OXYGEN
PLANT
INTRODUCTION:
The
oxygen required in various steel making-processing units, e.g. basic oxygen
furnaces of DSP is produced in this plant by liquefaction and rectification of
air. Since the process deals here with temperatures below -1300C,
the technology used is often called Cryogenic Technology. The main products
obtained are 99.5% pure O2 and almost 100% N2.
Atmospheric air contains mainly the followings:
CONSTITUENTS VOLUME
1. Nitrogen gas
:
78.08
2. Oxygen gas : 20.96
3. Argon gas
: 0.93
5. Carbon-dioxide : 0.033
6. Carbon-monoxide : 0.1 ppm
7. Hydrocarbons, mainly C2H2 : 1.5 ppm etc.
7. Water vapor (H2O) : 0.0 – 4.0 and impurities like dust, oil traces
etc.
Air, after removal of impurities like dust,
moisture, CO2, CO, hydrocarbon & oil traces etc., is liquefied by cryogenic
refrigeration and liquefaction process and finally product gases like Oxygen,
Nitrogen & Argon are separated by the process of distillation (according as
their property of volatility). At 1 Kg/cm2 B.P. (boiling point) of
Oxygen, Argon & Nitrogen are -1830 C, -1860 C &
-1950 C. Thus, nitrogen is more volatile than argon and argon is
more volatile than oxygen.
CRITICAL
TEMPERATURE AND PRESSURE:
With increase in pressure, B.P. of a gas increases up
to a certain temperature, after which B.P. remains unchanged in spite of
increase in pressure. This particular temperature is known as Critical
Temperature and the corresponding pressure is known as Critical Pressure
(of that particular product gas).
AIR OXY NITRO
Critical Pressure, 0C : 38 51 35
Critical Temperature, kg/cm2 :
-141 -119
-147
PRINCIPLE OF CRYOGENIC refrigeration & LIQUIFACTION OF
GASES:
In general, refrigeration is the science and art of
producing and maintaining temperature below that of the surrounding atmosphere.
And in particular the subject of techniques, process and equipment, which deals
with the temperature at and below -1500 C are broadly termed as Cryogenic
Engineering and others are ordinarily called refrigeration. The
liquefaction of air for producing oxygen was first done in engineering
application of Cryogenics and still in use in the industry.
The
thermodynamically ideal liquefaction system is the Carnot cycle in which two
processes are involved, namely
a) Reversible isothermal
compression, and
b) Reversible expansion at
isenthalpic condition
which are shown in T-S
diagram in Fig.1.For any practical liquefaction system, an expansion valve is
used to produce low temperature at isenthalpic condition. For a perfect gas
Joule-Thomson co-efficient is zero and inversion curve exists for a real gas
from where Joule-Thomson co-efficient may be +ve or –ve.
Isothermal Compression
2
1
Isenthalpic Expansion
f 3 4
|
T
S P
Fig. 1 Fig.
2
The second method of producing low
temperature is adiabatic expansion of any gas through a work producing devices
e.g. expansion engine, expansion turbine etc. For an ideal gas adiabatic &
reversible expansion is true only in isentropic condition. Moreover it may be
noted that for any gas , isentropic expansion through an engine or expander
will always result a decrease in temperature whereas expansion through an
expansion valve may or may not result with drop in temperature. The another
important factor is that at a given pressure an isentropic expansion will
always result a lower final temperature than an isenthalpic expansion for the
same initial temperature.
LIQUIFACTION
OF GASES:
Low
temperature gas liquefaction process involves two basic principles
a)
on joule Thomson expansion which is usually called Linde process.
b)
on expansion engine which is commonly called Claude process.
In
Linde process the most key factor is that the pressure and the temperature after
compression and cooling should lie on or below the inversion curve , other wise
the temperature of the gas will increase
with decrease in temperature as in the
case of helium or hydrogen gas.
But
in Claude process , a combination of the joule Thomson expansion &
expansion engine is used for refrigeration of the gases.
The
followings are the different steps of cryogenic air separation process:
1. Filtration
2. Compression
3. Pre-cooling &
Purification
4. Distillation
1. Filtration:
Air with dust particles enters the augmented self Cleaning Filter (ASCF) system and is cleaned by passing
through the panel-pack paper filter elements. The clean filtered air exits
through the ventury into the compressor inlet. In this process only the solid
dust particles are removed whereas the moisture, impurities like CO2,
CO, C2H2 and oil
traces are left over.
2.
Compression:
To liquefy air for distillation, its enthalpy is
decreased by isothermal compression i.e. compression followed by after cooling
at every stage.
For
any fluid,
H
= E + PV
H Ã Enthalpy
E Ã Internal energy, a function of temp. only
PV Ã A function of temperature (PV = RT)
For
any ideal gas E & PV remain const. at const. temperature and are
independent of pressure and this H is const. But, for real fluids, because of
non-ideality, the enthalpy (H) is decreased with decrease of E & PV,
which are affected due to variation of pressure at constant
temperature. Here, the variation of internal energy (E) isothermally with
pressure is much higher than the variation in PV.
PROCESS:
The production of
high-purity gaseous oxygen from air via the cryogenic process is illustrated in
the flow sheet on the next page. The process is based on the principle of the
Joule-Thomson effect, i.e. sudden decrease to pressure results in the lowering
of temperature. Linde first applied it in produced O2 industrially.
This method is distillation of liquid O2 & liquid N2,
is still in use but with modifications, most important of which is Claude,
which significantly decreased the power consumption (as the expansion in his
method is isenthalpic instead of isothermal) and increased the efficiency.
Filtered air is compressed
to about 5kg/cm2 pressure in a centrifugal compressor and after
cooled. After separating out any liquid water, the air enters the reversing
heat exchanger and is cooled to near its dew point in countercurrent exchange
with the outgoing gas products. Carbon dioxide along with remaining moisture or
other HCs’ is removed by the adsorption process in Driers where activated
alumina and molecular sieves adsorbs the last traces of CO2. The
cleaned air is then fed to the bottom tray of the double column rectifier,
consisting of two columns one operating at low pressure (0.5 kg/cm2)
and the other operating at medium pressure (5 kg/cm2). In the double
column rectifier, nitrogen being more volatile is collected as vapor from top
of the low pressure column, where liquid oxygen is collected from the bottom of
the low pressure column. A stream of water waste nitrogen (containing around
4.5% O2) is vented out after proper heat transfer to the refrigerant
unit.
Gaseous oxygen is produced in this plant as per
DSP’s requirement and sometimes the excess liquid pure oxygen is sold to
outside parties. In one unit, 9700 Nm3/hr of gaseous O2
and 650 Nm3/hr of liquid O2 is produced normally, the
excess being storage vessels as liquid O2 at 40 kg/cm2
pressure and around -1840C, the capacity of which is 1000 tons of
liquid O2. Proper insulating materials like glasswool are provided
at various places while in storage vessels pearlite powder is used as
insulating material. The plant is fully automated, however time-to-time
analysis of various outlet streams is to be done to ensure production at
desired rate.
Air is compressed from atmospheric pressure to plant
operating pressure (7.1 kg/cm2) by centrifugal air turbo compressor.
For tonnage air separation Plant centrifugal
compressors are used as it can handle a very large amount of air at low and
medium pressure.
Filtered air enters the
compressor first stage intake chamber through inlet guide vane. The high-speed
impeller blades increase the velocity of air and impart kinetic energy to air.
Part of this kinetic energy results in a static pressure rise of gas (air, in
this case) in the impeller and the remaining kinetic energy results in velocity
head which causes additional pressure rise in the compressor diffuser. After
air passes through impeller, it enters into the diffuser and reduces velocity
by increasing the flow area rapidly. The KE imparted to the air by the impeller
is converted to pressure rise by reduction of velocity in the diffuser.
As,
K.E. α V2 (impeller tip speed),
due
to increase in K.E. and increased pressure ratio of air, temperature of air
also increases.
Air is compressed basically polytropically rather than
isothermally. To enable to approach for isothermal compression, air leaving
each stage of compression is cooled into the intercoolers up to the inlet
temperature of gas and this reduces compressor’s power consumption also. After
leaving the first stage diffuser, air enters the return bend channel and
finally into next stage impeller through the intercooler. Thus air leaving the
last stage compression enters into the piping system through after cooler.
Each
ATC,
Air delivery = 61,000 NM3/Hr.,
at 7.1 kg/cm2 (abs) and 400 C.
3. Pre-cooling & Purification:
The impurities of air like moisture, oil traces, CO2,
CO, hydrocarbons are eliminated through this process. It may be noted that
constituents like moisture & CO2 forms deposit in the piping
system at cryogenic temperature and can cause obstruction of passage for flow.
C2H2 and oil traces are dangerous and can cause explosive
reaction with liquid oxygen.
PRECOOLING:
PRE-COOLING
SYSTEM removes a major portion of moisture content. With increase in pressure
the moisture carrying capacity of air decreases and in the pre-cooling system
part of moisture content condenses after getting separated from gas by the
process of cooling at a constant pressure.
The
pre-cooling unit consists of:
i)
Air-water tower (E10)
ii) Nitrogen-water tower (E11)
iii) Cooling water circulating pump (P01/02)
iv) Refrigeration Unit (X01/02)
i)
Air-Water Tower (E10):
This is a single stage, counter flow, direct contact
type heat exchanger with polypropylene packing. The compressed air (400
C) is cooled to 100 C by chilled water (30 M3/Hr, 8.1 kg/cm2,
50 C) in the Air-Water tower. A major part of moisture content is
extracted along with the cooling water and finally air is passed to the Driers
for further purification.
ii)
Nitrogen-Water Tower (E11):
The
cooling water at 330 C (30 M3/Hr., 3.5 kg/ cm2)
is cooled to 150 C (1.15 kg/cm2) in Nitrogen-Water tower
by utilizing available cold energy from waste nitrogen (21,500 NM3/Hr,
1.25 kg/cm2, 130 C). This is also a single stage, counter
flow, direct contact type heat exchanger.
iii) Cooling water circulating pump (P01/02) &
refrigeration Unit (X01/02):
The
cold water leaving E11 at 150 C is pumped to E10 through
refrigeration unit where it is further cooled to 50 C and after E10
it returns water (400 C, 2.5 kg/cm2) to cooling tower.
PURIFICATION:
Air is further purified through physical adsorption
process to eliminate CO2, CO, hydrocarbons and leftover moisture,
oil traces etc. Physical adsorption involves the attraction and retention of
layers of molecules of a gas on a solid surface into the adsorber vessels.
Air
purification unit consists of:
i) Adsorber Vessels (RO1/RO2)
ii) Regeneration Heater (E16)
iii)
Post Filter (F11/F12)
iv)
Silencer (FO5)
The adsorber vessels are double adsorbent bed type.
The bottom bed containing activated
alumina eliminates and adsorbs moisture & oil traces. The top
bed containing MOLECULAR SIEVES adsorbs CO2, CO and
hydrocarbons. While one vessel remains in operation, the other gets
regenerated.
Air at 100 C enters R01/R02 through the
bottom of the vessel and passes through
activated alumina and molecular sieve beds respectively and leaves at 150 C at the top of the vessel
eliminating all impurities.
During adsorption, the
operating vessel gets saturated gradually and then the vessel is changed over
for regeneration and the other vessel is put into operation. In order to
prevent possibility of dust particles coming from adsorbents with the leaving
air, it is passed through post filter where dust particles up to 50 microns is
removed, and bone dry, hydrocarbon free air is fed into the cold box.
The waste nitrogen coming from cold box is
heated to required temperature by a regeneration heater and passes
through drier in the reverse direction of process air. After gradual heating,
the adsorbed moisture, oil traces, CO2, CO, hydrocarbons etc. are
released from the surface of adsorbents and finally these are carried with WN2
and vented to the atmosphere. The total bed is reactivated again and cooled to
process temperature by purging with simple waste nitrogen.
4.
DISTILLATION:
Cryogenic separation is
achieved by iterative distillation.
Principles:
Distillation
is carried out by using the property of volatility where two fundamental
principles are followed.
I) If a liquid
mixture is heated and partially evaporated then the remaining liquid will be
richer in less volatile component and
the resulting vapour will be richer in more volatile component.
II) Conversely, if a vapour mixture
is partially condensed then the remaining vapour will be richer in more
volatile component and the resulting liquid will be richer in less volatile
component.
In
cryogenic separation process, distillation is carried out in perforated trays
(or in packed columns) inside the column where a liquid trickles down from top
to bottom and a vapour goes up from bottom to top. Both the fluid make their
passage through trays (as in Oxygen Plant, DSP) and come in direct contact with
each other in each tray. In air separation process, these liquid & vapour
are basically a binary mixture of oxygen and nitrogen. The point to be
remembered is that the liquid is at its vaporization point and the vapour is at
its condensation point.
Now,
let us consider one single tray T vapour lower temperature
zone
where
liquid trickles down through the runner
Column
of Tt and vapour comes out from Tb
. Liquid Tt
liquid
is held in the tray by
the vapour pressure below T
T and overflows the weirs and runs down the Tb Weir
Runner
runner of T to Tb. Vapour
from Tb, on the other
hand, gets through the perforations of T and
then
higher temperature zone
bubbles
through the liquid in T to reach the bottom of Tt. During
this process, the vapour, being a little warmer than the liquid, gives away
heat and condenses partially. And hence becomes richer in N2 and
poorer in O2 (Principle II). Conversely, the liquid,
gains heat & vaporises partially, hence becoming richer in O2
and poorer in N2 (Principle I). The result is
I) Vapour of T is richer in N2 but
poorer in O2, than vapour of
Tb.
II) Liquid of Tb
is richer in O2 but poorer in
N2, than liquid of T.
This
way, as the vapor moves up N2 concentration increases and as the
liquid goes down O2 concentration increases.
The distillation unit
consists of the following equipments:
(1) Main Exchanger (E01): (four
blocks)
GAS
|
MASS FLOW
NM3/hr
|
PRESSURE
Kg/cm2(a)
|
TEMPERATURE, 0 C
|
INLET
|
OUTLET
|
Air
|
59,000
|
6.71
|
15
|
-133, -170
|
Oxygen
|
9,700 (11,000max)
|
1.68
|
-178
|
13
|
Nitrogen
|
15,000
|
1.34
|
-180
|
13
|
Waste Nit.
|
33,565
|
1.39
|
-180
|
13
|
(2)
Expansion Turbine (D01 & D02):
Type : Reaction inward flow
Mass
flow : 12,915 NM3/hrmax
Inlet
air : 6.65 kg/cm2(a) at –1330
C
Outlet
air : 1.5 kg/cm2(a) at –1730 C
Speed : 16,521 r.p.m.
Power
generation : 200 KW
(3) Medium Pressure column (K01):
No. of
trays : 44 (Al)
Operating
Pressure : 6.53 kg/cm2
Operating
Temp. :
-1700 C
(4) Low Pressure column (K02):
No. of
trays : 85 (Al)
Operating
Pressure : 1.68 kg/cm2
Operating
Temp. :
-1780 C
(5) Pure Nitrogen Column (K03):
No. of trays : 15 (Al)
Operating
Pressure : 1.44 kg/cm2
Operating
Temp. :
-1930 C
(6) Main Vaporizer, or Condenser cum Reboiler (E03):
(four blocks)
Fluid
|
MASS FLOW
NM3/hr
|
PRESSURE
Kg/cm2 (a)
|
TEMPERATURE, 0 C
|
INLET
|
OUTLET
|
Oxygen (shell)
|
33,948
|
1.80
|
-178
|
-178
|
Nitrogen (tube)
|
47,746
|
6.76
|
-176
|
-176
|
(7) Liquid Oxygen Filters (R03/04):
Mobil
Sorbid : 3-5 mm balls
(8) Rich Liquid Sub-cooler (E04): (one blocks)
Fluid
|
MASS FLOW
NM3/hr
|
PRESSURE
Kg/cm2 (a)
|
TEMPERATURE, 0 C
|
INLET
|
OUTLET
|
Liq. Oxygen
|
5,900
|
1.68
|
-178
|
-178
|
Rich Liquid
|
27,995
|
6.50
|
-176
|
-176
|
(9) Sub-cooler (E05):
Fluid
|
MASS FLOW
NM3/hr
|
PRESSURE
Kg/cm2
(a)
|
TEMPERATURE, 0 C
|
INLET
|
OUTLET
|
LOX
|
650
|
1.53
|
-178
|
-185
|
LIN
|
9,750
|
6.36
|
-176
|
-189
|
Pure N2
|
15,000
|
1.39
|
-193
|
-180
|
WN2
|
33,565
|
1.44
|
-191
|
-180
|
PL
|
8,340
|
6.48
|
-175
|
-186
|
(10) Liquid Oxygen Pumps
(P03/P04):
Mass Flow : 1050 NM3/hr
Inlet Pressure : 2.80 kg/cm2 (a)
Outlet Pressure : 3.16 kg/cm2 (a)
Speed :
1450 r.p.m.