https://doi.org/10.1016/j.jece.2017.08.024
“Vapour-Liquid-Equilibrium (VLE) of the system CO2-MEA-H2O is of high importance to predict the desorption [35], [37]. Greer et al. [38] validated the simulation results from their Matlab model with VLE results, where temperature dependent variables were modelled to obtain the dynamic performance of the desorber, at rich loading of 0.46 mol CO2/mol amine. In this work, the ENRTL-RK rate-based model, available in ASPEN PLUS v8.6 for MEA, was used as a basis for the simulations and the discretization of the film was considered only on the liquid phase, along 10 segments. No film was considered in the gas phase, as there is not reaction in the gas phase. The VLE model in ASPEN PLUS was found to be valid by comparing the simulated partial pressure of CO2 to experimental VLE data [39]. The temperature, composition and flow rate of the rich amine solution were used as inputs and the rich flux was considered one phase solution entering the desorber for all the cases. Similar approach was used in Tobiesen et al. [14] who, in simulations, assumed that the rich solution was one liquid phase. In many of the pilot campaigns the reality is that at high loading, the solution could flash before going into the desorber. This would give differences in the enthalpy of the solution, what would cause under-predicted values of temperature and deviations in CO2 mass transfer [14]. The reboiler duty, pressure at the top of the desorber and pressure drop were used to define the operation conditions. The composition, temperature and flow rate of the lean solution as well as the CO2 concentration and gas flow at the top of the desorber and reboiler temperature were obtained from the simulations.
The data used in this work is collected in Table 2. A simulation model in ASPEN PLUS was validated with the data in Tobiesen et al. [15] and then used to represent data from Enaasen et al. [41], Pinto et al. [42] and Notz et al. [25], using the desorbed CO2 and the temperature of the lean flux in the reboiler as performance parameters.
Table 2. Overview of the literature data used in this work.
| Resources | [15] | [41] | [42] | [25] |
|---|---|---|---|---|
| Desorber Diameter (m) | 0.1 | 0.1 | 0.15 | 0.125 |
| Packing height (m) | 3.89 | 3.57 | 3.57 | 2.52 |
| Packing type | Sulzer Mellapak 250Y | Sulzer Mellapak BX 500 | Sulzer Mellapak BX 500 | Sulzer Mellapak 250Y |
| Rich solution Loading (mol CO2/mol MEA) | 0.30–0.45 | 0.41–0.49 | 0.25–0.53 | 0.31–0.51 |
| Lean solution Loading (mol CO2/mol MEA) | 0.18–0.45 | 0.22–0.34 | 0.21–0.34 | 0.146–0.36 |
| Temperature Rich Solution (°C) | 105–115 | 98–110 | 99–112 | 97–117 |
| Temperature Lean Solution (°C) | 101–121 | 106–119 | 112–118 | 102–125 |
| Flux Rich solution (L/min) | 3.0–9.0 | 2.43–4.1 | 3.3–3.43 | 1.32–6 |
| Condenser temperature (°C) | 15 | 15–25 | 15–34 | 14–20 |
| Reboiler Duty (KW) | 3.9–13.8 | 6.13–10.35 | 4–8.4 | 5.95–17.5 |
| Runs | 19 | 8 | 7 | 47 |
As seen in Table 2, the pilot plant campaigns have different desorber diameters, packing heights and different structured packings. Moreover, the runs were performed under different conditions of CO2 loading, temperatures and rich solvent flow. Due to this variability, the results allowed a comprehensive understanding of the desorption process and its representation. Table 2 shows that data sets covers in CO2 loadings from 0.15 to 0.53 molCO2/molMEA. In full height pilots the lean loading is typically 0.2–0.25 molCO2/molMEA and with rich loadings around 0.45–0.48 molCO2/molMEA. Thus, the data from the pilot plants used here covers the full range of loadings. This is important since the pilot don’t have enough packing heights to allow full recovery of the solvent in one run (from loading 0.48 to 0.2 molCO2/molMEA). Similarly, the lean solvent temperature (temperature of the liquid in the reboiler) and the rich solvent temperature vary more than 20 °C between the runs. The liquid flow rate is relatively contant in the works of Pinto et al. [42] and Enaasen et al. [41]. However, in Tobiesen et al. [15] and Notz et al. [25] the largest liquid flow is up to 4.5 times higher than the smallest flow used covering very different operating conditions for the packing.
The results in Zakeri [43] were incorporated through the Stilmchair correlation to characterize the packing Mellapack 250Y, used in Tobisen et al. [15] and Notz et al. [25]. Same behaviour as reported by Zakeri [43] in case of Mellapack 250Y was assumed in the case of Sulzer 2× packing (used in Pinto et al. [42] and Enaasen et al. [41]).
In this work, the deviations between experimental data and simulation results are given by the percentage of average absolute relative deviation (%AARD):
(1)ARD=|xsim−xexp|/xexp
(2)%AARD=∑1nARD/n*100
As mentioned by Weiland et al. [35], the heat effects cannot be ignored in the stripping process. Tobiesen et al. [15] carried out few runs with pure water to check the heat balance and estimate the heat loss, stayed as 0.5 kW along the stripper. In this work, these heat losses were incorporated as liquid losses. Enaasen et al. [41] and Pinto et al. [42] did not mention explicitly the numerical quantification of the heat loss along the column. Although the experiments were carried out in the same facilities used by Tobiesen et al. [15], the insulation of the pilot plant was improved. Since Enaasen et al. [41] and Pinto et al. [42] did not give a value of the heat losses, the results are given based on simulations without heat loss. However, at the end of this work, the effect of heat loss is presented. Notz et al. [25] reported that the heat losses varied from −0.247 (adding heat from outside instead of losing it) to 1.05 KW, with dependence on the running conditions, and those values were incorporated in this work and are commented below.
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