To examine the relationship between fermenter design and operating conditions, oxygen transfer capability and microbial growth.

SECTION ONE: GROWTH CHRACTERISTICS OF THE ORGANISM

The important information that needs to be obtained from the laboratory studies will enable us to evaluate if it is possible to reproduce the process in the larger scale vessel. In this first section we need to evaluate the mass balance for growth since we know we would like to achieve 22 gL^-1 biomass in the large scale process. This mass balance will give us the following key information:

- Amount of glucose required
- Amount of oxygen required for growth
- yield of biomass on substrate (g biomas . g^-1glucose)
- yield of biomass on oxygen (g biomas . g^-102)

Calculate the above values now. This information will be utilised in subsequent sections of the case study.

In this section laboratory data has also been provided on the growth kinetics of the organism. For the purposes of the design the maximum specific growth rate needs to be evaluated. Calculate it now using the batch fermentation data given in the table below.

SECTION TWO: MECHANICAL DESIGN AND OXYGEN REQUIRMENTS

The second stage of the design process is to evaluate the oxygen requirement for the organism. This is necessary so that we can establish if the vessel we construct will be capable of attaining the biomass concentration we require.

Calculate the maximum oxygen uptake rate given the following information:

-maintenance requirement for oxygen is 0.04 g (oxygen) g^-1 (biomass) hr^-1

You will find it necessary to establish the physical characteristics of the vessel first. It is best to complete the entire mechanical design for later sections where this information is required. For the physical dimensions ensure that you calculate/specify:

(i) vessel height and diameter

(ii) impeller diameter

(iii) liquid height

(iv) impeller type and locations

(v) number, size and position of baffles

SECTION THREE: POWER REQUIREMENTS AND OXYGEN TRANSFER

The next step is to estimate the oxygen mass transfer coefficient and ultimately the overall rate of oxygen transfer. In order to calculate this we need to first consider the mixing conditions in the fermenter and the power requirements for mixing.

(i) Evaluate the mixing conditions by calculating the Reynolds number given:

Density of broth: 1000 Kgm^-3

Viscosity of broth: 0.01 Nsm^-2

Rotational speed: 150 rpm

(ii) Evaluate the gassed (and ungassed) power requirements of the vessel. Note that the power rating of the motor specified for the vessel in 3.5 kW.

(iii) Knowing the gassed power requirement it is now possible to estimate the oxygen mass transfer coefficient for your vessel using an appropriate literature correlation such as:

SECTION FOUR: MINIMUM DOT LEVEL

Given our knowledge so far we now need to evaluate if it is feasible to run the process in the designed vessel. We therefore need to compare the oxygen uptake rate and the oxygen transfer rate. It is essential that the transfer rate exceeds the uptake rate by quite a margin but remember we are evaluating the worst case senario so we need to consider the result carefully.

In Section Two we calculated the maximum oxygen uptake rate of the culture for the conditions specified. We also need to check that the level of oxygen in the outlet gas is satisfactory for effective performance:

(i) Evaluate the exit gas oxygen concentration at the maximum oxygen uptake rate given that the vessel will probably operate with an over pressure of 0.25 bar g.

The final test is to evaluate if the required biomass consumption can be achieved in the vessel. If we assume that at the maximum oxygen uptake rate and the oxygen transfer rate are just matching we can evaluate the likely minimum dissolved oxygen tension that
will be attained at this point. We can then determine if this is acceptable.

(ii) Calculate the minimum dissolved oxygen tension likely to occur under the conditions described. Give reasons for any assumptions you make.

SECTION FIVE: DESIGN SENSITIVITY

(i) Make a list of places in the calculation where you judge the design process to be most sensitive.