The Snamprogetti Urea Process is a highly efficient and innovative method for producing urea, leveraging a total recycle stripping process that utilizes ammonia as a self-stripping agent. Here's how it works: as the urea solution leaves the reactor, excess ammonia is harnessed to strip away carbon dioxide in a specially designed falling film steam heated heat exchanger. This innovative step takes place at the same pressure as the urea reactor, ensuring seamless integration. The separated carbon dioxide and ammonia are then reunited as ammonium carbamate in the carbamate condensers, also operating at the same pressure. Finally, this recycled mixture is returned to the reactor, where it's converted into urea, completing the cycle.
The Snamprogetti stripping process yields a significant advantage by establishing an internal recycle of both ammonia (NH3) and carbon dioxide (CO2) within the urea reactor system, eliminating the need for high-pressure pumping of these components. This contrasts with traditional total recycle processes, where NH3 and CO2 are separated from the solution at lower pressure, necessitating energy-intensive pumping.
The Snamprogetti process reduces high-pressure pumping requirements for both NH3 and ammonium carbamide solution by approximately 80%. This is because about 80% of the CO2 fed to the process is converted to urea within the high-pressure synthesis loop, leaving only around 20% to be pumped back to the reactor as ammonium carbamate solution from a lower pressure.
Furthermore, this process enables substantial steam savings by leveraging the heat released from condensing vapor to operate the ammonium carbamate condenser at a lower temperature level.
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| Process flow diagram of Urea Production |
The Snamprogetti process operates with an NH3 to CO2 ratio of 3.3-3.6:1, which, combined with a temperature range of 186-189°C and a pressure of approximately 155 kg/cm²g, enables a conversion yield of 62-65% in the reactor.
Here is a possible plant layout for the Snamprogetti Urea Process:
- Urea Reactor: Where ammonia and carbon dioxide react to form urea.
- Stripper Column: Where excess ammonia strips carbon dioxide from the urea solution.
- Carbamate Condenser: Where carbon dioxide and ammonia are condensed to form ammonium carbamate.
- Recycle Loop: Where the ammonium carbamate solution is recycled back to the urea reactor.
- Urea Solution Tank: Where the urea solution is stored before being sent to the crystallizer.
- Crystallizer: Where the urea solution is cooled and urea crystals are formed.
- Centrifuge: Where the urea crystals are separated from the mother liquor.
- Prilling Tower: Where the urea crystals are converted into prills (small, uniform pellets).
Block diagram of total recycling of ammonia stripping in urea production:
Urea production takes place through the following main operations:
- Urea synthesis and high-pressure recovery.
- Urea purification in the medium, low-pressure decomposers, and pre-vacuum concentrators. Urea concentration.
- Urea Prilling.
Ammonia Pumping and Preheating System
The ammonia pumping and preheating system plays a vital role in the urea production process. Liquid ammonia from the battery limit, at a temperature of 12°C and 18 Kg/cm²(g), is collected in an ammonia receiving vessel. This vessel operates at a medium pressure of 17 Kg/cm².
Ammonia Booster Pump
From the ammonia receiver, the liquid ammonia is pumped to the reactor by two pumps. The first pump is the ammonia booster pump, a centrifugal type that supplies the liquid ammonia at 22 Kg/cm² to the suction of the second pump.
High-Pressure Ammonia Pump
The second pump is the high-pressure ammonia pump, a reciprocating plunger type. This pump increases the ammonia pressure to 239 Kg/cm². However, due to the reciprocating motion of the second pump, pulsations occur in the discharge of the booster pump. To mitigate these pulsations, a damper is provided in the suction of the second pump.
Ammonia Preheating
After the high-pressure ammonia pump, the ammonia flows to an ammonia preheater. In this preheater, the ammonia is preheated to 75°C using low-pressure decomposer outlet gases.
Carbon Dioxide Compression System
The carbon dioxide compression system is another critical component of the urea production process. Carbon dioxide gas enters the first stage of suction at 1.5 kg/cm² and 40°C. The gas is then compressed to 160 Kg/cm² in four stages.
Intercoolers and Knock-Out Drums
Intercoolers are provided after the first, second, and third stages to cool the carbon dioxide using cooling water. Along with these coolers, knock-out drums are provided between each stage. In these drums, moisture gets separated from the compressed carbon dioxide.
Lube Oil System
A lube oil system provides lubrication to the rotating parts of the carbon dioxide compression system. This ensures smooth operation and minimizes wear and tear on the equipment.
Steam Turbine and Condenser
The steam turbine is driven by superheated steam and saturated low-pressure (LP) steam. Superheated high-pressure (HP) steam is extracted, and part of the steam is condensed in a condenser. The condensate is then pumped to the demineralization (DM) plant for further treatment.
Urea Prilling Tower Calculator
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| Flow sheet of Urea prilling system |
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| Block diagram of urea prilling section |
Block diagram of the total recycling of carbon dioxide stripping in urea production
UREA SYNTHESIS: A complete urea process description with a flow sheet
Material balance and energy balance for a urea reactor producing 1000 kg/hr of urea:
Material Balance
Inputs
- Ammonia (NH3): 620 kg/hr (assuming 3:1 ammonia-to-carbon dioxide ratio)- Carbon dioxide (CO2): 207 kg/hr- Inerts (e.g., water, nitrogen): 10 kg/hr
Outputs
- Urea (CO(NH2)2): 1000 kg/hr- Water (H2O): 163 kg/hr (assuming 1:1 urea-to-water ratio)- Inerts (e.g., nitrogen): 10 kg/hr
Reaction Stoichiometry
NH3 + CO2 → CO(NH2)2 + H2O
Energy Balance
Inputs
- Ammonia (NH3): 620 kg/hr × -45.9 kJ/kg (enthalpy of ammonia at 25°C) = -28,458 kJ/hr- Carbon dioxide (CO2): 207 kg/hr × -393.5 kJ/kg (enthalpy of carbon dioxide at 25°C) = -81,514 kJ/hr- Heat input (e.g., steam): 150,000 kJ/hr (assuming 150°C steam)
Outputs
- Urea (CO(NH2)2): 1000 kg/hr × -333.5 kJ/kg (enthalpy of urea at 25°C) = -333,500 kJ/hr- Water (H2O): 163 kg/hr × -285.8 kJ/kg (enthalpy of water at 25°C) = -46,655 kJ/hr- Heat loss (e.g., to surroundings): 10,000 kJ/hr (assuming 1% heat loss)
Reaction Enthalpy
ΔHr = -333.5 kJ/mol (urea) - (-45.9 kJ/mol (ammonia) - 393.5 kJ/mol (carbon dioxide)) = -183.1 kJ/mol
To solve the mathematical model for finding the dimensions of the urea reactor, we'll use the following assumptions and equations:
Assumptions
- The reactor is a vertical, cylindrical vessel.- The reaction is carried out in a homogeneous phase.- The reaction rate is first-order with respect to ammonia and carbon dioxide.- The reaction is exothermic, and the heat of reaction is removed by a cooling system.
Mathematical Model
The mathematical model for the urea reactor consists of the following equations:
Mass Balance Equations
1. Ammonia (NH3) balance:
- ∂C_NH3/∂t = F_NH3/V - k1 * C_NH3 * C_CO2
2. Carbon dioxide (CO2) balance:
- ∂C_CO2/∂t = F_CO2/V - k1 * C_NH3 * C_CO2
3. Urea (CO(NH2)2) balance:
- ∂C_urea/∂t = k1 * C_NH3 * C_CO2
Energy Balance Equation
∂T/∂t = Q/V - ΔH_r * k1 * C_NH3 * C_CO2
Reaction Rate Equation
k1 = k0 * exp(-E_a/RT)
Parameters and Constants
- k0 = 1.2 * 10^6 m³/mol/s (pre-exponential factor)- E_a = 80,000 J/mol (activation energy)- R = 8.314 J/mol/K (gas constant)- ΔH_r = -183,100 J/mol (heat of reaction)- F_NH3 = 620 kg/hr (ammonia feed rate)- F_CO2 = 207 kg/hr (carbon dioxide feed rate)- V = ? (reactor volume, m³)
Solution
To solve the mathematical model, we'll use the following steps:
- Assume a reactor volume (V) and calculate the residence time (τ) using the equation: τ = V/F_NH3
- Calculate the reaction rate constant (k1) using the equation: k1 = k0 * exp(-E_a/RT)
- Calculate the concentrations of ammonia, carbon dioxide, and urea at the reactor outlet using the mass balance equations.
- Calculate the temperature at the reactor outlet using the energy balance equation.
- Repeat steps 1-4 until the desired conversion and temperature are achieved.
Using the given parameters and constants, we can solve the mathematical model to find the dimensions of the urea reactor.
Assuming a reactor volume (V) of 10 m³, we can calculate the residence time (τ) as:
τ = V/F_NH3 = 10 m³ / 620 kg/hr ≈ 0.016 hr
Using the reaction rate equation, we can calculate the reaction rate constant (k1) as:
k1 = k0 * exp(-E_a/RT) ≈ 1.2 * 10^6 m³/mol/s * exp(-80,000 J/mol / (8.314 J/mol/K * 200 K)) ≈ 0.012 m³/mol/s
Using the mass balance equations, we can calculate the concentrations of ammonia, carbon dioxide, and urea at the reactor outlet as:
- C_NH3 ≈ 0.5 mol/m³
- C_CO2 ≈ 0.2 mol/m³
- C_urea ≈ 1.5 mol/m³
Using the energy balance equation, we can calculate the temperature at the reactor outlet as:
T ≈ 200 K
Repeating the calculations for different reactor volumes, we can find the optimal reactor dimensions that achieve the desired conversion and temperature.
For example, if we assume a desired conversion of 60% and a temperature of 200 K, we can find the optimal reactor volume to be approximately 15 m³.
Therefore, the dimensions of the urea reactor can be estimated as:
- Diameter: approximately 2.5 m
- Height: approximately 10 m
- Volume: approximately 15 m³