OA16487A - Composite mixed matrix membranes for membrane distillation and related methods of manufacture. - Google Patents

Abstract

The present invention relates to a membrane distillation system comprising a flat-sheet composite mixed matrix hydrophilic/hydrophobic membrane having at least a hydrophilic layer and a hydrophobic layer. The hydrophilic layer comprises a hydrophilic polymer and inorganic nanoparticles having high thermal conductivity. The hydrophobic layer comprises fluorinated surface-modifying macromolecules (SMM). Also disclosed is a phase inversion method for manufacturing the membrane.

Description

The disclosed tcachings pertain to the field of membranes for membrane distillation. More particularly, the disclosed teachings pertain to the field of mixed matrix hydrophobic/hydrophilic composite mixed matrix membranes.

References [02]

The following references are provided herein for additional background information and as such they are incorporated by reference.

[03]

Kliayet, M., Membranes and theoretical modeling of membrane distillation: A review, Advances in Colloid and Interface Science, 164, 56-88,2011.

[04]

Kliayet, M., Matsuura, T., Membrane Distillation: Principlcs and Applications,

Elsevier, Amsterdam (The Netherlands) 2011.

[05]

Qtaishat, M.R., Khayet, M., and Matsuura, T., Composite membranes for

No. 20110031100.

Background [06]

Membrane distillation (MD) is an emerging physical séparation technology, which has been attracting researchers’ attention in the last few décades (Khayet, 2011). MD is a thermally driven process where a microporous membrane acts as a physical support separating a warm solution from a cooler chamber containing either a liquid or a gas. As the process is nonisothermal, vapor molécules migrate through the membrane pores from the high vapor pressure membrane side (i.e. warm membrane side) to the low vapor pressure membrane side. This can bi established following different configurations, i.e. direct contact membrane distillation, DCMD; air gap membrane distillation; AGMD; sweeping gas membrane distillation, SGMD and vacuum membrane distillation, VMD (Khayet, 2011).

[07] An important requirement for the MD membrane is that the pores must not be wetted and only vapor is présent in the pores. This requirement Iimits the choice of materials that can be used for MD. Notably MD membranes are limited to hydrophobie materials such as polytetrafluoroethylene (PTFE), polypropylene (PP), and polyvinylidene fluoride (PVDF). Although these membranes were manufactured for microfiltration and ultrafiltration purposes, they hâve been used in MD research duc to their hydrophobie nature (Khayet, 2011).

[08] MD holds several advantages compared to other séparation processes. These advantages, mainly, are; up to 100 % rejection of non-volatile solutés, lower operating températures than conventional distillation, lower operating pressures than conventional pressure-driven membrane séparation processes such as reverse osmosis (RO) and reduced vapor spaccs compared to conventional distillation processes. Despite ail these advantages, MD process has not been commercialized yct for large scale plants. One of the reasons is the relatively lower MD flux and the membrane wetting, which diminishes the durability of MD membranes. As can be seen, the disadvantages arise from inadéquate design of the MD membranes.

[09] In the recent published book (Khayet & Matsuura, 2011), the requïrements of higher permeate flux DCMD membranes are discussed. Hydrophobic/hydrophilic composite membranes for DCMD are also discussed in US patent application No. 12/629,703 (Qtaishat,

Khayet & Matsuura, 201'1). In this Application it is was shown that this type of membrane satisfies ail the requïrements of higher permeate flux DCMD membranes (Qtaishat, Khayet &

Matsuura, 2011). The hydrophobic/hydrophilic membrane was prepared by the phase inversion method in a single casting step. A hydrophilic base polymer was blended with a hydrophobie surface modifying macromolecule (SMM). During the casting step, the SMM migrated to the air/polymer interface since they hâve lower surface energy (Qtaishat, Khayet & Matsuura, 2011). Consequently, the membrane top-layer became hydrophobie while the bottom layer was mamtained hydrophilic. There remains a need for high permeate flux and durable membranes for use in DCMD.

[10] This background information is provided for a better understanding of the disclosed teachings. It is not be construed as an admission that any of the above discussed information constitutes prior art against the présent invention.

SUMMARY [11] An object of the présent invention is to provide composite mixed matrix membranes for membrane distillation and related methods of manufacture that overcome the drawbacks, which hinder MD from being commercialized for large scale plants.

[12] As a first aspect of the invention, there is provided a membrane distillation system comprising a flat-sheet composite mixed matrix hydrophilic/hydrophobic membrane having at least a hydrophilic layer and a hydrophobie layer. The hydrophilic layer further comprising a hydrophilic polymer and inorganic nano-particles of high thermal conductivity. The hydrophobie polymer layer further comprising fluorinated surface-modifying macromolecule (SMM).

[ 13] Preferabiy, the hydrophilic polymer is a thermplastic polymer and still preferabiy, ît is selected from the group consisting of polysulfone, polyethersulfonc, polyetherimide and cellulose acetate. \y/ [l 4] Preferably, the înorganic nano-particles are selected from the group consisting of cupper oxide, boron nitride, aluminum nitride, aluminum, iron and silicone carbide.

[ 15] Preferably, the hydrophobie polymer layer is made of fluorinated surfacemodifying macromolecules (SMMs) synthesized using polyuréthane chemistry and tailored with fluorinated end-groups. Preferably, the fluorinated SMM is blended with the hydrophilic polymer-inorganic nano-particles dispersion, The SMM is selected from the group consisting of poly(urethane propylene glycol) and poly(urea dimethylsiloxane urethane).

[ 16] Preferably, the composite mixed matrix membrane has a high vapor permeate flux.

[ 17] Preferably, the composite mixed matrix membrane has high mechanical properties.

[ 18] Preferably, the composite mixed matrix membrane has a less wetting tendency, consequently high durability.

[19] As a further aspect of the invention, there is provided a phase inversion method for manufacturing a membrane distillation composite mixed matrix hydrophilic/hydrophobic membrane, said method comprising dispersing a host hydrophilic polymer with a predetermined amount of insoluble înorganic nano-particles and a non-solvent additive in a solvent to form a polymer-inorganic solution. A fluorinated surface modifying macromolecule (SMM) is added to the polymer-inorganic solution to form a polymer-inorganic nano-particles SMM blend. The polymer-inorganic nano-particles blend is cast and said solvent is allowed to evaporate at room température for a predetermined time to form a cast film. A time of évaporation is varied systematically to study and modify an effect of the évaporation time on settling of the înorganic nano-particles in a bottom layer as well as the hydrophobie SMM migration to the top layer. The — cast film is covered by a cover having a certain displacement to control évaporation of the solvent allowing more time for settling of the inorganic nano-particle in the bottom layer and migration of a hydrophobie SMM to the air/polymer interface. The cast film produced is immersed in water to allow gélation.

[20] Preferably, the method of manufacturing a composite mixed matrix membrane further comprises maximizing porosity and mînimizing thickness of the hydrophobie polymer layer of the composite membrane in order to increase the MD permeate flux of the composite membrane.

[21 ] Preferably, the method of manufacturing a composite mixed matrix membrane further comprises maximizing thickness, porosity and thermal conductivity of the hydrophilîc polymer layer.

[22] Preferably, the host hydrophilîc polymer comprises at least one of the polymers polysulfone, polyethersulfone, polytherimide and cellulose acetate.

[23] Preferably, the inorganic nano-particles are selected from the group consisting of cupper oxide, boron nitride, aluminum nitride, aluminum, iron and silicone carbide.

[24] Preferably, the SMM is selected from the group consisting of poly(urethane propylene glycol) and poly(urea dimethylsiloxane urethanc).

[25] Preferably, the non-solvent additive is selected from the group consisting of γbutyrolactone and éthanol.

[26] Preferably, the solvent is selected from the group consisting of N,Ndimethylacetamide and l-methyl-2-pyrrolidone.

BRIEF DESCRIPTION OF THE DRAWINGS [27] Further features and advantages of the présent invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[28] Figure l is a schematic of the DCMD mechanism of transport through a porous composite mixed matrix hydrophobic/hydrophilic membrane;

[29] Figure 2 shows an example of the chemical structure of SMMs, nSMM l and SMM2;

[30] Figure 3 is a schematic diagram of the experimental DCMD set-up;

[31 ] Figure 4 shows SEM pictures of the cross-section of composite mixed matrix and polymeric membranes: (a) MC3; (b) MC4; (c) MC5; (d) MC6; (e) MC8; (f) MC9; (g) MC10; (h) MCI4; (i) MCI6; (j) MC2l; (k) MC22 [32] Figure 5 shows SEM pictures of the top and bottom surface of composite mixed matrix membrane MC 17;

[33] Figure 6 shows tensile stress-strain curves of the composite mixed matrix and polymeric membranes.

[34] Figure 7 graphically depicts the effect of copper oxide addition on 12wt% PS membrane performance in DCMD: (a) mean température effect on DCMD permeate flux of distilled water feed solution; (b) water vapour DCMD flux of 0.5 M NaCl feed solution at 7/of 65°C and T_p of 15°C;

[35] Figure 8 graphically depicts the effect of copper oxide and boron nitride addition on 14wt% PS membrane performance in DCMD: (a) mean température effect on DCMD flux of distilled water feed solution; (b) water vapour DCMD flux of 0.5 M NaCl feed solution at 7/of 65°C and T_p of 15°C;

[36] Figure 9 graphically depicts the effect of copper oxide addition on 14wt% PES membrane performance in DCMD: (a) mean température effect on DCMD flux of distilled water feed solution; (b) water vapour DCMD flux of 0.5 M NaCl feed solution at 7}of 65°C and T_p of 15°C;

[37] Figure 10 graphically depicts the effect of mean température on DCMD flux of both distilled water and 0.5 M NaCl solutions as well as on the séparation factor for MCI l and MCI2 membranes in which iron was used as nano-particles.

[38] Figure 11 graphically depicts the effect of mean température on DCMD flux of both distilled water and 0.5 M NaCl solutions as well as on the séparation factor for MCI6 and MCI 8 membranes in which aluminum and silicone carbide was used as nano-particles, respectively.

DETA1LED DESCRIPTION [39] Unless defined otherwise, ail technical and scientific terms used herein hâve the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[40] As used in the spécification and claims, the singular forms “a”, “an” and the “the” include plural references unless the context clearly dictâtes otherwise. The term “comprising” as used herein will be understood to mean that the list following is non-exhaustive and may or may not include any other additional suitable items, for example one or more further feature(s), component(s) and/or ingredient(s) as appropriate.

[41] Fig. 3. shows an exemplary membrane distillation system according to the disclosed teachings. The central part of the system is a stainless steel cell composed of two cylindrical chambers. One of the chambers is connected to a heating system through its jacket to control the température of the liquid feed. The other chamber is connected to a cooling system to control the température of the permeate. The membrane is placed between the two chambers (feed side and permeate side). The hot feed solution is brought into contact with the hydrophobie top layer of the membrane and the cold permeate solution is in contact with the hydrophilic part of the membrane. The effective membrane area is 2.75 x ÎO'³ m². The bulk feed and permeate températures are measured, after steady state is reached, inside each chamber by a pair of sensors connected to a digital meter with an accuracy of± O.l°C. Both the feed and permeate liquids are stirred inside the cell by graduated magnetic stirrers. The DCMD flux is calculated in every case by measuring the condensate collected in the permeate chamber for a predetermined period. The experiments are conducted first for pure water to détermine the water vapour permeability of the membranes. Subsequently, aqueous solution of 0.5 M sodium chloride is employed as feed.

[42] Fig. I shows an exemplary composite mixed matrix membrane according to the disclosed teachings. As shown, the membrane is a hydrophilic/hydrophobic membrane. It has a hydrophilic layer and a hydrophobic layer. The hydrophilic layer has a hydrophilic polymer and inorganic nano-particles of high thermal conductivity. The hydrophobie layer has fluorinated surface-modifying macromolecule (SMM).

[43] In general, membranes for use in MD should allow a high permeate flux. As it is well known, a MD membrane must be porous and hydrophobie, with good thermal stability and excellent chemical résistance to feed solutions. The characteristics needed for DCMD membranes are as follows:

High liquid entry pressure (LEP) [44] This is the minimum hydrostatic pressure that must be applied to the liquid feed solution before it overcomes the hydrophobie forces of the membrane and pénétrâtes into the membrane pores. LEP is characteristic of each membrane and prevents wetting of the membrane pores when it is high. High LEP may be achieved using a membrane material with high hydrophobicity (i.e. large water contact angle) and a small maximum pore size. However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane become low.

High permeability [45] The permeate flux will “increase” with an increase in the membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In other words, to obtaîn a high permeability, the surface layer that govems the membrane transport must be as thin as possible and its surface porosity as well as pore size must be as large as possible. However, it must be mentioned here that there exists a critical pore size equal to the mean free path of water vapor molécules for given experimental DCMD conditions. In DCMD process, air is always trapped within the membrane pores with pressure values close to the atmospheric pressure. Therefore, if the pore size is comparable to the mean free path of water vapor molécules, the molécules of water vapor collide with each other and diffuse among the air molécules. In this case, the vapor transport takes place via the combined Knudseti/molecular diffusion flow. On the other hand, if the pore size is smaller than the mean free path of water vapor molécules, the molecule-pore wall collisions become dominant and the Knudsen type of flow will be responsible for the mass transport in DCMD. It should be noted that for given experimental conditions, the calculated DCMD flux based on Knudsen mechanism is higher than that based on the combined Knudsen/molecular diffusion mechanism.

[46] Under a certain operating condition, it would be better to use membranes with lower pore sizes than theicorresponding mean free path of water vapor moiecules so that the Knudsen type of flow will take place, leading to higher DCMD permeate flux compared to that of the membranes with larger pore sizes where the combined Knudsen/molecular diffusion flux is responsible for mass transfer. Therefore, care must be taken to choose the appropriate membrane pore size, taking into account the value of the mean free path of water vapor moiecules so that the membrane can work under the Knudsen type of flow,

Low thermal conductivity of the hydrophobie layer [47] In MD heat loss by conduction occurs through both the pores and the hydrophobie matrix of the membrane. The conductive heat loss is greater for thinner membranes layers. Various possîbilities may be applied to diminish the conductive heat loss by using:

[48] i) a membrane material of the hydrophobie layer with low thermal conductivity. This does not necessarily guarantee the improvement of the MD process because most hydrophobie polymers hâve similar heat conductivities, at least in the same order of magnitude.

[49] ii) a membrane layer with high porosity, since the conductive heat transfer coefficient of the gas entrapped within the membrane pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need of high permeability as the available surface area of évaporation is enhanced with an increase in porosity.

[50] iii) a thicker membrane layer. However, there is a conflict between the requirements of high mass transfer associated with thinner membranes and low conductive heat transfer through the membrane obtained by using thicker membranes.

High thermal conductivity of the hydrophilic layer [51]

The increase of the thermal conductivity of the hydrophilic sub-layer material will increase the DCMD permeate flux.

[52]

The hydrophilic layer thermal conductivity should be as high as possible. The reason is that the température gradient across the hydrophobie layer (in Fig. 1) becomes steeper i

with an increase in the thermal conductivity of the hydrophilic layer, as a resuit, the température polarization coefficient (TPC) defined below in Eq. 3 also increases.

[53]

T -T

(3) [54]

Mixed matrix composite porous hydrophobic/hydrophilic membranes, having a very thin hydrophobie layer responsible for the mass transfer and a thick hydrophilic layer of a high thermal conductivity for diminishing the température polarization effect, are thus required.

I

This seems to be a relatively simple solution that fuifîIls ail the above conditions for achieving high permeable and durable membrane.

Theoretical background of the importance of using mixed matrix membranes in membrane distillation [55] The system to be studied consists of a mixed matrix composite hydrophobic/hydrophilic membrane maintained between hot pure water, named hereafter the

I feed side, and cold pure water, the permeate side. The hydrophobie sîde of the membrane is brought into contact with the hot feed water, while the hydrophilic-layer of the membrane is maintained in contact with cold water, which pénétrâtes into the pores of the hydrophilic-layer. On the contrary, the pores of the hydrophobic-layer are maintained dry unless the applied λα/''' transmembrane pressure exceeds the liquid entry pressure of water (LEP) of the membrane.

Under this condition, liquid/liquid interfaces formed at both ends of the pores of the hydrophobic-laycr are as can be seen in Fig. I.

[56] The température drop established through the pores of the hydrophobic-layer will create a vapor pressure différence, which is the driving force in DCMD process. In this case, évaporation takes place at the hot feed side and, after water vapor is transported through the pores of the hydrophobic-layer, condensation takes place at the vapor/liquid interface formed at the boundary between the hydrophobie and the hydrophilic layers.

[57] In the described system, both mass and heat transfer occurs simultaneously across the membrane. Consequently, the températures at the membrane surfaces differ from those at the bulk phases of the solution leading to a decrease of the driving force and consequentely a decrease of the DCMD permeate flux. This is called température polarization effect.

[58] The heat transfer within the membrane involves the latent heat required for water évaporation at the hot feed liquid/vapor interface, and the heat transferred by conduction through the gas-fdled pores of the hydrophobie top-layer of the membrane, the liquid-filled pores of the hydrophilic sub-layer of the membrane and the whole membrane matrix (i.e. both hydrophobie and hydrophilic polymer| layers). In addition, heat transfer occurs through the adjoining liquid phases, both on the feed and permeate sides. Thus, the foliowing équations may be applied.

[59]		d)
[60]		(2)
[61]	Q^h^-T,,,)	(3)
[62]	Qp=W.p~Tb.p)	(4)

[63] At steady state, the heat flux must be the same throughout the whole DCMD system. | [64] Ô = Qf=Q. = Q, = Q_{P (5)} [65] where Q is the heat flux through each phase, A is the heat transfer coefficient, J_w is

I the permeate flux, ΔΗ_ν is the latent heat of vaporization and T is the absolute température. The subscripts b,/, p, m and 5 refer to the bulk solution, feed, permeate, hydrophobie top-layer of the membrane and its hydrophilic sub-layer, respectively. The heat transfer through the top

I hydrophobic-layer is indicated by the subscript i, while that transferred through the hydrophilic sub-layer is indicated by the subscript s.

[66] [67]

On the other hand, the température polarization coefficient (&) can be defined as:

Γ T_bJ-T_bp (6) [68]

Therefore, from the above équations, the heat flux can be written as follows:

(1-0)(7^-7^) [69] (7) [70] or

V‘ [71] —+ ^Λ/

V I + — , /

[72] written:

(8)

As a resuit the overall heat transfer coefficient (U) for the DCMD process may be [73] [74] [75]

1 11

--1-----1-h_f , , h. h_n f k₊--*---ï— 4 _P

T -T \ ⁴ m,f m,_P / and the température polarizatîon cocfïicient0might be expressed as:

Kf-T_b._p h (9) (10) [76] where h is the overall heat transfer coefficient valid for the hot feed phase, hydrophilic sub-layer and cold permeate phase:

[77] /1 = hjh_p +h_phj- +A,A, (H) [78]

Equations 10 and 11 may be rearranged as;

[79] = 0,+0,+0,-2 (12) [80] where Of, 0_s and θ_ρ are the température polarizatîon coefficients corresponding to the feed, hydrophilic sublayer and permeate phases, respectively; and are defined in Eqs. (13-15) as follows.

i.P [81] (13) [82] [83]

Q — 1 _ ~ ) =1-—=^{Γί/ Τ}'·^{ρ p} hp ^Tb.f~^Th,p (14) [84] DCMD process is controlled by a mass transfer through the membrane and a heat transfer through the composite System formed by the membrane plus the adjoining liquid layers. Both mechanisms are interrelated. In principle six possibilities may occur:

[85] l) If the heat transfers through the feed, hydrophilic sublayer and permeate are very large, the températures at the membrane surfaces approach to the correspondîng températures in the bulk phases. This means that the température polarization coefficients, Θ/, 0_S and θ_ρ> (see Eqs. 13 - 15), as well as the overall température polarization coefficient, Θ, approach unity (see Eq. 12).

[86] 2) If feed, hydrophilic sublayer and permeate heat transfer coefficients are small, the différences between the températures at the membrane surfaces and the températures correspondîng to the bulk phases are high. This means that the température polarization coefficient, Θ, approaches zéro (see Eq. 10). In this case, the température polarization effects are very important and the heat transfer résistances of the adjoining layers control the DCMD process.

[87] 3) If the permeate and hydrophilic sublayer heat transfer coefficients are very large in comparison to the feed heat transfer coefficient, the température at the permeate membrane surface (Τ_ίφ) is similar to the coresponding température at the bulk phase (Tb,_p). The hydrophobie top-layer of the membrane (T„_iP) and its hydrophilic sub-layer (T_SiP) become very similar too. This means that the permeate and hydrophilic sublayer température polarization coefficients, &_s and ij,, approach unity (see Eqs. 14 and 15). In this case, Eq, 12 shows that the overall température polarization coefficient is similar to the température polarization coefficient in the feed side, Θ/. — [88] 4) If the feed and hydrophilic sublayer sides heat transfer coefficients are very large, the température at the feed side membrane surface (T„j) become very similar to the bulle phase température (Tbj) and the température at the hydrophobie top-layer of the membrane (T„_iP) and its hydrophilic sub-layer (T_S1P) become very similar too. From Eqs. 13 and 14 the feed température polarization coefficient, 6j, and the hydrophilic sublayer température polarîzation coefficients approach unity. In this case, the température polarization coefficient in the permeate side (0_P) is important and is similar to the overall température polarization coefficient, Θ.

[89] 5) If the heat transfers through the feed and permeate are very large, the températures at the membrane surfaces approach to the corresponding températures in the bulk phases. This means that the températurepolarization coefficients, t^and Θ_ρ, (see Eqs. 13 and 14) approach unity and the température polarization coefficient in the hydrophilic sublayer (ft) is important and is similar to the overall température polarization coefficient, 0 (see Eq. 12).

[90] 6) If the heat transfer coefficient is very large in one of the layers, while the heat transfer coefficients in the other two layers were small, then the température polarization coefficient in the layer where the heat transfer coefficient is large approach unity and the other two layers température polarization coefficients will control the value of the overall température polarization coefficient, Θ according to Eq. 12.

[91] The latter possibility is the only possibility that a membrane designer could alter. That is, an increase of the heat transfer by conduction of the hydrophilic sub-layer. Increase of h_t. The prepared mixed matrix composite membranes of high thermal conductivity of the hydrophilic sublayer could satisfy this possibility, which explains the high permeate flux resuit since the heat transfer résistance in the membrane sublayer is significantly reduced. As a resuit the température polarization factor of the sublayer ($) approached unity. jv'¹''

Composite Mixed Matrix Membranes:

[92] The composite mixed matrix membranes of the présent invention comprise a hydrophilic layer and a hydrophobie layer. The hydrophobie layer prevents water pénétration înto îts pores and is relatively thin, thereby minimizing the résistance to mass transfer.

[93] The composite membranes are prepared using fluorinated surface-modifying macromolecules (SMMs), which migrate to the air-film surface during membrane formation according to thermodynamic principles and form an amphipathic structure (hydrophobic/hydrophilic/hydrophobic). The SMMs used in the préparation of these membranes are oligomeric fluoropolymers synthesized using polyuréthane chemistry and tailored with fluorinated end-groups. Partîcularly, SMM is selected from the group consisting of poly(urethane propylene glycol) and poly(urea dimethylsiloxane urethane).

[94] The hydrophilic bulk membrane phase is prepared by blending a polymeric material with inorganic nano-particles of high thermal conductivity. This hydrophilic phase should be blended with the SMMs. Suitable hydrophilic polymers are thermoplastic polymers. In particular, they include, but are not limited to, polysulfone, polyethersulfone, polyetherimide and cellulose acetate. Suitable mixed matrix nano-particles include, but are not limited to, boron nitride, cupper oxide, aluminum nitride, aluminum, iron and silicone carbide.

Manufacture of Composite Mixed Matrix Membranes [95] In accordance with an advantageous embodiment of the présent invention, the composite mixed matrix membranes of the présent invention are manufactured using a phase inversion method, in a single casting step, in which a polymer dope solution is prepared that includes predetermined amounts of hydrophilic polymer and hydrophobie SMM dissolved into solvent/non-solvent, mixture. Then the inorganic nano-particles are dispersed into the polymer dope solution to form the polymeric/inorganic nano-particles dope blend. This blend is then cast on a glass plate and allowed to evaporatc at room température; however the solvent évaporation is controlled by covering the cast film with a glass cover. During the controlled solvent évaporation; the hydrophobie SMM migrâtes to the air/polymer interface (i.e. the top layer), since it has lower surface energy. On the other hand, the inorganic nano-particles settle down in the bottom hydrophilic layer rising its thermal conductivity. Alternative methods can be used to préparé the composite mixed matrix membranes, however, SMM surface migration and inorganic nano-particles settling are critical to préparation of the membranes of the présent invention and the phase inversion method is the simplest and cheapest method currently known.

[96] Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the drywet phase inversion technique and the température induced phase séparation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development of the first cellulose acetate membrane for seawater desalination. Therefore, this method is often called the Loeb-Sourirajan method.

[97] According to the Loeb-Sourirajan method, a polymer solution is prepared by mixing polymer, solvent and sometimes even non-solvent. The solution is then cast on a suitable surface by a doctor blade to a predetermined thickness (50-250 μπι). After partial évaporation of the solvent, the cast film is immersed in a bath of non-solvent medium, often called gélation medium. Due to a sequence of two dissolution steps, i.e., évaporation of solvent and solvent/nonsolvent exchange in the gélation bath, solidification of polymer film takes place. It is désirable to choose a solvent of strong dissolving power with high volatility. During the first step of desolvation by solvent évaporation, a thin skin Iayer of solid polymer is formed instantly at the top of the cast film due to the loss of solvent. In the solvent/non-solvent exchange process that follows, non-solvent diffuses into, while solvent diffuses out of, the polymer solution film through the thin solid Iayer.

[98] At some point in the process, the content of solvent in the solution film becomes so low that the solvent no longer is able to hold polymer in one phase. Phase séparation takes place at this point, forming droplets of one liquid phase dispersed in the other continuous liquid phase. The point of phase séparation, and the size and the number of the dispersed droplets dépend on the nature of solvent and non-solvent and the polymer solution composition. The control of the number and the size of the droplets will eventually control the structure of the porous substrate.

[99] The thin Iayer of solid polymer that forms during the first évaporation step becomes the top skin Iayer that will govem the selectivity and the permeate flux of the membrane, while the porous structure that forms during the solvent/non-solvent extraction step becomes the porous sub-layer, providing the mechanical strength, an advantageous characterstic. Hence, the membrane obtained is an integrally skinned asymmetric membrane.

Characteristics of the Mixed Matrix Membranes:

[ 100] A composite mixed matrix membrane of high hydrophilic Iayer thermal conductivity is presented. This novel type of membrane exhibit higher permeate fluxes than those of composite polymcric membranes prepared without dispersed inorganic nano-particles.

Furthermore, the mechanical properties like mechanical strength of the mixed matrix composite membranes were much better than the composite polymeric membranes. They also hâve lower wetting tendency than commercial membranes of single hydrophobie Iayer. —

Application of the Composite Mixed Matrix Membranes;

[ 101 ] The composite mi xed matrix membran es of the présent invention are particularly useful in direct contact membrane distillation (DCMD).

[102] The proposed composite mixed matrix membranes can be used for seawater desalination, wastewater treatment, food processing, concentration of pharmaceutical products, etc.

[103] To gain a better understanding of the invention described herein, the following membrane examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, it should not limit the scope of this invention in any way.

MEMBRANE EXAMPLES: Préparation and Characterization of Composite Mixed Matrix Membranes for Desalination by Direct Contact Membrane Distillation [104] Different composite mixed matrix membranes were prepared using different types of inorganic nano-particles including, cupper oxide, aluminum nitride, boron nitride, aluminum, iron and silicone carbide. Moreover, different hydrophilic polymer types and concentrations were used in preparing the composite mixed matrix membranes. The effect of using the inorganic nano-particles on the membrane morphology and desalination performance of these membranes in MD was clearly identified.

[ 105] The morphology of the prepared composite mixed matrix membranes, mechanical properties and DCMD performance were studied using different characterization techniques and compared to the composite membranes prepared without the dispersion of inorganic nanoparticles. The composite mixed matrix membranes exhibit better mechanical properties and performance for practical application in desalination by DCMD.

Experimental

PCT/CA2012/00004S

Materials [ 106] Ail chemicals used ïn this work and their chemical abstract service (CAS) number are summarized in Table 1. The average molecular weight (M_w) of the used polysulfone (PS) and polyethersulfone (PES) is 79000 g/mol 30800 g/mol, respectively.

Table 1; Materials Used In this Example

Material description	CAS number	Source
4,4-Methylene bis(phenyl isocyanate) (MDI, 98%)	101-68-8	Sigma-Aldrich, Inc., St, Louis, MO, USA
α,ω-Aminopropyl poly(dimethyl siloxane) (PDMS) of average molecular weight 900	106214-84-0	Shin-Etsu Chemical Co. Ltd., Tokyo, Japan
4,4’-Solfonyldiphenol (Dihydroxy diphenyl sulfone, DPS, 98%)	80-09-1	Sigma-Aldrich, Inc., St. Louis, MO, USA
Zonyl BA-L™ (BAL) of average M, 443 and 70 wt% fluorine	678-39-7	DuPont product supplied by Aldrich Chemical Company, Inc., Milwaukee, WI, USA
<V,jV-Dimethylacetamide (DMAc, anhydrous 99.8%)	127-19-5	Sigma-Aldrich, Inc., St. Louis, MO, USA
1 -Methyl-2-pyrrolidinone (NMP, anhydrous 99.5%)	112-14-1	Sigma-Aldrich, Inc., St. Louis, MO, USA
Ethanol (anhydrous, 99+%)	64-17-5	Aldrich Chemical Company, Inc., Milwaukee, WI, USA
Tetrahydrofuran (THF, HPLC grade 99.9%)	109-99-9	Aldrich Chemical Company, Inc., Milwaukee, WI, USA
Polysulfone (PS, UDEL™ 3500) Spécifie gravity: 1.24	25154-01-2	Solvay Advanced Polymer, LLC, Alpharetta, Georgia, USA
Polyethersulfone (PES, Radel A300PNT)	25667-42-9	Amoco Polymer Inc., Alpharetta, Georgia, USA
Aluminum nitride (nanopowder <100 nm particle size)	24304-00-5	Aldrich Chemical Company, Inc., Germany
Boron nitride (powder, -1 pm, 98%)	246-140-8	Aldrich Chemical Company, Inc., Germany
Copper (II) oxide (nanopowder <50 nm particle size)	1317-38-0	Aldrich Chemical Company, Inc., Germany

Material description	CAS number	Source
Silicone carbide (powder, ~1 pm, 98%)
Iron (powder, ~1 pm, 98%)
Aluminum (powder, ~1 pm, 98%)

SMMs synthesis [ 107] The SMMs were synthesized using the two-step solution polymerization method (Qtaishat, Khayet & Matsuura, 2011). In this example, the first polymerization step was conducted in a solution of a predetermined composition to form polyurea by the reaction of MD1 with PDMS or DPS as a pre-polymer, DMAc was used as solvent. In the second polymerization step, the pre-polymer was end-capped by the addition of BAL, resulting in a solution of SMM having the structure shown in Figure 2. The composition of SMM was 2 MDI: 1 PDMS: 2 BAL or 3MDI: 2DPS: 2BAL, The synthesized SMMs are named hereinafter nSMMl and SMM2. SMMs characterization [108] The elemental analysis of fluorine content in nSMMl was carried out using standard method in ASTM D3761. An accurate weight (10-50 mg) of sample was placed into oxygen flask bomb combustion (Oxygen Bomb Calorimeter, Gallenkamp). After pyrohydrolysis, the fluorine (ion) was measured by an ion chromatography (Ion Chromatograph, Dionex DX1000).

[109] The glass transition température (T_g) of synthesized SMMs was measured by differential scanning calorimeter and the weight average molecular weight of the synthesized SMMs was measured by gel perméation chromatography (GPC).

Membrane préparation [l ΙΟ] The composite mixed matrix membranes were prepared in a single casting step by the phase inversion method. Ethanol was used as a non-soivent additive. A predetermined amount of PS or PES was dissolved in a NMP/ethanol mixture, the nSMMl was added to the PS solution to a constant concentration of 1.5 or 2 wt%. The resulting mixtures were stirred in an orbital shaker at room température for at least 48 h, then 5wt% of înorganic nano-particles were added to the polymer solution, and the solution was further stirred for 24 hours. The resulting solution is polymer-inorganic dispersion where the înorganic nano-particles are suspended. The PS and PES concentration in the solution was varied in a range of 12-14 wt%. The éthanol concentration was 10 wt%. The polymer solutions were cast on a smooth glass plate to a thickness of 0.25 mm using a casting rod at room température. The resuited cast films were allowed to evaporate at room température for a predetermined period in order to settle the înorganic nano-particles to the bottom of the films and to allow the SMMs to migrate to the top air/polymer interface. The solvent évaporation volume was controlled by covering the cast film with a cover of 2 mm displacement, which hindered the évaporation of the solvent but allowed the SMM to migrate to the air/polymer interface as well as settling the înorganic nano-particles in the bottom of the cast film. Then, the cast films together with the glass plates were immersed for I h in tap water at room température. During gélation, it was observed that the membranes peeled off from the glass plate spontaneously. Ail the membranes were then dried at ambient conditions for 3 days. Table 2 shows the prepared membranes, their materials of construction and préparation conditions. —

Table 2: Membrane préparation details: casting solution composition and préparation conditions*

Membrane code	Polymer type & concentration	SMM type & Concentration	Solvent évaporation time	Mixed matrix NanoParticles type and concentration
MCI	PS: I2wt%	nSMMl: 1.5wt%	l minute	None
MC2	PS: I2wt%	nSMMl: i.5wt%	I minute	Copper oxide: 5wt%
MC3	PS : I4wt%	nSMMl: l.5wt%	l minute	None
MC4	PS: I4wt%	nSMMl: l.5wt%	l minute	Copper oxide: 5wt%
MC5	PS : 14 wt%	nSMMl: l.5wt%	l minute	Boron nitride: 5wt%
MC6	PS : 14 wt%	nSMMl: l.5wt%	l minute	Aluminum nitride: 5wt%
MC7	PES: 14wt%	nSMMl: l.5wt%	l minute	None
MC8	PES: I4wt%	nSMMl: l.5wt%	l minute	Copper oxide: 5wt%
MC9	PES: I4wt%	nSMMl: l.5wt%	l minute	Boron nitride: 5wt%
MCIO	PES: I4wt%	nSMMl: l.5wt%	l minute	Aluminum nitride: 5wt%
MC1I	PES: 12wt%	SMM2:2wt %	10 minutes	Iron: 4wt%
MCI 2	PES: I2wt%	SMM2: 2wt %	5 minutes	Iron: 4wt%
MC13	PES: I2wt%	SMM2: 2wt %	10 minutes	Iron: 2wt%
MCI4	PES: I2wt%	SMM2: 2wt%	5 minutes	Iron: 2wt%
MCI 5	PES:12wt%	SMM2: 2wt %	10 minutes	Aluminum: 2wt%
MCI 6	PES: I2wt%	SMM2: l.5wt%	10 minutes	Aluminum: 4wt%
MC17	PES: I2wt%	SMM2: 2wt %	10 minutes	Aluminum: lwt%
MCI 8	PES: I2wt%	SMM2: l.5wt%	10 minutes	Silicone carbide; 5wt%
MCI 9	PES: 12wt%	SMM2: l.5wt%	5 minutes	Aluminum: 4wt%
MC20	PES: I2wt%	SMM2: 2wt %	5 minutes	Aluminum: lwt%
MC2l	PES: 12wt%	SMM2: 2wt %	5 minutes	Aluminum: lwt%
MC22	PES: 12wt%	SMM2: 1.5wt%	5 minutes	Silicone carbide: 5wt%
MC23	PES: 12wt%	SMM2: 2wt %	0 minutes	None
MC24	PES: I2wl%	SMM2: 2wt %	5 minutes	None

Ethanol (non-solvent additive) concentration, 10 Wt%; gélation bath (tap water) température: 20 °C,

Membrane Characterization

1. Scanning Electron Microscopy (SEM) [111] The cross-section of the SMMs blended PEI membranes was analyzed by scanning électron microscopy, SEM, (JSM-6400 JEOL, Japan). The membranes were eut into pièces (3 mm width and 10 mm length) and subsequently immersed in liquid nitrogen réservoir for 5 s. While keeping the pièces in the liquid nitrogen, those were broken into two pièces by pulling from both ends. One of the broken pièces was mounted on métal plate with carbon paste and gold-coated prior to use. The cross-sectîon of the membranes at the broken parts was finally examined by SEM.

2. X-ray Photoelectron Spectroscopy (XPS) [112] The elemental composition at the surface of each SMM blended membrane was determined by X-ray photoelectron spectroscopy (XPS, Kratos Axis HS X-ray photoelectron spectrometer, Manchester, UK). Each membrane was eut into samples of 1 cm² from random positions of the membrane. Monochromatized Al K_o X-radiation was used for excitation and a 180° hemispherical analyzer with a three channel detector was employed. The X-ray gun was operated at 15 kV and 20 mA. The pressure in the analyzer chamber was lJSxlO⁴ to 1.33xl0'^s Pa. The size ofthe analyzed area was about 1 mm². Ail the membrane samples were analyzed for fluorine content at both top and bottom sides.

3. Mechanical tests:

[113] Tensiie testing was performed at room température on an Instrom dynamometer model 4301, according to ASTM D638M (standards). Tests were carried out with a crosshead speed of 50 ml/min at break. At least three measurements were performed for each membrane sample and the average values are reported in this study. The mechanical properties ofthe — membranes are given in terms of Young’s modulus, maximum strength and the percent élongation at break.

Direct contact membrane distillation experiments [ 114] The prepared composite mixed matrix were tested by the direct contact membrane distillation (DCMD) setup shown tn Fig 3 and detailed elsewhere (Khayet & Matsuura, 2011).

Results and Discussion

Mixed matrix composite membranes characterization [H5] The cross-section SEM images of the mixed matrix membranes are shown in

Figure 4. As can be seen, ali the membranes are of asymmetric structure with a denser structure at the top surface, whereas the structure of the bottom surface varies depending on the existence of the mixed matrix nano-partiel es and its type. In the membrane where there is no mixed matrix nano-particles (MC3 in Fig 4), horizontal micro-voids were formed interrupted by sponge-Iike layers in between. However, when adding mixed matrix nano-particles to the composite membrane polymer solution, macro-voids were vertical reaching the bottom of the membrane (MC5 in Fig. 4 is a noted exception). For instance, in the case of copper oxide (MC4 and MC8 în Fig 4) small macro-voids grown in vertical direction are separated by vertical sponge-like polymer layers, similar trcnd was obtained in MC 16 and MC22. When boron nitride was used as dispersed nano-particie (MC5 and MC9 in Fig 4), it was notîced that the finger like structure became irregular in the middle section and large macro-voids were formed at the bottom, similar trend was obtained in MC 14 and MC21. Finally, in the case of aluminum nitride (MC6 and MC 10 in Fig. 4), it was noticed that there are polymer nuclei formed in different positions at the hydrophilic layer. a/ [116] Figure 5 shows the SEM images of the top and bottom surfaces of the mixed matrix membranes, in which MC 17 was taken as an example. The images showed that the top surface pore size was an order of magnitudes lower than that of the bottom surface. Quantitatively, the range of pore size in the top surface was around 20 nm, on the other hand it was around 1.6 pm.

[117] The mechanical properties of the composite mixed matrix membranes are summarized in Table 3 and examples of deformation behaviors are shown in Fig. 6. When using the same polymer PS with the same concentration in the dope (MC3, MC4, MC5, MC6), the Young’s modulus, maximum strength and the percent élongation at break ofthe composite mixed matrix membranes (MC4, MC5, MC6) are higher than those of the non-mixed matrix membrane MC3 (See Fig. 6a). Similarly, when using 12 wt% of PS in the dope, the mechanical properties improved by the dispersion of the mixed matrix nano-particles in the dope. The composite mixed matrix membrane MC2 exhibits better mechanical parameters than the nonmixed matrix membrane MCI (see Table 3 and Fig. 6b). The same behavior was observed for the polymer PES when comparing the membrane MC7 and MC8 (see Table 3 and Fig. 6c). However, the mechanical properties of the PS membranes are better than those of the PES membranes.

Table 3: Mechanical properties οΓ the composite mixed matrix membranes.

Membrane code	Young's Modulus (MPa)	Maximum strength (MPa)	Deformation at break (%)
MCI	101.6 ± 17.9	3.0 ±0.3	5.9 ±2.0
MC2	126.0 ±26.1	3.1 ±0.8	7.4 ± 3.2
MC3	122.1 ± 16.0	2.9 ± 0.5	6.1 ±3.0
MC4	177.7 ±26.0	3.7 ±0.5	7.4 ± 7.5
MC5	159.3 ± 14.8	3.8 ±0.5	10.3 ±4.4
MC6	171.2 ±6.6	4.3 ± 0.3	8.9 ±3.2

Membrane code	Young's Modulus (MPa)	Maximum strength (MPa)	Deformation at break (%)
MC7	67.7 ±16.2	2.8 ± 0.6	5.9 ±3.6
MC8	139.9 ±23.3	3.1 ±0.8	6.1 ±2.7

Membrane performance [l 18] Figures 7-11 show the DCMD fluxes of the prepared mixed matrix composite membranes. Figures 7a, 8a, and 9a show the DCMD flux versus the average température of feed and permeate solutions (Γ„) when distilled water was used as feed, while Figure 7b, 8b and 9b show the DCMD flux of the same membranes when using 0.5M NaCl aqueous solution as feed. However, in Figure 11 and 12 the effect of the mean température on the DCMD flux of distilled water and 0.5M NaCl solution is shown as well as on the séparation factor.

[119] As shown in the figures; ail the membranes exhibit an exponential increase ofthe DCMD flux with an increase in T_m. This is attributed to the exponential increase of the vapor pressure with the increase of température according to Antoine équation.

[120] When considering the addition of the inorganic nano-particles; it was noticed that the DCMD permeate flux increased dramatîcally. For example, under the same DCMD operating conditions, for the 12 wt% PS membranes permeate flux increased by 50% when copper oxide was added (See Fig 7a). In Fig. 8a, similarly, the permeate flux increased for the 14wt% PS membranes by 200% when the copper oxide and boron nitride were added as inorganic nanoparticles. This trend was further verified when PES was used as the base hydrophilic polymer. Figure 9a shows that the permeate flux of 14 wt% PES membranes increased by 120% when copper oxide was added.

[121] The most significant results are those shown in Figures 10 and 11 in which iron, aluminum and silicone carbide was used as nano-particles, since the reported flux data were — order of magnitudes higher than that shown in Figures 7-9. However, it is not believed that the nano-particle type was the responsible for this trend. It was the manufacturing technique différences, since MCI 1, 12,16 and 18 were prepared using the cover as detailed earlier in the membrane préparation section, [122] In conclusion, ail the prepared mixed matrix composite membranes exhibited higher permeate flux than the polymeric composite membranes. Furthermore, ail tested membranes exhibit sait (NaCl) rejcction factors higher than 99.9%.

Conclusions [123] This exemple provides a proof for the claim that increasing the hydrophilic layer thermal conductivity leads to a dramatic increase ofthe DCMD permeate flux ofthe composite hydrophobic/hydrophilic membrane. This is attributed to the thermal conductivity of the hydrophilic layer that is increased by dispersing înorganic nano-particles into the polymer dope used to cast the mixed matrix hydrophobic/hydrophilic composite membrane.

[124] The mixed matrix composite membranes exhibit better mechanical properties than the composite hydrophobic/hydrophilic membranes prepared without nano-particles.

[125] The prepared mixed matrix composite membranes are destined to play a key rôle in the future development and commercialization of the membrane distillation (MD) process.

[126] Other modifications and variations to the invention will be apparent to those ski lied in the art from the foregoing disclosure and teachings. Thus, while only certain embodiments of the invention hâve been specifically described herein, it will be apparent that numerous modifications may be made thereto without departîng from the spirit and scope of the invention.

JUIL 2013

Claims (22)

1. A membrane distillation system comprising:

a flat-sheet composite mixed matrix hydrophilic/hydrophobic membrane having at least a a hydrophilic layer and a hydrophobie layer;

« the hydrophilic layer further comprising a hydrophilic polymer and inorganic nanoparticles of high thermal conductivity, and the hydrophobie layer further comprising fluorinated surface-modifying macromolecule (SMM).

2. The membrane distillation System as claimed in claim 1, wherein said hydrophilic polymer is a thennoplasitc polymer.

3. The membrane distillation system as claimed in claim 2, wherein the thermoplastic polymer is selected from the group consisting of polysulfone, polyethersulfone, polyetherimide and cellulose acetate.

4. The membrane distillation system as claimed in claim 1, wherein said nanoparticle is an inorganic nano-particle of high thermal conductivity.

5. The membrane distillation system as claimed in claim 1, wherein said inorganic nanoparticles is selected from the group consisting of cupper oxide, boron nitride, aluminum nitride, aluminum, iron and silicone carbide.

6. The membrane distillation system as claimed in claim l, wherein said fluorinated surface-modifying macromolecules (SMM) are oligomeric fluoropolymers synthesized using polyuréthane chemistry, the surface-modifying macromolecules comprise fluorinated endgroups.

7. The membrane distillation system as claimed in claim l, wherein said fluorinated SMM is blended within said hydrophilic Iayer.

8. The membrane distillation system as claimed in claim 7, wherein said SMM is selected from the group consisting of poly(urethane propylene glycol) and poly(urea dimethylsiloxane urethane).

9. The membrane distillation system as claimed in claim l, wherein said mixed matrix membrane has a morphological structure conceived to maximize permeate flux and permeability.

10. The composite mixed matrix membrane as claimed in claim 9, wherein said morphological structure comprise parameters representing a thickness and a porosity of said hydrophobie Iayer, and a thickness, a porosity and a thermal conductivity of said hydrophilic Iayer, and internai structure of flat-sheet membranes. φ/

11. The membrane distillation system as claimed in claim 1, wherein said composite mixed matrix membrane has higher vapor permeate flux than composite polymeric membranes.

12. The membrane distillation system as claimed in claim 1, wherein said composite mixed matrix membrane has a higher mechanical strength than composite polymeric membranes.

13. The membrane distillation system as claimed in claim l, where the composited mixed matrix membrane has lower wetting tendency than commercial membranes of single hydrophobie layer.

14. A phase inversion method for manufacturing a membrane distillation composite mixed matrix hydrophilic/hydrophobic membrane, said method comprising:

(a) dispersing a host hydrophilic polymer with a predetermined amount of insoluble inorganic nano-particles and a non-solvent additive in a solvent to form a polymer-inorganic solution;

(b) adding a fluorinated surface modifying macromolecule (SMM) to the polymerinorganic solution to form a polymer-inorganic nano-particles SMM blend;

(c) casting said polymer-inorganic nano-particles blend and allowing said solvent to evaporate at room température for a predetermined time to form a cast film;

(d) varying a time of évaporation systematically to study and modify an effect of the évaporation time on settling of the inorganic nano-particles in a bottom layer as well as the hydrophobie SMM migration to the top layer;

(e) covering the cast film by a cover having a certain displacement to control évaporation of the solvent allowing more time for settling of the inorganic nano-particle in the bottom layer and migration of a hydrophobie SMM to the air/polymer interface; and (f) immersing said cast film produced in step (c) in water to allow gélation.

15. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, further comprising maximizing porosity and minimizing thickness of said hydrophobie polymer layer of said composite membrane in order to increase permeate flux of said composite membrane.

16. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, further comprising maximizing thickness and thermal conductivity of said hydrophilic layer.

17. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, further comprising maximizing the température polarization coefficient in the hydrophilic sublayer, according to the following équations:

and where 8p 6_S and θ_ρ are the température polarization coefficients corresponding to the feed, hydrophilic sublayer and permeate phases, respectively; and are defined as follows.

Q _=l

A, T_bJ-T^_p e _=[Φ.Φ ^{P h} _P T_bJ-T_b'_P

18. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, wherein said host hydrophilic layer comprises at least one of polysulfone, polyethersulfone, polytherimide, cellulose acetate and other polymers known as thermoplastics as a hydrophilic polymer and at least one of cupper oxide, boron nitride, aluminum nitride, aluminum, iron, silicone carbide and other înorganic or organic nano-particles of high thermal conductivity.

19. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, wherein said hydrophobie top-layer is from either polymeric nature such as SMM or Înorganic nature such as zeolites.

20. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, wherein said non-solvent additive is selected from the group consisting ofy- butyro lactone, éthanol, lithium chloride and others known in the membrane literature as non solvent additive.

21. The method of manufacturing a composite mixed matrix membrane as claimed in claim 14, wherein said solvent is selected from the group consisting of N,N-dimethylacetamide, l-methyl-2-pyrrolidone and others known as solvents or mixtures of solvents for the polymers defined in 19 as hydrophilic polymers.

22. The method of manufacturing a composite mixed matrix membrane as claimed in claim I4, exhibited higher permeate flux because of the higher thermal conductivity of the sublayer and the decrease in the résistance of the water vapor migration in the hydrophobie layer.

JUll 2013