Factors affecting the phase and morphology of CaCO3 prepared by a bubbling method

Precipitated calcium carbonate (PCC) was prepared by bubbling a CO₂/N₂ mixed gas into a CaCl₂ solution. The influence of preparation conditions on the phase and morphology of PCC was discussed with the help of XRD and SEM measurements. The results showed that the initial CaCl₂ concentration, flow rate and temperature play an important role on the morphology of PCC. At low initial CaCl₂ concentration or high flow rate, spherical vaterite was preferably formed. Otherwise, the rhombic calcite was ready to form. Temperature is a determining factor on the formation of aragonite. Needle-like aragonite was precipitated at 60 °C. The results also indicated that both the bubbling time and stirring rate have a minor effect on the phase and morphology of PCC.     

Modeling and parametric analysis of hollow fiber membrane system for carbon capture from multicomponent flue gas

The modeling and optimal design/operation of gas membranes for postcombustion carbon capture (PCC) is presented. A systematic methodology is presented for analysis of membrane systems considering multicomponent flue gas with CO₂ as target component. Simplifying assumptions is avoided by namely multicomponent flue gas represented by CO₂/N₂ binary mixture or considering the co/countercurrent flow pattern of hollow‐fiber membrane system as mixed flow. Optimal regions of flue gas pressures and membrane area were found within which a technoeconomical process system design could be carried out. High selectivity was found to not necessarily have notable impact on PCC membrane performance, rather, a medium selectivity combined with medium or high permeance could be more advantageous. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

CO2 Capture Using Monoethanolamine in a Bubble‐Column Scrubber

A continuous bubble‐column scrubber, capturing CO₂ gas by monoethanolamine (MEA) solution in a pH‐stat operation, is used to search for optimum process parameters by means of the Taguchi method. The process variables are the pH of the solution, gas flow rate, concentration of CO₂ gas, and temperature. From the measured outlet CO₂ gas concentrations, the absorption rate and overall mass transfer coefficient can be determined with the support of a steady‐state material balance equation as well as a two‐film model. According to the signal‐to‐noise ratio, the significance sequence influencing the parameters and optimum conditions can be determined. CO₂ concentration and pH value proved to be decisive parameters, while temperature and gas flow rate were minor. Five sets of optimum conditions were obtained and could be further verified by empirical equations.     

Experiment and mathematical modeling of a bench-scale circulating fluidized bed gasifier

The gasification of two different coals and chars with CO₂ and CO₂/O₂ mixture in a 48-mm-i.d. circulating fluidized bed (CFB) gasifier is investigated. The effects of operation condition on gas composition, carbon conversion and gasification efficiency were studied. A simple CFB coal gasification district mathematical model has been set up. The effects of coal type and CFB operating conditions on CFB coal gasification are discussed based on the CFB gasification test and model simulation. The main operation parameters in CFB gasification system are coal type, gas superficial velocity, circulating rate of solids and reaction temperature. It is found that CO concentration and carbon conversion increase with increasing solids circulating rate and decreasing gas velocity due to the increase in gas residence time and solids holdup in the CFB. The carbon conversion increases with increasing temperature and O₂ concentration in the inlet gas. The experimental results prove that the CFB gasifier works well for high volatile, high reactivity coal.     

Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

In this article, the industrial process of CO₂ capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO₂ loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO₂ concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO₂ absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO₂ mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO₂ absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.     

Flue gas desulfurization by citrate process and optimization of working parameters

In this study, model flue gas was bubbled into 0.25 L tribasic sodium citrate (TSC) solution being in 0.5 L glass absorber to remove its SO₂ content. Size of gas bubbles, absorption temperature, gas flow rate, solution concentration and stirring rate were taken as working parameters to investigate their effect on SO₂ removal from flue gas. The Taguchi's experimental design method was used to obtain optimum values of working parameters for SO₂ saturation time of the TSC solution selected as a quality characteristic. The optimum levels of parameters to maximize the SO₂ saturation time of TSC solution were coarse bubbles for gas delivery, 35 °C for absorption temperature, 1.5 slm for gas flow rate, 0.5 M for TSC solution concentration and 500 rpm for stirring rate. Under these conditions, the SO₂ saturation time of the TSC solution was achieved as 511 min in average. The most effective parameters on the absorption of SO₂ in TSC solutions were ranked to the least as solution concentration, gas flow rate, size of gas bubbles, absorption temperature and stirring rate.     

The effect of carbon dioxide during the desulfurization of flue gas with Mardin–Mazıdagı phosphate rock

The effects of temperature, CO₂ concentration and particle size on simultaneous calcination/sulfation of Mardin–Maz?dag? phosphate rock in fluidized-bed reactor were investigated. For this, a raw sample was exposed to calcination and sulfation processes in a fluidized-bed reactor to determine the effects of parameters by using a model gas mixture similar to the flue gas composition. The calcination ratio increased with increasing temperature and decreasing particle size, but decreased with increasing CO₂ concentration. In sulfation process, however, sulphate conversion ratio increased with increasing CO₂ ratio and decreased with decreasing particle size. The sulfation reaction is well represented by the shrinking core model and can be divided into two regions with different rate controlling step. For low conversions, the controlling step was found to be chemical reaction at the interface, but the diffusion through the product layer for high conversion. The activation energies for the chemical reaction at the interface and diffusion through the product layer cases were calculated as 100 and 296 kJ mol^?1, respectively.     

Pilot plant study of post-combustion carbon dioxide capture by reactive absorption: Methodology, comparison of different structured packings, and comprehensive results for monoethanolamine

Post-combustion carbon capture (PCC) from fossil fuel power plants by reactive absorption can substantially contribute to reduce emissions of the greenhouse gas CO₂. To test new solvents for this purpose small pilot plants are used. The present paper describes results of comprehensive studies of the standard PCC solvent MEA (0.3 g/g monoethanolamine in water) in a pilot plant in which the closed cycle of absorption/desorption process is continuously operated (column diameters: 0.125 m, absorber/desorber packing height: 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow: 30-110 kg/h, CO₂ partial pressure: 35-135 mbar). The data establish a base line for comparisons with new solvents tested in the pilot plant and can be used for a validation of models of the PCC process with MEA. The ratio of the solvent to the flue gas mass flow is systematically varied at constant CO₂ removal rate, and CO₂ partial pressure in the flue gas. Optimal operating points are determined. In the present study the structured packing Sulzer BX 500 is used. The experiments with the removal rate variation are carried out so that the results can directly be compared to those from a previous study in the same plant that was carried out using Sulzer Mellapak 250.Y. A strategy for identifying the influence of absorption kinetics on the results is proposed, which is based on a variation of the gas load at a constant L/G ratio and provides valuable insight on the transferability of pilot plant results.     

CO2 mass transfer and conversion to biomass in a horizontal gas–liquid photobioreactor

This study deals with CO₂ mass transfers and biomass conversion in an industrial horizontal tubular photobioreactor. An analytical approach is used to determine an expression modeling the influence of CO₂ mass transfers on the overall biomass conversion efficiency for a given culture broth, heat and light conditions. Fluid mechanics and mass transfer are predicted with a classical two-phase flow approach (Taitel and Dukler, 1976) combined with a dissolution correlation developed and tested in the laboratory (Valiorgue et al., 2011). The influence of the stripping gas, removing the excess of oxygen in the liquid, on the conversion to biomass efficiency is shown to be not negligible. The expression is used to evaluate how the photobioreactor's design and process parameters can be tuned in order to improve biomass conversion efficiency. The biomass conversion efficiency evolution with the photobioreactor's length was found to behave asymptotically and it was explained by the relative orders of magnitude of gas dissolution and gas stripping. It has been shown that the gas flow rate for stripping and therefore the oxygen removal will be limited when further increasing the industrial photobioreactor's length for a given objective of CO₂ conversion to biomass efficiency.     

A Novel Process for Renewable Methane Production: Combining Direct Air Capture by K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/Alumina Sorbent with CO<Subscript>2</Subscript> Methanation over Ru/Alumina Catalyst

CO₂ methanation over supported ruthenium catalysts is considered to be a promising process for carbon capture and utilization and power-to-gas technologies. In this work 4% Ru/Al₂O₃ catalyst was synthesized by impregnation of the support with an aqueous solution of Ru(OH)Cl₃, followed by liquid phase reduction using NaBH₄ and gas phase activation using the stoichiometric mixture of CO₂ and H₂ (1:4). Kinetics of CO₂ methanation reaction over the Ru/Al₂O₃ catalyst was studied in a perfectly mixed reactor at temperatures from 200 to 300 °C. The results showed that dependence of the specific activity of the catalyst on temperature followed the Arrhenius law. CO₂ conversion to methane was shown to depend on temperature, water vapor pressure and CO₂:H₂ ratio in the gas mixture. The Ru/Al₂O₃ catalyst was later tested together with the K₂CO₃/Al₂O₃ composite sorbent in the novel direct air capture/methanation process, which combined in one reactor consecutive steps of CO₂ adsorption from the air at room temperature and CO₂ desorption/methanation in H₂ flow at 300 or 350 °C. It was demonstrated that the amount of desorbed CO₂ was practically the same for both temperatures used, while the total conversion of carbon dioxide to methane was 94.2–94.6% at 300 °C and 96.1–96.5% at 350 °C.     

Energy Minimization in Cryogenic Packed Beds during Purification of Natural Gas with High CO2 Content

Minimization of energy consumption was explored for countercurrent switched cryogenic packed beds in which separation of CO₂ and other components of natural gas can be achieved based on differences in freezing or desublimation points. Highly pure CO₂ and methane were obtained after separation. An experimental setup for CO₂ removal from natural gas was constructed and a detailed experimental study was conducted by changing different operating parameters. Compared to other cocurrent or jacket‐cooled constant‐temperature configurations, countercurrent switched beds provided optimal separation and energy efficiencies. The effects of important process parameters like initial temperature profiles of the cryogenic bed, feed composition, and feed flow rate on energy requirement, bed saturation, bed pressure, and cycling times were investigated. The energy requirement for cryogenic packed beds was compared with the conventional cryogenic distillation process.     

非平衡碘负离子转化二氧化碳

以直流脉冲负高压电晕放电形式,通过加入电子亲和能较高的碘气,在完全电负性离子体条件下资源化处理CO₂。考察了进气流量、高压放电频率和原料摩尔比对CO₂转化率的影响。结果表明:70℃时,利用碘负离子成功将CO₂还原生成了CI₄,其产率随着CO₂流量的增加而减少。当进气流量0.06 L·min^-1、放电频率9.608 kHz,n(I₂)/n(CO₂)为2.5时,CO₂转化率在碘负离子作用下达到88.71%。另外,对碘负离子和CO₂还原反应机理进行了初步探讨。     

A model on an entrained bed-bubbling bed process for CO2 capture from flue gas

A simplified model has been developed to investigate effects of important operating parameters on performance of an entrained-bed absorber and bubbling-bed regenerator system collecting CO₂ from flue gas. The particle population balance was considered together with chemical reaction to determine the extent of conversion in both absorber and regenerator. The calculated CO₂ capture efficiency agreed with the measured value reasonably well. Effects of absorber parameters — temperature, gas velocity, static bed height, moisture content of feed gas on CO₂ capture efficiency — have been investigated in a laboratory scale process. The CO₂ capture efficiency decreased as temperature or gas velocity increased. However, it increased with static bed height or moisture concentration. The CO₂ capture efficiency was exponentially proportional to each parameter. Based on the absolute value of exponent of the parameter, the effect of gas velocity, static bed height, and moisture content was one-half, one-third, and one-fourth as strong as that of temperature, respectively.     

HIGH-TEMPERATURE SOLAR CHEMICAL REACTORS FOR HYDROGEN PRODUCTION FROM NATURAL GAS CRACKING

This study deals with the thermal cracking of natural gas for the coproduction of hydrogen and carbon black from concentrated solar energy without CO₂ emission. A laboratory-scale solar reactor (1 kW) was tested and modeled successfully. It consists of a tubular graphite receiver directly absorbing solar radiation, in which a mixture of Ar and CH₄ flows. A temperature increase or a gas flow rate decrease results in chemical conversion increase. Methane conversion higher than 75% was obtained. Reaction occurred near the wall where temperature is maximal and gas velocity is minimal due to the laminar flow profile. The work focused also on the design of a medium-scale tubular solar reactor (10 kW) based on the indirect heating concept. A reactor model including gas hydrodynamics and heat and mass transfers coupled to the chemical reaction was developed in order to predict the reactor performances. Temperature and species concentration profiles and final chemical conversion were quantified. According to the results, temperature was uniform in the tubular reaction zone and the predicted chemical conversion was 65%, neglecting the catalytic effect of carbon particles.     

Reaction kinetics of the CO2 reforming of methane

The kinetics of the CO₂ reforming of methane was investigated in the temperature range 700–850°C at normal pressure with a 1:1 mixture of CH₄ and CO₂ on Ir/Al₂O₃ catalysts. The feed composition was kept constant to avoid a change in mechanism associated with composition changes. Various rate models were fitted to the experimental data by numerically integrating the rate equations. All rate models included the reverse water gas shift reaction as the most important side reaction at these reaction conditions. The best agreement was obtained with a rate model based on the stepwise mechanism, where in the rate-determining step methane is decomposed to hydrogen and active carbon followed by the direct and fast conversion of this active carbon with CO₂ to 2 CO. This model is also the first and only one containing a complete subset of reactions necessary to describe the network of reactions known to occur at these reaction conditions. Comparable fit quality was obtained with a simple first order model and with a model based on a Langmuir-Hinshelwood rate expression, where the latter provided physically meaningless parameters. Values of the reaction parameters are given for the 5 best rate models studied.     

Reduction of amine mist emissions from a pilot-scale CO2 capture process using charged colloidal gas aphrons

In the CO₂ capture process from coal-derived flue gas where amine solvents are used, the flue gas can entrain small liquid droplets into the gas stream leading to emission of the amine solvent. The entrained drops, or mist, will lead to high solvent losses and cause decreased CO₂ capture performance. In order to reduce the emissions of the fine amine droplets from CO₂ absorber, a novel method using charged colloidal gas aphron (CGA) generated by an anionic surfactant was developed. The CGA absorption process for MEA emission reduction was optimized by investigating the surfactant concentration, stirring speed of the CGA generator, and capture temperature. The results show a significant reduction of MEA emissions of over 50% in the flue gas stream exiting the absorber column of a pilot scale CO₂ capture unit.     

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO₂ ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO₂ into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed for simulating dynamic process of CO₂ ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO₂ absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO₂ gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO₂. The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO₂ concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO₂ into seawater in the CO₂ ocean disposal.     

A breakthrough technique for the preparation of high-yield precipitated calcium carbonate

Precipitated calcium carbonate (PCC) is conventionally produced through the gas-solid-liquid carbonation route, which consists on bubbling gaseous CO₂ through a concentrated calcium hydroxide (Ca(OH)₂) slurry. However, atmospheric carbonation processes are slow and have low carbonation efficiency. A novel technology based on the combination of supercritical carbon dioxide (scCO₂) and ultrasonic agitation is described here for the preparation of high-yield PCC. The combination of both techniques has demonstrated to produce outstanding improvement for the conversion of Ca(OH)₂ to the stable calcite polymorph of calcium carbonate (CaCO₃). These experiments were carried out at 313 K and 13 MPa using a high-pressure reactor immersed in an ultrasounds cleaner bath. The process kinetics and the characteristics of the precipitated particles using ultrasonic agitation were compared with those obtained under similar experimental conditions using mechanical stirring and non-agitated systems. The crystal characteristics of the samples obtained using the three different agitation techniques were characterized by X-ray diffraction and scanning electron microscopy.     

The deactivating effect of carbon dioxide on iron-oxide catalyst in the dehydrogenation of methylbutenes

Characteristics of the most energy-intensive second stage of the two-stage production of isoprene from isopentane, i.e., dehydrogenation of methylbutenes are studied to improve the technical and economic performance of the process. The effect of the carbon dioxide formed during self-regeneration of the ironoxide catalyst (according to the reaction C_coke + 2H₂O → 2H₂ + CO₂) on the conversion of methylbutenes and selectivity with respect to isoprene is investigated. It is found that the presence of CO₂ in the reaction batch has a considerable effect on the conversion of methylbutenes; when the content of CO₂ in the raw material feed is 1.5 wt %, conversion of methylbutenes is reduced by 5–6%. It is demonstrated that CO₂ reversibly deactivates the catalyst and the catalyst activity is restored when its influx is discontined (the yield of isoprene returns gradually to its original value). The recovery rate depends on the concentration and duration of exposure to carbon dioxide. Treatment of the catalyst by steam in the absence of the reaction mixture leads to rapid regeneration of the catalyst. It is concluded that measures to continuously monitor CO₂ in the contact gas during the first stage of dehydrogenation, and to select the optimum modes (temperature, steam/raw materials ratio, etc.) for reducing carbon residue during the operation of iron-oxide catalyst in order to implement the two-stage technology for the dehydrogenation of methylbutenes (the main goal of which is to raise the conversion of methylbutenes to 35–40%) are of special importance.     

Mass transfer performance and correlations for CO2 absorption into aqueous blended of DEEA/MEA in a random packed column

The mass transfer performance of CO₂ absorption into blended N,N‐diethylethanolamine (DEEA)/ethanolamine (MEA) solutions was investigated using a lab‐scale absorber (H = 1.28 m, D = 28 mm) packed with Dixon ring random packing. The mass transfer coefficient K_Ga_v, the unit volume absorption rate Φ, outlet concentration of CO₂ (y_CO2), and the bottom temperature T_bot of CO₂ in aqueous DEEA/MEA solutions were determined over the feed temperature range of 298.15–323.15 K, lean CO₂ loading of 0.15–0.31 mol/mol, over a wide range of liquid flow rate of 3.90–9.75 m³/m²‐h, by using inert gas flow rate of 26.11–39.17 kmol/m²‐h and 6–18 kPa CO₂ partial pressure. The results show that liquid feed temperature, lean CO₂ loading, liquid flow rate, and CO₂ partial pressure had significant effect on those parameters. However, the inert gas flow rate had little effect. To allow the mass transfer data to be really utilized, K_Ga_v and y_out correlations for the prediction of mass transfer performance were proposed and discussed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3048–3057, 2017