The fermentation factor

Fermentation processes impose a number of specific demands on mixing technology. Mary Anne Johnson, of SPX Process Equipment, considers the criteria

Multiphase processes are among the more demanding of mixing applications, making it essential that the correct type of impeller is selected to achieve good performance. In the specific case of fermentation, the process demands a number of mixing objectives, all of which influence the selection of the impeller. Achieving a uniform distribution of the three phases, together with gas dispersion, is crucial to achieving process performance.

Blending, solids suspension, temperature uniformity and homogeneity are all classed as flow controlled applications, and to ensure efficient bulk fluid movement throughout the tank an axial flow impeller is the most effective. However, its design does not accommodate the effects of gas dispersion. In contrast, radial impellers that have traditionally been used for gas dispersion are inefficient producers of flow.

The development of high efficiency axial flow impellers suitable for gas dispersion applications has led to increased take-up of these impellers in fermentation systems, but to ensure the maximum productivity, the mixer must provide several key functions.

energy concerns

Dispersion of the gas is essential both for uniform distribution at its point of entry and to reduce the size of the individual gas bubbles, which results in a greater interfacial surface area for gas-liquid mass transfer. Good contact between the three phases, namely gas, liquid and solid, accomplishes mass transfer. Blending ensures that concentrations of nutrients and organisms are distributed uniformly throughout the broth.

As the organisms are very small and settle at a slow rate, maintaining uniformity is a function of blending rather than the suspension of solids due to mixing.

Exothermic respiration demands that the broth is cooled, so the mixer must provide a turbulent fluid regime at the heat transfer side in order to maximise the mixer side heat transfer coefficient. In addition, all the energy imparted to the broth from the mixer is eventually converted to heat due to viscous shear, making heat input a consideration.

improved mixing

One impeller design that fulfils all these requirements is the high solidity, axial flow hydrofoil impeller from SPX.

This new technology employs multiple up-pumping impellers, which drive the contents of the vessel upwards through each of the impellers to the top where a strong re-circulating flow is created. The high velocity of this recirculation forces the contents, including entrained gas bubbles, back down the sidewall to the bottom of the vessel to the lowest impeller, which reverses the flow and drives the contents upwards again.

This method creates a single continuous loop flow that improves mixing. Other benefits include shorter blend time, an even distribution of solids, temperature gradients reduced to less than +/-2ºC and increased interface between phases. By encouraging gas bubbles to circulate in their natural direction, up-pumping technology prevents a surface forming on which the bubbles can coalesce.

The continuous dispersion of the gas bubbles minimises coalescence, increases gas hold up and maximises the gas-liquid interface. As a result, there is both a faster absorption of the gas bubbles and mass transfer rate that can be up to twice that of traditional systems at the same power per unit volume.

Better utilisation of motor power is ensured: as the impellers are not working against the gas stream and do not flood, there is little drop off in power upon aeration. The positive outcome of this is the removal of any need for expensive variable frequency drives, two-speed motors or additional controls.

vessel stresses

Recorded evidence from commercial installations shows that significant improvements in productivity can be achieved when using axial flow hydrofoil impellers compared with traditional systems. SPX Process Equipment's Lightnin A340 has recorded:

• a 20-40% increase in a fermentation system yield capability;

• chemical reaction times reduced from 12 to 3 hours;

• a fourfold decrease in blend time;

• better use of nutrients;

• and improved uniformity and purity.

Users have reported improved mechanical reliability, resulting from the impeller pushing against a free surface as opposed to the tank bottom. This effectively dampens any vibration and reduces stresses on the tank's structure.