Laser analyses of mixture formation and the influence of solute on particle precipitation in the SAS process

Laser based Raman and elastic light scattering measurements were performed to study the process of mixture formation and the influence of the solute paracetamol onto the phase behaviour of the pseudo-binary system ethanol/CO₂ in the supercritical antisolvent process. From the Raman based technique, mole fraction and partial density distributions of CO₂ were obtained. The mole fraction distributions indicate a rapid mixture formation with fast supersaturation of the solute. At the same time, the increase of the CO₂ partial density at conditions considerably above the mixture critical point (MCP) indicate a change from a homogeneous supercritical to a multi-phase subcritical flow. This phase change goes along with particle precipitation. Thus, the results of our investigations proof, why past approaches failed to generate amorphous paracetamol nanoparticles with the system paracetamol/ethanol/CO₂ above the MCP. Process parameters like injection pressure (20.0–35.0 MPa), chamber pressure of CO₂ (7.5–17.5 MPa), temperature (313–333 K) and solute concentration (0–5 wt%) were varied.     

The enantioselective hydrolysis of racemic naproxen methyl ester in supercritical CO2 using Candida rugosa lipase

The enantioselective hydrolysis of racemic naproxen methyl ester by Candida rugosa lipase (CRL) was studied in aqueous buffer solution/isooctane reaction system in the presence of supercritical CO₂. The effects pressure (75–160 bar), temperature (32–42 °C) and reaction time (0.5–12 h) on the enantiomeric excesses of the product (ee_p) and the substrate (ee_s), enantiomeric ratio (E), conversion (x) and enzyme activity were investigated in a batch reactor system. The highest enantiomeric ratio achieved at 120 bar of pressure, 37 °C of temperature and 2 h of reaction time was E = 193 with x = 41.3%, ee_p = 97.9% and ee_s = 68.8%. CRL remained active at least for 12 h at 37 °C and 120 bar in supercritical CO₂ medium. Furthermore, enantiomeric ratio increased with increasing reaction time and reached the value of E = 236 with ee_p = 98.2%, ee_s = 70.0% and x = 41.6% after 12 h of hydrolysis.     

Supercritical CO2 extraction of glycosides from Stevia rebaudiana leaves: Identification and optimization

The aim of this work was to optimize the glycoside composition of Stevia rebaudiana leaves using supercritical fluid extraction (SFE). A Box-Behnken statistical design was used to evaluate the effect of various values of pressure (150–350 bar), temperature (40–80 °C) and concentration of ethanol-water mixture (70:30) as co-solvent (0–20%) by CO₂ flow rate of 15 g min⁻¹ for 60 min. The most effective variables were co-solvent concentration (P < 0.005) and temperature (P ≤ 0.005). Evaluative criteria for both dependent variables (stevioside and rebaudioside A yields) in the model was assigned maximum. Optimum extraction conditions were elicited as 211 bar, 80 °C and 17.4% which yielded 36.66 mg/g stevioside and 17.79 mg/g rebaudioside A. Total glycosides composition were close to those obtained using conventional water extraction (64.49 mg/g) and a little higher than ethanol extraction (48.60 mg/g) demonstrating challenges for industrial scale application of SFE.     

The phenol steam reforming reaction towards H2 production on natural calcite

The steam reforming of phenol towards H₂ production was studied in the 650–800 °C range over a natural pre-calcined (air, 850 °C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h⁻¹) were investigated. The increase of reaction temperature in the 650–800 °C range and water feed concentration in the 40–50 vol% range were found to be beneficial for catalyst activity and H₂-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000 h⁻¹. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO₂ adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO₂ adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO₂/H₂O/He gas mixtures at 600 °C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and OH species was observed when the CaO surface was exposed to CO₂/H₂O/H₂/He gas mixtures at 600 °C, result that probes for the inhibiting effect of H₂ on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated “carbonaceous” species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of “carbonaceous” species with reaction time (650–800 °C range) was evidenced, in particular at 800 °C (4.7 vs. 6.7 mg C/g solid after 5 and 20 min on stream, respectively).     

High pressure CO2 inactivation of food: A multi-batch reactor system for inactivation kinetic determination

A multi-batch apparatus was developed for investigating food inactivation by high-pressure CO₂, both for performing preliminary studies on CO₂–substrate interactions and for measuring the inactivation kinetics of the microorganisms suspended in food matrices. Experiments were carried out for inoculated liquid growth medium and fruit juices, using the yeasts Saccharomyces cerevisiae ATCC 9763 and Pichia awry wild type as test microorganisms. Different combinations of temperature (38 and 32 °C), pressure (90 and 75 bar) and treatment time were investigated. The logarithmic inactivation kinetic curves showed a quite linear behavior with a slope change in some cases. It was also shown that the pasteurization degree of the considered foodstuffs depends on the physical–chemical properties of the treated substrate.The proposed multi-reactors system allows to save both working time and materials, giving a better collection of experimental data in terms of reliability and homogeneity.     

Particle formation of an edible fat (rapeseed 70) using the supercritical melt micronization (ScMM) process

The supercritical melt micronization (ScMM) process, also known as particles from gas saturated solutions (PGSS) was applied, in a continuous operated pilot plant, for the particle formation of the edible fat, rapeseed 70 (RP70). The effect of variables like the CO₂ concentration, the melt temperature and the atomization pressure were studied in order to investigate particle morphology, density and the particle size distribution. The experiments were performed at CO₂ concentrations between 0 and 50 wt%, atomization pressure between 70 and 180 bar and melt temperature between 60 and 100 °C. Particles obtained as a function of the CO₂ concentration, showed completely solid, spherical–hollow and aggregated particles with a decrease in particle mean size as the concentration of CO₂ was increased. The results obtained as a function of atomization pressure showed no significant influence on particle morphology and size distribution. Experiments carried out as a function of the melt temperature showed distorted, spherical–hollow and aggregated particles. Furthermore, a theory was developed to explain the mechanism for particle formation as a function of the CO₂ concentration and the melt temperature. The crystallinity of the final product of RP70, showed an alpha polymorph with a crystallinity of 84%.     

Development of cobalt–copper nanoparticles as catalysts for higher alcohol synthesis from syngas

The synthesis of higher alcohols from syngas has been studied over different types of Cu-based catalysts. In order to provide control over the catalyst composition at the scale of a few nanometers, we have synthesized two sets of Co–Cu nanoparticles with novel structures by wet chemical methods, namely, (a) cobalt core–copper shell (Co@Cu) and (b) cobalt–copper mixed (synthesized by simultaneous reduction of metal precursors) nanoparticles. These catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The catalysts were tested for CO hydrogenation at temperatures ranging from 230 °C to 300 °C, 20 bar and 18,000 scc/(hr.gcat). It was observed that the Co–Cu mixed nanoparticles with higher Cu concentration exhibit a greater selectivity towards ethanol and C₂₊ oxygenates. The highest ethanol selectivity achieved was 11.4% with corresponding methane selectivity of 17.2% at 270 °C and 20 bar.     

Comparison of different extraction methods for the extraction of major bioactive flavonoid compounds from spearmint (Mentha spicata L.) leaves

Different bioactive flavonoid compounds including catechin, epicatechin, rutin, myricetin, luteolin, apigenin and naringenin were obtained from spearmint (Mentha spicata L.) leaves by using conventional soxhlet extraction (CSE) and supercritical carbon dioxide (SC-CO₂) extraction at different extraction schemes and parameters. The effect of different parameters such as temperature (40, 50 and 60 °C), pressure (100, 200 and 300 bar) and dynamic extraction time (30, 60 and 90 min) on the supercritical carbon dioxide (SC-CO₂) extraction of spearmint flavonoids was investigated using full factorial arrangement in a completely randomized design (CRD). The extracts of spearmint leaves obtained by CSE and optimal SC-CO₂ extraction conditions were further analyzed by high performance liquid chromatography (HPLC) to identify and quantify major bioactive flavonoid compounds profile. Comparable results were obtained by optimum SC-CO₂ extraction condition (60 °C, 200 bar, 60 min) and 70% ethanol soxhlet extraction. As revealed by the results, soxhlet extraction had a higher crude extract yield (257.67 mg/g) comparing to the SC-CO₂ extraction (60.57 mg/g). Supercritical carbon dioxide extract (optimum condition) was found to have more main flavonoid compounds (seven bioactive flavonoids) with high concentration comparing to the 70% ethanol soxhlet extraction (five bioactive flavonoids). Therefore, SC-CO₂ extraction is considered as an alternative process compared to the CSE for obtaining the bioactive flavonoid compounds with high concentration from spearmint leaves.     

Feasibility of Na-based thermochemical cycles for the capture of CO2 from air—Thermodynamic and thermogravimetric analyses

Three Na-based thermochemical cycles for capturing CO₂ from air are considered: (1) a NaOH/NaHCO₃/Na₂CO₃/Na₂O cycle with 4 reaction steps, (2) a NaOH/NaHCO₃/Na₂CO₃ cycle with 3 reactions steps, and (3) a Na₂CO₃/NaHCO₃ cycle with 2 reaction steps. Depending on the choice of CO₂ sorbent – NaOH or Na₂CO₃ – the cycles are closed by either NaHCO₃ or Na₂CO₃ decomposition, followed by hydrolysis of Na₂CO₃ or Na₂O, respectively. The temperature requirements, energy inputs, and expected products of the reaction steps were determined by thermodynamic equilibrium and energy balance computations. The total thermal energy requirement for Cycles 1, 2, and 3 are 481, 213, and 390 kJ/mol of CO₂ captured, respectively, when heat exchangers are employed to recover the sensible heat of hot streams. Isothermal and dynamic thermogravimetric runs were carried out on the pertinent carbonation, decomposition, and hydrolysis reactions. The extent of the NaOH carbonation with 500 ppm CO₂ in air at 25 °C – applied in Cycles 1 and 2 – reached 9% after 4 h, while that for the Na₂CO₃ carbonation with water-saturated air – applied in Cycle 3 – was 3.5% after 2 h. Thermal decomposition of NaHCO₃ – applied in all three cycles – reached completion after 3 min in the 90–200 °C range, while that of Na₂CO₃ – applied in Cycle 1 – reached completion after 15 min in the 1000–1400 °C range. The significantly slow reaction rates for the carbonation steps and, consequently, the relatively large mass flow rates required, introduce process complications in the scale-up of the reactor technology and impede the application of Na-based sorbents for capturing CO₂ from air.     

Development of dissolved carbon dioxide-driven-and-controlled repeated batch fermentation process for ethanol production

We previously observed that as glucose is completely exhausted during ethanol fermentation, the dissolved carbon dioxide (DCO₂) level in the fermenter will suddenly decline. This observation was implemented to design and develop a DCO₂-driven-and-controlled repeated batch fermentation process for ethanol production. The process was tested at four different glucose concentrations (~150 g/L, ~200 g/L, ~250 g/L, and ~300 g/L), and each glucose concentration was controlled under three respective DCO₂ control levels (without DCO₂ control, and DCO₂ controlled at either 1000 mg/L or 750 mg/L). The results show that reported process features complete glucose utilization and is self-driven. For glucose concentration less than 200 g/L, ~41%-50% of fermentation time per batch was saved during the repeated batch operation. It took 12.1 ± 1.1 hours-14.9 ± 1.9 hours to complete a batch with glucose feed at ~250 g/L and 21.7 ± 6 hours-31.5 ± 7 hours to complete a batch with glucose feed at ~300 g/L. The reported process is time saving and stable, but the ethanol yield is ~20% lower than the operation without DCO₂ control. Dissolved CO₂ control became essential for glucose concentrations greater than 250 g/L if zero glucose discharge in each batch during the operation is desired.     

A novel process for precipitation of ultra-fine particles using sub-critical CO2

Supercritical CO₂ has been utilized as solvent, cosolvent or antisolvent in several processes for production of ultra-fine solid particles with narrow size distribution. The key to the precipitation of such particles is to produce a very large, rapid and uniform supersaturation in the solution of a solid substance. This can be achieved either by a rapid and large reduction in the temperature of solution or by drastically increasing the CO₂ solubility for imparting the antisolvent effect. Most of these CO₂ processes require high-pressure pumps, specially designed nozzles and accurate control of process parameters. In order to obviate these requirements, a simple technique of precipitation by pressure reduction over the gas-expanded liquids (PPRGEL), such as CO₂-expanded organic solutions has been utilized to impart a large, uniform and rapid reduction of temperature in the solution for instantaneous precipitation of ultra-fine particles. This process utilizes sub-critical CO₂ at relatively low pressures of 40-70 bar and near ambient temperature of 303 K for creating a temperature drop of 30-70 K in the solution within seconds, without using any specially designed nozzle or high-pressure pumps. The present paper validates the process principle for precipitation of Zinc acetate (ZnAc) nanoparticles from its organic solution in a mixed solvent of acetone and dimethyl sulfoxide (DMSO). Nanoparticles are produced with the average size of 20-250 nm (from 100 ml of solution in a high-pressure vessel of 1.09 L working volume), and vary in shapes such as long needles, rods and near spherical depending on pressure (40-70 bar at 303 K), solid concentration (0.01-0.05 g/ml) and addition of stabilizer.     

Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–Ru/γ-Al2O3 catalyst in slurry phase reactor

The rate of Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al₂O₃ catalyst was studied in a laboratory well mixed, continuous flow, slurry reactor under the conditions relevant to industrial operations as follows: temperature of 200–240 °C, pressure of 20–35 bar, H₂/CO feed ratio of 1.0–2.5, gas hourly space velocity of 500–1500 N cm³ g_cat^− 1 h^− 1 and conversions of 10–84% of carbon monoxide and 13–89% of hydrogen. The ranges of partial pressures of CO and H₂ have been chosen as 5–15 and 10–25 bar respectively. Five kinetic models are considered: one empirical power law model and four variations of the Langmuir–Hinshelwood–Hougen–Watson representation. All models considered incorporate a strong inhibition due to CO adsorption. The data of this study are fitted fairly well by a simple LHHW form − R_H2 + CO = ap_H2^0.988p_CO^0.508 / (1 + bp_CO^0.508)² in comparison to fits of the same data by several other representative LHHW rate forms proposed in other works. The apparent activation energy was 94–103 kJ/mol. Kinetic parameters are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Also, the performance of the catalyst for Fischer–Tropsch synthesis and the hydrocarbon product distributions were investigated under different reaction conditions.     

Analysis of cosolvent effect on supercritical carbon dioxide extraction for α-pinene and 1,8-cineole

The solubilities of α-pinene and 1,8-cineole in supercritical carbon dioxide with and without cosolvent were measured by using a circulation type apparatus coupled with an on-line Fourier transform infrared (FT-IR) spectroscope at 323 K and 8.0 MPa. The cosolvents interested in this work were ethanol, water, acetone and hexane. The solubilities were measured under vapor–liquid two phases. The effects of cosolvents on the solubilities and selectivities of α-pinene and 1,8-cineole in supercritical CO₂ were investigated. The selectivities at feed concentration of cosolvent of 0.056 mol L⁻¹ were increased 1.23 times by ethanol and decreased 0.82 times by hexane. Peng–Robinson equation of state with a quadratic mixing rule was used for correlations of the solubilities for α-pinene and 1,8-cineole in supercritical CO₂ with and without cosolvent. The correlated results reproduce the experimental data of cosolvent effects on the solubilities and selectivities of α-pinene and 1,8-cineole in supercritical CO₂.     

Supercritical CO2 extraction of essential oil from yarrow

Essential oil was extracted from yarrow flowers (Achillea millefolium) with supercritical CO₂ at pressure of 10 MPa and temperatures of 40–60 °C, and its composition and yield were compared with those of hydrodistillate. The yield of total extract, measured in dependence on extraction time, was affected by extraction temperature but not by particle size of ground flowers. CO₂-extraction of cuticular waxes was lowest at 60 °C. Major essential oil components were camphor (26.4% in extract, 38.4% in distillate), 1,8-cineole (9.6% in extract, 16.2% in distillate), bornyl acetate (16.7% in extract, 4.3% in distillate), γ-terpinene (9.0% in extract, 9.4% in distillate), and terpinolene (7.6% in extract, 3.9% in distillate). Compared to hydrodistillation, the yield of monoterpenes was lower due to their incomplete separation from gaseous CO₂ in trap but the yield of less volatile components like monoterpene acetates and sesquiterpenes was higher. Hydrolysis of γ-terpinene and terpinolene, occuring in hydrodistillation, was suppressed in supercritical extraction, particularly at extraction temperature of 40 °C.     

Ethanol production by solid state fermentation of sweet sorghum using thermotolerant yeast strain

Solid state fermentation of chopped sweet sorghum particles to produce ethanol was studied statically using thermotolerant yeast. The influence of various process parameters, such as yeast cell concentration, particle size and moisture content, on the ethanol yield was investigated. Optimal values of these parameters were 4 × 10⁶ cells/g raw sorghum, Dp = 1.5 mm and 75%, respectively. Addition of reducing agent H₂SO₃ into the fermentation medium provided anaerobic condition, and obtained the maximum ethanol yield of 7.9 g ethanol per 100 g fresh stalks or 0.46 g ethanol/g total sugar, which was 91% of the theoretic yield.     

Separation of CO2/CH4 mixtures with the MIL-53(Al) metal–organic framework

Adsorptive separation of CH₄/CO₂ mixtures was studied using a fixed-bed packed with MIL-53(Al) MOF pellets. Such pellets of MIL-53(Al) were produced using a polyvinyl alcohol binder. As revealed by N₂ adsorption isotherms, the use of polyvinyl alcohol as binder results in a loss in overall capacity of 32%. Separations of binary mixtures in breakthrough experiments were successfully performed at pressures varying between 1 and 8 bar and different mixture compositions. The binary adsorption isotherms reveal a preferential adsorption of CO₂ compared to CH₄ over the whole pressure and concentration range. The separation selectivity was affected by total pressure; below 5 bar, a constant selectivity, with an average separation factor of about 7 was observed. Above 5 bar, the average separation factor decreases to about 4. The adsorption selectivity is affected by breathing of the framework and specific interaction of CO₂ with framework hydroxyl groups. CO₂ desorption can be realised by mild thermal treatment.     

Propolis extracts obtained by low pressure methods and supercritical fluid extraction

Propolis is a natural product used for centuries by human kind, due to several evidenced biological activities: antioxidant, antimicrobial, anti-inflammatory, antitumor and anti-HIV. Extracts from propolis, used in food, pharmaceutical and cosmetic industries, present quality and composition related to the extraction method applied. Natural compounds with biological activity can be obtained by conventional techniques, such as Soxhlet and Maceration, or by alternative methods such as supercritical fluid extraction (SFE). Thus, the aim of this work was to compare propolis extraction yields obtained by different procedures, for instance, SFE in one stage, with CO₂ and CO₂ plus co-solvent, and SFE in two stages, as well as Soxhlet and Maceration as low pressure extraction methods using ethanol, ethyl acetate, chloroform, n-hexane, water and mixtures of water/ethanol. The operational conditions for SFE in one stage with pure CO₂ were: 30, 40 and 50 °C and from 100 to 250 bar. The SFE with co-solvent was performed at 150 bar and 40 °C and ethanol concentrations of 2, 5 and 7% (w/w). The highest yield was obtained by chloroform Soxhlet extraction (73 ± 2%, w/w) whereas for SFE the maximum yield was 24.8 ± 0.9%, using 5% ethanol as co-solvent. For SFE in two stages, 100 and 150 bar were used in the first stage while 250 and 300 bar were applied in the second stage, at 40 °C. The yields were 8.4 ± 0.7 (150 bar) and 5.1 ± 0.7 (250 bar), for stages 1 and 2, respectively. The chemical composition of the propolis material was determined by HPLC analysis. The experimental data were correlated using four models based on differential mass balance equations: (1) the Sovová’s model; (2) the logistic model (3) the diffusion model and (4) the simple single plate model (SSP). The logistic model provided the best adjustment for propolis SFE curves.     

High-pressure phase equilibria for the synthesis of ionic liquids in compressed CO2 for 1-hexyl-3-methylimidazolium bromide with 1-bromohexane and 1-methylimidazole

The use of carbon dioxide in the synthesis of ionic liquids (ILs) has many advantages over conventional solvents. Here, the high-pressure phase equilibria (including CO₂ solubility, volume expansion, and mixture critical points) are measured and modeled for the system involved in the synthesis of a model imidazolium ionic liquid 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) from 1-bromohexane and 1-methylimidazole. The global phase behavior of 1-methylimidazole was investigated and found to be a Type V system (or potentially IV) from the classification of Scott and van Konynenburg with regions of vapor–liquid equilibrium, vapor–liquid–liquid equilibrium, liquid–liquid equilibrium, an upper and lower critical endpoint and mixture critical points. The solubility and volume expansion of CO₂ in 1-methylimidazole, 1-bromohexane, a 1:1 mixture of 1-methylimidazole and 1-bromohexane and [HMIm][Br] was determined at 313.15 K and 333.15 K for pressures ranging from 10 to 160 bar. The solubility of CO₂ and the volume expansion increases in the order of [HMIm][Br]  1-methylimidazole < 1:1 mixture of reactants < 1-bromohexane. The Peng–Robinson equation of state with van der Waals 2-parameter mixing rules was used with estimated critical properties to well correlate the vapor–liquid equilibrium. The results have important ramifications on the kinetics and process constraints of an actual IL synthesis with CO₂.     

Electrodeposited Cu–ZnO and Mn–Cu–ZnO nanowire/tube catalysts for higher alcohols from syngas

Cu–ZnO and Mn–Cu–ZnO catalysts have been prepared by electrodeposition and tested for the synthesis of higher alcohols via CO hydrogenation. The catalysts were prepared in the form of nanowires and nanotubes using a nanoporous polycarbonate membrane, which served as a template for the electrodeposition of the precursor metals from an aqueous electrolyte solution. Electrodeposition was carried out using variable amounts of Zn(NO₃)₂, Cu(NO₃)₂, Mn(NO₃)₂ and NH₄NO₃ at different galvanostatic conditions. A fixed bed reactor was used to study the reaction of CO and H₂ to produce alcohols at 270 °C, 10–20 bar, H₂/CO = 2/1, and 10,000–33,000 scc/h g_cat. In addition to methane and CO₂, methanol was the main alcohol product. The addition of manganese to the Cu–ZnO catalyst increased the selectivity toward higher alcohols by reducing methane formation; however, CO₂ selectivity remained high. Maximum ethanol selectivity was 5.5%, measured as carbon efficiency.