The main mechanism for mass transfer is osmosis, which follows Equation (1).

1. Microscale Mass-Exchanger for Forward Osmosis Bradley Eagleson, Micah Houck, Nikhil Prem Abstract Mass Transfer Coefficient This analysis uses Equation 1, solving for A as the overall mass transfer coefficient. Once again, the slower flow velocity in the deep channels has poor performance. The faster flow velocity runs were significantly better for both channel depths. The microchannels appear better, but this cannot be said to 95% confidence, as they have nearly equal upper limits. The purpose of this project was to determine the effect of channel depth on mass flux in a microchannel forward osmosis (FO) system with a supporting theoretical model for the system. If microchannels were found to be more efficient, they could be used in the development of a water purification device. Channel depths of 60 and 300 μm were used. Dextrose was used as the draw solution with concentrations between 1 and 2.5 M, while brine concentration was held constant at 0.1 M. One set of experiments were completed with a constant flow rate of 0.3 mL/min, and one set with a constant velocity of 2.0 µm/s. Higher fluxes were found with smaller channels at a fixed volumetric flow rate. Little difference in flux was found when the flow velocity was held constant at different channel depths. Findings Fig. 6: Comparison of the overall mass transfer coefficient for each set of runs with 95% confidence intervals. Background Model Development Analytical model prediction of FO behavior in a cross flow microchannel unit. Economic Considerations Theoretical model based upon expected pressure drop from Equation (3), and an assumed 70% pumping efficiency The main mechanism for mass transfer is osmosis, which follows Equation (1). J α A (T ΔC - P) (1) The system is further complicated when the bulk concentrations of the liquids are taken into account. As the depth of the channels increases, the system develops a concentration gradient where the concentration near the membrane is not the same as in the liquid. This effect can be seen in Figure (1). Resistances within the system can be further described using Equation (2). Resistance increases in the bulk brine and dextrose solutions due to deeper channels, so the overall resistance should decrease. (2) The system pressure variation also need to be taken into account, which can be described by Poiseuille’s Equation (3). (3) • J = Flux (m3/m2-s) • A = Hydraulic Permeability (m/s-Pa) • T= Temperature (K) • ∆C = Concentration Difference (mol/m3) • P= Pressure (Pa) Fig. 7: Graph depicting effective diameter vs. Power usage per flow rate at various flow rates. The pressure drop was found using Equation (3), and an assumed 70% efficiency. While pumping cost appears low, it increases at an exponential rate making very small channels economically infeasible, especially at higher flow rates. Conclusions Fig. 1: Diagram A represents microchannel fluidic behavior that has less concentration polarization. Diagram B represents macrochannel fluidic behavior RB RM • Higher fluxes seen with shallower channels at constant volumetric flowrate. • - Shallower channels may reduce bulk transport resistance • Little difference seen when flow velocity constant. • - Shallower channels may be more efficient because lower concentrations generate the same flux • Larger volumes within deep channels maintain higher concentration differentials. • Larger pressure drop in shallow channels may increase the P term in Equation 1, thus decreasing the flux in the shallow channels • Research suggests microchannelsmore effective, but also have significant increase in energy requirements. • Future work could involve increased flowrates in shallow channels to investigate the effects of pressure drop. RD (A) (B) Fig. 2: Model predictions of the brine channel and its crossing draw channel concentration profiles Fig. 3: Model predictions of the draw channel and its crossing brine channel concentration profiles • RB= Resistance in the brine solution • Rm= Resistance in the membrane • RD= Resistance in the draw solution • R = Overall resistance Experimental Flux was measured between four higher concentrations of dextrose and a lower concentration of brine at two different channel depths. Two comparisons were made, one where the flow rate was held constant (Fig. 4), and one where the flow velocity was kept constant (Fig. 5). The flow rates were 0.3 mL/min for the 60 micron depth and 1.5 mL/min for the 300 micron depth to achieve a constant velocity. This flow velocity was kept low enough so that the highest pressure drop experienced by either of the channels would be approximately 40 kPa. Membranes Conductivity Probes 60 µm Channel Plate Acknowledgements Dr. Philip Harding, Dr. Todd Miller, Dr. Ed Beaudry, Matthew Bertram, Oregon State University, MBI (Microproducts Breakthrough Institute), Hydration Technologies, ONAMI (Oregon NanoscienceMicrotechnologies Institute References Fig. 4: Holding volumetric flow rate constant at 0.3 mL/min. This shows the performance of the two different channel depths compared to a theoretical model. A mean with a 95% confidence interval is shown for the 60 micron depth Fig. 5: Holding flow velocity constant. This shows how there is very little difference between the different channel depths McCutcheon, J. and McGinnis, R.L. and Elimelech, M. 2006. Desalination by ammonia-carbon forward osmosis: Influence of draw and feed solution concentrations on process performance. Journal of Membrane Science. 278: 114-123. Bertram, M. and Cunningham, M. 2008. Examination of a microscale mass-exchanger for forward osmosis. Chemical Engineering 416 at Oregon State University. Yeh, H. and Hsu, Y. 1999. Analysis of membrane extraction through rectangular mass exchangers. Chemical Engineering Science. 54: 897-908.