Page 319 - Biosystems Engineering

296 Cha pte r Ni ne

attempts have been made to overcome the difficulty of separating the
mycelium from broth on an industrial scale. In the recovery of strep-
tomycin, resistance of the mycelium to filtration has been reduced by
heating the fermentation broth. Coagulating mycelia protein with
heat to accelerate filtration may have wide application in the fermen-
2
tation industry. For this purpose, a filter of 110-cm area was used.
The filter aid was diatomaceous earth, and pressure was applied con-
2
stantly at 2 kg/cm . The sample of broth used was taken from a batch
fermenter (60 m ), which had been operated for 3 to 4 days. The orig-
3
inal culture medium consisted of glucose and soybean powder, sup-
plemented by inorganic salts and dried yeast. In the above experi-
ments, coagulation of mycelia protein was apparently achieved after
30 to 40 minutes at 100°C, but a longer exposure of the broth to heat
adversely affected the filtration rate, presumably due to disintegra-
tion of the coagulated protein.
To summarize, industrial filtration of streptomyces for the recovery
of streptomycin via that of the cells was successful with the pH at 3.7 to
4.3 and the temperature at 80 to 90°C for 30 to 60 minutes to raise the
broth to the desired temperature using 2 to 3 percent of the filter aid.

9.3.6 Evaporation
The objective of evaporation is to concentrate a solution containing a
nonvolatile solute and a volatile solvent. In the overwhelming major-
ity of evaporations, the solvent is water. Evaporation is conducted by
vaporizing a portion of solvent to produce a concentrated solution, or
thick liquor. The operation falls under the category of heat transfer to
boiling liquids.
In evaporation, consideration should be given to the physical
properties of the liquid to be concentrated, such as specific gravity,
specific heat, thermal conductivity, and the like, solid concentration
(initial and final), heat sensitivity of the material (very important in
pharmaceuticals manufacture, preserving foodstuffs, etc.), and scale
formation, and the like. Proper boiling temperature should be arrived,
taking into account the possible elevation of the boiling point of the
solution. The material of construction is an important criterion.
Evaporator performance is dependent on various parameters.
Thus, the capacity is influenced by the area of heating surface, tem-
perature drop, overall heat transfer coefficient, and heating load. The
economy is governed by load, heats of vaporization of steam and liq-
uid, heat of dilution of liquid, amount of superheat in steam, heat
loss, and so on. The multiple effects can be operated in various ways:
forward feed, backward feed, parallel feed, and mixed feed. Each of
these feeds has its own distinctive features. For example, forward
feed is very simple in operation and capital cost is less, but heat trans-
fer in the mixed feed is extremely poor. In backward feed, this diffi-
culty can be overcome, no doubt, but more pumps are required for
the transfer of solutions.