FORMATION FROM PROPENE OF CARBONACEOUS COMPOUNDS RESPONSIBLE FOR THE DEACTIVATION OF A 5A ADSORBENT - REMOVAL THROUGH PYROLYSIS AND OXIDATIVE TREATMENT

The formation of carbonaceous compounds (coke) from propene was invest igated by microgravimetry at 423 K on a 5A adsorbent (80% zeolite, 20% binder) for propene pressures of 6.5, 30 and 97 kPa. First, oligomers and their position and skeletal isomers are formed on the protonic si tes of the 5A zeolite. These branched compounds are trapped in the alp ha cages. They can be removed as propene (after cracking) by simple va cuum treatment at 423 K. These compounds (reversible coke) undergo aft erwards other reactions, in particular hydrogen transfer and cyclizati on with the formation of dienes, trienes and naphthenes, as shown by g as chromatographic-mass spectrometric (CC-MS) analysis. Part of these latter compounds can only be cracked at high temperatures leading to o lefinic products which rapidly transform into aromatics. These aromati cs cannot be transformed into linear products, which explains that par t of the coke cannot be removed from the adsorbent by thermal vacuum t reatment. All the carbonaceous compounds can be removed through treatm ent under oxygen at 773 K. The adsorption capacity decreases linearly with the increase in the irreversible coke content. This decrease is i ndependent of the propene, pressure used for coke formation. The total blockage of the access of nitrogen to the zeolite pores occurs for a number of coke molecules close to the number of alpha cages.