Filter Press

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General Description

The filter press is used to separate the solids from the liquids in the feed stream. There is the facility to wash the solids using wash water. This water will displace a percentage of the liquid in the cake from the filter press.

The user defines the following parameters:

  • The amount of moisture retained in the filter cake,
  • The amount of solids reporting to the filtrate,
  • The fraction of wash water that bypasses the filter cake, and
  • The required wash efficiency. The wash efficiency may be specified as either an overall wash efficiency, or as a single volume wash efficiency.

If the user specifies an constant wash efficiency, then the model will use this value to calculate the percentage of the feed liquid remaining in the washed cake. For example, an constant wash efficiency of 100% represents complete displacement of the feed liquor from the filter cake. If the constant wash efficiency is set at 80%, then 20% of the cake moisture will consist of feed liquor.

If the user specifies a single volume wash efficiency then the model will use this value together with the wash ratio to calculate the percentage of feed liquid remaining in the cake.

If there is no Washings stream connected to the Filter Press, then all of the filtrate and the wash water from the unit will report to the Filtrate stream. In this case the wash water will dilute the Filtrate stream.

If the user connects the 'Washings' stream to the unit, then the liquor from the washing stage will report to the washing stream.

Diagram

Filter-Press-Image001.gif

The diagram shows the default drawing of the Filter Press, with all of the streams that are available for operation of the unit. The user does not have to connect a stream to the 'Wash Water' or 'Washings' connections, as the Filter Press will operate without wash water. The user may have a wash water stream without a washings stream, as the wash water which does not remain in the cake will report to the Filtrate stream.

The physical location of the streams connecting to the Filter Press 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 Filter Press
Wash Water Optional In 0 10 Wash water to the Filter Press.
Filtrate Required Out 1 1 Filtrate outlet from the unit.
Cake Required Out 1 1 Solids outlet from the unit.
Washings Optional Out 0 1 Wash stream outlet.
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:

  • The field MassFracToCake becomes visible and the user may set the fraction of the feed stream that reports to the 'Cake' outlet. The default is 100%.
  • All streams connected to the 'Feed' inlet will flow out of the 'Cake' outlet, unless the user has changed the above field from 100%. In which case the user defined fraction of the Feed will report to the Filtrate stream;
  • All streams connected to the 'Wash Water' inlet will flow straight out of the 'Washings' outlet, if connected, otherwise out of the 'Filtrate' outlet;
  • Normally no mixing will occur. If the user sends some, or all, of the Feed to the Filtrate stream, then this will mix with the Wash stream, if the Washings stream is not connected.

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


Model Theory

The Filter Press is based on user defined variables that define the solids / liquids split, as well as the efficiency of the washing action.

The user may specify a fraction of the wash water to bypass the filter cake. This amount of wash water will not be involved in the displacement of feed liquor from the filter cake and will not be used in any of the wash efficiency calculations.

The user may specify the wash efficiency of the filter in two ways:

(1) Constant Wash Efficiency

In this case, the calculation used to determine the amount of feed liquid remaining in the washed cake is simply:
[math]R = 1 - E[/math]
where
R = Fraction of Feed liquid remaining in Washed Cake
E = Wash Efficiency
If there is sufficient wash water to replace the required amount of feed liquor, then the amount of wash water does not affect the wash efficiency.

(2) Based on Wash Ratio - Reference 1

In this case the washing efficiency of the filter depends on both the single volume wash efficiency and the ratio of wash water to residual cake moisture. The formula used is:
[math]\frac{R}{100} = \left(1 - \frac{E}{100}\right)^n[/math]
where
R = Percentage of Feed liquid remaining in Washed Cake
E = Percentage Single Volume Wash Efficiency
n = wash ratio = [math] \frac {\mbox{Volume of Wash Water}}{\mbox{Volume of Cake Liquor prior to washing}}[/math]
Note: If there are no solids in the Wash Water stream then the wash ratio is the same for the washed cake.

This model is designed to emulate a Filter Press, which is normally operated on a batch basis:

Step 1. The feed stream containing liquids and solids is sent to the Filter Press for a given length of time. During this time, the filtrate is removed and sent on to the next process. The filter cake, with retained moisture, remains in the unit.
Step 2. The feed to the Filter Press is stopped and the cake is washed with a wash water stream. The washings stream may either be combined with the filtrate, or sent to a different destination.
Step 3. The washed cake is removed from the unit.

Therefore, the liquid in the filtrate stream from the Filter Press will consist entirely of feed liquor. The washings stream will contain the feed liquor that was displaced from the cake as well as the excess wash water.

The solids reporting to the filtrate will be divided between the filtrate and washings streams, in proportion to the stream flows.

Example

The Filter Press shown in the Diagram in 2 is configured to give 10% moisture in the cake, 1% of the feed solids to the filtrate, and 90% constant washing efficiency. The table below shows the material flows around the Filter Press with this configuration.

Material Feed Wash Water Cake Filtrate Washings
H2O(l) 9.4 5.0 1.09 8.37 4.94
NaCl(aq) 0.6 0.0 0.01 0.53 0.06
Total Liquid 10.0 5.0 1.1 8.9 5.0
Solids 10.0   9.9 0.064 0.036
Total Mass 20.0 5.0 11.0 8.964 5.036

The amount of liquor from the feed stream which reports to the Filtrate is reduced by the percentage moisture retained in the cake, in this case 1.1t/h. The wash water stream then displaces 90% of this moisture into the washings stream.

References

1. Merritt, R.C, The Extractive Metallurgy of Uranium

Flowchart

Filter-Press-Image003.gif

Data Sections

The default sections and variable names are described in detail in the following tables. The default Filter Press access window consists of 3 sections:

  1. FilterPress tab - Contains general information relating to the unit.
  2. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  3. Links tab, contains a summary table for all the input and output streams.
  4. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Filter Press Page

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

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

Requirements

On Tick box Switch the model on or off. If the unit is 'Off' then by default all Feed reports to the Cake outlet and all wash streams report to the Filtrate stream. If the user specifies a value (less than 100%) in the MassFracToCake field, then some feed may report to the Filtrate stream.
MassFracToCake_WhenOff / MassFracToCake Input This field becomes visible when the unit is switched off. The user may set the fraction of the feed stream that reports to the Cake outlet. The default is 100%.
 
Method SolidsToFiltrateFraction Specify the percentage of solids in the feed that report to the filtrate. If a washings stream is connected then this fraction of solids will be the total which reports to the filtrate and the washings stream in proportion to their flows.
FiltrateConcAt25C Specify the required filtrate solids concentration at 25oC.
SolidsFractionInFiltrate Specify the solids fraction in the filtrate.
SolidsToFiltrate Input The required percentage of the feed solids that reports to the filtrate - visible with SolidsToFiltrateFraction method.
Note: If the user specifies 1%, then the filtrate stream will contain 1% of the solids in the feed.
FiltrateSolidConc25Reqd Input The required filtrate solids concentration, measured at 25°C - visible with FiltrateConcentration method.
FiltrateSolidFracReqd Input The required fraction of solids in the filtrate - visible with SolidsFractionInFiltrate method.
CakeMoistReqd Input The required percentage of liquids in the filter cake.
WashWaterBypass Input The fraction of wash water that bypasses the filter cake. This will emulate wash water "channeling" through the filter cake.

Wash Efficiency

WashEfficiencyMethod / WashEffMethod Constant Efficiency Use a constant wash efficiency for the filter.
Based on Wash Ratio Use the single volume wash efficiency, together with the ratio of wash water to cake moisture, wash ratio, to determine the actual wash efficiency of the filter.
WashEfficiencyReqd / WashEffReqd Input The constant wash efficiency - visible with the Constant Efficiency wash method. For example, 90% wash efficiency means that the liquid in the cake consists of 90% wash water and 10% feed liquor by mass.
SingleVolWashEfficiency / SingleVolWashEff Input The single volume wash efficiency - visible with the Based on Wash Ratio wash method. In this case, 90% single volume wash efficiency may give a final wash efficiency of more or less than 90% depending upon the ratio of wash water to cake moisture.
WashComponent / WashComp Input Choose Component/species from the drop down list for the calculation of Wash efficiency on a concentration base. This calculation is for display purposes only.

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 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).

Options

TrackStatus Tick Box Allows warning messages be displayed if requirements are not met. For example, if the required and actual wash efficiencies are different.

Results

Cake.SolidFrac / Cake.Sf Calc The calculated percentage of solids in the filter cake.
Cake.SolidConc25 Calc Displays the concentration of solids in the final Cake from the Filter Press at 25 deg C.
Filtrate.SolidFrac / Filtrate.Sf Calc The calculated percentage of solids in the filtrate.
Filtrate.SolidConc25 Calc Displays the concentration of solids in the filtrate from the Filter Press at 25 deg C.
Calculated Wash Values
WashRatio Calc The ratio of Wash Water to Feed Liquor in the un-washed cake.
Note: If there are no solids in the Wash Water, then this is also the ratio of Wash Water to Feed Liquor in the final washed cake.
SolidsWashRatio / SolWashRatio Calc The ratio of Wash Water to Solids in the feed to the Filter Press.
WashEfficiency / WashEff Calc The calculated wash efficiency, based on the mass fraction of feed liquor in the cake moisture. If this number is different to the required efficiency, and the TrackStatus box is ticked, then the unit will flag a warning.
RemainingLiquor / RemainLiq Calc The calculated fraction of feed liquor remaining in the final washed cake. This equivalent to the 'R' value in the wash ratio calculation above.
WashComponentUsed / WashCompUsed Display The Component/species chosen for the calculation of Wash efficiency on a concentration base.
CompWashEfficiency / CompWashEff Calc The calculated wash efficiency of the unit.

CompWashEff = (Conc Feed - Conc Cake) / (Conc Feed - Conc wash)

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

Separation.dll

Units/Links

Separation: Filter - Filter Press

OR

Group:

Mass Separation

Units/Links

Separation : Filter - Filter Press

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

Insert into Project

 

Insert Unit

Separation

Filter - Filter Press

See Insert Unit for general information on inserting units.