Energy Fuels 2010, 24, 213–219 Published on Web 07/28/2009
: DOI:10.1021/ef900564x
Reduction of Energy Requirement of CO2 Desorption by Adding Acid into CO2-Loaded Solvent† Bo Feng,* Min Du, Timothy James Dennis, Kim Anthony, and Marc Jarrod Perumal School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, Qld 4072, Australia Received June 2, 2009. Revised Manuscript Received July 19, 2009
The effect of the addition of weak acids into CO2-loaded solvents or rich solvents on the energy requirement of CO2 desorption was investigated experimentally. The commercially available CO2 solvent monoethanolamine (MEA) was used to absorb different amounts of CO2. Subsequently, a few weak acids such as suberic acid, phthalic acid, and oxalic acid were added into the solvents to study the effect of acid amount on the rate of CO2 release. It was found that CO2 could be released much faster and in much larger quantity with the addition of weak acids while the other desorption conditions were maintained the same. The amount of CO2 released was found to be proportional to the amount of acid added. Acid addition could be potentially used to reduce the energy requirement for CO2 desorption from solvent.
The solvent technology has been investigated extensively.2-24 Briefly, carbon dioxide is captured first in an absorption column (at 40-60 °C) using physical or chemical solvent. The CO2-saturated solvent (or rich solvent) is subsequently pumped to a desorption tower (or stripper) to release CO2 and regenerate the solvent (at 100-150 °C). The fresh solvent is then sent back to the absorption tower for reuse (see Figure 1). In spite of a mature technology, the solvent technology faces three major limitations:6,14,16 • Solvent degradation. A significant practical problem in the operation of a solvent absorption system is solvent losses due to degradation. The degradation of amine-based solvents typically occurs in three different ways.18,19 The flue gas stream generally contains 5-10% O2, which induces a series of oxidation reactions between the solvent and the O2, resulting in the formation of unwanted products. Oxidative degradation occurs in the presence of metal ions such as iron or copper, which act as catalysts to the process. The oxidation of the solvent produces organic acids and NH3, which occurs most heavily in the aqueous loaded solvent mixture built up at the bottom of the absorption column. Another form of solvent degradation is caused by a reaction of the solvent with the CO2 molecules in what is called carbamate polymerization. Finally the solvent is also susceptible to thermal degradation resulting in changes to the chemical composition when subjected to temperatures higher than roughly 200 °C. • Corrosion. The conventional solvent absorption system is contained within columns made from carbon steel and, as such, is very susceptible to corrosion. Corrosion occurs through standard oxidization of the iron in the steel and results in uniform deterioration of the material
1. Introduction The current CO2 capture methods can be classified into three categories: precombustion, postcombustion and oxyfuel,1 which are under various development stages. So far the only technology that is commercially available is the solvent technology, which is in the postcombustion category. † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ61 7 3346 9193. Fax: þ61 7 3365 4799. E-mail:
[email protected]. (1) Beck, R. A.; Hoag, K. J. Energy 1997, 22 (2/3), 115–120. (2) Al-Baghli, N. A.; Pruess, S. A.; Yesavage, V. F.; Selim, M. S. Fluid Phase Equilib. 2001, 185, 31–34. (3) Alie, C.; Backham, L.; Croiset, E.; Douglas, P. L. Energy Conversion and Management 2005, 46, 475–487. (4) Barreau, A; Blanchon le Bouhelec, E; Habchi Tounsi, K. N.; Mougin, P; Lecomte, F. Oil Gas Sci. Technol. - Rev. IFP 2006, 61 (3), 345–361. (5) Bavbek, O; Alper, E. G. Turk J Chem 1999, 23, 293–300. (6) Chakravarti, S; Gupta, A; Hunek, B. Advanced Technology for the Capture of Carbon Dioxide from Flue Gases. In First National Conference on Carbon Sequestration, Washington, DC, 2001, 1-10. (7) Chiu, L; Li, M. J. Chem. Eng. Data 1999, 44 (6), 1396–1401. (8) Dijkstra, J. W.; Jansen, D. Energy 2004, 29, 1249–1257. (9) Dupart, M. S.; Bacon, T. R.; Edwards, D. J. Hydrocarbon Process. 1993, 72 (5), 89–90. (10) Erga, O. Ind. Eng. Chem Fundam. 1989, 25 (4), 692–695. (11) Bekassy-Molnar, E.; Marki, E.; Majeed, J. G. Chem. Eng. Process. 2005, 44, 1039–1046. (12) Glasscock, D. A.; Rochelle, G. T. AIChE J. 1993, 39 (8), 1389– 1397. (13) Eimer, D.; Sjøvoll, M.; Eldrup, N.; Heyn, R.; Juliussen, O.; McLarney, M.; Swang, O. New thinking in CO2 removal. Nordic Symposium; Oct 2003. (14) Kohl, A; Nielsen, R. Gas Purification, 5 ed.; Gulf Publishing Company: Houston, TX, 1997. (15) Lee, J. W. A Novel Strategy for CO2 Sequestration and Clean Air Protection. In First National Conference on Carbon Sequestration, Oak Ridge, 2001; pp 1-15. (16) Lin, S. H.; Shyu, C. T. Waste Manage. 1999, 19, 255–262. (17) Ramachandran, N; Aboudheir, A; Idem, R; Tontiwachwuthikul, P. Ind. Eng. Chem. Res. 2006, 45 (8), 2608–2616. (18) Rochelle, G. T. Oxidative Degradation of Monoethanolamine. In First National Conference on Carbon Sequestration, Washington, DC, 2001. (19) Strazisar, B. R.; Anderson, R. R.; White, C. M. Energy Fuels 2003, 17 (4), 1034–1039. (20) Svendsen, H. F.; Hoff, K. A.; Poplsteinova, J.; da Silva, E. F. Absorption as a Method for CO2 Capture. In Second Nordic Minisymposium on Carbon Dioxide Capture and Storage, Goteborg, 2001, 24-29.
r 2009 American Chemical Society
(21) Thuy, L. T.; Weiland, R. H. Ind. Eng. Chem. Fundam. 1976, 15 (4), 286–293. (22) Tomcej, R. A.; Otto, F. D. AIChE J. 1989, 50 (5), 861–864. (23) Weiland, R. H.; Dingman, J. C.; Cronin, B. J. Chem. Eng. Data 1997, 42 (5), 1004–1006. (24) Weiland, R. H.; Rawal, M; Rice, R. G. AIChE J. 1982, 28 (6), 963–973.
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: DOI:10.1021/ef900564x
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Figure 1. pH swing process in conventional solvent absorption system.13
absorption prior to regeneration. It is expected that the pH control agent will dissolve in the solution with the temperature increase and will remain dissolved in the solvent until being passed through the heat exchanger on return to absorption. Removal of the pH control agent from the system flow can be achieved by a drop in temperature and drop in solubility, bringing about crystallization from out of the solvent before returning to the absorption column. The idea of reducing the energy requirement to regenerate the CO2 capture solvent by reducing the pH is a relatively new theory and, as such, there is no specific information available from previous research. The most relevant link to current developments in pH swing is the sodium citrate and sodium adipate processes10-12 used for SO2 extraction from flue gas streams in a near identical fashion to CO2 extraction. The sodium citrate process operates like a conventional system with selective absorption of SO2 taking place through flue gas contact with an aqueous solvent followed by an energy input in the form of steam to regenerate the solvent and liberate the SO2. The SO2 is extracted with the steam and separated through condensation of the water vapor to leave the pure SO2 ready for storage. The process offers many advantages to industrial applications for flue gas desulfurization, which is an important environmental requirement in light of the contribution of SO2 to acid rain. The sodium citrate process offers several advantages, including a high gas loading capacity and low solvent oxidation losses, however these attributes are beset by the high energy requirement of the system, used to produce the steam. The large energy requirement presents itself as a significant cost item in the sodium citrate method. The sodium adipate process10 is a modification of the sodium citrate process that is able to achieve a significant decrease in energy usage through the introduction of adipic acid to the system. The sodium adipate process operates in the same way as the sodium citrate process but involves the addition of adipic acid to the SO2-loaded solvent solution prior to steam stripping. The lowering of the pH in this solution results in a significant saving in steam required for regeneration while maintaining all of the inherent advantages of the sodium citrate processes. The relatively low cost of the adipic acid (C6H10O4) together with the
over all surface areas. Corrosion occurs naturally, however in this system corrosion rates are increased by higher solvent concentration, higher acid gas loading, higher operating temperatures, and a higher pH. Research has shown that it is not the amines themselves that increase the rate of corrosion, however it is the free acid gas that is released from the amines.9 • High regeneration energy. The chemical absorption process is hindered by several factors that limit the process efficiency and ease of operation. The main limitation arises from the need to use a solvent with high reaction energy to extract the dilute carbon dioxide in the flue gas stream. The unavoidable flipside to this is that same amount of energy is needed to put back into the system in regeneration at significant cost to the operator. The amount of energy required for regeneration accounts for roughly 70-80% of the total operating cost of the system. The large amount of low-grade heat is supplied to the system via the regeneration steam cycle and partially conserved through use of heat exchange where possible. The practicality of the process is often limited to facilities that can easily supply bulk amounts of low-grade heat, such as power plants. The high regenerative energy requirement and associated high operating cost is the single greatest barrier to chemical absorption being a widespread and effective CO2 capture solution. A range of work is ongoing to address the above limitations, for example new solvents are being developed to reduce degradation and corrosion.6,8 A new method, that is, pH swing method,13 has also been proposed to reduce the energy requirement of regeneration. This work intends to evaluate the effectiveness of the pH swing method in energy savings. The principal feature of the pH swing process is the addition of a pH control agent or acid to the system to lower the pH of the solution to be regenerated in order to reduce the amount of energy needed compared to conventional methods.13 The integration of the pH swing process to the conventional solvent absorption system (as shown in Figure 1) involves the addition of the pH control agent to the saturated solvent-CO2 mixture in transit from the bottom of the 214
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: DOI:10.1021/ef900564x
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savings in energy from the reduced steam requirement found experimentally mean that this modification is economically beneficial. Apparently, in order to evaluate the feasibility of the pH swing method for CO2 capture, the following questions need to be answered: • Are there any acids that can reduce the regeneration energy significantly? • Can the acids be recovered easily? • Would the residue acids in the solvent affect the absorption performance largely?
2.3. Absorption Stage. The purpose of the absorption experimentation stage was to prepare a large volume of sufficiently CO2-loaded solvent solution to then be divided up into samples and individually used in regeneration. The first step in the absorption experimental process was the calculating and measuring 160 mL of solvent solution based on the solvent concentration selected, either 1.1 M (30 wt %) or 2 M. The volumes of MEA and distilled water were measured and poured into an absorption unit flask, and the whole absorption system was flushed with nitrogen until the CO2 concentration reached zero. A mixture of CO2 and nitrogen (CO2 concentration of 20 000 ppm) was then introduced, and the concentration of CO2 was monitored continuously until the concentration reached 20 000 ppm. The amount of CO2 absorbed by the solvent was calculated based on the CO2 concentration profile. Three solvents were prepared with different CO2 loadings at room temperature (25 °C). 2.4. Regeneration Stage. Experimentation was conducted prior to the regeneration experimentation regarding the behavior of the three acids when mixed with the loaded solvent solution. It was noted that while suberic acid and phthalic acid showed little reaction and dissolved in the solution readily, oxalic acid produced a violent reaction when mixed with the solution, which involved violent fizzing, generation of heat, and the release of a sweet smelling gas, inferred to be an ester. The instability of the oxalic reaction justified the discontinuance of regeneration experimentation using that acid. The regeneration experiments were carried out using both concentrations of solvent solutions in 15 mL samples by heating the samples from room temperature to 95 °C, first with control runs with no acid added and then with suberic (2, 4, or 6 g and 0.5, 1, 2, 3, 4, or 6 g) and phthalic acid (2, 4, or 6 g) of varied amounts. The first step of experimentation was to pipet out a 15 mL volume of the loaded solvent and transfer it to the regeneration unit. The required amount of acid was then weighed out and added to the solvent solution, and the regeneration unit was sealed. Once the regeneration unit was sealed and connected in with the rest of the apparatus, nitrogen gas was passed through the system to bring the CO2 concentration readings to a minimum level as a reference point. When the CO2 level had dropped to the reference point, the regeneration unit was submerged into an oil bath controlled at 95 °C. The regeneration unit was left for a period of time in the oil bath while the CO2 levels from the outflow and temperature were monitored. At the end of the period of time given to regeneration, the CO2 and temperature data logging was stopped and the nitrogen flow was closed off. At this point the regeneration unit was opened up and, while still in the oil bath maintaining the solution at 95 °C, the pH of the solution was measured using a pH meter. The regeneration unit containing the partially regenerated solvent was then cooled and the volume of the solution inside was measured. The time period for the experiments was kept at 10 or 20 min. Randomly selected experiments were repeated, and it was found the difference was within 10%. The reported results are the average values in the case multiple tests were run under the same conditions.
As the initial step of the pH swing method project, this work attempts to answer the first question, that is, whether we can find any acids that can reduce the regeneration energy requirement for CO2 desorption. A few weak acids were selected and tested for the effect on the regeneration energy requirement of a conventional CO2 solvent. 2. Experimental Section The experimental steps are as follows. First of all, typical solvents of different concentrations were prepared. Subsequently the solvents were subject to CO2 absorption. Third, selected acids were added into the solvents, and the energy consumed for CO2 desorption for the solvents was then obtained and compared with the case when no acid was added. 2.1. Solvent Selection. The chemical solvents commercially available by order were alkanolamines: monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA). There are currently solvent absorption facilities in three power stations in the U.S and six other heavy industries worldwide that all use MEA as the chemical solvent.6 It is apparent from this that MEA is the industry standard, making it the most suitable solvent to experiment with. There is far more research literature related to the properties of MEA than any other solvents as well, therefore MEA was selected as the solvent. MEA is a good solvent to use also because of its high absorption performance and high regenerative energy requirement. These properties allow for the most effective absorption and will serve to highlight the regenerative energy savings. 2.2. pH Control Agents. Three weak acids selected in this work were suberic acid (C6H12(COOH)2), phthalic acid (C6H4(COOH)2), and oxalic acid (HOOC-COOH). The selection criterion is based on the properties of adipic acid, in particular, the wide pH buffering capacity, the steep solubility, and the monetary cost. The three acids were selected because they are dicarboxylic acids similar in chemical structure to adipic acid. The basic structure of dicarboxylic acids is HOOC-RCOOH where R consists of a hydrocarbon chain of varying length. In solution the acid dissociates through ionization of the carboxyl groups on either end of the molecule in the form HOOC-R-COOH f HOOC-R-COO- þ Hþ. Ionization of the second carboxyl group occurs less readily than the first. The steep solubility curve is a key requirement for the pH swing process so that the acid can be effectively removed through crystallization by a drop in temperature. The three acids exhibit the desired solubility over the operating temperatures of the solvent absorption process. At the high temperatures the acids are highly soluble in solution. However this solubility drops steeply with decreasing temperature, so that at the low temperatures of the process the acids are far less soluble and precipitate out as crystals. The final criterion, which is important from a commercialization stand point, is the price and availability of the acids. All three acids are relatively inexpensive and are available by order in small quantities; however, a detailed cost analysis for bulk supply is required to confirm the financial viability on an industry scale.
3. Results and Discussion 3.1. Absorption Results. The following observations were noted during absorption experimentation: • Heat is released when water is mixed with MEA. • Solution increases in viscosity with absorption. • Solution changes color and texture with greater absorption; Small bubbles are evident throughout solution. • If left over longer periods of time, the loaded solvent solution changes color to deep yellow/amber. 215
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Table 1. Volume of CO2 Absorbed into Solution in Samples 1-3
sample ID sample 1 sample 2 sample 3
MEA total concen- volume absorption tration (mL) time (min) 2M 1.12 M 2M
160 160 160
125 90 170
volume of CO2 absorbed in solution (mL)
moles of CO2 absorbed in solution
5872.4 3312.8 6725.9
0.2622 0.1479 0.3003
Table 3. Regeneration of Sample 2 acid none suberic suberic suberic phthalic phthalic phthalic
mass of acid start volume finish volume finish (g) (mL) (mL) pH 0 2 4 6 2 4 6
15 15 15 15 15 15 15
10.5 11.5 13 14.5 12.2 12 14
N2 flow reading
9 8.66 8.44 8 8.7 8.6 7.8
150 150 150 150 150 150 150
Table 2. Regeneration of Sample 1 acid none suberic suberic suberic none phthalic phthalic phthalic
mass of acid start volume finish volume finish (g) (mL) (mL) pH 0 2 4 6 0 2 4 6
15 15 15 15 15 15 15 15
8 10 11 12 8.5 15 10.5 11.5
10.45 9.1 8.75 8.28 9.48 8.9 8.7 8.6
Table 4. Regeneration of Sample 3
N2 flow reading
mass of acid start volume finish volume finish acid (g) (mL) (mL) pH
150 150 150 150 150 150 150 150
none suberic suberic suberic suberic suberic suberic
The observations were consistent with the previous work,20-22 indicating the reaction between CO2 and MEA. Three samples were produced with different MEA concentrations and CO2 loadings, as listed in Table 1. It can be noted that the volume of CO2 absorbed into the solution is consistent with the molar fraction of the solvent MEA in solution and the amount of time given to the absorption process. The calculation of moles of gas is under the assumption that CO2 is an ideal gas. This data can be used to determine the amount of CO2 contained within each 15 mL sample used in regeneration and compared to the amount released in each experiment to find the regeneration efficiency. 3.2. Regeneration Results. The three CO2-loaded solvent samples were divided into 15 mL samples and tested for regeneration. The following observations were noted during regeneration experimentation: • A significant amount of heat was released by the solution from the addition of the acid. • There is an instant release of CO2 from the solution corresponding to the heat release from the addition of the acid. • Solution increases in viscosity throughout regeneration. • Solution changes color to deep amber throughout regeneration. • Acid appeared to be fully dissolved at higher operating temperatures. • Appeared to be some crystallization of acid on cooling of the solution after regeneration. • The amount of water was reduced during regeneration.
0 0.5 1 2 3 4 6
15 15 15 15 15 15 15
10.5 11.5 12 13.5 12 14.5 15
N2 flow reading
9 8.9 8.85 8.7 8.5 8.37 8
160 160 160 130 160 160 160
Figure 2. Desorption of CO2 from sample 1 (2 M MEA).
absorbed CO2, was calculated based on the above results and is shown in Figure 8. It can be seen clearly from the figure that: • The regeneration efficiency increases with the increase of acid amount in the solvent, for both acids and three samples. For sample 1, the increase is more than 400% with 6 g of suberic acid added. • It appears that suberic acid is more effective than phthalic acid when the same amount of acid is added. • It appears that the regeneration efficiency is higher for the solvent with lower CO2 loading, if we compare sample 1 with sample 3.
The major results are shown in Tables 2-4 and Figures 2-6. It can be observed immediately that there is an increase in CO2 output with an increase of acid in the solution, or a decrease of pH level in the solution. The apparent trend shows in all cases except for 6 g of suberic acid in sample 2. It was also noticed the each experiment resulted in an almost identical temperature curve for the regeneration process, as shown in Figure 7. There appeared only to be a slight difference in the maximum temperature achieved by the control samples and the samples containing the acid. 3.3. Regeneration Efficiency. The regeneration efficiency, defined as the ratio of the amount of desorbed CO2 to that of
The mechanism of the effect of acid addition on the increase of regeneration efficiency is most likely to be attributable to the shift of the overall reaction between CO2 and the solvent toward the release of CO2 as discussed below, though no detailed quantitative work has been conducted to confirm this. The predominant reversible reactions that take place in the aqueous solvent solution to absorb the CO2 using MEA are as follows:2,21 216
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Figure 5. CO2 desorption from sample 2 (1.12 M MEA).
Figure 3. Desorption of CO2 from sample 1 (2 M MEA).
Figure 6. CO2 desorption from sample 3 (2 M MEA). Figure 4. Desorption of CO2 from sample 2 (1.12 M MEA).
Deprotonization of the MEA molecule to form a stable carbamate ion. CO2 þ RNH2 f Hþ þ RNHCOO -
ð1Þ
Further deprotonization to form another stable ion RNHCOO - þ RNH2 f RNH3 þ þ RNCOO2 -
ð2Þ
Reaction of carbamate ion with water molecule RNHCOO - þ H2 O f H3 Oþ þ RNCOO2 -
ð3Þ
Formation of carbonate ion from the reaction of carbon dioxide with water CO2 þ H2 O f Hþ þ HCO3 -
ð4Þ
The overall affect of these reactions is the formation of carbamate and carbonate ions from reactions with CO2 with the release of heat. All of these reactions are reversible, and the reverse reactions are activated with the addition of heat into the system in the regeneration process. When an acid is added into a solution, dissociation occurs to produce hydrogen ions (Hþ), the concentration of which is measured on the pH scale. The addition of an acid to this system of reactions has the effect of increasing the concentration of Hþ ions and causing a disturbance to the
Figure 7. Temperature variation of solvent during regeneration.
equilibrium of reactions 1, 3, and 4. In all of these reactions, the equilibrium shift brought about by the Hþ concentration increase favors the reverse reactions that take place to release the CO2 molecules. In effect, the addition of the acid shifts the equilibrium to provide a driving forcing for the 217
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in absorption. The heat of reaction is the energy transferred into the chemical reactions for the regeneration and release of the CO2 molecules. The heat of reaction term can be calculated from available data relating to the chemical energy requirement per unit volume of CO2. This heat of reaction term can be multiplied by the volume of CO2 calculated for each experiment. The heat of reaction term was found from research literature to be 65 KJ/mol.24 In the regeneration process a significant amount of heat is transferred into heating water into steam. The latent heat of vaporization is the amount of heat required to produce the steam, the amount of which will vary depending on the concentration of the solvent mixture, for example, 30 wt % MEA will have a smaller latent heat of vaporization than 10 wt % MEA. The latent heat of vaporization can be calculated from the standard heat of vaporization value for water given as 2.26 KJ/g and the amount of water vaporized in each experiment. Because of the high boiling point of MEA at roughly 170 °C and the fact that the regeneration temperature only reached 90 °C, the assumption can be made that the volume loss in solution over regeneration is solely water. It would be expected that a very small amount of solvent would be vaporized; however, it is not practically possible to determine the volume fraction of the solvent and the water in the solution loss. In a similar way, the latent heat of vaporization of solvent is heat absorbed in the vaporizing of usually a very small amount of the solvent. While this is unintentional in the regeneration process, it can be presumed that a small fraction of the liquid solvent changes phase to gas. The total energy transferred into the solvent solution in the regeneration process is calculated by summing each of these terms together for each experiment sample. From this total energy input value and data for the corresponding molar volume of CO2 released, the energy required per mole of CO2, and the energy efficiency, can be found for each sample. The detailed calculation results are shown in Table 5. It can be seen that the major energy consumption is in heat of vaporization, and the addition of acid reduces this term significantly. The effect of the type and the amount of added acid on the experimental sample’s total energy and energy efficiency is shown in Figure 9. It can be observed that: • There is a general trend that the total energy consumption decreases with the increase of acid amount for both suberic acid and phthalic acid, whereas the energy efficiency increases with the increase of acid amount. For sample 3, the decrease in energy consumption is more than 1400% with 6 g of suberic acid added. • The suberic acid is more effective in reducing the energy consumption, or the energy efficiency is higher for suberic acid, compared with phthalic acid. • The energy consumption is very high, with an average value of 3 MJ/mol CO2. This value is significantly higher than the reported value for the energy requirement for CO2 desorption from solvent, about 80 KJ/mol CO2. A detailed analysis of the energy consumption items reveals that most of the energy is consumed to generate steam in our experimental system (Table 5). It is expected that the figure will be much smaller when steam is used directly to heat up the solvent. • The energy efficiency is found to be low, less than 10% in most of the cases. This is again due to that most of the input energy is used to generate steam rather than for the desorption reaction. It is expected that the energy
Figure 8. Regeneration efficiency, that is, the ratio of the amount of CO2 desorbed to that of absorbed CO2, as a function of acid amount in the solvent.
regeneration reactions, and this in turn reduces the energy barrier to initiate these reactions. The regeneration efficiency is lower than 40% in most of the tests. This is not unexpected because the energy transfer in the experiment is mainly through conduction, which is a slow process. In a practical system, heat will be transferred mainly through convection and mass transfer, and thus the regeneration efficiency should be much higher within the same time period. 3.4. Energy Efficiency. The total energy absorbed by each solvent and the corresponding energy efficiency that is defined as the ratio of the energy used for CO2 desorption (heat of reaction) to the total energy absorbed by the sample per mole of CO2 released are calculated. The total energy absorbed by the solution in the regeneration process can be calculated from summing the energy components in the solution that are the sensible heat of solution, the heat of reaction, the latent heat of vaporization of water, and the latent heat of vaporization of solvent, that is, total energy ¼ heat of reaction þ latent heat of vaporization of water þ latent heat of vaporization of solvent þ sensible heat of solution Subsequently, the energy efficiency is calculated as, heat of reaction energy efficiency ¼ total energy The sensible heat is the heat required to raise the rich CO2-solvent mixture to the right temperature for stripping to take place. The sensible heat term is calculated using the heat capacity of the solution and the temperature increase observed in the experimentation. From available specific heat capacity curves for the two different molar concentrations of solution 2 and 1.12 M7,23 and the experimental temperature data, the sensible heat can be calculated. Because all of the experiments were subject to the same temperature increase, the only difference affecting the sensible heat term is the molar composition of the solvent solution. The heat of reaction is the heat absorbed in the endothermic reverse reactions to those that have taken place 218
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Table 5. Energy Consumption during Regeneration of Solvent Samples acid
mass of acid (g)
heat of reaction energy term (KJ)
heat of vaporization energy term (KJ)
sensible heat energy (KJ)
total energy (KJ)
released CO2 (mmoles)
energy cons. (MJ/mol CO2)
3.59 3.59 3.59 3.59 3.59 3.59 3.59
19.58 15.33 13.11 11.04 17.50 14.00 11.84
2.62 6.81 7.31 10.36 5.32 3.64 5.27
7.48 2.25 1.79 1.07 3.29 3.84 2.25
3.83 3.83 3.83 3.83 3.83 3.83 3.83
14.09 11.93 8.64 5.17 10.32 10.78 8.56
1.39 2.86 4.47 3.14 2.43 2.57 3.14
10.12 4.17 1.94 1.65 4.24 4.19 2.72
3.59 3.59 3.59 3.59 3.59 3.59 3.59
13.87 11.63 10.59 9.42 10.64 5.06 4.09
1.65 2.08 3.37 2.73 4.24 5.20 7.66
8.39 5.60 3.15 3.44 2.51 0.97 0.53
Sample 1 none Suberic Suberic Suberic Phthalic Phthalic Phthalic
0 2 4 6 2 4 6
0.17 0.44 0.48 0.67 0.35 0.24 0.34
15.82 11.30 9.04 6.78 13.56 10.17 7.91
none Suberic Suberic Suberic Phthalic Phthalic Phthalic
0 2 4 6 2 4 6
0.09 0.19 0.29 0.20 0.16 0.17 0.20
10.17 7.91 4.52 1.13 6.33 6.78 4.52
0 0.5 1 2 3 4 6
0.11 0.13 0.22 0.18 0.28 0.34 0.50
10.17 7.91 6.78 5.65 6.78 1.13 0.00
Sample 2
Sample 3 none Suberic Suberic Suberic Suberic Suberic Suberic
that the heat transfer rate is quite different from that in practical systems, as discussed before. This causes both the regeneration efficiency and the energy efficiency to be much lower than the expected values in practical systems. However, the results reported here do show without a doubt that acid addition has a pronounced effect on the regeneration and energy efficiency. Therefore, further work is being conducted to investigate in more detail the pHS method, including testing of more acids to identify the most suitable one, crystallization of the acids, chemistry of pH swing, effect of pH swing on corrosion, economics of the method, and application of the method for other solvents, etc. It should also be pointed out that the addition of acid might have less effect on CO2 desorption in a practical system than it was shown here because of the uncertainties discussed above. Further work is needed to test relatively larger systems that represent practical conditions. 4. Conclusions
efficiency will be much faster in practical systems in which the heat transfer rate is higher.
The concept of utilizing a swing in pH in the solvent absorption process for CO2 capture was investigated. The experimental results proved to substantiate the theory of pH swing and proved that the concept was indeed valid. The experimental evidence showed that a significant energy saving could be achieved through a swing in pH in the solution in the regeneration process. Experimentation that was carried out was able to achieve a maximum energy efficiency increase of 1400%, in terms of KJ of energy per mole of CO2 released, relative to the conventional process. The increase in energy efficiency was contributed to by the increase in volume of CO2 release and the decrease in heat energy consumption. The decrease in heat energy consumption was primarily due to a drop in water vaporization. It is considered beneficial to continue the investigation of the pH swing method in more detail.
3.5. Discussion. A few uncertainties exist in the preliminary experiments conducted in this work. The main uncertainty is
Acknowledgment. The financial support of the University of Queensland is gratefully acknowledged.
Figure 9. Effect of acid type and amount on total energy consumption and energy efficiency.
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