Simulation and Comparison Between the Real and Supplied Oxygen Demand of a Fermentation Process

Simulation and Comparison between the real and supplied oxygen demand of a fermentation process

Serna Sebastián¹, Sánchez Julio C.¹, García Carlos A.¹, Cardona Carlos A.^A1

¹ Instituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química. Universidad Nacional de Colombia Sede Manizales. Cra. 27 No. 64-60, Manizales, Colombia

^A1 Tel.: +57 6 8879400 Ext 55354. Corresponding author E-mail: ccardonaal@unal.edu.co

Acetic acid bacteria (AAB) derive their energy from the oxidation of ethanol to acetic acid during fermentation. Acetic acid bacteria are airborne and are ubiquitous in nature. They are actively present in environments where ethanol is being formed as a result of fermentation of sugars. They can be isolated from the nectar of flowers and from damaged fruit. Other good sources are fresh apple, cider and unpasteurized beer that have not been filter sterilized. In these liquids, they grow as a surface film due to their aerobic nature and active motility [1].

This work analyzes by simulation the physic-chemical conditions of fermentation that affect both viscosity and oxygen solubility of acetic acid fermentation using Acetobacter pasteurianus. Concentration profiles of biomass, substrate (ethanol) and product (acetic acid) through the fermentation were tested [2]. A. pasteurianus was chosen due to available data (kinetic parameters and microorganism characteristics). 

The aim of this analysis is to demonstrate the relation between concentrations of biomass, substrate and product with the viscosity of the fermentation media, in order to show the dependence of those with the oxygen supply. The real proficiency of fermentation was analyzed in terms of oxygen supply respect to oxygen demand of the microorganisms, and required agitation for full oxygen supply.

The viscosity of the fermentation is function of substrate, product and biomass concentrations, previously calculated by separate. Biomass viscosity was calculated using Vand's equation [3]. For the calculation of the viscosity of the ethanol and the acetic acid on an aqueous solution it was used the method of Teja and Rice [4], [5]. Mass transfer coefficient was calculated with the equation proposed for stirred fermenters containing non-coalescing non-viscous media [6]. The power dissipated and the amount of oxygen in the liquid was calculated with the model proposed by [7], and dissolved oxygen supply was calculated from [8].

It was concluded that viscosity of the fermentation solution and oxygen solubility are important variables to analyze. Both variables were directly related with the conditions that allow the proper functionality of the microorganism and therefore they are directly related with performance of the fermentation for acetic acid production. This analysis can be extended to any fermentation process. Results also reveal that agitation is an important variable on fermentation and affect directly the amount of available oxygen in fermentation media and therefore satisfy the microorganism oxygen demand.  Two cases were found, one in which the agitation and the oxygen supply exceeds the real requirements and another one doing the exact opposite, supplying less than the real requirements. In the first case we talk about over-costs and in the second one we talk about lower productivity. Therefore, the real equilibrium point between costs and productivity must be found within these two cases, with the aim of proposing an optimized performance of the fermentor.

Finally, this approach to the fluid dynamics of fermentation and its relation with the energetic demands opens a new perspective in which it is necessary to deepen the functioning of the associated processes of fermentation.

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