Reactor for Urea Production and Urea Process Parameters

Imagine a machine so massive it produces 2100 metric tons of urea every single day. That's the scale we're talking about when we delve into the design of a modern urea reactor. Urea, a crucial component of fertilizers, plays a vital role in global agriculture. But its production is a complex feat of engineering. In this post, we'll pull back the curtain on the design of a giant urea reactor, exploring the challenges and triumphs of building such a complex piece of equipment.

Low-Pressure Decomposer

 The Challenge: Meeting Global Demand: The world's growing population relies heavily on fertilizers to boost crop yields. Urea, with its high nitrogen content, is a cornerstone of this effort. Designing a reactor capable of producing such vast quantities of urea is a significant engineering challenge.   After the high-pressure synthesis of urea, the reaction mixture still contains unconverted ammonium carbamate. This isn't ideal because we want to maximize urea yield and recycle the valuable ammonia and carbon dioxide. That's where the low-pressure decomposer (LPD) comes in. Think of it as the cleanup crew, responsible for breaking down the remaining carbamate at a lower pressure.

Decomposing ammonium carbamate at high pressure is energy-intensive. By reducing the pressure, we can make the decomposition process more efficient and recover the valuable ammonia and carbon dioxide for reuse in the synthesis loop. This not only improves overall urea yield but also reduces energy consumption - a win-win!

Diagram of Low Pressure Urea Section

The urea solution from the medium-pressure decomposer enters the LPD. Here, at a lower pressure, the unconverted ammonium carbamate is decomposed into ammonia (NH3) and carbon dioxide (CO2). These gases are then separated from the urea solution and recycled back to the high-pressure synthesis section.

The output from the high-pressure section isn't pure urea just yet. It's a mixture containing urea, unconverted ammonium carbamate, ammonia, and carbon dioxide. This is where the low-pressure urea section steps in – a critical part of the process dedicated to maximizing urea yield and recycling valuable resources.

The Core Function: Decomposition and Recovery

The primary task of the low-pressure section is to decompose the unconverted ammonium carbamate back into ammonia (NH3) and carbon dioxide (CO2). 

 This is crucial for two reasons: 

1) It increases the overall urea yield, making the process more efficient, and 

2) It allows us to recover and reuse the ammonia and carbon dioxide, reducing raw material consumption and minimizing waste.

A complete urea process description with flow sheet

Brief equipment design of a reactor for producing 2100 MTPD of Urea:

  Inside the Beast: Reactor Design:

Let's take a look inside this industrial giant. The heart of the operation is the reactor itself, a pressure vessel operating at high temperatures and pressures. Here's a glimpse at some key design parameters:

Internal trays

Sieve trays :

480 hot trays: equispaced triangular pitch
Number of trays	: 15 equispaced , 666.67 cm diameter

Feed distribution nozzle : 

 

CO₂ inlet 265 holes of 8 mm diameter NH₃ inlet 440 holes of 8 mm diameter Operating/ Design temperature 188/210^oC Operating/ Design pressure 155/170 Kg/cm² g Design pressure 170 Kg/cm² g Joint efficiency = j 0.85 Allowable stress = f 22.5 Kg/cm² g Capacity 2100 MTPD Density of NH₃/CO₂ at 188^oC 881.5387/809.29 Kg/m³

 

Concentration Vs Rate of reaction data for carbon dioxide:

Concentration CA, Kgmole/m3	18.39	16.55	14.71	12.87	11.03	9.19	7.35	5.51
Rate of reaction, -rA Kgmole/hr m3	27.05	21.92	17.31	13.25	9.74	6.76	4.33	2.43

Calculation:

 τ/C_Ao  =  V/f_Ao  =  ΔX_A/ -r_A

From material balance :
f_Ao = 2278.645 Kg mole/hr
C_Ao  = f_Ao /V_o
V_o = (inlet flow of CO₂)/(Density of CO₂) = 100260.42 / 809.29 = 123.886 m³/hr
C_Ao  = 2278.645/123.886 = 18.39 Kg mole/m³

Plotting graph (1/-r_A) Vs C_A :

Concentration CA Kgmole/m3	18.39	16.55	14.71	12.87	11.03	9.19	7.35	5.51
Rate of reaction  -rA Kgmole/hr m3	27.05	21.92	17.31	13.25	9.74	6.76	4.33	2.43
1/-rA	0.037	0.046	0.058	0.075	0.102	0.148	0.231	0.411

From graph :

Area = 137.8×2×0.02 = 5.512 hr
Area = τ = 5.512 hr
Now V = τ× f_Ao/C_Ao
V = (5.512×2278.65)/18.39 = 682.974 m3
Assuming height to be 18 meters
V = pi R²H
R² = (683)/(π×10) = 12.075m²
R = 3.475 m
Diameter = 6.95 m

Relation of reaction rates to concentration of components in urea reactor

Urea Reactor Design Calculator

Tray Diameter (m):

Number of Trays:

CO2 Inlet Holes:

NH3 Inlet Holes:

CO2 Density (kg/m³):

NH3 Density (kg/m³):

Production Rate (MTPD):

Design Pressure (kg/cm²g):

Allowable Stress (kg/cm²g):

Joint Efficiency:

CO2 Concentration (Kgmole/m3)