Читать книгу Engineering Solutions for CO2 Conversion - Группа авторов - Страница 51

The applications of CCS using proton conducting membrane technologies has emerged as a hot topic to provide an industrial solution for the mitigation of the greenhouse effect. H₂‐related membranes can operate at intermediate and high temperatures (400–900 °C), and the processes in which they have been integrated to can be divided into (i) CO₂ reduction into valuable chemicals such as methane or methanol (catalytic membrane reactors [CMR]), (ii) conversion of chemicals into electrical energy (fuel cells), and (iii) generation of H₂ as a fuel.

SMR has proven to be one of the most energy‐efficient way to produce hydrogen from methane from an industrial point of view. SMR converts methane and water into syngas in an endothermic reaction (Eq. (3.8)); this step can be subsequently combined with water gas shift reaction (WGSR) to produce extra H₂ together with CO₂ (see Eq. (3.9)).

(3.8)

(3.9)

Using hydrogen‐selective membranes, a pure H₂ stream is obtained, resulting in the shift of the thermodynamic equilibrium and hence in process intensification. The majority of the reported membranes are metal‐based membranes, i.e. Pd or Pd/Ag alloys [71]. However, these membranes do not achieve full CH₄ conversion nor H₂ permeation [94–96].

WGSR at temperatures ranging from 700 to 900 °C have been performed using MPEC‐based membranes, SrCe_0.9Eu_0.1O_3−δ‐ and SrCe_0.7Zr_0.2Eu_0.1O_3−δ‐supported tubular membranes, yielding interesting results [97, 98]. By using the SrCe_0.7Zr_0.2Eu_0.1O_3−δ‐membrane, an increase of 77% in the CH₄ conversion as compared with thermodynamic conversion was obtained at 900 °C (H₂O/CO ratio = 1/1).

The selective conversion of natural gas to higher hydrocarbons and aromatics remains an important industrial challenge. Non‐oxidative coupling of methane to produce olefins and aromatics (see Eq. (3.10)) are reactions limited thermodynamically. The selective extraction of H₂ will allow the shift of the thermodynamic equilibrium, i.e. the conversion toward the product side, giving rise to a significant improvement in the reaction yield. However, the H₂ extraction accelerates coking and catalyst deactivation.

(3.10)

Methane dehydroaromatization (MDA) reaction was performed by Li and coworkers [99] using a 2 μm dense membrane made of SrCe_0.95Yb_0.05O_3−δ and 4 wt% Mo/HZSM‐5 catalyst. A modest increase of the CH₄ conversion as compared with the conventional reactor was obtained at 720 °C. However, a slightly higher catalyst deactivation was also observed.

Caro and coworkers studied the MDA reaction by using a U‐shape La_5.5W_0.6Mo_0.4O_11.25−δ [100] membrane and 6 wt% Mo/HZSM‐5 as a catalyst at 700 °C. Higher aromatics yield than that without membrane was obtained during the first five hours on the stream because of the important H₂ extraction, reaching 40–60% of the H₂ produced in the reaction. In this case, a catalyst deactivation was also observed, giving rise to aromatic yield lower than that without H₂ extraction after 10 hours on the stream.