(142f) A Study on NGL Recovery for FLNG

Nguyen Van Duc Long^a and Moonyong Lee^†

^aSchool of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea

Email: allenthelong@yahoo.com

^†School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea

Email: mynlee@yu.ac.kr

Increasing global demand for natural gas is supporting the rapid growth of the worldwide LNG production capacity. While onshore LNG facilities have traditionally focused on power efficiency as a key criterion for process design and equipment selection, offshore floating LNG (FLNG) would require not only energy efficiency but also safety and compactness. For onshore plant, liquid hydrocarbons recovered from NGL are typically separated into relatively pure ethane (C₂), propane (C₃), isobutane (iC₄), normal butane (nC₄), and gasoline (C₅₊) products. This is conventionally done by the sequential distillation of C₂, C₃ and C₄ from gasoline, followed by further distillation of iC₄ from nC₄. Because isobutane and normal butane have similar boiling points, their separation requires a large number of trays and a high reflux ratio. Due to height restrictions, deisobutanizer should not constructed in FLNG. In addition, the number of products and product tanks also reduce because of the limitation of space. In this study, a NGL recovery sequence is proposed for FLNG.

Furthermore, operating cost of NGL part could be reduced most significantly via feed split. Dividing wall columns, capable of reducing the energy and space required for the separation of ternary mixtures, were also explored for the energy-efficient integration of depropanization and debutanization. Response surface methodology was used to optimize both the structure of the top dividing wall column and the operating variables, simultaneously. Using a top dividing wall column was shown to reduce total annual cost considerably compared with conventional distillation sequence. An integrating of NGL and LNG was proposed to reduce the investment costs, space, as well as the refrigeration costs.