Production of urea and melamine

A system for the production of urea and melamine, said system comprising a urea production zone;

• said urea production zone comprising a high pressure urea synthesis section and, downstream thereof and in fluid communication therewith, a urea recovery section adapted to separately obtain a urea solution and a carbamate recovery stream;
• the system further comprising a melamine production zone;
• said melamine production zone comprising a high pressure melamine synthesis section, said production zones being connected with each other so as to allow transport of melamine off-gas obtained from the melamine synthesis section to the urea production zone, whereby the urea production zone comprises a medium or low pressure treatment section adapted to receive said melamine off-gas, said treatment section comprising a flashing unit and at least one ammonia and carbon dioxide liberating unit, said flashing unit having an inlet for urea synthesis solution connected to an outlet of the high pressure synthesis section via a pressure reducing device, an outlet for flash vapour and an outlet for flashed liquid, said ammonia and carbon dioxide liberating unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flashing unit, an outlet for carbamate recovery vapour and an outlet for a urea product stream, said outlets for flash vapour and carbamate recovery vapour each being connected via a condensation and recirculation circuit to the urea synthesis section, wherein an outlet for melamine off-gas vapour from the melamine synthesis section is in fluid communication with an inlet of at least one ammonia and carbon dioxide liberating unit, allowing the melamine off-gas vapour to be contacted with flashed liquid in or upstream of the ammonia and carbon dioxide liberating unit.

A method for the modification of a pre-existing urea plant comprising a high pressure urea synthesis section and, downstream thereof and in fluid communication therewith, a urea recovery section adapted to separately obtain a urea solution and a carbamate recovery stream, the method comprising connecting the urea plant with a melamine plant comprising a high pressure melamine synthesis section so as to allow recirculation of melamine off-gas from the melamine off-gas treatment section to the urea plant, the method comprising adding a medium or low pressure treatment section adapted to receive said melamine off-gas, said treatment section comprising a flashing unit and at least one ammonia and carbon dioxide liberating unit, said flashing unit having an inlet for urea synthesis solution connected to an outlet of the high pressure synthesis section via a pressure reducing device, an outlet for flash vapour and an outlet for flashed liquid, said ammonia and carbon dioxide liberating unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flashing unit, an outlet for carbamate recovery vapour and an outlet for a urea product stream, said outlets for flash vapour and carbamate recovery vapour each being connected via a condensation and recirculation circuit to the urea synthesis section, wherein an outlet for melamine off-gas vapour from the melamine synthesis section is in fluid communication with an inlet of the ammonia and carbon dioxide liberating unit, allowing the melamine off-gas vapour to be contacted with flashed liquid in or upstream of the ammonia and carbon dioxide liberating unit.

A method for the modification of a pre-existing system for the production of urea and melamine, said pre-existing system comprising a urea production zone and a melamine production zone;

• said urea production zone comprising a high pressure urea synthesis section in fluid communication with a urea recovery section adapted to separately obtain a urea solution and a carbamate recovery stream, said recovery section comprising a medium or low pressure treatment section comprising a flashing unit and an ammonia and carbon dioxide liberating unit, said flashing unit having an inlet for urea synthesis solution connected to an outlet of the high pressure synthesis section via a pressure reducing device, an outlet for flash vapour and an outlet for flashed liquid, said ammonia and carbon dioxide liberating unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flashing unit, an outlet for carbamate recovery vapour and an outlet for a urea product stream, said outlets for flash vapour and carbamate recovery vapour each being connected via a condensation and recirculation circuit to the urea synthesis section, said melamine production zone comprising a high pressure melamine synthesis section and, downstream thereof and in fluid communication therewith, a melamine off-gas treatment section for obtaining melamine off-gas of reduced pressure;
• said production zones being connected with each other so as to allow transport of melamine off-gas from the melamine off-gas treatment section to the urea production zone, and optionally also to allow transport of urea from the urea production zone to the melamine synthesis section, the method comprising providing a connection between an outlet for melamine off-gas vapour from the melamine synthesis section to an inlet of the ammonia and carbon dioxide liberating unit, allowing the melamine off-gas vapour to be contacted with flashed liquid in or upstream of the ammonia and carbon dioxide liberating unit.