Membrane Technology in Production of Biofuels

Tested technology improves new biofuel processes
By Kamla Jevons | March 10, 2011

Membrane filtration technology, a tried and proven method used in many industrial process streams, is now being adopted in biofuels production and integrated biorefineries. In particular, membrane filtration technology shows promise to improve second-generation cellulosic ethanol processes. Operators of second generation processes seek to optimize fuel recovery and secondary products from the feedstock. In addition, these operators want to obtain a better value fuel than first-generation bioethanol processes, which focused exclusively on fuel production and simply disposed of everything else as waste or animal feed material. Companies can utilize membranes to improve bioprocesses, lower overall energy costs and increase valuable product recovery.

Membranes are highly engineered, physical barriers that are used in processes for liquid/liquid and liquid/solid separation. They permit the passage of materials only up to a certain size, shape or character. Membrane filtration has been used extensively in industry for years, especially in the dairy, sugar, food, starch, wine and enzyme manufacturing sectors.

For example, membrane use is rising in biodiesel processes that facilitate water reuse, particularly in areas where water is scarce. Membrane technology shows promise for concentrating and purifying organic acids, commonly used as the base for a variety of new biodegradable plastics. In addition, integrated biorefineries are using a variety of 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. In the next few years, dozens of second-generation cellulosic ethanol and integrated biorefinery pilot and demonstration plants will be placed online as the biofuel industry seeks to improve its processes before implementing full-scale production facilities.

Optimizing Recovery

In North America, the conventional starch hydrolysis process is used throughout cereal-growing regions to convert corn and wheat into first-generation ethanol. In the first-generation process, owners focus almost exclusively on producing fuel, and aside from distillers grains, supplied for animal feed, consider all nonfuel material as waste to be disposed or used for low-value products.

Enter the new generation of biofuel hopefuls. Spurred by concerns about the environmental impact of these early biofuels, as well as massive federal spending for energy-related research and development, second-generation biofuel developers are seeking to optimize the recovery process to get better value and reduce waste.

Makers of second-generation cellulosic ethanol are taking corn stover and other agricultural residues and using acid/alkali pretreatments and enzymes to extract fermentable material. During the process, they are using a variety of 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 feed stock utilization, 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 it into sugars during the saccharification process.

Some processes use acid/alkali pre-treatment along with nanofiltration to recover and concentrate useful sugars from the hemi-cellulose that are also used in the fermentation process. Reverse osmosis is used for recovery of high quality water for reuse. Membrane filtration is also used for the production of fermentation derived organic acids that are used  as  base material for biodegradable plastic.

Future Adaptations

Membranes are being used in continuous and batch fermentation processes to produce biofuels, organic and amino acids.

The adaptations being considered include:

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

Using membrane technology has the potential to greatly reduce operating costs compared to the traditional method of using energy-intensive evaporators to recover or remove water.

Companies can use membrane technology to supplement their process to reduce energy costs. Reverse osmosis can offer about 75 percent lower capital and operational costs 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 sugars, while allowing salts to pass through the membrane.

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

The next few years will see a great increase in demonstration plants working on continuous fermentation, and membranes are poised to play a significant role in this important advancement.

Wastewater Treatment

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 wastestream 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 are trying to encourage water conservation and reuse rather than disposal.

Anaerobic digestion is often used to remove biochemical oxygen demand and chemical oxygen demand from the waste stream. Ultrafiltration may be used to concentrate the biosludge, followed by a water recovery and reuse reverse osmosis system. This is especially important in areas where water is limited. For example, Koch Membrane Systems’ membrane filtration technology is being used as part of a membrane bioreactor at an Australian ethanol facility where the waste streams go through biological treatment and use membranes to recover water for reuse.

More R&D Ahead

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

According to 2009 data from the U.S. DOE, 19 second-generation pilot and demonstration cellulosic bio-refinery facilities are either on the ground or in the works. These integrated biorefinery projects will receive up to $564 million for pilot, demonstration, and commercial scale facilities. Many of them are looking to gain the benefits of membrane technology. Pilot plant process development will feed into the demonstration plant installations.

As the membrane technology processes are improved in preparation for integrating future process scale and production facilities, they can provide biofuel manufacturers with cost-effective solutions and significant benefits. Although new to biofuel developers, membrane technology for filtration and separation has been used effectively for years in other starch- and sugar-based industries, and while new processes must be developed to take advantage of this promise, but they will be building on successful technology.

Author: Kamla Jevons
European Business Development Manager,
Koch Membrane Systems Inc.
(978) 694-7000