1
Flowsheet Development and Simulation of ethane
production from Synthesis Gas by using Hysys
Ruhul Amin, F. Enam, Golam Mohiuddin, Masud Rahman, Liton Kumar, Syeda
Ahammad
Department of Chemical Engineering
Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh
Abstract: Ethane is produced to give final product named ethylene which is used to produce
polyethylene a widely used plastic material. Several processes have been developed for optimum
production of ethane. Ethane is produced from synthesis gas. Synthesis gas is a fuel gas mixture
consisting mainly of hydrogen, carbon monoxide, and very often some carbon dioxide [1].
Synthesis gas is produced from natural gas, air and steam at high temperature. In our work
simulation of ethane synthesis process is done on Aspen Hysys 2006. In this simulation process
we used 4454 kmole/hr natural gas, 4536 kmole/hr reformer steam, 2245 kmole/hr air and
produced 1.633×104 kmole/hr ethane. It has also found that ethane production increases with
molar flow of natural gas, molar flow of air and temperature of air.
Keywords:
Ethane,
Synthesis
gas,
1. Introduction: Ethane is a hydrocarbon
and consists of hydrogen and carbon with
the formula of C2H6. At standard
temperature and pressure, ethane is a
colorless,
odorless
gas.
At
room
temperature, ethane is a flammable gas.
When mixed with air at 3.0%–12.5% by
volume, it forms an explosive mixture.
Ethane is isolated on an industrial scale from
natural gas, and as a byproduct of petroleum
refining. Ethane was first synthesised in
1834 by Michael Faraday, applying
electrolysis of a potassium acetate solution.
He mistook the hydrocarbon product of this
reaction for methane, and did not investigate
it further [2]. During the period 1847–1849,
in an effort to vindicate the radical theory of
Simulation,
Polyethylene,
Natural
gas.
organic chemistry, Hermann Kolbe and
Edward Frankland produced ethane by the
reductions of propionitrile (ethyl cyanide)
[3] and ethyl iodide [4] with potassium
metal, and, as did Faraday, by the
electrolysis of aqueous acetates. They,
however, mistook the product of these
reactions for methyl radical, rather than the
dimer of methyl, ethane. This error was
corrected in 1864 by Carl Schorlemmer,
who showed that the product of all these
reactions was in fact ethane [5]. The
principal use of ethane is in the chemical
industry in the production of ethene
(ethylene) by steam cracking. When diluted
with steam and briefly heated to very high
temperatures (900 °C or more), heavy
2
hydrocarbons break down into lighter
hydrocarbons, and saturated hydrocarbons
become unsaturated. Ethane is favored for
ethylene production because the steam
cracking of ethane is fairly selective for
ethane. Ethane can be used as a refrigerant
in cryogenic refrigeration systems. In this
experiment a detailed study is performed
about the process by means of simulation in
Aspen Hysys 2006 [8]. Though simulation
does not give the real world performance or
the real life production environment but if
the basic process is known and related data
are available, it surely is the best way by
which an individual can get ideas of an
industrial process without conducting any
experiment.
2.
Methodology:
The process of
producing ethane from synthesis gas is
simulated in simulation software Aspen
Hysys 2006. Aspen Hysys process simulator
is an essential element of Aspen
Tech’sAspenOne engineering applications.
It provides quite accurate results which
makes it an efficient simulator. It provides
comprehensive thermodynamics for accurate
determination of physical properties,
transport properties and phase properties. It
can be used to determine outlet process
conditions if the inlet conditions like
temperature, pressure and composition are
quantified. In this simulation for the
production of ethane, Peng-Robinson model
was used as fluid package.
2.1.
Process
Description:
The
production of ethane gas from synthesis gas
can be divided into two main processes:
production of synthesis gas from natural gas
and production of ethane from catalytic
reaction of ethylene. The production
synthesis can be divided into some main
process: steam treatment or primary
treatment of natural gas in a reformer,
conversion of residual natural gas in a
secondary reformer, catalytic conversion of
CO to CO2 and CO and CO2 removal. Here,
the simplified block diagram of ethane
production from synthesis gas is given
below.
3
Figure 01: Block diagram of ethane production from synthesis gas.
4
Figure 02: HYSYS Process Flow diagram for ethane synthesis from synthesis gas.
2.2. Reactions Involved:
In the
production of ethane from synthesis gas,
first synthesis gas was produced from
natural gas. This process includes three
conversion reactions and one equilibrium
reaction. The reactions in the reformer are as
follows:
CH4 + H2O = CO + H2
CH4 + 2H2O = CO2 + 4H2
CO2, converted to water and CO2 by the
following conversion reaction:
CO + ½ O2 = CO2
H2 + ½ O2 = H2O
Finally, the hydrogen gases react with
ethylene and produce ethane as gas in a
catalytic conversion reactor. The catalytic
hydrogenation of ethylene reaction is given
below:
The combustion reaction is as follows:
C2H4 + H2 = C2H6
CH4 + 2O2 = CO2 + 2H2O
The equilibrium water-gas shift reaction is
as follows:
CO + H2O ↔ CO2 + H2
After the production of synthesis gas, the
flue gas are treated in another conversion
reactor so that all of the hydrogen gas and
CO gas that are not converted to water and
Ethylene
2.2.1.
Hydrogen
Ethane
Production:
Hydrogen is produced by the reaction of
natural gas (mainly methane) with water.
But at first the sulfurous component must be
removed from the gas so that they cannot do
the catalyst poisoning [6]. The production of
hydrogen from the methane and water
reaction is a conversion reaction. This
5
reaction is carried out in a conversion
reactor that is called primary reformer. The
natural gas and superheated steam are fed to
the reformer at 770˚C in the presence of
15% Ni alloy catalyst. As a result, methane
converts to hydrogen, CO2 and CO. Here, as
a catalyst alumina or calcium aluminate can
also be used. Optimum temperature and
pressure are 750-850˚C and 5-40 bar
respectively. In the secondary reformer,
combustion of unconverted methane is
occurred. Methane reacts with oxygen in a
conversion reactor at 1000˚C in the presence
of nickel catalyst and produces CO2 and
H2O. Optimum pressure is lower than the
primary reformer. Here, the higher the
temperature the more rapid the conversion
process [11].
2.2.2.
Removal
of
Carbon
Monoxide: The carbon monoxide is
converted to carbon dioxide in a reaction
known as the water gas shift reaction. The
mixture of carbon monoxide and hydrogen
is known as water gas. The water-gas shift
reaction describes the reaction of carbon
monoxide and water vapor to form carbon
dioxide
and
hydrogen.
With
the
development of industrial processes that
required hydrogen, the demand for a cheaper
and more efficient method of hydrogen
production was needed [9]. This reaction is
performed in an equilibrium reactor at
temperature almost 1000˚C and pressure at
10 bars. Carbon monoxide reacts with steam
and produces carbon dioxide and hydrogen
[7]. But, the industrial scale water gas shift
reaction is conducted in multiple adiabatic
stages consisting of a high temperature shift
followed by a low temperature shift with
intermediate cooling system [10].
2.2.3. Synthesis of Ethane: The
remaining CO from the equilibrium reactor
is further converted to CO2 gas in a
converter [12]. Then the CO and CO2
produced are fed in a cooler with the
hydrogen gas. And the CO and CO2 are
separated with the help of mechanical
separation (that is not shown in the hysys
simulation). After mechanical separation,
hydrogen rich gas stream is fed in a catalytic
hydrogenation reactor along with ethylene.
Ethylene reacts with hydrogen at
temperature 1000˚C and pressure at 1 bar.
Then hydrogenation reaction occurs and
ethane is produced in the reactor. Here all of
the ethane is in vapor form.
3. Results and Discussions: By using
4454 kmole/hr natural gas, 4536 kmole/hr
reformer steam, 2245 kmole/hr air we have
produced 1.633×104 kmole/hr ethane.
From figure: 03 we see that ethane
production increases with increasing natural
gas molar flow rate till 12500 kmole/hr after
reaching this point ethane production rate
decreases with increasing molar flow rate of
natural gas and come to a steady rate when
natural gas molar flow rate is 3200
kmole/hr.
Form figure: 04 it has been seen that with
increasing temperature of product, ethane
production rate increases till the product
temperature is -100ºC further increase in
temperature does not affect the flow rate of
ethane and come to a stable position.
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Figure 03: Ethane-Molar flow vs. natural
gas-Molar Flow graph.
Figure 05: Product Molar Flow vs. Reformer
steam molar flow plot.
Figure 04: Ethane-Molar Flow vs. productTemperature graph.
Figure 06: Ethane molar flow vs. air molar
flow plot.
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Figure 07: Product-Molar Flow vs. natural
gas-Molar Flow graph.
Figure 09: Product-Molar Flow vs. reformer
steam-Molar Flow graph.
Figure 08: Mid-combust Temperature vs.
air-Molar Flow graph.
Figure 10: E-100 DUTY vs. natural gasMolar Flow graph.
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From figure: 05 in the case of increasing
reformer steam flow rate also increases
production flow rate. When reformer steam
flow rate reaches 8400 kmole/hr the
production flow rate is quite stable, in
further increase of reformer steam flow rate
does not affect product molar flow rate very
much.
Form figure: 06 ethane molar flow rate
increases with increasing air molar flow
rate.
From figure: 07, product molar flow rate
increases with increasing natural gas molar
flow rate till the value of natural gas molar
flow rate reached 4050 kmole/hr then further
increase in natural gas molar flow rate
decreases the product molar flow rate till
natural gas molar flow was 8450 kmole/hr
further increase in natural gas flow rate
increase the product flow rate very little.
From figure: 08, Mid-combust temperature
increases with air molar flow till 822.78
kmole/hr. Further increase in air molar flow
rate decreases the Mid-combust temperature.
From figure: 09 it has been seen that product
molar flow rate increases with reformer
steam molar flow rate.
From figure: 10, E-100 duty decreases with
increased molar flow rate of natural gas. E100 duty have its minimum value when
natural gas molar flow rate is 7653.6
kmole/hr, beyond this point E-100 duty is
remained steady.
4. Conclusion: Ethane is avery important
product. It is used in various purposes.
Ethane production rate depends on supplies
of natural gas (mainly methane), air flow
rate, inlet air temperature and superheated
steam flow rate. Hysys helped us a lot to
understand and to design of ethane
production from synthesis gas. The work
discussed in this project was focused
primarily to get optimum level of ethane
production and the factors affecting the
ethane production rate. In hysys simulation
we varied different factors and tried to fine
the optimum point where ethane production
rate is maximum for a given system. It is
quite similar to the real plant operation so
we can predict quite accurately about the
process by simulating it in Hysys.
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