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ePaper created 2017-03-14, 10:25:37 | version 1.49.03
ENVIRONMENT AND ENERGY 1 1 μm USE OF MEMBRANES FOR PROCESSING BIOBUTANOL BY OSMOSIS Christopher Hänel, Thomas Schiestel The need for energy-optimized processing Replacing fossil fuels with renewable energy sources is ab- solutely essential in the long term. Apart from the economic factors involved, the breakthrough of biofuels depends on the CO2 and energy balances. For second-generation fuels pro- duced by fermentation such as butanol, the energy balances still have to be signiicantly improved to make them competi- tive and environmentally compatible [1]. Yet the downstream processing required is an energy-intensive step and therefore also cost-intensive. The aim is to develop a process that enables dehydration of the product low with a signiicantly reduced energy input by means of the combined use of optimized gas stripping and an osmosis-driven membrane process. The use of forward osmosis to recover ethanol has already been described [2]. So far, no studies are known for butanol, which only requires concentration to 7.3 percent by weight, at which point a phase separation occurs. New forward osmosis membranes In the joint project “Innovative Process Combination for the Downstream Processing of Biobutanol” it was the task of Fraunhofer IGB to develop suitable membranes and a membrane process for the concentration of biobutanol. Tests were carried out on various commercial reverse osmosis membranes and the institute’s own membranes for forward osmosis (FO), both on a cellulose acetate base (CA) and also thin-ilm composite membranes (TFC) [3 – 5]. The membranes were examined with a view to their water permeation as well as the butanol and salt retention. The feed solution used was an aqueous butanol solution with 5.7 percent by weight. The draw solution used was an aqueous NaCl solution with a concentration of 300 g / L NaCl. Best separation performance The CA membrane was applied to a woven-type fabric with a doctor blade, is 60 µm thick and has a distinct asymmetrical structure with a very thin separating layer (approx. 100 nm). The matrix of the TFC membrane is a 110 µm thick microiltra- tion membrane with a pore size of 100 nm (Fig. 1). With the CA membranes, the separation performance can be controlled via the temperature of the heating bath. The higher the temperature, the lower the water low and the greater the butanol retention. At 90°C the CA membranes exhibit the same butanol retention as commercial reverse osmosis mem- branes, while the water low of 1.23 L / (h m2) is 75 percent higher (Fig. 2). With the TFC membranes the separation performance can be controlled via the reaction time of the surface polymeriza- tion. For example, a reaction time of 480 seconds results in membranes with a water low of 4.35 L / (h m2) and a butanol retention of 97 percent. A further variable that was used to compare the membranes was the ratio of water to butanol low. In the case of the commercial membranes this was in the region of 20, with the best CA membranes 40 was measured and with the best TFC membranes 60 was achieved (Fig. 2). Subsequently the membranes were transferred for 500 hours to an ABE solution (26 g / L acetone, 80 g / L butanol, 22 g / L ethanol). Generally speaking, the membranes were stable in this solution. Over time, however, the water low continuously 1 0 0