Problem No. 53: Restriction orifices in VOP exchangers and their influence on biuret formation

Vertical one pass (VOP) heat exchangers are used in many urea plants. As the residence time in these heat exchangers is small, unwanted side reactions like biuret formation and hydrolysis of urea are limited. Liquid enters the tubes of the heat exchanger at the bottom side via a control valve. Due to the pressure drop over this control valve some flashing will occur. Furthermore the heat input from the shell side will also result in gas formation. The gas causes a turbulent environment at the tube side increasing the heat transfer co-efficient.

Some phenomena, however, can limit or reduce the performance of these heat exchangers. One problem can be bad distribution of the inlet liquid/gas mixture over the tubes, with some tubes receiving more feed than others, recirculation from the outlet to the inlet may also occur, all limiting the performance. Another problem can be fouling on the tube side, which causes limitations of the heat exchanger.