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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, Simulation, Polyethylene, Natural 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

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

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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.

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Figure 01: Block diagram of ethane production from synthesis gas.

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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

The combustion reaction is as follows:

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

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:

C2H4 + H2 = C2H6

Ethylene Ethane

2.2.1. Hydrogen 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

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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 04: Ethane-Molar Flow vs. product-Temperature graph.

Figure 05: Product Molar Flow vs. Reformer steam molar flow plot.

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 08: Mid-combust Temperature vs. air-Molar Flow graph.

Figure 09: Product-Molar Flow vs. reformer steam-Molar Flow graph.

Figure 10: E-100 DUTY vs. natural gas-Molar 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. E-100 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.

References:

[1] Beychok, M.R., “Process and environmental technology for producing SNG and liquid fuels”, U.S. EPA report EPA-660/2-75-011, May 1975

[2] Faraday, Michael (1834). "Experimental researches in electricity: Seventh series". Philosophical Transactions 124: 77–122.

[3] Kolbe, Hermann; Frankland, Edward (1849). "On the products of the action of potassium on cyanide of ethyl". Journal of the Chemical Society 1: 60–74.

[4] Frankland, Edward (1850). "On the isolation of the organic radicals". Journal of the Chemical Society 2: 263–296.

[5] Schorlemmer, Carl (1864). Annalen der Chemie 132: 234.

[6] Twygg, Martyn V. (1989). Catalyst Handbook (2nd Edition). Oxford University Press.ISBN 1-874545-36-7.

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[7] Z Kowalczyk, S Jodiz and J Sentek, Applied Catalysis A: Genaral, 138, 83, 1996.

[8] HYSYS. Aspen HYSYS user guide. Aspen Technology Inc. www.aspentech.com.

[9] Lamm, editors, Wolf Vielstich, Hubert Gasteiger, Arnold (2003). Handbook of fuel cells : fundamentals, technology, applications (Reprinted ed.). New York: Wiley. ISBN 0-471-49926-9.

[10] Smith R J, Byron; Muruganandam Loganthan, Murthy Shekhar Shantha (2010). "A Review of the Water Gas Shift Reaction". International Journal of Chemical Reactor Engineering 8: 1–32.

[11] Drysdale, Dougal (2008). "Physics and Chemistry of Fire". In Cote, Arthur E. Fire Protection Handbook 1 (20th ed.). Quincy, MA: National Fire Protection Association. pp. 2–18. ISBN 978-0-87765-758-3.

[12] G Austin, Shreve’s Chemical Process Industries, 5th edition, McGraw-Hill International Edition, 1984.