Skip to main content
P & ID thumbnail, click for bigger image.

Piping and Instrumentation Diagram (P&ID): Overhead
Click image to enlarge and see location of component.

Overhead reservoir or overhead drum is a distillation process vessel that plays the following important roles in distillation equipment design and operations:

  • Holds a desired level of overhead liquid to prevent vapour escaping from condenser.
  • Stores overhead product and the storage level serves as an input for overhead flow control.
  • Serves as a decanter when overhead product is a two-phase mixture.

(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 5.1 shows a typical diagram of a distillation overhead reservoir with process level and flow controls. Liquid level signal measured by a level sensor (LS), typically differential pressure transducer, is used as an input signal to a level controller (LC) to maintain a constant liquid level in the reservoir. This ensures the liquid flow rate out of the reservoir is the same as the overhead flow rate coming out of the condenser. Thus, the condenser doesn’t get flooded or dried out during the continuous operations.

Schematic diagram of overhead reservoir and flow control for distillation process.

Figure 5.1. Schematic diagram of overhead reservoir and flow control for distillation process with (a) pump, (b) pneumatic valves, level sensor (LS), level controller (LC), flow meter (FM), and flow controller (FC). | Description

Site Author(s) and University of Waterloo

Photo of overhead reservoir.

Overhead reservoir
Click image to enlarge

Concept Check

What are the instruments used to control the overhead flow in the 360° interactive VR Tour?

Please add an answer and try again.

It is important to note that the overhead flow out of the overhead reservoir should be constant under specified operation condition (e.g. constant stream flow rate). The overhead flow is then split into reflux and distillate product streams. Thus, the operation reflux ratio can be conveniently set and controlled by manipulating the reflux flow rate.

UWaterloo Chemical Engineering Virtual Learning. (2022, Feb 7). 5 Distillation Product (Overhead Reservoir) [Video]. YouTube. https://www.youtube.com/watch?v=Zdao8M6kAOA | Transcript

Concept Check

1. The main purposes of an overhead reservoir for distillation operations are to?

2. A distillation column operation with a total condenser shows a constant overhead flow rate of 25 L/h. If the desired operation molar reflux ratio needs to be adjusted to 1.5, what volumetric flow rate should be set on the reflux flow controller to achieve the operation reflux ratio?

Please add an answer and try again.

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