Dry sample conditioning and transport
The eductor reliably extracts a representative sample for on-line analysis, from dry streams with flow rates in the range
The sample tube is located in the process powder flow (1). The powder from the process stream is aspirated by the air venturi (2), which dilutes the powder, and cools it (if required). The powder exits the venturi as a fast moving turbulent air jet, an energetic process that applies sufficient shearing force on the particles to break up any loosely bound aggregates that would impact data quality. The particle-laden jet passes through the measurement zone (3), where the laser beam is located, before being returned to the process line (4). The optics of the system are kept free from powder by an air purge system (5) which provides a curtain of air across the laser / photodetector windows. Valves (6) are used to both isolate the system in the case of routine maintenance or in order to take a background measurement. The valves can be either manual or automatic (so as to allow a periodic background to be taken automatically).
The eductor system has the following advantages:
- Disperses particles - For most samples, sample dispersion is required before measuring the powder. If dispersion is not carried out, loosely bound aggregates, held together by electrostatic van der waals forces, are measured leading to a artificially larger size being reported by the instrument. The key is to add enough shear to as to disperse these aggregates into their primary constituents, but so much shear that the particles are milled and broken into smaller particles than were present in the original sample
- Allows accurate sampling to be set up
- Has no moving parts
- Provides aspiration for sampling
- Dilutes sample where required
- Cools sample where required
- Can be easily isolated from process stream for routine maintenance
- Measurement cell and sampler are independent of the process line diameter, allowing much more standard components to be used
- Smaller "process footprint" than in-line systems, requiring minimal installation time (in-line systems require a section to be cut out of the process line and replaced with a spool piece containing the analyzer)
- Background measurements can be carried out independently of process being onstream
- Routine tasks such as background measurement and system cleaning can be automated
- Inert gas such as nitrogen can be used as motive and purge gas instead of air for explosive applications
- Prevents exposure of analyzer to process conditions, protecting instrumentation
When installing an eductor system on plant, there are several points to bear in mind :
- The line length from the sampling point to the instrument should be kept to a minimum
- In some applications, powder may arch across the sample inlet. In this case, for those systems using the automated Malvern Link cleaning system, the analyzer can detect that it is no longer measuring powder and initiate an automated cleaning sequence to clear the line and sample inlet and then continue measuring
- Multiplexing several lines to one instrument is to be avoided where possible as automation is complex, and results in a system which is no longer capable of real-time measurements (e.g. if there are three lines, each line may be measured only every 10 minutes - in comparison, a dedicated instrument on each line is capable of measurements every second).
Malvern also supply a booster probe which reduces the over sampling of the fines. The probe is more stable and improve the dispersion of the powder.