Heat pump operated freeze concentration system with tubular heat exchanger for seawater desalination

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Abstract

Desalination of seawater can provide an almost inexhaustible source of freshwater if it can be made affordable. Distillation and filtration, the commonly used processes, have high operating and maintenance cost. Low operating temperature and low latent heat of fusion of water makes freezing technology worth considering for further development. Here, water is selectively frozen, in the form of ice from sea water, and melted after isolating it from the concentrated seawater to generate pure water which can be made potable. However, desalination processes based on freezing have not been exploited because of the operational difficulties in ice–water separation, high cost of equipment and high parasitic power requirement. This paper discusses a patented layer freezing based technology which has competitive initial and operating cost and eliminates operational difficulties of conventional freezing system. It is scalable and is coupled with a heat pump which selectively freezes water from seawater in the evaporator and melts the ice in the subsequent phase when it serves as a condenser. The condenser optimally utilises the latent heat of melting of ice to partially condense the refrigerant and the excess heat is rejected to ambient. It avoids the need of ice scraper/separation mechanisms. Use of vented-double-wall tube–tube heat exchanger, TT_HE, enables refrigerant and seawater/potable water to exchange heat without the use of intermediate fluids, while keeping the inclusion low. TT_HE is a reliable heat exchanger which ensures that refrigerant and seawater/water do not mix. Operating COP of the heat pump is in the range of 8 to 12, which results in specific energy consumption in the range of 9 to 11 kWhe/m3 of water produced. Comparison of features with other freezing desalination processes is also presented.

Research Highlights

► Paper summarizes the potential of freezing process for desalination. ► Heat Pump is judiciously deployed to reduce power consumption. ► Difficulty of ice-brine separation is eliminated using fewer components. ► It also can reduce inclusion and improve techno-economic viability.

Introduction

Desalination is the process of separation of salt from water. It is mainly carried out using evaporation, membrane separation and freezing technologies. In evaporation technology, seawater is heated above its boiling point (normally 100.8 °C and 101.7 °C at 3.5% and 7% salt concentrations) and the generated vapour is condensed to get water. Multistage flashing (MSF), multiple effect evaporation (MEE), mechanical vapour compression (MVC), humidification and dehumidification (HDH), membrane distillation (MD), pervaporization and solar distillation are the major techniques which come under evaporation category. MSF, MEE and MVC are suitable for 100 to 500,000 m3/d fresh water production capacity plant. The specific energy consumption of MSF and MEE varies from 150 to 300 kJ/kg with additional electrical input of 2 to 4 kWhe/m3 for pumping liquor and water, while energy consumption of MVC is 8 to 15 kWhe/m3 (Cipollina et al., 2005). These technologies require pre-treatment of seawater, corrosion resistant material and permanent qualified maintenance staff (Al-Enezi et al., 2006).

HDH, MD, pervaporization and solar distillation are low operating temperature technologies and thus minimise corrosion related issues. They are mostly used for small capacity water generation, e.g. 100 l/d. HDH technology is simple in operation, but availability of low mass transfer coefficient in existing humidifier, 1 to 3 kg/s·m3·(kgw/kgda), (Amara et al., 2004) and in dehumidifier, increases the plant size and energy required for air circulation. In MD and pervaporization transport of water vapour from seawater occurs through microporous hydrophobic membranes. The specific energy consumption of MD can be low in the range of 73 to 75 kJ/kg when operated with low grade heat between 50 and 100 °C. It has potential to generate potable water with lower water cost, less than 0.5 $/m3. However, approximately after 800 h of use membrane may get damaged due to biofouling (Meindersma et al., 2006).

Solar desalination has been reported by many researchers. It can be used directly or coupled with other technologies such as MSF, MEE, vapour compression, RO, MD and electro dialysis, where solar collectors are used for heat generation. Direct solar desalination method includes use of solar still, which has no moving parts and is simple in operation. It is competitive to indirect desalination plant in small scale production. However, its productivity is low, 2 to 5 l/d·m2 compared to indirect technologies. Indirect solar technology operating with multiple effects HDH techniques has resulted in water generation of 12 l/d·m2 (Farid and Al-Hajaj, 1996). Although this is three times higher than simple solar still, the cost associated with air circulation is high due to large air side pressure drop in humidifier and dehumidifier. Solar collectors used for seawater desalination could be flat plate collector, evacuated tube collector or parabolic trough collector (Qiblawey and Banat, 2008).

In membrane separation technology, salt is separated from seawater through semi permeable membrane by reverse osmosis (RO) process. This process is most extensively used due to ease of operation, and is applicable to a wide range of capacities, from household level to entire cities. However requirement of high pumping pressure of seawater through membrane, 55 to 70 bar, increases specific energy consumption and maintenance cost (Avlonitis et al., 2003). The high pumping pressure of seawater is due to its high osmotic pressure (22.7 bar at 3.5% salt concentration), which increases further with increased salt concentration (Spiegler, 1966). The specific energy consumption for large capacity plant without energy recovering device is 8 to 12 kWhe/m3. This specific energy consumption increases over a period of time due to fouling/ageing of the membrane, which also affects product water quality. The specific energy consumption of RO can be reduced by installing brine energy recovery devices such as turbo, Pelton turbine or pressure exchanger. The use of turbo and Pelton turbine have resulted in specific energy consumption of 3.7 to 5.3 kWhe/m3 (Stover, 2007), while a pressure exchanger reported specific energy consumption of 3.02 kWhe/m3 in 14,888 m3/d capacity plant (Avlonitis et al., 2003). However, high cost of energy recovery device may restrict their use in small capacity plants. Further RO requires high pre-treatment of seawater to reduce fouling rate of membrane. In case fouling is excessive, membrane can no longer be cleaned. This increases membrane replacement and pre-treatment costs. The pre-treatment cost for 100 l per day capacity plant was found to be 10 €/m3 (700 Rs/m3) while it is 0.3 to 0.9 €/m3 (21 to 63 Rs/m3) in large capacity plant (Elfil et al., 2007). Thus, for small production capacities of the range of 1 to 100 l per day of production using sea water as the feed, RO process may not be an optimum choice.

Distillation and membrane separation technologies thus have difficulties mainly due to scaling of equipment, plant size, space requirement and reliability. The major problems in the systems discussed are scaling of the equipment and related operational cost. Freezing technologies being operated at lower temperature obviates the scaling related issues and increases equipment life.

Section snippets

Freezing technologies

In freezing technology, seawater is cooled below its freezing temperature, which is a function of its salinity. As salt concentration in normal seawater is much less than its eutectic composition as 23.3 wt.% NaCl, ice crystals of pure water are formed when it starts freezing. Depending on equipment design, ice could form in suspension with the mother liquor or as a frozen layer on the heat transfer surface.

Common methods of freezing include direct, indirect or vacuum freezing and their

Two stage freeze concentration system with tubular heat exchanger

The method proposed here suggests the use of a patented vented double wall Tube–Tube Heat Exchanger (TT_HE) as a heat transfer surface instead of open-to-atmosphere HTS (Rane and Tandale, 2002, Rane and Padiya, 2010). The double wall feature of TT_HE addresses the potability issues and avoids accidental mixing of water and refrigerant. TT_HE has already proven its reliability in various applications such as recovery of heat from refrigeration and air conditioning system, potable water chilling

Conclusions

Desalination of seawater as a source of potable water is likely to continue in the future as available pure water sources are reducing. Conventional desalination technologies such as reverse osmosis, multiple effect evaporator, multistage flashing and mechanical vapour compression have equipment scaling related issues which increase the operational and maintenance cost. Freeze desalination technique operates at low temperature and has less scaling problems. Conventional freezing methods include

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