E. I. du Pont de Nemours & Co., Inc.
In the ethylene plant, there are many oily water waste sources connected to a concrete "API oil/water separator". These sources include heavy by-products removed from the cracked gas quench tower, blowdowns from the amine and caustic columns and miscellaneous equipment, drainage from curbed areas and water decanted from aromatic distillate storage tanks. In the API separator, most of the oil overflows into a chamber and is recovered as heavy aromatic distillate product. The water, along with soluble organics, solids and some oil, underflows to a second chamber. Prior to 1988, this water was pumped to a settling basin where the water and soluble organics underflowed to a biopond. Floating oil and solids were removed for costly off site waste disposal. Nuisance odors were experienced at the basin, and also at the atmospheric vented API separator because of evaporation of volatile organics (including benzene).
To reduce waste disposal cost, improved recovery of oil and solids from the oily water stream was needed prior to sending the stream to the basin. To accomplish this, a $1 MM capital project was approved for installation of the following in mid 1988:
* Facilities to combine blowdowns of the amine and caustic system and allow neutralization of this stream prior to discharge to the API separator. CO2 is used to reduce the pH of the caustic rich stream with the help of a pH control loop. This improved oil separation in the API separator and provided adequate pH control of the water stream exit the separator. Sulfuric acid was considered as an alternate neutralizing agent but was ruled out because sulfates degrade to hydrogen sulfide in the biopond. * A dissolved gas flotation (DGF) unit to improve oil separation of the wastewater exiting the API separator. The DGF unit uses nitrogen for oil and solids removal. This unit consists of a flocculator, flotation cells with corrugated plates pack, and high pressure nitrogen saturation system. Water from the API separator is first routed to the flocculator which is designed for 45 seconds retention time under mild turbulent conditions. In this section, oil and solids particles flocculate and grow under gentle agitation. The water then enters a mixing loop where high pressure nitrogen saturated water is injected. Because of lower pressure in the mixing loop, the nitrogen is released to form small bubbles. The water stream then enter the flotation cell where nitrogen bubbles float to the surface carrying the floc particles of oil and solids with it. Diagonal corrugated plates provide surface area to improve nitrogen/floc contact. Floating oil and solids are skimmed off from the top of the water. Clean water exits the bottom of the DGF cell and is pumped to the settling basin.
Operation of the DGF allows reduction of over 2MM lb/yr of oily water waste that would otherwise have to be disposed of off-site. Furthermore, it prepares the waste water stream for further recovery of benzene utilizing the steam stripping process. The new vent system on the API separator reduced approximately 8 M lb/yr of organic vapor, including 4M lb/yr benzene, vented to atmosphere.
Details of Reductions
Additional Information :
After start up of the DGF project, waste disposal cost associated with the oily water settling basin dropped significantly. Although this $1MM project was justified as environmental necessity project, gross savings on disposal cost was realized at $1.1MM per year, much higher than expected.