https://doi.org/10.1007/s13369-015-2008-z
“The effect of varying the L/G ratio on CO2 recovery and gross reboiler heat duty for three different stripper heating
powers is illustrated in Figs. 2, 3 and 4.”
“At constant heater power, within L/G ratio changes, the CO2 recovery curves create reversed parabolic trends. Standard parabolic regularity can be observed for reboiler heat duty curves where some visible minimum at a specific L/G
ratio can be observed. This regularity is especially visible for lower stripper power adjustments—2000W and Fig. 2. The
reboiler heat duty parabolic trends as a function of L/G ratio were also reported by several authors: Han [12], Knudsen
[15], Artanto [16], Mangalapally [26]. The figures show that to obtain higher CO2 recovery and the lowest energy consumption, a specific liquid-to-gas ratio is required. Further, for a given gas flow rate, there is a specific or optimal solvent flow rate to operate the CO2 absorption process. This regularity can be explained because for a liquid ratio that is too low, there is not enough free amine in the small circulating volume of solvent to absorb a constant amount of carbon dioxide in the absorption column, despite the low
solvent loading leaving the stripper. However, a liquid flow rate that is too high results in a higher solvent loading leaving the stripper, so the circulating solvent cannot absorb the same amount of carbon dioxide in the absorption column.
Thus, there is also insufficient free amine to absorb the CO2 from the gas. The effect on the decrease in CO2 removal is
similar.
The reboiler heat duty is correlated with the energy supplied to the process and the mass of the CO2 removed, so that
both values are closely related to each other. The reboiler heat duty values do not include heat losses to the ambient air.
Figure 2 shows the effect on the CO2 recovery of the L/G ratio associated with the reboiler heat duty. At this setting
of the heater power, the lowest regeneration energy demand (approximately 7.70MJ/kgCO2 ) can be observed within a
5.9–7.3 kg/kg L/G ratio. The maximum achieved for the CO2 capture efficiency is in the range of 77.6–78.3%. For the
heat power setup at 2500W (Fig. 3), the highest CO2 recovery amounts to 7.1–8.8 kg/kg L/G ratio. The lowest reboiler heat duty can be observed in the range of 8.64–9.50MJ/kg CO2. Further increase in the setting of the heater power (3000W), as shown in Fig. 4, increases the CO2 recovery level (up to 90.7%). Figure 4 might also suggest that with the L/G ratio, the carbon dioxide efficiency will trend to infinity, but according to the minimum reboiler heat duty obtained, we can assume that the maximum CO2 recovery has been achieved. Further increasing the L/G ratio was not possible due to solvent flow meter limitations. The lowest reboiler heat duty can be seen in the 7.9–8.4 kg/kg L/G ratio range.”
“Comparing the effect of the L/G ratio on CO2 recovery for every stripper power setting (Fig. 5), we can see from the
graph that the stripper heater power set at 3000W exhibits the highest capture efficiency. However, CO2 recovery at the
L/G ratio in the range from 2.6 to 5.5 is almost the same between 3000 and 2500W heater power setting, implying
that minimal solvent loading and excess heat utilized in the condenser and/or wasted to the ambient air have been
achieved at 3000W. For an L/G ratio higher than 5.5, the efficiency is decreasing. This figure also indicates that, for
the stripper heater set at 3000 and 2500W, CO2 recovery is not significantly affected by L/G ratio at 2000W because the
lean solvent for 2000W has a higher loading than the others, so with the different liquid flow rates the regeneration impact
on the solvent loading is higher. From the graph, we can also see that for every stripper heater power setting, the optimal
L/G ratio is different. An optimal L/G ratio increases with the stripper heater increase.
Low CO2 recovery translates into a low amount of CO2 separated from the gas stream, which is reflected in the regeneration heat duty related to the unit of mass. Comparing reboiler heat duties for different stripper heater power settings, the same regularity with optimal L/G ratio can be observed (Fig. 6). The lowest regeneration energy consumption and the optimal L/G ratio are naturally the same as in Fig. 5, due to the CO2 removal rate. Direct comparison of the values of the reboiler heat duty shows clearly that stripper heat power set at 2000W is superior to other heater power settings, giving the lowest heat duty.
In the range of removal efficiencies (>85%) that we are interested in, by comparison with other publications, the
reboiler heat duty results that we report seem quite high [15,16,26,27]. Our results can be explained by the significant stripper heat losses to the ambient air. An estimation of the stripper heat losses to the ambient air revealed that even 30% of the electric heat that is supplied is wasted. A confirmation may be found in the literature [28,29], where similar laboratory-scale installations exhibit a similar range of reboiler heat duties. From our other research, we can see that with an increase in the process scale, the reboiler heat
duty decreases. The lowest desorption heat requirement can be observed for the pilot plant test, where similar results were
achieved for all units. As suggested, this result is related mainly to the unit heat exchangers. The comparison can be found in [30].”