Organic Sulfur Hydrogenation Catalyst Attrition Rate Testing: Methods, Standards, and Evaluation

Organic sulfur hydrogenation catalyst attrition rate testing is a critical mechanical integrity evaluation for Co-Mo and Ni-Mo based catalysts that convert organic sulfur compounds (COS, CS2, mercaptans, thiophenes) into H2S in hydrogen-rich process streams. This test quantifies the fines generated during handling, loading, and operation, providing essential quality control data for fixed-bed desulfurization systems. This article explains the test principle, sample requirements, standard methods such as ASTM D4058, calculation procedures, and typical acceptance criteria for industrial catalyst applications.

Technical Overview

Organic sulfur hydrogenation catalysts, typically cobalt-molybdenum (Co-Mo) or nickel-molybdenum (Ni-Mo) formulations supported on activated alumina, are designed to convert refractory organic sulfur compounds—such as carbonyl sulfide (COS), carbon disulfide (CS2), mercaptans, organic sulfides, and thiophenes—into hydrogen sulfide (H2S) through hydrogenation and hydrolysis reactions. The resulting H2S is subsequently removed by downstream zinc oxide polishers. These catalysts operate under demanding conditions in fixed-bed reactors, including elevated temperatures (300–400 °C), hydrogen-rich atmospheres, and cyclic pressure and thermal loading.

The attrition rate test measures the mechanical durability of the catalyst by subjecting a conditioned sample to standardized tumbling or agitation and determining the mass percentage of fines generated. A high attrition rate indicates poor mechanical strength, which can lead to several serious operational problems: increased bed pressure drop, migration of fines into downstream equipment, fouling of heat exchangers and valves, uneven gas flow distribution, catalyst inventory loss, and premature reactor unloading. Attrition testing is therefore an essential element of incoming catalyst inspection, manufacturing quality control, and supplier qualification programs.

Testing Samples

Cylindrical Extrudates

Cylindrical extrudates are the most common commercial form of organic sulfur hydrogenation catalyst, with typical diameters of 1.5–5 mm and lengths of 4–15 mm. Extrudates are particularly susceptible to edge chipping and longitudinal splitting, making attrition evaluation especially important. A minimum of 100–200 g of representative sample is generally required for each determination, with three parallel samples recommended for statistical reliability.

Spheres and Pellets

Spherical catalysts and pellets, typically 3–6 mm in diameter, are produced by oil-drop or granulation processes. Because spheres lack sharp edges, they usually exhibit lower fines generation than extrudates, but they can still fracture under impact or fatigue loading. The same conditioning and testing protocols apply.

Shaped Forms (Rings, Trilobes, and Multilobes)

Raschig rings, trilobe, and multilobe extrudates offer higher geometric surface area and reduced diffusion limitations. Their complex geometry, however, creates stress concentration points, so attrition behavior must be verified for each specific shape rather than extrapolated from cylindrical data.

Sample Conditioning Requirements

Testing is normally performed on fresh catalyst in the oxide form. Samples should be dried to constant mass (commonly 2 hours at 105–120 °C), cooled in a desiccator to prevent moisture pickup, and pre-sieved to remove adherent fines so that only mechanically generated fines are measured. Batch identification, manufacturing date, and physical dimensions must be documented before testing begins.

Testing Items

A comprehensive attrition evaluation of organic sulfur hydrogenation catalysts typically includes the following measurable parameters:

• Attrition rate (loss on attrition), expressed as the mass percentage of fines generated under standardized mechanical stress
• Particle size distribution before and after testing, determined by sieve analysis
• Mass percentage retained on the designated specification sieve
• Change in average particle diameter or characteristic length
• Visual and photographic documentation of broken, chipped, or cracked particles
• Supplementary mechanical indicators such as side crush strength per ASTM D4179 or radial crush strength of extrudates per ASTM D6175, bulk density, and specific surface area, which help interpret attrition results

Testing Methods

Rotating Drum Attrition Method (Primary Method)

The rotating drum method, consistent with the principle of ASTM D4058, is the standard approach for formed (shaped) catalysts. A conditioned, weighed sample is placed in a cylindrical drum fitted with an internal baffle or lifter bar. The drum is rotated at a defined speed for a specified duration, typically in the range of 30–60 minutes. The tumbling action causes particles to cascade, collide with the drum wall and with each other, simulating the mechanical stresses of transport and reactor loading. After rotation, the sample is discharged and sieved through a mesh selected according to the lower size limit of the product specification, and the retained mass is weighed.

Air Jet Attrition Method (Supplementary Method)

The air jet method, aligned with ASTM D5757, exposes a powder or fluidizable catalyst sample to a high-velocity jet of humidified air, causing particle-to-particle and particle-to-wall collisions. While this method is primarily intended for powdered or fluidized-bed catalysts, it may be applied to fine-particle hydrogenation catalysts or to comparative research studies. It is not the routine method for fixed-bed extrudates.

Calculation and Reporting

The attrition rate is calculated as the mass loss relative to the initial conditioned sample mass:

Attrition rate (%) = (m0 − m1) / m0 × 100, where m0 is the initial dry sample mass and m1 is the mass retained on the designated sieve after the test. The test report should state the method used, drum geometry, rotation speed and duration, sieve aperture, individual and mean results of replicate determinations, and the standard deviation. Testing should be performed by qualified in-house laboratories or accredited third-party laboratories following documented quality procedures.

Acceptance Criteria and Interpretation

For high-quality organic sulfur hydrogenation catalysts, typical commercial specifications limit attrition loss to approximately 0.5–2.0%, depending on shape and size. Results should be interpreted together with crush strength data: a catalyst with high crush strength but high attrition loss may indicate surface dust or weak particle edges, while simultaneous failure of both tests suggests fundamental formulation or processing defects. Relevant international and national frameworks, including ASTM standards and Chinese national (GB/T) and chemical industry (HG/T) standards for catalyst mechanical properties, should be consulted when defining contractual acceptance limits.

Applications

Attrition rate testing of organic sulfur hydrogenation catalysts supports quality assurance and process reliability across a wide range of industries:

• Ammonia synthesis gas purification, protecting downstream zinc oxide desulfurization beds and synthesis catalysts
• Methanol and hydrogen production plants, where feed gas total sulfur must be reduced to sub-ppm levels
• Natural gas processing and feed pretreatment for liquefaction and petrochemical units
• Refinery fuel gas, hydrogen unit, and pre-reforming feed purification systems
• Coal chemical and coke oven gas treatment, where COS and CS2 concentrations are elevated
• Catalyst manufacturing quality control, batch release, supplier qualification, and independent verification by accredited third-party laboratories

FAQ

What attrition rate is considered acceptable? Most industrial specifications for extruded organic sulfur hydrogenation catalysts require attrition loss below 1–2%, with premium spherical products often specified below 0.5%. The exact limit should be agreed between purchaser and supplier based on reactor design and handling conditions.

How does attrition rate differ from crush strength? Crush strength (ASTM D4179, ASTM D6175) measures the resistance of an individual particle to a static point load, while attrition rate measures the collective generation of fines under dynamic tumbling stress. Both properties are complementary indicators of overall mechanical integrity.

How much sample is required for the test? Typically 100–200 g of representative catalyst per determination, with three replicate samples recommended. Additional material is needed if supplementary particle size distribution or crush strength testing is required.

Can a sulfided or used catalyst be tested? Attrition testing is normally performed on fresh oxide-form catalyst. Used or sulfided samples may evolve toxic H2S during handling and drying and require special safety precautions; testing of such samples should only be undertaken by laboratories equipped for hazardous materials.

How can attrition losses be minimized in practice? Use gentle conveying, drop heights below recommended limits during loading, proper drum loading with dense loading or sock loading techniques, controlled ramping of temperature and pressure during startup, and verified attrition and crush strength results at catalyst receipt.