Counter Current Washer

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Contents 1 General Description 1.1 Diagram 2 Inputs and Outputs 3 Model Theory 4 Flowchart 5 Data Sections 5.1 Requirements 5.2 Results 6 Adding this Model to a Project 7 Hints and Comments

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 model assumes there are no solids in the overflow.
• The model allows reactions to be specified via a reaction file. The reactions occur before washing and any solids generated by the reaction(s) report to the underflow.
• The temperature of the solids and liquids is calculated after any reactions and/or heat loss has occurred.

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

Diagram

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	Input / Output	No. of Connections		Description
		Min	Max.
Mud	In	1	20	The slurry (or mud) feed to the Counter Current Washer
Wash Water	In	1	20	The dilute wash stream
Side Stream	In	0	10	Any input other than the Mud stream or the wash water. These streams are combined with the wash streams.
Overflow	Out	1	1	Counter Current Washer overflow (liquids only)
Underflow	Out	1	1	Counter Current Washer underflow
Vent	Out	0	1	Vent Stream (Vapour Only)

Model Theory

The Scandrett mixing efficiency is used to determine the concentration of the ALL aqueous species in the underflow and overflow from the unit.

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

C_{under/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"

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. The first tab has the same name as the unit class and contains general information relating to the unit.
2. RB - Optional tab, only visible if the Reactions are enabled.
3. EHX - Optional tab, only visible if the EnvironHX is enabled.
4. Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
5. Links tab, only visible in SysCAD 9.2, contains a summary table for all the input and output streams.
6. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Class: Washer-1 - The first tab page in the access window will have this name.

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Common First Data Section
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.
ScanEffSpecie	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.
RqdScandrettEff	Input	The required Scandrett mixing efficiency of the unit. At 100% efficiency, all of the incoming streams will be perfectly mixed.
Method	List	Options are: %_Solids or g/L_Solids. This allows the user to specify the solids in the washer underflow stream as either % solids or g/L solids.
RqdUFSolConc25	Input	Visible with g/L_Solids method - user specified solids concentration in the underflow stream in g/L at 25oC.
RqdUFSolids	Input	Visible with %_Solids method - user specified percentage solids in the underflow stream.
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.
EnvironHX	List	This can be used to switch on Environmental Heat Exchanger (EHX). If this is 'On' then the associated page, EHX may become visible and may be configured.
OperatingP... (available in SysCAD 9.2 or later)
Method	List	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.
		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.
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).
Results
UFSolids	Calc	The actual % solids in the under flow
UFSolConc25	Calc	The under flow solids concentration measured at 25°C.
ScandrettEff	Calc	The calculated Scandrett Efficiency of the unit.
UFSpecie	Calc	The concentration of the user defined species in the under flow.
OFSpecie	Calc	The concentration of the user defined species in the over flow.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

	DLL:	Separ1.dll	→	Units/Links	→	Separation: Washer (Counter Current)
OR	Group:	Mass Separation	→	Units/Links	→	Separation: Washer (Counter Current)

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

Insert into Project

Insert Unit

→

Separation

→

Washer (Counter Current)

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 specie. This specie must be set by the user to a relevant specie, i.e. NaOH(aq) in an alumina project. Do NOT leave this value as the default specie, H2O(l), as the resulting values will be slightly incorrect.
3. The model assumes no solids reports to the overflow stream.
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.