TREATMENT OF SODIUM SULFIDE BEARING WASTE WATER

Abstract

Petroleum as processed in a typical refinery has acid components, which must be reduced or eliminated. These acid components may be organic or inorganic in nature, which are components of crude oil or created during refining process. The inorganic components may be sulfur and hydrogen sulfide whereas the organic components may be phenol, cresylic and naphthenic acids, mercaptans and carbon dioxide. These acid gases are removed by scrubbing with caustic to form sulfides, carbonates, mercaptides, phenols and other soluble or emulsified organic compounds. This stream is called the spent caustic stream and its strength in terms of chemical oxygen demand (COD) is quite high ranging from 20000 --400000 mg/I and pH in between 13 and 14. Spent caustic streams are highly odorous and contain very complex organic species. The majority of chemical oxygen demand for olefin plants caustic stream is from inorganic sulfides as sodium sulfide, sodium bisulfide, mercaptans with 1000 mg/I of sufides as S-2 equivalent to 2000 mg/1 of chemical oxygen demand. This spent caustic stream containing sulfides are not biodegradable and hence it must be subjected to pretreatment before making it biodegradable. The pretreatment technologies available are wet air oxidation (WAO), incineration, neutralization followed by steam stripping, catalytic wet air oxidation, and supercritical oxidation. Wet air oxidation is ideally suited for spent caustics since it is too dilute to incinerate and too refractory to be handled by biological oxidation. Wet air oxidation is also suitable from economic point of view. So wet air oxidation was used to treat sodium sulfide, which is a major component of spent caustics. Sine the intermediates like sodium thiosulfate have significant residual chemical oxygen demand and pose design and operational problem for biological treatment unit, the formation of sodium sulfate was given special attention. The experiments were carried out in a new high-pressure reactor and synthetic mixture was prepared in tune with the amount of sulfide coming out from the refinery. The sulfide content of mercaptans and sodium bisulfide was accounted for by taking in terms of sulfide content of sodium sulfide as a whole. Experiments were carried out for various parameters and samples taken during oxidation were analyzed for sulfate, thiosulfate and sulfide content by gravimetric and iodometric methods. Both air and oxygen were used as oxidants and the effect of all possible parameters were studied for each of them. The parameters that were varied are temperature, pressure, catalyst loading, residence time and initial concentration of sodium sulfide. Since oxidation of sodium sulfide is known to proceed through a free radical mechanism, the effect of free radical initiators like hydrogen peroxide was also studied. The effect of phenol, which is a spent caustic constituent, was also studied. The rate of oxidation increases with the use of heterogeneous copper sulfate catalyst over non catalytic oxidation. The increase in temperature and pressure and residence time increases rate and hence conversion. However it was observed that the effect of temperature and pressure were not much pronounced at high initial concentration of sodium sulfides of 50 kg/m3. The increase in catalytic loading increases conversion for any initial concentration. Initial concentration of sodium sulfide entering the reactor has a drastic effect on the conversion. Low concentration of sulfides results in greater conversion to sulfate whereas high concentration of sulfides results in more thiosulfate formation. More than 70% of sodium sulfide was oxidized irrespective of the initial concentration, in the first one hour of operation. Free radical initiators like hydrogen peroxide decreases catalytic conversion of sulfide due to high pH of the solution. The presence of any trace organics like phenol was found to decelerate the oxidation rate. Oxygen was found to be more effective than air due to increased solubility in the liquid phase. The spent caustic from Indian Oil Corporation, Paniphat was analyzed and treated by wet oxidation using oxygen. The high sulfide content was oxidized completely at lower temperature by a high catalyst loading.