Evaluating Reboiler Heat Duties Using a Shortcut Approach

After passing through the cross-heat exchanger, the rich amine stream is fed to the reboiler, where it is heated with compressed steam. This heating is responsible for three distinct tasks.


Increasing the temperature from that of the reboiler feed (TX) up to that of desorption (TR), typically around TR = 120 °C for aqueous MEA, often called sensible heat.


Vaporizing the diluent, which is not only inevitable (as some of the diluent will surely volatilize at high temperatures) but also important for helping stripping CO2, often called latent heat.


Reversing the reaction between CO2 and the amine, thus desorbing CO2 from the solvent, often called absorption heat.

Oexmann and Kather (16) have arrived at eq 44 for the calculation of the reboiler heat duty of a CO2 capture plant.


Equation 44 can be further simplified if one defines Δq as the mass cyclic capacity of CO2 in the solvent following eq 45. This results in eq 46.

Equation 46 shows how the reboiler duty per mass of CO2 captured can be obtained as a function of the solvent heat capacity CP; the latent heat and partial pressure Li and pi, respectively, of all diluents at stripper temperature; the CO2 partial pressure and molar mass pA and MA, respectively; the heat of absorption ΔH; and the difference between the temperatures TR at the reboiler and TX at the cross-heat exchanger. The partial pressure of the diluents can be calculated using Raoult’s law with the saturation pressure psat of the pure diluents at 120 °C, which have been calculated with the parameters presented in Section S1 and shown in Table 6.
Table 6. Comparison between Reboiler Duties Using Different Hypothetical Solvents Based on 30 wt % MEAa
Name psat at 120 °C (kPa) case C: QR increment (%) case D: QR increment (%)
acetone 620.2 +39 +24
benzaldehyde 17.6 –25 –43
butanol 110.7 +1 –24
2-butanol 211.6 +10 –13
tert-butyl alcohol 363.5 +33 +5
cyclohexanol 25.7 –12 –39
cyclohexanone 35.4 –20 –40
cyclopentanone 73.5 –23 –35
dimethyl sulfoxide 2.0 –24 –44
dimethylformamide 37.5 –22 –39
ethanol 413.1 +45 +22
ethylene chloride 1454.5 +69 +57
ethylene glycol 4.3 –15 –41
glycerol 0.0 –10 –42
heptanol 14.9 –4 –37
hexanol 29.4 –7 –36
isoamyl alcohol 68.7 –1 –30
isobutyl alcohol 155.5 +14 –17
isopropyl alcohol 360.9 +41 +11
methanol 642.3 +71 +56
methyl ethyl ketone 319.4 +10 –8
nitrobenzene 5.9 –27 –45
N-methyl-2-pyrrolidone 5.9 +2 –35
pentanol 54.8 –2 –32
propanol 219.6 +18 –8
propionitrile 199.9 –2 –17
propylene carbonate 3.7 –26 –45
pyridine 115.8 –16 –31
sulfolane 0.5 –23 –45
water 199.0 +0 +0


TX varies in case C, TX = 105 °C in case D, TR = 120 °C, pA = 102 kPa.

Wanderley et al. (10) have shown that the cyclic capacity in water-lean solvents can, in theory, be the same or even higher than that of aqueous solvents. Also following that work, and just as we did in Section 3, the heat of absorption of CO2 is fixed for both aqueous and water-lean solvents at ΔH = 85 kJ·mol·CO2–1. We have also assumed that CO2 can be recovered in the reboiler at 1 bar (pA = 105 Pa), which can theoretically be achieved either by heating or by flushing with an inert gas. A discussion on these two hypotheses will be carried out in the next section. For now, if one assumes that Δα = 0.3 mol CO2·mol MEA–1 (or conversely 65 g CO2·kg solution–1 in a 30 wt % MEA solvent), Table 6 shows how the reboiler heat duties will decrease or often increase when shifting from aqueous to water-lean solvents.
Table 6 shows that water-lean solvents with low volatility (i.e., based on organic diluents less volatile than water) might often offer benefits in terms of reboiler duties when a specific heat exchanger with a large area AHX is designed to recover energy from the hot lean amine (case D). Such is the case with ethylene glycol, N-methyl-2-pyrrolidone, propylene carbonate, and sulfolane. This is even the case for excessively viscous solvents such as glycerol. However, when the issues regarding heat transfer in the PHE are taken into account (case C), many of these solvents underperform because of the extra amount of sensible heat required to compensate the shortcomings of the cross-heat exchanger.
Overall, however, the results presented in Table 6 are quite positive for water-lean solvents. Even diluents that have clear issues in terms of heat transfer in the PHE, such as sulfolane (TX = 76 °C), might be able to provide savings in reboiler heat duties following case C just by force of their low volatility. This is because sensible heat in the reboiler is a small fraction of the total heat expenditures, with latent heat playing a more significant part. Therefore, the diluent psat has a stronger impact on reboiler duty analyses than any other variable. A more in-depth discussion of the impact of each variable can be seen in Section S5.

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