Modelling Flare Gas Recovery System for Recovery and Utilization of Stranded Associated Gas in the Niger Delta

Abstract

In this paper, the modeling of a flare gas recovery system (FRGRS) for the recovery and utilization of stranded associated gas in the Niger Delta region of Nigeria was investigated. The gas recovery system is a novel modular plant with several integrated units and operational features. The FGRS was modeled for the recovery, treatment, processing of stranded associated gas. Two cases were considered, one was the compression of the treated gas from the FGRS to pipeline as sales gas while the other was the conversion of the treated gas to premium transport liquids by gas-to-liquids technology. 25 MMScfd of recovered stranded associated flare gas was used as feedstock and it yielded 23.22 MMscfd of treated natural gas and 1.77 Mscfd of acid gas. The treated gas met all pipeline sales gas specifications. In modeling the GTL plant, an autothermal reforming method of synthesis gas production was used and an H₂/CO ratio of 2.33 was recovered which was acceptable for Fisher-Tropsch reaction downstream. The entire GTL plant simulation was modeled in Honeywell Unisim with Peng-Robinson as the fluid property package. Gas-to-liquids (GTL) product produced from the 23.22 Mscfd treated gas fed to the GTL plant were 2350 b/d of liquid transport fuels comprising 1100 b/d of diesel and 1250 b/d of gasoline. Economic analyses revealed thata net present value (NPV) of US$ 109.9 million was realized from the sales of GTL liquids while an NPV of US$58.5 million was realized from the sales of pipeline quality gas. Thus, the sales of GTL products represent an increase in NPV of 87.8% when compared with that of pipeline gas. However, the Pay-out time (POT) for pipeline sales gaswas 1.16 years, the internal rate of return (IRR) was 86% while the profit-per-dollar invested was 16.18. Furthermore, the pay-out time for GTL product sale was 5.29 years, the internal rate of return was 18.3% and the profit-per-dollar invested was 2.78. The project showed that the gas may be sold outright (as pipeline gas) if the market was available and in the absence of a ready market for the gas, the gas could be converted to liquids that are easier to store and have greater market potentials in the long run.