TECHNOLOGY FOCUS

Elimination of Oily Wastes 
Using Supercritical Water Oxidation

UNIVERSITY OF CADIZ, ANDALUISA, SPAIN

July 2006:
Introduction

Hydrothermal Oxidation is a high-performance ecological waste treatment method which is finding commercial application in the treatment of aqueous organic residues, and is particularly relevant for breaking down synthetic oils (including industrial cutting fluids and engine oil).

Such effluents are almost impossible to treat thoroughly using traditional techniques due to the high rupture strength of the emulsion, whereas Supercritical Water Oxidation (SCWO) achieves close to 100% destruction of toxic content at very high processing rates.

The technique exploits the extreme oxidation characteristics of supercritical water, (defined as 221 bar and 374 ºC). Under these conditions oils become soluble, and a single homogeneous reaction phase is obtained in which the organic compounds and oxygen are brought into contact. This results in an almost instantaneous break down of organic substances into harmless products, essentially water and CO2 (without the formation of NOx, SOx, CO or other products of incomplete oxidation).

The underlying technology offers improved operational flexibility, including treatment of insoluble wastes, and is now available for commercial exploitation.

The University of Cadiz, a leading center of research for ecological waste treatment, recently conducted a comprehensive study of the hydrothermal treatment of a wide variety of compounds representing the principal effluents generated by industrial machining (classified as dangerous waste under current regulations).

The research found the design achieved greater than 99% elimination of the contamination in only a few seconds. On the basis of these results, we consider the system to represent a significant breakthrough in the elimination of dangerous wastes, offering a commercially viable, ecological alternative to simply storing up toxic waste in tanks or ponds.

System Design

Researchers at the University of Cadiz have patented an innovative hydrothermal oxidation plant design with two independent liquid feed lines. The new design offers some major advantages in operational versatility compared to alterative hydrothermal systems, including total oxidation of insoluble industrial waste, which would otherwise be impossible to break down sufficiently using other techniques.

The first feed-line transports aqueous waste solution to the reactor, while the second line feeds insoluble waste. The first line is pressurized and preheated to the correct operating conditions before introduction into the reactor. The second feed-line is simply pressurized and introduced into the reactor without preheating.

Supercritical operating conditions are maintained inside the reactor where oils become completely soluble. The oxidizing agent, a stream of pressurized / preheated air, is then introduced and the oxidation reaction commences.

The plant consists of a thermally insulated tubular adiabatic reactor, fitted with concentric heat exchangers to take advantage of the energy generated during the reaction process. The plant operates in the auto-thermal regime, without the need for an external supply of heat, which reduces operational costs considerably.

Also, as the stream of insoluble residues is fed to the reactor in the absence of water, the power needed for the high pressure pump is reduced significantly, permitting greater control over the reaction process. In the event of excessive pressures or temperatures, the control system arrests the insoluble feed line, while the aqueous feed is maintained. This allows the system to rapidly return to ideal operating conditions, without needing to shut down the plant down completely.

After the reaction process has completed, the output stream is cooled and depressurized to ambient conditions and passed through a gas-liquid separator in which a harmless aqueous phase and a gaseous phase containing mainly carbon dioxide and nitrogen are produced.

Industrial Applications / Advantages

• The Cadiz design is suitable for treating any type of oily waste. Its greatest advantage is in the direct treatment of oily phases, lubricating oils, cutting fluids etc. The process eliminates down-stream physico-chemical processing, and produces a final effluent with a DQO of much less than 500 mg/L • Oxidation in supercritical water eliminates the contaminants completely, so from the environmental point of view it presents enormous advantages over conventional processes which produce toxic sludges which need to be rendered inert and then stored in dumps • Conventional approaches to hydrothermal oxidation do not allow insoluble waste to be directly injected into the reactor. The Cadiz system design therefore eliminates the difficulty associated with pumping biphasic blends • The final effluent of the process complies easily with the most demanding environmental standards, and can therefore be specified with confidence to anticipate any (more stringent) future regulations • The process developed provides an approach to the treatment of numerous types of industrial waste, including those not soluble in water. In addition, by feeding the wastes in concentrated form, without dilution in water, the equipment needed for high pressure pumping is simplified. • The existence of two independent feed lines significantly improves process regulation and eliminates the problems associated with attempting to pump by-phase waste • The automated control system permits both the performance and safety of the plant to be improved considerably. In the event of over-heating or excessive pressure build-up, it is not necessary to shut down the plant completely. Restoring operating conditions is therefore faster, simpler, and avoids costs associated with unnecessary shut-downs • The process is self-maintaining thermally, and can generate surplus energy capable of being exploited on an industrial scale • The process plant size is very small and therefore offers considerably improved space efficiency than traditional treatments.