Scale-up of Bioprocesses
513
Example 11.6. Loop-reactor design for production of single-cell protein
In a production plant close to Trondheim, Norway, about 9000 ton/year of protein is produced from methane.
The methane is obtained from natural gas in the North Sea and the methanotrophic microorganism
Methylococcus capsulatus
is used as production organism. The overall reaction stoichiometry for production
of biomass from methane is approximately given by
-C H A
-1.5202
~ 0.09NH3 + 0.546CHiSOQ5N 02 + 0.544CO2+\.12H 20
(
1
)
The protein content in the biomass is quite high (70%), which gives a protein yield of about 0.5 kg (kg
methane)"1
. The reaction takes place at atmospheric conditions and at a temperature of 45 °C.
The bioreactor design for this process is rather demanding for several reasons. The solubility of methane (and
oxygen) is low in water. The saturation concentrations are approximately 1 mM for both compounds at
process conditions (see Table 10.1 and 10.8). The oxygen yield is, furthermore, high since a very reduced
substrate is used. This in turn gives a high heat yield. The heat of reaction is approximately given by (see
Section 4.2)
-
AH
= 1
'52'46Q1Q
= 1.28 ■
10
6
J (C-mol biomass
)’ 1
= 52 kJ (g biomass
) 1
(2)
0.546
At a biomass production rate of one ton per hour, the heat effect is thus approximately 14.4 MW. To obtain a
stoichiometric gas phase composition according to Eq. 1
using air at atmospheric pressure, the partial
pressures of CH
4
and 0
2
should be 0.116 and 0.177 atm, respectively.
The volumetric mass transfer
coefficient,
kfi,
obtained in a mechanically stirred bioreactor can be approximated to about 600 h'1. We can
therefore estimate the required reactor volume based on the mass transfer capacity (oxygen will be the
limiting component) from Eq.3.
k . a c y ^ Y ^ q y
(3)
Assuming that c
0
is ~ 0 and that c* = 0.177 mol m'3, the volume required for production of 1000 kg biomass
hf1
, is
V>
1
600-0.177
1.52-106
^0.456-24.6
=1276m3
(4)
The reactor volume in the actual process is, however, only 300 m3. Instead of a stirred tank reactor, a loop
reactor (see Fig 11.2), with two long horizontal tubes and two short vertical tubes is used. The liquid is
circulated at a linear flow rate of about 1 ms'1. Furthermore, pure oxygen is used instead of air. (The plant is
located very close to a major methanol production facility, which makes it easy to obtain pure oxygen in
sufficient quantities.) The gas containing methane and oxygen (partial pressures 0.397 and 0.603 atm,
respectively) is injected in the liquid flowing downwards in one of the vertical tubes. The tube is equipped
with regularly spaced static mixers, giving a high volumetric mass transfer rate
(k/a
> 900 h'1). Produced C0
2
and the remainder of 0
2
and CH
4
is separated from the liquid phase in the top, horizontal part of the loop. The
considerable cooling requirement is met by the use of external, very efficient, heat exchangers. Part of the
liquid is pumped out of the main loop to these external heat exchangers at several places along the loop.