Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-04-12
2002-09-24
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S180000
Reexamination Certificate
active
06454944
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of methane and other combustible gasses useful for generating power by converting non-toxic biodegradable organic materials and high solids such as sugar, starch and/or carbohydrates into a product gas. The gas is composed primarily of methane, carbon dioxide and hydrogen. Power may be produced by further burning the gas in a boiler or other suitable engine or generator to create electricity. The electricity may be used for operation of the gas-production plant itself, and any excess electricity made available for sale to others where it may be used in engines, cars, trucks, busses, etc. The gas may also be used as fuel for gas fired engine generators for peaker plants. Alternatively, the gas may be scrubbed and sold as clean gas to offset the use of natural gas.
2. Description of the Prior Art
The production of methane and other usable biogases by anaerobic digestion of various organic wastes, particularly sewage sludge organic waste, is well known. The organic feed mixture which provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources. Many digester designs, feed stocks, mixtures and additives have been proposed to increase the methane yield and to provide greater conversion efficiency of organic materials to useful products.
The production of biogas was discovered in the seventeenth century. Today, biogases such as methane are commonly produced at municipal wastewater treatment facilities in a controlled environment using anaerobic digesters. Most of these digesters are completely mixed digesters. Treatment of wastewater in municipal facilities is very expensive, and the cost is covered by charging a fee to residents or businesses for disposing off their waste in the sewer. Anaerobic treatment requires substantial quantities of heat, but this heat may be compensated for by the methane gas produced. There are many variations of anaerobic digestion that are very successful and very basic. The conversion of organic materials into biogas is performed by the microorganisms in the digester. These microorganisms do the work so long as the organic content and temperature of the digester are maintained within certain ranges.
Anaerobic filter-type reactors promote the retention of bacteria in the digester by attaching bacteria to fixed inert materials in the digester. Anaerobic filter-type digesters are also limited to primarily liquid feedstocks containing less than about 1 percent (1%) solids since they become plugged when solids concentration in the digester increases due to higher solids loading or accumulation of solids over longer periods of operation.
Horizontal plug flow digester designs have been implemented, but horizontal plug flow reactors are limited to use of homogeneous solids feed materials (such as manure), which do not tend to settle by gravity. The horizontal plug flow reactor design encourages rapid disengagement of gas from the liquid phase. Horizontal plug flow reactors generally have poor conversion efficiencies of the biodegradable fraction, on the order of about 40 to 60 percent due to biologically unreactive zones within the digester, short circuiting of the feed material, and bacterial washout.
Anaerobic digestion of terrestrial plant material to produce methane gas been recognized as exemplified by D. L. Klass and S. Ghosh, “Methane Production by Anaerobic Digestion of Bermuda Grass,” presented at Symposium on Biomass as a Non-Fossil Fuel Source, ACS/Chem. Soc. of Japan Joint Chemical Congress, Honolulu, Hi., Apr. 1-6, 1979. Likewise, the anaerobic digestion of aquatic plant material to produce methane has been recognized as exemplified by R. P. Lecuier and J. H. Marten, “An Economic Assessment of Fuel Gas from Water Hyacinths,” Symposium papers, Clean Fuels from Biomass, Sewage, Urban Refuse, Agricultural Wastes, Orlando, Fla., Jan. 27-30, 1976.
U.S. Pat. No. 4,329,428 teaches production of methane gas in higher yields and at higher rates by thermophilic and mesophilic anaerobic digestion of a mixture of plant material of terrestrial or aquatic origin and organic waste. U.S. Pat. No. 4,424,064 teaches production of methane gas with higher yields and at higher rates by thermophilic or mesophilic anaerobic digestion of aquatic plant material, at least a portion or all of which has been grown in organically polluted water. U.S. Pat. No. 4,316,961 teaches higher yields of methane gas at higher rates by thermophilic or mesophilic anaerobic digestion of plant material and/or organic waste of normally low biodegradability in the presence of an extract of different plant material.
An Upflow Anaerobic Sludge Blanket (UASB) process has been developed for bioconversion of feedstocks which contain primarily soluble organic waste wherein small amounts of solids, ordinarily less than 1 percent of the feedstock. The bacterial mass are allowed to settle in the reactor. The Upflow Anaerobic Sludge Blanket process and reactor are described in the following publications: G. Lettinga, et al, “Anaerobic Treatment of Methanolic Wastes,” Water Research, Vol. 33, pp. 725-737, Pergamon Press Ltd. 1979; and G. Lettinga, et al, “Upflow Sludge Blanket Processes,” 3rd International Symposium on Anaerobic Digestion, 1983, Cambridge Mass.; and G. Lettinga, et al, “Anaerobic Treatment of Raw Domestic Sewage” at Ambient Temperatures Using a Granular Bed UASB Reactor, Biotechnology and Bioengineering, Vol. XXV, pp. 1701-1723, 1983. This reactor design is limited to liquid feedstocks containing less than about 1 percent solids, and it requires effective gas/liquid separators, recycle for bed expansion, and means for distributing the feed over the bottom of the reactor.
Continuous flow fluidized bed fermenters embodying a tower design or a supported film reactor are described in G. F. Andrews, “Fluidized-Bed Fermenters: A Steady-State Analysis,” Biotechnology and Bioengineering, Vol. XXIV, pp. 2013-2030, 1982. This article teaches that stratification tends to occur in tower fermenters, and solids concentration varies along the height of the tower fermenter, with a low cell concentration in the upper parts of the tower fermenter leading to a low volumetric productivity.
U.S. Pat. No. 4,208,279 teaches anaerobic digestion of animal waste which is fed to the top and one end of an unstirred digestion volume which is about five times as wide as it is high. Effluent sludge is removed at the opposite end of the reactor. Solids movement in the digester is essentially horizontal and the liquid volume is not agitated, except by gas formation. Suitable solids residence times are one month and over, and the solids feed concentration is about 5 percent.
U.S. Pat. No. 4,311,593 teaches anaerobic digestion of waste water in a digester volume which is about four times as wide as it is high. Microorganisms are stabilized on high surface area media. Agitation of the microorganism biomass on the media is provided by gas formation bubbling up through the reactor liquid. U.S. Pat. No. 4,388,186 teaches mechanical condensation of sludge prior to anaerobic digestion of sludge in a vertically elongated, stirred digester tank. The '186 patent also teaches conducting an acid fermentation stage and an acid regression stage separately prior to carrying put an alkaline fermentation stage in the elongated, stirred digester tank. U.S. Pat. No. 1,806,698 teaches a sludge digester wherein solids collect at the bottom. Supernatant liquid accumulating in the upper portion of the digester is recycled to the surface of the digester contents to reduce foam scum. U.S. Pat. No. 1,880,773 also teaches anaerobic digestion of sewage sludge in a digester wherein solids settle to the bottom of the tank. Liquid supernatant from the upper portion of the digester is recirculated to prevent the accumulation of scum or foam at the top surface of the digester contents.
SUMMARY OF THE INVENTION
The present invention provides both a process and apparatus for supplying one or more digesters with
Barry Chester T.
Miller Mark D.
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