Pre-Formed Spray Scrubbers Optimization
Balancing Absorption and Particulate Removal
It is easy to dismiss the pre-formed spray type scrubber as simply a cyclonic separator outfitted with spray nozzles. It is more complicated than that.
This type scrubber has two-design situations that need to coexist. Small droplets are needed to achieve the desired particulate removal and gas absorption while larger droplets are needed to provide adequate liquid separation from the gas stream. As a result, these type scrubbers incorporate two zones distinctive zones of operation. One is the mass transfer zone, and the other is the droplet separation zone. To complicate things further, these zones sometimes overlap.
Pre-formed spray scrubbers vary in size however the primary functional zone is where the nozzles and nozzle assemblies are located. The largest diameter portion of the design is for separation of droplets and for a lesser extent, gas absorption. Figure 28.1 shows a pre-formed scrubber installation.
To optimize mass transfer (as with other wet scrubber designs), the general goal is to provide an adequate surface area of liquid to afford the mass transfer. For economy, that surface area is usually applied at the minimal vessel volume (usually the gas inlet area). To optimize separation, however, the gas and liquid droplets are separated in a larger vessel wherein the distance between the droplets and gas is increased (the droplets are thrown to the vessel wall). Sometimes these requirements are in disharmony. The designer usually separates the problem by focusing on the mass transfer (upfront) zone since performance guarantees usually are met or missed given the operation of that zone. The droplet separation is often relegated to simple centrifugal separation however the performance of the mass transfer zone can have a direct effect on the separation zone. To optimize, these zones must perform in harmony.
Some things can be done however to optimize a pre-formed spray scrubber that is not functioning at the desired level of performance.
Preformed spray scrubber. (Bionomic Ind.)
Some Problems to Consider
Problems may include the following:
- 1. Poor gas absorption
- 2. Poor particulate removal
- 3. Insufficient droplet separation
- 4. Excessive power input.
To optimize the performance, some techniques can be applied:
- 1. As mentioned above, the mass transfer zone determines the gas absorption performance of this type scrubber. The reasons are many. In this zone (at the inlet of the scrubber) the ratio of the surface area of the liquid to that of the gas is greatest. In addition, the "density" of the liquid droplets is also the greatest. This means that the probability of the contaminant gas into the liquid is also the greatest. After the gas stream leaves this zone, the high surface area droplet regime is forced (thrown by centrifugal force to the vessel wall) converted to a low surface area film of liquid. That film of liquid drains to the scrubber sump however its role as a gas absorption media is effectively ended. Thus, to optimize gas absorption, one must endeavor to provide the greatest surface area per unit volume at the mass transfer zone. Obviously, using a finer liquid spray in that zone will achieve that result. The caveat, however, is that if the spray generates droplets that are too small for centrifugal separation, liquid entrainment from the device could occur. The nozzle selection thus becomes critical. To optimize, sometimes the nozzles can be changed to ones that provide a denser spray of liquid but whose residual droplet size is greater than about 200 microns so that those droplets may be separated efficiently. Nozzle suppliers can be consulted regarding possible nozzle substitutions. Also, quite often the nozzles used are "fan spray" types that provide a flat spray pattern. This pattern typically relies on the shearing action of the moving gas to break up the liquid into smaller droplets. An additional header can possibly be added to supplement the fan sprays that use a full cone spray pattern that provides a greater density of smaller droplets. Another improvement could be increasing the residence time of the spray in the mass transfer zone. More contact time can improve the mass transfer particularly if the absorbed gases react with chemicals in the liquid spray. If the scrubber, say, uses a 90-degree gas inlet, it may be possible (though likely not inexpensive) to change the gas inlet to an involute type that extends perhaps 180 degrees around the vessel. A further improvement for gas absorption may be to add wall boxes outfitted with supplemental sprays aimed in the direction of the cyclonic rotation of the gases in the separator. These sprays must have a residual droplet size large enough to be affected by the cyclonic action of the separator (i.e., large residual droplets are needed if wall box sprays are used. In general, one can improve the mass transfer to an extent limited by the residual droplet size of the applied spray.
- 2. For particulate removal, the "standard" pre-formed spray scrubber is configured to have the gas and liquid moving in the same direction. This arrangement is simple and adequate particulate removal can be achieved. As learned in other sections of this book, one wants to impact the particulate into the liquid. Thus, what is really needed is that the target particle be moving faster than the liquid droplet. Though large (above 10-20 microns) particles can be removed using the concurrent method, to boost the removal of finer particulate may require the application of a counter-current spray pattern. Sometimes a full cone spray pattern can be applied "into the wind" to increase the relative velocity between the particle and droplet. This header (or a plurality of headers) might be able to be fitted into the mass transfer zone. If not, perhaps an additional "pre-scrubber" zone can be added ahead of the gas inlet. This technique has been applied successfully to dissolving tank vent scrubbers in the pulp and paper industry. As mentioned above, the caveat is that the residual spray droplet size must be large enough to be cyclonically separated (too fine a spray may not separate). Yet another optimization technique to consider for particulate removal is to, if possible, increase the pressure drop on the scrubber. The best place to increase the pressure drop is often the area just before the gas stream enters the cyclonic separator. The reason is that the droplet density is greatest at that point thus the probability of a particle being impacted into or being intercepted by a droplet is greatest at that point. Assuming the prime mover can overcome an increase in pressure drop, adding a baffle (fixed or movable) at that point can provide the required increase in pressure drop. The original scrubber vendor or perhaps a consulting engineer can provide the approximate open area required to achieve the pressure drop increase.
Insufficient droplet separation often occurs if the interior surfaces of the vessel are too rough and allow for liquid accumulation and entrainment or the incoming droplets are too small to be centrifu- gally separated. As mentioned above, if the applied spray droplet size in the mass transfer zone is too small then those droplets may avoid separation. The remedy if that is the case is to "tune" the mass transfer zone spray back to generate slightly larger droplets (usually the spray droplet size decreases with nozzle pressure). If decreasing the flow does not result in better liquid separation, then a change in nozzle(s) may be required. An additional technique to investigate involves adding a chevron type separator module to the vessel. One must have adequate vessel height for its installation, however, and the face velocity into the chevron must be within the chevron module vendor's requirements (often about 8-10 ft/s) and any swirling of the gases must be stopped. Sometimes the upper part of the cyclonic separator can be increased in height to accommodate the chevron. If a high solids environment is expected, often clean in place headers that spray the module are included as part of the optimization. A further improvement could be the addition of a crossflow separator after the cyclonic separator. These crossflow devices use chevrons (either vertically oriented or inclined on an angle) to separate the entrained droplets from the gas stream. Suppliers such as Munters and Coastal Technologies provide such devices.
3. Excessive power input usually is a result of the scrubber pressure drop. In a pre-formed spray scrubber, the pressure drop primarily occurs as the tangential location where the gas stream enters the cyclonic separator. This pressure drop, however, is required for proper particulate removal and that removal may be part of a permit to operate and/or a performance guarantee. To optimize the pressure, drop, the original scrubber vendor should be contacted to see what pressure drop is the lowest that can be used to meet any contract or compliance requirements. Once that pressure drop is known, the open area at the entrance to the separator can be calculated and that area be adjusted to suit. Also, if excessive liquid is being applied to the device, the pressure drop may, in turn, be excessive. As above, the device vendor should be consulted to determine the lowest acceptable liquid rate. Adjustments to the liquid rate on this type device may provide a pressure drop adjustment of about 5%.