NO331537B1 - Gas hydrate inhibitors and methods for controlling gas hydrate formation and clogging of gas hydrate forming fluids - Google Patents

Abstract

The present invention relates to compounds of formulas (1) and (II) for controlling gas hydrate formation and clogging of gas hydrate forming fluids, O. wherein R 1 is selected from the group consisting of H, acetyl and carbonyl derivative; R 2 is H, -OR 3 or -NR 4 R 5, wherein: R 3 is H, C 1-6 alkyl or carbonyl derivative, R 4, R 5 is selected from the group consisting of H, C 1-18 alkyl, alkanol, alkoxy, cyclic / aromatic or alkylene . The invention further encompasses compositions and processes for producing the compounds of formula (I) and (II), a process for controlling gas hydrate formation and clogging of gas hydrate forming fluids, and the use of compounds of formula (I) and (II).

Description

The present invention relates to gas hydrate inhibitors which comprise citric acid trialkylamides (citramides), citric acid esters and derivatives thereof used to control gas hydrate formation and clogging of gas hydrate forming fluids. The invention further relates to a method for controlling gas hydrate formation and clogging of gas hydrate forming fluids using derivatives of citramides and citric acid esters as low-dose environmental hydrate inhibitors.

The oil and gas industry uses various methods to prevent the formation of hydrate blockages in pipelines. These methods include heating the pipe, reducing the pressure, removing the water and adding antifreeze agents (Thermodynamic Hydrat Inhibitors; THIs) such as methanol, ethylene glycols (MEG, DEG, TEG) and salts. However, these THIs must be added in high concentrations, typically 10-50% by weight of the water present, to be effective. Recovery of the THIs is also usually necessary and involves an expensive procedure. Consequently, there is a need for alternative, low-cost methods of preventing hydrate blockages in oil and gas drilling and production.

An alternative to the above methods is to control the gas hydrate formation process by using inhibitors against nucleation and crystal growth and/or other chemicals that prevent the formed hydrate crystals from agglomerating. These types of chemicals are called Low Dose Hydrate Inhibitors (LDHIs). LDHIs have only recently been applied to the field, but provide significant cost savings { e.g. applied at concentrations of 0.01 to 2 wt%, which is much lower than the THIs) and other advantages compared to non-chemical hydrate methods or the more traditional THIs. There are two types of LDHIs: Kinetic Hydrate Inhibitors (KHIs) and Anti-Agglomerants (AAs). The best commercial AAs can operate at higher subcoolings than KHIs. For deep water applications, AAs are therefore the only class of LDHI that can be used.

Conventional KHI formulations are based on high molecular weight water-soluble polymers such as poly(vinylpyrrolidone) (PVP), poly(vinylcaprolactam)

(PVCap), copolymers of vinylpyrrolidone/vinylcaprolactam (VP/Vcap) and terpolymers of vinylpyrrolidone/vinylcaprolactam/dimethylaminoethyl methacrylate (VP/Vcap/ DMAEMA). These polymers contain pendant groups, namely lactam rings, with an amide group (-N-C=0) on top of each ring adjacent to the polymer backbone. Another class of effective kinetic inhibitors contains

the amide group as a pendant group without a ring. For example, poly(/V-ethylacrylamide) (PEA), poly(A/-methyl-A/-vinyl acetamide) (PNMVA) and poly(ethyloxazoline) (PEOX) act as hydrate growth inhibitory synergists that enhance the performance of poly(vinyl lactams) , especially PVCap. Hyperbranched poly(ester amides) are another type of KHIs.

Mechanism of KHIs.

KHIs delay hydrate nucleation (and usually crystal growth) so that there is enough time to transport the fluids to the production facilities before hydrates build up in the line. As previously mentioned, they are typically water-soluble polymers where the inhibitor polymer side groups are adsorbed on the hydrate crystal surfaces with hydrogen bonding and force the crystal to grow around and between the polymer threads, with a small crystal radius of curvature. Essentially, they delay the nucleation and growth of hydrate crystals, by interfering with the formation of the cage structure or lattice growth of the hydrates. Inhibitors also block steric, nonpolar solutes such as methane from entering and completing a hydrate cavity. Main limitations of KHIs.

Vinyl caprolactam-based polymers are successfully used to prevent gas hydrate formation in oil and gas field operations carried out in locations such as the UK sector of the North Sea, the Gulf of Mexico, South America and the Middle East. However, these polymers have four main disadvantages: Limited subcooling (< 10°C). Application of KHIs is limited by the degree of subcooling; if this is greater than approximately 12°C, the delay time for nucleation (induction time) is shorter, i.e. the higher the undercooling, the shorter the induction time to hydrate nucleation. This has implications for downtime.

Low biodegradability (< 20% in 28 days; persistent substances). The discharge of oilfield chemicals into the environment has become more and more investigated and legislated. Currently, the environmental authorities in Norway require at least 20% biological degradation (OECD 306 test) for new oilfield chemicals and that the products from the degradation are also environmentally acceptable.

Low fog point (e.g. - 30-40°C). This can cause problems in applications where the chemical injection point temperature is high (e.g. 95°C) or when the chemical comes into contact with high temperature liquids.

High viscosity (> 1 Pa sec (1000 eps) at 20°C). Product viscosity is of particular importance where a product is used underwater.

The aim for all operators is to use "gold" chemicals without substitution notices in the UK sector and "yellow" chemicals in the Norwegian sector. For the products to be yellow in the Norwegian sector, they must meet all of the following criteria:

1. Biological degradation >60% and Log Pow>3 (if not MW > 700)

2. Biological degradation 20-60%, toxicity >10 mg/l and Log Pow <3 (if not

MW > 700)

3. Biological degradation >60% and toxicity >10 mg/l

4. Inorganic and toxicity >1 mg/l.

order that the products are to be environmentally acceptable in the UK sector they must demonstrate

20% biodegradation based on the OECD 306 test and meeting two or more

of the following criteria:

a) biodegradation (OECD 301 B/C/F or OECD 306) within 60% in 28 days or biodegradation (OECD 301 A/E) within 70% in 28 days,

b) toxicity (LC5o or EC5o) above 10 mg/l and

c) biological accumulation (Log Pow) below 3, if not MW > 600.

KHIs should show supercoolings greater than 10°C, with high fog points

(crystallization points) and low viscosities of approximately 33 eps at 20°C or 100 eps at 4°C (underwater conditions).

Anti-agglomerants (AA) are chemicals that are effective as surface-active chemicals (surfactants) that provide a relatively stable water-in-oil emulsion. They are hydrophobic and are added at dosage rates of less than 0.5-2.0% by weight based on the water phase. AAs can be used over a wide range of subcooling (up to 25°C) and can even prevent problems during a shutdown with a subsequent cold start. The best performing AAs are quaternary ammonium-based surfactants where the main ammonium group (R2) has two or three butyl or pentyl groups attached to the quaternary nitrogen.

The mechanism of AAs.

Anti-Agglomerants (AAs) do not interfere with early crystal formation, but prevent their agglomeration and deposition for a period of time by forming a transportable hydrate slurry in a liquid hydrocarbon phase. There are two mechanisms by which a surfactant can act as an anti-agglomerant: A: The main mechanism by which AAs are thought to work is by introducing steric repulsion between adsorbed surfactants on the hydrate particles to prevent contact. The surfactant interacts via its polar head group with the hydrate surface to influence the crystal clustering process. The hydrophobic tail of the molecule "oil-wets" the hydrate particles and prevents their agglomeration and disperses them in the liquid hydrocarbon phase.

B: The surfactant forms a dense water-in-oil emulsion with the produced fluids. Hydrate formation occurs inside the small water droplets and is limited to the small droplets. This prevents agglomeration into larger masses that can clog pipelines and provides a dispersion of hydrate particles. The surfactant has no significant interaction with the hydrate crystal surface.

Main limitations of AAs. Due to their chemistry, most of the AAs are toxic, but a few of them are biodegradable. Anti-agglomerants have six main limitations: 1. Limited to lower water fractions. Their main limitation is that they require a continuous hydrocarbon phase (oil, gas condensate) and are therefore only applicable at water fractions of up to approximately 60 vol%. Above this value, the hydrate slurry rheology changes significantly, which can cause flow problems and unwanted pressure loss in the line. 2. Depending on the type of oil/condensate and the salt content of the formation water. 3. Depending on the operation of the pipeline. Dispersions of small hydrate crystals will be favored by higher velocities (e.g. above 1 m/s fluid velocity), while at low flow velocities crystals may deposit and agglomerate.

4. Toxic. Quaternary surfactants are known to be toxic.

5. Low biodegradation. (Tetraalkyl) quaternary AAs have low biodegradation, so the toxic molecule will survive in the sea for a considerable time and could possibly bioaccumulate.

6. High bioaccumulation.

In the last 10-15 years, LDHIs containing amide groups have been developed. For example, some polymeric amides with MW less than 1000 have been reported by Kelland et. a/.in GB 2 349 889 to inhibit gas hydrate formation. Other types of amides, ie hydroxycarbylamides and polyhydroxycarbylamides of substituted carboxylic acids are also described in US 4,973,775 to delay the formation and/or reduce the agglomeration tendency of hydrates.

These amide products behave as anti-agglomerates at water fractions of approximately 15-20 vol.%.

The main purpose of the invention is to provide a method for controlling gas hydrate formation and clogging using Low Dose Hydra t inhibitors (LDHIs), both kinetic and anti-agglomerants, which are effective (high subcooling and long induction time) and environmentally acceptable. The inhibitors must delay the formation and/or reduce the agglomeration tendency of hydrates at conditions where a hydrate can form. The LDHIs should: • provide high supercooling (<>>10°C for KHIs and > 13-25X for AAs) and a long induction time (10-100 h) at dosage rates of 0.5-3.0% by weight, • preferably be environmentally friendly acceptable (not on the substitution list, i.e. yellow in the Norwegian sector or gold in the UK sector) since the commercial products are red, • exhibit low viscosity (approximately 100 eps at 4°C and 33 eps at 20°C;

values given by Shell) and high cloud point (> 30-40X),

be cost competitive

in the case of anti-agglomerants, be effective at water fractions (vol.% of water compared to total liquid) up to 60 vol.% or even higher.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides gas hydrate inhibitors to control gas hydrate formation and clogging of gas hydrate forming fluids, characterized in that the inhibitor comprises at least one compound according to formula (I) or (II),

where

R 1 is selected from the group consisting of H, acetyl and carbonyl derivative;

R 2a , R 2b , R 2c are each independently selected from H, -OR 3 or -NR 4 R 5 , wherein:

R3 is H, C1-6 alkyl or carbonyl derivative,

R 4 , R 5 are selected from the group consisting of H, C 1-1a alkyl, alkanol, alkoxy, cyclic/aromatic or alkylene.

A second aspect according to the invention is a method for controlling gas hydrate formation and clogging of gas hydrate forming fluids, where at least one gas hydrate inhibitor according to the invention is added in concentrations of 0.01 to 6% by weight of the water present in the fluid.

The last aspect according to the invention is the use of a gas hydrate inhibitor according to the invention for gas hydrate inhibition in oil and gas drilling and production.

FIGURES

Figure 1 shows the IR spectra of the starting materials used in the synthesis of citramides: Citrofol® Al (1a, Fig. a), Citrofol® All (1b, Fig. b), n-propylamine (2a, Fig. c), n-butylamine ( 2b, Fig. d), tert-butylamine (2c, Fig. e) and sec-butylamine (2d, Fig. f); trimethylcitrate (Fig. g); methanol (Fig. h). Figure 2 shows the IR spectra for the reactions of Citrofol® Al/All 1a/b with amines 2 to synthesize citric acid tri-alkylamides/citric acid ester derivatives such as RD-0838-0708 (3aa, Fig. a), RD-0840-0708 (3ab , Fig. b), RD-0839-0708 (citric acid derivatives, Fig. c), RD-0841-0708 (citric acid derivatives, Fig. d), RD-0842-0708 (3ba, Fig. e), RD-0843-0708 (3bb, Fig. f), RD-0844-0708 (citric acid derivatives, Fig. g) and RD-0845-0708 (citric acid derivatives, Fig. h). Figure 3 shows the IR spectra for the reaction of Citrofol® Al 1a with amine 2a/2b/2c/2d at t = 0 hours, 48 hours, 72 hours, 92/138 hours and 160 hours and tFp (time final product, after removal of methanol on heating) for the synthesis of 3ab/3ab/3ac/3ad. Figure 4 shows the IR spectra for the reaction of Citrofol® Al 1a with amine 2a in isopropanol at room temperature/50°C to synthesize 3ab, for the reaction of Citrofol® Al 1a/Citrofol ® Al 1b with isopropanol at room temperature/50°C to synthesize isopropyl citric acid esters and for the reaction of Citrofol ® Al 1b with amine 2c in isopropanol at 50°C with the intention of also synthesizing isopropyl citric acid esters. tFp = time final product, after removal of isopropanol with rotavapor. Figure 5: Pressure and temperature profile formed during the hydrate tests of GT-7588 (Fig. a), RD-0849-0808 (3aa, Fig. b), RD-0850-0808 (Fig. c), RD-0851-0808 ( 3ba, Fig. d) and RD-0852-0808 (Fig. e). Testing was carried out using a high-pressure sapphire cell autoclave equipment located at the University of Stavanger. Figure 6. Pressure and temperature profile formed during the hydrate tests of Luvicap 55W (commercial product, Fig. a), Luvicap ECO Pluss (commercial product, Fig. b), GT-7588 (Fig. c), RD-0849-0808 (3aa , Fig. d), RD-0855-0908 (3ab, Fig. e), RD-0850-0808 (Fig. f), RD-0854-0908 (Fig. g), RD-0851-0808 (3ba, Fig. . h), RD-0861-1108 (3bb, Fig. i), RD-0852-0808 (Fig. j), RD-0856-0908 (Fig. k), Citrofol® Al (Fig. I) and RD- 0862/3/4-1108 (Citrofol® AII/BI/BII, Fig. m). Testing was carried out using a roller ball apparatus located at MI-PT Stavanger; GC = Green Canyon. Figure 7. Subcooling in relation to active content of the RD mixtures containing RD-0838-0708 RD-0845-0708 shown in the experimental results of Table 3. Figure 8. Pressure and temperature profile formed during the hydrate tests of different combinations of citric acid esters (Citrofol ® Al, trimethylcitrate, RD-0852-0808), citramides (RD-0849-0808 3aa, RD-0851-0808 3ba) and PV/PVCap-based polymers (Luvicap 55W, Luvicap ECO Pluss). Testing carried out using an RBA located at M-l PT in Stavanger. Figure 9. Assumed hydrate dissociation curve for a synthetic natural gas Mixed gas 500562/ Green Canyon gas in a system [tap water-synthetic natural gas]. Hydrate dissociation curve was calculated by considering a 25 ml "rolling ball apparatus" pressurized to 70 bar (at 20°C) where 15 ml was occupied by gas and 10 ml was occupied by tap water. B = thermodynamic point; C = hydrate formation.

Figure 10. GC/MS analysis of RD-0838-0708.

Figure 11: GC/MS analysis of RD-0840-0708.

Figure 12: GC/MS analysis of RD-0839-0708.

Figure 13: GC/MS analysis of RD-0841-0708.

Figure 14: GC/MS analysis of RD-0842-0708.

Figure 15: GC/MS analysis of RD-0843-0708

Figure 16: GC/MS analysis of RD-0844-0708.

Figure 17: GC/MS analysis of RD-0845-0708.

DETAILED DESCRIPTION OF THE INVENTION

Citric acid tri-alkylamides and citric acid esters are environmentally friendly raw materials, which have not previously been used as gas hydrate inhibitors. Their current applications are as food additives, plasticizers, emulsifiers and surfactants.

The first type of product, citric acid tri-alkylamides (also called citramide derivatives) are new compounds used as: (a) finishing agents (e.g. N,N',N"-tri-(hydroxymethyl)citramide and N,N' N"-trimethylcitramide, (b) (anionic) surfactants, (c) bactericides and fungicides, (d) smoothing agents, release and dispersing agents for processing thermoplastic and thermoset resins and synthetic rubber, (e) gelling agents and (f) plant growth-regulating agents.

Most common LDHIs and citramides show structural similarities, as shown in Scheme 1. For example, PVP, PVCap, PEA, PNMVA, PEOX and citramides contain amide groups (-N-C=0) which can participate in hydrogen bonding with water (or other protic solvents). The presence of a C=0 dipole and, to a lesser extent, an N-C dipole, allows these groups to act as H-bond acceptors. In addition, the presence of N-H dipoles in citramides allows these compounds to also act as H-bond donors. Thus, the oxygen and nitrogen atoms can accept H-bonds from water and N-H hydrogen atoms can donate H-bonds. Citramides will primarily interact via the alkylamide head groups with the surfaces of the hydrate crystals and thereby affect the growth process of the hydrate crystals. In addition, three terminal alkylamide groups can provide a stronger interaction with the hydrate surfaces than one such group. As a result of these interactions, citric acid tri-alkylamides are expected to adsorb on the hydrate crystal surfaces by hydrogen bonding and inhibit gas hydrate formation.

Furthermore, in Scheme 1, it is shown that the presence of other polar end groups, such as hydroxyl (R2 = H) and acetoxyl (R2 = COCH3) groups, makes the citramid molecules more water-soluble and clearly more willing to interact with hydrated surfaces. The groups R3 would prevent agglomeration of hydrate crystals by making the surface hydrophobic.

The other type of product, citric acid esters (also known as alkyl ether citrates), provide a wide range of advantages when used as plasticizers, solvent alternatives, ingredients in cosmetic products, smoothing agent additives, and performance enhancers in aqueous and solvent-based polymer formulations. They have a long history of use in food and beverage flavoring and in medical and pharmaceutical applications. Citric acid esters also give manufacturers a unique combination of environmentally friendly features. They are neither "volatile organic compounds" (VOCs) nor "hazardous air pollutants" ("Hazardous Air Pollutants" HAPs) and combine low viscosity with low toxicity, high flash points and rapid biodegradation.

In terms of structure, many of these compounds are amphiphilic, i.e. they have hydrophilic or polar groups (ester, carboxylic acid, hydroxyl, acetyl) that can interact with hydrate surfaces and hydrophobic or oleophilic groups (hydrocarbon chains such as ethyl, butyl, pentadecyl, heptadecyl) connected directly or indirectly to the polar groups. The amphiphilic nature of these additives is such that they can be concentrated at the water-hydrocarbon interfaces where hydrate formation is most prominent, disperse the gas hydrates in the fluid and prevent their agglomeration. Citric acid esters can interact primarily via the polar groups with the surfaces of hydrate crystals and thereby influence the growth process of the hydrate crystals. The alkyl groups will prevent agglomeration of hydrate crystals by making the surface hydrophobic. The additives can also stick to pipe walls to a certain extent and thereby reduce the adhesion of hydrate crystals to the pipe walls.

Citric acid esters such as trialkyl citrates (Citrofol® Al, Citrofol® Bl) and acetyl-trialkyl citrates (Citrofol® All, Citrofol® BM, Citrofol® AHII) are commercial products, available at low prices from, for example, Jungbunzlauer Ladenburg GmbH; see Table 1.

Citric acid tri-alkylamides (also called citramid derivatives) are not commercially available. They can be prepared by reacting primary amines with citric acid esters according to the method described by Slon et al, Air Products and Chemicals, Inc. EP 1174026, 2002. (b) Rieko, Y.; Shigemijp, T. Lion Corp. JP 08302387, 1996. (c) Abramitis Walter W., A. Akzona Inc. US 3946074,1976. Other methods for synthesizing citric acid amides: Ciba Ltd. US 944515, 1963. The method involves the reaction of a citric acid ester 1 with 3 equivalents of a primary amine 2 in a protic solvent such as methanol (ethanol, isopropanol, etc.) and under atmospheric pressure and temperature conditions (Scheme 2). Addition of an additional equivalent of amine 2 can in some cases accelerate the reaction. When the reaction was complete, after 3-5 days, the mixture was heated to 70-100°C for a few hours to remove the solvent (methanol), produced ethanol and any unreacted amine 2.

Citric acid esters used in the preparation of citramides were triethyl citrate (1a; Citrofol® Al) and acetyl triethyl citrate (1b; Citrofol® All), although citric acid or polymers containing citric acid and/or citric acid esters can also be used. The amines 2 that were used contained C3-C4 alkyl substituents (R3) because increased carbon number would reduce the water solubility of the material and, consequently, reduce the effectiveness of these materials as KHIs. Other amines suitable for this reaction are, for example, primary and secondary alkanolamines (e.g. monetanolamine, monopropanolamine, monoisopropanolamine, cyclohexanolamine, n-methylethanolamine, n-butylethanolamine, etc), polyalkanolamines, polyalkoxyglycolamines, alkoxyamines, aliphatic amines, alkylenediamines ( eg N,N-dimethylamino trimethylenediamine, ethylenediamine and diethylenetriamine) cyclic/aromatic amines (eg benzylamine, pyrrolidine, piperidine, pyrrolidone, caprolactam) and hydrazides, but they were not used in the present synthetic study. The alkyl groups R3 in the amines were linear and branched: n-propyl, n-butyl, tert-butyl and sec-butyl. The primary amines used in the preparation of citramides were thus: n-propylamine (2a), n-butylamine (2b), tert-butylamine (2c) and sec-butylamine (2d). Citramid derivatives of primary amines containing more than 4 carbon atoms { e.g. C5-18 alkylamines, tert-octyl amine) were not evaluated as LDHIs, although their citramides would be expected to be more soluble in condensate and consequently may be more effective as AAs.

The reaction processes and reaction products were studied by IR spectroscopy and GCMS. Both starting materials 1 and 2 (Figure 2) and final products (Figure 3) were analyzed by IR. In the synthesis of 3aa, 3ab, 3ba and 3bb, the characteristic C=0 extended sharp band of (acetyl)triethylcitrate 1 at 1732 cm"<1>disappeared almost completely after addition of amine. This band was replaced by different C=0 absorption bands at a lower frequency, in the range 1500-1700 cm"<1>and characteristic of amides. In addition, NH stretching vibration of secondary amides occurred in the 3500-3200 cm"<1> range. Also several C-H stretching bands that became more intense than in 2 and typical of the aliphatic R3 groups of the amides were observed. In the synthesis of 3ac, 3ad, 3bc and 3bd the intense C=0 extending band of esters 1 in the 1700-1750 cm"<1>range did not disappear during the course of the reaction, indicating that the final products were mainly mixtures of citric acid esters (e.g. trimethylcitrate, dimethylethylcitrate, diethylmethylcitrate, triethyl citrate, etc) and derivatives thereof. We hypothesize that the bulky hindrance of tert-butylamine 2c and sec-butylamine 2d could be the reason why reaction rates for route (b) were much slower compared to those with n-propylamine 2a and n-butylamine 2b for route (a), Scheme 2. These results were confirmed by GC-MS, which showed high conversion (> 90%) to citramides 3aa, 3ab, 3ba and 3bb and mixtures of citric acid derivatives and decarboxylated/de-O-acetylated compounds as end products in the reactions to synthesize 3ac , 3ad, 3bc and 3bd.

The appearance of citric acid esters such as trimethylcitrate, dimethylethylcitrate and diethylmethylcitrate, during the synthesis of 3ac, 3ad, 3bc and 3bd (route {b)) indicated that methanol (solvent) reacted with 1 faster than tert-butylamine 2c and sec-butylamine 2d. The reaction leading to decarboxylated/de-O-acetylated citric acid derivatives was also faster than the reaction of (sterically hindered) amines with citric acid esters. If the reaction is carried out in isopropanol/tert-butanol/etc. as a solvent and in the presence of sterically hindered amines (e.g. tert-butylamine) as indicated in column (c), isopropyl/tert-butyl/etc citric acid esters and citric acid derivatives could be obtained. These products may be more effective as gas hydrate inhibitors than the current methyl/ethyl citric acid esters and citric acid derivatives.

To get a better understanding of the main factors { e.g. steric hindrance of amines 2, concentration, reaction temperature, solvent, further reactions during heating to remove methanol) affecting the conversion of 1 to citramides 3 or citric acid derivatives, we decided to monitor the progress of the 3aa, 3ab, 3ac and 3ad syntheses by IR (Figure 3).

The structure of the alkyl tail (R3) of amines was found to be critical in the synthesis of citric acid tri-alkyl amides. When R3 = n-propyl (3aa) and n-butyl (3ab), the reaction occurred much faster than for R3 = tert-butyl (3ac) and iso-butyl (3ad). In an attempt to increase the reaction rate in the synthesis of RD-0839-0708 and RD-0841-0708, the mixtures were heated to 35°C for 5 hours, but no C=0 amide band was observed. Addition of acid catalysts (H2SO4, HCI) to accelerate the reactions did not result in IR changes.

When the concentration was doubled, the same products were obtained at the end of most reactions and only small changes in the results were observed when both batches were tested (see Table 3). It seems that the reactions proceed faster at higher concentrations, although this has not been confirmed.

By comparing the FT-IR spectra (Figure 3) at t.FT and t = 160 hours, it can be clearly seen that the heating process (75-100°C for 3-8 h) to remove methanol together with produced ethanol and unreacted amine , affected the transformation and composition of the final products. The solvent used also affected the composition of the resulting citric acid derivatives RD-0839/41/44/45-0708 since methanol acted as both solvent and reagent. The presence of hindered amines appeared to be necessary in the synthesis of RD-0839/41/44/45-0708.

In order to evaluate the importance of heating, type of solvent and the presence of amines in the synthesis of citramides and citric acid esters, various reactions have been run. The synthesis of 3aa occurred at room temperature (rt) in the presence of isopropanol as solvent, but at lower rates than those observed using methanol (Figure 3). Addition of an acid (H2SO4) does not accelerate the reaction. When the same reaction was carried out at 50°C, total conversion to 3aa was achieved approximately 12-fold faster, after 12 h. In both finished mixtures, isopropanol was removed by rotavapor (to avoid changes in the composition of the final products), a system that gave similar product compositions based on IR analysis (Figure 10).

In the reactions Citrofol® Al/Citrofol® All with isopropanol at room temperature/50°C and in the absence of amine, no variations in IR were observed. GC-MS analyzes of the final samples confirmed that no reaction to isopropyl citric acid esters occurred either at room temperature or 50°C. These analyzes were compared with the one obtained from the reaction of Citrofol® All with tert-butylamine 3c in isopropanol at 50°C and showed that the presence of a sterically hindered amine is detrimental to the formation of alkyl citric acid esters and citric acid derivatives. In addition, it was observed that in the synthesis of 3bc in isopropanol at 50°C, yellow-orange crystals appeared during the process. Some changes in IR were also detected. GC-MS analysis showed that the starting materials had been consumed and unknown citric acid derivatives had been formed.

Evaluation of products at UiS (Safir-Celle Autoclave)

Some of the KHI performance tests were carried out in a high-pressure sapphire cell autoclave equipment located at the University of Stavanger (UiS). In all experiments, tap water was used as the aqueous phase and a synthetic natural gas (SNG, designated AGA Spesialgass) which gives structure II hydrates, simulated a natural gas and was used to pressurize the cell at approximately 60 bar. The additives to be tested were dissolved in the aqueous phase to the desired concentration. The sapphire cell was cooled using a temperature-controlled bath and the cell contents were agitated at 120 rpm to simulate a similar agitation to M-l PT rolling cells (typically UiS uses an agitation of 600 rpm which gives a wall velocity of about 1 m/s, typical for fluid flow in a pipeline). The products were tested following a temperature /' ratio W time staircase method: cooling from 20°C to 9°C for 1 hour, 9°C for 8 hours, cooling from 9°C to 8°C for 1 min, 8° C for 8 hours, cooling from 8°C to 7°C for 1 min, 7°C for 8 hours and further. Criteria for the inhibitor performance are the temperature and time to hydrate formation and reproducibility of the results (most experiments were repeated twice). Multiflash 3.5 software was used to predict the theoretical hydrate dissociation curve for the system [tap water-synthetic natural gas]

(see curve in Table 2) and calculate the subcooling for each chemical.

UiS first performed sapphire cell tests on our commercial product GT-7588, a sample that showed a subcooling of 10.6°C at 1.1% by weight using rolling

cells. The purpose was to compare GT-7588 with the performance of different citramid derivatives (RD-0838-0708 3aa, RD-0842-0708 3ba) and citric acid esters (RD-0839-0708, RD-0844-0708) formulated in the same solvent package (MEG + EGMBE + Dl H 2 O) as GT-7588. The subcooling and induction time results of the KHI tests on GT-7588 and RD-0849/50/51/52-0808 in a dose of 1.0-2.0% by weight are summarized in Table 2 and shown in Figure 6.

In the case of GT-7588, hydrates begin to form approximately 2 hours after 8°C was reached, then a value of 10.3°C subcooling was achieved when the chemical was used at 1.0% by weight dosage rate. This value, 10.3°C subcooling, is actually very close to our previously observed experimental value at M-1 PT (10.6°C) and within statistical error for a stochastic process. This commercial inhibitor is clearly better than the no additive (tap water) or solvent only pack, where the hydrates formed almost immediately when the temperature reached 9°C. In the case of the RD products, it was found that citramides with an acetyl group, 3ba, gave a better performance than those without a polar group in R2, such as 3aa (with R2 = H). RD-0849-0808 appeared to perform slightly better than RD-0850-0808, although both products performed poorly at 9.4°C subcooling with induction times of 5.1 hours and 1.7 hours, respectively. Best subcooling (12.2°C) was obtained for RD-0852-0808 which, according to IR (Figure 3) and GCMS analysis, contained a mixture of citric acid esters and (O-acetyl) citric acid derivatives. It appears as predicted that the presence of this type of product has an important effect on hydrate inhibition performance.

Table 2. Evaluation of citric acid derivatives RD-0849-0808 (3aa), RD-0850-0808, RD-0851-0808 (3ba) and RD-0852-0808 as kinetic hydrate inhibitors (KHIs); carried out at UiS and using a synthetic gas (SNG; AGA Spesialgass) in a system [tap water-synthetic natural gas]. B = dissociation PT (thermodynamic point); C = hydrate formation.

Evaluation of products internally ("Rolling ball apparatus")

To screen all products at different doses, KHI performance tests were also performed internally in a rolling ball apparatus (RBA; located at M-l PT Stavanger) immersed in a temperature-controlled bath and rocked around its axis. In all experiments, a [tap water-synthetic natural gas] system was used that was shaken to simulate flow and pressurized to 70 bar with Blandgas 500562. The use of Green Canyon gas in some cases to simulate a natural gas did not significantly modify the performance of the products, only 1-2°C difference in subcooling. A T in relation to t ramping method was typically used for the screening of the samples: cooling from 20°C to 1°C for 48 hours. Figure 9 illustrates the PT equilibrium curve for the system.

It is important to note that some of the results obtained at UiS (Table 2) and at M-1 PT (Table 3) differ significantly. This is due to the many factors that affect the performance of these products, such as type of equipment (sapphire cell autoclave, RBA), T vs. t program (stepwise, gradual), stirrer type and speed (magnetic stirrer, moving balls), composition of synthetic gas (AGA Spesialgass, Blandgass 500562, Green Canyon), etc. It is therefore important to remember that these results only give us an indication on how the products behave under certain simulated conditions. The optimal solution is to test these products under real conditions.

The products obtained in the synthesis of citric acid tri-alkylamides were tested as KHIs at different concentrations from approx. 1 to 6 percent by weight, based on the total amount of water in the system (Table 3). Most of the products were soluble or slightly soluble in water, except for the citramides with R3 = n-butyl such as RD-0840-0708 (3ab) and RD-0843-0708 (3bb). To achieve lower viscosities and test the products using a commercial formula, RD-0838-0708, RD-0839-0708, RD-0841-0708, RD-0842-0708, RD-0844-0708 and RD-0845-0708 were formulated using the same solvents as in GT-7588, which gives RD-0849-0808, RD-0850-0808, RD-0854-0908, RD-0851-0808, RD-0852-0808 and RD-0856-0908 respectively. RD-0840-0708 and RD-0843-0708 were dissolved in isopropanol, yielding RD-0855-0908 and RD-0861-1108 and added to the system. Other solvents such as MEG, EGMBE, etc. can be used to formulate the products and can also act as synergists for the citric acid derivatives.

The results, which are represented in Figure 6 and summarized in Table 3, indicate that all products worked better than without additive (blank test: tap water), with subcooling in the range ~3-6°C in a concentration of 1% by weight. Even products such as RD-0850-0808, RD-0854-0908, RD-0852-0808 and RD-0856-0908, which contained mainly citric acid esters and decarboxylated/de-O-acetylated derivatives based on IR and GC-MS analysis, showed better subcooling (~3-5°C) at 1 wt% than that without additive. This undercooling can be entirely due to the ability of citric acid ester derivatives to inhibit gas hydrate formation. To confirm this idea, two somewhat water-soluble citric acid esters such as trimethylcitrate and triethylcitrate (Citrofol® Al) were tested as KHI at 0.5% by weight and gave 3-5°C undercooling. These products were better than without additive and can consequently be used as KHIs and/or synergists for other KHIs { e.g. citramides). The same happened with Citrofol® All (RD-0862-1108), which after being dissolved in isopropanol and tested as KHI in water, showed a subcooling of approx. 5°C. On the other hand, RD-0863-1108 (Citrofol® Bl) and RD-0864-1108 (Citrofol® BM) tested in water as KHIs did not show an improvement over the base case without additive.

Furthermore, the concentration of the products in the system was varied to see if this had an effect on the subcooling, as indicated in Table 3 and Figure 7. RD-0852-0808 worked well at 1-2% by weight, and produced no deposits in the cell throughout the test. The concentration of RD-0852-0808 was then raised from 1 wt% to 6 wt% and the test repeated. This time the product worked excellently (14.3°C subcooling and 12 hours induction time) compared to the trials at 1-2% by weight. The same happened with RD-0849-0808 (3ab) and RD-0851-0808 (3ba), where the undercooling was doubled (from 4.4°C to 8°C and from 4.2°C to 8.7°C) when the concentration was increased from 1 wt% to 6 wt%. These results indicate that concentration (active content) of derivatives of citric acid tri-alkylamides and citric acid esters is in some cases critical to achieve high performance and because their interaction with hydrate crystals is different from VP/VCap based polymers, they are needed in higher concentrations in the system. The most effective amount of these products appears to depend on the solubility of the molecules in water (and/or water condensate) and the solvent package in which these products are formulated.

In any case, it has been observed that some of the citric acid derivatives synthesized here perform better than commercial products such as Luvicap 55W (9.7°C subcooling) and Luvicap ECO Plus (8.3°C subcooling).

KHI combination trials with citric acid esters (Citrofol® Al, trimethylcitrate, RD-0852-0808), citramides (RD-0849-0808 3aa, RD-0851-0808 3ba) and PV/PVCap-based polymers (Luvicap 55W, Luvicap ECO Plus) were performed to investigate their interactions (Table 4 and Figure 8). When these products were mixed together, important variations in subcooling were observed, in the range of 1-4°C depending on the combination. This indicates that citramides and citric acid esters (and derivatives thereof) can be used individually and/or together as KHIs (AAs) as well as synergists for other additives and/or polymers (e.g. PVP, PVCap, etc) that inhibit nucleation and crystal growth of gas hydrates.

Furthermore, we intend to carry out a simulation study on how the citramid and the citric acid ester derivatives adsorb on ice surfaces and which pendant groups (R3) are better suited to the ice crystal steps that block ice growth. Based on the results, we can selectively design new citramide/citric acid ester molecules that bind to hydrate surfaces in preference to the bulk water. The hydrate-surface interaction of each molecule will affect the nucleation and growth rate of the hydrate as well as the morphology, aggregation and agglomeration tendencies. Five-ringed cyclic chemical structures such as N-substituted succinamides/succindiamides can be useful as hydrate inhibitors because they interact with one or more of the pentagonal faces of a hydrate cavity and thereby prevent the pentagonal faces from participating in the formation of a larger clathrate hydrate mass.

Environmental properties of citric acid trialkylamides and citric acid esters

In order to assess the environmental impact of citramid-based products and citric acid ester derivatives, a series of toxicity, biodegradation (OECD 306) and bioaccumulation tests were carried out on most of the above products, see table 5. It is important to note that bioaccumulation calculations (log Pow ) was performed on the identified components in the mixtures (see GC/MS analyzes in Figures 10-17). From the results of the preliminary investigations, it can be concluded that citric acid esters and decarboxylated/de-O-acetylated derivatives (RD-0839-0708 and RD-0844-0708, Citrofol® Al and Citrofol® All) and citric acid tri-alkylamides (RD- 0842-0708 (3ba) and RD-0843-0708 (3bb)) met the criteria for classification as environmentally acceptable in the North Sea, i.e. they are yellow.

Any QSAR ("Quantitative Structural Activity Relationship") calculation program such as KOWWIN, SPARC or ALOGPS will be allowed in Norway to calculate the physical-chemical parameters (e.g. Log Pow) for these substances. For the UK sector (CEFAS), however, some empirical data is needed

(such as BCF = bioconcentration factor), where new compounds are compared with known substances and trends can be shown. Another possibility for the products to be environmentally acceptable in the UK sector is that they show over 20% biodegradation based on the OECD 306 test and meet two or more of the following criteria: (a) biodegradation (OECD 301 B/C/F or OECD 306) above 60% in 28 days or biodegradation (OECD 301 A/E) above 70% in 28 days, (b) toxicity (LC50 or EC50) above 10 mg/l and (c) bioaccumulation (Log Pow) below 3, if not MW > 600. OECD 117 was performed on RD-0838-0708, RD-0842-0708, RD-0843-0708 and RD-0844-0708 to confirm the Log Pow values obtained by QSAR calculations and their yellow environmental profile. The OECD 306 test for RD-0838-0708 has been repeated with the hope of achieving biodegradation above 20% and is therefore classified as yellow.

EXPERIMENTAL

Evaluation of products in "Rolling Ball Apparatus" (RBA)

A rolling ball apparatus containing 3 cells and located at M-1 PT Stavanger, was used for the determination of hydrate inhibitor performance for different products. Each cell consists of a sapphire tube enclosed in a stainless steel cylinder and contains two balls. The total volume of sample injected into each cell was 10 ml, the total volume of each cell being 25 ml. The data acquisition system uses a SquirrelView Grant 1000 Series (Version 1.03; SQ1001 logger) program or most recently a Darca Plus program to collect the data. Each data set contains information on sample time, pressure in each cell and cooling bath temperature.

In the hydrate test, the pressure falls during cooling according to the equation of state for ideal gases: PV = nRT, where: R = the universal gas constant; T = temperature; n = number of moles of gas; V = system volume; P = pressure). Since the volume of the system and the number of moles of gas are constant, a lower temperature results in a lower pressure in the system. When gas hydrates start to form, gas molecules are removed from the gaseous phase by forming clathrates with the available free water. Thus, the number of moles of gas in the gas phase is quickly reduced and thus the pressure in the system. Sample screening methodology for KHIs: the inhibitor at a specified concentration of 0.46-6.00% by weight (based on the amount of water) was dissolved in tap water and 10 ml of the resulting solution was added to the cell. The cell was pressurized to approximately 70 bar with a gas, Mixed Gas 500562, containing methane (84.049%), N2(7.625%), ethane (5.039%), propane (1.567%), CO2(1.080%), n-butane (0.369 %) and iso-butane (0.271%). In other tests, Green Canyon became gas with the following composition: methane (87.2%), N2(0.4%), ethane (7.6%), propane (3.1%), n-butane (0, 8%), iso-butane (0.5%), n-pentane (0.2%) and iso-pentane (0.2%) used to simulate natural gas. A standard 48 hour test protocol (cooling from 20°C to 1°C for 48 hours) was typically used for the initial screening of the samples. The temperature was regulated with a digital recirculating cooling bath. Criteria for the inhibitor performance are the temperature and time to hydrate formation and reproducibility of the results (some of the experiments were repeated twice). Often one of the cells was filled with untreated water as the control sample. In some cases, hydrate formation was visualized through the sapphire window and a sharp pressure drop was recorded. The pressure of the gas in the cells was monitored for the duration of the test. Furthermore, Multiflash 3.5 software was used to predict the theoretical hydrate dissociation curve for the system and calculate the undercooling for each chemical.

External evaluation of products in a high-pressure sapphire cell autoclave

A high-pressure sapphire cell autoclave located at the University of Stavanger (UiS) was used for the determination of hydrate inhibitor performance of different products that behaved as KHIs and AAs.

Sample screening methodology for KHIs: the total volume of solution injected into the cell was 8 ml, the total volume of the cell was 23 ml. In all experiments, tap water was used as the aqueous phase. The cell was pressurized to approximately 60 bar at 20°C using a synthetic natural gas (AGA Spesialgas) containing methane (80.67%), N2 (0.10%), ethane (10.20%), propane (4, 90%), C02(1.84%), n-butane (0.76%) and isobutane (1.53%). The additives to be tested were dissolved in the aqueous phase to the desired concentration. The sapphire cell was cooled using a temperature controlled bath and the cell contents were agitated at 120 rpm to simulate a similar agitation to M-1 PT rolling cells. A standard 44 hour test protocol (cooling from 20°C to 9°C for 1 hour, 9°C for 8 hours, cooling from 9°C to 8°C for 1 min, 8°C for 8 hours, cooling from 8° C to 7°C for 1 min, 7°C for 8 hours and so on) were typically used for screening the samples.

Criteria for the inhibitor performance are the temperature and time to hydrate formation and reproducibility of the results (the experiments were repeated twice). Multiflash 3.5 software was used to predict the theoretical hydrate dissociation curve for the system and calculate the undercooling for each chemical.

Synthesis of citric acid trialkylamides and citric acid ester derivatives

Step-up process for the synthesis of citric acid tri-alkylamides and citric acid derivatives. Citrofol®-xx (0.4 mol) and methanol (80 ml) were added to a 500 ml round bottom flask. Then amine (1.2 mol, 3 equiv) was slowly poured into the solution and the mixture was stirred for 5-7 days at room temperature. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 6-8 h), to give the final products.

Synthesis of RD-0838-0708. Citrofol® Al 1a (27.6 g, 0.1 mol, 1 eq., MW = 276, triethyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, n-propylamine 2a (17.7 g, 0.3 mol, 3 equiv., MW = 59) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), giving mainly 2-hydroxy-N<1>,N<2>,N<3- >tripropylpropane-1>2>3-tricarboxamide 3aa (29 g, 92% yield; pale-yellow solid; soluble in water and alcohol); C15H29N3O4; MW 315.42; MS ( m/ e) 315 (M+>), 257, 239, 229 (70%), 180, 170 (100%), 144, 128, 101, 86, 60, 43. Scaling: 121 g 3aa (pale- yellow solid, soluble in water and alcohol); 96% yield. Synthesis of RD-0844-0708 in isopropanol at room temperature/50°C: the formation of 2-hydroxy-N<1>,N<2>,N<3->tripropylpropane-1,2,3-tricarboxamide 3aa occurred at room temperature in the presence of isopropanol as solvent, but at lower rates than when using methanol. The same reaction carried out at 50°C gave 3aa after 12 hours, approximately 12 times faster than at room temperature, as indicated in Figure 4.

Synthesis of RD-0840-0708 Citrofol® Al 1a (27.6 g, 0.1 mol, 1 eq., MW = 276, triethyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, n-butylamine 2b (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), giving almost pure N<1>,N<2>,N<3->tributyl- 2- hydroxypropane-1,2,3-tricarboxamide 3ab (33 g, 93% yield; pale-yellow solid; insoluble in water and synthetic condensate, but soluble in alcohol; C18H35N3O4; MW 357.50; MS ( m/ e ) 357 (M<+>>), 285, 257 (75%), 207, 184 (100%), 158, 144, 115, 100.74(40%), 57, 43.

Synthesis of RD-0839-0708. Citrofol® Al 1a (27.6 g, 0.1 mol, 1 eq., MW = 276, triethyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then tert -butylamine 2c (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution and the clear pale yellow mixture was stirred for 3 days at room temperature. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), yielding a crystalline white solid (25 g; soluble in water and alcohol) containing trimethylcitrate, dimethyl ethyl citrate, diethyl methyl citrate and triethyl citrate 1a. Escalation: 100 g of a crystalline white solid (soluble in water and alcohol).

Figure 12 shows GC/MS analysis of RD-0839-0708

Synthesis of RD-0841-0708. Citrofol® Al 1a (27.6 g, 0.1 mol, 1 eq., MW = 276, triethyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, sec-butylamine 2d (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution. The solution was stirred for 3 days at room temperature. After a few hours of reaction time, the reaction mixture became clear yellow, which during the reaction changed to clear orange-red. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), to give an orange-red solid (27 g; slightly soluble in water) containing mainly trimethylcitrate, dimethylethylcitrate, diethylmethylcitrate, triethylcitrate 1a and small amounts of other citric acid derivatives. Escalation: 98 g of a yellow liquid (soluble in water and alcohol).

Figure 13 shows GC/MS analysis of RD-0841-0708

Synthesis of RD-0842-0708. Citrofol® All 1b (31.832 g, 0.1 mol, 1 eq., MW = 318.32, acetyl triethyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, n-propylamine 2a (17.7 g, 0.3 mol, 3 equiv., MW = 59) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. The solution gradually turned clear brown. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), giving 1,2,3-tris(propylcarbamoyl)propan-2-yl-acetate 3ba together with other decarboxylated/de-O-acetylated amide/citric acid derivatives (33 g, 92% yield; brown solid, soluble in water); C 17 H 3 i N 3 O 5 ; MW 357.45; MS ( m/ e) 357 (M<+>>), 297, 268, 238, 211 (70%), 141, 126 (100%), 98, 84, 68, 56, 43. Escalation): 134 g 3ba (brown solid; 93.7% yield; soluble in water and alcohol).

Figure 14 shows GC/MS analysis of RD-0842-0708

Synthesis of RD-0843-0708 Citrofol® All 1b (31.832 g, 0.1 mol, 1 eq., MW = 318.32, acetyl tributyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, n-butylamine 2b (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. The solution gradually turned clear brown. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), giving 1,2,3-tris(butylcarbamoyl)propan-2-yl-acetate 3bb together with other amide derivatives (37 g, 93% yield; viscous dark brown liquid; insoluble in water and condensate, but soluble in alcohol); C19H35N3O5; MW 385.51; MS ( m/ e) 385 (M<+>'), 339, 266, 239 (75 %), 207, 155, 140 (100 %), 112, 96, 84, 68, 57, 41. Escalation: 153 g 3bb (brown liquid, 99.2% yield; insoluble in water but soluble in alcohol).

Figure 15 shows GC/MS analysis of RD-0843-0708

Synthesis of RD-0844-0708. Citrofol® All 1b (31.832 g, 0.1 mol, 1 eq., MW = 318.32, acetyl tributyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, tert-butylamine 2c (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. The solution gradually turned clear brown. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h) to give a brown solid (31 g; soluble in water) containing mainly trimethylcitrate, triethylcitrate 1a and other citric acid and amide derivatives. Escalation: 128 g of a very viscous brown liquid (soluble in water and alcohol). There was not enough methanol in the mixture to dissolve all the product formed during the reaction. Consequently, the reaction must be carried out at a lower concentration of the reactants in methanol. Synthesis of RD-0844-0708 in isopropanol at 50°C: when the reaction was carried out in isopropanol at 50°C for 12 hours, yellow crystals appeared during the reaction process. Isopropanol was removed with rotavapor, which gives a brown, solid substance (see IR analyzes in Figure 4) which must be analyzed by GCMS.

Figure 16 shows GC/MS analysis of RD-0844-0708

Synthesis of RD-0845-0708. Citrofol® All 1b (31.832 g, 0.1 mol, 1 eq., MW = 318.32, acetyl tributyl citrate) and methanol (40 mL) were added to a 250 mL round bottom flask. Then, sec-butylamine 2d (22 g, 0.3 mol, 3 equiv., MW = 73) was slowly poured into the solution and the clear yellow mixture was stirred for 3 days at room temperature. The solution gradually became clear orange-red. Methanol along with produced ethanol and unreacted amine was removed from the resulting solution by heating (75-100°C for 3-4 h), yielding an orange-red liquid (31 g) containing trimethylcitrate, dimethylethylcitrate, diethylmethylcitrate, triethylcitrate 1a and other decarboxylated/de-O-acetylated amide and citric acid derivatives. Escalation: 120 g of a red liquid (slightly soluble in water and soluble in alcohol).

Figure 17 shows GC/MS analysis of RD-0845-0708.

Claims (6)

1. Gas hydrate inhibitor to control gas hydrate formation and clogging of gas hydrate forming fluids, characterized in that the inhibitor comprises at least one compound according to formula (I) or (II), where R 1 is selected from the group consisting of H, acetyl and carbonyl derivative; R2a, R2b, R2c are each independently selected from H, -OR3 or -NR4R5, where: R3 is H, C1-6 alkyl or carbonyl derivative, R4, R5 are selected from the group consisting of H, C1-1a alkyl, alkanol, alkoxy, cyclic /aromatic or alkylene. 2. Gas hydrate inhibitor according to claim 1, where R1 is H or acetyl, R2 is independently -NR4R5 or -OR3, where R4 is H, R5 is CH2CH2CH3 or C(CH3)3, R3 is H and C1-4-alkyl. 3. Gas hydrate inhibitor according to claims 1 to 2, where said inhibitor comprises a mixture of compounds according to claim 1. 4. Gas hydrate inhibitor according to claims 1 to 3, where the inhibitor further comprises one or more other gas hydrate inhibitors and/or solvents. 5. Method for controlling gas hydrate formation and clogging of gas hydrate forming fluids, where at least one gas hydrate inhibitor according to claims 1 to 4 is added in concentrations of 0.01 to 6% by weight of the water present in the fluid. 6. Use of a gas hydrate inhibitor according to claims 1 to 4 for gas hydrate inhibition in oil and gas drilling and production.