Aqueous absorption medium for removing acid gases

Abstract

An aqueous absorbent medium for removing acidic gases from a gaseous stream comprises at least one tertiary alkanolamine and at least one secondary alkanolamine. The total weight of the at least one tertiary alkanolamine and the at least one secondary alkanolamine accounts for a percentage of the amine weight of the aqueous absorbent medium. The amine weight of the aqueous absorbent medium is 25% to 65% by weight based on the total weight of the aqueous absorbent medium, and the at least one secondary alkanolamine is present in an amount of 11% to 45% by weight based on the amine weight of the aqueous absorbent medium.

Description

Technical Field

[0001] The embodiments relate to an aqueous absorbent medium for removing acidic gases, comprising at least one tertiary alkanolamine and at least one secondary alkanolamine; and a method for removing acidic gases, comprising providing the aqueous absorbent medium. Background Technology

[0002] The gaseous stream may contain acidic gases such as carbon dioxide, hydrogen sulfide, sulfur dioxide, carbon disulfide, hydrogen cyanide, carbonyl sulfide, and / or thiols as impurities. The gaseous stream may originate from natural gas, petroleum, syngas, biogas, or coal processing. For example, the gaseous stream may include natural gas, refinery gas, hydrocarbon gases from shale pyrolysis, ammonia syngas, flue gas, and / or liquefied natural gas.

[0003] Aqueous amine-based absorbents are the most common absorbents for removing acidic gases. However, there is a search for improved amine-based aqueous absorbents for removing acidic gases in an attempt to minimize costs and maximize acidic gas removal. Summary of the Invention

[0004] The implementation scheme can be achieved by providing an aqueous absorption medium for removing acidic gases from a gaseous stream, the aqueous absorption medium comprising (a) at least one tertiary alkyl alcohol amine having the general formula (I).

[0005]

[0006] R1 is a C1 to C4 alkyl group.

[0007] (b) at least one secondary alkyl alcohol amine having the general formula (II)

[0008]

[0009] Wherein R2 is a C1 to C4 alkyl group, and

[0010] (c) Water.

[0011] The total weight of at least one tertiary alkanolamine and at least one secondary alkanolamine accounts for a percentage of the total amine weight of the aqueous absorbent medium. The total amine weight of the aqueous absorbent medium is 25% to 65% by weight based on the total weight of the aqueous absorbent medium, and the at least one secondary alkanolamine is present in an amount of 11% to 45% by weight based on the total amine weight of the aqueous absorbent medium. Attached Figure Description

[0012] The features of the embodiments will become more apparent to those skilled in the art from the detailed description of exemplary embodiments thereof with reference to the accompanying drawings, in which:

[0013] Figure 1 An exemplary flowchart is shown; and

[0014] Figure 2 Performance data for working examples 1 and 2, as well as comparative examples A and B, are shown. Detailed Implementation

[0015] Aqueous absorbent media containing tert-alkanolamines for acid gas removal may contain a sufficient amount of at least one activator to help accelerate the reaction rate and thus remove acid gases such as carbon dioxide from the gaseous stream. Not intended to be theoretically sound, it is believed that aqueous absorbent media containing tert-alkanolamines may not be effective at removing carbon dioxide without a sufficient amount of activator. Furthermore, for cost management reasons, to minimize process problems, it is ideal that the aqueous absorbent media does not require a relatively high concentration (by weight and / or molar percentage) of activator compared to at least one tert-alkanolamine and / or water. Therefore, an activator that allows for the effective removal of acid gases such as carbon dioxide at relatively low concentrations is sought.

[0016] It is desirable to keep the amount of activator relative to at least one tertiary alkanolamine as low as possible while still being effective. For example, the weight ratio of activator to at least one tertiary alkanolamine can be less than 1 (e.g., in a 55 wt% amine solution, if the total activator concentration is 25 wt%, the tertiary alkanolamine concentration can be 30 wt%). In this regard, it may be desirable to use a relatively small amount of activator because the heat of absorption of activators tends to be higher than that of tertiary alkanolamines such as MDEA. This high heat of absorption can lead to increased costs for regenerating the aqueous absorbent medium (e.g., OPEX), for example, because the regeneration process may require more energy and / or may result in high amine loss during the absorption process. The reaction rate refers to the number of moles of carbon dioxide consumed per unit volume per unit time in the aqueous absorbent medium, and it should be understood that the presence of activator can significantly affect the reaction rate.

[0017] For aqueous absorbent media, when the media contains tertiary alkanolamines such as methyldiethanolamine (MDEA), piperazine is a commonly used activator. Furthermore, secondary amines are preferred as activators because primary amines such as aminoethoxyethanol (AEE) can be corrosive and degraded in gas treatment applications, and tertiary amines are generally not good activators. However, the reaction rate of piperazine in end-use gas treatment applications may be too fast and / or piperazine itself may be too volatile. When piperazine reacts too quickly, an alternative choice is methylethanolamine (NMEA).

[0018] MDEA / NMEA blends are commonly used in refinery gas treatment applications, but this is not the optimal choice due to the low boiling point of NMEA. For example, the low boiling point is often a problem in these gas treatment applications because it can lead to significant activator loss relative to the MDEA base, particularly during absorption, in solvent / aqueous absorbent media. This high activator loss relative to the alkali amine can cause significant process problems, such as difficulty in maintaining optimal aqueous absorbent media compositions. Therefore, alternatives are sought that allow relatively high reaction rates while minimizing process problems.

[0019] Furthermore, blends containing diethanolamine (DEA), such as MDEA / DEA, have been proposed because DEA has a similar average standard boiling point to MDEA. However, it is known that the reaction rate of DEA is significantly lower than that of NMEA, thus making DEA less than ideal for removing carbon dioxide from gas mixtures in efficient methods.

[0020] According to an exemplary embodiment, the activator for the aqueous absorbent is a secondary amine having a structure similar to methylaminopropanediol (MAPD). This activator allows for high reaction rates at low concentrations, relatively low processing costs, and addresses the greater concerns regarding corrosion / degradation issues that are unknown. MAPD and similar structures are considered effective aqueous absorbents for removing acidic gases from gas mixtures in blends with MDEA and similar structures. Unlike DEA and MDEA, MAPD and similar structures are similar to NMEA, with hydroxyl groups at one end of the molecule. This structure allows for an alkyl nitrogen group structure at the other end of the molecule. Not intended to be theoretically constrained, it is believed that the interaction between the combination of multiple hydroxyl groups at one end of the molecule and the alkyl nitrogen group structure at the opposite end makes MAPD and similar structures a very suitable choice for use as activators.

[0021] Aqueous alkanolamine media

[0022] According to an exemplary embodiment, an aqueous absorbent medium for removing acidic gases from a gaseous stream comprises at least one tertiary alkanolamine for removing the acidic gases and at least one secondary alkanolamine as an activator in an amount sufficient to increase the reaction rate for improving the removal of the acidic gases, while minimizing other problems compared to using at least one tertiary alkanolamine without using sufficient amounts of such activator or when using less desirable amines. The aqueous absorbent medium can exclude other amines that are too volatile, such as piperazines, and / or other amines with low standard boiling points, such as NMEA.

[0023] At least one tertiary alkyl alcohol amine has the general formula (I):

[0024]

[0025] R1 is a C1 to C4 alkyl group, such as a straight-chain C1 to C4 alkyl group. C1 to C4 alkyl means an alkyl group containing 1 to 4 carbon atoms. Examples include -CH3, -CH2CH3, -(CH2)2CH3, and -(CH2)3CH3 (e.g., straight-chain). The aqueous absorbent may contain one or more different tertiary alkyl alcohol amines, each having the general formula (I). The aforementioned general formula (I) includes MDEA and similar structures believed to have similar properties for removing acidic gases from gas mixtures.

[0026] At least one secondary alkanolamine acting as an activator has the general formula (II):

[0027]

[0028] R2 is a C1 to C4 alkyl group, such as a straight-chain C1 to C4 alkyl group. C1 to C4 alkyl means an alkyl group containing 1 to 4 carbon atoms. Examples include -CH3, -CH2CH3, -(CH2)2CH3, and -(CH2)3CH3 (e.g., straight-chain). The aqueous absorbent may contain one or more different secondary alkanolamines, each having the general formula (II). The aforementioned general formula (II) includes a MAPD-like structure, which is considered to have similar properties as an activator in the removal of acidic gases from gas mixtures.

[0029] Exemplary embodiments include R1 in general formula (I) being a C1 alkyl group and / or R2 in general formula (II) being a C1 alkyl group.

[0030] The aqueous absorbent medium also comprises water. The aqueous absorbent medium may also comprise at least one physical solvent, such as those known in the art. Exemplary physical solvents include methoxytriethylene glycol (MTG), sulfolane, glycerol, methanol, ethylene glycol, diethylene glycol, and triethylene glycol.

[0031] The total weight of at least one tertiary alkanolamine and at least one secondary alkanolamine present in the aqueous absorbent medium accounts for a portion of the amine weight of the aqueous absorbent medium (e.g., the dry amine weight of the aqueous absorbent medium). In other words, the amine weight of the aqueous absorbent medium consists of the total weight of at least one tertiary alkanolamine and at least one secondary alkanolamine present in the aqueous absorbent medium, and does not include any other additives, water, or physical solvents. In an exemplary embodiment, the remainder of the total weight of the aqueous absorbent medium, excluding the amine weight of the aqueous absorbent medium, is water.

[0032] The amine in the aqueous absorbent medium comprises 25% to 65% by weight of the total weight of the aqueous absorbent medium (e.g., 25% to 60% by weight, 30% to 55% by weight, 30% to 50% by weight, 40% to 55% by weight, 45% to 55% by weight, etc.). The amount of at least one secondary alkanolamine can be based on the total weight of the amine in the aqueous absorbent medium. In an embodiment, based on the weight of the amine in the aqueous absorbent medium, at least one secondary alkanolamine may be present in an amount of 11% to 45% by weight (e.g., 11% to 40% by weight, 11% to 35% by weight, 11% to 30% by weight, 11% to 25% by weight, 12% to 20% by weight, etc.).

[0033] The amount of at least one secondary alkanolamine may be further based on the total weight of the aqueous absorbent medium. For example, based on the weight of the amine in the aqueous absorbent medium, at least one secondary alkanolamine may be present in an amount of 5% to 25% by weight (e.g., 5% to 20% by weight, 5% to 15% by weight, 6% to 20% by weight, 6% to 15% by weight, 6% to 11% by weight, etc.). When at least one secondary alkanolamine is present as an activator, it is present in an amount less than that of at least one tertiary alkanolamine and / or water. For example, the total weight percentage of at least one secondary alkanolamine in the aqueous absorbent medium is less than the total weight percentage of at least one tertiary alkanolamine and the total weight percentage of water (based on a total of 100% by weight of the aqueous absorbent medium).

[0034] In an exemplary embodiment, based on the amine weight of the aqueous absorption medium, at least one tertiary alkanolamine may be present in an amount of 25% to 60% by weight (e.g., 25% to 55% by weight, 30% to 55% by weight, 35% to 55% by weight, 35% to 50% by weight, 39% to 45% by weight, etc.).

[0035] application

[0036] Acid gases can be removed from the primary gaseous stream in gas-liquid contact devices such as pressure swing adsorption (PSA) and temperature swing adsorption (TSA) using liquid absorbents. Amine loss due to volatility can be a problem in gas-liquid separation. Volatility may be of particular concern when the components of the aqueous absorbent medium, such as the activator, have relatively low boiling points compared to commonly used base tertiary alkanolamines. Therefore, it has been found that for aqueous absorbent media containing at least one tertiary alkanolamine and at least one secondary alkanolamine as an activator, both having relatively high boiling points (e.g., similar boiling points of at least 230 °C) can be useful. It should be noted that while a low boiling point does indeed imply high volatility in gas handling applications, it does not necessarily imply high solubility in liquid hydrocarbon media. In other words, volatility and hydrocarbon co-solubility are different principles, and in this case, volatility should be the focus. In this respect, highly volatile amines are often impractical and expensive for the treatment of acid gases in gaseous streams.

[0037] Methods for removing acidic gases from a gaseous stream include providing a gaseous stream for acidic gas removal treatment and an aqueous absorbent medium. In methods for removing acidic gases, the aqueous absorbent medium may be countercurrently contacted with a gas mixture containing acidic gases in an absorption tower at low temperature and high pressure to form an acid-rich aqueous absorbent medium. Circulating adsorption methods may utilize high-rate gas-liquid exchange, large liquid stock transfers between absorption and regeneration steps, and the high energy requirements of amine solution regeneration. Such methods may take advantage of large temperature differences in the gas stream between the absorption and desorption (regeneration) sections of the cycle. For example, an aqueous amine washing method may use relatively low temperatures, such as less than 50°C, for acidic gas absorption, while the temperature increases to above about 100°C, such as 120°C or higher, for desorption.

[0038] Regeneration can at least partially remove acidic gases from an aqueous absorbent medium rich in acidic gases, thus creating an aqueous absorbent medium depleted of acidic gases. Regenerating large quantities of aqueous absorbent medium to create a depleted acidic absorbent medium can be carried out at temperatures above 100°C, which can cause significant amine loss from many commonly used amines due to evaporation during the temperature variation process. Therefore, it is desirable to use amines that minimize such losses. After regeneration, the depleted acidic absorbent medium can be recycled back into the acidic gas removal method for reuse in removing acidic gases from the gaseous stream.

[0039] Gas streams that can be processed to remove acidic gases include industrial gases, such as natural gas from hydrocarbon reservoirs, coal processing, biogas, and refinery gas.

[0040] Example

[0041] Regarding the illustrative working examples, comparative examples, and information used in the reported results of the working examples and comparative examples, the general properties, features, parameters, etc. are provided below.

[0042] The method for handling gaseous streams is based on Figure 1 The method shown involves feeding an aqueous absorbent medium via feed line 5 into the upper part of a gas-liquid countercurrent packed bed (1 / 4” ceramic saddle packing) absorber tower 2. A gaseous stream is introduced into the lower part of absorber tower 2 via feed line 1 at a gas flow rate of approximately 17 standard liters per minute. The dry feed composition is 90 mol% nitrogen and 10 mol% carbon dioxide. The pressure in the absorber tower is set to approximately 250 psig. Clean gas (i.e., reduced CO2) is discharged at the top of absorber tower 2 via outlet line 3, and the residual CO2 level is measured by gas chromatography. The CO2-rich aqueous absorbent medium is then directed to the lower part of absorber tower 2 and exits via line 4.

[0043] The abundant aqueous absorbent medium in line 4 is depressurized via level control valve 8 and flows through line 7 to heat exchanger 9, which heats the abundant aqueous absorbent medium. The heated abundant aqueous absorbent medium then enters the upper part of regenerator 12 via line 10. Regenerator 12 is equipped with random packing (1 / 4") capable of desorbing H2S and CO2. The pressure of the regenerator is set to approximately 27 psia. Gas from regenerator 12 then enters condenser 14 via line 13, where cooling and condensation of any residual water and amines occur. The gas then enters separator 15, where the condensed liquid is separated from the gas phase. The condensed aqueous solution can be pumped to the upper part of regenerator 12 via pump 22 through line 16. The remaining gas after condensation can be removed via line 17 for final collection and / or treatment. The regenerated aqueous solution flows downwards through regenerator 12 and a closely connected reboiler 18. Reboiler 18, equipped with an electrically heated device, can evaporate a portion of the aqueous solution to remove any residual gas. The reboiler temperature can be set to approximately 125°C. Vapor can rise from the reboiler and return to regenerator 12, where it can mix with the descending liquid and then exit via line 13 to enter the condensation stage of the method. The regenerated aqueous absorbent from reboiler 18 exits via line 19 and is cooled in heat exchanger 20. Then, the regenerated (i.e., acid-poor) aqueous absorbent medium can be pumped back to the absorber 2 via the solvent feed line 5 through pump 21.

[0044] The following materials are mainly used in the embodiments:

[0045] MDEA refers to a solution of approximately 98% tert-alkylaminomethyl diethanolamine, which is available from The Dow Chemical Company or its affiliates and has the following structure:

[0046]

[0047] MAPD refers to a solution of approximately 98% secondary alkanolamine methylaminopropanediol, which is available from Dow Chemical Company or its affiliates and has the following structure:

[0048]

[0049] NMEA refers to a solution of approximately 98% secondary alkanolamine methylethanolamine, which is available from Dow Chemical Company or its affiliates and has the following structure:

[0050]

[0051] Referring to the examples in Table 1 below, the acid gas removal performance of MDEA-MAPD blends is compared with that of commonly used MDEA-NMEA blends. However, although NMEA is a monoalkylolamine, it has a significantly lower mean standard boiling point compared to MAPD (which is a dialkylolamine). Furthermore, the examples are set with 50% water by weight, as this is a typical target for maximizing acid gas removal capacity while minimizing corrosion and processing problems; however, it is well known that the total amine content (and therefore the amount of water) can vary based on the intended use and the type of material used in a particular gas treatment facility. Activator / total amine refers to the weight percentage of the activator (i.e., MAPD or NMEA) relative to the weight of the amine (i.e., excluding the weight of water) in the aqueous absorbent medium.

[0052] Table 1

[0053]

[0054] The examples in Table 1 are based on the treatment of feed gas entering absorption tower 2 with a carbon dioxide content of 10 mol% (i.e., Figure 1 The feed line 1 in the absorption tower 2 aims to remove sufficient carbon dioxide in the treated gas (i.e., the feed line 1 in the absorption tower 2). Figure 1 The outlet pipeline 3) achieves a carbon dioxide concentration of 2 mol%. (Reference) Figure 1 Regarding the flow rate for CO2 removal, it is desirable to achieve the removal of 2 mol% CO2 from the gaseous stream while using a relatively small amount of activator and at the lowest possible flow rate of the aqueous absorbent medium (i.e., solvent) (e.g., less than 2.5 kg / h).

[0055] Reference Figure 2Regarding working examples 1 and 2, it is shown that the desired low flow rate can be achieved. Conversely, for comparative examples A and B, the flow rate is higher than 2.5 kg / h. Specifically, as... Figure 2 The data provided regarding solvent flow rate (i.e., in solvent feed line 5) and treated gaseous CO2 (i.e., in the gaseous stream in outlet line 3) show that working examples 1 and 2 (i.e., data for MDEA-MAPD (43:7) and MDEA-MAPD (40:10)) both achieved a treated gaseous CO2 threshold of 2.0 mol% (20,000 ppmv) at flow rates less than 2.5 kg / h. Specifically, Figure 2 This study demonstrates the potential results of removing CO2 from a feed gas stream consisting of 90 mol% nitrogen and 10 mol% CO2. The goal is to achieve a CO2 specification of 2 mol% or lower in the processed gas stream (i.e., the gas being processed) at the lowest possible solvent circulation rate.

[0056] Reference Figure 2 The amount of carbon dioxide removed is directly proportional to the rate at which it is absorbed, which can be determined by: (i) the reaction rate of carbon dioxide with the activator, (ii) the concentration of the activator, and (iii) the solvent flow rate. Further, it is shown that for equimolar 2 mol% solutions (5 wt% NMEA and 7 wt% MAPD), at the same flow rate, improved carbon dioxide removal can be achieved using MAPD / MDEA compared to using NMEA / MDEA. Therefore, MAPD is considered to have a relatively fast reaction rate. Surprisingly, for examples containing equimolar amounts of NMEA and MAPD, the MAPD solution will perform better than the NMEA solution, as the additional hydroxyl groups are expected to slightly reduce performance.

[0057] Furthermore, the only solvent blend capable of achieving sufficient carbon dioxide removal at relatively low flow rates is the MDEA / MAPD blend with 7 wt% and 10 wt% MAPD. While the MDEA / NMEA blend containing 5 wt% NMEA could potentially achieve a solvent circulation rate of approximately 3.0 kg / h, its use is not desirable in practice because NMEA is too volatile under process conditions, having an average boiling point of approximately 158 °C, which can lead to high solvent loss rates and high operating costs. In contrast, MAPD does not have the same volatility problem and has a significantly higher average boiling point of approximately 249 °C. It is also noted that the average boiling point of MDEA is similarly approximately 247 °C. Furthermore, referring to Working Example 2, it is shown that increasing the MAPD concentration also provides similar benefits.

[0058] Regarding the above, it can be seen that some MDEA / MAPD blends can perform better than MDEA / NMEA blends, for example, by using aqueous absorbent media with lower circulation rates to meet specifications for removing acidic gases such as carbon dioxide. It is believed that, compared to NMEA, MAPD, as an activator, may not hinder the reaction with acidic gases such as CO2, and may result in a reaction rate fast enough for MAPD to serve as a high-performance absorbent media for acidic gas removal.

[0059] Therefore, it has been found that MDEA / MAPD blends in aqueous absorbent media (where MAPD is present in an amount greater than 5% by weight for 50% by weight of amine solvent) can provide baseline performance for carbon dioxide removal at flow rates less than 2.5 kg / h, while minimizing the amount of the higher-cost MAPD activator required in the blend and minimizing concerns about volatility.

Claims

1. An aqueous absorption medium for removing acidic gases from a gaseous stream, comprising: (a) At least one tertiary alkyl alcohol amine having the general formula (I): (I) R1 is a C1 to C4 alkyl group. (b) at least one secondary alkyl alcohol amine having the general formula (II) (I) Wherein R2 is a C1 to C4 alkyl group, and (c) Water, The amine weight of the aqueous absorbent consists of the total weight of at least one tertiary alkanolamine and the total weight of at least one secondary alkanolamine present in the aqueous absorbent, wherein the amine weight of the aqueous absorbent is 25% to 65% by weight based on the total weight of the aqueous absorbent, and the at least one secondary alkanolamine is present in an amount of 11% to 45% by weight based on the amine weight of the aqueous absorbent.

2. The aqueous absorbent medium according to claim 1, wherein the at least one tert-alkanolamine is present in an amount of 25% to 55% by weight based on the total weight of the aqueous absorbent medium.

3. The aqueous absorbent medium according to claim 1 or claim 2, wherein the amount of the at least one secondary alkanolamine is from 5% to 20% by weight based on the total weight of the aqueous absorbent medium.

4. The aqueous absorbent medium according to any one of claims 1 to 3, wherein the amount of the at least one secondary alkanolamine is 6% to 11% by weight based on the total weight of the aqueous absorbent medium.

5. The aqueous absorbent medium according to any one of claims 1 to 4, wherein R1 is a C1 alkyl group.

6. The aqueous absorbent medium according to any one of claims 1 to 4, wherein R2 is a C1 alkyl group.

7. The aqueous absorbent medium according to any one of claims 1 to 4, wherein R1 is a C1 alkyl group and R2 is a C1 alkyl group.

8. The aqueous absorbent medium according to any one of claims 1 to 7, wherein piperazine and methylethanolamine are excluded from the aqueous absorbent medium.

9. A method for removing acidic gas from a gaseous stream, comprising providing the gaseous stream and an aqueous absorbent medium according to any one of claims 1 to 8.

10. The method of claim 9, further comprising: An aqueous absorbent medium rich in acidic gases is formed by contacting the gaseous stream with the aqueous absorbent medium. At least partially removing acidic gases from the acid-rich aqueous absorbent medium to form an acid-poor aqueous absorbent medium, and The aqueous absorbent medium, which is deficient in acidic gases, is recycled to further remove acidic gases from the gaseous stream.

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