Chemical Engineering August 2015 - 44

A consumable refrigerant is produced at a
central plant and delivered by truck to the
process site for use. Low temperatures are
generated by evaporating the consumable
refrigerant and releasing its vapors to the atmosphere.
At the point of use, consumable
refrigerant systems are typically simpler than
mechanical refrigeration systems, and easier
to install and operate. This simplicity means
that consumable refrigerant systems can
easily be designed to be very reliable.
Liquid nitrogen and liquid carbon dioxide
are the most common consumable refrigerants
for extremely low-temperature
processes. Liquid helium can be used as a
consumable refrigerant for certain high-value
applications, but care must be taken due to
the high unit cost of helium.
Liquid carbon dioxide, with a triple point
of -109.3°F (-78.5°C), can be used at the
upper end of the extreme temperature range.
Solid carbon dioxide, also known as dry ice,
can be used to make laboratory and pilotproduction-scale
cold baths by mixing it with
Chemical name
Dichlorofluoro methane
1,1-Difluoro-1-chloroethane
Chlorodifluoro
methane
Water (5)
1,1,1,2-Tetrafluoroethane
Ammonia
Propane
Hexafluoroethane
Carbon
dioxide
(as dry ice)
Carbon dioxide
(liquid at 95 psia)
Trifluoromethane
Ethane
Ethylene
Tetrafluoromethane
Methane
Argon
Nitrogen
Neon
Helium
refrigerant
number
21
142b
22
718
134A
717
290
116
744
744
23
170
1150
14
50
740
728
720
704
Category
(1)
MP HCFC
MP HCFC
MP HCFC
H2O/CO2/
NH3
HFC
H2O/CO2/
NH3
HC
HFC
H2O/CO2/
NH3
H2O/CO2/
NH3
HFC
HC
HC
HFC
HC
Element
Element
Element
Element
Molecular
weight
102.92
100.50
86.47
18.02
102.03
17.03
44.10
138.01
44.01
44.01
70.01
30.07
28.05
88.00
16.04
39.95
28.01
20.18
4.00
Normal
boiling
point, °F
48.0
15.4
-41.2
212.0
-14.9
-28.1
-43.7
-108.8
-109.3
-59.8
-115.9
-127.4
-154.6
-198.5
-258.6
-302.2
-320.5
-410.9
-452.1
Critical temperature,
°F
353.2
278.9
204.8
704.9
214.0
270.1
206.6
67.4
87.9
87.9
78.7
90.4
49.8
-50.2
-116.2
-188.4
-232.5
-379.8
-450.3
a low-freezing-point solvent such as acetone
or limonene. Such bath temperatures are
limited to about -109°F (-78°C).
Liquid nitrogen refrigeration
Liquid nitrogen, with a normal boiling point
of -320.5°F (-195.8°C), is the most widely
used consumable refrigerant for generating
temperatures below -110°F (-79°C). Figure
4 illustrates a typical liquid nitrogen refrigeration
system. There are typically five major
pieces of equipment:
Liquid nitrogen storage tank. This holds
the inventory of liquid nitrogen for use in the
process. A cryogenic liquid storage tank is
a sophisticated tank-within-a-tank. The inner
tank holding the liquid nitrogen is suspended
from a carbon steel outer tank by a carefully
engineered support system to minimize the
heat transfer by conduction. The space between
the inner and the outer tanks is held
at high vacuum to minimize heat transfer by
convection. The space between the tanks
can also be filled with powder insulation, or
Table 1. Some Common RefRigeRanTS
Triple
point, °F
-210.8
-203.4
-251.4
32.0
-149.8
-107.9
-305.8
-149.3
-70.0
-70.0
-247.3
-297.9
-272.5
-298.5
-296.5
-308.8
-345.9
-415.5
-455.8
Heat of
vaporization,
BTu/lb (2)
102.8
95.9
100.5
973.8
93.5
592.4
183.2
49.3
237.0
145.0
103.8
210.1
207.5
57.5
219.4
69.2
85.5
36.8
8.9
Flammability
range, % (3)
NF
6.3-17.9
NF
NF
NF
15-30
2.2-9.5
NF
NF
NF
NF
3-16
2.7-34
NF
5-15
NF
NF
NF
NF
refrigerant type
(4)
Working fluid
Working fluid
Working fluid
Example of consumable
Working
fluid
Working fluid
Working fluid
Working fluid
Consumable only
Both
Working fluid
Working fluid
Working fluid
Working fluid
Working fluid
Both
Both
Both
Both
(1) MP HCFC = an HCFC refrigerant that is subject to the Montreal Protocols; HFC = hydrofluorocarbon; HC = hydrocarbon; H2O/CO2/NH3 = compounds
(2) Carbon dioxide will sublime (solid-to-vapor transition) rather than vaporize (liquid-to-vapor transition) at atmospheric pressure
(3) Lower and upper flammable limits, respectively, in air; NF = non-flammable
(4) A working fluid is a refrigerant used in a mechanical refrigeration cycle
(5) Water is included as an example of a consumable refrigerant when used in a cooling tower
42
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