Aqueous absorbent medium for removing acid gases

Abstract

The aqueous absorbing medium for removing acid gases from a gas stream comprises at least one tertiary alkanolamine and at least one secondary alkanolamine. The combined weight of the at least one tertiary alkanolamine and the at least one secondary alkanolamine constitutes the amine weight of the aqueous absorbing medium. The amine weight of the aqueous absorbing medium is from 25% to 65% by weight based on the total weight of the aqueous absorbing medium, and the at least one secondary alkanolamine is present in an amount from 11% to 45% by weight based on the amine weight of the aqueous absorbing medium.

Description

Technical Field

[0004]

[0001] Embodiments relate to an aqueous absorption medium for removing acid gas comprising at least one tertiary alkanolamine and at least one secondary alkanolamine, and a process for removing acid gas comprising providing this aqueous absorption medium.

[0002] Introduction Gas streams may contain acid gases such as carbon dioxide, hydrogen sulfide, sulfur dioxide, carbon disulfide, hydrogen cyanide, carbonyl sulfide, and / or mercaptans as impurities. Gas streams can result from natural gas, petroleum, synthesis gas, biogas, or coal processing. For example, gas streams may include natural gas, refinery gas, hydrocarbon gas from shale pyrolysis, ammonia synthesis gas, flue gas, and / or liquefied natural gas.

[0003] Aqueous amine-based absorption media are perhaps the most common absorbents for removing acid gas. However, improved amine-based aqueous absorption media for removing acid gas are sought in both attempts to minimize cost and maximize acid gas removal.

Summary of the Invention

[0004] Embodiments can be realized by providing an aqueous absorption medium for removing acid gas from a gas stream, the aqueous absorption medium comprising: (a) at least one tertiary alkanolamine having the general formula (I),

[0005]

Chem.

[0006] [[ID=​​​ (In the formula, R2 is a C1-C4 alkyl group.) (c) Water and,

[0007] The combined weight of at least one tertiary alkanolamine and at least one secondary alkanolamine accounts for 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 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. [Brief explanation of the drawing]

[0008] The features of the embodiments will become more apparent to those skilled in the art by describing the exemplary embodiments in detail with reference to the accompanying drawings. [Figure 1] A diagram illustrating an exemplary process is shown. [Figure 2] Performance data for Examples 1 and 2, and Comparative Examples A and B are shown below. [Modes for carrying out the invention]

[0009] Aqueous absorbent media containing tertiary alkanolamines for removing acid gases may contain at least one activator in sufficient quantities to help accelerate the reaction rate and, consequently, the removal of acid gases such as carbon dioxide from the gas stream. While not intended to be theoretically bound, it is believed that without sufficient amounts of activator, aqueous absorbent media containing tertiary alkanolamines cannot efficiently remove carbon dioxide. Furthermore, for cost management reasons and to minimize process problems, it is desirable for optimal performance that the aqueous absorbent media does not require relatively high concentrations (by weight and / or mole percent) of activator compared to at least one tertiary alkanolamine and / or water. Therefore, there is a need for activators that enable efficient removal of acid gases such as carbon dioxide at relatively low concentrations.

[0010] 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 may be less than 1 (for example, in a 55 wt% amine solution, if the total activator concentration is 25 wt%, the tertiary alkanolamine concentration may be 30 wt%). In this regard, the absorption heat of activators tends to be higher than that of tertiary alkanolamines such as MDEA, and as a result of this high absorption heat, the cost of regenerating the aqueous absorbent medium (e.g., OPEX) may increase because more energy may be required in the regeneration process and / or a large amount of amine may be lost during the absorption process. Therefore, it may be desirable to use a relatively small amount of activator. The reaction rate refers to the number of moles of carbon dioxide consumed per unit time per unit volume in the aqueous absorbent medium, and it is understood that the presence of activators can greatly affect the reaction rate.

[0011] Regarding aqueous absorbent media, when the media contains tertiary alkanolamines such as methyldiethanolamine (MDEA), piperazine is a common activator. Furthermore, primary amines such as aminoethoxyethanol (AEE) may corrode and decompose in gas treatment applications, and tertiary amines are typically not good activators, so a secondary amine is preferred as the activator. However, the reaction rate of piperazine may be too fast and / or piperazine itself may be too volatile in the final gas treatment application. If the reaction of piperazine is too fast, another option is methylethanolamine (NMEA).

[0012] While MDEA / NMEA blends are frequently used in refinery gas treatment applications, they are not the optimal choice due to the low boiling point of NMEA. For example, the low boiling point can lead to high activator loss in the solvent / aqueous absorption medium, particularly during absorption, which is often problematic in these gas treatment applications. Such higher activator loss in the base amine can cause significant process problems, such as difficulties in maintaining an optimal aqueous absorption medium composition. Therefore, there is a need for alternatives that allow for relatively high reaction rates while minimizing process problems.

[0013] Furthermore, blends containing diethanolamine (DEA), such as MDEA / DEA, have been proposed because DEA has a mean standard boiling point similar to that of MDEA. However, the reaction rate of DEA is known to be significantly lower than that of NMEA, and therefore DEA is not very desirable for removing carbon dioxide from gaseous mixtures in an efficient process.

[0014] According to exemplary embodiments, the activator for an aqueous absorbent medium that enables high reaction rates at low concentrations and relatively low processing costs, and for which greater concerns regarding corrosion / decomposition issues are not known, is a secondary amine having a structure similar to methylaminopropanediol (MAPD). MAPD and similar structures are considered usable in blends with MDEA and similar structures as efficient aqueous absorbent mediums for removing acid gases from gaseous mixtures. MAPD and similar structures are similar to NMEA in that the hydroxyl group is located at one end of the molecule, in contrast to DEA and MDEA. This structure allows for an alkylated nitrogen group structure at the other end of the molecule. While not intended to be bound by this theory, the interaction between both the multiple hydroxyl groups at one end of the molecule and the alkylated nitrogen group structure at the opposite end is considered to make MAPD and similar structures a very suitable choice for use as an activator.

[0015] Aqueous alkanolamine medium According to an exemplary embodiment, an aqueous absorption medium for removing acid gas from a gas stream includes at least one tertiary alkanolamine in an amount sufficient to increase the reaction rate for improved acid gas removal while minimizing other problems compared to when at least one tertiary alkanolamine is used without a sufficient amount of an activator or when an amine that is less desirable is used, and at least one secondary alkanolamine as an activator. The aqueous absorption medium may exclude other amines that may be too volatile, such as piperazine, and / or other amines having a low normal boiling point, such as NMEA.

[0016] At least one tertiary alkanolamine has the general formula (I): [

[0017] [ [ [

Chemical formula

[0018] [ [ At least one secondary alkanolamine as an activator has the general formula (II): [

[0019] [ [ [

Chemical formula

[0020] The aqueous absorption medium further contains water. The aqueous absorption medium may further contain at least one physical solvent such as a physical solvent known in the art. Exemplary physical solvents include methoxytriglycol (MTG), sulfolane, glycerol, methanol, ethylene glycol, diethylene glycol, and triethylene glycol.

[0021] The total weight of at least one tertiary alkanolamine and at least one secondary alkanolamine present in the aqueous absorption medium accounts for the amine weight of the aqueous absorption medium (for example, the dry amine weight of the aqueous absorption medium). In other words, the amine weight of the aqueous absorption medium 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 absorption 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 absorption medium excluding the amine weight of the aqueous absorption medium is water.

[0022] The amine weight of the aqueous absorbent medium accounts for 25% to 65% by weight of the total weight of the aqueous absorbent medium (e.g., 25% to 60%, 30% to 55%, 30% to 50%, 40% to 55%, 45% to 55%, etc.). The amount of at least one secondary alkanolamine may be based on the total weight of amines in the aqueous absorbent medium. In embodiments, at least one secondary alkanolamine is present in an amount of 11% to 45% by weight (e.g., 11% to 40%, 11% to 35%, 11% to 30%, 11% to 25%, 12% to 20%, etc.) based on the amine weight of the aqueous absorbent medium.

[0023] The amount of at least one secondary alkanolamine may further be based on the total weight of the aqueous absorbent medium. For example, at least one secondary alkanolamine may be present in amounts of 5% to 25% by weight (e.g., 5% to 20%, 5% to 15%, 6% to 20%, 6% to 15%, 6% to 11%) based on the total weight of the aqueous absorbent medium. Since at least one secondary alkanolamine is present as an activator, it is present in amounts less than 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 (for a total of 100% by weight of aqueous absorbent medium).

[0024] In exemplary embodiments, at least one tertiary alkanolamine may be present in amounts 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.) based on the total weight of the aqueous absorbent medium.

[0025] Purpose The removal of acid gases from primarily gaseous flows can be performed using gas-liquid contactors (such as pressure swing absorption (PSA) and temperature swing absorption (TSA) using liquid absorbents). Amine loss due to volatility can be a concern in gas-liquid separation. Volatility can be particularly problematic when components of the aqueous absorbent, such as activators, have relatively low boiling points compared to commonly used tertiary alkanolamines. Therefore, it has been found that for aqueous absorbents containing at least one tertiary alkanolamine and at least one secondary alkanolamine as an activator, it can be beneficial for both to have relatively high boiling points (e.g., similar boiling points of at least 230°C). It should be noted that low boiling points imply high volatility in gas processing applications, but do not necessarily imply high solubility in liquid hydrocarbon media. In other words, volatility and hydrocarbon cosolubility are different principles, and in this case, volatility is the concern. In this regard, highly volatile amines tend to be impractical and costly for gaseous acid gas treatment.

[0026] A process for removing acid gas from a gas stream includes providing a gas stream and an aqueous absorbent medium for acid gas removal treatment. In a process for removing acid gas, the aqueous absorbent medium and a gaseous mixture containing the acid gas may be brought into countercurrent contact at low temperature and high pressure in an absorption tower to form an aqueous absorbent medium rich in acid gas. A cyclic adsorption process can utilize high-speed gas-liquid exchange, the transfer of a large liquid inventory between the absorption and regeneration steps, and high energy requirements for the regeneration of the amine solution. Such a process can take advantage of a large temperature difference in the gas stream between the absorption and desorption (regeneration) parts of the cycle. For example, an aqueous amine scrubbing method may use relatively low temperatures for acid gas uptake, e.g., below 50°C, and raise the temperature to over 100°C, e.g., above 120°C, for desorption.

[0027] Regeneration allows for the removal of acid gases, at least partially, from an aqueous absorbent medium containing a high concentration of acid gases, thereby forming an aqueous absorbent medium with a low concentration of acid gases. Regeneration of large quantities of aqueous absorbent medium to form an aqueous absorbent medium with a low concentration of acid gases may be carried out at temperatures exceeding 100°C, and as a result, many commonly used amines may suffer significant amine losses due to evaporation during the temperature swing process. Therefore, it is desirable to use amines that minimize such losses. After regeneration, the aqueous absorbent medium with a low concentration of acid gases may be recycled and returned to the acid gas removal process for reuse in removing acid gases from the gas stream.

[0028] Examples of gas streams that can be treated for acid gas removal include natural gas produced from hydrocarbon reservoirs, coal processing, biogas, and industrial gases such as refinery gases.

[0029] Examples Approximate characteristics, properties, parameters, etc., with respect to the information used in exemplary examples, comparative examples, and reported results for the examples and comparative examples are provided below.

[0030] The process for handling the gas flow is based on the one shown in Figure 1, where an aqueous absorbent medium can be supplied to the top of a gas-liquid counterflow packed bed (1 / 4-inch ceramic Intalox saddle) absorption column 2 via supply line 5. The gas flow can be introduced to the bottom of absorption column 2 through supply line 1 at a gas flow rate of approximately 17 standard liters / min. The composition of the dry feed is 90 mol% nitrogen and 10 mol% carbon dioxide. The pressure in the absorption column is set to approximately 250 psig. The clean gas (i.e., with reduced CO2 content) is discharged at the top of absorption column 2 through outlet line 3, and the residual CO2 level is measured by gas chromatography. The CO2-rich aqueous absorbent medium flows towards the bottom of absorption column 2 and exits via line 4.

[0031] The CO2-rich aqueous absorbent medium in line 4 is depressurized by the level control valve 8 and can flow through line 7 to the heat exchanger 9, where the CO2-rich aqueous absorbent medium can be heated. The heated CO2-rich aqueous absorbent medium can then enter the upper part of the regenerator 12 via line 10. The regenerator 12 is equipped with random packing (1 / 4 inch Pro-Pak®) that can desorb H2S and CO2. The pressure in the regenerator is set to approximately 27 psia. The gas from the regenerator 12 can then be sent through line 13 to the condenser 14, where any residual water and amines can be cooled and condensed. The gas can then enter the separator 15, where the condensed liquid can be separated from the vapor phase. The condensed aqueous solution can be pumped through line 16 via pump 22 to the upper part of the regenerator 12. The gas remaining from condensation can be removed through line 17 for final recovery and / or disposal. The regenerated aqueous solution flows downward through the regenerator 12 and the close-coupled reboiler 18. The reboiler 18, equipped with an electric heating device, can vaporize a portion of the aqueous solution to expel any residual gases. The reboiler temperature may be set to approximately 125°C. The vapor may rise from the reboiler and return to the regenerator 12, where it mixes with the falling liquid and then exits through line 13 to enter the condensation stage of the process. The regenerated aqueous absorbent medium from the reboiler 18 exits through line 19 and can be cooled in the heat exchanger 20. The regenerated (i.e., low in acid gas) aqueous absorbent medium can then be pumped via pump 21 to return to the absorber 2 through the solvent supply line 5.

[0032] In the examples, the following materials may be used as the main materials.

[0033] MDEA is available from The Dow Chemical Company or its affiliates and refers to a roughly 98% solution of tertiary alkanolamine methyldiethanolamine having the following structure:

[0034] [ka]

[0035] MAPD is available from The Dow Chemical Company or its affiliates and refers to a solution of approximately 98% of a secondary alkanolamine methylaminopropanediol having the following structure:

[0036] [ka]

[0037] NMEA is available from The Dow Chemical Company or its affiliates and refers to a roughly 98% solution of the secondary alkanolamine methylethanolamine having the following structure:

[0038] [ka]

[0039] The examples in Table 1 below compare the acid gas removal performance of MDEA-MAPD blends with commonly used MDEA-NMEA blends. NMEA, while a monovalent secondary alkanolamine, has a significantly lower average standard boiling point compared to MAPD, a divalent secondary alkanolamine. Furthermore, the examples are set to 50% by weight of water, which is a typical target for maximizing acid gas capacity while minimizing corrosion and processing problems. However, it is well known that the total amine content (and therefore the amount of water) may vary depending on the intended use and 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 in the aqueous absorption medium (i.e., excluding the weight of water).

[0040] [Table 1]

[0041] The examples in Table 1 are based on the treatment of a feed gas entering absorption column 2 with a carbon dioxide content of 10 mol% (i.e., feed line 1 in Figure 1). The goal is to remove sufficient carbon dioxide in absorption column 2 to achieve a carbon dioxide concentration of 2 mol% in the treated gas (i.e., outlet line 3 in Figure 1). Referring to Figure 1, regarding the flow rate for CO2 removal, it is desirable to achieve 2 mol% CO2 removal from the gas stream with the lowest possible flow rate of aqueous absorption medium (i.e., solvent) (e.g., less than 2.5 kg / hour) while using a relatively small amount of activator.

[0042] Referring to Figure 2, it is shown that desirable low flow rates are achievable for Examples 1 and 2. In contrast, for Comparative Examples A and B, the flow rates are higher than 2.5 kg / hour. In particular, as shown in the data provided in Figure 2 regarding solvent flow rate (i.e., in solvent supply line 5) versus treated gas CO2 (i.e., in the gas flow in outline line 3), both Examples 1 and 2 (i.e., data for MDEA-MAPD(43:7) and MDEA-MAPD(40:10)) can achieve a threshold of 2.0 mol% (20,000 ppmv) of treated gas CO2 at flow rates of less than 2.5 kg / hour. In particular, Figure 2 shows promising results for the removal of CO2 from a supply gas flow consisting of 90 mol% nitrogen and 10 mol% CO2. It is desirable to reach the specification of 2 mol% or less of CO2 in the treated gas flow (i.e., treated gas) at the lowest possible solvent circulation rate.

[0043] Referring to Figure 2, the amount of carbon dioxide removed is directly proportional to the rate at which carbon dioxide is absorbed, which can be determined by (i) the reaction rate between carbon dioxide and the activator, (ii) the concentration of the activator, and (iii) the flow rate of the solvent. Furthermore, it has been shown that for equimolar 2 mol% solutions (5 wt% NMEA and 7 wt% MAPD), using MAPD / MDEA can achieve improved carbon dioxide removal than using NMEA / MDEA at the same flow rate. Therefore, MAPD is considered to have a relatively fast reaction rate. In the examples containing equimolar amounts of NMEA and MAPD, it is surprising that the MAPD solution performs better than the NMEA solution, as it was expected that the additional hydroxyl would slightly degrade the performance.

[0044] Furthermore, the only solvent blend that can achieve sufficient carbon dioxide removal at relatively low flow rates is the MDEA / MAPD blend containing 7% and 10% by weight of MAPD. Additionally, while an MDEA / NMEA blend containing 5% by weight of NMEA may achieve a solvent circulation rate of approximately 3.0 kg / hour, the use of the MDEA / NMEA blend is undesirable in practice because NMEA has an average boiling point of approximately 158°C, making it too volatile under process conditions, potentially leading to high solvent loss 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 should also be noted that the average boiling point of MDEA is similarly approximately 247°C. Furthermore, as shown in Example 2, increasing the MAPD concentration provides similar benefits.

[0045] In relation to the above, it can be seen that certain MDEA / MAPD blends may perform better than MDEA / NMEA blends by using aqueous absorbents with lower circulation rates to meet standards for the removal of acidic gases such as carbon dioxide. Compared to NMEA, MAPD as an activator may not hinder the reaction with acidic gases such as CO2, and it is thought that this may result in a reaction rate fast enough to enable MAPD to act as a high-performance absorbent for acidic gas removal.

[0046] Therefore, it was found that an MDEA / MAPD blend in an aqueous absorption medium containing more than 5% by weight of MAPD per 50% by weight amine solvent can provide baseline performance for carbon dioxide removal at flow rates of less than 2.5 kg / hour while minimizing the amount of more expensive MAPD activator required for blending and minimizing volatility issues. This application provides, for example, the following inventions: [1] An aqueous absorbent medium for removing acid gas from a gas stream, (a) at least one tertiary alkanolamine having general formula (I), [ka] (In the formula, R 1 C 1 ~C 4 (It is alkyl.) (b) At least one secondary alkanolamine having general formula (II), [ka] (In the formula, R 2 C 1 ~C 4 (It is alkyl.) (c) Water and, An aqueous absorbent medium in which the total weight of the at least one tertiary alkanolamine and the at least one secondary alkanolamine accounts for 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. [2] The aqueous absorbent medium according to [1], wherein at least one tertiary 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 [1] or [2] above, wherein the amount of the at least one secondary alkanolamine is 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 the above [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] R 1 However, C 1 An aqueous absorbent medium as described in any one of the above [1] to [4], wherein the absorbent medium is alkyl. [6] R 2 However, C 1 An aqueous absorbent medium as described in any one of the above [1] to [4], wherein the absorbent medium is alkyl. [7] R 1 However, C 1 It is alkyl, R 2 However, C 1 An aqueous absorbent medium as described in any one of the above [1] to [4], wherein the absorbent medium is alkyl. [8] The aqueous absorbent medium according to any one of the above [1] to [7], wherein piperazine and methylethanolamine are excluded from the aqueous absorbent medium. [9] A process for removing an acid gas from a gas stream, comprising providing the gas stream and an aqueous absorbent medium as described in any one of [1] to [8] above.

[10] By bringing the gas flow and the aqueous absorbent medium into contact, an aqueous absorbent medium containing a large amount of acid gas is formed, To remove at least partially the acid gas from the aqueous absorbent medium containing a large amount of acid gas, thereby forming an aqueous absorbent medium that contains little acid gas, In order to further remove the acid gas from the gas stream, the aqueous absorbent medium containing little of the acid gas is recirculated, The process described in [9] above, further including the process described above.

Claims

1. An aqueous absorbent medium for removing acid gas from a gas stream, (a) at least one tertiary alkanolamine having general formula (I), 【Chemistry 1】 (In the formula, R １ C １ ~C ４ (It is alkyl.) (b) At least one secondary alkanolamine having general formula (II), 【Chemistry 2】 (In the formula, R ２ is a linear C １ ~C ４ (It is alkyl.) (c) Water and, An aqueous absorbent medium wherein the total weight of the at least one tertiary alkanolamine and the at least one secondary alkanolamine accounts for 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.

2. The aqueous absorbent medium according to claim 1, wherein the at least one tertiary 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, wherein the amount of the at least one secondary alkanolamine is 6% by weight to 11% by weight based on the total weight of the aqueous absorbent medium.

4. A process for removing acid gas from a gas stream, comprising providing the gas stream and the aqueous absorbent medium described in claim 1.

5. By bringing the gas flow and the aqueous absorbent medium into contact, an aqueous absorbent medium containing a large amount of acid gas is formed. The acid gas is removed from the aqueous absorbent medium containing a large amount of the aforementioned acid gas to form an aqueous absorbent medium from which the acid gas has been removed. In order to further remove the acid gas from the gas flow, the aqueous absorbent medium from which the acid gas has been removed is recirculated. The process according to claim 4, further comprising:

Images