Biomedical Engineering Reference
In-Depth Information
10
16
for
Eqn (18.47)
, we can solve with Excel for k
d
s
¼
(a) Setting p(
)
¼
7.59
63.91.
s
Therefore, L
¼
0.1356 m. A short section of the tube will be sufficient to meet the
requirement.
1
2
k
d
s
ln
4ðk
d
s
þ2Þ
k
d
s
ðk
d
s
þ8Þ
lnðC
X0
VÞ
t
S
¼
63
:
91
2
ln
4ð63:91þ2Þ
63:91ð63:91þ8Þ
lnð2:510
11
¼
10Þ¼6:267
(b) Setting P
1
(
)
¼
0.001897 for
Eqn (18.48)
, we can solve with Excel for k
d
s
¼
63.91.
s
Therefore, L
¼
0.1356 m. We arrived practically at the same answer as that for
10
16
.
setting p(
)
¼
7.59
s
18.7.3. Sterilization of Liquids
Thermal inactivation is preferred for large-scale sterilization of equipment and liquids.
It is the most reliable method of sterilization.
Sterile filtration is often used where elevated temperatures can be detrimental to the
process fluid. Liquid streams are steam or filter sterilized. Filtration with 0.2
e
0.45
m
m
membrane filter can be used to remove microbial components. The membrane filter itself
must be sterilized before use, via thermal sterilization. The use of membrane between
0.2
m
mand0.45
m
m is to ensure microorganism not passing through the membrane
(needing finer pore sizes) while maintaining high throughput (for larger pore sizes). Sterile
filtration is described in further detail for gases. Materials of construction for the filter
membranes are application dependent. Liquid media filtration is generally not as effective
or reliable as heat or thermal sterilization. Viruses andmycoplasma are able to pass through
membrane filters. Care must be taken to prevent holes or tears (leaks) in the membrane.
Usually, filter-sterilized medium is incubated for a period of time before use to ensure its
sterility.
Heat-sensitive equipment can be sterilized with chemical agents or radiation. Chemical
agent for sterilization must leave no residue that would be toxic to the culture or otherwise
adulterate the product. Typical chemical agents used are: ethylene oxide, 70% ethanol water
acidified to pH 2 with HCl, formaldehyde, 3% sodium hypochlorite.
Radiation sterilization can be applied to equipment surfaces as well as processing liquids.
Radiation sterilization is usually used for sterilization of filtration equipment but not
commonly used for large equipment due to operational difficulties involving applying radi-
ation to large areas. The radiation sources can be applied include
g
-ray, electron beam, or UV
light.
g
-radiation has better penetration into materials but is much more dangerous to work
with. Electron beam systems are not commonly used for sterilizing the insides of fermentors
but are becoming more prevalent in the sterilization of presterilized components such as
filters. UV light is the simplest of the radiations to be used, as it can be provided by basic
UV light bulbs. These are often encased in quartz tubes to protect them, as glass is insuffi-
ciently transparent in the UV range. UV can sterilize surfaces but cannot penetrate
fluids easily.
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