WO1999067409A1

WO1999067409A1 - Method of treating biomass material

Info

Publication number: WO1999067409A1
Application number: PCT/US1999/013915
Authority: WO
Inventors: David L. Brink; Scott Lynn; Michael M. Merriman
Original assignee: The Regents Of The University Of California
Priority date: 1998-06-23
Filing date: 1999-06-21
Publication date: 1999-12-29
Also published as: AU4700199A

Abstract

Hemicellulosic and cellulosic components in biomass material are hydrolyzed in a single-stage digester by using a dilute mineral acid such as sulfuric acid or nitric acid, at a temperature above 200 °C and a residence time of less than ten minutes. Two-stage processes of the prior art are avoided, as are the intervening sensitization stages associated with such processes. Various water-soluble organic compounds, in addition to C5 and C6 sugars, are produced during the digestion process. These organics are separated from the sugar solution by a combination of solvent extraction and distillation, and are recovered as a second product.

Description

METHOD OF TREATING BIOMASS MATERIAL

This invention resides in the field of chemical treatments of hemicellulosic and cellulosic materials, and relates in particular to the hydrolysis of these materials to monosaccharides and other products.

BACKGROUND OF THE INVENTION

Biomass materials that contain hemicellulosic and cellulosic components are generated in large quantities as forest- and wildland-grown materials and as waste from municipalities, agricultural operations, and various processing plants. Examples of such materials are waste wood materials from paper mills, forest waste from clearing brush, logging residues, or other unwanted growth or from dead wood, orchard and vineyard waste, and agricultural waste from crops and grasses. While the presence of these materials is undesirable at the sites where they are generated, the materials are rich in chemical values, and a variety of recovery processes have been developed for converting these materials to useful substances. Of interest in the present invention are hydrolysis processes, in which the lignin, hemicellulosic and cellulosic contents of the biomass materials are converted to sugars, organic acids, furfural, 5-hydroxymethyl- furfural (HMF), acid-soluble lignin (ASL), levulinic acid, and other useful chemicals. At this scale of operation the sugars can be fermented to make anhydrous ethanol, which is used as a component of gasoline for automotive fuel. Paul (United States Patent No. 5,697,987) has shown that the compound 2-methyl tetrahydrofuran, which can be derived from the processing of biomass, can be used, together with ethanol and C₅+ hydrocarbons, to form an alternative automotive fuel. Furfural, HMF, ASL, and other water-soluble organics are similarly useful for the production of fuels, either as such or after further processing, and hence are valuable byproducts in the hydrolysis of biomass by the method of this invention.

Dilute-acid hydrolysis processes for wood and similar biomass materials are of two general types. The first type of process is a percolation process, in which a hot dilute solution of sulfuric acid is percolated through the wood chips in a digester. Solubilized sugars are drawn from the bottom of the digester as a solution. The acid concentration and the temperature are continuously increased during the process so that the more labile hemicelluloses are hydrolyzed and removed in the early part of the cycle, and the less labile celluloses are hydrolyzed at the end. A difficulty with this process is the lengthy percolation time, and the need to use materials of construction that are resistant to sulfuric acid. Also, the process uses lime to neutralize the acid, and this causes the precipitation of calcium sulfate, which becomes less soluble at higher temperatures and causes equipment maintenance problems.

The second type of hydrolysis process is a two-stage process in which the hemicellulosic components are hydrolyzed with dilute acid in the first stage, then washed from the residue, and either enzymes or more acid are added for the second stage to hydrolyze the cellulosic components. Enzymes are costly, however, and require a long residence time, of the order of 24 hours. In either of these two types of hydrolysis process, solids are separated from the liquid between the stages. The result is a complex and capital-intensive process.

SUMMARY OF THE INVENTION

It has now been discovered that biomass material containing both hemicellulosic and cellulosic components can be hydrolyzed to convert both such components to C₅ and C₆ fermentable sugars (pentoses and hexoses), furfural, and HMF, in high yield in a single-stage hydrolysis reaction. A part of any lignin that may be present in the biomass is converted to ASL. The hydrolysis is performed in the presence of a dilute mineral acid at a temperature above 200°C and requires less than ten minutes of residence time, avoiding the expense and prolonged residence time required of enzyme systems and the expense, system cleaning, and disposal problems associated with the use of a two-stage acid reactor system. The hydrolysis of both components occurs in a single- stage digester with no need for intervening separations of solid from liquid, and no need for two reaction stages with different temperatures and different acids or acid strengths. Preferred mineral acids for use in this invention are nitric acid and sulfuric acid, although nitric acid offers the advantage of being less corrosive to metals and less hazardous to plant operators.

Upon leaving the single hydrolysis stage, the sugars are readily concentrated by flashing or other conventional means to remove water. Furfural, HMF, and other organics are recovered by a combination of solvent extraction and distillation. The solids are likewise removable by conventional means. The solid residue from the biomass material is useful as a fuel in a variety of applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of one example of a hydrolysis plant in accordance with this invention.

FIG. 2 is a process flow diagram of a separation system for recovering the C₅ and C₆ sugars and the furfural, HMF, and other organics as separate, concentrated, aqueous solutions.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Biomass material that can be treated in the practice of this invention includes any material that contains both hemicellulosic and cellulosic components that can be converted to fermentable sugars by hydrolysis. Such components are found in a wide variety of sources, many of which are waste materials from industrial plants, farms and agricultural processing operations, woody municipal wastes, landscaping operations, and the like. The invention is of particular interest in the treatment of lignocellulosic material such as "hog fuel" (i.e., a range of particles made from waste wood and bark), tree trunk chips that are a waste product from pulp and paper mills, forest waste such as thinnings, roots, branches and foliage, and orchard and vineyard trimmings. Also of interest are materials such as the stalks and leaves of cotton plants and grasses such as bamboo, rice, wheat, and corn as well as waste rice, wheat and corn, agricultural processing byproducts such as bagasse and hemp, and waste paper such as discarded newspapers, corrugated boxes, and computer printout sheets. The acid used in the process of this invention is generally used as a dilute aqueous solution, and preferably sulfuric or nitric acid. The pH of the solution may vary while still achieving the beneficial effects of the invention. The choice of pH will depend on the types and levels of hemicellulosic and cellulosic materials present in the feed as well as the presence of lignin or other materials that may require liberation from the hemicellulosic and cellulosic materials. A lower pH will generally permit a shorter residence time in the hydrolysis reactor. The quantity of nitric acid that corresponds to the desired pH is between 0.1% and 5% of the bone-dry weight of the biomass in the feed, preferably between 0.25% and 2.5%, and most preferably at about 1%. When sulfuric acid is used, the molar quantities are the same as those for nitric acid. The reaction conditions in the single-stage hydrolysis are selected to be severe enough to cause hydrolysis of both the hemicellulosic and cellulosic components simultaneously. The temperature will thus be above 200°C, preferably from about 200°C to about 250°C, more preferably from about 210°C to about 230°C, and most preferably from about 210°C to about 220°C. A presently preferred temperature is approximately 220°C. The pressure will be elevated to a value at or above the vapor pressure of water at the temperature of the reaction.

The hydrolysis is performed in a digester such as those known for use in pulping mills. Both batch and continuous digesters can be used. Continuous digesters are preferred. One example of a continuous digester is a Pandia digester of the type available from Beloit Pulping, Nashua, New Hampshire, USA, or from Voest- Alpine

Industrieanlagenbau Pulp Biomass Technologies, Austria, or from Black Clawson Co., Fulton, New York, USA. The Pandia digester is a horizontal tube, and typical dimensions are a diameter of 5 feet (1.5 m) and a length of 40 feet (12 m). The solid biomass material and the nitric acid are advanced through the digester by a rotating screw, causing a tumbling motion at elevated temperature. The temperature is typically raised to the desired level by saturated steam. As one example, steam can be fed to the digester at 235°C and 31 atmospheres (all pressures are absolute). Using nitric acid at this temperature and other temperatures within the ranges cited above, the material of construction of the digester can be 304 or 316 stainless steel or other materials that have a similar level of corrosion resistance.

The relative amounts of solid and liquid entering the digester will vary depending on the compositions and relative amounts of the incoming streams, including the water content of the biomass material, the concentration of the nitric acid, the amount of steam added, and any recycle streams that are included. The relative amounts of solid and liquid are not critical to the invention and may vary, and operating conditions can be adjusted accordingly to achieve optimal yields and conversions. In most cases, best results will be obtained with a solids content of from about 10% to about 50% by weight of the slurry, and preferably from about 25% to about 40% by weight of the slurry.

Using the above operating conditions, the combined hydrolysis reactions will occur in a relatively short residence time when compared to prior art processes. High yields and a favorable operation from an economic standpoint based on the cost of equipment and raw materials and the value of the product are obtainable with a residence time in the digester of less than ten minutes. Residence times of about two minutes to about ten minutes are preferred, and the most preferred are those ranging from about two minutes to about five minutes.

The initial hydrolysis products of the hemicellulosic materials are monosaccharides, including both pentoses (C₅ sugars) and hexoses (C₆ sugars). The hydrolysis product of the cellulosic materials is glucose. Each of these products may be further hydrolyzed, decomposed, or otherwise converted, either in the digester or elsewhere in the system. For example, part of the pentoses may be converted to furfural, and part of the hexoses may be converted to water and HMF or levulinic acid plus formic acid. Part of the lignin is converted to ASL in the digester. In subsequent processing, fermentation may be performed, converting one or more of the sugars to ethanol or other alcohols.

To facilitate the recovery of the hydrolysis products from the digester effluent, it is preferable to concentrate the product stream by removing a portion of the water. This is readily done by methods known to those skilled in the art, notably various forms of distillation, ranging from simple, single-stage distillations to multistage distillations in distillation columns. Flash evaporation is particularly convenient, since the slurry emerging from the digester is at high pressure and temperature. The vapor produced by flash evaporation can be used as a source of heat for other parts of the system or for other systems or processes occurring nearby. Flash evaporation may occur in a single stage or in multiple stages, preferably reducing the pressure to near- atmospheric pressure in the final stage. During flash evaporation of the slurry, a major fraction of the furfural in the hydrolysate will be volatilized along with the water. The streams recovered from the flash evaporations are processed to recover the furfural by solvent extraction and/or distillation; a part may be recycled through the process before furfural recovery.

The residue in the bottoms of the last flash drum will contain the concentrated solution of digester reaction products and the hemicellulose- and cellulose- depleted, friable solids. To maximize the recovery of monosaccharides and other reaction products, the solids can be size-reduced to facilitate the extraction and separation of liquids from them. Conventional crushing and grinding equipment will serve this purpose. Examples are gyratory crushers, roll crushers, roll presses, impact breakers, pan crushers, tumbling mills, stirred mills, hammer mills, ring roller mills and disk attrition mills. Hammer mills such as Rietz disintegrators (Rietz Div., Bepex Corporation, Minneapolis, Minnesota, USA), particularly the in-line Rietz disintegrator, are preferred.

Separation of the liquid solution from the solids is then achieved by conventional means, such as decantation or filtration, both under partial vacuum and elevated pressure, screening, flotation, and centrifugation. Centrifugal filtration is a particularly useful method in the process of this invention. The filtration may be combined with a water wash to enhance the recovery of water-soluble materials, and the residual depleted solids can be recovered by conventional means. Further dewatering of the solids can be achieved by conventional means. Examples are disk presses, roll presses, belt presses, filter presses and tube presses.

The liquid solution that is separated from the solids contains C₅ and C₆ sugars, the unflashed furfural, HMF, levulinic acid, ASL and various other organic compounds in minor amounts. HMF, levulinic acid, and ASL have very low volatilities, but can be extracted from aqueous solutions using a relatively volatile, polar, water- immiscible solvent such as 1-butanol. Such a solvent will also extract some of the water, together with residual furfural and some of the other minor organic solutes, but the amount of sugars extracted will be negligible. The solvent and co-extracted water can then be separated from the low-volatility organics by conventional distillation for recycle to the recovery process, leaving furfural, HMF, ASL and other organics, which can be sold as a fuel or as a chemical feedstock. The water separated from the sugar solution both during flash evaporation and solvent extraction may be recycled as wash water to the process or used as boiler feed water, the sugar solution and the non-sugar organics solutions can be recovered as separate products for various uses that will be apparent to the organic chemist. The solids can likewise be recovered as a separate product.

The following example and accompanying flow diagrams are offered for purposes of illustration, and are not intended to limit the scope of the invention in any manner.

EXAMPLE

The flow diagram shown in FIG. 1 and the corresponding stream chart in Table I depict a processing plant for the continuous single-stage hydrolysis of 600 metric tons per day (on a bone-dry basis) of a bark-free, mixed-softwood feedstock consisting of Douglas fir, white fir, and Ponderosa pine.

TABLE I

Stream Flows in kg/min for FIG. 1 for Throughput Rate of 600 Metric Tons of Biomass (bone-dry basis)/Day

The wood chips 11 (Stream A of Table I), which have a water content of 50%) by weight, are mixed with dilute aqueous nitric acid 12 (Stream B) at relative rates such that the HNO₃ content of the acid is 1% by weight relative to the bone-dry wood. The combined stream 14 (Stream D) is fed to a Pandia single-tube digester 13 measuring 1.5 m (5 feet) in diameter and 12 m (40 feet) in length. All parts of the digester (as well as the other items of equipment in the flow diagram) that come in contact with the process stream are made of either 304 or 316 stainless steel, which are inert to dilute nitric acid at the temperatures of the process. The gross feed 14 (Stream D) to the digester is about 33%o solids by weight and is advanced into and through the tube by a screw feeder, which imparts a tumbling motion to the slurry. Steam 15 (Stream K) at 235°C and 30 atmospheres is added to raise the temperature to 210-220°C; the corresponding saturation pressure is 18-23 atmospheres. The residence time in the digester is 5 to 10 minutes.

The hemicellulose is hydrolyzed rapidly in the digester 13 to C₅ and C₆ sugars, primarily C₆ sugars. The digester also converts a large fraction, about 90%, of the C$ sugars to furfural and water. After a residence time of approximately 5 minutes, about 75% of the cellulose in the solids is also hydrolyzed. The cellulose hydrolysis product is glucose, and its yield is equivalent to about 50% of the original cellulose. The remainder of the hydrolyzed cellulose is converted to HMF or levulinic acid, and water. Small fractions of both the furfural and the HMF are further degraded during the hydrolysis. The initial lignin content of the wood is about 28% by weight. During the hydrolysis, a significant fraction of this lignin is converted to soluble organic compounds, primarily ASL.

The digester effluent 16 (Stream E) is flashed to atmospheric pressure in two flash stages 21, 22. Optionally, three flash stages can be used. The steam from the first flash stage 21, although at a relatively low pressure (135°C, 3 atmospheres), has a heat content that exceeds the heat required for distillation and azeotrope-splitting of the alcohol that is subsequently formed by fermentation of the C₆ sugars produced in the digester. The steam 17 is therefore condensed 18 to provide process heat. (The fermentation stage in which this process heat is used is not shown in this flow diagram.) The vapor streams from both flash stages contain furfural at concentrations of approximately 1-2 percent by weight since some of the furfural from the digester is volatilized in these flash stages. This concentration is low enough to permit use of the vapor from the first flash stage 21 for process heating. Once the process heat has been extracted, the condensate 19 (Stream M) is either returned to the process as diluent water (as shown in FIG. 1) or treated to recover the furfural and then used as boiler feed water. The steam 23 (Stream N) released in the second flash stage 22 and in any subsequent stages is of lower pressure than the steam released from the first flash stage 21, and is used in the dissolved organics recovery section described below and shown in FIG. 2. The chips leaving the digester 13 retain much of their original shape but are very friable. During the flash stages 21, 22, the chips disintegrate to some extent into particles. Chips that are still too large for effective washing during the filtration process are pulverized further by directing the chips slurry 24 (Stream G) from the second flash drum 22 to a Rietz disintegrator 25. The disintegrator may be unnecessary however since the chips may be reduced to fine particles by the explosive conditions in the flash stages. The solids content of the slurry 24 (Stream G) leaving the flash stage 22 at atmospheric pressure is 11-12 weight percent, and the C₆-sugar content of the solution is about 1 1 weight percent. The slurry is passed through a centrifugal filtration system 26, which is either an automated basket-type centrifuge or a pusher-type centrifuge, to separate the solids 27 (Stream Y) from the liquid 28 (Stream H), and a water wash 29 (Stream N) incorporated into the system serves to recover further sugars. The used wash water 31 (Stream Z) is drawn off separately. The solids 27 are passed through a Rietz V- Press dewatering unit 32 which produces ligneous solids 33 (Stream J) and further wash water 34 (Stream AA). The wash water streams are combined in a wash water tank 35, then pumped back to the digester 13 as a recycle stream 36 (Stream C). Approximately 95% of the residual liquid 24 (Stream G) from the second flash stage 22 is collected with minimal dilution as the raw sugar solution 28 (Stream H). The remainder is in the recycle stream 36 (Stream C). The solids content of the stream 27 leaving the centrifuge 26 is 25-33%.

Because the solid particles, unlike wood fibers, have a relatively high settling velocity, the higher number may be more accurate. The pressed solids stream 33 (Stream I) contains about 50% solids (by weight). This stream can be fed to a waste-heat boiler as fuel, where the stream will generate substantially more steam than that required as incoming steam 15 by the process. A stream of 33% solids will also make a suitable fuel. The raw sugar solution 28 (Stream H) contains residual unflashed furfural, HMF, levulinic acid, ASL, and small amounts of other dissolved organic compounds. Extraction of sugar from this solution can be achieved in a dissolved organics recovery section 36, an example of which is shown in FIG. 2 and the corresponding stream chart of Table II. TABLE II

Stream Flows in kg/min for FIG. 2

In the dissolved organics recovery section, most of the dissolved organics other than sugar are removed, together with some of the water, by a combination of solvent extraction and distillation. The refined sugar solution produced by this section is then ready for fermentation to produce ethanol. The water that is removed by the unit is returned to the process, while the furfural, HMF, levulinic acid, ASL, and other extracted organics are recovered as a product that may be used directly for boiler fuel or for other uses.

Referring to FIG. 2, the raw sugar solution 28 (Stream H) enters the top of a tray-type solvent extraction column 51 in which the sugar solution flows countercurrently to a lean solvent stream 52 (Stream Q) that enters at the bottom. Suitable solvents are polar, yet immiscible with water and relatively volatile. This type of solvent will form a heterogeneous azeotrope with water. An example is 1-butanol, boiling point 117°C. The units downstream of the extraction column 51 include three strippers ~ a stripper 61 for the extractate 53 (Stream R) leaving the top of the column, a stripper 62 for the raffmate 54 (Stream P) leaving the bottom of the column, and a third stripper 63 to receive the overhead effluents of the first two strippers for further solvent removal. All three strippers remove dissolved solvent from the solvent-water mixtures, and the raffmate stripper 62 produces a finished sugar solution 54 (Stream J). Both the extraction column 51 and the raffmate stripper 62 also remove some of the water originally present in the raw sugar solution 28. As a result, the finished sugar solution 55 is more concentrated than the raw solution 28.

The extractate 53 from the solvent extraction column 51 (Stream R) is a rich solvent stream, and is passed through a steam heater 57 that vaporizes about two- thirds of the solvent before the stream enters the extractate stripper 61. Water vapor 18 released from the second flash stage 22 of the digestion plant of FIG. 1 enters the bottom of the extractate stripper 61 and strips the remaining solvent to produce a stream of dissolved organics 56 (Stream K) which is drawn from the bottom of the column. The steam 18 entering the bottom of the column contains furfural (boiling point 162°C) which is absorbed in the lower section of the column and is included in the dissolved organics stream 56 drawn from the bottom of the column. The dissolved organics 56 (Stream K) contain about 25% water. As an optional addition, part of this water can be removed by placing a reboiler on the stripper. Liquid solvent 52 drawn from the middle of the extractate stripper 61 serves as the solvent feed to the extraction column 51. Since water is more volatile than the solvent in the composition range within the extractate stripper 61, the exiting solvent stream 52 is partially dried.

The vapor streams 71, 72, 73 from the three strippers (Streams T, U, and V, respectively) are combined and flow through a condenser 74 to a decanter 75 where the condensate forms aqueous and organic phases. The organic phase 76 (Stream S) is pumped to the extractate stripper 61, and the aqueous phase 77 (Stream W) is pumped to the third stripper 63 to allow recovery of the dissolved solvent. Emerging from the bottom of the third stripper is solvent- free water 78 (Stream X), which can be returned to the system as wash water, for example to the wash water stream 29 entering the centrifugal separator 26 of FIG. 1.

The foregoing is offered primarily for purposes of illustration. It will be readily apparent to those skilled in the art that the process equipment, flow schemes, flow rates, stream compositions, operating conditions, and other parameters of the process described herein may be further modified or substituted in various ways without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A process for the hydrolysis of biomass material containing both hemicellulosic and cellulosic compounds, said process comprising: (a) combining said biomass material with a dilute mineral acid to form a slurry, and heating said slurry in a single stage hydrolysis reaction to a temperature in the range of from about 200┬░C to about 250┬░C while pressurizing said slurry to a pressure at or above the vapor pressure of water at the highest temperature in the reactor for a residence time of from about two minutes to about ten minutes, to hydrolyze both said hemicellulosic compounds and said cellulosic compounds in said single stage whereby said hemicellulosic compounds are converted to monosaccharides selected from the group consisting of pentoses and hexoses and said cellulosic compounds are converted to glucose: and (b) recovering an aqueous solution of hydrolysis products from said slurry thus hydrolyzed.

2. A process in accordance with claim 1 further comprising recovering the solids from said slurry as a separate product.

3. A process in accordance with claim 1 in which said dilute mineral acid is a member selected from the group consisting of nitric and sulfuric acids.

4. A process in accordance with claim 1 in which said dilute mineral acid is dilute aqueous nitric acid.

5. A process in accordance with claim 1 in which said temperature is from about 210┬░C to about 230┬░C.

6. A process in accordance with claim 1 in which said temperature is from about 210┬░C to about 220┬░C.

7. A process in accordance with claim 1 in which said residence time is from about two minutes to about five minutes.

8. A process in accordance with claim 1 in which said dilute mineral acid is nitric acid and the mass of said nitric acid is from about 0.1 to about 5.0 percent by weight of the mass of said biomass material on a dry basis.

9. A process in accordance with claim 1 in which said dilute mineral acid is nitric acid and the mass of said nitric acid is from about 0.25 to about 2.5 percent by weight of the mass of said biomass material on a dry basis.

10. A process in accordance with claim 1 in which said slurry prior to heating and pressurizing has a solids content of from about 10% to about 75%o by weight.

11. A process in accordance with claim 1 in which said slurry prior to heating and pressurizing has a solids content of from about 25% to about 60% by weight.

12. A process in accordance with claim 1 further comprising recycling a portion of said aqueous solution of hydrolysis products of step (b) to said slurry of step (a).

13. A process in accordance with claim 12 in which said portion of said aqueous solution of hydrolysis products thus recycled is from about 2% to about 25% thereof.

14. A process in accordance with claim 12 in which said portion of said aqueous solution of hydrolysis products thus recycled is from about 3% to about 10% thereof.

15. A process in accordance with claim 12 in which the product of step (a) is a hydrolyzed slurry, and said hydrolyzed slurry is reduced in pressure to release water therefrom as steam, thereby concentrating said slurry, prior to step (b).

16. A process in accordance with claim 15 in which said hydrolyzed slurry is reduced in pressure in a plurality of stages to approximately atmospheric pressure, releasing steam at each stage.

17. A process in accordance with claim 1 in which said biomass material is lignocellulosic material.

18. A process in accordance with claim 1 in which said biomass material is wood material.

19. A process in accordance with claim 1 further comprising separating said aqueous solution of hydrolysis products into (a) a solution of sugars and (b) a solution of non-sugar organics comprising one or more members selected from the group consisting of furfural, 5-hydroxymethylfurfural, acid-soluble lignin, and levulinic acid, and recovering said solutions as separate products.