Department of Mechanical Engineering, K. N. Toosi University of Technology


 liquefied natural gas (LNG) is the most
economical way of transporting natural gas (NG) over long distances.
Liquefaction of NG by using vapor compression refrigeration system requires
high operating and capital cost. Due to lack of systematic design methods for
multistage refrigeration cycles, the conventional approaches of determining
optimal cycle configuration are mostly based on trial-and-error and are very
time consuming. Here a new Mixed Integer Non-Linear Programming (MINLP) model
is introduced for selecting optimal synthesis of refrigeration system which
would reduce both the OPEX and CAPEX of a LNG plant through Pinch technology. A
superstructure of a complex refrigeration system is developed to select and
optimize the key decision variables in refrigeration cycles (i.e. partition
temperature, compression configuration, refrigeration features, refrigerant
flow rate). This method reduces calculation time and increases design options
that could be mounted over a multistage refrigeration cascade. Here a program
(LNG-Pro) is developed that integrates the VBA, Refprop and Excel MINLP Solver
to automate the methodology. Design of an LNG plant is described to illustrate
advantages in adopting this method. Conceptual diagrams of pinch technology are
shown to visualize the stepwise optimization procedure of compressor’s
shaftwork. The proposed cascade by this method reduces compressor shaftwork of
an LNG cascade from 1255 Kj/Kg to 1141.9Kj/Kg. 


Main Subjects