Operational experience with a running Ultra-Low Energy urea plant
By Rahul Patil, Senior Process Engineer at Stamicarbon
The Ultra-Low Energy (ULE) Design was first introduced in 2012, aiming to reduce the energy consumption of urea plants. Especially in areas with high energy costs, this design offered an alternative for more energy-efficient urea production. The first operational experiences have shown that, indeed, applying ULE can help reduce steam consumption by about 35% and cooling water consumption by about 16% compared to traditional CO2 stripping processes.
The first ULE plant went into operation in 2021 in China for Xinlianxin (XLX) Chemicals Group, with a nameplate capacity of 2334 MTPD urea. A number of other ULE plants have been licensed since then and are currently in the construction and engineering phases. This article shares the highlights of the ULE technology and the first experiences with the operational ULE plant in China.
Highlights of the ULE Design
In the traditional urea CO2 stripping process, the energy supplied to the plant is used two times (also known as the "N=2" concept). First, the steam is used as a heating agent in the high-pressure stripper, then the energy is recovered by condensing the stripped gas in the high-pressure carbamate condenser. The resulting low-pressure steam is then used in the downstream sections of the plant.
ULE reduces energy consumption by using heat three times ("N=3" concept). By adding a medium-pressure section to the plant design, it is possible to achieve heat integration between the pool condensation zone, the medium-pressure section and the evaporation section. The stripped gas from the pool condensation zone is condensed in the medium-pressure carbamate condenser-evaporator and is used to concentrate the urea solution in the evaporation section.
ULE pool reactor
The synthesis section only includes two high-pressure equipment items for lower-capacity plants, a pool reactor and a high-pressure stripper. For larger capacities, a vertical reactor is added to a pool condenser plant. The pool condenser/reactor’s U-tube bundle has two separate sections that handle steam and carbamate. The inner part is the steam bundle for generating low-pressure steam to use in the urea plant. The outer part, the carbamate bundle, integrates heat with the medium-pressure section. On the shell side, stripped ammonia and CO2 vapors are condensing, releasing heat. On the tube side, carbamate takes this heat and decomposes into ammonia and CO2, acting as a medium-pressure rectifying heater. Below is a process diagram of a typical ULE plant.
XLX-I in operation
XLX Jiujiang in China, the first Ultra-Low Energy urea plant, started up in February 2021. To ensure a smooth start-up, plant operators were trained on the Stamicarbon’s Operator Training Simulator to get a better understanding of the expected plant behavior. The plant was initially operated at a turndown production rate, after which the rate was increased to above 100% within the first week.
Low steam consumption
As the plant went into operation, several advantages were noted. Firstly, as anticipated, the Ultra-Low Energy Design brought high energy savings, using 587 kg of steam per ton of urea (saturated steam, 23 bar), compared to 870 kg/ton (23 bar, 320°C, traditional steam quality), which is equivalent to 940 kg/ton (saturated steam, 23 bar) in the Pool Condenser Design. This figure can be lowered even further by another 20-25 kg/ton urea by optimizing the process conditions of the ammonia feed temperature to the synthesis.
Easy to operate
Secondly, plant operators found it easier to operate the Ultra-Low Energy plant compared to traditional stripping plants and noticed fewer disturbances thanks to the addition of the carbamate bundle and medium-pressure section.
High product quality
Furthermore, the final product quality of urea was better due to lower biuret content. The biuret concentration in the final product was reduced by >0.05 wt% compared to the traditional stripping process, over 6% less than in traditional Pool Condenser and Pool Reactor Designs. This was achieved with lower steam temperatures on the shell side of the CO2 stripper (196-198°C compared to 210-215°C in the traditional stripper), also increasing the vessel’s lifetime.
At the Stamicarbon Symposium in May 2022, Mr. Weipeng from Jiangxi XLX shared his experiences via a video message. Mr. Weipeng said, “Our plant runs at 106%. Saturated steam consumption of 25 bar per ton of urea is less than 600 kg, and product biuret content is less than 0.8%, and the operation is very convenient. We are very satisfied with the urea plant.”
For more operational highlights of the ULE Design, watch Rahul Patil's Symposium presentation below.
ULE is Stamicarbon’s latest process technology that brings energy savings to unprecedentedly low levels. Learn more by clicking on the button below.