A comprehensive summary of reboiler knowledge!

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A comprehensive summary of reboiler knowledge! 12 types of reboilers. A reboiler (also known as a boiling reactor) is, as the name implies, a device that vaporizes a liquid once again. Its structure is similar to that of a condenser, but while a condenser is used to cool down, a reboiler is used to heat up and vaporize substances. Reboilers can be divided into cross-flow and axial-flow types. In the cross-flow type, the boiling process occurs entirely in the shell side; common types include kettle reboilers, internal reboilers, and horizontal thermosyphon reboilers. In the axial flow type, the boiling fluid flows axially, and the most common form is the vertical thermosyphon reboiler. 1. Kettle reboiler: A kettle reboiler features an enlarged shell, within which the vapor-liquid separation process takes place. The liquid level is maintained by a vertical baffle to ensure that the tube bundle is completely submerged in the liquid. The tube bundle is usually a U-tube structure with two tube passes, or it can also be a floating-head structure with multiple tube passes. Advantages of the kettle reboiler: The hydrodynamic effects have little impact on the kettle reboiler; as a result, its performance is relatively reliable. Its performance is even better under high vacuum conditions. By increasing the spacing between tubes, a very high heat flux density can be achieved, allowing for good operational performance under conditions of small temperature differences. Disadvantages: Kettle-type reboilers are prone to scaling; they are generally the most susceptible to scaling among all types of reboilers. In addition, they have a large footprint and are more expensive to manufacture. The optimal application of kettle reboilers is for clean fluids under low pressure, with a narrow boiling point range, and in conditions of either small or large temperature differences. Under near-critical pressure conditions, although the shell is large and costly, its performance is relatively reliable. 2. Internal reboiler in the tower: The characteristic of an internal reboiler in the tower is that the tube bundle is directly inserted into the liquid pool at the bottom of the distillation tower. Advantages: Similar to the kettle-type reboiler, it is minimally affected by hydraulic effects. By eliminating the shell and connection pipelines, etc., the built-in reboiler is the cheapest type of reboiler among all types. Disadvantages: Apart from the lack of a shell, the internal reboiler has the same disadvantages as a kettle-type reboiler; in addition, its heat transfer surface is also very limited. Its application scenarios are similar to those of kettle-type reboilers. 　　3. Internal reboiler in the tower; Horizontal thermosyphon reboiler. In a horizontal thermosyphon reboiler, the feed is introduced into the reboiler through the downcomer at the bottom of the tower. The liquid boils in the shell side and vaporizes, forming a vapor-liquid mixture with lower density. Due to the difference in density between the liquid in the feed pipe and that in the discharge pipe, a static pressure difference is created, which serves as the driving force for the natural circulation of the fluid. The heating medium flows inside the tubes; the tube side can be of single-pass or multi-pass type. Advantages: It has a high cycle rate, resulting in a high flow velocity and a low outlet dryness; this prevents the accumulation of high-boiling-point components and reduces the rate of scaling. Since the tube bundle is arranged horizontally and the flow area is easy to control, a lower static head is required. Disadvantage: It is difficult to clean once scaling occurs in the shell side. Due to the effect of baffle plates and support plates, local drying out can occur under high heat flux conditions. For large thermal siphon reboilers, multiple nozzles and connecting fittings are required to achieve a uniform flow distribution, which inevitably increases the cost of the reboiler. 　For horizontal thermal siphon reboilers used with fluids over a wide boiling range, horizontal baffle plates should be installed to prevent the light components from flashing at the inlet and the heavy components from concentrating at the outlet. To prevent flow blockage and flow instability, the maximum heat flux density should be limited. This type of reboiler is suitable for applications with medium pressure, medium temperature difference, and low static head. 4. Vertical tube-side thermosyphon reboiler: The boiling process in a vertical tube-side thermosyphon reboiler takes place within the tube side, while the heating medium is located on the shell side; the two-phase mixture flows at a high velocity from the discharge pipe into the tower. The flow cross-sectional area of the discharge outlet should be at least as large as the total flow area of the tube bundle, and the pressure drop across the discharge pipe should be less than 30% of the total pressure drop. The discharge pipe can be connected to the tower either through a large-diameter elbow along the axis or via a side opening. Tests show that the structure of the outlet tube has little effect on the performance of the reboiler, but the minimum flow area of the outlet tube has a significant impact on its performance. The driving head for the flow cycle is provided by the liquid level height in the liquid pool inside the tower. Usually, the liquid level in the tower and the upper tube sheet of the reboiler are at the same horizontal level. 　Advantages of vertical tube-side thermosyphon reboilers: Vertical thermosyphon systems have a high circulation speed; not only is the heat transfer coefficient higher than that of horizontal systems, but they also offer excellent anti-scaling properties, making them particularly suitable for polymer materials. Disadvantage: Vertical tubes are difficult to disassemble, clean, and repair. Furthermore, the liquid level at the bottom of the tower is approximately at the same level as the upper tube sheet of the reboiler, which raises the elevation at the tower bottom and increases the construction cost. The vertical tube-side thermosyphon reboiler is most suitable for applications with pure components, moderate pressure, moderate temperature differences, moderate heat flux, and conditions prone to scaling. 5. Vertical shell-side thermosyphon reboiler: The boiling process in a vertical shell-side thermosyphon reboiler takes place on the shell side. Baffles are installed on the shell side to enable longitudinal flow of the fluid. Vertical shell-side reboilers are suitable for special situations where it is not appropriate to place the heating medium on the shell side. Vertical shell-side thermosyphon reboilers: The design of such reboilers should ensure a uniform flow distribution on the boiling side, in order to avoid the formation of dead zones and prevent the accumulation of vapor and high-boiling-point components. However, there is very little design information in this area. Field tests have shown that the main problem is local overheating caused by the accumulation of vapor, which leads to failures in the upper tube sheet; therefore, during design, the two-phase mixture should be made to flow through the tube sheet at a uniform high velocity. The optimal application for this type of reboiler is the evaporation of pure components under medium pressure and medium temperature differences, with the heating medium having to be placed on the inside of the tubes. 6. Forced-flow horizontal reboiler The boiling process takes place on the inside of the tubes, with the fluid circulation being driven by a high-capacity pump. Generally, it is ensured that the evaporation rate is less than 1%, and the fluid will flash completely after passing through the valve at the outlet pipe. The optimal application for forced-flow reboilers is fluids with severe scaling and extremely high viscosity. Under conditions where the fluid maintains a high flow rate and a very low evaporation rate, the rate of scaling can be **reduced**; however, this requires an effective flow rate of 5–6 m/s, which results in high costs for the pumps as well as high energy consumption. 7. Vertical thermosyphon reboiler Vertical thermosyphon reboiler ▲Circulation driving force: The density difference between the kettle liquid and the gas-liquid mixture in the heat exchanger tubes. 　　▲Compact structure, small footprint, and high heat transfer coefficient. 　　▲The shell side cannot be mechanically cleaned, and it is not suitable for heat transfer fluids with high viscosity or that are dirty. 　　▲The tower bottom provides a space for gas-liquid separation and a buffer zone. 8. Horizontal thermosyphon reboiler Horizontal thermosyphon reboiler ▲Circulation driving force: The density difference between the kettle liquid and the gas-liquid mixture in the heat exchanger tubes. 　　▲It has a large footprint, a moderate heat transfer coefficient, and is easy to maintain and clean. 　　▲The tower bottom provides a space for gas-liquid separation and a buffer zone. 9. Forced-circulation reboiler Forced-circulation reboiler ▲Suitable for materials with high viscosity and heat sensitivity, solid suspensions, as well as systems with a long heat absorption section and a high resistance due to low evaporation ratios. 10. Membrane ring reboiler: In a membrane ring reboiler, 2/3 of the reboiler’s steam generators feature annular channels with variable cross-sections, allowing the two-phase fluid to flow upward. In 1/3 of the tube, the liquid flows downward in a film form and sinks into the space at the bottom of the tube–enabling a closed-loop circulation. The membrane-loop clean process can significantly enhance the heat transfer process (by about a factor of 2), especially under small and medium heat fluid conditions. At 2/3 of the heating surface of the reboiler, inserts with variable cross-sections having diameters of 0.027 m and 0.029 m (equivalent to 27–29 mm) are placed, thereby enabling the formation of a two-phase flow of gas-liquid mixture rising within the annular channel. While there is no vaporization at 1/3 of the heated surface. Studies on reboilers have shown that reboilers under such operating conditions enhance the heat transfer process significantly compared to the optimal heat transfer in conventional steam generator tubes, resulting in an increase in the reboiler’s production capacity by as much as 42%. 11. Forced-flow vertical reboiler. In addition to the forced-horizontal reboiler, the forced-flow vertical reboiler is also commonly used; it can be employed for the cyclic evaporation and extraction of the liquid from alcohol distillation columns. The liquid at the bottom of the tank can be used in the circulation of the reboiler, while another portion can be discharged under suction pressure and fed into subsequent equipment with a certain pressure head, such as a secondary mash preheater. In such a device, if it is used in special situations, its top portion can also be designed to have a certain separation space; once the steam is separated through the steam guide tube, some of the fluid can flow out from this area and enter the lower storage tank. 12. External circulation inert gas distillation reboiler: The external circulation inert gas distillation reboiler introduces an inert gas into a vertical tube flow boiling system, which significantly enhances heat transfer; this has led to the development of the \"carrier gas evaporation\" technique. This carrier gas evaporation method can clearly also be used in the reboilers of inert gas distillation to enhance heat transfer. The fluid enters the heating tubes of the reboiler from the bottom of the distillation tower, and the inert gas is also introduced from the bottom of the heating tubes. The introduction of an inert gas can significantly enhance heat transfer, as at the initial moment of contact between the inert gas bubbles and the boiling liquid, the liquid at the gas-liquid interface vaporizes rapidly, drawing the latent heat of vaporization from the surrounding liquid. Lower the liquid temperature at the gas-liquid interface to form an “interfacial vaporization well”