Economically viable co-production of methanol and sulfuric acid via direct methane oxidation

Methane is the most abundant energy source and a crucial feedstock for various fuels and chemicals. Increased methane production from shale gas exploitation necessitates efficient utilization methods beyond the current energy-intensive indirect conversion process (syngas production followed by refinement). This indirect method is unsuitable for small-scale facilities, leading to significant flaring of natural gas and associated greenhouse gas emissions. Direct methane conversion to methanol has been extensively researched but remains uncommercialized due to low selectivity and yields. The direct conversion process faces challenges due to the high energy required to break the strong C-H bond in methane, often resulting in over-oxidation of methanol to carbon monoxide and dioxide. Previous attempts to address this via synthesis of more stable methanol derivatives like methyl bisulfate, methyl bromide, and methyl trifluoroacetate have shown promise but lacked the scalability and cost-effectiveness for industrial applications. Methyl bisulfate, produced by oxidizing methane with SO3 in H2SO4, offers high yields but presents separation challenges from sulfuric acid, requiring energy-intensive distillation or depressurization. This paper proposes a novel approach to overcome these limitations and create a commercially viable process for methanol production.

Numerous studies have explored direct methane conversion to methanol, employing various methods including gas-phase reactions using Cu-zeolite catalysts, plasma without catalysts, in-situ H2O2 generation, and diverse catalytic systems. However, these have not been commercialized due to low methanol selectivity and yields. Research on using methyl bisulfate (MBS) as an intermediate is promising due to its cost-effective synthesis and high yield. Periana et al. demonstrated MBS synthesis using a bipyrimidyl-bonded platinum catalyst, but its turnover number was insufficient for industrial application. Recent advancements with Pt-black, K2PtCl4, and (DMSO)2PtCl2 catalysts have achieved high performance under milder conditions, but the separation of MBS from sulfuric acid remains a significant hurdle. Membrane separation has been suggested, but its functionality under strongly acidic conditions is questionable. Hydrolyzing MBS to methanol in the presence of sulfuric acid leads to significant dilution and loss of sulfuric acid. Direct Me-TFA synthesis from methane has also been investigated, but low conversion yields and solvent decomposition hamper its practicality. This paper builds upon these existing studies, aiming to create a process that efficiently tackles the challenges of methane oxidation and product separation.

The proposed method involves a sequential three-step process: methane oxidation to methyl bisulfate (MBS), esterification of MBS with trifluoroacetic acid (TFA) to form methyl trifluoroacetate (Me-TFA), and hydrolysis of Me-TFA to methanol. The esterification step is crucial because Me-TFA's lower boiling point (43.5 °C) compared to MBS (>170 °C) simplifies separation from sulfuric acid. The TFA is then recycled. The study employed Gaussian process Bayesian optimization (GPBO) to optimize the methane oxidation reaction parameters, including temperature and oleum concentration, using Pt-black as a catalyst. Equilibrium constants for the esterification and hydrolysis reactions were determined experimentally at various temperatures. A superstructure-based process design and optimization methodology was used, evaluating numerous process alternatives using a hybrid method combining genetic algorithm and Bayesian optimization. This hybrid approach aimed to optimize the process configuration and operating conditions. Aspen Plus and MATLAB were used for process simulation and economic analysis, respectively. The economic analysis calculated the net present value (NPV) considering capital expenditure (CAPEX), operating expenditure (OPEX), and revenue projections over 15 years. Sensitivity analysis using Fourier amplitude sensitivity testing (FAST) was conducted to assess the influence of uncertainties in operating variables and economic parameters on the NPV. Carbon footprint (CFP) analysis compared the proposed co-production process to conventional methanol and sulfuric acid production methods.

Experimental results showed that Pt-black catalyzed methane oxidation achieved high methane conversion (81.3% at 3h) and MBS yield (73% at 40°C for esterification). The GPBO successfully identified optimal reaction conditions for maximizing MBS yield. The esterification reaction was exothermic, and a reactive distillation column significantly enhanced MBS conversion (to 86%). Hydrolysis reaction efficiency increased with temperature, reaching over 30% methanol and TFA at 100 °C. Superstructure optimization identified an optimal process configuration using a reactive distillation column for esterification and a CSTR reactor for hydrolysis. This optimization resulted in a levelized methanol cost of $203 per ton, significantly lower than the current market price ($270–$450), while also co-producing sulfuric acid. The economic analysis revealed that the proposed process is economically viable even at small production scales (2000 kg yr⁻¹), making it suitable for smaller gas fields. The sensitivity analysis showed that oxidation reaction conversion and esterification significantly affect NPV, while sulfuric acid co-production mitigates the impact of reduced methanol yield. The analysis also highlighted the importance of efficient light gas recovery and optimized reflux ratio in the reactive distillation column. Carbon footprint analysis demonstrated that the co-production process significantly reduces the carbon emissions associated with methanol production compared to conventional methods, even when considering the emissions associated with sulfuric acid production. The main source of CO2 emissions in the proposed process was electricity usage for steam generation and unit operations.

The findings demonstrate the successful development and optimization of a commercially viable process for methanol production via direct methane oxidation coupled with sulfuric acid co-production. The innovative three-step process, incorporating esterification and hydrolysis reactions, effectively addresses the longstanding challenges of low selectivity and energy-intensive product separation. The significant cost reduction achieved, along with adaptability to smaller-scale operations, opens possibilities for utilizing flared methane from remote gas fields. The economic viability is further enhanced by the co-production of sulfuric acid, reducing the overall production cost. This approach offers a more sustainable alternative to conventional methanol production methods, mitigating greenhouse gas emissions. The results of the sensitivity analysis underscore the crucial aspects of the process design and operation that must be carefully controlled to maintain economic profitability and minimize environmental impact.

This study presents a novel and economically viable process for co-producing methanol and sulfuric acid from direct methane oxidation. The process addresses critical challenges associated with traditional methods, achieving significant cost reductions and enhanced sustainability. Its adaptability to smaller gas fields offers potential for mitigating methane flaring and promoting a more environmentally friendly methanol industry. Future research could focus on further optimization of reaction parameters, exploring alternative catalysts for enhanced performance, and investigating integration with renewable energy sources for minimizing carbon emissions.

While the proposed process offers significant advancements, some limitations exist. The sensitivity analysis reveals that the process performance is highly sensitive to the oxidation reaction conversion and the esterification reaction efficiency. The high energy consumption associated with the separation of sulfuric acid contributes to a noticeable carbon footprint of sulfuric acid production. Further research and development could focus on reducing energy consumption in this step, perhaps by exploring more energy-efficient separation methods or integrating with renewable energy sources. The economic analysis relies on current market prices for raw materials and products, and changes in these prices could impact the overall economic viability. While the model incorporates uncertainty, other unforeseen technical challenges or unexpected variations in feedstock characteristics may affect real-world implementation.