Direct Methane Aromatization Research Articles

Multilayer Operability Framework for Process Design, Intensification, and Modularization of Nonlinear Energy Systems

An optimization-based multilayer operability framework is introduced for the process design of nonlinear energy systems that are challenged by complexity and highly constrained environments. In the first layer of this framework, a mixed-integer linear programming (MILP)-based iterative algorithm considers the minimization of the footprint and the achievement of process intensification targets. Then, in the second layer, an operability analysis is performed to incorporate into the approach key features for optimality and feasibility, accounting for the system operation with changeable input conditions. The outcome of the framework consists of a set of modular designs, considering the aspects of both size and process operability. For this study, the nonlinear system is represented by multiple linearized models, resulting in low computational expense and efficient quantification of operability regions. The developed framework is applied to a membrane reactor for direct methane aromatization conversion to hyd...

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

AbstractTo secure future supply of aromatics, methane is a commercially interesting alternative feedstock. Direct conversion of methane into aromatics combines the challenge of activating one of the strongest C−H bonds in all hydrocarbons with the selective aromatization over zeolites. To address these challenges, smart catalyst and process design are a must. And for that, understanding the most important factors leading to successful methane C−H bond activation and selective aromatization is needed. In this review, we summarize mechanistic insight that has been gained so far not only for this reaction, but also for other similar processes involving aromatization reactions over zeolites. With that, we highlight what can be learnt from similar processes. In addition, we provide an overview of characterization tools and strategies, which are useful to gain structural information about this particular metal‐zeolite system at reaction conditions. Here we also aim to inspire future characterization work, by giving an outlook on characterization strategies that have not yet been applied for the methane dehydroaromatization catalyst, but are promising for this system.

Modeling and Design Optimization of Multifunctional Membrane Reactors for Direct Methane Aromatization.

Due to the recent increase of natural gas production in the U.S., utilizing natural gas for higher-value chemicals has become imperative. Direct methane aromatization (DMA) is a promising process used to convert methane to benzene, but it is limited by low conversion of methane and rapid catalyst deactivation by coking. Past work has shown that membrane separation of the hydrogen produced in the DMA reactions can dramatically increase the methane conversion by shifting the equilibrium toward the products, but it also increases coke production. Oxygen introduction into the system has been shown to inhibit this coke production while not inhibiting the benzene production. This paper introduces a novel mathematical model and design to employ both methods in a multifunctional membrane reactor to push the DMA process into further viability. Multifunctional membrane reactors, in this case, are reactors where two different separations occur using two differently selective membranes, on which no systems studies have been found. The proposed multifunctional membrane design incorporates a hydrogen-selective membrane on the outer wall of the reaction zone, and an inner tube filled with airflow surrounded by an oxygen-selective membrane in the middle of the reactor. The design is shown to increase conversion via hydrogen removal by around 100%, and decrease coke production via oxygen addition by 10% when compared to a tubular reactor without any membranes. Optimization studies are performed to determine the best reactor design based on methane conversion, along with coke and benzene production. The obtained optimal design considers a small reactor (length = 25 cm, diameter of reaction tube = 0.7 cm) to subvert coke production and consumption of the product benzene as well as a high permeance (0.01 mol/s·m2·atm1/4) through the hydrogen-permeable membrane. This modeling and design approach sets the stage for guiding further development of multifunctional membrane reactor models and designs for natural gas utilization and other chemical reaction systems.

Techno-Economic Assessment of Benzene Production from Shale Gas

The availability and low cost of shale gas has boosted its use as fuel and as a raw material to produce value-added compounds. Benzene is one of the chemicals that can be obtained from methane, and represents one of the most important compounds in the petrochemical industry. It can be synthesized via direct methane aromatization (DMA) or via indirect aromatization (using oxidative coupling of methane). DMA is a direct-conversion process, while indirect aromatization involves several stages. In this work, an economic, energy-saving, and environmental assessment for the production of benzene from shale gas using DMA as a reaction path is presented. A sensitivity analysis was conducted to observe the effect of the operating conditions on the profitability of the process. The results show that production of benzene using shale gas as feedstock can be accomplished with a high return on investment.

An optimization-based operability framework for process design and intensification of modular natural gas utilization systems

This paper introduces a novel operability-based framework for process design and intensification of modular systems. This framework is based on nonlinear programming concepts, including bilevel optimization, and extends the application domain of classical operability approaches to process simulator runs (Aspen Plus) for obtaining the relationship between input and output spaces, in addition to first-principles models. As potential modular applications of the proposed approach, a catalytic membrane reactor (MR) for the direct methane aromatization (DMA) conversion to fuels and chemicals, and a natural gas combined cycle (NGCC) system for power generation, are addressed. These applications are especially motivated by the advent of the shale gas revolution that allowed the recent discovery of stranded shale/natural gas deposits at different shale formations in the U.S. The utilization of such deposits could be capitalized by employing modular technologies for the production of power, value-added chemicals and fuels at the site. However, the design and intensification of modular energy systems is a challenging task as these processes are represented by complex and nonlinear models. The obtained results indicate that the proposed framework has the potential to accelerate the realization of the concept of modular manufacturing with reduced footprint and maximized efficiency, resulting in future cost savings. In particular for the NGCC process, the results provide the gas and steam turbine cycle conditions that could enable modular plants that are about three orders of magnitude smaller in terms of power generation than typical operating processes.

Effect of Bed Height on the Performance of a Fixed Mo/HZSM‐5 Bed in Direct Aromatization of Methane

AbstractThe catalyst‐overloading strategy is often employed to extend the catalytic lifetime of fixed catalyst bed reactors in chemical processes. The applicability of this strategy to the Mo/HZSM‐5 catalyst for non‐oxidative methane dehydroaromatization was confirmed for the first time by examining the influence of bed height on the catalytic stability and lifetime aromatic productivity of an integral fixed Mo/HZSM‐5 bed at a defined temperature and different CH4 superficial velocities. Increasing the bed height proved to have a very limited effect on extending either the stable period of aromatics production of the bed or its lifetime aromatics productivity. Simultaneous coke formation over all layers of the bed was also confirmed to be responsible for rapid deactivation of the whole bed, and pyrolysis of the intermediate C2H4 was found to be the dominant route to coke formation.

Novel operability‐based approach for process design and intensification: Application to a membrane reactor for direct methane aromatization

This article introduces a novel operability‐based approach for process design and intensification of energy systems described by nonlinear models. This approach is applied to a membrane reactor (MR) for the direct methane aromatization (DMA) conversion to benzene and hydrogen. The proposed method broadens the scope of the traditional path of the operability approaches for design and control, mainly oriented to obtain the achievable output set (AOS) from the available input set, and compare the computed AOS to a desired output set. In particular, an optimization algorithm based on nonlinear programming tools is formulated for the calculation of the desired input set that is feasible considering process constraints and intensification targets. Results on the application of the operability method as a tool for process intensification show reduction of the DMA‐MR footprint (≈77% reactor volume and 80% membrane area reduction) for an equivalent level of performance, when compared to the base case. This case study indicates that the novel approach can be a powerful tool for process intensification of membrane reactors and other complex chemical processes. © 2016 American Institute of Chemical Engineers AIChE J, 63: 975–983, 2017

Tuning external surface of unit-cell thick pillared MFI and MWW zeolites by atomic layer deposition and its consequences on acid-catalyzed reactions

The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) zeolites are 2-dimensional (2D) catalytically active materials made by pillaring of layered MCM-22(P) and multilamellar MFI precursors, respectively. The single- or near single-unit-cell thick 2D microporous layers in PMWW and PMFI expose comparable number of external surface acid sites (i.e., acid sites enclosed in mesopores between zeolitic layers) to those in micropores, which become important or dominant contributor to the catalytic properties. Although the acidity and catalytic activities of PMWW and PMFI have been studied, modification of their external surfaces and its implications on catalytic reactions are not available. In the present study, we report the tailoring of external surfaces of PMWW and PMFI zeolites by atomic layer deposition (ALD) of silicon (ALD-Si) and aluminum (ALD-Al), respectively. The textural, acidic and catalytic properties of the ALD modified pillared zeolites were investigated using a variety of characterization methods. ALD-Al and ALD-Si modifications kept micropore almost intact, but resulted in significant reduction in mesopore volume and considerable changes in external surface composition and acidity. The catalytic tests showed that intrinsic catalytic behavior of Brønsted acid sites in ALD modified pillared zeolites was similar to their parent counterparts. In diffusion constrained parallel reactions, ALD of Al- or Si-species altered catalyst selectivity. In addition, ALD of Si-species on PMFI zeolite deactivated surface active sites, which resulted in improved catalytic activity in direct methane aromatization reactions under optimal ALD cycles. The study exemplified for the first time that ALD is an effective tool for tuning the surface properties of 2D unit-cell thick zeolites.

Spatial distribution and catalytic performance of metal–acid sites in Mo/MFI catalysts with tunable meso-/microporous lamellar zeolite structures

The lamellar MFI zeolite with tunable meso-/microporosity was prepared by a dual template synthesis method. Implications of the tunable meso-/microporosity on the spatial distribution and catalytic performance of metal–acid sites in Mo/lamellar MFI catalysts were studied. The number of free Brønsted acid sites increases linearly, accompanied by a linear decrease in Mo dispersion, with decreasing external surface areas in catalysts. The Mo–acid active sites coexisted on the external surface or mesopores of the catalysts linearly depends on the hierarchy factor, being the product of relative micropore volume and relative external surface area. The reaction rate and product selectivity of Mo/lamellar MFI in direct methane aromatization reactions are consistent with the trends of changes in zeolite porosities. These results imply that a balance between the meso- and microporosity in zeolite and the resultant active site distribution can be realized for desirable catalysis by simple one-step dual template synthesis of zeolites.

Nonlinear Operability of a Membrane Reactor for Direct Methane Aromatization

This paper presents the first operability studies for the computation of catalytic membrane reactor feasible regions for process design and control. The equilibrium-limited direct methane aromatization conversion process to hydrogen and benzene is addressed as the application. For the operability analysis, a nonlinear model that describes the membrane reactor system is derived. This analysis is performed for different reactor designs, membrane characteristics and flow configurations. The obtained results demonstrate the capabilities of the nonlinear operability approach to provide novel findings on the design of emerging energy systems.

Textural and catalytic properties of Mo loaded hierarchical meso-/microporous lamellar MFI and MWW zeolites for direct methane conversion

The consequences of meso-/microporous zeolite structures on distribution and activity of active sites (Brønsted acid and metal sites) in molybdenum (Mo) impregnated lamellar MFI and lamellar MWW zeolites were investigated in direct methane aromatization (DMA) reactions. In comparison with their microporous analogies (Mo/MFI and Mo/MWW), the mesopores in the Mo/lamellar MFI and Mo/lamellar MWW zeolites facilitated the dispersion of Mo in zeolite micro-channels. The catalytic DMA reactions showed that the Mo loaded lamellar MFI and lamellar MWW zeolites enabled efficient methane conversion, aromatic product formation, and coke accumulation in the initial stage of reaction, while their performances were similar to those of Mo/MFI and Mo/MWW catalysts in the long term run. A plausible explanation to this scenario is that active sites in Mo/lamellar MFI and Mo/lamellar MWW can be accessed by reactants through both micropores and mesopores. The facilitated transport and accessibility to active sites by mesopores accounted for initial fast reaction and deactivation rates of Mo loaded lamellar zeolites. After mesopores were clogged by deposited coke in DMA reactions, micropores of zeolites were responsible for the long term catalyst performances, which were similar to Mo loaded microporous zeolites.

Some technological aspects of methane aromatization (direct and via oxidative coupling)

The investigations on transformation of methane to benzene and naphthalene have been carried out in aim to verify and supplement earlier reported data and on this basis to estimate real industrial perspectives of the CH 4 aromatization concept, the main challenges and barriers. Methane aromatization (direct and via oxidative coupling) has been studied over Mo/HZSM-5 catalyst used both for direct methane dehydroaromatization and for aromatization of methane oxidative coupling (OCM) products. The effects of Mo content in the catalyst, temperature, space velocity, the presence of CO 2, CO, H 2O, C 2H 4, C 2H 6 and their mixtures in the feed have been studied. The effectiveness of the catalyst regeneration in the air was also examined. All results were confronted with the literature data and analyzed from technological point of view. It was confirmed that direct CH 4 aromatization process was characterized by a low CH 4 single-pass conversion, low single-pass yields of the main products (benzene, hydrogen and naphthalene) and a low catalyst stability (rapid catalyst deactivation). Various possible technological schemes were analyzed. It was concluded that real industrial chances of direct methane aromatization or aromatization via OCM would depend largely on the advancement in the cost-effective separation techniques. The methane aromatization concept was also confronted with other methane conversion processes.