Improving Odor Control Scrubber Performance

“Smells like money” is what you often hear as you walk into a rendering plant, normally accompanied by the speaker taking a deep breath. To the uninitiated, the smell is nothing like the paper bills in your wallet. It is a deeper richer smell - much like the inside of a pet food bag. More than the smell of "money," it is the smell of recycling and sustainability.

The rendering industry is the original recycler, transforming waste materials from farmers, restaurants, food producers, etc. into products for pharmaceuticals, cosmetics, animal feed, renewable fuels, and more. The process of converting this material into proteins and oils is accompanied by vapors that create odors, environmental concerns, health hazards, and damage to equipment and facility infrastructure. Odor control programs provide treatment for these vapors, helping renderers to:

  • Be good neighbors
  • Increase cooker throughputs
  • Protect the health and improve the happiness of employees
  • Maintain environmental compliance
  • Prevent corrosion and damage to equipment

To achieve all these benefits, an odor control program utilizes a combination of equipment and chemistry.

Odor Control Equipment

Collection System
Odor control equipment starts at the sources with carefully designed ductwork and collection points. This network of ducts and openings needs to be engineered and balanced to ensure the correct volume of vapors is removed from each point in the process. The seeming simplicity of these giant ducts often leads them to be underestimated, but they are important to protect employees' health from toxic levels of VOCs, hydrogen sulfide, and ammonia. The proper collection of vapors also prevents their damaging effects on buildings, paint, electronics, and other equipment. Collection points can include:

  • Product Receiving Bay
  • Cookers
  • Milling and Grinding
  • Conveyors
  • Presses

Pretreatment
After vapors are collected, they may require pretreatment which is typically used with vapors that are highly concentrated and/or contain excessive heat. They can include:

  • Cooker Entrainment Traps: prevent valuable proteins & oils from leaving the cooker
  • Air Condensers: utilize air cooling to remove heat and condenses moisture and solids in the vapors
  • Heat Exchangers: provide similar benefits to air condensers utilizing water instead of air while generating 160°F water for use throughout the facility.

Maintaining and operating pretreatment units is important to prevent problems that result in a domino effect reducing the performance of downstream equipment. For example, pulling vapors too quickly through a cooker entrainment trap may result in plugging and fouling of the condenser or heat exchanger and the loss of yields in the cooker itself.  Dirty or plugged condensers and heat exchangers cause poor heat transfer which reduces the condensables removed from the vapor streams. This increases deposits in the scrubbers and reduces the airflow and removal efficiency in these units.  This reduction in airflow increases cook times as water vapors are not being removed from the cooker.

Scrubbers
Following pretreatment are the scrubbers. The two main types of scrubbers are Thermal Oxidizers and Wet Scrubbers.

Thermal Oxidizers
Thermal Oxidizers are primarily designed to remove VOCs and can utilize a variety of designs to ultimately oxidize VOC vapors in a combustion chamber using a natural gas or propane burner. To increase the burner efficiency of these units, they are often configured as a Regenerative Thermal Oxidizer (RTO) where heat from the combustion chamber is redirected and stored in ceramic media in the RTO inlet.  This media preheats the incoming vapors reducing the amount of energy needed in the combustion chamber to maintain the minimum operating temperature.

Dirty vapor streams can create issues for RTOs as they can coat the ceramic media resulting in poor airflow and heat transfer.  Over time this can reduce the VOC removal efficiency of the RTO. For this reason, one or more wet scrubbers are often installed in front of the RTO to "clean" up the vapors before they are oxidized.  Maintaining and optimizing the performance of these wet scrubbers is as important as regular cleaning and maintaining the thermal oxidizer itself.

Wet Scrubbers
Wet scrubbers come in a variety of configurations and can include:

  • Venturi: utilizes a constricting throat to briefly accelerate the vapor stream and increase turbulence as it contacts water spray. As the vapors leave the throat, the stream is rapidly decelerated increasing droplet sizes and causing particulate matter and highly soluble gases to fall out.
  • Spray Tower: utilizes a vessel in which the vapor stream enters from the bottom of the vessel and passes through several layers of water spray nozzles before exiting the top of the unit.
  • Packed Bed: utilizes a vessel containing media to increase the turbulence and interaction with the water spray in the unit. The vapor stream enters the bottom of the vessel passing through the media and water spray before exiting the top.
  • Cross Flow: can be similar to Spray Towers or Packed Beds depending on the configuration. In these units, the vapor stream passes horizontally through the water spray chamber.

Maintaining Wet Scrubbers
Whether a wet scrubber is being used in front of a Thermal Oxidizer or as the final treatment step before the vapors are discharged to the atmosphere, maintenance is critical to the performance of the scrubber and plant.  Regular maintenance will ensure proper air and water flow throughout the scrubber.

Airflow is critical to the individual unit, the system, and the plant performance. Scrubbers are designed for a specific range of airflow, often measured in CFMs. If CFMs are too low, cook times will increase and plant throughputs will decrease. If CFMs are too high, contact time will be reduced with the spray water and overall treatment and removal efficiency will decrease. To maintain correct airflow, differential pressure should be tracked and trended for each unit as an indicator of deposits forming in throats, demisters, media, and other parts of a scrubber. Visual inspections are also important to maintaining good airflow. The type of deposit present in a scrubber determines the correct cleaning process.

Wet scrubbers cannot function without water. To prevent the solids and other contaminants from over-concentrating in the recirculating scrubber water, most manufacturers recommend a certain amount of fresh make-up water be added continuously.  Typically a small amount of water is allowed to overflow removing floating material with it.  It is also a good idea to regularly drain water off the bottom of sumps to remove any solids present there.

A low flow rate can result in a poor spray pattern and reduced odor removal. The flow rate of the recirculated spray water should be regularly tracked and trended.  While a pressure gauge and pump curve can work, a flowmeter is recommended to ensure good flow is maintained and help diagnose any problems with spray nozzles and the distribution system.  Visual inspection should also be performed regularly to ensure proper spray patterns and coverage inside the scrubbers.

Chemical Treatment
The ability of wet scrubbers to capture and neutralize odors with water alone is very limited.  This is why most wet scrubbers are operated using a chemical program.  The three main types of chemical programs are pH control, oxidation, and capture programs.

pH
pH control in scrubbers is primarily used to control odors related to ammonia and hydrogen sulfide.  Both of these odor-causing compounds form water soluble ions in certain pH ranges. Controlling the pH allows these compounds to be captured and removed from the air.  While pH can be one component of an odor control program, it often is used in conjunction with an oxidative or capture program.

Chemical Oxidation
When odors cannot be controlled with pH adjustments an oxidation program can be used to improve the wet scrubber's performance. Oxidation programs treat odor-causing compounds by chemically stripping away parts of the molecule changing them from odor-causing to odorless compounds. While there are a variety of oxidizing products marketed for odor control it is important to consider:

  • The effectiveness against a particular odor stream
  • The storage or generation requirements
  • The safety regulations and restrictions
  • The application and monitoring of the program

Chemical Capture
As an alternative to an oxidation program, a chemical capture program can also have great success when controlling odors in wet scrubbers.  Chemical capture programs work by increasing the efficiency of the wet scrubber's ability to solubilize odor-causing compounds with water and remove them from the system. As an added benefit, these programs can prevent deposits from forming on the media, reducing costs, labor, and downtime associated with cleaning.  They can also be safer for employees and the environment.

Maintaining the Program
Chem-Aqua's odor control programs lead the industry in terms of effectiveness and flexibility to control odors and deposits. Our multiple product lines allow Chem-Aqua's engineers to design a program tailored to each customer's application. Our trained consultants provide regular service and inspection as a part of the program to ensure it is running at peak efficiency.

For more help with improving odors contact Chem-Aqua today.

Written by: Tim Daniels and Nicholas Valente

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