Strategies for Mitigating Fouling in Ethylene Plants

This article reviews the fouling mechanisms affecting distillation towers in ethylene plants and presents design and operation strategies for preventing fouling.

A study conducted by HSB Solomon Associates from 1999 to 2011 suggests that the three main factors that affect ethylene plant production are related to problems with cracking furnaces, major rotating equipment, and fouling, where fouling is a subset of the problems experienced from cracking furnaces and major rotating equipment (1).

While subject to other modes of failure, distillation tower performance and capacity are primarily affected by fouling. For instance, fouling of the primary fractionator frequently dictates the plant run length and, thus, turnaround frequency. Caustic towers are another application where fouling may cause unplanned outages. With ethylene plants striving for longer run lengths, understanding and mitigating the fouling of process equipment, such as distillation towers, is paramount.

Fundamentally, distillation towers are intended for vapor/liquid separation; solids in the feed stream should be avoided. However, in practice, dissolved solids may precipitate inside the tower, and chemical reactions may occur, resulting in the formation and accumulation of oligomers and/or polymeric materials. In most cases, the symptoms of fouling are associated with high pressure drops and premature flooding.

Mitigating the impact of fouling starts at the design stage. It requires understanding the process objectives and fouling mechanisms such that appropriate internals are selected. It may also require treating feed streams, when possible, to minimize the amount of fouling material entering the tower. Design operating conditions should also be specified to slow the rate of known fouling reactions. Other strategies for inhibiting fouling include injecting chemicals into the process or dispersing the reaction products so that they do not accumulate in the tower. In some instances, understanding the symptoms associated with the initial fouling stage may prevent irreversible loss of capacity and performance. Turnaround and unit commissioning practices may also influence the fouling propensity of distillation towers.

This article explores the known fouling mechanisms in distillation services of ethylene plants, and discusses strategies for minimizing fouling through strategic design, operation, and turnaround practices...