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Membrane Filtration Rules Biofuels

Membrane filtration technology improves biofuel processes by saving both costs and water.

Membrane filtration technology, a tried and proven method used in many industrial process streams, is now being adopted in biofuels production. In particular, membrane filtration shows promise in improving second-generation cellulosic ethanol processes. Operators of second-generation processes seek to optimize fuel recovery and secondary products from feedstock. In addition, these operators want to obtain a better value fuel than first-generation processes, which focus exclusively on fuel production and simply dispose of everything else as waste or animal feed material.

Koch membrane filtration technology

Koch Membrane Systems’ reverse osmosis system reduces operating costs.

Membranes are highly engineered physical barriers used in processes for liquid/liquid and liquid/solid separation. They permit the passage of materials up to a certain size, shape or character.

Membrane filtration has been used extensively in industry for years. For example, use is rising in biodiesel processes in which membranes facilitate water reuse, especially in areas where water is scarce. Membrane technology also shows promise for making organic acids, commonly used as the base for a variety of new biodegradable plastics. So-called integrated biorefineries are additionally using various membrane technologies, including microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Particular attention is being paid to using membrane filtration techniques that facilitate continuous, rather than batch, fermentation.

Makers of second-generation cellulosic ethanol are taking corn stover (cobs and other agricultural waste products), and using enzymes and acid/alkali to extract fermentable material, then converting cellulose to sugars, which are being fermented to produce alcohol. During the process, they incorporate several membrane filtration techniques to improve recovery, reduce waste and lower energy costs.

Because first-generation bioethanol facilities were focused on producing fuel rather than optimizing fuel recovery, there was very little use of or demand for membranes. With second-generation cellulosic bioethanol, a tremendous amount of work is going into using membrane filtration. For example, ultrafiltration is used for clarifying the process stream after turning feedstock into sugars during the saccharification process.

Some processes use acid/alkali, and nanofiltration can be used to recover and concentrate some of the useful sugars like hemi-cellulose, which also is used in the fermentation process. Reverse osmosis recovers high-quality water for reuse. Membrane filtration is also used for making organic acids as the base material for biodegradable plastic.

Membranes are being used in continuous and batch fermentation to produce biofuels, and organic and amino acids, using mesophilic and thermophilic organisms. In general:

  • Ultrafiltration and microfiltration are used for biomass retention, or fermentation broth clarification and product purification.
  • Nanofiltration is used for low molecular weight component fractionation and desalting applications, in which the process streams may have high concentrations of acids and base chemicals, and organic solvents/water mixture.
  • Reverse osmosis is used for product concentration and water recovery for reuse.

Moreover, membrane technology has the potential to reduce operating costs compared to the traditional method of using an evaporator to recover or remove water, which requires very high energy use. Companies can also use membrane technology to supplement their process to reduce energy costs. Reverse osmosis can offer about 75 percent lower cost of ownership compared to a five multi-effect evaporator with thermal vapor recompression. Reverse osmosis, which can be applied as a stand-alone process, or in conjunction with an evaporator, offers recovery of high-quality water for reuse. Nanofiltration allows the recovery of proteins, peptides, amino acids, and partial rejection of some salts and sugars.

One interesting application driving the interest in membrane technology is its ability to operate at higher temperatures. Operators frequently seek ways to run continuous fermentation, rather than a batch process. With a thermophilic process (micro-organisms, which display optimal activity around 70º to 78ºC), operators could continually run product and feedstock to the alcohol distillation columns. In that type of process, the membrane plays a critical role as part of a membrane bioreactor because it helps retain microbial biomass in the fermenter, while allowing liquid to be continuously drawn out of fermenters. This has potential to reduce the hardware investment cost (by reducing the number of cyclic fermenters), as well as operating costs.

Membrane technology has long been used in wastewater treatment plants, and it is now being used more and more in biodiesel production, which produces wash water very high in contaminants. In the past, facility operators could spread this waste stream on land or discharge it untreated to a wastewater treatment plant or local water source, such as a river. Today, this is not permitted in many areas. In addition, water-constrained areas encourage water conservation and reuse rather than disposal.

In some cases, operators are using anaerobic digestion to remove biochemical and chemical oxygen demand in the waste stream by installing ultrafiltration membranes to concentrate the biosludge, and then using reverse osmosis to recover water and reuse it in the facility. This is especially important in areas where water is limited.

A tremendous amount of government research and investment is being done on biofuels and integrated biorefineries, much of it focused on increasing product recovery, reducing waste, lowering energy costs and improving the greenhouse gas profile of biofuels.

Membrane technology has an opportunity to provide biofuel manufacturers with cost-effective solutions. New processes must be developed to take advantage of this promise. While new to biofuel developers, using membranes for filtration and separation has been around for years. It has great potential for providing benefits as developers improve processes in preparation for integrating process-scale and production facilities. 

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