Urea Production Plant (EMS³)

1. Reactor:

The reactor is operated at 190 °C and 150 bar. NH₃:CO₂ molar feed ratio to the reactor is 4:1. One pass conversion rate of CO₂ to urea is about 68%. Following reactions occurs inside the reactor.

2NH₃ + CO₂ ↔ NH₂COONH₄ + heat                             ∆H = +15.5 kJ/mol

NH₂COONH₄ + heat ↔ NH₂CONH₂ + H₂O                   ∆H = -117 kJ/mol

Where

NH3 = ammonia

NH₂COONH₄ = ammonium carbamate

NH₂CONH₂ = urea

• Stripper:

The stream leaving the reactor contains 34 wt% urea and the rest comprises ammonium carbamate, water and ammonia. To obtain a product that meets purity specifications, further purification steps are needed, the first of these is the stripper. The stripper is a falling film heat exchanger which facilitates the decomposition of ammonium carbamate into ammonia and CO₂. These gasses help strip additional ammonia from the liquid thereby purifying the product stream to the point where it contains 42 wt% urea. The urea solution coming from the stripper makes its way to the medium pressure decomposer.

                                      Fig: Diagram of a stripper

• Medium-Pressure and Low-Pressure Decomposers

The medium pressure decomposer, operating at 18 bar purifies the urea solution to 62 wt% urea. The purified urea liquor then makes its way to the low-pressure decomposer, operating at 4.5 bar where the resulting urea solution is further purified to 72 wt%. All ammonia and ammonium carbamate are removed by the Decomposers before it is transferred  to the vacuum evaporation section.

                          Fig: Diagram of a Decomposer

• Vacuum Evaporator:

The vacuum evaporators concentrate the urea solution through evaporation of any remaining water. The preceding steps (namely, stripper and decomposers) sufficiently reduce the ammonia and ammonium carbamate content, making water the biggest contamination of the product. The urea liquor is transported into the first vacuum evaporator through a heating element. The lower pressure of 0.3 bar will ensure that enough water evaporates to increase the purity to 96 wt% urea. The urea solution from the first vacuum evaporator then goes into the subsequent vacuum evaporator. The last evaporation step consists of a similar setup, with a vacuum evaporator and a heating exchanger; only the pressure is even lower (0.03 bar) in order to evaporate the last remnants of water and increase the purity of the urea up to 99 wt%. The evaporated water is brought to atmospheric conditions and condensed in two heat exchangers before it is sent to the wastewater treatment section.

Fig: Vacuum Evaporator

To determine the most appropriate technology, each of the 3 processes we looked at (StamiCarbon, Once Through, and SnamProgetti) were compared using the Kepner-Tregoe (KT) Decision Analysis technique. This system ranks a series of chosen criteria by allocating weight points with higher values being the most advantageous, and then by taking the highest summed score as the most favorable design. Upon completion, it was seen that SnamProgetti scored the highest overall. This was considered to be the best process out of the three because of the high urea yield in the final product, less toxic emissions, and amount of operational information available.

Some of the key changes we performed to the SnamProgetti design were:

1. ) Replacing reactor from a PFR to a CSTR design. This design change was implemented to improve controllability and mixing due to the fact that this reactor is well mixed. It also eliminates the need for the plug flow assumption validity therefore the need to further verify the axial dispersion is minimal. Also heat integration calculations were performed by designing a heat exchanger for the reactor. The heat exchanger was designed as a shell and tube heat exchanger and used water at 25°C as the service fluid. The implementation of the heat exchanger will help to reduce the costs associated with the utilities of this unit.
2. ) The decomposers were designed as a vertical separator as a decomposer unit is both a Separator and heat exchanger. Calculations were done for both designing the decomposers as either heat exchangers or separators and after review the calculations it was decided that designing them as separators was the safest route to take as the area and volume calculated was larger this way. The choice of a vertical separator was picked over that of a horizontal separator since yes the initial investment may be higher however, the utility, upkeep, and safety of them is greater than there counterpart.
3. ) The vacuum evaporator design was altered from 2 independent evaporators to a 2 effect evaporation system. This in turn will reduce the amount overall steam required and reduce the utilities costs associated. It will do this by using using a heat source for the first effect and then using the vapors from the first effect as a heat source for the second effect.
4. ) For the condenser the outer diameter was changed from 30mm to 20mm while keeping the internal diameter constant and reducing the baffle spacing. This in turn will reduce the overall area of the condenser and in turn reduce the amount of cooling water needed.
5. ) The addition of three scrubbers in the vacuum section will help decrease the amount of urea in the stream going to the wastewater. This will lead to less steam being needed to purify the wastewater stream. Therefore decreasing the utility costs associated with this process.
6. ) The last solution that makes our design effective is installing three condensers in the vacuum section as well. This will lead to another decrease in steam required for the wastewater effluent. Once again further decreasing the utility costs associated with wastewater effluent.

Symmetry software was used to validate these changes for each process and the entire design. Hand calculation were done as well to help validate the data obtained. Also the use of reliable papers were used to help guide, make assumptions and validate these changes performed.

While around 80-85% of urea produced is used as a fertilizer, it has other uses, which include the manufacture of the melamine, used in melamine-methanol resins. However, a staggering 40% of all food grown in the world is fertilized using urea. Worldwide, there were 218.21 million tons of Urea produced in 2019. Based on a survey done between 2018-2019, the amount of urea produced globally increased from 210 million metric tons to 218.12 million metric tons. 15 It’s estimated that by 2030, the production of urea will increase a considerable amount to 300 million metric tons. The top 5 producers were China, India, Russia, Indonesia, and Pakistan. The global market cap of Urea was 40.55 billion USD and is expected to reach 49.67 billion USD by the year 2027. It is also expected to have a compound annual growth rate of 2.8%.

The above figure which tracks the price per ton of urea since 2006 to present day shows urea is starting to get back to some of the highs it hasn’t seen since 2011/2012 this is good news for our plant as it will increase the economic feasibility of succeeding. Also since COVID-19 the demand of urea has began to skyrocket, which once again only adds to the feasibility of this design and project.

Another critically important part of the feasibility of this design is the pie chart above both the raw material and utilities costs will take up a combined 60% of our projects operating costs. Therefore the going price of ammonia, carbon dioxide, and electricity can affect how profitable this operation will be going forward.

It was determined from DCF that this project will have a net payout time (NPT) of 10 years and a positive net present value (NPV) of CAD$ 132 million after 20 years of operation. The internal rate of return (IRR) for this project is at 21.0%. This is very good news for our design as this shows that the economic feasibility for success is there.