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Ind. Eng. Chem. Res. 1999, 38, 2044-2050
Behavior of the Mass-Transfer Coefficient of Structured Packings in CO2 Absorbers with Chemical Reactions Adisorn Aroonwilas, Amornvadee Veawab, and Paitoon Tontiwachwuthikul* Process Systems Laboratory, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2
The present study provides comprehensive experimental data on the performance of structured packings in CO2 absorption application. Over 90 runs of the absorption experiments were conducted in 3 different scale absorption units (laboratory-scale, pilot-scale, and industrial-scale units). The structured packings used in this study were Sulzer EX, Gempak 4A, and Sulzer BX. Aqueous solutions of sodium hydroxide (NaOH), monoethanolamine (MEA), and 2-amino-2methyl-1-propanol (AMP) were employed as absorption solvents. The performance of the structured packings was evaluated in terms of the volumetric overall mass-transfer coefficient (KGav) as functions of the process operating parameters including gas load, CO2 partial pressure, liquid load, liquid temperature, solvent concentration, solvent type, and structured packing type. To emphasize the superior performance of the structured packings, performance comparisons between the used structured packings and common random packings are also given. 1. Introduction Carbon dioxide (CO2) is considered to be a major greenhouse gas, causing the temperature of the atmosphere to rise. Over the past few years, use of fossil fuels has contributed to the steady increase in the CO2 level in the world’s atmosphere, creating a global warming problem. Since this problem has a significant impact on the earth’s environment, there has been worldwide attention to the reduction of greenhouse gas emissions from related sources. During the United Nations Framework Convention on Climate Change (COP-3) in Dec 1997, an international attempt to cope with the global warming problem was made by reaching an agreement to reduce global greenhouse gas emissions by at least 5% below 1990 levels by the period 2008-2012. To achieve the agreement target, separation of CO2 from industrial waste gases, which would otherwise be vented to the atmosphere, becomes essential. With current technologies, CO2 separation can be performed by several approaches including absorption into liquid solvents, adsorption on solids, permeation through membranes, and chemical conversion. For removing CO2 from high-volume waste gas streams, absorption into liquid solvents is the suitable process approach.1 The CO2 absorption process generally consists of (i) an absorption unit where CO2 is removed from a gas phase into a liquid solvent and (ii) a regeneration unit where the absorption capability of the used solvent is recovered before being reintroduced to the absorption unit. Commonly used absorption solvents are alkanolamines, which were discovered in the late 1920s.2,3 Examples of these alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA). Besides the alkanolamines, another class of acid gas absorption solvents, so-called sterically hindered amines, has also been receiving a great deal of attention. * To whom correspondence should be addressed. Telephone: (306) 585-4726. Fax: (306) 585-4855. E-mail: paitoon@ uregina.ca.
According to Sartori et al.,4 the sterically hindered amine-based solvents had been used in 31 commercial plants between 1983 and 1992 under license from EXXON. The most recognized amine of this class is 2-amino-2-methyl-1-propanol (AMP). Since CO2 absorption fundamentally takes place when a gas stream containing CO2 contacts a liquid absorption solvent, the performance of the absorption process is therefore determined by the degrees of gas-liquid contact provided by absorption column internals. At present, there are many different types of column internals developed for the separation purposes. One of the most commonly used column internals is tower packing, which could be classified into random (dumped) and structured (ordered) categories. It is recognized that structured packings generally offer excellent masstransfer performance with a lower pressure drop in comparison with random packings. Due to the outstanding characteristics, the structured packings have been gaining a great deal of attention from many industries. Use of structured packings can be found almost exclusively in distillation application.5-8 In CO2 absorption application, structured packings show a great potential for replacing conventional random packings. According to our previous studies,9,10 the column fitted with structured packings yields significantly superior performance in terms of the masstransfer coefficient to the column using random packings. Due to the potential for using structured packings in CO2 absorption application, understanding the performance behavior of the structured packings is necessary for designing columns accurately and economically. Therefore, the objectives of this study are (i) to report experimental data on the performance of structured packings in CO2 absorption application, (ii) to investigate the performance behavior of structured packings affected by process operating parameters, and (iii) to compare CO2 absorption performances of structured packings with those of random packings. The CO2 absorption performance is represented in terms of the volumetric overall mass-transfer coefficient (KGav). The
10.1021/ie980728c CCC: $18.00 © 1999 American Chemical Society Published on Web 04/01/1999
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2045
interested process operating parameters in this study are gas load, CO2 partial pressure, liquid load, liquid temperature, solvent concentration, solvent type, and packing type. 2. Volumetric Overall Mass-Transfer Coefficient (KGav) Gas absorption is the phenomenon that a component A in a gas phase transfers across a gas-liquid interface into a liquid phase. The mass-transfer rate of component A is influenced by the concentration gradient of that component in the direction of mass transfer within each phase. The mass flux of component A (NA) across the gas-liquid interface at a steady state can be represented in terms of the gas-side mass-transfer coefficient (kG) and driving force (yA - yA,i) as follows:11
NA ) kGP(yA - yA,i)
(1)
where P, yA, and yA,i represent the total pressure, mole fraction of component A in the bulk gas, and mole fraction of component A on the gas side of the gas-liquid interface, respectively. Since the mass-transfer driving force takes place over extremely small distances, the determination of the component A concentration at the gas-liquid interface is particularly difficult. With this situation, the mass flux is therefore expressed in terms of the overall mass-transfer coefficient and the mole fraction of component A in the gas phase (yA*) in equilibrium with the concentration of A in the bulk liquid as follows:
NA ) KGP(yA - yA*)
(2)
Then, KG can be expressed as
KG ) NA/[P(yA - yA*)]
(3)
In a gas-absorption apparatus such as a packed column, the effective gas-liquid interfacial area per unit volume of the column (av) is considered to be another important parameter in the mass-transfer process besides the mass-transfer coefficients. Therefore, it is more useful to present the mass-transfer coefficient based on a unit volume of the absorption column rather than based on an interfacial area unit as follows:
KGav ) NAav/[P(yA - yA*)]
(4)
To evaluate KGav, the term NAav can be simply determined by performing absorption experiments in packed columns where the concentration profile of the absorbed component A in the gas phase can be measured along the column height. Considering an element of a packed column with height dZ in Figure 1, the mass balance of the element can be given as
NAav dZ ) GI d[yA/(1 - yA)]
(5)
where GI represents the inert gas molar flow rate per cross-sectional area of the column. From eqs 4 and 5, KGav can be defined as
KGav ) {GI/[P(yA - yA*)]}(dYA/dZ)
(6)
In this study, CO2 absorption experiments were carried out in tested columns packed with structured
Figure 1. Determination of the volumetric overall mass-transfer coefficient (KGav).
packings. The CO2 concentrations in the gas phase along the columns were measured and subsequently plotted as the CO2 concentration profile, which is a relationship between the mole ratio of CO2 (YA) and the column height (Z) as also shown in Figure 1. The slope of the profile, expressing the concentration gradient (dYA/dZ) at a particular yA, is then used for evaluating the KGav value according to eq 6. In addition, it should be noted that yA* in eq 6 could be evaluated from published solubility data.1,12 3. Experiment 3.1. Experimental Setup. The CO2 absorption experiments were conducted in three different scale absorption units: (i) laboratory-scale, (ii) pilot-scale, and (iii) industrial-scale units. Details on each unit are given below. (a) Laboratory-Scale Absorption Unit. The main component of the unit is an absorption column with an internal diameter of 0.019 m. The column made of acrylic plastic was packed with Sulzer-EX (Sulzer Brothers Ltd.) wire gauze laboratory structured packings of which the geometric area and element height are 1700 m2/m3 and 0.055 m, respectively. The total height of the packing section was 1.10 m (20 packing elements). To achieve maximum mass-transfer performance, the packing elements were staggered at 90° with respect to the previous one. Since the gas-phase CO2 concentration profile is required for the KGav determination, five gas sampling points were installed along the column height. (b) Pilot-Scale Absorption Unit. The absorption experiments also took place in a 0.10-m internal diameter acrylic column packed with Gempak 4A stainless steel (Glitsch, Inc.) structured packings. The height of the packing section was varied between 0.98 (four packing elements) and 2.21 m (nine packing elements) depending upon the desired CO2 removal target. An orifice-type liquid distributor with a maximum drippoint density of 1528 points/m2 was installed at the top
2046 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 1. Operating Conditions for the CO2 Absorption Experiments laboratory scale (Sulzer EX)
condition absorpn solvent gas phase CO2 concn, % inert gas load, kmol/(m2 h) liquid phase solvent concn, kmol/m3 liq load, m3/(m2 h) liq feed temp, °C
pilot scale (Gempak 4A)
industrial scale (Sulzer BX)
NaOH
AMP
NaOH
MEA
MEA
up to 15.0% 46.1-97.5
up to 15.2% 46.2-96.8
up to 15.4% 38.6
up to 15.1% 38.6
up to 12.4% 30.0
1.1-2.0 4.9-14.2 24
1.1, 2.0, 3.0 5.9-14.6 24
1.84-2.04 11.4-29.3 20 and 37
3.0-5.2 7.6-22.9 20 and 37
3.0 7.9 34-37
of the column. The absorption column was designed in such a way that the CO2 concentration in the gas phase and the temperature of the liquid solvent could be measured every 0.20 m along the column. (c) Industrial-Scale Absorption Unit. The absorption unit consisted of both absorption and regeneration sections. The experimental results in this study were generated from the absorption section of which the main component is an industrial-scale absorption column made of a 0.25-m (10-in.) internal diameter stainless steel pipe. The column was packed with six elements of Sulzer BX gauze structured packings (Sulzer Brothers Ltd.). The total height of the packing section was 1.02 m. Five gas sampling points were installed along the column to allow for CO2 concentration measurement. 3.2. Experimental Procedure. Prior to the CO2 absorption experiments, an aqueous solution of the absorption solvent was prepared at a given concentration. A mixture of air and CO2 was initially introduced into the bottom of the absorption column. The flow rate of the introduced gas stream was adjusted to a desired value by using calibrated flowmeters. Then, the prepared solution was circulated through the absorption column countercurrently to the gas stream. The circulation rate of the liquid solution was gradually increased until it reached a desired value. At this point, the CO2 absorption had already taken place in the column. However, samples from both gas and liquid phases could not yet be taken until the absorption reached a steady state indicated by constant values of temperature at given gas sampling points along the column. Details on the gas and liquid sample analyses can be found in our previous work.10,13 4. Results and Discussion The performance of the CO2 absorption in columns packed with structured packings was investigated by conducting absorption experiments over 90 runs under the operating conditions summarized in Table 1. The experimental results were plotted as profiles of the CO2 concentration, which is subsequently used for evaluating KGav. To observe the performance behavior of the structured packings, the values of KGav are reported as functions of the process operating parameters including gas load, CO2 partial pressure, liquid load, liquid temperature, solvent concentration, solvent type, and structured packing type. Comparisons of the CO2 absorption performance between the tested structured and common random packings are also given. 4.1. Mass-Transfer Behavior of Structured Packings. (a) Effect of Gas Load. The experimental results suggest that KGav of the tested structured packings is insensitive to changes in the flow rate of the gas passing through the packing elements. According to Figure 2, the KGav value remains constant as the gas load
Figure 2. Effect of the gas load on KGav (liquid load ) 9.73 m3/ (m2 h)). (a) [OH-] ) 0.9 kmol/m3; PCO2 ) 2.0 kPa; packing ) Sulzer EX. (b) [OH-] ) 0.9 kmol/m3; PCO2 ) 5.6 kPa; packing ) Sulzer EX. (c) [AMP]free ) 1.0 kmol/m3; PCO2) 8.0 kPa; packing ) Sulzer EX. (d) [OH-] ) 1.0 kmol/m3 PCO2) 3.5 kPa; packing ) Gempak 4A.
Figure 3. Effect of CO2 partial pressure on KGav (liquid load ) 9.73 m3/(m2 h); [OH-] and [AMP]free ) 1.0 kmol/m3; packing ) Sulzer EX).
increases. Logically, an increase in the gas load allows more CO2 molecules to travel from gas bulk to the gasliquid interface, which would result in higher masstransfer performance. However, the rate of gas absorption is not exclusively dependent upon the mass-transfer phenomenon in the gas phase. The mass-transfer behavior in the liquid phase also plays an important role. In the case when the KGav value is unaffected by an increasing gas load, the liquid-phase mass transfer is considered to be the major factor controlling the absorption process. At this point, diffusion of solvent molecules within the liquid phase is restricted in comparison with that of CO2 from the gas phase to the gas-liquid interface, thus causing a constant amount of CO2 absorbed regardless of the gas load values. (b) Effect of CO2 Partial Pressure. The KGav value is apparently affected by the CO2 partial pressure in the gas stream; i.e., it decreases as the CO2 partial pressure increases. From Figure 3, KGav is reduced by 30-45% when the CO2 partial pressure is raised from 3 to 10 kPa. The restricted diffusion of solvent molecules in the liquid phase is speculated to be the cause of this behavior. As mentioned previously, the restricted diffusion in the liquid phase basically results in a constant amount of CO2 absorbed. This means the term NAav in eq 4 becomes constant. Therefore, higher CO2 partial pressures lead to a reduction in the KGav value.
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2047 Table 2. Second-Order Reaction Rate Constant and Surface Tension of CO2 Absorption Solvents parameter m3/(kmol
k2, s) surf tension at 30 °C, N/m
NaOH aq soln °C14
13 190 at 20 70.9 × 10-3 at 2 kmol/m3 16
MEA aq soln °C15
5545 at 25 61.8 × 10-3 at 3.0 kmol/m3 17
AMP aq soln 520 at 25 °C15 52.9 × 10-3 at 1.1 kmol/m3 17
Figure 4. Effect of liquid load on KGav in the CO2-AMP system (solvent concentration ) 2.0 kmol/m3; PCO2 ) 10.0-12.5 kPa; packing ) Sulzer EX).
Figure 5. Effect of liquid load on KGav in CO2-NaOH and CO2MEA systems (Gempak 4A).
(c) Effect of Liquid Load. Figure 4 shows that the liquid load or flow rate has an influence on the value of KGav; i.e., an increase in liquid load generally yields a greater KGav value. The possible reason for this behavior is that a higher liquid load leads to (i) a greater liquidside mass-transfer coefficient (kL), which is directly proportional to the overall KG in the case of liquid-phase controlled mass transfer, and (ii) a greater effective area, which is caused by more liquid spreading on the packing surface. It should be noted that the sensitivity of KGav to changes in liquid load is variable depending upon the characteristics of the absorption systems. As illustrated in Figure 5, the KGav in the CO2-MEA system appears to be less sensitive to the liquid load changes than that in the CO2-NaOH system. This is probably due to the differences in the surface tension of the absorption solvents in the two systems. A lower surface tension generally provides liquid with a greater capability for spreading on the packing surface. According to Table 2, the surface tension of the MEA aqueous solution is lower than that of the NaOH solution. Therefore, a relatively greater effective area could be expected in the CO2-MEA system at a specific liquid load. Because of the better spreading, the CO2-MEA system seems to reach the maximum area limited by packing geometry before the CO2-NaOH system does. This perhaps results in less sensitivity to the changes in liquid load in the CO2-MEA system. (d) Effect of Liquid Temperature. Liquid temperature also has an effect on the absorption performance of the structured packings. From Figure 6a, increasing the feed temperature of the liquid solvent from 20 to 37 °C results in shifting the CO2 concentration profile down to the lower portion of the column. This indicates the higher mass-transfer performance, which can be presented by the increasing KGav value in Figure 6b. The increasing KGav value might be a consequence of
Figure 6. Effect of liquid temperature on the absorption performance in the CO2-MEA system (solvent concentration ) 3.0 kmol/ m3; liquid load ) 15.27 m3/(m2 h); packing ) Gempak 4A).
an increase in the second-order CO2 absorption rate constant (k2). In general, temperature is an essential parameter influencing reaction kinetics. Raising the temperature of the MEA solution yields a greater reaction rate constant, which can be evaluated from eq 7.18 This basically enhances the performance of the absorption phenomenon.
log(k2) ) 10.99 - 2152/T
(7)
Even though an increase in the liquid temperature generally yields a greater absorption performance as mentioned, too high a liquid temperature could as well deteriorate the absorption performance. As shown in Figure 7, raising the liquid temperature from 40 to 65 °C (middle of the column) reduces the KGav value from 0.40 to 0.37 kmol/(m3 h kPa). This behavior is probably caused by a great increase in Henry’s law coefficient (H in units of kPa/(kmol/m3)) of the MEA solution with temperature, limiting the capability of CO2 for traveling from the gas phase to the liquid. (e) Effect of Solvent Concentration. A change in the solvent concentration obviously has an impact on the absorption performance. According to Figure 8, an increase in the solvent concentration induces a higher KGav. It should be noted that only active molecules of solvent were taken into account for calculating the concentrations in Figure 8. The possible explanation for
2048 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 9. KGav comparison between CO2 absorption using aqueous solutions of NaOH and AMP (liquid load ) 9.73 m3/(m2 h); [OH-] or [AMP]free ) 0.8 kmol/m3; packing ) Sulzer EX).
Figure 7. Deterioration of KGav caused by increasing the liquid temperature in the CO2-MEA system (solvent concentration ) 3.0 kmol/m3; liquid load ) 7.64 m3/(m2 h); packing ) Gempak 4A). Figure 10. KGav comparison between CO2 absorption using aqueous solutions of NaOH and MEA (liquid load ) 15.3 m3/(m2 h); [OH-] or [MEA]free ) 1.5 kmol/m3; packing ) Gempak 4A).
Figure 8. Effect of solvent concentration on KGav (Gempak 4A).
this behavior is that increasing the solvent concentration reflects higher amounts of solvent molecules per unit volume available for absorbing more CO2 at the gas-liquid interface. As a result, the performance of CO2 absorption is improved and KGav is greater. (f) Effect of Solvent Type. Solvent type is considered another key factor affecting the efficiency of the CO2 absorption process. A particular absorption solution basically yields a specific range of KGav values. Considering Figure 3, absorbing CO2 with a NaOH aqueous solution gives KGav values between 1.55 and 4.04 kmol/ (m3 h kPa), while, at the same operating conditions, KGav values of the CO2 absorption using an AMP aqueous solution vary between 0.55 and 0.77 kmol/(m3 h kPa). The KGav comparison between the two systems at a specific operating condition is also illustrated in Figure 9. From Figure 9, the CO2-NaOH system provides approximately 3 times higher KGav than the CO2-AMP system does. The difference between the KGav values of the two absorption systems is primarily influenced by k2. The greater the rate constant, the higher the KGav value would be expected. According to Table 2, the k2 of the CO2-NaOH system is greater than that of the CO2-AMP system, thus leading to higher KGav.
Besides the influence of k2, the surface tension of the liquid solvents also plays an important role on KGav. In general, a lower surface tension would allow a greater effective mass transfer area, resulting in a greater KGav. According to Table 2, the surface tension of the AMP aqueous solution is lower than that of the NaOH aqueous solution, which would otherwise induce superior KGav in the CO2-AMP system. However, in this case, the influence of k2 seems to dominate over the influence of surface tension, thus resulting in a lower KGav in the CO2-AMP system. In addition to Figure 9, the effect of the solvent type on the absorption performance is also given in Figure 10, where KGav values of the CO2 absorption systems using NaOH and MEA aqueous solutions are compared. From the figure, the KGav value in the CO2-MEA system is greater than that in the CO2-NaOH system. According to Table 2, the MEA aqueous solution possesses less k2, which would result in less KGav in the system, while it also has a lower surface tension in comparison with the NaOH aqueous solution, which would result in a greater KGav in the system due to greater av. However, the experimental results suggest that the influence of lower surface tension might have overcome the influence of less k2, thus yielding higher KGav values in the CO2-MEA system. (g) Effect of Structured Packing Type. In general, structured packing can be classified into gauze and sheet metal categories. According to the literature,19 sheet metal structured packing yields a lower masstransfer performance in comparison with the gauze type in the distillation application. A lower effective area in the case of the sheet metal packing is believed to be responsible for its lower performance.
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2049 Table 3. Geometric Surface Areas of the Structured and Random Packings21 packing Gempak 4A Pall rings 1 in. 2 in. 3.5 in. IMTP No. 25 No. 50 No. 70
Figure 11. KGav comparison between absorption columns packed with Gempak 4A and Sulzer BX (conditions: 1 PCO2 ) 5.3-5.6 kPa; CO2 loading ) 0.31 mol/mol) (conditions: 2 PCO2 ) 8.2-8.8 kPa; CO2 loading ) 0.36 mol/mol).
surface area, m2/m3 446 205 115 92 230 98 56
provide a mass-transfer performance equivalent to that of the gauze packing in the case of the CO2 absorption application. 4.2. Performance Comparison with Random Packings. To emphasize the superior mass-transfer performance of structured packings in the CO2 absorption application, the KGav values produced from this study were then compared with the data of random packings summarized by Strigle.20 Gempak 4A is used as representative of structured packings, while Pall rings and IMPT (Norton Co.) are chosen as representatives of stainless steel random packings. With conversion factors in the literature,20 the KGav values of the random packings at the original conditions were converted to correspond with the conditions used for the structured packings in this study. Both CO2-NaOH and CO2-MEA systems were used for the comparisons. According to Figure 12, it is apparent that structured packing (Gempak 4A) provides almost 2-fold superior mass-transfer performance (KGav) to random packings (No. 25 IMTP and 1-in. Pall rings). This is perhaps due to a considerably higher geometric surface area per unit volume of the structured packing in comparison with those of random packings as shown in Table 3. 5. Conclusions
Figure 12. KGav comparison between absorption columns packed with Gempak 4A and random packings.
In this study, the mass-transfer performance of the gauze and sheet metal structured packings for the CO2 absorption application is compared. From Figure 11, the KGav values of Sulzer BX (gauze packing) and Gempak 4A (sheet metal packing) are generally comparable. This indicates that the sheet metal structured packing could
The following principal conclusions can be drawn from the present study: (1) Gas load has no influence on KGav since liquidphase mass transfer is the major factor controlling the absorption process. (2) CO2 partial pressure has an effect on KGav; i.e., increasing the CO2 partial pressure causes KGav to decrease. (3) An increase in liquid load generally yields a greater KGav. This is caused by increasing liquid-side mass-transfer coefficient (kL) and greater effective masstransfer area (av). (4) The liquid temperature has an influence on KGav in two different manners. First, an increase in the feed temperature of the liquid solvent results in a greater KGav caused by enhancements of the CO2 absorption rate constant (k2). Second, because of an increase in Henry’s law coefficient of the solvent, raising the liquid temperature beyond a specific temperature can reduce the KGav value. (5) An increase in solvent concentration induces a higher KGav. This is due to a greater amount of active solvent molecules available for absorbing CO2. (6) Solvent type also has an effect on KGav. The CO2NaOH system yields a greater KGav than the CO2-AMP system does but provides a lower KGav in comparison with the CO2-MEA system. The second-order CO2
2050 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
absorption rate constant (k2) and the surface tension of the solvents are speculated to play roles in this behavior. (7) The KGav of the sheet metal structured packing appears to be comparable to that of the gauze structured packing in the CO2 absorption application. (8) Structured packing offers superior performance to random packings. Because of its greater geometric surface area per unit volume, structured packing (Gempak 4A) provides almost 2 times greater KGav than stainless steel random packings (No. 25 IMTP and 1-in. Pall rings). Despite the superior performance of the structured packing, it should be noted that the cost of structured packing is generally higher than that of random ones. To acquire the optimal CO2 absorber, both performance and cost should be considered in the design process. Acknowledgment Support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Saskatchewan Power Corporation, Saskatchewan Energy and Mines, Prairie Coal Ltd., Wascana Energy Inc., Fluor Daniel Canada Inc., Sulzer Chemtech (Switzerland), Fluor Corporation, the Canada Centre for Mineral and Energy Technology (CANMET) is gratefully acknowledged. Nomenclature av ) effective interfacial area per unit volume of packing (m2/m3) AMP ) 2-amino-2-methyl-1-propanol [AMP]free ) concentration of active AMP molecules in the liquid phase (kmol/m3) CA,L ) concentration of component A in the liquid phase (kmol/m3) GI ) inert molar gas load (kmol/(m2 h) KG ) overall mass-transfer coefficient (kmol/(m2 h kPa)) kG ) gas-side mass-transfer coefficient (kmol/(m2 h kPa)) kL ) liquid-side mass-transfer coefficient (kmol/(m2 h (kmol/ m3))) k2 ) second-order reaction rate constant (m3/(kmol s)) MEA ) monoethanolamine [MEA]free ) concentration of active MEA molecules in the liquid phase (kmol/m3) NA ) mass-transfer flux of the absorbed component A (kmol/(m2 h)) NaOH ) sodium hydroxide [OH-] ) concentration of hydroxyl ions in the liquid phase (kmol/m3) P ) total pressure (kPa) PCO2 ) partial pressure of CO2 over solution (kPa) T ) absolute temperature (K) YA ) mole ratio of component A in gas bulk (mol/mol) yA ) mole fraction of component A in gas bulk (mol/mol) yA,i ) mole fraction of component A on the gas side of the gas-liquid interface (mol/mol) yA* ) gas-phase mole fraction of component A in equilibrium with the concentration of component A in the liquid phase (mol/mol) Z ) packing height (m)
Literature Cited (1) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Company: Houston, 1997. (2) Maddox, R. N. Gas Conditioning and Processing, 3rd ed.; Campbell Petroleum Series: Norman, OK, 1985; Vol. 4. (3) Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with Chemical Solvents; John Wiley & Sons: New York, 1983. (4) Sartori, G.; Ho, W. S.; Thaler, W. A.; Chludzinski, G. R.; Wilbur, J. C. Sterically hindered Amines for Acid Gas Absorption. In Carbon Dioxide Chemistry: Environmental Issues; Paul, J., Pradier, C., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1994. (5) Zanetti, R.; Short, H.; McQueen, S. Structured is the Byword in Tower-packing World. Chem. Eng. 1985, March 4, 22-25. (6) Roy, P.; Mercer, A. C. The Use of Structured Packing in a Crude Oil Atmospheric Distillation Column. Inst. Chem. Eng. Symp. Ser. 1987, No. 104, A103-A113. (7) Emma, C. Packing it in. Process Eng. 1987, 68, 51. (8) Hausch, G. W.; Quotson, P. K.; Seeger, K. D. Structured Packing at High Pressure. Hydrocarbon Process. 1992, 71 (4), 6770. (9) Aroonwilas, A. High Efficiency Structured Packing for CO2 Absorption Using 2-Amino-2-methyl-1-propanol (AMP). M.A.Sc. Thesis, University of Regina, Regina, Saskatchewan, Canada, 1996. (10) Aroonwilas, A.; Tontiwachwuthikul, P. High-efficiency Structured Packing for CO2 Separation Using 2-Amino-2-methyl1-propanol (AMP). Sep. Purific. Technol. 1997, 12, 67-79. (11) Treybal, R. E. Mass-Transfer Operations, 3rd ed.; McGrawHill Book Company: Singapore, 1980. (12) Tontiwachwuthikul, P.; Meisen, A.; Lim, C. J. Solubility of CO2 in 2-Amino-2-methyl-1-propanol Solutions. J. Chem. Eng. Data 1991, 36, 130-133. (13) Aroonwilas, A.; Tontiwachwuthikul, P. Mass Transfer Coefficients and Correlation for CO2 Absorption into 2-Amino-2methyl-1-propanol (AMP) Using Structured Packing. Ind. Eng. Chem. Res. 1998, 37, 569-575. (14) Pohorecki, R.; Moniuk, W. Kinetics of Reaction Between Carbon Dioxide and Hydroxyl Ions in Aqueous Electrolyte Solutions. Chem. Eng. Sci. 1988, 43, 1677-1684. (15) Alper, E. Reaction Mechanism and Kinetics of Aqueous Solutions of 2-Amino-2-methyl-1-propanol and Carbon Dioxide. Ind. Eng. Chem. Res. 1990, 29, 1725-1728. (16) Billet, R. Packed Towers. In Processing and Environmental Technology; VCH Publishers: New York, 1995. (17) Vazquez, G.; Alvarez, E.; Navaza, J. M.; Rendo, R.; Romero, E. Surface Tension of Binary Mixtures of Water + Monoethanolamine and Water+2-Amino-2-methyl-1-propanol and Tertiary Mixtures of These Amines with Water from 25 °C to 50 °C. J. Chem. Eng. Data 1997, 42, 57-59. (18) Blauwhoff, P. M. M.; Versteeg, G. F.; Van Swaaij, W. P. M. A Study on the Reaction Between CO2 and Alkanolamines in Aqueous Solutions. Chem. Eng. Sci. 1984, 39, 207-225. (19) Fair, J. R.; Bravo, J. L. Prediction of Mass Transfer Efficiencies and Pressure Drop for Structured Tower Packings in Vapor/Liquid Service. Inst. Chem. Eng. Symp. Ser. 1987, No. 104, A183-A201. (20) Strigle, R. F., Jr. Random Packing and Packed Towers; Design and Applications; Gulf Publishing Company: Houston, 1987. (21) Perry, R. H.; Green, D. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill Book Company: New York, 1997.
Received for review November 17, 1998 Revised manuscript received February 2, 1999 Accepted February 2, 1999 IE980728C
