142
C. Li et al.
5.2.6 Continuous Transesterification Process for Biodiesel
Production
The continuous transesterification process is a typical operation that includes vegetable oils as raw material for biodiesel production. It has a high yield and produces biodiesel in a continuous fashion. The continuous transesterification process
involves two stages. The first is the operation to produce biodiesel with transesterification (workshop), and the second is re-esterification, methanol recycling, and
glycerol refining operation (workshop). The basic flow diagram of the continuous
transesterification process for biodiesel production is presented as Fig. 5.5.
The key points of operation are highlighted as follows.
a. Transesterification reaction and separation. First, preheated vegetable oil (about
35 °C), methanol (molar ratio of methanol and oil is 6:1 to 4:1), and catalysts
(KOH or NaOH, 1–2%) are continuously added in a given proportion to the
first esterification tank for reaction with stirring. The reaction time is determined
based on the solution level (10–20 min). This process adopts a two-stage reaction
operation. Following the reactions in the first reaction tank, the reactants enter
a specially designed separation column, so that the rapeseed oil methyl ester
(RME) produced can be continuously separated from glycerol and water. RME
then continues to enter the second reaction tank for further and sufficient reaction.
After that, the reactions take place in the second separation column to separate
the glycerin phase (which flows into the storage tank for recycling).
b. Continuous water rinse, acid cleaning, and separation. The RME separated from
the glycerin phase enters the water rinse reactor to be rinsed with hot water. The
residues of catalyst, potassium soap, and glycerol in RME are rinsed and then
separated in the third separation column. Lastly, RME enters the acid cleaning
reactor to be washed with dilute acid, before entering the fourth separation column
to separate acid solution from the relatively pure biodiesel.
c. Vacuum drying and desolventizing. Since RME has a high viscosity, the remaining methanol and water needs to go through a two-stage diaphragm vacuum
drying process. This is to ensure that the flash point (higher than 110 °C) and
moisture content (300 × 10
−6 ) meet the required standard.
(2) Recycling of glycerol and methanol. This is a four-stage operation including the neutralization of crude glycerol, re-alcoholysis of oil phase, recycling and
evaporation of methanol, and rectification of methanol.
a. Neutralization process of crude glycerol. The by-products from transesterification (glycerol, acid solution, KOH the catalyst, methanol) are placed in the
mixture tank for neutralization. If more acid is needed for neutralization, add
96% sulfuric acid to ensure that all KOH is transformed into K 2 SO 4 . Following
that, separate potassium sulfate by pressure leaf filters (wash the filter cake, and
dry and recycle methanol, as well as potassium sulfate powder to be used as
C. Li et al.
5.2.6 Continuous Transesterification Process for Biodiesel
Production
The continuous transesterification process is a typical operation that includes vegetable oils as raw material for biodiesel production. It has a high yield and produces biodiesel in a continuous fashion. The continuous transesterification process
involves two stages. The first is the operation to produce biodiesel with transesterification (workshop), and the second is re-esterification, methanol recycling, and
glycerol refining operation (workshop). The basic flow diagram of the continuous
transesterification process for biodiesel production is presented as Fig. 5.5.
The key points of operation are highlighted as follows.
a. Transesterification reaction and separation. First, preheated vegetable oil (about
35 °C), methanol (molar ratio of methanol and oil is 6:1 to 4:1), and catalysts
(KOH or NaOH, 1–2%) are continuously added in a given proportion to the
first esterification tank for reaction with stirring. The reaction time is determined
based on the solution level (10–20 min). This process adopts a two-stage reaction
operation. Following the reactions in the first reaction tank, the reactants enter
a specially designed separation column, so that the rapeseed oil methyl ester
(RME) produced can be continuously separated from glycerol and water. RME
then continues to enter the second reaction tank for further and sufficient reaction.
After that, the reactions take place in the second separation column to separate
the glycerin phase (which flows into the storage tank for recycling).
b. Continuous water rinse, acid cleaning, and separation. The RME separated from
the glycerin phase enters the water rinse reactor to be rinsed with hot water. The
residues of catalyst, potassium soap, and glycerol in RME are rinsed and then
separated in the third separation column. Lastly, RME enters the acid cleaning
reactor to be washed with dilute acid, before entering the fourth separation column
to separate acid solution from the relatively pure biodiesel.
c. Vacuum drying and desolventizing. Since RME has a high viscosity, the remaining methanol and water needs to go through a two-stage diaphragm vacuum
drying process. This is to ensure that the flash point (higher than 110 °C) and
moisture content (300 × 10
−6 ) meet the required standard.
(2) Recycling of glycerol and methanol. This is a four-stage operation including the neutralization of crude glycerol, re-alcoholysis of oil phase, recycling and
evaporation of methanol, and rectification of methanol.
a. Neutralization process of crude glycerol. The by-products from transesterification (glycerol, acid solution, KOH the catalyst, methanol) are placed in the
mixture tank for neutralization. If more acid is needed for neutralization, add
96% sulfuric acid to ensure that all KOH is transformed into K 2 SO 4 . Following
that, separate potassium sulfate by pressure leaf filters (wash the filter cake, and
dry and recycle methanol, as well as potassium sulfate powder to be used as