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#250: Humidification-dehumidification desalination system using a state of art hollow fibre membranes

Omar RAMADAN1, Siddig OMER2, Mahmoud SHATAT3, Mona NAIM4, Yasser DESSOUKY5, Mahmoud ELEWA6, Saffa RIFFAT7

1 University of Nottingham, Nottingham, omar.ramadan@nottingham.ac.uk 2 University of Nottingham, Nottingham, siddig.omer@nottingham.ac.uk

3 Oxford Policy Management, UK, Mahmoodshatat@hotmail.com 4 Alexandria University, Egypt, monanaim66@gmail.com

5 Arab Academy for Science and Technology and Maritime Transport, Egypt, ygd@aast.edu 6 Arab Academy for Science and Technology and Maritime Transport, Egypt, mahmoud.elewa@gmail.com

7 University of Nottingham, Nottingham, saffa.riffat@nottingham.ac.uk

The increase in global population growth and population density has limited the ability of many nations to sustain fresh water supply. Sea or brackish water desalination provides a sustainable and nonconventional solution for the world’s water scarcity and the rapid increase in fresh water demand. This article investigates humidification-dehumidification water desalination system, using a state of art hollow fibre membranes, which are employed in the humidification chamber of the experimental rig, to humidify and heat the ambient air. The humidified air is then cooled via heat transfer in dehumidification chamber resulting in condensation of the water vapour to produce fresh potable water. A number of experimental tests were carried out using the test rig to investigate its water production capacity. A synthetic seawater water solution was prepared in the lab and used for the tests and its total dissolved solids (TDS) and electrical conductivity were measured before and after the distillation process. The aim of this work is to investigate and evaluate the performance of hollow fibre membranes as a humidification medium for the ambient air and optimise the operational performance of the system, using different mass flow rates of saline water and air. A water quality analysis showed that levels were well within the World Health Organization guidelines for drinking water. Further research is being performed to improve the performance of the installation.

Keywords: Water desalination, humidification, dehumidification, sea water

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INTRODUCTION

The rapid global growth in population has led to the increase in demand for fresh water, which reduced the ability of many local supplies to sustain fresh water (Lee et al, 2011). Where people are struggling to balance declining per capita water supplies with the rapid demands rising populations (Shatat et al, 2013). Even though over 71% of the earth's surface is covered with water, however around 97.5% of water on the earth is sea or brackish water, which contains large amounts of salt (Zhang et al 2017). It is forecasted that by the year 2025 water demand will exceed supply by 56%, due to persistent regional droughts, shifting of the population to urban coastal cities. In the next 50 years, water scarcity is expected to affect 2 to 7 billion people according to the United Nations (Teow et al, 2017). Water desalination system purifies brackish water or seawater, hence producing fresh water with total dissolved solids within the permissible limit of 500 ppm or less. Production of fresh water using desalination technologies driven by renewable energy presents a viable solution to the water scarcity at remote areas characterised by lack of potable water and conventional energy sources. However, conventional water desalination systems have some disadvantages, for example, electrodialysis, reverse osmosis and distillation are capital and energy intensive with hard maintenance regularly required (Wang et al, 2012, Welgemoeda et al, 2005, Yang et al, 2005). There are different types of water desalination systems, including thermal processes with phase change, membrane processes including Reverse osmosis (Gabelich et al, 2007), hybrid processes including phase change and membranes (Zhang et al 2017). However, such plants are not economically viable in remote areas, even those near a coast, and an electricity supply can also be lacking. The development of alternative, compact, small-scale water desalination systems is imperative for the population in such areas. In this article, experimental investigation is carried out for humidification-dehumidification based on water desalination system using hollow fibre membranes as a humidification medium under the ambient air conditions.

2. EXPERIMENTAL SETUP

Hollow fibre membrane bundles are used in the humidification chamber to humidify and heat the air, via heat and mass transfer with hot salty water. The resulting hot humid air is then cooled via heat exchange with cold salty water, in a shell and tube heat exchanger, leading to condensation of the air on the walls of the tube, and producing fresh potable water.

2.1. System Description

In the proposed system, a synthetic solution of sodium chloride with concentration of 35000 mg/l salinity is used to simulate the seawater, which is measured by Kern Salt Refractometer. The hollow fibre membrane bundles used in the humidification chamber are polypropylene with pore size of 0.1 × 0.5 µm. Air is blown from an environmental chamber under controlled conditions (temperature and humidity) using a fan. Heat and mass transfer occurs between the air on the shell side of the hollow fibre membrane humidifier and the hot salty water flowing on the inside of the fibre bundles. Under trans-membrane vapour partial pressure differences, water vapour from saline water inside the fibres can pass through the pores. The water vapour is absorbed by the air stream outside the fibre, resulting in the outlet air becoming hot and humid, while the outlet salty water becomes cooler with slightly higher salinity. The hot humid air then is condensed on the on the walls of the shell and tube heat exchanger dehumidification unit via heat transfer with the cold salty water, and the produced fresh water is collected in fresh water tank. The new hot salty water from the heat exchanger with the hot humid air in the dehumidification unit is pumped to a stainless steel tank where it is further heated using an immersion heater before it enters the humidification unit. The schematic diagram for the design is shown in Figure 1.

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Figure 1: Schematic diagram of the proposed system process

2.2. Experimental Design

The aim of the experimental investigation is to optimise the system operation using different flow rates of the salty water and the air on the system’s performance. The system components include the humidification chamber consisting of 18 bundles of hollow fibre membranes provided by Zena with 1385 fibres per bundle, while a shell and tube heat exchanger is used as the dehumidification chamber. An immersion heater is used to heat the salty water to the required temperature (after initial heating in the shell tube heat exchanger). Two Grundfos pumps are used to pump the salty water, the first circulating the salty water from the cold salty water storage tank to the shell and tube heat exchanger while the second pump is used to circulate the hot salty water from the storage tank with the immersion heater to the humidification unit. An in-line ducted fan integrated with a speed controller is used to circulate air at ambient temperature and humidity to the humidification chamber. Atrato Ultrasonic Flow Meter is used to monitor the liquid flow rates. Sensirion EK-H4 evaluation kit was used to monitor the humidity and temperature levels of the air at different locations of the system. Thermocouples were used to measure the temperature of the air and the salty water at different locations and the measured data was logged into a data logger (Data Taker DT85). The experimental rig developed is shown in Figure 2. Initially, the environmental chamber (source of air), is set to temperature of 26°C and 65% relative humidity for all the tests. The immersion heater temperature in the stainless steel tank is set to 60°C before starting the operation. The system was then operated for 1 hour for each flow rate of air and salty water. The air temperature and relative humidity levels were measured every second including controlled ambient temperature (from the environmental chamber) and at the outlet of the humidification chamber. While the salty water temperature was measured every second at the inlet to the dehumidification chamber (shell and tube heat exchanger), at the outlet of the stainless steel tank (with immersion heater) and at the outlet of the humidification chamber. The inlet ambient air speed was varied from 0.4m/sec to 1.4m/sec while maintaining the flow rate of the salty water at a constant value of 1litre/minute. Then the salty water flow rates were varied from 0.5litres/minute to 1.5litres/minute, while maintaining the air speed at a constant value of 0.9m/sec.

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Figure 2: System experimental test rig

2.3. Experimental Results and Discussion

The results for the temperature and humidity levels investigated for the two tests carried out for variable flow rates for the salty water and variable speed of inlet air from the environmental chamber. The level of produced fresh water varied from 550ml/hour to 650ml/hour for the conducted tests. The produced water quality was measured, and showed electrical conductivity levels of 15 µs/m and 10 mg/l, which is close to the value of distilled water. The amount of produced fresh water was not compared in this study, because the temperature of the cold salty water flowing to the dehumidification chamber was not controlled at a constant temperature (even though a chiller was used to maintain the salty water in the cold tank at a lower temperature), due to the closed loop nature of the designed system, which allowed variable temperature of salty water leaving the humidification chamber, which is the inlet to the dehumidification unit. Therefore, a comparison of the freshly produced water would not have been accurate for the closed loop designed system.

Variation of flow rates of salty water

As the inlet temperature of air was controlled for all the conducted tests via the environmental chamber, it was set at 26°C. Figure 3, shows the impact of varying the flow rate of the salty water, on temperature drop of the salty water inside the humidification chamber, after heat and mass transfer with the incoming ambient air in the fibre membrane bundles. The figure shows that there is a temperature decrease of salty water in all the conducted tests. This means that, heat and mass transfer occurs with the ambient air inside the humidification chamber. Furthermore, lower the flow rate of the salty water, resulted in higher temperature drop, which means that there is more heat transfer with the incoming air. The reason is that, the salty at lower flow rate, has more time inside the fibre membranes in contact with the air, which allows for added heat exchange compared to the higher flow rates.

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Figure 3: Effect of flow rates of salty water on temperature drop inside the humidification chamber

Figure 4 show the impact of varying the flow rates of the salty water, on the relative humidity of air at the outlet of the humidification chamber. The results show that the performance of the hollow fibre membrane as a humidification medium is very efficient, as the relative humidity levels after heat and mass transfer in the chamber varies from 95.5% to over 99%. Hence, the ambient air absorbs the moisture as it leaves the pores in the f ibre membranes to increase its humidity levels. This means, that relative humidity has increased in the chamber by around 30% (from 65% set at the environmental chamber).On the other hand, higher flow rate resulted in higher relative humidity of air at the outlet of the humidification chamber.

Figure 4: Effect of different flow rates of salty water on relative humidity of air

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Variation of ambient air speeds

Figure 5 shows the effect of varying the inlet ambient air speed from 0.4m/sec to 1.4m/sec. The air inlet temperature was controlled at 26°C, hence the results indicate that there is an increase in the temperature of air at the outlet of the humidification chamber by 14°C -22°C (at the steady state of the system after 30 minutes of running), depending on the air speed, due to the heat transfer with the hot salty water. In addition, the results show that the air outlet temperature was highest when the inlet air speed was lowest (0.4m/sec). While, at the start of operation, at air speed of 0.9m/sec, the air outlet temperature was initially 30°C, however, the temperature of the air at the outlet increased to over 40°C at the steady state, hence becoming higher than that of the temperature of the outlet air at air speed of 1.4m/sec (compared to the start of the operation). These outcomes indicates that lower air speed leads to higher temperature gradient between the air temperature at the inlet and outlet of the humidification chamber. This reason is that, the air remains in the humidification for a longer time, leading to higher heat transfer with the hot salty water flowing through the fibre membrane bundles.

Figure 5: Effect of inlet air speeds on air outlet temperature from the humidification chamber

Figure 6, shows the effect of inlet air speeds on the relative humidity of the air at the outlet of the humidification chamber. The results indicate that the relative humidity was lowest at the highest air speed (1.4m/sec). On the other hand, relative humidity at air speeds of 0.4m/sec and 0.9m/sec, seem to produce similar relative humidity levels.

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Figure 6: Effect of air speeds on relative outlet air humidity from the humidification chamber

3. CONCLUSION

This paper presented results of an experimental evaluation, which was carried out to investigate the performance of a humidification-dehumidification water desalination unit employing fibre membrane bundles as a humidification unit and a shell and tube heat exchanger as a dehumidification unit. The temperature of the salty water and humidity of the air were monitored at different locations in the experimental test rig. The purpose of the tests was to assess the impact of the input parameters including the flow rates for the salty water and the ambient inlet air speeds on the system performance. The tests demonstrated that the fibre membranes has significant potential to be used in the heat and mass transfer in humidification-dehumidification water desalination systems, as air relative humidity levels were increased significantly, reaching relative humidity of 99%, by absorbing the moisture which is leaked from the pores of the hollow fibre membrane bundles. Similar results in terms of temperature increase for the air were achieved when using different flow rates of salty water, while using lower flow rates of salty water resulted in higher temperature drop of the salty water in the humidification chamber; hence, it enhances heat transfer with the incoming inlet air. On the other hand, the lowest air speed used (0.4m/sec), led to the highest temperature of the air at the outlet of the humidification chamber, and displayed the highest value of relative humidity (98.5%).

4. ACKNOWLEDGEMENT

The authors would like to gratefully acknowledge that this work is funded by the Newton Fund Institutional Links

(Award ref 261839879)

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