Band 27, Heft 3-4

In the 21st century, the mission of chemical engineering is to promote innovative technologies that reduce or eliminate the use or generation of hazardous materials in the design and manufacture of chemical products. The sustainable use of renewable resources, complying with consumer health and environmental requirements, motivates the design, optimisation, and application of green benign processes. Supercritical fluid extraction is a typical example of a novel technology for the ecologically compatible production of natural substances of high industrial potential from renewable resources such as vegetable matrices that finds extended industrial application. The present review is devoted to the stage of development of supercritical fluid extraction from vegetable material in the last 20 years. Without the ambition to be exhaustive, it offers an extended, in comparison with previous reviews, enumeration of extracted plant materials, discusses the mathematical modelling of the process, and advocates a choice for the appropriate model that is based on characteristic times of individual extraction steps. Finally, the attention is focussed on the elements of a thermodynamic modelling framework designed to predict and model robustly and efficiently the complex phase equilibria of the systems solute+supercritical fluid.

The integration of reaction and separation in catalytic membrane reactors is gaining much attention in both academic and industrial research due to its huge potential for a wide range of chemical reaction applications. The use of inorganic membranes in catalytic membrane reactors is being widely considered because the chemical reactions to be carried out in such reactors are normally conducted in harsh conditions concerning both temperature and the chemical environment. There are a number of chemical processes being employed using catalytic membrane reactors, such as dehydrogenation, water-gas shift reaction, methane steam reforming, partial oxidation of methane to synthesis gas, and oxidative coupling of methane to C 2 hydrocarbons. All these processes are evaluated and discussed in this article. The future prospects of catalytic membrane reactors are also presented.

Industrial demands for more efficient boilers and evaporators as well as economic incentives have encouraged the development of methods to increase boiling heat transfer coefficients, critical heat fluxes, and, where possible, to obtain the highest heat flux by applying the smallest wall superheat. The goal may be to reduce the heat exchanger size or pumping power required for a specified heat duty, and also to prevent excessive temperature, or even system destruction, in systems where heat generation rates are fixed – for instance, in nuclear fuel assemblies or in chemical reactors. Numerous enhanced surfaces have been developed to intensify flow as well as pool boiling heat transfer. Excellent reviews on that topic have been provided in textbooks. Porous coated surfaces, which this article deals with, belong to the efficient categories of enhanced boiling surfaces. This article reviews the selected results of a comprehensive study of flow and pool nucleate boiling on porous coated surfaces completed in our laboratory. Particularly, this article deals with the following: (1) nucleate pool boiling on flat circular plates and horizontal tubes covered with porous coatings. Particularly, the influence of coating thickness and porosity on heat transfer coefficient and burnout heat flux for distilled water at atmospheric pressure was studied; (2) nucleate pool boiling on horizontal tube partially coated with a porous metallic layer. The possibility of porous coating application to local heat transfer enhancement, for instance in order to smooth and alleviate circumferential temperature distribution during nucleate pool boiling on horizontal, electrically heated tube, was examined; (3) nucleate pool boiling from small horizontal tube bundles built of porous coated tubes. The effect of the tube pitch and operating pressure on local, i.e., for a single tube or selected row of tubes, and average, i.e., for the whole tube bundle, heat transfer coefficient for three working fluids (distilled water, methanol, and refrigerant R141b) was studied. The bundle factor and bundle effect were determined. Second, practical application of porous coated tube bundle as an evaporator in prototype two-phase thermosyphon heat exchanger was investigated; (4) flow boiling of pure refrigerants and refrigerant/oil mixture inside porous coated tubes. The average evaporation heat transfer coefficient and simultaneous pressure drop during evaporation of R22, R134a, and R407C and their oil mixture of different concentrations was studied. A correlation equation for heat transfer coefficient calculation during flow boiling of pure refrigerants inside a tube with porous coating is proposed.

Investigation of heat transfer at supercritical pressures began as early as the 1930s, with the study of free-convection heat transfer to fluids at the near-critical point. In the 1950s, the concept of using supercritical “steam” to increase the thermal efficiency of fossil-fired power plants became an attractive option. Currently, using supercritical “steam” in fossil-fired power plants is the largest industrial application of fluids at supercritical pressures. Near the end of the 1950s and at the beginning of the 1960s, several studies were conducted to investigate the potential of using supercritical water as a coolant in nuclear reactors. The USA and the former USSR extensively studied supercritical heat transfer during the 1950s until the 1980s. Research primarily focused around circular water-cooled tube flow geometry. The primary objectives for using supercritical water as a coolant in nuclear reactors are: (1) to increase the thermal efficiency of modern nuclear power plants, which is currently 30–35%, to approximately 45% or higher, and (2) to decrease the operational and capital costs by eliminating steam generators, steam separators, steam dryers, etc. In support of the development of a supercritical water-cooled nuclear reactor, it is necessary to perform a heat-transfer analysis. As a first step in this process, heat-transfer to supercritical water in bare vertical tubes can be investigated as a conservative approach (in general, heat-transfer in fuel bundles will be enhanced with various types of appendages, i.e., bearing pads, end plates, fins, ribs, spacers, etc.). A comparison of selected supercritical-water heat-transfer correlations has shown that their results may differ from one another by more than 200%. Based on these comparisons, it became evident that there is a need to use a reliable, accurate and wide-range supercritical-water heat-transfer correlation. For this reason, the new or recently updated Mokry et al. (2009) correlation was used in the presented analyses. Other areas exist in which supercritical fluids are used, or will be implemented in the near future. These include, for example, using SuperCritical Water Oxidation (SCWO) technology for the treatment of industrial and military wastes, using carbon dioxide in the Supercritical Fluid Leaching (SFL) method for the removal of uranium from radioactive solid wastes and in the decontamination of surfaces, and using supercritical fluids in chemical and pharmaceutical industries, in such processes as supercritical fluid extraction, supercritical fluid chromatography, polymer processing, and others. The objective of this review is to assess the work performed concerning thermophysical properties and the associated heat transfer and pressure drop at supercritical pressures, based on examples of water and carbon dioxide.