Piping and Instrumentation Diagram (P&ID): Bottom
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The bottom product flow of a distillation column involves devices and instruments to withdraw liquid product from the bottom of reboiler, and is, therefore, an important part of reboiler design and operation. (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.)
Figure 6.1 shows a typical diagram for the bottom flow control. Just as the overhead flow control, a level controller is typically used to control the bottom product flow rate to ensure:
- All the heating tubes/elements are submerged in liquid.
- The overall mass balance for the column operation is maintained so that the reboiler doesn’t get flooded or dried out.
- The bottom product is at equilibrium concentration.
Figure 6.1. Schematic diagram of bottom level and product flow control for distillation process with (a) pump, (b) pneumatic valve, level sensor (LS), Level controller (LC), and flow meter (FM). | Description
Site Author(s) and University of Waterloo
Bottom Product Flow and Control
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Concept Check
The flow rates of overhead product and bottom product are both controlled by level controllers. Which flow would exhibit more fluctuation based on 360 VR video?
FEEDBACK
Bottom product flow, since the liquid level in the reboiler on the video shows much more fluctuation, that would translate to more fluctuation in the flow rate as the level controller takes liquid level as the input.Note that when a thermosiphon reboiler, as shown in Figure 1.1 and the VR Tour video on the Reboiler component page, is in use, the liquid in the reboiler is well mixed due to the liquid circulation around the reboiler. As well, the liquid-vapour equilibrium should be better attained in the reboiler, compared to vapour-liquid equilibrium on trays along the column. This is even true for systems with lower distillation efficiency such as alcohol solutions due to the well-mixed condition and sufficient contact between liquid and vapour in the thermosiphon reboiler. As such, the concentration of bottom product is more representative of the equilibrium concentration resulting in more consistent concentration or quality for the bottom product.
UWaterloo Chemical Engineering Virtual Learning. (2022, Feb 7). 6 Bottom Product [Video]. YouTube. https://www.youtube.com/watch?v=dQAITXwTcl0 | Transcript
Concept Check
2. A distillation column with a total condenser is operated at a feed flow rate of 120 L/h and feed temperature of 25°C. The operation reflux ratio and boilup ratio (ratio of vapour out of the reboiler to bottom product flow) are 1.5 and 1.25, respectively. If the flow meter for bottom flow rate shows a flow rate 86 L/h for some reason, how is the liquid level in the reboiler going to change? What could be the possible reasons for the operation problem?
FEEDBACK
The subcooled feed (F) means that vapour flow inside the column is about constant, with V/B=1.25, L/D=1.5, V=L+D, and B+D=120, the bottom flow rate should be 80 L/min, which is smaller than the actual flow rate. This means that the liquid level in the reboiler is going to drop. Possible reason for this could be the controller setting, or instability of the level sensor.References
Coker, A. K. (2010). Ludwig's applied process design for chemical and petrochemical plants. Volume 2, 4th edition. Elsevier. Electronic version available at: http://app.knovel.com/web/toc.v/cid:kpLAPDCP02?filter=table
Gorak, A., Olujic, E. (2014). Distillation: Equipment and Processes. Elsevier.
Green, D. W., Southard, M. Z. (2019). Perry’s Chemical Engineers’ Handbook”, 9th ed., McGraw-Hill