Production of ethanol by fermentation

The production of ethanol, particularly ethanol used as a fuel, optionally in conjunction with gasoline (petrol) may be produced from carbohydrate containing material, such as starch-containing material or sugar-containing material, by a solid phase fermentation process.

This invention relates to the production of ethanol particularly ethanol 
for use as a fuel, by fermentation of a sugar-containing material or of a 
starch-containing material, or of a material containing both substances. 
In particular, it relates to the production of fuel ethanol in high yields 
by a process which is economical on a much smaller scale than is the case 
with existing technology and which requires only the use of simple and 
relatively inexpensive equipment. 
Attention has recently been focused on the use of ethanol, particularly 
when blended with petrol (or "gasoline"), as a fuel for internal 
combustion engines of motor vehicles. The conversion of sugar into ethanol 
by yeast fermentation is well known, and many sugar-containing materials 
have been investigated for use in this method of production of ethanol. In 
general, these processes are based on the initial production of a 
sugar-containing liquid, followed by liquid-phase yeast fermentation 
thereof. 
Sugar beets are a well-known and widely-used source of sugars, particularly 
sucrose, and in one known process for the extraction of sugar therefrom, 
the beets are sliced into long thin strips or "cossettes" prior to 
extraction of sugar therefrom by a diffusion process. The cossettes are 
then fed to a continuous sloped diffuser through which they are carried 
upwardly from the lower end. Hot water is fed to the diffuser at the upper 
end, flowing down counter-current to the direction of cossette movement 
and leaving the lower end of the diffuser as a sugar-containing liquid. 
As far as ethanol production from starch containing materials is concerned, 
the normal method starting with starch crops is to mill and cook the 
starchy material to gelatinise the starch, to liquefy and hydrolyse the 
starch to sugars with malt, fungi or enzymes, and then to ferment the 
sugars to alcohol by means of liquid-phase yeast fermentation. 
It is an object of the present invention to provide an improved process 
whereby ethanol, particularly fuel ethanol, can be produced in high yield 
from a sugar-containing material, particularly, sugar beet, sugar-cane, 
fodder beet, mangolds, or the like, without the necessity of prior 
extraction of the sugar from the sugar containing material. It will be 
appreciated that if the necessity for prior extraction of the material can 
be avoided, the overall process of ethanol production will be simplified 
and made more economic. 
It is a further object of the present invention to provide an improved 
process for producing ethanol, particularly fuel ethanol, from 
starch-containing crops such as vegetables and cereal grains, for example 
potatoes, cassava, wheat, barley, corn, triticale, grain sorghum, 
vegetable waste and the like without the necessity for prior cooking of 
the starch-containing material or for separately converting starch into 
sugar prior to the fermentation of the sugar, or for the prior extraction 
of the sugar produced by the starch. 
According to the present invention there is provided a process for the 
production of ethanol by fermentation of a carbohydrate-containing 
material, comprising 
(a) crushing or pulping said carbohydrate-containing material to produce a 
pulp containing substantially no free liquid and comprising particles of 
said material having diameters of the range up to about 10 mm; 
(b) where necessary saccharifying and, if desired, heating said pulp to 
convert non-sugar carbohydrates in said material to sugars; 
(c) mixing a suspension of yeast with said pulp (simultaneously with or 
subsequent to said saccharification, if used) and maintaining said mixture 
under fermentation conditions to allow said yeast to convert sugars in 
said pulp to ethanol; and 
(d) extracting ethanol from said fermented pulp. 
According to a first aspect of the present invention there is provided a 
process for the production of ethanol by fermentation of a 
sugar-containing material which comprises: 
(a) crushing or pulping said sugar containing material to produce a pulp 
containing substantially no free liquid and comprising particles of said 
material having diameters of the range up to about 10 mm; 
(b) mixing a suspension of yeast with said pulp; 
(c) maintaining said mixture under fermentation conditions to allow said 
yeast to convert sugar in said particles to ethanol; and 
(d) extracting ethanol from said fermented pulp. 
According to the second aspect of the present invention, there is provided 
a process for the production of ethanol by fermentation of a 
starch-containing material, which comprises: 
(a) crushing or pulping said starch-containing material to produce a pulp 
containing substantially no free liquid and comprising particles of said 
material having diameters of the range up to about 10 mm; 
(b) saccharifying and, if desired, heating said pulp to convert starch in 
said material to sugars; 
(c) simultaneously with or subsequent to said saccharification, mixing a 
suspension of yeast with said pulp and maintaining said mixture under 
fermentation conditions to allow said yeast to convert sugars in said pulp 
to ethanol; and 
(d) extracting ethanol from said fermented pulp. 
Preferably, in both aspects of the present invention the ethanol is 
extracted from the fermented pulp by pressing or squeezing the pulp to 
express the ethanol-containing juice. The juice also contains most of the 
yeast from the fermented pulp, together with some fine fibres from the 
sugar-containing material or the starch-containing material. The yeast and 
fibres may be separated from the juice by known methods such as filtration 
or more preferably centrifugation, and, then may be recycled to the 
fermentation stage if desired. It is to be noted at this stage that the 
recycling of the fine fibres recovered with the yeast to the fermentation 
stage does not give rise to any problems since the fibre volume is small 
and should reach an equilibrium value in a fairly short time. 
Essentially, the process of the second aspect of the present invention, 
directed towards the use of starch-containing starting material, comprises 
a further development of the process of the first aspect of the present 
invention directed to the use of sugar-containing material as the starting 
material, in that the further development comprises the additional process 
step of saccharifying or hydrolysing the starch in the starch-containing 
material to sugars prior to or simultaneously with fermentation. 
Saccharification or hydrolysis may be carried out by any known process, 
for example by acid hydrolysis, however, it is preferably carried out by 
enzymatic means, particularly by the addition of amylase. If desired, the 
rate of saccharification of the starch may be promoted by heating the pulp 
during this process step. 
The saccharification or hydrolysis of starch to sugars by chemical or 
enzymatic means is, of course, well known and further description of this 
step at this stage is considered unnecessary. As noted above, the 
saccharification of the starch-containing material may be performed prior 
to the yeast fermentation of the sugars. However, the simultaneous 
saccharification and fermentation of the starch-containing material offers 
distinct advantages from the point of view of simplification of the 
process technology. In addition, such simultaneous performance of these 
steps may be advantageous in that the fermentation of sugars as they are 
produced may assist in the more complete saccharification of the starch. 
It is an important aspect of the present invention that the sugar 
containing material or the starch-containing material or material 
containing both sugar and starch be crushed or pulped to produce small 
particles of up to about 10 mm. diameter, however the precise shape and 
thickness of the particles is not essential to this invention. Where the 
sugar-containing material is sugar beet or fodder beet, it is preferred 
that the particles be of diameter up to about 5 mm., however, the particle 
size can be varied as desired for other sugar-containing materials, and 
any suitable pulping or crushing equipment may be used in order to effect 
this pulping of the sugar-containing material. 
Since the pulp contains little or no free liquid, it is generally, as a 
rule, relatively stiff in consistency and cannot flow of its own accord. 
Nevertheless, it has been found that in accordance with the present 
invention it can be added directly to a fermenter so as to effect a "solid 
phase" fermentation of the pulp. The term "solid phase" fermentation as 
used in the present specification is generally used to describe microbial 
attack, usually by fungi, on moist solid particles. The term is used for 
the process of the present invention since it involves fermentation of 
moist solid particles without the addition of further liquid (apart from 
the very small amount associated with the acid and yeast suspension). It 
has been found that baker's yeast was able to ferment the sugars in pulped 
sugar crops without the need for prior extraction of the sugar or 
agitation of the pulp. Moreover, the solid phase fermentation proceeded 
more rapidly than those in the liquid phase and the yield of ethanol was 
about the same. 
Preferably, in order to achieve appropriate conditions for fermentation of 
the pulp, an acid such as sulphuric acid or an alkali may be added to the 
pulp to adjust the pH to a range of between about 4 and 6. Preferably, the 
pH is adjusted to about 4.5 in the case where sugar-containing starting 
material is used in the process of the invention. Prior sterilization of 
the raw pulp is not required. 
As noted above, a suspension of yeast is added to the pulp and, by way of 
example, the suspension may contain up to 20% by weight of yeast, 
preferably about 10%. Preferably, 10 gm dry weight of yeast per kg of wet 
pulp is added to the pulp. Typically, in the production of fuel ethanol by 
this process, the yeast used is the strain Saccharomyces cerevisiae in the 
form of active dry baker's yeast or compressed yeast. It is to be noted 
that high alcohol tolerant strains of yeast or thermophilic strains of 
yeast may be used. The proportion of yeast suspension added to the pulp 
will depend upon the rate of fermentation required and can be adjusted as 
desired. Growth nutrients for the yeast may also be added to the mixture, 
however it is found that in many instances such nutrients will not be 
necessary. In addition, enzymes such as pectinases or cellulases to 
hydrolyze pectin and cellulose, respectively, in the pulp may be added if 
desired. The yeast suspension is then well mixed with the pulp together 
with any other materials added to the fermenter and the mixture maintained 
under fermentation conditions. Preferably, these fermentation conditions 
include a temperature of about 25.degree.-50.degree. C. in order that the 
fermentation will proceed rapidly. Although water contained in the yeast 
suspension is added to the pulp, it is found that this is taken up by the 
pulp and does not remain in the mixture as free liquid. 
On an industrial scale, mixing of the pulp and yeast suspension in the 
fermenter and heating of the mixture (or cooling as required) may be best 
achieved by circulating the pulp through an external heat exchanger with a 
suitable pump. Throughout the fermentation, the contents of the fermenter 
remain substantially solid and at no stage are able to flow on their own 
accord. Nevertheless, the particle size of the pulp is sufficiently small 
to make it readily pumpable. 
On completion of the fermentation, the fermented pulp is treated to extract 
the ethanol therefrom. In one such method for extraction of the ethanol 
which is suitable for use in practice, the fermented pulp is pumped to a 
fibre separator where the alcoholic juice is squeezed out of the pulp. It 
has been found, for example, that the use of a two-stage roll with a small 
water wash between the stages can remove about 95% of the ethanol from the 
fermented pulp. By way of comparison, the same treatment applied to raw 
pulp prior to fermentation has been found to remove only about 65% of the 
sugar from the raw pulp. 
As noted above, the alcoholic juice obtained by squeezing of the fermented 
pulp is found to contain almost all of the yeast, together with some fine 
fibres. Both the yeast and the fibres can be recovered by centrifuging the 
juice, and the yeast thereby recovered may be recycled to the fermenter, 
together with some fresh yeast. The liquor obtained after centrifuging the 
alcoholic juice may be distilled in a fractionation column to obtain the 
95% ethanol azeotrope in the usual way. Since very little, if any, water 
has been added throughout the process of the present invention, and the 
liquor distilled in the fractionation column is free of suspended solids 
due to the prior contrifuging step, the volume and BOD of the waste liquor 
or "slops" from the distillation column are relatively low, hence reducing 
effluent and environmental problems.

The attached block diagram schematically illustrates by way of example, the 
steps of a process in accordance with the present invention. The process 
is further illustrated by the following Examples. 
EXAMPLE 1 
Effect of Particle Size and Yeast Concentration 
Sugar beet was pulped in a Bauer defibrator to two particle sizes, one 
about 3 mm diameter (referred to as "coarse" pulp) and another about 0.5 
mm diameter (referred to as "fine" pulp). The dry matter content was found 
to be 28.9% in the coarse pulp and 30.8% in the fine pulp. Both were 
acidified with a little dilute sulphuric acid to lower the pH from the 
usual value of 6.4 to about 4.5, the optimum value for the alcohol 
fermentation. No nutrients or enzymes were added in this experiment. 
50 gm samples of the coarse (17.0% sucrose) and fine (18.9% sucrose) pulp 
were weighed into 250 ml Erlenmeyer flasks and various volumes of yeast 
suspension (active dry baker's yeast or compressed yeast made up to 40 
gm/liter in distilled water) were mixed thoroughly with each sample so as 
to give final yeast concentrations of 3,6 and 9 gm/liter. The flasks were 
then fitted with a stopper and gas release valve, weighed and incubated at 
30.degree. C. for a total of 29 hours, the flasks being weighed at 
intervals to check the loss of weight due to the escape of carbon dioxide 
produced by the fermentation. 
The results showed that the fermentation was completed in 38 hours with 3 
gm/liter yeast, 24 hours with 6 gm/liter and 16 hours with 9 gm/liter. The 
use of dry or compressed yeast made no noticeable difference to the rate 
of the fermentation but a more dilute yeast suspension does make it easier 
to mix thoroughly with the pulp. The water contained in the yeast 
suspension is readily soaked up by the pulp, so there is no change in its 
consistency or any evidence of free liquid separating during the 
fermentation. 
The fermented pulps were distilled under vacuum and the ethanol contents 
measured by gas chromatography. It was found that with a yeast 
concentration of 6 gm/liter, the yield of alcohol was 4.2 gm per sample 
(91-92% of the theoretical yield) with the coarse pulp and 4.0-4.3 gm 
(79-85%) with the fine pulp. Some variability in the results was 
experienced due to small size of the samples tested, but the coarse pulp 
was clearly no worse than the fine pulp, despite the lack of fluidity and 
mixing of the pulp solids after the start of the fermentation. 
EXAMPLE 2 
1 kg of coarse sugar beet pulp was acidified with dilute sulphuric acid to 
lower the pH to 4.5. Again, no nutrients or enzymes were added. Then 9 gm 
(dry weight) yeast in 100 ml water was added to 955 gm pulp (22.59% 
solids, 8.92% sucrose) and mixed well, the water being soaked up by the 
pulp to leave substantially no free liquid in the mixture. The mixture was 
allowed to ferment in a 2 liter Erlenmeyer flask fitted with a water 
condenser and the volume of carbon dioxide evolved was measured with a wet 
test gas meter (35.1 liters measured). As a check on the gas volumes, the 
loss of weight of the flask contents was measured periodically over a 
period of 20 hours. Of an initial weight of 1094.1 gm, 72.4 gm was lost 
during the fermentation, leaving 1021.7 gm fermented pulp. A 50.8 gm 
sample was taken to check the alcohol content by distillation and a 933.4 
gm sample was pressed in a rubber-covered steel rolls under heavy pressure 
so as to squeeze out the alcoholic juice. Of this later sample, 451.4 gm 
was collected as juice and 448.7 gm as fibre. Then 157.9 gm water was 
sprinkled onto the pressed fibre and it was pressed again to recover some 
more liquor. This time, 168.0 gm liquor and 417.4 gm fibre were collected. 
Due to the small scale of this experiment, there was an unavoidable loss 
of some material (about 3.5% in each pressing). The liquors from each 
pressing were analysed and found to contain 27.0 gm ethanol, 0.125 gm 
yeast and 0.017 gm fibre (fine particles only) in the first liquor and 5.9 
gm ethanol, 0.075 gm yeast and 0.010 gm fibre in the second liquor. 
Overall, this represented a 95% recovery of alcohol and 85% recovery of 
yeast from the fermented pulp in the two pressings. An additional pressing 
increases the yeast recovery to 95%. 
EXAMPLE 3 
Stems of fresh sweet sorghum were cut into small pieces about 1 cm long and 
milled to produce a coarse pulp. The pulp was acidified with dilute 
sulphuric acid to lower the pH to 4.5 and no nutrients or enzymes were 
added. Then 9 gm (dry weight) yeast in 90 ml water was added to 1000 gm of 
the acidified pulp (22.97% solids, 7.99% sucrose) and mixed well, the 
water being soaked up by the pulp to leave substantially no free liquid in 
the mixture. The mixture was allowed to ferment in a 2 liter flask fitted 
with a water condenser and a dry-ice trap. The volume of carbon dioxide 
evolved was measured with a wet test gas meter (16.29 liters measured). As 
a check on the gas volumes, the loss of weight of the flask contents was 
measured periodically over a period of 24 hours. Of an initial weight of 
1093.6 gm, 47.3 gm was lost during the fermentation, leaving 1046.3 gm 
fermented pulp. Small samples were taken to check the sucrose (0.09% 
measured), ethanol (3.68% w/w measured) and dry matter (13.47% measured), 
and a 700 gm sample was pressed in a rubber-covered steel roll under heavy 
pressure so as to squeeze out the alcoholic juice. Of this larger sample 
486.6 gm was collected as juice and 190.1 gm as fibre. Then 140.0 gm water 
was sprinkled onto the presssed fibre and it was pressed again to recover 
some more liquor (141.7 gm). The liquors from both pressings were analysed 
and found to contain 22.4 gm ethanol in the first liquor and 2.3 gm 
ethanol in the second liquor. Overall, this represented a 95.7% recovery 
of ethanol in the two pressings. 
Ethanol (as the 95% azeotrope) and a yeast/fine fibre mixture are recovered 
from the liquors by centrifugation and distillation as described above. 
In general the process of the present invention is found to provide an 
unexpectedly rapid and efficient fermentation of a sugar-containing 
material in the "solid phase" (i.e. in the absence of any substantial 
amounts of free liquid), together with a highly efficient removal of the 
ethanol and yeast from the pulp after fermentation. These factors enable 
much more economic production of fuel ethanol, such that the process 
becomes economical on a much smaller scale than is the case with the 
existing technology. Moreover, since most of the equipment required to 
carry out the process is fairly small and simple, it can be constructed in 
a modest engineering workshop and would be inherently less expensive than 
the equipment required in the known ethanol production processes. 
Turning now to the use of starch-containing raw material it has been found 
that instead of cooking the starch crops to make a mash as is usual 
practice in all breweries and distilleries, immobilised enzymes and yeast 
may be added to the raw milled crop material, thus allowing the ethanol 
fermentation to proceed simultaneously with the hydrolysis of the starch 
at a low temperature (e.g. 25.degree.-50.degree. C.). Amylase used in 
conjunction with the conversion of starch-containing materials requires a 
similar pH to that required for the yeast fermentation. The pH may be 
adjusted by adding either alkali or acid to establish the optimum pH 
value. The alcoholic liquor may then be squeezed from the residual solids, 
and the immobilised enzymes and yeast may be recycled. Ethanol is 
recovered in the normal way be distillation of the alcoholic liquor. 
The immobilization of the enzymes on small solid particles in order that 
they can be recovered together with the yeast provides an additional 
advantage wherein no substantial modification is necessary to the process 
or apparatus in relation to sugar crops. In particular, no vacuum 
distillation is required. 
The following example illustrates the conversion of starch-containing 
materials to ethanol. 
EXAMPLE 4 
To 49.87 gm raw milled cassava, 0.2% w/w each of three enzymes (a 
cellulase, a fungal amylase and an amyloglucosidase) and 0.9% w/w (dry 
matter) baker's yeast were added and mixed well in a small flask. A 
condenser was fitted to the flask and the flask was incubated in a water 
bath at 30.degree. C. The weight of the flask was checked at intervals in 
order to determine the amount of carbon dioxide produced. After incubation 
for 19 hours, 3.52 gm had been lost and since the cassava contained 28% 
starch, the weight loss corresponded to a fermentation yield of 46.3%. 
Owing to the small scale of this experiment, the residue was not pressed. 
It is to be noted that the actual alcohol concentration was not determined 
in this particular example. Larger experiments are to be conducted after 
conditions have been optimised, which include the use of special enzymes 
which are more potent with raw starch substrate. However, this experiment 
illustrates that the simultaneous hydrolysis and fermentation of raw 
starch can occur in a reasonable time to a low temperature, such as 
30.degree. C. 
The described arrangement has been advanced merely by way of explanation 
and many modifications may be made thereto without departing from the 
spirit and scope of the invention as defined in and appended claims.