Emerging Process Optimization Opportunities for Ethanol Facilities

By Philip A. Marrone, Kenneth R. Liberty and David J. | September 08, 2008
An increased demand for corn and product capacity in the market makes it critical for ethanol plants to ensure that operations are fully optimized to avoid profit erosion from increasing feed and decreasing product prices. As a result, it is prudent for ethanol producers to consider whether their plants are being operated as efficiently as possible. This means maximizing ethanol yield and productivity, and the number and value of coproducts, while minimizing waste and energy use.

Optimization approaches can be divided into three categories 1) process—feedstock preparation, fermentation or downstream separation as related either to physical or chemical processes, 2) energy—related primarily to heating, power or drying options, or choice of plant fuel type, and 3) wastes/coproducts—minimizing, reusing or finding new applications for unwanted products and increasing the value or yield of desired coproducts.

This article, the first in a three-part series, focuses on this first category and reviews options through which established ethanol plants can achieve process-related optimization goals. While they cover a range of different approaches, these options all strive to improve ethanol yield and/or productivity by
1) allowing operation with a higher sugar feed concentration,
2) allowing operation with a higher ethanol product concentration, or
3) decreasing process time. All of the options presented have been either demonstrated or proven to some extent but are not currently in widespread use or considered standard.

The most notable are discussed below. A more detailed discussion of these and additional options can be found in the full-length version of this three-part series, available at http://www.ethanolproducer.com/articles/EmergingOptimizationOpportunities/.

Fractionation Possibilities
The wet mill process inherently includes technology that fractionates or separates corn into its components upfront, thereby achieving more efficient fermentation and a greater number of higher-value coproducts. However, the wet mill process is more complex and costly relative to the more common dry grind process. As a result, several modifications have been developed to incorporate some of the wet mill fractionation benefits into the less expensive dry grind process. The simplest option is to employ dry fractionation. Dry fractionation is a process by which the more easily removable germ and pericarp fiber are mechanically separated from the corn kernel without soaking. Other fractionation approaches such as quick germ, enzymatic milling and Maize Processing Innovator's Quick Germ/Quick Fiber process have also been developed to improve the purity of separated coproducts relative to that of dry fractionation, but at a cost and complexity still lower than that of a wet mill process. Another alternative is to incorporate fractionation at the backend of the process to separate purer coproducts from distillers dried grains with solubles. An example is the Elusieve Process used to recover pericarp fiber through a sieving step. The local market and coproduct demand must be evaluated to determine whether fractionation makes sense and which version is most economical for a given plant.

Increased Enzyme Thermal Stability/Efficiency
The alpha-amylase and glucoamylase enzymes used in the liquefaction and saccharification steps of ethanol production, respectively, have different preferred process conditions (i.e., temperature, pH and concentration). Adjustment of these conditions during production to accommodate each step costs time and money in the form of added chemicals and energy, and can impact the performance of microorganisms downstream. At the same time, it would be preferable to operate all steps at as high a temperature as possible to minimize reaction time and kill contaminant bacteria.

Discovery or development of enzymes that are stable and active at high temperatures under the same process conditions (e.g., pH of 4.5 and temperature of 194 degrees Fahrenheit or higher) could significantly improve the overall rate of starch hydrolysis, and thus ethanol production, by combining fast kinetics with no or minimal batch adjustments. With advances in molecular biology techniques, many researchers and enzyme manufacturers have not only been able to screen large numbers of microorganisms for potentially useful enzymes, but also further alter these selected enzymes to better function under the conditions desired. For example, researchers have recently reported discovery of a glucoamylase enzyme from a bacterial source that was optimally active at 194 degrees Fahrenheit and a pH of 5.5 to 6, which would allow saccharification to take place under the same conditions as liquefaction without any parameter adjustment.

Self-Contained Enzymes
With the advent of genetic modification techniques, it is now possible to custom engineer both feedstock and microorganisms needed for ethanol production to self-produce starch hydrolysis enzymes or other desired attributes. Having the necessary enzymes already present would streamline the hydrolysis and fermentation processes. Work is being performed to develop hybrid corn strains that will contain starch-hydrolyzing enzymes in the endosperm and/or an increased percentage of starch. Researchers are also trying to develop strains that include the genome responsible for producing cellulase, which would help with the critical need to reduce the cost of cellulosic ethanol production.

With respect to microorganisms, genetically modified yeast cells have also been developed to produce hydrolytic enzymes for either starch or cellulose. These organisms are able to hydrolyze a starch or cellulose feedstock directly with less pretreatment, followed by immediate fermentation of the released glucose. As with all genetically modified organisms, the downside is the unknown long-term risks and associated negative public perception.

Contaminant Bacteria Reduction Without Antibiotics
Many of the same operating conditions that provide an optimal growth environment for fermenting yeast are also good for the growth of undesirable microorganisms such as lactic acid bacteria, which utilize valuable glucose. While antibiotics have traditionally been used in the ethanol industry to control contaminant bacteria populations, this approach is becoming increasingly unpopular for several reasons including promotion of increasingly resistant bacterial strains, more stringent government regulations or bans, and high expense.

In the absence of antibiotic use, options for controlling contaminant bacteria populations include more frequent batch sterilization, operation of liquefaction, saccharification and fermentation steps in minimal time or simultaneously, or addition of disinfectants or natural substances known to inhibit bacterial growth without fostering tolerance. Recently, several ethanol plants have utilized a commercial version of key acid extracts from hops shown to stop bacterial growth and achieve promising results, particularly in handling bacterial strains that have become antibiotic-resistant.

Improved Yeast Performance
There is significant incentive to increase the thermal, ethanol and osmotic tolerance of yeast cells to decrease fermentation time, increase final ethanol concentration and increase feed concentration, respectively, while also limiting production of unwanted byproducts. An increase in the thermal tolerance up to 140 degrees Fahrenheit (compared to the current maximum of 95 degrees Fahrenheit) is predicted to substantially decrease fermentation time by about 25 percent due to faster kinetics alone. If combined with improvements in enzyme thermal tolerance, it may be possible to operate hydrolysis and fermentation steps at a single, high temperature, thereby saving energy costs involved in adjusting conditions from one step to the next and, in turn, decreasing process time.

Further improvement in ethanol tolerance has been hindered due to the discovery that several genes must be manipulated to alter this trait. Nevertheless, progress continues to be achieved in this regard. For example, genetically modified versions of Saccaromyces cerevisiae have exhibited statistically significant improved cellular viability in tests even up to ethanol concentrations as high as 20 percent, compared to the standard 13 percent to 15 percent. Genetic engineering may also be able to "reprogram" cells to produce less undesirable byproducts or even to metabolize five- and six-carbon sugars, which is of interest for cellulosic ethanol process development.

Very High Gravity Fermentation
Developed originally at the University of Saskatchewan, the very high gravity (VHG) fermentation process utilizes a higher concentration of solids and can achieve higher ethanol concentrations than the standard process. VHG requires feeds containing 27 percent by weight dissolved solids or greater. At these high solids concentrations, most feeds are too viscous to handle efficiently. Thus, an important part of the VHG process involves preparing a mash with a sufficiently high solids content and an acceptable viscosity. This can be achieved by employing one or more of several options: addition of enzymes, double mashing, or addition of adjuncts (a high-sugar feed supplement such as grain hydrolysate, sugarcane juice or molasses). Ethanol concentrations as high as 23 percent by volume have been seen with a feed concentration of 38 percent by weight solids at 68 degrees Fahrenheit, with lower ethanol concentrations at higher temperatures. While the conditions necessary for the highest ethanol concentrations may not be practical on a large scale from an economic perspective, it is reasonable to expect to achieve ethanol concentrations in the range of 18 percent by volume using VHG.

There are several clear advantages to VHG fermentation relative to conventional fermentation, including increased plant throughput, lower water and energy usage, and decreased reactor size. The insoluble non-starch material also has a higher value since it is separated before fermentation and free of yeast biomass. On the other hand, VHG fermentation is more complex than the standard process, requires equipment designed to handle the higher viscosity feed, and may require yeast strains that have an inherently higher ethanol and osmotic pressure tolerance for the best results. Over the past decade, VHG fermentation has been successfully tested at the pilot-scale and is currently being incorporated in some full-scale plant designs focused on achieving high ethanol concentrations.

Extractive Fermentation
The toxicity of ethanol to yeast limits its final concentration during fermentation and thus the amount of feed that can be fermented per batch. If ethanol is removed from the fermentation reactor as it is formed, however, its concentration can be kept at a value that would permit cell activity and fermentation to continue unhindered. Such a concept is referred to as extractive fermentation. To achieve this goal, a way must be found to selectively remove ethanol from the fermentation broth in the reactor without impeding the fermentation reaction.

Early attempts to continuously remove ethanol from fermentation broth focused on vacuum fermentation or liquid-liquid extraction methods. In recent years, a more effective means of ethanol removal has involved the use of semi-permeable membranes. Membranes have the significant advantage of keeping the broth and solvent liquid phases separate, allowing the transfer of ethanol without contaminating either liquid phase. Impregnated polymer-based membranes, zeolite membranes and microporous hollow-fiber membranes are examples that have been successfully tested in recent years. Most of these options for extractive fermentation have been explored only at a bench-scale size and would clearly increase the complexity of the fermentation system relative to the current standard simple tank reactor. While scale-up is not trivial, the concept holds great potential. Results verify the improved performance expected when the cell-inhibiting ethanol concentration is restricted to an acceptable level.

Yeast Immobilization
In an immobilized cell system, the yeast cells are constrained to a limited, defined volume. Typically, they are imbedded or bound to a solid support structure, either on the surface or within a porous medium, and used in a continuous flow system. The most widely used method of immobilization is through physical entrapment within a spherical, porous polymer matrix.

Many studies have shown improved ethanol productivity and higher feed concentration tolerance in immobilized cell reactors versus traditional batch reactors with freely suspended yeast cells. This may be a result of decreased ethanol concentration near the cells due to constant removal and/or alteration of the cell membrane through the immobilization process to allow for enhanced transfer of substrates into and out of the cell. Other advantages include no cell washout, higher productivity due to higher cell density and better protection, and better control of the microbial environment. However, it is also clear that mass transfer constraints can significantly hinder ethanol production kinetics and nutrient transfer, depending on the material and design of the support structure and the particular conditions of operation. Whether the potential for greater ethanol productivity and avoidance of cell washout balances the greater system design and operational complexity of an immobilized cell system depends on the specifics associated with a given ethanol plant and therefore needs to be evaluated on a case-by-case basis.

Summary of Process Optimization Options
The options presented here are all currently at various levels of maturity, with some beginning to be implemented now and others still at a research/bench-scale level. With some, it is just a matter of time before widespread full-scale use, while others may never fully mature to the extent necessary to be economically viable except under limited circumstances. Each has its own unique advantages and disadvantages that must be weighed with respect to the design and economics of a specific plant to determine whether it is worthwhile to implement. However, all of these options represent the best process improvement modifications currently available and hold sufficient potential and promise to warrant either consideration or careful observation of further development to determine whether they may be applicable for a particular ethanol plant.

Philip A. Marrone is a chemical engineer, Kenneth R. Liberty is a biochemical engineer and David J. Turton is a civil engineer with Science Applications International Corp. Reach Marrone at marronep@saic.com or (617) 618-4686. Reach Turton at david.j.turton@saic.com or (443) 402-9209.