- Email: [email protected]
Evaluation of energy requirements in membrane distillation
Available online at www.sciencedirect.com
Chemical Engineering and Processing 47 (2008) 1098–1105
Evaluation of energy requirements in membrane distillation Alessandra Criscuoli a,∗ , Maria Concetta Carnevale a , Enrico Drioli a,b a
b
Institute on Membrane Technology, ITM-CNR, Via P. Bucci Cubo 17/C, Rende (CS), Italy Department of Chemical Engineering and Materials, University of Calabria, Via P. Bucci Cubo 42/A, Rende (CS), Italy Received 11 December 2006; received in revised form 13 February 2007; accepted 9 March 2007 Available online 16 March 2007
Abstract This paper presents a study of energy requirements of membrane distillation (MD) for different lab-made flat module designs of 40 cm2 membrane area. The MD runs were carried out in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) mode with a 0.2 m polypropylene membrane and in all tests pure water was fed to the system. In DCMD, the effect of the operating temperatures and streams flow rates on the flux, the evaporation efficiency and the energy consumption, was studied, whereas in VMD the parameters analyzed were the feed flow rate, the feed temperature and the vacuum applied at the permeate side. The VMD performed better than the DCMD and the cross-flow module resulted to be the most efficient design for obtaining high fluxes with moderate energy consumptions. The highest flux (56.2 kg/m2 h) was achieved with the cross-flow module working in VMD at a feed flow rate of 235 L/h, feed temperature of 59.2 ◦ C and a permeate pressure of 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h−1 ) (longitudinal-flow membrane module) and 1.1 kW/(kg h−1 ) (cross-flow membrane module) for DCMD and VMD tests, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Membrane distillation; Energy requirements; Module design
1. Introduction Membrane distillation has been, and is still, widely studied for production of pure water, concentration of solutions, purification of wastewater streams, etc. [1–3]. Although the potentialities of the technique are well recognised, its application on industrial scale is mainly limited because of the energy requirements associated. For a good performance of membrane distillation, in fact, high fluxes must be obtained with moderate energy consumptions. The performance of membrane distillation mainly depends on: • the membrane properties; • the operating conditions; • the module design. In literature many studies on the effect of membrane properties and operating conditions are reported [4–25]. Usually, a more ∗
Corresponding author. Tel.: +39 0984 492118; fax: +39 0984 402103. E-mail address: [email protected] (A. Criscuoli).
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.03.006
porous membrane with a higher thickness helps in reducing the heat loss by conduction; high pore sizes lead to high fluxes but also to high liquid entry pressures. The design of adequate membranes for membrane distillation is of great importance for its further development [26,27]. Concerning the operating conditions, the feed temperature has the most significant influence on the trans-membrane flux, followed by the feed flow rate and the partial pressure established at the permeate side (this last depending on the distillate temperature for DCMD and on the vacuum applied for VMD). Trapped air in the pores leads to a resistance to vapor flow through the membrane and higher fluxes can be obtained when the involved streams are deareated; however, wetting phenomena are increased when deareation occurs [2]. Module design plays also an important role in the overall performance of the technique and several works appeared in literature on this matter [28–31]. The main points to consider in designing a membrane distillation module are: • the uniform distribution of flow; • the temperature polarization;
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
• the pressure drops; • the liquid entry pressure. The module configuration has to be carefully optimized also in order to reduce the temperature change of the solutions along the membrane module (reduction of the heating and cooling power costs). In membrane distillation there is a heat transfer from the feed to the distillate: • from the feed bulk to the feed–membrane interface (heat transfer through the feed boundary layer); • from the feed–membrane interface to the membrane–distillate interface (heat transfer across the membrane); • from the membrane–distillate interface to the distillate bulk (heat transfer through the distillate boundary layer). The heat transfer through the boundary layer is due to temperature polarization phenomena. For an efficient process, the boundary layer resistances have to be minimized. Concerning the heat transfer across the membrane, it occurs by conduction through the membrane material and the gas/vapor in the pores (heat loss), as well as by transfer of the latent heat of vaporization with mass flux. The heat transfer which occurs in the membrane distillation module leads to a cooling of the hot stream and, in DCMD configuration, to a heating of the distillate. Therefore, in order to work with a constant driving force, it is necessary to supply heat to the hot stream and to remove heat from the distillate. The heating and cooling steps represent the energy requirements of the DCMD process. When VMD is considered, the cooling energy is substituted by the energy consumption of the vacuum pump. To date, the studies reported in literature on membrane distillation mainly investigate the temperature polarization phenomena, heat efficiency/heat transfer [32–37] and only few studies refer to the energy requirements [2,3,29,38]. Concerning this point, several authors propose the internal heat recovery as a way to reduce the external heat supply for DCMD. The aim of this work is to compare different lab-made flat module designs in terms of trans-membrane fluxes, energy consumption and evaporation efficiency for DCMD and VMD experiments. A preliminary study has been reported in [39]. The choice of the flat configuration was related to the fact that, although hollow fiber modules have the advantage of a high area/volume ratio, they can present problems of fiber vibration during the process and non uniform packing of fibers and/or polydispersity of the fiber inner diameter (both imply a bad distribution of flow). In particular, DCMD runs were carried out on two different configurations: longitudinal and transversal membrane modules, while the VMD tests were performed on the longitudinal and cross-flow membrane modules. The results achieved for the different configurations are compared considering the final target of reaching high fluxes with moderate energy consumptions. The energy consumption considered in the work made referred only to the external heat supply/removal needed, as
1099
well as to the vacuum application at the distillate side in VMD, and did not include the energy consumption of pumps used for re-circulating streams. In this way we compared the “energy efficiency” of the module designs studied in terms of energy required for keeping the necessary driving force during the process (after the start-up of the system). 2. Experimental procedures and set-up The simplified schemes of the set-up used for carrying out the DCMD and VMD tests are shown in Figs. 1 and 2, respectively. The DCMD set-up includes: one cryostat (RT-300, Neslab) for the regulation of the cold stream temperature (1 kW); one thermostatic bath for the regulation of the hot stream temperature (1.5 kW); two flow meters (E5-2600/H, asa) for the regulation of the flow rate of the two streams; a membrane module, in which the flat-sheet membrane is sandwiched and the hot stream (sent to the lower side of the module) and the cold stream (sent to the upper side of the module) flow counter-currently; one analytical balance (Europe 6000, Gibertini) used for weighting the distillate; two pumps (NOVAX 14-M OIL, Rover POMPE) for the re-circulation of the streams (250 W per pump); two manometers for registering the module inlet pressures of the two streams; four thermocouples (HD9214, Delta OHM, accuracy ±0.1 ◦ C) for evaluating the module inlet and outlet temperatures of both streams. The volume tank of the hot stream was of 3 L.
Fig. 1. Simplified scheme of the DCMD set-up.
Fig. 2. Simplified scheme of the VMD set-up.
1100
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
The VMD set-up is similar to that of DCMD. It mainly differs for the presence of: one vacuum pump (Water aspirator pump made in polypropylene, Sigma–Aldrich for low vacuum (60 W) and EDWARDS pump for high vacuum (250 W)); one trap for condensing and recovering the water vapor (the trap in refrigerated by ice when a low vacuum is applied; whereas liquid nitrogen is used as refrigerator for high vacuum tests); a membrane module in which the hot stream flows in the lower side/upper side of the module, while the vacuum is applied at the upper/lower side; one analytical balance (Europe 6000, Gibertini) weighting the quantity of liquid that evaporates. Furthermore, only two thermocouples (HD9214, Delta OHM) for the evaluation of the module inlet and outlet temperatures of the hot stream were included. Membrane distillation experiments were carried out by feeding distilled water to the system, in order to eliminate the effect of the concentration on the overall performance. No deareation of water was performed. The system was properly insulated, so that the heat lost to the environment was significantly reduced: by feeding the only hot stream no temperature variation along the line was detected by thermocouples (accuracy, ±0.1 ◦ C). In order to check the presence of leakages in the system, as well as the membrane hydrophobicity, the variations registered on the balance display when only the hot stream was re-circulated were observed for at least 30 min before starting the experiment. Each experiment was, then, initiated only when no variation were reported on the display for the time of observation. During the experiments, the feed was at atmospheric pressure and its flow rate was varied between 100 and 300 L/h. We did not operate at higher feed flow rates because of the increase of the inlet feed pressure. The feed temperature was varied between 40 and 60 ◦ C. In DCMD tests the distillate flow rate was kept at about 200 L/h and the distillate temperature was in the range of 13–14 ◦ C, whereas in VMD the pressures at the permeate side were of the order of 10–60 mbar. Concerning the lab-made membrane modules used, for the DCMD tests two different configurations were realized: longitudinal (see Fig. 3) and transversal (see Fig. 4). For the VMD tests, beside the longitudinal-flow module, a cross-flow membrane module was also designed and realized (see Fig. 5). For all three configurations a flat-sheet membrane was sandwiched between two rectangular plates made in nylon. In both longitudinal and transversal-flow modules, the hot stream flowed in the bottom plate and the cold stream/vacuum was sent/applied through the top plate. A net was also located on the membrane (upper side). The streams entered into a “water collector” channel and, then, flowed along the module thanks to channels, which ensured that all the membrane surface was in contact with the streams (channels were created on an area located 1 mm under the plate surface, so that all the membrane was wetted). At the end of the module streams were “re-collected” into another “water collector” channel, from which they left the module. In the cross-flow membrane module the hot stream flowed perpendicularly to the membrane surface from the top plate (it entered in
Fig. 3. Scheme of the longitudinal-flow membrane module. (a) Picture of a plate.
a central inlet pipe and was sent, by means of a “holed” plate, perpendicularly to the membrane surface: “shower effect”), while vacuum was applied through the bottom plate. The bottom plate was similar to those used for the other two modules. In this case the support net was located under the membrane (lower
Fig. 4. Scheme of the transversal-flow membrane module. (a) Picture of a plate.
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
1101
Fig. 6. Distillate flux as function of the hot stream flow rate and temperature. Longitudinal-flow module (Qd = 215 L/h; 13.4 ◦ C < Td < 14.3 ◦ C).
3. Results and discussion 3.1. DCMD tests Fig. 5. Scheme of the cross-flow membrane module.
side). For all tests, a flat commercial membrane (purchased from Membrana, Germany) with pore size of 0.2 m and a thickness of 91 m was used. The performances of the three modules were compared by operating in the same conditions (the same type of feed, the same feed flow rate and temperature, the same permeate pressure or distillate flow rate and temperature). The energy consumptions were calculated for DCMD tests considering the heating and cooling of the hot and cold stream, respectively; whereas, for VMD runs the heating of the hot stream and the vacuum application at the permeate side were taken into account. The equations used for obtaining the heating and cooling energy are reported below: Qh = Qf Cpf (Tf,in − Tf,out )
(1)
Qc = Qd Cpd (Td,in − Td,out )
(2)
with Qh , Qc the heating and cooling energy (W), Qf the feed flow rate (kg/h), Qd the distillate flow rate (kg/h), Cpf the feed specific heat (J/(kg K)), Cpd the distillate specific heat (J/(kg K)), Tf,in , Td,in the feed and distillate temperature at the module inlet (K), and Tf,out , Td,out the feed and distillate temperature at the module outlet (K).
Fig. 6 shows the distillate flux obtained in the longitudinalflow membrane module for different values of the hot stream flow rate (Qf ) and temperature (Tf ). As expected, the flux is strongly dependent on the hot stream temperature, whereas by varying the stream flow rate, the flux enhancement achieved is lower. Moreover, for flow rate values higher than 200 L/h, no improvements in flux were registered, index of absence of fluid dynamic control on the mass transfer. Fig. 7 reports the trend of the water vapor flux with time for the same experiments shown in Fig. 6. It can be observed that for all tests the steady state was reached within the first 50 min. Table 1 shows a comparison, in terms of permeate flux, energy consumption and evaporation efficiency among the different
Fig. 7. Trend of the water vapor flux with time. Longitudinal-flow module (Qd = 215 L/h) [ (Qf = 100 L/h; Td = 14.3 ◦ C; Tf = 54 ◦ C); (Qf = 200 L/h; Td = 14.1 ◦ C; Tf = 54 ◦ C); (Qf = 200 L/h; Td = 13.4 ◦ C; Tf = 39.8 ◦ C); 䊉 (Qf = 300 L/h; Td = 14.3 ◦ C; Tf = 54.3 ◦ C)].
Table 1 Comparison among the different tests carried out on the longitudinal-flow membrane module Qf (L/h)
Tf (◦ C)
Td (◦ C)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
100 200 200 200 300
54 59 54 39.8 54.3
14.3 14.3 14.1 13.4 14.3
15.6 25.4 18.9 7.81 19.1
227.7 360.3 287.6 143.0 274.2
3.65 3.55 3.80 4.58 3.60
30 39 29 20 27
1102
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
Fig. 8. Evaporation efficiency as function of the feed flow rate and temperature. Longitudinal-flow module (Td , 13.4–14.3 ◦ C).
tests carried out on the longitudinal-flow membrane module. From the Table, it can be noticed that the best results in terms of energy consumption/permeate flow rate (QP ) ratio and evaporation efficiency were obtained at hot stream flow rate and temperature of 200 L/h and 59 ◦ C, respectively. Concerning the evaporation efficiency (defined as the heat which contributes to the evaporation versus the total heat exchanged between the feed and distillate), the data reported in Table 1 were plotted as function of the hot stream flow rate and temperature in Fig. 8. The evaporation efficiency is independent on the stream flow rate and increases with the temperature: at high operating temperatures the heat effectively used for the evaporation is higher. This result is in agreement with what reported by Ref. [12]. Tests were performed also with the transversal-flow membrane module. Table 2 shows the comparison between the longitudinal and transversal-flow modules. The transversal-flow membrane module behaves similarly to the longitudinal-flow membrane module, but it leads to a distillate flux slightly lower than that obtained in the longitudinal-flow configuration, operating at the same conditions. This result is probably attributable to “entrance effects” that do not allow to have velocity profiles completely developed inside the module. For this reason, the transversal-flow membrane module was not used for further tests.
Fig. 9. Optimization of the VMD process. Longitudinal-flow module: Qf = 200 L/h; Tf ∼ 59 ◦ C; Pd , 60 mbar; Test 1 without distillate tank and module insulation; Test 2 with distillate tank in ice and without module insulation; Test 3 with distillate tank in ice and module insulation.
tillate is not required, but the consumption related to the use of a vacuum pump must be considered. Fig. 9 shows how the flux varied during the optimization of the VMD set-up when a water aspirator pump was used for making vacuum. The cooling of the distillate trap with ice facilitates the water vapor condensation with a consequent increase of the driving force across the membrane and, therefore, of the flux. Concerning the module insulation, it ensures that the permeate side is at the same temperature of the feed side avoiding any possible condensation of the permeate stream on the membrane and leading, then, to a high flux. The comparison between DCMD and VMD in the longitudinal-flow membrane module is shown in Fig. 10. VMD leads to higher fluxes with respect to DCMD and the achievable flux increases with the degree of vacuum applied at the permeate side. The comparison between DCMD and VMD was made also
3.2. VMD tests The longitudinal-flow membrane module was also tested in VMD. In VMD the energy supply needed for cooling the dis-
Fig. 10. Trend of the water vapor flux with time. Comparison between DCMD and VMD. Longitudinal-flow module (Qf = 200 L/h; Tf ∼ 59 ◦ C).
Table 2 Comparison between the longitudinal and transversal-flow membrane modules Longitudinal-flow module Tf (◦ C) Td (◦ C) Qd (L/h) Qf (L/h) J (kg/m2 h) Energy consumption (W) Energy consumption/QP (kW/(kg h−1 ))
54 14.3 215 100 15.6 227.8 3.65
39.8 13.4 215 200 7.81 142.9 4.58
Transversal-flow module 54 14.1 215 200 18.9 287.7 3.80
54.3 14.3 215 300 19.1 274.3 3.60
53.5 14.3 215 100 15.5 239.4 3.85
40.5 13.6 215 200 7.5 119.7 4.00
54.2 14.4 215 200 16.5 262.7 3.98
54.4 14.4 215 300 17 214.3 3.15
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
1103
Table 3 Comparison between DCMD (Tf ∼ 59 ◦ C; Qd = 100 L/h; Td = 13.4 ◦ C) and VMD (Tf ∼ 59 ◦ C) in the longitudinal-flow membrane module Pd (mbar)
Qf (L/h)
J (kg/m2 h)
Energy consumptiona (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
60 10 1000
200 200 200
43.7 48.8 25.4
199.5 389.5 360.3
1.15 2.00 3.55
79 88 39
a
The energy required for the high vacuum pump is 250 W. For the water aspirator pump a consumption of 60 W is considered.
in terms of energy consumption and evaporation efficiency and the results are reported in Table 3. In VMD the energetic consumption/permeate flow rate ratio is lower than that of DCMD; moreover, it increases with the degree of vacuum applied. The evaporation efficiency also increases with vacuum and it should be noted that, by operating at 10 mbar of vacuum at the permeate side, an evaporation efficiency of almost 90% was achieved. VMD tests were also performed on the cross-flow membrane module. Fig. 11 reports the trend of the water flux with time in the cross-flow membrane module for two different values of vacuum at the permeate side. Also in this case, the flux increases with vacuum. When operating at lower vacuum values (60 mbar) there is a decrease of the flux with time, due to the partial melting of the ice (because of the high permeate flux). The effect of the hot stream flow rate and temperature on the flux is shown in Fig. 12. The increase of the flux with the feed flow rate is more evident at 54 ◦ C. At higher feed temperatures, the temperature polarization increases, because of
the higher fluxes through the membrane. In this case, variations in the feed flow rate from 150 to 235 L/h did not lead to a significant reduction of the temperature polarization and, therefore, the flux slightly varied. For the same tests, Fig. 13 reports the temperature variation of the hot stream along the module. By re-circulating the stream at higher flow rate values, there is a reduction of the temperature difference along the module because of the lower residence time of the stream in the module and, therefore, the lower heat exchange. Moreover, higher is the feed temperature, higher is the T along the module due to the higher trans-membrane fluxes achieved. Similar results are reported in Refs. [30,31]. Table 4 summarizes the results obtained with the cross-flow membrane module in terms of energy consumption/permeate flow rate ratio. From the Table it can be noticed that the highest value of distillate flux was 56.2 kg/m2 h at 235 L/h, 59.2 ◦ C and a pressure at the permeate side of 10 mbar. Considering the energy consumption/permeate flow rate ratio, the lowest value was obtained at 235 L/h, 59.2 ◦ C and 60 mbar; by working at these operating conditions, an elevate flux (50.5 kg/m2 h) can be also gained. Table 5 compares the energy consumption/permeate flow rate ratio and the evaporation efficiency of the longitudinal-flow membrane module and of the cross-flow membrane module. At parity of energy consumption/permeate flow rate ratio and evaporation efficiency, the cross-flow configuration led to a higher distillate flux. This is probably due to a reduction of the temperature polarization because of the stream sent perpendicularly to the membrane surface. The good performance of this type of configuration has been also demonstrated by Sirkar and coworkers [30,31] who investigated it for a hollow fiber membrane module.
Fig. 11. Trend of the water vapor flux with time. Cross-flow module (Qf = 150 L/h; Tf ∼ 59 ◦ C).
Fig. 12. Distillate flux as function of the hot stream flow rate and temperature. Cross-flow module (Pd = 10 mbar).
Fig. 13. Temperature difference of the feed stream along the module as function of the feed flow rate and temperature. Cross-flow module (Pd = 10 mbar).
1104
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
Table 4 Summary of the results achieved with the cross-flow membrane module Qf (L/h)
Tf,in (◦ C)
Pd (mbar)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
150 150 200 200 235 235 150 200 235
53.9 59.3 54.0 59.4 54.3 59.2 59.1 59.1 59.2
10 10 10 10 10 10 60 60 60
29.7 51.5 38.1 51.6 42.9 56.2 46.9 49.9 50.5
354.6 441.8 366.2 412.7 386.6 441.2 199.5 222.7 223.9
2.98 2.15 2.40 2.00 2.25 1.98 1.08 1.13 1.10
Table 5 Comparison between the longitudinal and the cross-flow module Module
Qf (L/h)
Tf,in (◦ C)
Pd (mbar)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
Cross-flow Longitudinal-flow
200 200
59.1 59.3
60 60
49.9 43.7
222.7 199.5
1.13 1.15
77 79
4. Conclusions In this work three different lab-made flat modules – longitudinal, transversal and cross-flow membrane module – were realized for carrying out DCMD and VMD experiments and their performance were compared in terms of trans-membrane fluxes, energy consumption/permeate flow rate ratios and evaporation efficiency. For all tests, a commercial polypropylene membrane with a pore size of 0.2 m was used. The transversal-flow module behaved similarly to the longitudinal-flow module; fluxes slightly lower were probably due to entrance effects. VMD performed better than the DCMD in terms of transmembrane fluxes, energy consumption/permeate flow rate ratios, evaporation efficiency. The cross-flow module led to higher fluxes than the longitudinal-flow module and similar energy consumption/permeate flow rate ratios and evaporation efficiency. The highest flux achieved with the cross-flow module was 56.2 kg/m2 h, working at 235 L/h, Tf 59.2 ◦ C and Pvacuum 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h−1 ) (longitudinal-flow membrane module; hot and cold stream flow rate, 200 L/h; hot stream temperature, 59 ◦ C; cold stream temperature, 14.3 ◦ C; flux, 25.4 kg/m2 h) and 1.1 kW/(kg h−1 ) (cross-flow membrane module; hot stream flow rate, 235 L/h; hot stream temperature, 59.2 ◦ C; vacuum pressure, 60 mbar; flux, 50.5 kg/m2 h) for DCMD and VMD tests, respectively. References [1] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [2] A.M. Alklaibi, N. Lior, Membrane-distillation: status and potential, Desalination 171 (2004) 111–131.
[3] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [4] P. Godino, L. Pena, J.I. Mengual, Membrane distillation: theory and experiments, J. Membr. Sci. 121 (1996) 83–93. [5] M.P. Godino, L. Pena, C. Rincon, J.I. Mengual, Water production from brines by membrane distillation, Desalination 108 (1996) 91–97. [6] K.W. Lawson, D.R. Lloyd, Membrane distillation. II. Direct contact MD, J. Membr. Sci. 120 (1996) 123–133. [7] S. Bandini, A. Saavedra, G.C. Sarti, Vacuum membrane distillation: experiments and modeling, AIChE J. 43 (2) (1997) 398–408. [8] M. Gryta, M. Tomaszewska, A.W. Morawski, Membrane distillation with laminar flow, Sep. Purif. Technol. 11 (1997) 93–101. [9] L. Pena, M.P. Godino, J.I. Mengual, A method to evaluate the net membrane distillation coefficient, J. Membr. Sci. 143 (1998) 219–233. [10] M. Gryta, M. Tomaszewska, Heat transport in the membrane distillation process, J. Membr. Sci. 144 (1998) 211–222. [11] L. Martinez-Diez, F.J. Florido-Diaz, M.I. Vazquez-Gonzalez, Study of evaporation efficiency in membrane distillation, Desalination 126 (1999) 193–198. [12] L. Martinez-Diez, M.I. Vazquez-Gonzalez, A method to evaluate coefficients affecting flux in membrane distillation, J. Membr. Sci. 173 (2000) 225–234. [13] L. Martinez-Diez, F.J. Florido-Diaz, Desalination of brines by membrane distillation, Desalination 137 (2001) 267–273. [14] L. Martinez, F.J. Florido-Diaz, Theoretical and experimental studies on desalination using membrane distillation, Desalination 139 (2001) 373–379. [15] S.T. Hsu, K.T. Cheng, J.S. Chiou, Seawater desalination by direct contact membrane distillation, Desalination 143 (2002) 279–287. [16] L. Martinez, F.J. Florido-Diaz, A. Hernandez, P. Pradanos, Characterization of three hydrophobic porous membranes used in membrane distillation. Modelling and evaluation of their water vapour permeabilities, J. Membr. Sci. 203 (2002) 15–27. [17] D. Wirth, C. Cabassud, Water desalination using membrane distillation: comparison between inside/out and outside/in permeation, Desalination 147 (2002) 139–145. [18] V.V. Ugrozov, I.B. Elkina, Mathematical modeling of influence of porous structure a membrane on its vapour-conductivity in the process of membrane distillation, Desalination 147 (2002) 167–171.
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105 [19] J. Phattaranawik, R. Jiraratananon, A.G. Fane, Heat transport and membrane distillation coefficients in direct contact membrane distillation, J. Membr. Sci. 212 (2003) 177–193. [20] V.V. Ugrozov, I.B. Elkina, V.N. Nikulin, L.I. Kataeva, Theoretical and experimental research of liquid-gap membrane distillation process in membrane module, Desalination 157 (2003) 325–331. [21] F. Banat, F.A. Al-Rub, K. Bani-Melhem, Desalination by vacuum membrane distillation: sensitivity analysis, Sep. Purif. Technol. 33 (2003) 75–87. [22] M. Khayet, K.C. Khulbe, T. Matsuura, Characterization of membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process, J. Membr. Sci. 238 (2004) 199–211. [23] T.Y. Cath, V.D. Adams, A.E. Childress, Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement, J. Membr. Sci. 228 (2004) 5–16. [24] P. Peng, A.G. Fane, X. Li, Desalination by membrane distillation adopting a hydrophilic membrane, Desalination 173 (2005) 45–54. [25] M. Khayet, J.I. Mengual, T. Matsuura, Porous hydrophobic/hydrophilic composite membranes. Application in desalination using direct contact membrane distillation, J. Membr. Sci. 252 (2005) 101–113. [26] M. Khayet, T. Matsuura, Application of surface modifying macromolecules for the preparation of membranes for membrane distillation, Desalination 158 (2003) 51–56. [27] M. Khayet, T. Matsuura, J.I. Mengual, M. Qtaishat, Design of novel direct contact membrane distillation membranes, Desalination 192 (2006) 105–111. [28] K.W. Lawson, D.R. Lloyd, Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121.
1105
[29] P.J. Foster, A. Burgoyne, M.M. Vahdati, Improved process topology for membrane distillation, Sep. Purif. Technol. 21 (2001) 205– 217. [30] B. Li, K.K. Sirkar, Novel membrane and device for direct contact membrane distillation-based desalination process, Ind. Eng. Chem. Res. 43 (2004) 5300–5309. [31] B. Li, K.K. Sirkar, Novel membrane and device for vacuum membrane distillation-based desalination process, J. Membr. Sci. 257 (2005) 60–75. [32] Z. Wang, F. Zheng, Y. Wu, S. Wang, Membrane osmotic distillation and its mathematical simulation, Desalination 139 (2001) 423–428. [33] K. Schneider, H¨olz, R. Wollbeck, S. Ripperger, Membranes and modules for transmembrane distillation, J. Membr. Sci. 39 (1988) 25–42. [34] J. Phattaranawik, R. Jiraratananon, A.G. Fane, C. Halim, Mass flux enhancement using spacer filled channels in direct contact membrane distillation, J. Membr. Sci. 187 (2001) 193–201. [35] M. Gryta, Heat efficiency of the capillary modules for membrane distillation process, Inz. Chem. Proc. 27 (2006) 305–314. [36] F.A.A. Al-Rub, F. Banat, K. Beni-Melhim, Parametric sensitivity analysis of direct-contact membrane distillation, Sep. Sci. Technol. 37 (2002) 3245–3271. [37] Y. Xu, B.-K. Zhu, Y.-Y. Xu, Pilot test of vacuum membrane distillation for seawater desalination on a ship, Desalination 189 (2006) 165–169. [38] C. Cabassud, D. Wirth, Membrane distillation for water desalination: how to chose an appropriate membrane? Desalination 157 (2003) 307–314. [39] A. Criscuoli, M.C. Carnevale, E. Drioli, Energy requirements in membrane distillation: evaluation and optimization, Desalination 200 (2006) 586–587.
Chemical Engineering and Processing 47 (2008) 1098–1105
Evaluation of energy requirements in membrane distillation Alessandra Criscuoli a,∗ , Maria Concetta Carnevale a , Enrico Drioli a,b a
b
Institute on Membrane Technology, ITM-CNR, Via P. Bucci Cubo 17/C, Rende (CS), Italy Department of Chemical Engineering and Materials, University of Calabria, Via P. Bucci Cubo 42/A, Rende (CS), Italy Received 11 December 2006; received in revised form 13 February 2007; accepted 9 March 2007 Available online 16 March 2007
Abstract This paper presents a study of energy requirements of membrane distillation (MD) for different lab-made flat module designs of 40 cm2 membrane area. The MD runs were carried out in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) mode with a 0.2 m polypropylene membrane and in all tests pure water was fed to the system. In DCMD, the effect of the operating temperatures and streams flow rates on the flux, the evaporation efficiency and the energy consumption, was studied, whereas in VMD the parameters analyzed were the feed flow rate, the feed temperature and the vacuum applied at the permeate side. The VMD performed better than the DCMD and the cross-flow module resulted to be the most efficient design for obtaining high fluxes with moderate energy consumptions. The highest flux (56.2 kg/m2 h) was achieved with the cross-flow module working in VMD at a feed flow rate of 235 L/h, feed temperature of 59.2 ◦ C and a permeate pressure of 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h−1 ) (longitudinal-flow membrane module) and 1.1 kW/(kg h−1 ) (cross-flow membrane module) for DCMD and VMD tests, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Membrane distillation; Energy requirements; Module design
1. Introduction Membrane distillation has been, and is still, widely studied for production of pure water, concentration of solutions, purification of wastewater streams, etc. [1–3]. Although the potentialities of the technique are well recognised, its application on industrial scale is mainly limited because of the energy requirements associated. For a good performance of membrane distillation, in fact, high fluxes must be obtained with moderate energy consumptions. The performance of membrane distillation mainly depends on: • the membrane properties; • the operating conditions; • the module design. In literature many studies on the effect of membrane properties and operating conditions are reported [4–25]. Usually, a more ∗
Corresponding author. Tel.: +39 0984 492118; fax: +39 0984 402103. E-mail address: [email protected] (A. Criscuoli).
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.03.006
porous membrane with a higher thickness helps in reducing the heat loss by conduction; high pore sizes lead to high fluxes but also to high liquid entry pressures. The design of adequate membranes for membrane distillation is of great importance for its further development [26,27]. Concerning the operating conditions, the feed temperature has the most significant influence on the trans-membrane flux, followed by the feed flow rate and the partial pressure established at the permeate side (this last depending on the distillate temperature for DCMD and on the vacuum applied for VMD). Trapped air in the pores leads to a resistance to vapor flow through the membrane and higher fluxes can be obtained when the involved streams are deareated; however, wetting phenomena are increased when deareation occurs [2]. Module design plays also an important role in the overall performance of the technique and several works appeared in literature on this matter [28–31]. The main points to consider in designing a membrane distillation module are: • the uniform distribution of flow; • the temperature polarization;
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
• the pressure drops; • the liquid entry pressure. The module configuration has to be carefully optimized also in order to reduce the temperature change of the solutions along the membrane module (reduction of the heating and cooling power costs). In membrane distillation there is a heat transfer from the feed to the distillate: • from the feed bulk to the feed–membrane interface (heat transfer through the feed boundary layer); • from the feed–membrane interface to the membrane–distillate interface (heat transfer across the membrane); • from the membrane–distillate interface to the distillate bulk (heat transfer through the distillate boundary layer). The heat transfer through the boundary layer is due to temperature polarization phenomena. For an efficient process, the boundary layer resistances have to be minimized. Concerning the heat transfer across the membrane, it occurs by conduction through the membrane material and the gas/vapor in the pores (heat loss), as well as by transfer of the latent heat of vaporization with mass flux. The heat transfer which occurs in the membrane distillation module leads to a cooling of the hot stream and, in DCMD configuration, to a heating of the distillate. Therefore, in order to work with a constant driving force, it is necessary to supply heat to the hot stream and to remove heat from the distillate. The heating and cooling steps represent the energy requirements of the DCMD process. When VMD is considered, the cooling energy is substituted by the energy consumption of the vacuum pump. To date, the studies reported in literature on membrane distillation mainly investigate the temperature polarization phenomena, heat efficiency/heat transfer [32–37] and only few studies refer to the energy requirements [2,3,29,38]. Concerning this point, several authors propose the internal heat recovery as a way to reduce the external heat supply for DCMD. The aim of this work is to compare different lab-made flat module designs in terms of trans-membrane fluxes, energy consumption and evaporation efficiency for DCMD and VMD experiments. A preliminary study has been reported in [39]. The choice of the flat configuration was related to the fact that, although hollow fiber modules have the advantage of a high area/volume ratio, they can present problems of fiber vibration during the process and non uniform packing of fibers and/or polydispersity of the fiber inner diameter (both imply a bad distribution of flow). In particular, DCMD runs were carried out on two different configurations: longitudinal and transversal membrane modules, while the VMD tests were performed on the longitudinal and cross-flow membrane modules. The results achieved for the different configurations are compared considering the final target of reaching high fluxes with moderate energy consumptions. The energy consumption considered in the work made referred only to the external heat supply/removal needed, as
1099
well as to the vacuum application at the distillate side in VMD, and did not include the energy consumption of pumps used for re-circulating streams. In this way we compared the “energy efficiency” of the module designs studied in terms of energy required for keeping the necessary driving force during the process (after the start-up of the system). 2. Experimental procedures and set-up The simplified schemes of the set-up used for carrying out the DCMD and VMD tests are shown in Figs. 1 and 2, respectively. The DCMD set-up includes: one cryostat (RT-300, Neslab) for the regulation of the cold stream temperature (1 kW); one thermostatic bath for the regulation of the hot stream temperature (1.5 kW); two flow meters (E5-2600/H, asa) for the regulation of the flow rate of the two streams; a membrane module, in which the flat-sheet membrane is sandwiched and the hot stream (sent to the lower side of the module) and the cold stream (sent to the upper side of the module) flow counter-currently; one analytical balance (Europe 6000, Gibertini) used for weighting the distillate; two pumps (NOVAX 14-M OIL, Rover POMPE) for the re-circulation of the streams (250 W per pump); two manometers for registering the module inlet pressures of the two streams; four thermocouples (HD9214, Delta OHM, accuracy ±0.1 ◦ C) for evaluating the module inlet and outlet temperatures of both streams. The volume tank of the hot stream was of 3 L.
Fig. 1. Simplified scheme of the DCMD set-up.
Fig. 2. Simplified scheme of the VMD set-up.
1100
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
The VMD set-up is similar to that of DCMD. It mainly differs for the presence of: one vacuum pump (Water aspirator pump made in polypropylene, Sigma–Aldrich for low vacuum (60 W) and EDWARDS pump for high vacuum (250 W)); one trap for condensing and recovering the water vapor (the trap in refrigerated by ice when a low vacuum is applied; whereas liquid nitrogen is used as refrigerator for high vacuum tests); a membrane module in which the hot stream flows in the lower side/upper side of the module, while the vacuum is applied at the upper/lower side; one analytical balance (Europe 6000, Gibertini) weighting the quantity of liquid that evaporates. Furthermore, only two thermocouples (HD9214, Delta OHM) for the evaluation of the module inlet and outlet temperatures of the hot stream were included. Membrane distillation experiments were carried out by feeding distilled water to the system, in order to eliminate the effect of the concentration on the overall performance. No deareation of water was performed. The system was properly insulated, so that the heat lost to the environment was significantly reduced: by feeding the only hot stream no temperature variation along the line was detected by thermocouples (accuracy, ±0.1 ◦ C). In order to check the presence of leakages in the system, as well as the membrane hydrophobicity, the variations registered on the balance display when only the hot stream was re-circulated were observed for at least 30 min before starting the experiment. Each experiment was, then, initiated only when no variation were reported on the display for the time of observation. During the experiments, the feed was at atmospheric pressure and its flow rate was varied between 100 and 300 L/h. We did not operate at higher feed flow rates because of the increase of the inlet feed pressure. The feed temperature was varied between 40 and 60 ◦ C. In DCMD tests the distillate flow rate was kept at about 200 L/h and the distillate temperature was in the range of 13–14 ◦ C, whereas in VMD the pressures at the permeate side were of the order of 10–60 mbar. Concerning the lab-made membrane modules used, for the DCMD tests two different configurations were realized: longitudinal (see Fig. 3) and transversal (see Fig. 4). For the VMD tests, beside the longitudinal-flow module, a cross-flow membrane module was also designed and realized (see Fig. 5). For all three configurations a flat-sheet membrane was sandwiched between two rectangular plates made in nylon. In both longitudinal and transversal-flow modules, the hot stream flowed in the bottom plate and the cold stream/vacuum was sent/applied through the top plate. A net was also located on the membrane (upper side). The streams entered into a “water collector” channel and, then, flowed along the module thanks to channels, which ensured that all the membrane surface was in contact with the streams (channels were created on an area located 1 mm under the plate surface, so that all the membrane was wetted). At the end of the module streams were “re-collected” into another “water collector” channel, from which they left the module. In the cross-flow membrane module the hot stream flowed perpendicularly to the membrane surface from the top plate (it entered in
Fig. 3. Scheme of the longitudinal-flow membrane module. (a) Picture of a plate.
a central inlet pipe and was sent, by means of a “holed” plate, perpendicularly to the membrane surface: “shower effect”), while vacuum was applied through the bottom plate. The bottom plate was similar to those used for the other two modules. In this case the support net was located under the membrane (lower
Fig. 4. Scheme of the transversal-flow membrane module. (a) Picture of a plate.
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
1101
Fig. 6. Distillate flux as function of the hot stream flow rate and temperature. Longitudinal-flow module (Qd = 215 L/h; 13.4 ◦ C < Td < 14.3 ◦ C).
3. Results and discussion 3.1. DCMD tests Fig. 5. Scheme of the cross-flow membrane module.
side). For all tests, a flat commercial membrane (purchased from Membrana, Germany) with pore size of 0.2 m and a thickness of 91 m was used. The performances of the three modules were compared by operating in the same conditions (the same type of feed, the same feed flow rate and temperature, the same permeate pressure or distillate flow rate and temperature). The energy consumptions were calculated for DCMD tests considering the heating and cooling of the hot and cold stream, respectively; whereas, for VMD runs the heating of the hot stream and the vacuum application at the permeate side were taken into account. The equations used for obtaining the heating and cooling energy are reported below: Qh = Qf Cpf (Tf,in − Tf,out )
(1)
Qc = Qd Cpd (Td,in − Td,out )
(2)
with Qh , Qc the heating and cooling energy (W), Qf the feed flow rate (kg/h), Qd the distillate flow rate (kg/h), Cpf the feed specific heat (J/(kg K)), Cpd the distillate specific heat (J/(kg K)), Tf,in , Td,in the feed and distillate temperature at the module inlet (K), and Tf,out , Td,out the feed and distillate temperature at the module outlet (K).
Fig. 6 shows the distillate flux obtained in the longitudinalflow membrane module for different values of the hot stream flow rate (Qf ) and temperature (Tf ). As expected, the flux is strongly dependent on the hot stream temperature, whereas by varying the stream flow rate, the flux enhancement achieved is lower. Moreover, for flow rate values higher than 200 L/h, no improvements in flux were registered, index of absence of fluid dynamic control on the mass transfer. Fig. 7 reports the trend of the water vapor flux with time for the same experiments shown in Fig. 6. It can be observed that for all tests the steady state was reached within the first 50 min. Table 1 shows a comparison, in terms of permeate flux, energy consumption and evaporation efficiency among the different
Fig. 7. Trend of the water vapor flux with time. Longitudinal-flow module (Qd = 215 L/h) [ (Qf = 100 L/h; Td = 14.3 ◦ C; Tf = 54 ◦ C); (Qf = 200 L/h; Td = 14.1 ◦ C; Tf = 54 ◦ C); (Qf = 200 L/h; Td = 13.4 ◦ C; Tf = 39.8 ◦ C); 䊉 (Qf = 300 L/h; Td = 14.3 ◦ C; Tf = 54.3 ◦ C)].
Table 1 Comparison among the different tests carried out on the longitudinal-flow membrane module Qf (L/h)
Tf (◦ C)
Td (◦ C)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
100 200 200 200 300
54 59 54 39.8 54.3
14.3 14.3 14.1 13.4 14.3
15.6 25.4 18.9 7.81 19.1
227.7 360.3 287.6 143.0 274.2
3.65 3.55 3.80 4.58 3.60
30 39 29 20 27
1102
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
Fig. 8. Evaporation efficiency as function of the feed flow rate and temperature. Longitudinal-flow module (Td , 13.4–14.3 ◦ C).
tests carried out on the longitudinal-flow membrane module. From the Table, it can be noticed that the best results in terms of energy consumption/permeate flow rate (QP ) ratio and evaporation efficiency were obtained at hot stream flow rate and temperature of 200 L/h and 59 ◦ C, respectively. Concerning the evaporation efficiency (defined as the heat which contributes to the evaporation versus the total heat exchanged between the feed and distillate), the data reported in Table 1 were plotted as function of the hot stream flow rate and temperature in Fig. 8. The evaporation efficiency is independent on the stream flow rate and increases with the temperature: at high operating temperatures the heat effectively used for the evaporation is higher. This result is in agreement with what reported by Ref. [12]. Tests were performed also with the transversal-flow membrane module. Table 2 shows the comparison between the longitudinal and transversal-flow modules. The transversal-flow membrane module behaves similarly to the longitudinal-flow membrane module, but it leads to a distillate flux slightly lower than that obtained in the longitudinal-flow configuration, operating at the same conditions. This result is probably attributable to “entrance effects” that do not allow to have velocity profiles completely developed inside the module. For this reason, the transversal-flow membrane module was not used for further tests.
Fig. 9. Optimization of the VMD process. Longitudinal-flow module: Qf = 200 L/h; Tf ∼ 59 ◦ C; Pd , 60 mbar; Test 1 without distillate tank and module insulation; Test 2 with distillate tank in ice and without module insulation; Test 3 with distillate tank in ice and module insulation.
tillate is not required, but the consumption related to the use of a vacuum pump must be considered. Fig. 9 shows how the flux varied during the optimization of the VMD set-up when a water aspirator pump was used for making vacuum. The cooling of the distillate trap with ice facilitates the water vapor condensation with a consequent increase of the driving force across the membrane and, therefore, of the flux. Concerning the module insulation, it ensures that the permeate side is at the same temperature of the feed side avoiding any possible condensation of the permeate stream on the membrane and leading, then, to a high flux. The comparison between DCMD and VMD in the longitudinal-flow membrane module is shown in Fig. 10. VMD leads to higher fluxes with respect to DCMD and the achievable flux increases with the degree of vacuum applied at the permeate side. The comparison between DCMD and VMD was made also
3.2. VMD tests The longitudinal-flow membrane module was also tested in VMD. In VMD the energy supply needed for cooling the dis-
Fig. 10. Trend of the water vapor flux with time. Comparison between DCMD and VMD. Longitudinal-flow module (Qf = 200 L/h; Tf ∼ 59 ◦ C).
Table 2 Comparison between the longitudinal and transversal-flow membrane modules Longitudinal-flow module Tf (◦ C) Td (◦ C) Qd (L/h) Qf (L/h) J (kg/m2 h) Energy consumption (W) Energy consumption/QP (kW/(kg h−1 ))
54 14.3 215 100 15.6 227.8 3.65
39.8 13.4 215 200 7.81 142.9 4.58
Transversal-flow module 54 14.1 215 200 18.9 287.7 3.80
54.3 14.3 215 300 19.1 274.3 3.60
53.5 14.3 215 100 15.5 239.4 3.85
40.5 13.6 215 200 7.5 119.7 4.00
54.2 14.4 215 200 16.5 262.7 3.98
54.4 14.4 215 300 17 214.3 3.15
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
1103
Table 3 Comparison between DCMD (Tf ∼ 59 ◦ C; Qd = 100 L/h; Td = 13.4 ◦ C) and VMD (Tf ∼ 59 ◦ C) in the longitudinal-flow membrane module Pd (mbar)
Qf (L/h)
J (kg/m2 h)
Energy consumptiona (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
60 10 1000
200 200 200
43.7 48.8 25.4
199.5 389.5 360.3
1.15 2.00 3.55
79 88 39
a
The energy required for the high vacuum pump is 250 W. For the water aspirator pump a consumption of 60 W is considered.
in terms of energy consumption and evaporation efficiency and the results are reported in Table 3. In VMD the energetic consumption/permeate flow rate ratio is lower than that of DCMD; moreover, it increases with the degree of vacuum applied. The evaporation efficiency also increases with vacuum and it should be noted that, by operating at 10 mbar of vacuum at the permeate side, an evaporation efficiency of almost 90% was achieved. VMD tests were also performed on the cross-flow membrane module. Fig. 11 reports the trend of the water flux with time in the cross-flow membrane module for two different values of vacuum at the permeate side. Also in this case, the flux increases with vacuum. When operating at lower vacuum values (60 mbar) there is a decrease of the flux with time, due to the partial melting of the ice (because of the high permeate flux). The effect of the hot stream flow rate and temperature on the flux is shown in Fig. 12. The increase of the flux with the feed flow rate is more evident at 54 ◦ C. At higher feed temperatures, the temperature polarization increases, because of
the higher fluxes through the membrane. In this case, variations in the feed flow rate from 150 to 235 L/h did not lead to a significant reduction of the temperature polarization and, therefore, the flux slightly varied. For the same tests, Fig. 13 reports the temperature variation of the hot stream along the module. By re-circulating the stream at higher flow rate values, there is a reduction of the temperature difference along the module because of the lower residence time of the stream in the module and, therefore, the lower heat exchange. Moreover, higher is the feed temperature, higher is the T along the module due to the higher trans-membrane fluxes achieved. Similar results are reported in Refs. [30,31]. Table 4 summarizes the results obtained with the cross-flow membrane module in terms of energy consumption/permeate flow rate ratio. From the Table it can be noticed that the highest value of distillate flux was 56.2 kg/m2 h at 235 L/h, 59.2 ◦ C and a pressure at the permeate side of 10 mbar. Considering the energy consumption/permeate flow rate ratio, the lowest value was obtained at 235 L/h, 59.2 ◦ C and 60 mbar; by working at these operating conditions, an elevate flux (50.5 kg/m2 h) can be also gained. Table 5 compares the energy consumption/permeate flow rate ratio and the evaporation efficiency of the longitudinal-flow membrane module and of the cross-flow membrane module. At parity of energy consumption/permeate flow rate ratio and evaporation efficiency, the cross-flow configuration led to a higher distillate flux. This is probably due to a reduction of the temperature polarization because of the stream sent perpendicularly to the membrane surface. The good performance of this type of configuration has been also demonstrated by Sirkar and coworkers [30,31] who investigated it for a hollow fiber membrane module.
Fig. 11. Trend of the water vapor flux with time. Cross-flow module (Qf = 150 L/h; Tf ∼ 59 ◦ C).
Fig. 12. Distillate flux as function of the hot stream flow rate and temperature. Cross-flow module (Pd = 10 mbar).
Fig. 13. Temperature difference of the feed stream along the module as function of the feed flow rate and temperature. Cross-flow module (Pd = 10 mbar).
1104
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105
Table 4 Summary of the results achieved with the cross-flow membrane module Qf (L/h)
Tf,in (◦ C)
Pd (mbar)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
150 150 200 200 235 235 150 200 235
53.9 59.3 54.0 59.4 54.3 59.2 59.1 59.1 59.2
10 10 10 10 10 10 60 60 60
29.7 51.5 38.1 51.6 42.9 56.2 46.9 49.9 50.5
354.6 441.8 366.2 412.7 386.6 441.2 199.5 222.7 223.9
2.98 2.15 2.40 2.00 2.25 1.98 1.08 1.13 1.10
Table 5 Comparison between the longitudinal and the cross-flow module Module
Qf (L/h)
Tf,in (◦ C)
Pd (mbar)
J (kg/m2 h)
Energy consumption (W)
Energy consumption/QP (kW/(kg h−1 ))
Evaporation efficiency (%)
Cross-flow Longitudinal-flow
200 200
59.1 59.3
60 60
49.9 43.7
222.7 199.5
1.13 1.15
77 79
4. Conclusions In this work three different lab-made flat modules – longitudinal, transversal and cross-flow membrane module – were realized for carrying out DCMD and VMD experiments and their performance were compared in terms of trans-membrane fluxes, energy consumption/permeate flow rate ratios and evaporation efficiency. For all tests, a commercial polypropylene membrane with a pore size of 0.2 m was used. The transversal-flow module behaved similarly to the longitudinal-flow module; fluxes slightly lower were probably due to entrance effects. VMD performed better than the DCMD in terms of transmembrane fluxes, energy consumption/permeate flow rate ratios, evaporation efficiency. The cross-flow module led to higher fluxes than the longitudinal-flow module and similar energy consumption/permeate flow rate ratios and evaporation efficiency. The highest flux achieved with the cross-flow module was 56.2 kg/m2 h, working at 235 L/h, Tf 59.2 ◦ C and Pvacuum 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h−1 ) (longitudinal-flow membrane module; hot and cold stream flow rate, 200 L/h; hot stream temperature, 59 ◦ C; cold stream temperature, 14.3 ◦ C; flux, 25.4 kg/m2 h) and 1.1 kW/(kg h−1 ) (cross-flow membrane module; hot stream flow rate, 235 L/h; hot stream temperature, 59.2 ◦ C; vacuum pressure, 60 mbar; flux, 50.5 kg/m2 h) for DCMD and VMD tests, respectively. References [1] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [2] A.M. Alklaibi, N. Lior, Membrane-distillation: status and potential, Desalination 171 (2004) 111–131.
[3] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [4] P. Godino, L. Pena, J.I. Mengual, Membrane distillation: theory and experiments, J. Membr. Sci. 121 (1996) 83–93. [5] M.P. Godino, L. Pena, C. Rincon, J.I. Mengual, Water production from brines by membrane distillation, Desalination 108 (1996) 91–97. [6] K.W. Lawson, D.R. Lloyd, Membrane distillation. II. Direct contact MD, J. Membr. Sci. 120 (1996) 123–133. [7] S. Bandini, A. Saavedra, G.C. Sarti, Vacuum membrane distillation: experiments and modeling, AIChE J. 43 (2) (1997) 398–408. [8] M. Gryta, M. Tomaszewska, A.W. Morawski, Membrane distillation with laminar flow, Sep. Purif. Technol. 11 (1997) 93–101. [9] L. Pena, M.P. Godino, J.I. Mengual, A method to evaluate the net membrane distillation coefficient, J. Membr. Sci. 143 (1998) 219–233. [10] M. Gryta, M. Tomaszewska, Heat transport in the membrane distillation process, J. Membr. Sci. 144 (1998) 211–222. [11] L. Martinez-Diez, F.J. Florido-Diaz, M.I. Vazquez-Gonzalez, Study of evaporation efficiency in membrane distillation, Desalination 126 (1999) 193–198. [12] L. Martinez-Diez, M.I. Vazquez-Gonzalez, A method to evaluate coefficients affecting flux in membrane distillation, J. Membr. Sci. 173 (2000) 225–234. [13] L. Martinez-Diez, F.J. Florido-Diaz, Desalination of brines by membrane distillation, Desalination 137 (2001) 267–273. [14] L. Martinez, F.J. Florido-Diaz, Theoretical and experimental studies on desalination using membrane distillation, Desalination 139 (2001) 373–379. [15] S.T. Hsu, K.T. Cheng, J.S. Chiou, Seawater desalination by direct contact membrane distillation, Desalination 143 (2002) 279–287. [16] L. Martinez, F.J. Florido-Diaz, A. Hernandez, P. Pradanos, Characterization of three hydrophobic porous membranes used in membrane distillation. Modelling and evaluation of their water vapour permeabilities, J. Membr. Sci. 203 (2002) 15–27. [17] D. Wirth, C. Cabassud, Water desalination using membrane distillation: comparison between inside/out and outside/in permeation, Desalination 147 (2002) 139–145. [18] V.V. Ugrozov, I.B. Elkina, Mathematical modeling of influence of porous structure a membrane on its vapour-conductivity in the process of membrane distillation, Desalination 147 (2002) 167–171.
A. Criscuoli et al. / Chemical Engineering and Processing 47 (2008) 1098–1105 [19] J. Phattaranawik, R. Jiraratananon, A.G. Fane, Heat transport and membrane distillation coefficients in direct contact membrane distillation, J. Membr. Sci. 212 (2003) 177–193. [20] V.V. Ugrozov, I.B. Elkina, V.N. Nikulin, L.I. Kataeva, Theoretical and experimental research of liquid-gap membrane distillation process in membrane module, Desalination 157 (2003) 325–331. [21] F. Banat, F.A. Al-Rub, K. Bani-Melhem, Desalination by vacuum membrane distillation: sensitivity analysis, Sep. Purif. Technol. 33 (2003) 75–87. [22] M. Khayet, K.C. Khulbe, T. Matsuura, Characterization of membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process, J. Membr. Sci. 238 (2004) 199–211. [23] T.Y. Cath, V.D. Adams, A.E. Childress, Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement, J. Membr. Sci. 228 (2004) 5–16. [24] P. Peng, A.G. Fane, X. Li, Desalination by membrane distillation adopting a hydrophilic membrane, Desalination 173 (2005) 45–54. [25] M. Khayet, J.I. Mengual, T. Matsuura, Porous hydrophobic/hydrophilic composite membranes. Application in desalination using direct contact membrane distillation, J. Membr. Sci. 252 (2005) 101–113. [26] M. Khayet, T. Matsuura, Application of surface modifying macromolecules for the preparation of membranes for membrane distillation, Desalination 158 (2003) 51–56. [27] M. Khayet, T. Matsuura, J.I. Mengual, M. Qtaishat, Design of novel direct contact membrane distillation membranes, Desalination 192 (2006) 105–111. [28] K.W. Lawson, D.R. Lloyd, Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121.
1105
[29] P.J. Foster, A. Burgoyne, M.M. Vahdati, Improved process topology for membrane distillation, Sep. Purif. Technol. 21 (2001) 205– 217. [30] B. Li, K.K. Sirkar, Novel membrane and device for direct contact membrane distillation-based desalination process, Ind. Eng. Chem. Res. 43 (2004) 5300–5309. [31] B. Li, K.K. Sirkar, Novel membrane and device for vacuum membrane distillation-based desalination process, J. Membr. Sci. 257 (2005) 60–75. [32] Z. Wang, F. Zheng, Y. Wu, S. Wang, Membrane osmotic distillation and its mathematical simulation, Desalination 139 (2001) 423–428. [33] K. Schneider, H¨olz, R. Wollbeck, S. Ripperger, Membranes and modules for transmembrane distillation, J. Membr. Sci. 39 (1988) 25–42. [34] J. Phattaranawik, R. Jiraratananon, A.G. Fane, C. Halim, Mass flux enhancement using spacer filled channels in direct contact membrane distillation, J. Membr. Sci. 187 (2001) 193–201. [35] M. Gryta, Heat efficiency of the capillary modules for membrane distillation process, Inz. Chem. Proc. 27 (2006) 305–314. [36] F.A.A. Al-Rub, F. Banat, K. Beni-Melhim, Parametric sensitivity analysis of direct-contact membrane distillation, Sep. Sci. Technol. 37 (2002) 3245–3271. [37] Y. Xu, B.-K. Zhu, Y.-Y. Xu, Pilot test of vacuum membrane distillation for seawater desalination on a ship, Desalination 189 (2006) 165–169. [38] C. Cabassud, D. Wirth, Membrane distillation for water desalination: how to chose an appropriate membrane? Desalination 157 (2003) 307–314. [39] A. Criscuoli, M.C. Carnevale, E. Drioli, Energy requirements in membrane distillation: evaluation and optimization, Desalination 200 (2006) 586–587.