Raschig Rings: Materials, Sizes, Application, and Evolution

Author: Michael Klepik, Chief Executive Officer

Due to their simple and stable cylindrical shape, rings come in sizes ranging from 5-6 mm to 200 mm, allowing their use in a vast array of mass transfer equipment. This versatility enhances their efficiency and broadens their applicability in various industrial projects.

The selection of Raschig ring diameter depends on several factors, including the scrubber size, the type of process, and operating conditions (temperature, pressure, composition of gases and liquids). Proper selection of ring diameter is important to ensure optimal mass transfer, minimize flow resistance, and improve scrubber efficiency. Here are some general recommendations for selecting ring diameter:

• For smaller scrubbers (up to 0.5 meters in diameter), rings with diameters of 10–20 mm are typically selected.
• For medium and large columns (1 meter in diameter or more), the optimal choice would be to use rings with a diameter of 25 mm or larger.
• In processes with high gas flow rates or those requiring rapid heat and mass transfer, it is better to choose rings with a diameter of 10–20 mm.
• For low-temperature or low-flow processes, selecting rings with diameters up to 10 mm can maximize efficiency with minimal pressure drop.

Raschig elements are used in:

• Columns (steam, refrigerant, rectification);
• Converters;
• Extractors (for mixture separation);
• Distillation apparatus;
• Absorbers and scrubbers (for gas absorption and component separation).

Raschig demonstrated through experiments that the total surface area of the rings was sufficient for effective chemical processes, and their production was simple and inexpensive.

1. They provide a larger surface area, leading to more efficient mixing in comparably sized scrubbers.
2. They have lower aerodynamic resistance, reducing energy consumption.
3. Their low hydraulic resistance allows for faster chemical processes inside column apparatus.
4. They are cheaper to produce, reducing capital investment costs.

At the time of their invention, Raschig rings were a revolutionary type of packing for chemical equipment compared to trays and hollow scrubbers. While many modern materials (such as Pall rings, Intalox saddles, and Tri-Packs) have since been developed, the accumulated research and widespread adoption of Raschig elements still make them a leading choice for many applications.

The maintenance of Raschig rings in a scrubber necessitates systematic inspection and cleaning to sustain optimal operational efficiency. Periodic assessment of the packing's condition is crucial, including evaluating the uniformity of gas and liquid distribution, identifying potential fouling, and detecting mechanical degradation. In cases of clogging due to particulate deposition or scaling, appropriate cleaning methods—such as water flushing, chemical treatment, or steam purging—should be employed, contingent on the nature of the contamination. For severe fouling, mechanical cleaning or ultrasonic decontamination may be required.

As part of routine upkeep, degraded or fractured rings should be periodically replaced to maintain consistent packing density and prevent localized flow restrictions. Proper installation of replacement media is essential to ensure uniform distribution and to mitigate compaction effects. Additionally, continuous monitoring of critical scrubber parameters—such as differential pressure, gas and liquid flow rates, and liquid holdup—is imperative to prevent the formation of stagnant zones and excessive pressure drop. The integration of upstream filtration and pre-treatment systems can further reduce particulate loading on the packing, thereby enhancing durability and prolonging service life.

A study published in The Influence of Random Packed Column Parameters on the Liquid Holdup and Interfacial Area (Yucheng Fu et al., March 14, 2022) highlights how Raschig rings create a dense packing arrangement that enhances liquid-gas mass transfer processes. Compared to foam and hollow scrubbers, Raschig elements provide more precise efficiency calculations in wet scrubbers and column apparatus. This predictability makes them a reliable choice for chemical and petrochemical projects, ensuring that commercial benefits can be accurately estimated.

Raschig rings are often compared to Pall rings and Berl saddles. While they may be less efficient in mass transfer and have higher aerodynamic and hydraulic resistance, their simplicity and corrosion resistance make them a suitable choice for many applications, especially where chemical durability is critical.

Many competitors attempted to modify the original Raschig ring design to bypass the patent, leading to the development of numerous new types of packings.
For example, Pall rings surpass Raschig elements in efficiency for certain chemical processes, and Dixon rings, which consist of woven mesh structures twisted into rings, operate on the same principle as Raschig elements but with enhanced performance.
Despite this, Raschig’s original invention remains a brilliant and simple solution.

✅ Chemical Resistance: Due to the use of ceramics and glass, Raschig rings resist corrosion, making them widely used in laboratory scrubbers.
✅ Simple Production: Their manufacture requires no advanced technology, making them more accessible than other types of packing.
✅ Versatility: Decades of experience and well-developed chemical models allow them to be used in a broad range of industrial processes with predictable efficiency.

❌ Limited Efficiency: Compared to modern packings, Raschig rings have lower mass transfer efficiency due to their basic shape.
❌ High Pressure Drop: The high density of packing within a column can lead to significant pressure drop, particularly in taller layers.
Despite their age, Raschig rings remain an essential tool in chemical engineering due to their simplicity, durability, and versatility. Their continued use demonstrates that even the earliest technological innovations can stay relevant for decades.

Later, Super Raschig Rings were developed. Although they lost the original cylindrical shape, they retained the name in honor of the great chemist.

Their wavy structure creates an advanced packing design with an open, high-surface-area geometry, improving liquid-gas mass transfer. Super Raschig Rings offer 30% more surface area and nearly 70% lower pressure drop compared to traditional elements.