Precipitation3

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Contents 1 General Description 2 Diagram 2.1 Cooler Options 3 Thermal Modelling 3.1 Thermal Override 3.2 Thermal Loss Method 3.3 Ambient Interactions 3.4 Bypassing 3.5 Evaporation 4 Inputs and Outputs 5 Model Theory 6 Data Sections 6.1 Precipitator3 Tab 6.2 Precip Tab 6.3 Cooler Tab 7 Hints and Comments 7.1 General Configuration Hints:

General Description

The Precipitator is modelled as a continuous well-mixed tank reactor with Gibbsite precipitation. The model may be configured either as a simple tank with reactions, or it may use a Yield Model to calculate the Gibbsite Precipitation. If the Yield Model is selected, the unit uses Rosenberg-Healy[1] equation to calculate the alumina equilibrium concentration at the tank conditions.

The Precipitator expects the feed to be Bayer Liquor and precipitates Gibbsite, Al[OH]₃(s). There is the option to include the precipitation of bound soda either as NaOH*(s) or Na₂O(s). With bound soda precipitation it is also possible to have a fraction of the total bound soda precipitate as bound organics, Na₂C₅O₇*(s).

The model calculates yield, residence time, mass of bound soda precipitated with the Alumina, and the change in species concentrations and temperature in the tank. The Precipitator receives up to 10 feed streams and calculates a single product stream. The model does not take into account by-pass flow.

The model has options to include heat transfer to coolers, evaporation and environmental heat loss. The user may also specify additional reactions using the Reaction Block. Heat of crystallization for Gibbsite precipitation, heats of reaction for any additional reactions and heat transfer with evaporation, environment loss or coolers are all solved simultaneously to calculate outlet stream properties.

It is also possible to bypass some fraction of the input stream directly to the product stream: this simulates a poorly mixed tank.

Notes:

• The Precipitation3 model is designed to operate with the Alumina 3 Bayer Species Model and will NOT operate with the Generic Bayer Species Model (Alumina1).
• The Precipitation Projects, which are distributed with SysCAD in the Examples Folder, demonstrate the use of this model in a SysCAD project.

Diagram

The diagram shows the default drawing of the Precipitator, with the required connecting streams.

The physical location of the connections is not important; the user may connect the streams to any position on the drawing. When the user inserts a Precipitator into a flowsheet, he may choose a different drawing from a pull down menu.

Cooler Options

User can select from the following 3 options:

1. None - no cooling is applied.
2. Internal - Internal cooling models the use of draft tube coolers, where cooling water is used to directly cool the contents of the tank. For internal cooling, at least one Cool_in and one Cool_out streams must be connected for cooling water supply and return.
   • This option also includes the case where there is an external heat exchanger (such as a cooler mounted outside the tank) but considers this integral to the model, so that the cooling calculations are done in parallel with the precipitation calculations. In this case the liquor flow to the heat exchanger needs to be specified.
3. External - External cooling models the use of external heat exchangers taking a sidestream of the tank contents and cooling this sidestream. User connects a side stream (Cool_out IO) from the Precipitator to an external heat exchanger, where this side stream is cooled then returned to the Precipitator via the Cool_in IO.

NOTES:

1. The difference between the two cooling methods: in internal cooling, an external coolant stream is connected to and from the unit model, taking water, while in external cooling, the stream takes liquor from, and returns it to the unit model.
2. Both internal and external cooling require that the cooling connections be made. The unit will not evaluate unless they are present. If a simplified cooling model is required, just use direct thermal loss via ThermalLossMethod
3. The external cooling method ignores the actual contents of the incoming stream, ie it does not mix it with the tank contents. It simply determines the cooling load based on the outgoing and incoming temperature difference. If this stream is changed, say by mixing with cool liquor from another source, then the results will be unreliable.
   • In this case you should take a sidestream from a splitter on the outlet stream, cool this, then return this to the tank inlet along with any added liquid. External cooling allows you to use a specific exchanger model in the simple case of a single stream being returned directly to the tank so that you can get reporting information from the exchanger.

Thermal Modelling

There are a number of options related to thermal energy balance. Normally, the unit model will calculate HOR, cooler losses and so on, then apply a correction to the tank temperature based on these determinations. However in some circumstances it may be desirable to simply specify the final tank temperature directly, either as a temperature drop from the liquor inlet temperature, or even as a fixed final temperature. These cases may be physically unrealistic and should be used with care, however the ability to override the internal thermal calculations is there.

Thermal Override

If Thermal Override is selected, then other thermal calculations have no effect. Cooler calculations are still performed, so that the performance of the coolers is still available, but they do not affect the actual final temperature.

Thermal Loss Method

An additional thermal loss (or gain) may be specified by this selection, which is only available if Thermal Override is disabled. It is applied in addition to any cooler or reaction heats. Either a fixed temperature drop, fixed thermal loss, or ambient loss may be specified. A fixed thermal loss can be used to model internal or external cooling in a simplified fashion.

Ambient Interactions

The unit also allows thermal and mass loss to the environment. Thermal losses are due to convective cooling on the tank and free surface. There can also be mass losses due to evaporation at the free surface for an open tank; this mass loss is accompanied by a thermal loss due to evaporative latent heat.

There is a simple ambient loss model, where evaporative losses are proportional to the temperature difference, and a more detailed model which accounts for wind speed effects as well, for those refineries located in places exposed to icy winds from the North Sea. The more detailed model calculates a heat or evaporation loss per unit area, so the actual tank geometry is needed. For evaporation, the exposed surface area is required (this might not be the cross sectional area, since covered walkways may reduce wind driven evaporation.) For thermal loss, the total exposed tank surface area is needed, this includes both the top and side areas. These can be entered directly or calculated from tank radius and height in a PGM. The correlation is developed from observations on a large number of tanks in different plants.

If evaporation loss is implemented a stream may optionally be connected to accumulate the lost vapor.

Bypassing

If a precipitator is poorly mixed, then some of the input stream may never enter the tank and simply bypass directly to the outlet. This typically occurs when row tanks are connected by launder weirs which are simply open overflow channels running from one tank to the next. The model allows for bypassing by specifying a fraction of the incoming flow to pass directly to the outlet.

Evaporation

Losses of water due to evaporation are import and these also lead to significant overall thermal losses because of the latent heat of evaporation. Evaporation may be specified as a constant mass rate, proportional to ambient temperature difference. We have added a new model which accounts for effects of wind speed as well.

Inputs and Outputs

Label	Input / Output	No. of Connections		Description
		Min	Max
Feed	In	1	10	Precipitator Feed stream(s).
Product	Out	1	1	The product stream from the Precipitator.
Cool In	In	0	5	Optional Cooling Water input(s) to the Precipitator cooler / Cooled side stream return to Precipitator.
Cool Out	Out	0	1	Optional Cooling water return from the Precipitator cooler / Hot side stream drawn from Precipitator .
Evap	Out	0	1	Optional Evaporation outlet.

Model Theory

This has been moved to a separate section on its own:

Alumina3 Model Theory

Data Sections

The default access window consists of two sections,

1. The Precipitator3 tab allows the user to switch on optional sub models, such as cooling, additional reactions, evaporation or heat loss. It also contains a summary table for the precipitator.
2. Precip Tab allows the user to select and enter parameters for the growth method. It also contains a table for Thermal and Mass Balance.
3. Cooler - Optional Tab, allows the user to specify cooling options.
4. Evap - Optional Tab, allows the user to specify simple evaporation options.
5. RB - Optional tab, only visible if the Reactions are enabled in the Evaluation Block.
6. QFeed - Optional tab, only visible if ShowQFeed is enabled. This page shows the properties of the mixed stream (see Material Flow Section) as the feed to the tank.
   • This is before any Evaluation Block models or the HX are evaluated.
7. QProd - Optional tab, only visible if ShowQProd is selected. This page shows the properties of the mixed output stream (see Material Flow Section) as the product before separation to outlet streams of the tank.
   • This is after any Evaluation Block models or the HX are evaluated, but before flow splits.
   • This tab is only available for ProBal mode.
8. The Info section contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents. This is fully described in Common Data Sections.
9. Links tab, only visible in SysCAD 9.2, contains a summary table for all the input and output streams.
10. Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Precipitator3 Tab

Tag / Symbol	Input / Calc	Description/Calculated Variables / Options
Common First Data Section
Requirements
On	Tick Box	This can be used to take a precipitator off line. When a precipitator is not ON, the input stream will act as though it has bypassed the precipitator, thus no change will occur in this unit. This option is useful for feasibility studies of flowsheet configuration.
Cooling	None	No cooling required.
	Internal	Allows internal cooling, a Cooler tab becomes visible and can be configured. See Cooler Options for more information.
	External	Allows external cooling, a Cooler tab becomes visible and can be configured. See Cooler Options for more information.
Reactions	List box	This can be used to switch on reactions in the unit. If this is On, RB becomes visible and may be configured. Note: The user does not have to configure a reaction file, even if this block is On.
UseLastConverged	Tick Box	If selected, will restart solution iteration at last converged state. If the solution is nearly converged, this will speed up the iteration. If the solution was converged, and nothing else is changed it should immediately converge.
ShowQFeed	Tick box	Switches on the QFeed tab pages to display the total feed stream properties into the Precipitator. This is useful if more than one Feed streams are connected to the precipitator. See Material Flow Section.
ShowQProd	Tick box	Switches on the QProd tab pages to display the product stream properties from the Precipitator. See Material Flow Section.
Bypass	Tick box	Allows bypassing of feed directly to outlet.
BypassFraction	Input	How much of the feed to bypass, visible if Bypass is on.
ShowTankContents	Tick box	If ticked, the tank contents rather than the actual outlet stream will be displayed in the QProd tab
ThermalOverride	None	Don't override.
	TempDrop	Overall heat loss based on Temperature Drop. Note that calculated HOR and cooling effects are ignored as this temperature drop is applied as an overall override.
	ProductT	Specify the product temperature.
ThermalLossMethod	None	No additional heat loss to the environment or thermal balance overrides.
	TempDrop	Additional heat loss based on Temperature Drop. In this case the temperature drop is applied on top of HOR and cooling effects.
	FixedLoss	Additional heat loss is expressed as a fixed amount of energy.
	Ambient	Additional heat loss is expressed as Energy/degree of temperature difference to ambient.
	Wind	More detailed ambient model accounting for wind speed
Temp_Drop	Input	The temperature drop required, visible with the TempDrop Method.
ThermoLossRqd	Input	The amount of energy to be lost to the environment, visible with the FixedLoss method.
ThermoLossAmbient	Input	The amount of energy per degree to be lost to the environment, visible with the Ambient method.
Tank Surface Area	Input	Tank external surface area including both top and side , visible if WindAmbient option for thermal loss is selected.
Evaporation	None	No evaporation loss to the environment.
	Fixed	Fixed evaporation rate. Suggested values are in the range 0.25 to 1.0 t/h.
	Ambient	Fixed evaporation rate per degree of temperature. Suggested values are in the range of 0.005 to 0.025 t/h.K.
	Wind	Detailed Evaporation model accounting for wind and temperature.
Evap.Rate	Input	The evaporation rate required, visible with the Fixed Method.
Evap.Per.degK	Input	The evaporation rate required per degree of temperature, visible with the Ambient method, or overall constant for detailed model.
WindEvapRateK	Input	The evaporation rate required per unit area, visible if Wind option for evaporation is selected.
Surface Area	Input	Exposed tank surface area, visible if Wind option for evaporation is selected.
TankVol	Input	The precipitation tank volume, used to work out the residence Time.
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	Calc	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 Tank
Residence Time	Calc	The calculated residence time of the slurry in the unit.
SuperSat	Calc	The Supersaturation = Product A/C divided by Equilibrium A/C = (A/C) / (ASat@ProdcutT / C@25).
Yield in grams Al2O3 per liter liquor @ 25C
Yield	Calc	The calculated Yield = Gibbsite precipitated as equivalent Alumina per unit volume of feed liqour at 25°C.
THA.Precip	Calc	The mass of Trihydrate Alumina Al[OH]3 precipitated in the unit.
Solids.Precip	Calc	The mass of solids precipitated in the unit, includes THA and bound soda.
Solids.Conc	Calc	The solids concentration in the unit referenced to 25°C.
Results
MassFlowIn	Calc	The mass flowrate at the inlet conditions.
MassFlowOut	Calc	The mass flowrate at the outlet conditions.
VolFlowIn	Calc	The volumetric flowrate at the inlet conditions.
VolFlowOut	Calc	The volumetric flowrate at the outlet conditions.
TempIn	Calc	The inlet temperature.
TempOut	Calc	The outlet Temperature.
ACin	Calc	The A/C ratio at the inlet conditions.
ACout	Calc	The A/C ratio at the outlet conditions.
ACequil	Calc	The Asat/C ratio at the outlet conditions.
BoundSodaFrac	Calc	The bound soda precipitation rate as Na2O / THA precipitation rate as Al2O3.
BoundSodaPrecip	Calc	The soda precipitation rate less organic portion as NaOH*(s)
BoundOrganicsPrecip	Calc	The soda precipitation rate - organic portion as Na2C5O7*(s)
Precip Tab
Precip.Method	List box	(1) User SSA - User specified seed surface area for the growth calculations. (2) Stream SSA - Uses the specified seed surface area estimated from the input stream conditions.
Hydrate Precipitation
GrowthMethod	Fixed	User specifies a fixed alumina precipitation rate.
	White-Bateman	The alumina precipitation rate is calculated using the white correlations [2]
	SSA Yield	The alumina precipitation rate is calculated taking into account factors of free caustic, total organic carbon, soda concentration and SSA.
BoundSodaMethod (Not visible with the Fixed GrowthMethod)	original	Uses the Ohkawa, Tsuneizumi and Hirao [5] correlation, see Bound Soda Theory for information.
	Hunter	Armstrong, Hunter, McCormick and Warren. [7] correlation, see Bound Soda Theory for information.
	Fixed %	calculates a fixed percentage of soda co-precipitation with the precipitated hydrate.
Convergence.Limit	Input	Global tolerance for testing convergence for iterative calculation in all precipitator tankss. Default is 1.0e-8.
Iterations	Calc	Number of iterations solved.
Thermal.Damping	Input	Damping for energy convergence. Default of 0.
Mass.Damping	Input	Damping for mass convergence. Default of 0. This value may need to be increased, often significantly 80% plus, for a precip tank with a significant change. Try increasing this when failed to converge error message is shown.
Volume.Damping	Input	Damping for volume flow convergence. Default of 0.
UserHOR	tick box	Option for User to enter Heat of Reaction for Gibbsite Precipitation reaction (kJ/kg-Gibbsite). The default value is -252.3 kJ/kg-Gibbsite at 0°C. NB this is an exothermic reaction and energy is released as Gibbsite precipitates. For reference, please refer to Heat of Dissolution of Gibbsite and Boehmite
User.GibbsiteHOR@0C	Input	The Gibbsite precipitation HOR value to be used. Only available when UserHOR is selected. NB a positive entry generates a warning message as the HOR shold be negative. For reference, please refer to Heat of Dissolution of Gibbsite and Boehmite
GibbsiteHOR@0C	Calc	The Gibbsite precipitation HOR value used. -252.3 kJ/kg-Gibbsite at 0°C if UserHOR is not selected. For reference, please refer to Heat of Dissolution of Gibbsite and Boehmite
AdjustProdSSA	tick box	Adjust product stream SSA to account for particle growth (using spherical model). If unchecked, the product SSA will be the same as the feed SSA.
SSA	Input	The seed surface area, visible when UserSSA is ticked.
SSAin	Calc	The current SSA value from feed stream.
SSAused	Calc	The SSA value used in the calculations as per the above inputs.
Variables for the Fixed GrowthMethod
Precip.Rate	Input	The user specified precipitation rate.
BoundSodaFrac	Input	The user specified bound soda fraction.
Variables for the White-Bateman GrowthMethod
ER_White	Input	The Activation Energy (E) divided by the Gas Constant (R).
K_White	Input	The Constant used in the Growth Rate Factor correlation.
gF_White	Input	This allows the user to tune the growth rate based on a factor.
Variables for the SSA Yield GrowthMethod
ActivationEnergy/R	Input	The Activation Energy (E) divided by the Gas Constant (R).
K_0	Input	The Constant used in the Growth Rate Factor correlation.
k_TOC	Input	Organics term =
n_s	Input	Total Soda effect = Sn_s. The default value for n_s is 1.0. Using zero for ns will remove the precipitation dependence on soda.
n_fc	Input	Free Caustic – Free caustic is the sodium hydroxide in solution that is not associated with dissolved alumina. KFreeCaustic = FCn_fc. The default value for n_fc is 0.5. Using zero for n_fc will remove the precipitation dependence on free caustic.
n_C	Input	Caustic term Cn_C. The default value is zero.
n_AC	Input	This is the n factor in the equation . The default value is 2
n_ssa	Input	Specific Surface Area, SSA – this correction accounts for the fact that the effective surface area for precipitation may not scale linearly with SSA. Kssa = SSAnssa. The default value for nssa is 1.0 (making precipitation rates linearly proportional to SSA).
Bound Soda Calculations
K_tuneBS	Input	The tuning factor for the bound soda calculation.
K1	Input	The soda factor. The default value is 1.27*10-3.
E_SODA	Input	Constant used in the bound soda calculation, default is 2535 K-1
Bound Soda Fraction as Organics
BoundSodaSpecies	Text	Which soda species (Na2O or NaOH*) is used for occlusion.
BoundSoda_OrgPart	Input	The percentage of bound soda precipitated as organics.
Thermal and Mass Balance
Mass.Flow.In	Calc	The mass flow in to the precipitator
Mass.Flow.Out	Calc	The mass flow out of the precipitator
Evap.Mass.Loss	Calc	The evaporation mass rate
Evap.Thermal.Loss	Calc	The amount of energy lost through evaporation.
Env.Thermal.Loss	Calc	The amount of energy lost to the environment.
Cooler.Thermal.Loss	Calc	The amount of energy transferred through the cooler.
Total.Thermal.Loss	Calc	The sum of all thermal losses.
ReactionHeat@0	Calc	The amount of energy released by the precipitation reaction at the 0dC temperature.
ReactionHeat(@T)	Calc	The amount of energy released by the precipitation reaction at the product temperature.
Total.Thermal.Loss	Calc	Total amount of energy loss: Evap + Env + Cooler.
Stream Enthalpy
HzIn	Calc	Enthalpy flux into Precipitator.
HzEvap	Calc	Enthalpy flux to Evaporation.
HzOut	Calc	Enthalpy flux out of Precipitator.
HzBal	Calc	Enthalpy out minus enthalpy in. This is the net of heat transfer, evaporation loss and reaction heat (NB Rxn heat measured at 0°C reference temperature).
FeedHf	Calc	The total energy of the feed.
ProdHf	Calc	The total energy of the product.
EvapHf	Calc	The total energy of the material evaporated from the precipatator, if evaporation is used.
Cooler Tab
FOR INTERNAL COOLING OPTION
Cooler.On	tick box	Switches the cooler on/off
Cooling.Type	Fixed.dQ	User specifies the energy change required.
	Fixed.dT	User specifies the temperature change required.
	HeatExchange	User specifies the HTC, Area and flow for heat exchange.
Variables for the Fixed.dQ method
dQ	Input	The user specified change of energy.
Variables for the Fixed.dT method
dT	Input	The user specified change of temperature.
Variables for the HeatExchange method
HX.Area	Input	The user specified heat transfer area.
HX.HTC	Input	The user specified heat transfer coefficient.
By.Vol.Flow	tick box	Slurry to "cooler" can be specified in mass or volumetric flows
Heat Exchanger
Cooling.Flow	Input / Calc	The slurry mass flow to be cooled by the "cooler".
Int.Vol.Flow	Input / Calc	The slurry volumetric flow to be cooled by the "cooler".
Hx.UA	Calc	heat exchanger UA.
Hx.LMTD	Calc	heat exchanger log mean temperature difference.
Coolant
Water.Flow	Calc	Displays the CW mass flow into the "cooler" via the Cool_in connection.
Water.Vol.Flow	Calc	Displays the CW volumetric flow into the "cooler" via the Cool_in connection.
Water.Tin	Calc	Displays the CW temperature into the "cooler" via the Cool_in connection.
Water.Tout	Calc	Displays the CW temperature out of the "cooler" via the Cool_out connection.
Liquor.Tin	Calc	The slurry temperature into the "cooler".
Liquor.Tout	Calc	The slurry temperature out of the "cooler".
Cooling.Rate	Calc	The rate of cooling.
FOR EXTERNAL COOLING OPTION
By.Vol.Flow	tick box	Slurry to "cooler" can be specified in mass or volumetric flows
Ext.Vol.Flow	Input / Calc	Input or displays the volumetric flow into the external cooler via the Cool_in connection, depends on the By.Vol.Flow option.
Ext.Cooling.Flow	Input / Calc	Input or displays the mass flow into the external cooler via the Cool_in connection, depends on the By.Vol.Flow option.
Ext.Cooling.Temp	Calc	The temperature of the "cooled" side stream returning to the precipitator.
Ext.Cooling.TotHzOut	Calc	The total energy of the side stream leaving the precipitator.
Ext.Cooling.TotHzIn	Calc	The total energy of the side stream returning to the precipitator.
Cooling.Rate	Calc	The rate of cooling.

Hints and Comments

General Configuration Hints:

1. The species model, which the Precipitator uses, is important for the calculations. The feed streams to the precipitator must be configured with a Bayer3 species model for the calculations to be valid. For documentation on Bayer3 properties, see Alumina 3 Bayer species model
2. The user may find it more efficient not to use the SSA Yield model when first setting up a complex Alumina Flowsheet, but to set reaction extents for the Gibbsite and soda precipitation. Once the flowsheet is controlled and the Alumina and Caustic concentrations around the plant are close to the expected values, the Yield model may be implemented.
3. If using the yield method, the feed to the first precipitator in a precipitation train (either fresh or recycle) should contain some seed. Using the SetData method can modify the seed surface Area. See below for Hints on Setting the Alumina Particle Size Information.
4. Compound Al[OH]₃(s) must be present as this is the solid precipitated by the SSA Yield model.
5. Compounds NaOH*(s) and Na₂C₅O₇*(s) must be present as these are the bound soda compounds.
6. A precipitator tank may be turned off or effectively removed from a precipitation circuit by unchecking the "on" tick box in its access window. The tank will then pass any feed through with no changes to it. Alternatively, the user may set the volume of the Precipitator (V) to 0.0 to effectively remove it from the circuit.
7. Occasionally a Precipitator3 tank will return the error "No convergence of precip routine: try increased damping". In the first one or two tanks where there is the most precipitation, damping is sometimes required to achieve convergence. To add damping - go to the access window for the tank, turn on "show all fields" and at the top of the precip tab you will see three damping options. Set the mass damping to 50% and that should take care of the problem.
8. If you have a tank with large residence time, high supersaturation and lots of cooling, the calculated precipitation rate may remove most of the aluminate from solution, causing numerical instability. We are looking at other ways of stabilizing the solution, but mass damping allows the precipitation to move the tank to the actual saturation in several steps rather than overshooting. In extreme cases, try setting damping as high as 99%.