Patent Application
20230088039
The disclosure relates to the purification of gases, and more specifically to the removal of hydrogen sulfide from natural gas using a chemical scavenger.
Hydrogen sulfide (H2S) is a light, volatile compound that is a poisonous and corrosive contaminant of natural gas and crude petroleum. While only relatively small amounts of H2S occur in crude petroleum, natural gas can contain up to 40% by volume. As a result, H2S must be removed to acceptable levels prior to delivery to a refinery or main gas distribution system. Generally, in order to meet governmental, technical and natural gas sales specifications, H2S concentrations must be at very low levels (usually <4 ppm).
Natural gas containing a high concentration of sulfur is commonly referred to as sour gas. Removing H2S from sour gas is referred to as gas sweetening. Natural gas is generally considered sour if there are more than 5.7 mg of H2S per cubic meter of natural gas, which is equivalent to approximately 4 ppm by volume under standard temperature and pressure.
Hydrogen sulfide is a covalent hydride structurally related to water (H2O) as oxygen and sulfur occur in the same periodic table group. However, hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H+ and the hydrosulfide anion HS−l :
H2S→HS−+H+
Hydrogen sulfide reacts with many metals cations to produce the corresponding metal sulfides.
In oil and gas processing, chemical scavengers are often used for removing H2S from natural gas. Chemical scavengers are broadly divided into regenerative and non-regenerative H2S scavengers.
Regenerative scavengers comprising an amine solution are commonly used in large production facilities for sweetening sour natural gas. These processes are known simply as the ‘amine processes’, or alternatively as the Girdler process, and are commonly used in North American gas sweetening operations. Generally, the sour gas is run through a tower, which contains the amine solution. The amine solution has an affinity for sulfur and absorbs it much like glycol absorbing water. There are several amine solutions that are commonly used, including monoethanolamine (MEA), methyldiethanolamine (MDEA), and diethanolamine (DEA), each of which in their liquid form will absorb sulfur compounds from natural gas as it passes through the column. The effluent gas or sweet gas is virtually free of H2S compounds. Like the process for NGL extraction and glycol dehydration, the amine solution used can be regenerated (that is, the absorbed sulfur is removed), allowing it to be reused to treat more sour gas. This technology is relatively capital intensive and is generally more suitable for larger scale operations.
In other systems, non-regenerative H2S scavengers are used to sweeten natural gas. Non-regenerative scavengers include triazine, solid scavengers (generally zinc or iron-based materials), oxidizing chemicals (e.g. NaClO2, NaBrO3, NaNO2, etc.) aldehydes (e.g. glyoxal), and metal carboxylates and chelates. Scavengers react with H2S to eliminate it and sweeten the gas, typically in a contact tower or by direct in-line injection. The resulting “spent” scavenger is then separated from the sweetened gas and disposed of.
Triazine-based scavengers are the most commonly used non-regenerative liquid scavengers for sweetening natural gas. A triazine is one of three organic chemicals with a 6-member cyclic molecule with the formula C3H3N3. The three isomers of triazine, as shown below, differ based on the positions of their nitrogen atoms and are referred to as 1,2,3-triazine, 1,2,4-triazine, and 1,3,5-triazine.
Variations of triazine exist involving substitutions of the hydrogen atoms with other functional groups to result in different reactivity with H2S and changes in solubility of triazine and/or the reactant products. Triazine-based scavengers are generally composed of a reacted product of an aldehyde and ethanolamine. The most common triazine scavenger is an amine-aldehyde condensate manufactured by an exothermic reaction of monoethanolamine (MEA) and formaldehyde. Water and methanol are usually required to keep the formaldehyde in solution and prevent polymerization. The resulting “scavenger” product is a hexahydrotriazine and is usually just referred to as “triazine” in the industry. The “triazine” is typically offered in a water-based solution.
Triazine-based scavengers react specifically with H2S and light mercaptans, using a substitution reaction that is not reversible and has a limited uptake capacity. Triazine may reduce H2S concentrations to as low as 0 ppm and partially remove some light mercaptans (methyl, ethyl and propyl).
One mole of triazine scavenger has three reaction sites, each of which can react with one mole of H2S. The first two reactions occur very fast to form dithiazine, and the third reaction is somewhat slower to forms trithiazine. The three reactions are shown in
During gas sweeting with triazine, the optimal reaction is for two H2S molecules to react with 100% of the triazine to form dithiazine. This is referred to as a 100% spent reaction or a reaction with 100% efficiency, wherein the triazine is “fully consumed” or “fully spent”. If the reaction goes beyond this to the third reaction where a third H2S molecule reacts with triazine to form trithiazine, the triazine is considered “overspent”. “Overspending” is generally avoided because trithiazine precipitates out of solution and settles in equipment which creates problematic build-up and plugs line in gas processing systems and downstream systems. Trithiazine can be difficult to remove from equipment and lines, and generally needs to be removed with mechanical means.
Triazine scavenger systems for removing H2S have a relatively low capital equipment cost, but the cost of the chemical scavenger is relatively high, for example $3 to $10 per gallon. This results in the overall process cost of H2S removal from about $5 to $15 USD per pound of H2S removed. The triazine scavenger system is generally the preferred system for offshore gas treatment and onshore sites where there is a relatively small amount of H2S that needs to be treated.
There continues to be a need for a technology that improves the efficiency of utilization of scavenger reagent, such that the overall process economics can be improved.
In accordance with the disclosure, there are provided methods and systems for removing hydrogen sulfide from natural gas and for retrofitting existing gas sweetening systems.
In accordance with one aspect of the disclosure, there is provided a method for removing hydrogen sulfide from natural gas comprising the following steps in any order:
The second partially consumed scavenger produced in step d) may be used as at least part of the partially consumed triazine-based scavenger for step a). In step a), a second fresh triazine-based scavenger may be reacted with the sour natural gas along with the partially consumed triazine-based scavenger.
In step c), separating any fully consumed scavenger may include separating out byproducts created during the reaction of the scavenger with hydrogen sulfide. After the fully consumed scavenger has been separated from the partially and/or fully consumed scavenger in step c), any remaining partially consumed scavenger may be re-used in step a) for reacting with the sour natural gas.
The amount of the fresh triazine-based scavenger provided in step d) may be controlled based on a hydrogen sulfide concentration of the partially sweetened natural gas to prevent the fresh triazine-based scavenger from being fully consumed when the sweetened natural gas is produced.
In one aspect of the disclosure, there is provided a system for removing hydrogen sulfide from natural gas comprising:
a first vessel for receiving a sour natural gas and a partially consumed triazine-based scavenger, wherein the partially consumed triazine-based scavenger reacts with the sour natural gas to produce a partially sweetened natural gas and a partially and/or fully consumed scavenger, wherein in fully consumed scavenger, three reaction sites of triazine molecules have undergone a sulfur molecule substitution to form trithiazine;
a second vessel operatively connected to the first vessel for receiving the partially sweetened natural gas from the first vessel and a fresh triazine-based scavenger, the fresh triazine-based scavenger reacting with the partially sweetened natural gas to produce a sweetened natural gas and a second partially consumed scavenger;
a first separation vessel operatively connected to the first vessel for receiving the partially and/or fully consumed scavenger from the first vessel and separating any fully consumed scavenger, including any trithiazine that has precipitated out from the consumed scavenger;
a scavenger delivery system operatively connected to the first vessel and the second vessel for delivering the partially consumed triazine-based scavenger to the first vessel and the fresh triazine-based scavenger to the second vessel; and
a control system controlling a flow of the fresh triazine-based scavenger to the second vessel based on a hydrogen sulfide concentration in the partially sweetened natural gas to prevent the fresh triazine-based scavenger in the second vessel from being fully consumed when the sweetened natural gas is produced.
The system may comprise a second separation vessel operatively connected to the second vessel for receiving and separating the sweetened natural gas and the partially consumed scavenger. The sweetened natural gas and the partially consumed scavenger may be separated from each other in the second vessel. The second partially consumed scavenger produced in the second vessel may be introduced into the first vessel as at least part of the partially consumed triazine-based scavenger via the scavenger delivery system.
In the system, separating the fully consumed scavenger may include separating any byproducts formed by the reaction of the partially consumed triazine-based scavenger with hydrogen sulfide.
In the system, the control system may comprise a hydrogen sulfide sensor operatively connected between the first vessel and the second vessel for measuring the hydrogen sulfide concentration in the partially sweetened natural gas exiting the first vessel, and wherein the control system is responsive to the hydrogen sulfide concentration exiting the first vessel to increase or decrease the flow of the fresh triazine-based scavenger to the second vessel. The control system may be responsive to the hydrogen sulfide concentration exiting the first vessel to increase or decrease the flow of the partially consumed triazine-based scavenger to the first vessel.
In the system, the first vessel may receive a second fresh triazine-based scavenger for reacting with the sour natural gas along with the partially consumed triazine-based scavenger.
Any partially consumed scavenger in the first separation vessel may be recirculated back into the first vessel for further reaction with the sour natural gas.
The first vessel may comprise:
a first inlet for receiving the sour natural gas;
a second inlet for receiving the partially consumed triazine-based scavenger;
a contacting member inside the first vessel, wherein the contacting member includes a cavity for reacting the sour natural gas with the partially consumed triazine-based scavenger;
a separation member inside the first vessel for causing the separation of the partially sweetened natural gas from the partially and/or fully consumed scavenger;
a gas outlet through which the partially sweetened gas exits the first vessel; and
a second outlet through which the partially and/or fully consumed scavenger exits the first vessel.
The contacting member may include a wall separating the cavity from a channel, wherein the partially and/or fully consumed scavenger flows from the separation member to the second outlet through the channel. The contacting member may be a tube-shaped member, wherein the cavity is inside the wall and the channel is outside the wall. The separation member may be a perforated plate.
In one aspect of the disclosure, there is provided a system for retro-fit connection to an existing gas sweetening system, the system for pre-treating natural gas prior to the natural gas entering the existing gas sweetening system, the system comprising:
a first vessel for operative connection to the existing gas sweetening system, the first vessel for receiving a sour natural gas and a partially consumed triazine-based scavenger, wherein the partially consumed triazine-based scavenger reacts with the sour natural gas to produce a partially sweetened natural gas and a partially and/or fully consumed scavenger, wherein the partially sweetened natural gas is transported to the existing gas sweetening system for further treatment; wherein in fully consumed scavenger, three reaction sites of triazine molecules have undergone a sulfur molecule substitution to form trithiazine;
a first separation vessel operatively connected to the first vessel for receiving the partially and/or fully consumed scavenger from the first vessel and separating any fully consumed scavenger, including trithiazine that has precipitated out from the partially and/or fully consumed scavenger; and
a scavenger delivery system operatively connected to the first vessel for delivering the partially consumed triazine-based scavenger to the first vessel.
The partially consumed triazine-based scavenger received in the first vessel may be received from the existing gas sweetening system where it became partially consumed during operations in the existing gas sweetening system.
The operative connection of the first separation vessel and the first vessel may allow for any partially consumed scavenger from the first separation vessel to be recirculated back to the first vessel for further reaction with the sour natural gas.
In one aspect of the disclosure, there is provided a method for retrofitting an existing gas sweetening system to include a pre-treatment system, comprising the following steps in any order:
providing a first vessel and operatively connecting the first vessel to the existing gas sweetening system, the first vessel for receiving a sour natural gas and a partially consumed triazine-based scavenger to react the partially consumed triazine-based scavenger with the sour natural gas to produce a partially sweetened natural gas and a partially and/or fully consumed scavenger, wherein in consumed scavenger, three reaction sites of triazine molecules have undergone a sulfur molecule substitution to for trithiazine; and
providing a first separation vessel and operatively connecting the separation vessel to the first vessel, the first separation vessel for receiving the partially and/or fully consumed scavenger and separating any fully consumed scavenger, including trithiazine that has precipitated out from the partially and/or fully consumed scavenger;
wherein the operative connection of the first vessel to the existing gas sweetening system allows for the flow of partially sweetened natural gas from the first vessel to the existing gas sweetening system for further treatment.
The partially consumed triazine-based scavenger may be provided from the existing gas sweetening system.
Various objects and features of the disclosure will be apparent from the following description of particular embodiments of the disclosure, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the disclosure. Similar reference numerals indicate similar components.
With reference to the figures, systems and methods for removing H2S from natural gas are described.
In past systems, the reaction of triazine and H2S was ideally controlled to consume the triazine scavenger as much as possible without overspending it, which meant reacting two moles of H2S with one mole of triazine to form dithiazine, to the fullest extent possible, while avoiding any reaction of a third mole of H2S to form trithiazine, since trithiazine would precipitate out of the solution and cause problems. Practically speaking, it can be very challenging to obtain the ideal extent of the reaction at all times due to fluctuations in H2S levels in natural gas. In order to avoid overspending the triazine, the triazine is often underreacted, leaving some reaction sites in the triazine unreacted (i.e. not substituted with sulfur molecules). Since triazine is not reusable and is typically the most expensive part of the natural gas sweetening process, not fully utilizing the triazine can unnecessarily increase processing costs.
The inventor has realized the need to improve the systems and methods for sweetening natural gas with triazine scavenger to fully utilize the capacity of the scavenger to provide a more cost-effective system and method. The systems and methods are now described with reference to the figures.
For the purposes of illustration, components depicted in the figures are not necessarily drawn to scale. Instead, emphasis is placed on highlighting the various contributions of the components to the functionality of various aspects of the disclosure. A number of possible alternative features are introduced during the course of this description. It is to be understood that, according to the knowledge and judgment of persons skilled in the art, such alternative features may be substituted in various combinations to arrive at different embodiments of the present disclosure.
A typical prior art system used for sweetening natural gas using a triazine scavenger is illustrated in
In the prior art system, the triazine scavenger is added in significant excess to ensure that the H2S is fully removed and that the scavenger does not overreact and form a solid precipitate, but instead stays in liquid form. Due to normal fluctuations in the H2S concentration entering the column 10, and to provide an appropriate margin to ensure the gas is fully sweetened but the scavenger is not overreacted, significant amounts of scavenger delivered to the consumed scavenger tank 16 are not fully reacted.
The inventor previously came up with a “polishing system” that is described in U.S. Pat. No. 7,935,323 and is used in conjunction with the prior art system described above to more efficiently utilize the scavenger. One embodiment of the polishing system is illustrated in
The subject system adds a pre-treatment system to an existing gas sweetening system. The pre-treatment system is designed to “overspend” the triazine scavenger to force a third mole of H2S to react with the scavenger to form trithiazine which is insoluble and precipitates out of the liquid. Overspending a scavenger to form a precipitate has been avoided in prior art triazine scavenger systems, as discussed above, due to the problems the buildup of precipitate causes in equipment and pipelines. However, the inventor has come up with a system that forces overspending of the scavenger and which can handle the precipitate without negatively impacting the existing gas sweetening system and downstream piping. This allows for triazine scavenger to be more fully utilized for removing H2S, thereby decreasing the amount of scavenger required to remove a certain amount of H2S from gas, and decreasing costs.
The improved system is illustrated in
The contactor column 10 may include internal screens and random packing to force contact between the gas and scavenger in the column. Gas and scavenger enter the column at the bottom, flow up through the column as they react to sweeten the gas, then exit the top of the column.
The contactor column 10 and separator 12 have been illustrated and described as being two separate vessels, however they may also be combined into one vessel which acts as both a contactor and a separator.
The pre-treatment system 40 includes a first vessel 42 which is also referred to as a pre-treatment contactor herein, and a first separation vessel 44, which is also referred to as a settling tank. In the pre-treatment contactor 42, sour gas is reacted with fresh triazine scavenger and/or partially consumed triazine scavenger to partially sweeten the gas before it flows to the existing gas sweetening system 30. The partially-sweetened gas is then fully sweetened in the existing gas sweetening system 30 by contacting it with fresh scavenger.
The existing gas sweetening system 30 uses a large excess of fresh scavenger to ensure that the scavenger is not fully consumed (i.e. not overspent) in the existing gas sweetening system 30, thereby preventing precipitate from forming which prevents problematic build-up in the existing gas sweetening system and downstream systems.
The pre-treatment system 40 reacts scavenger and sour gas in the pre-treatment contactor 42 to force the third reaction between the scavenger and H2S, creating trithiazine. The trithiazine is then precipitated out in the separation vessel 44 of the pre-treatment system, which may be a settling tank, after which the trithiazine precipitate is processed and disposed of.
In reference to the subject system shown in
Referring to
The sour gas 42a reacts with the scavenger 42c, 42d in the pre-treatment contactor 42 to produce partially sweetened gas 42b. The reacted scavenger then flows to the settling tank 44 as partially/fully consumed scavenger 44a. The partially/fully consumed scavenger 44a may include partially consumed scavenger and/or fully consumed scavenger, depending on the extent it has reacted in the pre-treatment contactor. The partially/fully consumed scavenger 44a is preferably in liquid form but may include solids where trithiazine from fully consumed scavenger has already precipitated out. The settling tank 44 may also receive partially consumed scavenger 12d in liquid form from the existing gas sweetening system 30 (for example, from the separator 12) and/or from another source, which is then directed into the pre-treatment contactor where it can be fully consumed. Alternatively, or in addition, the partially consumed scavenger 12d from the existing gas sweetening system 30 and/or from another source can be directly introduced into the pre-treatment contactor 42 without going through the settling tank 44.
In the settling tank 44, the partially/fully consumed scavenger 44a from the pre-treatment contactor 42 and any partially consumed scavenger 12a that has entered the settling tank 44 from another source reside to allow trithiazine to precipitate out from the liquid scavenger. The solids 44c (i.e. the precipitated trithiazine) are then removed from the settling tank 44, for example through an outlet/drain at the bottom of the tank, after which they can flow to a solids processing system 48 where they are treated (typically dehydrated) and disposed of. The residence time of the scavenger in the settling tank 44 varies depending on the volume of scavenger in the settling tank 44 and the extent to which it is reacted, i.e. the ratio of fully consumed scavenger to partially consumed scavenger. In a typical system, the residence time is from about 30 minutes to about 6 hours. If the scavenger in the settling tank comprises only partially consumed scavenger, no or very low residence time in the settling tank may be needed.
The liquid scavenger in the settling tank 44 is separated based on density, with the less dense liquid byproducts 44b (e.g. water, MEA) of the partially/fully consumed scavenger 44a exiting the settling tank, preferably through an outlet at or near the top of the settling tank, and going to a liquid disposal 50. The denser liquid, being partially consumed scavenger 42d that hasn't been fully consumed, is circulated back to the pre-treatment contactor 42 for further reaction with natural gas in the pre-treatment contactor. By circulating the scavenger between the pre-treatment contactor 42 and the settling tank 44, the scavenger can be fully consumed in the pre-treatment contactor 42 while removing precipitate in a controlled manner from the settling tank 44 to prevent precipitate build-up in the system.
The pre-treatment system 40 includes one or more pumps to move the scavenger through the system. The pre-treatment system also includes a monitoring and control system 52 to monitor the level of H2S in the partially sweetened gas 42b leaving the pre-treatment contactor and to control the pumps, which may include pumps 46a and 46c for pumping fresh scavenger 42c and partially consumed scavenger 42d, respectively, into the pre-treatment contactor 42.
The pre-treatment contactor 42 may include a level controller 54 to monitor and maintain a certain level of scavenger in the pre-treatment contactor 42. The control may be done via a control valve 56 in the line in which partially/fully consumed scavenger 44a exits the pre-treatment contactor.
Inside the vessel is a contacting member 64 having an inner cavity 64a and a wall 64b. The contacting member 64 may be a tube-shaped member which is preferably positioned above the first and second inlets 66, 68. Natural gas and scavenger contact each other and react in the cavity 64a of the contacting member as they flow upwards through the contacting member cavity 64a.
The vessel also includes a separation member 72 for separating reacted scavenger from the natural gas. In the illustrated embodiment, the separation member 72 is a perforated plate positioned above the contacting member 64. When the partially sweetened gas 42b and partially/fully consumed scavenger 44a reach the separation member 72, the gas 42b flows through the perforations in the plate and out of the vessel 42 via a gas outlet 76, which is preferably located at the top of the vessel.
Inside the vessel 42, there is a channel 74 through which partially/fully consumed scavenger 44a flows after it has been separated from the partially sweetened gas 42b by the separation member 72. In the illustrated embodiment, the channel 74 is the annulus between the wall 60 of the vessel and the wall 64b of the contacting member 64. The partially/fully consumed scavenger 44a flows from the separation member 72 through the channel 74 and out a second outlet 78, which may be a drain located at the bottom of the vessel.
The vessel 42 may include one or more large passageways 80 to allow personnel access to the interior of the vessel for inspection, maintenance, repairs and cleaning.
The vessel 42 may also include an auxiliary outlet 86 that can be opened if needed to remove partially consumed scavenger 44a from the vessel to maintain a desired fluid level in the vessel. The opening of the auxiliary outlet 86 may be controlled by the level controller 54.
Use of the vessel 42 as the pre-treatment contactor will now be described with reference to
The subject system can be used to retrofit existing gas sweetening systems 30 with the pre-treatment system 40. To do so, the pre-treatment contactor 42 and separation vessel 44 are provided and operatively connected to the existing gas sweetening system as illustrated in
Although the present disclosure has been described and illustrated with respect to certain embodiments and uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the disclosure as understood by those skilled in the art.
The present application is related to and claims the priority benefit of U.S. Provisional Patent Application No. 63/243,797 filed Sep. 14, 2021, the content of which is hereby expressly incorporated by reference in its entirety into this disclosure.
| Number | Date | Country | |
|---|---|---|---|
| 63243797 | Sep 2021 | US |
