ACID GAS REMOVAL WITH AN ABSORPTION LIQUID THAT SEPARATES IN TWO LIQUID PHASES

Abstract

An apparatus and method for acid gas removal using an absorption liquid comprising a chemical solvent and a non-chemical solvent, each absorbing acid gas, wherein in embodiments regeneration of the absorption liquid involves separating the two components from each other in separate streams, and causing desorption from each stream using different desorption conditions.

Claims

1. A method for reducing the content of at least one gaseous component selected from the group consisting of CO.sub.2 and H.sub.2S in a gaseous mixture comprising such component, the method comprising contacting in an absorption zone said gaseous mixture with absorption liquid, wherein said absorption liquid comprises a chemical solvent and a non-chemical solvent, thereby causing absorption of at least some of said gaseous component from said gaseous mixture into said chemical solvent and into said non-chemical solvent, yielding a stream of absorption liquid comprising a thus absorbed component, wherein said stream of absorption liquid is phase separated in a first phase predominantly comprising said chemical solvent and a second phase predominantly comprising said non-chemical solvent, separating said first phase and said second phase at least partly from each other to yield a first liquid stream comprising chemical solvent and absorbed component and a second liquid stream comprising non-chemical solvent and absorbed component, and desorbing under first desorption conditions said absorbed component from said first liquid stream in a first desorption step yielding a first released desorbed component and a regenerated first stream, and desorbing under second desorption conditions which are different from said first desorption conditions said absorbed component from said second liquid stream in a second desorption step yielding a second released desorbed component and a regenerated second stream, wherein the chemical solvent and the non-chemical solvent have different desorption characteristics for said gaseous component.

2. The method according to claim 1, wherein said chemical solvent comprises one or more amines.

3. The method according to claim 1, wherein said chemical solvent comprises one or more amines selected from the group consisting of 2-amino-ethanol, diisopropylamine, diethanolamine, diethylethanolamine, triethanolamine, aminoethoxyethanol, 2-amino-2-methyl-1-propanol, dimethylaminopropanol, methyldiisopropanolamine, aminoethylpiperazine, piperazine, 2-amino-1-butanol, methyldiethanolamine, and combinations thereof.

4. The method according to claim 1, wherein said non-chemical solvent comprises one or more selected from the group consisting of sulpholane, 3-methylsulpholane, dimethylsulphoxide, thiodiglycol, dithiodiglycol, N-methylpyrrolidone, methanol, tributyl phosphate, N--hydroxyethylmorpholine, propylene carbonate, methoxytriglycol, dimethyl ether of polyethylene glycol, and mixtures of polyethylene glycol dialkyl ethers.

5. The method according to claim 1, wherein said chemical solvent comprises water.

6. The method according to claim 1, wherein said chemical solvent comprises three or more different molecules.

7. The method according to claim 1, wherein said chemical solvent comprises four or more different molecules.

8. The method according to claim 1, wherein the absorption liquid further comprises one or more modifiers for promoting phase separation between the two phases upon acid gas absorption.

9. The method according to claim 8, wherein said one or more modifiers comprise one or more compounds selected from the group consisting of neutral salts, hydrotropes, alcohols, organic liquid additives, and combinations thereof.

10. The method according to claim 1, wherein said gaseous mixture further comprises a hydrocarbon and/or hydrogen.

11. The method according to claim 1, wherein said first desorption step comprises thermal regeneration and said second desorption step comprises flashing.

12. The method according to claim 1, wherein said first liquid phase and said second liquid phase have a different volumetric density and wherein the separation of said first liquid phase and second liquid phase comprises separation by gravity.

13. The method according to claim 1, wherein the separation of said first liquid phase and second liquid phase comprises decanting, or centrifugation.

14. The method according to claim 1, further comprising providing said regenerated first stream and said second stream to said absorption zone, wherein 60% or more by total weight of said first liquid stream is provided closer to the inlet of said absorption zone for the gaseous mixture than 60% or more by total weight of said second liquid stream.

15. The method according to claim 1, wherein said second desorption step involves no heating of said second liquid stream, or heating said second liquid stream by a temperature increase at least 20 C. less than a temperature increase of said first liquid stream during said first desorption step.

16. The method according to claim 1, wherein said first released desorbed component comprises CO.sub.2 and H.sub.2S, wherein the method further comprises submitting said first desorbed component to a Claus process without further reduction of the CO.sub.2 concentration of said first desorbed component.

17. The method according to claim 1, wherein said method further reduces the content of at least one gaseous component selected from the group consisting of organosulphur compounds and mercaptanes that are comprised in the gaseous mixture.

18. The method according to claim 1, wherein said absorption liquid comprises water.

19. An apparatus for performing a method according to claim 1, the apparatus comprising: a contactor comprising an inlet and an outlet for a gaseous stream and at least one inlet and outlet for absorption liquid, a separation unit for separating said absorption liquid, comprising an inlet for absorption liquid having a connection to the outlet for absorption liquid of said contactor, a first outlet for a first separated stream and a second outlet for a second separated stream of said absorption liquid, a first regeneration unit, having an inlet for said first separated stream, said inlet having a connection to said first outlet of said separation unit, and having a first outlet for a gaseous stream and a second outlet having a connection to an inlet for absorption liquid of said contactor, and a second desorption unit, having an inlet for said second separated stream, having a connection to said second outlet of said separation unit, and having a first outlet for a gaseous stream and a second outlet for a first regenerated stream of said absorption liquid, said second outlet having a connection to an inlet for absorption liquid of said contactor.

20. The apparatus according to claim 19, wherein said first regeneration unit is a thermal stripper.

21. The apparatus according to claim 19, wherein said second regeneration unit is a pressure swing vessel.

22. The apparatus according to claim 19, further comprising a flash vessel between the separation unit and the first regeneration unit.

Description

EXAMPLES

Example 1

[0088] FIG. 2 shows photographs of absorption liquids. FIG. 2A shows a homogenous mixture of a mixture of chemical solvents namely mono-ethanolamine (MEA) and methyldiethanolamine (MDEA) and a non-chemical solvent, namely sulpholane in water. FIG. 2B shows the mixture after phase separation thereof upon CO.sub.2 absorption, with the physical solvent at the bottom and the chemical solvent on top. Some of the characteristics of the two phases for a mixture, partially loaded with CO.sub.2, are given in the table 1. The chemical solvent is richer in CO.sub.2, has a lower density and contains more water. It is important to note that at higher CO.sub.2 partial pressures, the CO.sub.2 concentration and the density of Phase 2 will increase.

TABLE-US-00001 TABLE 1 Property Phase 1 Phase 2 CO.sub.2 concentration [mol/l] 3.7 0.14 Density [g/ml] 1.21 1.25 Water content [%] 22 5

[0089] FIG. 3 shows gas chromatography spectra of the loaded mixture, the phase split top and the phase split bottom part, respectively. One can observe that the top part is indeed depleted of sulpholane and the bottom part depleted of the amines.

Example 2

[0090] The equilibrium behaviour of a solvent in the presence of different acid gases indicates the performance of the solvent. In FIG. 4, the equilibrium solubility of both the acid gases (CO.sub.2 and H.sub.2S) at 40 C. for a typical homogeneous absorption solvent consisting of MDEA, aminoethylpiperazine (AEP), sulpholane and water, is shown. It is important to note that already at low partial pressure of gases, the solvent has high capacity for acid gases. Moreover, at equilibrium there is no preference for absorption of either of the acid gas.

[0091] However, when considering the kinetics of absorption, the solvent absorbs H.sub.2S selectively over CO.sub.2. The homogeneous absorption solvent contains methyl diethanol amine (MDEA) as the major chemical solvent. This behaviour is similar to that expected from a H.sub.2S selective chemical solvent such as MDEA. This is depicted in FIG. 5 stating the faster absorption of H.sub.2S over CO.sub.2 from the gas space during a gas-liquid contacting step starting with equal amounts of H.sub.2S and CO.sub.2. A similar preference for H.sub.2S over CO.sub.2 is expected for the phase containing the chemical solvent.

[0092] A volumetric split of about 3:1 of the chemical solvent to non-chemical solvent was obtained from this example of homogeneous absorption solvent. Therefore, only three quarters of the solvent needs to be thermally regenerated, thereby translating in lower steam consumption and thus, lower operating costs. A higher net capacity for acid gases over typically used chemical solvent is expected and will lead to further decrease in reboiler steam consumption. Furthermore, an improved H.sub.2S/CO.sub.2 ratio, due to a preference for H.sub.2S in the chemical phase, in the acid gas increases the efficiency of the Claus unit further lowering the operational costs.