Development of novel refrigeration cycles for small scale LNG processes

UoM administered thesis: Master of Philosophy

  • Authors:
  • Héctor Almeida Trasviña

Abstract

Demand for liquefied natural gas (LNG) is continuously increasing. Commercial exploitation of small gas reserves is thus becoming accordingly attractive. Natural gas liquefaction processes are both capital- and energy-intensive. Refrigeration cycles are used to liquefy the natural gas to temperatures around -160°C; the shaft power demand for refrigerant compression dominates operating costs. Energy efficiency is usually achieved in large-scale commercial processes with complex configurations. However, the complexity of refrigeration cycles in small-scale LNG processes (where production rate is up to 1 million t per annum) should be low to keep the capital costs relatively low. A trade-off exists between energy efficiency and capital costs in the design of refrigeration cycles.In addition, optimisation of the operating variables of the refrigeration cycle is difficult. Optimisation aims to find the combination of operating variables (including mixed refrigerant composition) that minimises the shaft power demand for refrigerant compression in a process with a given configuration and a given liquefaction duty. However, because of the relatively large number of degrees of freedom available in the refrigeration cycle and the complex interactions between the operating variables, the optimisation becomes challenging.A limited range of refrigeration cycles is studied in the open research literature for the production of LNG at small scales. Single mixed refrigerant cycles are commonly studied because they have 'simple configurations', although 'complexity' of refrigerant cycle configurations has not been clearly defined. The PRICO cycle is the simplest commercial refrigeration cycle for LNG production. The so-called 'CryoMan' process, developed at the University of Manchester, modified the structure of the PRICO cycle and achieved significant shaft power savings (around 8%), compared to the PRICO cycle.In this work, further structural modifications to the CryoMan process are proposed, resulting in three novel refrigeration cycles (namely the 'Bypass' design, the 'Two Flash Levels' design and the 'Mixing After Precooling' design). Design constraints, related to the number of refrigerant compression stages and the number of streams in the multi-stream heat exchanger, are defined in this work to limit the complexity of the novel cycles.To illustrate the benefits of the structural modifications, the configurations are optimised in an industrially-relevant case study. Sensitivity studies and optimisation are employed to explore thoroughly the complex interactions between operating variables; a Genetic Algorithm is applied, to search the solution space and to avoid local optima. The case study demonstrates that the structural modifications proposed can bring shaft power savings of up to 3.2% in the case of the Bypass configuration (equivalent to operating cost savings of £0.69 million per annum for a natural gas feed of 0.75 million t per annum) with relatively minor increases in the complexity of the refrigeration cycles.

Details

Original languageEnglish
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Award date31 Dec 2016