KUMAR SINGH. Index Terms - Pressure Exchanger, Energy Recovery Turbine, Desalination Plant, Reverse Osmosis

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1 PERFORMANCE ANALYSIS OF RO PLANT USING PRESSURE EXCHANGER AS AN ENERGY RECOVERY DEVICE 1 SAYED MUJTABA ALMUHAFDAH, 2 GOLLAPUDI L NARAYANA, 3 DIVYA RYALI, 4 ANIL KUMAR SINGH 1,2 School of Engineering, Bahrain Polytechnic, Kingdom of Bahrain 3 Asst. Professor, 4 Asso. Professor, Dept. of Mechanical Engg., Sphoorthy Engg. College, Hyderabad, India Abstract - The objective of this paper is to investigate the possibility of improving the performance of Ras Abu Jarjur Reverse Osmosis (RO) plant in Kingdom of Bahrain by using Pressure Exchanger (PX) as a replacement for Energy Recovery Turbines (ERT) to reduce the power consumption of desalination process. The performance of original energy recovery turbines of Ras Abu Jarjur has been compared with the performance of pressure exchanger devices with the power model selector as main aspect. It has been found that the pressure exchanger device had reduced the power consumption of plant more than energy recovery turbine with the performance around 95.17% and recovery rate of 40%. It has also been concluded that there is an improvement in the performance of desalination plant that saves the expenses around 879,464 USD per year. The aims of this project were successfully achieved with high level of power reduction (around 46.23%) from 5.3 (KWh/m3) to 2.85 (KWh/m3) by installing pressure exchanger devices as an alternative energy recovery element to the present energy recovery turbines. Index Terms - Pressure Exchanger, Energy Recovery Turbine, Desalination Plant, Reverse Osmosis I. INTRODUCTION Ras Abu Jarjur plant is one of Bahrain water production plants that operate on the principle of RO employing membrane of DuPont s B-10 and Toray (TM ). These membranes are used in order to desalinate the brackish ground water that contains 12,500-19,000 mg/litre TDS. This water is pumped from 65m (approximately) below the ground level by using 19 submersible well pumps. There are four main stages that brackish water goes into before pumping it to blending station which are Pre- Treatment, RO Desalination, Post-Treatment, and the last stage is Storage and Forwarding as shown in Figure 1. (Post-Treatment), and the Brine which is rejected to an Energy recovery Turbine, which otherwise goes to waste. The energy of rejected water is transferred back to the electric motor of through the shaft that linked with the motor and high pressure pump shaft by coupling as shown in Figure 2. Figure 2: A typical arrangement showing Pelton Turbine Table 1 shows the key information of Ras Abu Jarjur desalination plant Figure 1: Simplified schematic of Ras Abu Jarjour RO desalination plant The heart of Ras Abu Jarjur plant is RO desalination where the feed water comes from the pre-treatment stage with 10 bar pressure and this water is made to pass through ten different high pressure pumps to pressurize up to 60 bar (approx.) before being sent to the first stage of membranes, each feeding an RO block comprising 70% recovery. The feed of RO in the Permeators is separated into two different streams, the permeate which is transferred through three of Draw-Back Tanks to the next stage Table 1: Ras Abu Jarjur desalination plant data. Many of Brackish water RO system and sea water RO system use energy recovery device (ERD) in order to save power and minimize the cost. ERD can be categorized into three main groups as discussed below. The first group is based on the centrifugal principle where the centrifugal force used to convert the energy of reject stream (hydraulic energy) into the 40

2 mechanical energy of the shaft (rotational energy that is used to generate electricity. This group considered as an add-on package. Second Group is commonly referred to as turbo chargers that are used to convert the energy of reject water into the mechanical energy of the shaft, and then it is transferred back as pressure energy to the feed stream. The commercial example of this ERD is Pressure Booster. This type of ERD is a stand-alone package (unlike first group). Third group of ERD uses the principle of positive displacement in order to pressurize filtered RO feed water by direct contact with the reject water that comes from membranes. The commercial example of positive displacement recovery device is pressure exchanger which is mainly focused in this paper and to be as a replacement for Pelton Wheel. From the previous experiments that were conducted on Ras Abu Jarjur RO Plant and results so obtained, it has been understood that its performance has been reduced drastically. This paper focuses on investigations if there is any chance for the improvement in the performance of recovery system thereby reducing the cost of desalination process. In last few years, many problems have been found on ERT system such as vibrations, misalignment in shaft, unwanted sound, and bearing problems. In last three months, it has been found that there are more problems in four of ERT blocks such as Block-A, Block-C, Block-D, Block-H. Sometimes all block lines have to be shut down in order to fix the problem such as misalignment problem that arises due to coupling of HPP, motor and ERT. This project also investigates the possibility to reduce the level of maintenance due to involvement of more number of rotational parts in ERT system and there is no standby turbine in each block that will take more time for fixing the problem. II. ANALYSIS AND DESIGN A. Analysis of current energy recovery system Ras Abu Jarjur RO Plant uses a Pelton wheel as energy recovery turbine (ERT) that saves power by converting the hydraulic energy of the reject water from membranes into the mechanical power of the motor and high pressure pump (HPP) shafts which are directly coupled. The brine water that is rejected from membranes would be fed to the turbine through one nozzle that is attached to the intake pipe. The flow energy of brine water could be used to reduce the power of the motor to reduce the wastage. Table 2 shows the specifications and manufacturer manual data sheet of Energy Recovery Turbine that is currently being used in Ras Abu Jarjur desalination plant. Table 2: Specifications and manufacturer manual data sheet of Energy Recovery Turbine at Ras Abu Jarjur desalination plant data B. Optimization Criteria for PX Isobaric Energy Recovery Figure 3 shows various elements of Pressure Exchanger device (PX) as an isobaric energy recovery device that uses the principle of positive displacement in order to transfer the pressure energy from brine water (reject water) that comes from membranes into a low pressure feed water stream. Figure 3: Various Elements of Pressure Exchanger (PX) device Figure 4 shows a typical flow schematic for reverse osmosis equipped with PX device. Actually, there are two different paths flow in the pressure exchanger systems which are high pressure (HP) side (Block- D to Block-F) and low pressure (LP) side (Block-A to Block-E). The brine water that is rejected from membranes with high pressure is made to pass through the pressure exchanger device or an array of devices which are operating in parallel directions. Figure 4: Typical Flow Path of a RO System with PX Technology The feed water that pressurized by the supply pumps passes through the low pressure ducts of pressure exchanger that leads to expel brine from the low pressure ducts at the right. With every rotation of rotor, this process of two-stroke repeated for each duct like the ducts are continuously discharging and filling. 41

3 The reverse osmosis feed water and the brine water are separated during each cycle by a liquid piston barrier. There is direct contact between the feed of reverse osmosis and the liquid piston resulting in contamination of small amount of mixing of brine and feed water. This volumetric mixing which is independent of the membrane rate of recovery can be calculated by using the following: efficiencies are based on Energy Recovery s factory performance test standards. The salinity is determined at the outlet and inlet pressure exchanger device connection or array of pressure exchanger devices. Volumetric mixing in PX device has been found to be around 6% when the low and high pressure flow rates are equal. Practically, it must be accounted for during the modeling process of RO in order to measure the impact to feed pressure and permeate total dissolved solids because when the feed water salinity increased that leads to increase both feed pressure and permeate. The following formula can be used in order to approximate the percent of increase in salinity at the feed to the membranes of RO: where, SI = increase in salinity; R = membrane recovery (affects concentration difference between feed and brine); M = volumetric mixing (approx. balanced flows). There is a difference between mixing and lubrication (or leakage) in pressure exchanger isobaric recovery device which are independent and unrelated. The leakage or lubrication flow happens at the seals that are located at rotor ducts end. High pressure of flow leaks to low pressure resulting in a slight loss from inlet of high pressure flow into the outlet of high pressure flow and a corresponding gain from the inlet low pressure flow to the outlet flow of low pressure. The rate of lubrication flow might be changed only when the seals become damaged; however, mixing will not increase with wear or time. III. SELECTION OF PRESSURE EXCHANGER (PX) A. The Family of PX Energy Recovery Company (Energy Recovery, Inc.) among other few pressure exchanger manufacturing companies around the world has been chosen for this analysis because of the reasons: constant high efficiency over entire operating range; highest availability; advanced technology that is smart, and elegantly simple; designed for lifetime; and supreme flexibility. There are two main series in the PX family such as Q and S series. Table 3 shows a list of PX series family that is suitable for selection and available in the market for purchase. The minimum guaranteed Table 3: List of available PX series family Figure 5 shows functional diagram of Ras Abu Jarjur R.O plant with PX energy recovery device. Figure 5: Functional diagram of Ras Abu Jarjur R.O plant with PX device PX Q300 is the latest generation of PX family which has the following benefits and features: Benefits: Highest guarantee of efficiency 97.2%. Lowest cost of life Best return on Investment (ROI). 99.8% Uptime -zero planned down-time. Ease of Installation-minimal footprint. Features: Quietest PX technology Below 81dB All other nice features of the PX device family. B. Circulation Pump with PX Devices RO system with pressure exchanger needs a circulation pump in order to circulate water through membranes and PX device or array. The high pressure flow that comes from circulation pumps can be controlled by using either control valve at the discharge end of circulation pump or a variable frequency motor drive in order to operate the pump. Energy Recovery Company recommends using a slightly oversized pump of circulation for the projected range of head requirements and concentrating flow. Many process variations have to be taken into account during sizing of circulation pump such as seasonal variations, membrane fouling, and other variations of process. Energy recovery device carries a circulation pumps line with the capacities of 4.5 m 3 /hr up to 500 m 3 /hr. Observations on some basic conditions of operating and precautions require successful PX-Q300 operation to ensure a long service life with safe 42

4 operation. Table 4 provides summary of the system performance limits. Table 4: System Performance Limits IV. PRESSURE EXCHANGE SELECTION METHODOLOGY Case 1: RO System with Pelton Wheel Recovery Turbines B. Finding the power output of the turbine The average values of three phase electric current of the motor with and without working of turbine and recalculate it when the turbine is in the shutdown status have been calculated in the beginning. The readings have been recorded for every 4 hours per day in three different days in the week in order to get very accurate values as shown in Tables 7 and 8. Table 8: Three phase electric current data of the motor with turbine Table 9 shows the readings that have been recorded for every 4 hours per day in three different days in the week in order to get very accurate values with the pressure exchanger as energy recovery device. Figure 6: Functional diagram of Ras Abu Jarjur RO plant with Pelton wheel Figure 6 shows the functional diagram of Ras Abu Jarjur RO plant with Pelton wheel. During process 1-2, a supply pump has been used to pressurize the water to the HP, and then HPP pressurizes the water with high pressure to the RO membranes wherein the water has been separated into two different streams; permeate which is transferred through 3 draw-back tanks to the post-treatment, and the brine rejected to an ERT. Steady operating conditions, no heat losses, and constant temperature (30 o C) at all stages are the main assumptions that are made in this analysis. A. Finding the power input of the turbine Average values of pressure and flow rate of brine water have been calculated using the pressure and current flow data shown in Tables 5 and 6 that were recorded for every 4 hours/day in three different days in the week. The values were recorded when the ERT device was replaced after returning back into the service. The Block-C data has not been taken into account due to misalignment problems. C. Economic Impacts 0.04 USD per kwh cost of electricity (as per EWA Authority, Kingdom of Bahrain); discount interest rate for the present value analysis; 24 operation hours per day; and 360 operational days of plant in a year have been considered to be the key points for 43

5 calculating energy recovery cost. Therefore, total cost of power generated per year has been found to be equal 105,708 USD. The total spare parts cost considering the estimated prices has been calculated to be around 19,078 USD in a year. Therefore, the total operating cost is equal 86,630 dollars per year. Case 2: RO System with PX energy recovery device Power model selector as shown in Figure 5 finds that the best PX type for Ras Abu Jurjur Plant is PX-Q300 with seven units in each block, so the total units of PX devices will be 56 units for all eight blocks. The performance of PX technology system as the power model selector has been shown in Table 10. recovery (around 95.13%), this block has been skipped from the comparison process with PX. Figure 8 shows the variation of the motor current with and without ERT for Block-B. Even though there is small reduction in the power of motor, it is not enough because there is a huge gap with the expected current which is varying in the rage of 180 amps and 185 amps. Figure 8: Current variation graph for Block-B Table 10: PX Technology performance A. Economic Impacts Considering the same key points as that of case 1, total cost of power generated per year has been estimated to be equal 973, 054 USD and the total spare parts cost has been estimated to be 6,960 USD per year. Therefore, the total operating cost has been found to be 966,094 USD per year. Energy saving rate for Block-B is around 4.688% with 71.87% errors. Due to very high percentage error this block has been skipped from the comparison process with PX. Figure 9 shows the status of Block-C which is found to be better than other blocks where there are acceptable reductions in the motor power. The gap between the variation of current with ERT and expected current variation is less with percentage error of 55.01% while the energy recovery rate of Block-C being 7.49%. V. RESULTS AND DISCUSSIONS A. Block wise analysis Figure 7 shows that the current value with ERT and without ERT are very close to each other due to many leakage problems in the membranes of Block-A. So, the rejected water loses most of energy before entering the recovery device that leads to a gap between the expected current and the real current of motor with ERT. Figure 7: Current variation graph for Block-A Figure 9: Current variation graph for Block-C The efficiency of this block ERT has been found to be 51.6% while the expected efficiency for this block is 71%. The percentage error between actual and expected value has been found to be 27.32% which is acceptable due to more service life operation of this block. This block recovery system has been considered to be the best in Ras Abu Jarjur plant. Therefore, when there are huge differences between this block and the performance of PX device, it will be more for other blocks in this case. The saving rate of energy for block A is around 0.811% which is very low percent and this reflects the status of this block. Due to very high percentage error between theoretical and actual 44

6 Figure 10: Current variation graph for Block-E Figure 10 shows that the variation of current with ERT for Block-E is stable and reduction in the power of motor more than that of other blocks except Block- C. The recovery rate of this block has been found to be 6.13% and the percentage error to be 63.2%. The efficiency of this ERT block is 44.29% while the expected efficiency for this block is 70%. So, the percentage error between actual and expected value has been found as 36.7% which is acceptable due to the service life operation of this block (more than 35 years). It is true that the variation of current in this block is more stable than other blocks, but the efficiency and recovery rate are still less than Block- C. Figure 11 shows that there is small power reduction between the current of motor with turbine and without turbine with the recovery rate of 4.11% with percentage error of 75.95% for Block-F. Figure 12: Current variation graph for Block-G The reduction rate of power in this block has been found to be around 1.89% with percentage error of 88.63%. This block has been found to be second worst block in the plant where the efficiency of this block has been found to be 14.4% while the expected efficiency to be 64% and 77.49% percentage error. These large values of percentage error are due to 30% opening of Q valve. B. Combined Block Analysis Table 11: General results of combined blocks for various criteria Table 11 shows the general results obtained from the analysis for various blocks in the system for various criteria considered. Figure 11: Current variation graph for Block-F It is found that difference between the variation of current with ERT and the expected theoretical current variation is very reasonable because the efficiency of Block-F ERT equals to 29.78% while the expected ERT efficiency is around 70%. So, the percentage error between two values equals 57.45%. The variation of current in this block is stable, but the efficiency and recovery rate are still low. Figure 12 shows the current variation graph of Block- G. The status of ERT is not in a good condition because the current variation with ERT is very close to that without ERT. Figure 13: Comparison graph of current variation for all blocks Figure 13 shows that there is huge reduction in the power of motor between ERT and PX. It is shown from the graph that the power reduction has not been matched with the huge amount of problems and maintenance that needs to applied frequently in few recent years. Even though that all blocks works with maximum efficiency, it is not enough when comparing with the current reduction in PX system where the differences in current reduction between two system is around 39.2%. 45

7 Figure 16 shows that the performance of recovery system has found to be increased by The percentage of recovery rate differences is more if actual recovery rates of ERT and the PX are compared which will be around 81.26%. CONCLUSION AND RECOMMENDATIONS Figure 14: ERT current reduction against PX current reduction Figure 14 shows the differences in current reduction between ERT with maximum efficiency of saving power and PX expected reduction values. There are a huge gap between the efficiency of PX and actual ERT efficiency as shown in Figure 15 where the differences between the best ERT block (Block-C) and the efficiency of PX is around 45.78%. It has been concluded that the performance of RO plant could be upgraded at this stage to decrease the cost of power by enhancing the system of energy recovery in the plant that can be implemented by using pressure exchanger as a replacement for ERT. It is clear that the recovery system devices are being used to reduce the power consumptions and increase the performance of the desalination process. It has been concluded that the performance of RO units by using PX recovery device could be increased by 45.78% that could save power consumption. It has also been concluded that the recovery rate ratio has been increased by 81.26% that saves 879,464 USD per year for total operating cost and 21,986,600 USD for the assumed 25 years of life. ACKNOWLEDGMENTS Figure 15: Recovery system performance comparison Even though the comparison is between the theoretical ERT efficiency that provided in the manufacture manual and the efficiency of PX device, the results will supporting the pressure exchanger system with a differences of 13%. Actually, the theoretical efficiency value is impossible to achieve because the importing data shows that the expected efficiency for each block is less than theoretical one. The authors would like to acknowledge Ras Abu Jarjur plant, Kingdom of Bahrain for providing all necessary facilities. Authors would also thank Mr. Chindambaram Nachiappan (EWA, Bahrain) for his continuous support, valuable guidance and moral support given to us in completing this paper successfully. REFERENCES [1] Nuri M. Eshoul, Brian Agnew, Mohammed A. Al- Weshahi and Mohanad S. Atab, "Exergy analysis of a twopass reverse osmosis desalination unit with and without an energy recovery turbine and pressure exchanger," Energies, Energies, 2015, 8, pp ; ISSN: [2] John P. MacHarg and Stuart A. McClellan, Pressure exchanger helps reduce energy costs in brackish water RO system, TechTalk, Journal AWWA, November [3] Bradley Sessions, Wen-Yi Shih, John MacHarg, Steve Dundorf, Jorge A. Arroyo, Optimizing brackish water reverse osmosis for affordable desalination, [4] T. M. Jamaluddin, Ata M. Hassan, Ali R. Al-Reweli, Abdullah Al-Rubaian, Leif J. Hauge, An efficient energy recovery prospect in SWRO process, Proceedings of 6th Saudi Engineering Conference, King Fahd University of Petroleum & Minerals, December 14-17, 2002, V-lV. Figure 16: Recovery rate comparison graph 46