METHOD OF PRODUCING ETHANOL BY CONTINUOUS FERMENTATION

METHODE DE PRODUCTION D'ETHANOL PAR FERMENTATION CONTINUE

English Abstract

ABSTRACT OF THE DISCLOSURE


For continuous production of ethanol by fermenta-
tion, fermenting liquor is fed directly from a fermentor to
a simple evaporator unit where a vacuum is maintained and
where the liquor is separated into a first vapor flow en-
riched in ethanol, which is processed to the desired ethanol
quality, and a yeast-containing bottom flow. Of the latter
flow, part is recirculated to the fermentor while the rest
is separated into a yeast concentrate flow, which is recir-
culated to the fermentor, and a yeast-free flow which is fed
to a stripping unit. The second vapor flow, enriched in
ethanol, which is thus obtained from the stripping unit can
be fed to the simple evaporator unit directly or indirectly.
The method is especially advantageous if the substrate is
made by enzymatic break-down of cellulose or starch, as the
enzymes in this case are not subjected to excessive tempera-
tures but can be recovered.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:

1. In the production of ethanol by fermentation
of a carbohydrate-containing substrate in a fermentor, the
method which comprises feeding fermenting liquor directly
from the fermentor to a simple evaporator unit, maintaining
a vacuum in the evaporator unit while separating said liquor
therein partly into a first liquid yeast-containing bottom
flow, treating said first vapor flow to produce ethanol of
a desired quality, recirculating part of said first bottom
flow to the fermentor, separating the remaining part of said
first bottom flow into a yeast concentrate flow and a yeast-
free flow, recirculating said yeast concentrate flow to the
fermentor, and feeding said yeast-free flow to a stripping
unit and there separating the yeast-free flow partly into a
second vapor flow enriched in ethanol and partly into a
second liquid bottom flow exhausted of ethanol.

2. The method of claim 1, which comprises also
feeding said second vapor flow to said simple evaporator
unit.

3. The method of claim 1, in which said separating
of the remaining part of said first bottom flow is effected
by centrifugal separation.


4. The method of claim 2, in which said separating
of the remaining part of said first bottom flow is effected
by centrifugal separation.

Description

37Z

This invention relates to a method for producing
ethanol by fermentation of a substrate containing carbo-
hydrate,a yeast-containing flow being separated into a yeast-
concentrate flow and a yeast-free flow.
Such methods are known in which the total flow of
fermenting liquor is fed to a centrifugal separator for said
separation, the yeast-concentrate flow being recirculated to
the fermentor. The centrifugal separator utilized must have
a capacity for separation of the total flow of fermenting
liquor, from which the ethanol is obtained continuously,
which can be a drawback. A method in which no centrifugal
separator is needed, but in which the yeast is still pro-
tected when recovering ethanol from fermenting liquor with
continuous fermentation, is disclosed in Swedish Patent
15 Application No. 7801134. In this method, the fermentation is
carried out at normal pressure, but the ethanol is recovered
by distillation from a flow of fermenting liquor including
yeast at vacuum pressure. Even if this method is suitable in
many cases, it has certain drawbacks. The apparatus is
rather complicated; and considering the amount of yeast
present, the extent to which a flow enriched in ethanol can
be evaporated is limited.
The principal object of this invention is to pro-
vide a method of the type mentioned above in which the
capacity of the utilized separation unit can be relatively
low, wherein the yeast is recirculated without being damaged,
and wherein there is great freedom in the choice of suitable
operation data for the recovery of ethanol by distillation.
According to the invention, fermenting liquor is
fed directly to a simple evaporator unit corresponding to
one or only a few distilling stages and in which a vacuum is




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maintained. In the evaporator unit, the fermenting liquor is
separated partly into a first vapor flow enriched in ethanol,
which is fed to a plant for production of the desired ethanol
quality, and partly into a first liquid, yeast-containing
bottom flow. Part of the latter flow is recirculated to the
fermentor while the residual part is separated into at least
one yeast concentrate flow, which is recirculated to the fer-
mentor, and into one yeast-free flow which is fed to a
stripping unit, where it is separated into a second vapor
flow enriched in ethanol and partly into a second liquid
bottom flow, exhausted of ethanol.
In a preferred embodiment of the new method, the
second vapor flow enriched in ethanol is fed to the simple
evaporator unit either directly or as an indirect source of
heat. The yeast is not exposed to any excessive temperature,
but the temperature in the stripping unit may be higher than
what could be tolerated by the yeast. According to the in-
vention, heating surfaces in the evaporator unit may be dis-
pensed with. Also, no high separator capacity is needed, and
the operation conditions for the ethanol recovery may be
chosen optimally.
If the substrate has been manufactured by enzy-
matic break-down of cellulose or starch raw material, it is
an advantage that the simple evaporator unit operate at a low
pressure and thus at a low temperature. In that case the
enzymes are not damaged but can be recovered. Also, the ten-
dency of deposit formation is less in the simple evaporator
unit if it is operated at a low temperature.
It is suitable to separate the yeast-containing
bottom flow by a centrifugal separator into a yeast concen-
trate flow and a yeast-free flow.




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The invention will now be described more in detail,
reference being made to the accompanying drawing in which
the single illustration is a schematic view of a system for
carrying out the method.
The system as illustrated comprises at least one
fermentor 1, a simple evaporator unit 2 corresponding to one
or a few distilling stages, a centrifugal separator 3 and a
simple stripping unit 4. A plant for production of the de-
sired ethanol quality is indicated with a condenser 5 and a
distilling column 6. A pipe 7 connects the fermentor 1 to
the evaporator unit 2, from which a bottom line 8 divides
into two branches 9 and 10. Branch 9 is connected to the
centrifugal separator 3, and branch 10 is connected to the
fermentor 1. The centrifugal separator 3 is provided with a
sludge discharge 13. A line 14 connects the simple stripping
unit 4 with the simple evaporator unit 2, from which a line
15 leads to the condenser 5. A feed pipe 16 delivers sub-
strate to the fermentor, and a discharge pipe 17 is provided
to discharge carbon dioxide formed in the fermentor. The
simple evaporator unit 2 and the simple stripping unit 4 are
provided with heating coils 18 and 19, respectively.
In the operation of the illustrated system, sub-
strate is fed continuously through feed pipe 16, and a suit-
able yeast concentration is maintained in fermentor 1. Fer-

menting liquor including yeast is discharged continuously
~' through line 7 and is evaporated at a relatively low tem-
.' perature under vacuum in the simple evaporator unit 2. The
bottom flow is discharged through line 8 and is divided so
. that one part is recirculated to fermentor 1 through line
10, and the residual part is fed to the centrifugal separa-

tor 3 through line 9. In the centrifugal separator 3, the



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incoming flow is separated into a yeast concentrate flow,which is recirculated to fermentor 1 through line 12, a
yeast-free flow which is fed to the simple stripping unit 4
through line 11, and an intermittent sludge flow which is
discharged through pipe 13. The vapor flow (enriched in
ethanol) from the simple stripping unit 4 is conducted
through line 14 to the simple evaporator unit 2 and contri-
butes to the heat feed to this unit. Vapor is fed as needed
through heating coil 18 as well as through heating coil 19,
which heats the simple stripping unit 4. The combined vapor
flow, enriched in ethanol, from the simple evaporator unit 2
is discharged through line 15 to condenser 5 and is then fed
to the distilling column 6 for production of the desired
ethanol quality.
The following is an example of the operation of a
system for carrying out the method according to the inven-
tion:
Example
Cane sugar molasses, 40Bx, is fed continuously
through feed pipe 16 to fermentor 1, where a yeast concentra~
tion of 2.5% (weight) is maintained by recirculation. Under
these conditions, a residence time of 6 hours is needed to
obtain complete fermentation. A flow of fermenting liquor,
containing 0.1-0.5% (weight) of non-fermented sugar and about
5% (weight) ethanol, is pumped continuously to simple evapo-
rator unit 2, where a pressure of 50 mm Hg is maintained.
The fermented mash boils at 35 C, and vapor containing
30-35% (weight) ethanol is driven off and condensed in con-
denser 5. The diluted ethanol is concentrated and further
purified in distilling column 6.




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The bottom flow from simple evaporator unit 2 is
divided into two flows so that about 80% is recirculated
through pipe 10 to fermentor 1, and 20% is centrifuged in
centrifugal separator 3, which operates at a high rotational
speed. The yeast concentrate is pumped to fermentor 1
through line 12 while the yeast-free flow is fed through
pipe 11 to simple stripping unit 4, which is designed as a
short column. The vapor, enriched in ethanol, from stripping
unit 4 is led either directly to simple evaporator unit 2 or
indirectly for heating of same, whereby the energy losses can
be minimized. The bottom flow from simple stripping unit 4
contains about 35% (weight) soluble dry solids and can be
treated further in a simple way, as in a combustion oven.




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