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Exergy Analysis of Amine-Based CO2 Removal Technology
Greenhouse Gas Control Technologies, Volume I J. Gale and Y. Kaya (Eds.) © 2003 Elsevier Science Ltd. All rights reserved
121
EXERGY ANALYSIS OF AMINE-BASED CO2 REMOVAL T E C H N O L O G Y F.H. Geuzebroek L.H.J.M. Schneider and G.J.C. Kraaijveld TNO-Environment, Energy and Process Innovation Laan van Westenenk 501, 7325 NE Apeldoorn, the Netherlands.
ABSTRACT An exergy analysis is conducted of a gas-fired CO2 capture process based on MEA absorption technology. It is shown that with the combination of AspenPlus and ExerCom it is possible to get a reasonable result of the exergy loss per unit operation. To enable this, certain modifications were made in the thermodynamics of the reaction of CO2 with MEA. In this reaction ionic species are formed and a approach enabling the use of electrolytes is necessary. The results show that the major sources of exergy loss are the absorber, the flasher and the flue gas blower. INTRODUCTION Absorption in alkanolamines like MonoEthanolAmine (MEA) is the currently preferred option for the removal of low-concentration C02 from flue gases using the following reaction: CO2 + 2MEA-> M E A C O O + MEAH + The MEA process is well-established, but the costs and the energy consumption of CO2 removal is substantial, typically 4 - 5 MJ/kg CO2. This energy consumption leads to a substantial parasitic power loss when applied in the capture of CO2 from flue gases of power plants. This is a strong driving force to optimise the process or to find alternative ways of capturing the CO2. A tool for optimising the energy consumption is the use of exergy analysis [ 1,2]. Exergy is defined as the actual work potential or maximum work available from a certain gas or liquid stream. Loss of exergy means loss of work potential and this loss has to be generated elsewhere by eventually primary energy. As a process analysis tool, exergy analysis has the advantage that it makes clear what makes a process efficient or inefficient. It should be pointed out that an analysis using second law type of consideration for the MEA system has been given by Leitas [3]. EXERGY ANALYSIS Exergy analysis can be performed when the composition and the physical properties of all the relevant streams of the capturing process are available. For that purpose the AspenPlus flow sheet program is used. ExerCom by Jacobs Engineering can perform exergy analysis as an add-on. It uses the output of AspenPlus and calculates the chemical, physical and mixing exergy for each gas and liquid stream in the process. The exergy AEx is a measure of the work potential at a certain state, relative to the reference state: AEx = AH- ToAS The choice of the reference by ExerCom is that defined by Szargut [ 1], i.e. the reference temperature To = 298.15 K and pressure Po = 101.325 kPa. For chemical equilibrium as a reference state the mean
122 composition of the earth atmosphere, the mean composition of seawater and the mean composition of the earth's crust is taken. By calculating the difference between the exergy of the input and output streams of a unit's operation, the exergy loss of each individual unit can be calculated. For the overall flow sheet, the exergy analysis has already been made and total exergy loss of about 1 MJ/kg CO2 has resulted [4]. When an exergy analysis is made on the level of individual unit's operations, it appears that the standard thermodynamics to describe the reaction of C02 with MEA are incomplete due to the incorporation of electrolytes.
Exergy analysis using AspenPlus and ExerCom: modification to the thermodynamic system Thus, to be able to make the calculation of the exergy of systems with the reaction CO2 with MEA, it is necessary that certain modifications are made to the thermodynamic system supplied by AspenPlus for this reaction. This is due to the fact that in the reaction, ionic species are formed, which have to be treated as Electrolytes. To the database of chemical exergy provided by ExerCom the component MEA has to be added. In more detail these modifications are: To calculate the mixing exergy by ExerCom in AspenPlus it is necessary to know the so-called infinite dilution aqueous phase Gibbs Free Energy of Formation (DGAQFM) of the ionic species MEAH + and MEACOO-, which are not present in the MEA data package of AspenPlus. These values can be derived from the equilibrium constants of the reaction, given by ApenPlus, if sufficient data are known: G* = ]~(Gproducts) - ~](Greactans)=- RT. In(K)
However, for the MEA system three necessary values of DGAQFM are not known (MEA, MEAH +, MEACOO), while there are only two independent reaction equations available. To circumvent this problem, an alternative way to calculate the DGAQFM for MEA is proposed using data generated by AspenPlus for a mixture of water and MEA, according to standard thermodynamics rules. Using this value, the two unknown DGAQFM values are obtained from the two independent reaction equation constants. DGAQFM for MEAH + is-500.504 kJ/mol and for MEACOO- the value is-196.524 kJ/mol. With the modifications to the AspenPlus model it is possible to perform sound calculation fulfilling not only the first law of thermodynamics, but also the second law of thermodynamics. A second addition is the chemical exergy of the MEA molecule in the liquid phase. Without this value ExerCom cannot make the calculation. Although there is substantial information in the literature, MEA could not be found. For this molecule, the group contribution method was used [ 1]. The value used is 1536 kJ/mol.
AspenPlus calculations The AspenPlus calculations were performed using so-called apparent components. This means that the results are reported in terms of neutral components (MEA and CO2) and not in terms of the electrolytes. This mode of calculation enables much faster convergence, without loss of quality of the result. It should be noted that the change in exergy of a stream containing electrolytes will be found in the mixing exergy, in stead of in the chemical exergy. A manual calculation revealed that this approach is a reasonable assumption. REFERENCE CASE. As a reference case, field data from a CO2 capture plant has been used. The data has been taken by Fluor from the plant at Bellingham [5] producing 320 ton/day of CO2. The plant uses flue gas from a gas-fired plant containing about 3.5 % o f CO2. To establish the validity of our AspenPlus modelling, a validation of an AspenPlus flowsheet of the process is made (see Figure 1).
123
~-----I CW-F:INW "C
~
CW-F-OUT
I"-F~s" I °
I°FF~s~ I
H2o-scR8
I
OF OA - I ~
' C
O
~ - t FLUEGAS
~
°
[ S~ER
,so*;E.
11 F~T
z--~ _
1
A oOO'E"W
]-[ HX
PUMP IW-CF-OUT I"
Figure 1" Flow sheet used in the AspenPlus calculation As can be seen in Table 1, the AspenPlus flow sheet does predict reasonably well the plant data. The main difference is the higher steam consumption. Another deviation is the higher loading of the solvent. This can be explained by the fact that in AspenPlus, equilibrium data is used that slightly deviates from reality, as has already been shown [5]. We consider the results such, that a good impression of the abilities of the exergy analysis is obtained by using this data. In a next step, the AspenPlus flow sheet should be further optimised. TABLE 1 VALIDATION OF AspenPlus FLOWSHEET STREAM Gas Solvent
Steam
CO2 in C02 out Flow C02 in CO2 out Net loading Flow Energy consumpt.
FIELD DATA 3.13% 0.54% 241.6 m3/h 0.42 mol/mol 0.16 mol/mol 0.26 mol/mol 27000 k~hr 4.3 MJ/kg C02
AspenPlus 3.13% 0.51% 241.6 m3/h 0.48 mol/mol 0.21 mol/mol 0.27 mol/mol 30000 kg/hr 4.9 MJ/kg CO2
RESULTS OF EXERGY ANALYSIS Based on the data of the reference case, ExerCom is used to determine the Exergy losses of the several unit operations in the flow sheet. The result of the total exergy loss is depicted in Table 2 and Figure 2. A crucial matter is how to treat the utilities in terms of where the system boundaries are set. Two eases are given: 1) DUTY/CARNOT. In this case the exergy loss in the utilities is ignored, i.e. a 100% efficiency is taken. For the different utilities this means: • Steam: the duty calculated by the AspenPlus unit operation is corrected for by the Carnot efficiency at the steam temperature of 120 °C.
124 •
•
Cooling water: same as for steam. In practice this means that the outgoing cooling water stream has a higher exergy than the incoming stream. In practice this stream usually has no significant value. Electricity: the efficiency of electricity generation is ignored.
The results reveal that the absorber (32%) and the flasher (24%) are the main sources of exergy loss. The feed cooler (18%) and the blower (16.5%) are also contributing strongly. An interesting result is that the exergy loss in the stripper (with reboiler) is very small. The total exergy loss is a little more than 1.4 MJ/kg CO2. Unit operations like the pump, the scrubber and the solvent cooler are minor contributors to the exergy loss. 2) WITH UTILITY LOSSES. The part of the exergy losses taking place at the utility side of the operation are now taken into account: • Steam: steam of 140°C is condensed at 130°C. It is assumed that the resulting stream can still be reused. The exergy loss to generate the steam of 140°C is ignored. It depends on the local situation how this should be treated (steam available from power station or generated locally by boiler). For the current exercise we ignore this part. • Cooling water. The outgoing cooling water stream usually represents a certain amount of exergy, but is without any real value. This is taken into account by calculating the exergy of the real streams of the cooling water streams of the cooler and the flasher (CW-F-IN/OUT and CW-CIN/OUT). Now the external streams result in an exergy loss instead of a gain, like in the situation when using duty. • Electricity: in this paper we ignored further exergy losses combined with electricity power generation The exergy loss increases about 0.3 MJ/kg CO2 by taking into account the losses in the utilities. The main shift is in the flasher, which now accounts for about 8% of the exergy loss. The flasher increases to 28%, while the contribution of the absorber has decreased to 26%. The feed cooler and the blower with 13-15% are still considerable loss factors, while the other unit operation leads to losses well below 10% of the total. A separate item are the exergy losses due to the consumption of MEA. We have assumed a consumption of 1.5 kg MEA per ton CO2 [7]. Using the chemical exergy of MEA in the liquid phase of 1536 kJ/mol, an exergy loss of about 0.03 MJ/kg CO/results, i.e. about 2 % of the total loss. Exergy loss divided per component Exergy consists of three table components: chemical, physical, and mixing exergy. To observe how the exergy loss is split up into these components, the exergy losses of the individual unit operations have been added. Chemical and mixing exergy are combined. The exergy loss attributed to electrical power is given as a separate item. External losses associated with the other utilities are mainly added to the physical exergy loss. The result is given in Table 3, using the two methods given above. As is clear, the exergy loss due to the electrical power loss is a major contributor to the exergy loss, contributing about 30-40% when the efficiency of generation is taken into account. Physical exergy loss contributes about 40%-50%, while the chemical exergy loss is actually rather small (20%). DISCUSSION The AspenPlus flow sheet overestimates the energy consumption of steam with about 0.5 MJ/kg CO2.In exergy terms this corresponds to an additional exergy loss of about 0.1 MJ/kg CO2. We assume that this difference does not effect the major observations of this study. The exergy analysis gives a clear insight where the actual exergy loss takes place. An interesting observation is that in the stripper and reboiler, the exergy losses are rather small, while in the flasher, the loss is substantial. This situation is similar to the case of distillation, as has been pointed out by, for instance ref [6]. To minimize the total exergy loss of a process, the exergy loss should be evenly divided over and in
125 the unit operations. In fact, large exergy loss in certain parts of the equipment corresponds to a large local driving force. This is unfavorable for total loss of exergy. To optimize the exergy loss the driving force should be lowered and equally divided over the reboiler, stripper and flasher. This can for instance be realized by integrating heat and mass transport in the stripper column (see [3,6]). On the other hand, lower driving force means a larger area for mass transfer and thus additional cost for internals. To optimize these two counteracting factors, an economic evaluation is necessary. Another observation is that the contribution of the power needed for the flue gas blower has a large contribution in exergy terms. It should be noted that the energy consumption of the blower in this plant is about 0.16 kWh/kg CO2, while in literature values of 0.12 to 0.14 kWh/kg CO2 are reported for gas fired systems [7]. Thus, a lowering of about 25% can be realized by applying state-of-the-art technology. Nevertheless, the blower and thus the pressure drop over the absorber remains a major item for optimization. It should be noted that obviously the CO2 compressor is an additional source of exergy loss. Values of the same order as the flue gas blower are expected to occur. Finally, it should be noted that the contribution of the chemical/mixing exergy loss is relatively small. Thus it may be prudently concluded that changing the chemical used in the capture process does not have the highest priority. Physical and electrical should be treated first. We want to stress that this hypothesis should be verified first by applying the analysis to other types of amines. An important remark is that the exact interpretation of the results is influenced by the exact definition of the system boundaries. In this paper, two cases are treated, but to get a good picture, the MEA capture system should be integrated with the other processes such as electricity generation, a chemical plant or whatever CO2 emitting system is evaluated. Another factor is the source of the flue gas. For coal-fired plant, the situation is quite similar and the analysis can be applied easily. FINAL R E M A R K S The combination AspenPlus-ExerCom is now accessible to perform optimising studies using exergy analysis for amine-based CO2 removal technology. It can be used to find ways to lower the energy consumption and coupled parasitic power loses. Options are the combination of heat and mass transfer in the stripper and absorber, as recently suggested for distillation, the use of amines with lower binding energy, the applications of split-flow, the use of the heat available in the flue gas for regeneration and vapour recompression. Integration of the CO2 capture unit with the process it is part of, can be easily realised. TABLE 2 TOTAL EXERGY LOSS BY UNIT OPERATION DUTY/CARNOT MJ/kg CO2 0.259 0.233 0.459 0.005
Pump Heatex Cooler
18.36% 16.51% 32.53% 0.35%
WITH UTILITY LOSSES MJ/kg CO2 0.259 0.233 0.459 0.005
14.94% 13.44% 26.47% 0.29%
0.002 0.093 0.003
0.14% 6.59% 0.21%
0.002 0.093 0.046
0.12% 5.36% 2.65%
Stripper Flasher
0.014 0.343
0.99% 24.31%
0.140 0.497
8.07% 28.66%
Total MEA losses Total with MEA losses
1.411 0.030 1.441
100.00 o
1.734 0.030 1.764
100.00% 1.75%
FeedCooler Blower Absorber Scrubber
..
.
2.15%
126
0.5
0.25
"
"
0
,
-~
l
i
~
I
I
I
I
l
]
I
ll DUTY/CARNOT [] WITH UTILITY LOSSESJ
I
<.Q mo
Total exergy loss per unit operation of the system of the flow sheet in figure 1. Black bars correspond to the case were the utilities are taken into account as duties corrected by Carnot efficiency. White bars take into account the external losses in the utilities. Figure 2:
TABLE 3 EXERGY LOSS SPLIT UP IN COMPONENTS (IN MJ/kg CO2)
DUTY/CARNOT Chemical/mixing 0.32 Physical 0.56 Power/Electric 0.57 Total 1.44
22% 39% 39%
WITH UTILITY LOSSES 0.35 0.85 0.57
20% 48% 32%
1.76
REFERENCES
[1] [2]
[3] [4] [51 [6] [7]
J. Szargut, D.R. Morris, F.R. Steward (1988), Exergy analysis of thermal, chemical and metallurgical processes, Hemishpere Publishing Corp., New York T.J. Kotas, (1995), The exergy method of thermal plant analysis, Krieger Publishing Company, Malabar, Florida I.L. Leitas, (2000) Some thermodynamic basis for design of energy saving chemical processes, Proceeding of ECOS 2000, 1235 B. Smit, (1993) Chemische absorptie van CO2 uit rookgassen (in Dutch), Report 93023, Department of Science, Technology and Society, Utrecht University K.W. Won, J.R. Scherffius, P. Condorelli, (1999) Improved Econamine FG Process sm and Economical Evaluation, information provided by Fluor P. Le Goff, T. Cachot, R. Rivero, (1996) Exergy analysis of Distillation Processes, Chem. Eng. Technol. 19, 478 D. Chapel, J. Ernst, C. Mariz (1999) Recovery of CO2 from Flue Gases: Commercial trends, Proceeding of Canadian Society of Chemical Engineers, paper 340.
121
EXERGY ANALYSIS OF AMINE-BASED CO2 REMOVAL T E C H N O L O G Y F.H. Geuzebroek L.H.J.M. Schneider and G.J.C. Kraaijveld TNO-Environment, Energy and Process Innovation Laan van Westenenk 501, 7325 NE Apeldoorn, the Netherlands.
ABSTRACT An exergy analysis is conducted of a gas-fired CO2 capture process based on MEA absorption technology. It is shown that with the combination of AspenPlus and ExerCom it is possible to get a reasonable result of the exergy loss per unit operation. To enable this, certain modifications were made in the thermodynamics of the reaction of CO2 with MEA. In this reaction ionic species are formed and a approach enabling the use of electrolytes is necessary. The results show that the major sources of exergy loss are the absorber, the flasher and the flue gas blower. INTRODUCTION Absorption in alkanolamines like MonoEthanolAmine (MEA) is the currently preferred option for the removal of low-concentration C02 from flue gases using the following reaction: CO2 + 2MEA-> M E A C O O + MEAH + The MEA process is well-established, but the costs and the energy consumption of CO2 removal is substantial, typically 4 - 5 MJ/kg CO2. This energy consumption leads to a substantial parasitic power loss when applied in the capture of CO2 from flue gases of power plants. This is a strong driving force to optimise the process or to find alternative ways of capturing the CO2. A tool for optimising the energy consumption is the use of exergy analysis [ 1,2]. Exergy is defined as the actual work potential or maximum work available from a certain gas or liquid stream. Loss of exergy means loss of work potential and this loss has to be generated elsewhere by eventually primary energy. As a process analysis tool, exergy analysis has the advantage that it makes clear what makes a process efficient or inefficient. It should be pointed out that an analysis using second law type of consideration for the MEA system has been given by Leitas [3]. EXERGY ANALYSIS Exergy analysis can be performed when the composition and the physical properties of all the relevant streams of the capturing process are available. For that purpose the AspenPlus flow sheet program is used. ExerCom by Jacobs Engineering can perform exergy analysis as an add-on. It uses the output of AspenPlus and calculates the chemical, physical and mixing exergy for each gas and liquid stream in the process. The exergy AEx is a measure of the work potential at a certain state, relative to the reference state: AEx = AH- ToAS The choice of the reference by ExerCom is that defined by Szargut [ 1], i.e. the reference temperature To = 298.15 K and pressure Po = 101.325 kPa. For chemical equilibrium as a reference state the mean
122 composition of the earth atmosphere, the mean composition of seawater and the mean composition of the earth's crust is taken. By calculating the difference between the exergy of the input and output streams of a unit's operation, the exergy loss of each individual unit can be calculated. For the overall flow sheet, the exergy analysis has already been made and total exergy loss of about 1 MJ/kg CO2 has resulted [4]. When an exergy analysis is made on the level of individual unit's operations, it appears that the standard thermodynamics to describe the reaction of C02 with MEA are incomplete due to the incorporation of electrolytes.
Exergy analysis using AspenPlus and ExerCom: modification to the thermodynamic system Thus, to be able to make the calculation of the exergy of systems with the reaction CO2 with MEA, it is necessary that certain modifications are made to the thermodynamic system supplied by AspenPlus for this reaction. This is due to the fact that in the reaction, ionic species are formed, which have to be treated as Electrolytes. To the database of chemical exergy provided by ExerCom the component MEA has to be added. In more detail these modifications are: To calculate the mixing exergy by ExerCom in AspenPlus it is necessary to know the so-called infinite dilution aqueous phase Gibbs Free Energy of Formation (DGAQFM) of the ionic species MEAH + and MEACOO-, which are not present in the MEA data package of AspenPlus. These values can be derived from the equilibrium constants of the reaction, given by ApenPlus, if sufficient data are known: G* = ]~(Gproducts) - ~](Greactans)=- RT. In(K)
However, for the MEA system three necessary values of DGAQFM are not known (MEA, MEAH +, MEACOO), while there are only two independent reaction equations available. To circumvent this problem, an alternative way to calculate the DGAQFM for MEA is proposed using data generated by AspenPlus for a mixture of water and MEA, according to standard thermodynamics rules. Using this value, the two unknown DGAQFM values are obtained from the two independent reaction equation constants. DGAQFM for MEAH + is-500.504 kJ/mol and for MEACOO- the value is-196.524 kJ/mol. With the modifications to the AspenPlus model it is possible to perform sound calculation fulfilling not only the first law of thermodynamics, but also the second law of thermodynamics. A second addition is the chemical exergy of the MEA molecule in the liquid phase. Without this value ExerCom cannot make the calculation. Although there is substantial information in the literature, MEA could not be found. For this molecule, the group contribution method was used [ 1]. The value used is 1536 kJ/mol.
AspenPlus calculations The AspenPlus calculations were performed using so-called apparent components. This means that the results are reported in terms of neutral components (MEA and CO2) and not in terms of the electrolytes. This mode of calculation enables much faster convergence, without loss of quality of the result. It should be noted that the change in exergy of a stream containing electrolytes will be found in the mixing exergy, in stead of in the chemical exergy. A manual calculation revealed that this approach is a reasonable assumption. REFERENCE CASE. As a reference case, field data from a CO2 capture plant has been used. The data has been taken by Fluor from the plant at Bellingham [5] producing 320 ton/day of CO2. The plant uses flue gas from a gas-fired plant containing about 3.5 % o f CO2. To establish the validity of our AspenPlus modelling, a validation of an AspenPlus flowsheet of the process is made (see Figure 1).
123
~-----I CW-F:INW "C
~
CW-F-OUT
I"-F~s" I °
I°FF~s~ I
H2o-scR8
I
OF OA - I ~
' C
O
~ - t FLUEGAS
~
°
[ S~ER
,so*;E.
11 F~T
z--~ _
1
A oOO'E"W
]-[ HX
PUMP IW-CF-OUT I"
Figure 1" Flow sheet used in the AspenPlus calculation As can be seen in Table 1, the AspenPlus flow sheet does predict reasonably well the plant data. The main difference is the higher steam consumption. Another deviation is the higher loading of the solvent. This can be explained by the fact that in AspenPlus, equilibrium data is used that slightly deviates from reality, as has already been shown [5]. We consider the results such, that a good impression of the abilities of the exergy analysis is obtained by using this data. In a next step, the AspenPlus flow sheet should be further optimised. TABLE 1 VALIDATION OF AspenPlus FLOWSHEET STREAM Gas Solvent
Steam
CO2 in C02 out Flow C02 in CO2 out Net loading Flow Energy consumpt.
FIELD DATA 3.13% 0.54% 241.6 m3/h 0.42 mol/mol 0.16 mol/mol 0.26 mol/mol 27000 k~hr 4.3 MJ/kg C02
AspenPlus 3.13% 0.51% 241.6 m3/h 0.48 mol/mol 0.21 mol/mol 0.27 mol/mol 30000 kg/hr 4.9 MJ/kg CO2
RESULTS OF EXERGY ANALYSIS Based on the data of the reference case, ExerCom is used to determine the Exergy losses of the several unit operations in the flow sheet. The result of the total exergy loss is depicted in Table 2 and Figure 2. A crucial matter is how to treat the utilities in terms of where the system boundaries are set. Two eases are given: 1) DUTY/CARNOT. In this case the exergy loss in the utilities is ignored, i.e. a 100% efficiency is taken. For the different utilities this means: • Steam: the duty calculated by the AspenPlus unit operation is corrected for by the Carnot efficiency at the steam temperature of 120 °C.
124 •
•
Cooling water: same as for steam. In practice this means that the outgoing cooling water stream has a higher exergy than the incoming stream. In practice this stream usually has no significant value. Electricity: the efficiency of electricity generation is ignored.
The results reveal that the absorber (32%) and the flasher (24%) are the main sources of exergy loss. The feed cooler (18%) and the blower (16.5%) are also contributing strongly. An interesting result is that the exergy loss in the stripper (with reboiler) is very small. The total exergy loss is a little more than 1.4 MJ/kg CO2. Unit operations like the pump, the scrubber and the solvent cooler are minor contributors to the exergy loss. 2) WITH UTILITY LOSSES. The part of the exergy losses taking place at the utility side of the operation are now taken into account: • Steam: steam of 140°C is condensed at 130°C. It is assumed that the resulting stream can still be reused. The exergy loss to generate the steam of 140°C is ignored. It depends on the local situation how this should be treated (steam available from power station or generated locally by boiler). For the current exercise we ignore this part. • Cooling water. The outgoing cooling water stream usually represents a certain amount of exergy, but is without any real value. This is taken into account by calculating the exergy of the real streams of the cooling water streams of the cooler and the flasher (CW-F-IN/OUT and CW-CIN/OUT). Now the external streams result in an exergy loss instead of a gain, like in the situation when using duty. • Electricity: in this paper we ignored further exergy losses combined with electricity power generation The exergy loss increases about 0.3 MJ/kg CO2 by taking into account the losses in the utilities. The main shift is in the flasher, which now accounts for about 8% of the exergy loss. The flasher increases to 28%, while the contribution of the absorber has decreased to 26%. The feed cooler and the blower with 13-15% are still considerable loss factors, while the other unit operation leads to losses well below 10% of the total. A separate item are the exergy losses due to the consumption of MEA. We have assumed a consumption of 1.5 kg MEA per ton CO2 [7]. Using the chemical exergy of MEA in the liquid phase of 1536 kJ/mol, an exergy loss of about 0.03 MJ/kg CO/results, i.e. about 2 % of the total loss. Exergy loss divided per component Exergy consists of three table components: chemical, physical, and mixing exergy. To observe how the exergy loss is split up into these components, the exergy losses of the individual unit operations have been added. Chemical and mixing exergy are combined. The exergy loss attributed to electrical power is given as a separate item. External losses associated with the other utilities are mainly added to the physical exergy loss. The result is given in Table 3, using the two methods given above. As is clear, the exergy loss due to the electrical power loss is a major contributor to the exergy loss, contributing about 30-40% when the efficiency of generation is taken into account. Physical exergy loss contributes about 40%-50%, while the chemical exergy loss is actually rather small (20%). DISCUSSION The AspenPlus flow sheet overestimates the energy consumption of steam with about 0.5 MJ/kg CO2.In exergy terms this corresponds to an additional exergy loss of about 0.1 MJ/kg CO2. We assume that this difference does not effect the major observations of this study. The exergy analysis gives a clear insight where the actual exergy loss takes place. An interesting observation is that in the stripper and reboiler, the exergy losses are rather small, while in the flasher, the loss is substantial. This situation is similar to the case of distillation, as has been pointed out by, for instance ref [6]. To minimize the total exergy loss of a process, the exergy loss should be evenly divided over and in
125 the unit operations. In fact, large exergy loss in certain parts of the equipment corresponds to a large local driving force. This is unfavorable for total loss of exergy. To optimize the exergy loss the driving force should be lowered and equally divided over the reboiler, stripper and flasher. This can for instance be realized by integrating heat and mass transport in the stripper column (see [3,6]). On the other hand, lower driving force means a larger area for mass transfer and thus additional cost for internals. To optimize these two counteracting factors, an economic evaluation is necessary. Another observation is that the contribution of the power needed for the flue gas blower has a large contribution in exergy terms. It should be noted that the energy consumption of the blower in this plant is about 0.16 kWh/kg CO2, while in literature values of 0.12 to 0.14 kWh/kg CO2 are reported for gas fired systems [7]. Thus, a lowering of about 25% can be realized by applying state-of-the-art technology. Nevertheless, the blower and thus the pressure drop over the absorber remains a major item for optimization. It should be noted that obviously the CO2 compressor is an additional source of exergy loss. Values of the same order as the flue gas blower are expected to occur. Finally, it should be noted that the contribution of the chemical/mixing exergy loss is relatively small. Thus it may be prudently concluded that changing the chemical used in the capture process does not have the highest priority. Physical and electrical should be treated first. We want to stress that this hypothesis should be verified first by applying the analysis to other types of amines. An important remark is that the exact interpretation of the results is influenced by the exact definition of the system boundaries. In this paper, two cases are treated, but to get a good picture, the MEA capture system should be integrated with the other processes such as electricity generation, a chemical plant or whatever CO2 emitting system is evaluated. Another factor is the source of the flue gas. For coal-fired plant, the situation is quite similar and the analysis can be applied easily. FINAL R E M A R K S The combination AspenPlus-ExerCom is now accessible to perform optimising studies using exergy analysis for amine-based CO2 removal technology. It can be used to find ways to lower the energy consumption and coupled parasitic power loses. Options are the combination of heat and mass transfer in the stripper and absorber, as recently suggested for distillation, the use of amines with lower binding energy, the applications of split-flow, the use of the heat available in the flue gas for regeneration and vapour recompression. Integration of the CO2 capture unit with the process it is part of, can be easily realised. TABLE 2 TOTAL EXERGY LOSS BY UNIT OPERATION DUTY/CARNOT MJ/kg CO2 0.259 0.233 0.459 0.005
Pump Heatex Cooler
18.36% 16.51% 32.53% 0.35%
WITH UTILITY LOSSES MJ/kg CO2 0.259 0.233 0.459 0.005
14.94% 13.44% 26.47% 0.29%
0.002 0.093 0.003
0.14% 6.59% 0.21%
0.002 0.093 0.046
0.12% 5.36% 2.65%
Stripper Flasher
0.014 0.343
0.99% 24.31%
0.140 0.497
8.07% 28.66%
Total MEA losses Total with MEA losses
1.411 0.030 1.441
100.00 o
1.734 0.030 1.764
100.00% 1.75%
FeedCooler Blower Absorber Scrubber
..
.
2.15%
126
0.5
0.25
"
"
0
,
-~
l
i
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I
I
I
I
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I
ll DUTY/CARNOT [] WITH UTILITY LOSSESJ
I
<.Q mo
Total exergy loss per unit operation of the system of the flow sheet in figure 1. Black bars correspond to the case were the utilities are taken into account as duties corrected by Carnot efficiency. White bars take into account the external losses in the utilities. Figure 2:
TABLE 3 EXERGY LOSS SPLIT UP IN COMPONENTS (IN MJ/kg CO2)
DUTY/CARNOT Chemical/mixing 0.32 Physical 0.56 Power/Electric 0.57 Total 1.44
22% 39% 39%
WITH UTILITY LOSSES 0.35 0.85 0.57
20% 48% 32%
1.76
REFERENCES
[1] [2]
[3] [4] [51 [6] [7]
J. Szargut, D.R. Morris, F.R. Steward (1988), Exergy analysis of thermal, chemical and metallurgical processes, Hemishpere Publishing Corp., New York T.J. Kotas, (1995), The exergy method of thermal plant analysis, Krieger Publishing Company, Malabar, Florida I.L. Leitas, (2000) Some thermodynamic basis for design of energy saving chemical processes, Proceeding of ECOS 2000, 1235 B. Smit, (1993) Chemische absorptie van CO2 uit rookgassen (in Dutch), Report 93023, Department of Science, Technology and Society, Utrecht University K.W. Won, J.R. Scherffius, P. Condorelli, (1999) Improved Econamine FG Process sm and Economical Evaluation, information provided by Fluor P. Le Goff, T. Cachot, R. Rivero, (1996) Exergy analysis of Distillation Processes, Chem. Eng. Technol. 19, 478 D. Chapel, J. Ernst, C. Mariz (1999) Recovery of CO2 from Flue Gases: Commercial trends, Proceeding of Canadian Society of Chemical Engineers, paper 340.