Effects Of Supercritical Conditions Research Articles

Hydropyrolysis of freshwater macroalgal bloom for bio-oil and biochar production: Kinetics and isotherm for removal of multiple heavy metals

In this study, hydropyrolysis was carried out using sodium carbonate to convert the green algal bloom into bio-oil, biochar, aqueous solution, and gases. The effect of supercritical conditions (400, 450, 500 °C) on the product yield, bio-oil composition, and structure, and functionalities of the biochar was determined. The high yield of biochar and bio-oil was reported at 400 °C. A significant reduction in bio-oil and increment in hydrocarbon content was reported on the elevation of temperature from 450 °C to 500 °C. After that, kinetic and isotherm analysis was investigated simultaneously to remove four heavy metals viz. Cu(II), Ni(II), Co(II), and Cd(II) from the mixture solution. Results show that kinetics data follow a pseudo-second-order kinetics model and adsorption isotherm is in better agreement with the Langmuir model, not with the Freundlich model. The maximum adsorption capacity was found 10.90, 5.74, 5.80, and 16.28 mg/g with the biochar prepared at 500 °C for Cu(II), Ni(II), Co(II), and Cd(II) metals, respectively. The current investigation provided a promising way for the utilization of freshwater algal bloom biomass for renewable products and simultaneously heavy metal removal from the water.

Kinetic modeling and experimental validation on the effect of KCl and SO2 concentration on corrosion of pure Fe under simulated municipal solid waste combustion

Thermal treatment of municipal solid waste (MSW) in waste-to-energy plants (WtE) has gained worldwide acceptance as an environmentally preferred method for sustainable disposal of waste and energy recovery. However, superheater corrosion induced by large amounts of alkali chloride (KCl) and SO2 released from MSW during combustion remains a major challenge. In this study, a novel kinetic model was developed to study the corrosion behavior of pure Fe-metal and the reaction pathways using chemkin-PRO and Gaussian 09. The model relies on detailed high-temperature interactions between gaseous potassium, O/H radicals, chlorine/sulfur species, and the Fe-metal. The rate coefficients for the surface reactions were estimated based on density functional theory computations using several Fe clusters that represented the Fe metal surface. The model was validated by the experimental results obtained from well-controlled simulated corrosion tests conducted in an electrically heated furnace at 500°C with each test lasted for 50hours. The exposure conditions consisted of 0-600mg KCl which was preheated and its gas was injected into the furnace, 0-1700ppm SO2, 20vol.% H2O, 8vol.% O2, and N2 as the carrier gas. Comparison of the kinetic model and the experimental data showed there was a good agreement. The results revealed that increasing KCl concentration accelerated the corrosion rate of Fe with a parabolic rate law. The concentration of SO2 > 500ppm enhanced the rate of corrosion of Fe, concentrations between 250-500ppm reduced the corrosion rate, and the concentration < 250ppm had less effects. The reaction series that replenished the Cl radicals and KSO3Cl promoted the high corrosion rates while reaction path that forms sulfate suppresses it. The effect of supercritical conditions on SO2 and KCl induced corrosion rate were presented and discussed.

Effect of supercritical conditions upon catalyst deactivation in the hydrogenation of naphthalene

The hydrogenation of naphthalene in cyclohexane solvent was studied over a NiMo/Al2O3 catalyst operated in a fixed bed reactor. The effect of operating under sub- and supercritical conditions was investigated at temperatures 250–300°C and pressures 20–90bar. Naphthalene/cyclohexene flows of 0.5–2.5mLmin−1 and hydrogen flows of 2–10mLmin−1 were investigated. When operating in the supercritical region with high hydrogen/liquid feed ratio, no significant deactivation occurred over 6h of operation. A strong effect of pressure upon conversion was observed, with naphthalene conversion close to 100% at 60bar, 300°C. Trans-decalin was the favoured product over most of the range of conditions investigated. When operating at low hydrogen/liquid feed ratio, initial conversion of naphthalene was 90–95%, declining to 58% at supercritical conditions and 35% at subcritical conditions after 7h. Coke deposition was 38% lower at supercritical conditions. The ability of supercritical fluids to delay catalyst deactivation is considered, together with interesting changes to product distribution that occur under these conditions.

Photocatalytic activity of porous titania nanocrystals prepared by nanoscale permeation process in supercritical CO2: Effects of supercritical conditions

Porous TiO 2 nanocrystals (PTN) were synthesized using activated carbon templates with supercritical CO 2 by using the nanoscale permeation (NP) process. The photoactivity of PTN was tested by methylene blue (MB) degradation. Compared with the commercially available P-25, all PTN exhibited significant photocatalytic degradation of MB mainly due to their porous structure with high surface area, high hydroxy concentration and small crystalline size. The optimum temperature and pressure are found to be 60 °C and 26 MPa, under which obtained PTN-1 shows the highest photoactivity and slow deactivation for MB degradation after 15 trials.

Effect of supercritical conditions on the transalkylation of diethylbenzene with benzene

The transalkylation of diethylbenzene with benzene to produce ethylbenzene (EB) was studied in order to establish the effect of the reaction conditions, pressure, temperature and contact time, as well as the influence of media reaction phase, supercritical or subcritical on catalysts performance. Both CO 2 and free-solvent systems, using an acidic commercial Beta zeolite as catalyst, have been explored. The results showed that both temperature and contact time (W/F) had a positive effect on the ethylbenzene yield, while pressure had a negative one that was more significant at low temperatures. Supercritical conditions only improved the catalyst performance in the free-solvent system. Catalysts tested in SC phase with CO 2 showed deactivation while the catalyst tested in SC phase without CO 2 maintained the activity. These results have been explained in terms of the different densities and diethylbenzene diffusion coefficients of the reaction mixture that determine the solubility of the coke precursors compounds, and thus catalyst deactivation.

Determination of solid products from the de-polymerization of poly(trimethylene terephthalate) in supercritical methanol

A method based on high-resolution size-exclusion chromatography (SEC) was established to analyze the solid products from the de-polymerization of poly(trimethylene terephthalate) (PTT) in supercritical methanol. In the qualitative analysis, four factors (chromatographic retention time, qualitative multi-wavelength ultraviolet spectra, linear internal-insert SEC and qualitative IR spectra) were considered. The main solid products from the process were dimethyl terephthalate (DMT), methyl-(2-hydroxypropyl) terephthalate (MHPT), bis(2-hydroxypropyl) terephthalate (BHPT), methyl-(2-hydroxyethyl) terephthalate (MHET), bis(2-hydroxyethyl) terephthalate (BHET), and hydroxyethyl-(2-hydroxypropyl) terephthalate (HEHPT). It is found that the method is of a high resolution among the solid products and has a fine repeatability. In addition, the solid products from the de-polymerization of poly(ethylene terephthalate) (PET) in similar process were also analyzed by this method. Furthermore, the effects of supercritical conditions on the distribution of the products were also discussed.

Effect of supercritical conditions on the oxidation of isobutane

The paper examines the kinetics of isobutane oxidation in the liquid phase and under supercritical conditions. Isobutane oxidation has gained importance because of the applications of the oxidation products such as tert-butyl hydroperoxide and tert-butyl alcohol in the manufacture of the important chemicals like propylene oxide and MTBE. An interesting aspect of this reaction is that, the reaction conditions can be conveniently manipulated in such a way that isobutane exists in supercritical state. The influence of supercritical conditions on the rate and selectivity of the reaction is investigated and this permits a comparison of liquid phase and supercritical phase oxidation. The reactions were also performed in a glass-lined continuous reactor. This mode of operation allows kinetic studies without an intervention of gas–liquid mass transfer and in addition eliminates the catalytic effect, if any, of the stainless-steel walls. The reaction is autocatalytic and the selectivity towards the hydroperoxide decreases with an increase in overall conversion. Under supercritical conditions, the rates and selectivity were significantly high compared to those obtained in liquid phase oxidation. In both, subcritical and supercritical oxidations, temperature has an adverse effect on selectivity towards tert-butyl hydroperoxide. A proposed kinetic model successfully explains the kinetic data and allows the rate constants and the activation energy to be determined. It is shown that the supercritical-phase oxidation shows all features present in the liquid-phase oxidation. Thus, this work provides a unified treatment for the supercritical- and the liquid-phase oxidation of isobutane.

Lipase-catalyzed kinetic resolution of 3-hydroxy esters in organic solvents and supercritical carbon dioxide

The lipase-catalyzed kinetic resolution of different 3-hydroxy esters in organic solvents and supercritical carbon dioxide (SCCO 2) was studied. Similar enantiomeric excesses (ee, up to 99%) as found for transesterifications in organic solvents were achieved also in SCCO 2. The influence of various reaction parameters was investigated for the resolution of 3-hydroxy octanoic acid methyl esters with lipase from Pseudomonas cepacia in SCCO 2. Optimum conditions have been found at 110 bar, 40°C, and in the presence of 1.5 g molecular sieve by using vinyl acetate as acyl donor. The addition of different cosolvents had only a small effect on the reaction. Immobilization of the lipase on VA-epoxy resulted in similar optical purities as found for the crude lipase, but at halved reaction times. The effect of supercritical conditions on the stability of the lipase was also investigated.