane area results in higher demineralization per stack, so fewer membrane stacks are needed to demineralize a given volume of water to a specific TDS level. The high-performance membrane stack is being used to design systems with a reduction in membrane area, capital costs, DC power consumption, and operating costs. These improvements in electrodialysis stack efficiency are accompanied by improvements in system hydraulics and control designs, all of which lead to smaller footprint, simpler control, and lower pumping power consumption. The parameters that determine the design of an EDR plant are reviewed. The performance of the conventional and the new high-performance membrane stacks, operating side-byside, are compared. introduction Electrodialysis (ED) has been a practical technology for desalination of brackish waters since the 1950 s. 1 Electrodialysis is an electrochemical separation process in which ions are transferred through ion exchange membranes by means of an electrical driving force. The components at the heart of an electrodialysis system are ion-exchange membranes, which selectively permit the transfer of ions, and flow spacers, which seprate the membranes and allow for the distribution of water across the surface of the membranes. The new high performance screen spacer, which forms the basis of this article, is already being used to desalt over 10 mgd (50,000 m 3 /day) of brackish water for potable, industrial, and agricultural applications. electrodialysis reversal Electrodialysis Reversal (EDR) is an automatic selfcleaning version of electrodialysis in which the polarity of the DC voltage is reversed two to four times per hour. 2 When DC voltage is applied across a pair of electrodes, positively charged ions such as sodium move towards the cathode, and negatively charged ions such as chloride move towards the anode. Membranes are placed between the electrodes to form several compartments. Flow spacers are placed between the membranes to support the membranes and to create turbulent flow. Water flows along the spacers flowpaths across the surface of the membranes rather than through the membranes as in Reverse Osmosis (RO). Charged ions travel through the membranes under the influence of an applied DC voltage. By alternating cation transfer membranes and anion transfer Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. TP1087EN.docx Mar-10

2 membranes, alternating flow streams dilute streams and concentrate streams are created. Cell pairs consist of a cation transfer membrane, a dilute flow spacer, an anion transfer membrane and a concentrate flow spacer. Up to 600 cell pairs are stacked on one another to make up a membrane stack as shown in Figure 1. Figure 1: Electrodialysis Stack system configurations Each membrane stack can desalt a nominal amount of water depending on the number of cell pairs. A typical 600-cell pair membrane stack can desalt about 200,000 gpd (757 m 3 /day). The amount of salt removal, or cut, is a function of several factors that are discussed in this paper. A nominal cut might be 50%. For applications requiring a greater amount of desalting, stacks are placed in series, or stages. Each additional stage removes 50% of the remaining salt. For larger volume applications, stacks are placed in parallel lines. salt removal rate The amount of salt that can be removed in a stack depends on the feedwater quality, the water temperature, the properties of the ion-exchange membrane, and the spacer properties. This paper focuses on the spacer properties, but discusses other factors where relevant. current for one second will transfer one gram equivalent of salt. This law is used as the basis for calculating the amount of electric current needed in an ED system to transfer a specific quantity of salts. 3 For ED calculations, Faraday s Law is: (N feed -N product ) = I * #cp * ce / (F * Q) N feed = feed normality n product =product normality I = current #cp = number of cell pairs F = Faraday s constant ce = current efficiency for ion transport Q = dilute flowrate Current efficiency for ion transport is a function of the ion-exchange membranes and the stream concentrations and is in the range of 70-90%. Faraday s law says that, for a membrane stack with a fixed number of cell pairs and flowrate, the greater the DC current applied to the stack, the greater the salt removal. It would appear that the best way to operate an EDR stack would be to apply as high a current as possible to make a very high cut and minimize capital equipment cost. There is, however, a mass transfer limitation on the amount of current that can be applied to the stack. At a certain limiting current density, polarization occurs. polarization Consider the transfer of chloride ions from the dilute stream through an anion-exchange membrane. Figure 2 illustrates the mechanisms for mass transfer from the dilute stream through the membrane. If the transfer number of chlorides in the membrane is t m and the transfer number of chloride ions in solution is t s, then the rate of transfer from the membrane surface through an ion-exchange membrane due to a direct current of density CD is given by tm CD/F, and the rate of transfer to the membrane surface through the solution due to a direct current is given by t s CD/F. Faraday s Law Faraday s Law, as related to the ED process, states that the passage of 96,500 amperes of electric Page 2 TP1087EN.docx

3 anion transfer membrane. This will damage the membranes over time, so it is desirable to operate at a current density below polarizing conditions. The current density at which polarization occurs is defined as the limiting current density and is mathematically described by the condition that N m = 0. Hence, (CD/N) lim = D F/((t m - t s ) ). Figure 2: Mass Transfer Through an Ion-Exchange Membrane t m is greater than t s, so the concentration of ions at the membrane drops, forming a concentration boundary layer between the membrane surface and the bulk dilute stream. The concentration gradient causes ions to diffuse from the bulk dilute stream towards the membrane surface. The ion flux is determined by the diffusion coefficient for ion transport through the solution, the concentration difference between the bulk stream and the membrane surface, and the boundary layer thickness. The mass balance at the membrane surface is given by: 4 (tm ts) CD/F = D(Nb Nm) / where D = diffusion coefficient N b = normality in bulk solution N m = normality in solution at membrane surface = boundary layer thickness As the current density is increased, the concentration at the membrane surface, Nm, decreases until there are no ions at the membrane surface to transfer. At this point, water splitting, or polarization occurs. Water dissociates into hydrogen (H + ) and hydroxyl (OH - ) ions. Experimentally, polarization is found to occur at the anion transfer membrane first. Since the anion transfer membrane transfers negatively charged ions, the hydroxyl ions travel through the membranes. The ph of the dilute stream decreases, and the ph of the concentrate stream increases. This rise in ph in the concentrate stream increases the likelihood of precipitation of scaling salts in the The factor, (CD/N) lim, is related to the mass transfer coefficient. The limiting current density is always normalized by the dilute stream normality because the more concentrated the dilute stream, the greater the current required to reach polarization. To minimize the membrane area required for an application, (CD / N) lim must be maximized by changing the boundary layer thickness, the other parameters being physical properties of the system. flow spacers The boundary layer is a function of velocity and flow spacer geometry. The boundary layer thickness may be minimized by promoting turbulent flow, and thus increasing the limiting current density. 5,6,7 The amount of turbulence promotion is a function of the velocity in the flow spacer the higher the velocity, the greater the turbulence. The operating velocity in an ED stack is limited by the pressure drop along the flow spacer and through the stack. There is a maximum inlet pressure at which to operate a membrane stack to prevent external leakage. If the pressure drop is too great, then the number of stages that can be utilized in series is limited. Hence, it is desirable to find a turbulence promoter that optimizes the performance by maximizing limiting current density and minimizing pressure drop. The type of flow spacer that has had the most commercial success in terms of the total installed capacity of ED/EDR plants is the tortuous path spacer. 8 This spacer is manufactured by using two sheets of low-density polyethylene with die-cut flow channels. The two sheets of polyethylene are glued together to form an under/over flow path that creates turbulence. Since the under/over straps that create the turbulence are fairly infrequent, the amount of turbulence promotion is limited. At higher velocities, however, the turbulence is sufficient to obtain reasonable limiting current densities. The pressure drop per unit length for this design is low, so a long flowpath can be used without having excessive pressure drops. By utilizing a long tortuous flowpath and operating at velocities in the cm/s range, TP1087EN.docx Page 3

4 this design optimizes turbulence promotion and stack pressure drop. The disadvantage of this approach is that the tortuous design has significant membrane area wasted as sealing area. In a typical commercial design, 36% of the membrane area is shadowed by sealed area, so only 64% of the membrane area is usable for desalting. Figure 3 illustrates a tortuous path flow spacer, designated Mark III by the manufacturer. Figure 3: Mark III Flow Spacer Several manufacturers of electrodialysis equipment have employed screen spacers to promote turbulence. In these spacers, woven or non-woven netting is used in the flow path to create turbulence. In the 1950 s the author s company looked into the development of screen spacers and found that promoting turbulence with a screen spacer was better than with a tortuous path spacer. The use of screen spacers was not practical at the time, because netting was relatively expensive, and methods for producing screen spacers in large volumes were limited. For practical long-term operation of large electrodialysis systems, the thickness tolerances of flow spacers are critical. Since a thousand or more spacers are piled on top of one another in an electrodialysis stack, even slight variations in component thickness can lead to large variations in the stack heights. The thickness of the sealing area around the flowpath must be very close to the thickness of the flow spacer. If the sealing area is thicker than the turbulence promoters, the turbulence promoters do not touch the membrane surface. This can cause laminar flow at the membrane surface, which in turn reduces limiting current density significantly. In the 1990 s, with the development of the spiralwound RO element, the technology for manufacturing non-woven netting has improved dramatically. As with flow spacers in an ED system, the retentate spacer in an RO element needs to promote turbulence to minimize the thickness of the concentration polarization boundary layer at the membrane surface. These retentate spacers are Page 4 manufactured from non-woven polypropylene netting. The RO industry s requirements for high volume production, as well as high tolerances to maximize the amount of membrane area in an RO element, have led to improved manufacture of nonwoven netting. It has become useful to take a new look at screen spacers for EDR systems. comparison of spacer properties Bench scale spacers were manufactured, and their performance, in terms of limiting current density and pressure drop, was measured in the laboratory for a sodium chloride solution at 70ºF. The limiting current density was defined as the current density at which the current efficiency for hydroxyl ion transfer was 2 x 10-5 in an all-anion stack. Experience has shown that this amount of increase in ph in the concentrate stream would be needed to significantly increase the risk of precipitation. Figure 4 shows a graph of limiting current density versus velocity for conventional tortuous path spacers and a screen spacer manufactured with nonwoven netting. This netting is similar, but not identical, to the netting used in many commercial RO elements. For a given velocity, the limiting current density for the screen spacer is about three times that of the tortuous path spacer. The limiting current density defines the maximum amount of salt that can be removed per unit length. For example, for a 0.01 N solution of sodium chloride at 70ºF operating at 70% of the limiting current density, the salt removal per unit length is shown in Figure 5. The screen spacer removes more salt per unit length than the tortuous path spacer, so a shorter flowpath can be used to obtain a desired level of salt removal. For both spacers, the lower the velocity, the greater the salt removal. The lower the velocity, however, the lower the flowrate per unit width, and so a wider flowpath is needed to desalt a given volume of water. The pressure drop per unit length is much greater for the screen spacer than the tortuous path spacer, as shown in Figure 6. Since the frequency of turbulence promoters is much higher for the screen spacer than for the tortuous path spacer, these results are not surprising. To minimize the pressure drop, the optimum velocity for the screen spacer is in the 6-12 cm/s range, and a short flowpath length is selected. TP1087EN.docx

5 Figure 4: Limiting CD/N versus Velocity Figure 7: Mark IV Flow Spacer The screen spacer thickness is 0.03 vs (0.08 vs. 0.1 cm) for the conventional tortuous path spacers. The maximum stack height of a Mark III stack was 500 cell pairs. This height is determined by the maximum safe DC voltage that can be applied to a stack and the dimensions of a standard shipping container into which the stack can fit. With the new spacer, the maximum stack height is 600 cell pairs. Figure 5: Salt Removal per Unit Length versus Velocity for 0.01 N NaCl at 70 F big boost in membrane area The combination of more usable area per membrane and more cell pairs per stack has led to an increase of 38% usable membrane area in an electrodialysis stack. The increase in membrane area per stack combined with the increased current density that can be applied to a stack means that fewer stacks are required to desalt a given volume of water. This leads to capital and building cost savings for the customer. Figure 6: Pressure Drop per Unit Length versus Velocity high performance screen spacer The outside dimensions of 18 x 40 (46 x 102 cm) for the high performance screen spacer were based on the dimensions of pre-existing ion-exchange membrane production lines and the requirement for membranes and spacers to be easy to handle by one person for stack maintenance. A U- shaped flowpath was developed to fit the optimum flowpath length and width into the 18 x 40 configuration. This spacer design, known as the Mark IV, has about 74% usable area in contrast to 64% for the Mark III spacers (see Figure 7). power consumption In electrodialysis plant design, there is a trade off between power consumption and capital cost. For any particular spacer, the greater the number of cell pairs, the lower the DC power consumption and vice versa (Figure 8). In developing the new spacer, it was important to make sure that capital cost savings from using fewer stacks and cell pairs were not accompanied by increases in power consumption. The use of the thinner spacer means that the electrical resistance of each cell pair is reduced, so that the overall DC power consumption for a given number of cell pairs decreases. Therefore, at constant DC power, fewer cell pairs can be used with the screen spacer, providing savings in capital cost without increases in operating costs. TP1087EN.docx Page 5

6 III stacks. This test demonstrated the ability of the Mark IV stack to consistently achieve the same salt removal as two stages of conventional stacks. Figure 10 shows that the DC power consumption was equal to or lower than the Mark IV stack, even though there was half the number of cell pairs. Figure 8: Power Consumption versus Membrane Area The overall system pressure drop is lower since fewerstages are required, and this in turn reduces pumping power. Use of the new spacer offers significant savings in capital costs and can reduce the overall power consumption of an EDR system. EDR system The new spacer has been incorporated into a new system design that minimizes pumping power and simplifies control. 9 Variable Frequency Drives (VFDs) on the feed and concentrate recycle pumps are used to control the pump speeds. The feed pump is controlled by product flow rate, and the concentrate recycle pump is used to maintain a constant differential pressure across the membranes. Patented 4-way valves reduce both the piping required to reverse the flow and the amount of time the system operates off-spec at polarity reversal. The control system is PLC (Programmable Logic Controller) based with a flat touch screen operator interface that can be integrated with SCADA (Supervisory Control and Data Acquisition) systems, if desired. These features reduce both capital and operating costs when compared with earlier systems. comparisons of performance Lake Granbury Surface Water and Treatment System,Texas For almost three years, a single Mark IV stack of 500 cell pairs has been operating in parallel with seven lines of two stages of 500 cell pair Mark III stacks at the Lake Granbury Surface Water and Treatment System (LGSWATS). Each line of stacks was operated with the same flowrate of 106 gpm (gallons per minute). Approximately 8,000 hours of operating data was collected. Figure 9 shows conductivity versus time of the feed to the EDR plant and the product from the Mark IV stack and from the average performance of the product from two stages of Mark Figure 9: Conductivity versus Time at LGSWATS Figure 10: DC Power Consumption versus Time at LGSWATS Foss Reservoir,Oklahoma A comparison between a 240 cell pair Mark IV prototype stack and a 450 cell pair Mark III conventional stack was begun in October 1994 at the Foss Reservoir water treatment plant, where a 3 mgd (11,000 m 3 /day) ED system has been in operation since The prototype stack had the equivalent of two hydraulic stages of Mark IV spacers. The conventional stack had four hydraulic stages. Both stacks were operated at a flowrate of 26 gpm (0.1 m 3 /h). The applied voltage and approach to polarization for both stacks were varied in order to compare power consumption at different salt removal rates. An initial period of testing compared the two stacks over 1,500 hours. Since the initial comparison, the prototype stack has been used for various research projects not presented here, and the screen spacers have been in operation for over 20,000 hours. Page 6 TP1087EN.docx

7 Figure 11: Pressure Drop versus Time at Foss Reservoir Figure 11 shows pressure drop and temperature for both stacks versus time. For the first 500 hours or so, the feed water temperature was fairly warm between 59 F and 68 F (15 and 20ºC). The pilot test was suspended over Christmas, but when it was restarted, the feed temperature was much lower at an average of 43 F (6ºC). that the systems are not operating at points of equal DC power consumption on their operating curves, as illustrated in Figure 8. The reduction from four stages of tortuous path spacers to two stages of screen spacers was such a large decrease that the power consumption increased to compensate. A comparison of four stages of tortuous path to three stages of screen spacer would have placed both spacers at the point of equal DC power consumption on their operating curves. Table 1: Foss Reservoir Stack Comparison Mark IV Prototype Mark III # cell pairs Conductivity cut 70% 70% DC power 2.5 kwh/kgal 1.7 kwh/kgal Stack pressure drop 25 psi 43 psi Pumping power based on new system design with VFDs Total power consumption 0.8 kwh/kgal 1.1 kwh/kgal 3.3 kwh/kgal 2.8 kwh/kgal The pressure drop of both stacks increased as the temperature decreased, and the Mark IV prototype stack had almost half of the pressure drop of the Mark III stack. Assuming these stacks would be used on a new EDR system that uses VFDs to conserve energy, the reduced pressure drop represents a 27% saving in pumping power, as shown in Table 1. Figure 12 shows the DC power consumption versus percent salt removal for both stacks during the period from operating hours when the average temperature was 6ºC and the feed conductivity was constant at 2,100 µs/cm. Although the pressure drop was lower, and a 46% savings in cell pairs was achieved, the DC power for the prototype Mark IV stack was greater than the DC power for the Mark III stack. The reason for this is Figure 12: DC Power versus Salt Removal at Foss Reservoir A summary of the performance comparison is shown in Table 1 for both stacks at 70% salt removal. The comparison is based on operating data between 500 and 1,500 hours when the feed water conductivity was 2,100 µs/cm, and the temperature was about 6ºC. The comparison demonstrates a 46% savings in the total membrane area with only an 18% increase in total power consumption. At Foss Reservoir, where power is quite inexpensive, this is an attractive trade-off. Suffolk,Virginia The Robert G. House Water Treatment Plant in Suffolk, Virginia, installed an EDR plant in This system treats 3.8 mgd and consists of three EDR units, each with 8 lines and 3 stages of conventional Mark III stacks. 10 The first stage of one line was recently replaced with a Mark IV stack. The Mark IV stack is the same size as the Mark III stack, although it includes 600 cell pairs rather than 500 cell pairs in the Mark III stacks. The Mark IV stack is operating at the same conditions as the other Mark III stacks in parallel lines. Each line has the same flowrate of approximately 110 gpm (0.4 m 3 /h), and the first stage applied voltage was 300 V in each case. TP1087EN.docx Page 7

8 Table 2 compares the salt removal for the Mark IV stack and a Mark III stack in parallel. This data shows the increase in salt removal that is made possible by the Mark IV stack. The Mark IV stack achieves a cut of 50% while the Mark III stack achieves a cut of 30% under the same operating conditions. Table 2: Suffolk Stack Performance Feed (mg/l) Product (mg/l) Mark IV Stage 1 Product (mg/l) Mark III Stage 1 % cut Mark IV Stage 1 % cut Mark III Stage 1 Sodium Calcium Magnesium Potassium Chloride Bicarbonate Sulfate Phosphate Fluoride TDS summary The new high performance spacers make it possible to reduce the overall life cycle costs of an EDR plant. At Lake Granbury, the comparison demonstrated minimizing cell pairs to reduce capital cost at constant operating costs. At Foss Reservoir, the comparison demonstrated reduced capital costs with only a small increase in power cost. At Suffolk, the comparison demonstrated better product quality from a single membrane stack. Page 8 TP1087EN.docx

9 references 1. Mason, E.A. and Kirkham, T.A., Design of Electrodialysis Equipment, Chem. Eng. Prog. Symposium Ser. No. 24, Vol 55, Siwak, L.R., Here s How Electrodialysis Reverses... and Why EDR Works, Int l Desalination & Water Reuse Quarterly, Vol. 2/4, Meller, F.H., Ed., Electrodialysis (ED) & Electrodialysis Reversal (EDR) Technology, Ionics, Incorporated, Rosenberg, N.W. and Tirrell, C.E., Limiting Currents in Membrane Cells, Ind. Eng. Chem., 49, 1957, p Belfort, G. and Guter, G.A., An Experimental Study of Electrodialysis Hydrodynamics, Desalination, 10, 1972, pp Chiapello, J.M. and Bernard, M., Improved Spacer Design and Cost Reduction in an Electrodialysis System, Journal of Membrane Science, 80, 1993, pp Zhong, K.W.; Zhang, W.R.; Hu, Z.Y. and Li, H.C., Effect of Characterizations of Spacer in Electrodialysis Cells on Mass Transfer, Desalination, 46, 1983, pp Wangnick, K., 1996 IDA Worldwide Desalting Plants Inventory Report, No. 14, pp von Gottberg, A.J.M. and Siwak, L.R., Re- Engineering of the Electrodialysis Reversal Process, Int l Desalination & Water Reuse Quarterly, Vol. 7/4, Feb./Mar Thompson, M.A. and Robinson, Jr., M.P., Suffolk Introduces EDR to Virginia, Proceedings, American Water Works Assoc. Membrane Conf., Orlando, FL, TP1087EN.docx Page 9