Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF TREATING SOUR GAS
AND LIQUID HYDROCARBON STREAMS
CASE NO: ~XXON 40
BACKGROUNp OF THE INVENTION
This invention relates generally to the treatment of sour
gas and liquid hydrocarbon streams to remove or reduce the
levels of hydrogen sulfide therein. In one aspect, the
invention relates to the treatment of sour gas and oil streams
flowing in a flow line. In another aspect, the invention
relates to kh~ use of nonregenerative scavengers to reduce the
levels of hydrogen sulfide in natural gas and liquid hydro-
carbon streams.
The toxicity o~ hydrogen sulfide in hydrocarbon streams
is well known in the industry and considerable expense and
efforts are PxpPnded annually to reduce its content to a safe
level. Many regulations require pipeline gas to contain no
more than 4 ppm hydrogen sulfide.
In large production facilities, it is generally more
economical to install a reganerative system for treating sour
gas streams. These systems typically employ a compound used
in an absorption tower to contact the produced flu:ids and
selectively absorb the hydrogen sulfide and possibly other
toxic materials such as carbon dioxide and mercaptans. The
absorption compound is then r~genera~ed and reused in the
~ystem. 'rypical hydrogen sul~ide absorption materials include
alkanolamines, PEG, hindered amines, and the like.
.
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However, during a development stage of a field or in
small producing fields where regenerative systems are not
economical, it is necessary to treat the sour hydrocarbon
production with nonregenerative scavengers.
Based on an article appearing in the OiI & Gas JournaI,
January 30, 1989, nonregenerative scavengers for small plant
hydrogen sulfide removal ~all into four groups: aldehyde
based, metallic oxide based, caustic based, and other pro-
cesses. In the removal of hydrogen su:Lfide by nonregene-
rative compounds, the scavenger reacts with the hydrogen
sulfide to form a nontoxic compound or a compound which can
be removed from the hydrocarbon. For example, in the formal-
dehyde type reaction, the reaction produces a chemical complex
known as formthionals (e.g., trithiane).
As described in detail below, the present invention
employs a nonregenera~ive scavenger which may be of the
aldehyde type. These include low molecular weight aldehydes
and ketones and adducts thereof. The low molecular weight
aldehydes may also be combined with an alXyl or alkanolamine
as disclosed in U.S. Patent 4,748,011. Other aldehyde derived
scavengers include the reaction product of low molecular
weight alkanolamines and aldehydes disclosed in U.S. Patent
4,978,512.
SUMMARY OF THE INVENTION
In accordance with the method of the present invention,
an H2S sour gas or liquid hydrocarbons arP treated with 1,3,5-
- 2~2~5~3
,~
trimethyl-hexahydro-1,3,5 triazine to reduce the level of H2S
and mercaptans therein. The 1,3,5-trime~hyl-hexahydro - 1,3,5
triazine may be represented by the following formula (FORMULA
I) CH3 ~ N N _ CH3
~J
CH3
The triazine is prepared by reacting trimethyl amine with
formaldehyde.
The method of the present invention involves adding the
triazine scavenger described above to any gas or liquid hydro-
carhon containing H2S and/or mercaptans in a sufficient quan-
tity to effectively reduce the levels of reactive S therein.
The method may also be employed by passing the sour gas
through an absorption containing a solution of the scavenger.
DESCRIPTIONQF THE PREFERRED hMBODIMENTS
The method of the present invention may be used in the
treatment of sour gas and oil production streams, as well as
in petroleum (e.g. crude oil and re~ined products) contained
in storage tanks, vessels, pipelines. etc.
As mentioned above, the scavenging composition use~ul in
the method of the present invention is 1,3,5-trimethyl-hexa-
hydro-1,3,5-triazine. (For convenience, this compound will
s:imply be re~ rred to as "triazine" unless otherwise indicated
212~
to distinguish between other triazines.) The triazine
(Formula I) i5 prepared by the condensation reaction of a
trimethylamine and formaldehyde:
3 C~I3NH2 ~ 3CHzO C~3 N N - CH3
CH3
The ~ormaldehyde may be in the ~orm of formalin or para-
formaldehyde~ with the ~ormer being preferred.
Other compounds such as hydrocarbon solvents may be
present in the final product. These include xylenes, aromatic
naphtha and alcohols.
In carrying out the reaction, an aqueous solution of
methylamine is added slowly to a concentrated aqueous
methanol-free solution of formaldehyde and the stoichiometry
is maintained so that there is a slight excess of methylaminP
at the end of the reaction, maintaining a molar ratio of at
least 1.01 ~e.g. about 1.02 moles3 o~ methylamine to 1.00
moles of formaldehyde for the overall process. Free formalde-
hyde is minimized to <1000 ppm in the liquid. Slow addition
is desirable to control the reaction t~mperature to below
1~0F. For climatization purposes, methanol or other solv~nts
can be added back without ad~ersely affecting the fo~mclldehyde
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level. Thus, an essentially quantitative yield of 1,3,5-
trimethyl-hexahydro-1,3,5-triazine can be formed under
conditions which minimize the presence of objectionable
amounts of free formaldehyde.
The triazine may also be manufactured by the reverse
addition of formaldehyde to methylamine to produce the same
result, provided the temperature is maintained below 105F to
minimize methylamine loss by evaporation and provided the
stoichiometry o~ the overall process is as described above.
The manufacture of the triazine by the method described
above produces highly desirable scavengers for use in treat-
ment o~ hydrocarbon streams because of the absence of formal-
dehyde. The reasons for this are believed to be due to the
following ~actors:
(1) The slight excess of methylamine drives the triazine
formation to completion.
(2) Methylamine is a small molecule and ~trong base and
as such does not require an additional base to form
a stable triazine.
(3) The absence (or minimization) of methanol removes
the possibility that formaldehyde is tied up as an
acetal or hemiacetal of ~ormaldehy~e and methanol.
These materials, if present, would be competing with
methylamine and hindering triazine formation.
(4) Methylamine is a monofunctional primary amine unlike
ethanolamine, which contains a hydroxy yxoup.
Methylamine cannot ~orm an oxazolidine, bis or
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~.
otherwise, thus clearly distinguishing the tri-
methyl hexahydro triazina from the tri-(2 hydroxy-
ethyl) hexahydro S triazines of the prior art.
The requirement to form such a structure as taught
by U.S. Patent No. 4,978,572 is a 2-aminoalcohol
such as monoethanol amine.
O~erations
In carryiny out the method of the present in~ention, the
scavenging composition is added to the gas or oil strPam in
a concentration sufficient to substantially reduce the levels
of H2S and/or mercaptans therein. ~n gas, generally from 0.01
to 0.12, preferably from 0.02 to 0.10, most preferably from
0.04 to 0.08 gallons of scavengPr product (34.5% active~ per
MMSCF for each ppm of HzS removed will be sufficient for most
applications. The treatment may also be based on weight of
H2S in the gas. From .05 to 1.0, pre~erably 0.1 to .4 pounds
of triazine per MMSCF per ppm H2S removed will normally be
required.
In treating hydrocarbon streams, the scavenging compound
contained in a solvent, such as water or alcohol, may be
injected by conventional means such as a chemical injection
pump or any other mechanical means ~or disp2rsing chemicals
in the stream. The injection may be in the ~low lines or the
gas may be pa~sed through an absorption tower containing a
solution of the triazine.
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For sour oil from .5 to 5 pounds, preerably from 1.0 to
4.0 pounds, and most preferably from 1.5 to 3.0 pounds of
triazine per pound of H2S removed will be sufficient.
In addition to the triazines described above, the
chemical formulations may also contain other compounds such
as ethoxylated alcohols, ethoxylated phenols, sulfates of
ethoxylated alcohols and phenols, quaternary amines, corrosion
inhibitors, and the like. The preferred scavenger formulation
comprlses 10-50 wt% actives (triazines).
The H2S scavenging ability of the 1,3,5-tri~methyl-
hexahydro~1,3,5 triazine is believed to be due to its reaction
with hydrogen sulfide to produce sulfur containing organic
compounds such as dithiazines.
EXPERIMENTS
Field Test. Comparative tests were run on a commercial gas
gathering system with gas flow through a 6" pipeline:
Gas Flow Rate - 6.5 ~MSCFD
H2S present - 250 ppm
The scavengers used to treat the facility were as
follows:
Formula I Product: 34.5 wt% 1,3,5-trimethyl-
hexahydro-1,3,5 triazi~e
tFormula I):
65.5 wt% solvent ~waker)
Commerci~l Scavenger: 34.5 wt% 1,3,5-tri-(2-hydroxy-
ethyl)-hexahydro-1,3,5-triazine.
65.5 wt% o~ a solvent.
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The treatment with the Commercial Scavenger involved con~
tinuous injection into the pipeline at a rate of 75 gallons
per day, and a 55 gallon sllig treatment twice a week.
This treatment successfully maintained the H2S level in
the gas at the 4 ppm limit, but experienced severe buildup of
reaction by-products, requiring cleanout every other day.
The treatment with the Formula I Product involved injec-
tion into the 6" pipeline at a rate of 73 gallons per day with
no need for any slug treatments. The use of the Formula I
Product limited the H2S content of the gas to 4 ppm. In a
four month treatment, only one cleanout was required.
Performance EfficiencY Tests
Experiment 1: Side stream bubble tower tests were per-
formed at a commercial facility to determine the absorption
efficiency and capacity of the Formula I Product in the
removal of hydrogen sulfide (H2S) from a natural gas stream.
The procedure was as follows: A 2-liter absorption
column was used. Three milliliters of the Formula I Product
were diluted in 500 milliliters of distilled water. The inlet
concentration of H2S was determined, the cylinder ~as filled,
and the flow rate of the natural gas stream was set at 3.0
liters of gas per minute. The flow rate was checked every 7
to 8 minutes and the outlet H2S concentration was determined
every 15 minutes. The test was continued until the outlet H2S
concentration was near the inlet le~el. The results are pre-
~ented in Table I.
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TABLE I
Elap~ed H2S ~2S Liters H2S
Time Inlet OutletPa~sed R i~n~d
~Hours) (ppm) ~ppm)(in interval) (gram~)
_ _ _ _ _ _ _ _ _ _ _ _ _ .
. oo 860 0
.25 8~0 0 45 .060
.50 860 5 45 059
.75 860 10 45 .059
1.00 950 45 45 .063
1.25 950 130 45 .057
1.50 950 220 45 .051
1.75 950 300 45 .04
2.00 950 350 45 .042
2.25 g50 400 ~5 .03
2.50 950 400 45 .038
2.75 950 700 ~5 o 017
The total H2S removed was 1.467 pounds per gallon of the
Formula I Product (34.5% active).
Experiment 2: A second side stream bubble tower test was
performed at a second commercial facility.
The procedure was as follows: A 2-liter absorption
column was used. Fifty milliliters of Formula I Product were
diluted in 400 milliliters of distilled water. The inlet
concentration of H2S was determined, the cylinder was filled,
and the flow rate was set at 3.0 litPrs of gas per minute.
The flow rate was checked every 10 minutes and the outlet H2S
concentration was determined every 15 minutes. ~he test w~s
continued until the outlet H25 conrentration was approximately
forty percent (40%) o~ the inlet level. The test results are
presented in TABL~ II.
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TABLE II
Elapsed ~2S ~2S Liters H2S
Ti~e Inlet Outlet Pas~ed R3Knld
(Hour~) (ppm) (ppm)(in inter~al)(gr~ms)
_~_________._________________________________________________ ___________
.00 30000 0 0 .000
.25 30000 0 45 2.078
.50 30000 5 45 2.077
.75 30000 50 45 2.074
1.00 30000 7800 45 1.537
1.25 30000 8200 15 .503
1.50 30000 l0000 15 .462
1~75 30000 11800 15 420
Total: 9.152
The total H2S remoYed was 1.526 pounds/gallon of Formula
I Product (34.5% active).
Com~arative Tests 1 and 2: A side stream hubble tower
test was performed at the commercial facility tested in
Experiment 2 to determine the a~sorption e~ficiency and
capacity of the commercial scavenger used in the Field Test
described abovP except the active triazine was between 45 and
50 wt%.
In one test procedure, a 2-lit~r absorption column was
used. The cylinder was charged with 100 milliliters of the
commercial scavenger and 500 milliliters of water. A gas 10w
rate of 4.0 liters per minute was passed through the cylinder.
In the second test procedure, a 250 milliliter cylinder
absorption column was used. The cylinder was charged with
100 milliliters of the commercial scavenger~ A gas flow rate
o~ 1.0 to 1.5 liters per minute was passed throuyh the
cylinder.
2 ~ 2 ~ ~ ~ 3
The inlet and e~fluent hydrogen sulfide (H2S) concen-
trations were determined by Gastec tubes.
The test results for the two tests are presented in
TABLES III and IV.
TABLE III
Elapsed H2S H2S
Time Inlet Outlet
(~ours) (ppm) (ppm) Test Comments
_____ __. __________________________________________________
.00 55000 0 Test Started
.17 55000 0 Added 0.5 ml
.25 55000 0 antifoam agent
.50 55000 10
.75 55000 600 Ended Test
A total of 1O15 pounds of H2S per gallon of the scavenger
(45-50~ active) were removed.
TABLE IV
Elapsed ~2S H2S
Time Inlet Outlet
(Hours) (ppm) (ppm) Test Comments
_______.______________ ____________________________________
.oo 550000 Test Started
.25 550000 Added lo0 ml
.50 550000 antifoam "E-22"
.75 550000
1.00 550000
1.25 55000o
1.50 550000
1.75 5500010
2.00 55000100
2.25 550001200
A total of 1.22 pounds of H2S per gallon of the commer-
cial scavenger (~5-50% active) were removed.
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Comparison of the Performance of Yormula _I and the
Commercial Scavenqer: The composition of the Commercial
Scavenger is 45.0% to 50.0% by weight of 1,3,5-tri(2-hydroxy-
~thyl)-hexahydro-1,3,5-triazine and the Formula I Product is
34.4% by weight of 1,3,5-trimethyl-hexahydro-1,3,5-triazine.
The efficiency based on the weight of the actives
ttriazines) in the 4 tests described above were as follows:
Pounds of H2S Removed per pound of Formula I - 0.514
Pounds of H2S Removed per pound of commercial scavenyer
(actives~ - 0.27
Based on the average results, the Formula I treatments
resulted in a 52% improvement over the commercial scavenger
in removing H2S~
Solubility Tests: Laboratory tests have shown that the
solubility characteristics of the reaction products of
hydrogensulfidewithl,3,5-trimethyl-hexahydro-1,3,5-triazine
are more soluble in hydrocarbon medium than the reaction
products of hydrogen sulfide with 1,3,5-tri-(2-hydroxyethyl)-
hexahydro-1,3,5-triazine. This is a highly desirable ~esult,
because it reduces plugging or fouling by reaction products
as demonstrated in the ~ield tests using the commercial
scavenger.
Summary of ExPeriments: The above experiments demon-
strate that the Formula I soavenger (1,3,5-tr;.methyl-
hexahydro~l,3,5 triazine) resulted in imp:roved per~ormance
~: , : :
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513
over the closest prior art scavenger (1,3,5(2-hydroxyethyl)-
hexahydro-1,3,5 triazine), in terms of H2S removal.
In addition, the Formula I scavenger did not result in
by-products that required frequent cleaning.
Also in addition, the manufacture and use of the
scavenger in accordance with the present invention offers the
advantage that it is ecologically acceptable since it is sub-
stantially free of ~oxmaldehydes.
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