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High water recovery of RO brine using multi-stage air gap membrane distillation
Desalination 355 (2015) 178–185
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High water recovery of RO brine using multi-stage air gap membrane distillation Hongxin Geng, Juan Wang, Chunyao Zhang, Pingli Li ⁎, Heying Chang Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
H I G H L I G H T S • • • •
A multi-stage AGMD process for further concentrating RO brine is investigated. The maximum value of JD and GOR can reach 6.8 kg/m2 h and 7.1. 24.7% of water recovery for the 4-stage AGMD process by experiment is obtained. 88.2% of water recovery for the 14-stage AGMD process is theoretically analyzed.
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 7 October 2014 Accepted 27 October 2014 Available online 5 November 2014 Keywords: Multi-stage AGMD RO brine High water recovery Energy recovery
a b s t r a c t A multi-stage AGMD process for further concentrating RO brine and obtaining a high water recovery is investigated. One-stage AGMD, using artificial RO brine as feed and an improved AGMD module with energy recovery, is implemented in this study. The maximum value of JD and GOR could reach 6.8 kg/m2 h and 7.1 respectively. The relationship between water recovery (R), gained output ratio (GOR), permeate water flux (JD) and temperature difference at the top of the AGMD module (ΔTtop) was studied. To get a high water recovery, a 4-stage AGMD process was experimentally studied. JD decreased by 5.5% in the case of equal Fb,i. The multi-stage AGMD process was designed in a series of AGMD stages for large scale plants. A theoretical analysis of such stages has been carried out and the relationship between overall water recovery (Rall) and the number of stages was generated. 88.2% of water recovery for the 14-stage AGMD process was obtained. This could have important implications for the use of AGMD in treating high salinity solutions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Water scarcity is one of the most important problems nowadays, which is becoming more and more acute with the population expansion and industrial development [1]. Now over one-third of the world's population lives in water-stressed areas and one billion people in developing countries have no access to clean drinking water [2]. However, Earth is rich in seawater resources. Now the total global desalination capacity is around 66.4 million m3/d [3,4]. Commercial desalination technologies are categorized into two groups: thermal process and membrane process [5–7]. Thermal desalination technology is based on the evaporation process where water evaporates from the brine. Now the most important thermal processes are multi-effect evaporation (MED), multi-stage flash evaporation (MSF) and vapor compression distillation (VC), while membrane desalination technology is based on separating the membrane and ⁎ Corresponding author at: Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (P. Li).
http://dx.doi.org/10.1016/j.desal.2014.10.038 0011-9164/© 2014 Elsevier B.V. All rights reserved.
mainly includes reverse osmosis (RO) and electrodialysis (ED). RO desalination technology is now the most economical way to get fresh water from seawater [8], but the recovery of the RO process is only 35%–50% [4]. The by-product of RO seawater desalination containing large quantities of chemical substances produced in the seawater pretreatment process is hard to be treated compared to domestic sewage. However RO brines discharged to the environment directly can cause heavy environmental pollution [9]. Proper separation technology combined with RO technology should be employed in seawater desalination to realize zero-emissions. Some workers have studied the performance of MD for highly concentrated solutions especially RO brine [10–12]. Membrane distillation (MD) has been known to all since the 1960s, which is derived by the vapor pressure difference led by the existing temperature difference across the micro-porous hydrophobic membrane. MD has developed into four different configurations: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) and vacuum membrane distillation (VMD). The main benefits of MD technology compared to other separation processes stem from the following [13–15]: (1) MD can be
H. Geng et al. / Desalination 355 (2015) 178–185
operated at normal pressure, which makes the membrane pores not easily blocked and the membranes not easily polluted compared to the separation process driven by pressure such as the RO process; (2) MD can use low-grade energy including industrial waste heat and solar energy; and (3) MD is not restricted by salt concentration because water vapor pressure is reduced slowly as the salt concentration increases, which enables MD to treat high salt wastewater such as RO brines. MD is a promising separation technology in the 21st century which is being investigated worldwide [16]. Recent studies on MD technology focus on the MD membrane, MD module and membrane separation technology integration. GOR generally ranged from 0.1 to 0.65 which varied the membrane used in the DCMD process and 0.7 to 0.98 in the traditional AGMD process. The goals of these studies are to reduce the seawater desalination cost and increase the fresh water recovery [17–22]. The idea of this study is to provide a feasible method for further concentrating RO brine and proposing a high water recovery integrated with other separation processes. The implementation process of the scheme is shown in Fig. 1. In all membrane separation processes, very significant membrane fouling occurs in the MD process because of the adherence of sediment including CaSO4 and CaCO3 onto the membrane surface and membrane wetting, which leads to the decrease of permeate flux with time [23,24]. In the pretreatment process, Ca2 + and Mg2 + should be removed because CaSO4 and CaCO3 precipitations formed in the multi-stage AGMD process with the continuous increases of feed concentration. Then seawater is sequentially concentrated in the RO process, multi-stage AGMD process and post-treatment process. The RO process and multi-stage AGMD process are the main processes to recover most of fresh water where more than 75% fresh water can be recovered. The solid–liquid separation for RO brine is realized in the post-treatment process such as evaporation and crystallization. Hereby we focus on the AGMD process because the other separation processes are relatively mature. We do some corresponding improvements to the previous MD module [25,26]. In the previous experiments, we found that enhancing heat transfer of hollow fibers and keeping a good distribution of hollow fibers and membranes were the key of further increasing the performance of the MD module. In multi-stage AGMD modules, a series of hollow fiber-membrane distributors are installed at the same distance to restrict the movement of hollow fibers and membranes (Fig. 2). In the previous MD experiment, the one-stage AGMD module can only evaporate at the most around 4–8% of the water. If we have to recover 70% we can adopt two methods: batch recirculation and concentration stages. In batch recirculation, the brine is circulated in the same AGMD module again for high water recovery till the desired water recovery is achieved. We have studied such a
Feed
Pretreatment
RO
Fig. 2. Schematic drawing of hollow fiber-membrane distributor.
process using 70 g/L NaCl solution wherein the NaCl solution was concentrated to 308 g/L [26]. Such a mode of operation may not be conducive to continuous operation in industry unlike the operating mode with concentration stages where a number of AGMD modules have been connected in series and the brine concentration keeps on increasing down the stage is easily operated because its change in each stage is limited. We can specify the operating conditions of each stage and therefore achieve predictable operation to get high water recovery. 2. Development of equations 2.1. One-stage AGMD process equations and water recovery The details of the aforementioned AGMD module are illustrated in Fig. 3. At the macroscopic level, we can derive the following equation based on overall energy and mass balances: mb;i ¼ mb;o ¼ md;i ¼ md;o þ mp
ð1Þ
mb;i Hb;i þ md;i Hd;i ¼ mp Hp þ mb;o Hb;o þ md;o Hd;o þ Q loss
ð2Þ
where mb,i, mb,o, md,i, md,o and mp are the mass flow rates of brine at the inlet of hollow fibers, outlet of hollow fibers, inlet of membranes, outlet of membranes and outlet of permeate, respectively; Hb,i, Hb,o, Hd,i, Hd,o and Hp are the brine enthalpies at the inlet of hollow fibers, outlet of hollow fibers, inlet of membranes, outlet of membranes and outlet of permeate, respectively; Qloss is the heat loss from the module to the surrounding environment. md;i Hd;i −Hb;o þ mp Hd;0 −Hp −Q loss ¼ mb;i Hd;0 −Hb;i
ð3Þ
md;i cp;b Td;i −Tb;o þ mp cp;p Td;o −Tp −Q loss ¼ mb;i cp;b Td;o −Tb;i
ð4Þ
mp cp;p Td;o −Tp Td;i −Tb;o Q loss − ¼1 þ Td;o −Tb;i m c T −T m c b;i p;p d;o b;i b;i p;b Td;o −Tb;i
RO Brine
MD
179
ð5Þ
ΔTtop ¼ Td;i ¼ Tb;o
ð6Þ
ΔTend ¼ Td;o −Tb;i
ð7Þ
Post-treatment
Fig. 1. Schematic drawing of the combined RO and MD systems.
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△Ttop
Td,i
Tb,o
△Tstage,b
△T
membranes
stage,d
Tp,mp
hollow fibers Td,o md,o
Tb,i mb,i
MD Module
△Tend
Fig. 3. One-stage AGMD process with energy recovery.
R¼
mp mb;i
ð8Þ
where ΔHv is the heat of vaporization of water vapor, which is a function of temperature. ΔHv is calculated by [17]: 3
where ΔTtop and ΔTend are the temperature difference at the top and the bottom of the module, respectively. During the experiment, good thermal insulation is introduced to diminish the heat loss from the membrane distillation system to the surrounding environment, so the heat loss to the surrounding environment can be neglected. Then it will be ΔTtop ≤ ΔTend. If the heat loss to the surrounding environment cannot be negligible, it may be ΔTtop ≥ ΔTend. R is the water recovery and we found that R usually was less than 10%. In the AGMD process, permeate flux (JD), gained output ratio (GOR), evaporation efficiency (η) and water recovery (R) are the most important performance indicators.
JD ¼
mp s
ΔHv ¼ 2:245 10 þ 2:475ð373:0−ðT þ 273:15ÞÞ:
The heat input provided from the external heat source, Qin, and the heat recovered by the brine in the hollow fibers, Qrec, can be calculated as: Q in ¼ mb;i cp;b Td;i −Tb;o ¼ mb;o cp;b Td;i −Tb;o
ð12Þ
Q rec ¼ mb;i cp;b Tb;o −Tb;i :
ð13Þ
ð9Þ
mp ΔHv mp ΔHv ≅ η¼ md;i Hd;i −md;o Hd;o md;i cp;d Td;i −md;o cp;d Td;o
(Tb,o)1
ð10Þ
(Td,i)1
(Tb,o)2
permeate
(Tb,i)1
(Td,o)1
In the AGMD process, the specific heat input can be: cp;b ΔTtop Q in mb;i cp;b Td;i −Tb;o ¼ ¼ mp R mp
(Td,i)2
(Tb,o)n
(Td,o)2
ð14Þ
(Td,i)n
permeate
permeate
(Tb,i)2
ð11Þ
(Tb,i)n
RO Brine
Fig. 4. High water recovery with a series of AGMD stages.
(Td,o)n
H. Geng et al. / Desalination 355 (2015) 178–185 Table 1 Characteristics of AGMD modules used in the experiments.
181
Table 2 Composition of an artificial RO brine.
Characteristics
AGMD module
NaCl
CaCl2
MgSO4
KCl
Na2SO4
NaHCO3
MgCl2
Membrane ID/OD (mm) Membrane average pore size (μm) Membrane porosity Hollow fiber ID/OD (mm) Number of membranes Effective length of module (m) Effective internal membrane area (m2) Number of hollow fibers Effective internal hollow fiber area (m2) Air gap width (mm)
0.55/0.7 0.23 71% 0.4/0.52 550 1.0 0.95 1100 1.38 0.5
45.5
1.20
1.30
1.26
7.07
0.32
6.95
Tb;i ¼ Tb;i ¼ … ¼ Tb;i 1
2
Td;i ¼ Td;i ¼ … ¼ Td;i 1
and then the GOR can be: GOR ¼
mp ΔHv mp ΔHv ΔTstage;b R ΔHv ¼ ¼ ¼η Q in cp;b ΔTtop ΔTtop mb;o cc;b Td;i −Tb;o
ð15Þ
2
ð20Þ
n
The brine concentration keeps on increasing down the stage, which influences the JD and GOR of each stage. Then ΔTstage, ΔTtop and water recovery (Ri) of each stage may not be equal. Ri ¼
ΔTstage;b ¼ Tb;o −Tb;i
ð19Þ
n
mp mb;i
ð16Þ
! ð21Þ i
Let the overall water recovery be R. Then R can be calculated by: ΔTstage;d ¼ Td;i −Td;o
ð17Þ
where ΔTstage,b and ΔTstage,d are the temperature drop of the cold brine side in the hollow fibers and hot brine side in the membranes, respectively.
The details of the multi-stage AGMD process containing a series of countercurrent-flow AGMD modules operated in continuous mode are illustrated in Fig. 4. In the multi-stage AGMD experiment, cold brine enters the AGMD module of stage 1 at (Tb,i)1, (mb,i)1 and leaves at (Td,o)1, (md,o)1. Then the concentrated brine is introduced into the cooling heat exchange and enters the AGMD module of stage 2 at (Tb,i)2, (mb,i)2. Tb,i and Td,i of each stage are kept the same.
j
ð18Þ
jþ1
Tb,o
n X
mp ΔHv
j¼1
n X cp md;i Td;i −Tb;o j¼1
Cold RO brine
j
Table 1 shows the characteristics of hollow fiber membranes, hollow fibers and membrane modules used in the experiments. The microporous-hydrophobic PP hollow fiber membranes and heatexchange hollow fibers were kindly provided by Tianjin Chemical
Td,i hot RO brine
Thermostat B
Membranes permeate water
Magnetic pump
ð23Þ
3.1. Membrane and membrane module
Rotameter A
Tb,i
:
3. Experimental
Rotameter B Hollow Fibers
ð22Þ
The overall value of GOR will be:
GOR ¼
2.2. Multi-stage AGMD process equations and water recovery
md;o ¼ mb;i
R ¼ R1 þ R2 þ … þ Rn :
Td,o Concentrate Tank
Thermostat A Fig. 5. Schematic diagram of one stage AGMD experimental apparatus.
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H. Geng et al. / Desalination 355 (2015) 178–185
8
0.09
7
Td,i=95ć
Td,i=85ć
Td,i=85ć
6
0.07
5 R
GOR
Td,i=95ć
0.08
0.06
4 0.05
3 0.04
2 0.03
1 2
4
6
8
10 12 ƸTtop ć
14
16
2
18
4
6
8
10
12
14
16
18
ƸTtop ć Fig. 8. Experimental results of R as a function of ΔTtop at F = 80 L/h.
Fig. 6. Experimental results of GOR as a function of ΔTtop at F = 80 L/h.
Separating Technologies Co. Ltd., Tianjin. Each AGMD module has 10 layers of hollow fiber membranes and 11 layers of heat-exchange hollow fibers. In the new AGMD module, the rate among membranes and heat-exchange hollow fibers remained unchanged. The heat transfer area of hollow fibers of 1.38 m2 is enough within the operating conditions. In order to keep the distribution of membranes and hollow fibers unchanged, 10 hollow fiber-membrane distributors made of ABS are installed in the AGMD module with equal distances from each other.
Na2SO4 and NaHCO3 were prepared together in tap water. Before the two solutions were mixed in the brine tank, each was stirred sufficiently to make sure that each chemical was dissolved. However, the solubility of CaSO4 (Ksp (25 °C) = 2.5 × 10−5) and CaCO3 (Ksp (25 °C) = 4.8 × 10−9) is very low. To avoid causing membrane fouling in the multi-stage AGMD process, CaCO3 and CaSO4 should be removed in the pretreatment process. The total dissolved solid (TDS) and electrical conductivity of the permeate water were monitored by a conductivity meter (DDSJ-308A, Leici, China).
3.2. Experimental apparatus 4. Results and discussion The one-stage AGMD experimental apparatus consisting of a thermostat, magnetic pump, rotameter, AGMD module and thermometric indicator is schematically depicted in Fig. 5. The suitable insulation must be taken in order to reduce heat loss from the membrane distillation system to the surrounding environment. The temperature of the feed is monitored at the inlets and outlets of hollow fibers and membranes using thermocouples with sensitivity of 0.1 °C. The one-stage AGMD experiment is introduced to investigate the performance of the new AGMD module. Then the fairly good operating conditions at which the values of JD and GOR are both large are chosen as the operating conditions of the multi-stage AGMD experiment. The experiments are conducted with an artificial 62.5 g/L RO brine supplied by Tianjin Chemical Separating Technologies Co. Ltd., Tianjin. The composition of the artificial RO brine showed in Table 2 included and major ions in RO brine, such as Na+, Cl−, Mg2+, Ca2 +, K+, SO2− 4 HCO− 3 . The feed solution of RO brine was prepared as follows: NaCl, MgCl2, CaCl2 and KCl were dissolved together in tap water; MgSO4,
4.1. Experimental study of the one-stage AGMD process The performance of the new AGMD module in one-stage was evaluated under various operating conditions. The relationship between water recovery (R), gained output ratio (GOR), permeate water flux (JD) and temperature difference at the top (ΔTtop) of AGMD module was fully studied. GOR as a function of ΔTtop (Td,i = 95 °C and 85 °C, F = 80 L/h) is illustrated in Fig. 6. GOR decreases as ΔTtop increases. Based on Eq. (15), there is an inverse correlation between GOR and ΔTtop. GOR reaches the value of 7.4 for ΔTtop of 4.2 because the heat input is recovered by the cold brine in the heat-exchange hollow fibers. When a high value of GOR is obtained, the value of ΔTtop is quite small, such as GOR of 13.8 with ΔTtop of 2.9 °C as illustrated in Ref. [17]. Without the heat recovery, the GOR of MSF and MED processes cannot be more than 1 [27]. The latent heat of water vapor is used to preheat the cold feed. Thus,
6.5
20
6.0
Td,i=95ć
18
Td,i=85ć
16
5.5
Td,i=85ć
14
2
ƸTend(ć)
5.0
JD(L/m h)
Td,i=95ć
4.5 4.0
12 10 8 6
3.5
4
3.0
2 0
2.5
2
4
6
8
10 12 ƸTtop ć
14
16
18
Fig. 7. Experimental results of JD as a function of ΔTtop at F = 80 L/h.
0
2
4
6
8
10
12
14
16
18
20
ƸTtop ć Fig. 9. Experimental results of ΔTend as a function of ΔTtop at F = 80 L/h.
H. Geng et al. / Desalination 355 (2015) 178–185
0.09
16
183
6.0
6.4
ƸTtop
14
0.08
6.0
0.07
5.6
10 8
0.06
R
GOR JD
5.6
2
5.2
5.2 4.8 4.8
6 0.05 4
40
50
60
70
80 90 F(L/h)
0.04 100 110 120 130
4.0
only a little heat is needed to be supplied externally for the following evaporation. However, JD increases as ΔTtop increases as shown in Fig. 7, but JD increases slowly at the high value of the ΔTtop. JD and GOR were substantially greater in experiments conducted with Td,i = 95 °C than Td,i = 85 °C because the water vapor pressure exponentially increases with temperature according to the Antoine equation. The driving force of mass transfer is the vapor pressure difference across the membranes resulting from the transmembrane temperature difference. A high temperature difference results in a high water vapor pressure difference across the membrane and a strong driving force for mass transfer in the AGMD process. When ΔTtop is 5.8 °C at Td,i = 95 °C and Tb,i = 45 °C, GOR and JD are 6.5 and 5.1 kg/m2 h, respectively, while as described in Ref. [26], GOR and JD are 5.7 and 4.3 kg/m2 h, which means that the performance of the new AGMD module has improved (Fig. 12). The porosity and average pore size of the membrane were increased in the AGMD module used in all experiments. As described in many papers [28,29], the mass-transfer coefficient depends on the characteristics of porous fibers including the porosity and average pore size. Hollow fiber membranes with high porosity and average pore size were prepared and used to increase the performance of the AGMD module. In the AGMD process, the heat transfer resistance across hollow fibers is the main resistance and a good heat transfer capacity has a positive effect on the mass transfer. It is essential for enhancing heat transfer to keep the distribution of membranes and hollow fibers stable over time. Fig. 8 shows that R increases with the increase of ΔTtop. In order to get a high water recovery, a high inlet temperature Td,i is expected. The maximum value of R is only 7.3% at Td,i = 95 °C and Tb,i = 25 °C, but the GOR was only 2.6. It is not strictly comparable to compete
4.2. High water recovery with a series of AGMD stages In order to get a high water recovery, we study on a 4-stage AGMD process and design for AGMD stages. In the 4-stage AGMD experiment,
9.0
0.25
Rall 0.15
0.060 0.10
ƸTtop(ć)
0.20
0.055
ƸTtop
8.0
0.070 0.065
4
commercially with MSF, MED and RO processes. Using a series of AGMD stages was preferable to get a high water recovery. ΔTtop is a little larger than ΔTend as shown in Fig. 9 partly because of heat loss to the environment. Meanwhile the permeate water carried a little energy flowing out of the hot brine in the membranes continuously. ΔTstage,d is a little larger than ΔTstage,b and it is much larger than ΔTtop. However, we found in experiments that ΔTstage,d was smaller than ΔTtop when the length of the membrane module was short. During all experiments, the electrical conductivity of permeate water was below 25 μS/cm and salt rejection was greater than 99.9% (the concentration of 0.1 g/L is around 118 μS/cm). Fig. 10 shows the variation of ΔTtop and R with F at Tb,i = 45 °C and T d,i = 95 °C. ΔT top increases with the increase of F, but R decreases slowly, which mainly because of the shorter retention time of the feed. JD increases as F increases, and it is conducive to increase the heat transfer coefficient and reduce the temperature and the concentration polarization effect. The measured experimental JD and GOR at different operating parameters are repeatable. The maximum experimental error in JD is + 0.12 kg/m2 h (2.7%) at ΔTtop = 4.9 °C, Td,i = 95 °C, and F = 80 L/h, and the maximum experimental error in GOR is + 0.10 (4.2%) at ΔTtop = 14.2 °C, Td,i = 85 °C, and F = 80 L/h (Fig. 13).
8.5
Ri Rall
0.075
2 3 Number of stages
Fig. 12. Experimental results of GOR and JD as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
0.30
0.080
4.0 1
Fig. 10. Experimental results of ΔTtop and R as a function of F at Td,i = 95 °C and Tb,i = 45 °C.
Ri
4.4
4.4
2
JD(kg/m h)
GOR
R
ƸTtop(ć)
12
7.5 7.0 6.5 6.0 5.5
0.050
0.05 1
2 3 Number of stages
4
Fig. 11. Experimental results of Ri and Rall as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
5.0 1
2 3 Number of stages
4
Fig. 13. Experimental results of ΔTtop and R as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
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H. Geng et al. / Desalination 355 (2015) 178–185
0.068
6.2
0.9 Ri Rall
GOR=-0.00955c+6.0714 0.064
2
6.0
R =0.976
0.8
0.060
0.7
0.056
0.6
5.6
0.5
0.052
5.4
0.048
5.2
0.044
Rall
Ri
GOR
5.8
0.4 0.3
0
20 40 60 Feed concentration(g/L)
4
80
6
8
10
12
14
Number of stages
Fig. 14. Dependence of linear GOR coefficient on the feed concentration at Td,i = 95 °C and Tb,i = 45 °C.
cold brine enters the AGMD module of stage 1 at (Tb,i)1 = 45 °C and (Fb,i)1 = 80 L/h. Then the concentrated brine is introduced into the cooling heat exchange and enters the AGMD module of stage 2 at (T b,i ) 2 = 45 °C. Tb,i and T d,i of each stage are kept the same. The multi-stage AGMD process can be operated in two modes. In the first mode, the feed flow rate Fb,i of each stage remains equal. In the second mode, the feed flow rate Fb,i of this stage is equal to the former stage. We now analyze the relation between the water recovery of each stage (Ri), water recovery of 4-stage AGMD (Rall), GOR, JD and ΔTtop of each AGMD module at Tb,i = 45 °C and Td,i = 95 °C. 4.2.1. Design for the 4-stage AGMD process 4.2.1.1. Case of the equal Fb,i of each stage. The water recovery of each stage and overall water recovery as a function of the number of stages are illustrated in Fig. 11. We can see that the general trend of Ri decreased for 4-stage continuous operations, but Rall always increases. Ri of one stage in a continuous operational mode is limited to 6–7% and the average Ri of 4-stage is 6.2%. Ri of the fourth stage is 6.1% and only decreases by 3.1%. Rall can reach 24.7% after 4-stage AGMD concentration, where the brine concentration is 84.5 g/L. It is necessary to put a number of AGMD stages in series to get a high water recovery. JD and GOR decrease as the number of stages increases. In the 4-stage AGMD process, the brine concentration increases continuously. The increase of brine concentration reduces the partial water vapor pressure and consequently reduces the driving force of the AGMD process, which leads to a decrease of the driving force of mass transfer in the AGMD process. Meanwhile the increase of brine concentration can reduce the diffusion coefficient of sodium chloride in feed liquid, which
Fig. 16. Ri and Rall as a function of number of stages in multi-stage AGMD process.
enhances the effect of concentration polarization. The heat loss of the hot brine in the membranes decreases because of the decrease of JD. Therefore the outlet temperature Td,o increases slightly and ΔTtop increases. 4.2.2. Design for multi-stage AGMD stages If we have to get a higher water recovery than 80%, we need more AGMD stages. It is difficult for us to get the multi-stage results by experiment because in the experiment it needs many external heat exchangers to keep Tb,i and Td,i constant. In the multi-stage AGMD process, we analyzed the case of equal Fb,i of each stage. In the multi-stage AGMD process, Tb,i and Td,i were kept the same for each stage. The values of Tb,i and Td,i were 45 °C and 95 °C, respectively. For the equal Fb,i of each stage, the feed concentration varied with the stage and it was the only adjustable variable. JD and the feed concentration changes in the relationship are linear as shown in many studies. In the 4-stage AGMD process, their linear relevance is not high. This result may be caused by fine distinctions of different AGMD modules during the preparation process. To get a good linear regression, the relationship between JD, GOR and c was studied in the same AGMD module. As can be seen in Fig. 14, there is a linear relationship with R2 of 0.976 between GOR and c. Similarly, JD is given by a linear equation that is strongly dependent on the feed concentration. If the model was applied in the 4stage AGMD process, the maximum predicted error of GOR was only 0.1 and the maximum predicted error of JD was only 0.1 kg/m2 h, which means that the model could predict the multi-stage AGMD process to a great extent. The minimum value of JD is 3.9 kg/m2 h and it is only slightly lower than that of the fourth stage in the 4-stage AGMD process. Rall can be up to 82.2% after the 14-stage AGMD process and the feed concentration is
JD= -0.00683c+5.775
5.7
R =0.970
5.2
5.6
5.2
GOR JD
2
4.8
4.8
GOR
4.4
5.4
4.4
4.0
4.0
3.6
3.6
2
5.5
5.3 5.2 5.1 0
20 40 60 Feed concentration(g/L)
80
JD(kg/m h)
5.8
2
JD(kg/m h)
5.9
3.2 4
6
8
10
12
14
3.2
Number of stages Fig. 15. Dependence of linear JD coefficient on the feed concentration at Td,i = 95 °C and Tb,i = 45 °C.
Fig. 17. GOR and JD as a function of number of stages in multi-stage AGMD process.
H. Geng et al. / Desalination 355 (2015) 178–185
352 g/L. The solubility of sodium chloride is 366 g/L at 40 °C. Then the concentrated brine can be used to prepare caustic soda in chlor-alkali industry and realize comprehensive utilization of seawater resources. 5. Conclusions In this study, we provided a multi-stage AGMD process for further concentrating RO brine and getting a high water recovery of seawater desalination. In experiment, the improved AGMD module with a series of hollow fiber-membrane distributors to keep the distribution of membranes and hollow fibers uniform and stable was designed. The following conclusions were drawn: (1) In the one-stage AGMD process, the maximum value of JD and GOR could reach 6.8 kg/m2 h and 7.1 respectively. The water recovery of one-stage was limited to 5–8% and the maximum value of R is only 7.3% at Td,i = 95 °C and Tb,i = 25 °C (Fig. 15). (2) Two different continuous operational modes of the 4-stage AGMD process were demonstrated experimentally and a high water recovery of 24.7% was obtained in the case of equal Fb,i of each stage. JD decreased by 22.2% from 5.4 kg/m2 h of the first stage to 4.2 kg/m2 h of the fourth stage in the case of not equal Fb,i of each stage (Fig. 16). (3) In the multi-stage AGMD process, Tb,i, Td,i and Fb,i of each stage were kept the same. The linear relationship between JD, GOR and c was studied through a single factor experiment. The maximum value of Rall and the minimum value of JD were 82.2% and 3.9 kg/m2 h respectively after the 14-stage AGMD process (Fig. 17). Notation Cp GOR H ΔHv JD mp mb,i mb,o md,i md,o Qin Qrec R Ri Tp Tb,i Tb,o Td,i Td,o ΔTend ΔTstage,b ΔTstage,d ΔTtop η
specific heat, kJ/kg/°C gained output ratio specific enthalpy, kJ/kg enthalpy of water evaporation, kJ/kg water permeate flux, kg/m2/h permeate water flow out of AGMD unit, kg/h brine flow rate into AGMD unit, kg/h brine flow rate out of AGMD unit, kg/h distillate flow rate into AGMD unit, kg/h distillate flow rate out of AGMD unit, kg/h brine heat input, kJ heat recovery, kJ water recovery water recovery in stage i permeate water temperature, °C brine inlet temperature, °C brine outlet temperature, °C distillate inlet temperature, °C distillate outlet temperature, °C Td,o − Tb,i Tb,o − Tb,i Td,o − Td,i Td,i − Tb,o thermal efficiency
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2014.10.038.
185
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Desalination journal homepage: www.elsevier.com/locate/desal
High water recovery of RO brine using multi-stage air gap membrane distillation Hongxin Geng, Juan Wang, Chunyao Zhang, Pingli Li ⁎, Heying Chang Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
H I G H L I G H T S • • • •
A multi-stage AGMD process for further concentrating RO brine is investigated. The maximum value of JD and GOR can reach 6.8 kg/m2 h and 7.1. 24.7% of water recovery for the 4-stage AGMD process by experiment is obtained. 88.2% of water recovery for the 14-stage AGMD process is theoretically analyzed.
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 7 October 2014 Accepted 27 October 2014 Available online 5 November 2014 Keywords: Multi-stage AGMD RO brine High water recovery Energy recovery
a b s t r a c t A multi-stage AGMD process for further concentrating RO brine and obtaining a high water recovery is investigated. One-stage AGMD, using artificial RO brine as feed and an improved AGMD module with energy recovery, is implemented in this study. The maximum value of JD and GOR could reach 6.8 kg/m2 h and 7.1 respectively. The relationship between water recovery (R), gained output ratio (GOR), permeate water flux (JD) and temperature difference at the top of the AGMD module (ΔTtop) was studied. To get a high water recovery, a 4-stage AGMD process was experimentally studied. JD decreased by 5.5% in the case of equal Fb,i. The multi-stage AGMD process was designed in a series of AGMD stages for large scale plants. A theoretical analysis of such stages has been carried out and the relationship between overall water recovery (Rall) and the number of stages was generated. 88.2% of water recovery for the 14-stage AGMD process was obtained. This could have important implications for the use of AGMD in treating high salinity solutions. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Water scarcity is one of the most important problems nowadays, which is becoming more and more acute with the population expansion and industrial development [1]. Now over one-third of the world's population lives in water-stressed areas and one billion people in developing countries have no access to clean drinking water [2]. However, Earth is rich in seawater resources. Now the total global desalination capacity is around 66.4 million m3/d [3,4]. Commercial desalination technologies are categorized into two groups: thermal process and membrane process [5–7]. Thermal desalination technology is based on the evaporation process where water evaporates from the brine. Now the most important thermal processes are multi-effect evaporation (MED), multi-stage flash evaporation (MSF) and vapor compression distillation (VC), while membrane desalination technology is based on separating the membrane and ⁎ Corresponding author at: Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (P. Li).
http://dx.doi.org/10.1016/j.desal.2014.10.038 0011-9164/© 2014 Elsevier B.V. All rights reserved.
mainly includes reverse osmosis (RO) and electrodialysis (ED). RO desalination technology is now the most economical way to get fresh water from seawater [8], but the recovery of the RO process is only 35%–50% [4]. The by-product of RO seawater desalination containing large quantities of chemical substances produced in the seawater pretreatment process is hard to be treated compared to domestic sewage. However RO brines discharged to the environment directly can cause heavy environmental pollution [9]. Proper separation technology combined with RO technology should be employed in seawater desalination to realize zero-emissions. Some workers have studied the performance of MD for highly concentrated solutions especially RO brine [10–12]. Membrane distillation (MD) has been known to all since the 1960s, which is derived by the vapor pressure difference led by the existing temperature difference across the micro-porous hydrophobic membrane. MD has developed into four different configurations: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) and vacuum membrane distillation (VMD). The main benefits of MD technology compared to other separation processes stem from the following [13–15]: (1) MD can be
H. Geng et al. / Desalination 355 (2015) 178–185
operated at normal pressure, which makes the membrane pores not easily blocked and the membranes not easily polluted compared to the separation process driven by pressure such as the RO process; (2) MD can use low-grade energy including industrial waste heat and solar energy; and (3) MD is not restricted by salt concentration because water vapor pressure is reduced slowly as the salt concentration increases, which enables MD to treat high salt wastewater such as RO brines. MD is a promising separation technology in the 21st century which is being investigated worldwide [16]. Recent studies on MD technology focus on the MD membrane, MD module and membrane separation technology integration. GOR generally ranged from 0.1 to 0.65 which varied the membrane used in the DCMD process and 0.7 to 0.98 in the traditional AGMD process. The goals of these studies are to reduce the seawater desalination cost and increase the fresh water recovery [17–22]. The idea of this study is to provide a feasible method for further concentrating RO brine and proposing a high water recovery integrated with other separation processes. The implementation process of the scheme is shown in Fig. 1. In all membrane separation processes, very significant membrane fouling occurs in the MD process because of the adherence of sediment including CaSO4 and CaCO3 onto the membrane surface and membrane wetting, which leads to the decrease of permeate flux with time [23,24]. In the pretreatment process, Ca2 + and Mg2 + should be removed because CaSO4 and CaCO3 precipitations formed in the multi-stage AGMD process with the continuous increases of feed concentration. Then seawater is sequentially concentrated in the RO process, multi-stage AGMD process and post-treatment process. The RO process and multi-stage AGMD process are the main processes to recover most of fresh water where more than 75% fresh water can be recovered. The solid–liquid separation for RO brine is realized in the post-treatment process such as evaporation and crystallization. Hereby we focus on the AGMD process because the other separation processes are relatively mature. We do some corresponding improvements to the previous MD module [25,26]. In the previous experiments, we found that enhancing heat transfer of hollow fibers and keeping a good distribution of hollow fibers and membranes were the key of further increasing the performance of the MD module. In multi-stage AGMD modules, a series of hollow fiber-membrane distributors are installed at the same distance to restrict the movement of hollow fibers and membranes (Fig. 2). In the previous MD experiment, the one-stage AGMD module can only evaporate at the most around 4–8% of the water. If we have to recover 70% we can adopt two methods: batch recirculation and concentration stages. In batch recirculation, the brine is circulated in the same AGMD module again for high water recovery till the desired water recovery is achieved. We have studied such a
Feed
Pretreatment
RO
Fig. 2. Schematic drawing of hollow fiber-membrane distributor.
process using 70 g/L NaCl solution wherein the NaCl solution was concentrated to 308 g/L [26]. Such a mode of operation may not be conducive to continuous operation in industry unlike the operating mode with concentration stages where a number of AGMD modules have been connected in series and the brine concentration keeps on increasing down the stage is easily operated because its change in each stage is limited. We can specify the operating conditions of each stage and therefore achieve predictable operation to get high water recovery. 2. Development of equations 2.1. One-stage AGMD process equations and water recovery The details of the aforementioned AGMD module are illustrated in Fig. 3. At the macroscopic level, we can derive the following equation based on overall energy and mass balances: mb;i ¼ mb;o ¼ md;i ¼ md;o þ mp
ð1Þ
mb;i Hb;i þ md;i Hd;i ¼ mp Hp þ mb;o Hb;o þ md;o Hd;o þ Q loss
ð2Þ
where mb,i, mb,o, md,i, md,o and mp are the mass flow rates of brine at the inlet of hollow fibers, outlet of hollow fibers, inlet of membranes, outlet of membranes and outlet of permeate, respectively; Hb,i, Hb,o, Hd,i, Hd,o and Hp are the brine enthalpies at the inlet of hollow fibers, outlet of hollow fibers, inlet of membranes, outlet of membranes and outlet of permeate, respectively; Qloss is the heat loss from the module to the surrounding environment. md;i Hd;i −Hb;o þ mp Hd;0 −Hp −Q loss ¼ mb;i Hd;0 −Hb;i
ð3Þ
md;i cp;b Td;i −Tb;o þ mp cp;p Td;o −Tp −Q loss ¼ mb;i cp;b Td;o −Tb;i
ð4Þ
mp cp;p Td;o −Tp Td;i −Tb;o Q loss − ¼1 þ Td;o −Tb;i m c T −T m c b;i p;p d;o b;i b;i p;b Td;o −Tb;i
RO Brine
MD
179
ð5Þ
ΔTtop ¼ Td;i ¼ Tb;o
ð6Þ
ΔTend ¼ Td;o −Tb;i
ð7Þ
Post-treatment
Fig. 1. Schematic drawing of the combined RO and MD systems.
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H. Geng et al. / Desalination 355 (2015) 178–185
△Ttop
Td,i
Tb,o
△Tstage,b
△T
membranes
stage,d
Tp,mp
hollow fibers Td,o md,o
Tb,i mb,i
MD Module
△Tend
Fig. 3. One-stage AGMD process with energy recovery.
R¼
mp mb;i
ð8Þ
where ΔHv is the heat of vaporization of water vapor, which is a function of temperature. ΔHv is calculated by [17]: 3
where ΔTtop and ΔTend are the temperature difference at the top and the bottom of the module, respectively. During the experiment, good thermal insulation is introduced to diminish the heat loss from the membrane distillation system to the surrounding environment, so the heat loss to the surrounding environment can be neglected. Then it will be ΔTtop ≤ ΔTend. If the heat loss to the surrounding environment cannot be negligible, it may be ΔTtop ≥ ΔTend. R is the water recovery and we found that R usually was less than 10%. In the AGMD process, permeate flux (JD), gained output ratio (GOR), evaporation efficiency (η) and water recovery (R) are the most important performance indicators.
JD ¼
mp s
ΔHv ¼ 2:245 10 þ 2:475ð373:0−ðT þ 273:15ÞÞ:
The heat input provided from the external heat source, Qin, and the heat recovered by the brine in the hollow fibers, Qrec, can be calculated as: Q in ¼ mb;i cp;b Td;i −Tb;o ¼ mb;o cp;b Td;i −Tb;o
ð12Þ
Q rec ¼ mb;i cp;b Tb;o −Tb;i :
ð13Þ
ð9Þ
mp ΔHv mp ΔHv ≅ η¼ md;i Hd;i −md;o Hd;o md;i cp;d Td;i −md;o cp;d Td;o
(Tb,o)1
ð10Þ
(Td,i)1
(Tb,o)2
permeate
(Tb,i)1
(Td,o)1
In the AGMD process, the specific heat input can be: cp;b ΔTtop Q in mb;i cp;b Td;i −Tb;o ¼ ¼ mp R mp
(Td,i)2
(Tb,o)n
(Td,o)2
ð14Þ
(Td,i)n
permeate
permeate
(Tb,i)2
ð11Þ
(Tb,i)n
RO Brine
Fig. 4. High water recovery with a series of AGMD stages.
(Td,o)n
H. Geng et al. / Desalination 355 (2015) 178–185 Table 1 Characteristics of AGMD modules used in the experiments.
181
Table 2 Composition of an artificial RO brine.
Characteristics
AGMD module
NaCl
CaCl2
MgSO4
KCl
Na2SO4
NaHCO3
MgCl2
Membrane ID/OD (mm) Membrane average pore size (μm) Membrane porosity Hollow fiber ID/OD (mm) Number of membranes Effective length of module (m) Effective internal membrane area (m2) Number of hollow fibers Effective internal hollow fiber area (m2) Air gap width (mm)
0.55/0.7 0.23 71% 0.4/0.52 550 1.0 0.95 1100 1.38 0.5
45.5
1.20
1.30
1.26
7.07
0.32
6.95
Tb;i ¼ Tb;i ¼ … ¼ Tb;i 1
2
Td;i ¼ Td;i ¼ … ¼ Td;i 1
and then the GOR can be: GOR ¼
mp ΔHv mp ΔHv ΔTstage;b R ΔHv ¼ ¼ ¼η Q in cp;b ΔTtop ΔTtop mb;o cc;b Td;i −Tb;o
ð15Þ
2
ð20Þ
n
The brine concentration keeps on increasing down the stage, which influences the JD and GOR of each stage. Then ΔTstage, ΔTtop and water recovery (Ri) of each stage may not be equal. Ri ¼
ΔTstage;b ¼ Tb;o −Tb;i
ð19Þ
n
mp mb;i
ð16Þ
! ð21Þ i
Let the overall water recovery be R. Then R can be calculated by: ΔTstage;d ¼ Td;i −Td;o
ð17Þ
where ΔTstage,b and ΔTstage,d are the temperature drop of the cold brine side in the hollow fibers and hot brine side in the membranes, respectively.
The details of the multi-stage AGMD process containing a series of countercurrent-flow AGMD modules operated in continuous mode are illustrated in Fig. 4. In the multi-stage AGMD experiment, cold brine enters the AGMD module of stage 1 at (Tb,i)1, (mb,i)1 and leaves at (Td,o)1, (md,o)1. Then the concentrated brine is introduced into the cooling heat exchange and enters the AGMD module of stage 2 at (Tb,i)2, (mb,i)2. Tb,i and Td,i of each stage are kept the same.
j
ð18Þ
jþ1
Tb,o
n X
mp ΔHv
j¼1
n X cp md;i Td;i −Tb;o j¼1
Cold RO brine
j
Table 1 shows the characteristics of hollow fiber membranes, hollow fibers and membrane modules used in the experiments. The microporous-hydrophobic PP hollow fiber membranes and heatexchange hollow fibers were kindly provided by Tianjin Chemical
Td,i hot RO brine
Thermostat B
Membranes permeate water
Magnetic pump
ð23Þ
3.1. Membrane and membrane module
Rotameter A
Tb,i
:
3. Experimental
Rotameter B Hollow Fibers
ð22Þ
The overall value of GOR will be:
GOR ¼
2.2. Multi-stage AGMD process equations and water recovery
md;o ¼ mb;i
R ¼ R1 þ R2 þ … þ Rn :
Td,o Concentrate Tank
Thermostat A Fig. 5. Schematic diagram of one stage AGMD experimental apparatus.
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H. Geng et al. / Desalination 355 (2015) 178–185
8
0.09
7
Td,i=95ć
Td,i=85ć
Td,i=85ć
6
0.07
5 R
GOR
Td,i=95ć
0.08
0.06
4 0.05
3 0.04
2 0.03
1 2
4
6
8
10 12 ƸTtop ć
14
16
2
18
4
6
8
10
12
14
16
18
ƸTtop ć Fig. 8. Experimental results of R as a function of ΔTtop at F = 80 L/h.
Fig. 6. Experimental results of GOR as a function of ΔTtop at F = 80 L/h.
Separating Technologies Co. Ltd., Tianjin. Each AGMD module has 10 layers of hollow fiber membranes and 11 layers of heat-exchange hollow fibers. In the new AGMD module, the rate among membranes and heat-exchange hollow fibers remained unchanged. The heat transfer area of hollow fibers of 1.38 m2 is enough within the operating conditions. In order to keep the distribution of membranes and hollow fibers unchanged, 10 hollow fiber-membrane distributors made of ABS are installed in the AGMD module with equal distances from each other.
Na2SO4 and NaHCO3 were prepared together in tap water. Before the two solutions were mixed in the brine tank, each was stirred sufficiently to make sure that each chemical was dissolved. However, the solubility of CaSO4 (Ksp (25 °C) = 2.5 × 10−5) and CaCO3 (Ksp (25 °C) = 4.8 × 10−9) is very low. To avoid causing membrane fouling in the multi-stage AGMD process, CaCO3 and CaSO4 should be removed in the pretreatment process. The total dissolved solid (TDS) and electrical conductivity of the permeate water were monitored by a conductivity meter (DDSJ-308A, Leici, China).
3.2. Experimental apparatus 4. Results and discussion The one-stage AGMD experimental apparatus consisting of a thermostat, magnetic pump, rotameter, AGMD module and thermometric indicator is schematically depicted in Fig. 5. The suitable insulation must be taken in order to reduce heat loss from the membrane distillation system to the surrounding environment. The temperature of the feed is monitored at the inlets and outlets of hollow fibers and membranes using thermocouples with sensitivity of 0.1 °C. The one-stage AGMD experiment is introduced to investigate the performance of the new AGMD module. Then the fairly good operating conditions at which the values of JD and GOR are both large are chosen as the operating conditions of the multi-stage AGMD experiment. The experiments are conducted with an artificial 62.5 g/L RO brine supplied by Tianjin Chemical Separating Technologies Co. Ltd., Tianjin. The composition of the artificial RO brine showed in Table 2 included and major ions in RO brine, such as Na+, Cl−, Mg2+, Ca2 +, K+, SO2− 4 HCO− 3 . The feed solution of RO brine was prepared as follows: NaCl, MgCl2, CaCl2 and KCl were dissolved together in tap water; MgSO4,
4.1. Experimental study of the one-stage AGMD process The performance of the new AGMD module in one-stage was evaluated under various operating conditions. The relationship between water recovery (R), gained output ratio (GOR), permeate water flux (JD) and temperature difference at the top (ΔTtop) of AGMD module was fully studied. GOR as a function of ΔTtop (Td,i = 95 °C and 85 °C, F = 80 L/h) is illustrated in Fig. 6. GOR decreases as ΔTtop increases. Based on Eq. (15), there is an inverse correlation between GOR and ΔTtop. GOR reaches the value of 7.4 for ΔTtop of 4.2 because the heat input is recovered by the cold brine in the heat-exchange hollow fibers. When a high value of GOR is obtained, the value of ΔTtop is quite small, such as GOR of 13.8 with ΔTtop of 2.9 °C as illustrated in Ref. [17]. Without the heat recovery, the GOR of MSF and MED processes cannot be more than 1 [27]. The latent heat of water vapor is used to preheat the cold feed. Thus,
6.5
20
6.0
Td,i=95ć
18
Td,i=85ć
16
5.5
Td,i=85ć
14
2
ƸTend(ć)
5.0
JD(L/m h)
Td,i=95ć
4.5 4.0
12 10 8 6
3.5
4
3.0
2 0
2.5
2
4
6
8
10 12 ƸTtop ć
14
16
18
Fig. 7. Experimental results of JD as a function of ΔTtop at F = 80 L/h.
0
2
4
6
8
10
12
14
16
18
20
ƸTtop ć Fig. 9. Experimental results of ΔTend as a function of ΔTtop at F = 80 L/h.
H. Geng et al. / Desalination 355 (2015) 178–185
0.09
16
183
6.0
6.4
ƸTtop
14
0.08
6.0
0.07
5.6
10 8
0.06
R
GOR JD
5.6
2
5.2
5.2 4.8 4.8
6 0.05 4
40
50
60
70
80 90 F(L/h)
0.04 100 110 120 130
4.0
only a little heat is needed to be supplied externally for the following evaporation. However, JD increases as ΔTtop increases as shown in Fig. 7, but JD increases slowly at the high value of the ΔTtop. JD and GOR were substantially greater in experiments conducted with Td,i = 95 °C than Td,i = 85 °C because the water vapor pressure exponentially increases with temperature according to the Antoine equation. The driving force of mass transfer is the vapor pressure difference across the membranes resulting from the transmembrane temperature difference. A high temperature difference results in a high water vapor pressure difference across the membrane and a strong driving force for mass transfer in the AGMD process. When ΔTtop is 5.8 °C at Td,i = 95 °C and Tb,i = 45 °C, GOR and JD are 6.5 and 5.1 kg/m2 h, respectively, while as described in Ref. [26], GOR and JD are 5.7 and 4.3 kg/m2 h, which means that the performance of the new AGMD module has improved (Fig. 12). The porosity and average pore size of the membrane were increased in the AGMD module used in all experiments. As described in many papers [28,29], the mass-transfer coefficient depends on the characteristics of porous fibers including the porosity and average pore size. Hollow fiber membranes with high porosity and average pore size were prepared and used to increase the performance of the AGMD module. In the AGMD process, the heat transfer resistance across hollow fibers is the main resistance and a good heat transfer capacity has a positive effect on the mass transfer. It is essential for enhancing heat transfer to keep the distribution of membranes and hollow fibers stable over time. Fig. 8 shows that R increases with the increase of ΔTtop. In order to get a high water recovery, a high inlet temperature Td,i is expected. The maximum value of R is only 7.3% at Td,i = 95 °C and Tb,i = 25 °C, but the GOR was only 2.6. It is not strictly comparable to compete
4.2. High water recovery with a series of AGMD stages In order to get a high water recovery, we study on a 4-stage AGMD process and design for AGMD stages. In the 4-stage AGMD experiment,
9.0
0.25
Rall 0.15
0.060 0.10
ƸTtop(ć)
0.20
0.055
ƸTtop
8.0
0.070 0.065
4
commercially with MSF, MED and RO processes. Using a series of AGMD stages was preferable to get a high water recovery. ΔTtop is a little larger than ΔTend as shown in Fig. 9 partly because of heat loss to the environment. Meanwhile the permeate water carried a little energy flowing out of the hot brine in the membranes continuously. ΔTstage,d is a little larger than ΔTstage,b and it is much larger than ΔTtop. However, we found in experiments that ΔTstage,d was smaller than ΔTtop when the length of the membrane module was short. During all experiments, the electrical conductivity of permeate water was below 25 μS/cm and salt rejection was greater than 99.9% (the concentration of 0.1 g/L is around 118 μS/cm). Fig. 10 shows the variation of ΔTtop and R with F at Tb,i = 45 °C and T d,i = 95 °C. ΔT top increases with the increase of F, but R decreases slowly, which mainly because of the shorter retention time of the feed. JD increases as F increases, and it is conducive to increase the heat transfer coefficient and reduce the temperature and the concentration polarization effect. The measured experimental JD and GOR at different operating parameters are repeatable. The maximum experimental error in JD is + 0.12 kg/m2 h (2.7%) at ΔTtop = 4.9 °C, Td,i = 95 °C, and F = 80 L/h, and the maximum experimental error in GOR is + 0.10 (4.2%) at ΔTtop = 14.2 °C, Td,i = 85 °C, and F = 80 L/h (Fig. 13).
8.5
Ri Rall
0.075
2 3 Number of stages
Fig. 12. Experimental results of GOR and JD as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
0.30
0.080
4.0 1
Fig. 10. Experimental results of ΔTtop and R as a function of F at Td,i = 95 °C and Tb,i = 45 °C.
Ri
4.4
4.4
2
JD(kg/m h)
GOR
R
ƸTtop(ć)
12
7.5 7.0 6.5 6.0 5.5
0.050
0.05 1
2 3 Number of stages
4
Fig. 11. Experimental results of Ri and Rall as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
5.0 1
2 3 Number of stages
4
Fig. 13. Experimental results of ΔTtop and R as a function of the number of stages at Td,i = 95 °C, Tb,i = 45 °C, and F = 80 L/h.
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H. Geng et al. / Desalination 355 (2015) 178–185
0.068
6.2
0.9 Ri Rall
GOR=-0.00955c+6.0714 0.064
2
6.0
R =0.976
0.8
0.060
0.7
0.056
0.6
5.6
0.5
0.052
5.4
0.048
5.2
0.044
Rall
Ri
GOR
5.8
0.4 0.3
0
20 40 60 Feed concentration(g/L)
4
80
6
8
10
12
14
Number of stages
Fig. 14. Dependence of linear GOR coefficient on the feed concentration at Td,i = 95 °C and Tb,i = 45 °C.
cold brine enters the AGMD module of stage 1 at (Tb,i)1 = 45 °C and (Fb,i)1 = 80 L/h. Then the concentrated brine is introduced into the cooling heat exchange and enters the AGMD module of stage 2 at (T b,i ) 2 = 45 °C. Tb,i and T d,i of each stage are kept the same. The multi-stage AGMD process can be operated in two modes. In the first mode, the feed flow rate Fb,i of each stage remains equal. In the second mode, the feed flow rate Fb,i of this stage is equal to the former stage. We now analyze the relation between the water recovery of each stage (Ri), water recovery of 4-stage AGMD (Rall), GOR, JD and ΔTtop of each AGMD module at Tb,i = 45 °C and Td,i = 95 °C. 4.2.1. Design for the 4-stage AGMD process 4.2.1.1. Case of the equal Fb,i of each stage. The water recovery of each stage and overall water recovery as a function of the number of stages are illustrated in Fig. 11. We can see that the general trend of Ri decreased for 4-stage continuous operations, but Rall always increases. Ri of one stage in a continuous operational mode is limited to 6–7% and the average Ri of 4-stage is 6.2%. Ri of the fourth stage is 6.1% and only decreases by 3.1%. Rall can reach 24.7% after 4-stage AGMD concentration, where the brine concentration is 84.5 g/L. It is necessary to put a number of AGMD stages in series to get a high water recovery. JD and GOR decrease as the number of stages increases. In the 4-stage AGMD process, the brine concentration increases continuously. The increase of brine concentration reduces the partial water vapor pressure and consequently reduces the driving force of the AGMD process, which leads to a decrease of the driving force of mass transfer in the AGMD process. Meanwhile the increase of brine concentration can reduce the diffusion coefficient of sodium chloride in feed liquid, which
Fig. 16. Ri and Rall as a function of number of stages in multi-stage AGMD process.
enhances the effect of concentration polarization. The heat loss of the hot brine in the membranes decreases because of the decrease of JD. Therefore the outlet temperature Td,o increases slightly and ΔTtop increases. 4.2.2. Design for multi-stage AGMD stages If we have to get a higher water recovery than 80%, we need more AGMD stages. It is difficult for us to get the multi-stage results by experiment because in the experiment it needs many external heat exchangers to keep Tb,i and Td,i constant. In the multi-stage AGMD process, we analyzed the case of equal Fb,i of each stage. In the multi-stage AGMD process, Tb,i and Td,i were kept the same for each stage. The values of Tb,i and Td,i were 45 °C and 95 °C, respectively. For the equal Fb,i of each stage, the feed concentration varied with the stage and it was the only adjustable variable. JD and the feed concentration changes in the relationship are linear as shown in many studies. In the 4-stage AGMD process, their linear relevance is not high. This result may be caused by fine distinctions of different AGMD modules during the preparation process. To get a good linear regression, the relationship between JD, GOR and c was studied in the same AGMD module. As can be seen in Fig. 14, there is a linear relationship with R2 of 0.976 between GOR and c. Similarly, JD is given by a linear equation that is strongly dependent on the feed concentration. If the model was applied in the 4stage AGMD process, the maximum predicted error of GOR was only 0.1 and the maximum predicted error of JD was only 0.1 kg/m2 h, which means that the model could predict the multi-stage AGMD process to a great extent. The minimum value of JD is 3.9 kg/m2 h and it is only slightly lower than that of the fourth stage in the 4-stage AGMD process. Rall can be up to 82.2% after the 14-stage AGMD process and the feed concentration is
JD= -0.00683c+5.775
5.7
R =0.970
5.2
5.6
5.2
GOR JD
2
4.8
4.8
GOR
4.4
5.4
4.4
4.0
4.0
3.6
3.6
2
5.5
5.3 5.2 5.1 0
20 40 60 Feed concentration(g/L)
80
JD(kg/m h)
5.8
2
JD(kg/m h)
5.9
3.2 4
6
8
10
12
14
3.2
Number of stages Fig. 15. Dependence of linear JD coefficient on the feed concentration at Td,i = 95 °C and Tb,i = 45 °C.
Fig. 17. GOR and JD as a function of number of stages in multi-stage AGMD process.
H. Geng et al. / Desalination 355 (2015) 178–185
352 g/L. The solubility of sodium chloride is 366 g/L at 40 °C. Then the concentrated brine can be used to prepare caustic soda in chlor-alkali industry and realize comprehensive utilization of seawater resources. 5. Conclusions In this study, we provided a multi-stage AGMD process for further concentrating RO brine and getting a high water recovery of seawater desalination. In experiment, the improved AGMD module with a series of hollow fiber-membrane distributors to keep the distribution of membranes and hollow fibers uniform and stable was designed. The following conclusions were drawn: (1) In the one-stage AGMD process, the maximum value of JD and GOR could reach 6.8 kg/m2 h and 7.1 respectively. The water recovery of one-stage was limited to 5–8% and the maximum value of R is only 7.3% at Td,i = 95 °C and Tb,i = 25 °C (Fig. 15). (2) Two different continuous operational modes of the 4-stage AGMD process were demonstrated experimentally and a high water recovery of 24.7% was obtained in the case of equal Fb,i of each stage. JD decreased by 22.2% from 5.4 kg/m2 h of the first stage to 4.2 kg/m2 h of the fourth stage in the case of not equal Fb,i of each stage (Fig. 16). (3) In the multi-stage AGMD process, Tb,i, Td,i and Fb,i of each stage were kept the same. The linear relationship between JD, GOR and c was studied through a single factor experiment. The maximum value of Rall and the minimum value of JD were 82.2% and 3.9 kg/m2 h respectively after the 14-stage AGMD process (Fig. 17). Notation Cp GOR H ΔHv JD mp mb,i mb,o md,i md,o Qin Qrec R Ri Tp Tb,i Tb,o Td,i Td,o ΔTend ΔTstage,b ΔTstage,d ΔTtop η
specific heat, kJ/kg/°C gained output ratio specific enthalpy, kJ/kg enthalpy of water evaporation, kJ/kg water permeate flux, kg/m2/h permeate water flow out of AGMD unit, kg/h brine flow rate into AGMD unit, kg/h brine flow rate out of AGMD unit, kg/h distillate flow rate into AGMD unit, kg/h distillate flow rate out of AGMD unit, kg/h brine heat input, kJ heat recovery, kJ water recovery water recovery in stage i permeate water temperature, °C brine inlet temperature, °C brine outlet temperature, °C distillate inlet temperature, °C distillate outlet temperature, °C Td,o − Tb,i Tb,o − Tb,i Td,o − Td,i Td,i − Tb,o thermal efficiency
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2014.10.038.
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