Optimal Design of Acid Gas Removal Unit In LNG Plant

Acid gases such as carbon dioxide and hydrogen sulfide in raw natural gas need to be removed before the main liquefaction process of natural gas to avoid corrosion, solidification and plugging problems. Amine based chemical solvents are the most generally accepted and widely used method for removal of acid gases. Amine processes including gas absorber and solvent regenerator require large energy consumption and therefore optimization in design of such processes are essential.

First, conceptual design and simulation of gas sweetening process using aqueous amines are performed in Aspen HYSYS. AMINES property package is selected for the thermodynamic properties. The goal is to lower carbon dioxide and hydrogen sulfide concentration to 50 ppmv and 4 ppmv, respectively.

Then, optimization formulation of the sweetening process is developed to minimize total capital and operational cost. HYSYS simulation is linked with MATLAB and GAMS to find the optimal design. Main design variables are liquid solvent to gas ratio (L/G ratio) and lean amine solvent loading value. Total trays of absorber and regenerator including the feed location of rich solvent in the regenerator are integer variables. The optimization formulation is a mixed-integer nonlinear programming (MINLP) problem. Design alternatives such as split-flow scheme are also analyzed and compared. 

This research was supported by a grant from the LNG Plant R&D Center funded by the Ministry of Land, Transportation and Maritime Affairs(MLTM) of the Korean government.