Hydroxyurea
Paolo Cardillo; Angelo Lunghi
San Donato Milanese, Italy
Chemical & Engineering News (1 Jun 1998) Vol. 76, No. 22, pp. 6.
Hydroxyurea is used as an antineoplastic and in the treatment of
AIDS, but one can find little data about it in the literature. For
example, in the 12th edition of the Merck Index, only its melting
point (133-136 
C) is reported. In particular, no data are
available about its thermal stability. Recently, in a fine chemicals
factory, a vessel (3 cubic meters) exploded during the concentration
step of hydroxyurea production.
Concentration of an aqueous solution was carried out at 50 C
while heating the vessel by hot water in a stirred tank connected to
a vacuum line (50 mm Hg) and equipped with a condenser. A stirrer
failure occurred on Friday afternoon and it was impossible to empty
the vessel because of a solid bulk precipitated at the bottom of the
tank. Operators tried to dissolve the solid hydroxyurea by heating,
but this attempt failed. They left the plant for the weekend after
filling the tank with fresh water. On Tuesday morning, an explosion
occurred, with no injury to personnel, but with serious damage to
structures.
Our institute was put in charge of studying the thermal stability of
hydroxyurea and its aqueous solutions, in order to understand the
causes of the accident. Several tests performed with differential
scanning calorimetry, thermogravimetric calorimetry, heat-flux
calorimetry, and adiabatic calorimetry techniques showed that solid
hydroxyurea decomposition starts at 85 C, while aqueous
solutions decompose at a lower temperature (70 C). In both
cases, a considerable heat evolution (2,200 joules per g) and a
buildup of pressure were observed.
Several ARC runs on an aged sample showed a variable induction time;
for example, at 50 C, the induction time was 80 hours. The
probable autocatalytic mechanism of decomposition, together with the
long stay, caused a temperature rise to boiling point, which, in
addition to decomposition gases, led to a pressure increase in the
sealed tank. The self-accelerating heat evolution and the rate of
pressure buildup caused the vessel to burst.
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