Tilt-compliant column may solve storage dilemma on FPSO vessels
Battling tilt-and motion, channeling problems
Packed columns are being used increasingly offshore for duties varying from gas sweetening and de hydration to stripping or recovery of volatile organic com-pounds. They also can play a key role in the emerging gas to liquids conversion packages for float ing production, storage, and off-loading (FPSO) vessels. However, there are two problems emerging from the process: the tilt and motion influences, and severe liquid and gas channeling.
- Tilt and motion: Column performance is known to be seriously impaired by these factors on floating vessels. Operators considering a gas-to-methanol package are facing the prospect of total shutdown of the integral tall methanol distillation column, should heavy seas strike an FPSO. Conoco devised a solution whereby the methanol is stored onboard until the weather eases, allowing distillation to resume. The downside is the infringe-ment of space and huge increase in weight imposed by the storage requirement.
- Channeling motion: The second problem arising from tilt and motion in packed columns is severe liquid and gas channeling. Liquid tends to channel through one side of the column, while gas channels through the other side. This reduces the total surface area of contact between the liquid and gas phases, and also the degree of contact between the two phases in the column.
Performance comparison between conventional packed column and HiperColumn, both 1 meter ID and 6.5 meters high.
The normal counter-measure is to install multiple liquid or gas redistributors. These range from simple annular rings to more complex devices incorporating a perforated tray to collect and redistribute liquid, with risers for the ascending gas stream.
Altra Consultants in Aberdeen and British Technology Group (BTG) are promoting an alternative technique which employs a helical redistributor to counteract the effects of tilt. Inventor Salah Baker of Altra Consultants claims that the HiPerColumn could operate efficiently on an FPSO, even in very bad weather, thus eliminating the need for extra storage capacity for converted liquids. This could in turn cut millions of dollars off capital and operating expenditure.
The HiperColumn features one or more elements in a helical arrangement. At any position up the column, liquid reaching the element from above is given some rotational movement and also an inward radial component of velocity as it spins off the inner edge. This process continues down the length of the helix which can extend over the full height of the packing. This way, redistribution occurs continuously over an extended section of column rather than intermittently at specific locations, as is the case with other designs.
Some liquid also passes through the holes in the redistributor (up to 2,000 holes per square meter), providing efficient irrigation of the packing. Up-flowing gas passes through a helical route that typically travels three times the vertical length, thus enhancing gas and liquid contact. Beneath the redistributor, gas flow suffers minimum gas-back mixing as the flow is partly in cross mode. With conventional packed columns, the flow is constantly in counter-current mode.
Altra Consultants' helical redistributor concept.
If a column is tilted from the vertical, conventional redistributors tend to promote preferential liquid flow on the column's lower side. With the new design, the helical element is always at an angle far below 90° to the axis - typically 25° - thereby allowing liquid to flow in a direction more parallel to the column's central axis.
Several column prototypes have been successfully developed and tested over the past decade, for varying applications. The first prototype featured a 400-mm internal diameter (ID) column packed to a depth of 2.45 meters with polypropylene Super Intalox saddles and Pall Rings. To assess the influence of the redistributor on mass transfer, (stripping of dissolved water) was studied experimentally.
Mains water feed was distributed at the top through a drip pan distributor with 204 holes. Nitrogen from a membrane generator was used as a stripping gas. Fluid flow was measured using turbine meters and rotameters. Water feed oxygen content was measured using an electrochemical dissolved oxygen meter with a resolution of 0.1 ppb oxygen. Temperatures of water and nitrogen were recorded in feed and outlet lines.
To aid water flow behavior observation, the column was made of clear acrylic. The helical redistributors were made from PVC sheet - one version was plain while the other had 7 mm diameter holes occupying 1.2% of the redistributor area. The entire column rig could be tilted to set angles measured with an inclinometer. Oxygen removal was measured for the column with packing only and for the two versions fitted with redistributors.
Better mass transfer
The tests revealed that with a helical redistributor, mass transfer improved significantly over virtually the entire range of gas/liquid ratios studied - on average by around 36% at the low liquid loading, 21% at the higher liquid loading, and over 90% in certain conditions. In all cases, an improvement was noted as the gas flow rate increased. Under tilt conditions, the two versions with helical redistributors also performed better. The perforated version achieved the bigger improvements for both lower and higher liquid loading.
Between 1989-97, several North Sea operators including Amoco, BP, Chevron, Elf, Marathon, Occidental and Texaco co-sponsored studies on the effect of tilt and motion on the perfomance of a water de-aeration process using a conventional packed column and the HiperColumn. The second prototype column, made from perspex, measured 1 meter in diameter and 6.5 meters total height. It was tested in vertical, tilted, and sinusoidal motion applications at a university in Scotland. Tests revealed improved stripping performance using the HiperColumn of 60% in static vertical applications compared with a conventional column, and 113% for a 3% tilt angle.
More recently, as part of a de-bottlenecking study, Altra reviewed the design of the water de-aerator for a fixed platform in an emerging offshore arena. This study revealed that the HiperColumn could maintain target outlet water quality (10 ppb dissolved oxygen) while reducing the volume of the stripping fuel gas by 60%, or 230 MMcf/year (this amount would otherwise have to be flared). Notional fuel gas savings realized would be around $440,000/year.
In summary, Baker and Altra believe that the HiperColumn can cut costs of offshore projects by:
- Simplifying the design and operation of pro-duction system
- Reducing equipment size and weight
- Increasing the production rate and improving product quality
- Increasing reliability and availability of the production system.