Ethylene syngas based production

Scheme 6.114 Syngas-based production of ethylene glycol.

The appeal of an acetic acid process, based on ethane oxidation, lies mostly in the absence of the need for the energy demanding step for syngas production. On the other hand, it has to compete not only with the well established methanol carbonylation (Section 4.2), but also with the current utilization of ethane in steam crackers for ethylene manufacture. In fact, ethane feedstock becomes attractive for acetic acid production if it is locally abundant and can be supplied at minimal cost, e.g., in a petrochemical complex close to a large gas field. The construction of a semi-commercial plant of 30 kt/a in the Persian Gulf region has been announced. 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

Hydroformylation was discovered in 1938 at the company Ruhrchemie in Ober-hausen, Germany by Otto Roden (1897-1993) in the context of investigations to optimize the Fischer-Tropsch synthesis (FT synthesis) for fuel production from coal based syngas (Section 6.11.1). Otto Roden realized at the time that the formation of C3-oxoproducts in the FT synthesis was due to the reaction of ethylene with syngas. 

All catalyst formulations, in fact, lead to the unselective formation of deep oxidation products and of syngas (H2 and CO). But cracking processes (here represented by simple dehydrogenation) may also occur at high temperature in the empty volumes of the reactor, with the formation of ethylene, methane, C4+ species, but also C (which is an issue when using a Pd-based catalyst, for instance). C gasification, as well as water-gas shift, CO and H2 post combustions, are usually also involved in the process surface kinetics. 

A particularly significant and useful contribution of transition metals in fine organic synthesis as well at the industrial level is based on their use as catalysts. This aspect is of course particularly important with expensive transition metals (Rh, Os, Pd, etc.). Indeed, there are numerous examples of selective processes which have never been developed up to the industrial stage because of catalyst costs, especially when some (even minor) loss of the catalyst could not be avoided. This was, for example, the case for palladium-catalyzed benzylic acetoxylation reactions, and several rhodium-catalyzed reactions, such as the direct ethylene glycol production from syngas (prohibitive pressures being an additional major drawback in this latter case). 

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