Content area
Full text
Understanding of waste components needed for proper spent caustic disposal
Spent caustic handling, treatment and disposal are major concerns for refining and olefins (ethylene) production facilities due to its hazardous nature and noxious properties. As sources for spent caustic generation are diverse, they do produce characteristically different waste streams consisting of inorganic and organic acidic components such as carbon dioxide (CO 2 ) sulfides, carbonates, mercaptans, phenolics, cresylics and naphthenates. These components are acidic and must be removed to avoid corrosion of downstream equipment and to prevent poisoning catalysts. The conditions can be further complicated by neutral oil carryover. Several treatment methods can be used to manage spent caustic and they include chemical precipitation, neutralization, chemical reagent oxidation, wet oxidation, catalytic oxidation and incineration.
Improved processes are now available. Each method offers certain advantages while its application would depend on the waste stream composition, size and the configuration of the total process facility and toxicity threshold limits of downstream biological treatment systems. Spent caustic is toxic to bacteria used in the wastewater treatment unit. When zero discharge or stringent limits are stipulated in environmental permits, high dissolved solids present in the spent caustics add to treatment loading in desalination plants.
Selecting the best treatment method is a critical task, especially in meeting total waste management needs. It is often driven by environmental regulatory limits associated with emissions and discharges. There are essential elements in dealing with spent caustic streams that will be presented in this article. They involve applying the principles of waste management hierarchy, with merits and demerits of each possible solution, as shown in Fig. 1. 1-2
Fig. 1. Waste management hierarchy.
Source characterization and segregation.
Typical processes in the hydrocarbon industry where spent caustic is generated are caustic scrubbing of straight-run light hydrocarbons, feed streams to isomerization and polymerization units; cracked gases from thermal/catalytic cracking units; and caustic washing of middle distillates, followed by mercaptans extraction/sweetening operations. The caustic converts the acidic components into their respective inorganic/organic salts of sodium such as sulfides, carbonates, mercaptides, disulfide oil, phenolates, cresolates, xylenolates and naphthenates. Caustic strength used (typically 5-20%) in these processes is governed by feed type and nature of caustic treatment; accordingly, the residual caustic strength will differ. Since acidic components have...