MAXBLEND Fermentor, for High Viscosity Fermentation.

Due to the increasing demands for a utilization of natural resources in consideration of the global environmental issues, research and development of biopolymers and bioplastics have been promoted in recent years, and practical applications as an industrial material have been widely expanded. But, focusing on the fermentation process of biopolymers, there is a problem that a viscosity of fermentation liquid (broth) is remarkably increased, and as a result, uniform mixing becomes insufficient, which causes reduced productivity. Similarly, in the fermentation process of bioplastics, a liquid viscosity decreases as the fermentation progresses, but in an initial stage, the viscosity of the raw material liquid is very high that is difficult to mix, so the fermentation does not proceed well. Both cases lead to a decrease in productivity and are one of the challenges in scale-up the size of fermentors for industrial use. In traditional fermentation industries, a fermentor with agitator using turbine impeller has been generally used. However, it is not suitable for the high viscosity fermentation system because of the poor mixing performance. Therefore, it’s very important to develop the fermentors suitable for the high viscosity fermentation broth, responding to the demand in fermentation industries of biopolymers and bioplastics expected further developments in the future.

We, Sumitomo Heavy Industries Process Equipment has already commercialized a mixing reactor “MAXBLEND” that exhibits excellent mixing performance in a wide viscosity range including the high viscosity zone. Based on this mixing technology, “MAXBLEND Fermentor” equipped with MAXBLEND impeller suitable for the high viscosity broth have been developed. This report mainly describes the results of the mixing performance test under aeration-mixing conditions and the biopolymer fermentation test conducted in a laboratory-scale fermenter.

Test

For the performance test for aeration-mixing, tap water and CMC (Carboxy Methyl Cellulose) aqueous solution were used as simulated liquid of the fermentation broth.

Table 1 shows the specifications of the test fermentors. As the fermentor vessel, a glass tank having an inner diameter of 130 mm and an effective liquid volume of 2.8 liter was used. As for the agitator, two types of mixing impellers were tested with shaft torque detectors to measure the mixing power, one is a two stages disk turbine impeller as a conventional fermentor (hereinafter 6DT), and the other is MAXBLEND impeller (hereinafter MBF).

Test Results and Considerations

Comparison of mixing performance

In order to know the mixing performance in the high viscosity fermentation liquid without aeration, a mixing test was conducted by the alkali addition method using high viscosity simulated liquid. The results measuring pH values at the three points are shown in Figure 2. This means MBF is particularly excellent in mixing up and down. In other words, MBF requires less time for the uniform mixing, and higher mixing performance at the high viscosity than 6DT.

Comparison of mixing performance under the aeration

Figure 3 shows the test results to compare the mixing performance of MBF using Spiral Sparger and 6DT using Conventional Ring Sparger under the aeration. Whether the viscosity (μ) of the CMC solution is 1.0 Pa·s or 4.6 Pa·s, the Mixing Time(θm) of MBF was 1/2 shorter than that of 6DT at the same Mixing Power (Pv). The mixing performance of MBF is superior to 6DT even in the aerated condition.

Oxygen uptake capacity (KLa)

The results are shown in Figure 4. In water, KLa of MBF was higher than that of 6DT, and the smaller the Pv(kW/m³), the greater the difference. It was confirmed that KLa of MBF was high even in the high viscosity CMC solution, and the difference was remarkable under the low mixing power condition.

Examination for shear stress

Many types of microorganisms are easily damaged by shear stress due to agitation. One of the characteristics of MBF is that the same mixing performance can be obtained in a mild mixing condition than 6DT. To evaluate this property, the effect of shearing stress by the mixing impeller of MBF and 6DT on the cutting of filamentous hyphae was examined by using a suspension of hyphae of “Agnomycetes sp” by comparing nucleic acid leakage rate(⊿E260). The relationship between ⊿E260 and KLa for MBF and 6DT was summarized as shown in Figure 5. That is, the under mixing conditions where the same KLa can be obtained, MBF has less impact damage to microorganisms than 6DT.

Biopolymer fermentation test

The difference in fermentation performance of 6DT and MBF in the high-viscosity fermentation system was investigated using hyaluronic acid as the fermentation target. Hyaluronic acid is a biopolymer used as a material for cosmetics and is currently produced by fermentation with lactic acid bacteria. But, in this process, it is presumed that the fermentation productivity is relatively suppressed because the viscosity of the fermentation broth becomes high and it is difficult to control the pH value. Figure 6 shows a comparison of the fermentation progress of 6DT and MBF under the mixing condition keeping the pH value at 6.8 and the DO (dissolved oxygen concentration) at 1ppm. In the case of MBF, the amount of cells (OD 660) was lower than that of 6DT, but the amount of hyaluronic acid produced was clearly (maximum 20%) higher than that of 6DT. There are several possible reasons for the increased productivity with MBF. The most likely reason was that the pH value in the tank was kept more uniform than 6DT.

Conclusion

We conducted performance tests using lab-scale fermentors with a charging capacity of 2 liters for MBF (MAXBLEND impeller) and 6DT (two stages disk turbine impeller, as a conventional fermentor). The results are summarized below.

(1) MBF showed higher mixing performance under the same mixing power condition as compared with 6DT (conventional fermentor having turbine impeller).

(2) Regarding Oxygen uptake capacity, KLa of MBF was higher than that of 6DT in both water and the high

viscosity liquid (CMC solution), and particularly excellent in the high viscosity liquid even in the low mixing power

consumption.

(3) MBF had the advantage of less damage to microorganisms under the same KLa conditions than 6DT.

(4) When MBF was applied to hyaluronic acid fermentation, productivity was about 20% higher than that of 6DT,

showing that MBF was suitable for the high viscosity fermentation such as biopolymer fermentation.