WO2008084490A1

WO2008084490A1 - A process for preparation of ethanol by hydrolysis of celluloses of ligno-cellulosic bio-mass

Info

Publication number: WO2008084490A1
Application number: PCT/IN2007/000139
Authority: WO
Inventors: Mahesh Kumar Khaitan
Original assignee: Mahesh Kumar Khaitan
Priority date: 2007-01-11
Filing date: 2007-04-05
Publication date: 2008-07-17

Abstract

The present invention relates to a process for preparation of ethanol by hydrolysis of celluloses of ligno-cellulose bio-mass which introduces an alkaline pulping stage after the hemi-cellulose hydrolysis & before cellulose hydrolysis to remove lignin. A pre- soaking step is provided in the various hydrolysis and pulping steps for better and thorough hydrolysis of cellulose under milder conditions.

Description

"A PROCESS FOR PREPARATION OF ETHANOL BY HYDROLYSIS OF CELLULOSES OF LIGNO-CELLULOSIC BIO-MASS"

FIELD OF THE INVENTION

5 The present invention relates to a process for preparation of ethanol by hydrolysis of celluloses of ligno-cellulosic bio-mass.

BACKGROUND OF THE INVENTION

Ethanol is increasingly becoming an important substitute and additive to motor fuel 10 (petrol). Currently, most of the ethanol is produced from starches (e.g. corn starch - the biggest source in the U.S.) and sugar (in Brazil). These sources, although well established, have problems that these complete with human/animal food demand and hence are highly priced. In the conversion process, these sources depend upon fossil fuels for energy sources (power and steam). 15

The energy content of ethanol to energy required for its production, is almost equal to or at some time even less. Hence, from bio-fuel point of view, ethanol is strictly not non-polluting .

20 The other potentially important source of ethanol is ligno - cellulosic bio - mass waste like rice straw, wheat straw, corn (maize) straw_s cobs, barley straw and other such waste.

The ligno-cellulosic bio-mass is basically composed of cellulose, lignin and hemi -

25 cellulose in varying proportions. There are other ingredients like ash (inorganic compounds) and other minor organic compounds of very little consequence in the cellulose hydrolysis and ethanol production. The Celluloses are polymers of C₆

(glucose, hexose etc.) sugars while hemi- celluloses are polymers of C₅ (xylose, pentose etc.) sugars. Lignin is an organic complex acting as a protective end binding

30. sheath around the fibrous cellulose. Hydrolysis of cellulose is quite difficult due to lignin sheath, while the hemi - cellulose can be and is easily hydrolyzed using acid as catalyst. The conventional process for the hydrolysis of celluloses of ligno - cellulosic bio - mass recommends acid hydrolysis of hemi - cellulose by mixing cut and shredded straw with about from 10 to 25% ( of straw) of sulphuric acid by weight into the straw and diluting it with appropriate quantity of water. The such mixed mass is then heated to 130 - 150°C under pressurized conditions and maintained (held) at that temperature for 2-4 hours to achieve as near as possible complete hydrolysis of hemi - cellulose. The heated mass is then suddenly released into a vessel maintained at atmospheric pressure. This process is also known as 'steam explosion'. This 'steam explosion' is also understood to loosen lignin sheath from the cellulosic fibrous mass. After separation of hydrolyzed C₅ SUGAR solution, the balance mass is subjected to cellulose hydrolysis either through acid hydrolysis or enzymatic hydrolysis. In acid hydrolysis, the balance mass is mixed with either dilute sulphuric acid (10-15% acid on total mass) and heated a temperature more than 220°C for more than 3 hours to achieve the cellulose hydrolysis and obtain lignin as precipitate or the mass is mixed with 30-60% of concentrated sulphuric acid and heated to 150-170⁰C for 2.5 -3 hours to achieve the cellulose hydrolysis and obtain lignin as precipitate. In enzymatic hydrolysis, the balance mass is inoculated with a saccharifying enzyme and maintained at around 35- 45⁰C (depending upon the properties of enzyme used) for a suitably long period of time (ever running into days) to attain near complete hydrolysis of cellulose and here again obtain lignin as precipitate.

The above process suffers due to interfering nature of lignin resulting in deployment of harsh processing conditions with respect to temperature and time for hydrolysis of cellulose.

The present invention overcomes the above drawbacks b^*y introducing an alkaline pulping stage after the hemi-cellulose hydrolysis & before cellulose hydrolysis to remove lignin. A pre soaking step is provided in the various hydrolysis and pulping steps for better and thorough hydrolysis of cellulose under milder conditions.

The present invention is a pollution free process as it handles all wastes and recovers energy in the process OBJECTS OF THE INVENTION

Principal object of the present invention is to increase the yield due to better conversion of cellulose into sugar.

Another object of present invention is to recover the pulping chemical (caustic soda) for re-use and at the same time generate energy in the form of steam and electric power (for various hydrolysis processes use) and sell it to the electric power grid.

Another object of the present invention is to provide a non-polluting and eco-friendly process.

A yet further object of the present invention is to use post fermentation and distillation 'spent wash' as energy source and eliminate this pollution source.

A yet further object of the present invention is to avoid any premature cellulose hydrolysis.

A yet further object of the present invention is to make the process more, controllable.

STATEMENT OF THE INVENTION

According to the invention, there is provided a process for preparation of ethanol by hydrolysis of celluloses of ligno-cellulosic bio-mass, said process comprises the following steps:

(a) mixing thoroughly cut and shredded ligno- cellulosic bio-mass with 15- 20% w/w of an acid (100% basis) and 200-500% w/w of water and allowing title acid to soak into the bio mass for 10-15 hours,

(b) ■ heating the soaked bio-mass of step (a) to 115-125⁰C₅ holding at that temperature for 60-120 minutes preferably 60-90 minutes, thereby accomplishing hydrolysis ofhemi-cellulose; (c) allowing the soaked bio-mass after hydrolysis of step (b) to cool at a ^■ reasonable temperature and squeezing ' to expel C₅ sugar solution, thereby recovering a squeezed bio-mass;

(d) adding water equivalent to 1.5 times the weight of bio-mass of step (c) mixing thoroughly and squeezing again to expel C₅ sugar solution;

(e) repeating the previous step (d) with water 1 time the weight of squeezed bio-mass of step (d), totally removing and recovering the C₅ sugar (xylose + sugars) solution;

(f) adding the squeezed liquid of step (e) counter - currently as wash diluent repeatedly, mixing and squeezing to obtain concentrated xylose sugar solution having xylose content 12-15%;

(g) raising the pH of the said xylose solution for fermentation to around 4 to 4.5 by treating with lime, generating calcium sulphate as precipitate and filtering out said precipitate;

(h) cooling the neutralized xylose solution to temperature around 35°C and inoculating with suitable yeast (e.g. S. Pemba etc.) for fermentation;

(i) distilling off the ethanol from post fermentation solution through a multi-stage distillation, leaving behind spent wash;

(J) ' subjecting said spent wash which contains around 8-10% solids to a process for recovery caustic soda from black liquors and heat from organics and disposing waste effluent;

(k) mixing the hemi - cellulose free mass with around 10% of caustic soda on w/w basis of the original straw as a 150-250 grams/litter solution for pulping to recover pulped mass; (1) subjecting the said pulped mass to squeezing, diluting, re-squeezing two to three times including continuous counter current operation as described above in step (f); recovering squeezed liquor ; which is black in color containing all of the lignin;

(m) collecting and forwarding the said squeezed black liquors to process for recovery of caustic soda and heat from residue organic matter and disposing waste effluent;

(n) performing hydrolysis of the balance semi-dry mass containing cellulose for recovering hydrolyzed solution;

(o) . raising the pH of the said hydrolyzed solution of step (n) for fermentation to about 4 to 4.5 by treating with lime;

(p) cooling the hydrolyzed solution having (C₆) sugar solution to temperature around 35⁰C without removing the precipitate, inoculating with suitable yeast and fermenting the sugar solution;

(q) distilling said ethanol from post fermentation solution of step (p) through a multi-stage distillation to get ethanol, the remaining spent wash; containing around 8-10% solids.

DETAILED DESCRIPTION OF THIS INVENTION According to one of the embodiment of the invention which comprises the pulping stage after the he^*mi-cellulose hydrolysis and before cellulose hydrolysis to remove lignin. This new additional step helps in achieving following advantages :-

1. By removal of lignin, it 'exposes' the cellulosic fiber mass to catalytic reactions with either acid hydrolysis or enzymatic hydrolysis in a more complete way and thus, get the cellulose hydrolyzed much more easily and completely.

2. Due to ease of hydrolysis, the cellulose can be hydrolyzed under milder conditions of acid addition, temperature required and time duration. A similar advantage would also be applicable towards enzyme hydrolysis in terms of lower enzyme inoculation time needed for hydrolysis.

3. The 'black liquor' containing alkaline lignin in solution form is easily subjectable to a chemical recovery process to recover the pulping chemicals (caustic soda) for re-use and at the same time, generate energy in the form of steam and electric power from residue organic matter (for various hydrolysis process use). In fact, the electric power generated is so much that surplus can be sold to the grid. Such process already exists for which a patent (Indian Patent Application No. 618/DEL/2002 dated 07.06.2002 with -a title "a process for recovery of caustic soda from black liquors and the apparatus thereof) is pending in the patent office.

4. In fact, in 'a process for recovery of caustic soda from black liquors and the apparatus thereof, it would also be possible to use post fermentation and distillation 'spent wash' as energy source as well as a suitable method of elimination of this pollution source.

Further according to the present invention it comprises a pre-soaking stage for hemi- cellulose hydrolysis. In a pre-soaking stage, the cut and shredded straw~or other bio mass is well mixed with the desired quantity of acid along with sufficient quantity of water and left to a soak for a certain length of time (10-15 hours). Then the soaked mass is subjected to elevated temperature (110-125°C) for 90-120 minutes to achieve near complete hemi-cellulose hydrolysis. The milder conditions also prevent any premature cellulose hydrolysis. This pre-soak stage can also be advantageously employed in pulping stage (soak with alkali) and cellulose hydrolysis stage.

The invention constitutes the following steps. These steps are being mentioned as applicable to various agricultural wastes like rice straw, wheat straw, barley straw, millet straw, corn straw, baggasse, fallen leaves, sarkanda and other grasses and other such bio mass. Its applicability to other woody bio mass- like trees, twigs,. etc. would be equally valid. The steps are:- Cutting and shredding of straw into 3-8 mm long pieces. (a) mixing thoroughly cut and shredded ligno- cellulosic bio mass with 15-20% w/w of an acid (100% basis) and 200-500% w/w of water and allowing the acid to soak into the bio mass for 10-15 hours,

(b) heating the soaked bio-mass of step (a) to 115-125°C, holding at that temperature for 60-120 minutes preferably 60-90 minutes, thereby accomplishing hydrolysis of hemi-cellulose;

(c) allowing the soaked bio-mass after hydrolysis of step (b) to cool at a reasonable temperature and squeezing to expel C₅ sugar solution, thereby recovering a squeezed bio-mass;

(d) adding water equivalent to 1.5 times the weight to biό-mass of step (c) mixing thoroughly and squeezing again to expel C₅ sugar solution; ■ . . • .

(e) repeating the previous step (d) with water 1 time the weight of squeezed bio- mass of step (d), totally removing and recovering the C₅ sugar (xylose + sugars) solution;

(f) adding the squeezed liquid of step (e) counter - currently as wash- diluent repeatedly, mixing and squeezing to obtain concentrated xylose sugar solution having xylose content 12-15%;

(h) cooling the neutralized xylose solution to a temperature around 35⁰C and inoculating with suitable yeast (e.g. S. Pemba etc.) for fermentation ; •

(i) distilling off the ethanol from post fermentation solution through a multi-stage distillation, leaving behind spent wash; (j) subjecting said spent wash which contains around 8-10% solids to a process for recovery of caustic soda from black liquors and heat from organic and disposing waste effluent;

(k) mixing the hemi - cellulose free mass with around 10% of caustic soda on w/w basis of the original straw as a 150-250 grams/liter solution for pulping to recover pulped mass;

The pulping is obtained in two ways:- • one way -the mixed mass of step (k) is cooked in a digester at about 160° C for about 3 hours and steam exploded into blow tank.

• second way - the mixed mass is allowed to rest as a pre - soak for 10-15 hours and then autoclaved at 120 - 125 ⁰C for 3-4 hours.

(1) subjecting the said pulped mass to squeezing, diluting it, re-squeezing two to three times including continuous counter current operation as described above hi step (f); recovering squeezed liquor ; which is black in color containing all of the lignin;

(n) performing hydrolysis of the balance semi-dry mass of step (m) containing cellulose for recovering hydrolyzed solution; wherein step (n) the hydrolysis is performed by acid hydrolysis of cellulose and /or by enzymatic hydrolysis of cellulose.

For acid hydrolysis, the balance semi-dry mass of step (m), which is now mostly cellulose, is mixed with 25-35% of sulphuric acid. Water is also added to this mix after taking into account the water already present in the cellulose post pulping squeeze. Enough water is added so that the total water is about 2-3 times the cellulose mass. The acid hydrolysis can again be carried out in two ways. One way - the acidulated mass is heated to 150° and held at that temperature for 4-5 hours to complete hydrolysis . Second way- the acidulated mass is allowed to rest as pre - soak for 10-15 hours and then heated to 125°C for 3-4 hours to complete hydrolysis. It is important to note that the second way although time wise lengthier - offers better control over the process.

For enzymatic hydrolysis, the semi - dry mass from step of step (m) above is brought to enzymatic hydrolysis temperature and pH (the values depending upon the particular enzyme properties) inoculated with a suitable enzyme and rested to hydrolyze the cellulose. . .

(o) raising the pH of the said hydrolyzed solution of step (n) for fermentation to about 4 to 4.5 by treating with lime;

(p) cooling the hydrolyzed solution having (C₆) sugar solution to temperature around 35⁰C without removing the precipitate, inoculating with suitable yeast, fermenting to recover ethanol;

(q) distilling said ethanol from post fermentation of step (p) through a multi-stage distillation still remaining spent wash; containing around 8-10% solids; ^{■ •}

The mixed mass (hemi - cellulose free mass mixed with around 10% of caustic soda or w/w basis of the original straw as a 150-250 grams/liter solution) can be subjected to pulping in two ways:- . ^{■ •} .

• The mixed mass is cooked in a digester at a temperature about 16O⁰C for around 3 hours and steam exploded into a blow tank in one way. or

• The mixed mass is allowed to rest as a pre-soak for 10-15 hours and then autoclaved at Ϊ2O-125°C for 3-4 hours.

The.ligno-cellulosic bio-mass (raw material) may be agricultural waste like rice straw, wheat straw, barley straw, millet straw, corn stalks, bagasse, fallen leaves, sarkanda, • other grasses, other such bio-mass end woody bio-mass like trees, twigs and so on. The present process enables removal of lignin from the ligno-celMosic bio-mass by pre-soaking the bio-mass, whereby it is possible to carry out hydrolysis at a temperature of around 125⁰C in about 3 hours. In prior art process lignin is not 5 removed and therefore those process required heating much higher at temperature of 16O⁰C to 24O⁰C for at least about 4 hours to achieve equivalent results. Hence the present process is simpler and economical in working and is inventive.

■ EXAMPLE

10 A laboratory experiment was performed to demonstrate the effectiveness of the proposed process vis a vis hydrolysis of the hemi-cellulose and cellulose. Market procured cut wheat straw was used as raw material. Accordingly :-

Hemi-cellulose hydrolysis:

15. A. 100 grams of cut wheat straw, having a moisture content of 6% (bone dry straw 94 grams) was put into a 5 liter poly propylene beaker and 11.5 ml concentrated sulphuric acid diluted in 300 ml water was added to it and mixed. The mixture was allowed rest for 15 hours.

20 B. The beaker containing the mixed mass was put in an autoclave over a water bath. The autoclave was set at 15 psig pressure (equivalent to about 121⁰C). The autoclave was turned on and the mixed mass was treated for 90 minutes after attaining temperature 121⁰C. After the time, the autoclave heat was shut down and the mixed mass was allowed to cool.

25 . - _. . . -

C. The cooled mixed mass was squeezed mrough a felt like cloth. The filtered solution obtained measured 450 ml.

D. The semi dry mass left behind was mixed with 500 ml water and squeezed again to 30 . yield further 560 ml solution. The semi dry mass was again mixed with 400 ml water and squeezed to yield 380 ml solution. The solution had a cloudy appearance possibly due to unhydrolyzed hemi-cellulose particles passing through filtering medium. E. 10 ml of above was neutralized to a pH of 7 to 7.5 and it used to titrate against 'Fehling' solution (A & B) and titration compared "with that of a 1% xylose solution. While xylose solution used was 1.9 ml, the solution obtained in D above used was 0.8 ml. Thus xylose percentage in the obtained solution was 1.9/0.8 or 2.375 % indicating

5 a xylose of 33 grams from 100 grams straw.

F. The final squeezed mass was weighed and was found to be 430 grams. 10 grams of it was put into a drying oven and weighed periodically till last three constant weights. The dried solid weighed 1.5 grams indicating a total solids content of 64.5 grams or

10 conversion (hydrolysis) of 35.5 grams of wheat straw hemi-cellulose (37.76 % on bone dry straw). The xylose to hemi- cellulose ratio was 92.9 %.

ALKALINE PULPING

A. 430 grams of the semi -dry mass, which is cellulose - lignin mass from above is 15 . taken in a 5. liter flask. 10 grams of caustic soda is dissolved in 20. ml water to give 200 grams/litters solution end mixed with this mass. The mixture is allowed to rest for 15 hours as in hemi-cellulose hydrolysis.

B. The beaker along with the mass is put into an autoclave as above and heated as 20 above. After attaining temperature, the mass is continuously heated for 4 hours.

C. After the mass is cooled, it is put through squeeze step, yield 180 ml of black liquor. The balance mass is washed on a fire sieve under running water and squeezed semi dry. The solid content of the squeezed black liquor is found (through drying over or 10

25 ml sample) to be 6.5% indicating a solid content of 11.7 grams.

D. The balance semi-dry mass is weighted and found to be 250 grams. The moisture content is found as 80% indicating dry mass of 50 grams, of the loss of 14.5 grams

. (mostly lignin along with same ash), the solid recovery in black liquor is only 10,7 30. grams, part of which is caustic soda as Na₂O .The balance loss is washed off which is recovered in a counter current multi-stage operation. CELLULOSE HYDROLYSIS:

A. Of the 250 grams, 200 grams (solid content 40 grams) is taken in a 3 litters beaker. To it , is added 8 ml of sulphuric acid (equivalent to 14.4 grams on 100% of basis which works out to be around 35.9% on the dry mass) dissolved in 300 ml water.

B. The mixed mass is allowed to rest for 15 hours as above. Then.it is transferred to . the autoclave as above and heated to 121⁰C and kept at that temperature for 3 hour.

C. After cooling, the mass is squeezed filtered to yield 470 ml of C₆ (hexose or glucose) sugar solution. The balance mass, weighting 42 grams, is found to have a solid content of 20% indicating solid content of 8.4 grams, consisting mostly of some un-removable lignin, ash and some un - hydrolyzed cellulose .

D.. 10 ml of this solution is neutralized to 7 to 7.5 and titrated against 'Fehling' solution (A & B) and titration is compared with that of^'1% glucose solution. While glucose solution used is 2.1 ml, the neutralized solution used is 0.3 ml .• Thus the glucose percentage obtained in the solution is 2.1/0.3 or 7% or total glucose content of

32.9 grams. Since only around 85% of the equivalent of straw material is- used (the balance being discarded as measurement and other experimental waste) , this yield tantamounts to 38.7% of total straw (40.3% on bone dry straw) of which the cellulose equivalent would be 38.7/1.11 or 34.86% of straw (36.3% on bone dry straw).

Since many apparently different embodiment and example of the present invention could be made without departing the spirit & scope thereof, it is intended that the description of present invention herein be interpreted as being illustrative only & not limiting in any manner.

Claims

I Claim:

1. A process for preparation of ethanol by hydrolysis of celluloses of ligno - cellulosic bio-mass , said process comprising the following steps :

(a) mixing thoroughly cut and shredded ligno- cellulosic bio-mass with 15-20% w/w of an acid (100% basis) and 200-500% w/w of water and allowing the acid to soak into the bio-mass for 10-15 hours,

(b) heating the soaked bio-mass of step (a) to 115-125⁰C, holding at that temperature for 60-120 minutes preferably 60 -90 minutes, thereby accomplishing hydrolysis of hemi-cellulose;

(d) adding water equivalent to 1.5 times the weight to bio-mass of step (c) mixing thoroughly and squeezing again to expel C₅ sugar solution;

(1) adding the squeezed liquid of step (e) counter - currently as wash diluent repeatedly, mixing and squeezing to obtain concentrated xylose sugar solution having xylose content 12-15%;

(g) raising the pH of the said xylose solution for fermentation to around 4 to 4.5 by treating with lime, generating calcium sulphate as precipitate and filtering out said precipitate; (h) cooling the neutralized xylose solution to a temperature around 35°C and inoculating with suitable yeast (e.g. S. Pemba etc.) for fermentation ;

(j) subjecting said spent wash which contains around 8-10% solids to a process for recovery of caustic soda from black liquors and heat from organics and disposing waste effluent; ■^• .

(k) mixing the hemi - cellulose free mass with around 10% of caustic soda on w/w ^■ basis of the original straw as a 150-250 grams/liter solution for pulping to recover pulped mass;

(1) subjecting the said' pulped mass to squeezing,, diluting, re-squeezing two to three times including continuous counter current operation as described above in step (f); recovering squeezed liquor ; which is black in color containing all of the lignin;

(n) performing hydrolysis of the balance semi-dry mass containing cellulose for recovering hydrolyzed solution; ■ - . . - ^{■ ■ '}

(o) ^•' raising the pH of the said hydrolyzed solution of step (n) for fermentation to about 4 to 4.5 by treating with lime;

(p) cooling the hydrolyzed solution having (C₆) • sugar solution to temperature around 35°C without removing the precipitate, inoculating with suitable yeast and fermenting the sugar solution; (q) distilling post fermentation solution of step (p) through a multi-stage distillation to get ethanol, the remaining spent wash; containing around 8-10% solids.

2. A process for preparation of ethanol by hydrolysis of celluloses according to claim 1, wherein the acid is sulphuric acid.

3. A process for preparation of ethanol by hydrolysis of celluloses according to 1, wherein water added is 200 to 300% w/w of the cellulose mass.

4. . A process for preparation of ethanol by hydrolysis of celluloses according to claim 3, wherein the mixed mass is cooked in a digester at a temperature about 16O⁰C for around 3 hours and steam exploded into a blow tank. .

5. A process for preparation of ethanol by hydrolysis of celluloses according to claim 3, wherein the mixed mass. is allowed to soak' for 10-15 hours as a pre soak and then autoclaved at 120-125°C for 3-4 hours.

6. .A process for preparation of ethanol by hydrolysis of celluloses according to claim 1 , wherein the balance semi-dry mass, which is mostly cellulose, is mixed with 25-35% of sulphuric acid, water is added to this mix taking into account the water already present in the cellulose post pulping squeeze and enough water is added so that total water is around 200 to 300% of w/w the cellulose mass.

7. A process for preparation of ethanol by hydrolysis of celluloses according to claim 6, wherein the acidulated mass is heated to a temperature around 150°C and held at that temperature for around 4-5 hours to complete hydrolysis. ^*

8. _. A process for preparation of ethanol by hydrolysis of celluloses according to claim 6, wherein the acidulated mass is allowed to rest as pre-soak for 10-15 hours and then heated to temperature of around 125°C^'for 3-4 hours to complete hydrolysis.

9. A process for preparation of ethanol by hydrolysis of celluloses according to ^• claim 5, wherein the semi dry mass, which is mostly cellulose, is brought to enzymatic hydrolysis temperature, pH (the values depending upon the particular enzymatic properties) inoculating with a suitable enzyme and rested to hydrolyze the cellulose.

10. A process for preparation of ethanol by hydrolysis of celluloses according to claim 1 , wherein the ligno-cellulosic bio-mass is agricultural waste like barley straw, millet straw, corn stalks, bagasse, fallen leaves, sarkanda, other grasses , or other such bio-mass or woody bio-mass like trees, twigs and the like mixture thereof.