Cell Mass from Fermenters as Nutrient Source in Biomass-to-Ethanol Conversion

The United States Government has rights in this invention under Contract No. DE-AC02- 83CH 10093 between the United States Department of Energy and the National Renewable Energy Laboratory, a Division of the Midwest Research Institute.

Techniςal Field

The present invention relates generally to introducing cell mass from fermenters as a source of nutrients in biomass-to-ethanol conversion processes. Specifically, the present invention relates to a process for recycling part or all of the effluent stream of fermenters, including distillation residues back to the fermenters to serve as a source of nutrients for the needs of fermentative organisms in biomass-to-ethanol conversion processes; wherein nutrients present in the cell mass and in the surrounding solution retain their nutritive value and can be taken up by the ethanol-producing organisms for cell growth and product formation purposes.

Background Prior Art

In conventional biomass-to-ethanol conversion processes, the design format of the fermenter is such that, various nutrients including carbon, nitrogen, sulfur, magnesium, and phosphorous sources, vitamins, cofactors, amino acids, and trace elements are provided exogenously to support cell growth and propagation with concomitant formation of ethanol. However, these nutrients become incorporated in the cell mass and are discarded at the end of the conventional process. As a consequence, the continuous addition of fresh nutrients to the fermenters increases the operating expenses of the biomass conversion process and contributes significantly to the final cost of the product, ethanol.

U.S. Patent 4,436,814 discloses recovery of enzymes used for the production of fermentation alcohol. The enzyme contained in wash water after solids separation is led back into the first enzymatic degradation stage simultaneously with a portion of the slop coining from the distillation. In essence, streams from the enzymatic treatment tanks and the distillation column contain active amylase enzymes, that are recycled to the front end of the process in order to minimize enzyme supplementation cost.

A process for fermentation of glucose with recycle of non-fermented components is disclosed in U.S. Patent 4,497,896. In this patent, there is disclosed, in particular, glucose fermentation where at least a part

of the distillation residue is recycled as part of the feedstock slurry of starch and acid to a hydrolyzer to allow unfermented feedstock to be hydrolyzed to a fermentable glucose-containing fluid. Thus, a portion of the distillation residue of a starch-to-ethanol plant, which contains carbohydrates, is recycled to the front end of the process in order to hydrolyze the unconverted carbohydrate. U.S. Patent 4,578,353 is also directed to fermentation of glucose with recycle of non-fermented components, wherein at least part of the distillation residue is recycled as part of the feedstock slurry of starch and acid to a hydrolyzer to permit unfermented feedstock to be hydrolyzed to a fermentable glucose- containing fluid.

A process for alcohol recovery by continuous fermentation is disclosed in U.S. Patent 5,036,005, and represents a system that involves the recycling of solvent used in an extraction process. More specifically, this is an extractive distillation followed by vacuum stripping (for solvent regeneration) in order to reduce energy requirements for production of ethanol by continuous fermentation.

U.S. Patent 5,110,319 discloses a process for extracting ethanol from fermentation broths for direct blending into gasoline while preserving cell viability broth for recycling. A solvent that is not toxic to cell mass is used to extract ethanol from the fermentation broth. The solvent-ethanol mixture is blended with gasoline, whereas the cells in the broth are returned to the fermenter to enrich the feedstream.

A process for conversion of biomass to ethanol using the organic acids present in biomass or carbon dioxide recycled from the fermentation units as catalysts for the hydrolysis of biomass is disclosed in U.S. Patent 5,135,861. In this process, a portion of the distillation stillage is recycled to reduce the amount of make-up water used.

In these prior art processes, there is no recognition of the nutritional value of cell mass in such streams, and there is no cognizance of or proposals for recycling of cell mass-rich streams as a means to reduce or minimize nutrient supplementation and/or cost.

Disclosure of Invention

Accordingly, one object of the present invention is to provide a process that introduces the recycling of part or all of the effluent streams of fermenters back to the fermenters to serve as a source of nutrients for the needs of the fermentative organisms in a biomass-to-ethanol process.

Another object of the invention is to provide a method for introducing the recycling of part or all of the effluent streams of fermenters, including the distillation residues, back to the fermenters to serve as a source of nutrients for the needs of the fermentative organisms in a biomass-to-ethanol process.

A yet further object of the present invention is to provide a process for introducing the recycling of part or all of the effluent stream of fermenters back to the fermenters to serve as a source of nutrients for the fermentative organisms, so that nutrients present in the cell mass and in the surrounding solution retain their

nutritive value and will be taken up by the ethanol-producing organisms for cell growth and product formation purposes.

A still further object of the present invention is to provide an improved method for a process of converting hgnocellulosic biomass-to-ethanol, which minimizes exogenous addition of nutrients to the fermenters of the process, wherein recycling the cell mass present in the effluent of the fermenters back to the fermenters is utilized.

A yet further object of the present invention is to provide a reduction of nutrient cost and the volume of process effluents that require waste treatment in a biomass-to-ethanol process by recycling not only the unfermented substrate, but also the attendant nutrients such as carbon, nitrogen, sulfur, magnesium and phosphorous sources, as well as vitamins, cofactors, amino acids, and trace elements.

These and other objects of the invention will be more fully appreciated by referring to the detailed description of the invention.

Detailed Description of Invention Microorganisms (yeasts, bacteria and fungi) useful in the fermentation of the process of the invention are those that are substantially resistant to contamination and capable of withstanding wide variations in pH. By the use of such microorganisms, their tolerance will prevent contamination by other microorganisms in the fermentation broth, and such microorganisms are easily and quickly cultured.

As mentioned, a wide variety of microorganisms may be utilized in the fermentation; however, the microorganisms most commonly used are the yeasts Saccharomyces cerevisiae and Brettanomyces custersii and the bacteria Zymomonis mobilis, Clostridium thermocellum, and Thermoanaerobacter ethanolicus. Numerous strains of S. cerevisiae are available and these numerous strains differ in the amount of ethanol they can tolerate, the rate of fermentation achievable by each, and the type of saccharides they are capable of fermenting. In general, with the use of these microorganisms, the best yields of ethanol are provided within a pH range of about 3.0 to about 7.0 at commencement of fermentation and at a temperature range of between about 20 °C and about 50 °C. Suitable microorganisms will grow adequately on a variety of amino acids, amines, purines and pyrimidines as sources of nitrogen. Moreover, some natural sources of biomass may have adequate quantities of these nutrients to meet the growth requirements of the microorganisms. Preferably, Saccharomyces cerevisiae is the yeast of choice, as its tolerance to wide ranges of pH will eliminate contamination of microorganisms from the fermentation broth. Further, this type of yeast is very easily and quickly cultured.

The biomass starting material for the process of the present invention may be a carbohydrate material, a chemically modified carbohydrate material, a derivative carbohydrate material or mixtures thereof.

These materials will include, but are not limited to cellulose and hemicellulose containing materials, such as energy crops, forestry residues, agricultural residues, municipal solid waste (including waste paper), pulp and paper industry wastes and by-products, and starch-to-ethanol industry water and by-products or combinations. Glucose is produced from the carbohydrate material by enzymatic, chemical, physical, mechanical

(or a combination) treatment (hydrolysis). The hydrolysis fluid in addition to glucose may contain other monomeric and oligomeric sugars. This fluid is led to the fermentation vessels for sugar conversion to ethanol by ethanologenic microorganisms. Ethanol is recovered from the fermenter effluents using distillation or extractive methods with the help of appropriate solvents and/or membrane systems. Part of the fermenter effluent prior to or after ethanol recovery (including distillation) is recycled back to the fermenters to serve as a source of nutrients for the needs of the fermentative organisms. This has been highly beneficial, as the nutrients present in the cell mass and in the surrounding solution of the carbohydrate material retain their nutritive value and are taken up by the ethanol-producing organisms for cell growth and product formation purposes. The ethanol concentrations in the fermentor will be kept below about 15%, and preferably between around 4 to about 9% so that inhibition of the yeast or other ethanologenic organism is avoided. Moreover, the ethanol containing broth may be continuously removed from the fermentator in order to keep low concentrations of ethanol in the fermentator.

The apparatus for carrying out the invention will include a standard fermenter or cascade of fermentors in series or parallel operating in batch, fed-batch or continuous mode, which receives fermentation broth that includes a carbon source and microorganisms which are maintained at conventional fermentation temperatures in the production of ethanol from nutrient broths and microorganisms.

The broth will be a water solution of glucose, other monomeric and oligomeric sugars, enzymes, unconverted polymeric carbohydrates and their derivatives and degradation products, lignin, and its derivatives and degradation products, nutrients, metabolic products including ethanol and cells of the fermentative microorganism.

The broth containing ethanol is led to a distillation column where ethanol is removed by evaporation, or to an extraction column for solvent extraction and then removal by evaporation or ethanol is recovered using membranes. By virtue of the improved method for converting Hgnocellulosic biomass-to-ethanol of the present invention, exogenous addition of nutrients to the fermenters of the process are either minimized or eliminated.

While the invention has been described with respect to certain preferred embodiments, it will be obvious to those skilled in the art that equivalent alterations and modifications will occur upon reading and

understanding the specification, however, the application will include such equivalent alterations and modifications, and is limited only by the scope of the appended claims.