Patent Application
20060177916
This invention relates to citrus waste processing and, more particularly, a process for the conversion of simple and complex carbohydrates contained in citrus waste into ethanol for use as bio-fuel and to yield other high-value byproducts.
Currently, the amount of citrus waste, consisting primarily of peel, membranes, and seeds, which result from processing citrus fruit for juice, is an environmental problem. The problem exists particularly in areas where the bulk of citrus is grown to produce juice, such as in the State of Florida and the country of Brazil. For example, in 2003 Florida had approximately 103 million citrus trees on 800,000 acres and produced 297 million boxes of citrus, 85% of which was processed into juice. The waste from such processing was approximately one-half of the citrus fruit, yielding approximately 5 million tons of wet waste, which reduces to 1.2 million tons of dry waste.
Traditionally, such waste has been converted into cattle feed, which currently does not have sufficient value to cover the production and transportation costs associated therewith. A further drawback of converting current waste into cattle feed is that the waste contains a high amount of d(+)-limonene (referred to simply as limonene). Volatilization of the limonene during the drying process causes air pollution to the extent that limonene vapors are exhausted into the atmosphere at the processing plants because it would require very expensive equipment to trap the limonene from the drier exhaust. Although citrus waste materials do create an environmental problem, these materials are rich in pectin and other polysaccharides that can be hydrolyzed into sugars for use in the production of ethanol.
Currently ethanol is used as a bio-fuel that is mixed with gasoline to increase the octane rating and improve the environmental characteristics of gasoline. Although another gasoline octane enhancer referred to as MTBE (Methyl Tertiary Butyl Ether) is also used, MTBE is controversial since it is believed to result in ground water pollution and is not biodegradable. Field corn (maize) is currently the primary feedstock for ethanol production in the USA. As the State of Florida has no cultivation of field corn, Florida must look to other sources for producing ethanol. The conversion of citrus processing waste into ethanol would reduce waste and provide a regional source of ethanol as a viable alternative octane enhancer to MTBE. The conversion of citrus processing waste in 2003 of approximately 5 million tons could result in potentially 100 million gallons of ethanol.
Unfortunately, one of the major problems that prevents processing citrus waste into ethanol is limonene. Limonene is a terpene-based liquid that is contained in citrus peel. Limonene provides a natural defense for the fruit against bacteria, viruses, molds, and other organisms. Accordingly, limonene protects the citrus waste from microbial buildup and fermentation by normal processes that would yield ethanol. It is also desirable to recover the limonene as a high value co-product. For efficient fermentation, limonene in the citrus waste must be reduced to a level below 3000 parts per million (preferred level below 1500 ppm). Thus, a need exists for processes that will decrease the amount of limonene in citrus processing waste in order to produce ethanol for use as a bio-fuel and other high value products, including cattle feed, limonene, five carbon sugars and galacturonic acid monomers and polymers.
The new processes disclosed herein, for processing citrus waste to ethanol, utilize enzyme mixtures of pectinase, hemicellulases, cellulases and beta-glucosidases for efficient hydrolysis of the complex carbohydrates in citrus waste residue into simple sugars. Two different processes, steam stripping or centrifuging, can be used to lower the limonene content in the citrus waste to a sufficiently low level whereby fermentation of the waste can efficiently produce ethanol. The fermentation utilizes traditional ethanol producing yeast, E. coli strain KO11, or other bacteria or fungi, followed by distillation to recover ethanol. The solids residue remaining may still be utilized as a cattle feed product and will have higher protein content than the citrus-based cattle feed currently being produced. The residue after distillation may also be pressed and filtered with optional recovery of acetate, five carbon sugars, or galacturonic acid monomers/polymers from the filtrate. Both jet cooking and centrifugation processes work more efficiently if the raw citrus processing waste is ground to a particle size of less than one inch (preferably less than one-half inch) using a hammer mill, grinding pump or similar shredding/chopping/grinding apparatus capable of handling and reducing said waste to the required size. A progressing cavity pump or similar pump (or conveyor) capable of pumping/moving raw or ground peel slurries with dry solids content up to thirty-five percent is then used to feed and mix the high viscosity mixture during the enzymatic hydrolysis and fermentation.
Once the particle size of solids in the raw citrus waste is reduced to a size sufficient for further processing, then in a preferred processing embodiment the ground peel is first heated to a range of 60°-240° Celsius (preferred range 90°-190° C.) by steam injection, passage through a heated hollow shaft screw conveyer, or other direct or indirect heating device. Heating by steam injection or extrusion has the benefit of a simultaneous or sequential shearing and disintegration action which is beneficial to the hydrolysis process. The heating causes the limonene content to be decreased through evaporation and steam stripping. The limonene is then recovered by condensation of the removed steam and decanting (or centrifuging) the recovered liquid. The citrus waste solids slurry is then cooled and adjusted for pH, followed by simultaneous hydrolysis and fermentation using an enzyme mixture and fermentation organisms such as yeast, E. coli strain KO11, or other bacteria or fungi, all while being continually mixed using high solids pumps or high solids mixers. After fermentation, the ethanol is separated by distillation and the resulting residue can then be pressed and dried for use as cattle feed or further processed with fermentation using E. coli KO11, to produce more ethanol and acetate, or the unfermented galacturonic acid monomers/polymers and five carbon sugars may be recovered as additional products.
In another embodiment of the process, the ground citrus peel is first directly hydrolyzed using an enzyme mixture (the enzymes are not significantly inhibited by the limonene) with controlled pH and temperature levels to maximize simple sugar content and then the limonene content is lowered using either a decanter (or tricanter) centrifuge or filtration device to remove the solids which are high in limonene content. Recovery of limonene from the solids cake or filtrate is accomplished by solvent extraction, or alternatively by steam stripping as described in the preferred embodiment described above. The liquid solution obtained from the centrifuge or filtration process is high in sugars and low in limonene content. The solution is adjusted for pH and temperature, and fermentated using either traditional fermentation yeast, genetically engineered E. coli KO11, or other microorganisms to produce ethanol. The ethanol is separated by distillation. Following fermentation and distillation, the resulting residue may be pressed and dried for use as cattle feed with optional recovery of acetate, five carbon sugars, and galacturonic acid monomers/polymers.
The relevant prior art includes the following patent documents:
None of the above patents discloses a process like the present invention for yielding ethanol and other byproducts from citrus processing waste.
The primary object of the present invention is to provide a system and method of producing ethanol from citrus processing waste.
Another object of the present invention is to increase the recovery of limonene from citrus waste in order to reduce pollution from limonene that results when raw citrus waste is converted into cattle feed by current drying processes.
A further object of the present invention is to increase the recovery of limonene from citrus waste in order be sold as a high value byproduct.
A further object of the present invention is to provide such a process that produces ethanol and byproducts for a lower cost than ethanol produced from corn.
An even further object of the present invention is to provide a process that yields other byproducts including five carbon sugars, galacturonic monomers/polymers, and a citrus based feed product for cattle and pets that has higher protein content and value than the citrus based cattle feed made from current processes.
The present invention fulfills the above and other objects by providing a system and method for producing ethanol from citrus waste that reduces limonene in the citrus peel in order that fermentation can take place to yield ethanol. This system includes means for reducing the particle size of citrus waste solids in a citrus waste slurry to a predetermined size when necessary for processing, utilizing a hammer mill, grinding pump or similar shredding/chopping/grinding apparatus.
Limonene is then removed using one of two techniques:
After the limonene has been reduced to a sufficiently low level, fermentation is accomplished in a fermentation tank using yeasts, E. coli/KO11, or other ethanol producing organisms such as fungi, E. coli, or Z. mobilis, and enzymes which may be mixed by circulation with high solids pumps or high solids mixers. Finally, ethanol can be distilled from the fermented citrus waste/beer. Optionally and additionally, the resulting residue can be further processed into solids and pet or cattle feed using a centrifuge and/or press and drying devices. Furthermore, the residue may also yield acetate, galacturonic acid monomers and polymers, and five carbon sugars.
The pH of the citrus waste is controlled throughout the process in the range of pH 1 to 13 (preferred range pH 2 to 11) by addition of acids/bases to optimize the hydrolysis by enzymes and/or cooking and to optimize fermentation outputs.
The above and other objects, features and advantages of the present invention should become even more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.
In the following detailed description, reference will be made to the attached drawings in which:
For purposes of describing the preferred embodiment, the terminology used in reference to the numbered components in the drawings is as follows:
With reference to the drawings, a preferred embodiment of the ethanol production process is shown in
In
