Absorbent system and method for capturing CO2 from a gas stream

Abstract

The present invention relates to an absorbent, an absorbent system and a process for removing acidic gas such as CO.sub.2 from exhaust gases from fossil fuel fired power stations, from natural gas streams, from blast furnace oven off-gases in iron/steel plants, from cement plant exhaust gas and from reformer gases containing CO.sub.2 in mixtures with H.sub.2S and COS. The liquid absorbent, a mixture of amine and amino acid salt is contacted with a CO.sub.2 containing gas in an absorber and CO.sub.2 in the gas stream is absorbed into the liquid. The absorbed CO.sub.2 forms more than one type of solid precipitate in the liquid at different absorption stages. In a first absorption stage solid precipitate of amine bicarbonate is formed and is withdrawn as slurry from the bottom of a first absorber section. In a second absorption stage solid precipitate of alkali metal bicarbonate is formed and withdrawn as slurry at the bottom of a second absorber section. The slurry withdrawn from the first absorption section is heated to dissolve the precipitate with CO.sub.2 release in an amine flash regeneration tank. The slurry from the second precipitation stage is withdrawn from the bottom of the second absorber section and sent to a regenerator for desorption with CO.sub.2 release. The lean amine and amino acid salt mixture from the flash regenerator and desorber are mixed and returned to the top of the absorber. This absorbent system improves carbon dioxide removal efficiency due to its higher CO.sub.2 removal ability per cycle when compared with conventional amine, absorbent from organic acid neutralized with inorganic base and carbonate based absorbent system. It exhibits less solvent vaporization loss because part of the absorbent is in salt form.

Claims

1. A system for capturing CO.sub.2 from an exhaust gas comprising: (a) an absorber having at least two sections containing an absorbent, wherein the absorbent consists of an aqueous absorbing mixture consisting of 0.5 to 10.0 mol/kg of 2-amino-2-methylpropanol, 0.5 to 8.0 mol/kg an amino acid salt of sarcosine or glycine, and water, wherein a first section of the absorber is configured to form a first precipitate of amine bicarbonates, wherein a second section of the absorber is configured to form a second precipitate of metal bicarbonates, wherein the first section comprises an outlet configured to remove a slurry comprising the first precipitate of amine bicarbonates from the first section, and wherein the second section comprises an outlet configured to remove a slurry comprising the second precipitate of metal bicarbonates from the second section, (b) a flash regenerator configured to dissolve the first precipitate of amine bicarbonates and release CO.sub.2, (c) a desorber configured to regenerate the aqueous absorbing mixture and release CO.sub.2, and (d) a reboiler configured to supply heat to the desorber.

2. The system according to claim 1, wherein the two sections are two sections within one absorber or are two separate absorbers in series.

3. The system according to claim 1, wherein the outlet in the first section is configured to remove the slurry of the first precipitate of amine bicarbonates from the first section and supply the slurry of the first precipitate of amine bicarbonates to the flash generator.

4. The system of according to claim 1, further comprising a first heat exchanger (HX1) arranged between the absorber and the desorber.

5. The system of according to claim 1, further comprising a second heat exchanger (HX2) arranged between the absorber and the flash regenerator.

6. The system according to claim 1, wherein the absorbent consists of an aqueous absorbing mixture consisting of 1.0 to 6.0 mol/kg of 2-amino-2-methylpropanol, 1 to 4 mol/kg an amino acid salt of sarcosine or glycine, and water.

Description

FIGURES

(1) FIG. 1 is a simplified sketch of the slurry-slurry process for CO.sub.2 capture.

(2) FIG. 2 is a simplified sketch of the slurry-slurry process for CO.sub.2 capture with two separate absorber sections.

(3) FIG. 3 shows XRD analysis result.

(4) FIG. 4 shows an example vapour liquid solid equilibrium (VLSE) at 40 C. and 120 C.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention came out of the desire of the inventors to shorten the time required for the formation of solid precipitate in certain amino acid salt systems and increase the absorption rate of such system. Precipitation of CO.sub.2 as solid in a process will increase the CO.sub.2 driving force into the liquid and result in increased loading capacity. Certain amino acid salt systems such as potassium sarcosinate have very good absorption kinetics, (Aronu U E, Ciftja A F, Kim I, Hartono A ; Understanding precipitation in amino acid salt systems at process conditions, Energy procedia 37 (2013) 233-240), but formation of solid potassium bicarbonate (KHCO.sub.3) occurs very late at the point when the absorption kinetic is very slow making it unfavorable precipitating system candidate since the benefit of precipitation at process condition cannot be fully exploited. The inventors therefore searched for a method to enhance early precipitation for such a system. It is known that certain amines, such as 2-amino-2-methylpropanol (AMP), at high CO.sub.2 loading or high concentration can precipitate in a process. An absorbent that can make the fast reacting amino acid salt to form an early precipitate will be ideal. Various blends of this AMP and different amino acid salt solutions were therefore prepared and CO.sub.2 absorption behavior monitored in rapid screening set up with a possibility to monitor the precipitation behavior at process conditions. It was also found that in some cases the mixture of the amino acid salt and AMP forms a complete homogeneous mixture, while in other cases the solution separates into two phases before CO.sub.2 absorption. The solution forming two phases gradually reverts to a single phase as CO.sub.2 absorption progresses before precipitation sets in. As CO.sub.2 absorption proceeded, and it was found that solid precipitation occurred early when the reaction rate is still very high, and it was found that the morphology of the precipitate is not the same as that of the metal bicarbonate, KHCO.sub.3. The morphology which was crosschecked by using X-Ray Diffraction (XRD) analysis, conforms to that of the bicarbonate of the amine, AMP. Further, as the experiment continued at a later stage a new type of crystals forming in the mist of the first crystals was found. These second crystals were found to be KHCO.sub.3 by using XRD. The first precipitate requires low temperature dissolution and amine regeneration with CO.sub.2 release, while a higher temperature dissolution and absorbent regeneration is required in the desorber for the second stage precipitate. A test of the dissolution temperature of the precipitates shows that the first precipitate dissolves from 50 C. and completely with CO.sub.2 release at 75 C., while the second precipitate dissolves from 70 C. to 100 C. An example of an overall initial reaction is described in Eq. 1:
RNHCOO.sup.M.sup.++RNH.sub.2+2CO.sub.2.sup.OOCRNHCOO.sup.M.sup.++RNHCOO(1)
The overall stepwise reaction in such a system can be described as follows: Step 1: Formation of Solid Precipitate of Amine Bicarbonate
RNH.sub.2+CO.sub.2RNHCOO(2)
RNHCOO+H.sub.2ORNH.sub.2+HCO.sub.3.sup.(3) Step 2: Formation of Solid Precipitate of Metal Bicarbonate
RNHCOO.sup.M.sup.++CO.sub.2.sup.OOCRNHCOO.sup.M.sup.+(4)
.sup.OOCRNHCOO.sup.M.sup.++H.sub.2O.sup.OOCRNH.sub.2+MHCO.sub.3(5)
R and R represent hydrogen, C.sub.1-4alkyl, C.sub.1-4alkanol, or a straight chain, cyclic or aromatic amine groups, wherein at least one of R and R is an C.sub.1-4alkyl, C.sub.1-4 alkanol or a straight chain, cyclic or aromatic amine group, wherein the straight chain contains up to 7 carbon atoms, and the cyclic or aromatic amine groups contain from 3 to 6 carbon atoms. M can be selected from K, Li or Na

(6) The amino acid salt used in the present invention is the product of neutralization between an amino acid and an inorganic base or organic base. The amino acids that can be used include but are not limited to glycine, taurine, sarcosine, proline, alanine, lysine, serine, pipecolinic acid, arginine, threonine and cysteine.

(7) The inorganic bases that can be used for amino acid neutralization in the present invention include but are not limited to potassium hydroxide, sodium hydroxide and lithium hydroxide.

(8) The organic bases that can be used for amino acid neutralization include amines; such amines include but are not limited to: methylaminopropylamine (MAPA), piperazine (PZ), N-2-hydroxyethylpiperazine, N-(hydroxypropyl)-piperazine, diethanol triamine (DETA), 2-((2-aminoethyl)amino)ethanol (AEEA), piperidine, pyrrolidine, dibutylamine, trimethyleneimine, 1,2-diaminopropane, 1,3-diaminopropane, 2-amino-2-methylpropanol (AMP), 2-(diethylamino)-ethanol(DEEA), 3-amino-1-cyclohexylaminopropane (ACHP), 3-aminopropanol (AP), 2,2-dimethyl-1,3-propanediamine (DMPDA), 1-amino-2-propanol (MIPA), 2-methyl-methanolamine (MMEA), piperidine (PE), monoethanolamone (MEA), diethanolamine (DEA), diisopropanolamine (DIPA).

(9) Where an organic base such as amine is used for amino acid neutralization, it is preferred that the pKa of the amine is at least greater than the pKa of amino acid used.

(10) The amine blended with the amino acid salt is preferably a strong or high bicarbonate forming amine like a sterically hindered and/or a tertiary amine. Such amines include but are not limited to 2-amino-2-methylpropanol (AMP), 2-amino-2-methyl-1,3-propanediol (AMPD), 2-(diethylamino)-ethanol(DEEA), N,N-dimethyl-ethanolamine (DMMEA), methyl diethanolamine (MDEA), triethanolamine (TEA), 1-(diethylamino)-2-propanol, 3-(diethylamino)-1-propanol, tripropylamine, 2-pyrrolidino-ethanol, 3-(diethylamino)-1,2-propanediol , N-piperidineethanol, 1-methylpiperidine-2-ethanol and 1-piperidinepropanol.

(11) In FIG. 1, a simplified process diagram of the method used to capture CO.sub.2 using the absorbent according to the invention is disclosed. The CO.sub.2-containing gas stream 1 enters the absorber A1 in the bottom and flows upwards. It meets a liquid absorbent stream 12 in two stages. At the first contact stage in section 2, at the bottom of the absorber A1, stream 12 is a stream containing a slurry of water, a mixture of the metal bicarbonate of amine (Li, Na, or K); carbamates of amine, amino acid; amino acid salt and amine. At the second contact stage in section 1 of A1, at the point of withdrawal of stream 3, stream 12 is stream containing a slurry of water, a mixture of the bicarbonate of amine; carbamate of amine, amino acid; amino acid salt and amine. This implies that at each point of contact the aqueous phase is partially or fully saturated with the bicarbonates such that the flow contains both solids and liquid. In addition to the bicarbonates, the aqueous solution contains a precipitation promoting amine and an absorption rate promoting amino acid salt or a precipitation promoting amino acid salt and an absorption rate promoting amine.

(12) In FIG. 2, where the two sections of absorber A1 are in two different columns, CO.sub.2-containing gas stream 1b from absorber section 2 enters the bottom of absorber section 1 and flows upwards in contact with the downward flowing stream 12. The withdrawn slurry, stream 3 is passed through a solid-liquid separator where more concentrated slurry, stream 4 is sent to the flash regenerator, while the liquid phase, stream 3b is returned to the absorber section 2 for contacting the upcoming CO.sub.2-containing gas stream 1.

(13) The operating temperature of the absorber will depend on the inlet flue gas temperature and will typically be from 30 C. to 80 C., preferably from 40 C. to 60 C. Further cooling or pre-treatment of the flue gas may be required to remove/reduce fly ash in cases with high temperature and water content, some cooling and water removal might be necessary. Some cooling may be required in the bottom region of section 1 before withdrawal of stream 3 to further enhance solid precipitation in this region. The stream 3 region in the absorber is preferably maintained at 30 C. to 50 C. In the absorber, the CO.sub.2 is absorbed into the aqueous slurry and the exhaust, stream 2 with reduced CO.sub.2 content leaves the absorber, after a water wash section. This water wash is only needed to retain the amine, depending on its volatility. The absorption tower is preferably a plate tower that can handle slurries. A spray tower, packed tower or any other suitable tower able to handle slurries can also be used. In the aqueous phase, the additional reactions described in Eq. 2 and 3 take place in section 1 before the withdrawal of stream 3 while the additional reactions described in Eq. 4 and 5 takes place in section 2 before the withdrawal of stream 7 at the bottom of the absorber.

(14) The entering stream 12 will typically be high in amino acid salt and amine content. During contact with CO.sub.2, the fast reacting amino acid salt enhances the transport of gaseous CO.sub.2 into the liquid absorbent. The CO.sub.2 in the liquid phase is further stored away into the amine as bicarbonate. As the gas liquid contact continues down the absorber, amine bicarbonate formed grows until the solubility limit is exceeded resulting in precipitation as solid amine bicarbonate slurry in the bottom of section 1, where the bicarbonate slurry is withdrawn as stream 3 at a temperature from 30 C. to 60 C. Withdrawal of stream 3 has a significant benefit; complete withdrawal of CO.sub.2 saturated slurry from the absorber will shift equilibrium further to the right enhancing more CO.sub.2 containing liquid phase products. The slurry withdrawal will also significantly reduce the viscosity of the absorbent system at this stage. This combined effect will accelerate mass transport of CO.sub.2 into the liquid phase even at the middle of the absorption resulting in enhanced absorption capacity. Further, a withdrawal of the first precipitate from the solution as stream 3 will result in a less volume of solution for regeneration in the desorber. The amino acid salt containing CO.sub.2 in low proportion and amine saturated with CO.sub.2 as carbamate and/bicarbonate will remain in solution and continue contacting CO.sub.2 in the absorber section 2. Ability of the amino acid salt for fast reaction with CO.sub.2 and increased gas to liquid in this section allows further uptake of CO.sub.2 along the absorber in section 2 forming more amino acid carbamate, which undergoes hydrolysis to produce KHCO.sub.3 as in Eq. 5. At the solubility limit, KHCO.sub.3 will precipitate and form a slurry containing amine bicarbonate/carbamate as well as amino acid carbamate/amino acid. This second stage precipitation in absorber section 2 will also result in another absorption rate and capacity enhancement since CO.sub.2 bound as precipitate will not participate in the equilibrium backpressure over the solution. Depending on the flue gas temperature, the slurry leaves the bottom of the absorber as stream 7 at a temperature from 40 to 70 C. The slurry in stream 7 is passed through the cross exchanger HX1 and is heated up like in a conventional amine process by heat transfer with the lean absorbent stream 9 from the desorber. According to the invention, the filtration and/or crystallization often proposed for a precipitating process is not required. The slurry is sufficiently saturated with solids and a smaller liquid volume will be treated in the desorber because part of the total liquid has been withdrawn as stream 3 as precipitate slurry. The heated slurry stream 8 is delivered into the desorber D1 from the top where the solid precipitate is completely dissolved with CO.sub.2 released in stream 13. CO.sub.2 desorption is enhanced on further contact with upcoming stripping vapour from a reboiler R1. The precipitate in the slurry may be completely dissolved before in enters the reboiler through stream 15 depending on the desorber operating conditions. Hot vapour from the desorber is returned into the bottom of the desorber column by stream 16. The typical temperature range in the desorber is 100 C. to 200 C. In the desorber the KHCO.sub.3 decomposes with CO.sub.2 release likewise the bicarbonate/carbamate of the amine and amino acid resulting in the regeneration of the amino acid salt and amine in the absorbent.

(15) The desorber can be a packed tower, a plate tower, a spray tower, a flash tank or any other suitable tower.

(16) Stream 9 emerges the cross changer as stream 10 with lower temperature 60 C. -110 C. but with sufficient heat to transfer at the cross exchanger HX2 to stream 3. Stream 3 emerges from HX2 as stream 4 at higher temperature 60-110 C., sufficient to completely dissolve the precipitate and further strip off CO.sub.2 from the liquid phase. The stream 4 is fed into the flash regenerator F1 where CO.sub.2 is released in stream 6 and combined with stream 13 to form stream 14 containing CO.sub.2 for storage. In addition, it could be possible to use some waste/low heat streams to heat up stream 3 and/or 4. CO.sub.2 pressure for compression will vary significantly based on the mode of operation of the desorber. The produced CO.sub.2 pressure can be in the range 3-50 bar. Stream 10 emerges from HX2 as stream 11 at lower temperature and is combined with stream 5, lean absorbent from the flash regenerator, F1 to form stream 12; the lean absorbent is returned to the top of the absorber A1. Stream 5 can also be combined with stream 8 delivered into the desorber D1. The process can also be configured such that the lean stream 9 is first used to heat up stream 3 in HX2 before using it to heat up stream 7 in HX1.

EXAMPLE 1

(17) Absorption test experiments were carried out on a 7 mol/kg solution containing 3 mol/kg potassium sarcosine (KSAR) and 4 mol/kg 2-amino-2-methylpropanol (AMP) charged in a jacket glass reactor. The reactor is equipped with Particle Vision and Measurement (PVM) and Focused Beam Reflectance Measurement (FBRM) probes for monitoring precipitation. Absorption at 40 C. starts after calibration of CO.sub.2 analyser with CO.sub.2-N.sub.2 gas mixture containing 10 vol % CO.sub.2 with flow rate 2.5 NL.min.sup.1. Same gas mixture is then bubbled through a 375 mol/kg of the absorbing solution while the solution is agitated using a stirrer at 300 rpm. The gas phase leaving the reactor is cooled and CO.sub.2 content is analysed online by IR. The absorption test gives fast relative comparison of absorption rate, it also allows the possibility to study the precipitate behaviour, crystal formation and dissolution during the experiment. The absorption process terminates when the concentration of CO.sub.2 in the effluent reaches 9.5 vol % representing about 9.5 kPa partial pressure of CO.sub.2 or when the absorption rate becomes too low. A liquid sample containing both the rich liquid and precipitated crystal is collected for analysis at the end of absorption. In addition, a similar sample is collected, filtered and dried for precipitate analysis by XRD. Precipitate dissolution was monitored by heating up the solvent in the range 40 C. to 90 C. while bubbling pure N.sub.2 gas at 2.25 NL/min through the solution in the reactor bottle. It was observed that the CO.sub.2 content of the effluent gas increases as N.sub.2 bubbles through the solution while the precipitate dissolution is monitored and logged. Gas phase analysis was used to determine the liquid phase CO.sub.2 concentration during the experiment.

EXAMPLES 2-3

(18) An absorption experiment was carried out in the same manner as in Example 1, except that a 7 mol/kg solution containing 3mol/kg potassium glycine (KGLY) and 4 mol/kg 2-amino-2-methylpropanol (AMP) as an absorbent was used as an absorbing solution in Example 2. This absorbent was found to form two liquid phase before CO.sub.2 absorption, but forms one phase as the loading progresses before precipitation start. In example 3, a 7 mol/kg solution containing 3 mol/kg sodium glycine (NaGLY) and 4 mol/kg 2-amino-2-methylpropanol (AMP) was the absorbent used. The results obtained are shown in Table 1.

(19) TABLE-US-00001 TABLE 1 Amino 1st Precipitation 2nd Precipitation Crystal acid salt Amine Initial abs rate Absorption rate Loading Absorption rate Loading Loading Dissolution [mol/kg] [mol/kg] [mol/m3/min] [mol/m3/min] [molCO2/kg] [mol/m3/min] [molCO2/kg] [molCO2/kg] Start ( C.) Example 1 3 m KSAR 4 m AMP 34.3 29.6 0.81 6.5 2.54 3.06 55 Example 2 3 m KGLY 4 m AMP 25.5 19.0 0.99 9.2 2.62 3.34 50 Example 3 3 m NaGLY 4 m AMP 23.0 23.2 0.30 7.7 2.07 2.39 65 Comparative 5 m KSAR 36.0 3.9 2.89 3.23 Example 1 Comparative 4 m AMP 13.4 3.9 1.91 2.39 Example 2 Comparative 4.9 m MEA 24.3 2.68 Example 3

COMPARATIVE EXAMPLES 1-3

(20) An absorption experiment was carried out in the same manner as in Example 1, except that an aqueous solution containing 5 mol/kg potassium sarcosine (KSAR) as an absorbent in comparative example 1 while in comparative example 2-3 aqueous solution containing 4 mol/kg 2-amino-2-methylpropanol (AMP) and 4.9 mol/kg of monoethanolamine (MEA) was used, respectively.

(21) From the result in Table 1, it can be seen that the type of absorbent used in Examples 1-3 forms more than one precipitate before absorption is terminated at 9.5 vol % CO.sub.2 out of the reactor. Here precipitation occurred in two steps. In comparative example 1-2, it can be observed that only one precipitate is formed. In addition, it can be observed when looking at the absorption rate that the first precipitate in Example 1-3 occurs when the absorption rate is still high and the loading is low, and the second precipitate occurs when the absorption rate is low and loading is high. In the comparative example 1-2, the only precipitate occurs when the absorption rate is low and the loading is high. In the comparative example 3, no precipitate is formed.

EXAMPLE XRD

(22) The slurry formed after absorption experiment was filtered and the filtrate dried at room temperature. X-ray diffraction (XRD) analysis was conducted on the filtered solid cake. The XRD analysis results are shown in FIG. 3. The filtrate from the comparative example 1 was identified as KHCO.sub.3 by XRD analysis, while the filtrate from comparative example 2 is identified as the bicarbonate of AMP. From the figure, it can be observed that the solid phase spectra of Example 1 and 2 contain spectra of comparative example 1 and 2 at variable degrees. This shows that two precipitate types are found in the solution and these two precipitates are a mixture containing the precipitate of comparative example 1 and 2. During visual observation and monitoring using PVM, the experiment shows that the first precipitate in Example 1 and 2 is needle-like as was observed in comparative example 2. The spectra confirm that formation of first precipitate of AMP bicarbonate and as loading further increases a second precipitate, KHCO.sub.3 is formed. The absorption solid phase of Example 3 appears somewhat different from the rest of the spectra. Although two precipitation stages occurred, precipitates with different morphology are formed. Example 3 is the only solution containing sodium salt; the other precipitates contain potassium salt.

EXAMPLE VLSE

(23) An example vapour liquid solid equilibrium (VLSE) at 40 and 120 C. is shown in FIG. 4 for a precipitating absorbent according to the invention. The figure shows that precipitate formation enables increase in the driving force of CO.sub.2 into solution with the formation of a flat plateau region at 40 C. where the increase in CO.sub.2 loading does not result in a corresponding increase in CO.sub.2 partial pressure. This enables higher CO.sub.2 loading in this precipitating system when compared to systems that do not precipitate.