Process for carbon dioxide recovery

Abstract

3-(Methylamino)propane-1,2-diol (MAPD) and its homologs are potentially useful alkanolamine components in aqueous absorbent that is used in carbon dioxide recovery from gas streams. MAPD can be generated in situ in the process by reaction of 3-(dimethylamino)propane-1,2-diol with a primary or secondary amine. A side reaction of MAPD and carbon dioxide can form an oxazolidinone side product, but the oxazolidinone side product can be decomposed to recover the MAPD by contact with alkali metal hydroxide.

Description

[0001] PROCESS FOR CARBON DIOXIDE RECOVERY

[0002] FIELD

[0003] This invention relates to the field of carbon dioxide recovery.

[0004] INTRODUCTION

[0005] Recovery of carbon dioxide from gas streams is a widely practiced process. Many industrial processes produce air streams that contain carbon dioxide, including power production, metal refining, and chemical flaring; producers want to reduce the carbon dioxide before returning the air stream to the atmosphere. The water shift gas process produces a mixture of hydrogen gas and carbon dioxide, which are often separated for use. Natural gas streams coming from the well may contain up to 30 mole percent carbon dioxide, which must be removed to achieve US pipeline specifications of 2 mole percent or less.

[0006] In normal carbon dioxide recovery, carbon dioxide is captured using a liquid absorbent. The liquid absorbent cycles between an absorption step and a regeneration step.

[0007] In the absorption step, a gaseous feed stream that contains high levels of carbon dioxide (called a “rich feed stream”) is contacted with a liquid absorbent that contains low levels of carbon dioxide (called “lean absorbent”). The contact takes place under low temperature and high pressure, as compared to the regeneration step. The conditions cause carbon dioxide to be absorbed from the feed stream into the absorbent, creating a lean feed stream that contains less carbon dioxide than the rich feed stream and a rich absorbent that contains more carbon dioxide than the lean absorbent. The lean feed stream and the rich absorbent stream are separated. The rich absorbent is sent to the regeneration step. Usually the absorption step takes place in an absorption column.

[0008] In the regeneration step, the rich absorbent is subjected to higher temperatures and lower pressures than the absorption step, causing carbon dioxide to desorb from the absorbent. The regeneration process yields lean absorbent and a carbon dioxide stream. The carbon dioxide is recovered, and the lean absorbent is sent back to the absorption step. The regeneration step is normally carried out in a column, which is sometimes called a stripper, but in some cases one or more streams of carbon dioxide may be flashed off from the rich absorbent before the regeneration column.

[0009] In the transition from the absorption step to the regeneration step, the temperature of the rich absorbent is increased by heat exchangers and supplemental heating in a reboiler associated with the regeneration column. The pressure is lowered, such as by pressure reducing valves.

[0010] In the transition from the regeneration step to the absorption step, the temperature of the lean absorbent is lowered by heat exchangers and optionally supplemental cooling. The pressure is increased, such as by compressors.

[0011] Absorbents used in the process may be physical absorbents or chemical absorbents. Physical absorbents dissolve carbon dioxide but do not chemically react with it. Chemical absorbents undergo a reversible chemical reaction with carbon dioxide in the absorption step, and the reaction reverses to release the carbon dioxide in the regeneration step. Chemical absorbents are often aqueous solutions containing 40 to 70 weight percent (wt%) water and 30 to 60 wt% water-soluble amines. Some of the amines most commonly used in carbon capture are monoethanolamine (MEA), methyldiethanolamine (MDEA), 2-amino-2-methylpropanol (AMP), piperazine (PIP), aminoethoxyethanol (AEE), diethanolamine (DEA), and di-isopropanolamine (DIPA).

[0012] The water and amines can undergo several reaction paths with carbon dioxide, but the main reactions are:

[0013] • Water reacts with carbon dioxide to form carbonic acid and bicarbonate salts. Primary, secondary and tertiary amines can all catalyze this reaction. This reaction proceeds slowly but, because the amines are not consumed in the reaction, each amine molecule can participate in the capture of one or more carbon dioxide molecules.

[0014] • Primary and secondary amines react with carbon dioxide in the absorption step to form carbamates. Usually, the carbamates can be decomposed with water in the regeneration step to recover the amine and release carbon dioxide as a bicarbonate salt. The reaction of carbon dioxide with amines to form carbamate proceeds quicker than the reaction of carbon dioxide with water to form carbonic acid, so that primary and secondary amines normally get consumed before catalyzing carbonic acid formation. The carbamate formation consumes two amine molecules for each molecule of carbon dioxide that is captured. After the carbamate is formed, the amine will not participate in capture until it is recovered in the regeneration step.

[0015] Frequently, both reactions are desired, and so an aqueous combination of tertiary amines with primary and secondary amines are used in absorbents for carbon dioxide.

[0016] The amines are often selected to be strongly hydrophilic, so that they are not stripped out of the absorbent by the feed stream. Amines with a high ratio of amine groups and hydroxyl groups to carbon atoms are usually more hydrophilic, and so alkanolamines and diamines are commonly used.

[0017] Primary and secondary amines are also selected to have high reactivity with carbon dioxide to form carbamates in the absorption step, and to have high reactivity to release carbon dioxide in the regeneration step. Steric hinderance from the organic groups bonded to the amine nitrogen can encourage the carbon dioxide to dissociate in the regeneration step, which is desirable. On the other hand, steric hinderance from the organic groups bonded to the amine nitrogen can also slow or prevent formation of the carbamate in the absorption step. The sizes of steric-hindering groups in the amines are balanced to balance the performance in each step.

[0018] There is a need to identify alkanolamines that have a good hydrophilicity and good balance of activity in the absorption and regeneration steps.

[0019] SUMMARY

[0020] Recently, 3-(methylamino)propane-l,2-diol (MAPD) and its homologs have been identified as alkanolamine components in aqueous absorbent. MAPD is hydrophilic and has high reactivity in both the absorption and regeneration steps. However, two problems discourage commercial adoption of MAPD. First, MAPD is costly to make. A related compound, 3-(dimethylamino)propane-l,2-diol (DMAPD) is less costly to make, but is a tertiary amine that does not form carbamates.

[0021] Second, in addition to forming the carbamate, MAPD can undergo a side reaction with carbon dioxide to form MAPD oxazolidinone (MAPD-OX) according to the reaction mechanism in Formula 1 :

[0022] Unlike the carbamate, MAPD-OX does not readily break down to release carbon dioxide under normal conditions in the regeneration step.

[0023] We have discovered that in a heated aqueous solution, 3-(dimethylamino)propane-l,2-diol (DMAPD) will transfer a methyl group to a primary or secondary amine, to make MAPD and a tertiary amine in situ, as shown in Formula 2.

[0024] We have further discovered that MAPD-OX will react with alkali metal hydroxide in an aqueous solution to regenerate MAPD and form alkali metal bicarbonate, as shown in Formula 3.

[0025] MAPD oxazolidin-2-one MAPD

[0026] These two reactions can enable the practical and economical use of MAPD and similar compounds in carbon capture.

[0027] This invention is a process to recover carbon dioxide from a gaseous rich feed stream that contains carbon dioxide, comprising the following steps: a) an absorption step in which (1) the gaseous rich feed stream is contacted with a lean aqueous absorbent under conditions such that the absorbent absorbs carbon dioxide from the feed stream to produce a gaseous lean feed stream that contains less carbon dioxide than the rich feed stream and a rich aqueous absorbent that contains more carbon dioxide than the lean absorbent, and (2) the rich aqueous absorbent is separated from the gaseous lean feed stream; and b) a regeneration step in which (1) the rich aqueous absorbent from the absorption step is subjected to conditions such that carbon dioxide desorbs from the rich aqueous absorbent to produce a carbon dioxide stream and a lean aqueous absorbent, and (2) the lean aqueous absorbent is returned to the absorption step; wherein (and characterized in that): i. the aqueous absorbent contains 30 to 70 wt% water and 70 to 30 wt% of an amine mixture that contains 3-(dialkylamino)propane-l,2-diol (DAkAPD) and a primary or secondary amine; ii. in the aqueous absorbent, the DAkAPD and the primary and secondary amine react to transfer an alkyl group from amine nitrogen in the DAkAPD to the amine nitrogen in the primary and secondary amine, such that the aqueous absorbent comprises DAkAPD and 3-(alkylamino)propane-l,2-diol (AkAPD) and primary or secondary amine and secondary or tertiary amine; and iii. at least some of the AkAPD reacts with carbon dioxide in the absorption step to form an oxazolidinone reaction product; and iv. the oxazolidinone reaction product formed in (iii) is contacted with aqueous alkali metal hydroxide under conditions such that at least some of the oxazolidinone reaction product decomposes to release the AkAPD and a carbonate or bicarbonate salt.

[0028] The process generates MAPD and related compounds in situ in the carbon dioxide recovery system, and breaks up the oxazolidinone reaction product in situ to recover MAPD for further use.

[0029] DETAILED DESCRIPTION

[0030] The process of this invention recovers carbon dioxide from a carbon dioxide-rich gaseous feed stream using an aqueous liquid absorbent. In some embodiments, the rich feed stream may contain air or hydrogen or methane or ethane or propane or natural gas or ethylene or propylene or combustion products. In some embodiments, the rich feed stream is effluent from an industrial process. In some embodiments, the rich feed stream is the product of a water-shift gas process. In some embodiments, the rich feed stream comprises natural gas.

[0031] Absorption Step

[0032] In the absorption step, the rich feed stream is contacted with lean absorbent under conditions such that the lean absorbent absorbs carbon dioxide from the rich feed stream to form a lean feed stream and a rich absorbent. Then the rich absorbent is separated from the lean feed stream. The rich absorbent moves to the regeneration step. The lean feed stream may be recovered, or it may be further processed, or it may be vented.

[0033] In some embodiments, the absorption step takes place in an absorption column. In some embodiments, the absorbent flows through the absorption column counter-current to the feed stream. The rich feed stream is introduced into a lower portion of the column and flows toward the top. Lean absorbent is introduced into an upper portion of the column and flows toward the bottom.

[0034] The lean absorbent contains a lower loading of carbon dioxide than the rich absorbent produced in the absorption step. (“Loading” means the total moles of carbon dioxide divided by the total moles of amine in the absorbent.) In some embodiments, the lean absorbent contains no more than a 0.2 moles- per-mole loading of carbon dioxide or no more than 0.1 or no more than 0.08. There is no minimum desired loading for the lean absorbent; the loading of carbon dioxide may be undetectable (essentially 0). In some embodiments, the lean absorbent contains at least 0.01 moles-per-mole loading of carbon dioxide or at least 0.02 or at least 0.05.

[0035] Conditions in the absorption step are selected to encourage absorption of the carbon dioxide from the rich feed stream into the lean absorbent.

[0036] • In some embodiments, the pressure in the absorption step (absorption pressure) is at least 8 bar or 10 bar or at least 12 bar or at least 14 bar. In some embodiments, the absorption pressure is at most 100 bar or at most 80 bar or at most 70 bar or at most 60 bar or at most 50 bar.

[0037] • In some embodiments, the temperature of absorbent in the absorption step (absorption temperature) is at most 100°C or 90°C or at most 80°C or at most 70°C or at most 60°C or at most 55°C or at most 50°C. In some embodiments, the absorption temperature is greater than 0°C or at least 10°C or at least 20°C or at least 30°C.

[0038] • In some embodiments, the absorption step takes place in an absorption column that has packing and / or trays to extend the contact of the feed stream and the absorbent. Examples of useful packing include random packing or structured packing.

[0039] In some embodiments, the absorption step captures at least 90 percent of the carbon dioxide in the rich feed stream, or at least 95 percent or at least 98 percent or at least 99 percent or at least 99.5 percent or at least 99.8 percent. There is no maximum desired capture of carbon dioxide, but in some embodiments, the lean feed stream may retain up to 0.001 percent of the carbon dioxide in the rich feed stream (99.999 percent capture) or up to 0.01 percent (99.99 percent capture).

[0040] The rich absorbent leaving the absorption column contains a higher loading of carbon dioxide than the lean absorbent that enters the absorption column. In some embodiments, the rich absorbent contains at least 0.3 moles-per-mole loading of carbon dioxide or at least 0.4 loading of carbon dioxide or at least 0.5 loading of carbon dioxide. In some embodiments, the rich absorbent contains no more than 1.0 moles-per-mole loading of carbon dioxide or no more than 0.8 loading or no more than 0.7 loading.

[0041] The rich absorbent proceeds from the absorption step to the regeneration step.

[0042] Regeneration Step

[0043] In the regeneration step, the rich absorbent is subjected to raised temperature and / or lowered pressure, as compared to the absorption step, in order to desorb carbon dioxide, producing a lean absorbent and a carbon dioxide stream. The regeneration step may be carried out in a single stage or in a plurality of stages that have successively higher temperature and / or lower pressure. The regeneration step may be carried out in known equipment such as in distillation columns and / or in flash drums.

[0044] In some embodiments, the regeneration step is carried out in one stage in a single regeneration column. In some embodiments, a flash drum removes a first stream of carbon dioxide before a main regeneration column removes a second stream of carbon dioxide.

[0045] In some embodiments, at least part of the regeneration step is carried out at a regeneration pressure of at most 5 bar, or at most 4 bar or at most 3 bar or at most 2 bar or at most 1 bar. In some embodiments, the regeneration pressure is at least 0.5 bar or at least 0.8 bar or at least 0.9 bar or at least 1 bar. In some embodiments, at least part of the regeneration step is carried out at a regeneration temperature of at least 90°C or at least 100°C or at least 110°C. In some embodiments, the regeneration temperature is at most 150°C or at most 140°C or at most 135°C. In some embodiments, the regeneration temperature is maintained low enough to avoid damaging the alkanolamines in the absorbent. On the other hand, partial vaporization of water may improve recovery of carbon dioxide.

[0046] In order to maintain an appropriate temperature in the regeneration step, the absorbent may receive supplemental heat before or during the regeneration step. Additional heat may be supplied by known means, such as a reboiler, a heating jacket or a heating coil. In some embodiments, a reboiler associated with a regeneration column heats a portion of the absorbent leaving the regeneration step and recycles it to the regeneration step.

[0047] The regeneration step produces a lean absorbent stream and one or more carbon dioxide streams. The lean absorbent is separated from the carbon dioxide and returned to the absorption step. The carbon dioxide streams may be recovered for use, storage or further processing.

[0048] Transition between Absorption and Regeneration

[0049] As previously described, the regeneration step takes place at a higher temperature than the absorption step. Rich absorbent leaving the absorption step is heated to prepare for regeneration. Lean absorbent leaving the regeneration step is cooled to prepare for the absorption step. At least part of the heating and cooling can be accomplished by heat exchangers that transfer heat from the lean absorbent to the rich absorbent. Examples of suitable heat exchangers include shell & tube, plate & frame, or plate & shell exchangers. Further heating and cooling may be provided by supplemental heaters and coolers.

[0050] As previously described, the regeneration step takes place at a lower pressure than the absorption step. Pumps can raise the pressure of the lean absorbent to the absorption pressure before it enters the absorption step. Examples of suitable pumps include centrifugal, positive displacement, regenerative turbine, axial flow, and ejector pumps. Likewise, pressure-reducing valves can reduce the pressure of the rich absorbent from the absorption pressure before it enters the regeneration step. Examples of suitable pressure-reducing valves include globe valves, diaphragm valves, gate valves, and needle valves.

[0051] Aqueous Absorbent Solution

[0052] In this invention, the absorbent used in the process steps described above comprises an aqueous absorption solution that contains (i) from 30 to 70 wt% water, (ii) from 30 to 70 wt% of an amine mixture. In some embodiments, the aqueous absorbent contains at least 35 wt% amine mixture or at least 40 wt% or at least 45 wt%. In some embodiments, the aqueous absorbent contains at most 75 wt% amine mixture or at most 70 wt% or at most 65 wt% or at most 60 wt% or at most 55 wt% or at most 50 wt%. For the purposes of these ratios reaction products of the amines, including those described above, are counted as part of the amine mixture.

[0053] In some embodiments, the aqueous absorbent contains at least 35 wt% water or at least 40 wt% or at least 45 wt% or at least 50 wt%. In some embodiments, the aqueous absorbent contains at most 65 wt% water or at most 60 wt% or at most 55 wt%. The amine mixture contains 3-(dialkylamino)propane-l,2-diol (DAkAPD) and a primary or secondary amine.

[0054] In some embodiments, the 3-(dialkylamino)propane-l,2-diol (DAkAPD) in the amine mixture meets Formula 4: wherein each R1is independently an alkyl group, selected such that the DAkAPD is water soluble. Each R1group contains at least 1 carbon atom. In some embodiments, each R1group independently contains at most 6 carbon atoms or at most 5 carbon atoms or at most 4 carbon atoms or at most 3 carbon atoms or at most 2 carbon atoms. For example, in some embodiments, the alkyl groups in R1are independently selected from methyl, ethyl, propyl or butyl groups. In some embodiments, the alkyl groups are ethyl. In some embodiments, the alkyl groups are methyl, and the DAkAPD is 3-(dimethylamino)propane-l,2-diol (DMAPD).

[0055] Some DAkAPD are commercially available. Others can be made by known processes, such as by reacting a secondary amine (R'zNH) with glycidol.

[0056] In some embodiments, the primary or secondary amine in the amine mixture meets Formula 5 : (5) HNR2R3wherein R2is an alkyl group and R3is hydrogen or an alkyl group. In some embodiments, R3is hydrogen, and the amine is a primary amine. In some embodiments, R3is an alkyl group, and the amine is a secondary amine. In some embodiments, R2and / or R3are further linked to an amine group to form a diamine or polyamine. In some embodiments, R2and / or R3are linked to each other to form a cyclic structure, which may be more stable in the process.

[0057] In some embodiments, R2and R3are selected such that the amine is water-soluble. In some embodiments, R2and R3are selected such that the primary or secondary amine contains at most 10 carbon atoms or at most 8 carbon atoms or at most 6 carbon atoms or at most 5 carbon atoms or at most 4 carbon atoms. In some embodiments, R2and R3are selected such that the primary or secondary amine contains at least 2 carbon atoms or at least 3 carbon atoms or at least 4 carbon atoms. In some embodiments, the primary amine contains no more than 6 carbon atoms per nitrogen atom or no more than 5 carbon atoms or no more than 4 carbon atoms or no more than 3 carbon atoms or no more than 2 carbon atoms. The primary amine contains at least 1 carbon atom per nitrogen atom, and in some embodiments contains at least 2 carbon atoms per nitrogen atom.

[0058] Examples of suitable amines include piperazine, piperidine, pyrrolidine and hydroxyethylpiperazine. In some embodiments, the primary or secondary amine may comprise a single amine, and in some embodiments, the primary or secondary amine may comprise a mixture of amines.

[0059] In some embodiments, the molar ratio of nitrogen atoms in the primary or secondary amine to nitrogen atoms in the DAkAPD is at least 30 percent or at least 35 percent or at least 40 percent or at least 45 percent or at least 50 percent or at least 55 percent. In some embodiments, the molar ratio of nitrogen atoms in the primary or secondary amine to nitrogen atoms in the DAkAPD is at most 110 percent or at most 100 percent or at most 90 percent or at most 80 percent or at most 75 percent or at most 70 percent or at most 60 percent. For the purpose of these ratios, it is recognized that both the DAkAPD and the primary or secondary amine may form reaction products, and those reaction products are counted as contributing to the numbers of DAkAPD and the primary or secondary amine. For example:

[0060] • The primary or secondary amine can react with carbon dioxide to form carbamates, and these carbamates are treated as part of the primary or secondary amine for calculating the ratios above.

[0061] • The DAkAPD and the primary or secondary amine react with each other as described above to transfer an alkyl group from the DAkAPD to the primary or secondary amine, making 3- (alkylamino)propanc-l,2-diol (AkAPD) and an alkylated amine. In calculating the ratios above, AkAPD is counted as part of the DAkAPD, and alkylated amine is counted as part of the primary or secondary amine.

[0062] As previously described, the primary or secondary amine reacts the DAkAPD in the conditions of the process to transfer an alkyl group from the nitrogen atom in DAkAPD to the nitrogen atom in the primary or secondary amine. The products are AkAPD and a secondary or tertiary amine. In some embodiments, AkAPD is illustrated by Formula 6: wherein R1has the meaning and embodiments previously described.

[0063] The reaction does not consume all of the DAkAPD or the primary or secondary amine, so the amine mixture contains both AkAPD and DAkAPD and amine mixture contains both the primary or secondary amine and the corresponding secondary or tertiary amine product. In some embodiments, the molar ratio of AkAPD to DAkAPD, based on the quantity of DAkAPD, is at least 0.5 mole percent or at least 1 mole percent or at least 3 mole percent or at least 4 mole percent or at least 5 mole percent or at least 7 mole percent or at least 8 mole percent or at least 10 percent. In some embodiments, the molar ratio of AkAPD to DAkAPD, based on the quantity of DAkAPD, is at most 100 percent or at most 80 percent or at most 60 percent or at most 50 percent or at most 40 percent or at most 30 percent or at most 20 percent or at most 18 percent or at most 16 percent or at most 15 percent. In some embodiments, the lean aqueous absorbent contains at least 0.05 wt% AkAPD or at least 0.1 wt% or at least 0.3 wt% or at least 0.5 wt% or at least 0.7 wt% or at least 0.9 wt% or at least 1 wt% or at least 3 wt% or at least 5 wt% or at least 8 wt% or at least 10 wt% or at least 12 wt% or at least 15 wt%. In some embodiments, the lean aqueous absorbent contains at most 20 wt% AkAPD or at most 18 wt% or at most 15 wt% or at most 12 wt% or at most 10 wt% or at most 8 wt% or at most 5 wt% or at most 4 wt% or at most 3 wt% or at most 2 wt%. For the purposes of these ratios, reaction products of the DAkAPD (other than AkAPD) counted as part of the DAkAPD, and reaction products of the AkAPD, such as carbamates and oxazolidinones, are counted as part of the AkAPD. Conversion of DAkAPD to AkAPD may not happen immediately, but rather in some embodiments AkAPD may accumulate in the aqueous absorbent over time.

[0064] In some embodiments, the amine mixture contains tertiary amines in addition to the tertiary amines formed in the reactions described above. Examples of such tertiary amines include MDEA and triethanolamine.

[0065] It is expected that most of the AkAPD reacts with carbon dioxide in the absorption step to form carbamates by the mechanism previously described. However, over time some of the AkAPD also undergoes side reactions with carbon dioxide to form oxazolidinone products, as previously described in (iii). In the process of this invention, described in (iv), the oxazolidinone products are broken down to recover AkAPD and carbon dioxide (in the form of alkali metal bicarbonate) by contact with alkali metal hydroxide. In some embodiments, this contact is performed outside the absorption step. In some embodiments, this contact is performed before or during the regeneration step. In some embodiments, the oxazolidinone products of AkAPD are contacted with alkali metal hydroxide in the regeneration step. In some embodiments, the oxazolidinone products of AkAPD arc contacted with alkali metal hydroxide in the transition from the absorption step to the regeneration step.

[0066] Any alkali metal hydroxide may be used. In some embodiments, the alkali metal hydroxide is sodium hydroxide. In some embodiments, the alkali metal hydroxide is potassium hydroxide. In some embodiments, the alkali metal hydroxide is dissolved in a solvent, such as in an aqueous solution. In some embodiments, the alkali metal hydroxide is added continuously to the aqueous absorbent. In some embodiments, the alkali metal hydroxide is added periodically to the aqueous absorbent. For example, the aqueous absorbent may be analyzed, and alkali metal hydroxide may be added when oxazolidinone levels become undesirably high.

[0067] In some embodiments, the molar ratio of alkali metal hydroxide to oxazolidinone during this step is at least 50% or at least 60% or at least 0% or at least 80% or at least 90% or at least 100% or at least 110%. In some embodiments, the molar ratio of alkali metal hydroxide to oxazolidinone during this step is at most 200% or at most 180% or at most 160% or at most 140% or at most 120% or at most 110% or at most 100%.

[0068] In some embodiments, the lean absorbent entering the absorption step contains no more than 50 wt% oxazolidinone products of AkAPD, based on the weight AkAPD, or no more than 40 wt% or no more than 30 wt% or no more than 20 wt% or no more than 10 wt%. In some embodiments, the lean absorbent entering the absorption step contains no measurable oxazolidinone products of AkAPD, which is 0 wt% based on the weight AkAPD.

[0069] Recovery of AkAPD from the oxazolidinone products makes alkali metal bicarbonates. Carbon dioxide can be released from the bicarbonates in the regeneration step, along with the bicarbonates formed in the absorption step. Alkali metal ions in the aqueous absorbent can be removed by known means, such as by electrodialysis. In some embodiments, the aqueous absorbent further contains physical absorbent that is miscible with water. Examples of physical absorbents include:

[0070] • low molecular weight polyalkylene glycols and their mono-ethers, dilpropylene glycol), tri(propylene glycol), difethylene glycol), tri(ethylene glycol), tetra(ethylene glycol) and their monomethyl, monoethyl, and mono-t-butyl ethers such as methoxytriglycol;

[0071] • cyclic sulfones such as sulfolane,

[0072] • thiodiglycol and

[0073] • glycerin.

[0074] In some embodiments, the aqueous absorbent contains at least 5 wt% physical absorbent or at least 10 wt%. In some embodiments, the aqueous absorbent contains at most 50 wt% physical absorbent or at most 40 wt% or at most 30 wt% or at most 20 wt% or at most 10 wt%. In some embodiments, the aqueous absorbent 0 contains wt% physical absorbent.

[0075] Test Methods

[0076] Gas Chromatography:

[0077] The contents of solutions are determined by gas chromatography with a flame ionization detector. Commercially-purchased standards are used to confirm the retention time of components in the solution and the degradation products identified by the flame ionization detector. The GC-FID identifies MAPD and MAPD-OX using the purchased standards. Concentration of MAPD and MAPD-OX (in wt%) is determined using at least a 5-point standard calibration curve. (Samples are diluted in methanol; the dilution factor, area count, and standard calibration curve are used to determine the concentration in wt%.)

[0078] Examples

[0079] The following examples illustrate specific embodiments of the invention, but do not limit the broadest scope of the invention.

[0080] The materials in Table 1 are used for the Examples:

[0081] Tabic 1

[0082] In-situ formation of MAPD

[0083] One liter of an aqueous blend of 41.2 wt% DMAPD and 8.8 wt% PIP is charged into a 2 L corrosion-resistant pressure vessel. The head space is charged with 30 psi of 99.9%carbon dioxide over a period of 1 hour to achieve equilibrium at 30 psi. The carbon dioxide is stopped, the vessel is isolated, and the temperature is ramped up to 150 °C where it is held for 7 days. The resulting solution is analyzed by gas chromatography, and the results are shown in Table 2.

[0084] In Table 1

[0085] • DMAPD is 3-dimethylamino-l,2-proanediol (Formula 4 wherein each R1is methyl);

[0086] • PIP is piperazine;

[0087] • MPIP is methylpiperazine;

[0088] • MAPD is 3-methylamino-l,2-proanediol (Formula 6 wherein R1is methyl); and

[0089] • MAPD oxazolidinone (MAPD-OX) is the reaction product of MAPD and carbon dioxide illustrated in Formula 1 wherein R1is methyl.

[0090] The conditions provide an accelerated test of reactions that take place over time in a carbon dioxide recovery system. From these results we conclude that (1) the combination of DMAPD and piperazine in an aqueous solution results in transfer of a methyl group to piperazine to form MAPD and MPIP; and (2) MAPD reacts with carbon dioxide to form MAPD oxazolidinone (MAPD-OX).

[0091] Decomposition of Amine Oxazolidinone

[0092] A 0.7 wt% solution of MAPD oxazolidinone (MAPD-OX) in water is analyzed by GC to confirm the contents of the solution. The solution is put into a 20-mL scintillation vial. A 0.5 mole-equivalent quantity of 40% aqueous NaOH is added to vial. The solution is stirred at room temperature for 30 minutes using a magnetic stir bar and stir plate. The solution is analyzed again, and the results are shown in Table 3:

Claims

CLAIMS:We claim:

1. A process to recover carbon dioxide from a gaseous rich feed stream that contains carbon dioxide, comprising the following steps:(a) an absorption step in which (1) the gaseous rich feed stream is contacted with a lean aqueous absorbent under conditions such that the absorbent absorbs carbon dioxide from the feed stream to produce a gaseous lean feed stream that contains less carbon dioxide than the rich feed stream and a rich aqueous absorbent that contains more carbon dioxide than the lean absorbent, and (2) the rich aqueous absorbent is separated from the gaseous lean feed stream; and(b) a regeneration step in which (1) the rich aqueous absorbent from the absorption step is subjected to conditions such that carbon dioxide desorbs from the rich aqueous absorbent to produce a carbon dioxide stream and a lean aqueous absorbent, and (2) the lean aqueous absorbent is returned to the absorption step; wherein(i) the aqueous absorbent contains 30 to 70 weight percent water and 70 to 30 weight percent of an amine mixture that contains 3-(dialkylamino)propane-l,2- diol (DAkAPD) and a primary or secondary amine;(ii) in the aqueous absorbent, the DAkAPD and the primary and secondary amine react to transfer an alkyl group from amine nitrogen in the DAkAPD to the amine nitrogen in the primary and secondary amine, such that the aqueous absorbent comprises DAkAPD and 3-(alkylamino)propane-l,2-diol (AkAPD) and primary or secondary amine and secondary or tertiary amine; and(iii) at least some of the AkAPD reacts with carbon dioxide in the absorption step to form an oxazolidinone reaction product; and(iv) the oxazolidinone reaction product formed in (iii) is contacted with aqueous alkali metal hydroxide under conditions such that at least some of the oxazolidinone reaction product decomposes to release the AkAPD and a carbonate or bicarbonate salt.

2. The process of Claim 1 wherein the lean aqueous absorbent contains at least 0.05 weight percent of AkAPD.

3. The process of Claim 2 wherein the molar ratio of AkAPD to DAkAPD in the aqueous absorbent is from 1 mole percent to 80 mole percent, based on the quantity of DAkAPD .

4. The process of Claim 2 wherein the primary or secondary amine is water soluble and the molar ratio of nitrogen atoms in the primary or secondary amine to nitrogen atoms in the 3-(dialkylamino)propane-l,2-diol (DAkAPD) and the 3-talky I ami no (propane- 1 ,2-diol (AkAPD) is from 50 to 80 percent.

5. The process of Claim 2 wherein the DAkAPD meets Formula 4wherein each R1independently contains from 1 to 6 carbon atoms.

6. The process of Claim 5 wherein each R1is independently a methyl, ethyl, propyl or butyl group.

7. The process of Claim 5 wherein each R1is a methyl group.

8. The process of Claim 1 wherein:(a) the DAkAPD is 3-(dimethylamino)propane-l,2-diol; and(b) the AkAPD is 3-(methylamino)propane- 1 ,2-diol, and the aqueous absorbent contains from 1 to 80 mole percent of AkAPD, based on the quantity of DAkAPD; and(c) the primary or secondary amine is water soluble and the molar ratio of nitrogen atoms in the primary or secondary amine to nitrogen atoms in the 3-(dialkylamino)propane-l,2- diol (DAkAPD) and the 3-(alkylamino)propane-l,2-diol (AkAPD) is from 50 to 80 percent; and(d) the aqueous absorbent contains at least 45 weight percent water and at least 40 weight percent amine mixture.

9. The process of Claim 8 wherein the primary or secondary amine comprises secondary amine.

10. The process of Claim 8 wherein the primary or secondary amine comprises piperazine.

11. The process of any one of Claims 1 through 10 wherein in (iv) the oxazolidinone reaction product is contacted with aqueous alkali metal hydroxide outside the absorption step.

12. The process of Claim 11 wherein in (iv) the oxazolidinone reaction product is contacted with aqueous alkali metal hydroxide before or during the regeneration step.

13. The process of Claim 11 wherein in (iv) the molar ratio of oxazolidinone reaction product aqueous alkali metal hydroxide is from 0.9 to 1.5.

14. The process of Claim 11 wherein the carbon dioxide is recovered from carbonate or bicarbonate salts produced in (iv) in the regeneration step.

15. The process of Claim 11 wherein alkali metal ions are removed from the aqueous absorbent by electrodialysis.

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