Gas separation: processes – Liquid contacting – Hydrate inhibitor
Reexamination Certificate
2000-04-12
2001-04-10
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
Hydrate inhibitor
C585S015000
Reexamination Certificate
active
06214091
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and compound for inhibiting the plugging by gas hydrates of conduits containing a mixture of low-boiling hydrocarbons and water. In particular, it relates to a method and compound for continuing the flow of a mixture of hydrocarbons and water through a conduit after a shutdown which caused separation of hydrate crystals from the mixture.
The problem of the formation of gas hydrates (clathrates of gases in a lattice consisting of water molecules) is well known in the art Low-boiling hydrocarbons, in particular methane, may under conditions of elevated pressure and reduced temperature tend to form gas hydrate crystals with water present in natural gas or crude oil.
Such gas hydrate crystals, when allowed to form and grow inside a conduit such as a pipeline, tend to block or even damage the conduit. A number of methods has been suggested to prevent such blocking of which the use of crystal growth inhibitors is considered to be very attractive.
In International Patent Application Publication WO 96/34177 the use of a class of hydrate formation inhibitors is described having the general formula (R
1
)(R
2
)(R
3
) (R
4
)A
+
Y
−
wherein two of R
1
-R
4
are independently normal or branched alkyls having 4 or 5 carbon atoms, two of R
1
-R
4
are independently representing organic moieties having at least eight carbon atoms, A represents a nitrogen or a phosphorus atom and Y represents an anion.
Attractive results have been obtained when using the di-ester of di-butyl-di-ethanol ammonium bromide and coconut fatty acid, both with respect to decreasing the temperature at which hydrates tend to be formed (as described in experiment A 3b of said patent specification) and with respect to restarting the flow of the medium after stopping (as described in experiment B 1b of said specification). In the experiment B 1b it is described-that after a temperature decrease of 11° C., followed by stopping the circulation of the medium used which resulted in the slow separation of a layer of very fine hydrate crystals from the hydrate forming medium, the circulation could be restarted whereby the layer of loose powder hydrates became readily resuspended into the hydrocarbon liquids resulting in the formation of the hydrate suspension which was observed prior to shutdown.
One of the problems still to be solved in managing the flow of suspended gas hydrates in conduits is being able to restart the flow after an interruption, either scheduled or unscheduled, when powder hydrates have been formed under rather severe conditions. Even with the preferred di-ester of di-butyl-di-ethanol ammonium bromide and coconut fatty acid restarts at rather severe conditions appear to be problematic.
SUMMARY OF THE INVENTION
It has now surprisingly been found that a specific group of esters, in addition to having the attractive properties as described in WO 96/34177 also exhibits an improved restart performance when subjected to rather severe conditions which makes them very attractive for such duties.
The present invention thus relates to a method for inhibiting the plugging of a conduit, the conduit containing a mixture comprising an amount of hydrocarbons having from one to eight carbon atoms and an amount of water wherein the amounts of hydrocarbons and water could form hydrates at conduit temperatures and pressures, the method comprising the steps of:
adding to the mixture an amount of a hydrate formation inhibitor component of formula
(R
1
)(R
2
)(R
3
)(R
4
)A
+
Y
−
(I)
wherein two of R
1
-R
4
are independently
normal or branched alkyls having 4 or 5 carbon atoms,
two of R
1
-R
4
are independently representing organic moieties having at least 8 carbon atoms,
A represents a nitrogen or phosphorus atom,
Y represents an anion
and wherein at least one of R
1
-R
4
represents a
—(CH
2
—CHR
5
—O)
p
—(CH
2
)
q
—(CHR
6
—CH
2
)
r
—(CH
2
-CHR
7
)
s
—(CHR
8
)
t
—O—C(O)—R
9
moiety wherein R
5
-R
8
each independently represent a hydrogen atom or a C
1
-C
4
alkyl group and at least one of R
6
-R
8
is not a hydrogen atom; p represents 0 or an integer of up to 50; q, r, s and t are 0, 1 or 2 and together at least 1 and not more than 4 and wherein when q represents 1 or 2 the sum of r, s and t is at least 1 and, wherein when t and the sum of q, r, s and t represent 2, R
8
does not represent a methyl group; and R
9
represents an (cyclo)alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, alkylaryl or alkenylaryl group having at least a carbon chain of 6 atoms,
the amount of hydrate formation inhibitor component being effective to inhibit formation of hydrates in the mixture at conduit temperatures and pressures, and
flowing the mixture containing the hydrate formation inhibitor component through the conduit.
DETAILED DESCRIPTION OF THE INVENTION
Preferably, A represents nitrogen and two of R
1
-R
4
independently contain between 8 and 20 carbon atoms, preferably in the range from 10 to 16 carbon atoms. Suitably, compounds are used wherein two of R
1
-R
4
contain the same number of carbon atoms, each being at least 8. Preferably, use can be made of components wherein two of R
1
-R
4
represent cocoyl moieties (i.e. the alkyl chain present in coconut fatty acids or similar compounds). Preference is given to compounds wherein R
6
, R
7
and R
8
independently represent hydrogen or a methyl or ethyl group.
Preferred ammonium compounds in accordance with the present invention are those wherein p is zero, the sum of q, r and s is 2 and R
9
represents an alkyl or alkenyl group of between 9 and 18 carbon atoms. Very good results have been obtained with compounds for which q and t are 1 and R
8
represents a methyl or ethyl group.
Suitably, the anion (Y
−
) represents a hydroxide, a carboxylate, a halide such as chloride or bromide, a sulphate or an organic sulphonate. Preferably, Y
−
represents a chloride, bromide or sulphate.
The presence of the carboxylic moiety in the compounds in accordance with the invention is of great advantage because of their biodegradable properties which renders them eminently suitable for the envisaged use. A further advantage in the envisaged use is that such compounds are sparingly soluble in water which allows discarding production water containing only marginal concentrations of such compounds.
The amount of hydrate formation inhibitor compound to be used lies suitably between 0.05 and 11% wt, based on the water content of the hydrocarbon/water mixture. Preferably, the amount is between 0.1 and 5% wt, in particular between 0.1 and 0.5% wt, based on the water content as defined.
If desired, corrosion inhibitors may be added to the hydrocarbon/water mixture. Corrosion inhibitors known to those skilled in the art can be suitably applied. If desired, polymers of ethylenically unsaturated N-heterocyclic carbonyl compounds can be added to the hydrocarbon/water mixture. Reference is made to the polymers or copolymers of N-vinyl-2-pyrrolidone as described in International Patent Application Publication WO 93/25798.
Compounds according to formula I, in which two of R
1
-R
4
are independently representing organic moieties having at least 8 carbon atoms, A represents a nitrogen or a phosphorus atom, Y represents an anion, and wherein at least one of R
1
-R
4
represents a —(CH
2
—CHR
5
—)
p
—(CH
2
)
q
—(CHR
6
-CH
2
)
r
—(CH
2
-CHR
7
)
s
—(CHR
8
)
t
—O—C(O)—R
9
moiety wherein R
5
-R
8
each independently represent a hydrogen atom or a C
1
-C
4
alkyl group and at least one of R
6
-R
8
is not a hydrogen atom; p represents 0 or an integer of up to 50; q, r, s and t are 0, 1 or 2 and together at least 1 and not more than 4 and wherein when q represents 1 or 2 the sum of r, s and t is at least 1 and, wherein when t and the sum of q, r, s and t represent 2, R
8
does not represent a methyl group; and R
9
represents an (cyclo)alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, alkylaryl or alkenylaryl group having at least a carbon chain of 6 atoms, are believed to be novel.
Suitable compounds are those in which A re
Muller Kim
Shell Oil Company
Smith Duane S.
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