Gas hydrate formation inhibition using low dosage hydrate ...conf.semnan.ac.ir/uploads//nicgh1392/articles/7252.pdf · Gas hydrate formation inhibition using low dosage hydrate inhibitors

2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University

Gas hydrate formation inhibition using

low dosage hydrate inhibitors

Amir Erfani*

Farshad Varaminian

Milad Muhammadi School of chemical,gas and petroleum,Semnan university

Email: [email protected]

Abstract Development and utilization of Low Dosage Hydrate Inhibitors (LDHI) have been attracted

researchers and industry for almost two decades. These inhibitors are known to be more effective,

more environmental friendly, less corrosive and havelower capital and operational expenses.

These inhibitors are usually classified to Kinetic Inhibitors (KI) and Anti-Agglomerants (AA).

While kinetic inhibitors prevent hydrate formation by prolonging induction time of hydrate

formation more than the residence time of free water in pipeline, anti agglomerants inhibits

pipeline plugging acting as a hydrate emulsifier. Environmental aspects of commercial LDHI’s

have encouraged researches to look for more environmentally friendly LDHI’s. Considerable

efforts have been made to develop non-ionic surfactants e.g. Alkylamide or zwitterionic

surfactants, which can be effective inhibitors or to utilize environmentally friendly materials like

starch or anti-freeze proteins. In present paper, these research activities, patents and industrial

reports are reviewed, including: chemicals with kinetic or anti agglomeration effects, their

mechanism of acts, Compatibility, chemical screening and selecting methods like molecular

Dynamics simulations, experimental procedures, data reproductivity, stochasticity, inhibitors

synergism, multifunction inhibitors, inhibitors surfactant effects and surface interfacial properties,

biodegradability, toxicity and environmental impacts.

Keywords: Gas hydrate, Kinetic inhibitors, Anti-agglomerants, inhibition mechanisms, synergism

Gas hydrate formation inhibition using low dosage hydrate inhibitors

1. Introduction

Gas Hydrates are crystalline solids wherein guest (generally gas) molecules are tapped in

cages formed from hydrogen bonded water molecules (host). CH4, H2S, CO2, C2H6, C-C3H6,

(CH2)3O, C3H8, i-C4H10, n-C4H10 are some of these hydrate former gases[1]. Gas hydrates

could form in pipelines causing serious operational and safety problems[2]. Gas expansion

and cooling effect, start up and shut down, well clean-up and testing, subsea separators and

deepwater production are some of the scenarios for hydrate formation[3].

For hydrate to be stable necessary conditions are Presence of water, suitably sized gas/liquid

molecules, Suitable temperature and pressure conditions. To avoid hydrate problems,

injecting inhibitors have been utilized as the most economical method. this chemicals based

on their operational concentration are classified to: 1) thermodynamic inhibitors e.g.

Methanol, ethanol, glycols, 2) Low Dosage Hydrate Inhibitors which are classified to 1-

Kinetic hydrate inhibitors (KHI), 2- Anti-Agglomerants (AA).

A widely used thermodynamic method is based on methanol injection. Thermodynamic

methods using methanol and glycol are costly in offshore developments and onshore

processing facilities because of the high treatment amounts required (10–50% of the water

phase). Thermodynamic inhibitors prevent hydrate formation by shifting the equilibrium

conditions so hydrate form in lower temperatures and higher pressures. Although there are

opportunities to optimize thermodynamic inhibitor requirements[4, 5], the high cost of

thermodynamic inhibitors has stimulated the search for kinetic inhibitors. The flow assurance

industry is progressively moving away from such avoidance of hydrate formation, towards

risk management. The risk management philosophy allows hydrates to form, but prevents

hydrates from agglomerating and forming a plug, or delays hydrate formation within the

timescale of the water residence in the hydrate-prone section of the flow line. Kinetic

inhibition methods are based on the injection of polymer-based chemicals at low dosages in

the water phase. Consequently, these chemicals are called low dosage hydrate inhibitors

(LDHI). LDHI interfere with hydrate nucleation, growth and agglomeration of hydrate

particles Thus, they are subdivided into so-called kinetic inhibitors (KI) and anti-agglomerates

(AA) [6]. For a successful LDHI design many parameters must be considered, most important

issues are as follows: hydrate stability zone and maximum degree of subcooling, water cut

and other important fluid parameters, salinity and composition, whether to use KI or AA, ,

fluids residence times, inhibitor limitations(low temperatures, high pressures, etc),

Economical evaluations, Safety, operational and environmental issues, Initial laboratory

testing, corrosion, scale, inhibitor dosage optimization at lab conditions, field tests,

monitoring and re-evaluation.

2. Chemicals with Kinetic inhibition effects Kinetic inhibitors are able to slow down the hydrate formation rather than make it

thermodynamically impossible, they decrease the rate at which hydrates form, and

delays expand hydrate formation induction time period longer than the free water

residence time, in a hydrocarbon transport line. The amount of the Kinetic inhibitor

used is generally from 0.01% to 0.1% with its molecular weight ranging from several

thousands to millions [7]. 2.1. First and second generation of kinetic inhibitors

First generation of kinetic inhibitor was Polyvinylpyrrolidone (PVP) a commercially

available water soluble polymer with lactam rings [8].PVP as illustrated in fig 2.1


consists of five member lactam rings attached to a carbon backbone, lactam rings are

amide group attached to polymer backbone. Molecular weights for commercially

available PVP are between 10,000 and 350,000.

Fig2.1, PVP polymer molecular structure

Having a polymer with a lactam ring, as an efficient inhibitor, three other commercially

available polymers with lactam rings were tested and proofed to be more effective. These

three polymers were called second generation kinetic inhibitors. Names and acronyms for

these polymers are as follows: 1) Polyvinylcaprolactam (PVCAP) , 2) a terpolymer,N-

vinylpyrrolidone/N-vinylcaprolactam/N- dimethylaminoethlmethacrylate (VC-713) , 3) a

copolymer, N-vinylpyrrolidone-co- N-vinylcaprolactam (VP/VC).fig2.2 illustrates these

polymers.

Fig 2.2, Second generation kinetic inhibitors, a) PVCAP, b) VC-713 c) VP/VC

2.2. hydrate formation and inhibition mechanisms

First study on effective kinetic inhibitors for natural gas hydrates was carried out by Lederhos

et al. [8] in their hypothesis it was imagined that hydrate formation is a autocatalytic reaction.

Fig 2.3 illustrates this reaction.


Fig2.3 Autocatalytic reaction mechanism for hydrate formation[8]

In their hypothesis the essence of kinetic inhibition is to extent the metastable period before

catastrophic nucleation by inserting a crystal stabilizer between species B, C or D. The size of

lactam ring in kinetic inhibitors is similar to five and six member of hydrate structure.

Because of electro negativity of Nitrogen and Oxygen in amide group Hydrogens of lactam

ring can be adsorbed on the hydrate crystal and sterically block the hydrate growth. Koh et al.

[9] studied hydrate formation and inhibition mechanisms using neutron diffraction,

differential scanning calorimetry and a multiple cell photo-sensing instrument. It was shown

that gas hydrate formation generally occurs at the gas-liquid interface; their study also

revealed that polymeric kinetic inhibitors are capable of controlling both surface and bulk

nucleation.

It has been shown that polymeric kinetic inhibitors are effective for SI and SII. It has been

also concluded that adsorption of polymers on hydrate crystal surface is practically

irreversible and as showed by experiments this adsorption is fairly rapid[10, 11]. Makagon

and Sloan [12] have utilized molecular dynamics and Monte Carlo Simulations to investigate

mechanism of kinetic inhibitors, they suggested that inhibition mechanism consist of two

main components. At first Inhibitor lactam ring adsorb on the hydrate crystal surface by

hydrogen bonding. By adsorbing on the hydrate crystal, the polymer forces the crystal to grow

around and between the polymer strands. Inhibitors also sterically block non-polar solutes

such as methane from entering and stabilizing a hydrate cavity. It is noteworthy that in their

study it was observed that there is a weak interaction between hydrate former solute (a non

polar gas), and hydrophobic part of the polymer which affect adsorption of methane in

hydrate cavity. Different polymers had different effects on the methane adsorption. The

largest decrease in methane adsorption was caused by PVCAP. In their article a case study on

effect of monomer size on inhibitor’s effectiveness was carried out. It was concluded that 160

A3 is the optimized value. 2.3. Kinetic inhibitor experiments on-the-spot

Kinetic inhibitors have been utilized for hydrate inhibition in many wells. Wu et al [7]

performed a field test by the addition of VC-713 inhibitor to a sea well mouth in Beihai.

The well produces 0.566 million m3 of natural gas, 1.59 million m3 of congealed oil,

and 0.64 million m3 of water per day. The produced fluid is transported to a platform,

where it is separated, compressed, and dehydrated through a 9.4 km long and 0.2 m

diameter pipeline.VC-713 is a terpolymer, when added with a solution of concentration

of less than 2%, its concentration in the water phase of pipeline is approximately

between 0.25% and 0.5%. There are four steps for field measurements. First step is the


determination of the operating conditions of the pipeline. Second step analyzes the

formation of hydrates. The highest degree of super cooling is measured in the third step,

and the forth step evaluate the effect of low concentration on the formation of hydrates.

The formation rate of hydrate can be judged by detecting the decrease inflow and by the

increase in pressure. Fluid temperature and pressure of the terminal point are used to

estimate super cooling of the fluid in the pipeline. Under the field test condition of

adding 0.5% VC-713 inhibitor it is concluded that the difference between the flat

temperature and the melting point at the cooling curve during the course of

crystallization is defined as super cooling. It was found from the experiments that VC-

713 inhibitor did not reduce the effort of clearing away hydrate of methanol under the

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

successful field tests are reported by Notz et al. [13] and Bloys et al. [14]. 2.4. Effect of non-aqueous condensate on induction time, in presence of kinetic inhibitors

The timescale between establishment of thermodynamically suitable temperaure and

pressure and formation of first macroscopic hydrate crystal is called induction time[15].

The essence of a kinetic inhibitor is to prolonge induction time more than residence

time of water phase in system. Lee et al[16] have reported induction times in specified

solutions with and without presence of a non aqueous condensate phase and it have

been observed that non aqueous condensate phase lowers induction time by a factor of

two. Our hypothesis is that non-aqueous condensate which is in touch with water, can

act as a nucleation site and favors hetrogeneous nucleation. 2.5. Effect of subcooling on kinetic inhibitors effectiveness

Generally any mass transfer phenomenon at least in a definite range can be kinetically

expressed as: rate=constant*driving force. If we assume hydrate formation as

simultaneous reaction and mass transfer of gas molecules to hydrate crystal then we

have temperature difference between bulk and termodynamic themperature of hydrate

formation as driving force of the system. A kinetic inhibitor must lower the constant in

the rate equation. with temperture getting cooler, drving force increases and inhibitor

effectivness will be lowered. As mentioned in literatures the higher the subcooling

(driving force for hydrate formation) the shorter the induction time to hydrate

nucleation [6, 8, 17]. 2.6. patented kinetic hydrate inhibitors

High capital and operational expenses of methanol injection inhibition encourage the

development of competitive commercial LDHI chemistries by the inhibitor

manufacturers. Many Patents have been granted for kinetic and anti agglomerants

inhibitors [18-24]. Supplier and commercial name for some of these chemical are

presented in fig 2.4. Molecular structure of Poly(N-methyl N-vinyl acetamide) (VIMA),

b) Poly(N-vinylvalerolactam) (PVVam), c) monomer of poly(n-acryloyl morpholine)

(PAMOR) as patented KIs are presented in fig 2.5.


Fig2.4 , some of patented chemical as effective kinetic inhibitors[6].

Fig 2.5, three different patented kinetic inhibitors, a) Poly(N-methyl N-vinyl acetamide) (VIMA), b)

Poly(N-vinylvalerolactam) (PVVam), c) monomer of poly(n-acryloyl morpholine) (PAMOR)

2.7. kinetic inhibitors compatibility

In industry there are multiple inhibitors acting in a pipeline. Anti corrosion, anti wax

and anti asphaltene are three of those. Before using any new chemical as a kinetic

inhibitor in a pipeline, compatibility of the chemical with other other available

Inhibitors should be tested. Compatibility of effective kinetic inhibitors with other

industrial inhibitors have been merely reported in articles and patents.

3. Chemicals with anti agglomeration effects As mentioned above kinetic inhibitors effectiveness is affected by subcooling, and they

don’t perform well at higher subcooling and are not suitable for conditions with

transient conditions like pipeline/well shut-in. Anti-agglomeration, is intended to be

effective at very high subcooling or at shut-in conditions. Anti-agglomerants (AA’s) as

described by Koh et al. [9] are emulsifying agents which suspend hydrate crystals in

condensate because the ends of AA molecules have qualities attractive to both hydrates

and oil. This combination leaves hydrates dispersed as small masses in oil and prevents

the accumulation of hydrate under proper water/oil ratios. This method, while not

preventing hydrate formation, prevents hydrate blockages in pipelines.


3.1. Commercially available surfactants as anti agglomerants

Surfactant are used as emulsifiers for many applications [25]. Common application of a

surfactant is based on its hydrophilic–lipophilic balance (HLB). HLB provides an

approximation of the emulsion type made by a surfactant. HLB values from 3 to 6 are

common to obtain water in oil (W/O) emulsion [26].Sprit of using emulsifier surfactants

as anti agglomerants indicates that presence of oil phase is essential. Huo et al.[27]

having appropriate HLB on their mind, focused on non-ionic surfactants because they

usually are non-toxic, their performance is not a function of water hardness, and they

work well in low dielectric liquids. As concluded by Huo et al. [27] non of

commercially available surfactants are suitable as a anti agglomerants, among all, Span

chemicals were the best commercially tested chemical but did not work well at high

subcooling. Span stays in the water–oil interfacial area with or without emulsions

because of their hydrophilic and lipohilic characteristics. All successful

Span chemicals e.g. Span 60 (Sorbitan monostearate) (illustrated in fig 3.1) , gave stable

water in oil emulsions. With failure of commercially available surfactant, studies have

been led to synthesizing surfactant to be used as anti agglomerants.

Fig 3.1, Span 60(Sorbitan monostearate)

3.2. Non-ionic Synthesized surfactants as anti agglomerants

Huo et al. [27] Hypothesized that the reason why most surfactants do not work may be due to

their inability to attach to the hydrate surface, because they are not hydrate-philic.for

emulsifier to be effective it is essential that surfactant adsorb dispersed particles. As

mentioned above lactam ring of PVCAP polymer fits the hydrate partially completed crystal.

This kinetic inhibition inspired Huo et al. [27] to design synthesized surfactants with lactam

ring or other functional group, which could possibly interact with hydrate surface. Among all

synthesized surfactants the chemicals dodecyl-2-(2-caprolactamyl) ethanamide (CDDA, with

general structure shown in Fig 3.2 with operating concentration of as low as 0.75 wt% is most

effective.

Fig 3.2, Molecular structure of CDDA (27)


3.3. Ionic surfactants AAs

Quaternary ammonium based surfactants AAs provided by Shell are nowadays fully

commercial and injected on a number of fields [24] These AAs are based on quaternary

ammonium surfactants with two or 3 butyl or pentyl groups attached to the quaternary

ammonium nitrogen atom. These quaternary groups attach strongly to hydrate crystal

surfaces (shown by experimental work) with the hydrates.The major problem with this class

of AAs is that they are not Environmental friendly. Molecular structure of Quaternary

ammonium based surfactant is presented in fig 3.3.

Fig 3.3 fhe structures of mono-tail (left) and twin-tail (right) quaternary ammonium surfactant AAs. R is

butyl or pentyl, R’ is a long alkyl chain and X- is an anion(24, 28).

3.4. Zwitterionic surfactant AAs

With respect to effectiveness of quaternary ammonium based surfactant Kelland et al.

investigated Zwitterionic surfactant. They incorporated the anion found in quaternary

ammonium surfactant AAs into the structure of the surfactant, and not to leave it as a

separated ion. In zwitterionic surfactants as illustrated by fig3.4, the negative and a positive

charge in the same molecule which was hoped to be more environmental friendly than

cationic surfactants.

Fig 3.4, structure of zwitterionic surfactant AAs

In the study the most important surfactants investigated and their code names include are:

• FX-BETA-EDS: R1 = C12H23, R2 = COOH, R3 = H and

R4 = n − C4H9.

• FX-BETA-PDS: R1 =C12H23, R2 =COOH, R3 =CH3 and

R4 = n − C4H9.


Fig 3.5 illustrated molecular structure of these surfactants. In fig 3.6 we can see two

resonance structures in aqueous solution. It is Noteworthy that the zwitterion can be formed

when the surfactant is added to water, The process of becoming zwitterionic in aqueous

solution may be too slow to reach equilibrium if the solid product or non-aqueous solution is

added to the system [29].Fig 3.6 illustrates structure of first class of Alkylamide surfactants.

This class passed the tests for biodegradability an toxicity test.

Fig 3.5, The structure of betaine surfactant AAs

Fig 3.6, the two resonance structures for FX-BETA-PDS in aqueous solution

Kelland et al. concluded that zwitterionic surfactants tested with ethyl dicarboxylate dibutyl

ammonium head groups were poor AAs. However, if a second carboxyl group is placed in the

molecule further from the head group the performance is improved substantially. Examples

are FX-BETA-PDS and FX-BETA-EDS. These were shown to perform well as AAs in

sapphire cell experiments at up to 15.9 ◦C subcooling but not as well as commercial

quaternary ammonium AAs. It seems an important parameter in effectiveness of zwitterionic

molecule is that charges stand wide separated. For a quaternary ammonium surfactant the

charges are not in the same molecule and are therefore easily separated in aqueous solution. 3.5. Alkylamide surfactant gas hydrate AAs Kelland and co workers [29] placed active groups of kinetic inhibitors (lactam ring) into AA

non ionic surfactants analogous to quaternary ammonium group to find effective

environmentally friendly AAs. Initially the designed was surfactants containing the

caprolactam head group next to a quaternary centre to drive the surfactant toward hydrate

surfaces. However, these were found to be toxic. In design of surfactants number of carbon

atoms was designed to obtain a suitable HLB for W/O emulsions. Fig 3.7 illustrates structure

of first class alkylamide surfactants. FX-OTMAA/LTMAA (refer to fig 3.7) 5/1 blend showed

good performance up to 13° C subcooling and also passed biodegradability and toxicity test

(described in section 8).


Fig 3.7, First class of alkylamide surfactants and their code names [29]

Kelland and co workers [29] having shown the good performance of first class alkylamides,

plus its good environmental properties, suggested related structures with improved

performance. Their idea was to place two alkylamide groups in the head of the surfactant with

the aim of getting both groups to interact with the hydrate surface. If both groups interact this

should reduce the hydrate growth rate. Fig 3.8 illustrates molecular structure of this

surfactant. This class of surfactant has showed lower solubility and not capable of self-

ionization in aqueous solutions thus didn’t provide a magnificent improvement.

Fig 3.8, structure of second class alkylamide surfactant

3.6. Mechanisms for anti-agglomeration

A hypothetical mechanism for anti-agglomerants is described by Makagon and Sloan

[12],in their mechanism effectiveness of hydrate inhibitors could be due to a distorted

hydrate lattice formation. The distorted hydrate nuclei promote the formation of

hydrate, but limit the size of particles as crystal defects make further growth

unfavorable. Simultaneously, the hydrocarbon radicals of anti agglomerants form an

oleophilic barrier on the crystal and block the diffusion of water to the hydrate crystal.

If this hypothesis is correct, one should be able to control the hydrate particle size

distribution with the concentration of anti agglomerants inhibitor. For quaternary

ammonium sulfonate zwitterions Stor and Rodger [30] showed preferred adsorption

locations and proposed a lock-and key mechanism for these inhibitors. Klomp et al. [24]

in their patent for Shell company described how patented surfactants are designed to

attach to hydrate crystal surfaces. The head of the surfactant is “hydrate philic”, the tail

(or tails) are hydrophobic. As described, Attachment of the polar head group to the

hydrate crystal surface disrupts the hydrate growth process slowing down crystal

growth. The tails make the crystals oil-wet making them easily dispersed in the liquid

hydrocarbon phase.


3.7. Anti agglomerants compatibility

Kelland et al. [17] studied compatibility of the best zwitterionic surfactant AAs (FX-

BETA-PDS and FX-BETA-EDS). They were found to be compatible with commercial

corrosion inhibitors except a betaine corrosion inhibitor.

In Summary anti agglomerants prevent hydrate agglomeration and blockage, Formation

of small hydrate particles, Need a condensate phase, May need downstream treatment,

are suitable for transient flow regimes, and have the advantage of highest degree of

subcooling. Quaternary ammonium based surfactants AAs are efficient and fully

commercial but not environmental friendly The goal of the researches on AAs is to find

an AA preferably non ionic surfactant ,that is as good as commercial quaternary AAs,

which would be economically competitive and more environmentally friendly[17, 27,

29].

4. Chemical screening and selecting methods 4.1. Edisonian experimental approach

When searching for kinetic inhibitors began Lederhos et al. [8] found a Edisonian approach

successful. 1500 commercially available chemical-concentrations combinations were

screened and tested by rigorous high-pressure hydrate tests.

4.2. Molecular dynamics simulation for selection of kinetic hydrate inhibitors

Testing of potential kinetic inhibitors involves expensive manipulation of hydrates at high

pressures. Numerical simulations can be used to cull the experimental candidates [31, 32];

molecular dynamics may prove to be a valuable research tool. Another advantage of computer

simulations is that one can modify the model molecule, and see how it will affect its

performance. In a study performed by Kvamme et al. [31] based on computer simulations,

studies of the model systems concluded that PVCap will outperform PVP as a kinetic hydrate

inhibitor. A modified version of the PVCap monomer, with a hydroxyl group added to the

ring, increased its potential for attachment to the hydrate surface even further. Simulations

involving the third active unit of polymer VC-713 (PVP and PVCap being the other two

active groups attached to the polymer’s backbone) indicated a favorable interaction with the

hydrate water as well.

5. Experimental procedures, data reproductivity and stochasticity

5.1. Experimental Apparatus

Typical apparatuses for assessing kinetic inhibitor effectiveness are rocking cell chambers,

autoclave cells[8], flow loops [33], or differential scanning calorimetry (DSC) [34]. Each of

these traditional methods uses one sample at a time to measure the induction time of hydrate.

To get a good statistical analysis of the effects of the kinetic inhibitor, many experiments need

to be performed. Sapphire cell, autoclave and wheel loop AA tests do not give perfect

correlation, although the cell (or a rocking cell) is a good and economic first screening device.

The lack of correlation is most apparent in shut-in/start-up experiments. The small size of the

sapphire cell makes it difficult to build up a substantial amount of hydrates at the

hydrocarbon-water interface during shut-in making observations at startup hard to


interpret[17]. As mentioned the objective of using anti-agglomerant is to disperse hydrate

particles into condensate, so motor current or torque have been used as a criterion for anti-

agglomerates effectiveness.

5.2. determining gas hydrate kinetic inhibitor effectiveness using emulsions

Hydrate formation is a stochastic phenomena and Kinetic inhibitors tend to delay the average

hydrate nucleation time and also causes hydrate nucleation to become more stochastic[34] so

induction time for different experiments with same operational conditions can vary in results.

In a study carried out by Lachance et al. [34] it was proposed that effect of hydrate kinetic

inhibition can be measured using emulsions. Because hydrate nucleation is stochastic, many

experiments are normally needed to obtain accurate analysis of the effectiveness of kinetic

inhibitors. Using differential scanning calorimetry (DSC), it was shown that emulsions can

reduce the number of kinetic samples needed to obtain a statistical analysis of the

effectiveness of kinetic inhibitors thus lowering material and experimental time compared to

traditional methods.

5.5. Pendant Bubble cell

Most kinetic experiments to evaluate the inhibition performances of KIs are performed in

stirred reactors where the nucleation was progressive, meaning that new nuclei form

continuously during the growth of old ones and the measured apparent hydrate formation rate

depends on both the nucleation rate and the growth rate. Therefore, it is impossible to

determine which stage of hydrate formation was inhibited in these cases, pendant bubble cell

described by Peng et al. [35] (illustrated in fig 5.1) is designed to measure interfacial

properties of gas and liquid phases and lateral growth rate of hydrate film, on the surface of a

bubble suspended in aqueous solution.

Fig 5.1, the schematic diagram of the experimental apparatus: 1, pendant-bubble cell; 2, thermostat; 3,

sample cylinder; 4, JEFRI 10-1-12-NA pump; 5, gas cylinder; 6, JEFRI 100-1-10-HB pump; 7,

microscope; 8, video camera; 9, computer


5.6. Model systems analogue to natural gas hydrates using refrigerants as hydrate formers

In natural gas because of the presence of propane, SII of hydrate are more stable. Normally

natural gas hydrate forms at pressures about 50 bar. In some scientific researches to avoid

high pressures, refrigerants were used as hydrate formers and it is a common belief that

inhibitors effectiveness is not related to hydrate former gas[11, 36]. It is noteworthy that in

formation of gas hydrate in presence of promoters, promoter effectiveness have been proofed

to be highly dependent on hydrate former gas.

5.7. Data reproductivity

As mentioned above gas hydrate formation is a stochastic phenomenon, most of the efforts in

inhibition experiments have been on evaluating different experimental conditions and not on

the reproducibility of a single experiment. Urdahl and Kirkhorn [37] have therefore looked at

the reproducibility of experimental method with emphasis on nucleation time, subcooling and

hydrate macrostructure build up. Their study has suggested that if deposits, agglomeration or

plugging by hydrates are obtained, it should be unnecessary to reproduce the experiments in

order to confirm the results for a given hydrocarbon gas-water-model oil system, on the other

hand, when no deposits by hydrates are obtained, all significant system parameters should be

carefully examined.

6. Synergism and multifunction inhibitors

6.1. Enhancement of the performance of gas hydrate kinetic inhibitors with Polyethylene oxide:

Polyethylene oxide (PEO) is a non-ionic, water-soluble, linear polymer. It has a general

chemical formula of –(–CH2–CH2–O–)n–,Hydrate formation experiments have shown that

the induction time is prolonged in the presence of the PEO by an order of magnitude in some

cases compared to the inhibitor only[6, 15, 38]. PEO is not a kinetic inhibitor by itself .The

mechanism of the action of PEO is unknown.

Fig 6.1 PEO molecular structure


Fig 6.2, Methane uptake (consumption) curves with pure water, inhibitor

(INH1) and inhibitor + polyethylene oxide (INH1 + P)

With respect to fig 6.2 it is interesting to observe that the rate of gas consumption after the

nucleation point is the same in spite of the dramatic difference in induction time. This

indicates that the inclusion of PEO in the inhibitor solution influences the induction time for

nucleation and not the growth of the hydrate crystals.

6.2 Enhancement of the performance of gas hydrate kinetic inhibitors with 2-butoxyethanol:

The performance of certain KIs was found to be enhanced by the presence of small molecular

weight materials like 2-butoxyethanol [39]. In particular addition of 0.75 mass% of 2-

butoxyethanol prolonged the induction time from 40 to 1200 min in a system containing 0.5

mass% of a kinetic inhibitor (Gaffix VC-713, a terpolymer of vinylcaprolactam,

vinylpyrrolidone and (dimethylamino) ethyl methacrylate). 6.3. Synergistic properties of small Cationic and Anionic ions with Kinetic inhibitors

having effectiveness of quaternary ammonium surfactants in mind, Shell researchers found

effects of small cationic ammonium ions on hydrate inhibition. As described by Klomp et al.

[40] cationic tetra butyl ammonium or phosphonium ions and tetra pentyl ammonium ions,

inhibit natural gas hydrate crystal growth and have been used as synergists for commercial

kinetic hydrate inhibitor polymers. Thus, addition of a salt such as tetra butyl ammonium

bromide (TBAB) or tetra- pentyl ammonium bromide (TPAB) to a vinyl caprolactam polymer

such as PVCAP significantly increased the kinetic hydrate inhibition performance of the

polymer. It is noteworthy that, TBAB or TPAB alone showed no effect on inhibiting the

nucleation of gas hydrates. As described by Sefidroodi et al. [41].

“The reason for the synergistic effect of TBAB or TPAB with vinylcaprolactam

polymers has been discussed. Using the synergistic effect of TPAB with PVCap as an

example, it was postulated that one of the pentyl groups in TPAB enters an open cavity

on the hydrate surface, and two of the other pentyl groups lay in channels on the

hydrate surface where new cages would normally be formed. These cages could

partially form trapping or imbedding the pentyl groups in the hydrate surface and thus

trapping TPAB on the hydrate surface preventing normal SII hydrate growth at this site.


PVCap, on the other hand, is polymeric and can attach at several sites on the hydrate

nuclei, probably via three or more caprolactam rings. Once PVCap is attached to the

hydrate surface, it will actually prevent declustering of the sub-critical nuclei .As the

nuclei reach the critical nuclear size, crystal growth will become energetically

favorable. At this point, any TPAB molecules attached to the crystal surface, can more

easily become embedded on the surface and prevent crystal growth. Thus the hydrate

nuclei have to find other sites where they can grow above the critical nuclear size.

Hydrate growth will probably not be detected until after both PVCap and TPAB cannot

prevent further crystal growth” (Sefidroodi et al. [41] p. 2051).

Sefidroodi et al. [41] investigated anionic molecules to inhibit hydrate growth. None of the

anionic molecules were as effective as the best tetra alkyl ammonium salts it was found that

non-ionic or cationic molecules with a single alkyl group were just as effective as the best

anionic molecules, suggesting that the charge on the head group makes no difference to the

performance when only one alkyl group is present in the molecule.

6.4 Ionic liquids: Dual function thermodynamic and kinetic inhibitors

Like Ethanol, Sodium Chloride is a thermodynamic inhibitor but it is obvious that adding

highly electrolyte, inorganic salts like Sodium Chloride would led to corrosion of pipelines.

From experiences with Ethanol, Sodium Chloride and even PVCAP, it could be concluded

that materials with electrostatic charges or forming hydrogen bonds with water can inhibit

gas hydrate formation, Xiao and Adidharma [42] suggested that ionic liquids are fit for this

purpose. It has been showed that these materials have both kinetic and thermodynamic

inhibition effects. Ionic liquids studied by Xiao Adidharma are presented in fig 6.3. Kinetic

and thermodynamic inhibition of 10 wt% of these inhibitors are respectively presented in fig

6.4 and 6.5.

Fig6.3, Studied Ionic liquid [42]


Fig 6.4, Kinetic inhibition of ionic liquids [42]

Fig6.5, thermodynamic inhibition of ionic liquids

The mean value of induction time of methane hydrate formation from samples containing

1wt% EMIM–BF4 is about 5.7h. The performance of EMIM–BF4 is also found to be much

better than PVCap, which is considered as an effective kinetic inhibitor.

7. Inhibitors surfactant effects and surface interfacial properties

From studies related to surface properties related too inhibition or promotion of hydrate

formation [32, 35, 36, 43] it seems that most of inhibitors and promoters are surface active

agents. Having that in mind one must differ characteristics of these additives to obtain a

reasonable conclusion why some of surfactant promotes while others inhibit formation of

hydrates. Peng et al. [35] measured the interfacial tensions between methane and aqueous

solutions of different concentrations of VC-713 at different temperatures and pressures in the


hydrate formation region. Fig 7.1 illustrates effect of VC-713 on interfacial tension between

gas and liquid phase.

Fig 7.1, the interfacial tension between methane and aqueous VC-713 solution at 278.2 K.

VC-713 cannot form micelles in the bulk aqueous phase. Without the balance of CMC, the

concentration of VC-713 at the interface may increase with the increase of that in the bulk

aqueous phase continuously. Its inability to form micelles in water is of great significance for

VC-713 as a kinetic inhibitor because the micelle may solubilize gas into it and therefore

promote hydrate formation[35]. It is also noteworthy that no micelle formation was observed

for any surfactants in the concentration range where strong hydrate promotion was previously

reported[44].

8. Biodegradability, toxicity and environmental impacts

The importance of biodegradability and hazardous associated with toxic ionic surfactants are

known. Researches on inhibition chemicals are geared toward synthesis and use of more

environmentally friendly materials. Surfactants toxicity test is carried out on a marine alga

called skeletonema costatum [29] The biodegradability test is carried out in seawater using

the closed bottle method (OECD 306).

8.1. Antifreeze proteins as kinetic hydrate inhibitors

One class of green inhibitors is antifreeze proteins[45]. in a study Zeng et al.[46] have shown

that Type I antifreeze protein (AFP) from winter flounder has a significant effect on the

formation of propane hydrate and methane hydrate. It was shown that the formation of both

hydrates is inhibited significantly, with both nucleation and crystal growth being affected.

Also, AFP showed the so-far unique ability to eliminate the “memory effect” in the

reformation of gas hydrate. It is noteworthy that proposed mechanism for inhibition effects

involves the interference of AFP with heterogeneous nucleation and subsequent growth of the

hydrates [5].


8.2. Starch as a kinetic hydrate inhibitor

A class of inhibitors explored by Lee et al. [3] is natural polymers like starch. Starch is the

most abundant polysaccharide and is a mixture of two polymers of anhydroglucose units,

amylase and amylopectin. Amylose is a linear polymer, whereas amylopectin is highly

branched. Starch is usually cationized and then used in industrial applications and the

motivation for its use is that it is non-toxic and biodegradable [16]. The kinetic inhibiting

effect of a number of cationic starches in hydrate formation experiments with methane and

methane/ethane and methane/propane gas mixtures have been investigated and found that

they are hydrate formation inhibitors. That synergism effect of PEO on kinetic inhibitors has

been proved to be valid for starches[16]. The proposed mechanism for inhibition effects of

starch is that the anhydroglucose unit of starch fits within the hydrate structure in a manner

similar to that for the hydrophilic pendant lactam group. Cationic starches are known to be

highly hydrophilic and have high capacity to create hydrogen bonds with other entities in

solution. It is also noteworthy that promotion effect of starches on gas hydrate formation rate

have been reported by Fakharian et al [47].

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