A laminar flow reactor (LFR) is a type of

chemical reactor A chemical reactor is an enclosed volume in which a chemical reaction takes place. In chemical engineering, it is generally understood to be a process vessel used to carry out a chemical reaction, which is one of the classic unit operations in chem ...

that uses laminar flow to control reaction rate, and/or reaction distribution. LFR is generally a long tube with constant diameter that is kept at constant temperature. Reactants are injected at one end and products are collected and monitored at the other. Laminar flow reactors are often used to study an isolated

elementary reaction An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state. In practice, a reaction is assumed to be elementary if no reaction ...

or multi-step reaction mechanism.

Overview

Laminar flow reactors employ the characteristics of laminar flow to achieve various research purposes. For instance, LFRs can be used to study fluid dynamics in

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

s, or they can be utilized to generate special chemical structures such as carbon nanotubes. One feature of the LFR is that the

residence time The residence time of a fluid parcel is the total time that the parcel has spent inside a control volume (e.g.: a chemical reactor, a lake, a human body). The residence time of a set of parcels is quantified in terms of the frequency distributi ...

(The time interval during which the chemicals stay in the reactor) of the chemicals in the reactor can be varied by either changing the distance between the reactant input point and the point at which the product/sample is taken, or by adjusting the velocity of the gas/fluid. Therefore the benefit of a laminar flow reactor is that the different factors that may affect a reaction can be easily controlled and adjusted throughout an experiment.

Means of analyzing reactants in LFR

Means of analyzing the reaction include using a probe that enters into the reactor; or more accurately, sometimes one can utilize non-intrusive optical methods (e.g. use

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

to identify and analyze contents) to study reactions in the reactor. Moreover, taking the entire sample of the gas/fluid at the end of the reactor and collecting data may be useful as well. Using methods mentioned above, various data such as

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number concentration'', ...

flow velocity In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the f ...

etc. can be monitored and analyzed.

Flow velocity in LFR

Fluids or gases with controlled velocity pass through a laminar flow reactor in a fashion of laminar flow. That is, streams of fluids or gases slide over each other like cards. When analyzing fluids with the same

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

("thickness" or "stickiness") but different velocity, fluids are typically characterized into two types of flows: laminar flow and

turbulent flow In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between t ...

. Compared to turbulent flow, laminar flow tends to have a lower velocity and is generally at a lower Reynolds number. Turbulent flow, on the other hand, is irregular and travels at a higher speed. Therefore the

of a turbulent flow on one cross section is often assumed to be constant, or "flat". The "non-flat" flow velocity of laminar flow helps explain the mechanism of an LFR. For the fluid/gas moving in an LFR, the velocity near the center of the pipe is higher than the fluids near the wall of the pipe. Thus, the velocity distribution of the reactants tends to decrease from the center to the wall.

Residence time distribution (RTD)

The velocity near the center of the pipe is higher than the fluids near the wall of the pipe. Thus, the velocity distribution of the reactants tends to be higher in the center and lower on the side. Consider fluid being pumped through an LFR at constant velocity from the inlet, and the concentration of the fluid is monitored at the outlet. The graph of the residence time distribution should look like a negative slope with positive concavity. And the graph is modeled by the function: E(t)=0 if t is smaller than τ/2; E(t)=τ^2/2t^3 if t is greater than or equal to τ/2. Notice that the graph has the E(t) value of zero initially, this is simply because it takes sometime for the substance to travel through the reactor. When the material is starting to reach the outlet, the concentration drastically increases, and it gradually decreases as time proceeds.

Characteristics

The laminar flows inside of a LFR has the unique characteristic of flowing in a parallel fashion without disturbing one another. The velocity of the fluid or gas will naturally decrease as it gets closer to the wall and farther from the center. Therefore the reactants have an increasing

in the LFR from the center to the side. A gradually increasing residence time gives researchers a clear layout of the reaction at different times. Besides, when studying reactions in LFR, radial gradients in velocity, composition and temperature are significant. In other words, in other reactors where laminar flow is not significant, for instance, in a plug flow reactor, velocity of the object is assumed to be the same on one cross section since the flows are mostly turbulent. In a laminar flow reactor, velocity is significantly different at various points on the same cross section. Therefore the velocity differences throughout the reactor need to be taken into consideration when working with a LFR.

Research

Various researches pertaining to the modeling of LFR and formations of substances within a LFR have been done over the past decades. For instance, the formation of Single-walled carbon nanotube was investigated in a LFR. As another example, conversion from methane to higher hydrocarbons have been studied in a laminar flow reactor.

References

{{Reflist, group=Combustion

External links

* https://www.youtube.com/watch?v=VOemsoVSPWI Chemical reactors