Safe working with solvents in microwaves

If highly flammable solvents are heated in microwave systems, special safety precautions must be observed. The solutions are heated either individually or in several vessels in a microwave oven to temperatures well above their boiling points. The containers are tightly sealed, causing a pressure build up corresponding to the vapour pressure of the solution. The containers must therefore contain an overpressure safety device to prevent the vessels from bursting. Today, this is usually done with special overpressure valves, which open the container briefly at a defined maximum working pressure and close it again. Solvent vapours escape and the pressure drops back below the maximum working pressure of the vessel.

By this or through poorly sealed vessels (leaks) solvent vapours escape into the oven chamber. If solvent vapours now accumulate in the furnace chamber, in rare cases sparks can cause explosions in the furnace chamber. This is effectively prevented by preventing accumulation above the lower explosion limit of the solvent. This is ensured by

1. continuous ventilation of the oven chamber. The air in the oven chamber is exchanged between 62 and 100 times per minute, depending on the load of containers. This prevents enrichment even in the lower percentage range.
2. A gas flow monitor monitors the ventilation of the oven compartment. If this fails, the microwave is switched off in an emergency
3. A solvent sensor monitors the concentration of organic solvents in the exhaust air. Above a threshold value, the microwave is switched off in an emergency. The threshold value is less than one tenth of the lower explosion limit for all common solvents.

These safety precautions also exceed the requirements of the current EPA, ASTM, ... standard procedures for microwave extractions.