BACKGROUND OF THE INVENTION In the present, rapidly worsening energy crisis, ethanol has taken on a critical significance because of its potential use as a motor fuel — either by itself or mixed with gasoline. The importance of ethanol is heightened because of the ready availability of the raw materials needed for its production. These include the fermentable carbohydrates, such as starches and sugars, produced by the agricultural industry, as well as other sources, such as, municipal wastes (i.e., garbage).

Historically, the recovery of ethanol, whether intended for chemical and industrial uses, or as a beverage, has been by distillation. However, notwith¬ standing the increase in sophistication of distillation apparatus in general, the recovery processes basically comprised the familiar distillation procedure, namely, boiling the fermented liquid to vaporize the ethanol and then condensing the vapors to recover the former. Of course, as is well known, ethanol forms a constant boiling mixture with water at about 95% (v/v) ethanol, thus rendering it impossible, except by subsequent chemical procedures, to separate the remaining 5% water from the water-ethanol azeotrope. One such prior procedure is to add benzene to the 95% ethanol — 5% water binary azeo¬ trope. Benzene forms a ternary azeotrope with ethanol and water. When this is distilled, all of the benzene,

a small proportion of the ethanol, and substantially all of the water come off, leaving only a trace of. water in. t residual ethanol. However, where even the slightest trac of water found in commercial absolute ethanol must be removed, this was accomplished, in accordance with one known procedure, by _, reacting the commercial absolute ethanol with metallic magnesium. The latter reacts with the water to form insoluble magnesium hydroxide from whic the ethanol was then distilled. Furthermore, distillatio of the ethanol from the fermentation broth by the appli¬ cation of external heat to a boiler is wasteful of energy, particularly if. carried out at atmospheric pressure, as compared with low pressure distillation, because of the higher boiling temperatures. Methods and apparatus have been proposed in the past, without specific reference to the production of ethanol but generally having as their objects more efficient and economical recovery of products by the distillation or the evaporative concentration of liquids. Typical of such proposals are the methods and apparatus disclosed in U. -S. Patents 1,150,713 (Sδderlund) , 1,461,640 (Wirth-Frey) , and 3,294,649 (Powell, Jr.). All of these operate on the principle of compressing the vapors leaving the body of liquid being distilled or concentrated and_passing the compressed (and thereby, heated) vapors back through the body of liquid in heat exchange relationship with the latter. Soderlund passes the compressed vapors through coils, which must be preheated by steam to initiate evaporation, over which coils the liquid to be evaporated is then flowed downward in the form of a thin film. irth-Frey compresses ^" distil led vapors to heat them and then reintroducespart of thes vapors directly into the body of liquid being evaporated. The balance of the compressed vapors are passed through

coils submerged in the body of liquid. Powell, Jr. , discloses -a process for the evaporative desalinization of water which involves using, as a direct heating medium, a volatile liquid which is immiscible with the saline solution being evaporated as* well as with the condensed water. Mixed vapors of heating medium and of ^' water are compressed, passed through heat exchange means placed in the body of mixed liquids, and condensed in the heat exchanger to water and liquid heating medium. After standing in a settling tank where the immiscible liquids separate into two layers, the two are separately d ^' rawn off and recovered.

Although the above described procedures render some of the basic distillation processes more energy efficient, there do not appear to have been any attempts to apply them to the recovery of ethanol,' particularly in high concentrations, from fermentation broths of yeast and fermentable sugar-containing substances.

More relevant to the present invention, Boecker, in U. S. Patent 2,440,925, disclosed continuous and quasi-continuous processes for increasing the amount of ethanol obtained from a fermentation mash by stripping the produced ethanol before its concentration in the mash became high enough to poison the yeast. This was done under reduced pressure, either in successive steps, whereby the stripped mash was then pumped to a new fermentation tank for further production of alcohol or in a cyclic process whereby the stripped mash was re¬ turned to its original fermentation tank. In either case, stripping was accomplished by distillation under reduced pressure; but no attempt was made to remove all of the ethanol during any given stripping step and some of the ethanol was always returned to the fermenter with the stripped mash. **•-■ More recently, Cysewski and ilke, (Biotechnology and Bioengineering, Vol. XX, pp. 1421-1444; 1978) dis¬ closed cell recycle and vacuum fermentation processes

for the continuous production of ethanol. The entire fermentation, as well as the distillation, was carried out under vacuum and was designed to produce 95% ethanol. A single stage distillation was performed which required a large rise in pressure to recover the product at atmospheric pressure. Furthermore, no provisions appear to have been made for the separate removal of C0 ₂ produced during the fermentation, making it necessary to have a pumping system with a large enough capacity to pump all of the produced CO,, and alcohol, as well as any residual air or oxygen from that originally introduced into the ^' fermenti mash.

It is one object of the present invention to provide a novel energy-e ficient method for distilling ethanol from yeast-fermented-, sugar compositions. Another object is to provide such a method which can readily be combined with a continuous fermentation process. A further object is to provide a method for recovering ethanol of ig concentrations through distillation. More particularly, it is- an object of this invention to recover, through the novel distillation procedure, ethanol having a con¬ centration greater than about 95%, that is, greater than the constant-boiling azeotrope normally obtainable at atmospheric pressure. Yet another object is to provide a novel distilling system and apparatus for the economical and efficient recovery of ethanol from yeast-fermented, sugar-containing broths. A further object is to provide a novel distilling apparatus for use in conjunction with a continuously produced fermentation broth. Other objects will become apparent to those skilled, in the art from the description of the novel method and apparatus which follow and from the accompanying drawings. SUMMARY OF THE INVENTION In general, in accordance with the present invention, the novel process broadly comprises fermenting a yeast- containing solution of sugar or other yeast-fermentable substance, capable of being converted to ethanol by the enzyme action of yeast, under subatmospheric pressure;

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withdrawing ethanol ^' vapors and carbon dioxide from the . fermenting solution; compressing the ethanol vapors and carbon dioxide gas. to raise their temperature; passing the compressed vapor and gas in heat exchange relation- 5 ship through the fermenting liquid to raise the tempera¬ ture of the latter and distill off more ethanol vapors - while causing the compressed ethanol vapors to condense and liquify; withdrawing the condensed, liquified ethanol and carbon dioxide gas; and continuously replenishing

10 _. fermentable solution as ethanol vapors and carbon dioxide gas are withdrawn.

More particularly, in ^' accordance with a first, simplified aspect of the invention, an aqueous solution containing yeast and sugar, or other fermentable material

15 capable of being converted to ethanol by the enzyme action of yeast, is permitted to ferment in a closed vessel until the optimum ethanol content (determined by the maximum amount of ethanol which the yeast can produce before the concentration of ethanol becomes so high as to slow further

20 fermentation) is reached. When a vacuum pump reduces the pressure sufficiently over the fermenting liquid, the ethanol begins to boil off. The ethanol vapors so produced, together with the carbon dioxide also produced curing fermentation, are pumped, under a pressure sufficient to

25 raise the temperature of the vapors above the boiling point of. the ethanol under the reduced pressure in the tank, but insufficient to liquify the vapors, to a heat exchanger immersed in the fermenting liquid. Under these conditions, the heat exchanger functions as a condenser

30 so that the ethanol gives up some of its heat to the cooler body of - fermenting liquid surrounding the heat exchanger and itself is liquified. Then, still together with the carbon dioxide gas, the liquid ethanol flows to a collection tank from which it is separated from the _^

35 carbon dioxide gas and both are separately recovered. As soon as the distillation process has been started, fresh fermentable liquid is added to the fermentation tank to make up for yeast and sugar used up and to replenish water

lost as vapor during distillation.

In another aspect, the invention comprises a novel apparatus which, in general, features the use of a closed tank that serves the double purpose of fermentation tank and boiler. An external pump has its low. pressure side connected to the fermentation tank and the high pressure side connected to a heat exchanger submerged in the fermenting solution.

In another aspect of the present invention, referring to both the process and apparatus, advantage is taken of the exothermic nature of the fermentation reaction by carrying out the latter on a large scale in a large, ^' closed vessel, -which serves as a reservoir, and immersing the boiler, previously described, in the main body of fermenting liquid. External pumping and circulating means keep the immersed boiler supplied with liquid from the reservoir so that the fermentation produces ethanol both in the large body of liquid-in the reservoir and in the boiler. The apparatus just referred to, in its broadest aspects, comprises a large vessel for holding a large body of fermenting liquid; a smaller vessel mounted within the larger vessel; heat exchange means within the smaller vessel; circulating and conducting means for supplying the smaller vessel with fermenting liquid from the large body of. fermenting liquid and for returning liquid to the large .body; a distillation .column extending upward from the smaller vessel to which are connected mean for withdrawing vaporous and gaseous fermentation products to the heat exchange means; reflux means for returning a portion of the effluent from the heat exchange means to the top of the distillation column; and means for trans¬ ferring cooled fermentation products from said heat exchange means to collection and separation means. In the preferred aspect of the invention, again referring to both the process and apparatus, fermentation is also carried out on a large scale in a large, first

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vessel, which serves as a reservoir, and also in the smaller boiler. However, in the preferred form, the boiler is not mounted inside the reservoir. External pumping and- circulating means keep the boiler supplied with liquid mash from the reservoir so that fermentation produces ethanol both in the large body of liquid within the reservoir and in the boiler, as previously described.

Thus, the preferred form of apparatus, in its broadest aspects, comprises a large vessel for holding a large body of fermenting liquid; a smaller vessel in which fermentation also takes place; heat exchange means within the smaller vessel; circulating and conducting means for supplying the smaller vessel with fermenting liquid from the large body of fermenting liquid and ^' for returning liquid to the large body; a distillation column extending upward from the smaller vessel to which are connected means for withdrawing vaporous and gaseous fermentation products from the smaller vessel arid for externally pumping and conducting said vaporous and gaseous fermentation products to the heat exchange means; reflux means for returning a portion of the effluent from the heat exchange means to the top of the distillation column; and means for transferring cooled fermentation products from said heat exchange means to collection and separation means.

In order that the invention may be more readily understood, reference is made to the detailed description below and to the accompanying drawings in which like numerals refer to the same or similar parts. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a simplified form of the apparatus for the purpose of explaining the principles on which the present invention is based; ^ Figure 2 is a schematic representation of one form of the novel apparatus used for carrying out the process of the present invention; and

Figure 3 is a schematic representation of the

preferred form of the novel apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to -Figure 1, it will be seen that a closed tank or boiler 31 is provided with filling means 39 through which the tank is supplied with a mash 34 which comprises an aqueous composition containing sugar (or any other fermentable material which can be fermented by yeast to produce ethanol) and live yeast. Tank 31 is' maintained at a proper fermentation temperature of 35°C (95°F) by conventional temperature control means (not shown)-'.

A pipe 37 connects tank 31 to the low pressure side of a compressor or vacuum pump 32, the high pressure side of which is connected by pipe 38 to the inlet end of heat exchanger 33 submerged in ^" the fermenting solution 34 The other end of heat exchanger 33 is connected by pipe 35 to a collection and separating tank 36 where ethanol and carbon dioxide, also produced during the fermentation reaction, are s.eparated and independently recovered. The purpose of the vacuum pump 32 is to place a partial vacuum on the fermenting solution in tank 31 to lower the boiling point of the produced ethanol below the temperature of fermenting solution 34 to cause the ethanol to boil off. Since the fermentation reaction is exothermi the distillation becomes self-sustaining and, at the selected pressure, the temperature is sufficient to cause the ethanol to boi-l off or evaporate just as it would in a conventional distillation at atmospheric pressure at a temperature of 81?C (178°F) . Pump 32 is operated at such a pressure that, while lowering the pressure ^~ in boiler 31 (e.g., to ^' about 100 mmKg) , the ethanol vapors and carbon dioxide are compressed only sufficiently to raise their temperature a few degrees. Hov/ever, the pressure at the high end of the pump is adjusted to be -**■* _. . low enough so that, at the temperature to v/hich the ethan vapors and CO- were raised by their compression, the pressure would be insufficient to cause the ethanol to liquify. Under these conditions, by selecting an

priate pressure at the high pressure side of pump 32, heat exchanger 33, which is immersed in the cooler liquid 34, acts as a condenser for the compressed ethanol vapors. The heat given up by the condensing ethanol vapors to liquid 34 is* thus made to drive the vaporization process ■ in fermenting liquid 34 by making up for* the heat of - vaporization lost in the evaporation of the ethanol from the body of liquid 34. Since the rates of condensation andevaporation are the same, the heat added by condensation theoretically balances the heat lost by evaporation.

Actually, there is a certain amount of surplus hea't, namely, the heat added to the system by the work of the pump or compressor 32. ^' _

A valve 50 is provided in boiler 31 to allow draining the latter if desired, or for any other purpose.. The simplified process described above, is more energy-efficient than distillation carried out at atmos¬ pheric pressure and is capable of producing an ethanol product of higher ethanol concentration than would normally be realized in a simple (1-stage) distillation done at atmospheric pressure. Hov/ever, in accordance with the refinements embodied in the form of the apparatus shown in Figure 2, it is possible to recover an even higher concentration of ethanol and also to render the overall process highly energy efficient by avoiding the necessity of compressing and pumping all of the carbon dioxide produced during the fermentation, as in the case of the embodiment of Figur 1. The process carried out by the apparatus of Figure 2 is capable of producing a distillation product having an ethanol con¬ centration at least as high as 99% by volume. Until the present _^ invention, the production of ethanol of higher concentration than the 95% azeotrope required that the remaining 5% water be removed by chemical .procedures , as? already described.

Referring now to Figure 2, it v/ill be seen that a large, closed, first fermentation vessel 1, provided with supply means 20, serves as a reservoir to contain t .- _^ _

ain body of mash or fermenting liquid 21. Mounted in¬ side vessel 1, at a level so as to be completely sub¬ merged in the main body of fermenting liquid 21, is a second fermentation vessel 2, generally similar to boiler 31 of Figure 1, and of similar function. ^■ Thus, since the vessel 2 functions both as a fermentation vessel and as a boiler it will hereinafter be referred to either as "fermentation vessel 2" or "boiler", as required by the context. Fermenting liquid 52 is supplied to vessel 2 from the main body of liquid 21 drawn off through pipe 40 in the side of vessel 1 and pumped by means of turbine pump 7 through pipe 41 extending through vessel 1 to vessel 2 where it is designed as 52. Mechanically couple to turbine pump 7, for reasons to be discussed below, is second pump 6 which drav/s fermenting liquid from vessel 2 through pipe 42 and recirculates it through pipe 43 to the main body of liquid 21. The directions of flow of liquid through pipes 40, 41, 42 and 43 are shown in Figure 2 by the directional arrows. Extending from boiler 2 is a rectifying column

4 which is connected through pipe 44 and throttle valve 9 to vacuum pump or compressor 5 which performs the same function as pump 32 in the embodiment shown in Figure 1. Under the conditions selected in the operating plant described below, compressor 5 is extremely energy efficie since it need be operated at an overall compression factor of only 2 to 4. Ethanol vapors, carbon dioxide, and water vapor boil off from liquid 52 and pass up through column 4 where the water vapor is condensed and returned to fer- menting liquid 52. The remaining ethanol vapors and carbo dioxide gas are compressed and pumped through pipe 45 in the direction shown by the arrows to heat exchanger 3 sub¬ merged in the liquid 52 in boiler 2. The parameters of temperature and pressure are, again, such that heat exchanger 3 acts as a condenser so that the ethanol vapors condense, give up their heat to the liquid 52 and flow as a liquid, together with the carbon dioxide gas through

pipe 46 into a tee, splitting the flow between lines 63 and 64. The flows in lines 63 and 64 are controlled by valves 62 and 61, respectively. Adjusting these valves sets the reflux ratio, that is, the rate of' flow in line 63 divided by the rate of flow in line ^' 64. The flow in line 63 goes to line 60, the reflux line, to provide reflux for column 4. The flow in line 64 passes into collection tank 12. From tank 12, carbon dioxide is pumped out through pipe 47 by means of. pump 11 and liquid ethanol is pumped out through pipe 48 by means of pump 13. A drainage valve 51 may be provided to permit emptying tank 1 for cleaning or any other purpose.

The construction and apparatus just* described avoid the necessity of pumping all of the carbon dioxide, produced during the fermentation, through the system together with the ethanol. It will be immediately apparent that there is no direct contact between the atmosphere above the liquid 21 in vessel 1 and that above liquid 52 in boiler 2. Accordingly, there is provided, at the top of vessel 1, a valve 14a to remove carbon dio¬ xide produced by the fermentation taking place in the main body of .-liquid 21. In this way, the only carbon dioxide which pump 5 must move is that which is produced by the fermentation taking place in boiler 2 and that which comes out of solution when the fermenting liquid in boiler 2 is subjected to the reduced pressure. Since there is no contact between the atmospheres above liquids 21 and 52, valve 14a- can be opened to release carbon- dioxide without breaking the vacuum in boiler 2. To fully understand and appreciate the invention, as illustrated by the aspect shown in Figure 2, considera¬ tion must be given-to the conservation of heat (that is, the heat balance) and to the conservation of mass in the system. These are discussed below. __ To recapitulate, and still referring to Figure 2, ethanol and water 52 evaporate in boiler 2, and the vapors pass up through rectifying column 4 where the water vapors

condense and drip back into boiler 2. Dissolved carbon dioxide also evolves from the liquid in boiler 2. Ethanol vapors, with very little water vapor, along with the carbon dioxide, are compressed by compressor 5, and flow still as vapors, into heat exchanger 3, which acts as a condenser at the higher pressure. ^' The heat exchanger returns the heat released by the condensing ethanol vapors to the evaporating liquid 52 in boiler 2. As the rate of evaporation is the same as the rate of condensa- tion, only for the ethanol, this part of the heat balances The power input (work) into the compressor 5 will .add heat to replace that carried out of. boiler 2 by the carbon dioxide. Carbon dioxide and a fraction of the liquid ethanol pass from heat exchanger 3 into tank 12, where pump 13 pumps out the liquid ethanol and compressor 11 removes the carbon dioxide. The remaining fraction of the liquid ethanol returns to column 4 as reflux. Pump 13 has to overcome the hydrostatic head to pump the ethaήol from the reduced pressure in tank _. 12, and compressor 11 acts as a vacuum pump, maintaining the pressure ^" in- tank 12 at the desired subatmospheric level.

-The heat to boil the water component is made up by the reflux fraction of. the ethanol condensed in heat exchanger 3. All water evaporated either recondenses, boiling ethanol, or passes through the heat exchanger

(less than 1%/v.). Thus all heat lost to boiling water is passed on to boiling ethanol and returns when the ethanol is condensed or passed through with water vapor, which is also condensed in- heat exchanger 3. Considering now.- the conservation of mass in the system shown in. Figure 2, material enters boiler 2 at rate R^ and leaves the boiler at a rate R_+R--+R.. Thus if R ₁ can.be made equal to 'R +R +R.. , the condition will be met. R3 is the rate that ethanol leaves column 4, and R is the rate that carbon dioxide leaves the boiler. R„ is the rate that pump 6 removes material from boiler 2 to recirculate it into vessel 1. Rate R, is made up of tv/o components, R ₅ , the rate through turbine 7, and R , ^' the

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rate through valve 8. If turbine 7 is a positive dis¬ placement hydraulic motor and pump 6 is a positive dis¬ placement pump, and if the tv/o are mechanically coupled so that rate R_ is mechanically made to equal rate R., this will make the volume rates equal, but the error in the mass rates will be very small. This error, plus the rate R-ι+R ₄ is made up by the flow through valve 8 at rate R_. Thus R, , which is identical to R.+R^. equals ^κ 2 3 4" Rectifying column 4 consists of a packed column, bubble plates, or other multiple effect distillation device. The criterion for selecting among these is an acceptable pressure drop at the required flow rate and concentrating effectiveness. Throttle 9 regulates the pressure in column 4 and thus in boiler 2. Throttle 10 regulates the pressure in tank 12 and thus the pressure at the outlet of compressor 5.

Although the aspect of- the invention shown in Figure 2 was described in detail above, Figure 3 repre- sents the preferred form.

The essential structural differences between the apparatus ,of Figures 2 and 3 are that, in the latter, the small fermentation vessel 2 is placed outside the large main reservoir of fermenting liquid and fermenting liquic is fed to the small vessel by means of a pump connected between the reservoir and rectifying column 4.

Referring now specifically to Figure 3, in which reference characters* similar to those in Figure 2 repre¬ sent similar structural elements, it v/ill be seen that, just as in the case of the apparatus of Figure 2, a large, closed first fermentation vessel 1, provided with supply means 20, serves as a reservoir to contain the main body of fermenting liquid 21. A second fermentation vessel 2, generally similar to boiler 31 of Figure 1, is..-. of similar function. Thus, since the vessel 2 in Figure 3 also functions both as a fermentation vessel and as a boiler it v/ill also hereinafter be referred to either as "fermentation vessel 2" or "boiler", as requ

the context. Fermenting liquid 52 is supplied to column 4 from the main body of liquid 21 drawn off through pipe 40 in the side of vessel 1 and pumped by means of turbine pump 7 through pipe 41 to column 4 from which it flows down to vessel 2 where it is designated as 52. Mechan¬ ically couplied to turbine pump 7, for reasons similar to those already discussed above, is a second pump 6 which draws fermenting liquid from vessel 2 through pipe 42 and recirculates it through pipe 43 to the main body of liqui 21. The directions of flow of liquid through pipes 40, 41, 42, and 43 are shown in Figure 3 by the directional arrows. •

Extending from boiler 2 is a rectifying column 4, similar to that of Figure 2, which is connected through pipe 44 and throttle valve 9 to vacuum pump or compressor 5 which performs the same function as pump 5 in the embodiment shown in Figure 2. Under the conditions selected in the operating plant described in the example below, compressor 5 is extremely energy efficient, since it need be operated at an overall compression factor of only 2 to 4. Ethanol vapors, carbon dioxide, and water vapor boil off from liquid 52 and pass up through column 4 where the water vapor is condensed and returned to fermenting liquid 52. The remaining ethanol vapors and carbon dioxide gas are compressed and pumped through pipe 45 in the direction shown by the arrows to heat exchanger 3 submerged in the liquid 52 in boiler 2. The parameters of temperature and pressure are, again, such that heat exchanger 3 acts as a consenser so that the ethanol vapors condense, give up their heat to the liquid 52 and flow as a liquid, together v/ith the carbon dioxide gas through pipe 46 into a tee, splitting the flow between lines 63 and 64. The process now closely resembles the operation of a heat-pump with the hot coil (condenser) heat-coupled as closely as possible to the cold coil (evaporator) . Greatest efficiency is achieved by providing the best possible heat coupling. The flows in lines 63 and 64 are controlled by valves 62 and 61, respectively. Adjusting these valves sets the reflux ratio, that is, the rate^r^

flow in _. line 63 divided by the rate of flow in line 64. The flow in. line 63 goes to line ^" 60, the reflux line, to provide re ^' flux for column 4. The flow in line 64 passes into collection tank 12. From tank 12, carbon dioxide is pumped out through pipe 47 by- means of pump 11 and liquid ethanol is* pumped out 'through pipe 48 by means of pump 13. A drainage valve 51 may be -provided to permit emptying tank 1 for_ cleaning or- any. other purpose.

As in the case of Figure 2, construction of the apparatus of Figure 3, just described, avoids the necessity of pumping all of the carbon dioxide, produced during the fermentation, through the system together with the ethanol. There is provided, at- the top of _. vessel 1, a valve 14a to remove carbon dioxide produced by the fermentation taking place in the main body of liquid 21. In this way, the only carbon dioxide which pump 5 must move- is that which is produced by the fermentation taking place -in ' boiler 2 and that v/hich comes out of solution when the fermenting liquid in boiler 2 is subjected to the reduced pressure. Since there is no contact between the atmos¬ pheres above liquids 21 and 52, valve 14a can be opened to release car.bon dioxide without breaking the vacuum in ^' boiler 2.

The general considerations involving the conservation of -both mass -and heat, are the same as those discussed above with respect to the system of Figure 2 . To 'recapitulate, and still referring to Figure 3, ethanol and water-52 evaporate in boiler 2, and the vapors pass up through rectifying column 4 where the water vapors- condense and drip back into boiler 2. Dissolved carbon dioxide also evolves from the liquid in boiler 2. Ethanol vapors, with very little water vapor, along with the carbon dioxide, are compressed by compressor 5, and flow still as vapors, into heat .exchanger 3, v/hich acts _^ as a condenser at the higher pressure. The heat exchanger returns the heat released by the condensing ethanol vapors to the evaporating liquid 52 in boiler 2. As the rate of evaporation is the same as the rate of condensation, only

for the ethanol, this part of the heat balances. The power input (work), into the compressor 5 will add.enough heat to replace that carried out of boiler 2 by the carbon dioxide. Carbon dioxide and a fraction of the liquid ethanol pass from heat exchanger 3 into tank 12, where pump 13 pumps out the liquid ethanol and compressor 11 removes the carbon dioxide. The remaining fraction of the liquid ethanol returns to column 4 as reflux. Pump 13 has. to overcome the hydrostatic head to pump the ethanol from.the reduced pressure in tank 12, and compressor 11 acts as a vacuum pump, maintaining the pressure in tank 12 at the desired subatmospheric level.

The heat to boil the water component is made up by the reflux fraction of the ethanol condensed in heat exchanger 3. All water evaporated either recondenses boiling ethanol, or passes through the heat exchanger (less than 1%/v.). Thus all heat lost to boiling water is passed on. to. boiling ethanol and returns when the ethanol is -condensed or passed through with water vapor, v/hich is also condensed in heat exchanger 3.

Rectifying column 4 consists of a packed column, bubble plates-, or other multiple effect distillation device. The criterion for selecting among these is an acceptable pressure drop at the required flow rate and concentrating effectiveness. Throttle 9 regulates the pressure in column 4 and thus in boiler 2. Throttle 10 regulates the pressure in tank 12 and thus the pressure at the outlet of compressor 5.

Among the advantages of the preferred apparatus described above is greater economy and simplicity in manufacture. Another advantage is that the apparatus of Figure 3 can be made highly energy efficient. Still another advantage is the fact that exact operating points and parameters do not actually matter since, by placing***- the apparatus of Figure 3 under computer control (either digital or analog) in a manner well known to those skilled in the art, not only can a plant be operated more easily by a minimum staff, but a proper computer progr

will allow the unit to operate at optimum efficiency, taking into account variations in atmospheric and other conditions effecting proper operation, such as changes in size of the apparatus and the development ^' and use of high temperature tolerant yeast.

As an illustration of an ethanol-producing plant constructed in accordance with the invention described above, there is presented, in the following example, the characteristics of a plant having a capacity of 50,000 gallons of ethanol per day.

EXAMPLE ^' .

The boiler 2 is operated at 90°F (32.4°C) and 1.74 PSIA (90 mmHg) . Inlet conditions for compressor 5 are 85°F (29.4°C) and 1.35 PSIA (70 mmHg) . Outlet conditions for compressor 5 are 115°F (46.2°C) and 2.7 PSIA (114 mmHg) . Rate R, (volume) is set at 30.8 gallons/second, and rate R.-, is 30.2 gallons/second. R_ is 0.6 gallons/second, or 2124 cubic feet/second at the inlet of compressor 5, which compresses an additional 45 cubic feet/second of carbon dioxide. Compressor 11 compresses 22.5 cubic feet per second from 2.7 PSIA (114 mmHg) to-atmospheric pressure (760 mmHg) . Compressor 5 takes about 725 H.P.; compressor 11 ^' ab.out 40 H.P.; pump 6, considering that more than half of its driving power comes from turbine 7, only about 7 H.P.; and pump 13 only about 1/2 H.P. This is about 772.5 H.P. total, or about 618 Kilowatts of electricity, which amounts to 14832 Kv/h daily. In ^' the area where this invention was made, electricity costs about $.03/Kwh, which would amount to about $444.96 daily to operate this plant in this area.

To achieve a capacity of 50,000 gallons of ethanol a day, under the operating conditions described above, the fermentation tank 1 should have a capacity of- 50,000 gallons and boiler 2 a capacity of 500 gallons.

Because the proportion of ethanol in the ethanol-water azeotrope increases with a decrease in the pressure under which distillation takes place, the process and apparatus described above are capable of yielding a product having more than 99% by volume of the alcohol.

In fact, as shown by Table 1, below, which gives the compo tions and boiling points of several ethanol-water azeotrop at various pressures, a pressure of 100 mmHg over the fer¬ menting liquid 52 in boiler 2 (Figure 3) will permit disti lation of an ethanol-water azeotrope having 99.6 mole % ethanol which represents substantially water-free ethanol in terms of percent by volume,- as commonly expressed.

TABLE I

COMPOSITIONS AND - BOILING POINTS OF

ETHANOL-WATER AZEOTROPES AT VARIOUS PRESSURES -'

Mole%

Pressure (mmHg) Boiling Point C°C) Ethanol

100 3*4.2 99.6

150 42.0 96.2

200 47.8 93.8

400 62.8 91.4

760 78.1 90.0

1100 87.8 89.3

1450 95.3 89.0

— Source: International Critical Tables, Volume III, page 322.

Although the invention was illustrated by describing the construction and operation of a plant havin a capacity of about 50,000 gallons per day, it will be obvious that the process and apparatus are both capable of either upscaling or dov/nscaling within the scope of the invention. It will also be obvious to those skilled in the art that therecan be technical variation without departing from the spirit of the invention and that the invention is not limited except as defined in the claims v/hich follow.