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.

1.	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.
2.	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.
3.	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 C_P; the latent heat and partial pressure L_i and _pi, respectively, of all diluents at stripper temperature; the CO₂ partial pressure and molar mass p_A and M_A, respectively; the heat of absorption ΔH; and the difference between the temperatures T_R at the reboiler and T_X at the cross-heat exchanger. The partial pressure of the diluents can be calculated using Raoult’s law with the saturation pressure p^sat of the pure diluents at 120 °C, which have been calculated with the parameters presented in Section S1 and shown in Table 6.

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 (p_A = 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 A_HX 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.