Large urea plants require large high-pressure equipment that is difficult and costly to manufacture and transport. To reduce urea fabrication costs, Stamicarbon has developed a Mega Urea Plant concept for single-line urea plants that produce capacities of 5000 mtpd.
In the Stamicarbon Mega Urea Plant concept, a proportion of the liquid effluent from the reactor is diverted to a medium-pressure recycling section, thereby reducing the size of the high-pressure vessels needed. In fact, thanks to the Mega Urea Plant concept, the size of the required high-pressure equipment and lines will not exceed the size of equipment needed for a 3250 mtpd pool condenser type CO2-stripping urea plant! A Mega Urea Plant can be built with both the Avancore® or the Urea 2000plus™ technology.
The Mega Urea Plant process works as follows: About 70% of the urea solution leaving the urea reactor flows to the high-pressure CO2 stripper, while the remainder is fed into a medium-pressure recirculation section. This reduced liquid feed to the stripper in turn reduces not only the size of the stripper needed, but also the heat exchange area of the pool condenser. The degree of stripping efficiency is adjusted to ensure that as much low-pressure steam is produced by the carbamate reaction in the pool condenser as is needed in the downstream sections of the urea plant. About 30% of the urea solution that leaves the reactor is expanded and enters a gas/liquid separator in a recirculation stage operating at a reduced pressure. After expansion, the urea solution is heated by medium-pressure steam. By heating the urea solution, the unconverted carbamate is dissociated into ammonia and carbon dioxide.
Our Mega Urea Plant Concept does not need the ammonia recycle section or the ammonia hold-up steps that are commonly seen in competitors total recycle urea plants. This is because the low ammonia-to-carbon dioxide molar ratio in the separated gases allows for easy condensation as carbamate only. The operating pressure in this medium-pressure recirculation stage is about 20 bars. After the urea solution leaves the medium-pressure dissociation separator, it flows into an adiabatic CO2 stripper, which uses carbon dioxide to strip the solution.
View Mega Plant lay out
As a result of this process, the ammonia-to-carbon dioxide molar ratio in the liquid leaving the medium-pressure recirculation section is reduced, facilitating the condensation of carbamate gases in the next step. The vapors leaving the medium-pressure dissociation separator, together with the gases leaving the adiabatic CO2 stripper, are condensed on the shell side of the evaporator. The carbamate formed in the low-pressure recirculation stage is also added to the shell side of this evaporator. The heat released by condensation is used to concentrate the urea solution. Further concentration of the urea solution is achieved using low-pressure steam produced in the pool condenser.
The remaining uncondensed ammonia and carbon dioxide leaving the shell side of the evaporator are sent to a medium-pressure carbamate condenser. The heat released by condensation in this condenser is dissipated into a tempered cooling water system. This process forms medium-pressure carbamate that contains only 20-22wt% water. The carbamate is transferred via a high-pressure carbamate pump to the high-pressure scrubber in the urea synthesis section. The urea solution leaving the adiabatic CO2 stripper and the high-pressure stripper are expanded together in the low-pressure recirculation section.
