Robust synthesis of amine-based natural gas sweetening plants

Abstract

The natural gas processing and refining industry uses acid gas removal units to improve the quality of their sour gas feed to a certain level. Various disturbances such as sour gas feed conditions or solvent quality can influence the operation of such plants. A framework is developed using linear control tools which can be used to assess the flexibility of the operating condition of a sweetening plant against such disturbances. A quantitative measure of the flexibility and controllability of a design is provided as a systematic tool for the designer to evaluate different process structures or conditions. The framework can also be used to calculate the maximum tolerable disturbances by the process. The framework is illustrated in a plant that removes acid gas from a raw gas stream using MDEA as the solvent. The parameter σ1 as the maximum singular value of the steady state transfer matrix of the plant is considered as the sensitivity of the outlet H2S and CO2 purities against external disturbances where a higher value means more sensitivity. The results show that higher solvent concentrations are preferred considering the robustness of the absorption column against the sour gas feed disturbances and the uncertainties of the solvent properties. For example, at the solvent flow rate of 40,000 kg/h, the value of σ1 is 2.817, 0.736 and 0.148 at the wt% of 20%, 30% and 40%, respectively. Also, it is shown that for a plant designed with the sweet gas H2S purity of 2 ppm, when a solvent with 20 wt% and flow of 88897.3 kg/h is used, the feed quality disturbance of 0.59 kg/h and 505.19 kg/h in the H2S and CO2 mass flows relative to their nominal can be tolerated before reaching the maximum allowable value of 3 ppm. Such calculations at the 45% point with solvent flow of 44015.8 kg/h lead to the maximum tolerable disturbance of 1.79 kg/h and 967.36 kg/h in the H2S and CO2 flows. The rationale behind this behavior can be well understood if the plot of acid gas loading of rich amine is considered. A lower acid gas loading is observed at higher solvent wt% which means that more free solvent molecules are available in the solvent stream to absorb new acid gases. Accordingly, it has a better capability of confronting external disturbances. Also, the effect of the number of stages of the absorption column on the maximum tolerable disturbance is studied. It is concluded that the robustness of the process will decrease against both types of disturbances as the number of stages increases and lower disturbances can be handled without H2S outlet constraint violation.