Heat Transfer Enhancement and Optimization of Lean/Rich Solvent Cross Exchanger for Amine Scrubbing

Abstract

The lean/rich amine cross exchanger is one of the cost centers in the amine scrubbing process, and accounts for 20–30% of the capital cost. To minimize the cross exchanger cost, shortcut methods that determine optimum LMTD and fluid velocity were developed. The optimum LMTD is a function of heat transfer coefficient, temperature change and the capital cost of heat exchanger. A greater LMTD should be used to prevent excessive capital cost when the number of heat transfer units (NTU) is large and the heat transfer coefficient is small. The heat transfer performance can be enhanced by increasing pressure drop and reducing solvent viscosity. The corrugation angle is the primary design geometry for plate-and-frame exchanger (PHE). Based on the empirical correlations for PHE, the heat transfer coefficient at 60° is almost double that at 30°; however, the pressure drop at a large corrugation angle is also greater. The dependence of the pressure drop per unit length on the heat transfer coefficient is 0.35–0.40, which implies that the heat transfer coefficient will increase 30% by doubling the pressure drop per unit length. The cost associated with the optimization of the cross exchanger has been developed as a function of the fluid velocity, the physical properties, the exponents of the empirical correlations and the pricing parameters. The optimum velocity is independent of the solvent rate, the temperature change of the cross exchanger, and the cross exchanger LMTD. To stay at optimum fluid velocity, the plate number needs to increase as the solvent rate increases while the plate length will increase as the NTU increases. Viscous solvent will result in a lower optimum velocity since it causes higher pressure drop. Typical optimum fluid velocity is at 0.32–0.42 m/s for 8 m PZ. It is worthwhile to utilize higher fluid velocity and pressure drop when the heat transfer can be effectively enhanced by turbulence.