离子液体与动力学抑制剂作用下混合气体水合物生成特性研究

利用等温恒容法实验考察了一种高性能离子液体N-丁基-N-甲基吡咯烷四氟硼酸盐([BMP][BF₄])、两种动力学抑制剂(KHIs)[聚乙烯基吡咯烷酮(PVP)和聚乙烯己内酰胺(PVCap)]及[BMP][BF₄]与KHI的二元混合物对甲烷/乙烷/丙烷三元混合气体水合物生成动力学过程的影响规律。通过分析压力和气相组分变化规律，发现混合气体水合物呈现两步骤生长模式。在高过冷度(>10℃)和高搅拌速率(1000 r/min)条件下，单一添加剂基本失效，而[BMP][BF₄]可较好协助增强PVCap的抑制性能。粉末X射线衍射和激光拉曼光谱测试结果均显示，所有体系中形成的水合物样品结构同时存在sⅠ型和sⅡ型，抑制剂的添加主要影响两种晶体结构的相对含量和各客体分子的笼子占有率。最后探讨了协同抑制机理。     

一种可生物降解水合物动力学抑制剂的研究

目前用于天然气水合物防治的工业动力学抑制剂主要是水溶性聚合物，如聚乙烯基吡咯烷酮（PVP）、聚乙烯基己内酰胺（PVCap）、Gaffix VC-713等，然而生物降解性低限制了其工业应用。因此，开发环保型的抑制剂具有重要意义。实验采用易生物降解的海藻酸钠与PVCap的单体接枝共聚，合成一类新型水合物动力学抑制剂NaAlg-g-PVCap，结合最大过冷度及耗气量评价了新型抑制剂在水合物生成过程中的抑制性能，并通过BOD₅/COD值评价了新型抑制剂的生物降解性。结果表明低剂量[0.25%（质量）]下NaAlg-g-PVCap的最大耐受过冷度优于PVP K90，但低于PVCap，且随着添加剂量增大而微弱降低；在其最大耐受过冷度以下（ΔT_sub=5℃），NaAlg-g-PVCap表现出较好的水合物成核和生长抑制作用，其体系水合物初始生长速率值约只为纯水体系的 1/10，也远高于PVP体系，且总耗气量相比纯水及PVP体系减少了60%以上，与PVCap体系接近，但随着过冷度增大，NaAlg-g-PVCap成核抑制作用下降明显，这可能是共聚物中两部分共同作用的结果；同时，NaAlg-g-PVCap相比PVCap其生物降解性提高了26%， 倾向于易降解。说明PVCap与NaAlg共聚后优化了整体的性能，表现出较好的水合物抑制性能和生物降解性。     

水合物动力学抑制剂室内实验及现场应用

在天然气开采及储运中、水合物导致阀门阻塞、气井停产、管道停输等严重事故。为了满足低耗、高效、环保等要求,本文对四种水合物动力学抑制剂进行了不同浓度下的抗冻性性能评价以及不同浓度、不同过冷度下抑制剂流动实验研究,结果表明在抗冻数据可知,随着抑制剂浓度的增加,抑制剂溶液的抗冻温度点逐渐降低;并且在相同浓度下的抗冻效果VC-713抑制剂最好,1^#抑制剂次之,PVP抑制剂溶液最差。不同浓度、不同过冷度下抑制剂流动实验中,在0℃上下的环境温度,浓度在1%～2%之间的这四种水合物抑制剂是可以起到抑制效果的;并且在此浓度范围内的这四种抑制剂,其浓度对水合物抑制效果影响不大。2.0%的四种抑制剂在过冷度均小于15℃,即环境温度在-5℃以上时,可以起到抑制水合物生成的作用,而环境温度在-5～-25℃时,这四种抑制剂都起不到抑制效果;VC-713抑制剂作用最好,1#抑制剂次之,PVP抑制剂溶液最差。不同浓度、不同过冷度下抑制剂流动实验中,在0℃上下的环境温度,浓度在1%～2%之间的这四种水合物抑制剂是可以起到抑制效果的;并且在此浓度范围内的这四种抑制剂,其浓度对水合物抑制效果影响不大。2.0%的四种抑制剂在过冷度均小于15℃,即环境温度在-5℃以上时,可以起到抑制水合物生成的作用,而环境温度在-5～-25℃时,这四种抑制剂都起不到抑制效果;VC-713抑制剂作用最好,1^#抑制剂作用次之,PVP抑制剂作用最差。并在苏里格井区中一口井进行了现场试验,效果明显。     

动力学抑制剂与乙二醇协同作用下甲烷水合物再生成

在500mL的高压反应釜中,实验研究了乙二醇（MEG）与动力学抑制剂PEO-co-VCap-1在细砂存在下对甲烷水合物再生成过程的协同抑制作用。实验过程中,控制MEG的质量分数范围为0~5%,PEO-co-VCap-1的质量分数范围为0~0.5%,形成4种的抑制剂配伍组合,进行了12组实验。实验结果表明,PEO-co-VCap-1在单独作用时,可以延缓水合物的成核阶段,但可能导致水合物在生长阶段短时间内大量生成的灾难性生长现象。其与MEG复配可在延缓水合物成核的同时,有效减少灾难性增长现象的出现,降低油气管输的堵管风险。当MEG质量分数为5%、PEO-co-VCap-1质量分数为0.5%时,协同抑制效应极为明显,可将甲烷水合物诱导期延长至2800min以上。MEG同PEO-co-VCap-1的协同抑制效果与提高温度的抑制作用相似。这一发现表明,如果在使用PEO-co-VCap-1的同时使用MEG等良好的增效剂,有助于动力学抑制剂用于更高的过冷度环境,为高效解决高过冷条件下油气生产中的水合物防控问题提供新的可能。     

SDS对甲烷水合物生成动力学和微观结构的影响

表面活性剂是促进水合物生成的有效手段之一。在高压反应釜中研究了十二烷基硫酸钠（SDS）对水合物生成过程的动力学影响,利用XRD和拉曼光谱探究了SDS存在条件下水合物的微观结构。宏观结果表明SDS缩短了诱导时间,加快了水合物生长速率。微观结果表明SDS没有影响sI型水合物的晶型结构,晶面间距与理想sI型水合物及纯水甲烷对比误差在千分之几。水合物中甲烷在大笼小笼中的拉曼位移分别为2904和2915 cm^-1,SDS没有改变大笼小笼结构。大笼绝对占有率（q_L）接近饱和时,SDS可以进一步提高小笼绝对占有率（q_S）,从微观角度证明了SDS可以减少水合数,提高储气率。     

聚乙烯己内酰胺链端改性及其对甲烷水合物形成的抑制作用研究

注入动力学抑制剂是一种有效缓解天然气水合物管道堵塞的方法。本文以动力学抑制剂聚乙烯基己内酰胺(PVCap)结构为基础,将氧乙基和酯基引入PVCap的分子链端,合成了新抑制剂PVCap-XA1,在高压定容反应釜内评价了PVCap-XA1对甲烷水合物形成的抑制作用,并采用粉末X射线衍射、低温激光拉曼光谱和冷冻扫描电子显微镜研究了抑制剂对甲烷水合物结构和形态的影响。实验结果表明,相同实验条件下PVCap-XA1比PVCap具有更好的抑制作用;微观测试表明PVCap-XA1的加入没有改变甲烷水合物的晶体结构,但会使甲烷水合物晶面扭曲变形,可以降低水合物大小笼占有比(I_L/I_S),使得甲烷分子更难进入水合物大笼,同时PVCap-XA1的加入使甲烷水合物的微观形貌由多孔有序变得更致密而不利于气体通过。     

甲烷水合物生成分解的实验研究

海底存在着大量可燃冰,1 m3可燃冰能够储存160 m3的天然气。因此,可燃冰的开采与利用可燃冰储存与运输天然气具有重要意义。在改变搅拌、过冷度及低浓度动力学抑制剂的条件下,对甲烷水合物生成量与生成速率进行了实验研究。将甲烷水合物进行升温分解,分析水合物分解时的压力变化情况。结果表明:搅拌对甲烷水合物生成的促进效果最好,其次是过冷度,最后是超低浓度动力学抑制剂;水合物生成的传质过程最终被阻碍,采取将水与天然气的上下位置交换的方法,可以生成更多水合物。水合物升温可以得到相平衡曲线;改变初始时刻压力,可以得到不同温度区间的相平衡曲线;降低水合物分解时的升温速度,可以得到更长温度区间的相平衡曲线。     

TBAB水合物晶体生长过程的实验研究

通过对常压下9%(w) TBAB溶液降温生成水合物晶体的过程进行观察,以过冷度作为生长驱动力,研究了TBAB水合物晶体的生长特性. TBAB水合物刚形成时,晶体呈高透光度及规则柱体外形,随晶体继续生长逐渐变得不规则,透光度下降. 当恒温浴的过冷度分别为6.0, 8.1和9.6 K时,水合物晶体长度/宽度方向生长时间依次为183 min/140 min, 85 min/65 min, 70 min/37 min,同时反应前的等待时间即生成时间分别为83, 53和55 min. 生长时间和生成时间随过冷度增加逐渐降低,幅度逐渐减小,表明过冷度增加能有效减少水合反应时间,促进水合物的快速生成. 过冷度增加会增加TBAB水合物晶体成核数量,因此晶体与溶液的总体接触面积增加,有利于水合物的快速形成.     

动力学型天然气水合物抑制剂研究进展

介绍了动力学型天然气水合物抑制剂研究进展、水合物生成动力学,分析了动力学抑制剂的抑制机理。动力学抑制剂的代表性产品有PVP、PVCap、P(VP/VC)、VC-713,抑制效果VC-713PVCapVP/VCPVP,与其他醇类、醚类等复配使用时抑制效果更好。     

水合物动力学抑制剂的作用机理研究进展

水合物动力学抑制剂作为低液量抑制剂,其可应用于深水流动保障风险控制水合物冻堵问题,受到国内外研究者的广泛关注。本文重点阐述了动力学抑制剂的可承受最大过冷度和对诱导时间的延长这两个评价指标,同时梳理了动力学抑制剂对水合物生成及分解过程影响的研究成果。总体而言,可承受最大过冷度越大、延长诱导时间程度越强的动力学抑制剂,抑制水合物生成并保障流动安全的可靠性越高;动力学抑制剂对水合物生成与分解过程存在复杂的影响规律。本文将其对气体消耗速率、气体消耗量和形态,分解温度、时间和分解速率,“记忆效应”等影响进行了分析。结合上述研究成果,总结了动力学抑制剂对水合物的影响机理,特别是提出了化学型动力学抑制剂对水合物吸附抑制机理的概念示意图。最后,给出了未来深入开展动力学抑制剂研究的建议。     

Effect of complex inhibitors containing ionic liquids and PVP K90 on formation of methane hydrate

Injecting inhibitors is the most commonly used method in the oil and gas industry to solve the problem of blockage caused by hydrate formation during pipeline transportation. However, most of the kinetic hydrate inhibitors (KHIs) are strictly limited by weak inhibition performance and low subcooling. Ionic liquids, a kind of green solvent, have been recognized to act as excellent thermodynamic inhibitors on methane hydrate formation. So, it is proposed to add the ionic liquids into KHIs to improve their overall performance. In this paper, the kinetic effects of an ionic liquid N-butyl-N-methylpyrrolidine tetrafluoroborate ([BMP][BF₄]), a commercial kinetic inhibitor polyvinyl pyrrolidone (PVP K90) and their mixtures with different mass ratios on the methane hydrate formation were experimentally studied at 8.0 K subcooling and two concentrations [1.0%(mass) and 2.0%(mass)]. The best mass ratio of the compound inhibitor was determined. Moreover, the crystal structures and cage occupancy characteristics of methane hydrates formed without and with inhibitors at different mass concentrations and composition ratios were measured by using powder X-ray diffraction (PXRD) and low-temperature Laser Raman spectrometers. It was found that the addition of inhibitors did not change the crystal structure of methane hydrate, but affected the cage occupancies and hydration numbers. Based on the results from macroscopic kinetics and microscopic structure tests, the inhibition mechanism of compound inhibitors was proposed.     

Synthesis and evaluation of poly (N-vinyl caprolactam)-co-tert-butyl acrylate as kinetic hydrate inhibitor

Low dosage kinetic hydrate inhibitors (KHIs) are a kind of alternative chemical additives to high dosage thermodynamic inhibitors for preventing gas hydrate formation in oil & gas production wells and transportation pipelines. In this paper, a new KHI, poly (N-vinyl caprolactam)-co-tert-butyl acrylate (PVCap-co-TBA), was successfully synthesized with N-vinyl caprolactam (NVCap) and tert-butyl acrylate. The kinetic inhibition performances of PVCap-co-TBA on the formations of both structure I methane hydrate and structure II natural gas hydrate were investigated by measuring the onset times of hydrate formation under different conditions and compared with commercial KHIs such as PVP, PVCap and inhibex 501. The results indicated that PVCap-co-TBA outperformed these widely applied inhibitors for both structure I and structure II hydrates. At the same dosage of KHI, the maximum tolerable degree of subcooling under which the onset time of hydrate formation exceeded 24 hours for structure I hydrate was much lower than that for structure II hydrate. The inhibition strength increased with the increasing dosage of PVCap-co-TBA; The maximum tolerable degree of subcooling for the natural gas hydrate is more than 10 K when the dosage was higher than 0.5% (mass) while it achieved 12 K when that dosage rose to 0.75% (mass). Additionally, we found polypropylene glycol could be used as synergist at the dosage of 1.0 % (mass) or so, under which the kinetic inhibition performance of PVCap-co-TBA could be improved significantly. All evaluation results demonstrated that PVCap-co-TBA was a very promising KHI and a competitive alternative to the existing commercial KHIs.     

Kinetic Modeling of Methane Hydrate Formation in the Presence of Low‐Dosage Water‐Soluble Ionic Liquids

The kinetic and thermodynamic effects of three typical low‐dosage imidazolium‐based ionic liquids (ILs) on methane hydrate formation and dissociation were investigated, considering the anion nature and subcooling and/or overpressure driving forces. Isochoric hydrate formation and dissociation data were obtained by the modified slow step‐heating method. ILs proved to have a dual effect on both formation and dissociation of methane hydrate including thermodynamic and kinetic inhibition. Kinetic modeling of methane hydrate inhibition by low‐dosage ILs was performed. Kinetic analysis showed that IL inhibitors mainly cause a delay in the nucleation or hydrate growth step. The related inhibition mechanism was resolved regarding the ionic nature and electrostatic interactions of ILs with water molecules. Two binomial exponential kinetic relations were derived and used for simple methane hydrate formation in the presence of ILs as kinetic hydrate inhibitors. The proposed relations can serve for a quick estimation of the nature, extent, strength, and effectiveness of ILs on various gas hydrates.     

An investigation into the kinetic hydrate inhibitor properties of two imidazolium-based ionic liquids on Structure II gas hydrate

Kinetic hydrate inhibitors (KHIs) are used to prevent gas hydrate formation in gas and oilfield operations. All KHIs discovered to date are water-soluble polymers. However, their performance can be enhanced by certain non-polymeric organic molecules. Recently, it was claimed that certain imidazolium-based ionic liquids could have a dual function, acting as both thermodynamic inhibitors and KHIs (Xiao, C., Adidharma, H., 2009. Chem. Eng. Sci. 64, 1522). As the KHI experimental work was carried out at a temperature of –12 °C, giving a very high subcooling of about 25 °C, we reinvestigated two of these ionic liquids at more typical subsea temperatures and subcoolings. We find that these ionic liquids are very poor KHIs when used alone at 5000–10000 ppm, but they are fairly good synergists for commercial KHIs based on vinyl lactam polymers and hyperbranched poly(ester amide)s. Both ionic liquids showed only weak growth inhibition of tetrahydrofuran hydrate crystals. Finally, both ionic liquids were poorly biodegraded in the OECD306 seawater 28 day biodegradation test.     

Tetrahydrofuran hydrate crystal growth inhibition by hyperbranched poly(ester amide)s

Kinetic hydrate inhibitors (KHIs) are water-soluble polymers designed to delay gas hydrate formation in gas and oilfield operations. Inhibition of growth of gas hydrate crystals is one of the mechanisms by which KHIs have been proposed to act. One class of commercial KHIs is the hyperbranched poly(ester amide)s. We have investigated the ability of a range of structurally different hyperbranched poly(ester amide)s to inhibit the crystal growth of tetrahydrofuran (THF) hydrate which forms a Structure II clathrate hydrate, the most common gas hydrate structure encountered in the upstream oil and gas industry. The results indicate that there is an optimum size of hydrophobic groups attached to the succinyl part of the polymer, which gives best crystal growth inhibition. However, total inhibition was impossible to achieve even at a concentration of 8000 ppm of one of the best polymers at a subcooling of 3.4 °C, tentatively suggesting that polymer adsorption onto natural gas hydrate crystal surfaces is probably not the primary mechanism of kinetic inhibition operating in field applications with this class of KHI.     

Experimental and density functional theory computational evaluation of poly(N-vinyl caprolactam-co-butyl methacrylate) kinetic hydrate inhibitors

Natural gas hydrate inhibitor has been serving the oil and gas industry for many years. The development and search for new inhibitors remain the focus of research. In this study, the solution polymerization method was employed to prepare poly(N-vinyl caprolactam-co-butyl methacrylate) (P(VCap-BMA)), as a new kinetic hydrate inhibitor (KHI). The inhibition properties of P(VCap-BMA) were investigated by tetrahydrofuran (THF) hydrate testing and natural gas hydrate forming and compared with the commercial KHIs. The experiment showed that PVCap performed better than copolymer P(VCap-BMA). However, low doses of methanol or ethylene glycol are compounded with KHIs. The compounding inhibitors show a synergistic inhibitory effect. More interesting is the P(VCap-BMA)-methanol system has a better inhibitory effect than the PVCap-methanol system. 1% P(VCap-BMA) + 5% methanol presented the best inhibiting performance at subcooling 10.3 ℃, the induction time of natural gas hydrate was 445 min. Finally, the interaction between water and several dimeric inhibitors compared by natural bond orbital (NBO) analyses and density functional theory (DFT) indicated that inhibitor molecules were able to form the hydrogen bond with the water molecules, which result in gas hydrate inhibition. These exciting properties make the P(VCap-BMA) compound hydrate inhibitor promising candidates for numerous applications in the petrochemical industry.     

The gas-adsorption mechanism of kinetic hydrate inhibitors

Molecular dynamics was employed to study the inhibition mechanism of vinyl lactam-based kinetic hydrate inhibitors (KHIs). By comparing the inhibition functions of the same KHIs at different initial locations, we found that the KHI molecules on the surface of hydrate nuclei could obviously prolong the hydrate induction time and exhibited the best inhibition effect. The impacts of KHIs on the methane migration and the arrangement of H₂O molecules were analyzed at the molecular level. A gas-adsorbing mechanism for KHIs (i.e., the KHIs with an excellent gas adsorption ability could reduce the supersaturation of methane in the aqueous solution, reinforce the migration resistance of methane to the nucleus, and further inhibit the hydrate growth) was proposed. In addition, the conformations of KHI polymer molecules in the aqueous solution are closely related to their inhibitory effect, that is, stretched skeletons and well-organized structures would maximize their inhibitory effect.     

Reviews of gas hydrate inhibitors in gas-dominant pipelines and application of kinetic hydrate inhibitors in China

During the development and application of natural gas, hydrate plugging the pipelines is a very important issue to solve. Currently, adding thermodynamic hydrate inhibitors (THIs) and kinetic hydrate inhibitors (KHIs) in gas-dominated pipelines is a main way to prevent hydrate plugging of flow lines. This paper mainly reviews the efforts to develop THIs and KHIs in the past 20 years, compare the role of various THIs, such as methanol, ethylene glycol and electrolyte, and give the tips in using. The direction of KHIs is toward high efficiency, low toxicity, low pollution and low cost. More than a hundred inhibitors, including polymers, natural products and ionic liquids, have been synthesized in the past decade. Some of them have better performance than the current commercial KHIs. However, there are still few problems, such as the complex synthesis process, high cost and low solubility, impeding the commercialization of these inhibitors. The review also summarized some application of KHIs in China. Research of KHIs in China began late. There are no KHIs used in gas pipelines. Only a few field tests have been carried out. In the end of this paper, the field test of self-developed KHIs by China is summarized, and the guidance is given according to the application results.