Filter Press

From SysCADWiki

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

---

Contents 1 General Description 1.1 Diagram 2 Inputs and Outputs 3 Model Theory 3.1 References 4 Flowchart 5 Data Sections 5.1 First Section- Tag Name

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

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	Input / Output	No. of Connections		Description
		Min	Max.
Feed	In	1	20	The slurry feed to the Filter Press
Wash Water	In	0	1	Wash water to the Filter Press - optional
Filtrate	Out	1	1	Filtrate outlet from the unit
Cake	Out	1	1	Solids outlet from the unit
Washings	Out	0	1	Wash stream outlet - optional
Vent	Out	0	1	Vent Stream (Vapour Only) - optional

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:

Constant Wash Efficiency

In this case, the calculation used to determine the amount of feed liquid remaining in the washed cake is simply:

R = 1 − E

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.

Based on Wash Ratio

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:

where

R = Percentage of Feed liquid remaining in Washed Cake

E = Percentage Single Volume Wash Efficiency

n = wash ratio =

Reference 1

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	0.01	0.53	0.06
Total Liquid	10.0	5.0	1.1	8.9	5.0
Solids	10.0	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

Data Sections

The default access window consists of three sections,

1. The first tab has the same name as the model tag, contains general information relating to the unit.
2. The Info tab contains general settings for the unit, fully described in Common Data Sections.
3. The last tab Audit, fully described in Audit Section. See Model Examples for enthalpy calculation Examples.

First Section- Tag Name

Tag / Symbol	Input / Calc	Description
Common First Data Section
Requirements
Method	List Box	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.
RqdSolToFilt	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.
RqdFiltSolConc25	Input	The required filtrate solids concentration, measured at 25°C - visible with FiltrateConcentration method.
RqdSolInFilt	Input	The required fraction of solids in the filtrate - visible with SolidsFractionInFiltrate method.
RqdCakeMoist	Input	The required percentage 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
WashEffMethod	List Box	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.
RqdWashEff	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.
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.
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... (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
CakeSolids	Calc	The calculated percentage of solids in the filter cake.
FiltSolids	Calc	The calculated percentage of solids in the filtrate.
FiltSolConc25	Calc	The filtrate solids concentration measured at 25°C.
Calculated Wash Values
WashRatio	Calc	The ratio of wash water to liquor in the un-washed cake.
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.
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.
WashCompUsed	Display	The Component/species chosen for the calculation of Wash efficiency on a concentration base.
CompWashEff	Calc	The calculated wash efficiency of the unit. CompWashEff = (Conc Feed - Conc Cake) / (Conc Feed - Conc wash)
TrackStatus	Tick Box	Allows warning messages be displayed if requirements are not met. For example, if the required and actual wash efficiencies are different.