Analytical Model of Shell-and-Tube Dehydrator Using Tubular Zeolite Membrane

Isopropanol (IPA) is an important chemical used as a solvent for dye paint and cleaning agent for precision machinery and semiconductor electronics.  Dehydration of IPA has been usually carried out with multi-step distillation process because IPA-water mixture forms azeotrope at IPA concentration of 87.4 wt%, thereby requiring a large amount of energy.  Energy saving will be expected by the IPA dehydration with vapor permeation (VP) when direct dehydration of IPA-water azeotrope vapor discharging from top of a distillation tower is carried out.  Recently, a hybrid process of distillation and zeolite membrane separation has been attracting a lot of attention in terms of lower energy consumption compared with the conventional process and higher separation performance.  Concentration polarization, well known as a phenomenon reducing separation performance significantly, should be eliminated by introducing a membrane separation process into the conventional process.  In this study, an effect of concentration polarization on permeation flux in laminar flow condition is analyzed by comparing numeric calculation and experimental results. 

A porous α-alumina tube with outer diameter of 2.5 mm and average pore size of 0.15µm was selected as support, on which polycrystalline zeolite layer was synthesized by the secondary growth method.  The zeolite membrane was observed with a scanning electron microscope and identified with an X-ray diffraction.  VP test was carried out using the tubular membrane at IPA concentration of 80 wt% in laminar flow condition to determine the separation performance of membrane such as permeation flux and separation factor.  The composition of permeate was analyzed with a gas chromatograph.

In order to evaluate the dehydration performance, mathematical modeling and its numeric calculation were carried out for the shell-and-tube dehydrator.  In addition, plug flow model as ideal flow was developed and compared with experimental results.

As the result of VP tests with the synthesized membrane, it was found that separation factor and permeation flux of water was reduced as feed rate of mixture decreased, while the permeation flux of IPA increased. It made clear that permeation flux of water decreased because water concentration was significantly reduced toward the membrane surface by selective water permeation through the membrane.