Journal of Natural Gas Chemistry 16(2007)8185

Article

A Study on Inhibitors for the Prevention of Hydrate

Formation in Gas Transmission Pipeline

Ming Wu1, Shumiao Wang2, Hongbo Liu3

1. 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. China

Petroleum Pipeline Engineering Corporation, Langfang 065000, Hebei, China

[Manuscript received August 18, 2006; revised October 27, 2006]

Abstract: Gas Hydrate is usually formed during the transportation and treatment of oil and gas,resulting 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 T 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 formation.Kinetic 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 discussed.Polymer 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 betterresult.

Key words: natural gas hydrate; thermodynamic inhibitor; kinetic inhibitor; polymer; surface-activeagents

1. Introduction

Gas hydrate is easily formed during the trans-portation of oil and gas when it contains a certainamount of water, resulting in the damage to the oiland gas industry [1,2]. Since Hammerschmidt [3] dis-covered in 1934 that gas hydrate would block the gaspipeline, the prevention of hydrate formation has be-come an important matter. At present, researches ongas hydrate inhibitors have been performed in manycountries. During the transportation and processing,especially when the product gases contain saturatedwater steam and under cold weather conditions, gashydrate will plug the pipeline, valve, and equipment.The miscible fluid of oil and gas will be transportedto certain distance before it is subjected to dehy-dration and thus, the gas hydrate is easily formed

in the offshore pipeline. Gas hydrate may also beformed during the liquefied separation process underultralow temperature. Therefore, the investigation onan effective method for preventing and eliminatingthe formation of gas hydrate has aroused significantinterest.

In this article, different types of inhibitors, theircharacteristics, and their effect on hydrate forma-tion are described. The capacity of methanol andmono-ethylene glycol as gas hydrate thermodynamicinhibitors was tested in a reactor. The results showthat the addition of 10% inhibitors to the reactor canprevent the formation of hydrate. The field applica-tion of VC-713 inhibitor is also given in this article.

2. Thermodynamic inhibitor and calculationformula of natural gas hydrate (NGH)

Corresponding author. E-mail: wuming0413@sohu.com

82 Ming Wu et al./ Journal of Natural Gas Chemistry Vol. 16 No. 1 2007

Study on the gas hydrate thermodynamic in-hibitor has become widespread. The most exten-sively used thermodynamic inhibitors are methanol,mono-ethylene glycol, diethylene glycol, and someother electrolytes. Inhibitor molecule or ion will com-pete with the water molecule, changes the thermo-dynamic equilibrium of water and hydrocarbon mole-cule (changing the chemical potential of hydration),and prevent the formation of hydrate by moving thephase equilibrium curves to lower temperature andhigher pressure. The hydrate will become instableand decomposed and can be easily separated.

Some calculation formulas of the gas hydrate in-hibitor are as follows:

T =K x

M (100 x) (1)

In Equation (1), T represents the temperature low-ering for the formation of hydrate, K is a constantspecific to each inhibitor, M is the molecular weightof the inhibitor; x is the mass concentration of theinhibitor.

Hammerschmidt [4] gives the empiricalformula (1) for the calculation of lowering of gas hy-drate temperature, and the K values for some of theinhibitors are listed in Table 1.

Table 1. The K values of some gas hydrate inhibitors

Inhibitor K values

Methanol, ethanol, cymene, ammonia 1228

Sodium chloride 1220

Glycol, propyl 2195

Sulphonal 2425

It should be pointed out that Equation (1) doesnot consider the content of inhibitor in the saturatedgas phase, and it is just enough to lower the satura-tion steam pressure. This equation is not applicablefor the inhibitors that are not tested.

Pieroen gives a formula for the calculation of low-ering of gas hydrate formation temperature and X3(Pieroen equation):

T =nRT 20H

X3 (2)

Where X3 represents the mole fraction of dilute non-electrolyte solution; T0 represents the hydrate forma-tion temperature without inhibitors denoted by K;H means the heat of formation of one mole hydratewith n mole water at T0, denoted by J/mol; R is thegas constant and its value is taken as 1.987/(molK);n represents the number of water molecules in the

hydrate; T gives the lowering of hydrate formationtemperature, denoted by K.

It should be pointed out that for derivingequation (2), the following two assumptions are made:(1) the component of inhibitor will not dissociate andgenerate hydrate itself; (2) R, n, and H are con-stant. Further refinement may be necessary. Fromthe above two formulas, it can be seen that theinhibitors with smaller molecular weight are moreeffective to lower the hydrate formation temperature.Methanol and ammonia are considered to be the de-sirable organic reagent and mineral substance, respec-tively. Now much research is focused on the studyof new inhibitors. Glycol is not poisonous, its boil-ing point is much higher than methanol, and evap-oration loss is negligible. Glycol is suitable for thestation where plenty of natural gas be treated. Fromthis study, it is concluded that the thermodynamic in-hibitor is effective only when its content is less thanup to 6% of the gas to be treated.

Besides the above-mentioned organic inhibitors,inorganic acid solution (dilute electrolyte solution),including those of sodium chloride, calcium chloride,rough niter, and lithium chloride, can also be used.As far as effectiveness, nonpoisonous nature and lowcost are concerned, calcium chloride is the best choice.Sodium chloride is also frequently used. But the cor-rosivity of its dilute electrolyte solution restricts itsapplications under many conditions.

According to the thermodynamic derivation, it isconcluded that the theoretical formula for calculatingthe lowering of hydrate formation temperature Tfrom the lowering of inhibitor freezing point T

is

given by

T =n

(T 0T 0

)2T (3)

Where T 0 represents the freezing point of pure wa-ter, K; represents the solidification heat of purewater, K/kg; represents the solidification heat ofinhibitor, K/kg; T 0 gives the freezing point of in-hibitor, K.

Among the formulas above, Equation (1) is anempirical formula and is not suitable for untested in-hibitors. Equation (2) is a theoretical formula, whichis confined only to dilute nonelectrolyte inhibitor so-lution. Equation (3) is applicable for all inhibitors re-gardless of its types and concentrations. In addition,using this equation, the lowering of freezing point canbe easily measured and therefore, it is widely used.

Journal of Natural Gas Chemistry Vol. 16 No. 1 2007 83

3. Kinetic inhibitor

The addition of certain quantity of kinetic in-hibitors limit or delay the growth of gas hydrate andthus prevent the formation of hydrate [5]. The addedconcentration of this kind of inhibitor is low. These in-hibitors prevent the agglomeration of hydrate crystalgrain and plugging by decreasing the rate of formationof hydrate. This is accomplished by the addition ofsmall quantity of chemical adjunct, which reduces thenucleation rate of hydrate as well as delays and evenprevents the formation of critical nucleus and thus in-terferes the first growth direction and directional sta-bility of hydrate crystal to prevent the formation ofhydrate. This method has two advantages: only smallquality of inhibitor is required and has high efficiency.It has become the hot point of present research [6].

On the basis of the different mechanism of mole-cular reaction, Kinetic inhibitor can be divided intothree: Hydrate Growth Delay inhibitor, Hydrate Ag-glomeration Inhibitor, and Dual-Purpose Inhibitor.Hydrate Growth Delay inhibitor will delay the growthrate of hydrate nucleus and prevent their rapidgrowth, and when the hydrate is stagnated in thefluid, it leads to their deposition. Hydrate Agglom-eration Inhibitor restricts the assembling tendency ofthe hydrate crystal and leads to suspension of the hy-drate in fluid and then flow out with the fluid withoutblocking.

The content of the Kinetic inhibitor used is gen-erally from 0.01% to 0.5% with its molecular weightranging from several thousands to millions. Its costcomparing with the thermodynamic inhibitor is morethan 50%. It significantly decreases the stocking vol-ume and pour volume. Its use and maintenance arevery convenient.

Kinetic inhibitor generally contains polymer andsurface-active agents, and only small amount of thisinhibitor is required for preventing the formation ofhydrate. Emulsification between the water phase andthe oil phase will occur to prevent the agglomerationof the hydrate crystal before the formation of the hy-drate. The advantage of this inhibitor is that its per-formance is not influenced by temperature [7].

The 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)ethyl methacrylate, PVCap, N -acyl polyolefine imine,polyisopropyl methyl orange, N,N -alkyl acrylamide,

2-propyl-2-imidazoline, acrylate, N -methyl N -ethylacetamine, etc. The mechanism of their operation isvia the formation or adsorption of eutectic crystals toprevent the growth of hydration nucleus so that thehydration particles are scattered and do not gather,thus the hydrate formation is prevented. Many hy-drate dynamic inhibitors have been developed, andamong them, PVP, PVCap, VC-713, and P (VP/VC),which are composed of PVP and PVCap in the ratio1:1. PVP, are considered as the first generation ki-netic inhibitors.

In 1972, Yukiev first introduced the idea that-surface-active agents can be assembled as hydrate de-fence agents. Recently, French Petroleum Institute(I.F.P.) has listed surface-active agents in a series ofpatents, and proved that nonionic amphiprotic com-pound could restrict the formation of gas hydrate inthe gas pipe. Some of them are amide compounds.The most efficient surface-active reagents are hydroxycarboxylic acid amide, (in which the carboxyl groupwith 336 atoms are better, 820 best), alkoxy dihy-droxy carboxylic acid amide (or polyalkoxy dihydroxyamide), and N,N -dihydroxy carboxylic acid amide.The common surface-active agents used in industryfor this purpose are SDS, SAS, DLS, and decyl ben-zene amine. They all contain weak electronegativeoxygen atoms.

At present, the polymer inhibitor gives better re-sults and is applied more extensively. The most im-portant function of Kinetic inhibitor is the effectiveprevention of hydrate formation. Any fault associatedwith the pouring system and the well not closed reg-ularly or the insufficient use of inhibitor will causehydrate blocking, and under these conditions, Kineticinhibitor cannot be applied. The hydrate blockingcan be prevented by the addition of methanol or byadopting the pressure drop method [8]. Therefore, inactual application a combination of kinetic inhibitorand thermodynamic inhibitor are used to solve thehydrate-plugging problem.

We have tested the influence of different concen-trations of thermodynamic inhibitor on the hydrateformation temperature. Here we take methanol andglycol as examples. We also take VC-713 as the sam-ple of kinetic inhibitor and test its effect on the spot.

4. Experimental

4.1. Sample analysis

4.1.1. Experimental equipment

84 Ming Wu et al./ Journal of Natural Gas Chemistry Vol. 16 No. 1 2007

Figure 1 shows the experimental equipment. Itis composed of six parts. They are high-pressuresapphire cell, churn-dasher, constant temperature airbath, pressure increase system, temperature and pres-sure test system, and data collection system.

Figure 1. Schematic diagram of the experimental ap-

paratus for hydrate formation

1Inlet valve, 2Air bath, 3Sapphire cell, 4Magnetic stirrer,

5U magnet, 6Floating piston, 7O-ring, 8Pressure gauge

DSDriving system, DPTDifferential pressure transducer,

TTTemperature transmitter, DASData acquisition system,

PDPPositive displacement pump, MC-Methane cylinder

4.1.2. Experimental results

According to the mole composition data of thenatural gas transported in a ground pipeline, we simu-late the field condition in our laboratory as presentedin Table 2. The environment temperature is 6 . Af-ter the addition of the inhibitor, hydrate formation inthe reactor is shown in Figures 2 and 3.

Table 2. Mole composition data of natural

gas transported in the reactor

Component of Composition

natural gas (mol%)

CH4 85

CO2 2.5

C2H6 11

C3H8 1

H2O 0.5

Figure 2. Hydrate formation in the reactor after

adding different content of methanol

(1) field pipeline, (2) 0% methanol solution, (3) 10% methanol

solution, (4) 20% methanol solution

Figure 3. Hydrate formation in the reactor after

adding different content of glycol

(1) field pipeline, (2) 0% glycol solution, (3) 10% glycol solu-

tion, (4) 20% glycol solution

Where curve (1) indicates the temperature andpressure of the field pipeline; % indicates the per-centage of glycol addition.

After adding the inhibitor, under the same pres-sure, the hydrate formation temperature clearlydrops. The more the inhibitor content, the higherthe temperature drop. At the same temperature, thehydrate formation pressure obviously ascends withthe inhibitor content. After adding the inhibitor, itsuncharged cluster produces some type of interactionforce with the water molecule and destroys the liquidwater molecule grid generated by the hydrogen bond.Thus, the water molecule needs to surmount this in-teraction for the formation of grid. So the gas hydrateneeds some extra energy to change the conditions ofhydrate formation temperature and pressure from the

Journal of Natural Gas Chemistry Vol. 16 No. 1 2007 85

actual operating condition.From the figures, it can be seen that by the ad-

dition of 10% methanol or glycol, the hydrate gen-eration curve and temperature pressure curve of theground pipeline do not intersect, and the formationof hydrate in the reactor can be efficiently prevented.The traditional thermodynamic inhibitor has the dis-advantages that large quantity of inhibitor is requiredfor the prevention of hydrate formation. It alsorequires huge storage and injection equipment andcauses environmental pollution. Its use is inconve-nient and is not economical. Therefore, much interestis paid on the development of kinetic inhibitors in-stead of the traditional thermodynamic inhibitor.

4.2. Kinetic inhibitor experiment on-the-spot

A field test was performed by the addition of VC-713 inhibitor to a sea well mouth in Beihai. Thiswell produces 0.566106 m3 of natural gas, 1.59 m3of congealed oil, and 0.64 m3 of water per day. Theproduced fluid is transported to a platform, where itis separated, compressed, and dehydrated through a9.4 km long and 0.2 m diameter pipeline.

VC-713 is a terpolymer, when added with a solu-tion of concentration of less than 2%, and when in-fused with a viscosity lower than 45 MPas, its concen-tration in the water phase of pipeline is approximatelybetween 0.25% and 0.5%. There are four steps forfield measurements. First step is the determinationof the operating conditions of the pipeline. Secondstep analyzes the formation of hydrates. The highestdegree of supercooling is measured in the third step,and the forth step evaluate the effect of low concentra-tion on the formation of hydrates. The formation rateof hydrate can be judged by detecting the decrease inflow and by the increase in pressure. Fluid temper-ature and pressure of the terminal point are used toestimate supercooling of the fluid in the pipeline.

Under the field test condition of adding 0.5% VC-713 inhibitor (see Table 3) it is concluded:

The difference between the flat temperature andthe melting point at the cooling curve during thecourse of crystallization is defined as supercooling. Itwas found from the experiment that VC-713 inhibitordid not reduce the effort of clearing away hydrate of

methanol under the condition of hydrate formation.VC-713 is more economical than methanol.

Table 3. Hydrate generation conditions after adding

0.5% VC-713 inhibitor

Number of days Supercooling ( ) Hydrate generation

34 4 not formed

4 2 not formed

3 1 not formed

8 12 formed

5. Conclusions

Two types of inhibitors are described in this ar-ticle, they are thermodynamic inhibitors and kineticinhibitors. Three thermodynamic calculation formu-las were analyzed, and the lowering of freezing pointby the addition of inhibitors is used for the calcula-tion of the formation temperature of the natural gashydrates. From the results of testing methanol andmono-ethylene glycol as the inhibitors in a pipeline itis concluded that using 10% inhibitors for the pipelinecould restrict the hydrate formation. Kinetic in-hibitors were also studied. They are divided into poly-mers and surface-active agents. The result of applyingVC-713 in the field showed that polymer inhibitorshad better efficiency and good application prospect.In practice, it is better to use the thermodynamic in-hibitors together with the kinetic inhibitors.

References

[1] Long J. The 73th Annual GPA Convention, New Or-

leans. LA. 1994

[2] Sloan E D. Gas Research Institute Topical Report.

GRI-91/0302, 1992, 6

[3] Hammerschmidt E G. Ind Eng Chem, 1934, 26(8):

851

[4] Makogon Y F. Hydrates of Natural Gas. Wang M Sh

trans. Beijing: Petroleum Industry Press, 1987. 10

[5] Xu Y J, Yang X X, Ding J, Ye G X. Natural Gas

Industry, 2004, 24(12): 135

[6] Wu D J, Hu Y F, Yang J T. Natural Gas Industry,

2000, 20(6): 95

[7] Zhao Y, Ding J. Natural Gas Industry, 2004, 24(12):

132

[8] Chen G L. Natural Gas Industry, 2004, 24(8): 89