H.J. Van Der Kooi

Exergy analysis with a flowsheeting simulator—I. Theory; calculating exergies of material streams

Abstract This paper presents a method for calculating the absolute exergy of multicomponent liquid, vapour or two-phase flows. The method is different from those described by most authors in that it enables a clear division of the total exergy of a material stream into three terms such that the exergy change of mixing is calculated separately from the chemical and the physical exergy. The method has been implemented in a set of external subroutines which have been integrated with a flowsheeting simulator, in order to calculate exergies of material streams along with the traditional energy- and mass-balance calculations. Except for an external database containing values of standard chemical exergies, the exergy calculations require no additional input data. Exergies are calculated as an extensive stream property and are therefore accessible to various procedures supported by the flowsheeting simulator, e.g. optimization. Furthermore, this also enables the determination of exergy losses by simple accounting. A complete exergy analysis, of course, must also incorporate the interpretation of these exergy losses and a suggestion or indication of ways to reduce them. These will be discussed in part 2 of this article with the case of synthesis gas production from natural gas, showing the exergy method as a useful diagnostic tool for analyzing chemical processes.

Illustrations towards quantifying the sustainability of technology

This paper attempts to quantify the sustainability of technological processes. It is based on thermodynamics with energy carriers and materials (products, waste, etc.) expressed in the same calculable quantity–exergy (Joule). The results have three considerations. One factor reflects to that extent renewable resources are used. In addition, the technological efficiency has to be accounted for as it affects sustainability. Finally, the results take into account the generation of waste products and the exergy required for converting the waste into products which are harmless or assimilable in the ecosphere. The proposed measure of sustainability has been illustrated for two types of products. In the first illustration, ethanol production was studied. Two routes were investigated, one starting from fossil oil and the other from agricultural products. Additionally, a route based on the synthesis from carbon dioxide and hydrogen was examined, in which hydrogen was generated by splitting water with electricity from photovoltaic solar energy conversion. The second product studied was electricity, generated from the combustion of natural gas or from photovoltaic solar energy conversion. The merit of the obtained results are that they treat technological sustainability not only in qualitative but also in quantitative terms. The insights obtained can help to account for sustainability in the development of new concepts of chemical technology.

Measurements and calculations of phase equilibria for (ethane + tetracosane) and (p, Vm′∗, T) of liquid tetracosane

Abstract For (ethane + tetracosane) partial miscibility in the liquid phase occurs in the vicinity of the critical point of ethane. As the three-phase locus (solid tetracosane + l + g) intersects the three-phase locus (l + l + g) a quadruple point (solid tetracosane + l + l + g) will be present. The four three-phase loci which must originate in this quadruple point were determined experimentally. In addition bubble-pressure and crystallization-pressure curves as well as molar volumes of liquid tetracosane were determined. The experimental work was carried out in a temperature region from about 280 to 360 K and pressures up to 12 MPa were applied. To test the possibility of representing (l + g) equilibria in this mixture by means of a simple cubic equation of state the original Redlich-Kwong equation of state was used.

Exergetic life cycle analysis of hydrogen production and storage systems for automotive applications

In this paper, the results of an exergetic well-to-wheels analysis of a number of hydrogen production and hydrogen storage systems for automotive applications are given. A total of eight different fuel chains is exergetically analysed. Exergy analysis is shown to have considerable additional value compared to conventional energetic well-to-wheels analyses based on the lower or higher heating value of fuels under consideration. Exergy can be used for both fuel and non-fuel resources and can play an important role in the quantification of resource depletion in fuel chains. With exergy analysis, it is possible to determine thermodynamic limits of processes and to locate and interpret process losses. Exergy analysis is therefore a useful tool in process improvement and process comparison. Furthermore, exergy can play a role in the quantification of the effort it would take to abate or recycle waste streams.

The search for tricriticality in binary mixtures of near-critical propane and normal paraffins

Abstract This paper reports on three-phase equilibria liquid + liquid + vapor of binary mixtures of near-critical propane and higher normal paraffins. Special attention has been given to locate the (pseudo-)binary first showing partial miscibility in the liquid phase. For that purpose the three-phase equilibria liquid + liquid + vapor, including critical endpoints, have been determined experimentally in the binaries C 3 + C 32 , C 3 + C 34 , C 3 + C 36 , C 3 + C 38 , C 3 + C 40 C 3 + C 44 , C 3 + C 46 and C 3 + C 50 . Extrapolation of the results obtained shows that for a carbon number between 29 and 30 tricriticality has to be expected. In order to model the measured three-phase equilibrium data, an algorithm has been developed on the basis of the simplified perturbed hard chain theory. The calculations are still in progress.

Exergy analysis with a flowsheeting simulator—II. Application; synthesis gas production from natural gas

Abstract Several processes, producing synthesis gas from natural gas, have been analyzed by the exergy method, showing exergy analysis as a valuable diagnostic tool. In addition, a generally applicable and systematic way of performing exergy analyses and dealing with their results is illustrated. Exergy calculations have been carried out by user-defined subroutines, which are integrated with a flowsheeting simulator. The method of calculating exergies has been described in part 1 of this article. First, in order to systematically perform exergy analyses, the overall exergy loss for each process is determined. Absolute exergy losses based on final product yield, which is chosen to be methanol, are used for process comparison and diagnosis rather than exergetic efficiencies. Compared to the conventional steam reforming process, giving an overall exergy loss of approximately 8.5 GJ/t methanol, the exergy loss can be reduced to about 4.9 GJ/t methanol by application of the convective reforming option in combination with partial oxidation. Secondly, by considering progressively smaller subsystems within the overall process, locations of major exergy loss are revealed and their potential for improvement can be indicated. Finally, the minus value of the standard Gibbs energy of the overall reaction of each process, denoted as available reaction exergy, is compared to the exergy loss associated with this overall reaction. This comparison demonstrates that available reaction exergies should always be minimized to reduce exergy losses associated with chemical reactions. They cannot however, be eliminated completely when reactions are only thermally coupled. Further improvement can be attained by direct coupling of chemical reactions such that the overall Gibbs energy of reaction is still reasonably negative. This latter conclusion will hopefully result in a reconsideration of the chemical paths along which important chemicals are produced nowadays.

The sustainability of resource utilization

There is a need for a method with which the sustainability of technological processes can be described quantitatively, and earlier work by De Wulf et al. in cooperation with our laboratory has yielded a useful method based on thermodynamic principles. New insights however have led to a revision of this earlier work, and considerable improvements have been introduced. Most progress is made in quantitatively describing the sustainability of resource utilization. Instead of classifying resources either as renewable or as non-renewable, the new method is based on the depletion rates of resources, which account for many grades of resource renewability and which include also the size of natural reserves of resources. Also, this paper reviews the quantification of other aspects of process sustainability, explains the vital role of exergy, and discusses how the overall sustainability of a process should be determined.

Thermodynamic optimization of energy transfer in (bio)chemical reaction systems

The engineering science literature on energy-transfer processes (heat engines, heat pumps, etc.) for the production of mechanical energy, electricity, cold or heat has produced some remarkable results on maximum power and efficiency optimization. We have wondered in how far these results can be extended to include chemical reaction systems to describe living and possibly future-industrial energy-transfer systems. With elements of non-linear irreversible thermodynamics and chemical kinetics, we have arrived at some interesting results on optimizing the performance of chemical energy transfer. We discuss thermodynamic efficiency, energy-transfer rate, entropy-generation rate, and optimum-performance characteristics of (bio)chemical energy transfer.

Phase equilibria in binary mixtures of ethane and hexadecane

Abstract This paper reports experimental results of a study of the phase behaviour of binary mixtures of ethane + hexadecane. In the near-critical region of ethane liquid + vapour and solid hexadecane + liquid two-phase boundaries have been measured. Also the three-phase equilibrium solid hexadecane + liquid + vapour has been determined experimentally. The experimental data cover the complete mole fraction range. Pressures up to 18 MPa were applied and the investigation was performed in a temperature region from about 260 K up to 450 K.

Simulation and thermodynamic analysis of conventional and oxygen permeable CPO reactors

Abstract A one-dimensional steady-state heterogeneous model has been used to simulate the conventional CPO reactor. With the mechanism of O 2 permeable membrane, the model has been developed to simulate O 2 membrane reactor. The output temperature and the mole flow rates of different species in the tube side and the shell side can be calculated. They are the basis for the exergy analysis of the conventional CPO reactor with air, the conventional CPO reactor with pure O 2 , and the O 2 permeable membrane CPO reactor. The simulation and exergy analysis results indicate that when the inlet conditions are the same, for a given methane conversion, the exergy efficiencies η 2 and η 1 of conventional CPO reactor with pure oxygen is lowest among the three reactors, because of the large amount of accumulative exergy required for obtaining pure oxygen. The exergy efficiencies η 1 and η 2 of membrane reactor are comparable with conventional CPO reactor with air and much higher than conventional CPO reactor with pure oxygen. As the membrane reactors can carry out simultaneous separation and reaction, in the mean time, removal of nitrogen from the product stream can be accomplished; the membrane reactor has advantages compared to other types of reactors. The operation of the membrane CPO reactor is more favourable when the inlet temperature is increased and the operation pressure is decreased from a thermodynamic point of view.