An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation

Journal Pre-proof An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation

Junming Zhou, Faming Wang, Nuruzzaman Noor, Xiaosong Zhang PII:

S0360-5442(19)32586-1

DOI:

https://doi.org/10.1016/j.energy.2019.116891

Reference:

EGY 116891

To appear in:

Energy

Received Date:

25 June 2019

Accepted Date:

31 December 2019

Please cite this article as: Junming Zhou, Faming Wang, Nuruzzaman Noor, Xiaosong Zhang, An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation, Energy (2019), https://doi.org/10.1016/j.energy. 2019.116891

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Journal Pre-proof

An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation Junming Zhou1,2, Faming Wang2,3, Nuruzzaman Noor2,Xiaosong Zhang1 1School of Energy and Environment, Southeast University, Nanjing , China1 2Materials Synthesis and Processing Lab, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China. 3Shool of Architecture and Art, Central South University, Changsha, China

Abstract： In this paper, a liquid regeneration method by vacuum membrane distillation (VMD) is proposed for the liquid desiccant air conditioning system(LDACs), and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on the membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The results show that the error between experimental and calculation results is reduced from less than 15% to less than 5% by the optimized calculation model. The temperature of feed solution has a great influence on the regeneration performance of VMD, and the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8-1.2% with the increase of regeneration temperature. The VMD regeneration process of LiCl solution is the result of Poiseuille flow and Knudsen diffusion, and Poiseuille flow dominates. Under the same regeneration capacity, the regeneration temperature of 30wt% LiCl solution is about 7 ℃ higher than that of 20wt% LiCl solution, and this temperature difference also increases as the target regeneration amount increases. In order to improve the regeneration effect of high concentration solution, the regeneration temperature can be increased appropriately. The water flux of the membrane decreases with the increase of the length of the membrane. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1-1.7 times higher than Type 2. Further, both the water flux and regeneration ability of the solution decrease first and then increase with the increasing number of membranes. Therefore, the reasonable selection of number of fiber membranes can significantly save materials and also improve the regeneration ability. Keywords Liquid dehumidification; experiment; VMD; Regeneration ability

1

Corresponding author E-mail address:[email protected](XS.Zhang)

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1. Introduction Rising comfort requirements and increasing population have led to a significant increase in the energy consumption for air-conditioning systems in the last few decades. Heating, Ventilating & Air Conditioning (HVAC) system consumes over 50% of building energy in hot and humid climates [1]. Liquid desiccant air conditioning systems (LDACs) have shown promising potential and application in refrigeration and air conditioning by lowering energy consumption and improving indoor humidity comfort level[2]. Compared with conventional air conditioners, LDACs has a higher air dehumidification capacity, which can effectively purify the treated air and has almost no pollution to the environment[3]. LDACs are usually composed of collectors, regenerators, dehumidifiers, liquid storage tanks, solution pumps, etc. [4]. The regenerator is one of the most important heat and mass transfer devices in the LDACs [5], and the efficient solution regeneration method can effectively improve the efficiency of the solution dehumidification air conditioning system. Existing thermal regeneration methods can be roughly divided into the air type and the boiling type. The principle of the air type regeneration is usually to contact the air and hot dilute solution in the packed tower, and the air takes away the water in the solution to realize the concentrated regeneration of the solution[6]. The temperature of heat source is relatively low. The concentration and efficiency of solution regeneration in this regeneration method are low, and the regeneration process is susceptible to environmental impact. Yin et al. [4, 7] carried out an experimental study on the solution desiccant evaporative cooling air conditioning system of a packed tower regenerator. It was found that the heat source temperature of the regenerator had an important influence on the regeneration performance. The results showed that the internal heat regenerator had a higher regeneration rate and energy utilization efficiency than the traditional adiabatic regenerator. Air-type solution regeneration depends heavily on the surrounding environment. Under high temperature or humid climate conditions, the regeneration effect often fails to meet the needs of dehumidification [8]. In contrast, the boiling type solution regeneration can reduce the dependence of the air conditioning system on the outdoor environment. Boiling type regeneration equipment usually distills its water by boiling the dilute solution. This regeneration method is little affected by environmental factors, but requires a high regeneration temperature. Membrane distillation (MD) is a membrane separation technology combination of thermal evaporation and membrane separation[9]. The hydrophobicity of the membrane prevents mass transfer of the liquid, where by a gas-liquid interface is created.Vapor pressure difference caused by liquid temperatures on both sides of the membrane or vacuum pump on one side, where by volatile components in the supply mix evaporate through the pores (10nm –1µm). For feed solutions that only contain non-volatile substances, such as salts, water vapour will be transported through the membrane whereby the demineralised water is obtained on the distillation-side and a further concentrated salt flow on the feed side[10]. In addition, in comparison with Reverse Osmosis(RO), MD is less susceptible to flux limitations caused by concentration polarisation, whereby a higher concentration of matter is obtained on the feed side. Theoretically, MD offers about 99% retention for non-volatile dissolved substances, whereby there is no limit of the supply concentration[11].At present, MD technology has four basic operating modes, namely direct contact membrane distillation(DCMD), air gap membrane distillation(AGMD), scavenging membrane distillation(SGMD), and vacuum membrane distillation(VMD)[12]. Among them, the

Journal Pre-proof water flux of VMD is usually higher than that of other MDs [13]. In addition, VMD has the advantage of low heat loss, because heat does not pass through the membrane through vacuum. Therefore, the heat transfer through the membrane can be neglected [14, 15]. VMD is a process driven by both heat and pressure. The membrane separation of the solute and the solvent is achieved by using the steam pressure difference on both sides of the hydrophobic membrane as the driving force[16]. Compared with the traditional thermal regeneration process, MD not only needs a lower heat source temperature (40-80 ℃), but also can be operated stably in high temperature and humidity environments[17]. Membrane distillation can evaporate at a lower temperature (than the boiling temperature), so it can utilize industrial waste heat, geothermal, solar energy and other cheap types of energy[10]. At present, research of membrane distillation technology is basically in the process of seawater desalination and wastewater treatment, but there is a lack of research in LDACs. Membrane distillation regeneration in LDACs is different from seawater desalination in drinking water production, which involves removing only a small amount of water in a high concentration solution. Bodell et al. first introduced membrane distillation in 1963, using microporous hydrophobic membrane evaporation to provide clean water[14]. Duong et al.[18] examined the application of membrane distillation (MD) in the regeneration of the air-conditioned lithium chloride liquid desiccant. The process can increase the concentration of lithium chloride to 29 wt.% at the feed temperature of 65 °C without any significant loss of lithium chloride. Zhou et al. [19] studied the regeneration performance under various operating parameters and climatic conditions through model simulation, and compared the solar energy consumption of regenerating 1 kg desiccant between VMD regeneration and TH regeneration. The results showed that VMD regeneration is more suitable for solution regeneration in wet areas than the TH regeneration. Lefers et al [20]. Described the testing of a vacuum membrane distillation system for liquid dehumidification. It was found that 30 wt% magnesium chloride solutions obtained an average flux of 8 kgm-2h-1 at a temperature of 50 ℃. Previous studies have mainly discussed the feasibility of solution regeneration by the membrane distillation technology. However, few studies have been conducted on the influence of different factors on the performance of VMD liquid regeneration. Therefore, in this work, we performed an experimental study to examine the various factors on the VMD liquid regeneration performance and it is believed that our work will certainly promote the application of VMD liquid regeneration in LDACs.

2. Methods 2.1 Materials and test equipment 2.1.1 Materials Membrane material: Fiber membrane is PTFE membrane (manufacturer: Nanjing Bidun New Membrane Co., Ltd., Nanjing, China). The specific parameters of the membrane are shown in Table 1. Table 1 The specific parameters of the membrane Parameter

Value

Parameter

Value

Membrane type No. of fibers Inner diameter (mm) Outer diameter (mm)

PTFE 4,6,10 0.70 1.55

Porosity ε (%) Effective fiber length, Type 1 (m) Effective fiber length, Type 2 (m) Average aperture

65 0.52 0.24 0.2um

Journal Pre-proof 2.1.2 Experimental device The experimental device consists of a VMD module, constant temperature water tank, peristaltic pump, condenser, collection bottle and a water ring vacuum pump. The VMD regeneration system is shown in Fig.1. The PTFE film assembly is sealed by high temperature resistant epoxy resin. Major equipment and instruments used for the experiments are listed in Table 2. Table 2. Major equipment and instruments Device name

Number

Specification

Remark

Circulating Water Vacuum Pump

1pc

Pressure sensor

1pc

Shanghai Lichen Bangxi Instrument Technology Co., Ltd. Asmik

Thermocouple

5m

Pumping capacity:10L/min,Pow er:180W MIK-P300,measuring range:-100kPa–0kPa, Output：:4-20mA TT-T-36

Constant temperature water tank Liquid densitometer Conductivity Measuring Instrument Blender Electronic balance Data acquisition instrument

1pc

HH-2, Power:600W

1pc 1pc

DA-130N MP515

Shanghai Jiangxing Instrument KEM Shanghai Sanxin

1pc 1pc 1pc

JJ-1 TS20001, range 0-2kg 34970A

Lichen Shanghai Jiming Agilent

Blender

Snake tube condenser Computer

Electronic balance Constant temperature water tank Vacuum Pump

Pressure regulating bottle

Condensate collection bottle

Schematic diagram

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Experiments diagram PTFE membrane module Fig.1 The schematic diagram of VMD liquid regeneration. 2.2 Test procedures and analytical methods 2.2.1 Test procedure The glass bottle containing lithium chloride solution is placed into a water tank which is maintained at constant temperature to control the temperature. Then the PTFE membrane module is put into the glass bottle and the solution is stirred by a blender. When the solution temperature reaches the set temperature, the water ring vacuum pump is turned on. The vacuum pressure in the PTFE film is adjusted and maintained at a constant value through the vacuum pump and the valve. Agilent Data Acquisition Instrument (34970A) was used to collect the data of pressure sensor (error: ±0.01 kPa, range: -100~0kPa), temperature sensor (error: ±0.5℃, range: 0 ~150℃ ). The dilute solution is regenerated in a glass bottle, and the density of the solution is measured by densitometer (KEM). The water vapor through the membrane distillation unit is cooled by a snake tube condenser and then becomes condensate and enters the collection bottle. The quality of condensate water is measured by an electronic balance (error: ±0.01g, range: 0~2kg), and the conductivity of condensate water is measured by a conductivity meter. Every time a certain operating condition (solution temperature, solution concentration, vacuum, membrane parameters, etc.) is changed, the sample will be taken after the system is stabilized. After one hour, the regenerated solution will be taken out from the tank and is then tested at room temperature. 2.2.2 Analytical methods The experiment of VMD solution regeneration using PTFE membrane was carried out. The experimental indexes were defined as follows: membrane flux, mass transfer coefficient, regeneration ability and rejection rate.

J

m AAt

(1)

Where, J is the water flux of VMD process (kg/m2·h), m is the mass difference before and after regeneration(kg), A is the effective area of the membrane (m2). The water flux of VMD is proportional to the vapor pressure difference on both sides of the membrane, and the mass transfer coefficient can be expressed as:

Km =

J ( Pfm  Pp )

(2)

Journal Pre-proof Where, Km is the mass transfer coefficient of the membrane (kg/m2·h·Pa), Pfm and Pp are the vapor pressure of the solution on the membrane side and vacuum pressure, respectively. The vapor pressure of Milli-Q water can be calculated by the Antoine equation[21].

P0  exp(23.238 

3841 ) T  45

(3)

Lithium chloride solution has a non-volatile solute, which reduces the vapor pressure of water. If the solution concentration is high, it can be modified according to the following Eq(4)[22]:

P  P0(1  x)(1  0.5 x  10 x 2 )

(4)

Where, x is the molar fraction of the solution. The regeneration ability of VMD process is evaluated by the increase of LiCl concentration, and can be obtained by using the Eq(5)[23]:

X  X after -X before 

X A(mafter -mbefore ) mafter

(5)

Where, X,m are the concentration (%) and mass (kg) of the solution respectively. Subscripts after and before are the States before and after the VMD process, respectively. In this paper, the conductivity of raw material and condensate is measured by a conductivity meter, and the ion interception rate of VMD process is calculated by the following equation



h  c h

Where,  h is the conductivity of the raw liquid (μS/cm) and

(6)

c is the conductivity of the

distillate(μS/cm).

3 Results and discussion 3.1 Optimization of membrane flux calculation Different mechanism models have been proposed from different perspectives, such as the Knudsen flow model and the viscous flow model, but their application is limited to a certain extent. According to the characteristics of vacuum membrane distillation, some researchers combined the above two models to obtain membrane flux formulas in different ranges. The diffusion mechanism of the gas in the membrane during VMD regeneration is determined by the mean free path λ of the molecular motion and the size of the membrane pores. The mean free path can be calculated by formula



k BT P 2 2

(7)

When the pore radius r<0.05λ, the molecule-pore wall collisions are dominant in comparison with the molecule-molecule collisions, and Knudsen-type diffusion of the vapour molecules through the membrane pores is applied using the following equations[24, 25].

J

2 8M 12  r ( ) ( Pfm  Pp ) 3  RT 

(8)

Journal Pre-proof Where, τ is pore tortuosity of the membrane. The most successful correlation was suggested by Macki-Meares [26]



(2   ) 2



(9)

When 0.05λ
 2 8M 12  r 1  r 2 MPavg  J  ( )   ( Pfm  Pp )  3  RT  8  RT 

(10)

The above membrane flux is divided into different zones by molecular free path, but the effect of molecular free path on flux is not considered in different calculation intervals. When the operating conditions change, the average temperature and average pressure of gas in the membrane will change. This makes the average movement free path of gas molecules change, and the mass transport mechanism in the membrane will also change. In order to optimize the formula, we introduce different coefficients, i.e.,

J  (Ck K k  C p K p )( Pfm  Pp ) Where, Ck 

 (r   )

is the weight coefficient of Knudsen flow, C p 

(11)

r is the (r   )

weight coefficient of Poiseuille flow. It should be noted that the theoretical calculation of J can be excluded because water flux calculation using C involves the water vapour pressure at the membrane surfaces[28]. Because there are concentration polarization and temperature polarization in VMD process, the main body temperature cannot be used to replace the film surface temperature, nor can the saturated vapor pressure of the main body be used to replace the saturated vapor pressure of the film surface. In order to verify the formula, we use Milli-Q water to carry out VMD experiments. In the process of VMD using Milli-Q water, concentration polarization cannot be considered. In the experimental process, the blender is used to agitate quickly to reduce the temperature polarization of membrane surface. Therefore, concentration polarization and temperature polarization are not considered in this process. VMD experiments were carried out on four type 1 and type 2 membranes. Vacuum pressure was maintained at 4.7 kPa by vacuum pump. As can be seen from Fig. 2, the calculated results are basically the same as the experimental results under different types of membranes, and the maximum error is less than 15%. This shows that both traditional and optimized formulas can be applied to the VMD membrane flux calculation to obtain accurate results.

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14

Calculated value(kg/m2.h)

12 10 8 6 4

Traditional calculated value(Type1) Optimized calculated value(Type1) Traditional calculated value(Type2) Optimized calculated value(Type2)

2 0

0

1

2

3

4 5 6 7 8 9 10 11 12 13 14 Experimental value(kg/m2.h)

Fig. 2 Comparisons between calculated and experimental values of water flux In this paper, the regeneration of VMD solution by the PTFE fiber membrane was studied. The set values of parameters during the experiment are shown in Table 3. Table 3.

The set values of parameters during the experiment

parameter

value

parameter

value

Solution temperature/℃ solution concentration Vacuum pressure/kpa

55-85

Membrane length/m

0,20%,30%

Number of membranes

0.52(Type1),0.24(Typ e2) 4,6,10

4.7,range:3.7-7

Fig. 3(a) displays the flux and mass transfer coefficients of VMD process in Milli-Q water were analyzed based on four Type1 membrane materials. When the temperature of Milli-Q water rises from 55 to 85 °C the VMD water flux increases exponentially. This is because with the increase of solution temperature, the vapor pressure of solution increases exponentially, the vapor pressure difference on both sides of the membrane increases, and the flux of the membrane increases exponentially. However, with the increase of temperature, the error of the traditional formula will also increase. When the temperature is 55 °C, the experimental flux, the calculation results of the traditional formula and the optimization formula are close to 3.1 kg/m2h. When the temperature is 85 °C, the experimental flux is the lowest, which is 10.1 kg/m2h. The calculated results of traditional formula and optimized formula are 12 and 10.7 kg/m2h, respectively. Hence, the optimized formula can improve the accuracy of calculation. Fig. 3 (b) shows the effect of liquid temperature on the membrane mass transfer coefficient. In Fig. 3 (b), it can be seen that the mass transfer coefficient of the membrane calculated by the traditional method has little effect on the change of temperature. The optimized results are quite close to the experimental results and they decrease with the increasing temperature of Milli-Q water. As mentioned above, the transport of water vapor molecules in micropores can be divided into Poiseuille flow and Knudsen diffusion. When the transfer of vapor molecules in the pore is

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14 13 12 11 10 9 8 7 6 5 4 3 2 50

Milli-Q water Traditional calculated value Optimized calculated value

55

60 65 70 75 Liquid temperature(°C)

80

85

Mass transfer coefficient(10-4kg/m2.h.Pa)

Calculated value(kg/m2.h)

dominated by the Knudsen diffusion, Km decreases with the increase of temperature. When the transfer of vapor molecules in the pore is dominated by the Poiseuille flow, the corresponding Km increases exponentially with the increase of temperature. According to the calculated Km value, the experimental Km decreases with the increase of temperature, so the mass transfer process is dominated by the Knudsen diffusion. 2.8 2.7 2.6

Milli-Q water Traditional calculated value Optimized calculated value

2.5 2.4 2.3 2.2 2.1 2.0 50

55

60 65 70 75 Liquid temperature(°C)

80

85

(a) Water flux and (b) Mass transfer coefficient Fig. 3. (a) Water flux and (b) process mass transfer coefficient (Km) of the VMD process with Milli-Q water at various feed temperature. 3.2 Effect of Temperature and Concentration on Regeneration Performance In practice, it is difficult to measure the surface temperature of the membrane. It is more practical to calculate the water flux by using the feed steam pressure and the vacuum pressure. With this method, the water flux of VMD can be calculated as follows [29]:

J  K m' ( Pf  Pp )

(12)

'

Where, K m is process mass transfer coefficient (L/m2 h Pa), Pf is water vapor pressure (Pa) of feed. VMD experiments were carried out with four fibrous membranes (Type1) and the vacuum was maintained at 4.7 kPa. Selection of lithium chloride (LiCl) solution as dehumidifier. Fig. 4 (a) illustrates the variation of experimental and calculated values of VMD membrane flux with the feed temperature and concentration. The membrane flux of Milli-Q water, 20wt% and 30wt% LiCl solutions increased with the increase of feed temperature. When the concentration of solution '

is 0, the experimental value of membrane flux in VMD process were measured and hence, K m can '

be solved by Eq 12. Based on the K m value obtained from Milli-Q water as the feed solution, the water flux of other LiCl solutions in VMD process was calculated using Eq 12 and comparisons with the experimental value were made. VMD water flux with 30 wt% LiCl solution feed is significantly lower than the initial Milli-Q water flux obtained during the process using Milli-Q water feed under the same operating conditions, which is also lower than the calculated value from Eq 12. It is also noted that the difference between experimental and calculated values increases with the increase of liquid temperature. The main reason for this phenomenon is that the

Journal Pre-proof partial pressure of water vapor in the feed solution is used to replace the partial pressure of water vapor on membrane surface in Eq 10, and the effect of concentration polarization is neglected. This reduction reveals the importance of concentration polarization effect during the VMD regeneration of LiCl liquid desiccant. The effect of temperature and concentration on mass transfer coefficient of VMD is '

'

demonstrated in Fig.4(b). The K m value calculated from the Eq. 12 shows that the K m value '

of Milli-Q water decreases with the increase of liquid temperature, whereas the K m value of 20wt% and 30wt% LiCl solution increases with the increase of liquid temperature. As mentioned '

above, the Knudsen diffusion dominates the process of VMD in Milli-Q water. However, K m of 20wt% and 30wt% LiCl solution is not exponentially correlated with the increasing liquid temperature, thus, it is not a single Poiseuille flow process. In VMD process, there are very few air molecules in the membrane micropores, and the corresponding molecular diffusion has little effect on mass transfer process. Comprehensive analysis shows that the Knudsen diffusion plays a role in the mass transfer process. Therefore, it is considered that the mass transfer process is the result of the interaction of Poiseuille flow and Knudsen diffusion, and the Poiseuille flow is dominant. It can be seen from Fig. 4 (c) that the membrane rejection rate is little affected by the feed temperature and concentration. Due to the good hydrophobicity of PTFE fiber membrane, the conductivity of condensate produced by VMD is less than 1000 uS/cm, and the retention rate of lithium chloride is over 94%. The interception effect is obvious. It can also be seen from Fig. 4 (c) that the regeneration ability of VMD solution varies with the feed temperature and concentration. When the solution temperature increases from 55 to 85 ℃, the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8-1.2% with the liquid temperature. This is because the surface vapor pressure increases with the increase of solution temperature, which increases the driving force of heat and mass exchange between two sides of the PTFE film, and this makes the regeneration effect enhance the concentration of solution. It also directly leads to the increase of the concentration difference between the outlet and the inlet of the solution and the enhancement of the regeneration ability. At the same temperature, the regeneration ability of LiCl with 20 wt% concentration is higher than that with 30 wt% concentration. This is because with the increase of the inlet concentration of regenerator, the partial pressure of surface water vapor decreases, the driving force of regenerator solution regeneration decreases, and the latent heat required for water evaporation decreases as well. Thus, the solution temperature increases with the increase of concentration. It is not difficult to find that the regeneration temperature of the 30 wt% solution is about 7℃ higher than the regeneration temperature of the 20 wt% solution, and this temperature difference also increases as the target regeneration amount increases.. In order to regenerate high concentration solution, the regeneration temperature can be increased appropriately.

11 10 9 8 7 6 5 4 3 2 1 0 50

Mass transfer coefficient(10-4kg/m2.h.Pa)

Water flux(kg/m2.h)

Journal Pre-proof Milli-Q water(Experimental) 20wt% solution(Experimental) 30wt% solution(Experimental) 20wt% solution(Calculated) 30wt% solution(Calculated)

55

60

65 70 75 80 Liquid temperature(°C)

85

(a) Water flux

90

2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 50

Milli-Q water 20wt% solution 30wt% solution

55

60

65 70 75 80 Liquid temperature(°C)

85

(b) Mass transfer coefficient 1.6

100

Regeneration ability(wt%)

Interception rate(%)

1.4 80

1.2

Interception rate(20wt%) Interception rate(30wt%) Regeneration ability(20wt%) Regeneration ability(30wt%)

60

1.0 0.8

40

0.6 0.4

20

0.2 0

55

60

65 70 75 80 Liquid temperature(°C)

85

90

0

(c) Interception rate and Regeneration ability Fig. 4. Variation of experimental and calculated values of VMD process with feed temperature and concentration 3.3 Effect of Fiber Membrane Parameters In the process of fabrication, the thinner the membrane wall is, the higher the requirement of fabrication technology and the lower the compressive capacity. Therefore, the fiber membrane is related to the fabrication process in the factory. In this paper, the effects of parameters such as length and quantity on the performance of VMD process are studied. 3.3.1 Effect of fiber membrane length The VMD regeneration performance of four fibre membranes with lengths of 0.52m (Type1) and 0.24m (Type2) were compared at solution concentration of 30wt%. Fig. 5 (a) shows the flux and mass transfer coefficients of two different length fiber membranes varying with temperature. As can be seen from Fig. 5(a), when the regeneration temperature is the same, the longer the membrane length, the smaller the water flux and also, the smaller the mass transfer coefficient. For example, when the regeneration temperature is 65 °C, the water flux of Type 1 long membrane and Type 2 short membrane are 0.86 kg/m2h, 1.26kg/m2h, and the mass transfer coefficients are 8.16×10-5 and 1.2×10-5kg/m2hPa, respectively. This is because when the dilute solution is regenerated on the membrane surface driven by the vacuum pump, the water vaporizes on the membrane surface and the water vapor is transferred to the tube through the membrane. When the length of the fiber membrane tube increases, the ability of the vacuum pump to maintain

90

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Water flux(Type1) Water flux(Type2) Mass transfer coefficient(Type1) Mass transfer coefficient(Type2)

Water flux(kg/m2.h)

5 4

1.8 1.6 1.4

3

1.2 1.0

2

0.8

1 0

0.6

55

60

65 70 75 80 Liquid temperature(°C)

85

0.4

90

(a) Water flux and Mass transfer coefficient

1.6

100

1.4 80

1.2

Interception rate(Type1) Interception rate(Type2) Regeneration ability(Type1) Regeneration ability(Type2)

60

1.0 0.8

40

0.6 0.4

20

0.2 0

55

60

65 70 75 80 Liquid temperature(°C)

85

90

0

(b) Interception rate and Regeneration ability

Fig. 5. Effect of Fiber Membrane Length on VMD Performance. 3.3.2 Effect of the number of fiber membranes When the concentration of solution was 30wt%, the water flux and regeneration ability of 4, 6 and 10 fibrous membranes at 55-85 °C were compared by using Type2 fibrous membranes. When the number of membranes increased from 4 to 10, other parameters remained unchanged as shown in Table 3. The results of flux, mass transfer coefficient, interception rate and regeneration ability of VMD regeneration process are shown in Fig. 6(a). At the same regeneration temperature, if the temperature is 75 ℃, the water flux of 4, 6 and 10 fibrous membranes are 2.21, 1.3 and 1.48 kg/m2h, respectively. It can be seen that when the number of fiber membranes is 4, the moisture passing through the membranes per unit area of time is the most. When the number of membranes increases from 4 to 10, the water flux decreases first and then rises. This is mainly due to the fact that when the number of membranes is the minimum, the fibre membranes do not affect each other independently and more amount of water is allowed to pass through the membrane per unit area of time. When the number of fibrous membranes increases, the interaction between the membranes reduces the water flux and mass transfer coefficient per unit area of time. However, as the number of membranes continues to increase, the interaction between the fibre membranes is limited, which forces the water flux of the membranes rise. Fig. 6 (b) shows that when the solution temperature is 75 ℃, the regeneration ability of the fiber membrane solution with the number of 4, 6 and 10 is 0.27%, 0.24% and 0.41%, respectively. It is also well understood that the larger the number of fibrous membranes, the larger the effective area of the membrane surface. However,

Regeneration ability(wt%)

2.0

Interception rate(%)

6

Mass transfer coefficient(10-4kg/m2.h.Pa)

the vacuum pressure in the fiber membrane tube decreases, hence, the amount of water passing through the membrane per unit area of time decreases. In Fig. 5 (b), it is not difficult to find that the longer the membrane length, the greater the regeneration capacity of VMD. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1-1.7 times higher than Type 2. This can be explained that when the length of the fiber membrane increases, the larger the effective area of the inner surface of the membrane, the more the total heat and mass transfer of VMD regeneration, and the higher the concentration of solution after regeneration. However, when the regeneration temperature is low, both types of regeneration capacity are reduced and their differences are not significant. The interception rate of solute by fibrous membranes changed little with the length of membranes, and remained above 93%.

Journal Pre-proof the regeneration capacity does not increase linearly with the number of fiber membranes due to the change of membrane flux. With the increase of the number of fibre membranes, the regeneration ability of VMD decreased slightly at first and then increased. Therefore, according to different processes, reasonable selection of the number of fiber membranes can not only save materials but also improve the regeneration ability. In addition, when the regeneration temperature is low, different types of regeneration capacity are reduced and tend to be the same. The number of fiber membranes has little effect on the retention of solute, and the retention rate is still above 93%.

1.6 1.4

4

1.2

3

1.0 0.8

2

0.6

1

0.4

0

0.2 55

60

65 70 75 80 Liquid temperature(°C)

85

90

0

1.8 1.6

80

Interception rate(4 fibers) Interception rate(6 fibers) Interception rate(10 fibers) Regeneration ability(4 fibers) Regeneration ability(6 fibers) Regeneration ability(10 fibers)

60

1.4 1.2 1.0 0.8

40

0.6 0.4

20

0.2 0

55

60

65 70 75 80 Liquid temperature(°C)

85

0 90

(a) Water flux and Mass transfer coefficient (b) Interception rate and Regeneration ability Fig. 6 The influence of the number of fibre membranes on the performance of VMD 3.4 Effect of Regeneration Pressure Four Type2 fiber membranes were used to regenerate 30wt% LiCl solution. The regeneration effect under different vacuum pressure was analyzed when the average feed temperature was heated to 66 ℃. Vacuum pressure creates a negative pressure environment for the regeneration process of membrane distillation, so that the regeneration process of VMD is no longer affected by the surrounding high temperature and humidity air environment. Therefore, the vacuum pressure will have an important impact on the regeneration process. Fig. 7 (a) illustrates the effect of vacuum pressure on the membrane flux and mass transfer coefficient. The experimental results show that when vacuum pressure increases from 3.1 kPa to 7.1 kPa, the vapor pressure difference on both sides of the membrane decreases with the increase of vacuum pressure, and the driving force of membrane mass transfer decreases. Thus, the water flux of the membrane decreases from 1.46 to 0.82 k/m2h. Membrane flux is proportional to the pressure difference, which is also shown by theoretical inference. The effect of vacuum pressure on the mass transfer coefficient of the membrane is small. In addition, as shown in Fig. 7(b), the regeneration capacity of VMD decreases from 3wt% to 2.1wt% with the decreasing vacuum pressure. It can be deduced that with the decrease of vacuum pressure, the trans-membrane driving force (i.e. the pressure difference between the gas-liquid interface layer and the main body of the gas phase) decreases, which shows that vacuum pressure directly affects the regeneration process of the membrane. For the interception rate, with the increase of the cold side vacuum, the change of the interception rate is very small. This is mainly because the rejection rate depends on the structure and performance of the membrane itself. In contrast, the pressure has little effect on the rejection rate.

Regeneration ability(wt%)

5

1.8

100

Interception rate(%)

6

Water flux(kg/m2.h)

2.0

Water flux(4 fibers) Water flux(6 fibers) Water flux(10 fibers) Mass transfer coefficient(4 fibers) Mass transfer coefficient(6 fibers) Mass transfer coefficient(10 fibers)

Mass transfer coefficient(10-4kg/m2.h.Pa)

7

100

1.4

1.6

1.3

1.4 1.2

1.2

1.0

1.1

0.8

1.0

0.6

Water flux Mass transfer coefficient

0.9

0.4 0.2

0.8 3.5

4.0

4.5

5.0

5.5

6.0

Vacuum pressure(kPa)

6.5

7.0

0

7.5

80

Interception rate(%)

Water flux(kg/m2.h)

1.8

Mass transfer coefficient(10-4kg/m2.h.Pa)

2.0

1.5

60 40 20 0

Water flux Mass transfer coefficient

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 7.5

Vacuum pressure(kPa)

(a) Water flux and Mass transfer coefficient (b) Interception rate and Regeneration ability Fig. 7 Effect of Vacuum Pressure on VMD Performance

4. Conclusions In this paper, a liquid regeneration method by VMD is proposed for the LDACs, and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The specific conclusions are as follows: 1.Through analysis, the mathematical model of VMD membrane flux was optimized, and its accuracy was verified according to the VMD experimental data. The results show that the error between the experiment and the calculation results is reduced from less than 15% to less than 5% by optimizing the calculation model through the VMD experiment of Milli-Q water. 2.The temperature of feed solution has a great influence on the regeneration performance of VMD, and the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8-1.2% with the increase of regeneration temperature. The VMD regeneration process of LiCl solution is the result of Poiseuille flow and Knudsen diffusion, and Poiseuille flow dominates.Under the same regeneration capacity, the regeneration temperature of 30wt% LiCl solution is about 7 ℃ higher than that of 20wt% LiCl solution, and this temperature difference also increases as the target regeneration amount increases. In order to improve the regeneration effect of high concentration solution, the regeneration temperature can be increased appropriately. 3.The water flux of the membrane decreases with the increase of the length of the membrane. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1-1.7 times higher than Type 2. Further, both the water flux and regeneration ability of the solution decrease first and then increase with the increasing number of membranes. Therefore, the reasonable selection of number of fiber membranes can significantly save materials and also improve the regeneration ability. Vacuum pressure has a great influence on membrane water flux and solution regeneration capacity, but has little effect on retention rate.

Acknowledgments The work of this paper is financially supported by the National Natural Science Foundation of China (No. 51520105009), the Scientific Research Foundation of Graduate School of Southeast University (No. YBPY1912), Postgraduate Research & Practice Innovation Program of Jiangsu

Regeneration ability(wt%)

Journal Pre-proof

Journal Pre-proof Province(No. KYCX19_0100). The supports are gratefully acknowledged.

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Journal Pre-proof We have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Journal Pre-proof Highlights   

A liquid regeneration method by vacuum membrane distillation is proposed for LDACs. The effects of various influence parameters on regeneration ability were studied. The mathematical model of membrane flux was optimized, and its accuracy was verified.