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Spray Tower Optimization

Spray towers basically disperse by using pressurized spray a high surface area liquid stream counterflow (primarily) into a gas stream containing contaminants and/or heat. The purpose of the design is to efficiently affect the transfer of mass and/or heat into the liquid. The liquid is then constantly drained from the spray tower. The word "primarily" is used because some of the liquid is inherently carried upward in the vessel (sometimes called "back-mixing") given the movement of the gases. Unless the contaminant is absorbed quickly and is rapidly neutralized (retained in solution by being converted to a low partial pressure salt or oxide), movement of the liquid upward can reduce the net efficiency of the spray tower. The optimization goal is therefore to minimize the amount of liquid moving up with the gas stream.

The spray tower shown in Figure 31.1 (Bionomic Industries) was installed on a power boiler.

Possible Problems

Some possible problems are given below:

  • 1. Excessive vessel vertical velocity thus aggravating back-mixing
  • 2. Improper liquid distribution
  • 3. Single zone application of chemicals, if used, is insufficient
  • 4. Local spray nozzle wear
  • 5. Poor droplet control.

Optimization Techniques

Some optimization techniques to consider are given below:

1. Ensure the proper vertical velocity of the gases in the tower to minimize back-mixing

FIGURE 31.1

Spray Tower, Bionomic Industries.

  • 2. Provide a uniform dispersion of the liquid across the entire vessel
  • 3. Use "staging," if needed
  • 4. Maintain the integrity of any spray nozzles
  • 5. Control any droplet carryover that could reduce the tested performance.

Vertical Gas Velocity

Regarding the vertical gas velocity, spray towers can operate as low as about 4-5 ft/s vertical velocity (based upon the actual gas volume being moved) up to 10-15 ft/s. In general, if the designer uses high-pressure spray nozzles that generate exceptionally fine droplets, the vessel velocity is often kept in the low rate to minimizing back-mixing. Conversely, if large droplets were applied using lower pressure nozzles, the vertical velocity chosen would be in the higher range. Obviously, much of the cost of the scrubber is in the vessel thus the designer needs to balance the requirement of adequate performance versus the cost. Spray towers thus tend to be designed in the higher gas velocity range. What can occur is that to achieve greater performance, smaller droplet nozzles are used. What can occur is that doing so actually reduces the performance since the high-pressure nozzles produce smaller droplets that are carried upward. Often a better optimization approach is to increase the liquid to gas ratio (more liquid) but while using nozzles that do not generate fine residual spray. Contacting the nozzle supplier may allow a switch to a different nozzle design (though a pump change may be needed). One must keep the vertical gas velocity within the range as required by the designer. It is usually better to "tweak" the nozzle selection.

Spray Nozzles and Location

The amount of the spray and the spray's uniform location in the tower are the key to success with a spray tower. The designer typically makes the nozzle location graphically and in doing so there are inherent overlaps. Thus, on a "per unit volume" basis, there will be more sprayed liquid in some areas than in others. Compensation is often made by using multiple spray zones (levels) separated by a distance required to disengage the spray per stage. If the nozzle patterns merge (stage to stage) one can aggravate the situation wherein an excess of liquid occurs in some areas of the tower. To optimize, the spray headers may need to be adjusted to minimize overlap. Some firms provide CFD analysis of such towers. Nozzle vendors may also be able to supply possible location of sprays. In addition, nozzles can often be changed to a different spray angle (rather than changing the header) to alter the coverage. Some stages may use different spray angle nozzles. For example, the upper most spray header may use 120-degree pattern nozzles whereas lower levels may use 90-degree nozzles. As the sprayed liquid descends in the tower, the effect of the nozzle pattern typically becomes less important, but the patterns should be investigated.

When Chemicals Are Used

In applications that use a reactive chemical to neutralize the absorbed gases, many systems simply administer the chemical to the main recycle header. Though that may be enough for many applications, often the performance can be optimized by also injecting the chemical into a lower header. The latter may involve some relatively minor piping changes. What happens is that the pH in the tower can be better controlled. Fewer products of reaction will drain from the upper spray zone and the driving force will likely be improved at the lower spray zones. A disadvantage in a tower equipped with only a few spray zones could be an increase in unreacted chemical in the scrubber sump. What is usually done is that the gross amount of chemical is kept the same. If the operation reveals greater unreacted chemical in the sump, the amount of chemical going to the lower grid is trimmed back until the system is in balance.

Spray Nozzle Wear

Spray nozzle wear is a common problem with spray towers but is controlled through proper monitoring of the nozzle performance. If the spray header(s) can be monitored for both pressure and flow, one can interpret from the data the condition of the nozzles. A reduction in pressure and increase in flow usually reveal excessive nozzle wear. An increase in header pressure with a decrease in flow usually indicates plugging. If the pressure and flow is data logged, the trend can be a resource for maintenance personnel regarding the nozzles.

Back-Mixing Issues

As mentioned earlier, residual spray upward (back-mixing) can reduce the performance of a spray tower. Some back-mixing is likely to occur, but one must not permit those droplets to exit the tower otherwise, during testing, the solids those droplets contain may be counted as an emission, even though the solids are in a droplet. Thus, the droplet control stage (usually a chevron type) is particularly important, Single stage droplet control chevrons may be optimized by converting to two stages. An "interface" tray such as a weeping sieve tray could possibly be added below the main chevron to help agglomerate the spray (make the droplets larger and thus more easily controlled) or a more open, baffle type chevron could be used below the primary chevron to reduce the gross loading of droplets to the primary chevron. If excessive solids are building up on the chevron, a clean in place (CI)P header configuration could possibly be applied to the tower. If used the CIP header usually receives the cleanest water in the system. If scale is experienced, sometimes the CIP circuit is doped with an acid or other descaling chemical.

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Tray Scrubber (Tray Tower) Optimization

Tray-type scrubbers have been used extensively for gas absorption, gas cooling, and particulate removal. The designs go back to the early 20th century and their characteristics have been well explored over the years. That is not to say that their operation cannot be optimized, however.

Possible Problems

Some problems that may occur are given below:

  • 1. Inconsistent operation
  • 2. Unusual wear or solids build-up on the trays
  • 3. Weeping (when weeping is not desired)
  • 4. Sudden change in pressure drop
  • 5. Decrease in efficiency (particularly if a change in the application has occurred)
  • 6. Solids build up on the underside of the lowest tray
  • 7. Poor droplet separation.

Optimization Possibilities

Some things to investigate given the need to improve the operation are the following:

Mounting the Vessel True Vertically

There is a particularly good reason that the scrubber vendor requires that the scrubber be mounted vertically. By "vertically" they mean as measured at the tray level (not, necessarily at the vessel). Inside the scrubber, the scrubbing liquid is flowing across the tray. That liquid flow is influenced by gravity and must be uniform if the tray is to operate properly. If the tray is not level, inconsistent operation can occur. The liquid depth above certain portions of the tray may be lower than at other portions and therefore the gas flow will take the path of least resistance through that lower resistance zone. Higher gas velocities can reduce the mass transfer at that portion of the tray. A tray that is not level could experience a loss in efficiency given poor liquid distribution. Thus, it is imperative that the trays be level. The vessel, however, may not be "true" vertical, thus the tray "decks" should be checked internally to ensure that the trays are level. The remedy may include shimming the tray or even the entire vessel. Another "remedy"' may include adding an "end weir" on the tray (at the point where the liquid exists the tray) to basically "dam up" some of the liquid to force a uniform distribution of the liquid. One should check with the tray vendor and/or consultant, however, since adding liquid depth will increase the pressure drop across that tray. Usually, any end weir is only about 1" (2.5 cm) tall or less. The pressure drop across a tray is a function of the liquid depth and the frictional losses through the openings (usually perforated holes) through the tray. Thus, increasing the liquid depth can directly increase the tray pressure drop. The tray vendor usually applies empirical correction factors to calculate the overall tray pressure drop. It is therefore best to check with the tray supplier prior to adding any end weirs.

 
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