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L. Lin et al.
➂ Electric purification [24, 25]
Chemical purification does not necessarily disrupt the emulsion in glycerol solution. The addition of extra chemicals normally increases the ash content of the
solution and leads to glycerol loss.
By using direct current to purify glycerol waste solution, the emulsion of the
solution is disrupted with the presence of electric field. The particles in disperse
phase float on the surface layer of the solution in the state of “bubble cap”, while the
clear, purified glycerol solution is left on the lower layer.
Lab experiments have been made to study the effect of the following factors on
glycerol solution purification. The parameters have been determined as follows:
Electric field strength: no less than 30 V/cm
Electrode material: aluminum
Electrode spacing: the spacing should be as small as possible and avoid danger
Temperature: not lower than 70 °C
Glycerol liquid layer thickness: 70 cm.
On-site experiments using these parameters in factories function well. One ton
of 10–12% glycerol wastewater has an average electricity consumption level of
18.6 kW·h and 0.23 kg aluminum consumption. On the anode attach aluminum
paste and other residues that need to be cleaned regularly. This is a major weakness
of this approach. There must be accompanying methods for precipitation prevention
or cleaning.
6.3 Concentration of Purified Glycerol Wastewater
After purification, most impurities in glycerol solution have been removed [26]. But
the glycerol concentration does not increase as much water still remains. As to the
dilute glycerol solution made from wastewater treated by oils and fats saponification
and hydrolysis, it still contains much salts and thus has to be dehydrated and concentrated. In this way, the salts are gradually crystallized and precipitated to the point
that the glycerol level in solution reaches about 80%, known as crude glycerol.
(1) Concentration theories [27, 28]
The theoretical basis of dilute glycerol solution concentration is the evaporation
theory. As to the evaporation process, the glycerol solution is first heated to its
boiling point so that part of the water is gasified and escapes, and the glycerol
concentration increases. In fact, during the evaporation of dilute glycerol solution,
a tiny amount of glycerol is gasified along with water. Since the boiling point of
glycerol is way higher than that of water, the amount is too tiny to be taken into serious
consideration. Therefore, glycerol is regarded as nonvolatile during the evaporation
and concentration of dilute glycerol solution. As water keeps vaporizing, salts are
crystallized and precipitated, so that the dilute glycerol solution is concentrated.
L. Lin et al.
➂ Electric purification [24, 25]
Chemical purification does not necessarily disrupt the emulsion in glycerol solution. The addition of extra chemicals normally increases the ash content of the
solution and leads to glycerol loss.
By using direct current to purify glycerol waste solution, the emulsion of the
solution is disrupted with the presence of electric field. The particles in disperse
phase float on the surface layer of the solution in the state of “bubble cap”, while the
clear, purified glycerol solution is left on the lower layer.
Lab experiments have been made to study the effect of the following factors on
glycerol solution purification. The parameters have been determined as follows:
Electric field strength: no less than 30 V/cm
Electrode material: aluminum
Electrode spacing: the spacing should be as small as possible and avoid danger
Temperature: not lower than 70 °C
Glycerol liquid layer thickness: 70 cm.
On-site experiments using these parameters in factories function well. One ton
of 10–12% glycerol wastewater has an average electricity consumption level of
18.6 kW·h and 0.23 kg aluminum consumption. On the anode attach aluminum
paste and other residues that need to be cleaned regularly. This is a major weakness
of this approach. There must be accompanying methods for precipitation prevention
or cleaning.
6.3 Concentration of Purified Glycerol Wastewater
After purification, most impurities in glycerol solution have been removed [26]. But
the glycerol concentration does not increase as much water still remains. As to the
dilute glycerol solution made from wastewater treated by oils and fats saponification
and hydrolysis, it still contains much salts and thus has to be dehydrated and concentrated. In this way, the salts are gradually crystallized and precipitated to the point
that the glycerol level in solution reaches about 80%, known as crude glycerol.
(1) Concentration theories [27, 28]
The theoretical basis of dilute glycerol solution concentration is the evaporation
theory. As to the evaporation process, the glycerol solution is first heated to its
boiling point so that part of the water is gasified and escapes, and the glycerol
concentration increases. In fact, during the evaporation of dilute glycerol solution,
a tiny amount of glycerol is gasified along with water. Since the boiling point of
glycerol is way higher than that of water, the amount is too tiny to be taken into serious
consideration. Therefore, glycerol is regarded as nonvolatile during the evaporation
and concentration of dilute glycerol solution. As water keeps vaporizing, salts are
crystallized and precipitated, so that the dilute glycerol solution is concentrated.