The method
The cryogenic recovery of volatile compounds was developed to efficiently and simultaneously purge liquid gas storage tanks with nitrogen, and recover the remaining gas heel on specification, without emission to the atmosphere.
The process is based upon a supply of liquid nitrogen, where the liquid nitrogen is heat-exchanged with the outgoing gas vapors in such a way that the gas is efficiently recovered at cryogenic temperatures, on specification, ready for further processing. Therefore, the vaporized nitrogen is used as a purging gas.
The process development
The process was developed in close co-operation with Sintef Research Institute in Norway, and with the funding of I.M.Skaugen Norway, Statoil Norway and by governmental programs. The development led to various patents within the cryogenic field of technology.
The gas recovery unit
The gas recovery unit is a compact unit built into a standard 10-foot container frame. It consists of a series of configured copper brazed heat exchangers containing heat transfer fluid, and pneumatic and electronic control systems.
The GRS unit is remotely controlled from a control station during operations, where the GRS unit is positioned in hazardous zone 1 area, hooked up by hoses to the tank being gas freed, and to the tank into which the recovered product is being pumped. The same technology can be used during the recovery of petrochemical gases such as:
• Ethylene • Ethane
• Propylene • Propane
• Butadiene • Butane
• Halons • VCM
• Chlorofluorocarbons • Isporenemonomer
• Freons • Isobutane
VOC emissions during operation.
The heat exchange circuit of GRS enables the recovery of each product close to its freezing point. Typically, in a VCM operation the recovery temperature is at -153.8°C, at which temperature the product has no vapor pressure and thus the emission is reduced to a couple of ppm only.
A typical VCM recovery operation was conducted at Vopak Terminal in Immingham, in which a 6,647m3 sphere was decontaminated. The results were as follows:
1. VCM liquid recovered from system: 13.020 kg
2. VCM recovered during purging operation: 89.130 kg
3. VCM emissions to atmosphere total: 5 kg
4. Total liquid nitrogen consumption: 120.172 kg
5. Recovery efficiency: 99.98 %
As we can see, using this method of cryogenic recovery for volatile compounds is almost 100 percent efficient, with little emission to the atmosphere. In short, the system can be said to be one of the most effective available today.
