Cryogenic Process Principle.ppt

Cryogenic Process

1. By : Mehdi Sibtain
Cryogenic processes

2. Introduction
Program aimed at customized training for better
understanding of cryogenic process.

3. Agenda Day One
Introduction to cryogenic process
Cooling in a cryogenic process.
Joule-Thomson effect.
Inversion Temperature.
Expansion in work producing process.
Thermodynamic analysis of cryogenic
process.

4. Agenda Day
Two(morning)
Thermodynamic cooling paths.
Comparison of J-T & expander process.
Safety in cryogenic process.
Cold box configuration.
Effect on metals.

5. Agenda Day Two
(Evening)
Heat exchangers.
Gas expanders.
Separation system.
PLANT VISIT

6. Agenda (Day-Three)
He & N
2separation in NG System.
Insulation.
Cryogenic control valves.
Instrumentation.
Precautions during pre-commissioning.
Operational precautions.

7. Agenda (Day-Four)
•Inert gas & Hydrogen recovery plants.
FFC experience with cold boxes.
Plant Visit.
Conclusion.

8. Introduction to
cryogenics
Cryogenic means the generation of cold.
Considered to attain at <-101C
temperature.
Based on the principle of liquefaction
separation of gases by decreasing the
temperature.

9. Benefits
Low capital investment.
Compact units requiring minimum space
for installation.
Best possible recovery of valuable
components
Production of higher heating value fuels.
Economical production of rare gases .

10. Applications
Separation of air for production of Oxygen
& Nitrogen.
Production of liquid Hydrogen for Nuclear
weapons & space programs.
Liquid Helium for space programs.
Liquefied natural gas (LNG) plants.

11. Applications Contd..
Liquid N
2for food preservation.
In Biological application utilization of liquid
nitrogen cooled containers.
In surgery for Parkinson's disease.
Recovery of valuable feedstock from
natural gas streams, upgrading the heat
content of fuel gas.
Purification of various gas streams

12. Properties of Cryogenic Fluids
Boiling points less than 0
o
C.
Critical temp. normally below ambient.
Gases with high Joule-Thomson
coefficient.
Higher non-ideality (polarity) of the gas
better it is for the cryogenic cooling.
Inversion temperature above ambient
temperature.

13. Properties Contd..
Shouldn’t solidify at the operating
temperature.
Principles of mechanics &
thermodynamics at ambient temperature
also apply for cryogens.
Except for H
2 & He all have thermal
conductivities that increase as the
temperature is decreased.

14. Liquefaction and the
Liquid States
Molecules in the gaseous state exhibit
opposing tendencies-dispersion &
aggregation..
The intermolecular attractive forces
increase to definite maxima as the
distances between molecules are
diminished.

15. Liquefaction Contd..
These conditions are brought about when
the temperature of a substance is
lowered,or
When the molecules are crowded close
together I.e pressure is increased.

16. Liquefaction Contd..
When the energy of attraction of one
molecule for another exceeds its kinetic
energy of translation, the molecules will
form a dense aggregation which is termed
a liquid.

17. Cooling in Cryogenic Process
May be accomplished in several ways:
–Cooling at constant pressure, as in a heat
exchanger.
–Application of Joule Thomson effect by
expansion valve or throttling process.
–Expansion in an expansion engine.
–Cooling & Purification by evaporation

18. Cooling In A Heat Exchanger
Requires a heat sink at a lower temperature.
An external refrigeration system is required if
necessary.
This method is used for pre-cooling the gas &
recovery of the low temperature from out going
streams.
Operation at low temperature demands special
techniques for fabrication.

19. Joule-Thomson Effect
Joule-Thomson studied the behavior of
gasses during expansion..
It can be utilized in a cycle till the dew
point of a gas.
Expansion causes decrease in the kinetic
energy of translation;the net result is
decrease in the temperature.

20. Throttling Process
Processes at continuous flow in a pipe occurring
at condition approaching constant enthalpy.
Flow through a porous plug or throttling valve
causes decrease in pressure due to resistance.
If device is insulated & no shaft work is
produced then process is one of constant
enthalpy.

21. Joule-Thomson Coefficient
It is the measure of temperature behavior of a
fluid during a throttling process.
At constant enthalpy, it is ratio of the change in
temperature & pressure.
This coefficient is a property of the gas.
The value can be positive, negative, or zero.
For ideal gas, = 0 this means that there is
negligible change in temperature when ideal
gas flows through an insulated valve.

22. Coefficient Contd..
For most gases is positive at moderate
pressures & temperatures; that is, the gas
becomes cooler as it expands.
Joule-Thomson coefficient represents the slope
of h = constant lines on a T-P diagram.
Can be determined from the data of constant
pressure specific heat & P-V-T behavior.

23. Inversion Temperature
Temperature at which J-T coefficient is zero.
It is the temperature at point where a constant-
enthalpy line intersects the inversion line.
The temperature at the intersection of P = 0 line
the upper part of the inversion line is called the
maximum inversion temperature.
Slopes of the η= constant lines are negative
(η<0) at states to the right of the inversion line
and positive (η>0) to its left side.

24. Inversion Temp. Contd..
Temperature of a fluid will increase during a
throttling process that takes place on the right-
hand side of the inversion line.
Temperature will decrease during a throttling
process that takes place the left-hand side of
the inversion line.
Cooling effect cannot be achieved by throttling
unless the fluid is below its maximum inversion
temperature.

25. Expansion in a Claude Process
It is an isentropic process.
Temperature of the fluid always decreases.
Cooling does not depend on being below the
inversion temperature prior to expansion.
Temperature drop is much higher for this
process than in isenthalpic expansion.
Increasing the pressure drop ratio increases the
amount of heat removed.

26. Cooling By Flashing
Flashing causes decrease in Partial pressure of
the component & temperature decreases as the
distance of the molecule increases.
Pressure letdown effect on liquids serves
following benefits:
–Avoidance of two phase flow in piping / heat
exchangers.
–Better purification of due to high solubility.

27. CryogenicSeparation
The constant-pressure path (1) approaches
the two-phase region.
The isenthalpic expansion (3) will not result in
liquefaction unless the initial state is at a high
enough pressure and low enough
temperature for the constant-enthalpy line to
cut into the two-phase region.
The isentropic process (2) does not require
an initial state at as high a pressure (at a
given temperature) for liquefaction.

28. Liquefaction of Air
Can be explained from its enthalpy-pressure
data by reference to the TS diagram for air.

29. Achievement of Steady -State
Maximum liquefaction is not obtained until
steady-state conditions are reached.
which time an energy balance around the
separator, valve, and cooler gives
H
6z + H
8 (1-z) = H
3
where the enthalpy quantities refer the
positions numbered in figure.

30. Claude -Process
The flow scheme is the same as for the Joule-
Thomson expansion, except that an expansion
engine replaces the expansion valve.
The energy balance becomes
 H
6z + H
8(1 –z) + WS = H
3
If the operation is reversible and adiabatic, the
work is given by the expansion.
 WS = -(H
5 –H
4)

31. Thermodynamic Paths
Liquefaction by joule-Thomson valve.
Liquefaction by expansion engine.
Liquefaction by expansion engine &
expansion valve .

32. Comparison
Turbo-expander process has a far better
capacity for recovering liquids.
Process design is complicated with
expanders
Initial & operating cost are higher for
expander process.

33. Item
Specified
Condition
Heat Exchanger
Pressure drop (psia) shell side 5
Tube 5
Minimum Approach
Temperature,
O
F 15
Flash Separator
Pressure, psia 400
Turbo-expander
Discharge Pressure, psia 400
Efficiency, % 75
Specified Process Conditions For Joule-Thomson/Turbo-expander Process Comparison

34. Item
Calculated Process Conditions
Joule-Thomson
Turbo-
expander
Primary Separator
-Temperature, F -48 -45
-Liquid rate, moles/hr 119 182
Secondary Separator
(Turbo-expander)
-Temperature F - -112
-Liquid rate, moles / hr - 463
Heat Exchanger
-Temperature of
Feed out, F
-9 -45
-Power Generated in
Turbo Expander, hp
- 1316

35. Component Turbo-expender Joule-Thomson Turbo-expander
Incremental
Rec Moles/hrMols/hr %Rec Moles/hr%Rec
N
2
0.88 0.80 0.07 0.06 0.81
C
1
353.42 3.42 30.60 0.30 322.82
C
2
103.18 30.26 10.22 3.00 92.96
C
3
92.09 76.11 17.11 14.27 74.82
iC
4
30.92 93.70 11.48 34.79 19.44
nC
4
21.27 96.68 10.33 46.95 10.94
iC
5
10.94 99.45 8.15 74.09 2.79
nC
5
10.97 99.73 8.98 81.64 1.99
C
6
+ 22.00 100.00 21.74 98.92 0.26
645.68 118.85 526.83
C4+ Recovery 97% 61%

36. Physiological Hazards
Severe cold "burns" may be inflicted if the
human body comes in contact.
Damage to the skin or tissue is similar to tan
ordinary burn because the body is composed
mainly of water.
The low temperature effectively freezes the
tissue damaging or destroying it.

37. Physiological Hazards Contd..
Body can normally adjust for a heat loss of 95
J/m
2
s for an area of limited exposure.
Freezing of facial tissue will occur in about
100 sec. if the heat loss is 2300 J/m
2
s.

38. Materials and Construction
Hazards
Consider the ductility of the material
since low temperatures have the effect
of making some construction materials
brittle or less ductile.
Exposure of low temperature stream on
to ordinary metals may result in material
failure.

39. Typical protections of cold box
On high ΔP, closure of inlet valve(During
S/UP)
Trip of unit at high inlet temperature to protect
exchanger.
Closure of J-T valve / expander nozzle on low
flow of inlet gas to protect temperature
gradient.
Trip of unit on low exit gases temperature.

40. Flammability and
Explosion Hazards
Explosion requires an oxidant, a fuel and an
ignition source.
The oxidizer will be oxygen available from a
leakage or condensation of air on cooled
surfaces and buildup as a solid with water.
The ignition source may be a mechanical or
electrostatic spark, flame, impact heat by
kinetic effects, friction, chemical reaction.

41. Mixture Flammability
Limits (mol%)
Detonability Limits
H
2
–air 4-75 20-65
H
2
–O
2
4-95 15-90
CH
4
–air 5-15 6-14
CH4 –O2 5-61 10-50
Flammability & Detonability Limits of Hydrogen & Methane Gas

42. High –Pressure Hazards
High pressure is obtained by gas
compression during liquefaction (if required).
If this confined gas is suddenly released a line
a significant thrust may be experienced.
Force generated by rupturing a 2.5 cm
diameter valve on a 13.9-Mpa pressurized
gas cylinder would be over 6670N.
Adequate PSVs should be provided.

43. Cold Box
Cold box is a group of multi fluid exchanger
cores assembled in a single structure.
Designed for cooling upto very low
temperatures.
These must be properly insulated for
approaching the adiabatic conditions.

44. Components of cold box
Piping and liquid separators with low
temperature metallurgy.
Coiled or plate-fin exchangers.
Isenthalpic valves/ Expanders or both
Separation or rectification columns(if
required).
Instrumentation.
Insulation.

45. Cryogenic Metallurgy
Metals used for vessels and piping vary
with temperature.
As temperature decreases most
mechanical properties—hardness, yield,
tensile and fatigue strength increase.
Corrosion is of less concern.
The problem is ductility.
Linear expansion is important as well.

46. Ductile Properties
Usual way of characterizing ductile properties
of metal is Charpy-notch or keyhole test.
Notch sensitivity depends on crystal structure
of the metal.
Austenitic stainless steels, high nickel steels,
aluminum, and copper all have face-centered
crystal structures.
Aluminum is usually preferred over copper
because of its lighter weight.

47. Linear Expansion
Coefficient of linear expansion for several
metals varies with temperature.
The higher coefficients for aluminum and copper
raise the problem of providing for Expansion by
means of expansion loops, bellows type
connections, and the like.

48. Structural Properties
Face-centered-cubic (fcc) metals and their
alloys are most often used in the construction of
cryogenic equipment.
Al, Cu, Ni, their alloys and the austenitic
stainless steels are “fcc” and do not exhibit an
impact ductile-to-brittle transition at low
temperatures.
The body-centered-cubic (bcc) metals and
alloys areundesirable for low temperature.

49. Structural PropertiesContd..
Fe, the martensitic steels (low carbon and the
400 series stainless steels), Mo and Nb. exhibit
a ductile-to-brittle transition at low temperatures.
Hexagonal-close-packed (hcp) metalsexhibit
mechanical properties intermediate between
those of the “fcc” and “bcc” metals.

50. Structural PropertiesContd..
Zr and pure Ti alloys with a “hcp” structure
have been used where weight reduction
and reduced heat leakage through the
material have been important.
Small impurities of O, N, H, and C can
have a detrimental effect on the low
temperature ductility properties of Ti and
its alloys.

51. Properties non Metals
Plastics increase in strength as the temperature
is decreased, but this is also accompanied by a
rapid decrease in elongation in a tensile test
and a decrease in impact resistance.
Teflon and glass-reinforced plastic retain
appreciable impact resistance.
The glass-reinforced plastics also have high
strength-to-weight and strength-to-thermal
conductivity ratios.

52. Properties non Metals Contd..
Elastomers,become brittle at low
temperatures.
Nevertheless,rubber, Mylar, and nylon
can be used for static seal gaskets
provided they are highly compressed at
room temperature prior to cooling.

53. Thermal Properties
High-purity aluminum and copper exhibit high
thermal conductivity from 20 to 50 K.
These peaks are rapidly suppressed with
increased impurity levels and cold work.
Monel, Inconel, stainless steel & aluminum
alloys show a steady decrease in thermal
conductivity.
All cryogenic liquid except hydrogen and helium
have thermal conductivities that increase as the
temperature is decreased.

54. Electrical Properties
The electrical resistivity of most pure metallic
elements at ambient and moderately low
temperatures is approximately proportional to
the absolute temperature.
At very low temperatures it approaches a
residual value almost independent of
temperature.
For alloysit is largely independent of
temperature.

55. Heat Exchangers
Selection
Small temperature difference between
inlet and exit streams to enhance
efficiency.
Large surface area-to-volume ratio to
minimize heat leak.
High heat transfer to reduce surface area.
Low mass to minimize start-up time.

56. Selection Contd..
Multi-channel capability to minimize the
number of exchangers.
High-pressure capability to provide design
flexibility.
Low or reasonable pressure drop to
minimize compression requirements.
High reliability with minimal maintenance
to reduce shutdowns.

57. Tubular-fin Heat
Exchangers
These heat exchangers are used as
gas-to-liquid heat exchangers.

58. Plate-fin Heat
Exchangers
Plate-fin are about nine times as compact as
conventional shell-and-tube heat exchangers.
Commonly fabricated from aluminum.
These are stack of layer.
Each layer consists of corrugated aluminum
sheets (fins) between flat aluminum
“separator” plates to form individual passage.

59. Plate-fin Exch. Contd..
The stack, including the sidebars, is bonded
by a carefully controlled brazing process.
Passages can be arranged for either cross
flow, countercurrent flow, concurrent flow, or
multi pass flow.
The number of passages provided for each
stream can be varied to yield the required flow
rates and pressure drop.

60. Plate-fin Exch. Contd..
Up to eight separate streams can be
accommodated in one core.
Heat transfer area per unit volume is
around 2000 m
2
/ m
3
.

61. Applications
The plate-fin type is primarily used for gas-to-
gas application and tube-fin exchangers for
liquid-air heat exchangers.
In most of the application (in trucks, cars, and
airplanes), mass and volumes reduction are
particularly important.
Widely used in cryogenic, energy recovery,
process industry, refrigeration, and air-
conditioning system.

62. Forms of Corrugated Sheets
The plates are typically 0.5 to 1.0 mm thick and
the fins 0.15 to 0.75 mm thick.
Fins give extra heat transfer area and structural
support to the flat plates.
Mostcommon types of fins are:
–Plain fin
–Plain-perforated fin
–Serrated fin (also called “lanced”, “interrupted”,
“louver” or “militantly”)
–Herringbone or wavy fin

63. Limitations
The flow channels are small, which means
that the mass velocity also has to be small
(10 to 300 kg/[m
2
.s]) to avoid excessive
pressure drops.
Plate-fin exchangers are restricted to
clean fluids.

64. Limitations Contd..
Operation at low temperatures requires
removal of essentially all impurities that
are to be cooled.
High operating temperature & differential
temperature are not allowed due to
brazing.

65. Gas Expanders
There are four general types of expanders.
Mostly expanders in low temperature services
are of the single-stage, radial wheel impeller.
High efficiency of a turbo-expander requires
operation at near optimum speed.
Optimum efficiencyliesin the speed range of
10,000 to over 50,000 rpm.

66. Flow Control
Flow through the turbo-expander is controlled
by variable stationary nozzles.
These are mechanically arranged for an
outside manual or diaphragm control to vary
the clearances between the nozzles.

67. Shaft Seal
The “oil-free” arrangement is widely used.
An outward leakage of gas is permitted out of
each labyrinth seal.
A continual stream of pressurized gas ( seal
gas) is introduced at a midpoint of each
labyrinth seal which leaks to atmosphere.
This seal gas is injected at a rate slightly higher
than that leaking out through the seal thus
retaining the valuable process gas

68. Turbo-Expander Materials
Expanders should have bearings
approx.10 times as strong as in similar
machines.
Blade materials are selected based on
erosion resistance and ability to
withstand low temperature.
Aluminum alloys are used in the
manufacture of expander wheels.

69. Separation System
Utilized if the mixture to be separated is
essentially binary with a difference in
boiling points.
Tray columns are utilized for the desired
separation.

70. Single column Separation
It is a J-T liquefaction system with a
substitution of a rectification column for
the liquid reservoir.
In a simple single-column process,
although the bottoms purity is high, the
top effluent stream is impure.

71. Linde Double-Column System
Two rectification columns are placed one on top
of the other.
Feed is introduced at an intermediate point.
Condenser-evaporator at the top of the lower
column makes the arrangement a complete
reflux distillation column.
Almost pure top & bottom products are
produced.

72. Linde System Contd..
Condenser must condense light gas
vapor by evaporating liquid heavy
component.
It is necessary to operate the lower
column at a higher pressure, while the
upper column at lower pressure.

73. Helium And Nitrogen Separation In
N G System
Helium content of the natural gas plants
normally has varies upto 02 %
Nitrogen content varies from 12 to 80%.
The remainder of the natural gas is
methane, ethane, and heavier
hydrocarbons.

74. He & N
2Separation In NG
System Contd..
Major constituents of NG have boiling
points very much different from that of
He, a distillation column is unnecessary.
High separation can be accomplished
with condenser-evaporators.
Expanders give better recovery but at
the expense of complexity of process.

75. Insulation
To minimize radiative heat transfer, minimize
convective heat transfer and use only a
minimum of solid conductance media.
In selection consider ruggedness, convenience,
volume, weight, ease of fabrication and
handling, thermal effectiveness and cost.
Experimentally obtained apparent thermal
conductivity is used to characterize the thermal
effectiveness of various insulations.

76. Types of Insulation
Cryogenic insulations have generally been
divided into five general categories:
High vacuum, multilayer insulation,
powder, foam, and special insulations.

77. Vacuum Insulation
Heat transport across an evacuated space
dependson the emissivity.
The insertion of low-emissivity floating shields
within the evacuated space reduce the heat
transport by radiation.
Radiant heat transfer is reduced to around one-
half of the rate without the shield, two shields
can reduce this to around one-fourth of the rate
without the shield.

78. Multilayer Insulation
Consists of alternating layers of highly reflecting
material, such as aluminum foil or aluminized
Mylar, and a low-conductivity spacer material or
insulator, such as fiberglass mat or paper, glass
fabric, or nylon net, all under high vacuum.
Degradation in thermal performance is caused
by presence of edge exposure to isothermal
boundaries, gaps, joints, fill and vent lines.

79. Powder Insulation
It has the benefit of multiple floating
shields without incurring the difficulties of
awkward structural complexities.
Amount of heat transport due to radiation
through the powders can be reduced by
the addition of metallic powders.

80. Foam Insulation
Thermal conductivity is dependent upon
the bulk density of the insulation.
Of all the foams, polyurethane and
polystyrene have received the widest use
at low temperatures.
The major disadvantage is that they tend
to crack upon repeated thermal cycling
and lose their insulation value.

81. Selection of Insulation
Many of the vessels contained in a “cold box”
usegranular perlite.
Wool felt might be used around maintenance
entrances.
On transfer lines use urethane, perlite or
possibly vacuum jacketed lines.
Polyurethane foam requires a vapor proof
outer jacket.

82. Cryogenic Control Valves
The principal function of a J-T valve is to
obtain isenthalpic cooling.
These valves are generally needle-type valves
modified for cryogenic operation.
Normally these are made of SS-304 or SS-
321 material
Have a protection sleeve around the body.
The plugs are tapered & installed in a clean &
polished body for minimum friction.

83. Cryogenic Instrumentation
Metals become brittle at low temperatures, so
the instrument literally falls apart.
Elastomeric gasket and seals contract faster
than the surrounding metal parts, and the seal
often is lost.
Even hermetically sealed instruments can
develop pin holes or cracks
For cryogenic service, check integrity under
liquid N
2.

84. Pressure Instrumentation
Measured by the flush mounted pressure
transducer
Consists of a force-summing device (below,
diaphragm, bourdon tube), an analog device
(strain gage, piezoelectric crystal, variable
distance between capacitor plates, and the like).
Elements are likely to be made of different
materials (bronze diaphragm, stainless-steel
case, semiconductor strain gage), each will
react to the temperature change differently.

85. Pressure Instrumentation Contd..
Very small pressure sensing elements
from a single semiconductor chipmay be
used to reduce or eliminate temperature
gradients across the device.
The single element nature of the pressure
gage assembly reduces differences in
materials of construction.

86. Liquid Level
The measurements are made in the
conventional CPI approach using floats.
Sight glasses cannot be used since cryogenic
fluid in it shall boil.
The dielectric constant of cryogens is related to
their density.
As liquid level rises, greater dielectric constant
of liquid causes capacitance to vary linearly.
For best accuracy, should be calibrated in place.

87. Flow
The measurement of cryogenic fluids is most
troublesomesince it is a derived quantity.
Normally these are avoided & flow is measured
by the liquid level transfer & measurement at
high temperature exit streams.
Calibration has accuracy limitation.

88. Temperature
The level & range of measurement need
consideration.
Up to 20K favorite choice is platinum resistance
thermometer (PRT).
Below 20k, semiconductor thermometers
(germanium, C, or Si based) are preferred.
Their resistance increases as the temperature is
lowered, semiconductors are usually chosen for
temperatures from about 1 to 20 K.

89. Temperature Contd..
For large range of say 1 to 400 K, diode
thermometers are recommended.
Diode thermometers are very much
smaller and faster.
Thermocouples are not used as the
thermoelectric power drops to a few µV/K.

90. Installation and Operation
Transport and storage.
Erection.
Operation.

91. Transport and Storage
Plate-fin heat exchangers are shipped
hermetically sealed to avoid ingress of moisture
or dust.
Without N
2-filling, silica gel will be inserted into
the headers to absorb any moisture.
Inactive or dummy layers are normally closed
by self-sticking aluminum foils.
Lifting and transportation only allowed on the
marked points and under use of suitable
devices. Protect the exchanger edges.

92. Measures at Arrival on-site
Check for damages of the packing& block.
Check the nitrogen pressure level.
Check for any defects of stuck aluminum foils
and possible renewal.
In case of any leakages or mechanical damage,
Vendor shall be informed.
Carry out washing / drying if exchanger is not
sea worthy packed for sea freight batteries.

93. Storage
Block should be stored within the original
packing. Otherwise plate-fin exchangers may be
stored upon square timbers.
Protect from ambient influences (rain, dirt) by
suitable covering.
Storageshall be in a position where aluminum
foils are seen on vertical.
Should be stored in-house.
Monitor pressure of nitrogen gas regularly.
Check Sealing of inactive sections.

94. Installation
The covers on nozzles or flanges shall be
removed only immediately before the
connection of nozzles to the piping system.
Consider Exact Alignment .
Piping has to be carefully cleaned and dried
before connecting to the heat exchangers.
Install strainers at the inlet pipes.
Before insulation work any existing temporary
nipples or gauges on dummy layers should be
removed

95. OPERATION
Process fluids should be in steady flow.
Deviations from the specified operation
conditions are only allowed, if there are no
appreciable changes in stream
compositions, temperatures and
pressures.
Before starting, Nitrogen filling has to be
purged out by the process fluids.

96. Cooling & Warming up
Avoid building up of thermal stresses.
Avoid sudden temperature shocks and
under changes of stream temperatures.
Cooling is allowed with designed gas only.
Warming up with gas is allowed only after
draining all liquid out of the exchanger.

97. Allowable limits & Precautions
Temperature difference between two streams in
the heat exchanger must not exceed 50 K.
The temperature change of any stream must not
exceed 60 K/h and 2 K/min.
Mass velocity on any side should never exceed
300 Kg/m
2
.s.
Pressurization / de-pressurization rate to be
kept lower than 2 Kg/cm
2
/min.

98. Allowable limitsContd..
Cold box cooling down speed should not
exceed 25oC / hr.
During deriming transfer the cold only in one
direction to avoid excessive temperature
difference.
Degree of drying is sufficient if equipment
temperature is 30
o
C and dew point is <–40
o
C.
Strictly follow the design limits for temperature
and pressure.

99. Allowable limitsContd..
Differential pressure of any side of plate-
fin exchanger should be controlled. Follow
vendor instructions about the same.
Analyze Insulation chamber periodically.
If leakage from cold box is detected,
inform Vendor, & shutdown/ preserve it.

100. General Operational Consideration
The major design problem is assembling
accurate process & enthalpy data.
The operating problems stem from In-adequate
design& temperature involved are:
–Freezing of liquids
–Protection of metals at low temperature
–Handling of highly volatile liquids
–Safety
–Mechanical failure

101. Freezing of Liquids
Hydrocarbons heavier than propane are solids
below –130
0
C.
CO
2& water freeze at –80
0
C & O
0
C resp. H
2S &
other contaminants solidifying on cold surface.
Insulation should be continuously purged to
protect freezing on outer sides.
Control dew point from upstream section .
Perform warming up operations.

102. Hydrogen Recovery Unit
Purpose:
Removal of Ar, CH
4& N
2
from synthesis gas.

103. Nitrogen Separation Plant
The Nitrogen plant produces inert gas
(nitrogen) in purity, using the cryogenic
process.
The source from which nitrogen is
producedis air.

104. Major Problems Encountered in HRU
Fouling/choking of Cold Box Exchangers.
–Low regeneration of adsorbers
–High temperature of inlet gas.
–Upsets in NH
3recovery unit.
–Bellow leakages at V-4581A/B.
–Passing of inlet valve TV-45083.
Inlet Strainer Choking.
Failure of PV-45022 (Letdown Valve).

105. Major Problems in HRU Contd..
Insulation Shell Pressure Monitoring and
Sampling.
Condensation and Icing on External
Surfaces.
No specialty for maintenance of fin
exchangers on site.

106. Deriming Operation Problems
•Deriming process is very slow.
–Drain valve #45112 line occasionally chokes.
–Warming rate control is sometimes
problematic.
–Temperature gradients limits werenot
followed (occasionally).
–Procedure revised to improve the
understanding.

107. Areas of Concern
There is no procedure available to carryout
following activities:
–Removal and refilling of perlite.
–Leak test of HRU cold box.
–Inspection of E-4552A/B and V-4583 etc.
–Repair of any damaged part including brazing
procedure and alignment of exchangers.

108. Areas of ConcernContd..
Flow / Pressure Variations During
Adsorber Changeover.
Sampling Points Sensing Tubing Blockage

109. N
2PlantProblems Encountered
Turbine Flow Controlnozzles sticking during
startup; resolved by repacking of valve.
Nitrogen Plant Evaporation Pit Y-3919:
–Icing and condensate formation around the
pit.
–Seepage of water inside the pit and
solidfication.
–LS tracing around the pitwas provided.

110. Nitrogen PlantProblemsContd..
After T/A-2000 liquid N
2 inventory dropped to
25.4% & level rise rate was slow. Probable
reason is low insulation level.
Plant Tripping at Low Lube Oil Temperature.
Refrigeration Unit Performance
Deterioration.
Leakage From Storage Liquid Inlet Line
during commissioning.

112. Compounds
Critical Temperature
T
c
(K) to
Critical Pressure
P
c
(Atm)
Ammonia 405.5 111.3
Carbondioxide 304.2 72.9
Hydrogen 33.3 12.8
HydrogenSulfied 373.6 88.9
Nitrogen 126.2 33.5
Oxygen 154.4 49.7
Sulfurdioxide 430.7 77.8
Water 647.3 218.2
Methane 190.7 45.8
Ethane 305.43 48.2
Propane 369.9 42.01

134. Plate-Fin Exchanger Parts

142. Separation Column

149. CRYOGENIC VALVES

151. Cold Box-Exchanger

153. Condenser-Evaporator