Modeling and simulation of a hybrid separation process for the carbon dioxide removal of the oxidative coupling of methane process

Modeling and simulation of a hybrid separation process for the carbon dioxide removal of the oxidative coupling of methane process

19th European Symposium on Computer Aided Process Engineering – ESCAPE19 J. JeĪowski and J. Thullie (Editors) © 2009 Elsevier B.V. All rights reserved...

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19th European Symposium on Computer Aided Process Engineering – ESCAPE19 J. JeĪowski and J. Thullie (Editors) © 2009 Elsevier B.V. All rights reserved.

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Modeling and simulation of a hybrid separation process for the carbon dioxide removal of the oxidative coupling of methane process S. Stünkel1, O. Litzmann, J.-U. Repke, G. Wozny Berlin Centre of Technology, Department of Process Engineering, Chair of Process Dynamics and Operation , 10623 Berlin, Germany, 1 E-mail: [email protected]

Abstract The oxidative coupling of methane (OCM) is a promising alternative for the oil-based production of olefins. The aim is to convert methane-containing natural gas catalytically to ethylene and open up a new feedstock for olefins and further organic synthesis products [1] , [2] . The whole process is designed modular and built up in a miniplant to investigate different new approaches. For realization in a short time period, but in a more efficient way, the entire process is divided into three units: reaction unit, purification unit and separation unit, which are designed simultaneously. Particular requirements for process conditions on the transitions had to be defined and were done by laboratory screenings and literature study. Due to the novel process design strategy, downstream process conditions affect the design specification for the catalyst and the reaction unit. In the article the purification section is discussed particular and a novel hybrid separation process for the CO2 removal is presented. An efficient and modern carbon dioxide separation process of a membrane and an amine unit was developed. The membrane unit has been modeled with Aspen Custom Modeler® (ACM), and was integrated in the Aspen Plus® process simulation. The amine unit was modeled with a rate-based absorption model, including an electrolyte NRTL approach [3] and concentration-based reaction kinetics [4] . The simulation results of the conventional amine process, the single membrane unit and the improved novel hybrid process are presented in this paper. Keywords: hybrid systems, amine, membrane, downstream OCM, gas permeation

1. Introduction Oxidative coupling of methane (OCM) is a novel technology for the conversion of natural gas to ethylene, reaching widespread attraction among various research groups in the last decade. OCM is a surface induced gas phase reaction and their overall yield is still limited up to 30%. Beside new catalysts, a concept for an integrated downstream process is necessary to overcome this limitation [5] . To hit this target in a more efficient way, the downstream process, the reactor and the catalyst are designed simultaneously. This novel strategy causes particular interactions during the design period between the downstream process, the reactor and the catalyst. Various processes for the OCM with integrated downstream concepts were proposed like the OXCO Process, the UCC Process, the ARCO Process, the Suzuki Process, the Turek-Schwittay Process or the Co-Generation Process [7] . All processes have the importance of the product separation under high pressure and the recycling of unreacted

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methane in common, which have major impact to the process economics. Investigation and process synthesis regarding to separation efficiency, energy consumption, operating and investment cost are rare but essential for industrial application of the OCM Process. A roughly process flow diagram of the OCM Process is presented in Figure 1. Recycle of CH , ( CO, C H

O CH Diluton Gas

OCM Reactor

Water Separation

)

Carbon Dioxide Separation

HO

Ethylene Separation

CO

Figure 1: Flowsheet of the OCM Process [5]

2. Process Synthesis in the OCM miniplant The OCM Process has not been applied in the industry yet. Besides general information on the reaction kinetics, the heat and mass transfer efficiency, fundamental studies of the process possibility, catalyst life time, and the effect of recycles and efficiencies of each unit are crucial for process implementation. This can only be gained by investigation of the real process. The miniplant technique is a well known technique in the scope of process synthesis, to obtain fundamental information experimentally. A flow sheet of the generalized miniplant layout is presented in Figure 2. Due to the simultaneous process design and caused by economical reasons, the downstream gives requirements to the reaction unit and the catalyst, especially to the yield, the C2 and CO2 selectivity, the methane conversation rate and dilution concentration.

Figure 2: synthesis Simplified–process flow diagram of and the OCM Process [6] Process simultaneous design construction

For the process synthesis, the whole OCM Process is divided into three general unit operations: the reaction unit, the purification unit and the separation unit (Figure 2). All of them are linked and investigated simultaneously. Therefore a design case for each unit has been defined for the composition, which is presented in table 1. The range of the process and stream conditions for the units is presented in table 2. Those conditions are defined for now by literature study and limited by our laboratory conditions, but have to determined and evaluated.

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Table 1: Defined feed gas concentration in [vol%] for the process section Unit

CH4

O2

C2H4

C2H6

CO2

H2O

Inert gas

Reaction

60 – 70

20

-

-

-

-

20 – 10

Purification

45

-

10

10

25

-

20 -10

Separation

60

-

13

13

-

-

14

Table 2: Process conditions for each unit Unit

Pressure range

Temperature range

Reaction Purification Separation

1 to 5 bar 1 to 35 bar Up to 35 bar

30 to 900 °C 30 to 100 °C Down to -100 °C

3. The downstream process The downstream process of the OCM consists of a phase separation unit, a carbon dioxide removal unit and a product separation unit, as recommended by various authors [7] . Concerning the simultaneous design and construction of the miniplant, the state of the art separation processes are taken as a base and is shown in Figure 2. The purification unit consists of an amine based absorption process for the carbon dioxide separation. The separation unit consists of a cryogenic distillation for the product separation. Due to the high energy consumption of the cryogenic distillation, the pressure is increased up to 35 bar to increase the boiling point of the hydrocarbons. Considering the limitation by the laboratory conditions, the pressure increase is limited too. Regarding the idea of using carbon dioxide as an inert gas for the dilution in the OCM reactor, the carbon dioxide removal step becomes even more important in the downstream process. Therefore, the purification section as the first downstream unit is picked out and the carbon dioxide removal is investigated. The purification unit The specification of this unit is to remove the carbon dioxide from the product stream totally as given in table 1. Such request is not unusual in the process industry, but attracts wide interest nowadays. Different approaches are known for those separation units like: absorption processes, adsorption processes, cryogenic distillation or membrane processes. This processes based on different physical and chemical principles: Absorption: physical or chemical absorption in liquids, caused by the gas solubility in the liquid or in combination with a superimposed chemical reaction. Adsorption: physical or chemical adsorption on a particle surface of the sorbent. Cryogenic separation: caused by the different condensation points of the gas Membranes: selective solubility and diffusion or molecular sieves caused by different molecule dimensions and Knudsen diffusion. The best developed and industrial applied technique is the absorption process with chemical or physical absorption liquids. A modern technique is the membrane separation, which has low selectivity yet and is mainly implemented as stand alone units for biogas or natural gas cleaning. The adsorption is available only for small gas streams and is hard to handle, concerning the sorbent regeneration. The cryogenic separation is applicable in combination with liquefaction and storage of the carbon dioxide. The range of the operation conditions are up to 60 bar for the pressure and from -20 °C to

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20 °C for the temperature, that is uneconomicaly without the combination of liquifaction and storrage. Only the membrane and the absorption techniques can be an opportunity for the OCM miniplant. Absorption processes The absorption technique for the carbon dioxide separation is well developed and industrial available. Physical absorption processes like the UOP Selexol® or the Lurgi Rectisol® Process are known, which is using dimethyl ether and cold methanol respectively. Those physical absorption processes causes high product losses of more than 30 vol%, due to a nearly similar solubility of the product and the carbon dioxide in the liquid. Therefore only chemical solvents like Monoethanolamine (MEA), Diethanolamine (DEA) and Methyldiethanolamine (MDEA) or a mixture of them are applicable for the purpose in the OCM miniplant. Those chemicals are used in amine scrubbing processes like the aMDEA® Process in different concentration ranges [8] . Standalone rigorous simulations for the absorption process of the miniplant were carried out in Aspen Plus®. As detergent 15 wt% MEA and 30 wt% MDEA solution were compared. Table 2 summarizes the basic engineering details for the column design, limited by laboratory conditions. Table 3: Technical and hydrodynamic operation conditions of the absorption process

Packing Column Gas Packing Packing Fheight diameter factor feed section capacity [m] [m] [-] [Pa0,5] kg/h] [m²/m³] 5 0.04 0.8 21 50 450

Maximum Liquid Top liquid load stream pressure [m³/m²h] [kg/h] [bar] 55 70 35

The in-built ELECNRTL model is used, with activity coefficients of the electrolyte NRTL approach for the liquid phase [3] and the Redlich-Kwong equation of state (EoS) for the gas phase is applied. Furthermore, concentration-based reaction kinetics is used and a rigorous rate-based model for absorption in packed columns could apply [4] . The carbon dioxide concentration can reduced to 15 vol% with 25 kW energy demand using MEA solution, whereas with MDEA the carbon dioxide concentration can reduced down to 7 vol% with only 5 kW energy input. This separation efficiency caused obviously by the solvent concentration, but they are corrosion limited [8] . Neither MEA nor MDEA as solvent can remove the carbon dioxide totally in a standalone absorption process for the given conditions. Membrane unit The advantages of a membrane unit are the easy operation and a short start up and shut down time caused by their small size [10] . Those units are very flexible in use, due to the modular design. For vapor/gas membrane separation different kinds of materials are available: Polymeric membranes: rubbery or glassy polymers, with different solubility and diffusion properties for carbon dioxide and hydrocarbons. Molecular sieves: absorption effects, separation by the different molecule dimensions. Glassy and rubbery polymeric membrane preferred for the carbon dioxide separation and hydrocarbon recovery [12] . In the investigated membrane unit a carbon dioxide selective membrane is applied. The membrane unit is modeled with the solubility ® diffusion model in Aspen Custom Modeler as a one dimensional, dense membrane. The Peng-Robinson EoS is used for the fugacity. As further non-ideal effects concentration polarization, the Joule-Thomson effect and pressure loss for low

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Reynolds numbers are considered. The membrane unit is calculated using geometry and permeability data for a GKSS flat sheet membrane module [11] . The carbon dioxide concentration could reduce with a one stage membrane unit of an area of 0.5 m² down to 14 vol %. The product losses in this unit are in the range of 30 vol%, similar to those of the physical absorption and for the purpose in the OCM miniplant not applicable. The two stage membrane process The application of a two stage membrane unit can reduce the product losses, whereas the carbon dioxide reduction is of the same range as for one stage membrane system: down to 14 vol%. The dimension and process conditions of the two stage process are shown in Table 3. Neither with the one stage membrane unit nor with the two stage membrane unit the carbon dioxide can be removed totally. Table 3: Technical requirements of the two stage membrane process

Membrane surface for the 1st Stage 1 m²

Membrane surface for the 2nd stage 0.5 m²

Pressure second stage 6 bar

Product losses 10 vol%

4. Conclusion – The hybrid separation process Facing the required purity of the product stream and the lack of an unlimited absorption column, the use of a membrane section for the pre - separation of carbon dioxide was applied successfully. Table 4: Energy balance of the hybrid process

Compressor power

Cooling power

Heating power

Pumping power

1.5 kW 3.5 kW 5 kW 1 kW This hybrid membrane amine process, Figure 3, can remove the carbon dioxide in the product stream totally. A two stage membrane system shows the lowest hydrocarbon losses and was combined with an absorption column to form a hybrid process. The overall energy consumption is listed in table 4. The ethylene loss could reduced to 10 vol% with an over all energy demand of 11 kW without consuming Figure 3: Hybrid carbon dioxide separation process auxiliary materials. The whole process has to be optimized economically and the simulation results have to be validated in the miniplant by experiments.

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5. Acknowledgements The authors acknowledge the support from the Cluster of Excellence "Unifying Concepts in Catalysis, coordinated by the Berlin Institute of Technology and funded by the German Research Foundation (DFG).

References [1] Kent R. Hall, "A new gas to liquids or gas to ethylene technology", Catalysis today, 2005, 106, 243 [2] Hugill et. al., "Feasibility study on the co-generation of ethylene and electricity through oxidative coupling of methane", Applied Thermal Engineering 25 (2005) 1259–1271 [3] Austgen et. al., "Model of Vapor-Liquid Equilibria for Auqeous Acid GasAlkanolamine Systems Using the Electrolyte-NRTL Equation", Ind. Eng. Chem. Res., 1989, 28, p.P. 1060 – 1073 [4] Aspen Plus Example Library “ Rate-Based Model of the CO2 Capture Process by MEA/MDEA using Aspen Plus” – 2008,Aspen Technolgy Inc. [5] M. Driess et.al., "Unifying Concepts in Catalysis" Evaluation Presentation of the Cluster of Excellence for the German Research Foundation , Bad Honef, 6. Juni 2007 [6] Stuenkel et. al., “Ethylene Production via Oxidative Coupling of Methane (OCM) – Investigation of alternative separation Processes”, 17th International Conference on Process Engineering and Chemical Plant Design, October 2008, Crakow, Poland [7] E.E. Wolf (Ed), Methane Conversion by Oxidative Processes, Fundamental and Engineering Aspects. Van Nostrand Reinhold, New York, 1992 [8] Kohl A. L., Nielsen R., „Gas Purification“, Gulf Pub Co, 5th edition, 1997 [9] Deibele, Dohrn “Miniplant-Technik”, WILEY-VCH Verlag, Weinheim 2006 [10] Brinkmann et al. “Membranverfahren in der Erdgasaufbereitung”, Chemie Ingenieur Technik, 2003, Vol. 75, Iss. 11, pP 1607 – 1611 [11] GKSS-Forschungszentrum Geesthacht in der Helmolz-Gesellschaft, Germany [12] Baker, R.W., Membranes for vapor/gas separation, Membrane Technology and research, Inc, 2006