Modeling of CO2 Freeze-Out in the Processing of CO2-Rich Natural Gas

Abstract

Equations of state (EOS) have been developed for the petroleum and natural gas (NG) industries to predict phase behavior and properties of gas and liquid hydrocarbons on wide pressure–temperature domains. However, the processing of CO2-rich NG may create semi-cryogenic conditions leading to precipitation of solid CO2 (dry- ice), which is problematic in expanders, demethanizers, distillations, cold boxes, liquefied NG (LNG) facilities, and supersonic separators for CO2 removal. This phenomenon is denominated CO2 freeze-out. Semi-cryogenic separation of hydrocarbons and CO2 from CO2-rich NG can be designed to operate as desired, provided the CO2 solubility limit is not exceeded; otherwise, CO2 freezes out due to its high triple-point temperature relatively to hydrocarbons. Although EOS’s can predict vapor–liquid equilibrium (VLE), liquid–liquid equilibrium (LLE), and vapor–liquid–liquid equilibrium (VLLE), they must be complemented by thermodynamic models of solid CO2 and adequately managed by algorithms to predict CO2 freeze-out boundaries of constant total composition (CTC) streams on plane P × T. To accomplish this, it is necessary to solve solid–liquid equilibrium (SLE), solid–vapor equilibrium (SVE), and solid–vapor–liquid equilibrium (SVLE) with CO2 and light hydrocarbons. This chapter discusses property modeling of solid CO2 and numerical strategies for SLE-SVE-SVLE problems to predict freeze-out P × T boundaries of CTC CO2-rich NG streams.