In-depth analysis of microchannel reactors

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Microchannels, also known as microchannel heat exchangers, are heat exchangers whose equivalent channel diameter ranges from 10 to 1000 μm. The flat tubes of this heat exchanger contain dozens of tiny flow channels, with the ends of the flat tubes connected to circular collectors. Baffles are installed inside the manifold to divide the heat exchanger flow channels into several passes. Compared with traditional chemical production, microchannels hold great development potential and broad application prospects in the field of fine chemicals. So let’s take a look at microchannels from several negative perspectives. I. Understanding Microchannel Reactors: A microchannel reactor is, in essence, a type of continuous-flow tubular reactor. It includes mixers, heat exchangers, reactor controllers, and other components required for chemical processing units. Currently, the overall structure of microchannel reactors can be divided into two types: one is the monolithic structure, which takes the form of a cross-flow or counter-flow heat exchanger and allows for high-throughput operation per unit volume. Within the overall structure, only one operation step can be carried out at a time, and ultimately these respective components are connected to form a complex system. Another type is the layered structure; such systems consist of a stack of modules with different functions, with one operation being performed in one layer of modules and another operation in another layer. The flow of fluid through the various modular layers can be controlled by intelligent flow-dividing devices. For higher throughputs, certain microchannel reactors or systems are typically operated in parallel. II. Principles of microchannel reactors Microreactors refer to small, multi-channel microstructured reactors that are manufactured using surface science and microfabrication techniques, through microprocessing and precision engineering; the channel sizes of these microreactors are on the order of sub-microns and sub-millimeters. Furthermore, since microreactors possess heat/mass transfer properties that are 1–3 orders of magnitude better than those of traditional chemical equipment, they are particularly suitable for experiments involving highly exothermic and rapid reactions. And the concept of microreactors is surely something that many people want to understand. The idea behind microchemical engineering stems from the heat transfer mechanisms at conventional scales. For laminar flow inside a circular tube, when the wall temperature remains constant, equation (1) shows that the heat transfer coefficient h is inversely proportional to the tube diameter d; in other words, the smaller the diameter, the greater the heat transfer coefficient ； For laminar flow in a circular tube, when the concentration of component A at the wall remains constant, the mass transfer coefficient kc is inversely proportional to the tube diameter (Equation (2)); that is, the smaller the tube diameter, the larger the mass transfer coefficient. Since the flow within microchannels is mostly laminar, mixing between fluids relies primarily on molecular diffusion. As can be seen from equation (3), the mixing time t is proportional to the square of the channel size. Reducing the characteristic size of the channels not only **increases the specific surface area but also** enhances the transfer properties of the process. Nu=hd/k=3.66(1) Sh=kc/DAB=3.66(2) t=d²/DAB(3), where Nu is the Nusselt number, Sh is the Sherwood number, and D is the diffusion coefficient. The chemical reactions that occur in chemical processes are controlled by the rate of transport or by the intrinsic reaction kinetics, or by both. In terms of instantaneous and rapid reactions, when they take place in conventional scale reaction devices, the rate is governed by the transfer rate; whereas in microscale reaction systems, since the transfer rate increases by several orders of magnitude, the rate of such reaction processes increases significantly ； Such as the excited oxygen generator in oxyiodine chemical lasers (reaction of chlorine with an alkaline hydrogen peroxide solution), and direct fluorination of hydrocarbons. Slow reactions are primarily controlled by the intrinsic reaction kinetics, and one of the key ways to enhance their rate is to increase the intrinsic reaction speed, which can usually be achieved by raising the reaction temperature or altering the process conditions ； Medium-speed reactions are influenced by both the transfer and reaction rates, and similar measures to those for slow reaction processes can also be adopted. Currently, most hydrocarbon nitration reactions used in industrial applications are medium- to slow-speed processes, with reaction times ranging from dozens of minutes to several hours. In microreactors, adiabatic nitration can be employed; by simultaneously altering the process conditions, the reaction time can be reduced to just a few seconds. Therefore, theoretically, process intensification can be achieved for almost all current reaction processes. III. Classification of microchannel reactors Microreactors can be further divided into gas-solid phase catalytic microreactors, liquid-liquid phase microreactors, gas-liquid phase microreactors, and gas-liquid-solid three-phase catalytic microreactors, among others. 1. Gas-solid phase catalytic microreactors: Due to the characteristics of microreactors that make them suitable for gas-solid phase catalytic reactions, research on microreactors has primarily focused on such reactions to date; as a result, there are the most types of gas-solid phase catalytic microreactors. The simplest gas-solid phase catalytic microreactor is a microchannel with catalyst fixed on its walls. Complex gas-solid phase catalytic microreactors generally integrate one or more functions such as mixing, heat transfer, sensing, and separation. The most widely used is gas-solid catalytic oxidation of toluene. 2. Liquid-liquid phase reactors To date, compared to gas-solid phase catalytic microreactors, there are very few types of liquid-phase microreactors. A key factor affecting liquid-liquid phase reactions is thorough mixing; therefore, liquid-liquid phase microreactors are either coupled with micromixers or are themselves micromixers. There are few examples of microreactors designed specifically for liquid-liquid phase reactions and coupled with other functional units such as micromixers. There are mainly microreactors for the synthesis of vitamin precursors designed by BASF, and microreactors for carrying out the Dushman chemical reaction designed by MIT. 3. Gas-liquid phase microreactors: One type features gas and liquid flowing into a single microchannel from two separate microchannels, giving the overall structure a T-shaped appearance. Since in the gas-liquid two-phase flow, the flow pattern is similar to that in a bubble column, typical flow regimes such as bubble flow, slug flow, annular flow, and jet flow emerge as the flow rates of gas and liquid change, and this type of gas-liquid microreactor is referred to as a microbubble column. Another type is the sedimentation film microreactor, in which the liquid phase flows in a film-like manner from top to bottom, allowing thorough contact between the gas and liquid phases at the surface of the film. The rate and conversion of gas-liquid reactions often depend on the contact area between the gas and liquid phases. Both types of gas-liquid phase reactors have very large gas-liquid contact areas; their internal surface areas are both close to 20,000 m²/m³, which is an order of magnitude larger than that of conventional gas-liquid phase reactors. 4. Gas-liquid-solid three-phase catalytic microreactor
Gas-liquid-solid three-phase reactions are also quite common in chemical reactions; there are many types of such reactions. In most cases, the solid serves as the catalyst, while the gas and liquid act as reactants or products. The Massachusetts Institute of Technology in the United States has developed a micro-packed bed reactor for gas-liquid-solid three-phase catalytic reactions. Its structure is similar to that of a fixed-bed reactor: the reaction chamber (microchannels) is filled with fixed catalyst particles. The gas phase and liquid phase are divided into several streams, which then converge in the reaction chamber to undergo the catalytic reaction. IV. Introduction to the Advantages and Disadvantages of Microchannel Reactors I. First, let’s take a look at the advantages of microchannel reactors. Compared with conventional reaction vessels such as flasks, their structural characteristics confer upon them specific advantages, which are mainly reflected in the following aspects: (1) Precise control over reaction temperature; for exothermic reactions, if the mixing and heat transfer efficiency is low, local overheating can occur. The interior of the microchannels exhibits excellent heat and mass transfer characteristics, preventing excessive accumulation of reaction temperature and allowing it to be precisely controlled within a certain range. (2) Large specific surface area, high transfer rate, short contact time, and fewer by-products: The characteristic scale of micro-reaction channels is small, with a micro-channel ratio generally ranging from 5000 to 50,000 mm; as a result, the heat and mass transfer capacity per unit area is significantly enhanced. (3) Rapid and direct amplification: Traditional amplification processes involve a scaling effect, where amplification is achieved by increasing the size and scale of production equipment. This process is time-consuming and labor-intensive, and it is not possible to respond immediately to market demands, resulting in delays. The microreactor system has a multi-channel structure, with each channel acting as an independent reactor. When scaling up production, it is not necessary to increase the size of the reactors; instead, it is sufficient to add more of them in parallel, a concept known as \"scaling up by quantity\". (4) High safety: A large amount of heat can be removed promptly, thereby keeping the reaction temperature within the set range and minimizing the likelihood of accidents. (5) Good operability: The microreaction system is a parallel system with a modular structure, featuring portability. It enables decentralized construction and on-site production and supply at the location where the product is used, truly making chemical plants portable. Moreover, it allows for adjustments in production by increasing or decreasing the number of channels and replacing modules according to market conditions, thus offering high operational flexibility. Due to its many advantages, the successful application of microchannel reactor technology in the chemical industry has attracted increasing attention. II. Disadvantages of microreactors Compared to traditional batch reactors, their disadvantages mainly lie in four aspects. ⑴There are already many studies on the issue of channel blockage, utilizing microreactors to produce nanomaterials; due to their very high mixing efficiency, microreactors result in particles with a narrow size distribution. However, the micron-scale channel sizes and highly complex internal structure of microreactors make it extremely easy for the reactor channels to become clogged, and cleaning them is also very difficult. Currently, the clogging problem of microreactors has become the biggest obstacle to their replacement of batch reactors. ⑵The pulsation problem of pumps: Microchannel reactors generally use mechanical pumps to drive the fluid, but most mechanical pumps generate pulsating flow, leading to instability of the fluid within the microreactor. One solution that can achieve a stable continuous flow at present is electroosmotic flow. ⑶The issue of equipment corrosion: the corrosion of the microreactor channels by the fluids involved in the reaction is also a major problem. Due to the very high specific surface area and extremely small characteristic dimensions of the microchannels in microreactors, even minimal corrosive degradation can have a significant impact on them. This necessitates stringent anti-corrosion requirements for the materials used in the channels. Undoubtedly, this increases the manufacturing costs of microreactors and hinders their large-scale industrial application. ⑷In industry, complex microreactors utilize “number scaling” to increase production capacity. Although this method can effectively reduce scaling costs, the processing capacity is also significantly limited. Secondly, scaling up microreactors seems simple, but achieving it is a huge challenge. As the number of microreactors **increases**, the complexity of their monitoring and control also **rises**; furthermore, the operating costs become **higher** in actual production. V. Application scope of microchannel reactors We all know that microreactors have many advantages—for instance, excellent mass and heat transfer capabilities—which significantly reduce the likelihood of accidents ； Rapid and direct scaling capabilities, cost and time savings, etc. Of course, it also has certain limitations, and it is for these reasons that many experiments cannot be conducted using microreactors. Therefore, it is essential for chemical enterprises to understand the applicable scope of microchannel reactors. Firstly, strictly speaking, it is currently difficult to determine which reactions are suitable for microchannel reactors, as the characteristics of each reaction vary, and there are also a great many types of microchannel reactor devices. However, it is generally believed that 20-30% of the existing synthesis reactions can be upgraded using microchannel reactors. At the same time, by using microchannel reactors, we can implement approximately 20%-30% of the process flows that were previously considered dangerous. In other words, it appears that currently around 30–50% of chemical processes can be upgraded using microchannel reactors. In terms of their structural characteristics, microchannel reactors can currently be used for the following types of reactions: 1. Reactions in which the reaction rate itself is high, but the overall reaction speed is low due to limitations in the transfer processes; such reactions are mainly liquid-liquid multiphase reactions, as well as physical processes like liquid-liquid extraction. The characteristic of this process is that the reaction itself occurs rapidly, but the overall reaction rate is low due to the diffusion of the substrate within the liquid phase. In traditional reaction vessels, stirrers are generally used for mixing during reactions; this approach is inefficient as it fails to ensure thorough mixing between the two liquid phases, resulting in low reaction efficiency. In microchannel reactors, the reduced diffusion scale due to the small size of the channels enables such reactions to proceed rapidly. 2. These are reactions that are fast in themselves, but they are intense and involve strong heat release; the products formed are prone to degradation. Such reactions include nitration, diazotization, as well as certain hydrolysis and alkylation reactions. The nitration and diazotization reactions themselves are very rapid and intense, but in actual industrial operations, the reaction time is often measured in hours. This is because the heat transfer capacity of the reactor is limited; to prevent the temperature inside the system from rising uncontrollably, the reagents need to be added gradually. It can be said that the reaction speed is entirely determined by the heat transfer capacity. If a microchannel reactor with strong heat transfer capacity is used, reagents can be introduced quickly and the reaction can proceed steadily. It can be said that this type of reaction has the greatest potential for industrialization and should be given priority as a process to consider. 3. Reactions that require strict control of the flow pattern inside the reactor. Such reactions mainly involve the synthesis of nanoparticles, etc. These processes have been introduced previously; they primarily utilize the regular flow patterns within microchannels to produce materials with a narrow particle size distribution, thereby increasing the added value of the products. For such reactions, the product yield is generally low, but the added value is very high. In some cases, combining several experimental setups can result in a production facility, and their application prospects are quite promising. 4. Some gas-liquid reactions can, in principle, be carried out using microchannel reactors; however, no optimal reactor design for such reactions has yet been developed. Hydrogenation is the most prominent example of this situation. There are many types of hydrogenation reactions, and while some of them have high reaction rates, they are limited by the diffusion of hydrogen into the liquid phase, which results in a lower overall reaction rate. Under such conditions, it is of course possible to utilize the mixing properties of microchannel reactors for the reaction, similar to type 1 reactions; however, what is emphasized here is the gas-liquid mass transfer process. However, gas-liquid processes have their particularities, mainly in terms of fluid distribution and control, which results in the absence of suitable scaled-up gas-liquid microchannel reactors. Therefore, experimental research in this area is very active; in industrial applications, it is not feasible unless the production volume is low enough to allow the use of experimental equipment. 5. Microchannel reactors are not used for reactions containing solids whose particle size is 10% or more of the characteristic size of the microchannels and whose solid content exceeds 5%. VI. Technical Requirements for Microchannel Reactor Systems 1. ★General Requirements: The synthesis reaction system shall include reactant channels that can operate independently of each other, with no fewer than 6 such independent reactant channels. 2. The ★ reactor frame allows for flexible configuration of the number of reaction modules (not less than 4), with at least 8 feeding and collection interfaces, as well as 4 heat exchange fluid interfaces. 3. ★ The reactor can be divided into two temperature zones using two thermostatic circulators and sealed insulation panels; the control parameters for each of these two temperature zones can be set flexibly. 4. ★ The reaction module has a three-layer structure: the upper layer is the base plate, the middle layer serves as the mixing or reaction channel, and the lower layer functions as the heat exchange channel. All modules are made of silicon carbide, and the molding process utilizes diffusion welding to achieve a unified structure that ensures gas tightness and high-pressure resistance. To prevent contamination due to metal leaching, no metal connectors may be installed within the modules. 5. The ★reactor contains multiple sets of silicon carbide modules, including mixing modules and reaction modules, which can carry out reactions of the type A+B→P or A+B→P’+C→P. The mixing modules can also be used as quenching modules to stop the reaction or lower its temperature. 6. ★ The structural design of the reaction channel should enable enhanced mass transfer while reducing backmixing, thereby ensuring consistent residence time of the material within the reactor; an internal structure diagram is required. 7. Thermal conductivity: ≥ 100 W/mK (within a temperature range of 200°C). 8. Corrosion resistance: The material in contact with the liquid inside the reactor is capable of withstanding substances such as sulfuric acid, hydrofluoric acid, hydrobromic acid, and strong alkalis at the operating temperature of the reactor. 9. Annual loss rate: ≤0.1 mm/year (tested at 120°C under 1:1 HF/HNO3 conditions). 10. Operating temperature range on the process side: -20–150°C; operating temperature range on the heat exchange side: -20–150°C. 11. Process side pressure range: 0–25 bar; test pressure: 75 bar; a pressure measurement certificate is provided ； The pressure range on the heat exchange side is 0-5 bar. 12. Flux: 0.2–20 mL/min. 13. ★ Volume inside the reactor: 0.95–13.5 ml. The minimum liquid holding capacity per plate is not more than 1 ml, while the maximum liquid holding capacity per plate is not more than 4.8 ml. 14. ★ The size of the reaction channel should not exceed 1.4×1.4 mm, while the size of the preheating channel should not exceed 1×1 mm. 15. Residence time: 2.7sec-60min. 16. Requirements for reactor accessories: The inlet and outlet pipelines as well as the backpressure system must be made of corrosion-resistant and pressure-resistant materials to ensure the proper progression of gas-liquid reactions and liquid-liquid reactions.