Plug-flow reactor and completely mixed flow reactor

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Backmixing is an important concept in reaction engineering; it refers to the mixing between fluids with different residence times in a continuously flowing system, that is, the mixing of materials that enter the reactor at different moments. Backmixing causes the temperature and concentration distributions within the reactor to become more uniform, resulting in an increase in the concentration of products and a decrease in the concentration of reactants; this affects the conversion rate of the reaction as well as the distribution of the reaction products. A plug flow reactor, also known as a piston flow reactor, has a large length-to-diameter ratio, low fluid viscosity, and high flow velocity. 1. There is no velocity distribution in the cross-section perpendicular to the flow direction ； 2. At each cross-section of the reactor, the concentration of the material does not change over time ； 3. There is no mixing of fluid particles in the direction of flow, and no backmixing occurs ； 4. All fluid particles have the same residence time t, which is equal to the reaction time t. Due to its low backmixing, large specific surface area, and high volumetric efficiency, the plug-flow reactor is suitable for processes that require high conversion rates or involve sequential side reactions. However, for slow reactions, long pipelines are required, which results in a large pressure drop within the reaction and affects its efficiency. A fully mixed flow reactor, also known as an ideal flow reactor or continuous stirred-tank reactor, has continuous operations at the inlet and outlet, and can be operated with a single tank or multiple tanks in series. 1. The physical properties at all locations within the reactor are uniform, and equal to those of the material at the reactor outlet; that is, the concentration and temperature of the material inside the reactor are uniform, and identical to those of the material at the outlet. 2. Fresh materials are mixed uniformly in an instant, resulting in mixing between materials with different residence times, namely backmixing. Moreover, the degree of reverse mixing is the greatest, resulting in the strongest dilution effect due to reverse mixing. 3. Under isothermal operating conditions, all parameters of the system inside the reactor do not change with time nor with position.