A Pressure Reactor, sometimes referred to as a pressure tube, or a sealed tube, is a chemical reaction vessel which can conduct a reaction under pressure. A pressure reactor is a special application of a pressure vessel. The pressure can be caused by the reaction itself or created by an external source, like hydrogen in catalytic transfer hydrogenation.
A pressure reactor can offer several advantages over the conventional round-bottom flask. Firstly, it can conduct a reaction above the boiling point of a solvent. Secondly, the pressure can reduce the reaction volume, including the liquid phase, and in turn increase concentration and collision frequency, and accelerate a reaction.
Increase in temperature can speed up the desired reaction, but also speed up the decomposition of reagents and starting materials. However, pressure can speed up the desired reaction and only impacts decomposition when it involves the release of a gas or a reaction with a gas in the vessel. When the desired reaction is accelerated, competing reactions are minimized. Pressure generally enables faster reactions with cleaner reaction profiles.
The above benefits from a pressure reactor has been shown in microwave chemistry. E.g., if a Suzuki Coupling takes 8 hours at 80°C, it only takes 8 minutes at 140°C in a microwave synthesizer. The microwave effect is a controversial topic. Later experiments show some of these early reports to be artifacts and rate enhancement is strictly due to thermal effects.
If a pressure reactor is engineered properly, it can meet 4 out of 12 green chemistry principles
- 1, less solvent and cleaner reaction profile result in less waste
- 5, less solvent is needed
- 6, short reaction time can save up to 92 percent electricity and 200 gallons of cooling water per refluxed reaction
- 12, closed vessel can prevent releasing toxic gas and explosions.
Types of Pressure Reactors
Standard glass pressure reactor
One of the drawbacks of a standard glass pressure reactor is the potential explosions due to hard-to-predict excessive internal pressure and lack of relief mechanism.
A Fisher-Porter tube or Fisher-Porter vessel is a glass pressure reactor used in the chemical laboratory. Manufactured by Andrews Glass Co. of Vineland NJ
There is a new pressure reactor, called Q-Tube. It features a simple and safe pressure release and reseal mechanism which can prevent a glass pressure reactor from explosions and retain the solvent.
Metal pressure reactor
The drawbacks of a metal pressure reactor (bomb) are set-up, maintenance, and corrosiveness.
- Biotage Synthesis and Purification Catalog 2008
- Nonthermal Microwave Effects Revisited, On the Importance of InternalTemperature Monitoring and Agitation in Microwave Chemistry M. A. Herrero, J. M. Kremsner, C. O. Kappe J. Org. Chem. 2008, 73, 36-47.
- Microwave Chemistry in Silicon Carbide Reaction Vials: Separating Thermal from Nonthermal Effects. D. Obermayer, B. Gutmann, C. O. Kappe Angew. Chem. Int. Ed. 2009, 48, 8321-8342.
- manufacturer's website