Recovering CH4 from natural gas hydrate with CO2 in porous media below the freezing point

The recovery of methane from gas hydrate layers that have been detected in permafrost regions is a promising perspective in the future. In order to study the replacement characteristics of CO₂–CH₄ hydrate in permafrost environments, simulation experiment was carried out in this work. The results indicated that the replacement rate and efficiency increased with the increasing of injection pressure of CO₂ gas. And the replacement rate and efficiency reached up to 0.403?mmol/h and 13.20% during the experiment. Furthermore, the results also showed that the replacement rate of CO₂–CH₄ hydrate was slower below the freezing point.     

Kinetics of (TBAF + CO2) semi-clathrate hydrate formation in the presence and absence of SDS

In this communication, the impacts of adding SDS (sodium dodecyl sulfate), TBAF (tetra-n-butylammonium fluoride) and the mixture of SDS + TBAF on the main kinetic parameters of CO₂ hydrate formation (induction time, the quantity and rate of gas uptake, and storage capacity) were investigated. The tests were performed under stirring conditions at T = 5 ℃ and P = 3.8 MPa in a 169 cm³ batch reactor. The results show that adding SDS with a concentration of 400 ppm, TBAF with a concentration of 1–5 wt%, and the mixture of SDS + TBAF, would increase the storage capacity of CO₂ hydrate and the quantity of gas uptake, and decrease the induction time of hydrate formation process. The addition of 5 wt% of TBAF and 400 ppm of SDS would increase the CO₂ hydrate storage capacity by 86.1% and 81.6%, respectively, compared to pure water. Investigation of the impact of SDS, TBAF and their mixture on the rate of gas uptake indicates that the mixture of SDS + TBAF does not have a significant effect on the rate of gas uptake during hydrate formation process.     

Influence of hydrogen sulfide on the efficiency of xenon recovery from natural gas by gas hydrate crystallisation

A mathematical simulation of the gas hydrate formation based on a gas mixture approximated to natural gas composition – CH₄+H₂S?+?CO₂+Xe at a normalized increase in hydrogen sulfide (H₂S) concentration in gas mixture from 3.08·10^?4?vol.% to 4.88?vol.%, at changes in the gas hydrate formation temperatures from 273.15?K to 283.15?K. It is shown that xenon (Xe) distribution coefficient decreases from 12.37 to 5.90, and is more dependent on the change in H₂S concentration than on the change in the gas hydrate formation temperature. Effective Xe recovery from natural gas at the gas hydrate formation temperature is 273.15?K, and at a minimum impurity concentration with a dissociation pressure close to Xe.     

Dissociation behavior of （CH4 ＋ C2H4） hydrate in the presence of sodium dodecyl sulfate

Separation of a mixture of CH4+C2H4 gas by forming hydrate in ethylene production has become of interest,and the dissociation behavior of(CH4+C2H4) hydrate is of great importance for this process. The hydrate formation rate could be increased by adding a small amount of sodium dodecyl sulfate(SDS) into water. In this work,the kinetic data of CH4(18.5 mol%) +C2H4(81.5 mol%) hydrate decomposition in the presence of 1000 mg·L-1 SDS at different temperatures and pressures were measured with the depressurizing m...     

NH3作为CO2置换CH4水合物促进剂的分子动力学模拟研究

针对天然气水合物开采中CO₂置换面临的渗透性差、置换效率低的问题,采用分子动力学模拟方法,将对水合物相具有强穿透能力的NH₃作为促进剂,分别模拟了CO₂单组分和CO₂/NH₃混合组分置换水合物过程。结果表明:在模拟设定的温度压力范围内,245 K和255 K条件下,NH₃对CO₂置换水合物过程起到正向促进作用,而当温度升高至265 K时,则会对置换过程起到抑制作用;温度相同时,升高压力可以提高置换效率,但不会改变NH₃对置换过程的促进/抑制作用。该研究结果可为提高CO₂置换法的置换效率提供新的思路。      

Thermodynamic modeling of equilibrium conditions of CH4/CO2/N2 clathrate hydrate in presence of aqueous solution of sodium chloride inhibitor

In this work, the effect of sodium chloride (NaCl) on thermodynamic properties of CH₄+CO₂+N₂ hydrate formation and equilibrium condition has been studied. The three-phase (hydrate–liquid–gas) equilibrium calculation has been carried out using the Peng–Robinson equation of state (PR EoS) and Universal Quasi Chemical (UNIQUAC) activity coefficient models. The PR EoS coupled with classic mixing rule is applied for the vapor phase. The calculations of the gas hydrate formation pressures are performed in the absence and presence of sodium chloride inhibitor for the gas hydrate systems. The Chen–Guo model has been used for the hydrate phase and the UNIQUAC activity coefficient is applied for non-ideality of the liquid phase. To obtain higher accuracy, the solubility of the gases in the aqueous phase is also taken into account using pressure corrected Henry's law. Finally, the stepwise procedure has been followed to obtain the results and compared with the experimental results. The addition of 2% (by volume) sodium chloride to water results in large shifts in phase equilibrium boundary to increase pressure for the same temperature condition.     

液态CO2原位置换整形天然气水合物的形态研究

CO_2置换法在开采天然气(CH_4)水合物资源的同时,能将温室气体(CO_2)以稳定的水合物形式永久封存并保持海底地质结构的稳定。探究液态CO_2原位置换整形天然气水合物的过程形态,验证水合物的形态结构是否会破坏,对实际开采技术的研发具有关键意义。利用自行设计的可视化反应釜对液态CO_2置换整形天然气水合物进行实验观测研究。结果表明:①在置换过程中,原位CH_4水合物分解的同时生成CO_2水合物,并保持过程中水合物的整形结构稳定的第二类原位置换过程是可以实现的;②在实际工程应用中,采用液态CO_2在相应的相平衡压力条件下,对大规模的天然气水合物藏进行第二类原位置换,可望获得较高的水合物藏置换开采效率。实验结果为进一步研究液态CO_2置换天然气水合物开采工艺提供了直观的过程现象。     

A new empirical correlation for prediction of carbon dioxide separation from different gas mixtures

Carbon dioxide (CO₂) emission from different systems such as fuel gas (H₂+CO₂), flue gas (N₂+CO₂), and biogas gas (CH₄+CO₂) is one of the main factors of global warming and environmental problems. So, CO₂ separation from different systems is essential. Low energy consumption, environmental friendliness, and low operational cost of hydrate-based gas separation (HBGS) process show the high potential of this approach in separation of some gases such as CO₂. Hydrate phase equilibrium data are required for designing the separation process. So far numerous models has been proposed for prediction of hydrate formation/dissociation conditions in various systems with/without promoters or inhibitors. This study attempts to present a simple and comprehensive model for fast prediction of hydrate formation conditions to separate CO₂ from biogas, fuel gas, and flue gas systems in the presence of promoters such as tetra-n-butylammonium bromide, tetra-n-butylammonium chloride, tetra-n-butylammonium fluoride, tetra-n-butyl ammonium nitrate, and tetra-n-butylphosphonium bromide. According to the error analysis results, this point can reach the new proposed correlation has better estimation capability in comparison with Sayyad Amin et al. model. On the other hand, hydrate formation temperature can be predicted in the presented correlation with high accuracy.     

Modelling of CO2 capture and separation from different gas mixtures using semiclathrate hydrates

In this work we present a model for predicting hydrate formation condition to separate carbon dioxide (CO₂) from different gas mixtures such as fuel gas (H₂+CO₂), flue gas (N₂+CO₂), and biogas gas (CH₄+CO₂) in the presence of different promoters such as tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylammonium fluoride (TBAF), tetra-n-butyl ammonium nitrate (TBANO₃), and tetra-n-butylphosphonium bromide (TBPB). The proposed method was optimized by genetic algorithm. In the proposed model, hydrate formation pressure is a function of temperature and a new variable in term of Z, which used to cover different concentrations of studied systems. The study shows experimental data and predicted values are in acceptable agreement.     

含杂质CO2管道输送水合物形成规律研究

在延长油田产生的CO₂气体输送过程中,管线会发生水合物冰堵,影响气体输送流量,为了探究CO₂水合物在管道输送过程中的形成规律,利用PVTSIM软件生成了CO₂水合物的相平衡曲线,并通过OLGA软件对水平管和弯管输送的水合物形成规律进行了模拟分析。结果表明:在低温高压条件下,水平管和弯管输送过程中均会有水合物形成,其生成过程是一种类似于盐类的结晶过程,通常包括成核和生长两个阶段,然后依靠流体颗粒之间的黏附力致使水合物聚集,与直管段相比,弯管段更容易产生水合物;水合物生成速率均由小到大,然后快速进入稳定阶段,最后趋于0。现场管线的水合物也多发生在弯管处,从而进一步验证了CO₂水合物的形成规律。因此,在管道输送过程中应避免高压出口和低温入口条件,保证管道安全运营。      

The Induction Time of Hydrate Formation From a Carbon Dioxide-Methane Gas Mixture

Kinetics of hydrate formation from CO₂?CH₄ gas mixture has been investigated. Eight experiments in various pressures, gas compositions, and load factors (volume of injected water/volume of reactor) were performed in a 460 CC vessel. For each gas mixture, the induction time of hydrate formation has been measured and the pressure-temperature-time diagram has been plotted. The results of the experiments show that by increasing the composition of carbon dioxide in the gas, the induction time of hydrate formation decreased and by increasing the load factor, the hydrate formation rate increased.     

N-甲基二乙醇胺溶液中CO2气体吸收与水合物生成特性实验研究

目的酸性气田开发气流中往往含有二氧化碳(CO₂)和硫化氢(H₂S)等酸性组分,易形成天然气水合物(简称水合物),引起管道堵塞。解决酸性组分吸收剂对水合物作用机制不明确问题,为脱除酸性气体组分并防治水合物生成提供理论依据。 方法 采用恒温恒容法研究了N-甲基二乙醇胺(MDEA)溶液质量分数、搅拌状态及初始压力对CO₂气体吸收规律的影响、MDEA溶液对CO₂水合物生长速率和宏观晶体形态的影响,并与传统热力学抑制剂乙二醇(EG)效果进行对比。 结果CO₂气体吸收量随MDEA溶液质量分数的增加表现为先增后减的趋势。开启搅拌和降低压力可加快CO₂气体的吸收速率,增加气体吸收量。在气液界面,水合物晶体以二维模式生长,并且MDEA可改变CO₂水合物的宏观形貌,增加其质量分数可显著增加CO₂水合物覆盖溶液表面的时间、降低CO₂水合物的生长速率。与EG相比,MDEA水合物的动力学抑制效果较差,但水合物膜覆盖时间较长,生长速率较慢。 结论MDEA可与溶液中水分子形成氢键,与水合物竞争水分子,减少水的活性,同时,MDEA分子可与CO₂分子结合,与水合物竞争CO₂,显著降低水合物生长速率。研究结果对酸性气体的分离捕获和天然气流动的安全保障具有理论指导意义。      

Prediction of hydrate formation conditions to separate carbon dioxide from fuel gas mixture in the presence of various promoters

In this study estimation of hydrate formation conditions to separate carbon dioxide (CO₂) from fuel gas mixture (CO₂+H₂) was investigated in the presence of promoters such as tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium fluoride (TBAF), and tetra-n-butyl ammonium nitrate (TBANO₃). The emission of CO₂ from the combustion of fuels has been considered as the dominant contributor to global warming and environmental problems. Separation of CO₂ from fuel gas can be an effective factor to prevent many of environmental impacts. Gas hydrate process is a novel method to separate and storage some gasses. In this communication, a feed-forward artificial neural network algorithm has been developed. To develop this algorithm, the experimental data reported in the literature for hydrate formation conditions in the fuel gas system with different concentrations of promoters in aqueous phase have been used. Finally, experimental data compared with estimated data and with calculation of efficiency coefficient, mean squared error, and mean absolute error show that the experimental data and predicted data are in acceptable agreement which demonstrate the reliability of this algorithm as a predictive tool.     

水合物法天然气脱酸的影响因素

为了促进水合物法分离CO₂在天然气脱酸工艺中的应用,以CH₄+CO₂混合气为例,采用CO₂分离率、CH₄损失率及分离因子作为水合物法脱酸的评价指标,利用自主设计的水合物法气体分离实验装置考察了初始压力、操作温度、四氢呋喃（THF）及实验用水量对水合物法分离CH₄+CO₂混合气的影响。结果表明,随初始压力增大和操作温度降低,平衡时气相CO₂浓度降低,CO₂分离率和CH₄损失率同时增大,分离因子小幅减小;随THF浓度增大,平衡时气相CO₂浓度增加,在THF摩尔分数为1.0%时,体系具有较大的CO₂分离率和分离因子,较低的CH₄损失率;随实验用水量增加,平衡时气相CO₂浓度降低,CO₂分离率增大,CH₄损失率减小,分离因子增大。综合分析,可通过提高初始压力、降低操作温度、增加实验用水量,并添加摩尔分数1.0%THF来促进水合物生成,提高分离效率。     

CO2置换水合物CH4的统一动力学模型

文章对水合物分解、生成的主要动力学模型进行介绍,包括Kim-Bishnoi模型、Englezos模型、Jamaluddin模型,然后对CO2置换甲烷水合物CH4的Ota模型进行重点介绍。通过研究发现,这些模型的驱动力都可用逸度差表示,反应面积可用气液相界面面积或分解表面积表示,并可以写成一个水合物分解生成双过程统一的动力学模型。基于CO2置换水合物CH4是一个分解生成过程同时发生的自然统一,最后提出一个统一的动力学模型。     

冰点以下甲烷水合物等压分解实验研究

开展了甲烷水合物在2.25MPa、2.08MPa和1.88 MPa 3种压力和不同温度条件下等压分解实验,研究温度驱动力对甲烷水合物分解过程及其自保护效应的影响。实验结果表明,在等压分解过程中,温度推动力ΔT对甲烷水合物的分解产生较大影响,当温度推动力ΔT>|T₀| 时,分解速率与ΔT具有正相关关系;当ΔT≤|T₀|时,甲烷水合物存在明显的自保护效应,分解速率与ΔT的关系不明显。     

不同类型体系下复合型添加剂对水合物生成的影响

通过实验研究了纯水和盐类体系下复合型添加剂对水合物生成的影响,选用甲基环己烷(MCH)与NaCl分别和阴离子表面活性剂十二烷基苯磺酸钠(SDBS)混合溶液作为实验溶液。研究发现,两种类型体系相比较,NaCl+SDBS混合溶液在SDBS浓度为255mg/kg时诱导时间最短,为复合型添加剂的最佳反应浓度,此时的溶液体系能很好地优化水合物形成过程,且比单一的NaCl溶液、MCH溶液或SDBS溶液的诱导时间都有明显缩短,此时的氯化钠溶液浓度为海水浓度。此数据对今后储存运输水合物有着重要的指导意义。     

The Equilibrium Data and Thermodynamic Modeling of Hydrate Formation for CO2 and Mixtures of CO2 and CH4 in the Presence of Methanol

Equilibrium data for the novel formation of hydrates of carbon dioxide and mixtures of carbon dioxide and methane in 20 wt% aqueous methanol solution were measured by the constant-volume method. For CO₂, these data were taken at the temperature and pressure ranges of 264.7–270.7 K and 1,470–3,160 kPa, respectively. For mixtures of carbon dioxide and methane, these data were taken at the temperature and pressure ranges of 262.9–273.7 K and 1,370–5,100 kPa, respectively. The data obtained for CO₂ in 20 wt% aqueous methanol solution were in disagreement with previously published data, but there was good agreement between our data and the predictions of thermodynamic models. The Peng-Robinson equation of state (PR EOS) coupled with the Wong-Sandler (WS) mixing rule was used to obtain the fugacities of the components in the gas and aqueous liquid phases. The PR EOS was then coupled with van der Waals-Platteeuw (vdW-P) hydrate model and applied to predict hydrate-formation conditions in the system containing methanol. The model predictions demonstrated good agreement with the experimental data.     

Experimental study of separation of ammonia synthesis vent gas by hydrate formation

Termodynamic data on methane hydrate formation in the presence of ammonia are very important for upgrading of ammonia synthesis vent gas using hydrate formation. This paper is focused on the formation conditions of methane hydrate in the presence of ammonia and the effects of gas-liquid ratio and temperature on the separation of vent gas by hydrate formation. Equilibrium data for methane hydrate within an ammonia mole concentration range from 1% to 5 % were obtained. The experimental results indicated that ammonia has an inhibitive effect on hydrate formation. The higher the ammonia concentration, the higher is the pressure reguired for methane hydrate formation would be. The primary experimental results showed that when volume ratio of gas to liquid was 80：1 and temperature was 283.15 K, total mole fraction of （H2＋N2） in gas phase could reach 96.9 %.