A PROCESS FOR THE PRODUCTION OF ETHANOL

Abstract: Process for the production of ethanol by continuous fermentation of a carbohydrate containing substrate in presence of yeast in a fermentor (1) during simultaneous recovering of carbon dioxide of high purity, more specifically of a purity exceeding 95% by volume. High yeast concentration is maintained in the fermentor by maintaining the yeast in a circuit comprising the fermentor (1) and a separation step (11), in which a stream (10) of fermentation liquor from the the fermentor is divided into a yeast impoverished stream (13), which is fed to an ethanol recovering step, and a yeast enriched stream (12), which is returned to the fermentor, and further by adding to the fermentation liquor in said circuit an oxygen containing gas for achieving a growth of yeast that balances the yeast losses in the circuit. The characterizing feature of the process is that a liquid stream (8, 12) discharged from the fermentor (1) is atomized in an oxygen containing gas stream, whereafter the liquid phase is separated from the gas stream and returned to the fermeture.

Description: A process for the production of ethanol The present invention relates to a process for the production of ethanol through continuous fermentation of a carbohydrate containing substrate in a fermentor during simultaneous recovering of carbon dioxide of a purity exceeding 95 % by volume, in which process a high yeast concentration is maintained in the fermentor by retaining the yeast in a circuit comprising the fermentor and a separation step, in which a stream of fermentation liquor from the fermentor is divided into a yeast impoverished stream which is fed to an ethanol recovering step, and a yeast enriched stream, which is returned to the fermentor, and further by adding to the fermentation liquor in said circuit an oxygen containing gas for achieving ja growth of yeast that balances the yeast losses in the circuit. In existing processes of the kind mentioned above for the production of ethanol, as for example is described in the Swedish patent 430 171, high productivity in ethanol and carbon dioxide is achieved in a fermentor by maintaining high yeast concentration in the fermentor. This requires the combined measures of continuous recirculation of the yeast from a separation step outside the fermentor and the maintenance of a yeast growth which balances the yeast losses. The yeast growth is controlled by the distribution of air at the bottom of the fermentor so that the air passes through the fermentation liquor in the form of small bubbles. The supply of oxygen required for the yeast growth causes that the carbon dioxide formed in the fermentor will be too polluted with air for commercial use, since successive purification processes for converting the carbon dioxide to the purity required would be awkward enough to make the recovery of the carbon dioxide uneconomic. To reduce the air content in the carbon dioxide leaving the r fermentor, the addition of air to the fermentation liquor outside the fermentor has also been suggested. Due to the limited oxygen solubility in the fermentation medium, a relatively large liquid flow between a separate aeration tank and the fermentor should be required, whereby an essential part of the carbon dioxide produced would be lost with the gas mixture leaving the aeration tank. Alternatively, pure oxygen gas could be supplied to a stream of fermentation liquor, which after passage of a required holding loop is returned to the fermentor. However, calculations show that a very large recirculation flow would be required, and nevertheless a substantial amount of oxygen gas will reach the fermentor and pollute the carbon dioxide. The cost for oxygen gas, additional pumps and additional holding coil equipment makes the method less attractive, particularly due to the fact that the carbon dioxide to some extent is still polluted by oxygen gas. The object of the present invention is to achieve the supply of oxygen required for the yeast growth in the fermentation process concerned so that the carbon dioxide discharged from the fermentor has a purity exceeding 95 % by volume and so that the additional operation costs and plant costs required for this purpose will be considerably lower than the value of the produced carbon dioxide. According to the invention this object has been reached in a process of the kind introductively mentioned, which process is primarily characterized in that a liquid stream from the fermentor is atomized in an oxygen containing gas stream, whereafter the liquid phase is separated from the gas stream and returned to the fermentor. According to the invention the oxygenation of the fermentation liquor occurs by dispersing the fermentation liquor in the gas phase in contrast of conventional technique according to which the gas is dispersed in the form of bubbles in the liquid phase. Surprisingly this modification has proved to give a much larger effect on the oxygen assimilation and therefore also to require a considerably smaller liquid flow than could be expected from known values of the oxygen solubility. A probable explanation herefore is that the dispersing of the liquid phase not only facilitates that equilibrium with respect to dissolved oxygen in the liquid is reached, but also drastically reduces the oxygen diffusion distance into the yeast cells, thereby enabling the yeast cells to assimilate a substancial part of the available oxygen during its transport through a gas-liquid contact zone. Oxygenation tests carried out according to the invention have shown that as much as 60 - 80 % of the total oxygen absorbed by the liquid has been assimilated by the yeast cells. In all circumstances the recirculation flow of fermentation liquor can now according to the invention be reduced so that the operation costs and plant costs associated with oxygen supply outside the fermentor can be restricted well below the value of the carbon dioxide produced in the fermentor. Since the assimilation of the yeast cells probably accounts for a substantial part of the oxygen absorption in the oxygenation step, the concentration of yeast cells in the liquid phase treated in the gas-liquid contact step is of importance. Therefore, according to a preferred embodiment of the invention the stream enriched in yeast cells from the yeast separation step is primarily used for the oxygenation step. If an additional flow of fermentation liquor is required for achieving sufficient yeast growth in the circulation loop, a further stream of fermentation liquor is discharged from the fermentor and supplied to the gas-liquid contact step. This stream should prior to introduction into the oxygenation step be screened from particles in a screening step. Since the liquid stream discharged from the fermentor for ethanol recovering at said known fermentation process is usually freed from particles in a screening step before it is supplied to the yeast separation step, the same screening equipment can suitably be used for the stream of fermentation liquor recirculated to the gas-liquid contact step. In many cases no screening of the fermentation liquor Is required for the oxygenation process according to the invention, and a stream of fermentation liquor can be led directly from the fermentor to be dispersed in an oxygen stream or air stream. Since temperature control in the fermentor is often achieved by continuous discharge from the fermentor of a stream of fermenta- tion liquor for cooling in a heat exchanger, the same stream can suitably also be used for the oxygenation before the cooled stream is returned to the fermentor. Hereby additional pumping costs for pumping the fermentation liquor to the gas-liquid contact step is saved. As oxygen containing gas either air or oxygen gas can be used. Since the gas supplied to the gas-liquid contact step Is separated from fermentation liquor before it is returned to the fermentor, the nitrogen in the air does not cause any disadvantage, left out that the required apparatus volume increases in inverse proportion to the oxygen concentration of the gas used. The profitability of using oxygen gas is thus determined by the cost for available oxygen gas and the reduction in plant costs due to reduced apparatus volume. In case the use of oxygen gas is found to be advantageous, according to a specific embodiment of the invention it can however be optimal to first disperse the liquid stream to be oxygenated in an air stream, since a part of the oxygen containing gas can be considered as consumed for driving off the carbon dioxide contained in the fermentation liquor. In a successive contact zone oxygen gas can thereafter be supplied. The gas-liquid contact apparatus required for the process accor¬ ding to the invention can be arbitrarily selected among known equipment for dispersing liquid in a gas phase. According to a preferred embodiment of the invention the oxygenation is carried out in a packed column, to which fermentation liquor is supplied in the top and streams downwards by gravity through the packing in counter-current with oxygen or air introduced at the bottom of the column. The oxygenated fermentation liquor is collected on the bottom of the column and returned to the fermentor. The invention will now be further illustrated by means of a preferred embodiment of the same, reference being made to the accompanying drawing, in which Fig. 1 is a flow sheet of the • part of a continuous ethanol fermentation process of interest for the invention, and Fig. 2 shows an alternative embodiment of the gas-liquid contact step of the process shown in Fig. 1. A fermentor 1 is continuously supplied with a hydrocarbon con- taining substrate through a line 2. Carbon dioxide produced in the fermentor is continuously discharged from a gas outlet 3, and a stream of fermentation liquor is continuously discharged through a line 4 from the bottom of the fermentor. Stream 4 is supplied to a screening device 5. From the screening device a stream 6 enriched in solid material is discharged, and a part 6a of said stream is returned to the fermentor and the remaining part 6b is discharged or supplied to a distillation unit (not shown) after possible washing with stillage recirculated from the distillation unit for reducing the yeast losses. The liquid stream 7 which passes through the screening device 5 is divided into a stream 8, which is led directly to a packed column 9 for oxygenation, and a stream 10 which is supplied to a centrifugal separation step 11. In the separation step 11 the stream 10 is divided into a yeast enriched stream 12, which together with the stream 8 is fed into the packed column 9, and a yeast impoverished stream 13, which is led to a distillation unit (not shown) for recovering of ethanol. The stream 13 is so dimensioned that the ethanol content in the fermentor 1 is maintained at a desired value in the range 3-7 % J> by weight. The stream of fermentation liquid 8 and the yeast enriched stream 12 is introduced in the top of the packed column 9 and flows by gravity downwards in the column through a packing of filler bodies 14 in counter-current with air or oxygen gas supplied through a gas inlet 15 in the lower part of the column. During passage of the packing 14 the downward streaming liquid is dispersed in the upward streaming gas phase, whereby the dispersed liquid elements offer a large surface for oxygen absorbtion. Unabsorbed gas and carbon dioxide driven off from the liquid stream is discharged in the top of the column through a gas outlet 16, while the downward streaming liquid Is collected in a collecting zone 17 in the bottom of the column, wherefro it is returned to the fermentor through line 18. The gas liquid contacting apparatus shown in Fig. 2 consists as in Fig. 1 of a packed column 20. The liquid stream 21 supplied in the top of the column for oxygenation is dispersed in a first gas-liquid contact zone 22, consisting of an upper part of a packing of filler bodies, where the dispersed liquid meets an upwards streaming air stream, which is fed into the column at an intermediate -level through line 23. In a second contact zone 24, constituting the lower part of the packning, the downwards streaming liquid dispersion then meets an oxygen gas stream, which is fed into the lower part of the column through line 25. From a collecting zone 26 in the bottom of the column fermentation liquid is returned to the fermentor through line 27. Example 3 In a fermentor with a liquid volume of 2.8 m continuous fermentation of a hydrolysed wheat substrate was carried out. The substrate was supplied through line 2 in a fermentation plant of the kind schematically shown in Fig. 1. The yeast species used was Saccaromyces Serevisiae. From the fermentor a stream of fermentation liquid was discharged, and the stream was dimensioned so that a sufficient liquid stream remained after screening through a bow sieve 5 to discharge from the separation step 11 about 23 kg/h ethanol with the yeast-free stream 13 to a distillation unit and so that the yeast concentrate stream 12 together with the remaining partial stream 8, which is recirculated directly to the packed column 9, was 1.92 m /h. A part of the sieve reject 6 is returned through 6a to the fermentor, while the remaining part 6b is washed in a further screening step (not shown) with recirculated stillage from the distillation unit to reduce yeast losses. The sieve reject obtained thereby was supplied to the stripping unit of the distillation plant and the liquid stream freed from solid material was returned to the fermentor. The united streams 8 and 12 were discharged in the top of a packed column having a diameter of 200 mm and containing a 2.5 m packing of intalox plastic saddles. At the lower part of the packing an oxygen gas stream of 0.74 m /h was introduced. The gas flow leaving the column was measured and analysed with respect of oxygen and carbon dioxide. The following test data were obtained: Stream No. m 3 /h % CO. ^ Temp. °C 3 10. 1 98.2 8 1.44 32° 10 0.88 12 0.48 1155 00..7744 99 16 0.88 43 53 From the above values can be calculated that the oxygen 3 assimilated by the fermentation liquid was 0.274 m /h or 0.358 kg/h, which means that in average 186 ppm oxygen had been absorbed in the stream of 1.92 /h fermentation liquor, which passed through the packed column. Since the equilibrium concentration for oxygen solved In pure water at 32°C is 36 ppm, it is realized that the oxygen amount absorbed by the yeast cells is several times larger than the amount of oxygen physically dissolved in the fermentation liquid. 10 The loss of carbon dioxide with the gas leaving the packed column tthhrroouugghh lliinnee 1166 wwaass 00..4433 xx 00..8888 == 00..337788 mm 33 //h, which constituted 3.7 % of the total carbon dioxide produced. ' *

Claims: Claims 1. A process for the production of ethanol and carbon dioxide of a purity exceeding 95 % by volume through continuous fermentation of a carbohydrate containing substrate in the presence of yeast during formation of ethanol and carbon dioxide in a fermentor (1), in which process a high yeast concentration is maintained in the fermentor by essentially retaining the yeast in a circuit comprising the fermentor (1) and a separation step (11), in which a stream of fermentation liquor (10) from the fermentor is divided into a yeast impoverished stream (13), which is fed to an ethanol recovering step, and a yeast enriched stream (12), which is returned to the fermentor, and further by adding to the fermentation liquor in said circuit an oxygen containing gas for achieving a growth of yeast that balances the yeast losses in the circuit, c h a r a c t e r i z e d i n that a liquid stream (8, 12) is atomized in an oxygen containing gas stream, whereafter the liquid phase is .separated from the gas stream and returned to the fermentor. 2. A process according to claim 1, c h a r a c t e r i z e d i n that a liquid stream (8, 12) from the fermentor (1) is atomized and transported through a gas-liquid contact zone (14) in counter-current with an oxygen containing gas to a collecting zone (17), from which the liquid is returned to the fermentor. 3. A process according to claim 2 , c h r a c t e r i z e d i n that a stream (4) of fermentation liquid is discharged from the fermentor (1) and divided in a screening step (5) into a stream (6) enriched in solid material and a stream (7) freed from solid material, of which at least a partial stream (10) is sent to a centrifugal separation step (11) in which said partial stream is divided into a yeast enriched stream (12) and a yeast impoverished stream (13), and that said yeast enriched stream (12) and the possible remaining part of said stream (8) freed from solid material is brought to pass through said gas-liquid contact zone (14), whereafter the liquid so enriched in oxygen is collected and returned to the fermentor. 5 4. A process according to any of the preceding claims, c h ¬ r a c t e r i z e d i n that a liquid stream (8, 12) discharged from the fermentor is brought in contact with an air stream in a first gas-liquid contact zone (22) and then is 10 brought in contact with an oxygen gas stream in a second gas- liquid contact zone (24) before the liquid phase is collected and returned to the fermentor (1). 5. A process according to any of the preceding claims, c h a- 15 r a c t e r i z e d i n that a liquid stream (8, 12) discharged from the fermentor is introduced in the top of a column (9) provided with means (14) to achieve fine division of liquid while streaming by gravity downwards in the column in counter-current with an oxygen containing gas which is 20 supplied on one or several levels in the column and discharged at the top of the column together with carbon dioxide driven off from the liquid, whereafter the liquid stream so enriched in oxygen is collected in the lower part (17) of the column and returned to the fermentor. 25 6. A process according to claim 5, c h a r a c t e r i z e d i n that said means (14) consists of a packing of filler bodies. 7. A process according to any of the claims 5 or 6, c h a - 30 r a c t e r i z e d i n that an oxygen gas (25) is fed into the lower part of the column (9), and that an air stream (23) is fed on an intermediate-level of the column (9). J

Inventors: HOFSTEDT ANDERS, HOFSTEDT, ANDERS
Applicants: AC BIOTECHNICS AB
Application year: 1984
Main classification: C12P7
IPC codes: C12P7