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Vessels: flash drums, surge tanks and accumulators

There are a variety of places that vessels are used in chemical plants and oil refineries, e.g.:

Flash drums. Vessels into which flow a mixture of liquid and vapor. The goal is to separate

the vapor and liquid. For design calculations it is normally assumed that the vapor and liquidare in equilibrium with one another and that the vessel is adiabatic (no heat lost or gained).

One must simultaneously satisfy a material balance, a heat balance, and equilibrium. InHYSYS/UniSim, this can be done using either a Separator or a Tank.

Surge tanks. These are storage tanks between units, and can serve a variety of purposes. Theycan dampen fluctuations in flow rate, composition or temperature. They can allow one unit to

be shut down for maintenance without shutting down the entire plant. Use a Tank inHYSYS/UniSim.

Accumulators. These are storage tanks following distillation column condensers. For partiacondensers, this flow may be a mixture of vapor and liquid. The outlet flow may be regulated

by a level controller in order to avoid the tank either flooding (liquid out the top) or going dry(vapor out the bottom). Use a Separator or Tank in HYSYS/UniSim.

Sizing of vesselsTwo classic works are accessible here: Vesselsand Vessels2. These are the basis of the HYSYSvessel sizing utility. There are two methods to access this. Double click on the vessel icon in your

pfd. On Rating Sizing click on Quick Size. This uses the vessel sizing utility with the defaulsettings of L/D = 3 and 5 minutes liquid residence time. (These are the same as in the heuristicsforprocess vessels.) Record the volume, diameter and height. Go to Tools, Utilities, Vessel Sizing.Make any desired changes on Design Sizing and Design Construction. The results are a

Performance Sizing Results. This utility will also estimate costs, at Design Costing. The

"Chemical Engineering Index" shown at Design Construction and Index & "CE Fab Index" atDesign Costing, Cost Equation Help are the Chemical Engineering Plant Cost Index (CEPCI). Toupdate to present costs, you change the index on the Design Construction page to the currenCEPCI value in Chemical Engineering magazine.

The following additional information is courtesy of Mohammad Kabir, Technical SupportConsultant at Aspen Technology in 2005: In the Vessel Sizing utility in HYSYS, the maximum allowable velocity is estimated based onthe steady-state results, using the Souders- Brown equation. With this velocity known plus otherinfo, an adequate diameter, height and volume of the separating vessel can then be obtained. Toachieve an equilibrium between different phases, an adequate residence time for both light liquid

and heavy liquid is assumed and it is then achieved by adjusting the vessel size. The droplet size isnot a factor in the equations used in our Vessel Sizing Utility. The Vessel Sizing utility is based on the following references (linked above):

- Evans, Frank L., "Equipment Design Handbook for refineries and chemical plants -Volume 2", 2nd Edition, p. 154, Gulf Publishing Company, Houston, Texas, 1980.

- Watkins, R.N., "Sizing Separators and Accumulators", Hydrocarbon Processing,November 1967, p. 253.

LLSD is meant for Liquid level shut down. You can define that value in HYSYS by yourself.Default calculation method should be found in above references.Updated 20 July 2009. Please email questions, comments and suggestions to W.R. Wilcox

7/27/2019 Vessels_ Flash Drums, Surge Tanks and Accumulators

2/2

10/4/2014 Vessels: flash drums, surge tanks and accumulators

http://people.clarkson.edu/~wwilcox/Design/veslsize.htm

Wilcox homeChE design homeProfessionGeneralPropertiesEquipmentSeparationAspen +HYSYS &

UniSimCostsSafetyCase studiesExcelMATLAB