ABSORPTION OF HYDROGEN SULFIDE

Abstract: A process for the preferential absorption of H₂S from a gas which contains H₂S and CO₂ comprises contacting of the gas with a sorbent solution comprising alkali metal ethylenediaminetetraacetate and/or alkali metal nitrilotriacetate, in the substantial absence of oxidizing agents which can oxidize H₂S.

Description: Background of the Invention This invention relates to a process for absorbing H₂S from a gas. In another aspect, this invention relates to the selective absorption of H₂S from a gas which also contains CO₂. Many industrial gas streams, in particular natural gas and refinery gases, contain both H₂S and CO₂ as impurities. Numerous processes for the preferential removal of H₂S from these gas streams, as well as for the desorption of absorbed H₂S, are known, some of which employ triethanolamine (TEA) or methyldiethanolamine (MDEA) as absorbents (DD-A-234 027 and US-A-4 147 754). However, there is an ever present need to develop new H₂S absorbing agents which are more effective in separating H₂S and CO₂ than those commonly employed. Summary of the Invention It is an object of this invention to provide a process for the preferential removal of H₂S from gases which contain both H₂S and CO₂. It is another object of this invention to provide a process for absorbing and thereafter desorbing H₂S. Other objects and advantages will be apparent from the detailed disclosure and the appended claims. These objects have been accomplished by a process for absorbing H₂S from gas containing H₂S and CO₂, characterized by contacting said gas for not more than 60 minutes with a sorbent solution comprising at least one dissolved sorbent selected from alkali metal ethylenediaminetetraacetate and alkali metal nitrilotriacetate, in the absence of oxidizing agents for H₂S. In a preferred embodiment, the process of this invention comprises the additional step of (b) substantially desorbing H₂S, which has been absorbed by said solution comprising said at least one sorbent in step (a). Preferably the desorption of H₂S is carried out by applying heat and/or reduced pressure to the sorbent solution containing absorbed H₂S and CO₂, more preferably followed by step of (c) removing desorbed H₂S from the gas (vapor) phase above the thus regenerated sorbent solution. The thus regenerated sorbent solution can be recycled to the absorption zone of step (a) and reused for the preferential absorption of H₂S in accordance with the process of this invention. Brief Description of the Drawings FIGURE 1 shows the rates of absorption of H₂S and CO₂ by a 1.14 molar aqueous solution of trisodium nitrilotriacetate. FIGURE 2 shows the rates of absorption of H₂S and CO₂ by a 1.14 molar aqueous solution of tetrapotassium ethylenediaminetetraacetate. FIGURE 3 shows the rates of absorption of H₂S and CO₂ by a 1.14 molar aqueous solution of methyldiethanolamine. FIGURE 4 compares the CO₂ absorption rates of 1.14 molar aqueous solutions of trisodium nitrilotriacetate, tetrapotassium ethylenediaminetetraacetate, methyldiethanolamine and triethanolamine. FIGURE 5 depicts the H₂S absorption capacity of fresh and regenerated aqueous 1.14 molar solutions of trisodium nitrilotriacetate. Detailed Description of the Invention The present invention can be used to purify any suitable gas feed (also herein referred to as "gas mixture", "gas stream", "gas feed stream" or "gaseous feed stream") which contains both hydrogen sulfide and carbon dioxide. It is understood that the term "stream", as used herein, applies to continuous processes, as well as batch processes. The H₂S content in these gas streams generally is in the range of from about 0.001 to about 10 volume percent H₂S, and preferably is in the range of from about 0.1 to about 3 volume percent H₂S. The CO₂ content in these gas streams generally is in the range of from about 0.1 to about 20 volume percent CO₂, and preferably is in the range of from about 1 to about 10 volume percent CO₂. Strong oxidizing gases (such as ozone) which can oxidize absorbed H₂S to elemental sulfur or to compounds of sulfur in a positive oxidation state, e.g., H₂SO₃ or H₂SO₄, should be absent from the gas feed. Non-limiting examples of suitable gas feed streams are natural gas, fractions of natural gas, gas streams from the distillation of crude oil, gases formed by the pyrolysis of coal (including lignite), gas streams obtained in processes for liquefying coal (including lignite), industrial waste gases, and the like. It is understood that these gas streams may contain other undersirable impurities besides H₂S and CO₂, such as CO, SO x (sulfur oxides), NO x (nitrogen oxides), arsine, and the like. However, the amounts of these other impurities should not exceed levels at which they significantly interfere with the preferential absorption of H₂S in accordance with this invention. The sorbent of this invention can be any dissolved alkali metal ethylenediaminetetraacetate or alkali metal nitrilotriacetate or mixtures of any of these compounds (also referred to as salts). The preferred alkali metal salts are sodium or potassium salts. Presently particularly preferred are tetrapotassium ethylenediaminetetraacetate (K₄EDTA) and trisodium nitrilotriacetate (Na₃NTA). The above-recited sorbent compounds can be dissolved in any suitable solvent, such as water, lower aliphatic alcohols (e.g., those having 1-6 C atoms per molecule), esters such as ethyl acetate, ketones such as acetone, and the like, and mixtures of the above. Preferably water is used as a solvent. Any suitable concentration of dissolved sorbent compounds in these solutions (preferably aqueous) can be employed. Preferably, the concentration of alkali metal ethylenediaminetetraacetate or alkali metal nitrilotriacetate is in the range of from about 0.5 to about 3 mol/l, more preferably in the range of from about 1 to about 2 mol/l. Since the absorbed hydrogen sulfide is to be removed from the absorbent solution (so as to regenerate the sorbent solution for re-use), oxidizing agents that can oxidize absorbed H₂S to elemental sulfur (or to compounds with sulfur in a positive oxidation state) should be absent from the sorbent solution. In particular, hydrogen peroxide and ferric compounds (i.e., compounds of Fe in the oxidation state +3, such as ferric complexes of ethylenediaminetetraacetic acid, or of nitrilotriacetic acid, or of salts of these two acids) should be absent from the sorbent solution. Also oxidizing compounds of other polyvalent metals (in particular transition metals) in their higher oxidation states, such as compounds of Sn⁺⁴, V⁺⁵, Cr⁺⁶, Mo⁺⁶, Mn⁺⁷, Mn⁺⁶, Co⁺³, and the like, should be absent from the sorbent solution. It is understood that trace amounts of these oxidizing compounds may be present as long as they do not significantly interfere with the absorption and desorption of H₂S in accordance with the process of this invention. Since the absorption equipment generally is made of metallic materials, the sorbent solution of this invention preferably contains an effective amount of at least one suitable metal corrosion inhibitor, such as those described in Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 7, pages 135-142. It is understood that the corrosion inhibitor should not adversely interfere with the H₂S absorption process of this invention. A presently preferred corrosion inhibitor is 2-mercaptobenzothiazole (C₇H₅NS₂), present at an effective level, more preferably at a concentration of about 0.02-0.2 g/l. Any suitable contacting conditions can be employed in the process of this invention. In the H₂S absorption step, the temperature can be in the range of from about 4 to about 93°C (preferably about 10-32°C), and the pressure can be in the range of from about 0 to 0.69 MPa (preferably about 0 Pa). The time of contact between feed gas and absorbent solution can vary over a range (depending on the type of equipment) of from about 0.1 minute to 60 minutes. In the step for desorbing H₂S from the spent sorbent solution, the temperature is generally in the range of from about -1 to about +105°C (preferably about 10-32°C), and the pressure is generally in the range of from about 10-2x10⁵ Pa, preferably from about 10-10⁵ Pa. The desorbed H₂S gas is generally swept away from the gas phase above the thus regenerated sorbent solution by a gas stream (e.g., an inert gas stream) or is pumped off. Any suitably absorption apparatus and equipment can be employed in the H₂S absorption process of this invention. Commercial absorption and desorption (regeneration) equipment for contacting a gas and a liquid sorbent and techniques for operating such equipment are well known to those skilled in the art, and are described in many technical publications, such as in "Encyclopedia of Chemical Technology", Kirk-Othmer, Volume 1, 1978, pages 53-55 and 88-96. The most common gas absorption technique is that of using countercurrent columns, equipped with bubble-trap plates, or sieve trays, or spray nozzles or the like, or filled with suitable packing materials, as described in the above-cited publication. Feed rates of feed gas and sorbent solution depend on specific column configuration, composition of the feed gas, desired efficiency of H₂S absorption and H₂S/CO₂ separation, and the like, and are readily optimized by those skilled in the art. The sorbent solution of this invention can be regenerated by desorption of H₂S as many times as a satisfactory H₂S absorption capacity and H₂S/CO₂ separation efficiency of the regenerated sorbent solution is attained. The desorbed H₂S can be utilized for a variety of end uses, e.g., for making mercaptans, for converting H₂S to elemental sulfur in a Claus process, and the like. The following examples are presented to further illustrate the invention and are not to be considered unduly limiting the scope of this invention. Example I This example illustrates the experimental setup for measuring the rates of absorption of H₂S and of CO₂ employing different aqueous absorbents. A glass apparatus, which could be evacuated by means of a pump to a pressure of about 10 Pa and then filled with a gas (H₂S or CO₂) up to a pressure of 10⁵ Pa, was used for the absorption tests. The main parts of the glass apparatus were a gas reservoir bulb of about 100 cc capacity, an absorption flask of about 60 cc capacity with built-in rubber septum (through which liquid absorbent could be injected by means of a hypodermic syringe), and a MKS/Baratron pressure sensor with attached pressure gauge. The entire glass apparatus could also be isolated from the vacuum pump by means of a glass valve. Four aqueous solutions were tested for their rates of absorption of H₂S and CO₂, respectively: (A) 1.14 molar (1.14 M) methyldiethanolamine (MDEA), (B) 1.14 M triethanolamine (TEA), (C) 1.14 M trisodium nitrilotriacetate (Na₃NTA) and (D) 1.14 M tetrapotassium ethylenediaminetetraacetate (K₄EDTA). A sample of about 5 cc of one of these solutions was injected through the rubber septum into the absorption flask, which had been evacuated. The solution was stirred by means of a magnetic stirring bar placed inside the absorption flask, and was deaerated by means of a vacuum pump. Then the gas, H₂S or CO₂, was introduced into the evacuated reservoir bulb through a gas inlet valve, which thereafter was closed again. The total gas pressure generally ranged from about 2x10⁴ to about 8x10⁴ Pa. All parts of the glass apparatus were at room temperature (about 26°C). The valve to the absorption flask was then opened and the resulting pressure was measured. Pressure measurements were generally made at time intervals of 1-2 minutes in H₂S absorption tests and 10-20 minutes in CO₂ absorption tests. Example II This example illustrates the rates of absorption of H₂S and CO₂ by the two preferred absorbents of this invention, Na₃NTA and K₄EDTA (see Example I), compared with those by two commercially used absorbents, MDEA and TEA (see Example I). FIGURES 1-3 show the absorption of H₂S and CO₂, respectively, by Na₃NTA, K₄EDTA and MDEA, as a function of time. FIGURE 4 depicts the rate of absorption of CO₂ exhibited by Na₃NTA, K₄EDTA, MDEA and TEA. The absorption curves in FIGURES 1-4 were obtained in representative test runs, carried out in accordance with the procedure of Example I. Test results depicted in FIGURES 1-3 show that the rates of absorption of H₂S were quite high and were approximately the same for invention absorbents Na₃NTA and K₄EDTA and for control absorbent MDEA. However, the rates of absorption of CO₂ were considerably lower in invention tests with Na₃NTA and K₄EDTA than in control tests with MDEA and TEA (see especially FIGURE 4). FIGURES 1-3 show that at the time when substantially all H₂S was absorbed, less than about 10% of available CO₂ had been absorbed by Na₃NTA and K₄EDTA, whereas more than 10% of available CO₂ had been absorbed by MDEA. Thus, Na₃NTA and K₄EDTA were more effective than MDEA in separating H₂S and CO₂. Example III This example illustrates that H₂S can be desorbed from one of the absorbent solutions of this invention and that the absorbent solution can thus be regenerated. FIGURE 5 shows the H₂S absorption capacity of a fresh 1.14 M Na₃NTA solution , expressed as millimoles H₂S absorbed at a given H₂S pressure. The H₂S-saturated Na₃NTA solution was then cooled to 0°C, and a vacuum was applied for about 10 minutes so as to desorb as much H₂S as possible. The thus regenerated Na₃NTA absorption solution was then used again for H₂S absorption. The second curve in FIGURE 5 depicts the H₂S absorption capacity of the regenerated Na₃NTA solution. The H₂S absorption capacity of the regenerated Na₃NTA solution was about 80% of that of the fresh Na₃NTA solution. When the H₂S-saturated regenerated Na₃NTA solution was again subjected to a vacuum, the absorption capacity of the thus twice regenerated Na₃NTA solution was essentially the same as that of the once regenerated Na₃NTA solution. Thus, after the first regeneration step, which left about 20% of the absorbed H₂S in the Na₃NTA absorbent solution, essentially all absorbed H₂S could be desorbed in a subsequent regeneration (desorption) step. This result indicates that the H₂S absorbents of this invention can be repeatedly regenerated and reused for H₂S absorption.

Claims: A process for absorbing hydrogen sulfide from gas containing hydrogen sulfide and carbon dioxide, characterized by contacting said gas for not more than 60 minutes with a sorbent solution comprising at least one dissolved sorbent selected from alkali metal ethylenediaminetetraacetate and alkali metal nitrilotriacetate, in the absence of oxidizing agents for hydrogen sulfide. The process of claim 1, characterized in that said gas contains from 0.001 to 10 volume percent H₂S and from 0.1 to 10 volume percent CO₂. The process of claim 1 or 2, characterized in that said alkali metal is selected from sodium and potassium. The process of any of the preceding claims, characterized in that said sorbent is selected from tetrapotassium ethylenediaminetetraacetate and trisodium nitrilotriacetate. The process of any of the preceding claims, characterized in that said solution is an aqueous solution. The process of any of the preceding claims, characterized in that the concentration of said sorbent in said solution is in the range of 0.5 to 3 mol/l. The process of any of the preceding claims, characterized in that said contacting is carried out in the absence of oxidizing compounds of polyvalent metals. The process of claim 7, characterized in that said contacting is carried out in the absence of ferric coumpounds. The process of any of the preceding claims, characterized in that said contacting is carried out at a temperature in the range of 4 to 93°C and at a pressure in the range of 0 to 0.69 MPa gauge. The process of any of the preceding claims, characterized in that said sorbent solution additionally contains a metal corrosion inhibitor. The process of claim 10, characterized in that said metal corrosion inhibitor is 2-mercaptobenzothiazole. The process of claim 1, characterized in that the following compounds are absent from said sorbent solution: hydrogen peroxide, iron compounds containing Fe in the oxidation state +3, tin compounds containing Sn in the oxidation state +4, vanadium compounds containing V in the oxidation state +5, chromium compounds containing Cr in the oxidation state +6, molybdenum compounds containing Mo in the oxidation state +6, manganese compounds containing Mn in the oxidation state +6, manganese compounds containing Mn in the oxidation state +7, and cobalt compounds containing Co in the oxidation state +3. The process of claim 1, characterized in that oxidizing gases, which can oxidize hydrogen sulfide to elemental sulfur or to compounds containing sulfur in a positive oxidation state, are absent from said gas. The process of claim 1 comprising the additional step of desorbing hydrogen sulfide from used sorbent solution thus forming a regenerated solution.

Inventors: CYMBALUK, TEDDY HENRY, JOHNSON, MARVIN MERRILL, NOWACK, GERHARD PAUL
Applicants: PHILLIPS PETROLEUM COMPANY
Application year: 1988
Main classification: B01D53
IPC codes: C01B3, B01D53, C10L3, C10G1, C10G7