20th European Symposium on Computer Aided Process Engineering – ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) © 2010 Elsevier B.V. All rights reserved.

Simulation of commercial dimethyl ether production plant Ik Hyun Kima, Seunghyok Kima, Wonjun Chob, En Sup Yoona a School of chemical and biological Engineering, Seoul National University, San 56-1, Sillinm-dong Gwanak-Gu, Seoul 151-744, Korea bDME Project, R&D Division, Korea Gas Corporation, Incheon, Korea

Abstract The process of Dimethyl ether(DME) production consists of the four parts which are syngas synthesis from natural gas, absorbing CO2 from syngas, DME synthesis reactor and DME separation/purification. KOGAS has developed a process in which syngas is produced from natural gas and converted to DME using a single reactor[1]. For the construction of commercial scale DME plant, the modeling of one-step DME synthesis reactor is required prior to beginning the construction. Since then, the simulation of DME production process should represent actual operation data of pilot or demo scale plant. The simulations of reactor had been conducted using a one-dimensional steady-state model of a shell-and-tube type fixed-bed reactor[2]. Using the result of a reactor analysis, we have conducted simulations of all processes using steady-state models in Aspen Plus®. The simulation of process in this paper reflects the result of a reactor simulation and the real operation data of demo scale DME plant. And the simulation results are satisfied with the requirements for the basic design and engineering of commercial DME plant construction. Keywords: dimethyl ether, single step process, simulation, Aspen Plus

1. Introduction Dimethyl ether(DME, CH3OCH3) is the simplest ether and is considered a leading alternative to petroleum-base fuels and liquefied natural gas. Its physical properties are similar to liquefied petroleum gas(LPG) and can be stored and delivered using existing infrastructures with minor modifications. And DME is considered as a substitute for diesel fuel because it has a cetane number of between 55 and 60. Korea Gas Corporation(KOGAS) has developed a process in which syngas is produced from natural gas in proprietary auto-thermal reformer(Tri-Reformer) and then converted to DME in a single reactor. The 3,000 metric ton per day KOGAS DME plant will be built close to a remote natural gas field and will convert natural gas to dimethyl ether. For the basic design of commercial DME production plant, a commercial steady state process simulator, Aspen Plus, has been used to model the KOGAS DME process and a number of cases were run and reviewed with KOGAS. The previous simulation of process reflects only conceptual design of pilot and demo scale DME plants. In this paper, we have conducted simulations improved to reflect the data of experimental vapor-liquid equilibrium in the literature[3-7] and the test operation data of the KOGAS’ demo scale plant. This work will provide theoretical supports for the basic engineering of commercial plant construction.

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2. Process Description The process producing dimethyl ether will have four main sections: Reforming, CO2 Absorption and Recycle, DME Synthesis, and DME Separation and Purification. Within each of these sections various technologies might be used and design decisions made.

Figure 1. Simple block diagram of dimethyl ether production

In this process, natural gas and CO2 are combined with a recycle carbon dioxide stream and heated in a fired heater. The recycle CO2 comes from two locations: the first point is after the clean-up of the syngas and the second point is after the DME synthesis reaction. In the same fired heater steam is heated in a separate convection coil. The natural gas and recycled CO2 stream combines with the steam and flows to the burner of the Tri-Reformer. Oxygen is heated in a separate heater that uses high pressure steam for the heating function. The temperatures are maintained such that reaction occurs instantaneously and a significant flame front is established. The syngas exits the Tri-Reformer at temperatures of around 1000°C and pressures of about 3000 kPa. This hot gas is cooled down and compressed to 6000kPa and routed through the CO2 Absorber which is designed the extract the CO2 out of this stream using cold methanol. The decarboxylated syngas along with unconverted reaction gas is reheated up to temperatures of around 230°C and directed into the DME synthesis reactors. The gases leaving the DME synthesis reactors are cooled down in a series of sequential heat exchangers. The final heat exchanger is designed to cool the syngas down to about - 40°C. Most of the DME is condensed along with all of the methanol and water exiting the DME Reactor. The uncondensed gases, primarily comprising CO and hydrogen are recycled back to the DME synthesis reactors. A small amount of purge is maintained to control the inert levels in the recycle loop. The liquid DME/CO2/methanol/water stream is then depressurized down to 3500 kPa and flows into the CO2 Column. All the CO2 is rejected in the overheads. The bottoms are depressurized down to 1800 kPa and directed into a DME Column. The product DME is recovered in the overheads and directed to storage tanks. The bottoms are a small stream of methanol with water. This is combined with a portion of methanol solvent from the CO2 stripper and directed into the Methanol Dehydration Columns where the methanol is recovered. A portion of recovered methanol is recycled back to the CO2 absorber and the rest is directed into the Methanol Dehydration Reactor for converting methanol into DME. A large portion of CO2 stream from CO2 absorption and DME purification section is recycled back to the Reforming section and the rest is vented.

Simulation of commercial dimethyl ether production plant

3. Simulation Description KOGAS has constructed and operated demo scale plant since 2008. The primal simulation studies of the pilot or demo scale plant have been accomplished in terms of conceptual design of DME processes. So they still have limits for the basic design of commercial scale DME plant. This means that the primal simulation does not represent sufficiently detailed utilities and process considerations like steam generation rate in the fired heater, significant heat exchangers, pressure drop, reflecting demo plant experiences of the key units, and so on. Consequently, the purpose of simulation in this paper is the basic engineering of commercial DME plant. It should contain significant changes driven by the new process knowledge gained during the operation of the KOGAS DME Demo plant. The first, physical property model is improved by using Aspen Plus®’ SRK(Soave-Redlich-Kwong equation of state) for vapor phase system and RKSMHV2(Redlich-Kwong-Kwong equation of state with modified Huron-Vidal mixing rules) for two phase vapor-liquid system. The Mathias-Copeman and UNIFAC molecular Rv and Q parameters from the article are used where available[3]. For components which are not covered by this literature, we regressed data from another literatures[4-7]. The second, we conduct the modification of DME synthesis reactor. The model of DME reactor represented by a single yield reactor block(RYIELD) in Aspen Plus® with a FORTRAN user subroutine that determines the reactor output composition based on KOGAS’ specified CO conversion and product composition profile. It can accept and calculate five values which are one conversion and four selectivities, which is different from traditional definition of selectivity. It can also reflect the considered inert. The third, we conduct the modification of Tri-Reformer model changing from Gibbs reactor block(RGIBBS) to yield reactor block(RYIELD). It can reflect the experimental data of demo scale Tri-Reformer operation more precisely. The fourth, we conduct the adding heat exchangers, heaters, and coolers for realistic heat integration strategy. This represents good engineering practice, and makes it possible to properly calculate the operation costs. It can also represent the generation rate of high pressure, medium pressure, and superheated high pressure steam so that we can decide the specifications of fired heater. Finally, the flowsheet layout and simulation convergence is improved for good engineering practice and leading to accurate results.

4. Simulation Result The following Aspen Plus Modeling areas are being considered:

Component List Physical Properties Flowsheet Layout and Unit Operation Blocks Convergence Reactors

4.1. Overall Design and Documentation Issues There are significant design differences between the former case and the improved case as indicated in the table below:

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Table 1. Overall design and documentation issues

Design Element Former Case Improved Case Concern

Columns 5 5 Considerable impact on capital & operating costs

Process-Process Heat Exchangers 0 8 Increases process efficiency

Air Coolers 0 3 Decreases cooling water consumption

Compressors 1 2 Feasible utility costs 4.2. Component List The component list is the same as that used former case:

H2, O2, N2, H2O, CH4, CO, CO2, C2H6, C3H8, n-C4, i-C4, i-C5, Methanol, DME Our understanding is that most catalytic reactors producing methanol or DME also produced small amounts of higher molecular weight alcohols and ethers. But there is no document that KOGAS have measured these compounds during the Demo Plant operation before.

4.3. Physical Properties The improved model uses the same physical properties as the former case for the vapor-only situation but modifies three different physical property packages for vapor-liquid situations.

Table 2. Physical properties

Situation Former Case Improved Case Concern

Vapor-Only Flash SRK (A+ default)

SRK (A+ default) None

Vapor-Liquid Flash SRK, NRTL, RKSMHV2 (A+ default)

RKSMHV2 (Literature parameters)

Validated with the literature data

4.4. Unit Operation Blocks and Flowsheet Layout The number of unit operation block in the improved model flowsheet is significant different from the former case as shown in the table below. This is the result of design changes, the addition of all process heat integration, and the addition of a number of blocks used to characterize real process constitution.

Table 3. Unit operation blocks

CASE RadFrac Heater Compr MCompr FLASH2 FSPLIT Pump Former 5 15 1 0 4 3 3 Improved 5 44 1 1 6 4 5

CASE Mixer RGIBBS RYIELD RSTOIC SEP VALVE (Total) Former 7 2 1 1 1 2 (45) Improved 7 2 2 1 0 0 (78)

The lack of heat integration is unrealistic, does not represent good engineering practice, and makes it impossible to properly calculate the operation cost. So the ten heat exchangers and the three waste heat boilers are added in the improved case, it will impact the reduction of the steam generation and uses on the all process.

Simulation of commercial dimethyl ether production plant

4.5. Convergence The former simulation model contains three TRANSFER blocks. This has been for easy convergence when the model calculations of the various case studies or analysis are performed. But this use of TRANSFER blocks is unnecessary in basic engineering process and is not good simulation practice since they can lead to inaccurate results. In essence, it means that the model recycle streams are not fully connected and may not be properly converged. In addition there will be no warning from Aspen Plus that the results are inaccurate since the TRANFER blocks are outside of the normal mass balance checks. This is modified by creating a Broyden convergence block and making a partial revision of convergence options in Aspen Plus®. 4.6. Reactors The former and improved models contain the reactors shown in below. And all of reactors reflect new process knowledge from Demo Plant operation.

Table 4. Reactor type

Reactor Former Case Improved Case Concern

Sulfur Guard Bed none RGIBBS

Added for representing capital cost and pressure drop

Pre- Reformer RGIBBS RGIBBS

Tri- Reformer RGIBBS RYIELD

Modified for representing exactly real experiment data

DME Reactor

RYIELD (with Fortran subroutine)

RYIELD (with Fortran subroutine)

Methanol Dehydration Reactor

RSTOIC RSTOIC

The Sulfur Guard Bed that would be required in an actual plant is included in this case. This is a relatively minor improvement and means that the pressure drop of this equipment is taken into account. The Tri-Reformer using an RGIBBS as does in former case does not reflect actual operation condition or operation data of reaction. In the improved case, the Tri- Reformer satisfies the operation condition and represents the actual Demo plant reformer data more exactly. The former and improved model represent the DME reactor using the RYIELD/Fortran subroutine method developed during the feasibility study. So the reaction calculation in the DME synthesis reactor is same as before.

5. Conclusion The process simulation of the commercial DME plant, which has 3,000 metric ton per day capacity, has been completed. The former study on the DME synthesis reactor modeling provides the important specifications of overall process, and can help to reflect the real operation data more precisely. A commercial steady-state chemical process simulator, Aspen Plus®, is used to simulate the KOGAS DME plant in order to ensure the accuracy and reliability. Aspen Plus® includes the largest database of

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physical properties and provides easy application of modifying its property models from literatures. So we modify the built-in model of SRK and RKSMHV2 so that simulation results can be entirely consistent with the experimental vapor-liquid equilibrium data of the literatures. It is also conducted to modify essential unit operation blocks in Aspen Plus®’ in order to reflect the real data and behavior of DME demo plant. The improvement of convergence strategy and flowsheet layout has been conducted to make good simulation practices and lead to accurate results. Based on the result of this simulation, basic design and engineering will be in progress.

6. Acknowledgements This work was supported by ‘The study of reactor design and simulation for DME demo plant’ from Korea Energy Management Corporation(KEMCO) and Korea Gas Corporation(KOGAS).

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Analysis of a Pilot-scale Fixed-Bed Reactor for Dimethyl Ether(DME) Synthesis, Ind. Eng. Chem. Res, 47(13), 4553-4559

[3] Soren Dahl, Aage Fredenslund, and Peter Rasmussen, 1991, The MHV2 Model: A UNIFAC Based Equation of State Model for Prediction of Gas Solubility and Vapor-Liquid Equilibria at Low and High Pressure, Ind. Eng. Chem. Res., 30, 1936-1945

[4] Takashi Katayama, Kazunri Ohgaki, Goro Maekawa, Motojiro Goto, and Tamon Nagano, 1975, Isothermal vapor-liquid equilibria of acetone-carbon dioxide and methanol-carbon dioxide systems at high pressure, Journal of Chemical Engineering of Japan, Vol. 8, No. 2

[5] Torben Laursen and Simon Ivar Anderson, 2002, High-Pressure Vapor-Liquid Equilibrium for Nitrogen+Methanol, J. Chem. Eng. Data, 47, 1173-1174

[6] Elaine Chang, Jorge C.G. Calado, and Willian B. Streett, 1982, Vapor-Liquid Equilibrium in the System Dimethyl Ether/Methanol from 0 to 180 C and at Pressure to 6.7 MPa, J. Chem. Eng. Data, 27, 293-298

[7] E. Brunner, W. Hultenschmidt, and G. Schlichtharle, 1987, Fluid mixtures at high pressures; .Isothermal phase equilibria in binary mixtures consisting of (methanol+hydrogen or nitrogen or methane or carbon monoxide or carbon dioxide), J. Chem. Thermodynamics, 19, 273-291