(118a) Scale-Down of a Condensation Polymerization Process in a Millifluidic Device

We have developed a flexible millifluidic device to acquire thermodynamic and kinetic data on a condensation polymerization reaction. Because studied reaction is an equilibrium limited reaction, water (reaction by-product) must be eliminated in order to shift the chemical equilibrium to the products side and obtain a high molecular weight polymer. A two step millifluidic process has been developed: a gas-liquid segmented flow allows water to be stripped from the liquid phase to the gas phase; then a gas selective membrane enables the gas-liquid separation. Both steps are later repeated.

Condensation polymerizations are step growth polymerization reactions that produce a low molecular weight molecule as a by-product. Studied reaction produces water as by-product and is equilibrium limited. To achieve high molecular weight, high extent and therefore extremely small water quantities are needed. The aim of this project is to develop a millifluidic device to acquire thermodynamic and kinetic data on a condensation polymerization reaction, the polyesterification of adipic acid with ethylene glycol.

Millifluidics (process miniaturization to the millimetre scale) offers several advantages to the development of a flexible research device: laminar flow and thus a narrow residence time distribution (RTD), great surface/volume ratio, excellent heat transfer, low reagents consumption and a small investment. Developed process consists of a series of two unit operations ? stripping of formed water followed by gas-liquid separation ? repeated several times. The millifluidic set-up consists of an assembly of stainless steel capillaries (inner diameter 0.75 mm) and gas injection and separation stages. The whole is placed inside an oven. Liquid is injected with a pressurized heated pump (by TopIndustrie) containing the preheated reactants and a mass flowmeter (by Bronkhorst), allows the gas flow rate control. A sampling system consisting of two electric actuated valves (by Vici Valco) is also implemented in the set-up.

Water stripping is achieved thanks to a gas-liquid segmented flow. This kind of flow can be obtained by choosing the right couple of gas and liquid flow rates (1). Gas is injected with a controlled flow rate through a small injector inside the liquid flow and monodisperse gas bubbles are easily obtained. Gas volume fraction can be adjusted. It has been shown that segmented flow induces a better mixing within the liquid phase as well as a narrower RTD (1). Injected gas is an inert gas such as argon or nitrogen. In the presence of the dry gas a fraction of the water evaporates from the liquid to the gas phase until the thermodynamic equilibrium is reached. The gas phase is the separated from the liquid flow thanks to a membrane process described below.

The gas fraction of the flow ? evaporated water and injected inert gas ? is separated from the liquid fraction using a gas selective membrane. The principle of using a water selective membrane within a microreactor has been used in condensation reactions to shift the reaction equilibrium (2). The authors succeed in selectively removing water (reaction by-product) increasing reaction conversion. They have also shown that the great ratio surface/volume of small scale reactors improves the membrane separation performance. The separator consists of a stainless steel module with engraved channels where the segmented flow is injected, covered by a supported selective membrane through which gas passes selectively. Developed supported membrane consists of a 1 mm thick porous support (by Mott Corp, nominal pore size 0.1 µm) coated with a thin film of a selective layer of Teflon®AF 2400 (by DuPont). Teflon®AF 2400 is an amorphous fluoropolymer highly permeable to low molecular weight gases and particularly to water vapour (3). It is not said that Teflon®AF 2400 is more permeable to water vapour than to liquid water (4), but besides improving mixing in the liquid phase, introducing the gas phase allows to extract the water from the liquid phase before the membrane separation. Thus, we are not limited by water diffusion in the liquid phase towards the membrane, reducing needed membrane surface. Membranes are prepared by slowly evaporating a 1%w/w polymer solution in Fluorinert® Electronic Liquid FC-40 (by 3M) on a flat surface. This gives out thin films (~20 µm). In order to get a good adhesion between the film and the substrate, porous metal surface is soaked with FC40, the membrane is put over it and the whole is let to dry at 60ºC for 24 h. Membranes are later characterized by 2 different ways: measurement of volumetric flow rate at constant pressure or monitoring of pressure drop over time. These methods were found to be equivalent. The determined permeation rate allowed us to design the stainless steel separator.

Samples are taken at different residence times (by changing fluids flow rates) with the help of two electric actuated valves. They are later analysed by gel permeation chromatography (GPC) and by titration of the acid function. Performance of the stripping/permeation process is evaluated by comparing the results obtained in a simple continuous millireactor.

Acquired data will be helpful to identify parameters of a rate-based model accounting for kinetics, interface transfer and permeation. This could thus improve our knowledge of involved phenomena and allow the optimisation of the process.

(1) Gunther, A.; Khan, S. A.; Thalmann, M.; Trachsel, F.; Jensen, K. F. Transport and reaction in microscale segmented gas-liquid flow. Lab Chip 2004, 4, 278

(2) Lai, S. M.; Ng, C. P.; Martin-Aranda, R.; Yeung, K. L. Knoevenagel condensation reaction in zeolite membrane microreactor. Microporous and Mesoporous Materials 2003, 66, 239

(3) Brandrup, J.; Immergu, E. H.; Grulke, E. A.; Polymer Handbook, 4th Edition, John Willey & Suns, Inc, 1999, Volume 1

(4) Uchytil, P.; Petrickovic, R. Vapour permeation and pervaporation of propan-1-ol and propan-2-ol in polyethylene membrane, Journal of membrane Science 2002, 209, 67