Impact of Minimum Temperature Approach on Heat Recovery and on the Required Heat Exchanger Surface Area

The minimum temperature approach of the lean/rich heat exchanger of a solvent-based CO₂ absorption and desorption process determines how much heat can be recovered by the rich stream from the lean stream flowing from the desorber. This is shown in Table 9, where the heat recoveries at the ΔTmin of 5 °C, 15 °C and 20 °C are compared with the heat recovery of the base case ΔTmin of 10 °C. The results obtained are compared with the results also calculated for a 400 MW NGCC power plant exhaust gas in the book of Dag Eimer [23]. Negative values represent relative less heat recovery, while positive values show how much more heat is recovered compared to ΔTmin of 10 °C.

Even though the estimated amount of heat recovery in the base case ΔTmin in [23] is 6% less than the result in this work, both works calculated 7% more heat recovery at ΔTmin of 5 °C compared to the reference process at ΔTmin of 10 °C for the NGCC system. The heat recovery is higher in this work because the cold rich stream enters the cross-heat exchanger at 46 °C, while 50 °C was assumed by [23]. At ΔTmin of 15 °C, 8% less heat recovery was obtained in this work in the CO₂ capture from the NGCC power plant’s exhaust gas, while [23] also calculated this value to be 7%. In this work, in the NGCC system, if ΔTmin of 20 °C is specified, the heat recovery will decrease by 16%.

No work was found to compare the heat recovery results for the cement system. However, the heat recovery at ΔTmin of 5 °C is about 10% higher than at ΔTmin of 10 °C in the CO₂ absorption and desorption in the cement plant scenario. At ΔTmin of 15 °C and 20 °C, heat recovery decreases by approximately 9% and 20%, respectively.

The comprehensive results of heat duties of the cross-exchanger, reboiler and lean MEA coolers at the different ΔTmin are presented in Table 10, Table 11, Table 12 and Table 13 for the NGCC exhaust gas cleaning process and cement plant flue gas purification systems. These four tables also show the resulting heat exchange surface area required in the lean/rich heat exchanger for a ΔTmin range of 5–20 °C for the STHXs and a ΔTmin range of 3–20 °C for the PHE. The relative increase and decrease in the heat transfer area needed in the lean/rich heat exchanger for both the STHXs and PHE are also computed and presented in Table 9, Table 10, Table 11 and Table 12.

In the NGCC power plant standard CO₂ capture process, if a ΔTmin of 5 °C is specified in the lean/rich heat exchanger instead of 10 °C, the required heat exchange area becomes 76% larger if any of the STHXs are employed as the lean/rich heat exchanger. If the PHE is used, then the calculated surface area becomes 95% larger at 5 °C instead of 10 °C. According to Karimi et al. [11] and Eimer [23], the required heat exchanger area doubles if a ΔTmin of 5 °C is used instead of 10 °C. The analysis of Eimer [23] is based on the same 400 MW NGCC process as was completed in this work. The work of [11] regards a 90% CO₂ absorption and desorption from a 150 MW bituminous coal power plant’s exhaust gas. In the NGCC system, especially for the PHE case, the results of this work are close to two times the heat transfer area required if ΔTmin of 5 °C is used instead of the reference ΔTmin of 10 °C. The difference is simply due to the overall heat transfer coefficients used. In this work, an overall coefficient (U-value) of 732 W/m²·K [14] was used to estimate the required heat transfer area for the STHXs. Eimer [23] used 1250 W/m²·K, which is considerably higher than the U-values in CO₂ capture studies such as [14,24,31,47]. Since the analysis of Karimi et al. [11] was based on data from [61,62], the overall heat transfer coefficient used for the STHX surface calculation should be considerably higher than the U-value in this work and in [14,24,31,47].

In the cement plant standard CO₂ capture process, decreasing the ΔTmin from 10 °C to 5 °C resulted in a 97% and 100% increase in the heat exchanger area needed for the cases of STHXs and PHE, respectively.

In the lean vapour compression cases, using ΔTmin of 5 °C instead of 10 °C caused the heat exchanger area to increase by 128% for both the STHXs and PHE in the NGCC power plant CO₂ capture process. Meanwhile, in the case of the cement plant, the increase is 138% and 139% for the STHXs and PHE, respectively.

The number of heat exchanger units required are significantly fewer if the PHE is selected for the CO₂ capture operations instead of the STHX. These also lead to a lower area or volume requirement as well as less capital cost, as also shown in Figure 2 and Figure 3. The reboiler duty increases by 1–2 MW with an increase of 1 °C of ΔTmin of the lean/rich heat exchanger. The duty of the lean MEA cooler also increases at approximately 2–3 MW for every 1 °C increase in the ΔTmin of the lean/rich heat exchanger. The specific heat demand by the reboiler increases mainly between 0.03–0.04 GJ/tCO₂ with each 1 °C increase in the ΔTmin of the lean/rich heat exchanger. Table 14 provides a summary of comparison between the standard and the lean vapour compression configuration CO₂ capture processes.

The results in this section show that the ΔTmin of the lean/rich heat exchanger has significant influence on important economic variables in a CO₂ absorption and desorption process. An increase in the reboiler duty implies an increase in the amount of steam needed. The amount of cooling water needed also increases with increase in the lean amine duty. As the reboiler and lean MEA cooler duties increases with an increase in the ΔTmin of the lean/rich heat exchanger, the corresponding required heat transfer area also increases; therefore, to arrive at the minimum cost of the process, a trade-off analysis is required using Equations (8)–(11) and (13), as also stated by [11]. In this work, Equation (10) is mostly used for the trade-off analysis, which also depends on Equations (8) and (9).