Achieving Good Packed Tower Efficiency

Achieving Good Packed Tower Efficiency

In air and water pollution control equipment, as well as in odor control equipment such as scrubbers and air strippers, efficiency is essential to performance. Some packed tower wet scrubbers, for instance, can achieve 99% removal efficiencies or higher for inorganic compounds and volatile organic compounds (VOCs). Figures like that require careful calibration of various internal factors. Though a separation tower may contain a high volume of packing, that packing can’t provide efficient mass transfer and separation unless the tower’s layout and liquid distribution allow it.

When operators of separation towers experiment with how to improve packed tower efficiency, liquid distribution is generally one of the main areas of assessment. Achieving correct, balanced liquid distribution is one of the best ways to improve packed tower efficiency and maximize the benefits of its separation capacities.

Evaluating Liquid Distribution

Uniform liquid distribution is key to obtaining expected performance from a packed bed. Proper liquid distribution enhances efficiency by ensuring adequate wetting and optimal performance throughout the packing.

Why is uniform liquid distribution so critical? One answer lies in the tower design. Generally, a packed tower contains a distributor that disperses liquid down into the random or structured packing below. The distributor disperses the liquid through a set of individual orifices. Liquid from the orifices sometimes enters a pipe or runs across a spreader plate that facilitates distribution. The distributor may use gravity to propel the liquid through the orifices or use pumps to pressurize the liquid.

When the distributor and its orifices are working properly, they distribute equal amounts of liquid to all parts of the packed bed — if not, maldistributions may pose challenges. Different types of maldistributions include:

  • Point-to-point maldistributions: If the liquid distribution is uneven from point to point, the maldistribution can distort the liquid-to-vapor (L/V) ratio in some parts of the packing. Variations in the L/V ratio create what is known as equilibrium pinching. The packing appears not to function, even though more depth is available, because no further separation is possible.
  • Whole packing bed maldistributions: On a larger scale, if the maldistribution occurs across much of the packing bed, the liquid flows unevenly through the packing, generally pooling at the wall and leaving the interior of the packing unwetted. This scenario also makes the packing inefficient and reduces its separation capabilities.

For these reasons, evaluating liquid distribution in a packed tower often means monitoring for lack of separation that could indicate packing inefficiency. It also means focusing on L/V ratio changes that could signal poor liquid distribution.

Key Criteria for Proper Liquid Distribution

When you’re trying to optimize your packed tower’s performance and efficiency by ensuring adequate liquid distribution, you’ll need to address several different factors that add up to optimal distribution. Below are a few key criteria to keep in mind:

Uniformity of Flow

1. Uniformity of Flow

Maintaining uniform flow is essential to proper liquid distribution in a packed tower. Regardless of the tower’s flow rate, the distributor must disperse the liquid uniformly throughout the entire tower bed.

When the liquid flow in a separation tower is not uniform, the tower may not achieve its maximum separation potential. Even if more packing is available for use, further separation becomes impossible because of the uneven flow.

One potential culprit in a packing tower’s uneven flow is the rate at which the liquid distributor operates. If the distributor runs at rates lower than its specified turndown ratio, the lowered liquid head levels above the orifices disrupt the liquid flow both from point to point and across the entire cross-section of the tower.

2. Appropriate Pour-Point Density

Maintaining proper pour-point density is also critical for optimizing liquid distribution in a packed tower. Proper pour-point density requires having a sufficient number of pour points for the amount of packing and a design that allows for the unimpeded flow of vapor. If a tower has a large volume of packing, but only a few pour points through which to distribute the liquid, most of the liquid will remain concentrated within a few channels while the rest of the packing receives too little.

3. Proper Irrigation Along the Column Wall Area

The design of the column wall area in a packed tower must allow for adequate packing irrigation — and it must also ensure the packing near the wall does not receive too much irrigation. Too little irrigation along the column wall leads to poorly functioning packing and inefficient separation. Too much irrigation is also problematic because it leads to underirrigation elsewhere in the packing, which diminishes separation efficiency. Excessive irrigation near the column wall can also lead to other problems like short-circuiting in that area.

4. Sufficient Open Area for Vapor Flow

Because impeded vapor flow in the distributor can interfere with the tower’s hydraulic capacity, the tower design must allow a sufficient open area for vapor flow through the distributor. In most designs, it’s standard for the distributor or redistributor to be located several inches above the packing bed to create a substantial open area.

5. Entrainment Prevention

Preventing liquid entrainment is essential to facilitating adequate liquid distribution in a packed tower. Entrainment occurs when vapor effluent carries liquid into the tray above instead of allowing it to flow into the tray below. It is undesirable because it can lead to flooding.

Two types of distributors are particularly vulnerable to creating entrainment — notched trough and weir flow distributors. These models promote entrainment because they discharge their liquid directly into the vapor riser region. This location is where the vapor velocity in the tower is at its highest and has the greatest potential to cause liquid back-mixing and reduce separation efficiency.

Effects of Flow Variation on Proper Liquid Distribution

Effects of Flow Variation on Proper Liquid Distribution

Flow variation can have adverse effects on the liquid distribution in a packed tower. If flow variations are substantial, they will likely have detrimental effects on liquid distribution throughout the entire tower. Reducing flow variation is essential for enhancing liquid distribution to improve packed tower efficiency.

Flow variation can occur at the macro-level — affecting a large portion of the packed tower, between different areas of the distributor — or at the micro-level, affecting flow distribution among the orifices. You may need to address both large- and small-scale flow variations to achieve your tower’s optimum results.

To prevent the flow variation that can lead to improper liquid distribution in a packed tower, you’ll need to know what causes it in the first place. Below are a few common causes for variation in the liquid flow through a packed tower:

1. Feed Pipe Construction

The construction of the tower’s feed pipe directly affects variation in the tower’s liquid flow — the design must work to keep the liquid flowing out of the feed pipe from unbalancing the distributor’s hydraulics. The liquid flowing out of the feed pipe also should not interfere with the function of the distributor’s orifices. The feed pipe design generally becomes more complex as the tower diameter increases. Engineers must design carefully to gain the correct ratio of pressure drop to liquid velocity and minimize flow variability.

2. Liquid Head Above the Distributor Orifices

The volume of liquid above the orifices also helps determine the variability of the liquid flow. At very low heads — often about an inch or less — the liquid flow becomes incredibly sensitive to even minor changes in the depth of the liquid head. That’s because even minor modifications represent a large proportion of the total volume. Adding another inch or two of additional head over the tops of the distributor orifices can make the liquid flow more resistant to volume fluctuations.

3. Liquid Velocity Through the Orifices

The velocity of the liquid moving through the distributor’s orifices can also cause variation in the liquid flow. The liquid’s velocity directly affects how much liquid the orifice will discharge in a given time. The discharge coefficient generally depends on the Reynolds number — the ratio of the liquid’s inertial forces to its viscous forces — which in turn depends on the orifice diameter. Some calculations will likely be necessary to determine the proper liquid velocity and discharge coefficient for your tower’s distributor orifices.

4. Liquid Velocity Through the Liquid Head

The velocity of the liquid flowing above the orifices likely differs from the velocity of the liquid flowing through the orifices. The horizontal velocity of the liquid flowing over the orifices should remain low — otherwise, it will cause unwanted variations in the liquid head. Maintaining an adequate liquid head depth is often a useful strategy for reducing horizontal liquid velocity and minimizing flow variation.

5. Orifice Density

How closely packed the distributor orifices are also affects flow variation throughout the tower. With lower density, if the orifices are too far apart, a substantial portion of the packing will remain dry. Using many small orifices significantly improves flow uniformity, liquid distribution and tower efficiency, though the smaller orifices used in high-density layouts can sometimes be more susceptible to fouling.

6. Orifice Edges

The manufacturing quality of the orifice edges is significant because of the way most edges are created. The manufacturing process uses a punch to create these edges, and the direction of the liquid flow should always follow the direction the punch followed through the material.

The reason is that the punch makes a smooth, sharp, precise edge where it enters the material and a rough, rounded edge where it exits, and the liquid flows best if it moves from the smooth edge to the rough edge. Variations in the quality of the edges can lead to variations in the liquid flow.

7. Orifice Layout Plan

The layout of the distributor orifices also matters to flow variation. Regardless of orifice density, the layout plan design should ensure that the liquid entering the tower distributes itself evenly across the packing bed. The orifices themselves should have a more or less even distribution. Installing fewer orifices in a particular area — for instance, to make room for a tray ring, gas riser or other tower internals — can cause uneven distribution, with that area of the packing receiving less liquid than it requires.

8. Orifice Shape

The shape of the orifices also substantially impacts flow variation. Most orifices are round to allow for greater flow volume per cross-sectional area. However, some drilled holes have a more pointed shape, and some applications use noncircular orifices. These orifices require different calculations to ensure even flow and proper liquid distribution.

Utilizing Packed Tower Internal Innovations

Utilizing Packed Tower Internal Innovations

Advances in packed tower design over the years have enhanced performance to provide better, more efficient separation. A few innovative technologies are available to improve packed tower efficiency, such as the ones below.

Enhanced Baffle Plate Distributors

Enhanced baffle plate distributors use distinctive baffle plates to intervene between the orifices and the packing bed. The baffle plates spray out the liquid into a fan or curtain shape to disperse it more evenly through the packing and prevent entrainment.

Enhanced baffle plate distributors include a few different features designed to preserve the integrity of the fanned-out shape of the liquid. These include a shield plate placed next to the first baffle plate to keep the liquid and vapor separate and a design that lets the baffle plates rest directly on the packing to minimize the disruptive impact from free fall. Enhanced baffles also have special surface treatments to improve the liquid’s spread and serrations to minimize liquid tracking.

Mixing Collectors

Some towers, particularly those with large diameters and specific composition requirements, use mixing drums to churn the distributed water from one level before redistributing it to the level below. The benefit of this mixing is that it ensures the liquid composition and L/V ratio are identical across all tower cross-sections. However, one drawback of the mixing drum is the amount of room it takes up in the tower, which minimizes the area available for packing.

Specialized mixing collectors neutralize this disadvantage by combining the mixing drum with the tower’s collector. Using these mixing collectors can save up to a meter of height at each distribution point and reduce the height of a new tower by 15%. Alternatively, it can increase the number of tower stages by 15% to boost efficiency, reduce energy consumption and increase purity and throughput.

Latest Generation of Structured Packing

Original structured packing contained innovative geometry — part of the flow channel turned vertically so the layers rested one on top of the other. This arrangement functioned to increase packing capacity while maintaining optimum efficiency.

The latest generation of structured packing improves on the original by boosting efficiency and providing up to 15% more capacity. It can also increase the feed rate by up to 30% while raising utility consumption only 10%.

Contact MACH Engineering to Identify the Best Tower Packing Options for Your Project

Contact MACH Engineering to Identify the Best Tower Packing Options for Your Project

To receive assistance in determining the best packaging options for your tower, partner with MACH Engineering. We are a full-service company that can address your project requirements from design to fabrication and installation and everything in between. Our quality components, including an extensive catalog of random tower packing and tower internals like distributors, redistributors and grids will also help you meet your air and water pollution control needs. We are happy to work with you to identify the best solutions for your project.

Contact us today to request a consultation or learn more.