Dynamic simulation of the liquefaction section in baseload LNG plants

摘要

The thesis demonstrates modelling and simulation of a cryogenic heat exchanger in a baseload LNG plant within two main areas. The first one concerns the development and description of the transient liquefaction models. The second one looks at the verification and studies the process dynamics. Three transient models, model A, B and C, were developed. Models A and B were simplified models based on the energy balance, with model B additionally considering an accumulation of liquid in separator drums. Model C was a complete model which also handled accumulation of mass and momentum in heat exchangers and drums. Each unit in the plant was modeled separately and linked in a process flow sheet. The greatest challenge during design of the simulation system was to model the coil-wound heat exchanger. A strong coupling between the variables and the nonlinear equations led to converge problems in certain cases. The equation system was particularly sensitive in the region where the fluid was on the border of single and two-phase flow. A model of the digital control system was implemented, and start-up, shutdown, steady-state operation and change over in the production capacity were described. Simulation of these different operation models were performed, and particular effects due to changes in control parameters or input variables such as pressure, temperature, flow rate and compositions, were discussed. Small alteration of these parameters caused displacement in the heat-exchanger load, and prevented the process from running optimal. As a result, the process could not manage to produce the specified LNG-quantity. Consequences could also be fluctuation in the tube-wall temperature, which might eventually destroy the equipment through material stress. 72 refs., 48 figs., 30 tabs.