Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD FOR SEPARATING HYDROGEN SULPHIDE
FROM EFFLUENT GAS
Field Of Invention
The invention relates to a method for treating effluent gas containing
hydrogen
sulphide that is generated from a gas treating process such as a gas
sweetening
process. More particularly, the invention pertains to a method for selectively
separating hydrogen sulphide from the effluent gas stream using sea water.
Background Of The Invention
Natural gas as well as other hydrocarbon gases from the oil and gas production
usually contain significant amounts impurities, particularly hydrogen sulphide
and
carbon dioxide. Gases containing hydrogen sulphide are known as sour gas and
they
are undesirable due to their toxic and corrosive properties. Therefore, the
sour gas
must be treated to remove the hydrogen sulphide and carbon dioxide to meet
downstream production or export requirements or venting to atmosphere.
Hydrogen sulphide and carbon dioxide are commonly removed from the sour gas by
absorption in an alkaline solution. The sour gas is subjected to an amine gas
treating
process, where the sour gas is fed into a tower containing an amine solution
which
absorbs hydrogen sulphide and carbon dioxide as it passes through. The
solution most
widely used is an aqueous solution of alkanolamine, such as monoethanolamine,
diethanolamine and methyldiethanolamine. The resultant gas stream is
substantially
free of the impurities, whilst the amine solution leaving the absorption tower
is rich in
the absorbed gases (which are hydrogen sulphide and carbon dioxide).
The absorbed gases are removed from the amine solution using a regenerator by
preheating the solution and stripping hydrogen sulphide and carbon dioxide,
from the
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solution in a reboiler. The effluent gas therefrom, which contains high
concentrations
of hydrogen sulphide and carbon dioxide, has to be disposed in a safe manner
to avoid
negative impacts on either workers or the environment. Where the concentration
of
the gases components in the effluent gas meets the environmental and safety
regulatory limits for atmospheric disposal, it is possible to vent the
effluent gases.
However, if the gases are present in high concentration, it is not safe and
environmentally acceptable to vent the gases into the atmosphere. Therefore,
there
exists a need for a method for selectively and effectively removing or
reducing
hydrogen sulphide content prior to disposal of the effluent gas.
Dissolved hydrogen sulphide is known to react readily with dissolved oxygen in
the
seawater and decomposes into various products such as sulphates, sulphites
etc. which
have less negative impact to the marine environment. It is thus possible to
extract the
hydrogen sulphide in a single-stage or a multistage scrubbing system using sea
water
from the ocean and then dispose the scrubber effluent into the ocean. Further
decomposition of the dissolved hydrogen sulphide into various products will
then
take place in the ocean which is rich in dissolved oxygen.
Summary Of Invention
One of the objects of the present invention is to provide a method for
treating sour
effluent gas generated from a sweetening process. Specifically, the method
provided is
capable of selectively separating hydrogen sulphide from the effluent gas
stream to
substantially reduce the hydrogen sulphide content therein, thus eliminating
air
pollution problems.
Another object of the invention is to introduce an effective absorbing agent
for
removing hydrogen sulphide from the effluent gas stream generated from a gas
processing unit such as a gas sweetening process. Fresh sea water that
contains rich
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dissolved oxygen is preferred as the absorbing agent as it is effective in
dissolving and
decomposing the hydrogen sulphide.
Still another object of the invention is to provide a method for further
reducing
hydrogen sulphide content in an effluent gas stream released from a gas
sweetening
process. It can be achieved by introducing dissolved alkaline agent, such as
carbonates, into the sea water to increase alkalinity and thereby enhance the
efficiency
of hydrogen sulphide removal.
Yet another object of the invention is to provide a method for further
reducing
hydrogen sulphide content in an effluent gas stream by introducing catalyst,
such as
transition metals in form of dissolved salts, into the sea water to increase
the rate of
decomposition of dissolved hydrogen sulphides into products such as sulphates,
sulphites etc. that have less negative impact to the marine environment
At least one of the preceding objects is met, in whole or in part, by the
invention, in
which one of the embodiments of the invention describes a method for
separating
hydrogen sulphide from effluent gas released from a gas sweetening process,
the
method comprising the steps of feeding the effluent gas into at least one
scrubber, the
scrubber operating at a pressure less than 100 psig (or 700 kPa); and feeding
sea water
into the scrubber counter-currently to the direction of the gas flow, so that
the effluent
gas is contacted with the sea water thereby to reduce the hydrogen sulphide
content in
the effluent gas stream to less than 0.50% by weight by dissolution in the sea
water.
The sea water may contain dissolved alkaline agent or transition metals which
can act
as catalyst. If required, an additional metal catalyst may be added into it
prior to the
scrubber process, in order to further reduce the hydrogen sulphide content in
the
effluent gas stream.
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In another embodiment of the invention, the method further comprises the step
of
discharging the sea water with dissolved hydrogen sulphide from the scrubber
back
into the ocean.
The method may also comprise additional step of treating the contaminated sea
water
prior to disposal to the ocean.
The method according to the preceding embodiment further comprises the step of
releasing the effluent gas stream with reduced hydrogen sulphide content into
atmosphere or for use as a fuel gas or for further processing of the remaining
stream
as necessary.
One skilled in the art will readily appreciate that the invention is well
adapted to carry
out the aspects and obtain the ends and advantages mentioned, as well as those
inherent therein. The embodiments described herein are not intended as
limitations on
the scope of the invention.
Brief Description Of Drawings
For the purpose of facilitating an understanding of the invention, there is
illustrated in
the accompanying drawing the preferred embodiments from an inspection of which
when considered in connection with the following description, the invention,
its
construction and operation and many of its advantages would be readily
understood
and appreciated.
Figure 1 is the schematic diagram of the process flow showing a method
for
separating hydrogen sulphide from an effluent gas stream by using a
single scrubber, in accordance with a preferred embodiment.
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Figure 2 is the schematic diagram of the process flow showing a method
for
separating hydrogen sulphide from an effluent gas stream by using
more than one scrubber, according to another preferred embodiment.
5 Detailed Description Of The Invention
Hereinafter, the invention shall be described according to the preferred
embodiments
of the present invention and by referring to the accompanying description and
drawings. However, it is to be understood that limiting the description to the
preferred
embodiments of the invention and to the drawings is merely to facilitate
discussion of
the present invention and it is envisioned that those skilled in the art may
devise
various modifications without departing from the scope of the appended claim.
The invention provides a method for treating effluent gas generated from a gas
sweetening process. More particularly, the method utilizes fresh sea water as
an
absorbing agent to effectively and selectively separate hydrogen sulphide
present in
the effluent gas stream.
Although the method is depicted and described herein for treating effluent gas
generated from a gas sweetening process, it should be noted that the effluent
gas to be
treated shall not be limited thereto or thereby, but it also includes the
waste gas from a
sour gas field and those associated with production and refining of crude
petroleum
oil or any such processes as to generate an effluent of sour gas composition
that
requires further sweetening treatment. Correspondingly, it should be
appreciated to
note that the term "effluent gas", "acid gas", "waste gas", "exhaust gas" or
any like
term can be used interchangeably herein throughout the description and shall
refer to
a gas stream containing 0.50 wt% (which is equivalent to 5000 ppm) or more
hydrogen sulphide with the remaining made up of hydrocarbon, carbon dioxide,
moisture, nitrogen and other possible impurities in gaseous form, that the
hydrocarbon
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is present in a minor amount therein, preferably a few percent.
Referring now to the present invention, the method for separating hydrogen
sulphide
from an effluent gas stream comprises the steps of feeding the effluent gas
into at least
one scrubber, the scrubber operating at a pressure less than 100 psig (or 700
kPa); and
feeding sea water into the scrubber counter-currently to the direction of the
gas flow,
so that the effluent gas is contacted with the sea water thereby to reduce the
hydrogen
sulphide content in the effluent gas stream to less than 0.5% by weight by
dissolution
in the sea water.
In accordance with the preceding description, at least one scrubber which can
either
be a packed bed type, a structured packing type or a type configured to have
flow
distributors positioned to at least one of its bottom, top and intermediate
locations is
used to remove hydrogen sulphide from the effluent gas. Nevertheless, the
number of
scrubbers is dependent on flow rate or amount of the effluent gas to be
treated. In the
industrial plant, the effluent gas stream flow rate is usually ranging from 10
to 200
MMSCFD (or approximately 3 to 7 m3/s). At higher flow rate, the exhaust gas
stream
is preferably divided into several parallel streams and routed to a
predetermined
number of scrubbers. The scrubbers can be arranged in series, in parallel to
each other
or in a combination thereof to ensure efficient design of the overall system
for
removing hydrogen sulphide therefrom to the required emission specification.
Reference is now made specifically to Figure 1 when a single scrubber is used
to
separate hydrogen sulphide from the effluent gas. The scrubber is configured
to
receive the effluent gas stream feeding near bottom of the scrubber whilst the
absorbing agent is routed to the opposite end of the scrubber to the entry
point of the
gas stream, such that the gas stream and the liquid stream (i.e. the absorbing
agent)
are fed counter-currently. The gas and the liquid streams can also be fed to
the
scrubber co-currently, continuously or intermittently, depending on operating
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preference. It should be noted that the absorbing agent referred herein is
fresh sea
water taken from the surrounding ocean. Fresh sea water is used due to its
rich
dissolved oxygen concentration, as dissolved oxygen is capable of dissociating
and
decomposing the dissolved hydrogen sulphide into products such as sulphates,
sulphites etc. which have less negative impact to the marine environment.
Inside the scrubber, the hydrogen sulphide in the effluent gas stream is first
absorbed
into the sea water based on Henry's Law. Mass transfer of hydrogen sulphide
from
gas stream to liquid stream is further enhanced by using counter-current flow
in the
scrubber, where the fresh sea water is exposed to lower hydrogen sulphide gas
concentration to enhance hydrogen sulphide removal. The hydrogen sulphide
absorbed into the sea water then begins to interact with the dissolved oxygen
and
other constituents naturally present in the sea water to produce various
compounds
such as sulphates, sulphites etc., thereby reducing the hydrogen sulphide
content in
the effluent gas.
If more than one scrubber is used as illustrated in Figure 2, the scrubbers
may be
configured to receive the fresh sea water feeding at the top of each scrubber,
whilst
the effluent gas stream is fed into the first scrubber and the treated
effluent gas stream
from the first scrubber will then be fed into the second scrubber.
Correspondingly, it is
worth noting that the treated gas stream from the N-th scrubber will be the
inlet feed
of the (N+1)-th scrubber. It should also be appreciably noted that the
effluent gas or
the treated effluent gas stream is fed to the respective scrubber near the
bottom
thereof, so that the gas flows upwards and counter-currently to the direction
of the sea
water.
According to the preferred embodiment of the invention, the effluent gas
stream flows
into the scrubber (or scrubbers) at a temperature close to but not exceeding
by 30 C
than that of the fresh sea water. In addition, the effluent gas can be pre-
processed
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before being subjected to the scrubber or scrubbers using an acid gas removal
unit or
an acid gas enrichment unit, where the unit can be solvent based, sorbent
based or
membrane based, in order to prepare the effluent gas stream in the desired
operating
conditions for sea water scrubbing. Further, the effluent gas stream may also
require
adequate conditioning by utilizing a knock-out scrubber, a heater or a
combination
thereof, so as to minimize any liquid drop-out within the scrubber.
Whilst the sea water is filtered and treated if necessary, it facilitates to
ensure that the
scrubber is not severely fouled during operation. The amount of sea water
required
should be such that there is adequate sea water to absorb the hydrogen
sulphide in the
incoming gas stream and the seawater contains enough dissolved oxygen for
hydrogen sulphide oxidation to be initiated.
In another preferred embodiment of the invention, the sea water can include
dissolved
alkaline agent therein, such as carbonates, hydroxides, bicarbonates and other
buffer
solutions. Such alkaline agent can be added to the sea water stream either at
the inlet
or at predefined positions of the scrubber. It should be appreciably noted
that presence
of the alkaline agent in the sea water substantially improves the efficiency
in
removing hydrogen sulphide from the effluent gas by converting it to
bisulphide ions
(HS) and hydrosulphides and forming products such as sulphates, sulphites,
etc., thus
reducing the dissolved hydrogen sulphide content to a concentration, either as
required by the environmental regulation set forth by the respective country
or
according to the system setup such as number of scrubbers. However, selection
of the
alkaline agent is not limited thereto or thereby. Transition metals in form of
salts can
also be added to and dissolved in the sea water prior to scrubbing, for
example, iron
(II) ions (Fe2'). These metal ions are known to enhance or catalyze the
dissociation
reaction of hydrogen sulphide to other compounds in the sea water. In
addition, the
sea water can further be aerated by bubbling air into the sea water prior to
scrubbing,
so as to substantially increase the dissolved oxygen content to enhance
hydrogen
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sulphide dissociation by oxidizing and converting the dissolved hydrogen
sulphide to
products such as sulphates, sulphites, etc. which has less negative impact to
environment.
It is also desirable in the present invention that the scrubber is configured
to operate at
low pressure preferably less than 100 psig (approximately 700 kPa or 6 bar).
One
skilled in the art will appreciate the fact that high operating pressure is
not required in
the scrubber depicted herein, because of the absence of large amounts of
hydrocarbon
components in the exhaust gas. High operating pressure is required when
significant
amount of hydrocarbon is present in the gas stream, so as to create a partial
pressure
sufficiently large to separate hydrogen sulphide preferably from the exhaust
gas
stream.
In still another preferred embodiment of the invention, the method further
comprises
the step of discharging the sea water contaminated with dissolved hydrogen
sulphide
from the scrubber back to the ocean at a distance and depth sufficient in
order to avoid
the sea water contaminated with dissolved hydrogen sulphide from contaminating
the
fresh sea water intake for the scrubber, the processing facility, the seabed
or the ocean
surface. More specifically, the contaminated sea water is first routed to a
collector
where the sea water from the scrubber (or the scrubbers) are comingled and
then
routed to the subsea discharge system. The subsea discharge system preferably
comprises a single discharge pipe or a network of pipes with each discharge
pipe
being connected to a subsea diffuser outlet. The diffuser outlet is designed
such that it
allows maximum entrainment of fresh sea water and dissolved oxygen to
accelerate
and continue dissociation of hydrogen sulphide and its reactions. Ambient
ocean
currents assist to dissipate the dissolved hydrogen sulphide for interaction
with more
fresh sea water in the ocean, thereby effectively reducing the concentration
of
dissolved hydrogen sulphide in the discharge stream. Typically, the seawater
discarded contains a negligible increase in dissolved sulphates therein than
in the
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fresh seawater used. The resulting subsea plume presents a localized but
transitory
impact to the ocean environment due to constant changes in current direction
and
strength in the open ocean and hence, gives no detrimental environmental
effects on
the ecosystems of the ocean. Further, the increase of the seawater effluent
temperature
5 caused by
the scrubbing operation is very slight due to the high heat capacity of sea
water and thus, any potential thermal pollution can be avoided.
Meanwhile, the effluent gas with reduced hydrogen sulphide content from the
scrubber can be disposed by releasing into atmosphere, depending on the
10
environmental and safety regulatory requirement. Alternatively, the treated
effluent
gas from the scrubber is routed to a combustor for producing energy upon
combustion
with a fuel gas and to oxidize any remnants of contaminants. However, the
treated
effluent gas may still contain the less absorbable components such as carbon
dioxide,
nitrogen, hydrocarbon and other impurities. Therefore, the treated exhaust gas
may
also be routed to other post-treatment system for further processing prior to
disposal
so as to ensure that the gas to be disposed is no longer detrimental to the
environment
and personnel.
In a further embodiment of the invention, the scrubber or scrubbers can be
equipped
with appropriate monitoring and control instrumentation, such as pressure and
temperature monitoring as well as level, flow control and dissolved oxygen
monitoring. The scrubber can also include internal parts facilitating for
effective
distribution of the streams such as distributor trays and sprays. Other
internal
compartments such as demisters or other liquid droplet removal mechanisms may
also
be included to prevent sea water from being carried over into the exiting
treated gas
stream.
Further, since the scrubber operates in low pressure and that the scrubber is
used to
separate hydrogen sulphide using sea water, the scrubber is preferably
constructed
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using a material resistant to corrosion caused by the sea water as well as the
corrosive
components in the effluent gas (hydrogen sulphide and carbon dioxide in
particular).
The material used can be any one or a combination of stainless steel, glass,
polymeric
plastic, glass reinforced epoxy and fiber reinforced plastic. However, the
material
used should not be limited thereto or thereby, as other appropriate materials
could be
used for the design and operating conditions.
Though Figure 1 shows that the scrubber is positioned and operating at above
sea
level, it shall be noted that this does not limit the position of the scrubber
thereto, but
the scrubber can be positioned and operating at subsea level (or below sea
level).
The disclosure includes as contained in the appended claims, as well as that
of the
foregoing description. Although this invention has been described in its
preferred
form with a degree of particularity, it is understood that the disclosure of
the preferred
form has been made only by way of example and that numerous changes in the
details
of construction and the combination and arrangements of parts may be resorted
to
without departing from the scope of the invention.
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