Flash Tank

NOTE: This is a LEGACY model, available in the current build for backwards compatibility. It is being phased out and may have limited functionality. Documentation may be outdated. Please consider changing to the most recent version of the model.

Navigation: Models ➔ Energy Transfer Models ➔ Flash Tank

Related Links: Flash Tank 2, Flash Train

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General Description

The newer Flash Tank 2 can be used. The Flash Tank is used to 'flash' a feed stream to produce a liquid and vapour product at a set pressure. The liquids and vapours leave the unit separately.

There are two operational modes for the flash tank, a) as a stand-alone unit or b) as part of a Flash Train. The operational mode is decided by the overall configuration of the flowsheet in which the Flash Tank is located. For example, if the Flash Tank is connected to other flash tanks, Barometric Condensers and / or heat exchangers, the model will become part of the entire Flash Train. The user does not have to specify that the unit is part of the Flash Train, SysCAD will do this automatically. If the Flash Tank is connected to units other than the previous three, i.e. Feeders, tanks, etc; then the Flash Tank is a stand-alone unit and can be configured as such.

If the Flash Tank is inserted as part of a Flash Train, see Flash Train for a description of the theory and variables. This documentation will only discuss the variables for a 'stand alone' Flash Tank.

The Vapour Liquid Equilibrium section allows the user to configure which species to flash and / or the flashing method, fully described in Vapour Liquid Equilibrium (VLE).

The Reaction Block (RB) allows the user to configure any number of reactions within the flash tank, fully described in Reaction Block (RB).

Diagram

The diagram shows the default drawing of the flash tank, with the required connecting streams. When the user inserts a flash tank into a flowsheet, he may choose a different drawing from a pull down menu.

Inputs and Outputs

Label	Required Optional	Input Output	Number of Connections		Description
			Min	Max
Feeds	1 Required	In	1	20	Flash tank feed
Liquor	Required	Out	1	1	Liquor outlet
Vapour	Required	In	1	1	Vapour outlet

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

• All liquids and solids in streams connected to the 'Feed' inlet will flow straight out of the 'Liquor' outlet;
• All vapours in streams connected to the 'Feed' inlet will flow out of the 'Vapour' outlet;
• The pressure of the outlet streams will be the pressure of the input stream, or the minimum pressure of all input streams if more than one inlet;
• No Flashing or reactions will occur.

So basically, the unit will be 'bypassed' without the user having to change any connections.

Model Theory

The unit is configured to achieve vapour : liquid equilibrium at a required pressure (or temperature) using the user defined VLE method. Any solids that may be in the stream entering the unit are ignored in the flash calculations. However the enthalpy balance does include the solids.

For further information on the theory for the VLE calculations see Vapour Liquid Equilibrium (VLE).

Flowchart

The following shows the sequence of events if sub model options are switched on. See next heading for more information.

Data Sections

The default access window consists of several sections:

1. FlashTank1 tab - This first tab contains general information relating to the unit.
2. RB - Optional tab, only visible if the Reactions are enabled.
3. VLE - Only visible if the VLEquilibrium is enabled (If it is NOT enabled, then the Flash Tank will operate as a pipe)
4. QFeed - Optional tab, only visible if ShowQFeed is enabled. This page shows the properties of the mixed stream as the feed to the tank.
   • This is before any Evaluation Block models are evaluated.
5. QProd - This page shows the properties of the Flash Tank discharge as a single stream.
   • This is AFTER the flash calculations, but before the split to liquid and vapour streams.
6. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
7. Links tab, contains a summary table for all the input and output streams.
8. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Flash Tank Page

Unit Type: FlashTank1 - The first tab page in the access window will have this name.

Tag (Long/Short)	Input / Calc	Description/Calculated Variables / Options
Tag	Display	This name tag may be modified with the change tag option.
Condition	Display	OK if no errors/warnings, otherwise lists errors/warnings.
ConditionCount	Display	The current number of errors/warnings. If condition is OK, returns 0.
GeneralDescription / GenDesc	Display	This is an automatically generated description for the unit. If the user has entered text in the 'EqpDesc' field on the Info tab (see below), this will be displayed here. If this field is blank, then SysCAD will display the UnitType or SubClass.
Requirements
On	Tickbox	This variable in used to enable flashing in the unit. If this not selected, the material will flow out of the liquor outlet with no change in state, i.e. the unit acts as a pipe.
OperatingP
Method	Atmospheric	All flash calculations will be done at Atmospheric Pressure. The atmospheric pressure is calculated by SysCAD based on the user defined elevation (default elevation is at sea level = 101.325 kPa). The elevation can be changed on the Environment tab page of the Plant Model.
	RequiredP	All flash calculations will be done at the user specified pressure.
	Saturated	All flash calculations will be done at the saturated pressure. (This option is only valid if the flash tank is part of a Flash Train configuration.)
	RequiredT	All flash calculations will be done at the user specified temperature.
PressureReqd / P_Reqd	Input	The Required Flash Pressure - This is only visible if the RequiredP Method is selected.
TemperatureReqd / T_Reqd	Input	The Required Flash Temperature - This is only visible if the RequiredT Method is selected.
Result	Calc	This field displays the operating pressure of the flash Tank.
VapSpModel	Input	Standard - The default species model. This sets the species model to be used in the vapour exit stream. This selection has no effect on the model operation.
		More options may be available, depending on the user configurations.
Reactions	List Box	Reaction Block (RB) is enabled here. If this is 'On' then the associated page, RB becomes visible and may be configured. Note: The user does not have to configure a reaction file, even if this block is checked.
VLEquilibrium	List Box	Vapour Liquid Equilibrium (VLE) is enabled here. This MUST be enabled for the unit to operate as a Flash Tank. When it is enabled the associated page, VLE becomes visible and may be configured. If this field is disabled then the flash tank will operate as a pipe and all feed material will report to the Liquor outlet.
ShowQFeed	Tick Box	QFeed and associated tab pages (e.g. Qm) will become visible if this is enabled. These tabs will show the properties of the combined feed stream to the Unit. These values are BEFORE any flashing calculations or reactions occur.
TrackVapFd	Tick Box	Selecting this will add a warning message and error status colour to the flash tank if vapour is present in the feed stream.
Vapour Entrainment
FlashVapToLiquid	Input	This field allows the user to set a proportion of the flash vapour bypassing to the liquid stream. The default is 0%.
OtherGasToLiquid	Input	This field allows the user to set a proportion of any non-condensable gas bypassing to the liquid stream. The default is 0%.
Results
VapourCalcs	Calc	This displays the compound(s) used to calculate the vapour pressure.
TemperatureIn / Ti	Calc	The flash tank feed temperature.
BPE	Calc	The Boiling Point Elevation of the Flash Tank discharge. This is the BPE value that is used for the Flash calculations.
WaterSatT	Calc	The water saturation temperature at the flash tank pressure. This is used to calculate the BPE. (BPE = Saturation T - WaterSatT)
FlashT	Calc	The calculated temperature at the operating pressure of the Flash Tank.
FlashP	Calc	The calculated operating pressure of the Flash Tank.
VapourMassFlow / VQm	Calc	The mass flow of vapour leaving the Flash Tank (this includes any vapour entrained in the liquor stream).
Flash Train Macro Model These extra fields are only visible if the unit is part of a Flash Train.
VapourMassFlow / VQm	Calc	The mass flow of flashed vapour.
PressureReqd / P_Reqd	Calc	The required flash pressure calculated by the Flash Train.
TotalPressChange / TotaldP	Calc	The total pressure change calculated by the Flash Train.
FeedSatP	Calc	The saturation pressure of the feed.
FlashTrain	Display	A unique tag assigned to the flash train by SysCAD. Each unit in the flash train will have the same tag in this block.
FlashTearBlock	Display	Displays the name of the tear block that is part of the Flash Train.
FlashTrainEqp	List	This contains a list of all of the equipment tags in this flash train. the list might be as follows: FT_FT_001 FT_ST2_001
PAdvBase	Input	Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * PAdvBase when PAdvExtra=0.
PAdvExtra	Input	Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * (PAdvBase + PAdvExtra*(VQmReqd-VQm)/Max(VQmReqd,VQm)).

Adding this Model to a Project

Add to Configuration File

Sort either by DLL or Group:

	DLL:	HeatXch1.dll	→	Units/Links	→	Heat Transfer: Flash Tank(1)
or	Group:	Energy Transfer	→	Units/Links	→	Heat Transfer: Flash Tank(1)

See Model Selection for more information on adding models to the configuration file.

Insert into Project Flowsheet

Insert Unit

→

Heat Transfer

→

Flash Tank(1)

See Insert Unit for general information on inserting units.

NOTE: This model is being phased out and will be removed in future versions (refer to Discontinued Models and Dlls). In Build 139, to add this model to the project (not recommended), please scroll to the bottom of the Insert Unit dialog box and find it under the group {Old}.

Hints and Comments

1. There may be mass entering or leaving the unit via the Reaction Block source or sink, so if the outgoing mass is not the same as the incoming mass, this may be a place to check first.
2. If the VLE check box is not ticked the Flash Tank operates as a pipe only. All the material entering the unit leaves in the same state via the Liquor outlet.
3. The information in the VLE Section must be correct for the Flash Tank to operate as required. If the VLE configuration requires information that is not available, then the flashing will not occur.