The contaminant solubility, vapor pressure characteristics, and the scrubbing liquid's capacity for that contaminant control the actual amount of packing needed in a packed tower. Packing selection is covered in detail in books specifically devoted to mass transfer (see Appendix A) and is beyond the scope of this book.
A method has been developed to compare various packing types. This parameter is called the packing factor, and you will see specific packing factors published for various packing types. Most packing vendors, however, will provide for you the estimated packing quantity for their specific packing after you submit the gas flow and scrubbing liquid characteristics to them. Some will even design towers for you. It is advised, however, that you solicit the assistance of an experienced packed tower vendor before committing to a tower selection. These devices are more complicated than they appear to be on the surface.
Gas inlet velocities are usually 40-55 ft/s in packed towers. The inlet velocity is usually dictated by common ventilation system design practice. In vertical counterflow tower designs, the vessel gas velocity is 3-8 ft/s. The upper limit is dictated by the flooding characteristics of the packing.
Any packing can flood. Flooding occurs when the gas kinetic energy is sufficient to hold up all the scrubbing liquid. The liquid spreads out across the tower seeking some means to drain but cannot. The pressure drop of the tower starts to swing or surge and the hydraulics become unstable. For most gas absorption problems at near-ambient conditions, at approximately 8 ft/s, the tower might flood. Packing vendors perform tests on their packing and determine flooding velocities and gas mass flow rates for their various packing types. The designer sizes the vessel to stay below that flooding point.
Ironically, most mass transfer operations reach their peak efficiency just before flooding occurs. Mechanically, however, the stability of the tower decreases as one approaches flooding. A compromise is needed. Most towers are designed for less than 80% of predicted flooding.
To support the packing, flat or curved injection type grids are used. Figure 12.7 is a rendering of an injection type grid. The curved surfaces allow the ascending gas to be injected into the packing not on one plane but over a deep zone. The gas can enter the packing at an angle, thereby allowing the liquid to drain more readily.
If dumped-type packing is used, hold-down grids are often used to hold the packing within the required absorption zone.
The liquid itself is distributed by spray headers as shown in Figure 12.8 or by distribution weirs as shown in Figure 12.9. Care is taken with spray-type distributors to make certain that the spray patterns overlap but do not impact the vessel wall excessively. If the liquid hits the wall, it forms sheets of liquid
Injection type packing support (RVT Process Equipment, Inc.).
Retractable liquid distribution headers (Bionomic Industries, Inc.).
Liquid distribution weir boxes (RVT Process Equipment, Inc.).
on the wall that are largely ineffective in absorption because it only attains the area of the vessel wall itself. Many vendors of packed tower internals offer proprietary liquid distributors. These designs often have their roots in distillation towers and are highly engineered (and tested) to produce a uniform liquid loading. If spray headers are used, the liquid velocity is 4-8 ft/s. Free-flow fittings to distributor trays are in the 3-4 ft/s range, sometimes lower.
Packing is usually irrigated at a minimum of about 6-8 gallons per minute (gpm) of liquid per square foot of packing. It is not unusual to irrigate at over 20 gpm per square foot to make certain that all the packing is wetted. If the packing is not fully wetted, the performance of the scrubber will be reduced.
If a packing depth of more than about 10 feet is required, redistributors or rosettes are used to pull liquid from the wall toward the center. The gas velocity through the packing causes the pushing of the liquid toward the wall. The velocity tends to be slightly higher at the center than the wall, thus the liquid is ejected toward the wall. The rosettes act as baffles to direct the liquid back toward the vessel center, thereby keeping all the packing wetted.
The upper surface of the packing and its liquid distributor generate residual droplets that are controlled by a mist eliminator. Mesh pads are often used when the gas stream is clean (no solids) or chevrons when some particulate may be present. Mesh pads require a gas velocity of about 10 ft/s or less. Chevrons permit higher gas velocity (10-12 ft/sec), but this would require a change in vessel diameter. As a result, the packed tower vessel is usually designed for about 8 ft/s or less. If a chevron is used, its face area is reduced using a blank-off plate.
Gas inlet velocities are in accordance with the counterflow designs. Because the gas flows side to side in the crossflow design, the liquid is draining out of the gas stream, so the packing resists flooding. As a result, the crossflow orientation can run at higher gas velocities.
The box velocity is usually 5-10 ft/s. The liquid loading can be higher than that used in the counterflow packed tower. This higher liquid capacity can be an advantage where the gas is only slightly soluble in water (you can use more water).
The droplet eliminator in the crossflow also rejects liquid out of the air stream, but it ejects it out and down, rather than back into the gas stream. If a chevron is chosen, it can operate at 12-15 ft/s. If a mesh pad is used, velocities of 8-12 ft/s and sometimes higher are possible. The mesh pads are often inclined to enhance draining along its element (the gas drains at an angle given the gas velocity pushing the liquid to the side). Either of these devices is often mounted in a containing box with a flanged service cover.
With crossflow designs, a reduction in efficiency can occur if the gas short- circuits over the top of the packing. To prevent this, vendors use baffles or extend the packing up into the box area above the packing. Others place a layer of mesh pad above the packing to offer greater resistance to gas flow. Still others use two or three different packing sizes so that the gas is pushed lower in the tower. The more resistant packing is placed at the top, near the irrigation headers.
The liquid is distributed much like in a counterflow packed tower at similar velocities. The liquid header pressure need not be high since the liquid nozzles are within a foot or so of the packing. Pressures of less than 10 psig are used firing full cone nozzles. Some designs use pipes with holes in them, thereby eliminating the nozzles.
Packed towers, particularly vertical gas flow types, need to be installed vertically. A plumb line is often used to help set the vertically of the unit.
If the tower is made from fiberglass-reinforced plastic and is installed on a concrete pad, roofing felt (tar paper) is placed under the tower to compensate for pad irregularities. If the towers are installed on steel plates, roofing felt is also used to allow the plastic packed tower to expand and contract with minimum chafing on the plate.
Always plan the surrounding area for packing removal and installation. Sometimes height constraints eliminate the possibility of using access doors above and below the packed bed. In this case, the whole top of the tower may have to be flanged and bolted for removal.
On towers equipped with liquid headers, to access the nozzles, retractable, flanged headers are suggested. If these headers are plastic and are less than 3-ft long, they can be cantilevered. If they are longer, they typically need to be extended fully across the tower diameter and be retained in a socket or similar support. Why? When the header is pressurized, the reaction force of the liquid ejecting from the nozzle tends to push the header upward. When unpressurized, the header tends to sag. The end support reduces both effects.
If caustic is used in packed towers, it should be thoroughly mixed. One way to do this is to inject it into the liquid recirculation circuit ahead of an inline mixer. Another way is to take part of the recycle liquid and divert it into a submerged sparger located in the scrubber sump. The caustic is injected into this sparger and is thoroughly mixed in the sump.
Using a differential pressure gauge or transmitter monitoring the bed pressure drop can reveal the condition of the packed tower. All things being equal, if the pressure drop rises, the bed may be plugging.
Acid washing a scaled-up packed bed can be difficult. It is much like trying to clean both sides of an umbrella by sending liquid down on it. You can possibly clean the upward-facing side, but what about the underside?
The only truly effective method is to totally flood the tower with descaling chemical (usually acid for carbonate scale and caustic for silicate scale). The other method is to remove the packing and wash or replace it. The latter is the most common method.
Care should be taken in packed towers using spray nozzles to provide strainers to remove or trap solids that could plug the nozzles. Some vendors use removable perforated plates that trap solids. Others use single or duplex basket strainers.
Packed towers offer efficient control of soluble gases in environments in which solids plugging, either by solids in the gas stream or by-products of the gas/liquid reaction, is minimal.