166
L. Lin et al.
its color. Furthermore, it also consumes more fuel (to increase the concentration from
6 to 65%, 1.3–1.7 tons of coal is needed for every ton of glycerol waste solution) and
the process takes a long time. Therefore, this approach is applicable only in small
factories with limited conditions for the pre-concentration of glycerol solution. Due
to the high value of glycerol, every bit of loss is costly. Therefore, vacuum evaporation
is a more common method.
• Vacuum evaporation. Vacuum evaporation can be categorized into single-effect
evaporation and multi-effect evaporation. The glycerol concentration process can
be categorized into single-effect evaporation, multi-effect evaporation, and the
combination of the two.
For single-effect evaporation, it requires a smaller equipment investment. But it
consumes more thermal energy because the secondary steam cannot be reused. The
condensation of the secondary steam requires a large amount of cooling water, which
increases the production cost.
Multi-effect evaporation takes advantage of secondary evaporation to improve
thermal efficiency, saving fuel as well as cooling water. Multi-effect evaporation is
similar to single-effect evaporation in terms of evaporation efficiency. For a given
temperature difference, the multi-effect evaporator efficiency, or the amount of steam
generated from the heated area per square meter is smaller than that of single-effect
evaporator. However, single-effect evaporation costs less.
Multi-effect evaporation requires several evaporators that increase the equipment
cost. Meanwhile, it is not feasible to increase the concentration of crude glycerol
using multi-effect evaporator. This is because the boiling point of glycerol solution
increases with its concentration, so does the amount of heat required for evaporation,
which exceeds the heat provided by secondary steam. Therefore, vacuum state is
required for the process to accelerate the evaporation speed to match the first-effect
evaporation operation (using forward feeding).
At present, glycerol factories utilize double-effect evaporation in the first stage.
When the glycerol concentration reaches a certain level, they continue to use singleeffect evaporation (Fig. 6.4).
(3) Main equipment for dilute glycerol solution concentration
The vacuum evaporation system consists of three major components: vacuum
evaporator with a trap, condenser, and vacuum pump.
(1) Vacuum evaporator. Circulating evaporator is used for glycerol solution evaporation. For this type of evaporator, the solution is circulating in the evaporator, which
improves heat transfer efficiency. At present, most glycerol factories are using central
circulating evaporator (standard-type), suspending-basket-type evaporator, and the
most commonly used external heating evaporator.
Central circulating evaporator (standard-type). Its structure is shown in Fig. 6.5.
The heating chamber contains a vertical heating tube (or “boiling tube”) bundle. In
the center of the bundle there is a tube with a large diameter, known as the central
circulating tube. Since the central circulating tube has a large cross section area, the
heat transfer area of the solution per unit volume is smaller than that in the other
L. Lin et al.
its color. Furthermore, it also consumes more fuel (to increase the concentration from
6 to 65%, 1.3–1.7 tons of coal is needed for every ton of glycerol waste solution) and
the process takes a long time. Therefore, this approach is applicable only in small
factories with limited conditions for the pre-concentration of glycerol solution. Due
to the high value of glycerol, every bit of loss is costly. Therefore, vacuum evaporation
is a more common method.
• Vacuum evaporation. Vacuum evaporation can be categorized into single-effect
evaporation and multi-effect evaporation. The glycerol concentration process can
be categorized into single-effect evaporation, multi-effect evaporation, and the
combination of the two.
For single-effect evaporation, it requires a smaller equipment investment. But it
consumes more thermal energy because the secondary steam cannot be reused. The
condensation of the secondary steam requires a large amount of cooling water, which
increases the production cost.
Multi-effect evaporation takes advantage of secondary evaporation to improve
thermal efficiency, saving fuel as well as cooling water. Multi-effect evaporation is
similar to single-effect evaporation in terms of evaporation efficiency. For a given
temperature difference, the multi-effect evaporator efficiency, or the amount of steam
generated from the heated area per square meter is smaller than that of single-effect
evaporator. However, single-effect evaporation costs less.
Multi-effect evaporation requires several evaporators that increase the equipment
cost. Meanwhile, it is not feasible to increase the concentration of crude glycerol
using multi-effect evaporator. This is because the boiling point of glycerol solution
increases with its concentration, so does the amount of heat required for evaporation,
which exceeds the heat provided by secondary steam. Therefore, vacuum state is
required for the process to accelerate the evaporation speed to match the first-effect
evaporation operation (using forward feeding).
At present, glycerol factories utilize double-effect evaporation in the first stage.
When the glycerol concentration reaches a certain level, they continue to use singleeffect evaporation (Fig. 6.4).
(3) Main equipment for dilute glycerol solution concentration
The vacuum evaporation system consists of three major components: vacuum
evaporator with a trap, condenser, and vacuum pump.
(1) Vacuum evaporator. Circulating evaporator is used for glycerol solution evaporation. For this type of evaporator, the solution is circulating in the evaporator, which
improves heat transfer efficiency. At present, most glycerol factories are using central
circulating evaporator (standard-type), suspending-basket-type evaporator, and the
most commonly used external heating evaporator.
Central circulating evaporator (standard-type). Its structure is shown in Fig. 6.5.
The heating chamber contains a vertical heating tube (or “boiling tube”) bundle. In
the center of the bundle there is a tube with a large diameter, known as the central
circulating tube. Since the central circulating tube has a large cross section area, the
heat transfer area of the solution per unit volume is smaller than that in the other