Composition of spent caustic

The phenol content, present in the form of Na phenolates, of course characterizes sweetening of feeds that are rich in these compounds. The main one is from the FCC despite a reaction temperature of around 500°C. In the SC at a higher temperature (around 800°C ) phenol production is theoretically more massive. However the phenols are immediately reextracted by the considerable steam recirculation. 

The relative concentration in phenols used to be the basis for dividing spent caustic into two main types phenolic and sulfidic. This distinction was made for a long time in the U.S. and M.R. Beychok (Aqueous Wastes from Petroleum Plants, John Wiley and Sons) in particular characterized phenolic spent caustic as coming from alkaline scrubbing of cracked gasoline that has been debutanized. It contains mainly thiophenols and mercaptans and very little Na2S. 

Later the definition seems to have been broadened to cover spent caustic whose phenols are extractable (steam or solvent) and marketable. This presupposed a total phenol concentration of around 5-6 g l .  

Since the current market value of reclaimed phenols is almost zero, the treatment of spent caustic is now based first of all on desulfurization and secondly on phenol removal. No attempt is made at reclaiming phenols so the treatment can be destructive. 

Having buffer tanks to hold eight to 15 days worth of stored spent caustic ensures a supply of relatively homogeneous and constant spent caustic for treatment. 

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The composition of spent caustic cresolates varies in the range of 20-25% of Ce-Cs phenols and 10-15% sulfur compounds. 

By way of an illustration, orders of magnitude of die composition of the two types of spent caustic are given (Table 36). 

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