Study on the performance of double-pipe air gap membrane distillation module

Desalination 396 (2016) 48–56

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Desalination journal homepage: www.elsevier.com/locate/desal

Study on the performance of double-pipe air gap membrane distillation module Zhiyu Liu a,b,c, Qijun Gao a,b,⁎, Xiaolong Lu a,b,c,⁎⁎, Lihua Zhao a,b,c, Song Wu a,b,c, Zhong Ma a,b,c, Hao Zhang a,b,c a b c

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, PR China Institute of Biological and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A novel double-pipe air gap membrane distillation module was developed successfully. • The air gap width of the module can be evenly distributed and easily regulated. • The concept of equivalent membrane distillation flux (JAGMD) was firstly introduced. • The maximum value of J, GOR and JAGMD could reach 11.4 kg/(m2·h), 6.6 and 29.6 kg/(m2·h), respectively.

Configuration of DP-AGMD-M.

a r t i c l e

i n f o

Article history: Received 28 January 2016 Received in revised form 22 March 2016 Accepted 25 April 2016 Available online xxxx Keywords: Air gap membrane distillation Double-pipe module Air gap width Equivalent membrane distillation flux PVDF hollow fiber membrane

a b s t r a c t Air gap width has an important influence on the performance of air gap membrane distillation (AGMD) process. In this study, a novel double-pipe AGMD module (DP-AGMD-M) consisted of PVDF hollow fiber membrane and heat exchange capillary copper tubes was successfully developed. The evenly distribute and easily control of the air gap width in DP-AGMD-M can be implemented. The effect of the air gap width (da), hot feed temperature (T1), hot feed flow rate (Q), temperature difference (ΔT) between the hot feed outlet temperature and the cold feed inlet temperature, and the effective membrane module length (L) on the performance of DP-AGMD-M were experimental studied. The concept of the equivalent membrane distillation flux (JAGMD) was firstly introduced in this paper and used to evaluate the comprehensive performance of DP-AGMD-M. The optimal performances, including membrane distillation flux (J), gained output ratio (GOR) and JAGMD were obtained. Within the experimental range, the maximum J arrived at 11.4 kg/(m2 · h), the maximum GOR reached 6.6 and the maximum JAGMD was 29.6 kg/(m2 · h). © 2016 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Correspondence to: Q. Gao, State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, PR China. ⁎⁎ Correspondence to: X. Lu, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. E-mail addresses: [email protected] (Q. Gao), [email protected] (X. Lu).

http://dx.doi.org/10.1016/j.desal.2016.04.025 0011-9164/© 2016 Elsevier B.V. All rights reserved.

Membrane distillation (MD) technique is a kind of membrane separation technology combines with traditional distillation process. The driving force for mass transfer is the vapor pressure difference of volatile molecules across the hydrophobic micro-porous membrane [1]. Compared to other separation techniques, MD has some unique advantages:

Z. Liu et al. / Desalination 396 (2016) 48–56

higher rejection and concentrating multiple, and lower operating temperature. With these advantages, MD has great potential on the application for wastewater resource utilization, water conservation and emission reduction, especially in the field of deep concentration of industrial wastewater [2–6]. MD process has be divided into five basic configurations depending on the ways to collect or remove the transported vapor from the permeate side: (i) direct contact membrane distillation (DCMD), (ii) air gap membrane distillation (AGMD), (iii) vacuum membrane distillation (VMD), (iv) sweeping gas membrane distillation (SGMD) and (v) osmotic membrane distillation (OMD) [7–12]. However, the biggest barrier in the large-scale application of MD process is the high thermal energy consumption required for evaporation. The energy efficiency of an evaporation-based separation process (such as multiple effect distillation (MED) and MD) is commonly measured as “gained output ratio” (GOR). The GOR of typical thermal desalination techniques (such as MED) can reach more than 15.0 [13,14]. Unfortunately, the value of GOR for traditional MD process is just 0.2– 1.0 [15,16]. Besides, MD needs to consume a large amount of cooling water to condense vapor into fresh water completely. The high consumption of energy and cooling water severely restricts the development and industrialization of MD technique. Therefore, it became the focus of research to find ways of recovering the vapor latent heat to improve the energy efficiency of MD process in recent years. Lu [17–19] referred the principle of vapor latent heat recovery of MED process and developed a new MD process, namely multiple effect membrane distillation (MEMD). MEMD possesses two mainly technical advantages of efficient separation (MD) and effective heat recovery (MED), and thus has been became a hot-spot of MD technology research. In AGMD configuration, a thin stagnant air gap is interposed between the membrane and the cooling surface, which can weaken the heat loss by conduction [20,21]. Therefore, AGMD has higher thermal efficiency than other MD configurations [22]. Compared with flat-sheet membrane, hollow fiber membrane has higher specific surface area and packing density. The advantage has encouraged researchers to design several tubular AGMD modules by using parallel polypropylene (PP) hollow fiber membranes and PP hollow fiber heat exchange tubes [23–26]. And experiment results proved that these modules have exhibited excellent performance of heat recovery. The efficiency of vapor latent heat recovered by cold flow in the heat exchange tubes plays a decisive role in AGMD module, which can both determine the total energy efficiency of whole AGMD process (i.e., GOR) and the vapor pressure difference across the membrane (i.e., mass transfer driving force). Obviously, AGMD is a synergic process composed of heat and mass transfer, and which is mainly controlled by heat transfer. However, the stagnant air gap increases the resistance to the heat transfer. Consequently, the reduction of the heat transfer resistance (i.e., air gap width) plays an important role in enhancing the heat transfer efficiency of AGMD process. But there exists the problem that

Fig. 1. Schematic diagram of the cross-section of (a) tubular and (b) double-pipe AGMD modules.

49

the air gap width distributed unevenly and controlled difficultly in tubular AGMD modules. The cross-section of tubular AGMD module shown in Fig. 1a, it is obviously that the distance between porous membrane and dense wall tubes are uncertainly. On the one hand, larger air gap width leads to larger heat transfer resistance. On the other hand, condensed water vapor in limited space can easily form a water bridge [22,27] when air gap width is too small. Both can weaken the performance of AGMD process significantly. The purpose of this paper is to introduce a novel double-pipe AGMD module (DP-AGMD-M). The DP-AGMD-M consisted of hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes and heat exchange capillary copper tubes. In this design, each membrane was inserted into the corresponding copper tube to constitute an independent AGMD unit. Then a certain number of the units were assembled to make up a DP-AGMD-M. And the gap between the outer surface of porous membrane and the inner surface of copper tube acted as the air gap. The air gap width can be evenly distributed and easily regulated in DP-AGMD-M, as shown in Fig. 1b. Thus the heat and mass transfer resistance in AGMD process can be controlled effectively. Meanwhile, another problem in AGMD process is that high membrane distillation flux (J) and high GOR cannot be obtained simultaneously, the true performance cannot be reflected by only J or GOR. So the equivalent membrane distillation flux (JAGMD) was firstly introduced in this paper and used to evaluate the comprehensive performance of AGMD process. Tap water was used as feed solution in the experiments. The effect of various parameters on the performance of DP-AGMD-M were experimental investigated. The parameters including the air gap width (da), hot feed temperature (T1), temperature difference (ΔT) between the hot feed outlet temperature and the cold feed inlet temperature, hot feed flow rate (Q) and the effective length of membrane module (L). J, GOR and JAGMD were used to evaluate the performance of DP-AGMD-M. 2. Experimental 2.1. Membranes and modules The hydrophobic PVDF porous hollow fiber membranes with an average pore size of 0.16 μm, porosity of 85% and contact angle of 78° were produced by our research group. The detail information of the membrane properties and preparation process have been described in the previous reports [12,28]. The inner and outer diameters of the membrane are 0.8 mm and 1.1 mm, respectively. The capillary copper tubes were provided by Shanghai Yongkun Refrigeration Equipment Co., Ltd., China. Table 1 shows the parameters of DP-AGMD-Ms. The configuration of the module is shown in Fig. 2. Tap water had an electrical conductivity of 530 μS/cm. 2.2. Experimental apparatus The schematic diagram of the AGMD experimental apparatus is depicted in Fig. 3. And Fig. 4 shows the mass and heat transfer process in DP-AGMD-M unit. The whole feed circulatory system was wrapped with thermal insulation cotton, in order to reduce heat loss to surrounding. The feed was heated to a constant temperature T1 in the thermostat, then pumped into the inlet of hollow fiber membranes at the top of the module. The flow rate Q was adjusted by the rotameter 1. After the vapor evaporated from the feed and diffused across the porous membrane wall, the temperature of the hot feed dropped to T2. Then the feed leaving the module and flowed into an external condenser, and thus its temperature dropped to T3. And then the feed was allowed to flow into the shell side of the copper tubes at the bottom of the module as cold feed. The cold feed flowed counter-currently with the hot feed in the module, as shown in Fig. 4. The cold feed recovered the vapor heat and gradually warmed up to T4. At the same time, the vapor was condensed into distillate water. Finally, the feed with T4 flowed back into the thermostat.

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Table 1 Parameters of DP-AGMD-Ms. Parameters

Module #1

Module #2

Module #3

Module #4

Module #5

Module #6

Effective length of module, m Shell inside diameter, mm Number of membranes Number of copper tubes Inner/outer diameters of copper tube, mm Effective evaporation surface areaa, m2 Effective condensation surface areaa, m2 Air gap width, mm Packing densityb, % Cross-section area of the cold feed channel, m2

0.55 27 24 24 2.0/3.0 3.3 × 10−2 8.3 × 10−2 0.45 4.0 2.3 × 10−4

0.55 27 24 24 3.0/4.0 3.3 × 10−2 12.4 × 10−2 0.95 4.0 2.3 × 10−4

0.55 20 24 24 1.6/2.0 3.3 × 10−2 6.6 × 10−2 0.25 7.3 2.3 × 10−4

0.40 27 27 27 2.0/3.0 2.7 × 10−2 6.3 × 10−2 0.45 4.5 3.7 × 10−4

0.55 27 27 27 2.0/3.0 3.7 × 10−2 9.3 × 10−2 0.45 4.5 3.7 × 10−4

0.70 27 27 27 2.0/3.0 4.8 × 10−2 11.9 × 10−2 0.45 4.5 3.7 × 10−4

a b

The effective evaporation and condensation surface area were calculated based on the inner diameter of membrane and copper tube, respectively. Packing density is the ratio of total membrane cross-section area (based on outer diameter) to shell cross-section area (based on inner diameter).

After all the parameters (such as T2, T3) were running at steady state, the distillate was collected and weighted every 6 min by the electronic balance. The conductivity meter was used to measure the conductivity of the distillate for monitoring any leakage in the module. Finally, the distillate was back into the feed tank. Each test was repeated 3 times so as to reduce experimental errors, and the average value were reported. 2.3. Performance of AGMD process The performance of AGMD process is mainly characterized by J, GOR and JAGMD. (i) J J is an important parameter to characterize the production capacity of DP-AGMD-M, kg/(m2 · h), and it can be calculated by: J¼

W St

respectively, °C, and ΔH is the evaporation enthalpy of the hot feed that can be calculated by [29], kJ/kg: ΔH ¼ 2258:4 þ 2:47½373:0−ðT þ 273:15Þ

ð3Þ

where T is the mean temperature of the hot feed in the module, °C. (iii) JAGMD High J and GOR cannot be obtained simultaneously in AGMD process, the true performance cannot be reflected by only J or GOR. Therefore the current work introduced the concept of the equivalent membrane distillation flux (JAGMD) as a term to evaluate the comprehensive performance of DP-AGMD-M. JAGMD, kg/(m2 · h), can be calculated by: J AGMD ¼ J  GOR:

ð4Þ

ð1Þ 3. Results and discussion

where W is the weight of distillate, kg, S is the effective surface area based on the inner diameter of the hollow fiber membrane, m2, and t is the operating time, h. (ii) GOR GOR is used to measure the energy consumption of AGMD process and it can be described by [23,24]: GOR ¼

J  S  ΔH J  S  ΔH ¼ Q in Q  C p  ðT 1 −T 4 Þ

ð2Þ

where Qin is the heat input from the external thermostat, kJ, Cp is specific heat of hot feed, kJ / (kg · °C), Q is the flow rate of hot feed, L/h, T1 and T4 are the temperature of the hot feed inlet and the cold feed outlet,

The electrical conductivity of distillate was under 15 μS/cm in all experiments showed that there was not leakage in the module. Thus it was not mentioned in following sections. The effect of da, T1, Q, ΔT and L on J, GOR and JAGMD were studied. The optimization performances of DPAGMD-M were carried out to explore the feasibility of the novel module for AGMD process. 3.1. Effect of the air gap width (da) Module #1, #2 and #3 were used to investigate the effect of da on AGMD process under the experimental conditions of T1 = 90.0 °C, ΔT = 10.0 °C and Q = 12.0 L/h which corresponded to the flow

Fig. 2. Configuration of DP-AGMD-M.

Z. Liu et al. / Desalination 396 (2016) 48–56

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Fig. 3. Schematic diagram of the AGMD experimental apparatus.

velocities was 0.25 m/s. Table 2 shows the variations of J, GOR and JAGMD with da. As shown in Table 2, J, GOR and JAGMD all increased first with the increment of da and then decreased when da reached 0.95 mm. The air gap width was considered as the control factor of AGMD process, due to its obvious influence on the mass and heat transfer resistance. At atmospheric pressure, the mass transfer mechanism of water vapor molecule in air gap is molecular diffusion. The molecular diffusion resistance is proportional to the width of air gap. Larger air gap width leads to larger mass transfer resistance. So the mass transfer resistance was relatively small when da = 0.25 mm. However, the distillate water in limited space could form a water bridge (i.e., the gap bridging effect). In the meantime, the hollow fiber membranes could contact directly with copper tubes when da was small, which led to a large heat conduction loss from the feed side to the permeate side. Hence, small air gap width was not good for AGMD module. When da increased to 0.45 mm, the mass transfer resistance in air gap was still small, and the gap bridging effect could be diminished even removed completely. Most of the vapor heat in air gap could be recovered via exchange with the cold feed, which was verified by the increment of the cold feed outlet temperature T4, as shown in Table 3. As a result, both J and GOR improved with the increase of da.

When da further increased, J dropped significantly because of the growing mass transfer resistance in air gap. This is commonly found in AGMD process. The probability of membranes contacting with copper tubes decreased dramatically, and the water bridge could be removed completely while da was 0.95 mm. However, due to large heat transfer resistance, the vapor in air gap could not be condensed timely, which reduced the vapor pressure difference across the membrane and thus decreased the mass transfer driving force accordingly. Therefore, AGMD process provided a lower J. The vapor latent heat produced by membrane evaporation reduced with the decrease of J. As mentioned above, a considerable part of heat was brought out the module by the distillate water and thus T4 dropped as shown in Table 3. Therefore, GOR decreased significantly as da increased. Since the effect of da on J and GOR was consistent, it was evident that JAGMD increased first and then decreased with the increase of da. Therefore, a suitable da could improve the performance of AGMD process. The trend of JAGMD, in this study, demonstrated that the da of 0.45 mm was optimal.

3.2. Effect of the hot feed temperature (T1) Module #5 was used to study the effect of T1 on AGMD process under the experimental conditions of ΔT = 10.0 °C, Q = 4.0 L/h, 8.0 L/ h and 12.0 L/h (the corresponding flow velocities were 0.08 m/s, 0.16 m/s and 0.24 m/s, respectively). Fig. 5 illustrated the variations of J, GOR and JAGMD with T1. As can be observed, the value of J, GOR and JAGMD showed positive correlation with the increase of T1 within the range of 70.0–90.0 °C. It is well known that the mass transfer driving force of AGMD process was the vapor pressure difference across the membrane. According to Antoine equation [23], the saturated vapor pressure of water increased exponentially with its temperature. The increase of T1 improved the vapor pressure difference across the membrane, and thus enhanced the mass transfer driving force of AGMD Table 2 Effect of da on the performance of DP-AGMD-M at T1 = 90.0 °C, ΔT = 10.0 °C and Q = 12.0 L/h. da (mm)

J (kg/(m2 · h))

GOR

JAGMD (kg/(m2 · h))

10.2

1.5

15.3

12.2

1.9

23.2

9.3

1.3

12.4

0.25 0.45 Fig. 4. Schematic diagram of mass and heat transfer in DP-AGMD-M unit.

0.95

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Table 3 Effect of da on the T4 of the AGMD process at T1 = 90.0 °C, ΔT = 10.0 °C and Q = 12.0 L/h. da (mm)

T1 (°C)

T4 (°C)

90.0

79.0

0.25 90.0

79.5

90.0

78.8

Table 4 Effect of T1 on T4 of the AGMD process at ΔT = 10.0 °C. T4 (°C)

Q (L/h)

0.45

T1 (°C)

4.0 8.0 12.0

70.0

75.0

80.0

85.0

90.0

59.6 59.8 59.8

64.8 64.7 64.7

69.8 69.7 69.8

74.8 74.6 74.4

79.5 79.4 79.3

0.95

process. Besides that, the viscosity of feed decreased with the increase of T1, which reduced the thickness of boundary layer, and thus weakened the effect of temperature polarization in MD process. Therefore, the heat and mass transfer were strengthened, and thus contributed to a higher J. AGMD is a synergic process composed of heat and mass transfer. J went up when T1 increased, which can enhance the vapor transported into the permeate side and that is to say more heat was recovered by cold feed. So the cold feed outlet temperature T4 increased, as illustrated in Table 4. Like the influence of T1 on membrane evaporation process, the increase of T4 enhanced the heat recovery process. And thus higher GOR can be obtained. Because both J and GOR increased with T1, it can be seen from Eq. (4) that JAGMD also improved with T1. The results pointed that T1 had a significantly influence on the comprehensive performance of AGMD process.

3.3. Effect of the hot feed flow rate (Q) As the above experiment, module #5 was used to investigate the effect of Q on AGMD process under the experimental conditions of T1 = 90.0 °C and ΔT = 10.0 °C. Fig. 6 shows the effect of various hot feed flow rate.

As shown in Fig. 6a, J increased from 4.9 kg/(m2 · h) to 11.4 kg/ (m · h) as Q was increased from 2.0 L/h to 12.0 L/h at the same hot inlet temperature and temperature difference. There are two reasons for the promotion of J. Firstly, the boundary layers of hot and cold feed became thinner with the increase of Q, and thus weakened the influence of temperature polarization on membrane evaporation and heat recovery process. As a consequence, the temperature difference across the air gap improved. Secondly, the retention time of feed in the module became shorter when Q increased, and thus the temperature of bulk hot feed increased. The reasons not only increased the evaporation temperature of hot feed but also improved the vapor pressure difference across the membrane, which intensified the mass transfer of AGMD process, and thus contributed to the increase of J. Fig. 6a also illustrates the variation of GOR with Q. Obviously, GOR dropped with Q increasing. On the one hand, J improved with the increase of Q meant that more vapor heat need to be recovered by the cold feed. On the other hand, the increase of Q shortened the retention time of cold feed in module. As a consequence, the vapor heat could not be recovered completely, resulting in that a considerable part of heat was brought out the module by the distillate water. And thus GOR showed a negative correlation with Q. It can be seen from Fig. 6b that JAGMD increased first and then decreased with the increment of Q. When Q was little, JAGMD increased significantly with the increase of Q. 2

Fig. 5. Effect of T1 on the performance of DP-AGMD-M at ΔT = 10.0 °C.

Z. Liu et al. / Desalination 396 (2016) 48–56

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Fig. 6. Effect of Q on the performance of DP-AGMD-M at T1 = 90 °C and ΔT = 10.0 °C.

Therefore, a suitable Q could improve the performance of AGMD process. The trend of JAGMD, in this study, demonstrated that the Q of 4.0 L/h was optimal. When Q = 4.0 L/h, the maximum value of JAGMD could reach 29.6 kg/(m2 · h). It must be pointed out that, when the operate condition has opposite effect on J and GOR, the good performance of AGMD process and the optimal operate condition can be obtained directly from the variation trend of JAGMD. It can be confirmed that JAGMD is very efficient to evaluate the comprehensive performance of AGMD process.

3.4. Effect of the temperature difference (ΔT) This experiment utilized the module #5 to investigate the effect of ΔT on AGMD process under the conditions of T1 = 90.0 °C, Q = 2.0 L/

h, 4.0 L/h, 8.0 L/h and 12.0 L/h. Fig. 7 illustrated the variations of J, GOR and JAGMD with ΔT. As shown in Fig. 7a, J increased correspondingly with the increase of ΔT. It was because that when T1 remained constant, according to Antoine equation, larger temperature difference ΔT between hot feed and cold feed meant larger vapor pressure difference across membranes. Thus the mass transfer driving force of AGMD process increased accordingly, leading to the promotion of J. Fig. 7b shows the decreasing trend of GOR with the increase of ΔT. The increase of J meant more heat of hot feed used to evaporate and thus the hot feed outlet temperature T2 dropped, so the cold feed inlet T3 and outlet temperature T4 decreased accordingly. Therefore T1–T4 increased which led to more input of external energy to keep T1 constant. When Q = 4.0 L/h and ΔT increased from 3.0 °C to 10.0 °C, T1–T4 doubled from about 5.0 °C to 10.5 °C, and J increased by 62% from 4.5 kg/(m2 · h) to 7.3 kg/(m2 · h). Fig. 7c shows

Fig. 7. Effect of ΔT and Q on the performance of DP-AGMD-M at T1 = 90.0 °C.

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Fig. 8. Effect of L on the performance of DP-AGMD-M at T1 = 90.0 °C, ΔT = 10.0 °C.

that JAGMD has a positive correlation with ΔT. It was because that with the increase of ΔT, the degree of J increased was more obviously than the decrease of GOR. 3.5. Effect of the effective length of membrane module (L) Module #4, #5 and #6 were used to investigate the effect of L on AGMD process under the experimental conditions of T1 = 90.0 °C, ΔT = 10.0 °C and Q = 4.0 L/h, 8.0 L/h and 12.0 L/h. The results are presented in Fig. 8. As shown in Fig. 8a, J decreased gradually with the increase of L. The effective membrane surface area will be large when L was long, which will cause the total weight of distillate increased inevitably. As water evaporated and transferred from the hot feed to the air gap, heat was also transferred from the feed to the air gap which reduced the outlet temperature of hot feed. Consequently the average temperature of hot feed (i.e. (T1 + T2) / 2) in the module dropped, and J decreased eventually, just like the effect of T1 above discussed. Fig. 8b illustrated that GOR increased with the increase of L. Low J meant that less latent heat of vapor need to be recovered in unit surface area of porous membrane. When T1, Q and ΔT remained stable, latent heat of vapor could be recovered sufficiently, which was verified by the increment of T4, as shown in Table 5. As a result, GOR improved

L (m)

3.6. Comparison with other AGMD modules Table 6 shows the optimal performances of DP-AGMD-M with effective length of 0.55 m. The corresponding operate conditions were also summarized in Table 6. As shown in Table 6, the maximum J (i.e., Jmax) of 11.4 kg/(m2 · h), the maximum GOR (i.e., GORmax) of 6.6 and the max2 imum JAGMD (i.e., Jmax AGMD) of 29.6 kg/(m · h) were gained. It is obviously that Jmax and GORmax cannot be obtained simultaneously in AGMD process, similar to the conclusion in Ref. [23]. The main reason is that the effect of operate conditions on J and GOR are somewhat differently, even opposite, such as the feed flow rate Q. Table 7 lists a performance comparison between the current work and the previous investigations. And Table 8 shows the module parameters and operate conditions of DP-AGMD-M and other hollow fiber AGMD modules. It is found that the newly developed DP-AGMD-M produced in this study have comparable or even better performance Table 6 The optimal performances of DP-AGMD-M.

Table 5 Effect of L on T4 of the AGMD process at T1 = 90.0 °C, ΔT = 10.0 °C. T4 (°C)

when L increased. It can be seen from Fig. 8c that JAGMD increased first and then decreased with the increment of Q, and the higher JAGMD always can be obtained when L was 0.55 m under various flow rates. Therefore, a suitable L could acquire the optimal performance of DPAGMD-M.

Index

T1 (°C) (70.0–90.0)

Q (L/h) (2.0–12.0)

ΔT (°C) (3.0–10.0)

da (mm) (0.25–0.95)

Value

Jmax (kg/(m2 · h)) GORmax Jmax AGMD (kg/(m2 · h))

90.0

12.0

10.0

0.45

11.4

90.0 90.0

2.0 4.0

3.0 10.0

0.45 0.45

6.6 29.6

Q (L/h)

0.40 0.55 0.70

4.0

8.0

12.0

78.8 79.5 79.8

79.1 79.4 79.8

79.2 79.3 79.8

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Parameters

This study

Ref. [23]

Ref. [24]

Ref. [30]

was experimentally investigated under various parameters (i.e., da, T1, Q, ΔT and L). The following conclusions can be drawn from this study: (1) In this design, each membrane was inserted into the corresponding copper tube to constitute an independent AGMD unit. The air gap width can be evenly distributed and easily regulated in DP-AGMD-M. (2) JAGMD is very efficient to evaluate the comprehensive performance of AGMD process, especially when the operate condition has opposite effect on J and GOR. (3) A suitable air gap width da could improve the performance of AGMD process. Increasing hot feed temperature T1 could increase J and GOR significantly. Hot feed flow rate Q, temperature difference ΔT and module length L have opposite effect on J and GOR. (4)The maximum J arrived at 11.4 kg/(m2 · h), the maximum GOR reached 6.6 and the maximum JAGMD was 29.6 kg/(m2 · h). The results demonstrate that double-pipe air gap membrane distillation module is very suitable for AGMD process.

Membrane type Inner/outer diameters of membrane, mm Average pore size, μm Porosity, % Heat exchanger tube Inner/outer diameters of tube, mm Effective length of module, m Air gap width, mm Hot feed temperature, °C Feed flow rate, L/h Hot feed flow velocity, m/s Cold feed temperature, °C

PVDF 0.8/1.1

PP 0.4/0.6

PP 0.33/0.63

PTFE 0.8/1.6

Acknowledgement

0.16 85 Copper 2.0/3.0 0.55 0.45 70.0–90.0 2.0–12.0 0.04–0.25 –

0.22 68 PP 0.4/0.52 0.9 0.5 70.0–95.0 10.0–30.0 0.13–0.38 25.0–50.0

0.2 65 PP 0.38/0.48 1.07 0.685 70.0–90.0 16.0–48.0 0.09–0.28 25.0–45.0

0.4 60 PTFE 0.6/1.0 1.15 0.5 70.0–90.0 20.0–50.0 0.07–0.17 25.0–45.0

Table 7 Performance comparison between DP-AGMD-M and other hollow fiber AGMD modules. Type

T1 (°C)

Jmax (kg/(m2 ·

GORmax

h)) This study Ref. [23] Ref. [24] Ref. [30]

Double-pipe Tubular Tubular Spiral-wound

90.0 90.0 95.0 90.0

2 Jmax AGMD (kg/(m ·

h))

11.4 5.8 7.0 5.9

6.6 13.8 6.4 5.4

29.6 30.8 23.9 23.4

Table 8 Comparison of module parameters and operating conditions between DP-AGMD-M and other hollow fiber AGMD modules.

compared to the tubular or spiral-wound modules in the previous reports [23,24,30]. As shown in Table 7, under the same hot feed temperature, the value of Jmax obtained in this study was much higher than all of the published reports. It is considered that high Jmax was mainly decided by the structure of DP-AGMD-M. The double-pipe shape of AGMD unit can evenly distribute the air gap width and effectively weaken the gap bridging effect. It was demonstrated the feasibility of DP-AGMD-M for AGMD process. However, the GORmax of DP-AGMD-M wasn't high enough, which was ascribed to the reduction of its packing density. Casting the copper tubes by epoxy resin was so hard that the number of copper tubes had to be reduced, which cause the packing density reduced seriously. The reduction of packing density made the cross-section area of flow channel of the cold feed became large, which led to, on the one hand, the decline of flow velocity of cold feed, on the other hand, the ascent of distance between the outer surface of copper tubes and the bulk cold feed. Both can strengthen the temperature polarization and weaken the heat transfer driving force severely. All in all, the reduction of packing density has a serious negative effect on the heat recovery of DPAGMD-M. Most importantly, it can be clearly observed that a noticeable JAGMD has been achieved in the current work and the JAGMD is almost equal or even better than most of the previous reports. It is believed that DP-AGMD-M has potential to be applied for AGMD process. Since the difference of Jmax or GORmax among the hollow fiber AGMD modules were very large, the real performance cannot be evaluated by only J or GOR. So JAGMD seems to be a necessary and effective method to evaluate the performance of AGMD modules.

4. Conclusions In this study, a novel DP-AGMD-M with capillary copper tubes as heat exchange parts was successfully developed. The concept of equivalent membrane distillation flux (JAGMD) was firstly introduced in this paper. The mainly performance (i.e., J, GOR and JAGMD) of DP-AGMD-M

The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant No. 21176188, 21106100 and 21306135), Tianjin Science and Technology Support Program of China (Grant No. 15ZCZDSF00070) and The Science and Technology Plans of Tianjin (No.15PTSYJC00230).

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