Industrial Gases
LIQUID NITROGEN
REACTION COOLING
Choosing the right refrigeration system is critical
to a safe and economical process.
By Jon Trembley and
Svetlana Ivanova, Air Products
A
range of chemical reactions and
processes can benefit from the use
of low temperature technologies,
particularly in the chemical and pharmaceutical industries. For example, in organic and
organometallic synthesis, low temperature
operation is important to achieve the balance
between reactivity, selectivity and yield of
the reaction. In the case of highly exothermic
reactions, proper cooling is critical to control
heat release and avoid reaction runway.
Two common cooling methods can be
used to keep temperatures sufficiently low
and control an exothermic reaction: lowtemperature mechanical refrigeration and
cryogenic refrigeration via sublimation of
carbon dioxide or evaporation of liquid
nitrogen (LIN). Thorough selection of the
refrigeration method is critical to providing
a safe and economical operation process.
For reaction cooling, mechanical refrigeration must work at the limit of its lower tem-
perature range, which is not only challenging
but also expensive. In addition, if the equipment's maximum potential is only occasionally
utilized, then a lot of costly capital equipment
may be left sitting around doing nothing.
Mechanical refrigeration is provided by a
chiller that includes a compressor — sometimes single-, but often two-stage — as the
heart of the system. When the temperature
requirements drop lower than -58°F (-50°C),
the chiller system becomes more complex
with a multi-stage cascade design and multiple
refrigerants.1 It also is known that mechanical
refrigeration provides a limited ability to cope
with widely or rapidly varying heat loads.2
Cryogenic refrigeration provides much more
flexibility in terms of temperature ranges and
cooling regime variability when compared
to mechanical refrigeration. Cryogenic refrigeration by means of sublimation of carbon
dioxide can be a viable option for controlling
some cold chemistry reactions and can reach
temperatures as low as -108°F (-78°C) at atmospheric pressure. Alternately, LIN can achieve
much lower temperatures of -320°F (-195°C)
Figure 1. Theoretical Refrigeration Capacities Available from Liquid Carbon Dioxide
Figure 2. Theoretical Refrigeration Capacities Available from
Liquid Nitrogen
and a higher refrigeration capacity per mass unit than CO2 (figures 1 and
2). Liquid nitrogen may provide a design engineer more confidence in
the cooling system's control as well as allow a chemist the full flexibility
needed when choosing which synthesis route to follow.
There are three primary options for cooling reaction vessels with LIN:
• Direct.
• Semi-indirect.
• Indirect.
Each method has advantages and disadvantages (figure 3).
Direct injection of LIN achieves maximum efficiency and is inexpensive
to install, but solvent entrainment, foaming and localized freezing can
occur. This option often is used in emergency situations because vaporizing LIN can quickly provide rapid cooling to the reaction process should
an unsafe or runaway situation occur.
In a semi-indirect method, heat transfer takes place by flowing liquid
nitrogen either through a coil inside a reactor or through a reactor cooling
jacket. The main benefits of this method include accurate temperature
control, the ability to reuse the nitrogen, and simplicity. Drawbacks include
reduced efficiency, demand on reactor volume, and the expense of the
cryogenic construction and corrosion-resistant materials.
Indirect heat transfer occurs in systems where LIN is exchanged with a
suitable heat transfer fluid in an external heat exchanger. This approach
has the most system flexibility, provides accurate temperature control
and accommodates large heat loads. Many reaction processes already
utilize a heat transfer fluid/medium that is either cooled or heated
accordingly to provide flexibility in multi-step reaction processes.
Regardless of the method used, one of the important considerations
when using LIN for cooling purposes is a properly designed supply system that minimizes nitrogen loss at all the stages and ensures good qual-
Figure 3. Liquid Nitrogen Cooling Methods
ity liquid nitrogen for a specific application.
While reaction cooling requirements may vary significantly — from
maintaining a steady low temperature to fast heat removal during reaction
content additions — LIN refrigeration is capable of providing a flexible,
reliable, environmentally-friendly and economic cooling option. In many
applications, cryogenics can be seen as specialized; however, cryogenics
provide distinct advantages in many instances.
The most common use of nitrogen in a chemical plant is as an inert
blanketing gas.3 If the evaporated LIN from the cooling process can be
recovered and used elsewhere in the plant, the running costs of the
system can be reduced dramatically.
Liquid nitrogen often is portrayed as an expensive option, but when
improved reaction yields and selectivity, reduced unwanted byproducts
and the relatively low capital costs involved are taken into consideration,
LIN becomes an economically attractive choice. PC
John Trembley is technology manager, cryogenic applications at
Air Products in Basingstoke, U.K., and Svetlana Ivanova, Ph.D.
is marketing and applications manager, merchant gases at Air
Products in Allentown, Pa. To learn more about liquid nitrogen
reaction cooling, call (800) 654-4567 or (610) 706-4730 or visit
www.airproducts.com.
References
1. ASHRAE Refrigeration Handbook, “Ultra-low Temperature
Refrigeration,” Chapter 39 (2002).
2. “Capacitance: Key Design Consideration for Refrigerated Central
Cooling Systems for Batch Process Plants,” Pharmaceutical Engineering,
Vol. 21, No. 4 (2002).
3. “Nitrogen: A Security Blanket for the Chemical Industry,” Chemical
Engineering Progress, 107 (11), pp. 50-55 (Nov. 2011).
For More Information
Corporate Headquarters
Air Products and Chemicals, Inc.
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Tel 800-654-4567 or 610-706-4730
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China
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Reprinted with permission from Process Cooling, October 2012 © 2013, BNP Media.
312-13-005-US