Question

The purpose of a plug-flow reactor is to

• A. Minimize reactor volume
• B. Maximize contact time between substrate and microorganisms
• C. Allow continuous addition and removal of substrate
• D. Enhance heat transfer

Hint:

• Plug Flow Reactor (PFR): Idealized reactor with no axial mixing, perfect radial mixing.
• All fluid elements have the same residence time.
• Concentration gradients exist along the length of the reactor.
• Often achieves higher conversion per unit volume than CSTR for many reaction types.
• Purpose is to provide a controlled environment for reactions to proceed over a defined path and time.

Answer 1

The Correct Option is B

A Plug-Flow Reactor (PFR), also known as a tubular reactor, is a type of chemical reactor or bioreactor characterized by the following idealized flow pattern:

• Fluid particles pass through the reactor in an orderly manner without overtaking or mixing with earlier or later entering fluid (no axial mixing, perfect radial mixing).
• Each fluid element has the same residence time in the reactor.
• Concentration of reactants and products changes along the length of the reactor, but is uniform across any given cross-section.

Purposes and characteristics:

• Defined Residence Time and Contact Time: PFRs provide a well-defined residence time for all fluid elements. This ensures that all parts of the fluid (containing reactants, substrates, microorganisms) spend a specific amount of time in the reactor, allowing for sufficient contact and reaction to occur.
• High Conversion (for many reaction orders): For many reaction kinetics (especially positive order reactions), PFRs can achieve higher conversion per unit volume compared to a CSTR (Continuous Stirred-Tank Reactor) of the same volume because reactants enter at high concentration and are gradually consumed, maintaining a higher average reaction rate along the length.
• Continuous Operation: PFRs are typically operated continuously, with reactants fed at one end and products removed at the other.

Let's analyze the options: (a) "Minimize reactor volume": For a given conversion, a PFR is often smaller in volume than a CSTR, so this can be an advantage, but not the sole "purpose". (b) "Maximize contact time between substrate and microorganisms": While PFRs ensure a specific contact time (residence time), "maximize" might be too strong unless compared to a system with severe short-circuiting. The purpose is more about providing a *controlled and sufficient* contact time for the reaction to proceed to the desired extent. As fluid flows along the PFR, substrate and microorganisms are in contact for the duration of their passage. (c) "Allow continuous addition and removal of substrate": This describes continuous operation, which is a mode PFRs operate in, but "substrate" removal is usually as product or unreacted substrate in the effluent. This is more a characteristic of continuous reactors in general. (d) "Enhance heat transfer": Heat transfer can be managed in PFRs (e.g., with jacketing), but it's not their primary defining purpose over other reactor types. The high surface-area-to-volume ratio of tubular reactors can be advantageous for heat transfer. Considering the options, option (b) "Maximize contact time between substrate and microorganisms" (interpreted as ensuring sufficient and uniform contact time for all fluid elements) is a key functional aspect directly related to the plug-flow ideal. Because there's no backmixing, all particles experience the full length of the reactor for reaction. If we rephrase (b) to "Ensure all fluid elements have a specific, defined contact time for reaction to occur effectively along the reactor length," it becomes a strong descriptor. Given the options, (b) appears to be the most relevant purpose related to the reaction process itself. \[ \boxed{\text{Maximize contact time between substrate and microorganisms}} \]