Oxygen Transfer

Oxygen Transfer

Introduction The oxygen transfer process The oxygen transfer model

o Factors affecting kL Interfacial area and oxygen transfer

o Factors affecting the saturation concentration of oxygen

o Oxygen uptake

Introduction

The aerobic fermentation is the primary method of product formation in Biotechnology. Most enzymes, antibiotics, biochemicals, diagnostics and therapeutics are produced using aerated bioreactors. In contrast, there very few commercially important products produced by anaerobic fermentation processes; products of lactic acid bacteria being one example.

Supplying oxygen to aerobic cells has always represented a significant challenge to fermentation technologists. The problem derives from the fact that oxygen is poorly soluble in water.

Sucrose, for example, is soluble to 600 g.l-1. The solubility of oxygen at 4oC in pure water is only 8 mg.l-1. To make matters worse, the solubility of oxygen decreases as with increasing temperature and increasing concentration of solutes in the solution.

In this section we will look more deeply into factors that affect oxygen transfer and how fermentation systems can be designed to maximize dissolved oxygen concentration in bioreactors.The supply of oxygen is often the rate limiting step in an aerobic fermentation and satisfying oxygen demands can often constitute a large proportion of the operating and capital of a industrial scale fermentation system.

The oxygen transfer process

Oxygen transfer involves the movement of oxygen from the gas phase to the liquid phase..... Nothing new here.

Let's review how oxygen moves from a bubble to an immobilized cell system.

Note that where the cells are in suspended form, then step 5-7 are not involved.

If only suspended cells are involved and if the level of mixing in the bulk liquid is sufficiently high, then the rate limiting step in the oxygen transfer process is the movement of the oxygen molecules through the bubble boundary layer.

The oxygen transfer model

When bulk mixing levels are high and suspended cell cultures are involved, the rate limiting step in above process will generally be the diffusion of oxygen through the bubble boundary layer .

Therefore, it is possible to use the interphase oxygen transfer equation to describe the oxygen transfer rate (OTR):

where

CO is the concentration of oxygen dissolved in the bulk liquidkL is the mass transfer coefficient for the bubble boundary layera is the interfacial area per unit volumeCO

* is the concentration of oxygen in the bubble boundary layer.

kLa and CO*

Oxygen transfer coefficient (kL) and interfacial area (a)

Because it not possible to accurately measure the total interfacial area of the gas bubbles (a), kL and a are combined into single term, referred to kLa.

The kLa represents the oxygen transfer rate per unit volume.

The are a number of methods of measuring kLa. You should refer to Wang et al. (1978) for further details.

kLa and CO*

Oxygen concentration at the gas-liquid interface(Co*).

There is no probe small enough to directly measure the average dissolved oxygen concentration at the gas-liquid interface (Co

*).

However, the value of Co* can be approximated by indirect methods. During the aeration

of an uninoculated fermenter, the dissolved oxygen concentration will eventually reach a steady value which is equivalent to the maximum solubility of oxygen in the liquid.

This value is also approximately equal to Co* as can be seen as follows:

Since

We see that at steady state

And therefore

The maximum solubility of oxygen in a fluid can also be estimated using Henry's equation:

where Po is the partial pressure of oxygen in the gas phase and Ho is Henry's constant.

Factors affecting kL

The mass transfer coefficient (kL) represents the rate at which oxygen molecules move through the boundary layer to the bulk liquid.

The value of kL can be increased by

reducing the size of the boundary layer

increasing the rate at which molecules travel through the boundary layer

As discussed in the previous chapter,

the size of the boundary layer is determined by the level of mixing. the diffusivity of the molecule through the boundary layer is determined by

o Medium viscosity o Temperature

Increasing temperature also reduces the medium viscosity but it will also decrease the solubility of oxygen.

Factors affecting the interfacial area (a)

The primary method of enhancing the value of kLa is by increasing the area of the gas-liquid interface (a).

Increasing the interfacial area, increases the value of kLa and increases the oxygen transfer rate.

The interfacial area is determined by the

aeration rate and the bubble diameter


Effect of bubble size

In the previous chapter we saw that bubble size plays a major role in determining the total interfacial area.

The smaller the bubble size the larger the interfacial area.

Smaller bubble sizes can be achieved through the use of appropriately designed and operated gas sparging systems and agitation systems.

As we shall see, an average bubble size which is too small can have detrimental effects on the oxygen transfer rate.


Impeller design and stirrer speed

As is discussed in the previous chapter, impellers designed for the maximization of oxygen transfer rates typically produce high shear conditions.

The high shear conditions are used to reduce the bubble size.

One such impeller is the Rushton turbine.

Shear rates increase with the stirrer speed or the impeller tip speed.

Factors such as operating costs, the capital cost of the agitation system and the sensitivity of the cells to shear will determine the impeller diameter and stirrer speed used in practice.


Flooded impeller

If the agitation speed is too low or the air flow rate is too high, then a phenomenon known as a flooded impeller will occur.

When the impeller is flooded, bubbles will accumulate underneath the impeller and coalesce.

This leads to the formation of large bubbles and poor oxygen transfer efficiencies.


Sparger design

The sparger design also plays an important role determining bubble size. In air-lift and bubble column reactors, the sparger hole size is the sole determinant of the size of the bubbles.

In stirred tank reactors, the bubble size is determined by the sparger design and the shear generated by the stirrer.

To maximize the efficiency of bubble breakup, the sparger is normally designed such that the holes are directly under the impeller blades.

This ensures that as the bubbles rise from the sparger, they encounter the high shear conditions surrounding the impeller.


Detergents and antifoams

Detergent-like and and oil-like substances have dramatic effects on bubble size.

Both these agents are surface active agents, that is they affect the surface properties of the liquid.

Both classes of chemicals affect the surface tension properties of culture medium. However their effects on oxygen transfer are dramatically different.


Detergents

Detergents and detergent like molecules typically have a hydrophobic and hydrophilic end.

Because they act at the surface of liquids, they tend to accumulate in the bubble-liquid interface and thus cover the bubbles.

The hydrophobic end will face the bubble (as bubbles are full of air which is hydrophobic). The hydrophilic end will face the liquid. If the hydrophilic end is charged (as with ionic detergents), the bubbles will then repel each other and thus prevent bubble coalescence.

During a fermentation, bacteria and fungi tend to produce detergent like molecules. These molecules include proteins and long chain fatty acids.

This can lead to the formation of foams. Excessive foam levels can lead to loss of fermenter contents and the blockage of air filters.


Antifoams

Antifoaming agents are most often used to control foam levels. Anting foaming agents include vegetable oils and silicone oils. Vegetable oils are often used by the cells as a substrate. Silicone oils are biologically inert and thus can cause problems in downstream processing. They are however much more effective antifoaming agents than vegetable oils.

Antifoaming agents act by binding up detergents preventing and thus preventing them from aggregating around bubbles.


Effect of antifoams on the interfacial area

The accumulation of antifoam at the bubble surface also leads to an increased tendency for bubbles to coalesce. Unlike as happens with ionic detergents, these bubbles do not repel each other since the antifoams are not charged. In fact, high hydrophobicity of the antifoams increases the attraction between the bubbles.

This effect is shown in the following photograph showing the effect of the addition of antifoam on bubble size in a bubble column.

With no antifoam added With antifoam added

Note that following addition of antifoam, the bubble average bubble size has increased and that the number of bubbles has decreased.


Effect of antifoams on kL

Because antifoams accumulate around bubbles, they block the movement of oxygen through the interfacial boundary.


Effect of antifoams on oxygen transfer

Summary

Excess antifoam levels decrease oxygen transfer rates by

decreasing kL by hindering the movement of oxygen through the gas-liquid interface

decreasing the interfacial area (a) by encouraging bubble coalescence.

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Oxygen Transfer