oyebaba,
The sizing of a demister pad is subject to specific design calculations, based on the mixture of phases, the primary and secondary separation efficiencies, the gas flow upwards velocity and the expected size of the droplets carried over, towards the demister. At this point, you have to know what is the most suitable configuration of the mesh, the material of the mesh (polymeric fibre or metallic wire), in order to provide the best chance of the droplets to coalesce on and around the fibre / wire. Then you have to calculate the best density of that demister (which typically is a proprietary secret of the demister fabricators), which the will give you the most economic size (diameter and thickness) of demister for that particular operation. Providing that you have behind you 10+ years of practical experience in designing, testing and developing new ways of improving your demister efficiency, in the extremely competitive demister market, all what you need is a small detail;- the proprietary software developed by the various demister specialised fabricators or a method of calculation developed by companies like Shell.
Assuming that all the above is accomplished and you have generated a design for a demister;- is your manager prepared to accept the design of a pressure vessel and the demister inside, without any guarantees that the separator will work?
Past this stage, how do you figure out how to actually make the demister, will you buy a specialised machine costing hundred of thousand of dollars to spin the wire in a very special way, to generate the mesh density calculated by yourself? Would you still have a job after all this?
good luck to you,
gr2vessels
Give your process conditions and vessel dimensios to 2 or 3 or 4 demister vendors (they are plenty around) and compare their quotations and make them justify them especially if one comes in vastly different to the others. The more detail you give the more you tie them down to your parameters.
Then you can take the best from each of their quotes and write a hard specification for what you determine that you really need for your situation.
Then make them all bid against your hard specification and see what they come up with especially compared to their initial offering.
oyebaba,
You can (and in my opinion should) easily do a preliminary sizing of your demister using the Sauders-Brown equation. The procedure for doing so has appeared repeatedly on this forum and in ChEResources. Furthermore, the design includes calculations for minimum height and residence times. You would do well to become familiar with these practices. After your preliminary sizing, you can present your information to a demister vendor for final confirmation of the size. I would expect your sizing to be adequate or more than adequate.
Doug
djack77494 i agree with you (and my first post wasnt meant to put oyebaba down) just to tell him that for now - maybe he didnt even need to do it - just assume its there.
If he need to do so e.g. if the preliminary sizing has shown that height of the vessel is critical - then he could also do as you suggest and use e.g. the procedure posted earlier.
I think that the precautions listed by gr2vessels while being true if he was to actually design the thing may be right - in most cases they will however be over the top. At an early stage it can often be difficult to get the vendor to reply (if they dont smell a sale - or if you are a consultant who only specifies not buys (sic)). Then preliminary sizing may be the only option.
