Mathematical modelling of immobilized living yeast cell reactor for sugar bioconversion to ethanol

In the present work, a mathematical model was developed regarding the immobilized living yeast cell reactor for sugar bioconversion to ethanol. The model, composed of a system of ordinary differential equations (ODEs) enables the computation of the paramters involved in the steady state reactor behaviour. Comparing the values computed through the integration of this mathematical model with the experimental data, it has been shown its capacity to describe sufficiently accurate the steady state behaviour of the continuous fixed film bioconversion reactor.     

Effect of oxygen on ethanol fermentation in packed-bed tapered-column reactor

Summary In ethanol production with immobilized yeast a major problem is the provision of nutrients to these highly concentrated cells. O₂ being one of the nutrients of utmost importance to yeast cells, was fed into a column packed with beads with a cell loading of more than 40 g/l. Since addition of large volume of air or O₂ to a cylindrical column reactor would aggravate the problems of pressure build up and channelling caused by the evolving CO₂ gas, a tapered-column reactor and pulsed flow of oxygen gas was used. The supplement of O₂ gas to the tapered column increased the productivity from 21.1 g ethanol x (l gel x h)^-1 to 26.7 g x (l gel x h)^-1, when the ethanol concentration at the outlet was about 80 g/l. The yield coefficient of ethanol was also increased from 0.41 g ethanol/g glucose to 0.43 after O₂ supplement was started. The effects of frequency and duration of O₂ supplement were also determined.     

Continuous ethanol production by immobilized yeast reactor

Summary Saccharomyces cerevisiae yeast immobilized in calcium alginate gel beads was employed in packed-bed column reactors for continuous ethanol production from glucose or cane molasses, and for beer fermentation from barley malt wort. With properly balanced nutrient content or periodical regeneration of cells by nutrient addition and aeration, ethanol production could be maintained for several months. About 7 percent (w/v) ethanol content could be easily maintained with cane molasses diluted to about 17.5 percent (w/v) of total reducing sugars at about 4 to 5 h residence time. Beer of up to 4.5 percent (wv) of ethanol could be produced from barley wort at about 2 h residence time without any addition of nutrients.     

Continuous ethanol fermentation using immobilized yeast cells

Growing cells of Saccharomyces cerevisiae immobilized in calcium alginate gel beads were employed in fluidizedbed reactors for continuous ethanol fermentation from cane molasses and other sugar sources. Some improvements were made in order to avoid microbial contamination and keep cell viability for stable long run operations. Notably, entrapment of sterol and unsaturated fatty acid into immobilized gel beads enhanced ethanol productivity more than 50 g ethanol/L gel h and prolonged life stability for more than one-half year. Cell concentration in the carrier was estimated over 250 g dry cell/L gel. A pilot plant with a total column volume of 4 kL was constructed and has been operated since 1982. As a result, it was confirmed that 8-10%(v/v)ethanol-containing broth was continuously produced from nonsterilized diluted cane molasses for over one-half year. The productivity of ethanol was calculated as 0.6 kL ethanol/kL reactor volume day with a 95% conversion yield versus the maximum theoretical yield for the case of 8.5% (v/v) ethanol broth.     

Continuous ethanol production by immobilized yeast in a fluidized reactor

Summary In order to minimize the adverse effect of CO₂ gas in a packed bed immobilized yeast reactor, a fluidized bed reactor was used for the continuous production of ethanol from glucose. Immobilized yeast was prepared by entrapping whole cells of Saccharomyces cerevisiae within a Caalginate matrix. It was found that the efficiency of the ethanol production in a fluidized bed reactor was 100% better than that for a packed bed reactor system. The alcohol productivity obtained was 21 g/l/hr in a fluidized bed reactor at 94% of conversion level.     

Nonisothermal immobilized enzyme reaction in a packed-bed reactor

Summary This work investigates the reaction behavior of immobilized enzymes in a packed-bed reactor. The effect of heat generation due to exothermic enzyme reaction is considered. Conservations of substrate and energy constitute two coupled nonlinear partial differential equations which are simultaneously solved by a numerical method. It is found that substrate conversion is generally increased at higher temperature. However, the extent of temperature heavily depends on the magnitude of the dimensionless Michaelis constant which is defined as the ratio of Michaelis constant to inlet substrate concentration. At low dimensionless Michaelis constant, substrate conversion is considerably affected by temperature, but at high dimensionless Michaelis constant, the temperature effect is negligibly small. It is also found that maximum bulk temperature of reaction mixtures occurs around a dimensionless reactor length of 1.3 for the case with high substrate conversion.     

Production of ethanol by a stirred catalytic basket reactor with immobilized yeast cells

A stirred catalytic basket reactor with immobilized yeast cells was used for the batchwise production of ethanol. Fractional conversions up to 0.99 in 10 h were attained, depending on the agitation rates, initial glucose, and cell densities. The volumetric productivity of the reactor was considerably better than that of conventional stirred tank reactors. Productivities were strongly dependent on the stirred speed.     

Rapid ethanol fermentation with immobilized Zymomonas mobilis in a three stage reactor system

Summary Using the Zymomonas mobilis NRRL B 14023 strain for ethanol fermentation with immobilized cells the combination of two external loop reactors followed by a plug flow reactor was the most effective reactor configuration. A maximal productivity of 92 and 108 g/l·h at practically complete sugar consumption was obtained with Carrageenan and Alginate catalysts respectively. Due to the high dilution rate nonsterile operation for extended periods of weeks was possible.     

Continuous ethanol production by immobilized yeast reactor coupled with membrane pervaporation unit

A system comprised of an immobilized yeast reactor producing ethanol, with a membrane pervaporation module for continuously removing and concentrating the produced ethanol, was developed. The combined system consisted of two integrated circulation loops: In one the sugar-containing medium is circulated through the membrane pervaporation module. The two loops were interconnected in a way allowing for separate parameter optimization (e.g., flow rate, temperature, pH) for each loop.The fermentation unit was 2.0 L bioreactor with five equal segments, packed with 5-mm beads of immobilized yeasts. The bead matrix was a crosslinked polyacrylamide hydrazide gel coated with calcium alginate. The fast circulation loop of the bioreactor allowed for efficient liberation of CO(2) at the top of the immobilized yeast reactor. Continuous operation of the uncoupled reactor for over 50 days with inflowing defined medium or dilute molasses at a residence time of 1.25 h yielded ethanol at a rate of about 10 g/L h.The pervaporation unit was constructed from four 60-cm-long tubular membranes of silicone composite on a polysulfone support. The output from the fermentor was circulated through the inside of the tubes of a unit with a total surface area of 800 cm(2), having an average flux of 150 mL/h, and selectivities to ethanol vs. water up to 7. A vacuum of 30 mb was applied to the outside of the tubes, removing 20-30 g of ethanol per hour, which was collected in condensors. The continuous removal of ethanol, avoiding inhibition of the fermentation process, resulted in an improved productivity and allowed the use of high sugar concentrations (40% wt/vol) offering the potential of a compact system with reduced stillage.The combined system of ethanol production and removal enabled an operative steady state at which the liquid volume of the system, and the concentrations of ethanol within the reactor ( 4% wt/vol), as well as within the flux crossing the pervaporation membrane (17%-20% wt/vol) were kept constant. At the steady state, a 40% wt/vol sugar solution could be continuously added to the fermentor when 12%-20% wt/vol clear ethanol solution was continuously removed by the pervaporation unit. Membrane fouling was reversed by short washing steps, and continuous step operation was maintained by working with two different modules that were interchanged. In this manner, long term continuous operation (over 40 days) was achieved with a productivity of 20-30 g/L h, representing over a twofold increase relative to the continuously operated reactor uncoupled from the membrane and a fivefold increase in comparison with the value obtained fro a corresponding batch fermentation.     

Dynamic modeling of immobilized cell reactor: application to ethanol fermentation

A mathematical model has been developed for the unsteady-state operation of an immobilized cell reactor. The substrate solution flows through a mixed-flow reactor in which cells immobilized in gel beads are retained. The substrate diffuses from the external surface of the gel beads to some internal location where reaction occurs. The product diffuses from the gel beads into liquid medium which flows out of the reactor. The model combines simultaneous diffusion and reaction, as well as cell growth, and it can predict how the rates of substrate consumption, product formation, and cell growth vary with time and with initial conditions. Ethanol fermentation was chosen as a representative reaction in the immobilized cell reactor, and numerical calculations were carried out. Excellent agreement was observed between model predictions and experimental data available in the literature.     

Comparative study of ethanol production by an immobilized yeast in a tubular reactor and in a multistage reactor

Summary The productivity of continuous ethanol fermentation has been increased using fixed bed reactors where a high density of yeast cells was maintained on a packing of wood chips. Two different systems have been used: 1. A tubular reactor which produced alcohol solutions containing up to 13.5% (V/V) ethanol. High CO₂ retention and a poor mass transfer between bulk medium and immobilized biomass prevented production rates higher than 2.2 g/l·h. 2. A multistage reactor where a better utilisation of the reactor volume led to improved performances. Solutions containing 132 g/l of ethanol (16.5% V/V) were produced with a productivity increased up to 4.8 g/l·h. A better distribution of the active biomass and a lower gradient of alcohol concentration between support and bulk medium are possible reasons for this improvement.     

Performance of a continuous bioreactor with immobilized yeast cells in the ethanol fermentation of molasses-stillage medium

Summary It is feasible to produce ethanol by continuous fermentation of molasses-stillage medium, without any supplementation, employing calcium alginate immobilized cells of Saccharomyces cerevisiae. High flow rates generated high values for the productivity; however, the percent substrate conversion decreases with the dilution rate.     

Denaturable immobilized enzymatic reaction in a packed-bed reactor with and without substrate inhibitions

Summary An immobilized enzymatic reaction in a packed-bed reactor is investigated in this paper. The thermal denaturation of immobilized enzyme caused by excessive reacting temperature rise is considered. An unsteady state dispersion model is employed to examine the dynamic behaviors of the substrate concentration, temperature and enzyme activity along the reactor. Also included in the present paper is the effect of substrate inhibition which occurs rather frequently in many enzymatic reactions. Comparison of results of the immobilized enzymatic reactions with and without substrate inhibitions are made to show the extent the substrate inhibition affects the enzymatic reaction. Furthermore, the effects of heat reaction and the Peclet number which characterize the reaction and flow behaviors, respectively, on the system considered are analyzed in detail.     

Studies on immobilized Saccharomyces cerevisiae. I. Analysis of continuous rapid ethanol fermentation in immobilized cell reactor

Rapid fermentation of cane molasses into ethanol has been studied in batch, continuous (free-cell and cell-immobilized systems) by a strain of Saccharomyces cerevisiae at temperature 30 degrees C and pH 5.0. The maximum productivity of ethanol obtained in immobilized system was 28.6 g L(-1) h(-1). The cells were immobilized by natural mode on a carrier of natural origin and retention of 0.132 g cells/g carrier was achieved. The immobilized-cell column was operated continuously at steady state over a period of 35 days. Based on the parameter data monitored from the system, mathematical analysis has been made and rate equations proposed, and the values of specific productivity of ethanol and specific growth rate for immobilized cells computed. It has been established that immobilized cells exhibit higher specific rate of ethanol formation compared to free cells but the specific growth rate appears to be comparatively low. The yield of ethanol in the immobilized-cell system is also higher than in the free-cell system.     

Ethanol fermentation in a yeast immobilized tubular fermentor

In this article, a mathematical model describing the kinetics of ethanol fermentation in a whole cell immobilized tubular fermentor is proposed. Experimental results show reasonable agreement with the proposed model. A procedure for treating the fermentation data for determining the ethanol inhibition constants k(1) and k(2) is described. The ethanol productivity of the immobilized cell fermentor is compared with those of traditional fermentors. Experimental studies indicate that with Saccharomyces cerevisiae (NRRL Y132) culture, ethanol productivity in the range 21.2-83.7 g ethanol L(-1) h(-1) at ethanol concentration of 76-60 g/L can be achieved. This is comparable to or higher than those reported in the literature for yeast. The product yield factor of 0.5 g ethanol/g glucose was obtained. The immobilized cell fermentor does not show washout at dilution rates of 7 h(-1) and shows good stability over a 650-h operating period.     

Continuous ethanol fermentation in a tower reactor with flocculating yeast recycle: scale-up effects on process performance, kinetic parameters and model predictions

An unsegregated and unstructured model developed for a small-scale process of ethanol production in a tower reactor with cell recycle was applied to describe the experimental data obtained in a large-scale process. The model was developed considering the following points: reactor hydrodynamic behavior analogous to that of ideal CSTR, substrate limitation, inhibition phenomena linked both to ethanol and to biomass, absence of fermentation in the settler, and no loss of cell viability. The scale-up criterion consisted in maintaining an identical relation settler volume/fermentor volume on the two scales. All large-scale experiments were carried out using a flocculating yeast strain IR-2, isolated from fermented food, and identified as Saccharomyces cerevisiae. Sugarcane juice was used as the substrate source with sugar concentrations of 150?g/l. Different values of dilution rate and recycle ratio were employed (D?=?0.11–0.33?h^?1, α?=?5.4–18.0) and the temperature was of 32?°C. The kinetic parameters were similar on both scales and the model predictions agreed well with the large-scale experimental data.