Simulation of a Multi Functions Methane Steam Reforming Reactor [MSRR] Tomadir A. I. Hamed, Chemical Engineering Department, Sudan University of Science and Technology, Khartoum, Sudan [SUST]
Babiker K. Abdalla, Chemical Engineering Department, Karary University, Khartoum, Sudan Correspondence author
[email protected]
and 3, and the moderately exothermic water-gas shift (WGS) reaction 2 [2].
Abstract: One promising alternative to fossil fuels is hydrogen, which can mitigate the problem of energy supply and the ill effects of using hydrocarbons. The reaction of hydrogen with oxygen can release energy explosively in heat engines and quietly in fuel cells to produce water as the byproduct. In 2004, 50 million metric tons of hydrogen equivalent to 170 million tons of oil, were produced. Since, hydrogen storage and transportation is expensive, most hydrogen is currently produced locally, and used immediately. In this investigation data of an existing MSRR were used to simulate the process. Simulation of the process was performed using Aspen-HYSYS. Simulation results showed that as the reaction temperature increases the production of hydrogen increases until the temperature reaches 1370 oC then the production is slightly change with temperature.
CO+ H2OCO2 + H2 [ΔH = -41 kJ.mol-1]
(2)
III.
Partial Oxidation [PO]:
An alternative to the steam reforming is the partial oxidation, which is also endothermic. It may be carried out through Catalytic Partial Oxidation (CPO) or by Non-Catalytic Partial Oxidation (POX) or by an AutoThermal Reforming (ATR) [3, 4]. a.
(Non-Catalytic) (POX)
Partial
Oxidation
In non-catalytic partial oxidation reaction (4) is dominant. The absence of any catalyst means that the process is tolerant of a small degree of carbon formation and allows even higher operating temperatures. It is thus possible to operate partial oxidation without any steam addition. The resulting gas is carbon monoxide [CO] rich [5].
Introduction
More than 95% of the hydrogen for refinery use is nowadays produced via hydrocarbon steam reforming. A hydrogen economy has become an interest of the world. Steam reforming reactions will play a key role in new applications of synthesis gas and in a future hydrogen economy [1]. In broad terms, steam reforming (SR) is a process of producing hydrogen by combining steam and hydrocarbon; reacting in a reformer at temperatures above 500oC in the presence of a metal-based catalyst.
2CH4 + O2
b.
2CO + 4H2
(4)
Auto Thermal Reforming (ATR)
In catalytic auto-thermal reforming oxygen is added to the feed. The heat requirement for reaction (5) is largely met by the partial oxidation reaction (6) thus producing a lower H2/CO ratio in the syngas product. For this reactor considerable amounts of steam are required to suppress carbon formation. The absence of the metallurgical limitations of the catalyst tubes of a steam reformer allows higher operating temperatures thus reducing methane slip. At these higher temperatures the CO shift equilibrium is also more favorable to CO than in the case of the tubular steam reformer [5].
In principle, there are three types of reforming processes: II.
(1)
CH4 + 2H2OCO2 + 4H2 [ΔH = +165 kJ.mol-1] (3)
Keywords: Aspen-HYSYS, MSRR, Simulation, Fuel Cells
I.
CH4+H2O CO +3H2 [ΔH = +206 kJ.mol-1]
Steam Reforming:
Steam reforming of methane consists of three reversible reactions. Which are strongly endothermic; reactions, 1
1
Reforming (strongly endothermic) CH4 + H2 O CO + 3H2 (5)
IV.
Combustion (strongly exothermic) 2 CH4 + O2 2CO + 4H2 (6)
The Steam Reforming Plant
For many years, steam methane reforming (SMR) has been leading technology for generation of hydrogen in refining and petrochemical complexes.
Often times, auto thermal reforming is grouped under partial oxidation, because partial oxidation may be carried by a combination of non-catalytic oxidation and steam reforming. The advantage of partial oxidation and auto thermal reforming is that these processes are self-sustaining and do not require external provision of heat. However, they are less efficient in producing hydrogen [1].
Increase in vessel size/throughout.
The steam reforming plant (Fig (1)) consists of four basic sections:
The first is feedstock treatment where sulfur and other contaminants are removed. The second is the steam methane reformer, which converts feed stock and steam to syngas (mainly hydrogen and carbon monoxide) at high temperature and moderate pressure. The third section is the syngas heat recovery and incorporates CO shift reactor to increase the hydrogen yield. The final section is the raw hydrogen purification, in which modern plants employ a pressure swing adsorption (PSA) unit to achieve the final product purity. The reforming reaction between steam and methane is highly endothermic and is carried out using specially formulated nickel catalyst contained in vertical tubes situated in the radiant section of the reformer [2].
Limitations: Must use a clean, light hydrocarbon feed Cost of oxygen Limitation in H2 pressure Limitation in exit temperature Excess steam production Needs waste heat boiler to limit Bouduard carbon formation Challenges Reduce steam to carbon ratio, Increase CH4 conversion by increasing temperature, Carbon free burner operation,
Fig 1: Block Diagram of the Steam Methane Reforming (SMR) Plant (4 Main Steps) V. Reactor Specifications An equilibrium reactor (Fig (2)) is used in this simulation as the reactor for the reforming section, the feed which contains methane and steam is fed to the reactor at high temperature (816oC) because the
reaction is highly endothermic reaction, the steam/ methane ratio is chosen to be 7/1. The pressure drop in the reactor is about 1.73 bar. The feed conditions are shown in Table I.
2
Fig (2): Schematic Daigram of the Reforming Reactor Table II : Product Specification at 816 oC as feed temperature [6]. Temperature 603 oC
Table I : The Reforming Reactor Feed Condition [6]. Temperature 816 oC pressure
24.15 bar
Molar flow rate
2268 kgmole/hr
Methane/steam ratio
1/7
Methane%
12.5%
Steam%
87.5%
Pressure
22.05 bar
Molar flow rate
2464.0l kgmole/hr
Methane%
7.5%
Steam%
76.6%
CO%
3.9%
H2%
11.8%
Table III : Production of Hydrogen (%) at different Feed Temperatures Temperature (oC) Hydrogen Production (%) 0815.56 11.80 0626.67 15.50 1037.78 19.30 1148.89 22.90 1260.00 26.10 1371.11 28.70 1482.22 29.80 1593.33 29.98 1704.44 29.99 1815.56 29.99 1926.67 30.00
VI. Simulation Results and Discussion Simulation of the process was performed using Aspen HYSYS, Simulation results shows that as the temperature of the reactor increases the production of hydrogen increases until the temperature reaches 1370 oC then the production is slightly changed with temperature as shown in Table III and Fig (3). Also changing methane/steam ratio in different feed temperature (816 oC - 1370 oC) have changed hydrogen ratio in the outlet stream. Hydrogen production increases with the increase of the ratio till 4:1 ratio is reached then the production of hydrogen decreases. These results are shown in Table IV and Fig (4).
3
Hydrogen ratio (%)
35 30 25 20
15 10 5 0 0
1000 Feed
2000
3000
4000
Temperature (oC)
Fig (3): Variation of H2 production with temperature changes (H2 (%) vs T (oC))
Methane ratio (%) 05 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Table IV : Change in hydrogen production with change in methane/steam ratio at different feed Temperature H2 ratio(%) @ H2 ratio(%) @ H2 ratio(%) @ H2 ratio(%) @ H2 ratio(%) @ T=1500 oC T=1750 oC T=2000 oC T=2250 oC T=2500oC 09.85 12.75 13.58 13.63 13.64 11.32 15.76 20.00 23.46 24.85 12.07 16.88 21.70 26.36 30.64 12.61 17.55 22.76 27.93 32.86 13.06 18.20 23.60 29.02 34.25 13.45 18.76 24.47 29.93 35.38 13.80 19.34 24.96 30.74 36.36 14.10 19.79 25.56 31.32 37.10 14.40 20.20 26.10 31.99 37.90 14.63 20.57 26.58 32.59 38.63 14.84 20.89 27.02 33.13 39.27 15.01 21.17 27.39 33.59 39.82 15.15 21.39 27.69 33.96 40.24 15.24 21.54 27.90 34.20 40.47 15.24 21.58 27.96 34.22 40.33 15.15 21.47 27.76 33.79 39.19 14.88 21.04 26.96 31.96 34.33 14.20 19.73 23.81 24.91 24.99 11.81 13.51 13.36 13.64 13.64
4
H2 ratio@ T=1500 F H2 ratio@ T=1750F H2 ratio@ T=2000F H2 ratio@ T=2250F H2 ratio@ T=2500F 100
50
0
methane ratio in feed stream (%)
Hydrogen ratio in outlet stream(%)
45 40 35 30 25 20 15 10 5 0
Fig (4): change in hydrogen production with change in methane/steam ratio at different feed Temperature VII.
[3] S.P. Teh, B.K. Abdalla and J. Zaman, ‘Membrane Reactors for the oxidative coupling of natural gas’ RSCE’97, 13-15 Oct. 1997, UTM, Malaysia.
Conclusions:
The simulation results showed that the production rate of the syngas is very sensitive to both the reaction temperature and the hydrocarbon steam ratio. There is a temperature degree at which the reaction reaches a stagnant point; where no production increase is achieved. Also a critical hydrocarbon to steam ration exists where the production rate severely drops. The simulation also showed that the best solution for this multiple reaction system is the use of multifunction reactor (ATR) where the at least to reactions system will be carried in the reactor i.e. CPO and MSR will take place simultaneously.
[4] B.G. Teoh, B.K. Abdalla and J. Zaman ‘Optimize Reactor Development for the Partial Oxidation of Methane (POM) to Methanol’. RSCE’97, 13-15 Oct. 1997, UTM, Malaysia. [5] Christopher Higman ‘Synthesis Gas Processes for Synfuels Production’, June 1990, Trondheim,
presented at EUROGAS '90, [6] B. K. Abdalla, S.S.E.H. Elnashaie, 'A Modified Model for the Fluidized Bed Reactor for the Catalytic Steam Reforming of Methane'. J. King Saud Univ. 7, [Engng. Sci.]. (Special Issue), 271-282, (1995).
REFERENCES [1] Liu, J. A. “Kinetics, catalysis and mechanism of methane steam reforming”, Thesis Submitted to the Faculty of the Worcester Polytechnic Institute, Department of Chemical Engineering 2006. [2] Collodi, G. and Wheeler, F., “Hydrogen production via steam reforming with CO2 capture”, Chemical Engineering Transaction, volume 19, 2010.
5
