Ekin Kıpçak

Treatment of whey wastewater by supercritical water oxidation

Whey wastewater is a by-product of cheese industry, which causes environmental pollution problems due to its containment of heavy organic pollutants. Conventional methods such as biological treatment and physico-chemical treatment are insufficient or ineffective. In this paper, the treatment of cheese whey wastewater has been carried out by supercritical water oxidation, using hydrogen peroxide as oxidant. The reaction conditions ranged between temperatures of 400-650°C and residence times of 6-21 s under a pressure of 25 MPa. Treatment efficiencies based on TOC removal were obtained between 75.0% and 99.81%. An overall reaction rate model, which consists of the hydrothermal and the oxidation reactions, was determined for the hydrothermal decomposition of the wastewater with an activation energy of 50.022 (±1.7) kJmol(-1) and a pre-exponential factor of 107.72 (±4.1) s(-1). The oxidation reaction rate orders for the TOC and the oxidant were 1.2 (±0.4) and 0.4 (±0.1) respectively, with an activation energy of 20.337 (±0.9) kJmol(-1), and a pre-exponential factor of 1.86 (±0.5) mmol(-0.6)L(0.6)s(-1) in a 95% confidence level.

In situ gas fuel production during the treatment of textile wastewater at supercritical conditions

Supercritical water gasification has recently received much attention as a potential alternative to energy conversion methods applied to aqueous/non-aqueous biomass sources, industrial wastes or fossil fuels such as coal because of the unique physical properties of water above its critical conditions (i.e. 374.8 °C and 22.1 MPa). This paper presents the results obtained for the hydrothermal gasification of textile wastewater at supercritical conditions. The experiments were carried out at five reaction temperatures (between 450 and 650 °C) and five reaction times (between 30 and 150 s), under a constant pressure of 25 MPa. It was found that the gaseous products contained considerable amounts of hydrogen, carbon monoxide, carbon dioxide, and C(1)-C(4) hydrocarbons, such as methane, ethane, propane and propylene. The maximum amount of the obtained gaseous product was 1.23 mL per mL textile wastewater, at a reaction temperature of 600 °C, with a reaction time of 150 s. At this state, the product comprised 13.02% hydrogen, 38.93% methane, 4.33% ethane, 0.10% propane, 0.01% propylene, 7.97% carbon monoxide, 27.22% carbon dioxide and 8.00% nitrogen. In addition, a 62.88% decrease in the total organic carbon (TOC) content was observed and the color of the wastewater was removed. Moreover, for the hydrothermal decomposition of the textile wastewater, a first-order reaction rate was designated with an activation energy of 50.42 (±2.33) kJ/mol and a pre-exponential factor of 13.29 (±0.41) s(-1).

Hydrogen Production by Supercritical Water Gasification of Biomass

It is widely accepted that hydrogen energy can sustainably provide the world’s growing energy needs. Currently, hydrogen is produced from mainly nonrenewable feedstocks through biochemical and thermochemical technologies. However, to achieve a genuinely sustainable, economic, viable, and environmentally benign technology, hydrogen will need to be produced from renewable energy sources through innovative production processes. This chapter focuses on one of these technologies that provide a novel approach for hydrogen production: supercritical water gasification of biomass. The process has significant potential for the conversion of biomass to produce hydrogen and other combustible gases. It also has major advantages when compared with other processes, such as eliminating the necessity for drying of the feedstock, providing high gasification efficiency and hydrogen selectivity, enabling the formation of clean gaseous products, and producing much lower amounts of tars and chars. To increase the hydrogen selectivity, the use of catalysts is common, with the preferred catalysts being alkaline salts, some metals, and metal oxides. The supercritical water gasification process can exhibit different gas compositions or activities with respect to the feedstock, reaction conditions, or catalyst used. Therefore, in this chapter, hydrogen production from various biomass sources by supercritical water gasification is comparatively discussed with examples from the literature. The term biomass covered in this chapter includes model compounds (such as glucose, cellulose, and lignin), alcohols, and real biomass (such as industrial wastewaters and sewage sludge). The effects of reaction time, system temperature and pressure, biomass concentration, oxidant concentration, catalyst use, and the kind of catalyst on the hydrogen yield are investigated.