(92d) Process Water Treatment in Ethylene Plants

The term “Process Water”, when used in the context of an ethylene plant, refers mainly to the condensed dilution steam produced in the Quench Section of the plant.  Although dilution steam condenses in the Quench Water Tower (QWT), process water differs from quench water in that the latter is used to recover the process waste heat against other suitable process streams and is maintained in circulation.  Driven by environmental stewardship and energy savings, among other factors, many ethylene producers are using the process water either to saturate the pre-heated feed to the furnaces, to produce dilution steam, or both. 

Water from the QWT is contaminated with free, dispersed, and dissolved hydrocarbons.  Depending on the process scheme arrangement of the Quench Section and whether a gas or liquid feedstock is being cracked, this water may also contain tar, polymer oils, and coke fines.  It must therefore be treated for the removal of hydrocarbon and other particulates prior to any further uses in the process scheme.  Process water clean-up will have an impact in plant reliability, ease of operations, and maintenance frequency.

This paper provides an overview of process water clean-up (or treatment) in ethylene plants and some of the most common challenges associated with it.  Different technologies available for process water treatment are discussed and aspects to be considered for the selection as well as the integration of these into typical quench water and dilution steam systems are identified.