Ralf Dohrn

Experimental measurements of phase equilibria for ternary and quaternary systems of glucose, water, CO2 and ethanol with a novel apparatus

Abstract Vapor-liquid equilibrium data have been measured for the ternary glucose-water-CO 2 system and for the quaternary glucose-water-ethanol-CO 2 system at temperatures from 323 to 343 K and pressures up to 30 MPa. Since the solubility of glucose in the vapor phase is small, a special high pressure apparatus has been designed and developed to ensure reliable sampling and analysis. The heart of the apparatus is a autoclave with a volume of 500 cm 3 equipped with capillary tubes for liquid samples. The vapor phase is recirculated by an air-driven piston pump. The novel feature of our design is a sample bomb of 300 cm 3 capacity placed in the vapor phase loop which can be blocked off and the content of which can be analyzed without disturbing equilibrium. Results indicate that while the solubility of glucose in pure supercritical CO 2 is very small (10 −5 -10 −4 mol%), it shows a remarkable increase (2*10 −4 -5*10 −2 mol%) with increasing pressure when ethanol is added as a polar co-solvent.

An algorithm for calculating critical points in multicomponent mixtures which can easily be implemented in existing programs to calculate phase equilibria

Abstract This paper describes an add-on procedure for the calculation of critical points in multicomponent mixtures according to the method by Heidemann and Khalil [1] . The benefit of this approach is that all equations of state (EOS) and mixing rules of an existing program for calculating phase equilibria, which can be a commercial product or self-made, are available for the calculation of critical points. The method by Heidemann and Khalil requires the first and second partial derivatives of the fugacities with respect to the male numbers ( ∂ ln f k / ∂ n i ) T,V,n i≠j ( ∂ 2 ln f k / ∂ n l ∂ n j ) T,V,n l≠i,j . In our work the first partial derivatives are obtained numerically with a four-point differencing scheme and the second partial derivatives are obtained from the first partial derivatives with a numerical directional derivative as suggested by Michelsen [2] . This new approach allows the combination of the method by Heidemann and Khalil with any program for calculating phase equilibria and eliminates the need for the cumbersome determination of the analytical first partial derivatives of the fugacity with respect to the mole numbers. The implementation into the commercially available process simulator ASPEN PLUS Version 9.2 [3] is described and some calculation results are shown.

Experimental Methods for Phase Equilibria at High Pressures

Knowledge of high-pressure phase equilibria is crucial in many fields, e.g., for the design and optimization of high-pressure chemical and separation processes, carbon capture and storage, hydrate formation, applications of ionic liquids, and geological processes. This review presents the variety of methods to measure phase equilibria at high pressures and, following a classification, discusses the measurement principles, advantages, challenges, and error sources. Examples of application areas are given. A detailed knowledge and understanding of the different methods is fundamental not only for choosing the most suitable method for a certain task but also for the evaluation of experimental data. The discrepancy between the (sometimes low) true accuracy of published experimental data and the (high) accuracy claimed by authors is addressed. Some essential requirements for the generation of valuable experimental results are summarized.

Phase equilibria in systems containing hydrogen, carbon dioxide, water and hydrocarbons

Vapor-liquid (VLE) and Vapor-liquid-liquid equilibrium (VLLE) data have been measured for the ternary mixtures water + n-hexadecane + carbon dioxide (CO2), benzene + n-hexadecane + CO2 and benzene + n-hexadecane + hydrogen and for the binary mixture CO2 + n-hexadecane at pressures between 10.1 to 30.1 MPa and at temperatures between 473.15 and 573.15 K. The applied experimental procedure is based on the static-analytical method. For equilibration, an autoclave (volume: 1000 cm3) with an electromechanically driven stirrer was used. Samples could be withdrawn from the autoclave through thermostated capillaries. For the analysis of the samples, the use of an on-line gaschromatograph was successful only for light components up to n-heptane. Therefore a conventional technique using cold traps to separate the samples into a liquid and a gaseous part including a subsequent analysis was applied. The experimental results could be correlated with simple equations of state (e.g. Peng-Robinson or Dohm-Prausnitz) using van der Waals mixing rules with one interaction parameter kij for each binary pair. For the determination of the kij values, a robust optimization routine was developed. It works on the operating-system level of the computer (e.g. by using shell scripts on UNIX machines) and can handle program stops automatically.

A simple perturbation term for the Carnahan-Starling equation of state

A simple perturbation term is presented for the Carnahan-Starling (CS) hard-sphere reference equation of state (EOS). This perturbed CS EOS is compared with seven other two-parameter EOSs; it represents the critical isotherms of eight fluids with the lowest deviations in density and pressure. After a generalized temperature dependence is introduced for parameters a and b, the perturbed CS EOS is compared with the well-known Peng-Robinson equation. For 11 non-polar pure fluids, the perturbed CS EOS represents liquid densities significantly better, but it is not superior for vapor pressures. For mixtures, the CS reference term is given by the Boublik-Mansoori hard-sphere mixture EOS. Some calculations for binary mixtures are given, using conventional mixing rules for parameters a and b in the simple perturbation term.

Thermophysical properties—Industrial directions

Changing conditions in the companies have necessarily led to changes in the work in thermodynamics groups in industry during the last 20 years. While many companies have reduced activities in the field of applied thermodynamics or even have closed their thermodynamics group, other companies, including Bayer, have put more emphasis in the field of applied thermodynamics. Here, ten industrial directions in the field of applied thermophysical properties are discussed. It is shown, where and when expertise in applied thermodynamics within the company is needed. At Bayer, this expertise can be delivered most efficiently by a group of thermodynamicists who use either internal sources, e.g. a laboratory, a database or estimation methods, or coordinate and supervise the use of external sources.

Phase equilibria in ternary and quaternary systems of hydrogen, water and hydrocarbons at elevated temperatures and pressures.

Abstract VLE and VLLE for ternary mixtures of hydrogen-water-hexadecane, hydrogen-water-benzene, hydrogen-benzene-hexadecane and the quaternary hydrogen-water-benzene-hexadecane system were experimentally determined at temperatures of 200 to 350°C and pressures up to 300 bar. The applied experimental procedure is based on the analytical method. A 1000-cm3 autoclave with an electromagnetically driven stirrer was used for equilibration. Samples could be taken at three different heights. Experimental results show that at lower temperatures three-phase equilibria prevail, while at higher temperatures two-phase VLE are more common. For the representation of the experimental data a special graphic program for a Hewlett-Packard microcomputer was developed. With a modified Redlich-Kwong equation of state some of the measured systems could be correlated successfully.

Measurement and correlation of vapor-liquid-liquid equilibria in the glucose + acetone + water + carbon dioxide system

Abstract Phase equilibria in the glucose + acetone + water + carbon dioxide system have been investigated experimentally in the three-phase region at temperatures of 313, 323, and 333 K and pressures of 4, 6, and 8 MPa. The experimental method was checked by measuring phase equilibria in the binary and ternary subsystems acetone + carbon dioxide and acetone + water + carbon dioxide and comparing the results with literature data. The experimental data for the quaternary system have been correlated using the Soave-Redlich-Kwong equation of state (EOS) with the mixing rule proposed by Mathias, Klotz and Prausnitz (1991). The binary interaction parameters for the binary subsystems without glucose have been regressed from phase equilibria of the binary subsystems, and the interaction parameters for glucose have been regressed from the glucose K-factors determined in the quaternary system investigated. With these interaction parameters the extent of the three-phase region can be well predicted and the partioning of glucose between the three phases can be reproduced.

High pressure liquid-liquid equilibria in ternary systems containing water, benzene, toluene, n-hexane and n-hexadecane

Abstract Liquid-liquid equilibria have been measured in the ternary systems water-n-hexane-n-hexadecane, water-toluene-n-hexadecane and water-benzene-n-hexadecane at 573 K and 30 MPa. A new apparatus for determination of high pressure liquid-liquid equilibria by the static-analytical method was constructed and set into operation. The pressure drop during sampling could be avoided by a gaseous expansion volume in contact with the equilibrium cell. The experimental data have been correlated using the Peng-Robinson-, Redlich-Kwong-Soave, Hederer-Peter-Wenzel- EOS. For the Peng-Robinson-EOS, the original as well as modified parameters were used. Good representation of the data could be obtained, though no EOS was able to reproduce the data in all the three systems equally well.

Vapor-phase thermal conductivity of 1,1,1,2,2-pentafluoropropane, 1,1,1,3,3-pentafluoropropane, 1,1,2,2,3-pentafluoropropane and carbon dioxide

Abstract The insulation efficiency of polyurethane foam is mainly (60 to 65%) determined by the thermal conductivity of the cell gas, which is a mixture of carbon dioxide (CO 2 ) and a blowing agent. Until recently CFC-11 (trichlorofluoromethane) was the most widely used blowing agent. To find adequate substitutes for CFC-11, the vapor-phase thermal conductivity of several potential blowing agents has been measured. The experimental procedure is based on the transient hot-wire method. The apparatus was designed in cooperation with the University of Stuttgart (Prof. K. Stephan). The measuring cell (stainless steel, 1.4571) with a length of 270 mm and a diameter of 48 mm consists of two parallel chambers with platinum wires of different lengths. In this paper experimental results for the vapor-phase thermal conductivity of carbon dioxide (CO 2 ), 1,1,1,2,2-pentafluoropropane (HFC-245cb), 1,1,1,3,3-pentafluoropropane (HFC-245fa), and 1,1,2,2,3-pentafluoropropane (HFC-245ca) are given. Experiments have been performed at pressures between 0.2 MPa and 1.5 MPa and temperatures between 298 K and 419.91 K.