F. Recasens

Free liquid-to-supercritical fluid mass transfer in packed beds

Abstract In the supercritical fluid extraction of seeds and other naturally occurring porous solids, as much as 35–90% of the solute is extracted by evaporation of the free liquid into the dense gas, during the so-called fast regime. In this work we characterize this fast extraction period by evaporating 1,2-dichlorobenzene deposited on a shallow bed of non-porous, non-adsorbing glass spheres arranged in a packed bed with through circulation of supercritical carbon dioxide. Measurements performed over the ranges of pressure and temperature of 8–25.5 MPa and 310–320 K, respectively, provided information on the rate of mass transfer and on solubility. Mass conservation equations for solute were solved using the mixed-flow and the plug-flow assumptions. The two models involve two parameters: a vapour-liquid partition constant and a free liquid-to-fluid mass transfer coefficient. These were simultaneously determined using time-domain curve-fitting of the extraction curves with the solution to the model equations. The measured global mass transfer coefficients are put into perspective and compared with published transfer data for packed columns and fixed beds of solids.

Use of neural net computing for statistical and kinetic modelling and simulation of supercritical fluid extractors

An empirical kinetic model was developed for the extraction of black cumin (Nigella sativa L.) seed oil with supercritical carbon dioxide as solvent. Extraction conditions were up to 20 MPa and 333 K. Kinetic measurements provided by a neural-regressive hybrid prediction system allowed the simulation and design of a supercritical fluid extraction unit using differential laboratory data modelled with neural networks. Differential extraction yields were correlated with the factors that affected the rate (particle size, degree of milling, fluid flow rate, pressure, temperature, moisture, and time). Their influence on the extracted mass of oil (or system response) was determined by a statistical design of experiments. Response surface data were obtained by using non-parametric statistics. Thus, a neural net model was developed to predict extraction yields. It consisted in a feedforward multilayer neural network, conveniently trained with the backpropagation algorithm. Inputs to the neural net were: pressure, temperature and time. The system provided a single output (the amount extracted) as the only system response. The network architecture was kept to minimum complexity by carefully choosing the number of units of the hidden layer. The neural net computing program was intended as a design and simulation tool for SCF extractors. The bed was assimilated to a pile of layers, each corresponding to a neural net so that its extraction rate could be predicted by the associated neural network as a function of time. In the design stage, the neural-net system operates with numerical routines that carry out the integration of the fluid-phase mass balance equation. The intra-bed mass conservation equation takes into account solute accumulation, convective transport and transport by axial dispersion. Solution to the mass balance equation at each bed height and time was necessary for obtaining the breakthrough curve. By using certain scale-up rules, no fitting parameters were necessary to describe the actual breakthrough behaviour. A sensitivity analysis clearly pointed out the key process parameters, and how to use the model for design purposes. Predictions made with the present hybrid computing system (neural network in combination with pseudohomogeneous mass balance) successfully agreed with data.

Studies of a pervaporation reactor: Kinetics and equilibrium shift in benzyl alcohol acetylation

Abstract This work is centred on the coupling of a pervaporation module to a discontinuous esterification reactor using a commercial GFT membrane and analysing its possible application on an industrial level. The reaction chosen for the pervaporation study was that of the esterification of benzyl alcohol with acetic acid. This study was used to determine the kinetic parameters of the esterification and the applicability of pervaporation to esterification by determining the membrane permeability and its selectivity under the conditions of 80°C and a membrane surface area of 170 cm 2 . The results showed a flux of 0.54 kg/m 2 h, 96% selectivity in water and 99% conversion. A theoretical model was developed that satisfactorily agrees with the obtained experimental results, thus allowing the prediction of the conversion variation with the pervaporation time.

What's New in Industrial Polymerization with Supercritical Solvents? A Short Review

This review attempts to answer the following questions: what are the interest and benefits of polymerizing in supercritical media and what type of polymer and polymerization process can be used? Supercritical fluids (SCFs) in theory offer many advantages since they have intermediate propertics between liquids and gases. Also, at a time when the environment is undergoing such great impacts as the depletion of the ozone layer by chlorofluorocarbons (CFCs), the use of CO 2 seems an attractive a ternative for sustainability and conservation. This is why heterogeneous polymerization methods, particularly suspension and dispersion processes, ofter great potential with an SCF as a suspending medium. Unfortunately, polyolefins and other commodities, such as poly(vinyl chloride) (PVC), may not benefit from SCF technology today for profitability reasons, but acrylics, styrenics and other fluorinated plastics are open to this new technology. Furthermore. SCFs may have an impact on polyamides, polycarbenates and other speciality poly mers.

Safe storage temperature of peroxide initiators: prediction of self-accelerated decomposition temperature based on a runaway heuristics

Abstract Organic peroxides are slightly stable substances that can self-decompose under favourable conditions. During storage there can be a loss of activity if the temperature is above the recommended storage temperature. At higher temperatures this decomposition is self-accelerating. Many peroxides, are extremely shock sensitive or unstable in pure form and are, therefore, only commercially available in diluted solution. In this work, the thermal stability of 44 compounds is evaluated. The method suggested here for predicting the self-accelerating decomposition temperature (SADT) is based on the numerical solution of the equations derived from the mass and energy balances coupled to a heuristics to recognise parameter sets that produce runaway.

Endothermic character of toluene adsorption from supercritical carbon dioxide on activated carbon at low coverage

Abstract Recasens, P., Velo, E., Larrayoz, M.A. and Puiggene, J., 1993. Endothermic character of toluene adsorption from supercritical carbon dioxide on activated carbon at low coverage. Fluid Phase Equilibria, 90: 265-287. Heat effects and volumetric properties are analyzed for the adsorption of toluene from supercritical carbon dioxide onto activated carbon at the limit of zero coverage, based on existing data for the system. Using values of the adsorption equilibrium constant at different temperatures as a function of fluid density, large, negative partial molar volumes for toluene in the fluid were obtained, which were previously unavailable. Numerical integration of the differential equation that expresses the isobaric temperature dependence of the equilibrium constant, coupled with parameter optimization, enabled us to estimate the differential enthalpy of toluene adsorption onto the surface from the ideal gas at the same pressure and temperature, in addition to the enthalpy of transfer from the fluid to the surface. This is found to be large and positive near the critical conditions. Using the thermodynamic analysis of Kelley and Chimowitz, our results show that in terms of the enthalpy of transfer, the isothermal adsorption from a supercritical fluid is an endothermic process, thus explaining the retrograde behavior experimentally observed for the regeneration of carbon with supercritical CO2 at conditions not far from the solvents critical point.

PREDICTING THE APPROXIMATE SOLUBILITIES OF SOLIDS IN DENSE CARBON DIOXIDE

Abstract The aim of this paper is the prediction of solid solubilities in supercritical fluids (SCF) on an order-of-magnitude basis. A further aim is to clarify some of the computational aspects of the Boublik and Mansoori equations for the perturbed hard-sphere used in the estimation of solubility. Modelling the solubilities of heavy hydrocarbons and other organics provides an average value for the pair-potential integral function a 12 . The latter varies by only ±30% from one solute to the other over relatively wide ranges of pressure ( P ) and temperature ( T ). Thus, it is possible to use a mean value to estimate approximate solubilities in SCF, which are useful in some stages of engineering work where order-of-magnitude data is sufficient. On the other hand, the (one-parameter) Peng–Robinson equation of state with the interaction parameter optimized for each solute gives no clear guidance.

Near-critical solvent extraction of wool with modified carbon dioxide — experimental results

The extraction of a natural wax from raw wool was carried out with modified near-critical solvent in order to develop a cleaner, less polluting extraction process compared with the conventional soap-and-water washing process. As shown previously, natural wax is soluble in near supercritical carbon dioxide but only at very high pressures. Thus, carbon dioxide would be a very interesting non-aqueous solvent provided that lower pressures could be used. It has been found that the addition of a modifier increases the solvent polarity, hence increasing the solvating power of the mixture. In the present study, solutions of carbon dioxide and ethanol were employed, either in subcritical or supercritical conditions at moderate pressure (<200 bar). The experiments were done on a Separex 200 bench-scale unit. An experimental study was conducted to see the effects of pressure (70–200 bar), temperature (30–80°C) and wool packing densities ranging from 106 to 318 kg/m3. Specific solvent flowrates ranged from 10 to 25 kg/h per l of extractor. Also, the approximate lanolin solubilities were measured in separate experiments in order to interpret extraction rates and yields. Under the conditions of the study it is possible to extract 90% of the lanolin from raw wool.

Estimation of copolymer composition from online headspace analysis in batch emulsion polymerization

Abstract A new method has been developed for calculating instantaneous copolymer compositions from sampled measurements of component fractions in the reactor vapor-phase by a modified online headspace technique. The methodology is successfully applied to batch emulsion copolymers of Styrene and Acrylonitrile. Reliable values of monomer reactivity ratios as well as equilibrium (L/L/V) parameters are used in the calculation.

Quasi-static measurement of equilibrium solubilities in SC fluids: a mass transfer criterion

Abstract In this paper a continuous quasi-equilibrium method was used to determine the solubilities of α and β-naphthol in supercritical carbon dioxide. For the case of a packed bed extractor in which pure solid slabs were contacted with the flowing fluid, the approach to equilibrium is judged in terms of a modified Stanton number, in the region where free convection effects are dominant (low Reynolds number). Analysis of data from Tan and Weng [C.-S. Tan, J. Weng, Fluid Phase Equilib. 34 (1987) 37–48] and Anistescu-Tavlarides [G. Anitescu, L.L. Tavlarides, J. Supercrit. Fluids 10 (1997) 175–189;] for several solutes, as well as own data, show that quasi-static experiments provide the equilibrium solubility with negligible error.