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Piping and Instrumentation Diagram (P&ID): Utilities
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Utilities are essential components of distillation operation and design. They include electricity, high pressure air for pneumatic valves, and more notably, process steam and cooling water. (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.) The flow control for steam and cooling water is an important part of the distillation design because they are primary cost components when operating a distillation process. The steam heat flow, $ Q_{steam} $, supplied by a steam mass flow rate, $ \dot{m} $, can be determined by,

$$ Q_{steam} = \dot{m}\lambda_{steam} $$

where $ \lambda_{steam} $ is the heat of condensation of steam. In an ideal distillation operation, the heat flow of steam should be the same as heat flow required to generate a specified vapour flow rate in the reboiler, namely the process-side reboiler duty (refer to Equation 1 on the Reboiler component page), but this is hardly the case due to the heat loss of steam to the environment. In general, about 25%~45% of the energy supplied by the steam flow is lost. As well, since the process side reboiler duty is determined first when designing a distillation column, the process side reboiler duty needs to be added by the proper percentage to determine the actual steam flow rate from Equation 1 for a reboiler design and operation cost estimation.

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Steam and Cooling water Flow and Control
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Concept Check

What is the purpose of high pressure air in the 360° interactive VR Tour?

Please add an answer and try again.

Similarly, the heat removed by a cooling medium in the condenser, $ Q_{c} $, at a mass flow rate of cooling medium, $ \dot{m}_{c} $, can be determined by

$$ Q_{c} = \dot{m}_cC_{p,c}(T_{out}-T_{in}) $$

where $ C_{p,c} $ is the heat capacity of the cooling medium, and $ T_{in} $ and $ T_{out} $ are inlet and outlet temperatures of the cooling medium, respectively. In most cases, process water is used as a cooling medium, and in general, a temperature difference of over 25°C should be applied to ensure the efficient usage of the resource. The heat flow from the utility-side should also be similar to that from the process side or condenser duty (Equation 1 on the Condenser component page). This relationship is normally used to estimate the operation cost of the cooling and condenser design.

UWaterloo Chemical Engineering Virtual Learning. (2022, Feb 7). 7 Utilities [Video]. YouTube. https://www.youtube.com/watch?v=Jm4ZKsyHTdo | Transcript

Concept Check

1. For distillation operation, the reflux stream needs to be heated back to the saturation temperature before being introduced to the top tray. For the VR column, this was done by running the reflux pipe inside of the vapour pipe. What is the purpose for this equipment design?

2. If the reboiler duty from process side for a distillation operation is 16.0 kJ/s, and desired operation temperature difference for cooling water is 25°C, what volumetric flow rate should you set on the flow controller of the cooling water? The heat capacity for water is 4184 J/kg°C.

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

Geankoplis, C. J. (2003). Transport Processes and Separation Process Principles. 4th edition. Elsevier.

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