Efficient pollutant removal is critical for the performance of any sized separation column, from scrubbers and air strippers to other water and air pollution control equipment. A tower’s internal components ensure an even flow of liquid and gas to maximize mass transfer and aid with removing pollutants and contaminants from the gas or liquid undergoing purification. The tower internals used directly impact packed tower efficiency and must match the application for peak performance.
In the past several decades, tower internals have undergone significant advancement and come in many constructions designed for different uses. Understanding the effect different tower internals can have on packing performance enables your company to select the best tower internals for its application.
Understanding How Tower Internals Impact Packing Performance
Tower internals are essential components of a packed tower and include liquid and gas distributors, liquid redistributors and supporting and hold-down grids. Each component performs a unique role in the separation process and the tower’s integrity.
Consider the impact of tower internals on packing performance in the following areas:
Liquid-to-Vapor Ratio
Effective liquid and gas distribution within the packed tower is critical for optimal performance. A packed tower’s liquid distributor disperses liquid onto the packing below, using gravity or pumps to propel the liquid through the orifices. Proper functioning of the liquid distributor results in evenly distributed liquid throughout the packed tower. The distribution quality of the liquid refers to the liquid-to-vapor (L/V) ratio inside the tower. This ratio must be ideal for the tower to function as expected.
If one side of the distributor is overloaded while the other is underloaded, a maldistribution of liquid can result. Liquid maldistribution may lead to an unequal L/V ratio, which can create equilibrium pinching. If the maldistribution occurs across the entire packing bed, the efficiency is further reduced, and the tower’s packing performance suffers.
The tower internals used in the separation column impact the L/V ratio. Care should be taken to ensure the liquid distributor operates at the correct rate and not lower than its turndown ratio.
Liquid Distributor Design
The design of the tower internals used is another critical component of a tower’s packing performance. For example, liquid distributors must perform uniform liquid distribution. A liquid distributor’s ability to achieve this function depends on several factors, including its number of drip points compared to the column’s area. This figure indicates its irrigation density.
Another crucial consideration in liquid distributor design is the liquid load required for the separation process. Different liquid distributors have varying liquid loads they can handle. The required liquid load determines the number of outlets needed for the right flow velocity.
Another important liquid distributor design consideration is the placement of the feed points in the liquid distributor. The location of the feed points impacts liquid distribution quality, and distributors with a base hole create the best distribution pattern. Internal distribution hydraulics, distance from the packing bed and the distributor’s orientation to the packing are also essential considerations that impact distributor efficiency.
Packing and Inner Surface Area
The surface area within a packed tower influences the types of tower internals to use. Maximizing the tower surface area is critical for effective mass transfer, as increased surface area puts the liquid and gas into greater contact. This process improves packing performance and mass transfer efficiency. The internal surface area in a tower also affects the tower’s pressure drop, impacting its separation performance.
The type of tower packing media impacts its surface area, with pall rings providing enhanced surface area within the tower. This feature is due to the pall ring’s design. This type of ring packing has tabs that increase the internal surface area over which mass transfer can occur and decrease the pressure drop within the tower.
The greater the surface area within a tower and its packing, the more efficiently tower internals must work to distribute liquid evenly. An increased area also facilitates gas flow and reduces pressure drop.
Type of Liquid Distributor
Several factors influence tower internals selection. Liquid distributors sit above packing, and they spread liquid evenly when it enters a packed column. A packed column can perform effectively when a liquid distributor has the following qualities:
- Fouling or plugging resistance
- Low liquid residence time
- Even liquid dispensation
- Proper turn-down range operation
- Minimal vapor phase pressure drop
- Optimal vessel height usage
- Proper mixing capability to redistribute water to the next packing bed
The right liquid distributor for your business’s application depends on the following factors:
- Presence of solids in the liquid: In any application where solids are present in the liquid being separated, liquid orifice and trough distributors are ideal because they allow solids to settle as liquids pass through.
- Heavy liquid or gas flows: Trough distributors have good turndown characteristics, making them ideal for heavy gas and liquid flows.
- Bed heights: For towers with a short bed height, spray nozzle distributors ensure uniform distribution. Orifice pan distributors with drip tubes also provide higher turndown ratios while maintaining a shorter vapor riser height.
Tower Pressure Variations
The pressure variations within a tower vary significantly and may influence the optimal design of tower internals used in the tower. Tower internals must meet the specific tower’s pressure needs to provide optimized performance.
When selecting tower internals, your company must consider how to minimize the pressure drop and uniformly distribute liquid across the tower. Here are some of the recommended tower internals depending on a tower’s pressure needs:
- Low pressure: Spray nozzle distributors and trays may work best in low-pressure applications. Structured packing also works best in low-pressure applications, which may require structured packing support grids.
- High pressure: Pressure liquid distributors provide higher pressure when needed. These types of liquid distributors are used in applications with moderate to high liquid rates. Random packings function best with higher liquid flows, meaning the tower might require packing support grids with flat bars instead of structured support grids.
- Pressure drop or vacuum: Orifice plate distributors and trough distributors provide low pressure drop while distributing liquid in certain applications. A system’s pressure dropping capability will also impact the gas distributor sparger used in the application.
Tower Internal Options to Consider
Tower internals are available in various designs and materials and can also be customized to an application’s specific needs. Here are a few tower internal options your company may choose from when optimizing performance for your applications:
- Liquid orifice distributors: The types of liquid distributors work differently to spread a mist of liquid over the packing media. Liquid orifice distributors allow gas to pass through risers as the liquid travels through openings in the pan floor.
- Liquid trough distributors: In a liquid trough distributor, a parting box diverts liquid onto the laterals.
- Liquid ladder distributors: A pressure-driven ladder-type distributor directs the liquid through metering orifices in the branch pipes.
- Liquid spray nozzle distributors: These tower internals ensure uniform distribution in towers with a short bed height.
- Orifice liquid redistributors: These liquid redistributors collect liquids from one section of a tower and redistribute them to another. An orifice liquid redistributor has hats or caps that prevent liquid from bypassing the collector.
- Liquid draw-off collector: A liquid draw-off collector features tall vapor risers.
- Gas distributor spargers: A gas distributor sparger delivers consistent distribution of vapor feed over a tower area through small pores.
- Hold-down grids: A hold-down grid, also called a bed limiter, maintains a level surface on the top of the packing bed by preventing its movement.
- Packing support grid: A supporting grid reinforces the tower packing bed and works with structured and random packing processes.
- Bed limiter: A bed limiter is an anti-migration screen that separates different packing sizes in a bed.
Contact MACH Engineering for More Information About Tower Internals for Your Business
For best performance, tower internals should precisely fit the desired application. When your business needs high-performing tower internals for a range of applications, turn to MACH Engineering. We take special care in designing and engineering various styles of tower internals to match our clients’ specific needs. Our process engineers can assist you in determining the best tower internals for your application. To learn about our tower internals options, contact us today.
