(246m) Maintaining Higher Efficiency of LPG Butane Boil-Off Gas Re-Liquefaction in LPG Butane Storage Tanks Using Unsteady State Process Simulation

Badak LNG is a natural gas liquefaction plant, which operates LNG trains to produce LNG and refrigerated LPG. The recent annual LPG production amount is 500,000 ton/year (equivalent with 500 Million USD/year productions value). During production and storage of LPG, boil-off gas (BOG) is generated and then re-condensed in a LPG BOG Butane Condenser unit. LPG BOG Re-liquefaction system uses centrifugal compression system with total maximum electric power duty of 1.4 MW (equivalent with 2.4 Million USD/year operating cost). Cooling medium flow rate in condenser unit is controlled by using Butane Tank pressure controller or cooling medium temperature outlet controller. Normally, the pressure controller dictated the cooling medium flow rate. However, during accumulations of Non-Condensable Gas (NCG) occurrence in the condenser unit hot stream side (LPG BOG Butane) will lead to cooling medium temperature outlet controller dictation which indicates less LPG BOG Butane condensing rate. Furthermore, this less condensing rate will lead to LPG Butane tank overpressure flaring (LPG Butane product losses) accounted as LPG Butane storage efficiency for just 91.42%.

Therefore accumulation of this NCG makes LPG Butane tank pressure fail to be controlled. Hypothesis was made that by continuously venting small amount NCG, higher reliability of this tank pressure control performance could be achieved. The strategy to improve the NCG Butane BOG venting control system strategy were also formulated using unsteady state analysis of LPG Butane Tanks and Butane BOG re-liquefaction. Hysys simulator software in dynamic condition is employed to analyze unsteady state tank behavior. Activation of Fidelity feature in the simulation is critical to simulate the hydrostatic pressure of Butane BOG re-liquefaction unit in order to approach as close as possible the simulation with actual process condition during unsteady state condition.

Approach of NCG accumulation occurrence on Butane BOG re-liquefaction system can be successfully simulated in the dynamic condition simulation software. By introducing NCG disturbance into Butane BOG re-liquefaction system, the tank pressure control is failed. While tank pressure control strategy improvement by continuously venting NCG as much as 5 Nm³/hr (0.0044 MMSCFD), LPG Butane Storage and Loading efficiency can be also dramatically improved from 91.42% to 99.96. This improvement is equivalent with annual production loss only 200,000 USD of 500 Million USD LPG Product value.