Basis for Heat Exchange Equipment Sizing and Assumptions

Initial cost estimation of heat exchangers is mainly based on the required heat exchange area. This is the surface area needed to effectively recover a reasonable amount of heat from the returning lean amine stream from the desorber to heat up the rich amine stream. The estimation of the required heat exchange area is relatively simple compared to columns and vessels during initial cost estimation. This is simply completed using Equation (1):

where Q˙LRHX is the thermal load, and U is the overall heat transfer coefficient. “A” refers to the required heat exchange area, and ΔTLMTD is the log mean temperature difference (LMTD). Subscript “STHX” stands for shell and tube heat exchanger type, while subscript “PHE” represents the plate heat exchanger. Since the LMTD is only slightly higher than the minimum temperature approach (ΔTmin), some studies simply assume LMTD ≈ ΔTmin [14,23]. In this study, LMTD is calculated as shown in Equation (3).

where Thot,in and Thot,out are the temperature of the returning lean amine stream at the hot side and cold side, respectively. The temperature of the cold stream, rich amine at the cold side and hot side are represented with TCold,in and TCold,out, respectively.

In the literature, constant overall heat transfer coefficients are typically used in techno-economic studies (initial cost estimates) of carbon capture processes [46]. The following values can be found for the overall heat transfer coefficients, U for the lean/rich heat exchanger in an MEA CO₂ capture process with a shell and tube heat exchanger (STHX): 500 W/m²·K [24], 550 W/m²·K [47], 710 W/m²·K [48], 732 W/m²·K [14] and 760.8 W/m²·K [4]. The U-value in [14] is used this work. If we assume LMTD = ΔTmin, as done in [14,23], Equation (1) becomes:

or

The overall heat transfer coefficient of the plate heat exchanger is much higher than that of the shell and tube heat exchangers. Thus, they exhibit an order of magnitude higher surface area per unit volume in comparison with the STHXs. The overall heat transfer coefficient for the PHE is 2–4 times of the STHXs [32,49,50]. Based on that, a conservative overall heat transfer coefficient of 1500 W/m²·K was assumed in this work. Therefore, Equations (4) and (5) for the PHE become:

or

Equations (5) and (7) simply indicate that the required heat transfer surface area is directly proportional to the thermal load and inversely proportional to the minimum temperature approach (ΔTmin). The inverse relationship between the required heat exchange area and the minimum temperature approach (ΔTmin) shows that decreasing ΔTmin implies increasing the required heat exchange surface area, and thus, an increase in capital cost. On the other hand, the lower the ΔTmin, the higher the Q˙LRHX. An increase in Q˙LRHX implies a decrease in the reboiler heat demand for desorption, which in turn means lower energy costs.