Membrane Processes for Sustainable Energy Applications

Constantly growing world population leads to ever-increasing energy demand and is stretching the available energy supplies. Consequently, new energy sources must be identified and developed. Economic, environmental and social impact must be considered when evaluating alternative energy sources. An ideal source would be locally distributed and renewable with minimal economic and environmental impact on our current lifestyle.Membrane processes have gained importance in a wide spectrum of applications due to their low energy requirements. Process technologies for alternative energy production can be intensified with the application of membranes. This dissertation is focused on the application of membrane science for advancing the efforts to develop sustainable energy alternatives. We developed two membrane processes, one for application in a bio-refinery based on ligno-cellulosic biomass and another for electricity generation from salinity gradient.The first process development attempts to solve a separation issue related to a bio-refinery that operates with a ligno-cellulosic biomass feedstock. Production of bio-ethanol from ligno-cellulosic biomass requires pretreatment of the biomass to hydrolyze hemi-cellulose, separate lignin and reduce cellulose crystallinity. During the pretreatment by dilute acid hydrolysis (or other method of hydrolysis), several grams of acetic acid per liter of hydrolysate are generated. Acetic acid needs to be removed from the hydrolysate since it can dramatically reduce the efficiency of the subsequent fermentation step by inhibiting enzyme activity. The conventional method of hydrolysate detoxification, lime treatment, consumes significant quantities of calcium hydroxide, involves solids handling and does not produce any valuable products.We propose to remove the acetic acid with an alternative process whereby acetic acid is extracted into 1-octanol in a membrane extractor and is reacted with 1-octanol in situ with a catalyst present on the surface of membrane contactor. This esterification reaction yields 1-octyl acetate and water as the reaction products. Acetic acid concentration in the hydrolysate will be reduced from 12.5 g/L to less than 1 g/L. The product ester will be removed periodically from the organic phase. The product has very good solvent properties and can be sold to improve the process economics. In our study, we identify a suitable membrane and combine it with a catalyst (strong acidic ion exchange resin) to achieve extraction and reaction in a single step. Thus, the membrane extractor/reactor removes acetic acid from the hydrolysate and produces a valuable product (ester) in an environmentally friendly process.The second membrane process development was aimed at harnessing the energy of mixing of two salt solutions differing in their salinities. Significant interest exists in using the salinity difference between fresh water and sea water to produce power from mixing the two streams. Membrane processes such as Reverse Electrodialysis (RED) and Pressure Retarded Osmosis (PRO) can be used to produce electricity using salinity gradients. The less-developed alternative of RED was chosen in this work.We demonstrate the optimization of power output using RED. The effects of several different operational parameters such as current density, solution concentrations, channel thickness, flow rates were evaluated to improve the power output. I also examined the relationship between electrons and ion transfer to determine the efficiency of RED. The Nernst equation indicates that a temperature gradient between the salt solutions will increase the driving force for ion exchange in RED and also increase the conductance (or reduce resistance) of the salt solutions. We investigated the improvement in output power density due to superimposition of temperature gradients on concentration gradients.A theoretical and experimental study was carried out to understand the effect of spacer design on the performance of RED. The computational fluid dynamics package COMSOL was used to evaluate flow distribution within the RED flow channels for a standard spacer and designs that improve flow uniformity. The simulations results were then verified by flow visualization studies. The results show that there is significant scope for improvement in flow distribution to improve the overall RED performance and is an important area requiring dedicated development efforts in future work.