Underflow Washer

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Navigation: Main Page -> Models -> Mass Separation Models

This model is only available in SysCAD 9.3 Build 137 and later versions.


General Description

The Underflow Washer is used to separate the solids from the liquids in the feed stream and then wash the solids with a wash stream. This unit is similar to the Washer and the Counter Current Washer.

  • The user defines the percentage solids in both the underflow and the overflow from the unit.
  • If there is insufficient solids in the feed to the Underflow Washer to satisfy both the under and over flow requirements, the unit will try to satisfy the underflow solids / liquids requirements first.
  • The wash stream combines with the underflow solids and 'washes' the feed liquid out of the solid stream.
  • The model allows the user to set sub-models in the Feed section, Underflow and Overflow sections. The following sub-models are avaialble in each section:
    • The Feed section, EB, allows the user to set Solubility, Reactions, Makeups, Environmental Heat Exchange and Evaporation;
    • The Overflow section, OFEB, allows the user to set Solubility, Reactions, Environmental Heat Exchange and Evaporation; and
    • The Underflow section, UFEB, allows the user to set Solubility and Reactions.
  • The temperature of the solids and liquids is calculated after any reactions and/or heat loss has occurred.


Diagram

Underflow Washer.png

The diagram shows the default drawing of the Underflow Washer, with all of the streams that may be connected for the unit to operate.

The physical location of the streams connecting to the Underflow Washer is unimportant. The user may connect the streams to any position on the unit.

Inputs and Outputs

Label Required
Optional
Input
Output
Number of Connections Description
Min Max.
Feed 1 Required In 1 20 The slurry feed to the Underflow Washer.
Wash Optional In 0 20 The Washing Stream to the Underflow Washer.
Overflow Required Out 1 1 Underflow Washer overflow
Underflow Required Out 1 1 Underflow Washer underflow
Vent Optional Out 0 1 Vent Stream (Vapour Only)

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then they may specify:

  • The fraction of the Feed streams that will report to the 'Underflow' outlet;
  • The fraction of the Wash streams that will report to the 'Underflow' outlet.

This means:

  • All of the streams to the unit could report to the underflow, if both fractions are set to 100%; or
  • All of the streams to the unit could report to the overflow, if both fractions are set to 0%; or
  • The Feed and Wash streams report to different locations and do not mix at all if the user sets one fraction to 100% and the other to 0%.
  • Any combination of the above.

Model Theory

The liquid associated with the solids in the Feed stream is washed from underflow stream, based on the user defined washing efficiency. For example, if the washing efficiency is set at 90%, then 10% of the liquid in the underflow stream will come from the Feed streams.

NOTES:

  1. The user may enable sub-models in the Feed, Overflow and the Underflow sections of the unit. This does mean that the solid composition may be different in the over and underflow streams.

Flowchart

The following shows the sequence of events if sub model options are switched on.

Underflow Washer Flow Chart.png

Data Sections

The default access window consists of several sections,

  1. UnderflowWasher tab - Contains general information relating to the unit.
  2. PC - Optional tab, only visible if the Solubility is enabled in the Feed section.
  3. RB - Optional tab, only visible if the Reactions are enabled in the Feed section.
  4. EHX - Optional tab, only visible if the EnvironHX is enabled in the Feed section.
  5. Evap - Optional tab, only visible if the Evaporation is enabled in the Feed section.
  6. MUx - Optional tab, only visible if the Makeups have been added in the Feed section.
  7. UFPC - Optional tab, only visible if the Solubility is enabled in the Underflow section.
  8. UFRB - Optional tab, only visible if the Reactions are enabled in the Underflow section.
  9. OFPC - Optional tab, only visible if the Solubility is enabled in the Overflow section.
  10. OFRB - Optional tab, only visible if the Reactions are enabled in the Overflow section.
  11. OFEHX - Optional tab, only visible if the EnvironHX is enabled in the Overflow section.
  12. OFEvap - Optional tab, only visible if the Evaporation is enabled in the Overflow section.
  13. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  14. Links tab, contains a summary table for all the input and output streams.
  15. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Underflow Washer Page

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

Symbol / Tag
Input / Calc
Description/Calculated Variables
Common Data on First Tab Page

Requirements

On Tick Box This allows the user to enable or disable the unit. The user may choose the fraction of flows reporting to the Underflow of both the Feed and Wash streams. See Behaviour when Model is OFF.
FeedFracToUF_WhenOff / FeedFracToUF Input Only visible when the unit is NOT On. This lets user select how much of the Feed streams report to the Underflow.
WashFracToUF_WhenOff / WashFracToUF Input Only visible when the unit is NOT On. This lets user select how much of the Wash streams report to the Underflow.
WashEfficiencyReqd / WashEffReqd Input The required washing efficiency of the underflow solids by the Wash streams.
UFSolidFracReqd Input The required percentage of solids in the underflow stream.
OverFlowMethod / Method OverFlowSolidsFraction The user specifies the fraction of solids in the overflow.
OverFlowSolidsConcentration25 The user specifies the overflow solids concentration at 25oC.
OverFlowSolidsConcentration The user specifies the overflow solids concentration at the stream temperature.
OFSolidFracReqd Input The required percentage of solids in the overflow stream. This is only visible if the user has chosen OverFlowSolidsFraction above.
OFSolidConcReqd25 Input The required overflow Solids concentration at 25oC. Visible only with the OverFlowSolidsConcentration25 method.
OFSolidConcReqd Input The required overflow Solids concentration. Visible only with the OverFlowSolidsConcentration method.
OFSolidsRecoveryUsed Calc Shows the value used by SysCAD for the O/F solids in the solver.
WashFracToUF Calc The fraction of the wash streams reporting to the underflow stream.
WashEfficiencyUsed/WashEffUsed Calc Shows the actual wash efficiency achieved in the unit.

Operating Pressure

Method Atmospheric Outlet streams will be 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 Species tab page of the Plant Model.
AutoDetect If there are any liquids AND no vapours present in the feed, outlet streams will take the highest pressure of the feeds. Else (eg. some vapours present) outlet streams will take the lowest pressure of the feeds.
LowestFeed Outlet streams will take the lowest pressure of the feeds.
HighestFeed Outlet streams will take the highest pressure of the feeds.
RequiredP Outlet streams will be at the user specified pressure.
IgnoreLowQm Tick Box This option is only visible if the AutoDetect, LowestFeed or HighestFeed methods are chosen. When calculating the outlet pressure and temperature of the tank, SysCAD will ignore the low flow feed streams should this option be selected. The low flow limit is set in the field below.
LowQmFrac Input This field is only visible if the IgnoreLowQm option is selected. This is the amount any stream contributes to the total flow. For example, if the total feed to the tank is 10 kg/s, and this field is set to 1%. Then any feed streams with less than 0.1 kg/s will be ignored in the pressure calculations.
PressureReqd / P_Reqd Input This field is only visible if the RequiredP method is chosen. This is user specified pressure.
Result Display The actual pressure used for the sum of the feeds which will also be the outlet pressure (unless further model options change the pressure).

EB (Feed Evaluation Block)

This allows the user to enable sub-models in the Feed section of the unit.
Solubility.On Tickbox Allows the user to switch on any predefined solubility curve to adjust the composition of material 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 sequence followed will be MU1, MU2, etc., RB, EHX, Evap, VLE.
Makeups Input The number of makeup blocks required. Extra dropdown options Makeup1, Makeup2, etc will be added to allow these to be switched on.
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 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 becomes visible and may be configured.
Note: The user does not have to configure an environmental heat exchange, even if this block is checked.
Evaporation List This can be used to switch on the Evaporation Block (Evap). If this is 'On' then the associated page, Evap becomes visible and may be configured.
Note: The user does not have to configure an evaporator, even if this block is checked.

UFEB (Underflow Evaluation Block)

This allows the user to enable sub-models in the underflow, or wash, section of the unit.
Solubility.On Tickbox This can be used to enable Solubility. If this is ticked, then the associated page, UFPC becomes visible and may be configured.
Reactions list This can be used to enable Reaction Block (RB). If "On" is selected then the associated page, UFRB becomes visible and may be configured. Note: The user does not have to configure a reaction file, even if reactions are turned On.

OFEB (Overflow Evaluation Block)

This allows the user to enable sub-models in the Overflow section of the unit.
Solubility.On Tickbox Allows the user to switch on any predefined solubility curve to adjust the composition of material 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 sequence followed will be MU1, MU2, etc., RB, EHX, Evap, VLE.
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 becomes visible and may be configured.
Note: The user does not have to configure an environmental heat exchange, even if this block is checked.
Evaporation List This can be used to switch on the Evaporation Block (Evap). If this is 'On' then the associated page, Evap becomes visible and may be configured.
Note: The user does not have to configure an evaporator, even if this block is checked.

Options

ShowQFeed Tick Box QFeed and associated tab pages (eg Sp) will become visible, showing the properties of the combined feed stream. See Material Flow Section. This will be prior to any sub-model (eg reactions) actions.
TrackStatus Tick Box This is used to enable warning messages.

Separation Results

WashEfficiency/WashEff Calc The actual wash efficiency achieved in the unit.
UFSolidsRecovery Calc The fraction of solids in the feed streams reporting to the underflow stream.
UFLiquidRecovery Calc The fraction of liquids in the feed streams reporting to the underflow stream.
The following fields are may be presented in a table showing the values for the Under and Over flow streams, if the 'GridDisplay' box is ticked.
MassFlow/Qm Calc The total mass flow in the stream.
VolFlow/Qv Calc The total volume flow in the stream.
SolidMassFlow/SQm Calc The solid mass flow in the stream.
LiquidMassFlow/LQm Calc The liquid mass flow in the stream.
SolidFrac/Sf Calc The solids fraction in the stream.
Density/Rho Calc The total density of the stream.
SolidConc Calc The solids concentration in the stream.
SolidConc25 Calc The solids concentration at 25C in the stream.
GridDisplay Tick Box If this is ticked then the above fields are displayed in a table. Otherwise, they are displayed in individual fields.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

Separation.dll

Units/Links

Separation: Thickener - Underflow Washer

OR

Group:

Mass Separation

Units/Links

Separation : Thickener - Underflow Washer

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

Insert into Project

 

Insert Unit

Separation

Thickener - Underflow Washer

See Insert Unit for general information on inserting units.

Hints and Comments

  1. There may be mass entering or leaving the Underflow Washer 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 reaction block is enabled in the model, there may be small errors resulting in the calculation of Overflow/underflow solids due to the feed composition change.