Ward E. TeGrotenhuis
Micro chemical separations systems offer a flexible platform in which to perform waste treatment and chemical processing. As envisioned, these systems will consist of separations hardware that will have a much higher throughput per unit volume of space than conventional chemical separations equipment. In addition, the reduced equipment size and enhanced mass transport can improve energy efficiency and reduce capital and operating costs over conventional chemical processes.
The initial platform for microchemical separations is a contactor that can be used for solvent extraction. Eventually, the platform can be adopted for other separations, such as gas absorption, membrane separations, and facilitated transport. When integrated with PNNL's micro-heat exchanger technology, the multi-channel contactor architecture also has the potential to be adapted for distillation. The objectives are to develop capabilities to fabricate specialized micro-machined contactors, to characterize contactors, and to design and test devices for performance in solvent extraction. In addition, mass transfer models will be developed to support enhancements in device design and for evaluating full-scale applications.
The microchannel architecture to be used for solvent extraction consists of thin channels separated by porous contactor plates. Two immiscible fluids come into contact as they flow cocurrently or counter-currently through the channels, transferring solute from one to the other. Reducing the thickness of the channels minimizes mass transfer resistance. In addition, the decreasing thickness, increasing porosity, and reducing tortuosity of the plate also improve mass transfer. By reducing length-scales to 10-100 µm, the residence time can be reduced to on the order of seconds and improving throughput per unit volume by orders of magnitude over conventional equipment.
Technical progress has been made in FY98 in contactor fabrication, device design, in operating microchannel contactors, and in mathematical modeling of performance.
A major accomplishment in FY98 in contactor fabrication was reducing the size of the holes from 30 (m to 5-10 µm, while achieving a porosity of 39%. These contactors improved the breakthrough pressure--the maximum allowable pressure drop across the contactor plate--from typically 2 inches water column to about 6 inches when contacting cyclohexane and water. This improvement in breakthrough pressure facilitated counter-current testing for the first time in these devices.
Figure 1 shows test results for extraction of cyclohexanol from water using cyclohexane as the solvent. Results are shown for both cocurrent and countercurrent flow. The ratio of aqueous to organic concentrations at equilibrium is 1.3, so the equilibrium limiting aqueous effluent concentration is 0.57 times the influent concentration for co-current operation at equal flowrates. Figure 1 illustrates that a microchannel device can achieve 50% of a theoretical stage in less than two minutes residence time. This is a substantial improvement over conventional technologies, such as a mixer settler.
Figure 1 also shows curves predicting theoretical performance of the devices for co-current operation at conditions corresponding to the experimental conditions. These curves were obtained by numerical solution of the convective-diffusion equation using a finite difference method. The model accounts for convection only in the flow direction and diffusion in the transverse direction. The model is two-dimensional with three different regions for the two flow channels and the contactor plate. The regions are coupled by boundary conditions that include an interfacial mass transfer correlation for the interface. The effective diffusivity of the solute in the contactor plate is assumed to be equivalent to the wetting fluid diffusivity times the porosity and divided by the tortuosity, which is one for the laser drilled plates. The pores are not resolved in the model.
Comparing experimental data to theoretical curves in Figure 1 indicates that the devices are not performing as well as expected. In addition, counter-current operation does not improve performance significantly. These observations indicate resistance to mass transfer is unexpectedly high, and suggest the potential for significant improvements.
Continued developments in contactor plate fabrication and testing are anticipated. Performance enhancements are anticipated by improving device design to obtain more uniform and thinner films.
TeGrotenhuis, W.E., R. Cameron, M.G. Butcher, P.M. Martin, R.S. Wegeng, "Micro Channel Devices for Efficient Contacting of Liquids in Solvent Extraction", (accepted for publication in Separation Science and Technology).
DW Matson, PM Martin, WD Bennett, DC Stewart, and JW Johnston. 1997 "Laser Micromachined Microchannel Solvent Separator." Proceedings of SPIE Conference on Micromachining and Microfabrication, Vol. 3223, 1997.
TeGrotenhuis, W.E., R. Cameron, M.G. Butcher, P.M. Martin, R.S. Wegeng, "Micro Channel Devices for Efficient Contacting of Liquids in Solvent Extraction", 10th Symposium on Separation Science and Technology for Energy Applications, Gaitlinburg, TN, October 1997.
TeGrotenhuis, W.E., R. Cameron, M.G. Butcher, P.M. Martin, R.S. Wegeng, "Micro Channel Devices for Efficient Contacting of Liquids in Solvent Extraction", AIChE 1998 Spring National Meeting, New Orleans, LA, March 1998
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