Process – Page 27 – What Is Piping

What is a Reboiler? Reboilers are typical heat exchanger that produces vapor to drive fractional distillation separation. In classical fractional distillation services, all the vapor to drive the separation comes from the reboiler. (Alternatively, externally generated vapor, feed preheat, or inter-reboiler systems may be used). Proper reboiler operation is vital to effective distillation. Types of Reboilers Reboilers are classified depending on the orientation of the reboiler and the type of circulation used. Depending on the reboiler orientation there are two types of reboilers: Vertical Reboiler (Fig. 2) and Horizontal Reboiler (Fig. 1 and 3) Depending on the types of circulation, the following five types of reboilers are widely used Fired Reboilers Force Circulation Reboilers Kettle Reboilers Thermosyphon Reboilers Internal reboilers The selection of the proper type of reboiler for any specific service is the most difficult job of reboiler design. Reboilers normally are shell-and-tube exchangers that heat up the fractionation column fluid utilizing the heat transfer from steam. Specific services may use other specialized designs including stab-ins, plate-fins, spiral-plate, and others. The purpose here is not to go into the design details of each specific type but rather to examine the selection criteria that favor one configuration over another. Since shell-and-tubes are so common, most of the discussion focuses on them but some factors favoring other designs are covered as well. Factors influencing Reboiler Type Selection Many factors influence reboiler type selection. In the end, all these factors reduce economics. Every plant will weigh the trade-off between these factors differently. No one-size-fits-all selection exists. Major factors include: Plot space available Total duty required The fraction of tower liquid traffic vaporized Fouling tendency Temperature approach available Temperature approach required Corrosion of the fluid. Design temperature and Pressure. Characteristics of the reboiler boiling fluid. Heating medium requirements. Reboiler Configurations All of the above mentioned factors affect the desired configuration of the Reboiler. The major configuration selections include: Forced reboiler versus natural circulation reboiler Tube side versus shell side vaporization inside reboiler Once-through versus process recirculation reboiler Single-shell versus multiple-shell systems Vertical versus horizontal reboiler Stab-in bundles Other types Figures 1 to 4 show common types of reboilers. Figure 1 shows two horizontal, shell-side boiling configurations. Figure 1A is a recirculating thermosyphon reboiler. Figure 2A is a once-through thermosyphon reboiler. Figure 2 shows vertical configurations. Figure 2C is a tube-side boiling configuration with the once-through flow and Figure 2D is a shell-side boiling recirculating thermosyphon. Figure 3E shows a kettle reboiler and Figure 3F shows a forced-circulation reboiler. Figure 4G illustrates a stab-in bundle and figure 4H shows a forced-circulation, fired heater. Fig.1: Horizontal Shell Side Boiling Reboilers Fig. 2: Vertical Reboilers Fig. 3: Flooded bundle reboilers Fig. 4: Stab-in and fired heater reboilers Reboiler Type Selection Table Table 1 includes the major factors in making a choice for the reboiler type selection. FactorFavored typesDisfavored typesLow bottoms product fraction compared to boil-upRecirculatingKettleOnce-throughHigh bottoms product fraction compared to boil-upOnce-through reboilerRecirculatingLow relative volatility systemsRecirculating High relative volatility systemsOnce-throughRecirculatingLarge exchanger size or high-duty requirementsHorizontal reboilerVertical reboilerSmall exchangersVerticalStab-in Leaks are hazardous or difficult to deal withStab-in Exotic materialsStab-in Tight temperature approachSpiral-platePlate-finShell-and-tubeSolids presentKettleSpiral-platePlate-finThermally unstable productsRecirculating (no baffle)KettleOnce throughTight plot planVerticalHorizontalAmple plot planHorizontal High temperaturesFired heaters High heat fluxesForced circulationFlooded bundlesNatural circulationTable. 1: Reboiler-type selection table Other systems in addition to the ones shown here are also possible. Of course, every reboiler system's final choice will depend upon specific design details involved. Many reboiler systems have specific characteristics that favor designs that might not be immediately apparent. Few more resources for you… Piping Design and Layout Piping Stress Analysis Piping Materials Piping Interface Further Studies https://en.wikipedia.org/wiki/Reboiler https://www.chemicalprocessing.com/articles/2012/select-the-right-reboiler/