Pressure variation due to heat shock of CO2hydrate desserts

CO₂ hydrate desserts are carbonated frozen desserts in which the CO₂ is trapped in a crystalline water‐carbon dioxide structure called a CO₂ clathrate hydrate. The CO₂ concentration of the dessert enables strong perception of carbonation, but CO₂ hydrate dissociation during heat shock can cause high package pressures during storage and distribution. In this work, a model is developed for package pressure as a function of temperature, CO₂ content, package volume, dessert mass, and recipe. The model is validated by comparison with an experimental measurement of the pressure and mass of a CO₂ hydrate dessert subjected to heat shock. It is shown that during heat shock a sealed package can reach pressures greater than the ice‐CO₂ hydrate equilibrium pressure. At pressures above the ice‐CO₂ hydrate equilibrium pressure, the fraction of water crystallized in the dessert can be increased, potentially mitigating heat shock damage. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

Gas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the hydrate stoichiometry calculations are inaccurate. This study investigated the CO₂ hydrate formation process in porous media directly and quantitatively. The characteristics of the time-variable consumption of hydrate formation indicated a two-stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO₂ hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO₂·6.90H₂O, CO₂·6.70H₂O, and CO₂·6.49H₂O, respectively.     

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.     

Prediction of induction time for natural gas components during gas hydrate formation in the presence of kinetic hydrate inhibitors in a flow mini‐loop apparatus

The objective of this work is the prediction of induction time (t_i) for simple gas hydrate formation in the presence or absence of kinetic hydrate inhibitors at various conditions based on the Kashchiev and Firoozabadi model in a flow mini‐loop apparatus. For this purpose, the t_i model is developed for simple gas hydrate formation in batch system for natural gas components during hydrate formation in a flow mini‐loop apparatus. A laboratory flow mini‐loop apparatus is designed and built up to measure the t_i for simple gas hydrate formation when a hydrate former (such as C₁, C₃, CO₂ and i‐C₄) is contacted with water in the absence or presence of dissolved inhibitor, such as poly vinylpyrrolidone, PVCap and L ‐tyrosine. In each experiment, a water blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of the required gas make‐up. The average absolute deviation (AAD) of the predicted t_i values from the corresponding experimental data are found to be about 11% and 9.4% for gas hydrate formation t_i in the presence or absence of kinetic hydrate inhibitors, respectively. © 2012 Canadian Society for Chemical Engineering     

Conversion of carbohydrates into 5‐hydroxymethylfurfural in a green reaction system of CO2‐water‐isopropanol

In this work, a green reaction system of CO₂‐water‐isopropanol was developed for 5‐hydroxymethylfurfural (HMF) production. The conversion of fructose in a CO₂‐water system was first investigated, and the results showed this system could promote the formation of HMF compared to a pure water system. Then, isopropanol was introduced into the CO₂‐water system and the HMF formation became better because the solvent effect of isopropanol increased the tautomeric composition of fructofuranose, which was easy to form HMF. The existence of isopropanol was found to greatly suppress secondary reactions where HMF was converted to levulinic acid and insoluble humin. Meanwhile, the effects of reaction parameters on the conversion of fructose to HMF in the CO₂‐water‐isopropanol system were analyzed, and a high HMF yield of 67.14% was obtained. Finally, to further illustrate the merits of CO₂‐water‐isopropanol system, productions of HMF from other carbohydrates were tested and satisfactory yields were achieved. © 2016 American Institute of Chemical Engineers AIChE J, 63: 257–265, 2017     

Effects of Graphene Oxide Nanosheets and Al2O3 Nanoparticles on CO2 Uptake in Semi‐clathrate Hydrates

Gas hydrate/clathrate hydrate formation is an innovative method to trap CO₂ into hydrate cages under appropriate thermodynamic and/or kinetic conditions. Due to their excellent surface properties, nanoparticles can be utilized as hydrate kinetic promoters. Here, the kinetics of the CO₂ + tetra‐n‐butyl ammonium bromide (TBAB) semi‐clathrate hydrates system in the presence of two distinct nanofluid suspensions containing graphene oxide (GO) nanosheets and Al₂O₃ nanoparticles is evaluated. The results reveal that the kinetics of hydrate formation is inhibited by increasing the weight fraction of TBAB in aqueous solution. GO and Al₂O₃ are the most effective kinetic promoters for hydrates of (CO₂ + TBAB). Furthermore, the aqueous solutions of TBAB + GO or Al₂O₃ noticeably increase the storage capacity compared to TBAB aqueous solution systems.     

Application of Water‐Swollen Thin‐Film Composite Membrane in Flue Gas Purification

A water‐swollen thin‐film composite membrane, which was a reverse osmosis membrane with a thin polyamide layer, was used to separate a model mixture of N₂, CO₂, and SO₂. The polyamide swells with water, and thus, becomes more permeable to polar gases. The flue gas contains water vapor, which must be removed before it is subjected to SO₂ removal. Here moisture is employed to keep the membrane swollen. Using the model mixture, the humidified feed stream is brought to the membrane, where it is cooled below the dew point, so that water condenses on the membrane to keep the polyamide swollen. The membrane showed high CO₂ and SO₂ permeance, but low selectivity, so it could be applied to separate these two gases from N₂, and thus, is suitable for flue gas purification.     

Technico‐economical assessment of MFI‐type zeolite membranes for CO2 capture from postcombustion flue gases

A detailed survey of the effect of moisture on the CO₂/N₂ permeation and separation performance of Mobile Five (MFI) zeolite membranes in view of downstream postcombustion CO₂ capture applications in power plants and incinerators is presented. The membranes, displaying a nanocomposite architecture, have been prepared on α‐alumina tubes by pore‐plugging hydrothermal synthesis at 443 K for 89 h using a precursor clear solution with molar composition 1 SiO₂:0.45 tetrapropylammonium hydroxide:27.8 H₂O. The synthesized membranes present reasonable permeation and CO₂/N₂ separation properties even in the presence of high water concentrations in the gas stream. A critical discussion is also provided on the technico‐economical feasibility (i.e., CO₂ recovery, CO₂ purity in the permeate, module volume, and energy consumption) of a membrane cascade unit for CO₂ capture and liquefaction/supercritical storage from standard flue gases emitted from an incinerator. Our results suggest that the permeate pressure should be kept under primary vacuum to promote the CO₂ driving force within the membrane. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3183–3194, 2012     

Stabilized Performance of Al‐Decorated and Al/Mg Co‐Decorated Spray‐Dried CaO‐Based CO2 Sorbents

The sharp loss‐in‐capacity in CO₂ capture as a result of sintering is a major drawback for CaO‐based sorbents used in the calcium looping process. The decoration of inert supports effectively stabilizes the cyclic CO₂ capture performance of CaO‐based sorbents via sintering mitigation. A range of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were synthesized via an easily scaled‐up spray‐drying route. The decoration of Al‐based and Al/Mg‐based supports efficiently enhanced the cyclic CO₂ capture capability of CaO‐based sorbents under severe testing conditions. The CO₂ capture capacity losses of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were alleviated, representing more stable CO₂ capture performance. The stabilized CO₂ capture performance is mainly attributed to the formation of Ca₁₂Al₁₄O₃₃, MgAl₂O₄, and MgO that act as the skeleton structures to mitigate the sintering of CaCO₃ during carbonation/calcination cycles.     

Experimental study of CO2 hydrate formation in porous media with different particle sizes

To investigate the effect of the particle size of porous media on CO₂ hydrate formation, the formation experiments of CO₂ hydrate in porous media with three particle sizes were performed. Three kinds of porous media with mean particle diameters of 2.30 μm (clay level), 5.54 μm (silty sand level), and 229.90 μm (fine sand level) were used in the experiments. In the experiments, the formation temperature range was 277.15–281.15 K and the initial formation pressure range was 3.4–4.8 MPa. The final gas consumption increases with the increase in the initial pressure and the decrease in the formation temperature. The hydrate formation at the initial formation pressure of 4.8 MPa in 229.90 μm porous media is much slower than that at the lower formation pressure and displays multistage. In the experiments with different formation temperatures, the gas consumption rate at the temperature of 279.15 K is the lowest. In 2.30 and 5.54 μm porous media, the hydrate formation rates are similar and faster than those in 229.90 μm porous media. The particle size of the porous media does not affect the final gas consumption. The gas consumption rate per mol of water and the final water conversion increase with the decrease in the water content. The induction time in 5.54 μm porous media is longer than that in 2.30 and 229.90 μm porous media, and the presence of NaCl significantly increases the induction time and decreases the final conversion of water to hydrate.     

Hydrogen and carbon dioxide adsorption with tetra‐n‐butyl ammonium semi‐clathrate hydrates for gas separations

Gas adsorption rates of H₂, CO₂, and H₂‐CO₂ gas mixture (H₂/CO₂ = 3.4) with tetra‐n‐butyl ammonium salt (bromide, chloride, and fluoride) semi‐clathrate hydrate particles were measured at 269 K to assess their properties for gas separation. Equilibrium gas occupancies in the S‐cages of the particles were in order of (high to low) for hexagonal structure‐I, tetragonal structure‐I, and superlattice of cubic structure‐I structures with the maximum fractional occupancy by CO₂ being about 40%. The CO₂ diffusion rate depended on the anion size of the salt, which is attributed to distortion of the S‐cage that is close to the molecular size of CO₂. Simulations of semi‐clathrate hydrate particles with theory showed that H₂/CO₂ selectivities could be as high as 36 (3.0 mol% TBAF) and that selectivities for an ideal membrane (3.3 mol% TBAF) could be >100 (269 K, 0.3–4.5 MPa). Semi‐clathrate hydrates have wide application as separation media for gas mixtures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 992–1003, 2015     

CO2 removal by single and mixed amines in a hollow‐fiber membrane module—investigation of contactor performance

This work investigates CO₂ removal by single and blended amines in a hollow‐fiber membrane contactor (HFMC) under gas‐filled and partially liquid‐filled membrane pores conditions via a two‐scale, nonisothermal, steady‐state model accounting for CO₂ diffusion in gas‐filled pores, CO₂ and amines diffusion/reaction within liquid‐filled pores and CO₂ and amines diffusion/reaction in liquid boundary layer. Model predictions were compared with CO₂ absorption data under various experimental conditions. The model was used to analyze the effects of liquid and gas velocity, CO₂ partial pressure, single (primary, secondary, tertiary, and sterically hindered alkanolamines) and mixed amines solution type, membrane wetting, and cocurrent/countercurrent flow orientation on the HFMC performance. An insignificant difference between the absorption in cocurrent and countercurrent flow was observed in this study. The membrane wetting decreases significantly the performance of hollow‐fiber membrane module. The nonisothermal simulations reveal that the hollow‐fiber membrane module operation can be considered as nearly isothermal. © 2014 American Institute of Chemical Engineers AIChE J, 61: 955–971, 2015     

Development of formation and growth models of CO2 hydrate film

This study aims to develop models to estimate the CO₂ hydrate film formation and growth for different temperature and flow velocity conditions. First, the CO₂ hydrate film thickness at the initial stage of its formation is experimentally measured under different temperature and flow velocity conditions using laser interferometry. Based on the results, the CO₂ hydrate film thickness was found to decrease with increasing temperature and flow velocity. Next, the CO₂ hydrate film formation model and growth model are developed, and the models are verified using the present experimental data. Finally, the long term growth of CO₂ hydrate film thickness is estimated by the proposed growth model of CO₂ hydrate film thickness. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4078–4089, 2016     

CO2 Adsorption Kinetics of K2CO3/Activated Carbon for Low‐Concentration CO2 Removal from Confined Spaces

K₂CO₃ supported on activated carbon (K₂CO₃/AC) is a promising means to remove low‐concentration CO₂ from confined spaces. In this removal process, physical adsorption plays an important role but it is difficult to quantify the amount of CO₂ adsorbed when both H₂O and CO₂ are present. The linear driving force mass transfer model is adopted to study the CO₂ adsorption kinetic characteristics of K₂CO₃/AC by analyzing the experimental data. The effect of K₂CO₃ and H₂O on the adsorption of CO₂ in K₂CO₃/AC was also evaluated. K₂CO₃ loaded on the support is found to increase the mass transfer resistance but decrease the activation energy required for the physical adsorption process. The presence of water vapor is disadvantageous to achieve high physical adsorption capacity since it enhances the chemical sorption in the competitive dynamic sorption process.     

STUDY OF CAPTURING EMITTED CO2 IN THE FORM OF HYDRATES IN A TUBULAR REACTOR

Stabilizing atmospheric CO₂ concentration requires the development of novel methods for capturing it in the form of permanent reservoirs. Among the proposed methods is CO₂ storage in the form of hydrate. In this study a method was established for CO₂ conversion to hydrate. This method can be applied to bioethanol plants, which produce CO₂ as a by-product of ethanol fermentation. In this regard, a tubular recirculating flow reactor was developed for the study of CO₂ hydrate formation. The experiments were carried out at 279 K and 3.5–5 MPa to determine the rate of CO₂ hydrate formation. Further, a model was developed for prediction of the rate of hydrate formation based on the mass transfer, crystallization, and thermodynamic concepts. The predicted hydrate formation rate was compared to the experimental data in order to validate the model prediction. The predicted results were in good agreement with the experimental data at different operating conditions.     

A Study on CO2 Absorption Kinetics by Aqueous Solutions of N,N‐Diethylethanolamine and N‐Ethylethanolamine

N‐Ethylethanolamine (EEA) and N,N‐diethylethanolamine (DEEA) represent promising candidate alkanolamines for CO₂ removal from gaseous streams, as they can be prepared from renewable resources. In this work, the reaction rate constant for the reaction between CO₂ and EEA was determined from the absorption rate measurements of CO₂ in a blend comprising DEEA, EEA and H₂O. A stirred‐cell reactor with a plane, horizontal gas‐liquid interface was used for the absorption studies. While the DEEA concentration in the formulated solution was varied in the range of 1.5–2.5 kmol/m³, the initial EEA concentration was 0.1 kmol/m³. A zwitterion mechanism for EEA and a base‐catalyzed hydration mechanism for DEEA were used to describe the reaction kinetics. At 303 K, the second‐order reaction rate constant for the CO₂ reaction with EEA was found to be 8041 m³/(kmol s). The liquid‐side mass transfer coefficient was also estimated, and its value (0.004 cm/s) is in line with those typical of stirred‐cell reactors.     

Impact of the fluid flow conditions on the formation rate of carbon dioxide hydrates in a semi‐batch stirred tank reactor

CO₂ hydrate formation experiments are performed in a 20 L semi‐batch stirred tank reactor using three different impellers (a down‐pumping pitched blade turbine, a Maxblend?, and a Dispersimax?) at various rotational speeds to examine the impact of the flow conditions on the CO₂ hydrate formation rate. An original mathematical model of the CO₂ hydrate formation process that assigns a resistance to each of its constitutive steps is established. For each experimental condition, the formation rate is measured and the rate‐limiting step is determined on the basis of the respective values of the resistances. The efficiencies of the three considered impellers are compared and, for each impeller, the influence of the rotational speed on the rate‐limiting step is discussed. For instance, it is shown that a formation rate limitation due to heat transfer can occur at the relatively small scale used to perform our experiments. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4387–4401, 2015     

Determination of kinetics of CO2 absorption in solutions of 2‐amino‐2‐methyl‐1‐propanol using a microfluidic technique

The kinetics for the reactions of carbon dioxide with 2‐amine‐2‐methyl‐1‐propanol (AMP) and carbon dioxide (CO₂) in both aqueous and nonaqueous solutions were measured using a microfluidic method at a temperature range of 298–318 K. The mixtures of AMP‐water and AMP‐ethylene glycol were applied for the working systems. Gas‐liquid bubbly microflows were formed through a microsieve device and used to determine the reaction characteristics by online observation of the volume change of microbubbles at the initial flow stage. In this condition, a mathematical model according to zwitterion mechanism has been developed to predict the reaction kinetics. The predicted kinetics of CO₂ absorption in the AMP aqueous solution verified the reliability of the method by comparing with literatures’ results. Furthermore, the reaction rate parameters for the reaction of CO₂ with AMP in both solutions were determined. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4358–4366, 2015     

Mass‐transfer rate enhancement for CO2 separation by ionic liquids: Theoretical study on the mechanism

To promote the development of ionic liquid (IL) immobilized sorbents and supported IL membranes (SILMs) for CO₂ separation, the kinetics of CO₂ absorption/desorption in IL immobilized sorbents was studied using a novel method based on nonequilibrium thermodynamics. It shows that the apparent chemical‐potential‐based mass‐transfer coefficients of CO₂ were in three regions with three‐order difference in magnitude for the IL‐film thicknesses in microscale, 100 nm‐scale, and 10 nm‐scale. Using a diffusion‐reaction theory, it is found that by tailoring the IL‐film thickness from microscale to nanoscale, the process was altered from diffusion‐control to reaction‐control, revealing the inherent mechanism for the dramatic rate enhancement. The extension to SILMs shows that the significant improvement of CO₂ flux can be obtained theoretically for the membranes with nanoscale IL‐films, which makes it feasible to implement CO₂ separation by ILs with low investment cost. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4437–4444, 2015     

A superstructure‐based mixed‐integer programming approach to optimal design of pipeline network for large‐scale CO2 transport

Pipeline transport is the major means for large‐scale and long‐distance CO₂ transport in a CO₂ capture and sequestration (CCS) project. But optimal design of the pipeline network remains a challenging problem, especially when considering allocation of intermediate sites, like pump stations, and selection of pipeline routes. A superstructure‐based mixed‐integer programming approach for optimal design of the pipeline network, targeting on minimizing the overall cost in a CCS project is presented. A decomposition algorithm to solve the computational difficulty caused by the large size and nonlinear nature of a real‐life design problem is also presented. To illustrate the capability of our models. A real‐life case study in North China, with 45 emissions sources and four storage sinks, is provided. The result shows that our model and decomposition algorithm is a practical and cost‐effective method for pipeline networks design. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2442–2461, 2014