Elsevier

Journal of Hazardous Materials

Volume 137, Issue 2, 21 September 2006, Pages 965-971
Journal of Hazardous Materials

Destruction of an industrial wastewater by supercritical water oxidation in a transpiring wall reactor

https://doi.org/10.1016/j.jhazmat.2006.03.033Get rights and content

Abstract

The supercritical water oxidation (SCWO) is a technology that takes advantage of the special properties of water in the surroundings of critical point of water to completely oxidize wastes in residence times lower than 1 min. The problems caused by the harsh operational conditions of the SCWO process are being solved by new reactor designs, such as the transpiring wall reactor (TWR). In this work, the operational parameters of a TWR have been studied for the treatment of an industrial wastewater. As a result, the process has been optimized for a feed flow of 16 kg/h with feed inlet temperatures higher than 300 °C and transpiring flow relation (R) between 0.2 and 0.6 working with an 8% (w/w) isopropanol (IPA) as a fuel. The experimental data and a mathematical model have been applied for the destruction of an industrial waste containing acetic acid and crotonaldehyde as main compounds. As the model predicted, removal efficiencies higher than 99.9% were obtained, resulting in effluents with 2 ppm total organic carbon (TOC) at feed flow of 16 kg/h, 320 °C of feed temperature and R = 0.32. An effluent TOC of 35 ppm under conditions feed flow of 18 kg/h, feed inlet temperatures of 290 °C, reaction temperatures of 570 °C and R = 0.6.

Introduction

Water above its critical point (374 °C and 22.1 MPa) exhibits excellent heat and mass transfer properties making supercritical water oxidation (SCWO) a powerful process for the treatment of industrial wastes and sludges. Supercritical water is completely miscible with organic solvents and with oxygen creating a homogeneous reaction media. This behavior joined with high temperatures allows high reaction yields. The resulting effluent complies with the most stringent environmental regulations and can be disposed without further treatment. Several papers have reviewed the main characteristics of the process [1], [2]. In the last few years, some commercial applications have been studied [3], [4] and computer models have been developed in order to scale-up [5].

Our group have been working in the SCWO process since 1994, developing a cool wall reactor for SCWO in a 30 kg/h pilot plant and tested it with industrial wastes such as cutting oils and PET effluents [6], [7], [8] obtaining excellent results. In addition, a demonstration plant [9], [10] located in an industrial site in Santovenia de Pisuerga (Valladolid, Spain) was constructed to treat 200 kg/h of waste. Previous results show that the suitable operation conditions in order to achieve high TOC removal efficiencies are not influenced by waste chemical composition. Temperatures above 650 °C, residence times around 50 s and stoichiometric amount of oxygen lead to TOC removal efficiencies over 99.9% [11].

Corrosion and salt deposition are still the two main challenges of SCWO. In order to overcome these two problems a number of reactor designs have been developed so far. One of the most recent designs has been the transpiring wall reactor (TWR), which consists of a reaction chamber limited by a porous wall through which clean water flows continuously. This cold water creates a thin layer that protects the wall against corrosive agents, salt deposition and extreme temperatures. During the last few years, a number of TWRs have been developed by several organisms. A complete description of these reactors, their operating characteristics and the wastes treated so far can be found in [12].

In the University of Valladolid a TWR was designed and constructed in order to deal with high salt content wastes. The aim of this paper is to study of the main operational parameters and conditions for the treatment of an industrial waste, to evaluate the feasibility of the new reactor design, in a previous step before dealing with high salt concentration wastes. The data obtained will be used to scale up the process to a demonstration plant in the near future.

For doing so, the study of the effect of the main operational parameters in TOC removal working with a synthetic feed with the goal of applying the results to the treatment of an industrial waste was carried out. The results of operation with a real wastewater from a chemical industry are given as an example. The low degradability of the selected waste makes it an excellent candidate to be treated by SCWO technology.

Section snippets

Experimental device

The transpiring wall reactor consists of a stainless steel high pressure shell with a volume of 10 L. It contains a reaction chamber limited by a sintered porous alloy wall through which clean water circulates. More information about the reactor performance and dimensions can be found in [13]. A complete description of the transpiring wall and of the of the operation problems is presented in the transpiring wall design 1 of [12].

In order to accurately follow the reaction, temperature is measured

Operational parameters

The objective is to study the effect of the main operational parameters in TOC removal working with a synthetic feed with the goal of applying the results to the treatment of an industrial waste. TOC removal is defined as the mass of organic carbon eliminated divided by the organic carbon introduced in the reactor in Eq. (1):TOCRem=FTOC0(F+FFT)TOCefFTOC0The main operational parameters are feed flow, fuel concentration, feed inlet temperature, transpiring flow, transpiring flow temperature and

Conclusions

A transpiring wall reactor has been developed in the SCWO pilot plant of the University of Valladolid. Experiments have been performed to study the operational conditions of the reactor. To accomplish this, the reactor was tested at progressively higher feed flows ranging from 2.5 to 18 kg/h, and progressively lower feed temperatures in order to increase the difficulty of TOC removal. Pressure was always maintained at 23 MPa. For each feed flow, transpiring flow was changed. It is only possible

Acknowledgments

Authors wish to thank CETRANSA for providing technical and financial support. This project has been supported by “Supercritical Fluids and Materials Network. SUPERMAT Interreg Atlantic III” funds. M.D.B. wants to thank the Ministerio de Educación y Ciencia for the FPU Grant.

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