412
Chapter 9
Problem 9.4 Heat transfer limitation in SCP production.
a.
In an SCP production 2.5 N m
3
CFL, (N= 273 K, latm) is used to produce 1 kg of biomass
X=
CH
18
O0.j N
0.2
by continuous fermentation in a stirred tank. Determine
Y30
and thereafter the
total stoichiometry of the reaction, assuming that biomass, C 0
3
and H20 are the only products
and that NH
3
is used as nitrogen source.
Calculate the heat of reaction
Q
in kJ(kg biomass)"1.
b.
The bioreactor is cylindrical with diameter
d
and height
h. h =
3
d.
The inner cylinder area A
(nr), but not the bottom of the reactor is covered with heat exchange surface {see Figure 11.4).
The effective temperature driving force is
&T=
30°C, and the heat transfer coefficient
hT
is 300
kj (m
2
h K ) '. The heat transferred is consequently
Q ‘ = hTAT A
ki h"1. Show that -due to heat
transfer limitation- the maximum productivity of biomass is
{qx)m3x
= 0.60 V‘m kg m
‘3
h
"1
, where
V
is the reactor volume.
(1)
For a sterile feed, and
x =
20 g L l determine the maximum possible dilution
rate ,
for
V=
1 L and for
V=
50 m3.
c.
Laboratory experiments are conducted in a small reactor where heat transfer is no problem. The
liquid feed is sterile and contains 5 g L
"1
NH
3
and a sufficient supply of minerals. The gas feed
is 1 N m
3
per m
3
reactor volume and per min. The composition is 25 vol% CH
4
and 75 vol% 0 3.
With this much CL in the feed the reaction is always limited by CH
4
following data are obtained:
D (h ‘)
CH» (vol% in exit gas)
Biomass in
0.05
20.53
36.17
0.1
14.16
36.17
0.2
13.18
19.20
0.3
9.626
15.20
0.5
25.00
0
Assume that no water is transferred from gas to liquid phase or vice
versa.
Show that -
? C//4
in mol CH
4
consumed (L reactor rain
)"1
and the gas-phase concentrations in the
second column of the table are related by:
vol %
0-25 • 0.044642 +
q CH^
100 _ 0.044642 + (1
+ Yto)qCHt-Y x qca4
qCnA
to the table.
For
D =
0.2 and 0.3 h
'1
show that the yield coefficient
Y„
calculated in (a) is also found from the
data of the table. Calculate the effluent NH
3
concentration. No NH
3
is stripped to the gas phase.
Finally explain the data for
D =
0.05 and 0.1 h'1. What happens at
D =
0.5 h 1?