An intelligent optimization–based prediction model for natural gas hydrate formation in a deepwater pipeline

Temperature, pressure, and composition of gas mixtures in deepwater pipelines promote rapid formation of gas hydrates. To avert this dilemma, it is more significant to find out the temperature and pressure limits in gas hydrates formation of the deepwater pipeline. The objective of this research is to develop an optimization method that finds the optimal temperature and pressure profile for natural gas hydrate formation conditions and an error calculation method to find the realistic approach of the hydrate formation prediction model. A newly developed correlation model is computing the hydrate formation pressure and temperature for a single component of methane (CH₄) gas. The proposed developed prediction model is based on the 2 and 15 constant coefficients and holds a wide range of temperature and pressure data about 2.64 to 46°C and 0.051 to 400 MPa for pure water and methane, respectively. The reducing error discrepancies are 1.2871, 0.35012, and 1.9052, which is assessed by GA, PSO, and GWO algorithms, respectively. The results show the newly developed optimization algorithms are in admirable compliance with the experimental data and standards of empirical models. These correlations are providing the capability to predict gas hydrate forming conditions for a wide range of hydrate formation data.     

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.     

Experimental and Theoretical Investigations on the Enhancement of Methane Gas Hydrate Formation Rate by Using the Kinetic Additives

Effects of the most important influencing parameters on the methane gas hydrate formation kinetics at the onset of hydrate formation in the absence or presence of sodium dodecyl sulfate (SDS) kinetic promoter are investigated. At the SDS concentration of 500 ppm, the maximum methane effective diffusion coefficient and the highest conversion rate are obtained. The methane hydrate-liquid interfacial tension according to the suggested mechanism is about 18.6 mJm^–2 for all cases. The overall average absolute deviations between predicted and measured methane consumption rates are about 0.71% and 0.83% regarding nonpromoted and SDS-promoted gas hydrate formation processes, respectively.     

Experimental and Modeling Study of Kinetics for Methane Hydrate Formation with Tetrahydrofuran as Promoter

The kinetics behavior of methane hydrate formation in the presence of tetrahydrofuran(THF)as promoter was studied.A set of experimental equipment was designed and constructed.A series of kinetics data for the formation of methane hydrate in the presence of THF were measured with the isochoric method.The influences of temperature, pressure and liquid flow rate on the methane consumption rate were studied respectively.Based on the Chen-Guo hydrate formation mechanism,a kinetics model for the formation of methane hydrate in the presence of THF by using the dimensionless Gibbs free energy difference of quasi-chemical reaction of basic hydrate formation,??GRT,as the driving force was proposed.The model was used to calculate the rate of methane consumption and it was shown that the calculated results were in good agreement with the experimental results.     

低甲烷浓度煤层气的水合物法提纯实验

低甲烷浓度煤层气提纯是煤层气资源开发利用的一个重要发展方向,但是甲烷回收率低是低甲烷浓度煤层气提纯技术急需解决的关键问题。为此,采用气体水合物方法对低甲烷浓度煤层气进行提纯实验研究,向反应体系引入环戊烷降低气体水合物相平衡条件并提高甲烷回收率。采用等温压力搜索法测定了低甲烷浓度煤层气在环戊烷—水体系中的水合相平衡数据,并在等温等压条件下研究了生长驱动力和环戊烷浓度对甲烷回收率的作用规律。结果表明:1环戊烷对低甲烷浓度煤层气生成气体水合物的相平衡条件具有显著的促进作用。2甲烷回收率随着生长驱动力的升高而减小,随着环戊烷浓度的升高而增大。压力升高后,氮气将与甲烷竞争进入气体水合物晶体,导致甲烷回收率下降;在283.4K、2.6MPa和13%环戊烷浓度的实验条件下,甲烷回收率高达46.1%。3经过二级水合分离后,煤层气的甲烷浓度从30%提高到了72%。该项成果为低甲烷浓度煤层气提纯技术的发展提供了基础数据和实验依据。     

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.     

Development of a hydrate formation prediction model for sub-sea pipeline

In this research work, a novel model is developed to the hydrate formation pressure and hydrate formation temperature for a single component of methane (CH₄) gas. This research model holds at a temperature and pressure of real-time data for pure water, methane, and other mixtures of gases, respectively. Furthermore, the modelling and statistical analysis conducted in this research were divided into three major segments; the assessment of the computability of the mathematical models, the performance of the proposed optimization techniques, and the comparison of the proposed techniques with real-time data of gas pipeline. Results showed that the improved optimization algorithms are in admirable compliance with the real-time data. These correlations are providing the capability to predict gas-hydrate-forming conditions for a wide range of hydrate formation data.     

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...     

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.     

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.     

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。     

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.     

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。      

天然气水合物抑制过程中甲醇用量的影响

采用可视化高压流体测试装置,考察了甲醇含量对陕北某气田天然气水合物生成条件的影响。实验结果表明,气体组成对其水合物的生成条件有较大的影响,大分子气体或液态烃的存在可显著降低水合物的生成压力;甲醇含量对水合物生成的温度降有较大影响,甲醇含量越高,水合物生成温度降越大;水合物生成压力的对数(lgp)与温度(t_e)呈线性关系,不同甲醇含量时水合物生成条件的lgp～t_e曲线相互平行;甲醇质量分数小于30%时,Hammerschmidt方程和Nielsen-Bucklin方程对水合物生成温度降的预测偏差较小,但甲醇质量分数大于30%时,预测偏差较大;采用Nielsen-Bucklin修正式,预测甲醇用量的偏差小于2%。     

混合介质对降温法形成甲烷水合物性质的影响

目前,对沉积物中天然气水合物形成与分解性质的研究主要是在单一介质中进行,但自然界中的天然气水合物主要赋存于混合介质沉积物中。因此,有必要考察不同介质类型对天然气水合物形成的影响。为此,将粗砂、细砂、粉土3种介质按不同方法混合,搭配出6种混合型介质,并采用降温法在其内生成甲烷水合物,研究介质类型对甲烷水合物形成性质的影响及在降温过程中不同介质消耗甲烷气体的特点,为研究介质内水合物形成机制提供理论基础。实验装置由供气、反应和数据采集3个系统组成。结果表明:①不同单一介质对水分的吸持力差别很大,介质混合后水分在其内的分布状态及水分子在介质表面的吸附排列存在较大差异,从而使不同混合型介质内甲烷水合物的最终生成形态不同;②介质类型不仅会影响其内甲烷水合物的形成过程,而且会影响水合物的含气率;③不同介质内甲烷水合物生长过程所处的时间阶段不同。     

Vapor-hydrate phases equilibrium of （CH4＋C2H6） and （CH4＋C2H4） systems

Separation of the (C₁ + C₂) hydrocarbon system is of importance in natural gas processing and ethylene production. However it is the bottleneck because of its high refrigeration energy consumption, and needs to be urgently addressed. The technology of separating gas mixtures by forming hydrate could be used to separate (C₁ + C₂) gas mixtures at around 0 °C and has attracted increasing attention worldwide. In this paper, investigation of vapor-hydrate two-phase equilibrium was carried out for (C₁ + C₂) systems with and without tetrahydrofuran (THF). The compositions of vapor and hydrate phases under phase equilibrium were studied with model algorithm when structure I and structure II hydrates coexisted for the (methane + ethane) system. The average deviation between the modeled and actual mole fractions of ethane in hydrate and vapor phases was 0.55%, and that of ethylene was 5.7% when THF was not added. The average deviation of the mole fraction of ethane in vapor phase was 11.46% and ethylene was 7.38% when THF was added. The test results showed that the proposed algorithm is practicable.     

Exploitation of methane in the hydrate by use of carbon dioxide in the presence of sodium chloride

The replacement process of CH4 from CH4 hydrate formed in NaCl solution by using pressurized CO2 was investigated with a self-designed device at temperatures of 271.05,273.15 and 275.05 K and a constant pressure of 3.30 MPa.The mass fraction of the NaCl solution was either 0.5 wt% or 1.0 wt%.The effects of temperature and concentration of NaCl solution on the replacement process were investigated.Experimental results showed that high temperature was favorable to the replacement reaction but high NaCl concentration had a negative effect on the replacement process.Based on the experimental data,kinetic models of CH4 hydrate decomposition and CO2 hydrate formation in NaCl solution were established.The calculated activation energies suggested that both CH4 hydrate decomposition and CO2 hydrate formation are dominated by diffusion in the hydrate phase.     

The effect of a TEG additive on hydrate formation

In gas industry, gas hydrate formation has both advantages and disadvantages. The best advantage of gas hydrate is Persian Gulf water sweetening, carbon dioxide capture, and gas storage, and its drawbacks are pressure drop, plugging, and explosion in pipelines. In recent years, using the inhibitors to prevent hydrate formation is being considered among researchers. In this study, the new equilibrium data for hydrate formation of inlet natural gas to Gachsaran NGL-1200 refinery with addition of Tri ethylene glycol (TEG) with mass concentration of 5% and 15% in distilled and Persian Gulf waters were measured by constant-volume method. The experimental results show that the hydrate formation conditions will be hard with the increase of TEG concentration in distillated and Persian Gulf waters. In other words, TEG addition to Persian Gulf water had more inhibitory effect in hydrate formation than TEG addition to distilled water. The hydrate formation temperature, in pressure range of 28–29 bars, reduced 0.3°C and 2.7°C for distilled water and 2.8°C and 4.6°C for Persian Gulf water in presence of TEG with mass concentration of 5% and 15%, respectively.     

Modelling of methane hydrate formation pressure in the presence of different inhibitors

Formation of gas hydrates is one of the problems in the production, processing, and transportation of natural gas. Hence, an understanding of conditions where hydrates form is necessary to overcome hydrate-related issues. The aim of this study was to develop an effective relation between the methane hydrate formation pressure based on the temperature, weight fraction of inhibitor, and molecular weight of inhibitor using the least square support vector machine. This computational model indicates the great ability of predictions for determining hydrate pressure in the presence of different inhibitors such as the methanol, ethylene glycol, diethylene glycol, and triethylene glycol. The values of R-squared (R²) and mean squared error obtained for this model are 0.9925 and 0.2325, respectively. This developed predictive tool can be applied as an accurate estimation of methane hydrate formation pressure.