Fermenter for the production of alcohol
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 07/384,860 filed Jul. 21, 1989 now U.S. Pat. No. 4,992,370 which was a continuation application of application Ser. No. 06/774,184, filed on Sep. 9, 1985, now abandoned, which in turn was a continuation application of application Ser. No. 06/483,130, filed Apr. 8, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a process for the production of alcohol (ethanol) by the continuous fermentation of a must being circulated in a closed circuit by a mammoth (air lift) pump, with fresh broth being continuously injected into the circuit and fermented broth being continuously removed from the circuit. 
     This invention also relates to a fermenter for the continuous production of alcohol, comprising a fermentation vat and a mammoth pump which are connected in a closed circuit, a device for injecting fresh broth and a device for removing fermented broth. 
     Several processes and fermenters for the continuous production of alcohol are known, in which the circulation of a culture medium rich in fermentable sugar and which is designated in the present specification by the term &#34;must&#34; is assured either by means of an ordinary pump, or by a mammoth pump fed via a compressor. A process is known in which the circulation is assured solely by the release of carbon dioxide gas produced during fermentation. 
     In this last-mentioned known process, the fermentation conditions vary considerably the further one moves up the fermentation vat, because of the expansion and the coalescence of ascending bubbles of carbon dioxide gas and, even if the yield in weight of alcohol obtained with respect to the weight of fermentable sugar used is good, the productivity is low compared to the size of the installation. 
     In the other processes mentioned above, it is possible to obtain suitable productivities as well as good yields which approach the theoretical maximum. The dilution rates, that is the ratio between the quantity of fresh must which is injected per hour and the quantity of must contained in the circuit, are fairly high, of the order of ten percent or multiples thereof. The active yeast content of the must is about 50 g of dry matter of yeast per liter, the fresh must which is injected contains from about 100 to 150 g of fermentable sugars per liter and the fermented must which is removed contains from about 6.5 to 8.5% of alcohol. All these quantities are substantial and are to be found in the best of these known processes and fermenters. 
     However, the dimensions of these fermenters and the quantities of must which are treated by these known processes are relatively modest, i.e., from a few liters to a few m 3 . The reason for this lies in the fact that it is very difficult to maintain homogeneous conditions favourable for alcoholic fermentation in a fermenter of several tens, even hundreds, of m 3  without running into serious technical and economic difficulties. In effect, if good transfer to the yeast solely of the oxygen necessary for its anaerobic metabolism is to be supplied, it is necessary to assure on the one hand good agitation of the medium and, on the other hand, an adequate oxygen concentration in the gaseous phase. By increasing the dimensions of the fermenter, the inhomogeneity of the conditions prevailing in the fermentation medium are increased, notably as a function of the height of the vat. The restoration of this homogeneity would imply for the known fermenters and processes complicated devices for the injection of gas under a variable pressure and/or differential stirring devices distributed regularly over the complete height of the vat. The maintenance of this homogeneity could only be ensured by the expenditure of a very great stirring energy, at a prohibitive cost. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a process and a fermenter for the continuous production of alcohol which may be realized on a large scale without running into the technical and economic difficulties which have been mentioned above. 
     To this end, the process according to the present invention is characterised in that the must in the lower part of a fermentation vat is stirred by the injection of a descending flow of recycled must, a back pressure is applied at the top of the vat so that the carbon dioxide gas which is released by the fermentation remains compressed in the upper part of the vat, the carbon dioxide gas is allowed to exert the mammoth pumping effect in a pumping column located above the vat, the must is recycled by being allowed to descend in a turbulent flow in a return pipe and a mixture of oxygen and inert gas is injected into the upper part of the return pipe. 
     Likewise, the fermenter according to the present invention is characterised in that the vat has in its lower part a hydraulic stirring device which is connected to a return pipe for the recycled must, a back pressure valve is provided at the top of the vat to prevent the expansion in the vat of the carbon dioxide gas which is released during fermentation, a mammoth pump is positioned above the back pressure valve to be actuated by means of said carbon dioxide gas in expansion, the return pipe connected to the top of the mammoth pump has in its upper part a device for injecting gas to supply the must with oxygen, and this return pipe is disposed so that it ensures a turbulent flow of the recycled must. 
     The process and fermenter according to the present invention make it possible to achieve the object which has been set and they also have numerous advantages. In particular, their design is such that solely the quantity of oxygen necessary for the anaerobic action of the yeast need be injected into the must. Thus, it is possible to inject per hour a quantity of said gaseous mixture containing less than 0.15 ml of oxygen under atmospheric pressure per g of dry weight of yeast which is present in the must. These quantities are lower than those which are recommended in the known processes. It is by radically improving the conditions under which the oxygen is introduced into the must as well as the conditions prevailing in the vat that it is thus possible to dispense with any surplus oxygen and to avoid the risks of local triggering of aerobic fermentation and the risks of local inhibition of anaerobic fermentation. 
     Thus, the must can contain in solution in the fermentation vat the exact quantity of oxygen which is necessary for the anaerobic action of the yeast, and the agitation in the vat is sufficient for all of this oxygen to be effectively taken up by the yeast. In this manner, the agglutination phenomena of the yeast around the gas bubbles in the fermetation vat are avoided which would directly result in an overfeeding of oxygen to the agglutinated cells. Dilution of the oxygen in the gas bubbles is also avoided, for these would rise and expand while ascending in the fermentation vat, the adverse consequence of which would be a reduction in the transfer rate of the oxygen into the must. 
     The process and fermenter of the present invention are thus adapted to be realized on a large scale, in particular implying a quantity of must treated in the circuit of a volume of at least 50 m 3 . The larger the vat, the more the realization of the favourable conditions is facilitated without necessitating any expenditure of energy for the stirring and circulation. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to carry out the present process, it is possible to use as the starting material a must preferably having a content of fermentable sugar of from 100 to 280 g/l, in addition to the nutrients which are required by yeast. The latter may be selected from among yeasts known for their suitability for the production of alcohol on the one hand and for the formation of aggregates which facilitate their sedimentation on the other hand. 
     Fresh must may be injected into the bottom of the circuit, for example, into the bottom of the return pipe, to obtain the most benefit from the stirring effect of the flow of recycled must on the vat. A quantity of fresh must corresponding to from 0.2 to 0.5 times the quantity of must present in the circuit is preferably injected per hour into the circuit, the must in the circuit being able to contain on average from 50 to 100 g in dry weight of yeast per liter. 
     It has been particularly noted above that it is possible to inject oxygen into the circuit in very moderate quantities which only cover the actual oxygen requirement of the yeast for its anaerobic action. In order to ensure a complete resorption of the oxygen in the must, steps may be taken for the residence time of the must in the return pipe to be long enough, and to provide a turbulence of the must in the upper part of the return pipe which is strong enough for the diameter of the bubbles of said gaseous mixture to remain fairly small. Thus, in a preferred embodiment of the present process, it is provided, on the one hand, that the turbulence of the must in the upper part of the return pipe is strong enough for the diameter of the bubbles of said gaseous mixture injected into the must not to exceed about 4 mm at the time of their formation and, on the other hand, for the residence time of the must in the return pipe to be at least 10 seconds. 
     The reason for the injection of a gaseous mixture instead of, for example, pure oxygen is that it is also possible to use an inert gas which is immediately resorbed in the must, leaving in the form of smaller bubbles the oxygen which it contained. Due to the fact that no more oxygen than necessary is injected, these residual bubbles remain separate and are resorbed for the most part before colliding and forming larger bubbles by coalescence which would not be resorbed before entering the fermentation vat. 
     Therefore, use is made of the double effect of the turbulence of the must which imposes a limit on the size of the injected bubbles, and of the dilution of the oxygen in an easily resorbable gas which makes it possible to rapidly reduce the diameter of the bubbles to a fraction of this limit size. A gaseous mixture which is very suitable may be produced with ambient air and with the carbon dioxide gas released during fermentation, and it preferably comprises one part by volume of air and at least five parts by volume of carbon dioxide gas. 
     Thus, the recycled must is charged with oxygen which is practically completely resorbed and, optionally, it entrains the fresh must when it enters at the bottom of the fermentation vat at a high speed due to the height of the return pipe. This kinetic energy may be effectively transmitted to the must which is located in the vat by an appropriate design of the shape of the bottom of the vat and the mouth of the return pipe. 
     The must may rise slowly, at a speed of about 1-2 cm/s, for example, in the vat where it is possible to arbitrarily distinguish three successive zones, i.e., a stirring zone, a sedimentation zone and a CO 2  concentration zone. In the stirring zone, the fresh must is thus initially mixed with the recycled must and with the must which is in the bottom of the vat. In the sedimentation zone which represents the largest part of the vat, the yeast acts efficiently without being hindered by the stirring at the bottom of the vat or by the release of CO 2 . The yeast may sediment faster than the must rises and thus it is possible to observe a concentrating effect on the yeast at the bottom of the sedimentation zone. Consequently, the recycled must contains less yeast per liter than the must in the fermentation vat, which facilitates the operation of a decanter which may be provided outside the fermentation vat, connected to the return pipe, for example, and which reduces the exposure of the yeast to oxygen during the passage of the yeast along the return pipe. In the CO 2   concentration zone, as in all of the fermentation vat, the release of CO 2  in the form of gas bubbles is hindered by the back pressure applied at the top of the vat, which guarantees homogeneous and optimal fermentation and sedimentation conditions for the yeast in the sedimentation zone. 
     Thus, it is only above the vat, in a pumping column, that the carbon dioxide gas is allowed to be released from the must, to form bubbles, to expand and to exert the mammoth pumping action. The circulation speed in this column may be very high, in the region of at least a few m/s, for example. At this speed, the foam which is formed is broken up and separation of the gases, in other words the degassing operation which has already been produced in part in the pumping column, may be completed in an efficient device such as a cyclone. 
     The design of the present process also makes it possible to realize a particular embodiment in which a pressure lower than atmospheric pressure is maintained at the top of the circuit in order to remove the alcohol from the fermented must be evaporation. Thus, it is possible to benefit from the advantages of an evaporative system, i.e., a higher yield and less inhibition due to the alcohol, without having the attendant disadvantages, i.e., the necessity of introducing large volumes of air under reduced pressure. 
     Once it has been degassed, the must runs out in a turbulent flow into the return pipe. In order to ensure this turbulent flow along the complete length of the return pipe, it should be possible to absorb pressure drops. This is one of the reasons why the process and the fermenter of the present invention are most advantageously produced on a large scale so that the necessary velocities, flow rates and heights may be obtained. 
     It is in this return pipe that the recycled must is preferably subjected to the cooling operation which is necessary to counterbalance the heating caused by the fermentation in the vat. Likewise, it is from this pipe, in particular from its lower part that, in one embodiment without an evaporative system, the fermented must charged with alcohol is preferably removed. The present process allows in this pipe a particularly efficient decanting due to the fact that the must has previously been degassed and that the small quantity of carbon dioxide gas which might then have been injected into the must cannot be desorbed due to the pressure exerted by the height of the return pipe. Thus it is possible to obtain decanting speeds of the yeast of about 20 cm/min. A decanting operation of this type makes it possible, on the one hand to remove must which is freed from the yeast and charged with alcohol and, on the other hand to remove from the circuit, if necessary, excess yeast. The decanted yeast which is not in excess may be recycled into the pumping column, for example solely under the effect of the pressure due to the height of the return pipe, without the aid of an auxiliary pump. The decanted must which is removed may have an alcohol content of, for example, from about 50 to 80 g/l. 
     The fermenter according to the present invention is characterised as indicated above. The hydraulic stirring device which is provided in the lower part of the vat is preferably of the Venturi type. To this end, the mouth of the return pipe may be located above the bottom of the vat, directed downwards and surrounded coaxially by a length of tube of a larger diameter than its own diameter and flaring at its two ends. The bottom of the vat may itself have a semi-spherical shape, for example, which reinforces the action of the Venturi device. The centre part of the vat may then have a cylindrical shape extending from the hemisphere of the bottom and may be capped by a hemisphere complementary to that of the bottom so that it is possible to channel the fermented must towards the back pressure valve. 
     This valve preferably has an adjustable flow rate so that it is possible to adjust, at will, and to control the flow rate of the mammoth pump and thus the circulation speed of the must in the circuit. The diameter and the height of the mammoth pump are selected as a function of the volume of the fermenter and of the flow rate of the must to be circulated. This flow rate is itself a function of the heat released by the reaction, and it should be fairly high so that the temperature gradient does not exceed a certain limit within the fermentation vat. This limit is in the region of a difference of about 3° C. between the top and the bottom of the vat. 
     Likewise, the dimensioning of the return pipe is effected as a function of the circulation speed of the must and in order to preferably observe the turbulence and residence time conditions described above. The turbulence may be influenced not only by the dimensioning of the pipe, but also by providing therein diaphragms or baffles, for example. Care should simply be taken that the fermenter is high enough to overcome the pressure drops related thereto which may exceed the equivalent of 2 m of water column. 
     The present fermenter may be provided at its top with a degassing device of, for example, the cyclone type. In effect, the circulation speed of the must in the fermenter upper part is such that it allows the use of this particularly efficient device. 
     The present fermenter preferably has a cooling device on a part, even on the largest part of its return pipe. This device may, for example, be in the form of a tubular heat exchanger in which the highly turbulent flow of must is reflected in a very high heat transfer coefficient. 
     Moreover, the present fermenter may advantageously be provided with an external decanter which is connected upstream to the bottom of the return pipe and downstream to the middle of the mammoth pump, for example. The decanter thus connected is capable of operating very efficiently without the aid of an auxiliary pump. On the one hand, its function is to provide fermented must which is charged with alcohol but freed from its yeast and, on the other hand, it is to recycle the decanted yeast, while allowing, if necessary, any excess yeast to be removed. 
     Finally, it is at the bottom of the return pipe that a pipe for supplying fresh must is preferably connected, so that this must may be directly entrained in the stirring device by the recycled must. 
     In a particular embodiment of the present fermenter, the top of the fermenter is connected, on the one hand, to a gas pumping device for producing a pressure lower than atmospheric pressure and, on the other hand, to a device for collecting alcohol vapours. 
     In this particular embodiment, it is also possible to provide a device for heating the top of the mammoth pump to provide for more effective release of the alcohol vapours. These vapours will be mixed with CO 2  which is released from the must and will have to be separated therefrom. Hence, the device for collecting the alcohol vapours preferably comprises a combination of condensation means and means for separating the alcohol and the CO 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The fermenter according to the present invention is described in the following with reference to the accompanying drawing FIG. 1 which schematically illustrates one preferred embodiment thereof. 
     FIG. 2 illustrates a means for influencing turbulence in the return pipe of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In this embodiment, a fermentation vat 1 has in its lower part a stirring device 2 of the Venturi type which is connected to a return pipe 3 for the recycled must. The vat has at its top an adjustable back pressure valve 4 to prevent the expansion in the vat of the carbon dioxide gas which is released during fermentation. A mammoth pump, 5 i.e., pumping column, activated by means of the carbon dioxide gas delivered by said valve. The return pipe 3 is connected to the top of the mammoth pump 5 by means of a degassing device 6 of the cyclone type. A small compressor 7 is connected to a CO 2  removal pipe 8 for injecting into the top of the return pipe 3 a mixture of air and CO 2 . Air is introduced into the gas injection device by a mixing valve 9. The return pipe 3 passes through a heat exchanger 10 for cooling the recycled must. A pipe 11 for the injection of fresh must is connected to the return pipe 3 just before it enters the vat 1. Likewise, a pipe 12 for removing must charged with alcohol and yeast is connected to the return pipe below the heat exchanger 10. This pipe 12 discharges into a pressurized decanter 13. A valve 14 for removing the fermented and decanted must makes it possible to adjust the quantity of must which is removed. A drainage valve 15 is provided for removing any possible excess of active yeast. Finally, a recycling valve 16 is provided for recycling, in the mammoth pump 5 under the effect of the pressure due to the height of the recycling pipe 3, a must charged with yeast recovered in the decanter 13. 
     As illustrated in FIG. 2, return pipe 3 includes baffles 17 which assist in influencing turbulence of the flow in the return pipe. 
     EXAMPLES 
     The fermentation process according to the present invention is illustrated in the following by two examples, in which the yeast which is used is Saccharomyces cerevisiae CBS 2961 yeast, the flocculation qualities of which may be considered as relatively good. 
     EXAMPLE 1 
     An embodiment of the process according to the present invention is implemented in a fermenter corresponding to the embodiment illustrated in the drawing. The fermenter has a total volume of 300 m 3  and a total height of 23.5 m. The fermentation vat has a hemispherical bottom and a hemispherical top, a height of 8.5 m and a diameter of about 7.5 m. The average diameters of the mammoth pump and of the return pipe are respectively about 75 and 60 cm. 
     60 m 3  /h of fresh must containing 150 g/l of fermentable sugar are injected into the bottom of the return pipe. 6 m 3  of air per h mixed with 60 m 3  per h of CO 2  recovered at the outlet of the degassing cyclone are injected into the top of the return pipe. The content of active yeast in themust in the fermenter is 65 g of dry matter per 1. 
     The flow rate through the back pressure valve on the vat is adjusted so that the average circulation speed of the must is about 2 m/s in the mammoth pump, about 6 m/s in the degassing cyclone and about 1.5 m/s in the return pipe. The average residence time of the must in the vat is 10 min. The residence time of the must in the return pipe is on average 15 s. The turbulence in the return pipe is such that the bubbles of the injected gas mixture have a diameter which hardly exceeds 3.5 mm in the upper part of the pipe. 
     60 m 3  per h of must having an alcohol content of 8.5% by volume are removed from the fermenter. 
     EXAMPLE 2 
     An embodiment of the process according to the present invention is implemented in a fermenter similar to the one which is schematically illustrated in the drawing, but which also has at its top a device for pumping the alcohol vapours. The fermenter is higher by 10 m than the one of Example 1. 
     135 m 3  per h of fresh must containing 265 g/l of fermentable sugar are injected into the bottom of the return pipe. 9 m 3  of air per h mixed with 90 m 3  per h of CO 2  recovered at the outlet of the pumping device are injected into the top of the return pipe. The content of active yeast in the must in the fermenter is 50 g of dry matter per l 
     A pressure of 70 mbar is applied to the top of the fermenter and the upper part of the mammoth pump is heated to a temperature of 36° C. The alcohol vapours are separated from the CO 2  by any suitable device comprising a condensation trap and a water separator. 90% of the alcohol are recovered by this means and the remaining 10% are recovered by the decanter. A total productivity of 120 g of alcohol per l of fresh injected must is obtained.