PART ONE: BATCH STERILISATION
A pilot scale batch fermentation broth is to be sterilised. The broth will be inoculated with a filamentous micro-organism. The vessel has a working volume of 3m3 and is operated at 30oC.
The medium consists of: (g L-1)
Yeast extract 6.0
Glycerol 3.0
(NH4)2 SO4 2.0
K2HPO4 2.0
MgSO4 1.0
FeSO4 0.01
CaCl2 0.01
A small quantity of synthetic anti-foam is added to minimise foaming during sterilisation.
(a) Are there likely to be any problems during the sterilisation?
(b) The broth is to be sterilised at 121oC. Calculate the holding time if:
(i) the heating from 100oC to 121oC takes 32 minutes
(ii) the cooling from 121oC to 100oC is achieved at a rate of 2.5oC min-1
(iii) the acceptable level of contamination has been determined as 1 in 1000 operations.
(iv) the initial spore concentration is 5 x 106 spores ml -1
(c) A failure in the steam generator means that pressure cannot be maintained and the maximum temperature which can now be achieved is 119oC. The heating from 100oC to 119oC now takes 50 minutes. The cooling rate is unaffected. What is the new holding time? What other parameters change as a result?
(d) Process development is indicating that an improved medium is required to boost product and biomass levels. The new medium is complex and includes starch, rape seed oil and soya flour. Discuss the implications for the sterilisation and suggest any changes you may think appropriate particularly if some money is available for capital investment.
PART TWO: CONTINUOUS STERILISATION
The production scale process for the fermentation process described in part one operates using a continuous steriliser. On the plant, steam is available at 130oC and the medium needs to be produced at a rate of 10 m3 hr-1
(a) Calculate the following for the system:
(i) the total Del factor
(ii) the holding pipe residence time
(iii) the length of the holding pipe of the diameter of the pipes is 5 cm.
assuming that the details given for part one also apply here, where appropriate.
(b) For the system calculate the heating and cooling loads if the initial feed is available at 20oC.
(c) For the system calculate the heating and cooling loads if a heat recovery system is used to pre-heat the fresh feed to 70oC.
PART THREE: STERILISATION ASSUMPTIONS
In batch and continuous sterilisation calculations a number of assumptions have been made in order to simplify the calculations. In this section we will examine the validity of these assumptions and assess the likely consequences.
(a) What is the effect of assuming that the ‘rate of killing' below 100oC is insignificant?
(b) What is likely to actually occur in the temperature - time profile during heating and cooling when:
(i) the temperature is raised from 20oC to 100oC
(ii) the temperature is raised from 100oC to 121oC
(iii) the temperature is cooled from 121oC to 100oC
(iv) the temperature is lowered from 100oC to the operating temperature
(c) For the continuous steriliser, what are the likely consequences of assuming plug flow?
(d) Is the assumption of a failure rate of 1 in 1000 reasonable?
PART FOUR: FERMENTER STERILISATION PROBLEMS
In any process the situation is unlikely to be perfect every cycle, that is, not every sterilisation will be identical and ideal. In this section we will look at some possibilities and assess the consequences for the sterilisation process.
What are the consequences if:
(a) the bioreactor has a slight leak from the top plate during a batch sterilisation above 100oC
(b) the filter which cleans the steam before steam arrives at the pilot plant fails during a batch sterilisation. Sterilisation of the bioreactor is carried out via the heating jacket.
(c) a problem means that the steam filter (see b) has to be by-passed for 3 months due to pipe work alterations. The heating jacket is again used for batch sterilisation.
(d) the plant steam pressure for a continuous steriliser drops leading to a lower temperature of 129oC.