Light-Naphtha End Point

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Operators may attempt to maximize heavy naphtha production at the expense of light naphtha by increasing the fractionator top reflux rate, which drops the tower top temperature. The water vapor in the overhead hydrocar­bon vapors begins condensing at its dew point. If the tower top temperature is too low, water will condense on the top trays. This water is corrosive and will eat holes in the tray decks. Over a period of months, the degree of separation between light and heavy naphtha will consequently deteriorate.

If the light-naphtha end point is high and the tower has been operating within 10°F of its calculated water dew point, corrosion of the top trays is indicated. The procedure to calculate water dew point is given in the Appen­dix.

Replacing tray decks and downcomers with Monel steel (high nickel content) will help prevent this corrosion. The area underneath the downcom­ers are especially prone to this type of leakage due to hydrochloric acid attack.

 

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