ACID GAS SCRUBBING COMPOSITION AND PROCESS

Abstract: An aqueous acid gas scrubbing composition comprises (a) at least one alkali metal salt, suitably potassium carbonate, and (b) at least one defined primary sterically hindered amino acid. Examples are 2-aminoisobutyric acid; 4-amino- 4 methyl pentanoic acid; and 1-amino-cyclopentane carboxylic acid. A pentavalent vanadium compound may also be present as a corrosion inhibitor.

Description: Field of the Invention The present invention relates to the use of primary hindered aminoacids, such as 2-amino isobutyric acid, as promoters for alkali metal salts in acid gas scrubbing. Background of the Invention The treatment of gaseous streams for removal of acid gases, such as hydrogen sulfide and carbon dioxide, is an essential processing step in petroleum refining, natural gas production, and the petrochemical industry. Myriad technologies have achieved commercial status, each satisfying for a particular set of circumstances a required balance among operability; process requirements, flexibility, and economic factors. Such processes include the use of physical solvents, aqueous solutions of chemical agents (amines, carbonates, redox systems), solvent/chemical mixtures, and solid adsorbents, etc. Historically, gas treating problems have been of three main types--hydrogen sulfide removal, the simultaneous removal of hydrogen sulfide and carbon dioxide, and carbon dioxide removal with little or no hydrogen sulfide present. The present invention pertains to the removal of acid gases, particularly to carbon dioxide, from gaseous streams containing little or no sulfur gases, such as in gaseous streams used in the manufacture of hydrogen and ammonia. One leading type of process for the removal of CO₂ from gaseous streams which has met with commercial success is the so-called "hot pot" process. The hot pot process is based on the use of a hot aqueous potassium carbonate solution to convert the CO₂ to potassium bicarbonate. An activator, or promoter, is usually used to improve the absorption rate and/or capacity of the solution and a V⁺⁵ salt is often used as a corrosion inhibitor. Non-limiting examples of promoters used in the hot pot process include alkanol-amines, particularly diethanolamine (DEA), sterically hindered polyamines, such as N-cyclohexyl 1,3-propane-diamine (CHPD) and sterically hindered amino acids such as N-secondary butyl glycine (SBG). While all of these promoters have met with varying degrees of commercial success, they are all faced with shortcomings. For example, DEA has a relatively low absorption rate and capacity and is not very stable. CHPD requires a cosolvent and undergoes degradative reactions, and SBG undergoes oxidative degradation in the presence of vanadium. Consequently, there exists a need in the art for promoters for hot pot processes which not only have relatively high absorption rates and working capacities, but which are not susceptible to degradation under process conditions or in the presence of vanadate corrosion inhibitors. Summary of the Invention In accordance with the present invention there is provided an aqueous gas scrubbing composition consisting of: (a) one or more alkali metal salts; (b) one or more primary sterically hindered aminoacids, being (i) represented by the formula: where R₁ and R₂ are independently selected from CH₃, C₂H₅, and C₃H₇; R₃ and R₄ are independently selected from hydrogen and CH₃; and n is 0, 2, or 3; or (ii) 1-amino-cyclopentane carboxylic acid; and (c) water Preferably, R₁ and R₂ are chosen independently from CH₃ and C₂H₅. More preferably R₁ and R₂ are CH₃ and n=0. Preferably, the aqueous gas scrubbing solution is comprised of 10 to 40 wt %, more preferably 15 to 30 wt %, of alkali metal salt and 2 to 20 wt %, more preferably 2 to 15 wt % of sterically hindered primary amino acid. Optionally, an effective amount of a vanadium compound to provide corrosion protection may be present. In both cases the balance is substantially wholly water. Potassium carbonate is a preferred alkali metal salt. Also in accordance with the present invention, there is provided a process for the removal of CO₂ optionally in combination with any other acid gas from a normally gaseous mixture, which process comprises contacting the normally gaseous mixture in an absorption step with said aqueous gas scrubbing composition and describing at least a portion of the absorbed CO₂ and optionally any other acid gas from the solution. In a more preferred embodiment, the solution also contains a V5⁺ corrosion inhibitor at least an amount which will result in the inhibition of corrosion of the metal of the process unit apparatus. Vanadium salts can be used at a concentration of 0.001 to 10 wt.% based on vanadium metal, preferably 0.1 to 5 wt.%, more preferably from 0.1 to 1 wt.%. Brief Description of the Drawings The sole figure hereof is a diagrammatic flow sheet illustrating an experimental reaction apparatus for removing carbon dioxide from gas streams. Detailed Description of the Invention The term, acid gas, includes CO₂ alone or in combination with H₂S, SO₂, SO₃, CS₂, HCN, COS and the oxides and sulfur derivatives of C₁ to C₄ hydrocarbons. These acid gases may be present in trace amounts within a gaseous mixture or in major proportions. The above acids are removed from gaseous streams by use of one or more alkali metal salts (which include hydroxides), and one or more primary sterically hindered amino acids. The primary sterically hindered amino acid can be 1-amino-cyclopentane carboxylic acid or can be an amino acid represented by the formula: where R₁ and R₂ are independently selected from CH₃, C₂H₅, and C₃H₇; R₃ and R₄ are independently hydrogen and CH₃; and n is 0, 2, or 3. Preferably R₁ and R₂ are chosen independently from CH₃ and C₂H₅, more preferred is when both R₁ and R₂ are CH₃ and n is 0. It is noted that n cannot be 1 because beta amino acids in aqueous mediums undergo deamination, whereas alpha and gamma amino acids do not. Potassium is the preferred alkali metal for use in the absorbent solutions of the present invention. Although other potassium salts of weak acids may be used, potassium carbonate, potassium hydroxide, potassium borate, and their mixtures, are preferred. It is understood herein that the term alkali metal salts also includes alkali metal hydroxides. Vanadium corrosion inhibitors suitable for use herein are those vanadium compounds which contain vanadium at an oxidation state of +5 or which can undergo oxidation to result in vanadium being at an oxidation state of +5. The preferred V⁺⁵ compound is V₂O₅. Preferred vanadium salts which can result in V⁺⁵ compositions upon oxidation include the alkali metal vanadates. The contacting of the absorbent mixture and the acid gas may take place in one or more suitable contacting vapor/liquid zones. In such processes, the gaseous mixture from which the acid gases are to be removed may be brought into intimate contact with the absorbing solution using conventional means, such as a tower packed with, for example, metallic or ceramic rings or with bubble cap plates or sieve plates, or a bubble reactor. In a preferred mode of practicing the invention, the absorption step is conducted by feeding the gaseous mixture into the base of the tower while fresh absorbing solution is fed into the top. The gaseous mixture freed largely from acid gases emerges from the top. Preferably, the temperature of the absorbing solution during the absorption step is in the range from 25 to 200°C, more preferably from 50 to 150°C. Pressures in the absorber may vary widely; acceptable pressures are between 5 and 2000 psia (34.5 to 13790 kPa), preferably 100 to 1500 psia (689.5 to 10342 kPa), and most preferably 200 to 1000 psia (1379 to 6895 kPa). In the desorber, the pressures may range from 5 to 100 psia (34.5 to 689.5 kPa). The partial pressure of the acid gas, e.g., CO₂, in the feed mixture will preferably be in the range from 0.1 to 800 psia (0.7 to 5516 kPa), and more preferably in the range from 1 to 400 psia (7 to 2758 kPa). Absorption takes place under conditions such that the acid gas, e.g., CO₂, is absorbed by the absorption solution. Generally, countercurrent contacting to remove the acid gas will last for a period from 0.1 to 60 minutes, preferably 1 to 10 minutes. During absorption, the solution is maintained in a single phase. The aqueous absorption solution comprising the alkaline material, the activator system comprising the primary sterically hindered amino acid, which is saturated or partially saturated with gases, such as CO₂, may be regenerated so that it can be recycled back to the absorber. Preferably, the regeneration should take place in a single liquid phase. Therefore, the presence of the highly water soluble amino acid provides an advantage in this part of the overall acid gas scrubbing process. The regeneration, or desorption, may be accomplished in one or more vapor/liquid contacting stages and is performed by conventional means, such as pressure reduction, which causes the acid gases to flash off and by-passing the solution into a tower of similar constuction to that used in the absorption step, at or near the top of the tower, and passing an inert gas, such as air, nitrogen, or preferably steam, up the tower. The temperature of the solution during the regeneration step may be the same as in the absorption step, i.e., 25 to 200°C, and preferably 50 to 150°C. The absorbing solution, after being cleansed of at least a portion of the acid moieties, may be recycled back to the absorbing tower. Makeup absorbent may be added as needed. As a typical example, during desorption, the acid gas, e.g., CO₂-rich solution from the high pressure absorber, is first sent to a flash chamber where steam and some CO₂ are flashed from the solution at low pressure. The amount of CO₂ flashed off will, in general, be 30 to 60% of the net CO₂ recovered in the flash and stripper. This is increased somewhat, e.g., to 50 to 80%, with the high desorption rate promoter system of this invention owing to the enhanced CO₂ loading of the rich solution. Solution from the flash chamber is then steam stripped in the packed or plate tower, stripping steam having been generated in the reboiler in the base of the stripper. Pressure in the flash chamber and stripper is usually 16 to 100 psia (110 to 689.5 kPa), preferably 16 to 30 psia (110 to 207 kPa), and the temperature is in the range from 25 to 200°C, preferably 35 to 150°C, and more preferably 100 to 140°C. Stripper and flash temperatures will, of course, depend on stripper pressure, thus at 16 to 25 psia (110 to 172 kPa) stripper pressures, the temperature will preferably be 100 to 140°C during desorption. In the most preferred embodiment of the present invention, the acid gas, e.g., CO₂, is removed from a gaseous stream by means of a process which comprises, in sequential steps, (1) contacting the gaseous stream with a solution comprising 10 to 40 weight percent, preferably 15 to 30 weight percent of potassium carbonate, an activator or promoter system comprising 2 to 20 weight percent, preferably 2 to 15 weight percent, more preferably 2 to 10 weight percent of the sterically hindered primary amino acid, the balance of the solution being comprised of water, the contacting being conducted at conditions whereby the acid gas is absorbed in the solution, and preferably at a temperature ranging from 25 to 200°C, more preferably from 50 to 150°C and a pressure ranging from 100 to 1500 psia (689.5 to 10342 kPa), and (2) regenerating the solution at conditions whereby the acid gas is desorbed from the solution. By practicing the present invention, one can operate the above described process at conditions whereby the working capacity, which is the difference in moles of acid gas absorbed in the solution at the termination of steps (1) and (2), based on the moles of potassium carbonate originally present, is greater than obtained under the same operating conditions for removing acid gases from gaseous streams, wherein said same operating conditions include conventional promoters such as DEA. In other words, working capacity is defined as follows: It should be noted that throughout the specification where working capacity is referred to, the term may be defined as the difference between CO₂ loading in solution at absorption conditions (step 1) and the CO₂ loading in solution at regeneration conditions (step 2), each divided by the initial moles of potassium carbonate. The working capacity is equivalent to the thermodynamic cyclic capacity, that is, the loading is measured at equilibrium conditions. This working capacity may be obtained from the vapor-liquid equilibrium isotherm, that is, from the relation between the CO₂ pressure in the gas and the acid gas, e.g., CO₂ loading in the solution at equilibrium at a given temperature. To calculate thermodynamic cyclic capacity, the following parameters must usually be specified: (1) acid gas, e.g., CO₂ absorption pressure, (2) acid gas, e.g., CO₂ regeneration pressure, (3) temperature of absorption, (4) temperature of regeneration, (5) solution composition, that is, weight percent of aminoacid and the weight percent of the alkali metal salt or hydroxide, for example potassium carbonate, and (6) gas composition. The skilled artisan may conveniently demonstrate the improved instant process which results by use of the sterically hindered primary aminoacid of the instant invention by a comparison directly with a process wherein the sterically hindered primary aminoacids are not included in the aqueous absorption solution composition, (at similar pressure and temperature conditions). When the sterically hindered primary aminoacids of the present invention are utilized, the difference between the amount of acid gas, e.g., CO₂ absorbed at the end of step 1 (absorption step) defined above, and step 2 (desorption step) defined above, is significantly greater than with conventional promoters such as DEA. It has been found that the use of the sterically hindered primary aminoacids of the present invention provide a working capacity which is at least 15% greater than the working capacity of commercial promoters, such as diethanolamine (DEA). Working capacity increases of about 20 to 60% may be obtained by use of the sterically hindered primary aminoacids compared to DEA. Besides increasing working capacity, the use of aminoacids of the present invention leads to higher rates of absorption and desorption. Rate of absorption is defined as the amount of CO₂ absorbed in a fixed time period. The combined higher rate and capacity leads to a lower steam consumption during desorption. Steam requirements are the major part of the energy cost of operating an acid gas, e.g., CO₂ absorption unit. Substantial reduction in energy, i.e., operating costs, will be obtained by the use of the process utilizing the aminoacids of the present invention. Additional savings from new plant investment reduction and debottlenecking of existing plants may also be obtained by the use of the aminoacids of the present invention. The removal of acid gases, such as CO₂, from gas mixtures is of major industrial importance, particularly in systems which utilize potassium carbonate activated by the aminoacids of the present invention. While sterically hindered amines, as shown in US-A- 4112050, provide unique benefits in their ability to improve the working capacity in the acid absorbing, or scrubbing process, their efficiency decreases in alkaline "hot pot" (hot potassium carbonate) scrubbing systems at high temperatures and at low concentrations of the acid gas owing to phase separation. Therefore, full advantage of the highly effective sterically hindered amines cannot always be utilized at these operating conditions. The addition of an aminoacid, as a cosolvent and copromoter, as shown in US-A- 4094957, solves the problem of phase separation and enables a more complete utilization of sterically hindered amines as the alkaline materials activator or promoter. Many of the aminoacids disclosed in that patent, while soluble in these alkaline systems, when used alone, are not as effective as activators in acid gas scrubbing processes as the other sterically hindered amines, or are incompatible with vanadium corrosion inhibitors. Therefore, it was not expected that the sterically hindered primary aminoacids of the present invention, as the sole promoter, would provide high working capacity, high rates of CO₂ absorption, and stability at process temperatures in the presence of V⁺⁵ salts. The primary hindered aminoacids of this invention have been surprisingly found to be very stable to chemical oxidation by vanadium +5 corrosion inhibitor, and also to provide excellent absorption rates of carbon dioxide, when used in the aqueous acid gas scrubbing compositions of this invention. More particularly, the primary hindered aminoacids of this invention had been found to undergo no substantial chemical oxidation in the presence of such vanadium +5 corrosion inhibitor over extensive time periods at elevated temperatures, and in particular, according to NMR analysis, at least about 95%, and more preferably at least about 98%, of the amount of vanadium +5 corrosion inhibitor charged to an aqueous acid gas scrubbing composition of this invention will remain in the +5 oxidation state after at least 7 days, and preferably at least 30 days, at 100°C when tests are conducted in sealed glass tubes. The absorbing solution of the present invention, as described above, is comprised of a major proportion of two materials, e.g., alkali metal salts or hydroxides, and a minor proportion of the sterically hindered primary aminoacid activator system. The remainder of the solution is comprised of water and/or other commonly used additives, such as anti-foaming agents, corrosion inhibitors, etc. The following invention is illustrated further by the following examples which, however, are not to be taken as limiting in any respect. All parts and percentages, unless expressly stated to be otherwise, are by weight. Example 1 (Comparative) The experimental reaction apparatus used is shown in Fig. 1. It is a reaction vessel V having a capacity of about 2.5 liters and a diameter of 10 cm, equipped with a heating jacket. The stirrer shaft carries two three-blade propellers, of which the upper one pushes the liquid downward and the lower one pushes the liquid upward. Pump P₁ removes liquid from the bottom of the reaction vessel and feeds it back to the gas-liquid interface through stainless steel sparger S₁. Vertical baffles further increase the contact between liquid and gas. Thermocouple T permits the reading of the temperature of the liquid. The top of a reflux condenser C is connected to a U-shaped, open ended manometer M. The apparatus can be evacuated by means of pump P₂ through tap T₁. Nitrogen and CO₂ can be fed to the bottom of the reaction vessel through sparger S₂, using tap T₂; CO₂, coming from a cylinder, goes first through the reservoir tank R acting as a ballast, then through a 3-liter wet test meter, then through bubbler B₁, where it is saturated with water. Hg-bubbler B₂ insures that no air is sucked into the reservoir tank R. Constrictions such as narrow tubings and taps, have been carefully avoided in the CO₂ path. Tap T₂, which is the only one inserted in such a path, has a key with large holes (8 mm). The following reagents, representative of a typical commercial gas-treating composition, were put into a 2-liter Erlenmeyer flask: When all was dissolved, the resulting solution was put into the absorber and brought to 80°C. The apparatus was closed and evacuated until the liquid began to boil. At this point, CO₂ was admitted until the solution was saturated with CO₂. The CO₂-rich solution was then transferred to the desorber for regeneration, in which it boiled for one hour to desorb CO₂. The regenerated solution was transferred back to the absorber and cooled to 80°C. The apparatus was closed, and evacuated until the liquid began to boil. At this point, CO₂ was readmitted. At saturation, 23.6 liters of CO₂ was absorbed, 4 liters of which were absorbed in the first minute. The 23.6 liters of CO₂ is a measure of the cyclic capacity, whereas 4 liters in the first minute is a measure of the CO₂ absorption rate. Example 2 (Comparative) The following solution was prepared: The CO₂ absorption-desorption-reabsorption cycle was repeated as described in Example 1. 20.2 liters of CO₂ was reabsorbed, of which 4 liters was reabsorbed in the first minute. Glycine-promoted potassium carbonate solutions are used commercially for gas treating. Example 3 Example 1 was repeated, using a solution having the following composition: It was found that the regenerated solution reabsorbed 30 liters of CO₂, 8.6 liters of which was absorbed in the first minute. This Example shows that a sterically hindered primary aminoacid, such as AIBA, has improved capacity and rate for CO₂ absorption, when compared to a commercial standard, such as DEA or glycine. Example 4 Example 1 was again repeated, this time using a solution having the following composition: The CO₂ absorption-desorption-reabsorption cycle was repeated as described in Example 1. 26.8 liters of CO₂ was reabsorbed, of which 10.5 liters was reabsorbed in the first minute. Example 5 Again, Example 1 was followed, except the solution had the following composition: The CO₂ absorption-desorption-reabsorption cycle was repeated as described in Example 1. 26.8 liters of CO₂ was absorbed, of which 9 was absorbed in the first minute. Example 6 Example 1 was repeated using a solution having the following composition: The absorption-desorption-reabsorption cycle was repeated: 31.4 liters of CO₂ was reabsorbed, of which 10.5 liters were reabsorbed in the first minute. While tertiary aminoacids such as MSBG are not very useful as promoters alone, they can provide some synergism when combined with aminoacids as promoters of the instant invention. They may also provide improvement in solubility of the aminoacid components. Furthermore, they are also compatible with V⁺⁵ salts. Example 7 A total of about 3g of a mixture of the following ingredients was charged into a pyrex tube: The tube was sealed and put into a metal heating block at 100°C. All solids dissolved. After three weeks, the yellow color of V⁺⁵ was still present, indicating no degradation or reaction of AIBA with V ⁺⁵. Example 8 (Comparative) Experiment 7 was repeated, replacing AIBA with the same amount of N-sec. butyl glycine. After 24 hours the solution was green, indicating formation of V⁺⁴. Example 9 The following ingredients were charged into a pyrex tube: The tube was sealed and put into a metal heating block at 100°C. All solids dissolved. After 3 weeks the yellow color of V⁺⁵ was still present, indicating no degradation or reaction of 2-amino-2-methyl - butyric acid with V⁺⁵. Example 10 (Comparative) The following ingredients were put into a pyrex tube: The tube was sealed and put into a metal heating block at 100°C. All solids dissolved. After 4 days the color of the solution had turned from yellow to green, indicating reduction of V⁺⁵ by glycine. Example 11 A quantitative V⁺⁵ stability test was conducted. Two NMR tubes were prepared with the same composition as in example 7, in which AIBA was used. One tube was kept at ambient temperature, and the other tube was heated in a metal block at 100°C. Periodically, the tube in the block was removed and ⁵¹V NMR spectra (spectrometer probe temperature was 80°C) were obtained with both this tube and the tube which remained at ambient temperature (control sample). Since V⁴⁺ is paramagnetic, it gives no NMR signal. The NMR signal is therefore due only to V⁵⁺ and can be used to measure the amount of V⁵⁺ in the heated sample when the sample remaining at ambient temperature is used as a calibration sample. The results were: These results confirm the visual observations of Example 7, indicating no degradation or reaction of AIBA with V⁵⁺. Example 12 According to the method of Example 1 (evaluating rate and capacity), and according to the method of Example 7 (evaluating stability to V⁺⁵), samples of primary unhindered aminoacids (e.g., glycine), secondary unhindered aminoacids (e.g. N-n-butyl glycine), secondary moderately hindered aminoacids (e.g. N-sec. butyl glycine), secondary, severely hindered aminoacids (e.g. N-sec. butyl alanine) are compared in Table 1 below with primary sterically hindered aminoacids of the present invention. Variations will be obvious to those skilled in the art, e.g. potassium, or other salts, of the aminoacids can be substituted for the aminoacids themselves. Also, various blends and mixtures of suitable aminoacids may be utilized instead of a single component. Aminoacid precursors, which upon hydrolysis yield the desired primary hindered aminoacids under the absorber-desorber process conditions are also suitable for use in the present invention. It is noted that only the primary aminoacids of the present invention have all three of the above characteristics. That is, relatively high rates of absorbtion, working capacity, and stability in the presence of V⁺⁵ salt.

Claims: An aqueous acid gas scrubbing composition consisting of: (a) at least one alkali metal salt; (b) at least one primary sterically hindered amino acid selected from: (i) those having the formula where R₁ and R₂ are independently selected from CH₃, C₂H₅, and C₃H₇; R₃ and R₄ are independently selected from hydrogen and CH₃; and n is 0, 2, or 3; and (ii) 1-amino-cyclopentane carboxylic acid; and (c) water. A scrubbing composition as claimed in claim 1, wherein the alkali metal salt is a Potassium salt. A scrubbing composition as claimed in claim 2, wherein the salt is potassium carbonate. A scrubbing composition as claimed in any preceding claim, wherein R₁ and R₂ are methyl, and n is 0. A scrubbing composition as claimed in any preceding claim, which also contains a V⁵+ compound corrosion inhibitor. A scrubbing composition as claimed in any preceding claim, containing from 10 to 40 wt % in total of said alkali metal salt(s) and from 2 to 15 wt % in total of said aminoacid(s), the balance being substantially wholly water or water plus said corrosion inhibitor. A scrubbing composition as claimed in any preceding claim, in which component (b) derives from a salt thereof or a hydrolysable precursor thereof. A process for removing CO 2 optionally in combination with any other acid gas from a normally gaseous mixture, which process comprises: (a) contacting the normally gaseous mixture under acid gas absorption conditions with an aqueous scrubbing solution claimed in any preceding claim, and (b) desorbing at least a portion of the absorbed CO₂ and any other acid gas from the solution. A process as claimed in claim 8, wherein the contacting step is conducted at a temperature ranging from 25 to 200°C and at a pressure ranging from 5 to 2000 psia (34.5 to 13790 kPa). A process as claimed in claim 8 or claim 9, wherein the desorption step is conducted by heating the solution to a temperature ranging from 50 to 150°C and at a pressure of 16 to 100 psia (110.3 to 689.5 kPa).

Inventors: HO, WIN-SOW WINSTON, MILLIMAN, GEORGE ELMER, SARTORI, GUIDO, SHULIK, LARRY JOSEPH, THALER, WARREN ALAN
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1987
Main classification: B01D53
IPC codes: B01D53, C07C67, C07C227