Heterogeneous Catalysis in Supercritical Media: 2. Near‐Critical and Supercritical Water

It was demonstrated in part I, also published in this journal, that supercritical carbon dioxide may be advantageous for the use as a reaction medium in heterogeneously catalyzed reactions. Due to its miscibility with hydrogen, hydrogenations of increased selectivity can be carried out or carbonization of catalysts can be reduced by the dissolution and removal of coke precursors. The drawback of supercritical H₂O (T_crit = 374 °C) as compared to supercritical CO₂ (T_crit = 31 °C) is its relatively high critical point. At these temperatures, organic substances decompose within minutes. Consequently, many reactions are carried out below this critical point, in the so‐called “near‐critical water” or “hot compressed water.” Reaction conditions in near‐critical and supercritical water are also referred to as “hydrothermal.” In spite of the relatively high pressures and temperatures required and the associated high experimental expenditure, many types of reactions have already been studied, as the strong dependence of physical and chemical properties on pressure and temperature makes near‐critical and supercritical water a unique and extraordinary solvent.     

Convection Heat Transfer of CO2 at Supercritical Pressures in Microtubes

Convection heat transfer of supercritical pressure CO₂ in microtubes as well as characteristics of fluid flow and heat transfer were investigated. Alterations of physical properties of supercritical CO₂ and the impact of buoyancy on the heat transfer were analyzed when the inlet Reynolds (Re) number of CO₂, diameter of heat exchanger, and inlet Re of cooling water were changed. The temperatures of hot fluid in numerical simulations from several classical turbulence calculation models were compared with experimental results from the literature. The physical properties of CO₂ changed significantly around the critical point, causing a distinct decline of buoyancy and strengthening of heat transfer.     

Fluorescence studies on local density change in supercritical CO2 mixtures using the order parameter model

Supercritical CO₂ has many unusual physicochemical properties in or near the critical region, which is mostly due to the local density enhancements and specific molecular interactions. Compared to pure CO₂, the study on CO₂ mixed with co-solvent is attractive in wide applications but it is hard to quantitatively describe the local density change and intermolecular clusters in CO₂ mixtures. The fluorescence response of a sensitive pyrene probe, empirical Py scale, was determined in CO₂ mixed with pentane at 323.15 K in different phase regions. A variable called the order parameter was introduced to account for possible correlations in the first derivative of Py scale and fluid pressure (dPy/(d(P/P₀ − 1))). This model, avoiding the hard choice of the reference line in CO₂ mixtures, was effective in calculating the local density change and it showed the specific molecular interaction in the critical region. The behaviors of the local density change and the isothermal compressibility of supercritical CO₂ mixtures correlated well with microscopic and macroscopic observations.     

Prediction of phase equilibrium of CO2/cyclic compound binary mixtures using a rigorous modeling approach

Vapor liquid equilibrium (VLE) data has significant role in designing processes which include vapor and liquid in equilibrium. Since it is impractical to measure equilibrium data at any desired temperature and pressure, particularly near critical region, thermodynamic models based on equation of state (EOS) are usually used for VLE estimating. In recent years due to the development of numerical tools like artificial intelligence methods, VLE prediction has been find new alternatives.In the present study a novel method called Least-Squares Support Vector Machine (LSSVM) used for predicting bubble/dew point pressures of binary mixtures containing carbon dioxide (CO₂) + cyclic compounds as function of reduced temperature of the system, critical pressure, acentric factor of the cyclic compound, and CO₂ composition. A 333 binary equilibrium data points of CO₂ and six cyclic compounds within temperature and pressure ranges of 308.15–473.15 K and 0.5–27.71 MPa were used to develop the model. Results show that the proposed model is able to predict VLE data for binary systems containing supercritical or near-critical CO₂/cyclic compounds with an acceptable average absolute relative deviation percent (AARD%) of 3.9381% and the coefficient of determination (R²) value of 0.9980. For detection of the probable doubtful experimental data, and applicability of the model, the Leverage statistical approach performed on the data sets. This algorithm showed that the proposed LSSVM model is statistically valid for VLE prediction and the whole phase equilibrium data points are in applicability domain of the model.     

Supercritical antisolvent precipitation of andrographolide from Andrographis paniculata extracts: Effect of pressure, temperature and CO2 flow rate

Andrographis paniculata extracts were precipitated using the so-called supercritical antisolvent (SAS) technique. Ethanol was used as the solvent and compressed CO₂ as the antisolvent. The effects of process operating conditions (pressure: 5-24 MPa, temperature: 308-328 K and CO₂ flow rate: 0.5-1.5 g/min) on particle size and morphology of precipitated andrographolide were evaluated. X-ray diffraction (XRD) patterns showed significant changes in andrographolide morphology depending on process operating conditions; both column-like and slice-like crystals were observed depending on operating conditions. Crystals with mean diameters of 3.30-228.35 μm were produced, smaller crystals were obtained at high pressure, low temperature and high CO₂ flow rate and vice versa for large crystals. In addition, SAS process also produced high precipitation yields, since solubility of andrographolide is small in the supercritical CO₂ plus ethanol. When operating under subcritical conditions, amorphous particles were produced.     

Miscibility in blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(?-caprolactone) induced by melt blending in the presence of supercritical CO2

Blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(?-caprolactone) (PCL) have been produced by melt blending in the presence of supercritical CO₂. Infrared spectroscopy has shown that supercritical CO₂ can induce melting in PHBV at temperatures below the melting point. The miscibility of the PCL-PHBV blend system produced by both mechanical and supercritical means has been characterised by a combination of differential scanning calorimetry and dynamic mechanical thermal analysis. It has been shown that PHBV-PCL blends produced using mechanical means were immiscible, whereas the same blends produced using supercritical methods were found to be miscible as evidenced by a decrease in the glass transition temperature of the PHBV component. The development of miscibility is discussed in terms of enhanced interdiffusion resulting from the action of supercritical CO₂. In addition, the infrared spectrum of the blends produced using supercritical CO₂ showed negligible levels of the degradation product crotonic acid. Whereas in the samples produced using mechanical blending without supercritical CO₂, there was a significant increase in the level of crotonic acid, which was interpreted as evidence of degradation.     

Numerical investigation of cooling heat transfer to supercritical CO2 in a horizontal circular tube

Cooling heat transfer to supercritical CO₂ in a horizontal circular tube has been numerically investigated using CFD code FLUENT in the present study. The purpose is to provide detailed information on heat transfer behavior which is hard to be observed in experimental studies and to help to better understand the heat transfer mechanism. Simulation starts with five key issues, including physical model, mathematical models, mesh independency, boundary conditions and solution methods. The results demonstrate that almost all models are able to reproduce the trend of heat transfer characteristics qualitatively, and LB low Re turbulence model shows the best agreement with the experimental data, followed by standard k-? model with enhanced wall treatment. After the validation, further studies are discussed on velocity and turbulence fields, buoyancy effect, and heat transfer mechanism. It concludes that buoyancy significantly affects the turbulent flow, and evidently enhances the cooling heat transfer of supercritical CO₂, especially in the vicinity of pseudo-critical point. The mixed convection is the main heat transfer mechanism during supercritical CO₂ cooling process.     

Supercritical carbon dioxide essential oil extraction of Lamiaceae family species: Mathematical modelling on the micro-scale and process optimization

In this paper the essential oil supercritical carbon dioxide extraction from leaves of Lamiaceae family species was studied. Recent investigations of Lamiaceae family essential oil storage have shown that most of the oil is found in peltate glandular trichomes on the leaf surface. The effect of supercritical CO₂ on the peltate glands was investigated by Scanning Electron Microscopy. It was observed that exposure to supercritical CO₂ led to disruption of the peltate glands and essential oil release. This phenomenon was used as a basic hypothesis of the mathematical model of the supercritical fluid extraction with CO₂. The model was applied to simulate basil, rosemary, marjoram and pennyroyal supercritical CO₂ extraction on the existing experimental data. An average deviation from the experimental data was less than 0.83%. The model results indicated a possibility of a decrease in the supercritical CO₂ consumption by modified and optimized processing of Lamiaceae family herbaceous material.     

Diffusion coefficients of diesel fuel and surrogate compounds in supercritical carbon dioxide

Facilitating a new concept of clean diesel combustion using supercritical fluids requires a better understanding of thermophysical properties of the diesel fuel/diluent system. Mass diffusivity is one such property that is important to understand diesel fuel/diluent mixing and spray and combustion of supercritical fuel mixtures. In this work, diffusion coefficients of diesel fuel and surrogate compounds in supercritical carbon dioxide were experimentally determined by the Taylor dispersion method at temperatures from 313.15 to 373.15 K and pressures up to 30 MPa. Difficulties were encountered to measure diffusion coefficients using the Taylor dispersion method near the critical region of CO₂ which resulted in curve-fitting errors greater than 5%. Predictive correlations including Wilke-Chang, Scheibel, and He-Yu were examined. Diffusivity data were also fitted by D₁₂/T − η and correlations. Results showed that the He-Yu correlation has the best prediction performance while the D₁₂/T − η correlation best fits the data with AAD% < 8%.     

Micronization and characterization of drug substances by RESS with supercritical CO2

A RESS (rapid expansion of supercritical solution) process for the preparation of ultra-fine drug particles with no organic solvent has been developed with supercritical CO₂. Three drug substances with different solubility in supercritical CO₂ were used, and orifice disks and capillary tubes were adapted as an expansion device. The solubilities of drug substances in supercritical CO₂ and the effects of various operating parameters on the characteristics of the particles prepared by RESS process were experimentally investigated. The solubility of the drug substance in supercritical CO₂ had a major effect on the average diameter of the particle prepared by RESS process, and the particle diameter decreased with the solubility for all the drugs and operating conditions. The particle diameter also decreased with preexpansion temperature and increased with the hole diameter of the orifice nozzle and aspect ratio (L/D) of the capillary tube.     

Thermal behaviors of polystyrene plasticized with compressed carbon dioxide in a sealed system

The thermal behavior of polystyrene (PS) plasticized with compressed carbon dioxide (CO₂) was studied using differential scanning calorimetry with a high‐pressure stainless steel pan in a sealed system. The technique proved to be a simple and convenient way to study the thermal behavior of a polymer plasticized with compressed CO₂ at pressures up to 100 atm, which covers both the gas and supercritical states. A sharp fall in the decrease rate of the glass transition temperature (T_g) under conditions near the critical point of compressed CO₂ was firstly observed, which corresponded with the solubility of CO₂ in PS. Since the system is scaled, which results in a stable pressure at a certain temperature, it is more suitable to study the effect of annealing. An endotherm was detected after the PS was annealed at a temperature below its T_g under compressed CO₂. The enthalpy of this endotherm increased linearly with increasing logarithm of annealing time under a certain pressure. The endotherm was affected by two thermodynamic equilibrations at a temperature below its T_g: (i) enthalpy relaxation of the PS; and (ii) the absorption/desorption of CO₂. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

A numerical study of supercritical forced convective heat transfer of n-heptane inside a horizontal miniature tube

Supercritical convective heat transfer of hydrocarbon propellants plays a key role in the regenerative cooling technology development in aerospace applications. In this paper, a numerical study of the supercritical forced convective heat transfer of a typical hydrocarbon fuel, n-heptane, has been conducted based on a complete set of conservation equations of mass, momentum, and energy with accurate evaluations of the thermophysical properties. The present fundamental numerical study focuses on the effects of many key parameters, including the inlet pressure, inlet velocity, wall heat flux, and the inlet fluid temperature, on the supercritical heat transfer processes. Results indicate that under supercritical heat transfer processes, heat transfer deterioration could occur once the wall temperature or the fluid temperature in a large near-wall region reaches the pseudo-critical temperature, and increasing the fluid pressure would enhance heat transfer. The conventional empirical Gnielinski expression could only be used for supercritical heat transfer predictions of n-heptane under very limited operational conditions. It is found in the present numerical study that a supercritical heat transfer expression for CO₂, H₂O, and HCFC-22 applications can generally be employed for predicting the supercritical heat transfer coefficient of n-heptane when the inlet velocity is higher than 10 m/s.     

Activity and stability of lipases from different sources in supercritical carbon dioxide and near‐critical propane

The stability and activity of lipases from Pseudomonas fluorescens, Rhizopus javanicus, Rhizopus niveus, porcine pancreas and Candida rugosa in a non‐solvent system at atmospheric pressure, in supercritical carbon dioxide (SC CO₂), and near‐critical propane at 100 bar and 40 °C were studied. Esterification of n‐butyric acid with ethanol and isoamyl alcohol was used as a model system. In supercritical carbon dioxide there was a great loss in activity of the examined lipases. Decreased relative activity of lipases in SC CO₂ was attributed to the interactions between CO₂ and the enzyme. The second reason for this effect was the differences in water partitioning between the enzyme and its surroundings. In contrast, the use of near‐critical propane improved the activity of lipases in the comparison to the non‐solvent system by four‐ (porcine pancreas lipase) to nine‐times (Rhizopus javanicus lipase). The use of near‐critical propane also improved the thermal stability of porcine pancreas lipase compared with the non‐solvent system. The calculated deactivation constant for esterification between butyric acid and isoamyl alcohol, catalyzed by porcine pancreas lipase, showed that there was more than twice as much inactive as active enzyme in the non‐solvent system studied whereas the ratio in propane was 1. © 2001 Society of Chemical Industry     

Lipase-catalyzed esterification of stearic acid with ethanol, and hydrolysis of ethyl stearate, near the critical point in supercritical carbon dioxide

The effect of pressure on the lipase-catalyzed reaction in supercritical carbon dioxide (SCCO₂) was investigated for the esterification of steric acid (SA) with ethanol and the hydrolysis of ethyl stearate (ES) near the critical point, ranging from 6 to 20 MPa in pressure and 35 to 60°C in temperature. The esterification rate of SA began to increase near the critical point and kept increasing steadily with an increase in pressure, reflecting the increase in SA solubility in SCCO₂. The hydrolysis rate of ES showed a maximum at a pressure near the critical point. When the reaction was carried out with an initial overall ES concentration below its solubility limit in SCCO₂, the maximum pressure shifted along the extended line of the gas-liquid equilibrium in the supercritical region in the pressure-temperature phase plane. This seems to be related to the singular behavior of some properties in SCCO₂ along this line reported in the literature. These results show the importance of pressure, as well as temperature, as a parameter to control enzyme reactions in SCCO₂.     

Gasification of aqueous biomass in supercritical water: A thermodynamic equilibrium analysis

The aim of this work is to give insights into the gasification of aqueous biomass in supercritical water. More precisely, a mathematical model based on the thermodynamical equilibrium assumption is derived in this study. This model allows the computation of the solid, liquid and gas phases produced in a process composed of a gasification reactor and a separator. The composition of these three phases is computed in terms of fractions of CH₄, H₂, H₂O, CO, CO₂, H₂S, NH₃, C₆H₅OH, CH₃COOH, CH₃CHO, C_(s) and minerals. In the reactor, this composition is computed according to the derivation of balance equations on atoms and to the derivation of equations translating chemical equilibrium between species. Because of the specific conditions prevailing in this device (above the critical point of pure water), the computation of the activity of chemical species in the reacting media is performed using Peng-Robinson equation of state. The modelling of the separator is performed using mass balances as well as equations translating physical equilibrium of species between liquid and gaseous phases. The numerical predictions of this mathematical model are compared to experimental results obtained in the case of gasification of methanol and glucose in supercritical water. The gasification of sewage sludge is also investigated in this study.     

Guava (Psidium guajava L.) seed oil obtained with a homemade supercritical fluid extraction system using supercritical CO2 and co-solvent

In this work we designed and built a homemade supercritical fluid extraction (HM-SFE) system, in which pure CO₂ and CO₂ with co-solvents were used. The HM-SFE was made by means of thermal dilatation-contraction (TDC). This HM-SFE system was used for obtaining guava (Psidium guajava L.) seed oil, using supercritical CO₂ adding ethanol as co-solvent (CO₂ SC/EtOH), extractions were performed at 313 K and different pressures (10, 20 and 30 MPa), each one in four stages of 30 min, the extract with higher yield was subjected to transesterification and high-resolution gas chromatography (HRGC) analysis. The highest extraction yield was obtained at 30 MPa (17.30% w/w), this yield was higher than one observed in a previous work using SC-CO₂, and near to the one obtained by Soxhlet extraction (20.2% w/w). HRGC enabled the identification of components of the derivatized extract as methyl esters of palmitic, oleic, linoleic, and stearic fatty acids. The results obtained with HM-SFE system was compared with a commercial SFE system, obtained very similar results. In this work was possible to construct a low cost and simple manner HM-SFE system which was employed for obtaining guava seed oil, using CO₂ SC/EtOH.     

Extrema of isochoric heat capacity of water and carbon dioxide

The experimental and predicted loci extrema behavior of the isochoric heat capacity C_v was examined for water and carbon dioxide along the subcritical and supercritical isotherms and along the liquid and vapor isochores. The studies were based on a nonanalytical Helmholtz energy-volume-temperature equation (AVT, fundamental equation of state), the IAPWS-95 formulation for water, and scaling-type crossover equations of state (CREOS). The measured isochoric heat capacity data for these fluids near the critical point were analyzed to study the behavior of loci of C_v maxima and to compare these with predictions by the equations of state. A CREOS was applied to study the behavior of the isochoric heat capacity maxima in the immediate vicinity of the critical point. Good agreement with the CREOS prediction and experimental isothermal C_v maxima loci was observed near the critical point. The basic characteristic points on the C_v extrema loci curves in the P-T and 𝜌-T planes were determined on the basis of detailed analysis of the experimental and prediction of C_v extrema loci behavior. Qualitative explanations are given for the nature of isochoric and isothermal C_v maxima-minima curves. The role of C_v extrema loci behavior in developing high-accuracy equations of state in the supercritical region and in the study of supercritical phase-transition phenomena are discussed.     

Phase equilibria in the H2/CO2 system at temperatures from 220 to 290 K and pressures to 172 MPa

Phase equilibria in the system H₂/CO₂ have been studied experimentally using a vapor-recirculating apparatus. Measurements of vapor and liquid phase compositions were made for ten temperatures ranging from 220 to 290 K and pressures to 172 MPa. The mixture critical line and the pressure-temperature trace of the three-phase region solid-liquid-vapor have been located. The three-phase region and the critical line intersect at T ? 234.8 K and P ? 198 MPa to form an upper critical end point. Experimental results have been compared with previous studies and with predictions of three equations of state, Peng-Robinson, Redlich-Kwong and Deiters. Expressions have been derived for temperature dependent critical parameters that account for quantum effects in H₂, in calculations for H₂/X systems using the Peng-Robinson equation.     

High-pressure phase equilibrium for the hydroformylation of 1-octene to nonanal in compressed CO2

The hydroformylation reaction in supercritical carbon dioxide or CO₂-expanded liquids (CXLs) has many advantageous properties. However, accurate phase behavior and equilibrium must be known to properly understand and engineer these systems. In this investigation, the vapor-liquid equilibrium and mixture critical points of CO₂ systems with 1-octene, nonanal, 1-octene and nonanal mixtures, and mixtures of 1-octene, nonanal and syngas (CO/H₂) were measured at 60 °C up to 120 bar of pressure. The Peng-Robinson equation of state with van der Waals two-parameter mixing rule was employed successfully to correlate the binary mixture data and predict the ternary mixture data. The presence of CO/H₂ pressure increased the mixture critical points and decreased the volume expansion at any given pressure. In an actual reaction, the mixture critical point would increase throughout the reaction, while the volume of the liquid phase would decrease. These data will aid the understanding and reaction engineering for the hydroformylation reaction in CO₂-expanded liquids and supercritical fluids.