Investigation of the Fouling Mechanisms in the Heat Exchangers of a Hydrotreater
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The problem of fouling on heat exchanger surfaces has been one of the major unresolved problems of the hydrotreatment unit at the Homs oil refinery in Syria. In this unit, a feed stream of straight-run naphtha is preheated in a series of four heat exchangers prior to the hydrotreater. Severe fouling conditions led to an overall reduction in the performance of the unit as a result of lower temperatures at the heat exchangers outlet and increased pressure drop. An investigation of the cause or causes of this problem was called for. For this purpose, deposits from the heat exchanger surfaces were analyzed in order to identify the fouling mechanism(s). Such analysis pointed unmistakably to fouling due to polymerization in general and oxygen-initiated polymerization in particular. Various laboratory tests were also conducted on the naphtha feed stream, which provided indications of the stream's polymerization potential. Since the naphtha feed stream was drawn from blending tanks that are not very well blanketed, the naphtha must clearly be exposed to atmospheric oxygen. It was concluded that a gaseous protective layer, composed of nitrogen or refinery gas, is essential for the amelioration of the fouling conditions encountered in the heat exchangers.