Acrolein . Acrolein (2-propanal; CH 2 = CHCHO), is the simplest unsaturated aldehyde. The primary characteristic of acrolein is its high reactivity due to the conjugation of the carbonyl group with a vinyl group.
Summary
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- 1 Features
- 2 Storage
- 3 Obtaining
- 4 Catalytic oxidation
- 5 Stages of absorption
- 6 Main side reactions
- 7 Uses
- 8 Sources
characteristics
- Freezing point: – 87 ºC
- Boiling point: 52.7 ºC
- Density: 0.8427
Acrolein is a highly toxic compound with very strong tear properties. At room temperature, acrolein is a liquid with a volatility and flammability somewhat similar to that of acetone , but, unlike acetone , its solubility in water is limited.
Storage
Commercially, acrolein is always stored with hydroquinone [C 6 H 4 (OH) 2 ] and acetic acid (CH 3 CO 2 H), which act as inhibitors.
Obtaining
The first commercial process carried out to obtain acrolein was based on the vapor phase condensation of acetaldehyde (CH 3 CH = O) and formaldehyde (HCH = O). The development of catalysts brought with it the performance of vapor phase processes for the production of acrolein, in which propylene (CH 3 CH = CH 2 ) is the starting material.
Catalytic oxidation
The catalytic vapor phase oxidation of propylene is generally carried out in a multitubular fixed-bed reactor at pressures close to atmospheric and high temperatures (350 ºC); molten salts are used to control the temperature. Air is used as the common oxygen source , and water vapor is added to suppress the formation of flammable gas mixtures. The operation can be single-stage or a recycle stream can also be used.
Absorption stages
The reactor outlet gases are cooled to condense and separate acrolein from unreacted propylene, oxygen, and other low-boiling compounds (predominantly nitrogen ). This is commonly carried out in two absorption stages, where:
- The acrylic acid aqueous (CH 2= CHCO 2 H) is condensed and the reaction effluent is then absorbed in an aqueous stream
- Acrolein (CH 2= CHCHO) is condensed and absorbed in water to separate it from propylene (CH 3 CH = CH 2 ), nitrogen (N 2 ), oxygen (O 2 ) and carbon oxides (CO and CO 2 ).
If desired, acrylic acid can be recovered from the aqueous product stream. Applying consecutive refining stages by distillation separates water (H 2 O) and acetaldehyde (CH 3 CHO) of crude acrolein. In another distillation column, refined acrolein is recovered as an azeotrope together with water.
Main side reactions
The main side reactions produce:
- Acrylic acid (CH 2= CHCOOH)
- Acetaldehyde (CH 3CHO)
- Acetic acid (CH 3CO 2 H)
- Carbon monoxide (CO)
- Carbon dioxide (CO 2), and small amounts of a wide variety of other aldehydes and acids will form .
