Synthesis and characterization of a PVA/LiCl blend membrane for air dehumidification

Abstract

A novel vapor-permeable composite membrane for air dehumidification has been fabricated. The membrane used porous polyethersulfone (PES) as the support layer, and dense polyvinylalcohol (PVA) as the active separating layer. To facilitate moisture permeation, lithium chloride (LiCl) salts were used as an additive to the PVA solution before it was cast onto the support layer. The effects of the LiCl concentration on the water vapor permeability are investigated. The permeation tests indicate that the addition of LiCl in PVA cast solution can improve vapor permeability substantially. The observations of the microstructures and the characterization of the synthesized material disclosed that the reason behind increased moisture rates is that the addition of LiCl increased the hydrophilicity.

Introduction

It is often necessary to control and modify the water vapor content in the air. The water vapor content of atmospheric air is small: some tens of grams per kilo of air. Nonetheless, due to the very high heat of vaporization, the latent heat load in air conditioning is as high as the sensible load. In hot and humid regions like South China, it is quite usual that ambient air humidity stays above 80–90% continuously for a dozen of days, leading to mildew growth on surfaces of building surroundings and furniture. In such cases, air dehumidification becomes the number-one issue in air conditioning [1], [2].

The most widely practiced method of removing water vapor from a conditioned space involves condensing the moisture by cooling the atmosphere below the dew point. This method is mature and widely used. However, it requires a large amount of energy to operate the cooling/refrigeration system to cool the gas, condense the moisture and remove the latent heat. Other methods include the use of hygroscopic agents/salts to remove moisture from the atmosphere. The technique has been most often associated with the removal of water vapor at lower humidities to achieve a relatively dry state. The salts, of course, must be regenerated or discarded after absorbing quantities of water. Desiccant wheels, while increasingly being used, have coherent problem of cross-over between the dehumidified air and regenerating air.

On the other side, with the developments in membrane technology, polymer membranes have been used in air dehumidification. Hydrophilic polymer membranes that are permeable to vapor, but impermeable to air, have been considered. Various materials have been tested: for instance Nafion [3], [4], regenerated cellulose [5], cellulose triacetate [6], sulfonated poly(phenylene oxide) [7], polyether-polyurethane [8] siloxane-amide copolymer [9], polystyrene-sulfonate [10], polyvinylidene fluoride and polyethersulfone [11], and cellophane [12].

Vapor diffusion in dense layers is rather small. To increase vapor permeation rates, composite membranes have been used. According to this scheme, a thin active layer is cast onto a thick porous support layer. The porous support layer provided the necessary mechanical strength while the thin active layer provided the permselective separating effect. The permeation rates can be greatly improved due to the reduction in resistance [13].

It is accepted that vapor permeations through dense membranes are based on solution-diffusion mechanism. The more hydrophilic the material is, the more moisture it can adsorb, and more moisture can permeate through membrane. According to this theory, materials that have large quantities of hydrophilic groups such as SO₃H, NH₂, COOH, OH are required to have a strong hydrophilicity.

Polyvinyl alcohol (PVA) is a material that has large quantities of OH groups. It has ideal vapor selectivity since other unwanted gases like CO₂ are hard to be dissolved in and permeated through it. Besides, it has good membrane-forming properties, good chemical and thermal stability. Furthermore, most importantly it is cheap, which is the prerequisite condition for commercial applications. These characteristics indicate that it is a good candidate for moisture permeation. PVA membranes have been tested in pervaporation applications [14], [15]. They were also used as the base film in facilitated gas transportation [16], [17]. PVA is water soluble. To make them into membranes which are insoluble to liquid water and with good tractility, some degree of cross-linking is required during membrane fabrication. However, unfortunately, during this cross-linking, its hydrophilicity is sacrificed since some OH groups are cross-linked and reduced. This will then affect its vapor permeability properties. On the other hand, LiCl salts are very hydrophilic since they can build hydrogen bonds with water molecules. When dispersed in the PVA membrane, they can off-set the effects of PVA cross-linking and make the membrane very hydrophilic and thus facilitate the transport of water vapor.

Upadhyay and Bhat [18] have investigated the effect of addition of alkali salt on separation of water–isopropyl alcohol mixture. They proved that LiCl modified PVA membranes have improved water flux. In their research, the surface of a single-layer PVA membrane is cross-linked by exposure to low-pressure nitrogen plasma. This research uses an easier and cheaper way of cross-linking: l-malic acid is used as the cross-linker and glacial acetic acid is used as the catalyst. PVA is cross-linked during the membrane fabrication. LiCl salts are added in the PVA solution before it is cast onto the support layer. Furthermore, to make the active layer thinner, a composite membrane approach is used.