WO2022177420A1 - An acid gas removal/capture system and process for converting carbon dioxide to chemical compounds - Google Patents

An acid gas removal/capture system and process for converting carbon dioxide to chemical compounds Download PDF

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Publication number
WO2022177420A1
WO2022177420A1 PCT/MY2022/050009 MY2022050009W WO2022177420A1 WO 2022177420 A1 WO2022177420 A1 WO 2022177420A1 MY 2022050009 W MY2022050009 W MY 2022050009W WO 2022177420 A1 WO2022177420 A1 WO 2022177420A1
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mixture
solvent
carbon dioxide
photoreactor
acid gas
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PCT/MY2022/050009
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French (fr)
Inventor
Zhe Phak CHAN
Ven Chian QUEK
Masniroszaime M ZAIN
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Petroliam Nasional Berhad (Petronas)
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Publication of WO2022177420A1 publication Critical patent/WO2022177420A1/en

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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C29/00—Preparation of compounds having hydroxy or O-metal groups bound to a carbon atom not belonging to a six-membered aromatic ring
    • C07C29/15—Preparation of compounds having hydroxy or O-metal groups bound to a carbon atom not belonging to a six-membered aromatic ring by reduction of oxides of carbon exclusively
    • C07C29/159—Preparation of compounds having hydroxy or O-metal groups bound to a carbon atom not belonging to a six-membered aromatic ring by reduction of oxides of carbon exclusively with reducing agents other than hydrogen or hydrogen-containing gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
    • B01J19/08—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor
    • B01J19/12—Processes employing the direct application of electric or wave energy, or particle radiation; Apparatus therefor employing electromagnetic waves
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C29/00—Preparation of compounds having hydroxy or O-metal groups bound to a carbon atom not belonging to a six-membered aromatic ring
    • C07C29/74—Separation; Purification; Use of additives, e.g. for stabilisation
    • C07C29/76—Separation; Purification; Use of additives, e.g. for stabilisation by physical treatment
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C29/00—Preparation of compounds having hydroxy or O-metal groups bound to a carbon atom not belonging to a six-membered aromatic ring
    • C07C29/74—Separation; Purification; Use of additives, e.g. for stabilisation
    • C07C29/76—Separation; Purification; Use of additives, e.g. for stabilisation by physical treatment
    • C07C29/78—Separation; Purification; Use of additives, e.g. for stabilisation by physical treatment by condensation or crystallisation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/18—Carbon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/16—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J23/20—Vanadium, niobium or tantalum
    • B01J23/22—Vanadium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/16—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J23/24—Chromium, molybdenum or tungsten
    • B01J23/28—Molybdenum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
    • B01J27/04—Sulfides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/02—Sulfur, selenium or tellurium; Compounds thereof
    • B01J27/057—Selenium or tellurium; Compounds thereof
    • B01J27/0573—Selenium; Compounds thereof
    • B01J35/23—
    • B01J35/39—

Definitions

  • the present invention generally relates to acid gas removal/capture processes, and more particularly to a process for regenerating absorbents in an acid gas removal process.
  • Natural gas although an extremely important source of energy, contains acid gases such as Hydrogen Sulfide (H2S) and Carbon Dioxide (CO2) along with other impurities, which are considered as poisonous pollutants, if not treated or processed properly. Accordingly, removal of acid gases from natural gas streams is crucial particularly in hydrocarbon producing or gas processing plants to comply with sales product standards. This process is carried out in acid gas removal units (AGRU).
  • acid gas removal units AGU
  • An Acid Gas Removal Unit is generally designed to remove acidic components to meet gas sulphur and CO2 specifications.
  • the primary stages in a typical AGRU setting include an absorption stage for the absorption of acid gases by means of an absorber - usually a solvent, in which the choice of solvent is dependent on its capability to absorb or remove the acid gases; and the solvent or absorbent regeneration stage, designed to produce lean solvent (free of acid gases) to be reused in the absorber.
  • Aqueous CO2 absorbent is typically used for the removal of acidic gases but the regeneration of aqueous absorbents consumes a lot of energy while releasing CO2 to the atmosphere. Increased CO2 concentration in the atmosphere contributes to excessive heating of earth’s atmosphere, leading to climate change and affecting living beings.
  • an acid gas removal/capture system for converting carbon dioxide to at least one chemical compound
  • an absorption section for adding a solvent to an acid gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; a regeneration section for separating the chemical compound and the solvent from the mixture; characterised in that: the regeneration section comprises a photoreactor including at least one catalyst for initiating a photoreaction to convert the carbon dioxide in the mixture to said chemical compound.
  • the solvent absorbs the carbon dioxide which can then be converted into useful chemical compounds with the photoreactor, typically by reduction.
  • the amount of carbon dioxide emitted into the atmosphere is reduced.
  • the regeneration section comprises a liquid-gas phase separator for separating the chemical compound from the mixture.
  • the regeneration section includes a pervaporation membrane for separating the chemical compound from the mixture.
  • the regeneration section includes a distillation means for separating the chemical compound from the mixture.
  • the regeneration section includes a flash drum for recovery of light or volatile hydrocarbons from the mixture before it is received by the photoreactor.
  • the catalyst comprises at least one of (graphene, graphene oxide, magnesium oxide, zinc, copper, B12S3 QDs, CdS(Bi 2 S 3 ), CdSe/Pt, ZnPc, silver or nickel) immobilised/doped with T1O2 nanoparticles, T1O2 anatase, ZnO-rGO, molybdenum disulfide-TiC hybrid, amine functionalized T1O2, porous T1O2, anatase T1O2 nanocrystals, fluorinated anatase T1O2 nanosheet, AgBr/C3N4/N-Graphene, atomically thin single-unit- cell B12WO6 layers, B12WO6 hollow microspheres, zinc oxide, lamellar BiVO t , conducting polymer-modified B12WO6 hierarchical hollow microspheres, Bi2S3/CdS, RuC -modified CuxAgylnzZn
  • the photoreactor irradiates the mixture with a light source having a wavelength of about 200 to 700nm.
  • the temperature during irradiation is 20- 65°C, preferably 40-50°C.
  • methanol is produced as the compound under such conditions.
  • the light source has a power of 0.1 to 50mW/cm 2 or any suitable power for intensifying the process.
  • the catalyst and photoreactor wavelength can be adjusted to determine the compound that the carbon dioxide is converted to.
  • the regeneration section includes a pump to facilitate transfer of the regenerated solvent to the absorption section.
  • a process for converting carbon dioxide to one or more chemical compounds in an acid gas removal/capture system comprising: an absorption step including adding a solvent to a gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; and a regeneration step including separating said compounds and regenerating the solvent from said mixture; characterised in that: in the regeneration step the mixture is fed to a photoreactor comprising at least one catalyst; and a photoreaction is initiated to convert the carbon dioxide into said compounds.
  • the compounds include carbon monoxide, methane, methanol, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like.
  • the process further includes feeding the mixture to a flash drum for recovery/removal of volatile and/or light hydrocarbons prior to the photoreactor.
  • the pressure in the photoreactor may be adjusted. Typically the pressure may be up to 80bar before entering the photoreactor, but is reduced before, during or after absorbent regeneration.
  • the solvent is any or any combination of amines suitable for capturing or removing CO2.
  • the amine is an alkanolamine or piperazine.
  • the alkanolamine is diethanolamine (DEA) or methyldiethanolamine (MDEA).
  • the solvent is H 2 O, NaOH, NaHCCh, KrCCh.NarCCE,
  • FIG. 1 provides a schematic diagram of the system in accordance with an embodiment of the present invention
  • FIG. 2 shows a flowchart for the process in accordance with an embodiment of the present invention
  • FIG. 3 shows the experimental examples of preparing the catalyst in accordance with an embodiment of the present invention
  • FIG. 4 shows the experimental examples of preparing the CO2 reduction test in accordance with an embodiment of the present invention
  • FIG. 5 shows the experimental result for methanol production based on two samples
  • FIG. 6 shows the methanol concentration with standard solution MeOH quantified using Gas Chromatography (GC) with Flame-Ionization Detection (FID) Headspace;
  • FIG. 7 shows the experimental results providing the methanol concentration based on three different photocatalysts with MDEA as medium quantified using Gas Chromatography (GC) with Flame-Ionization Detection (FID) Headspace;
  • FIG. 8 shows the experimental results providing the methanol yield based on three different photocatalysts;
  • FIG. 9 shows the assessment results of methanol yield in different temperatures
  • FIG. 10 shows the assessment results of methanol yield under different UV lamp settings.
  • the present invention provides a process whereby acid gases, in particular carbon dioxide (CO2) that is normally produced during regeneration of absorbents for acid gas removal or capture systems, is converted into useful compounds such as those suitable for producing fuels, by way of a photocatalysis process.
  • acid gases in particular carbon dioxide (CO2) that is normally produced during regeneration of absorbents for acid gas removal or capture systems
  • useful compounds such as those suitable for producing fuels, by way of a photocatalysis process.
  • the invention provides a process for converting carbon dioxide to chemical compounds in the presence of a solvent as an absorbent; in an acid gas removal system comprising: feeding a solvent loaded with carbon dioxide to a photoreactor thereby initiating a photoreaction in the presence of at least one catalyst; and reducing the carbon dioxide contained in the solvent into compounds useful for making fuels or value-added products.
  • the absorbent used in the process disclosed herein is Methyldiethanolamine (MDEA) which is widely used in industry due to high absorption capacity, especially for acid gases including CO2.
  • MDEA Methyldiethanolamine
  • the aqueous solution - having water as the solvent to prepare the alkanolamine solution may contain a predetermined percent by weight of the alkanolamine, which may be determined by the type and quantity of gases to be treated or removed.
  • the catalysts used to facilitate or accelerate the conversion of CO2 in the process disclosed herein includes graphene oxide immobilised/doped with T1O2 nanoparticles (rGO-TiCh) and magnesium oxide immobilised/doped with T1O2 nanoparticles (Mg-TiCh).
  • the catalysts may be prepared by various methods known in the art. In one example, the catalysts may be prepared or synthesized by a combination of methods or reactions - sonothermal, hydrothermal and calcination reactions under predetermined conditions.
  • FIG. 1 provides an example of the system for converting carbon dioxide to one or more chemical compounds in an acid gas capture process in accordance with an embodiment of the present invention.
  • the system comprises: an absorption section (100) to receive acid gas stream and adding solvent to said gas stream for carbon dioxide absorption to form a mixture; and a regeneration section (200) to receive the mixture and separating the compounds from the mixture.
  • the regeneration section (200) the mixture is fed to a photoreactor comprising at least one catalyst for initiating a photoreaction to reduce the carbon dioxide into said chemical compounds and to form regenerated solvent.
  • the gas inlet feeds acid gas or natural gas stream into the absorber (12) of the absorption section (100) wherein the absorber (12) includes an inlet for feeding lean solvent solution and an outlet for rich solvent solution, referred herein as lean solvent stream (14A) and rich solvent stream (16A), respectively.
  • lean solvent stream (14A) and rich solvent stream (16A) A portion of gases exits from the absorber (12) by way of an outlet as purified or treated gas.
  • the rich solvent stream (16A) flows into the flash drum or flash tank (22) for recovery of light or volatile hydrocarbons (which can be flashed) prior to entering the photoreactor (24) in the regeneration section (200).
  • the rich solvent containing or loaded with absorbed gases including carbon dioxide
  • lean solvent stream (14A) which is removed from the photoreactor (24) via the designated outlet and flows into a pervaporation membrane (32) prior to entering absorber (12) of the absorption section (100) as regenerated absorbent or solvent.
  • the pervaporation membrane (32) separates the compound of interest from the solvent.
  • the system may further include solvent pumps (40, 42) to facilitate the flow of the regenerated solvent from the regeneration section (200) to the absorption section (100) and a solvent tank (44). It is anticipated that in other embodiments, the pervaporation membrane (32) may be replaced with a distillation system or equivalent apparatus.
  • the pressure in the photoreactor (24) may be adjusted before, during or after the absorbent regeneration process.
  • the photoreactor (24) may be operated at high pressure such that the solvent can be regenerated without reducing the pressure in the absorber (12). Further, if the photoreactor is operated at high pressure, the flash drum and amine pump may be excluded from the system.
  • the chemical compounds or species stream (18A) produced as a result from the photocatalytic reaction of carbon dioxide gas for the solvent regeneration process flows into a liquid-gas phase separator (34) to enable separation of gases from the compounds of interest, such as, but not limited to, methanol, methane, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like.
  • gases from the compounds of interest such as, but not limited to, methanol, methane, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like.
  • Gases produced from the liquid-gas separation process enters a heater (36) to release oxygen (18B), which is removed via an outlet. Examples of these compounds are listed in TABLE 1 below:
  • the present invention provides a process for converting carbon dioxide (CO2) to one or more chemical compounds in an acid gas removal/capture system comprising: adding solvent to a gas stream to absorb CO2 to form a mixture (S200); feeding the mixture to a solvent regeneration section (S201); separating said compounds from said solvent (S202); characterised is that: in the solvent regeneration section the mixture is fed to a photoreactor (S201A) comprising at least one catalyst; and; initiating a photoreaction (S201B) to reduce the carbon dioxide into said compounds and produce regenerated solvent (S203).
  • the photoreactor may be configured to allow pressure adjustments between or during the photoreaction processes.
  • FIG. 3 and FIG. 4 An experimental example of preparing the catalyst and CO2 reduction test are shown in FIG. 3 and FIG. 4 respectively, while the assessment results in relation to CO2 reduction test and production of methanol under varying parameters are shown in FIG. 5 to FIG. 10.
  • MDEA the photoreduction medium
  • FIG. 5 to FIG. 10 An experimental example of preparing the catalyst and CO2 reduction test are shown in FIG. 5 to FIG. 10.
  • the concentration of solvents and the type of catalysts used may be varied to determine the output compounds.
  • the parameters and conditions of the photoreaction process may be calculated using known or conventional methods. Examples of varying conditions and catalysts that can be used, and methanol yield based on said varying conditions and catalysts are shown in TABLE 2 below:

Abstract

An acid gas removal/capture system and process for converting carbon dioxide to chemical compounds such as methanol comprising: an absorption section (100) for adding a solvent to an acid gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; a regeneration section (200) for separating the chemical compounds and the solvent from the mixture, wherein the regeneration section (100) comprises a photoreactor (24) including at least one catalyst for initiating a photoreaction to convert the carbon dioxide in the mixture to said chemical compounds.

Description

AN ACID GAS REMOVAL/CAPTURE SYSTEM AND PROCESS FOR CONVERTING CARBON DIOXIDE TO CHEMICAL COMPOUNDS FIELD OF INVENTION
[0001] The present invention generally relates to acid gas removal/capture processes, and more particularly to a process for regenerating absorbents in an acid gas removal process. BACKGROUND
[0002] Natural gas, although an extremely important source of energy, contains acid gases such as Hydrogen Sulfide (H2S) and Carbon Dioxide (CO2) along with other impurities, which are considered as poisonous pollutants, if not treated or processed properly. Accordingly, removal of acid gases from natural gas streams is crucial particularly in hydrocarbon producing or gas processing plants to comply with sales product standards. This process is carried out in acid gas removal units (AGRU).
[0003] An Acid Gas Removal Unit (AGRU) is generally designed to remove acidic components to meet gas sulphur and CO2 specifications. The primary stages in a typical AGRU setting include an absorption stage for the absorption of acid gases by means of an absorber - usually a solvent, in which the choice of solvent is dependent on its capability to absorb or remove the acid gases; and the solvent or absorbent regeneration stage, designed to produce lean solvent (free of acid gases) to be reused in the absorber.
[0004] One of the key challenges for acid gas removing processes is regenerating the solvent for reuse in the absorber. Aqueous CO2 absorbent is typically used for the removal of acidic gases but the regeneration of aqueous absorbents consumes a lot of energy while releasing CO2 to the atmosphere. Increased CO2 concentration in the atmosphere contributes to excessive heating of earth’s atmosphere, leading to climate change and affecting living beings.
[0005] Accordingly, there is a need to find a more sustainable and environmentally friendly way of removing acid gases from natural gas streams. SUMMARY
[0006] In one aspect of the present invention, there is provided an acid gas removal/capture system for converting carbon dioxide to at least one chemical compound comprising: an absorption section for adding a solvent to an acid gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; a regeneration section for separating the chemical compound and the solvent from the mixture; characterised in that: the regeneration section comprises a photoreactor including at least one catalyst for initiating a photoreaction to convert the carbon dioxide in the mixture to said chemical compound.
[0007] Advantageously the solvent absorbs the carbon dioxide which can then be converted into useful chemical compounds with the photoreactor, typically by reduction. In addition, the amount of carbon dioxide emitted into the atmosphere is reduced.
[0008] In one embodiment, the regeneration section comprises a liquid-gas phase separator for separating the chemical compound from the mixture.
[0009] In an alternative embodiment, the regeneration section includes a pervaporation membrane for separating the chemical compound from the mixture.
[0010] In a further alternative embodiment, the regeneration section includes a distillation means for separating the chemical compound from the mixture.
[0011] After the chemical compound is separated from the mixture the remainder is regenerated solvent, which is typically directed to the absorption section.
[0012] In one embodiment, the regeneration section includes a flash drum for recovery of light or volatile hydrocarbons from the mixture before it is received by the photoreactor.
[0013] In one embodiment the catalyst comprises at least one of (graphene, graphene oxide, magnesium oxide, zinc, copper, B12S3 QDs, CdS(Bi2S3), CdSe/Pt, ZnPc, silver or nickel) immobilised/doped with T1O2 nanoparticles, T1O2 anatase, ZnO-rGO, molybdenum disulfide-TiC hybrid, amine functionalized T1O2, porous T1O2, anatase T1O2 nanocrystals, fluorinated anatase T1O2 nanosheet, AgBr/C3N4/N-Graphene, atomically thin single-unit- cell B12WO6 layers, B12WO6 hollow microspheres, zinc oxide, lamellar BiVOt, conducting polymer-modified B12WO6 hierarchical hollow microspheres, Bi2S3/CdS, RuC -modified CuxAgylnzZn m solid solutions, 3% Ni0x-Ta205-1% immobilised on reduced graphene, core-shell Ni/NiO-loaded N-InTaOt, and reduced Titanium dioxide such as grey T1O2 or black T1O2 .
[0014] In one embodiment the photoreactor irradiates the mixture with a light source having a wavelength of about 200 to 700nm. Typically, the temperature during irradiation is 20- 65°C, preferably 40-50°C. Typically methanol is produced as the compound under such conditions.
[0015] In one embodiment the light source has a power of 0.1 to 50mW/cm2 or any suitable power for intensifying the process.
[0016] Advantageously the catalyst and photoreactor wavelength can be adjusted to determine the compound that the carbon dioxide is converted to.
[0017] Typically, the regeneration section includes a pump to facilitate transfer of the regenerated solvent to the absorption section.
[0018] In a further aspect of the invention, there is provided a process for converting carbon dioxide to one or more chemical compounds in an acid gas removal/capture system comprising: an absorption step including adding a solvent to a gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; and a regeneration step including separating said compounds and regenerating the solvent from said mixture; characterised in that: in the regeneration step the mixture is fed to a photoreactor comprising at least one catalyst; and a photoreaction is initiated to convert the carbon dioxide into said compounds.
[0019] In one embodiment, the compounds include carbon monoxide, methane, methanol, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like.
[0020] In one embodiment, the process further includes feeding the mixture to a flash drum for recovery/removal of volatile and/or light hydrocarbons prior to the photoreactor. [0021] In one embodiment, the pressure in the photoreactor may be adjusted. Typically the pressure may be up to 80bar before entering the photoreactor, but is reduced before, during or after absorbent regeneration.
[0022] In one embodiment, the solvent is any or any combination of amines suitable for capturing or removing CO2. Typically, the amine is an alkanolamine or piperazine.
[0023] In one embodiment, the alkanolamine is diethanolamine (DEA) or methyldiethanolamine (MDEA). [0024] In an alternative embodiment, the solvent is H2O, NaOH, NaHCCh, KrCCh.NarCCE,
KOH, KHCO3 or other inorganic bases suitable for capturing or removing CO2
BRIEF DESCRIPTION OF DRAWINGS [0025] The invention will be more understood by reference to the description below taken in conjunction with the accompanying drawings herein:
[0026] FIG. 1 provides a schematic diagram of the system in accordance with an embodiment of the present invention;
[0027] FIG. 2 shows a flowchart for the process in accordance with an embodiment of the present invention;
[0028] FIG. 3 shows the experimental examples of preparing the catalyst in accordance with an embodiment of the present invention;
[0029] FIG. 4 shows the experimental examples of preparing the CO2 reduction test in accordance with an embodiment of the present invention; [0030] FIG. 5 shows the experimental result for methanol production based on two samples
- using Gas Chromatography - mass spectrometry (GC-MS) Headspace;
[0031] FIG. 6 shows the methanol concentration with standard solution MeOH quantified using Gas Chromatography (GC) with Flame-Ionization Detection (FID) Headspace; [0032] FIG. 7 shows the experimental results providing the methanol concentration based on three different photocatalysts with MDEA as medium quantified using Gas Chromatography (GC) with Flame-Ionization Detection (FID) Headspace; [0033] FIG. 8 shows the experimental results providing the methanol yield based on three different photocatalysts;
[0034] FIG. 9 shows the assessment results of methanol yield in different temperatures; [0035] FIG. 10 shows the assessment results of methanol yield under different UV lamp settings.
DETAILED DESCRIPTION [0036] In line with the above summary, the following description of a number of specific and alternative embodiments is provided to understand the inventive features of the present invention. It shall be apparent to one skilled in the art, however that this invention may be practiced without such specific details. Some of the details may not be described at length so as not to obscure the invention. For ease of reference, common reference numerals will be used throughout the figures when referring to the same or similar features common to the figures.
[0037] Embodiments of the invention are described by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various respects, all without departing from the scope of the present invention. It should be noted that the drawings include schematic description of how the process in accordance with the preferred embodiments can be carried out. The necessary pumps, valves, and other standard equipment or components may have not been illustrated since they are known in the art.
[0038] In one embodiment, the present invention provides a process whereby acid gases, in particular carbon dioxide (CO2) that is normally produced during regeneration of absorbents for acid gas removal or capture systems, is converted into useful compounds such as those suitable for producing fuels, by way of a photocatalysis process. [0039] More specifically the invention provides a process for converting carbon dioxide to chemical compounds in the presence of a solvent as an absorbent; in an acid gas removal system comprising: feeding a solvent loaded with carbon dioxide to a photoreactor thereby initiating a photoreaction in the presence of at least one catalyst; and reducing the carbon dioxide contained in the solvent into compounds useful for making fuels or value-added products.
[0040] In this embodiment, the absorbent used in the process disclosed herein is Methyldiethanolamine (MDEA) which is widely used in industry due to high absorption capacity, especially for acid gases including CO2. The aqueous solution - having water as the solvent to prepare the alkanolamine solution, may contain a predetermined percent by weight of the alkanolamine, which may be determined by the type and quantity of gases to be treated or removed.
[0041] Further in this embodiment, the catalysts used to facilitate or accelerate the conversion of CO2 in the process disclosed herein includes graphene oxide immobilised/doped with T1O2 nanoparticles (rGO-TiCh) and magnesium oxide immobilised/doped with T1O2 nanoparticles (Mg-TiCh). The catalysts may be prepared by various methods known in the art. In one example, the catalysts may be prepared or synthesized by a combination of methods or reactions - sonothermal, hydrothermal and calcination reactions under predetermined conditions.
[0042] FIG. 1 provides an example of the system for converting carbon dioxide to one or more chemical compounds in an acid gas capture process in accordance with an embodiment of the present invention. In this embodiment, the system comprises: an absorption section (100) to receive acid gas stream and adding solvent to said gas stream for carbon dioxide absorption to form a mixture; and a regeneration section (200) to receive the mixture and separating the compounds from the mixture. In this embodiment, in the regeneration section (200) the mixture is fed to a photoreactor comprising at least one catalyst for initiating a photoreaction to reduce the carbon dioxide into said chemical compounds and to form regenerated solvent. [0043] The gas inlet feeds acid gas or natural gas stream into the absorber (12) of the absorption section (100) wherein the absorber (12) includes an inlet for feeding lean solvent solution and an outlet for rich solvent solution, referred herein as lean solvent stream (14A) and rich solvent stream (16A), respectively. A portion of gases exits from the absorber (12) by way of an outlet as purified or treated gas. The rich solvent stream (16A) flows into the flash drum or flash tank (22) for recovery of light or volatile hydrocarbons (which can be flashed) prior to entering the photoreactor (24) in the regeneration section (200). In the photoreactor (24), the rich solvent, containing or loaded with absorbed gases including carbon dioxide, is regenerated by way of photoreaction in the presence of a catalyst to produce lean solvent stream (14A) which is removed from the photoreactor (24) via the designated outlet and flows into a pervaporation membrane (32) prior to entering absorber (12) of the absorption section (100) as regenerated absorbent or solvent. The pervaporation membrane (32) separates the compound of interest from the solvent. The system may further include solvent pumps (40, 42) to facilitate the flow of the regenerated solvent from the regeneration section (200) to the absorption section (100) and a solvent tank (44). It is anticipated that in other embodiments, the pervaporation membrane (32) may be replaced with a distillation system or equivalent apparatus.
[0044] During the photoreaction process, desorption of acid gas, in particular carbon dioxide to produce lean solvent solution or regenerated solvent is achieved by way of photo irradiation after which the gas is reacted via photocatalysis and reduced into value-added chemical compounds and species, instead of releasing the gas into the environment. The CO2 contained in the alkanolamine solvent is utilised during the photoreaction process.
[0045] In one embodiment, the pressure in the photoreactor (24) may be adjusted before, during or after the absorbent regeneration process. For example, the photoreactor (24) may be operated at high pressure such that the solvent can be regenerated without reducing the pressure in the absorber (12). Further, if the photoreactor is operated at high pressure, the flash drum and amine pump may be excluded from the system.
[0046] The chemical compounds or species stream (18A) produced as a result from the photocatalytic reaction of carbon dioxide gas for the solvent regeneration process flows into a liquid-gas phase separator (34) to enable separation of gases from the compounds of interest, such as, but not limited to, methanol, methane, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like. Gases produced from the liquid-gas separation process enters a heater (36) to release oxygen (18B), which is removed via an outlet. Examples of these compounds are listed in TABLE 1 below:
TABLE 1 Redox Potential of Various Species Against Normal Hydrogen Electrode for Carbon
Dioxide Conversion into Chemical Fuels
Figure imgf000010_0001
Mote: H O water, h* protons, O Oxygen, H Hydrogen, e~ electron, CO carbon dioxide, CO - carbon dioxide anion, HCOOH formic acid, HCHO formaldehyde, CO carbon monoxide, CH methane, CH COOH acetic acid, C H OH ethanol, CHCOCH acetone, CH CH CHO propionaldehyde, V voltage, and NHE normal hydrogen electrode
[0047] With further reference to FIG. 2, the present invention provides a process for converting carbon dioxide (CO2) to one or more chemical compounds in an acid gas removal/capture system comprising: adding solvent to a gas stream to absorb CO2 to form a mixture (S200); feeding the mixture to a solvent regeneration section (S201); separating said compounds from said solvent (S202); characterised is that: in the solvent regeneration section the mixture is fed to a photoreactor (S201A) comprising at least one catalyst; and; initiating a photoreaction (S201B) to reduce the carbon dioxide into said compounds and produce regenerated solvent (S203). The photoreactor may be configured to allow pressure adjustments between or during the photoreaction processes. [0048] An experimental example of preparing the catalyst and CO2 reduction test are shown in FIG. 3 and FIG. 4 respectively, while the assessment results in relation to CO2 reduction test and production of methanol under varying parameters are shown in FIG. 5 to FIG. 10. [0049] With reference to the experimental examples, it is observed that the photoreduction of CO2 is optimised with MDEA as the reaction medium. It is further observed from the experimental results that the highest methanol yield was obtained using Mg-TiCh as catalyst for photocatalytic reaction conducted within the temperature range of between 40°C to 50°C. In the experimental examples, 0.1L medium (MDEA) was used with 0.1 - lg catalyst, in which the operating hours ranged from 6h to 144 hours.
[0050] In accordance with the preferred embodiment of the present invention, the concentration of solvents and the type of catalysts used may be varied to determine the output compounds. The parameters and conditions of the photoreaction process may be calculated using known or conventional methods. Examples of varying conditions and catalysts that can be used, and methanol yield based on said varying conditions and catalysts are shown in TABLE 2 below:
TABLE 2 Methanol Max Yield Based on Varying Conditions
Figure imgf000011_0001
Figure imgf000012_0001
Figure imgf000013_0001
[0051] While the invention has been described as required in terms in preferred embodiments and specific operating ranges and conditions, those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.

Claims

1. An acid gas removal/capture system for converting carbon dioxide to at least one chemical compound comprising: - an absorption section (100) for adding a solvent to an acid gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; a regeneration section (200) for separating the chemical compound and the solvent from the mixture; characterised in that: - the regeneration section (100) comprises a photoreactor (24) including at least one catalyst for initiating a photoreaction to convert the carbon dioxide in the mixture to said chemical compound.
2. The system according to claim 1, wherein the regeneration section (200) comprises a liquid-gas phase separator (34) for separating the chemical compound from the mixture.
3. The system according to claim 1, wherein the regeneration section (200) includes a pervaporation membrane (32) for separating the chemical compound from the mixture.
4. The system according to claim 1, wherein the regeneration section (200) includes distillation apparatus for separating the chemical compound from the mixture.
5. The system according to claim 1, wherein the regeneration section (200) includes a flash drum for recovery of light or volatile hydrocarbons from the mixture before it is received by the photoreactor (24).
6. The system according to any claim 1, wherein the catalyst comprises at least one of (graphene, graphene oxide, magnesium oxide, zinc, copper, B12S3 QDs, CdS(Bi2S3),
CdSe/Pt, ZnPc, silver or nickel) immobilised/doped with T1O2 nanoparticles, T1O2 anatase, ZnO-rGO, molybdenum disulfide-TiCh hybrid, amine functionalized T1O2, porous T1O2, anatase T1O2 nanocrystals, fluorinated anatase T1O2 nanosheet, AgBr/C3N4/N-Graphene, atomically thin single -unit-cell BhWCL layers, BriWCL hollow microspheres, zinc oxide, lamellar BiVCri, conducting polymer-modified B12WO6 hierarchical hollow microspheres, Bi2S3/CdS, RuC -modified CuxAgylnzZn Sm solid solutions, 3% Ni0x-Ta205-1% immobilised on reduced graphene, core-shell Ni/NiO-loaded N-InTaCri , and reduced Titanium dioxide such as grey T1O2 or black T1O2 .
7. The system according to claim 1 , wherein the photoreactor (24) irradiates the mixture with a light source with wavelengths between 200nm to 700nm.
8 The system according to claim 7, wherein the temperature during irradiation is 20- 65°C in order to produce methanol as the compound.
9. A process for converting carbon dioxide to one or more chemical compounds in an acid gas removal/capture system comprising: an absorption step including adding a solvent to a gas stream to form a mixture, said solvent capable of absorbing carbon dioxide; and a regeneration step including separating said compounds and regenerating the solvent from said mixture; characterised in that: in the regeneration step the mixture is fed to a photoreactor comprising at least one catalyst; and a photoreaction is initiated to convert the carbon dioxide into said compounds.
10 The process according to claim 9, wherein the compounds include carbon monoxide, methane, methanol, ethanol, formic acid, formaldehyde, ethanoic acid, acetone, propionaldehyde and/or the like.
11. The process according to claim 9, wherein the process further includes feeding the rich solvent solution to a flash drum for recovery/removal of volatile and/or light hydrocarbons prior to the photoreactor.
12. The process according to claim 9, wherein the solvent is any or any combination of amines.
13. The process according to claim 12, wherein the amines include an alkanolamine and/or piperazine.
14. The process according to claim 13, wherein the alkanolamine includes methyldiethanolamine and/or diethanolamine.
15. The process according to claim 9, wherein the solvent is ThO, NaOH, NaHCCb,
Na2C03, K2CO3, KOH or KHCO3
16. The process according to claim 9, wherein the pressure in the photoreactor may be adjusted.
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