158
L. Lin et al.
6.1.1 Physicochemical Characteristics and Uses of Glycerol
Pure glycerol [C 3 H 5 (OH) 3 ] is a colorless, sweet, and viscous liquid with a relative
density of 1.261/l(20 °C). By measuring the relative density of glycerol solution,
we can calculate the amount of glycerol. At normal pressure, the boiling point of
glycerol is 290 °C. Glycerol has a high viscosity. Pure glycerol has a viscosity 777
times more than that of water, and 50% glycerol solution has a viscosity 5.41 times
more than that of water. Glycerol can be mixed with water, methanol, and aniline in
any proportion. It is soluble in acetone, alcohol and ether mixtures, or chloroform
and ethanol mixtures of a certain mass ratio [1, 2]. Meanwhile, it is hardly soluble in
anhydrous ether, and slightly soluble in oils and fats, gasoline, benzene, chloroform,
carbon disulfide, and other organic solvents. Glycerol is a preferable solution for
many organic compounds, inorganic salts, caustic soda, and heavy metal soaps.
Glycerol is a trihydric alcohol with typical chemical characteristics of that type.
It can be involved in many chemical reactions and produce many derivatives.
Since glycerol has a large number of important physicochemical properties, it has
become a crucial chemical raw material [3–5] Today in China, glycerol is mainly
used in the production of paint, food, medicines, dynamite, toothpaste, cellophane,
and insulating materials.
6.2 Production of Crude Glycerol
Since biodiesel production is yet to form a large-scale industry in China, recycling
by-products of the production is not a meaningful approach [6]. But with the developing oils and fats manufacture industry, biodiesel is steadily heading toward a
mass-production industry. In this sense, the recycling and reuse of by-products are
becoming more important [7, 8]. In fact, the recycling and reuse of glycerol in
biodiesel are similar to that in oils and fats hydrolysis wastewater and in saponification wastewater [9–11]. We may adapt the glycerol recycling approach in both
cases to biodiesel production. Below is a brief introduction to this glycerol recycling
approach:
(1) Raw materials of glycerol and their origins
Glycerol products can be divided into chemically synthesized glycerol and natural
glycerol. Most synthesized glycerols are made through chemical synthesis of petrochemical products. Natural glycerol widely exists in nature mainly in the form of
triglycerides (vegetable oils and fats and animal fats). It is produced by saponification,
hydrolysis or alcoholysis of oils and fats. Biodiesel production by transesterification
falls under the alcoholysis category. With oils and fats as industrial raw material, a
huge amount of wastewater is produced in the production of fatty acids, soaps, and
biodiesel. Such wastewater contains useful substances including glycerol. If directly
emitted, they not only contaminate the environment, but also waste resources. When
recycled and reused, they can create significant social and economic benefits.
L. Lin et al.
6.1.1 Physicochemical Characteristics and Uses of Glycerol
Pure glycerol [C 3 H 5 (OH) 3 ] is a colorless, sweet, and viscous liquid with a relative
density of 1.261/l(20 °C). By measuring the relative density of glycerol solution,
we can calculate the amount of glycerol. At normal pressure, the boiling point of
glycerol is 290 °C. Glycerol has a high viscosity. Pure glycerol has a viscosity 777
times more than that of water, and 50% glycerol solution has a viscosity 5.41 times
more than that of water. Glycerol can be mixed with water, methanol, and aniline in
any proportion. It is soluble in acetone, alcohol and ether mixtures, or chloroform
and ethanol mixtures of a certain mass ratio [1, 2]. Meanwhile, it is hardly soluble in
anhydrous ether, and slightly soluble in oils and fats, gasoline, benzene, chloroform,
carbon disulfide, and other organic solvents. Glycerol is a preferable solution for
many organic compounds, inorganic salts, caustic soda, and heavy metal soaps.
Glycerol is a trihydric alcohol with typical chemical characteristics of that type.
It can be involved in many chemical reactions and produce many derivatives.
Since glycerol has a large number of important physicochemical properties, it has
become a crucial chemical raw material [3–5] Today in China, glycerol is mainly
used in the production of paint, food, medicines, dynamite, toothpaste, cellophane,
and insulating materials.
6.2 Production of Crude Glycerol
Since biodiesel production is yet to form a large-scale industry in China, recycling
by-products of the production is not a meaningful approach [6]. But with the developing oils and fats manufacture industry, biodiesel is steadily heading toward a
mass-production industry. In this sense, the recycling and reuse of by-products are
becoming more important [7, 8]. In fact, the recycling and reuse of glycerol in
biodiesel are similar to that in oils and fats hydrolysis wastewater and in saponification wastewater [9–11]. We may adapt the glycerol recycling approach in both
cases to biodiesel production. Below is a brief introduction to this glycerol recycling
approach:
(1) Raw materials of glycerol and their origins
Glycerol products can be divided into chemically synthesized glycerol and natural
glycerol. Most synthesized glycerols are made through chemical synthesis of petrochemical products. Natural glycerol widely exists in nature mainly in the form of
triglycerides (vegetable oils and fats and animal fats). It is produced by saponification,
hydrolysis or alcoholysis of oils and fats. Biodiesel production by transesterification
falls under the alcoholysis category. With oils and fats as industrial raw material, a
huge amount of wastewater is produced in the production of fatty acids, soaps, and
biodiesel. Such wastewater contains useful substances including glycerol. If directly
emitted, they not only contaminate the environment, but also waste resources. When
recycled and reused, they can create significant social and economic benefits.