Pervaporation: Study on Different Membranes to Separate Butanol from Aqueous Solutions

  • Posted on: 11 June 2018
  • By: mmiltner
TitlePervaporation: Study on Different Membranes to Separate Butanol from Aqueous Solutions
Publication TypeConference Paper
Year of Publication2013
AuthorsRom A, Gimeno DEsteve, Friedl A
Conference Name7th International Conference on Environmental Engineering and Management ICEEM2017, Vienna, Austria

Many researchers focus on butanol as a „new and better” biofuel. But ABE-Fermentation (Acetone-Butanol-Ethanol) is one of the oldest industrial processes and was invented 1916. From that time on butanol was produced by the fermentation process until in the early 50s the emerging petrochemical industry has overtaken the field.

Still in 2013 topics such as increasing CO2 emissions, climate change and ongoing rise of energy demands have lost none of its importance. Since about 15 years efficient fermentative butanol production is again the aim of several research projects. Biobutanol with its advantages compared to ethanol has a high potential to become an important biofuel.

N-Butanol is a four carbon alcohol. This makes it an alcohol with higher energy content than ethanol and increasing the mileage/gasoline blend ratio. It is less corrosive, which has effects on the existing infrastructure. Other than with ethanol butanol can be transported and stored in existing tanks and pipelines. It has lower volatility and lower saturation pressure, which result in a saver handling. Butanol is not hygroscopic and therefore can be blended in any concentration with gasoline without engine modification. Compared to ethanol, butanol can also be blended with diesel and kerosene Biobutanol can be produced from the same biomass as ethanol. Also existing ethanol production plants only need a few modifications to produce this biofuel with several advantages.

In the biotechnological production of butanol by using ABE-Fermentation, the product inhibition of microorganisms at 13 g/l is a major problem. Due to the low butanol concentration present in the broth, state of the art distillation results in a highly uneconomically energy demand.

State of the art process is a multicolumn distillation. Due to the relatively diluted fermentation broth energy consumption is double the value observed during ethanol production. Additionally the process results in a bad wastewater footprint (L stillage/L alcohol), which is 10.7 l for ethanol and over 40 for butanol.

To counteract the limited factor of the product inhibition an on-line butanol separation is desirable. With e.g. a membrane separation step butanol concentration in the separated aqueous solution can be raised to a desired range before entering the distillation section of the process. This two-step separation process leads to a low overall energy demand and an optimized efficiency. An attractive option to put this into practice is the product separation with organophilic pervaporation.
Several recovery processes have been tested: gas stripping, vacuum membrane distillation, liquid-liquid extraction, adsorption, reverse osmosis, perstraction, but pervaporation is seen to be one of the most potential recovery processes. The utilization of waste heat as well as the use of renewable energy resources for pervaporation offers promising process combinations on an industrial scale.

In pervaporation the liquid stream to be separated is in contact with a dense polymeric or tight inorganic membrane. Due to the applied vacuum on the other side of the membrane the targeted compound sorb into the membrane, permeate through it and evaporate into the vapour phase. The driving force for this separation process is achieved by differential partial pressures. The butanol-water mixture is a highly non-ideal mixture with an immiscible gap between 7 and 80 w%. In addition butanol, in very low concentrations as present in fermentation broth, has an activity coefficient of about 54, compared to Ethanol with about 13. These characteristics make the ABE-fermentation broth striking for a process driven by partial pressure difference. The aim of the pervaporation step is to concentrate the mixture up to 7 w% to have a 2-phase permeate, which results in a lower energy demand compared to distillation starting from a lower concentration range.

The second important factor for the effectiveness of this thermal separation process is the membrane itself. Different to other membrane separation processes in pervaporation the dense membrane has a selective layer. For Butanol recovery a hydrophobic layer, like PDMS; POMS; PTMSP, several compound as well as ceramic or zeolite membranes offer, are used. The perfect membrane for pervaporation summarizes characteristics such as: long term stability, high selectivity, simple performing, no harm to the broth and low production costs. Special membrane materials, which lead to selectivities like 100-140, offer high potential for pervaporation to become an outstanding separation process. Nevertheless commercially available membranes like PDMS still select butanol selectively from fermentation broth and with an alpha of 20 energy demand decreases compared to distillation.

In this work new commercially available membranes for organophilic pervaporation were investigated in terms of vacuum pressure, feed concentration, feed temperature and feed flow rate.