Separation and Recovery of Oxo Catalysts

The separation and recovery of the oxo catalysts are very essential steps in the process. The ease and completeness of this operation is of great economic importance to oxo production units. Cobalt hydrocarbonyl and dicobalt octacarbonyl are not only highly soluble in organic solvents but are also very volatile and are taken out of a solution with gas streams. This 

There are a number of different methods applied. Some of these cannot be recommended for plant operation but only for laboratory work. 

In case of low boiling aldehydes (propionaldehyde, butyraldehyde, etc) and carbonyls of low volatility (Rh-carbonyls, phosphine modified cobalt or rhodium catalysts), the aldehydes may be separated from the reaction product by flash distillation [150]. In this operation the metal carbonyls remain in the bottoms. Depending on temperature, length of treatment and stability of the carbonyls, varying amounts of them are decomposed. The corresponding metals formed also remain in the bottoms. This method is limited to laboratory operations (with the exception of phosphine modified catalyst — see chapter on modified catalysts). It cannot be recommended for cobalt carbonyls since they are too volatile. 

This method was already applied in the first technical oxo reaction unit in the so-called two-tower-process (hydroformylation reactor and deco-balting reactor) in which both reactors were filled with cobalt on carrier the first operating with high, the second with low pressure. In the first reactor the solid cobalt was converted into hydrocarbonyl, in the second reactor the dissolved carbonyls were decomposed (because the pressure was too low to keep them stable) and precipitated on the carrier. When most of the cobalt had been transferred from the first to the second reactor the functions of the two reactors were reversed [151, 152]. 

The method may also be applied on the laboratory scale. In this case pressure is released at the end of the reaction from the reaction vessel — e. g. an autoclave — and the gas vented while heating is continued, resulting in decomposition of the carbonyls and precipitation of the corresponding metals. The big disadvantage of this type of catalyst removal is the fact that the metal not only decomposes on the carrier but also on the wall of the reactor, thus reducing heat transfer through the wall and clogging the reactor. 

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