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Chapter 2 Pressure Vessels Definition of pressure vessels: (1) Maximum operating pressure P ≥ 0.1 MPa (gauge pressure)； (2) Inner diameter ≥ 0.15 m, and volume ≥ 0.025 m3 ； (3) The medium contained is a gas, a liquefied gas, or a liquid with a maximum operating temperature higher than its standard boiling point. Classification by pressure: (1) Low-pressure vessels: 0.1MPa≤P＜1.6MPa; (2) Medium-pressure vessels: 1.6MPa≤P＜10MPa ； (3) High-pressure vessels: 10MPa ≤ P < 100MPa ； (4) Ultra-high pressure vessels: P ≥ 100 MPa Section 1: Various reaction tanks I. Reaction equipment (reactors) Chemical production consists of three stages: raw material processing, chemical reactions, and product separation/purification. The chemical reaction process is a crucial part of chemical production, playing a decisive role in its success or failure. Since chemical reactions take place within reaction equipment, this equipment becomes essential for chemical production. At present, the most common type of reaction equipment in our company is batch reactors. Therefore, the following mainly focuses on understanding kettle-type reaction equipment. Due to the wide variety of types of chemical reaction equipment, there is a great diversity in such equipment; it includes tubular reactors, column reactors, and fluidized bed reactors as well. The kettle reactor is the most commonly used reaction equipment for liquid-liquid or liquid-solid phase reactions. A kettle-type reactor is mainly composed of three parts: the reactor vessel, the agitator, and the heat exchanger. The reactor vessel forms the core of the reactor; it is cylindrical in shape, with a height-to-diameter ratio generally ranging from 1 to 3. The upper and lower covers are mostly oval in shape ； The material used for kettle-type reactors is usually ordinary carbon steel or stainless steel. Stirrer: To ensure uniform mixing of the materials inside the reactor as well as efficient heat transfer, reactors are usually equipped with stirrers; different reaction requirements call for stirrers of various shapes. Heat exchanger: To enable the materials in the reaction vessel to react at the most suitable temperature, it is often necessary to heat or cool these materials. The most common heat exchange devices for reaction vessels are jackets, coiled tubes (spiral coils), and reflux condensers. II. Tower Reactors 1. Bubble Column Reactor The bubble column reactor is widely used in medium- and slow-speed reactions as well as reactions that release large amounts of heat, where the liquid phase also participates in the reaction. For example, this type of bubble column reactor is used in reactions such as the oxidation of various organic compounds, the chlorination of various paraffins and aromatics, various biochemical reactions, aeration oxidation in wastewater treatment, and the carbonation of ammonia water to produce solid ammonium bicarbonate. The bubble column reactor has the following advantages in practical applications: Ø The gas is evenly distributed in the form of small bubbles, passing continuously through the gas-liquid reaction layer, which ensures good contact between the gas and liquid and facilitates thorough mixing, leading to efficient reactions. Ø It has a simple structure, is easy to clean, operates stably, and has low investment and maintenance costs. Ø Bubble column reactors have a very high liquid holding capacity and interphase contact area, resulting in high mass and heat transfer efficiency; they are suitable for slow chemical reactions and highly exothermic processes. Ø Heat exchange devices can be installed both inside and outside the tower. Ø Compared with packed towers, bubble columns can handle suspended liquids. Bubbling towers also have some difficult-to-overcome disadvantages when in use, which are mainly as follows: Ø To ensure a uniform distribution of gas across the cross-section, the diameter of the bubbling tower should not be too large, generally staying within 2–3 meters. Ø Axial mixing of the liquid phase in the bubble column reactor is very severe; when the height-to-diameter ratio is not too large, the liquid phase can be considered to be in an ideally mixed state. As a result, it is difficult to achieve a high conversion rate of the liquid phase in a single continuous reactor. Ø The bubble column reactor requires a high pressure drop during bubbling. Simple bubble column (upper diagram) 1–Column body ； 2- Jacket ； 3-Gas distributor ； 4-Tower ； 5- Baffle ； 6-Extra-tower heat exchanger ； 7-Liquid catcher ； 8-Expansion section Mainly consists of the tower body and gas distributor. The tower can be equipped with jackets or other types of heat exchangers, as well as expansion sections and droplet catchers ； Filler can be placed in the liquid layer inside the tower ； Horizontal porous partitions can be installed inside the tower to improve gas dispersion and reduce liquid backmixing. In a simple bubble column, the liquid phase can be approximated as an ideal mixed-flow regime, while the gas phase can be approximated as an ideal displacement-flow regime. The optimal gas velocity in the empty tower must meet two conditions: (1) ensuring optimal selectivity in the reaction process ； (2) Ensure the reactor volume is minimized. Factors affecting mass transfer: When the gas empty tower velocity is below 0.05 m/s, the structure of the gas distributor determines the dispersion of the gas and the size of the bubbles, which in turn influences the gas holdup and the value of the liquid-phase mass transfer coefficient. When the gas empty tower velocity is greater than 0.1 m/s, the structure of the gas distributor becomes unimportant. At this stage, the bubbles are formed due to the impact and friction between the gas flow and the liquid. The size of the bubbles and their distribution depend mainly on the gas linear velocity in the empty tower. Gas-lift bubble column (upper figure) 1 – Cylinder ； 2-Air lift tube ； 3-Gas distributor: The tower is equipped with gas lift pipes, which induce a regular cyclic flow of the liquid; this enhances the mass transfer within the reactor and facilitates the suspension of the solid catalyst. Feature: In this bubble column, the agitation of the airflow is much more intense than in a simple bubble column. In a simple bubble column, the gas void velocity does not exceed 1 m/s; in a gas-lift bubble column, the gas void velocity within the gas-lift bubbles can reach up to 2 m/s, while the void velocity across the entire column cross-section is around 1 m/s. The liquid circulation velocity can range from 1 to 2 m/s. Transfer characteristics of bubble columns: Hydrodynamic characteristics of bubble columns. The most fundamental phenomenon in bubble columns is that gas exists in the form of bubbles. Basic characteristics: The shape, size, and movement of bubbles directly influence the macroscopic reaction process. 1. Flow state and bubble characteristics Industrial bubble column reactors typically operate in two flow states, namely the calm zone and the turbulent zone. The so-called quiet zone operation refers to a situation in which the gas flow rate in the bubble column is low, the bubble sizes are relatively uniform and rise in an orderly manner, with little stirring of the liquid. Operating in a quiet zone allows for achieving a certain gas flow rate while preventing axial backmixing of the gas, making it very suitable for slow reactions that are controlled dynamically. The so-called turbulent zone operation occurs when the gas flow rate is high; in this condition, the movement of bubbles becomes irregular, the liquid experiences intense turbulence, and the materials inside the tower mix thoroughly. The mechanism by which bubbles exert their effect is quite complex, and this situation is referred to as the turbulent zone. In the turbulent zone, the bubble sizes are uneven; large bubbles rise quickly while small bubbles rise more slowly, resulting in different residence times. Coupled with the lack of directional mixing, this not only leads to significant liquid-phase backmixing but also gas-phase backmixing. 2. Bubble size The size of the bubbles is directly related to the gas-liquid mass transfer area. At the same empty tower gas velocity, the smaller the bubbles, the better the dispersion, and the larger the gas-liquid contact area. In the quiet zone, since the rising speed of bubbles is slow, the gas velocity at the small holes has little impact on their size; it is mainly related to the diameter of the distributor holes and the properties of the gas-liquid mixture. In the turbulent zone, bubbles are formed due to ejection, impact, and friction between the airflow and the liquid. Therefore, in such a bubble column, the shape, size, and movement of the bubbles are varied, constantly changing, and random, resulting in groups of bubbles of different sizes. 3. Gas content rate The gas content rate refers to the volume fraction of gas in a gas-liquid mixture, and it can be expressed by the following formula: http://web.czie.net/jpkc/hx/WLKC/content/ch7/7_2.htm – Taide.t’s blog. In this formula, εG represents the gas content rate ； VG — gas volume, m3 ； VL — liquid volume, m3 ； VGL — volume of the gas-liquid mixture, m3. For cylindrical towers, since the cross-section remains constant, the value of the gas holdup indicates the extent of expansion of the packed bed inside the tower before and after aeration. For mass transfer and chemical reactions, the gas holdup is very important, as it is related to the residence time and the area of the gas-liquid interface. The factors affecting the gas content rate mainly include equipment structure, physical property parameters, and operating conditions. The properties of ordinary gases have little impact on the gas content and can be ignored. The surface tension σL, viscosity μL, and density ρL of the liquid all have an impact on the gas content. The presence of electrolytes in the solution alters the gas-liquid interface, resulting in bubbles that rise more slowly, which increases the gas content by 15% to 20% compared to that in pure water. As the gas velocity in the empty tower increases, εG also increases; however, when μOG reaches a certain value, the bubbles coalesce and εG decreases instead. εG decreases as the tower diameter D increases, but when D > 0.15 m, D has no effect on εG. When μOG < 0.05 m/s, εG is independent of the tower diameter D. (Therefore, the diameter of laboratory testing equipment should generally be greater than 0.15 m; only when μOG < 0.05 m/s can a smaller tower diameter be used.) 4. Gas-liquid ratio interfacial area The gas-liquid ratio interfacial area refers to the surface area of bubbles per unit volume of the gas-liquid mixed bubble bed. The value of α is directly related to the mass transfer rate and constitutes an important parameter. It is difficult to determine the value of α; therefore, people often use the mass transfer relationship NA=kLαΔcA to directly determine the value of kLα for practical purposes. 5. Gas resistance ΔP in the bubble column The gas resistance within a bubble column consists of two components: one is the resistance due to the gas distributor, and the other is the resistance resulting from the static pressure head in the bed. 6. Backmixing: There is backmixing in the liquid phase within the bubble column; therefore, the liquid phase in industrial bubble column reactors is generally assumed to be ideally mixed. The backmixing of gases within the tower is generally not significant, and it is often assumed to be a plug flow; the calculation error for this assumption is about 5%. However, when strict calculations are required, especially at high gas conversion rates, backmixing must be taken into account. Mass transfer in bubble columns: In the mass transfer process within a bubble column reactor, the gas film mass transfer resistance is generally low and can be ignored, while the magnitude of the liquid film mass transfer resistance determines the speed of mass transfer. When the bubble column operates in a quiescent zone, the factors affecting the liquid-phase mass transfer coefficient are mainly bubble size, empty-tower gas velocity, liquid properties, and diffusion coefficient ； When operating in the turbulent region, the diffusion coefficient of the liquid, the properties of the liquid, the equivalent specific surface area of the bubbles, and the surface tension of the gas become the main factors affecting the mass transfer coefficient. Heat transfer in bubble columns Heat transfer in bubble columns generally occurs in three ways: by using the vaporization of the solvent, liquid reactants, or products to carry away heat ； The reaction heat is removed using a liquid circulation external cooler. ( ； Use jacketed, coiled-tube, or shell-and-tube coolers. In a bubble column, the movement of bubbles causes intense disturbance of the liquid in the bed. The heat transfer coefficient of a fluid to the heat exchanger wall is more than 10 times greater than that of natural convection; therefore, it generally does not constitute a major obstacle in heat exchange. The overall heat transfer coefficient of the bubble column is typically 694–915 W/(m2·K). Semi-continuous bubble column: A semi-continuous bubble column reactor involves the addition of liquid in one go, with gas being continuously introduced at the bottom of the reactor; this gas passes through the bed in the form of bubbles and then exits from the top. The gas supply is stopped once the composition of the liquid phase meets the desired requirements, and the liquid is then removed from the reactor as the finished product. In this type of reactor, gas is fed continuously while liquid is fed intermittently; hence it is called a semi-continuous bubble column reactor. Like the homogeneous batch reactor, each operating cycle consists of a reaction time τ and a auxiliary time τ'. III. Packed Tower Reactor The packed tower reactor is a device widely used for gas absorption; it can also be used as a gas-liquid phase reactor. As the liquid flows down along the surface of the packing, a liquid film is formed on this surface, allowing it to come into contact with the gas phase and react with it, resulting in a relatively small amount of liquid phase. Suitable for instantaneous reaction, fast, and medium-speed reaction processes. For example, catalytic thermal alkaline absorption of CO2, the absorption of NOX by water to form nitric acid, the absorption of HCl by water to produce hydrochloric acid, and the absorption of SO3 to form sulfuric acid – all these processes typically use packed tower reactors. Packed tower reactors have the advantages of simple structure, low pressure drop, ease of adaptation to various corrosive media, and minimal tendency to cause foaming in the solution. Packed tower reactors also have many disadvantages. Firstly, it is unable to remove heat directly from the tower; when the heat of reaction is high, it is necessary to increase the amount of liquid spray in order to carry away the heat in the form of sensible heat ； Secondly, due to the issue of a minimum wetting rate, self-circulation often has to be employed in many cases to ensure basic wetting of the filler, but such self-circulation violates the principle of counterflow. Nevertheless, the packed tower reactor remains a common device for gas-liquid reactions and chemical absorption. Filler tower reactors are particularly suitable, especially at normal and low pressures when pressure drop becomes the main issue and when the reaction solvent tends to foam. Packed tower with regular packing; regular packing. IV. Plate tower reactor: In a plate tower reactor, the liquid is the continuous phase while the gas is the dispersed phase; chemical reactions take place as the gas passes through the tray, breaking down into small bubbles that come into contact with the liquid on the tray. Plate tower reactors are suitable for fast and medium-speed reactions. The use of multiple plates can minimize axial backmixing, and it allows operation at very low liquid flow rates, thereby enabling extremely high liquid-phase conversion rates to be achieved directly in a single tower. At the same time, the gas-liquid mass transfer coefficient in plate tower reactors is high, allowing cooling or heating elements to be installed on the plates to meet the requirements for maintaining the desired temperature. However, plate tower reactors have disadvantages such as a high pressure drop in the gas phase and a small mass transfer surface. Plate tower tray types V. Spray tower reactors Spray tower reactors have a relatively simple structure; the liquid is dispersed in the gas in the form of fine droplets, with the gas acting as the continuous phase and the liquid as the dispersed phase. This configuration offers advantages such as a large contact area between the phases and a low pressure drop in the gas phase. Suitable for instantaneous, interfacial, and rapid reactions, as well as reactions that produce solids. The spray tower reactor has the disadvantages of a low liquid holdup, an excessively low mass transfer coefficient on the liquid side, and severe mixing between the gas and liquid phases. VI. Tubular reactor: A continuously operating reactor with a tubular shape and a very high length-to-diameter ratio. Such reactors can be very long; for example, the reactor tubes used in propylene dimerization can be several kilometers in length. The structure of the reactor can be a single tube or multiple tubes in parallel; it can be an empty tube, such as in a tubular pyrolyzer, or it can be a packed tube with granular catalyst filled inside it to carry out multiphase catalytic reactions, such as a tubular fixed-bed reactor. Generally, when the reaction stream is in a turbulent state, the length-diameter ratio of the pipe is greater than 50; the ratio of the length of the packed section to the particle size is greater than 100 for gases or 200 for liquids, and the flow of the material can be approximated as plug flow. VII. Fluidized bed reactor: A type of reactor in which gas or liquid is passed through a layer of particulate solids, causing the solid particles to be in a suspended state, thereby enabling gas-solid phase reactions or liquid-solid phase reactions to take place. When used in gas-solid systems, it is also known as a bubbling bed reactor. The early application of fluidized bed reactors in modern industry was the Winkler furnace for coal gasification, which appeared in the 1920s (see coal gasification furnace) ； However, the development of modern fluidized reaction technology was represented by petroleum catalytic cracking in the 1940s. Currently, fluidized bed reactors have been widely used in industries such as chemicals, petroleum, metallurgy, and the nuclear industry. Based on the applications of fluidized bed reactors, classifications can be divided into two categories: one category deals mainly with solids, such as the roasting of ores, and is referred to as a solid-phase processing process ； Another category of processing subjects are primarily fluids, such as catalytic processes like petroleum catalytic cracking and enzyme-mediated reactions; these are referred to as fluid-phase processing processes. There are two types of structures for fluidized bed reactors: ① Those equipped with devices for continuous feeding and discharging of solid materials, which are used in solid-phase processing processes or in fluid-phase processing processes where the catalyst becomes inactive rapidly. For example, in the catalytic cracking process, the catalyst becomes significantly deactivated within a few minutes, and it must be continuously separated using the aforementioned devices before it can be regenerated. ②A device for continuous feeding and discharging of solid materials, used in reaction processes where the properties of the solid particles do not change significantly over an extended period of time (such as half a year or a year). Compared with fixed-bed reactors, the advantages of fluidized-bed reactors are: ① It is possible to achieve continuous input and output of solid materials ； ②The movement of fluids and particles enables the bed to exhibit good heat transfer properties, results in a uniform temperature within the bed, and makes it easy to control; it is particularly suitable for exothermic reactions. On the other hand, severe backmixing can have a certain impact on the efficiency of the reactor and the selectivity of the reaction. Furthermore, the presence of bubbles in the gas-solid fluidized bed reduces gas-solid contact, resulting in incomplete gas reaction. Therefore, it is generally not suitable for reactions that require a very high one-way conversion rate. Furthermore, the wear of solid particles and the entrainment of dust in the gas flow also impose certain limitations on the use of fluidized beds. To limit backmixing, a multi-layered fluidized bed or internal components can be used within the bed. This allows a certain concentration difference or temperature difference to be created within the bed. Furthermore, as the gas is redistributed, the contact between the gas and the solid can also be improved. In recent years, turbulent fluidized beds with fine particles and high gas velocities, as well as high-speed fluidized beds, have been put into industrial use. When the gas velocity is higher than the particle entrainment velocity, the bed is maintained through the circulation of solids; since the contact between the gas and solid phases is enhanced, this is particularly advantageous in situations where the interphase mass transfer resistance plays a significant role. On the other hand, since a large amount of solid particles are entrained by the gas and need to be separated and recycled back to the bed, high requirements are placed on gas-solid separation.