Continuous ethanol production using immobilized yeast cells entrapped in loofa-reinforced alginate carriers

Abstract

A culture of Saccharomyces cerevisiae M30 entrapped in loofa-reinforced alginate was used for continuous ethanol fermentation in a packed-bed reactor with initial sugar concentrations of 200-248 g/L. Maximum ethanol productivity of 11.5 g/(L·h) was obtained at an ethanol concentration of 57.4 g/L, an initial sugar concentration of 220 g/L and a dilution rate (D) of 0.2 h(-1). However, a maximum ethanol concentration of 82.1 g/L (productivity of 9.0 g/(L·h)) was obtained at a D of 0.11 h(-1). Ethanol productivity in the continuous culture was 6-8-fold higher than that in the batch culture. Due to the developed carrier's high biocompatibility, high porosity, and good mechanical strength, advantages such as cell regeneration, reusability, altered mechanical strength, and high capacity to trap active cells in the reactor were achieved in this study. The immobilized cell reactor was successfully operated for 30 days without any loss in ethanol productivity. The average conversion yield was 0.43-0.45 throughout the entire operation, with an immobilization yield of 47.5%. The final total cell concentration in the reactor was 37.3 g/L (17.7 g/L immobilized cells and 19.6 g/L suspended cells). The concentration of suspended cells in the effluent was 0.8 g/L.

Keywords: alginate; continuous; ethanol; immobilization; loofa.

Figure 1

Schematic diagram of the immobilized-cell PBR.

Figure 2

(A) Cross-section of the ALM after 72 h of ethanol fermentation. The ALM consisted of loofa fiber (I), a hollow space between the loofa fiber and alginate gel (II), and alginate gel (III). (B) A higher-magnification view of the hollow space.

Figure 3

Yeast cells in the alginate gel layer of the ALM after 72 h of ethanol fermentation

Figure 4

Yeast cells in the hollow core fibers of the ALM after 72 h of ethanol fermentation.

Figure 5

Batch fermentations using ALM carriers with dimensions of 9 × 9 × 3 mm³ (open symbols) and 20 × 20 × 3 mm³ (filled symbols); ∆, ▲= sugar: □,■= ethanol.

Figure 6

Ethanol concentration at dilution rates of 0.1–0.3 h^- and initial sugar concentrations of 200 g/L (□), 220 g/L (■), and 248 g/L (▲).

Figure 7

Ethanol productivity at dilution rates of 0.1–0.3 h-¹ and initial sugar concentrations of 200 g/L (□), 220 g/L (■), and 248 g/L (▲).

Figure 8

The steady-state ethanol concentration profiles at initial sugar concentrations of 200 g/L (A), 220 g/L (B), and 248 g/L (C) and at superficial velocities of 4.8 cm/h (♦), 6.9 cm/h (▲), 8.7 cm/h (○), and 13.0 cm/h(●).

Figure 9

Suspended cells in the PBR (A) and in the effluent (B) after 30 days of continuous fermentation.