Counter Current Washer

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


General Description

The Counter Current Washer is used to separate the solids from the liquids in the feed stream. This unit is similar to the Thickener and the Washer.

  • The counter current washer perfectly mixes the incoming wash water and side streams.
  • Then based on the feed stream (Mud), Scandrett efficiency and underflow solids content, the amount, temperature and composition of the underflow and overflow are calculated.
  • The Scandrett efficiency is based on the user defined species concentrations within the counter current washer.
  • The Counter Current Washer was revised in SysCAD9.2 Build 135.14140 to allow solids in the overflow. Previous version of SysCAD assume no solids in the Overflow.
  • The model allows reactions to be specified via a reaction file. The reactions occur to the Mud stream only, before mixing with the wash and side streams.
  • The EHX sub model applies to the Mud Stream only, before mixing with the wash and side streams.

Note: The Counter Current Washer Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this model.

Diagram

Counter-Current-Washer-Image001.gif

The diagram shows the default drawing of the Counter Current Washer, with all of the streams that can be connected to the unit. The Side Stream does not have to be connected for the unit to operate.

The physical location of the streams connecting to the Counter Current 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.
Mud Feed Required In 1 20 The slurry (or mud) feed to the Counter Current Washer.
In SysCAD 9.2 and previous this port is called 'Mud'.
Wash Water Required In 1 10 The dilute wash stream
Side Stream Optional In 0 10 Can represent Flocculant addition, miscellaneous dilution etc. These streams are combined with the wash streams.
Not available in SysCAD 9.3. Use the Wash Water connections.
Overflow Required Out 1 1 Counter Current Washer overflow
(Revised in SysCAD 9.2 Build 135.14140 to allow solids in the overflow.
Previous versions of SysCAD assume no solids in the Overflow.)
Underflow Required Out 1 1 Counter Current 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 the following actions occur:

  • All streams connected to the 'Mud Feed' inlet will flow out of the 'Underflow' outlet;
  • All streams connected to the 'Wash Water' inlet will flow straight out of the 'Overflow' outlet;
  • No mixing will occur.

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

Model Theory

The Scandrett mixing efficiency [math]\mathbf{\mathit{\boldsymbol{\varepsilon}}}[/math] is used to determine the concentration of the ALL aqueous species in the underflow and overflow from the unit.

[math]\mathbf{\mathit{\boldsymbol{\varepsilon}=\frac{C_{mud}-C_{underflow}}{C_{mud}-C_{overflow}}}}[/math]

Where: Cmud - concentration of the user defined species in the mud stream

Cunder/over flow - concentration of user defined species in the under and over flow respectively

The user must specify the Scandrett mixing efficiency and a representative aqueous species, e.g. Caustic, for the unit to perform the required mixing calculations.

The model will calculate the solid and liquid splits based on the Scandrett mixing efficiency and the user defined solids to underflow split.

References

H.F. Scandrett, "Equations for Calculating Recovery of Soluble Values in a counter current Decantation Washing System"

Limits and Notes

The Counter Current Washer was revised in SysCAD9.2 Build 135.14140 to allow solids in the overflow. Previous version of SysCAD assume no solids in the Overflow.

  • In the case where there is no solids present in the Feed, all flow will report to the Overflow.
  • In the case where there not enough solids to satisfy both overflow and underflow solids requirements, the Overflow solids conditions will be met first.
  • Limits apply to the overflow solids concentration, this equates to 5000g/L or 99% solid split to the overflow. A reasonable overflow solids concentration should be much lower than this.

Flowchart

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

CounterCurrentWasher.png

Data Sections

The default access window consists of several sections,

  1. CCWasher tab - Contains general information relating to the unit.
  2. PC - Optional tab that is visible if EB.Solubility.On is enabled on the first tab.
  3. 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.
  4. RB - Optional tab, only visible if the Reactions are enabled.
  5. EHX - Optional tab, only visible if the EnvironHX is enabled.
  6. Evap - Optional tab that is visible if Evaporator is enabled.
  7. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  8. Links tab, contains a summary table for all the input and output streams.
  9. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Counter Current Washer Page

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

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

Requirements

On Tick Box This allows the user to enable or disable the unit. If the unit is disabled, this is equivalent to bypassing the unit, as the following behaviour will occur:
  • All material in the streams connected to the Mud connections will flow out of the underflow;
  • All material in the streams connected to the Side Stream and Wash Water connections will flow out of the overflow.
EffMethod Input The required Mixing Efficiency method. Currently the only method implemented is the Scandrett Mixing Efficiency method.
ScanEffSpecies Input The selected species is used to calculate the Scandrett efficiency. (e.g. for Alumina industry, this would be Caustic.)
Note: The default species is H2O(l), as water is always present in a SysCAD project. However, the user MUST change this to a different aqueous species for the calculations to be performed correctly.
ScandrettEffReqd Input The required Scandrett mixing efficiency of the unit. At 100% efficiency, all of the incoming streams will be perfectly mixed.
Method UF Solids Fraction User to specify the required percent solids in the washer underflow stream
UF Solids Conc User to specify the solids concentration at Stream Temperature in the washer underflow stream
UF Solids Conc25 User to specify the solids concentration @25°C in the washer underflow stream
UFSolidFracReqd Input Visible with UF Solids Fraction method - user specified percentage solids in the underflow stream.
UFSolidConcReqd Input Visible with UF Solids Conc method - user specified solids concentration in the underflow stream in g/L at temperature.
UFSolidConc25Reqd Input Visible with UF Solids Conc25 method - user specified solids concentration in the underflow stream in g/L at 25°C.
OFSolidsMethod OF Solids Fraction User to specify the required percent solids in the washer overflow stream
OF Solids Conc User to specify the solids concentration at Stream Temperature in the washer overflow stream
OF Solids Conc25 User to specify the solids concentration @25°C in the washer overflow stream
OFSolidFracReqd Input Visible with OF Solids Fraction method - user specified percentage solids in the overflow stream.
OFSolidConcReqd Input Visible with OF Solids Conc method - user specified solids concentration in the overflow stream in g/L at temperature.
OFSolidConc25Reqd Input Visible with OF Solids Conc25 method - user specified solids concentration in the overflow stream in g/L at 25°C.
OperatingP...
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 (Flow Evaluation Block)

Solubility.On Tickbox Allows the user to switch on any predefined solubility curve to adjust the composition of material in the unit. This sub model has been added to this unit operation in SysCAD 9.3.
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. This sub model has been added to this unit operation in SysCAD 9.3.
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. This sub model has been added to this unit operation in SysCAD 9.3.
 

Results

ScandrettWashEff / ScandrettEff Calc The calculated Scandrett Efficiency of the unit.
VapourMassFlow / VQm Calc Only available in SysCAD 9.3 Build 137 or later. The total flow of vapour to the Underflow and Overflow streams. (This does NOT include the vapour to the Vent)
Slurry Separation Results
UF.SpConc Calc The concentration of the user defined species in the underflow.
OF.SpConc Calc The concentration of the user defined species in the overflow.
The following table displays the flows and compositions of the Underflow and Overflow streams. Note: These are for slurry only, vapours are excluded.
MassFlow / Qm Display The total mass flow in each of the Underflow and Overflow streams.
VolFlow / Qv Display The total volume flow in each of the Underflow and Overflow streams.
SolidMassFlow / SQm Display The solids mass flow in each of the Underflow and Overflow streams.
LiquidMassFlow / LQm Display The liquid mass flow in each of the Underflow and Overflow streams.
SolidFrac / Sf Display The solid fraction in each of the Underflow and Overflow streams.
Density / Rho Display The total density of each of the Underflow and Overflow streams.
SolidConc Display The solid concentration (mass of solids/density of slurry) in each of the Underflow and Overflow streams.
SolidConc25 Display The solid concentration at 25°C (mass of solids/density of slurry at 25°C) in each of the Underflow and Overflow streams.
GridDisplay Tick Box This is a global selection, if this is selected, then where available, variables will display in a table format, if not selected, they will be listed in separate groups as a single list.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

  DLL: Separation.dll Units/Links Separation: Thickener - CC Washer
or Group: Mass Separation Units/Links Separation: Thickener - CC Washer

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


Insert into Project

  Insert Unit Separation Thickener - CC Washer

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 Scandrett efficiency is based on the concentration of a Liquid or Aqueous species. This species must be set by the user to a relevant species, i.e. NaOH(aq) in an alumina project. Do NOT leave this value as the default species, H2O(l), as the resulting values will be slightly incorrect.
  3. The Counter Current Washer was revised in SysCAD9.2 Build 135.14140 to allow solids in the overflow. Previous version of SysCAD assume no solids in the Overflow.
  4. If vapour is present in the feed streams, or is produced in the reaction block, the vent line must be connected to collect the vapour. Otherwise, the vapour will be lost resulting in mass imbalance.