- Understanding Tower Layouts
- What Are Tower Internals?
- How to Select Tower Internals Based on Pressure and Applications
The internal components directly impact the efficiency of a tower, and different applications require specialized internals. Use this tower internal selection guide to learn more about the components in a tower and how to choose the best ones for specific applications.
Understanding Tower Layouts
While knowing what tower internals do is important to understanding the operation of the tower, they only make up a small part of the tower. Seeing how the internals fit into the overall tower layout can make understanding the system easier.
First, the operation of the tower is to intermingle liquid and gas. By maximizing the surface area through a variety of packing media and distribution channels, the tower can increase the surface area of the liquid that contacts the gas. While the internals play critical roles in the even dissemination of the liquid throughout the gas, the method of adding the liquid to the top plays an equally important part in the process as the choice of internal packing.
Inside the tower, packings or trays, supports, and distributors ensure the even flow of liquid and gas. Through mass transfer between liquids and gases, towers may conduct absorption or distillation tasks. Multiple industries use these structures, such as water treatment, wastewater processing, chemical processing and scrubbing pollution from gaseous outputs from plants.
The general layout of a tower includes an inlet at the top for the liquid and an inlet at the bottom for the gas. The gas moves up through a distributor and the packing material to intermingle with the liquid. The liquid sprays down from a distributor in the top over packing material. Supports hold up each layer of packing media. A liquid redistributor in the middle prevents any liquid from collecting along the walls of the tower.
The distribution of these masses ensures the efficiency of the tower. The treated liquid flows out of the bottom of the tower, and the gas that has picked up volatile organic compounds (VOCs) from the water discharges from the top.
Several factors influence tower design, such as:
- Type, depth and size of packing media
- Rate of surface loading
- The ratio of the volume of air to water
The tower itself should have a construction that does not react with the materials inside. Therefore, common materials include plastic with glass reinforcement, steel with a rubber lining or polyethylene. Towers have a cylindrical shape to allow for the even distribution of liquid from the top.
What Are Tower Internals?
Each of the tower internals works to aid in evenly distributing liquid and gas to allow for the maximum mass transfer. Each component plays a vital role and must match the needs of the pressure, liquids and gases it works with. The wrong parts could contribute to tower problems, such as foaming and fouling. The following are common tower internals necessary for efficient operation:
1. Liquid Distributor
The critical liquid distributor must spread an even mist of liquid over the packing media. Clogs, uneven pressure and other problems that prevent the most widespread dispersion of the liquid will impact the efficiency of the entire tower. Liquid distributors come in two main forms operated by gravity or pressure. The force behind the liquid indicates which category the distributor falls into.
Gravity distributors use the force of gravity to spread a uniform layer of liquid into the tower. While these can handle larger quantities of liquid than pressure types, they don’t have as many outlet points as pressure liquid distributors.
Types of gravity distributors include trough, drip tube and pan. To use a trough distributor, first, pour the liquid into a parting box. From this box, the liquid will flow evenly into the troughs and out of the bottom of the distributor. This model works well with high-volume applications or those that require fouling prevention.
Drip tube and pan are both closely related types of gravity distributors. Both have a design that allows gas to move up through the distributor via stepped openings of varying heights. Orifices or drip tubes allow the liquid to pass through. For fouling applications, drip tube designs work better than pan distributors.
Pressure liquid distributors are more cost-effective than gravity models and use pressure to force the liquid through the openings. These permit more vapor to flow up into the distributor. However, they may corrode more than gravity models.
Pressure liquid distributors include ladder-type and spray nozzle. Ladder-type distributors feature branching pipes arranged at 90-degree angles from the source. Openings in the branches allow the liquid to flow out. While ladder-type pipes accommodate large or medium volumes of liquid, their openings can clog in fouling systems or when liquids have solids dissolved in them. Spray nozzle distributors best send smaller volumes of liquid over short beds for heat transfer or refineries. These distributors do not work well for distillation use.
2. Supporting Grid
The supporting grid can handle either random or structured packing materials. These grids must hold up the packing components without letting any fall through while permitting adequate gas and liquid flow through them. When using support grids with random packing, the grids need bars on them to attach to the support rings.
Well-made supporting grids will ensure a maximized open area, offer adequate support for packing beds of any height and ensure high performance of the tower. Support grids have a variety of materials for construction, including fiberglass reinforced plastics (FRP), metal or plastic.
In applications with low or medium gas flow, packing support grids may also double as hold-down grids.
3. Hold-Down Grid
A hold-down grid, also known as a bed limiter, sits atop the packing material to keep it level and in place. For structured packing, movement of the tower or flooding could displace the packing. However, a bed limiter atop the packing prevents the material from moving.
For random packing, a hold-down grid performs an even more critical task of preventing fluidizing of the top of the bed in case of a flood in the tower. The hold-down grid ensures more control over the system, even if a flood happens. This grid distributes the pressure more evenly across the bed, reducing the impact of random packing floods or other incidents.
These grids will either sit atop the packing bed material or install on the walls of the tower.
4. Liquid Redistributor
Not all towers require the use of liquid redistributors. However, for applications that have multiple liquid feeds, need side draws of liquid or require better mixing of unalike substances, a liquid redistributor will help. These devices sit under a bed and collect liquid and redistribute it evenly into the bottom portion of the tower. The main types are liquid collectors for draw off and orifice liquid redistributors.
Collectors for draw-off include vapor risers and pipes that can divert the liquid to sumps. Orifice liquid distributors catch the liquid from the bed above, mix it and redistribute it in a uniform pattern through orifices into the bottom of the tower. For optimum liquid distribution throughout the tower and mixing of different fluids, liquid redistributors should pair with distributors. This pair improves the uniformity of the liquid spread and efficiency of the tower.
5. Gas Distributor
A gas distributor sparger delivers a uniform vapor into the tower. To maximize tower height, install one of these at the bottom of the tower. Another option for towers with multiple beds is to install a sparger between the beds. Therefore, fresh vapor mixes better with the liquid and vapor from the bottom of the tower.
Sizing gas distributor spargers depends on the air pressure, gas flow, temperature, compressor strength and vapor exit velocity.
How to Select Tower Internals Based on Pressure and Applications
The tower internals depend heavily on the pressure of the system and the types of packing media. However, these factors are only some of the many considerations when purchasing tower internals. Flow rates of the vapor and liquid also play parts in the choice of internals.
Tower Internals Pressure Types
Pressure variations in towers can be high, low or a significant drop. The internal devices inside the tower must meet the pressure needs of the vapor or liquid flow to avoid impeding performance:
- Low pressure: Low-pressure application of liquids via a spray nozzle may operate better with trays, as this is one of the trays’ two main uses. Structured packing works best when pressure of the system is below 2 atmospheres (atm) and at low liquid rates.
- High pressure: In high-pressure distillation applications, it’s best to use media that function better with higher liquid flows, such as trays or random packings. Excessively high pressures can damage some random packings. Consulting with a design engineer using specific parameters of a high-pressure system may help in finding the ideal packing material.
- Vacuum or pressure drop: In applications that require minimizing pressure drop, use structured packing to reduce the effect. If structured packing will not suffice, use random packing, which has lower pressure drops than trays.
Factors That Influence Tower Internals Selection
The types of media used for packings inside a tower also matter. The contents of the liquids passing through the media and bed height will both factor into the final selection:
- Solids in liquid: For any situation that involves solids passing through the tower in the treated liquids, orifice pan liquid distributors and trough distributors are ideal. They allow solids to settle while allowing liquids to pass through.
- Heavy gas or liquid flows: Trough distributors are ideal for applications that require mass transfer with heavy gas or liquid flows. Thanks to the tapered-notch design, turndown characteristics are good.
- Wastewater treatment: For wastewater treatment, packing material that enhances the growth of beneficial bacteria will improve the processing of the wastewater. For instance, MACH Engineering Bio-Media random packings have a design and material that encourages protozoa and fungi that will decompose organic matter in the wastewater. MACH Cascade Bio-Media packings have a shape that allows solids to pass through, reducing fouling.
- Corrosive materials: Acids and other corrosive materials in the liquid or vapor require resistant materials that do not break down. Ceramic materials are inert and can resist damage from acids. However, some metals, such as stainless steel, may suffice for some high salt, alkaline or acidic operations.
- Bed heights: For packed towers that need lower bed heights to achieve the same results, orifice pan distributors with drip tubes can provide higher turndown ratios without increasing riser height.
- Multiple feeds and products: Liquid redistributors can accommodate towers that must handle the mixing of dissimilar liquids or multiple feeds. For cross mixing of liquids, improper distribution of the material increases greatly when not using liquid redistributors.
Request a Quote From MACH Engineering for Your Tower Internals
Get the components to improve the mass transfer efficiency inside a tower with the right internals for the application. You can find these internals and other components at MACH Engineering. While we distribute products at affordable prices, we also assist in custom designs.
With more than a quarter of a century of experience in tower engineering and mass transfer technology, we can help you select the ideal tower internals for your application. If you need assistance with how to select tower internals, let us know. You can also contact us today to request a quote for high-quality tower packing materials and other tower components if you already know what you need.