The overall goal of this methodology is to measure extrudate catalyst bending strength and to predict how well catalysts resist breakage caused by either a collision with a surface or stress in a fixed bed. I originally had the thought that perhaps the rupture force in bending mode was the appropriate length to relate catalyst strength to the severity of handling of the catalyst to describe catalyst breakage. The main advantage of this technique is that it measures how catalyst extrudates hold up to breakage in quantitative simulations of real life events.
This technique accurately measures the length to diameter ratio of a catalyst and therefore quantitatively measures the extent of the breakage caused by collisions or stress. This method's evaluation of extruded resilience to breakage in fixed beds can also be applied to other shapes such as a spherical catalyst. The bending strength of an extrudate is difficult to determine without visuals because of the various stress regions during bending.
To begin the procedure, obtain at least 30 catalyst extrudates by riffling an appropriately thermally-pretreated catalyst sample of interest. Then turn on and calibrate the bending test frame. Attach a 10 Newton load cell to the bending test frame.
Once the test frame is ready to use, select an anvil speed of 0.2 millimeters per second and a five millimeter support span. Choose modulus of rupture and maximum force in the results tab. Configure the test to end when the load force drops to a sufficient extent to indicate extrudate breakage.
Fill in any required sample and specimen information and ensure the software is ready to start the test. Then clamp a five to six centimeter diameter Buchner funnel with one millimeter pours to a lab stand. Direct a stream of nitrogen gas upwards through the filter funnel to create a dry gas blanket.
Transfer the catalyst extrudate sample from a desiccator to the filter. Then pick up one specimen with tweezers and quickly place the specimen across the support beams of the test frame. Center the specimen as best as possible while still working swiftly to avoid moisture pick up.
Start the test once the sample is in place. During the test ensure that the extrudate does not get lodged between the anvil and the support points because that may throw off the rupture force reading. Ensure that the crosshead returns to the starting position after experiencing a 40%drop in load force.
Clean the sample area and supports with a soft brush before placing another sample on the supports. When the new specimen is in place, click next in the software to start the next test. Once all specimens have been measured in this way, end the test to obtain the automatically generated report of the extrudate strength properties.
To begin the aspect ratio measurement procedure, riffle a catalyst sample of interest to a representative size of 50 to 250 particles. Gently sieve the sample to remove particles with a length to diameter ratio of less than or equal to one. Then wipe the glass of a scanner with a microfiber cloth to remove any dust or debris.
Place a clean transparency sheet on the scanner glass. Randomly distribute the extrudates across the transparency within a rectangular area of no more than 10 centimeters by 20 centimeters. Use tweezers to separate particles in contact with each other or to move particles to more open areas.
Close the scanner lid and prepare the scan and particle imaging software. Scan the particles and review the results to confirm that all particles with the appropriate dimensions and placement are included in the scan. Once the scan is satisfactory, save the results and record the average diameter, average length, and number of particles.
Prior to the drop test measure the aspect ratio of a sample of about 50 riffled catalyst extrudate specimens as previously described. To begin the test, assemble a drop tube with a collision plate and recovery pan made of type 316 stainless steel. Leave about a one centimeter gap between the tube and the collision plate.
Set the vibratory feeder discharge height accordingly for the drop tube length and center the feeder shoot outlet over the drop tube. Measure and record the drop tube height as the vertical distance from the catalyst feeder release point to the collision plate. Then load the sample into the feed hopper.
Set the feeder frequency to 250 hertz or less. Monitor the collision plate and adjust the rate of discharge as needed to ensure that the extrudates do not drop on top of each other. Allow each specimen to fall freely into the drop tube, one by one.
Then turn off the feeder and transfer the particles from the recovery pan to a sieve. Gently sieve the sample to remove the catalyst finds. Measure the post collision aspect ratio of the sample.
Repeat the drop test and aspect ratio measurements up to 10 times this way. Compare the results to initial measurements. To begin the bulk crush testing procedure, fill a tared sample container to overflowing with a representative sample obtained by riffling of properly heat treated catalyst.
Use a metal straight edge to carefully level the sample without over packing the resulting particle bed. Record the mass of the sample in the cup. Then place the sample cup in a load block and piston assembly.
Gently rest the load block on top of the sample ensuring that the extrudates are not crushed. Place the assembly ball bearing in the center of the load block. Guided by a carpenter's level, adjust the lock arm to be level over the ball bearing at the appropriate height.
Lock the arm in place. Confirm that the pressure regulator is set appropriately for the catalyst sample. Verify that the bleed valve is open and that the pressure valve is closed.
Then close the bleed valve and open the pressure valve. Monitor the assembly as the load block rises to the set pressure. Allow the sample to equilibrate at the set pressure for 60 seconds.
Then close the pressure valve and open the bleed valve to release the pressure. Once the load block has returned to its original position unlock the lock arm and carefully remove the ball bearing and the load block. Measure and record the indentation of the sample.
Gently sieve the sample over a wide container to collect the catalyst finds. Measure the post crush test aspect ratio of the catalyst sample and record the quantity of catalyst finds collected. High speed photography of catalyst extrudates impacting a polycarbonate surface indicate that the times of impact and hence the forces experienced by the extrudate during impact as a function of time are spiked and irregular.
The aspect ratio of catalyst samples subjected to repeated drop tests quickly approached the asymptotic aspect ratio for multiple impacts for that catalyst. The difference in aspect ratio became smaller at greater drop heights indicating that the extrudates were impacting at or close to their terminal velocity. A single drop test of extrudates with large aspect ratios clearly showed the asymptotic aspect ratio for a single drop of long extrudates for that catalyst.
Bulk crush tests indicated that the catalyst extrudates only began to break once a critical pressure had been reached at which point the aspect ratio decreased with increasing pressure. The relationship between the starting aspect ratio and the critical pressure was then described in terms of the modulus of rupture, the extrudate shape factor, and the bed properties. This method can help answer key questions about catalyst scale up and enables the technology transfer from laboratory to commercial plants regarding the length to diameter ratio of catalysts.
Once mastered the technique for measuring the modulus of rupture can be done in 10 to 15 minutes if it is performed properly. While attempting this procedure remember to riffle the catalyst to select a representative sample. After watching this video you should have a good understanding of how to measure the catalyst aspect ratio, the modulus of rupture, and breakage by collision and stress.
Don't forget that working with the materials and equipment shown here can be hazardous. Appropriate PPE should always be used for these procedures.
