A municipal wastewater treatment plant in Germany designed a new H₂S scrubber to treat 6,000 m³/hr of biogas at 35°C with an inlet concentration of 1,500 ppm H₂S and a target outlet below 25 ppm (98.3% removal required). The engineering specification called for 25 mm Raschig rings at a packing cost of $3,800, based on the plant’s historical preference for the traditional packing geometry. During commissioning, the scrubber achieved 92% removal — short of the 98.3% target. Performance testing revealed that the Raschig ring bed was operating at 75% of flood with a pressure drop of 1.4 in wc/ft, and the HETP was 0.85 m — significantly higher than the 0.55 m that a Pall ring bed of the same nominal size would have achieved in the same service. Replacing the Raschig rings with 25 mm PP Pall rings cost an additional $2,100, but reduced the pressure drop by 45% to 0.8 in wc/ft and improved the removal efficiency to 97%. The plant was still 1.3% short of the target and needed to increase the bed height by 0.6 m — a $5,000 modification to the column shell — which they could have avoided entirely by specifying Pall rings from the start. The total financial impact of the incorrect random packing selection: $7,100 in change orders plus a 3-week commissioning delay. The incident underscores a fundamental principle of random packing selection: the choice of packing geometry determines not only the mass transfer efficiency but also the pressure drop, the flood point, and ultimately the operating cost of the scrubber over its entire service life.
The selection of random packing type determines the scrubber’s efficiency, pressure drop, fouling resistance, and service life. This guide compares each type with quantified data — Pall ring, Raschig ring, saddle, Tri-Pack, and Tellerette — and provides material selection, specification rules, and installation guidance for scrubber applications.
For the general comparison between random and structured packing see our tower packing types guide. For the complete packing media selection methodology see the scrubber packing media selection guide.
Key Takeaways
- Pall rings outperform Raschig rings by 40-60% lower pressure drop and 20-30% better mass transfer efficiency in scrubber service. A 25 mm PP Pall ring provides 209 m²/m³ specific surface area with 91% void fraction and a packing factor of 176 m⁻¹. A Raschig ring of the same nominal size offers approximately 190 m²/m³ surface area with only 65% void fraction. The packing factor difference — 176 m⁻¹ versus approximately 550 m⁻¹ for ceramic Raschig rings — means that Pall rings can operate at nearly double the gas velocity before flooding.
- The packing factor ranges from 315 m⁻¹ for 16 mm Pall rings to 51 m⁻¹ for 76 mm Pall rings — a sixfold difference that directly determines column diameter requirements. For a typical HCl scrubber treating 10,000 m³/hr, switching from 25 mm Pall rings (Fₚ = 176) to 50 mm Pall rings (Fₚ = 80) increases the required bed height from 2.5 m to 3.3 m but allows a 15-20% reduction in column diameter due to the lower packing factor. Specifying 76 mm rings instead of 16 mm increases hydraulic capacity by approximately 60% but reduces available surface area from 318 m²/m³ to 68 m²/m³, requiring approximately 2.3× the bed height for the same removal efficiency.
- PP random packing in acid-gas service lasts 10-15 years at temperatures up to 80°C, covering 85-90% of all scrubber applications. PVDF extends the temperature range to 120°C with 12-18 year lifespan. SS316 fails within 2-5 years in chloride service — PP in the same environment lasts 10-15 years at less than one-third the material cost. Ceramic handles up to 900°C but cracks under thermal cycling, dissolves in HF, costs 4-7× PP, and requires wet packing installation at $800-1,500 additional labor.
- Tellerette rings at 92-94% void fraction resist fouling 2-3 times longer than Pall rings in particulate-laden gas streams. For electroplating and chemical batch processing exhaust with intermittent particulate spikes, Tellerettes maintain stable pressure drop longer than any other random packing geometry. The trade-off is HETP of 0.58-0.82 m for 50 mm Tellerettes versus 0.45-0.65 m for 25 mm Pall rings.
- The D/8 rule governs random packing size selection: nominal packing diameter ≤ column diameter ÷ 8. For a 600 mm column, maximum is 75 mm — 50 mm is the largest standard. For 1.2-3.0 m columns, 25 mm for clean gas and 50 mm for particulate-laden gas. Dry packing takes 2-4 hours for PP and metal at $400-800 labor; wet packing for ceramic takes 6-12 hours at $800-1,500 additional labor.
What Is Random Packing for Scrubbers?
Random packing is a type of tower packing media consisting of individual pieces — rings, saddles, or spheres — that are dumped into a scrubber column to create an irregular packed bed through which gas and liquid flow in intimate contact for mass transfer. The random orientation of the pieces creates a large wetted surface area for gas-liquid contact, enabling pollutants to transfer from the gas stream into the scrubbing liquid through absorption or chemical reaction. The irregular arrangement creates statistically consistent flow paths across the full bed cross-section — while the orientation of any individual piece is unpredictable, the bulk behavior of the bed is repeatable and can be modeled using standard engineering correlations developed over decades of packed column research.
The performance of random packing is quantified by three fundamental parameters: specific surface area expressed in square meters per cubic meter of packed volume (m²/m³), void fraction expressed as a percentage of open space in the packed bed (%), and packing factor expressed in inverse meters (m⁻¹). The specific surface area determines the available wetted surface for mass transfer — higher values generally provide better efficiency but at the cost of higher pressure drop and greater susceptibility to fouling. The void fraction determines the open area available for gas flow — higher values mean lower resistance and higher capacity before the column floods. The packing factor combines both parameters into a single empirical value used in the generalized pressure drop correlation (GPDC) to predict the hydraulic performance of the packed bed at design conditions. For PP Pall rings, the most common random packing material and geometry, specific surface area ranges from 318 m²/m³ for the 16 mm nominal size down to 68 m²/m³ for the 76 mm nominal size, with void fractions from 88% to 96% and packing factors from 315 m⁻¹ to 51 m⁻¹ — a sixfold variation that directly determines the hydraulic capacity of the column and must be accounted for during the design process.
Random packing for scrubber applications differs from random packing used in distillation columns in two important respects that influence the selection of both geometry and material. First, scrubber packing must resist chemical corrosion from the scrubbing solution — typically NaOH at pH 8-12 for acid-gas scrubbing, or H₂SO₄ at pH 2-4 for ammonia scrubbing — which limits the compatible materials to corrosion-resistant plastics and ceramics rather than the carbon steel and stainless steel commonly used in distillation. Second, scrubber packing must tolerate the particulate loading common in industrial exhaust gas streams — dust, fumes, mists, and reaction byproduct solids — that would not be present in a clean distillation feed and that can rapidly foul structured packing or small-size random packing geometries. These differences drive scrubber random packing selection toward PP as the default material (handling pH 0-14 at temperatures up to 80°C) and toward larger nominal sizes (50 mm or greater) or open geometries (Tellerettes at 92-94% void fraction) when the gas stream contains significant particulate matter.
Types of Random Packing in Scrubber Service
Five random packing geometries dominate the scrubber market, each offering a different balance of specific surface area, void fraction, packing factor, pressure drop, and fouling resistance that determines its best-fit application range. The selection among these five types is the first and most important decision in random packing specification, as it determines the fundamental mass transfer and hydraulic characteristics of the packed bed.
Pall Rings
Pall rings are the dominant random packing geometry for scrubber applications, accounting for approximately 60% of all random packing installations worldwide. A Pall ring is a cylindrical piece with rectangular window openings cut into the cylinder wall and internal tabs or fingers that point inward toward the center of the ring. This design, patented in the 1950s as an improvement over the earlier Raschig ring, creates three distinct functional advantages that translate into measurable performance benefits. The window openings allow gas to flow laterally through the ring rather than being forced to flow around the outside of the cylinder, reducing the pressure drop through the packed bed by 40-60% compared to Raschig rings of the same nominal size. The internal tabs break up the falling liquid into additional drip points that improve liquid distribution across the ring surfaces, increasing the effective wetted area by 20-30% compared to the smooth inner wall of a Raschig ring. The combined effect of the windows and tabs increases the void fraction of the packed bed from approximately 65% for a ceramic Raschig ring to 91% for a PP Pall ring of the same 25 mm nominal size.
Standard PP Pall rings are manufactured in nominal sizes of 16 mm, 25 mm, 38 mm, 50 mm, and 76 mm. The 25 mm size provides a specific surface area of 209 m²/m³ with 91% void fraction and a packing factor of 176 m⁻¹, making it the default specification for most HCl, SO₂, and H₂S scrubbing applications where the gas stream is relatively clean and the primary design objective is maximizing mass transfer efficiency per unit bed height. The 50 mm size provides 100 m²/m³ specific surface area with 95% void fraction and a packing factor of 80 m⁻¹, and is the preferred specification when the gas stream carries particulate loading above 50 mg/Nm³ because the larger void channels resist plugging and maintain stable pressure drop 2-3 times longer between cleaning cycles compared to 25 mm Pall rings in the same service. Pall rings are available in PP (up to 80°C operating temperature), PVDF (up to 120°C), SS304 and SS316 (up to 500°C, but not recommended for chloride service), and ceramic (up to 900°C), with PP accounting for approximately 80% of scrubber Pall ring sales.
Raschig Rings
Raschig rings are the original random packing design — simple hollow cylinders with a length approximately equal to the outer diameter, without any internal features, window openings, or surface texturing. The design was patented by German chemist Friedrich Raschig in 1914 and remained the standard random packing for packed columns in distillation, absorption, and scrubbing service for over four decades until the development of the Pall ring in the 1950s. The fundamental hydraulic limitation of the Raschig ring design is its low void fraction — a 25 mm ceramic Raschig ring has a void fraction of approximately 65%, compared to 91% for a PP Pall ring of the same nominal size — because the gas must navigate around solid cylinders without the benefit of lateral openings. This low void fraction translates directly into 40-60% higher pressure drop and approximately 50% lower capacity before flooding compared to Pall rings, making the Raschig ring a significantly less efficient choice for new scrubber designs.
Raschig rings remain in production for three specific applications where their unique characteristics provide advantages over Pall rings. First, high-temperature ceramic service above 120°C — ceramic Raschig rings operate up to 900°C with a lifespan of 5-8 years in steady-temperature service, making them the only option for absorption columns in hot H₂SO₄ and HNO₃ service where plastic Pall rings cannot survive and ceramic Pall rings are not available in all sizes. Second, replacement of existing columns originally designed using Raschig ring hydraulic data — re-rating an existing column for Pall rings would require re-verification of the column hydraulics and possibly the liquid distributor design, which some operators choose to avoid during a routine packing replacement. Third, laboratory and pilot-scale columns with diameters below 150 mm that require packing sizes below 16 mm — Pall rings are not manufactured below 16 mm nominal size, while Raschig rings are available in sizes as small as 6 mm for laboratory applications.
Saddle Rings (Intalox Saddles)
Saddle rings are curved, saddle-shaped packing pieces with textured surfaces that prevent nesting — the failure mode where cylindrical packing pieces settle into tight, interlocked configurations that block liquid flow and create preferential gas channels through the bed. The saddle geometry maintains open channels even when packed randomly, providing 15-25% better liquid spreading than Pall rings at the same nominal surface area of approximately 210 m²/m³ for the 25 mm size. Intalox saddles are the preferred random packing type for polishing scrubbers where the outlet concentration must remain below 5 mg/Nm³, because the superior liquid distribution reduces the statistical probability of untreated gas bypassing through dry zones in the packing bed. They are available in PP (up to 80°C) and ceramic (up to 900°C), with the ceramic variant being the preferred choice for hot H₂SO₄ service in sulfuric acid plants where the operating temperature exceeds the limits of PP packing.
Tri-Packs
Tri-Packs are spherical random packing pieces with multiple internal ribs and open window openings, designed specifically to address the settling and compaction problem that occurs with cylindrical packing geometries over time. Cylindrical packing pieces — both Pall rings and Raschig rings — settle under their own weight and the dynamic load of gas and liquid flow, causing the void fraction of the packed bed to decrease by 10-20% over the first 12-24 months of operation as the pieces shift into tighter configurations. The spherical geometry of Tri-Packs prevents this settling because spheres cannot interlock in the same way that cylinders can, maintaining consistent void fraction throughout the bed life. A 25 mm PP Tri-Pack offers a specific surface area of 185 m²/m³ with 90% void fraction, and is commonly specified for retrofit projects where the existing fan has limited static pressure capacity and the conservation of void fraction over time translates directly into longer intervals between pressure-drop-related maintenance shutdowns.
Tellerettes (Rosette Packing)
Tellerettes are multi-ring rosette-shaped packing pieces with the highest void fraction of any random packing type at 92-94%, with specific surface area ranging from 125-180 m²/m³ depending on the nominal size. The open rosette structure, consisting of concentric rings joined by radial spokes, resists fouling from particulate-laden gas streams better than any other random packing geometry because the large open spaces between the rings provide fewer locations where solid particles can accumulate and bridge across flow channels. In electroplating exhaust systems handling chrome mist and in chemical batch processing operations with intermittent particulate spikes, Tellerette packing maintains stable pressure drop 2-3 times longer between cleaning cycles than Pall rings and 5-10 times longer than structured packing in the same service. The trade-off for this fouling resistance is lower mass transfer efficiency per unit bed height — the HETP for 50 mm Tellerettes is 0.58-0.82 m compared to 0.45-0.65 m for 25 mm Pall rings in the same acid-gas scrubbing service — which means that achieving the same removal efficiency with Tellerettes requires a taller packed bed by a factor of approximately 1.3-1.5x depending on the specific operating conditions and removal requirements.
Random Packing Performance Comparison Table
The table below compares the key performance parameters of the most common PP random packing sizes used in industrial scrubbers. The packing factor (Fₚ) is the most important value in this table for column sizing because it determines the flooding velocity and pressure drop characteristics through the generalized pressure drop correlation (GPDC).
| Parameter | 16 mm | 25 mm | 38 mm | 50 mm | 76 mm |
|---|---|---|---|---|---|
| Specific surface area (m²/m³) | 318 | 209 | 127 | 100 | 68 |
| Void fraction (%) | 88 | 91 | 94 | 95 | 96 |
| Packing factor (m⁻¹) | 315 | 176 | 107 | 80 | 51 |
| PP weight (kg/m³) | 110 | 69 | 52 | 45 | 38 |
| Pieces per m³ | 213,000 | 49,360 | 12,120 | 5,960 | 1,720 |
| Relative pressure drop | High | Moderate | Low-Moderate | Low | Very Low |
To illustrate how this performance data translates into real-world column dimensions, consider a typical HCl scrubber treating 10,000 m³/hr of exhaust gas at 25°C with an inlet concentration of 500 ppm and a target outlet concentration of 5 ppm (99% removal). For an acid-gas system at dilute concentrations, the required number of transfer units is approximately 4.6. Using 25 mm Pall rings with an HETP of 0.55 m at 70% of flood, the required packed bed height is 4.6 × 0.55 = 2.5 m. Using 50 mm Pall rings with an HETP of 0.72 m for the same duty increases the required bed height to 4.6 × 0.72 = 3.3 m — 0.8 m taller. However, the 50 mm rings have a packing factor of 80 m⁻¹ versus 176 m⁻¹ for the 25 mm size, which means the column diameter can be reduced by approximately 15-20% for the same gas flow rate because the lower packing factor allows a higher gas velocity before flooding. The trade-off between column diameter and bed height is a classic packed column design optimization that depends on the relative costs of the column shell (diameter-driven) and the packing media (volume-driven). For the step-by-step calculation procedure using these values, see our scrubber pressure drop calculation guide.
Material Selection for Random Packing
| Material | Max Temp | Acid-Gas Lifespan | Cost vs PP | Best For |
|---|---|---|---|---|
| PP (Polypropylene) | 80°C | 10-15 years | 1× | HCl, H₂SO₄, H₂S, SO₂; 85-90% of all scrubbers |
| PVDF | 120°C | 12-18 years | 2.5-3.5× | HF service, hot acids 80-120°C |
| SS316 | 500°C | 2-5 years (chloride) | 3-5× | HNO₃, hot gases; NOT for chloride service |
| Ceramic | 900°C | 5-8 years | 4-7× | High-temperature acids above 120°C |
PP is the default random packing material covering 85-90% of all scrubber operating conditions at temperatures up to 80°C with a lifespan of 10-15 years. PP resists HCl at all concentrations, H₂SO₄ up to 50%, NaOH across pH 0-14, and most organic acids, making it the most versatile and cost-effective material for the vast majority of industrial acid-gas scrubbing applications. PVDF is required when the process stream contains HF above trace levels or when the operating temperature exceeds 80°C, with a lifespan of 12-18 years at 2.5-3.5× the cost of PP. SS316 should never be specified for chloride-containing service — pitting corrosion begins within months in wet HCl service above 60°C, and the dissolved iron from corroding SS316 rings contaminates the scrubbing liquor discharge. Ceramic random packing handles temperatures up to 900°C but cracks under thermal cycling in intermittent service, dissolves in HF, costs 4-7× PP, and requires wet packing installation (filling the column with water before adding the media) to prevent breakage rates of 5-15% during the installation process.
How to Specify Random Packing
Specifying random packing for a scrubber requires four sequential decisions — geometry, nominal size, material, and packed bed depth — each determined by different aspects of the process conditions. The geometry determines the mass transfer efficiency and fouling resistance. The size determines the pressure drop and hydraulic capacity via the packing factor. The material determines the service life and temperature range. The bed depth determines the removal efficiency via the HETP method.
Step 1: Select Geometry
Choose the packing geometry based on the gas stream characteristics. For clean acid gas with particulate loading below 50 mg/Nm³ at moderate temperature below 80°C, specify 25 mm PP Pall rings — the default for 60% of all scrubber installations. For particulate-laden or fouling gas streams, specify Tellerette rings at 92-94% void fraction or 50 mm Pall rings at 95% void fraction, both of which maintain stable pressure drop significantly longer than smaller Pall rings in dirty service. For high-purity polishing scrubbers requiring outlet concentration below 5 mg/Nm³, specify Intalox saddles for their superior liquid distribution characteristics. For high-temperature service above 120°C where plastic packing cannot survive, specify ceramic Raschig rings.
Step 2: Determine Size by the D/8 Rule
The nominal packing diameter must not exceed one-eighth of the column inner diameter — this is a fixed design constraint, not a recommendation. For a 600 mm diameter scrubber, the maximum nominal packing size is 75 mm, making 50 mm Pall rings the largest practical standard choice. For most industrial scrubbers with diameters of 1.2-3.0 m, 25 mm Pall rings are the standard specification for clean gas service and 50 mm Pall rings for gas streams with particulate loading above 50 mg/Nm³. Smaller packing sizes such as 16 mm provide 52% more specific surface area than 25 mm (318 m²/m³ versus 209 m²/m³) but create 2-3× higher pressure drop due to the higher packing factor (315 m⁻¹ versus 176 m⁻¹) and foul 3-6× faster in the presence of particulate matter.
Step 3: Choose Material
Select the packing material based on the peak operating temperature plus a 10°C safety margin — not on the average temperature, because PP creep failure at sustained temperatures above 80°C occurs within 12-18 months regardless of how many hours the column spends at the average. PP for gas temperatures below 80°C and no HF. PVDF for HF service or operating temperatures between 80-120°C. Ceramic for temperatures above 120°C. SS316 for high-temperature non-chloride service only — never specify SS316 for HCl or wet chloride service.
Step 4: Calculate Bed Depth
Calculate the required packed bed height using the HETP method: Bed Height = NTS × HETP × SF, where NTS is the number of theoretical stages determined from the inlet and outlet concentrations using the absorption factor method for dilute gas systems, HETP is taken from the performance table for the selected packing size at the design gas velocity, and SF is a safety factor of 1.15-1.25 to account for uncertainties in the mass transfer model, future process variability, and potential fouling over the packing life. For the complete calculation procedure, see our packed bed scrubber design calculation guide.
Random Packing Installation Methods
Dry packing is the standard installation method for PP and metal random packing. The packing media is poured or dumped directly into the column through the top manway and carefully distributed to maintain a level bed surface. The rings should be dropped from a height of no more than 1.5 m above the bed surface to prevent deformation of PP rings at the bottom of the bed from the accumulated impact load. Dry packing takes 2-4 hours for a 1.5 m diameter tower with 3.0 m of packed height, with a typical labor cost of $400-800 depending on the site conditions and crew size. Wet packing is required for ceramic random packing to prevent breakage. The column is filled with water before the packing media is added, and the water cushions the fall of the ceramic pieces as they settle into the bed. After the packing is installed, the water is drained and the bed is allowed to dry thoroughly before the column is returned to service. Wet packing takes 6-12 hours total for a 1.5 m diameter tower, adding $800-1,500 in labor compared to dry packing.
Random vs Structured Packing for Scrubbers
Random packing differs from structured packing in three fundamental ways that determine which is the better choice for a given scrubber application. Random packing costs 30-55% less per unit volume, installs in 2-4 hours versus 6-12 hours for a 1.5 m diameter column, and handles particulate-laden gas streams significantly better because the irregular bed geometry creates natural traps for solids that can be flushed out during high-flow irrigation cycles without causing permanent plugging. Structured packing achieves 25-40% lower HETP (0.30-0.48 m versus 0.45-0.65 m for 25 mm Pall rings) and 40-60% lower pressure drop per theoretical stage, but the precise corrugated channels plug within 3-6 months in gas streams with particulate loading above 50 mg/Nm³, requiring a complete packing replacement at a cost of $4,000-8,000 for a 1.5 m diameter tower. For scrubber applications, the choice depends primarily on the gas cleanliness and the annual operating hours — random packing is the appropriate choice for any gas stream with particulate regardless of operating hours, while structured packing is justified only for clean gas streams with operating hours above 4,000 per year where the energy savings from lower pressure drop offset the 1.8-3.0× cost premium within 3-5 years.
FAQ
What is the difference between Pall rings and Raschig rings?
Pall rings have rectangular window openings and internal tabs in the cylinder wall that reduce pressure drop by 40-60%, increase capacity by 50-100%, and improve mass transfer efficiency by 20-30% compared to Raschig rings of the same nominal size. Pall rings are the standard for modern scrubber designs; Raschig rings are used primarily for high-temperature ceramic service above 120°C.
What size random packing should I use for my scrubber?
Apply the D/8 rule: nominal packing diameter must not exceed one-eighth of the column diameter. For most industrial scrubbers of 1.2-3.0 m diameter, use 25 mm Pall rings for clean gas and 50 mm Pall rings for gas streams with particulate loading above 50 mg/Nm³.
What material is best for random packing in HCl service?
PP is the standard material for HCl service at temperatures below 80°C with a lifespan of 10-15 years. PVDF is required for temperatures between 80-120°C. SS316 must be avoided — pitting corrosion begins within months in wet HCl service above 60°C.
How long does random packing last in a scrubber?
PP random packing lasts 10-15 years in acid-gas scrubbing service at temperatures below 80°C. PVDF lasts 12-18 years. SS316 in chloride service fails within 2-5 years. Ceramic lasts 5-8 years in steady service but 2-4 years under thermal cycling.
How is random packing installed?
Dry packing for PP and metal: poured into the column, 2-4 hours for a 1.5 m tower, labor cost $400-800. Wet packing for ceramic: column filled with water first, 6-12 hours total, $800-1,500 additional labor.
Conclusion
Random packing is the workhorse of scrubber mass transfer, with Pall rings as the dominant geometry accounting for approximately 60% of all installations. The selection of random packing type, size, and material determines the scrubber’s removal efficiency, fan energy consumption, and service life. Correctly specifying Pall rings over Raschig rings typically improves efficiency by 20-30% and reduces pressure drop by 40-60% at a marginal cost difference that pays back within 6-18 months of operation. The packing factor range from 315 m⁻¹ for 16 mm rings to 51 m⁻¹ for 76 mm rings allows the column designer to trade hydraulic capacity against mass transfer surface area based on the specific process requirements. Random packing handles particulate-laden gas streams that would rapidly plug structured packing, making it the default choice for the majority of industrial scrubber applications where some level of particulate matter is present in the exhaust gas.
XICHENG EP LTD manufactures random packing in PP, PVDF, metal, and ceramic for scrubber applications, with over 2,600 scrubber systems shipped to 60+ countries since 2008.
Contact XICHENG EP for random packing →