CO2 Mass Transfer Coefficient Research Articles

Numerical study of structural optimization for improving gas-liquid flow and microalgae carbon fixation rate in air-lift photobioreactors

In this paper, the gas-liquid flow pattern and CO2 dissolution transport process of microalgae solution in an air-lift photobioreactor (PBR) are investigated by numerical simulations. In order to promote the gas-liquid mass transfer efficiency to enhance the carbon fixation, we propose different optimization schemes for the structure. Firstly, a two-phase flow model and a mass transfer model are employed to investigate the fluid flow characteristics. The flow state is evaluated by velocity in light direction, turbulent kinetic energy (TKE), gas hold-up and CO2 mass transfer coefficient. The results show that the gas flow rate of 0.2 V/V/min is the optimum value for the inlet. Secondly, CO2 mass transfer and carbon fixation rates are analyzed for different inlet CO2 volume fractions. With the increase of CO2 volume fraction, the carbon fixation rate first increases and then decreases, the highest fixation rate reaching 2.21 × 10−4 mol m−3 s−1 at 6 %. Finally, based on numerical simulations, three optimized structures with different spacing of baffles are proposed and evaluated the performance in comparison with the original. The results show that the gas distribution and CO2 mass transfer are significantly improved, with the one-sided baffle structure is the best. In summary, this study provides a new and useful reference to evaluate the gas-liquid mixing and improve the microalgae carbon fixation rate.

A mass transfer study of CO2 absorption in aqueous solutions of isomeric forms of sodium alaninate for direct air capture application

Direct air capture (DAC) of CO2 is a critical emerging technology required to achieve ambitious net-zero emissions. Aqueous amino acid absorbents are ideal candidates for DAC and this work describes the investigation of the CO2 absorption performance of sodium salts of β- and α-alaninate under relevant conditions for DAC by utilising synthetic CO2 gas streams at concentrations levels found in the atmosphere (200–600 ppm). The effects of solution concentration, temperature, gas flowrate, liquid flowrate, and dissolved CO2 concentration (i.e., CO2 loading) on CO2 mass transfer coefficients were investigated. The overall mass transfer coefficient was found to be independent of the gas and liquid flowrates, with larger values determined for β-alaninate solutions (0.75–1.16 mmol m−2 s−1 kPa−1) than that of α-alaninate (0.65–0.87 mmol m−2 s−1 kPa−1), under similar experimental conditions. The results showed that the overall mass transfer coefficient increases with increasing temperature, conversely decreasing steadily with increasing CO2 loading. The maximum CO2 loading capacity of the 5.0 M solutions was ∼10 % higher for β-alanine (0.651 mol CO2/mol alaninate) than α-alanine (0.607 mol CO2/mol alaninate) at 25.0 °C. Precipitation was found to play an important role in the CO2 absorption processes here. Interestingly, beyond CO2 loadings >0.12 mol CO2/mol α-alaninate, the 5.0 M solution of α-alaninate begins to develop precipitate(s) while precipitation in the β-alaninate solution is only initiated at much higher CO2 loadings >0.52 mol CO2/mol β-alaninate. This study provides valuable insights into the suitability of β-alanine and α-alanine for CO2 capture directly from the ambient air.

Analytical first-principles-based model for sprays-based CO2 capture

CO2 removal from flue gases or air is directly proportional to the overall contact area between the solvent and gas flow. Spraying the solvent offers possibilities for large area/volume ratios resulting from the small size of solvent droplets. The overall mass transfer (KGAv) is a crucial indicator of the performance of the system and indicates the overall CO2 capture rate per unit volume at given working pressure. The maximum capture rate of spray systems considered in this study is 608 kmol/m3-hr. This study develops an analytical model to predict mass transfer in spray-based systems for CO2 capture by evaluating KGAv and capture efficiency. Model predictions are validated using two experimental studies for a range of liquid flow rate, inlet solvent loading, different nozzle types, and inlet CO2 partial pressure. The relative importance of the incorporation of equilibrium partial pressure of CO2 (P*) into amine with respect to CO2 partial pressure is also studied. It is found that its effect on KGAv is prominent in high liquid flow rate and low solvent loading regimes where the overall CO2 capture is high. Finally, the variation of key parameters such as KGAv, total mass transfer coefficient (KG), effective area (Av), solvent loading (α), and mole fraction of CO2 in the gas stream (XCO2) along the channel height is studied, which gives insights on optimization of the channel height for specified operating conditions.

CO2 capture using superhydrophobic ceramic membrane: Preparation and performance analysis

The application of membrane absorption for CO2 capture has been limited by membrane wetting. In this paper, tubular Al2O3 ceramic membranes (marked as M1 and M2) are hydrophobically modified by sol-gel and dip-coating method and then used for CO2 capture. The surface morphology, surface composition, wettability and flux are characterized in detail. The mass transfer performance of M1 and M2 is studied at different gas and liquid Reynolds numbers. After modification, the surface contact angle can reach as high as 170.7°. The carbon capture performance of the modified M1 and M2 is significantly improved. Under laminar flow condition, CO2 mass transfer coefficient increased from the initial 1.03 × 10−4 m/s to 14.59 × 10−4 m/s, and the CO2 capture efficiency increased from the initial 28.55%–98.55%. The mass transfer resistance of ceramic membrane contactor is also analyzed. The resistance of liquid phase plays a leading role in the mass transfer performance in laminar flow state. For non-wetting membrane, membrane thickness has little effect on the mass transfer performance. In addition, it is shown that the superhydrophobic M1 and M2 exhibit excellent corrosion resistance. The research in this paper shows that superhydrophobic ceramic membrane is more suitable for CO2 capture in industrial production.

Enhancement of CO2 Absorption Process Using High-Frequency Ultrasonic Waves

The advancement of efficient carbon capture technology is vital for the transition to a net-zero carbon future. Critical developments in ultrasonic irradiation can be used to enhance the conventional CO2 absorption process. For example, sonophysical effects such as acoustic streaming, acoustic cavitation, acoustic fountain and atomization induced by the propagation of high-frequency ultrasonic waves in a liquid medium can enhance the mixing and create a larger interfacial area for gas–liquid mass transfer. In this study, the performance of a continuous ultrasonic-assisted CO2 absorption process using MDEA was investigated. The design of experiment (DOE) was used to study the effect of the gas flowrate, liquid flowrate and ultrasonic power on CO2 absorption performance. Based on the findings, ultrasonic power was the most significant parameter affecting the CO2 outlet concentration, liquid-to-gas ratio (L/G) and mass transfer coefficient (KGa), which confirmed that ultrasonic irradiation has a significant impact on the intensification of the CO2 absorption process. The optimum condition to achieve the target CO2 absorption performance was numerically determined and validated with experimental tests. The results from the verification runs were in good agreement with the predicted values, and the average error was less than 10%.

Losses of Yeast-Fermented Carbon Dioxide during Prolonged Champagne Aging: Yes, the Bottle Size Does Matter!

When it comes to champagne tasting, dissolved CO2 is a key compound responsible for the very much sought-after effervescence in glasses. Nevertheless, the slow decrease of dissolved CO2 during prolonged aging of the most prestigious cuvees raises the issue of how long champagne can age before it becomes unable to form CO2 bubbles during tasting. Measurements of dissolved CO2 concentrations were done on a collection of 13 successive champagne vintages stored in standard 75 cL bottles and 150 cL magnums showing prolonged aging ranging from 25 to 47 years. The vintages elaborated in magnums were found to retain their dissolved CO2 much more efficiently during prolonged aging than the same vintages elaborated in standard bottles. A multivariable exponential decay-type model was proposed for the theoretical time-dependent concentration of dissolved CO2 and the subsequent CO2 pressure in the sealed bottles during champagne aging. The CO2 mass transfer coefficient through the crown caps used to seal champagne bottles prior to the 2000s was thus approached in situ with a global average value of K ≈ 7 × 10-13 m3 s-1. Moreover, the shelf-life of a champagne bottle was examined in view of its ability to still produce CO2 bubbles in a tasting glass. A formula was proposed to estimate the shelf-life of a bottle having experienced prolonged aging, which combines the various relevant parameters at play, including the geometric parameters of the bottle. Increasing the bottle size is found to tremendously increase its capacity to preserve dissolved CO2 and therefore the bubbling capacity of champagne during tasting. For the very first time, a long time-series dataset combined with a multivariable model indicates that the bottle size plays a crucial role on the progressive decay of dissolved CO2 experienced by champagne during aging.

Optimization Study of CO2 Gas Absorption with NaOH Absorbent Continuous System in Raschig Ring Packing Column Using Box–Behnken Design

Increasing CO2 gas emissions results in climate change by increasing air temperature and worsening environmental problems. It is necessary to control CO2 gas in the air to overcome this. This research aims to optimize the absorption of CO2 gas in the air with 0.1 M NaOH absorbent in the column of the Raschig ring stuffing material using the response surface methodology (RSM). This research was conducted using a continuous system of three independent variables by varying the contact time (10–80 min), the flow rate of NaOH absorbent (2–5 L/min), and the flow rate of CO2 gas (1–5 L/min). The response variables in this study were the absorption rate (L/min) and mass transfer coefficient, while the air flow rate was constant at 20 L/min. Air and CO2 gas mix before absorption occurs and flow into the Raschig ring packing column so that contact occurs with the NaOH absorbent. Mass transfer of CO2 gas occurs into the NaOH absorbent, resulting in absorption. The results showed that the effect of contact time (min), the flow rate of NaOH absorbent (L/min), and CO2 gas flow rate individually and the interaction on CO2 absorption rate and mass transfer coefficient were very significant at a p-value of 0.05. Chemical absorption of CO2 also occurred due to the reaction between CO2 and OH- to form CO32− and HCO3−, so the pH decreased, and the reaction was a function of pH. Optimization using Design Expert 13 RSM Box–Behnken Design (BBD) yielded optimal conditions at an absorption time of 80 min, NaOH absorbent flow rate of 5 L/min, CO2 gas flow rate of 5 L/min, absorption rate of CO2 gas of 3.97 L/min, and CO2 gas mass transfer coefficient of 1.443 mol/min m2 atm, with the desirability of 0.999 (≈100%).

Modeling of NO mass transfer characteristics absorbed in sodium persulfate solution with a bubble reactor

ABSTRACT: Sodium persulfate solution is considered as an effective wet denitrification medium, however, it is unclear that the influence of the operating conditions on mass transfer characteristics parameters during the absorption of NO with sodium persulfate solution. To determine the key mass transfer characteristics parameters, the specific interfacial area and the mass transfer coefficients were determined based on the Danckwerts method during CO2 absorption in a bubble column. and were calculated by correlations between the mass transfer coefficients of NO and CO2. Results showed that the specific interfacial area increased 77.64 m−1, the liquid phase mass transfer coefficient increased 2.49 × 10−4 m·s−1, and the gas phase mass transfer coefficient increased 0.71 × 10−5 mol·Pa−1·s−1·m−2 with superficial gas velocity increasing from 0.6 to 1.4 L·min−1. With the temperature of sodium persulfate solution increasing from 293 to 333 K, the specific interfacial area decreased 42.66 m−1, while the liquid phase mass transfer coefficient and the gas phase mass transfer coefficient increased 3.89 × 10−4 m·s−1 and 1.18 × 10−5 mol·Pa−1·s−1·m−2, respectively. The experiments results determined the correlations of and with the temperature of the absorption phase and the superficial velocity of the gas. It can serve as a guide to the enhancement of the sodium persulfate wet denitrification process.

Characteristics of CO2 adsorption and desorption on activated carbon in comparison with zeolite 13X and carbon molecular sieve and applications in biogas upgrading using vacuum pressure swing adsorption

AbstractBACKGROUNDBiogas upgrading is the process that increases the fraction of methane (CH4) in biogas, making the biogas suitable for use as vehicle fuel or as fuel with high and stable heating values for combustion engines. The advantages and disadvantages of activated carbon (AC) in CO2/CH4 adsorptive separation should be well understood before application for biogas upgrading. In the first part of this study, characteristics of CO2 adsorption and desorption on AC were investigated using Aspen Adsorption. In the second part, vacuum pressure swing adsorption (VPSA) was modeled, and the performances of the VPSA process using zeolite 13X, carbon molecular sieve (CMS) and AC were compared.RESULTSThe simulations showed that the advantages of AC for CO2 adsorption include relatively high dynamic adsorption capacity and high mass transfer coefficient of CO2 (KLDF,CO2) in AC and consequently relatively high CO2 desorption rates. However, due to the relatively high mass transfer coefficient of CH4 (KLDF,CH4) in AC, the molecular sieving characteristics for CO2 that were estimated from the ratio between KLDF,CO2 and KLDF,CH4 of AC were not significantly higher than that of zeolite 13X and significantly lower than that of CMS (9.85, 8.41 and 20.92 for AC, zeolite 13X and CMS, respectively).CONCLUSIONSThe modeling results predicted that the VPSA process using zeolite, CMS and AC adsorbents gives a biomethane product of around 98%, 97% and 93% CH4, respectively. © 2023 Society of Chemical Industry (SCI).

Effect of incorporating hydrophilic silica on carbon dioxide volumetric mass transfer coefficient using a fabricated membrane as the diffuser

In this work, 0 %,1.5 wt%, and 3 wt%, of the hydrophilic silica loading were prepared respectively (later denoted as N-0, N-1.5 and N-3), and their effect on the membrane surface properties, equivalent bubble diameter, and volumetric CO2 mass transfer coefficient (kLa) were investigated at a different flow rate (0.05 – 20 ml/s). The results show a slight decrease in pore size diameter of N-3, and N-1.5 without significant change in contact angle. At a low flow rate of 0.1 ml/s the bubble size of N-3 decreased by 50 %, which led to an increase of 143 % in the kLa compared to N-0.

CO2 absorption performance of ChCl-MEA deep eutectic solvent in microchannel

Deep eutectic solvent (DES) has been a new type of absorbent for CO2 capture due to the advantages of environmental friendliness, simple synthesis and excellent selectivity. A series of choline chloride (ChCl)-monoethanolamine (MEA) deep eutectic solvents with various water contents were prepared. The effects of gas-liquid flow rate, molar ratio of ChCl-MEA, water content and temperature on CO2 absorption performance and mass transfer were investigated. The addition of an appropriate amount of water can effectively reduce the viscosity of DES and improve its CO2 capture ability. The results show that the CO2 absorption efficiency, absorption rate, loading and mass transfer coefficient of ChCl-MEA deep eutectic solvent are optimal when the water content is 70 wt% for the DES with the molar ratio of ChCl and MEA 1:6. When the water content exceeds 70 wt%, the hydrogen bond network of DES would be destroyed, and its CO2 capture ability would be weakened. According to the Arrhenius equation, the reaction activation energy of CO2 absorption is as low as 7.54 kJ·mol−1 when the water content is 70 wt% in DES with the molar ratio of ChCl and MEA 1:6. This study can provide technical support for the application of deep eutectic solvents in CO2 capture.

Experimental study and modified modeling on effect of SO2 on CO2 absorption using amine solution

Understanding the competitive mechanism in absorption process between SO2 and CO2 is essential for simultaneous absorption of SO2 and CO2. In this study, the effect of SO2 on CO2 absorption using amine solution was studied experimentally and theoretically. The CO2 absorption rates in MEA/MMEA/MDEA solutions were determined using wetted wall column. Results showed that the reduction of flue gas temperature was beneficial for CO2 absorption at the typical coal-fired flue gas CO2 concentration. On this basis, the effect of SO2 on CO2 absorption performance was studied. Firstly, the overall CO2 mass transfer coefficient decreased under exposure to SO2. Secondly, in the presence of SO2, pH variation of amine solution increased. In addition, the saturated CO2 capacity largely decreased when SO2 was continuously bubbled into the solution. Furthermore, the quantum chemical calculation showed that CO2 had little effect on the absorption performance of SO2 whereas SO2 greatly weakened the interaction between CO2 and amines. The overall CO2 mass transfer coefficient was modified considering the effect of SO2. The competitive absorption factor, χ, was introduced to modify the CO2 liquid phase diffusion coefficients. The average deviation of overall CO2 mass transfer coefficient narrowed to 1.76 % (0.5 mol/L MEA), 1.13 % (1.0 mol/L MEA), 1.88 % (1.0 mol/L MMEA) and 1.43 % (1.0 mol/L MDEA) respectively, which agreed well with the experimental results.

Development of biphasic solvent for CO2 capture by tailoring the polarity of amine solution

A facile screening method for amine/phase splitter/H2O biphasic solvents was proposed. The polarity difference between the high polarity absorption products and the low polarity phase splitter was recognized as the decisive factor for phase separation. In this study, biphasic solvent was developed by tailoring the polarity of absorption products. It’s revealed that the amine/1-propanol/H2O solvents with a dipole moment difference above 6.0 Debye featured phase separation. After screening, TETA/1-propanol/H2O biphasic absorbent with high CO2-rich loading and low regeneration energy was developed. The phase separation behavior was affected by TETA concentration, 1-propanol concentration, and the absorption temperature. Remarkably, the CO2 capacity and volume distribution of the rich phase can be simply tuned by altering the 1-propanol concentration. For the optimal composition of 30 wt% TETA and 50 wt% 1-propanol, a rapid phase separation in 2 min was observed at 60 °C. And the rich phase exhibited a CO2 capacity as high as 6.06 mol/L with a volume proportion of 38.9%. With the promotion of 1-propanol, the CO2 mass transfer coefficient in TETA/1-propanol reached 21.4 × 10−11 mol/cm2·s·Pa, 42.7% higher than 30 wt% TETA and 110.2% higher than 30 wt% MEA. The estimated regeneration heat was 2.06 GJ/t CO2, 48.4% lower than 30 wt% MEA. This research provided a facile screening method and a promising alternative absorbent for CO2 capture.

Mass transfer and capture of carbon dioxide using amino acids sodium aqueous solution in microchannel

Capture and storage of CO2 are important to restrain the greenhouse effect. The amino acid salt solution as a new type of CO2 absorbent shows good application potential. Besides, microchannel has a unique enhancement to mass transfer, which is conducive to promote CO2 capture. The absorption performance of CO2 into aqueous solutions of sodium salts of l-threonine, l-valine, l-alanine and glycine in the microchannel was investigated. It was found that the CO2 absorption rate of four sodium amino acid aqueous solutions under the same condition follows in the order: ñ (sodium glycinate) > ñ (sodium l-alaninate) > ñ (sodium l-valinate) > ñ (sodium l-threoninate). The sodium glycinate aqueous solution has maximum CO2 absorption ability, thus the mass transfer and capture of CO2 absorbed into the sodium glycinate aqueous solution were studied systematically. The effects of operating conditions including gas flow rate, liquid flow rate, and sodium glycinate concentration on the specific surface area, overall volumetric mass transfer coefficient, overall mass transfer coefficient and absorption fraction of CO2 were explored. Furthermore, a new correlation was proposed to predict the volumetric mass transfer coefficient.

Determination of mass transfer coefficient from biogas purification process by mono ethanol amine (MEA)

Biogas is a renewable energy source, which can be produced from organic waste such as palm oil mill effluent (POME), through an anaerobic fermentation process. Biogas requires a purification process to remove the content of hydrogen sulfide (H2S) and carbon dioxide (CO2). The purpose of this study is to calculate the value of mass transfer coefficient in biogas purification process, namely in the process of chemical absorption of H2S and CO2 gases in raw biogas by monoethanolamine (MEA) absorbent. Biogas purification was carried out using a bubble column absorber, where raw biogas was flowed into a column containing a MEA absorber of 0.678 L. The biogas content from the column was analyzed using biogas content measuring device (Biogas 5000 ©). The overall mass transfer coefficient values in gas phase obtained from this study are 0.182–1.027 s−1 for H2S – MEA system, and 0.674–3.993 s−1 for CO2 –MEA system. The overall mass transfer coefficient of CO2 – MEA from this study is greater than that of previous studies.

Characterisation of a simple ‘hanging bag’ photobioreactor for low‐cost cultivation of microalgae

AbstractBACKGROUNDMicroalgae are a diverse group of photosynthetic microorganisms of significant interest to the biotechnology industry, either as a sustainable source of natural compounds, or as light‐driven cell factories to produce recombinant metabolites and proteins. Their ability to utilise light, CO2, and basic nutrients leads to a simple and low‐cost phototrophic cultivation process. This is particularly relevant to low‐ and middle‐income countries, all of which require a cultivation system that is cheap, technically simple to operate, readily scalable, and can meet basic Good Manufacturing Practice requirements. A disposable ‘hanging bag’‐type photobioreactor operated as a bubble column fits these criteria.RESULTSIn this study, the characterisation and design modifications to improve the performance of a 15 L hanging bag is reported. The bubble behaviour using different sparger designs was investigated together with gas hold‐up, mixing time, and mass transfer coefficient of CO2. A gas flow rate of 5 L min−1 using a new sparger design and a modified height‐to‐diameter ratio of the bag led to a two‐fold improvement in algal biomass productivity when culturing the green microalga Chlorella sorokiniana. The cultivation of a luciferase‐expressing Chlamydomonas reinhardtii strain in the modified hanging bag also demonstrated an 11% increase in luciferase content.CONCLUSIONThis is the first attempt to characterise this simple hanging bag system that brings the industry‐favoured single‐use bag concept into the research field of photobioreactor technology. The hanging bag with modified sparger and dimensions improves microalgal biomass productivity and demonstrates the potential of simple and low‐cost systems for microalgal cultivation. © 2021 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

The Effect of Temperature on Solvent Recycle Process using Stripping of CO2 from MDEA, Ethylene Glycol, and Water

Stripping is a process to separate dissolved gas in the saturated solvent to regenerate the absorption solvent. In this study, N2 gas was used to strip dissolved CO2 gas in MDEA, ethylene glycol, and water. The experiment was conducted with three variations of temperature, namely 28°C, 35°C, and 50°C, to determine the effect of solvent’s temperature entering the stripper column on the value of the mass transfer coefficient (Kla). The stripper effluent was connected to the KANE 457 Flue Gas Analyzer to measure the concentration of CO2. Data retrievals were carried out at the 0; 30; 60; 90; 120; 150; 180th second. The experimental results show the trend of mass transfer coefficient of CO2 (Kla) is higher with the increase of solvent’s temperature entering the stripper.

Integration of a calcium carbonate crystallization process and membrane contactor–based CO2 capture

This research work presents the integration of polypropylene hollow fiber membrane contactor–based CO2 absorption unit using aqueous sodium hydroxide (NaOH) as the absorbent solution, and precipitation of high-quality calcium carbonate by addition of calcium chloride. The integrated crystallization process bypasses the need for cost-intensive energy regeneration of the absorbent solution. Given the lifetime of high added value precipitated calcium carbonate, the proposed approach for integrating solid particle formation and a carbon dioxide mitigation technique can be considered as a potential post-combustion technology for carbon capture, utilization and sequestration (CCUS). The overall CO2 mass transfer coefficient in the membrane contactor and subsequent crystallization process was investigated by conducting the experiments at different operating conditions. For a maximum hydroxide concentration of 5.01molL-1, an overall mass transfer coefficient of 2.25·10-4ms-1 and CO2 flux of 2.45·10-4molm-2s-1 were obtained for a 0.5Lmin-1 of gas flow rate. The proposed liquid–liquid crystallization process produces a narrow size distribution of micron-sized calcium carbonate with a mean diameter of 3–8 µm that can be used in the paper, coatings, food and biomedical industries.

Mass transfer coefficients of carbon dioxide in aqueous blends of monoethanolamine and glycerol using wetted-wall column

There is an urgent need for CO2 capture development because of the global warming crisis. Recently CO2 absorption by the mixture of monoethanolamine (MEA) and glycerol, as an eco-friendly solvent, has been considered due to its promising performance and low technical and environmental impacts. However, more aspects of this process, especially mass transfer coefficients, need to be studied further. In this work, a bench-scale wetted-wall column was used to find the CO2 mass transfer coefficients in the aqueous blends of MEA (25 wt%) and glycerol (5–20 wt%). The experiments were performed nearly to the industrial conditions of flue gas at atmospheric pressure and three different temperatures (313, 323, and 333 K). The gas flow rate was maintained around 0.17 ± 0.01 stdL/s, and the CO2 partial pressure was in the range of 1–15 kPa. The findings revealed that increasing the glycerol to 10 wt% improves the overall mass transfer (KG), and adding more glycerol up to 20 wt% decreases the KG. The gas-side mass transfer resistance (1/kg) found to be negligible. Thus, the primary mass transfer resistance was in the liquid phase. It is also found that the solution with 10 wt% glycerol and 25 wt% MEA (10G25M) had the highest liquid-side mass transfer coefficient (kg′) among the other solutions. The 10G25M showed a comparable and even better absorption rate than solutions with a higher concentration of MEA studied in the literature. Compared with industrial-grade, the kg′ of the 10G25M was over two times higher than the 30 wt% MEA solution.

Biogas upgrading by adsorption onto activated carbon and carbon molecular sieves: Experimental and modelling study in binary CO2/CH4 mixture

Biomethane, a renewable form of natural gas, can be produced by biogas upgrading and its utilization is incentivized to mitigate CO2 emissions. In this work, biogas upgrading via adsorption on commercial carbonaceous adsorbents, an activated carbon (AC) provided by Desotec and two carbon molecular sieves (CMSs) produced by Carbotech and Xintao, is experimentally investigated in a fixed-bed column, testing CO2/CH4 mixtures at different concentrations as feed gas. The experimental campaign highlights that AC has a higher adsorption capacity for both CO2 and CH4, as well as better kinetic performances (i.e. higher bed usage efficiency and lower desorption times), than CMSs. On the other hand, the CMSs show a much lower CH4 adsorption capacity than the AC due to their combined thermodynamic/kinetic sieving properties, which emerge from dynamic breakthrough curves and equilibrium adsorption data alike. Therefore, the CMSs show a much higher selectivity for the separation of the CO2/CH4 mixture, and Xintao sample shows a better overall performance thanks to its faster kinetics; moreover, experimental cyclic adsorption/desorption runs on Xintao confirmed its complete regenerability. Breakthrough curves modelling, performed in MATLAB environment and aimed at evaluating the CO2 mass transfer coefficients, points out that intraparticle diffusion is the rate-limiting step for CO2 adsorption process. It can be concluded that CMSs show better performances for biogas upgrading due to their high selectivity and despite their lower CO2 adsorption capacity, which cannot be taken as the main controlling parameter in the adsorbent selection devoted to this specific application.