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Experimental investigation on an adsorption desalination system with heat and mass recovery between adsorber and desorber beds
Desalination 446 (2018) 42–50
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Experimental investigation on an adsorption desalination system with heat and mass recovery between adsorber and desorber beds Hongting Maa,b, Jingyu Zhanga,b, Chaofan Liua,b, Xueyin Lina,b, Yuexia Suna,b, a b
T
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School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China MOE Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Adsorption desalination Heat and mass recovery Experimental analysis Running performance Desalted water quality
An experimental equipment for adsorption desalination system with falling-film evaporator and heat recovery between adsorber and desorber beds was designed and established. The running performance of the system was investigated experimentally in steady condition. The yield and quality of product water were tested. The results show that the specific daily water production and performance ratio are 4.69 and 0.766, respectively. The heat transfer between tube and shell side in falling-film evaporator is mainly influenced by the amount of vapor desorbed from adsorbents. The utilization of heat recovery cannot increase the water production, but it can save energy. The heat recovery efficiency is 74.7% in every process of heat recovery. All indexes of produced water meet the standard of drinking water in China.
1. Introduction Desalination is an enforceable solution to water-shortage problem due to the growth of population. Methods for conventional desalination include distillation, membrane method and crystallization. The commercial desalination technologies include membrane-based reverse osmosis (RO), multi-stage flashing (MSF) and multi-stage distillation (MED) [1]. However, the expense for initial investment and operation of conventional desalination technologies are quite high [2,3]. The energy costs for unit water production by MSF or RO are higher than that of potable water produced from surface water and underground water resources. The cost of desalination is site specific. In 2016, the cheapest cost of seawater reverse osmosis was 0.5 US$/m3 [4]. The desalination by traditional energy demands large consumption of natural resources. Therefore, the utilization of solar, wind, geothermal and other renewable pollute-free energy is a new developing direction for desalination. In Egypt, with solar-driven adsorption desalinationcooling system, the average specific cooling power was 112 W/kg and average specific daily water production was 4 m3 per ton silica gel with a coefficient of performance (COP) of 0.45 [5]. As for solar desalination system using heat pump, the performance ratio (PR) ranged from 0.43 to 0.88, the average COP was 8 and the highest distillate production was 1.38 kg/h [6]. Porous silica gel with intense adsorption of water vapor can desorb vapor at a relatively low temperature (< 100 °C), which makes
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adsorption desalination (AD) driven by low temperature heat resource possible. The adsorbent based on zeolite, i.e., AQSOA-Z01 and AQSOAZ02 [7], was reported to be suitable for the utilization of thermal energy up to 150 °C. Other advantages with AD are lower evaporating temperature, more production on pure water and less generation of hydrogen halide gas. AD systems have drawn considerable attention due to the lower temperature required for driving heat source [8]. AD system cycle can produce two basic products simultaneously, namely cooling power and potable water [9,10]. However, there are problems about the interaction between solid adsorbent and vapor as well as heat losses. A better solution needs to be proposed to improve the performance of AD system. Researchers investigated the influences of various AD structures and operation modes on system performance. Wang et al. [11] reported that internal mass recovery was achieved by pressures equalization between adsorber and desorber beds. Thu et al. [12] studied the internal heat recovery from condenser to evaporator in AD system cycles and proposed two possible schemes such as (i) combining evaporator and condenser, namely evaporation-condensation device, and (ii) heat recovery pipes, namely cooling water pipes crossing through condenser and evaporator, respectively. In addition, Thu et al. [13] built an experimental four-bed single-effect adsorption desalination refrigeration system. This system was operated on two-bed model and four-bed model, which showed that the evaporator-condenser heat recovery circuit between evaporator and condenser doubled the SDWP
Corresponding author at: School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China E-mail address: [email protected] (Y. Sun).
https://doi.org/10.1016/j.desal.2018.08.022 Received 31 December 2017; Received in revised form 13 August 2018; Accepted 29 August 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.
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Nomenclature
mhw h(Tcond) Qdes Qrec Msg Mco Mal MAl,alloy Mw cp,hw csg cco cal cAl, alloy Cw
Abbreviations AD RO MSF MED SDWP PR TDS
adsorption desalination reverse osmosis multi-stage flashing multi-stage distillation specific daily water production (kg of fresh water/kg of silica gel per day) performance ratio (−) total dissolved solids
Symbols x P T U Tsg Tw Tcond TA,init TB,init Thw,in Thw,out ΔTA ΔTB ΔTdes UT n t tcycle mwater
ratio of the equilibrium adsorbed amount to maximum adsorbed amount (−) pressure (Pa) temperature (K) uncertainty temperature of silica gel (K) temperature of water vapor (K) temperature of condensation (K) initial temperature of bed A (K) initial temperature of bed B (K) inlet temperature of hot water (K) out temperature of hot water (K) temperature difference of bed A before and after the heat recovery (K) temperature difference of bed B before and after the heat recovery (K) temperature rise of desorber bed before and after the heat recovery (K) uncertainty of temperature (K) number of cycles per day (−) time (s) period of a cycle (s) mass flow rate of desalted water (kg/s)
mass flow rate of hot water (kg/s) laten heat of condensation at temperature Tcond (J/kg) energy required for desorption process (J) energy recovered during heat recovery process (J) mass of silica gel (kg) mass of copper (kg) mass of aluminum (kg) mass of aluminum alloy (kg) mass of water adsorbed in the bed (kg) specific heat of hot water (J/(kg·K)) specific heat of silica gel (J/(kg·K)) specific heat of copper (J/(kg·K)) specific heat of aluminum (J/(kg·K)) specific heat of aluminum alloy (J/(kg·K)) specific heat of aluminum water (J/(kg·K))
Greek letters ε
efficiency of heat recovery (−)
Subscripts sg w cond A B init water des rec in out hw co al Al, alloy
silica gel water vapor condensation bed A bed B initial desalted water desorption recycled inlet outlet hot water copper aluminum aluminum alloy
source. Du et al. [21] focused on the area optimization of solar collectors for adsorption desalination and concluded that the unit costs of producing fresh water can be reduced up to 20% after collector area optimization when the auxiliary energy was expensive. Besides, the overall cost of solar heating adsorption desalination was around 0.03–0.04 CNY/MJ, which was lower than conventional energy. Currently, an innovative combination between reverse osmosis and adsorption desalination system has been proposed [22]. The main advantage of the reverse osmosis and adsorption desalination system was that it increased the system recovery with low potential permeate salinity by about 25% compared to a single stage RO system. Compared with AD system, the mechanical vapor compression adsorption desalination system increased specific daily water production in range of 10%–45% at designated driving temperature [23]. The hybridization of MED and AD extended the limited temperature range of the MED, typically from 65 °C at top-brine temperature to a low-brine temperature of 40 °C and to a lower low-brine temperature of 5 °C, while the topbrine temperature remained the same [24,25]. The seawater thermocline-driven MED system, powered by the seawater thermocline energy with zero global warming potential, had the ability to operate efficiently within a 20 °C temperature difference between the warm surface seawater and the cold seawater from deep sea-bed [26]. Multi-effect solar humidification-dehumidification desalination system had a good productivity performance due to re-utilization of the latent heat of condensation of vapor between the two desalination loops, and solar
approximately. Wang et al. [14] found that SDWP and PR were improved by 15.7% and 42.5%, respectively, by using delayed closing valves and pressure equalization between adsorber and desorber beds. Wu et al. [15,16] discussed the thermal performance of adsorption desalination and refrigeration system when adsorption desalination and refrigeration system only produced water. They also discussed the demand of evaporating temperature and cooling water temperature for two conditions, (i) only producing water and (ii) producing potable water and cooling power simultaneously. In the study of adsorption desalination and cooling system with integrated evaporator and condenser, Youssef et al. [17] found that decreasing condenser temperature upgraded cycle outputs of desalinated water and cooling. And the system outputs were sensitive to changes in the heat source temperature. At present, there are few studies on AD in China. Wang and He [18] discovered that the exergy efficiency of adsorption single-effect evaporation desalination cycle is 0.40–0.71 by different adsorbents. Wang et al. [19], using silica gel and silica gel-CaCl2 as adsorbents, found that energy saving rate of cogenerated adsorption refrigeration and seawater desalination system was at 9.9%–26.2% and 22.4%–35.4% for the two adsorbents, compared with refrigeration-only and water-only systems. Lu et al. [20] studied the adsorption performance of watersilica gel. They concluded that the inlet hot water temperature has significant impact on mass recovery. The mass recovery time for hightemperature heat source was less than that for low-temperature heat 43
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the adsorber bed and is adsorbed. The non-evaporating seawater is collected in the brine storage. The heat used to drive vaporization is from the condensation of vapor in the tube side of falling-film evaporator. At the same time, the vapor is condensed into water and collected in the tank. The adsorption process is an exothermal process and cooling water is needed to take away heat to reduce the temperature of adsorber bed. A cooling tower is utilized to provide cooling water. During the experiment, the temperature of cooling water is about 27 °C. The hot water that drives water evaporation in the desorption process is supplied by a hot water tank. The heat can be from solar collections or a ground source heat pump system. In this experiment, due to the restriction of experimental conditions, an electric heating water tank is used as heat resource instead of solar collectors and ground source heat pump. The temperature of outlet water can be easily controlled with a proportion integration differentiation (PID) controller. Before the operation, the pressure of shell side of falling-film evaporator is reduced to 6.7 kPa (All pressure values in this paper are absolute pressure). A mass recovery is conducted at the beginning of a complete cycle. In this process, bed A has just finished desorption but still in a higher temperature and dry status, while bed B has just finished adsorption but still in a lower temperature and saturated status. At this time, the outlet gas valves of two beds keep open and the inlet gas valves of two beds are closed. The pressures between the two beds are balanced, leading to that water desorbed from bed A flows into bed B and it is adsorbed by bed B. During mass recovery, the flowing states of hot water and cooling water are same as they were before mass recovery. Subsequently, the heat recovery is achieved by the liquid circulation and heat exchange between the two beds. In this process, bed B is heated and bed A is cooled. After that, hot water passes through bed B to initiate desorption process. When the vapor pressure of bed B is greater than that in the falling-film evaporator, the vapor flows into the tubes of falling-film evaporator and is condensed. At the same time, the seawater in the shell of the evaporator is heated and evaporated by the latent heat of vapor in the tubes. Meanwhile, cooling water passes through bed A to initiate adsorption process. When the vapor pressure of bed A is less than that in the shell of falling-film evaporator, the vapor produced from seawater evaporating flows into bed A and is adsorbed by the adsorbent. The remaining concentrated brine in the shell of falling-film evaporator is collected in brine storage and discharged. At the same time, the desalted water condensed in the tubes of falling-film evaporator is collected in the collection tank. The states of all valves in different period of the cycle are shown in Table 1. During the whole process, hot water and cooling water pass through
energy could be used in this kind of system [27,28]. Types of AD systems have been studied, including the type employing integrated evaporator-condenser device. However, there is no such an AD system that achieves heat and mass recovery between adsorber and desorber. This was the motivation behind the present study. Our study aims to (i) design and set up an experimental installation to investigate a “new” AD system, (ii) test performance of this system, (iii) test the quality of produced water. The “new” AD system employed in our study has the following originality: (i) a falling-film evaporator for seawater evaporation and fresh water condensation is operated in our experiment (ii) a heat and mass recovery circuit between adsorber and desorber bed is installed and operated. The present study has a great potential to guide applications of AD systems. Results obtained in our study provide valuable data for the design and evaluation of AD systems. 2. Experimental apparatus and methods The schematic diagram of our adsorption desalination system is shown in Fig. 1. The system comprises two major parts, (i) two beds where water vapor is adsorbed and desorbed and (ii) falling-film evaporator, an integrated evaporator-condenser, where the seawater evaporates and water vapor is condensed. The bed employs the fin-tube heat exchanger, and the gaps between fins are filled with silica gel. The two beds work as adsorber and desorber alternately. The falling-film evaporator is a shell-and-tube type heat exchanger. The seawater flows in the shell side, while the vapor in the tube side is condensed. The condensation of vapor is as the heat source for the evaporation of seawater in the shell side. In addition, the heat generated in adsorption is removed by circulating cooling water, while the hot water supplied to the desorption drives the vapor desorbed out of the adsorbent. The design of our AD system is referred to the study of Thu et al. [29], where the performance of an advanced AD cycle with condenserevaporator heat recovery scheme was modeled and predicted. However, their system used hot or cooling water to pre-heat the desorber bed or pre-cool the adsorber bed in switch interval between the half cycles. In our study, a heat and mass recovery circuit between two beds is utilized to reduce the waste of heat and increase the production of desalted water. The water tank in heat recovery circuit is used to eliminate the influence of water volume variation on pipes caused by temperature variation. The vapor is adsorbed and desorbed in the beds during the operation. The seawater is pumped from seawater tank into the falling-film evaporator and sprayed by the array of nozzles. The seawater evaporates in the shell side of falling-film evaporator. The vapor flows into
Fig. 1. Schematic diagram of the adsorption desalination system. 44
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In the present study, 12 indexes of desalted water are tested, such as chromaticity, pH, total hardness and volatile phenol, etc. The results are compared with Drinking Water Health Standards of China and desalted water produced by other traditional desalination technologies.
Table 1 State of valves in different period. Valve
GV-1 GV-2 GV-3 GV-4 LV-1 LV-2 LV-3 LV-4 LV-5 LV-6 LV-7 LV-8 LV-9 LV-10 LV-11
Period Mass recovery
Heat recovery
A as adsorber B as desorber
B as adsorber A as desorber
Off On Off On No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state Off Off Off
Off Off Off Off Off Off Off Off Off Off Off Off On On On
On Off Off On Off On Off On On Off On Off Off Off Off
Off On On Off On Off On Off Off On Off On Off Off Off
3. Performance analysis The adsorber bed is filled with graininess pore spherical silica gel (Type A silica gel). Silica gel is an amorphous substance, insoluble in water. And it does not react with other substances except hydrofluoric and strong base. In the process of designing adsorber bed, the thermodynamic equilibrium equation for silica gel-water adsorption is selected according to references [30, 31], and is given by B (Tsg )
P (Tw ) ⎤ x = A (Tsg ) ⎡ ⎢ P (Tsg ) ⎥ ⎣ ⎦
where A(Tsg) and B(Tsg) are given by
Table 2 Desired operation parameters of the AD system.
Hot water inlet (desorber) Cooling water inlet (adsorber) Seawater spraying Temperature difference of hot water inlet and outlet Temperature difference of cooling water inlet and outlet Temperature of vapor desorbed from bed
Temperature
Flow rate
83 ± 1 °C 27 °C 30 °C 5 °C 5 °C
1.17 m3/h 1.68 m3/h 0.18 m3/h
46 °C
Mass
Specific heat
29.17 kg 7.77 kg 5.27 kg 18.13 kg 36.72 m2
0.92 kJ/(kg·K) 0.384 kJ/(kg·K) 0.903 kJ/(kg·K) 0.896 kJ/(kg·K)
A (Tsg ) = A0 + A1 Tsg + A2 Tsg2 + A3 Tsg3
(3)
B (Tsg ) = B0 + B1 Tsg + B2 Tsg2 + B3 Tsg3
(4)
where x is the ratio of the equilibrium adsorbed amount to maximum adsorbed amount, Tsg is the temperature of silica gel, Tw is the temperature of water vapor, A0-A3, B0-B3 are all constant, whose fitting values are given by A0 = −14.2904, A1 = 0.1546 K−1, A2 = −5.5498 × 10−4 K−2, A3 = 6.7512 × 10−7 K−3, B0 = 36.1487, B1 = −0.3820 K−1, B2 = 1.3016 × 10−3 K−2, −6 −3 B3 = −1.4150 × 10 K . In this experiment, Type A silica gel was used as the adsorbent and the physical properties of that are shown in Table 5. For assessment of this AD system, two parameters are calculated, i.e., specific daily water production (SDWP) and performance ratio (PR). Both of them are given by
Table 3 Parameters of adsorber bed.
Silica gel Copper Aluminum alloy Aluminum Heat exchange area
(2)
SDWP = n
PR =
1 tcycle
∫0 ∫0
tcycle
tcycle
m water dt Msg
(5)
m water × h (Tcond ) dt Qdes
(6)
bed A and bed B alternately, and the adsorption process and desorption process are operated alternately. The desired operation parameters of the AD system are shown in Table 2, while parameters for bed are shown in Table 3. The data measured and recorded in this experiment includes three types, namely temperature, pressure and flow rate. The inlet and outlet temperatures and flow rates of beds are measured by heat meters, while other temperatures are measured by Pt100 thermometers. The uncertainty of the Pt100 thermometers is calculated by
where tcycle is the period of a cycle, n is the number of cycles per day in the continuous operation, mwater is the mass flow rate of desalted water, Msg is the total mass of silica gel in the bed, h(Tcond) is the latent heat of condensation at the temperature Tcond determined according to reference [32], Qdes is the energy required for desorption process. The adsorbent bed is enclosed in a cubic container with a 50 mm insulation layer of rubber and plastic material. Because of the good insulation effect, the heat transfer between the bed and ambient air can be neglected. Therefore, Qdes is the heat transfer between the desorber bed and hot water, and it can be calculated by Eq. (7)
UT = (0.15 + 0.002T ) K
Qdes = mhw cp, hw (Thw, in − Thw, out )
(1)
(7)
where mhw is the flow rate of hot water, cp,hw is the specific heat capacity of hot water, Thw,in and Thw,out are the inlet and outlet temperature of hot water, respectively. In heat recovery process, the beds are pre-heated and pre-cooled by the circulating water flowing between two beds, without power input by hot or cooling water. Therefore, both heat power for pre-heating the desorber bed and cooling power for pre-cooling the adsorber bed are taken into account in the calculation of recycled energy. It is noted that there is water adsorbed in the desorber bed while there is no water existing in the adsorber bed. The energy pre-heating the water in the desorber bed is also considered in the calculation of recycled energy. The energy recycled from heat recovery is calculated by Eq. (8). The efficiency of heat recovery is the ratio of maximum allowable heat
where T is the temperature measured by it. The pressure is measured by pressure gauge, whose range is from −100 kPa to 0 kPa and relative uncertainty is 0.5%. The relative uncertainty of heat meters is 2.5%. The test data is recorded under stable operation condition. Fig. 2 demonstrates the physical diagram of the experimental device. The seawater used in the experiment is produced by mixing local tap water with dried sea salt. The salinity of simulated seawater is 3.5%. Table 4 shows the properties of simulated seawater. In order to prevent non-condensable gas from interfering normal operation of the system, the seawater must be de-aerated before it is poured into the tank. The dried sea salt is sterilized before drying. In the whole process of seawater preparation, the ambient air is filtered by an air cleaner. 45
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Falling-film evaporator
Collection
Adsorber
Hot water tank
Fig. 2. Physical diagram of AD system.
Table 4 Properties of simulated seawater. Parameters
Value
Unit
pH Total dissolved solids (TDS) Total hardness Na Al Fe Cu Chloride Sulfate Sulfide Boron
8.5 30,071 6270 9993 0.209 0.177 0.004 18,440 2110 < 0.01 1.57
/ mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Table 5 Properties of Type A silica gel. Property
Value
Granular average diameter (mm) Average pore diameter (nm) Surface area (m2 g−1) Porous volume (cm3 g−1) Specific heat (kJ/kg K−1) Thermal conductivity (W m−1 K−1) Apparent density (kg m−3) True density (kg m−3)
0.5–1.5 2.0–3.0 650–800 0.35–0.45 0.92 0.175 790 2200
Fig. 3. Variation of pressure with time in each part during a complete cycle.
transfer rate between two beds to the actual heat transfer rate. The efficiency of heat recovery is calculated by Eq. (9).
Qrec = (Msg csg + Mco cco + Mal cal + MAL, alloy cAL, alloy )(|ΔTA| + |ΔTB|) + Mw c w ΔTdes
Table 6 Values of the parameters used in the performance calculation.
ε=
Parameters
Value
Unit
tcycle n h(Tcond) cp,hw mhw
0.5 48 2358.62 4.18 1136.63
h / kJ/kg kJ/(kg·K) kg/h
Qrec (Msg csg + Mco cco + Mal cal + MAL, alloy cAL, alloy )|TA, init − TB, init |
(8)
(9)
where Msg, Mco, Mal, MAl,alloy, Mw are the mass of silica gel, copper, aluminum, aluminum alloy, water adsorbed in bed respectively. Mw is equal to the water production of a half cycle. csg, cco, cal, cal, alloy, cw are the specific heat of corresponding materials. ΔTA and ΔTB are the temperature differences of bed A and bed B before and after the heat recovery, and TA,init, TB,init are the initial temperature of two beds in the process of heat recovery. ΔTdes is the temperature increase of desorber bed. The values of all the coefficients and the parameters used in the performance calculation are listed Tables 3 and 6. 46
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4. Results and discussion In the present study, the cooling water pump operates at a constant rotational speed, so the temperature and flow rate of cooling water remain constant during the experiment. In order to achieve the continuous coupling operation of the experimental AD system, the temperature and flow rate of hot water are also unchanged. In addition, the circulation of hot or cooling water will not be stopped until the adsorber bed is almost saturated. During two months of experiment, many attempts and adjustments were made to meet the requirements of the experiment. Nevertheless, the limited conditions lead to that there is only one condition achieving the coupling operation of the whole system. And in this condition, the system can operate stably for 5–6 h. Each cycle in stable operation condition is extremely same. Therefore, our discussion is aimed at a single cycle of 30 min. In the stable operation condition, seawater temperature, seawater spray flow rate, hot water inlet temperature and cooling water inlet temperature are 30 °C, 180 L/h, 83 ± 1 °C and 27 °C, respectively. In the present work, a complete cycle is 30 min. In a half cycle, the first 13 min is a heating/cooling process, then a 30 s of mass recovery process, finally a 90 s of heat recovery process. The schedule of first half cycle is the same as that of the second half cycle.
Fig. 4. Variation of temperature with time at each part of bed in a complete cycle.
4.1. Variation of pressure in each part When the system is in stable operation, the variations of pressure in each part are measured and presented in Fig. 3 (At the beginning of the cycle, bed A is adsorber and bed B is desorber). From Fig. 3, because of the intense adsorption of adsorber bed to water vapor, the pressure of adsorber bed is less than that in the shell side of falling-film evaporator. What's more, the seawater is easy to evaporate in the shell of falling-film evaporator because of that the pressure is much lower than standard atmospheric pressure. Then, water vapor flows into the adsorber bed. In addition, due to the cooling effect of seawater, the vapor in the tube is condensed into liquid water, leading to the pressure in the tube lower than that in desorber bed. This makes the vapor desorbed and flow into the tube consecutively. It is the variation of pressure in different parts that the AD system can operate persistently and stably. During the mass recovery process, the pressure of bed A and bed B tends to be balanced rapidly. In the followed heat recovery process, part of water vapor is desorbed from bed A which is heated by circulatory hot water, while the pressure of bed B begins to decrease due to the cooling water circulation in bed B.
Fig. 5. Variation of temperature with time in the falling-film evaporator in a complete cycle.
4.2. Variation of temperature at each part During the experiment, the variation of temperature at each part is measured and shown in Fig. 4. According to Fig. 4, in the end of heat recovery, the temperature of bed A increases from 30.8 °C to 44 °C, while the temperature of bed B decreases from 68.2 °C to 55 °C. This phenomenon indicates that part of heat from the desorber bed can be recycled by heat recovery, and cooling power as well as heating power can be saved. During heat recovery, the temperature of bed A has increased by 13.2 °C, while the bed B has decreased by 13.2 °C. According to Eq. (8), the mass and specific heat of different materials in Table 3 and the water production of a half cycle discussed in Section 4.3, the energy recycled from each heat recovery cycle Qrec is 1435.4 kJ. The heat recovery efficiency ε is 74.7%. The difference between inlet and outlet hot/cooling water temperature has decreased to 0.3–2 °C during 10–13 min, indicating that adsorption and desorption have approximately completed. The heat transfer between circulatory water and beds is very poor. However, the temperature difference between desorber bed and hot water approaches to 10–12 °C, indicating that the heat transfer between silica gel and the
Fig. 6. Variation of water production with time at each part in a complete cycle.
47
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Table 7 Comparison between the present and previous studies for SDWP. Present
Youssef et al. [17]
Thu et al. [29]
Alsaman et al. [5]
Ng et al. [9]
Wang et al. [33]
Thu et al. [34]
Method
Experiment
Simulation
Simulation
Experiment
Experiment
Experiment
Experiment
SDWP
4.69
6.64
23
3.95
7.2
4.7
8.2
decreases while the pressure of adsorber bed increases, indicating a second desorption and a second adsorption. Nevertheless, the vapor generating from second desorption is directly adsorbed by adsorber bed, which means that the water vapor of second adsorption/desorption is accounted as water production in next cycle. It is noted that the water production of the system cannot be improved by heat recovery process. In the steady condition of operation, when the time of adsorption/desorption is > 10 min, the increment of cycle period will lead to a decrease of water production per unit time. In the present experimental condition, the specific daily water production (SDWP) is 4.69 and the performance ratio (PR) is 0.766. Under the same experimental condition, the PR of Thu et al. [29] is around 0.7, from 0.698 to 0.708. It can be seen that the utilization of heat recovery circuit between adsorber bed and desorber bed can increase PR significantly. However, due to the fact that the ideal simulation conditions cannot be achieved in real experiment, the SWDP is lower than that in numerical simulation. Table 7 shows the comparison between the present and previous studies for SDWP. It is found that the result of present study is in agreement with the previous experimental study. However, due to the fact that the ideal simulation conditions are difficult to achieve in real experiment, SDWP of present study is lower than the numerical simulation results.
Table 8 The relative uncertainty of indirect measured value. Parameter
SDWP
PR
Qdes
Qrec
ε
Relative uncertainty
1.44%
2.46%
2.00%
0.54%
1.26%
fins of bed is poor. It is necessary to improve the heat transfer performance of the bed in the future work. The temperature variation with time in the falling-film evaporator is shown in Fig. 5. the adsorption and desorption processes are from 2 min to 15 min. At the beginning of adsorption and desorption (the second minute), the seawater is sprayed in the shell and there is not too much vapor is desorbed and flows in the tubes. Therefore, the heat transfer between seawater and vapor is poor. It is the time that temperature in tube and shell side is dominated by the original temperature of vapor and seawater. And it is the time that a minimum temperature in shell side appears, while a maximum temperature in tube side appears. After that, the temperature in shell side rises sharply resulted from the strong heat transfer and evaporation at lower pressure (4–6 kPa). Along with the heat transfer weakening, the temperature decreases in both sides and the temperature difference decreases. In the whole process, it can be seen that the temperature variation range of tube side is larger than that of shell side, manifesting that the heat transfer is mainly influenced by the amount of vapor in tubes.
4.4. Uncertainty analysis The system error mainly consists of three parts: system device error, data processing error and random error. Among those, the data processing error mainly derives from the calculation of formula, and it can be ignored in the present experiment. The random error can be eliminated by repeated measurements. SDWP, PR, Qdes, Qrec and ε are all indirect measured value, the uncertainties of which are from the measurement of temperature and water flow rate. The relative uncertainty of indirect measured value can be calculated by Eq. (10) and the results are shown in Table 8.
4.3. Variation of water production Water production rate is an important technical index of AD system. Fig. 6 shows accumulation of water production in 30 min. The total water production in 30 min is 2.85 kg. Therefore, the water production rate of this AD system is 5.7 kg/h. In Fig. 6, it shows that the water production is relatively stable during 10–13 min and 25–28 min, which means that water production per unit time is very small. As shown in Fig. 4, during the same period, the temperature difference of circulatory water is small, which indicates the adsorption or desorption process has basically completed. These two periods (10–13 min; 25–28 min) cause a reduction in average water production per unit time. At the stage of mass recovery, the pressure of desorber bed
Uy y
n
=
2 ∂f Uxi ⎞ ⎝ i y ⎠
∑ ⎛ ∂x ⎜
i=1
⎟
(10)
where y is a kind of indirect measured value and y = f (x1, x2, x3, …… xn,). x1, x2, x3, …… xn are direct measured value. Uxi is the uncertainty
Table 9 Comparison of water quality for different desalination methods and standard in China. Test items
Chromaticity pH TDS Total hardness Na Al Fe Cu Chloride Sulfate Sulfide Boron
Unit
Degree / mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Standard for drinking water quality
15 6.5–8.5 1000 450 200 0.2 0.3 1 250 250 0.02 0.5
Results The present method
Dang-gang power plant MSF
Guohua Cang-dong MED
Shengsi county RO
Tianjin Science Committee RO
5 6.98 13 10 0.78 0.0156 0.00633 0.00759 < 1.0 1.60 < 0.01 0.011
<5 6.5 < 10 <1 0.29 < 0.025 < 0.002 0.030 0.53 0.10 < 0.01 < 0.020
<5 6.7 < 10 1.02 0.42 < 0.025 < 0.002 < 0.002 0.73 0.15 < 0.01 < 0.020
<5 7.04 238 14.5 – < 0.010 < 0.3 < 0.010 110 6.22 – 0.72
0 6.5 308 26.09 – < 0.03 0.019 – 38.09 0.328 – 0.46
48
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References
of xi.
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Hawlader, Analysis of solar desalination system using heat pump, Renew. Energy 74 (2015) 116–123 https://doi.org/10.1016/j.renene.2014. 07.028. [7] A. Myat, K.C. Ng, K. Thu, Y.D. Kim, Experimental investigation on the optimal performance of zeolite–water adsorption chiller, Appl. Energy 102 (2013) 582–590 https://doi.org/10.1016/j.apenergy.2012.08.005. [8] K. Srinivasan, P. Dutta, B.B. Saha, K.C. Ng, Realistic minimum desorption temperatures and compressor sizing for activated carbon + HFC 134a adsorption coolers, Appl. Therm. Eng. 51 (2013) 551–559 https://doi.org/10.1016/j. applthermaleng.2012.09.028. [9] K.C. Ng, K. Thu, B.B. Saha, A. Chakraborty, Study on a waste heat-driven adsorption cooling cum desalination cycle, Int. J. Refrig. 35 (2012) 685–693 https://doi.org/ 10.1016/j.ijrefrig.2011.01.008. [10] K.C. Ng, K. Thu, S.J. Oh, L. Ang, M.W. Shahzad, A.B. Ismail, Recent developments in thermally-driven seawater desalination: energy efficiency improvement by hybridization of the MED and AD cycles, Desalination 356 (2015) 255–270 https:// doi.org/10.1016/j.desal.2014.10.025. [11] R.Z. Wang, Performance improvement of adsorption cooling by heat and mass recovery operation, Int. J. Refrig. 24 (2001) 602–611. [12] K. Thu, H. Yanagi, B.B. Saha, K.C. Ng, Performance investigation on a 4-bed adsorption desalination cycle with internal heat recovery scheme, Desalination 402 (2017) 88–96 https://doi.org/10.1016/j.desal.2016.09.027. [13] K. Thu, B.B. Saha, A. Chakraborty, W.G. Chun, K.C. Ng, Study on an advanced adsorption desalination cycle with evaporator–condenser heat recovery circuit, Int. J. Heat Mass Transf. 54 (2011) 43–51 https://doi.org/10.1016/j. ijheatmasstransfer.2010.09.065. [14] X.L. Wang, K.C. Ng, A. Chakarborty, B.B. Saha, How heat and mass recovery strategies impact the performance of adsorption desalination plant: theory and experiments, Heat Transf. Eng. 28 (2) (2007) 147–153 https://doi.org/10.1080/ 01457630601023625. [15] J.W. Wu, M.J. Biggs, E.J. Hu, Thermodynamic analysis of an adsorption-based desalination cycle, Chem. Eng. Res. Des. 88 (2010) 1541–1547. [16] J.W. Wu, E.J. Hu, M.J. Biggs, Thermodynamic cycles of adsorption desalination system, Appl. Energy 90 (2012) 316–322 https://doi.org/10.1016/j.apenergy. 2011.04.049. [17] P.G. Youssef, S.M. Mahmoud, R.K. Al-Dadah, Numerical simulation of combined adsorption desalination and cooling cycles with integrated evaporator/condenser, Desalination 392 (2016) 14–24 https://doi.org/10.1016/j.desal.2016.04.011. [18] Y.Q. Wang, H.Z. He, Thermodynamic analysis of an adsorption desalination cycle, Acta-Energiae Solar Sinica 36 (2015) 11. [19] Y.Q. Wang, H.Z. He, Performance Analysis of Combined Adsorption Refrigeration and Seawater Desalination System, S2 CIESC, 2014, pp. 115–121. [20] Z.S. Lu, R.Z. Wang, Z.Z. Xia, Performance and optimization of adsorption chiller with mass recovery and heat recovery, J. Refrig. 31 (4) (2010), https://doi.org/10. 3969/j.issn.0253-4339.2010.04.007. [21] B.W. Du, J. Gao, L.J. Zeng, X. Su, S.L. Yu, H.T. Ma, Area optimization of solar collectors for adsorption desalination, Sol. Energy 157 (2017) 298–308 https://doi. org/10.1016/j.solener.2017.08.032. [22] E.S. Ali, A.S. Alsaman, K. Harbyc, A.A. Askalany, M.R. Diab, S.M.E. Yakoot, Recycling brine water of reverse osmosis desalination employing adsorption desalination: a theoretical simulation, Desalination 408 (2017) 13–24 https://doi.org/ 10.1016/j.desal.2016.12.002. [23] A.A. Askalany, Innovative mechanical vapor compression adsorption desalination (MVC-AD) system, Appl. Therm. Eng. 106 (2016) 286–292 https://doi.org/10. 1016/j.applthermaleng.2016.05.144. [24] M.W. Shahzad, K.C. Ng, K. Thu, B.B. Saha, W.G. Chun, Multi effect desalination and adsorption desalination (MEDAD): a hybrid desalination method, Appl. Therm. Eng. 72 (2014) 289–297 https://doi.org/10.1016/j.applthermaleng.2014.03.064. [25] M.W. Shahzad, K. Thu, Y.D. Kim, K.C. Ng, An experimental investigation on MEDAD hybrid desalination cycle, Appl. Energy 148 (2015) 273–281 https://doi. org/10.1016/j.apenergy.2015.03.062. [26] M.W. Shahzad, M. Burhan, N. Ghaffour, K.C. Ng, A multi evaporator desalination system operated with thermocline energy for future sustainability, Desalination 435 (2018) 268–277 https://doi.org/10.1016/j.desal.2017.04.013. [27] Z.H. Chang, H.F. Zheng, Y.J. Yang, Y.H. Su, Z.C. Duan, Experimental investigation of a novel multi-effect solar desalination system based on humidification-dehumidification process, Renew. Energy 69 (2014) 253–259 https://doi.org/10.1016/j. renene.2014.03.048. [28] G. Wu, H.F. Zheng, H.F. Kang, Y.J. Yang, P. Cheng, Z.H. 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4.5. Analysis of the quality of desalted water Table 9 presents the laboratory test results of desalted water quality. The comparison with Standard for drinking water quality of China (GB 5749-2006) and desalted water produced by typical desalination devices operating in China is also shown in Table 9. Table 9 shows that there is no significant difference of water quality between AD system and other desalination technologies, except for the RO in Tianjin and Shengsi. According to the results, TDS, chloride and boron of AD are better than that of RO apparently. Comparing Tables 4 and 9, it can be seen that all elements in the water has been significantly reduced, reaching a very low level. The pH of the AD method is closer to 7.0, much better than other methods. It is noteworthy that no silicon is detected in the product water. That's also a verification for the chemical stability of silica gel in the AD system. All results demonstrate that the quality of water produced by AD method has obvious advantages compared with other methods. However, the good desalination performance of the AD system means that the pollution of silica gel caused by impurity ions and solid particles is inevitable. In the silica gel-water adsorption system, the adsorption deterioration of silica gel due to the pollution is verified [35]. In the present work, the adsorption deterioration has not been detected after several experiments. Furthermore, it can be found that there is less sulfate of AD and other technologies except for RO. It is mainly due to that the water used for making up seawater comes from underground water that contains hydrogen sulfide, the hydrogen sulfide might be dissolved in the water and converted into sulfate. The comparing result shows that the desalted water produced by the AD system is up to the Standard for drinking water quality, and the main indexes are in consistent with that of the desalted water produced by conventional distillation desalination.
5. Conclusions The paper is performed to present a new advanced adsorption desalination system. The performance of this system has been investigated experimentally. The following conclusions can be drawn in our study: (i) The heat transfer between tube and shell side in falling-film evaporator is mainly influenced by the amount of vapor desorbed from adsorbents. The adsorption and desorption processes in bed is priority to control for the coupling operation of adsorption desalination system with integrated evaporator-condenser. (ii) Heat recovery cannot increase water production, but it can recycle energy. During a heat recovery process, the efficiency of heat recovery is 74.7%. The specific daily water production and performance ratio of this system is 4.69 and 0.766, respectively. The utilization of heat recovery circuit between adsorber bed and desorber bed can increase PR significantly. (iii) Compared to the Standard for drinking water of China (GB 57492006),it is clear that the quality of desalted water produced by the adsorption desalination system meet the Standard. The main characteristics is in consistent with that of the desalted water produced by conventional distillation desalination.
Acknowledgement This research was supported by the National Natural Science Foundation of China [grant number 51576137]. The authors would like to thank Professor G.Y. Chen for his collaboration and insightful discussions. 49
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[32] C.J. Chen, Thermal Design of Silica Gel-Water Adsorption Refrigeration, Shanghai Jiao Tong University, 2010. [33] X.L. Wang, K.C. Ng, Experimental investigation of an adsorption desalination plant using low-temperature waste heat, Appl. Therm. Eng. 25 (2005) 2780–2789. [34] K. Thu, K.C. Ng, B.B. Saha, A. Chakraborty, S. Koyama, Operational strategy of adsorption desalination systems, Int. J. Heat Mass Transf. 52 (2009) 1811–1816. [35] D.C. Wang, J.P. Zhang, Y.Z. Xia, Y.P. Han, S.W. Wang, Investigation of adsorption performance deterioration in silica gel–water adsorption refrigeration, Energy Convers. Manag. 58 (2012) 157–162 https://doi.org/10.1016/j.enconman.2012. 01.013.
investigation of a multi-effect isothermal heat with tandem solar desalination system based on humidification–dehumidification processes, Desalination 378 (2016) 100–107 https://doi.org/10.1016/j.desal.2015.09.024. [29] K. Thu, A. Chakraborty, Y.D. Kim, A. Myat, B.B. Saha, K.C. Ng, Numerical simulation and performance investigation of an advanced adsorption desalination cycle, Desalination 308 (2013) 209–218 https://doi.org/10.1016/j.desal.2012.04.021. [30] J.Q. Hu, System Design and Performance Simulation of a New Silica Gel-Water Adsorption Chiller, Shanghai Jiao Tong University, 2008. [31] B.B. Saha, E.C. Boelman, T. Kashiwagi, Computational analysis of an advanced adsorption-refrigeration cycle, Energy 20 (1995) 983–994 https://doi.org/10. 1016/0360-5442(95)00047-K.
50
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Desalination journal homepage: www.elsevier.com/locate/desal
Experimental investigation on an adsorption desalination system with heat and mass recovery between adsorber and desorber beds Hongting Maa,b, Jingyu Zhanga,b, Chaofan Liua,b, Xueyin Lina,b, Yuexia Suna,b, a b
T
⁎
School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China MOE Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Adsorption desalination Heat and mass recovery Experimental analysis Running performance Desalted water quality
An experimental equipment for adsorption desalination system with falling-film evaporator and heat recovery between adsorber and desorber beds was designed and established. The running performance of the system was investigated experimentally in steady condition. The yield and quality of product water were tested. The results show that the specific daily water production and performance ratio are 4.69 and 0.766, respectively. The heat transfer between tube and shell side in falling-film evaporator is mainly influenced by the amount of vapor desorbed from adsorbents. The utilization of heat recovery cannot increase the water production, but it can save energy. The heat recovery efficiency is 74.7% in every process of heat recovery. All indexes of produced water meet the standard of drinking water in China.
1. Introduction Desalination is an enforceable solution to water-shortage problem due to the growth of population. Methods for conventional desalination include distillation, membrane method and crystallization. The commercial desalination technologies include membrane-based reverse osmosis (RO), multi-stage flashing (MSF) and multi-stage distillation (MED) [1]. However, the expense for initial investment and operation of conventional desalination technologies are quite high [2,3]. The energy costs for unit water production by MSF or RO are higher than that of potable water produced from surface water and underground water resources. The cost of desalination is site specific. In 2016, the cheapest cost of seawater reverse osmosis was 0.5 US$/m3 [4]. The desalination by traditional energy demands large consumption of natural resources. Therefore, the utilization of solar, wind, geothermal and other renewable pollute-free energy is a new developing direction for desalination. In Egypt, with solar-driven adsorption desalinationcooling system, the average specific cooling power was 112 W/kg and average specific daily water production was 4 m3 per ton silica gel with a coefficient of performance (COP) of 0.45 [5]. As for solar desalination system using heat pump, the performance ratio (PR) ranged from 0.43 to 0.88, the average COP was 8 and the highest distillate production was 1.38 kg/h [6]. Porous silica gel with intense adsorption of water vapor can desorb vapor at a relatively low temperature (< 100 °C), which makes
⁎
adsorption desalination (AD) driven by low temperature heat resource possible. The adsorbent based on zeolite, i.e., AQSOA-Z01 and AQSOAZ02 [7], was reported to be suitable for the utilization of thermal energy up to 150 °C. Other advantages with AD are lower evaporating temperature, more production on pure water and less generation of hydrogen halide gas. AD systems have drawn considerable attention due to the lower temperature required for driving heat source [8]. AD system cycle can produce two basic products simultaneously, namely cooling power and potable water [9,10]. However, there are problems about the interaction between solid adsorbent and vapor as well as heat losses. A better solution needs to be proposed to improve the performance of AD system. Researchers investigated the influences of various AD structures and operation modes on system performance. Wang et al. [11] reported that internal mass recovery was achieved by pressures equalization between adsorber and desorber beds. Thu et al. [12] studied the internal heat recovery from condenser to evaporator in AD system cycles and proposed two possible schemes such as (i) combining evaporator and condenser, namely evaporation-condensation device, and (ii) heat recovery pipes, namely cooling water pipes crossing through condenser and evaporator, respectively. In addition, Thu et al. [13] built an experimental four-bed single-effect adsorption desalination refrigeration system. This system was operated on two-bed model and four-bed model, which showed that the evaporator-condenser heat recovery circuit between evaporator and condenser doubled the SDWP
Corresponding author at: School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China E-mail address: [email protected] (Y. Sun).
https://doi.org/10.1016/j.desal.2018.08.022 Received 31 December 2017; Received in revised form 13 August 2018; Accepted 29 August 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.
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Nomenclature
mhw h(Tcond) Qdes Qrec Msg Mco Mal MAl,alloy Mw cp,hw csg cco cal cAl, alloy Cw
Abbreviations AD RO MSF MED SDWP PR TDS
adsorption desalination reverse osmosis multi-stage flashing multi-stage distillation specific daily water production (kg of fresh water/kg of silica gel per day) performance ratio (−) total dissolved solids
Symbols x P T U Tsg Tw Tcond TA,init TB,init Thw,in Thw,out ΔTA ΔTB ΔTdes UT n t tcycle mwater
ratio of the equilibrium adsorbed amount to maximum adsorbed amount (−) pressure (Pa) temperature (K) uncertainty temperature of silica gel (K) temperature of water vapor (K) temperature of condensation (K) initial temperature of bed A (K) initial temperature of bed B (K) inlet temperature of hot water (K) out temperature of hot water (K) temperature difference of bed A before and after the heat recovery (K) temperature difference of bed B before and after the heat recovery (K) temperature rise of desorber bed before and after the heat recovery (K) uncertainty of temperature (K) number of cycles per day (−) time (s) period of a cycle (s) mass flow rate of desalted water (kg/s)
mass flow rate of hot water (kg/s) laten heat of condensation at temperature Tcond (J/kg) energy required for desorption process (J) energy recovered during heat recovery process (J) mass of silica gel (kg) mass of copper (kg) mass of aluminum (kg) mass of aluminum alloy (kg) mass of water adsorbed in the bed (kg) specific heat of hot water (J/(kg·K)) specific heat of silica gel (J/(kg·K)) specific heat of copper (J/(kg·K)) specific heat of aluminum (J/(kg·K)) specific heat of aluminum alloy (J/(kg·K)) specific heat of aluminum water (J/(kg·K))
Greek letters ε
efficiency of heat recovery (−)
Subscripts sg w cond A B init water des rec in out hw co al Al, alloy
silica gel water vapor condensation bed A bed B initial desalted water desorption recycled inlet outlet hot water copper aluminum aluminum alloy
source. Du et al. [21] focused on the area optimization of solar collectors for adsorption desalination and concluded that the unit costs of producing fresh water can be reduced up to 20% after collector area optimization when the auxiliary energy was expensive. Besides, the overall cost of solar heating adsorption desalination was around 0.03–0.04 CNY/MJ, which was lower than conventional energy. Currently, an innovative combination between reverse osmosis and adsorption desalination system has been proposed [22]. The main advantage of the reverse osmosis and adsorption desalination system was that it increased the system recovery with low potential permeate salinity by about 25% compared to a single stage RO system. Compared with AD system, the mechanical vapor compression adsorption desalination system increased specific daily water production in range of 10%–45% at designated driving temperature [23]. The hybridization of MED and AD extended the limited temperature range of the MED, typically from 65 °C at top-brine temperature to a low-brine temperature of 40 °C and to a lower low-brine temperature of 5 °C, while the topbrine temperature remained the same [24,25]. The seawater thermocline-driven MED system, powered by the seawater thermocline energy with zero global warming potential, had the ability to operate efficiently within a 20 °C temperature difference between the warm surface seawater and the cold seawater from deep sea-bed [26]. Multi-effect solar humidification-dehumidification desalination system had a good productivity performance due to re-utilization of the latent heat of condensation of vapor between the two desalination loops, and solar
approximately. Wang et al. [14] found that SDWP and PR were improved by 15.7% and 42.5%, respectively, by using delayed closing valves and pressure equalization between adsorber and desorber beds. Wu et al. [15,16] discussed the thermal performance of adsorption desalination and refrigeration system when adsorption desalination and refrigeration system only produced water. They also discussed the demand of evaporating temperature and cooling water temperature for two conditions, (i) only producing water and (ii) producing potable water and cooling power simultaneously. In the study of adsorption desalination and cooling system with integrated evaporator and condenser, Youssef et al. [17] found that decreasing condenser temperature upgraded cycle outputs of desalinated water and cooling. And the system outputs were sensitive to changes in the heat source temperature. At present, there are few studies on AD in China. Wang and He [18] discovered that the exergy efficiency of adsorption single-effect evaporation desalination cycle is 0.40–0.71 by different adsorbents. Wang et al. [19], using silica gel and silica gel-CaCl2 as adsorbents, found that energy saving rate of cogenerated adsorption refrigeration and seawater desalination system was at 9.9%–26.2% and 22.4%–35.4% for the two adsorbents, compared with refrigeration-only and water-only systems. Lu et al. [20] studied the adsorption performance of watersilica gel. They concluded that the inlet hot water temperature has significant impact on mass recovery. The mass recovery time for hightemperature heat source was less than that for low-temperature heat 43
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the adsorber bed and is adsorbed. The non-evaporating seawater is collected in the brine storage. The heat used to drive vaporization is from the condensation of vapor in the tube side of falling-film evaporator. At the same time, the vapor is condensed into water and collected in the tank. The adsorption process is an exothermal process and cooling water is needed to take away heat to reduce the temperature of adsorber bed. A cooling tower is utilized to provide cooling water. During the experiment, the temperature of cooling water is about 27 °C. The hot water that drives water evaporation in the desorption process is supplied by a hot water tank. The heat can be from solar collections or a ground source heat pump system. In this experiment, due to the restriction of experimental conditions, an electric heating water tank is used as heat resource instead of solar collectors and ground source heat pump. The temperature of outlet water can be easily controlled with a proportion integration differentiation (PID) controller. Before the operation, the pressure of shell side of falling-film evaporator is reduced to 6.7 kPa (All pressure values in this paper are absolute pressure). A mass recovery is conducted at the beginning of a complete cycle. In this process, bed A has just finished desorption but still in a higher temperature and dry status, while bed B has just finished adsorption but still in a lower temperature and saturated status. At this time, the outlet gas valves of two beds keep open and the inlet gas valves of two beds are closed. The pressures between the two beds are balanced, leading to that water desorbed from bed A flows into bed B and it is adsorbed by bed B. During mass recovery, the flowing states of hot water and cooling water are same as they were before mass recovery. Subsequently, the heat recovery is achieved by the liquid circulation and heat exchange between the two beds. In this process, bed B is heated and bed A is cooled. After that, hot water passes through bed B to initiate desorption process. When the vapor pressure of bed B is greater than that in the falling-film evaporator, the vapor flows into the tubes of falling-film evaporator and is condensed. At the same time, the seawater in the shell of the evaporator is heated and evaporated by the latent heat of vapor in the tubes. Meanwhile, cooling water passes through bed A to initiate adsorption process. When the vapor pressure of bed A is less than that in the shell of falling-film evaporator, the vapor produced from seawater evaporating flows into bed A and is adsorbed by the adsorbent. The remaining concentrated brine in the shell of falling-film evaporator is collected in brine storage and discharged. At the same time, the desalted water condensed in the tubes of falling-film evaporator is collected in the collection tank. The states of all valves in different period of the cycle are shown in Table 1. During the whole process, hot water and cooling water pass through
energy could be used in this kind of system [27,28]. Types of AD systems have been studied, including the type employing integrated evaporator-condenser device. However, there is no such an AD system that achieves heat and mass recovery between adsorber and desorber. This was the motivation behind the present study. Our study aims to (i) design and set up an experimental installation to investigate a “new” AD system, (ii) test performance of this system, (iii) test the quality of produced water. The “new” AD system employed in our study has the following originality: (i) a falling-film evaporator for seawater evaporation and fresh water condensation is operated in our experiment (ii) a heat and mass recovery circuit between adsorber and desorber bed is installed and operated. The present study has a great potential to guide applications of AD systems. Results obtained in our study provide valuable data for the design and evaluation of AD systems. 2. Experimental apparatus and methods The schematic diagram of our adsorption desalination system is shown in Fig. 1. The system comprises two major parts, (i) two beds where water vapor is adsorbed and desorbed and (ii) falling-film evaporator, an integrated evaporator-condenser, where the seawater evaporates and water vapor is condensed. The bed employs the fin-tube heat exchanger, and the gaps between fins are filled with silica gel. The two beds work as adsorber and desorber alternately. The falling-film evaporator is a shell-and-tube type heat exchanger. The seawater flows in the shell side, while the vapor in the tube side is condensed. The condensation of vapor is as the heat source for the evaporation of seawater in the shell side. In addition, the heat generated in adsorption is removed by circulating cooling water, while the hot water supplied to the desorption drives the vapor desorbed out of the adsorbent. The design of our AD system is referred to the study of Thu et al. [29], where the performance of an advanced AD cycle with condenserevaporator heat recovery scheme was modeled and predicted. However, their system used hot or cooling water to pre-heat the desorber bed or pre-cool the adsorber bed in switch interval between the half cycles. In our study, a heat and mass recovery circuit between two beds is utilized to reduce the waste of heat and increase the production of desalted water. The water tank in heat recovery circuit is used to eliminate the influence of water volume variation on pipes caused by temperature variation. The vapor is adsorbed and desorbed in the beds during the operation. The seawater is pumped from seawater tank into the falling-film evaporator and sprayed by the array of nozzles. The seawater evaporates in the shell side of falling-film evaporator. The vapor flows into
Fig. 1. Schematic diagram of the adsorption desalination system. 44
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In the present study, 12 indexes of desalted water are tested, such as chromaticity, pH, total hardness and volatile phenol, etc. The results are compared with Drinking Water Health Standards of China and desalted water produced by other traditional desalination technologies.
Table 1 State of valves in different period. Valve
GV-1 GV-2 GV-3 GV-4 LV-1 LV-2 LV-3 LV-4 LV-5 LV-6 LV-7 LV-8 LV-9 LV-10 LV-11
Period Mass recovery
Heat recovery
A as adsorber B as desorber
B as adsorber A as desorber
Off On Off On No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state No change of valve state Off Off Off
Off Off Off Off Off Off Off Off Off Off Off Off On On On
On Off Off On Off On Off On On Off On Off Off Off Off
Off On On Off On Off On Off Off On Off On Off Off Off
3. Performance analysis The adsorber bed is filled with graininess pore spherical silica gel (Type A silica gel). Silica gel is an amorphous substance, insoluble in water. And it does not react with other substances except hydrofluoric and strong base. In the process of designing adsorber bed, the thermodynamic equilibrium equation for silica gel-water adsorption is selected according to references [30, 31], and is given by B (Tsg )
P (Tw ) ⎤ x = A (Tsg ) ⎡ ⎢ P (Tsg ) ⎥ ⎣ ⎦
where A(Tsg) and B(Tsg) are given by
Table 2 Desired operation parameters of the AD system.
Hot water inlet (desorber) Cooling water inlet (adsorber) Seawater spraying Temperature difference of hot water inlet and outlet Temperature difference of cooling water inlet and outlet Temperature of vapor desorbed from bed
Temperature
Flow rate
83 ± 1 °C 27 °C 30 °C 5 °C 5 °C
1.17 m3/h 1.68 m3/h 0.18 m3/h
46 °C
Mass
Specific heat
29.17 kg 7.77 kg 5.27 kg 18.13 kg 36.72 m2
0.92 kJ/(kg·K) 0.384 kJ/(kg·K) 0.903 kJ/(kg·K) 0.896 kJ/(kg·K)
A (Tsg ) = A0 + A1 Tsg + A2 Tsg2 + A3 Tsg3
(3)
B (Tsg ) = B0 + B1 Tsg + B2 Tsg2 + B3 Tsg3
(4)
where x is the ratio of the equilibrium adsorbed amount to maximum adsorbed amount, Tsg is the temperature of silica gel, Tw is the temperature of water vapor, A0-A3, B0-B3 are all constant, whose fitting values are given by A0 = −14.2904, A1 = 0.1546 K−1, A2 = −5.5498 × 10−4 K−2, A3 = 6.7512 × 10−7 K−3, B0 = 36.1487, B1 = −0.3820 K−1, B2 = 1.3016 × 10−3 K−2, −6 −3 B3 = −1.4150 × 10 K . In this experiment, Type A silica gel was used as the adsorbent and the physical properties of that are shown in Table 5. For assessment of this AD system, two parameters are calculated, i.e., specific daily water production (SDWP) and performance ratio (PR). Both of them are given by
Table 3 Parameters of adsorber bed.
Silica gel Copper Aluminum alloy Aluminum Heat exchange area
(2)
SDWP = n
PR =
1 tcycle
∫0 ∫0
tcycle
tcycle
m water dt Msg
(5)
m water × h (Tcond ) dt Qdes
(6)
bed A and bed B alternately, and the adsorption process and desorption process are operated alternately. The desired operation parameters of the AD system are shown in Table 2, while parameters for bed are shown in Table 3. The data measured and recorded in this experiment includes three types, namely temperature, pressure and flow rate. The inlet and outlet temperatures and flow rates of beds are measured by heat meters, while other temperatures are measured by Pt100 thermometers. The uncertainty of the Pt100 thermometers is calculated by
where tcycle is the period of a cycle, n is the number of cycles per day in the continuous operation, mwater is the mass flow rate of desalted water, Msg is the total mass of silica gel in the bed, h(Tcond) is the latent heat of condensation at the temperature Tcond determined according to reference [32], Qdes is the energy required for desorption process. The adsorbent bed is enclosed in a cubic container with a 50 mm insulation layer of rubber and plastic material. Because of the good insulation effect, the heat transfer between the bed and ambient air can be neglected. Therefore, Qdes is the heat transfer between the desorber bed and hot water, and it can be calculated by Eq. (7)
UT = (0.15 + 0.002T ) K
Qdes = mhw cp, hw (Thw, in − Thw, out )
(1)
(7)
where mhw is the flow rate of hot water, cp,hw is the specific heat capacity of hot water, Thw,in and Thw,out are the inlet and outlet temperature of hot water, respectively. In heat recovery process, the beds are pre-heated and pre-cooled by the circulating water flowing between two beds, without power input by hot or cooling water. Therefore, both heat power for pre-heating the desorber bed and cooling power for pre-cooling the adsorber bed are taken into account in the calculation of recycled energy. It is noted that there is water adsorbed in the desorber bed while there is no water existing in the adsorber bed. The energy pre-heating the water in the desorber bed is also considered in the calculation of recycled energy. The energy recycled from heat recovery is calculated by Eq. (8). The efficiency of heat recovery is the ratio of maximum allowable heat
where T is the temperature measured by it. The pressure is measured by pressure gauge, whose range is from −100 kPa to 0 kPa and relative uncertainty is 0.5%. The relative uncertainty of heat meters is 2.5%. The test data is recorded under stable operation condition. Fig. 2 demonstrates the physical diagram of the experimental device. The seawater used in the experiment is produced by mixing local tap water with dried sea salt. The salinity of simulated seawater is 3.5%. Table 4 shows the properties of simulated seawater. In order to prevent non-condensable gas from interfering normal operation of the system, the seawater must be de-aerated before it is poured into the tank. The dried sea salt is sterilized before drying. In the whole process of seawater preparation, the ambient air is filtered by an air cleaner. 45
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Falling-film evaporator
Collection
Adsorber
Hot water tank
Fig. 2. Physical diagram of AD system.
Table 4 Properties of simulated seawater. Parameters
Value
Unit
pH Total dissolved solids (TDS) Total hardness Na Al Fe Cu Chloride Sulfate Sulfide Boron
8.5 30,071 6270 9993 0.209 0.177 0.004 18,440 2110 < 0.01 1.57
/ mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Table 5 Properties of Type A silica gel. Property
Value
Granular average diameter (mm) Average pore diameter (nm) Surface area (m2 g−1) Porous volume (cm3 g−1) Specific heat (kJ/kg K−1) Thermal conductivity (W m−1 K−1) Apparent density (kg m−3) True density (kg m−3)
0.5–1.5 2.0–3.0 650–800 0.35–0.45 0.92 0.175 790 2200
Fig. 3. Variation of pressure with time in each part during a complete cycle.
transfer rate between two beds to the actual heat transfer rate. The efficiency of heat recovery is calculated by Eq. (9).
Qrec = (Msg csg + Mco cco + Mal cal + MAL, alloy cAL, alloy )(|ΔTA| + |ΔTB|) + Mw c w ΔTdes
Table 6 Values of the parameters used in the performance calculation.
ε=
Parameters
Value
Unit
tcycle n h(Tcond) cp,hw mhw
0.5 48 2358.62 4.18 1136.63
h / kJ/kg kJ/(kg·K) kg/h
Qrec (Msg csg + Mco cco + Mal cal + MAL, alloy cAL, alloy )|TA, init − TB, init |
(8)
(9)
where Msg, Mco, Mal, MAl,alloy, Mw are the mass of silica gel, copper, aluminum, aluminum alloy, water adsorbed in bed respectively. Mw is equal to the water production of a half cycle. csg, cco, cal, cal, alloy, cw are the specific heat of corresponding materials. ΔTA and ΔTB are the temperature differences of bed A and bed B before and after the heat recovery, and TA,init, TB,init are the initial temperature of two beds in the process of heat recovery. ΔTdes is the temperature increase of desorber bed. The values of all the coefficients and the parameters used in the performance calculation are listed Tables 3 and 6. 46
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4. Results and discussion In the present study, the cooling water pump operates at a constant rotational speed, so the temperature and flow rate of cooling water remain constant during the experiment. In order to achieve the continuous coupling operation of the experimental AD system, the temperature and flow rate of hot water are also unchanged. In addition, the circulation of hot or cooling water will not be stopped until the adsorber bed is almost saturated. During two months of experiment, many attempts and adjustments were made to meet the requirements of the experiment. Nevertheless, the limited conditions lead to that there is only one condition achieving the coupling operation of the whole system. And in this condition, the system can operate stably for 5–6 h. Each cycle in stable operation condition is extremely same. Therefore, our discussion is aimed at a single cycle of 30 min. In the stable operation condition, seawater temperature, seawater spray flow rate, hot water inlet temperature and cooling water inlet temperature are 30 °C, 180 L/h, 83 ± 1 °C and 27 °C, respectively. In the present work, a complete cycle is 30 min. In a half cycle, the first 13 min is a heating/cooling process, then a 30 s of mass recovery process, finally a 90 s of heat recovery process. The schedule of first half cycle is the same as that of the second half cycle.
Fig. 4. Variation of temperature with time at each part of bed in a complete cycle.
4.1. Variation of pressure in each part When the system is in stable operation, the variations of pressure in each part are measured and presented in Fig. 3 (At the beginning of the cycle, bed A is adsorber and bed B is desorber). From Fig. 3, because of the intense adsorption of adsorber bed to water vapor, the pressure of adsorber bed is less than that in the shell side of falling-film evaporator. What's more, the seawater is easy to evaporate in the shell of falling-film evaporator because of that the pressure is much lower than standard atmospheric pressure. Then, water vapor flows into the adsorber bed. In addition, due to the cooling effect of seawater, the vapor in the tube is condensed into liquid water, leading to the pressure in the tube lower than that in desorber bed. This makes the vapor desorbed and flow into the tube consecutively. It is the variation of pressure in different parts that the AD system can operate persistently and stably. During the mass recovery process, the pressure of bed A and bed B tends to be balanced rapidly. In the followed heat recovery process, part of water vapor is desorbed from bed A which is heated by circulatory hot water, while the pressure of bed B begins to decrease due to the cooling water circulation in bed B.
Fig. 5. Variation of temperature with time in the falling-film evaporator in a complete cycle.
4.2. Variation of temperature at each part During the experiment, the variation of temperature at each part is measured and shown in Fig. 4. According to Fig. 4, in the end of heat recovery, the temperature of bed A increases from 30.8 °C to 44 °C, while the temperature of bed B decreases from 68.2 °C to 55 °C. This phenomenon indicates that part of heat from the desorber bed can be recycled by heat recovery, and cooling power as well as heating power can be saved. During heat recovery, the temperature of bed A has increased by 13.2 °C, while the bed B has decreased by 13.2 °C. According to Eq. (8), the mass and specific heat of different materials in Table 3 and the water production of a half cycle discussed in Section 4.3, the energy recycled from each heat recovery cycle Qrec is 1435.4 kJ. The heat recovery efficiency ε is 74.7%. The difference between inlet and outlet hot/cooling water temperature has decreased to 0.3–2 °C during 10–13 min, indicating that adsorption and desorption have approximately completed. The heat transfer between circulatory water and beds is very poor. However, the temperature difference between desorber bed and hot water approaches to 10–12 °C, indicating that the heat transfer between silica gel and the
Fig. 6. Variation of water production with time at each part in a complete cycle.
47
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Table 7 Comparison between the present and previous studies for SDWP. Present
Youssef et al. [17]
Thu et al. [29]
Alsaman et al. [5]
Ng et al. [9]
Wang et al. [33]
Thu et al. [34]
Method
Experiment
Simulation
Simulation
Experiment
Experiment
Experiment
Experiment
SDWP
4.69
6.64
23
3.95
7.2
4.7
8.2
decreases while the pressure of adsorber bed increases, indicating a second desorption and a second adsorption. Nevertheless, the vapor generating from second desorption is directly adsorbed by adsorber bed, which means that the water vapor of second adsorption/desorption is accounted as water production in next cycle. It is noted that the water production of the system cannot be improved by heat recovery process. In the steady condition of operation, when the time of adsorption/desorption is > 10 min, the increment of cycle period will lead to a decrease of water production per unit time. In the present experimental condition, the specific daily water production (SDWP) is 4.69 and the performance ratio (PR) is 0.766. Under the same experimental condition, the PR of Thu et al. [29] is around 0.7, from 0.698 to 0.708. It can be seen that the utilization of heat recovery circuit between adsorber bed and desorber bed can increase PR significantly. However, due to the fact that the ideal simulation conditions cannot be achieved in real experiment, the SWDP is lower than that in numerical simulation. Table 7 shows the comparison between the present and previous studies for SDWP. It is found that the result of present study is in agreement with the previous experimental study. However, due to the fact that the ideal simulation conditions are difficult to achieve in real experiment, SDWP of present study is lower than the numerical simulation results.
Table 8 The relative uncertainty of indirect measured value. Parameter
SDWP
PR
Qdes
Qrec
ε
Relative uncertainty
1.44%
2.46%
2.00%
0.54%
1.26%
fins of bed is poor. It is necessary to improve the heat transfer performance of the bed in the future work. The temperature variation with time in the falling-film evaporator is shown in Fig. 5. the adsorption and desorption processes are from 2 min to 15 min. At the beginning of adsorption and desorption (the second minute), the seawater is sprayed in the shell and there is not too much vapor is desorbed and flows in the tubes. Therefore, the heat transfer between seawater and vapor is poor. It is the time that temperature in tube and shell side is dominated by the original temperature of vapor and seawater. And it is the time that a minimum temperature in shell side appears, while a maximum temperature in tube side appears. After that, the temperature in shell side rises sharply resulted from the strong heat transfer and evaporation at lower pressure (4–6 kPa). Along with the heat transfer weakening, the temperature decreases in both sides and the temperature difference decreases. In the whole process, it can be seen that the temperature variation range of tube side is larger than that of shell side, manifesting that the heat transfer is mainly influenced by the amount of vapor in tubes.
4.4. Uncertainty analysis The system error mainly consists of three parts: system device error, data processing error and random error. Among those, the data processing error mainly derives from the calculation of formula, and it can be ignored in the present experiment. The random error can be eliminated by repeated measurements. SDWP, PR, Qdes, Qrec and ε are all indirect measured value, the uncertainties of which are from the measurement of temperature and water flow rate. The relative uncertainty of indirect measured value can be calculated by Eq. (10) and the results are shown in Table 8.
4.3. Variation of water production Water production rate is an important technical index of AD system. Fig. 6 shows accumulation of water production in 30 min. The total water production in 30 min is 2.85 kg. Therefore, the water production rate of this AD system is 5.7 kg/h. In Fig. 6, it shows that the water production is relatively stable during 10–13 min and 25–28 min, which means that water production per unit time is very small. As shown in Fig. 4, during the same period, the temperature difference of circulatory water is small, which indicates the adsorption or desorption process has basically completed. These two periods (10–13 min; 25–28 min) cause a reduction in average water production per unit time. At the stage of mass recovery, the pressure of desorber bed
Uy y
n
=
2 ∂f Uxi ⎞ ⎝ i y ⎠
∑ ⎛ ∂x ⎜
i=1
⎟
(10)
where y is a kind of indirect measured value and y = f (x1, x2, x3, …… xn,). x1, x2, x3, …… xn are direct measured value. Uxi is the uncertainty
Table 9 Comparison of water quality for different desalination methods and standard in China. Test items
Chromaticity pH TDS Total hardness Na Al Fe Cu Chloride Sulfate Sulfide Boron
Unit
Degree / mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Standard for drinking water quality
15 6.5–8.5 1000 450 200 0.2 0.3 1 250 250 0.02 0.5
Results The present method
Dang-gang power plant MSF
Guohua Cang-dong MED
Shengsi county RO
Tianjin Science Committee RO
5 6.98 13 10 0.78 0.0156 0.00633 0.00759 < 1.0 1.60 < 0.01 0.011
<5 6.5 < 10 <1 0.29 < 0.025 < 0.002 0.030 0.53 0.10 < 0.01 < 0.020
<5 6.7 < 10 1.02 0.42 < 0.025 < 0.002 < 0.002 0.73 0.15 < 0.01 < 0.020
<5 7.04 238 14.5 – < 0.010 < 0.3 < 0.010 110 6.22 – 0.72
0 6.5 308 26.09 – < 0.03 0.019 – 38.09 0.328 – 0.46
48
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4.5. Analysis of the quality of desalted water Table 9 presents the laboratory test results of desalted water quality. The comparison with Standard for drinking water quality of China (GB 5749-2006) and desalted water produced by typical desalination devices operating in China is also shown in Table 9. Table 9 shows that there is no significant difference of water quality between AD system and other desalination technologies, except for the RO in Tianjin and Shengsi. According to the results, TDS, chloride and boron of AD are better than that of RO apparently. Comparing Tables 4 and 9, it can be seen that all elements in the water has been significantly reduced, reaching a very low level. The pH of the AD method is closer to 7.0, much better than other methods. It is noteworthy that no silicon is detected in the product water. That's also a verification for the chemical stability of silica gel in the AD system. All results demonstrate that the quality of water produced by AD method has obvious advantages compared with other methods. However, the good desalination performance of the AD system means that the pollution of silica gel caused by impurity ions and solid particles is inevitable. In the silica gel-water adsorption system, the adsorption deterioration of silica gel due to the pollution is verified [35]. In the present work, the adsorption deterioration has not been detected after several experiments. Furthermore, it can be found that there is less sulfate of AD and other technologies except for RO. It is mainly due to that the water used for making up seawater comes from underground water that contains hydrogen sulfide, the hydrogen sulfide might be dissolved in the water and converted into sulfate. The comparing result shows that the desalted water produced by the AD system is up to the Standard for drinking water quality, and the main indexes are in consistent with that of the desalted water produced by conventional distillation desalination.
5. Conclusions The paper is performed to present a new advanced adsorption desalination system. The performance of this system has been investigated experimentally. The following conclusions can be drawn in our study: (i) The heat transfer between tube and shell side in falling-film evaporator is mainly influenced by the amount of vapor desorbed from adsorbents. The adsorption and desorption processes in bed is priority to control for the coupling operation of adsorption desalination system with integrated evaporator-condenser. (ii) Heat recovery cannot increase water production, but it can recycle energy. During a heat recovery process, the efficiency of heat recovery is 74.7%. The specific daily water production and performance ratio of this system is 4.69 and 0.766, respectively. The utilization of heat recovery circuit between adsorber bed and desorber bed can increase PR significantly. (iii) Compared to the Standard for drinking water of China (GB 57492006),it is clear that the quality of desalted water produced by the adsorption desalination system meet the Standard. The main characteristics is in consistent with that of the desalted water produced by conventional distillation desalination.
Acknowledgement This research was supported by the National Natural Science Foundation of China [grant number 51576137]. The authors would like to thank Professor G.Y. Chen for his collaboration and insightful discussions. 49
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