Ethanol: The original renewable green fuel

The ethanol boom of the 21st century can trace its roots back to environmental concerns about reducing automobile emissions and the "Clean Air" legislation of the 1970s and 1980s that boosted the potential market for ethanol as a gasoline additive. Anhydrous ethanol is used as an octane enhancer for gasoline/petrol, thus eliminating hazardous octane boosters such as lead, benzene, MTBE and the like. It promotes efficient burning of the fuel, which can considerably reduce carbon monoxide emissions and result in cleaner air.

Today, the impetus is to reduce dependence on fossil fuels and imported petroleum. In the US, ethanol is produced almost exclusively from corn. From a small beginning in the early 1970s, annual ethanol production capacity in the US was 1.5 billion gallons (5.7 million cubic meters) in 2000, 5.7 billion gallons (21.5 million cu.m.) in 2006 and 13 billion gallons per year (48 million cu.m.) by 2011. New technologies will be needed to meet the increase in production.

Commercially, fuel ethanol is produced by fermentation of sugars (from corn, cane, cheese whey, molasses) by the yeast Saccharomyces cerevisiae. The ethanol obtained from current fermentation technology is relatively expensive. The fermentation-related factors are:

  • Most fermenters are opened in a batch or semi-continuous manner.
  • Long fermentation times: The process is slow, taking typically 20-60 hours
  • High capital costs due to large fermentation volume
  • High energy costs for separating and purifying the ethanol to meet fuel-grade standards

Our Laboratory at the University of Illinois (UI) developed a high-performance fermentation process that overcomes many of these problems. As shown in the diagram, the Continuous Membrane Bioreactor (CMB) uses synthetic semi-permeable membranes to separate and recycle the yeast, while simultaneously removing the ethanol as it is formed. This has several advantages over current technology:

  • The continuous separation and recovery of the yeast will reduce yeast costs and the cycle time of the fermenters, since there will be little or no time lost due to start-up and shut down as in present batch fermenters.
  • The recycle of yeast will allow us to obtain much higher cell densities than currently practiced. Laboratory studies have shown a 1000-fold increase in yeast cell numbers in the CMB during operation. The high concentration allows us to pump the feedstock through the fermenters much faster.
  • "Cell wash-out" is eliminated, thereby allowing operation at dilution rates greater than the specific growth rate of the organism.
  • The continuous removal of ethanol -- which is inhibitory to the yeast -- allows us to maintain the fermenter at just below the alcohol level which inactivates the yeast. Thus the yeast cells are always viable and producing ethanol.

The graph illustrates the improvements to be expected with the membrane-based fermenters. In laboratory trials, productivities 10-50 times higher than batch fermenters have been obtained. Other benefits have been observed:

  • The membrane bioreactors are very flexible, allowing a range of outputs that can be matched very easily to the demands of the ethanol refinery.
  • The "beer" from the fermenter is crystal clear, containing no suspended matter. This will improve the subsequent distillation process, with a further reduction in cost.
  • The membrane units are available as modular systems, making expansion easy.
  • Due to the high productivity, floor space requirements for the membrane bioreactor system are much less than with present-day batch fermenters.

This concept went through several stages of research and development in our laboratory, starting with preliminary experiments which were first published in 1983. From 1983 to 1993, the research focused on understanding the interactions between the microorganisms used for the fermentation -- especially their behavior under high stress conditions -- and the process engineering factors controlling the performance of the membrane modules (until then, mainly polymeric hollow fiber devices).

In the 1994-97 period, the CMB concept was demonstrated under industrial conditions, using ceramic microfiltration membranes. This work was conducted in two phases and in two locations:
(1) Design phase: This occurred at our laboratory at the University of Illinois. The CMB would be scaled up from 10 liters (the largest size we had done until then) to 7000 liters. Additional screening and optimization of membranes would occur during this period, together with preliminary engineering and fouling studies with industrial feed streams. Selection of an engineering company to construct this first-of-its-kind fermentation system would be an important part of this phase.
(2) Operation phase: Installation and evaluation of the CMB side-by-side with the present fermentation process at a large midwest corn-based ethanol plant. Further optimization and fine-tuning of the system would continue during this phase.

For the in-plant trials, a 7000-liter CMB was designed based on original concepts developed by the Principal Investigator, Professor Munir Cheryan. It consisted of a 2m dia x 2.4m high fermentation vessel coupled to ceramic membranes in a computer-controlled system with its own automated CIP system, as shown in the pictures below.

View of the CMB from above: the fermentation vessel is at the back, the membrane modules skid is in front and the CIP+PLC skid is on the right rear.

View of the membrane module skid, showing the Membralox ceramic modules and the prefilters below them. The fermentation vessel can be seen on left

The CMB was operated on-site at what was then the world's second-largest ethanol producer. The trials confirmed the value of the CMB in improving productivity by reducing fermentation time, and the potential to substantially reduce downstream stillage handling, improve heat transfer in the beer still and reduce evaporation or waste treatment costs. This should help lower the cost of producing ethanol, thus ensuring its role in providing clean air, reducing our dependence on imported oil and better utilizing our agricultural resources in the 21st century.

The unique element of this project was the partnership between industry (ethanol producers, membrane manufacturers, engineering and design companies), university (through our laboratory), farmers (through the Illinois Corn Marketing Board) and government (Illinois Department of Energy and Natural Resources, Council of Great Lakes Governors). Such partnerships are the key to facilitating rapid transfer of technology from the laboratory to the marketplace.

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