Abstract:
This invention provides a method for the removal of carbon dioxide present in gases which comprises bringing a CO 2  -containing gas into contact with an aqueous solution containing at least one amine compound of the general formula ##STR1## wherein R 1  to R 8  may be the same or different and each represent a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, and m is 0 or 1. The method of the present invention makes it possible to remove carbon dioxide efficiently. In particular, since carbon dioxide can be easily desorbed by heating the aqueous solution having absorbed carbon dioxide, the thermal energy required for regeneration of the aqueous solution can be reduced.

Description:
This application is a divisional of application Ser. No. 08/816,646, filed Mar. 13, 1997 now U.S. Pat. No. 5,904,908. 
    
    
     FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     This invention relates to a method for the removal of CO 2  (carbon dioxide) present in CO 2  -containing gases such as combustion exhaust gas. More particularly, it relates to a method for the efficient removal of CO 2  present in gases by using an aqueous solution containing a specific amine compound. 
     Conventionally, investigations have been made on the recovery and removal of acid gases (in particular, CO 2 ) contained in gases (i.e., gases to be treated) such as natural gas, various industrial gases (e.g., synthesis gas) produced in chemical plants, and combustion exhaust gas, and a variety of methods therefor have been proposed. In the case of combustion exhaust gas taken as an example, the method of removing and recovering CO 2  present in combustion exhaust gas by bringing the combustion exhaust gas into contact with an aqueous solution of an alkanolamine or the like, and the method of storing the recovered CO 2  without discharging it into the atmosphere are being vigorously investigated. 
     Although useful alkanolamines include monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine and the like, it is usually preferable to use monoethanolamine (MEA). 
     However, the use of an aqueous solution of such an alkanolamine, typified by MEA, as an absorbent for absorbing and removing CO 2  present in combustion exhaust gas is not always satisfactory in consideration of the amount of CO 2  absorbed for a given amount of an aqueous amine solution having a given concentration, the amount of CO 2  absorbed per mole of the amine in an aqueous amine solution having a given concentration, the rate of CO 2  absorption at a given concentration, and the thermal energy required for regeneration of the aqueous alkanolamine solution having absorbed CO 2 . 
     Now, many techniques for separating acid gases from various mixed gases by use of an amine compound are known, and examples thereof are given below. 
     In Japanese Patent Laid-Open No. 100180/&#39;78, there is described a method for the removal of acid gases wherein a normally gaseous mixture is brought into contact with an amine-solvent liquid absorbent composed of (1) an amine mixture comprising at least 50 mole % of a hindered amine having at least one secondary amino group forming a part of the ring and attached to a secondary or tertiary carbon atom, or a primary amino group attached to a tertiary carbon atom, and at least about 10 mole % of a tertiary amino-alcohol, and (2) a solvent for the aforesaid amine mixture which serves as a physical absorbent for acid gases. It is stated therein that useful hindered amines include 2-piperidine-ethanol [i.e., 2-(2-hydroxyethyl)piperidine], 3-amino-3-methyl-1-butanol and the like, and useful solvents include sulfoxide compounds which may contain up to 25% by weight of water. As an example of the gas to be treated, a normally gaseous mixture containing high concentrations of carbon dioxide and hydrogen sulfide (e.g., 35% CO 2  and 10-12% H 2  S) is described therein. Moreover, CO 2  itself is used in some examples of this patent. 
     In Japanese Patent Laid-Open No. 71819/&#39;86, an acid gas scrubbing composition comprising a hindered amine and a nonaqueous solvent such as sulfolane is described. In this patent, the usefulness of hindered amines for the absorption of CO 2  is explained with the aid of reaction formulas. 
     The carbon dioxide absorption behavior of an aqueous solution containing 2-amino-2-methyl-1-propanol (AMP) as a hindered amine is disclosed in Chemical Engineering Science, Vol. 41, No. 4, pp. 997-1003. CO 2  and a CO 2  -nitrogen mixture at atmospheric pressure are used as gases to be treated. 
     The rates of CO 2  and H 2  S absorption by an aqueous solution of a hindered amine (such as AMP) and an aqueous solution of a straight-chain amine (such as MEA) in the vicinity of ordinary temperature are reported in Chemical Engineering Science, Vol. 41, No. 2, pp. 405-408. 
     U.S. Pat. No. 3,622,267 discloses a technique for purifying synthesis gas obtained by partial oxidation of crude oil or the like and having a high partial pressure of CO 2  (e.g., synthesis gas containing 30% CO 2  at 40 atmospheres) by use of an aqueous mixture containing methyldiethanolamine and monoethylmonoethanolamine. 
     Deutsche Offenlegungschrift Nr. 1,542,415 discloses a technique for enhancing the rate of CO 2 , H 2  S and COS absorption by the addition of a monoalkylalkanolamine or the like to physical or chemical absorbents. Similarly, Deutsche Offenlegungschrift Nr. 1,904,428 discloses a technique for enhancing the absorption rate of methyldiethanolamine by the addition of monomethylethanolamine. 
     U.S. Pat. No. 4,336,233 discloses a technique for the purification of natural gas, synthesis gas and gasified coal by use of a washing fluid comprising an aqueous solution containing piperazine at a concentration of 0.81-1.3 moles per liter or an aqueous solution containing piperazine in combination with a solvent such as methyldiethanolamine, triethanolamine, diethanolamine or monomethylethanolamine. 
     Similarly, Japanese Patent Laid-Open No. 63171/&#39;77 discloses a CO 2  absorbent comprising a tertiary alkanolamine, monoalkylalkanolamine or the like to which piperazine or a piperazine derivative such as hydroxyethylpiperazine is added as a promoter. 
     As described above, an efficient method for the removal of CO 2  from various CO 2  -containing gases is desired. In particular, it is a pressing important problem to choose a CO 2  absorbent (amine compound) which, when a gas is treated with an aqueous solution containing the absorbent at a given concentration, can give a large amount of CO 2  absorbed per mole of the absorbent, a large amount of CO 2  absorbed per unit volume of the aqueous solution, and a high absorption rate. Moreover, it is desirable that the absorbent requires less thermal energy in separating the absorbed CO 2  to regenerate the absorbing solution. It may be difficult to meet all of these requirements by using a single amine compound. However, if an amine compound meeting some requirements is found, it may be possible to meet a more desirable combination of requirements, for example, by mixing it with one or more other amine compounds. That is, if an amine compound capable of giving, for example, a large amount of CO 2  absorbed per mole of the absorbent, it may be possible to improve its absorption rate and other properties separately. 
     In view of the above-described existing state of the prior art, it is an object of the present invention to provide an efficient method for the removal of CO 2  from CO 2  -containing gases by using a novel amine compound which can give a large amount of CO 2  absorbed per mole of the absorbent and has the property of liberating the absorbed CO 2  easily. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-described problems, the present inventors made intensive investigations on absorbents used to remove CO 2  present in combustion exhaust gas and have now discovered that an aqueous solution of a specific amine compound has great CO 2  -absorbing power and permits the absorbed CO 2  to be easily liberated. The present invention has been completed on the basis of this discovery. That is, the present invention has the following two aspects. 
     According to a first aspect of the present invention, there is provided a method for the removal of CO 2  present in gases which comprises bringing a CO 2  -containing gas into contact with an aqueous solution containing at least one amine compound of the general formula [1] ##STR2## wherein R 1  to R 8  may be the same or different and each represent a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, and m is 0 or 1. 
     According to a second aspect of the present invention, there is provided a method for the removal of CO 2  present in gases which comprises bringing a CO 2  -containing gas into contact with an aqueous solution containing at least one amine compound of the above general formula [1] and at least one other amine compound having great CO 2  -absorbing power. 
     When an aqueous solution containing an amine compound of the general formula [1] is used as an absorbing solution according to the method of the present invention, the amount of CO 2  liberated per mole of the absorbent is increased as compared with the case where a conventional absorbing solution is used. Thus, CO 2  can be removed more efficiently. 
     Moreover, since the amine compound of the general formula [1] permits the absorbed CO 2  to be easily desorbed by heating the absorbing solution having absorbed CO 2 , less thermal energy is required to regenerate the absorbing solution. Thus, a process having a smaller overall energy consumption for he recovery of CO 2  can be constructed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram illustrating an exemplary process for the removal of CO 2  present in combustion exhaust gas to which the method of the present invention can be applied; and 
     FIG. 2 is a graph showing changes with time of the CO 2  concentration in the absorbing solution as observed in the CO 2  desorption tests of Example 1 and Comparative Example 1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the amine compounds of the general formula [1] which can be used in the present invention, R 1  to R 8  may be the same or different and each represent a hydrogen atom or an alkyl group of 1 to 4 carbon atoms. Specific examples of the alkyl group of 1 to 4 carbon atoms include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl groups. Among others, it is preferable to use a combination of alkyl groups in which the sum of the numbers of carbon atoms of R 1  and R 2  and the sum of the numbers of carbon atoms of R 5  and R 6  are 4 or less and the sum of the numbers of carbon atoms of R 3  and R 4  and the sum of the numbers of carbon atoms of R 7  and R 8  are 2 or less. Amine compounds of the general formula [1] may be used alone or in admixture of two or more. 
     Specific examples of amine compounds of the general formula [1] include 2-aminopropionamide [H 2  NCH(CH 3 )CONH 2  ], 2-amino-2-methylpropionamide [H 2  NC(CH 3 ) 2  CONH 2  ], 3-amino-3-methylbutylamide [H 2  NC(CH 3 ) 2  CH 2  CONH 2  ], 2-amino-2-methyl-N-methylpropionamide [H 2  NC(CH 3 ) 2  CONH(CH 3 )], 3-amino-3-methyl-N-methylbutylamide [H 2  NC(CH 3 ) 2  CH 2  CONH(CH 3 )], 3-amino-3-methyl-N,N-dimethylbutylamide [H 2  NC(CH 3 ) 2  CH 2  CON(CH 3 ) 2  ], 2-ethylaminoacetamide [(H 5  C 2 )NHCH 2  CONH 2  ], 2-(t-butylamino)acetamide [(tert-H 9  C 4 )NHCH 2  CONH 2  ], 2-dimethylamino-N,N-dimethylacetamide [(CH 3 ) 2  NCH 2  CON(CH 3 ) 2  ], 2-ethylamino-2-methylpropionamide [(H 5  C 2 )NHC(CH 3 ) 2  CONH 2  ], 3-ethylaminopropionamide [(H 5  C 2 )NHCH 2  CH 2  CONH 2  ], 3-ethylaminobutylamide [(H 5  C 2 )NHCH(CH 3 )CH 2  CONH 2  ], 3-ethylamino-3-methylbutylamide [(H 5  C 2 )NHC(CH 3 ) 2  CH 2  CONH 2  ], 2-diethylaminoacetamide [(H 5  C 2 ) 2  NCH 2  CONH 2  ], 2-diethylaminopropionamide [(H 5  C 2 ) 2  NCH(CH 3 )CONH 2  ], 2-diethylamino-2-methylpropionamide [(H 5  C 2 ) 2  NC(CH 3 ) 2  CONH 2  ] and 3-diethylamino-3-methylbutylamide [(H 5  C 2 ) 2  NC(CH 3 ) 2  CH 2  CONH 2  ]. 
     In the aqueous solution containing at least one amine compound as described above (hereinafter also referred to as the absorbing solution), which is used for contact with a CO 2  -containing gas according to the present invention, the concentration of the amine compound is usually in the range of 15 to 65% by weight and preferably 30 to 50% by weight. The temperature at which the absorbing solution is brought into contact with a CO 2  -containing gas is usually in the range of 30 to 70° C. 
     If necessary, the absorbing solution used in the present invention may further contain corrosion inhibitors, deterioration inhibitors and the like. 
     Moreover, in order to enhance the CO 2  -absorbing power (e.g., the amount of CO 2  absorbed and the absorption rate) of the absorbing solution, one or more other amine compounds having great CO 2  -absorbing power may be used in addition to the amine compound of the above general formula [1]. Preferred examples of the other amine compounds used for this purpose include monoethanolamine, 2-methylaminoethanol, 2-ethylaminoethanol, 2-isopropylaminoethanol, 2-n-butylaminoethanol, piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, piperidine and 2-piperidine-ethanol. Where these other amine compounds are used, they are usually used at a concentration of 1.5 to 50% by weight and preferably 5 to 40% by weight, provided that they are soluble in water together with the amine compound of the general formula [1]. 
     The gases which can be treated in the present invention include natural gas, various industrial gases (e.g., synthesis gas) produced in chemical plants, combustion exhaust gas and the like. Among others, the method of the present invention can be applied to gases under atmospheric pressure and, in particular, combustion exhaust gas under atmospheric pressure. As used herein, the term &#34;atmospheric pressure&#34; comprehends a deviation from atmospheric pressure which may be caused by using a blower or the like to feed combustion exhaust gas. 
     The present invention is more specifically explained below in connection with an illustrative case in which the gas to be treated comprises combustion exhaust gas. Although no particular limitation is placed on the process employed in the removal of CO 2  present in combustion exhaust gas according to the method of the present invention, one example thereof is described with reference to FIG. 1. In FIG. 1, only essential equipment is illustrated and incidental equipment is omitted. 
     The equipment illustrated in FIG. 1 includes a decarbonation tower 1, a lower packed region 2, an upper packed region or trays 3, a combustion exhaust gas inlet port 4 to the decarbonation tower, a decarbonated combustion exhaust gas outlet port 5, an absorbing solution inlet port 6, a nozzle 7, an optionally installed combustion exhaust gas cooler 8, a nozzle 9, a packed region 10, a humidifying and cooling water circulating pump 11, a make-up water supply line 12, a CO 2  -loaded absorbing solution withdrawing pump 13, a heat exchanger 14, an absorbing solution regeneration tower (hereinafter abbreviated as &#34;regeneration tower&#34;) 15, a nozzle 16, a lower packed region 17, a regenerative heater (or reboiler) 18, an upper packed region 19, a reflux water pump 20, a CO 2  separator 21, a recovered CO 2  discharge line 22, a regeneration tower reflux condenser 23, a nozzle 24, a regeneration tower reflux water supply line 25, a combustion exhaust gas feed blower 26, a cooler 27 and a regeneration tower reflux water inlet port 28. 
     In FIG. 1, combustion exhaust gas is forced into combustion exhaust gas cooler 8 by means of combustion exhaust gas feed blower 26, humidified and cooled in packed region 10 by contact with humidifying and cooling water from nozzle 9, and then conducted to decarbonation tower 1 through combustion exhaust gas inlet port 4. The humidifying and cooling water which has come into contact with the combustion exhaust gas is collected in the lower part of combustion exhaust gas cooler 8 and recycled to nozzle 9 by means of pump 11. Since the humidifying and cooling water is gradually lost by humidifying and cooling the combustion exhaust gas, make-up water is supplied through make-up water supply line 12. 
     In the lower packed region 2 of decarbonation tower 1, the combustion exhaust gas forced thereinto is brought into counterflow contact with an absorbing solution having a predetermined concentration and sprayed from nozzle 7. Thus, CO 2  present in the combustion exhaust gas is removed by absorption into the absorbing solution supplied through absorbing solution inlet port 6. The decarbonated combustion exhaust gas passes into upper packed region 3. The absorbing solution supplied to decarbonation tower 1 absorbs CO 2  and the resulting heat of reaction usually makes the absorbing solution hotter than its temperature at absorbing solution inlet port 6. The absorbing solution which has absorbed CO 2  is withdrawn by CO 2  -loaded absorbing solution withdrawing pump 13, heated in heat exchanger 14, and then introduced into absorbing solution regeneration tower 15. The temperature of the regenerated absorbing solution can be regulated by heat exchanger 14 or cooler 27 which is optionally installed between heat exchanger 14 and absorbing solution inlet port 6. 
     In absorbing solution regeneration tower 15, the absorbing solution is regenerated through heating by regenerative heater 18. The regenerated absorbing solution is cooled by heat exchanger 14 and optionally installed cooler 27, and returned to the absorbing solution inlet port 6 of decarbonation tower 1. In the upper part of absorbing solution regeneration tower 15, CO 2  separated from the absorbing solution is brought into contact with reflux water sprayed from nozzle 24, cooled by regeneration tower reflux condenser 23, and introduced into CO 2  separator 21 where CO 2  is separated from reflux water obtained by condensation of water vapor entrained thereby and then conducted to a CO 2  recovery process through recovered CO 2  discharge line 22. Part of the reflux water is recycled to absorbing solution regeneration tower 15 through nozzle 24 by means of reflux water pump 20, while the remainder is supplied to the upper part of decarbonation tower 1 through regeneration tower reflux water supply line 25. 
     The present invention is further illustrated by the following examples. 
     EXAMPLE 1 
     A glass reactor placed in a thermostatic chamber was charged with 50 milliliters of a 1 mole/liter (13 wt. %) aqueous solution of diethylaminoacetamide [DEAAA; (H 5  C 2 ) 2  NCH 2  CONH 2  ] as an absorbing solution. While this absorbing solution was being stirred at a temperature of 40° C., CO 2  gas was passed therethrough under atmospheric pressure at a flow rate of 1 liter per minute for 1 hour. During this test, CO 2  gas was supplied through a filter so as to facilitate bubble formation. 
     After 1 hour, the amount of CO 2  contained in the absorbing solution was measured with a CO 2  analyzer (or total organic carbon analyzer), and the degree of CO 2  absorption (i.e., the molar ratio of CO 2  to the absorbing solution) was determined. Next, the reactor holding the absorbing solution was heated at 100° C. to examine the ease of desorption of CO 2  from the absorbing solution at 100° C. To this end, small amounts of samples of the absorbing solution heated at 100° C. were taken with the lapse of time and their CO 2  contents were measured with a CO 2  analyzer. 
     COMPARATIVE EXAMPLE 1 
     An absorption/desorption test was carried out with a 1 mole/liter (12 wt. %) aqueous solution of 2-diethylaminoethanol [DEAE; (H 5  C 2 ) 2  NCH 2  CH 2  OH] having an analogous chemical formula. The results thus obtained are shown in Table 1 and FIG. 2. 
     
                       TABLE 1______________________________________Component     CO.sub.2 content                    CO.sub.2 content                              Amount of  of     after absorption     after heating        CO.sub.2 libera-                               absorbing   (A)                (B)                                        ted (A-B)  solution  [mole %]       [mole %]       [%]______________________________________Example 1   DEAAA     62.9       2.3     60.6  Comparative    DEAE    99.9    44.8    55.1  Example 1______________________________________ 
    
     It can be seen from the results shown in Table 1 and FIG. 2 that, when an aqueous solution of diethylaminoacetamide (DEAAA) that is an amine compound in accordance with the present invention is used as an absorbing solution for CO 2  gas, the amount of CO 2  absorbed per mole of the absorbent is somewhat smaller than when an aqueous solution of DEAE is used, but the amount of CO 2  liberated is larger than when an aqueous solution of DEAE is used because of the ease of desorption of CO 2  from the absorbing solution, thus making it possible to remove CO 2  efficiently. 
     EXAMPLES 2-3 AND COMPARATIVE EXAMPLE 2 
     Absorption/desorption tests for CO 2  gas were carried out in the same manner as in Example 1, except that the aqueous solution of DEAAA was replaced by a 1 mole/liter (10 wt. %) aqueous solution of 2-(t-butylamino)acetamide [t-BAAA; (tert-H 9  C 4 )NHCH 2  CONH 2  ] (Example 2) or a 1 mole/liter (13 wt. %) aqueous solution of 2-dimethylamino-N,N-dimethylacetamide [DMADMAA; (CH 3 ) 2  NCH 2  CON(CH 3 ) 2  ] (Example 3). Moreover, an absorption/desorption test was carried out with a 1 mole/liter (9 wt. %) aqueous solution of 2-ethylaminoethanol (EAE) (Comparative Example 2). The results thus obtained are shown in Table 2. 
     
                       TABLE 2______________________________________Component     CO.sub.2 content                    CO.sub.2 content                              Amount of  of     after absorption    after heating       CO.sub.2 libera-                               absorbing   (A)               (B)                                       ted (A-B)  solution  [mole %]       [mole %]       [%]______________________________________Example 2   t-BAAA    89.3       10.9    78.4  Example 3  DMADMAA      86.2      8.5       77.7  Comparative   EAE   92.5     39.1           53.4  Example 2______________________________________ 
    
     It can be seen from the results shown in Table 2 that, when an aqueous solution of 2-(t-butylamino)acetamide (t-BAAA) or 2-dimethylamino-N,N-dimethylacetamide (DMADMAA) that is an amine compound in accordance with the present invention is used as an absorbing solution for CO 2  gas, the amount of CO 2  absorbed per mole is somewhat smaller than when an aqueous solution of EAE is used, but the amount of CO 2  liberated is larger than when an aqueous solution of EAE is used because of the ease of desorption of CO 2  from the absorbing solution, thus making it possible to remove CO 2  efficiently.