Production of Chemicals and Fuels by Immobilized Cell Fermentation

Keywords

Yeast, ethanol, packed bed reactor, continuous fermentation, alternate control strategy, bioethanol, cell immobilization, dilute acid hydrolysis.

Introduction

Immobilized cell fermentation techniques discuss the effects of immobilization on cell physiology, metabolism, genetics, and fermentation behavior, and various types of immobilized cell bioreactor systems and their applications in fermentation. Bioethanol is a renewable fuel from different kinds of renewable feedstock such as sugar cane, corn, wheat, cassava, and cellulose biomass.

A variety of immobilized cell bioreactors have been developed to optimize the fermentation processes. Immobilized cells are currently being used industrially for vinegar, organic, and amino acid production, as well as in wastewater treatment. The application of various methods of cell immobilization employed in ethanol production.

Ethanol production by immobilized yeast and bacterial cells has been attempted in various bioreactor types. The substantial improvements in ethanol productivity by immobilized cell systems is indicative of the fact that future developments in the production of ethanol and alcoholic beverages will be directed towards the use of immobilized cell systems.

Yeast cell immobilization was carried out in a reactor to investigate the effects of the volumetric capacity of carriers as well as the different fermentation modes on fuel ethanol production. The productivity of immobilized cell fermentation was 16% higher than that of suspended-cell fermentation.

An effective method of ethanol production with a new immobilized approach, and by switching the flow directions, traditional continuous fermentation can be greatly improved, which could have practical and broad implications in industrial applications

Cell Immobilization

Cell immobilization was carried out in a packed bed reactor (PBR) that included two main parts: immobilized fillers and a 20 L recirculation tank, with a diameter of 14 cm and a height of 33 cm. Previously, spiral wound carriers have been used to immobilize cells; however, as the liquid flowed vertically there was less contact with the carrier surface, and thus the rate of immobilization was reduced.

The sheet cotton fibers were packed into porous hollow balls, which increased the contact area of free cells, while also reducing the use of steel wire mesh. The free cell concentration varied depending upon the adsorption time and amount of carrier used

The number of adsorbed cells on the carriers was roughly equal to the reduced concentration of suspended cells as previously described. The concentration of suspended cells in all experimental groups was found to decrease during the initial 30 h, with this reduction seemingly related to the different amounts of carrier added.

Batch and Repeated-Batch Fermentation

Repeated-batch simultaneous saccharification and fermentation (SSF) by immobilized cells was also carried out in a 20 L PBR after the immobilization process, where 16 L of the same fermentation broth was added to the reactor. The fermentation conditions were the same as those used for free cell fermentation. At the end of each batch, which was defined by the level of reducing sugar dropping below 1 g/l, the fermented broth was removed, the same amount of fresh medium was added, and the next batch was initiated.

The immobilized cells were used for 6 successive batches. Samples were collected at regular intervals and analyzed for ethanol production yield and reducing sugars. To assess the value of the application of immobilized yeast cells, we compared it to traditional fermentation technology.

We found that the consumption rates of residual total sugar and reducing sugars, as well as the production rate of ethanol in immobilized yeast fermentation were faster than those in the free cell fermentation, especially in the early stage. The ethanol yield in the first batch, with the immobilized cells, was like that of the free cell fermentation, while in later batches, the mean yield from immobilized cell fermentation was higher than that obtained from free cell fermentation, most likely due to the lower concentrations of residual total sugars and free cells.

This result can be explained by the fact that the immobilized yeast cells in the production of ethanol may overcome the product inhibition as previously reported and that the sugar can be consumed more completely, resulting in improved ethanol concentration.

Continuous Ethanol Productions

Continuous ethanol production using immobilized cells was carried out in a series-wound bioreactor. The concentrations of the substrate, ethanol, and free cells as well as the cellular death rate were found to vary as the dilution rates were varied.

This phenomenon may be explained by the mass transfer being enhanced and the cell activity improved by the lower ethanol concentration as the dilution rate increased. The maximum ethanol yield obtained was 97.63% at a dilution rate of 0.058 h-1.

It was observed that low stirring and higher cell concentrations had a positive effect on ethanol production. Stirring is a parameter of great importance in a fermentation process, interfering directly in the nutrients uptake rate and oxygen transfer. The stirring provides better medium homogenization and maintains the gradient between the inside and outside of the cell, facilitating the entry of nutrients and gases exit and other products of cellular catabolism.

Feasibility of Continuous Ethanol Production

This immobilization increases the contact area for free cells and improves the adsorption of cells, ensuring efficient continuous ethanol production. In addition, it also avoids the use of steel wire mesh, which reduces the cost of production. By switching the flow direction of the bioreactors, the activity of cells was enhanced throughout the whole process, thus maintaining a long-term stable fermentation without contamination. In summary, the application of continuous ethanol production using immobilized cells in PBRs is feasible for industrial production.

Effects of Control Strategy

Despite continuous ethanol production enhancing productivity and yield, questions surrounding the sustainability and stability of the fermentation still exist, limiting its application. Long-term cultures with little nutrition and production inhibition, in the last bioreactor, not only caused the decline of yeast cell activity, but also increased the risk of bacterial infection. To solve this problem, some people have improved their aeration rate to maintain the activity of the strain, while others have supplemented nutrition.

Conclusion

The stirring in the fermentation process is the crucial factor that influences the production of ethanol by immobilized cells. This parameter shows the extreme importance of oxygen availability to the microorganisms and stability of the immobilization support. A new type of immobilized medium with high strength, great adsorption, and perfect mass transfer was used as a cell immobilized carrier.

In batch fermentation, the ethanol yield using immobilized cells was like that obtained by free cell fermentation. Continuous ethanol production with four bioreactors showed a higher productivity with a dilution rate (0.092 h-1). Switching the flow direction of the bioreactors was used to improve the activity of cells and maintain a long-term stable fermentation without contamination and an effective method for ethanol production.