Week 3 Blended Learning Mass Transfer

Question 1

Mass Transfer

Answer 1

1. Transfer of nutrients from the aqueous phase into the microbial cells during fermentation is relatively straight forward as the nutrients are provided in excess and typically are readily dissolved in the liquid
2. Oxygen transfer is more complex and is diminished as water can dissolve only a relatively low concentration of oxygen. Oxygen transfer is considered on its own and is associated with the rate of transfer. It is considered separately as the basis for scaling-up/ scaling-down fermentors

Question 2

Aeration

Answer 2

Oxygen is only sparingly soluble in aqueous solution– at 25*C, the maximum concentration in pure water is 8.4 mg/L; with each addition of a dissolved reagent, this concentration is diminished.

-Large volumes of biomass in industrial-sized fermentors require large oxygen inputs. With these two facts considered, oxygen is most often the rate-limiting factor.

-In order to try and maximize the transfer of oxygen, it is delivered through a sparger that is located directly under the impeller. This helps to deliver the smallest air bubble possible and to keep the oxygen level more uniform throughout. Sparger with pore sizes too small are prone to breakage and require more energy input.

-Sterile air flow into a fermentor is delivered at rates as high as 0.5 to 1.0 volumes of air per volume of media per minute (0.5-1.0 vvm)

Question 3

The actual transfer of oxygen from the gas phase to the liquid phase is affected by the following factors

Answer 3

-Physical conditions: temperature, pressure, surface area of air bubbles (temp increases- solubility of gas decreases, pressure increases- solubility of gas increases

-Chemical composition of medium
-Volume of gas delivered per unit volume of reactor per unit of time
-Type of sparger
-Agitation rpm
-Combination of all these factors

Question 4

7 steps involved in oxygen transfer?

Answer 4

1. The bulk gas phase in the bubble
2. The gas-liquid interface
3. The liquid film around the bubble
4. The bulk liquid culture medium
5. The liquid film around the microbial cell
6. The cell-liquid interface
7. The intracellular oxygen transfer resistance

Question 5

Rate controlling step?

Answer 5

The movement of oxygen from the gaseous phase to the gas-liquid phase boundary,

Particularly more difficult in more viscous liquids. This is because the oxygen can move freely and rapidly in the gaseous phase because of their high levels of kinetic energy.

To get through to the liquid phase, they need to pass through a layer of gas molecules inside the bubble and then a layer of water molecules outside of the bubble

The diffusion process is influenced by temperature and the presence of other solutes and surfactants

Question 6

The rate of transfer of oxygen from an air bubble to the liquid phase is described by the equation?

Answer 6

dCL/dt= KLa(C-CL) or ln (C-CL)=-KLa *t

Here CL= oxygen concentration at time “t” (mmol/dm^3)
C*= saturated dissolved oxygen concentration (mmol/dm^3)
t= time (h)
KL= mass transfer coefficient (cm/h)
a= gas-liquid interface area per liquid volume (cm^2/cm^3)

Because both “KL” and “a” are difficult to measure individually, they are often linked as “KLa”- the volumetric mass transfer coefficient (hr^-1 or 1/hr)

Question 7

Volumetric mass transfer coefficient

Answer 7

Is a measure of the aeration capacity of the fermenter and must be maintained above a certain minimum level to supply the oxygen to the microorganism

Question 8

The determination of KLa

Answer 8

Is important in the scaling up of fermentors- that is, it will be determined for a smaller fermenter and then used to determine the design and operating condition of a larger fermentor

ln (C*-CL)= -KLa *t