Alternative Feedstocks for Bioprocessing
INTRODUCTION
Bioprocessing is an inexact term often incorrectly limited to the single context of combining a fermentable sugar with an organism for the purpose of making a chemical. This is a common misconception and one that must be dispelled because bioprocessing includes all aspects of the use of renewable and sustainable materials for the production of chemical products, fuels, and power. Alternative feedstocks is a term more easily understood, and for the purposes of this discussion, will mean biomass-derived raw materials and building blocks, i.e., renewables. Bioprocessing deals with three primary issues: supply, separation, and conversion.
DISCUSSION
Supply issues are primarily associated with the growing and collection of biomass feedstock. Bioprocessing deals with the source of the starting feedstock, its availability, the best geographic location for production, its supply and sustainability, collection, densification, and processing. In a direct analogy to crude oil, biomass feedstocks are complex mixtures of different materials that must be separated to obtain the primary raw materials for conversion into other products. Bioprocessing involves developing the best methods for separation and isolation of these renewable building blocks, such as lignocellulosic components (cellulose, hemicellulose, lignin), oils (soy oil, canola, etc.), protein, extractives, and higher value chemicals.
Once isolated, renewable building blocks must be converted into chemical intermediates and final products. Bioprocessing investigates the development of the best technology to carry out these transformations. The very limited view of bioprocessing being only a sugar interacting with an organism more correctly fits as a subset of conversion. It is important to realize that bioprocessing does not require biotechnology. Bioprocessing also includes the use of conventional chemical technology to carry out transformations on renewable building blocks, as well as hybrid transformations such as combining nonrenewable building blocks with biochemical technology. Lichtenthaler has written extensively about the use of conventional chemical technology to convert carbohydrates into new products.
A comparison between a bioprocessing industry (the ‘‘biorefinery’’) and the existing petrochemical industry is appropriate because each faces three primary issues. For the most part, issues of supply and separation are generally understood for both bioprocessing and petrochemical refining. However, conversion processes are not as well understood for bioprocessing as they are for petrochemical processing. The primary difference between the two industries is technology development. The petrochemical industry has gained amazing transformational control over the behavior of its many crude oil-derived building blocks.
In contrast, the analogous use of renewables suffers from a much narrower range of discrete building blocks, fewer methods to convert those building blocks to other materials, and a lack of information about the properties and performance available from those products. We are faced with the puzzle of possessing an almost limitless source of raw material in the United States while being unable to effectively convert it to a wide range of useful products. This discussion cannot cover all the specifics surrounding these three very broad areas. Accordingly, this brief overview will describe only the major concepts involving bioprocessing and alternative feedstocks for the production of new chemical intermediates and products.
THE CASE FOR RENEWABLES
Well into the 20th century, bioprocessing of renewable feedstocks supplied a significant portion of the United States' chemical needs. The chemurgy movement of the 1930s, led by such notables as William Hale and Henry Ford, promoted the use of farm products as a source of chemicals, with the belief that ‘‘anything that can be made from a hydrocarbon could be made from a carbohydrate.’’ It is only in the period of time between 1920 and 1950 that we have witnessed the transition to a nonrenewable-based economy. A vast amount of renewable carbon is produced in the biosphere.
About 77109 tons are fixed annually, an amount that could supply almost all domestic organic chemical needs, currently about 7–8% of our total nonrenewables consumption. When measured in energy terms, the amount of carbon synthesized is equivalent to about ten times the world consumption. Cellulose, the most abundant organic chemical on Earth, has an annual production of about 109 tons. Yet our chemical feedstock supply is overwhelmingly dominated by nonrenewable carbon. Only about 2% comes from renewable sources, thus, relatively few examples of large-scale industrial bioprocessing exist.
Two notable exceptions are the pulp and paper and the corn wet milling industries. Both convert huge amounts of renewable feedstocks into market products. The experience of these industries would seem to indicate that renewables hold considerable promise as feedstocks, complementary to those used by the chemical industry. However, the pulp and paper industry devotes only a small part of its production to chemicals, while the corn wet milling industry is focused largely on starch and its commercial derivatives, ethanol, and corn syrup.
Several advantages are frequently associated with the bioprocessing of alternative feedstocks:
The use of biomass has been suggested as a way to mitigate the buildup of greenhouse CO2 in the atmosphere. Because biomass uses CO2 for growth through photosynthesis, the use of biomass as a feedstock results in no net increase in atmospheric CO2 content when the products break down in the environment.
It is generally acknowledged that increased use of biomass would extend the lifetime of the available crude oil suppliesc. A chemical industry incorporating a significant percentage of renewable materials is more secure because the feedstock supplies are domestic, leading to a lessened dependence on international ‘‘hot spots.’’.
Biomass is a more flexible feedstock than is crude oil. For example, the advent of genetic engineering has allowed the tailoring of certain plants to produce high levels of specific chemicals. Moreover, increased use of renewable feedstocks could address broader issues: . Global Feedstock Needs: Recent work has attempted to model when world oil production will peak and has concluded that a decline will begin sometime in the next 5–10 years.[18] Demand will not decrease in line with production. The United States' energy consumption has increased by more than 28% (about 21 quadrillions BTU) during the last
25 years, but more than half of the overall energy growth of the last 25 years (about 11 quadrillions BTU) has occurred during the last 6 years. Domestic Energy Consumption: The United States annually consumes about 94 quads of energy. Of this, 35 quads are used by industry in general, almost 8 quads of which are used in the production of chemicals and paper. This is a significant energy target and one that could be addressed by the greater use of renewables.
EXAMPLES OF BIOPROCESSING IN THE PRODUCTION OF CHEMICALS
Conventional Processing
Conventional transformation of renewable feedstocks to products is not common in the chemical industry. In many cases, the products are those that can be isolated from existing renewable resources without further structural transformation. Examples include extractives from the pulp and paper industry used in the production of turpentine, tall oils, and rosins, the production of oils from corn or other oil crops, or starch-based polymers.
Other products are made by simple derivatization of the materials found naturally occurring in biomass. The pulp and paper industry produces a number of chemicals, including a wide range of cellulose derivatives such as cellulose esters and ethers, rayons, cellophane, etc. A few well-known routes exist for the conversion of renewables to low-molecular-weight monomeric products. Glucose is converted to sorbitol by catalytic hydrogenation, and to gluconic acid by oxidation. Furfural is manufactured by the acidic dehydration of corn byproducts. Xylitol is also produced from xylose by hydrogenation.
Vanillin and DMSO have been commercially produced from lignin. Some renewables-based materials under development have promise, but their production is not yet commercialized. These products include levulinic acid and its derivatives; methyl tetrahydrofuran, an automobile fuel extender; aminolevulinic acid, a broad spectrum biodegradable herbicide, insecticide, and cancer treatment; and phenolic acid, a material for the production of polymers and other materials. Levoglucosan and levoglucosenone are products of sugar pyrolysis. Hydroxymethylfurfural is made by acid treatment of sugars. Lignin has also been investigated as a chemical feedstock (for example, in the production of quinones. and has been widely suggested as a component in graft copolymers or polymer blends.
Biocatalysis
The most successful route so far for introducing renewables to the chemical industry has been through biotechnology, although its use for the production of large-volume chemicals is only starting to be realized. Most examples of the use of organisms or enzymatic steps in the production of chemicals have been limited to low-volume, high-value fine chemicals and pharmaceuticals such as oligosaccharides, amino acids, purines, vitamins, nicotine, or indigo.
This is a sensible first application, given the strict structural requirements of many of these specialty materials. Biocatalysts are generally unchallenged in their ability to provide the stereo-, regio-, and enantioselectivity required by these specialty products. The development of more robust biological systems that can operate in extreme conditions (temperature, low water levels, in organic solvents, under high hydrostatic pressure) will also broaden their applicability.
An important new example of industrial biotechnology is the Mitsubishi Rayon process for acrylamide, currently operating on a 3104 metric ton/yr basis by the treatment of nonrenewable acrylonitrile with nitrile hydratase. Several examples of large-scale biotechnological processes are known. Some operations have been used for many years because there is no equivalent nonbiological route. Ethanol and lactic acid are of particular interest because they represent chemicals whose original nonbiological production has been almost totally replaced by biochemical manufacturing. In addition, the pulp and paper industry has started to incorporate enzyme treatments into their pulping and bleaching sequences, and low-lactose milk (up to 250,000 liters daily) is produced by treating milk with b-galactosidase.
CONCLUSION
The United States possesses sufficient renewable resources to supply all domestic organic chemical needs without sacrificing traditional applications of renewables in the production of food, feed, and fiber. Bioprocessing of renewables will play an important role in the future evolution of the chemical industry. Progress in the use of conventional chemical processing and catalysis for the conversion of renewables to products will see significant growth as nonrenewable crude oil feedstocks diminish and the world turns its attention to new carbon sources.