Obtaining Representative Samples of FCC Catalysts
The task of obtaining a representative sample of fluidized cracking catalyst from a fresh catalyst shipment is not an easy one. It is very difficult to maintain a homogeneous mixture of different sized particles when they are maintained in a non-fluidized state. If an FCC catalyst with a typical particle size distribution is subjected to vibration, either horizontal or vertical, an initially homogeneous mixture will separate into distinct particle size layers, ranging from coarse at the top of the bed to fine at the bottom.
This separation should be considered when sampling truck or railcar shipments since an improperly collected sample when analyzed can lead to misleading characterizations of shipment quality. To obtain a representative sample for particle size analysis from a railcar or truck may be a difficult task unless one has invested in a more sophisticated sampling technique, such as a continuous sampler, which has the capability to sample and combine representative portions of a complete cross section of the entire load during the unloading process.
The following experiment which was performed at BASF's research laboratories helps illustrate the above comments .
A sample of FCC catalyst was separated into four particle size fractions on a Ro-Tap Testing Sieve Shaker. The fractions were
then dyed different colors, and re-screened to remove coarse and fine materials produced in the coloring phase. The following fractions were then used in this study:
The fractions were then weighed in equal proportions and blended to produce a homogeneous mixture. The blend was then subjected to either vertical or horizontal vibration. Vertical vibration was generated by the "volume weight machine" used in the compacted bulk density test. Horizontal vibration was generated by a Ro-Tap. For the Ro-Tap study, a special container was made from a standard 100 mesh screen by sealing the bottom of the screen and cutting a viewing hole in the wall.
Illustration 1 shows the results of shaking for three hours on the "volume weight machine." As the blend is vibrated, the coarse particles move to the surface while the finer particles move downward. While the purple and green fractions separate into distinctive bands, it is much harder to distinguish between the pink and white (0-75 microns) fractions which do not readily separate. These separations only occur in the upper two-thirds of the bed. It appears that the lower third of the bed is sufficiently compacted during the test so that the particles cannot move and remain blended. Additional vibrating time does not increase the depth of the separations. It was also noted that the very coarse particles (purple) separate extremely rapidly and form a surface layer almost immediately upon vibration. When the vibrated sample is seen from above, the entire cross-section is a bright purple, indicating that the separation is not just a wall effect.
Illustration 2 shows the results of the Ro-Tap study. Once again, the coarse particles move very rapidly to the surface. Horizontal vibration does not create a distinct green layer, and the three finer fractions are difficult to distinguish. Extended vibration time does not improve the separation.
While both forms of vibration are responsible for some separation by particle size, vertical vibration is more severe. This separation should be considered when sampling trucks or railcars.
Representative Retain Samples
BASF uses a continuous catalyst sampling device for taking representative sample of all customer shipments. During the loading process the catalyst sample is taken automatically by a device that is designed to sweep through the catalyst flow from the storage silo to the vehicle. The device is designed to make multiple sweeps per minute throughout the time of loading. Thus it insures that a more representative cross section of the load has been looked at during the analysis.
These retain samples, after being taken, are generally kept for a year for any possible future reference work after which time they are disposed of. Sufficient quantities are taken so that a customer can obtain a retain sample upon request to their local BASF technical sales or technical service representative.
BASF recommends to customers who are taking samples of catalyst shipments that a continuous sampling method or similar technique be used to capture a representative sample of the entire cross section of the shipment.