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Types of reboilers and usage considerations: The combination of a reboiler and a distillation tower is a common process configuration in petrochemical plants. The reboiler is installed at the bottom of the distillation tower; it heats the liquid in the bottom of the tower (the distillation vessel), causing part of it to vaporize and form rising steam. This steam provides the heat necessary for heat and mass transfer in the distillation section, the stripping section, and between the vapor and liquid phases on the tower trays. Vertical thermosyphon reboiler: A vertical thermosyphon reboiler utilizes the density difference between the single-phase liquid in the bottom of the tower and the vapor-liquid mixture inside the heat exchange tubes to generate a driving force for circulation, thereby enabling the flow of the process fluid between the bottom of the distillation tower and the reboiler. Vertical and horizontal thermosyphon reboilers do not have their own gas-liquid separation space or buffer zone; these are provided by the bottom of the tower. This type of reboiler has the following characteristics: 1. Circulation driving force: the density difference between the kettle liquid and the gas-liquid mixture in the heat exchanger tubes ； 2. Compact structure, small footprint, high heat transfer coefficient ； 3. The shell side cannot be mechanically cleaned, and it is not suitable for heat transfer media with high viscosity or that are dirty ； 4. The tower bottom provides a space for gas-liquid separation and a buffer zone ； 5. The equipment is installed directly next to the tower; due to the simple piping system, its cost is low. Disadvantage: The pipe length is usually limited by the height of the tower skirt and the heat transfer area. It is difficult to maintain and clean, and cannot be used in systems where there may be high flow rates or sudden fluctuations; a higher boiling point results in a lower steam generation rate. Horizontal thermal siphon reboiler: The heating medium flows inside the tubes, and the tube side can be of single-pass or multi-pass type. 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. This type of reboiler has the following characteristics: 1. Circulation driving force: the density difference between the liquid in the kettle and the gas-liquid mixture in the heat exchanger tubes. 2. It has a large floor area, a moderate heat transfer coefficient, and is easy to maintain and clean. 3. The tower bottom provides a space for gas-liquid separation and a buffer zone. 4. It has a high circulation rate, resulting in a high flow velocity and a lower outlet dryness; this prevents the accumulation of high-boiling-point components and reduces the rate of scaling. 5. Horizontal reboiler: The process stream flows in the shell side; the heat transfer coefficient is relatively high, the investment cost is moderate, it occupies a large area, the skirt height is low, and the vaporization rate ranges from 3% to 35%. Disadvantage: To achieve a good fluid distribution, multiple nozzles are usually used, which complicates the piping system and increases the cost of the equipment. Forced-circulation reboiler: A forced-circulation reboiler relies on a pump to supply mechanical power for the circulation of the fluid. In most systems, external boiling tube bundles are generally suitable. Typically, the evaporation rate is kept below 1%, and the fluid will flash completely after passing through the valve at the outlet pipe. Additionally, the circulation rate is controlled by the pump; therefore, the cost associated with pump power consumption must be taken into account. The circulation rate can be optimized through considerations such as the volume of fluid circulated, initial investment costs, and operating expenses. Applicable scenarios include materials with high viscosity or heat sensitivity, solid suspensions, as well as high-resistance systems with long heat transfer paths and low evaporation ratios. The optimal application for forced-flow reboilers is fluids with severe scaling and extremely high viscosity. Kettle reboiler: A kettle reboiler consists of a shell with a gas-liquid separation space and an extractable tube bundle, with an overflow weir at the end of the tube bundle to ensure that it is effectively submerged in the liquid. The space outside the overflow weir serves as a buffer for the discharged liquid. The packing factor of the liquid in the reboiler is 80% for systems that do not tend to foam, and shall not exceed 65% for systems that are prone to foaming. Features: Insensitive to fluid dynamics parameters, high reliability, capable of operation in high vacuum, and easy to maintain and clean. Disadvantages: Low heat transfer coefficient, large shell volume, large floor space, high cost; the liquid in the tower bottom stays in the heating section for a long time, making scaling likely. Built-in reboiler: The characteristic of a tower-mounted built-in reboiler is that the tube bundle is directly inserted into the liquid pool at the bottom of the distillation tower. Like other kettle-type reboilers, it also has the same advantages as kettle-type reboilers. Features: An internal reboiler is one in which the reboiler’s tube bundle is placed directly inside the reactor vessel, eliminating the need for a casing and connecting pipelines. It has a simple structure, and its cost is lower than that of a vessel-type reboiler. Disadvantage: Due to its built-in tower design, the length of the tube bundle is limited by the diameter of the distillation tower, resulting in limited dimensions. The volume of the tower bottom space is limited, so the heat transfer area cannot be too large, resulting in suboptimal heat transfer efficiency. Requirements for the use of reboilers: (1) The reboiler should be placed as close as possible to the tower; in fact, its outlet ports can be directly connected to those of the tower. If the reboiler is connected directly to the tower, and there is a instrumentation interface between the two pipe ends on the PID diagram, then this instrumentation interface should be arranged at either of the connected pipe ends. (2) For a column with dual reboilers, it is advisable to arrange the reboilers symmetrically. (3) The distance between the center of the vapor line connecting the bottom of the tower to the reboiler and the reboiler tube sheet should not be too large, to avoid poor thermosiphon effect and thus affect the efficiency of the reboiler. (4) The support position of the reboiler should be determined such that the upward displacement of the connection port between the reboiler and the tower is the same. Once the support location is determined, it should be promptly communicated to the equipment team. For vertical siphon reboilers, the normal liquid level in the reactor bottom is usually at about the level of the upper tube sheet of the reboiler. To accommodate fluctuations in the liquid level, an appropriate margin should be provided for the height of the reboiler’s tube sheet and for the lower end of the pipe connecting it to the gas phase of the tower. (5) When multiple reboilers are arranged together with a tower, the location and installation height of the reboilers should not only meet the process requirements but also satisfy the layout requirements for the inlet and outlet manifolds, as well as facilitate operation and maintenance. (6) Horizontal reboilers heated by steam or a heat carrier should be placed near the tower, at a certain height above it; the distance between them shall meet the requirements for pipeline layout. One end of the reboiler’s tube bundle should provide space and access for maintenance. For the initial startup of the heat exchange unit system, the pipes attached to the housing (vent pipes and drain pipes) are first used to remove any gas or liquid present inside the heat exchanger, in order to prevent water hammer or air blockages; thereafter, all the exhaust valves are opened ； First, introduce the low-temperature fluid (gas or liquid). If the low-temperature fluid is a liquid, close the vent valve once the liquid has filled the heat exchanger ； The high-temperature fluid (gas or liquid) should be introduced slowly, to avoid thermal shock caused by the rapid inflow of the fluid due to a large temperature difference ； As the temperature rises to the normal operating temperature, the externally connected bolts should be retightened to prevent leaks due to inadequate sealing. For a proper shutdown of the heat exchange system, the high-temperature fluid should be stopped first, and after a certain interval, the low-temperature fluid should be stopped as well, to ensure safe shutdown. For long-term parking, the accumulated liquid should be drained, air should be removed, and in some cases inert gas should be used for replacement to ensure safety during equipment maintenance.