A Study on Inhibitors for the Prevention of Hydrate Formation in Gas Transmission Pipeline

Availableonlineatwww.sciencedirect.comScienceDirectJoumal of Natural Gas CemistryJournal of Natural Gas Chemistry 16(2007)81-85SCIENCE PRESSArticlea Study on Inhibitors for the Prevention of HydrateFormation in Gas Transmission pipelineMing Wu, Shumiao Wang, Hongbo Liu1. Department of Storage and Architecture Engineering, Liaoning University of Petroleum and Chemical Technology,Fushun 113001, Liaoning, China; 2. R &D Center, Xinao Group Langfang 065001, Hebei, China; 3. ChinaPetroleum Pipeline Engineering Corporation, Langfang 065000, Hebei, ChinaManuscript received August 18, 2006; revised October 27, 2006Abstract: Gas Hydrate is usually formed during the transportation and treatment of oil and gasresulting in the plugging of gas pipeline and equipment. Three thermodynamic calculation formulas areanalyzed to deal with this problem. The lowering of the freezing point of the inhibitors AT is used tocalculate the formation temperature of natural gas hydrates. This is considered to be a good approachbecause it is not limited by what kind and what concentration of inhibitors one uses. Besides, the rate oflowering of the freezing point could be easily measured. The result of testing methanol and mono-ethyleneglycol in a reactor shows that adding 10% inhibitors to the reactor can prevent the hydrates formationKinetic inhibitors are favored in the present research. They are divided into two types, polymer andsurface-active agents. Their characteristics, mechanisms, and application prospect are separately discussedPolymer inhibitors exhibit better efficiency. The result of field application of vC- 713 inhibiter is also givenin this article. In practice, the combination of thermodynamic inhibitors and kinetic inhibitors gives betterKey words: natural gas hydrate; thermodynamic inhibitor; kinetic inhibitor; polymer; surface-active1. Introductionin the offshore pipeline. Gas hydrate may also beformed during the liquefied separation process underGas hydrate is easily formed during the trans-ultralow temperature. Therefore, the investigation onportation of oil and gas when it contains a certain an effective method for preventing and eliminatingamount of water, resulting in the damage to the oilthe formation of gas hydrate has aroused significantand gas industry (1, 2]. Since Hammerschmidt(3) dis- erescovered in 1934 that gas hydrate would block the gasIn this article, different types of inhibitors, theirpipeline, the prevention of hydrate formation has be- characteristics, and their effect on hydrate forma-come an important matter. At present, researches on tion are described. The capacity of methanol andgas hydrate inhibitors have been performed in many mono-ethylene glycol as gas hydrate thermodynamicountries. During the transportation and processingespecially when the product gases contain saturated that the addition of 10% inhibitors to the reactor canwater steam and under cold weather conditionsprevent the formation of hydrate. The field applicahydrate will plug the pipeline, valve, and equipment凵中国煤化工 en in this artiThe miscible fluid of oil and gas will be transporteCNMHGto certain distance before it is subjected to dehy. 2L ammar and calculationdration and thus, the gas hydrate is easily formed formula of natural gas hydrate(NGH)orrespondingauthorEr-mail:wuming0413@sohu.comMing Wu et al. Journal of Natural Gas Chemistry Vol. 16 No. I 2007Study on the gas hydrate thermodynamic in- hydrate; AT gives the lowering of hydrate formationhibitor has become widespread. The most exten- temperature, denoted by Ksively used thermodynamic inhibitors are methanolIt should be pointed out that for derivingmono-ethylene glycol, diethylene glycol, and some equation(2), the following two assumptions are madeother electrolytes Inhibitor molecule or ion will com-(1) the component of inhibitor will not dissociate andpete with the water molecule, changes the thermo- generate hydrate itself; (2)R, n, and AH are con-dynamic equilibrium of water and hydrocarbon mole- stant. Further refinement may be necessary.Fromcule(changing the chemical potential of hydration), the above two formulas, it can be seen that theand prevent the formation of hydrate by moving the inhibitors with smaller molecular weight are morephase equilibrium curves to lower temperature and effective to lower the hydrate formation temperatrurehigher pressure. The hydrate will become instable Methanol and ammonia are considered to be the de-and decomposed and can be easily separatedsirable organic reagent and mineral substance, respecSome calculation formulas of the gas hydrate in- tively. Now much research is focused on the studyhibitor are as followsof new inhibitors. Glycol is not poisonous, its boil-△TM(100-x)(1) mg point is much higher than methanol, and evap-oration loss is negligible. Glycol is suitable for theIn Equation( 1), AT represents the temperature low- station where plenty of natural gas be treated. Fromering for the formation of hydrate, K is a constant this study, it is concluded that the thermodynamic inspecific to each inhibitor, M is the molecular weighteffective only when its content is less thanof the inhibitor: r is the mass concentration of the upof the gas to be treatedinhibitorabove-mentioned organic inhibitorsHammerschmidt [4 gives the empirical morganIcolution(dilute electrolyte solutionformula(1)for the calculation of lowering of gas hyincluding those of sodium chloride, calcium chloridedrate temperature and the K values for some of the rough niter, and lithium chloride, can also be usedinhibitors are listed in Table 1As far as effectiveness, nonpoisonous nature and lowcost are concerned, calcium chloride is the best choiceTable 1. The K values of some gas hydrate inhibitors Sodium chloride is also frequently used. But the cor-Inhibitorrosivity of its dilute electrolyte solution restricts itsMethanol, ethanol, cymene, ammoniaapplications under many conditionsSodium chlorideAccording to the thermodynamic derivation, it isGlycol, propyl2195concluded that the theoretical formula for calculatingSulphonal2425the lowering of hydrate formation temperature ATIt should be pointed out that Equation(1)does from the lowering of inhibitor freezing point ATisnot consider the content of inhibitor in the saturatedven bygas phase, and it is just enough to lower the saturaA7÷nx/m)2tion steam pressure This equation is not applicablefor the inhibitors that are not testedPieroen gives a formula for the calculation of low- Where To represents the freezing point of pure wa-ering of gas hydrate formation temperature and X3 ter, K; X represents the solidification heat of pure(Pieroen equationwater,K/kg; Represents the solidification heat ofinhibitor,K/kg: To gives the freezing point of in-4T nRT2X2(2) hibitor, K△HAmong the formulas above, Equation (1)Where X3 represents the mole fraction of dilute non- empirical formula and is not suitable for untested in-electrolyte solution; To represents the hydrate forma- hibitors. Equation(2)is a theoretical formula, whichion temperature without inhibitors denoted by K; is中国煤化 T ctrolyte inhibitor sAH means the heat of formation of one mole hydrate lutie for all inhibitors re-with n mole water at To, denoted by J/mol; R is the gardYHC Rations. In additiongas constant and its value is taken as 1.987/(molK); using this equation, the lowering of freezing point cann represents the number of water molecules in the be easily measured and therefore, it is widely usedJournal of Natural Gas Chemistry Vol. 16 No. I 20073. Kinetic inhibitor2-propyl-2-imidazoline, acrylate, N-methyl N-ethylacetamine, etc. The mechanism of their operation isThe addition of certain quantity of kinetic in- via the formation or adsorption of eutectic crystals tohibitors limit or delay the growth of gas hydrate and prevent the growth of hydration nucleus so that thethus prevent the formation of hydrate[5]. The added hydration particles are scattered and do not gather,concentration of this kind of inhibitor is low. These in- thus the hydrate formation is prevented. Many hyhibitors prevent the agglomeration of hydrate crystal drate dynamic inhibitors have been developed, andgrain and plugging by decreasing the rate of formation among them, PVP, PVCap, VC-713, and P(VP/c)of hydrate. This is accomplished by the addition ofwhich are composed of PVP and PVCap in the ratiosmall quantity of chemical adjunct, which reduces the 1: 1. PVP. are considered as the first generation ki-nucleation rate of hydrate as well as delays and even netic inhibitorsprevents the formation of critical nucleus and thus in-In 1972. Yukiey first introduced the idea that-terferes the first growth direction and directional sta- sUsurface-active agents can be assembled as hydrate de-bility of hydrate crystal to prevent the formation of fence agents. Recently, French Petroleum Institutehydrate. This method has two advantages: only small (I.F. P )has listed surface-active agents in a series ofquality of inhibitor is required and has high efficiency. patents, and proved that nonionic amphiprotic comIt has become the hot point of present research[)pound could restrict the formation of gas hydrate inOn the basis of the different mechanism of mole- the gas pipe. Some of them are amide compoundscular reaction, Kinetic inhibitor can be divided into The most efficient surface-active reagents are hydroxythree: Hydrate Growth Delay inhibitor. Hydrate Ag- carboxylic acid amide, (in which the carboxyl groupglomeration Inhibitor, and Dual-Purpose Inhibitor.with 3-36 atoms are better, 8-20 best), alkoxy dihHydrate Growth Delay inhibitor will delay the growth droxy carboxylic acid amide(or polyalkoxy dihydroxyrate of hydrate nucleus and prevent their rapidoid amide), and N, N-dihydroxy carboxylic acid amidegrowth, and when the hydrate is stagnated in theThe common surface-active agents used in industryfluid, it leads to their deposition. Hydrate Agglom- for this purpose are SDS, SAS, DLS, and decyl bereration Inhibitor restricts the assembling tendency of zene amine. They all contain weak electronegativethe hydrate crystal and leads to suspension of the y- oxygen atomsdrate in fluid and then flow out with the fluid withoutAt present, the polymer inhibitor gives better reblockingsults and is applied more extensively. The most im-o, The content of the Kinetic inhibitor used is gen- portant function of Kinetic inhibitor is the effectiveally from 0.01%to 0.5% with its molecular weight prevention of hydrate formation. Any fault associatedranging from several thousands to millions. Its coswith the pouring system and the well not closed reg-ularly or the insufficient use of inhibitor willcomparing with the thermodynamic inhibitor is more hydrate blocking, and under these conditions, Kinetiume and pour volume. Its use and maintenance areinhibitor cannot be applied. The hydrate blockingcan be prevented by the addition of methanol or byvery convenientadopting the pressure drop method [8. Therefore, inKinetic inhibitor generally contains polymer and actual application a combination of kinetic inhibitorsurface-active agents, and only small amount of this and thermodynamic inhibitor are used to solve theinhibitor is required for preventing the formation of hydrate-plugging problemhydrate. Emulsification between the water phase ande have tested the influence of different concen-the oil phase will occur to prevent the agglomeration trations of thermodynamic inhibitor on the hydrateof the hydrate crystal before the formation of the hy- formation temperature. Here we take methanol anddrate. The advantage of this inhibitor is that its pglycol as examples. We also take VC-713 as the sam-formance is not influenced by temperature [7]ple of kinetic inhibitor and test its effect on the spotThe characteristic of this type of polymer mole-cule chain is that it contains many water-solublegenes and has long fatty carbon chain. The polymermonomers are usually PVP, (N, N-2 dimethylamine) 4.1TYH中国煤化工CNMHGethyl methacrylate, PVCap, N-acyl polyolefine imine,polyisopropyl methyl orange, N, N-alkyl acrylamide, 4.1.1. Experimental equipment84Ming Wu et al. Journal of Natural Gas Chemistry Vol. 16 No. 1 2007Figure 1 shows the experimental equipment. Itis composed of six parts. They are high-pressuresapphire cell, churn-dasher, constant temperaturebath, pressure increase system, temperature and prsure test system, and data collection system(4(3)Figure 2. Hydrate formation in the reactor afterdding different content of methanol(1)field pipeline, (2)0% methanol solution,(3)10% methanolsolution,(4)20% methanol solutionFigure 1. Schematic diagram of the experimental ap-paratus for hydrate formation1-Inlet valve, 2-Air bath, 3-SapphireMagnetic stirrer,5-U magnet, 6-Floating piston, 7-O-ring, 8-Pressure gaugeDS-Driving system, DPT-Differential pressure transducer,Temperature(℃)TT-Temperature transmitter, DAS-Data acquisition systemFigure 3. Hydrate formation in the reactor afterPDP-Positive displacement pump, MC-Methane cylinderadding different content of glycol(1)field pipeline, (2)0% glycol solution,(3)10% glycol solu-4. 1.2. Experimental resultstion, (4)20% glycol solutionAccording to the mole composition data of theWhere curve(1)indicates the temperature andatural gas transported in a ground pipeline, we simu- pressure of the field pipeline; indicates thlate the field condition in our laboratory as presented centage of glycol additionin Table 2. The environment temperature is 6C.Af-After adding the inhibitor, under the same pres-ter the addition of the inhibitor, hydrate formation in sure, the hydrate formation temperature clearlythe reactor is shown in Figures 2 and 3.drops. The more the inhibitor content, the higherthe temperature drop. At the same temperature, theTable 2. Mole composition data of naturalhydrate formation pressure obviously ascends withgas transported in the reactorthe inhibitor content. After adding the inhibitor, itsComponent ofuncharged cluster produces some type of interactionnatural gasforce with the water molecule and destroys the liquidYH中国煤化工 y the hydrogen bondCNMHGto surmount this in.id. So the gas hydrateC3Heeds some extra energy to change the conditions ofhydrate formation temperature and pressure from theJournal of Natural Gas Chemistry Vol. 16 No. 1 2007actual operating conditionmethanol under the condition of hydrate formationFrom the figures, it can be seen that by the ad- VC-713 is more economical than methanoldition of 10% methanol or glycol, the hydrate generation curve and temperature pressure curve of theTable 3. Hydrate generation conditions after adding0.5% VC-713 inhibitorground pipeline do not intersect, and the formationof hydrate in the reactor can be efficiently preventedNumber of days Supercooling (C) Hydrate generationThe traditional thermodynamic inhibitor has the dis-not formeddvantages that large quantity of inhibitor is requirednot formedfor the prevention of hydrate formation. It alsoequires huge storage and injection equipment andcauses environmental pollution. Its use is inconve-nient and is not economical. Therefore. much interest 5. Conclusionsis paid on the development of kinetic inhibitors in-stead of the traditional thermodynamic inhibitoTwo types of inhibitors are described in this ar-ticle, they are thermodynamic inhibitors and kineti4.2. Kinetic inhibitor experiment on-the-spot inhibitors. Three thermodynamic calculation formu-las were analyzed, and the lowering of freezing pointA field test was performed by the addition of ve by the addition of inhibitors is used for the calcula-713 inhibitor to a sea well mouth in Beihai. This tion of the formation temperature of the natural gaswell produces 0.566x10 m of natural gas, 1.59 m hydrates. From the results of testing methanol andof congealed oil, and 0.64 m of water per day. TImono-ethylene glycol as the inhibitors in a pipeline itproduced fluid is transported to a platform, where it is concluded that using 10% inhibitors for the pipelineis separated, compressed, and dehydrated through a could restrict the hydrate formation. Kinetic in-9.4 km long and 0.2 m diameter pipeline.hibitors were also studied. They are divided into polyVC-713 is a terpolymer, when added with a solu- mers and surface-active agents. The result of applyingtion of concentration of less than 2%, and when in- vC-713 in the field showed that polymer inhibitorsused with a viscosity lower than 45 MPa-s, its concen- had better efficiency and good application prospecttration in the water phase of pipeline is approximately In practice, it is better to use the thermodynamic in-between 0.25% and 0.5%. There are four steps for hibitors together with the kinetic inhibitorsfield measurements. First step is the determinationof the operating conditions of the pipeline. Second Referencesstep analyzes the formation of hydrates. The highestdegree of supercooling is measured in the third step, (1 Long J. The 73th Annual GPA Convention, New Orand the forth step evaluate the effect of low concentra-leans. LA. 199tion on the formation of hydrates. The formation rate [2] Sloan E D. Gas Research Institute Topical Repoof hydrate can be judged by detecting the decrease inGRI91/0302,1992,6flow and by the increase in pressure. Fluid temper- [3] Hammerschmidt E G. Ind Eng Chem, 1934, 26(8)ature and pressure of the terminal point are used toestimate supercooling of the fluid in the pipeline4 Makogon Y F. Hydrates of Natural Gas. Wang M ShUnder the field test condition of adding 0.5% vctrans. Beijing: Petroleum Industry Press, 1987. 10713 inhibitor (see Table 3) it is concluded5 Xu Y J, YangX X, Ding J, Ye GX. Natural Gasndustry,2004,24(12):13The difference between the flat temperature and(6 Wu D J, Hu Y F, Yang J T. Natural Gas Industthe melting point at the cooling curve during the2000,20(6):95course of crystallization is defined as supercooling. It (7 Zhao Y, Ding J. Natural Gas Industry, 2004, 24(12)was found from the experiment that vc-713 inhibitorid not reduce the effort of clearing away hydrate of 8 Chen G L. Natural Gas Industry, 2004, 24(8): 89中国煤化工CNMHG