Design of scrubber system for emergency relief of a large-scale petrochemical plant

Difficulties and practical, compromise solutions to design an emergency scrubber system for a large-scale petrochemical plant are presented via a case study. The case studied is a paraxylene oxidation process in which acetic acid is used as the solvent. The heat of the oxidation reaction is removed from the reactor by evaporating the solvent. The solvent vapor is condensed and refluxed to the reactor. During process upsets such as cooling failure, the solvent vapor can no longer be condensed and the overpressure generated within the reactor and condensers must be relieved through a pressure safety valve (PSV). The vapor relieved to the atmosphere raises significant environmental concern to the nearby communities. Thus, a proper disposal of the relieving vapor is desired. To completely dispose of the relieving vapor would require a replica of the entire vapor processing unit of the oxidation process which is generally impractical. Instead, a single scrubber (absorber) using water as an absorbing medium is chosen for the service. Owing to the large amount of relieving vapor, fire water is chosen for the water supply. The action of the scrubber is triggered by now switches located in the relief line. The size and layout of the relief line are determined by the allowable back pressure of existing PSVs. Additional constraints to the layout of the relief line are stresses arising from the thermal expansion and reaction force of the line during relieving conditions. Dynamic and static stress analyses are performed to ensure both the transient and static stresses are within allowable ranges. Design methodologies concerning the setting of the flow switches, scrubber drainage, internal structure and final operation and maintenance are given in detail. The result is a safe and effective design that Rill reduce the concentration of relieving acetic acid vapor from 30% to 650 ppm. Unawareness of the back pressure and stress constraints will result in high risk of the reactor.