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Structured Packing Media

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STRUCTURED PACKING SURFACE AREA

Structured packing is today widely available from a number of manufacturers. Typically the packing consists of assemblies or packs of individual corrugated, flat, or both type sheets bound together in some form, e.g. glued together. Plastic and metal are probably the most popular materials of construction employed. Typical applications include evaporative cooling, biological treatment, gas cleaning, and aquaculture.

Vendor catalogs frequently indicate surface area, say 30 sq.ft./cu.ft, and void space, say 96 %. In practice, the actual or effective surface area can be augmented by an aggregate of sprays, bubbling, dripping, droplet splash, among others. Effective or actual surface area can be as well diminished by fouling/plugging/blocking, and unused or idle sections. 

TRANSFER RATES

It is generally understood that greater surface areas promote greater transfer rates. However this will happen as long as whatever contemplated greater surface area is actually put to use, thus the actual or effective surface area nuance (versus geometric or catalog surface area). It is wise to scrutinize active to plain geometric surface area ratios.

It must be noted that all too dense, small flute media while proffering highest surface area per cu.ft. may incur so high pressure drops so as to render the media inapplicable. It may also give rise to exacerbated plugging. 

 

PRESSURE DROP

Whatever the liquid gas stream arrangement - counterflow, crossflow, cocurrent - packing introduces a level of resistance to the passage of the fluid and pressure drops through the packing media should be appropriately accounted for. In general one would treasure packing proffering lowest pressure drops naturally assuming full performance can be delivered all other things considered equal. 

In many applications, liquid side pressure drops can be overlooked as it's just some liquid, frequently water, trickling down/gravity flow. 

FOULING

Fouling refers to the undue accumulation and plugging of the media. Not only will performance noticeably suffer but there can be structural problems, even media/bed collapse as well. Fouled packing can weight far more than first day, original media. 

GAS AND LIQUID DISTRIBUTION

Probably most if not all structured packing applications require uniform gas and liquid distribution within the unit. To a certain extent this can translate into a make it or break it situation. Aside from things like faulty liquid distributors or flow design, media related culprits may include instances of fouling and channeling/by-pass. Uneven "rain" zones, thermal gradients, dry spots, media holes all add up to under performing packing. 

 

WETTING 

As with most packings, structured media achieves optimum utilization when most if not all exposed surface area is thoroughly wetted. So-called wetting point is defined as the recommended or required minimum liquid loading so as to insure complete media surface "coating" thus permitting design mass transfer rates. Packings with lower wetting points would configure the more desirable designs. 

FLOODING

During operation most all packings hold up some given amount of the circulating liquid. When gas or air stream velocity is unduly high, liquid will not be able to trickle or flow through the packing bed at all, a situation called flooding point. It should be apparent that for certain applications one will want this value to be as high as possible - we would then able to move larger gas phase volumes through smaller face areas. It should be noted that pressure drops increase as operation, willingly or unwillingly, approaches flooding conditions.

 

 

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Need Some Help? answers@engineeringfundamentals.com - James C. Young Environmental - Balestie, Irwin & Balestie