In several sections of a urea plant easily pipelines can become blocked when temperatures become too low and crystallization can take place. In the evaporation section urea melt is present which crystallizes at about 133 oC when in pure form. Furthermore urea can also polymerize to biuret, triuret and polyurea with high crystallization temperatures when urea is kept for a longer period at high temperatures. Not only pipelines can become blocked, also internals of several equipment items can suffer from these problems (refer to picture) In the recirculation section ammonium carbamate is present which crystallizes at about 153 oC when in pure form. And in the high pressure synthesis section urea and carbamate mixtures are present. During normal operation blocking risks of lines is small, however during a shut-down of the plant, blocking can occur when not the right measures are taken. This Round Table discussion discusses which measures to take and what to do when a line is blocked.

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