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Acetone-Butanol-Ethanol Fermentation
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Acetone-Butanol production from renewable resources

 

Objective: To study the growth & A-B-E production kinetics of Clostridium acetobutylicum under batch cultivation conditions in bioreactor

 

Theory: Basic concepts & need of Acetone – Butanol - Ethanol production from renewable resources

 

Fluctuations and continuous upward trend in the price as well as uncertainty of a reliable steady supply of hydrocarbon feed stocks have a profound effect on petrochemical markets. Alternative manufacturing processes are being sought based preferably on renewable raw materials. Fermentation-based fuel processes & industrial solvent production processes are being re-examined for their potential in supplying these materials “competitively priced” & in an “environmentally” safe manner. While anaerobic product oriented fermentation processes have been known since the beginning of this century their industrial exploitation reached its peak between the World wars. New hopes are placed in the bioconversion processes and based on the rapidly changing hydrocarbon feedstock situation, recent breakthroughs in the development of superior microbial catalysts and on significant improvements in fermentation technologies coupled together with advanced process control and product recovery techniques. Last but not the least; it is the renewable resources which they are capable of utilizing which makes bioconversion processes so attractive. Not only can the renewable resource materials be utilized but a number of dilute waste streams originating from other manufacturing sectors constitute perfect raw materials for production of desirable compounds based on fermentation processes.

 

During the recent era of pressing energy shortage the process of acetone-butanol biosynthesis the fermentative production of higher alcohols (butanol, iso-propanol) and ketones (acetone) by bacterial strains has received relatively little attention as yet. Indeed, bacterial anaerobic product yielding processes in general are not so well understood and received only very scattered contemporary scientific attention, particularly when it comes to their more sophisticated industrial scale-up and utilization. This is partly due to the difficulties associated with the investigation of strictly anaerobic microbial strains.

 

This bacterium is characterized by easy growth on a variety of substrate, synthesizing three major liquid end products (A-B-E) through two intermediate acids (acetic and butyric) giving off a gaseous mixture of CO2 and H2 (Rogers, 1986). The culture features a complex metabolism (at pH 4.5, Temperature 37ºC by rod shaped bacteria) and an intriguing physiology reflected in a stage-wise fermentation and a spore forming life cycle. A normal batch fermentation is characterized by acid production during its first acidogenic phase which is followed by a solventogenic phase when most of the acids are reabsorbed and solvents accumulate to the point of process termination (2 %) due to their toxicity. The young growing culture in the acid fermentation phase does not possess the enzymatic apparatus for reducing the acids. The most important productive second phase of the culture features a low growth rate solvent production and acid uptake. The long time it takes for the development of the solvent production phase is partially responsible for the low over all productivity of the batch fermentation. Appropriate conditions for solvent production are sought in order to improve the solvent productivity. All of these features and the sensitivity of the strain to environmental factors make it a challenging target for in-depth study.

 

However, apart from the theoretical challenges of the process, there exist very strong practical and economic stimuli for increased scientific attention to the anaerobic product-oriented microbial processes. Some of these stimuli are listed below:

 

a)  High and increasing DEMAND for industrial solvents.

b)  Applicability of solvents as industrial chemical synthesis FEED STOCKS.

c)  Applicability of butanol as a FUEL (high heating value and octane number, miscible with gasoline).

d)  Recovery of butanol from the fermentation broth is easier than that of ethanol.

e)  Associated production of hydrogen gas.

f)  The production microbial strains are capable of utilizing abundant and cheap complex carbohydrates (STARCH) and (PENTOSE) sugars directly with-out pretreatments.

 

It is extremely difficult and time consuming to investigate, elucidate and optimize a multi parameter system of this nature by using a conventional empirical and experimental approach. Due to wide available choices the conventional empirical procedure is highly labor intensive and generally inefficient. In order to reduce the amount of experimental labor necessary to design &/or optimize a (bio-) process, a mathematical model (Fredrickson ) of the culture system can not only computer simulate experiments and serve as a guidance for designing an effective experimental program It can also serve as a basis for designing an optimal system configuration which can provide a basis for dynamic on-line optimal process control.

 

 

Mathematical Modelling of Acetone – Butanol – Ethanol Fermentation

 

Following were the assumptions for the development of the model

 

1.     Glucose is the only limiting substrate in the batch cultivation.

2.     There is no process limitation by other nutrients in the fermentation broth.

3.     The culture inhibition is caused by the Butanol only.

4.     The culture pH is known and controlled at a constant value throughout the modeled period.


The equation of sp growth rate of biomass is shown to dependent on availability of limiting nutrient by Monod type of growth kinetics and is inhibited by higher concentration of butanol. The death of the culture occurs at high concentrations of butanol & is linearly to the term Kd B.

 

The specific substrate consumption rate features contributions for growth associated substrate consumption and also its for maintenance functions of the cell.

 

The net accumulation of specific acetic acid rate features difference of growth associated production by limiting nutrient (S) and its consumption for the production of Acetone in the fermentation broth. Both production and consumption sp rates were inhibited by accumulating butanol concentrations.

 

The net accumulation of specific butyric acid rate features difference of growth associated production by limiting nutrient (S) and its consumption for the production of butanol in the fermentation broth. Both production and consumption sp rates were inhibited by accumulating butanol concentrations.

 

The net accumulation of specific Butanol production rate features difference of growth associated production by limiting nutrient (S) and the contribution resulting from conversion of butyric acid in the fermentation broth. Both production and consumption sp rates were inhibited by accumulating butanol concentrations.

 

The net accumulation of specific Acetone production rate features difference of growth associated production by limiting nutrient (S) and the contribution resulting from conversion of acetic acid in the fermentation broth. Both production and consumption sp rates were inhibited by accumulating butanol concentrations.

 

The net accumulation of specific Ethanol production rate features difference of growth associated production by limiting nutrient (S) The production sp rates were inhibited by accumulating butanol concentrations.

 

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