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Exergy analysis: A CO2 removal plant using a-MDEA as the solvent
Energy 118 (2017) 77e84
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Energy journal homepage: www.elsevier.com/locate/energy
Exergy analysis: A CO2 removal plant using a-MDEA as the solvent Vafa Feyzi a, Masoud Beheshti a, b, *, Abolfazl Gharibi Kharaji a a b
Department of Chemical Engineering, University of Isfahan, Isfahan, Iran Process Engineering Institute, University of Isfahan, Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2016 Received in revised form 16 November 2016 Accepted 6 December 2016
An exergy analysis was conducted for CO2 removal process from syngas using methyldiethanolamine activated by piperazine (a-MDEA). The process was simulated with Aspen HYSYS and the validation of the results was performed using the field data of Fanavaran Petrochemical Company. The process was divided into five separate sections. In order to determine the exergy loss, exergy balance equation was established around each section. The results showed that the highest exergy loss amongst five sections returns to the flasher (contributes 31.5% of total exergy loss); followed by the absorber column, stripper column, heat recovery section and the pump. In order to decrease the exergy loss in the process, piperazine concentration in a-MDEA solvent was increased which led to reduction in amine solvent flow rate. According to the results, the total exergy loss is decreased from 3.25 MW to 2.63 MW. When the amine flow rate decreased by 27%, the exergy loss, total energy consumption and the total utilities cost decreased by 19%, 18% and 25%, respectively. © 2016 Published by Elsevier Ltd.
Keywords: Energy Exergy CO2 removal a-MDEA Utilities cost
1. Introduction The carbon monoxide is an important substance in the chemical industry as secondary feedstock for acetic acid, methanol, phosgene and formic acid production [1]. Its production process from natural gas is composed of four steps [2]: the first step is the pre-treatment of raw materials to remove impurities, such as Sulphur compounds contained in the feedstock; the second step is the production of raw syngas mainly composed of hydrogen and carbon monoxide (CO) by steam reforming reactions of feedstock hydrocarbons; the third step is the separation of carbon dioxide from syngas and the final step is cryogenic separation. Considerable amount of carbon dioxide is produced as a byproduct during the methane reforming reaction. First priority in this condition is to separate the produced carbon dioxide before the cryogenic system, due to negative impact on cryogenic system and downstream catalytic processing [2]. There are several and different available technologies for CO2 removal. Main technologies include: amine processes [3], hot potassium carbonate [4,5] Physical solvents and Membranes [6]. Generic amines, especially, activated methyldiethanolamine (a-MDEA) have been used for carbon
* Corresponding author. Department of Chemical Engineering, University of Isfahan, Isfahan, Iran. E-mail addresses: [email protected], [email protected] (M. Beheshti). http://dx.doi.org/10.1016/j.energy.2016.12.020 0360-5442/© 2016 Published by Elsevier Ltd.
dioxide removal in recent decades [7e9]. MDEA is widely used since it has low vapor pressure which means that it can be used in high concentration without noticeable evaporation loss. MDEA is highly resisted toward degradation and is practically non-corrosive [2]. It also has a very low heat of reaction compared to other alkanolamines. The heat of reaction is an important parameter since it can cause the largest operating cost. MDEA, compared to other alkanolamines has a low rate of reaction with CO2; therefore, MDEA is often activated by adding primary or secondary amines and recently piperazine (PZ) as a promoter [10e12]. Recently, some studies investigated the modeling, simulation and kinetics of CO2 absorption in MDEA and Piperazine solutions [8,13e20]. The study of Afkhamipour and Mofarahi [16] showed that proper selection of mass transfer and kinetics models have a dominant role in modeling and simulation of the CO2 capture process by MDEA. New models are proposed for Aspen Plus rate-based simulation of absorber column in CO2 capturing by MDEA and piperazine (PZ), where Eddy diffusivity theory instead of film theory is taken into account [14,20]. Although it is a well-established separation method, the energy consumption and the costs associated with CO2 separation are substantially high [40]. Reducing the energy consumption in solvent-based CO2 capture processes can be addressed through improved solvents and process design [21]. Many publications are focused on developing new
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solvents to reduce the energy consumption of the CO2 capture process [22e27]. Cousins et al. [21] reviewed different flow sheet modifications, which reduced the energy penalty associated with the solvent-based to capture the CO2 from a high pressure gas stream, either natural gas or syngas mixtures. It can be appreciated that improvements to the standard process are brought with increased complexity and an increase in the number of unit operations (which would tend to increase the capital cost). In order to increase the energy efficiency and prevent extra costs and energy consumption, it is necessary to optimize the process and evaluate the performance of the whole system by an exergy analysis. Exergy Analysis is a method of thermodynamic analysis that commonly is used in the investigated processes, with the aim of calculating the second thermodynamic law efficiency of the process [28e30]. The thermodynamic performance of a process is best evaluated by exergy analysis [31,32]. As a process analysis tool, exergy analysis has the advantage of revealing what makes a process efficient or inefficient. Clearly the energy consumption in amine processes increases as the CO2 recovery increases [7]. Different CO2 capturing technologies are analyzed with exergy method. Atsonios et al. [33,34] performed exergitic analysis for two pre-combustion CO2 capturing schemes, including H2 selective membranes and cryogenic separation schemes. They investigated the effects of different process parameters on the exergitic efficiency through sensitivity analysis. Two other technologies used in CO2 capturing, investigated with exergy analysis method are the calcium looping process and the oxyfuel combustion which are commonly applied in fossil fuel power plants. Studies performed on the calcium looping process revealed that most of the exergy loss during the process returns to the calcination step [35,36]. Several different studies have been performed on the exergy analysis of the oxyfuel combustion for CO2 capture [37e39]. These studies show that the main exergy losses are located in the boiler, steam generation, steam turbine, distillation unit and the compression step. Several studies have been done exergy analysis on the process of CO2 removal from flue gas, using MEA as amine, and have calculated the amounts of exergy loss and exergy efficiency in different sections of the process [40e45]. These studies revealed that exergy is mainly dissipated in the absorption column, the stripping column and the heat exchangers. Odejobi et al. [43] had investigated the process for CO2 capture from the exergy viewpoint. They have studied the effects of several operating parameters on exergy efficiency and recovery of CO2. Valenti et al. [46] performed an exergy analysis of the CO2 capturing process using chilled aqueous ammonia as the adsorption medium, the study revealed that exergy efficiency of this process is more than amine processes. Some studies performed a comparison between different CO2 capture processes from the exergy viewpoint [47,48]. Lara et al. [48] compared the results of the exergitic analysis of six different CO2 capture technologies to identify the weakness of any schemes to reduce inefficiencies, energetic penalties, and hence the operational cost of the systems. Atsonios et al. [47] compared three carbon capture technologies, the amine scrubbing, the calcium looping and the oxyfuel combustion in terms of exergy losses, the results revealed that the calcium looping process is the most efficient process with the lowest exergy penalty. In this study, we apply the exergy analysis method in the process of CO2 capture from syngas by a-MDEA amine in order to evaluate the amounts of exergy loss and exergy efficiency in different sections of the process and reduce exergy loss in the process by proposing some modifications in operating parameters. There are mainly two differences between the process investigated in this study and other CO2 removal studies. The first one is the difference in the feed gas to process. In this process CO2 is removed
from a syngas stream while in the other studies [35e45] the feed gas is a flue gas stream. With the syngas as the feed, due to negative impact on the system and downstream catalytic and cryogenic processing the CO2 content must decrease to less than five ppm, i.e. CO2 recovery is very close to 100% but for a flue gas stream, CO2 content is not completely removed. Desiring higher recovery of CO2 requires more energy to be supplied and more exergy loss is expected. The second difference is the type of amine solvent; the amine used in this study is a-MDEA while in the other studies on amine processes, MEA solvent was used as the absorption medium. 2. Base case As a base case, field data from a CO2 capture plant have been used. This plant is located in Fanavaran petrochemical company, Mahshahr-Iran, producing about 46000 Nm3/hr. Syngas. Inlet feed stream is the effluent stream from water gas shift reactors of upstream plant; this stream contains about 3.5% CO2. Carbon dioxide in the feed stream is absorbed by a-MDEA solvent in the absorber column. The composition of the feed stream and the amine solvent are illustrated in Tables 1 and 2, respectively. In this process, methyldiethanolamine (MDEA) have been used as solvent to absorb CO2 from the feed stream. Some amount of piperazine as the activator is added to the water-MDEA solution to enhance the absorption ability of the solvent. Absorber column and stripping column are the main process equipment in the CO2 removal process. Both absorber and stripper columns are packed bed type. Table 3 shows a summary of the key process specification of the absorber and stripper columns. We know that the CO2 removal mechanism in amine processes is through chemical reaction between CO2 and amine solvent. The following reactions may occur during MDEA process for CO2 removal (Kierzkowska and Chacuk, 2010): CO2 þ R1R2R3N þ H2O 4 (R1R2R3NH)þ þ (HCO3)
(1)
CO2 þ (OH) 4 (HCO3)-
(2)
(HCO3) þ (OH) 4 (CO3)2 þ H2O
(3)
(R1R2R3NH)þ þ (OH) 4 R1R2R3N þ H2O
(4)
2H2O 4 (OH) þ (H3O)þ
(5)
in above equations denotes MDEA, where R1R2R3N R1 ¼ R2 ¼ CH2CH2OH and R3 ¼ CH3. The reactions considered between piperazine and CO2 are as follow (Norouzbahari et al., 2015): CO2 þ PZ þ B 4 (PZCOO) þ (BH)þ
(6)
CO2 þ (PZCOO) þ B 4 PZ(COO)2 þ (BH)þ
(7)
where B might be any base present in the solution (PZ, PZHþ,
Table 1 The composition of the feed stream. Component
Composition (mole %)
Methane H2O N2 H2 CO2 CO
2.68 0.34 0.91 69.55 3.33 23.19
V. Feyzi et al. / Energy 118 (2017) 77e84 Table 2 The composition of the amine solvent (a-MDEA). Component
Composition (mass %)
MDEA Piperazine H2O
26.39 3.48 69.72
Table 3 Key process specification of absorber column and stripper column. Parameter
Absorber
Stripper
Type of packing Total height of packing(m) Diameter of column (m) Packing dimension Type of vendor No. of equilibrium stages Operating pressure (barg)
FLEXIPAC 34 1.7 250Y KOCH 36 15.6
FLEXIPAC 19.85 15.5 3X KOCH 11 0.5
PZCOO, H2O and OH). A steady state model of the plant has been developed by using Aspen HYSYS (v.8.4) simulator. Calculation type of the simulation is equilibrium, at which any tray is considered as an equilibrium stage. The selected property package for this model is DBR-Amine package (v2012.1) which is a well-established package for amine systems and reasonably predicts its thermodynamic properties. The DBR-Amine package considers all the possible reactions between acid gases and amine solvent. The process flow diagram of the base case simulation has been illustrated in Fig. 1.
79
determines where more focusing is needed to improve the system efficiency. It also help to make an informed design decisions. Exergy (also called Availability and Work Potential) is the maximum useful work can be obtained from a system at a given state and environment; in other words, the most work you can get out of a system. In the absence of potential and kinetic energy change during the process, exergy of the stream is divided into physical exergy and chemical exergy. Physical exergy, Exph, is the maximum useful work obtained by passing the unit of mass of each substance from present state (T, P), to the environmental state (T0, P0) through reversible processes [49,50]. Physical exergy of a stream is determined by the enthalpy and entropy of the stream as Equation (8).
Exph ¼ ½hðT; pÞ hðT0 ; p0 Þ T0 ½SðT; pÞ SðT0 ; p0 Þ
(8)
Chemical exergy is the maximum useful energy which would be attained from the environmental state to the dead state, by means of chemical processes with reactant and products at the environmental pressure and temperature, when the stream composition is not in chemical equilibrium with the environment. Standard chemical exergy of various substances is reported in literature [49]. If standard chemical exergy of all constituents of the stream are available, chemical exergy of the mixture can be calculated by Equation (9).
Exch ¼
X
xi 3 0i þ RT0
X
i
xi lnxi
(9)
i
is the standard chemical exergy and xi is the molar composition of component i. So the total exergy of the stream is:
3 0i
3. Exergy analysis Although quantitative evaluation of the energy in a process can be done by the first law of thermodynamics, it is equally important to assign the quality of energy. The second law of thermodynamics determines the direction of work or heat; therefore, it cannot indicate the quality of energy. The exergy analysis integrates the first and second laws of thermodynamics in a particular environmental condition to evaluate the quality of energy, thus this analysis is very important in the optimization of a chemical process. By applying the exergy analysis method, the amount of destroyed exergy in each section of the process can be evaluated. It
Ex ¼ Exph þ Exch
(10)
Equipment Exergy loss (irreversibility) is calculated by establishing the exergy balance around a hypothetical control volume that encircles that equipment.
I_ ¼
X
m_ i Exi
m_ k Exk þ
out
in
Term
X
P _ Q j 1 TT0j j
Fig. 1. Process flow diagram of CCP.
X j
T Q_ j 1 0 Tj
! _ W
(11)
! in equation (11) is the amount of exergy
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V. Feyzi et al. / Energy 118 (2017) 77e84
transferred to the equipment due to heat exchanges with the heat sources of the system at temperature Tj . The Exergy efficiency of an equipment in the process is defined as the ratio of change of exergy in the sinks to the change of exergy in the sources.
hEx ¼
DExsink DExsources
(12)
The exergy analysis can be performed when the composition and the physical properties of all the relevant streams are available, which is why the Aspen HYSYS program is used. The Szargut et al. [51] method is applied to calculate the chemical exergy of each stream. Fifteen user variables and eleven user properties in Aspen HYSYS have been defined as the reference of partial pressure of components, ambient pressure and temperature. Visual basic codes are developed to define each user variable, which is imported to Aspen HYSYS software. These codes are produced according to the procedure presented by Abdollahi-Demneh et al. [52]. The databank of Aspen HYSYS is used to calculate the chemical, physical and mixing exergy for each gas and liquid stream in the process. Although there is substantial information in the literature, the standard chemical exergy of MDEA and piperazine in the liquid phase were not available. Therefore, Visual basic code is defined in User Variable of Aspen HYSYS. The calculated values for standard chemical exergy of MDEA and piperazine are 3748.1 kJ/mol and 3116.7 kJ/mol respectively. The calculated values of standard chemical exergy for other components presented in the process are compared with the values reported in literature (Kotas, 1995), Table 4 shows this comparison: Theoretically different methods were proposed for increasing exergy efficiency in the CO2 absorption by amine process to decrease exergy losses. Some of these propositions are as follow: - Combination of heat and mass transfer in the stripper and absorber [32,53]. - The use of amines with lower binding energy [54]. - Applying splitting flow on amine stream [55]. - Recovering of the heat available in the syngas stream for regeneration [41]. - Overhead vapor recompression in the stripping column [41].
4. Results Field data of the plant for thirteen cases that was operated at different conditions of feed flow, feed composition, amine flow and amine composition have been extracted from control room of CCP. These data were applied to validate the simulation. Relative error was defined for the temperature of the top and the bottom streams of the absorber and the stripper columns. The results for relative error in thirteen cases are shown in Fig. 2. According to the results, all defined relative errors are less than 10. Therefore, there is a good agreement between simulation and industrial data, and the simulation can be applied to exergy analysis of the mentioned plant.
Fig. 2. Relative error between simulation result and field data for all thirteen cases.
Table 5 Validation of Aspen HYSYS simulation. Parameter
Field data
Aspen HYSYS
Syngas stream flow rate Lean amine flow rate Concentration of CO2 in feed stream Concentration of CO2 in Syngas stream Concentration of CO2 in CO2 stream
46149 Nm3/hr 74884 kg/hr 3.33 mol% 4.7 ppm 93 mol%
46170 Nm3/hr 74900 kg/hr 3.33 mol% 4.8 ppm 92 mol%
Table 5 compares some key process data from the simulation with the real field data. As it mentioned before, the process in the current study is divided to five sections, including: absorption column, stripping column, heat recovery section, flasher and pump. Input and output exergy streams of these sections are shown in Fig. 3. The choice of the ambient conditions is that defined by Szargut et al. [51], i.e. the ambient temperature T0 ¼ 298.15 K and pressure P0 ¼ 101.325 kPa. Exergy values of all process streams are shown in Table 6. Exergy of the streams, exergy efficiency and exergy losses for each section were calculated from equation (11) and equation (12), these results are presented in Table 7. The results show that the section with highest exergy loss is flasher; this is followed by absorption column, stripping column and heat recovery section. Net exergy loss of the process is about 3.25 MW. In the flasher section effluent hot stream from the top stage of the stripper column is cooled by cooling water in exchanger E102 and turns to a two phase stream. The vapor and the liquid phases are separated in flash drum D-2001. Due to finite temperature difference between process stream 5 and cooling water, also low Carnot number for cooling water as cooling medium in this exchanger (about 0.048), considerable amount of thermal exergy of stream 5 is destroyed in the flasher section. One of the main reasons for exergy destruction in chemical processes is the chemical reactions take occur in the processes [49]. During a chemical reaction, materials with higher chemical exergy convert to materials with lower chemical exergy and considerable
Table 4 Standard chemical exergy. Component
Calculated standard chemical exergy (kJ/kmol)
standard chemical exergy reported by Kotas (kJ/kmol)
CH4 H2O H2 O2 CO2 CO N2
840310 116875 238520 3985 21004 276354 734
836510 11710 238490 3970 20140 275430 720
V. Feyzi et al. / Energy 118 (2017) 77e84
81
Fig. 3. Input and output exergy streams for each section.
Table 6 The exergy values of the streams. Stream
Temperature ( C)
Pressure (bar-g)
Mass Flow (ton/hr)
Physical Exergy (Mj/hr)
Chemical Exergy (Mj/hr)
AMINE BFW FEED ABSTOP ABSBOTT 2 SIN GAS SEPLIQ rich 5 4 CO2 16 6 9 11 lean
37.02 126 50 37.6 49.85 35 34.95 34.95 100.5 94.17 50 50 50 120.8 72.1 36.5 37.02
15.6 50 15.8 15.6 15.8 15.6 15.4 15.4 15.8 0.36 0.36 0.36 0.36 0.94 0.94 0.94 15.6
75 0.159 21.47 18.38 78.25 18.38 18.36 0.0185 78.25 4.795 4.795 3.243 1.556 75.03 75.03 75.03 75.03
3368.014 67.31 2668.2 1883.3 6560.9 1691.8 1968.2 0.782 21427.6 4133.4 427.4 269 1.587 133432.9 13046.4 3116.4 3368.4
654551.4 70.23 526935.7 526134.5 656085.6 526080.2 400770.9 0.882 656207.1 1674.7 1674.7 15556.9 0.134 655262.3 655262.3 654554.2 654554.2
in the syngas stream. The results are presented in Fig. 4. Piperazine concentration in a-MDEA solvent is normally in the
Table 7 Exergy analysis results. Section
Exergy efficiency
Irreversibility (Mj/hr)
Sections contribution to total exergy loss (%)
Absorber column Stripper column Heat recovery section Flasher Pump Total
0.33
3152
26.9
0.42
2933.3
25
0.87
1934.3
16.5
0.41 0.93
3690 17.2 11726.8
31.5 0.14 100
amount of heat is released. The exergy loss for the absorber column is relatively high, because of the reactions between CO2 and MDEA solvent and the heat released in the reactions. In this study we investigate the effect of the piperazine concentration in a-MDEA solvent on the CO2 removal. Simulation results show that increasing the piperazine concentration increases the absorption ability of the a-MDEA and reduces the CO2 amount
Fig. 4. Effect of piperazine concentration on the CO2 concentration in syngas stream.
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V. Feyzi et al. / Energy 118 (2017) 77e84
0.2
The results of the exergy analysis showed that decreasing the amine flow rate from 75 ton/hr to 54.5 ton/hr. decreases the total exergy loss to 2.63 MW (see Fig. 6). Finally, to have a better view on the influence of the modifications, it will be interesting to investigate the change of energy and costs in unit. Therefore, in continue total utilities cost and energy consumption of the base and modified processes are shown in Table 9. The results indicate that total utilities cost and total energy consumption decrease by 25% and 18%, respectively.
0
5. Discussion
1.2
0.87
0.91
1 0.8 0.6
0.53 0.41 0.44
Pump
0.42
Flasher
Heat recovery section
Modified case
0.33 0.35
Stripper
0.4
Exergy Efficiency
0.93
0.97
Absorber
Base case
Fig. 5. Comparison of exergy efficiency for the base case and the modified case.
Table 8 Exergy analysis results for the modified plant.
Absorber column Stripper column Heat recovery section Flasher Pump
Irreversibility (Mj/hr)
0.35 0.53 0.91 0.44 0.97
3091 1917.7 963.3 3520 4.8
4000 3520 3152 2933.3
3091
3500 3000 2500
1934.3
1917.7
2000 1500
963.3
1000
Exergy Loss (kj/hr)
3690
Exergy efficiency
500 17.2
4.8
Pump
0 Flasher
Heat recovery section
Stripper
Modified case
Base case
Absorber
Fig. 6. Comparison of exergy loss for the base case and the modified case.
range of 3e5 wt% [56]. This value in the a-MDEA solvent using in Fanavaran petrochemical company is about 3.5 wt% and amount of CO2 in syngas stream is 4.8 ppm (see Fig. 5). By increasing the piperazine concentration from 3.5% to 4.7% the amine circulation rate decreases from 75 ton/hr. to 54.5 ton/hr, while CO2 composition in the syngas stream remains at 4.8 ppm. By declining the rate of amine circulation the exergy losses in the process decrease due to reduce the require power of pump and the reboiler duty in stripping column. The results of the exergy analysis which was done on the simulation of modified plant are shown in Table 8.
Table 9 Total utilities cost and total energy consumption for the base and modified cases. Total utilities costs (USD/Yr) Total energy consumption (kw) The base case 2.26Eþ06 The modified case 1.69Eþ06
7.75Eþ03 6.33Eþ03
The simulation of Aspen HYSYS shows that the energy consumption is about 1.3 MJ/kg CO2; this value is in agreement with its industrial value. In exergy terms, the total exergy loss is about 0.55 MJ/kg CO2. According to the results, an interesting observation is that in the pump and heat recovery the exergy losses are rather small, while in the flasher the loss is substantial. This situation is similar to the study of Geuzebroek et al. [41]. Although the exergy loss of the stripper in this study (0.14 MJ/kg CO2) is the same of their study, there is a big difference between the exergy losses of absorbers. The absorber exergy loss in this study is 200% smaller than their study. In fact, large exergy loss in certain parts of the equipment corresponds to a large local driving force. Therefore, the driving force of MEA is further than a-MDEA. It corresponds to the intrinsic properties of MEA as a primary amine and MDEA as a tertiary amine. Thus, contrast to the Geuzebroek et al. conclusions [41], it can be concluded that changing the chemical used in the capture process is effective on the exergy loss. According to the obtained results, for the same equipment and process the amount of amine flow rate is a very impressive parameter to reduce the exergy loss. Our investigation indicates when the amine flow rate decreases by 27%, the exergy loss, total energy consumption and utilities cost decrease by 19%, 18% and 25%, respectively. Therefore, it may be prudently concluded that changing the solvent and operation conditions in order to decrease the amine flow rate is very effective to reduce the exergy loss, the total energy consumption and consequently the total utilities cost. Due to the high amounts of exergy loosed in the stripping column and heat recovery section it can be mentioned that one of the main sources of exergy destruction in this process is heat transfer through finite temperature difference in heat transferring equipments such as heat exchangers and reboiler of stripping column. As flow rate of amine is decreased, amount of required heating in reboiler and amount of heat transferred between hot and cold steams in heat exchangers decreases, this fact is the reason for the largest improvement observed in heat recovery section and stripping column. 6. Conclusions In this study, we have simulated a CO2 capture plant in CO production unit of Fanavaran Petrochemical Company with Aspen HYSYS simulator. In this plant, CO2 is removed from a synthesis gas stream by using a-MDEA as the absorption medium. The simulation validation is performed by comparison of field data from the plant in thirteen different operational conditions (such as feed flow, feed composition and composition of amine solvent) with the results from the simulation. This comparison shows that the simulation can predict the actual data with a reasonably good approximation. Exergy analysis has applied to achieve a clear insight where the actual exergy loss takes place. Exergy analysis result shows that the flasher had highest rate of exergy loss, it followed by the absorber column, stripping column and heat recovery section. The main
V. Feyzi et al. / Energy 118 (2017) 77e84
reasons for exergy loss in this process return to chemical reactions in absorber column, finite temperature differences in heat exchangers, mixing of streams at different thermodynamics condition, composition and friction. Exergy efficiency of the process is increased by increasing piperazine concentration in amine solvent, which leads to decline the amine flow rate. Finally, the modifications reduce the total exergy loss from 3.25 MW to 2.63 MW.
[17] [18] [19]
[20]
Nomenclature [21]
T P T0 P0 Exph h S ExCH xi R 3 0i
Ex I_ m_ Q_
Temperature, K Pressure, kPa Ambient temperature, K Ambient pressure, kPa Stream's specific physical exergy, kJ/kmol Specific enthalpy, kJ/kmol Specific entropy, kJ/kmol.K Stream's specific chemical exergy, kJ/kmol Component i molar composition in stream Universal gas constant taken as 8.314 kJ/kmol.K Component i standard chemical exergy, kJ/kmol Stream's specific total exergy, kJ/kmol Exergy loss rate, kJ/hr Stream's molar flow rate, kmol/hr
Heat transfer rate, kJ/hr Heat source temperature, K Shaft work, kJ/hr hEx Rational exergy efficiency DExsink Total rate of exergy transfer to sinks of exergy, kJ/hr DExsources Total rate of exergy transfer from sources of exergy, kJ/hr
Tj w_
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Exergy analysis: A CO2 removal plant using a-MDEA as the solvent Vafa Feyzi a, Masoud Beheshti a, b, *, Abolfazl Gharibi Kharaji a a b
Department of Chemical Engineering, University of Isfahan, Isfahan, Iran Process Engineering Institute, University of Isfahan, Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2016 Received in revised form 16 November 2016 Accepted 6 December 2016
An exergy analysis was conducted for CO2 removal process from syngas using methyldiethanolamine activated by piperazine (a-MDEA). The process was simulated with Aspen HYSYS and the validation of the results was performed using the field data of Fanavaran Petrochemical Company. The process was divided into five separate sections. In order to determine the exergy loss, exergy balance equation was established around each section. The results showed that the highest exergy loss amongst five sections returns to the flasher (contributes 31.5% of total exergy loss); followed by the absorber column, stripper column, heat recovery section and the pump. In order to decrease the exergy loss in the process, piperazine concentration in a-MDEA solvent was increased which led to reduction in amine solvent flow rate. According to the results, the total exergy loss is decreased from 3.25 MW to 2.63 MW. When the amine flow rate decreased by 27%, the exergy loss, total energy consumption and the total utilities cost decreased by 19%, 18% and 25%, respectively. © 2016 Published by Elsevier Ltd.
Keywords: Energy Exergy CO2 removal a-MDEA Utilities cost
1. Introduction The carbon monoxide is an important substance in the chemical industry as secondary feedstock for acetic acid, methanol, phosgene and formic acid production [1]. Its production process from natural gas is composed of four steps [2]: the first step is the pre-treatment of raw materials to remove impurities, such as Sulphur compounds contained in the feedstock; the second step is the production of raw syngas mainly composed of hydrogen and carbon monoxide (CO) by steam reforming reactions of feedstock hydrocarbons; the third step is the separation of carbon dioxide from syngas and the final step is cryogenic separation. Considerable amount of carbon dioxide is produced as a byproduct during the methane reforming reaction. First priority in this condition is to separate the produced carbon dioxide before the cryogenic system, due to negative impact on cryogenic system and downstream catalytic processing [2]. There are several and different available technologies for CO2 removal. Main technologies include: amine processes [3], hot potassium carbonate [4,5] Physical solvents and Membranes [6]. Generic amines, especially, activated methyldiethanolamine (a-MDEA) have been used for carbon
* Corresponding author. Department of Chemical Engineering, University of Isfahan, Isfahan, Iran. E-mail addresses: [email protected], [email protected] (M. Beheshti). http://dx.doi.org/10.1016/j.energy.2016.12.020 0360-5442/© 2016 Published by Elsevier Ltd.
dioxide removal in recent decades [7e9]. MDEA is widely used since it has low vapor pressure which means that it can be used in high concentration without noticeable evaporation loss. MDEA is highly resisted toward degradation and is practically non-corrosive [2]. It also has a very low heat of reaction compared to other alkanolamines. The heat of reaction is an important parameter since it can cause the largest operating cost. MDEA, compared to other alkanolamines has a low rate of reaction with CO2; therefore, MDEA is often activated by adding primary or secondary amines and recently piperazine (PZ) as a promoter [10e12]. Recently, some studies investigated the modeling, simulation and kinetics of CO2 absorption in MDEA and Piperazine solutions [8,13e20]. The study of Afkhamipour and Mofarahi [16] showed that proper selection of mass transfer and kinetics models have a dominant role in modeling and simulation of the CO2 capture process by MDEA. New models are proposed for Aspen Plus rate-based simulation of absorber column in CO2 capturing by MDEA and piperazine (PZ), where Eddy diffusivity theory instead of film theory is taken into account [14,20]. Although it is a well-established separation method, the energy consumption and the costs associated with CO2 separation are substantially high [40]. Reducing the energy consumption in solvent-based CO2 capture processes can be addressed through improved solvents and process design [21]. Many publications are focused on developing new
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V. Feyzi et al. / Energy 118 (2017) 77e84
solvents to reduce the energy consumption of the CO2 capture process [22e27]. Cousins et al. [21] reviewed different flow sheet modifications, which reduced the energy penalty associated with the solvent-based to capture the CO2 from a high pressure gas stream, either natural gas or syngas mixtures. It can be appreciated that improvements to the standard process are brought with increased complexity and an increase in the number of unit operations (which would tend to increase the capital cost). In order to increase the energy efficiency and prevent extra costs and energy consumption, it is necessary to optimize the process and evaluate the performance of the whole system by an exergy analysis. Exergy Analysis is a method of thermodynamic analysis that commonly is used in the investigated processes, with the aim of calculating the second thermodynamic law efficiency of the process [28e30]. The thermodynamic performance of a process is best evaluated by exergy analysis [31,32]. As a process analysis tool, exergy analysis has the advantage of revealing what makes a process efficient or inefficient. Clearly the energy consumption in amine processes increases as the CO2 recovery increases [7]. Different CO2 capturing technologies are analyzed with exergy method. Atsonios et al. [33,34] performed exergitic analysis for two pre-combustion CO2 capturing schemes, including H2 selective membranes and cryogenic separation schemes. They investigated the effects of different process parameters on the exergitic efficiency through sensitivity analysis. Two other technologies used in CO2 capturing, investigated with exergy analysis method are the calcium looping process and the oxyfuel combustion which are commonly applied in fossil fuel power plants. Studies performed on the calcium looping process revealed that most of the exergy loss during the process returns to the calcination step [35,36]. Several different studies have been performed on the exergy analysis of the oxyfuel combustion for CO2 capture [37e39]. These studies show that the main exergy losses are located in the boiler, steam generation, steam turbine, distillation unit and the compression step. Several studies have been done exergy analysis on the process of CO2 removal from flue gas, using MEA as amine, and have calculated the amounts of exergy loss and exergy efficiency in different sections of the process [40e45]. These studies revealed that exergy is mainly dissipated in the absorption column, the stripping column and the heat exchangers. Odejobi et al. [43] had investigated the process for CO2 capture from the exergy viewpoint. They have studied the effects of several operating parameters on exergy efficiency and recovery of CO2. Valenti et al. [46] performed an exergy analysis of the CO2 capturing process using chilled aqueous ammonia as the adsorption medium, the study revealed that exergy efficiency of this process is more than amine processes. Some studies performed a comparison between different CO2 capture processes from the exergy viewpoint [47,48]. Lara et al. [48] compared the results of the exergitic analysis of six different CO2 capture technologies to identify the weakness of any schemes to reduce inefficiencies, energetic penalties, and hence the operational cost of the systems. Atsonios et al. [47] compared three carbon capture technologies, the amine scrubbing, the calcium looping and the oxyfuel combustion in terms of exergy losses, the results revealed that the calcium looping process is the most efficient process with the lowest exergy penalty. In this study, we apply the exergy analysis method in the process of CO2 capture from syngas by a-MDEA amine in order to evaluate the amounts of exergy loss and exergy efficiency in different sections of the process and reduce exergy loss in the process by proposing some modifications in operating parameters. There are mainly two differences between the process investigated in this study and other CO2 removal studies. The first one is the difference in the feed gas to process. In this process CO2 is removed
from a syngas stream while in the other studies [35e45] the feed gas is a flue gas stream. With the syngas as the feed, due to negative impact on the system and downstream catalytic and cryogenic processing the CO2 content must decrease to less than five ppm, i.e. CO2 recovery is very close to 100% but for a flue gas stream, CO2 content is not completely removed. Desiring higher recovery of CO2 requires more energy to be supplied and more exergy loss is expected. The second difference is the type of amine solvent; the amine used in this study is a-MDEA while in the other studies on amine processes, MEA solvent was used as the absorption medium. 2. Base case As a base case, field data from a CO2 capture plant have been used. This plant is located in Fanavaran petrochemical company, Mahshahr-Iran, producing about 46000 Nm3/hr. Syngas. Inlet feed stream is the effluent stream from water gas shift reactors of upstream plant; this stream contains about 3.5% CO2. Carbon dioxide in the feed stream is absorbed by a-MDEA solvent in the absorber column. The composition of the feed stream and the amine solvent are illustrated in Tables 1 and 2, respectively. In this process, methyldiethanolamine (MDEA) have been used as solvent to absorb CO2 from the feed stream. Some amount of piperazine as the activator is added to the water-MDEA solution to enhance the absorption ability of the solvent. Absorber column and stripping column are the main process equipment in the CO2 removal process. Both absorber and stripper columns are packed bed type. Table 3 shows a summary of the key process specification of the absorber and stripper columns. We know that the CO2 removal mechanism in amine processes is through chemical reaction between CO2 and amine solvent. The following reactions may occur during MDEA process for CO2 removal (Kierzkowska and Chacuk, 2010): CO2 þ R1R2R3N þ H2O 4 (R1R2R3NH)þ þ (HCO3)
(1)
CO2 þ (OH) 4 (HCO3)-
(2)
(HCO3) þ (OH) 4 (CO3)2 þ H2O
(3)
(R1R2R3NH)þ þ (OH) 4 R1R2R3N þ H2O
(4)
2H2O 4 (OH) þ (H3O)þ
(5)
in above equations denotes MDEA, where R1R2R3N R1 ¼ R2 ¼ CH2CH2OH and R3 ¼ CH3. The reactions considered between piperazine and CO2 are as follow (Norouzbahari et al., 2015): CO2 þ PZ þ B 4 (PZCOO) þ (BH)þ
(6)
CO2 þ (PZCOO) þ B 4 PZ(COO)2 þ (BH)þ
(7)
where B might be any base present in the solution (PZ, PZHþ,
Table 1 The composition of the feed stream. Component
Composition (mole %)
Methane H2O N2 H2 CO2 CO
2.68 0.34 0.91 69.55 3.33 23.19
V. Feyzi et al. / Energy 118 (2017) 77e84 Table 2 The composition of the amine solvent (a-MDEA). Component
Composition (mass %)
MDEA Piperazine H2O
26.39 3.48 69.72
Table 3 Key process specification of absorber column and stripper column. Parameter
Absorber
Stripper
Type of packing Total height of packing(m) Diameter of column (m) Packing dimension Type of vendor No. of equilibrium stages Operating pressure (barg)
FLEXIPAC 34 1.7 250Y KOCH 36 15.6
FLEXIPAC 19.85 15.5 3X KOCH 11 0.5
PZCOO, H2O and OH). A steady state model of the plant has been developed by using Aspen HYSYS (v.8.4) simulator. Calculation type of the simulation is equilibrium, at which any tray is considered as an equilibrium stage. The selected property package for this model is DBR-Amine package (v2012.1) which is a well-established package for amine systems and reasonably predicts its thermodynamic properties. The DBR-Amine package considers all the possible reactions between acid gases and amine solvent. The process flow diagram of the base case simulation has been illustrated in Fig. 1.
79
determines where more focusing is needed to improve the system efficiency. It also help to make an informed design decisions. Exergy (also called Availability and Work Potential) is the maximum useful work can be obtained from a system at a given state and environment; in other words, the most work you can get out of a system. In the absence of potential and kinetic energy change during the process, exergy of the stream is divided into physical exergy and chemical exergy. Physical exergy, Exph, is the maximum useful work obtained by passing the unit of mass of each substance from present state (T, P), to the environmental state (T0, P0) through reversible processes [49,50]. Physical exergy of a stream is determined by the enthalpy and entropy of the stream as Equation (8).
Exph ¼ ½hðT; pÞ hðT0 ; p0 Þ T0 ½SðT; pÞ SðT0 ; p0 Þ
(8)
Chemical exergy is the maximum useful energy which would be attained from the environmental state to the dead state, by means of chemical processes with reactant and products at the environmental pressure and temperature, when the stream composition is not in chemical equilibrium with the environment. Standard chemical exergy of various substances is reported in literature [49]. If standard chemical exergy of all constituents of the stream are available, chemical exergy of the mixture can be calculated by Equation (9).
Exch ¼
X
xi 3 0i þ RT0
X
i
xi lnxi
(9)
i
is the standard chemical exergy and xi is the molar composition of component i. So the total exergy of the stream is:
3 0i
3. Exergy analysis Although quantitative evaluation of the energy in a process can be done by the first law of thermodynamics, it is equally important to assign the quality of energy. The second law of thermodynamics determines the direction of work or heat; therefore, it cannot indicate the quality of energy. The exergy analysis integrates the first and second laws of thermodynamics in a particular environmental condition to evaluate the quality of energy, thus this analysis is very important in the optimization of a chemical process. By applying the exergy analysis method, the amount of destroyed exergy in each section of the process can be evaluated. It
Ex ¼ Exph þ Exch
(10)
Equipment Exergy loss (irreversibility) is calculated by establishing the exergy balance around a hypothetical control volume that encircles that equipment.
I_ ¼
X
m_ i Exi
m_ k Exk þ
out
in
Term
X
P _ Q j 1 TT0j j
Fig. 1. Process flow diagram of CCP.
X j
T Q_ j 1 0 Tj
! _ W
(11)
! in equation (11) is the amount of exergy
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V. Feyzi et al. / Energy 118 (2017) 77e84
transferred to the equipment due to heat exchanges with the heat sources of the system at temperature Tj . The Exergy efficiency of an equipment in the process is defined as the ratio of change of exergy in the sinks to the change of exergy in the sources.
hEx ¼
DExsink DExsources
(12)
The exergy analysis can be performed when the composition and the physical properties of all the relevant streams are available, which is why the Aspen HYSYS program is used. The Szargut et al. [51] method is applied to calculate the chemical exergy of each stream. Fifteen user variables and eleven user properties in Aspen HYSYS have been defined as the reference of partial pressure of components, ambient pressure and temperature. Visual basic codes are developed to define each user variable, which is imported to Aspen HYSYS software. These codes are produced according to the procedure presented by Abdollahi-Demneh et al. [52]. The databank of Aspen HYSYS is used to calculate the chemical, physical and mixing exergy for each gas and liquid stream in the process. Although there is substantial information in the literature, the standard chemical exergy of MDEA and piperazine in the liquid phase were not available. Therefore, Visual basic code is defined in User Variable of Aspen HYSYS. The calculated values for standard chemical exergy of MDEA and piperazine are 3748.1 kJ/mol and 3116.7 kJ/mol respectively. The calculated values of standard chemical exergy for other components presented in the process are compared with the values reported in literature (Kotas, 1995), Table 4 shows this comparison: Theoretically different methods were proposed for increasing exergy efficiency in the CO2 absorption by amine process to decrease exergy losses. Some of these propositions are as follow: - Combination of heat and mass transfer in the stripper and absorber [32,53]. - The use of amines with lower binding energy [54]. - Applying splitting flow on amine stream [55]. - Recovering of the heat available in the syngas stream for regeneration [41]. - Overhead vapor recompression in the stripping column [41].
4. Results Field data of the plant for thirteen cases that was operated at different conditions of feed flow, feed composition, amine flow and amine composition have been extracted from control room of CCP. These data were applied to validate the simulation. Relative error was defined for the temperature of the top and the bottom streams of the absorber and the stripper columns. The results for relative error in thirteen cases are shown in Fig. 2. According to the results, all defined relative errors are less than 10. Therefore, there is a good agreement between simulation and industrial data, and the simulation can be applied to exergy analysis of the mentioned plant.
Fig. 2. Relative error between simulation result and field data for all thirteen cases.
Table 5 Validation of Aspen HYSYS simulation. Parameter
Field data
Aspen HYSYS
Syngas stream flow rate Lean amine flow rate Concentration of CO2 in feed stream Concentration of CO2 in Syngas stream Concentration of CO2 in CO2 stream
46149 Nm3/hr 74884 kg/hr 3.33 mol% 4.7 ppm 93 mol%
46170 Nm3/hr 74900 kg/hr 3.33 mol% 4.8 ppm 92 mol%
Table 5 compares some key process data from the simulation with the real field data. As it mentioned before, the process in the current study is divided to five sections, including: absorption column, stripping column, heat recovery section, flasher and pump. Input and output exergy streams of these sections are shown in Fig. 3. The choice of the ambient conditions is that defined by Szargut et al. [51], i.e. the ambient temperature T0 ¼ 298.15 K and pressure P0 ¼ 101.325 kPa. Exergy values of all process streams are shown in Table 6. Exergy of the streams, exergy efficiency and exergy losses for each section were calculated from equation (11) and equation (12), these results are presented in Table 7. The results show that the section with highest exergy loss is flasher; this is followed by absorption column, stripping column and heat recovery section. Net exergy loss of the process is about 3.25 MW. In the flasher section effluent hot stream from the top stage of the stripper column is cooled by cooling water in exchanger E102 and turns to a two phase stream. The vapor and the liquid phases are separated in flash drum D-2001. Due to finite temperature difference between process stream 5 and cooling water, also low Carnot number for cooling water as cooling medium in this exchanger (about 0.048), considerable amount of thermal exergy of stream 5 is destroyed in the flasher section. One of the main reasons for exergy destruction in chemical processes is the chemical reactions take occur in the processes [49]. During a chemical reaction, materials with higher chemical exergy convert to materials with lower chemical exergy and considerable
Table 4 Standard chemical exergy. Component
Calculated standard chemical exergy (kJ/kmol)
standard chemical exergy reported by Kotas (kJ/kmol)
CH4 H2O H2 O2 CO2 CO N2
840310 116875 238520 3985 21004 276354 734
836510 11710 238490 3970 20140 275430 720
V. Feyzi et al. / Energy 118 (2017) 77e84
81
Fig. 3. Input and output exergy streams for each section.
Table 6 The exergy values of the streams. Stream
Temperature ( C)
Pressure (bar-g)
Mass Flow (ton/hr)
Physical Exergy (Mj/hr)
Chemical Exergy (Mj/hr)
AMINE BFW FEED ABSTOP ABSBOTT 2 SIN GAS SEPLIQ rich 5 4 CO2 16 6 9 11 lean
37.02 126 50 37.6 49.85 35 34.95 34.95 100.5 94.17 50 50 50 120.8 72.1 36.5 37.02
15.6 50 15.8 15.6 15.8 15.6 15.4 15.4 15.8 0.36 0.36 0.36 0.36 0.94 0.94 0.94 15.6
75 0.159 21.47 18.38 78.25 18.38 18.36 0.0185 78.25 4.795 4.795 3.243 1.556 75.03 75.03 75.03 75.03
3368.014 67.31 2668.2 1883.3 6560.9 1691.8 1968.2 0.782 21427.6 4133.4 427.4 269 1.587 133432.9 13046.4 3116.4 3368.4
654551.4 70.23 526935.7 526134.5 656085.6 526080.2 400770.9 0.882 656207.1 1674.7 1674.7 15556.9 0.134 655262.3 655262.3 654554.2 654554.2
in the syngas stream. The results are presented in Fig. 4. Piperazine concentration in a-MDEA solvent is normally in the
Table 7 Exergy analysis results. Section
Exergy efficiency
Irreversibility (Mj/hr)
Sections contribution to total exergy loss (%)
Absorber column Stripper column Heat recovery section Flasher Pump Total
0.33
3152
26.9
0.42
2933.3
25
0.87
1934.3
16.5
0.41 0.93
3690 17.2 11726.8
31.5 0.14 100
amount of heat is released. The exergy loss for the absorber column is relatively high, because of the reactions between CO2 and MDEA solvent and the heat released in the reactions. In this study we investigate the effect of the piperazine concentration in a-MDEA solvent on the CO2 removal. Simulation results show that increasing the piperazine concentration increases the absorption ability of the a-MDEA and reduces the CO2 amount
Fig. 4. Effect of piperazine concentration on the CO2 concentration in syngas stream.
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V. Feyzi et al. / Energy 118 (2017) 77e84
0.2
The results of the exergy analysis showed that decreasing the amine flow rate from 75 ton/hr to 54.5 ton/hr. decreases the total exergy loss to 2.63 MW (see Fig. 6). Finally, to have a better view on the influence of the modifications, it will be interesting to investigate the change of energy and costs in unit. Therefore, in continue total utilities cost and energy consumption of the base and modified processes are shown in Table 9. The results indicate that total utilities cost and total energy consumption decrease by 25% and 18%, respectively.
0
5. Discussion
1.2
0.87
0.91
1 0.8 0.6
0.53 0.41 0.44
Pump
0.42
Flasher
Heat recovery section
Modified case
0.33 0.35
Stripper
0.4
Exergy Efficiency
0.93
0.97
Absorber
Base case
Fig. 5. Comparison of exergy efficiency for the base case and the modified case.
Table 8 Exergy analysis results for the modified plant.
Absorber column Stripper column Heat recovery section Flasher Pump
Irreversibility (Mj/hr)
0.35 0.53 0.91 0.44 0.97
3091 1917.7 963.3 3520 4.8
4000 3520 3152 2933.3
3091
3500 3000 2500
1934.3
1917.7
2000 1500
963.3
1000
Exergy Loss (kj/hr)
3690
Exergy efficiency
500 17.2
4.8
Pump
0 Flasher
Heat recovery section
Stripper
Modified case
Base case
Absorber
Fig. 6. Comparison of exergy loss for the base case and the modified case.
range of 3e5 wt% [56]. This value in the a-MDEA solvent using in Fanavaran petrochemical company is about 3.5 wt% and amount of CO2 in syngas stream is 4.8 ppm (see Fig. 5). By increasing the piperazine concentration from 3.5% to 4.7% the amine circulation rate decreases from 75 ton/hr. to 54.5 ton/hr, while CO2 composition in the syngas stream remains at 4.8 ppm. By declining the rate of amine circulation the exergy losses in the process decrease due to reduce the require power of pump and the reboiler duty in stripping column. The results of the exergy analysis which was done on the simulation of modified plant are shown in Table 8.
Table 9 Total utilities cost and total energy consumption for the base and modified cases. Total utilities costs (USD/Yr) Total energy consumption (kw) The base case 2.26Eþ06 The modified case 1.69Eþ06
7.75Eþ03 6.33Eþ03
The simulation of Aspen HYSYS shows that the energy consumption is about 1.3 MJ/kg CO2; this value is in agreement with its industrial value. In exergy terms, the total exergy loss is about 0.55 MJ/kg CO2. According to the results, an interesting observation is that in the pump and heat recovery the exergy losses are rather small, while in the flasher the loss is substantial. This situation is similar to the study of Geuzebroek et al. [41]. Although the exergy loss of the stripper in this study (0.14 MJ/kg CO2) is the same of their study, there is a big difference between the exergy losses of absorbers. The absorber exergy loss in this study is 200% smaller than their study. In fact, large exergy loss in certain parts of the equipment corresponds to a large local driving force. Therefore, the driving force of MEA is further than a-MDEA. It corresponds to the intrinsic properties of MEA as a primary amine and MDEA as a tertiary amine. Thus, contrast to the Geuzebroek et al. conclusions [41], it can be concluded that changing the chemical used in the capture process is effective on the exergy loss. According to the obtained results, for the same equipment and process the amount of amine flow rate is a very impressive parameter to reduce the exergy loss. Our investigation indicates when the amine flow rate decreases by 27%, the exergy loss, total energy consumption and utilities cost decrease by 19%, 18% and 25%, respectively. Therefore, it may be prudently concluded that changing the solvent and operation conditions in order to decrease the amine flow rate is very effective to reduce the exergy loss, the total energy consumption and consequently the total utilities cost. Due to the high amounts of exergy loosed in the stripping column and heat recovery section it can be mentioned that one of the main sources of exergy destruction in this process is heat transfer through finite temperature difference in heat transferring equipments such as heat exchangers and reboiler of stripping column. As flow rate of amine is decreased, amount of required heating in reboiler and amount of heat transferred between hot and cold steams in heat exchangers decreases, this fact is the reason for the largest improvement observed in heat recovery section and stripping column. 6. Conclusions In this study, we have simulated a CO2 capture plant in CO production unit of Fanavaran Petrochemical Company with Aspen HYSYS simulator. In this plant, CO2 is removed from a synthesis gas stream by using a-MDEA as the absorption medium. The simulation validation is performed by comparison of field data from the plant in thirteen different operational conditions (such as feed flow, feed composition and composition of amine solvent) with the results from the simulation. This comparison shows that the simulation can predict the actual data with a reasonably good approximation. Exergy analysis has applied to achieve a clear insight where the actual exergy loss takes place. Exergy analysis result shows that the flasher had highest rate of exergy loss, it followed by the absorber column, stripping column and heat recovery section. The main
V. Feyzi et al. / Energy 118 (2017) 77e84
reasons for exergy loss in this process return to chemical reactions in absorber column, finite temperature differences in heat exchangers, mixing of streams at different thermodynamics condition, composition and friction. Exergy efficiency of the process is increased by increasing piperazine concentration in amine solvent, which leads to decline the amine flow rate. Finally, the modifications reduce the total exergy loss from 3.25 MW to 2.63 MW.
[17] [18] [19]
[20]
Nomenclature [21]
T P T0 P0 Exph h S ExCH xi R 3 0i
Ex I_ m_ Q_
Temperature, K Pressure, kPa Ambient temperature, K Ambient pressure, kPa Stream's specific physical exergy, kJ/kmol Specific enthalpy, kJ/kmol Specific entropy, kJ/kmol.K Stream's specific chemical exergy, kJ/kmol Component i molar composition in stream Universal gas constant taken as 8.314 kJ/kmol.K Component i standard chemical exergy, kJ/kmol Stream's specific total exergy, kJ/kmol Exergy loss rate, kJ/hr Stream's molar flow rate, kmol/hr
Heat transfer rate, kJ/hr Heat source temperature, K Shaft work, kJ/hr hEx Rational exergy efficiency DExsink Total rate of exergy transfer to sinks of exergy, kJ/hr DExsources Total rate of exergy transfer from sources of exergy, kJ/hr
Tj w_
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