4 replies to this topic

[umesh2329]

I am working on adsorption (biosorption) of heavy metal ions by natural adsorbent i.e. powder of orange peels (average size 0.450 mm). I had succesfully done the batch adsorption study but facing difficulties with contiuous system. I have following doubts,

1) while performing fixed bed experiment in a glass column with i.d. 3 cm and about 16 c.m. bed height. the volumetric flow rate was maintained at 11 ml/min (its the minimum my perstaltic pump can give) in upflow manner, behyond which the bed start lifting upward.???
how can I keep it in place? should I chane diameter or height of bed? what is ideal column diameter to length ratio.

2) Do i need to modify sorbent to get more void space or increase particle size?

I have read Concluson & Richerdson too but problem wot solved.

plz guide me its very cruical for my expt. work............

[Zauberberg]

For all industrial adsorbers I have seen (or reading about), downflow has been the design path of feed gas (feed stream in general). During regeneration cycles, the flow is in upward direction but the flow rate is much smaller when compared to active/adsorption cycle so no bed fluidization can occur. In the same way like you have bottom support screens/grids, you may consider hold-down grids on top of adsorbent. Actually I don't know how this works on a pilot-scale.

This should solve your problem. For proper gas/liquid distribution inside the adsorbent, consider minimum pressure drop of 0.01 PSI/ft of packing height - the value recommended by GPSA.

[Art Montemayor]

In prior years, I have written several posts on adsorption beds as related to direction of gas flow for the purposes of adsorption and bed regeneration. If you do a Search on the subject you should find the posts.

I have always employed downward flow for the adsorption bed and - depending on the type and/or condition of the regeneration gas stream - and upward (counterflow) or downward (concurrent) flow. For example, I can regenerate an adsorption dryer unit filled with either Activated Alumina or Silica Gel by using either dry regeneration gas or humid (a side stream of the wet feed gas) gas. I have designed, built, and operated both kinds of regeneration systems. When I regenerate with humid gas I MUST employ a CONCURRENT stream of regeneration gas. The regeneration gas must not flow countercurrent to the adsorption flow direction when the regen gas is humid.

I would never use a pilot adsorption column as small as 3 cm in internal diameter. This is just too small to do a decent distribution and scale-up later. I recommend you employ a pilot adsorber with a diameter of 7 to 10 cm.

I also strongly recommend you employ hold-down SS screens on top of your adsorbent together with a sturdy support - distribution grid at the bottom of the bed. You will always have a situation of high gas flow upward through the bed - for example, when a PSV activates at the top of the bed.

[umesh2329]

Dear friends,
Thank ou very much for giving me very valuable sugesstions.
I will definatel implant them & will revert with my experience.

Only thing that isnt clear to me is,
Wheather their is any ideal ratio for diameter of the column to its length for a smooth operations?

thank you.

[Art Montemayor]

There is no "ideal" L/D ratio for an adsorption bed. That kind of thinking is reserved for interior decorators, artists, architects, or people interested only in aesthetics. Engineers are trained and paid to make things work - and work safely, efficiently, and consistently.

The most important mechanical and process factor is to ensure a conservative superficial velocity through the bed so that you obtain even, stable, consistant distribution through the entire length of the bed and you don't provoke bed movement of any kind. The best way I know of to ensure that is to employ downward adsorption flow and a conservative superficial velocity as recommended by the adsorbent manufacturer.