Journal of Engineering (JOE) ISSN: 2325-0224
Vol. 2, No. 1, 2013, Pages: 81-84
Copyright © World Science Publisher, United States
www.worldsciencepublisher.org
81
Simulation of Separation of nitrogen gas from air
Ruhul Amin*, Masrul Huda, Latiful khabir
Department of Chemical Engineering,
Bangladesh University of Engineering and Technology, (BUET) Dhaka-1000, Bangladesh.
*E-mail: [email protected]
Abstract: Nitrogen is the common element of the universe, estimated at about seventh in total abundance in our galaxy and
the Solar System. The greatest single commercial use of nitrogen is as a component in the manufacture of ammonia,
subsequently used as fertilizer and to produce nitric acid and liquid nitrogen is used as a refrigerant for freezing and
transporting food products. Different methods are being used for separating of nitrogen gas from air. Linde-Hampson cycle
of Cryogenic process is the most used process for separation of nitrogen. This thermodynamic cycle has been simulated
using Aspen HYSYS simulation software. The objective of this paper is to present analysis of thermodynamic cycle
used for liquefaction of Nitrogen (N2) under given set of operating condition and efficiencies. The liquefying
temperature of Nitrogen being -200oC was taken into consideration. The final composition of nitrogen obtained is
approximately 92%.
Keywords: Nitrogen, Separation, Cryogenic, HYSYS, Linde- Hampson cycle.
1. Introduction
Earth is surrounded by air, where nitrogen is one of the
main components of air. Nitrogen is very vital chemical for
different chemical industries. It has very versatile
applications such as- In ordinary incandescent light bulbs
nitrogen is used as an inexpensive alternative to
argon[1],During The manufacturing of stainless steel [2] , to
Fill automotive and aircraft tires[3] , It is also used to
concentrate and reduce the volume of liquid samples during
chemical analysis [4] etc. Air is the main source of Nitrogen.
Production levels of nitrogen and argon have increased by
50 to 60% during the last decade. Significant improvements
have been made to cryogenic processes to reduce the
energy consumption of nitrogen separation process and
increase argon recovery [5]. Nitrogen can be separated from
air by liquefaction of air. Different processes can be used to
separate nitrogen from air. Among these cryogenic
process is the most used process for nitrogen
separation. The cryogenic separation process requires a
very tight integration of heat exchangers and separation
columns to obtain a good efficiency and all the energy for
refrigeration is provided by the compression of the air at
the inlet of the unit [6]. the National Bureau of Standards
at Boulder, Colorado considers temperature below 123K
as cryogenic operating temperature. This can be accepted
because boiling temperature of permanent gases like
helium (He), hydrogen (H), neon (Ne) etc are below 123K
Simple Linde- Hampson cycle and CLAUDE cycle are the
main cryogenic processes. The Linde- Hampson cycle is
based on the Joule-Thomson effect and is used in the
liquefaction of gases. William Hampson and Carl von
Linde independently filed for patent of the cycle in
1895[7] .It is very simple and operating cost is very low.
CLAUDE cycle is mainly used for achieving highly pure
product. In 1902 George Claude invented Claude system
for liquifying air [8] The system enabled the production of
industrial quantities of liquid nitrogen, oxygen, and argon;
Claude's approach competed successfully with the earlier
system of Carl von Linde (1895) [9] .Computer based
simulation has been popular nowadays for different
chemical engineering purposes. This paper contains
HYSYS simulation and description of N2 production using
Simple LINDE-HAMPSON cycle. It will help to get the
basic idea of N2 plant.
2. Methodology
To simulate the process of Separation of nitrogen gas
from air renowned Aspen HYSYS v7.1 is used. For this
problem, Peng-Robinson fluid package have been used as
it is the most enhanced fluid package in Hysys with
highest T & P range. Making some assumptions and using
hypothetical units the N2 production simulation have been
performed. Though it does not give the real world
performance or the real life production environment,
but it can give relief from making wide range of
experiment without making the small scale reactors or
plant .
2.1 Process descriptions:
82
Following assumptions can be made while producing
nitrogen using Linde- Hampson cycle.
No irreversible pressure drop occurs during the
process
All equipments are thermally insulated so that no
heat loss occurs during the process
The heat exchanger used in the process was 100%
effective
The primary components of air are nitrogen and oxygen.
Though small amounts of carbon dioxide and some
inert along with water vapor and dust may also found
in the air.
used to cool the gas stream to room temperature that is
26.85oC. The gas stream coming out from the cooler
(E-100) is used as the hot stream inlet of LNG heat
exchanger which is actually a counter current shell and
tube heat exchanger where the hot fluid flows through the
tube side and the cold fluid through the shell side.
Figure 2: Block Diagram of Separation of Nitrogen from
Air
Figure 1: Composition of dry atmospheric air [10]
Air must be pre-filtered to make it clean from dust
and it must be dried before entering it into the
nitrogen separation system. Otherwise, the production rate
of nitrogen will decrease. The clean, dry air containing
mostly nitrogen and oxygen is fed to a mixer at a rate
of 1kg/hr where it mixes with the recycle stream at
the same intermediate and equilibrium state. The
temperature and pressure of both streams entering the
mixer were kept at 26.85oC and 1bar. The mixed gas
stream is then fed to a compressor where the gas is
compressed to 250 bar. The temperature of the gas
stream coming out from the compressor rises to a very
higher level as the pressure increases. So, a cooler is
The stream coming out from the cooler is used as the hot
fluid inlet in our process. A pressure drop of 1 bar in hot
stream and 0.2 bars in the cold stream occur. Then an
isentropic J-T valve is used which works at a constant
enthalpy and is such that with decrease in pressure a
significant drop in temperature is brought about. For the
purpose of flashing the vapor-liquid mixture that comes
out of the J-T valve, a phase separator is used. The liquid
product from the separator is withdrawn. Meanwhile the
vapor product is fed to the LNG heat exchanger as the
cold fluid inlet. The outlet stream from the LNG heat
exchanger is passed through a tee and the stream is split
into two different streams. One of the two streams is used
as the recycle stream in the mixer while the other one
is our product stream. We have to recycle almost 96%
of the stream coming out from heat exchanger to
achieve maximum purity of nitrogen. We get 92%
nitrogen product. Further separation can be done by using
Claude process.
Figure 3: Process flow diagram of Separation of Nitrogen by Hysys Simulation.
2.2 Reactions
83
Separation of nitrogen from air is a non-reactive process.
So, no reactions occur in this process. Air is Simplycooled
to a very lower temperature and then both liquid nitrogen
and liquid oxygen are separated from air.
3. Result
Final composition of nitrogen obtained from Cooler
E-101 is 92% but it is expected to obtain around 99%
pure nitrogen from air. Unavailability of process
condition is the main reason for getting less percentage of
nitrogen products from air.
Figure 5: Composition of N2 in product Vs Percent
recycled from LNG Heat exchanger.
3.1.2 Temperature Dependency of First Compressor:
Figure 4: Results workbook of Aspen Hysys v7.1 of
Cooler E-101(product unit)
The temperature difference of first compress depends on
the temperature of inlet feed to mixture. Temperature
difference of first cooler increases approximately linearly
with the increase of inlet feed temperature. Higher
temperature difference indicates higher outlet temperature
of Compress resulting higher cooling duty in chiller.
3.1 Discussion
Efficiency of cryogenic systems are much less. Work
input per unit mass of gas liquefied is still very high. A
huge scope lies in the improvement of efficiency of
cryogenic cycles. User friendly ASPEN HYSYS can be
used for the optimization and simulation of these cycles.
The main problem with these cycles is energy
exchange with the surroundings (ambient condition).
But advancement has been made in the field of design of
heat insulators. Some salient facts are discussed below:
3.1.1 Recycling of LNG stream
In this process it was needed to recycle 96% product
to achieve better purity. From Graph it is apparent that
product purity is increased with the increases of Recycling
of LNG stream. But this higher recycling decreases amount
of product withdrawn the product purity is low with regard
to expectation. This problem can be solved by adding
more LNG exchanger or using CLAUDE process. In
that case we can get 99% pure N2 but operating cost will be
lot higher.
Figure 6: Graphical representation of temperature
difference of first compressor vs. inlet temperature.
3.1.3 Duty of First cooler
Duty of first cooler is very high and it will be very
expensive in practical life. We can use more than one
cooler in practical life to solve this problem.
3.1.4 Impurity Concern of Feed Air
84
In this process only two main components of air were
considered. But there exist some other component such as
Ar, He, H2, CO and CO2 etc.
4. Conclusion
Nitrogen is very important for the balance of our Ecosystem, besides this, it is very important element of modern
chemical industries, especially Fertilizer and food
reservation industries. To meet the huge demand of large
quantity of nitrogen for attaining food Autarky, production
of pure nitrogen has become key concern of chemical
engineers. This paper contains simulation of Simple
LINDE-HAMPSON process is the most basic process of
nitrogen separation. Simulation is the imitation of the
operation of a real world process or system over time. A
computer simulation is an attempt to model a real-life or
hypothetical situation on a computer so that it can be
studied to see how the system works. By changing
variables in the simulation, predictions may be made about
the behavior of the system. It is a tool to virtually
investigate the behavior of the system under study.
Traditionally, the formal modeling of systems has been
via
a mathematical model, which attempts to find
analytical solutions enabling the prediction of the
behavior of the system from a set of parameters and
initial conditions. Another important aspect of computer
simulations is that of reproducibility of the results,
meaning that a simulation model should not provide a
different answer for each execution.
Acknowledgement:
Authors are very much grateful to chemical Engineering
Department, Bangladesh University of Engineering and
technology for giving opportunity to use computer
simulation lab during their research work.
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