WO1989005860A1

WO1989005860A1 - The continuous process of removing ethanol from mash during ethanol fermentation (crem process)

Info

Publication number: WO1989005860A1
Application number: PCT/AU1988/000480
Authority: WO
Inventors: Siang-Chung Yu; Pinnan Soong
Original assignee: Yu Siang Chung; Pinnan Soong
Priority date: 1987-12-18
Filing date: 1988-12-14
Publication date: 1989-06-29
Also published as: AU2900689A; AU631527B2

Abstract

A continuous process of ethanol fermentation and concurrent entrainment of ethanol from the mash has been achieved. This comprises dual fermenter vessels with continuous recycling of carbon dioxide through the first fermenter. The stream of carbon dioxyde entrains aqueous ethanol vapour which is subsequently separated from the gaseous carbon dioxide by condensation and recovered as aqueous ethanol at concentrations up to 39% v/v. In the second fermenter, the mash undergoes final intensive fermentation and is then subjected to distillation. The separated carbon dioxide can be recycled back into the first fermenter for further entrainment.

Description

TITLE: THE CONTINUOUS PROCESS OF REMOVING ETHANOL FROM MASH DURING ETHANOL FERMENTATION (CREM PROCESS)

A.. INTRODUCTION

In current, conventional ethanol fermentation processes, the fermentation ceases when the ethanol concentration reaches 8-12%. At. this stage, the mash may, for example, be diluted or else withdrawn from the fermenter for distillation to remove ethanol. The reason is that the activity of yeast is hindered or even demolished at this level of ethanol concentration. Extension of the fermentation time does not help. The situation is even worse in solid state fermentation. The Chinese Kao-Liang wine, for instance, is a high ethanol content liquor (up to 50%) and is the product of solid state fermentation of sorghum. Its fermentation process must be interrupted three times for distillation to remove ethanol from the mash when the concentration reaches 8%. At each stoppage, the mash is cooled, mixed with koji and yeast to allow fermentation to restart. Therefore, the whole process needs 24-30 days to completion. This is an inefficient and very costly operation and highlights the major technical problem in ethanol production that is presented by inhibition of fermentation due to the increase in ethanol concentration.

In order to overcome the high ethanol concentration problem, selection of ethanol-tolerant yeast strains or lower fermentation temperatures has been utilised in current industrial processes. However, the improvements are limited, whilst there are undesirable consequences as well, such as the longer fermentation time at low temperature.

Ethanol has for a long time been used as a replacement for petroleum products in automotive fuel, as long as its cost competitiveness and economic advantages generally are adequate. Unless the above described yeast-ethaπol barrier can be overcome, however, the fLiture of ethanol in this field woLtld be doubtful. This patent introduces a process which is successful in overcoming this barrier by removing ethanol continuously out of mash without interruption of the fermentation process. B. BASIC PRINCIPLES AND APPLICATION

Experience tells us that under normal conditions fermentation stops as ethanol concentration in mash rises towards 12%. There are exceptions in particular cases, such as the Chinese Shau-Hsin wine (a fermented alcoholic beverage made without distillation), which has ethanol content of l6-18%. However, to reach this ethanol level, the fermentation temperature is cooled to 15°C and it requires three weeks to complete the fermentation. This kind of operation obviously is not suitable for economical ethanol production.

Assuming that the ethanol concentration in mash is at its optimal level, the activity of yeast is uninterrupted and the fermentation rate remains unchanged. Any excess ethanol above this optimal level must be removed from mash or diluted. Dilution of the ethanol would create more difficulty in the final distillation step which is highly energy-cosuming. On the other hand, removal of ethanol from mash is much easier to achieve. Various approaches for ethanol removal are known, such as illustrated by the following examples.

1. Reverse osmosis:

Ethanol is forced through a special type of membrane which prevents diffusion of water, sugar, and yeast, while ethanol is concentrated on the other side of the membrane. However, manufacture of this type of membrane is a complex technology with high costs of production as well.

2. Vacuum evaporation:

One process for vacuum removal of ethanol, developed at University of California, removes ethanol vapor from mash by a vacuum process. A major disadvantage is that equipment and installation investment is high, so that the total cost reduction is limited when compared with conventional processes. Furthermore, the yeast activity may be adversely affected under vacuum conditions.

The underlying principle of the process set out in the present patent is likewise the removal of ethanol from mash during fermentation, but by continuous recycling of carbon dioxide (CO₂), generated as the by-product of fermentation using yeast or other suitable microorganisms (with sugars being fermented to ethanol and CO₂). In conventional processes the CO₂ produced is expelled from the fermenter spontaneously and collected, usually carrying with it a small amount of saturated mixed ethanol amd water vapors. The entrained ethanol may be recovered by washing with water in an absorber. Our proposed patented process as set out in this patent is derived from this concept.

In applying this concept, the basic principle of our process consists of continuously recycling the by-product CO₂ back to the mash to carry out more aqueous ethanol vapor which is condensed in a cooling unit, while the CO₂ is separated and recycled. Such a concept would involve concurrent continuous addition of the fermentation substrate to sustain continuous operation. This substrate may consist of various sugar and starch based raw materials e.g., molasses, cereal grains, etc..

This conceptual system, however, cannot function continuously due to accumulation of impurities from the raw material in the mash, leading to eventual poisoning of the fermentation process. This impairs the efficiency of the system and limits its practical application. Consequently, in order to achieve a practically feasible process with high degree of productive efficiency and commercial viability, the concept of ethanol entrainment with CO₂ has been combined with techniques of preventing accumulation in the mash of the impurities from the fermentation substrate.

Among the various methods to achieve this one may cite, as particular examples, the use of special membrane filters or the application of co-fermenter technology. These techniques can be applied either separately or in combination.

In particular, the application of co-fermenter technology, consisting of a main fermenter and co-fermenter set up in series, in conjunction with CO₂ entrainment of ethanol from the main fermenter forms the basis of our CREM process, enabling it. to attain advanced standards of production and cost efficiency.

The use of a co-fermenter (CF) in conjunction with the main fermenter (MF) involves linking up the two vessels by pipework, allowing mash from MF to overflow continuously into CF, while concurrently fresh mash is fed into MF. By this means, there is no process of accumulation in MF of impurities introduced from the fermentation substrate. In the CF the fermentation of sugars in the mash is completed, while at the same time the fully fermented mash is continually withdrawn and subjected to final distillation. We have found that this procedure can be optimised by special operating techniques. First of all, the mash from MF is introduced at the bottom of CF through a mushroom type stainless steel distributor which ensures slow, even and turbulence-free spread of the liquid in the bottom layers of the mash in the CF. This allows the yeast to settle and concentrate in the bottom layers of the CF. This technique confers several advantages which include: increased efficiency of ethanol fermentation in the CF mash by the concentrated yeast in the bottom layers; the accumulation of the yeast at the bottom of the CF facilitates and reduces costs associated with the withdrawal of the yeast (from the bottom of the CF), this yeast being recycled to the MF for fermentation of mash, or put into storage for subsequent use. The transfer of mash from MF to CF proceeds at a slow, controlled rate, so as to ensure efficiency and a high degree of conversion of sugar to ethanol in the CF. Thus, whereas the CF has a capacity of about 1/5-1/20 of the MF, the volume of overflow from MF to CF every hour is of the order of 1/100-1/20 of the load of mash in the MF.

As will be seen, the entrainment with CO₂ of the aqueous ethanol in the MF of the CREM process normally retrieves most but not all of the ethanol recoverable from the mash, which is generally subjected to the intensified final fermentation in the CF, followed by distillation to recover the remaining ethanol. However, the mash volume requiring distillation in the CREM process is considerably less than that obtaining in conventional batch or continuous processes because, during the fermentation/ethanol entrainment operation in the MF, a large amount of aqueous ethanol is continuously removed from the mash (while at the same time fermentation substrate is continuously added to the fermentation system). All this results in a much more concentrated mash liquor and therefore the mash volume requiring distillation is much smaller than for conventional processes and requires much less distillation capacity and energy.

Also, as a direct consequence of the above, the waste final stage stillage is likewise reduced several-fold compared to conventional processes and thus creates far less environmental pollution problems. These are far more manageable and far less costly for the CREM process than with conventional processes, whether using batch or continuous operation.

As indicated above, the entrainment of ethanol by CO₂ in the main fermenter is an important factor in achieving greater efficiency in the overall CREM process. This phase of the process enables the production of a large part of the total ethanol yield in a relatively concentrated form as demonstrated in practical trials. One such experiment has indicated that when ethanol concentration in mash was 8% and the mash temperature was 35°C, the partial pressure of ethanol vapor in the vapor mixture entrained by the CO₂ was 100 mm Hg and the water vapor at the same time had a partial pressure of 40 mm Hg. After cooling down to 0°C, CO₂ still remained in the gas phase, but ethanol and water were converted to liquid phase. Thus, CO₂ was completely separated from the ethanol. The ethanol concentration in the condensed liquid phase was more than 30% (thus approaching the theoretical value of 35.5% calculated for the conditions of the trial). During the conduct of the trial, the fermentation activity of the yeast remained unhindered.

The CREM process is flexible in application and can be modified for adaptation or combination with various types of conventional operations, batch or continuous.

It is likewise possible to adapt existing systems to implement the CREM process by modifying some features of the existing plant. As examples of such modifications, a blower and a cooling unit may be connected to a conventional fermenter with the blower recycling the CO₂ , while bubbling devices can accomplish dispersion of CO₂ through the mash. The mixed vapor which is entrained can be collected by gas collectors and then chilled by a cooling unit, whose operation uses a principle similar to that of dehumidifiers condensing water from air. Likewise, the stream of CO₂ generated during the process may be amplified by supplementary CO₂ obtained from other sources. Furthermore, supplementary or alternative gases or volatile liquids may be used to blend with or replace the CO₂ used for entraining ethanol as described above, provided they do not cause undesirable chemical or other interactions with any of the contents of the mash or the entrained vapors. These materials may include, among others, helium, nitrogen, hydrogen, air, as well as volatile liquids boiling below a temperature range of 25-45°C, such as various aerosol propellants. For each specific material used, naturally, the operating conditions and results will vary.

The ethanol obtained by the CREM process can provide any grade of ethanol required, whether for industrial or potable purposes.

As a special application of the principle of the CREM process, it can also be used in solid state fermentation, but in this case with exclusion of the co-fermenter. Thus, in the solid state fermentation of Chinese Kao-Liang wine, our process starts to entrain ethanol with CO₂ from the mash when the ethanol concentration in the mash reaches 3-10% and continues until the end of fermentation. In this way, the first and second stoppages for distillation (mentioned earlier in Section A) are eliminated. The fermentation time is also reduced to half, in other words the productivity of the fermenter is doubled.

To adapt our system to the solid state fermentation, similar equipment modifications may be applied as indicated above for liquid state fermentation (i.e. blower, gas collector, cooling unit). C. CONCLUSION

As a consequence of the novel features of our process described in the foregoing, this innovative process provides considerable advantages which are listed in the following summary, compared to conventional industrial processes, whether batch or continuous.

1. Greater productivity with more ethanol obtained in the same period of time for the same size of fermenter.

2. Reduced investment for the same yield of ethanol.

3. Ready adaptation to any type of plant, whether new or existing.

4. Reduction in waste residues with minimised pollution problems and costs.

5. Reduced overall fixed and variable production costs, mainly related to labour, services/energy, equipment.

D. EXAMPLES

The following examples illustrate the effect of entrainment of ethanol by CO₂ or other gases, as described in the preceding text.

1. A container with 2 openings was loaded with 10 litres of 8% aqueous ethanol solution. One opening was connected to a blower, the other to a cooling condenser. The container was maintained at 35°C, while air and CO₂ gas, first preheated to 35°C, were alternatively in separate experiments injected through the blower and bubbled continuously through the 8% ethanol solution. The entrained condensate which was collected showed a 35% concentration of ethanol.

2. A cooling condenser was connected to the outlet of the CO₂ collector in a 50 M³ fermenter, in which the mash had attained an ethanol concentration of 9.8%. The mash temperature was 35°C. The condensate obtained by CO₂ entrainment gave an ethanol concentration of 39%.

Claims

Cl ai ms :

Claim 1. A continuous ethanol fermentation process, utilising various sugar base or starch base raw materials which are converted to fermentable sugars by malt, koji or other microbial enzymes, the fermentable sugars being subsequently converted to ethanol by various microorganisms such as yeast, Zymomonas mobilis, or Thermoanaerobacter ethanol i cus . The conti nuous f ermentati on process compr i ses a combination of a dual fermenter system (main fermenter and co-fermenter mounted in series) with continuous entrainment of ethanol by a stream of carbon dioxide (CO₂) from the main fermenter (MF). The CO₂ is largely by-product from the fermentation. The dual fermenter system is essential in preventing accumulation of impurities from the fermentation substrate and resulting inhibition of fermentation and interruption of the process.

The ethanol generated in MF is continuously entrained as mixed ethanol /water vapors which are then chilled in a cooling unit and condensed as aqueous ethanol containing 20-60%, but preferably 30-40% of ethanol, v/v. The CO₂ gas, which is separated from the condensed aqueous ethanol, is continuously recycled to the fermenter and sparged through the mash, thus maintaining the continuous entrainment of ethanol from the mash.

At the same time sugar solution is continuously fed to the mash in MF so as to maintain continuous fermentation to ethanol. The aqueous ethanol from the cooling unit can be distilled into high percent ethanol. Alternatively, it can also be mixed with fermented mash and the mixture distilled into high percent ethanol.

Concurrently, mash is continually withdrawn from the MF to the co-fermenter (CF) to prevent accumulation of various impurities in the MF mash which would poison the fermentation process. By this means maximum production and cost efficiency is achieved.

The CF has a capacity of 3-50%, but preferably 5-20% of MF. The mash is fed slowly into CF from MF through a special distributive device to ensure settling and concentration of yeast in the bottom layers of CF.

The hourly rate of overflow of mash from MF to CF is in the range 1-5%, but preferably 2-3%, of the amount of mash in the MF. Such a flow rate also prevents accumulation of soluble impurities in the mash, their concentration consequently remaining approximately constant.

This flow rate to CF is equivalent to a column of mash in the co-fermenter of no more than 2 meters per hour, such a rate of inflow ensuring that the yeast in the mash settles and becomes concentrated in the bottom layers of mash in the CF. According to this procedure, as the settled yeast accumulates and rises to the mid-levels of the CF, it is bled off until the level is reduced to about 1/10-1/3 of the full level of mash in the CF. The mash after passing through the CF overflows and is directed to distillation to produce high percent ethanol. The period of residence of mash in the CF is preferably 2-4 hours.

The CREM process results in considerable reduction of stillage residue and consequently pollution problems.

Claim 2. In the process according to Claim 1, another method to prevent inhibition of fermentation on account of accumulation of various impurities, and thus to achieve maximum efficiency, is to subject the fermentation substrate to filtration through special membrane filters which remove the salt and/or non-fermentable organic impurities. This method may be combined with the dual fermenter system.

Claim 3. The sugar substrates used in the method of Claim 1 are obtained from cane sugar, cane juice, cane molasses, syrup, high test molasses, sorghum juice, sorghum molasses, corn syrup, corn molasses, beet sugar, beet molasses, maple sugar, maple juice, palm juice, various kinds of fruit juice such as grape juice, orange juice, pineapple juice, mango juice, lichee juice, longan juice, or other materials from which fermentable sugars are derived.

Claim 4. The starch used in the method of Claim 1 is extracted from various sources of starch, including grains such as barley, oats, wheat, rye, rice, corn, sorghum, millet, sago; tuberous roots, such as potato, sweet potato, cassava, yam; seeds, such as garden pea, lima bean, green lentils, cowpea; banana, plantain, taro; or artichoke, chicory, dahlia, etc.

Claim 5. Fermentable sugar solutions used in the liquid state fermentation of the method of Claim 1 may have a sugar concentration of 5-25%, but preferably 12-18%.

Claim 6. In the process according to Claim 1, the ethanol content of mash may be maintained in the range of 3-12% and preferably of 5-9% .

Claim 7. The temperature of the mash in the process according to Claim 1 may be in the range 20-75°C, depending on operating conditions and particularly on the type of microoganisms used for fermentation. When using yeasts or

Zymomonas, the preferred temperature range is 25-35°C.

Claim 8. In the method of Claim 1, the ratio of the CO₂ used to entrain ethanol vapor from the mash may be from 0.2:1 to 8:1 v/v/minute in relation to the mash.

Claim 9. In the method of Claim 1, the CO₂ from the cooling unit is pre-warmed through a heat exchanger utilising surplus heat from the CREM process, prior to sparging through the mash.

Claim 10. The CO₂ generated by the process of Claim 1 and used for entrainment of ethanol may be replaced wholly or partly by CO₂ from other sources, or by other gases like helium, nitrogen, hydrogen, air, as well as volatile liquids boiling below a temperature range of 25-45°C, such as aerosol propellants. Such materials are selected so as to be inert in relation to the mash and surrounding vapor.

Claim 11. In the method of Claim 1, the ethanol liquid either collected from the cooling unit, or after distillation to high percent ethanol may be used for various industrial or potable uses.

Claim 12. The ethanol liquid collected from the cooling unit in the method of Claim 1 can be mixed with the filtrate from the fermented mash after filtration for preparation of, or mixing into, alcoholic beverages.

Claim 13. The ethanol liquid collected from the cooling unit in the method of Claim 1 can be mixed with distilled ethanol liquid for preparation of, or mixing into, alcoholic beverages.