Juan R. Portela

Kinetic comparison between subcritical and supercritical water oxidation of phenol

Wet air oxidation (WAO) and supercritical water oxidation (SCWO) processes have been studied by numerous researchers, proving their effectiveness to treat a wide variety of wastes and presenting the kinetics involved in each case. As a result, a substantial amount of kinetic information describing organic reactions in those environments has been accumulated. In most cases, predictions from kinetics models obtained below and above the critical point of water are completely different. Furthermore, predictions from kinetic expressions obtained in the same range of operating conditions vary considerably. Phenol is a model pollutant that has been the subject of numerous studies both in subcritical and supercritical conditions. In this work, both batch and continuous flow reactors have been used to compare the kinetics obtained for phenol oxidation at subcritical and supercritical conditions. Moreover, most of the rate expressions available in the literature have been compared in order to find the reasons for the discrepancies found.

Elimination of cutting oil wastes by promoted hydrothermal oxidation

Cutting oils are emulsionable fluids widely used in metalworking processes. Their composition is normally oil, water, and additives (fatty acids, surfactants, biocides, etc.) generating a toxic waste after a long use. Generally, it is a waste too dilute to be incinerated and it is difficult to treat biologically. Other conventional treatment methods currently used are not satisfactory from the environmental point of view. Wet air oxidation (WAO) and supercritical water oxidation (SCWO) are two forms of hydrothermal oxidation that have been proved to be effective processes to treat a wide variety of industrial wastes, but hardly tested for oily wastes. In the case of refractory wastes, WAO process is not efficient enough due to the moderate temperatures used. SCWO is a more powerful process since operating temperatures are usually around 600 degrees C, but the use of severe conditions leads to major disadvantages in the commercialization of the technology. In order to enhance WAO and SCWO efficiency at mild conditions, the use of free radical promoters has been studied in this work. Both normal and promoted hydrothermal oxidation have been tested to treat cutting oil wastes in a continuous flow system operating at 300-500 degrees C. Hydrogen peroxide has been used both as a source of oxygen and as a source of free radicals by introducing it into the reactor with or without previous thermal decomposition, respectively. Organic material is easily oxidized in both cases, obtaining more than 90% TOC reduction in less than 10s at 500 degrees C. At lower temperatures, the use of promoters clearly enhances the oxidation process. Activation energies have been estimated for normal and promoted oxidation processes.

Advanced thermal hydrolysis: optimization of a novel thermochemical process to aid sewage sludge treatment.

The aim of this work was to study in depth the behavior and optimization of a novel process, called advanced thermal hydrolysis (ATH), to determine its utility as a pretreatment (sludge solubilization) or postreatment (organic matter removal) for anaerobic digestion (AD) in the sludge line of wastewater treatment plants (WWTPs). ATH is based on a thermal hydrolysis (TH) process plus hydrogen peroxide (H(2)O(2)) addition and takes advantage of a peroxidation/direct steam injection synergistic effect. On the basis of the response surface methodology (RSM) and a modified Doehlert design, an empirical second-order polynomial model was developed for the total yield of: (a) disintegration degree [DD (%)] (solubilization), (b) filtration constant [F(c) (cm(2)/min)] (dewaterability), and (c) organic matter removal (%). The variables considered were operation time (t), temperature reached after initial heating (T), and oxidant coefficient (n = oxygen(supplied)/oxygen(stoichiometric)). As the model predicts, in the case of the ATH process with high levels of oxidant, it is possible to achieve an organic matter removal of up to 92%, but the conditions required are prohibitive on an industrial scale. ATH operated at optimal conditions (oxygen amount 30% of stoichiometric, 115 °C and 24 min) gave promising results as a pretreatment, with similar solubilization and markedly better dewaterability levels in comparison to those obtained with TH at 170 °C. The empirical validation of the model was satisfactory.

Remediation of PAH spiked soils: concentrated H2O2 treatment/continuous hot water extraction-oxidation.

Artificially contaminated soil with four different polynuclear aromatic hydrocarbons (acenaphthene, phenathrene, anthracene and fluoranthene) has been separately treated by two different processes: (A) concentrated hydrogen peroxide at mild conditions of temperature (343-393 K) and pressure (0.5 MPa) and (B) hot water extraction at relatively high temperature (523-657 K) and pressure (10 MPa). Both methods achieve acceptable PAH removal percentages from soil. Acenaphthene (the most soluble PAH) is completely removed with treatment A regardless of the operating conditions used. Under optimum conditions, the rest of PAHs are also eliminated to a high extent with both technologies. Temperature and hydrogen peroxide amount seem to play a major role in process A. Similarly, temperature and water flowrate are the most influencing parameters in process B. In the latter case, a post-stage for the extracting water cleaning is required.

Supercritical Water Oxidation for Wastewater Destruction with Energy Recovery

Abstract Supercritical water oxidation (SCWO) is a promising technology that respecting the environment destroys wastes definitely and allows an energy recovery. This process has been applied to many model compounds and real wastewaters at laboratory scale. However SCWO treatments at pilot plant scale of real wastewaters are much more limited in literature. Furthermore, the application of this technology to industrial wastewaters has some drawbacks as corrosion, salt deposition, management of biphasic wastes, presence of suspended solids and high costs, so nowadays the industrial scale-up is scarce and it is being delayed. As an attempt to reduce process costs, energy recovery from the effluent of the reactor has been studied by several authors. In this chapter, the main aspects of the SCWO are briefly described and the studies regarding energy recovery are summarized.

Supercritical CO2 extraction of PAHs on spiked soil: co-solvent effect and solvent regeneration by ozonization.

The supercritical CO(2) extraction of four PAHs (acenaphthene, phenanthrene, anthracene and fluoranthene) from an artificially contaminated soil has been investigated. The effect of temperature (40-60 degrees C), pressure (300-500 bar) and extraction time (90-150 min) has been assessed by conducting a Box-Behnken experimental design. The results suggest the existence of perturbation variables other than the aforementioned controlled variables leading to a significant dispersion of extraction recoveries. With the exception of anthracene, an optimum in temperature (50 degrees C) is envisaged when extracting the PAHs. Analogously, with the exception of anthracene (positive effect), pressure does not have a significant influence. The recovery yield increases as extraction time is increased to a value of 120 min. No further improvement is experienced thereafter. If a co-solvent is used (H(2)O(2) aqueous solution) a beneficial effect can be noticed. Hydrogen peroxide concentration did exert no significant influence in the process. Methanol used to collect the extracted PAHs could be regenerated by gaseous ozone and reused in several consecutive runs.

Chapter 10 – Supercritical Water Gasification of Organic Wastes for Energy Generation

Nowadays, numerous studies are focused on finding nonpolluting energy sources. Among others, wet biomass waste is one of the most promising renewable resource and its conversion into a suitable form of energy, usually electricity or fuel, can be achieved using different routes, each one with specific advantages and disadvantages. A review of the main conversion processes is presented, among which it is possible to highlight those which use high pressure. In Supercritical Water Gasification, the supercritical water is not only a solvent for organic materials but also a reactant. In this way, besides the destruction of wastewaters, it is aimed to harness their energy potential by burning the gas effluent generated in the process to produce electrical power, due to its high content in hydrogen and light hydrocarbons.

Supercritical water gasification: a patents review

Abstract Supercritical water gasification (SCWG) is a very recent technology that allows conversion of organic wastewaters into a fuel gas with a high content of hydrogen and light hydrocarbons. SCWG involves the treatment of organic compounds at conditions higher than those that define the critical point of water (temperature of 374°C and pressure of 221 bar). This hydrothermal process, normally operated at temperatures from 400 to 650°C and pressures from 250 to 350 bar, produces a gas effluent with a high hydrogen content. SCWG is considered a promising technology for the efficient conversion of organic wastewaters, mainly wet biomass, into fuel gas. This technology has received extensive worldwide attention, and many research groups have studied the effect of operation conditions, reaction mechanisms, kinetics, etc. There are some recent reviews about the research works carried out in the last decades, but there is no information or analysis of almost 100 patents registered in relation with this new technology. A revision of the current status of SCWG patents and technologies has been completed based on the Espacenet patent database. The objective of this revision was to set down the new perspectives toward the improvement of this technology efficiency. Patents have been published with regard to process or device improvements as well as to the use of different catalysts. More than 71% of these patents were published since 2009, and a substantial climb in the number of patents on SCWG is expected in the coming years. One of the most important aspects where research is particularly interesting if the integration of renewable energy recovery systems with SCWG processes.

Energy Production by Hydrothermal Treatment of Liquid and Solid Waste from Industrial Olive Oil Production

This work studies the use of olive oil mill waste ( OMW ) treated as subcritical or supercritical water to produce both, a biofuel by liquefaction and a gas fuel by gasification. The increasing amount of OMW , both liquid and solid, is becoming a serious environmental problem. This wastewater is highly resistant to biodegradation and contains a wide variety of compounds such as polyphenols, polyoils, organic acids, etc, that require depuration treatments to remove the odour and pollutant load before being discharged. This work studies both, liquefaction and gasification of OMW streams in subcritical and supercritical water in different batch reactors at temperatures between 200 and 530 ÂoC and pressures between 150 and 250 bar. This study also tests the effectiveness of various types of homogeneous (KOH 0.01 g/g sample dry ) and heterogeneous catalysts (TiO 2 , V 2 O 5 and Au-Pd 0.1-0.5 g/g sample dry ) for supercritical water gasification (SCWG) and studied the way they affect biomass conversion yields. It also covers the effect that the use of different organic compound concentrations (23, 35, and 80 g O 2 /l of chemical oxygen demand concentration (COD)) and compositions (mixtures of solid and liquid OMW ) has on energy production results. A maximum of 82% oil yield was obtained from the hydrothermal liquefaction of OMW under optimum conditions (330 ÂoC, 150 bar, 23 g O 2 /l as initial concentration and 30 minutes reaction time). Meanwhile, a yield of 88.6 mol H 2 /kg OMW dry was obtained when Au-Pd was used as a catalyst for the gasification of OMW supercritical water.

Supercritical Water Oxidation

Abstract Supercritical water oxidation (SCWO) is a powerful green technology to treat hazardous wastewaters. This process has been extensively studied for decades from laboratory scale studies of model compounds to several demonstration and industrial plants under construction worldwide. However, SCWO treatments at pilot plant scale of real wastewaters are much less extensive in literature. Furthermore, the application of this technology to industrial wastewaters has two main drawbacks such as corrosion and salt deposition, and some other problems to be solved related to management of biphasic wastes, presence of suspended solids, high costs, etc.; and therefore, currently, the industrial scale-up and commercialization of this process is still subject to difficulties.