Synergism of tert-Heptylated Quaternary Ammonium Salts with Poly(N

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Synergism of t-Heptylated Quaternary Ammonium Salts with Poly(N-vinyl caprolactam) Kinetic Hydrate Inhibitor in High Pressure and Oil-Based Systems Mohamed F. Mady, and Malcolm A. Kelland Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00110 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Synergism of t-Heptylated Quaternary Ammonium Salts with Poly(N-vinyl caprolactam) Kinetic Hydrate Inhibitor in High Pressure and Oil-Based Systems

Authors: Mohamed F. Madya,b,*, Malcolm A. Kellanda,* a

Department of Chemistry, Biological Science and Environmental Technology, Faculty of

Science and Technology, University of Stavanger, N-4036 Stavanger, Norway b

Department of Green Chemistry, National Research Centre, 33 El Bohouth st. (former El

Tahrir st.), Dokki, Giza, Egypt, P.O. 12622

*Corresponding authors: Prof. Dr. Malcolm A. Kelland (M.A. Kelland) E-mail: [email protected] Ass. Professor. Mohamed F. Mady (M.F. Mady) E-mail: [email protected]

Abstract: Quaternary ammonium ionic liquid salts (QAILs), are well-known as synergists for kinetic hydrate inhibitor (KHI) polymers such as poly(N-vinyl caprolactam) (PVCap). Earlier work showed that branching of the pentyl tails of tetra(n-pentyl)ammonium bromide to make isohexyl groups and t-heptyl groups gave a significant improvement in the synergistic KHI performance. We have now evaluated in more detail the KHI performance of tris(t-heptyl)-Npentyl-1-ammonium bromide (tris(t-heptyl)PeAB) or tris(t-heptyl)-N-propyl-1-ammonium

1 ACS Paragon Plus Environment

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bromide (tris(t-heptyl)PrAB) blended with poly(N-vinyl caprolactam) (PVCap) in steel rocking cells at 130 bar. Due to tris(t-heptyl)PeAB being sparingly soluble in water we used a water-gas-decane system in order to get this compound dissolved in the fluids. These QAILs were also compared to the known QAILs, tetra(n-butyl)ammonium bromide (TBAB), tetra(npentyl)ammonium bromide (TPAB), tetra(n-hexyl)ammonium bromide (TnHexAB), tetra(isohexyl)ammonium bromide (TiHexAB), tetra(n-heptyl)ammonium bromide (TnHepAB) and hexa-n-butylguanidinium chloride (n-Bu6GuanCl),. Under constant cooling test conditions it was found that both t-heptyl-based QAILs as well as TiHexAB and n-Bu6GuanCl gave particular good synergistic performance results. To further differentiate the ranking of these QAIL synergists, we also conducted long-term isothermal KHI performance tests on the best synergists. When PVCap was mixed with tris(theptyl)PeAB it gave better performance than when mixed with TPAB, TiHexAB or nBu6GuanCl in tests at two different isothermal test conditions. These results highlight the excellent synergy of t-heptylated quaternary ammonium salts and that sparingly water-soluble but oil-soluble quaternary ammonium salts, with the correct functional groups, can still be good synergists for KHI polymers such as PVCap.

Key words: low dosage hydrate inhibitors, kinetic hydrate inhibitors, ionic liquids, highpressure system

1. Introduction


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Production chemicals are broadly used in the petroleum industry to deal with production problems that can occur as a result of changes in chemical and physical properties.1 Flow assurance is a term used in the industry which describes the goal of keeping the produced fluids flowing to avoid production upsets and maximize revenue. One aspect of flow assurance is to prevent solid deposits (fouling), including gas hydrates, asphaltene, wax, scale and naphthenates.2 Gas hydrates belong to the class of clathrates where water molecules form a lattice of cages that are filled with small molecules.3 Gas hydrate is known to form several structures. In practice a typical natural gas mixture containing C1-C4 components will form Structure II hydrates as the structure which is most thermodynamically stable. Deposits gas hydrates can form blockages in flow lines and are therefore a menace to the oil and gas industry if they form. This can also occur during deep water drilling and in other types of well operations. Low dosage hydrate inhibitors (LDHIs) have been used for over decades to prevent gas hydrate plugging of flow lines. In some field scenarios these products may be significantly cheaper than other hydrate plug prevention methods, and are therefore useful tools in the hydrate management toolbox.4 Two main classes of LDHI are known: (a) Kinetic Hydrate Inhibitors (KHIs), (b) Anti-Agglomerants (AAs). As the LDHI name suggests these additives are added at low dosages, usually at about 0.1-1.5 wt.% concentration in solvents. This compares to much higher dosages, often 10-50 wt.%, which is needed for thermodynamic inhibitors such as methanol, ethanol or small glycols. KHIs delay hydrate formation but there is evidence that they may even give permanent kinetic hydrate inhibition up to certain subcoolings.5 AAs control hydrate crystal formation and disperse the particles in a carrier fluid, usually a liquid hydrocarbon phase.


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Quaternary ammonium and phosphonium salts have been known for over two decades as gas hydrate crystal growth modifiers.1,6 With small alkyl or aryl groups, these salts are part of the larger class of ionic liquids. The temperature range at which a salt can be classed as an IL is unclear but is often restricted to salts whose melting point is below about 100 °C. Thus, quaternary ammonium ionic liquids (QAILs) are ionic compounds that have the following general structural formula as shown in Figure 1.7-11 This includes tetra(n-butyl)ammonium bromide (TBAB) and tetra(n-pentyl)ammonium bromide (TPAB), tetra(n-hexyl)ammonium bromide (TnHexAB) and tetra(n-heptyl)ammonium bromide (TnHepAB) as shown in Figure 2.

Figure 1. Typical structures of quaternary ammonium ionic liquids (QAILs).

QAILs have been investigated for many oilfield chemical applications their use as scale, gas hydrate and corrosion inhibitors as well as demulsifier, and desalting agents.12-14 Some QAILs and tetraalkylphosphonium salts form their own clathrate hydrate structures.15,16 This was the clue to their discovery as hydrate crystal growth modifiers as the QAIL clathrate hydrates have common structural features to natural gas clathrate hydrates. 17-19


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Br

Br N

N

TBAB

TPAB

Br

Br N

N

TnHexAB

TnHepAB

Figure 2. Structures of commercial quaternary ammonium salts tested as THF hydrate crystal growth inhibitors and as synergists with N-vinylcaprolactam-based KHI polymers..

The discovery that small tetraalkyl quaternary ammonium salts are able to inhibit hydrate crystal growth led to use of TBAB and other related QAIL salts in blends with kinetic hydrate inhibitor (KHI) polymers, where they act as synergists. Typical polymers used in the field with TBAB and related quaternary ammonium salts include VCap-based polymers.20 In addition, QAILs modified to contain one or two long hydrophobic tails became the flagship surfactant class for commercial AAs.21


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Since then other QAILs with improved performance have been found. Our group has carried out several studies of potentially novel types of synergists related to the original QAILs. Figure 3 showed our promising QAILs incorporating different functional groups such as isohexyl, fluoro, guanidinium and cyclopropenium, which all show excellent performance as Structure II hydrate crystal inhibitors.22-25

Figure

3.

Structures

of

hexa-n-butylguanidinium

chloride

(n-Bu6GuanCl),

tris(dialkylamino)cyclopropenium chloride, tetra(iso-hexyl)ammonium bromide (TiHexAB), tri-fluoro quaternary ammonium bromide Pe[F(CH2)4]3NBr (TFAB).

Tetra(iso-hexylammonium) bromide (TiHexAB) was shown to be a much better inhibitor of THF hydrate crystals and synergist for PVCap compared to TPAB.25 This was assumed to be related to the isoalkyl group giving a better Van der Waals interaction with the cavities on the Structure II hydrate surfaces, compared to a straight pentyl chain. 6 ACS Paragon Plus Environment

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An obvious continuation of these studies would be to investigate if t-hepylated (or 4,4dimethylpentylated) ammonium salts can give improved synergy with PVCap compared to less branched tetraalkyl quaternary salts. These salts have the alkyl chains split at the end into three methyl groups, as you get in t-butyl groups also (Figure 4). Since it is known that neopentane (2,2-dimethylpropane) can enter 51264 cages in Structure II hydrate (with smaller help gases), this means that the t-butyl group is small enough to enter the 51264 open cavities on the Structure II hydrate surface.26 This observation led to our first study on quaternary ammonium salts with t-heptyl groups in which we discovered they were excellent THF hydrate crystal growth inhibitors and synergists for VCap-based kinetic hydrate inhibitor (KHI) polymers.27 Figure 4 shows the structures of tris(t-heptyl)-N-propyl-1-ammoniumbromide (tris(t-heptyl)PrAB) and tris(theptyl)-N-pentyl-1-ammonium bromide (tris(t-heptyl)PeAB). It is assumed that PVCap and these quaternary salts make good synergistic blends because they inhibit hydrate crystal growth by attacking different sites of the growing hydrate structures. The quaternary salts have a negligible effect on the cloud point of this polymer, which might have been a possible reason for synergy.


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Figure 4. Structures of tris(t-heptyl)-N-propyl-1-ammoniumbromide and tris(t-heptyl)-Npentyl-1-ammonium bromide.

In order to compare the relative performance of these t-heptylated salts in Figure 4, and with other alkylated quaternary ammonium salts, we have now carried out constant cooling and isothermal KHI tests with a gas mixture that forms Structure II hydrate as the most thermodynamically stable phase. Test were carried out at high pressure, 130 bar. We have also carried out synergy tests with added liquid hydrocarbon phase (decane) since tris(theptyl)(n-pentyl)ammonium bromide (tris(t-heptyl)PeAB) has negligible solubility in water but is easily soluble in hydrocarbon solvents. The KHI performance has been compared to other quaternary ammonium salts that are not soluble in water, such as tetra-nheptylammonium bromide.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods Poly(N-vinylcaprolactam) (PVCap) was supplied as Luvicap EG high molecular weight (HM) (41.4 wt % PVCap in monoethylene glycol) from BASF. It was used as received still containing the monoethylene glycol. All references to PVCap in this study refer to the active polymer concentration (Mw= 7000-12000 g/mole). Besides PVCap and the chemicals synthesised for this study, all chemicals were obtained from Fluka, VWR or Sigma-Aldrich and were used without further purification. NMR spectra were recorded on a Bruker Ascend NMR 400 MHz spectrometer at ambient temperature unless otherwise stated.27 Highresolution mass spectra (HRMS) were recorded on an ESI-MS Thermo LTQ Orbitrap XL (Infusion 5 µL/min, resolution: 100 000 at m/z 400, ca. 10 scans/sample averaged). 8 ACS Paragon Plus Environment

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Tris(t-heptyl)PrAB and tris(t-heptyl)PeAB were synthesized by standard quaternization procedures from the reaction of either 1-propylamine or 1-pentylamine with 1-bromo-4,4dimethylpentane in reasonable yields and have been reported previously.27

TiHexAB was prepared from the reaction of 3.3 mole equivalent of iso-hexyl bromide with 3 mole equivalent of powdered K2CO3, and 1 mole equivalent of iso-hexylamine hydrochloride under reflux in isobutyronitrile as previously reported.25

n-Bu6GuanCl was synthesized from guanidinium chloride and n-butyl bromide as previously reported.22

All synthesized compounds were characterized by IR, 1H NMR,

13

C NMR and HRMS

analysis and have been reported previously.22, 25,27

2.2. High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods The method for testing the kinetic hydrate inhibition performance of additives has been reported previously.22-25,27 All experiments used a set of five high pressure 40 mL steel rocking cells, each containing a steel ball as shown in Figure 5. The instrument was manufactured by PSL Systemtechnik, Germany. The gas used was a synthetic blend of natural gases (SNG) as shown in Table 1. Two kinds of KHI test methods were carried out, i.e., the “slow constant cooling” test and the “isothermal” test to evaluate the performance of novel quaternary ammonium salts as synergists for PVCap.


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Figure 5. The rocking cell equipment showing the steel cells in the water bath (left) and a view inside of a rocking cell with steel and glass balls (right).

Table 1. The composition of Synthetic Natural Gas (SNG). Component

Mole %

Methane

80.67

Ethane

10.20

Propane

4.90

Isobutane

1.53

n-butane

0.76

N2

0.10

CO2

1.84

2.2.1. Constant cooling test procedure The test procedure was a constant cooling test method. At the beginning of each experiment, the pressure was 75 or 130 bar as detailed below. The equilibrium temperature (Teq) at 75 bar pressure with water only (i.e. no decane) was determined by dissociation experiments, 10 ACS Paragon Plus Environment

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whereby formed hydrates were melted using a heating rate of 0.025 °C/h for the last 3-4 °C.28 Five repeat tests were carried out, which gave 20.2 °C ± 0.05 °C as Teq. This value was in agreement with a Teq value of 20.5 °C at 76 bar calculated using Calsep’s PVTSim software. For the water-decane system (20 vol. % decane) at 130 bar the Teq value was calculated to be 20.3 °C. The constant cooling test procedure was as follows: 1. 20mL of distilled water in which the various additives had been dissolved was placed in each cell. In addition, some experiments were carried out with water and decane at 80% water cut, using 16ml water plus 4ml decane in which all KHIs were dissolved in water first before decane was added. This was to solve the issue that some KHI components were not very water-soluble. 2. Vacuum pumping and filling with SNG to 2 bar was used to remove air from the cells. The procedure was repeated twice. 3. SNG was led into each cell to give 75 or 130 bar pressure. The cells were rocked at 20 rocks per minute with an angle of 40°. 4. The cooling bath was activated which led to the cells being cooled from 20.5 °C to 2 °C, at a rate of 1 °C/h. If after reaching the minimum temperature no catastrophic hydrate formation had taken place, we held the temperature at 2 °C until this event had occurred. 5. Temperature and pressure data for each cell, as well as the temperature in the cooling bath, were recorded throughout the test.

As an example of the data obtained, we shown in Figure 6 the temperature and pressure data for four rocking cells containing tetra(n-pentyl)ammonium bromide (TBAB) in blends with PVCap (2500+ 2500 ppm) at 75 bar, i.e. gas only with no addition of decane. The onset of hydrate formation is determined as a temperature (To), which is when the first deviation from 11 ACS Paragon Plus Environment

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the pressure drop that is not due to the temperature drop occurs. It should be noted that true nucleation may have occurred somewhat earlier. In this set of tests in Figure 6, it was found that the average onset temperature when the pressure drops due to hydrate formation was 7.6°C, and the average rapid hydrate formation temperature (Ta), when the rate of hydrate formation reaches a maximum, was 6.8°C. In addition, Figure 7 shows a typical graph obtained from a single experiment of the constant cooling test for the synergistic mixture tris(t-heptyl)PrAB with PVCap (2500+2500ppm) at 75 bar. The average To value is 3.7oC, with the average Ta for rapid hydrate formation at 3.6oC.

90

60

80 50

60

40

50

P2

P3

P4

P5

40

T2

T3

T4

T5

30

30

20

Temperature (oC)

70

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 10 10 0

0 0

200

400

600

800

1000

1200

Time (mins)

Figure 6. Temperature and pressure data versus time for a constant cooling KHI test involving four cells of TBAB blended with PVCap (2500+2500ppm).


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90

60

80 50 60

40

50

P3

T3

30

40 30

20 Ta= [Y-VERDI] C

To= 3.7 C

20

Temperature (oC)

70

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 10 0

0 0

200

400

600

800

1000

1200

Time (mins)

Figure 7. Determination of To and Ta values for a standard constant cooling test for 2500ppm PVCap + 2500 ppm tris(t-heptyl)PrAB.

2.2.2. Isothermal Test Procedure In this method, the cells were pressurized to 130 bar SNG at 20.5°C and 80 % water cut using decane (16ml water plus 4ml decane). The fluids were cooled rapidly over about 1 h to the experimental temperature (4 and/or 2°C) whilst rocking and then held at this temperature for 24 hours with continual rocking. At 4oC the subcooling (∆T) was calculated to be =15.9oC and at 2oC the subcooling was 17.8oC. The start of the induction time period (time zero) is taken as the time when cooling was begun. From the pressure vs time graph as shown in Figure 8, the induction time period (Ti) is taken as the start of detectable hydrate formation (by first observable pressure drop), and the fast, catastrophic hydrate formation, Ta. As with the constant cooling test, nucleation might have begun before the first observed pressure drop.


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130

60

120

50

110

40

100

30 P2

P4

P5

90

20 T2

T4

Temperature (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Pressure (bar)

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T5

80

10 T= [Y-VERDI]oC

70

0 0

200

400

600

800

1000

1200

1400

Time (min) Figure 8. Typical pressure and temperature graphical data obtained from an isothermal test using 2500 ppm of three different QAILs + 2500ppm PVCap at 130 bar and 4 oC. P2= tris(theptyl)PeAB, P4= TiHexAB, P5= TPAB.

Figures 9, 10 and 11 show the evaluation of the long-term gas hydrate inhibitory performances of PVCap combined with tris(t-heptyl)-PrAB, TiHexAB

and TPAB

respectively at 80 % water cut, decane, 130 bar and 4 oC, utilizing the long-term isothermal experimental method. The gas consumption and the first observed hydrate formation were determined by the pressure drop curve, from which the observed induction time (Ti) was identified. In Figure 9 no pressure drop due to hydrate formation can be seen throughout the 1600 minutes experimental time. In contrast, Figures 10-11 show a first pressure drop at approximately 1180 and 900 minutes respectively.


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130

60

No pressure drop due to hydrates

50

Pressure (bar)

40 110 30 100 P2

T2

20

90

Temperature (oC)

120

10

T= 4oC

80

0 0

200

400

600

800

1000

1200

1400

Time (min)

Figure 9. Temperature versus pressure data from an isothermal test using 2500 ppm tris(theptyl)PeAB + 2500 ppm PVCap at 4 oC. 130

60 50

120

Ti= 19 h 40 30

100 P4

T4

20 T= 4oC

90

10

80

Temperature (oC)

110

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0

200

400

600

800

1000

1200

1400

Time (min)

Figure 10. Temperature versus pressure data from an isothermal test using 2500 ppm TiHexAB + 2500 ppm PVCap at 4 oC. 15 ACS Paragon Plus Environment

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130

60

50

120

Ti= 15 h 40

110 30 100 P5

T5

20

90

Temperature (oC)

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

80

0 0

200

400

600

800

1000

1200

1400

Time (min) Figure 11. Temperature versus pressure data from an isothermal test using 2500 ppm TPAB + 2500 ppm PVCap at 4 oC. 3. RESULTS AND DISCUSSION Constant Cooling KHI studies: We studied the synergistic effect of PVCap with watersoluble tris(t-heptyl)(n-propyl)ammonium bromide tris(t-heptyl)PrAB and compared this PVCap mixed with tetra(n-butyl)ammonium bromide (TBAB), tetra(n-pentyl)ammonium bromide (TPAB) and tetra(iso-hexyl)ammonium bromide (TiHexAB). This was initially done using an aqueous phase without using hydrocarbon (pure decane) solvent. The constant cooling test method was used with a Structure II hydrate-forming gas mixture with a starting pressure of 75 bar. The average results from 6-8 experiments on each mixture are shown in Table 2. In general, for the results in this paper, we observe a maximum scattering of 15−20% in the To data points. This is in line with many previous rocking cell studies from our group, including the first study on t-heptyl quaternary ammonium salt synergists. The result with no


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additive is also added for comparison. However, tris(t-heptyl)PeAB was not tested as it is only sparingly soluble in water, certainly much less than 2500ppm from visual observations. Tris(t-heptyl)PrAB, TiHexAB and TPAB gave fairly similar and higher synergistic performance than TBAB. For example, with 2500 ppm each of tris(t-heptyl)PrAB and PVCap the onset of hydrate formation (To) occurred at an average value of 4.2 °C, with rapid hydrate formation (Ta) at 3.9 °C. Figure 7 discussed earlier shows a graph of a constant cooling test for a single cell of the synergistic solution tris(t-heptyl)PrAB / PVCap (2500+2500 ppm).

Table 2. Constant Cooling KHI Tests in the in the rocking cell equipment: water and SNG only, at 75 bar.a To (°C)b

Ta (°C)c

17.8

17.4

2500

10.9

9.3

TBAB + PVCap

2500 + 2500

7.6

6.8

TPAB + PVCap

2500 + 2500

4.8

4.3

TiHexAB + PVCap

2500 + 2500

4.2

3.1

Tris(t-heptyl)PrAB + PVCap

2500 + 2500

4.2

3.9

Chemical

Concentration (ppm)

No additive PVCap

a

All constant cooling tests were run at 100% water cut at 75 bar. bTo, average onset temperature. cTa, average rapid hydrate formation.

Some earlier published results of the tris-t-heptylated QAILs blended with PVCap showed that no hydrates formation at 2 oC in tests with a liquid hydrocarbon phase at 75 bar.27 In order to discern the ranking of these chemicals, we conducted constant cooling tests at a higher pressure, 130 bar, decane as liquid hydrocarbon and 80% water cut, to give more subcooling (∆T = 17.8 oC at the minimum system temperature of 2 oC, calculated by 17 ACS Paragon Plus Environment

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commercial software). In addition to tris(t-heptyl)PeAB and tris(t-heptyl)PrAB, we also evaluated

TBAB,

heptyl)ammonium

TPAB, bromide

tetra(n-hexyl)ammonium (TnHepAB)

and

bromide

(TnHexAB),

hexa-n-butylguanidinium

tetra(n-

chloride

(n-

Bu6GuanCl) as synergists for PVCap in the hydrocarbon solvent. The structures of all these QAILs are presented in Figures 2, 3 and 4. For tests with no additive, we obtained an average To value of 14.3 °C, and average Ta value of 13.4 °C. For tests using 2500 ppm of PVCap, the average To and Ta values were lowered to 10.3 and 9.9 °C, respectively, indicating that this polymer has good KHI efficacy. When either 2500 ppm of TBAB or TnHepAB was added to 2500 ppm PVCap the average To values were 9.3 oC and 9.4 oC respectively as tabulated in Table 3. These results indicate only minor synergism from these QAILs. TnHexAB shows stronger synergy with PVCap, and TPAB stronger synergy still, but the best results obtained were for n-Bu6GuanCl, tris(t-heptyl)PrAB, TiHexAB and tris(t-heptyl)PeAB. The latter two QAILs showed no pressure drop down to the minimum temperature of 2oC.

As an example, the graph in Figure 12 illustrates the

determination of the onset temperature (To) and the temperature of fast hydrate formation (Ta) for one of a series of the synergistic solution TnHepAB/PVCap (2500+2500 ppm).

Table 3. Constant cooling KHI tests in the rocking cell equipment: water, decane and SNG, at 130 bar.a

Chemical

To (°C)b

Ta (°C)c

14.3

13.4

2500

10.3

9.9

2500 + 2500

9.3

8.1

Concentration (ppm)

No additive PVCap TBAB + PVCap


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TPAB + PVCap

2500 + 2500

4.2

4.2

TnHexAB + PVCap

2500 + 2500

7.9

4.7

TnHepAB + PVCap

2500 + 2500

9.4

9.1

n-Bu6GuanCl + PVCap

2500 + 2500

2.6

2.3

TiHexAB + PVCap

2500 + 2500

-

Tris(t-heptyl)PeAB + PVCap

2500 + 2500

Tris(t-heptyl)PrAB + PVCap

2500 + 2500

No pressure drop No pressure drop 2.5

a

b

2

c

All tests were run at 80 % water cut, decane, and at 130 bar. To. Ta.

140

60

120

50

100 P5

80

T5 30

60 Ta= [Y-VERDI] C

To= [Y-VERDI] C

20

40

Temperature (oC)

40

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

20

0

0 0

200

400

600

800

1000

1200

Time (min)

Figure 12. Pressure and temperature curves versus time during a standard constant cooling test using 2500 ppm TnHepAB + 2500ppm PVCap at 130 bar.

The results can be rationalized as follows: TnHepAB is only sparingly soluble in water and a very poor THF hydrate crystal growth inhibitor. Both these factors lead to poor synergism 19 ACS Paragon Plus Environment

Energy & Fuels

with PVCap in a gas hydrate system. TnHexAB and TBAB are water-soluble but TBAB shows much better THF hydrate inhibition, but in a gas hydrate system the more hydrophobic TnHexAB can inhibit nucleation of hydrates more than TBAB.25 The four best synergists for PVCap that were tested are also excellent THF hydrate crystal growth inhibitors.22,25,27 Tris(theptyl)PrAB and n-Bu6GuanCl showed very good hydrate inhibition performances, with average onset temperatures To of 2.5 oC and 2.6 °C, respectively. Even more impressively, tris(t-heptyl)PeAB and TiHexAB blends with PVCap showed no pressure drop even at 2 oC. For example, Figure 13 shows a constant cooling graph for the synergetic solution of tris(theptyl)PeAB and PVCap at 130 bar. It was observed that the onset temperature (To) was not investigated after 18 h. TnHepAB shows poor synergy but tris(t-heptyl)PeAB shows excellent synergy, yet both compounds are sparingly soluble in water. This highlights the fact that the correct alkyl group size and shape is critical for optimal performance.

No pressure drop due to hydrates

140

60

120

50

40 P3

T3

80 30 60 20 40 10

20

0

0 0

200

400

600

800

Time (min)


1000

1200

Temperature (oC)

100

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 13. Temperature and pressure data versus time for a standard constant cooling test using 2500 ppm tris(t-heptyl)PeAB + PVCap at 130 bar.

Isothermal KHI Studies: In addition, long-term isothermal KHI performance tests have been carried out to further clarify the KHI performance ranking of PVCap in synergist blends with tris(t-heptyl)PrAB, tris(t-heptyl)PeAB, n-Bu6GuanCl, TPAB and TiHexAB also at 130 bar. The cells were cooled over about 1h to the start temperature of 2 oC. The cells were then rocked at 2 oC for 24h (subcooling ∆T =17.8oC). Table 4 shows the results of the synergy of different QAs with PVCap in high-pressure isothermal KHI rocking cell tests. Three tests were carried out for each blend. Figures 14 and 15 show examples of graphical results of isothermal KHI tests for the synergistic solutions of PVCap combined with tris(t-heptyl)PeAB (20h) and TiHexAB respectively. The ranking and average time (mins) to the first observed pressure drop was as follows: tris(t-heptyl)PeAB (1200) > TiHexAB (360) > tris(t-heptyl)PrAB (330) > n-Bu6GuanCl (300) > TPAB (30) The percentage error in these times was about 15-20%, meaning that the blends with times of 300-360mins (5-6h) could not be distinguished with statistical significance with regards to the performance. Clearly, tris(t-heptyl)PeAB was the best synergist, which may be considered surprising given that it is sparingly soluble in water but partitions mainly to the hydrocarbon phase.

Table 4. Induction time Ti (min) for QAILs blended with 2500 ppm PVCap at 2.0 oC in isothermal tests at 130 bars. 21 ACS Paragon Plus Environment

Energy & Fuels

Synergist

Concentration

Induction time

(ppm)

Ti (min)

TPAB

2500

30

TiHexAB

2500

360

Tris(t-heptyl)PeAB

2500

1200

Tris(t-heptyl)PrAB

2500

330

n-Bu6GuanCl

2500

300

140

60

Ti = 20 h 120

50

100 40 80 P5

60

30

T5

Temperature (oC)

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

20 40 20

10

T= 2oC

0 0

200

400

600

800

1000

1200

1400

0 1600

Time (min)

Figure 14. Isothermal test data for 2500 ppm tris(t-heptyl)PeAB + 2500 ppm PVCap at 2oC.


Page 23 of 27

60

140

Ti = 6 h 120

50

40 80 P2

T2

30

60 20 40 T= 2oC

10

20 0 0

200

400

600

Temperature (oC)

100

Pressure (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

0 1000

Time (min) Figure 15. Isothermal test data for 2500 ppm TiHexAB + 2500 ppm PVCap at 2 oC.

Finally, to confirm the ranking results at 2oC we repeated the isothermal KHI performance test of PVCap in synergistic blends with tris(t-heptyl)PeAB, TiHexAB and TPAB at 130 bars but this time the fluids were cooled to a temperature of 4 oC (subcooling ∆T = 15.9 oC). Two tests per blend were carried out. It was observed that the time to the first observed pressure drop for TiHexAB was 1000-1180 mins (16-19h) for 2 tests, and for TPAB the first pressure drop was at 900-950mins (15-16 h) whereas for tris(t-heptyl)PeAB no drop in pressure was detected after 24 hours, again for 2 tests as presented in Table 5 and Figures 9 and 10. We are aware that only two tests for such a stochastic system are a low number of parallel tests. Assuming the percentage error is the same as in the first set of isothermal tests (15-20%, Table 4), the error is small enough that the main observation from these tests is still statistically significant, i.e. to show that a sparingly-soluble quaternary ammonium salt can still be an excellent KHI synergist.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Table 5. Induction time Ti (h) for QAILs with 2500 ppm PVCap at 4.0 oC in isothermal experiments at 130 bars. 2 tests for each synergist mixture. Synergist

Concentration

Induction time

(ppm)

Ti (mins)

TPAB

2500

900, 950

TiHexAB

2500

1000, 1180

Tris(t-heptyl)PeAB

2500

>1440, >1440

4. Conclusion

As a continuation of our studies to explore new quaternary ammonium ionic liquids (QAILs) as KHI additives, we have evaluated the performance of tris(t-heptyl)-N-pentyl-1-ammonium bromide (tris(t-heptyl)PeAB) or tris(t-heptyl)-N-propyl-1-ammonium bromide (tris(theptyl)PrAB) blended with poly(N-vinyl caprolactam) (PVCap) in steel rocking cells. Due to tris(t-heptyl)PeAB having negligible water-solubility we moved from a water-gas system to a water-gas-decane system in order to get this compound dissolved. The pressure was also increased to 130 bar to obtain sufficient subcooling in order to get hydrate formation in the majority of the KHI tests. These QAILs were also compared to the known QAILs, TBAB, TPAB, THexAB, TiHexAB and THepAB and n-Bu6GuanCl. Under constant cooling test conditions it was found that both t-heptyl-based QAILs as well as TiHexAB and nBu6GuanCl gave particular good synergistic results with To values at 2.6 oC or lower.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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As mixtures of tris(t-heptyl)PeAB or TiHexAB with PVCap gave no detectable hydrate formation down to 2 oC in the constant cooling tests, we conducted long-term isothermal KHI performance tests. This was done only for the QAILs with best synergistic performance. At 4oC no hydrates formed for the mixture of PVCap with tris(t-heptyl)PeAB in 1440min (24h), whereas with TPAB or TiHexAB we obtained induction times (Ti) of 900-1180min (15-19h). Further isothermal tests at 2oC showed that tris(t-heptyl)PeAB was still the best QAIL synergist giving average Ti of 1200min (20h) compared to 30min for TPAB and 300-360min for TiHexAB or n-Bu6GuanCl or tris(t-heptyl)PrAB. These results suggest that a tertiary carbon with three methyl groups at the end of the alkyl group (t-butyl) gives a better Van der Waals interactions with Structure II hydrate surfaces than a secondary carbon with 2 methyl groups (iso-propyl) or just a straight chain alkyl group. For quaternary ammonium salts we have demonstrated this for t-heptyl groups (i.e. t-butylCH2CH2CH2-) compared to iso-hexyl and n-pentyl groups. Designing t-butyl groups into other LDHIs, not necessarily as t-heptyl groups, may lead to improved performance. This could be for KHI polymers and synergists, AA surfactants or synergistic solvents. (These heptylated quaternary salt synergists are unlikely to act as AAs by themselves unless they contain a longer hydrocarbyl tail). In addition, the results in this study highlight that sparingly water-soluble but oil-soluble quaternary ammonium salts, with the correct functional groups, can still be good synergists for KHI polymers containing the VCap monomer.

References 1. Kelland, M. A. Production Chemicals for the Oil and Gas Industry; 2nd edition, CRC Press (Taylor & Francis Group): Boca Raton, FL, 2014.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2. Gudmundsson, J. S., Flow assurance solids in oil and gas production. CRC Press (Taylor & Francis Group): Balkema: London, UK, 2017. 3. Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2008. 4. Kelland, M.A. Energy Fuels 2006, 20, 825. 5. Mozaffar, H.; Anderson, R. Tohidi, B. Energy Fuels 2016, 30, 10055. 6. Li, X.-S.; Zhan, H.; Xu, C.-G.; Zeng, Z.-Y.; Lv, Q.-N.; Yan, K.-F., Energy Fuels 2012, 26, 2518. 7. Welton, T. Chem. Rev. 1999, 99, 2071. 8. Endres, F.; Zein El Abedin, S. Chem. Phys. 2006, 8, 2101. 9. Michael, F. An Introduction to Ionic Liquids. Royal Society of Chemistry (RSC), ISBN 978-1-84755-161-0, 2009. 10. MacFarlane, D. R.; Golding; J.; Forsyth, S.; Forsyth, M.; Deacon, G. B. Chem. Commun. 2001, 16, 1430. 11. Freemantle, M. Chem. Eng. News 1998, 76, 32. 12. Tariq, M.; Rooney, D.; Othman, E.; Aparicio, S.; Atilhan, M.; Khraisheh, M., Ind. Eng. Chem. Res. 2014, 53, 17855. 13. Handy, S. Progress and Developments in Ionic Liquids", InTech, ISBN 978-953-512902-8, 2017. 14. Cao, S.; Liu, D.; Zhang, P.; Yang, L.; Yang, P.; Lu, H.; Gui, J. Scientific Rep. 2017, 7, 8773. 15. Jeffrey, J.; Academic Press: 1984, 1, 159. 16. Dyadin, Y. A.; Udachin, K. A. J. Macrocyclic Chem. 1984, 2, 61. 17. Klomp, U. C.; Kruka, V. C.; Reijnhart, R. International Patent Application WO95/17579, 1995.


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18. Klomp, U. C.; Reijnhart, R. International Patent Application WO96/34177, 1996. 19. Klomp, U. C.; Kruka, V. C.; Reijnhart, R. Low Dosage Hydrate Inhibitors and How They Work, Proceedings of the Symposium Controlling Hydrates, Waxes, and Asphaltenes, IBC Conference, Aberdeen, Oct 1997. 20. Duncum, S.; Edwards, A. R.; Osborne. C. G. International Patent Application WO96/04462, 1996. 21. Tian, J.; Walker, C. “Non-emulsifying, New Anti-agglomerant Developments,” Offshore Technology Conference, Houston, TX, 30 April-3 May 2012. 22. Kelland, M. A.; Moi, N.; Howarth, M. Energy Fuels 2013, 27, 711. 23. Kelland, M. A.; Reyes, F. T.; Trovik, K. W., Chem. Eng. Sci. 2013, 93, 423. 24. Mady, M. F.; Kelland, M. A. Energy Fuels 2013, 27, 5175. 25. Chua, P. C.; Kelland, M. A. Energy Fuels 2012, 26, 1160. 26. Tohidi, B.; Danesh, A.; Todd, A.C.; Burgass, R.W.; Ostergaard, K.K. Fluid Phase Equilibria 1997, 138, 241-250. 27. Mady, M. E.; Kelland, M. A. Chem. Eng. Sci. 2016, 144, 275. 28. Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722.


Synergism of tert-Heptylated Quaternary Ammonium Salts with Poly(N

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