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Materials testing is a fundamental aspect of engineering and manufacturing. It plays a pivotal role in ensuring the reliability and efficiency of a wide range of products and processes. Material property calculation is commonly associated with tensile testing, which assesses a material’s response to stretching forces, sometimes to high elongation. However, while tensile testing garners significant attention, compression testing is equally crucial. In this article, we narrow the focus to the specific domain of crush testing for granules, pellets, and similar test specimens.

Subject matter

The need to test granules and pellets for crush resistance arises in various industries where these small particles play a vital role in product performance, safety, and quality. Applications include chemical processing where catalysts, in the form of granules or pellets, are essential to the chemical reaction, and not to mention agriculture, food & beverage and pharmaceuticals sectors where similar types of products are used as additives.

Such test specimens contain no moving parts and have little physical function other than to stay intact when in use, so as not to contaminate. Brittle materials exhibit little or no plastic deformation before failure, and their fractures are typically sudden and catastrophic.

Bulk catalyst crush testing

As all industries strive for greater sustainability and efficiency, the performance of catalysts and pellets has never been more critical, particularly in the fields of gasoline, plastics, polyesters and artificial fertilizer (ammonia).

Bulk crush testing of packed-bed catalysts is a valuable tool in their development and evaluation. This testing method serves as a critical quality control measure to assess the mechanical strength and durability of catalyst particles. By subjecting catalysts to controlled compression, it enables researchers and engineers to determine the point at which the catalyst bed begins to deform or crush.

PPT Group ASTM D7084 fixture on OmniTest
The test standard ASTM D7084 covers catalyst particles of 0.8 – 4.8 mm diameter, such as granules. Particles are contained in a cylindrical sample holder and crushed with a piston. Pressure is applied at a uniform rate to the bed of particles, held for 30 seconds and slowly released. The pressure which causes 1% of fines (particles which pass through a sieve of half-size of the pellet) is recorded and typically lies between 0.1 – 0.35 MPa for granules and 1 – 3.5 MPa for larger formed materials.

This quality assurance testing strategy is crucial for designing and selecting catalysts for various industrial processes, such as refining, petrochemicals, and environmental applications. It ensures that catalysts can withstand the harsh conditions within reactors, minimizing the risk of particle breakage or attrition, which can lead to reduced catalyst performance and increased operational costs.

This bulk crush testing of packed bed catalysts is an indispensable tool in optimizing catalyst formulations, ensuring their long-term stability, and ultimately enhancing the efficiency and sustainability of chemical and petrochemical processes.

Variation on a theme

Potassium sorbate is widely used as a food preservative to inhibit the growth of molds and yeasts in various food and beverage products. It is typically produced from a solution which is cooled to induce crystallization. The resulting potassium sorbate crystals are separated from the liquid and dried to remove any remaining moisture.

Crush resistance testing is also a fundamental aspect of quality control for potassium sorbate production. The material properties of this compound must ensure that the particles are resistant to crushing or breakage during packaging, handling, storage, and transportation. This is crucial to guarantee the practical suitability of the product in the supply chain. This performance benchmark tests that when in real use in tablet form, the potassium sorbate maintains its structural integrity until it dissolves to deliver the active ingredient or remains intact to be evenly dispersed within its matrix, or base material.

Regulatory compliance with industry standards and regulations may require crush resistance testing to ensure the product meets specified quality and safety standards. This is particularly important in industries where the product comes into direct contact with food, drugs, or other consumer products. 

The crystals are crush tested in a similar manner to the catalyst test—again implemented by a universal tester and a plunger/cylinder fixture. The bulk test sample is levelled in the container, the compaction plunger is inserted, and the sorbate displaced to its maximum under 2400N pressure. The displacement distance is plotted against the pressure with the area under the curve being directly related to the hardness of the sorbate (the smaller the number the harder the sorbate). The assessment is repeated multiple times per batch source of sorbate, with data being statistically analyzed based on the normality of the particle size data.

Single pellet crush standards

Commercial catalysts are optimized for energy release in chemical reactions and come in diverse shapes, including pellets, granules, tablets, spheres, rings, and extrudates. This diversity poses challenges in testing, leading to the creation of various integrity test standards tailored to different catalyst geometries and types.

Individual results

When catalytic materials consist of regular-shaped particles, the implementation of single pellet strength testing methods is possible.

PPT Group, OmniTest compression test single catalyst pellet
ASTM D4179, titled "Single Pellet Crush Strength of Formed Catalysts and Catalyst Carriers," focuses on evaluating the compressive ‘side crush strength’ (SCS) of individual pellets in regular forms such as spheres, short cylinders, or tablets. Specifically, it allows for testing as a radial or axial crush. The test applies a uniform force to the pellet until it crushes or collapses, recording the maximum crush strength at the point of initial collapse.

In such a test, a small diameter compression plate or flat-faced probe descends to compress an individual sample of the product. The load at which the material’s structural integrity fails is measured—often requiring a high data sampling rate to detect any sudden collapse—and this break force value is reported.

ASTM D6175, “Radial Crush Strength of Extruded Catalyst and Catalyst Carrier Particles” also covers measuring the compressive ‘side crush strength’ (SCS). However, in this case, the relevance is only for radial crush.

Increasing efficiency

The single pellet crush test can be time-consuming when there is the essential need to calculate representative data for a whole batch. A software-controlled materials tester augmented with a degree of automation can vastly increase repeatability, throughput and confidence in the recorded hardness data.

PPT Group. Pellet auto tester single frame closeup
An automated pellet crush testing system. Many individual pellets are loaded at one time onto a rotating platen, into which has been machined suitably dimensioned depressions to locate each single specimen. Operators are spared the repetitive error-susceptible (and tiresome) task of manually loading the sample to perform an individual material crush resistance test.

Software is an integral part of a modern materials testing system, controlling the mechanical movement of the fixtures to apply appropriate loading to the test piece in tension or compression. It also enables the repetition of tests from which a representative hardness (kg/granule) for the supply is easily and reliably obtained. The software performs extensive statistical analysis of results across sample batches and can present data in easily digestible report formats.

Conclusions

Crush testing of particles, pellets and granules is a vital aspect of ensuring the reliability and performance of a wide range of processes in many industries. Accurately, efficiently and repeatedly characterizing the materials against international standards provides confidence in various applications that rely on the physical integrity of these products in their dependent processes.