Evaporator

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Related Links: Solid-Liquid Separator, Potash Evaporator

This model is only available in SysCAD 9.2 Build 134 and later.


General Description

The Evaporator is used to 'flash' a feed stream to produce a liquid and vapour product at a set pressure or temperature. The unit may also have connections to an Embedded or External Heat Exchanger.

There are two operational modes for the Evaporator:

  1. as a stand-alone unit; or
  2. as part of a Flash Train.

The user sets the operational mode of the Evaporator. However, if the user sets the mode as Flash Train and the Evaporator is NOT connected to another Evaporator (Embedded Shell), Barometric Condenser, Heat Exchangers of any Flash Train aware destination unit, the model will go to an error state.

Crystallization

The Evaporator model may be used as a Crystallizer, as it behaves in the same way. Vapour is flashed off and the remaining solution is then supersaturated with the required species. This species then precipitates out as the crystal product.

If the user has solubility data for the species, then they may enable Solubility and SysCAD will automatically calculate the amount of species that precipitates. This will work for the standard solubility data and for Potash solubility.

Alternatively, the user may use the Reaction Block (RB) to set the precipitation reaction and extent.

Available Sub-Models

  1. Solubility may be enabled in an Evaporator, if this is NOT enabled globally.
  2. Makeup Blocks (one or more) may be enabled in a Evaporator. This allows the user to control the addition of streams such as reagents, water, air, etc to the Evaporator, often without having to use a PID or General controller.
  3. The Reaction Block (RB) allows the user to configure any number of reactions within the Evaporator.
  4. The Environmental Heat Exchanger Block allows the user to add or remove energy from the Evaporator using a number of different methods.
  5. The Vapour Liquid Equilibrium Block is ALWAYS enabled in an Evaporator. This will attempt to ensure that the vapours and liquids of the selected species (normally H2O) will be in equilibrium in the Tank.

Heat Exchange Options

The user may select any of the following 3 options for heat exchange:

  1. None - no Heat Exchange is required.
  2. Embedded - The Embedded Heat Exchange model uses a Shell and Tube style heat exchanger. The user connects the Steam (or other cooling or heating medium) to the Embedded HX Shell connection and the Condensate (or return heat exchange medium) to the Embedded HX Shell. The recycle flow to the heat exchanger must to be specified, but this is handled internally within the unit.
  3. External - External Heat Exchange models the use of external heat exchangers taking a recycle stream from the Evaporator contents and heating, or cooling, this recycle stream. The user must connect the Recycle stream from the Evaporator to an external heat exchanger. This recycle stream is either heated or cooled then returned to the Evaporator via the Recycle stream. The recycle flow to the heat exchanger must to be specified, and this is sent to the Recycle connection.

NOTES:

  1. The difference between the two heat exchange methods:
    • With Embedded heat exchange, the steam and condensate streams (or other heat exchange medium) are connected directly to Evaporator via the Embedded HX Shell connections. The Recycle stream to the heat exchanger is handled internally within the unit.
    • With external heat exchange, the recycle stream is sent to an external heat exchanger via the Recycle output connection and returned to the Evaporator via the Recycle input connection.
  2. For both Embedded and external heat exchange the user must set the recycle flow to the heat exchanger.

Diagram

Evaporator3.png

  • The diagram shows default drawings of the Evaporator with connecting streams, with heat exchanger connections.
  • The Heat Exchanger connections are optional.
  • The user does not have to connect a stream to the 'Overflow' connection, as the Evaporator will operate without this connection.
  • The physical location of the streams connecting to the Evaporator is unimportant. The user may connect the streams to any position on the unit.
  • When inserting an Evaporator into a flowsheet, the user may choose a different symbol from the menu.

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max.
Feed 1 Required In 1 20 Feed to the Evaporator.
Embedded HX Shell Optional In 0 10 Inlet stream to the Shell side of internal Shell and Tube Heat Exchanger (often the steam inlet).
Recycle Optional In 0 1 Recycle stream from an external Shell and Tube Heat Exchanger to the Evaporator.
Product Required Out 1 1 Product Slurry outlet - this stream normally contains the majority of the solids.
Vent Required Out 1 1 Vent outlet containing all Vapours from the unit.
Overflow Optional Out 0 1 Overflow liquor outlet.
Embedded HX Shell Optional Out 0 1 Outlet stream from the Shell side of internal Shell and Tube Heat Exchanger (often the condensate).
Recycle Optional Out 0 1 Recycle stream from the Evaporator to an external Shell and Tube Heat Exchanger.

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' and 'Recycle' inlets will flow straight out of the 'Product' outlet, with no change in phase or energy exchange with material flowing through the internal heat exchanger;
  • All material in streams connected to the 'Embedded HX Shell' connections will have no change in phase or energy exchange with material flowing through the Evaporator;
  • No sub-models will be called.

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


Model Theory

Vapour Liquid Equilibrium

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).

Pressure used for Evaluation Block Calculations

For an Evaporator in the Stand Alone mode the calculations, including the Solubility calculations, in the Feed Evaluation Block (FEB) or the Body Evaluation Block (EB) may be performed at either:

  • The Feed pressure; or
  • The final pressure; or
  • The pressure into the Body of the Evaporator if the user has chosen Final Temperature as the mode.

The user enables the 'UseFeedP' tickbox on the first tab of the Evaporator to use the Feed pressure.

Using the Feed or other pressure may produce small differences in the final results from the Evaporator.

Solubility

If Solubility is enabled in the Evaporator, either Globally or in the individual unit, then the solubility calculations will be performed simultaneously with the VLE calculations. Therefore, the product from the Evaporator will contain liquid at, or below, the saturation value for any relevant species.

Note: If the user enables Solubility and has an embedded heater using the 'Duty' recycle converge method, then the Feed and EB Feed temperatures displayed on the 'PC' tab will not be valid. This is because the Duty method adds the energy directly to the Evaporator contents, rather than using a recycle stream. However, the final product temperature and solubility value will be correct.

Flow Chart

Evaporator Flow Diagram Rev 1.jpg

Data Sections

The default access window consists of several sections:

  1. Evaporator - The first tab contains general information relating to the unit and allows the user to set the Heat Exchange type and flows.
  2. Results - This tab contains the results fields for the Evaporator.
  3. Separ - The tab allows the user to specify the split between the Product and Overflow streams.
  4. PC - Optional tab, only visible if the Solubility is enabled in the Evaluation Block.
  5. RB - Optional tab, only visible if the Reactions are enabled in the Evaluation Block.
  6. EHX - Optional tab, only visible if the EnvironHX is enabled in the Evaluation Block.
  7. VLE - Always visible.
  8. 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.
  9. HX - Optional tab, only visible if an Internal Heat Exchanger is selected. (In this case the Internal HX connections should be connected).
  10. QFeed - Optional tab, only visible if ShowQFeed is enabled. This page shows the properties of the mixed stream as the feed to the Evaporator.
    • This is before any Evaluation Block models are evaluated.
  11. QProd - This page shows the properties of the Evaporator discharge as a single stream.
    • This is AFTER the flash calculations, but before the split to slurry, overflow and vapour streams.
  12. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  13. Links tab, contains a summary table for all the input and output streams.
  14. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Evaporator Page

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

Tag (Long/Short)
Input / Calc
Description
Common Data on First Tab Page

Requirements:

On Tickbox This variable in used to turn the unit ON or OFF. If this not ticked, the material will flow out of the Product outlet with no change in state, i.e. the unit acts as a pipe.
Mode Stand Alone (Manual) The Evaporator will act as a 'Stand Alone' unit and the user may set the required operating Pressure or Temperature.
Flash Train The Evaporator will act as part of a Flash Train. In this mode the Evaporator vent must be connected to a steam consumer, such as a Barometric Condenser, Shell and Tube Heat Exchanger, Shell and Tube Heat Exchanger 2 or a Direct Contact Heater. The steam requirements of the steam consumer will drive the operating pressure of the Evaporator, therefore the user may NOT set the operating temperature or pressure.
OperatingP

The user may only set the operating pressure or temperature of the Evaporator if it is NOT part of a Flash Train.

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.
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 Display This field displays the operating pressure of the Evaporator.
MinFlashP Input The minimum Pressure at which the Evaporator may operate. This is visible if the Evaporator is configured as part of a Flash Train, as it will not allow the unit to drop to an unrealistic pressure.
UseFeedP Tickbox This is only visible if the mode = Stand Alone. If this field is enabled then all of the Evaluation Block calculations will be performed at the Feed pressure. Otherwise, if it is not enabled, the calculations will be performed at the final pressure. See also Model Theory.

Feed Evaluation Block

EvalSequence Display 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 sequence followed will be MU1,MU2, etc., RB, EHX.
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 then 0. If there is one makeup then X=1. If there are two makeups, then X=1 and X=2, etc.
Reactions List This can be used to switch on the Reaction Block (RB). 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 This can be used to switch on the Environmental Heat Exchanger (EHX). If this is 'On' then the associated page, EHX may become visible and may be configured.
Note: The user does not have to configure an environmental heat exchange, even if this block is checked.

Body Evaluation Block

Solubility.On Tickbox The user may enable the solubility in the body only if Global solubility is disabled.
Note: If the user enables Solubility and has an embedded heater using the 'Duty' recycle converge method, then the Feed and EB Feed temperatures displayed on the 'PC' tab will not be valid. This is because the Duty method adds the energy directly to the Evaporator contents, rather than using a recycle stream. The final product temperature is correct.
EvalSequence Display This will always only display VLE.
VLEquilibrium Display Vapour Liquid Equilibrium (VLE) is always ON in the Evaporator - the user may NOT turn it off.

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%.

Evaporator Heat Exchanger Options

HXType None The unit is not connected to a Heat Exchanger. No other fields are visible for the Heat Exchanger if this option is chosen.
External The Evaporator is connected to an External Heat Exchanger unit. This external unit may be any type of SysCAD model.
Embedded Heater The Evaporator contains an Embedded Heater. If this option is chosen, an additional page HX becomes visible and the user configures the Heat Exchanger within the Evaporator.
Embedded Cooler The Evaporator contains an Embedded Cooler. If this option is chosen, an additional page HX becomes visible and the user configures the Heat Exchanger within the Evaporator.
HXOut.SplitMethod Off No flow is sent to the Heat Exchanger.
Ratio to Feed MassFlow The flow to the Heat Exchanger (either Embedded or External) is a Ratio of the Feed mass. The composition and temperature of the stream to the Heat Exchanger is the same as the Combined Feed to the Evaluation Block.
Ratio to Prod MassFlow The flow to the Heat Exchanger (either Embedded or External) is a Ratio of the Product mass. The composition and temperature of the stream to the Heat Exchanger is the same as the Product from the Evaluation Block.
Note: The Product mass includes both the Overflow and the Product streams.
MassFlow The flow to the Heat Exchanger (either Embedded or External) is a fixed mass flow. The composition and temperature of the stream to the Heat Exchanger is the same as the Product from the Evaluation Block.
Note: The Product mass includes both the Overflow and the Product streams.
VolumeFlow The flow to the Heat Exchanger (either Embedded or External) is a fixed volumetric flow. The composition and temperature of the stream to the Heat Exchanger is the same as the Product from the Evaluation Block.
Note: The Product mass includes both the Overflow and the Product streams.
MassFlow Per Heater This option can be used for the Embedded Heater option, where user can define the number heaters used. The flow to each Embedded Heater is mass based. The composition and temperature of the stream to the Heat Exchanger is the same as the Product from the Evaluation Block.
Note: The Product mass includes both the Overflow and the Product streams.
VolumeFlow Per Heater This option can be used for the Embedded Heater option, where user can define the number heaters used. The flow to each Embedded Heater is volume based. The composition and temperature of the stream to the Heat Exchanger is the same as the Product from the Evaluation Block.
Note: The Product mass includes both the Overflow and the Product streams.
HXOut.RatioToFeedQm Input The flow to the Heat Exchanger (either Embedded or External) is Feed Flow times by Ratio.
HXOut.RatioToProdQm Input The flow to the Heat Exchanger (either Embedded or External) is Product Flow times by Ratio.
Note: The Product mass includes both the Overflow and the Product streams.
HXOut.MassFlowRqd Input The required Product mass flow to the Heat Exchanger (either Embedded or External).
HXOut.VolumeFlowRqd Input The required Product volumetric flow to the Heat Exchanger (either Embedded or External).
HXOut.EachHeaterQmRqd Input The required Product mass flow to the each Heater (Embedded Heater method only).
HXOut.EachHeaterQvRqd Input The required Product volumetric flow to the each Heater (Embedded Heater method only).

Options

VapStandardSpModel Tickbox This forces the vent stream to revert to the Standard species model. If this is NOT ticked, then the species model in the vent will be the same as the species model used in the Evaporator.
Normally this is left ticked, as the vent contains steam and the Standard species model will calculate all of the properties of steam correctly.
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 the Evaluation Block is processed.
ShowQBodyFeed Tick Box QBodyFeed and associated tab pages (e.g. Qm) will become visible if this is enabled. These tabs will show the properties of the feed to the Evaporator Body.
ShowQRecycle Tickbox This is only visible if an Embedded Heat Exchanger is used. If this option is ticked then the recycle stream to the Heat Exchanger is displayed as 'QRecycle' and the user may access properties of the stream feeding the Heat Exchanger.
ShowQProd Tick Box QProd and associated tab pages (eg Qm) will become visible if this is enabled. These tabs will show the properties of the product stream from the Unit, AFTER the Evaluation Block is processed, but BEFORE the stream is split to overflow and product.

Results Page

The page contains the results and the user may enable the Heat Exchanger function and set the type of Heat Exchanger (Internal or External) and the flow to the Heat Exchanger.

Tag(Long/Short) Input / Calc Description
Operating Conditions
TemperatureIn / Ti Calc The temperature of the combined Feed stream to the Evaporator.
PressureIn / Pi Calc The pressure of the combined Feed stream to the Evaporator.
VapFlashed.MassFlow / VapFlashed.Qm Calc The quantity of flashed vapour.
BPE Calc The Boiling Point Elevation used in the Flash calculations in the Evaporator.
FlashT Calc The final temperature (the Flash Temperature) in the Evaporator.
FlashP Calc The final pressure (the Flash Pressure) in the Evaporator.
Flash Train Macro Model
These extra fields are only visible if the unit is part of a Flash Train.
VapourMassFlow / VQm Calc The quantity of flashed vapour.
PressureReqd / P_Reqd Calc The required flash pressure calculated by the Flash Train.
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. For example, the list might be as follows:

Heat_Exchanger_2

Evaporator_1

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)).
Summary of input and output streams
Tag Feed Vapour OF Prod
Temperature / T Feed Temperature Vapour Temperature Overflow Temperature Product Temperature
MassFlow / Qm Feed Mass Flow Vapour Mass Flow Overflow Mass Flow Product Mass Flow
SolidMassFlow / SQm Feed Solids Mass Flow Vapour Solids Mass Flow Overflow Solids Mass Flow Product Solids Mass Flow
SolidFrac / Sf Feed Solids Fraction Vapour Solids Fraction Overflow Solids Fraction Product Solids Fraction
Results
BFeed.Temperature / BFeed.T Calc The temperature of the Feed to the Evaporator evaluation block.
BFeed.Pressure / BFeed.P Calc The Pressure used in the calculations in the Evaporator evaluation block. If the user has ticked the 'UseFeedP' then this will be the feed pressure to the Evaporator.
BFeed.MassFlow / BFeed.Qm Calc The mass flow of Feed to the Evaporator evaluation block.
BFeed.SolidMassFlow /BFeed.SQm Calc The mass flow of solids to the Evaporator evaluation block.
Body.SolidsYield Calc The mass flow of solids in the Product - mass flow of solids in the Feed.
SolidsYield Calc The mass flow of solids in the Product - mass flow of solids in the Feed.
Evaporator to Embedded or External Heater / Cooler
HXOut.MassFlow / HXOut.Qm Calc The mass flow to the specified Heat Exchanger.
HXOut.EachHeaterQm Calc The mass flow to the specified Heat Exchanger (per heater if more than one is used).
HXOut.VolFlow / HXOut.Qv Calc The volume flow to the specified Heat Exchanger.
HXOut.EachHeaterQv Calc The volume flow to the specified Heat Exchanger (per heater if more than one is used).
HXOut.Temperature / HXOut.T Calc The temperature of the stream to the specified Heat Exchanger.
Embedded Heater Recycle Loop convergence
RecycleConvergeMethod FullStream User can select to solve the recycle loop using Full stream data (loop has to solve and converge all mass and energy terms.)
Stream Mass Only User can select to solve the recycle loop using mass only. Loop will be solved when mass is converged, ignoring energy.
Duty User can select to solve the recycle loop using energy only. Duty from "shell" side of heat exchanger is added directly to the feed (recycle mass flow is not actually added). Loop will be solved when duty is matched, ignoring mass balance.
MaxIter(Global) Input The maximum number of iteration allowed within the recycle solver loop. Only shown for Stream Recycle Converge Methods.
IterSteps Calc The number of iteration used in the current step to solve the recycle loop. Only shown for Stream Recycle Converge Methods.
IterLastStep Calc The number of iteration used in the last step to solve the recycle loop. Only shown for Stream Recycle Converge Methods.
IterMaxInAStep Calc The maximum number of iterations (in a step) used to solve the recycle loop. Only shown for Stream Recycle Converge Methods.
IterTotal Calc The total number of iterations used to solve the recycle loop. Only shown for Stream Recycle Converge Methods.
FeedMaxDuty Calc The maximum amount of Duty that can be added to feed before temperature exceeds water critical temperature. Only shown for Duty Recycle Converge Method.
IterDuty Calc The number of iterations to apply duty and flash. When this is greater then 1 then the Duty from the condensing steam is applied in iterations, the values displayed on the VLE page reflect this. Only shown for Duty Recycle Converge Method.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

HeatExchange.dll

Units/Links

Heat Transfer: Evaporator

or

Group:

Energy Transfer

Units/Links

Heat Transfer: Evaporator

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

 

Insert Unit

Heat Transfer

Evaporator

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 Evaporator to operate as required. If the VLE configuration requires information that is not available, then the flashing will not occur.