Hydrocarbon partial pressure and the role of steam

Moreover, the condensation side-reaction rate b much more heavily influenced by the hydrocarbon content of the reaction mixture than the rate of the primary reactions, which are substantially of the first order with respect to the reactants. A decrease in the partial pressure of the hydrocarbons, by dilution with steam, for example, reduces the overall reaction rate, but also helps to enhance the selectivity of pyrolysb substantially in favor of the light olefins desirecL Apart from thb specifically kinetic role, steam exerts a number of other beneficial effects  

Given the veiy high temperature (l(X)0 C) required, this conversion nevertheless plays only a minor role in the cleaning of pyrolysis tubes. 

The use of steam also involves a number of draw-backs which impose a limit value to its content in the feedstock. Since the steam must be heated to the reaction temperature, its presence increases the reaction volume required and hence the furnace investment. And its separation from the hydrocarbon effluents requires vezy large condensation areas and results in high utility consumption. 

The amount of steam employed, which is normally expressed as the weight of steam per weight of feedstock, depends on the molecular weight of the hydrocarbons treated. 

This ratio is 0.25 to 0.40 for ethaae, but may be as high as 0.5 to 1 t/t of feed for petroleum cuts which display a much more pronouDced tendency to produce heavy by-products. 

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