Question

The best combination of reactors for an exothermic reaction is:

• A. A CSTR
• B. CSTR in series
• C. A Plug flow reactor followed by CSTR
• D. CSTR followed by a Plug Flow Reactor

Hint:

Think about controlling the "intensity" of the reaction and heat release. Starting with a well-mixed reactor can help in diluting the reactants and distributing the heat, preventing localized hot spots. The plug flow reactor then allows for efficient conversion at potentially higher but more controlled temperatures.

Answer 1

The Correct Option is D

Step 1: Understand the characteristics of an exothermic reaction. 
An exothermic reaction releases heat. In a reactor, this heat release can lead to an increase in temperature, which in turn affects the reaction rate and selectivity. Managing the temperature profile is crucial for optimizing the performance of an exothermic reaction. 
Step 2: Consider the temperature and concentration profiles in different reactor types. 
Continuous Stirred Tank Reactor (CSTR): Perfect mixing leads to uniform temperature and concentration throughout the reactor, equal to that of the outlet stream. For an exothermic reaction, the heat released is immediately distributed, potentially leading to a higher overall temperature compared to a plug flow reactor for the same conversion. Plug Flow Reactor (PFR): Reactants flow through the reactor without axial mixing, resulting in a concentration gradient along the length. In an exothermic reaction, the heat released at a particular point affects the temperature at that point as the reaction progresses down the reactor. This can lead to a temperature profile along the length, potentially with hot spots if heat removal is not efficient. 
Step 3: Analyze the impact of reactor configuration on exothermic reactions. 
The choice of reactor or reactor combination for an exothermic reaction depends on factors like reaction kinetics, desired conversion, temperature sensitivity, and heat removal capabilities. Single CSTR: Can be used for exothermic reactions, but the uniform high temperature due to immediate mixing and heat distribution might not always be optimal for selectivity or to prevent runaway reactions, especially for highly exothermic reactions. CSTRs in series: Using multiple CSTRs can approximate the behavior of a PFR with better temperature control. The temperature in each stage can be managed more effectively by controlling the heat removal in each reactor. PFR followed by CSTR: The PFR allows for a high reaction rate at the initial high reactant concentration. However, the temperature rise along the PFR can be significant and might lead to undesirable side reactions or instability if not controlled. The subsequent CSTR would operate at a lower reactant concentration and a temperature determined by the effluent of the PFR. CSTR followed by a PFR: The initial CSTR operates at a lower average reactant concentration compared to the inlet of a PFR (for the same conversion). For exothermic reactions, operating at a lower initial reactant concentration in the first stage can help in moderating the initial heat release and temperature rise. The effluent from the CSTR then enters the PFR, where the reaction proceeds further with a more gradual change in concentration and temperature along the length. This combination can offer a balance between the advantages of initial mixing for temperature control and the high conversion achievable in a PFR at higher concentrations. 
Step 4: Evaluate the suitability of each combination for exothermic reactions. 
For exothermic reactions, controlling the temperature to avoid runaway and maximize selectivity is crucial. A single CSTR might have temperature control issues for highly exothermic reactions.
CSTRs in series offer better temperature staging.
A PFR first might lead to significant temperature increases early in the reaction.
A CSTR first can moderate the initial temperature rise, and the subsequent PFR can then achieve high conversion under more controlled temperature conditions.
Therefore, a CSTR followed by a PFR is often considered a good combination for exothermic reactions as the initial mixing in the CSTR helps in distributing the heat and moderating the temperature, while the PFR then drives the reaction to higher conversions with a more gradual temperature profile. 
Step 5: Select the best combination. 
The best combination of reactors for an exothermic reaction is often a CSTR followed by a Plug Flow Reactor to manage the heat release and achieve high conversion.