Robin J. White

Poly(oxymethylene) dimethyl ether synthesis – a combined chemical equilibrium investigation towards an increasingly efficient and potentially sustainable synthetic route

Polyoxymethylene dimethyl ethers (denoted hereon as OME) are potential sustainable fuels (e.g. as a diesel substitute). In this paper, the fundamental analysis of a potentially, sustainable synthetic OME system is presented (i.e. based on CH3OH synthesised from H2 and recycled CO2). In this context, a multicomponent thermodynamic vapour–liquid equilibrium model, based on CH3OH as the educt and source of H2CO for OME synthesis, is described. A thermodynamic equilibrium mathematical model for this complex (i.e. a 29 reaction network) CH3OH–H2CO equilibrium system is presented, capable of solving the sequential chemical and phase equilibrium, importantly considering all components in the reaction system including poly(oxymethylene) hemiformals and poly(oxymethylene) glycols. A theoretical efficiency evaluation indicates that the proposed anhydrous route is potentially more attractive than the conventional synthesis (i.e. based on dimethoxymethane and trioxane). To substantiate these theoretical investigations, a complimentary experimental batch OME synthesis is also presented, providing validation for the presented thermodynamic model. An initial kinetic analysis of the OME synthesis over different commercial catalysts is also highlighted. Our presented findings reliably describe the synthesis equilibrium with respect to our experimentally obtained results. The presented work supports further an operating OME synthesis framework based on CH3OH and H2CO and highlights the requirement for innovative process design regarding feed preparation, reactor technology, and product separation/fractions recycling.

Highly correlated ab initio thermodynamics of oxymethylene dimethyl ethers (OME): formation and extension to the liquid phase

Oxymethylene dimethyl ethers, of the structure CH3(OCH2)nOCH3, denoted as OMEn are receiving increasing interest (where n = 2–5) in a range of important applications including as sustainable fuels and solvents (e.g. as derived from green methanol). However, limited thermodynamic information from computational studies exists in the literature regarding their formation in the gas and liquid phases. In this context, this report describes the principal thermodynamic functions of gaseous and liquid phase OME formation derived from B3LYP-D3(BJ)/def2-TZVPP optimised structures and a series of CCSD(T) and MP2 calculations. The generated total energies are almost of CCSD(T)/A′VQZ quality, the “gold standard” of computational chemistry. Thermal corrections to enthalpy and entropy were included on the basis of analytical BP86-D3(BJ)/def-TZVP frequencies and empirical corrections for low anharmonic C–O–C–O torsional vibrations/hindered rotations and due to the neglect of other conformers/enantiomers. This yielded corrected values for the standard entropy S° of gaseous OMEn (n = 2–7). With the well-established experimental formation enthalpies of dimethyl ether (i.e. OME0) and OME1, the formation enthalpies of OME2–7 were obtained from those and the isodesmic reaction enthalpy of nOME1 → OMEn + (n − 1)OME0. Overall, an error bar on those gas phase values of <1 kJ mol−1 is assigned. From the known and extra- or interpolated phase change thermodynamics, the standard formation enthalpy H°, and the standard entropy S°, as well as the heat capacity cp were established for the liquid mixture of OME2–7. The internal consistency of these data was assessed based on the plots of H°/S° vs. n, presenting linear regressions and correlation coefficients very close to unity. Data quality was also evaluated against published combustion energies, suggesting our values are currently the most reliable, internally consistent dataset that should be used in future investigations for the design of sustainable ether-based fuels and chemicals.

Economics & carbon dioxide avoidance cost of methanol production based on renewable hydrogen and recycled carbon dioxide – power-to-methanol

The synthesis of sustainable methanol based on renewable electricity generation, sustainable hydrogen (H2) and recycled carbon dioxide (CO2) represents an interesting sustainable solution to integrated renewable energy storage and platform chemical production. However, the business case for this electricity based product (denoted hereafter as eMeOH) under current market conditions (e.g. vs. conventional fossil methanol (fMeOH) production cost) and the appropriate implementation scenarios to increase platform attractiveness and adoption have to be highlighted. The aim of the following study was to perform a dynamic simulation and calculation of the cost of eMeOH production (where electricity is generated at a wind park in Germany), with comparison made to grid connected scenarios. Consideration of these scenarios is made with particular respect to the German energy market and potential for the reduction in fees/taxes (i.e. for electrolyser systems). This evaluation and indeed the results can be viewed in light of European Union efforts to support the implementation of such technologies. In this context, CO2 is sourced from EU relevant sources, namely a biogas or ammonia plant, the latter profiting from the resulting credit arising from CO2 certificate trading. Variation in electricity cost and the CO2 certificate price (in the presented sensitivity study) demonstrate a high cost reduction potential. Under the energy market conditions of Germany it is found that eMeOH production costs vary between €608 and 1453 per tonne based on a purely grid driven scenario, whilst a purely wind park supplied scenario results in €1028–1067 per tonne. The reported results indicate that the eMeOH production cost in Germany is still above the present (although variable) market price, with the economical evaluation indicating that electrolyser and H2 storage represent the lion share of investment and operational cost. Substitution of fMeOH results in CO2 avoidance costs between €365 and 430 per tonne of CO2eq avoided for green methanol produced in Germany. The presented assessment indicates that the eMeOH production cost in Germany will reach market parity in ca. 2030–2035 with the price for the avoidance of CO2eq turning from a cost to a benefit at around the same time. Optimistically, the cost is predominantly influenced by rapidly reducing renewable electricity costs as is already the case in South American and Arabic countries offering the potential for methanol production at a cost of <€500 per tonne.

Describing oxymethylene ether synthesis based on the application of non-stoichiomsetric Gibbs minimisation

The synthesis of short chain poly oxymethylene dimethyl ethers, also known as oxymethylene ethers (OME; molecular formula: H3CO–(CH2O)n–CH3 where n = 1–8) is described through the application of non-stoichiometric Gibbs minimisation (NSGM) to a synthesis based on methanol and anhydrous formaldehyde. The presented approach shows several synthesis efficiency and economic advantages as demonstrated through a simulation platform based on MATLAB® (where the two main reactors models are implemented) and the NSGM, which utilises stochastic global optimisation (SGO) to perform an unconstrained minimisation and convergence of the complex OME reaction system (comprising >31 reactants and also recycling of non-reacted components). A complimentary experimental validation is provided for the OME reaction equilibrium model based on the use of different feeds, namely 1) CH3OH/CH2O and 2) H3C–O–(CH2O)1–CH3/(CH2O)3. The presented results demonstrate the robustness of the applied NSGM for this multi-reaction system. With regard to the overall evaluation of the presented process, key performance indicators (KPIs) are discussed based on the material balance results of the simulation platform. A cost model for the OME synthesis process based on different feeds is also presented based on an annual production of one million metric tonnes of OME3–5. The cost of 571 € per tonne demonstrates the economic potential of the presented OME production process.

A hybrid description and evaluation of oxymethylene dimethyl ethers synthesis based on the endothermic dehydrogenation of methanol

Concerning oxymethylene dimethyl ethers (e.g. a class of potential oxygenated diesel substitutes; denoted as OME), this work utilises a hybrid process model based on methanol (MeOH) and its partially selective conversion to anhydrous formaldehyde (FA, target MeOH conversion ≥67% and target FA selectivity ≥93%), which in turn is used as the feed for OME synthesis. The model couples the merits of algorithms available in the commercial software CHEMCAD® together with self-developed reactor models as implemented through Matlab® and the coupling node implemented in Visual Basic for Applications (VBA) software. This is followed by process heat integration using PinCH 2.0 software. This modelling is complemented by experimental investigations and results concerning the synthesis of the anhydrous FA/MeOH feed through a designed and developed annular counter current reactor, with the use of Na2CO3 as an inexpensive and sustainable dehydrogenation catalyst. The process material and energy balance of the proposed process have also been used to evaluate the key performance indicators (KPIs). An overall process yield of 80.3% at 71.7% process energy efficiency and production cost of 951 US

Bio-electrochemical conversion of industrial wastewater-COD combined with downstream methanol synthesis – an economic and life cycle assessment

per ton of OME3–5 at small production capacity (35 kt per annum) demonstrates the technical and the economic potential of the described process.

Towards a Sustainable Synthesis of Oxymethylene Dimethyl Ether by Homogeneous Catalysis and Uptake of Molecular Formaldehyde

Herein, a techno-economic and environmental performance evaluation (i.e. Life Cycle Assessment (LCA)) of a 45 kW Microbial Electrolysis Cell (MEC) system is presented in the context of industrial wastewater remediation. This system produces H2 and CO2 – suitable for downstream CH3OH synthesis – based on the bio-electrochemical conversion of chemical industry wastewater with an organic content of 3.9 g(COD) L−1. A cost–benefit analysis indicates that the MEC system hardware costs, share of CO2 captured from the MEC and MEC operating current density (i.e. 1.0 mA cm−2) are crucial parameters influencing the total cost and represent areas for potential cost reductions. It was established based on the present study that MEC system operation with renewable electricity leads to H2 production costs of 4–5.7€ kg(H2)−1 (comparable to H2O electrolysis) and CH3OH production costs of 900€ t(CH3OH)−1. At the current CH3OH market prices, however, the production is currently not profitable. In turn, the cost-efficient construction of the MEC system and the use of less expensive materials could lead to improved CH3OH production economics based on this route. Our results indicate that the use of low-cost materials has greater potential with regard to cost reduction compared to reducing the internal resistance and polarization losses via the use of expensive high-performance materials in MEC construction. A complementary LCA of the proposed system, based on a “cradle-to-gate” definition, indicates that waste-based is superior to fossil-based CH3OH production with respect to global warming potential and cumulated fossil energy demand, provided the system is operated with 100% renewable electricity and CO2 sourced only from the MEC. However, with regard to the impact categories Metal Depletion and Freshwater Eutrophication Potential, the system was found to perform less satisfactorily (i.e. in comparison with fossil-based CH3OH production).

Structure, stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

Oxymethylene dimethyl ethers (OMEn ; CH3 (-OCH2 -)n O-CH3 , n=3-5) are a novel class of sustainable synthetic fuels, which are of increasing interest due to their soot-free combustion. Herein a novel anhydrous OMEn synthesis route is presented. Catalyzed by trimethyloxonium salts, dimethoxymethane takes up monomeric gaseous formaldehyde instantaneously and forms high purity OMEn at temperatures of 25-30u2009°C. This new anhydrous approach using molecular formaldehyde and catalytic amounts of highly active trimethyloxonium salts represents a promising new step towards a sustainable formation of OMEn emanating from CO2 and H2 .