(140a) Design Criteria for Mitigation of Fouling in Reboilers

Significant progress has been made in understanding the underlying mechanisms of hydrocarbon fouling and characterizing the effects of physical and chemical parameters. However, the interactive effects of two-phase flows and fouling mechanisms are poorly understood. The performance of these exchangers directly impacts the overall performance of the process unit, such as distillations columns, hydrotreaters and also reactors. This is because design guidelines are inadequate to minimize localized process conditions that induce severe fouling condition, which could also lead to localized corrosion by concentrating corrosion species. This paper presents design criteria that should be followed to evaluate two-phase flow conditions under different operating conditions and design the exchanger to minimize severity conditions that would promote localized fouling under two-phase conditions. Such a design approach may not be optimum for heat transfer and pressure drop considerations under clean conditions, but it will minimize fouling and localized corrosion. The paper focuses on improved design of for minimizing fouling and under-deposit corrosion and a case study of revamping to minimize fouling propensity. The result is lower lifecycle ownership cost of the exchanger. Industrial heat exchangers in refining and petrochemical plants operate under multi-phase flow conditions, and fouling and localized corrosion are common problems in such heat exchangers. The major heat transfer equipment, not including fired heaters, in petroleum processing that encounter fouling and coking in a two-phase flow environment are: a) reboilers operating as thermosiphon or under forced feed conditions: localized fouling is generally caused by two-phase flow maldistribution on shell side and slug flow on tube side creating regional environment for chemical reaction fouling and particulate deposition of corrosion products that might be generated in the overhead condenser/separation drums, transfer pipelines, and/or in the distillation column; and b) feed/effluent (F/E) exchangers: F/E in hydrotreating and reforming processes are prone to fouling, mostly caused by chemically active species, such as iron-sulfide with non-uniform two-phase flows on shell side. The interactive effects of fouling precursors derived from blending products streams such as diolifins, reactive sulfur, dissolved metals reactive species like iron-naphthenate combined with two-phase flow pattern significantly enhances the fouling propensity. The chemical treatment to interrupt the interactive effects fouling precursors must take into account of two-phase flow for targeting these precursors. Chemical treatments developed using single-phase laboratory testing may not be effective in two-phase conditions. This presentation will provide a comprehensive approach of design criteria to follow for effective fouling mitigation by maintain the process parameters below threshold fouling conditions and effective chemical treatment.