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
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:
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Single 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
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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.
- Find out the product we need (its concentration, properties, and amount to be produced in a day).
- Search some of the literature and patent information regarding the desired product reaction mechanism.
- Does the process have side and undesired reactions?
- The operating conditions like temperature and pressure required for the reaction to take place.
- Go through the reaction kinetics and catalyst suitable to the reaction.
- Now find out the material and energy balance of the whole process.
- By using the various combinations of reactors and separation equipment draw the optimum flow sheet.
- Include all the environmental and safety aspects concerning chemicals and equipment.
- Simulate the startup and shutdown of the plant designed.
- 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):
- CPi: Concentration of propylene at the gas-liquid interface (dissolved in liquid).
- CPL: Concentration of propylene in the bulk liquid.
- kmP: Mass transfer coefficient for propylene in the gas-liquid film.
Liquid-Solid Film (Propylene):
- CPL: Concentration of propylene in the bulk liquid.
- CPS: Concentration of propylene at the catalyst surface.
- ksP: Mass transfer coefficient for propylene in the liquid-solid film.
Surface Reaction:
- CPS: 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 = kmP * (CPi - CPL) --- (1)
- Liquid-solid film transfer (Propylene): r = ksP * (CPL - CPS) --- (2)
- Surface reaction: r = k * CPS * a --- (3)
3. Express Concentrations in Terms of the Rate (r):
- From (1): CPL = CPi - r / kmP --- (4)
- From (2): CPS = CPL - r / ksP --- (5)
- From (3): CPS = r / (k * a) --- (6)
4. Substitute and Solve for r:
- Substitute (4) into (5): CPS = (CPi - r / kmP) - r / ksP
- Substitute (6) into this equation: r / (k * a) = CPi - r / kmP - r / ksP
- Rearrange to solve for r: r * (1 / (k * a) + 1 / kmP + 1 / ksP) = CPi
r = CPi / (1 / (k * a) + 1 / kmP + 1 / ksP)
Final Rate Expression for Cumene Production:
r = CPi / (1 / (k * a) + 1 / kmP + 1 / ksP)
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 = FP0 * (XP)
Where:
- V: Reactor volume.
- FP0: Molar flow rate of propylene into the reactor.
- XP: Conversion of propylene.
- Solving for Conversion (XP): XP = (V * r) / FP0
Initial Conditions and Numerical Solution
To get a numerical value for conversion, we need initial conditions or values for the parameters:
- CPi: 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).
- kmP: Mass transfer coefficient for propylene (m/s or similar units).
- ksP: Mass transfer coefficient for propylene (m/s or similar units).
- V: Reactor volume (L or m³).
- FP0: 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 XP.