Plug Flow Reactor : Working, Derivation, Characteristics & Its Applications

Plug flow is a significant characteristic of these reactors, so any two molecules can be entered into the reactor in less time and exit at the same time. Plug flow reactor provides an efficient controlling reaction time when optimizing the division of reactants as well as products. So, good plug flow is necessary for good performance in reactors. So reactors that use plug flow chemistry are normally called plug flow reactors or PFR reactors. The Plug Flow Reactor or PFR is a third general type reactor where the nutrients are introduced continuously to the reactor & move throughout the reactor as a “plug”. This article discusses an overview of a plug flow reactor, its working, and its applications.

What is a Plug Flow Reactor?

The plug flow reactor or piston flow reactor is a rectangular-type idealized flow reactor that uses a continuous fluid flow for processing materials throughout a tube. This reactor is used to depict chemical reactions within a cylindrical pipe such that all the chemical reaction combinations will be supplied at a similar speed along the flow direction, thus; there is no integration or backflow.

This reactor includes a cylindrical pipe with openings on every end for reactants as well as products through which reactants supply. For maintaining a uniform reaction in this reactor, water at a fixed temperature is provided to the reactor. The plug flow is produced in this reactor by introducing material continuously from one end to and other end, it removes the materials continuously. The frequently produced materials in PFR, are; petrochemicals, polymers, pharmaceuticals, etc. These reactors have a wide range of applications in either liquid or gas phase systems.

Plug flow reactor provides an outstanding residence time control as well as reaction conditions. So they provide high levels of conversion & are compatible with reactions through high heat release (or) sensitivity to concentrations of reactant. However, they have some limitations without radial mixing & simply axial mixing.

Key Features

The key features of a plug flow reactor include the following.

Unidirectional Flow

In PFR, the reactants as well as products travel in a single direction along the reactor’s length without back-mixing.

Concentration Gradient

The reactant concentration & products in this reactor change with the reactor’s length although it is consistent across any section vertical to the flow.

Residence Time

Residence time a separate reactant volume that is spent within the PFR is called residence time and is stable for all volumes.

Plug Flow Reactor Working Principle

Plug flow reactor works by oxidizing alcohols & other organic compounds to produce fine chemicals like; pigments & dyes. The fluids in this reactor move in a continuous & uniform manner throughout a pipe or tube. The reactants enter at one end of the reactor to flow throughout the reactor and exist at the other end.

The plug flow nature in this reactor ensures that the chemical reactants are exposed to similar conditions through the PFR & that every reactant resident time is the same. So, a plug flow reactor is an outstanding choice for main reactions that need exact control of resident time, temperature & pressure.

Plug Flow Reactor Diagram

The design of a plug flow reactor can be done with some type of a capillary which is a small tube (or) a channel fixed into a plate. This is a continuous reactor set with an inlet of reactants & an outlet of the reactor content which are continuously done throughout the reactor operation.

A plug flow reactor (PFR) doesn’t have an agitator which has a cylindrical shape that allows the fluid to develop with a minimum quantity of back mixing, as a result, all the fluid particles that go into the reactor have a similar residence time. This reactor can certainly considered as a series of thin fluid slices, comprising a tiny batch reactor, completely stirred in the slice to move ahead within the reactor like a piston.

The equation for general mass balance can be expressed like the following for one of the fluid slices within the reactor:

Inlet = Outlet + Consumption + Accumulation

The units of every component of the above expression are a material run rate like mol/sec.

Plug flow Reactor Equation Derivation

Plug-flow reactor is an idealized reactor where all particles in a particular section have the same velocity & motion direction. In a plug flow reactor (PFR) there is no backflow or mixing, thus the flow of a fluid like a plug from the inlet side to outlet is shown in the below figure.

This reactor is created depending on mass balance as well as heat balance within a differential amount of fluid. If we imagine that the procedure is isothermal, then mass balance is only considered.

If we imagine steady-state conditions reactant concentrations do not vary eventually. It is a typical method of operation of PFR. The mathematical equation for PFR can be written simply as;

udCi/dx = vir

Ci(0) = Ci(f)

0≤ x ≤ L

Where ‘Ci’ is the reactant, ‘i’ is concentration, ‘u’ is the velocity of the fluid, ‘νi’ is the stoichiometric coefficient, ‘r’ is the reaction rate & ‘x’ is the position within the reactor. ‘Caf’ is reactant A concentration at the reactor inlet & ‘L’ is the reactor length. The velocity of fluid ‘u’ is measured depending on the Fv (m3/s) volumetric flow rate & the cross-section region of the reactor S (m^2):

u=Fv/S

In an ideal PFR, all the liquid particles have been in the reactor for precisely the same quantity of time which is called a mean residence, measured as;

T =L/u

The residence time data is normally used within chemical reactor engineering for making predictions of change & exit concentrations.

First-order Irreversible Reaction

Let us consider a simple decomposition reaction:

A–>B

Whenever the reaction is irreversible & first order, we have:

udCa/dx = -kCa

Where ‘k’ is a kinetic constant. Generally, kinetic constant mainly depends on temperature. Generally, an Arrhenius equation can be used to describe this relationship. Here, we assume isothermal conditions, so will not use this dependency.

The model for first-order irreversible reactions can be solved logically. So the solution follows as;

Ca = Cafexp(-x*k/u)

Second-order Irreversible Reaction

The second-order irreversible reaction example let us utilize the below one:

2A–>B

Once the reaction is irreversible & second-order, we have:

udCa/dx = -2k*(Ca)^2

Plug Flow Reactor Characteristics

The characteristics of a plug flow reactor include the following.

• The reactants in a plug-flow reactor flow throughout the reactor in a continuous flow with little to no mixing.
• The reaction in PFR occurs when the reactants move with the reactor length.
• The concentration of reactants changes with the reactor’s length and the rate of reaction is generally higher at the entry.
• These reactors are frequently used for reactions wherever a high amount of change is necessary and wherever the reaction speed is not responsive to absorption changes.
• The residence time within the PFR is normally short.
• The biofilm forms close to the air-liquid interface simulating environments such as the oral cavity, wet rock surfaces, and shower curtains.
• This type of reactor generates a consistent biofilm in low shear that can be utilized like the static glass coupon reactor to check microbicide effectiveness.
• The biofilm of this reactor is analyzed easily with different methods like viable plate counts, determination of thickness & light microscopy.
• The reactants in PFR are consumed continually because they flow down the reactor’s length.
  A typical PFR could be a tube packed through some solid material.

Advantages and Disadvantages

The plug flow reactor advantages include the following.

• The PFR advantage over CSTR is that this reactor has a low volume for a similar space-time & conversion level.
• The reactor needs less space & that the quantity of conversion is high within PFR as compared to CSTR for a similar reactor volume.
• This reactor is used frequently to decide the gas-phase catalytic kinetics process.
• These reactors are very effective in handling reactions & for a large group of “typical” reactions effect within higher conversion rates for each reactor volume as compared to CSTR (Continuous Stirred-Tank Reactors)
• The reactors are very well suitable for quick reactions
• Heat transfer in PFR can be managed fairly better as compared to tank reactors which leads to an excellent fit for extremely exothermic systems
• Because of the plug flow character & not having back-mixing, there is a consistent residence time on behalf of all reactants, which leads to reliable product quality particularly where huge residence times lead to contamination formation and charring, and many more.
• Plug flow reactor maintenance is easy because there are no moving elements.
• These are simple mechanically.
• Its conversion rate is high for every reactor volume.
• Product quality has not changed.
• Excellent to study quick reactions.
• Reactor volume is used very efficiently.
• Excellent for huge capacity processes.
• Fewer pressure drops.
• There is no back-mixing
• Direct scalability
• Efficient time control of residence, temperature control, efficient mixing, batch-to-batch variation is limited, etc.

The plug flow reactor disadvantages include the following.

• In a PFR, exothermic response performance is hard to control due to the broad range of temperature profiles.
• For a PFR, maintenance & operational expenditures are costly as compared to the CST.
• Temperature control is difficult for a reactor.
• Hot spots occur in the reactor whenever used for exothermic reactions.
• It is hard to control because of composition & temperature variations.
• PFRs are expensive to design & maintain because of their complex design and assembly.
• PFRs are designed typically for precise reactions & may not be able to accommodate changes within feedstocks or reaction conditions.
• These are difficult to maintain and clean because of their narrow and long design.
• The reactants in PFR can flow unevenly which leads to hot spots or incomplete reactions.
• It is very significant to keep in mind that plug flow reactors cannot fit in all applications. So one must analyze carefully the residence time, kinetics, selectivity issues, etc to decide on what type of reactor is suitable for an application.

Applications

The applications of plug-flow reactors include the following.

• PFRs are used commonly in fertilizer, large-scale chemical, petrochemical & pharmaceutical production.
• These reactors are used within polymerization processes like polypropylene & polyethylene production.
• Plug flow reactors are suitable for liquid-solid & gas-solid reaction systems.
• These are suitable for heterogeneous or homogeneous reactions like; oil & fat hydrogenation.
• PFRs are used for oxidizing alcohols & other organic compounds & to generate fine chemicals like pigments & dyes.

Thus, this is an overview of plug flow reactor, working, advantages, disadvantages, and applications. Design & selection of a good flow reactor is still an art & years of knowledge make you improve at making selections. Sometimes, a plug flow reactor is also known as a CTR (continuous tubular reactor). In an idealized form, the shape of the reaction combination can be measured to be made up of some plugs & every plug has a uniform concentration. This PFR has a supposition that there is no axial mixing so there is no back mixing in the reactor. Here is a question for you, what is a reactor?