Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
This invention relates to a method for the recovery
of a volatile organic compound by fermentation of a carbo-
hydrate material in a fermentor, the gases formed during the
fermentation being allowed to escape from the fermentor from
the space above the liquid surface. In the fermentation
process alcohol and other volatile organic compounds act on
the yeast inhibiting the fermentation, with the result that
the fermentation process will stop when a certain,
relatively low, concentration of these compounds has been
achieved in the fermenting liquid. In order to eliminate
this drawback and to make possible the utilization of
higher than normal concentrations of fermentable carbo-
hydrates, with a higher yield of produce from the fermenta-
tion, it has been suggested that the alcoholic fermentation
and other fermentations be carried out under a reduced
pressure, making possible a recovery of the formed volatile
compounds in vapor form from the fermenting liquid or mash.
Preferably, the vaporization should be carried out at, or
near the fermentation temperature. For an alcoholic
fermentation, in which ethyl alcohol is the predominating
volatile product of fermentation, reduced pressures between
20 and 50 mm. mercury absolute should be employed.
In alcoholic fermentations to produce ethyl
alcohol, approximately 95.6 grams of carbon dioxide gas are
produced for every 100 grams of ethyl alcohol formed.
Consequently, when the fermentor vessel is subjected to a
reduced pressure, the carbon dioxide gas will constitute a
high proportion of the total vapors and gases being
removed. Since the carbon dioxide gas is not condensed at
a later stage, it will be drawn into the equipment used for
producing the necessary reduced pressure conditions. There-
fore, an appreciable energy requirement is necessary to
achieve and maintain the reduced pressure level.
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When ethyl alcohol and water vapors are being
condensed, the presence of the carbon dioxide gas will also
have an adverse effect upon the condensing conditions.
In alcoholic fermentations, the heat of fermentation
liberated is in the order of 287 kilocarlories per kilogram of
ethyl alcohol formed. Under a reduced pressure at normal
fermenting temperatures, the vapor-liquid equilbirium con-
ditions for vaporizing an ethyl alcohol and water mixture will
require an additonal heat input. This heat input will vary
depending upon the concentration of ethyl alcohol and water
in the fermenting liquid.
The normal method of introducing heat into a fer-
mentor to achieve vaporizing conditions is with hot water
inside coils immersed in the fermenting liquid. In order not
to destroy the viability of the yeast, it is generally
necessary not to allow the warm water temperature to exceed
40~ centig~rade. By employing sensible heat from a warm
water supply, or any other warm media, to provide the heat
input, only a small temperature difference is possible. This
condition necessitates large volumes of warm heating media
and subsequently a large surface area to achieve sufficient
heat transfer rates.
An object of the present invention is to provide a
method of the type mentioned which permits the utilization
2~ of a substrate with a relatively high concentration of
fermentable carbohydrates, and which is not impaired by such
drawbacks as are characteristic in the prior art, i.e., a
high energy consumption for maintaining a low pressure in the
fermentor, that is requisite when using a heating medium with
a relatively low temperature, relatively large size equipment
requisite because of the large heat transfer surface
required, and relatively long residence time of the fermenta-
tion material in the apparatus. According to the present
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invention, the method is characterized in that part of the
fermentation liquid which is conveyed from the fermentor to
a separate vessel, standing under vacuum (herein referred to
as a separate vacuum vessel) wherein the organic compound
formed is removed in the form of a vapor under reduced pressure
at or near the fermentation temperature.
According to the present invention, the bulk of the
carbon dioxide gas formed during fermentation is allowed to
escape from the fermentor vessel in the normal way from the
gas space above the surface of the liquid. Since only the
fermentation liquid itself is to be subjected to a reduced
pressure, the fermentation liquid is transported from the
fermentor to a separate vessel wherein the organic compounds
formed are removed in the form of vapor under a reduced
pressure at or near the fermentation temperature. Part of
the gases, like carbon dioxide, contained in the fermentation
liquid as absorbed gases, are also conveyed with the
fermentation liquid to the separate vessel where these gases
are desorbed under vacuum.
Because most of the carbon dioxide gas was removed
prior to subjecting the fermenting liquid to a reduced
pressure in the separate vessel, it is possible for the
equipment for condensing the vapors back to a liquid to be
smaller in surface area due to an improvement in the heat
transfer condensing film conditions. Also, the dew point for
the mixed vapors and gases will be elevated in temperature
by the absence of a large proportion of the carbon dioxide
gases.
At the same time, the size of the equipment for
producing and maintaining the required reduced pressure level
will be much smaller, together with the energy consumption
for operation.
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With the present invention, by the use of a separate
vessel for the evaporation of vapors and gases from the
liquid under a reduced pressure it is possible to achieve a
constant head level at the liquid-vapor interface by using
overflow weirs or downcomer pipes. Also, by transporting the
liquid continuously from the fermentor vessel to this vapor
disengaging vessel and retur~ng the residual liquid back -
again to the fermentor (or another vessel), the residence
time in the separate vessel can be short. In consequence,
the depth for the heating surface media within the liquid can
be suitable and optimum. Also, as the boiling point of the
liquid is a function of its composition, surface pressure and
liquid depth, the form of heating need not be from a sensible
heat source.
In a fermentor vessel where the liquid contents
have a specific gravity of about 1.150, a 5 centigrade rise
in boiling point would represent a liquid depth of about
14 cm. Boiling of the liquid throughout this depth could
occur without the fermentative capacity of the yeast being
seriously altered.
If the boiling time is of short duration, such as
30 seconds, boiling temperatures of up to 50 centigrade are
permissible without the yeast's fermentative ability being
seriously impaired, once the temperature of the yeast is
2S returned to its normal fermenting temperature range.
High heat transfer rates can he achieved by the
use of condensing steam as the heat source; or some other
form of heat input, preferably with a condensing character-
istice can be used. In order to prevent scaling and/or
yeast degradation at the outside wall of the heat transfer
surface media, low pressure steam is recommended at a
temperature not exceeding 75C. However, depending upon the
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overall heat transfer coefficient and the nature of the liquid
being evaporated, higher steam temperatures may be used.
In one preferred embodiment, fermentation liquid is
transported continuously to the separate vacuum vessel and is
returned to the fermentor and/or a second fermentor, or is
discharged from the system.
Alternatively, the fermention can be carried out
batchwise.
In this context, "fermentor" means, either one single
fermentor tank, or a plurality of fermentor tanks, coupled
in series.
A tubular fermentor reactor for "plug-flow"
operation can be used as well.
In said embodiment of the method, part of the
fermenting liquid is withdrawn close below the liquid surface
in the fermentor and is transported to the separate vacuum
vessel, in order that the fermentation liquid fed to the
separate vessel shall contain only the absorbed gases which
are formed by fermentation in the fermentor and in the
fermenting liquid during transport.
In another embodiment, part of the fermenting
liquid is hydraulically removed from the fermentor by
intimate contact with an inert gas, or a gas less soluble
than those gases formed by fermentation and absorbed in the
fermenting liquid, said inert or less soluble gas being
separated from the transported part of the fermenting liquid
before this is exposed to a reduced pressure in the separate
vessel.
Suitably, the apparatus for carrying out the method
according to the invention is designed in order that the
difference in elevations of the liquid surface in the
fermentor and in the separate vessel is slightly greater
than the barometric height of the liquid needed to balance
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the reduced pressure in the separate vessel and the pressure
in the fermentor.
Thus the liquid surface is kept on a relatively
constant level by the utilization of overflow weirs or down-
comer pipes, through which the fermenting liquid flows down,after the vapor of organic compound has been removed.
In production of ethyl alcohol, the pressure in the
separate vessel is suitably maintained at 20 to 50 mm. of
mercury absolute.
The usage in the separate vessel of a heating
wall, in contact with low pressure steam with a condensing
temperature comprised between 40 and 75 centigrade, is
advantageous.
The residence time of the fermenting liquid in the
separate vessel should not, suitably, be longer than 60
seconds, and the temperature at the contact with the heating
surface at its lowest point should not be higher than 55C.
In one preferred embodiment of the method according
to the invention, the fermentation is carried out in order
than an ethanol concentration is maintained in the part of
the lqiuid or liquid mixture that is transported to the
separate vessel, at 1.0 to 8.0 grams ethanol/100 mls.
Other objects and advantages of the invention will
appear as the invention is described in connection with the
accompanying drawings.
In the drawings,
FIGURE 1 is a schematic view of apparatus for
carrying out the invention~
FIGURE 2 is a similar view of an alternative part
of the apparatus shown in Figure 1.
The apparatus in Figure 1 comprises two process
stages, coupled in series, each of them provided with a
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fermentor vessel, Al and A2, respectively, and with
separate evaporators Cl and C2, standing under vacuum.
Each fermentor vessel is connected, via a pump, Bl and B2,
respectively, and pipe such as F-l to its evaporator. In
this case evaporators Cl and C2 are designed as hairpin
evaporators for low pressure or vacuum steam, condensing on
the inside of the tubes. Evaporators Cl and C2 are placed on
such an elevation above the fermentor vessels, that the
difference in elevations of the liquid surface in the
fermentor and in the separate vessel (as shown by the broken
line) is slightly greater than the needed barometric height
of the liquid; in this case the difference is 10 meters.
The evaporators are provided with overflow weirs 20,
in order to maintain the liquid head level constant.
Reduced pressure is supplied by vacuum apparatus G
which in this instance is a mechanical unit employing exhauste~s
and ring pumps. Mixed vapors from evaporators Cl and C2 pass
throught pipes 14 to "vacuum"-condenser D, where the mixture
is condensed and is conveyed to an atmospheric receiver E via
a barometric seal leg. This product is pumped out by pump F
under level control LC for further rectification. The sealing
liquid of the vacuum apparatus should be returned for substrate
dilution, as it will contain most of the uncondensed ethyl
alcohol that passes through the condenser D.
Control of the steam for each evaporator Cl and C2,
is an individual temperature controller TC, which senses the
li~uid temperature and controls the steam to each evaporator.
The tubes 21 are provided with baffles 23 for the liquid flow
along their length to the overflow of the weir 20. The steam
condensate is discharged to and through a cooler and a ring
pump, system H, by pipes 11', 11", in order to maintain the
proper temperature of the steam condensing in the tubes.
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The liquid may be withdrawn from fermentors Al and
A2, which are at or slightly below atmospheric pressure, by a
suction swing arm 10 through which liquid is withdrawn close
below the liquid surface to pump Bl and B2, respectively.
Alternatively, the equipment shown in Figure 2 may be used.
Since air has a much lower solubility in water and
water solutions than carbon dioxide gas, the displacement of
carbon dioxide gas by air in the liquid will be an advantage
in improving the dew point for the vapors being condensed at
a later stage under reduced pressure and in reducing the size
and the energy requirements at the reduced pressure producing
equipment.
It is known that liquids can be transported by means
of a compressed gas by the system generally called an air
left, when compressed air is employed. Also, it is known that
stripping or negative gas absorption can be carried out by
bringing a liquid containing a dissolved gas into contact
with an inert or less soluble gas in order to remove the dis-
solved gas fromthe liquid.
In Figure 2 we have shown an air lift system with
excess gas and foam removal returning back to the fermentor,
together with any recirculated liquid not being taken into the
suction of pump P. As there shown, compressed air from a foot
piece jet 16 is delivered to a lift pipe J immersed in the
fermentor vessel K and lifts the liquid to chamber L located
externally of the fermentor. This chamber L is fitted with an
agitator 17 rotating at an optimum speed sufficient to assist
the disengagement of entrained gas from the liquid. Gases,
foam and any surplus liquid overflow through pipe M back to the
fermentor vessel K above the operating liquid level. At the
same time, the optimum oxygen tension in the fermenting
substrate can be achieved by regulating the air flow to the
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lift pipe J in order that an excess of liquid returns to the
fermentor vessel in whi_h air is absorbed.
Liquid under gravity head flows through pipe O to
pump P, from which it is transported to the separate vessel Cl
under a reduced pressure, as illustrated in Figure 1.
In a plant of the type shown in Figure 1, comprising
two fermentor and two evaporator stages, coupled in series,
improved operation economy is possible compared with a ~lant,
consisting of one single fermentor stage. A relatively high
substrate concentration may be employed in the flow, fed to
fermentor vessel Al, while the second stage with fermentor
vessel A2 is utilized for recovering residual alcohol through
a final fermentation. By using a circulating flow, the flow
through the evaporator can be greater than the flow between
the fermentor vessels, excess of liquid being returned to its
original fermentor vessel. Generally, this excess flow will be
in the order of up to 90% of the liquid circulating in the
evaporative loop. Hence, there will be little chance of
fermentable sugars bypassing the fermentors and being discharged
in the final fermentor effluent.
Should too much water be evaporated with the ethyl
alcohol vapor and the solids concentrations increase too much
in the fermentor vessel, diluent water can be added to
establish optimum fluidity.
As an example of operational data for the new
process, it may be mentioned, that in a two stage plant as
shown in Figure 1 when fermenting molasses, a reduced pressure
of 32 mm. mercury absolute in evaporators Cl and C2, and low
pressure steam, condensing at 75C. are used for the
evaporation. The depth of the overflow weir is set at 275 mrn.
and the height of the tube bundle allows a 25 mTn. cover of
liquid at the liquid-vapor interface.
Atmospheric pressure is prevailing in fermentor Al,
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and the yeast concentration is 40 grams pressed yeast per
liter of liquid. The liquid is at a specific gravity of 1.130,
containing 5 grams ethyl alcohol per 100 milliliters and
having a 4.9 pH. Carbon dioxide, that is evolved during
the fermentation is prmitted to be discharged from the upper
part of fermentor vessel Al.
The flow rate through evaporator Cl in this case is
set so that one gram of ethyl alcohol per 100 milliliters is
removed by evaporation from the liquid. For an evaporation
rate of 100 kilograms per hour of ethyl alcohol, this would
equate to a pumping rate for pump Bl of 10 cubic meters of
liquid per hour.
Hence, pump Bl is only required to overcome friction
losses and a small static head of approximately 1.2 meters of
liquid.
It is advantageous to maintain a reasonably high
ethanol content in the liquid before and after evaporation in
order that the alcohol to water ratio in the distillate is
maintained at a reasonable level. Should an excess of water
be removed with the ethyl alcohol, a higher heat load is
necessary for evaporation.
Depending upon the alcohol contents in the fermentors
and the removal rates of ethyl alcohol in the evaporators,
alcohol concentrations up to 30~ by weight can be expected
in the product.
As the spirit is pure, there will be no problems
with deposits on trays and in the evaporators, in further
rectification.
A~ the end of the fermentation, when the alcohol
concentration level starts to sink in the fermenting liquid,
it will be necessary to remove more water vapor with the
ethyl alcohol vapor in order to maximize the alcohol yield.
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In this instance, a continuous fermentation and evaporation
system in cascade flow is envisaged. Only two stages are
shown in Figure 1, but more stages may be used.
The residence time for ~he liquid through evaporator
Cl is set to be less than 30 seconds. The overflowing liquid
is recirculated by passing down through another barometric leg
to a seal chamber 12, where it overflows to next fermentor
vessel A2, the excess being recirculated to fermentor Al.
Where molasses is used in fermentation at a high
initial concentration, the effluent will be rich in solids,
possibly at four or five times the solids contents found in
normal molasses distillery effluents. Hence the effluent has
an economic value if it is to be further evaporated to a syrup
or used for other purposes, such as a liquid fertilizer.
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