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Venturi Scrubber Optimization

The moniker "workhorse" certainly applies to the venturi-type wet scrubber for air pollution control. It got its reputation (and application in thousands of wet scrubber applications) given its simplicity, reliability, and low-capital cost. Like its packed tower brethren, its simplicity makes it sometimes difficult to optimize. Optimization can result in some added cost that is therefore in conflict with its reputation for low cost. Also, as mentioned in the chapter on Venturi scrubbers, improving its removal efficiency often results in an increase in power consumption whether that increase occurs at the prime mover (usually a fan) or at the pump.

There are some things to be considered however to optimize and integrate a venturi scrubber with a process. Usually the first thing to do is to check the pressure (or vacuum) capability of the prime mover (fan, educator, etc.) since optimizing the performance of a venturi type scrubber may require some additional energy input (i.e. pressure drop). The goal, however, is to get the best performance at the lowest energy consumption.

A multiple throat venturi and separator is shown in Figure 33.1 in a typical configuration wherein the venturi section (in this case multiple throats) is in one section and the droplet control is in another. In this case, the droplet control section uses chevron type control thus the containing vessel is enlarged to reduce the "face" velocity of the droplet-laden gas stream.

Possible Problems

Some typical problem areas may be as follows:

  • 1. Less than desired fine particulate removal
  • 2. Insufficient turndown with respect to gas volume
  • 3. Unstable draft control (noticeable on draft sensitive applications)
  • 4. Plugging in the liquid circuit (particularly associated with designs that use spray nozzles)
  • 5. Wear
  • 6. Poor absorption of soluble gases

FIGURE 33.1

Venturi scrubber, BACT.

Possible Remedies

Possible remedies to consider include the following:

Preconditioning the Gas Stream

For improving fine particulate removal, "preconditioning" the particulate prior to the venturi throat may help. Preconditioning essentially involves growing the particles so that, given their increased size, they are easier to remove. The "growth" may sometimes be achieved using an air atomized clean water mist or fog as far ahead of the venturi throat as possible. The distance is needed because the particulate merges with the fog in a random fashion so extra time is needed to contact the fog with the particulate. If the liquid added prior to spraying can be converted to an electrolytic solution (for example by adding a halide), the mist or fog can possibly be administered with a static charge (electrostatic spray) that can encourage the fine particulate to combine (agglomerate). A water fog followed by rapid cooling in the venturi (say, using colder recycled liquid) can possibly harness ther- mophoretic forces that tend to move particulate and condensate towards a colder surface. Though these forces are weak, such application may be worth exploring particularly if the gas stream exhibits high humidity (large amounts of water vapor to be utilized). Another possibility is to saturate, and sub cool the gases to induce the water vapor to condense on the fine particulate thus making the particle aerodynamically larger (the so-called “flux force condensation" technique described elsewhere in this book and other literature). Though usually of modest positive effect, surfactants can sometimes be used to "wet" the particulate. The surfactant can be administered by air or hydraulically atomized sprays applied well in advance of the venturi throat. Another possibility, though not common, is that the venturi throat length is insufficient. What can happen is that the liquid in the venturi throat does not reach a point wherein the maximum density of droplets is not achieved. The gases are moving so fast that a "blow hole" is formed in the throat area thus providing a highway through which particulate may pass and escape capture. That zone is usually located at the point where the discharge of the throat zone expands, and the net gas velocity is reduced (sometimes called the "velocity pressure recovery" zone). A possible optimization method is to install a spool piece at the discharge portion of the venturi throat that increases the effective throat length so that the "blow hole" closes within the throat thus providing the needed high droplet density zone. It is beyond the scope of this book to explain the intricacies of the throat length design however the original scrubber vendor and/or consulting engineering firm may be able to optimize the venturi scrubber if the throat length is suspect. If a rectangular, fixed venturi throat exceeds about 8" width and the liquid enters using a "dentist bowl" type (swirling liquid path) approach to the venturi, sometimes simply installing a vertical plate in the center of the throat can improve performance. What happens is the throat is in effect converted to two parallel narrower throats. The swirling liquid is forced to move through the narrower passageway and thus is distributed more uniformly. Also, the wetted surface of the throat is increased thus providing a slower moving film of liquid on the plate to enhance interception of particulate. Given the greater surface, the pressure drop will increase slightly but that is often compensated by a reduction in the gross liquid rate.

Turndown and Pressure Drop Maintenance

For turndown with respect to gas volume, there are a variety of techniques to explore. One common method is to use an adjustable venturi throat if the venturi scrubber was not so configured initially. Usually the original scrubber vendor (or consulting firm) can recommend and supply an adjustable venturi throat that can replace the fixed throat. The adjustable throat could be "manual" in that one must move a lever (either by hand or using an positioner) to change the throat pressure drop or "automatic" wherein a signal such as the pressure drop across the throat is sent to an electric, hydraulic or pneumatic positioner to adjust the pressure drop. Another method involves sending some previously scrubbed gases back to the venturi scrubber inlet. The gas flow is modulated using an opposed blade damper equipped with a positioner. The control signal is usually the pressure drop across the venturi throat (often defined in the permit to operate). If control over a fine range (say less than 10% peak gas volume to minimum gas volume) is required, the liquid recycle rate can sometimes provide the needed control. The liquid passing through the throat zone influences the pressure drop across the throat. The greater the liquid rate, the higher the pressure drop. If a draft signal is available (and the Permit to Operate allows it), the liquid flow rate can be used to fine tune the scrubber pressure drop and process draft. The draft signal is used to actuate a control valve that varies the liquid rate (within permitted bounds) to provide the control. The scrubber vendor should be contacted first however to determine the maximum and minimum recycle rates required to meet their performance guarantee. If a modulating damper and a variation in the liquid rate is used for extreme fine-tuning, the damper is used for "gross" control and the liquid rate is used for "fine" control. The control algorithm is configured to modulate the damper less frequently than the liquid rate. Very precise draft or pressure drop control can be obtained. If the facility building is heated or cooled, and the source is located inside the building, precise draft control indirectly minimizes ambient air infiltration that, perhaps at a cost, needs to be heated or cooled. Some operating costs can potentially be obtained.

Somewhat related to the above, for draft sensitive application like hazardous waste incinerators, it often is best to leave the venturi scrubber "fixed" and just finely control the gas volume. The venturi scrubber hydraulic characteristics usually do not respond linearly to adjustments. Adjustable surfaces rotate or swing in arcs yielding a somewhat sinusoidal response instead. If the draft on the source is monitored, however, cleaned gases can often be recycled back to the scrubber inlet based upon that draft signal. The scrubber in effect "sees" a constant gas volume as part of that recycle loop even though the inlet and outlet gas volume may change. Very precise draft control can be achieved if an opposed blade damper is used to regulate the loop volume. Opposed blade dampers provide a more linear response to control versus parallel blade dampers. To reduce the unnecessary "hunting" motion of the damper blades (excessive modulation), the output signal of the draft or pressure drop transmitter is often dampened by configuring the controller to send output signals to the damper positioner less frequently. Less frequent output signals can also accommodate to some extent the compressibility of the gas stream thus smoothing the gas flow through the system. Some installations also use a variable frequency drive on the prime mover to set the "coarse" volume adjustment and use the opposed blade damper configuration to provide the required “fine" adjustment. There are dozens of installations using this technique. Adjusting the liquid rate (mentioned previously) can provide even finer adjustment for extremely draft sensitive applications.

 
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