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Oct-2016

How can waves significantly impact the performance of an amine unit installed on a FLNG?

Among the challenges to overcome and complete successfully a FLNG project, the design of the acid gas removal unit might be of great importance: the experts shall address the question “What effect does the motion of the ship cause on the gas sweetening process?”.

G. Perdu, L. Normand, C. Salais and V. Carlier, PROSERNAT
P. Alix and M. Fourati, IFP Energies nouvelles
C. Weiss and T. Maubert, TOTAL S.A.

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Article Summary

It is necessary to build up accurate knowledge of the impact of the movements on the towers operation, to first secure then to optimise the design of the processing units. Indeed both flooding and internals efficiency significantly impacts towers sizing (diameter and height) and thus the whole FLNG project.

Taking into account the multitude of factors such as liquid viscosity, height of packing, diameter of column, acceleration, liquid/gas flowrate, type of packing, and motions imposed on the system for columns design is very complex. Thus, Total, Prosernat and IFP Energies nouvelles have conducted a four-year research and development program at Heriot-Watt University to model mass transfer and hydraulics of a tilting and moving tower. In order to better understand the impact of the column diameter on the results and to be able to scale-up the results, two different column sizes were used and will be described.

The first phase of tests included static and pitch tests with varying periods and angles, at different liquid loads to map the distortion of liquid distribution at several heights of packing. The data acquired have showed, depending on operating conditions, a significant impact on liquid distribution in the packing bed, leading to well differentiated wetted zones. This work will give some examples to illustrate the impact of this phenomenon on the tower efficiency and the corresponding impact on towers design.

In perspective, a second test phase has been launched at the beginning of 2015 with a column installed on a hexapod. The machine, which has six degrees of freedom, will allow quantifying the impact of accelerations and 3D motions on the hydraulics and mass transfer of a gas/liquid contacting tower, especially under surge and heave.

Introduction
Acid gas absorption is a critical step in the chain of processing stages for natural gas liquefaction. Acid gases need to be removed from the natural gas down to very low levels (typical LNG specifications are: CO2 <50 ppmv, H2S <4 ppmv) corresponding to the design requirement of the downstream cryogenic section. Portions of the acid gas shall never slip the Acid Gas Removal Unit (AGRU) absorber in order to avoid crystallisation problems in the downstream liquefaction unit, to prevent shut down and to avoid subsequent long restart-up periods. For any project, a safe and reliable design shall be reached within an acceptable compromise between overdesign, moderate equipment number, weight, and operating costs. FLNG Projects face more sharply those constraints than any other gas development project.

The motions of a hull add specific design constraints to all the units installed on the topside, and their performance can be affected. In order to secure those performances, the design may include additional capacity in processing equipment or CO2 polishing safeguarding equipments that increases constraints on the floating structure itself. This means that the platform size increases which may jeopardise the economic feasibility of the project.

The impact of motion on the performance of AGRU columns is not well-understood. Industrial feedback for design of absorption/fractionation towers on floating platforms is still limited and does not allow for precise predictions on the actual loss of efficiency of such columns.

Extrapolation from past experience on floating vessels is difficult to apply to AGRU columns due to the very different constraints than other processing units including dehydration applications or fractionation columns (Cullinane, Yeh, Grave, 2011). Meanwhile, Literature (Kobayashi et al., 1999; Yoshinaga et al., 1981; Berger et al., 1983; Tanner et al. 1996) indicates that the performance of packed absorbers could be decreased by up-to 60%. This performance loss is highly dependent on the system (distillation, absorption), the gas/liquid contactor (packing, tray) and the overall geometry and location of the tower on board the floating platform.

Therefore, it is critical to identify the controlling parameters and understand the impact of motion on the performance of an acid gas absorption column with amine solvents to secure and optimise the design of a floating Acid Gas Removal Unit on FPSO and FLNG vessels.

In an onshore environment, the design of acid gas absorption requires the development of accurate simulation tools to calculate the acid gas absorption rate through chemical reaction with different type of amines on given internals used within an absorption tower (Weiss, Perdu, Alix et al 2014). This is not the usual approach taken for such columns, where an equivalent height type calculation is often used requiring some sort of stage efficiency to determine the overall packing height. Accurate prediction can only come from a detailed understanding of both the liquid flow paths and the chemical reaction between gas-phase solute and liquid absorbent.

Figure 1 shows that the acid gas absorption rate within a tower depends on many factors such as:
• Hydraulic conditions with effective area developed by the selected internal,
• Thermodynamics and G/L equilibrium of phases,
• Mass transfer coefficient on gas side
• Mass transfer coefficient on liquid side
• Kinetics of the reaction between acid gas and amines.

The in-house proprietary simulator named Desulfo developed by Total, IFP Energies nouvelles (IFPEN) and PROSERNAT integrates State of the Art mass transfer models for the calculation of amine systems and absorption of acid gas by amine solvents. With a rate-based simulator like Desulfo, the calculation of acid gas absorption is not performed based on a theoretical stage approach including a mass transfer efficiency coefficient. Desulfo calculates the absorption of acid gas through a stage after stage approach along the column, and incorporates the type and arrangement of internals selected for the design. When the column is a packed bed, Desulfo provides a clear picture of the absorption profile within the tower, including slippage if the solvent flow is not sufficient. After each step of development based on R&D program, the simulator is tested and continuously upgraded based on the feedback recovered from the operating and design experience aggregated by Total over its 50 years experience in the field of H2S and CO2 removal units.

Armed with this knowledge, Total, PROSERNAT and IFPEN completed an extensive review of bibliographic data available on the impact of motion on liquid/gas distribution in columns to check how motions of columns could affect the performance calculated by Desulfo. The analysis of moving environment determined that two main parameters could distort the calculation of absorption performance predicted by Desulfo. The first one is the permanent tilt of the column from verticality. Whatever the oscillations amplitude, whatever the period of oscillations can be, the tilt diverts the liquid from its axial route normally expected in onshore absorbers/regenerators. The distortion created by non verticality generates accumulation of liquid in some places, and drought in other places of the column section. The second key parameter is the acceleration generated by the movement of hull, amplified in some places by the large distance between the upper beds of AGRU absorber and the centre of rotation which can be of the order of 350 meters or higher for a ship. Radial forces imposed by accelerations make the liquid deviate from its conventional onshore distribution map normally modelled in simulator. Analysis then raises the simple question: “are the models, introduced in figure1 determined on vertical and static systems, still valid when the systems are submitted to movement?”


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