Combined Brayton-JT cycles with refrigerants for natural gas liquefaction

Abstract

Thermodynamic cycles for natural gas liquefaction with single-component refrigerants are investigated under a governmental project in Korea, aiming at new processes to meet the requirements on high efficiency, large capacity, and simple equipment. Based upon the optimization theory recently published by the present authors, it is proposed to replace the methane-JT cycle in conventional cascade process with a nitrogen-Brayton cycle. A variety of systems to combine nitrogen-Brayton, ethane-JT and propane-JT cycles are simulated with Aspen HYSYS and quantitatively compared in terms of thermodynamic efficiency, flow rate of refrigerants, and estimated size of heat exchangers. A specific Brayton-JT cycle is suggested with detailed thermodynamic data for further process development. The suggested cycle is expected to be more efficient and simpler than the existing cascade process, while still taking advantage of easy and robust operation with single-component refrigerants.

Nitrogen expander cycles for large capacity liquefaction of natural gas

“…These existing and modified cycles are simulated with commercial software (Aspen HYSYS) for comparison in terms of thermodynamic efficiency, liquefaction capacity, and system size. Similar studies have been performed for various combined cycles of C3-JT, C2-JT, and N2-Brayton cycles by the present authors [5], and the same approach is utilized in this work.…”

“…This study is part of ongoing efforts [5][6][7][8] under a Korean governmental project supported by the LNG Plant R&D Center, which was established in 2008 with an eight-year program to prepare the competition for worldwide LNG plant market in the near future. Many of newly proposed LNG cycles are patented domestically or internationally, and two reduced scale test beds are under operation or construction at the Incheon LNG site of the Korea Gas Corporation.…”

Ship systems for natural gas liquefaction

“…Since natural gas is dominantly a mixture of hydrocarbon gases, the specific heat is variable with temperature at various points in the liquefaction process and it is therefore important to reduce the entropy generated by the temperature difference (ΔT) in a cryogenic heat exchanger [6].…”

“…Brayton closed nitrogen refrigeration cycle abatic compression of nitrogen in three stages with heat removal in coolers after each stage (1-7), additional cooling of gas at constant pressure (7)(8), adiabatic expansion in the cryogenic expansion turbine (8-9) and expansion of cargo at constant pressure (9-1). After the first and second stage of compression gas passes through intermediate coolers (2-3; 4-5), and after the third compression stage through a cooler (6)(7). Compander consists of a threestage compressor driven by an electric motor on the same axis with the cryogenic turbine, in which the expansion of gas performs a partial recuperation of consumed energy and thereby reduces the consumption of electricity for running the compressor.…”

“…Thermodynamic efficiency of the primary Brayton cycle can be improved by installing an additional expansion turbine and a pre-cooler [6]. Joule-Thomson expansion is suitable for the cycle in which it is necessary to achieve lower temperatures, while the Brayton principle with the expansion turbine is thermodynamically more efficient.…”