Gerd Maurer

High-pressure (vapor+liquid) equilibrium in binary mixtures of (carbon dioxide+water or acetic acid) at temperatures from 313 to 353 K

Abstract Experimental results for the high-pressure (vapor+liquid) equilibrium of binary systems of carbon dioxide+(either water or acetic acid) are reported for temperatures from 313 to 353 K and pressures between 1 and 14 MPa. The experimental results are correlated using the modification of Melhem et al. [G.A. Melhem, R. Saini, B.M. Goodwin, A modified Peng–Robinson equation of state, Fluid Phase Equilibr. 47(1989) 189] for the Peng–Robinson equation of state and the mixing rule of Panagiotopoulos and Reid [A.Z. Panagiotopoulos, R.C. Reid, Equations of state: theories and applications, in: K.C. Chao, R.L. Robinson (Eds.), ACS Symposium Series, vol. 300, American Chemical Society, Washington, DC 1986 pp. 571]. The correlation of the experimental data for (carbon dioxide+water) required temperature-dependent binary interaction parameters. Correlating the data for (carbon dioxide+acetic acid) was not very successful as long as the dimerization of acetic acid was neglected. However, including dimerization resulted in a reliable correlation and even a successful prediction of the high-pressure (vapor+liquid) equilibrium from pure component data alone.

On the derivation and extension of the uniquac equation

Abstract The UNIQUAC equation is derived by phenomenological arguments based on a two-fluid theory. This derivation is free of the inconsistencies which arise when a lattice one-fluid theory is used to derive UNIQUAC or any other local-composition equation. The essential step in the derivation is the adoption of Wilsons assumption that local compositions can be related to overall compositions through Boltzmann factors. In UNIQUAC, the energy terms in the Boltzmann factors are assumed identical to those in the excess energy of mixing. When the assumption of identity is replaced by the much less stringent assumption of proportionality, a three-parameter UNIQUAC equation is derived. This equation often gives a better fit of experimental data whenever all three binary paramters are freely adjusted. Upon assigning a universal value to the third parameter, only marginal improvement is obtained.

Vapor+liquid equilibrium of water, carbon dioxide, and the binary system, water+carbon dioxide, from molecular simulation

Abstract NVT- and NpT-Gibbs ensemble Monte Carlo (GEMC) simulations were applied to describe the vapor–liquid equilibrium of water (between 323 and 573 K), carbon dioxide (between 230 and 290 K) and their binary mixtures (between 348 and 393 K). The properties of supercritical carbon dioxide were determined between 310 and 520 K by NpT-MC simulations. Literature data for the effective pair potentials (for water: the SPC-, SPC/E-, and TIP4P potential models; for carbon dioxide: the EPM2 potential model) were used to describe the properties of the pure substances. The vapor pressures of water and carbon dioxide are calculated. For water, the SPC- and TIP4P models give superior results for the vapor pressure when compared to the SPC/E model. The vapor–liquid equilibrium of the binary mixture, carbon dioxide–water, was predicted using the SPC- as well as the TIP4P model for water and the EPM2 model for carbon dioxide. The interactions between carbon dioxide and water were estimated from the pair potentials of the pure components using common mixing rules without any adjustable binary parameter. Agreement of the predicted data for the compositions of the coexisting phases in vapor–liquid equilibrium and experimental results is observed within the statistical uncertainties of the simulation results in the investigated range of state, i.e. at pressures up to about 20 MPa.

Chemical equilibrium constants for the formation of carbamates in (carbon dioxide + piperazine + water) from 1H-NMR-spectroscopy

Abstract 1 H -NMR spectroscopic investigations were performed on aqueous solutions of carbon dioxide and piperazine at temperatures ranging from (283 to 333) K to determine quantitatively the speciation. The experimental results were used to determine the chemical equilibrium constants for the formation of piperazine carbamate, piperazine dicarbamate and protonated piperazine carbamate.

Vapor-liquid-solid equilibria in the system NH3CO2H2O from around 310 to 470 K: New experimental data and modeling

Abstract As part of an ongoing project dealing with experimental and theoretical work on the solubility of weak electrolyte gases like ammonia, carbon dioxide, sulfur dioxide, hydrogen sulfide or hydrogen cyanide in aqueous phases, the simultaneous solubility of ammonia and carbon dioxide in water was measured in the temperature range from 313 K to 353 K at total pressures up to about 0.7 MPa. The model proposed by Edwards et al. (1978) is used to correlate the new as well as literature data (Goppert and Maurer (1988), Muller et al. (1988)). Experimental and calculated results are reported and compared.

Aqueous two-phase systems of poly(ethylene glycol) and dextran — experimental results and modeling of thermodynamic properties

Abstract Experimental results for some properties (activity of water, change of enthalpy on mixing and diluting, compositions of coexisting liquid phases) of aqueous solutions of hydrophilic polymers poly(ethylene glycol) and dextran between 277 and 333 K are reported. The results are used to develop and test a semiempirical group contribution expression for the excess Gibbs energy of diluted aqueous polymer solutions. The model is applied to correlate some of the new experimental data, while the remaining experimental results are used to test its potential for predicting the influence of polymer molecular weight and temperature on the phase behavior of aqueous two-phase systems — a field where most previously known methods fail.

High-pressure multiphase behaviour of ternary systems carbon dioxide–water–polar solvent: review and modeling with the Peng–Robinson equation of state

Abstract Mixtures of water and a hydrophilic organic solvent, e.g. an alcohol, a ketone, or a carboxylic acid, reveal a complex phase behaviour when pressurized with near critical carbon dioxide at temperatures near the critical temperature of carbon dioxide. Although water and the hydrocarbonaceous solvent are completely miscible, by pressurization with carbon dioxide a liquid–liquid phase split is observed, resulting in a ternary liquid–liquid–vapor equilibrium. In some cases even a four-phase equilibrium is observed. Such equilibria are discussed for systems with the organic solvents methanol, ethanol, 1-propanol, 2-propanol, acetone and propionic acid. That phase behaviour is modelled using the Peng–Robinson equation of state with several mixing rules. A short description of the mathematical tools for such phase equilibrium calculations is given. From pure component and binary data alone, the calculations usually result in a qualitative agreement with the experimental data, i.e. typical effects like for example the existence of ternary critical endpoint lines and tricritical points can be predicted. However, in most cases the quantitative agreement with experimental results is poor. It can be considerably improved by fitting some interaction parameters to ternary high-pressure three-phase equilibrium data.

Partitioning of some amino acids and low molecular peptides in aqueous two-phase systems of poly(ethylene glycol) and dipotassium hydrogen phosphate

Abstract Aqueous two-phase systems are discussed for the extraction of biomolecules like, for example, amino acids, peptides and enzymes from aqueous phase. The development of such extraction processes requires experimental results for the partitioning of model substances, as well as methods for correlating and predicting such phase equilibria. The present contribution reports experimental results for the partitioning of small amounts (about 1 mg/g) of amino acids glycine, l -glutamic acid, l -phenylalanine and l -lysine and some of their low molecular peptides in aqueous two-phase system of dipotassium hydrogen phosphate and poly(ethylene glycol) of molecular mass of about 6000 and 35 000 at about 293 K. The experimental results for the partitioning of the amino acids and some of their peptides are used to determine interaction parameters of a group contribution model for the excess Gibbs energy. The remaining experimental data are used to test the model for predicting partitioning of peptides in PEG/K2HPO4 two-phase systems.

Solubility of Carbon Dioxide in Aqueous Solutions of N-Methyldiethanolamine and Piperazine in the High Gas Loading Region

A synthetic technique was used to measure the solubility of carbon dioxide in three aqueous solutions of 2,2′-methyliminodiethanol (N-methyldiethanolamine, MDEA) and piperazine (PZ) at around (313, 333, 353, and 393) K and pressures between about (0.2 and 10) MPa. The molalities of (MDEA + PZ) in the aqueous solutions were (2.2 + 1.97), (4.22 + 2.01), and (7.83 + 2.07), respectively. The loading of the amines by carbon dioxide (i.e., the molar ratio of (MDEA + PZ) and CO2 varied between 0.2 and 2.0 The new experimental results are to supplement previously published experimental data of our research group for a single solution of both amines {molalities of (MDEA + PZ) in the aqueous solutions: (2.0 + 2.0)}. The new experimental results are compared to predictions from a model that combines previously published models for (CO2 + MDEA + H2O) and (CO2 + PZ + H2O).

A SIMPLE EFFECTIVE PAIR POTENTIAL FOR THE MOLECULAR SIMULATION OF THE THERMODYNAMIC PROPERTIES OF AMMONIA

A new optimized effective pair potential model is proposed, which is appropriate for the prediction of thermodynamic properties of fluid ammonia including vapour—liquid coexistence data. The phase behaviour is determined using a recently developed version of the Gibbs ensemble Monte Carlo method. Furthermore, liquid structure characteristics, the dielectric constant and supercritical properties are determined by Monte Carlo simulations in the isothermal—isobaric ensemble. The second virial coefficient of the pair potential model is calculated over a broad range of temperature. All properties are compared with experimental data or results of a multi-parameter equation of state for ammonia. The new model is found to yield coexistence properties and second virial coefficients in good agreement with experimental data and the results of the equation of state, respectively.