Experimental Study of Regeneration Performance of Aqueous N,N

Jul 29, 2016 - Amine regeneration tests on MEA, DEA, and MMEA with respect to cabamate stability analyses. Huancong Shi , Yunlong Zhou , Mengyin Si , ...
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Experimental Study of Regeneration Performance of Aqueous N,N‑Diethylethanolamine Solution in a Column Packed with Dixon Ring Random Packing Bin Xu, Hongxia Gao,* Menglin Chen, Zhiwu Liang,* and Raphael Idem Joint International Center for CO2 Capture and Storage (iCCS), Hunan Provincial Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO2 Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China ABSTRACT: Carbon dioxide (CO2) stripping from CO2-loaded aqueous N,Ndiethylethanolamine solution (DEEA) was comprehensively investigated in a laboratory scale regeneration column packed with Dixon ring random packing. The regeneration heat duty (Qreb) of DEEA was evaluated as a function of various operating parameters, including rich CO2 loading, lean CO2 loading, solution flow rate, feed temperature, and DEEA concentration, as well as synergistic parameters (e.g., Δα × L and C × L). The experimental results showed that the Qreb was sensitive to these operating parameters. For example, the Qreb decreased significantly as the lean CO2 loading, rich CO2 loading, and DEEA concentration increased, indicating that the Qreb can be reduced by adjusting these operating parameters. In addition, CO2-loaded aqueous DEEA solution was observed to have a lower Qreb (of 2.17 GJ/t CO2) as compared to those of aqueous MEA and DETA solutions. Based on heat duty alone, DEEA can be considered to be an attractive solvent for amine-based postcombustion CO2 capture.

1. INTRODUCTION The prospect of worsening global warming and climate due to the elevated global emissions of carbon dioxide (CO2) has attracted considerable concern in recent years. The chief culprit for the drastic rise of CO2 emissions is the combustion of fossil fuels (coal, natural gas, and oil).1 The development of efficient strategies to reduce atmospheric CO2 emissions from the exhaust gas streams of power plants and other industrial processes has been receiving considerable interest. Amine-based postcombustion CO2 capture is a mature technology that has received much attention as a promising method for bulk removal of CO2 from flue gases.2−4 Aqueous amines such as monoethanolamine (MEA), diisopropanolamine (DIPA), diethanolamine (DEA), methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and piperazine (PZ) have been widely investigated for capturing CO2.5−10 However, processes using conventional amine solutions are energy intensive, and as such, not costeffective for CO2 removal. The major challenge of amine-based CO2 capture is to reduce the heat energy consumption for solvent regeneration, which accounts for approximately 70% of the operational cost.1,11 Energy consumption is of great importance and must be known for the design and operation of the industrial CO2 absorption process because it determines the economic benefits of operation.12−14 Therefore, there is the need for developing strategies to reduce the heat energy requirement for solvent regeneration. As shown in the literature,15−18 four main methods have been suggested for reduction of heat energy consumption in © XXXX American Chemical Society

the absorption-stripping process: (i) optimization of process configuration; (ii) development of process control strategies; (iii) improvement of packing performance; and (iv) development of alternative solvents with high absorption rate, high absorption capacity, high mass transfer performance, reasonable solvent cost, and low heat duty for regeneration. Considering the last method, the search for an innovative and efficient solvent system is a promising way to decrease energy requirements for regeneration. The heat energy consumption is generally accepted to be the sum of three main forms of heat: reaction heat for breaking the chemical bonds between amine solvents and CO2; sensible heat for elevating the temperature of CO2 rich solution to the boiling point; and vaporization heat for evaporation of water vapor to reduce the operating H2O pressure that is good for stripping CO2.12,19 Significant R&D efforts are devoted to finding innovative and efficient solvents that can provide improvements as means to tackle the high heat energy requirement in the amine-based CO2 capture process. Some researchers have focused on the reaction heat of different amines, which is considered to be directly related to regeneration energy requirements. Carson et al.20 measured absorption heat of MEA, DEA, MDEA, and MEA-MDEA solutions at 25 °C at very low CO2 loadings (0.01−0.25 mol/ Received: March 9, 2016 Revised: July 1, 2016 Accepted: July 12, 2016

A

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the desorption process.

mol). Xie et al.21 reported the heats of CO2 absorption into aqueous blended AMP-PZ solutions with the molar ratios of 2:1, 4:1, and 6:1 at the operation conditions of 40, 60, and 80 °C and CO2 loading ranging from 0 to saturation while Xu et al.22 investigated the CO2 absorption in 1, 3, and 4 M DEEA solutions at 40, 60, 80, and 120 °C. Kim et al.23 studied the effects of CO2 loading on the heats of CO2 absorption of 10 wt % aqueous MEA, PZ, 2-methylpiperazine (2-MPZ), homopiperazine (HomoPZ), and hexamethylenediamine (HMDA) solutions at 40 °C whereas Kim and Svendsen24 measured the heats of CO2 with different single amines (i.e., AEEA, AMP, DEEA, DETA, MAPA, MDEA, MEA, and PZ) and their blends (such as AMP-MAPA, AMP-MEA, DEEA-MAPA, MDEAMAPA, MEA-PZ, and PZ-K+) as a function of temperature, CO2 loading, and solvent composition. In addition, a few studies for heat capacities of aqueous amines have been reported in the literature. Welland et al.10 published the heat capacities of aqueous MEA, DEA, MDEA, MEA-MDEA, and DEA-MDEA as a function of CO2 loading and amine concentrations at 25 °C while Chiu et al.25 reported the heat capacities of aqueous MEA, DEA, diglycolamine,26 DIPA, triethanolamine (TEA), MDEA, AMP, and 2-piperidineethanol (2-PE) performed at 30 to 80 °C with a differential scanning calorimeter. Zhang et al.27 measured the heat capacities of aqueous MDEA and AMP at temperatures up to 90 °C. In principle, the heat energy consumption for solvent regeneration could be calculated by combining reaction heat, heat capacity, and vaporization heat through a rigorous process. However, the calculations are very difficult and complicated

since the heat-associated data is lacking. In addition, the reboiler heat duty for solvent regeneration can be greatly influenced by both amine type and operating conditions. Sakwattanapong et al.12 experimentally evaluated the reboiler heat duty for aqueous MEA, DEA, MDEA, and AMP and mixtures of MEA-MDEA, MEA-AMP, and DEA-MDEA in a bench-scale stripper. The reboiler heat duties of MEA and DEA at different operating parameters were experimentally measured in a lab-scale stripper by Galindo et al.28 Li et al.29 investigated the energy requirement of CO2 desorption from rich MEA, MDEA, and MEA-MDEA with molar ratios of 2:1 and 1:2 at various flow rates, total amine concentrations, rich amine solvent feed temperatures, rich CO2 loadings, reboiler temperatures, and pressures. However, research on experimental amine solvent regeneration in a stripper column is rather limited in the open literature. Therefore, it is still necessary to directly measure the reboiler heat duty to increase the database for heat duty and to supply the necessary data needed for the operation and design of industrial CO2 capture units. Among all the amines, tertiary amines have the lowest regeneration energy consumption, lower than primary and secondary amines since tertiary amines cannot directly react with CO2 to produce carbamate.30,31 Recently, N,N-diethylethanolamine (DEEA) has been considered as a promising alternative tertiary amine solvent to remove CO2 with better performance, in terms of absorption rate, absorption heat, and absorption capacity, than MDEA, and it can be prepared from cheap and/or renewable resources.22 Therefore, it is necessary B

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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sample of lean amine solution was taken out for determination of amine concentration and CO2 loading, and the steady-state values of the flow rates of heating oil and rich/lean amine solution as well as all the temperatures were recorded. After the completion of the steady-state data collection, the introduction of rich amine solution into the stripper column was stopped, while the supply of cooling water to the overhead condenser and heating oil were maintained until no stripped CO2 exited from the reboiler. At the same time, the temperatures and flow rate of heating oil were also recorded for calculating heat energy loss. The stripped CO2 determined by gas and liquid were used to calculate mass balance error for all experiments to ensure the accuracy and reliability of the results. The system was operated approximately at atmospheric pressure. Details of the experimental conditions and test parameters used are summarized in Table 1.

to measure the reboiler heat duty of aqueous DEEA solution under a variety of operating conditions. In the present work, reboiler heat duties for stripping CO2 from aqueous DEEA solutions were investigated as a function of rich CO2 loading, lean CO2 loading, solvent flow rate, stripper feed temperature, DEEA concentration, and CO2 cyclic capacity in a lab-scale stripper packed with Dixon ring random packing. Additionally, sensible heat, reaction heat, and vaporization heat for DEEA were compared to those of MEA and DETA. These results are presented and discussed in this paper.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Reagent grade MEA and DETA each with purity of ≥99% were purchased from Tianjin Hengxing Chemical Preparation Co. Ltd., China. DEEA also with purity ≥99% was purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. Commercial grade CO2 was obtained from Changsha Jingxiang Gas Co. Ltd., China, with purity of 99.9%. 2.2. Lab-Scale Stripper and Procedure. Figure 1 shows the schematic diagram of solvent desorption experimental apparatus. The experimental setup essentially consisted of a stripper column, a condenser, a reboiler, two constant flow pumps, two 10-L amine solution reservoirs, a preheater, a gas flow meter, and four temperature indicators. The stripper column (28.0 mm internal diameter and 0.5 m packing height) was made of a double-layer glass with a vacuum interlayer for heat insulation packed with Φ3 × 3 mm, 316L stainless steel Dixon ring random packing with specific surface area of 2275 m2/m3. Dixon ring is made of metal mesh with the same diameter and height and mainly used for laboratory and lowvolume, high-purity product separation processes. The reboiler was a triple-layer glass reaction kettle with heat supplied by heating oil. The two solvent flow pumps for amine solution circulation (model BT100-02) were supplied by Baoding Qili Precision Pump Co., Ltd., China. Four sensors were installed at different locations to measure the inlet and outlet temperatures of amine solution and heating oil. Details on the experimental setup for solvent regeneration are available from our previous work.32 Prior to the solvent regeneration experiments, the aqueous amine solutions were prepared by diluting the pure amine with deionized water to the desired concentration. Then, the desired rich CO2 loadings of the amine solutions were obtained by sparging pure CO2 gas into the solution. The amine concentration and CO2 loading were measured by titration with 1 mol/L HCl using methyl orange solution as the indicator. For a typical solvent regeneration experimental run, 2 L of CO2-loaded aqueous amine solution was first introduced into the reboiler and preheated to the reboiler temperature by circulating heating oil. Once the reboiler temperature reached the specified value, the desired flow rate of rich amine solution, which was heated to a given feed temperature by two waterbath heaters, was continuously pumped to the top of the stripper column. The mixture of CO2 and water vapor, which was liberated from the rich amine solution, exited through an overhead condenser with total reflux of water. The already regenerated amine solution was heated to its boiling point in the reboiler and then collected in the lean amine solution reservoir for recycling. The regeneration experiment ran for a reasonable period of time (about 1.5 h) to reach steady state, which was indicated by constant temperatures, flow rate of lean/rich amine solution, and lean CO2 loading. Then, a 10 mL

Table 1. Operating Parameters and Conditions operation parameter

conditions

lean CO2 loading, mol/mol rich CO2 loading, mol/mol solution flow rate, mL/min DEEA concentration, mol/L feed temperature, °C

0.10−0.40 0.55−0.80 40−80 1.7−3.8 65−85

2.3. Calculation of Reboiler Heat Duty. The reboiler heat duty (Qreb) can be determined based on the mass and energy balance in the whole regeneration system. The heat energy supplied by the heating oil in the reboiler (Hreb) was calculated as Hreb = moCp,o(t in,o − tout,o)

(1)

where mo, Cp,o, tin,o, and tout,o are the mass flow rate (kg/h), heat capacity (kJ/(kg·°C)), inlet temperature, and outlet temperature (°C) of the heating oil, respectively. The Qreb (kJ/kg CO2) is defined as the ratio of the effective power supplied at the reboiler and the CO2 mass flow rate (mCO2, kg/h), which is expressed as

Q reb =

Hreb − Hloss mCO2

(2)

where Hloss denotes the energy loss in the regeneration system (kJ/h), which inevitably exists during the experimental run, and is calculated by the eq 1. The CO2 mass flow rate was determined by the following equation: mCO2 = nAmine(αrich − αlean)MCO2

(3)

where namine is the molar flow rate of rich amine solution (kmol/h), αrich and αlean are the CO2 loadings of the inlet and outlet amine solutions from the stripper column, respectively, and MCO2 is the molecular weight of CO2 (g/mol). As mentioned earlier, the reboiler heat duty is made up of sensible heat (Qsen), absorption heat (Qabs), and vaporization heat (Qvap). Q reb = Q abs + Q sen + Q vap Q abs = ΔHabs,CO2 = R

(4)

d(ln PCO2) d(1/T )

(5)

Q sen = ρsolvent VCp(Treboil − Tinput)/mCO2 C

(6)

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(7)

where ΔHabs,CO2 is the heat of reaction (kJ/kg CO2), ρsolvent represents the density of the amine solution, V and Cp denote the volume flow rate and specific heat of the solution (kJ/ (kg·°C)), and Treb and Tin are the reboiler and feed temperatures (°C). In the present work, the unit of Qreb is converted from kJ/kg CO2 to GJ/t CO2.

3. RESULTS AND DISCUSSION 3.1. Experimental Validation. The accuracy and reliability of the experimental apparatus was effectively validated by comparing the Qreb obtained from the stripper column packed with stainless steel Dixon ring random packing to those from a bench-scale solvent regeneration system12 and a CO2 capture pilot plant.33 To ensure a fair comparison, it was essential to do the regeneration experiments at operational conditions very close to those in the reference. Based on the experimental conditions in the literature, the CO2 was stripped from 5 mol/L aqueous MEA solutions with 0.5 mol/mol rich CO2 loading in the different lean CO2 loadings. As shown in Table 2, Qreb values from this work are in good agreement with the pilot plant and bench-scale system data in the literature.12,33 Table 2. Experimental Validation Using 5 mol/L Aqueous MEA Solution

Figure 2. (a) Effects of lean CO2 loading on Qreb (C = 3 mol/L, solution flow rate = 50 mL/min, feed temperature = 75 °C). (b) Effects of rich CO2 loading on Qreb (C = 3 mol/L, solution flow rate = 50 mL/min, feed temperature = 75 °C).

Qreb, GJ/t CO2 lean CO2 loading, mol/mol

literature 1

literature 2

this study

0.20−0.24 0.24−0.29 0.29−0.35

5.4 4.8 3.8

5.203 (0.23) 4.849 (0.25) 3.767 (0.30)

5.027 (0.24) 4.566 (0.27) 3.675 (0.30)

increase the solvent circulation rate which leads to higher absorption costs. This behavior can be attributed to the fact that an increase in lean CO2 loading requires a reduction in H2O partial pressure for vapor−liquid equilibrium. When the DEEA solution was stripped to a lower lean CO2 loading, the operating and equilibrium lines tend to pinch at the lean CO2 loading end. Therefore, more heat energy is consumed in order to achieve a lower lean CO2 loading due to the difficulty of achieving a further reduction in CO2 partial pressure. On the issue of the value of rich CO2 loading, it is known that the value of H2O partial pressure is lower at low rich CO2 loading. This indicates that CO2 was stripped from DEEA solution with rich CO2 loading of 0.70 mol/mol, and it required more water vapor to be supplied by the reboiler than in the case of 0.80 mol/mol. The regeneration performance of aqueous DEEA solution was evaluated at solution flow rates in the range of 40 to 80 mL/min in 10 mL/min increments. The influences of solution flow rate on the Qreb are shown in Figure 3. It shows that Qreb first decreases and then begins to increase at solution flow rate of 70 mL/min, leading to a minimum value of Qreb at solution flow rate of 70 mL/min. In general, the effective interfacial area in the regeneration column increased with an increase in the solution flow rate. This shows that both mass transfer and heat transfer between solution and packing are enhanced, resulting in a reduction in energy consumption. In contrast, the opposing phenomenon is that, by increasing the solution flow rate, the residence time of the solution decreases due to the limited interfacial area of the stripper column with specific size, requiring additional heat consumption in the reboiler. These opposing phenomena are responsible for the exhibition of a minimum in the relationship between Qreb and solution flow rate.

3.2. Effects of Operation Parameters. As earlier mentioned, Qreb is a key parameter for the design, operation, and economic analysis of industrial CO2 capture plants. To evaluate the performance of DEEA solution in terms of heat of regeneration, a series of experiments was conducted at different operation conditions. Rich CO2 loading and lean CO2 loading in the stripping system are both very important variables in the reduction of the Qreb. The difference between these two parameters also determines the Δα (CO2 cyclic capacity) and the amine solution circulation rate for a given CO2 removal efficiency. The effect of lean CO2 loading was evaluated with 3 mol/L DEEA by varying the lean CO2 loading in the range from 0.10 to 0.40 mol/mol for rich CO2 loading of 0.65 and 0.70 mol/mol, respectively, and the rich CO2 loading, as shown in Figure 2. Figure 2a illustrates that the variation of Qreb with the lean CO2 loading is not linear. Qreb declines very rapidly with an increase in lean CO2 loading up to 0.27 mol/mol. Beyond these lean CO2 loadings, changes in Qreb become negligible and tend to exhibit minimum points. These results indicate that there are two Qreb operating regions for lean CO2 loading. The first Qreb region is more sensitive to lean CO2 loading. This leads to substantial heat energy consumption. The second region has little or insignificant changes in Qreb with further increase in lean CO2 loading. This region presents a favorable operating region. Also, Figure 2a,b shows that, at a given lean CO2 loading, an increase in rich CO2 loading results in a decrease in Qreb, as expected. The optimum lean CO2 loading based on the rich CO2 loading may not permit adequate absorption of CO2 in the absorber due to the need to D

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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results are shown in Figure 5. It shows that Qreb decreases with increasing amine concentration. An explanation for this could

Figure 3. Effects of solution flow rate on Qreb (C = 3 mol/L, rich CO2 loading = 0.70 mol/mol, feed temperature = 75 °C).

Generally, it is easy to release the CO2 from the tertiary amine solution, meaning that it starts to regenerate at relatively lower temperatures. The feed temperature of rich amine solution is a key parameter in the stripper system because it determines the amount of CO2 that is stripped prior to entry of rich amine into the stripper and the utilization of the heat energy of the lean solution. Figure 4 shows the variation of Qreb

Figure 5. Effects of DEEA concentration on Qreb (lean CO2 loading = 0.27 mol/mol, rich CO2 loading = 0.70 mol/mol, feed temperature = 75 °C).

be that an increase of amine concentration contributes in increasing the partial CO2 pressure in the stripper overhead gas and consequently decreases the partial pressure of water vapor. In addition, an increase in amine concentration from 1.7 mol/L to 3.8 mol/L provided a reduction in Cp of the amine solution as well as reboiler temperature, causing a reduction in Qsen. As a consequence, an increase in amine concentration results in the need to use a lower reboiler temperature, which significantly lowers the Qreb. However, it should be noted that the increase in amine concentration will lead to increases in the solution viscosity, as well as degradation and corrosion rates in the CO2 absorption−stripping process. Based on the experimental results in the present study, the optimum amine concentration was 3 mol/L. The total amount of captured CO2 is constant in most industrial plants, due to the fixed gas flow rate and composition of flue gas, as well as the outlet concentration of CO2 from the absorber. Therefore, it is necessary to perform a preliminary exploration of the synergistic effects on parameters such as Δα, C, and L on the reboiler heat duty Qreb while keeping the total amount of captured CO2 constant. In the present work, these were investigated by fixing both one parameter and the total absorption capacity at a constant value and varying the other two parameters. It is well-known that solution circulation rate is a very important variable which affects the sizes of the absorber column, stripper column, heat exchangers, pump, and piping and, thus, has a large influence on the capital cost of the CO2 capture plant. The solution circulation rate of an absorptionstripping process, in turn, depends on the lean CO2 loading. The solution circulation rate was therefore considered to be a critical factor in the study of Qreb. In this work, we experimentally evaluated the variation the solution circulation rate and lean CO2 loading at a fixed total absorption capacity into amine solution, that is, a fixed Δα × L for the present regeneration system. Other parameters, including rich amine stripper feed temperature, rich CO2 loading, and amine concentration were held constant. The results are given in Figure 6, which shows that Qreb for this regeneration system has a minimum point at the solution circulation rate of 70 mL/min. As was reported in our previous work, at low solution circulation rate, lean CO2 loading (i.e., high Δα), and a high Qvap as well as high Qreb are required due to the existence of

Figure 4. Effects of stripper feed temperature on Qreb (C = 3 mol/L, lean CO2 loading = 0.27 mol/mol, rich CO2 loading = 0.70 mol/mol, solution flow rate = 70 mL/min).

with rich amine stripper feed temperature. From Figure 4, it can be observed that the increments of feed temperature result in a decrease of Qreb to achieve the same CO2 stripping capacity when the reboiler temperature is about 90 °C. It clearly shows that the Qreb for the 5 °C temperature of approach (i.e., rich amine stripper feed temperature fixed at 85 °C) is lower than those for 10, 15, 20, and 25 °C temperatures of approach with lean-rich solutions. Although the reboiler temperature appears to be the same, the Qreb is greatly influenced by the rich amine stripper feed temperature. An increase in rich amine stripper feed temperature results in a reduced energy consumption because less energy is now required to strip CO2 from the amine. However, it needs to be mentioned that the heat energy required to increase the rich amine stripper feed temperature has to come from external sources, potentially nullifying the advantages mentioned earlier. Amine concentration is a very important parameter in the economics of industrial CO2 capture processes. The starting point for DEEA concentration in our present study was 1.7 mol/L since the amine concentration in a typical industrial CO2 capture process is around 30 wt %. Also, it is recommended that the amine solution be circulated to enter the tower at not less than 15 wt %. The influence of amine concentration on the regeneration performance of DEEA was investigated. The E

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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values for DETA and DEEA were compared against this benchmark. The Qreb of 2 mol/L DETA as obtained from the previous work was 3.44 GJ/t CO2 with stripper feed temperature of 90 °C and lean and rich CO2 loadings of 0.92 and 1.40 mol/mol. Based on the experimental results in the current work, the optimal regeneration performance of aqueous DEEA solution in terms of Qreb was approximately 2.17 GJ/t CO2, obtained at the DEEA concentration of 3 mol/L, solution flow rate of 70 mL/min, lean CO2 loading of 0.27 mol/mol, and rich CO2 loading of 0.70 mol/mol at a feed temperature of 75 °C. These results are given in Figure 8. Figure 6. Synergistic effect of cyclic CO2 capacity and solution flow rate on Qreb (C = 3 mol/L, rich CO2 loading = 0.70 mol/mol, feed temperature = 75 °C).

high H2O partial pressure under this condition; at high solvent circulation rate and lean CO2 loading, Qreb is mainly determined by Qsen. The calculation of Qreb for each experimental condition was performed using the mass flow rate of stripped CO2 both in stripper overhead gas and in the amine solution. These are both presented together in Figure 6 in order to evaluate the mass balance in the whole regeneration system. The results showed that the mass balance error determined for all the runs in the present work was under 10%. The synergistic effects of L and C on the Qreb were equally examined while the values of cyclic CO2 capacity (Δα) and L × C were kept constant. In contrast to the influence of single solution flow rate in Figure 4, Qreb increased continuously with an increase in the solution flow rate (L) while DEEA concentration (C) decreased from 3.8 mol/L to 1.7 mol/L, as illustrated in Figure 7. This is because of the decrease of CO2

Figure 8. Distribution of energy associated with Qreb for MEA, DETA, and DEEA.

Figure 8 also shows that the optimal Qreb value of DEEA is less than that of MEA and DETA, accounting for a reduction of 48.3% and 36.9%, respectively. It indicates the fact that more CO2 is actually released from DEEA solution during stripping than from DETA and MEA at the same Qreb and confirms that DEEA possesses a better regeneration performance than MEA and DETA. It can also be seen from Figure 8 that both MEA and DETA have similar heats of reaction (Qabs). In addition, the Qvap of DETA is much lower than that of MEA, whereas the Qvap of DEEA has the lowest value. This may be attributed to the fact that aqueous DETA solution has higher boiling temperature than MEA, as well as a higher CO2 loading in the stripper overhead, leading to a lower partial vapor pressure (i.e., a lower Qvap) for DETA solution. In comparison, DEEA as a tertiary amine has the lowest absorption heat (Qabs). A higher absorption heat (Qabs) generally corresponds to a higher reboiler heat duty (Qreb), which means a higher regeneration temperature resulting in a relatively high Qabs and Qvap. This is most likely due to the fact that the Qabs released in carbamate formation is higher than that of bicarbonate formation.

Figure 7. Synergistic effect of DEEA concentration and solution flow rate on Qreb (lean CO2 loading = 0.27 mol/mol, rich CO2 loading = 0.70 mol/mol, feed temperature = 75 °C).

4. CONCLUSIONS The regeneration experiments of aqueous DEEA solution were conducted in a lab-scale stripper packed with Dixon ring random packing. The influence of various operating parameters, i.e., lean CO2 loading (0.10−0.40 mol/mol), rich CO2 loading (0.55−0.80 mol/mol), flow rate (40−80 mL/min), feed temperature (65−85 °C), and DEEA concentration (1.7−3.8 mol/L), on the Qreb of DEEA was investigated. Additionally, synergistic effects of Δα × L and C × L on Qreb were also investigated. The experimental results from this study showed that Qreb decreased with the rich CO2 loading, rich amine stripper feed temperature, and DEEA concentration. The optimal operating points were obtained with the variation of lean CO2 loading and solution flow rate at about 0.27 mol/mol

being absorbed per unit volume of aqueous DEEA solution with an increase of L, thereby requiring a larger amount of energy for stripping CO2 in the regeneration system. This same phenomenon was also reported by Zhang et al.32 Based on the results, it implies that Qreb was dominated by DEEA concentration as compared with the solution flow rate. 3.3. Comparison of MEA, DETA, and DEEA. In order to evaluate the potential of aqueous DEEA solution for CO2 capture, its Qreb as well as the three component parts of Qreb (i.e., Qabs, Qvap, Qsen) were compared with those of MEA and DETA. In this work, the Qreb value of 5 mol/L aqueous MEA solution (4.20 GJ/t CO2) at lean and rich CO2 loadings of 0.28 and 0.50 mol/mol was regarded as the benchmark, and the F

DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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and 70 mL/min, respectively. By comparing with aqueous MEA and DETA solutions, DEEA was found to have a lower Qreb of 2.17 GJ/t CO2 because of its lower absorption heat (Qabs).

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-13618481627. Fax: +86-731-88573033. E-mail: [email protected] (Z. Liang). *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Nos. 21536003, 21476064, U1362112, 21376067, and 51521006), National Key Technology R&D Program (No. 2014BAC18B04), Innovative Research Team Development Plan-Ministry of Education of China (No. IRT1238), Specialized Research Fund for the Doctoral Program of Higher Education (MOE-No. 20130161110025), and China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No.2011-40) are gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research (32) Zhang, X.; Fu, K.; Liang, Z.; Rongwong, W.; Yang, Z.; Idem, R.; Tontiwachwuthikul, P. Experimental studies of regeneration heat duty for CO2 desorption from diethylenetriamine (DETA) solution in a stripper column packed with Dixon ring random packing. Fuel 2014, 136, 261−267. (33) Idem, R.; Wilson, M.; Tontiwachwuthikul, P.; Chakma, A.; Veawab, A.; Aroonwilas, A.; Gelowitz, D. Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 capture technology development plant and the Boundary Dam CO2 capture demonstration plant. Ind. Eng. Chem. Res. 2006, 45 (8), 2414−2420.

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DOI: 10.1021/acs.iecr.6b00936 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX