(546k) Simulation and Optimization of LNG Plant Hot Section

Mohamad M. Hussein^a Mary A. Katebah^a , Abdulla R. Al-Hajri^b, Easa, I. Al-musleh^a,*

^aChemical Engineering Department, Qatar University

^b Qatargas operating company

*Corresponding author. Tel. +974 44034148. E-mail address: e.almusleh@qu.edu.qa (E. Al-musleh)

The demand for natural gas has increased remarkably due to its clean-burning characteristics and its ability to meet stringent environmental regulations. Moreover, a strong growth rate is foreseen over the next couple of years. The most effective means for natural gas (NG) transportation over long distances is by liquefaction in liquefied natural gas (LNG) plants. Prior to its liquefaction, NG enters a condensate recovery and stabilization unit (CRS), which as its name indicates, aims to recover and stabilize condensate. Stabilization is essential for safe storage and transportation in addition to meeting typical crude oil specifications (e.g. vapor pressure). After leaving the CRS, multiple purification steps are required before liquefaction to remove water and other impurities such as: CO₂, H₂S, and mercaptans, also known as acid gases. Although the removal of these impurities is associated with high energy consumption, it is essential for safety and operational purposes. More than 98% of the removed H₂S is converted to elemental sulfur while incinerating the remaining portion in a sulfur recovery unit (SRU).

The overall objective of this work is to maximize on-spec condensate production while minimizing energy consumption of the acid gas removal and sulfur recovery steps without violating sulfur emissions (SO₂ in the tail gas) regulations and H₂S/CO₂ LNG content. The optimization was carried out for one of Qatargas LNG plants (3 million tonnes per annum of LNG production) which uses conventional CRS, SRU processes and a sequence of methyl diethanolamine (MDEA) and Sulfinol purification steps.^{1 2} ProMax^® and Aspen Plus^® process simulators were used to simulate and optimize the process using a combination of parametric analysis and sequential quadratic programming (SQP) techniques. Detailed operational constraints such as heat exchangers overall heat transfer coefficients (UAs), compressors curves, and column hydraulics were considered during optimization.

Initial results indicated that the energy consumption could be reduced while maintaining sweet gas composition at 1.4% CO₂ and 170 H₂S, and SRU tail gas at no more than 100 ppm H₂S. As for the CRS, negligible improvements were achieved, which motivated the synthesis of new retrofitting options that enabled the plant, after optimization, to increase condensate production by more than 7%.

References

1. Maddox, R. Gas conditioning and processing, volume 4: Gas and liquid sweetening, 3rd ed.; Campbell Petroleum Series, 1985.
2. Campbell, J. M.; Lilly, L. L.; Maddox, R. N. Campbell J.M. Gas Conditioning and Processing. Volume 2: The Equipment Modules; 1984.