Proceedings of the International Symposium on Current Research in Hydraulic Turbines CRHT VI March 14, 2016, Turbine Testing Lab, Kathmandu University, Dhulikhel, Nepal Paper no. CRHT2016-09 Design of a Portable Reverse Osmosis System Thea Karlsen Løken 1* and Ole Gunnar Dahlhaug 1 1 Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway * Corresponding author (theakarl@stud.ntnu.no) Abstract This paper presents the preliminary work of a master s thesis written at the Norwegian University of Science and Technology. The motivation for this thesis is to design a reverse osmosis system that is small, lightweight, durable and easy to transport. The intended application area of the system is to secure the supply of clean water in remote parts of the world where infrastructure is lacking, or in areas struck by natural disasters. The primary focus of the thesis is the high pressure pump supplying the feedwater to the system and the energy recovery device utilizing the high pressure energy of the exiting brine. These components and their influence on the performance of the entire system is to be evaluated by looking at four different solutions. The solutions evaluated include high pressure pumps of the centrifugal and reciprocating type, and energy recovery devices of the centrifugal and isobaric type. The most practical solution for the given application area is to be decided, and a model of the entire system is to be constructed using the computer-aided design software Autodesk Inventor. The expected findings of the thesis is that the most suitable solution for the given application area will be one comprising a reciprocating pump and an isobaric energy recovery device. This result is expected due to the low flow rate and high pressure requirements of the system. Keywords: Reverse Osmosis, Energy Recovery Device, Desalination 1. Introduction Access to freshwater is a worldwide, basic need. Only 2.5 percent of the total water volume on earth is freshwater, and of these 2.5 percent, nearly 70 percent is frozen in the glaciers and ice caps of the planet [1]. There are several regions of the world where adequate surface and ground water resources are scarce, or even unavailable. In these areas water treatment technologies are essential. Reverse osmosis (RO) is a leading desalination technology worldwide, both for small and large-scale applications. The focus of the thesis is to design a small-scale RO system. The intended application area of the system is to secure the supply of clean water in remote parts of the world where infrastructure is lacking, and in areas struck by natural disasters. For this reason, the system should be lightweight, small, durable and easy to transport. In an RO system, pressurized feedwater is forced through a membrane, and low pressure freshwater and high pressure brine exits. The feedwater may be seawater or brackish water. The process is very energy intensive, and the energy cost could represent up to 50 percent of the final costs of the water product [2]. By utilizing the energy of the exiting high pressure brine, the energy consumption of the process can be significantly reduced.
The components of the RO system receiving the most attention in the thesis are the high pressure pump supplying the feedwater and the device recovering the energy of the high pressure brine. Different solutions for these components are to be compared and evaluated in order to find the components that gives the best overall system performance. The most important parameters upon making this decision is size, weight, simplicity, durability, cost, energy requirement and efficiency. The different technical solutions investigated for the high pressure pump and the energy recovery device (ERD) are briefly described in the following. The first proposed solution comprises a high pressure centrifugal pump and an impulse turbine ERD. The turbine is coupled directly to the shaft of the pump, thereby reducing the required power input to the pump by transferring mechanical energy. The second proposed solution comprises a high pressure reciprocating pump and an impulse turbine ERD coupled directly to the shaft of the pump. The third proposed solution comprises a reciprocating pump and an isobaric ERD. In an isobaric ERD, the high pressure energy of the exiting brine is transferred to the feedwater directly, not through mechanical energy to the pump shaft. The theoretical efficiency of a system with an isobaric ERD is therefore higher than a system with a centrifugal ERD. The fourth proposed solution is a system comprising a high pressure reciprocating pump and no ERD. The positive effects of excluding the ERD on size and weight must be compared to the negative effects on the overall energy requirement of the system. In this paper, a brief theory of RO technology is covered, together with the working principles of the high pressure pumps and energy recovery devices considered for the system. Results are given for the design of the high pressure centrifugal pump and impulse turbine ERD, referring to the first proposed solution in the previous paragraph. These components were constructed by the use of the computer-aided design software Autodesk Inventor. A reciprocating pump and isobaric ERD have not yet been designed. The expected optimal solution for the system design is discussed based on the expected performance of the given components with regard to size, weight, simplicity, durability, cost, energy requirement and efficiency. 2. Method and Theory 2.1. Reverse osmosis Osmosis is a natural process that plays an important role in the metabolism of humans, plants and animals. Osmosis equalizes the difference in concentration of a solute between two solutions. If seawater and freshwater are separated by a semi-permeable membrane (meaning that only specific particles may travel through), the freshwater will travel through the membrane and tend to dilute the seawater, lowering the salt concentration on this side. This process will continue until the osmotic pressure is reached on the seawater side, and freshwater can no longer travel through the membrane. Reverse osmosis is the opposite process of osmosis, i.e., the water is forced in the opposite direction by applying an external pressure greater than the osmotic pressure. This is the working principle when producing drinking water from seawater or brackish water. By applying an external pressure greater than the osmotic pressure of the saline waters, freshwater can be forced through a membrane while the unwanted particles remain. The energy requirement, i.e., the external pressure required, depends on the salt content of the feedwater. A simple sketch showing the main components of an RO system for the production of drinking water is given in Figure 1.
Figure 1. The main components of an RO system [3]. The feedwater enters the membrane at high pressure, and low pressure freshwater and high pressure brine exits. The amount of freshwater produced depends on the recovery ratio of the membrane, defined as the volume of freshwater produced per unit volume of feedwater. The recovery ratio for seawater is in the range of 40-70 percent [4]. The sea or brackish water, here on out referred to as the feedwater, must be applied a pressure exceeding that of the osmotic pressure to force freshwater through the membrane. For seawater this pressure is in the range of 65 to 75 bar [5], and for brackish water in the range of 15-40 bar [3]. The energy consumption is a key factor that influences the production cost of freshwater produced by reverse osmosis. Typically 50-75 percent of the energy consumed by seawater RO desalination plants is used to drive the high pressure feed pumps [3]. The energy consumption can be reduced by improving the membrane technology, and by utilizing the high pressure of the exiting brine stream in energy recovery devices. 2.2. The Centrifugal Pump In a centrifugal pump, the mechanical energy of the impeller or runner is transformed into hydraulic energy in the fluid. In the radial type of centrifugal pump, the total head generated is produced by the action of centrifugal forces as well as the change in absolute velocities. The velocity diagram of a radial centrifugal pump impeller is given in Figure 2.
Figure 2. Velocity diagrams at the inlet and outlet of a radial centrifugal impeller. The inlet is denoted 1 and the outlet is denoted 2. An axial view of the impeller is given in the upper left corner and a radial view of the impeller is given in the lower left corner [6]. The theoretical head generated by a centrifugal pump is given by the famous Euler equation as stated below in Eq. (1). H t = u 2c u2 u 1 c u1 g (1) where H t is the theoretical head generated by the pump [m] u is the peripheral velocity of the impeller [m/s] c u is the peripheral absolute velocity of the water [m/s] g is the gravitational acceleration [m/s 2 ] In Eq. (1), the inlet and outlet of the pump impeller is denoted 1 and 2, respectively. To generate high head with a limited number of stages, the values of u 2, c u2, or both, must necessarily be high as well. A high value of u 2 is equivalent with a large diameter impeller. A high value of c u2 may lead to higher friction losses in the impeller because of longer passages, and difficulty in removing the rotational component of the flow in the casing between impeller stages.
2.3. The Reciprocating Pump The reciprocating pump comprises a piston or a plunger, a cylinder, inlet and outlet valves and a driving mechanism. The plunger is connected to a rotating shaft through a crankshaft that converts rotating motion into linear reciprocating motion. The inlet fluid is raised by a vacuum that arises when the plunger creates a cavity in the cylinder. When the plunger reverses, the inlet valve closes and the fluid is discharged through the outlet valve. The fluid pressure is raised by the force exerted by the plunger on the fluid. A simple sketch of a plunger pump is given in Figure 3. Figure 3. Simple sketch of a plunger pump [7]. 2.4. A Comparison between the Centrifugal and the Reciprocating Pump One of the main differences between the reciprocating and the centrifugal pump is that in reciprocating pumps mechanical energy is transferred periodically to the fluid, while in the centrifugal pump energy is transferred continuously. This means that in the reciprocating pump there is a periodic variation in the rate of pressure and flow with every stroke. In the centrifugal pump, the motion is uniform. The advantages a centrifugal pump holds over a reciprocating pump is the following. A centrifugal pump is very reliable due to the simplicity of the moving parts, the moving elements are self-lubricating by the fluid flowing through them and the pump is able to supply variable delivery. A disadvantage of a centrifugal pump is however low efficiency at high pressure and low design flow rate, because a low flow rate gives narrow flow passages which in turn lead to high friction losses. Another disadvantage of the centrifugal pump is the need to prime the pump before start-up. 2.5 Centrifugal Type Energy Recovery Device A centrifugal ERD might be an impulse turbine or a turbocharger. The first energy recovery devices were turbines, utilized in RO systems since the early nineteen-eighties. In a paper published in the international journal Desalination in 1981 [8], Woodcock suggests that the impulse turbine can be used to convert the kinetic energy of a brine jet to rotating mechanical energy which again can drive an electric generator. Woodcock also suggests that the turbine might be coupled directly to the shaft of the high pressure pump to reduce the electrical input to the motor. The simplicity as well as the flat efficiency curve of the impulse turbine is emphasized, making it well suited to operate at flows outside the best efficiency point (BEP). The efficiency of an impulse turbine ERD range from 70 percent to 90 percent [2]. A sketch of an RO system configuration including an impulse turbine ERD is provided in Figure 4.
Figure 4. RO system configuration including a centrifugal type energy recovery device [3]. 2.6 Isobaric Type Energy Recovery Device Figure 5. Schematics of a pressure exchanger. Arrows indicate flow direction and pressure. Red color for brine, blue for feedwater, high pressure white arrow, low pressure transparent arrow [9]. In an isobaric type ERD, the pressure energy of the brine is directly transferred to the low pressure feedwater. The pressure of the feedwater leaving the device will be equal to the brine inlet pressure minus
the loss in the ERD. The efficiency of an isobaric ERD can therefore be expected to be higher than the efficiency of a centrifugal ERD since the energy is not converted into an intermediate mechanical form. Indeed, the energy transfer efficiency of the isobaric ERDs can exceed 95 percent [3]. An example of an isobaric ERD is the rotary pressure exchanger given in Figure 5. This device comprises a rotating cylinder with ducts parallel to the axis of rotation. The cylinder rotates within a sleeve between two end covers and is turned by the flow itself. The high pressure of the brine is directly transferred to the low pressure feedwater [10]. 3. Results In section 1, four different solutions for the high pressure pump and the energy recovery device of the reverse osmosis were proposed. At the time of writing this paper, only the first solution comprising a high pressure centrifugal pump and an impulse turbine ERD has been thoroughly considered. The design of the centrifugal pump and the impulse turbine is presented here. The design basis for the centrifugal pump was a discharge pressure of 65 bar, a discharge flow rate of 100 liters per minute and feedwater with a density of 1020 kg/m 3. The design basis for the impulse turbine was an available pressure of 63 bar, an available flow rate of 30 liters per minute and a brine density of 1050 kg/m 3. The flow rate is limited by the capacity of the RO membranes. A higher flow rate would mean several membranes, and this would increase the total size and weight of the RO system. In order to make the components as small as possible, a rotational speed of 3000 rpm was chosen. To achieve the required discharge pressure in the centrifugal pump, several stages were necessary. To limit the size of the pump, a maximum of 7 stages were chosen. With the aid of the book Impeller Pumps by Łazarkiewicz and Troskolański [11], the design procedure for the impeller was performed. Some flow parameters had to be chosen. Often times, parameters such as this can be chosen on the basis of empirical values and charts as the ones developed by Stepanoff [12]. The charts are given for certain ranges of flow and head. The impellers of the centrifugal pump designed for the RO system were outside the range of the charts, and the parameters were therefore chosen within reasonable values given by empirical knowledge as found in the literature [11]. An Autodesk Inventor model of a single impeller is given in Figure 6. Figure 6. Impeller design. All dimensions given in [mm]. Figure 6 shows that the height between the front and back shroud of the impeller outlet is only 1.45 mm. This is a result of the high pressure and low flow rate requirements of the pump. The impulse turbine was chosen to be a Turgo turbine. A Turgo turbine is similar to a Pelton turbine, but the buckets does not have a splitter, and so the path of the water through the bucket is somewhat different.
The water must enter the bucket at an angle. This type of bucket was chosen because of the low flow rate available, giving a jet diameter of only 2.4 mm. It was thought that the interaction of the jet with a Pelton splitter would cause unacceptable incidence losses. The design of the Turgo turbine was based on hydrodynamic principles and partly on literature concerning the Pelton turbine [6][13]. The minimum number of buckets and the width of the buckets was found by requiring that no water should be lost. The shape of the buckets was found by requiring a shockless entry of water and a smooth deflection through the bucket, making the water leave with a relative outlet velocity opposite to the relative inlet velocity, and as small kinetic energy as possible. An Autodesk Inventor model of the Turgo turbine is given in Figure 7. Figure 7. Turgo model. All dimensions given in [mm]. 4. Discussion The intended application area of the reverse osmosis system is to secure the supply of freshwater in remote parts of the world where infrastructure is lacking, or in areas struck by natural disasters. When evaluating which high pressure pump and energy recovery device is best suited for the system, the intended application area must be kept in mind. Several parameters might be examined, such as size, weight, simplicity, durability, cost, energy requirement and efficiency. The most important parameters must be identified. Size and weight are obviously important parameters in order to have a portable system. Because the system is intended for use in areas where infrastructure is lacking, simplicity and durability become important parameters as well, seeing as the system should be easy to operate and that spare parts might be hard to come by. The energy requirement is important because the system might operate in areas where energy is a scarce resource. The four proposed solutions for the system, given in section 1, should be discussed in light of these parameters. Looking at the solution comprising the high pressure centrifugal pump and the Turgo turbine as presented in section 3, the Turgo turbine will contribute positively to the energy requirement of the system. The design of the Turgo as seen in Figure 7, is thought to be quite efficient. Assuming the design of the Turgo can give a hydraulic efficiency of 92 percent, the energy savings with a system recovery ratio of 40 percent could be as high as 42 percent. The size of the runner is also reasonable, and it is assumed that it can be manufactured in a lightweight material such as hard plastic, by considering the dimensioning forces on the bucket. The number of buckets is based on a minimum number required not to lose any water, found theoretically. The shape of the buckets is found by requiring a shockless entry of water. Although
the theoretical considerations suggests a high efficiency for the turbine designed, this should be confirmed through Computational Fluid Dynamics (CFD), experiments or both. The centrifugal pump would be a durable component, giving a continuous pressure rise to the water, and not requiring any valves or extra lubrication of the moving parts in contrast to the reciprocating pump. The centrifugal pump designed is however bound to give very high frictional hydraulic losses, due to the narrowness of the channels and the viscosity of water. The poor suitability of the centrifugal pump to the RO system was however expected, because the given flow rate and head specifications are well outside the range for centrifugal pumps. The flow rate is limited by the capacity of the RO membranes. Consequently, another type of pump should be considered for the RO system, a type more suitable for high head and low flow rate applications, such as the reciprocating pump. The second or third solution comprising a reciprocating pump and a Turgo ERD or an isobaric ERD is believed to be more suitable for the intended application area of the RO system. The advantages of the reciprocating pump is the ability to generate high discharge pressure at low flow rates. The need for several stages, as in the case of the centrifugal pump, is avoided, and therefore the size and weight of a reciprocating pump will be less than that of the centrifugal pump. The complexity of a reciprocating pump is somewhat higher due to the need for valves and lubrication of the moving parts, but the advantages outweighs the disadvantages. An isobaric ERD might give an energy recovery that is higher than a system utilizing a Turgo ERD, the deciding parameters will be the size and weight. If it is possible to construct an isobaric ERD of the same size range, weight and simplicity as the Turgo ERD, the isobaric type will be preferred. At the time of writing this paper, this is not yet uncovered. The fourth solution comprises only a reciprocating pump and no energy recovery device. This alternative needs also to be considered, though it is the author s belief that the reduced size and weight of the RO system could not be able to defend the highly increased energy requirement. 5. Conclusion At the time of writing this paper, a final recommendation for the optimal design of the reverse osmosis system can not yet be made. The options still need closer consideration. However, the centrifugal pump may already be deemed unsuitable as a high pressure pump due to its poor performance at low flow rates. The expected findings after further investigations is that the most suitable solution for the given application area is a high pressure reciprocating pump in combination with either an isobaric energy recovery device transferring the pressure energy of the brine stream directly to the feed stream, or a Turgo turbine energy recovery device coupled to the shaft of the reciprocating pump. The reciprocating pump is expected due to its ability to generate high pressure at low flow rate. Either the isobaric or Turgo turbine energy recovery devices are expected due to their high efficiency. The decision between the two will depend on the final size and weight of the isobaric energy recovery device. Acknowledgement I would like to thank Professor Ole Gunnar Dahlhaug for guidance, the students and PhD Candidates at the Waterpower Laboratory at the Norwegian University of Science and Technology and Julia Navarsete at Waterbox4Life Norway AS. References [1] U.S. Geological Survey, The World s Water, 2016. [Online]. Available: http://water.usgs.gov/edu/earthwherewater.html. [Accessed: 25-Feb-2016]. [2] Peñate, B. and Lourdes, L. García-Rodríguez, Energy optimisation of existing SWRO (seawater reverse osmosis) plants with ERT (energy recovery turbines): Technical and thermoeconomic assessment, Energy, vol. 36, no. 1, pp. 613 626, 2011. [3] Gude, V. G., Energy consumption and recovery in reverse osmosis, Desalin. Water Treat., vol.
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