Supercritical Water

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 a...

SCWRs are high-temperature, high-pressure water cooled reactors that operate above the thermodynamic critical point of water (374°C, 22.1 MPa). The SCWR offers many advantages compared to state-of the-art LWRs including the use of a single phase coolant with high enthalpy, the elimination of components such as steam generators and steam separators and dryers, a low coolant mass inventory resulting in smaller components, and a much higher efficiency (~44% vs. 33% in current LWRs). In these systems high pressure (25 MPa) coolant enters the vessel at 280ºC which is heated to about 500ºC and delivered to a power conversion cycle.

A theoretical and analytic expression for the first shell, and an analytic empirical expression for the whole radial distribution function (RDF) of water are introduced. All the asymptotic limits and functionalities of the RDF with tem­ perature and density are incorporated in these expressions. An effective Kihara pair potential function is presented for water intermolecular interactions which incorporates the hydrogen bonding by using the chain association theory. The intermolecular pair potential parameters are adjusted to the experimental x-ray diff raction data of water RDF at various temperatures. The predicted first-shell results for water near critical and in supercritical conditions compare satisfac­ torily with the available neutron diffraction RDF data, with the simulation RDF results, and with the empirical RDF curves. The empirical expression initially proposed for the RDF of the Lennard-Jones fluid is extended to predict the RDF and the isothermal compressibility of water to conditions where experimental or simulated data are not available. Comparison with the Lennard-Jones fluid shows that the height of the fi rst peak of water RDF changes much less at subcritical and supercritical conditions compared to that of the Lennard-Jones fluid which decreases appreciably going from subcritical to supercritical conditions.

In this report two approaches are presented to predict the structure and PVT behavior of associating fluids with emphasis on water. One approach is the development of equations of state based on the analytic chain association theory (ACAT).  An associating fluid is assumed to be a mixture of monomers, dimers, trimers, etc., for which the composition distribution is obtained. The resulting equations are simple enough to be used for PVT calculations. The second approach is the development of an effective Kihara pair potential for water which incorporates the hydrogen bonding using the ACAT. This potential function has been used in an analytical expression to predict the first shell of the radial distribution function Ž RDF. for water. The expression for RDF which was initially applied to simple potential energy functions, such as the Lennard–Jones and Kihara functions satisfies the general functionality of the RDF with respect to intermolecular potential, temperature and density as well as all the limiting values of RDF at high temperature and dilute gas, and infinite separation. The effective potential parameters are determined to predict the first shell of the RDF data for water at various subcritical temperatures and densities. The predicted results for water at near-critical and supercritical conditions are shown to be in agreement with the data obtained by neutron diffraction experiments and with the simulation data.

In this report, the Percus-Yevick and the Ornestein-Zernike integral equations are solved simultaneously for the radial distribution functions of water at various state conditions, including sub-and supercritical states. The intermolecular potential function used in this study consists of an effective Kihara potential, which is derived for associated fluids. For derivation of the effective potential function, water is considered as a mixture of associated species due to hydrogen bonding. The contribution of hydrogen bonding is considered in the formulation of the effective Kihara potential parameters through the application of the analytic chain association theory. There is a good agreement between the present calculations and the experimental data in predicting the oxygen-oxygen radial distribution function near the critical point and at supercritical conditions for which experimental data are available. It is also concluded that at supercritical conditions a considerable degree of hydrogen bonding may be still present in the form of linear chain association. Therefore, the chain association model is valid near the critical point and at supercritical conditions instead of other structure models for the investigations on molecular structure of water.

In this paper, two-dimensional laminar convective heat transfer of water at a supercritical pressure of 25 MPa in a horizontal rectangular duct has been investigated. The objectives were to understand the thermal and hydrodynamic behavior of fluid flow under different conditions. A set of governing equations containing the continuity, momentum and energy are solved simultaneously using CFD techniques. The finite volume method is employed to obtain the discretized forms of the governing equations. In the numerical solution of these equations, the SIMPLE algorithm is used for pressure-velocity computations. In all the cases studied in this paper, sub-cooled water at a supercritical pressure enters a duct with constant surface temperature which is near to the fluid pseudocritical temperature. For this type of fluid flow with forced convective heat transfer, the pressure, velocity and temperature fields are calculated. Numerical results show that as the fluid temperature becomes closer to the pseudocritical temperature (384.8 degrees C), the rapid variation of the fluid properties causes an unusual velocity profile for this type of convective flow. In the present work, the crucial influences of two main parameters, the Reynolds number and wall temperature on flow and heat transfer distributions, are thoroughly explored.

Keywords: supercritical water; heat transfer; computational fluid dynamics

The cellulose dissolution is an essential pretreatment process for the chemical conversion of lignocellulosic biomass into biofuels. Here, the dissolution of a 36-chain Ib cellulose model with a hexagonal crosssection (M36HCS) in water is analyzed by Molecular Dynamics (MD) with CHARMM36/TIP3P (C36/TIP3P) force field using gradual heating at 25 MPa. Our simulations showed that the dissolution of M36HCS starts to occur between 560 K and 600 K, which agrees with experimental observations. In our system, conditions near the critical point of water reveal that translational and rotational entropies decrease, while the low hydration level increases vibrational entropy. This investigation theoretically shows that C36/TIP3P adequately reproduces the dissolution of M36HCS even in high-pressure water as corroborated in reactors.