Nitrogen (N2), which makes up most of the air we breathe, is interesting and effective as a low-temperature condensed liquid for driving cooling processes. Not only does it have a more frigid boiling point than several other industrial gases, but in many cases, it also removes more heat per pound of liquid vaporized. This characteristic makes liquid nitrogen (LN2) a potent, cost-effective and environmentally friendly cooling agent that is capable of resolving some of the toughest process cooling challenges.
Challenge 1: Reactor CoolingQ: How do I reliably cool my reactor or process separations unit to temperatures below -75°F (-59°C)? I do not want to use direct injection of liquid nitrogen into a reaction vessel because volatiles can be stripped out, and nitrogen coils often freeze up with my ingredients.
A: The best way to prevent stripping of volatiles from the batch is to avoid contact between the liquid nitrogen and the reactor or separator contents. It is also a good idea to use an intermediate heat transfer fluid to buffer the aggressively cold nitrogen from the reactor systems.
Cooling/heating units (figure 1) contain heat exchangers to cool heat transfer fluids such as methanol, silicon oil or isohexane with liquid nitrogen as the fluid is circulated to and from the reactor. The system typically contains a heat-transfer-fluid loop, circulation pump, liquid nitrogen heat exchanger, thermal expansion tank and temperature controls. The amount of cooling power can range from 5 to 400 kW (1.5 to 116 tons of refrigeration), and such systems can control a reactor or heat exchange equipment accurately to -150°F (-101°C) or much lower. The flashed nitrogen gas can even be reused elsewhere in the plant.
Some systems use an all-solid-state aluminum-block heat exchange unit to cool the process without the use of any heat transfer fluid. This alternative can be even simpler and less costly than a cooling/heating unit.
Both of these cooling options have benefits over direct or indirect injection of liquid nitrogen into reaction vessels. They also are compact, have few moving parts and require minimal maintenance.
Challenge 2: Chemical AbatementQ: I have a waste stream containing several volatile chemicals that I need to recover for value or environmental reasons. How can I achieve high recovery efficiencies without thermal incineration or spray towers?
A: A good solution for recovery of both the volatile chemicals and the spent gaseous nitrogen used to cool the process is a volatile organic compound (VOC) abatement system. A typical VOC recovery unit employs liquid-nitrogen-powered cryocondensation of volatile compounds from the background gas phase to separate and recycle the chemicals as storable liquids.
The design involves two large condensers that cycle back and forth continuously to provide clean, cold heat transfer surfaces to condense the VOCs from the noncondensible gas stream. The long cycle times between condenser switching provide stable performance at a low system pressure drop. A high level of chemical removal performance is attainable due to the combined mechanism of VOC condensation and freezing. VOC recoveries of 90 to 99 percent or greater are typical, depending on the species and process conditions.
It is important to note that VOC recovery units are less regulated than incinerators and can offer better abatement performance and operating reliability than absorption-based systems.
Challenge 3: Environmental ChambersQ: I’m having trouble with the cost and cooling performance of my environmental chambers. Can nitrogen be used in place of mechanical cooling?
A: Converting a mechanically cooled chamber to one chilled directly or through a cooling coil by liquid nitrogen provides several advantages. High chamber-ramp rates (125°F [70°C] per minute) and low temperatures (-280°F [-173°C]) can be achieved, which allows for conformance to highly accelerated life testing (HALT) and highly accelerated stress screening (HASS) standards. Additionally, liquid nitrogen systems do not use chlorofluorocarbon (CFC)-based multistage mechanical refrigeration units. And, as with the reactor cooling and VOC recovery systems, the spent nitrogen can be reused for facility purging needs.
In conclusion, liquid nitrogen is the cooling force behind many other industrial processes. With its cost-effectiveness, high performance and low environmental impact, liquid nitrogen is becoming widely recognized as “the cool element.”