Carbon dioxide absorbent based on amine having nitrile functional group, and carbon dioxide absorption method and separation method using same

Abstract

The present invention relates to a method for using, as a carbon dioxide absorbent, a secondary amine having a nitrile group, that is, a 3-(alkylamino)propionitrile compound. The absorbent based on the 3-(alkylamino)propionitrile compound and the carbon dioxide absorption method and separation method using same, according to the present invention, not only have an excellent carbon dioxide absorption capacity and a rapid carbon dioxide absorption rate, but also allow absorbent regeneration even at a considerably low temperature compared with a conventional alkanolamine-based absorbent and thus can significantly reduce the entire energy consumption required for an absorption process, and can also prevent recovered carbon dioxide from being contaminated with moisture and absorbent vapor, owing to the low regeneration temperature.

Claims

1. A carbon dioxide (CO.sub.2) absorbent based on an amine having a nitrile functional group, wherein the carbon dioxide absorbent consists essentially of a mixture of a 3-(alkylamino)propionitrile compound represented by Formula 3 and a secondary alkanolamine represented by Formula 4,
R.sub.1—NH—CH.sub.2—CH.sub.2—C≡N  Formula 3, ##STR00004## wherein R.sub.1 represents a C1 to C6 alkyl group or a cycloalkyl group, R.sub.2 represents a C1 to C6 alkyl group, and R.sub.3 represents hydrogen or a methyl group.

2. The carbon dioxide absorbent of claim 1, wherein an amount of the secondary alkanolamine is 20 to 150 parts by weight with respect to 100 parts by weight of the 3-(alkylamino)propionitrile compound used as a main absorbent.

3. A method of absorbing carbon dioxide, the method comprising absorbing carbon dioxide by using the carbon dioxide absorbent of claim 1, the carbon dioxide absorbent being dissolved in water or in an organic solvent.

4. The method of claim 3, wherein the organic solvent is at least one selected from the group consisting of a C1 to C6 alcohol, an amide compound, a ketone compound, dimethyl sulfoxide (DMSO), N-methylpyrrolidinone (NMP), and sulfolane.

5. The method of claim 3, wherein an amount of the mixture of the 3-(alkylamino)propionitrile compound and the secondary alkanolamine is 10 to 150 parts by weight with respect to 100 parts by weight of the solvent.

6. A method of separating carbon dioxide, the method comprising: a first step of absorbing carbon dioxide from a gas mixture containing carbon dioxide by using the carbon dioxide absorbent of claim 1; and a second step of separating carbon dioxide absorbed by the carbon dioxide absorbent.

7. The method of claim 6, wherein a temperature of the absorption in the first step is 20° C. to 60° C.

8. The method of claim 6, wherein a pressure of the absorption in the first step is normal pressure to 30 atmospheres.

9. The method of claim 6, wherein a temperature of the separation in the second step is 70° C. to 120° C.

10. The method of claim 6, wherein a pressure of the separation in the second step is normal pressure.

11. The method of claim 6, wherein a temperature of the absorption in the first step is 20° C. to 60° C., wherein a pressure of the absorption in the first step is normal pressure to 30 atmospheres, wherein a temperature of the separation in the second step is 70° C. to 120° C., and wherein a pressure of the separation in the second step is normal pressure.

Description

DRAWINGS

(1) FIG. 1 is a schematic view illustrating an example of a device for carbon dioxide absorption and separation experiments.

MODE FOR INVENTION

(2) Hereinafter, the invention is described more fully with reference to illustrative embodiments and the accompanying drawings. However, it is understood that the embodiments are merely illustrative for explanation of the present invention, and the scope of the present invention is not limited thereto.

(3) In order to solve the problems of general carbon dioxide absorbents, inventors of the present invention have conducted research on an absorption and regeneration mechanism, and found out that by reacting a primary amine and a secondary amine with carbon dioxide, carbamates, bicarbonates, or a mixture thereof may be formed depending on the structure of amine compounds; and as more carbamates are formed, an absorption rate gets higher, and as more bicarbonates are formed, regeneration efficiency is increased.

(4) Based on the research, the inventors of the present invention have concluded that the problems of the general amine-based absorbents are mostly caused by a high regeneration temperature of absorbents, and have made efforts to develop an absorbent which may be regenerated even at a low temperature and exhibits excellent absorption capacity.

(5) The present invention may provide a method of significantly reducing energy consumption required to regenerate an absorbent, as compared to a general alkanolamine-based or alkali-carbonate-based carbon dioxide absorbent, in which the method of the present invention uses a 3-(alkylamino)propionitrile compound, which is an amine compound having a nitrile group and is represented by the following Formula 3, as a carbon dioxide absorbent alone or in combination with a secondary alkanolamine that is a rate enhancer and is represented by the following Formula 4, the 3-(alkylamino)propionitrile compound being produced by reacting a primary alkylamine represented by the following Formula 1 with acrylonitrile or meta-acrylonitrile represented by the following Formula 2. A nitrile group having a high electron-attracting effect is present in the 3-(alkylamino)propionitrile compound, such that basicity of the 3-(alkylamino)propionitrile compound may be substantially reduced as compared to the basicity of alkanolamine such as ethanolamine, thereby weakening an interaction force between an amino group of the 3-(alkylamino)propionitrile compound and carbon dioxide. As a result, thermal stability of a compound formed by reaction with carbon dioxide is reduced, thereby facilitating regeneration of an absorbent.
R.sub.1—NH.sub.2  [Formula 1]
CH.sub.2═CH—C≡N  [Formula 2]
R.sub.1—NH—CH.sub.2—CH.sub.2—C≡N  [Formula 3]

(6) ##STR00002##

(7) Examples of a C.sub.1 to C.sub.6 alkyl group or cycloalkyl group represented by R.sub.1 in the above Formulae 1 to 3 include, but are not limited to, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, n-amyl, i-amyl, t-amyl, n-hexyl, 2-hexyl, cyclohexyl, methylcyclohexyl, n-heptyl, n-octyl, and the like.

(8) Further, in the above Formula 4, R.sub.2 represents C.sub.1 to C.sub.6 alkyl group, and examples thereof include, but are not limited to, methyl, ethyl, n-propyl, n-butyl, n-amyl, n-hexyl, and the like. R.sub.3 is hydrogen or a methyl group.

(9) In the case where a secondary alkanolamine is used as a rate enhancer, the amount of the secondary alkanolamine may be preferably 20 to 150 parts by weight, and more preferably 30 to 100 parts by weight with respect to 100 parts by weight of the 3-(alkylamino)propionitrile compound used as a main absorbent. In the case where the amount of the secondary alkanolamine is less than 20 parts by weight with respect to the weight of the 3-(alkylamino)propionitrile compound, the effect of enhancing the CO.sub.2 absorption rate is significantly reduced, and in the case where the amount of the secondary alkanolamine is greater than 150 parts by weight with respect to the weight of the 3-(alkylamino)propionitrile compound, a CO.sub.2 absorption regeneration rate is significantly reduced with only a slight increase in the CO.sub.2 absorption rate.

(10) Further, according to the present invention, the absorbent based on the 3-(alkylamino)propionitrile compound may absorb carbon dioxide even without a solvent, but when considering an absorption capability and viscosity of an absorbent, it is preferable to dissolve the absorbent based on the 3-(alkylamino)propionitrile compound in water or in an organic solvent, so as to use the absorbent to absorb carbon dioxide.

(11) The absorbent based on the 3-(alkylamino)propionitrile compound has substantially different water solubility depending on the length of an alkyl group represented by R.sub.1 in Formula 3, and particularly in the case where R.sub.1 is an alkyl group having 6 or higher carbon atoms, the absorbent is almost insoluble in water.

(12) According to the present invention, examples of the organic solvent include, but are not limited to polar organic solvents containing a C1 to C6 alcohol, such as methanol, ethanol, propanol, butanol, hexanol, ethylene glycol, 2-methoxyethanol, 2-(2-methoxyethoxy)ethanol, and the like, an amide compound such as N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMAc), and the like, a ketone compound such as methylethylketone (MEK), methylisobutylketone (MIBK), benzophenone, and the like.

(13) In the case of using the 3-(alkylamino)propionitrile compound by dissolving the compound in water or in an organic solvent, the amount of the amine compound may be preferably 10 to 150 parts by weight, and more preferably 30 to 120 parts by weight with respect to 100 parts by weight of a solvent. In the case where the total amount of the amine compound is less than 10 parts by weight, a carbon dioxide absorption capability is significantly reduced. In the case where the total amount of the amine compound is greater than 150 parts by weight, viscosity of a solution after absorbing carbon dioxide is excessively increased with only a slight increase in the CO.sub.2 absorption rate and amount.

(14) Further, as illustrated in the following Reaction Equation 1, the CO.sub.2 absorbent based on the 3-(alkylamino)propionitrile compound, which is used with no solvent or used by being dissolved in an organic solvent to react with CO.sub.2, forms a carbamic acid that has much lower thermal stability than a carbamate compound which has high thermal stability and is formed when using a general alkanolamine. Further, as illustrated in the following Reaction Equation 2, even in the case of using water as a solvent, a bicarbonate (HCO.sub.3) compound that may be easily regenerated is formed, such that energy required for the regeneration may be significantly reduced as compared to a general alkanolamine absorbent.

(15) ##STR00003##

(16) Accordingly, by using the absorbent according to the present invention, carbamates may not be formed even after CO.sub.2 is absorbed, such that an absorbent may be regenerated even at a low temperature, thereby reducing the entire energy required for an absorption process, and solving problems of corrosion and absorbent loss caused at a high regeneration temperature.

(17) In addition, unlike the case where carbamates are formed by reaction of primary and secondary alkanolamines, which are not sterically hindered, with carbon dioxide at a ratio of 2:1, the absorbent based on the 3-(alkylamino)propionitrile compound according to the present invention reacts with carbon dioxide at a ratio of 1:1 (mol CO.sub.2/mol alkanolamine=1/1), such that the absorbent may absorb two times more CO.sub.2 per molar unit than general alkanolamine such as monoethanolamine (MEA), and accordingly, is economical since a circulated amount of an absorbent may be reduced, which reduces the size of an absorption device.

(18) Further, according to the present invention, a method of separating CO.sub.2 from a gas mixture containing CO.sub.2 by using the above-described CO.sub.2 absorbent includes: a first step of absorbing CO.sub.2 in a gas mixture containing CO.sub.2 by using the CO.sub.2 absorbent based on an amine having a nitrile functional group; and a second step of separating the absorbed CO.sub.2 from the CO.sub.2 absorbent based on an amine having a nitrile functional group.

(19) Examples of the gas mixture containing CO.sub.2 include exhaust gases, natural gases, and the like that are discharged from chemical plants, power plants, and large boilers.

(20) When CO.sub.2 is absorbed in the first step, the absorption temperature may be preferably in the range of 10° C. to 80° C., and more preferably in the range of 20° C. to 60° C.; and the pressure may be preferably in the range of normal pressure to 50 atmosphere, and more preferably in the range of atmospheric pressure to 30 atmosphere. In the case where the absorption temperature is above 60° C., separation is performed at the same time as the absorption such that the absorbed amount of CO.sub.2 is reduced, whereas in the case where the absorption temperature is below 10° C., additional refrigeration equipment is required, thereby causing economic inefficiency. Further, an exhaust gas has normal pressure, such that it is most economical to perform absorption at normal pressure. In the case where an absorption pressure is above 30 atmosphere, although an absorbed amount is increased, additional equipment, i.e., a compressor, is needed to increase the pressure, thereby resulting in economic inefficiency.

(21) When the absorbed CO.sub.2 is separated in the second step, the temperature may be preferably in the range of 60° C. to 140° C., and more preferably in the range of 70° C. to 120° C., and the pressure may be preferably normal pressure. In the case where the separation temperature is below 70° C., separation may not be performed, whereas in the case where the separation temperature is above 120° C., the condition is the same as in the case of using an MEA absorbent, such that the effects of the ternary absorbent according to the present invention may not be achieved. Further, it is difficult to perform separation at a high pressure, since a vapor pressure of water is required to be significantly increased to maintain such high pressure, thereby requiring high temperature, and resulting in economic inefficiency. Accordingly, separation is preferably performed at normal pressure.

(22) Among the terms used throughout the present invention, the term “normal pressure” refers to atmospheric pressure, i.e., 1 atmosphere.

(23) Hereinafter, configurations and effects of the present invention will be described in detail with respect to specific examples and comparative examples; however, these examples are merely illustrative to make the present invention better understood and do not limit the scope of the present invention.

(24) First, experiments on the CO.sub.2 absorption capacity were conducted by using the device for carbon dioxide absorption and separation experiments illustrated in FIG. 1. The device illustrated in FIG. 1 includes a 60 ml stainless steel absorption reactor R1 equipped with a thermometer T2, a high-pressure transducer P1 (0 to 70 atm), a 75 ml CO.sub.2 storage cylinder S2 equipped with a thermometer T1, and a stirrer 1, and is installed in an isothermal oven to measure the carbon dioxide absorption capacity at a constant temperature. Further, a CO.sub.2 supply cylinder S1 and a manometer P2 are installed on the outside of the isothermal oven.

(25) After weighing the entire weight of the stainless steel absorption reactor R1 into which a certain amount of absorbent was put along with a magnet bar, the absorption reactor was stirred at 40° C. to 80° C. for one hour to be dried under vacuum, and the temperature was reduced to 40° C. so that the absorption reactor and the isothermal oven were maintained at a constant temperature. After turning off a valve V4 connected to the stainless steel absorption reactor R1, carbon dioxide at a constant pressure (e.g., 10 to 50 atm) was put into the CO.sub.2 storage cylinder S2, and the pressure and temperature in equilibrium were recorded. Then, after the stirring of the absorption reactor R1 was stopped, the pressure of the absorption reactor R1 was maintained at a constant pressure by using the valve V4 and a pressure regulator, and the pressure and temperature of the CO.sub.2 storage cylinder S2 maintained in equilibrium were recorded, and then the CO.sub.2 storage cylinder S2 was stirred. After one hour, the final pressure and temperature were recorded (equilibrium values), and a change in the weight of the absorption reactor R1 was measured.

(26) During a separation test, after turning off the valve V4, and increasing the temperature of the absorption reactor R1 to 70° C. to 120° C., the valve V4, a valve V5, and a valve V6 were turned on, and 20 ml/min of nitrogen was introduced to the absorption reactor R1 to separate carbon dioxide, and then, the temperature was reduced to room temperature, and a change in the weight of the absorption reactor R1 before and after the separation was measured.

Examples 1 to 8

(27) Carbon dioxide absorption tests were performed in the following manner: after filling the absorption reactor R1 illustrated in FIG. 1 with 20 g of an absorbent containing 3-(alkylamino)propionitrile compound used as a main absorbent alone or in combination with 2(alkylamino)ethanol as a rate enhancer, the temperature of the isothermal oven was maintained at 40° C. After stirring of the absorption reactor R1 was stopped, the pressure was maintained at 1 atm by using the valve V4 and a pressure regulator, and the pressure of the CO.sub.2 storage cylinder S2 maintained in equilibrium was recorded, and then the stirring was resumed. After one hour, the final pressure and weight were measured to obtain amounts of CO.sub.2 absorbed per mole of amine, and the results are shown in Table 1 below.

(28) TABLE-US-00001 TABLE 1 CO.sub.2 absorption capability (mol CO.sub.2/mol Example Absorbent component amine) 1 3-(methylamino)propionitrile — 0.68 2 3-(ethylamino)propionitrile 2-(methylamino)ethanol 1.01 3 2-methyl-3- 2 (butylamino)ethanol 1.01 (isopropylamino)propionitrile 4 3-(butylamino)propionitrile 1-methyl-2-(ethylamino) 1.02 ethanol 5 2-methyl-3- 1-methyl-2-(butylamino) 0.97 (amylamino)propionitrile ethanol 6 2-methyl-3- 2-(pentylamino)ethanol 0.94 (hexylamino)propionitrile 7 3-(octylamino)propionitrile 2-(hexylamino)ethanol 0.94 8 3-(cyclohexylamino)propionitrile 1-methyl-2- 0.95 (propylamino)ethanol

Examples 9 to 17

(29) Carbon dioxide absorption tests were performed in the following manner: after filling the absorption reactor R1 illustrated in FIG. 1 with 30 g of an absorbent solution obtained by dissolving, in water or in an organic solvent, 35% by weight of an amine compound containing 70% by weight of 3-(alkylamino)propionitrile as a main absorbent and 30% by weight of 2(alkylamino)ethanol as a rate enhancer, the temperature of the absorption reactor R1 was maintained at 40° C. After stirring of the absorption reactor R1 was stopped, the pressure of the absorption reactor R1 was maintained at 1 atm by using the valve V4 and a pressure regulator, and the pressure of the CO.sub.2 storage cylinder S2 maintained in equilibrium was recorded, and then the stirring was resumed. After one hour, the final pressure and weight were measured to obtain amounts of CO.sub.2 absorbed per mole of amine, and the results are shown in Table 2 below.

(30) TABLE-US-00002 TABLE 2 CO.sub.2 absorption capability Type of absorbent Type of (mol CO.sub.2/mol Example Main absorbent Rate enhancer solvent amine) 9 3-(ethylamino)propionitrile — Water 0.69 10 3-(ethylamino)propionitrile 2- Water 1.03 (methylamino)ethanol 11 3-(ethylamino)propionitrile 2- Methanol 1.11 (methylamino)ethanol 12 2-methyl-3- 2-(butylamino) Ethanol 1.02 (isopropylamino)propionitrile ethanol 13 3-(butylamino)propionitrile 1-methyl-2- DMF 0.96 (ethylamino) ethanol 14 2-methyl-3- 1-methyl-2- NMP 0.94 (amylamino)propionitrile (butylamino) ethanol 15 2-methyl-3- 2-(pentylamino) MIBK 0.89 (hexylamino)propionitrile ethanol 16 3-(octylamino)propionitrile 2-(hexylamino) Sulfolane 0.91 ethanol 17 3- 1-methyl-2- DMAc 0.97 (cyclohexylamino)propionitrile (propylamino) ethanol

Examples 18 to 22

(31) Carbon dioxide absorption tests were performed in the same manner as in Example 1: by using a methanol solution obtained by dissolving 35% by weight of an amine compound containing 70% by weight of 3-(butylamino)propionitrile as a main absorbent and 30% by weight of 2(butylamino)ethanol as a rate enhancer; and by varying the absorption temperature while fixing the CO.sub.2 pressure at 1 atm. The results are shown in Table 3 below.

(32) TABLE-US-00003 TABLE 3 CO.sub.2 absorption capability Example Absorption temperature (° C.) (moll CO.sub.2/mol amine) 18 20 1.14 19 30 1.10 20 40 1.03 21 50 0.84 22 60 0.56

Examples 23 to 35

(33) Carbon dioxide absorption tests were performed in the same manner as in Example 1: by using an absorbent solution obtained by dissolving, in 65% by weight of an organic solvent, 35% by weight of an amine compound containing 70% by weight of 3-(butylamino)propionitrile as a main absorbent and 30% by weight of 2(butylamino)ethanol as a rate enhancer; and by varying the types of the organic solvent while fixing the CO.sub.2 pressure at 1 atm and the absorption temperature at 40° C. The results are shown in Table 4 below.

(34) TABLE-US-00004 TABLE 4 CO.sub.2 absorption capability Example Organic solvent (mol CO.sub.2/mol amine) 23 Ethanol 0.97 24 Propanol 0.95 25 Butanol 0.89 26 Hexanol 0.84 27 Ethylene glycol 1.01 28 2-methoxyethanol 0.99 29 2-(2-methoxyethoxy)ethanol 0.96 30 DMF 0.83 31 DMSO 0.83 32 NMP 0.86 33 Sulfolane 0.87 34 MEK 0.80 35 DMAc 0.85

Examples 36 to 40

(35) Carbon dioxide absorption tests were performed in the same manner as in Example 1: by using a methanol solution obtained by dissolving 35% by weight of an amine compound containing 70% by weight of 3-(butylamino)propionitrile as a main absorbent and 30% by weight of 2(butylamino)ethanol as a rate enhancer; and by varying the absorption pressure while fixing the temperature at 40° C. The results are shown in Table 5 below.

(36) TABLE-US-00005 TABLE 5 Absorption pressure CO.sub.2 absorption capability Example (atm) (mol CO.sub.2/mol amine) 36 Normal 0.99 pressure 37 5 1.23 38 10 1.34 39 20 1.56 40 30 1.87

Examples 41 to 46

(37) Carbon dioxide absorption tests were performed in the same manner as in Example 1: by using, as an absorbent, an aqueous solution obtained by dissolving, in water, 3-(butylamino)propionitrile as a main absorbent and 2(butylamino)ethanol as a rate enhancer at a weight ratio of 7:3; and by varying a total amount of amine with respect to the weight of water while fixing the temperature at 40° C. and the pressure at 1 atm. The results are shown in Table 6 below. As the amount of amine was increased, the amount of CO.sub.2 absorbed per mole of amine was reduced. The reason for this is considered that an increased amount of amine leads to an increase in the viscosity of an absorbent solution, thereby limiting delivery of materials.

(38) TABLE-US-00006 TABLE 6 Amine/water CO.sub.2 absorption capability Example (wt %) (mol CO.sub.2/mol amine) 41 20 1.01 42 40 1.00 43 50 0.98 44 60 0.93 45 80 0.89 46 100 0.83

Examples 47 to 52

(39) Carbon dioxide absorption tests were performed in the same manner as in Example 9: by using, as an absorbent, an aqueous solution obtained by dissolving, in water, 35% by weight of an amine containing 70% by weight of 3-(butylamino)propionitrile as a main absorbent and 30% by weight of 2(butylamino)ethanol as a rate enhancer; and by varying the composition (wt %) of the main absorbent and the rate enhancer while fixing the CO.sub.2 pressure at 1 atm and the absorption temperature at 40° C. The results are shown in Table 7 below.

(40) TABLE-US-00007 TABLE 7 CO.sub.2 absorption Absorption rate of CO.sub.2 capability during the initial 10 min. Rate enhancer/main (mol CO.sub.2/mol period (g CO.sub.2/Kg Example absorbent (wt %) amine) absorbent-min) 47 20 1.03 91.5 48 40 0.98 97.9 49 60 0.94 104.5 50 80 0.89 108.1 51 100 0.82 114.4 52 150 0.79 114.5

Examples 53 to 60

(41) Carbon dioxide absorption tests were performed in the same manner as in Example 9: by using, as an absorbent, an aqueous solution obtained by dissolving, in water, 35% by weight of an amine containing 3-(butylamino)propionitrile as a main absorbent and 2(butylamino)ethanol as a rate enhancer at a weight ratio of 7:3; and by fixing an absorption temperature at 40° C. and an absorption pressure at 1 atm. After measuring CO.sub.2 amounts absorbed during the CO.sub.2 absorption tests, separation tests were performed by introducing nitrogen at the rate of 20 ml/min. Upon completion of the first absorption and separation of carbon dioxide, CO.sub.2 absorption and separation processes were repeated five times under the same conditions. The comparison results between the first absorption capacity and the fifth absorption capacity are shown in Table 8 below.

(42) TABLE-US-00008 TABLE 8 CO.sub.2 absorption capability Separation (mol CO.sub.2/mol amine) temperature First Example Main absorbent Rate enhancer (° C.) absorption Fifth absorption 53 3- 2- 100 1.03 0.94 (ethylamino) (methylamino) propionitrile ethanol 54 3- 2- 100 1.01 1.00 (butylamino) (butylamino) propionitrile ethanol 55 2-methyl-3- 2- 100 1.00 0.98 (hexylamino) (ethylamino) propionitrile ethanol 56 2-methyl-3- 1-methyl-2- 100 1.02 1.01 (hexylamino) (butylamino) propionitrile ethanol 57 3- 2- 100 0.97 0.95 (octylamino) (ethylamino) propionitrile ethanol 58 3- 1-methyl-2- 120 1.02 1.02 (cyclohexylamino) (propylamino) propionitrile ethanol 59 3- 2-(butylamino) 90 1.01 0.98 (butylamino) ethanol propionitrile 60 3- 2-(butylamino) 70 1.01 0.76 (butylamino) ethanol propionitrile

Comparative Example

(43) The separation test was performed five times in the same manner as in Example 53. After absorbing carbon dioxide at 1 atm and at 40° C. by using an aqueous solution containing 35% by weight of monoethanolamine (MEA) as an absorbent, the absorbed carbon dioxide was separated at normal pressure and at 100° C.

(44) During the first absorption, 0.62 mol of carbon dioxide per mole of monoethanolamine was absorbed into the absorbent; however, during the fifth absorption, 0.24 mol of carbon dioxide per mole of monoethanolamine was absorbed into the absorbent, which resulted in about 63.0% reduction in the absorption capability of the absorbent.

(45) While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.