BUCHI SpeedDigester K-436 - Manual (Page 20)

4 Description of function
20 K-425 / K-436 Operation Manual, Version A
4 Description of function
This chapter explains the basic working principle of the SpeedDigester K-425 / K-436. It also shows
how the instrument is structured and provides a general functional description of its assemblies.
4 .1 Functional principle
The SpeedDigester K-425 / K-436 is a special heating device to digest samples. It can generate max.
temperatures of 580 °C typically. This allows e.g. acid digestion of organic sample components in a
concentrated sulfuric acid / catalyst mixture, known as 'Kjeldahl digestion'. During the processing the
sample material can reach peak temperatures of up to 370 °C at the bottom area of the sample tubes.
The maximum temperature of the sample highly depends on its composition i.e. the ratio of catalyst to
acid and can change during the digestion process.
Digestion process overview
3
2
1
4
Environmental
air inlet
dangerous vapors are drawn away
Condensing
zone
Infrared heating
Boiling / digesting
sample
Insulation plate
(fireclay insertion)
Acid vapor
outlet to
suction unit
(e.g. Scrubber)
( ≈ 5 cm)
Safety
zone
Processing steps:
The infrared heating • 1 generates high temperatures in the sample.
Inside the sample tubes the digestion process takes place • 2 while the acid is constantly boiling.
Hot acid fumes rise into the condensing zone • 3. After reflux, the acid flows back down to the
sample material, establishing a constant cycle.
Residual fumes which escape the condensation zone are highly corrosive (!) and hazardous to life
•
forms. It must be withdrawn by a suction unit that deliveres sufficient flow (i.e. with a Scrubber
B-414 or a water jet pump) 4.
Note
The condensation should not take place inside the safety zone in order to prevent digestion of the
sealing rings. In case the condensation is too close to the suction module (e.g. due to very high
temperature settings) the digestion fumes may escape and the sample can dry out!