Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Reexamination Certificate
2000-08-16
2002-07-16
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
C162S018000, C162S047000, C162S060000, C162S063000, C162S065000, C162S078000, C162S019000, C162S090000, C127S037000, C426S635000, C530S500000, C536S056000
Reexamination Certificate
active
06419788
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of cellulose from lignocellulosic biomass, and in particular to process whereby cellulose is separated from other constituents of lignocellulosic biomass so as to make the cellulose available as a chemical feedstock and/or accessible to enzymatic hydrolysis for conversion to sugar.
BACKGROUND OF THE INVENTION
The possibility of producing sugar and other products from cellulose has received much attention. This attention is due to the availability of large amounts of cellulosic feedstock, the need to minimize burning or landfilling of waste cellulosic materials, and the usefulness of sugar and cellulose as raw materials substituting for oil-based products.
Natural cellulosic feedstocks typically are referred to as “biomass”. Many types of biomass, including wood, paper, agricultural residues, herbaceous crops, and municipal and industrial solid wastes, have been considered as feedstocks. These biomass materials primarily consist of cellulose, hemicellulose, and lignin bound together in a complex gel structure along with small quantities of extractives, pectins, proteins, and ash. Due to the complex chemical structure of the biomass material, microorganisms and enzymes cannot effectively attack the cellulose without prior treatment because the cellulose is highly inaccessible to enzymes or bacteria. This inaccessibility is illustrated by the inability of cattle to digest wood with its high lignin content even though they can digest cellulose from such material as grass. Successful commercial use of biomass as a chemical feedstock depends on the separation of cellulose from other constituents.
The separation of cellulose from other biomass constituents remains problematic, in part because the chemical structure of lignocellulosic biomass is not yet well understood. See, e.g., ACS Symposium Series 397, “Lignin, Properties and Materials”, edited by W. G. Glasser and S Sarkanen, published by the American Chemical Society, 1989, which includes the statement that “[l]ignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived.”
The separation of cellulose from other biomass constituents is further complicated by the fact that lignin is intertwined and linked in various ways with cellulose and hemicellulose. In this complex system, it is not surprising that the “severity index” commonly used in data correlation and briefly described below, can be misleading. This index has a theoretical basis for chemical reactions (such as hydrolysis) involving covalent linkages. In lignocellulose, however, there are believed to be four different mechanisms of non-covalent molecular association contributing to the structure: hydrogen bonding, stereoregular association, lyophobic bonding, and charge transfer bonding. Bonding occurs both within and between components. As temperature is increased, bonds of different types and at different locations in the polymeric structure will progressively “melt”, thereby disrupting the structure and mobilizing the monomers and macro-molecules.
Many of these reactions are reversible, and on cooling, re-polymerization can occur with deposits in different forms and in different locations from their origins. This deposition is a common feature of various conventional high temperature cellulosic biomass separation techniques. Furthermore, at higher temperatures in acid environments, mobilization of lignin is in competition with polymer degradation through hydrolysis and decomposition impacting all lignocellulosic components. As a result, much effort has been expended to devise “optimum” conditions of time and temperature that maximize the yield of particular desired products. These efforts have met with only limited success.
Known techniques for the conversion of biomass directly to sugar or other chemicals include concentrated acid hydrolysis, weak acid hydrolysis and pyrolysis processes. These processes are not known to have been demonstrated as feasible at commercial scale under current economic conditions or produce cellulose as either a final or intermediate product.
Conventional processes for separation of cellulose from other biomass components include processes used in papermaking such as the alkaline kraft process most commonly used in the United States and the sulphite pulping process most commonly used in central Europe. There are additional processes to remove the last traces of lignin from the cellulose pulp. This is referred to as “bleaching” and a common treatment uses a mixture of hot lye and hydrogen peroxide. These technologies are well established and economic for paper making purposes, but have come under criticism recently because of environmental concerns over noxious and toxic wastes. These technologies are also believed to be too expensive for use in production of cellulose for use as chemical raw material for low value products.
The use of organic solvents in cellulose production has recently been commercialized. These processes also are expensive and intended for production of paper pulp.
Many treatments have been investigated which involve preparating crude cellulose at elevated temperature for enzymatic hydrolysis to sugar. Investigators have distinguished particular process variations by such names as “steam explosion”, “steam cooking”, “pressure cooking in water”, “weak acid hydrolysis”, “liquid hot water pretreatment”, and “hydrothermal treatment”. The common feature of these processes is wet cooking at elevated temperature and pressure in order to render the cellulosic component of the biomass more accessible to enzymatic attack. In recent research, the importance of lignin and hemicellulose to accessibility has been recognized.
Steam cooking procedures typically involve the use of pressure of saturated steam in a reactor vessel in a well-defined relationship with temperature. Because an inverse relationship generally exists between cooking time and temperature, when a pressure range is stated in conjunction with a range of cooking times, the shorter times are associated with the higher pressures (and temperatures), and the longer times with the lower pressures. As an aid in interpreting and presenting data from steam cooking, a “severity index” has been widely adopted and is defined as the product of treatment time and an exponential function of temperature that doubles for every 10° C. rise in temperature. This function has a value of 1 at 10° C.
It is known that steam cooking changes the properties of lignocellulosic materials. Work on steam cooking of hardwoods by Mason is described in U.S. Pat. Nos. 1,824,221; 2,645,633; 2,294,545; 2,379,899; 2,379,890; and 2,759,856. These patents disclose an initial slow cooking at low temperatures to glassify the lignin, followed by a very rapid pressure rise and quick release. Pressurized material is blown from a reactor through a die (hence “steam explosion”), causing defibration of the wood. This results in the “fluffy”, fibrous material commonly used in the manufacture of Masonite™ boards and Cellote™ insulation.
More recent research in steam cooking under various conditions has centered on breaking down the fiber structure so as to increase the cellulose accessibility. One such pretreatment involves an acidified “steam explosion” followed by chemical washing. This treatment may be characterized as a variant of the weak acid hydrolysis process in which partial hydrolysis occurs during pretreatment and the hydrolysis is completed enzymatically downstre
Alvo Steve
Hogan & Hartson LLP
PureVision Technology, Inc.
LandOfFree
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