Gharib Aly

New local composition model for electrolyte solutions: single solvent, single electrolyte systems

Abstract A new model for representation of the excess Gibbs energy of electrolyte solutions is proposed. The model is based on the Electrolyte nrtl model according to Chen. However, considerable modifications have been introduced. The new model contains four adjustable parameters per binary electrolyte–solvent system. A new local composition expression is used, in which an energy correction term has been introduced empirically, based on calculation results from statistical thermodynamics. The paper gives computed values of the new model parameters for 163 aqueous electrolyte systems at 298.15 K. The parameters were obtained by correlating data on molal mean ionic activity coefficients and osmotic coefficients from the open literature. A comparison of the performance of the model is made with the Chen model, and shows that the new model may be used to very high concentrations with very good accuracy. An investigation of the temperature dependence of the model parameters has been made for some electrolyte systems over a wide range of temperatures and concentrations. The results show a slight temperature dependence of two of the parameters, while the two other parameters may be assumed independent of the temperature.

Extraction of cadmium from phosphoric acid solutions with amines Part I. Extractant selection, stripping, scrubbing and effects of other components

Abstract Cadmium can be effectively removed from wet-process phosphoric acid (WPA) by solvent extraction using long-chain amines, preferably tertiary amines, as extractants. Stripping can be performed either with water or with acids such as phosphoric or sulphuric acid depending on the final conversion of the cadmium waste. Scrubbing could be necessary in some cases and can be performed with alkali. A factorial design experiment was performed to determine the effects of four different components (H2SO4, HCl, HF, H2SiF6) in WPA on cadmium extraction with Alamine 336. The largest influence on cadmium extraction is seen for the hydrochloric acid, drastically increasing the distribution coefficient. Variations of the concentrations of H3PO4, HCl, HF, Fe and Al were also studied. The extraction of cadmium was seen to be strongly dependent on phosphoric acid concentration. A simple extraction model is proposed indicating that CdCl42− ions are extracted.

On the efficiencies of absorption heat transformers

Abstract Although commonly used, the coefficient of performance (COP) is not always an adequate measure to describe the effectiveness of a sorption heat pump. Equations for four different efficiencies are derived, discussed and compared for absorption heat transformers. A flow-sheeting computer program, developed for both design and evaluation simulations of arbitrarily complex absorption cycles, is used to exemplify the derived equations. The working pair H2ONaOH is used in two heat transformer systems. The given examples clearly show that the COP can only be used to compare different heat transformers operated at the same circulation ratio. The COP can be considered as an indicator of the effectiveness of heat exchange within and thermal insulation of a heat transformer operated at a fixed circulation ratio. The thermodynamic efficiency, E th , is shown to be a more logical measure of the heat transformer efficiency, since it takes into account both heat losses, heat exchange and the temperature lift. The exergetic efficiency, E ex , is an alternative to E th since both are mathematically compatible. The possibility of taking into consideration the temperature level, at which heat energy may be considered economically worthless, is demonstrated to be a major advantage of the exergetic efficiency. However, its numerical value does not provide a clear interpretation of the importance of exergetic losses in the system. The exergetic index, I ex , is directly related to the exergetic efficiency but its numerical value is more significant for evaluating the performance of the heat transformer system.

Extraction of cadmium from phosphoric acid solutions with amines Part II. Diluent selection, flow-sheet configuration, pilot plant operation

A large number of diluents were screened to determine the optimum combination between diluent and extractant. Combinations of a C8 tertiary amine and aromatic diluents or of a C12 tertiary amine and aliphatic diluents do not lead to third-phase formation. A flow-sheet for a solvent-extraction process for treatment of various qualities of wet-process phosphoric acid solutions is proposed. Based on a cadmium purification level of 1 mg/kg, extraction, stripping and scrubbing sections of the process would contain two, two and one stages, respectively. A solvent pre-treatment stage may be necessary in some cases. Co-extraction of acids such as H3PO4, H2SO4, HCI, HF and H2SiF6 can be decreased to very low values by manipulating process variables such as pre-equilibration, phase ratios, diluent and amine concentration. Co-extraction of cations other than cadmium was also investigated. Iron, aluminium and magnesium are not co-extracted while zinc and calcium are co-extracted to some extent. The proposed process can be used either for pollution abatement or in combination with other purification processes to recover some of the valuable cations such as uranium, or to produce more refined phosphoric acid streams. (Less)

Solvent Extraction of Phosphoric Acid with Long Chain Tertiary Amines

Solvent extraction of phosphoric acid with Alamine 336 and tri-n-octylamine in toluene has been investigated. The activity of phosphoric acid is expressed as a polynomial of ionic strength and Bromleys model for electrolyte solutions is employed for calculation of ionic activity coefficients. Three species R3N (H3P04) n with n = 1, 2 and 3 in the organic phase are taken into account for explanation of the distribution equilibria. The extraction constants have been determined as log K11 = 2.05, log K12 = 3.61 and log K13 = 3.84 respectively. Aggregation and other non-ideal behaviour in the organic phase are described by letting the corresponding activity coefficient quotients be a function of the total concentration of acid in the organic phase.

Separation of Dilute CO2-CH4 Mixtures by Adsorption on Activated Carbon

The adsorption separation process of diluted gas mixtures containing carbon dioxide and methane on activated carbon has been investigated. Experimental breakthrough curves for both the adsorption and desorption processes have been obtained using mixtures of one or two adsorbable components and one nonadsorbable. A parametric study was conducted to determine the influence of temperature, inlet partial pressure, and superficial velocity on the performance of the adsorption process. The separation factor for the activated carbon was determined to be 3-3.5 at 40[degree]C and 2.7-2.8 at 60[degree]C. The experimental curves have been calculated using a mathematical model based on the approximations of local equilibrium and isothermal conditions. The system of differential equations was solved numerically using a Crank-Nicolson finite difference discretization scheme. The resulting nonlinear system of equations was solved iteratively using the Gauss-Seidel method. The parameters in the model have been estimated either by empirical correlations or using experimental data. The model simulations for both the adsorption and desorption processes are generally in good agreement with experimental data. The evident deviations that resulted in a few cases can be attributed to deficiencies in the Langmuir model itself. 39 refs., 15 figs., 4 tabs.

Separation of dilute CO[sub 2]-CH[sub 4] mixtures by adsorption on activated carbon

The adsorption separation process of diluted gas mixtures containing carbon dioxide and methane on activated carbon has been investigated. Experimental breakthrough curves for both the adsorption and desorption processes have been obtained using mixtures of one or two adsorbable components and one nonadsorbable. A parametric study was conducted to determine the influence of temperature, inlet partial pressure, and superficial velocity on the performance of the adsorption process. The separation factor for the activated carbon was determined to be 3-3.5 at 40[degree]C and 2.7-2.8 at 60[degree]C. The experimental curves have been calculated using a mathematical model based on the approximations of local equilibrium and isothermal conditions. The system of differential equations was solved numerically using a Crank-Nicolson finite difference discretization scheme. The resulting nonlinear system of equations was solved iteratively using the Gauss-Seidel method. The parameters in the model have been estimated either by empirical correlations or using experimental data. The model simulations for both the adsorption and desorption processes are generally in good agreement with experimental data. The evident deviations that resulted in a few cases can be attributed to deficiencies in the Langmuir model itself. 39 refs., 15 figs., 4 tabs.

On the efficiencies of absorption heat pumps

Abstract Although commonly used, the coefficient of performance COP is not always an adequate measure to describe the effectiveness of a sorption heat pump. Equations for four different efficiencies are derived, discussed and compared for absorption heat pumps (AHP). A flow-sheeting computer program, developed for both design and evaluation simulations of arbitrarily complex absorption cycles, is used to exemplify the derived equations. The working fluid pair H 2 OLiBr has been used in two different AHP configurations. The examples given clearly show that the COP can only be used to compare different AHPs operated at the same circulation ratio. The COP can be considered as an indicator of the effectiveness of heat exchange within and thermal insulation of a heat pump operated at a fixed circulation ratio. Nevertheless, it is an insufficient measure to compare different AHPs, even when they are operated at the same circulation ratio. On the other hand, the coefficient of performance for cooling Q e / Q g is better in this respect since it takes into account the real heat flow to the generator. The Carnot efficiency COP rev takes into consideration both the real heat outputs from the absorber and condenser, and the temperature of heat sources and heat sinks. The thermodynamic efficiency E th is shown to be a more logical measure of the heat pump efficiency, since it takes into account the real heat input the generator. The exergetic efficiency E ex can be considered as an alternative to the thermodyanamic efficiency E th but it offers a possibility to take into account any temperature level where heat energy may be considered worthless. However, both E ex and E th are not conventionally used since their numerical values are always less than 1.0. On the other hand, the exergetic index I ex is directly related to E ex but its numerical value shoul be considered as a more significant measure for evaluating the performance of AHP systems, since it properly takes into account the exergy losses which inevitably occur in the system. It may however be stressed that exergy analysis should be used as a compliment to the First Law analysis.

New local composition model for electrolyte solutions: multicomponent systems

The recently proposed model for single solvent, single electrolyte solutions has been modified and extended to multicomponent systems. Parameters for 22 electrolytes have been regressed to data on mean ionic activity coefficients and osmotic coefficients from the literature. A new assumption has made it possible to reduce the number of model parameters to two per electrolyte, and has also eliminated the need for a mixing rule to derive multicomponent parameters from binary parameters. In general, the model still compares favourably to the Electrolyte NRTL model according to Chen for binary electrolyte systems. A comparison between experimental and calculated osmotic coefficients has shown that the new model predicts osmotic coefficients of multicomponent electrolyte systems very well, using model parameters obtained by regressing data on only binary electrolyte systems. The regressed parameters were also used to calculate solubilities in some ternary and quaternary aqueous electrolyte systems at 298.15 K. The results show reasonable agreement with literature data.

Representation of vapor-liquid equilibria using selected equations of state

Abstract Ashour, I. and Aly, G., 1994. Representation of vapor-liquid equilibria using selected equations of state, Fluid Phase Equilibria, 98: 55-69. Four equations of state, two cubic and two perturbed hard-sphere equations, are examined with regard to their prediction of vapor-liquid equilibria. The computed predictions are compared with experimental data. Results of the comparison are reported for mixtures of polar, non-polar, and highly associated compounds. The experimental data base of the comparison includes fifteen binary systems covering a wide range of pressure, temperature, and molecular variety. The same computational technique was used throughout this study. The results indicate that both cubic equations give better predictions of the vapor-liquid equilibria for non-polar and symmetric mixtures, while both perturbed hard-sphere equations give significantly better predictions for the highly asymmetric and associated systems of CO2-fatty acid methyl esters. All equations failed to predict the vapor-liquid equilibria for some mixtures, especially the highly associated systems. However, the phase equilibrium behavior of these systems is greatly improved using mixing rules other than the simple quadratic mixing rule. Generally, no single equation of state currently exists that is equally suitable for the prediction of vapor-liquid equilibria of all classes of binary systems.