Air Pollution Control Innovations

Liliana Chen

Recent Posts

Improving Entrainment Separator Design

Posted by Liliana Chen on Tue, Mar 26, 2013 @ 08:38 AM


Venturi scrubbers are used to remove particulate from the exhaust gas of industrial sources.  They are highly efficient at removing particulate 1-micron in size and larger. Venturi scrubbers are used in solid waste incineration, waste-to-energy production, mining, biosolids sludge processing, plastics production and coal gasification.  In many of these applications, the Venturi is used on the back end of a dryer or thermal destruction device.
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In accordance with Bernoulli's equation, inlet gas accelerates at the converging section, increasing gas-liquid contact. As water is injected perpendicular to the gas flow, the accelerated gas particles are captured by water droplets upon collision. The resulting droplets aggregate through the diverging section and are separated from the process gas by the mist eliminator (ME) in the entrainment separator (ES). 

The ability of the mist eliminator to remove water droplets from the gas stream can have a significant impact on the scrubber performance. Any water droplets that "escape" the ME will carry entrained particulate which can foul a stack test and increase the measured outlet emissions. The amount of pressure drop consumed by the ME can impact scrubber performance.   Minimizing ME pressure drop allows a higher pressure drop across the Venturi  which increases the particulate capture efficiency in the Venturi throat.

The mist eliminator efficiency is heavily impacted by velocity.  Therefore, it is critical to achieve even flow distribution before reaching the ME. The ability to distribute the flow uniformly will allow the system to operate more effectively under a wider range of inlet gas flow rates.

Flow studies were performed to evaluate the capability of three different designs:

  • Envitech design
  • Design from an ES supplier (referred to as Config. 1)
  • Design from a customer based on past experience (referred to as Config. 2).

All three systems have their respective "plate" design to help improve the flow distribution.

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Differential pressure (∆P) across the mist eliminator is a direct indicator of how well distributed the flow is; the lower the ∆P, the more evenly the flow is distributed. The graph below compares the ∆P across the mist eliminator and the plate respectively for the three designs. The ideal ∆P across ME was obtained at uniform inlet gas flow through the ES. Envitech's design has the lowest ∆Ps, reflecting that the plate distributes the flow most effectively and as a result the ∆P across ME is the closest to ideal.

To provide a visual illustration, the cut plots below were obtained from flow studies showing the velocity(y) distribution prior to the plate, before and after mist eliminator. A zone with high velocity (red) was observed in all designs before entering the plate. The plate breaks up the hot spot and re-distributes the flow. The flow pattern before and after the ME for the Envitech design is the most homogenous among the three which is consistent with the pressure drop results.

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The flow studies enabled the Envitech Venturi Scrubber to be optimized three ways as follows:

  • Reduced the material cost wiwthout compromising scrubber ability to agglomerate and remove particles.
  • Refined the flow distribution to expand the process window in the entrainment separator.
  • Reduced the pressure drop across the ES which allows a higher pressure drop across the Venturi throat.

To read more about this particular application, please download the case study below.

Download Case Study

Topics: particulate control, Venturi scrubbers, Scrubbers

Wet Electrostatic Precipitator Insulator Design

Posted by Liliana Chen on Mon, Mar 18, 2013 @ 06:00 AM

The purpose of a wet electrostatic precipitator (WESP) is to remove particulates that are smaller than one micron from a gas stream.  One of the common industrial applications is downstream of standard pollution control equipment on power plants or pulp mill facilities. Power plants typically use wet electrostatic precipitators after all other pollution control equipment for polishing.  Pellet mill facilities use wet electrostatic precipitators upstream of RTOs to prevent buildup on the RTO media. Particulate removal is achieved by sending process gas through an array of high voltage electrodes and grounded collectors.  Particles in the gas are charged and then attracted to the collector surface;  clean gas continues up the WESP.

The outlet section of a WESP houses the insulator compartments.  Insulator compartments support a high voltage grid which holds the discharge electrodes in place. This arrangement separates the high voltage grid from the grounded section. The introduction of purge air through the insulator compartment reduces dust and moisture buildup on the walls of the compartments. This not only keeps the insulator bushing clean but also maintains a positive air pressure to prevent process gases from entering the compartment. 


The diagram above shows how the WESP is connected to the insultator compartment with a shroud. Operating the system at minimum purge air flow helps reduce operating cost as less energy will be required to heat up the purge air. The caveat here is that inadequate flow will result in wet process gases swirling into the insulator compartment through the shroud from the WESP. In an attempt to optimize our operating conditions, flow simulation was performed to analyze the flow pattern of purge air through the shroud at varying inlet flow rates.

The close-up view of the shroud shown on the bottom was taken facing the insulator compartment with WESP at the back. The green/blue color indicates air flow towards the WESP (into the page), while the red/yellow represents flow towards the insulator (out of the page). This illustrates a positive air flow into the WESP achieved at minimal flow rate. Under this operating condition, energy savings will be maximized while ensuring no swirling of wet process gas into the insulator compartment. 



To see how this modeling is applied, download our case study on treating a lead fume using a wet electrostatic precipitator.

Download Paper

Topics: wet electrostatic precipitators