https://doi.org/10.3390/pr9122184
“2.2.1. The Modeling Object
In the process of amine scrubbing, pumps, condensers, reboilers and compressors all consume energy. The two most energy-consuming parts are the reboiler and compressor. The compressor is used to compress the high-purity CO2 gas desorbed from the top of the regenerator. Therefore, its energy consumption depends only on the pressure of the regenerator. If the pressure of the regenerator is high, the compression energy consumption will be low, while the pressure of the regenerator depends on the high resistance to oxidative and thermal degradation of the absorbent. This is due to the fact that high desorption pressure will make the desorption temperature higher. Different absorbents have different resistance to degradation at high temperatures, and absorbents that can withstand higher temperatures can be desorbed at higher pressures. The degradation of absorbents is not within the scope of this work, and all absorbents are compared under the same desorption pressure, so their compression energy consumption is the same. The energy consumption of the reboiler is determined by the CO2 cyclic capacity and the heat of CO2 desorption of the absorbent, which is the main research content in this work. Firstly, an accurate thermodynamic model of the capture system was established using data from experiments and the literature. On this basis, a process simulation based on the thermodynamic-equilibrium model was established, and it was assumed that both the absorber and the regenerator reached thermodynamic equilibrium and the influence of kinetics was not considered. The lowest energy consumption of each absorbent was obtained by optimizing the process parameters. In order to evaluate the performance of HEPZ instead of PZ as a promotor, simulation models of HEPZ, PZ and MEA as absorbent systems were established in this work, and the energy consumption, cyclic absorption and absorbent loss of the three were compared, respectively.
2.2.2. Physical Parameter System
In the processing system, there are components H2O, CO2, N2, HEPZ, H3O+, OH–, HCO3−, CO32−, HEPZH+, HEPZH22+, HEPZCOOH and HEPZCOO−, and the physical property method is ENRTL-RK. Establishing a process simulation model requires a complete physical parameter system, including the necessary physical parameters of amines in the system, the interaction parameters of the nonrandom two-liquid model (NRTL) and the electrolyte nonrandom two-liquid (ENRTL) activity coefficient model, along with the standard state properties of amine ions. The physical property parameters and interaction parameters between the HEPZ molecule and various ions in the system are lacking in the Aspen database. It is necessary to perform regression fitting on the heat capacity, saturated vapor pressure and vapor–liquid balance and solubility of amine data. A rigorous thermodynamic model was established to obtain the physical property system, and the specific modeling method is described in detail in [29]. In our previous work, CO2 solubility was measured for HEPZ aqueous solutions at three concentrations—5, 15 and 30 wt.%, and four temperatures—313.15, 343.15, 373.15 and 393.15 K. The VLE data for HEPZ/H2O were obtained at a pressure of 30–100 pKa, within a whole mole-fraction range. By regressing and fitting data from experiments and the literature, the parameters of NRTL and ENRTL were obtained, and the standard thermodynamic parameters of the new substances HEPZH+ and HEPZH22+, which were not in the database of Aspen, were manually adjusted to fit the dissociation constant of HEPZ. The equilibrium constant can be calculated from the standard state properties of the reactants and products. Knowing the standard Gibbs free energy, the standard enthalpy of each component’s formation and the heat capacity in the reference state in the reaction equilibrium equation, the equilibrium constant of each reaction can be calculated. In the work of [30], we could obtain pKa1 and pKa2 of HEPZ, so the equilibrium constants of the hydrolysis reactions of HEPZ could also be obtained. Therefore, the calculated equilibrium constant can be used to verify whether the standard state properties of the reactants and products are correct. The pKa curve was compared with the experimental data in [30], and the value of the standard property is manually adjusted if there is any deviation. In this work, the obtained pKa curve agreed well with the experimental data. More detailed descriptions can be found in [31].
This accurate thermodynamic model can predict the thermodynamic properties and absorption performance of the CO2 capture system, which provides reliable physical properties for the establishment of process simulation. The physical parameters of the model are listed in the previous work [31].
2.2.3. Process Description and Main Process Parameter Setting
A schematic diagram of the amine scrubbing process simulation is shown in Figure 2. The main equipment in the process is an absorber and regenerator, and the auxiliary equipment includes pumps, heat exchangers, condensers and flash tanks. The absorber and regenerator are modeled by RadFrac in Aspen Plus® platform, using the equilibrium model. FLUEGAS with a temperature of 110 °C, a pressure of 1 bar and a flow rate of 499.8 kg/h is first flashed by a flash evaporator, the purpose of which is to reduce the flue gas temperature to 40 °C and ensure that the water in the flue gas reaches saturation. The composition of flue gas is (volume fraction): 8% H2O, 10% CO2, 76% N2 and 6% O2, which was referenced by the experimental values of the pilot plant. Then, GASIN enters from the bottom of the absorber, contacts with the absorbent in the countercurrent and is evacuated from the top of the absorber after condensation, separation and liquid reflux. The temperature of the condenser is 40 °C, and the temperature of LS1 is also set to 40 °C. After RICHOUT is pressurized to 2.5 bar by the pump, the HS1 exchanges heat with the high-temperature lean liquid from the regenerator. The lean-rich heat exchanger is modeled using the Aspen® Heatx model. The minimum heat transfer temperature difference is set to 10 °C, and the temperature of the condenser at the top of the desorption tower is set to 40 °C. The number of plates in the absorber is sufficient to ensure that the absorber reaches the thermodynamic equilibrium, which is generally 40. When operating the process, the converged calculation result of the operation for the absorption tower with 40 plates was basically the same as the result for the absorption tower with more plates. For the desorption tower, the thermodynamic equilibrium can be achieved with fewer plates by our simulation, and it is sufficient to set the number of plates in the regenerator to 10, as suggested by Oyenekan and Rochelle [32]. The top plate’s pressure of the absorber is 1 bar, and the pressure drop of 0.2 bar is considered for both towers. Some design specifications for the reboiler duty and the lean-rich heat exchanger are incorporated to converge the flow sheet in a closed loop. The design specifications require that the CO2 removal ratio reaches 90% and the CO2 loading of LS1 is equal to the CO2 loading of LS2. The value range of the flow rate of LS1 and reboiler duty is set and the program is run. The program can calculate the accurate values of the flow rate of LS1 and reboiler duty. The parameters of LS1 with the lowest reboiler duty are explored by changing the amine concentration and CO2 loading.
Figure 2. Schematic diagram of amine scrubbing process simulation.
2.3. Regeneration Energy Study
This work studied the regeneration energy consumption, absorbent loss and cyclic capacity of different absorbents in the process of capturing CO2. The goal of optimizing process parameters is to obtain the lowest energy consumption of the reboiler at 90% CO2 recovery. The adjustable process parameters include solvent concentration, lean loading of LS1 and liquid–gas ratio, L/G. When the reboiler heat duty reaches the minimum value, the cyclic capacity and absorbent loss of different absorbents for capturing CO2 can be obtained. The absorbent with the lowest regeneration energy consumption, the least absorbent loss and the largest cyclic capacity has the best economic benefits and potential for factory promotion.
The regeneration energy is determined by the CO2 cyclic capacity of the absorbent and the heat of CO2 desorption, which is the main research content of this study. The energy consumption of the reboiler is mainly used in three parts: water evaporation, CO2 desorption and liquid heating. The evaporation heat of water and the liquid heating heat are determined by the CO2 cyclic capacity. The higher the CO2 cycle absorption, the lower the amount of absorbent, which will reduce the water evaporation heat and heat for liquid heating. In order to analyze the total energy consumption, that is, the composition of the reboiler energy consumption, Qreb, the desorption part, including the heat exchanger, can be calculated as follows:
Qreb=HCO2+Hleanin2−Htoheatx1−Qcon
where Qcon is the heat of condensation of water, HCO2+Hleanin2−Htoheatx1 is the sum of the heat QT required to heat the rich liquid at the bottom outlet of the absorption tower and the heat Qabs required to desorb from the rich loading to the lean loading. QT is obtained directly from the heat capacity, and the calculation formula is as follows:
QT=Cptoheatx1×Fmass,toheatx1×(Tleanin2 − Ttoheatx1/Fmass,CO2
where Cptoheatx1 and Fmass,toheatx1 are the heat capacity and mass flow rate of rich liquid TOHEATX1, respectively, and Fmass,CO2 is the mass flow rate of CO2 at the top of the desorption tower. After obtaining QT, the value of Qabs can be calculated as:
Qabs=Qreb+Qcon−QT
This work studies the influence of absorbent concentration and process parameters on the total energy consumption of the reboiler and its three parts. By comparing energy consumption under the optimal operating conditions of each absorbent, the potential of HEPZ as a new absorbent is evaluated. In Section 3.1, the effect of these process parameters on the reboiler energy of each absorbent is discussed. According to the discussion, the optimal process parameters that could minimize the energy consumption of the reboiler were obtained, as presented in Section 3.2. When optimizing process parameters, the effect of a certain process parameter was studied while keeping other process parameters fixed. The optimal value of one process parameter was selected, then the optimal value of the next process parameter was studied.
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