Fig. 14 Distribution of CO₂ partial pressure and loading factor in stripper on reference calculation conditions

　

In the absorbing column, the operating lines get close to the equilibrium line at the terminal points of column but CO₂ transfer driving force does not result in a pinch state.

In the stripping column, the operating lines get close to the equilibrium line at a pinch point around the middle of column： they get closer to the equilibrium line, as the CO₂ recovery ratio increases, and eventually contact with the equilibrium line, which denotes that there is no driving force of CO₂ mass transfer. Therefore, in the range of the calculation conditions of this analysis, the attainable maximum CO₂ recovery ratios are determined by the equilibrium condition at the pinch point in the stripper.

When the operating lines for the stripper are assumed to be linear, they are explained by the pairs of the loading factor and the CO₂ partial pressure both at the top of the column, (φ _h P_T), and at the bottom of the column, (φ ₁ P₁). The lowest loading factor, φ₁ and the highest one,φ_h are related by Eq.1, which is derived from CO₂ mass balance in the absorber, that is, CO₂ mole difference between φ _h andφ ₁ stands for CO₂, moles absorbed in the absorber.

　

φ ₁ =φ _h - G_Ay_EXα_R/(L_AC_MEA/ρ_L(1)

　

where y_EX stands for CO₂ mole fraction of exhaust gas, C_MEA for MEA concentration of solution, and ρL for density of solution.

The CO₂ partial pressure at the bottom, p₁ can be regarded as 0 kPa.

The CO₂ partial pressure at the top, p_T, is approximately calculated by Eq. 2, which is derived according to the following：

1)　conservation exists between the CO₂, stripped and the steam condensed in the stripper, where it is taken into account that steam of 2 moles per 1 mole of stripped CO₂ condenses since the heat of the reaction between CO₂ and MEA per 1 mole of CO₂ is roughly equal to the latent heat of water of 2 moles：

2)　the total pressure in the stripper, P_tot, is almost the same of the pressure of the saturated steam multiplied by the mole fraction of water in the solution at the stripping temperature：

3)　C0₂ flow rate at the top equals that recovered from the exhaust gas：

4)　mass flow of solution is the same between the absorber and the stripper.

　

P_T=p_tot{(1/y_EX α_R (G_s/L_A)/(G_s/L_A)-1}^-1　(2)

　

By both Eq. 1 and Eq. 2, it is understood that the operating line gets nearer to the equilibrium line as the CO₂ recovery ratio, α_R, increases, since φ₁ becomes smaller and P_T gets larger： and as the ratio of the gas-to-liquid feed rate ratio of the stripper to that of the absorber, (G_s/L_s)/(G_AL_A), becomes smaller, the operating line gets nearer to the equilibrium line and the attainable maximum CO₂ recovery ratio becomes smaller.

　

5.2　Effect of Gas and Liquid Feed Rates on Output Power

Heat of the steam required by the reboiler, Q_RB, is equal to that of the steam used in the stripper which is the sum of the steam at the top of the stripper and the condensed steam used for stripping. The ratio of CO₂ and steam at the top of the stripper can be derived by using Eq. 2. Being taken into account that steam of 2 moles per 1 mole of the stripped CO₂ condenses, the heat balance of the steam gives Eq.3 for heat required by the reboiler, Q_RB.

　

Fig. 15　Maximum attainable CO₂ recovery ratio and net output power dependency on the gas-to-liquid feed ratio

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