(Draft of January 20, 2004)
Introduction:
Particle size analysis of powders using test sieves is, and has been, the standard for many years. The sieving procedure was formalized in 1925 with the establishment of ASTME-
11. The ASTM standards cover the manufacture of test sieves and the procedure for conducting a manual sieve test.
The manual method of test sieving is still practiced today, but mechanical shakers have become common and new shakers are even relatively quiet. Other methods of particle size analysis have been developed and are common for submicron analysis where sieves are difficult to use.
Quality control programs such as ISO 9000 have required the development of methods to calibrate and certify test sieve procedures. As practiced for decades, the sieve procedure can be outlined as follows:
1. Weigh a sample and record the starting sample weight.
2. Weigh and record the weight of each sieve and the bottom pan in a
A stack.
@
3. Place the sample on the top sieve of a stack of sieves and cover the top sieve with a flat cover.
4. Shake the stack (keeping it vertical) for a specified length of time at a predetermined speed.
5. When the shaking is complete, reweigh and record the weight of each sieve and the bottom pan.
6. Calculate the weight of sample on each sieve and the pan.
7. Calculate the percent of sample on each sieve and the pan.
The total sample recovery for this manual method is typically 95 to 98 percent. For more detailed information see Chapter 19, section 20 of Perry = s seventh edition. Also see the American Society for Testing and Materials A Guidelines for Establishing Sieve
Analysis Procedures (STP 447 B).
Sieving for Fluidized Bed Reactor Design:
One application of using particle/powder descriptions is to predict the behavior of a gassolids fluidized bed reactor. Fundamental parameters such as size and size distribution, shape, and density of particles influence explicitly and implicitly the behavior of fluidized beds. Most correlations that are used to predict parameters such as the minimum fluidization velocity require at least an average particle diameter, d p(ave)
.

For a particle of any size other than a sphere, there are many ways (>12) of defining its size. In fluidized bed reactor design, these four particle descriptions are the most common: sieve size, volume diameter, surface/volume diameter, and surface diameter.
They are all well defined in the literature.
A common and useful particle parameter is the Sauter Diameter which can be calculate from the sieve results as shown in Table 1. The fluidized bed design the calculation method gives A proper @ weight to the influence of smaller particles and fines. It is not however theoretically related to the surface/volume and volume means. Common approximations are used for d sv
and d v
where the former is 0.87 times the Sauter
Diameter and the latter is 1.13 times the Sauter Diameter. Table 1 is attached and is from the a Gas Fluidized Bed Technology training notebook prepared by Derek Geldart
(also see his 1986 John Wiley book).
The sphericity of powders has been defined and can also be measured, though not easily.
The ratio of the surface volume diameter to the volume diameter is defined as sphericity. Note that the approximations above yield a ratio of 0.77, which is very close to the measured value for ground coal (0.75).
Experimental Procedure:
Assume that you are considering the fluidized bed combustion of ground coal. In order to design the reactor = s gas distributor (only one of many design decisions that require d p
), you must know the average particle diameter. Therefore, as a team you are requested to determine the Sauter Diameter for a sample of ground coal. Assume that any material that ends up on the pan will not be used for the process. Since abnormal size distributions pose additional design problems, you should also determine and evaluate the particle size distribution.
We have prepared a A stack @ of 3" sieves for your use. Manual shaking should be for 10 -
15 minutes and can be done in shifts. In the spirit of ISO, you should document and fully describe the sieving procedure.
Note: Table 19-6, which is attached, is from Perry = s handbook and describes sieve specifications.