Reinforcing fins are installed in plate-fin heat exchangers

This post was last edited by Cuaili Yisu on 2019-2-22 at 16:09. Design, manufacture, and application of new types of thermal melting capture condensers: These new types of thermal melting capture condensers are heat exchange devices that involve phase changes, and they have characteristics that are quite different from those of ordinary heat exchangers. This product features a rational design, with highly finned finned tubes inside to ensure good heat transfer and a large heat exchange area. Compared with existing technologies, it has advantages such as a small size, the ability to avoid high thermal stress, a large number of cycles, and prevention of severe corrosion of the material on the welding areas of the equipment. This product meets the various functional requirements of hot-melt condensers in the production of phthalic anhydride and p-chlorobenzonitrile. It also outlines the heat and mass transfer mechanisms involved in the sublimation process of phthalic anhydride, as well as the structural features and optimization parameters of hot-melt capture condensers. Additionally, it presents the applications of the developments achieved in hot-melt capture condensers. Keywords: seamless steel pipe, weld-free wound product pipe, sublimation characteristics, wing height of 30 mm for increased productivity and reduced energy consumption, high-efficiency hot-melt capture condenser. The new type of hot-melt capture condenser is a device that captures materials through sublimation. Also known as: Switching condenser ； Thermosetting condensation box ； Partial condenser ； Condenser. Technical Field: This product specifically relates to an efficient device for collecting gaseous materials, particularly the reaction gases generated during the production of phthalic anhydride and p-chlorobenzonitrile. Background Technology: In many types of chemical production processes, a material collection device that is capable of both condensing and heating is required. However, due to the large temperature differences between these two operating modes, which often exceed 350°C, significant thermal stress is generated in the equipment. Additionally, the device has to switch between these two modes frequently. This is especially true in environments with high corrosivity during chemical production. Moreover, since heat exchange is needed to trap sublimated gases and to collect condensed solids, any uncollected material must be discharged or further processed. As a result, the equipment requires a large heat exchange surface area as well as a high overall heat transfer coefficient in order to meet the requirements. Currently, in the production processes of phthalic anhydride and p-chlorobenzonitrile, heat-melt condensers are required to collect the materials, and the efficiency of this collection process directly affects the yield of production. In existing technical solutions, in order to achieve a large heat exchange area, the size of the equipment is usually made very large, resulting in a large floor space and high investment costs. Moreover, the welding points of the equipment materials are prone to severe corrosion and material leakage, which can lead to production shutdowns and significant economic losses. The development of this product effectively addresses the shortcomings of the aforementioned issues. I. Working principle: 1. The high-temperature mixed gas containing the product and impurities enters the capture condenser from the top. 2. The cooling medium inside the finned tubes causes the desired product solids to sublimate and deposit on the finned tubes; unwanted impurity gases are discharged from the lower part of the collection condenser. 3. Once the fins are filled with solid product, the cooling medium inside the finned tube is replaced with a heating medium to melt the solid product, which then collects in the bottom area of the condenser. The bottom end cap of the product’s container features an inclined discharge opening, which facilitates the outflow of material and prevents blockages at the bottom of the container. 4. After discharging the liquid product, replace the heat transfer medium in the finned condenser tubes with a refrigerant medium to start a new cycle of product capture. Refrigerant media: circulating water, cold oil, hot water. Heat transfer medium: hot water, hot oil, steam. II. Uses: 1. Organic anhydrides (produced via catalytic oxidation process) ⑴ Phthalic anhydride (phthalic anhydride, phthaleic anhydride): C8H4O3 ； Molecular weight 148.12 ； CAS85-44-9 ； Melting point 129°C-132℃ ； Boiling point 284℃ ； Flash point 152°C. (2) Maleic anhydride: C4H2O3 ； Molecular weight 98.06 ； CAS108-31-6 ； Density 1.48 ； Melting point 52°C-55℃ ； Boiling point 200℃ ； Flash point: 102°C. 2. Organic benzonitriles (produced by ammonia oxidation process) ⑴ m-Chlorobenzonitrile (3-chlorobenzonitrile): C7H4ClN ； Molecular weight 137.57 ； CAS766-84-7 ； Density 1.14 ； Melting point 38°C-42℃ ； Boiling point 94℃ ； Flash point 97°C. 3. p-Chlorobenzonitrile (p-chlorobenzanilide, 4-chlorobenzonitrile): C7H4CIN ； Molecular weight 137.57 ； CAS623-03-0 ； Melting point 91°C-94.2℃ ； Boiling point 223℃ ； Flash point: 108°C. III. Design: The design prototype for the new capture condenser is based on the melt condenser used by Imperial Chemical Industries (ICI) in its large-scale phthalic anhydride production lines. After further research by the author’s company, it has been widely applied in the gas treatment processes of fluidized-bed reactors for organic anhydrides and organic nitriles. The product gas generated as a result of the reaction enters the capture condenser, where it is cooled and condensed into solid crystals on the cooling surface. Once the condensation process is complete, the operation is reversed: heating is applied to convert the solid products back into liquid products. Through years of experience in the design of capture condensers, the author has found that the capture efficiency of such condensers is primarily influenced by the following factors: 1. The average degree of sublimation of the reaction gas and its impact on capture by the condenser. Due to the different freezing points of the products that need to be condensed, as well as the varying amounts of heat released during sublimation, it is possible to determine the average degree of sublimation of the reaction gases for various products through experiments. Through the application of this parameter in design, the amount of heat that needs to be transferred is calculated based on the output rate of the fluidized bed continuous reactor per unit time and the output rate specified for the switching operation; the condensation time and switching cycle are determined, and appropriate safety factors are taken into account, ultimately leading to the calculation of the equipment’s volume and total heat exchange area. 2. The impact of reaction gas flow rate on capture condensation: Once the reaction gas enters the capture condenser, the process of condensation and deposition begins. If the chosen gas flow rate is too high, it is likely that the gas will not be fully condensed before reaching the gas outlet. If the gas flow rate is too low, the reaction gas that enters the condenser first will quickly form an aerosol-like substance; as this substance moves, it will accumulate upon encountering cooler surfaces. Once it has accumulated to a certain extent, it will block the gas channels, resulting in a short circuit within the equipment. After research, the author’s company developed an optimized flow rate measurement method derived from experiments on various materials, and its performance has been good in practical applications over the past few years. 3. The effect of temperature difference △t on capture condensation: The temperature difference △t between the cooling medium and the main stream of product gas can significantly affect the degree of deposition. Among these, there is a relationship among the cooling temperature, gas flow velocity, and product gas. Various experiments with different reaction gases have shown that when the temperature difference Δt falls below the maximum value at which aerosol formation occurs, the formation of needle-shaped crystals through sublimation gradually disappears. At this point, the bulk density of the gas condensate will gradually increase. When the temperature of the cooling medium rises to near that of the reaction gas, a dense sublimation crystal layer is formed. 4. The influence of equipment structure on condensation capture: After determining parameters such as gas flow velocity and design temperature difference, as well as calculating the total volume and heat exchange area, it is necessary to properly design the gas flow channels and the space for condensation. This can be achieved by adjusting fin tube parameters, tube spacing, row spacing, the number of tube rows, and by using parallel tube arrays to ensure optimal performance. IV. Manufacturing: Several considerations: 1. Ensure the highest product capture rate or the lowest exhaust emission concentration. 2. Extend the service life of equipment. 3. The equipment investment cost and operating expenses should be low. Therefore, the structural design should include optimal structural parameters, such as the best finned tube parameters, the optimal tube spacing and row spacing, as well as the optimal number of tube rows. The design, manufacturing technical solutions, and process requirements for the new type of capture condenser are as follows: 1. The exterior of the tank is equipped with a semi-circular tube jacket; this jacket ensures continuous circulation of cooling water during operation, allowing a layer of condensed crystalline material to form on the inner wall of the tank. This layer acts as a protective coating that prevents corrosion of the welds. At the same time, reducing the temperature of the enclosure slows down the rate of corrosion. 2. The interior of the box is composed of multiple groups of tube bundles in U-shaped format, which helps to avoid thermal stress resulting from temperature differences under condensation and heating conditions. The bending process is employed to reduce the diameter of the U-shaped part of the heat exchange tube, thereby bringing the fins on the upper and lower tubes of the U-tube closer together and allowing for a larger heat exchange area within the same volume. Each tube bundle in the shell is equipped with a specially designed sliding support structure, which allows the tube bundle to expand and contract freely during frequent temperature changes; moreover, the tube bundle can be withdrawn individually for maintenance or replacement. 3. The tubes in the tube bundle are all made by bending whole custom-made seamless steel tubes; they are wrapped around the coiled fins with a pre-tension, without any weld points, thereby avoiding stress corrosion caused by welding stresses. Our factory’s advanced fin winding technology increases the standard fin height from 25 mm to 30 mm, thereby maximizing the fin area. The spacing between the spirally wound fins can be adjusted according to the heat exchange area and the density of the material; it should generally be greater than 6 mm. 4. The U-tube is joined to the tube sheet by strength expansion and sealing welding, or by strength welding. Support plates are installed between the U-tubes; these plates, along with each finned U-tube, are fixed in place using tie rods and spacers. The finned U-tubes, tube sheets, support plates, spacers, and tie rods together form a tube bundle. Grooves are made in the bottom surfaces of each support plate, and flat steel bars are placed in these grooves so that they extend beyond the support plates, thereby creating sliding strips and forming a tube bundle with sliding strips. 5. Design the tube sheet with partitions according to the tube bundle requirements; the tube sheet is equipped with sealing surfaces that ensure sealing between it and the tube bundle via gaskets under the preload from the bolts. Determine the width of the enclosure based on the width of the finned tube. 6. A semi-circular tube jacket is welded outside the box, enhancing its strength. Pipe bundle insertion ports are provided on the left and right sides of the box; inside the box, slides are installed at the positions corresponding to the pipe bundles to ensure that the entire pipe bundle can slide smoothly along these slides. 7. Outside the tube bundle insertion port, matching flanges are installed based on the tube bundle tube sheet and the tube box flanges; gaskets are used between the box flanges, the tube bundle tube sheet, and the tube box flanges to ensure a good seal under the pre-tightening force of the flange bolts. 8. Install the gas feed inlet, non-condensable gas outlet, and inspection ports on the box; insert the finned U-tube bundle into the box, and connect the tube sheet to the tube bundle as well as to the box using bolts. To form a new type of hot-melt capture condenser. The tube box can also be welded directly to the box, which ensures no leaks.