Refrigerant Condensation in DMR Liquefaction Process for Natural Gas

Abstract

Dual Mixed Refrigerant (DMR) liquefaction process licensed by Air Products and Chemicals Inc. (APCI) utilizes 2 different sets of mixed refrigerant to precool and liquefy the natural gas. This technology is considered for medium-sized Liquefied Natural Gas (LNG) production facilities such as Floating LNG (FLNG) due to its interesting features. Unfortunately, there is no APCI DMR liquefaction process plant has been built and operated so far. One of the uncertainties is the risk of incomplete (differential) condensation of the mixed refrigerant (MR) of the first (warm) circuit. The objective of the Master thesis work was to analyze of the interaction between the WMR condenser and the DMR system, focusing on evaluation of the consequences for varying operating parameters and a realistic degree of differential condensation. The evaluations were performed by developing flow sheet models in Hysys in collaboration with HTRI program and literature references. The materials used as design basis for simulation were based on data from Statoil. Based on the simulation results, differential condensation model in Hysys would give a realistic number of about less than 3% uncondensed vapor (mole fraction) at the outlet of WMR condenser, which may result in higher temperature at design pressure 46.45 bar or lower pressure at design temperature 22 oC at the outlet of WMR condenser. As a consequence, the specific work would be slightly increased, which means the power efficiency of DMR process would be decreased. This 3% of uncondensed vapor due to differential condensation could be reduced and eliminated by one of these alternatives: increasing the condensing pressure from 46.95 bar to 47.95 bar, reducing 15% flow rate of WMR, feed NG and CMR, reducing methane fraction from WMR composition and lowering sub-cooling temperature by utilizing lower cooling water inlet temperature, if it is available. On the other hand, removing propane for safety reason apparently did not give a negative influence to the WMR cycle and showed a better result instead.