Alginate-coated microporous PTFE membranes for use in the osmotic distillation of oily feeds

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Abstract

Osmotic distillation (OD) has two main advantages over thermally driven concentration processes attributable to its ambient temperature operation. These are maintenance of the integrity of thermally labile components and minimisation of the loss of volatile flavour/fragrance components. However, a major disadvantage of osmotic distillation is the potential for wet-out of the hydrophobic membrane when fouled by surface-active agents such as citrus oils. In this work, sodium alginate hydrogel coatings were applied to PTFE membranes for protection against wet-out. The coating technique developed for this purpose resulted in a 10-fold increase in adhesion strength over that achievable by simple casting. This was effected by increased intrusion of the coating solution meniscus into the porous PTFE structure by surface tension adjustment with ethanol, precipitation of sodium alginate by the selective removal of water, and finally alginate crosslinking. Precipitation occurred both on the surface and in the void spaces between the PTFE fibres, thereby providing better anchorage for the coating. The coating decreased the overall OD mass transfer coefficient by less than 5%. OD flux measurements using coated membranes with 0.2, 0.4 and 0.8 wt.% orange oil–water mixtures over a period of 300 min indicated that the coating was successful in protecting the membrane against wet-out. An uncoated membrane was immediately wet out by a 0.2 wt.% orange oil–water OD feed. In a separate trial, a coated membrane retained its integrity after contact with a 1.2 wt.% oil–water mixture for 72 h.

Introduction

Osmotic distillation (OD) is unique among membrane separation processes in that it facilitates the dewatering of aqueous feeds to high levels of concentration under ambient temperature operation. Fruit juices including grape, orange, tomato and passion fruit juices have attracted considerable attention as OD feeds because of their propensity for discolouration, production of off-flavours, and loss of volatile organic flavour/fragrance components when subjected to thermally driven concentration processes [1], [2], [3], [4], [5], [6], [7], [8]. OD utilises a microporous hydrophobic membrane (pore diameter 0.01–1.0 μm) to separate the feed stream from the strip stream. The latter stream consists of an aqueous solution of a salt with high osmotic activity. The lower water vapour pressure of the strip stream relative to that of the feed stream provides a vapour pressure gradient as the driving force for water removal. Water evaporates at the feed-membrane interface, diffuses through the air-filled membrane pores, and then condenses at the membrane-strip interface [9].

Feed and strip pumping pressures in industrial plants are typically less than 150 kPa. The feed pressure is normally maintained at 20–40 kPa above that of the strip stream as a precaution against contamination of the feed by the strip solution in the event of membrane leakage, or wet-out. In most cases this pressure gradient is well below the critical liquid entry pressure of the membrane, ΔP. However, membrane wet-out can occur during the OD of feeds containing surface-active components [10]. OD membranes are particularly sensitive to wet-out by oily feeds such as orange juice [11].

ΔP depends on the liquid surface tension, γl, the solid (membrane)–liquid contact angle, θ (greater than 90° for hydrophobic membranes), the maximum value of the membrane pore radius distribution, rmax, and the pore geometry factor, B (unity for cylindrical shape), in accordance with the Laplace equation [12] (Eq. (1)).ΔP=−2Bγ1cosθrmax

The contact angle is related to the membrane surface tension, γs, and the membrane–liquid interfacial tension, γsl, in accordance with the Young equation [12] (Eq. (2)).γ1cosθ=γs−γsl

For most feed solutions, wet-out results from the deposition of amphiphilic proteins, lipids or fats on the membrane surface. The increased affinity between the liquid and membrane afforded by these surface-active agents reduces γsl, which in turn reduces θ (Eq. (2)) and ΔP (Eq. (1)). Liquid enters the membrane pores when ΔP falls below the operating pressure gradient. In practice, wet-out by proteins, lipids and fats can usually be avoided by the use of a regular membrane-cleaning regime. However, orange juice contains oil in various concentration levels, depending on the variety and method of juice extraction. The major oil component, the monoterpene limonene (74–97 wt.% [13]), has a high affinity for hydrophobic surfaces and promptly causes membrane wet-out, even in low concentrations [11]. Indeed, limonene is extracted separately from the rind and used widely as a cleaning agent [14].

Work undertaken in our laboratory on the preparation of crosslinked sodium alginate films has been reported previously [15]. Films prepared using optimum crosslinking conditions with respect to durability against water had an equilibrium water content of approximately 60 wt.%. The good durability and hydrophilicity of these films provided the potential for use as feed-side coatings for OD membranes for protection against wet-out by surface-active agents. The potential for solute rejection by these coatings was supported by the high separation factors obtained when non-crosslinked sodium alginate membranes were used for the pervaporation dehydration of ethanol–water mixtures [16], [17], [18].

The present work was undertaken in order to fabricate and test composite OD membranes consisting of a PTFE substrate membrane and a crosslinked sodium alginate coating on one side. PTFE membranes were chosen because of their inertness and durability. The initial focus of this work was on the development of a physical technique that would improve the inherently weak adhesion of the hydrophilic coating to the hydrophobic membrane. This is in contrast with previous work on the coating of polypropylene membranes with poly(vinylalcohol) crosslinked with glutaraldehyde in which adhesive forces were improved by etching of the membrane surface with chromic acid [11]. Other reports on the surface modification of PTFE membranes have involved reactive processes including plasma-graft polymerization [19], sodium etching [20] and gamma irradiation grafting [21]. The development of a physical coating technique as a potential replacement for these harsh chemical techniques was undertaken because of the potential for adaptation for in situ modification of membranes in existing commercially available modules. A physical coating technique would also have the advantage that membranes may be recoated as required.

The present coating technique was designed to facilitate improved adhesion between the coating and substrate membrane by physically interlocking their respective structures in the region of the pore entrances. This involved an initial reduction in the surface tension of the sodium alginate coating solution by ethanol addition. The resulting decreases in γsl and θ was expected to increase the extent of meniscus penetration and increase the degree of contact between the solution and the fibres forming the pore perimeter. Subsequent precipitation of the sodium alginate in the PTFE pores and on the surface by the selective removal of water using OD was expected to result in a structure that was more effectively anchored to the substrate than in the absence of ethanol. The structure of PTFE membranes as a system of thick fibres interconnected by fine fibrils and with no true pore walls was considered to be suited to this process.

Coated membranes were examined by scanning electron microscopy (SEM) in order to determine the effectiveness of the technique in promoting intrusion of the coating into the porous substrate. T-peel strength tests were used to determine the improvement in the adhesion strength between the coating and membrane. The membranes were tested for their OD flux performance and resistance to wet-out using orange oil–water mixtures with oil concentrations of 0.2, 0.4 and 0.8 wt.%. These concentrations are typical of industrial feeds [11]. A 1.2 wt.% oil mixture was used to test the durability of a coated membrane over a 72 h period.

Section snippets

Materials

Sodium alginate (Manugel GMB) was donated by Germantown International Limited (Sydney, Australia). The crosslinking agent, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, or water-soluble carbodiimide (WSC) was purchased from Sigma–Aldrich (Sydney, Australia). Ethanol (spectroscopic grade) was purchased from Aldrich Chemical Co. (Sydney, Australia). Calcium chloride dihydrate (AR grade) was purchased from Chem Supply (Brisbane, Australia). Hydrochloric acid (32 wt.%, AR grade) was purchased from

Membrane coating

SEM images of the surfaces of the uncoated PTFE membrane and partially coated membranes prepared using sodium alginate solutions containing 0, 10 and 20 wt.% ethanol without crosslinking are shown in Fig. 3. The image of the uncoated membrane showed the PTFE structure to consist of a system of thick, parallel fibres interconnected by fine fibrils. Coating in the absence of ethanol resulted in the deposition of smooth sections of coating on the highly porous substrate with no evidence of pore

Conclusions

This work has shown that a 10-fold increase in adhesion strength between a hydrophobic PTFE membrane and a protective hydrophilic sodium alginate coating can be achieved by surface tension adjustment of the coating solution. Enhanced penetration of the solution into the porous PTFE structure provided better anchorage of the precipitated coating material. The reduction in the overall mass transfer coefficient due to the coating was less than 5%. OD flux trials using feeds containing 0.2, 0.4 and

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