Vessels: flash drums, surge tanks and accumulators

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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 liquid are 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. In HYSYS/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. They can 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 in HYSYS/UniSim.

· Accumulators. These are storage tanks following distillation column condensers. For partial condensers, 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 vessels

Two classic works are accessible here: Vessels and Vessels2. These are the basis of the HYSYS vessel 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 default settings of L/D = 3 and 5 minutes liquid residence time. (These are the same as in the heuristics for process 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 at 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" at Design Costing, Cost Equation Help are the Chemical Engineering Plant Cost Index (CEPCI). To update to present costs, you change the index on the Design Construction page to the current CEPCI value in Chemical Engineering magazine.

The following additional information is courtesy of Mohammad Kabir, Technical Support Consultant at Aspen Technology in 2005:

“In the Vessel Sizing utility in HYSYS, the maximum allowable velocity is estimated based on the steady-state results, using the Souders- Brown equation. With this velocity known plus other info, an adequate diameter, height and volume of the separating vessel can then be obtained. To achieve 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 is not 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