3. Condenser

Piping and Instrumentation Diagram (P&ID): Condenser
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A condenser is a heat exchanger that is used to condense process vapour back to liquid with the help of a cooling medium. (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.) For a distillation column, a condenser is needed to transfer the latent heat from the vapour stream out of the top of the distillation column to a utility stream such as cooling water or air, so that the liquid condensate can be used as reflux and top product.

Most of the condensers for distillation processes are shell-and-tube, shell-and-coil, or simply double pipe heat exchangers (see Figure 3.1). Shell-and-tube heat exchangers, either arranged vertically or horizontally, are predominant in industrial processes because of their high efficiency and fast response for process control. Condensation on the shell side is much more common because of lower pressure drop and easier access of different cooling utilities on the tube side, particularly for systems that need to handle large vapour flow rates. As well, corrosion concern when using cooling water, the most commonly used cooling utility, nearly always favours the cooling medium on the tube side.

Figure 3.1. Schematic representation of distillation condensers: shell-and-tube heat exchanger (a) and shell-and-coil heat exchanger (b). | Description

Site Author(s) and University of Waterloo

Condenser
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UWaterloo Chemical Engineering Virtual Learning. (2022, Feb 7). 3 Condenser [Video]. YouTube. https://www.youtube.com/watch?v=tDC5vCNvb0c | Transcript

Designing an effective condenser for distillation process is in theory similar to the design of a reboiler (Green & Southard, 2019, Geankoplis, 2003, Gorak & Olujic, 2014, Coker, 2010). Not only must a condenser, such as a shell-and-tube heat exchanger, fully condense the vapour stream coming into the condenser, but also achieve this in no more than two-thirds of the condenser length in order to avoid vapour escaping through the condenser or column vent and ensure the operation safety. With the specified vapour flow rate, along with the defined dew point and vapour composition, the heat flow that needs to be removed from the vapour stream, also known as condenser duty, $ Q_c $, can be determined by,

$$ Q_c = V\lambda $$

where $ V $ is the vapour molar flow rate from the top of the distillation column, and $ \lambda $ is the molar heat of vapourization of the vapour mixture. The value of $ \lambda $ can be found from the enthalpy-composition chart or calculated from the relationship of $ \lambda $ values of pure substance and temperature. The condenser duty is the basis for sizing a condenser for a specific distillation separation, which involves using similar design equation to other heat exchangers such as,

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

where $ A_o $ is the required exterior tube area for condensation, $ h $ is the film heat transfer coefficient, $ T_w $ is the wall temperature of the condensation surface, and $ T_s $ is the saturation temperature or dew point of vapour in the condenser, defined by T-xy diagram similar to Figure 1.3 for the binary ethanol-water system. Note that $ T_w $ is realized through conduction heat transfer from the cooling medium. In general, wall temperature difference between cooling media side and condensation surface is small. If the difference becomes significant, the overall heat transfer coefficient, $ U $, can be used to determine the required condensation area,

$$ Q=UA_o\Delta T_{lm} $$

where $ \Delta T_{lm} $ is logarithmic mean temperature between saturated vapour and cooling media, defined as,

$$ \Delta T_{lm}= \dfrac{(T_s-T_{in})-(T_s-T_{out})}{ln\left(\dfrac{T_s-T_{in}}{T_s-T_{out}}\right)} $$

With the defined inlet temperature, $ T_{in} $, and outlet temperature, $ T_{out} $, of cooling medium, the area of the condenser required for specified distillation operation or design can be determined. The overall heat transfer coefficient is normally determined from film transfer coefficients such as convective heat transfer coefficient on cooling water side and condensation heat transfer coefficient on vapour side (Green & Southard, 2019, Geankoplis, 2003, Gorak & Olujic, 2014, Coker, 2010). As such, a condenser design with specified performance expectations must take into account the distillation operation, the thermodynamic nature of the separation system, and the cooling duty provided by the flow rate and operation temperature difference of the cooling medium.

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