Chemistry Reference
In-Depth Information
The centrifuge to be used operated on a similar principle. It was lined with
a cloth and spun. The slurry was then added to the spinning centrifuge. The
liquid passed though the cloth leaving the spun solids behind. A blade would
then scrape the solids off the walls of the centrifuge and they would exit the
centrifuge through a chute at the bottom. One of my tasks was to identify a
suitable cloth. Conceivably if the weave was too fine, the separation would
be too slow. If too coarse, then fine particulate solids could pass through.
Coincidentally, another member of our group, also a freshly minted
Ph.D. chemist, had the same task for a different process. We took different
approaches. I took a bottle of laboratory-prepared slurry and brought it to
the plant. I talked to the operators who ran the centrifuge and solicited their
advice. I also consulted with some of the experienced people in our group.
Based upon the advice given, I selected a cloth and trialed it on a laboratory
centrifuge. It worked well and when done in the plant, the separation went
smoothly.
The other chemist took a different approach. He did statistical laboratory
experiments with dozens of cloth materials. After weeks of research and some
very impressive weekly reports, he identified an optimum cloth. When the
process went to the plant, the separation was a disaster. The material would
not separate in the centrifuge. After several delays and switching cloths, the
separation was finally done. The delays put the plant behind schedule and did
not do much for the credibility of our group. All of this could probably have
been avoided by consulting experienced people prior to the scale-up.
On the topic of communication regarding scale-ups, let me relate another
true story. A scale-up I did early in my career involved the bromination of a
ketone. It was done in a pilot plant on a scale of 100 gallons. The reaction
proceeds through the enol so is acid catalyzed.
H
O
O
O
Br
2
H
+
R
′
R
′
+ HBr
R
′
R
R
R
Br
A solution of the ketone in water is made. A small amount of HBr is added
and then bromine gradually added. The reaction is rapid and exothermic but
can be controlled by the rate of bromine addition. For each equivalent of
bromine added, an equivalent of HBr is formed. Initially the HBr dissolves
in the water but eventually the water becomes saturated and the HBr evolves
as a gas from the reaction. In the lab, it is easy to contain the HBr. I passed
the gas through a flask containing a sodium hydroxide solution. In the pilot
plant, I planned to pass the gas counter-current through a water stream. The
water absorbs the HBr gas. Based upon HBr solubility in water and bromine
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