Note: Descriptions are shown in the official language in which they were submitted.
8~
This invention relates to a method for producing a
volatile organic compound, preferably ethanol, by continuous
fermentation of a substrate, containing carbohydrate, in a
fermentor. "Fermentor" in this context means either a
single fermentor or a number of fermentors coupled in series,
or possibly one tubular reactor for "plug-flow" operation
(i.e.l for a reaction with a stable reaction gradient). Con-
sidering ethanol of technical grade~ this is mainly manufac-
tured by exclusively synthetic methods from ethylene, which
in turn is obtained from petroleum or natural gas.
Due to the limited supply of fossil raw materials,
investigations have been started to assess the possibilities
of obtaining fuels and chemicals from renewable resources
(i.e., of a vegetable nature). Thus, the old method of pro-
ducing ethanol from carbohydrates, broadly defined, hasacquired renewed interest. It may be noted that the Swedish
organic chemical industry, developed after World War II, was
based upon sulphite-spirit, which was converted into
ethylene and further into ethylene oxide, etc. The changing
raw material situation has thus made such synthesis routes
interesting. If up to 20% anhydrous ethanol is mixed into
gasoline, the octane number is improved, and only minor
changes have to be made in combustion engines to make them
operate on such a mixed fuel. In many countries having great
quantities of cheap sugar or starch-based raw materials
available, a vigorous development work is under way along
those lines. Recent research results make it possible to
believe that the conversion of cellulose into fermentable
sugar will be economically feasible.
'
o~
Cellulose is available all over the world in seem-
ingly unlimited quantities and is a potentially ideal raw
material for the production of ethanol.
Most presently existing plants for the production
5 of ethanol by fermentation are based upon simple batchwise -~
operation, a relatively diluted raw material being utilized.
In this way, enormous amounts of waste water (i~e., slop) are
obtained which, if le~t untreated to the recipient, will mean
a very high biological load upon the recipient. Production
of ethanol on a great scale by fermentation is feasible only
if the waste water can be disposed of in an economically jus-
tifiable way without deterioration of the environment.
The drawbacks of the methods used hitherto for the
production of ethanol by fermentation are related primarily
to the diluted substrate, which gives a low fermentation rate
and severe problems with great volumes of diluted waste
water. In conventional ethanol fermentation, the substrate
concentration is such that the ethanol concentration will be
about 7~ (weight) in the fermentation liquor after termina-
tion of the fermentation. If molasses is used as a sub~
strate~ the original concentration of same is about 22 Bx,
and the molasses may be fermented completely to ethanol.
Higher concentrations of molasses cannot be fermented com-
pletely, as the ethanol formed will inhibit the fermentation.
In order to eIiminate the drawback due to the in-
hibiting ethanol accumulated in the fermentor, it has been
suggested to keep the fermentor under a reduced pressure to
make the ethanol formed ~oil away at the prevailing tempera-
ture, which is kept a level low enough not to deteriorate the
yeast. One serious drawback inherent in this method is that
all the carbon dioxide formed during the fermentation must
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s
be removed by the apparatus used for producing the reduced
pressure, which means a great energy consumption.
It has also been suggested that the ethanol which
is formed be removed during the fermentation from the sub-
strate with the aid of an azeotrop-former. In this case, the
ethanol will form with water and a solvent a so-called azeo-
trop, which has a lower boiling point than the corresponding
ethanol-water-mixture. The ethanol is then separated in a
subsequent step from the azeotrop. There are many drawbacks
inherent in such a process. Among other things, it is very
difficult to find an azeotrop-forming solvent which does not
influence the activity of the yeast in an unfavorable way.
The principal object of the present invention is to
provide a method of the type initially mentioned which allows
a substrate with a high concentration of carbohydrate to be
fermented and which gives rise to a concentrated waste flow
which can be rendered harmless or be utilized in an effi-
cient, economical way.
According to the invention, a method of the type
initially mentioned is characterized in that a flow of fer-
mentation li~uor is separated by centrifugal force into at
least one yeast concentrate flow and one yeast-free flow,
the yeast concentrate flow being recirculated to the fer-
mentor, the yeast-free flow being separated into one flow en-
~5 riched in volatile organic compound, which is discharged, andone residual flow which is at least in part recirculated to
the fermentor.
By recirculating the yeast concentrate to the fer-
mentor, the yeast is protected from being deteriorated when
separating the volatile organic compound. The recirculation
of yeast also means that a higher concentration of yeast may
be maintained, which in turn means a higher fermentation
rate.
It is advantageous to separate a flow of fermenta-
tion liquor into three flows, namely, the two already men-
tioned and one sludge flow containing impurities. This canbe carried out by a centrifugal separator which separates the
incoming flow of fermentation liquor partly into a continuous
yeast concentrate flow, partly into a continuous yeast-free
flow, and partly into an intermittent sludge flow. This
means that sludge is accumulated in the peripheral part of
the rotor of the centrifugal separator and is discharged
intermittently therefrom. In this way, impurities are pre-
vented from being accumulated in the system. In one conveni-
ent embodiment of such a centrifugal separator, the yeast
concentrate flow is discharged with the aid of a paring
tube, and the yeast-free flow by a paring disc.
As previously mentioned, the conventional ethanol
fermentation techniques utilize a substrate concentration
such that the ethanol concentration is about 7~ (weight)
after termination of the fermentation. If molasses is used
as substratel the original concentration of molasses is about
22 Bx. Such a substrate can be fermented completely.
Higher concentrations of molasses cannot be fermented com-
pletely, as they would give rise to higher ethanol concentra-
tions than 7% (weight), which act to inhibit the fermentationreactions. ~ccording to the present invention, molasses with
a concentration as high as 50-60 Bx may be fermented, if the
ethanol is prevented from growing over about 5% (weight) by
continuous removal of ethanol from the circulation circuit
of fermentation liquor.
S
In the method according to the invention, a carbo-
hydrate concentration of no more than 5% (weight) is main-
tained in the fermentation liquor.
According to one embodiment of the new method, the
yeast-free substrate flow obtained by centrifugal separation
is then separated by a thermal method into one flow enriched
in organic compound, such as ethanol, and one residual flow
which at least in part is recirculated to the fermentor,
while one partial flow is discharged from the circulation
circuit in the form of a slop. According to the present in-
vention, this slop is much more concentrated than slops ob-
tained in distilling methods hitherto operated together with
ethanol fermentation. "Thermal method" means essentially
distillation and fractionation, but other thermal methods
such as evaporation can be used as well. One great advantage
inherent in the present invention is that the distillation
curve for ethanol is considerably improved at the high sub-
strate concentrations that are utilized. The concentration
of ethanol in the vapor which stands in equilibrium with
fermentation liquor of a certain ethanol concentration thus
is much higher, provided that the concentration of carbo-
hydrate is higher in the fermentation liquor.
As an alternative for thermal methods for separat-
ing the ~olatile organic compound from the circulating fer-
mentation liquor, extractive methods may be used. That is,the ethanol is extracted by any suitable circulating sol-
vent which has a major affinity for the compound in question,
but a minor affinity for water. An example of such a sol-
vent is octanol, if the volatile organic compound is ethanol.
The latter is obtained from the octanol solution by frac-
tionation. Such an extractive method has a good heat econo~y.
s
The thermal or extractive separation of the vola-
tile organic compound may be carried out substantially at
atmospheric pressure. Indeed, thermal separation in a vacu-
um has the advantage that a lower temperature may be main-
tained, but the operational costs for maintaining a vacuumare a drawback.
The method according to the invention may be oper-
ated in one ~ermentation stage, but some advantages may be
gained by operating the method in a plurality of fermenta-
tion stages coupled in series, as a better heat economy maybe achieved.
In a suitable embodiment of the new method, the
residual flow from the thermal or the extractive separation
is pasteurized at a temperature of 60-100~ C before it ls re-
circulated to the fermentor.
Another great advantage with the new method is thatindependently of the way of obtaining the volatile organic
c.ompound, a waste flow is obtained with a high concentration
of substance which can be disposed of in an economically
reasonable way.
~n connection with the ethanol fermentation, a
slop is obtained which is 4-6 times more concentrated than
the slop obtained in connection with conventional ethanal
distillation.
Thus, according to the invention a slop with a
positive value of combustion heat is obtained, which contri-
butes to good operational economy of the method. Consider-
ing the high concentration of organic material, the slop
might be used as raw material for the manufacture of pro-
ducts such as furfurol.
:
S
The methcd of the invention will now be described
more in detail, reference being made to the accompanying
drawings, in which:
Fig. 1 is a principal flow sheet of the method
according to the invention;
Fig. 2 is a flow sheet of the method carried out
in one stage, with distillative separation of the volatile
organic compound;
Fig. 3 is a flow sheet of the method carried out
in two stages coupled in series, and
Fig. 4 is a flow sheet of the method carried out
in one stage, with extractive separation of the volatile
organic compound.
The system shown in Fig. 1 comprises a fermentor
F, a centrifugal separator C/ a unit RU for removal of a
volatile organic compound such as ethanol, and a mixer M.
These units are connected via lines 1, 2, 3 and 4 in a cir-
culation circuit. Centrifugal separator C is directly con-
nected to mixer M via a line 5. An inlet line 6 to the cir-
culation circuit is connected to mixer M via a line 5. Aninlet ~ine 6 to the circulation circuit is connected to
mixer M. Fermentor F is provided with a gas outlet 7,
centrifugal separator C with a sludge outlet 8, unit RU with
an outlet 9 for a flow enriched in volatile organic compound,
and line 3 with a bifurcation 10 for discharge from the cir-
culation circuit. Centrifugal separator C is of a type that
separates an incoming flow into two continuous liquid flows
and one intermittent sludge flow. Fermentor F is shown as
one sin~le fermentor tank. In practice, it is suitable to
use at least two fermentor tanks coupled in series in a
fermentation stage or to use a tubular reactor provided
with a reaction gradient.
One embodiment of the method is shown in Fig. 2,
where the volatile organic compound such as ethanol is sepa-
rated from the circulation circuit by distillation, eitherin the form of one simple stripping operation or by frac-
tional distillation. In this figure, the same reference
notations have been used as in Fig. 1 for corresponding
apparatus units and lines. Unit RU in Fig. 1 has been re-
placed by a distillation unit D. Line 2 to the distillationunit and line 3 from it are in heat exchange relationship by
a heat exchanger HE I. 1ine 3 passes a cooler HE II before
the connection to mixer M.
~hen producing ethanol in a plant of the type
shown in Fig. 2, a concentrated, clarified substrate is fed
via inlet line 6 and mixer M to fermentor F. The flow is
thus mixed partly with the yeast suspension from the cen-
trifugal separator and partly with the yeast-free flow from
the distillation unit, that is, the slop. In centrifugal
separator C, impurity sludge is separated intermittently.
This sludge would otherwise be accumulated in the plant.
Ethanol is separated from the yeast-free flow in distilla-
tion unit D, the heat required being provided either by in-
direct heating or by feed of live steam. In the latter
case, the dilution is compensated by an increase in the con-
centration of the substrate fed. The advantage of live
steam is that deposits on the heat transfer surfaces are
avoided. Ethanol is discharged through outlet 9, and the
slop leaves almost all its~available heat content to the
flow entering the distillation unit via heat exchanger HE I.
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A small part of the slop flow is discharged from
the circulation circuit via outlet 10 and is so concentrated
that is can be disposed of in an economical way~ The rest
of the slop flow is cooled by heat exchanger HE II and is
fed via mixer M to fermentor F. In the fermentation, there
is formed a gas, mainly carbon dioxide, which is discharged
through outlet 7. The method is carried out in such a way
that the ethanol concentration in the fermentor is main-
tained at a low level, i.e., about 4~ (weight), the sub-
strate thus being fermented to almost 100% by the processparameters chosen. The ethanol concentration in the slop
discharged from the distillation unit is quite low.
In order to improve the operation economy, it may
be advantageous to couple a number of fermentors in series
as shown in Fig. 3, which discloses schematically a plant
comprising two fermentor tanks F and F2. The plant com-
prises a double set of apparatus shown in Fig. 2. As in the
plant described above/ slop from distillation unit D is con-
veyed via a heat exchanger HE I and a cooler HE II and is
divided partly into one flow which is fed to fermentor F and
partly into one flow which is fed to fermentor tank F2 via
line 12 and mixer M 2. The ethanol flow coming from dis-
tillation unit D2 through line 13 is utilized for heating
distillation unit D. In a plant of this type, the operation
parameters are set in such a way that -the slop from distilla-
tion unit D has an ethanol concentration of about 4~ (weight),
whereas the ethanol concentration of the slop discharged from
unit D2 through an outlet 14 is quite low.
As an alternative to distillative separation of
the volatile organic compound from the circulation circuit,
extractive techniques may be used. In Fig. 4 a plant for
such a process is shown schematically. In addition to cer-
tain elements shown in Fig. 2, the plant comprises an ex-
traction unit E, such as a counter-current extraction column,
and a distillation unit FR such as a fractionation column.
5 Line 2 from the centrifugal separator C is connected to ex-
traction unit E, to which a solvent flow i5 also fed through
an inlet 15. This solvent flow streams through extraction
unit E, absorbs volatile compound, such as ethanol, and
streams through a line 16 to distillation unit FR, wherefrom
volatile organic compound is discharged through an outlet 17.
The bottom flow of solvent from distillation unit FR streams
via a line 18 to inlet 15, to which is also fed solvent
through an inlet 19. Part of the flow, exhausted in vola-
tile organic compound, which is discharged from extraction
unit E is discharged from the plant through line 20, whereas
the rest is fed to fermentor F via mixer M. The distilla-
tion unit is suitably heated by indirect heating 21.
Example
As an exmple of the performance of the method
according to the invention, continuous fermentation of mo-
lasses in a plant of the type shown in Fig. 2 will be
described.
In order to initiate the process, 10 kgs. of
bakers yeast were charged in 100 liters of clarified 20
Brix molasses in a fermenting tank provided with a stirrer.
A cooler was provided in the circulation circuit. The fer-
mentation temperature was controlled to 32 C, and the fer-
mentabIe sugar was converted to 90% into ethanol within 3
houxs. During the latter part of the fermentation process,
ethanol had to be removed continuously from the substrate
in order to maintain an ethanol concentration of about 4%
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(weight) in same. Therefore, fermentation liquor was circu-
lated through centrifugal separator C, a yeast concentrate
flow being recirculated to the fermentor, whereas a yeast-
free flow was fed to a simple distillation unit D where the
ethanol was separated at atmospheric pressure. When the
fermentation process of the original charge was compl~eted,
7-13 kgs/hour clarified 40 molasses were fed continuously
to the fermentor. In the fermentor a liquid volume of 100
liters was maintained. The fermentation was run for one
week, during which time an ethanol flow with an ethanol con-
centration of 25-35~ (weight) was discharged, whereas the
ethanol concentration in the fermentor was maintained at
about 4% (weight). A small slop flow with 25-30~ (weight)
of DS was discharged from the circulation circuit. Operation
data were noted for both feed rates in the table below.
Equilibrium Ethanol
Feed rate fermentahle sugarproductivity
40 Brix molasses F3/F6 in the fermentor ~ (100~)
_ ~gs/hour ~ (weight) kgs/100 litr./h.
7.0 10.0 1.0 1.0
13.0 5.5 2.5 1.7
F3/F6 means the relationship between the volume flows in
lines 3 and 6 in Fig. 2. It is obvious from the table that
the ethanol productivity increased with an increased feed
rate, but at the cost of the utilization of the sugar fed,
as a higher percentage of the latter remained non-utilized
in the latter case. It is obvious that the optimization of
the feed rate is determined by the relationship b~tween the
raw material cost and the investment and operational costs.
It may be noted that the raw material was not
sterilized in the fermentation described above. In spite of
this fact, no accumulation of bacteria in the system
.
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,.
occurred, probably because the flow was sterilized in the
distillation unit.
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