Particle Size and Shape of Catalyst Supports

Hydrotreating catalysts come in different sizes and shapes depending on the application and manufacturer. Common shapes include spheres, pellets, cylinders, trilobe, and quadralobe shaped pieces. The size and shape of the catalyst support are important specifications that require quantification. Older techniques including calipers and sieves have been replaced by dynamic image analysis using the CAMSIZER. The CAMSIZER provides fast, accurate, highly reproducible results of all physical definitions important to catalyst manufacturers and end users.

Extruded catalysts ready for measurement with the CAMSIZER
Extruded catalysts ready for measurement with the CAMSIZER (click to enlarge)

The Role of Particle Size and Shape

Catalysts are a chemical species that increase the rate of a chemical reaction by providing an alternative reaction pathway to the reaction product, typically by lowering the activation energy. A catalyst is different than a chemical reagent in that it participates in but is not consumed by the reaction. Catalysts are involved in more than 80% of all processes in the chemical industry. In most of these processes, a mixture of chemicals (liquid or gaseous) is induced into a reactor filled with catalyst material.  The reaction vessel is put to a certain pressure and temperature to start the reaction. The acceleration effect arises mainly from absorption and activation phenomena at the surface of the catalyst.

For effective catalytic reactions with a high yield it is important to have

  • Large surface area of the catalysts
  • Enough free volume in the reactor in order to achieve a high throughput - the more volume is occupied by the catalyst, the less reactant can be introduced to the reactor
  • The ratio of catalyst surface and reactant volume is crucial for controlling the reaction kinetics
  • High permeability within the reactor in order to have a high and constant flow and good mixing of the reactants

For elongated catalysts, size and shape distribution provides useful information on how the aims mentioned above can be achieved. The size and shape of the catalyst pieces is a trade off between the desire to minimize pore diffusion effects in the catalyst particles (requiring small sizes), and pressure drop across the reactor (requiring large particle sizes). Size analysis has to provide information on width as well as length and length distribution of the particles and to show the amount of dust in the sample. Shape analysis needs to give results on various dimensions as well as symmetry. Taking all these parameters into account, it is possible to predict the reaction behavior of the catalysts in a certain process. Except for spherical catalysts, these ambitious demands can never be fulfilled with a sieve analysis.  Measuring single rods with a caliper gives precise results but this method is time consuming and still it is only possible to measure a very small quantity of particles.


How the CAMSIZER Works (Video)

Watch an overview of the CAMSIZER Dynamic Image Analyzer (7:16) by clicking the play button.


Spherical Catalysts

Spherical catalysts can be measured with the CAMSIZER quite easily without special efforts.  Using xc min or xMa min the results should be in good agreement with sieve analysis.  With shape parameters like Symmetry it is possible to detect broken or defective particles.  Aspect ratio (b/l) or Sphericity can control the roundness of the beads.

Extruded Catalysts

It is possible to define the length and width of cylindrical catalysts several ways using the CAMSIZER. The length can be defined using xFe max, xlength or xstretch, depending on the preference of the user. xFe max displays the longest dimension (diagonal) of the particle projection, xlength and xstretch deliver the true height of the oriented extrudates – see the figure below.

CAMSIZER length definitions: xFemax, xStretch, xLength
Figure 1: CAMSIZER length definitions: xFemax, xStretch, xLength

 

The width for straight extrudates can be defined as xc min or xMa min. The width measurement for curved extrudates is better defined by xMa min as seen below in the figure below.

CAMSIZER width definitions: xMa min, xFe min
Figure 2: CAMSIZER width definitions: xMa min, xFe min

Special Cross Sections

Many catalyst extrudates have a trilobe or quadralobe cross section (Fig.3). Using the xc min or xMa min size definition it is possible to distinguish between the two different diameters of the quadralobe extrudates. They are reflected in the two maxima of the bimodal xc min or xMa min distribution (Fig.4). For a trilobal cross section xc min delivers a width distribution representing an average of all projections (Fig.5).

Quadralobe and trilobe catalyst supports
Figure 3: Quadralobe and trilobe catalyst supports (click to enlarge)
Quadralobe catalyst width and length measurements from the CAMSIZER. Note the ability of the CAMSIZER to distinguish each width.
Figure 4: Quadralobe catalyst width and length measurements from the CAMSIZER. Note the ability of the CAMSIZER to distinguish each width (click to enlarge)
Trilobe catalyst width and length measurements from the CAMSIZER
Figure 5: Trilobe catalyst width and length measurements from the CAMSIZER

 

Note: In both graphs the shorter green distribution shows the xlength length distribution. The taller peaks in maroon show the xc min width distributions, including the two different values for the quadralobe shape.


The Importance of Orientation

The CAMSIZER can provide width and length information comparable to hand caliper measurement because the particles are oriented to fall without tumbling. The key accessory to accomplish this feat is the motorized guidance sheet. Watch the video below for a short explanation of how this accessory makes catalyst measurements possible.


Summary of Benefits

The CAMSIZER provides significant gains in speed and reproducibility over sieves, hand calipers, and various homemade measurement systems. A CAMSIZER implementation will produce the following benefits:

  • Significant labor reduction due to measurement in less than 5 minutes
  • Excellent statistics with measurement of thousands of particles every minute -- more reliable indicator of good vs. bad batch quality
  • High reproducibility owing to method-driven software, automated measurement, and excellent statistics
  • Easily matches historic data with high resolution cameras
  • Measures from 30 microns to 30 millimeters -- quantifying dust, oversize particles, and agglomerates
  • Control coating process by measuring how thickness and shape change
  • Typical payback in less than one year

Particle Analyzers for Catalyst Support Applications

CAMSIZER Dynamic Image Analyzer

HORIBA's CAMSIZER features dynamic image analysis to provide rapid and precise particle size and particle shape distributions for dry powders and bulk material in the size range from 20µm to 30mm.