ACES21^®: Urea Synthesis

Summary

TOYO has established the most energy efficient process of ACES (Advanced Process for Cost and Energy Saving.) in early 1980’s. And using its own expertise, advanced technology and new thinking, TOYO has established the ACES21, which achieves energy saving and plant cost reduction with maintaining the excellent feature of ACES process such as high performance and high efficiency.　ACES21 has been developed together with P.T. Pupuk Sriwidjaja (PUSRI) of Indonesia as a Cost and Energy Saving version of the ACES Process.

Feature

ACES21 is advantageous in low investment cost and low energy consumption for urea production. Major features of this technology are given below:

Vertical Submerged Carbamate Condenser(VSCC)

Vertical Submerged Carbamate Condenser (VSCC) in ACES21 has functions to condense NH₃ and CO₂ gas mixture from the stripper to form ammonium carbamate in the shell side, synthesize urea by dehydration of ammonium carbamate in the shell side and remove the reaction heat of ammonium carbamate formation by generating 5 barG steam in boiler tubes.

Advantages of the vertical submerged configuration of carbamate condenser are:

• High gas velocity, appropriate gas hold up and sufficient liquid depth in the bubble column promote mass and heat transfer;
• An appropriate number of baffle plates distributes gas bubbles in the column effectively without pressure loss;
• A vertical design inevitably requires smaller plot area.

Optimum Selection of Synthesis Condition

Since urea synthesis reaction takes place in two steps, the different N/C ratios are employed for the VSCC and the reactor. VSCC is operated at N/C ratio of 2.8 – 3.0 where the equilibrium vapor pressure of urea-carbamate solution gives the lowest. This optimum N/C selection allows VSCC be operated at relatively high temperature (180 – 182 °C) and low pressure (152 barG), rendering efficient heat transfer between the shell and the tube and higher reaction rate of ammonium carbamate dehydration to form urea as high as 45% conversion from CO₂. The reactor N/C ratio is selected at 3.7 to maximize CO₂ conversion with appropriate excess pressure to that of equilibrium. As inert gas fed to the reactor is only 1/5 of that of conventional CO₂ stripping process, vapor fraction in the reactor decreases drastically and the reactor volume is fully utilized for urea synthesis reaction which takes place only in the liquid phase. Resultantly, high CO₂ conversion of 63 - 64% is achieved in the reactor at relatively low temperature and pressure, i.e. 182 - 184 °C and 152 barG. The higher CO₂ conversion at lower synthesis pressure requires less decomposition heat in HP stripper and less energy for CO₂ compression and liquid ammonia and carbamate solution pumping.