and Bis-Amine Oxides - ACS Publications

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Study of the Gas Hydrate Anti-Agglomerant Performance of a Series of Mono- and Bis-Amine Oxides: Dual AntiAgglomerant and Kinetic Hydrate Inhibition Behavior Pei Cheng Chua, and Malcolm A. Kelland Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03789 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Study of the Gas Hydrate Anti-Agglomerant Performance of a Series of Mono- and Bis-Amine Oxides: Dual AntiAgglomerant and Kinetic Hydrate Inhibition Behavior

Pei Cheng Chua and Malcolm A. Kelland*

Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, NORWAY

Abstract Anti-agglomerants (AAs) are surfactants used in the upstream oil industry to prevent gas hydrate plugging in flow lines. Most, if not all, AAs used in the field today are cationic quaternary ammonium surfactants and function using the “hydrate-philic” mechanism. In this study we have synthesized a series of butylated mono- and bis-amine oxide surfactants with aliphatic tails with chains of 9 to 17 carbon atoms and either amide or ester spacer groups. Their performance as hydrate-philic AAs has been investigated in a sapphire autoclave and sapphire rocking cells with a Structure II-forming natural gas mixture. There was generally good agreement regarding the performance and relative ranking of the surfactants between the two sets of equipment. The AA performance of the amine oxide surfactants depended on many factors, including the polar head and spacer groups, subcooling at hydrate onset, salinity and the composition of the hydrocarbon

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fluid. Amido amine oxide surfactants performed better than the equivalent ester amine oxide, probably due to the stronger hydrogen-bonding ability of the amide group. The bis-amine oxide surfactants were designed with optimum inter-amine distance for best Structure II crystal growth inhibition and performed better as AAs than the mono-amine oxide surfactants. The amido bisamine oxide surfactants showed reasonable seawater biodegradation rates over 28 days, giving biological oxygen demand values of 25-40%. The bis-amine oxide surfactants in particular also showed a strong kinetic hydrate inhibition effect which could be very useful for field applications. Thus, these surfactants could be used first as kinetic hydrate inhibitors (KHIs) and if for any reason hydrate formation does occur they could also function as AAs to prevent hydrate blockages under certain conditions.

Introduction Anti-agglomerants (AAs) are a class of low dosage hydrate inhibitors (LDHIs), the other class being kinetic hydrate inhibitors (KHIs).1 AAs allow the hydrate crystals to form but they modify and retard the crystal growth process and prevent hydrate agglomeration and plugging of multiphase flow lines. AAs are surfactants. There are two methods by which the result can be achieved. The first method was developed by the French Petroleum Institute (IFP) and required a special polymeric surfactant that emulsified all the produced water in the produced liquid hydrocarbon phase (condensate or crude oil).2 When hydrate formation occurs the water droplets are converted to hydrate particles but remain separate and do not agglomerate. This method has drawbacks such as requiring all the water to be emulsified before hydrate formation ensues in order to be successful. Water drop-out or top-of-the-line condensed water can also be a problem during shut-in and start-up. This method was field-trialed but never took off as a commercial product used in the oil industry.


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The second method has been used in the field since the late 1990’s and all commercial AAs for multiphase flow lines on the market today are based on this method. The basic premise of the method is to use a so-called “hydrate-philic” surfactant, i.e. a surfactant with a head group that interacts strongly with hydrate particle surfaces. The breakthrough to the development of commercially viable AAs came from research at Shell oil company when they discovered that certain tetraalkyl quaternary ammonium and phosphonium salts were excellent tetrahydrofuran (THF) hydrate crystal growth inhibitors.3-4 THF hydrate is a Structure II (SII) hydrate, the same hydrate structure most commonly formed with natural gases in oilfield operations. The best alkyl groups for these quaternary salts from Shell’s work were n-butyl and n-pentyl. Later work has shown that iso-hexyl and t-heptyl groups give even better inhibition performance.5-6 These cationic quaternary ammonium salts are therefore hydrate-philic. By lengthening one or two of the small alkyl groups to a fatty alkyl chain length (usually C12-18) the surfactant gave excellent performance as an AA. Quaternary ammonium surfactants based on these principles are now the main active ingredients in most commercial AAs. Examples are given in Figure 1.


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Figure 1. Examples of quaternary surfactant AAs.

In their patent, Shell summarized other features of the quaternary AAs besides their hydrate-philic nature that makes them so attractive for field use: •

concentrate the subject compound near the water-hydrocarbon interfaces, where hydrate formation is most pronounced, thereby raising the local concentration of ions (lowering the local hydrate equilibrium temperature);

•

modify the structure of water near the hydrocarbon-water interface in such a way that the formation of hydrate crystals is hindered;

•

impede further access of water molecules to the hydrate crystal after attachment of the subject compound to the hydrate crystals;

•

prevent agglomeration of hydrate crystals by making their surface hydrophobic;


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•

adhere the subject compound to the conduit wall, thereby preventing the adhesion of hydrates to the conduit wall. (This also means the AA also acts as a useful film-forming corrosion inhibitor)

Shell also added that the emulsifying ability of AA is useful, keeping the concentration of water available for hydrate forming at the conduit wall small. Actually, we now know this is not necessary for good AA performance and can in fact be detrimental to the overboard water quality. The service companies have now deliberately developed quaternary surfactant AAs with poor emulsifying ability in order to avoid undesirable high ppm concentrations of produced hydrocarbons in the overboard water that would otherwise need further treatment. Another feature not mentioned by Shell is that quaternary surfactants are often good biocides, and could therefore help prevent microbial corrosion, H2S production and bio-fouling.1

One drawback with commercial quaternary ammonium surfactant AAs is that they are only partially biodegradable at best in the sea water OECD306 test over 28 days.1,7 The tails may degrade off if they contains hydrolyzable groups such as ester or amide, but the ammonium head group is only very slowly degraded. This is because the nitrogen atom is surrounded by 4 alkyl groups greater then methyl size. Alkyl groups greater than methyl are difficult to be degraded. Therefore, we sought alternate surfactant classes for use as AAs that might have improved biodegradation. Amine oxides are neutral surfactants but somewhat related to quaternary ammonium salts and have also been shown to be excellent THF hydrate crystal growth inhibitors.89

Amine oxides are compounds that contain the functional group R3N+-O−, an N-O bond with three additional hydrogen and/or organic groups attached to the nitrogen atom (Figure 2). Alternative


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notations are R3N→O and R3N=O. It was determined that the best THF hydrate crystal growth inhibition was provided by amine oxides with n-butyl, n-pentyl or iso-pentyl groups, with tributylamine oxide being the best individual trialkylamine oxide.10-11

OR1

N+ R2

O R3

R1

N

R3

R2

Figure 2. Structure of trialkylamine oxides.

Therefore, we surmised that butylated amine oxide surfactants could be an alternative and nonionic class of gas hydrate anti-agglomerant (AA) surfactants to quaternary surfactant AAs. Some work on monoamine oxide AAs has been reported previously but no systematic study.2,12 Recently, we reported comprehensive studies on the clathrate hydrate crystal growth inhibition of monoamine and bisamine oxides using Structure II (SII) tetrahydrofuran (THF) hydrate as our model hydrate.5,8,13 Replacement of one of the three butyl groups in tributylamine oxide with another small or larger alkyl group lowered the inhibition significantly. We assumed this would have consequences for the use of mono-amine oxides as AAs as they would not be hydrate-philic enough. However, certain bis-amine oxides in this study gave excellent THF hydrate crystal growth inhibition, more so than tributylamine oxide. The best butylated bis-amine oxides had a chain of 5-6 carbon atoms between the nitrogen atoms.14 A related study on bis-quaternary ammonium salts also indicated that about 6 carbon atoms was optimal for best THF hydrate growth inhibition.13 Therefore we were interested to explore the AA properties of alkylated bis-


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amine oxide surfactants with about 5-6 carbon atoms between the amine oxide groups. We report here a study on the AA properties of butylated mono and bis-amine oxide surfactants with either ester or amide spacer groups between the hydrate-philic head group and the long alkyl tail.

Synthesis of amine oxides surfactants All chemicals were purchased from VWR, Tokyo Chemical Industry Co., Ltd., and SigmaAldrich. All solvents were used as purchased without further purification. Nuclear magnetic resonance (NMR) spectra were recorded on a 300 MHz Varian NMR spectrometer or a 400 MHz Varian NMR spectrometer. 1H and

13

C chemical shifts were obtained in deuterotrichloromethane

using tetramethylsilane as an internal standard. A purity level of at least 95% for all new amines synthesized in this work was determined by 1H and 13C NMR spectroscopy.

Five synthetic reactions were used in this work to make the mono- and bis-amine oxides outlined in Figures 3-5. These reactions are butylation of primary and secondary amines, synthesis of 3hydroxy-1,1,7,7-tetrabutyldiaminopentane, amidification and conversion of mono- or butylamino groups to mono- or dibutylamino oxide groups using hydrogen peroxide. An example of butylation, amidification and conversion to amine oxide is illustrated in the synthesis of the mixed amido bis-amine oxide products (AbAO-C11) as follows: diethylenetriamine was butylated by refluxing with 4 molar equivalents of n-butyl bromide and 4 equivalents of potassium carbonate in THF for 16 hours.15-16 After filtration volatiles were removed from the filtrate. The residue, which is a mixture of butylated diethylenetriamine products was dissolved in diethyl ether. One molar equivalent of triethylamine was added and the solution stirred. Lauroyl chloride (C11H23COCl) was slowly added dropwise. The solution was stirred for a further 1 hour and then filtered. Volatiles were removed from the filtrate and the residue dissolved in isopropyl alcohol (IPA). This


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solution was treated with 1.05 molar equivalents of 30% hydrogen peroxide for 16 hours with stirring.17 The solution was warmed to 70 oC to destroy excess hydrogen peroxide to leave C11amido-bisAO which is a mixture of products. Other surfactant mixtures with C13-C17 tails were prepared similarly to give the series of amido bis-amine oxides, pure and isomers mixture: AbAO-R (R = C9, C11, C13, C15, C17). 1,1,7,7-tetrabutyldiethylenetriamine was also converted in an identical way to give Amido bis-amine oxides, pure: AbAO-R (R = C9, C11, C13, C15, C17).

4-hydroxy-1,1,7,7-tetrabutyldiaminopentane was made by ring opening of epichlorohydrin with dibutylamine.

Esterification

of

3-hydroxy-1,1,5,5-tetrabutyldiaminopentane

or

dibutylaminoethanol with fatty acid chlorides in diethylether using triethylamine as base gave the ester mono- or bis-amine oxide surfactants EmAO-R (R = C11, C13, C15) and EbAO-R (R = C11, C13, C15) respectively. Similarly, amidification of dibutylaminoethylamine with fatty acid chlorides gave the mono amido amine oxide surfactants AmAO-R (R = C11, C13, C15).

In summary, the following products were synthesised: •

Ester mono-amine oxides: EmAO-R (R = C11, C13, C15) (Figure 3)

•

Amido mono-amine oxides: AmAO-R (R = C11, C13, C15) (Figure 3)

•

Ester bis-amine oxides: EbAO-R (R = C11, C13, C15) (Figure 4)

•

Amido bis-amine oxides, pure: AbAO-R (R = C9, C11, C13, C15, C17) (Figure 4)

•

Amido bis-amine oxides, isomers mixture: AbAO-R isomers (R = C9, C11, C13, C15, C17) (Figure 5)


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X

R

NBu2 X = O or NH

O

Figure 3. Structures of ester mono-amine oxides: EmAO-R (R = C11, C13, C15) and amido monoamine oxides: AmAO-R (R = C11, C13, C15).

O R

O

O O

O

O

R O

N Bu2N

NBu2

Bu2N

NBu2

Figure 4. Structures of amido bis-amine oxides, pure: AbAO-R (R = C9, C11, C13, C15, C17) (left) and.ester bis-amine oxides: EbAO-R (R = C11, C13, C15) (right).


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4BuBr + Base

H N H2N

H N Bu2N

NH2

NBu2

+ Bu N BuHN

NBu2

RCOCl + base

R

O

O

O

N

N Bu2N

Bu2N

NBu2

H2O2

Bu

O

Bu

N

N R

NBu2

N

NBu2

+

+ O

R

O

R

N

NBu2

O Bu

O

Bu

Figure 5. Synthesis of amido bis-amine oxides, isomers mixture: AbAO-R isomers (R = C9, C11, C13, C15, C17).

Experimental Methods: High Pressure Tests with Synthetic Natural Gas AA performance tests were carried out in a high-pressure stirred autoclave (Figure 6-7) and highpressure rocking cell equipment (Figure 8). Two experiments were conducted for each chemical at any one set of conditions. Generally, there was very good agreement between the two experiments,


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with the same ranking over 95% of the time. When there was a discrepancy in the performance we took the worst result as the performance level for the AA at that particular set of conditions.

The high-pressure autoclave consists of a sapphire cell with volume 23 ml. Since the stirring rate of the fluids in the cell was set to a constant value, an increase of the fluid viscosity due to hydrate formation and/or plugging will lead to an increase of the torque moment of the magnetic, flat steel stirrer blade. The torque moment, along with the temperature and pressure of the gas phase were measured and continuously monitored on the computer using the LabView® data acquisition program during the experiments.18

The rocking cell equipment, supplied by PSL Systemtechnikk, Germany, consists of six cells.19 A steel ball in each of the 20 ml sapphire tube gave agitation during the rocking. The time needed for the ball to travel between the run time sensors installed at both ends of each cell is recorded as the run time measurement. Using our standard rocking rate of 15 rocks per minute at an angle of 40°, the reference range of the run time without hydrate particles in the cells is 150 – 450 ms. The integrated computer system also records the temperature and pressure of the mixed fluids.18

Two kinds of test method were carried out, the “constant cooling” test and the “superheating isothermal” test. Our methods have only been used to determine a structure-performance analysis using specific conditions and these particular test methods.


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Figure 6: High pressure sapphire autoclave equipment.

Figure 7. Schematic for the high pressure sapphire autoclave equipment.


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Figure 8. High pressure sapphire rocking cell test equipment.

The initial procedures for preparation of the AA experiment and filling of the cells are as follows: 1. Each cell was filled with the AAs dissolved in the test solutions. European white spirits from Europris and Biltema as well as condensate from a North Sea field were used as the hydrocarbon phase. 2. Weight percentages (wt.%) of AA used are based on the water phase. In order to give 33% water cut, the amount of solutions used for the tests were: a. 2.5ml of aqueous phase and 5ml of hydrocarbon phase for autoclave; b. 3ml of aqueous phase and 6ml of hydrocarbon phase for rocking cells. 3. Air in the cells was removed using a vacuum pump and then the cells filled with 3 bar of Structure II hydrates forming synthetic natural gas (SNG, Table 1) and stirred / rocked. This procedure was repeated twice.


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4. The cells were charged with SNG to the test pressure. Immediately after the stirring / rocking had been started, the pressure dropped as the SNG dissolves partially into the hydrocarbon phase.

For constant cooling tests, the cells were saturated with SNG to about 75 bar at 20°C for autoclave and 79 bar at 19°C for rocking cells. The cells were cooled to 2°C with agitation. The cooling rate was set to be 1.0°C/h for autoclave and 1.3°C/h for rocking cells. If hydrate formation did not occur during the constant cooling procedure, the test fluids were held at 2°C with agitation. Upon completion of the hydrate formation, the cells were heated up to 25°C for 3 hours during the melting procedure.

This could be followed by the superheating isothermal tests, where the cell content from the constant cooling test was cooled without agitation to the experimental temperature 6oC. This led to a constant pressure drop in the closed system. The temperature and pressure in the cell were stabilized before the stirring / rocking was started. Further details and result evaluation of each test method were described in our previously published works.18, 20

The hydrate equilibrium temperature of our SNG-water systems were determined by means of calculations using Calsep’s PVTSim and laboratory experiments by standard slow hydrate dissociation.20-23 Experiments conducted by our group previously agreed very well with the calculated value (accuracy within 1°C). Assuming that this agreement is also valid for our SNGhydrocarbon-water system, the hydrate equilibrium temperature at 79 bar was calculated using Calsep’s PVTSim software to be 17.2°C in 1.5 wt.% sodium chloride (NaCl) solution, 16.0 °C in 3.6 wt.% NaCl brine and 14.7°C in 7.0 wt.% NaCl brine.


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Table 1. Composition of synthetic natural gas supplied by Yara Praxair. Component

mole %

Methane

80.67

Ethane

10.20

Propane

4.90

Iso-butane

1.53

n-butane

0.76

N2

0.10

CO2

1.84

The three liquid hydrocarbon phases used in the AA experiments were two white spirits from local companies Europris (E.w.s.) and Biltema (B.w.s.) with almost identical spectra from 1H n.m.r., and a North Sea condensate (M.c.). Supply of one white spirit was discontinued, which is why we used two white spirits. White spirit (or mineral spirits or naphtha solvent) is a distillation mixture of aromatic and aliphatic hydrocarbons. It is a cheap, easily accessible and clear solvent that makes a substitute for field condensates that can vary in quality during the field lifetime. The condensate was supplied by Statoil and was chosen as a typical light condensate to compare to the white spirits.

Performance Evaluations


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The AA ranking for the autoclave is evaluated mainly based on the visual observations in the cell during the slow growth and rapid hydrate formation periods. Photos of various observations are given in Figure 9. The AA rankings are as follows: I

Fine dispersed hydrates without any deposits.

II

Minor deposits on the wall with fine hydrate dispersion during slow growth period, small lumps during rapid hydrate formation. The cell is not plugged.

III

Hydrate deposits on the wall and/or lump at the bottom of the cell during rapid growth, which were then dispersed by the stirring.

IV

Fine hydrate dispersion during slow growth period, plugged immediately after rapid hydrate formation.

V

Plugged immediately after hydrate formation started.

(a)

(b)

(c)

(d)

(e)

Figure 9: Visual observations in the autoclave. (a): cell content before hydrate formation occurs, (b): minor deposit at the bottom of the cell during slow growth period, (c): fine dispersion during slow growth period, (d): fine dispersed hydrate after rapid hydrate formation and (e): hydrate plugging after rapid hydrate formation.


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Figure 10 shows an example of a constant cooling test in a stirred sapphire autoclave of an additive giving a rank A result. The torque on the stirrer stays roughly constant through the gas hydrate formation process which starts after 104 minutes at about 5.3 oC. With an additive giving a plug the torque on the graph would rise rapidly to a high peak and then drop vertically due to stopping of the stirrer.

Figure 11 shows an example of a rank A additive from a superheating isothermal sapphire autoclave test. In this graph gas consumption is plotted rather than pressure drop. In this test the pressure has dropped about 8 bar, from the standard starting pressure of 75 bar to about 67 bar. Stirring begins at time zero and the torque rises to a low and constant value during the period of gas dissolution in the liquid phases (a few minutes) and hydrate formation which occurs over about 30 minutes.


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Figure 10. A constant cooling test in a stirred sapphire autoclave giving a rank A result. The black line, the first half of which is superimposed over the pressure plot shows the extrapolation of the pressure plot assuming no hydrate formation.

Figure 11. An isothermal test in a stirred sapphire autoclave giving a rank A result.

Based on the run time measurement and visual observations, the AA performance in the rocking cells at the various test conditions was ranked from A-E. A photo of additives giving rankings A, C and E in the sapphire rocking cells is shown in Figure 12. The rankings are classified as follows: A. Fine dispersed hydrates without any deposits, no increase in run time. B. No plugging/deposits is observed in video, run time somewhat increased, generally indicating coarse hydrate particles.


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C. Build-up of significant deposits, but the ball is still moving slowly. Loose hydrates may be pushed up one or both ends of the sapphire tube resembling a plug. D. Plug of hydrate initially, but run time decreases as the steel ball gradually breaks up hydrate lumps. E. Hard plug of hydrate, ball stops moving.

Figure 12. Rocking cells showing additives giving rankings A (top), C (middle), E (bottom).

Figure 13 shows the graphical data for a constant cooling test in a sapphire rocking cell with an additive giving rank E. The temperature drops constantly until 8.3oC when hydrate formation takes place causing a much more rapid pressure drop. Simultaneously the run time increases significantly and eventually the steel ball stops moving because of a hydrate plug, indicated by a smooth horizontal line at about 370 minutes. Figure 14 shows the graphical data for a isothermal


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start-up test in a sapphire rocking cell at 6oC in which the additive gave a fairly good performance and a rank B. This particular test was for a bis-amine oxide and shows a long kinetic delay of 1240 minutes before the start of hydrate formation was first observed from the pressure drop. The pressure drop and therefore also the gas hydrate formation can be seen to be more gradual in this test compared to the test in Figure 13, even though the subcooling is higher at the start of the hydrate formation process. The hydrate particles were slightly coarser compared to the very best additives tested and a gradual small but permanent increase in the steel ball run time was observed.

Figure 13. Typical graphical data for a sapphire rocking cell constant cooling test with an additive giving rank E.


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Figure 14. Typical graphical data for a sapphire rocking cell isothermal start-up test with an additive giving rank B.

Marine Biodegradation Studies Biodegradability in seawater was determined using the method based on OECD 306 guidelines.7 The method has been outlined in previous publications.23 The biological oxygen demand (BOD) for each additive was measured using the OxiTop ® Control manometric closed system (WTW, Germany) over 28 days by measuring the oxygen consumption data via measurement of pressure loss. Three bottles were used for each additive. The percentage biodegradability was determined based on comparing the measured BOD and the calculated theoretical oxygen demand values. The test medium was seawater without inoculum, but nutrients were added to ensure non-limiting conditions for microbial activity and growth. The test flasks contained seawater (taken from 70 m down and at a temperature 12°C in Byfjord outside Stavanger, Norway), nutrients and the test chemicals. Control flasks were also used: (1)


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seawater with nutrients only, (2) negative controls with autoclaved seawater (which kills any organisms that were originally present), nutrients and the test chemicals at 60 mg/l concentration, (3) positive seawater controls with nutrients and sodium benzoate at 100 mg/l which is known to be readily biodegradable.

Results and Discussions

High pressure stirred autoclave experiments. We carried out high-pressure SII gas hydrate constant cooling and superheating isothermal experiments in the autoclave for each new amine oxide surfactant at 1.0 wt.% based on the water phase. Test with a commercial quaternary ammonium-based AA (supplied by NALCO-Champion) were carried out for comparison. The aqueous phase was 1.5 wt.% brine solution. The performance of all the amine oxide surfactants with butyl head groups and tails of varying alkyl chain length are summarized in Tables 2 and 3 for constant cooling and superheating isothermal tests respectively. “To” refers to the onset temperature for the first detectable sign of hydrate formation. Two experiments were conducted for each chemical. Therefore two values are given, which generally gave very good agreement.

Table 2. Constant cooling performance in autoclave using 1.0 wt.% active AA in 1.5 wt.% NaCl brine as water phase and Europris white spirit as hydrocarbon phase. AA EmAO AmAO EbAO AbAO AbAO isomers

C9 1.4 / 1.4 -

To (Hydrate onset temperatures) [°C] C11 C13 C15 2.5 / 2.2 4.8 / 4.4 5.4 / 3.7 2.2 / 1.3* 1.5 / 1.5 1.5 / 1.5 1.5 / 1.5

1.9 / 1.5 1.7 / 1.5 1.5 / 1.4 1.5 / 1.5


1.5 / 1.5* 1.4 / 1.5 1.5 / 1.5 1.3 / 1.5

C17 1.6 / 1.4 1.2 / 1.4

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* No hydrate formation during the constant cooling procedure. The temperature in the cooling bath was then dropped quickly and held at 1.0°C after constant cooling until hydrate formation occurred.

Hydrate formation was observed to initiate at about 11.0-11.5°C using 1.5 wt.% brine solution without AA addition in the constant cooling test. Rapid formation occurred and the cell was plugged immediately. By adding 0.75 wt.% of the commercial AA based on the water, slow growth of hydrate was observed at 4.7°C, without any rapid hydrate formation during the constant cooling procedure down to 1°C. The final result was a fine, dispersed hydrate slurry without any deposit or plugging.

The results in Table 2 show that in general only some of the bis-amine oxide surfactants perform reasonably well AAs at these conditions, particularly the amide derivatives, whereas all of the monomeric amine oxides performed poorly. The pure AbAOs performed better as the alkyl chain length was increased to C15 and C17, whereas the best performance of the isomer mixtures was given by the C11 derivative. An important observation is the low onset temperatures (giving as much as 15 – 16°C subcooling) during the constant cooling, showing the kinetic inhibition ability of the amine oxides for Structure II gas hydrate formation. This may be why no rank A results were observed due to the increased hydrate formation rate at high subcoolings (high driving force). It was therefore in our interest to investigate the AA performance of the amine oxides at lower subcoolings using the superheating isothermal test.

Table 3. Superheating isothermal test performance at 6°C in autoclave using 1.0 wt.% active AA in 1.5 wt.% NaCl brine as water phase and Europris white spirit as hydrocarbon phase.


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Additive EmAO AmAO EbAO AbAO AbAO isomers

C9 276 / 30 -

Induction times [min.] C11 C13 C15 0 / 277 0 / 165 0/0 233 / 163 177 / 105 190 / 36 94 / 725 137 / 449 0 / 576 496 / 14 355 / 266 327 / 46 221 / 131 191 /413 452 / 428

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C17 352 / 14 111 / 650

The results from superheating isothermal tests (two for each chemical) in the sapphire autoclave are summarized in Table 3. The induction times vary greatly highlighting the stochastic nature of hydrate formation. Now that the subcooling is about 11oC more of the surfactants gave positive results. For example, the amido mono-amine oxide surfactants, the AmAOs, all gave B rankings. In contrast the ester mono-amine oxide surfactants all behaved poorly at this subcooling. These results alone would suggest that there is some benefit in having an amide rather than an ester group in an amido amine oxide surfactant AA. The reason may be that the amide group gives stronger hydrogen-bonds than the ester group which may be useful for bonding to hydrate surfaces. In agreement with the results for the constant cooling tests, of all the new surfactants the amido bis-amine oxides performed best in the isothermal tests, several of them giving rank A. The C13, C15 and C17 amido bis-amine oxide derivatives performed better than the equivalent amido mono-amine oxide with same hydrocarbyl tail. This suggests that surfactants with 2 amine oxide groups rather than a single amine oxide is advantageous for the AA performance. The AbAOs isomer mixtures performed better than the pure AbAO products for the C15 and C17 derivatives, which may be due to the two products in the isomer mixture having butylated amine oxide groups at different distances. This mixture of amine oxide distances may be able to interact in different and beneficial ways with hydrate surfaces compared with a single bis-amine oxide. Many of the


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amine oxide surfactants also showed kinetic hydrate inhibition with the amido bis-amine oxides giving the longest delays times before hydrate formation was detected.

High Press Rocking Cell Experiments. The second set of experiments was carried out in the high pressure sapphire rocking cell test equipment with exactly the same gas composition used in the autoclave experiments. Duplicate experiments were run for each AA surfactant. We concentrated on the amido bis-amine oxide surfactants as these were purer compounds than the isomer mixtures and had given some of the best performances in the sapphire autoclave. Initially, we carried out constant cooling tests on the additives followed by superheating isothermal experiments using 1.5 wt.% actives based on the water phase in 1.5 wt.% NaCl and with three different liquid hydrocarbon phases. The three phases used were two white spirits from local shops Europris (E.w.s.) and Biltema (B.w.s.) and a North Sea condensate (M.c.), described in the experimental section. Practically all the constant cooling tests gave hydrate formation at the minimum temperature of 2oC showing a kinetic hydrate inhibition effect for the amido bis-amine oxide surfactants, as also seen in the sapphire autoclave. The low onset temperatures led to rapid hydrate plug formation due to the high subcooling and fast hydrate formation. Therefore it was hard to compare the ranking for these additives by this method. However, the superheating isothermal tests at 6oC (ca. 11oC subcooling) gave more varied performance between the surfactants as described below. The superheating isothermal tests were conducted at various concentrations (0.5-1.5 wt.% based on the water phase) to determine the performance difference between the isomer mixture AbAOs with C13, C15 and C17 tails. The aims were to determine the minimum AA concentration required to pass the tests at various salinities as well as determine the influence of the hydrocarbon phase.


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Duplicate experiments were run for each AA surfactant. Figure 15 shows the minimum AA concentration required to pass the test in the three hydrocarbon phases. The results in this work are presented according to the worst case observed if the two tests gave different performance rankings, which happened only twice. In agreement with the isothermal tests in the sapphire autoclave, we also observed significant induction times of 44-1100 minutes at 6oC before observing hydrate formation with the amido bis-amine oxides, indicating a significant kinetic hydrate inhibition effect. Commercial KHIs do not disperse gas hydrates (i.e. they are not AAs) and commercial AAs using the Shell “hydrate-philic” mechanism have only weak nucleation inhibition properties that they are not used as KHIs by themselves. Therefore, the dual role (i.e. both KHI and AA performance) is something sought after by some oil companies. For example, if a KHI is being used and the flow line is shut in for an unexpected length of time, there may be a risk that the KHI is being used outside of its operational range and a hydrate plug could form. But if the KHI also has sufficient AA properties (i.e. dual properties) then the risk is removed as the hydrates, if formed, will be dispersed.

The C13 and C15 derivatives performed poorly with North Sea condensate at 1 wt.% actives, but the C17 derivative gave a very different rank A. It has been noted previously that the efficacy of a good AA surfactant class is often enhanced by increasing the chain length of the surfactant.5 However, there will be a limit to tail length as the hydrophilic-lipophilic balance will be drawn more and more into the liquid hydrocarbon phase and away from the “action“ at the waterhydrocarbon interface where hydrate formation is expected to start. This rule of thumb did not fit with all tests we have carried out. For example, for the tests conducted using Biltema white spirit we obtained only ranking C for 1.5 wt.% AbAO-C15 even though the minimum efficient concentration of AbAO-C13 is 1.0 wt.%. The Europris white spirit appeared to be the most


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“friendly” hydrocarbon phase for testing as all the surfactants gave a rank A performance at 1 wt.% in this solvent. However, dropping the surfactant concentration to 0.5wt.% gave considerably lower rankings of D-E. In order to see if increased salinity would improve the performance relative to the tests with 1.5 wt.% NaCl solution illustrated in Figure 15, we investigated AbAO-C15 further as this surfactant gave mixed results at low salinity in the various hydrocarbon phases. Figure 15 shows results of the effect of increased salinity to 3.5 wt.% and 7 wt.% NaCl with the three hydrocarbon phases.

1.5

1

0.5 M.c. B.w.s.

0 C13

E.w.s. C15

C17

Figure 15. Minimum efficient concentration of AbAOs using superheating isothermal method in 1.5 wt.% NaCl brine solution and in various hydrocarbon phases.


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1.5

1

0.5 M.c. B.w.s.

0 1.5% NaCl

E.w.s. 3.6% NaCl

7.0% NaCl

Figure 16. Minimum efficient concentration of AbAO-C15 using superheating isothermal method in various salinities and various hydrocarbon phases.

If we compare the performance of AbAO-C15 in varying NaCl brine concentrations, the results in Figure 16 show that increased salinity helps to improve the performance, irrespective of the hydrocarbon phase. For example, the minimum efficient concentration in E.w.s decreases from 1.0 wt.% to 0.5 wt.% by increasing the salinity from 1.5 wt.% to 7.0 wt.% NaCl. A similar trend is also observed for B.w.s and the North Sea condensate. An AA performance enhancement by the addition of NaCl salt had been reported previously.25-26 The reason for this observation may be related to the gas hydrate thermodynamic equilibrium temperature, which decreases with increasing salinity brine. This lowers the subcooling and will therefore lower the hydrate growth rate.

Seawater Biodegradation of Amido Bis-Amine Oxide Surfactants Quaternary ammonium salts with alkyl or benzyl groups are known to be poorly biodegradable. There we were interested to see if amine oxide surfactants would fare any better using the standard


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OECD306 seawater biodegradation method. Table 4 summarizes the biological oxygen demand (BOD) for the calibration standard (seawater only), sodium benzoate, and the three amide bisamine oxides with C13, C15 and C17 tails. Sodium benzoate gave the expected rapid and full degradation with no significant lag time, giving a 28 days biodegradation of about 88% without assimilation included in the calculation. None of the amine oxide surfactants showed “ready biodegradability”, which is a term usually alllocated to chemicals giving greater than 60% biodegradation in 28 days. However, reaching the threshold value of 20% biodegradation is useful with regards to obtaining an approved ecotoxicological category for offshore use in the North Sea, an area considered to have the strictest offshore environmental regulations.1

Table 4. Average biodegradability activity (triplicate tests) measured by the OECD 306 procedure over 28 days. Inhibitor

% BOD by OECD 306

Seawater

0

Sodium benzoate

88

AbAO-C13

40.8

AbAO-C15

33.1

AbAO-C17

25.0


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Conclusion A series of butylated mono- and bis-amine oxide surfactants with aliphatic tails of various lengths were synthesized and tested for their performance as AAs in a sapphire autoclave and sapphire rocking cells with a Structure II-forming natural gas mixture. There was good agreement regarding the performance and relative ranking of the surfactants between the two set of equipment. Aliphatic hydrocarbyl tails of at least 13 carbon atoms gave best performance. In general, the amido amine oxides performed better than the equivalent ester amine oxide. This may be related to the stronger hydrogen-bonding ability of the amide group giving a greater “hydrate-philicity” for the amido-based surfactants. Most of the amido bis-amine oxide surfactants also performed better than the amido mono-amine oxide surfactants. This indicates that the bis-amine oxide is probably better at binding to the gas hydrate particle surface. In these surfactants the distance between the two amine oxide head groups had previously been optimized based on crystal growth studies with THF hydrate.

In summary, the AA performance of the amine oxide surfactants depends on many factors, including single versus double amine oxide head group, the correct spacer group (amide found to be better than ester), subcooling at hydrate onset, salinity and the composition of the hydrocarbon fluid. The amido bis-amine oxide surfactants showed reasonable seawater biodegradation rates over 28 days, giving BOD values of 25-40% for the C13-C17 surfactants which are all inside the 20-60% bracket for the ecotoxicological OSPARCOM rules for chemicals to be used in the North Sea.

The bis-amine oxide surfactants in particular showed a strong kinetic hydrate inhibition effect, which is why very low gas hydrate onset temperatures were observed in many of the constant


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cooling experiments. The high subcooling in these experiments often led to hydrate plugs meaning that these surfactants were not effective at very high subcoolings and low salinities. However, this kinetic inhibition property could be very useful for field applications. These bis-amine oxide surfactants could be used for both KHI and AA purposes. As long as the subcooling is not too high these surfactants could be used as KHIs, giving total prevention of hydrate formation. If for any reason hydrate formation did begin, for example due to a shut-in and a prolonged residence time for the fluids, the surfactant could then function as an AA, thus avoiding hydrate plugging. We are continuing to investigate the KHI properties of bis- and polyamine oxide surfactants for used as KHIs and AAs.

Acknowledgement We thank Statoil ASA for the purchase of the rocking cell equipment, the sample of field condensate and financial support of some of the research work.

References 1. Kelland, M. Production Chemicals for the Oil and Gas Industry; CRC Press: Boca Raton, FL: 2nd edition, 2014. 2. Kelland, M. A. Energy Fuels 2006, 20 (3), 825− 847. 3. Klomp, U. C.; Kruka, V. C.; Reijnhart, R. International Patent Application WO 95/17579, 1995. 4. Klomp, U. C.; Rejnhart, R. International Patent Application WO 96/34177, 1996. 5. Chua, P. C.; Kelland, M. A. Energy Fuels 2012, 26 (2), 1160−1168.


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6. Mady, M.F.; Kelland, M.A. Chem. Eng. Sci. 2016, 144, 275–282. 7. OECD Guideline for testing of chemicals, Biodegradability in Seawater, 2002. Adopted by the Council on 17th July, p. 27. 8. Kelland, M. A.; Kvæstad, A. H.; Langeland Astad, E. Energy Fuels 2012, 26, 4454−4464. 9. Klug, P. U.S. Patent 6,102,986, 2000. 10. Kelland, M.A.; Mady, M.F. Energy Fuels 2016, 30, 3934−3940. 11. Magnusson, C. D.; Kelland, M. A. Energy Fuels 2015, 29, 6347−6354. 12. Panchalingam, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent 7,264,653, 2007. 13. Norland, A. K.; Kelland, M. A. Chem. Eng. Sci. 2012, 69 (1), 483−491. 14. Kelland, M.A. Chem. Eng. Sci. 2016, 98, 1-6. 15. Perrine, T.D. J. Org. Chem. 1953, 18, 1137. 16. Shabanov, A.L.; Azizov, A.M.; Sultanzade, S.S.; Azizova, N.A., Alieva, N.T. Azerb. Khim. Zh. 1988, 1,67–70. 17. Smith, M.B., March, J., 2007. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. John Wiley & Sons Inc, New Jersey. 18. Chua, P.C.; Kelland, M.A. Energy Fuels 2013, 27 (3), 1285–1292. 19. https://www.youtube.com/watch?v=r1G4s732L4Y&t=23s 20. Chua, P.C.; Kelland, M.A.; Hirano, T.; Yamamoto, H. Energy Fuels 2012, 26 (8), 4961–4967. 21. Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722-734. 22. Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N. Y. Acad. Sci. 2000, 912, 924− 931. 23. Azarinezhad, R.; Chapoy, A.; Anderson, R.; Tohidi, B. "HYDRAFLOW: A Multiphase Cold Flow Technology for Offshore Flow Assurance Challenges", Offshore Technology Conference, Houston, Texas, USA, 5-8 May 2008.


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and Bis-Amine Oxides - ACS Publications

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