Modelling thixotropic behavior of fresh cement pastes from MRI measurements

From MRI measurements we show that in a flowing cement paste thixotropic effects dominate over short time scales while irreversible effects become significant over larger timescales. The steady and transient flows exhibit a yielding behavior which differs from usual yield stress model: the transition from the solid to the liquid regime is abrupt. We propose a simple thixotropic model based on these observations. The validation is done on the steady and transient state on local experimental tests. We build the “local” rheogram which is representative of the intrinsic rheological properties. Comparisons with “apparent” rheograms demonstrate that it is possible to use correction techniques from the literature to have access to the real behavior law of the material from standard measurements but that, in the case of the material studied in this paper, this would nevertheless lead to an underestimation of the yield stress.     

Kinetics of Film Diffusion Controlled Ion Exchange Processes,Theory and Applications

A differential equation describing the kinetics of film diffusion controlled ion exchange processes is derived on the basis of the theory of irreversible thermodynamics. This equation is solved subsequently with the boundary conditions of infinite solution volume and applied to some experimental results involving differential ion exchange reactions. In case of the boundary conditions of a finite solution volume, the kinetics of dissolution of a sparingly soluble salt by an ion exchanger is considered.     

Probabilistic analysis of transport-reaction processes over catalytic particles: Theory and experimental testing

We developed a probabilistic theory of transport-reaction processes over various types of configurations of catalytic particles, particularly for a single particle. The theory describes a single pulse-response experiment which is similar to the single pulse temporal analysis of products (TAP) experiment. The analysis is based on the general theory of Brownian motion with “killing” and the Feynman-Kac formula. Different diffusion-reaction problems, particularly the problem “diffusion-very fast reaction” (infinite rate reaction), have been analyzed. In the latter case, the probability for a reactant to be converted equals a purely geometric characteristic, namely, the probability for a reactant molecule to hit at least one catalyst particle in a configuration of particles. Solving a boundary value problem, the probability of conversion can be found as a function of the apparent kinetic parameter. Based on experimental data on exit flow and conversion this parameter can be extracted. Experimental data in studies of CO oxidation over a single particle of Pt catalyst show a qualitative agreement with data from computational experiments based on the developed probabilistic theory. For two-particle catalyst configurations, it was found computationally a non-trivial dependence of the reactant conversion on some geometric characteristics, especially the distance between particles. A distance was found for which conversion is highest.     

Thermodynamics,a good theory for the study of electrochemistry in non-isothermal systems. A specific application: thermal diffusion

We present here a short survey on general non-equilibrium thermodynamics when the external forces are especially of electrostatical nature. Such a development provides a very well-adapted frame to analyse many irreversible processes as thermal diffusion in ionic media. This phenomena, so-called Soret effect in fluid mixtures, has greatly been studied by various experimental methods. Among these, the electrochemical ones give valuable information on quantities such as entropies and heats of transfer.     

Dynamics of liquid-liquid systems based on linear thermodynamics of irreversible processes

The use of non-equilibrium models for integrated processes involving liquid-liquid systems has increased in recent years. These processes often exhibit complex dynamic behavior. These dynamical systems still pose many open questions, e.g. with regard to the sources of multiple steady states (MSS). This article analyzes the effect of mass transfer on the MSS of these systems. A generalized non-equilibrium modeling approach based on linear thermodynamics of irreversible processes (LTIP) is presented, and the dynamics of the system is studied systematically. It is shown that the non-linearity present in even the simplest non-ideal activity model acts as a source for MSS. The parameters that affect the solubility, e.g. temperature, can play a critical role on the existence of MSS in the system. A geometrical visualization of the MSS is also illustrated.     

Efficient hydrogen generation from organic chemical hydrides by using catalytic reactor on the basis of superheated liquid-film concept

Under boiling and refluxing conditions for catalytic dehydrogenation of organic chemical hydrides (decalin, methylcyclohexane and others) in a batch-wise reactor, either suspended states with excess amounts of substrate or sand-bath states with its scarce amounts were found to be inferior generally to the so-called “liquid-film states” with adequate amount ratios of substrate to catalyst, where the catalyst-layer temperatures were superheated or raised higher than the boiling point, and, consequently, reactivities became more favorable at higher heating temperatures in contrast to the boiling suspended states.Equilibrium shifts due to reactive distillation were well demonstrated under boiling and refluxing conditions in naphthene dehydrogenation. Moreover, desorption of hydrogen from the active sites to the bubble space was enhanced in the superheated liquid-film states, with large translational entropy endowed.Provided the extents of equilibrium deviation were large enough (Prigogine's approach), thermodynamic couplings among irreversible processes would be realized between heat transfer and mass transfer as an example of the extended De Donder's equation. Restriction of chemical equilibrium could be removed under temperature gradient conditions, as the consequence that its Gibbs energy change became more negative than that under iso-temperature conditions.Within the framework of irreversible thermodynamics, the decreased retardation constant K in the superheated liquid-film states was interpreted in terms of a vector-level coupling between temperature gradient and desorption. Moreover, vigorous bubble formation would give additional favor to the reaction rates owing to enlarged repeating frequencies of sequential non-microreversible processes in dehydrogenation catalysis.Organic chemical hydrides are attractive from the viewpoints of safe, economical, exergy-saving and large hydrogen contents for hydrogen storage and distribution. Their main defects have been pointed out hitherto that the endothermic reaction temperatures are too high. In this paper, a new concept on superheated liquid-film catalysis is explored for dehydrogenation temperatures to decrease, which would result in not only saving exergy for external heating but also avoiding catalyst deactivation due to carbon deposit.     

Non-equilibrium molecular dynamics simulation of gas separation in a microporous carbon membrane

We have carried out non-equilibrium molecular dynamics simulations of gas separation in a “selective surface flow” membrane. The gas mixture studied is hydrogen/methane, which is relevant to hydrogen purification in refineries. The simulations give insight into the separation mechanism, which is based on the transport of the more strongly adsorbing species (methane) in a dense layer near the pore wall, with the less strongly adsorbed species (hydrogen) diffusing through a less dense region close to the centre of the pore. Good agreement is obtained with experimental selectivity data. This work is also relevant to the study of the combined effects of adsorption and diffusion in microporous carbon adsorbents.     

ASR pessimum behaviour of siliceous limestone aggregates

Siliceous limestone aggregates have “pessimum” behaviours similar to those observed for pure siliceous aggregates such as flint or opal. For high alkali contents concretes based on fine and coarse reactive siliceous limestone aggregates swell less than concretes based on fine reactive siliceous limestone aggregates and non-reactive coarse aggregates. The reduction of the swelling is more significant for a “micritic” limestone containing highly reactive free silica than for a “sparitic” limestone containing less reactive free silica. The consumption of a part of the alkalis by non-expansive processes such as chemical sorption on silanol sites of reactive silica and pozzolanic C-S-H is expected to explain the reduction of the swelling.     

Mass-Transfer Equation in Multicomponent Mixtures

A multicomponent mass-transfer equation for the distillation of multicomponent mixtures is derived in terms of the thermodynamics of irreversible processes.__________Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 3, 2005, pp. 337–344.Original Russian Text Copyright © 2005 by Serafimov, Timoshenko.     

A New Theoretical Approach To Electroosmotic Dewatering (Eod) Based On Non-Equilibrium Thermodynamics☆

ABSTRACT

A novel theoretical approach to electroosmotic dewatering (EOD), with or without a pressure gradient, of clays, sludges and other colloidal suspensions is proposed. The treatment is based on nan-equilibrium thermodynamics as developed in the work of Overbeek, De Groot and others. The interpretation of electrokinetic phenomena in terms of the cancepts of irreversible thermodynamics when combined with Onsager's relations, it has been shown by Overbeek, provides a complete framework for understanding all electrokinetic phenomena.

We have applied this approach here to the electroosmotic dewatering. both in the presence and absence of applied hydrostatic pressure.

The approach provides much clarification on the nature and significance of currents and fluxes observed during EOD: these are composed of three components, during combined pressure electroosmotic dewatering: (i) electrochemicavelectrical current; (ii) hydrodynamic flux: (iii) electroosmotic current.

We have also shown the manner in which the proposed new approach to EOD based on irreversible thermodynamics can be connected to the conventional approach based on the Helmholzu-Smoluchowski equation.     

Studies with inorganic precipitate membranes—XI: Evaluation of the thermodynamically effective fixed charge density by various methods

Thermodynamically effective fixed charge densities of parchment supported membranes were estimated by methods of (a) Teorell—Meyer—Sievers, (b) Altug and Hair, and (c) the most recent one of Kobatake based on the thermodynamics of irreversible processes. The two limiting forms of Kobatake's equation for dilute and concentrated ranges gave identical values of charge densities. It is interesting to note that these two values of limiting cases are closer to the Teorell—Meyer—Sievers and Altug and Hair values. The theoretical prediction for membrane potential by the Kobatake's equation were borne out quite satisfactorily by experimental results obtained with both the membranes.     

Pressing dynamics of disperse mixtures in analytical,diagram, and nomogram interpretations

Physicomathematical methods of the thermodynamics of irreversible processes are used to create a new experimental and analytical model of the pressing process that makes it possible to combine results of investigations on the theory and practice of production of refractories into a unified orderly system of concepts of the technological process. Trabsfer of the energy and mass of a substance in space and time is described in the form of compound energy-mass-concentration nomograms. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 7, pp. 20 – 27, July, 1996.     

Applications of non-equilibrium thermodynamics to the theory of overvoltage

An unambiguous definition of the different overpotential terms is possible based on the theory of non-equilibrium thermodynamics. The overpotentials are a direct consequence of the thermodynamic irreversibility of the processes taking place in an electrochemical system, and the irreversible entropy production of a process occurring at a finite rate is a quantitative measure of this irreversibility. Therefore, the overpotential terms can be directly defined in terms of the entropy production rate of the process in question.

, with η_i = (T/I)(dS_i/dt). Two examples are given: the separation of ohmic and diffusion overpotentials and the separation of the charge transfer and diffusion overpotentials.     

非平衡态热力学理论与钢铁冶金过程

钢铁冶金过程具有典型的非平衡和非线形特征。本文分析讨论了基于非平衡态热力学、耗散结构和混沌理论来研究处理实际钢铁冶金过程。认为：为真实地定量描述实际冶金过程，应该加强、加速开展和进行冶金过程非平衡态热力学及其应用的研究。     

Permeation characteristics and modeling of barrier properties of multifunctional rubber nanocomposites

Nanocomposites used in various applications, like tires, diaphragms etc. must exhibit superior gas permeation behavior along with other properties. Oxygen permeability characteristics of such multifunctional styrene butadiene rubber based nanocomposites, including thermodynamics and kinetic aspects of transport have been discussed here. For the first time, these characteristics have been determined for nano, micro and dual filler based multifunctional nanocomposites. The permeability of the nanocomposites was remarkably decreased by the presence of high loadings of montmorillonite due to high aspect ratio and exquisite dispersion, ascertained from morphological studies. The results were explained by the increment in tortuosity and also correlated with the reduction in free volume. Relative permeabilities were compared to predictions of existing permeation models and a novel function was successfully introduced to address deviations. Finally, the enhanced barrier properties of dual filler based nanocomposites have been explained by the formation of zeta potential driven “nano-blocks” and “nano-channels”.     

An improved general kinetic analysis of non-linear irreversible polymerisations

A method to predict average molecular weights before and after gelation for general irreversible non-linear polymerisations forming tree-like molecules is described. Recently developed numerical methods for solving two point boundary value problems are essential for the success of these calculations after gelation and open the way to eventually be able to efficiently predicting chain length distributions. Anionic and free-radical polymerisation of vinyl monomers in the presence of divinyl monomers or with transfer to polymer are taken as case studies. Comparison to experimental data and with simulation results obtained through “numerical fractionation” confirms the usefulness of current approach.     

On some dynamic peculiarities of the charge transfer with adsorption and attractive interactions

Complex electrode reactions with adsorption of intermediate product and with attractive interactions accounted for by Frumkin adsorption isotherm may show up a variety of unusual steady states (ss) characterized by ss adsorption isotherms. Under symmetry of kinetic equations the single peculiarity to be expected in electrochemical adsorption and desorption reactions is hysteresis responsible for current discontinuities in potential sweep experiments and similar discontinuity of immittance data recorded for potential or frequency sweeps. For asymmetry of partial reactions, as for charge transfer coefficients different then 0.5 or unequal charge numbers in partial reactions, the steady-state isotherms get unusual forms described in the chemical dynamics literature as “mushrooms” or “isolas”, this last term referring to disjoined branches of the ss isotherm. In potential scan experiments, these isotherms present double hystereses and irreversible switches between steady states’ branches at high and at low surface coverage.