Basic information on fixed-bed reactors

Fixed-bed reactor_Baidu Baike. Its difference from fluidized-bed reactors and moving-bed reactors is that the solid particles remain stationary. Fixed-bed reactors are primarily used to carry out gas-solid phase catalytic reactions, such as ammonia synthesis towers, sulfur dioxide contact oxidizers, and hydrocarbon steam reformers. Used for... baike.baidu.com/view/951578.htm 18K 2009-8-28 -

This post was last edited by lxq700918 on 2009-12-29 at 13:04. A fixed-bed reactor, also known as a packed-bed reactor, is a type of reactor that contains solid catalysts or solid reactants in order to carry out multiphase reaction processes. Solids are usually in granular form, with particle sizes ranging from 2 to 15 mm, and they accumulate to form a bed of a certain height (or thickness). The bed remains stationary, with the fluid flowing through it to undergo the reaction. Its difference from fluidized bed reactors and moving bed reactors is that the solid particles remain stationary. Fixed-bed reactors are primarily used to carry out gas-solid phase catalytic reactions, such as ammonia synthesis towers, sulfur dioxide contact oxidizers, and hydrocarbon steam reformers. When used in non-catalytic gas-solid or liquid-solid reactions, the bed is filled with solid reactants. The trickle-bed reactor can also be classified as a fixed-bed reactor, with the gas and liquid phases flowing downward through the bed layer, resulting in contact between the gas, liquid, and solid phases. 　　1. Classification: Fixed-bed reactors come in three basic forms: ① Axial adiabatic fixed-bed reactor. The fluid flows axially from top to bottom through the bed, with no heat exchange between the bed and the outside environment. ②Radial adiabatic fixed-bed reactor. The fluid flows radially through the bed; centrifugal flow (Figure 2) or centripetal flow can be used, with no heat exchange between the bed and the outside environment. Compared to axial reactors, radial reactors have a shorter distance for fluid flow, a larger cross-sectional area of the flow channels, and a lower pressure drop for the fluid. However, the structure of a radial reactor is more complex than that of an axial reactor. Both of the above forms are adiabatic reactors, suitable for situations where the heat of reaction is not significant, or where the reaction system can withstand the temperature changes caused by the heat of reaction under adiabatic conditions. ③Tube-type fixed-bed reactor (Figure 3). It is composed of multiple reaction tubes connected in parallel. Catalysts are placed inside or between the tubes, and a heat carrier flows through the spaces between or inside the tubes to provide heating or cooling; the tube diameter is usually between 25 and 50 mm, with the number of tubes potentially reaching tens of thousands. Shell-and-tube fixed-bed reactors are suitable for reactions with large heat of reaction. In addition, there are reactors formed by connecting the aforementioned basic units in series, known as multi-stage fixed-bed reactors. For example, when the heat of reaction is large or temperature control on a step-by-step basis is required, multiple adiabatic reactors can be connected in series to form multi-stage adiabatic fixed-bed reactors. Heat exchangers or additional materials are placed between the reactors to regulate the temperature, thereby enabling operation under conditions close to the optimal ones. 　　2. Characteristics: The advantages of a fixed-bed reactor are: ① Low backmixing, allowing for effective contact between the fluid and the catalyst; this enables higher selectivity when the reaction is accompanied by sequential side reactions. ②The catalyst suffers minimal mechanical wear. ③Simple structure. The disadvantages of fixed-bed reactors are: ① poor heat transfer; when the heat released during the reaction is large, even tubular reactors may experience runaway temperature (the reaction temperature gets out of control and rises sharply, exceeding the allowable range). ②During the operation, the catalyst cannot be replaced; reactions that require frequent catalyst regeneration are generally not suitable for use, and fluidized bed reactors or moving bed reactors are often used instead. 　　Catalysts in fixed-bed reactors are not limited to granular form; mesh catalysts have long been used in industry. Currently, honeycomb and fibrous catalysts are also widely used. 　　Mathematical models: The fixed-bed reactor is a type of multiphase reactor that has been extensively studied. There are various mathematical models to describe fixed-bed reactors, which can be broadly classified into pseudo-homogeneous models (which do not take into account the differences in concentration and temperature between the fluid and the solid) and multiphase models (which do consider such differences). Each of these categories can further be divided into models with no backmixing and models with backmixing, depending on whether backmixing is taken into account. They can also be classified as one-dimensional models or two-dimensional models, depending on whether the radial concentration gradients and temperature gradients within the reactor are considered.