Question

The most suitable reactor to carry out an auto-thermal reaction is a:

• A. Back mix reactor
• B. Plug-flow reactor
• C. Batch reactor
• D. Semi-batch reactor

Hint:

Think about how a well-stirred pot on a stove maintains a more uniform temperature than a long, thin pipe heated at one end. The uniform conditions in a CSTR are beneficial for auto-thermal operation where internal heat management is crucial.

Answer 1

The Correct Option is A

Step 1: Understand the characteristics of an auto-thermal reaction.
An auto-thermal reaction is a chemical reaction that uses the heat generated by the reaction itself to maintain the reaction temperature, thereby minimizing or eliminating the need for external heating or cooling once the reaction is initiated. This typically involves a combination of exothermic and endothermic reactions, or an exothermic reaction with heat losses that are balanced by the heat generated. Step 2: Consider the operating characteristics of different types of reactors.
Back mix reactor (Continuous Stirred Tank Reactor - CSTR): Characterized by perfect mixing, which results in a uniform temperature and concentration throughout the reactor. The output stream has the same composition and temperature as the contents inside the reactor. This uniformity helps in efficient heat transfer and maintaining a stable operating temperature for auto-thermal reactions. Plug-flow reactor (PFR): Characterized by no axial mixing. The reactants flow through the reactor in a plug, and the composition and temperature vary along the length of the reactor. This can lead to hot spots or temperature runaways in highly exothermic auto-thermal reactions if heat removal is not perfectly managed along the reactor length. Maintaining a stable auto-thermal operation can be challenging due to the axial gradients. Batch reactor: A closed system where reactants are added, allowed to react for a certain period, and then the products are removed. Temperature control can be achieved through jackets or coils, but maintaining a uniform temperature throughout the batch as the reaction progresses can be difficult, especially for large-scale auto-thermal reactions. Semi-batch reactor: A reactor where one reactant is added in portions or continuously to another reactant present in the reactor, or where one product is continuously removed. This can offer some control over reaction rate and heat release, but it still operates in a transient state, making it less ideal for sustained auto-thermal operation compared to a continuous, well-mixed system. Step 3: Analyze the suitability of each reactor type for auto-thermal reactions.
The key to successful auto-thermal operation is efficient heat transfer and stable temperature control. Back mix reactor (CSTR): The excellent mixing ensures uniform temperature, facilitating the use of the heat generated to preheat the incoming feed and maintain the reaction temperature. This inherent feedback mechanism makes CSTRs well-suited for auto-thermal operation. Plug-flow reactor (PFR): The lack of axial mixing can lead to temperature gradients and potential instability in auto-thermal reactions. While PFRs can be designed with heat exchange, maintaining the required temperature profile for stable auto-thermal operation throughout the reactor can be complex. Batch reactor: Maintaining a uniform and stable temperature for a sustained auto-thermal reaction in a batch system can be challenging due to the transient nature and potential for temperature variations within the batch. Semi-batch reactor: Similar to batch reactors, the unsteady-state operation makes them less ideal for sustained auto-thermal operation compared to a continuous, well-mixed system like a CSTR. Step 4: Select the most suitable reactor.
The back mix reactor (CSTR) is generally considered the most suitable reactor for carrying out auto-thermal reactions due to its excellent mixing capabilities, which promote uniform temperature distribution and facilitate stable operation by effectively utilizing the heat generated by the reaction.