Experimental study of ammonia removal from water by membrane distillation (MD): The comparison of three configurations

Journal of Membrane Science 286 (2006) 93–103

Experimental study of ammonia removal from water by membrane distillation (MD): The comparison of three configurations Zhongwei Ding ∗ , Liying Liu, Zhaoman Li, Runyu Ma, Zurong Yang College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Received 17 July 2006; received in revised form 5 September 2006; accepted 11 September 2006 Available online 14 September 2006

Abstract This study tries to compare the separation performance of three kinds of membrane distillation (VMD, DCMD and SGMD) used in the removal of ammonia from water in terms of mass transfer coefficient (Ka ) and selectivity (β). For all three MD configurations, equations to determine their Ka and β values from experimentally obtained results were derived. Experiments of three MD configurations were conducted, respectively, to measure the ammonia concentration and transmembrane flux. So, on the ground of derived equations, Ka and β were evaluated. It was found that, in comparison of three MD configurations with each other under similar operation conditions, VMD shows the highest Ka but the lowest β, DCMD gives the highest β and moderate Ka , and SGMD has the moderate β and lowest Ka . Factors that may affect the separation processes, such as the membrane characteristics, the feed temperature, the feed and permeate velocity, and the initial concentration and PH of the feed were also experimentally examined. © 2006 Elsevier B.V. All rights reserved. Keywords: Membrane distillation; Ammonia removal; Mass transfer coefficient; Selectivity

1. Introduction While nitrogen element exits in nature in many different forms, its majority in water body is ammonia. It has been well known that excessive ammonia in water is an important source of eutrophication, the enrichment of water by nutrients causing an accelerated growth of algae, which often result in red water bloom in the lake or ocean. And fish often suffer from the concomitant depletion of dissolved oxygen in water. The ammonia contaminated water needs more medicine to be added in the process of treatment, and this may seriously damage the quality of water. The modern industries should be responsible for the expanding contamination of water resource caused by ammonia, and more and more authorities are convinced of the seriousness of this problem. However, conventional ammonia removal techniques, such as physical stripping and biological method, cannot satisfy the increasing effluent criteria, or they cannot solve the problem with high efficiency and acceptable

∗

Corresponding author. Tel.: +86 10 64454601; fax: +86 10 64436781. E-mail addresses: [email protected], [email protected] (Z. Ding). 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.09.015

cost. Therefore new and effective method needs to be developed to solve this problem. Since 1990s, the membrane-based stripping process, for its very high inter-phase area and its easiness to control in operation, has been recognized by more and more people. Studies pertinent to this topic, especially to the removal of volatile component from water or wastewater, have been widely conducted, and many volatile compounds, such as ammonia [1–3], chloroform [4–6], toluene, phenol, o-xylene [6–7], trichloroethylene [8], were tested in these studies. Usually, the permeated components are discharged out of the membrane module either by means of sweeping gas, or using some solvent, which may react very quickly with the permeated components. These studies discussed the separation process mainly in terms of mass transfer coefficient, permeate flux and removal efficiency. And factors affecting mass transfer, such as feed and sweep gas velocity, the concentration and pH of the feed, etc. have been determined. Although membrane distillation (MD) was discovered in late 1960s as a potential desalination technique, it may also be regarded a membrane-based stripping process if it is used to remove volatile component from water. Like other membranebased stripping process, MD makes use of micro-porous hydrophobic membrane to act as inter-phase supporter between

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liquid and gas phase, and also the volatile component is driven across the membrane by its concentration difference. Here, the particularity of MD lies on its deliberately introduced temperature difference between the two sides of the membrane to intensify the mass transfer process or improve the removal efficiency. The permeated species may be carried out of the membrane module in four different ways, corresponding to four different MD configurations: (i) the permeated component condenses directly in the liquid coolant flowing through the module at permeate side (direct contact membrane distillation, DCMD), (ii) the permeated component is aspirated into the vacuum system (vacuum membrane distillation, VMD), (iii) the permeated component is swept out of the module by gas stream flowing through the permeate side (sweeping gas membrane distillation, SGMD), (iv) the permeate component passes through a layer of static gas and condenses on a cold plate, and the condensate is drained out of the module by gravity (air gap membrane distillation, AGMD). DCMD, VMD and SGMD have been extensively used for stripping purpose, and from the published literatures [9–14] it can be found that attentions were mainly paid to evaluating the separation process in terms of permeate flux and selectivity. Certainly, that being in possession of high permeate flux and selectivity are desirable. Here, one important question may arise: for a certain mixture to be separated, which MD configuration is the most applicable? To the best of the authors’ knowledge, few studies have performed MD based stripping by one more configurations. Therefore this question is yet to be answered. In this study, the removal of ammonia from water by means of VMD, SGMD and DCMD are experimentally investigated, respectively. The separation performance of the three MD configurations will be compared in terms of experimentally obtained mass transfer coefficient and selectivity. So, based on the results of this study, the above question may be answered. 2. Theory 2.1. Mass transfer equation and mass transfer coefficient

(4)

The membrane employed in MD can exert its influence on transmembrane mass transfer in three ways. Firstly, ammonia molecules move only through the pores of the membrane, so the effective area for mass transfer is less than the total membrane area. Secondly, the membrane pores do not go straight through the membrane, so the path for ammonia transport is longer than the thickness of the membrane. Thirdly, the inside wall of pores also increases the resistance to diffusion. The definition of km [1,3] includes the above three factors km =

Dm ε τδ

(5)

As the mass transfer resistance of permeate side can be neglected, Eqs. (2) and (4) permit to rewrite the mass flux of ammonia for VMD and DCMD: N a = Ka c f

(6)

where 1 1 1 = + Ka kf Hkm



Kb 1+ [OH− ]

 (7)

And Ka is called the overall mass transfer coefficient of ammonia. But for SGMD, the ammonia mass transfer in permeate side is not negligible, and can be quantitatively described as

The following equation is applicable for the ammonia mass flux through the gas phase trapped within membrane pores: (3)

(8)

The equation of overall mass transfer is obtained from Eqs. (2), (3) and (8)    cp Kb Na = Ka cf − (9) 1+ [OH− ] H where

(1)

With the consideration of this dissociation, Semmens et al. [1] derived the following equation to describe the mass transfer in the feed side:   Kb c1,fm Na = kf cf − c1,fm − (2) [OH− ]

Na = km (Hc1,fm − c1,pm )

Na = km Hc1,fm

Na = kp (c1,pm − c1,p )

The ammonia removal from water by MD is a membranebased stripping process, and theoretically the process consists of three mass transfer resistances connected in series, existing in feed side, membrane and permeate side, respectively. The mass transfer in the feed side is complicated by the ionization of ammonia: NH3 + H2 O  NH4 + + OH−

In VMD of the present study, the vacuum degree is above 90 kPa at permeate side, and most of vacuum is occupied by water vapor. In DCMD, the 0.5 M sulfur acid is circulated at permeate side as tripping solution, and the permeated ammonia can reacts promptly with H2 SO4 to form non-volatile ammonium sulfate. Therefore, for the two MD configurations, the concentration of ammonia in the permeate side should be very close to zero. So Eq. (3) can be rewritten as

1 1 = + Ka kf



1 1 + Hkm Hkp



Kb 1+ [OH− ]

 (10)

2.2. Methods to determine mass transfer coefficient from experimental results This study tries to evaluate the separation performance in terms of two different physical indexes. One is the mass transfer coefficient and the other is the selectivity. This section shows how to determine the overall mass transfer coefficient from experimental results.

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2.2.1. For VMD and DCMD In VMD and DCMD, Eq. (6) is applicable. Considering a differential element of the employed membrane module, the mass balance of total ammonia for this element gives the following equation: −Ff dc = Ka cf dA

(11)

Integration of Eq. (11) under proper boundary conditions gives Ka A cf,0 = ln cf,L Ff

(12)

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where Fp is the flowrate of sweeping gas. The mass balance for an element in SGMD module gives     cp Kb −Ff dcf = Ka cf − 1+ dA (18) H [OH− ] Eqs. (17) and (18) permit to write dcf Ka dA =− cf (1 − B) + cf,L B Ff where  B = 1+

Kb [OH− ]



Ff HFp

(19)

(20)

where Ff is the flow rate of the feed, A the total membrane area, cf,0 and cf,L are the ammonia concentration in the feed flow at the module inlet and outlet, respectively. In this work, the ammonia removal experiments were performed in batch operation, the mass balance for the solution in the feed tank gives

The relationship of cf,0 with cf,L can be obtained by the integration of Eq. (19):

−d(Vc0 ) = Ff (cf,0 − cf,L ) dt

E=

(13)

The MD process reduces the feed volume in the tank there under the following equation: V = V0 −

JA t ρ

(14)

where V0 is the initial feed volume in the tank for each run, J the total mass flux through the membrane. The following can be obtained from the above three equations: −d(Vcf,0 ) = −cf,0 dV − V dcf,0   JA JA 0 = cf,0 dt − V − t dcf,0 ρ ρ    Ka A dt = Ff cf,0 1 − exp − Ff

0 /ct ) ln(cf,0 f,0

(15)

(21)

where 1−B exp[Ka A(1 − B)/Ff ] − B

(22)

Integration of Eq. (13), in which cf,L is replaced by Eq. (21), leads to   0 cf,0 JA JAt (23) ln t = − ln 1 − 0 ρFf (1 − E) − JA cf,0 m 0 /ct ) with −ln(1 − JAt/m0 ) will give a line, and Plotting ln(cf,0 f,0 from its slope the total mass transfer coefficient could be determined.

This study evaluates the separation performance in terms of selectivity as well as mass transfer coefficient. Selectivity represents the measure of the preferential transport of ammonia. Similar to the common definition, the selectivity in this study is defined as y/(1 − y) β= (24) x/(1 − x) As x and y are very close to zero in this study, β ≈ y/x. Therefore β=

(16)

−ρd(Vcf,0 ) cf,0 JA dt

(25)

Considering Eq. (14), one can obtain

−ln(1 − JAt/m0 )

Plotting with will give a line, and from its slope the total mass transfer coefficient could be obtained. 2.2.2. For SGMD In SGMD, both cf and cp vary along the axial direction of membrane module, and their relationship is represented by the mass balance equation for the membrane module: Fp cp = Ff (cf − cf,L )

cf,0 (1 − B) = Ecf,0 exp[Ka A(1 − B)/Ff ] − B

2.3. Selectivity

The integration of this equation under proper initial conditions gives c0 JA ln f,0 t ρ{Ff [1 − exp(−Ka A/Ff )] − JA/ρ} cf,0   JAt = − ln 1 − 0 m

cf,L =

(17)

JA(β − 1) dt dcf,0 = cf,0 JAt − m0

(26)

Integration of Eq. (26) under proper initial conditions gives   c0 JAt ln f,0 (27) = −(β − 1) ln 1 − t cf,0 m0 where m0 is the initial mass of the feed. The slope of the line 0 /ct ) with −ln1 − JAt/m0  gives the obtained by plotting ln(cf,0 f,0 selectivity.

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Table 1 The specifications of the employed membranes

Type and material Effective area (cm2 ) Thickness (␮m) Pore diameter (␮m) Porosity (%)

3.2. The flowsheet and equipment

No. 1

No. 2

Flat sheet PTFE 31 80 0.1 60

Flat sheet PTFE 31 60 0.2 60

3. Experimental 3.1. Membranes The membranes employed in this study were provided by Beijing Institute of Plastic Research, and their specifications are shown in Table 1. It is well known that hydrophobic membrane with pore size of 0.1–1.0 ␮m is suitable for MD. However, caution should be taken in choosing membrane (pore size) for MD, especially for VMD. VMD is usually operated under higher transmembrane pressure difference (up to 100 kPa) than any other MD configurations. The membrane with larger pore size has lower liquid entry pressure, therefore it is more easily wetted during VMD process. For this reason, this study selects membranes of 0.1 ␮m and 0.2 ␮m pore diameter for experiments. To observe the influence of membrane characteristics on separation performance, both 0.1 ␮m and 0.2 ␮m membranes were used in VMD. On this topic, the drawn conclusions in VMD should be qualitatively valid for the other two configurations, because the Knudsen diffusion is important for all three configurations in transmembrane mass transfer, and the gas trapped within membrane pores causes significant mass transfer resistance in all configurations too. The 0.2 ␮m membrane was chosen to be utilized in SGMD to partly offset the very high mass transfer resistance in gas flow. And, for its less possibility of membrane breakage, the 0.1 ␮m membrane was selected to perform DCMD.

The flow sheet of the MD equipment of this work is given in Fig. 1. The observed three MD configurations are very similar in the feed circulation. The feed solution was driven by a gear pump (MG213XTS17, Fluid-o-Tech.), which had been calibrated before experiments. A thermostatic bath of 2 kW (501A, Shanghai Rongfeng Sci Equipment Corp.) was used to provide the feed with heat energy to perform MD. The temperatures of the feed stream at the inlet and outlet of the membrane module were measured by two Pt-100 sensors with sensitivity ±0.1 K. The feed tank was placed on an electronic balance (ARD110, OHAUS Corp.) with the sensitivity of 0.1 g, and the total mass flux through the membrane was obtained from the weight change of solution in the tank divided by the membrane area and time interval. However, the three MD configurations are quite different in the permeate side. In VMD, the vacuum was pulled by a vacuum pump (SHZ-III, Shanghai Yarong Biochem. Equipment Corp.), whose water was cooled by a chiller (DTY-8A, Beijing Detianyou Corp.). A dumper was placed between the membrane module and the vacuum pump to stabilize the permeate side pressure, which was regulated by an electromagnetic valve and a vacuum transducer with the uniformity ±0.1 kPa. For SGMD, the air was aspirated from environment into membrane module by a fan (ACD-388D, Guangdong Haili Corp.). A tube packed with silica gel particle was installed on the air line to dry the air before it entered into the membrane module. The permeated species (NH3 and water vapor) was swept out of the module by the air flow. The air temperature at the inlet and outlet of the module were measured by another two Pt-100 sensors. In DCMD, another gear pump was employed to circulate the 0.5 M (initial concentration) sulfuric acid, and the circulation included the permeate side of the membrane module, the permeate tank and the coil in the chiller. The sulfur acid was used as not only a refrigerant, but also a reactant, which could

Fig. 1. Schematic of MD apparatus. (1) Thermostatic bath, (2) coil for heating, (3) gear pump for feed circulation, (4) feed tank, (5) electric balance, (6) flat sheet membrane module, (7) buffer tank, (8) vacuum pump, (9) chiller, (10) pipe for air drying, (11) air fan, (12) permeate tank for DCMD and (13) gear pump for permeate circulation in DCMD.

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promptly react with permeated ammonia to produce ammonium sulfate. 3.3. Experimental procedure Working on ammonia removal from water by VMD, SGMD and DCMD, respectively, the experiments were designed to obtain their mass transfer coefficient and selectivity. The influence of various factors, such as the membrane characteristics, the feed temperature, the feed and air flow velocity inside the membrane module, the pH of the feed and the feed concentration, were investigated. The experiments were conducted in batch mode. The feed liquid was prepared by pure ammonia and de-ionized water, and its pH was adjusted by adding HCl and NaOH to the feed tank. Each time before MD was started a test was performed with the feed of ambient temperature flowing through the membrane module. Under this condition, it could be made sure that there is no considerable leakage in the employed membrane if the measured weight by electric balance remains unchanged for more than 5 min. One sample of the feed solution was taken from the feed tank every other 30 min, and was analyzed by formaldehyde titration to give the feed concentration. The feed weight loss within a certain time was observed by the electric balance, so the total transmembrane mass flux was calculated. The obtained concentration and mass flux were used to determine mass transfer coefficient and selectivity on the ground of equations derived in above section. 3.4. Analysis method A titration method was used in this study to measure the concentration of ammonia in the feed tank. This method is based on the following reaction, and was proved to be effective with acceptable accuracy and convenience [15]: 4NH4 + + 6HCHO = (CH2 )6 N4 + 4H+ + 6H2 O By adding superfluous formaldehyde in the sample solution, the existing ammonium salts can react instantly and completely to yield acid in equimole with ammonium salts. This acid can be titrated by the solution of sodium hydroxide, using phenolphthalein as the indicator. So the concentration of ammonium salts can be determined from the amount of sodium hydroxide consumed.

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Table 2 The summary of operating conditions and results of the experiments Items

VMD

SGMD

DCMD

Pore size (␮m) uf (m/s) up (m/s) tf (◦ C) Vacuum degree (kPa) 0 (mol/L) cf,0 pH in the feed Ka (×105 m/s) β

0.1 and 0.2 0.15–1.12

0.2 0.35–0.70 0.35–2.94 50–65

0.1 0.29–0.74 0.29–0.70 50–61.2

0–0.2 mol/L 9–13 1–2.2 8–14

0–0.2 mol/L 10 1.2–2.3 16–23

42.4–55.7 90–97 0–1.2 mol/L 9–13 1–4.7 4–11

0 t Fig. 2. ln(cf,0 /cf,0 ) vs. −ln(1 − JAt/m0 ) in VMD.

β values were determined. Fig. 3 gives the linear relationships 0 /ct ) with −ln(1 − JAt/m0 ) of SGMD, from the slopes of ln(cf,0 f,0 their Ka and β values were obtained. 4.1. The variation of transmembrane flux with time During the separation process, both water and ammonia vaporized at membrane pore entrance of the feed side, trans-

4. Results and discussions To compare the separation performance of the three MD configurations, a summary of their operating conditions and experimentally obtained results is shown in Table 2. Discussions about these results will be given in detail in the following sections. For all three MD configurations, the measured ammonia concentration and transmembrane flux were used to determine total mass transfer coefficients of ammonia, Ka and selectivity, β. As 0 /ct ) an example, Fig. 2 shows the linear relationships of ln(cf,0 f,0 0 with −ln(1 − JAt/m ) of VMD, and from the slopes their Ka and

0 t Fig. 3. ln(cf,0 /cf,0 ) vs. −ln(1 − JAt/m0 ) in SGMD.

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Fig. 4. The variation of transmembrane flux with time.

Fig. 5. The influence of feed temperature on Ka .

ferred to the other side. Interestingly, it was found that, for one running under constant operating conditions, the obtained transmembrane flux, J, decreased with time, as shown in Fig. 4. This phenomenon can be explained by the positive effect on the water vapor pressure caused by the mixing of ammonia with water. Obviously, this effect is in favor of the mass transfer of water vapor across the membrane. As the concentration of ammonia in the feed side deceased with time, this positive effect was gradually mitigated in one running. This explanation may be argued by the fact that, as shown in Fig. 4, the flux decreased more slowly when a feed with lower initial concentration was used. The observed phenomena deeply coincide with the separation performance of the being discussed processes (please see the following sections).

the transfer resistance at permeate side, but the pressure within membrane pore was atmosphere. Although the membrane used in SGMD is of 0.2 ␮m pore diameter, its Ka value is still lower than DCMD (in which the 0.1 ␮m membrane was used), due to the extra resistance introduced by air flow at the permeate side of SGMD membrane module. Although the operating conditions in Fig. 5 were not identical for the three MD configurations, it cannot be a hindrance to drawing the above conclusion from this figure if the following are considered: (a) Fig. 6 shows that in both SGMD and DCMD Ka changes very little with uf . Although the uf of VMD in Fig. 5 is higher than that of SGMD and DCMD, Fig. 6 shows that under the same uf VMD still gives much higher Ka . (b) The studies carried out by other researchers show that there is very little influence of initial feed concentration on Ka [1,2], so the difference in initial feed concentration of the curves in Fig. 5 cannot demolish the conclusion based on Fig. 5. (c) Although PHf is an adjustable parameter that may affect Ka , it was not adjusted in the experiments of Fig. 5. In these experiments the influence of PHf is included in that of initial feed concentra-

4.2. The influence of membrane characteristics and MD configuration on Ka The influence of membrane characteristics on Ka can be seen in Fig. 5. Here, for two sets of VMD experiments conducted under similar conditions but different membrane characteristics, the Ka values obtained from the membrane of 0.2 ␮m pore diameter are much higher than that of 0.1 ␮m, indicating that the resistance caused by membrane is of great importance to ammonia mass transfer. In VMD, due to the very high vacuum degree in membrane pores, the collision of ammonia with pore wall dominates the transfer process within the membrane, so pore size should be an important factor. The 0.2 ␮m membrane has larger pores as well as smaller thickness; therefore its resistance to ammonia transport should be lower than that of 0.1 ␮m membrane. Under similar operating conditions but different configuration, VMD gives higher Ka value than the other two. In VMD, the vacuum not only eliminates the transfer resistance in permeate side, but also greatly reduce the collision of ammonia molecule with water vapor and air within membrane pores. The latter explains why the Ka value of VMD is higher than that of DCMD, in which the 0.5 M sulfa acid was used to eliminate

Fig. 6. The influence of feed velocity on Ka .

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tion, i.e., here, PHf should not be regarded as an independent parameter. 4.3. The influences of operating conditions on Ka 4.3.1. The feed temperature on Ka The effect of feed temperature on Ka can also be read in Fig. 5. Obvious positive effects were observed in VMD and SGMD, but to the exclusion of DCMD. As ammonia has very high solubility in water, it is the mass transfer in gas phase that dominates the overall mass transfer in all three configurations. The average temperature within membrane pore (tm ) or of the sweeping gas increases with feed temperature. Considering that higher diffusivity can be obtained under higher temperature, hot feed is in favor of ammonia diffusion in both membrane pores and sweeping gas. The less sensitivity of DCMD mass transfer to feed temperature may be attributed to the relative higher thermal capacity of the liquid in permeate side, which makes the tm less affected by the feed temperature. Another considerable contribution to the above mentioned positive effect comes from the more favorable thermodynamics under higher temperature. Both Eqs. (7) and (10) show that the Ka depends strongly on the dimensionless Henry constant, H. The higher H the higher the Ka is. H is dependent on the ammonia concentration but it increases always rather strongly with increasing temperature. The effect of feed temperature on Ka can also be illustrated by comparing the Ka value obtained in this study with that of other researchers. Using DCMD under ambient temperature, the value obtained by Semmens et al. [1] is lower than 1.0 × 10−5 m/s, and the highest one obtained by Zhu et al. [2] is 1.4 × 10−5 m/s; whereas, using DCMD under the temperature up to 61.5 ◦ C, this work obtained the value of 2.3 × 10−5 m/s. Although in this study the Ka of DCMD is not very sensitive to feed temperature, so big temperature difference between this study and that of Semmens et al. or Zhu et al. should remarkably contribute to the improvement in Ka . 4.3.2. The influence of feed velocity on Ka As mentioned above, the resistance caused by the gas phase, which is trapped within membrane pores or flows through permeate side, dominates the overall mass transfer. For this reason, the feed velocity should have little influence on Ka . In the present study, this was verified in SGMD and DCMD, but was not completely followed by VMD, as shown in Fig. 6. This exception may be due to the relatively low membrane resistance under high vacuum condition, so the resistance in the feed side of VMD becomes more important than that of DCMD and SGMD. In DCMD, slight increase of Ka with feed velocity was found when feed velocity was lower than 0.5 m/s. This may be the result of the very slow diffusion of hydroxide from the feed bulk to membrane surface (please see Section 4.3.4). 4.3.3. The influence of permeate velocity on Ka The permeate velocity refers to that of the sweeping gas in SGMD membrane module or that of the 0.5 M sulfa acid in DCMD membrane module. As shown in Fig. 7, the mass transfer

Fig. 7. The influence of permeate velocity on Ka .

in DCMD cannot be improved by increasing the velocity of sulfa acid at permeate side, but the sweeping gas velocity in SGMD is of great importance to mass transfer coefficient. Moreover, the SGMD with sweeping gas velocity higher than 1.5 m/s gave even higher Ka value than DCMD. Understanding these results can also resort to the analysis of mass transfer resistance in Sections 4.2 and 4.3.2. Here, the Ka of DCMD is independent of both feed and permeate velocity, indicating that the process is almost totally controlled by the mass transfer within membrane pores. This implies that a simplified equation may be obtained from Eq. (7) to determine the membrane characteristics from the measured Ka . 4.3.4. The influence of initial feed pH on Ka The mass transfer process of ammonia is complicated by its ionization in water, as shown in Eq. (1). The molecule of NH3 can be directly removed at the membrane interface, but NH4 + must react with hydroxide before it is removed. Although this reaction is a fast chemical step, the diffusion of hydroxide to the membrane surface may not be very fast. So the pH value of the feed may affect the mass transfer. The existence of hydroxide concentration in Eqs. (7) and (10), by which the Ka for VMD, DCMD and SGMD are defined, respectively, provides the evidence in theory. The experimental proofs are given in Fig. 8, in which the influence of initial pH value of the feed on Ka for VMD and SGMD are shown. As predicted by Eqs. (7) and (10), the experimentally obtained Ka increases with the initial pH of the feed in both VMD and SGMD. Interestingly, the shapes of the curves in Fig. 8 are very alike, which also resemble the results obtained by Zhu et al. [2] in the DCMD of ammonia removal. In physical sense, the relationship of PHf with Ka reveals the influence of ammonia ionization equilibrium on the mass transfer. Considering that the ionization occurs only in the feed, it is foreseeable that the general trend of PHf versus Ka should be very similar for all three configurations. This analysis may be verified by both the similarity of the results given in Fig. 8 with that of Zhu et al., and the universality of Eq. (7) for both VMD and DCMD.

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Fig. 8. The effect of initial pHf on Ka .

For each of these curves, although positive effect of pHf is attached to Ka , this increment is decelerated with rising pHf . Almost no remarkable increase can be obtained when pHf is higher than 11. Increasing the concentration of hydroxide in the feed is in favor of its diffusion from the feed bulk to membrane surface, and this make the dissociation equilibration, as shown by Eq. (1), move towards to yielding NH3 , the only form of ammonia that can be stripped. However, resistance caused by membrane and gas flow will gradually dominate the mass transfer process with the improvement of the feed side, this may be why pHf become less influential at higher value. 4.4. The influence of MD configuration and membrane characteristics on selectivity Fig. 9 shows the selectivity (as a function of feed temperature) obtained from the experiments of different MD configurations but conducted under similar operating conditions. It can be seen

Fig. 9. The influence of feed temperature on β.

that the selectivity of DCMD varies approximately from 17 to 22, that of SGMD varies from 10 to 13, and that of VMD from 8 to 10 for 0.1 ␮m membrane or from 4 to 7 for 0.2 ␮m membrane. It should be noticed that this order is quiet the reverse of that of transmembrane flux, as shown in Fig. 4. Theoretically, the selectivity depends on both thermodynamics effect (gas–liquid equilibrium, GLE) and kinetics effect (mass transfer) of concerned components. Although VMD has the highest mass transfer coefficient, i.e. the best kinetics for ammonia mass transfer, it is also most in favor of water evaporation, i.e. the best thermodynamics for water evaporation, thus producing much higher water flux than SGMD and DCMD. In SGMD and DCMD, the feed were operated under similar thermodynamics conditions, so their difference in selectivity should mainly come from their difference in kinetics. Therefore, with higher Ka , DCMD gives better selectivity than SGMD. Despite the fact that the operating conditions in Fig. 9 were not identical for three configurations, the conclusion drawn from this figure is reasonable. The influence of membrane characteristics on selectivity can also be seen in Fig. 9. For VMD, the 0.1 ␮m membrane shows better selectivity than the membrane of 0.2 ␮m. Although 0.2 ␮m membrane gives higher Ka , it prefers water vapor mass transfer too. Experimentally, the 0.2 ␮m membrane gave much higher transmembrane flux than 0.1 ␮m membrane, as shown in Fig. 4, implying that 0.2 ␮m membrane even more like water vapor to permeate through than 0.1 ␮m membrane. The selectivity of SGMD and DCMD should come to the same thing as VMD, because the membrane characteristics are of great importance to the mass transfer of all three configurations in this study. 4.5. The influence of operating conditions on selectivity 4.5.1. The influence of feed temperature The effect of feed temperature on selectivity for three configurations can also be seen in Fig. 9. In spite of the positive effect of tf on the Ka , negative effect on β was observed in all three configurations. Two impacts which contribute to this observed behavior are the increase of water vapor pressure and its diffusivity with rising temperature and in these ways the mass transfer of water vapor is intensified. The obtained results indicate that these effects preponderate over the positive influence of tf on Ka within the range of operating conditions used. 4.5.2. The influence of feed velocity As mentioned above, due to the very high resistance caused by membrane and flowing gas, the Ka cannot be effectively improved by increasing the feed velocity in SGMD and DCMD. The mass transfer of water vapor should be subject to this analysis. Therefore, there should be no noticeable effect of feed velocity on β in SGMD and DCMD. This inference was confirmed by the experimentally obtained results, as shown in Fig. 10. Likewise, the feed velocity in VMD also has no considerable influence on selectivity. In VMD, although ammonia mass transfer can benefit from increasing feed velocity (please see Fig. 6),

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Fig. 10. The effect of feed velocity on selectivity. Fig. 12. The influence of initial feed concentration on selectivity.

this contribution may be offset by the simultaneously improved mass transfer of water vapor. 4.5.3. The influence of permeate velocity Fig. 11 shows that, in DCMD, the selectivity is independent of the velocity of the sulfuric acid, a feature that may be in accordance with the independence of Ka . As has been shown in Fig. 7, almost no observable influence of sulfuric acid velocity on Ka can be found, so the mass transfer of water vapor in DCMD should come to the same thing. Therefore the selectivity is independent of permeate velocity. In SGMD, as a result of the domination of gas flow in ammonia mass transfer, gas velocity is of great importance to Ka (as shown in Fig. 7). So the selectivity can be improved by increasing gas velocity if the correspondingly intensified water vapor mass transfer cannot get ahead of that of ammonia. This was verified by the experimentally obtained results, as shown in Fig. 11.

Fig. 11. The effect of permeate velocity on selectivity.

4.5.4. The influence of initial feed concentration Of VMD and SGMD, Fig. 12 shows the variations of selectivity with initial concentration of ammonia in the feed. It can be found that the circumstance would become more adverse to the selectivity if a solution with higher ammonia concentration is used. Here, thermodynamic effect may play an important role. As mentioned in Section 4.1, the decline of transmembrane flux with time implies that the existence of ammonia in water makes the water vaporize more easily, i.e. the water vapor pressure is increased, and in this way, the driving force for water vapor mass transfer is improved. This effect would become more prominent with the increase of ammonia concentration in the feed, thus the negative influence of ammonia concentration is impacted on the selectivity. As this effect roots in the feed solution, it is believable that DCMD would also be subject to it. Therefore, although Fig. 12 gives no results of DCMD, it can be deduced that the negative effect of initial feed concentration on selectivity should also be found in DCMD. More rapid decease of selectivity was observed in VMD, as shown in Fig. 12, possibly as a result of its being operated under very low pressure. 4.5.5. The influence of initial pHf on selectivity The influence of initial pHf on selectivity may also be related with the thermodynamic effect. The increment of pHf in the feed will lead to the movement of dissociation equilibrium (as shown by Eq. (1)) towards to the formation of NH3 , producing a higher concentration of NH3 at the membrane surface of the feed side. Therefore selectivity may be improved as a result of the increased driving force of ammonia mass transfer. As shown in Fig. 13, the experimentally obtained results of VMD and SGMD configurations are consistent with this analysis. Similarly, as these effects also root in the feed solution, the general trend shown in Fig. 13 should also apply to DCMD.

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Fig. 13. The influence of initial pHf on selectivity.

5. Conclusions For ammonia removal from water by MD, this study tries to compare the separation performance of three configurations in terms of mass transfer coefficient (Ka ) and selectivity (β). In addition, factors that may affect the separation processes were experimentally examined. The following conclusions may be drawn from the obtained results: • The separation performance is greatly affected by the characteristics of the employed membrane. Those with larger pore size and less thickness give higher Ka , but lower β. • In comparison of three MD configurations with each other, it was found that, under similar operating conditions, VMD shows the highest Ka , but the lowest β, DCMD gives the highest β and moderate Ka , and SGMD has the moderate β and lowest Ka . • For all three MD configurations, higher Ka value but lower β are observed in the feed of higher temperature. Only in VMD does the feed flow velocity have noticeable influence on Ka , and almost no effect of feed flow velocity on selectivity can be found for all three configurations. In SGMD, increasing the sweep gas velocity can greatly increase its β as well as Ka ; however, neither Ka nor β is affected by the permeate velocity in DCMD. The feed pH is of great importance to process performance of all three configurations, both Ka and β can be greatly improved by increasing the feed pH. In all three MD configurations, lower selectivity can be found in the feed of higher ammonia concentration. Acknowledgements Financial supports from China National Natural Science Foundation (20206003) and the China National “973” Science and Technology Projects (2003CD 61570) are gratefully acknowledged.

A c D F H J k Ka Kb m N t u x y

membrane area (m2 ) concentration of ammonia (g m−3 ) diffusion coefficient (m2 s−1 ) flow rate (m3 s−1 ) dimensionless Henry constant transmembrane flux (kg m−2 s−1 ) mass transfer coefficient (m s−1 ) total mass transfer coefficient of NH3 (m s−1 ) equilibrium constant of Eq. (1) mass of the feed (kg) mass transfer rate of NH3 (kg m−2 s−1 ) time (min) or temperature (◦ C) velocity (m s−1 ) mass fraction of ammonia in the feed mass fraction of ammonia in the permeate

Greek letters β selectivity δ membrane thickness (m) ε membrane porosity ρ density of the feed (kg m−3 ) τ membrane tortuosity Subscripts a of ammonia f of the feed fm at the membrane surface of the feed side L at the outlet of membrane module m of the membrane p of the permeate pm at the membrane surface of the permeate side 0 at the inlet of membrane module 1 of NH3 Superscripts t at time of t 0 at initial time

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