A comparative study of the treatment of ethylene plant spent caustic by neutralization and classical and advanced oxidation

Performance of electro-Fenton process for the treatment of synthetic sulphidic spent caustic waste stream generated from petroleum refineries

Sulphidic spent caustic (SSC) is an alkaline waste stream which is generated during caustic scrubbing of liquefied petroleum gas and ethylene products. Due to presence of high concentrations of sulphides and phenols, the waste stream requires proper treatment before mixing with the low strength wastewater streams produced from other refinery operations. Electrochemical process is an emerging treatment method that can work efficiently at ambient conditions. The present study reports performance of electro-Fenton (EF) process for the treatment of synthetic SSC wastewater (sulphides = 10 g L-1, phenol = 2 g L-1 and pH = 12.9). The EF runs were carried out for 2 h duration in a reactor equipped with iron electrodes. The effects of H2O2 dose (0.26-1.3 M), current density (1-20 mA cm-2), pH (4.5-12.9) and stirring speed (100-1000 rpm) were investigated on removal of pollutants. The H2O2 was rapidly consumed in initial 30 min during which the significant fraction of the pollutants was degraded or removed. The optimum conditions for EF process were found to be as follows: pH = 4.5, H2O2 dose = 1.05 M, current density = 5 mA cm-2 and stirring speed = 500 rpm. At these conditions, the maximum sulphide and phenol removals from the wastewater were 98% and 91%, respectively. The results will be helpful to the wastewater treatment plant operators worldwide dealing with high concentrations of such pollutants.

Treatment of Olefin plant spent caustic by combination of Fenton-like and foam fractionation methods in a bench scale

Spent Merox caustic (SMC) is a hazardous waste that is produced during the Merox desulfurization process in the petroleum refinery industry and should be treated before discharging to environment. In the present study, treatment of SMC was investigated by three methods including Fenton-like process, foam fractionation, and a combination of both processes. Immobilized TiO₂/Fe⁰ on modified silica nanoparticles was used as a heterogeneous Fenton-like catalyst. The chemical and physical characteristics of the catalyst were determined using Fourier-transform infrared spectroscopy, X-ray diffraction, diffuse reflectance spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and transmission electron microscopy techniques. The treatment performance of the combined method was measured as a cost-effective method with chemical oxygen demand (COD) removal percentage. The effect of parameters including pH, gas flow rate, surfactant type and concentration of hydrogen peroxide, catalyst, and chelate were investigated. It is found that the prepared heterogeneous catalyst has high activity for the treatment of SMC. In addition, the results showed that the combined method achieved 97.6 ± 0.5% COD removal, while the measured values for Fenton or foam fractionation methods alone did not exceed 85.5 ± 1% and 47.2 ± 0.4%, respectively. The advantage of combination process over foam fractionation was the use of an advanced oxidation process in the separating column to eliminate or reduce the secondary phase contamination load. Besides, the role of the column in the effective contact of contaminants with the rising bubbles improved the degradation performance of the proposed process and reduced the consumption of hydrogen peroxide by 46% compared to the Fenton-like method.

Comparison of two methods of neutralization and wet air oxidation for treating wastewater spent caustic produced by oil refineries

Today, problems of treating wastewater spent caustic produced by refinery units with high toxic compounds and chemical oxygen demand (COD) become concerns of many managers of these industries and environmental experts. Hence, emission without application of treatment methods will have adverse environmental impacts. In this study, two direct acid neutralization (DAN) and wet air oxidation (WAO) processes were selected to treatment the wastewater of the Bandar Abbas oil refinery in southern Iran. The aim was to reduce COD and harmful substances, compare the two methods, optimize processes, and evaluate their performance. Experimental experiments were performed in a reactor system with different input variables related to two different methods. Parameter optimization was performed based on Box-Behnken (BBD) method and Design Expert software. The analysis of the results based on statistical methods and response procedure diagrams was used to evaluate the status of COD changes to the parameters. The best operating conditions of temperature, pressure, residence time, and stoichiometric coefficient of air were 148.269 °C, 15.716 bar, 3.563 h, and 8.415 l/h respectively in WAO with 68% reduction in COD, and for DAN process, temperature, pH, and agitation speed were 30.082 °C, 2.008, and 203.672 rpm, respectively, with 43% reduction in COD. Results of rapid impact assessment matrix (RIAM) showed that WAO process with a higher score is a more environmentally friendly method and DAN process has been considered by experts due to its popularity, ease of testing, less equipment requirements, and lower cost.

Electrochemical treatment of industrial sulfidic spent caustic streams for sulfide removal and caustic recovery

Alkaline spent caustic streams (SCS) produced in the petrochemical and chemical manufacturing industry, contain high concentrations of reactive sulfide (HS^-) and caustic soda (NaOH). Common treatment methods entail high operational costs while not recovering the possible resources that SCS contain. Here we studied the electrochemical treatment of SCS from a chemical manufacturing industry in an electrolysis cell, aiming at anodic HS^- removal and cathodic NaOH, devoid of sulfide, recovery. Using a synthetic SCS we first evaluated the HS^- oxidation product distribution over time, as well as the HS^- removal and the NaOH recovery, as a function of current density. In a second step, we investigated the operational aspects of such treatment for the industrial SCS, under 300 A m^-2 fixed current density. In an electrolysis cell receiving 205 ± 60 g S L^-1 d^-1 HS^- over 20 days of continuous operation, HS^- was removed with a 38.0 ± 7.7 % removal and ∼80 % coulombic efficiency, with a concomitant recovery of a ∼12 wt.% NaOH solution. The low cell voltage obtained (1.75 ± 0.12 V), resulted in low energy requirements of 3.7 ± 0.6 kW h kg^-1 S and 6.3 ± 0.4 kW h kg^-1 NaOH and suggests techno-economic viability of this process.

Characteristics and Treatment of Wastewater from the Mercaptan Oxidation Process: A Comprehensive Review

Sulfur compounds are removed from petroleum by the addition of sodium hydroxide at a very high concentration. As a result, a residue called spent soda or spent caustic is generated, being extremely aggressive to the environment. In this work, the chemical properties of this residue are described in detail. The sodium hydroxide remains that have not reacted, sulfur compounds, and organic matter are the primary pollutants reported. Additionally, the main characteristics of the methods of treatment used to reduce them are described. This review comes from comprehensive and updated research and bibliographic analysis about the investigation on the topic. The advantages and disadvantages of the different treatment methods are highlighted. We established some criteria to set out when assessing the application of each one of these treatments is considered.

Intensification of industrial wastewater treatment using hydrodynamic cavitation combined with advanced oxidation at operating capacity of 70 L

The present work deals with treatment of industrial wastewater with hydrodynamic cavitation (HC) combined with advanced oxidation processes (AOPs) at a pilot plant operating at a capacity of 70 L. Initially approach of only HC was studied with understanding into the effect of parameters like inlet pressure (range of 2-8 bar) and dilution factor (two cases with dilution ratio as 1:1 and 1:3). Only HC was found to give minimal chemical oxygen demand (COD) removal within 180 min and the further focus of the study was aimed to test efficiency of different oxidants in combination with HC. Oxidants used in treatment were hydrogen peroxide (H₂O₂), Fenton's reagent, air and oxygen. Effect of H₂O₂ loading was analyzed with variation of loading in the range of 5-15 g.L^-1 and maximum COD removal of 30% was achieved at 15 g.L^-1 loading of H₂O₂. Keeping 15 g.L^-1 as the constant loading of H₂O₂, optimization of Fenton based treatment was performed with variation of Fe²⁺/H₂O₂ molar ratio in the range of 1:10-1:30. Maximum COD removal of 42% was achieved at best molar ratio of 1:20. For the very first time, air and oxygen injection were tested in combination of HC giving 15.5% and 42% COD removal respectively. Combination study of oxygen + Fenton + HC was performed at H₂O₂ loading of 15 g.L^-1 and Fenton molar ratio as 1:20, which resulted in 63% COD removal within 180 min of treatment. Energy efficacy analysis in terms of COD reduction per unit energy and cost calculations were also performed and for process giving maximum of 63% COD removal in 180 min, cost of treatment obtained was 398 US$/m³. It was clearly established that HC in combination of Fenton and oxygen is the best approach for treatment of complex industrial wastewater.

A BiOCl/bipolar membrane as a separator for regenerating NaOH in water-splitting cells

NaOH regeneration from spent caustic using a BiOCl/BPM configuration.