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Chemical Engineering and Processing 50 (2011) 139–150
Contents lists available at ScienceDirect
Chemical Engineering and Processing:Process Intensification
journa l homepage: www.e lsev ier .com/ locate /cep
eview
owards practical implementations of membrane distillation
eru Susanto ∗
epartment of Chemical Engineering, Faculty of Engineering, Universitas Diponegoro, Jl. Prof. Sudarto, Tembalang-Semarang 50275, Indonesia
r t i c l e i n f o
rticle history:eceived 21 March 2010eceived in revised form 6 November 2010
a b s t r a c t
Membrane distillation, which combines thermal desalination and porous hydrophobic membrane asnon-wetting contact media, is currently gaining increasing important in membrane processes. However,the vast researches and reported publications of membrane distillation (MD) are less followed by its
ccepted 14 December 2010vailable online 23 December 2010
eywords:embrane distillation
ractical/industrial application
practical/industrial applications. This paper review analyzes the reasons for MD has not widely beingimplemented in practical/industrial applications. In addition, the strategies towards practical applicationare presented. Thus, this review will complement previous review of MD papers.
© 2010 Elsevier B.V. All rights reserved.
D energy sourceD “extreme” application
ontents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1392. Principle and historical development of membrane distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
2.1. Principle of separation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1402.2. A brief history of membrane distillation development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3. Principle of heat and mass transfer in MD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414. Strategy development towards practical/industrial implementations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.1. Improvement of permeate flux, evaporation and process efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.1.1. Fluid management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434.1.2. Module and spacer designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.1.3. Novel membrane preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.1.4. Solving flux reduction problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
4.2. Energy source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.3. More “extreme” application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5. Concluding remark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
. Introduction
Since its invention by Bodell in 1963 [1], membrane distilla-ion (MD) has continued to be an attractive desalination processith significant advantageous than more traditional desalina-
has not been widely implemented in practical/industrial appli-cation yet. While two previous comprehensive review paperswith focus on fundamental aspects to enhance the understand-ing of MD have been found [2,3], this review is an attempt toendorse practical/industrial implementations of MD. The funda-
ion process and reverse osmosis (RO) do not possess. Moreover,D has also shown great potential to be applied in many other
pplications. Unfortunately, despite the vast reported publica-ions, which showed that MD is very attractive technology; it
∗ Tel.: +62 247460058; fax: +62 247480675.E-mail address: [email protected]
255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2010.12.008
mental aspects will only be discussed briefly. In general, this papercovers brief principle and historical development of MD (Section2) and principle of heat and mass transfers (Section 3). The mainpart of this paper is organized into tools for strategy development
and practical/industrial applications as well as recent develop-ments with relevance to industrial/practical applications (Section4). This paper is not comprehensive review but rather discussesimportant aspects for practical applications. The author apolo-
140 H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150
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Fig. 1. Schematic illustration of m
izes for possible oversights of important findings in previousublications.
. Principle and historical development of membraneistillation
.1. Principle of separation process
At its core, MD is a separation process using a porous hydropho-ic membrane consisting of three main steps, i.e., (i) evaporationf volatile component in feed side, (ii) followed by migration ofolatile vapor molecules to permeate side through membrane porend (iii) condensation of vapor molecules in permeate side. Fig. 1
hows the separation principle of MD and its related configura-ions. The temperature difference between feed (Tf) and permeateTp) results in vapor pressure difference across the membranes the driving force of migration of vapor molecules from feedide to permeate side (Pf > Pp). Due to hydrophobic character ofne distillation separation process.
the membrane used, liquid solution is prevented for entering themembrane pores. However, in order to avoid pore wetting phe-nomenon, the transmembrane (hydrostatic) pressure differenceshould not be higher than the so-called liquid entry pressure (LEP)– a minimum pressure required for wetting the dry membranepores by pure water. This LEP is influenced by the membranematerial, membrane pore size and structure which are describedby Laplace (Cantor) equation [2,4]. One should differentiate MDwith osmotic membrane distillation (OMD). In the later case,the vapor pressure difference is created by the concentrationdifference.
The benefits of MD compared to other separation processes arederived from its characteristics, which include:
(i) It can be operated at low temperature meaning that low-gradeheat (such as solar energy, waste heat and geothermal) can beused. However, it should be noted that this does not mean therequired heat energy for MD is low.
H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150 141
1960 1970 1980 1990 2000 now
Initiationphase
Death phase Awakening phase
Development phase
Bodell introduces MD and Wehl published the first patent
No reported
Cheng and Wiersma prepare the first composite membrane for MD
- Efforts to commercialize by Gore, Enka and Swedish Development
- First workshop on membrane distillation in Rome, May 1985
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Fig. 2. Milestones in the deve
(ii) Modular design and it needs smaller space than conventionaldistillation.
(iii) High solute rejection can be achieved.(iv) It can work with high solute concentration of feed or with near
saturated solution.(v) Lower hydrostatic pressure than pressure driven membrane
processes is needed.(vi) Less pretreatment compared to pressure-based membrane
processes is needed.vii) Less sensitive to feed variations (e.g., pH, TDS, etc.)
Unfortunately, beside those benefits, the practical applicationsf MD are mainly limited by its low permeate flux and no commer-ially available membrane fabricated specifically for MD. Further,ompared to pressure driven membrane processes the transportrocess appears to be more complex, and high thermal energyonsumption are other disadvantages.
Depending on the way how vapor pressure difference as drivingorce and vapor condensation are provided, four different configu-ations of MD are currently known (Fig. 1), namely [2,3,5]: (1) directontact membrane distillation (DCMD). In this type, water or anqueous solution having lower temperature than liquid in feed sides used as condensing fluid in permeate side. The trans-membraneemperature difference results in vapor pressure difference as ariving force. Even though its heat loss is the greatest, this config-ration is the simplest among all configurations of MD. (2) Air gapembrane distillation (AGMD). In this configuration, to reduce heat
oss, a stagnant air gap is inserted between the membrane and aondensing surface. However, this stagnant air adds a mass transferesistant. (3) Sweeping gas membrane distillation (SGMD). To min-mize heat loss in DCMD and mass transfer resistant in SGMD, a coldas (inert) is used in permeate side to sweep the vapor moleculesnd carry to outside the membrane module for condensation. Nev-rtheless, operational cost will definetly increase due to externalondensation system. (4) Vacuum membrane distillation (VMD).n this type, permeate side is vacuumed yielding lower vapor pres-ure than in the feed side. Consequently, heat loss can be reducednd permeate flux can be increased (it should be noted that the
pplied vacuum pressure should not exceed the saturation pressuref volatile molecules). However, similar with SGMD, vapor conden-ation also occurs outside of the membrane module resulting inigher operational cost. Readers, who are interested in MD configu-ation, are directed to Lawson and Lloyd [2] and El-Bourawi et al. [3].Rapid growing but still no industrial scale
ent of membrane distillation.
2.2. A brief history of membrane distillation development
Fig. 2 shows a time line illustrating the historical developmentof MD. The first investigations about MD process can be tracedback to Bodell invention [1,6] followed by Weyl [7] and Findleyet al. [8,9]. Bodell [1,6] and Weyl [7] were interested in applyingMD process for desalination of saline water while Findley et al.[8,9] studied the occurring heat and mass transfer phenomena. Itis important to note that at that time high flux asymmetric cel-lulose acetate reverse osmosis (RO) membrane, which is knownas breakthrough discovery in membrane technology, was devel-oped by Loeb and Sourirajan [10]. Apparently, during 1970–1980the “death” phase of MD seemed to occur as indicated by noreported study can be found. This condition was most probablydue to the success of RO membrane as new desalination tech-nology. By the 1980 s, interest in MD was again started. This wassupported by advances in membrane manufacturing that made itpossible to prepare high performance membrane for MD. Compos-ite membranes consist of hydrophobic–hydrophilic polymer wereprepared to increase membrane performance [11,12]. In the 1985s,efforts to commercialize MD have been done by Gore and Asso-ciates (USA), Enka AG (Germany) and the Swedish DevelopmentCo. [2,13–15]. Unfortunately, the results showed that MD had notbeen commercially accepted as feasible process. In that year, thefirst workshop on membrane distillation was performed to respondincreasing research interest in membrane distillation [16]. One ofthe subjects in that workshop was nomenclature of the membranedistillation process. Since 1990, academic research and develop-ment on MD have rapidly increased but no industrial scale could befound.
3. Principle of heat and mass transfer in MD
Because of many previous publications have discussed heat andmass transfers for MD in detail, in this section the heat and masstransfers will be discussed briefly with the focus on direct con-tact membrane distillation (DCMD). The use of DCMD is based onthe fact that this configuration is the most commonly used in MD
researches, the simplest configuration due to condensation step iscarried out inside the membrane module and it has the least processparameters. Even though each configuration has its own specificheat and mass transfer mechanism, understanding of DCMD couldto some extent be used for other configurations. Readers with mass
142 H. Susanto / Chemical Engineering and
am
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Fig. 3. Temperature profile and polarization in MD.
nd heat transfers interest are directed to references [2,3,17] inore detail.Fig. 3 describes temperature profile as well as temperature
olarization as key points for heat and mass transfers in MD. Heatransfer occurs at liquid, solid and gas phases, while mass trans-er occurs at liquid and gas phases. In general, heat transfer in
embrane distillation includes: (1) heat transfer from the feedulk to the membrane interface across feed boundary layer (ıf),2) heat transfer for vaporization (latent heat), (3) heat transferrom the membrane surface at feed side to the membrane surfacet permeate side across the solid membrane (conductive) and theas filled pores, and (4) heat transfer from the membrane surfacet permeate side to the permeate bulk across permeate bound-ry layer (ıp). During heat transfer in the liquid phase (cf. (1)nd (4)), due to boundary layer formed, temperature polarizationTfm < Tfb and Tpm > Tpb) occurs, which will reduce significantly theet driving force of the mass transfer. One should note that thisolarization could not be avoided but it is possible to be mini-ized. It was reported that this temperature polarization could
ecrease more than 50% of driving force [2,18,19]. In this context,he heat transfer in liquid phase is influenced by operation condi-ion, fluid property, and hydrodynamic condition. In order to haven efficient heat transfer this polarization effect should be mini-ized. The heat used for vaporization (2) determines directly the
mount of vapor transported to the permeate side. This meanshe more heat used for vaporization the more permeate prod-ct will be produced. The amount of heat used for vaporizationepends on the extent of temperature polarization and heat lossy conduction. The heat transfer across the membrane (3) is in facteat loss; therefore it should also be minimized. This heat transfer
s influenced by both membrane porosity and membrane thick-ess. This heat transfer will decrease as the membrane porosity is
ncreased meaning less heat loss will be obtained. One should note
hat thermal conductivity of polymer membrane is significantlyigher than thermal conductivity of air/gas in the membrane pores.he heat loss by conduction can also be minimized by increasinghe thickness of membrane. However, the increase in membranehickness will decrease the resulting mass transfer. Therefore, thisProcessing 50 (2011) 139–150
trade-off phenomenon suggests that the membrane thicknessshould be optimized.
It is important to mention that two indicators for evaluatingheat efficiency in MD are known, i.e., evaporation efficiency (EE)and process efficiency (PE). EE is a parameter to characterize theefficiency of a MD operation, which can be defined as part of heatwhich contributes to evaporation divided by total heat input in themodule. PE is defined as heat which contributes to the evaporationof the distillate divided by total heat input of the process.
Mass transfer of volatile component in MD is driven by vaporpressure difference, which involves: (i) mass transfer from thebulk feed to the membrane surface, (ii) vapor transfer throughthe membrane pores and (iii) mass transfer from the membranesurface at permeate side to the bulk permeate. Three resistancesappear including both membrane surface in feed and permeateand the presence of gas (usually air) trapped within the mem-brane pores. The vapor permeation through the membrane porescan be described by vapor diffusion (Knudsen and molecular diffu-sion) and Poiseuille flow [2,3,17]. In general, the mass transfer inMD is dependent on the component of feed system. For aqueoussolution containing non-volatile solute, concentration polarization(solute) will take place. For feed containing volatile componentsuch as water–ethanol mixture concentration polarization willoccur for the solvent (water). Overall, the permeate flux of MD canbe described by the following equation:
J = C(Pfm − Ppm ) (1)
where C is membrane distillation coefficient, which depends onmembrane properties (e.g., porosity, pore size) and is independenton temperature and flow rate [20]. Pfm and Ppm are vapor pressureat membrane surface in feed and permeate sides, respectively.
An optimization guidelines for the materials and methods ofDCMD obtained by a comprehensive analysis on DCMD consideringtransport phenomena, membrane structural properties and mostsensitive process parameters has been developed [21].
4. Strategy development towards practical/industrialimplementations
Even though the development of membrane distillation hasbeen started since 1960s and many reported studies have been pub-lished, MD has gained little acceptance in industry or practice andis yet to be implemented. The reasons for this condition are the MDis less competitive than other (membrane) processes because MDwas used for general application such as desalination (it shouldbe noted that RO is very competitive in this application) and theenergy used for the operation is not free/wasted energy. Not allprevious works had potential to be economically applied althoughthey can improve the performance of MD and were very interestingfrom scientific point of view. In addition, the permeate flux of MD islower compared to other membrane processes. This is mainly dueto heat transfer inefficiency. Fig. 4 shows the schematic diagramfor strategy developments towards industrial or practical imple-mentations of MD. To be practically/industrially useful, MD shouldeither use free/cheap energy or should be applied in “extreme” areawhere other unit operations including pressure driven membraneprocesses are either not possible to be used or too expensive. Effortsto enhance both heat efficiency and mass transfer (via optimizingprocess condition and module and spacer design as well as man-ufacturing specific membrane for MD) and to solve the existing
problem such as fouling can only improve the performance of MD,but those results seem not determine practical applications. Never-theless, the results from these studies are very important to supportpractical implementation of MD. An illustration was given by Buiet al. [22], who suggested that operating DCMD process under the
H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150 143
Module design: • Provide high rate mass
transfer • Provide high turbulence for
feed and permeate • Optimum packing density • Low pressure drop • Efficient evaporation
High performance MD: High flux and energy efficient MD
Process conditions: • High feed temperature • Low permeate temperature • High flow rate and stirring
(turbulence condition) • Hydrostatic pressure < LEP • System isolation and heat
recovery
Membrane characteristics: • Hydrophobic and wetting
property • Optimum membrane thickness • High porosity • Optimum pore size • Uniform pore size • Low tortuosity (close to 1)
Free/cheap energy sources
Novel or “extreme” application
oAlternative energy: solar, geothermal
oWaste heat
o Single (alone) process
o Hybrid with other processes
PRACTICAL/INDUSTRIAL IMPLEMENTATIONS
nt tow
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Fig. 4. Schematic diagram for strategy developme
ptimal conditions could result in a reduction in energy consump-ion of up to 26.3%,
.1. Improvement of permeate flux, evaporation and processfficiencies
So far most of researches in MD are addressed to improveeat efficiency and enhance mass transfer flow. In this regard,he performance of MD can indeed be enhanced by managinghe fluid conditions (temperature, flow rate and concentration ofeed and permeate streams), designing module and spacer and
anufacturing of novel membrane. Modeling and computationre important tools to optimize those parameter effects on MDerformance.
.1.1. Fluid management
Feed and permeate temperature are important parameters forontrolling performance of MD (cf. Fig. 4). In general, the increasen feed temperature increases the permeate flux exponentially.his condition can be explained by the resulting vapor pressureifference as driving force. Although the resulting temperature
ards practical/industrial implementations of MD.
polarization will be higher, evaporation efficiency – the ratio ofheat used for evaporation and total heat input in the module – willbe in general more efficient for operating at high feed tempera-ture [20,23–25]. However, decreasing selectivity with increasingfeed temperature could be observed [24,26,27]. In contrast to feedtemperature, the increase in permeate temperature decreases per-meate flux. Higher vapor pressure difference can be obtained inprinciple by decreasing permeate temperature. However, this stepwill significant for the temperature higher than 40 ◦C. Nevertheless,this can be effectively done in DCMD and not for other configu-rations [28,29]. One should note that decreasing and increasingfluid temperatures along the membrane module length for feedand permeate, respectively, could not be avoided but they shouldbe minimized. Operating the permeate stream at room or ambienttemperature without any treatment is the most economic way forpractical implementation of MD. By contrast, feed stream should
be heated to certain temperature. Even though working at highfeed temperature will result in higher evaporation efficiency thisfeed temperature in practical application will mainly determinedby the heat source used. Therefore, this parameter is very limited tobe engineered. Economic analysis showed that to be economically
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44 H. Susanto / Chemical Engineeri
mplemented the discharge temperature of the hot solution shoulde 50 ◦C or higher.
The effect of feed concentration on performance of MD dependsery much on the characteristics of feed itself (volatile vs. non-olatile solute). For a feed having non-volatile solute (e.g., foresalination), the presence of solute will in principle not onlyecrease the vapor pressure, but also cause concentration polar-
zation. Both effects then eventually decrease the permeate flux.y contrast, for a feed having volatile solute (e.g., for water/ethanoleparation), the increase in solute concentration usually increaseshe permeate flux due to increasing vapor pressure. For a complexeed system containing both volatile and non-volatile solutes, theffect will be dependent on vapor pressure ratio and may increasehe selectivity for volatile component [30,31]. Application of MD forreatment of water–nonvolatile solute mixture has greater chanceo be practically applied. In addition, it is interesting to note thathe advantage of MD over pressure-driven membrane processes ists ability to work with near saturated solution.
Permeate flux of MD can also by increased via managing theeed and permeate flow rate. Feed velocity and stirring rate areddressed to decrease boundary layer thickness and reduce bothhe temperature and concentration polarization. This means theermeate flux will increase as the feed flow rate and stirring ratere increased (see e.g., [2,3,20,28,31]). However, a plateau valueay be achieved at a certain high feed flow rate. It is known that
t high feed as well as permeate flow rate, high turbulence ischieved and temperature polarization can be minimized and per-eate flux eventually increases. Nevertheless, one should note that,hile increasing feed and permeate flow rates the (hydrostatic)
rans-membrane pressure should be lower than LEP to avoid poreetting. Therefore, operating at medium velocity with Reynoldumber within the range 3000–5000 is the best option.
.1.2. Module and spacer designsThere are at least four modules, namely plate and frame, spi-
al wound, capillary and hollow fiber, have been used during MDesearches and studies. Ideal module for MD should provide highates of mass and heat transfer, high flow rates for both feed andermeate, high packing density-membrane surface area to moduleolume ratio, low pressure drop, and high evaporation efficiency.n addition, because MD is non-isothermal process and is used atelative high temperature, a module for MD should have high ther-al stability and heat recovery. A lot of efforts have been devoted
o develop membrane module for MD [3]. For examples, it wasecently identified that the permeate flux reduction of MD in hol-ow fiber module was within the range 12–58% (of ideal module)ue to flow mal distribution caused by polydispersity of lumeniameter and non-uniformly distribution of fiber in module shell32]. Preparing the fiber uniformly distributed could decrease theermeate flux reduction [33]. New membrane distillation configu-ation and module were developed to provide reduced temperatureolarization effects due to better mixing and increased mass trans-ort of water [34]. They claimed that comparison with previouslyeported results in the literature reveals that mass transport ofater vapors is substantially improved with the new approach.
Very recently, different hollow fiber module designs withaffles, spacers and modified hollow fiber geometries for fluxnhancement were investigated [35]. They claimed that the appli-ation of baffles can increase the fide-side heat transfer coefficient.urther, the application of different hollow fiber configurationsith wavy geometries caused flux enhancements as high as 36%
ithout inserting any external turbulent promoter. Another efforto support practical implementation from module design point ofiew is increasing the module size. Industrial application can con-ain several hundred thousand square meters of membrane; this
eans significant cost reduction can be achieved by increasing
Processing 50 (2011) 139–150
the size (area) of the individual membrane module in MD plants.An illustration, if a 16-in. diameter spiral wound module is usedinstead of an 8-in. diameter, the membrane area is increased fromapproximately 40 m2 to 150 m2.
In addition to membrane module, spacer can also have signif-icance influence on MD performance. Using spacers among thefibers might increase the effective membrane area within the range18–33% [35]. Addition of screen separator as channel spacer forhot and cold water in DCMD could increases the permeate fluxby increasing fluid turbulence [36]. Similar result was obtained byPhattaranawik et al. [37].
4.1.3. Novel membrane preparationUnfortunately, the increasing interest in MD is less followed by
development of membrane manufacturing, which is specificallyproduced for MD. So far, polymeric hydrophobic microfiltrationmembranes such as polypropylene (PP), polyvinylidene fluoride(PVDF), polytetrafluoro ethylene (PTFE) which are originally notdesigned for MD, are used. Ideal membrane for MD should havethe following characteristics: high mass transfer, high wettingpressure (LEP), low thermal conductivity, high resistant towardsfouling and scaling, good thermal stability and high chemical resis-tance. Beside polymeric membranes, inorganic membranes havealso been developed for MD. However, they are usually modifiedin order to face the characteristic needed for MD. Cerneaux et al.modified ceramic membrane with C8F17(CH2)2Si(OC2H5)3 perfluo-roalkylsilane molecule (C8) [38]. Recently, carbon nanotube basedmembranes have been prepared instead of polymeric membranefor DCMD. The membranes were found to have many propertiesfavorable for MD as a high contact angle (113◦), high porosity (90%),and relatively low thermal conductivity [39]. They claimed that BPmembranes are a promising alternative to current polymeric mem-branes. From the material point of view, that development maybe interesting but to the best of my knowledge it will be difficultto be practically applied. The reason is its thermal conductivity issignificantly higher than normal polymer used for MD. To achievethe above characteristics, important parameters include membranethickness, pore size and its distribution, porosity, tortuosity and(polymer) material. Because most of the established membranepolymers cannot meet all performance requirements for a mem-brane dedicated to a MD and trade off between one characteristicand other characteristic, manufacturing membrane for MD withhigh performance is not easy. For example a trade-off betweenmass flux and evaporation efficiency is found during determinationof membrane thickness. As membrane thickness is increased, themass flux will decrease but the heat efficiency will increase. There-fore, membrane thickness should be optimized in order to obtainoptimum mass flux and heat efficiency. Lagana et al. reported thatthe optimum thickness of membrane for MD is within the range30–60 �m [25]. Porosity should be as high as possible because highpermeate flux as well as low heat (conduction) loss can be achieved.Similar with membrane thickness, a trade-off effect is also observedfor membrane pore size. Large pore yields high permeate flux butsmaller LEP. The later means the membrane is easier to be wetted.Pore size distribution will influence uniformity of vapor permeationmechanism. Definitely uniform pore size is preferable rather thandistributed pore size. The permeate flux can also be increased byminimizing the membrane tortuosity. Other important parameterfor (polymeric) membrane applied in MD is its wetting property.Even though it can also be influenced by its pore size, the characterof material determines significantly this property. In this regard,
hydrophobic material should be used because this material can beused for manufacturing the membrane having big pore size withhigh LEP (can be operated at high pressure without wetting phe-nomenon). Very recently, Zhang et al. identified the material andphysical features of membrane for MD [40].
H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150 145
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ig. 5. SEM and EDX images of dual layer hydrophilic–hydrophobic hollow fiber meension (reprinted from [45]; with permission from Elsevier, 2007).
Composite membrane, a combination of two (or more)aterials with different characteristic to obtain synergetic prop-
rties, offers possibility to achieve the desired characteristicsf membrane for MD. Three approaches to prepare compos-te membranes have been proposed, i.e., (i) preparation ofydrophobic–hydrophilic composite membranes by blending, (ii)reparation of hydrophobic–hydrophilic composite membranes byrafting and (iii) preparation of hydrophobic–hydrophobic com-osite membranes by grafting. Nevertheless, composite membraneomposed of hydrophilic/hydrophobic has also been proposed [41].n addition, to obtain high mass transfer, composite membrane cane achieved using support layer with high porosity at the bottomide of the thin selective layer facing the feed.
To obtain stronger hydrophobicity, surface modification is usu-lly done. Matsuura and coworkers have developed compositeembrane for MD via blending fluorinated surface modifyingacromolecules (SMMs) with hydrophilic polymer (polyether sul-
one, polyether imide) during phase separation technique [42,43].he results showed that the membranes prepared with addi-ion of SMMs had higher contact angle and higher LEP thanhe membranes prepared without addition of SMMs. In addition,he membranes had similar permeate flux with commercial PTFE
embrane having larger pore size and higher porosity. Surfaceodification of macroporous cellulose nitrate membranes has been
erformed by plasma polymerization of octafluorocyclobutane torepare composite membrane having hydrophobic–hydrophilic
ayers [44]. This approach has been initiated by Cheng and Wiersma
11,12]. Typical MD behavior could be observed and good per-ormance was obtained. Another novel membrane specificallyesigned for MD was proposed by Li and Sirkar [45]. The mem-rane consisted of hydrophobic–hydrophobic layers prepared byoating of silicone polymer via plasma polymerization on top of PPe incorporating clay particles for enhancing mechanical property as well as surface
microfiltration membrane. Boyadi and Chung [46] fabricated duallayer hydrophobic–hydrophilic porous composite PVDF HF mem-branes by a co-extrusion spinning process. Hydrophobic andhydrophilic clay particles were incorporated into the outer andinner layer dope solutions, respectively, in order to enhancemechanical properties and modify the surface tension propertiesof inner and outer layers. They claimed that the examination ofthe resulting membranes for DCMD process yielded higher fluxthan the membranes used in most of the previous reports. Fig. 5shows the SEM and EDX micrographs of the composite membraneprepared by Boyadi and Chung [46].
4.1.4. Solving flux reduction problemBesides increasing mass transfer, industrial/practical imple-
mentations of MD will also be supported by solving the problemof fouling and scaling, which will decrease the permeate flux.Although considerable amount of debate in existing of scaling andfouling phenomena during MD operation [47–50] are found, butdecrease in flux is definitely observed especially for long term oper-ation. Therefore, it is important to consider this flux reduction forlong term or specific applications. For examples, Curcio et al. [51]reported that DCMD tests, carried out under a temperature gradientof 20 ◦C (feed temperature: 40 ◦C), showed a progressive deteri-oration of the system performance, and the initial distillate flux(2.05 ± 0.05 l/m2 h) was reduced by 45% after 35 h of uninterruptedoperation. He et al. analyzed the scaling potential in DCMD usinggypsum solution in hollow fiber module with crossflow mode [52].
They showed that scaling was really found during operation (Fig. 6)and the induction period for the gypsum precipitation decreasewith increasing feed brine temperature. However, no significantimpact on flux decrease was observed. This phenomenon may becaused by no organic matter in feed solution. It was reported that
146 H. Susanto / Chemical Engineering and
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ig. 6. SEM picture of gypsum crystals on the surface of hollow fibers evaporated toryness after an experimental run with Tb,i = 86.4 ◦C, Td,i = 22.0 ◦C (reprinted from51]; with permission from Elsevier, 2008).
rganic molecules can be deposited or flocculated in the presencef salts or due to concentration polarization [53,54]. On the otherand, NOM such as humic acid, even at low concentration, retardedhe nucleation and growth of vaterite crystals [48].
Scaling, deposition of inorganic salt or mineral on mem-rane surface due to crystallization phenomenon, is probablyound in long term application for desalination or treatingeed with high salt/solute concentration. Scaling is intensifiedn application of MD for treating feed containing solute withnverse temperature–solubility behavior such as gypsum andalcium carbonate and sodium sulfate. Fouling can be causedy deposition of organic substances that present as dissolvedrganic matter or by biological substances on the membraneurface [3].
In sum, fouling and scaling are two phenomena that couldeduce the resulting flux of the membrane. However, the behav-or and the extent of these problems will depend very much onhe characteristics of feed and permeate streams used (it should beoted that in all cases hydrophobic membrane is used in MD), theembrane material properties and the operating condition used.ifferent component and composition of feed and permeate will
esult in different fouling and scaling as well as their consequence.imilar results will definitely be observed for membrane charac-eristics and operation conditions. Readers, who interested in fluxeduction phenomenon during MD operation, are directed to Gryta48] and Khayet [3].
Rinsing and cleaning to remove all deposit solid on the mem-rane surface is usually done to bring back the flux into initial value.n example of cleaning procedure was given by Curcio et al. [51].wo-step cleaning with citric acid aqueous solution (20 min)/NaOHqueous solution (20 min) allowed to completely restore the trans-embrane flux and the hydrophobicity of the membrane. Another
echnique to minimize scaling is antiscalant. The use of antiscalantould extend the induction period for the nucleation of gypsumnd calcite, respectively; and slow down the precipitation rate ofrystals [55]. Nevertheless, one should note with the dosing ofntiscalant because antiscalant is organic compound, which couldasily wet the membrane [56].
.2. Energy source
One of the reasons for MD has not being practically/industriallymplemented is its high thermal energy consumption, which ishe main production cost. This occurs because membrane is added
Processing 50 (2011) 139–150
in evaporation process meaning addition of both mass and heatresistances and thus higher energy is needed. However, the advan-tage of MD is it can be operated at low temperature. Therefore, theuse of thermal energy, which is already available, e.g., waste heator renewable energy (geothermal, wind or solar energy), will real-ize practical/industrial implementations of MD even tough the fluxmay be smaller compared with other processes.
Solar energy is the most interesting alternative energy for MD.An interesting development of MD using solar and wind energyfor water supply was developed [57,58]. This development wasdesigned for small board and rural or coastal area, where electric-ity is not available. As presented in Fig. 7, the feed is pre-heated bypermeate stream in heat exchanger and further heated in solar cellcollector before entering the membrane module. All pumps used inthis device are powered by wind generator.
The small and large scale stand alone desalination system(SMADES system) were fabricated to provide potable water inremote areas with a lack of electricity and drinkable water but withhigh solar irradiation [59,60]. The plant integrates solar thermaland photovoltage (PV) energy with MD modules with internal heatrecovery function. Fig. 8 shows the main component of large scaleSMADES system.
Solar (pond) desalination by air gap membrane distillation aspart of a zero discharge system was also developed [61]. It wasreported that the resulting flux varies within the range 0–6 l/m2 h.Similar work the combination of solar still and MD was investigatedfor producing potable water by Banat et al. [62]. They showed thatsolar intensity was very important parameter to obtain high flux.The contribution of solar still on the resulting potable water wasless than 20% of the total water flux. As solar intensity increased,the water fluxes increased for both solar still and membrane mod-ule. An effort to realize practical implementation of MD for freshwater supply in arid and semi arid areas has been conducted bycoupling MD with solar energy via MEDESOL project [63]. Multisteps MD was constructed and to be implemented in solar platformof Almeria, Spain. Another example of solar thermal-driven mem-brane distillation was developed by Fraunhofer ISE [64]. Finally,an economic analysis of solar powered MD plant for the supply ofdomestic drinking water in the rural regions of Australia has beenmade [65].
In sum, the use of renewable energy would decrease theproduction cost significantly and realize practical/industrial imple-mentations of MD. Nevertheless, the investment cost especiallyfor the fabrication of solar sell and membrane module should fur-ther be decreased. For the membrane module, Fig. 4 shows the tipsfor the optimization to reduce the cost. The optimization of solarsell however, is beyond the scope of this paper. The economic ofMD integrated with power plant was also evaluated [66]. Recently,Banat and Jwaied reported an economic assessment of solar pow-ered MD for potable water production in arid area [67]. Based on thecalculations, the estimated cost of potable water produced by thecompact unit is $15/m3, and $18/m3 for the large unit. Membranelifetime and plant lifetime are key factors in determining the waterproduction cost. The cost decreases with increasing the membraneand/or the plant lifetime.
Beside solar energy, the use of geothermal will support realiza-tion of MD in practical applications. Unlike the use of solar energy,the use of geothermal for MD has not been widely developed. Anexample of MD using geothermal was given by Bouguecha andDhahbi [68] and El-Amin et al. [69]. Further, Bouguecha et al. [70]have made economic evaluation of MD using geothermal energy.
Using waste heat from a steam turbine, Tay et al. showed the use ofwaste heat for heating the feed stream during vacuum membranedistillation [71]. An interesting example was given by Xu et al. Theydesigned a pilot vacuum membrane distillation and installed ona ship [72]. Seawater as feed was heated by the waste heat gen-
H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150 147
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Fig. 7. Schematic of DCMD p
rated from the vessel engine. This waste heat could be used toncrease feed temperature up to 55 ◦C yielding permeate flux of.4 kg/m2 h.
.3. More “extreme” application
A wide range of applications of MD can be found in many pre-iously reported literatures. However, not all applications have
conomic feasibility to be practically/industrially implemented. Toy opinion, only for “extreme” applications, where other processannot be used or very expensive, MD will be competitive to beractically implemented. In this case, MD can be used as eitheringle process or coupled with other technologies.
ig. 8. Large SMADES autonomous desalination solar driven MD: solar collector field (lef59]; copyright (2007) Elsevier).
d by solar and wind energy.
One of the attractive applications for MD is for treating highsaline water (brine) such as concentrate of RO. In this case, RO mem-brane becomes more expensive, due to very high osmotic pressure,and is problematic due to scaling phenomenon. Disposal of RO con-centrate to the sea in huge amount will disturb the ecosystemand disturb the environment equilibrium. MD, which is not lim-ited by the concentration of solute and has ability to work withnear saturated solution, is interesting to be used in this applica-
tion. In relation with this application, MD has been integrated tothe crystallizer to produce supersaturation solution as well as purewater [73–75]. In this way, two product streams can be obtained,i.e., pure water from permeate and solid crystal from crystallizer.Fig. 9 shows the schematic representation of MD-crystallizationt panel) and main component of MD (right panel) (reprinted with permission from
148 H. Susanto / Chemical Engineering and Processing 50 (2011) 139–150
tion o
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Fig. 9. Schematic representa
ystem. The key success parameters in this application are min-mization of deposition and growth of crystal on the membraneurface and maximization of crystal production and removal in therystallizer. Thus, temperature and concentration are importantariables. Operating MD at appropriate condition such as moder-te trans-membrane flux could reduce the resulting concentrationolarization. Note that some salts such as Na2SO4 have a negativeemperature coefficient meaning that the solubility decreases withncreasing temperature.
The principle of MD has been adopted and has been usedor solvent distillation (to separate benzene and toluene) [76]. Inhis case hollow fiber membrane from alumina (ceramic mem-rane), which is the same material used to produce commercialandom packing, was used to increase the stability. The resultshowed that the process had high capacity, which allowed themo operate at many times above the flooding limit of conventionalquipment, owing to the absence of the flow constraints. Theylso had the exceptional high separation efficiency, as presentedy the very low height transfer unit (HTU) of 9 cm, which wasesulted from the very high surface area per unit volume. Never-heless, the effects of membrane thickness and porosity on the HTUere not so pronounced. Very recently, a novel micro-separator
ombining the sweep gas membrane distillation principle withicro-fluidic channels was designed and tested for the separa-
ion of a mixture of methanol and water with a low to highethanol concentration [77]. In addition, the selection of an appro-
riate membrane liquid–vapor/gas contactor was found to be anmportant design parameter for the reduction of temperature
Fig. 10. Schematic diagram of the NASA DOC wastewater treatment proce
f MD-crystallization system.
polarization effects. The novelty of this technique is the separa-tion of liquid mixtures with a high concentration of volatile organiccomponents. Compared to conventional membrane distillation,the use of micro-fluidic system reduces the external condensationeffort required for normal-scale sweep gas membrane distillationprocess and which constitutes the main drawback of this pro-cess.
Interesting application of MD concept was given by Cath et al.[78]. They incorporated MD into NASA direct osmosis concentration(DOC) for treating of combined hygiene and metabolic wastewaterin spacecraft (Fig. 10 shows the schematic diagram of NASA DOCwaste water treatment).
In the existing DOC #2 (DO/OD (direct osmosis/osmotic distil-lation) system), mass transport is carried out in three steps: waterdiffuses from the wastewater stream through the semi-permeablemembrane, evaporates through the hydrophobic microporousmembrane, and then condenses in the osmotic agent (OA). The driv-ing forces throughout the process are the osmotic pressure andpartial vapor pressure gradients across the two membranes cre-ated by the concentration difference between the feed wastewaterand the OA. The main limitation of this process is the capacity totreat wastewater is very low due to slow mass transport in OD pro-cess. Consequently, this process is not enough to treat the resultingwastewater. MD was then proposed to increase the existing mass
transfer (it should be noted that the temperature driving force isstronger than the concentration driving force). Two MD-enhancedconfigurations, i.e., DO/MD and DO/MOD (membrane osmotic dis-tillation) were evaluated.ss (obtained with permission from [77]; copyright (2005) Elsevier).
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In DO/OD system, the process is isothermal meaning that theriving force comes only from the concentration gradient acrosshe two membranes. The DO/MD configuration uses cold permeatetream with negligible solute concentration. Therefore, the drivingorce for mass transport is the vapor pressure gradient across the
embranes, which is resulted from the temperature gradient. Inhis configuration the concentration driving force opposes the tem-erature driving force, but because the temperature driving force isuch stronger than the concentration driving force, this opposition
lightly reduces the flux but does not stop the process. In order toeverse the concentration gradient and create a concentration driv-ng force, the permeate stream could be switched from ultrapure
ater to an OA (MOD). In this configuration (DO/MOD), the con-entration and temperature driving forces are shown to operaten the same direction. The experimental results from bench-scaleests showed that both DO/MD and DO/MOD were successful inchieving higher flux and complete urea rejection with the fluxesn DO/MD and DO/MOD were 4–20 and 8–25 times greater, respec-ively, than in conventional DO/OD system.
Other potential application of MD is in metallurgical industry.n this industry, a large amount of wasted heat is usually found.his wasted heat is very suitable for MD. In addition, solutionsn hydro-metallurgical processes frequently needed to be concen-rated (it should be noted that MD can be used to treat saturatedolution). Therefore, the industrialization of membrane distillationan greatly promote the progress of technology in the metallur-ical industry. Some interesting publications of MD applications inhis area have been reported (e.g., [79,80]). In general, the potentialpplication of MD in metallurgical industry includes (i) condensingaste acid, alkaline or salt solution, recovery of acid solutions from
are earth, acid recovery (pickle liquor) during pickling process.
. Concluding remark
Even though the development of MD has been started since960s, practical/industrial implementations have not been widelyealized. Along with its interesting benefits, MD requires high ther-al energy and results in relatively low permeate flux. A wide
ange of applications of MD has been reported in many previ-usly reported literatures, but not all applications have economiceasibility to be practically/industrially implemented. To be practi-ally implemented, MD should use free/cheap energy or should bepplied in “extreme” applications. Studies to improve membraneerformance via process conditions optimization, preparation ofovel membranes, module configurations as well as spacers sup-ort the practical implementation but they will not determineractical/industrial applications. Therefore, to be useful from prac-ical point of view, researches to find new energy which isree/cheap as well as finding extreme application should be pri-ritized.
cknowledgement
This work was financially supported by the Ministry of Nationalducation, Indonesia.
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