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Home / Blog / Pall Rings as Efficient Packing Solutions for Wet Scrubbers

Pall Rings as Efficient Packing Solutions for Wet Scrubbers

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Author: Michael Klepik, Chief Executive Officer
Pall rings are an irregular bulk packing designed to fill the working space in mass transfer columns and equipment, aimed at enhancing the intensity of heat and mass transfer processes in the equipment of the chemical, petrochemical, and oil refining industries.

The clearly and precisely defined struts give the packing high mechanical strength while maintaining high porosity. The low weight and high strength allow for a greater packing height without the need for intermediate support grids.

When replacing conventional packing materials with Pall rings in a column, productivity significantly increases, and energy consumption decreases.

They are the most popular packing material we use in our scrubbers. These elements are distinguished by their exceptional performance and versatility, making them an essential part of modern gas absorption processes. Everything stated in this article primarily applies to wet scrubbers.
Pall Rings
Pall Rings

Description and Materials

Pall rings are cylindrical bodies with cutouts and internal protrusions, and these cutouts ensure better wettability of the packed bed.
  • Metal Versions: Carbon steel, AISI 304, AISI 316, and other metals
  • Plastic Versions: Polypropylene, polyethylene, PVDF, and other plastics
  • Other Material Options: Ceramic
Types of Materials
Types of Materials

Comparison Table of Materials

Parameter

Plastic

Metal

Ceramic

Materials

Polypropylene, polyethylene, PVDF, and others

Carbon steel, AISI 304, AISI 316, and other metals

Ceramic (aluminum oxide, zirconium, and others)

Corrosion Resistance

Excellent in aggressive environments (acids, alkalis)

Good, but lower than plastic in some environments

Outstanding, resistant to most chemicals

Mechanical Strength

Low, limits use in harsh conditions

Strong, resistant to mechanical stress

Exceptional, withstands impact and heavy loads

Thermal Resistance

Up to 90-120°C (depending on material)

Withstands extreme temperatures (up to 800°C and higher)

Superior, up to 1000°C and beyond

Cost

Relatively low

Medium/high (depending on the material)

Expensive, especially for specialized ceramics

Applications

Chemical and food industries, water and gas treatment

Suitable for aggressive environments, high temperatures, petrochemical, metallurgy

Ideal for high-temperature and corrosive conditions (metallurgy, chemical industry)

Flexibility and Adaptability

Highly adaptable to various conditions

Moderate, requires adjustments for specific conditions

Limited, mainly used in specialized processes

Wear Resistance

Moderate, may degrade with frequent contact with abrasives

Excellent, resists mechanical wear

Durable, though brittle under strong impacts

Key Features

Unlike Raschig rings, Pall rings have a more complex structure due to additional struts, walls, and lamellae. These design elements significantly increase the contact area between the two phases (liquid and gas), leading to improved efficiency.

Their more open structure allows for increased liquid irrigation and reduced gas flow resistance through the mass transfer layer. This positively impacts reaction speed and energy consumption.

Pall rings are effective for removing the following contaminants:
  • Sulfur compounds (SO₂, H₂S)
  • Carbon dioxide (CO₂)
  • Ammonia (NH₃)
  • Nitrogen oxides (NOₓ)
  • Acid gases (HCl, HF)
  • Organic compounds (VOCs)
  • Heavy metals (mercury, lead)
Pall rings are continuously tested in both laboratory and field trials across various projects. This extensive experience has resulted in a robust mathematical and chemical framework that enables accurate project calculations and ensures the desired efficiency.
The influence of random packed column parameters on the liquid holdup and interfacial area
The influence of random packed column parameters on the liquid holdup and interfacial area
Reaction Rate
Thanks to their complex branched structure and increased phase contact area, Pall rings allow for greater flexibility in regulating the reaction rate inside the scrubber. This makes it possible to reduce the packing layer thickness while maintaining high process efficiency.
Bulk Density
The optimized shape of Pall rings allows for a reduction in the height of the packing layer and the column diameter compared to Raschig rings. This decreases the overall amount of material used and enhances mass transfer efficiency.
Aerodynamic Resistance
The more open structure of Pall rings provides significantly lower aerodynamic resistance compared to Raschig rings. This improves gas circulation and reduces energy consumption within the system.

Characteristics

Plastic Rings:
✔ Have low density, which reduces the load on equipment.
✔ Resistant to corrosion and aggressive chemical environments, including acids and alkalis.
✔ Have low hydraulic resistance and good throughput capacity.
✔ Limited in application due to temperature constraints (usually up to 150°C).
Plastic Rings
Plastic Rings

Performance Comparison Table of Plastic Pall Rings

Parameter

Value

Size, mm

16

25

38

50

90

Free space, %

88

91

94

95

96

Specific Surface Area,

m²m³

320

209

127

100

59

No. Per Unit Volume, m³

213,000

49,360

12,120

5,960

1,090

Packing factor

315

176

107

80

51

Specific Weight of Plastic Packings by Size

Parameter

Specific Weight, kg/m³

Packing Size, mm

16

25

38

50

90

PP

110

69

52

45

42

PVDF

198

128

99

84

67

PVC

165

107

82

70

56

Comparison of the Performance of Stainless Steel Pall Rings

Metal Rings:
✔ Have high mechanical strength, allowing them to be used under heavy loads.
✔ Withstand high temperatures (significantly higher than plastic).
✔ Possess excellent thermal conductivity, which can be advantageous in certain processes.
✔ Provide minimal hydraulic resistance due to thin walls and high throughput capacity.
✔ Are susceptible to corrosion unless made of stainless steel or other resistant alloys.
Unlike metal, plastic rings are lighter and corrosion-resistant, while ceramic rings are more chemically inert and heat-resistant but fragile.
Metal Rings
Metal Rings

Parameter

Value

Size, mm

16

25

38

50

90

Free Space %

93

94

95

96

97

Specific Surface Area,

m²m³

316

209

122

100

54

No. Per Unit Volume, m³

210,000

49,500

13,450

6,000

1,000



Stainless Steel

Specific Weight, kg/m³

Packing Size, mm

16

25

38

50

90

0.3

400

N/A

N/A

N/A

N/A

0.4

535

322

200

N/A

N/A

0.5

N/A

403

250

196

N/A

0.6

N/A

483

300

235

N/A

0.7

N/A

564

350

275

144

0.8

N/A

644

400

314

165

0.9

N/A

N/A

N/A

354

180

1.0

N/A

N/A

N/A

N/A

206



Physicochemical Properties of Ceramic Rings

Ceramic Rings:
✔ Withstand extremely high temperatures (significantly higher than plastic and metal).
✔ Completely resistant to corrosion and chemically inert, making them ideal for aggressive environments, including acids and alkalis.
✔ Provide excellent heat and mass transfer efficiency.
✔ Not subject to wear and aging, unlike plastic counterparts.
✔ Very fragile, requiring careful handling and limiting their use in mechanically stressed systems.

Comparison:
  • Plastic rings are lighter and resistant to mechanical impacts but have temperature limitations.
  • Metal rings are stronger and have high throughput capacity but may be susceptible to corrosion.
Ceramic Rings
Ceramic Rings

Property

Value

Water Absorption

No more than 0.5%

Thermal Resistance

At least 8 heat cycles (from 150°C to 20°C)

Compressive Strength

At least 1500 MPa

Acid Resistance

At least 99%

Alkali Resistance

At least 55%

Physical Density

300-2350 kg/m³


Bulk Properties of Packings by Size:

Size, mm

Bulk Density, kg/m³

Surface Area, m²/m³

Porosity, %

Quantity, pcs/m³

50x40x5

600

145

50

6,4

80x64x8

820

120

56

1,95

100x80x10

850

110

53

1

120x96x12

860

75

55

370

150x150x15

980

60

58

295

Commercial Efficiency

It is also important to consider that commercial efficiency depends on the resistance of the scrubber column, which directly affects your electricity costs. The higher the aerodynamic resistance of the column, the more powerful a fan is required, increasing the overall cost of processed material.
The size of the column is a crucial factor. Compared to Raschig rings, Pall rings allow for a narrower yet taller column while maintaining comparable resistance. This means that the overall efficiency of the scrubber is higher. As a result, we can reduce pressure drop while increasing throughput.
Pall ring sizes range from 14-16 mm to 150-200 mm. This flexibility allows us to design scrubbers of virtually any size and air handling capacity, offering a wide range of applications for Pall rings. Smaller sizes are used in columns with high packing density, while larger sizes are suitable for columns with low density.
Additionally, we can model the efficiency of columns with Pall rings, leveraging over a century of accumulated knowledge.
The packing effect inside the column is also significant. Due to their open structure and optimized surface distribution across the scrubber’s cross-section, Pall rings provide greater phase contact, directly improving the scrubber’s speed and efficiency.
Since liquids can more easily flow through the packing, fluid replacement on the packing surface occurs rapidly, enhancing mass transfer.
The ability of Pall rings to retain liquid on their surface increases the absorption effect. This feature allows engineers to design scrubbers that either slow down or accelerate reaction rates depending on process requirements.
The support strut structure of Pall rings ensures high mechanical strength, enabling the use of taller packing layers.
Finally, flow turbulence in the packing mass plays a crucial role. Due to the random arrangement of the packing inside the scrubber, Pall rings create a natural barrier between the liquid and gas phases. This interaction generates turbulent effects, further improving scrubbing efficiency.

Practical Application Examples

In the chemical industry, Pall rings are widely used in ammonia, carbon dioxide, and other gas absorption processes, where their superior performance significantly reduces waste. Recent studies show that using Pall rings in these processes increases mass transfer coefficients and reduces energy consumption by 10-15%.
In large manufacturing facilities such as chemical plants or petrochemical complexes, there is often a need to clean emissions from harmful gases such as ammonia, sulfur compounds, nitrogen oxides, and other toxic substances.
Studies show that their use in such installations significantly improves the absorption of pollutants. As gas passes through a column with Pall rings, it effectively interacts with the liquid phase, which helps to reduce the concentration of harmful substances in emissions to levels that meet strict environmental standards. This approach ensures a higher cleaning speed and reduces the load on equipment, lowering operational and maintenance costs.

Maintenance

Maintaining a packing element, such as Pall rings, requires regular inspection and care to ensure their effective operation in mass transfer systems, especially in devices like scrubbers. Here are some key aspects related to the maintenance and care of these rings:
1. Inspection of Packing Condition
The packing can undergo mechanical wear and degradation due to aggressive chemicals and elevated temperatures. Check for cracks, fractures, or deformations that could reduce the efficiency of the absorption or filtration process.
2. Cleaning from Contaminants
Over time, the elements may become covered with deposits or contaminants such as salts, oils, chemicals, or organic materials. Their accumulation reduces mass transfer efficiency and can lead to blockages, which in turn increases aerodynamic resistance and reduces gas flow speed. Regular cleaning using appropriate solvents or cleaning agents helps restore their operating performance.
3. Monitoring Liquid Condition
In wet scrubbers with Pall rings, an important element is maintaining the quality and level of the liquid. It is necessary to ensure that the liquid used in the absorption process is not contaminated and does not lose its cleaning properties. Periodic replacement of the liquid or addition of necessary chemicals can significantly extend the lifespan of the packing and improve the efficiency of the entire system.
4. Maintenance of Columns and Containers
In addition to the rings themselves, the columns or containers in which they are placed are an important part of the system. These structures should be regularly inspected for cracks, corrosion, or other damages. Leaks of gas or liquid can significantly reduce system performance. It is also essential to ensure that the ventilation and pumping systems are correctly set up to maintain the required gas and liquid flow parameters.
5. Corrosion Resistance
For metallic elements, it is important to regularly check the protective coatings for signs of corrosion. This is crucial for extending the lifespan in aggressive chemical environments. Anti-corrosion coatings or replaceable inserts can be used in areas with a significant risk of corrosion exposure.
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