6 By-Products of Biodiesel and Their Recycling
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environment, ferric chloride is transformed into ferric hydroxide or ferric oxalate
precipitate. Ferric hydroxide further absorbs impurities to be removed by filtration.
The aqueous solution containing barium carbonate (BaCO 3 ) is alkaline [23].
Adding barium carbonate in the first step will neutralize the acidic glycerol waste
solution and forms barium sulfate by reacting with calcium sulfate. As more barium
carbonate is added, the solution turns neutral or slightly alkaline. Barium carbonate
then reacts with calcium ions in the solution to form calcium carbonate, while ferric
chloride is transformed into ferric hydroxide, which further absorbs impurities to be
removed by filtration.
In the final step, soda or sodium oxalate is used for purification. The purified
waste solution contains water-soluble sodium sulfate. Sodium sulfate will not lead
to precipitate in the evaporation equipment, but it does not reduce the ash content of
glycerol. Therefore, barium carbonate is a better option to remove calcium sulfate
and reduce the ash content. However, due to the high cost of barium carbonate, many
factories use soda for the final purification step.
➁ Purification by ion exchange
Ion-exchange resin is a good option to purify dilute glycerol solution, and it is
widely used in the industry [18, 22].
Ion-exchange resin is a complicated, network-structured organic substance. With
many exchangeable active genes, it is stable in common acid, basic, or other solutions. Ion-exchange resins can be categorized into strong acid, weak acid, strong
base an weak base types. It depends on whether it contains acidic or alkaline genes
and how strong the acidity or alkalinity is. Ion-exchange resins can also be categorized into cation and anion types. The former is represented by the strong sulfonic
acid type, which is widely applicable in acidic, alkaline, and neutral solutions for
exchanging simple or complicated inorganic and organic cations. Common cation
exchange resins include sulfonated coal, sulfonated styrene-divinylbenzene copolymers, and sulfonated phenolic resin. The latter is strongly alkaline, and can be applied
in acid, basic, or neutral solutions. It can react with strong or weak acid for anion
exchange. Common anion exchange resins are those contain -N(CH 3 ) 3 OH
- , including
styrene-divinylbenzene-chloromethyl-trimethylamine resin.
The basic principle for the purification of dilute glycerol solution using ionexchange resin is as follows. Ionizing impurities such as sodium chloride, sodium sulfate, other salts and fatty acid, soap and pigment solutions are removed by exchange
and absorption through a set of cations and anions exchange equipment. Glycerol
is nonelectrolyte and will not transform in this process. Some impurities including
polyglycerol and esters are difficult to remove, but they are rare in glycerol. Therefore, with proper treatment, no other process except evaporation and concentration
is needed to produce refined glycerol.
After ion-exchange purification, the glycerol solution is concentrated to a certain
level. If the evaporation equipment is made of stainless steel, no bleaching process is
needed as the finishing step. For any other material, the solution should be discolored.
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environment, ferric chloride is transformed into ferric hydroxide or ferric oxalate
precipitate. Ferric hydroxide further absorbs impurities to be removed by filtration.
The aqueous solution containing barium carbonate (BaCO 3 ) is alkaline [23].
Adding barium carbonate in the first step will neutralize the acidic glycerol waste
solution and forms barium sulfate by reacting with calcium sulfate. As more barium
carbonate is added, the solution turns neutral or slightly alkaline. Barium carbonate
then reacts with calcium ions in the solution to form calcium carbonate, while ferric
chloride is transformed into ferric hydroxide, which further absorbs impurities to be
removed by filtration.
In the final step, soda or sodium oxalate is used for purification. The purified
waste solution contains water-soluble sodium sulfate. Sodium sulfate will not lead
to precipitate in the evaporation equipment, but it does not reduce the ash content of
glycerol. Therefore, barium carbonate is a better option to remove calcium sulfate
and reduce the ash content. However, due to the high cost of barium carbonate, many
factories use soda for the final purification step.
➁ Purification by ion exchange
Ion-exchange resin is a good option to purify dilute glycerol solution, and it is
widely used in the industry [18, 22].
Ion-exchange resin is a complicated, network-structured organic substance. With
many exchangeable active genes, it is stable in common acid, basic, or other solutions. Ion-exchange resins can be categorized into strong acid, weak acid, strong
base an weak base types. It depends on whether it contains acidic or alkaline genes
and how strong the acidity or alkalinity is. Ion-exchange resins can also be categorized into cation and anion types. The former is represented by the strong sulfonic
acid type, which is widely applicable in acidic, alkaline, and neutral solutions for
exchanging simple or complicated inorganic and organic cations. Common cation
exchange resins include sulfonated coal, sulfonated styrene-divinylbenzene copolymers, and sulfonated phenolic resin. The latter is strongly alkaline, and can be applied
in acid, basic, or neutral solutions. It can react with strong or weak acid for anion
exchange. Common anion exchange resins are those contain -N(CH 3 ) 3 OH
- , including
styrene-divinylbenzene-chloromethyl-trimethylamine resin.
The basic principle for the purification of dilute glycerol solution using ionexchange resin is as follows. Ionizing impurities such as sodium chloride, sodium sulfate, other salts and fatty acid, soap and pigment solutions are removed by exchange
and absorption through a set of cations and anions exchange equipment. Glycerol
is nonelectrolyte and will not transform in this process. Some impurities including
polyglycerol and esters are difficult to remove, but they are rare in glycerol. Therefore, with proper treatment, no other process except evaporation and concentration
is needed to produce refined glycerol.
After ion-exchange purification, the glycerol solution is concentrated to a certain
level. If the evaporation equipment is made of stainless steel, no bleaching process is
needed as the finishing step. For any other material, the solution should be discolored.