Note: Descriptions are shown in the official language in which they were submitted.
CA 02592926 2007-07-03
Method for Sour Gas Treatment
Field of the Invention
The present invention relates to a method of sour gas treatment, and in
particular, a
method for selectively removing hydrogen sulfide with minimal carbon dioxide
absorption from the sour gas by scrubbing the gases with a sodium hydroxide
solution. Further, this invention relates to a process for producing salts
such as
ammonium sulfate or ammonium thiosulfate.
Background of the Invention
A number of different processes are currently in use to remove or scrub
hydrogen
sulfide (H2S) and carbon dioxide (C09) from sour natural gas. Generally, these
processes are also capable of removing carbonyl sulfide (COS), carbon
disulfide
(CS2) and mercaptans (RSH ¨ where R is any radical). The processes include
chemical and physical processes, batch processes, molecular sieve processes
and
membrane separation processes.
Caustic (sodium hydroxide) has traditionally been used to carry out a fine
purification
of the gas after the coarse treatment of a natural gas stream by a chemically
or a
physically regenerable solvent. Conventionally, a caustic solution
countercurrently
contacts with a sour gas mainly containing H2S and CO2 in a packed or trayed
column. When CO2 is present, the sodium hydroxide solution will absorb CO2 as
well
as H2S without preference. This leads to high caustic consumption and the
spent
solution has to be neutralized by acid for disposal.
The chemical reactions involved in caustic scrubbing are as follows:
NaOH + CO2 _______ 00. NaHCO3
H2S + NaOH NaSH + 1120
NaSH + NaOH Na2S + H20
H2S + 2NaOH P Na,S + H20
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RSH + NaOH RSNa + H20
CO2 + 2NaOH -00. NaCO3 + H20
CS? +2NaOH -4. 2NaHS + CO2
The scrubbing liquors contain mixtures of sodium hydrosulfide, sodium
bicarbonate,
sodium carbonate and sodium carbamate in varying amounts. Depending on the
composition of the gas which has been scrubbed and the operating conditions of
the
scrubber, there may be anywhere from 0 to 100% carbonate salts in the
scrubbing
liquors.
Various options exist to treat the liquors. One option is to fully oxidize the
mixture to
a mixture of sodium sulfate and sodium bicarbonate. Another option is to
partially
oxide the scrubbing liquors to a mixture of sodium thiosulfate and sodium
bicarbonate. The mixture may then be acidified with sulfuric acid to decompose
the
bicarbonate to carbon dioxide. This forms a solution which is essentially pure
sodium
sulfate. The sodium sulfate formed in this way (or the sodium sulfate/sodium
bicarbonate mixture) may then be treated in a bipolar cell or an
electrochemical cell to
regenerate a solution of sodium hydroxide. The regenerated sodium hydroxide
may
then be recycled to a column or mixer for example to be used for further
scrubbing.
U.S. Patent 5,098,532 discloses a three compartment electrochemical cell that
can be
used to produce ammonium sulfate from sodium sulfate. Alternative
electrochemical
cells may also be used.
Another option for regeneration of caustic is countercurrent contacting of the
liquid
with 10% sodium hydroxide solution in a packed column. The caustic is
regenerated
in a stripping column by the addition of open steam or by steam internally
generated
by a column heating element. The condensate is returned to the stripping
column to
maintain caustic concentration.
Summary of the Invention
In accordance with a broad aspect of the invention, there is provided a
process for
removing hydrogen sulfide and carbon dioxide from a sour gas stream, the
process
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CA 02592926 2007-07-03
comprising: a) scrubbing the sour gas stream with sodium hydroxide using a
static
mixer while controlling residence time in the mixer b) separating the gas from
the
liquid; c) air stripping the liquid portion of the solution from b); d)
oxidizing the
solution from c) in an oxidizer vessel; e) electrodialysizing the solution
from d) to
produce a salt product and sodium hydroxide; and f) recirculating the sodium
hydroxide to step a).
In accordance with another aspect of the invention, there is a process for
removing
hydrogen sulfide and carbon dioxide from a sour gas stream, the process
comprising:
a) scrubbing the sour gas stream with sodium hydroxide; b) oxidizing the
solution
from a) to produce sodium sulfate and sodium thiosulfate; and c) passing the
solution
from b) through an electrolytic cell comprising bipolar membranes to produce
sodium
hydroxide and a salt product.
In accordance with a broad aspect of the invention, there is provided a
process for
removal of hydrogen sulfide from a sour gas stream and production of a salt
product
from the sour gas stream, the process comprising: a) removing free water from
a sour
gas stream; b) scrubbing the sour gas stream with sodium hydroxide using a
static
mixer while controlling residence time in the mixer; c) separating the gas
from the
liquid in the solution from step b); d) air stripping the liquid portion of
the solution
from c); e) oxidizing the solution from d) in an oxidizer vessel; f) filtering
the solution
from e); g) electrodialysizing the solution from f) to produce a salt product
and
sodium hydroxide; and h) recirculating the sodium hydroxide to step b).
It is to be understood that other aspects of the present invention will become
readily
apparent to those skilled in the art from the following detailed description,
wherein
various embodiments of the invention are shown and described by way of
illustration.
As will be realized, the invention is capable for other and different
embodiments and
its several details are capable of modification in various other respects, all
without
departing from the spirit and scope of the present invention. Accordingly, the
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drawings and detailed description are to be regarded as illustrative in nature
and not as
restrictive.
Brief Description of the Drawings
Referring to the drawings wherein like reference numerals indicate similar
parts
throughout the several views, several aspects of the present invention are
illustrated
by way of example, and not by way of limitation, in detail in the figures,
wherein:
Figure 1 is a schematic view of a process for treating sour gas;
Figure 2 is a schematic view of a bipolar membrane cell and a process for
producing
salt from a salt feed;
Figure 3 is a schematic representation of an arrangement for conducting
scrubbing
tests; and
Figure 4 is a graph showing outlet H2S concentration and
supplied/stoichiometric
sodium hydroxide ion to gas flow ratio.
Detailed Description of Various Embodiments
The detailed description set forth below in connection with the appended
drawings is
intended as a description of various embodiments of the present invention and
is not
intended to represent the only embodiments contemplated by the inventor. The
detailed description includes specific details for the purpose of providing a
comprehensive understanding of the present invention. However, it will be
apparent
to those skilled in the art that the present invention may be practiced
without these
specific details.
The present invention is intended to treat sour gas. The present process may
be
carried out in whole or in part at the wellhead of a sour gas well and may for
example
be used at an isolated gas well away from a central gas processing system in a
commercially viable manner. With that in mind, Figure 1 is a schematic
representation of an entire process, including both the portion of the process
that may
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occur at a well site (i.e. until the point of liquid knockout) and that which
may occur
in a regenerating plant (hydrocarbon removal to electrochemical separation).
With reference to Figure 1, sour gas 8 from a natural gas well containing
hydrogen
sulfide, carbon dioxide and mercaptans and other sulfur compounds such as
carbonyl
sulfide and carbon disulfide is typically saturated with formation water. The
excess
water may be separated from the gas in various ways for example with a free
water
knockout 14. Liquid slugs 10 may be sent to disposal or re-injection into an
abandoned gas well. Saturated gas with minimal free water 18 may then be sent
through a filter 22, which may be for example a coalescing filter, for maximum
gas
liquid separation. The gas leaving 26 the coalescing filter 22 may be
saturated, but
there is not expected to be any free water in the gas. Any liquid remaining
following
this filtration 10a may be removed for disposal or re-injection into an
abandoned gas
well.
Saturated sour gas 26 which may contain methane and heavier hydrocarbons,
hydrogen sulfide, carbonyl sulfide, carbon disulfide, carbon dioxide and
generally no
free water is contacted with either recycled sodium hydroxide solution 34 or
fresh
sodium hydroxide solution 6 or some combination thereof. The gas may be
contacted
with the sodium hydroxide for example in an in-line mixer 30 or other device
such as
a column, absorption tower, etc. The sodium hydroxide may be from various
sources.
As will be described in more detail, the sodium hydroxide may be regenerated
from
an electrochemical cell 90 for example. Alternately, fresh caustic 6 may be
added
directly to the mixer 30.
In the illustrated embodiments, the mixer 30 is set up to flash the sour gas
quickly
enough to be selective on hydrogen sulfide removal. The mixer may be selected
for
example, to introduce sodium hydroxide as a spray in liquid droplet form to
the gas
passing there through. The mixer acts as a scrubber, discouraging the CO2
reaction by
maximizing gas-liquid mass transfer rates while minimizing the retention time
for the
reaction. This combination strongly favors the reaction of H2S with caustic
over the
reaction of CO2 with caustic. Carbon dioxide reacts with sodium hydroxide
solution
which results in large sodium hydroxide usage. There is, however, a minimum
CO2
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pick up required to provide for the required sodium carbonate, sodium
bicarbonate
and sodium carbamate buffer for the electrochemical cell, as well as to
precipitate the
metal ions out at the filtration step.
The mixer may be different forms, provided there is a rapid reaction of
caustic and
sour gas. In one embodiment, the mixer 30 may be a static mixer and may
include a
series of stationary, rigid elements placed lengthwise in a pipe, for example.
These
elements form intersecting channels that split, rearrange, and recombine
component
streams into smaller and smaller layers until one homogeneous stream exists.
One
possibility is that the mixer is Sulzer SMV-type static mixer available from
Sulzer
Chemtech for example, although other mixers may also be used. In the mixer,
the
reaction of H2S with NaOH is very robust, due to the complete mixing of
scrubbing
solution with sour gas. There is instantaneous gas/liquid contact when the gas
enters
the blender.
The liquid to gas ratio of the mixer (L/G ratio) may be about 1) 0.13 to 0.27
L/Nm3
based on fresh caustic that must be added to complete the reaction; or 2)
¨0.30 L/Mm3
based on the total caustic rate (fresh and recirculated) to the mixer. This
value may
vary depending on the mixer dimensions and spray nozzle.
Generally, CO2 absorption may be in the range of about 12-17%. Generally, if
there
is inadequate NaOH to satisfy all the H2S, the CO2 pick up may be very low. A
very
high H2S/CO2 ratio may also result in lower CO2 pick up. The operation
conditions,
including gas-liquid mass transfer rate, retention time, and pH value of spent
sorbent,
may be optimized to ensure a minimal carbon dioxide reaction. Generally, if
the
sweet gas is not up to specification, more NaOH may be sprayed into the mixer.
The
amount of CO2 may be from about 0 to 30% in the inlet gas and from about 0 to
27%
in the outlet gas. The pH value of the spent scrubbing solution may be between
about
to 15. Retention time may be between about 0 and 10 seconds. The mass transfer
rate is dependent on gas volume, percentage of H2S and the percentage of CO2
for
example.
6
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The removal of H2S is mass transfer limited. The reaction rate for H2S in the
aqueous
phase may be faster than the mass transfer rate of H2S into the aqueous phase.
In one
embodiment, sufficient caustic may be added to stoichiometrically satisfy all
the H2S
in the sour gas. About 10% of the CO2 may be captured, with about 5% carry
over of
NaOH in the scrubber system. In this way, pipeline specifications of <16 ppm
may be
achieved. Operation at sub-stoichiometric conditions of caustic may reduce
consumption by up to 30%. In one embodiment, using the scrubbing technology, a
WS concentration at the inlet of approximately 1700 ppm may be reduced to
about 5-
20 ppm at the outlet of the static mixer over a wide range of operating
conditions.
The gas and liquid solution 42 is then separated with a second filter 46,
which may be
a coalescing filter, for example, to generate sweetened gas 38 and scrubber
solution
50. The filter may help to prevent contamination of the caustic. Also, the
filter 46
may remove liquid droplets of hydrocarbons (along with any other entrained
water,
etc.).
Sweetened gas 38 with at least a portion of and in some cases all hydrogen
sulfide,
mercaptans, carbonyl sulfide, carbon disulfide and some carbon dioxide removed
is
pipelined to market. A portion of the swet gas be analyzed in a gas analyzer
44 which
provides feedback 45 to fresh caustic input 6. Scrubber solution 50 containing
some
heavier hydrocarbons, sodium sulfide, sodium hydrosulfide, sodium bicarbonate,
sodium carbonate, sodium hydroxide, carbonyl sulfide, carbon disulfide and
miscellaneous contaminants is the liquid dump of filter 46. From the filter
46, part of
the scrubber solution may be carried forward, while part of the scrubber
solution may
be recycled through a caustic recirculation pump 32 for example and returned
to
mixer 30. This recycling helps to ensure greater reaction between the caustic
and sour
gas. Hydrocarbons and mercaptans may be removed from the scrubber solution 50
in
a stripper 58, which may be an air stripper. Thus, hydrocarbons may be removed
from the scrubber solution by stripper 58.
Once air sparged in stripper 58, the hydrocarbons and mercaptans 54 may be
scrubbed
in a scrubber or column 56 to produce additional sodium sulfate 62 which may
be fed
to an oxidizer 66. The air-stripped scrubber solution 60 may be pumped to the
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oxidizer 66 where oxygen 70 may be sparged into the oxidizer under controlled
temperature and pressure conditions. For example, pressure conditions of 75-
125 psig
and 100-160 C may be used. The solution may be partially oxidized to sodium
thiosulfate with sodium bicarbonate, sodium carbonate and sodium carbamate.
Alternatively, the scrubber solution may be fully oxidized to sodium sulfate
with
sodium bicarbonate/sodium carbonate and sodium carbamate. Initially, only
partial
oxidation from NaHS to NaS203 is achieved. With time, complete oxidation from
NaS203 to Na2SO4 is achieved. Thus, the same concentration of oxygen may be
used
whether oxidation is full or partial.
Positioning the oxidizer prior to any filtration steps may be useful because
the
impurities such as Fe, Mn, V, Ca and Mg in trace amounts appear to act as
catalysts
for oxidation sites, thereby helping to ensure more complete oxidation. The
bicarbonate/carbonate/carbamate co-products are important for electrochemical
cell
operability, as will be discussed below.
The oxidized scrubber solution 72 may undergo a series of filtration steps to
remove
all organic impurities and metal cations like calcium, magnesium and iron from
water
recovered in the saturated gas. The order of the filtration steps may be
varied, and
certain filtration steps such as the second filtration step are optional. The
first filter
may be a diatomaceous earth filter 74, which could remove any suspended solids
including the precipitates of Ca2+ and Mg2+and partially organics. A second
filtration
step 80, which is optional, may be an activated carbon filter 76 to remove any
light
organics and odors. A third filtration step 82 may be a chelating resin 78 to
remove
any metal cations. The filtered solution 84 may be pumped into an
electrodialysis
system 90, which may comprise for example an electrochemical cell and possibly
one
including a bipolar membrane. Ammonia 92 can be added to the electrodialysis
system. The introduced ammonia protects the membrane by preventing back
migration of protons into the feed compartment.
In one embodiment, depleted sodium sulfate or sodium thiosulfate 86 may be
withdrawn from the electrochemical cell. This depleted solution could be mixed
with
fresh sodium sulfate or sodium thiosulfate solution in a recycle step. Stream
88
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comprises a salt solution. In some embodiments, the salt solution may comprise
ammonium sulfate, ammonium thiosulfate, ammonium bicarbonate, ammonium
carbonate and ammonium carbamate and mixtures thereof.
In one embodiment, the electrochemical cell may be a bipolar electrochemical
cell.
Referring to Figure 2, the electrodialysis system 100 can be divided into
several
compartments, each compartment delineated by membranes or by either a cathode
or
an anode and a membrane. The solution 166 derived from the scrubbing process
can
be conveyed into compartments 108 and 126 of the electrochemical system. In
the
illustrated embodiment, compartment 104 is bounded by cathode 102 and cation
membrane (C) 106. Compartments 108 and 126 are bounded by an anion membranes
(A) 112, 130 and cation membranes 106, 124. Compartments 110 and 132 are
bounded by anion membranes 112, 130, respectively and bipolar membranes 118,
136, respectively. Compartments 116 and 138 are bounded by cation membranes
124, 142, respectively and bipolar membranes (BP) 118, 136, respectively.
Compartment 146 is bounded by cation membrane 142 and anode 148. For each
bipolar membrane, the anion layer of the bipolar membrane is oriented so that
it faces
the cathode, while the cation layer is oriented so that it faces the cathode.
To operate the cell, ammonia 154 may be fed into compartments 110 and 132. The
introduced ammonia protects the membrane by preventing back migration of
protons
into compartments 108 and 138. Sodium ions can be transported across cation
membranes 106, 124 into compartments 104 and 116. Sulfate ions, thiosulfate
ions,
bicarbonate ions, carbonate ions and carbamate ions can be transported across
anion
membranes 112 and 130. Water splitting occurs across the bipolar membranes 118
and 136 resulting in the production of hydrogen ions in compartments 110 and
132;
with hydroxide ions being produced in compartments 116 and 138.
The sodium bicarbonate, sodium carbonate, and sodium carbamate product acts as
a
buffer protecting the integrity of the cell membranes by preventing the
hydroxide
back migration across the cation membrane. The presence of
carbonate/bicarbonate/carbamate in the feed may also help with pH control and
ammonium contamination in the feed. First, the bicarbonate may help to buffer
the
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CA 02592926 2007-07-03
feed solution, keeping the hydroxide concentration low so that it will not
compete
with sulfate. Secondly, any carbonate in the feed may react with the ammonium
in
the feed, producing bicarbonate and ammonia. The ammonia may be stripped off
and
returned for pH control of the "acid" compartment. The key to running this
system is
to match the inefficiencies for the membranes or at least the flux of ammonium
from
"acid" to feed to the flux of hydroxide from the base.
In a typical salt split of sodium sulfate, sulfuric acid and sodium hydroxide
are
produced at low concentrations because of the back migration of protons from
the
acid compartment. However, by neutralizing the acid produced with ammonia,
there
is minimal build up of proton and high concentrations of ammonium sulfate can
be
produced.
Sodium hydroxide 160 may be withdrawn from compartments 104 and 116. The salt
product 162, which may be made of a mixture of ammonium sulfate, ammonium
thiosulfate, ammonium bicarbonate, ammonium carbonate, and ammonium carbamate
can be withdrawn from compartments 110 and 132. The amounts of ammonium
bicarbonate, ammonium carbamate and ammonium carbonate produced are dependent
on the amount of CO2 pick-up in the mixer. It is possible to halve the amount
of
bicarbonate in the cell feed by operating the oxidizer in such a manner that
the
bicarbonate is thermally decomposed to carbonate and carbon dioxide. The
percentage of ammonium sulfate or ammonium thiosulfate produced depends on the
completion of oxidation stage.
The depleted salt solutions 164 which may be either sodium sulfate or sodium
thiosulfate or some combination of these salts, can be withdrawn from
compartments
108 and 124. The sodium hydroxide can then be recycled to scrub hydrogen
sulfide
and carbon dioxide from sour gas. The salt product may contain one of more of
the
following salts: ammonium sulfate, ammonium thiosulfate, ammonium bicarbonate,
ammonium carbonate or ammonium carbamate. The salt product may be separated,
evaporated, dried, granulated and sold as a fertilizer product. Using this
method,
mixtures of 0-99% ammonium sulfate, 0-50% ammonium bicarbonate and 0-30%
ammonium carbonate may be produced.
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The reactions occurring inside the cell are as follows:
Na2SO4 2Na+ + S042-
Na2S203 ¨4 2Na+ + S2032-
NaHCO3 Na + + HCO3-
Na2CO3 2Na+- + C032NaH2NCO2 Na + + H2NCO2
The chemistry that takes place in the base side of the cell is:
Na + OH- = NaOH
The chemistry that takes place on the acid side of the cell is:
2NH3+ + 2H+ + S042- ¨4 (NH4)SO4 or
2NH3+ + 2H+ + S2032- (NH4)2S203 with
NH3 + + H+ + HCO3- NH4 HCO3 plus
2NH3+ + 2H+ + C032- ¨ (NH4)2 CO3 plus
NH3 + + H+ + H2NCO2- NH2COONH4
The combined result of the above processes may be the production of a salt
stream
comprising predominantly either ammonia sulfate or ammonium thiosulfate.
Additional components of the salt stream can be ammonium carbonate, ammonium
bicarbonate, and ammonium carbamate. The components of the salt stream may be
separated.
Referring to Figure 3, a possible arrangement for conducting scrubbing tests
is shown.
The test arrangement is described in more detail in Example 1. Sour gas 200
can be
fed into an inline mixer 212. Fresh sodium hydroxide 204 may be sprayed into
the
sour gas which ensures that the desired hydrogen sulfide reaction occurs with
minimal
carbon dioxide uptake. Recycled caustic 208 can also be sprayed into the in-
line
mixer. The solution 216 emerging from the in-line mixer can then be fed into a
coalescing filter 220. The spent scrubber solution 228 and treated gas 224
emerge
from the coalescing filter 220.
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Example 1
Operating Systems
Sour gas from an operating sour gas well was scrubbed. The scrubbing process
used a
static mixer as a scrubber in contrast to a trayed vessel traditionally used
for bulk H2S
removal with sodium hydroxide. An inlet caustic stream was sprayed into the
sour
gas, with minimal retention time allowed. Tests were conducted to determine if
a H2S
reaction occurs with minimal CO2 uptake, which may be due to a higher rate of
mass
transfer of 1-12S into the aqueous phase.
The major process variable values were recorded for each set of test
conditions. The
acid gas composition of the treated gas was recorded through sampling and on-
site
analysis. The gas flow rate was manipulated between 0.3 to 0.9 MMSCFD. The
fresh caustic flow rate was manipulated between 0.5 to 1.2 USGPM. The total
liquid
flow rate to scrubber may be between 0.7 to 1.4 USGPM. Once the supplied NaOH
to gas flow ratio goes beyond about 0.6 gmol OH/m3 gas, there may be little
benefit in
H2S removal. If the supplied NaOH to gas flow ratio is smaller than about 0.45
gmol
OH/m3 gas, sufficient H2S removal for pipeline specifications may be achieved
at
sub-stoichiometric conditions, which optimizes caustic consumption.
The scrubbing technology was able to reduce H2S concentration from
1700 ppm at the inlet to 5-20 ppm at the outlet of the static mixer over a
wide range
of operating conditions. The few tests wherein H25 exceeded the pipeline
specification of 16 ppm were a result of attempting to determine the operating
limits
for the technology. In order to reduce the level of H2S at the outlet to less
than 16
ppm, the L/G ratio was manipulated to about 0.3. CO, absorption was in the
range of
about 12-17%. The CO, pick up was adequate for providing the compounds
necessary for cell buffering.
The process proved to be very robust, which indicates it will be very
appropriate for
unmanned well sites. The scrubbing tests assessed the basic operability of the
equipment and provided the foundation for the "proof of concept".
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The present invention will be most economical when the circulation rate of
caustic
through all the process elements is minimized (i.e. a minimum amount of
caustic is
used). In one embodiment, the fresh caustic flow rate may be about 0.5 to 1.2
USGPM. The total liquid flow rate to the scrubber may be about 0.7 to 1.4
USGPM
for example. The mixer/scrubber technology tested in this work was designed to
discourage the CO2 reaction by maximizing gas-liquid mass transfer rates while
minimizing the retention time for the reaction. This combination strongly
favors the
reaction of H2S with caustic over the reaction of CO2 with caustic. There is,
however,
a minimum CO2 pick up required to provide for the required sodium carbonate,
sodium bicarbonate and sodium carbamate buffer for the electrochemical cell.
The goal of the scrubber was to selectively remove almost all of the H2S from
the feed
natural gas by reaction with NaOH solution. Almost all of the H2S reacted with
caustic (1700 ppmv in the inlet gas, down to 10 ppmv in the outlet gas), while
only
about 15% of the CO, reacted with caustic (2.78 mol% in inlet gas, 2.35 mol%
in the
treated gas). The H2S reduction was adequate to meet commercial sweet gas
specifications and the CO, pick up was enough to provide the carbonate,
bicarbonate,
and carbamate ions required for the electrochemical portion of the present
invention.
Table 1 shows the operating conditions, the resulting outlet H2S and CO2
concentrations and the amounts of Na2S, Na2CO3, NaHCO3 and NaHS produced
under those operating conditions before the solution is oxidized.
Table 1: Operating Conditions and Results Obtained in a Sample Test.
Test No. 5
Date and Time (Approx.) 1/19/06 12:10 PM
Gas Temperature C 6.8
System Pressure kPag 2385 kPag
Gas Flow Rate MMSCFD 0.59
gmol/min 3.876
Fresh Caustic Flow USGPM 0.46
(mean) gmol/min 3.876
Recycle Caustic Flow (mean) USGPM 0.92
M-900 Caustic Flow (total) USGPM 1.38
Inlet H2S Conc. PPm 1700
Inlet CO2 mol% 2.76
Outlet 112S Conc. PPm 13.03
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Outlet CO2 Conc. mol% 2.32
Outlet NaOH pH pH 10.95
NaOH mg/L mg/L 37
NaHS mg/L mg/L 26817
Na7S mg/L mg/L 33
Na2CO3 mg/L mg/L 52245
NaHCO3 mg/L mg/L 63768
pH 10.97
The results show that the system is robust enough to operate successfully at
an inlet
flow rate lower than 1 USGPM without having adverse effects on process
efficiency.
Further, the results show that comfortable H2S removal may be achieved while
optimizing fresh caustic consumption.
The intent of the sample test was to evaluate whether a larger liquid flow
rate through
the inlet spray nozzle and scrubber itself had any impact on the scrubbing
efficiency
of the system. However, the fact that H2S exceeded spec on the outlet gas
required an
adjustment to the fresh caustic rate to try to meet specifications. Thus, a
test was
conducted, the purpose of which was 2-fold:
= to operate at the optimized conditions as a result of the rest of the
experiments;
and
= to establish the effects of increased overall scrubbing liquor to the
nozzle
spray on Static Mixer/Scrubber M-900.
As shown in Figure 4, the outlet H2S concentration of the gas follows one
uniform
trend line across all of the data points that were collected in the field
study. This
reinforces the prediction that removal of H2S is mass transfer limited. The
reaction
rate for H2S in the aqueous phase was faster than the mass transfer rate of
H2S into the
aqueous phase. Once the supplied NaOH to gas flow ratio goes beyond
0.6 gmol OH/m3 gas, there was little benefit in H2S removal. If the supplied
NaOH to
gas flow ratio was smaller than 0.45 gmol OH/m3 gas, sufficient H2S removal
for
pipeline specifications can be achieved at sub-stoichiometric conditions,
which
optimizes caustic consumption. Operation at sub-stoichiometric conditions of
caustic
reduced consumption by up to 30% for the case that was investigated.
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Example 2
Oxidation System
Tests were designed to determine the basic process parameters for the partial
oxidation to a sodium thiosulfate solution and the total oxidation to sodium
sulfate
solution. The objectives were to determine a practical, high performance and
relatively cost effective process to oxidize the caustic scrubber solution to
form either
sodium sulfate (Na2SO4) or sodium thiosulfate (Na2S203) and to feed these
salts to an
electro dialysis process downstream.
Oxidation Data
Two oxidation tests were performed in a continuous stirred tank reactor (CSTR)
with
a Robin agitator at a pressure of 100 psig and a temperature of 130 C. The
CSTR
reactor is scalable to a commercial unit.
Test 1 was performed with a batch of synthetic liquor made from reagent grade
sodium carbonate, bicarbonate and sulfide. The pH of the starting solution was
13.
Oxidation of the synthetic solution produced the results is shown in Table 2
below.
Batch oxidation to sulfate was, within the limited of analytical accuracy,
complete and
rapid (less than 30 minutes).
Table 2: Synthetic Liquor Oxidation Results
Run Reaction Sodium Sodium Sodium Sodium Total
Time Sulfide Sulfite Thiosulfate Sulfate Sulfur
Na2S, g/L Na2S03, Na2S203, Na2SO4, S, g/L
g/L g/L g/L
2 0 31.6 <0.02 0.91 0.59 13.0
2 45 <0.04 <0.02 0.083 65.3 13.7
2 55 <0.04 <0.02 0.083 66.1 13.8
2 65 <0.04 <0.02 0.083 65.2 13.6
3 0 31.8 <0.02 0.87 0.64 13.1
3 10 <0.04 <0.02 14.5 44.7 14.1
3 20 <0.04 <0.02 0.083 67.7 14.1
3 30 <0.04 <0.02 0.083 68.0 14.0
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CA 02592926 2007-07-03
Test 2 was performed on a batch of "real" scrubbing liquor sampled from
Example 1.
Prior to the oxidation, additional caustic (72.5 gpl of 30% caustic) was
spiked to raise
the pH to 13Ø Results of the spiked liquor oxidation are shown in Table 3
below.
Table 3: Spiked Liquor Oxidation Test Results
Reaction Time Sodium Thiosulfate, Na2S203
g/L
0 0.83
17.9
4.78
1.59
40-A 0.91
40-B 0.83
The residual thiosulfate of 0.83 gpl at time 40 minutes is believed to
demonstrate that the reaction has gone to completion, particularly since the
starting
thiosulfate number also analyses to 0.83 g/L.
In order to shorten the required time, either multiple, continuous stirred
tank
reactors (CSTRs), or the combination of a front end CSTR with a downstream
plug
flow device may be needed.
The work has shown that complete oxidation may be obtained at the selected
temperature 130 C and pressure 100 psig with an appropriate reactor
combination.
Example 3
Filtration System
The overall objective of the research on filtration systems was to determine
the effect
of preliminary pre-treatment/filtration on the concentration of select
chemical
components in the scrubber solution. The most important components to remove
were the metal hardness cations: calcium, magnesium, and iron.
A two stage filtering regime, first using a diatamateous earth (DE) +
activated carbon
(AC) filter bed and second with a 0.45 micron polishing filter (CH) under
vacuum
DmsLega1\0570081.00001, 25992000 16
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was used to filter all solutions after treatment. In addition, Amberlite IRC-
747
treatment, either alone or with DE and AC was used.
The effect of the filtration system on the calcium, magnesium, iron,
manganese,
potassium, and sodium cation is included in Table 4. Amberlite IRC-747 was
used.
Amberlite IRC-747 is a polymer-based chelating resin widely used for metal
removal
and recovery in mining and chemicals, ground water remediation, waste water
treatment, and brine softening. Amberlite IRC-747 presents high selectivity
for Fe,
Sr, Ca, Ba and Mg, with very low leakage of barium.
Table 4: Effect of Filtration Systems on Metal Ions in Scrubber Solutions
Final Initial Precipitation CH IRC* DE+AC DE+AC+ DE+AC
Filtration Sample Sample Treated Treated Treated CH 100 +IRC
Results Untreated Untreated Treated 747
Treated
Analyte Unit Result Result Result Result Result
Result
Calcium mg/L 40 6.2 1 1 4.2 1 1
Hardness mg/L <200 <200 <200 <200 <200 <200
Iron mg/L 16 2.1 2.4 0.2 3.9 0.2 2.6
Magnesium mg/L 20 4 4 4 4 4 4
Manganese mg/L <1 <1 <1 <1 <1 <1 <I
Potassium _ mg/L <80 <80 <80 <80 <80 <80 <80
Sodium mg/L 62000 67100 64300 64600 58700 61100 61100
*IRC= Amberlite IRC-747
Without treatment, the select cations, calcium, magnesium and iron decreased
substantially (>80%) likely from precipitation overtime. With treatment, there
was a
significant additional decrease in calcium and iron for the single Amberlite
IRC 747
and the combination DE+AC+CH treatments. The magnesium concentration
remained constant in all treatment regimes.
Example 4
Electrochemical Cell System
Experiments using sodium sulfate have shown the concentrations of caustic and
ammonium sulfate that can be produced from the process along with estimates
for
current efficiencies and water transport properties using these
concentrations.
DIASLegal \ MUMOU00 I \ 2,992U9 17
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It was shown that a sodium thiosulfate salt split is possible with bipolar
membranes.
When acidified, thiosulfate decomposes into sulfur and sulfite (bisulfite) and
in the
experiments performed, some sulfur was produced in the cell near the bipolar
membrane, where the protons are formed. At lower current density, this seems
to
only occur in possible low flow areas of the cell, with sulfur deposits
occurring
mostly around the corners away from the flow inlet and outlet.
A laboratory scale electrodialysis (ED) setup was utilized. The ED stack was
an ESC
Model ED-I-BP electrodialysis cell with an individual active ion exchange
membrane
area of 100 cm2. The cell is a filter-press design comprising of steel
endplates plastic
end frames, a stainless steel 316 cathode, a platinized titanium anode, and a
series of
spacers with polypropylene turbulence mesh for the flow compartments. The cell
was
bolted together to properly seal the stack and avoid internal or external
leaks. Four
solution flows were fed to the cell: feed, acid, base and rinse. For these
experiments,
the feed was 4 liters of either sodium sulfate or sodium thiosulfate and was
standardized to an initial concentration of 1.5 M, which is close to
solubility limit of
sodium sulfate at room temperature.
The "acid" compartment produces the ammonium sulfate or thiosulfate product.
As
the splitting process goes into the electrochemical cell, the concentrations
of
ammonium sulfate or ammonium thiosulfate in the acid compartment increases
from
1.5 M to 3.0 M. Finally, the base compartment was normally started at low
concentrations (0.4 M/1.5%) and allowed to increase up to a maximum of 2.6 M
(9%).
The electrode reactions were as follows:
Anode: 2 H20 4 1-1+ 02 + 4e
Cathode: 2 e + 2 H20 2 OFF + H2
Overall: 2 H20 ¨+02 + 2H7
The cell had 5 sets of membranes. Neosepta AHA membrane was used for the anion
exchange membrane and BP-1 was used for the bipolar membrane for all
Ivosiesams-momooni 2599209v7 18
CA 02592926 2007-07-03
experiments. Two different cation membranes were used; Nafion 115 was used in
initial experiments and Neosepta CMB was used for comparison.
The rest of the electrodialysis setup consisted of four solution reservoirs
for feed,
"acid", base and electrode rinse streams, corresponding centrifugal drive
pumps for
solution recirculation, tubing, fittings and valves. Instrumentation included
pH,
temperature and flow measurement for the feed and "acid" and flow for the
base.
Feed and acid pHs were controlled by automatically using electricity activated
valves
to allow introduction of 50% sulfuric acid and ammonia, respectively. The acid
reservoir was water jacketed which allowed temperature control at 40 C. A DC
power supply capable for 10 amps and 25 volts output was used to control the
process
and a battery cycler was used to record data.
A series of experiments were performed to determine the effect of running the
salt
split at different concentrations of base and acid. In the first experiments,
sulfuric
acid was used and pretreated with ammonia to a specific pH set point, to
produce
ammonium sulfate. The first set of experiments also used Nafion 115 as the
cation
exchange membrane. The results are shown in Table 5 below, which gives current
efficiencies for the production of caustic and ammonium sulfate for runs,
along with a
calculation of the number of water molecules associated with the transport of
sodium
from the feed to the caustic or sulfate from the feed to "acid" compartment.
Table 5: Current Efficiencies for the Production of Caustic and Ammonium
Sulfate
Base Base Base "Acid" Acid Acid Water Water
Initial CE Water Initial CE Transport Transport
Final Transport Final Moles/mole From Feed
M / % Moles/mole M / % SO4 Moles/mole
Na Na2SO4
0/0 92 3.1 1.45/17.4 92 10.9 19.1
1.6/6.14 1.93/22.7 *(94%)
0.4/1.5 95 3.1 2.78/31.4 90 9.6 14.3
1.6/6.0 3.24/35.7 *(93%)
0.4/1.4 91 4.0 3.0/33.5 93 13.4 16.5
2.6/9.3 3.18/35.1 *(92%)
Current efficiencies (CE) and water transport for sodium sulfate salt split.
*Numbers in parenthesis are
efficiencies obtained by using sulfate loss from Feed.
Divi S Leg al 057008'a/00 1 2599209%17 19
CA 02592926 2007-07-03
Very high current efficiencies (>90%) were obtained for all experiments for
both
caustic and ammonium sulfate production. The efficiency of sulfate transport
across
the AHA membrane was also very good. Our research successfully proved a sodium
sulfate salt split using bipolar membrane electrodialysis with high efficiency
for the
production of 33% ammonium sulfate and 8% caustic at a current density of 100
mA/cm2. An estimated power consumption of 1680 kWhr/metric tonne of caustic
was found, which also produces 1.9 metric tonne of ammonium sulfate.
Preliminary
long-term data showed that the ammonium sulfate end product would contain
about
0.1% sodium, which would need to be re-supplied to the system as caustic.
Experiments were conducted on the salt splitting of sodium thiosulfate using a
bipolar
membrane cell to produce ammonium thiosulfate and caustic. A traditional salt
split
of sodium thiosulfate was not possible because thiosulfate is unstable in
acidic media.
It was hoped that by neutralizing the acid produced with ammonia, the pH would
be
kept high enough to prevent decomposition of thiosulfate to sulfur and
sulfite. A
similar technique was successful on the salt split of sodium sulfate.
A laboratory scale electrodialysis (ED) setup was utilized. The ED stack for
this
work was an Eurodia Eur2C-BIP electrodialysis cell with an individual active
ion
exchange membrane area of 200 cm2 (5 sets of membranes were used). The cell
was
a filter-press design comprising of steel endplates, plastic end frames, a
stainless steel
316 cathode, a platinized titanium anode, and a series of spacers with
polypropylene
turbulence mesh for the flow compartments. The cell was bolted together to
properly
seal the stack and avoid internal or external leaks. Four solution flows were
fed to the
cell: feed, acid, base and rinse. For these experiments, the feed was
typically 8 liters
of 0.75 M sodium thiosulfate/0.75 M sodium bicarbonate. The "acid" compartment
produced the ammonium thiosulfate product, and was mostly run at a start
concentration of 1.5 M (20%) ammonium thiosulfate with 0.4 M sodium
bicarbonate.
Finally, the base compartment was normally started at low concentrations (0.4
M) and
allowed to increase up to a maximum of 2.6 M. 2 L were used for both the
"acid" and
base start solutions.
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The electrode reactions are as follows:
Anode: 2 H20 --> 4 H+ + 02 + 4E
Cathode: 2 E + 2 H20 ¨> 2 OH- + ff)
Overall: 2 H2O --> 02 + 2H2
Combining the anolyte and catholyte maintains pH neutrality in the electrode
rinse
stream. The cell had 5 sets of membranes. Neosepta AHA membrane was used for
the anion exchange membrane, Nafion 115 for the cation membranes and BP-1 for
the
bipolar membrane for all experiments.
The rest of the electrodialysis setup consisted of four solution reservoirs
for feed,
"acid", base and electrode rinse streams, corresponding centrifugal drive
pumps for
solution recirculation, tubing, fittings and valves. Instrumentation included
pH,
temperature and flow measurement for the feed and "acid" and flow for the
base.
"Acid" pH was controlled by automatically using electrically activated valves
to
allow introduction of ammonia. The feed pH was not controlled. The "acid"
reservoir was water jacketed which allowed temperature control at 40 C. A DC
power supply capable of 10 amps and 25 volts output was used to control the
process
and an Arbin Battery Cycler was used to record data.
Samples of anolyte ((NH4)7S203) and feed (Na2S203) were taken at the start and
end
of each experiment and analyzed for ionic species using a Dionex Ion
Chromatograph
equipped with either the IonPac CS12A analytical column (for NH4+ and Na+
species) or the IonPac AS17 column (for S2032- and C032- species), CD25 (or
CD25A) conductivity detector, and Peaknet 6.3 data acquisition software.
Catholyte
samples were also analyzed for NH4 + in this manner.
The results of the tests are shown in Table 6.
Table 6: Current Efficiencies and Solution Concentrations for Sodium
Thiosulfate
Epdm Salt Split
Run Current Solution Base Base "Acid" "Acid"
Density Initial CE Initial CE
(mA/cm2) Final Final
M / % M / %
DNISI egah 05700M00001% 2599209v7 2 1
CA 02592926 2007-07-03
617-47 40 Feed: 1.5 M 0.4/1.5 83 1.5/20.3 91 thio
Na2S203 1.6/5.8 1.8/23.7
"Acid": 1.5 M
(NH4)2S203
617-51 40 Feed: 1.5M 0.4/1.5 76 1.5/20.2 63 thio
Na2S203 1.6/6.2 1.6/20.9 (12%
"Acid": 1.5 M bicarb)
(NH4)2S203 (14%
0.4 M NaHCO3 NH4)
to both
617-51A 50 Same as 617- 1.6/6.2 68 1.6/20.9 76 thio
51 2.6/9.6 1.9/24.4 (6%
bicarb)
(9%
NH4)
617-60 80 Feed: 1.5 M 0.4/1.5 82-86 3.0/38.4 73-
90
Na2S203 2.6/9.4 3.2/38.8 thio
"Acid": 3.0 M (6%
(NH4)2S203 bicarb)
0.4 M NaHCO3 (14%
to both NH4)
617-63 80 Feed: 0.75 M 0.4/1.6 86 1.4/18.9 64 thio
Na2S203/0.75 2.6/9.4 1.7/22.4 (24%
M NaHCO3 bicarb)
"Acid": 1.5 M (12%
(NH4)2S203/0.4 NH4)
M NaHCO3
N1215/AHA/BP1. *Numbers in parenthesis are percentages of current attributable
to transport of other
species. Anolyte pH control set point 8.35. Base start concentration 0.4 M.
Salt splitting experiments where both the feed solution and ammonium
thiosulfate
product solution were buffered with bicarbonate were performed. The
bicarbonate
prevented the feed pH from becoming too basic due to hydroxide back migration
across the cation membrane. The bicarbonate in the ammonium thiosulfate
product
was converted to ammonium carbamate (by addition of ammonia) which buffered
the
acid produced at the bipolar membrane. These additions allowed the production
of a
mixed ammonium thiosulfate, bicarbonate, and carbamate solution, at maximum
commercially allowed current density. Excellent current efficiencies (over 90%
for
combined thiosulfate and bicarbonate) were obtained.
The previous description of the disclosed embodiments is provided to enable
any
person skilled in the art to make or use the present invention. Various
modifications
to those embodiments will be readily apparent to those skilled in the art, and
the
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CA 02592926 2014-01-15
generic principles defined herein may be applied to other embodiments. Thus,
the
present invention is not intended to be limited to the embodiments shown
herein, but
is to be accorded the full scope, wherein reference to an element in the
singular, such
as by use of the article "a" or "an" is not intended to mean "one and only
one" unless
specifically so stated, but rather "one or more". Moreover, nothing disclosed
herein is
intended to be dedicated to the public.
WSLegal\037008\00001\2599209v8 23
