Table of Contents:
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:
To optimize the performance, some techniques can be applied:
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%.