Wet Air Oxidation

When the organic sludge is heated, heat causes water to escape from the sludge. Thermal treatment systems release water that is bound within the cell structure of the sludge and thereby improves the dewatering and thickening characteristics of the sludge. The oxidation process further reduces the sludge to ash by wet incineration (oxidation). Sludge is ground to a controlled particle size and pumped to a pressure of about 300 psi. Compressed air is added to the sludge (wet air oxidation only), the mixture is brought to a temperature of about 350° F by heat exchange with treated sludge and direct steam injection, and then is processed (cooked) in the reactor at the desired temperature and pressure. The hot treated sludge is cooled by heat exchange with the incoming sludge. The treated sludge is settled from the supernatant before the dewatering step. Gases released at the separation step are passed through a catalytic after-burner at 650 to 705° F or deodorized by other means. In some cases these gases have been returned through the diffused air system in the aeration basins for deodorization. The same basic processes is used for wet air oxidation of sludge by operating at higher temperatures (450 to 640° F) and higher pressures (1200 to 1600 psig). The wet air oxidation (WAO) process is based on the fact that any substance capable of burning can be oxidized in the presence of water at temperatures between 250° F and 700° F. Wet air oxidation does not require preliminary dewatering or drying as required by conventional air combustion processes. However, the oxidized ash must be separated from the water by vacuum filtration, centrifugation, or some other solids separation technique.

Wet-air oxidation (also called liquid-phase thermal oxidation) is not a new technology; it has been around for over forty years and has already demonstrated its great potential in wastewater treatment facilities. Despite this, there are some very important issues that remain to be addressed before a wet oxidation process can be scaled-up: the kinetics of oxidation of many important hazardous compounds are as yet unavailable, to mention only one among them. However, the kinetic models that predict solely the disappearance rate of mother compounds usually reported in the open literature are not enough; what is needed is a model capable of predicting complete conversion of all organic species present in a wastewater. Such models have to rely on the use of lumped parameters such as total organic carbon (TOC), chemical oxygen demand (COD), and biochemical oxygen demand (BOD). To point out a reaction engineering problem associated with the designing of a well established subcritical wet oxidation reactor, one can assume the TOC reduction to be linearly dependent on both reactant, organic compounds and oxygen. With a large excess of oxygen, the oxidation can be considered as a pseudo-first order reaction with the Hatta number defined as:

If Ha2 > > 1, the entire oxidation reaction occurs within the liquid film and when Ha2 < < 1, then most of the reaction occurs beyond the film, i.e. in the bulk liquid phase. In the latter case, the bulk liquid volume controls the rate with no benefit from an increase of the interfacial area. However, the Hatta number is a criterion which tells us whether the oxidation would occur in the bulk liquid-phase, therefore necessitating a large volume of liquid, or completely in the boundary layer, which calls for contacting devices that provide for a large interfacial area. In order to employ the principle of the Hatta number, in a system of complex reactions such as oxidation of organics in wastewater, a multidimensional space has to be reduced by lumping the species with similar reactivity. For kinetic analysis and design purposes, the species originally present in a wastewater or produced during the course of oxidation are conveniently divided at least into three lumps: (i) original compounds and relatively unstable intermediates, (ii) high molecular mass organic acids, and (iii) low molecular mass organic acids. As it has been shown for catalytic cracking, the lumped oxidation kinetics in many cases also obey the power-law form, with the exponent for a continuous-stirred tank reactor (CSTR) being lower than that for a plug-flow reactor (PFR) or a batch reactor (BR). It has been demonstrated that the kinetic behavior of a reactive mixture of organics in a batch system is governed by the most refractory lump, i.e. the lump of low molecular mass acids, while this is not the case with CSTR. Consequently, the lumped kinetics developed from BR data cannot be used for predicting TOC conversions in CSTR.

The Catalytic Wet Air Oxidation (CWAO) process is capable of converting all organic contaminants ultimately to carbon dioxide and water, and can also remove oxidizable inorganic components such as cyanides and ammonia. The process uses air as the oxidant, which is mixed with the effluent and passed over a catalyst at elevated temperatures and pressures. If complete COD removal is not required, the air rate, temperature and pressure can be reduced, therefore reducing the operating cost. CWAO is particularly cost-effective for effluents that are highly concentrated

(chemical oxygen demands of 10,000 to over 100,000 mg/Liter) or which contain components that are not readily biodegradable or are toxic to biological treatment systems. CWAO process plants also offer the advantage that they can be highly automated for unattended operation, have relatively small plant footprints, and are able to deal with variable effluent flow rates and compositions. The process is not cost-effective compared with other advanced oxidation processes or biological processes for lightly contaminated effluents (COD less than about 5,000 mg/Liter).

The CWAO process is a development of the wet air oxidation (WAO) process. Organic and some inorganic contaminants are oxidized in the liquid phase by contacting the liquid with high pressure air at temperatures which are typically between 120° C and 310° C.

In the CWAO process the liquid phase and high pressure air are passed co-currently over a stationary bed catalyst. The operating pressure is maintained well above the saturation pressure of water at the reaction temperatures (usually about 15-60 bar) so that the reaction takes place in the liquid phase. This enables the oxidation processes to proceed at lower temperatures than those required for incineration. Residence times are from 30 minutes to 90 minutes, and the chemical oxygen demand removal may typically be about 75% to 99%. The effect of the catalyst is to provide a higher degree of COD removal than is obtained by WAO at comparable conditions (over 99% removal can be achieved), or to reduce the residence time. Organic compounds may be converted to carbon dioxide and water at the higher temperatures; nitrogen and sulphur heteroatoms are converted to to molecular nitrogen and sulphates. The process becomes autogenic at COD levels of about 10,000 mg/1, at which the system will require external energy only at start-up. A simplified process diagram of the wet air oxidation process is shown in Figure 28. Typical wet oxidation applications have a feed flow rate of 1 to 45 ml/hr (5 to 100 gpm) per train, with a Chemical Oxygen Demand (COD) between 10,000 and 100,000 mg/Liter. Wet air oxidation can involve any or all of the following reactions:

Wet Oxidation is the oxidation of soluble or suspended oxidizable components in an aqueous environment using oxygen (air) as the oxidizing agent. When air is used as the source of oxygen the process is referred to as wet air oxidation (WAO). The oxidation reactions occur at elevated temperatures and pressures.

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