Question 5.28: The lime/limestone FGD process has been previously discussed...

1. First-generation Venturi scrubbers are presented in Fig. 18. Such systems typically used a fixed-throat design for the Venturi. As such, the throat opening used to form the Venturi remains fixed in these designs. Industry preference to use a single Venturi scrubber to process variable gas flow rates led to designs of Venturi scrubbers with adjustable throat openings. Examples of variable-throat Venturi scrubbers are presented in Fig. 18b–h.

Venturi scrubbers typically have a pressure drop of 10 to as high as 30 in. of water. As such, the Venturi scrubber is normally classified as a high-power-consumption unit operation. In addition to having a high energy demand, the choice of the Venturi scrubber is also limited by polluted gas–absorbent (the slurry) contact time within the tower.

2. Typical spray towers are presented in Fig. 19. In the spray tower, the absorbent (slurry) is injected into the polluted gas stream being treated through atomizing nozzles. The slurry is forced into a mist of fine microdroplets by the action of the nozzles. Droplet formation is also supported by the velocity of the gas being treated within the tower. The resultant extremely high surface area of the many droplets provides. for excellent contact between gas and liquid surfaces. In normal operations, slurry droplets are formed with diameters of 50–4000 mm.

3. In addition to promoting excellent gas–liquid contact, a spray tower accomplishes this with minimal pressure loss. This is a result of the fact that the spay tower has no internal components that will impede the upward flow of air as the slurry droplets pass downward through the tower countercurrent to the gas flow, as seen in Fig. 19a. This simple design allows for spray towers to operate with pressure losses in the range of 1–4 in. of water.

Spray towers are also sometimes designed using a crosscurrent flow scheme as presented in Fig. 19b. This design may be chosen over the countercurrent design as the result of height restrictions or other concerns regarding a vertical tower. As the result of the lower height, the slurry pump size will be reduced somewhat. A cross-flow tower will always require increased spatial area than a vertical tower. The need to have a larger tower fabricated will result in increased capital expense for a cross-flow tower versus a countercurrent flow spray tower.

4. A tray tower will always utilize the classic countercurrent flow scheme. As the name implies, the tray tower has one or more internal trays. These trays have openings to a certain open area per the tower’s design. As polluted gas being treated enters the
tower, the gas passes upward through the tower. The slurry liquid (the absorbent) is introduced into the top of the tower and flows downward. Therefore, the slurry flows across each tray and the liquid finds its way to the tray openings. As a result, the two opposing flows are forced to interact, with resultant gas–liquid surface contacts that allow for the pollutant present in the gas to absorb into the liquid, as seen in Fig. 20.

Tray towers known as sieve towers utilize a design gas velocity sufficient to force the gas to form bubbles as the gas passes through the tray openings. Figure 20 illustrates this method of forcing gas–liquid contacts. An alternate design for tray towers is the valve tray tower. These towers use a “bubble cap” on each tray opening. Each bubble cap is also surrounded with a cage intended to constrain the flow of liquid (see Fig. 20). As the polluted gas flows upward through the tray openings, these caps keep the downward flowing slurry in an agitated condition. This forces the gas to exit each cap at near Venturi scrubber velocity. Tray towers typically operate at a pressure drop below that of a Venturi scrubber but well above the pressure drop of a spray tower. A typical pressure loss for a tray tower is about 20 in. of water. The power consumption of such towers is therefore significant.

5. Most packed towers are of vertical design so as to utilize countercurrent flow between the gas and liquid (see Fig. 21). Inside the tower is a packed bed. The packing that comprises the packed bed is in the tower to force increased gas–liquid contact to improve absorption efficiency. Packings of a wide variety of shapes, sizes, and material of construction are available. Additionally, packings can form several structures. A fixed structure such as the honeycomb packing seen in Fig. 21a is possible. Also, a random yet fixed structure such as the glass spheres presented in Fig. 21b may be used. The packing may also be mobile, as illustrated in Fig. 21c. The lass spheres become fluidized with sufficient gas velocity through the tower. In normal operation of the fluidized-bed scrubber, the packing actually passes out of the tower, where it is normally collected for reuse. Finally, rods, decks, vanes, or some other fixed structure may be used inside the tower as in Fig. 21e,f. As such, in this last choice, there is actually no “packing” per se; the rods are used to force gas–liquid contact.

When properly designed, packed towers do not need high power. Packed towers are normally designed for pressure losses that overlap or are slightly higher than with tray towers. Apacked (wet scrubber) will normally operate with pressure drop in the range of 2–8 in. of water.

A combination tower, as implied by the name, is the use of two or possibly more of these absorption techniques in a single tower. As such, the combination tower will allow for targeted pollutant removal (absorption) and/or operational flexibilities not possible with a tower that utilizes only one absorption technique. In the combination tower, discrete chemical and physical conditions are possible in different sections of the tower. Thus, one unit installation may be used to accomplish multiple goals. A combination tower will obviously be larger than a single absorption technique tower. Therefore, initial capital costs will be greater for a combination tower versus the single absorption technique tower. However, the costs of the single tower may be favorable when compared to the costs of two individual absorption towers. Combination towers that have been successfully used in industry are spray/Venturi and spray/packed tower combinations.