CA1125200A

CA1125200A - Method and apparatus for maintaining the activity of yeast fermentation

Info

Publication number: CA1125200A
Application number: CA327,750A
Authority: CA
Inventors: Joseph Meyrath
Original assignee: Waagner Biro AG
Current assignee: Waagner Biro AG
Priority date: 1978-05-19
Filing date: 1979-05-16
Publication date: 1982-06-08
Also published as: CH639999A5; DE2917411A1; BR7903175A; ZA792289B; AU523054B2; NL7903589A; PH16304A; GB2021639A; FR2426085B1; TR20802A; IN150750B; GB2021639B; FR2426085A1; AU4715779A; JPS54151185A; BE876373A; SE7904261L; IT1114284B; IT7922818A0

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus are provided for maintaining the activity of yeast in a continuous fermentation of sugar-contain-ing mash to alcohol. In the continuous fermentation process fresh mash is continuously added and alcoholic solution contain-ing yeast is continuously drawn off. This solution is permitted to settle. At least a portion of the settled yeast is returned to the fermentation process while the non-settled or non-flocculated yeast cells and bacteria are drawn off along the alcohol-containing solution. This results in considerable lowering of the danger of infection from the bacteria as well as increase in concentration of yeast in the fermenter and shortening of the duration of time of the mash in the fermenter, thus increasing productivity. The apparatus for effecting this process comprises a fermentation vat and a yeast sedimentation container, with conduit from the fermentation vat to the sedimentation container of fermented mash with return conduit for settled yeast and a conduit our of the settling stage of the alcoholic solution containing non-settled yeast and bacteria.

Description

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It is well known that yeast cells, particularly of the genus saccharomyces, break down sugar into ethyl alcohol and carbon dioxide. This process is known as alcoholic fermentation and results in the production of an ethyl alcohol-containing solution ~ith about ~/O b~ volume of alcohol, depending on the sugar content of the mash. In this process it is important to inhibit the development of microorganisms other than the yeast, such as for exam~le lactic acid ~acteria.
In normal alcoholic fermentation, the growth of yeast is rather slow so that the fermentation takes a relatively long time.
In raw materials with a low nutrient content, such as spent sulfite liquor or cellulose hydrolysate, the fermentation time is even longer.
In order to reduce this inconvenience, in the prior art a centrifuge has been arranged behind the fermentation stage in order to separa~e the yeast after which the separated yeast cream was used to inocculate fresh mash. However, since yeast cells have only a slightly higher specific gravity than water, the centrifuge required very high speed with consequent costly operationO ~Another disadvantage of this method is that the yeast which is returned to f~sh mash carries with it bacteria which multiply rapidly in :
the fresh mash so that the fermentation product was severely damaged thereby. In order to eliminate this disadvantage, the outflowing yeast cream has in the past been subjected to treatment with mineral acids. This treatment also carried ~ith it considerable disadvantages.

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Generally speaking, in accordance with this invention mash is subjected to fermentation with yeast, produced alcoholic solution containing yeast is withdrawn and introduced into a settler wherein some of the yeast flocculat~sand settles while other yeast and also bacteria remain suspended in the solution, the settled yeast is continuously reintroduced into the fermentation stage along with fresh mash and the alcoholic solution containing non-settled yeast and bacteria is continuously withdrawn. This results in considerable reduction in the danger of infection from bacteria with simultaneously increase of yeast concentration in the fermenter resulting in shortening of the time of duration of the mash in the fermenter and increase in the productivity. The process is particularly advantageous for the industrial production of alcohol, for example for the alcohol addition to gasoline.
It is accordingly a primary object of the present invention to p~ovide for a continuous alcohol production with increased alcoholic content of the produced solution without undesired bacteria being introduced into the high nutrient low alcohol mash~
, It is a further object of the present invention to provide a method for fermentation of sugar-contalning mash to alcohol wherein the speed of fermentatlon is increased with c~nseguent increase in production.

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It is yet another object of the present invention to provide apparatus for effecting the method of this invention.
With the above and other objects in view, the method of the present invention mainly comprises subjecting a yeast-containing mash to fermentation to produce an alcohol-containing solution, continuously withdrawing the alcohol-containing solution which also contains yeast and introducing the same into a settler and permitting it to settle whereby flocculated yeast settles to the bottom while other yeast and bacteria remain in the liquid, withdrawing the settled yeast and reintroducing the same with fresh mash into the fermenter, and withdrawing the alcohol-containing solution with unsettled yeast and bacteria from the settler.
In accordance with a preferred embodiment of the in-vention the settled yeast is returned ln dosed amounts in homogenized state into the ermentation stage so that the fermentation stage contains at most 15~/o of the yeast necessary for fermenting of the mash during the time of duration in the fermenter, with the duration of time of the withdrawn alcoholic ~olution in the settler being not more than one half the time of stay of the mash in the fermenter.

Por a fuller understanding of the invention,~reference is had to the following descrlptlon taken in connection with the accompanying drawings, in which:

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FIG. 1 diagrammetically illustrates the carrying out of the process of the present invention;
FIG. 2 diagrammetically illustrates a simplified ~ermentation stage of the overall processi FIG. 3 diagrammetically illustrates a variant for the fermentation stage;
FIG. 4 diagrammetically illustrates a multi-stage process similar to that of FIG. l;
FIG. 5 shows a variant of a multi-stage process of FIG. 4;
and FIG. 6 diagrammatically illustrates a pilot plant.

Referring now to FIG. 1, this diagrammatically shows a process for the production of ethyl alcohol solution from sugar-containing mash. The mas~ is introduced through conduit 9 into a fermentation stage 1 where the mash is fermented. The requirèd air is supplied to the mash and fermentation vat through pipe 8. In order to avoid clump-in~ o the yeast cells, a storing means, such as stirrer 10, is provided for agitating the ma~h The fermented liquor is discharged from the fermentation stage ~0 1 through conduit 11 into a settling tank or degasifier 12. This is designed as a simple settllng tank or vacuum degasifier so that the yeast sedimentation vat 2 only receives the degassed, fermented mash. The sedimentation vat or settler 2 consists of two stages.

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Conduit 3 leads from the first stage of settler 5 into the fermenta-tion stage 1 for the r~turn of the yeast cream. The introduction of the yeast cream and dosing thereof is effected by pump 13, which also homogenizes the yeast cream. The pressure pipe to pump 13 can be fitted directly into the fermentation stage 1 or into the feed conduit 9 through which the fresh mash is introduced.
Another conduit 14 leads from the settler 5 to a second settler 5' which is designed as a thickener for the yeast cream.
The thickened yeast cream is liberated in washer 15 of alcoholic impurities and ed in purified sta~e to other settlers 6 and 6'.
The liquid discharge 16 from settlers 5 and 5' contains the desired alcoholic solution and is directed to the distillation plant (not shown).
The washed alcohol from the yeast crop is used for the production of new mash. Stirrers 7 and 7' are provided in settlers 5 and 5', respectively, for agglomerating the yeast. Any sugar-containing solution, e.g. diluted molasses, can be used as mash, even cellulose-containing waste waters or spent sulfite liquor can be used.
FIG. 2 shows in simplified representation a fermentation stage 1 as the first stage of a sedimentation plant 2. Fermentation stage 1 consists substantially of a cylindrical vat 17 provided with a tubular fitting 4 in which a feed pipe 18 is arranged. At the bottom of vat 17 there is connected a circulation pipe 19 through ' which a practically gas ~ree mash is ~ucked in through p~mp 20 , . .

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whose pressure pipe 21 leads into a nozzle 22 in which pipe 18 is connected. When li~uid is supplied through pipe 21, a vacuum is proauced in the area of narrowest cross section of nozzle 22, which is used to suck in gas, such as air, pure oxygen, oxygen-enriched air or fermentation gas, so that the mash flowing through nozzle 22 carries along with it the gas in finely divided bubbles and mixes the same at the outlet of feed pipe 18 with the contents of the fermentation stage.
The gas, which is not soluble in the mash, bubbles in the tubular fitting 4 causing an upward flow therein, thus pre-venting clumping of the yeast. The upward flow in chamber 14 causes a downward flow in the outer tubular chamber 23, so that fresh mash is fed to tubular chamber 4, and a circulating flow is effected. At the upper end of the vat 17 there is provided a conical fitting whose outer chamber 24 is designed as a steady-ing zone, while the interior chamber 25 represents the degassina chan~er. In the steadying zone, the yeast grows undisturbed and finally flocculates so that the yeast content of the discharge from the fermentation stage 1 is increased.
Outlet 11 leads to the settler 5 from which pipe 3 returns the yeast cream to the fermenting stage 1 while pipe 14 branches ; off therefrom. The homogeniæation of the returned yeast cream is effected by means of pumps 13 and 20, respectively and by the intensive flow in feed pipe 18. The introduction of fresh mash is effected either into the clr~ulation condult 19 or in the connecting .

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a-ea of circulation conduit 19 to vat 17 at the region of the outlet of the opening of conduit 18.

FIG. 3 shows another design of a fermentation stage 1 where the air necessary for the accelerated fermentation is injected close to the bottom of vat 17 inside the tubular cha~ber 4, so that here too the flow described in FIG. 2 is obtained. In contrast to FIG. 2, the returned yeast cream is introduced through pipe 3 by means of pump 13 directly in homogenized form into vat 17. The outgoing air is removed as shown by arrows 26.

In the embodiments of FIGS. 2 and 3, the individual settlers5 and 5' have a much larger volume than fermentation stage l, so that even in a multi-stage sedimentation plant a volumetric ratio of at most l:l is achieved. This is a great advantage over known embodiments of yeast separating plants since the volume of the settlers represents here a multiple of the fermentation vat.
FIGS. 4 and 5 show multi-stage fermentation processes.
In FIG. 4, a two stage fermentation process is shown, wherein settling stage 2 and 2', respectively is assigned to each fermenta-tion stage l and l', and wherein return pipes 3 and 3', areprovided respectively for the yeast cream, leading from each sedimentation stage into the feed pipe of the preceding fermenta-tion stage l and l', respectively. The excess yeast cream is removed through pipes 14 and 14', respectively and fed to a productiDn plant similar to FIG. l. In order to o~tain a `~

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uniformly high ethylene alcoh~ content in overflow 11', fresh mash is supplied to each fermentation stage 1, 1' through pipes 9, 9'. This is particularly important in multi-stage plants where more than two stages are used. In these plants the mash can be produced, for example, in the first stage from molasses, while pure sugar solutions are introduced through feed pipes 9, 9', so that the maximum alcohol content is not limited by the rising impurities from the molasses.
FIG. 5 shows a three stage fermentation plant with the individual stage being designated 1, 1' and 1", re~pectively.
These stages are provided with only a single common settler 2, in contrast to FIG 4, from which the yeast cream is returned through pipes 3, 3' and 3". For the exact dosing of the returned amounts of yeast cream pumps 13, 13' and 13" are pro-vided. In this embodiment too, fresh mash is supplied to each fermentation stage through pipes 9, 9' and 9", and the fermentation stages 1' and 1" also receives the discharge from the preceding fermentation stages 1 and 1' respectively.
FIG. 6 shows the arrangement of a pilot plant where the fermentation vat of fermentation stage 1 is provided with a " separate ventilation system 22, 90 that a low velocity of flow is obtained in the fermentation vat, which permits floccular settling of the yeast in the upper region of the fermentation vat.
The yeast-enriched mash is fed through p1pe 11 to thickener 12, which may be a centrifugal thickener, designed as a degasifier, ,, ~5~

as soon as a pxedetermined alcohol concentration has been obtained, and the waste gas escapes in the ~rection of arrow 26'. The de-gased mash is fed to settler 2 in ~ich the yeast settles to the bottom of the funnel shaped settler and is returned by way of dosing pump 13 into the fermentation vat. The excess yeast ob-tained is separated through pipe 14. The fermented mash which is freed of the settled yeast is removed through pipe 16. The use of the settler results in only flocculated yeast being returned to the fermentatinn vat while the non-flocculating yeast and bacteria are removed with the alcoholic mash and fed to the distillery. This results in a reduction of the risk of infection in fermentation stage 1 so that the alcohol produced is of high purity.
In tests which are carried out, the fermentation stage was charged with 12 1 molasses mash up to the overflow limit.
The concentration of fermentable sugar was 103 g/l. The circulation and simultaneous ventilation (arrow 8) were effected by means of pump 20. After about 10 hours, fresh mash was supplied continuously by means of a dosing pump, the mash having the above mentioned sugar concentration, through,pipe 9. The overflow rom fermentation stage 1 opened into the settling plant consisting of degasifier 12 and settler 5. The s~ttled yeast from the latter was returned by means of pump 13 into the fermentation stage 1. When the fllling level was reached in settler 5, the dosing of fresh mash was so adjusted that the discharge through 16 : . .. .
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of settler 5 was completely fermented. In this manner the yeast was constantly enriched in the entire system, and the supply of fresh mash could be constantly increased according to the pre-vailing yeast concentration until excess yeast was found in the discharge from 16 of the settler. The amount of fresh mash was so adjusted that the yeast just completely fermented the charged sugar. This dosing rate was reduced by not more than one third of the value required for complete ferrllentation, i.e. the minimum rate of feed in this case was 0.5 1 mash per liter of fermentation vat contents per hour. At this rate of feed the retention periods in the fermentation stage were 1.33 and 2 hours. The yeast con-centration in the fermentation stage attained a maximum of 15~/o of the concentration required for complete fermentation~ The optimum yeast concentration in the fermentation stage was 41 to 62 g dry ~ yeast per liter with a retention period of 2 hours in the fermenta-i tion stage. The removal of the excess yeast was effected either jointly with the overflow, or in concentrated form by means of the pump from the bottom part of the settler 5.
The fermentation was carried out over a longer period of time equilibrium state. Alcohol concentrations between 6.44 - 6.73%
by volume were measured in discharge 16. Thus, the yield was be-tween 625 and 653 lit~ers alc~ol per 100 kg fermentabLe sugar, which in this case was almost excIusively saccharose. The rate of feed of mash was 0.73 to 0.79 leters per liter fermentation stage contents per hour. Thus, the productivity is 47.5 - 50.9 ml alcohol `~ per liter fermentation stage contents per hour. The production o~

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$, -10-excess yeast was between 1.36 and 5 g dry yeast substance per liter of discharged mash. A compilation of the test values is found in the table below.
In another test series a two-sta~e pilot plant similar to th~t of FIG. 4 was used. The rate of feed of fresh mash was set at 6.75 liters of mash and the yeast concentration in the fermentation stage was about 55 g per liter. The overflow of settler 5 with the excess yeast produced was introduced into a second fermentation stage having a capacity of 48 liters, to which a settler was likewise connected to return the yeast into the second fermentation stage. To the discharge from the first settling stage 2 were continuously added 1.62 liters concentrated mash per hour in the form of a molasses solution containing about 400 g sugar per liter. The alcohol concentration in outlet 11' of the second settling stage was 10.5D/D by volume.
The productivity of the two fermentation stages together was thus about 15.3 ml alcohol per liter fermentation stage content per hour.
The following table aets forth the productivity and yield of ethyl alcohol and yeast in the continuous intensive process according to FIG. 6.

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Test period Sugar in Yield of Excess Yeast Productivity *) 24 hours each g/l mash Ethylalcohol T.S. -~) Ethylalcohol Yeast in in ml/kg g/l mash ml/l h g/l h vol% sugar 1 103 6.44 625 2.8 50.9 2.21 2 103 6.73 653 5.0 50.7 3.76

3 103 6.4~ 629 2.76 47.5 2.02

4 106 6.66 628 1.89 52.0 1.47.
110 6.80 618 1.85 5402 1.47 6 108 6.70 620 2.18 51.7 1.68 7 107 6.73 629 1.36 50.4 1.02 Average: 628.8 2.54 51.0 1.94 *) relates to ml and g respectively per liter fermentation stage contents per hour ~) dry substance While the invention has been illustrated in particular with respect to specific apparatus and process conditions, it is apparent that variations and modifications of the invention can be made without departing from the spirit or scope thereof.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method for the yeast fermentation of sugar-containing mash to alcohol, which comprises subjecting a yeast and sugar-containing mash to fermentation to produce an alcohol-containing solution, withdrawing the alochol-containing solution, which also contains yeast, from the fermenting mash, permitting the thus withdrawn alcohol-containing solution to settle, whereby flocculated yeast settles while non-flocculated yeast and bacteria remains suspended, withdrawing the settled yeast which is substantially free of the suspended bacteria, and reintroducing the same to the fermenting mash.

2. Method according to claim 1 wherein the withdrawing of the alcohol-containing solution, the settling of the withdrawn alcohol-containing solution, the withdrawing of the settled yeast and the reintroducing of the same into the fermentation stage are effected continuously.

3. Method according to claim 2 wherein the settled yeast is reintroduced into the fermenting mash along with fresh mash.

4. Method according to claim 3 wherein the withdrawn settled yeast is homogenized prior to being reintroduced into the fermenting mash.

5. Method according to claim 2 wherein the fermenting mash contains up to 15% of the yeast required for the fermentation of the mash and wherein the time of settling of the yeast from the withdrawn alcohol-containing solution is up to one half the time that the yeast remains in the fermenting mash.

6. Method according to claim 5 wherein the settled yeast is reintroduced into the fermenting mash in dosed amounts.

7. Method according to claim 2 wherein the mash fermentation is effected in successive passages through a plurality of fermentation vats and wherein fermented mash from each fermentation vat is subjected to settling.

8. Method according to claim 2 wherein a portion of the settled yeast is removed from the system.

9. Method according to claim 2 wherein the withdrawn alcohol-containing solution which also contains yeast is degassed prior to settling.

10. Method according to claim 9 wherein the degassing is effected to a concentration of alcohol of not more than 5% by volume.

11. Method according to claim 1 wherein oxygen in the form of air, pure oxygen or oxygen enriched gases is added to the fermenting mash.

12. Method according to claim 7 wherein oxygen in the form of pure oxygen, air or oxygen-enriched gases is introduced into the fermenting mash in each fermenting vat.

13. Method according to claim 1 wherein the hydrogen ion cencentration of the fermenting yeast is maintained at a pH
of less than 6.

14. Method according to claim 13 wherein the pH is 5.4.

15. Method according to claim 2 wherein the settling stage is subjected to an eddy current to accelerate the aggrega-tion of the yeast cells.

16. Apparatus for continuous yeast fermentation of sugar-containing mash to alcohol, comprising at least one fermentation vat in which yeast and sugar-containing mash is subjected to fermentation to produce an alcohol-containing solution; at least one settler in which alcohol-containing solution containing yeast is subjected to settling whereby flocculated yeast settles while non-flocculated yeast and bacteria remains suspended; first conduit means for withdrawing alcohol-containing solution from said fermentation vat and introducing the same into the settler; and second conduit means for withdrawing settled yeast from the settler and reintroducing the same into the fermentation vat.

17. Apparatus according to claim 16 wherein said fermenta-tion vat comprises a cylindrical reactor within which is located a cylindrical, concentrically arranged tube which acts as a riser for gases introduced into the reactor and which separates rising current from falling current in the reactor.