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Structured Packing Media

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EVAPORATIVE COOLING PACKING

Numberless structured packing variants have been used since the inception of tower-like, evaporative cooling constructs. Early arrangements included planks, boards, and bricks. At the risk of historical inaccuracy perhaps one can point to the 1936 ACB construct, basically assemblies of side-by-side, corrugated asbestos cement board (thus the name) plates, as breaking grounds as regards performance and somehow anticipating later field developments.

The '50s' and the '60s saw probably the greater strides namely Carl Munters 1957 patent combining corrugated and flat plates, Kohl and Fuller's 1963 vertically corrugated pack patent and ultimately Bredberg's 1966 cross-fluted design.patent which brought [successful] closure as it were to low-profile, highly efficient packing endeavors.

Cross-flute packing, noted for high efficiency and moderate pressure loss, has been used extensively. Illustrative media include Munters' CF-12060 and CF-19060, Brentwood's CF-1200 and CF-1900, and Marley's MC-67

Offset-flute packing, developed much later splits and reunites gas and liquid streams along the vertical fill section profile. Typical media include Hamon's ANCS and Balcke-Durrs' FB-20.

Vertical-flute packing, also referred at times as vertical flow packing and a far more recent design, proffers the largest flute openings available for structured packing today. By permitting higher liquid trickle down rates vertical flute packing helps maintain the packing walls and paths cleaner of foreign, undesirable material such as biofilms or debris. . 

The beauty of cross-flute structured packing is its ability to inherently achieve, impart or impose a high degree of auto or self- liquid phase distribution even though the latter role is frequently played by/assigned to specialty trays, pressurized nozzles or open, gravity flow distributor channels or basins. 

COUNTERFLOW AND CROSSFLOW FILL: Is it possible to use conventional counterflow media such as 1200 or 1900 fill for small crossflow cooling towers?

We have been asked now and then whether it's possible to use conventional counterflow media such as 1200 or 1900 fill for small crossflow cooling towers, say about 125 or 450 tons each, without major customization or field work. It's common practice to pile the packs standing on end and then the air passes through the honeycomb edge. There's also traditional mention to a 10° cut on each end (or built-in makes) but would involve a troublesome additional step/feature for these smallish or exceptional jobs. So one often wonders to what extent would it be workable enough to simply add some more air travel depth, say go to 36", and forget about the tilt or provision for inclination as water would still be in a valid crossflow arrangement, id est trickling downwards and air passing through the fill sideways. Would it make too much or neglible of an impact/make shift?

As long as all other system components are in good shape we would would expect it to be okay. However, use expanded metal or plastic grid between vertical layers for support -- do not just stack one layer on top of the one below. Use same air travel (horizontal air travel) as they have now. If you are using 1200 mm you may not want to add air travel because the extra pressure drop might put the fan in stall. Check the volts and amps to be sure the motor is drawing the correct fan power. Do not overload the motor. Change the fan speed as necessary. We would not worry about the slope of the fill, but if all the fill is not wet it might increase fouling. Look at it and stagger the vertical packs horizontally if necessary (move the bottom packs towards the fan just a little to follow the path of the falling water being drawn towards the fan).

 

BIOLOGICAL TREATMENT PACKING

Structured packing as currently utilized for either aerobic or anaerobic treatment can be similarly categorized.

Cross-flute or crossflow packing, proffering 31 sq.ft. per cu.ft surface area, is typically employed for trickling filters, including nitrification.

Vertical-flute or vertical flow packing, proffering 31 or 40 sq.ft. per cu.ft surface area, is typically employed for applications where media plugging can be a concern. However, vertical-flute media is not able to redistribute wastewater within each pack and only neglibly from layer to layer.

It is possible to combine both packing types in order to try get the best of each world, a concept and actual arrangement referred to as mixed media. One could/can aim at stacking the two upper filter media layers with higher reaction rate/higher performance cross-flute packing and utilize less fouling prone, vertical flute packing for all the other layers. The two upper layers would experiment the higher flushing rates which would tend to keep them clean. At the same time, the upper cross-flute packs will aid in uniformingly spreading or feeding incoming wastewater to the vertical-flute packs below.

Cross-flute structured packing media has been successfully employed in full-scale anaerobic treatment systems. In particular, Patrick et al document an anaerobic installation achieving COD removals ranging from 70 to 90% based upon loadings between 2 to 4 kgCOD/day per m3 and typical BOD removals in excess of 90%. Cross-flute media with a surface area of 30 sq.ft./cu.ft. provided a service life of nearly 20 years.

BIOTOWERS: SIZING GUIDELINES

While both aerobic and anaerobic biotowers have seen their market share increase so to speak, easily or readily transferable design/bioengineering formulas have yet to arrive if at all.  Let's discuss each type separately.

In the case of aerobic biotowers limitations arise because much of the available material/equations/models say derives from either

 - low strength, municipal type contexts

- installations or designs using just one type of media, say either cross-flute OR vertical-flute media, and/or

- general problem/solution formulations focusing or addressing soluble or dissolved biological oxygen demand, sBOD for short.

We're left sort of unaided when it comes to address high strength, mixed media, lump BOD/COD all at the same time scenarios.

For municipal wastewater, many reputable biotower media manufacturers and vendors provide some sort of computerized/courtesy design/ballpark sizing based on modified Velz formula variants.  It is also very common that they specify minimum and maximum wetting rates, basically hydraulic loading ranges 0.25 - 0.75 gpm/sq.ft. being possible figures, as well as maximum organic load per cubic feet, say 50 lbBOD/day per 1,000 cu.ft.  Taking this into account and sticking to predominantly/solely municipal wastewater aerobic biotowers find their way in  many engineering reports.  Even then, things can get tricky.

One can seldom can apply equations developed for municipal wastewaters (ie the Velz equation for TFs) to industrial wastewaters. The reason is that the sizing and performance equations for municipal systems almost always are related to hydraulic factors, ie HRT, wetting rates, etc, while industrial systems are almost always load controlled (ie SRT, organic loading rate, etc.). The problem with biotowers is that when even treating municipal wastewaters, say at 1 kg/m3/day, the BOD is spread out from around 400 mg/L at the top to 25+/- at the bottom of the tower. With industrial wastewaters for the same loading rate, the BOD at the top of the reactor is substantially higher even with direct recycle, so that oxygen transfer limits are reached rapidly in the top few feet or meters of depth.  Under these conditions, excessive growth occurs with associated plugging and odor production, usually occur.

WARNING: BEWARE OF LOW QUALITY MEDIA AND EXPLOSIVE GROWTH EXCURSIONS

Assuming media with a thick biofilm of 3-4 mm( including the water) it will have a weight of above 500 kg/m³. On 6 meters the bottom layer will have to sustain a weight of 3 tonnes! Media failure is around the corner.

DISTRIBUTION AND PUMP SIZING

Since the TF is open at the top, any recirculation pump will have to overcome the entire elevation head plus friction headloss for the influent and the recycle streams. So the elevation head is the height from the bottom of the sump for the recycle pump to the top of the highest point in the TF distributor.

ROUGHING FILTERS

Roughing filters are trickling filters employing relatively coarse media and operated at high rates to perform aerobic biological preliminary treatment, e.g. target soluble BOD 50-80 % removals.

It is very difficult to treat high strength industrial wastes with a TF of this type because of the high COD/BOD at the distributor. Usually excessive growth occurs at the surface with fairly rapid plugging.  Also, when the recycle ratio exceeds 3:1, the cost of pump power tends to negate the inherent advantages of a trickling filter.  Work out pump rough HPs and compare with aeration HPs were an aerobic approach considered.

STRUCTURED MEDIA COMBINED SERVICE: COOLING DUTY AND BIOLOGICAL TREATMENT

At 115°F the bacteria will still grow, but they will not stick to the surface of the media. So, until the cooling tower/media has dropped the temperature to below 105°F or so at least, the cooling tower would not function as a biological treatment unit. An explorable path would be to handle it as a cooling tower application and then consider a trickling filter after the water is below 95°F. The trickling filter would be large - many industries and in particular most paper mills have huge lagoons for treating these wastewaters.

 

 

Industrial Biotower

 

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