(248c) Integrated Cryogenic Upgradation Process for Baseload LNG Plant

Natural gas (NG) is the cleanest fossil fuel and is regarded as the bridge for the transition from non-renewable to renewable energy sources. Transport of natural gas over long distances as liquified natural gas (LNG) at -160°C and atmospheric pressure, is the most common and economical option. The raw natural gas feed from the field undergoes a series of energy-intensive up-gradation and liquefaction processes to produce the final LNG product, falling in with customer specifications. A realistic baseload LNG plant consists of hot and cold sections, with the former consisting of inlet processing, acid gas removal (AGRU), dehydration, and sulfur removal units, and the latter, operating at sub-ambient temperature, consists of natural gas liquid (NGL) recovery, liquefaction, nitrogen removal (NR), fractionation units, and storage tank.[1] The various processes produce hydrocarbon-rich waste streams, utilized as fuel gas (FG) in a gas turbine to compensate for the power requirements of the LNG plant. This study focuses on incorporating the acid gas removal unit in the cold section by solidifying the acid gases at freezing conditions. It aims to design and optimize an energy-integrated and self-sustained cryogenic process incorporating the AGRU, liquefaction unit, NGL recovery unit, NRU, and export terminal.

The present literature has studied the potential capital and energy saving by removing solidifiable acid gases from process gas stream by direct contact with a cold liquid. [2] The controlled freezing zone (CFZ^TM) process, patented by ExxonMobil, consists of an open section in a distillation column, operating at desublimation conditions for solidifying acid gases.[3] The NG vapor containing acid gases moves up through the open section of the column, where it is contacted with liquid methane that has been sprayed through nozzles, causing the acid gases to solidify and methane to vaporize. P S Northrop et al [4] discussed integrated NRU/CFZ^TM process operation at 40 bar for feed with a high acid gas content of 65mol% and obtained both pipeline and LNG quality product with acid gas concentration of <2mol% and <50ppm respectively. However, the CFZ^TM process for low acid gas content in feed (5 mol%) demands lower operating pressure (10-20 bar) to meet LNG specifications. The energy-intensive liquefaction of this low-pressure NG stream post-AGRU further increases the total energy consumption of the plant and hence the cryogenic AGRUfor low acid gas feed to meet LNG specifications remain unexplored.

The inclusion of AGRU in the cold section results in increased cold energy requirements and impacts the operating parameters of downstream processes including NGL recovery, NR, and end flash. The significance of managing boil-off gases (BOG), and balancing the FG streams generated from various processes against the total fuel requirement of the entire plant remain briefly addressed. Although many studies are addressing the two aforementioned problems exclusively, to the best of our knowledge, there is no literature discussing an integrated cryogenic process consisting of AGRU, liquefaction, NGL recovery, and NRU with BOG and hydrocarbon waste stream generation at various process steps accounting for the FG.

In this study, first, we investigate the feasibility of solidifying acid gases from a feed with low acid gas content (5mol%) to meet LNG specifications by incorporating AGRU in the cold section and operating in the freezing line. The AGRU process is integrated with other cold section processes like NGL recovery, NR, end flash, and the export terminal. The cold energy is provided by a refrigerant cycle and a parity between total FG requirement and generation in various processes is established. The NRU consists of one high-pressure column and one low-pressure column, developed in our previous study. The process simulation and thermodynamic calculations are carried out in Hysys, Petro-SIM, and MultiFlash. The feed components and fluid package are selected in MultiFlash. The fluid package selected is GERG-2008 for high accuracy and hydrocarbon components and acid gases are declared as freeze-out components. The material streams and equipment in Petro-SIM support the solid phase and hence the conditions for solidification of acid gases were detected in Petro-SIM. The thermodynamic calculations pertaining to solid formations were carried out by importing the declared MultiFlash file in Petro-SIM. The entire process was simulated in Hysys and the fluid package selected was GERG-2008. The simulation software Hysys and PetroSIM were connected to MATLAB using ActiveX control to allow the exchange of thermodynamic calculation.

Then a simulation-based optimization (SBO) approach deploying particle swarm optimization (PSO), coded in MATLAB, was implemented to minimize the specific power requirement of the plant and find optimal operating conditions of the processes while taking into account the various process and product constraints like higher heating value (HHV) of LNG product, NGL Reid vapor pressure (RVP), Wobbe index (WI), and the pressure of FG and parity between FG heating value and fuel requirements. The energy integration of the process requires identification of solid forming streams and separating them from the liquid and vapor flow stream to prevent the solid formation in the multi-stream heat exchanger (MSHE). The cold energy is provided by a refrigerant cycle and hence the stream providing cold energy for freezing out acid gases from liquids needs to be further identified. The stream for exchanging heat was divided into bundles and then converted to a two-stream heat exchanger network for liquid-vapor flow and liquid-solid flow. [5,6] The performance of the integrated cryogenic process, the design was compared with a conventional LNG up-gradation process from literature.

Keywords: AGRU, Cryogenic, Boil-Off Gas, LNG, NRU

References:

1. Katebah, Mary A., Mohamed M. Hussein, AbdurRahman Shazed, Zineb Bouabidi, and Easa I. Al-musleh. “Rigorous Simulation, Energy and Environmental Analysis of an Actual Baseload LNG Supply Chain.” Computers & Chemical Engineering 141 (October 4, 2020): 106993. https://doi.org/10.1016/j.compchemeng.2020.106993.
2. Kaminsky, Robert D., and Moses Minta. Systems and methods for using cold liquid to remove solidifiable gas components from process gas streams. SG182399A1, filed January 5, 2011, and issued August 30, 2012. https://patents.google.com/patent/SG182399A1/en.
3. Singh, Vikram, Edward J. Grave, Paul Scott Northrop, and Narasimhan Sundaram. Integrated controlled freeze zone (CFZ) tower and dividing wall (DWC) for enhanced hydrocarbon recovery. United States US8312738B2, filed November 20, 2007, and issued November 20, 2012. https://patents.google.com/patent/US8312738B2/en.
4. Northrop, P. Scott, and Jaime A. Valencia. “The CFZ^TM Process: A Cryogenic Method for Handling High- CO2 and H2S Gas Reserves and Facilitating Geosequestration of CO2 and Acid Gases.” Energy Procedia 1, no. 1 (February 2009): 171–77. https://doi.org/10.1016/j.egypro.2009.01.025.
5. Rao, Harsha Nagesh, and Iftekhar A. Karimi. “A Superstructure-Based Model for Multistream Heat Exchanger Design within Flow Sheet Optimization.” AIChE Journal 63, no. 9 (2017): 3764–77. https://doi.org/10.1002/aic.15714.
6. “Unified Heat Exchanger Network Synthesis via a Stageless Superstructure | Industrial & Engineering Chemistry Research.” Accessed April 9, 2021. https://pubs.acs.org/doi/10.1021/acs.iecr.8b04490.