Journal of Engineering (JOE) ISSN: 2325-0224 Vol. 2, No. 3, 2013, Pages: 118-122 Copyright © World Science Publisher, United States www.worldsciencepublisher.org
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Simulation of Methanol Production from Synthesis Gas
Ruhul Amin1, Imran Hassan 1, Avijit Das1, Fatima Enam1*, Taslima Rahman1, Tania Hossain1 1
Department of Chemical Engineering
Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh *Corresponding Author:
[email protected] Abstract: An innovative process scheme to produce methanol from carbon monoxide and carbon dioxide is presented and accessed via simulation. In this configuration, the synthesis gas stream, composed by CO, CO 2, and H2 and fed to the methanol synthesis reactor, is produced by means of a reverse-water – gas-shift gas-shift by which a CO stream is partially converted in carbon dioxide. A simulation simulati on model is applied to t o define the proper operating operat ing conditions to achieve a chieve synthesis gas composition targets. The simulation results show that the plant configuration represents a feasible way to produce produce methanol using carbon carbon dioxide, dioxide, competitively competitively with the traditional process process in which which the synthesis synthesis gas is produced produced by a natural gas steam reforming unit. Plotted graphs show the dependency dependency of simulation on variables variables (temperature, pressure etc).This is useful to understand the changes that will occur if properties of each unit are changed. Since pilot plants are expensive for this type of case, HYSYS gives us an opportunity to observe and realize the plant and the process. Keywords: Methanol; Synthesis Gas; Production of methanol; Purification; Simulation; Recycling
Journal of Engineering (JOE) ISSN: 2325-0224 Vol. 2, No. 3, 2013, Pages: 118-122 Copyright © World Science Publisher, United States www.worldsciencepublisher.org
1. Introduction The methanol industry spans the entire globe, with production in Asia, North and South America, Europe, Africa and the Middle East. Worldwide, over 100 methanol plants have a combined production capacity of about 100 million metric tons (33 billion gallons or 125 billion liters), and each day more than 180,000 tons of methanol is used as a chemical feedstock or as a transportation fuel (60 million gallons or 225 million liters). The global methanol industry generates $36 billion in economic activity each year, while creating over 100,000 jobs around the globe. Methanol is produced naturally in the anaerobic metabolism of many varieties of bacteria, and is commonly present in small amounts in the environment. As a result, there is a small fraction of methanol vapor in the atmosphere. Over the course of several days, atmospheric methanol is oxidized with the help of sunlight to carbon dioxide and water [1]. In a typical plant, methanol production is carried out in three steps. The first step is to convert the feedstock natural gas into a synthesis gas stream consisting of CO, CO2, H2O and hydrogen. This is usually accomplished by the catalytic reforming of feed gas and steam. Partial oxidation is another possible route. The second step is the catalytic synthesis of methanol from the synthesis gas. Each of these steps can be carried out in a number of ways and various technologies offer a spectrum of possibilities which may be most suitable for any desired application. And, the final step is distillation of the final product to obtain required product specification [2-4]. Computer based simulation has been popular now-adays for different chemical engineering purposes. Though our task was to represent production of methanol in renowned aspen HYSYS software, making some assumptions and using hypothetical reactors we have performed the methanol production simulation. 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. Here, a short literature review of methanol synthesis and its kinetics and modeling is presented. The simulators for the process are developed based on the models from the literature. The simulators are used to evaluate the steady state behavior of the process even though the mutual agreement about the reaction mechanism has not been established and thus the reliability of the results cannot be evaluated. The evaluation of the results will be carried out later when the applicability of the models found from the literature will be studied with the process data.
2. Methodology 2.1 Process Description
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The process chain for producing methanol is rather simple including the following phases • Production of synthesis gas • Conversion of synthesis gas to methanol and • Distillation of the final product to obtain required product specification [3].
2.2 Production of synthesis gas Synthesis gas is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. CO, CO2 and H2 produced from various sources through steam reforming, partial oxidation, CO2 reforming auto thermal reforming [3]. Originally, synthesis gas for the production of methanol came from coal. Today, synthesis gas is most commonly produced from the methane component in natural gas, because natural gas contains h ydrogen. Three processes are commercially practiced. At moderate pressures of 4 MPa (40 atm) and high temperatures (around 850 °C), methane reacts with steam on a nickel catalyst to produce synthesis gas according to the chemical equation: CH4 + H2O → CO + 3 H2 This reaction, commonly called steam-methane reforming or SMR, is endothermic, and the heat transfer limitations place limits on the size of and pressure in the catalytic reactors used. Methane can also undergo partial oxidation with molecular oxygen (at atmospheric pressure) to produce synthesis gas, as the following equation shows: 2 CH4 + O2 → 2 CO + 4 H2 This reaction is exothermic, and the heat given off can be used in-situ to drive the steam-methane reforming reaction. When the two processes are combined, it is referred to as autothermal reforming. The high pressures and high temperatures needed for steam-reforming require a greater capital investment in equipment than is needed for a simple partial oxidation process; however, the energy-efficiency of steam-reforming is higher than for partial-oxidation, unless the waste-heat from partialoxidation is used [5-6]. In methanol synthesis, either CO or CO2 or both hydrogenates to methanol. The reactions are [5] CO + 2H2↔CH3OH (1) CO2+ 3H2 ↔CH3OH + H2O (2)
Also the water gas shift reaction may occur [7].The water gas shift reaction is CO + H2O → CO2 + H2 (3)
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The production of methanol is strongly influenced by
5000 4000 3000
q e2000 K 1000 0 0
500
1000
-1000
Temperature (°C)
Figure 02: Keq2 vs. Temperature plot
thermodynamics. The thermodynamic equilibrium limits the process to a low conversion and thus the recycling of the outlet is required if a high conversion is desired. The overall reaction is also strongly exothermic and thus a significant cooling is required. The recycling and the cooling are the main causes of the investment costs. The methanol synthesis reactors have been designed based on three principles- the high cooling demand, the low pressure drop and the favourable economy of scale [2]. Equilibrium constant of reactions depend largely on temperatures. For equation (1) and (2) equilibrium constant vs. temperature graphs are shown:
Mixer: Mixes two or more fluid stream input and gives one output. Heater: This unit heats up the feed to desired temperature by specifying the pressure drop and outlet pressure. Methanol synthesis reactor (MSR): It is an equilibrium reactor chosen from the library. It is used for the equilibrium reaction sets only. This reactor is valid both for exothermic and endothermic reactions. It is also possible to maintain a constant temperature in the reactor by removing or supplying heat. Cooler: It is used to cool the process stream to desired temperature. Separator: It is used for separating liquid and vapor mixture in given process stream at a constant temperature or by inputting or removing heat. It is also possible to add reaction sets here. Distillation column: It is the most versatile tool in HYSYS. Any kind of distillation is possible in this unit. It works by separating most volatile components from a process feed. [8]
1.00E-01
Figure 03: Process Block Diagram
8.00E-02 6.00E-02
2 q e K
3. Result and Discussions
4.00E-02 2.00E-02 0.00E+00 0
200
400
600
8 00
1000
-2.00E-02
Temperat ure (°C) Figure 01: K eq1 vs. Temperature plot
The production of methanol from synthesis gas may take place under the low or high pressure. The high pressure process operates typically at 200 atm and 350°C while the low pressure process operates at 50 - 100 atm and 220 - 250°C. The low pressure process has such economical and operational benefits that almost all the methanol plants built after year 1967 operate at the low pressure [9]. Some of the units used in HYSYS are given below:
After performing the simulation we found that the production of methanol (purity and production rate) depends on different parameters of the process. So by controlling these parameters optimum methanol production can be obtained [11]. From the simulation result we have observed the effect of several parameters on the production. Those effects are discussed below.
3.1 Purity of methanol on Feed stage to distillation The relation between composition of methanol in final product and feed stage to distillation can be easily realized from the plot. purity of methanol increases in higher feed stage from top and reaches at maximum and then again decreases. For this synthesis, optimum feed stage is the 6th stage.
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) % ( l o n a h t e M f o n o i t i s o p m o C
93.6 93.4 93.2 93 92.8 92.6 92.4 92.2 92 91.8 0
0.2
0.4
0.6
0.8
1
Fraction of Recycle
3.2 Conversion of CO and CO2 on reactor feed Figure 04: Composition of methanol in product versus feed stage of distillation
temperature
Conversion of both carbon monoxide and carbon dioxide both decrease with temperature increase [10]. But the reaction rate will be low at low temperature. Considering these two contradicting aspects, the reaction is carried out at 200oC.
Table 06: Variation of purity of methanol with fraction of recycle
3.4 Conversion of CO &CO2 on Feed pressure
Conversion of both CO and CO2 increases with feed pressure i.e. reactor pressure. But considering the safety purpose and operational cost, optimum pressure is chosen at 39.48 atm.
Figure 05: Conversion versus reactor feed temperature Figure 07: Conversion of CO and CO 2 versus reactor pressure
3.3 Purity of distilled methanol on fraction of recycle
3.5 Purity of methanol on separator feed temperature
Purity of methanol increases with increase in fraction of recycle. But there may be inerts present in the feed and to control the buildup of inert in the product, some reactor outlet has to be purged after separation and before recycle.
Distilled methanol purity increases with increase in separator feed temperature. But after a certain temperature, it becomes constant.
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Methanol synthesis is an important chemical process. In our paper, we have generated data based on the simulation performed in HYSYS. This data can help us understand the process in various situations in industrial practice. By varying the different parameters in this simulation environment, the effects of these parameters on methanol production are observed and the results are shown in graphical and form. Using the plots, the optimum conditions for methanol production can be easily found out. References
Figure 08: Composition of distilled methanol versus separator feed temperature
3.7 Methanol production rate on Feed pressure Methanol production rate increases with feed pressure i.e. reactor pressure. But considering the safety purpose and operational cost, optimum pressure is chosen at 39.48 atm.
Figure 10: Methanol production rate versus feed pressure
4. Conclusion
[1] Kurtz M., Wilmer H., Genger T., Hinrichsen O. and Muhler M. (2003) Deactivation of supported copper catalysts for methanol synthesis. Catalysis Letters, vol. 86, p. 77-80. [2] Lange J.-P. (2001) Methanol synthesis: a short review of technology improvements. Catalysis Today, vol. 64, p. 3-8. [3] Lovik I. (2001) Modelling, estimation and optimization of the methanol synthesis with catalyst deactivation. Doctoral thesis, Norwegian University of Science and Technology, 127p. [4] Yang R., Yu X., Zhang Y., Li W. and Tsubaki N. (2008) A new method of low temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2. Fuel, vol. 87, p.443-450. [5] Klier K. (1982). Methanol synthesis. Advances in Catalysis, vol. 31, p. 243-313 [6] Skrzypek J., Lachowska M., Grzesik M., Słoczyński J. and Novak P. (1995) Thermodynamics and kinetics of low pressure methanol synthesis. The Chemical Engineering Journal, vol. 58, p. 101-108. [7] VandenBussche K.M. and Froment G.F. (1996) A steady-state kinetic model for methanol synthesis and the water gas shift reaction on a commercial Cu/ZnO/Al2O3 catalyst. Journal of Catalysis, vol. 161, p. 1 -10. [8] Tijm P.J.A.,Waller F.J. and Browne D.M. (2001) Methanol technology developments for the new millennium.Applied Catalysis A: General, vol. 221, p. 275 – 282. [9] Gallucci F. and Basile A. (2007) A theoretical analysis of methanol synthesis from CO2 and H2 in a ceramic membrane reactor. International Journal of Hydrogen Energy, vol. 32, p. 5050-5058. [10] Graaf G.H. and Beenackers A.A.C.M. (1996) Comparison of two-phase and three-phase methanol synthesis processes. Chemical Engineering and Processing, vol. 35, p. 413-427. [11] Felder R.M. And R.W. Rousseau (1939) Elementary principles of Chemical Processess, 3rd edition,p. 592-595.