To lower urea fabrication costs through means of economy of scale, Stamicarbon has developed its MEGA Plant concept for single line urea plants in excess of 4500 mtpd. Large plant capacities require large sized high pressure vessels and apart from equipment up scaling constraints it is certainly not easy to manufacture and transport such large and extremely heavy equipment. In this MEGA concept part of the liquid effluent from the reactor is diverted to a medium pressure recycle section, thus limiting the size of the high pressure vessels. The concept is based on proven process steps whereby the sizes of the required high pressure equipment will not go beyond sizes needed for a 3250 mtpd pool condenser type CO₂ stripping urea plant.

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The urea solution leaving the urea reactor flows for about 70% to the high pressure CO₂-stripper. The remaining solution is fed into a medium pressure recirculation section. The reduced liquid feed to the stripper results in a limited stripper size and consequently in a reduced amount of gases to be condensed in the pool condenser. The stripping efficiency is chosen in such a way that sufficient low-pressure steam is produced by the carbamate reaction in the pool condenser as needed in the downstream sections of the urea plant.
About 30% of the urea solution, leaving the reactor, is expanded into a medium pressure recirculation stage. This medium pressure recirculation stage is only needed to separate the non-converted carbamate from the urea solution.
After expansion, the urea solution is heated to approximately 165°C by means of 12-bar steam whereby the non-converted carbamate is dissociated into ammonia and carbon dioxide.
The ammonia recycle and ammonia hold-up as usually applied in conventional total recycle urea plants are not needed in this design because the ammonia to carbon dioxide molar ratio in the dissociated gasses allows for easy condensation after some CO₂ addition. The operating pressure in this medium pressure recirculation stage is about 20 bars.

The urea solution leaving the medium pressure dissociation separator flows to an adiabatic carbon dioxide stripper in which the solution is stripped with carbon dioxide. By doing so, the ammonia to carbon dioxide ratio in the liquid leaving the medium pressure recirculation section is reduced and so facilitates condensation of carbamate gasses in the next step. The vapors leaving the medium pressure dissociation together with the gasses leaving the adiabatic carbon dioxide stripper are condensed at the shell side of the evaporator.
Also the carbamate formed in the low-pressure recirculation stage is added to the shell side of this evaporator. The released condensation heat is used for concentrating the urea solution. Further concentrating of the urea solution is done by means of low-pressure steam, produced in the pool condenser.
The remaining non-condensed ammonia and carbon dioxide, leaving the shell side of an evaporator, are sent to a medium pressure carbamate condenser. The released condensation heat in this condenser is dissipated into a tempered cooling water system. The formed medium pressure carbamate contains only 20 to 22-wt% of water and is transferred via a high-pressure carbamate pump to the scrubber part of the urea synthesis.
The urea solution leaving the adiabatic carbon dioxide stripper, together with the urea solution leaving the high-pressure stripper are expanded into the low-pressure recirculation section. The formed low-pressure carbamate of this section contains about 30-wt% of water and is transferred via a low-pressure carbamate pump to the shell side of the evaporator.

For more information please consult our downloadable paper on the Urea 2000 plus™MEGA Urea concept [305 kb] or contact us.