Dry sampling

Several sampling techniques are available:

• Static samplers
• A simple eductor system to aspirate the sample from the sample tube and present it to the instrument.

Static samplers have several advantages:

• They sample continuously and present a very minimal delay before being measured by the instrument (in the order of 1-2 seconds for a standard line length). This is one of the most strong economic justifications for this instrument, as continuous measurement allows short infrequent process upsets to be monitored and characterized, so the process may be better understood and optimized, as well as safeguarding product quality
• They are extremely robust and reliable as they have no moving parts
• They are very simple to implement, requiring only two weld stubs (for the inlet to the instrument and the return line to the process) which generally requires very minimal down-time for the process, and very few modifications to the process line
• They are very repeatable in sampling
• Insertion and removal of the sampler can be done usually whilst the process is on-stream
• They take up very little room within the process line thus preventing any hindering of process flow
• Line - cleaning is simple by using a back-flush of the venturi air to clean the sample line and sampler
• They are independent of line diameter, meaning that the same standard parts can be used in several sampling locations for several instruments
• They only require air (or the appropriate process gas e.g. nitrogen) to operate the sampler

They do however rely on having the correct powder flow conditions with the process pipe. This is determined in a number of ways:

• By looking at what point in the process the sample is being measured - in a gravity chute after an airlock valve, the powder is available in good quantities, has relatively slow velocities and can be fairly well blended by the rotary airlock valve; sampling after a dynamic classifier can prove more challenging as the powder flow generally has a strong swirl and can be more dilute and possess higher velocities - in this case, a flow conditioner would be inserted before the sampling point which would make the powder flow more uniform and enable sampling to be carried out
• In the case of a J-tube sampler, a pipe traverse can be carried out which is plotted as a particulate loading graph automatically by the Insitec software. The sampler can be positioned at the point of greatest flow.
• For a given flow of powder in the process stream, the loading on the sampler is determined by the size and number of holes in the static sampler, and the flow rate of air to the venturi.

Representativity means that sample taken is statistically representative of the powder flowing in the process stream. For a powder to be representative:

• The amount recovered should be statistically significant
  Most laboratory techniques measure several grams of powder. As manual sampling is time consuming, most quality techniques call for at most a sample to be taken every two hours, and at worst once a day. In a high tonnage process e.g. 100 t/h, 5g of powder could be assumed to be representative of 200t/h. With an Insitec, 20kg/h can be measured of powder with update rates every second if required. In the same process, this would equate to 40 kg/h (a much more statistically significant sample), but it in a low tonnage process e.g. pilot-scale air jet mills with a flowrate of 10-20kg/h, the entire batch can be measured!
  
• The sampling technique should have little bias
  All sampling techniques will show some errors as opposed to measuring the entire batch. Malvern optimizes the sampling by:
  - first and foremost working with the client to select the optimal sampling point in the process (to both achieve success in the client's objectives and to maximize representativity of sampling).
  - aspirating sample at velocities higher than the process fluid velocity (i.e. up to twice the isokinetic sampling velocity)
  - understanding how spatial variations may affect sample representativity. For low to medium flowrates, if the powder is well dispersed and relatively mono-disperse, a J-tube will suffice, however if the powder is less well-dispersed and/or the powder is polydisperse, the flute is used.
  
• The sampling/measurement technique should be repeatable
  Manual sampling has an inherent disadvantage in that the sampling even from the same location can be open to random errors from the same operator, or even bigger errors induced with the use of several operators (as in the case of workshift changes). Insitec has the big advantage of continuing to deliver the same sampling performance 24/7, with efforts focused primarily in setting the technique up correctly, and on proving that the performance is acceptable through correlation with the client's current laboratory technique.

Malvern Insitec sampler configurations include:

• J-tube static sampler
  loading is determined by the diameter of the nozzle chosen. Useful, simple approach when powder flow is homogeneous and particles are small to medium sized.
  
• Flute static sampler
  this is a very flexible sampling strategy, where holes are drilled in the sampling tube to match the powder flow conditions. The loading is determined by the size and number of holes drilled. This has the advantage of being able to sample powder streams where the flow is not totally homogeneous (if the flow is spatially non-uniform, a flow conditioner upstream of the sampling point may prove useful).
  
• Dynamic samplers - Auger system
  For 10-100 t/h with abrasive particles. This is used in conjunction with the eductor system. The auger continuously takes a first cut sample from the process line, and is then sent to a gravity chute to be resampled by a flute or J-tube in the eductor system.

	More on augers...

• In-line units
  In-line units are less commonly installed than eductor systems, mainly due to the problems of correct sample dispersion and in taking the system off-line without affecting the process stream. However, in cases where powder dispersion is not a problem (for example in a very turbulent pneumatic convey stream), and the process stream is too dilute for effective sampling, an in-line unit can be of use.

	More on measurement zones...

• At-line systems
  For off-line near process measurement systems, there are a range of at-line sample feeders available.

	More on at-line systems....