Options for Combining Pervaporation Membrane Systems with Fermentors for Efficient Production of Alcohols from Biomass

Although several separation technologies are technically capable of removing volatile products from fermentation broths, distillation remains the dominant technology. In this presentation, the status of an emerging membrane-based technology called pervaporation for the recovery of biofuels such as ethanol is reviewed and options for integrating pervaporation units with fermentors will be discussed. Pervaporation is an attractive technology because of the potential to selectively separate alcohols and water. Several issues and research priorities which will impact the ability of pervaporation to be competitive for biofuel recovery from fermentation systems will be identified. Introduction The production of renewable biofuels has been receiving increased attention due to the phase out of methyl t-butyl ether (MTBE) as a fuel oxygenate, the reliance on sometimes problematic sources of fossil fuels, and the effect of non-renewable fossil fuel combustion on the earth’s climate. Currently, the conversion of corn to ethanol accounts for the vast majority of liquid biofuels produced in the United States. Only a few decades ago, the ethanol produced from corn contained less energy value than the energy required to produce that ethanol [1]. Over time, efficiency gains in the corn-to-ethanol conversion process have reversed this situation [1]. Unfortunately, some of these efficiencies are only possible at the large scale, thereby fueling the drive to large corn-to-ethanol plants. For example, the median capacity of corn-to-ethanol plants under construction in the year 2004 was 40 million gallons per year (MGY) (150 million liters per year) [Source: BBI International]. While corn will remain a sizeable fraction of the starting material for liquid biofuels for the foreseeable future, other carbon sources will be required if renewable biofuels are to make more significant inroads into the world’s energy portfolio. A variety of biomass materials are available for production of liquid biofuels, both intentionally grown for this purpose and that which is a side product or waste material from another process. Some of these waste materials, such as corn stover, are co-located with current large-scale ethanol production facilities. Many more sources of lignocellulosic material will be distributed in nature. The amount of this distributed biomass waste is significant, although often overlooked. In order to utilize this distributed biomass, a process train which can convert lignocellulosic materials to biofuels such as ethanol or butanol in an efficient, cost-effective manner at a small scale is needed. The stages of this process train must be reassessed individually and collectively in order to arrive at the most efficient small scale system. For example, recovery of ethanol from the fermentation broth has long been ceded to distillation for corn-to-ethanol operations. The economies of scale and the degree of heat integration which are achieved at the large scale make distillation economically and energetically efficient. However, the advantages of distillation over competing separations technologies for biofuel recovery fade as the scale of the operation is reduced, thereby opening the door for other technologies such as gas stripping, liquid-liquid extraction (perstraction), vacuum stripping, membrane distillation, vacuum membrane distillation (VMD), sorption, and pervaporation. Fundamentals of pervaporation Pervaporation is a process in which a liquid stream containing two or more miscible components is placed in contact with one side of a non-porous polymeric membrane or molecularly porous inorganic membrane (such as a zeolite membrane) while a vacuum or gas purge is applied to the other side. The components in the liquid stream sorb into/onto the membrane, permeate through the membrane, and evaporate into the vapor phase (hence the word “pervaporate”). The resulting vapor, referred to as “the permeate”, is then condensed. Due to different species in the feed mixture having different affinities for the membrane and different diffusion rates through the membrane, a component at low concentration in the feed can be highly enriched in the permeate. Thus, the permeate composition may widely differ from that of the vapor evolved after a free vapor-liquid equilibrium process. A schematic diagram of the pervaporation process is shown in Figure 1. The main process units of a pervaporation process: feed source, feed pump, heater, membrane module, condenser, and vacuum pump, are shown in Figure 2. Pervaporation = Permeation + Evaporation