Flash Tank 2

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Flash Tank	Evaporator	Falling Film Evaporator	Shell&Tube Heat Exchanger	Simple Heat Exchanger	Barometric Condenser	Direct Contact Heater	Simple Heater	Simple Evaporator	Simple Condenser

Related Links:Flash Tank 2 in Dynamic Mode

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General Description

The Flash Tank 2 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.

Flash Tank 2 in Steady State Mode

• There are two operational modes for the flash tank 2 model, a) as a stand-alone unit or b) as part of a Flash Train. The operational mode is chosen by the user, but in order for the Flash Train method to work, the Flash Tank needs to be connected to other flash train aware units such as other flash tanks, Barometric Condensers and/or heat exchangers.
• If the Flash Tank 2 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 2.

Flash Tank 2 in Dynamic Mode

• In Dynamic Mode, the flash tank will only operate in the Stand Alone mode, user will need to set/adjust the pressure of the flash tank either manually or via a controller. The Flash Train Control is recommended if the flash tank is to be part of a flash train (flash train units needs to be specified by the user, as the flash train will not be formed automatically in dynamic mode).
• In Dynamic mode, user is allowed to define volume and shape for the flash tank.
• See Flash Tank 2 in Dynamic Mode for more information.

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 VLE sub-model is always enabled and would usually be the last sub-model evaluated. Other sub-models allowed as part of the Evaluation Block (EB) are Makeup Block (MU), Reaction Block (RB) and Environmental Heat Exchanger (EHX).

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 material in streams connected to the 'Feed' inlet will flow straight out of the 'Product' outlet;
• No material 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 sub-models such as 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. FlashTank2 tab. This first tab contains general information relating to the unit.
2. Flash Tank 2 in Dynamic Mode tab. This section contains the variables that will only be visible in dynamic mode.
3. RB - Optional tab, only visible if the Reactions are enabled.
4. EHX - Optional tab, only visible if the EnvironHX is enabled in the Evaluation Block.
5. VLE - Only visible if the VLEquilibrium is enabled (If it is NOT enabled, then the Flash Tank will operate as a pipe)
6. MU - Optional tab, or multiple tabs if more than 1 Makeup is selected. Only visible if one of more Makeup is enabled in the Evaluation Block.
7. 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.
8. 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.
9. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
10. Links tab, contains a summary table for all the input and output streams.
11. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Flash Tank2 Page

Unit Type: FlashTank2 - 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.
Mode	Stand Alone (Manual)	The flash tank will not form part of a flash train.
	Flash Train	The flash tank will form part of a Flash Train if appropriate units are connected to it. The OperatingP - Method will be set to Saturated.
OperatingP The user may only set the operating pressure or temperature of the Flash Tank if it is NOT part of a Flash Train.
Method	Atmospheric	This method is only available if 'Stand Alone' Mode is chosen. 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	This method is only available if 'Stand Alone' Mode is chosen. All flash calculations will be done at the user specified pressure (and saturated temperature @ P).
	RequiredT	This method is only available if 'Stand Alone' Mode is chosen. All flash calculations will be done at the user specified temperature (and saturated pressure @ T).
	Saturated	The OperatingP - Method will be set to Saturated if 'Flash Train' Mode is chosen. All flash calculations will be done at the saturated pressure.
PressureReqd / P_Reqd	Input	This field is only visible if the RequiredP method is chosen. This is user specified flash pressure.
TemperatureReqd / T_Reqd	Input	This field is only visible if the RequiredT method is chosen. This is user specified flash temperature.
Result	Calc	The actual pressure used for the sum of the feeds which will also be the outlet pressure (unless further model options change the pressure).
MinFlashP	Input	This is only visible when 'Flash Train' Mode is chosen. The Minimum Flash Pressure.
EB (Evaluation Block)...
Solubility.On	Tickbox	Only visible if Solubility has been defined for at least one species in the project. Allows the user to switch on any predefined solubility curve to adjust the composition of material in the unit.
[email protected]	Tickbox	Only visible if Phase Change at Temperature has been defined for at least one species in the project and Plant Model - Species Tab - PhaseChange@T = OFF. Allows the user to switch on any predefined phase changes at temperature in the unit.
EvalSequence	Calc	The sequence in which the sub models (which are part of the evaluation blocks) will be calculated. The sequence is determined by the priority selection for the individual sub-models. Note: If the user chooses On-AutoSequence then SysCAD will determine the sequence of the sub-models. The auto evaluation sequence followed will be the order the sub models are listed below.
Makeups	Input	The number of Makeup Blocks required. Extra dropdown options Makeup1, Makeup2, etc. will be added to allow these to be switched on and off and prioritised in relation to the other sub-models.
MakeupX	List	This can be used to switch the Makeup Block (MU) on or off and prioritise it in relation to the other sub-models. If this is 'On' then the associated page, MUX becomes visible and may be configured. Note: This field is only visible if the entry for 'Makeups' is greater than 0. If there is one makeup then X=1. If there are two makeups, then X=1 and X=2, etc.
Reactions	List	Reaction Block (RB) - Enable or disable Reactions and set the sequence in relation to the other sub-models. 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.
EnvironHX	List	Environmental Heat Exchanger (EHX) - Enable or disable Environmental Heat Exchange and set the sequence in relation to the other sub-models. If this is 'On' then the associated page, EHX becomes visible and may be configured. Note: The user does not have to configure an environmental heat exchange, even if this block is checked.
VLEquilibrium	Display	This shows that the Vapour Liquid Equilibrium (VLE) is always on for the Flash Tank2 model. The associated page, VLE may be configured. Note: This option may automatically adjust the species make-up.
Options
VapStandardSpModel	Tick Box	If ticked then the Standard (default) species model will be used in the vapour exit stream. This selection has no effect on the model operation.
ShowQFeed	Tick Box	QFeed and associated tab pages (e.g. Sp) 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 other sub-models occur.
ShowQProd	Tick Box	QProd and associated tab pages (e.g. Sp) will become visible if this is enabled. These tabs will show the properties of the combined product streams from the Unit. These values are after any flashing calculations or other sub-models occur but before the final phase split is made.
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
TemperatureIn / Ti	Calc	The flash tank feed temperature.
PressureIn / Pi	Calc	The flash tank feed pressure.
BPE	Calc	The Boiling Point Elevation of the Flash Tank discharge. This is the BPE value that is used for the Flash calculations.
HeatFlow	Calc	Any environmental heat loss will be displayed here.
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 flashed vapour.
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:	HeatExchange.dll	→	Units/Links	→	Heat Transfer: Flash Tank(2)
or	Group:	Energy Transfer	→	Units/Links	→	Heat Transfer: Flash Tank(2)

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

Insert into Project Flowsheet

Insert Unit

→

Heat Transfer

→

Flash Tank(2)

See Insert Unit for general information on inserting units.

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. 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.