- Email: [email protected]
Screening of physical-chemical biphasic solvents for CO2 absorption
International Journal of Greenhouse Gas Control 85 (2019) 199–205
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Screening of physical-chemical biphasic solvents for CO2 absorption ⁎
T
Mimi Xu, Shujuan Wang , Lizhen Xu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Engineering Research Center for Ecological Restoration and Carbon Fixation of Saline-alkaline and Desert Land, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 absorption Biphasic solvent Physical-chemical solvent
21 pairs of physical-chemical biphasic solvents were screened out using a fast screening facility and the distribution of components was detected both in upper and lower phases after absorption. The physical solvents and amines were enriched in opposite phases due to the limited solubility of absorption products in the physical solvents, and the enrichment of components gets higher as the loading time goes by. Comparisons of the cyclic capacity showed that MEA and DETA were much better main solvents than BDA in biphasic solvents, and 2.5 DETA-4 sulfolane has the highest CO2 cyclic capacity of 4.486 mol L−1, which was much higher than that of 30 wt% MEA. The concentrations of physical solvents influence the cyclic capacity a lot rather than the types of physical solvents. Then the phase separation character and absorption performance were analyzed for solutions in different amine to physical solvent concentration ratios, and the results showed that increasing the concentration of the physical solvent within certain range benefits to raising cyclic absorption capacity and reducing the energy penalty. At last, the sensible heat and vaporization heat of two biphasic solvents were estimated with a flash module of Aspen Plus, which showed less energy penalty than that of 30 wt% MEA.
1. Introduction The global warming, one of the most environmental vital issues in the world, has attracted widespread attention (Keith, 2009). The emissions of greenhouse gases, especially CO2, have long been considered as the major cause of global warming (Boot-Handford et al., 2014). Therefore, various technologies are being developed to reduce CO2 emissions. Among them, amine scrubbing is one of the most promising technologies for its flexibility and easy implementation in existing power plants (Rochelle, 2009). Aqueous alkanolamine solutions such as monoethanolamine (MEA), a primary amine, has a fast reaction rate and high absorption capacity, so they have been widely researched (Ma’mun et al., 2005). However, the high energy penalty of the MEA regeneration process and high degradation rate hinder its extensive use. Researchers are looking for other solvents with better performance by screening all kinds of amines, such as piperazine (PZ), methyldiethanolamine (MDEA), and diethanolamine (DEA). Those amine solutions, known as first generation absorbents, have various advantages as large absorption capacity, lower degradation rate, or higher reaction rate. Second generation absorbents like MEA + PZ, PZ + 2-(Diethylamino)-ethanol (DEEA), and MDEA + PZ have then been proposed to combine the merits of the different amines by mixing them together in various ratios (Arshad et al., 2014;
⁎
Khan et al., 2017; Sutar et al., 2013). Another method to reduce the energy consumption is to use liquidliquid biphasic solvents that form two immiscible liquid phases after loading CO2 with the CO2 mainly in one phase. After separation, only the CO2 rich phase is sent to the desorber for regeneration and the other phase is sent directly back to the absorber, which significantly reduces the solution volume for regeneration and the energy penalty (Pinto et al., 2014a). Researchers have screened some existing organic amine solvents with the potential for phase splitting. Xu et al. (2013a) developed a biphasic solvent, 2B4D (2 M 1,4-Butanediamine + 4 M (Diethylamino)-ethanol) with a cyclic loading 46% higher, a cyclic capacity 48% higher and a cyclic efficiency 11% higher than 30 wt% MEA. The biphasic solvent, DMX™ was shown to have a low regeneration energy penalty of 2.1 GJ ton−1, but the authors did not indicate the exact compositions (Gomez et al., 2014; Raynal et al., 2014). Ciftja et al. (2013) and Pinto et al. (2014b) studied a blend of N-methyl1, 3-diaminopropane (MAPA) and DEEA which exhibited significant liquid-liquid phase separation after CO2 absorption and explained the mechanism. Ye et al. (2015) screened more than 50 mixed amine solutions to find that a good biphasic solvent should have absorption accelerators and regeneration promoters. Most existing studies have focused on mixed amine solvents. Though those biphasic solvents have better cyclic capacities than 30 wt% MEA, the CO2 rich phase tends to
Corresponding author. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.ijggc.2019.03.015 Received 13 April 2018; Received in revised form 20 February 2019; Accepted 14 March 2019 Available online 02 May 2019 1750-5836/ © 2019 Published by Elsevier Ltd.
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
The amine concentrations were determined by titration against 0.1 N H2SO4 using a Metrohm 809 Titrando auto titrator. The CO2 loading was determined by titration using the barium carbonate precipitation method (Hilliard, 2008). Hilliard estimated that the loading error of the precipitation titration method was ± 2% (Hilliard, 2008). An FTIR system from ABB was used to quantify the physical solvents in the upper and lower phases. Standard physical solvent solutions of various concentrations were prepared and detected by FTIR for calibration with an uncertainty of less than 10%.
have a higher viscosity which slows the operation and cause severe amine degradation with a high amine concentration in the stripper. Physical-chemical biphasic solvents, which are mixtures of traditional organic amines and physical solvents, have been proposed to improve the efficiency of amine utilization and reduce the water ratio in the solution. Various amines were chosen to act as the main absorption solvent to react with CO2 in biphasic solvents. Physical solvents have been screened to replace some of the water and to act as a phase splitting accelerator to enrich the carbamate and bicarbonate in one phase when CO2 is absorbed. Zhang et al. (2017) studied MEAalcohol-H2O solutions showing that mixed solvents with appropriate concentrations of n-propanol, isopropanol or tert butanol gave good phase separation. A physical-chemical biphasic solvent containing Diethylenetriamine (DETA), sulfolane, and water was reported to have a cyclic loading 35% higher than that of 30 wt% MEA aqueous solution (Luo et al., 2016). Anhydrous physical-chemical biphasic solvents, such as 2-(methylamino)ethanol (MMEA) + bis(2-ethoxyethyl) ether (DEGDEE), 2-(ethylamino)ethanol (EMEA) + bis(2-ethoxyethyl) ether (DEGDEE) and MEA-octanol have also been studied (Barzagli et al., 2017; Kim et al., 2014). This work tested 29 pairs of amines and physical solvents in various concentrations, and 21 pairs of them were screened out as biphasic solvents. Four physical solvents were demonstrated to be potential phase separation accelerators. The absorption capability of potential physical-chemical biphasic solvents and the phase separation process were then studied for these mixtures.
3. Results and discussion 3.1. Preliminary screening of physical-chemical solvents In the preliminary screening, DGM, DMSO, sulfolane and n-propanol that do not react with CO2 were chosen as the physical solvents, BDA, MEA, and DETA which have better CO2 absorption performance were selected as the main solvents. All the solvents structures are shown as following:
2. Experimental The chemicals used in this work were DETA (> 99 wt%), MEA (> 99 wt%), BDA (> 98 wt%), Diethylene glycol dimethyl ether (DGM) (> 99.5 wt%), Dimethyl sulfoxide (DMSO) (> 99 wt%), n-propanol (> 99.5 wt%) and sulfolane (analytical reagent) from the Aladdin Reagent Company and were used as received. CO2 (≥99.9% purity) and N2 (≥99.9% purity) were obtained from Air Liquide (China) Company. Distilled deionized water was used for preparing the solutions. The fast screening facility shown in Fig. 1 was built to measure the absorption and desorption capacities described by Xu et al. (2012). The absorption was conducted at 40℃ with the desorption conducted at 90℃ at atmospheric pressure. In the absorption mode, CO2 and N2 were used to simulate the flue gas with 12% CO2 by volume and a total gas flow rate of 657 ml min−1. Equilibrium was assumed to be reached when the outlet CO2 concentration reached 12%. In the desorption mode, N2 was used to sweep the desorbed CO2 through the condenser, acid washing, a drier and an analyzer at a flow rate of 694 ml min−1. Equilibrium was assumed to be reached when the outlet CO2 concentration was less than 0.1%.
Cyclic capacity (ΔR) , cyclic loading (Δα ) and cyclic efficiency (η) are important evaluation parameters for CO2 absorption solvents.
ΔR = Camine (αrich − αlean )
(1)
Δα = αrich − αlean
(2)
(αrich − αlean ) αrich
(3)
η=
Where ΔR is the cyclic capacity, mol L−1; Camine is the molar concentration of amine, mol L−1; αrich and αlean are the CO2 loading of rich and lean solvent, mol mol−1; Δα is the cyclic loading, mol mol−1; η is the cyclic efficiency. For a biphasic solvent, the cyclic capacity, cyclic loading, and cyclic efficiency were defined based on the desorption solution. As shown in Table 1, 29 pairs of physical solvent and aqueous amine mixtures were screened using various concentration ratios. The ratios were carefully chosen according to their viscosity, solubility and potential absorption capacity. The results showed that only 2BDA5DMSO, 3BDA-3.3DMSO, 2DETA-5DMSO, 3DETA-4DMSO, 3DETA3DMSO, 5MEA-5DMSO, 4MEA-3DMSO, and 5MEA-3DMSO solvents remain homogeneous after absorption, while the rest 21 solvents have a clear, rapid liquid-liquid phase separation after CO2 absorption. These physical-chemical biphasic solvents having rapid phase separation were Table 1 Solvent concentrations used for the preliminary screening tests (M/M).
BDA DETA MEA
DGM
DMSO
Sulfolane
n-propanol
2/2,2/2.5,2.5/2.5 2/2.5,2/3,2.5/ 2,2.5/2.5 5/2.5,4/2.5,6/ 2.5,6.5/2.5
2/5,3/3.3 2/5,3/4,3/ 3,2.6/4.8 5/5,4/3,5/3
2/4 2.1/3.6,2.5/ 3,2.5/4 5/4
2/7 2/7 4.6/4.7,5/7
Note: aM is mol L−1; bThe first number is the concentration of amine and the second number is the concentration of physical solvent.
Fig. 1. Schematic diagram of the absorption-desorption apparatus. 200
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Table 2 CO2 concentrations in the upper and lower phases of physical-chemical biphasic solvents. Solvent No.
Solvent
CO2 concentration in upper phase (mol L−1)
CO2 concentration in lower phase (mol L−1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
2DETA-2.5DGM 2DETA-3DGM 2.5DETA-2DGM 2.5DETA-2.5DGM 5MEA-2.5DGM 4 MEA-2.5DGM 6MEA-2.5DGM 6.5MEA-2.5DGM 2.6DETA-4.8DMSO 2.1DETA-3.6sulfolane 2.5DETA-3sulfolane 2.5DETA-4sulfolane 4.5MEA-4.7(n-propanol) 5MEA-7(n-propanol) 2BDA-4sulfolane 2BDA-7(n-propanol) 2DETA-7(n-propanol) 2BDA-2.5DGM 2BDA-2DGM 2.5BDA-2.5DGM 5MEA-4sulfolane
0.153 0.112 0.131 0.081 0.089 0.077 0.213 0.049 0.887 4.846 4.891 6.705 0.246 0.369 3.643 0.027 1.126 0.016 0.086 0.192 3.826
4.985 5.361 5.141 5.567 3.575 2.564 4.339 4.688 4.295 0.062 0.026 0.089 3.203 4.235 0.038 4.439 5.446 3.357 2.988 4.176 0.024
Note:
a
The number before each solvent name is the molar concentration in mol L−1.
then analyzed further. 3.2. Components distribution The component distributions in the upper and lower phases after absorption were then analyzed. The CO2 concentrations and amine concentrations were directly titrated, and the physical solvents were quantified with FTIR. Then the distribution mass fractions were calculated according to the concentrations and weights in the upper and lower phases, as following:
Du =
mu mu + ml
Dl = 1 − Du
(4) (5)
Du : distribution mass fraction of one component in the upper phase; Dl : distribution mass fraction of one component in the lower phase; mu : mass of one component in the upper phase; ml : mass of one component in the lower phase. The solvents screened out were numbered as shown in Table 2. The distribution mass fractions of the amines, physical solvents and water in the upper and lower phases of the loaded solvents are shown in Figs. 2–4. For all of these biphasic solvents, the amines were enriched in one phase after CO2 absorption with the distribution mass fractions in this phase all higher than 90%, which means that most CO2 were rich in the same phase since amines were the only chemical absorbent and then only this phase needed to be desorbed. This also benefits the amine utilization rate. Except for Solvents No. 10, 11, 12, 15 and 21, the amines were mostly concentrated in the lower phase due to the smaller density of the physical solvent in those solutions. And the density of the physical solvent-rich phase in Solvents No. 10, 11, 12, 15 and 21 were obviously higher than that of the amine-rich phase. The CO2 absorption capacity differs significantly in the upper and lower phases as shown in Table 2, which is consistent with the amine distribution mass fraction here. The water and physical solvents distribution mass fractions should also be considered because they influence the volume of the desorption solution. As shown in Fig. 3, most of the physical solvents were enriched in the opposite phase from the amines, but their enrichment ratios were lower than those of the amines. Table 2 shows the CO2 concentrations in two immiscible phases, which indicates that only a little reaction product of the amines and the CO2 was dissolved in the
Fig. 2. Amines mass fractions in the upper and lower phases of the CO2 loaded solutions.
physical solvents phase and the limited mutual solubility between them caused the liquid-liquid phase separation, which is consistent with the results of Luo et al. (2016) However, for Solvent No. 6 and 9, more than 70% of the physical solvent was in the amine-rich phase which was higher than the other solutions. The reason is that the concentration ratio of amine to physical solvent in these two fresh solution is a bit small, and the concentration of reaction products is lower so that much more physical solvent dissolved in the amine-rich phase. As shown in Fig. 4, water existed in both phases, though the water distribution mass fraction in amine-rich phase is higher than that in the other phase, the enrichment is not so high as that of amines. For biphasic solvents, only the amine-rich phase is sent to the stripper, so if there is less water and less physical solvent concentrated in the aminerich phase, the sensible heat and vaporization heat would be cut down. Because the sensible heat gets down with the decrease of the volume of CO2-rich solution, and the vaporization heat gets down with the decrease of the water content in solution which leads to lower partial pressure of water in the top of the stripper (Chakma, 1997; Leites, 1998; 201
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Fig. 5. Cyclic absorption property of the CO2-rich phase.
loading was measured with the titration method. The cyclic capacity, the cyclic loading and the cyclic efficiency of the CO2-rich phase are showed in Fig. 5, where Solvent No. 22 is 5 mol L−1 MEA from Xu et al. (2013b). When comparing different kinds of amine solvents in the same desorption conditions, those solvents with a higher cyclic capacity are potential to reduce sensible heat (Aronu et al., 2011; Rochelle, 2009). Furthermore, increasing the cyclic loading rather than amine concentration would be a better way to increase cyclic capacity, because high amine concentration leads to higher volatilization and severer corrosion. All of these biphasic solvents have greater cyclic capacities than 30 wt% MEA. Solvent No. 12 has the highest absorption capacity of 4.486 mol L−1, which is more than twice the capacity of 30 wt% MEA. The reason is that the amine concentration in the CO2-rich phase increases significantly due to the phase splitting. The cyclic loadings for the different amines solvents differ greatly since MEA has only one amino group, BDA has two, and DETA has three ones, which influence their total loadings. For those solvents with the same kind of amine and equal amine concentration but different physical solvents, the cyclic capacities were similar which indicates that the type of physical solvent has little effect on the absorption capacity. However, the physical solvents concentration was quite important as shown by comparing the results of Solvents No. 1 and 2, 3 and 4, 11 and 12, and 18 and 19, where the absorption capacity varies with various physical solvent concentrations. The additional experiments discussed in part 3.4 provide more evidence. Comparing the amine structures, the cyclic loading increases with the increasing number of amino groups in the amine molecule for both primary and secondary amines. These solvents with MEA (No. 5, 14, and 21) and the solvent with BDA (No. 20) had the same primary amino group concentrations, but those with MEA had higher cyclic capacities than that of BDA solution. The reason is that the amino group in BDA absorbs almost as much CO2 as MEA, but it desorbs less than MEA at the same operation conditions, which indicates that the primary amino group of BDA is more difficult to regenerate CO2 than MEA. As for the influence of the secondary amino group, Solvents No. 1 and 7 had the same total amino group concentration of 6 mol L−1 but a third of the amino groups were secondary amino groups in Solvent No. 1. The results showed that Solvents No. 1 and 7 had roughly the same cyclic absorption capacities, which indicates that the secondary amino group of DETA behaves as good as the primary amino group on the absorption. Thus, both MEA and DETA have good absorption and desorption characteristics, so further studies of the energy penalty and physical properties are needed to compare these two solvents.
Fig. 3. Physical solvents mass fractions in the upper and lower phases of the CO2 loaded solutions.
Fig. 4. Water mass fractions in the upper and lower phases of the CO2 loaded solutions.
Nwaoha et al., 2017). Comparing Solvent No. 1, 4, 5 and 6, with the amine concentration increases in the fresh solution, less physical solvent distributes in the amine-rich phase after loading CO2 but more water exists in the amine-rich phase. When the fresh solution contains more physical solvent, according to the results of Solvent No. 3, 4, 11, 12, 18 and 19, the amine-rich phase seems to have less physical solvent and less water after absorption. So when screening physical-chemical biphasic solvents, increasing the concentration of physical solvents properly in fresh solution can contribute to cutting down energy penalty, but the influence of increasing amine concentration is uncertain as the water and physical solvent concentration in amine-rich phase change in different ways.
3.4. BDA-DGM phase separation mechanism 3.3. Comprehensive cyclic absorption property The phase separation process of BDA and DGM mixtures was studied to further understand the driving mechanisms. Nine pairs of BDA-DGM
After absorption, the CO2-rich phase was desorbed and the CO2 202
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Fig. 6. Component distributions after CO2 absorption in BDA/DGM solutions with various concentrations.
solutions with different concentration ratios were chosen, with the same experiment process and detection methods as in the screening experiment. As shown in Fig. 6, DGM was enriched in the upper phase, but CO2 and BDA mainly existed in the lower phase after the absorption. The DGM concentration in the upper phase and the BDA concentration in the lower phase increased steadily as the DGM or BDA concentration in the fresh solvent increased, while the DGM concentration in the lower phase and the BDA concentration in the upper phase slightly decreased. The CO2 concentration in the amine-rich phase increased firstly with increasing DGM or BDA concentration, but then started to decrease after reaching a maximum at 3.5DGM or 2.5BDA. Thus, the CO2 absorption capacity depends greatly on the BDA to DGM concentration ratio. Low BDA concentration limits the total solution capacity, while a high ratio of BDA to DGM has a negative impact on the absorption because of the high viscosity. However, high absorption capacity doesn’t mean high cyclic capacity. As shown in Fig. 7, the 2BDA2.5DGM solution has the highest cyclic capacity of 2.536 mol L−1 and the highest cyclic loading of 0.796 mol mol−1 amine. The reason is that the amine concentration in amine-rich phase increased with the increasing concentration of physical solvent and amine in fresh solution which may cause an increase in viscosity. High viscosity is detrimental to absorption and desorption, so there must be an optimal concentration for best cyclic capacity. For biphasic solvents, the cyclic capacity is much more important as it stands for the solution utilization rate and
partly determines the regeneration penalty. Considering the influence of concentration ratio on energy penalty as stated in part 3.2, there is a suggestion for biphasic solvent screening: firstly choosing the proper concentration ratio by comparing cyclic capacity, then adjusting the physical solvent and amine concentrations according to part 3.2 to get an optimal energy consumption. Then 2BDA-2.5DGM solution was used to further characterize the phase separation process as the CO2 loading in the lower phase increased. Five groups of fresh 2BDA-2.5DGM solutions were used to absorb CO2 at the same absorption mode but for different absorption times. As time goes by, more CO2 would be absorbed and the loading gets higher. Indeed, the five groups of solutions with different absorption times have different CO2 loadings which were demonstrated by titration. The concentrations of BDA and DGM, and CO2 capacities in upper and lower phases were measured for each group respectively, as shown in Fig. 8. The BDA concentration and the CO2 capacity in the lower phase, and the DGM concentration in the upper phase increase with time, which means that the separation process kept happening in the whole period. The DGM migrated from the lower phase to the upper phase while the BDA migrated to the opposite direction as the loading increases. These results show that some of the DGM separated from the reaction products and migrated to the upper phase due to the limited solubility of DGM in the BDA and CO2 reaction products and its relatively smaller density.
Fig. 7. Cyclic absorption capacity for various BDA-DGM ratios.
Fig. 8. Phase separation of 2BDA-2.5DGM with time. 203
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
3.5. Estimation of the sensible heat and vaporization heat The lower desorption energy consumption is a key advantage of the biphasic solvents. The total regeneration energy requirement can be divided into the heat of reaction (qreac ), the sensible heat (qsens ), and the vaporization heat of water (qvap ). The heat of reaction mainly depends on the chemical reactions between the solvents and CO2, influenced by the temperature and CO2 loading (Kim and Svendsen, 2007). The sensible heat and the vaporization heat are considered here as following:
qsens =
ρCP ΔT (αrich − αlean ) Camine
qvap = ΔHvap·
ΔHvap PH2O nw nw = ΔHvap· = ∙ mCO2 MCO2·nCO2 MCO2 PCO2
(6)
(7)
The calculation of heat consumption here was based on the CO2-rich phase, where ρ is the mass density (kg m−3) and CP is the specific heat (J kg−1 K−1) of the CO2-rich phase which is to be heated to the desorption temperature, and ΔT is the temperature difference between the heat exchanger outlet and the desorbing temperature. The desorber temperature is normally 118℃ and the heat exchanger outlet temperature is 105℃, therefore ρ and CP were set at the average temperature of 111℃ to estimate the sensible heat (Xu, 2014). Camine is the amine concentration (mol L−1) in the CO2-rich phase; αrich and αlean are from the earlier experimental results; ΔHvap is the latent heat of water (kJ mol−1); n w is the total molar amount of evaporation water in desorber (mol); mCO2 is the weight of desorbed CO2 (g); MCO2 is the molecular weight (g mole−1) of CO2, nCO2 is the total molar amount of CO2 released in the desorber (mol); PH2 O and PCO2 are the gaseous phase partial pressures of water and CO2 at the top of the desorber. The qvap is the amount of energy needed to vaporize the water in the CO2 saturated amine solution to provide the stripping vapor and it was derived in Eq. (7) where the vaporization heat of water was expressed with the partial pressure of matters in the gas phase finally. In this work, a flash module was used to simulate the vapor-liquid equilibrium with the physical parameters based on the amine package ELECNRTL-Rate-Based-MEA-Model in Aspen Plus V8.4 (Sherman and Rochelle, 2017; Zhang et al., 2011). Oexmann (Oexmann and Kather, 2009, 2010) used Aspen Plus to create a rigorous and consistent model to predict the thermodynamic characteristics of the H2O-MEA-CO2 system and to describe the thermodynamics associated with the absorption-desorption process. The modeling method were used here to perform simple flash calculations to analyse the vapor-liquid-equilibria of the CO2-rich phase of CO2-loaded biphasic solvents. Figs. 9 and 10 show that the model predictions for the MEA solution are in good agreement with the data from Aronu et al. (2012), Jou et al. (1995) and Page et al. (1993). The model was then used to calculate the density, specific heat and vapor-liquid equilibrium in the CO2 rich phase for Solvent No. 7 and 21 because the main absorbent in these two solution is MEA and the physical solvents are less volatile. Accurate modeling of thermodynamic properties of CO2 absorption in aqueous alkanolamine solutions is essential for simulation and design of such CO2 capture processes (Zhang et al., 2011). As shown in Table 3, the densities and specific heats of these three solvents are similar, while the sensible heat of 6MEA-2.5DGM and 5MEA-4sulfolane are much lower, because the cyclic capacity of 5MEA is only about half of that of the others. The vaporization heat of the two mixtures are also much lower because the water and physical solvent are distributed into the CO2-lean phase with a lower water ratio in the CO2-rich phase. Thus, when desorbing the same amount of CO2 in the desorber, less volume of solution is heated and less water is evaporated with the biphasic solvents. The estimation indicates that the biphasic solvents have great potential to reduce the energy cost because the cyclic capacities and component distributions greatly influence qsens and qvap .
Fig. 9. Comparison of the model predictions to experimental data for the partial pressure of CO2 with 30 wt% MEA.
Fig. 10. Comparison of the model predictions with experimental data for the density and specific heat at 40℃.
4. Conclusions The present study screened out 21 pairs of physical-chemical biphasic solvents, the CO2 cyclic absorption capacity and the component distributions were analyzed. The results show that 2.5DETA-4sulfolane gave the best results with 4.486 mol L−1 cyclic capacity and 0.994 mol mol−1 cyclic loading, much higher than those of 30 wt% MEA. The physical solvents and the amines distributed in opposite phases and the distribution coefficients increased with increased CO2 loading. The cyclic capacities show that MEA and DETA behave better than BDA in biphasic absorption process, but the type of physical solvents shows little influence on the absorption capacity. Tests with various concentrations of solutions to absorb CO2 showed that much more physical solvent migrated to one phase while the amine migrated to the other phase for fresh solutions with higher physical solvent concentrations or higher amine concentrations. The component concentration ratio of biphasic solvent has great impact on the desorption volume and cyclic capacity which should be both considered in the screening of biphasic solvents. At last, the estimation of the sensible and vaporization heat for two solvents showed that the physical-chemical biphasic solvents have great potential for reducing the solvent regeneration energy.
204
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Table 3 Estimation result of parameters and energy penalty. Solvent
△R (mol L−1)
ρ (kg m−3)
CP (kJ kg−1 K−1)
PH2Oa (kPa)
PCO2a (kPa)
qsens (kJ kg−1)
qvap (kJ kg−1)
5MEA 6MEA-2.5DGM 5MEA-4sulfolane
1.55 3.34 3.13
945.82 933.62 967.91
3.96 3.56 3.48
160.17 103.17 113.62
29.07 82.27 74.29
714.42 294.45 317.99
533.35 121.39 148.05
Note:
a
△R is the Cyclic Capacity and PH2O and PCO2 are the partial pressures of H2O and CO2 in the top of the stripper.
Acknowledgments
purification technology. Energy Convers. Manage. 39, 1665–1674. Luo, W., Guo, D., Zheng, J., Gao, S., Chen, J., 2016. CO2 absorption using biphasic solvent: blends of diethylenetriamine, sulfolane, and water. Int. J. Greenh. Gas Control 53, 141–148. Ma’mun, S., Nilsen, R., Svendsen, H.F., Juliussen, O., 2005. Solubility of carbon dioxide in 30 mass % monoethanolamine and 50 mass % methyldiethanolamine solutions. J. Chem. Eng. Data 50, 630–634. Nwaoha, C., Idem, R., Supap, T., Saiwan, C., Tontiwachwuthikul, P., Rongwong, W., AlMarri, M.J., Benamor, A., 2017. Heat duty, heat of absorption, sensible heat and heat of vaporization of 2-Amino-2-Methyl-1-Propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) tri-solvent blend for carbon dioxide (CO2) capture. Chem. Eng. Sci. 170, 26–35. Oexmann, J., Kather, A., 2009. Post-combustion CO2 capture in coal-fired power plants: comparison of integrated chemical absorption processes with piperazine promoted potassium carbonate and MEA. Energy Procedia 1, 799–806. Oexmann, J., Kather, A., 2010. Minimising the regeneration heat duty of post-combustion CO2 capture by wet chemical absorption: the misguided focus on low heat of absorption solvents. Int. J. Greenh. Gas Control 4, 36–43. Page, M., Huot, J.Y., Jolicoeur, C., 1993. A comprehensive thermodynamic investigation of water ethanolamine mixtures at 10-degrees-C, 25-degrees-C, and 40-degrees-C. Can. J. Chem.-Revue Canadienne De Chimie 71, 1064–1072. Pinto, D.D.D., Knuutila, H., Fytianos, G., Haugen, G., Mejdell, T., Svendsen, H.F., 2014a. CO2 post combustion capture with a phase change solvent. Int. J. Greenh. Gas Control 31, 153–164. Pinto, D.D.D., Zaidy, S.A.H., Hartono, A., Svendsen, H.F., 2014b. Evaluation of a phase change solvent for CO2 capture: absorption and desorption tests. Int. J. Greenh. Gas Control 28, 318–327. Raynal, L., Briot, P., Dreillard, M., Broutin, P., Mangiaracina, A., Drioli, B.S., Politi, M., La Marca, C., Mertens, J., Thielens, M.-L., Laborie, G., Normand, L., 2014. Evaluation of the DMX process for industrial pilot demonstration – methodology and results. In: Dixon, T., Herzog, H., Twinning, S. (Eds.), 12th International Conference on Greenhouse Gas Control Technologies, Ghgt-12, pp. 6298–6309. Rochelle, G.T., 2009. Amine scrubbing for CO2 capture. Science 325, 1652–1654. Sherman, B.J., Rochelle, G.T., 2017. Thermodynamic and mass-transfer modeling of carbon dioxide absorption into aqueous 2-amino-2-methyl-1-propanol. Ind. Eng. Chem. Res. 56, 319–330. Sutar, P.N., Vaidya, P.D., Kenig, E.Y., 2013. Activated DEEA solutions for CO2 capture–a study of equilibrium and kinetic characteristics. Chem. Eng. Sci. 100, 234–241. Xu, Z., 2014. Absorption of CO2 With Aqueous BDA/DEEA Blended System. Tsinghua University, Beijing. Xu, Z., Wang, S., Liu, J., Chen, C., 2012. Solvents with low critical solution temperature for CO2 capture. In: Rokke, N.A., Hagg, M.B., Mazzetti, M.J. (Eds.), 6th Trondheim Conference on Co2 Capture, Transport and Storage, pp. 64–71. Xu, Z., Wang, S., Chen, C., 2013a. CO2 absorption by biphasic solvents: mixtures of 1,4butanediamine and 2-(diethylamino)-ethanol. Int. J. Greenh. Gas Control 16, 107–115. Xu, Z., Wang, S., Chen, C., 2013b. Experimental study of CO2 absorption by liquid-liquid biphasic solvents. J. Tsinghua Univ. (Sci. Tech.) 336–341. Ye, Q., Wang, X., Lu, Y., 2015. Screening and evaluation of novel biphasic solvents for energy-efficient post-combustion CO2 capture. Int. J. Greenh. Gas Control 39, 205–214. Zhang, Y., Que, H., Chen, C.-C., 2011. Thermodynamic modeling for CO2 absorption in aqueous MEA solution with electrolyte NRTL model. Fluid Phase Equilib. 311, 67–75. Zhang, W., Jin, X., Tu, W., Ma, Q., Mao, M., Cui, C., 2017. Development of MEA-based CO2 phase change absorbent. Appl. Energy 195, 316–323.
Financial supports from the National Key R&D Program (2017YFB0603301-2) and NSFC (51576108) is greatly appreciated. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ijggc.2019.03.015. References Aronu, U.E., Hoff, K.A., Svendsen, H.F., 2011. CO2 capture solvent selection by combined absorption-desorption analysis. Chem. Eng. Res. Des. 89, 1197–1203. Aronu, U.E., Gondal, S., Hessen, E.T., Haug-Warberg, T., Hartono, A., Hoff, K.A., Svendsen, H.F., 2012. Solubility of CO2 in 15, 30, 45 and 60 mass% MEA from 40 to 120 degrees C and model representation using the extended UNIQUAC framework. Chem. Eng. Sci. 78, 246–247. Arshad, M.W., Fosbol, P.L., von Solms, N., Svendsen, H.F., Thomsen, K., 2014. Equilibrium solubility of CO2 in alkanolamines. In: Rokke, N.A., Svendsen, H. (Eds.), 7th Trondheim Conference on Co2 Capture, Transport and Storage, pp. 217–223. Barzagli, F., Mani, F., Peruzzini, M., 2017. Novel water-free biphasic absorbents for efficient CO2 capture. Int. J. Greenh. Gas Control 60, 100–109. Boot-Handford, M.E., Abanades, J.C., Anthony, E.J., Blunt, M.J., Brandani, S., Mac Dowell, N., Fernandez, J.R., Ferrari, M.C., Gross, R., Hallett, J.P., Haszeldine, R.S., Heptonstall, P., Lyngfelt, A., Makuch, Z., Mangano, E., Porter, R.T.J., Pourkashanian, M., Rochelle, G.T., Shah, N., Yao, J.G., Fennell, P.S., 2014. Carbon capture and storage update. Energy Environ. Sci. 7, 130–189. Chakma, A., 1997. CO2 capture processes – opportunities for improved energy efficiencies. Energy Convers. Manage. 38, S51–S56. Ciftja, A.F., Hartono, A., Svendsen, H.F., 2013. Experimental study on phase change solvents in CO2 capture by NMR spectroscopy. Chem. Eng. Sci. 102, 378–386. Gomez, A., Briot, P., Raynal, L., Broutin, P., Gimenez, M., Soazic, M., Cessat, P., Saysset, S., 2014. ACACIA project – development of a post-combustion CO2 capture process. Case of the DMX (TM) Process. Oil Gas Sci. Technol.-Revue D Ifp Energies Nouvelles 69, 1121–1129. Hilliard, M.D., 2008. A predictive thermodynamic model for an aqueous blend of potassium carbonate, piperazine, and monoethanolamine for carbon dioxide capture from flue gas. Carbon Dioxide—Absorption and Adsorption. Jou, F.Y., Mather, A.E., Otto, F.D., 1995. The solubility of CO2 in a 30-mass-percent monoethanolamine solution. Can. J. Chem. Eng. 73, 140–147. Keith, D.W., 2009. Why capture CO2 from the atmosphere? Science 325, 1654–1655. Khan, A.A., Halder, G.N., Saha, A.K., 2017. Experimental investigation on efficient carbon dioxide capture using piperazine (PZ) activated aqueous methyldiethanolamine (MDEA) solution in a packed column. Int. J. Greenh. Gas Control 64, 163–173. Kim, I., Svendsen, H.F., 2007. Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(aminoethyl)ethanolamine (AEEA) solutions. Ind. Eng. Chem. Res. 46, 5803–5809. Kim, Y.E., Park, J.H., Yun, S.H., Nam, S.C., Jeong, S.K., Yoon, Y.I., 2014. Carbon dioxide absorption using a phase transitional alkanolamine-alcohol mixture. J. Ind. Eng. Chem. 20, 1486–1492. Leites, I.L., 1998. Thermodynamics of CO2 solubility in mixtures monoethanolamine with organic solvents and water and commercial experience of energy saving gas
205
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Screening of physical-chemical biphasic solvents for CO2 absorption ⁎
T
Mimi Xu, Shujuan Wang , Lizhen Xu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Engineering Research Center for Ecological Restoration and Carbon Fixation of Saline-alkaline and Desert Land, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 absorption Biphasic solvent Physical-chemical solvent
21 pairs of physical-chemical biphasic solvents were screened out using a fast screening facility and the distribution of components was detected both in upper and lower phases after absorption. The physical solvents and amines were enriched in opposite phases due to the limited solubility of absorption products in the physical solvents, and the enrichment of components gets higher as the loading time goes by. Comparisons of the cyclic capacity showed that MEA and DETA were much better main solvents than BDA in biphasic solvents, and 2.5 DETA-4 sulfolane has the highest CO2 cyclic capacity of 4.486 mol L−1, which was much higher than that of 30 wt% MEA. The concentrations of physical solvents influence the cyclic capacity a lot rather than the types of physical solvents. Then the phase separation character and absorption performance were analyzed for solutions in different amine to physical solvent concentration ratios, and the results showed that increasing the concentration of the physical solvent within certain range benefits to raising cyclic absorption capacity and reducing the energy penalty. At last, the sensible heat and vaporization heat of two biphasic solvents were estimated with a flash module of Aspen Plus, which showed less energy penalty than that of 30 wt% MEA.
1. Introduction The global warming, one of the most environmental vital issues in the world, has attracted widespread attention (Keith, 2009). The emissions of greenhouse gases, especially CO2, have long been considered as the major cause of global warming (Boot-Handford et al., 2014). Therefore, various technologies are being developed to reduce CO2 emissions. Among them, amine scrubbing is one of the most promising technologies for its flexibility and easy implementation in existing power plants (Rochelle, 2009). Aqueous alkanolamine solutions such as monoethanolamine (MEA), a primary amine, has a fast reaction rate and high absorption capacity, so they have been widely researched (Ma’mun et al., 2005). However, the high energy penalty of the MEA regeneration process and high degradation rate hinder its extensive use. Researchers are looking for other solvents with better performance by screening all kinds of amines, such as piperazine (PZ), methyldiethanolamine (MDEA), and diethanolamine (DEA). Those amine solutions, known as first generation absorbents, have various advantages as large absorption capacity, lower degradation rate, or higher reaction rate. Second generation absorbents like MEA + PZ, PZ + 2-(Diethylamino)-ethanol (DEEA), and MDEA + PZ have then been proposed to combine the merits of the different amines by mixing them together in various ratios (Arshad et al., 2014;
⁎
Khan et al., 2017; Sutar et al., 2013). Another method to reduce the energy consumption is to use liquidliquid biphasic solvents that form two immiscible liquid phases after loading CO2 with the CO2 mainly in one phase. After separation, only the CO2 rich phase is sent to the desorber for regeneration and the other phase is sent directly back to the absorber, which significantly reduces the solution volume for regeneration and the energy penalty (Pinto et al., 2014a). Researchers have screened some existing organic amine solvents with the potential for phase splitting. Xu et al. (2013a) developed a biphasic solvent, 2B4D (2 M 1,4-Butanediamine + 4 M (Diethylamino)-ethanol) with a cyclic loading 46% higher, a cyclic capacity 48% higher and a cyclic efficiency 11% higher than 30 wt% MEA. The biphasic solvent, DMX™ was shown to have a low regeneration energy penalty of 2.1 GJ ton−1, but the authors did not indicate the exact compositions (Gomez et al., 2014; Raynal et al., 2014). Ciftja et al. (2013) and Pinto et al. (2014b) studied a blend of N-methyl1, 3-diaminopropane (MAPA) and DEEA which exhibited significant liquid-liquid phase separation after CO2 absorption and explained the mechanism. Ye et al. (2015) screened more than 50 mixed amine solutions to find that a good biphasic solvent should have absorption accelerators and regeneration promoters. Most existing studies have focused on mixed amine solvents. Though those biphasic solvents have better cyclic capacities than 30 wt% MEA, the CO2 rich phase tends to
Corresponding author. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.ijggc.2019.03.015 Received 13 April 2018; Received in revised form 20 February 2019; Accepted 14 March 2019 Available online 02 May 2019 1750-5836/ © 2019 Published by Elsevier Ltd.
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
The amine concentrations were determined by titration against 0.1 N H2SO4 using a Metrohm 809 Titrando auto titrator. The CO2 loading was determined by titration using the barium carbonate precipitation method (Hilliard, 2008). Hilliard estimated that the loading error of the precipitation titration method was ± 2% (Hilliard, 2008). An FTIR system from ABB was used to quantify the physical solvents in the upper and lower phases. Standard physical solvent solutions of various concentrations were prepared and detected by FTIR for calibration with an uncertainty of less than 10%.
have a higher viscosity which slows the operation and cause severe amine degradation with a high amine concentration in the stripper. Physical-chemical biphasic solvents, which are mixtures of traditional organic amines and physical solvents, have been proposed to improve the efficiency of amine utilization and reduce the water ratio in the solution. Various amines were chosen to act as the main absorption solvent to react with CO2 in biphasic solvents. Physical solvents have been screened to replace some of the water and to act as a phase splitting accelerator to enrich the carbamate and bicarbonate in one phase when CO2 is absorbed. Zhang et al. (2017) studied MEAalcohol-H2O solutions showing that mixed solvents with appropriate concentrations of n-propanol, isopropanol or tert butanol gave good phase separation. A physical-chemical biphasic solvent containing Diethylenetriamine (DETA), sulfolane, and water was reported to have a cyclic loading 35% higher than that of 30 wt% MEA aqueous solution (Luo et al., 2016). Anhydrous physical-chemical biphasic solvents, such as 2-(methylamino)ethanol (MMEA) + bis(2-ethoxyethyl) ether (DEGDEE), 2-(ethylamino)ethanol (EMEA) + bis(2-ethoxyethyl) ether (DEGDEE) and MEA-octanol have also been studied (Barzagli et al., 2017; Kim et al., 2014). This work tested 29 pairs of amines and physical solvents in various concentrations, and 21 pairs of them were screened out as biphasic solvents. Four physical solvents were demonstrated to be potential phase separation accelerators. The absorption capability of potential physical-chemical biphasic solvents and the phase separation process were then studied for these mixtures.
3. Results and discussion 3.1. Preliminary screening of physical-chemical solvents In the preliminary screening, DGM, DMSO, sulfolane and n-propanol that do not react with CO2 were chosen as the physical solvents, BDA, MEA, and DETA which have better CO2 absorption performance were selected as the main solvents. All the solvents structures are shown as following:
2. Experimental The chemicals used in this work were DETA (> 99 wt%), MEA (> 99 wt%), BDA (> 98 wt%), Diethylene glycol dimethyl ether (DGM) (> 99.5 wt%), Dimethyl sulfoxide (DMSO) (> 99 wt%), n-propanol (> 99.5 wt%) and sulfolane (analytical reagent) from the Aladdin Reagent Company and were used as received. CO2 (≥99.9% purity) and N2 (≥99.9% purity) were obtained from Air Liquide (China) Company. Distilled deionized water was used for preparing the solutions. The fast screening facility shown in Fig. 1 was built to measure the absorption and desorption capacities described by Xu et al. (2012). The absorption was conducted at 40℃ with the desorption conducted at 90℃ at atmospheric pressure. In the absorption mode, CO2 and N2 were used to simulate the flue gas with 12% CO2 by volume and a total gas flow rate of 657 ml min−1. Equilibrium was assumed to be reached when the outlet CO2 concentration reached 12%. In the desorption mode, N2 was used to sweep the desorbed CO2 through the condenser, acid washing, a drier and an analyzer at a flow rate of 694 ml min−1. Equilibrium was assumed to be reached when the outlet CO2 concentration was less than 0.1%.
Cyclic capacity (ΔR) , cyclic loading (Δα ) and cyclic efficiency (η) are important evaluation parameters for CO2 absorption solvents.
ΔR = Camine (αrich − αlean )
(1)
Δα = αrich − αlean
(2)
(αrich − αlean ) αrich
(3)
η=
Where ΔR is the cyclic capacity, mol L−1; Camine is the molar concentration of amine, mol L−1; αrich and αlean are the CO2 loading of rich and lean solvent, mol mol−1; Δα is the cyclic loading, mol mol−1; η is the cyclic efficiency. For a biphasic solvent, the cyclic capacity, cyclic loading, and cyclic efficiency were defined based on the desorption solution. As shown in Table 1, 29 pairs of physical solvent and aqueous amine mixtures were screened using various concentration ratios. The ratios were carefully chosen according to their viscosity, solubility and potential absorption capacity. The results showed that only 2BDA5DMSO, 3BDA-3.3DMSO, 2DETA-5DMSO, 3DETA-4DMSO, 3DETA3DMSO, 5MEA-5DMSO, 4MEA-3DMSO, and 5MEA-3DMSO solvents remain homogeneous after absorption, while the rest 21 solvents have a clear, rapid liquid-liquid phase separation after CO2 absorption. These physical-chemical biphasic solvents having rapid phase separation were Table 1 Solvent concentrations used for the preliminary screening tests (M/M).
BDA DETA MEA
DGM
DMSO
Sulfolane
n-propanol
2/2,2/2.5,2.5/2.5 2/2.5,2/3,2.5/ 2,2.5/2.5 5/2.5,4/2.5,6/ 2.5,6.5/2.5
2/5,3/3.3 2/5,3/4,3/ 3,2.6/4.8 5/5,4/3,5/3
2/4 2.1/3.6,2.5/ 3,2.5/4 5/4
2/7 2/7 4.6/4.7,5/7
Note: aM is mol L−1; bThe first number is the concentration of amine and the second number is the concentration of physical solvent.
Fig. 1. Schematic diagram of the absorption-desorption apparatus. 200
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Table 2 CO2 concentrations in the upper and lower phases of physical-chemical biphasic solvents. Solvent No.
Solvent
CO2 concentration in upper phase (mol L−1)
CO2 concentration in lower phase (mol L−1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
2DETA-2.5DGM 2DETA-3DGM 2.5DETA-2DGM 2.5DETA-2.5DGM 5MEA-2.5DGM 4 MEA-2.5DGM 6MEA-2.5DGM 6.5MEA-2.5DGM 2.6DETA-4.8DMSO 2.1DETA-3.6sulfolane 2.5DETA-3sulfolane 2.5DETA-4sulfolane 4.5MEA-4.7(n-propanol) 5MEA-7(n-propanol) 2BDA-4sulfolane 2BDA-7(n-propanol) 2DETA-7(n-propanol) 2BDA-2.5DGM 2BDA-2DGM 2.5BDA-2.5DGM 5MEA-4sulfolane
0.153 0.112 0.131 0.081 0.089 0.077 0.213 0.049 0.887 4.846 4.891 6.705 0.246 0.369 3.643 0.027 1.126 0.016 0.086 0.192 3.826
4.985 5.361 5.141 5.567 3.575 2.564 4.339 4.688 4.295 0.062 0.026 0.089 3.203 4.235 0.038 4.439 5.446 3.357 2.988 4.176 0.024
Note:
a
The number before each solvent name is the molar concentration in mol L−1.
then analyzed further. 3.2. Components distribution The component distributions in the upper and lower phases after absorption were then analyzed. The CO2 concentrations and amine concentrations were directly titrated, and the physical solvents were quantified with FTIR. Then the distribution mass fractions were calculated according to the concentrations and weights in the upper and lower phases, as following:
Du =
mu mu + ml
Dl = 1 − Du
(4) (5)
Du : distribution mass fraction of one component in the upper phase; Dl : distribution mass fraction of one component in the lower phase; mu : mass of one component in the upper phase; ml : mass of one component in the lower phase. The solvents screened out were numbered as shown in Table 2. The distribution mass fractions of the amines, physical solvents and water in the upper and lower phases of the loaded solvents are shown in Figs. 2–4. For all of these biphasic solvents, the amines were enriched in one phase after CO2 absorption with the distribution mass fractions in this phase all higher than 90%, which means that most CO2 were rich in the same phase since amines were the only chemical absorbent and then only this phase needed to be desorbed. This also benefits the amine utilization rate. Except for Solvents No. 10, 11, 12, 15 and 21, the amines were mostly concentrated in the lower phase due to the smaller density of the physical solvent in those solutions. And the density of the physical solvent-rich phase in Solvents No. 10, 11, 12, 15 and 21 were obviously higher than that of the amine-rich phase. The CO2 absorption capacity differs significantly in the upper and lower phases as shown in Table 2, which is consistent with the amine distribution mass fraction here. The water and physical solvents distribution mass fractions should also be considered because they influence the volume of the desorption solution. As shown in Fig. 3, most of the physical solvents were enriched in the opposite phase from the amines, but their enrichment ratios were lower than those of the amines. Table 2 shows the CO2 concentrations in two immiscible phases, which indicates that only a little reaction product of the amines and the CO2 was dissolved in the
Fig. 2. Amines mass fractions in the upper and lower phases of the CO2 loaded solutions.
physical solvents phase and the limited mutual solubility between them caused the liquid-liquid phase separation, which is consistent with the results of Luo et al. (2016) However, for Solvent No. 6 and 9, more than 70% of the physical solvent was in the amine-rich phase which was higher than the other solutions. The reason is that the concentration ratio of amine to physical solvent in these two fresh solution is a bit small, and the concentration of reaction products is lower so that much more physical solvent dissolved in the amine-rich phase. As shown in Fig. 4, water existed in both phases, though the water distribution mass fraction in amine-rich phase is higher than that in the other phase, the enrichment is not so high as that of amines. For biphasic solvents, only the amine-rich phase is sent to the stripper, so if there is less water and less physical solvent concentrated in the aminerich phase, the sensible heat and vaporization heat would be cut down. Because the sensible heat gets down with the decrease of the volume of CO2-rich solution, and the vaporization heat gets down with the decrease of the water content in solution which leads to lower partial pressure of water in the top of the stripper (Chakma, 1997; Leites, 1998; 201
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Fig. 5. Cyclic absorption property of the CO2-rich phase.
loading was measured with the titration method. The cyclic capacity, the cyclic loading and the cyclic efficiency of the CO2-rich phase are showed in Fig. 5, where Solvent No. 22 is 5 mol L−1 MEA from Xu et al. (2013b). When comparing different kinds of amine solvents in the same desorption conditions, those solvents with a higher cyclic capacity are potential to reduce sensible heat (Aronu et al., 2011; Rochelle, 2009). Furthermore, increasing the cyclic loading rather than amine concentration would be a better way to increase cyclic capacity, because high amine concentration leads to higher volatilization and severer corrosion. All of these biphasic solvents have greater cyclic capacities than 30 wt% MEA. Solvent No. 12 has the highest absorption capacity of 4.486 mol L−1, which is more than twice the capacity of 30 wt% MEA. The reason is that the amine concentration in the CO2-rich phase increases significantly due to the phase splitting. The cyclic loadings for the different amines solvents differ greatly since MEA has only one amino group, BDA has two, and DETA has three ones, which influence their total loadings. For those solvents with the same kind of amine and equal amine concentration but different physical solvents, the cyclic capacities were similar which indicates that the type of physical solvent has little effect on the absorption capacity. However, the physical solvents concentration was quite important as shown by comparing the results of Solvents No. 1 and 2, 3 and 4, 11 and 12, and 18 and 19, where the absorption capacity varies with various physical solvent concentrations. The additional experiments discussed in part 3.4 provide more evidence. Comparing the amine structures, the cyclic loading increases with the increasing number of amino groups in the amine molecule for both primary and secondary amines. These solvents with MEA (No. 5, 14, and 21) and the solvent with BDA (No. 20) had the same primary amino group concentrations, but those with MEA had higher cyclic capacities than that of BDA solution. The reason is that the amino group in BDA absorbs almost as much CO2 as MEA, but it desorbs less than MEA at the same operation conditions, which indicates that the primary amino group of BDA is more difficult to regenerate CO2 than MEA. As for the influence of the secondary amino group, Solvents No. 1 and 7 had the same total amino group concentration of 6 mol L−1 but a third of the amino groups were secondary amino groups in Solvent No. 1. The results showed that Solvents No. 1 and 7 had roughly the same cyclic absorption capacities, which indicates that the secondary amino group of DETA behaves as good as the primary amino group on the absorption. Thus, both MEA and DETA have good absorption and desorption characteristics, so further studies of the energy penalty and physical properties are needed to compare these two solvents.
Fig. 3. Physical solvents mass fractions in the upper and lower phases of the CO2 loaded solutions.
Fig. 4. Water mass fractions in the upper and lower phases of the CO2 loaded solutions.
Nwaoha et al., 2017). Comparing Solvent No. 1, 4, 5 and 6, with the amine concentration increases in the fresh solution, less physical solvent distributes in the amine-rich phase after loading CO2 but more water exists in the amine-rich phase. When the fresh solution contains more physical solvent, according to the results of Solvent No. 3, 4, 11, 12, 18 and 19, the amine-rich phase seems to have less physical solvent and less water after absorption. So when screening physical-chemical biphasic solvents, increasing the concentration of physical solvents properly in fresh solution can contribute to cutting down energy penalty, but the influence of increasing amine concentration is uncertain as the water and physical solvent concentration in amine-rich phase change in different ways.
3.4. BDA-DGM phase separation mechanism 3.3. Comprehensive cyclic absorption property The phase separation process of BDA and DGM mixtures was studied to further understand the driving mechanisms. Nine pairs of BDA-DGM
After absorption, the CO2-rich phase was desorbed and the CO2 202
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Fig. 6. Component distributions after CO2 absorption in BDA/DGM solutions with various concentrations.
solutions with different concentration ratios were chosen, with the same experiment process and detection methods as in the screening experiment. As shown in Fig. 6, DGM was enriched in the upper phase, but CO2 and BDA mainly existed in the lower phase after the absorption. The DGM concentration in the upper phase and the BDA concentration in the lower phase increased steadily as the DGM or BDA concentration in the fresh solvent increased, while the DGM concentration in the lower phase and the BDA concentration in the upper phase slightly decreased. The CO2 concentration in the amine-rich phase increased firstly with increasing DGM or BDA concentration, but then started to decrease after reaching a maximum at 3.5DGM or 2.5BDA. Thus, the CO2 absorption capacity depends greatly on the BDA to DGM concentration ratio. Low BDA concentration limits the total solution capacity, while a high ratio of BDA to DGM has a negative impact on the absorption because of the high viscosity. However, high absorption capacity doesn’t mean high cyclic capacity. As shown in Fig. 7, the 2BDA2.5DGM solution has the highest cyclic capacity of 2.536 mol L−1 and the highest cyclic loading of 0.796 mol mol−1 amine. The reason is that the amine concentration in amine-rich phase increased with the increasing concentration of physical solvent and amine in fresh solution which may cause an increase in viscosity. High viscosity is detrimental to absorption and desorption, so there must be an optimal concentration for best cyclic capacity. For biphasic solvents, the cyclic capacity is much more important as it stands for the solution utilization rate and
partly determines the regeneration penalty. Considering the influence of concentration ratio on energy penalty as stated in part 3.2, there is a suggestion for biphasic solvent screening: firstly choosing the proper concentration ratio by comparing cyclic capacity, then adjusting the physical solvent and amine concentrations according to part 3.2 to get an optimal energy consumption. Then 2BDA-2.5DGM solution was used to further characterize the phase separation process as the CO2 loading in the lower phase increased. Five groups of fresh 2BDA-2.5DGM solutions were used to absorb CO2 at the same absorption mode but for different absorption times. As time goes by, more CO2 would be absorbed and the loading gets higher. Indeed, the five groups of solutions with different absorption times have different CO2 loadings which were demonstrated by titration. The concentrations of BDA and DGM, and CO2 capacities in upper and lower phases were measured for each group respectively, as shown in Fig. 8. The BDA concentration and the CO2 capacity in the lower phase, and the DGM concentration in the upper phase increase with time, which means that the separation process kept happening in the whole period. The DGM migrated from the lower phase to the upper phase while the BDA migrated to the opposite direction as the loading increases. These results show that some of the DGM separated from the reaction products and migrated to the upper phase due to the limited solubility of DGM in the BDA and CO2 reaction products and its relatively smaller density.
Fig. 7. Cyclic absorption capacity for various BDA-DGM ratios.
Fig. 8. Phase separation of 2BDA-2.5DGM with time. 203
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
3.5. Estimation of the sensible heat and vaporization heat The lower desorption energy consumption is a key advantage of the biphasic solvents. The total regeneration energy requirement can be divided into the heat of reaction (qreac ), the sensible heat (qsens ), and the vaporization heat of water (qvap ). The heat of reaction mainly depends on the chemical reactions between the solvents and CO2, influenced by the temperature and CO2 loading (Kim and Svendsen, 2007). The sensible heat and the vaporization heat are considered here as following:
qsens =
ρCP ΔT (αrich − αlean ) Camine
qvap = ΔHvap·
ΔHvap PH2O nw nw = ΔHvap· = ∙ mCO2 MCO2·nCO2 MCO2 PCO2
(6)
(7)
The calculation of heat consumption here was based on the CO2-rich phase, where ρ is the mass density (kg m−3) and CP is the specific heat (J kg−1 K−1) of the CO2-rich phase which is to be heated to the desorption temperature, and ΔT is the temperature difference between the heat exchanger outlet and the desorbing temperature. The desorber temperature is normally 118℃ and the heat exchanger outlet temperature is 105℃, therefore ρ and CP were set at the average temperature of 111℃ to estimate the sensible heat (Xu, 2014). Camine is the amine concentration (mol L−1) in the CO2-rich phase; αrich and αlean are from the earlier experimental results; ΔHvap is the latent heat of water (kJ mol−1); n w is the total molar amount of evaporation water in desorber (mol); mCO2 is the weight of desorbed CO2 (g); MCO2 is the molecular weight (g mole−1) of CO2, nCO2 is the total molar amount of CO2 released in the desorber (mol); PH2 O and PCO2 are the gaseous phase partial pressures of water and CO2 at the top of the desorber. The qvap is the amount of energy needed to vaporize the water in the CO2 saturated amine solution to provide the stripping vapor and it was derived in Eq. (7) where the vaporization heat of water was expressed with the partial pressure of matters in the gas phase finally. In this work, a flash module was used to simulate the vapor-liquid equilibrium with the physical parameters based on the amine package ELECNRTL-Rate-Based-MEA-Model in Aspen Plus V8.4 (Sherman and Rochelle, 2017; Zhang et al., 2011). Oexmann (Oexmann and Kather, 2009, 2010) used Aspen Plus to create a rigorous and consistent model to predict the thermodynamic characteristics of the H2O-MEA-CO2 system and to describe the thermodynamics associated with the absorption-desorption process. The modeling method were used here to perform simple flash calculations to analyse the vapor-liquid-equilibria of the CO2-rich phase of CO2-loaded biphasic solvents. Figs. 9 and 10 show that the model predictions for the MEA solution are in good agreement with the data from Aronu et al. (2012), Jou et al. (1995) and Page et al. (1993). The model was then used to calculate the density, specific heat and vapor-liquid equilibrium in the CO2 rich phase for Solvent No. 7 and 21 because the main absorbent in these two solution is MEA and the physical solvents are less volatile. Accurate modeling of thermodynamic properties of CO2 absorption in aqueous alkanolamine solutions is essential for simulation and design of such CO2 capture processes (Zhang et al., 2011). As shown in Table 3, the densities and specific heats of these three solvents are similar, while the sensible heat of 6MEA-2.5DGM and 5MEA-4sulfolane are much lower, because the cyclic capacity of 5MEA is only about half of that of the others. The vaporization heat of the two mixtures are also much lower because the water and physical solvent are distributed into the CO2-lean phase with a lower water ratio in the CO2-rich phase. Thus, when desorbing the same amount of CO2 in the desorber, less volume of solution is heated and less water is evaporated with the biphasic solvents. The estimation indicates that the biphasic solvents have great potential to reduce the energy cost because the cyclic capacities and component distributions greatly influence qsens and qvap .
Fig. 9. Comparison of the model predictions to experimental data for the partial pressure of CO2 with 30 wt% MEA.
Fig. 10. Comparison of the model predictions with experimental data for the density and specific heat at 40℃.
4. Conclusions The present study screened out 21 pairs of physical-chemical biphasic solvents, the CO2 cyclic absorption capacity and the component distributions were analyzed. The results show that 2.5DETA-4sulfolane gave the best results with 4.486 mol L−1 cyclic capacity and 0.994 mol mol−1 cyclic loading, much higher than those of 30 wt% MEA. The physical solvents and the amines distributed in opposite phases and the distribution coefficients increased with increased CO2 loading. The cyclic capacities show that MEA and DETA behave better than BDA in biphasic absorption process, but the type of physical solvents shows little influence on the absorption capacity. Tests with various concentrations of solutions to absorb CO2 showed that much more physical solvent migrated to one phase while the amine migrated to the other phase for fresh solutions with higher physical solvent concentrations or higher amine concentrations. The component concentration ratio of biphasic solvent has great impact on the desorption volume and cyclic capacity which should be both considered in the screening of biphasic solvents. At last, the estimation of the sensible and vaporization heat for two solvents showed that the physical-chemical biphasic solvents have great potential for reducing the solvent regeneration energy.
204
International Journal of Greenhouse Gas Control 85 (2019) 199–205
M. Xu, et al.
Table 3 Estimation result of parameters and energy penalty. Solvent
△R (mol L−1)
ρ (kg m−3)
CP (kJ kg−1 K−1)
PH2Oa (kPa)
PCO2a (kPa)
qsens (kJ kg−1)
qvap (kJ kg−1)
5MEA 6MEA-2.5DGM 5MEA-4sulfolane
1.55 3.34 3.13
945.82 933.62 967.91
3.96 3.56 3.48
160.17 103.17 113.62
29.07 82.27 74.29
714.42 294.45 317.99
533.35 121.39 148.05
Note:
a
△R is the Cyclic Capacity and PH2O and PCO2 are the partial pressures of H2O and CO2 in the top of the stripper.
Acknowledgments
purification technology. Energy Convers. Manage. 39, 1665–1674. Luo, W., Guo, D., Zheng, J., Gao, S., Chen, J., 2016. CO2 absorption using biphasic solvent: blends of diethylenetriamine, sulfolane, and water. Int. J. Greenh. Gas Control 53, 141–148. Ma’mun, S., Nilsen, R., Svendsen, H.F., Juliussen, O., 2005. Solubility of carbon dioxide in 30 mass % monoethanolamine and 50 mass % methyldiethanolamine solutions. J. Chem. Eng. Data 50, 630–634. Nwaoha, C., Idem, R., Supap, T., Saiwan, C., Tontiwachwuthikul, P., Rongwong, W., AlMarri, M.J., Benamor, A., 2017. Heat duty, heat of absorption, sensible heat and heat of vaporization of 2-Amino-2-Methyl-1-Propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) tri-solvent blend for carbon dioxide (CO2) capture. Chem. Eng. Sci. 170, 26–35. Oexmann, J., Kather, A., 2009. Post-combustion CO2 capture in coal-fired power plants: comparison of integrated chemical absorption processes with piperazine promoted potassium carbonate and MEA. Energy Procedia 1, 799–806. Oexmann, J., Kather, A., 2010. Minimising the regeneration heat duty of post-combustion CO2 capture by wet chemical absorption: the misguided focus on low heat of absorption solvents. Int. J. Greenh. Gas Control 4, 36–43. Page, M., Huot, J.Y., Jolicoeur, C., 1993. A comprehensive thermodynamic investigation of water ethanolamine mixtures at 10-degrees-C, 25-degrees-C, and 40-degrees-C. Can. J. Chem.-Revue Canadienne De Chimie 71, 1064–1072. Pinto, D.D.D., Knuutila, H., Fytianos, G., Haugen, G., Mejdell, T., Svendsen, H.F., 2014a. CO2 post combustion capture with a phase change solvent. Int. J. Greenh. Gas Control 31, 153–164. Pinto, D.D.D., Zaidy, S.A.H., Hartono, A., Svendsen, H.F., 2014b. Evaluation of a phase change solvent for CO2 capture: absorption and desorption tests. Int. J. Greenh. Gas Control 28, 318–327. Raynal, L., Briot, P., Dreillard, M., Broutin, P., Mangiaracina, A., Drioli, B.S., Politi, M., La Marca, C., Mertens, J., Thielens, M.-L., Laborie, G., Normand, L., 2014. Evaluation of the DMX process for industrial pilot demonstration – methodology and results. In: Dixon, T., Herzog, H., Twinning, S. (Eds.), 12th International Conference on Greenhouse Gas Control Technologies, Ghgt-12, pp. 6298–6309. Rochelle, G.T., 2009. Amine scrubbing for CO2 capture. Science 325, 1652–1654. Sherman, B.J., Rochelle, G.T., 2017. Thermodynamic and mass-transfer modeling of carbon dioxide absorption into aqueous 2-amino-2-methyl-1-propanol. Ind. Eng. Chem. Res. 56, 319–330. Sutar, P.N., Vaidya, P.D., Kenig, E.Y., 2013. Activated DEEA solutions for CO2 capture–a study of equilibrium and kinetic characteristics. Chem. Eng. Sci. 100, 234–241. Xu, Z., 2014. Absorption of CO2 With Aqueous BDA/DEEA Blended System. Tsinghua University, Beijing. Xu, Z., Wang, S., Liu, J., Chen, C., 2012. Solvents with low critical solution temperature for CO2 capture. In: Rokke, N.A., Hagg, M.B., Mazzetti, M.J. (Eds.), 6th Trondheim Conference on Co2 Capture, Transport and Storage, pp. 64–71. Xu, Z., Wang, S., Chen, C., 2013a. CO2 absorption by biphasic solvents: mixtures of 1,4butanediamine and 2-(diethylamino)-ethanol. Int. J. Greenh. Gas Control 16, 107–115. Xu, Z., Wang, S., Chen, C., 2013b. Experimental study of CO2 absorption by liquid-liquid biphasic solvents. J. Tsinghua Univ. (Sci. Tech.) 336–341. Ye, Q., Wang, X., Lu, Y., 2015. Screening and evaluation of novel biphasic solvents for energy-efficient post-combustion CO2 capture. Int. J. Greenh. Gas Control 39, 205–214. Zhang, Y., Que, H., Chen, C.-C., 2011. Thermodynamic modeling for CO2 absorption in aqueous MEA solution with electrolyte NRTL model. Fluid Phase Equilib. 311, 67–75. Zhang, W., Jin, X., Tu, W., Ma, Q., Mao, M., Cui, C., 2017. Development of MEA-based CO2 phase change absorbent. Appl. Energy 195, 316–323.
Financial supports from the National Key R&D Program (2017YFB0603301-2) and NSFC (51576108) is greatly appreciated. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ijggc.2019.03.015. References Aronu, U.E., Hoff, K.A., Svendsen, H.F., 2011. CO2 capture solvent selection by combined absorption-desorption analysis. Chem. Eng. Res. Des. 89, 1197–1203. Aronu, U.E., Gondal, S., Hessen, E.T., Haug-Warberg, T., Hartono, A., Hoff, K.A., Svendsen, H.F., 2012. Solubility of CO2 in 15, 30, 45 and 60 mass% MEA from 40 to 120 degrees C and model representation using the extended UNIQUAC framework. Chem. Eng. Sci. 78, 246–247. Arshad, M.W., Fosbol, P.L., von Solms, N., Svendsen, H.F., Thomsen, K., 2014. Equilibrium solubility of CO2 in alkanolamines. In: Rokke, N.A., Svendsen, H. (Eds.), 7th Trondheim Conference on Co2 Capture, Transport and Storage, pp. 217–223. Barzagli, F., Mani, F., Peruzzini, M., 2017. Novel water-free biphasic absorbents for efficient CO2 capture. Int. J. Greenh. Gas Control 60, 100–109. Boot-Handford, M.E., Abanades, J.C., Anthony, E.J., Blunt, M.J., Brandani, S., Mac Dowell, N., Fernandez, J.R., Ferrari, M.C., Gross, R., Hallett, J.P., Haszeldine, R.S., Heptonstall, P., Lyngfelt, A., Makuch, Z., Mangano, E., Porter, R.T.J., Pourkashanian, M., Rochelle, G.T., Shah, N., Yao, J.G., Fennell, P.S., 2014. Carbon capture and storage update. Energy Environ. Sci. 7, 130–189. Chakma, A., 1997. CO2 capture processes – opportunities for improved energy efficiencies. Energy Convers. Manage. 38, S51–S56. Ciftja, A.F., Hartono, A., Svendsen, H.F., 2013. Experimental study on phase change solvents in CO2 capture by NMR spectroscopy. Chem. Eng. Sci. 102, 378–386. Gomez, A., Briot, P., Raynal, L., Broutin, P., Gimenez, M., Soazic, M., Cessat, P., Saysset, S., 2014. ACACIA project – development of a post-combustion CO2 capture process. Case of the DMX (TM) Process. Oil Gas Sci. Technol.-Revue D Ifp Energies Nouvelles 69, 1121–1129. Hilliard, M.D., 2008. A predictive thermodynamic model for an aqueous blend of potassium carbonate, piperazine, and monoethanolamine for carbon dioxide capture from flue gas. Carbon Dioxide—Absorption and Adsorption. Jou, F.Y., Mather, A.E., Otto, F.D., 1995. The solubility of CO2 in a 30-mass-percent monoethanolamine solution. Can. J. Chem. Eng. 73, 140–147. Keith, D.W., 2009. Why capture CO2 from the atmosphere? Science 325, 1654–1655. Khan, A.A., Halder, G.N., Saha, A.K., 2017. Experimental investigation on efficient carbon dioxide capture using piperazine (PZ) activated aqueous methyldiethanolamine (MDEA) solution in a packed column. Int. J. Greenh. Gas Control 64, 163–173. Kim, I., Svendsen, H.F., 2007. Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(aminoethyl)ethanolamine (AEEA) solutions. Ind. Eng. Chem. Res. 46, 5803–5809. Kim, Y.E., Park, J.H., Yun, S.H., Nam, S.C., Jeong, S.K., Yoon, Y.I., 2014. Carbon dioxide absorption using a phase transitional alkanolamine-alcohol mixture. J. Ind. Eng. Chem. 20, 1486–1492. Leites, I.L., 1998. Thermodynamics of CO2 solubility in mixtures monoethanolamine with organic solvents and water and commercial experience of energy saving gas
205