3778
Ind. Eng. Chem. Res. 1997, 36, 3778-3782
Acid Gas Absorption by Means of Aqueous Solutions of Regenerable Phenol-Modified Polyalkylenepolyamine Gilberto Rinaldi Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, Facolta` di Ingegneria, Universita` di Roma, La Sapienza, Via Eudossiana 18, 00184 Rome, Italy
New absorbing products were prepared by reacting tetraethylenepentamine (TEPA) with formaldehyde (CH2O) and different amounts of phenol (TEPA: CH2O: phenol ) 1:1:X in moles, where X was 0.2, 0.4, 0.6). The products, called polyalkylenepolyaminophenols (PAP.X), were chemically and physically characterized. Tests with CO2, H2S, and SO2 in different conditions were carried out with different aqueous solutions of PAP.X products. The solution containing 25 g of PAP.4/100 cm3 was the most advantageous for both the absorption and desorption of CO2, the 10 g/cm3 of the same PAP.4 for H2S and SO2. The PAP.X products showed to be better than the alkanolamines both in absorption (total capacity, heat, and speed of reaction) and in desorption (faster regeneration at the same temperatures). Introduction Modified amines and polyamines are widely employed in the absorption of acid gases like CO2, H2S, and SO2; the modification is of primary importance in the reduction of the alkaline strength in order to obtain the regenerability of the product without compromising the absorption kinetics (Kohl and Riesenfeld, 1985). The use of alkanolamines and other variously modified aliphatic amines (Kohl and Riesenfeld, 1985; Giavarini et al., 1973; Giavarini and Rinaldi, 1977; Wirtz et al., 1964), some of which are sterically hindered (Sartori and Savage, 1983; Krieger, 1991), allows not only the purification of the gaseous atmospheres from the harmful acid components but also, when needed, their reutilization for further industrial uses: synthesis of urea and production of dry ice, sulfur, and sulfuric acid. Such operations, however, are interesting not only from the industrial engineering point of view (the acid gases can poison or inhibit many catalysts, hinder some reactions, and cause corrosion of equipments) but also from the environmental one (greenhouse effect and acid rain (Pieri, 1992; Regis, 1996)). The industrial use of aminic products for these kinds of processes is advantageous only if the product has special properties (Kohl and Riesenfeld, 1985; Kent and Eisenberg, 1976): high nitrogen content in the molecule (i.e., high capacity), water solubility, allowing the use of high concentrated absorbing solutions, low vapor pressure to minimize the losses of the product during the hot regeneration, high reactivity (i.e., brief time of contact), thermal stability; besides, of primary importance is the stability of “salts” formed with acid gases: it must be very high at absorption temperatures but as low as possible at regeneration temperatures. Finally, for the economy of depuration treatments, quite low costs are desired. Following former researches which have already given good results (Giavarini et al., 1973; Giavarini and Rinaldi, 1977) are now presented new products of modification of a polyalkylenepolyamine (tetraethylenepentamine, TEPA) with phenol, called polyalkylenepolyaminophenols (PAP), which highly have all the previous features and show to be far better than the ordinary alkanolamines. Experimental Section and Results Amine Modification (Marchetti et al., 1985; Giavarini and Rinaldi, 1989). One mole of formaldehyde S0888-5885(97)00057-2 CCC: $14.00
Figure 1. Evolution of chemical structure during preparation of PAP.X products from TEPA, phenol, and formaldehyde.
(40% aqueous solution, C. Erba) was gradually added drop by drop under continuous stirring and cooling, to a solution of phenol (Merck) in tetraethylenepentamine, TEPA (Fluka), containing 1 mol of TEPA for 0.2-0.40.6 mol of phenol. The mixture, after standing 24 h, was then gradually heated, up to 110 °C, to eliminate the water and to make possible the addition reaction of aldehyde to amine (Figure 1). The mixture, containing methylolamine and phenol (Dvorko et al., 1988), was then gradually heated up to 160 °C to let the condensation between the products happen, with the obtainment of polyalkylenepolyamine substituted with phenol (Rinaldi and Rossi, 1993; Giavarini and Rinaldi, 1989) and elimination of reaction water. The products, called PAP.2, PAP.4, and PAP.6 depending on the ratio of phenol in the initial reaction mixture, were yellowish viscous liquids and totally water soluble. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3779 Table 3. Stalagmometric Surface Tension (20 °C) of Some Solutions Experimented in Absorption Tests
Table 1. Properties of Modification Products in Comparison with Unmodified TEPA property
TEPA
PAP.2
PAP.4
PAP.6
mol wt, Mn (cryoscopic), average values for PAP.X specific gravity, 20 °C, kg/m3 viscosity, 20 °C, Pa‚s
189
240
385
430
910 11
1010 20
1025 24
1030 30
concn (g of concn (g of product/ surface product/ surface absorbing 100 cm3 tension absorbing 100 cm3 of tension product of solution) (N/m) solution) (N/m) product DEA PAP.4
25 10
0.0621 0.0602
PAP.4 PAP.4
25 40
0.0591 0.0552
Table 4. Aqueous Solutions of PAP.4 and Specific Gravity (20 °C) concn (g of product/100 cm3 of solution)
specific gravity (kg/m3)
concn (g of product/100 cm3 of solution)
specific gravity (kg/m3)
10 18
1060 1090
25 30
1120 1140
Table 5. pH (20 °C) of Aqueous Solutions of Absorbing Products (Glass Electrode), 10 g/100 cm3 of Solution Figure 2. Infrared spectrum of PAP.4 in the ranges 195O-165O cm-1 (a) and 900-600 cm-1 (b); thin film, NaCl disk.
Figure 3. Vapor pressure P of PAP.4 against temperature. Table 2. Products and Solutions Experimented in Absorption Tests
absorbing product
concn of aqueous absorbing solution (g of abs. prod./100 cm3 of solution)
MEA DEA TEA TEPA
25 25 25 10, 18, 25
absorbing product
concn of aqueous absorbing solution (g of abs. prod./100 cm3 of solution)
PAP.2 PAP.6 PAP.4
10, 18, 25 10, 18, 25 10, 18, 25, 30, 40
Characterization of Absorbing Products. Infrared spectra were obtained by using a Perkin Elmer Model 1124 spectrophotometer; average molecular weights (Mn) of the PAP products were determined cryoscopically in water; the specific gravity was determined by using a pycnometer; and the rotational viscosity was determined by by a Searle-type viscometer (viscotester Haake). TGA and TGD curves were also obtained (thermobalance Stanton Model 780); vapor pressures at various temperatures were determined by means of an effusion cell coupled with a torsional balance. The results (above all for the PAP.4 product) are in Table 1 and Figures 2 and 3. Characterization of Absorbing Solutions. Absorption tests were carried out with PAP.2, PAP.4, and PAP.6 products in aqueous solutions of different concentrations (Table 2); therefore, alkalinity (potentiometric titrations), surface tension (stalagmometer), viscosity (falling ball Ho¨ppler viscometer), and specific
absorbing product
pH
absorbing product
pH
TEPA PAP.2
11.2 10.7
PAP.4 PAP.6
10.4 9.8
Figure 4. Titration curves (potentiometric) of TEPA and PAP.4 in solution (5 g of product/100 cm3 of aqueous solution); 1.0 N HCl; electrodes glass/calomel.
Figure 5. Viscosity against temperature of aqueous solutions (10-25-40 g of PAP.4/100 cm3 of solution).
gravity (pycnometer for liquids) of the experimented solutions were preliminarily determined. Some tests were carried out in comparison with aqueous solutions of tetraethylenepentamine and of ethanolamines (mono-, di, and triethanolamine). The results are in Tables 3-5 and in Figures 4 and 5. Absorption Tests. The batch reactor (Figure 6) was a glass round-bottomed flask (P) connectable with a glass bulb (A) by means of a cock (R′); the volumes of the spheric vessel and of different bulbs were preliminarily and carefully measured at 20 °C. Absorption tests were carried out with CO2, H2S, and SO2 pure or
3780 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 6. Experimental setup (batch reactor) for absorption tests (R′′: to the mercury U-tube manometer).
Figure 9. SO2 absorption (20 °C): PAP.4 solutions (10-18-25 g/100 cm3).
Figure 7. CO2 absorption (20 °C): aqueous solutions of PAP.4 (10, 18, 25, 30 g/100 cm3 of solution). Figure 10. Absorption of CO2 (20 °C) diluted with air (v/v): aqueous solutions of PAP.4 (25 g/100 cm3). (a) CO2 90%. (b) CO2 80%. (c) CO2 40%. (d) CO2 15%. (e) CO2 7.5%.
Figure 8. H2S absorption (20 °C): solution of DEA (25 g/100 cm3) in comparison with PAP.4 solutions (10, 18, 25 g/100 cm3).
diluted by air. The round flask was filled with the acid gas (gas bottle), while the absorbing solution (initially in the upper bulb) was transferred into the round flask by opening the cock R′, which was then immediately turned off. In order to obtain (in different tests) several mixtures of acid gas and air, bulbs of known volumes were filled with pure gas, while the round flask, filled with air, contained the absorbing solution; the cock R′ was always turned on (and R off) during these tests. The process of absorption with time was evaluated by measuring the depression generated (the R′′ cock was connected with a mercury U-tube manometer). The volume ratio gas/solution was kept constant at 100:1 in the tests (about 2000:20 cm3). In order to increase the contact surface area between the solution and the gas, 20 glass balls (L 2 mm) were preliminarily introduced into the vessel, together with an iron cylinder coated with porcelain (L 4 mm, height 40 mm): an external magnetic device allowed stirring (100 rpm). Tests were always carried out at 20 ( 1 °C (thermostatic water bath). Examples of the results of absorption tests are in Figure 7 (CO2), Figure 8 (H2S), and Figure 9 (SO2). Tests with diluted CO2 were also carried out by using the PAP.4 product (Figure 10), 25 g/100 cm3 of solution.
Figure 11. CO2 desorption (90 °C): MEA, DEA, and TEA aqueous solutions (25 g/100 cm3).
Desorption Tests. The tests were carried out by employing a thermobalance (Gibertini Model Ecotherm, (0.1 mg). The saturated solution was put in a Petri dish (L 12 cm) over the plate of the balance, the oven being at selected temperature: desorption was experimented at 50, 60, 70, 80 and 90 °C ((1 °C). In order to verify that weight loss should be due to the desorption of previously absorbed gas, tests with nonsaturated solution were always carried out. No weight losses were recorded with nonsaturated solutions. In a few different tests at 90 °C, for further verification, the desorbed acid gases were trapped into a 0.1 N KOH solution and the total amounts of desorbed gas were measured by titration; the volumetric determination gave results in good agreement with thermogravimetry, the differences being lesser than 0.2% of the totally desorbed gas. Figure 11 reports the desorption of CO2 from saturated solutions of various PAP.X products at 90 °C. Cyclic absorption/ desorption tests were carried out by using the same PAP.4 solution (Figure 12). H2S and SO2 desorption at 90 °C from PAP.4 is represented in Figure 13. Heats of Absorption. Solubility curves (Dodge, 1986) at 20, 35, and 45 °C ((0.2 °C) were obtained by
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3781 Table 7. Absorption Rates of CO2 at 20 °C (First Stage of the Process): mg of CO2/g of Absorbing Product/s
Figure 12. CO2 absorption: same aqueous PAP.4 solution (25 g/100 cm3) cyclically regenerated. (I) After first regeneration. (II) After second regeneration. (III) After third regeneration.
Figure 13. Desorption (90 °C) of H2S and SO2 from PAP.4 aqueous solutions (25 g/100 cm3). Table 6. Estimated ∆H for Gas Absorption at Infinite Dilution absorbing absorbed ∆H∞ absorbing absorbed ∆H∞ product gas (kJ/mol) product gas (kJ/mol) MEA DEA TEA
CO2 CO2 CO2
45.21 44.37 42.69
PAP.4 PAP.4 PAP.4
CO2 H2S SO2
17.99 20.41 21.35
using mixtures of different concentrations of gas in air, protracting the contact time up to the equilibrium, without stirring; the experimental setup was the same as mentioned above. By employing the integrated van’t Hoff equation and reporting log(equilibrium pressure) vs 1/T (K) for different molar ratios of absorbed gas, it was possible to obtain a linear trend of heat of absorption (∆H) vs quantity of absorbed gas: H at infinite dilution was then extrapolated (Table 6). Corrosion Tests. The rate of corrosion of a specimen of steel (type 1040: C, 0.41 (Mn 0.75; Si, 0.38; S and P, e0.035) in 25 g/100 cm3 of a solution of PAP.4 was measured (corrograph M.P. Model 110). The value obtained was 68 × 10-6 m/yr, whereas a solution of DEA (same concentration) gave a value of 60 × 10-6 m/yr. Discussion of Results The condensation of tetraethylenepentamine with phenol by reaction with formaldehyde brings the obtainment of several water-soluble polyaminophenolic oligomers (PAP.X); in the adopted conditions of the reaction, the number of phenolic groups linked through a methylene bridge to the nitrogen atoms of the chain is determined by the molar ratio of the reagents (Figure 1). PAP.4, the best of the obtained products, has a
concn of absorbing solution (g of product/100 cm3 of solution):
absorbing product
10
18
25
TEPA PAP.2 PAP.4 PAP.6
3.89 2.33 1.21 0.41
4.12 2.06 1.09 0.33
1.89 1.00 1.13 0.29
dimeric structure (Table 1), with at least one phenolic group linked in ortho or para position to one of the aminic groups of the molecule: the IR spectrum of the product shows the bands and effects of the 1,2- and 1,4disubstituted benzene rings at 1020, 1810, 1700, and 1670 cm-1 (Figure 2a) and 820, 760, and 690 cm-1 (Figure 2b), respectively, while the absorption at 870 cm-1 refers to a 1,2,4-trisubstitution. The modification yields a molecule with several chemical and physical properties favorable for the absorption and desorption of acid gaseous products: great water solubility; thermal stability (decomposition starts at near 170 °C), low vapor pressure even at about 100 °C (Figure 3); average molecular weight not too high, but with a high amount of aminic groups (of primary importance for absorption capacity); viscosity not too elevated (Table 1) even in concentrated aqueous solutions (Figure 5). Furthermore, the surface tension (Table 3) and the specific gravity (Table 4) of solutions are on the same order as those of the diethanolamine solutions. The presence of a phenolic ring causes steric hindrance on aminic absorbing groups and decreases their alkaline strength (Figure 4 and Table 5), allowing the easier regenerability of the saturated products (complete desorption of CO2 at 90 °C in about 13 min) in comparison with ethanolamines (Figure 11). In the CO2 absorption stage (Tables 7 and 8 and Figure 7) the rate and capacity of PAP.X solutions are clearly lower than those of the equivalent solutions of unmodified tetraethylenepentamine (these last ones are not regenerable yet) but higher than those of ethanolamines in the same experimental conditions. PAP.2 and PAP.4 products are the most advantageous: they evidently exhibit the best combination of viscosity, basicity, and steric hindrance, with the optimum concentration being 25 g of product/ 100 cm3 of solution; the absorptions with higher concentrations are negatively affected by the enhancement of viscosity (Figure 7). The PAP.4 product has been positively experimented with CO2 diluted with air (Figure 10), while for the absorption of H2S and SO2 (Table 9 and Figures 8 and 9) PAP.4 is more suitable than DEA, but the optimum concentration is lower: 10 g/100 cm3 of solution, probably owing to the molecular chains conformation in diluted solution; the steric hindrance of phenol group and the threadlike structure of molecules in higher concentration could, in fact, interfere with the absorption of H2S and SO2 molecules in solution. In the desorption stage (regeneration of solutions saturated with acid gases) similar arguments can be considered, but the most advantageous product is PAP.4 (Figure 11), whereas the PAP.2 product is less rapidly regenerable (because of its lower basicity; Table 5), and so is PAP.6 (less alkaline, but more viscous; Table 1). It must be noted that every PAP.X product is fully regenerable, and the process is faster in comparison with ethanolamines in the same conditions. Desorption of H2S and SO2 (Figure 13) from PAP.4 solutions is equally fast and complete. The estimated heats of absorption of CO2, H2S, and SO2 (Table 6) are
3782 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 8. Absorption of CO2 (20 °C): Total Capacities (Stirring with Glass Balls) absorbing product
concn of absorbing solution (g of product/ 100 cm3 of solution)
total capacity (mg of CO2 adsorbed/g of absorbing product)
absorbing product
concn of absorbing solution (g of product/ 100 cm3 of solution)
total capacity (mg of CO2 adsorbed/g of absorbing product)
TEPA TEPA TEPA PAP.2 PAP.2 PAP.2 PAP.4 PAP.4
10 18 25 10 18 25 10 18
441.2 363.7 378.3 253.6 300.2 321.5 237.5 277.4
PAP.4 PAP.4 PAP.6 PAP.6 PAP.6 MEA DEA TEA
25 30 10 18 25 25 25 25
362.7 275.9 202.0 194.7 225.1 274.0 223.2 177.5
Table 9. Absorption of H2S and SO2 (20 °C): Total Capacities of PAP.4 Solutions (Stirring with Glass Balls) total capacity concn of absorbing solution (g of product/ 100 cm3 of solution)
mg of H2S absorbed/g of absorbing product
mg of SO2 absorbed/g of absorbing product
10 18 25
303.2 274.5 252.9
1110.8 908.7 875.2
very low for the PAP.4 product. Cyclic processes (absorption/desorption) seem not to affect the rate and the capacity of this product (Figure 12), while corrosion behavior of the solutions is acceptable. Conclusions Even if tests are still in progress, especially for evaluating the mechanisms and the kinetics of absorption/desorption reactions, it must already be noted that the PAP.4 product has several real properties: high rate and capacity of absorption, fast and complete regenerability, physical and chemical properties favorable for plants in industrial operations. Continuous-column absorption tests are in progress, and the results will be the object of a next publication, together with the model for the kinetic study of the whole process of absorption/ desorption of CO2, H2S, and SO2. Theoretical approximate calculations seem to indicate in column operations some advantages in using 25 g/100 cm3 of solutions of PAP.4 instead of DEA: saving 40% for the minimum feed of an absorbing solution, while for the same height of column (filled with Raschig rings) the savings for a liquid feed would be about 35%.
Acknowledgment I thank Luigi d’Aiello for the contribution of a part of the paper constituted his Chemical Engineering Doctorate Thesis, University of Roma, Italy, 1996. Literature Cited Dodge, M. Trans. Am. Inst. Chem. Eng. 1936, 32, 27. Dvorko, I.; Marchetti, M.; Maura, G.; Rinaldi, G. Eur. Space Agency 1988, 9, 351. Giavarini, C.; Rinaldi, G. Chim. Ind. (Milan) 1977, 59 (9), 621. Giavarini, C.; Rinaldi, G. Ind. Eng. Chem. Res. 1989, 28, 1231. Giavarini C.; Maura, G.; Rinaldi, G. Chim. Ind. (Milan) 1973, 55 (1), 23. Kent, R. L.; Eisenberg, B. Hydrocarbon Process. 1976, 2, 87. Kohl, A. L.; Riesenfeld, F. C. Gas Purification; Gulf Publishing Co.: Houston, 1985. Krieger, J. Chem. Eng. News 1991, 7, 7. Marchetti, M.; Maura, G.; Rinaldi, G., It. Patent 48283A85, 1985. Pieri, M. Dimensione Energia: Earth Summit 1992, Suppl. “Emissioni Gassose”, May 1992; p 40. Regis, V. Energ. Mater. Prime 1996, 115 (4), 14. Rinaldi, G.; Rossi, D. Polym. Int. 1993, 31, 227. Sartori, G.; Savage, D. W. Ind. Eng. Chem. Fundam. 1983, 22, 239. Wirtz, E.; et al. Erdoel Kohle 1964, 17 (6), 448.
Received for review January 21, 1997 Revised manuscript received May 9, 1997 Accepted May 9, 1997X IE9700573
X Abstract published in Advance ACS Abstracts, July 1, 1997.
