Most refineries use caustic treatment for various product streams to remove hydrogen sulfide, mercaptans, phenolic compounds, thiophenols (sulfur-bearing aromatics), and naphthenic acid impurities. Caustic treatment of cat-alytically and thermally cracked gasoline produces a spent caustic that is rich in phenolates and thiophenolates, with lesser amounts of sulfides and mercaptides, commonly referred to as phenolic caustic. Treatment of fuel gas, LPG, and straight run gasoline produces sulfidic caustic, rich in sulfides and mercaptides, with only small amounts of phe-nolates. Both types of spent caustics emit a foul odor and pose serious disposal problems. If dumped into a receiving stream, they will discolor water, impart an objectionable taste, exert a high chemical oxygen demand, and poison fish.
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