INSTITUTO POLITECNICO NACIONAL

ESCUELA SUPERIOR DE INGENIERIA QUIMICA

E INDUSTRIAS EXTRACTIVAS

Optimizacion y Simulacion de Procesos

Final ProjectProduction of Dimethyl Ether (DME) via the Dehydration of Methanol

1 Process Notes

Dimethyl ether (DME) is used primarily as a propellant. DME is miscible with most organic solvents,it has a high solubility in water, and it is completely miscible in water and 6% in ethanol.1 Recently,the use of DME as a fuel additive for diesel engines has been investigated due to its high volatility(desirable for cold starting) and high cetane number.The production of DME is via the catalyticdehydration of methanol over an acid zeolite catalyst. The main reaction is as follows:

2CH3OH (CH3)2O+H2O (1)

methanol DME

In the temperature range of normal operation, there are no significant side reactions.

A preliminary process flow diagram for a DME process is shown in Fig. 1, in which 50,000 metrictons per year of 99.5wt% purity of DME product is produced. Due to the simplicity of the process,a stream factor of 0.95 (8375 h/yr) is used. Preliminary equipment summaries and process streaminformation are given in Tables 1 and 2, respectively.

2 Process Description

Fresh methanol, Stream 1, is combined with recycled reactant, Stream 13, and vaporized prior tobeing sent to a fixed-bed reactor operating between 250 C and 368 C. The single-pass conversion ofmethanol in the reactor is 80%. The reactor effluent, Stream 7, is then cooled prior to being sent tothe first of two distillation columns, T-201 and T-202. DME product is taken overhead from the firstcolumn. The second column separates the water from the unused methanol. The methanol is recycledback to the front end of the process, while the water is sent to waste water treatment to remove traceamounts of organic compounds.

3 Reaction Kinetics and Reactor Configuration

The reaction taking place is mildly exothermic with a standard heat of reaction, Hreac(25C) =

11700 kJ/kmol. The equilibrium constant for this reaction at three different temperatures is given,by Bondiera andNaccache,2 below:

T Kp473 K (200C) 92.6573 K (300C) 52.0673 K (400C) 92.6

The corresponding equilibrium conversions for pure methanol feed over the above temperaturerange is greater than 92%. The equilibrium constants reported above appear to be higher than those

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Figure 1: Production of Dimethyl Ether from Methanol.

calculated using standard Gibbs free energy and heat of formation data. The single-pass conversionof 90% used above may not be attainable due to the equilibrium constraint. A single-pass conversionof 80% may be more realistic goal for this design.

The reaction takes place on an amorphous alumina catalyst treated with 10.2% silica. There areno significant side reactions below 400C. Above 250C, the rate equation is given by Bondiera andNaccache2 as:

rmethanol = k0 exp

[

Ea

RgTr

]pmethanol (2)

where k0 = 1.21 106 kmol/(m3 catalyst filled reactor h kPa), Ea = 80.48kJ/mol, and pmethanol =

partial pressure of methanol (kPa).

Significant catalyst deactivation occurs at temperatures above 400C, and the reactor should be de-signed so that this temperature is not exceeded anywhere in the reactor. The design given in Fig. 1uses a single packed bed of catalyst which operates adiabatically. The temperature exotherm of 118C,occurring in the reactor, is probably on the high side and gives an exit temperature of 368C. However,the single-pass conversion is quite high (80%), and the low reactant concentration at the exit of thereactor tends to limit the possibility of a runaway. In practice, the catalyst bed might be split into twosections with an intercooler between the two beds. This has the overall effect of increasing the volume(and cost) of the reactor and should be investigated if catalyst damage is expected at temperatureslower than 400C. In-reactor cooling (shell and tube design) and cold quenching by splitting the feedand feeding at different points in the reactor could also be investigated as viable alternative reactorconfigurations.

4 Vapor-Liquid Equilibrium (VLE)

The DME-water binary system exhibits two liquid phases when the DME concentration is in the range34% to 93%. However, upon addition of 7% or more alcohol, the mixture becomes completely miscibleover the complete range of DME concentration. In order to ensure that this nonideal behavior issimulated correctly, it is recommended that binary VLE data for the three pairs of components beused in order to regress binary interaction parameters (BIPs) for a UNIQUAC thermodynamics model.If VLE data for the binary pairs are not used, the UNIFAC can be used to estimate BIPs. The resultshown in Table 2 a were obtained using UNIQUAC/UNIFAC on the Aspen Plus simulator. The SRKthermodynamics package was used for the enthalpy calculations in this simulation.

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Table 1: Prelimary Equipment Summary Table for DME Process.

Equipment P-201 A/B P-202 A/B P-203 A/B V-201 V-202 T-201 T-202 R-201

MOC Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel

Power (kW) 7.2 1.0 5.2 - - - - -

Efficiency 60% 40% 40% - - - - -

Type/drive Reciprocating/ Centrifugal/ Centrifugal/ - - - - -

Electric Electric Electric

Temperaure (C) 25.0 46.0 121.0 - - - - -

Pressure in (bar) 1.0 10.3 7.3 - - - - -

Pressure out (bar) 15.5 11.4 16.0 - - - - -

Diameter (m) - - - 0.96 0.85 0.79 0.87 0.72

Height/length (m) - - - 2.89 2.53 15.8 14.9 10.00

Orientation - - - Horizontal Horizontal Vertical Vertical Vertical

Internals - - - - - 22 SS Trays 26 SS Trays Packed bed

24 in spacing 18 in spacing section 7.2 m

high filled

with catalyst

Pressure (barg) - - - 9.3 6.3 9.6 6.3 13.7

Equipment E-201 E-202 E-203 E-204 E-205 E-206 E-207 E-208

Type Float. Head Float. Head Float. Head Float. Head Fixed TS Float. Head Float. Head Float. Head

Vaporizer Partial Cond. Reboiler Condenser Reboiler Condenser

Duty (MJ/h) 14,400 2,030 12,420 2,490 3,140 5,790 5,960 1,200

Area (m2) 99.4 171 101.8 22 100.6 83 22.7 22.8

Shell side

Max temp (C) 154 250 280 153 46 167 121 167

Pressure (barg) 14.2 14.1 12.8 9.5 9.3 6.6 6.3 6.6

MOC Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel

Phase Boiling liq. V Cond. Vapor Boiling liq. Cond. Vapor Boiling liq. Cond. Vapor L

Tube side

Max temp (C) 184 368 40 184 40 184 40 40

Pressure (barg) 10 12.9 4 10 4 10 4 4

MOC Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel Carbon steel

Phase Cond. Steam V L Cond. Steam L Cond. Steam L L

Table 2: Flow Table for DME Process.Stream No. 1 2 3 4 5 6 7 8

Temperature (C) 25 25 45 154 250 364 278 100Pressure (bar) 1 15.5 15.2 15.1 14.7 13.9 13.8 13.4

Vapor fraction (molar) 0 0 0 1 1 1 1 0.0798Mass flow (tonne/h) 8.37 8.37 10.49 10.49 10.49 10.49 10.49 10.49Mole flow (kmol/h) 262.2 262.2 328.3 328.3 328.3 328.3 328.3 328.3

Component mole flow (kmol/h)Dimethyl ether 0 0 1.5 1.5 1.5 130.5 130.5 130.5

Methanol 259.7 259.7 323 323 323 64.9 64.9 64.9Water 2.5 2.5 3.8 3.8 3.8 132.9 132.9 132.9

Stream No. 9 10 11 12 13 14 15 16 17Temperature (C) 89 46 153 139 121 167 50 46 121

Pressure (bar) 10.4 11.4 10.5 7.4 15.5 7.6 1.2 11.4 7.3Vapor fraction (molar) 0.148 0 0 0.04 0 0 0 0 0Mass flow (tonne/h) 10.49 5.97 4.52 4.52 2.13 2.39 2.39 2.17 3.62Mole flow (kmol/h) 328.3 129.7 198.6 198.6 66.3 132.3 132.3 47.1 113

Component mole flow (kmol/h)Dimethyl ether 130.5 129.1 1.4 1.4 1.4 0 0 46.9 2.4

Methanol 64.9 0.6 64.3 64.3 63.6 0.7 0.7 0.2 108.4Water 132.9 0 132.9 132.9 1.3 131.6 131.6 0 2.2

Utility mps cw mps cw mps cw cwEquipment E-201 E-203 E-204 E-205 E-206 E-207 E-208

Temperature in (C) 184 30 184 30 184 30 30Temperature out (C) 184 40 184 40 184 40 40

Flow (tonne/h) 7.22 297.1 1.25 75.12 2.9 142.6 28.7

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5 Report

The solution of the problem should be undertaken in groups of no more than three students. A veryimportant point to success is to break the problem into subtasks and thus divide the labor.

Requirements:

1. Language: Your report MUST be written in the English language, otherwise it will not beaccepted. It is worth remarking that this is a technical report, nonetheless a good command ofyour English language is required.

2. Summary: A short summary outlining the calculations undertaken, summary of key results,and an outline of the rest of the report.

3. Process Description: description of the process flowsheet. Detail any changes to flowsheetstructure or operating parameters required to make the problem feasible.

4. Process Flow Diagram and Stream Tables: give flowrate, composition, pressure, temper-ature, enthalpy flow, and phase of all streams.

5. Model Formulation: discuss any assumptions and/or simplifications introduced in formulationof the process model with a justification. Discuss the physical property models employed, withsupporting rational and data.

6. Flowsheeting Calculations: decomposition strategy, initialization and convergence methodused, particular problems encountered. Brief mathematical analysis of results, e.g., check overallbalances, estimate numerical errors, are design specifications satisfied?

7. Appendix: Aspen Plus input file (.inp) and report file (.rep) for your final converged flowsheet.

The submission deadline is May 29th, 2014 at 16.00 hrs. As part of the grade, you will be questionedconcerning to the project you are handing in. Note that no extension in the deadline will be grantedunder any circumstances! and, for every hour of delay a 35% of your grade will be deducted.

6 Problem Decomposition

You have to solve this complex modeling task within a very limited period of time. The key of yoursuccess will lie in working as a team and divide the labor. It is vital to build the flowsheet model inan evolutionary manner. As general guidelines:

Start with a unidirectional flowsheet, and then add in recycle streams one at a time.

Do material balance only calculations, and then material and energy balance calculations (AspenPlus allows you to switch off energy balance calculations).

Use simple models (e.g., SEP2), and then more rigorous models (e.g., RADFRAC).

Use the results of earlier calculations as initial guesses for more rigorous calculations, and puttight bound on manipulated variables. You may want to use different flowsheet decompositionsfor simpler models.

Do simulation and/or sensitivity calculations first and then design calculations.

Use ideal physical property models and then more rigorous/appropriate models later.

You can also spend some time on thinking about how the overall task can be split into a series ofsubtasks that can be worked on by individuals in parallel. For example, initially the following threetasks can be done in parallel:

1. Develop and converge material balance only model of the flowsheet (simple models see Fig.2).

2. Start building material and energy balance model for part of flowsheet (see Fig. 3) to provideinitial guesses for later.

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At the end of this phase, you should have good initial guesses (10%) for the material flows in theflowsheet and a decision concerning a good physical property model. In the second stage:

1. With a good guess for liquid feed to distillation column train, get rigorous models working forthe distillation train.

2. Continue to add models and recycle streams to the material and energy balance model, eachtime using the results of the previous calculation as initial guesses for the next. Add rigorousdistillation models last.

(a) Material Balance 1. (b) Material Balance 2.

Figure 2: Production of DME Material Balance: Simple Models.

Figure 3: DME Process Energy Balances.

References

[1] DuPont, DuPont Talks about its DME Propellant, Aerosol Age, USA, May and June 1976.

[2] Bondiera, J., and C. Naccache, Kinetics of Methanol Dehydration in Dealuminated H-Mordenite: Model with Acid and Basic Active Centres, Applied Catalysis 69, 139-148, 1991.

[3] McKetta, J. J.; Cunningham. Encyclopedia of Chemical Processing and Design, Marcel-Dekker:New York, 1976.

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Process NotesProcess DescriptionReaction Kinetics and Reactor ConfigurationVapor-Liquid Equilibrium (VLE)ReportProblem Decomposition