Control of pure benzene crystallization point?
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The purity of pure benzene in our facility is 99.99%, and its melting point remains between 4.0 and 4.4 – it never reaches above 4.5. I also suspected that the purity of benzene was not up to standard; even after conducting a PONA test, the value was 99.93. The water content is around 300 ppm, and all other parameters are within the acceptable range; what could be the cause of this?
That’s impossible. The purity of my benzene is between 99.97 and 99.985, and its melting point is usually above 5.45; so how could the melting point be unsatisfactory for benzene with a purity of 99.99?
The water content isn’t very high; extract the feed material and take a sample to check whether there’s too much cyclohexane present. The PONA value you obtained is 99.93, which is quite low – generally, this value should be above 99.95.
For extraction, the benzene is considered qualified if its freezing point is above 5.45; in that case, its purity is also considered satisfactory. However, just because the purity is satisfactory does not necessarily mean that the freezing point is also within acceptable limits.
We’ve found the cause: we extract the raw material without toluene, so the toluene tower isn’t in use. But nothing is absolute, so some heavy compounds accumulated at the bottom of the benzene tower; they’ve now been removed and the crystallization parameters are satisfactory