Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon dioxide or hydrogen sulfide component
Reexamination Certificate
1996-11-04
2001-07-31
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon dioxide or hydrogen sulfide component
C423S228000, C048S127500, C208S20800M, C208S237000, C208S238000
Reexamination Certificate
active
06267938
ABSTRACT:
FIELD OF THE INVENTION
This invention features a method and composition for sweetening fluid streams. In particular, this invention relates to a method and composition for reducing the amount of hydrogen sulfide and organic sulfides in such streams, particularly natural gas streams.
BACKGROUND OF THE INVENTION
Natural gas is a mixture of hydrocarbons and non-hydrocarbon gases found in geologic formations beneath the earth's surface, often in association with petroleum. As obtained from the ground, raw gas contains a number of impurities which must be removed at some point. The principal impurities in natural gas are water, carbon dioxide, hydrogen sulfide, organic sulfides and condensable hydrocarbons, such as propane, butane and pentane.
Generally speaking, hydrogen sulfide and organic sulfides (collectively here “sulfides”), because of their corrosiveness and toxicity, are removed in the field prior to introduction to a pipeline for transport to a market or off-site processing plant. The maximum level of hydrogen sulfide, “H
2
S”, the primary sulfide impurity of natural gas, permitted to be introduced into a pipeline is often limited to 0.25 grains per 100 cubic feet of gas, that is, 4 parts per million (ppm), although in some instances, up to 1 grain per 100 cubic feet (16 ppm) is acceptable.
H
2
S, like hydrocarbon components of natural gas, exists in the gaseous state at normal temperatures and pressures. There is a variety of approaches to removing H
2
S from, or at least substantially reducing the amount of H
2
S in natural gas, i.e., “sweetening” natural gas. One general approach is to expose the raw natural gas to a treatment liquid containing an agent which chemically reacts with H
2
S, a so-called H
2
S scavenger. Usually, the natural gas is bubbled through the liquid and as the bubbles rise through the liquid, H
2
S comes into contact with the agent to react and form a non-gaseous, or relatively non-volatile, product. The H
2
S thus becomes trapped in the liquid and is removed from the gas.
One type of agent often used to react with H
2
S is the reaction product of an organic amine compound having an “active hydrogen” and an aldehyde. An active hydrogen herein is a hydrogen directly bonded to a nitrogen atom. It is this type of scavenging or trapping agent to which the present invention relates. Examples of such agents are described in U.S. Pat. No. 4,978,512 (issued Dec. 18, 1990 to Dillon) and U.S. Pat. No. 5,462,721 (issued Oct. 31, 1995 to Pounds et al.). The specifications of both of these earlier patents are incorporated herein by reference.
As described by Pounds et al., there is a number of characteristics that are considered desirable in treatments containing such active agents. Generally speaking, it is desirable that the agent be highly reactive, i.e., the H
2
S (or organic sulfides) should react readily with the agent as the H
2
S passes through the treatment liquid. At the same time, it is desirable that the agent remain effective even in the presence of a large excess of CO
2
, that is, it should be selective. The scavenging agent should be easy to handle, that is, be of appropriate viscosity, have a suitable pour point and low toxicity—it should have no free formaldehyde nor produce a vapor containing formaldehyde. In terms of performance, it is also desirable that the agent not foam excessively in use.
It is also desirable for the reaction product(s) of the H
2
S and the scavenging agent to have certain characteristics. It has generally been the approach to use single phase treatments. In particular, liquid treatments in which the products of the H
2
S trapping reaction are soluble in the liquid have thus been considered highly desirable because of the ease of disposal of the spent fluids.
SUMMARY OF THE INVENTION
The present invention, in one broad aspect, is a method for reducing the level of sulfide impurities present in a fluid stream. The method includes contacting the stream with a composition comprising the reaction product of an aldehyde and a aminoethylpiperazine (AEP) and a second aliphatic amine NHR
1
R
2
, wherein R
1
or R
2
can be hydrogen and wherein R
1
and R
2
contain a total of from 1 to about 6 carbons, for a period of time sufficient to reduce the level of the impurities in the stream.
According to particular embodiments of the invention, described below, the invention is a scavenger which separates into two liquid phases. Such separation may occur as the active agent of the scavenger reacts with sulfides such as H
2
S or organic sulfide(s) of a stream being treated. Usually the stream being treated contains hydrocarbon, and is typically an oil or gas stream, usually a natural gas stream. It is also possible for a scavenger composition to include two liquid phases prior to reaction with impurities of the stream.
As the active ingredient of a scavenger composition of the present invention generally includes a water soluble phase, the invention includes a method for reducing levels of sulfide(s) in an aqueous system, say, for example, sewage.
An advantage of the presence of two immiscible liquids, aside from the avoidance of solids formation, which is usually undesirable, is that the two liquids i.e., liquid phases can be readily separated from each other. According to a particular embodiment of the invention, described below, it is thus possible to isolate one phase, containing amine which is produced during reaction of the scavenger and react the amine with an aldehyde to produce more scavenger which can then be used to treat the stream. The other phase, containing spent scavenger, i.e., sulfided components, can be used as an anti-corrosion agent.
The performance of the embodiments of the invention subjected to testing, as described below, was found to be relatively unaffected by the presence of CO
2
, and hence scavenger compositions of the present invention are generally “selective”.
Generally speaking, the second amine, as described above, of a scavenger of the present invention is also referred to as an “enhancing amine”. The enhancing amine, NHR
1
R
2
, can be non-cyclic. R
1
can be hydrogen while R
2
is an aliphatic hydrocarbon group having between 1 and about 6 carbons.
An aldehyde from which a composition of the present invention is derived can be a monoaldehyde, is usually an aliphatic aldehyde, and usually has between 1 and 4 carbon atoms. The aldehyde of the preferred embodiment is formaldehyde.
Generally speaking, in the oil and gas industry, it is unacceptable to have unreacted formaldehyde present in a scavenger composition to any significant degree. For this reason, when formaldehyde is the aldehyde from which a scavenger composition is derived, the total amine content is sufficient to react with all of the formaldehyde.
On a molar basis, the aldehyde:AEP ratio is between about 1.0:1.6 and about 10:1 and is more preferably between 1:1 and 5:1 or 2:1 and 4:1. An even more preferred the ratio is between about 2.5:1 and 4.5:1, and most preferably, it is about 3:1.
In terms of the enhancing amine, the amount selected can depend upon the advantages of the invention that are desired. That is, it is possible for a person skilled in the art to modify the tested formulations of the invention to obtain scavenger compositions which vary somewhat in their properties and still remain within the scope of the invention. It is possible to vary proportions and retain to varying degrees at least one of the advantages of the present invention: formation of a biphasic liquid as the scavenger composition reacts; a biphasic composition prior to reaction with a sulfide-containing stream; avoidance of solid(s) formation; reduced foaming during the scavenging operation.
Generally, the ratio of AEP:enhancing amine, again on a molar basis, is between about 10:1 and 1:5, but more typically between 5:1 and 1:3, or even 4:1 and 1:2. More preferably still, the ratio is between 2:1 and 1:2 or between about 1:1 and 1:1.5. In a particular embodiment, described below, the molar ratio is 1:1.4 and in another, it is 1:1.
According to a meth
Cossar John
Warrender Neil Andrew
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
Griffin Steven P.
Stanchem, Inc.
Vanoy Timothy C.
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