Additives for inhibiting gas hydrate formation

Details Additives for inhibiting gas hydrate formation Additives for inhibiting gas hydrate formation

1999-01-29

2001-01-23

Michl, Paul R. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C095S153000, C585S015000, C585S950000

Reexamination Certificate

active

06177497

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an additive and a process for inhibiting nucleation, growth and/or agglomeration of gas hydrates by adding an effective amount of an inhibitor which contains 2-isobutoxyethanol to a multiphase mixture prone to hydrate formation and comprising water, gas and optionally condensate.
Gas hydrates are crystalline clathrate compounds of gas molecules in water which form under specific temperature and pressure conditions (low temperature and high pressure). The water molecules form cage structures around the corresponding gas molecules. The framework formed from the water molecules is by itself thermodynamically unstable and the framework is not stabilized until the inclusion of gas molecules, resulting in an icelike compound which can also exist above the freezing point of water (up to more than 25° C.), depending on pressure and gas composition. A review of the subject of gas hydrates is given in Sloan, Clathrate Hydrates of Natural Gases, M. Dekker, New York, 1990.
In particular, the gas hydrates which form from water and the natural gas components methane, ethane, propane, isobutane, n-butane, nitrogen, carbon dioxide and hydrogen sulfide are of major importance in the petroleum and natural gas industry. Particularly in natural gas production today, the existence of these gas hydrates presents a major problem, especially when wet gas or multiphase mixtures of water, gas and alkane mixtures under high pressure are exposed to low temperatures. Here, owing to the insolubility and crystalline structure, the formation of the gas hydrates leads to blockage of various transport means, such as pipelines, valves or production means, in which wet gas or multiphase mixtures are transported over relatively long distances at low temperatures, as occurs in particular in colder regions of the earth or at the bottom of the sea. Moreover, the gas hydrate formation can also give rise to problems under corresponding pressure and temperature conditions during the drilling process for developing new gas or petroleum deposits.
To avoid such problems, the gas hydrate formation in gas pipelines or during the transport of multiphase mixtures can be suppressed by using relatively large amounts (more than 10% by weight) of lower alcohols, such as methanol, glycol or diethylene glycol. The addition of these additives shifts the thermodynamic limit of gas hydrate formation to lower temperatures and higher pressures (thermodynamic inhibition). However, the addition of these thermodynamic inhibitors gives rise to major safety problems (flash point and toxicity of the alcohols), logistical problems (large storage tanks or recycling of these solvents) and accordingly high costs, especially in offshore production.
Today, attempts are therefore being made to replace thermodynamic inhibitors, in the temperature and pressure ranges in which gas hydrates can form, by adding additives (amount used<2% by weight) which either delay gas hydrate formation (kinetic inhibitors) or keep the gas hydrate agglomerates small and therefore pumpable, so that they can be transported through the pipeline (so-called agglomerate inhibitors or antiagglomerates).
In addition to the thermodynamic inhibitors, many classes of monomeric and polymeric substances which are kinetic inhibitors or agglomerate inhibitors have been described as gas hydrate inhibitors in the prior art.
WO-96/08636 describes surface-active substances as gas hydrate inhibitors which carry a polar head group and a hydrophobic radical, the hydrophobic radical containing not more than 12 carbon atoms. Sodium valerate, butanol, butyl sulfate and butyl sulfonate, alkylpyrrolidones and a zwitterion of the formula R
2
N(CH
3
)
2
—(CH
2
)
4
SO
3
−
are mentioned as examples.
WO-96/08456 describes synergistic mixtures of the substances disclosed in WO-96/08636 with water-soluble copolymers.
W. D. Young et al. (American Chemical Society, Vol. 42, No.2; Preprints of Papers presented at the 213th ACS National Meeting San Francisco, Calif., Apr. 13-17, 1997) describe synergistic mixtures of amide polymers with lower alcohols and glycol ethers. Here, 2-butoxyethanol and 2-(2-butoxyethoxy)ethanol (butyldiglycol) exhibit particularly good efficiency.
To be able to use gas hydrate inhibitors also at lower temperatures than is possible at present, i.e. further within the hydrate region, a further increase in the activity of the available hydrate inhibitors is necessary. It was therefore the object of the present invention to find improved additives which slow down the formation of gas hydrates (kinetic inhibitors) or which keep gas hydrate agglomerates small and pumpable (antiagglomerates), in order to be able to replace the thermodynamic inhibitors (methanol and glycols) which are still used at present and give rise to considerable safety problems and logistical problems.
SUMMARY OF THE INVENTION
As has now surprisingly been found, the glycol ether 2-isobutoxyethanol has substantially better efficiency as a gas hydrate inhibitor than the alcohols and glycol ethers described to date in the prior art.
As shown in the attached examples, 2-isobutoxyethanol in a low dose can suppress the formation of gas hydrates far more effectively than is possible using the same amount of 2-butoxyethanol which is known from the literature.
The invention relates to an additive for inhibiting gas hydrate formation, containing
a) from 5 to 90% by weight of 2-isobutoxyethanol,
b) from 5 to 60% by weight of a copolymer which has structural units which are derived from maleic acid, maleic anhydride or derivatives of maleic acid or of maleic anhydride, and
c) at least 10% by weight of water or a monohydric or polyhydric alcohol, except the 2-isobutoxyethanol, or mixtures of the stated substances,
based on the total weight of the additive, the sum of the contents of compounds a) and b) being from 10 to 90% by weight.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The additives according to the invention contain preferably from 10 to 80, in particular from 15 to 50, % by weight of 2-isobutoxyethanol and preferably from 10 to 50% by weight, in particular from 20 to 30% by weight, of a copolymer corresponding to b) as defined above. The sum of the contents is preferably from 20 to 80% by weight. Further components of the additive are, according to c), water and monohydric or polyhydric alcohols except for 2-isobutoxyethanol, in an amount of at least 10% by weight. Preferred further components apart from water are water-soluble monohydric alcohols having 1 to 4 carbon atoms. Also preferred are short-chain dihydric alcohols, such as, for example, ethylene glycol. The additive can be used alone or in combination with other known gas hydrate inhibitors. Typical concentrations of use which are reached in the aqueous phase in contact with the gas are 0.01-2% by weight of additive in water, especially concentrations of 0.02-1.0 % by weight (200-10 000 ppm).
The invention furthermore relates to a process for inhibiting the formation of gas hydrates, wherein 2-isobutoxyethanol is added to the aqueous phase in contact with the gas. 2-isobutoxyethanol is added to the aqueous phase preferably in an amount such that the concentration in the water is more than 0.005% by weight; in particular the 2-isobutoxyethanol concentration may be from 0.01 to 2, especially from 0.02 to 1, % by weight. In the process according to the invention, further gas hydrate inhibitors in addition to 2-isobutoxyethanol may be added to the aqueous phase.
The additive according to the invention and the process according to the invention are particularly suitable for inhibiting hydrates of natural gas.
2-isobutoxyethanol has scarcely any surfactant activity, which is advantageous particularly with respect to the foam effect since conventional surfactant additives often lead to serious foam problems.
If derivatives of maleic acid or of maleic anhydride are used as component b) of the additive, these are preferably semiamides, diamides, half-esters or diesters. Amides or esters preferably carry al

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