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Effect of heat loss for Aspen Plus based CO2 absorption

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

“As mentioned earlier, Enaasen et al. [41] and Pinto et al. [42] did not specify 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 over the years. To investigate the effect of heat losses, simulations were carried out with and without 0.5 KW of heat loss (Fig. 8Fig. 9). The deviations of both scenarios are shown in (Table 4) as AARD results of the loading in the lean amine, temperature of the lean amine and CO2 stripped.

In this study, it was found that generally, the temperature profile of the desorber was over-predicted, with exception of two runs in Pinto et al. [42]. This over-prediction was also observed by Luo et al. [11]. Typical profiles from Pinto et al. [42] and Enaasen et al. [41] are included in Fig. 8Fig. 9. The temperature profile in Enaasen et al. [41] improved with the addition of 0.5 KW of heat loss (Fig. 8). Additionally, the loading and temperature of the lean amine improved by 1.8 and 0.11% of AARD respectively. However, the CO2 stripped had a 4.6% higher AARD than not considering heat loss. The temperature profile in Pinto et al. [42] improved for three runs (Fig. 9). The loading of the lean amine is also enhanced by 0.34% of AARD. However, the temperature of the lean amine and the CO2 stripped had a higher AARD, 3.8 and 0.16% respectively, than the case without considering the heat loss. These results can be explained with Fig. 3, where an under-prediction in the lean amine loading is shown. Including 0.5 KW of heat loss decreased the desorption of CO2 and the lean amine loading was higher, compensating this under-prediction observed in the scenario without heat loss (Table 4). However, the CO2 stripped was also under-predicted and including the heat loss decreased the predicted amount of CO2 stripped and increased this under-prediction. Regarding these results, the values presented in this work did not include heat loss in the campaigns of Pinto et al. [42] and Enaasen et al. [41]. Overall it can be said, that an estimation of heat loss should be reported together with the experimental values. The simulation results improve when heat loss is taken into account in Tobiesen et al. [15] whereas for Pinto [42] and Enaasen [41], estimating the same heat loss does not give an overall improvement of the simulation results as seen from Table 4. This indicates that the in well insulated pilots, the heat loss is not influencing the results.

Fig. 8

Fig. 8. Runs 2,4 and 5 from Enaasen et al. [41]: Experimental results from Enaasen et al. [41](▲); results from this work without heat loss(▪▪▪) and incorporating heat loss (−).

Fig. 9

Fig. 9. Runs 1,2 and 7 from Pinto et al. [42]: Experimental results from Pinto et al. [42](*); results from this work without teat loss(▪▪▪) and incorporating heat loss (−).

Table 4. Simulation results of incorporating heat losses (0.5 KW) in Pinto et al. [42] and Enaasen et al. [41] compared with simulations without heat losses.

Empty Cell AARD (%)
Empty Cell Without Heat loss With heat loss (0.5 KW)
Empty Cell [42] [41] [42] [41]
Lean Loading 3.4 5.9 3.0 4.1
Lean Temperature 2.3 0.8 6.2 0.6
CO2 stripped 11.9 7.7 12.0 12.3

Notz et al. [25] indicated the heat losses in each run varying from −0.247 (adding heat from outside instead of losing it) to 1.05 KW. The heat loss was assumed to be heavily dependent on the running conditions. To see if this large variation in heat loss correlated with model predictions, the simulated temperature of the lean amine was plotted as a function of heat loss in Fig. 10. The figure showed that, overall, there was no correlation between the reboiler temperature and heat loss. However, four runs showed strongest deviation. The error of the simulated lean loading showed some dependency on the heat loss and increased as the heat losses increased. At heat losses between 0.6 and 1 KW, the simulated loading on the lean amine agreed with the experimental values. This could indicate that the heat loss is less dependent on the experimental conditions than what Notz et al. [25] reported.

Fig. 10

Fig. 10. Evolution of the Ratio SIM/EXP Lean temperature (left) and lean loading (right) with the heat loss reported in Notz et al. [25].

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