ACES21®: Urea Synthesis


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.


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

  • Low Investment Cost
    Reduction of construction cost (Low Elevation and Compact Layout)

    • Reactor on Ground Level
    • Vertical Submerged type Carbamate Condenser (VSCC)
    • Simple synthesis loop and no gravity flow

    Reduction of Equipment cost

    • Small reactor by two stage reaction
    • Less number of equipment in synthesis loop
  • Low Energy Consumption and running cost saving
    The operation condition of synthesis section is optimized under the lower operation pressure than the previous process. As a result, a remarkable reduction in energy consumption is achieved.

  • Easy Operation and High On-stream Factor
    Forced flow of synthesis loop circulation by ejector and no gravity flow

  • Maintenance Cost Saving

    • Milder operating condition in synthesis loop (Less corrosion risk)
    • Advanced and proven material developed by TOYO for high pressure vessel
    • Maintenance support service
    ACES21 Synthesis Loop
    ACES21® Synthesis Loop
    1,725 t/d ACES21 Urea plant, PT. Pupuk Kujang (Indonesia)
    1,725 t/d ACES21® Urea plant,
    PT. Pupuk Kujang (Indonesia)

Process Flow Diagram of ACES21®

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.

※ACES21® is a registered trademark of Toyo Engineering Corporation in Japan.
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