Structure de mise en forme 2 colonnes
Young Researchers

SP2 – Separation processes

An important criteria for the realisation of the OCMOL process concept has been to address the need for efficient and selective separation processes at the interface of the core process steps. Two aspects have been under focus:

  • Remove specific by-products that can impact or even "poison" the activity of the catalyst

  • Separate out key components for recycling in order to achieve the overall targets of the OCMOL process

Technologies based on sorbent and membrane separation have been investigated, with the core of the research involving the developing and modifying materials with potential functionality and modelling the process to gauge simulate the potential for the process of the materials.

Three key challenges were ultimately addressed in the project:

  1. Achievement of a purified stream of ethylene as a feed into the sensitive oligomerization
  2. Providing an oxygen feed stream for the reforming step
  3. Aiming at more energy efficient separation of methane prior to the oxygenates step.

The results of the research activities have been based on development and screening of materials with relevant properties, with further consideration of their potential for application in sorbent or membrane based separation.

With respect to achieving a purified stream of ethylene, a novel material with unusually high selectivity in the separation of the product ethylene  from by-products of similar size was discovered. As shown in Figure 4, a "molecular sieve" with silver as part of its composition (AgA) and pore dimensions similar to the size of the ethylene molecule was able to completely exclude the undesirable ethane molecule from entering the pores.

Figure 4: Adsorption isotherms of ethylene (closed symbols) and ethane (open symbols) measured by volumetric method on AgA (black squares) and AgX (red circles) at 303 K.

Regarding the oxygen feed stream, the enrichment of the CO2 stream by oxygen separated from air allowed to adjust the content of oxygen in the RM inlet stream in order to achieve heat balance between the endothermic RM and exothermic OCM, resulting in a global autothermal process. High oxygen permeability and stable performance of asymmetric supported membranes were achieved due to a positive role played by developed perovskite–fluorite interfaces with specific structure and composition.

For the separation of methane, the key challenge was to find a sorbent able with the affinity to separate the relatively inert methane molecule from a mixture with hydrogen and CO. Another "molecular sieve" (ZSM-5) was shown to in fact to have higher affinity for methane over the other two molecules, though not to the level desired. This has meant carrying out detailed modelling studies to see if a viable commercial process can be based on this level of affinity.

The key step in the demonstration of the separation processes, was the vacuum swing adsorption (VSA) for the separation of ethylene from the stream after removal of water, CO and CO2. This demonstration was carried out at in a larger scale pressure swing adsorption (PSA) unit conditioned to measure the ternary separation of CH4, C2H6 and C2H4. The results from the breakthrough curves in the unit indicated that it is possible to separate the olefin from the paraffin, though one of the main issues to be addressed is the regeneration pressure.

 

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