Subject:
Simulation of Production of Dimethyl Ether from Methanol
From:
Katelyn Pate and Tom Bertalan
To:
Dr. Stephen Ritchie
9 December 2011 Dr. Ritchie, The goal of this project was to reproduce via simulation in Chemstations CHEMCAD the process described in appendix B1 of Turton et al. (“Turton”) (1). In the reaction described for this process, two equivalents of methanol are combined via dehydration to produce dimethyl ether (DME) and water. The simulation typically matched Turton’s values within 1 -2%. Unit operation parameters (page 5) (page 5),, stream compositions (page 6) (page 6),, and a process flow diagram (PFD) (page 7) (page 7) are are attached. Katelyn Pate Tom Bertalan CHE 481, Chemical Process Design I
CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
Approach
We first assembled a PFD visually, while simplifying some combinations of unit operations into single black-box simulators. In particular, the two combinations of trayed tower, reboiler, condenser, and reflux drum were each combined into single SCD S Distillation Columns. Some bypass loops were also excluded, such as a flow control loop around P-201, and a short bypass line beside E-202. Control hardware was also excluded (such as LICs that manipulated tower product flow rates). However, because pressure in streams 13 and 10 (the two tower distillate product streams) was higher than that inside the towers, pumps P-202 and P-203 were used after the reflux tees (not before it, as shown in Turton, Figure B.2 (1)) to achieve this higher pressure.
Model Decisions and Parameters Thermodynamics
Because the DME-water-methanol system exhibits nonideal behavior (1), we used a UNIFAC global kvalue model, in which UNIQUAC was used to e stimate the binary interaction parameters since actual VLE data was not available. Soave's modification of the Redlich-Kwong equation of state (2) was used as the global enthalpy model because of its suitability for multicomponent mixtures, and its citation by Turton. Heat Transfer Coefficients
All heat exchangers were first specified in design mode, in which one emerg ing stream temperature is 2
supplied, and then later respecified in rating mode, in which heat transfer area ( , in m ) and heat transfer 2
coefficient (, in W/m K) are supplied. Areas were taken from equipment tables on pages 942-943 of Turton, while coefficients were initially estimated using the heuristic values on page 345. With input streams per exchanger then set at the temperature, pressure, and composition specified in Turton's stream tables, we then manipulated these coefficients until the exiting streams had the desired temperatures. Reactor
The reactor R-201 was initially specified only through stoichiometry and conversion. However, fo r the final simulation, we switched to CHEMCAD's "Kinetic Reactor (KREA)", which makes bett er use of the kinetics information supplied by Turton. A possible alternative would be to use CHEMCAD's "Equilibrium Reactor (EREA)", since Turton also cites some equilibrium constants (3). Although Turton calls for a simpler packed-bed reactor, we used CHEMCAD's nearest equivalent of a plug-flow reactor, without specifying any complicating details like number of parallel tubes. We specified
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CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
adiabatic thermal mode, with a conversion of 0.798 with respect to methanol to achieve the desired input/output flow stoichiometry. 3
We used Turton's units of kPa, kmoles, k g, kJ, m , and hours where possible, but activation energy w as converted to units of 80480 kJ/kmol, from Turton's units of kJ/mol. Literature values for this activation energy depend on catalyst used, but could be as high as 154 kJ/mol (4) (5). We used exponential factors of 1, 10-6, and -6
10 for methanol, DME, and water re spectively (to signify that the reaction was first order in methanol and zeroeth order in the products, as claimed by Turton). CHEMCAD calculated a heat of reaction of -3106 MJ/h, which was 20% lower than Turton's suggested value of -3868 MJ/h, but which achieved the desired output temperature of about 364 C while keeping the °
maximum temperature within the reactor below Turton’s 400 C catalyst-denaturation limit, as depicted in °
Figure 1. 400 350 ] 300 C ° [ 250 e r u t 200 a r e p 150 m e T
100 50 0 0
0.5
1
1.5
2
2.5
3
Position in Reactor [m 3] Figure 1. Profile of temperature through positions in the simulat ed plug-flow reactor.
Convergence
By default, CHEMCAD will iterate through recycle calculations 40 times before declaring that the recycle calculation did or did not converge. We found this to be insufficient—in about half of our simulation runs, CHEMCAD reported this failure of convergence. So, in Run > Convergence > Max recycle iterations, we raised this number to 150, which seemed to consistently allow enough computation time for CHEMCAD to find a satisfactory solution.
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CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
Works Cited
1. Turton, Richard, et al., et al. Analysis, Synthesis, and Design of Chemical Processes. Upper Saddle River, New Jersey : Pearson Education, Inc., 2003. ISBN 0-13-064792-6. 2. Equilibrium constants from a modified Redkh-Kwong equation of state. Soave, Giorgio. Milan : Pergamon Press, October 1, 1971, Chemical Engineering Science, Vol. 27, pp. 1197-1203. 3. Kinetics of Methanol Dehydration in Dealuminated H-Mordenite: Model with Acid and Basic Active Centres. Bondiera, J. and Naccache, C. 1991, Applied Catalysis, Vol. 69, pp. 139-148.
4. Synthesis of dimethyl ether (DME) from m ethanol over solid-acid catalysts. Xu, Mingting, et al., et al. 1997, Applied Catalysis A: General, Vol. 149, pp. 289-301. 5. Intrinsic and Global Reaction Rate of Methanol Dehydration over γ -Al2O3 Pellets. Berčič, Gorazd and Levec, Janez. 1992, Ind. Eng. Chem. Res., Vol. 31, pp. 1035-1040.
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CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
Unit Ops Report Name Output pressure Efficiency
P-201 A/B 15.5000 0.6000
P-202 A/B 11.4000 0.4000
P-203 A/B 15.5000 0.4000
Name 2nd Stream T Out C U W/m2-K Area/shell m2 Calc Ht Duty MJ/h
E-201 184.0000 580.0000 99.4000 14421.0508
E-202
E-203 40.0000 234.0000 101.8000 -11792.8711
Name Reactor type Reaction phase Thermal mode Reactor volume m3 Conversion Molar Flow Unit Activ. E/H of Rxn Unit Volume Unit Overall IG Ht of Rxn (MJ/h) Mass unit Partial P unit Include holdup flag
R-201 2 1 2 3.0714 0.7989 1 4 1 -3105.8335
bar
27.6000 171.0000 2035.9839
1 6 1
Reaction Stoichiometrics and Parameters RateConst = 1.2100e+006 Act.E = 8.0480e+004 Hrxn = Comp Stoich. Exp.factor AdsorbFac. AdsorbE 1 -2.00e+000 1.0000e+000 0.0000e+000 0.0000e+000 2 1.00e+000 1.0000e-006 0.0000e+000 0.0000e+000 3 1.00e+000 1.0000e-006 0.0000e+000 0.0000e+000 Name No. of stages 1st feed stage Condenser mode Condenser spec Cond comp i pos. Reboiler mode Reboiler spec. Est. T top C Est. T bottom C Est. T 2 C Calc cond duty MJ/h Calc rebr duty MJ/h Calc Reflux ratio
E-208 40.0000 250.0000 22.8000 -1183.5540
T-201 22 12 6 0.9954 2 6 0.0070 45.9657 148.8026 45.9699 -3505.9812 2135.5090 0.5290
T-202 26 14 6 0.9593 1 6 0.0053 118.1277 166.1348 124.5812 -5676.3052 5567.3657 1.5605
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0.0000e+000 AdsorbExp. 0.0000e+000 0.0000e+000 0.0000e+000
CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
Stream Compositions Stream No. Temp C Pres bar Vapor mole fraction Total kmol/h Methanol Dimethyl Ether Water
1 25.0000* 1.0000* 0.00000 262.2000 259.7000 0.0000 2.5000
2 25.3936 15.5000 0.00000 262.2000 259.7000 0.0000 2.5000
3 45.2647 15.2000 0.00000 328.6240 323.4192 1.4011 3.8036
4 155.7232 15.1000 1.0000 328.6240 323.4192 1.4011 3.8036
Stream No. Temp C Pres bar Vapor mole fraction Total kmol/h Methanol Dimethyl Ether Water
5 251.7531 14.7000 1.0000 328.6240 323.4192 1.4011 3.8036
6 367.1346* 13.9000* 1.0000 328.6241 65.0305 130.5956 132.9980
7 280.1164 13.8000 1.0000 328.6377 65.0370 130.5994 133.0013
8 99.3156 13.4000 0.23317 328.6377 65.0370 130.5994 133.0013
Stream No. Temp C Pres bar Vapor mole fraction Total kmol/h Methanol Dimethyl Ether Water
9 91.2244 10.4000 0.27670 328.6377 65.0370 130.5994 133.0013
10 46.2951 11.4000 0.00000 129.7984 0.6004 129.1977 0.0000
11 148.8344 10.5000 0.00000 198.8395 64.4366 1.4017 133.0013
12 137.5024 7.4000 0.035898 198.8395 64.4366 1.4017 133.0013
Stream No. Temp C Pres bar Vapor mole fraction Total kmol/h Methanol Dimethyl Ether Water
13 119.2164 15.5000 0.00000 66.4244 63.7192 1.4011 1.3036
14 166.1362 7.6000 0.00000 132.3980 0.7005 0.0000 131.6974
15 50.0237 1.2000 0.00000 132.3979 0.7005 0.0000 131.6974
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CHE 481-1 Katelyn Pate | Tom Bertalan
2011 12 09 Simulation of Production of Dimethyl Ether from Methanol
Process Flow Diagram (PFD)
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