Wet Scrubbers are devices that utilize some liquid (most often water) to remove contaminants from an exhaust gas stream. Wet scrubbers are most easily categorized by the type of contaminant that they are designed to remove: particulate or gaseous.

Wet Scrubbers for Particulate Removal

Selecting a device to separate particulate from a gas stream usually is based upon a simple question: What is the lowest cost method of removing the particulate to an allowable emission level? Scrubbers are commonly thought of as being very expensive. This assumption is often incorrect. Scrubbers themselves are very simple devices and are often the least expensive and most reliable device for removal of particulate from a gas stream. The long term capital and operating costs for any separation device is dependent on a number of variables that should be considered to make a proper evaluation. In the case of wet scrubbers the cost analysis is often dependent upon the availability of water and the infrastructure for waste water treatment and/or disposal.

Some of the common process reasons for using scrubbers for particulate separation are;

1. Scrubbers are simple to build and operate. They can be engineered easily for very severe and/or variable operating conditions.
2. The most efficient way to cool high temperature process gases for treatment is by quenching with water. Once this is done, scrubbing with water for particulate removal is often the most economical and operable method for particulate separation.
3. Wet scrubbers may be more compact than other dry devices.
4. Gaseous and particulate contaminants may be collected in the same device.
5. Wet scrubbing may be the safest way of handling potentially explosive, pyrophoric, or combustible dusts.
6. Wet scrubbers can operate reliably on sticky and/or highly hygroscopic materials

How Particulate Scrubbers Work

The primary method by which scrubbers operate is to cause dust particles to attach to liquid droplets or film, then to remove the liquid containing the particulate from the gas stream.

While many particles will grow in mass by absorption of water, the most significant mechanism for particle growth is contact between dust particles and water droplets or a liquid film. This contact occurs by the following basic mechanisms:

1. Inertial Impaction (Fig 1a)- This is the primary mechanism that wet scrubbers for particulate removal using venturis and other “power-contacting theory” devices utilize. Inertial impaction occurs when there is relative velocity between objects. As a faster particle entrained in the gas flow tries to go around a slower or stationary object (e.g. water droplet), it may break out of the flow stream due to its inertia and impact the slower object. This is inertial impaction.
2. Interception (Fig 1b) If the inertia of the faster particle is not great enough to directly impact it upon the slower one it still may contact it if the flow stream that the particle is traveling within passes close enough to the slower particle. This is Interception.
3. Diffusion (Fig. 1c)- Largely an effect of Brownian motion of very small particles ( less than .3 micron with SG=1), this mechanism provides higher collection than would otherwise be expected due to the Inertial Impaction and Interception. Diffusion causes these very small particles to contact larger water droplets.
4. Condensation- When condensation occurs, it first does so using any available dust particles as nuclei of condensation. Although, not exactly a “contacting” method, Condensation can be used to form water droplets around the dust particles. While droplets formed by condensation are relatively small, they are larger than the particles that form their nucleus.

Inertia-

There are two primary methods for contacting liquid droplets with particles. The first method uses a device that takes advantage of inertial impaction. These devices are typically spray towers or chambers, wet cyclones, or venturis (in order of increasing effectiveness). All of these devices utilize acceleration of the flow stream to cause inertial impaction and follow the power-contacting theory. The power-contacting theory in simple terms states that the amount of power utilized in contacting the particles is proportional to the contact efficiency.

In figure (2) we show a diagram of a conventional venturi contacting device. In the contraction portion of the device, the gas and droplet mixture accelerate as the cross sectional area is reduced. In this section the fine dust particles accelerate more rapidly than the larger water droplets creating a relative velocity between the two. In the throat section, the the particles and liquid droplets are further mixed and allowed to reach a maximum velocity. In the expansion or divergence section, the larger particles remain at a high velocity while the small un-collected dust particles slow with the gas flow as the cross sectional area increases. The success with which we contact dust particles with liquid droplets is a function of the power consumed across the venturi as measured by pressure drop plus the power consumed in injecting the liquid usually as measured as pump horse power. Once the relatively small dust particles are combined with larger water droplets we may easily remove the large dirty water droplets with inertial separators (cyclones) or with other entrainment separation devices.

Interception-

The second method for scrubbing particulate uses interception as the mechanism by passing the particle laden gas pass through a structure or media that is wetted with the scrubbing liquid. While not as common as venturi and wet wall cyclones, these devices can be particularly effective for the collection of soluble particulate with high efficiency with low power consumption. Figure (xxxx) shows a cross flow Interceptor Series Scrubber. When utilizing interception as the primary mechanism for scrubbing, the amount of surface area of liquid film, as well as the gas path, affects the collection efficiency. Collection efficiency can be increased by increasing the number of interception sites and/or the surface area of liquid film available for contact.

Interception devices are rarely utilized for insoluble particulate due to the probability of plugging or fouling within the media.

Particulate Scrubber Series:

Impactor Series

Utilizing inertial impaction, the Impactor Series of HEC Scrubbers include our fixed and adjustable inlet venturi scrubbers and cyclonic separators.

Nomenclature:

HPS-Size- Style-Tank

Size- Primary separator diameter in inches x 10 (e.g. 72” i.d.- Size 720)

Style- FV = fixed venturi/throat , AV = adjustable venturi

Tank- E= external or none, I= integral recycle tank

Available Features:

1. Blind Injection: Problems often occur in scrubbers at the point at which the particle laden gas stream comes in contact with wet surfaces. This is called the wet/dry zone. In this area, solids may build up because they are damp and sticky but there is not enough liquid flow to wash them away. HEC’s “blind injection” virtually eliminate the wet/dry zone and prevent solids build up.
2. Injection Tubes: Water may be introduced into a scrubber in a number of ways. Conventional spray nozzles typically provide the easiest and least expensive method of liquid distribution but often result maintenance problems due to nozzle plugging and erosion. Utilizing HEC’s unique understanding of how droplets are generated our engineers are able to provide low pressure liquid injection utilizing simple injection tubes (or pipe) without any loss in collection efficiency. By utilizing low pressure injection tubes we achieve the following benefits:
   • Reduced pump pressure which means lower pump cost, less pump horsepower, and more reliable pump operation
   • No nozzles to plug or erode away. One of the most common causes of a loss of collection efficiency in scrubbers is poor liquid distribution to the contactor as a result of nozzle pluggage or nozzle erosion. HEC eliminates this problem with its injection tube option.
   • Higher solids recirculation means lower bleed rates and reduced operating costs. By using low pressure, large diameter injection tubes, we can run our liquid recirculation systems at higher solids concentrations than our competitors. This means more concentrated and less liquid volume for secondary waste treatment.
3. Enhanced Cyclonic Separator design: HEC cyclonic separators provide the lowest liquid carry over, often exceeding 99.99% droplet removal.

Interceptor Series

Utilizing interception, HEC’s Interceptor Series cross flow scrubbers provide superior collection of soluble particulate at low power consumption with one or more stages of Kimre® media arranged perpendicular to the gas flow.