1. Reboiler

Piping and Instrumentation Diagram (P&ID): Reboiler
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A reboiler is a heat exchanger that is used to transfer heat from a heating fluid (often steam) to the liquid mixture undergoing distillation. This has the effect of vapourization a fraction of the liquid in the reboiler, which rises upward, driving further separation within the distillation column. (Click on the Piping and Instrumentation Diagram (P & ID) on the right in order to see the location of this component within the distillation column.) Reboilers come in a variety of designs, styles, and configurations, with the kettle type and the thermosiphon type being the most common. In a kettle-type reboiler, as shown in Figure 1.1, liquid flows from the the bottom tray of column into the shell side of the reboiler in which there is a horizontal tube bundle, boiling taking place on the outside of this bundle. In kettle-type reboilers, the heated liquid mixture is usually not circulated but can be circulated using a pump (forced circulation), whereas in thermosiphon reboilers it is the density difference between the inlet liquid and the outlet vapour/liquid mixture that drives the natural circulation of the liquid. The thermosiphon reboiler also comes in two primary configurations: vertical and horizontal, as illustrated in the diagrams in Figure 1.1.

Reboiler
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Another function of the reboiler is to serve as a reservoir for bottom product storage and flow control – a function that ties the reboiler to the overall mass balance for the entire distillation process. The selection of a suitable reboiler for a distillation process depends on many factors, such as the heat transfer and vapour flow rates, the type of heating media, system properties, cost, space, etc. The heating medium can be electrical power, heat transfer fluid, and condensing vapour or high pressure process steam. Industrial scale reboilers are mostly shell-and-tube heat exchangers due to their smaller volume, higher efficiency, and more uniform mixing and boiling, and the heating medium is most often process steam. Boiling can take place either in the tubes or on the shell side, depending on the type of the separation system and distillation design. In general, thermosiphon reboilers are advantageous over kettle reboilers for having smaller volume, faster response time, and more consistent performance, and hence, they offer a higher degree of flexibility for both distillation design and process control. Kettle-type reboilers are better suited for high viscosity or highly fouling liquids, or when high vapourization of vacuum service is required.

Figure 1.1. Schematic representation of kettle type reboiler as well as vertical and horizontal thermosiphon reboilers with bottom product control. | Description

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UWaterloo Chemical Engineering Virtual Learning. (2022, Feb 7). 1 Reboiler [Video]. YouTube. https://www.youtube.com/watch?v=MKL457Sjzrw | Transcript

The boiling of a liquid in a reboiler is closely related to the chemical nature of the system, reboiler design, and heat transfer. As depicted in Figure 1.2, the liquid boiling in the reboiler renders a two-phase mixture of liquid and vapour, which has a lower density than the liquid mixture at the bottom of the distillation column. The density difference creates a static head that drives the natural liquid circulation and a uniform boiling in the reboiler. The vapour bubble sizes and their distribution in the reboiler are related to heat transfer from the heating media through the temperature and heat flow rate. At the bottom of the boiling zone, small vapour bubbles coalesce to form large bubbles, and the large bubbles float up and break at top of the boiling zone, resulting in the unique profiles of vapour and temperature as shown in Figure 1.2.

By and large, a large driving temperature difference (ca. >10°) can usually result in transition and film boiling or the boiling instability (Heaslip, 2008), (Arneth, 2001).

Figure 1.2. Schematic representation of a working vertical thermosiphon reboiler in relation to heat transfer from steam condensation. | Description

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Many fundamental concepts go into reboiler design and operation. A reboiler itself is a separation stage because of its boiling condition and vapour-liquid equilibrium (VLE). The relationship among the boiling temperature, liquid composition, and vapour composition in a reboiler follows the thermodynamic laws such as Gibbs’ phase rule, Raoult’s law, and Dalton’s law. Take a binary system as an example, when an ethanol-water mixture is boiling at atmospheric pressure, as demonstrated in Figure 1.3, if we know the liquid composition, then the boiling temperature and the vapour composition will be automatically known, as per the Gibbs phase rule. We can find the boiling temperature from the intercept of liquid composition and bubble point curve, and vapour composition from the intercept of the boiling point and dew point curve. Evidently, the vapour has a greater ethanol mole fraction than the liquid (until the azeotropic point), which justifies the separation of ethanol from water by distillation.

Figure 1.3. T-xy diagram of vapour-liquid equilibrium for binary ethanol-water system at atmospheric pressure (the red line represents bubble point curve or liquid composition, and blue line represents dew point curve or vapour composition). | Description

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For a distillation column to achieve a specified separation, the reboiler must supply the required vapour flow rate to the bottom tray. Knowing the required vapour flow rate along with the boiling temperature and vapour composition in the reboiler, the amount of power required to generate the vapour flow rate, also known as reboiler duty, can be determined by the following formula,

$$ Q_R=\overline{V}\lambda $$

where:

$ \overline{V} $ is the vapour molar flow rate out of the reboiler, and

$ \lambda $ is the molar heat of vapourization of the liquid mixture.

The $ \lambda $ value can be found from the enthalpy-composition chart such as Figure 10-8 in the reference (Coker, 2010) or calculated from the relationship between the heat of vapourization of pure substances and temperature (Yaws, 2012).

The reboiler duty is the basis for sizing a reboiler for a specific separation by distillation. This involves using a design equation common to other heat exchangers,

$$ Q_R=hA_o(T_w - T_s) $$

where:

$ A_o $ is the required exterior heating area for boiling,

$ h $ is the boiling heat transfer coefficient,

$ T_w $ is the wall temperature of the heating surface, and

$ T_s $ is the saturation temperature of liquid in the reboiler, determined from the T-xy diagram (e.g., Figure 1.3 for the binary ethanol-water system).

Note that $T_w$ is realized through conduction heat transfer from the heating media side. In general, the difference between the wall temperature and the temperature of the heating media is small. If saturated steam is used as heating medium, then the steam pressure dictates the temperature, and the steam flow rate dictates the amount of heat available to be transferred for boiling and the vapour flow rate for the distillation process. As illustrated with a vertical thermosiphon reboiler in Figure 1.4, when the temperature difference or driving force is about 10°C or less, the boiling inside the reboiler exhibits uniform nucleate boiling, providing high degree of boiling stability and steam efficiency (Hagan and Kruglov, 2010). As such, the temperature difference is generally a baseline criteria for selecting the steam pressure for reboiler operation and design. Overall, a reboiler design with specified performance expectations must consider the distillation operation, the thermodynamic nature of the separation system, and the selection of heating utility.

Figure 1.4. Specific heat transfer rate or reboiler duty as a function of driving force in a vertical thermosiphone reboiler. | Description

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Concept Check

1. What type of reboiler needs larger liquid volume and more response time for a change in heating medium temperature or flow rate?

a. Kettle type.

b. Vertical thermosiphon type.

c. Horizontal thermosiphon type.

d. Both (b) and (c).

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2. Which of the following functions must a reboiler for a distillation column deliver?

a. Generate vapour.

b. Generate required vapour flow rate.

c. Serve as a reservoir for bottom flow control.

d. All the above.

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3. For an ethanol-water mixture boiling at the atmospheric pressure, if the mole fraction of ethanol in the boiling liquid is 0.14, what are the boiling temperature and the mole fraction of ethanol in vapour?

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FEEDBACK

On the T-xy diagram of Figure 1.3, draw a vertical line from x=0.14, and find the boiling temperature from the intercept of the line and bubble point curve to be 85°C (green dashed line in the diagram below). Similarly, draw a horizontal line from T= 85°C, and find the vapour composition from the intercept of the line and due point curve to be y=0.50 (purple dashed line). We can see a significant enrichment of ethanol in vapour with just one boiling stage.

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4. A distillation column is to be designed to separate a subcooled feed mixture with molar flow rate F and feed quality q, the design calls for a distillate flow rate D and a reflux ratio R. What is the reboiler duty for the design?

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FEEDBACK

Assuming the constant molal overflow stands for the system, the vapour molar flow in the enriching section is:
$V=(R+1)D$.
The vapour molar flow in the stripping section is:
$\overline{V}=V-(1-q)F=(R+1)D-(1-q)F$.
The reboiler duty is then:
$Q_{reb}=\overline{V}\lambda $.

5. For the distillation of an ethanol-water mixture from ethanol biofermentation, ethanol composition in the feed is 12 mol %. If the high pressure process steam (~100 psi) is used for a thermosiphon reboiler, what should the steam pressure of the reboiler be adjusted to if a smooth and stable boiling is desired?

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FEEDBACK

The boiling temperature in the reboiler should be 86~100°C according to Figure 1.3, which means that the steam temperature should be about 105°C to avoid film boiling or unstable boiling. From the steam table, steam pressure should be between 1.1 ~1.2 bar or around 17 psi.

References