(93e) Sulfide Refinery Waste Treatment

This paper describes the available technologies for the treatment and disposal of refinery or petrochemical plant spent caustic waste. If the caustic stream is disposed directly into a water influent stream, it will place a burdensome oxygen demand, both chemical and biological, on the receiving stream. The sulfide and bisulfide content of the stream represent a large immediate oxygen demand and, therefore require oxidation, neutralization, and/or stripping prior to their release to the environment. Fish and other aquatic populations are adversely affected, and sometime eliminated, for several miles down the stream from the plant discharge.

Several methods were tried to treat the caustic waste, including oxidation, electric cells, membrane separation, and innovative chemical precipitation reaction. On the other hand the reuse of the waste in other industries was investigated.

Waste samples were supplied by Qatar Petroleum. Qatar petroleum has adopted a zero discharge policy and has determined to solve all environmental problems in a safe efficient manner.

Membrane separation was tested however the reject stream (20%) will remain a problem. The electric cell using different electrode caused hydrogen sulfide creation. The oxidation using different oxidizing agent was successful but costly. The innovative way to solve the problem was the use of magnesium chloride or sulfide. It will react, under certain conditions, with sodium sulfide and sodium bisulfide and form magnesium sulfide or magnesium bisulfide. Both compounds are unstable and in the air they are easily oxidized to magnesium sulfate. Where the later could be used again and again leaving clean water behind.