Reaction principle of fixed-bed reactors

LZ should provide a description of the material properties, right?:lol

Fixed-bed reactor – Basic principles: Also known as a packed-bed reactor, it 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. 　　Classification 　Fixed-bed reactors come in three basic forms: ① Axial adiabatic fixed-bed reactor (Figure 1). 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 effect 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 series combination of the aforementioned basic forms, known as multi-stage fixed-bed reactors. For example, when the heat of reaction is large or temperature control in stages is required, multiple adiabatic reactors can be connected in series to form a multi-stage adiabatic fixed-bed reactor (Figure 4). Heat exchangers or additional materials are placed between the reactors to regulate the temperature, thereby enabling operation under conditions close to the optimal ones. Fixed-bed reactor – Characteristics The advantages of fixed-bed reactors are: ① Low backmixing, allowing 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 particulate 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 roughly divided 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. Source: http://www.hoodong.com/wiki/%E5%9B%BA%E5%AE%9A%E5%BA%8A%E5%8F%8D%E5%BA%94%E5%99%A8 Last edited by guo0425 on 2009-2-23 11:45]

In Zhu Bingchen’s \"Chemical Reaction Engineering\" (2nd edition), the calculation methods are described in some detail. The original poster might as well find it for reference and do the calculations by hand.

Can a gas-solid fluidized bed produce solid material continuously?； Can the amount of material carried away during discharge be controlled and adjusted?

First of all, it should be clarified that a fixed-bed reaction is not necessarily a batch reaction; the principle behind such reactions is explained clearly on the third floor, so no further details are needed.

　　 Basic gasification principle of atmospheric-pressure fixed-bed gas generators: The process of thermally processing solid fuel with a gasifying agent to produce combustible gases is known as the gasification of solid fuel, also referred to as gas production. The gases obtained in this process are called gasified gas, while the gas used to carry out the gasification reaction with the fuel is called the gasifying agent. 　　At normal pressure, fixed-bed gas generators generally use lump-shaped anthracite or bituminous coal as raw materials, and steam or a mixture of steam and air as the gasification agent, to produce gasified gas whose main combustible components are carbon monoxide and hydrogen.

Fixed-bed reactors can definitely operate continuously; this has already been achieved in industry