CA1189050A - Non-sterically hindered-sterically hindered amine co- promoted acid gas scrubbing solution and process for using same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to an alkaline promoter system comprising specific mixtures of non-sterically hindered amino compounds and sterically hindered amino acids and their use in acid gas scrubbing processes. The preferred promoter system comprises a mixture of (1) diethanol amine or 1,6-hexanediamine and (ii) N-secondary butyl glycine or pipecolinic acid.

Description

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1 The present invention relates to an alkaline

2 promoter system comprising specific mixtures of non-

3 sterically hindered amino compounds and sterically ~ hindered amino acids and their use in acid gas scrubbing processes, particularly in the "hot pot" type acid gas 6 scrubbing processes.

7 The present invention pertains to an improved 8 process for carrying out what is known as the aqueous g base scrubbing process or "hot potash" ("hot pot") process. In this process a relatively small level of an 11 amine is included as an activator for the aqueous base 12 used in the scrubbing solution. This type of process 13 is generally used where bulk removal of an acid gas, 14 such as CO2, is desired. This process also applies to situations where the CO2 and feed gas pressures 16 are high. In such processes, useful results are achiev-17 ed using aqueous potassium carbonate solutions and an 18 amine activator. Many industrial processes for removal 19 of acid gases, such as CO2, use regenerable aqueous alkaline scrubbing solutions, such as a potassium 21 carbonate and an activator comprising an amine, which 22 are continuously circulated between an absorption zone 23 where acid gases are absorbed and a regeneration zone 24 where they are desorbed, usually by pressure reduction and steam-stripping~ The capital cost of these acid gas 26 scrubbing processes is generally controlled by the size 27 of the absorption and regeneration towers, the size of 28 the reboilers for generating stripping steam, and the 29 size of the condensers which condense spent stripping steam so that condensate may be returned to the system 31 to maintain proper water balance. The cost of operating 32 such scrubbing plants is generally related to the amount 33 of heat required for the removal of a given amount o 3~ acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam 36 consumed. Means for reducing the costs in operating ~1 1 these industrial processes have focused on the use o~
2 absorbing systems or combinations of chemical absorbents 3 which will operate more efficiently and effectively in

4 acid gas scrubbing processes using existing equipmentO

There are a number of patents which describe 6 processes to improve the efficiency of the "hot potash"
7 process. Some of these improvement processes are 8 described below.

g In U.S. Patent No. 2,718,454, there is described a process for using potash and similar alkali 11 metal salts in conjunction with amines, such as mono-12 ethanolamine, diethanolamine and triethanolamine to 13 remove acid gases from a gas mixture. The combination 14 of the alkali metal compounds in conjunction with the designated amine yields higher capacity for acid gases 16 than systems with the amines alone.

17 In U~Sn Patent No. 3,144,301, there is dis-18 closed the use of potassium carbonate in conjunction 19 with monoethanolamine and diethanolamine to remove C02 from gaseous mixtures.

21 U.S. Patent Nos. 3,563,695; 3,563,696, and 22 3,642,430 to Benson et al. disclose processes for 23 removing C02 and H2S from gaseous mixtures by alkaline 24 scrubbing processes wherein at least two separate regeneration zones are provided. Alkanolamines and 26 amino acids such as glycine are described as activators, 27 but the use of sterically hindered amino compounds i5 28 not taught or disclosed in these patents.

29 In U.S0 Patent Nos. 3,637,345; 3,763,434, and 3,848,057 processes for the removal of acid gases by 31 means of aqueous carbonate scrubbing solutions activated 1 by an amino compound such as 1,6-hexanediamine, piperi-2 dine and their derivatives are described.

3 In U.S. Patent No. 3,356,921, there is dis-4 closed a process for removal of acid gases from fluids by use o~ a basic salt of an alkali or alkaline earth 6 metal and an amino compound activator, such as 2-methyl-7 aminoethanol, 2-ethylaminoethanol, morpholine, pyrroli-8 dine and derivatives thereof.

g Belgian Patent No. 767,105 discloses a process for removing acid gases from gaseous streams by contact-11 ing the gaseous streams with a solution comprising 12 potassium carbonate and an amino acid, such as substi-13 tuted glycines (e.g., N-isopropyl glycine, N-t-butyl-14 glycine, N-cyclohexylglycine, etc.). The data in Table IV of the patent indicates that the highly substituted 16 compounds, such as N-t-butylglycine, are inferior to the 17 straight chain compounds, such as N-n-butyl glycine, but 18 N-cyclohexyl glycine, a sterically hindered amine, has a 19 good rate of absorption. Similarly, British Patent No.
1,305,718 describes the use of beta- and gamma amino 21 acids as promoters for alkaline salts in the "hot pot"
22 acid gas treating process. These amino acids, however, 23 are not suitable because the beta- amino acids undergo 24 deamination when heated in aqueous potassium carbonate solutions. The gamma amino acids form insoluble lactams 26 under the same conditions.

27 Recently, it was shown in U.S. Patent No.
28 4,112,050 that sterically hindered amines are superior 29 to diethanolamine (DEA) and 1,6-hexanediamine ~IMDA) as promoters for alkaline salts in the "hot pot" acid gas 31 scrubbing process. U.S. Patent No. 4,094,957 describes 32 an improvement to the '050 patented process whereby 33 amino acids, especially sterically hindered amino acids, 34 serve to prevent phase separation of the aqueous solu-1 tion containing sterically hindered amines at high2 temperatures and low fractional conversions during 3 the acid gas scrubbing process. In these patents 4 "sterically hindered amines" are defined as amino compounds containing at least one secondary amino group 6 attached to either a secondary or tertiary carbon atom 7 or a primary amino group attached to a tertiary carbon 8 atom. At least one nitrogen atom w;ll have a sterically 9 hindered structure.

In some instances, where an existing commer-11 cial gas treating plant utilizes a non-sterically 12 hindered amine promoter such as diethanolamine or 13 1,6-hexanediamine, there is a need to increase the C02 14 scrubbing capacity due to increased levels of CO2 in the gas. The need to meet this increased C2 capacity 16 can be accomplished by increasing the size o the plant 17 (e.g., adding treating towers and the like) or by 18 replacing the non-sterically hindered amine with steri-19 cally hindered amines as proposed in U.S. Patent Nos.
4,094,957 and 4,112,050. In the case of the latter 21 approach, the preexisting scrubbing solution must be 22 removed and replaced with the fresh solution containing 23 potassium carbonate and sterically hindered amine. This 24 change-over procedure requires some "downtime" of the plant with consequent losses of production. Therefore, 26 increasing the size of the gas treating plant or chang-27 ing over the scrubbing solution can be costly.

28 It has now been discovered that one may add 29 sterically hindered amino acids to the non-sterically hindered amino compound-promoted carbonate scrubbing 31 solution and thereby increase the CO2 absorption rate 32 relative to that of the pre-existing non-sterically 33 hindered amino compound-promoted carbonate.

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1 Accordingly, in one embodiment o~ the present 2 invention, there is provided a process for the removal 3 of C2 from a gaseous stream containing C2 which com-~ prises contacting said gaseous stream as follows:

(1) in an absorption step absorbing said CO2 6 from said gaseous mixture with an aqueous absorbing 7 solution, comprising:

8ta) a basic alkali metal salt or hydrox-9ide selected from the group consist-10ing of alkali metal bicarbonates~
11carbonates, hydroxides, borates, 12phosphates and their mixtures, and 13(b) an activator or promoter system for 1~said basic alkali metal salt or 15hydroxide, comprising:

16(i) at least one non-sterically 17hindered amino compound, and 18(ii) at least one sterically hinder-19ed amino acid, and 20(2) in a desorption and regeneration step, 21 desorbing at least a portion of the absorbed CO2 from 22 said absorbing solution.

23 The mole ratio of the non-sterically hindered 24 amino compound to the sterically hindered amino acid 25 may vary widely, but is preferably 1:3 to 3:1, most 26 preferably 1:1. The sterically hindered amino acid may 27 be added to the scrubbing solution containing the 28 non-sterically hindered amino compound all at once or in ~9 increments during the gas scrubbing operation.

1 As another embodiment of the invention, there 2 is provided an acid gas scrubbing composition, compris-3 ing:

4 (a) 10 to about 40% by weight of an alkali metal salt or hydrox;de;

6 (b) 2 to about 20~ by weight of a non-steri-7 cally hindered amino compound;

8 (c) 2 to about 20% weight of a sterically g hindered amino acid; and (d) the balance, water.

11 The non-sterically hindered amino compound may 12 be any compound having amino functionality which is 13 water soluble in the presence of the sterically hindered 14 amino acid co-promoter. Typically, the non-sterically hindered amino compound will comprise those amino 16 compounds heretofore used or described in acid gas 17 treating processes. By the term "non-sterically hin-18 dered", it is meant those compounds that do not contain 19 at least one secondary amino group attached to either a secondary or tertiary carbon atom or a primary amino 21 group attached to a tertiary carbon atom. Typically, 22 the nitrogen atoms will not have a sterically hindered 23 structure. For example, such compounds will include 24 diethanolamine, monoethanolamine, triethanolamine, 1,6-hexanediamine, piperidine and their derivatives and 26 the like. Preferably, the non-sterically hindered amino 27 compound will be diethanolamine or 1,6-hexanediamine 28 which are presently used in commercial acid gas treating 29 plants throughout the world.

The sterically hindered amino acids may 31 include any amino acid which is soluble in the alkaline 1 aqueous solution to be used in thle acid gas treating 2 solution. Preferably, the amino acid will have 4 to 8 3 carbon atoms and contain one amino moiety. By the term 4 "sterically hindered amino acid", it is meant those S amino acids that do contain at least one secondary amino 6 group attached to either a secondary or tertiary carbon 7 atom or a primary amino group attached to a tertiary 8 carbon atom. At least one of the nitrogen atoms will 9 have a sterically hindered structure. Typical steri-cally hindered amino acids useful in the practice of 11 the present invention will include N-secondary butyl 12 glycine, pipecolinic acid, N-isopropyl glycine, N-2-amyl-13 glycine, N-isopropyl alanine, N-sec. butyl alanine, 14 2-amino-2-methyl butyric acid and 2-amino-2-methyl valeric acid.

16 In general, the aqueous scrubbing solution 17 will comprise an alkaline material comprising a basic 18 alkali metal salt or alkali metal hydroxide selected 19 from Group IA of the Periodic Table of Elements. More preferably, the aqueous scrubbing solution comprises 21 potassium or sodium borate, carbonate, hydroxide, 22 phosphate or bicarbonate. Most preferably, the alkaline 23 material is potassium carbonate.

24 The alkaline material comprising the basic alkali metal or salt or alkali metal hydroxide may be 26 present in the scrubbing solution in the range from 27 about 10% to about 40% by weight/ preferably from 20~
28 to about 35% by weight. The alkaline material, the 29 non-sterically hindered amine and the amino acid acti-vator or promoter system remain in solution throughout 31 the entire cycle of absorption of CO2 from the gas 32 stream and desorption of CO2 from the solution in the 33 regeneration step. Therefore, the amounts and mole 34 ratio of the non-sterically-hindered amines and the amino acids are maintained such that they remain in s~

1 solution as a single phase throughout the absorption and 2 regeneration steps. Typically, these criteria are met 3 by including from about 2 to about 20%, preferably from 4 5 to 15% more preferably, 5 to 1()% by weight of the non-sterically hindered amino compound and from 2 to 6 about 20% by weight, preferably 5 to about 15% by weight 7 of the sterically hindered amino acid.

8 The scrubbing solution may be premixed and g placed into use in the absorbing reactors. Alternative-ly, in an existing acid gas treating plant where the 11 non-sterically hindered amino compound is being used, 12 the sterically hindered amino acid may be added to the 13 scrubbing solution, preferably in increments.

14 The aqueous scrubbing solution may include a variety of additives typically used in acid gas 16 scrubbing processes, e.g., antifoaming agents, anti-17 oxidants, corrosion inhibitors and the like. The amount 18 of these additives will typically be in the range that 19 they are effectivel i.e., an effective amount.

Figure 1 graphically illustrates the capacity 21 for potassium carbonate solutions activated by diethano-22 lamine (DEA) and mixtures of diethanolamine/pipecolinic 23 acid, 1,6-hexanediamine (~MDA) and 1,6-hexane-diamine/
24 N-secondary butyl glycine (SBG) at 80C wherein the cumulative liters of C02 reabsorbed is a function of 26 time.

27 The term C02 includes C02 alone or in combi-28 nation with ~l2S, CS2, HCN, COS and the oxides and sulfur 29 derivatives of Cl to C4 hydrocarbons. These acid gases may be present in trace amounts within a gaseous mixture 31 or in major proportions.

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1 The contacting of the absorbent mixture and 2 the acid gas may take place in any suitable contacting 3 tower. In such processes, the gaseous mixture from 4 which the acid gases are to be removed may be brought in~o intimate contact with the absorbing solution using 6 conventional means, such as a tower packed with, for 7 example, metal or ceramic rings or with bubble cap or 8 sieve plates, or a bubble column reactor.

g In a preferred mode of practicing the inven-tion, the absorption step is conducted by feeding the 11 gaseous mixture into the base of the tower while fresh 12 absorbing solution is fed into the top. The gaseous 13 mixture freed largely from acid gases emerges from the 14 top. Preferably, the temperature of the absorbing solution during the absorption step is in the range from 16 about 25 to about 200C, and more preferably from 35 17 to about 150C. Pressures may vary widely; acceptable 18 pressures are between 5 (3.5) and 2000 psia (1906 kg/
19 cm2), preferably 100 (70.3) to 1500 psia (1054.5 kg/
cm2), and most preferably 200 (140.6) to 1000 psia 21 (703 kg/cm2) in the absorber. In the desorber, the 22 pressures will range from about 0 (0) to 1000 psig (703 23 kg/cm2). The partial pressure of the acid gas, e.g., 24 CO2 in the feed rnixture will preferably be in the range from about 0.1 (0.0703) to about 500 psia (351.5 26 kg/cm2), and more preferably in the range from about 1 27 (0.703~ to about 400 psia (281.2 kg/cm2). The contact-23 ing takes place under conditions such that the acid gas, 29 e.g., CO2, is absorbed by the solution. Generally, the countercurrent contacting to remove the acid gas 31 will last for a period of from 0.1 to 60 minutes, 32 preferably 1 to 5 minutes. During absorption, the 33 solution is maintained in a single phase. The amino 34 acid aids in reducing foam in the contacting vessels.

The aqueous absorption solution comprising the 1 alkaline material, the activator system comprising the 2 non-sterically hindered amino compound and the steri-3 cally hindered acid which is saturated or partially 4 saturated with gases, such as CO2 ancl H2S may be regen-erated so that it may be recycled back to the absorber.
6 The regeneration should also take place in a single 7 liquid phase. Therefore, the presence of the highly 8 water soluble amino acid provides an advantage in this g part of the overall acid gas scrubbing process. The regeneration or desorption is accomplished by conven-11 tional means, such as pressure reduction, which causes 12 the acid gases to flash off or by passing the solution 13 into a tower of similar construction to that used in the 14 absorption step, at or near the top of the tower, and passing an inert gas such as air or nitrogen or prefer-16 ably steam up the tower. The stripping steam may be 17 generated by boiling the solution. The te~perature of 18 the solution during the regeneration step may be the 19 same as used in the absorption step, i.e., 25 to about 200C, and preferably 35 to about 150C. The absorbing 21 solution, after being cleansed of at least a portion of 22 the acid bodies, may be recycled back to the absorbing 23 tower. Makeup absorbent may be added as needed. Single 24 phase is maintained during desorption by controlling the acid gas, e.g., CO2, level so that it does not fall 26 into the region where two liquid phases form. This, of 27 course, following the practice of the present invention 28 is facilitated by the use of the highly water soluble 29 amino acid in the mixture.

As a typical example, during desorption, the 31 acid gas, e.g., CO2-rich solution from the high pressure 32 absorber is sent first to a flash chamber where steam 33 and some CO2 are flashed from solution at low pressure.
3~ The amount of CO2 flashed off will, in general, be about 35 to 40% of the net CO2 recovered in the flash and 36 stripper. This is increased somewhat, e.g., to 40 to 1 50~, with the high desorption rate promoter system owing 2 to a closer approach to equilibrium in the flash. Solu 3 tion from the flash drum is then steam stripped in the 4 packed or plate tower, stripping steam having been generated in the reboiler in the base of the stripper.
6 Pressure in the flash drum and stripper is usually 16 7 (11.2) to about 100 psia (70.3 kg/cm2), preferably 16 8 (11.2) to about 30 psia (21.09 kg/cm2), and the temper-9 ature is in the range from about 25 to about 200C, preferably 35 to about 150C, and more preferably 100 11 to about 140C. Stripper and flash temperatures will, 12 of course, depend on stripper pressure, thus at about 16 13 (11.2) to 25 psia (17.6 kg/cm2) stripper pressures, 14 the temperature will be about 100 to about 140C
during desorption. Single phase is maintained during 16 desorption by regulating the amount of acid gas, e.g., 17 CO2, recovered.

18 In the most preferred embodiment oE the 19 present invention, the acid gas, e.g., CO2 is removed from a gaseous stream by means of a process which 21 comprises, in sequential steps, (1) contacting the 22 gaseous stream with a solution comprising 10 to about 40 23 weight percent, preferably 20 to about 30 weight percent 24 of potassium carbonate, an activator or promoter system comprising 2 to about 20 weight percent, preferably 5 to 26 about 15 weight percent, more preferably 5 to about 10 27 weight percent of at least one non-sterically hindered 28 amino compound as herein defined, 2 to about 20 weight 29 percent, and preferably 5 to about 15 weight percent of the sterically hindered amino acid as herein defined, 31 the balance of said solution being comprised of water, 32 said contacting being conducted at conditions whereby 33 the acid gas is absorbed in said solution, and prefer-34 ably at a temperature ranging from 25 to about 200C, more preferably from 35 to about 150C and a pressure 36 ranging from 100 (70) to about 1500 psig (1054.5 kg/cm2), 1 and (2) regenerating said solution at conditions whereby 2 said acid gas is desorbed from said solution. By 3 practicing the present invention, one can operate the 4 process above described at conditions whereby the working capacity, which is the difference in moles of 6 acid gas absorbed in the solution at the termination of 7 steps (1) and (2) based on the moles of potassium 8 carbonate originally present, is greater than that 9 obtained under the same operating conditions for remov-ing acid gases from gaseous streams, wherein said same 11 operating conditions do not include the mixture of the 12 non-sterical]y hindered amino compound and the ster-13 ically hindered amino acid co-promoter system. In other 14 words, working capacity is defined as follows:

15 CO2 in solution C2 in solution 16 at completion of less at completion of 17 absorption desorption 18 Which is:
19 Moles of CO2 Absorbed Moles Residual CO2 Absorbed less 21 Initial Moles K2CO3 Initial Moles K2CO3 22 It should be noted that throughout the speci-23 fication wherein working capacity is referred to, the 24 term may be defined as the difference between CO2 load-ing in solution at absorption conditions (step 1) and 26 the CO2 loading in solution at regeneration conditions 27 (step 2) each divided by the initial moles of potassium 28 carbonate. The working capacity is equivalent to the 29 thermodynamic cyclic capacity, that is the loading is measured at equilibrium conditions. This working 31 capacity may be obtained from the vapor-liquid equilib-32 rium isotherm, that is, from the relation between 33 the CO2 pressure in the gas and the acid gas, e.g., CO2 34 loading in the solution at equilibrium at a given tem-perature. To calculate thermodynamic cyclic capacity, L I ~

1 the following parameters must usually be specified: (1) 2 acid yas, e.g., CO2 absorption pressure, (2) acid gas, 3 e.g., CO2 regeneration pressure~ (3) temperature of 4 absorption~ (4) temperature of regeneration, (5) solu-tion composition, that is weight percent of the non-6 sterically hindered amino compound, weight percent 7 of the sterically hindered amino acid and weight percent 8 of the alkaline salt or hydroxide, for example potassium g carbonate, and (6) gas composition. The skilled artisan may conveniently demonstrate the improved process which 11 results by use of the non-sterically hindered amino-12 compound and the sterically hindered amino acid mixture 13 by a comparison directly with a process wherein the14 mixture is not included in the aqueous scrubbing solution. For example, it will be found when comparing 16 two similar acid gas scrubbing processes (that is 17 similar gas composition, similar scrubbing solution 18 composition, similar pressure and temperature condi-19 tions) that when the sterically hindered amino acid is utilized in the mixture the difference between the 21 amount of acid gas, eOg., CO2 absorbed at the end of 22 step 1 (absorption step) defined above and step 2 23 (desorption step) defined above is significantly greater 24 This significantly increased working capacity is observ-ed even though the scrubbing solution that is being 26 compared comprises an equimolar amount of a prior art 27 amine promoter, such as diethanolamine, 1,6-hexane-28 diamine, ~alone) etc. It has been found that the use of 29 the admixture of the non-sterically hindered amino compound and the sterically hindered amino acid of the 31 invention provides a working capacity which is greater 32 than the workiny capacity of a scrubbing solution which 33 does not utilize this new activator or promoter system.

34 Besides increasing working capacity and rates of absorption and desorption, the use of the admixture 36 of the non-sterically hindered amino compound and 3s~

1 sterically hindered amino acid leads to lower steam 2 consumption during desorption.

3 Steam requirements are the major part of the 4 energy cost of operating an acid gas, e.g., CO2 scrub-bing unit. Substantial reduction in energy, i.e., 6 operating costs will be obtained by the use of the 7 process wherein the mixture is utilized. Additional 8 savings from new plant investment reduction and debottle-g necking of existing plants may also be obtained by the use of the mixture of the invention. The removal of 11 acid gases such as CO2 from gas mixtures is of major 12 industrial importance, particularly the systems which 13 utilize potassium carbonate activated by the unique 14 activator or co-promoter system of the present invention.

The absorbing solution of the present inven-16 tion, as described above, will be comprised of a ma~or 17 proportion of alkaline materials, e.g., alkali metal 18 salts or hydroxides and a minor proportion of the 19 activator system. The remainder of the solution will be comprised of water and/or other commonly used additives, 21 such as anti-foaming agents, antioxidant~, corrosion 22 inhibitors, etc. Examples of such additives include 23 arsenious anhydride, selenious and tellurous acid, 24 protides, vanadium oxides, e.gD, V2O3, chromates, e.g., K2Cr2O7, etc.

26 Many of the sterically hindered amino acids 27 useful in the practice of the present invention are 28 either available commercially or may be prepared by 29 various known procedures.

Preferred sterically hindered amino acids 31 include N-secondary butyl glycine (CAS Registry Number 32 58695-42-4), N-2-amyl glycine, N-isopropyl glycine, 33 pipecolinic acid, N-isopropyl glycine, N-2-amyl-glycine, 1 N-isopropyl alanine, N-sec. butyl alanine, 2-amino-2-2 methyl butyric acid and 2-amino-2-methyl valeric acidO

3 A preferred method for preparing the preferred 4 sterically hindered amino acids comprises first reacting glycine or alanine under reductive conditions with a 6 ketone in the presence of a hydrogenation catalyst.
7 This reaction produces the sterically hindered monosub-8 stituted amino acid.

9 Preferred non-sterically hindered amino compounds include: diethanolamine, monoethanolamine, 11 triethanolamine, 1,6-hexanediamine, piperidine, etc.

12 The invention is illustrated further by the 13 following examples which, however, are not to be taken 14 as limiting in any respect. All parts and percentages, unless expressly stated to be otherwise, are by weicJht.

16 "Hot Pot" Acid Gas Treating Process 17 Example 1 ~Comparison) 18 The reaction apparatus consists of an absorber 19 and a desorber. The absorber is a vessel having a capac-ity of 2.5 liters and a diameter of 10 cm., equipped 21 with a heating jacket and a stirrer. A pump removes 22 liquid from the bottom of the reactor and feeds it back 23 to above the liquid level through a stainless-steel 24 sparger. Nitrogen and CO2 can be fed to the bottom of the cell through a sparger.

26 The desorber is a l-liter reactor, equipped 27 with teflon blade stirrer, gas sparger, reflux condenser 28 and thermometer.

29 The reaction apparatus is the same as shown in Figure 1 of U.S. Patent 4,112,050.

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1 The following reagents are put into a 2-liter 2 Erlenmeyer:

3 37.8g of diethanolamine 4 187.5g of K2CO3 525g of water 6 When all the solid has dissolved, the mixture 7 is put into the absorber and brought to 80C. The 8 apparatus is closed and evacuated until the liquid g begins to boil. At this point C02 is admitted. In total 29 liters oE C02 is absorbed.

11 The rich solution is transferred to the 12 desorber and boiled for one hour, during which time 21 13 liters of C02 is desorbed.

14 The regenerated solution so obtained is transferred back to the absorber and cooled to 80C.
16 The apparatus is closed and evacuated until the liquid 17 begins to boil. At this point C02 is admitted. 20.5 18 liters of CO2 is re-absorbed, of which 6 liters are 19 re-absorbed in the first minute.

Example 2 (Comparative) 21 The procedure described above in Example 1 is 22 repeated except that the solution charged into the 23 2-liter Erlenmeyer is as follows:

24 41.4g of 1,6-hexanediamine (HMDA) 187.59 of K2CO3 26 520g of water 27 A total of 30O7 liters of C02 is absorbed.
28 The rich solution transferred to the desorber and boiled ~

1 for one hour. The regenerated solution so obtained is 2 transferred back to the absorber and cooled to 80C.
3 The apparatus is closed and evacuated until the liquid 4 begins to boil. At this point C02 is admitted and 21.2 liters of CO2 is re-absorbed, of which 6 liters 6 is re~absorbed in the first Minute.

7 Example 3 8 The procedure of Example 1 is repeated for 9 several acid gas scrubbing solutions containing a mixture of the non-sterically hindered amino compound, 11 e.g. diethanolamine (DEA) or 1,6-hexanediamine (HMDA) 12 and the sterically hindered amino acid co-promoter of 13 the present invention. The results of these tests along 14 with the results of comparative Examples 1 and 2 are shown in Tables I and II.

1 ~ ~9~

2 EXPERIMENTS IN 25 WT% K2CO3 Liters C2 4 Amine TotalReabsorbed in

5 Activator(s) Liters CO2 Reabsorbed 1st Mir.ute -

6 .357 moles DEAl) 20.5 6

7 21.5 6 21.6 6 9 .357 moles HMDA2) 21.2 6 10 .714 moles DEA 22.5 8 11 22.7 8 12 .357 moles DEA +
13 .357 moles SBG3) 28.2 13 14 27.6 12 15 .357 moles HMDA +
16 .357 moles PA4) 25.2 8 17 .357 moles HMDA +
18 .357 moles SBG 25.9 10 19 l)DEA is diethanolamine 2)HMDA is 1,6-hexanediamine 21 3)SBG is N-sec. butyl glycine 4)PA is pipecolinic acid EXPERIMENTS IN 30 WT% K2C03 3 Amine 4 Activator(s~Liters C02 Reabsorbed 1st Minute -5 .357 moles DEAl) 24.2 7 6 24.2 5 7 .714 moles DEA 25.4 8

8 .357 moles DEA +

9 .178 moles SBG3) 29.4 11 11 .357 moles HMDA2) +
12 .178 moles PA4) 27.2 9 26.6 9 13 1)DEA is diethanolamine 14 2)HMD~ is 1,6-hexanediamine 3)SBG is N-sec. butyl glycine 4)PA is pipecolinic acid 17 It can be seen from the data in Tables I and 18 II that the addition of the sterically hindered amino 19 acid improves the capacity and rate of reabsorption of the C02 compared to the scrubbing solution containing 21 the non-sterically hindered amino compound without the 22 sterically hindered amino acid. These results are 23 illustrated in Figure 1 in the case of the DEA, DEA~
24 pipecolinic acid, HMDA-pipecolinic acid and ~MDA-N-secondary butyl glycine promoted solutions ~herein the 26 cumulative liters of C02 reabsorbed at 80C as a 27 function of time are plotted graphically. Here it is 2~ clear that the sterically hindered amino acids, particu-29 larly N-secondary butyl glycine enhance the reabsorption f C02.

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1 Example 4 2 (a) Aging Studies in CO2 ',crubbing Apparatus 3 The following experiments are carried out 4 to ascertain the stability of the amino acids under accelerated-simulated acid gas treating conditions.

6 The following reagents are charged into a 7 stainless-steel bomb:

8 121 g of N-sec. butyl glycine 9 433 g of KHC03 540 g of H2O

11 The bomb is put into an oven and heated at 12 120C for 1000 hours. Then the content is dischar~ed 13 into a 2 liter flask and refluxed for several hours.

14 750 g is taken and subjected to an absorption-desorption-reabsorption cycle as described in Example 1.
16 27.9 liters of CO2 is absorbed into the regenerated 17 solution, 10 liters being absorbed in the first minute.

18 Comparison of this result with that obtained 19 with the fresh solution of N-secondary butyl glycine and potassium carbonate shows that the aging process 21 does not lead to a significant loss of activity.

22 If the aging experiment is carried out after 23 replacing N-sec. butyl glycine with the equivalent 24 amount of N-cyclohexyl glycine, 145 g, and reducing the water to 516 g in order to have the same total weight, 26 a considerable amount of solid, identified as 1,4-bis-27 cyclohexyl-2,5-diketopiperazine is formed. An attempt L~

1 to carry out an absorption-desorption cycle causes 2 plugging of the unit.

3 (b) Aging Under CO2 and H2S
-4 The following reagents are charged into a stainless-steel bomb:

6 121 g of N-sec. butyl glycine 7 24 g of K2S
8 390 g of KHCO3 9 5~4 g of water The bomb is put into an oven and heated at 11 120C for 1000 hours. Then the content is discharged 12 into a 2 liter flask and refluxed for several hours.

13 765 g is taken and subjected to an absorption-14 desorption-reabsorption cycle as described in Example lo 28.9 liters of CO2 is absorbed into the regenerated 16 solution, 10 1 being absorbed in the first minute.

17 Comparison of this result with that obtained 18 with the fresh solution of N-secondary butyl glycine and 19 potassium ca~bonate shows that the aging process leads to only a slight loss of activity.

21 If the aging experiment is carried out after 22 replacing N-secondary butyl glycine with the equivalent 23 amount of N-cyclohexyl glycine, 145 g, and reducing the 24 ~ater to 516 g in order to have the same total ~eight, 2S a considerable amount of solid, identified as 1,4-bis-26 cyclohexyl-2,5-diketopiperazine is formed. An attempt 27 to carry out an absorption-desorption cycle causes 28 plugging of the unit.

1 The excellent stability under the aging 2 conditions shown above for the N-secondary butyl glycine 3 coupled with its good performance as a promoter demon-4 strates the desirability of using it in combination with S non-sterically hindered amines.

6 Examp~e 5 7 This example is given in order to show that 8 beta-amino acids are not stable under alkaline condi-9 tionsO The following solution is prepared in a 2-liter Erlenmeyer:

12 49.7 g of ~ N-CH2CH2CH2NH-CHCH2COOH

14 ~H3 CH3 64.0 g of ~ -CH2CH2CH2NH-CHCH2COOH

16 411 g H2O

17 225 g K2CO3 1~ When everything is dissolved, the solution 19 is put into the absorber described in Example 1. An absorption-desorption-reabsorption cycle as described 21 in Example 1 gives 32.2 liters of CO2 reabsorbed, of 22 which 11 liters are absorbed in the first minute.

23 The aged solution is prepared in the following 24 way. The following reagents are charged into a stain-less-steel bomb:

26 CH3~ ICH3 27 66.3 g of ~ N-CH2CH2CH2NH-CHCH2COOH

s~

1 ÇH3 CH3 2 85.3 g of ~ -CH2CH2CH2NH-CHCH2CO0H

3 391.5 g of KHCO3 4 23.9 g f K2S

503 g of H2O

6 The bomb is put into an oven at 120C and left 7 there for 1000 hrs~ After that~ the bomb content is put 8 into a 2-liter flask and boiled at reflux for some 9 hours. 750 g of the solution so obtained is used to carry out a standard absorption-desorption-reabsorption 11 test. Only 25 liters of CO2 is reabsorbed, of which 6 12 liters are reabsorbed in the first minute.

13 13C-NMR analysis of the aged solution shows 14 the presence of 32 peaks, whereas the fresh solution only has 18. The aged solution shows the presence of 16 olefin bonds, which indicates that aging has led to 17 decomposition of the diamino acids into diamines and 18 crotonic acid~

19 The results shown above with respect to the beta-diamino acids agree with those obtained by Corbett, 21 McKay and Taylor, J. Chem_Soc. 5041 (1961) on beta-22 mono-amino acids.

23 The excellent stability under the aging 24 conditions shown above for N-secondary butyl glycine compared with other amino acids coupled with its good 26 performance as a co-promoter for the non-sterically 27 hindered amino compounds demonstrates the desirability 28 of using this admixture as a co-promoter system, par~
29 ticularly in de-bottlenecking existing acid gas treating processes which employ non-sterically hindered amino 31 compounds.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the removal of C02 from a gaseous stream containing C02 which comprises contacting said gaseous stream (1) in an absorption step absorbing said C02 from said aqueous stream with an aqueous absorbing solution comprising (a) a basic alkali metal salt or hydroxide selected from the group consisting of alkali metal bicarbonates, carbonates, hydroxides, borates, phosphates and their mixtures, and (b) an activator or promoter system for said basic alkali metal salt or hydroxide, comprising:

(i) at least one non-sterically hindered amino compound, and (ii) at least one sterically hindered amino acid, and (2) in a desorption and regeneration step, desorbing at least a portion of the absorbed C02 from said absorbing solution.

2. The process of claim 1 wherein the aqueous solution contains 10 to about 40% by weight of said basic alkali metal salt or hydroxide.

3. The process of claim 1 wherein the aqueous solution contains 2 to about 20% by weight of said non-sterically hindered amino compound and 2 to about 20% by weight of said sterically hindered amino acid.

4. The process of claim 1 wherein the mole ratio of said non-sterically hindered amino compound to said sterically hindered amino acid ranges between 0.2 and 5.

5. The process of claim 1 wherein said non-sterically hindered amino compound is selected from the group consisting of diethanolamine and 1,6-hexanediamine and said sterically hindered amino acid is selected from the group consisting of N-secondary butyl glycine and pipecolinic acid.

6. The process of claim 1 wherein the absorb-ing solution additionally includes additives selected from the group consisting of antifoaming agents, anti-oxidants and corrosion inhibitors.

7. A process for the removal of CO2 from a gaseous stream containing CO2 in accordance with claim 1, which comprises, in sequential steps:

(1) contacting the gaseous stream with an aqueous absorbing solution comprising (a) from about 20 to about 30% by weight of potassium carbonate, and (b) an activator or promoter system for the potassium carbonate, comprising:

(i) from about 5 to about 15% by weight of diethanol amine or 1,6-hexane diamine, (ii) 5 to about 10% by weight of N-secon-dary butyl glycine or pipecolinic acid, (c) the balance of the aqueous solution comprising water and additives selected from the group consisting of antifoaming agents, antioxidants and corrosion inhibitors, wherein said contacting is conducted at conditions whereby CO2 is absorbed in said absorbing solution and the temperature of the absorbing solu-tion is in the range from about 35 to about 150°C, and the pressure in the absorber is in the range from about 100 (70.3) to about 1500 psig (1054.5 kg/cm2; and (2) regenerating said absorbing solution at conditions whereby CO2 is desorbed from said absorbing solution, wherein the regeneration takes place at temperatures ranging from about 35 to about 150°C
and at pressures ranging from about 0 (0) to about 100 psig (70.3 kg/cm2).

8. The process of claim 7 wherein the absorbing solution from the regeneration step is recycl-ed for reuse in the absorption step.

9. An absorption composition comprising:
(a) 10 to about 40% by weight of an alkali metal salt or hydroxide, (b) 2 to about 20% by weight of a non-sterically hindered amino compound, (c) 2 to about 20%
by weight of a sterically hindered amino acid.

10. The composition of claim 9 wherein said composition contains (a) 20 to about 30% by weight potassium carbonate; (b) 5 to about 10% by weight of diethanol amine; (c) 5 to about 10% by weight of N-secondary butyl glycine or pipecolinic acid, and the balance, water.

11. The composition of claims 9 or 10 wherein the composition additionally includes antifoam-ing agents, antioxidants and corrosion inhibitors.