fig1 shows a cross - sectional view of a packing element according to the invention in a scrubbing column . the packing element 1 , of moulded polypropylene , is of a substantially ellipsoidal basic shape with a recess 3 in which a fluid can collect , shown here containing captured liquid 2 . a gas stream to be treated is represented by the arrows 5 , flowing upwards through the column . a treatment liquid is introduced into the column from an inlet 6 in an initial direction indicated by arrows 7 . the vicinity around the element 1 is shown in schematic form . the element 1 has a displacement volume of 22 . 5 ml . the recess 3 , measured relative to an equivalent ellipsoid with an upper envelope of the same shape as the illustrated lower envelope , has a volume of 5 . 7 ml , i . e . about 25 % of the element volume . the density of the element 1 with no captured liquid in the recess 3 is 220 kg / m 3 , rising instantaneously to 260 kg / m 3 when the recess 3 is filled to capacity with liquid 2 . fig2 shows the tumbling action of the element 1 . at point a the element has its recess 3 uppermost . the element 1 then tumbles in a direction denoted by arrow 9 until it has turned through 180 ° to point b , when the recess is lowermost . when the recess 3 is at point a it gathers liquid from the down - flowing liquid 7 whilst being buffeted by the rising gas stream 5 . the captured liquid 2 causes an increase in the density of the upper part of the element which increases the pressure gradient . the element 1 tumbles from position a , the extra weight of the liquid 2 on the descending side of the element 1 increasing the tumble velocity . the liquid 2 in the recess 3 is displaced by the movement and by point b the recess 3 has entirely emptied of liquid . now at the bottom of the element 1 , the recess 3 acts as a lip for the rising gas stream 5 to push the element back rapidly towards its upright position a . fig3 shows a cross - sectional view of a second version of packing element according to the invention . this packing element 11 , also of moulded polypropylene , is of a substantially spherical basic shape but has two opposing recesses 13 and 14 . the recess 13 is shown containing captured liquid 12 . an upwardly - flowing gas stream to be treated is represented by the arrow 15 . treatment liquid passes downwards as indicated by the arrow 17 . the element 11 is thus simultaneously subjected to the effects of captured liquid 12 in recess 13 and captured gas in recess 14 . tumbling of the element 11 in the direction indicated by arrows 18 is thus increased by both these effects . the fig4 graph shows the performance ( measured as the inverse of the height of a transfer unit ( htu − 1 , with units of m − 1 ), against the pressure gradient ( δp / h o , with units of pa / m ) for elements in a fluidised bed . the total packing height requirement is the product of the height of a transfer unit ( htu ) and number of transfer units ( ntu ). the graph effectively compares the relative bed heights of the packing elements , including a perfect 2d ( two - dimensional ) ellipsoid ( oblate spheroid ), ovoidal ellipsoids such as ‘ birds eggs ’ and ‘ acorns ’ both with and without cog / cos offset , and an indented ‘ acorn ’ shape . the lower bed heights represent better transfer efficiency . by taking ratios of 1 / htu − 1 or htu the relative bed heights are apparent and it can be seen that the lowest bed heights were most easily attainable with the indented acorn ( turbopak ®) element having a 29 % cog / cos offset and an indentation of 5 . 6 % of the basic element . the next best element in terms of the δp / ntu ratio was the 0 % offset whole - acorn shape marketed as turboid ®, although this element was much inferior in terms of effective bed height ( htu − 1 ). fig5 shows the energy efficacy in a fluidised bed of the packing elements of fig4 by comparing their pressure drop / efficiency ( δp / ntu ) with their pressure gradient ( δp / h o ) measured in a system absorbing acetone at 20 ° c . it is clear that the 29 % offset , 5 . 6 % indented acorn is at or near a minimum point in terms of energy required to achieve a certain absorption efficiency . it is better than the 0 % offset non - indented 38 × 32 mm acorn and substantially better than the oblate spheroid and ovoidal ‘ egg ’ shapes tested . this shows that both bed heights and pressure drops are improved by the elements of the invention over existing fluidisable packing elements . tests were undertaken to compare a 38 mm × 32 mm 5 gram 29 % indented ‘ acorn ’ shape ( turbopak ®) against a 57 mm × 40 mm 7 gram ‘ bird &# 39 ; s egg ’ shape . whilst the latter larger element had the advantage of a longer semi - axis and 29 % cog / cos offset to increase its tumbling rate this was to some extent balanced by the higher density of the non offset outline smaller element . with a gas flow velocity of 3 . 8 m / s and a liquid to gas ratio of 3 litres / m 3 in a system absorbing acetone at 20 ° c . there was a surprising improvement in performance of 125 % over the ‘ egg ’ when measured in terms of mass or heat transfer coefficient ( htu − 1 )