General Separator

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General Separator	Thickener	Classifier	Washer	Counter Current Washer	Underflow Washer

Introduction

This model can be used as a general separator, especially for separating solids and liquids using the Solid-Liquid Separator sub-model. In addition, a number of sub-models are available which makes this model very flexible so that it can be used to model many types of separation units.

Diagram

The diagram shows the default drawing of the General Separator, with all of the streams that have to be connected for the unit to operate.

The physical location of the streams connecting to the General Separator 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
Label	Required Optional	Input Output	Min	Max	Description
Feed	1 Required	In	1	20	The feed to the separator
Underflow	1 Required	Out	1	1	The separator underflow.
Overflow	Optional	Out	0	1	The separator overflow. If this is NOT connected, all material flows out through the 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:

The field MassFracToUF becomes visible and the user may set the fraction of the feed stream that reports to the 'Underflow' outlet. The default is 100%.
All streams connected to the unit will flow out of the 'Underflow' 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 Overflow stream;
No sub-models will occur.

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

Model Theory

The following model theory is for the solid - liquid separator sub model used by the general separator.

The Solid-Liquid Separator allows the user to define the solid-liquid separation with a high degree of flexibility, including the ability to allow specified species to bypass the separation calculations.

Note: To minimise input fields, some requirement combinations for solid-liquid separation necessitate swapping the OF and UF definitions and connections.

e.g. A settler requiring a defined Recovery to OF and OF Solids Fraction may be defined by swapping the OF/UF connections and choosing "Recovery to UF" and "UF Solids Fraction".

Solids Density - Erf Cut Density Method

If this method is chosen, SysCAD uses the error function along with the solid species density, user specified SolidsCutDensity and α to determine how much of each solid species is sent to the Overflow and Underflow streams.

The fraction of a particular solid species which is sent to the Overflow stream is calculated as follows:

[math]\displaystyle{ Frac=0.5 * \left( 1- \operatorname{erf} \, \left( \alpha* \left( Rho \, - \, \operatorname{SolidsCutDensity} \right) /1000 \right) \right) }[/math]

where:	Frac	= Calculated fraction of the solid species reporting to Underflow
	α	= User specified separation curve efficiency parameter or sharpness (ErfAlpha)
	Rho	= The density of each solids species
	SolidsCutDensity	= User specified Solids Cut Density

The standard Error Function (erf) is used:

[math]\displaystyle{ \operatorname{erf}(z) = \dfrac{2}{\sqrt{\Pi}} \,\int\limits_{0}^{z} \, e^{-t^2} \, dt }[/math]

Solids Density - Logistic Cut Density Method

Recovery to underflow for a given solid species i:

[math]\displaystyle{ \mathbf{\mathit{R_i=\cfrac{exp\left(\alpha \, \cfrac{Rho_i}{SolidsCutDensity }\right)-1}{exp\left(\alpha \, \cfrac{Rho_i}{SolidsCutDensity }\right)+exp(\alpha)-2}}} }[/math]

where:	Rho_i	= Density of solids species i
	α	= User specified separation curve efficiency parameter or sharpness (LogisticAlpha)
	Rho	= The density of each solids species
	SolidsCutDensity	= User specified Solids Cut Density. For a species with this density, there is a 50% split to underflow.

Bypass Species

Often in a plant one, or more, of the solid species will behave differently in a unit operation to the other solid species.

	Example 1: A stream may contain a mixture of Salts species (KCl, NaCl, MgCl₂, etc.) and Gypsum. The gypsum will tend to float on the surface in thickeners, and hence the user may require the ability to send all of the gypsum to the thickener overflow. This model allows the user to do this by specifying the bypass Method = Bypass to OverFlow, Gypsum as a 'Bypass' species, with 100% reporting to the overflow.
	Example 2: A stream containing a number of species must sent to a solid-Liquid separator (e.g. a thickener, or filter), but the user wishes to specify the split to both the Underflow and the Overflow of one of the species - say 10% of species B reports to the Overflow and the remaining 90% reports to the Underflow. The user specifies the bypass Method = Bypass to Both, species B as the bypass species, Bypass Fraction = 100% and Fraction to Overflow = 10%. If the Bypass to Overflow method was used with Bypass Fraction = 10%, then 10% of species B would be bypassed to the overflow, with the remaining 90% of species B reporting to the Solid-Liquid calculation block. Hence, some of the remaining species B might be sent to the overflow, depending on how the Solid-Liquid calculation block was configured.
	Results Table: If the BypassSeparCalcs tickbox is enabled, the Slurry Separation Results table displayed on the Separ page is for the internal streams, before the addition of the bypassed species, it will be displayed with the text "(excluding bypass)". User should note that the composition of the actual Overflow and Underflow stream may be different to the results shown in the results table as they will include the bypassed species.

The Examples below demonstrate one of the Separation methods used with the Bypass species functionality and the use of the BypassSeparCals tickbox:

Solid-Liquid Separator Settings:
SplitMethod:	Solid Separation
SolidsSeparMethod:	Recovery to UF
SolidsToUFReqd:	95%
UFSolidsMethod:	UF Solids Fraction
UFSolidsFracReqd:	60%

Note: All values in the tables below are mass flows as t/h.

Description	Total Solids	Species A(s)	Species B(s)	Liquids
Feed	100	90	10	100
Example 1: No Bypass
Overflow	5	4.5	0.5	36.67
Underflow	95	85.5	9.5	63.33

Example 2: Overflow Bypass Species B Bypass = 10%
BypassSeparCalcs NOT enabled (Overflow satisfies requirements)
Overflow	5	3.64	1.36	36.67
Underflow	95	86.36	8.64	63.33
BypassSeparCalcs enabled (Internal stream 1A satisfies requirements) see Note in Bypass Species
Overflow	5.95	4.5	1.45	37.3
Underflow	94.05	85.5	8.55	62.7
Example 3: Underflow Bypass Species B Bypass = 10%
BypassSeparCalcs NOT enabled (Underflow satisfies requirements)
Overflow	5	4.55	0.45	36.67
Underflow	95	85.45	9.55	63.33
BypassSeparCalcs enabled (Internal stream 2A satisfies requirements) see Note in Bypass Species
Overflow	4.95	4.5	0.45	37.3
Underflow	95.05	85.5	9.55	62.7
Example 4: Bypass to Both Species B: Bypass Fraction = 100% and Fraction to Overflow = 50%
BypassSeparCalcs NOT enabled Under and Over flow streams satisfy requirements)
Overflow	5	0	5	36.67
Underflow	95	90	5	63.33
BypassSeparCalcs enabled (Internal streams 1A and 2A satisfy requirements) see Note in Bypass Species.
Overflow	9.5	4.5	5	43
Underflow	90.5	85.5	5	57

Data Sections

Summary of Data Sections

GeneralSeparator tab - Contains general information relating to the unit.
Separ - Contains configuration and results of separating material to the underflow and overflow outlets.
PC - Optional tab that is visible if EB.Solubility.On and/or [email protected] is enabled in the Evaluation Block.
RB - Optional tab, only visible if the Reactions are enabled in the Evaluation Block.
EHX - Optional tab, only visible if the EnvironHX is enabled in the Evaluation Block.
Evap - Optional tab that is visible if Evaporator is enabled in the Evaluation Block.
VLE - Optional tab, only visible if the VLEquilibrium is enabled in the Evaluation Block.
MU - Optional tab, or multiple tabs if more than 1 Makeup is selected. Only visible if one of more Makeups are enabled in the Evaluation Block.
DB - Optional tab, or multiple tabs if more than 1 Discard Block is selected. Only visible if one of more Discard Blocks are enabled in the Evaluation Block. Only available in Build 138 or later.
QFeed - Optional tab, only visible if ShowQFeed is enabled. This page shows the properties of the mixed stream as the feed to the separator.
- This is before any Evaluation Block models are evaluated.
QUF - Available from Build 139. Optional tab, visible if ShowQUF is enabled. This and subsequent tab pages, e.g. QUF.. and Sp, shows the properties of the Underflow stream. The tags in the QUF tab are valid even when the ShowQUF option is not selected.
QOF - Available from Build 139. Optional tab, visible if ShowQOF is enabled. This and subsequent tab pages, e.g. QOF.. and Sp, shows the properties of the Overflow stream. The tags in the QOF tab are valid even when the ShowQOF option is not selected.
Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
Links tab, contains a summary table for all the input and output streams.
Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.
Additional fields are shown in Dynamic mode. These are described in the Tank - Dynamic Mode.