Modelling of autogenerative high-pressure anaerobic digestion in a batch reactor for the production of pressurised biogas.

BACKGROUND: Pressurised anaerobic digestion allows the production of biogas with a high content of methane and, at the same time, avoid the energy costs for the biogas upgrading and injection into the distribution grid. The technology carries potential, but the research faces practical constraints by a.o. the capital investment needed in high-pressure reactors and sensors and associated sampling limitations. In this work, the kinetic model of an autogenerative high-pressure anaerobic digestion of acetate, as the representative compound of the aceticlastic methanogenesis route, in batch configuration, is proposed to predict the dynamic performance of pressurised digesters and support future experimental work. The modelling of autogenerative high-pressure anaerobic digestion in batch configuration, which is not extensively studied and simulated in the present literature, was developed, calibrated, and validated by using experimental results available from the literature. RESULTS: Under high-pressure conditions, the assessment of the Monod maximum specific uptake rate, the half-saturation constant and the first-order decay rate was carried out, and the values of 5.9 kg COD kg COD-1 d-1, 0.05 kg COD m-3 and 0.02 d-1 were determined, respectively. By using the predicted values, excellent fittings of the final pressure, the CH4 molar fraction and the specific methanogenic yield calculation were obtained. Likewise, the variation in the gas-liquid mass transfer coefficient by several orders of magnitude showed negligible effects on the model predictive values in terms of methane molar fraction of the produced biogas, while the final pressure seemed to be slightly influenced. CONCLUSIONS: The proposed model allowed to estimate the Monod maximum specific uptake rate for acetate, the half-saturation rate for acetate and the first-order decay rate constant, which were comparable with literature values reported for well-studied methanogens under anaerobic digestion at atmospheric pressure. The methane molar fraction and the final pressure predicted by the model showed different responses towards the variation of the gas-liquid mass transfer coefficient since the former seemed not to be affected by the variation of the gas-liquid mass transfer coefficient; in contrast, the final pressure seemed to be slightly influenced. The proposed approach may also allow to potentially identify the methanogens species able to be predominant at high pressure.

Enhancing Biomass and Lutein Production From Scenedesmus almeriensis: Effect of Carbon Dioxide Concentration and Culture Medium Reuse.

The main purpose of this study is to investigate the effects of operative parameters and bioprocess strategies on the photo-autotrophic cultivation of the microalgae Scenedesmus almeriensis for lutein production. S. almeriensis was cultivated in a vertical bubble column photobioreactor (VBC-PBR) in batch mode and the bioactive compounds were extracted by accelerated solvent extraction with ethanol at 67°C and 10 MPa. The cultivation with a volume fraction of CO2 in the range 0-3.0%v/v showed that the highest biomass and lutein concentrations - 3.7 g/L and 5.71 mg/g, respectively - were measured at the highest CO2 concentration and using fresh growth medium. Recycling the cultivation medium from harvested microalgae resulted in decreased biomass and lutein content. The nutrient chemical composition analysis showed the highest consumption rates for nitrogen and phosphorus, with values higher than 80%, while sulfate and chloride were less consumed.

Selective Extraction of ω-3 Fatty Acids from Nannochloropsis sp. Using Supercritical CO2 Extraction.

In this article, microalgae Nannochloropsis sp. was used for fatty acid (FA) extraction, using a supercritical fluid-carbon dioxide (SF-CO2) extraction method. This study investigated the influence of different pre-treatment conditions by varying the grinding speed (200-600 rpm), pre-treatment time (2.5-10 min), and mixing ratio of diatomaceous earth (DE) and Nannochloropsis sp. biomass (0.5-2.0 DE/biomass) on FAs extraction. In addition, the effect of different operating conditions, such as pressure (100-550 bar), temperature (50-75 °C), and CO2 flow rate (7.24 and 14.48 g/min) on eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) recovery, was analyzed. Experimental data evidenced that, keeping constant the extraction conditions, the pre-treatment step enhanced the FAs extraction yield up to 3.4 fold, thereby the maximum extracted amount of FAs (61.19 mg/g) was attained with the pre-treatment with a ratio of DE/biomass of 1 at 600 rpm for 5 min. Moreover, by increasing both SF-CO2 pressure and temperature, the selectivity towards EPA was enhanced, while intermediate pressure and lower pressure promoted DHA recovery. The highest amount of extracted EPA, i.e., 5.69 mg/g, corresponding to 15.59%, was obtained at 75 °C and 550 bar with a CO2 flow rate of 14.48 g/min, while the maximum amount of extracted DHA, i.e., ~0.12 mg/g, equal to 79.63%, was registered at 50 °C and 400 bar with a CO2 flow rate of 14.48 g/min. Moreover, the increased CO2 flow rate from 7.24 to 14.48 g/min enhanced both EPA and DHA recovery.

Supercritical Fluid Extraction of Lutein from Scenedesmus almeriensis.

Lutein has several benefits for human health, playing an important role in the prevention of age-related macular degeneration (AMD), cataracts, amelioration of the first stages of atherosclerosis, and some types of cancer. In this work, the Scenedesmus almeriensis microalga was used as a natural source for the supercritical fluid (SF) extraction of lutein. For this purpose, the optimization of the main parameters affecting the extraction, such as biomass pre-treatment, temperature, pressure, and carbon dioxide (CO2) flow rate, was performed. In the first stage, the effect of mechanical pre-treatment (diatomaceous earth (DE) and biomass mixing in the range 0.25-1 DE/biomass; grinding speed varying between 0 and 600 rpm, and pre-treatment time changing from 2.5 to 10 min), was evaluated on lutein extraction efficiency. In the second stage, the influence of SF-CO2 extraction parameters such as pressure (25-55 MPa), temperature (50 and 65 °C), and CO2 flow rate (7.24 and 14.48 g/min) on lutein recovery and purity was investigated. The results demonstrated that by increasing temperature, pressure, and CO2 flow rate lutein recovery and purity were improved. The maximum lutein recovery (~98%) with purity of ~34% was achieved operating at 65 °C and 55 MPa with a CO2 flow rate of 14.48 g/min. Therefore, optimum conditions could be useful in food industries for lutein supplementation in food products.

Eicosapentaenoic Acid Extraction from Nannochloropsis gaditana using Carbon Dioxide at Supercritical Conditions.

This research shows that carbon dioxide supercritical fluid (CO2-SF) is an emerging technology for the extraction of high interest compounds for applications in the manufacturing of pharmaceuticals, nutraceuticals, and cosmetics from microalgae. The purpose of this study is to recover fatty acids (FAs) and, more precisely, eicosapentaenoic acid (EPA) from Nannochloropsis gaditana biomass by CO2-SF extraction. In the paper, the effect of mechanical pre-treatment was evaluated with the aim of increasing FAs recovery. Extraction was performed at a pressure range of 250⁻550 bars and a CO2 flow rate of 7.24 and 14.48 g/min, while temperature was fixed at 50 or 65 °C. The effect of these parameters on the extraction yield was assessed at each extraction cycle, 20 min each, for a total extraction time of 100 min. Furthermore, the effect of biomass loading on EPA recovery was evaluated. The highest EPA extraction yield, i.e., 11.50 mg/g, corresponding to 27.4% EPA recovery, was obtained at 65 °C and 250 bars with a CO2 flow rate of 7.24 g/min and 1.0 g biomass loading. The increased CO2 flow rate from 7.24 to 14.48 g/min enhanced the cumulative EPA recovery at 250 bars. The purity of EPA could be improved by biomass loading of 2.01 g, even if recovery was reduced.

Extraction of Bioactive Compounds Using Supercritical Carbon Dioxide.

Microalgae Dunaliella salina contains useful molecules such as ß-carotene and fatty acids (FAs), which are considered high value-added compounds. To extract these molecules, supercritical carbon dioxide was used at different operative conditions. The effects of mechanical pre-treatment (grinding speed at 0⁻600 rpm; pre-treatment time of 2.5⁻7.5 min) and operating parameters for extraction, such as biomass loading (2.45 and 7.53 g), pressure (100⁻550 bars), temperature (50⁻75 °C) and CO2 flow rate (7.24 and 14.48 g/min) by varying the extraction times (30⁻110 min) were evaluated. Results showed that the maximum cumulative recovery (25.48%) of ß-carotene was achieved at 400 bars and 65 °C with a CO2 flow rate of 14.48 g/min, while the highest purity for stage (55.40%) was attained at 550 bars and 65 °C with a CO2 flow rate of 14.48 g/min. The maximum recovery of FAs, equal to 8.47 mg/g, was achieved at 550 bars and 75 °C with a CO2 flow rate of 14.48 g/min. Moreover, the lowest biomass loading (2.45 g) and the first extraction cycle (30 min) allowed the maximum extraction of ß-carotene and FAs.

Extraction of astaxanthin from microalga Haematococcus pluvialis in red phase by using generally recognized as safe solvents and accelerated extraction.

Solvent Extraction was tested to extract astaxanthin from Haematococcus pluvialis in red phase (HPR), by investigating effects of solvents, extraction pressure and temperature. Astaxanthin isomers were identified and quantified in the extract. The performances of acetone and ethanol, Generally Recognized As Safe (GRAS) solvents, were explored. Negligible effect of pressure was found, while with increasing extraction temperature astaxanthin recovery increased till a maximum value, beyond which thermal degradation seemed to be greater than the positive effect of temperature on extraction. Furthermore, to maximize the extraction yield of astaxanthin, mechanical pre-treatment of HPR biomass was carried out and several extraction runs were consecutively performed. Experimental results showed that after the mechanical pre-treatment the astaxanthin recovery strongly increased while a single extraction run of 20 min was sufficient to extract more than 99% of total astaxanthin extracted. After pre-treatment, maximum recovery of about 87% was found for acetone (pressure = 100 bar; temperature = 40 °C; total time = 60 min).

Development and Characterization of High Performance Shape Memory Alloy Coatings for Structural Aerospace Applications.

This paper presents an innovative approach, which enables control of the mechanical properties of metallic components by external stimuli to improve the mechanical behavior of aluminum structures in aeronautical applications. The approach is based on the exploitation of the shape memory effect of novel Shape Memory Alloy (SMA) coatings deposited on metallic structural components, for the purpose of relaxing the stress of underlying structures by simple heating at field-feasible temperatures, therefore enhancing their structural integrity and increasing their stiffness and rigidity while allowing them to withstand expected loading conditions safely. Numerical analysis provided an insight in the expected response of the SMA coating and of the SMA-coated element, while the dependence of alloy composition and heat treatment on the experienced shape memory effect were investigated experimentally. A two-phase process is proposed for deposition of the SMA coating in an order that induces beneficial stress relaxation to the underlying structure through the shape memory effect.

Discontinuous permeable adsorptive barrier design and cost analysis: a methodological approach to optimisation.

The following paper presents a method to optimise a discontinuous permeable adsorptive barrier (PAB-D). This method is based on the comparison of different PAB-D configurations obtained by changing some of the main PAB-D design parameters. In particular, the well diameters, the distance between two consecutive passive wells and the distance between two consecutive well lines were varied, and a cost analysis for each configuration was carried out in order to define the best performing and most cost-effective PAB-D configuration. As a case study, a benzene-contaminated aquifer located in an urban area in the north of Naples (Italy) was considered. The PAB-D configuration with a well diameter of 0.8 m resulted the best optimised layout in terms of performance and cost-effectiveness. Moreover, in order to identify the best configuration for the remediation of the aquifer studied, a comparison with a continuous permeable adsorptive barrier (PAB-C) was added. In particular, this showed a 40% reduction of the total remediation costs by using the optimised PAB-D.

Degradation of ibuprofen by hydrodynamic cavitation: Reaction pathways and effect of operational parameters.

Ibuprofen (IBP) is an anti-inflammatory drug whose residues can be found worldwide in natural water bodies resulting in harmful effects to aquatic species even at low concentrations. This paper deals with the degradation of IBP in water by hydrodynamic cavitation in a convergent-divergent nozzle. Over 60% of ibuprofen was degraded in 60 min with an electrical energy per order (EEO) of 10.77 kWh m(-3) at an initial concentration of 200 µg L(-1) and a relative inlet pressure pin=0.35 MPa. Five intermediates generated from different hydroxylation reactions were identified; the potential mechanisms of degradation were sketched and discussed. The reaction pathways recognized are in line with the relevant literature, both experimental and theoretical. By varying the pressure upstream the constriction, different degradation rates were observed. This effect was discussed according to a numerical simulation of the hydroxyl radical production identifying a clear correspondence between the maximum kinetic constant kOH and the maximum calculated OH production. Furthermore, in the investigated experimental conditions, the pH parameter was found not to affect the extent of degradation; this peculiar feature agrees with a recently published kinetic insight and has been explained in the light of the intermediates of the different reaction pathways.

Silver impregnated carbon for adsorption and desorption of elemental mercury vapors.

The Hg(0) vapor adsorption experimental results on a novel sorbent obtained by impregnating a commercially available activated carbon (Darco G60 from BDH) with silver nitrate were reported. The study was performed by using a fundamental approach, in an apparatus at laboratory scale in which a synthetic flue gas, formed by Hg(0) vapors in a nitrogen gas stream, at a given temperature and mercury concentration, was flowed through a fixed bed of adsorbent material. Breakthrough curves and adsorption isotherms were obtained for bed temperatures of 90, 120 and 150 degrees C and for Hg(0) concentrations in the gas varying in the range of 0.8-5.0 mg/m3. The experimental gas-solid equilibrium data were used to evaluate the Langmuir parameters and the heat of adsorption. The experimental results showed that silver impregnated carbon was very effective to capture elemental mercury and the amount of mercury adsorbed by the carbon decreased as the bed temperature increased. In addition, to evaluate the possibility of adsorbent recovery, desorption was also studied. Desorption runs showed that both the adsorbing material and the mercury could be easily recovered, since at the end of desorption the residue on solid was almost negligible. The material balance on mercury and the constitutive equations of the adsorption phenomenon were integrated, leading to the evaluation of only one kinetic parameter which fits well both the experimentally determined breakthrough and desorption curves.

Sulfite oxidation catalyzed by cobalt ions in flue gas desulfurization processes.

This paper presents an experimental study of calcium bisulfite oxidation, a key step in the wet limestone-gypsum flue gas desulfurization (FGD) process, in the presence of catalysts (e.g., cobalt ions and a mixture of ferrous and cobalt ions). A fundamental approach is followed, by reproducing a simplified synthetic FGD liquor in which both catalyst ions, alone or mixed together, are present. A laboratory-scale apparatus is used, in which sulfurous solution is contacted with a gas phase at a fixed oxygen partial pressure (21.3 kPa) and at different temperature levels (25, 45, and 55 degrees C). The experimental results are analyzed using the theory of gas-liquid mass transfer with chemical reaction, showing that the slow reaction regime is explored and the transition from the kinetic to the diffusional subregime is identified. The experimental results are compared with those obtained in the presence of other catalytic species (manganese and ferrous ions), showing that cobalt is effective in catalyzing the oxidation of calcium bisulfite to sulfate, but to a minor extent with respect to iron and manganese.