Pseudo-equilibrium Model Research Articles

Prediction of tar yield produced from devolatilisation of empty fruit bunch in a fluidised-bed reactor using pseudo-equilibrium model

In this work, a pseudo-equilibrium model (PEM) is developed for prediction of yield and product from devolatilisation of empty fruit bunch (EFB). The yield of individual species (H2, CO, CO2, CH4, bio-oil, tar, and char) from the primary decomposition of EFB is expressed empirically. Subsequently, they are the inputs to the secondary reaction region. In the secondary reactions zone, the water-gas shift reaction (WGSR) is treated under equilibrium conditions, but kinetically modified by a factor to predict the concentration of the four gases (H2, CO, CO2, and H2O) which participated in this reaction. Meanwhile, the methane, bio-oil and tar are modelled separately in a purely kinetic environment by applying a plug flow reactor (PFR) model to estimate the conversion of both species. The model is further to investigate the effects of changes in the operating conditions of devolatilisation such as temperature and carrier gas flow rate. The results showed a good prediction for tar yield with low root mean square (RMS = 0.003) compared to experiments, and conversion (59.7%) compared to experiments (51.5%) at 850 °C. A sensitivity analysis for the pyrolysis model was conducted to investigate the effects of process temperature and nitrogen gas flowrate. Tar yield was significantly reduced at high temperatures. Meanwhile, the change of nitrogen flowrate resulted in slight increase of tar yield. Reasonable predictions were obtained for other products.

Partitioning of Lead and Lead Compounds under Gasification-Like Conditions

An investigation of the chemical form, the concentration, and distribution of lead species between the gas and condensed phase was carried out under gasification-like conditions. The influence of hydrogen sulfide, hydrogen chloride, and steam on the speciation of lead was studied. The gaseous species were determined online by molecular-beam-mass-spectrometry. The condensates were analyzed by standard methods. The dominant chemical forms were metallic Pb, PbCl2, and PbS. The experimental data were compared with the results of thermodynamic Scheil–Gulliver cooling calculations. Further, the experimental data was used as an input for a pseudoequilibrium model aiming at the determination of kinetic information on the underlying transformations.

Study on the species of heavy metals in MSW incineration fly ash and their leaching behavior

This study aims to identify the chemical species and leaching behavior of Cd, Zn, Pb, Cu and Cr in a fly ash with high content of calcium collected from the incineration of municipal solid waste (MWS), based on the sequential leaching procedure. A thermodynamic pseudo-equilibrium model was also developed to evaluate the possible chemical compounds of these heavy metals in the fly ash. The results indicate that, approximate 30% of Cu was distributed in the organic bound fraction and very likely combined with some organic ligands. Pb exhibited the highest fraction among these five metals in water soluble fraction, accounting for about 7.5%. It thus potentially causes a menace to the surroundings. In terms of model calculation, the metallic chlorides in the fly ash were responsible for the leaching of Pb, Zn, Cd and Cu even under a rigorous environmental condition (pH=2) where the oxides and/or metallic ferrites were rarely mobile. The leaching of Cr and Cd in the fly ash was controlled by a dissolution mechanism whereas the fate of Pb, Zn and Cu was controlled by the precipitation/sorption. Cu and Zn in fly ash have been proven to associate with Ca-bearing compounds through precipitation/sorption during leaching test while Pb mainly exists as sulfate and phosphate.

Process simulation of the transport gasifier

The transport gasifier manufactured by Kellogg, Brown and Root (KBR) is reportedly capable of economically converting low rank coal (e.g. lignite) to syngas for the production of synthetic chemicals, fuels and energy. However no process simulation of the KBR transport gasifier yet exists in the public domain literature. In this work three alternative process simulation models of the transport gasifier were developed using a commercial process simulator combined with Excel/VBA routines. The first model determined gasification products on the basis of minimum Gibbs energy. The second model used pseudo-equilibrium approach and the third model used kinetic expressions. The simulation models were validated with real process data. The pseudo-equilibrium model was best able to replicate the data with reasonable process assumptions.

The dilution enthalpies of l-prolinol in N,N-dimethylformamide aqueous solutions at T = 298.15 K

Values of the enthalpy of dilution were measured for l-prolinol in pure water and N,N-dimethylformamide (DMF) aqueous solutions with various mass fractions of DMF at T=298.15K using a flow-mixing microcalorimeter. A pseudo phase equilibrium model was proposed to simplify the complex aggregation equilibrium and interpret the abnormality in the dilution enthalpy, which together with the McMillan–Mayer approach was used to fit the experimental data to obtain the enthalpic pairwise interaction coefficients and the molar aggregation enthalpies of l-prolinol in DMF aqueous solutions. The results are discussed in terms of the hydrophobic interaction and the interactions between the solvated solutes.

Silver loss during the oxidative refining of silver–copper alloys

The high temperature oxidative refining of molten silver–copper alloys is accompanied by silver losses to the slag. Typically, borosilicate slags are utilized as a flux and air or oxygen is injected into the molten alloy. The majority of the copper can be selectively oxidized to copper oxide, but some silver is also lost to the slag. In this research, the mechanism of silver loss has been investigated. Firstly, TGA and DTA studies of molten silver–copper alloys were performed, in order to elucidate the oxidation processes occurring at the gas–metal interface in the bubble. Secondly, the silver losses to the slag in the high temperature refining tests were quantified. Thirdly, the slag was characterized using XRD and the morphology of the silver in the slag was examined. Fourthly, a pseudo-equilibrium model of the refining process was developed and this was utilized to suggest methods for minimizing the silver loss to the slag. Finally, possible methods for recovering the silver from the slag are discussed.

Use of stirred batch reactors for the assessment of adsorption constants in porous solid catalysts with simultaneous diffusion and reaction. Theoretical analysis

A simple, pseudo-equilibrium model was derived for a catalytic system with a first order chemical reaction and simultaneous diffusive and adsorptive processes, in order to assess the corresponding kinetics and Henry law's-type adsorption parameters. Solutions from this model were compared to exact solutions from a more detailed, general model. It was shown that under most of the experimental conditions used in stirred batch reactors and the usual model considerations, it is only possible to assess apparent adsorption parameters. Also, we observed that a stable relationship between the concentrations in the gas and solid phases is reached. The error produced in assuming that the apparent adsorption constant is the real one was calculated to be very important. The value of the apparent adsorption constant depends on various system properties and experimental conditions, such as the Thiele modulus, the amount of catalyst and the contact time. The ratio between the apparent and real adsorption constants was shown to be the transient effectiveness factor at any moment. This ratio reaches a maximum value for the pseudo-equilibrium state, that is always larger than the steady-state effectiveness factor, becoming closer as long as the system's adsorption capacity decreases. The analysis determines the operative conditions to reduce the parametric correlation. Also a criterion for the applicability of usual approximations in the assessment of kinetics and equilibrium adsorption parameters in porous solid catalysts by means of pulse injection methods is established.

A modified pseudo-equilibrium model competing with kinetic models todetermine the composition of a two-temperature SF6 atmosphere plasma

This paper is devoted to calculation of the non-equilibriumcomposition in a SF6 thermal plasma at atmospheric pressure.Non-equilibrium thermal plasmas are characterized by heavy speciestemperatures Th below 9000 K with electron temperatures at the maximumthree times higher than Th when the latter is below 4000 K. Differenttheories have been used based on either multi-temperature plasmas,Saha-Potapov modified by André et al, van de Sanden et al, Cliteuret al, or kinetic calculations or the pseudo-equilibrium model, recentlydeveloped. This model gives results similar to those of kineticcalculations for N2 and H2 plasmas but with calculation times twoorders magnitude faster. Pseudo-equilibrium calculation takes into accountthe reactions with low activation energies instead of ionization reactions,while keeping all the species present in the kinetic calculation. First,the theories are compared in a case already studied in the literature byCliteur: a heavy species temperature Th at 6000 K, with the electrontemperature Te varying between 6000 and 15 000 K. Comparison of theresults shows that the multi-temperature calculations, except those ofCliteur, are far from kinetic especially for ne and nF-. Inaddition, the pseudo-equilibrium model fits rather well with the kineticcalculations as long as molecular species are present in the plasma.Second, to calculate the composition of non-equilibrium thermal plasmas theratio Te/Th is assumed to vary as the logarithm of the electrondensities ratio ne/ncmax, nemax being the electron densityover which equilibrium prevails, i.e. 1023 m-3. For kineticreactions where electrons are involved (in the direct reaction while heavyspecies intervene in the reverse reaction), a temperature T* betweenTe and Th is defined. T* is calculated as a function of theelectron flux to that of heavy species. The variation of T* with This smoother than that of Te. In such conditions again, there is anexcellent agreement between kinetic and pseudo-equilibrium calculationsperformed at T*, which is not the case for multi-temperaturecalculations. These results demonstrate that the pseudo-equilibriumcalculation developed for thermal plasma simple forming gases such as N2and H2 can also be applied to more complex gases such as SF6.

Modelling and prediction of the solubility of acid gases in diethanolamine solutions

The pseudo-equilibrium model proposed by Kent and Eisenberg is used in this study to model the absorption of acid gases (H 2 S and CO 2 ) in aqueous diethanolamine solutions at equilibrium. A wide range of experimental data available in the literature is used to obtain the values of the pseudo-equilibrium constants of the amine reactions. These data cover amine concentrations from 0.5 to 8 N, temperatures from 273 to 413 K, and acid gas loadings from zero to unity. New correlations for the pseudo-equilibrium constants and solution pH are obtained as a function of temperature, amine concentration, and acid gas loading for loading up to unity. The partial pressure of H 2 S is much better predicted by the new correlation compared to the work of Kent and Eisenberg. The predictions of the partial pressure of CO 2 are improved mainly at high temperatures and amine concentrations.

Dehydrogenation of Pure Cyclohexane in the Membrane Reactor and Prediction of Conversion by Pseudo Equilibrium Model.

Three kinds of membrane reactor, porous Vycor glass, ceramic Sol-gel, and Palladium-silver membrane reactors, are used to study the dehydrogenation of pure cyclohexane. The reactors are intended to be applied as chemical heat pumps, therefore they were fed with pure cyclohexane. The experimental results show that the higher conversions exceeding the equilibrium one, αeq = 6.4%, were achieved up to 27% and 11% for Pd-Ag (PT-2), and porous Vycor glass membranes respectively. A pseudo equilibrium (PE) model was proposed to predict the conversion achieved in the reactor. In the Damkohler number (Da) range employed of 1.05–4.48, the model is in good agreement with the experimental results. The prediction becomes less accurate with increasing pressure difference.

Model-based QSAR for ionizable compounds: Toxicity of phenols against Tetrahymena pyriformis

The previously described pseudo-equilibrium model for kinetics of distribution of extraneous chemicals in biological systems has been used for the development of the QSAR equation expressing toxicity of phenols against Tetrahymena pyriformis as a non-linear function of their hydrophobicity and acidity. The model assumes binding of all the tested compounds to a single class of the receptors (i.e., it does not discriminate between possible narcotic and respiratory uncoupling modes of action of phenols) with the association constant related to the electronic structure of the molecules rather than to their hydrophobicity. Non-linear regression analysis revealed satisfying agreement between the model and published biological data measured after a single dose.

Acidity of the environment and intracellular distribution of ionisable substances

The description of the bioactivity—hydrophobicity—acidity profiles for monovalent acids or bases represents an application of our published treatment of distribution of ionisable compounds undergoing elimination in biological systems. The approach is applied to the literature data on growth inhibition of Sarcina lutea by lincomycins. The model exhibits satisfactory agreement with the experimental data. The results indicate that incorporation of elimination into the pseudo-equilibrium model improves its quality.

Modelling spatial and temporal variability of denitrification

Denitrification shows both spatial and temporal variability. Any attempt to model the process must take this into account. A model has been developed in which the soil is treated as a large assembly of potentially denitrifying microsites (a modification of the “hot-spot” concept). Chemical and biological heterogeneity is represented by a log-normal distribution of microsite respiration potential. Structural heterogeneity (where present) is accommodated by associating individual microsites with soil aggregates, the radius of which varies log-normally. Spatial variation arises naturally from the existence of microsites. Model microsites are assumed to be in a state of pseudo-equilibrium (a “steady state”). This means that they respond rapidly to any perturbation; it does not imply that they are static. A pseudo-equilibrium model can readily encompass temporal variation provided that the response time of the system is relatively short. Examination of the response times of typical model microsites, by numerical solution of the partial differential equations governing the transient processes occurring within them, suggests that in soils with few large (greater than 10-2m in radius) denitrifying microsites the steady-state approximation is probably adequate. Where denitrification occurs predominantly in large microsites, however, a pseudo-equilibrium model is inappropriate.

An investigation of radium extraction from uranium mill tailings

Uranium mill tailings contain small amounts of radionuclides that may result in low level radiation exposures to the population and environment over many years. The possibility of extracting a substantial portion of the most toxic radionuclide ( 226Ra) from sulfuric acid-leach process tailings from the Elliot Lake area was investigated by contacting with several leachants such as HNO 3 and solutions of DTPA, EDTA and CaCl 2. Data on the distribution of radium between the leachants and tailings were obtained for various contact times. Data on crosscurrent and countercurrent extraction were also obtained for the more promising extractants, and interpreted in terms of a pseudo-equilibrium model. Up to 85% of the radium could be removed by crosscurrent or countercurrent contacting with nitric acid or DTPA solutions.

The Abrupt Junction Transition Region†

ABSTRACT The potential distribution in the vicinity of an abrupt junction between p-and n-type material is calculated using approximations to the pseudo-equilibrium model. Numerical results in normalized form are presented to illustrate the dependence of the transition capacitance on the applied voltage. A. useful formula for transition capacitance, valid for reverse bias and moderate forward bias, is derived. This formula reduces to the usual ½-power law for large reverse bias. The potential solution is used to estimate the imref drop across the transition region for the current flow case. It is shown that it is possible to use the emitter diode characteristic to deduce information about the structure of a p-n-p diffusion transistor since the imref drop for holes can be neglected at low emitter bias currents.