Paolo Cardillo; Angelo Lunghi
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.
page last revised December 7, 1998