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Desorbed CO2, lean loading and temperature – simulation results

https://doi.org/10.1016/j.jece.2017.08.024

Firstly, the standard ENRTL-RK ASPEN PLUS model available in ASPEN PLUS v8.6 was used to simulate the data from Tobiesen et al. [15]. The results are shown in Fig. 1Fig. 2. As seen in the figures, the model predicted the desorbed CO2 well, with AARD of 8.7%. The lean loading was predicted with AARD of 6.0%. The model was modified by taking into account the packing characterization obtained experimentally in Zakeri [43] and simulations were performed to see if the experimental data representation was improved. The enhanced model showed AARD% of 6.9% and 1.9%, for desorbed CO2 and lean loading, respectively. As seen in Fig. 1, most of the represented values were within the 10% of error with the current model while the base case had three additional points with higher error. Fig. 1 shows that the highest deviations were observed in the cases of high stripped CO2, while at low stripped CO2, below 6 kg/h, a good fit was observed. Fig. 2 shows a consistent prediction of the lean loading with AARD below 6%, with exception of one of the runs done with the standard ENRTL-RK ASPEN PLUS at low lean loading. The standard ENRTL-RK ASPEN PLUS model showed some under-prediction of the lean loading, what means that this model predicted a higher CO2 absorption than the reality.

Fig. 1

Fig. 1. Experimental values of CO2 absorbed [15] vs simulated values obtained in this work (▲) and with the ASPEN PLUS ENRTL-RK template (○). Dotted lines represents the ±10% prediction of experimental values and on the black the experimental and simulated desorbed CO2 (kg/h) are equal.

Fig. 2

Fig. 2. Experimental values of Lean Loading [15] compared to simulated values obtained in this work (▲) and with the ASPEN PLUS ENRTL-RK template (○). Dotted lines represents the ±6% prediction of experimental values and on the black line the experimental and simulated lean loadings (mol CO2/mol MEA) are equal.

The developed model was used for the campaigns from Enaasen et al. [41], Pinto et al. [42] and Notz et al. [25]. The results from the simulations with the model from this work are shown in Fig. 3Fig. 4 . Fig. 3 shows that the best fit with the new model was seen for loadings below 0.39 mol CO2/mol MEA with an AARD% of 10%. The better prediction of the desorbed CO2 automatically improved the prediction of lean loading because the CO2 entering the stripper was taken from the experimental data and what is not desorbed leaves with the lean solvent. As seen in Fig. 3 and Table 3, for the campaign of Tobiesen et al. [15] and Pinto et al. [42], there was a good agreement at low loadings and under-prediction at high loadings. Tobiesen et al. [15] also simulated the desorption at a wide range of CO2 loadings and temperatures of the rich solvent. Their in-house model was able to represent the CO2 concentration and temperature along the stripper column. The in-house model was able to represent the CO2 concentration and temperature along the stripper column. At low loadings (0.29–0.32 mol CO2/mol MEA) the simulations showed good agreement in loading and temperature compared to experimental data, while under-predicted lean loadings were observed at medium (0.33–0.39 mol CO2/mol MEA) and high loadings (0.4–0.46 mol CO2/mol MEA).

Fig. 3

Fig. 3. (Left) Ratio Simulated/Experimental CO2 stripped (Kg/h) vs Rich Loading (mol CO2/mol MEA) and (Right) Ratio Simulated/Experimental Lean Loading (mol CO2/mol MEA) vs Rich Loading (mol CO2/mol MEA) from the campaigns: from Enaasen et al. [41] (▲)), Pinto et al. [42] (*),Tobiesen et al. [15] (●) and Notz et al. [25] (▪). Line represent the case in which simulated and experimental values of CO2 stripped are equal.

Fig. 4

Fig. 4. Ratio Simulated/Experimental CO2 stripped (Kg/h) vs Rich Flux (Kg/h) from the campaigns: from [18 et al. [41] (▲), Pinto et al. [42] (*), Tobiesen et al. [15] (●) and Notz et al. [25] (▪). Line represent the case in which Simulated and Experimental values of CO2 stripped are equal.

Table 3. AARD (%) of the simulation (from this work) compared to the experimental campaigns.

Empty Cell AARD (%)
Empty Cell [15] [42] [41] [25]
Loading Range Stripped CO2 (Kg/h) Lean Loading (mol/mol) Stripped CO2 (Kg/h) Lean Loading (mol/mol) Stripped CO2 (Kg/h) Lean Loading (mol/mol) Stripped CO2 (Kg/h) Lean Loading (mol/mol)
0.25–0.32 4.7 2.2 4.9 3.1 17.2 6.3 7.8 5.6
0.33–0.4 4.0 2.5 9.4 3.4 11.8 5.6
0.41–0.55 12.4 1.2 11.5 3.8 6.4 5.9 18.9 12.0
Overall 6.9 1.9 8.6 3.4 7.7 5.9 13.9 8.2

However, the best agreements were observed at high loadings (0.4–0.45 mol CO2/mol MEA) in the simulation of the campaign of Enaasen et al. [41], with exception of two runs, while some under-prediction was shown at low loadings. Moreover, the CO2 stripped was over-predicted for the campaign of Notz et al. [25], at loading over 0.4 mol CO2/mol MEA. The AARD for all the four pilot campaigns (in total 78 experimental runs) was 9.2% and 4.9% for stripped CO2 and lean loading, respectively. This is a good result considering that, for example, the physical properties as Henrýs Law constant and diffusion for the CO2 into the solutions are typically not measured at the stripper and reboiler temperatures but the behaviour is extrapolated [40][44].

In this work, it was found that there is no dependency of the prediction of the CO2 desorbed on the temperature or flux of the rich solution in the cases of Pinto et al. [42], Enaasen et al. [41] and Tobiesen et al. [15]. However, the simulation model predicted the data of Notz et al. [25] better at rich flux temperatures above 115 °C, as seen in Fig. 3.

In addition to the desorbed CO2, the temperature of the lean solvent leaving the desorber was checked with the experimental data and the results of each pilot plant campaign were analysed separately by values of AARD. Overall, the simulation model slightly over-predicted the reboiler temperature. The simulation results in Tobiesen et al. [15] showed some over-prediction of the lean temperature (3.29% of AARD). This general over-prediction could indicate that the heat losses were even higher than it was reported in the experimental data. However, at the same time the simulation model of Tobiesen et al. [15] under-predicted the temperature along the stripper, which could be attributed to the presence of two phases, gas-liquid, in the rich amine entering the stripper. In the current work, however, the over-prediction together with the under-predicted stripped CO2 could indicate deviations on the desorption kinetics. Nevertheless, the simulated temperature results agreed well with the other pilots campaigns. In the case of the campaign from Pinto et al. [42], the temperature in the reboiler was slightly under-predicted (2.3% of AARD). Additionally, the experimental reboiler temperatures in Enaasen et al. [41] and Notz et al. [25] were well predicted. The AARD were 0.7 and 1.5% for Enaasen et al. [41] and Notz et al. [25] respectively, even though there were four experimental runs Notz et al. [25] that showed a greater deviation, up to 14% of AARD.

For further study, the temperature profile along the desorber was studied in this work and compared to the experimental data. Typical results are showed in Fig. 6, where data from Notz et al. [25] are represented. Overall, the temperature profiles were well predicted. The runs with the highest temperature deviations were the ones with the highest ratio between simulated and experimental values in Fig. 5 and Fig. 10.

Fig. 5

Fig. 5. Ratio Simulated (in this work)/Experimental values from the campaigns: from Enaasen et al. [41] (▲), Pinto et al. [42] (*), Notz et al. [25] (▪), Tobiesen et al. [15] (●). Line represent the case in which Simulated and Experimental values of Lean Temperature are equal.

Fig. 6

Fig. 6. Runs 3, 13 and 23 from Notz et al. [25]: Experimental results from Notz et al. [25](▪); results from this work (−).

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