Performance of absorption and distillation column for integration, for instance, in offshore natural gas liquefaction plants has successfully been tested during pilot scale experiments on the motion simulator at DET NORSKE VERITAS (DNV).
The efficiency of moving packed and tray columns was determined. A distillation column (height 5 m, diameter o. 7 m) was mounted on a random motion simulator that produced movement in all six degrees of freedom. The simulator was controlled by computer programs containing statistical data of sea conditions, combined with the motion response of the structure.
The results are used to predict the efficiency of offshore separation units, and column design can be based on these. The effectiveness of measures taken to guarantee reliable process operation are also shown.
Offshore processing of natural or oil associated gas is becoming a more and more attractive alternative to conventional pipeline transport. Marginal gas fields as found in offshore shelf areas can be economically worked by in-situ processing of the gas giving several products. Those offshore process plants on floating structures could be production units for LNG (liquefied natural gas), methanol or ammonia.
For economical production, the plant has to be designed for operation under sea conditions with irregular motion due to wave, wind and current forces. Absorption and distillation columns are considered to be critical components with regard to motion. For these units the uniform countercurrent gas and liquid flow is disturbed by motion leading to reduced heat and mass transfer. Their efficiency is often influenced by the deviations from the vertical position. For example, within a LNG plant (fig. 1) the effect of a large efficiency drop of the C02 absorption tower or of the fractionation unit could lead to a blockage due to freezing of, for instance, C02, H2S or heavy hydrocarbons in the low temperature heat exchangers.
For this reason both absorption and distillation units have been investigated in this study.
Until now, there has been little information concerning irregular column movement due to sea conditions. The few publications that refer to this problem are based mainly on theoretical estimates which, however, often lack experimental proof.
A permanent inclination is considered to be the most critical situation for a separation column. Several authors have described the efficiency drop of plate and packed columns in inclined positions.
For a distillation column (50 mm Ø, packing rectification of liquid air), Weedman1, has shown that, beginning at about 1° inclination, efficiency drastically drops to about 50 %at 2.5°.
