Effects of Tower Motion on Packing Efficiency

Abstract

This work reviews the major published studies, both theoretical and experimental, that address the impact of wave motion on packed tower performance. Current practice is to add excess packing to guarantee the required separation is obtained, though there is little data available to derive a safety factor for packing height. This work highlights deficiencies in the current knowledge base and analyzes general trends to address common misconceptions about tower design for floating production. Tilt and motions imposed on a fractionation column have a significant impact on product specifications due to reduced packing efficiency. Improved awareness of motion impacts will assist the analysis of tower design and allow feedback to the process design. Identifying gaps in available data shows limits in the current understanding and allows the development of appropriate simplifying assumptions. The current understanding of towers in motion allows only very basic design rules and the safety factor for packing height in literature varies from 1.1 to 2.0. This provides little confidence in the ability to predict packing efficiency for floating production. Static tilt is more detrimental to liquid distribution than motion at a given amplitude. Still, liquid maldistribution from motion approaches that of static tilt as the period increases. Liquid sloshing, often cited as a significant concern, is a relatively minor contribution to maldistribution except for short periods and tall towers. The relative bed size has a significant impact on liquid maldistribution, limiting the recommended maximum bed height:column diameter to 2 – 3 to maximize the efficiency. Approach to equilibrium is often overlooked as the major determinant of sensitivity of efficiency to maldistribution. Separations that operate near equilibrium are more sensitive to maldistribution than services with a large driving force. Thus, distillation towers are generally more sensitive to motion than absorbers and sensitivity may vary over column height. Recommendations are provided for a bed-by-bed analysis and feedback to the process design. Floating production units require additional complexity and conservatism in tower design. An improved awareness of motion effects on towers, and the factors involved, will lead to improved designs and a reduction in the cost of floating production facilities.

Influence of Rotational and Translational Oscillations on the Drainage of Liquid in Floating Packed Beds

“…Once setting the column in motion, the effect of non-tilting robot oscillations was minor (Figure b) whereas the rotational-frame accelerations in the form of roll and pitch were to impact the liquid drainage dynamics (Figure a). Hence, the relative contribution of gravity and acceleration due to column rotation (Figure a) on the liquid drainage can be described in terms of the inertia-to-gravity force ratio, R IG where g cos (θ) = effective streamwise gravitational acceleration component.…”

“…Reproducing offshore sea/swell movements under laboratory conditions is an essential step to ensure reliably functional unit-operation systems once commissioned for seagoing vessels. , As an example, marine applications of gas/oil processing have shaken up habits by contemplating gas–liquid-packed beds beyond their traditional use as stationary geometrical objects. Hence, the assessment of the scope of divergence in their operation under nonstationary/unstable sea conditions − has arisen as a legitimate query. Indeed, unlike their land-based analogs, the incessant movements induced by the erratic marine conditions inflict performance losses to the on-board fixed beds, depending on the extent of the oscillations of their host vessels in response to the swell and wind dynamics. ,− Safety considerations for emergency shutdown to be met in unfriendly sea areas with extreme weather conditions are an important aspect of the operation of packed-bed reactors. , Therefore, a key step to temporarily secure the processing units and to avert accidents onboards cannot afford to ignore the liquid drainage dynamics and how long it takes to free up in time the reactor fluid inventory.…”

Hydraulic Design of Thermally Coupled Columns and a DWC for NGL Fractionation Plants

“…The main challenge for floating applications stems from the fact that required separations are carried out at high to moderate pressures, which implies employment of tray columns. Trays, being prone to detrimental liquid and vapour maldistribution effects induced by motion of the floating facility, are generally avoided in offshore applications, where random and structured packings, being less sensitive in this respect, are considered an appropriate choice [6][7][8] . Here, a compact design (less weight and plot area), i.e., a DWC, though taller than any two columns in a thermally coupled direct sequence (TCDS) arrangement, may be a preferred option.…”

Marinization of Mass Transfer Columns for FLNG Applications

“…Other papers have referred to this conclusion and have limited studies to only structured packing performance when the column is under motion. A very valuable literature overview was published by Cullinane et al in 2011. Because of the above-described uncertainties, Raschig and UOP set up a new pilot plant to study the performance of various structured and random packings under identical column motion conditions The experimental test results have been validated by UOP in terms of Computational Fluid Dynamics (CFD) modeling.…”