Cryogenic condensation – theoretical basis
Cryogenic waste gas purification and solvent recovery is based on the principle that volatile organic compounds (VOCs) condense at low temperatures. By cooling the waste gas to temperatures below the dew point the solvent vapour condenses on a cooling surface and can be removed in liquid form. The lower the temperature the smaller the remaining load in the gas flow. In theory, this process can be described using the vapour pressure diagram of the respective compound with help of the Antoine equation.
Vapour pressure and load
The following graph shows the concentration of dichloromethane and methyl chloride (and water for comparison) in air as a function of temperature.
Waste gas sources (production reactors, storage vessels etc.) produce an atmosphere with typical VOC concentration in the range of 10 to more than 1,000 g/m3, and temperatures between ambient and very warm (e.g. 80°C). In order to reduce the VOC load so that it complies to TA Luft limits, cooling to usually far below -100°C is necessary.
When removing a mixture of organic compounds from waste gas, the minimum operating temperature is set by the most volatile constituent. Liquid nitrogen with its boiling point of -196°C is highly suitable as a cooling source for such tasks. Dichloromethane is a solvent with a melting (freezing) point of -96°C, however, in order to achieve the limits of the TA Luft a cleaning temperature of approx. -120°C is necessary. This means that DCM is not only condensed out in liquid form, it also has to be frozen out. As a result of this layers of (solvent-) ice or snow cover the cooling surface and lead to clogging of the condenser and interruptions of the operating process when the equipment has to be defrosted in order to melt the ice.
Some material data
The following table shows some relevant material data and the minimum cleaning temperatures necessary for water and many VOCs to achieve compliance to TA Luft. The actual clean gas temperatures are normally 10 to 20°C below these theoretical values, so that for common VOCs like alkanes, aromatics, ethers, and alcohols, temperatures of about −100°C and lower are necessary.