One-step CO2 hydrogenation to dimethyl ether via packed bed membrane reactors

Dimethyl ether (DME) has been recently proposed as a cleaner alternative to diesel for heavy transportation vehicles, which makes its production from CO2 even more attractive. 

Currently, DME is produced from fossil fuels via an energy intensive process, which has a high CO2 footprint. Therefore, the use of captured CO2, in combination with green H2 (i.e., H2 produced using renewable resources) as the feedstock for the DME production is a much cleaner alternative to the conventional route.

Thermodynamic limitation

Nevertheless, such a shift in the feedstock is not trivial. Indeed, one of the biggest challenges of the process is the strong thermodynamic limitation of the reaction system. In optimal conditions, approximately 30% of the CO2 is converted to DME, indicating a scarce efficiency in the conversion.

The thermodynamic limitations of this process mainly come from the high volume of water produced as a reaction by-product. Thus, the reaction system could benefit enormously from the in-situ removal of water from the reaction environment. A promising technology for such purpose is a membrane reactor: a unit where reaction and separation occur simultaneously. If membranes with a high affinity to water are incorporated in a conventional reactor for the CO2 hydrogenation to DME, water can be separated from the reaction mixture as soon as it is formed along with DME and shift the CO2 conversion towards much higher values (i.e., beyond the thermodynamic limitations).

Membrane reactor technology

In her thesis, Serena Poto studied the use of membrane reactor technology for the one-step CO2 hydrogenation to DME reaction under different perspectives, ranging from the selection and development of the membrane materials to the experimental demonstration of the technology.

First, Poto used reactor modeling to study the effect of the membrane properties, such as water permeability and separation factors, to shed light on suitable membrane materials as well as to set a target for the development of novel membranes. This study led to the selection of carbon molecular sieve membranes (CMSM), which offer superior performance than benchmark materials combined with high stability in a hot, humid environment. 

Thus, Poto developed CMSMs studied their use for the separation of water vapor from the reaction mixture (i.e., mainly CO2 and H2). Some of the CMSM synthesis parameters were manipulated in order to maximize the membrane performance, improving the hydrophilicity (i.e., affinity to water) of this material. Most importantly, Poto proposes a deep understanding of the phenomena involved in the water permeation/separation process, which is crucial and preparatory for the optimization of the membranes for a specific application.

Rigorous reactor model

Thereafter, Poto turned to a more rigorous membrane reactor model used for a more detailed calculation of the effect of using CMSMs to improve reaction performance. In optimal conditions, the CO2 conversion to DME was found to be more than 40% higher than in a conventional reactor (i.e., with no membranes). This improvement was later confirmed experimentally at the laboratory scale.

Finally, a tecno-economic evaluation of the one-step CO2 hydrogenation to DME was proposed by Poto to elucidate on the advantage of using a membrane reactor for improving the energy efficiency of the process, as well as to reduce the DME production cost.

Title of PhD thesis: One-step CO2 hydrogenation to dimethyl ether via packed bed membrane reactors. Supervisors: Fausto Gallucci and Fernanda Neira d’Angelo.