Gas hydrate inhibitors

Abstract

The present invention relates to a method to inhibit gas hydrate formation in the field of crude oil and natural gas extraction, transportation and processing.

Claims

1. A method for inhibiting the formation of gas hydrates in systems comprising mixtures of hydrocarbons and water, said method comprising adding to the mixture a quaternary ammonium ester salt of formula I: ##STR00004## wherein R.sub.1(CO)— is the residue of a saturated or unsaturated, linear or branched fatty acid containing from 6 to 24 carbon atoms; R.sub.2 and R.sub.3 are, independently of each other, a butyl or a pentyl group; R.sub.4 is linear or branched alkyl group having from 1 to 5 carbon atoms; X can be R.sub.4SO.sub.4.sup.−, R.sub.4O(CO)O.sup.−, bicarbonate and carbonate; a can be 1 or 2; with the proviso that at least 50% of the aliphatic carboxylic acid contains less than 16 carbon atoms.

2. The method of claim 1, wherein, in the quaternary ammonium ester salt of formula I, R.sub.1(CO)— is the residue of a saturated or unsaturated, linear or branched aliphatic carboxylic acid wherein at least 60% by weight of said acid contains less than 16 carbon atoms.

3. The method of claim 1, wherein, in the quaternary ammonium ester salt of formula I, R.sub.2 and R.sub.3 are the same and are a butyl group.

4. The method of claim 1, wherein, in the quaternary ammonium ester salt of formula I, R.sub.4 is linear or branched alkyl group having from 2 to 4 carbon atoms.

5. The method of claim 4, wherein R.sub.4 is linear or branched alkyl group having 2 or 3 carbon atoms.

6. The method of claim 1, comprising adding to the mixture of hydrocarbons and water the quaternary ammonium ester salt of formula I as a composition comprising between 20 and 95% % by weight of said salt, a solvent and other optional additives.

7. The method of claim 6, wherein the composition comprises between 45 and 90% by weight of said quaternary ammonium ester salt.

8. The method of claim 1, comprising adding to the mixture of hydrocarbons and water between 0.1 and 8.0% by weight of quaternary ammonium ester salt of formula I.

9. The method of claim 8, comprising adding between 0.5 and 5.0% by weight of said quaternary ammonium ester salt.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) Preferably, R.sub.1(CO)— is the residue of a saturated or unsaturated, linear or branched aliphatic carboxylic acid wherein at least 60% by weight of said acid contains less than 16 carbon atoms.

(2) In a preferred embodiment of the method of the invention, R.sub.2 and R.sub.3 are the same and are a butyl group.

(3) The quaternary ammonium ester salt of formula I of this invention can be prepared by quaternization of a tertiary amino ester of formula II:

(4) ##STR00003##

(5) wherein R.sub.1, R.sub.2 and R.sub.3 have the same meaning as reported above.

(6) The tertiary amino ester of formula II can be obtained by condensation of a saturated or unsaturated, linear or branched aliphatic carboxylic acid having formula R.sub.1COOH, and a N,N-substituted ethanol amine is of formula R.sub.2R.sub.3N—CH.sub.2CH.sub.2—OH, wherein R.sub.2 and R.sub.3 are, independently of each other, a butyl or pentyl group.

(7) Specific examples of saturated or unsaturated, linear or branched aliphatic carboxylic acids, suitable for the realization of the present invention, are 2-hexanoic acid, ethylhexanoic acid, n-octanoic acid, n-nonanoic acid, n-decanoic acid, n-undecanoic acid, n-dodecanoic acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and the like.

(8) Also mixtures of these saturated or unsaturated, linear or branched aliphatic carboxylic acids can be used for the realization of the present invention. Suitable examples are the mixtures of carboxylic acids derived from natural oils, such as coco fatty acids, palm kernel fatty acids and palm fatty acids.

(9) Preferred fatty acid are mixtures of fatty acids from natural oil and particularly preferred are palm kernel fatty acids and coco fatty acids, the latter being the most preferred.

(10) The preferred N,N-substituted ethanol amine is N,N-dibutyl ethanol amine.

(11) The preparation of carboxylic acid esters of alkanol amines is well known in the art. In an exemplary preparation process, the N,N-substituted ethanol amine is reacted with about 0.95 to about 1.1 molar equivalents of the carboxylic acid, ester or acid chloride, at a temperature ranging from about 90 to about 180° C. for about 1 to about 30 hours.

(12) The process of preparation of the quaternary ammonium ester salt of formula I requires a further reaction step wherein the amino groups present in the molecule are substantially all quaternized. Quaternization is a reaction type that is well known in the art: typically it contemplates the reaction of the substrate with an alkylating agent.

(13) For the quaternization step of the present invention, the alkylating agent can be selected from the group consisting of dialkyl sulfate and dialkyl carbonate, wherein the alkyl group has from 1 to 5 carbon atoms. Specific example of these alkylating agent are dimethyl sulfate, diethyl sulfate, dimethyl carbonate and diethyl carbonate, dipropyl sulfate, etc. The most preferred alkylating agents being diethyl sulfate and diethyl carbonate.

(14) In one embodiment of the present invention, the tertiary amino ester of formula II is melt or dissolved in a suitable solvent, such as a C.sub.1-C.sub.4 alcohol or diol, and quaternized with about 0.95 to about 1.5 molar equivalents of a dialkyl sulfate to form the quaternized ammonium ester salt. The temperature is normally comprised between 70 and 140° C. Isopropanol, propylene glycol are the preferred solvents for the quaternization as they exhibit the best ability at reducing the viscosity of the quaternary salt solution.

(15) The aforementioned quaternary ammonium ester salts of alkyl carbonates, carbonates and bicarbonates can be prepared by methods known in the art, such as those described in U.S. Pat. No. 5,438,034 and WO 03/006419.

(16) It must be pointed out that the quaternary ammonium ester carbonates and bicarbonates of the invention are in equilibrium. The ratio bicarbonates/carbonates varies depending on the pH of the solution in which they are contained.

(17) In one embodiment, the method of the present invention comprises the addition to the mixture of hydrocarbons and water of the quaternary ammonium ester salt as such, without any diluents or additives.

(18) A another embodiment, the method of the present invention comprises the addition of a gas hydrate inhibitor composition, comprising the quaternary ammonium ester salt as herein described, a solvent (e.g. a liquid solvent) and other optional additives.

(19) The gas hydrate inhibitor composition of the invention can comprise between 20 and 95% by weight, preferably between 45 an 90% by weight, more preferably 55 an 85% by weight of the quaternary ammonium ester salt.

(20) Representative solvents suitable for formulation with gas hydrate inhibitor include polar solvents such as water, alcohols (including straight chain or branched aliphatic alcohols such as methanol, ethanol, 2-ethoxyethanol, propanol, isopropanol, butanol, isobutanol, hexanol), glycols and glycol ether derivatives (including ethylene glycol, propylene glycol, hexylene glycol, ethylene glycol monobutyl ether, ethylene glycol dibutyl ether, or diethylene glycol monomethyl ether), ethers (e.g., tetrahydrofuran), amides (e.g., N-methyl-2-pyrrolidinone or N,N-dimethylformamide), ketones (e.g. methyl ethyl ketone, cyclohexanone, or diisobutyl ketone); apolar solvents, such as aromatic hydrocarbon solvents (e.g. toluene and xylene) and mixtures thereof.

(21) Preferred solvents are propylene glycol and isopropanol.

(22) Suitable optional additives are paraffin inhibitors, asphaltene inhibitors, scale inhibitors, corrosion inhibitors, oxygen scavengers, hydrogen sulfide scavengers, non emulsifiers and emulsion breakers.

(23) The quaternary ammonium ester salts, according to the present invention, are particularly suitable as gas-hydrate inhibitors when added to hydrocarbon fluids containing water.

(24) They may be used by simple addition to the hydrocarbon fluids to be treated.

(25) In the preferred procedure of this invention, the quaternary ammonium ester salt is added to a flowing hydrocarbon fluid which may contain both oil and water, at any point in a flow line upstream of the point or line that is intended to be protected. The dosage of gas hydrate inhibitor of the invention needed to obtain a sufficient protection varies with the application, but it is advantageously added in such an amount that the concentration is between 0.1 and 8.0% by weight, preferably between 0.5 and 5.0% by weight and more preferably between 1.0 and 3.5% by weight.

EXAMPLES

(26) Gas Hydrate Inhibition Test

(27) The performances of the gas hydrate inhibitors of the invention were evaluated with a Rocking Cell RC5 by PSL Systemtechnik.

(28) Test Fluids Hydrocarbon: Diesel Aq. Phase: 4% wt seasalt water or deionized water Gas: Mix of methane, ethane, propane and butane (various isomers)

(29) Test Procedure

(30) The sapphire test cells, containing a stainless steel ball, were filled with the fluids (see Table 1) and 2% by weight of inhibitor and pressurized with the gas mixture.

(31) TABLE-US-00001 TABLE 1 Inhibitor Fluid (v/v) Test 1 2% 50/50 4% wt seasalt water/diesel Test 2 2% 20/80 deionized water/diesel

(32) Each cell was the subjected to a cycle of cooling and rocking consisting of three steps: 1) flowing condition, 2) shut-in and 3) re-start flowing condition. 1) The pressurized cells were cooled down to 4° C. over a period of 5 hours while rocking. After reaching 4° C., the cells were rocked for 12 hours. 2) The rocking was stopped and the test cells were kept in horizontal position (shut-in) for 16 hours. 3) At the end of the shut-in period, rocking was re-started for 4 hours. Finally the cells were heated back up to 20° C.

(33) At the beginning of the third step, the content of the cells was visually evaluated.

(34) Each cycle was replicated three times and the results registered.

(35) Results

(36) The results of the gas hydrate inhibition tests (Table 2) are reported according to the following scale: FAIL: The ball is stuck and/or large agglomerations or solid crystals and/or visible deposits on the cell walls. PASS: The ball is free; solid crystals might be present, but agglomerates (large or small) break up with agitation.

(37) TABLE-US-00002 TABLE 2 Test 1 Test 2 benzylcocodimethyl ammonium chloride* FAIL FAIL N,N-dibutyl-N-Etyl-cocooxyethyl PASS PASS ammonium ethyl sulfate *Comparative

(38) Foaming Power Tests

(39) The foam volume (FV) and the foam stability (FS) were determined by stirring for 30 seconds at high speed (8000 rpm) with a Waring Blender 100 mL of a 1% by weight solution of the inhibitors in deionized water (Test 3) or in a 4% sea salt water solution (Test 4). The foamed composition is then transferred into a graded cylinder for the evaluation of the foam volume and the stability of the foam.

(40) FV represent the volume in mL of foam at the end of the stirring. FS is the time in seconds required to the foamed solution to regenerate 50 mL of liquid. The longer the time, the higher the stability of the foam.

(41) Table 3 shows the results of the foaming power test.

(42) TABLE-US-00003 TABLE 3 Test 3 Test 4 FV FS FV FS benzylcocodimethyl ammonium chloride* 480 260 430 215 N,N-dibutyl-N-ethyl-soyaoxyethyl 320 180 230 118 ammonium ethyl sulfate* N,N-dibutyl-N-ethyl-cocooxyethyl 210 97 172 59 ammonium chloride* N,N-dibutyl-N-ethyl-cocooxyethyl 185 70 160 46 ammonium ethyl sulfate *Comparative

(43) The results demonstrate that the quaternary ammonium ester salts of formula I of the invention produce less foam than a ammonium salt of the prior art.

(44) Corrosion Tests

(45) The Linear Polarization Resistance (LPR) measurements were made with a Gamry Electrochemical Instrument system.

(46) The LPR corrosion tests were conducted in 1 L Pyrex jacketed cells. 900 mL of synthetic brine (50/50 v/v 4.0% Seasalt water/Fresh Water) were loaded in the cell placed on a magnetic stirrer, deaerated overnight with CO.sub.2 and, finally, saturated with 200 ppm H.sub.2S gas just before testing. A clean C1018 Mild Steel rod was inserted in the corrosion cell assembly as sample probe. A graphite rod was used as the counter electrode. The temperature of the solution was brought to 80° C. for the duration of the tests and CO.sub.2 was continuously purged at a constant flow rate. The (Test 4) inhibitors were added at 10 ppm by volume of test solution.

(47) The results are reported in Table 4 as % of protection after a fixed period of time compared to the blank, the test solution without any inhibitor, which is considered 100% corrosion.

(48) TABLE-US-00004 TABLE 4 % Protection 1 hour 16 hours benzylcocodimethyl ammonium chloride* 79.7 89.5 N,N-dibutyl-N-ethyl-cocooxyethyl 78.9 87.9 ammonium chloride* N,N-dibutyl-N-ethyl-cocooxyethyl 81.4 90.2 ammonium ethyl sulfate *Comparative

(49) The results demonstrate that the quaternary ammonium ester salts of formula I of the invention produce less corrosion than a ammonium chloride salt of the prior art.