CRYOGENICS-LIQUEFACTION

CLASS NOTES FOR ENGINEERING

 

 Cryogenics  involves the study of

temperatures below -150⁰C. It is

of interest to know how to produce such

temperatures. These temperatures

are produced by liquefaction of gases. In

addition , it is of interest is know which

materials are suitable at these

temperatures.

There are many areas of interest where we

need cryogenic temperatures such as

(i)   Storage of large volumes of gases in

small space in the liquefied form

(ii)  Preservation of insemination

(iii) Very high vacuum

(iv) Fundamental research in

understanding more deeply about entropy

and sub-atomic structure of matter as the

motion of protons, electrons reduces

significantly at cryogenic temperatures.

 Liquefaction of gases involves one or two

types of expansions. In this, gases are

compressed, cooled and throttled. Gases

must at temperatures lower than its

inversion temperature before throttling.

In this, hydrogen and helium inversion

temperature are -77.8⁰C and -250⁰C

respectively. These gases are pre-cooled

with liquid nitrogen.

 

(i)          Throttling expansion

It is a must for liquefaction of each gas. This overcomes the practical difficulties associated with the reversible expansion. Throttling expansion of a gas eliminates the lubrication difficulties. It simplifies the equipment necessary.  But the efficiency becomes low. There is a complete loss from high pressure to low pressure. It is Joule Thomson Expansion.

Joule Thomson coefficient= μ =(∂T/∂p)_h=C

Joule Thomson Coefficient is a function of both temperature and pressure. Therefore this coefficient does not have an unique value. It can be positive, zero and negative.

 Joule Thomson Effect

It is related to expansion at constant enthalpy i.e. throttling process. In an expansion there is always a fall of pressure.

Three possibilities are possible in Joule Thomson Effect.

(a)  Heating (rise of temp) occurs after expansion if temperature before expansion is higher than INVERSION TEMPERATURE.

(b) No heating or no cooling if the temperature before expansion is INVERSION TEMPERATURE itself.

(c) Cooling occurs after expansion if the temperature before expansion is lower than the Inversion temperature.

Mathematically Joule Thomson Coefficient

µ = Change of temp/fall of pressure

   = (t₂ – t₁)/ (p_{1 –} p₂) = — value

shows heating after expansion

µ= 0 for no heating or no cooling

µ= +  for cooling effect          

(ii) Isotropic expansion (theoretical)/real expansion

It is apart of Claude’s process of air liquefaction. By this process alone, liquefaction is not feasible because of lubrication and erosion problems. This expansion is helpful in decreasing the temperature before throttling and ultimately increases the percentage yields of liquefied gas. Under this process, the temperature after expansion is as given below:

For a theoretical process        (T₂/T₁)= (p₂/p₁)^(γ–1)/γ  ,    γ=1.4

For a real process      (T₂/T₁)= (p₂/p₁)^(n–1)/n                n=1 to 1.4

INVERSION TEMPERATURE: Maximum temperature at which Joule Thomson coefficient μ =(∂T/∂p)_h=C  is zero. It represents neither heating nor cooling on expansion which is possible only for an ideal gas. The Inversion temperature for real gases is not unique.  It is a function of both temperature and pressure.  There is one maximum inversion temperature for each gas.

(i)        If the temperature is above the inversion temperature before expansion, then heating occurs upon expansion.

(ii)     If temperature is below the inversion temperature before expansion, cooling results on expansion.

(iii)   For cooling upon expansion of a gas, it is at a temperature below its inversion temperature.

(iv)   For such a case, fall of temperature with fall of pressure makes the Joule Thomson Coefficient positive.

(v)     Final liquefaction is practically possible only with throttling process.

TABLE: IMPORTANT PROPERTIES OF GASES

Gas	N.B.P. 0C	Freezing Point	Critical temperature 0C	Maximum inversion temperature 0C
Air	-1910C	–	-140.2	3300C
Oxygen	-1830C	-218.8	-118.8	6200C
Nitrogen	-1960C	-2100C	-147.0	347.80C
Hydrogen	-252.80C	-259.2	-239.9	-77.80C
Helium	-268.90C	-269.7	-267.9	-250.00C
Carbon dioxide	-78.30C	–	31.10C	12300C

  The inversion temperatures of hydrogen and helium are -77.8⁰C and -250⁰C.  Liquefaction happens when gas is below its inversion temperature before throttling.

(ii)   Advantages of liquefaction

1. Liquefaction of gases produces cryogenic temperatures required for fundamental research.

2. Gases in the Liquefied form can  store large volume in a small storage.

TABLE: REDUCTION IN VOLUME ON LIQUEFACTION

Gas	Wt.	Volume on liquefaction	Volume of gas	Reduction ratio
O2	1	0.031	26.62	858
N2	1	0.044	30.41	691
Air	1	0.040	29.50	737
H2	1	0.496	422.41	851.6
Helium	1	0.282	213.4	756.7

LIQUEFACTION OF AIR

Liquefaction of air is achieved in two steps.

1. Joule Thomson expansion alone

         as in Linde’s process

              or

        Isentropic expansion as well as

        Joule Thomson expansion as

       used in Claude’s process

3. Liquefaction of Hydrogen and nitrogen

need pre-cooling before liquefaction

 Air liquefaction

 Two processes do the liquefaction. First is by Linde’s process and second is Claude’s process respectively.

Linde’s (or Hampson) air liquefaction method

Line diagram & its representation on Temperature entropy chart

Underline principle – Uses only Joule Thomson expansion for liquefaction

In Linde’s process, compress air to 200 atm. The yield is about 10 percent.  Fractional distillation of liquid air give oxygen and nitrogen.

Analysis Basis – for a unit mass of air liquefaction

t₁       =       t₂       =       t₇

p₁      =       p₇      =       1 atm

p₂      =       200 atm

Mass balance

Firstly m₄     =       m₆+1 =m₇+1                                               (i)

Secondly  m₁     =       m₂     =m₃   =m₄

 _Thirdly m_{6         =}              m₇

  _Fourthly m₅         =       1

Energy balance around heat exchanger.

Entering energy  = energy leaving

m₂h₂+m₆h₆ =       m₃h₃+m₇h₇                                                                  (ii)

For air, h2, h6 & h7 are known from T-S chart. Unknowns are m₂ and h₃.

Use energy balance around separator

Energy entering = energy leaving

m₄ h₄ = m₅ h₅ + m₆ h₆                                                    (iii)

but  h₄= h₃                                                                     (iv)

m₄ & h₃ are unknown and are solved by using equations (i), ii),(iii) & (iv).

Disadvantage of Linde’s air liquefaction method

1. It results in a lower yield of around 10%.

2. The working pressure is very high resulting in high power consumption and requiring robust equipment.

3. Method is costly.

Advantage

This is a simple method.

CLAUDE’S AIR LIQUEFACTION METHOD

Principle – uses both isentropic and throttling expansions (Fig.2)

Line diagram and its representation on temperature entropy chart

In this liquefaction method, compress air to approximately 40 atm. Compressed air passes through heat exchanger I (HE1). It is then divided into two streams. 20 percent passes through the second heat exchanger and throttled to atmospheric pressure.  Remaining 80 %  goes to the turbine, expanded & cooled. This cooled air from turbine cools the air in heat exchanger II. The temperature of air before throttling is lowered. Hence, a higher percentage of liquefied air is obtained.

Analysis of Claude’s air liquefaction method

  For a unit mass of air liquefaction

t₁, t₂, t₁₁, t₃ are known

h₁, h₂, h₁₁, h₃ are known

h₃=h₃’=h₃”

t₃=t₃’=t₃”

Apply energy balance around H.E.I., II & Separator together

m₂ h₂ + m₈ h₈ = m₈ h₃ + (m₂-1) h₁ + 1.h₆

In this process, yield is much more than Linde’s process. But some lubrication difficulties are there in expander.

 

Liquefaction of Nitrogen

Fig. Liquefaction of Nitrogen

A modified Claude cycle is used for the liquefaction of nitrogen. It has the used advantage of both the turbo-ex pander and Joule Thomson valve [Fig.3(a)]. Three are three heat exchangers in Claude cycle.  Two heat ex changers are used in this liquefier. Last two heat exchangers of the Claude cycle are combined into a single heat exchanger. It reduces the cost of the liquefier.

LIQUEFACTION OF HYDROGEN

 Types of Hydrogen Atoms

Ordinary hydrogen is the simplest of atoms.  It consists of a proton in the center and an electron moving in the orbit around the nucleus. Both proton and electron are also spinning on their axes as well. As such there are four different types of hydrogen atoms (A,B,C and D). Firstly atoms A and C combine to form ortho-hydrogen molecule. In this, protons spinning in same directions but electrons spinning in the opposite directions. Secondly the atom A and D combine to form para molecule of hydrogen. In atoms A and D, the protons as well as electron are spinning in the opposite directions. Thus at room temperature, Hydrogen gas consists of two molecular varieties, Ortho-hydrogen 75% and Para-hydrogen 25%.

PECULIARITY OF HYDROGEN

Thus at room temperature, Hydrogen gas consists of two molecular varieties, Ortho-hydrogen 75% and Para-hydrogen 25%. This composition does not change even in freshly liquefied hydrogen. However, ortho- form changes to para form till equilibrium at 99% para form is achieved.  It is an extremely a slow process. This conversion process is highly exothermic and releases heat (1420 kJ/kg) of hydrogen. This tremendous heat evolution causes liquid hydrogen to boil. This is boil off loss. This loss reduces by employing catalysts like Cr ₂ O ₃ or Al ₂ O ₃ . This catalyst converts ortho to para even before complete liquefaction of hydrogen is achieved. The liquid  Jet aircraft use liquid hydrogen as a fuel. It has a number of environmental and technological advantages over conventional fuels. The liquefaction of hydrogen requires a large expenditure of energy.

 

Principle of Hydrogen liquefaction

• Pre-cooling by liquid nitrogen to a temperature below its inversion temperature of (-78⁰C)

• Throttling

The process is similar to the liquefaction of nitrogen.

Yield in this process is around 25%.

Helium Liquefaction by Collins’s Cryostat

Principle

(i)  Polytrophic expansion for pre-cooling

(ii) Throttling for liquefaction

Fig. HELIUM CRYOSTAT

Liquid nitrogen does the pre-cooling of Helium. Do precooling before the helium enters the first cascade heat exchanger.  Helium liquefies at a lower temperature than that of hydrogen. It uses more number of heat exchangers. It achieves significant amount of liquid helium with reasonable expenditure of energy. This liquefaction process requires about 45 KW to yield 35 to 40 liters of helium per hour.

Properties and Uses of Liquid Helium

In reality, there are two isotopes of Helium namely H4 and H3. Their boiling point, critical temperature are different.  The boiling point of H3 is -268⁰C.

Uses of Liquid Helium

(i) Producing  superconducting magnets

(ii) Magnetic Resonance Imaging

(iii) Nuclear Magnetic Resonance