Advantages, disadvantages, and application ranges of fixed-bed, moving-bed, and fluidized-bed reactors

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The following content is all sourced from the Internet, and no legal responsibility is assumed. Fixed bed: 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 uncatalyzed 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, either in a centrifugal or centripetal manner, 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. 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. Tube-type 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 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 temperature. 2. Features: The advantages of a fixed-bed reactor 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 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. A fixed-bed reactor is a widely used type of multiphase catalytic reactor. It is filled with stationary solid particles, which can be either solid catalysts or solid reactants. For example, in a tubular fixed-bed reactor, the catalyst is placed inside the tubes; the reaction material flows through the bed layer from top to bottom, while a heat carrier exchanges heat with the reactants inside the tubes in order to maintain the desired temperature conditions. Additionally, fixed-bed reactors can also be used for non-catalytic gas-solid and liquid-solid reactions. When the gas flows through the fixed bed at a low velocity, the upward resistance of the flowing gas does not cause any change in the motion state of the particles, and the bed height remains constant ； The bed pressure drop increases logarithmically with flow rate. Fluidized bed: In a fluidized bed, the fluid flows upward through a layer of fine particles (within a tower). When the flow velocity is low, the fluid merely passes through the gaps between the stationary particles; in this case, the bed is referred to as a fixed bed ； As the flow rate increases, the particles move apart from each other, and a small number of particles can be seen vibrating and moving within certain areas; this is known as an expanded bed ； When the velocity increases further to the point where all particles are just suspended in the upward-flowing gas or liquid, this is the starting point of the fluidized bed. Solid particles, like liquids such as water, can have a distinct interface within the device; even when the device is tilted, this interface remains horizontal ； The bed pressure drop does not change with flow rate (it remains essentially constant). Simply put, the phenomenon in which solid particles exhibit behavior similar to that of a fluid under the influence of a fluid is called fluidization. A fluidized bed is a device in which fluidization occurs. In a fluidized bed reactor, the catalyst is in a state of bubbling fluidization, and it is carried rapidly through the reactor by the reaction medium and the lifting gas, as in FCC lift tube reactors. Conveying bed: No distinct interface for solid particles within the equipment ； The bed pressure decreases as the flow rate increases. A bubbling bed is a type of fluidized bed; solids flow within the fluidized bed reactor, and the layer composed of fluid and solid particles behaves like boiling liquid. A distribution plate is located at the bottom of the bubbling bed reactor, with solid particles placed on this plate. The fluid is introduced beneath the distribution plate, and once the fluid velocity reaches a certain level, the solid particles begin to loosen; increasing the velocity further brings about a fluidized state. Internal components such as baffles, heat exchangers, and devices for separating fluid from solids are typically found within the reactor. Bubbling beds offer a large contact area, resulting in high efficiency in heat and mass transfer as well as high space-time yields, but they suffer from severe backmixing. In a fluidized bed, since the solids are in motion, the reaction or heat transfer efficiency is high, but power consumption is high, and a large amount of dust is generated during coal humidification. Moving bed: A moving bed is similar to a fixed bed, with the difference being that solid particles are continuously added from the top and removed from the bottom. A moving bed is an ion exchange device in which the ion exchange resin flows periodically between the exchanger, regenerator, and cleaning tower. The difference between a moving-bed and a fixed-bed reactor is that, during the reaction process, the catalyst moves slowly from the inlet to the outlet of the reactor. Fresh catalyst (or regenerated catalyst) enters from the reactor inlet, while the deactivated catalyst exits from the reactor outlet for regeneration. The catalyst moves at a slow speed and has not reached a fluidized state. Such as the moving-bed reactors in the UOP and IFP continuous reforming processes. Difference: Fixed-bed reactors can handle coal with high ash content and high ash fusion points; they require less investment but have poor environmental performance... Fluidized-bed reactors achieve the highest production rates, but they have certain requirements regarding the type of coal used ； The pressure in the sulfurization bed cannot be increased at present, limiting its widespread use. The most crucial aspect of these three types of beds is the design of their frames. Currently, in China, the design of bed frames relies mostly on empirical estimates and simulation experiments, which often lead to many shortcomings when applied in industrial production. Their main differences lie in their purpose, the properties of the materials involved, and whether it is a physical process or a chemical reaction. Fixed-bed and moving-bed reactors are more suitable for gas-gas, gas-liquid, and liquid-liquid reactions; the bed itself acts as a catalyst, with advantages such as low backmixing, less solid phase carried away, and simple separation. The bed configuration of the fluidized bed is very important in the design, as it requires a high degree of compatibility with the reaction system. Furthermore, the gas velocity during operation, the carry-away rate, and the design of the associated separation equipment such as cyclones are subject to strict requirements. Heat transfer in fluidized beds, as well as measures to break bubbles and prevent channeling, are also subjects of extensive research. For fluidized beds, it is important to avoid clogging the gas distributor, as that can cause serious problems. The distinction between fixed-bed, moving-bed, and fluidized-bed reactors is based on the amount of air supplied to the bed layer; as the amount of air increases, the order is fixed-bed, bubble bed (fluidized bed), turbulent bed, and plug flow bed. A moving bed strictly belongs to the category of fluidized beds, where the particles move downward while the height of the bed layer remains constant. The catalytic reforming process in oil refining is a typical example of a moving bed process. As for its application scope and advantages and disadvantages, fluidized bed technology offers good mass and heat transfer as well as high uniformity; it can be used on a large scale and is therefore the most widely applied. Examples include various gasification and combustion processes developed based on circulating fluidized beds. Fixed-bed and moving-bed systems are limited by mass and heat transfer, resulting in small scale, but they require low capital investment. For example, Lurgi’s gasified coal technology is a typical fixed-bed system; usually several furnaces are used in sequence to ensure continuous operation of the entire process.