Ind. Eng. Chem. Res. 2010, 49, 1041–1049
1041
CO2 Absorption in a Microstructured Mesh Reactor Achilleas Constantinou and Asterios Gavriilidis* Department of Chemical Engineering, UniVersity College London, Torrington Place, London, WC1 7JE, U.K.
Carbon dioxide absorption in sodium hydroxide solution was studied in a metal mesh microstructured reactor. The reactor comprised of a microstructured metal mesh placed between two acrylic plates. Channels were machined in the plates with 0.85 mm and 0.2 mm depth forming the areas where gas and liquid flowed, respectively. The reactor was 192 mm × 97 mm (length × width). Experimental data were obtained for 2 M NaOH and 20 vol % CO2 inlet concentrations, for various liquid and gas flow rates, while keeping the molar flow rate ratio CO2/NaOH at 0.6. Results showed that in less than 1.2 s gas residence time approximately 30% of the carbon dioxide was removed. A two-dimensional model of the reactor where the solid area of the mesh was neglected and its percentage open area was used to modify the effective length of the reactor (segregated model) was formulated. This model’s predictions gave better agreement with the experimental results compared to a pseudohomogeneous model where the diffusivities in the mesh were approximated with effective diffusivities based on mesh percentage open area. The model indicated that carbon dioxide was consumed within a short distance from the gas-liquid interface and the main mass transfer resistance was located in the mesh. Increasing the open area of the mesh increases CO2 removal as observed both theoretically and experimentally. 1. Introduction Membrane contactors are devices that allow two phases to come into direct contact with each other, for the purpose of mass transfer between them, without dispersion of one phase into the other. The concept of using membranes to bring two phases into contact covers many industrial processes such as extraction, pervaporation, stripping, and absorption. Their use is seen as part of process intensification trends boosting efficiency, saving energy, minimizing environmental impact, and increasing safety.1,2 Microfabricated meshes are the microengineered analogue of membranes. Recent developments in the area of microengineered structures for chemical processing3 have made it possible to manufacture micromeshes from various materials by techniques such as standard mask lithography or laser interference lithography.4 Thin meshes with straight pores, micrometerrange pore size, and a regular arrangement can be obtained.5 Such micromeshes combine the advantages of minimizing mass transfer resistance with high porosity and regular patterned pore structured having at the same time good mechanical strength. They can be easily incorporated in the design of microdevices for processing at microscale.6 Various investigators have studied CO2 absorption in membrane contactors. Qi and Cussler7,8 were the first to use microporous polypropylene membranes for CO2 absorption in aqueous solutions of NaOH and amines. They observed that the main resistance to mass transfer existed in the liquid phase, with membrane resistance being very small. For cases where membrane resistance is relatively small, hollow fiber membrane modules offer distinct advantages over conventional columns, due to lower HTUs (height of transfer unit) and independence of gas and liquid flows. Karoor and Sirkar9 conducted comprehensive studies for absorption of CO2, SO2, CO2-N2, and CO2-air mixtures using distilled water in microporous hydrophobic hollow fiber devices. For CO2 absorption in water, the wetted mode of operation increased * To whom correspondence should be addressed. E-mail: a.gavriilidis@ ucl.ac.uk.
the mass transfer resistance when compared with the nonwetted mode. The experimentally obtained KLa values for the membrane contactors were considerably larger than those for packed towers. Kreulen et al.10,11 investigated the effects of various factors such as porosity, hollow fiber dimension, liquid viscosity, chemical reaction on the mass transfer in a hollow fiber contactor using CO2-water/glycerol and CO2-NaOH aqueous solutions. Comparison between a membrane module and a bubble column, in terms of KLa demonstrated improved performance for the former only for viscous liquids. In this case membranes keep constant interfacial area and avoid stagnant bubble and foam formation. Furthermore, modeling of gas absorption with and without chemical reaction was performed to simulate the mass transfer occurring in the contactor and agreed well with experimental results. Rangwala12 observed that the overall mass transfer rates in a hollow fiber membrane contactor can be up to 9 times higher than those found in conventional packed columns. The membrane transfer coefficient for all the aqueous solutions studied (NaOH 2 M, DEA 0.5 M), were lower than those theoretically calculated for a completely nonwetted pore indicating that the pores were partially wetted. In addition they showed that even marginal (
