含环戊烷体系中二氧化碳水合物形成分解热特性

水合物法分离捕集二氧化碳（CO₂）是实现碳减排的重要技术。然而,受制于气体水合物形成分解微观机理的不明确,水合物形成速度慢及气体消耗量低两个关键问题还未得到解决,气体水合物CO₂分离捕集技术还未得到商业应用。为了揭示气体水合物形成微观机理,本研究利用激光Raman光谱仪对不同实验条件下生成的CO₂水合物进行测试,详细分析了生成的气体水合物的Raman峰,利用低温高压差式扫描量热仪（DSC）对含环戊烷（CP）体系中CO₂水合物形成分解进行热表征。研究发现,恒容条件下,初始压力为2.5MPa时,气体消耗量为0.0187 mol/mol,CO₂的Raman峰出现在1276.3 cm^-1和1379.6 cm^-1;初始压力为5.0MPa时,气体消耗量为0.744mol/mol,CO₂的Raman峰出现在1276.1 cm^-1和1379.6 cm^-1。CO₂水合物形成分解热结果表明,一方面,随着操作温度、压力条件的变化,形成水合物的种类与结构发生改变;另一方面,对于相同初始体系,最终形成的水合物不是单一的,而是多种水合物共存。此研究结果为进一步理清气体水合物形成微观机理提供了理论基础和重要的科学依据。     

液化条件对多孔介质中CO2水合物生成过程的影响

为了研究液化条件对多孔介质中CO₂水合物生成过程的影响机制及其规律,在初始压力为3.9、4.2、4.5、4.8和5.1 MPa,温度为273.5、274.5和275.5 K条件下研究粒径为700μm的石英砂介质中CO₂水合物的生成过程。结果表明：在相同条件下,随着初始压力的增加,多孔介质中CO₂水合物的生成速率逐渐增大;当压力低于液化压力时,随着初始压力的增加,CO₂水合物的生成速率逐渐增大,且温度越高,水合物生成速率增加的趋势越明显;当CO₂气体压力达到液化压力时,随着初始压力的不断升高,CO₂水合物的生成速率明显增大;多孔介质中CO₂水合物的最大生成速率达到了9.297×10^-3 mol·s^-1。研究结果进一步表明：液化可有效强化多孔介质中CO₂水合物的生成过程,提高CO₂水合物的生成速率。     

促进剂体系中二氧化碳水合物常压分解

目前,水合物法捕获二氧化碳时,都会添加不同的形成促进剂。而这些促进剂对后期二氧化碳水合物运输条件、封存条件的影响还未可知,因此本文对比研究了纯冰粉体系、单一促进剂体系和复配体系中CO₂水合物的稳定性,促进剂分别选用0.24g/L十二烷基酸钠（SDS）、0.288g/L四丁基溴化铵（TBAB）、5g/L纳米石墨（GN）及0.33g/L NaCl 4种促进剂。在常压、274.65K条件下利用定压分解方法研究不同体系中CO₂水合物的分解动力学特性。结果表明：单一促进剂体系中CO₂水合物的稳定性由强到弱的顺序为SDS>TBAB>GN≈纯冰粉>NaCl,并且SDS能够延长CO₂水合物分解时间,其平均分解速率相对于纯冰粉体系分别减少了29.6%、15.8%及18.5% 。在复配促进剂体系中TBAB+NaCl中水合物稳定性最强,TBAB占主导作用,NaCl＋GN中水合物稳定性最弱,NaCl占主导作用。此外文章还分析了促进剂体系中CO₂水合物稳定性提高的作用机理,主要与促进剂本身的性质有关。     

CO2水合物浆在水平圆管中的传热特性

在CO₂水合物浆流动传热特性测试实验系统上,采用套管式电加热的方法对CO₂水合物浆进行了分解实验,并对CO₂水合物浆的流动传热特性进行了分析。对CO₂水合物浆的相变特性进行了研究,得到CO₂水合物浆的相变温度在8～12℃。研究了在固相体积分数为13.2%以及流速为0.45m/s的条件下CO₂水合物浆在内径为8mm的水平不锈钢管中的传热特性,计算得到CO₂水合物浆在不锈钢水平圆管中的对流传热系数为1500～1800 W/(m²·K),并且其在流动传热过程中呈现先增大随后趋向平稳的趋势,在水合物的相变区相应的对流传热系数表现最大。研究了分解加热功率对管壁温和对流传热系数的影响,发现加热功率对管壁温度的影响较强。在实际应用中可利用CO₂水合物浆的相变作用来增强传热,提高传热效率。     

氨缓冲体系强化高纯重质碳酸锰的制备

以碳酸钠和高纯硫酸锰为原料,通过共沉淀法在氨缓冲溶液中制备高纯重质碳酸锰。当硫酸锰和碳酸钠溶液浓度都为1.5 mol·L^-1、碳酸钠过量系数为110%、溶液pH为8.5、温度为50℃、滴加速率为120 ml·h^-1时,得到的碳酸锰视密度达1.67 g·cm^-3,振实密度达2.15 g·cm^-3。氨缓冲体系增加了溶液的稳定性,抑制了溶液中氢氧化锰和偏氢氧化锰的生成,制备出的高密度碳酸锰形貌趋于球形,粒径分布均匀,D₅₀平均大小为30.32 μm。以本研究制备的碳酸锰为锰原料焙烧得到的四氧化三锰松装密度为1.09 g·cm^-3,振实密度为2.18 g·cm^-3,锰的含量可达71.85%。     

纳米粒子对CO2水合物导热性能的影响

试验研究了不同种类（Al₂O₃、Cu、SiO₂）、不同质量分数（0.05%、0.1%、0.15%）及不同粒径（10、30、50 nm）的纳米粒子对CO₂水合物热导率的影响。结果表明温度为-5~5℃时,纯CO₂水合物热导率为0.553~0.5861 W·m^-1·K^-1,具有玻璃体的变化特性。分散剂SDBS的加入,可改善CO₂水合物-纳米粒子体系的导热性能。在相同的质量分数和粒径下,纳米Cu粒子对CO₂水合物热导率的增强作用最好,但综合考虑水合物生成特性和溶液悬浮稳定性,选用纳米Al₂O₃粒子较合适。Al₂O₃粒子粒径越小,水合物热导率越大,15 nm比50 nm纳米粒子体系中CO₂水合物热导率的增长率平均提高了12.7%。此外,CO₂水合物热导率随Al₂O₃粒子质量分数的增大而增大,质量分数由0.05%增加到0.15%时,水合物热导率的增长率由4.2%提高到8.2%。     

碳毡负载Sn气体扩散电极CO2电化学还原性能

针对CO₂电化学还原中气体扩散电极可强化CO₂的传质,基于碳毡制备了负载锡-石墨烯催化层的新型气体扩散电极,研究了CO₂反应条件、电极厚度、催化剂载量及反应电位对CO₂电化学还原性能的影响。实验结果表明：与溶解态CO₂反应条件相比,采用气相CO₂反应条件电化学还原性能更好;一定范围内增加电极厚度和催化剂载量可以增加气-液-固三相反应界面,提升CO₂电化学还原性能;随着电解电位负移,甲酸产量增加,电流效率先增大后减小;实验中使用厚度为5 mm、载量为5 mg·cm^-2的电极,在-1.8 V（vs Ag/AgCl）条件下进行电化学还原时,平均电流密度为（12.79±1.27） mA·cm^-2,甲酸电流效率达到最佳为41.55%±2.50%。     

直接接触式制备CO2水合物的生长和蓄冷特性

采用直接接触式水合物反应釜,并在釜内初始水温为26℃和15℃以及充注压力为3.6～4.2 MPa的条件下制备CO₂水合物,研究其生长和蓄冷特性。实验结果表明:在初始水温为15℃、充注压力为3.6 MPa时,预冷时间较短(11 min),水合物开始生长时过冷度约为1.3℃,温升不明显,平均蓄冷速率和平均蓄冷效率分别为82.4 kJ·min^-1和4.34;在其他温压条件下,预冷时间较长(20～55 min),但随充注压力的升高而减小,过冷度较大(5.5～9.7℃),水合物生长迅速且密集,温升明显,并在充注压力为4.0 MPa时温升幅度最大,水合物平均生长速率、总蓄冷量、平均总蓄冷速率和效率随充注压力的升高而变大,其最大值分别为77和154 g·min^-1,3.725和3.791 MJ,64.1和99.5 kJ·min^-1,3.14和 4.91。水合物释冷分解是一个相变过程,相变温度一般在10～14℃。     

水合物法平衡级分离CO2/N2流程模拟分析

化石燃料燃烧排放烟气中CO₂的量占CO₂总排放量的75%,为了缓解CO₂导致的全球温室效应,需将CO₂/N₂中的CO₂分离出来。水合物法分离是一种高效、低能耗的CO₂/N₂分离技术。本文研究了水合物法平衡级分离CO₂/N₂过程中,进料CO₂体积分数、反应条件与反应特性三者间的关系,利用CPA-SRK方程+Chen-Guo模型对其进行平衡级分离流程模拟分析。经计算发现,进料干基CO₂体积分数对水合物法分离CO₂/N₂工艺的反应压力、平衡级级数均有较大影响。随着体积分数的增加,反应压力呈减小趋势,减小幅度随体积分数增加而减小,当进料CO₂体积分数小于20%时,压力下降较快,当体积分数大于50%时,压力降低幅度变小。温度为277K时,CO₂体积分数小于10%时,需四个水合物平衡级分离才能得到满足要求的气样;当体积分数为10%~20%时,需三个水合物平衡级分离;体积分数大于30%时为两个水合物平衡级分离。温度对水合分离的反应压力有较大影响,但对所需平衡级分离级数的影响并不大。随着温度的升高,水合反应压力呈增加趋势,增加幅度随进料干基CO₂体积分数的增加而降低。针对所研究气样,在不同温度下,均需三个水合物平衡级分离才能达到工艺要求。     

TMPAC-AlCl3离子液体中恒电流电沉积铝

合成了不同TMPAC和AlCl₃摩尔比的TMPAC-AlCl₃离子液体,并测定了其在不同温度下的电导率。在摩尔比为1:2的TMPAC-AlCl₃离子液体中进行了恒电流电沉积实验,研究了温度、电流密度、搅拌速度和添加剂甲苯等对沉积层的影响。结果表明:在相同条件下摩尔比为1:2的TMPAC-AlCl₃离子液体电导率最大,当温度为80℃,电流密度为20 mA·cm^-2,搅拌速度为500 r·min^-1时,沉积层质量较好且电流效率较高;甲苯的加入,增大了TMPAC-2AlCl₃离子液体的电导率,降低了槽电压,当添加甲苯的体积分数为50%,电流密度为23 mA·cm^-2时,可以得到较为致密、色泽良好、平整而均匀的沉积层。     

Metastable states during dissociation of carbon dioxide hydrates below 273 K

In this study, the dissociation of isolated carbon dioxide hydrate particles of sizes in the range 0.25–2.5 mm was investigated. It was found that below the ice melting point, the hydrates dissociated into supercooled water (metastable liquid) and gas. The formation of the liquid phase during CO₂ hydrate dissociation was visually observed, and the pressures of the hydrate dissociation into supercooled water and gas were measured in the temperature range 249–273 K. These pressures agreed well with the calculated data for the supercooled water–hydrate–gas metastable equilibrium (Istomin et al., 2006). In the P–T area on the phase diagram bounded by the ice–hydrate–gas equilibrium curve and the supercooled water–hydrate–gas metastable equilibrium curve, hydrates could exist for a long time because the metastable phase and their stability are not connected to the self-preservation effect. The growth of the metastable CO₂ hydrate film on the surface of supercooled water droplets formed during the hydrate dissociation was observed at pressure above the three-phase supercooled water–hydrate–gas metastable equilibrium pressure but still below the three-phase ice–hydrate–gas equilibrium pressure. It was found that the growth rate of the metastable CO₂ hydrate film was higher by a factor of 25 and 50 than that for methane hydrate and propane hydrate, respectively.     

Experimental investigation and modeling on the dissociation conditions of methane hydrate in clayey silt cores

As the majority of global natural gas hydrate reserve, the dissociation conditions of hydrate in clayey silts are of great significance for its efficient production. In this work, the dissociation conditions of methane hydrate in clayey silt cores were experimentally measured by step-heating method at the temperature range of 280.76–289.55 K and pressure range of 8.11–15.03 MPa, respectively. Various cores including quartz powder, montmorillonite, and South China Sea sediments at the water content range of 20%–33% were used for investigation. The results showed that the dissociation temperatures of methane hydrate in clayey silt cores depressed compared to bulk hydrate. The grain size, salinity, and lithology of clayey silt cores significantly affect the dissociation conditions of hydrate. In comparison to grain size, salinity, and lithology had a more significant influence on the equilibrium temperature depression. The dissociation temperature depression of methane hydrate was considered as a consequence of the water activity depression which is caused by the effect of capillary, salt, or clay. A water activity meter was used to measure the water activity in clayey silt cores. The influence of salt component and mineral characteristics on the water activity was investigated. By combining the measured water activity data with the Chen-Guo model, a novel water activity measurement (WAM) method for the hydrate dissociation conditions prediction was proposed. With the maximum deviation less than 12%, the predicted results are in good agreement with the experimental data. It demonstrated that the WAM method could effectively predict the dissociation conditions of methane hydrate in clayey silts with convenience and accuracy.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

Selective Permeation of Ammonia and Carbon Dioxide by Novel Membranes

Abstract

Experimental results are presented on membranes of novel composition which selectively permeate ammonia and carbon dioxide from mixtures containing hydrogen. The CO₂-selective membrane, which consists of a thin liquid film of the salt hydrate tetramethylammonium fluoride tetrahydrate, exhibits a CO₂ permeance of 4-1 × 10^?5 cm³/cm²·s·cmHg with selectivity, α(CO₂/H₂), ranging from 360-30. The NH₃-selective membrane, poly(vinylammonium thiocyanate), displays a high NH₃ permeance, 5?20 × 10^?5 cm³/cm²·s·cmHg, with α(NH₃/N₂) as high as 3600 and α(NH₃/H₂) as high as 6000. Such membranes, which retain H₂ at pressure in the feed stream, may offer new opportunities in the design of separation processes.     

Phase and kinetic behavior of the mixed methane and carbon dioxide hydrates

Large amounts of CH₄ are stored as hydrates on continental margins and permafrost regions. If the CH₄ hydrates could be converted into CO₂ hydrate, they would serve double duty as CH₄ sources and CO₂ storage sites in the deep ocean sediments. As preliminary investigations, both the phase behavior of CH₄ hydrates and kinetic behavior of CO₂ hydrate were measured at versatile conditions that can simulate actual marine sediments. When measuring three-phase equilibria (H-LW-V) containing CH₄ hydrate, we also closely examined pore and electrolyte effects of clay and NaCl on hydrate formation. These two effects inhibited hydrate nucleation and thus made the hydrate equilibrium line shift to a higher pressure region. In addition, the kinetic data of CO₂ hydrate in the mixtures containing clay and NaCl were determined at 2.0 MPa and 274.15 K. Clay mineral accelerated an initial formation rate of CO₂ hydrate by inducing nucleation as initiator, but total amount of formed CO₂, of course, decreased due to the capillary effect of clay pores. Also, the addition of NaCl in sample mixtures made both initial formation rate and total amount of CO₂ consumption decrease.     

Thermodynamic stability, spectroscopic identification and cage occupation of binary CO2 clathrate hydrates

The hydrate phase behavior of CO₂/3-methyl-1-butanol (3M1B)/water, CO₂/tetrahydrofuran (THF)/water and CO₂/1,4-dioxane (DXN)/water was investigated using both a high-pressure equilibrium viewing cell and a kinetic pressure-temperature measurement system with a constant volume. The dissociation pressures of CO₂/3M1B/water were identical to those of pure CO₂ hydrate, indicating that CO₂ is not acting as a help gas for structure H hydrate formation with 3M1B, thus the formed hydrate is pure CO₂ structure I hydrate. The CO₂ molecules could be encaged in small cages of the structure II hydrate framework formed with both of THF and DXN. For a stoichiometric ratio of 5.56 mol% THF, we found a large shift of dissociation boundary to lower pressures and higher temperatures from the dissociation conditions of pure CO₂ hydrate. From the measurements using the kinetic pressure-temperature system, it was found that the solid binary hydrate samples formed from off-stoichiometric THF and DXN aqueous solutions are composed of pure CO₂ hydrate with a hydrate number n=7.0 and THF/CO₂ and DXN/CO₂ binary hydrates with a molar ratio of xCO₂·THF·17H₂O and xCO₂·DXN·17H₂O, respectively. The X-ray diffraction was used to identify the binary hydrate structure and Raman spectroscopy was measured to support the phase equilibrium results and to investigate the occupation of CO₂ molecules in the cages of the hydrate framework.     

Molded micro- and mesoporous carbon/silica composite from rice husk and beet sugar

A molded carbon/silica composite with high micro- and mesoporosity, as well as a high bulk density, was fabricated by activating a disk-molded precursor made from carbonized rice husk (RH) and beet sugar (BS) at 875 °C in CO₂. The pore structure of the RH- and BS-based carbon/silica composite (RBC) was analysed in relation to the bulk density. An activation time of 2.0 h provided the largest BET specific surface area (1027 m²/g) and total pore volume (0.68 cm³/g) and a low bulk density (0.54 g/cm³). An RBC that was first activated for 1 h was immersed again in BS syrup and then activated in CO₂ for 1 h. This two-step activation process provided both a high bulk density (0.69 g/cm³) and a highly textured structure (BET specific surface area, 943 m²/g; total pore volume, 0.56 cm³/g). The immersion in BS syrup was useful for improving the texture without reducing the bulk density, in comparison to one-step activation for 1.0 h. The suspension of the RBCs was basic because of the residual inorganic compounds of potassium and calcium. However, the basicity of the suspension was alleviated by washing the RBCs with water.     

Gas hydrate concentration measurements on sucrose solutions using a new pilot test rig

A new 750 cm³ pilot test rig based on the “isochoric pressure method” was designed and commissioned for the hydrate measurements to concentrate sucrose solutions. The reactor included an improved agitation system and enabled sampling of the sucrose solutions. The experimental method was validated be performing dissociation measurements for the CO₂ + water system. Gas hydrate kinetic and sampling data were measured for the CO₂ + sucrose solutions at sucrose concentrations between (12–60) ^oBrix, within the temperature range of (274.65–276.15) K and at pressures up to 3.70 MPa. Results showed that sucrose is a kinetic inhibitor. The data were modeled to obtain hydrate formation rate, storage capacity, gas consumption and apparent rate constant. Stage-wise concentration measurements were performed with reactor conditions at 274.65 K, 3.70 MPa and 130 rpm mixer speed with liquid sample withdrawal. A final sucrose product of approximately 60 ^oBrix was obtained.     

Phase behavior of gas hydrates in nanoporous materials: Review

A precise understanding of phase behavior for a variety of both artificial and natural processes is essential to achieving scientific and technological goals. There has been growing research interest in gas hydrates confined in nanoporous media aiming to simulate and analyze the unique behavior of natural gas hydrates in sediments. Moreover, the appearance of peculiar properties due to the confinement effect stimulates research on gas hydrate technology for gas separation, such as CO₂ capture from versatile pre/post combustion emissions. In spite of their importance, reliable phase equilibrium data on gas hydrates confined at a nanoscale are scattered throughout the literature, while those in bulk state are abundant. Accordingly, we surveyed the previous studies on the phase behavior of gas hydrates in various nanoporous materials to include and provide valuable information and knowledge for start-up researchers in various gas hydrate fields.     

Density and phase equilibrium for ice and structure I hydrates using the Gibbs–Helmholtz constrained equation of state

A new, rigorous framework centered around the multi-scale GHC equation of state is presented for predicting bulk density and phase equilibrium for light gas–water mixtures at conditions where hexagonal ice and structure I hydrate phases can exist. The novel aspects of this new framework include (1) the use of internal energies of departure for ice and empty hydrate respectively to determine densities, (2) contributions to the standard state fugacity of water in ice and empty hydrate from lattice structure, (3) computation of these structural contributions to standard state fugacity from compressibility factors and EOS parameters alone, and (4) the direct calculation of gas occupancy from phase equilibrium. Numerical results for densities and equilibrium for systems involving ice and/or gas hydrates predicted by this GHC-based framework are compared to predictions of other equations of state, density correlations, and experimental data where available. Results show that this new GHC-based EOS framework accurately predicts the densities of hexagonal water ice and structure I gas hydrates as well as phase equilibrium for methane–water and CO₂–water mixtures.