Acid gas absorbent composition

Abstract: The present invention discloses an acid gas absorbent composition, comprising a diethylene glcyol dialkyl ether represented by the general formula I: R¹O(C₂H₄O)₂R² (I) wherein R¹ and R² are independently an alkyl group of 1 to 4 carbon atoms, and a polyethylene glycol dialkyl ether represented by the general formula II: R³O(C₂H₄O) n R⁴ (II) wherein R³ and R⁴ are independently an alkyl group of 1 to 4 carbon atoms and n is an integer in the range of 3 to 8.

Description: This invention relates to acid gas absorbent compositions. More particularly, it relates to improved acid gas absorbent compositions for separating by absorption such acid gases as carbon dioxide and hydrogen sulphide from a mixed gas. The acid gas absorbent compositions of this invention are particularly useful for the separation of acid gases which are contained in natural gas, synthetic gas, and coke oven gas. The practice of using organic solvents or aqueous solutions of such organic solvents as acid gas absorbents for the removal of such acid gases as carbon dioxide and hydrogen sulphide from natural gas and mixed gas has been widely known in the art. Various acid gas absorbents are mentioned in the third edition (1979) of "Gas Purification"' written by A.L. Kohl and F.C. Riesenfeld, for example. Generally, these gas absorbents are divided by the mechanism of absorption into chemical absorbents (causing absorption by chemical reaction) and physical absorbents (causing absorption by physical union). As chemical absorbents, aqueous solutions of such alkanolamines as ethanolamine and hot potassium carbonate solution are chiefly used. In all the physical absorbents available for the removal of such acid gases as carbon dioxide and hydrogen sulphide from such mixed gases as natural gas and synthetic gas, ethers of polyethylene glycol oligomers are used most widely. In JP-B-48(1973)-23,782, US-A-3,737,392, and US-A-4,581,154, for example, the use of dimethyl ether of ethylene glycol oligomer (dimer to octamer) as an absorbent for the removal of acid gases from mixed gases is disclosed. JP-B-59(1984)-45,034 discloses the use of methyl isopropyl ether of polyethylene glycol of the general formula: wherein n is an integer in the range of 2 to 8, as an absorbent for the removal of acid gases such as carbon dioxide and/or hydrogen sulphide from a mixed gas. JP-A-49(1974)-98,383 discloses the use of an alkylpolyethylene glycol tertiary butyl ether of the general formula: wherein R is a linear or branched alkyl group of one to four carbon atoms and n is an integer in the range of 2 to 10, preferably 2 to 5, as an absorbent for the removal of acid gases such as carbon dioxide and/or hydrogen sulphide from natural gas. US-A-2,139,375 discloses the use of an ether or ester or an ether-ester mixture of a polyhydric alcohol or polyhydric alcohol oligomer as an absorbent for the removal of sulphur-containing acid gases from a mixed gas. In this patent as examples of the absorbent applicable thereto, dipropyl ethers of diethylene glycol and dipropylene glycol are mentioned among various, other substances. In the specification of this patent, there is a mention to the effect that the ability of the absorbent to absorb the acid gases increases with the decreasing size of the alkyl group linked to the oxygen atom and methyl ether exhibits the highest ability to absorb the acid gases. It has no mention whatever about superiority of diisopropyl ether. US-A-3,877,893 discloses the use of a polyethylene glycol dialkyl ether possessing one to eight carbon atoms and three to eight ethylene units as an absorbent for the removal of impurities containing carbon dioxide from a mixed gas. In the specification, polyethylene glycol dimethyl ether is designated as the best of all the specific absorbents disclosed. This US-A- has absolutely no mention about superiority of diisopropyl ether. US-A-4,044,100 discloses the use of a mixture of diisopropanol amine and a polyethylene glycol dialkyl ether. This US-A suggests that dimethyl ether of tetraethylene glycol excels, though not exclusively, among other ethers. EP-A-248541 corresponding to US-A-4,741,745, a document relevant under Art. 54(3)EPC (published on 9 Dec. 1987 with priority of 07.05.86), discloses the use of polyethylene glycol diisopropyl ether as absorbent for the removal of the acid gases. EP-A-62789 discloses the use of dialkyl ethers of polyethylene glycols having 2 to 9, preferably 3-8 moles ethyleneoxide incorporated therein for the absorption of, among others, acid gases such as CO₂ and H₂S from mixed natural gases. Diisopropylethers are mentioned among others, but not prefered. US-A-4302220 is concerned with a process for the simultaneous removal of water and H₂S from gas mixtures. Diisopropylethers of polyethylene glycols are mentioned among various others as absorbents for the H₂S. US-A-2,926,751 discloses the use of propylene carbonate as an acid gas absorbent for the removal of such acid gases as carbon dioxide from a mixed gas. The chemical absorbents using aqueous solutions of alkanol amines have limits to their capacity for absorbing acid gases such as carbon dioxide and/or hydrogen sulphide and, therefore, prove to be disadvantageous for the treatment of a mixed gas having a high partial pressure of carbon dioxide. An aqueous solution of alkanol amine which has absorbed carbon dioxide gas is passed to a regeneration column to release the carbon dioxide gas and then returned for reuse to the carbon dioxide gas absorption column. For this regeneration, the aqueous solution must be heated. When the amount of the carbon dioxide gas absorbed in the aqueous solution is large, therefore, the heat required for the regeneration is fairly large. The susceptibility of the apparatus to corrosion by the absorbent also poses a problem. The physical absorbent is such that its capacity for absorption of carbon dioxide gas is proportional to the partial pressure of carbon dioixde gas in the mixed gas under the treatment. In the separation by absorption of carbon dioxide gas from a mixed gas having a high partial pressure of carbon dioixde gas, the ability of the physical absorbent to absorb the carbon dioixde gas is large as compared with that of the chemical absorbent. Further,in the step of regeneration, the physical absorbent is easily enabled to release the absorbed carbon dioxide gas by relief of pressure or dispersion with air or other inert gas. Thus, the physical absorbent is also advantageous in terms of energy consumption over the chemical absorbent. Such physical absorbents as dimethyl ether of polyethylene glycol and propylene carbonate heretofore disclosed to the art, however, may well not be regarded as possessing a fully satisfactory capacity for the absorption of carbon dioxide gas. When a physical absorbent possessing a still larger capacity for the absorption of carbon dioxide gas is developed, it will make a conspicuous contribution to economy by reducing the size of the apparatus and reducing energy consumption. As examples of known acid gas absorbents for the removal of acid gases such as carbon dioxide and hydrogen sulphide from mixed gases, dimethyl ether of polyethylene glycol, methyl isopropyl ether of polyethylene glycol, and alkyl polyethylene glycol-tertiary butyl ethers may be mentioned. These known acid gas absorbents, however, do not have a sufficient capacity for the absorption of acid gases. In all the polyethylene glycol diisopropyl ethers contemplated by earlier developments, diethylene glycol diisopropyl ether appears to excel in its ability to absorb acid gases. Since this particular compound possesses a relatively high vapour pressure, the operation of absorption by the use of this compound must be carried out at a sufficiently low temperature to avoid loss of the absorbent during the course of the operation. An object of this invention, therefore, is to provide novel acid gas absorbent compositions. Another object of this invention is to provide novel physical absorbents which possess a large capacity for the absorption of acid gases compared with conventional physical absorbents and possess a sufficiently low vapour pressure, while at the same tic advantageously provide for the absorption of acid gases. The objects are accomplished by a acid gas absorbent composition, comprising (a) diethylene glycol diisopropylether and either: (b) tetraethylene glycol diisopropylether in a molar ratio of (a) to (b) of from 95:5 to 60:40; or (c) a mixture of polyethylene glycol diisopropylethers having 3 to 8 mols of ethylene oxide incorporated therein, the average molecular weight of said mixture being from 234 to 400 and the molar ratio (a) to (c) ranging from 95:5 to 60:40; or (d) triethylene glycol diisopropylether in a molar ratio of (a) to (d) of 80:20; or (e) tetraethylene glycol dimethylether in a molar ratio of (a) to (e) of 80:20; or (f) tetraethylene glycol ethylisopropylether in a molar ratio of (a) to (f) of 80:20. It has been discovered as the result of considerable study that the addition of certain polyethylene glycol dialkyl ethers to diethylene glycol diisopropyl ether brings about the effect of notably lowering the vapour pressure of the latter ether to an extent greater than the drop in the vapour pressure calculated in accordance with the Raoult's law, and gives rise to a composition capable of retaining a fully satisfactory ability to absorb acid gases. The present invention has been perfected, based on this knowledge. Preferably, the average molecular weight of the above mixture (c) is up to 300. Typically the diethylene glycol diisopropyl ether and the polyethylene glycol dialkyl ether employed in this invention are produced by a method which comprises causing a glycol such as diethylene glycol, triethylene glycol, or tetraethylene glycol to react with an alkyl halide, an alkali hydroxide, or an alkali metal at a temperature in the range of 50° to 150°C or by a method which comprises causing an olefin such as propylene to react with polyethylene glycol in the presence of an acid catalyst, for example. As example of the olefin to be used for the etherification in the present invention, propylene may be mentioned. For the composition to be advantageously used as an absorbent for acid gases, high capacity for the absorption of carbon dioxide under high pressure, ample difference in the amount of absorption under low pressure and high pressure, sufficiently low viscosity particularly at low temperatures, stability to withstand hydrolysis, and prevention of the loss of absorbent due to low vapour pressure are important factors demanding due consideration. Examples of absorbent compositions which prove to be particularly desirable in terms of these factors are the above composition comprising diethylene glycol diisopropyl ether (a) and tetraethylene glycol diisopropyl ether (b) and the above composition comprising diethylene glycol diisopropyl ether (a) and polyethylene glycol diisopropyl ether (c) , i.e. a mixture having added thereto 3 to 8 mols of ethylene oxide, having an average molecular weight in the range of 234 to 400. The absorbent composition of this invention represents an acid gas absorbent enjoying ample capacity for absorption, low vapour pressure, and a wide range of working temperature as compared with a conventional absorbent, particularly when the molar ratio of the diethylene glycol diisopropyl ether (a) to the polyethylene glycol dialkyl ether (b) or (c) is in the range of 85 : 15 to 65 : 35. If the amount of the above polyethylene glycol dialkyl ether (b) or (c) to be incorporated in the composition exceeds 40 mol%, the composition possesses a still lower vapour pressure but suffers from a decline in the capacity for the absorption of acid gases. Thus, the composition is no longer advantageous over a conventional physical absorbent. As an example of the process for which this invention can be particularly advantageously utilized, the removal of carbon dioxide from a synthetic gas may be cited. Typically, this process is effected by causing the synthetic gas to contact the absorbent in a carbon dioxide absorption column which is operated under a pressure of about 2.7 MPa (27 atmospheres). The carbon dioxide absorbed in the absorbent and hence separated from the mixed gas flows downwardly inside the absorption column. The absorbent which has absorbed the carbon dioxide gas is forwarded through the bottom of the absorption column into a flash drum which is operated under a pressure in the range of 0.5 to 1.0 MPa (5 to 10 atmospheres). In the flash drum, the greater part of hydrogen, methane, and inert gas are separated from the absorbent. The separated gases are continuously compressed and then recycled to the absorption column or used for other purposes. Since the carbon dioxide is retained substantially wholly in the absorbent at this stage, the absorbent is introduced into a second flash drum which is operated under a pressure of about 0.1 MPa (1 atmosphere). In the second flash drum, 50 to 70% of the carbon dioxide is separated from the absorbent and recovered and forwarded to a carbon dioxide product line. When the carbon dioxide is recovered from the absorbent in a sufficiently large amount, the absorbent is regenerated in a diffusion column. Generally, air which is used as a diffusion medium is brought into counterflow contact with the absorbent in a packed column. The air now entraining a small amount of carbon dioxide is released from the top of the column into the ambient air. The absorbent which has undergone diffusion contains substantially no carbon dioxide and is recycled to the absorption column. The carbon dioxide which is released into the ambient air as entrained by the air escapes recovery in the present process. Where the amount of carbon dioxide to be used is large and the ratio of recovery of this gas is high as a necessary consequence, the absorbent which has been treated in the second flash drum operated under a pressure of 0.1 MPa (1 atmosphere) is generally introduced into a vacuum flash drum operated under a pressure in the range of 0.03 to 0.08 MPa (0.3 to 0.8 atmosphere) before it is advanced to the air diffusion column. When the third flushing operation is carried out under a sufficiently low pressure, the ratio of recovery of carbon dioxide increases to 90%. The several improvements in the process of separation described above fall in the scope of general practices known to the art. Optionally, the absorption may be carried out by any of the following methods. A method which comprises using an air diffusion column designed to effect the diffusion under a vacuum in place of the flash drum designed to introduce air into counterflow contact with the absorbent thereby decreasing the amount of air required for the treatment and increasing the concentration of carbon dioxide in the released gas,and a method which comprises effecting the diffusion with a synthetic gas having undergone the treatment of absorption in place of air are available, for example. In the case of the latter method mentioned above, the carbon dioxide is recovered practically wholly because the gas emanating from the diffusion column is recycled to the absorption column at the bottom. The acid gas absorbent compositions of this invention which comprise diethylene glycol diisopropyl ether and a certain polyethylene glycol dialkyl ether possess a high ability to absorb acid gases and exhibit a sufficiently low vapour pressure as compared with the conventional acid gas absorbent. The operation of absorption by the use of this acid gas absorbent compositions, therefore, need not to be carried out at low temperatures. This fact permits a reduction in the cost of equipment and the cost of operation. As acid gas absorbents for the separation by absorption of such acid gases as carbon dioxide gas and hydrogen sulphide gas from a mixed gas, this acid gas absorbent compositions show an outstanding performance. Now, the present invention will be described more specifically below with reference to working examples. It should be noted, however, that the present invention is not restricted by these examples. In the examples, the amount of carbon dioxide gas absorbed was determined at a temperature of 25°C under a pressure of 0.6 MPa (6 atmospheres) of carbon dioxide gas and reported in terms of the number of mols of carbon dioxide per kg of the relevant absorbent. Example 1 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of an acid gas absorbent composition comprising 80 mol% of diethylene glycol diisopropyl ether and 20 mol% of tetraethylene glycol diisopropyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Example 2 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of an acid gas absorbent composition comprising 80 mol% of diethylene glycol diisopropyl ether and 20 mol% of triethylene glycol diisopropyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide as released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Example 3 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of an acid gas absorbent composition comprising 70 mol% of diethylene glycol diisopropyl ether and 30 mol% of polyethylene glycol diisopropyl ether mixture having an average molecular weight of 280 (n= 3 to 8) was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide gas released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Example 4 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of an acid gas absorbent composition comprising 80 mol% of diethylene glycol diisopropyl ether and 20 mol% of tetraethylene glycol dimethyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Example 5 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of an acid gas absorbent composition comprising 80 mol% of diethylene glycol diisopropyl ether and 20 mol% of tetraethylene glycol ethylisopropyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide gas released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Control 1 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of diethylene glycol diisopropyl was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Control 2 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of diethylene glycol ethylisopropyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide gas released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Control 3 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of diethylene glycol dimethyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Control 4 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of diethylene glycol diethyl ether was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide gas released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. Control 5 In an autoclave of stainless steel possessing an inner volume of 300 ml and maintained at a fixed temperature (25°C), 100 g of polyethylene glycol dimethyl ether (average molecular weight 270, and n = 3 to 10) was placed and deaerated under a vacuum. From a carbon dioxide gas cylinder provided with a pressure gauge, carbon dioxide gas was introduced into the autoclave until a constant inner pressure was reached. The amount of the carbon dioxide gas absorbed was determined by comparing the amount of carbon dioxide released from the cylinder and the amount of carbon dioxide gas contained in the inner gaseous phase of the autoclave. The amount of the absorbed carbon dioxide gas is shown in Table 1. As can be seen, acid gas absorbent compositions according to the present invention have a large capacity for the absorption of acid gases and exhibit a sufficiently low vapour pressure. Thus, they serve advantageously as an acid gas absorbent for the separation by absorption of such acid gases as carbon dioxide and hydrogen sulphide from a mixed gas without any appreciable loss of the absorbent due to the impacts of the treatment of absorption.

Claims: An acid gas absorbent composition, comprising (a) diethylene glycol diisopropylether and either: (b) tetraethylene glycol diisopropylether in a molar ratio of (a) to (b) of from 95:5 to 60:40; or (c) a mixture of polyethylene glycol diisopropylethers having 3 to 8 mols of ethylene oxide incorporated therein, the average molecular weight of said mixture being from 234 to 400 and the molar ratio (a) to (c) ranging from 95:5 to 60:40; or (d) triethylene glycol diisopropylether in a molar ratio of (a) to (d) of 80:20; or (e) tetraethylene glycol dimethylether in a molar ratio of (a) to (e) of 80:20; or (f) tetraethylene glycol ethylisopropylether in a molar ratio of (a) to (f) of 80:20. A composition according to claim 1, characterized in that the molar ratio (a) to (b) or the molar ratio (a) to (c) ranges from 85:15 to 65:35. The use of a composition as claimed in any one of claims 1 and 2 for the absorption of acid gases from a mixed gas comprising natural gas, synthetic gas and/or coke oven gas. The use of claim 3, wherein said acid gases comprise carbon dioxide and/or hydrogen sulphide.

Inventors: KADONO YUKIO, URANO YOSHIAKI, WATANABE FUMIO, KADONO, YUKIO, URANO, YOSHIAKI, WATANABE, FUMIO
Applicants: UNION CARBIDE CHEM PLASTIC, UNION CARBIDE CHEMICALS AND PLASTICS COMPANY, INC.
Application year: 1988
Main classification: B01D53
IPC codes: C10L3, B01D53, C10K1, C01B31, C01B17