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Liquid biomethane (LBM), also referred to as liquid biogas (LBG), is a promising biofuel for transport that can be obtained from upgrading and liquefaction of biogas. With respect to fossil fuels, LBM is a renewable resource, it can be produced almost everywhere, and it is a carbon neutral fuel. LBM is 3 times more energy dense than compressed biomethane (CBM) and it allows longer vehicle autonomy. LBM has also a higher energy density than other transport biofuels, it is produced from wastes and recycled material without being in competition with food production, and it assures a high final energy/primary energy ratio. The low temperatures at which LBM is obtained strongly suggest the use of cryogenic/low-temperature technologies also for biogas upgrading. In this respect, since biogas can be considered as a “particular” natural gas with a high CO 2 content, the results available in the literature on natural gas purification can be taken into account, which prove that cryogenic/low-temperature technologies and, in particular, low-temperature distillation are less energy consuming when compared with traditional technologies, such as amine washing, for CO 2 removal from natural gas streams at high CO 2 content. Low-temperature purification processes allow the direct production of a biomethane stream at high purity and at low temperature, suitable conditions for the direct synergistic integration with biogas cryogenic liquefaction processes, while CO 2 is obtained in liquid phase and under pressure. In this way, it can be easily pumped for transportation, avoiding significant compression costs as for classical CO 2 capture units (where carbon dioxide is discharged in gas phase and at atmospheric pressure). In this paper, three natural gas low-temperature purification technologies have been modelled and their performances have been evaluated through an energy consumption analysis and a comparison with the amine washing process in terms of the equivalent amount of methane required for the upgrading, proving the profitability of cryogenic/low-temperature technologies. Specifically, the Ryan-Holmes, the dual pressure low-temperature distillation process and the anti-sublimation process have been considered. It has been found that the dual pressure low-temperature distillation scheme reaches the highest thermodynamic performances, resulting in the lowest equivalent methane requirement with respect to the other configurations. [ABSTRACT FROM AUTHOR]

Many correlations are available in the literature for computing the gas hold-up in bubble columns, mainly in pure liquids. Contrarily, very few works deal with the gas hold-up in liquid mixtures, giving different opinions about the successful application of Andrew’s dynamic surface tension model. This work further investigates this topic using more recent gas hold-up data presented in a previous work for the binary mixture comprising monoethylene glycol and water and measured for a large-diameter bubble column, which differs from the equipment used in the other few works that have pointed out the unusual behavior of the gas hold-up in binary liquid mixtures. The correlation proposed for representing the experimental data on the basis of the dynamic surface tension theory has been applied in the entire range of gas superficial velocities (0.004–0.20 m/s) and it has been also generalized to predict the gas hold-up enhancement due to the presence of electrolytes. The results suggest that the dynamic surface tension model allows to reproduce the experimental gas hold-up data in a qualitative way, but a quantitative agreement is still lacking since the maximum frothing ability has been experimentally observed for a monoethylene glycol concentration lower than that predicted by Andrew’s theory. [ABSTRACT FROM AUTHOR]

The production of sugars from lignocellulosic biomass is the key to a sustainable, renewable chemical industry. Glucose, xylose and other monosaccharides can be easily produced by hydrolyzing cellulose and hemicellulose, the primary polysaccharides in biomass. However, the hydrolysis of biomass generates byproducts that, together with the mineral acid normally added in the hydrolysis step, have to be removed before the downstream conversion processes. In this work, the recovery of monosaccharides from lignocellulosic hydrolysates by means of Ion Exclusion Chromatography (IEC) has been studied. The analyzed process relies on new pretreatment and hydrolysis steps, involving the neutralization of the hydrolysate with sodium hydroxide. The adsorption behavior of the main components involved in the separation has been experimentally investigated. Pulse tests at the high loading encountered in preparative conditions have been performed for a selected group of model components found in the hydrolysates. For all the electrolytes, the retention volume fraction was always between the interparticle porosity and the total column porosity, confirming that ion exclusion was the dominant retention mechanism. On the other hand, sugars eluted before the total column porosity, indicating partial steric exclusion from the resin pores. This observation was then confirmed by size-exclusion experiments with polyethylene glycol standards, from which the distribution coefficient of the studied sugars has been determined. The comparison between the elution profiles of the same sugars in pure form and as a mixture present in the hydrolysate showed differences in both peak shape and retention times. Therefore, an investigation of the influence of the main electrolytes contained in the hydrolysates on sugars adsorption has been performed through the pulse on a plateau method. The electrolytes were found to enhance the sugars retention by promoting their adsorption onto the resin. However, this effect was not sufficient to explain the observed differences, which were effectively explained in terms of viscous fingering, due to the high viscosity differences between the eluent and the sample. A previously developed model for IEC has been updated to take into account all the observed phenomena and applied to simulate the experimental results. The proposed model was in good agreement with the batch-column elution profiles both for the pure components and for the actual hydrolysate, allowing a quantitative description of the separation. [ABSTRACT FROM AUTHOR]

• Novel technologies have been developed for cryogenic separation of CO 2. • Cryogenic CO 2 separation from flue gas requires solving solid-vapor equilibria. • The classical thermodynamic approach is applied to solid-vapor equilibria. • Operating conditions can be determined for achieving a desired CO 2 capture rate. This work deals with the thermodynamic modeling of phase equilibria calculations in the presence of solid carbon dioxide for application to CO 2 capture from flue gas by desublimation. The proposed solid-vapor equilibria calculation method is based on the classical approach, namely on the equality of components' fugacities in the different phases and on the use of an expression for the fugacity of the freezing component in the solid phase. The results obtained with the proposed method are validated for the system CO 2- N 2 by comparison with literature experimental data and for the system CO 2- N 2- O 2 by comparison with the results reported in the literature and those obtained using another tool that makes use of Gibbs free energy minimization. The good agreement obtained in both cases suggests that the proposed approach can be used for determining suitable operating conditions for processes aimed at separating CO 2 out of flue gas by freezing it out. [Display omitted] [ABSTRACT FROM AUTHOR]

The production of acetone, n-butanol and ethanol (ABE) by fermentation has recently gained renewed attention to produce fuels and chemicals from renewable resources. The main problem associated with the ABE fermentation is the self-inhibition of the process due to butanol toxicity to the culture. The use of a separation method, which enables selective removal of toxic components from the fermentation broth, may contribute to improve the economics of such process. This work concerns the recovery of butanol from a batch fermenter, in which the ABE mixture is recovered from the broth by means of gas stripping. The aim of this work is to compare the performances of the integrated fermentation-gas stripping process with those of a conventional batch fermenter. For this purpose, both processes have been modeled and their energy requirements have been determined by means of an energy analysis. Results suggest that the continuous product removal from the fermenter allows to reduce the fermentation time by 50%. Therefore, the process productivity is almost double. At the same time, the total specific energy requirement for the integrated recovery process is almost three times greater than that of the traditional batch process. This is essentially due to the duty required at the condenser for products recovery from the gas stream, as a result of the low concentration of ABE in the recovered stream. Therefore, the application of an integrated recovery process allows to increase the process productivity, but at the expense of an increase in the whole process energy demand. [ABSTRACT FROM AUTHOR]

Some authors, in the last decades, have observed the dual effect of viscosity on gas holdup and flow regime transition in small-diameter and small-scale bubble columns. This work concerns the experimental investigation of the dual effect of viscosity on gas holdup and flow regime transition as well as bubble size distributions in a large-diameter and large-scale bubble column. The bubble column is 5.3 m in height, has an inner diameter of 0.24 m, and can be operated with gas superficial velocities in the range of 0.004–0.20 m/s. Air was used as the dispersed phase, and various water-monoethylene glycol solutions were employed as the liquid phase. The water-monoethylene glycol solutions that were tested correspond to a viscosity between 0.9 mPa s and 7.97 mPa s, a density between 997.086 kg/m 3 and 1094.801 kg/m 3 , a surface tension between 0.0715 N/m and 0.0502 N/m, and log 10 ( Mo ) between −10.77 and −6.55 (where Mo is the Morton number). Gas holdup measurements were used to investigate the global fluid dynamics and the flow regime transition between the homogeneous flow regime and the transition flow regime. An image analysis method was used to investigate the bubble size distributions and shapes for different gas superficial velocities, for different solutions of water-monoethylene glycol. In addition, based on the experimental data from the image analysis, a correlation between the bubble equivalent diameter and the bubble aspect ratio was proposed. The dual effect of viscosity, previously verified in smaller bubble columns, was confirmed not only with respect to the gas holdup and flow regime transition, but also for the bubble size distributions. Low viscosities stabilize the homogeneous flow regime and increase the gas holdup, and are characterized by a larger number of small bubbles. Conversely, higher viscosities destabilize the homogeneous flow regime and decrease the gas holdup, and the bubble size distribution moves toward large bubbles. The experimental results suggest that the stabilization/destabilization of the homogeneous regime is related to the changes in the bubble size distributions and a simple approach, based on the lift force, was proposed to explain this relationship. Finally, the experimental results were compared to the dual effect of organic compounds and inorganic compounds: future studies should propose a comprehensive theory to explain all the dual effects observed. [ABSTRACT FROM AUTHOR]

We experimentally investigate the effect of ethanol (0.05% in mass) on the hydrodynamics of a large-diameter and large-scale bubble column. The bubble column is 5.3 m in height, has an inner diameter of 0.24 m, and we consider gas superficial velocities in the range of 0.004–0.20 m/s. The experimental investigation consists of gas holdup measurements and image analysis. The gas holdup measurements are used to investigate the flow regime transition and the global bubble column hydrodynamics. The image analysis is used to investigate the bubble shapes and size distributions near the sparger and in the developed region of the column. The presence of ethanol increases the gas holdup, stabilizes the homogeneous regime and modifies the bubble shapes and size distributions. In particular, the addition of ethanol increases the mean bubble aspect ratio and decreases the bubble diameters. The results suggest that the addition of ethanol changes the bubble properties, which modifies the bubble size distribution and shapes, thus, stabilizing the homogeneous flow regime and, finally, increasing the gas holdup. [ABSTRACT FROM AUTHOR]

Advanced processes, which are alternatives to ordinary distillation, are essential to dehydrate azeotropic alcoholic mixtures for biofuel production. In that regard, this work focuses on the analysis of heterogeneous azeotropic distillation for the separation of a 2-propanol + water mixture in order to recover the alcohol with a sufficiently low water content. By comparing the performances of various entrainers on the basis of ternary maps, isooctane was selected for further process analysis. An advantage it poses is related to the fact that traces of it within the recovered dehydrated alcohol are highly welcome with a view to its subsequent use as a fuel. Aspen Plus® V11 software was employed for the simulation of the process, thus filling the gap existing in the literature due to the lack of studies on the process analysis of the heterogeneous azeotropic distillation of the 2-propanol + water system using isooctane as an entrainer. [ABSTRACT FROM AUTHOR]