Process for Increased Selectivity and Capacity for Hydrogen Sulfide Capture from Acid Gases

Abstract

A process for selectively separating H.sub.2S from a gas mixture which also comprises CO.sub.2is disclosed. A stream of the gas mixture is contacted with an absorbent solution comprising one or more amines, alkanolamines, hindered alkanolamines, capped alkanolamines, or mixtures thereof. The H.sub.2S/CO.sub.2 selectivity of the absorbent solution is preferably greater than about 4.0 for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] between about 0.2 and about 0.6, and is achieved by reducing pH of the absorbent solution.

Claims

1. A method for increasing the selectivity of an absorption process for H.sub.2S absorption from a gas mixture which also comprises CO.sub.2, the absorption process having an absorbent solution comprising one or more amines, the method comprising reducing the pH of the absorbent solution.

2. The method of claim 1, wherein the pH reducing step comprises diluting the absorbent solution.

3. The method of claim 2, wherein the diluted absorbent solution is less than 36 wt %.

4. The method of claim 2, wherein the diluted absorbent solution is less than 30 wt %.

5. The method of claim 1, wherein the pH reducing step comprises adding an acid to the absorbent solution.

6. The method of claim 3, wherein the acid is selected from phosphoric acid and sulfuric acid.

7. The method of claim 1, wherein the one or more amines is selected from the group consisting of amines, alkanolamines, sterically hindered akanolamines, and mixtures thereof.

8. The method of claim 7, wherein the sterically hindered alkanolamine is a capped alkanolamine.

9. The method of claim 7, wherein the amine is M3ETB.

10. The method of claim 7, wherein the amine is EETB.

11. The method of claim 7, wherein the amine is MDEA.

12. The method of claim 1, wherein the H.sub.2S/CO.sub.2 selectivity of the absorbent solution is greater than about 5.0 for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] range between about 0.2 and about 0.55.

13. The method of claim 1, wherein the H.sub.2S/CO.sub.2 selectivity of the absorbent solution is greater than about 6.0 for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] range between about 0.2 and about 0.55.

14. The method of claim 1, wherein the H.sub.2S/CO.sub.2 selectivity of the absorbent solution is greater than about 7.0 for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] range between about 0.2 and about 0.55.

15. A process for selectively separating H.sub.2S from a gas mixture which also comprises CO.sub.2, the process comprising: contacting a stream of the gas mixture with an absorbent solution comprising one or more amines, wherein the absorbent solution has a reduced pH.

16. The process of claim 15, wherein the pH of the absorbent solution is reduced by dilution.

17. The process of claim 16, wherein the diluted absorbent solution is less than 36 wt %.

18. The process of claim 16, wherein the diluted absorbent solution is less than 30 wt %.

19. The process of claim 15, wherein the pH of the absorbent solution is reduced by addition of an acid.

20. The process of claim 19, wherein the acid is selected from phosphoric acid and sulfuric acid.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0014] FIG. 1 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading with different concentrations of M3ETB aqueous solutions.

[0015] FIG. 2 is a plot of the CO.sub.2 uptake as a function of treatment time by aqueous solutions of M3ETB.

[0016] FIG. 3 is a plot of the H.sub.2S uptake as a function of treatment time by aqueous solutions of M3ETB.

[0017] FIG. 4 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total acid gas loading with different amine solutions as identified in Table 1.

[0018] FIG. 5 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total acid gas loading per volume of amine solution.

[0019] FIG. 6 is a plot of the CO.sub.2 and H.sub.2S uptake as a function of treatment time for the 36 wt % M3ETB solutions with and without phosphoric acid.

[0020] FIG. 7 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total loading with M3ETB aqueous solutions with and without phosphoric acid.

[0021] FIG. 8 is a plot of the CO.sub.2 and H.sub.2S uptake as a function of treatment time for the 36 wt % MDEA solutions with and without phosphoric acid.

[0022] FIG. 9 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total loading with MDEA aqueous solutions with and without phosphoric acid.

DETAILED DESCRIPTION

[0023] Conventional wisdom within the art of acid gas treating is that tertiary amines in general form bicarbonates preferentially in reaction with CO.sub.2 in aqueous solution. Although hindered secondary amines are also able to form a carbamate with CO.sub.2, the bicarbonate route becomes the preferred route as steric hindrance increases. For absorption systems using tertiary and sterically hindered amines, it is well known that there is a rapid equilibrium between bicarbonate and carbonate formation as shown below:

TABLE-US-00001 Stoichiometry Rate H.sub.2S (Acid Base) H.sub.2S + Amine ⇄ AmineH.sup.+ + HS.sup.− 1.0 Very fast CO.sub.2 (Acid Base) [00003] 0.5 1.0 Slow [00004]

[0024] Because of their 1:1 stoichiometry with the amine group, the bicarbonate route is more desirable because it favors enhanced selectivity for the absorption of CO.sub.2 and also for H.sub.2S as the mercaptide salt in acid gas removal processes. However, at the high pH of ˜12 of the initial loading of the amine in aqueous solution carbonate formation is favored, but that ties up two amine molecules for acid gas capture, providing a less favorable 0.5:1 stoichiometry. Furthermore, at higher amine concentrations, there is less water available facilitating higher pH.

[0025] Described herein is a method for controlling/reducing the pH of a hindered amine/alkanolamine absorbent system to favor bicarbonate versus carbonate and/or hydrosulfide formation to maximize the stoichiometry of absorption.

[0026] In a first aspect of the present invention, the pH of the hindered amine/alkanolamine absorbent system is reduced. As demonstrated herein below, this reduced pH favors bicarbonate formation, increases acid gas (H.sub.2S and CO.sub.2) loading and increases the selectivity of H.sub.2S over CO.sub.2 over a broad loading range.

[0027] In a second aspect of the present invention, the concentration of the hindered amine/alkanolamine absorbent is reduced, resulting in a diluted amine solution necessarily having a lower pH. As demonstrated herein below, this reduced pH favors bicarbonate formation, increases acid gas (H.sub.2S and CO.sub.2) loading, and increases the selectivity of H.sub.2S over CO.sub.2 over a broad loading range.

[0028] The effect of reducing the concentration of the hindered amine/alkanolamine absorbent is unexpected and counterintuitive. Traditionally, one strives to increase the absorbent concentration in order to increase absorption. As demonstrated herein below, decreasing the weight percentage of hindered amine/alkanolamine absorbent in solution effectively reduces the amount of carbonate that can be formed and thereby increases the amount of free amine available for acid gas recovery as bicarbonate and hydrosulfide. Because absorption of H.sub.2S is kinetically faster the selectivity for HS.sup.− formation is favored over HCO.sub.3.sup.− formation. Additional advantages of the present invention include reduced chemical costs (directly related to reduced hindered amine/alkanolamine usage), reduced viscosity resulting in reduced circulation energy, reduced corrosivity of the absorption system, and reduced energy required to regenerate the higher loaded (1:1 stoichiometry), but reduced volume of hindered amine/alkanolamine.

[0029] In order to provide a better understanding of the foregoing, the following non-limiting examples are offered. Although the examples may be directed to specific embodiments, they are not to be viewed as limiting the invention in any specific respect.

[0030] The following experimental and analytical methods were used in the examples. The process absorption unit (PAU) is a semi-batch system, comprising a water saturator, a stirred autoclave to which gas can be fed in up-flow mode, and a condenser. The autoclave is equipped with a pressure gauge and a type J thermocouple. A safety rupture disc is attached to the autoclave head. A high wattage ceramic fiber heater is used to supply heat to the autoclave. The gas flows are regulated by Brooks mass flow controllers and the temperature of the condenser is maintained by a chiller. The maximum PAU working pressure and temperature are 1000 psi (69 bar) and 350° C., respectively.

[0031] During runs at atmospheric pressure, the pH of the solution is monitored in situ by using a Cole-Parmer pH probe installed in the bottom of the autoclave. This pH probe is limited by a maximum temperature and pressure of 135° C. and 100 psi, respectively. Therefore, before doing experiments at pressure, the pH probe is removed and the autoclave is capped. In both atmospheric and pressure runs, liquid samples are collected directly to a vial (atmospheric runs) or to a stainless steel cylinder (pressure runs).

[0032] A custom LabVIEW program is used to control the PAU operation and to acquire experimental data (temperature, pressure, stirrer speed, pH, gas flow rate, and off-gas concentration).

[0033] The experiments described herein below were performed by flowing the test acid gas mixture through the autoclave in which the amine solution was previously loaded. The acid gas mixture was fed to the bottom of the reactor by-passing the water saturator. The gases leaving the autoclave were transferred through the condenser (maintained at 10° C.) in order to remove any entrained liquids. A slip-stream of the off-gas leaving the condenser was piped to a micron-GC (Inficon) for analysis while the main gas flow passed through a scrubber. After reaching breakthrough, nitrogen was used to purge the system.

[0034] The off-gas composition was measured using a custom-built micro GC. The micro GC is configured as a refinery Gas Analyzer and includes four columns (Mole Sieve, PLOT U, OV-1, PLOT Q) and four TCD detectors. A slip stream of the off-gas was injected into the micro GC approximately every 2 minutes. A small internal vacuum pump was used to transfer the sample into the micro GC. The nominal pump rate was ˜20 mL/min in order to achieve 10× the volume of line flushes between the sample tee and the micro GC. The actual gas injected into the micro GC was ˜1 μL. The PLOT U column was used to separate and identify H.sub.2S and CO.sub.2, and the micro TCD was used to quantify H.sub.2S and CO.sub.2.

[0035] Table 1 below identifies each amine solution prepared in water (MW=18 g/mol; density =1 g/cm.sup.3) for Example 1:

TABLE-US-00002 TABLE 1 MW Density Molarity Conc.sub.amine Conc.sub.Water Wt. % H.sub.2O:amine Amine (g/mol) (g/cm.sup.3) (mol.sub.amine/L) (g.sub.amine/L) (g.sub.Water/L) Amine (mole ratio) MDEA 119.2 1.04 2.17 258.66 736.20 26 18.8 MDEA 119.2 1.04 3.05 363.56 646.33 36 11.8 EETB 161.2 0.94 2.17 349.80 621.87 36 15.9 M3ETB 219.3 0.92 2.17 475.88 485.49 49.5 12.4 M3ETB 219.3 0.92 1.59 348.69 619.89 36 21.7 M3ETB 219.3 0.92 1.34 293.86 685.68 30 28.3 M3ETB 219 3 0.92 0.90 197.37 789.48 20 48.5

EXAMPLE 1

Selectivity Study

[0036] Three different amines were used to prepare amine absorbent solutions for Example 1. N-methyldiethanolamine (MDEA) is a commercially available, conventional amine treating absorbent having the following structure:

##STR00005##

[0037] Ethoxyethanol-t-butylamine (EETB) is a commercially available, highly sterically hindered amine treating absorbent having the following structure:

##STR00006##

[0038] Methoxyethoxyethoxyethanol-t-butylamine (M3ETB) is a methyl capped, sterically hindered amine:

##STR00007##

[0039] Test conditions for Example 1 were as follows: gas feed composition: 10 mol % CO2, 1 mol % H.sub.2S, balance N.sub.2; gas flow rate: 154 sccm; temperature: 40.8 ° C., pressure: 1 bar; volume: 15 mL; stirring rate: 200 rpm.

[0040] FIG. 1 is a plot of the selectivity of H.sub.2S over CO2 as a function of acid gas loading with different concentrations of M3ETB aqueous solutions. FIG. 2 is a plot of the CO.sub.2 uptake as a function of treatment time by aqueous solutions of M3ETB. FIG. 3 is a plot of the H.sub.2S uptake as a function of treatment time by aqueous solutions of M3ETB.

[0041] The following conclusions are readily apparent from the plotted data of FIGS. 1-3. The initial lower selectivity (up to 0.2 mol/mol amine) of the 30 wt % M3ETB solution in FIG. 1 is due to higher CO.sub.2 and H.sub.2S pickup (FIGS. 2 and 3, respectively) when compared to the 49.5 and 35.8 wt % M3ETB solutions. However, the higher selectivity (above 0.2 mol/mol amine) of the 30 wt % solution is primarily due to the significantly higher H.sub.2S pickup when compared to the 49.5 and 35.8 wt % M3ETB solutions. This increase in H.sub.2S pickup (FIG. 3) is directly related to increased initial bicarbonate formation versus carbonate formation, resulting in a higher amount of free amine for H.sub.2S capture. Importantly, FIG. 1 further demonstrates that the 30 wt % M3ETB solution yields an overall higher selectivity of H.sub.2S over CO.sub.2 for the commercially desirable acid gas loading range of 0.2 to 0.6, when compared to the 49.5 and 35.8 wt % M3ETB solutions.

[0042] FIG. 4 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total acid gas loading with different amine solutions as identified in Table 1. These selectivity curves show that MDEA selectivity towards H.sub.2S is in general lower than obtained from the more highly sterically hindered secondary amines EETB and M3ETB. This is expected since MDEA is a tertiary and less basic amine. As shown in FIG. 5, the observed MDEA acid gas loadings are also significantly lower than the loadings of the more sterically hindered amines EETB and M3ETB.

[0043] Furthermore, FIG. 4 allows one to compare the H.sub.2S selectivity of various M3ETB concentrations in water against EETB at 36 wt % concentration. The 49.5 wt % M3ETB solution provides higher H.sub.2S selectivity for lower acid gas loadings (up to ˜0.3 mol/mol amine) when compared with the EETB 36 wt % solution. The 36 wt % M3ETB solution provides an even higher H.sub.2S selectivity over a broader range of acid gas loadings (up to ˜0.55 mol/mol amine), and provides the highest H.sub.2S selectivity for lower acid gas loadings (up to ˜0.3 mol/mol amine). The 30 wt % M3ETB solution provides even higher selectivity up to higher acid gas loadings (up to ˜0.65 mol/mol amine). As can be readily understood by a person having ordinary skill in the art, each of these diluted M3ETB solutions provides an improved H.sub.2S selectivity up to higher acid gas loadings, and can be predictably tuned to meet specific acid gas removal applications.

[0044] FIG. 5 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total acid gas loading per volume of amine solution. The M3ETB solutions have a higher capability for removing H.sub.2S than MDEA and EETB up to a higher acid gas loading per volume (˜0.8 mol/L solution). As can be readily understood by a person having ordinary skill in the art, this represents potential energy savings from a reduced circulation rate when using diluted M3ETB solutions.

EXAMPLE 2

M3ETB pH Study

[0045] Two amine solutions were prepared using M3ETB, a first solution with 36 wt % M3ETB, and a second solution of 36 wt % M3ETB with 0.5 wt % H.sub.3PO4, both in water. Test conditions for Example 2 were as follows: gas feed composition: 10 mol % CO.sub.2, mol % H.sub.2S, balance N.sub.2; gas flow rate: 154 sccm; temperature: 40.8° C., pressure: 1 bar; volume: 15 mL; stirring rate: 200 rpm.

[0046] The effect of phosphoric acid on M3ETB acid gas absorption behavior can be observed in FIGS. 6 and 7. FIG. 6 is a plot of the CO.sub.2 and H.sub.2S uptake as a function of treatment time for the 36 wt % M3ETB solutions with and without phosphoric acid. FIG. 7 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total loading with M3ETB aqueous solutions with and without phosphoric acid. The presence of phosphoric acid increases the loading of both CO.sub.2 and H.sub.2S gases, thereby impacting the selectivity. This result is in line with the prior conclusion from Example 1 that bicarbonate formation is favored with lower pH, thereby increasing the free amine concentration in solution available for rapid H.sub.2S capture.

EXAMPLE 3

MDEA pH Study (Comparative)

[0047] Two amine solutions were prepared using MDEA, a first solution with 36 wt % MDEA, and a second solution of 36 wt % MDEA with 0.5 wt % H.sub.3PO.sub.4, both in water. Test conditions for Example 2 were as follows: gas feed composition: 10 mol % CO.sub.2, mol % H.sub.2S, balance N2; gas flow rate: 154 sccm; temperature: 40.8° C., pressure: 1 bar; volume: 15 mL; stirring rate: 200 rpm.

[0048] The effect of phosphoric acid on MDEA acid gas absorption behavior can be observed in FIGS. 8 and 9. FIG. 8 is a plot of the CO.sub.2 and H.sub.2S uptake as a function of treatment time for the 36 wt % MDEA solutions with and without phosphoric acid. FIG. 9 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of total loading with MDEA aqueous solutions with and without phosphoric acid. There is no significant difference observed because it is known that bicarbonate is preferentially formed during CO.sub.2-MDEA reactions. Nevertheless, it is observed that the acidified MDEA solution may lead to slightly higher bicarbonate formation. FIG. 9 in particular demonstrates a reduced H.sub.2S selectivity for the MDEA solution with phosphoric acid, which is the opposite effect as the highly sterically hindered M3ETB.

Additional Embodiments

[0049] According to certain teachings of the present invention, a process for selectively separating H.sub.2S from a gas mixture which also comprises CO2 is disclosed, the process comprising contacting a stream of the gas mixture with an absorbent solution comprising one or more amines. The H.sub.2S/CO.sub.2 selectivity of the absorbent solution is greater than about 4.0, and preferably greater than 5.0, for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] between about 0.2 and about 0.6. The one or more amines is selected from the group consisting of amines, alkanolamines, sterically hindered akanolamines, and mixtures thereof, and is preferably methoxyethoxyethoxyethanol-t-butylamine (M3ETB), ethoxyethanol-t-butylamine (EETB), or N-methyldiethanolamine (MDEA). The sterically hindered alkanolamine is preferably a capped amine.

[0050] Another embodiment of the present invention is a method for increasing the selectivity of an absorption process for H.sub.2S absorption from a gas mixture which also comprises CO.sub.2, the absorption process having an absorbent solution comprising one or more amines, the method comprising reducing the pH of the absorbent solution. This pH reducing step is accomplished by diluting the absorbent solution, or by adding an acid to the absorbent solution, the acid being selected from phosphoric acid and sulfuric acid. The one or more amines is selected from the group consisting of amines, alkanolamines, sterically hindered akanolamines, and mixtures thereof, and is preferably M2ETB, EETB, or MDEA.

[0051] Yet another embodiment of the present invention is a process for selectively separating H2S from a gas mixture which also comprises CO.sub.2, the process comprising contacting a stream of the gas mixture with an absorbent solution comprising M3ETB. The M3ETB concentration in the absorbent solution is less than about 36 wt %, and preferably between 25 and 30 wt %. The H.sub.2S/CO.sub.2 selectivity of the absorbent solution is greater than about 4.0, and preferably greater than 5.0, for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] range between about 0.2 and about 0.6.

[0052] Still another embodiment of the present invention is a system for selectively absorbing H2S from a raw gas stream which also comprises CO.sub.2, the system comprising an absorber tower for contacting the raw gas stream countercurrently with an aqueous amine stream to create a spent amine stream comprising at least a portion of the H.sub.2S from the raw gas stream, and a regeneration tower for creating a regenerated amine stream and a desorbed acid gas stream. The H.sub.2S/CO.sub.2 selectivity of the aqueous amine stream is greater than about 4.0, preferably greater than about 5.0, for an acid gas loading [mol(CO.sub.2+H.sub.2S)/mol(amine)] between about 0.2 and about 0.6. The aqueous amine stream preferably comprises methoxyethoxyethoxyethanol-t-butylamine (M3ETB) in a concentration less than about 36 wt %, and preferably between 25 and 30 wt %. The H.sub.2S/CO.sub.2 selectivity of the system is increased by reducing the pH of the aqueous amine stream, preferably by lowering the amine concentration in the aqueous amine stream.

[0053] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings therein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and sprit of the present invention. Unless otherwise indicated, all numbers expressing quantities of ingredients, properties, reaction conditions, and so forth, used in the specification and claims are to be understood as approximations based on the desired properties sought to be obtained by the present invention, and the error of measurement, etc., and should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Whenever a numerical range with a lower limit and an upper limit is disclosed, a number falling within the range is specifically disclose. Moreover, the indefinite articles “a” or “an”, as use in the claims, are defined herein to mean one or more than one of the element that it introduces.