A common characteristic of this type of scrubber is the use of spray nozzles to extend the liquid surface and produce target droplets.
At least one spray zone is produced in a spray tower using at least one spray nozzle in a containing vessel. In practice, however, most spray towers use multiple spray zones to achieve the required gas cleaning efficiency. Figure 14.1 shows a sectional view of a spray tower. The gas inlet is typically
Spray tower sectional view.
horizontally oriented into the containing vessel. A multiplicity of spray zones is used, each containing an array of nozzles. In FGD applications, these nozzles are wear-resistant designs (such as silicon carbide) since the scrubbing liquid is an abrasive slurry of lime or limestone.
The hydraulic pressure applied to the liquid acts as stored energy. When this pressurized liquid flashes from the spray nozzle, the energy stored is expended in producing a spray. The high relative velocity between the liquid and surrounding gas causes a shearing action that breaks the liquid into tiny droplets. The net effect is that the liquid surface area increases so that the contaminant gas or gases can be more readily absorbed.
After the spray is produced, the contaminant gas is absorbed through the liquid film. If a reactive chemical is contained in the droplet, the contaminant will react, forming a by-product (usually a salt) of lower vapor pressure. Therefore, the contaminant remains in the droplet.
Most droplets fall by gravity in counterflow designs to the sump. Quite often, the scrubber is mounted directly over the sump to facilitate this separation. A small portion of the spray goes overhead with the gas. This droplet dispersion is controlled using chevron-type droplet eliminator(s) in the case of a gas stream containing particulate, or mesh pads if the gas stream is low in or devoid of particulate. The chevron droplet eliminators are often sprayed constantly from below and on a timer basis from above for cleaning purposes.
Figure 14.2 shows a common application where a utility FGD spray tower is installed after a dry precipitator used for particulate control.
Utility FGD system (Babcock & Wilcox Co.).
Preformed spray scrubber (Bionomic Industries, Inc.).
With the preformed spray scrubber, the spray nozzles are generally installed in the gas inlet area of essentially a cyclonic separator. The spray dispersion is very intense and dense in the inlet zone. The gas is accelerated as the gas approaches the tangent point of the separator vessel. This action enhances particulate capture. The droplets are then spun from the gas stream using centrifugal force. Figure 14.3 shows a sketch of a preformed spray scrubber in elevation and plan view. Note how the gas inlet curves around the cylindrical separator vessel. This curved portion is called an involute and may extend from 90° to 270° of vessel circumference. Note also that the sprays are mounted on individual headers on the involute for simplified access. These headers usually are connected to a distribution pipe by hoses and are isolated by valves so that individual headers may be removed for servicing.
A preformed spray scrubber was used on the superphosphate fertilizer multistage scrubber application referenced in previous chapters. It forms the base of the stack as shown in the center of Figure 14.4. It was used to remove residual fluoride compounds and to concentrate the fluosilicic acid prior to
Preformed spray scrubber on superphosphate dryer (Bionomic Industries, Inc.).
the solution being sent to the filament/mesh pad scrubber (to the left) for further concentration. The fluosilicic acid recovery tanks are to the right in the picture.
Primary Mechanisms Used
The primary scrubbing mechanism used in a spray tower is absorption. To some extent, diffusion is in play as the contaminant gas moves toward the droplet surface. The droplets themselves can remove some particulate by impaction; however, the relative velocity between the gas and liquid is low (usually below 20-40 ft/s), so impaction is minor.
Spray scrubbers using cyclonic action do apply impaction and interception forces to the gas stream and therefore exhibit higher particulate removal rates.
Gas inlet velocities of spray towers are in the range of 50 to 60 ft/s as is common with other wet scrubbing systems. Sometimes, for gas distribution purposes, the gas is conveyed to the scrubber at this velocity to keep particulate entrained but is reduced to 40-45 ft/s at the scrubber itself.
Countercurrent spray towers normally operate at vertical gas velocities of 8-10 ft/s; however, in recent years, efforts have been made to operate them at up to 15 ft/s. At approximately 15-16 ft/s gas velocity, the descending spray tends to be held up or fluidize. At this point, the spray tower begins to transition to a fluidized bed scrubber. The spray nozzle method of liquid injection becomes of diminishing importance as the gas velocity rises since the spray is created by the ascending gas at these speeds.
The chevron zones of these designs usually use a face (open vessel) velocity of approximately 10-12 ft/s. Interface trays, much like weeping sieve trays, are used in some designs to suppress liquid carryover and isolate the dilute wash water spray that is applied to clean the chevrons.
If a top-mounted stack is used, the gas outlet velocity will be often under 45 ft/s to reduce the chance of entrainment. Speeds of 35-45 ft/s are common.
For FGD systems, the pH (and sometimes the density) of the scrubbing solution is controlled to operate within a window bounded by efficiency and scaling. For limestone slurry scrubbing, this results in a pH range of approximately 5.6-6.5 in the slurry and 5.4-6.2 in the sump. For lime, the slurry is approximately pH 7-8 going into the absorber and 5-5.5 in the sump.
Nozzle pressures of 30-60 psig are used, depending on the application.
Given that the liquid surface area of a spray decreases as the distance from the nozzle increases, high liquid-to-gas (L/G) ratios are used, that is, using multiple nozzles, to maintain the net surface area at a sufficiently high level. As a result, it is not uncommon to see L/G ratios of 50-100 gpm/1000 acfm treated being used in these designs. The pumping cost, therefore, becomes a significant design factor.
Given the open area of the vessel, however, the gas side pressure drop is quite low. Spray towers operate at pressure drops of only 1-3 inches water column. This keeps the fan horsepower low. This factor is significant for high gas volume applications.
Spray towers of more than 30 ft diameter have been built. The simple vessel design allows these large-diameter vessels to be made. For high gas volumes, multiple towers are used in parallel. On utility boiler systems, redundancy is often built-in by having the capability to switch between operating and standby vessels using suitable isolation dampers.
Over the past 40 years, operators of spray towers have developed specific methods for the best operation of these devices.
Some basic techniques include separation of solids that would be sufficiently large to plug the spray nozzles. Settling tanks and liquid cyclones are often used to separate the large solids. The nozzles themselves are designed for high solids throughput and wear resistance. Often of full cone design, the nozzles are arranged in patterns that cover the vessel but reduce zones where agglomeration of droplets (resulting in an undesirable reduction of surface area per unit volume) can occur. Multiple spray levels increase the probability that all zones are covered.
Once absorption occurs, the chemical reaction kinetics in the liquid may be slow. In FGD systems, the scrubbing liquid is often impounded in an agitated tank to allow crystal formation and settling. Residual crystals can recycle to help scour the scrubber interior and reduce hard scaling. Sometimes, chemical additives (such as adipic acid) are administered to improve the scrubbing performance. Oxygen is sometimes injected to oxidize the sulfite component of the scrubbing solution to sulfate so that the sulfate may be more easily settled and removed.
For materials of construction, the vessels are often mild steel with rubber lining for utility FGD application. If chlorides are present, alloys such as 904L, AL6XN, C-22, or C-276 are used.
Chevrons in many FGD designs are installed in stages given the high droplet loading. A coarse stage of widely spaced blades is used followed by more narrowly spaced chevrons in either vertical flow or horizontal flow configuration. Figure 14.5 shows a chevron set using multiple design configurations to arrest the spray.
Multiple chevron stages (Munters Corp.l.
Spray scrubbers have been made in a wide variety of materials, from carbon steel, to rubber-lined steel, to fiberglass-reinforced plastic, to exotic alloys. Some designs have even received food-grade interior polishing to handle explosive-type material. Preformed spray scrubbers usually are equipped with retractable spray headers and individual shut-off valves for nozzle servicing. Obviously, one must plan for sufficient pull space to remove such headers.
If large solids are anticipated that could plug the nozzles, strainers on the recirculation loop should be used. It is also advised to locate any vessel access door such that the worker can gain entrance to the scrubber easily. A common location is directly over the separator inlet duct.
Preformed spray scrubbers perform like Venturi scrubbers operating at 6-10 inches water column pressure drop. This means they are best suited for the control of particulate above 10 pm aerodynamic diameter. For gas absorption, an inlet spray type unit can achieve about 0.8-1.5 transfer units of separation. Ones with wall-mounted sprays can often achieve higher mass transfer rates but are more likely to entrain droplets.
Spray towers and spray scrubbers are popular devices for use in gas absorption applications and, in the case of preformed spray scrubbers, for particulate control on particles more than 10 pm diameter.