Types of Reactors used for chemical reactions and chemical process

Reactors are equipment used for carrying out some chemical reaction, they are broadly classified based on the application and involved in the energy, food, health dialysis categories, and much more. We know that the nuclear reactor is the heart of a nuclear plant in the same way, that biological chemical reactions in the human body in organic living molecules act as a complex reactor. Observing the nature when a raw material converts into another product, leads to conversion, which is done in a system with a boundary and separated from surroundings; this system is called a reactor.

People who work in the chemical plant will be familiar with this equipment, but knowing more about the reactors helps a lot in the shift work and in their career too. 

Without much discussion let's see the types of reactors used for chemical reactions.

The three basic classifications or chemical reactors are

1. Batch reactor (agitator and baffle design play an important role).
2. Continuous stirred tank reactor
3. Plug flow reactor

Based on heating and cooling reactors are named as

• Internal heat or cooled reactor
• External heat or cooled reactor
• Jacketed reactor

Most chemical reaction is carried out with a catalyst to improve the time and conversion of the reaction. Based on this parameter the subclass of the reactor can be distributed as:

Single fixed bed reactor

1. Fixed bed reactor
2. Multi-tubular reactor
3. Slurry reactor
4. Moving bed reactor
5. Fluidized bed reactor
6. Thin or shallow bed reactor
7. Dispersion reactor
8. Film reactor

Multitubular reactor

In designing, a chemical reactor one should have chemical reaction kinetic data and the equipment design data with these in hand we can estimate the approximate design without wasting time, following the below steps will lead to a sequence of the designing procedure.

1. Find out the product we need (its concentration, properties, and amount to be produced in a day).
2. Search some of the literature and patent information regarding the desired product reaction mechanism.
3. Does the process have side and undesired reactions?
4. The operating conditions like temperature and pressure required for the reaction to take place.
5. Go through the reaction kinetics and catalyst suitable to the reaction.
6. Now find out the material and energy balance of the whole process.
7. By using the various combinations of reactors and separation equipment draw the optimum flow sheet.
8. Include all the environmental and safety aspects concerning chemicals and equipment.
9. Simulate the startup and shutdown of the plant designed.
10. You will notice some of the drawbacks in material balance and energy balance, by rectifying them you are ready with the design of the chemical reactor and chemical process which can be used for production.

Let's apply the film theory and develop the rate expression for cumene production in a slurry reactor, and then solve for conversion with some initial conditions.

Cumene Production in a Slurry Reactor: Rate Expression

Reaction: Propylene (P) + Benzene (B) → Cumene (C)

Catalyst: Solid phosphoric acid (SPA) on a support.

Assumptions:

• Steady-state.
• First-order reaction with respect to propylene on the catalyst surface.
• Benzene concentration is high enough that it doesn't limit the rate (pseudo-first-order).

1. Resistances and Concentrations:

Gas-Liquid Film (Propylene):

• C_Pi: Concentration of propylene at the gas-liquid interface (dissolved in liquid).
• C_PL: Concentration of propylene in the bulk liquid.
• k_mP: Mass transfer coefficient for propylene in the gas-liquid film.

Liquid-Solid Film (Propylene):

• C_PL: Concentration of propylene in the bulk liquid.
• C_PS: Concentration of propylene at the catalyst surface.
• k_sP: Mass transfer coefficient for propylene in the liquid-solid film.

Surface Reaction:

• C_PS: Concentration of propylene at the catalyst surface.
• k: First-order reaction rate constant.
• a: Catalyst surface area per unit volume of liquid.

2. Rate Equations:

•  Gas-liquid film transfer (Propylene): r = k_mP * (C_Pi - C_PL) --- (1)
•  Liquid-solid film transfer (Propylene): r = k_sP * (C_PL - C_PS) --- (2)
•  Surface reaction: r = k * C_PS * a --- (3)

3. Express Concentrations in Terms of the Rate (r):

•  From (1): C_PL = C_Pi - r / k_mP --- (4)
•  From (2): C_PS = C_PL - r / k_sP --- (5)
•  From (3): C_PS = r / (k * a) --- (6)

4. Substitute and Solve for r:

•  Substitute (4) into (5): C_PS = (C_Pi - r / k_mP) - r / k_sP
•  Substitute (6) into this equation: r / (k * a) = C_Pi - r / k_mP - r / k_sP
•  Rearrange to solve for r: r * (1 / (k * a) + 1 / k_mP + 1 / k_sP) = C_Pi

r = C_Pi / (1 / (k * a) + 1 / k_mP + 1 / k_sP)

Final Rate Expression for Cumene Production:

r = C_Pi / (1 / (k * a) + 1 / k_mP + 1 / k_sP)

Solving for Conversion

To solve for conversion, we need to consider the reactor type. Let's assume a Continuous Stirred Tank Reactor (CSTR) for this example.

CSTR Design Equation: V * r = F_P0 * (X_P)

Where:

• V: Reactor volume.
• F_P0: Molar flow rate of propylene into the reactor.
• X_P: Conversion of propylene.
•  Solving for Conversion (X_P): X_P = (V * r) / F_P0

Initial Conditions and Numerical Solution

To get a numerical value for conversion, we need initial conditions or values for the parameters:

• C_Pi: Solubility of propylene in benzene at the operating conditions (mol/L).
• k: Reaction rate constant (L/(mol*s) or similar units).
• a: Catalyst surface area per unit volume (m²/m³ or similar units).
• k_mP: Mass transfer coefficient for propylene (m/s or similar units).
• k_sP: Mass transfer coefficient for propylene (m/s or similar units).
• V: Reactor volume (L or m³).
• F_P0: Molar flow rate of propylene (mol/s).

Once you have these values, plug them into the rate expression to calculate r, and then use the CSTR design equation to find X_P.

Cumene Production Conversion Calculator

C_Pi (mol/L):
k (L/(mol*s)):
a (m²/m³):
k_mP (m/s):
k_sP (m/s):
V (m³):
F_Po (mol/s):

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)