Sensible heat from process simulation

“According to Eq. (22), the sensible heat (qsens) is determined by the specific heat capacity of the solution (cp), the temperature difference at the hot side of the rich and lean heat exchanger (ΔTRLHX), and the CO2 loading capacity (Δα). As shown in Table 7, these values of the degraded solution were almost equivalent to the fresh solution. Note that the reason for the consistent values is different from property to property.

The cp is altered in two ways by the presence of HSS. Since the cp of the weak acids which generate HSS is relatively lower than that of water (e.g., formic acid=2.15, oxalic acid=1.299, water=4.2 kJ∙kg−1∙K−1), the solution cp at a given CO2 loading and amine concentration when HSS are present is lower than that of fresh solution. On the other hand, since cp generally increases as CO2 loading decreases (Hilliard, 2008) and the presence of HSS reduces the CO2 loading range, the result is an increase in solution cp when HSS are present. These two effects largely cancel one another out.

The ΔTRLHX is determined by the overall heat transfer coefficient (U) and the heat transfer area (A). Since A is likely to be constant in most systems, U only affects the ΔTRLHXTable 8 summarizes important values related to the heat exchanger in the simulation results. Increase in viscosity and decrease in specific heat capacity—which are expected due to the physical properties of weak acids themselves— are minor due to the reduced CO2 loading range. For density, two contrary effects of HSS similar to viscosity are present. All else being equal, the density of the solution increases due to the presence of HSS (Ju et al., 2018), but the lower CO2 loading range acts to decrease the density. Due to these counteracting effects for the solution properties, the U and ΔTRLHX are relatively stable between fresh and degraded solution.

Table 8. Important values related to the simulated heat exchanger.

Empty Cell Empty Cell Fresh Degraded
Empty Cell Empty Cell Lean Rich Lean Rich
Density kg∙m−3 1000 1065 992 1057
Viscosity mPa∙s 5.693 7.111 5.800 7.276
Thermal conductivity W∙m−1∙K−1 0.134 0.194 0.135 0.197
Specific heat capacity J∙kg−1∙K−1 3651 3339 3593 3299
Fluid velocity m∙s−1 0.354 0.367 0.356 0.369
Reynolds number 248.9 219.9 243.4 214.3
Prandtl number 154.9 122.6 154.9 121.8
Nusselt number 94.2 78.2 92.6 76.5
Heat transfer coefficient W∙m−2∙K−1 3160 3784 3117 3768
Overall heat transfer coefficient W∙m−2∙K−1 1722 1706
UA value W∙K−1 3.100 × 107 3.071 × 107

The Δα is set if the CO2 capture rate, the liquid mass flow rate, and the amine concentration are fixed. In this simulation, the CO2 capture rate and the amine concentration were fixed; the liquid mass flow rate was nearly unchanged between fresh and degraded solution. Therefore, Δα for fresh and degraded solution is almost identical.

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