Computer Aided Design for the Recovery of Boil-Off Gas from LNG Plant

Abstract

Boil - Off Gas creation and usage has been a source of worry in Liquefied Natural Gas value supply chain. BOG is generated when there is temperature gradient between the environment and LNG temperature within the carrier tank, process lines or vessels. In this work, Computer Aided Design for the recovery of BOG from flare in an LNG Plant considered the dynamic nature of the BOG with minimized total energy consumption. A rigorous simulation based optimization model using HYSYS V8.8 was presented. Possible BOG scenarios were formulated in this report and considerations taken from the BOG scenarios to form the basic scope of this work. An Aspen HYSYS Software was used to develop a Process Flow Scheme (PFS) which was simulated using the BOG scenarios formulated. The BOG scenario temperatures considered were - 15°C for Warm Ship analogy, - 90°C for Cold Ship and - 140°C for Normal Design Mode. Assumptions were also made on the feed into the developed PFS before quenching the various BOG temperatures. With HYSYS simulation at assumed constant inlet mass flow rate of 25 , 000 kg/s for BOG FEED, 6250 kg/s for LNG & LNG1 FEED, quenching at various BOG feed temperature - 15°C, - 90°C and - 140°C, gave a meaningful output. The Mass flow rate recovered from Warm Ship at - 15°C for Cold Product was 35 , 183 Kg/s and for Liquid Product 2317 Kg/s. For Cold ship at - 90°C, the Cold Product recovered was 32 , 174 Kg/s and Liquid Product was 5326 Kg/s. Also, for - 140°C, the Cold Product was 28 , 004 Kg/s and the Liq uid Product was 9496 Kg/s. The Energy stream for the Compressor, Cooler and Pump in the Process Flow Stream (PFS) were observed in Table 5 . At - 15°C , the Compressor energy was 3.22E+07KJ/h, while the Pump energy was 3412KJ/h , and the Cooler gave 1.90E+07KJ/h. The results above showed that excessive BOG from Warm ship can be quenched and recovered for other end users rather than undue flaring of the gases. Extra work needs to be done to ensure minimal energy utilisation, optimal recovery and high efficiency of this developed model.