تجزیه و تحلیل عملکرد از نمونه آزمایشی نمک زدایی جذب گرمای هدررفته حرارت محور در دمای پایین

This paper discusses the performance analysis of an advanced adsorption desalination (AD) cycle with an internal heat recovery between the condenser and the evaporator. The AD cycle employs the adsorption–desorption principles to convert sea or brackish water into high-grade potable water with total dissolved solids (TDS) less than 10 ppm (mg/L) utilizing low-temperature heat source. The salient features of the AD cycle are the utilization of low temperature waste heat (typically 55 °C to 85 °C) with the employment of an environment-friendly silica gel/water pair and the low maintenance as it has no major moving parts other than the pumps and valves. For improved performance of the AD pilot plant, the internal heat recovery scheme between the condenser and evaporator has been implemented with a run-about water circuit between them. The efficacy of the scheme is analyzed in terms of key performance indicators such as the specific daily water production (SDWP) and the performance ratio (PR). Extensive experiments were performed for assorted heat source temperatures ranging from 70 °C to 50 °C. From the experiments, the SDWP of the AD cycle with the proposed heat recovery scheme is found to be 15 m3 of water per ton of silica gel that is almost twice that of the yield obtained by a conventional AD cycle for the same operation conditions. Another important finding of AD desalination plant is that the advanced AD cycle could still be operational with an inlet heat source temperature of 50 °C and yet achieving a SDWP of 4.3 m3 – a feat that never seen by any heat-driven cycles.

Water is the basic substance of life on earth and is directly related to the sustainability and the development of an economy. Much water is needed for agriculture, industry, recreation and human consumption [1], [2] and [3]. Statistically, three-quarters of the earth’ surface is covered by water [4], however, most of the water available to human beings is unusable due to the high salinity, typically, total dissolved solids (TDS) higher than 35,000 mg/L. According to World Health Organization (WHO) drinking water standards, water with TDS less than 1000 ppm is considered to be drinkable [5] and [6]. However, over 97.5% of the earth’s water represents oceans or sea with TDS value much higher than the potable TDS range. Besides, 80% of the remaining 2.5% fresh water is frozen in the icecaps or combined as soil moisture [7]. On the other hand, Global potable water demand has been increasing with economical and industrial growth. Concurrently, fresh water requirement for food production increases exponentially with drastic increase in population [8], [9] and [10]. WHO report states that at least one billion people do not have access to clean and fresh water, and about 41% of Earth’s population live in the water-stressed areas, and this deprived population may climb to 3.5 billion by the year 2025 [11]. Meanwhile, fresh water demand of the world is expected to be at 2% growth rate to 6900 billion m3 (Bnm3) in 2030 from today’s 4500 Bnm3, while the existing sustainable supply from the Earth’s natural water cycle is only 400 Bnm3[12] and [13]. Desalination can be considered a practical solution to daily water crisis in the regions where the annual rainfall is significantly low and no fresh water is available within a reasonable distance. Desalination methods can be categorized into three major groups, namely: (i) thermally-activated systems which utilize thermal energy to produce fresh water from sea or brackish water by evaporation and then condensing the vapor, (ii) pressure-activated systems which extract potable water from the salt water applying excess osmotic pressure across a semi-permeable membrane which selectively allows the passage of fresh water but retarding the dissolved salts and (iii) chemically-activated desalination methods which include ion-exchange desalination, liquid–liquid extraction, and gas hydrate or other precipitation schemes [14], [15], [16], [17], [18], [19] and [20]. Among desalination technologies, the RO technology represents 59% of total 14,451 desalination plants with 60 million cubic meters per day desalination capacity of the world while the MSF has 27% share [21], [22] and [23]. Another type of desalination method that utilizes the electrochemical process includes the Electrodialysis (ED) and Electrodialysis Reversal (EDR) where ions migrate through ion-selective semipermeable membranes due to their attraction to electrically charged electrodes [24] and [25]. Besides, recent developments in the Multi-Effect Distillation (MED) system coupled with the Mechanical Vapor Compressor (MVC) or Thermal Vapor Compressor (TVC) have improved the performance. However, the improvement in the water production rate is subject to the available number of effects that depends on the temperature difference between the Top Brine Temperature (TBT) and the down condenser temperature which is limited by the condenser temperature where seawater is usually the case [26], [27] and [28]. Thus, the evaporation temperature of the saline cannot be below 35 °C in most cases. Moreover, almost all the desalination systems suffer from at least two inherent drawbacks i.e., high energy intensive and/or prone to fouling and corrosion to the evaporation or separation unit of the sea water [29] and [30]. Adsorption desalination (AD) cycles utilize the sorption phenomena of the adsorbate/adsorbent pair to produce potable water from sea or brackish water. Adsorption desalination cycles are designed to mitigate the shortcomings of conventional desalination methods [31]. The AD cycle mimics the evaporation in the ambient by low-temperature waste heat of the sun, and condensing the water vapor at high altitude, producing pure water with no input of fossil fuel or energy from carbon-based energy. In AD cycles, the evaporation of the saline water occurs at low-temperatures typically between 5 °C and 20 °C. The operating regime for the AD cycle on the calcium sulfate solubility verses temperature is given in Fig. 1[32]. It is noted that the low-temperature evaporation of the saline water allows AD cycle to achieve high recovery ratio while operating outside the solubility limit. Thus, scaling to the evaporating units can be reduced in AD cycle. Full-size image (23 K) Fig. 1. Operation regime of AD cycle on calcium sulfate solubility diagram [32]. Figure options The earliest development on adsorption-based desalination was reported by Broughton [33], using an ion-retarded resin for the vapor uptake, where a process with a thermally-driven two-bed configuration is simulated. Similar theoretical studies on the adsorption desalination plant were also proposed by Zejli et al. [34], Al-kharabsheh and Goswami [35] and [36]. Solar heat source was studied as a heat source for the desalination plant, combined with an open-cycle adsorption heat pump using zeolite as the adsorbent material. Wang and Ng investigated the performance of the AD cycle using a four-bed regeneration scheme and reported an optimal specific daily water production (SDWP) of 4.7 kg/kg silica gel [30]. A number of studies have been reported on the AD cycle highlighting the theoretical model, the numerical simulation, the operation strategy at various regeneration temperatures as well as the performances across assorted heat source temperatures [37], [38], [39], [40] and [41]. In this paper, the performance of the advanced AD cycle with the implementation of the internal heat recovery between the condenser and evaporator is presented. The internal heat recovery is achieved by a water run-about circuit between the evaporator and condenser. The results are augmented with a conventional AD cycle where the inlet temperatures to the evaporator and condenser are fixed depending on the ambient or external heatsource, and an advanced cycle with internal heat recovery between the condenser and the evaporator.

The performance of adsorption desalination (AD) cycles with different configurations namely (i) the advanced cycle with heat recovery between the condenser and the evaporator, and (ii) the conventional cycle for 2-bed mode are analyzed using different hot water inlet temperatures. For the same operating conditions, the experimental results showed that the specific daily water production (SDWP) of the advanced cycle is two-folds that of the conventional cycle with higher performance ratio due to the improved adsorption and desorption mechanisms resulted from the proposed heat recovery scheme. The pressurization on the adsorption from higher evaporation temperature and the cooler condensation process is achieved simultaneously by recovering the condensation heat to the evaporation. It is observed that the advanced cycle has been demonstrated to be operational at an unprecedentedly low inlet temperature, typically from 50 °C to 80 °C, a feat that cannot be matched by any other cycles.