Cryogenic condensation is a versatile process that is not VOC specific. Typically, condensation takes place with liquid nitrogen as the refrigerant in a straightforward heat exchange process. Non-toxic, non-corrosive, and non-flammable, liquid nitrogen is a versatile, zero ODS coolant with a normal boiling point of -196 degrees C.

As the organic-laden vapor stream is cooled, VOCs will condense when the dew point is reached. Fluctuations in VOC stream velocity or content are easily handled by quick response controls on liquid nitrogen injection. Typically, the only constraint on the VOC itself is that its freezing point should be below about -30 degrees C; otherwise, freezing is likely to occur.

Cryogenic condensation systems generally consist of one or a series of plate-fin or shell-and-tube heat exchangers. The VOC stream and the liquid nitrogen stream flow through the heat exchanger countercurrently, maximizing heat transfer. The VOC condenses on the shell side of the exchanger then drains into a collection tank, from which it can be recycled, reclaimed, recovered for reuse, or, at worst, accumulated for disposal.

During condensation, the presence of water vapor or VOCs with a high melting point can cause freezing on the external surface of the tubes inside a cryogenic condenser. A solid buildup will blanket the heat transfer area and thus reduce the efficiency of the process, unless suitable precautions are taken. One method of preventing buildup due to freezing is to periodically flush the pipes with the condensed phase. Another method is to use two condensers in parallel (continuous operation) so that one condenser is in operation while the other is out of service being cleaned or defrosted. Another option when excessive moisture is a concern is to arrange two condensers in series. In the first or pre-cooling stage, the VOC stream is cooled to about 1 degree C, condensing the majority of water so that it will not be present to freeze at condensing temperatures below 0 degrees C (system shutdown for defrosting might be required). In the second, or main, condensing stage, the temperature of the VOC stream can be lowered as needed to drop out the required/desired amount of VOCs without concern of ice formation.

For some VOCs, substantial cooling -40 to -50 degrees C below the compound’s dew point may cause fog to form. Fog occurs when the rate of heat transfer exceeds the rate of mass transfer to the liquid stage, causing the bulk of the gas to quickly cool below its dew point. This causes the nucleation of tiny droplets of the VOC which, instead of coalescing and condensing on the surface of the tube, become a colloidal suspension in the bulk gas stream. The formation of fog, like the formation of solids, can be minimized by splitting the cooling process into steps, allowing better control of temperature changes. Other options to minimize fog formation include the use of a mist elimination device or reduction of the gas stream velocity by using a larger diameter inlet pipe.

Air sweeps can sometimes be replaced with nitrogen sweeps to enhance the recovery of VOCs from the vent gas. Because the amount of VOC recovered is proportional to the fraction of VOC in the vapor (expressed as the partial pressure of the component), recovery rates are increased at higher vapor concentrations. For example, if a 5 percent concentration of VOC in a gas stream is technically feasible, but the vent stream concentration is controlled at 0.5 percent in air to eliminate the risk of explosion, that is, the concentration is controlled below the lower explosion limit (LEL), then the addition of nitrogen can render the atmosphere inert and at the same time allow a VOC concentration of 5 percent, without which the mixture would be explosive. Thus, VOC recovery is increased, while maintaining safe operating conditions.

Cryogenic condensation has been selected as the Best Achievable Control Technology (BACT) by environmental regulators for VOC control in a few processes.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDSs prior to implementing this technology.

• $50K for a 25 scfm unit;
• $150K to $200K for a 100 scfm unit;
• $500K for a 500 scfm unit; and
• Units larger than 500 scfm can cost up to several million dollars.

Operating costs are relatively low, provided a ready source of liquid nitrogen is used at the site for some other purpose such as tank blanketing. Nitrogen demand depends on type of solvents, concentration desired, control efficiency, inlet temperature and pressure. For facilities with no continuous source of liquid nitrogen, several options are available:

1. For an intermittent, single-use installation, e.g., VOC recovery from semi-annual ship off-loading, the required amount of liquid nitrogen could be brought in for the duration of off-loading.
2. For continuous use, combination systems with mechanical refrigeration (for the bulk condensation) and liquid nitrogen (only for polishing) can be assembled. This type of combination system is much more efficient if liquid nitrogen has to be purchased solely for this process.

A more meaningful economic analysis can only be done knowing the value of the VOC and the concentration, flowrate, and availability of liquid nitrogen at the facility. In certain cases, greater efficiency can be gained by designing the system for countercurrent contact of the streams using the cooling capacity of the “warm” liquid nitrogen from the main condenser to cool the incoming warm or ambient temperature VOC vapors in the pre-condenser.

Assumptions: Cryogenic condensation technology using liquid nitrogen is not a fractionation process. It will condense essentially all the components of a vapor stream with the exception of elemental gases like hydrogen, helium, and neon, including gases as light as methane. Recovery and reuse of pure VOC streams from a cryogenic condensation system is the ideal application of this technology. However, VOCs are often released or captured as mixtures in large volume gas streams. Although the cryogenic condensation process will recover these materials, condensed mixtures are typically not easily separated even by subsequent reclamation. Another possibility for mixtures includes sale for another application that tolerates a mixture. Cryogenic condensation systems can also accommodate different VOC streams if production processes allow batches, e.g., single contaminant streams. To maximize purity and minimize cross-contamination, batches may have to be separated by purging and flushing of the system.