Jtf Jos Keurentjes

Sustainable polymer foaming using high pressure carbon dioxide: a review on fundamentals, processes and applications

In recent years, carbon dioxide (CO2) has proven to be an environmentally friendly foaming agent for the production of polymeric foams. Until now, extrusion is used to scale-up the CO2-based foaming process. Once the production of large foamed blocks is also possible using the CO2-based foaming process, it has the potential to completely replace the currently used foam production process, thus making the world-wide foam production more sustainable. This review focuses on the polymer–CO2-foaming process, by first addressing the principles of the process, followed by an overview of papers on nucleation and cell growth of CO2 in polymers. The last part will focus on application of the process for various purposes, including bulk polymer foaming, the production of bioscaffolds and polymer blends.

Dissolved gas and ultrasonic cavitation : a review

The physics and chemistry of nonlinearly oscillating acoustic cavitation bubbles are strongly influenced by the dissolved gas in the surrounding liquid. Changing the gas alters among others the luminescence spectrum, and the radical production of the collapsing bubbles. An overview of experiments with various gas types and concentration described in literature is given and is compared to mechanisms that lead to the observed changes in luminescence spectra and radical production. The dissolved gas type changes the bubble adiabatic ratio, thermal conductivity, and the liquid surface tension, and consequently the hot spot temperature. The gas can also participate in chemical reactions, which can enhance radical production or luminescence of a cavitation bubble. With this knowledge, the gas content in cavitation can be tailored to obtain the desired output.

Description of dehydration performance of amorphous silica pervaporation membranes

The dehydration performance of a ceramic pervaporation membrane is studied for the separation of isopropanol/water mixtures. The membranes are provided by ECN (The Netherlands) and consist of a water selective amorphous silica top layer and four alumina supporting layers. For the system investigated, these membranes appear to combine high selectivities with high permeabilities. This results in a very high pervaporation separation index (PSI up to 6000 kg/m2 h at 80°C). A generalized Maxwell–Stefan model has been set up to model the fluxes. From this analysis it follows, that the water flux is only proportional to its own driving force. It is experimentally demonstrated that this holds for a wide range of operating conditions and feed compositions. From these data, values of various Maxwell–Stefan diffusivities are estimated.

Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures

In this paper, a set of performance data of a ceramic pervaporation membrane, provided by ECN, Petten, The Netherlands, is described. For the dehydration of alcohol/water mixtures, these membranes appear to combine high selectivities with high permeabilities, resulting in a high Pervaporation Separation Index (PSI). At 70°C the water flux and separation factor for the dehydration of isopropanol (water concentration varied between 1 and 7 wt.%) range from 0.45 to 2.8 kg/(m2 h), and 340–600, respectively. For the dehydration of n-butanol (water concentration varied between 1 and 5 wt.%) these values are between 0.4 and 2.3 kg/(m2 h) and 680–1340, respectively. These flux values are high as compared with the ceramic pervaporation membranes described in the literature.

Opportunities for process intensification using reverse micelles in liquid and supercritical carbon dioxide

Liquid and supercritical carbon dioxide can be used as a replacer for organic solvents, which potentially enables the development of clean processes. A general disadvantage of CO2, however, is that it is a very poor solvent for high molecular weight or hydrophilic molecules. By using reverse micelles to overcome this problem, a wide variety of novel processes can be thought of. In this review, it is shown that the use of micellar systems in supercritical CO2 leads to processes generating a minimum amount of waste and with a low energy requirement. It is also shown that especially downstream processing of biochemicals and polymerization reactions can benefit from this technique, as the number of consecutive process steps can be reduced significantly.

Calorimetric study of the energy efficiency for ultrasound-induced radical formation.

Energy conversion in sonochemistry is known to be an important factor for the development of industrial applications, however, the strong influence of the physical properties of the liquid on the ultrasound characteristics usually prevents an accurate determination of the chemical effects. In this study, the energy efficiency of the ultrasound-induced radical formation from methyl methacrylate has been investigated. The energy yield can be quantified by comparison of the ultrasonic power that is transferred to the liquid and the radical formation kinetics. Based on this method the influence of temperature and amplitude of the ultrasound horn on the energy efficiency has been determined. The energy yield for the formation of radicals from ultrasonic waves appears to be in the order of 5 x 10(-6) J/J. The energy conversion is the highest at low temperatures and at low amplitudes.

Design criteria for dense permeation-selective membranes for enantiomer separations

Dense enantioselective membranes can distinguish between two enantiomers by different mechanisms. At this moment, it is not clear which mechanism provides the best membranes for large-scale enantiomer separations. Therefore, we studied the design criteria for permeation-selective membranes combining literature data, experiments and model calculations. Literature data on dense permeation-selective membranes for enantiomer separation show that these membranes could be divided into two different classes: diffusion selective and sorption selective. Reviewing the literature on diffusion-selective membranes shows that these membranes have one main disadvantage: the inverse proportionality relation between the permeability and selectivity. This disadvantage is absent for sorption-selective membranes. As a model system, the diffusion of phenylalanine through a packed bed of polypropylene beads coated with N-dodecyl-l-hydroxyproline:Cu(II) was studied. The experiments showed that the material could selectively adsorb phenylalanine (Phe) with a selectivity (d/l) of 1.25. However, no permeation selectivity could be detected. With a dual sorption model these results could be interpreted. These model calculations showed that the permeation selectivity only approaches the intrinsic selectivity of the selector if the selectively adsorbed population is mobile and the non-selective permeation is minimized. Therefore, to our opinion more emphasis should be put on the development of sorption selective membranes.

Effect of resonance frequency, power input, and saturation gas type on the oxidation efficiency of an ultrasound horn

The sonochemical oxidation efficiency (η(ox)) of a commercial titanium alloy ultrasound horn has been measured using potassium iodide as a dosimeter at its main resonance frequency (20 kHz) and two higher resonance frequencies (41 and 62 kHz). Narrow power and frequency ranges have been chosen to minimise secondary effects such as changing bubble stability, and time available for radical diffusion from the bubble to the liquid. The oxidation efficiency, η(ox), is proportional to the frequency and to the power transmitted to the liquid (275 mL) in the applied power range (1-6 W) under argon. Luminol radical visualisation measurements show that the radical generation rate increases and a redistribution of radical producing zones is achieved at increasing frequency. Argon, helium, air, nitrogen, oxygen, and carbon dioxide have been used as saturation gases in potassium iodide oxidation experiments. The highest η(ox) has been observed at 5 W under air at 62 kHz. The presence of carbon dioxide in air gives enhanced nucleation at 41 and 62 kHz and has a strong influence on η(ox). This is supported by the luminol images, the measured dependence of η(ox) on input power, and bubble images recorded under carbon dioxide. The results give insight into the interplay between saturation gas and frequency, nucleation, and their effect on η(ox).

Design directions for composite catalytic hollow fibre membranes for condensation reactions

Pervaporation membrane reactors are ideal candidates to enhance conversion in reversible reactions generating water as a by-product. The equilibrium displacement can be enhanced by catalytic membranes due to the close integration of reaction and separation. In this paper, the viability of composite catalytic hollow fibre pervaporation membranes for condensation reactions is examined. The esterification reaction between acetic acid and butanol has been taken as a model reaction, for which a parametric model study was carried out to provide a fundamental understanding of the composite catalytic membrane reactor behaviour. With increasing catalytic layer thickness, the conversion becomes no longer limited by the amount of catalyst present in the reactor but by diffusion in the catalytic layer. External mass transfer was never found to be rate-limiting. An optimum catalytic layer thickness was found to be around 100 μm under the prevailing conditions, which is within practically reachable dimensions. At this optimum catalytic layer thickness, the performance of a catalytic membrane reactor exceeds the performance of an inert membrane reactor due to the close integration of reaction and separation. This shows the potential added value of such a membrane system compared with more usual reactor designs. The exact value of this optimum is a function of the reaction kinetics and the membrane permeability.

High-flux palladium-silver alloy membranes fabricated by microsystem technology

In this study, hydrogen selective membranes have been fabricated using microsystem technology. A 750 nm dense layer of Pd (77 wt%) and Ag (23 wt%) is deposited on a non-porous 1 mm thick silicon nitride layer by cosputtering of a Pd and a Ag target. After sputtering, openings of 5 μm are made in the silicon nitride layer to create a clear passage to the Pd/Ag surface. As a result of the production method, these membranes are pinhole free and have a low resistance to mass transfer in the gas phase, as virtually no support layer is present. The membranes have been tested in a gas permeation system to determine the hydrogen permeability as a function of temperature, gas flow rate, and feed composition. In addition, the hydrogen selectivity over helium has been determined, which appears to be above 1500. At 0.2 bar partial hydrogen pressure in the feed, the hydrogen permeability of the membranes has been found to range from 0.02 to 0.95 mol.H2/m2×s at 350 and 450°C, respectively. It is expected that by improving the hydrodynamics and increasing the operation temperature, substantially higher fluxes will be attainable.