Oxygen transmission rate

The volumetric mass transfer coefficient, also referred to as kLa value or volumetric mass transfer coefficient is a measure for the transport of gas from the gas phase into the liquid phase of a reaction system. It is a measure of the efficiency of the gas entry and is therefore of great importance for the planning of chemical or biological reactors. In the interpretation of the latter is especially the oxygen transfer and thus the kLa value of oxygen is of central importance.

• 2.1 sulfite method (with oxygen)
• 2.2 Continuous measurements with probes

Physical Basics

The kLa value in a medium

The derivation of the kLa value is based on the two-film theory of mass transfer. It is assumed here simplified, in that the transport of gas through the interfacial gas / liquid, the rate-determining step. The following relationship applies:

Wherein the dissolved gas concentration C and C * is the equilibrium concentration at the phase boundary, in other words, which is the maximum solubility. The equilibrium concentration can be calculated using Henry's law of partial pressure of each gas.

When considering the relationship of the oxygen is generally formulated as follows:

Where OTR the oxygen transport rate ( engl. oxygen transfer rate or sometimes oxygen transmission rate ) is.

The difference between the two concentrations is the driving force for mass transport through the phase interface.

The kLa - value is the product of the mass transfer coefficient ( kL ) of the liquid phase and of the volume- phase boundary surface (a ). These two quantities are not separately identify each other for sparged reactors, therefore, we speak of the kLa value.

Relevance of the kLa value for biotechnological processes

In a biotechnological process of kLa value is of great importance since it makes a statement about how well microorganisms in a biotechnological plant with gases can be supplied. In the cultivation of aerobic micro-organisms, it is important to achieve a high kLa value for oxygen in the bioreactor, while autotrophic microorganisms growing, such as the kLa - value for carbon dioxide, hydrogen, and play an important role. Possible measures to increase the kLa - value, for example, include an increase in the stirrer speed, more efficient type of agitation, the enlargement of the contact surface to the air ( for example by aeration with finer air bubbles) or to optimize the medium. The usefulness of these measures is limited, however, by the higher mechanical stress on the microorganisms.

Determination of kLa value

Sulfite method (with oxygen)

To determine the kLa value of the bioreactor is operated without microorganisms. For an approximately 0.8 Molar sodium sulfite solution is presented, which comprises, as a catalyst of copper sulfate. At the beginning is solely sodium sulphite Na2SO3 before, but now aerated by means of air or pure oxygen, the sodium sulfite into sodium sulfate aufoxidert Na2SO4. First, a null value is determined, then aerated for a defined time, and ultimately determines the concentration of sodium sulfate. The Oxdidationsgeschwindigkeit determined as follows:

One advantage is good reproducibility and independence of microbiological problems. The disadvantage is that an artificial oxidation system is used, which is too high kLa values ​​suggests.

Continuous measurements with probes

Agent dissolved gas probes ( for example, an oxygen sensor ) that can directly measure the concentration of a dissolved gas in liquid, can be calculated for any gas with the help of the formula, the product kLa.

Prerequisite is of course that an appropriate sensor exists, which is not currently the case for example with hydrogen.