CO2 Concentration In Gas Phase Research Articles

High temperature CO2 capture in lithium orthosilicate packed fluidized bed: Gamma ray densitometry based numerical model and its experimental validation

BackgroundLithium orthosilicate (Li4SiO4) is steadily emerging as the default solid sorbent for high temperature CO2 capture in industrial settings. Li4SiO4 is also one the few materials qualified to serve as both catalyst and sorbent in sorption enhanced steam reforming process for high purity carbon neutral H2 production. Application of packed fluidization technique to enhance CO2 uptake in packed bed of pure Li4SiO4 particles has not been reported until now. MethodsGamma ray densitometry (GRD) technique is employed to non-intrusively obtain average voidage of Li4SiO4 packed fluidized bed as a function of operating time and axial bed coordinate. Based on the experimental results of GRD technique, a numerical model is developed to obtain the transient cross-sectional profiles of internal bed temperature and gas phase CO2 concentration as high temperature carbonation of Li4SiO4 progresses in packed fluidized bed. The numerical model is validated using experimentally obtained internal bed temperature and breakthrough curves. Temporal variation of bed effective thermal conductivity has been evaluated from model temperature profiles. FindingsIt was observed that the time averaged effective thermal conductivity and net solid weight gain of packed fluidized bed is around 1.7–2.3 and 1.1–1.3 times that of unary packed bed respectively at specified process parameters for high temperature CO2 capture.

Soil CO2 in organic and no-till agroecosystems

Increased carbon dioxide (CO2) concentrations in soil profiles could increase carbon sequestration via improved residence times of soil organic carbon (SOC), greater nutrient availability and plant growth, and via other pathways, but the effect of agricultural management techniques on soil CO2 is poorly understood. We used nondispersive infrared (NDIR) CO2 sensors to measure gas-phase CO2 concentrations in 1.50 m soil profiles in paired no-till (NT) and organic (ORG) agricultural systems in the Palouse region of eastern Washington State, USA, to investigate the effect of management on soil CO2. CO2 concentrations were significantly greater in ORG than in NT. The CO2 efflux into the atmosphere estimated using Fick's first law was generally greater in ORG due to both greater diffusivity and stronger concentration gradients observed in ORG. Results demonstrate that organic agriculture may facilitate greater soil CO2 concentrations and production rates. Further research is needed to quantify and understand the role of increased CO2 in soil carbon cycling in agroecosystems.

Dynamic behavior of gas-phase NCl3 and CO2 in indoor pool facilities

The presence of gas-phase trichloramine (NCl3) in indoor swimming pool facilities is associated with adverse health outcomes among swimmers and other pool users, as well as degradation of pool infrastructure. Given their similarities in terms of volatility (i.e., Henry's law constant), the dynamic behaviors of NCl3 and carbon dioxide (CO2) in the air above indoor pools were postulated to be similar. Experiments were conducted to characterize and quantify the dynamic behavior of gas-phase NCl3 and CO2 in a mechanically ventilated swimming pool facility. The results of these time-course measurements allowed for examination of the effects of background water circulation and swimmer-induced mixing on the dynamics of both compounds in the air space above an indoor swimming pool. Measured gas-phase NCl3 concentrations exceeded the suggested guideline values of 300 μg/m3 or 500 μg/m3 during periods of heavy use. Measured gas-phase CO2 concentration followed a similar dynamic pattern as gas-phase NCl3; gas-phase CO2 concentrations often exceeded 1000 ppmv during swimming meets. Mass balance models for gas-phase NCl3 and CO2 were developed to relate the characteristics of the indoor pool environment to their dynamics. The results of these modeling efforts indicated that the similarity of CO2 transfer behavior to NCl3 may allow the use of CO2, which can be measured with low-cost infrared gas sensors, as a control parameter for NCl3. Moreover, the models that were developed to describe the dynamic behaviors of these volatile compounds may serve as tools for pool design, optimization of pool operations, and control of their mechanical ventilation systems.

Experimental and numerical study on the kinetics of CO2–N2 clathrate hydrates formation in silica gel column with dodecyltrimethylammonium chloride for effective carbon capture

The kinetics of carbon dioxide (CO2) and nitrogen (N2) gas mixtures hydrates formation in silica gel (SG) columns of varied pore sizes are studied both experimentally and numerically. Dodecyltrimethylammonium chloride (DTAC) solution with concentrations of 0–0.6 wt% was used to speed up the hydrate formation and improve the CO2 separation performance. A comprehensive shrinking-core model was established to reveal the roles of CO2 and N2 molecules diffusion and reaction in hydrate formation, and to investigate the effects of DTAC on kinetics. The experimental results show that the induction time is much shorter in larger pores of SGs, and is reduced remarkably with the addition of DTAC. The CO2 gas uptakes increase remarkably in the initial 200–270 min but change little later. The final CO2 concentration in gas phase decreased from 70 mol% to 32.6 mol% in the presence of DTAC, showing the CO2 separation factor raised by 36.6%–77.5%. However, N2 is also involved in the hydrate phase. The simulation results show that the combined reaction rate constants of both CO2 and N2 increase with the addition of DTAC. The lower combined rate constant in smaller pores indicates that the diffusion rate constant changes with pore size more strongly than the “reaction” rate constant. The dramatic decreasing trend of the effective diffusion coefficient reflects the higher porosity of the hydrate shell with more DTAC added.

A rapid, low-cost flocculation technology for enhanced microalgae harvesting

A rapid, low-cost, scalable flocculation technology, using either potash alum (for culture pH < 8.5), or potash alum with hydrochloric acid (for culture pH > 8.5), is employed to harvest algal biomass in less than thirty minutes. The gaseous CO2 (0.04–5%) and acetic acid glacial added during mixotrophic cultivation produce hydrogen and acetate ions that react with hydrolysed potash alum to form charged species capable of electrostatic interactions with the charged microalgal cell surface, leading to flocculation and gravity settling. The post-cultivation and post-harvesting pH, as well as the biomass settling time, decrease exponentially with increasing inlet gas-phase CO2 concentration. The biomass settles from 10 L to 2 L in 13–29 min, with a harvesting efficiency of 95–98.7%, for Tris, ammonium chloride, and urea-mediated algal growth. Rapid chemical flocculation with multiple recycling reduces the harvesting cost to 4.48–30.77 INR (0.06$-0.42$) per kg of dry algal biomass, making it suitable for commercialization.

How Does Gas-Phase CO2 Evolve in the Headspace of Champagne Glasses?

The chemical space perceived by a consumer of champagne or other sparkling wines is progressively modified all along tasting. Real-time monitoring of gas-phase CO2 concentration was performed, through a CO2-diode laser sensor, along a two-dimensional array of nine points in the headspace of three types of glasses poured with champagne. Two original glasses with distinct headspace volumes were compared with the standard INAO tasting glass. For each of the three glass types, a kind of temperature-dependent CO2 fingerprint was revealed and discussed as a function of the glass geometry and headspace volume. Moreover, a simple model was developed, which considers the rate of decrease of the concentration of gas-phase CO2 in the headspace of a glass after the pouring process as being mainly ruled by natural air convection in ambient air. The timescale which controls the rate of decrease of gas-phase CO2 was found to highly depend on the ratio of the headspace volume to the open aperture of the glass.

Effect of Carbon Dioxide (CO2) concentration in the gas feed on Carbon Dioxide Absorption Performance using Aqueous Potassium Carbonate promoted with Glycine

This paper presented the effect of CO2concentration in the gas feed to the CO2absorption performance using a green solvent, potassium carbonate promoted with glycine (PCGLY). Recently, the performance of this solvent (with precipitates) was hindered by its poor mass transfer performance due to the blockage in packings and piping. Therefore, this study focused to study the CO2 absorption performance of non-precipitating potassium carbonate promoted with glycine. This green solvent contains aqueous blend of 15wt% potassium carbonate and 3 wt% glycine. The absorption performance of the solvent was obtained by demonstrated a few experimental works using a bench scale packed absorption column. The packing type was Sulzer metal gauze and the column consisted of six sampling point which located equidistance along the packing.The effect of CO2 concentration at the gas feed was assessed in term of its CO2 removal efficiency and concentration profile along the packing. The study shows the decreasing trend of CO2 removal as CO2 inlet concentration in the gas feed increases. The reason of this behavior is due to the limited reactant of liquid phase to absorb high CO2concentration in gas phase. The main outcome of this study demonstrated the efficient absorption which can absorb up to 79.24 % of CO2from natural gas using non-precipitated PCGLY.

Studies on power plant algae: assessment of growth kinetics and bio-char production from slow pyrolysis process

ABSTRACT Algae can simultaneously be used for bio-capture of CO2, generation of biofuels and biochar. In the present study, the kinetics of autotrophic growth of a power-plant blue–green algae Leptolyngbya subtilis JUCHE1 have been determined along with the assessment of capability of lipid production. The gas phase concentrations of CO2 have been varied in the range of 5–20% (v/v). Illuminated 250 mL Erlenmeyer flasks have been used as small scale photobioreactors. The temperature and light intensity have been maintained at 37oC and 2.5 kLux respectively. Substrate inhibition has been observed beyond 15% CO2 concentration in gas phase. Haldane model explains the growth kinetics. The values of growth kinetic parameters namely μmax, Ks, KI have been determined to be 0.6064d−1, 0.0165 and 1.7429 g/L, respectively. The algal oil has been extracted using a mixture of chloroform and methanol 2:1(v/v) and the dry algal biomass has been pyrolysed in an isothermal semi-batch reactor in the temperature range of 300oC–700°C. The lipid content varies in the range of 4.0–12.5% (wt./wt.) and is strongly influenced by the concentration of CO2 and the culture period. The yield of bio-char increases from 29 to 44% with the decrease of pyrolysis temperature from 700°C to 300°C.

A first step towards the mapping of gas-phase CO2 in the headspace of champagne glasses

Key upgrades to a previously developed CO2-Diode Laser Sensor (CO2-DLS) were done to monitor as accurately as possible the level of gas-phase CO2 in champagne glasses. The upgraded CO2-DLS allows to access a wide range of CO2 concentrations (10–100%). An additional set of mirrors with two pairs of galvanometer scanners was developed in order to provide simultaneous measurements of gas-phase CO2 concentrations in the whole glass headspace. During the five minutes after pouring, the CO2-DLS unambiguously unveiled horizontal homogeneity in the glass headspace in terms of gas-phase CO2 concentrations. Nevertheless, a strong vertical gradient of gas-phase CO2 was highlighted, with ever increasing CO2 concentrations ranging from approximately 15% to 50% while moving away from the glass edge to the champagne interface. This strong vertical gradient was found to persist during the five next minutes following the pouring process, but with ever decreasing CO2 concentrations as time elapses.

Chlorella vulgaris cultivation in airlift photobioreactor with transparent draft tube: effect of hydrodynamics, light and carbon dioxide on biochemical profile particularly ω-6/ω-3 fatty acid ratio

Chlorella vulgaris is used for food and feed applications due to its nutraceutical, antioxidant and anticancer properties. An airlift photobioreactor comprising transparent draft tube was used for C. vulgaris cultivation. The effect of reactor parameters like hydrodynamics (0.3-1.5vvm), light intensity (85-400μmolm-2s-1), photoperiod (12-24h) and gas-phase carbon dioxide (CO2) concentration (5-15% v/v) were evaluated on microalgae and associated bacterial growth, biochemical profile; with special emphasis on ω-3, ω-6 fatty acids, and vitamin B12. The optimal growth of C. vulgaris without CO2 supplementation was observed at 1.2vvm, which was associated with higher algal productivity, chlorophyll, vitamin B12 content, and bacterial load along with 72% of nitrate removal. The higher light intensity (400μmolm-2s-1) and photoperiod (24:0) increased biomass productivity and ω-3 fatty acid content (in lipid) up to 2-3 fold. The elevated levels of gas-phase CO2 concentration (15% v/v) enhanced EPA content up to 7% and biomass productivity up to 171mgL-1day-1. However, the increase in CO2 concentration lowered vitamin B12 content (up to 30%) and bacterial load (2-3 log). Also, all the cultivation conditions favoured desirable ω-6/ω-3 ratio(in the range of 1-2).

Catalyst performance and experimental validation of a rigorous desorber model for low temperature catalyst-aided desorption of CO2 in single and blended amine solutions

In this study, experimental results of an integrated CO2 capture pilot plant utilizing blended MEA-MDEA solution and two solid acid catalysts (γ-Al2O3 and HZSM-5) in the desorber are presented, and a model developed in-house was validated against the experimental data. The model showed good agreement with the pilot plant data with an absolute average deviation (AAD) of 7.7% for CO2 production rates and could predict well the temperature profiles in the column. The model which has been previously validated against experimental data utilizing MEA and the two solid acid catalysts, was used to predict the performance (in terms of their contribution to the overall reaction rates) of the solid acid catalysts in both solvents (single and blended amine solutions). The performance results showed that the catalysts contributed more to the process as the temperature and amount of catalyst increased with HZSM-5 yielding as high as 95% increase to the overall rate of reaction in the single amine system. The results of the performance of the catalysts in the blended solvent were quite low compared to those of the single amine solution. It was observed that the predicted gas phase CO2 concentration profiles in the desorber, which was not readily available experimentally, was quite high and undesired. Thus, further steps should be taken to address this problem.

Isotopic evidence of passive mineral carbonation in mine wastes from the Dumont Nickel Project (Abitibi, Quebec)

Natural weathering of ultramafic rocks in mine tailings captures atmospheric CO2 through the formation of magnesium carbonates. The Dumont Nickel Project (DNP) is of particular interest as it will generate 1.7 Gt of ultramafic residues. A field experiment has been conducted at the DNP site in order to understand the process of natural CO2 sequestration. Two experimental cells were built using waste rock and mineral processing tailings and were instrumented with gas sampling ports and probes to monitor water saturation and suction. A decrease of the interstitial gas-phase CO2 concentration in both cells, from atmospheric values (∼390ppmv) near the surface to ∼100ppmv near the bottom, reflects active CO2 consumption by the residues. The total carbon content of the weathered DNP mine waste ranges from 0.2wt% to 6.5wt%C. Hydrotalcites supergroup minerals (pyroaurite-3R, brugnatellite, pyroaurite 2-H), aragonite, nesquehonite, dypingite and hydromagnesite were absent from the unweathered residues and precipitated in the cells during passive mineral carbonation. In situ measurements using Wavelength-Scanned Cavity Ring Down Spectroscopy reveal an increase of δ13C(air) from −8‰ near the surface of the cells to ∼2‰ near the bottom that is correlated with the decrease in CO2 concentration. This trend is explained by kinetic carbon isotope fractionation during dissolution of atmospheric CO2 in interstitial water (ΔDIC-CO2=−11.2‰). Secondary carbonates, precipitated from the interstitial water, are characterized by a moderately high δ18O and low δ13C. These isotopic compositions of the carbonates are consistent with precipitation in an evaporative environment where the kinetic carbon fractionation during atmospheric CO2 dissolution produces interstitial water depleted in 13C. Moreover, isotopic compositions of hydrotalcite supergroup minerals and other carbonate minerals are consistent with modern precipitation from the weathering of mining residue. These observations demonstrate the atmospheric source for the sequestered CO2 and help constrain a conceptual model of the carbonation reaction in the residues.

Field evidence of CO2 sequestration by mineral carbonation in ultramafic milling wastes, Thetford Mines, Canada

Two experimental small-scale cells have been constructed in the field to better understand passive mineral carbonation under natural atmospheric conditions in the ultramafic milling wastes of Thetford Mines (Québec, Canada). The magnesium-rich milling wastes mainly consist of poorly sorted grains and fibers of lizardite and chrysotile, with smaller amounts of antigorite, brucite and magnetite. These wastes could serve as significant and long-term CO2 sinks but the mechanisms and rates of mineral carbonation are not well understood. In this paper, the design of the experimental cells along with observations on gas composition, gas pressure, soil temperature, volumetric water content and mineral composition are presented with the objective to better understand the mineral carbonation processes under natural conditions and to propose a conceptual model for mineral carbonation at the field cell scale. Low CO2 concentrations (5–50ppm) measured in the experimental cells and the presence of hydromagnesite suggest that natural and passive mineral carbonation is a significant process occurring in the magnesium-rich wastes (4kg/m3/year of sequestrated CO2). In the proposed conceptual model, atmospheric CO2 (∼400ppm) dissolves in the hygroscopic water of the piles, where weathering of magnesium silicates forms magnesium carbonates. Water saturation in the cells is relatively stable over time and varies between 0.4 and 0.65, which is higher than optimal saturation values proposed in the literature, reducing CO2 transport in the unsaturated zone. Gas-phase CO2 concentrations along with gas flow rate measurements in the cells suggest that the reaction is most active close to the surface and that diffusion of CO2 is the dominant transport mechanism in the wastes. Although the carbonation reaction is exothermic, no evidence of thermal convection has been observed in the experimental cells. This conceptual model will serve to support a reactive transport model in order to quantify and optimize the amount of CO2 that can be captured in chrysotile milling waste piles.

Investigating dynamic processes in a porous substrate biofilm photobioreactor — A modeling approach

In the present study, a one dimensional kinetic model was developed for a porous substrate biofilm photobioreactor (PSBR), a biofilm bioreactor for the production of microalgae and other biotechnological applications. Light transfer was modeled with a radiative transfer equation (RTE), considering absorption, scattering and biomass pigment adaptation. Dissolved chemical species were modeled using mass balance equations (partial differential equations), with terms describing diffusion, convection, biomass growth, biomass consumption, and chemical conversions. pH was modeled as a state variable, and a novel approach of modeling biomass increase in a phototrophic biofilm was introduced. The model was solved using numerical methods and model parameters were acquired either from literature or from experimental work carried out in the framework of this study. The proposed mode was applied to simulated gradients of light, dissolved oxygen concentration, pH, etc. in a PSBR biofilm. The simulated results were compared with experimental data acquired in previous studies.Our results show that the proposed model can accurately predict light intensity distribution in the modeled PSBR biofilm, provided the optical properties of the biomass were measured experimentally. The prediction of the concentration of dissolved oxygen and pH profiles also reflected experimental data with high fidelity. The results also strongly suggest that facilitated CO2 transfer due to the presence of extracellular carbonic anhydrase in the biofilm matrix plays an important role in DIC transport in the modeled PSBR biofilm at low gas phase CO2 concentration; and the pH gradient along the depth gradient of the modeled biofilm was mainly caused by the uptake of dissolved inorganic carbon. In addition, the simulation can predict biomass growth in the modeled PSBR with minimal error, thus, with minor modifications, the model can also be applied to predict biomass growth in larger-scale PSBRs.

Biosequestration of CO2 using power plant algae (Rhizoclonium hieroglyphicum JUCHE2) in a Flat Plate Photobio-Bubble-Reactor – Experimental and modeling

Biosequestration of CO2 in a 1.8L Flat Plate Photobio-Bubble-Reactor using power plant algae (Rhizoclonium hieroglyphicum JUCHE2) has been studied. CO2 concentration and light intensity have been varied in the range of 5–25% and 82μmolm−2s−1–398.71μmolm−2s−1, respectively. Dependence of initial specific growth rate of algae has been explained by Haldane type kinetics and by Langmuir–Hinshelwood type kinetics including shape factor with respect to liquid phase CO2 concentration and the intensity of light, respectively. Half saturation constants for CO2 (=Ks) and light (=KE), inhibition constant for CO2 (=KI) and maximum specific growth rate (=μmax) have been determined, respectively as 1.25×10−7Mol/L, 120μmolm−2s−1, 0.00165Mol/L and 0.996d−1. A mathematical model of the Flat Plate Photobio-Bubble-Reactor has been developed incorporating effects of phenomena like gas to liquid mass transfer of CO2, stripping of dissolved CO2 to overhead gas space of the reactor, CO2 uptake by algal biomass. Population dynamics of long term growth of algal biomass have been incorporated by using a Gaussian time function. The model has been validated up to gas phase CO2 concentration of 25% through successful comparison of predicted data with corresponding experimental results.

Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation

Capacitive technologies, such as capacitive deionization and energy harvesting based on mixing energy (“capmix” and “CO2 energy”), are characterized by intermittent operation: phases of ion electrosorption from the water are followed by system regeneration. From a system application point of view, continuous operation has many advantages, to optimize performance, to simplify system operation, and ultimately to lower costs. In our study, we investigate as a step towards second generation capacitive technologies the potential of continuous operation of capacitive deionization and energy harvesting devices, enabled by carbon flow electrodes using a suspension based on conventional activated carbon powders. We show how the water residence time and mass loading of carbon in the suspension influence system performance. The efficiency and kinetics of the continuous salt removal process can be improved by optimizing device operation, without using less common or highly elaborate novel materials. We demonstrate, for the first time, continuous energy generation via capacitive mixing technology using differences in water salinity, and differences in gas phase CO2 concentration. Using a novel design of cylindrical ion exchange membranes serving as flow channels, we continuously extract energy from available concentration differences that otherwise would remain unused. These results may contribute to establishing a sustainable energy strategy when implementing energy extraction for sources such as CO2-emissions from power plants based on fossil fuels.

Evolution of Vehicle Exhaust Particles in the Atmosphere

Aerosol mass spectrometer (AMS) measurements are used to characterize the evolution of exhaust particulate matter (PM) properties near and downwind of vehicle sources. The AMS provides time-resolved chemically speciated mass loadings and mass-weighted size distributions of nonrefractory PM smaller than 1 μm (NRPM1). Source measurements of aircraft PM show that black carbon particles inhibit nucleation by serving as condensation sinks for the volatile and semi-volatile exhaust gases. Real-world source measurements of ground vehicle PM are obtained by deploying an AMS aboard a mobile laboratory. Characteristic features of the exhaust PM chemical composition and size distribution are discussed. PM mass and number concentrations are used with above-background gas-phase carbon dioxide (CO2) concentrations to calculate on-road emission factors for individual vehicles. Highly variable ratios between particle number and mass concentrations are observed for individual vehicles. NRPM1 mass emission factors measured for on-road diesel vehicles are approximately 50% lower than those from dynamometer studies. Factor analysis of AMS data (FA-AMS) is applied for the first time to map variations in exhaust PM mass downwind of a highway. In this study, above-background vehicle PM concentrations are highest close to the highway and decrease by a factor of 2 by 200 m away from the highway. Comparison with the gas-phase CO2 concentrations indicates that these vehicle PM mass gradients are largely driven by dilution. Secondary aerosol species do not show a similar gradient in absolute mass concentrations; thus, their relative contribution to total ambient PM mass concentrations increases as a function of distance from the highway. FA-AMS of single particle and ensemble data at an urban receptor site shows that condensation of these secondary aerosol species onto vehicle exhaust particles results in spatial and temporal evolution of the size and composition of vehicle exhaust PM on urban and regional scales.

CO2 Capture Using Activated Amino Acid Salt Solutions in a Membrane Contactor

An activated solution based on amino acid salt was proposed as a CO2 absorbent. Piperazine (PZ) was selected as an activating agent and added into the aqueous glycine salt to form the activated solution. A coupling process, which associated the activated solution with a PP hollow fiber membrane contactor, was set up. An experimental and theoretical analysis for CO2 capture was performed. The performances of CO2 capture by the coupling process were evaluated using the PZ activated solution and the non-activated glycine salt solution. A numerical model for the simulation of the hollow fiber membrane gas–liquid mass transfer was developed. Typical parameters such as outlet gas phase CO2 concentration, capture efficiency, and mass transfer coefficient for the activated solution were determined experimentally. The effects of operation temperature and liquid CO2-loading on mass transfer coefficient and capture efficiency were discussed in this work. Axial and radial concentration profiles of CO2 in the fiber lumen and mass transfer flux were simulated by the model. Results show that the performances of the PZ activated glycine salt solution are evidently better than that of the non-activated glycine salt solution in the membrane contactor for CO2 capture. Elevation of the operation temperatures can enhance the overall mass transfer coefficient. The activated solution can maintain higher capture efficiency especially in the case of high CO2-loadings. The gas phase CO2 concentration with the activated solution is lower than that with the non-activated solution whether along axial or radial distances in the fiber lumen. The model simulation is validated with experimental data.

The Electro-Reduction of Carbon Dioxide in a Continuous Reactor

This paper reports an investigation into the electro-reduction of CO2 in a laboratory bench-scale continuous reactor with co-current flow of reactant gas and catholyte liquid through a flow-by 3D cathode of 30# mesh tinned-copper. Factorial and parametric experiments were carried out in this apparatus with the variables: current (1–8 A), gas phase CO2 concentration (16–100 vol%) and operating time (10–180 min), using a cathode feed of [CO2 + N2] gas and 0.45 m KHCO3(aq) with an anolyte feed of 1 m KOH(aq), in operation near ambient conditions (ca. 115 kPa(abs), 300 K). The primary and secondary reactions here were respectively the reduction of CO2 to formate (HCOO−) and of water to hydrogen, while up to ca. 5% of the current went to production of CO, CH4 and C2H4. The current efficiency for formate depended on the current density and CO2 pressure, coupled with the hydrogen over-potential plus mass transfer capacity of the cathode, and decreased with operating time, as tin was lost from the cathode surface. For superficial current densities ranging from 0.22 to 1.78 kA m−2, the measured values of the performance indicators are: current efficiency for HCOO− = 86–13%, reactor voltage = 3–6 Volt, specific energy for HCOO− = 300–1300 kWh kmol−1, space-time yield of HCOO− = 2 × 10−4–6 × 10−4 kmol m−3 s−1, conversion of CO2 = 20–80% and yield of organic products from CO2 = 6–17%.

Effects of operating and design parameters on CO2 absorption in columns with structured packings

This paper highlights the importance of operating and design parameters to mass-transfer in CO2 absorption in columns with structured packings. The study compares the performance of four stainless steel packings, Gempak 4A, Mellapak 500Y, Mellapak 500X, and Optiflow. Mass-transfer efficiency of these packings was determined by carrying out absorption experiments in a pilot-scale absorption unit with aqueous solutions of sodium hydroxide (NaOH) and monoethanolamine (MEA) as test liquid. The overall mass-transfer coefficients (KGae) were calculated from experimentally determined gas-phase CO2 concentration profiles within the test columns. The results of this study indicate that mass-transfer coefficient of these structured packings tested varies significantly not only with operating parameters such as liquid load, liquid CO2 loading and liquid temperature but also with design parameters, i.e. the pattern of packing arrangement and initial liquid distribution.