ETHANOL FROM SUGAR BEET - Sugar Beets - page 17

Heat requirement to evaporate this quantity would be too huge, about 1.65 GJ · t − A .
Instead, the concentration occurs in a multi-effect evaporator, which consists in a succession
of liquid/gas separators. The first effect is heated by process steam. The evaporated water
in turn heats by condensation the following effect, and so on. Process steam consumption
is close to the amount of water to remove divided by the number of effects.
The process requires a strict control of pressure and temperature of each effect, and is
limited by liquid fluidity. Hence it is not amendable to all feedstock and final concentration
depends on the nature of the solid residue. Juice with dissolved sugar is easier to concentrate
than stillage residue, and beet pulp does not suit.
Multi-effects can be combined with a thermo-compressor. Part of the evaporated water
at the first effect is recompressed and heated owing to entrainment by high pressure process
steam (at about 4 bars) in an injector. The output heats the first effect. It is equivalent to
add another effect.
The drawback is the increased number of equipment and, subsequently, electricity con-
sumption.
A simple five effect evaporator in a typical Brazilian sugar mill requires r conc = 10.3 kJ
of electricity and 0.224 kg of process steam per kg of removed water [31].
A simple six effect one used to concentrate stillage consumes r conc = 51 kJ of electricity
and 0.176 kg of process steam per kg of removed water [?].
With the addition of thermo-compression we assume a consumption to concentrate the
juice of r conc = 55 kJ e · kg − 1

and 0.155 kg st · kg − 1 .

w w

4.6.4. Juice fermentation

In a large ethanol factory of 0.30 M.m 3 annual capacity, fermentation takes place in a series
of 5 to 6 tanks of about 2 million liters each at a temperature between 30-35 ◦ C [2]. Over a
total residence time of about 35 hours a beer at 10%vol of ethanol is produced. Thus 10 l
of beer contains 1 l of ethanol, or w f erm = 1 . 0 m beer · t − A .
The fermentation step requires electricity to pump the liquor between tanks, to drive
agitators, to compress and inject air and CO 2 for aeration, and to cool the tanks. The con-
sumption is assumed proportional to the quantity of beer. The local rate r f erm varies from
about 14.5 MJ e · m − 3

beer , according to two Swiss studies using manufacturer data [28, 29], to

22 MJ e · m − 3

beer in French factories [2]. This large difference is surprising for equipment and
procedures quite similar. The last value is retained as corresponding to actual operations in
France.

4.6.5. Beer distillation

A French study based on information from UNGDA indicates that the steam requirement
of a one column system is as high as r dist,th = 250 kg w · m − 3

beer , whereas operating with a
double effect and thermo-compression decreases r dist,th to 100 kg w · m − 3

beer [11].
Distillation columns and multi-effect evaporators share some similarities. They are two
systems of separation which intend to recover large part of the water latent heat. Unsur-
prisingly both systems present close performances in terms of consumptions per mass of
evaporated liquid.