Wednesday, June 6, 2012

Process Wednesday: Seeing through a glass, darkly

I'm going to guess a turbidity probe costs more, too.
Credit: directindustry.com, southernsportsman
Upon looking through a manway and seeing a hazy solution (suspension?), I was reminded of limnology (the study of inland waters) and one of their tools, the Secchi disk:
The Secchi disk, created in 1865 by Pietro Angelo Secchi SJ, is a circular disk used to measure water transparency in oceans and lakes. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water. 
Of course, one finds that chemists and chemical engineers have come up with something a little more advanced to measure turbidity inside reactors. From Dow crystallization engineers in the pages of Organic Process Research and Development [1]:
The fiber-optic probe measures backscattered light to generate a real-time turbidity signal indicative of the amount of solid-phase material present in the crystallizing slurry.  
A fiber-optic probe can be used for a broad range of applications because the output signal depends only on the probe’s ability to detect backscattered light; that is, no specific chemical properties such as molecular absorption are required. It also can be quite robust; numerous Dow probes have been in service for more than 10 years with no degradation in performance and without fouling problems. The basic Dow design includes a polished sapphire window, a spring-loaded gasket seal system to resist solvent infiltration, and fiber-retaining inserts (Figure 1). This design allows for expansion and contraction over a wide temperature range without breaching the process seal, yet it maintains precise optical alignment of the fibers and the window.
Well, I guess we'll have to turn that agitator back on and let it stir some more...

[1] Harner, R.S.; Ressler, R.J.; Briggs, R.L.; Hitt, J.E.; Larsen, P.A.; Frank, T.C. Org. Process Res. Dev., 2009, 13 (1), pp 114–124. 

1 comment:

  1. Some GL reactors have their own "Secchi disks" - wall or baffle graduations made with glass in contrasting color.
    You post gave me an idea to instruct the operators to note the lowest visible graduation rather than to note when all solids dissolved into the reaction mix. This should be a lot less subjective measure. Thanks!

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