CA1110985A

CA1110985A - Production of volatile organic compound by continuous fermentation

Info

Publication number: CA1110985A
Application number: CA320,434A
Authority: CA
Inventors: Lars K.J. Ehnstrom
Original assignee: Alfa Laval AB
Current assignee: Alfa Laval AB
Priority date: 1978-01-31
Filing date: 1979-01-29
Publication date: 1981-10-20
Also published as: FI66905B; NL7900803A; ZA787390B; FI66905C; FI790258A; JPS54110387A; GB2013716B; NZ189485A; SE430171B; DE2903273C2; DK37779A; SE7801133L; DK149782B; SU1303034A3; FR2416263B1; IN150767B; PH17707A; JPS6043117B2; AR222649A1; CA1140873B

Abstract

ABSTRACT OF THE DISCLOSURE

A flow of fermentation liquor from continuous fer-mentation of a carbohydrate-containing substrate is centrifu-gally separated into a yeast-free flow and a yeast concen-trate flow, the latter being recirculated to the fermentor.
The yeast-free flow is separated by a thermal method into a flow enriched in ethanol, which is discharged, and a residual flow which is recirculated at least in part to the fermentor.
The concentration of ethanol in the fermentation liquor is maintained no greater than ?% by weight.

Description

8~

This invention relates to a method for producing a volatile organic compound, preferably ethanol, by continuous fermentation of a substrate, containing carbohydrate, in a fermentor. "Fermentor" in this context means either a single fermentor or a number of fermentors coupled in series, or possibly one tubular reactor for "plug-flow" operation (i.e.l for a reaction with a stable reaction gradient). Con-sidering ethanol of technical grade~ this is mainly manufac-tured by exclusively synthetic methods from ethylene, which in turn is obtained from petroleum or natural gas.
Due to the limited supply of fossil raw materials, investigations have been started to assess the possibilities of obtaining fuels and chemicals from renewable resources (i.e., of a vegetable nature). Thus, the old method of pro-ducing ethanol from carbohydrates, broadly defined, hasacquired renewed interest. It may be noted that the Swedish organic chemical industry, developed after World War II, was based upon sulphite-spirit, which was converted into ethylene and further into ethylene oxide, etc. The changing raw material situation has thus made such synthesis routes interesting. If up to 20% anhydrous ethanol is mixed into gasoline, the octane number is improved, and only minor changes have to be made in combustion engines to make them operate on such a mixed fuel. In many countries having great quantities of cheap sugar or starch-based raw materials available, a vigorous development work is under way along those lines. Recent research results make it possible to believe that the conversion of cellulose into fermentable sugar will be economically feasible.

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Cellulose is available all over the world in seem-ingly unlimited quantities and is a potentially ideal raw material for the production of ethanol.
Most presently existing plants for the production 5 of ethanol by fermentation are based upon simple batchwise -~
operation, a relatively diluted raw material being utilized.
In this way, enormous amounts of waste water (i~e., slop) are obtained which, if le~t untreated to the recipient, will mean a very high biological load upon the recipient. Production of ethanol on a great scale by fermentation is feasible only if the waste water can be disposed of in an economically jus-tifiable way without deterioration of the environment.
The drawbacks of the methods used hitherto for the production of ethanol by fermentation are related primarily to the diluted substrate, which gives a low fermentation rate and severe problems with great volumes of diluted waste water. In conventional ethanol fermentation, the substrate concentration is such that the ethanol concentration will be about 7~ (weight) in the fermentation liquor after termina-tion of the fermentation. If molasses is used as a sub~
strate~ the original concentration of same is about 22 Bx, and the molasses may be fermented completely to ethanol.
Higher concentrations of molasses cannot be fermented com-pletely, as the ethanol formed will inhibit the fermentation.
In order to eIiminate the drawback due to the in-hibiting ethanol accumulated in the fermentor, it has been suggested to keep the fermentor under a reduced pressure to make the ethanol formed ~oil away at the prevailing tempera-ture, which is kept a level low enough not to deteriorate the yeast. One serious drawback inherent in this method is that all the carbon dioxide formed during the fermentation must s be removed by the apparatus used for producing the reduced pressure, which means a great energy consumption.
It has also been suggested that the ethanol which is formed be removed during the fermentation from the sub-strate with the aid of an azeotrop-former. In this case, the ethanol will form with water and a solvent a so-called azeo-trop, which has a lower boiling point than the corresponding ethanol-water-mixture. The ethanol is then separated in a subsequent step from the azeotrop. There are many drawbacks inherent in such a process. Among other things, it is very difficult to find an azeotrop-forming solvent which does not influence the activity of the yeast in an unfavorable way.
The principal object of the present invention is to provide a method of the type initially mentioned which allows a substrate with a high concentration of carbohydrate to be fermented and which gives rise to a concentrated waste flow which can be rendered harmless or be utilized in an effi-cient, economical way.
According to the invention, a method of the type initially mentioned is characterized in that a flow of fer-mentation li~uor is separated by centrifugal force into at least one yeast concentrate flow and one yeast-free flow, the yeast concentrate flow being recirculated to the fer-mentor, the yeast-free flow being separated into one flow en-~5 riched in volatile organic compound, which is discharged, andone residual flow which is at least in part recirculated to the fermentor.
By recirculating the yeast concentrate to the fer-mentor, the yeast is protected from being deteriorated when separating the volatile organic compound. The recirculation of yeast also means that a higher concentration of yeast may be maintained, which in turn means a higher fermentation rate.
It is advantageous to separate a flow of fermenta-tion liquor into three flows, namely, the two already men-tioned and one sludge flow containing impurities. This canbe carried out by a centrifugal separator which separates the incoming flow of fermentation liquor partly into a continuous yeast concentrate flow, partly into a continuous yeast-free flow, and partly into an intermittent sludge flow. This means that sludge is accumulated in the peripheral part of the rotor of the centrifugal separator and is discharged intermittently therefrom. In this way, impurities are pre-vented from being accumulated in the system. In one conveni-ent embodiment of such a centrifugal separator, the yeast concentrate flow is discharged with the aid of a paring tube, and the yeast-free flow by a paring disc.
As previously mentioned, the conventional ethanol fermentation techniques utilize a substrate concentration such that the ethanol concentration is about 7~ (weight) after termination of the fermentation. If molasses is used as substratel the original concentration of molasses is about 22 Bx. Such a substrate can be fermented completely.
Higher concentrations of molasses cannot be fermented com-pletely, as they would give rise to higher ethanol concentra-tions than 7% (weight), which act to inhibit the fermentationreactions. ~ccording to the present invention, molasses with a concentration as high as 50-60 Bx may be fermented, if the ethanol is prevented from growing over about 5% (weight) by continuous removal of ethanol from the circulation circuit of fermentation liquor.

S

In the method according to the invention, a carbo-hydrate concentration of no more than 5% (weight) is main-tained in the fermentation liquor.
According to one embodiment of the new method, the yeast-free substrate flow obtained by centrifugal separation is then separated by a thermal method into one flow enriched in organic compound, such as ethanol, and one residual flow which at least in part is recirculated to the fermentor, while one partial flow is discharged from the circulation circuit in the form of a slop. According to the present in-vention, this slop is much more concentrated than slops ob-tained in distilling methods hitherto operated together with ethanol fermentation. "Thermal method" means essentially distillation and fractionation, but other thermal methods such as evaporation can be used as well. One great advantage inherent in the present invention is that the distillation curve for ethanol is considerably improved at the high sub-strate concentrations that are utilized. The concentration of ethanol in the vapor which stands in equilibrium with fermentation liquor of a certain ethanol concentration thus is much higher, provided that the concentration of carbo-hydrate is higher in the fermentation liquor.
As an alternative for thermal methods for separat-ing the ~olatile organic compound from the circulating fer-mentation liquor, extractive methods may be used. That is,the ethanol is extracted by any suitable circulating sol-vent which has a major affinity for the compound in question, but a minor affinity for water. An example of such a sol-vent is octanol, if the volatile organic compound is ethanol.
The latter is obtained from the octanol solution by frac-tionation. Such an extractive method has a good heat econo~y.

s The thermal or extractive separation of the vola-tile organic compound may be carried out substantially at atmospheric pressure. Indeed, thermal separation in a vacu-um has the advantage that a lower temperature may be main-tained, but the operational costs for maintaining a vacuumare a drawback.
The method according to the invention may be oper-ated in one ~ermentation stage, but some advantages may be gained by operating the method in a plurality of fermenta-tion stages coupled in series, as a better heat economy maybe achieved.
In a suitable embodiment of the new method, the residual flow from the thermal or the extractive separation is pasteurized at a temperature of 60-100~ C before it ls re-circulated to the fermentor.
Another great advantage with the new method is thatindependently of the way of obtaining the volatile organic c.ompound, a waste flow is obtained with a high concentration of substance which can be disposed of in an economically reasonable way.
~n connection with the ethanol fermentation, a slop is obtained which is 4-6 times more concentrated than the slop obtained in connection with conventional ethanal distillation.
Thus, according to the invention a slop with a positive value of combustion heat is obtained, which contri-butes to good operational economy of the method. Consider-ing the high concentration of organic material, the slop might be used as raw material for the manufacture of pro-ducts such as furfurol.
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S

The methcd of the invention will now be described more in detail, reference being made to the accompanying drawings, in which:
Fig. 1 is a principal flow sheet of the method according to the invention;
Fig. 2 is a flow sheet of the method carried out in one stage, with distillative separation of the volatile organic compound;
Fig. 3 is a flow sheet of the method carried out in two stages coupled in series, and Fig. 4 is a flow sheet of the method carried out in one stage, with extractive separation of the volatile organic compound.
The system shown in Fig. 1 comprises a fermentor F, a centrifugal separator C/ a unit RU for removal of a volatile organic compound such as ethanol, and a mixer M.
These units are connected via lines 1, 2, 3 and 4 in a cir-culation circuit. Centrifugal separator C is directly con-nected to mixer M via a line 5. An inlet line 6 to the cir-culation circuit is connected to mixer M via a line 5. Aninlet ~ine 6 to the circulation circuit is connected to mixer M. Fermentor F is provided with a gas outlet 7, centrifugal separator C with a sludge outlet 8, unit RU with an outlet 9 for a flow enriched in volatile organic compound, and line 3 with a bifurcation 10 for discharge from the cir-culation circuit. Centrifugal separator C is of a type that separates an incoming flow into two continuous liquid flows and one intermittent sludge flow. Fermentor F is shown as one sin~le fermentor tank. In practice, it is suitable to use at least two fermentor tanks coupled in series in a fermentation stage or to use a tubular reactor provided with a reaction gradient.
One embodiment of the method is shown in Fig. 2, where the volatile organic compound such as ethanol is sepa-rated from the circulation circuit by distillation, eitherin the form of one simple stripping operation or by frac-tional distillation. In this figure, the same reference notations have been used as in Fig. 1 for corresponding apparatus units and lines. Unit RU in Fig. 1 has been re-placed by a distillation unit D. Line 2 to the distillationunit and line 3 from it are in heat exchange relationship by a heat exchanger HE I. 1ine 3 passes a cooler HE II before the connection to mixer M.
~hen producing ethanol in a plant of the type shown in Fig. 2, a concentrated, clarified substrate is fed via inlet line 6 and mixer M to fermentor F. The flow is thus mixed partly with the yeast suspension from the cen-trifugal separator and partly with the yeast-free flow from the distillation unit, that is, the slop. In centrifugal separator C, impurity sludge is separated intermittently.
This sludge would otherwise be accumulated in the plant.
Ethanol is separated from the yeast-free flow in distilla-tion unit D, the heat required being provided either by in-direct heating or by feed of live steam. In the latter case, the dilution is compensated by an increase in the con-centration of the substrate fed. The advantage of live steam is that deposits on the heat transfer surfaces are avoided. Ethanol is discharged through outlet 9, and the slop leaves almost all its~available heat content to the flow entering the distillation unit via heat exchanger HE I.

A small part of the slop flow is discharged from the circulation circuit via outlet 10 and is so concentrated that is can be disposed of in an economical way~ The rest of the slop flow is cooled by heat exchanger HE II and is fed via mixer M to fermentor F. In the fermentation, there is formed a gas, mainly carbon dioxide, which is discharged through outlet 7. The method is carried out in such a way that the ethanol concentration in the fermentor is main-tained at a low level, i.e., about 4~ (weight), the sub-strate thus being fermented to almost 100% by the processparameters chosen. The ethanol concentration in the slop discharged from the distillation unit is quite low.
In order to improve the operation economy, it may be advantageous to couple a number of fermentors in series as shown in Fig. 3, which discloses schematically a plant comprising two fermentor tanks F and F2. The plant com-prises a double set of apparatus shown in Fig. 2. As in the plant described above/ slop from distillation unit D is con-veyed via a heat exchanger HE I and a cooler HE II and is divided partly into one flow which is fed to fermentor F and partly into one flow which is fed to fermentor tank F2 via line 12 and mixer M 2. The ethanol flow coming from dis-tillation unit D2 through line 13 is utilized for heating distillation unit D. In a plant of this type, the operation parameters are set in such a way that -the slop from distilla-tion unit D has an ethanol concentration of about 4~ (weight), whereas the ethanol concentration of the slop discharged from unit D2 through an outlet 14 is quite low.
As an alternative to distillative separation of the volatile organic compound from the circulation circuit, extractive techniques may be used. In Fig. 4 a plant for such a process is shown schematically. In addition to cer-tain elements shown in Fig. 2, the plant comprises an ex-traction unit E, such as a counter-current extraction column, and a distillation unit FR such as a fractionation column.
5 Line 2 from the centrifugal separator C is connected to ex-traction unit E, to which a solvent flow i5 also fed through an inlet 15. This solvent flow streams through extraction unit E, absorbs volatile compound, such as ethanol, and streams through a line 16 to distillation unit FR, wherefrom volatile organic compound is discharged through an outlet 17.
The bottom flow of solvent from distillation unit FR streams via a line 18 to inlet 15, to which is also fed solvent through an inlet 19. Part of the flow, exhausted in vola-tile organic compound, which is discharged from extraction unit E is discharged from the plant through line 20, whereas the rest is fed to fermentor F via mixer M. The distilla-tion unit is suitably heated by indirect heating 21.
Example As an exmple of the performance of the method according to the invention, continuous fermentation of mo-lasses in a plant of the type shown in Fig. 2 will be described.
In order to initiate the process, 10 kgs. of bakers yeast were charged in 100 liters of clarified 20 Brix molasses in a fermenting tank provided with a stirrer.
A cooler was provided in the circulation circuit. The fer-mentation temperature was controlled to 32 C, and the fer-mentabIe sugar was converted to 90% into ethanol within 3 houxs. During the latter part of the fermentation process, ethanol had to be removed continuously from the substrate in order to maintain an ethanol concentration of about 4%

(weight) in same. Therefore, fermentation liquor was circu-lated through centrifugal separator C, a yeast concentrate flow being recirculated to the fermentor, whereas a yeast-free flow was fed to a simple distillation unit D where the ethanol was separated at atmospheric pressure. When the fermentation process of the original charge was compl~eted, 7-13 kgs/hour clarified 40 molasses were fed continuously to the fermentor. In the fermentor a liquid volume of 100 liters was maintained. The fermentation was run for one week, during which time an ethanol flow with an ethanol con-centration of 25-35~ (weight) was discharged, whereas the ethanol concentration in the fermentor was maintained at about 4% (weight). A small slop flow with 25-30~ (weight) of DS was discharged from the circulation circuit. Operation data were noted for both feed rates in the table below.

Equilibrium Ethanol Feed rate fermentahle sugarproductivity 40 Brix molasses F3/F6 in the fermentor ~ (100~) _ ~gs/hour ~ (weight) kgs/100 litr./h.
7.0 10.0 1.0 1.0 13.0 5.5 2.5 1.7 F3/F6 means the relationship between the volume flows in lines 3 and 6 in Fig. 2. It is obvious from the table that the ethanol productivity increased with an increased feed rate, but at the cost of the utilization of the sugar fed, as a higher percentage of the latter remained non-utilized in the latter case. It is obvious that the optimization of the feed rate is determined by the relationship b~tween the raw material cost and the investment and operational costs.
It may be noted that the raw material was not sterilized in the fermentation described above. In spite of this fact, no accumulation of bacteria in the system .

,.

occurred, probably because the flow was sterilized in the distillation unit.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the production of ethanol, the method of con-tinuously charging a fermentor with a carbohydrate-containing sub-strate comprising fermentable as well as non-fermentable substance, maintaining yeast in the fermentor to produce ethanol by fermenta-tion of said fermentable substance, releasing from the fermentor carbon dioxide formed during the fermentation, withdrawing continu-ously from said fermentor a discharge stream of fermentation liquor containing yeast, ethanol and both fermentable and non-fermentable substance, continuously separating said stream of fer-mentation liquor into at least a continuous yeast concentrate flow and a continuous yeast-free ethanol-containing flow, returning said yeast concentrate flow to the fermentor, separating said yeast-free flow, in a second continuous separating step, into a portion enriched in ethanol and a residual slop portion, continu-ously and separately discharging said ethanol-enriched portion and part of said residual slop portion, continuously returning to the fermentor a remaining part of said residual slop portion, main-taining the concentration of fermentable substance in said discharge stream from the fermentor at greater than 2.0 and not more than 2.5%
by weight, and regulating said discharge stream to keep it at a flow rate which exceeds the charging rate of said substrate into the fermentor by an amount sufficient to maintain the ethanol con-centration in said discharge stream at about 4 to 5% by weight.

2. The method of claim 1, in which said separation of said yeast-free flow is effected by a procedure selected from distillation and solvent extraction.

3. The method of claim 1, in which said substrate is a molasses substrate having a concentration of molasses not ex-ceeding 60° Bx.

4. The method of claim 1, in which said centrifugal separating step separates the flow of fermentation liquor into three flows, namely, said yeast concentrate and yeast-free flows and an impurity sludge flow.

5. The method of claim 1, in which said separation of the yeast-free flow is effected by heating.

6. The method of claim 5, in which said heating is carried out at substantially atmospheric pressure.

7. The method of claim 1, in which said separation of the yeast-free flow is effected by extraction with an organic solvent.

8. The method of claim 7, in which said extraction is carried out at substantially atmospheric pressure.

9. The method of claim 1, in which said fermenting is carried out in a plurality of stages coupled in series.

10. The method of claim 1, in which said residual por-tion is pasteurized at a temperature of 60°-100° C before being recirculated to the fermentor.