Vapor Pressure Behavior Research Articles

Vapor–liquid equilibria of vibrating square well chains

Vibrating square well (SW) 2-mer, 4-mer, and 8-mer with average reduced bond lengths of 0.97±0.03, 0.60±0.03, and 0.40±0.03 were studied by discontinuous molecular dynamics (DMD) simulation in the NVE ensemble. Average bond angles for the reduced bond length of 0.4 were constrained to 127±16° while the longer bond lengths were freely jointed. Vapor–liquid equilibria of the vibrating SW fluids were determined based on DMD simulation by isochoric integration and compared to that of rigid SW chains from Gibbs ensemble Monte Carlo (MC) simulation. The binodals of vibrating chains show a shift to higher temperatures relative to rigid chains, reflecting their less repulsive (more attractive) nature. Vapor pressures of the vibrating chains were computed through isochoric integration with Clausius–Clapeyron consistency to 5% or better. Vapor pressure behavior for each chain model was characterized in terms of critical temperature, critical pressure, and acentric factor. The trend in acentric factor vs. chain length showed that shorter bond lengths gave improved agreement with the experimental trend for n-alkanes. Nevertheless, the trends in acentric factor did not support any molecular model for alkanes which represented methylene segments as individual SW interaction sites. If SW chains are to be applied as models of alkanes, each interaction site must be assigned more than one methylene segment.

A simple non-classical equation of state for fluids and fluid mixtures

A method for improving the behavior of classical equations of state (EOS) in the critical region, originally proposed by Fox [J.R. Fox, Fluid Phase Equilibria 14 (1983) 45–53], has been modified in this work for the Patel–Teja (PT) EOS [N.C. Patel, A.S. Teja, Chem. Eng. Sci. 37, 463–473]. The application of the new equation (NPT) for predicting PVT and vapor pressure behavior of pure substances, as well as vapor–liquid equilibrium behavior of binary mixtures, is demonstrated. The NPT equation is simple to use and requires the same input information as the original PT equation. However, it reproduces the correct PVT behavior in the critical region. Limitations of both the PT and NPT equations in calculating the isochoric heat capacity are discussed.

Physicochemical properties as useful tools for predicting the environmental fate of organic chemicals

The distribution of a chemical between the environmental media air, water, and soil/sediment is largely determined by its physical and chemical properties. Relatively inexpensive (compared to field studies) measurements can be used in order to predict roughly the distribution of the chemicals. This is of greatest importance in testing new chemicals. In this case, nothing is known about the behavior of the substance in the natural environment. Models still remain to be developed for calculating the distribution on the basis of physicochemical data for different environmental conditions or, alternatively, by using globally averaged data. It is shown that—except in the case of highly persistent chemicals—distribution can only be considered in conjunction with (biotic and abiotic) degradation processes. According to our present knowledge, the key properties with regard to the environmental fate include vapor pressure, water solubility, adsorption and desorption behavior, partition coefficient (octanol/water), volatility from aqueous solution, and hydrolysis and photochemical reactivity in the air, in water, and when absorbed at surfaces (aerosol, soil, organic matter).

The vapor pressure behavior and association of mixtures of 1-hexanol and n-hexane between 293 and 373 K

Vapor pressure isotherms for mixtures of 1-hexanol and n-hexane determined between 293 and 373 K with the usual accuracy of ∼0.03 kPa can be described satisfactorily by the Wilson equations. The initial increase and final decrease of G max E with increasing temperature reveal that the thermodynamic behaviors associated with the more strongly self-associated compound methanol and with the relatively weakly self-associated compound methylamine are observed for the hexanol mixtures at the extremes of the temperature range studied. Correspondingly, the values for the Wilson coefficient λ 12 — λ 11 (1 = hexanol, 2 = n-hexane) as well as the values for the activity coefficients and the association energy of hexanol lie between those determined under similar conditions for methylamine and methanol. Within series of mixtures containing homologous alcohols a decrease of the difference (λ 12 — λ 11) — (λ 12 — λ 22) = λ 22 — λ 11 with increasing chain length reveals decreasing interaction between the alcohol molecules. A decrease of the coefficient λ 12 — λ 22 indicates increasing interaction between the alcohol and hexane molecules with increasing chain length. A two-constant model of association, one for dimerization and the other for formation of higher polymers, permits numerical description of the thermodynamic behavior of solutions of simple aliphatic amines, thiols and short-chain alcohols in hexane without large errors over nearly the entire composition range. Such a description is not possible for solutions of hexanol and other long-chain alcohols in hexane.

Generalized vapor pressure behavior of substances between their triple points and critical points

AbstractSeveral approaches are utilized for the estimation of the parameters associated with the normalized vapor pressure equation to develop a capability for the prediction of vapor pressures consistent with the theorem of corresponding states. In this development, conditions and restrictions associated with the critical point and the normal boiling point were used to establish the vapor pressure parameters α, β, γ, and m. For the evaluation of these parameters, five different approaches were applied, and vapor pressures calculated with each of them were compared with corresponding experimental values presented in the literature. These comparisons were further examined using the results of four other well‐known vapor pressure equations available in the literature.This evaluation included 138 substances of all types and involved a total of 7,633 literature reported vapor pressure values to show that improved results are possible through the involvement of the generalized vapor pressure equation of this investigation.

Generalized treatment for the vapor pressure behavior of polar and hydrogen‐bonding compounds

AbstractThe vapor pressure behavior of 25 polar compounds, including water and a number of alcohols, has been analyzed and subjected to a generalized treatment using the relationship, for a total of 1343 experimental measurements presented in the literature and covering the range included between their respective triple points and critical points. For each of these compounds, the parameters of this equation were established using a nonlinear regression numerical technique. The parameters resulting from this regression reproduce vapor pressures for all these polar compounds with an average deviation of 0.44 percent (1343 points).To extend the application of this vapor pressure relationship, the original parameters of these polar substances were correlated with their respective basic physical properties M, Tc, Pc and Tb. The resulting generalized parameters reproduce vapor pressures with an average deviation of 1.44 percent (1343 points) for all these polar and hydrogen‐bonding compounds. This generalized correlation can be applied for the reliable prediction of vapor pressures for polar substances for which inadequate information is available.

Benedict‐webb‐rubin parameters for ethanol

AbstractExperimental PVT measurements available in the literature for ethanol have been utilized to establish the BWR parameters for this substance using a nonlinear regression analysis involving 300 experimental points for reduced densities up to ρR = 1.80. The resulting parameters reproduce pressures for these 300 points, with an average deviation of 2.60% and predict the vapor pressure behavior between the normal boiling point and the critical point with an average deviation of 0.91%. This treatment is applied for the first time to the PVT behavior of an alcohol and indicates that the BWR equation of state could well be extended in application to include other oxygenated organic compounds receiving considerable interest in the currently changing energy emphasis.

A New Two-Constant Equation of State

The development of a new two-constant equation of state in which the attractive pressure term of the semiempirical van der Waals equation has been modified is outlined. Examples of the use of the equation for predicting the vapor pressure and volumetric behavior of singie-component systems, and the phase behavior and volumetric behavior of binary, ternary, and multicomponent systems are given. The proposed equation combines simplicity and accuracy. It performs as well as or better than the Soave-Redlich-Kwong equation in all cases tested and shows its greatest advantages in the prediction of liquid phase densities.

A vapor pressure equation for n-hydrogen between the triple and critical points

A relation between reduced vapor pressure and reduced temperature of the form ln P R = α + β/T R + γ ln T R + δT R n has been proposed to describe the vapor pressure behavior of n-hydrogen between the triple and critical points. Data obtained from fourteen sources available in the literature were used to establish the constants α, β, γ and δ and the exponent n of the proposed relationship. An error analysis indicates that n = 5.00 provides the best fit. Thus this represents a modification of the Riedel relationship which specifies that n = 6.00. Similarly, the calculation of α from the normal boiling point appears to have advantages over the use of the critical point. When applied to 281 data points this relationship produced an average deviation of 0.82%.

Dynamics and stability of a gas cavity in generalized viscoelastic liquids

A general theory is developed to determine the dynamic characteristics and stability of a gas cavity situated in viscoelastic liquids whose rheological state is described by the generalized model of the Kelvin or Maxwell type. The characteristic function of the bubble-liquid system is obtained by the first-order perturbation method, including the effects of surface tension, vapor pressure, and thermodynamic behavior of the gas confined in the cavity. The zeros of the characteristic function determine the types of bubble growth, bound on unbound. The Hurwitz (or Routh) stability is employed to determine the conditions for the onset of incipient cavitation. The applications of the theory are demonstrated in some liquids having simpler shear-stress-shear-strain relationship.

Bubble pulsation and cavitation in viscoelastic liquids

An analysis is performed to investigate free and forced oscillation of a gas bubble and cavitation in viscoelastic liquids of a three-parameter Oldroyd model. The first-order perturbation method for small amplitudes has been employed to obtain periodic solutions to the bubble dynamics equation. Consideration is given to the influence of surface tension, vapor pressure, and thermodynamic behavior of the gas inside the bubble. It is disclosed that transient behavior of the bubble in viscoelastic liquids is governed by five dimensionless parameters. Conditions for stable bubbles are obtained. Free oscillation consists of a decaying exponential component and a damped sinusoidal oscillation associated with the natural frequency of the bubble-liquid system. The effects of the five governing parameters on the natural frequency are determined. The radial motion of the bubble in an oscillating pressure field includes a sinusoidal oscillation associated with the forcing frequency, a decaying exponential component, and a damped sinusoidal oscillation associated with the natural frequency. Criteria for the onset of incipient cavitation are determined when a bubble is suddenly released into the liquid or is situated in an oscillating pressure field.

Thermodynamic behavior of polymethyl methacrylate solutions

Abstract The Maron theory of polymer solutions was used to examine the available osmotic pressure, light scattering, and heat of dilution data for polymethyl methacrylate (PMMA) in fourteen solvents. No vapor pressure or heat of solution data could be found for these systems. Wherever possible interaction parameters, μ, were evaluated as functions of volume fraction, temperature, and molecular weight, as well as X, the temperature coefficient of μ. The equations deduced allow in some instances the prediction of the vapor pressure behavior of solutions of noncrystalline PMMA and also the heat of mixing of random amorphous polymer with solvent. The data were not sufficient to establish the complete dependence of μ on concentration, molecular weight, and temperature for any PMMA-solvent system.

Arsenic Source Vapor Pressure Kinetics and Capsule Diffusion

After diffusion temperature and time, the most important parameters in capsule diffusion source behavior are the dopant vapor pressure characteristics in the capsule. The vapor pressure behavior is a function of the degree of homogenization of the Si-dopant system and hence of the source preparation technology. Various methods of preparing Si-As source material are discussed. Homogenized-source preparation is described as it relates to reproducibility and controlled capsule diffusion behavior. The measurement of the kinetics of arsenic vapor pressure development in a typical capsule system and the role of vapor pressure in determining capsule diffusion behavior are described. Finally, the dependence of arsenic vapor pressure characteristics, and hence of the diffusion, on source weight-to-capsule volume ratio is described.

Thermodynamics of polyisobutylene solutions. II. Vapor pressures and polymer order

Abstract The Maron theory of nonelectrolyte solutions has been used to examine the vapor pressure behavior of solutions of polyisobutylene in benzene and in cyclohexane between 25 and 65°C and in n-pentane between 25 and 55°C. The results show that the theory can predict the vapor pressures of these solutions over the full concentration range, provided the polymers involved are amorphous. When the polymers are not completely amorphous, then discrepancies between theory and experiment are observed at high solution concentrations which are due to persistence of polymer order in the highly concentrated solutions. A method is described by means of which these discrepancies can be used to obtain on an absolute basis both the degree of polymer order in solution and in the original polymer samples used.

Correlation of latent heats of vaporization

AbstractGeneralized equations for vapor pressure and PVT behavior are used to develop a functional relationship between latent heat of vaporization and reduced temperature and pressure. This function leads to a graphical correlation which utilizes the slope M of the vapor‐pressure curve at the critical point. The correlation gives the latent heat of vaporization at any temperature in the two‐phase region for any substance for which the critical point and one vapor‐pressure point are known. The correlation is particularly useful for prediction of latent heat of vaporization near the critical point by the use of information for removed from the critical. Included in the study are polar, nonpolar, organic, inorganic, and metallic substances. General predictions of latent heat as a function of temperature are within a few percent wherever comparisons can be made with experimental data.

The Cricondentherm and Cricondenbar Pressures of Multicomponent Hydrocarbon Mixtures

Abstract A method is presented for the accurate calculation of the cricondentherm and cricondenbar pressures of multicomponent hydrocarbon mixtures of known composition. The mixtures may contain six and quite possibly any number of components including paraffins, isoparaffins, olefins, acetylenes, naphthenes and aromatics. The approach is similar to that used for calculating critical pressures and cricondentherm and cricondenbar temperatures. The critical pressure, the normal boiling point and the approximate vapor pressure behavior of each component are all that are required. A stepwise calculation procedure is necessary for mixtures containing more than two components. From an analysis of 22 binary systems and 118 mixtures, the average deviation of calculated cricondentherm pressures from reported values is 2.4 per cent. For nine multicomponent mixtures the average deviation is 1 per cent. Considering 19 binary systems and 108 mixtures, the average deviation of calculated cricondenbar pressures from reported values is 1.7 per cent. For 15 multicomponent mixtures, the average deviation is 2.2 per cent. Introduction A knowledge of the phase behavior in the critical region of multicomponent hydrocarbon mixtures is of value both in industrial processing operations and for the optimum operations of gas condensate reservoirs. Accurate methods of calculating the critical temperatures and critical pressures and cricondentherm and cricondenbar temperatures of multicomponent hydrocarbon mixtures are available in the literature. If the cricondentherm and cricondenbar pressures could be calculated with equal accuracy, the entire phase diagram of a multicomponent hydrocarbon mixture could be well approximated. The work of Etter and Kay is limited to systems containing the normal paraffins and has not been tested on systems containing a heavier component than heptane. In addition, the development of their multicomponent equations is based upon a limited number of mixtures containing from three to six components. Silverman and Thodos have considered systems containing both paraffinic and non-paraffinic hydrocarbons, but their correlation is limited to binary systems and is highly inaccurate for methane systems. Eilerts has done extensive work on the cricondenbar pressure; he has produced an excellent correlation for binary systems. However, his procedure for multicomponent mixtures is chiefly useful for highly complex mixtures requiring a knowledge of the vapor-liquid equilibrium behavior of the mixtures; he has not considered the cricondentherm pressure. The objective of this study was the development of an accurate and rapid method for the calculation of the cricondentherm and cricondenbar pressures of multicomponent mixtures containing all types of hydrocarbons and having a wide volatility range. The approach that was adopted is similar to that used by Grieves and Thodos for cricondentherm and cricondenbar temperatures and for critical pressures. However, methane systems had to be considered separately and a modified stepwise calculation procedure was utilized for the cricondentherm pressure. The correlations were developed in a manner similar to those for critical pressures and cricondentherm and cricondenbar temperatures. Based upon binary data reported in the literature it was observed that the ratios of cricondentherm and cricondenbar pressure to the pseudocritical pressure (molar average), pt/ppc and pp/ppc respectively, in two-component systems depended upon the mole fraction of this low-boiling component and upon the diversity in properties of the two components. A dimensionless boiling-point parameter T'b/Tb was chosen to represent the diversity in properties of the components. For a binary system, T't, is the molar average of the normal boiling points of the two components involved. SPEJ P. 240ˆ

The Cricondentherm and Cricondenbar Temperatures of Multicomponent Hydrocarbon Mixtures

Abstract A method has been developed for the accurate calculation of the cricondentherm and cricondenbar temperatures of multicomponent hydrocarbon mixtures of known composition. The mixtures may contain any number of components including paraffins and isoparaffins, olefins, acetylenes, naphthenes and aromatics. This approach is based upon the mole fraction of the low-boiling component in the mixture and graphically presents the ratios of the cricondentherm and cricondenbar temperatures to the pseudocritical temperature as functions of a boiling-point parameter. The only requirements are the critical temperature, normal boiling point and the approximate vapor pressure behavior of each component. For mixtures of more than two constituents a stepwise calculation procedure is necessary where the pseudocritical temperature is based upon the critical temperature of the pure low-boiling component and upon the actual cricondentherm or cricondenbar temperature of the mixture of all the remaining higher-boiling components. From an analysis of 22 binary systems (123 compositions), the average deviation of calculated cricondentherm temperatures from reported values is 0.87 per cent, based on degrees Rankine; and for 10 multicomponent mixtures containing from three to six components, the average deviation is 0.98 per cent. From an analysis of 18 binary systems (104 compositions) the average deviation of calculated cricondenbar temperatures from reported values is 0.79 per cent; and for 15 multicomponent mixtures, 1.47 per cent. Equations, derived from the graphical relationships, are presented which enable rapid calculations for both binary and multicomponent systems. Introduction With the increasing tendency towards the use of high pressure processes, the need for an accurate method of calculation of the properties of a multicomponent hydrocarbon mixture in the critical region is becoming more essential. Critical region phase relations are also significant in gas condensate reservoirs, for which a knowledge of the fluid phase boundaries and regions of retrograde condensation make it possible to evaluate optimum operating conditions. The possibility of liquefaction in the underground structure may be established, and information on the feasibility of the use of repressurizing may be determined. A typical phase diagram for a multicomponent hydrocarbon mixture is presented as Fig. 1. This figure also shows the regions of retrograde behavior encountered by following lines AB and AC. If accurate methods of calculating the temperatures and pressures at the critical cricondentherm (maximum temperature) and cricondenbar (maximum pressure) points of multicomponent hydrocarbon mixtures of known composition were available, it would be possible to estimate closely the entire phase diagram of any mixture. Several methods have been presented in the literature for the estimation of the critical temperatures and pressures of multicomponent mixtures containing all types of hydrocarbons and including the fixed gases. Considering the cricondentherm and cricondenbar points, the work is more limited. SPEJ P. 287^

VAPOR–LIQUID EQUILIBRIA, DENSITIES, AND REFRACTIVITIES IN THE SYSTEM ACETIC ACID – CHLOROFORM – WATER AT 25 °C

The volume changes on mixing of the two binary mixtures acetic acid – water and acetic acid – chloroform and of the ternary mixture acetic acid – chloroform – water have been determined. In the system acetic acid – water a volume contraction occurs, showing a minimum excess molar volume at 50 mole%. Partial molal volume determinations suggest the formation of a mixed association of one molecule of water with one molecule of acetic acid. In the system acetic acid – chloroform the excess molar volume function is positive over the complete concentration range and this suggests the probability of a positive deviation from Raoult's law.The refractivities of the two binary systems show that the true molecular volumes of acetic acid, water, and chloroform are additive on mixing.Studies of the vapor–liquid equilibria show that in all three constituent binary systems there is positive deviation from Raoult's law but if the dimerization of acetic acid in the vapor phase is taken into account the vapor pressure behavior of the system acetic acid – chloroform is virtually ideal. The vapor pressures in the ternary system show strong positive deviations from Raoult's law over the complete concentration range. This shows that the line of pseudo-ideality in terms of zero excess molar volume does not really represent ideal behavior. Zero excess molar volume is caused by the counteracting effects of water and of chloroform molecules on acetic acid molecules.In the two binary systems investigated the experimental data satisfy the Gibbs–Duhem–Margules equation. This proves that treatment for dimerization of acetic acid in the vapor phase is not merely permissible but necessary.

The use of the mass spectrometer for the quantitative analyses of mixtures of amino acids

The quantitative analysis of a low vapor pressure unstable solid mixture of amino acids is possible by putting the solid mixture in a crucible which is then placed directly into the ionization chamber of a mass spectrometer. Relative sensitivities for the various acids in a mixture can be established by the proper use of a particular amino acid as an internal-calibrating standard. The success of the method depends upon: (1) the reproducibility of the mass spectra of the pure components, (2) the ideal vapor pressure behavior of the solid mixture, (3) the preparation of an intimate solid mixture by rapid freezing of the water solution of the acids with subsequent vacuum sublimation of the ice and ball-milling the dried mixture.

The Critical Temperatures and Critical Pressures of Binary Mixtures of the Fixed Gases and Aliphatic Hydrocarbons

Abstract A method has been developed for predicting the critical temperatures and critical pressures of binary mixtures of carbon dioxide, hydrogen sulfide, nitrogen, hydrogen, carbon monoxide and oxygen with the normal paraffin hydrocarbons. For carbon-dioxide and hydrogen-sulfide systems, relations are presented that take into account the peculiar behavior of mixtures with closely boiling components, such as carbon dioxide-ethane and hydrogen sulfide-propane mixtures which exhibit minimum critical temperature points. For hydrogen, nitrogen and carbon-monoxide systems, the extreme critical behavior caused by wide differences in pure component properties is established. In addition, those fixed gas-paraffin systems which resemble paraffin-paraffin systems are also accounted for. For a mixture of known composition, the pure component critical temperatures, critical pressures and normal boiling points are all that are required to determine its critical point. Graphical relations are presented relating Tc and Pc of the mixture to the pure component properties. From the treatment of 12 carbon-dioxide and hydrogen-sulfide systems reported in the literature (74 mixtures), the expected error for the critical temperature is approximately 1.5 per cent, and for the critical pressure, approximately 2 per cent. From the treatment of six hydrogen, nitrogen and carbon-monoxide systems reported in the literature (30 mixtures), the expected error for both the critical temperature and critical pressure is approximately 2.5 per cent. The relationships, which have been developed with only normal paraffins as the hydrocarbon components, may be extended to those isoparaffins and olefins which fall within the allowable volatility ranges. Introduction Many of the fixed gases - carbon dioxide, hydrogen sulfide, nitrogen, hydrogen, carbon monoxide and oxygen - occur in natural mixtures with hydrocarbons. Carbon dioxide and hydrogen sulfide are frequent components of the fluids produced from underground petroleum reservoirs. Nitrogen, carbon dioxide and hydrogen sulfide are present in varying quantities in most natural gases and gas-condensate well effluents. Hydrogen mixtures are of considerable interest in many phases of refining processes of petroleum. The determination of the critical temperatures and critical pressures of such mixtures is of value in vapor-liquid equilibrium studies, for the prediction of the characteristics of underground reservoirs, and for reduced-state correlations of PVT, transport and thermodynamic properties. The accurate estimation of the critical point for binary mixtures is an important initial step toward a complete analysis for the establishment of the critical temperatures and pressures of multicomponent mixtures. Methods for predicting the critical temperatures and critical pressures of binary hydrocarbon systems have already been presented in the literature. It is possible to apply these existing methods to fixed gas-paraffin mixtures but due to their unusual critical behavior, values calculated deviate considerably from experimental values. For systems containing trace quantities of the fixed gases, these methods are acceptable; however, for systems containing more than 5 mol per cent of the fixed gases, these utterly fail to produce reasonable critical values. Consequently, in this study a method has been developed for handling such binary mixtures over the entire composition range. CARBON-DIOXIDE AND HYDROGEN-SULFIDE SYSTEMS The critical behavior and the vapor pressure behavior of mixtures of carbon dioxide and hydrogen sulfide with paraffinic hydrocarbons may be quite similar or quite dissimilar to that of paraffin-paraffin mixtures, depending on the volatilities of the components involved. The critical temperature and normal boiling point of carbon dioxide are very close to the corresponding values for ethane, while its critical pressure is considerably higher than that of ethane. SPEJ P. 197^