Treatment and Recovery of Spent Caustic by Catalytic Oxidation
Spent caustic from olefin units in petrochemical complexes is a highly alkaline wastewater stream that may contain sulfides, hydrogen sulfide, mercaptans, sodium mercaptides, phenolic compounds and high chemical oxygen demand (COD). Because of its unpleasant odour, toxicity, corrosivity and high pollutant loading, spent caustic must undergo appropriate treatment before disposal, transfer to a wastewater treatment unit or reuse.
In a published study, the cobalt phthalocyanine catalyst IVKAZ was investigated for regeneration and treatment of spent caustic from an olefin unit. The proposed process consists of two main stages:
Catalytic air oxidation followed by chemical precipitation with lime.
Process structure
In the first stage, spent caustic enters a reactor. Compressed air is introduced into the liquid through a porous distributor at the bottom of the reactor, and IVKAZ catalyst is added to the reaction medium.
IVKAZ accelerates oxidation of sulfur compounds in the caustic, particularly sulfides and mercaptides.
The principal reactions in this stage include:
NaSH + 2O₂ + NaOH → Na₂SO₄ + H₂O
NaSH + 2O₂ → ½Na₂S₂O₃ + ½H₂O
And for sodium mercaptide oxidation:
2RSNa + ½O₂ + H₂O → RSSR + 2NaOH
In the third reaction, sodium mercaptides are converted into organic disulfides while some caustic soda is regenerated.
Second stage: precipitation and caustic recovery
Following oxidation, the first reactor effluent enters a stirred reactor. Lime, or calcium hydroxide (Ca(OH)₂), is added. The purpose of lime addition is to precipitate some of the oxidised compounds while increasing caustic soda recovery. The principal reactions reported for this stage are:
Na₂SO₄ + Ca(OH)₂ → CaSO₄(s) + 2NaOH
Na₂CO₃ + Ca(OH)₂ → CaCO₃(s) + 2NaOH
As a result, calcium sulfate and calcium carbonate form as solids, and some sodium hydroxide is produced again in solution.
The lower part of the second reactor can be used as a settling and decanting zone to separate precipitated solids from the regenerated caustic solution.
Laboratory process results
In the studies conducted, the initial spent caustic characteristics were approximately as follows:
| Parameter | Value |
|---|---|
| NaOH concentration | approximately 1 wt% |
| COD | approximately 15,000 mg/L |
| Sulfide concentration | approximately 670 mg/L |
| Phenol | approximately 300 mg/L |
| pH | approximately 13.5 |
Initial spent caustic properties
Among the conditions tested, one of the best results was obtained under the following conditions:
| Parameter | Value |
|---|---|
| Temperature | approximately 90°C |
| Air flow | 7 L/min |
| Reaction time | 30 minutes |
Experimental conditions
Under these conditions:
NaOH concentration increased from approximately 1 wt% to approximately 6 wt%, and sulfide concentration decreased from 670 to 362 mg/L. COD reached approximately 5,050 mg/L, and sulfide reduction was reported as approximately 45%.
Based on various articles in this field, it can be concluded that recovered caustic may be returned to the alkaline washing section of an olefin unit after its quality and operating conditions have been assessed.
Process flow diagram:
The role of IVKAZ catalyst in caustic regeneration
The main role of IVKAZ in this process is to accelerate oxidation of sulfur compounds in the alkaline medium.
Sulfides and mercaptides in spent caustic are converted into more stable compounds in the presence of IVKAZ catalyst and compressed air.
This process is significant in several respects:
Recovery of some caustic soda and the possibility of its reuse
Reduction of sulfide and mercaptide concentrations
Reduction of wastewater COD
Reduction of spent caustic odour and toxicity
Reduction of the load entering wastewater treatment
Use of milder operating conditions than some high-pressure and high-temperature air oxidation methods
Potential application in olefin and petrochemical units
In olefin units, caustic soda solution is used to remove acidic and sulfur-containing compounds from hydrocarbon streams. Over time, accumulation of sulfides, mercaptides, phenolic compounds and other pollutants converts this solution into spent caustic. An IVKAZ-based process can be investigated as an option for treating, regenerating and reducing the pollutant load of this stream.
Summary
Using IVKAZ catalyst in combination with compressed-air oxidation and lime precipitation is a method worth investigating for regeneration and treatment of spent caustic from olefin and petrochemical units. IVKAZ accelerates oxidation of sulfides and mercaptides, while the subsequent calcium hydroxide precipitation stage enables separation of some oxidised compounds and further caustic soda recovery. In addition to reducing pollutant loading and wastewater COD, this technology may reduce fresh caustic consumption and the volume of wastewater requiring final treatment. These studies were conducted at laboratory scale. Industrial design requires separate assessment of actual spent caustic characteristics; initial NaOH, sulfide, mercaptide, phenol and COD concentrations; flow rate; IVKAZ catalyst dosage; gas–liquid mass transfer; air distributor design; residence time; lime consumption; solids separation; temperature control; and the required recovered caustic quality.
Artin Azma Mehr, with exclusive representation for production of this catalyst under licence from VINIUS and IVKAZ of Russia, supplies IVKAZ catalysts and related technologies and provides specialist services in demercaptanization, hydrocarbon stream sweetening, treatment of sulfur-containing alkaline wastewater and assessment of spent caustic regeneration in refining, petrochemical and gas processing industries.
References
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