Behavior of Reboiler Heat Duty for CO2 Capture Plants Using

The reboiler heat duty for regeneration of aqueous single and blended alkanolamines used in the carbon dioxide (CO2) absorption process was evaluated ...
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Ind. Eng. Chem. Res. 2005, 44, 4465-4473

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Behavior of Reboiler Heat Duty for CO2 Capture Plants Using Regenerable Single and Blended Alkanolamines Roongrat Sakwattanapong, Adisorn Aroonwilas, and Amornvadee Veawab* Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2

The reboiler heat duty for regeneration of aqueous single and blended alkanolamines used in the carbon dioxide (CO2) absorption process was evaluated experimentally in a bench-scale gas stripping and solvent regeneration system under atmospheric pressure. The evaluation was done for a number of alkanolamines, including monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP), and the mixtures of MEA-MDEA, DEA-MDEA, and MEA-AMP. The experimental results of heat duty were compared with industrial data available in the literature and subsequently correlated with process parameters. The results indicate that the reboiler heat duty is dependent upon CO2 loading of lean and rich solutions, alkanolamine type and concentration, and composition of blended alkanolamines. MEA requires the highest reboiler heat duty, followed by DEA and MDEA. The reboiler heat duties of blended alkanolamines are between the heat duties of their parent alkanolamines. 1. Introduction Energy consumption for solvent regeneration is an important parameter that must be known for design and operation, economic analysis, and development of strategy for a cost-effective carbon dioxide (CO2) absorption process. The energy consumption is commonly referred to as reboiler heat duty because the total energy for solvent regeneration is provided by hot steam passing through a reboiler at the bottom of a regeneration column. The reboiler heat duty is essentially a sum of the energy utilized for three main purposes: raising the temperature of CO2-loaded solution to the boiling point, breaking the chemical bonds between CO2 and absorption solvent, and generating water vapor to establish an operating CO2 partial pressure needed for CO2 stripping. The level of reboiler heat duty relates directly to the quantity of CO2 stripped from the regeneration column and the quality of lean solution fed back to the absorption column. That is, a higher heat duty results in a larger amount of CO2 product and a leaner solution leaving the regeneration column. To date, very few data on reboiler heat duty for the CO2 absorption process using aqueous alkanolamine solutions have been reported in the literature. A book by Astarita et al.1 presents the reboiler heat duty of the most widely used monoethanolamine (MEA). The traditional 15-20 wt % (approximately 3 kmol/m3) MEA requires 90 000 Btu/lb-mol CO2 (4748 kJ/kg of CO2), while the 30 wt % (5 kmol/m3) MEA containing Amine Guard corrosion inhibitors requires a reduced heat duty in the range of 39 000-60 000 Btu/lb-mol CO2 (20573165 kJ/kg of CO2). These heat-duty values are, however, specific to certain operating conditions and exclude the dependence on process parameters. As such, using these fixed values in process design and economic evaluation would lead to inaccurate outcome. In principle, the reboiler heat duty for solvent regeneration could be calculated using a rigorous process * To whom correspondence should be addressed. Tel.: (306) 585-5665. Fax: (306) 585-4855. E-mail: [email protected].

design approach. However, such calculations are rather complicated and even formidable in some cases due to the lack of heat-associated information including heat of reaction or enthalpy of CO2 absorption, and the heat capacity of the CO2-alkanolamine-water system. At present, there are only a few data on the heat of reaction of CO2 with alkanolamines available in the literature, especially for blended alkanolamines. These data were reported as both approximate integral and differential values. Kohl and Nielsen2 summarized the integral heats of reaction as constant values for CO2 absorption in aqueous solutions of MEA, diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), triethanolamine (TEA), and diisopropanolamine (DIPA). Using these constant values in the calculations would lead to inaccurate results since the heat of reaction in fact varies with temperature and CO2 content in the alkanolamine solutions (CO2 loading).3 The differential heat of reaction, on the other hand, offers more accuracy in calculations due to its dependence on operating conditions. A number of differential heats of reaction were reported as a function of CO2 loading for DEA by Lee et al.,4 for MDEA by Jou et al.5 and Mathonat et al.,6 and for MEA, MDEA, and their mixtures by Jou et al.7 More recently, Carson et al.8 reported the heats of reaction for MEA, DEA, MDEA, and MEA-MDEA at 25 °C. The data by Carson et al. were obtained only at low CO2 loading (0.01-0.25 mol/mol). For the heat capacity of aqueous alkanolamine solutions, there are a number of data reported in the literature. However, most data were obtained at low temperatures and low CO2 loading, which do not represent the actual solvent regeneration conditions. Hayden et al.9 measured the heat capacities of aqueous MDEA solutions at 25, 50, and 75 °C. Chen, Shih, and colleagues10-13 published heat capacities of aqueous solutions of MEA-MDEA, MEA-AMP (2-amino-2methyl-1-propanol), DEA-AMP, and MEA-2PE (2piperidineethanol) at temperatures up to 80 °C. Zhang et al.14 reported the heat capacities of aqueous solutions of MDEA and AMP at temperatures up to 95 °C.

10.1021/ie050063w CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005

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Figure 1. Schematic diagram of bench-scale gas stripping and solvent regeneration system.

Weiland et al.15 reported the heat capacities of both single (MEA, DEA, MDEA) and blended alkanolamines (MEA-MDEA and DEA-MDEA) at 25 °C, under CO2loaded conditions. Kohl and Nielsen2 summarized the heat capacities of several alkanolamine solutions as a function of both temperature and CO2 loading. However, the heat capacity data at regeneration temperature (100-120 °C) are limited to only a few alkanolamines (i.e., MEA, DEA, and DIPA). Due to the limited heat-associated data together with the complicated calculation nature, direct measurements of the reboiler heat duty are necessary. The objectives of this work are (1) to generate reboiler heat duty values of both single and blended alkanolamine systems through experiments, (2) to correlate the relationship between the reboiler heat duty and process parameters, and (3) to provide a better understanding of reboiler heat duty behavior. In this work, the reboiler heat duty of the CO2 absorption process was measured directly from a bench-scale gas stripping and solvent regeneration system. The obtained data are presented here as a function of process parameters, including CO2 loading of lean and rich solutions, alkanolamine type and concentration, and also composition of blended alkanolamine. These data are essential for the design and operation of a CO2 capture unit because they indicate the requirement of the steam rate for CO2 stripping, which is key in determining sizes of the reboiler and regeneration column. The findings obtained from this work would contribute to economic analysis and the development of a strategy for cost-effective CO2 capture. 2. Experiments 2.1. Experimental Setup and Procedure. Figure 1 shows a schematic diagram of the experimental setup used for measuring the reboiler heat duty of aqueous alkanolamine solutions. The setup was designed to simulate the actual continuous operation of solvent regeneration in the typical CO2 absorption process. It is a flow-through system consisting of a series of process components including (i) a 20-L liquid feed reservoir,

(ii) a variable-speed liquid feed pump, (iii) a rich-lean heat exchanger, (iv) a glass regeneration column (2.4 × 10-2 m in diameter and 1.0 m in height) packed with high-performance Sulzer’s EX structured packing, (v) a 0.7-L stainless steel reboiler with built-in heating coils, (vi) a Friedrichs overhead condenser (0.25 m jacket length), and (vii) a 20-L liquid receiver. The EX structured packing provided 20-50 theoretical stages which were much greater than the number of stages required for the typical CO2 stripping operation (i.e., 5-9).16 Use of this large number of stages ensured that the column performance did not affect the values of reboiler heat duty obtained in this work. A series of temperature indicators (TI) and flowmeters (FI) were installed at various locations to measure inlet and outlet temperatures as well as flow rates of liquids. These measurements were subsequently used for analysis of reboiler heat duty. Prior to the experiments, an aqueous alkanolamine solution was prepared by diluting the concentrated alkanolamine with deionized water to a given concentration. The solution was then loaded with CO2 until a desired CO2 loading was obtained. Details on the analysis of alkanolamine concentration and CO2 loading can be found in our previous work.17 Each experimental run began by introducing about 500 mL of the CO2loaded alkanolamine solution (rich solution) to the reboiler where a heating fluid at 130 °C was circulated through the heating coils to bring up the reboiler temperature. The cooling water was then circulated through the condenser. Once the reboiler temperature reached boiling point and the condenser temperature increased to stabilization, the rich solution which was preheated to an elevated temperature (90 °C) by the rich-lean heat exchanger was pumped continuously at a given flow rate (up 6 m3/m2‚h) to the top of the regeneration column. As the rich solution traveled downward, it was heated to boiling. As a result, a mixture of CO2 and water vapor was released from the solution. The water vapor was then condensed in the overhead condenser and returned to the regeneration column with total reflux, allowing only the stripped CO2 to leave the system. The hot lean solution from the reboiler flowed through the rich-lean heat exchanger and was eventually collected in the liquid receiver. The system was left in operation for a reasonable period of time to allow for steady state, which was indicated by constant readings of temperatures at all locations and also constant CO2 loading of lean solution. It took about 3-8 h, depending upon the operating conditions. The steady values of all temperatures and flow rates were recorded, and also a sample of lean solution was taken from the liquid receiver for the analysis of alkanolamine concentration and CO2 loading. After data collections, the introduction of rich solution to the column was stopped, while the circulations of heating fluid through the reboiler and cooling water through the overhead condenser were maintained. The system was left in this mode of operation until there was no stripped CO2 stream leaving the system. At this point, temperatures and flow rates of heating fluid and cooling water were recorded for determining the energy supplied to the reboiler and the energy removed from the condenser under nonregeneration conditions. These data were used for estimating the energy loss and performing the energy balance around the system. Details of test parameters and conditions are listed in

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4467 Table 1. Summary of Test Parameters and Conditions test parameter

condition

single alkanolamine blended alkanolamine molar mixing ratio for blends, mol/mol total alkanolamine concentration, kmol/m3 CO2 loading in feed solution, mol/mol

MEA, DEA, MDEA MEA-MDEA, DEA-MDEA, MEA-AMP 1:2, 1:1, 2:1, 1:5 (only for MEA-AMP) 4.0, 5.0, 7.0 0.30, 0.50

Table 2. Experimental Validation at 5 kmol/m3 MEA Concentration lean CO2 loading (mol/mol)

reboiler heat duty (kJ/kg of CO2)

literaturea

this study

3800 4800 5400

0.28-0.35 0.23-0.29 0.20-0.24

0.30 (at 3767 kJ/kg of CO2) 0.25 (at 4849 kJ/kg of CO2) 0.23 (at 5203 kJ/kg of CO2)

a

Estimated values from the work by Wilson et al.18

Table 1. All experiments were carried out under atmospheric pressure. 2.2. Analysis of Reboiler Heat Duty. The reboiler heat duty is by definition a ratio of the power supplied by the heating fluid at the reboiler (H˙ reboiler) and the mass rate of CO2 released from the regeneration column (m ˘ CO2). The power supplied was determined by the following equation:

H˙ reboiler ) m ˘ fCP,f(tin,f - tout,f)

(1)

where m ˘ f, CP,f, tin,f, and tout,f denote the mass flow rate, heat capacity of the heating fluid, and temperatures of the heating fluid entering and leaving the reboiler, respectively. The CO2 mass rate was defined as

m ˘ CO2 ) n˘ amine(RCO2,rich - RCO2,lean)MWCO2

(2)

where n˘ amine and MWCO2 are the molar flow rate of alkanolamine and molecular weight of CO2, respectively. RCO2,rich and RCO2,lean are the CO2 loadings of rich and lean solutions. In this study, the reboiler heat duty in the units of kilojoules per kilogram of CO2 was calculated by using the ratio of H˙ reboiler and m ˘ CO2 and the system energy loss (H˙ loss) as shown by

heat duty )

H˙ reboiler - H˙ loss m ˘ CO2

(3)

3. Experimental Validation Validation of the experimental setup, experimental procedure, and data analysis was performed by comparing the data obtained from this work to those from a CO2 absorption pilot plant reported in the literature.18 The comparison shown in Table 2 was made for the CO2 stripping from 5.0 kmol/m3 MEA solutions containing 0.50 mol of CO2/mol of amine. It is apparent that the heat-duty values from this work are in good agreement with the industrial data in the literature. 4. Results and Discussion 4.1. Effect of Lean-CO2 Loading. The behavior of reboiler heat duty of single and blended alkanolamine solutions was investigated by conducting more than 150 runs of regeneration experiments under wide ranges of operating conditions. The general representation of experimental results is given as a plot between reboiler heat duty and CO2 loading of the lean solution (lean-

Figure 2. Reboiler heat duty of an aqueous DEA solution as a function of lean-CO2 loading (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

CO2 loading). The results indicate that the reboiler heat duty is in inverse relation to the lean-CO2 loading; i.e., it decreases with increasing lean-CO2 loading. For instance, the heat duty of DEA in Figure 2 reduces from about 9000 to 1500 kJ/kg of CO2 as the lean-CO2 loading increases from 0.06 to 0.22 mol/mol, illustrating a reduction in solvent regeneration efficiency. Figure 2 also shows that the reboiler heat duty does not have a linear correlation with lean-CO2 loading. It in fact manifests two distinct behaviors as depicted in two regions of lean-CO2 loading. In the first region where the lean-CO2 loading is below 0.11 mol/mol for DEA, the reboiler heat duty is highly sensitive to the change in lean-CO2 loading. A significant amount of additional heat duty is required for a small reduction in lean-CO2 loading. In some cases, the lean-CO2 loading could remain virtually unchanged regardless of the amount of energy supplied. This presents an unfavorable operating region that consumes excessive energy during solvent regeneration. In the second region, where the lean-CO2 loading is above 0.11 mol/mol, the reboiler heat duty becomes less and less sensitive to the change in lean-CO2 loading. This suggests that only a small amount of additional heat duty is required to achieve a substantial reduction in lean-CO2 loading, thus presenting a favorable operating region. Such progressive behavior of reboiler heat duty can be explained by considering McCabe-Thiele diagrams for a CO2 stripping operation shown in Figure 3. The equilibrium and operating lines were obtained from the simulation results of our in-house mechanistic model for packed column design17 which accounts for kinetics and vapor-liquid equilibrium for CO2-alkanolamine systems, mechanism of liquid flow distribution within the column, heat- and mass-transfer resistance in both gas and liquid phases, and water evaporation and condensation. To achieve a low lean-CO2 loading (Figure

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Figure 4. Effect of rich-CO2 loading on reboiler heat duty of 5 kmol/m3 MEA solution.

Figure 3. McCabe-Thiele diagrams for CO2 stripping operation at different lean-CO2 loadings: (a) low lean-loading region; (b) high lean-loading region.

3a), the regeneration column must be operated in the lean-pinch mode, where both equilibrium and operating lines are pinched at the CO2-lean end (bottom of regeneration column). The lean pinch clearly indicates that the operating CO2 partial pressure at the reboiler is limited to an extremely low equilibrium CO2 partial pressure of the lean solution. The operating CO2 partial pressure cannot be easily reduced further to achieve even a slight reduction in lean-CO2 loading without a penalty of excessive energy input. The energy penalty can be illustrated by the operating CO2 partial pressure at the rich end. As seen in Figure 3a, a small reduction in lean-CO2 loading from 0.08 to 0.07 mol/mol causes the operating CO2 partial pressure at the rich end to decrease significantly from 40 kPa to less than 10 kPa. This indicates a considerable increase in the amount of water vapor leaving the regeneration column, which in turn reflects a considerable increase in the reboiler heat duty. In the region of high lean-CO2 loading (Figure 3b), the regeneration column is operated at much higher CO2 partial pressures, compared to those in the region of low lean loading (Figure 3a). This simply demonstrates how the increase in lean-CO2 loading affects the amount of water vapor and energy required for CO2 stripping. Figure 3b also shows that, at relatively high lean-CO2 loading, the regeneration column is operated in the unpinch mode where the operating lines run parallel to the equilibrium line. These operating lines appear to be insensitive to the change in lean-CO2 loading. For instance, reducing the lean-CO2 loading from 0.20 to 0.15 mol/mol leads to a slight shift of operating line and a very small drop in CO2 partial pressure at the top of

regeneration column (rich end), which indicates a comparable amount of excess water vapor leaving the column and also a comparable reboiler heat duty. This supports the previous finding that the reboiler heat duty is not sensitive to the lean-CO2 loading in this region. 4.2. Effect of Rich-CO2 Loading. The CO2 loading of rich solution (rich-CO2 loading) was found to have a significant impact on the reboiler heat duty. It is apparent in Figure 4 that, at a given lean-CO2 loading, a reduction in rich-CO2 loading causes the reboiler heat duty to increase substantially. For instance, a rich solution of MEA containing 0.50 mol/mol CO2 loading requires a reboiler heat duty of 5200 kJ/kg of CO2 to achieve a 0.25 mol/mol lean-CO2 loading, while that containing 0.30 mol/mol consumes at least 13 000 kJ/ kg of CO2. This effect is mainly attributed to the differences in magnitude of equilibrium CO2 partial pressure at different rich-CO2 loadings. From the McCabe-Thiele diagrams in Figure 5, the equilibrium CO2 partial pressures for 0.30 mol/mol rich-CO2 loading (up to 4 kPa) are much lower than those for 0.50 mol/mol (up to 96 kPa). This suggests that stripping CO2 from 0.30 mol/ mol rich solution requires a much higher amount of water vapor generated from the reboiler, causing the heat duty to increase significantly. Note that, although the McCabe-Thiele diagrams for 0.50 mol/mol rich-CO2 loading clearly illustrate the lean-pinch operation with a significant excess water vapor leaving the top of regeneration column, the magnitude of water vapor is still much less compared to that for 0.30 mol/mol. 4.3. Effect of Alkanolamine Concentration. Concentration of aqueous alkanolamine solution is another process parameter influencing the reboiler heat duty for solvent regeneration. In Figure 6, the reboiler heat duty of aqueous MEA solutions at three different concentrations (i.e., 4.0, 5.0, and 7.0 kmol/m3) is plotted as a function of lean-CO2 loading. The figure shows that the alkanolamine concentration has a slight effect on the energy consumption for solvent regeneration. An increase in MEA concentration from 4.0 to 5.0 kmol/m3 results in a small reduction in the reboiler heat duty. This effect can perhaps be explained by the information on vapor-liquid equilibrium of CO2-alkanolamine systems. It is known that, at a given CO2 loading, the equilibrium CO2 partial pressure increases with the

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Figure 7. Relationship between reboiler heat duty and CO2 stripping rate.

Figure 5. McCabe-Thiele diagrams for CO2 stripping operation at different rich-CO2 loadings: (a) rich loading of 0.50 mol/mol; (b) rich loading of 0.30 mol/mol.

Figure 6. Effect of alkanolamine concentration on reboiler heat duty (0.50 mol/mol rich loading).

concentration of alkanolamine.2 This suggests that a more concentrated solution can be regenerated at a greater CO2 partial pressure, consuming less energy for evaporating a lower amount of water vapor at the reboiler. The effect of alkanolamine concentration appears to be insignificant as the concentration increases beyond 5.0 kmol/m3. From Figure 6, the reboiler heat duty for a 7.0 kmol/m3 MEA solution is comparable to that for a 5.0 kmol/m3 solution. A small difference in equilibrium CO2 partial pressure of these concentrated solutions2 is probably the cause of this behavior.

Although the alkanolamine concentration demonstrates a slight impact on the reboiler heat duty as discussed above, use of concentrated solutions does present opportunities for energy saving on a given basis of CO2 capture and stripping. This can be seen in Figure 7 from the plots between reboiler heat duty and relative CO2 stripping rate (the ratio of CO2 stripping rate at a given condition to the CO2 stripping rate at 5.0 kmol/ m3 reference point). It is clear that an increase in MEA concentration from 4.0 to 5.0 kmol/m3, or from 5.0 to 7.0 kmol/m3, offers more than 50% saving of reboiler heat duty at a given CO2 stripping rate. This is true based on the principle of mass conservation. That is, for a given rate of CO2 capture or stripping, use of a more concentrated solution at a fixed liquid circulation rate results in a smaller CO2 cyclic capacity (a difference between rich- and lean-CO2 loadings). The smaller CO2 cyclic capacity can be translated into a higher lean-CO2 loading, which requires less energy for solvent regeneration. Figure 7 also shows that, if the alkanolamine plant is to be operated at a fixed reboiler heat duty, use of more concentrated solutions will enhance the rate of CO2 capture. For instance, at a reboiler heat duty of 3100 kJ/kg of CO2, the 7.0 kmol/m3 MEA solution offers an approximately 2 times higher CO2 stripping rate than the 4.0 kmol/m3 solution does. This simply presents a greater plant capacity and a reduced cost of CO2 capture. 4.4. Effect of Alkanolamine Type. (a) Single Alkanolamines. Figure 8 presents the reboiler heat duty of three single alkanolamines (MEA, DEA, and MDEA) at 0.50 mol/mol rich-CO2 loading and 4.0 kmol/ m3 alkanolamine concentration. These alkanolamines were chosen in this work because of their widespread use in gas-treating applications. It is apparent that the reboiler heat duty of the three alkanolamines behaves in a manner similar to what was previously described in the effect of lean-CO2 loading. As the reboiler heat duty increases, the lean-CO2 loading decreases and eventually stabilizes at a minimum value. The minimum lean-CO2 loading is specific to each alkanolamine, i.e., 0.22 mol/mol for MEA, 0.06 mol/mol for DEA, and 0.02 mol/mol for MDEA. These values relate directly to the equilibrium CO2 solubility of each alkanolamine

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Figure 8. Reboiler heat duty of single alkanolamine solutions (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

under the test condition. The minimum lean-CO2 loading is an indication of the liquid circulation rate to be used in service. That is, a higher minimum lean-CO2 loading suggests a greater liquid circulation rate. For instance, MEA requires a greater liquid circulation rate than DEA and MDEA to capture a given amount of CO2. Figure 8 also provides a reboiler heat duty comparison of the tested single alkanolamines. MEA appears to require the highest reboiler heat duty followed by DEA and MDEA. For instance, MEA requires at least 9000 kJ/kg of CO2 to achieve 0.22 mol/mol lean-CO2 loading, while DEA consumes only 1500 kJ/kg of CO2. DEA in turn requires 9000 kJ/kg of CO2 to achieve 0.06 mol/ mol lean-CO2 loading, while MDEA consumes 1200 kJ/ kg of CO2. Such ranking is attributed to two main factors. The first factor is heat of reaction of alkanolamine with CO2. MEA has the highest heat of reaction with CO2 among the three alkanolamines. It requires as high as 85.6 kJ/mol of CO2 to break the chemical bonds, while DEA and MDEA require 76.3 and 60.9 kJ/ mol of CO2, respectively.2 The second factor is the heat of water vaporization associated with the operating CO2 partial pressure. MEA requires the lowest operating CO2 partial pressure compared to DEA and MDEA to establish the driving force for CO2 stripping. This suggests that the largest amount of water vapor must be produced for MEA, leading to the largest energy consumption for water vaporization. (b) Blended Alkanolamines. The reboiler heat duty of three blended alkanolamines, including MEAMDEA, DEA-MDEA, and MEA-AMP, was measured as a function of lean-CO2 loading. The measurement was carried out at 4.0 kmol/m3 total alkanolamine concentration, 0.50 mol/mol rich-CO2 loading, and various mixing ratios (2:1, 1:1, 1:2, and 1:5 mol/mol). The obtained heat duties of blends were plotted against the heat duties of their parent alkanolamines (MEA, DEA, and MDEA) as shown in Figures 9-11. It should be noted in Figure 11 that an aqueous solution of single AMP was not included as a parent alkanolamine for MEA-AMP. This is because the AMP solution underwent crystallization under the test condition. Therefore, the mixing ratio of 1:5 was tested instead. From Figures 9-11, the reboiler heat duties of blended alkanolamines are between those of the parent

Figure 9. Reboiler heat duty of MEA-MDEA blended solutions (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

Figure 10. Reboiler heat duty of DEA-MDEA blended solutions (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

alkanolamines, suggesting a combined energy requirement. For example, MEA-MDEA requires regeneration energy less than MEA, but more than MDEA. Similar behavior was also found in the cases of DEA-MDEA and MEA-AMP. This combined effect demonstrates that adding a tertiary (MDEA) or sterically hindered (AMP) alkanolamine to the primary (MEA) or secondary (DEA) alkanolamine can reduce the reboiler heat duty of the CO2 capture process. The magnitude of reduction in reboiler heat duty is dependent on the mixing ratio of the blends, or the concentration of MDEA or AMP. From Figures 12 and 13, the heat duties of MEAMDEA, DEA-MDEA, and MEA-AMP decrease significantly as the concentration of MDEA or AMP increases. It was noticed that the reduction in reboiler heat duty of these blended alkanolamines is not proportional to the increasing concentration of MDEA or AMP. From Figure 12, the heat duty of MEA-MDEA and that of DEA-MDEA decrease rapidly with the MDEA concentration. As the concentration exceeds 2.0 kmol/m3 (mixing ratio of 1:1), the reduction in heat duty recedes

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Figure 11. Reboiler heat duty of MEA-AMP blended solutions (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

Figure 12. Effect of MDEA concentration on reboiler heat duty of MEA-MDEA and DEA-MDEA blended solutions (4.0 kmol/ m3 alkanolamine concentration, 0.50 mol/mol rich loading).

Figure 14. Distribution of energy associated with reboiler heat duty for blended alkanolamines.

Figure 13. Effect of AMP concentration on reboiler heat duty of MEA-AMP blended solutions (4.0 kmol/m3 alkanolamine concentration, 0.50 mol/mol rich loading).

gradually and eventually levels off. A similar trend was also found in the case of MEA-AMP (Figure 13); i.e., the reboiler heat duty decreases and begins to stabilize

at about 2.7 kmol/m3 AMP concentration. This demonstrates a synergistic effect or nonlinear relationship between the mixing ratio and the reboiler heat duty, which can be explained by considering three energy components contributing to CO2 stripping, i.e., heat of reaction, sensible heat, and heat of water vaporization. The distributions of the three energy components are illustrated in Figure 14. Based on data available in the literature for single alkanolamines,2 heats of reaction for breaking the chemical bonds between CO2 and blended alkanola-

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Table 3. Equilibrium CO2 Partial Pressure (PCO2) at Different Compositions of Blended Alkanolamines19 solution composition

PCO2 (kPa)

0.54-0.56

4.2 kmol/m3 MEA 2.1 kmol/m3 MEA + 2.1 kmol/m3 MDEA 0.8 kmol/m3 MEA + 3.4 kmol/m3 MDEA 4.28 kmol/m3 MDEA

455 782 1306 1900

0.27-0.31

4.2 kmol/m3 DEA 2.1 kmol/m3 DEA + 2.1 kmol/m3 MDEA 4.28 kmol/m3 MDEA

93 265 558

CO2 loading (mol/mol)

mines (hR,blend) were estimated by using the following equation: m

hR,blend )

Ci

hR,i ∑ i)1 C

(4)

T

where hR,i, Ci, and CT denote the heat of reaction, molar concentration of ith alkanolamine in the blend, and total molar concentration of alkanolamine, respectively. The estimated hR,blend was plotted against the obtained heatduty values in Figures 12 and 13. It appears that heats of reaction of MEA-MDEA, DEA-MDEA, and MEAAMP are insensitive to the mixing ratio, compared to the magnitude of the reboiler heat duty; i.e., they remain virtually constant throughout the MDEA or AMP concentration range. This suggests that the heat of reaction does not contribute to the synergistic effect of the mixing ratio. The sensible heat for raising the solution temperature to the boiling point was also calculated using a series of heat capacity equations proposed by Chen and colleagues.10,11 It was found that the sensible heats of all blended alkanolamines are insignificant compared to the heat of reaction and reboiler heat duty. As such, the sensible heat does not contribute to the synergistic behavior. Note that the estimated sensible heats are not shown in Figures 12 and 13 due to their magnitude. The heat of water vaporization is the remaining portion of the reboiler heat duty illustrated in Figures 12 and 13 as the difference between heat duty and heat of reaction. The heat of water vaporization does not have a linear correlation with the MDEA or AMP concentration. This clearly indicates the contribution of water vaporization to the synergistic effect. It should be noted that the heat of water vaporization is a direct indication of how much water vapor is required from the reboiler to establish an adequate driving force for CO2 stripping, which fundamentally relates to the CO2 partial pressure of alkanolamine solutions. As such, the nature of equilibrium CO2 partial pressure plays a major role in the synergistic effect of the mixing ratio. This is evidenced by a nonlinear relationship between the equilibrium CO2 partial pressure and the composition of blended alkanolamines shown in Table 3. It is also interesting to note that, in the cases of MEA-MDEA and DEA-MDEA at MDEA concentration below 2.0 kmol/m3, the heat of water vaporization is considerable and predominant over the heat of reaction. This implies that regenerating these two blends under this condition requires a large amount of water vapor from the reboiler to achieve low equilibrium CO2 partial pressure, resulting in high reboiler heat duty. As the MDEA concentration increases, the heat of water vaporization becomes less and less significant, thus making the heat of reaction more predominant.

Only a small amount of water vapor is required for CO2 stripping as indicated by the close proximity between the reboiler heat duty and the heat of reaction. As such, the regeneration of MEA-MDEA or DEA-MDEA mostly takes place through CO2 flashing, resulting in low reboiler heat duty. This, however, is not the case for MEA-AMP. The heat of water vaporization is comparable to the heat of reaction at high AMP concentration. Thus, CO2 flashing is not a primary mechanism of CO2 stripping during MEA-AMP regeneration. 5. Conclusion This work advances the knowledge of reboiler heat duty for CO2 capture using aqueous solutions of alkanolamines. The reboiler heat duty and its behavior were evaluated and reported as a function of process parameters. The heat duty is greatly dependent upon process parameters; i.e., it relates inversely to lean-CO2 loading, rich-CO2 loading, and alkanolamine concentration. MEA requires the highest reboiler heat duty, followed by DEA and MDEA. This is primarily due to the heat of reaction with CO2 and the heat of water vaporization. Blended MEA-MDEA, DEA-MDEA, and MEA-AMP require the combined reboiler heat duty compared to their parent alkanolamines. A synergistic effect of their mixing ratio on the reboiler heat duty exists and is attributed to the heat of water vaporization. CO2 flashing is a primary stripping mechanism for regeneration of MEA-MDEA and DEA-MDEA with high concentrations of MDEA, but not for regeneration of MEA-AMP solution. Acknowledgment The authors gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council (NSERC) and the Natural Resources Canada (NRCan). Literature Cited (1) Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with Chemical Solvents; John Wiley & Sons: New York, 1983. (2) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Co.: Houston, TX, 1997. (3) Mathonat, C.; Majer, V.; Mather, A. E.; Grolier, J.-P. E. Use of Flow Calorimetry for Determining Enthalpies of Absorption and the Solubility of CO2 in Aqueous Monoethanolamine Solutions. Ind. Eng. Chem. Res. 1998, 37 (10), 4136. (4) Lee, J. I.; Otto, F. D.; Mather, A. E. Solubility of Carbon Dioxide in Aqueous Diethanolamine Solutions at High Pressures. J. Chem. Eng. Data 1972, 17 (4), 465. (5) Jou, F.-Y.; Mather, A. E.; Otto, F. D. Solubility of H2S and CO2 in Aqueous Methyldiethanolamine Solutions. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 539. (6) Mathonat, C.; Majer, V.; Mather, A. E.; Grolier, J.-P. E. Enthalpies of Absorption and Solubility of CO2 in Aqueous Solutions of Methyldiethanolamine. Fluid Phase Equilib. 1997, 140 (1-2), 171. (7) Jou, F.-Y.; Otto, F. D.; Mather, A. E. Vapor-Liquid Equilibrium of Carbon Dioxide in Aqueous Mixtures of Monoethanolamine and Methyldiethanolamine. Ind. Eng. Chem. Res. 1994, 33 (8), 2002. (8) Carson, J. K.; Marsh, K. N.; Mather, A. E. Enthalpy of Solution of Carbon Dioxide in (Water + Monoethanolamine, or Diethanolamine, or N-methyldiethanolamine) and (Water + Monoethanolamine + N-methyldiethanolamine) at T ) 298.15K. J. Chem. Thermodyn. 2000, 32, 1285. (9) Hayden, T. A.; Smith, T. G. A.; Mather, A. E. Heat Capacity of Aqueous Methyldiethanolamine Solutions. J. Chem. Eng. Data 1983, 28, 196.

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4473 (10) Chen, Y.-J.; Shih, T.-W.; Li, M.-H. Heat Capacity of Aqueous Mixtures of Monoethanolamine with N-Methyldiethanolamine. J. Chem. Eng. Data 2001, 46, 51. (11) Chen, Y.-J.; Li, M.-H. Heat Capacity of Aqueous Mixtures of Monoethanolamine with 2-Amino-2-methyl-1-propanol. J. Chem. Eng. Data 2001, 46, 102. (12) Shih, T.-W.; Li, M.-H. Heat Capacity of Aqueous Mixtures of Diethanolamine with 2-Amino-2-methyl-1-propanol. Fluid Phase Equilib. 2002, 202, 233. (13) Shih, T.-W.; Chen, Y.-J.; Li, M.-H. Heat Capacity of Aqueous Mixtures of Monoethanolamine with 2-Piperidineethanol. Thermochim. Acta 2002, 389, 33. (14) Zhang, K.; Hawrylak, B.; Palepu, R.; Tremaine, P. R. Thermodynamics of Aqueous Amines: Excess Molar Heat Capacities, Volumes, and Expansibilities of {Water + Methyldiethanolamine (MDEA)} and {Water + 2-Amino-2-methyl-1-propanol (AMP)}. J. Chem. Thermodyn. 2002, 34, 679. (15) Weiland, R. H.; Dingman, J. C.; Cronin, D. B. Heat Capacity of Aqueous Monoethanolamine, Diethanolamine, NMethyldiethanolamine, and N-Methyldiethanolamine-Based Blends with Carbon Dioxide. J. Chem. Eng. Data 1997, 42, 1004.

(16) Strigle, R. F., Jr. Random Packings and Packed Towers: Design and Applications; Gulf Publishing Co.: Houston, TX, 1987. (17) Aroonwilas, A. Mass-Transfer with Chemical Reaction in Structured Packing for CO2 Absorption Process. Ph.D. Thesis, University of Regina, Regina, Saskatchewan, Canada, 2001. (18) Wilson, M.; Tontiwachwuthikul, P.; Chakma, A.; Idem, R.; Veawab, A.; Aroonwilas, A.; Gelowitz, D.; Stobbs, R. Evaluation of the CO2 Capture Performance of the University of Regina CO2 Technology Development Plant and the Boundary Dam CO2 Demonstration Plant. The 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, Sept 5-9, 2004; peer-reviewed paper (ID No. 365). (19) Dawodu, O. F.; Meisen, A. Solubility of Carbon Dioxide in Aqueous Mixtures of Alkanolamines. J. Chem. Eng. Data 1994, 39, 548.

Received for review January 17, 2005 Revised manuscript received March 30, 2005 Accepted April 19, 2005 IE050063W