Method of inhibiting nitrosation of an aqueous amine solution used in a process of removing carbon dioxide from a flue gas

Abstract

A method is provided for inhibiting nitrosation of an aqueous amine solution used in a process of removing carbon dioxide from a flue gas. That method includes a step of adding an effective amount of an antioxidant to the aqueous amine solution being circulated between an absorber and a stripper of a carbon dioxide capture system.

Claims

1. A method of inhibiting nitrosation of an aqueous amine solution used in a process of removing carbon dioxide from a flue gas, comprising: adding an effective amount of an antioxidant to the aqueous amine solution being circulated between an absorber and a stripper of a carbon dioxide capture system whereby carbon dioxide is removed from flue gas and formation of undesirable nitrosamines and nitramines is inhibited, wherein the antioxidant is selected from the group consisting of vanillin, vitamin D, erythorbic acid, triphenylmethylmercaptan, 2-mercapto 1-methyl benzimidazole and 2,5-dimercapto 1,3,5-thiodiazole.

2. The method of claim 1, including maintaining said antioxidant at a concentration of between about 0.01 mM to about 1,000 mM in said aqueous amine solution.

3. The method of claim 1, including maintaining said antioxidant at a concentration of between about 0.1 mM to about 100 mM in said aqueous amine solution.

4. The method of claim 1, wherein said antioxidant includes a is vanillin.

5. The method of claim 1, wherein said antioxidant is vitamin D.

6. The method of claim 1, wherein said antioxidant is erythorbic acid.

7. The method of claim 1, wherein said antioxidant is triphenylmethylmercaptan.

8. The method of claim 1, wherein said antioxidant is 2-mercapto 1-methyl benzimidazole.

9. The method of claim 1, wherein said antioxidant is 2,5-dimercapto 1,3,5-thiodiazole.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The accompanying drawing figures incorporated herein and forming a part of the specification, illustrate several aspects of the method and together with the description serve to explain certain principles thereof. In the drawing figures:

(2) FIG. 1 is a schematic illustration of a process for removing acid gas, such as carbon dioxide, from a flue gas utilizing an aqueous amine solvent and thermal swing regeneration.

(3) FIG. 2 is a graph of amount of nitrosomorpholine (NSMO) versus reaction time illustrating inhibitor evaluation for morpholine nitrosation under reaction conditions: 5 M morpholine, 100 ppm sodium nitrite, 25 mM isoascorbic acid, 25 hours.

(4) Reference will now be made in detail to the present preferred embodiments of the method, examples of which are illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

(5) For amine-based CO.sub.2 capture, the presence of dissolved oxides of nitrogen has the potential to act as a nitrosating agent for reaction with secondary amines. Such reactions can lead to the formation of nitrosamines, which are potential undesired environmental pollutants. The present method relates to inhibiting NO.sub.x induced amine degradation during carbon dioxide (CO.sub.2) capture from a gas stream. The inhibitor is completely or partially soluble in the CO.sub.2 capture solution. The inhibitor is a chemical additive that presumably reacts with the NO.sub.x derived species and prevents the undesirable reaction with the amine. More specifically, the chemical additive presumably reacts with the radical species present in the solution from the dissolved NO.sub.x compounds present in the flue gas contacting the amine. The inhibitor is replenished in a CO.sub.2 capture process as needed by adding more to the solution. The inhibitor concentration could be monitored in the process using either online or offline process quality control methods.

(6) The described method is relevant to any post-combustion CO.sub.2 capture process that utilizes amines. The method is specifically relevant to the use of secondary amines (R.sub.2NH) as the amine solvent since secondary amines form stable nitrosation by-products. Secondary amines are particularly relevant to post-combustion CO.sub.2 capture as they are characterized by fast reaction rate (important at low driving force such as this) and lower energy of regeneration. Furthermore, the method can be applied to any amine based post-combustion process since the degradation pathways due to the process conditions of primary (RNH.sub.2) and tertiary (R.sub.3N) amines can lead to secondary amines. Typical secondary amines used for CO.sub.2 capture are, piperazine, diethanolamine, morpholine and/or mixture thereof. Depending on the amine solution used the amine concentrations are between 15 and 70 wt % with the balance typically as water.

(7) In the broadest sense the additives for the process can be characterized as antioxidants. However, as shown in the examples certain types of compounds are more effective at inhibition than others. The most effective inhibitors are characterized by the presence of a heteroatom (N, S, or O) within an aromatic ring. The most advantageous materials for the process are typically N containing heterocycles and/or thiol groups. It has been found that the presence of these compounds can serve a triple active role in simultaneously reducing nitrosation, oxidation, and corrosion. For example, of particular interest for the invention are the compounds 2-mercaptobenzthiazole, 2-mercapto 1-methyl benzimidazole, 2,5-dimercapto1,3,5-thiodiazole, and 2-mercaptobenzimidazole.

(8) The amount of inhibitor added to the CO.sub.2 capture solvent can vary depending on the solvent selected, the secondary amine products formed in degradation (if not present originally), process temperatures, and the amount of NO.sub.x compounds in the flue gas. A typical amount of inhibitor would be between 0.1 mM to 100 mM. It is possible that ranges from 0.01 mM to 1000 mM could be used.

(9) The inhibitors used in the present method maintain their activity for inhibiting nitrosation reaction at the elevated temperatures applied in typical CO.sub.2 capture process. This temperature is typically between 30-60 C. for scrubber and 100-160 C. for stripper.

(10) The advantage of the method is to prevent the nitrosation reaction presumably by increasing the rate of the competing reaction with inhibitor or by preventing the radical reaction of the nitrosating agent with amine. The prevention of the nitrosation reaction significantly reduces or eliminates potentially detrimental emissions into the environment for this reaction.

(11) A secondary benefit of the proposed inhibitors is that they can potentially prevent the degradation of amine solvent via other degradation pathways reducing amine degradation loss during the process which helps in process cost reduction.

(12) Consistent with this description, the current method may be broadly described as comprising the step of adding an effective amount of antioxidant to the aqueous amine solution being circulated between an absorber and a stripper of a carbon dioxide capture system. As a result, the carbon dioxide is removed from the flue gas and formation of undesirable nitrosamines and nitramines is inhibited.

(13) The method may include the step of maintaining the antioxidant at a concentration of between about 0.01 mM to about 1000 mM in the aqueous amine solution. In another possible embodiment, the method includes the step of maintaining the antioxidant at a concentration of between about 0.1 mM to about 100 mM in the aqueous amine solution.

(14) The antioxidant utilized in the method may be a food preservative. In one useful embodiment, the antioxidant is a nitrogen containing heterocycle. In another useful embodiment, the antioxidant is a sulfur containing heterocycle. In still another useful embodiment, the antioxidant includes a thiol group.

(15) Specific antioxidants used in the current method include, but are not limited to, vanillin, vitamin D, ascorbic acid, erythorbic acid, ethoxyquin, triphenylmethylmercaptan, 2-mercaptobenzthiazole, methylmercaptobenzimidazole, 2,5-dimercapto 1,3,5-thiodiazole, and 2-mercaptobenzimidazole. Any of the classes or specific antioxidants identified above may be used individually or in combination in the current method for inhibiting nitrosation of an aqueous amine solution or solvent used in a process of removing carbon dioxide from the flue gas.

(16) While numerous potential methods for practicing the invention could be described an example is given. An EOR facility requires CO.sub.2 delivered to extend the extraction efficiency of a well. An amine based CO.sub.2 scrubber is constructed at a nearby coal-fired power plant to capture a slipstream of the product flue gas. An aqueous diethanolamine solution is used to capture the CO.sub.2. The flue gas from the power plant, despite NO.sub.x reduction using selective catalytic reduction, contains 100 ppm of NO.sub.x species. Nitrosodiethanolamine, a regulated compound, and other nitroso compounds are monitored using analytical methods in the amine solvent and the absorber gaseous emissions. An inhibitor/additive such as 2-mercaptobenzthiazole is dosed into the liquid process stream to bring nitroso compound generation below a predefined limit. The concentration of the additive is also monitored and a makeup amount is dosed as needed.

(17) The following examples are presented to further illustrate but not limit the current method as set forth and described in the following claims:

EXPERIMENTAL

(18) For laboratory tests, the reactions are conducted in a 100 mL 4-necked round bottom flask fitted with a condenser at the gas outlet. The apparatus is immersed in an oil bath for heating control (2 C.). A K-type thermocouple is immersed in the solution to maintain the required temperature. The outgoing gas is purged into 2.5 M KOH bath to trap the NO.sub.x based components before connecting to the hood vent stack. 100 ppm NO.sub.2 gas cylinder is purchased from Scott-Gross. The incoming gas from the cylinder is passed into the reactor medium with specified flow rate using a flow meter (0.4-2 SCFH).

(19) For the high temperature studies, a solution containing amine solvent and sodium nitrite are placed in a stainless steel pressure rated reactor. These reactors are kept at the desired temperature in ovens and are taken out at regular time intervals from the ovens. Similar experiments are conducted with the solution containing inhibitor.

(20) The known amounts of samples are collected at periodic time intervals from each reaction. These samples are extracted in methylene chloride to analyze the organic layer by GC-MS for nitrosamine quantification. Nitrosopyrrolidone is used as an internal standard.

Examples 1-11

Experiments with NO2 Gas

(21) For a standard reaction, 100 mL of 5 M morpholine is taken into the flask. Inhibitor (5 mM) is added to the solution. The temperature is maintained at 30 C. 100 ppm NO.sub.2 gas in nitrogen is passed through the solution at the flow rate of 2 SCFH for 6 h. The inhibition activity of the different studied inhibitors is calculated based on the blank run (Example 7) as summarized in Table 1.

(22) TABLE-US-00001 TABLE 1 Effect of nitrosation inhibitors with NO.sub.2 gas Amount of Inhibition Nitrosomorpholine activity Example Inhibitor (g/g) (%) 1. No Inhibitor 88 2. Vanillin 54 39 3. Vitamin E 47 46 4. Triphenylmethylmercaptan 38.3 56 5. 2-mercaptobenzothiazole 15 83 6. Ascorbic acid 14 84 7. Erythorbic acid 12 86 8. 2-mercapto-1-methylimidazole 12.4 86 9. 2,5-dimercapto 1,3,4-thiodiazole 9.8 89 10. 2-Mercaptobenzimidazole 8.9 94 11. Ethoxyquin 4 95 Experimental conditions: Morpholine 5M, NO.sub.2 100 ppm with flow rate 2 SCFH, 30 C., 6 h, inhibitor 5 mM

Example 12-17

Experiments with Sodium Nitrite at High Temperature

(23) The reactions are conducted in 10 mL stainless steel reactors. The reactor was filled with 4 mL 5 M morpholine solution and 1000 ppm sodium nitrite. 25 mM erythorbic acid was added as inhibitor. The reactor was placed in a thermostatic oven at the temperature mentioned in Table 2. The inhibitor was found active and stable even at high temperature (FIG. 2).

(24) TABLE-US-00002 TABLE 2 Inhibitor quantity and different temperatures Inhibitor quantity Temperature Example (mM) ( C.) 12 0 125 13 0 135 14 0 145 15 25 125 16 25 135 17 25 145 Experimental conditions: Morpholine 5M, sodium nitrite 1000 ppm, 25 h

Example 18-23

Inhibition with Different Amines

(25) In the reaction vessel, 100 mL of 5 M solvent was charged. Erythorbic acid 5 mM was added as an inhibitor to it. The temperature was maintained at 30 C. 100 ppm NO.sub.2 gas in nitrogen was passed through the solution at the flow rate of 2 SCFH for 6 h. The inhibition activity of erythorbicacid with different amines is summarized in Table 3.

(26) TABLE-US-00003 TABLE 3 Nitrosation inhibition with isoascorbic acid over different amines Amount of Inhibition Inhibitor Nitrosomorpholine activity Example Amine (mM) (g/g) (%) 18 Morpholine 0 88 19 Morpholine 25 12 86 20 Diethanolamine 0 68 21 Diethanolamine 25 7 90 22 Piperazine 0 78 23 Piperazine 25 34 56 Experimental conditions: Amine 5M, NO.sub.2 100 ppm with flow rate 2 SCFH, 30 C., 6 h, inhibitor

Example 24-31

Demonstration of Multi-Use Inhibition

(27) Table 4 shows the ability of various amines to inhibit the oxidation. Specifically, a solution of the common CO.sub.2 capture amine MEA were exposed to air and CO.sub.2 mixture at accelerated high temperature conditions for 100 h. It was found that the inhibitors containing both a N-containing 5 or 6 membered ring and a thiol group were most effective at inhibiting MEA oxidation to formate.

(28) TABLE-US-00004 TABLE 4 Inhibitors tested for oxidative degradation and their activity % formate # Inhibitors screened inhibition 24 2,5-dimercapto-1,3,4-thiodiazole 100 25 2-mercapto-1-methyl imidazole 100 26 2-mercaptobenzothiazole 100 27 sodium sulfide 64 28 alpha-toluenethiol 76 29 Triphenylmethyl mercaptan 78 30 benzyl sulfide 0 31 2-mercaptobenzimidazole 100 Experimental conditions: 5M MEA, 25% CO.sub.2 + 75% air, 80 C., 5 mM inhibitor

(29) The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.