Sugar Fugal

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General Description

The sugar fugal unit operation is available with the Sugar add-on.

The sugar fugal is a type of centrifuge used to separate and wash molasses from sugar crystals. Massecuite is loaded into a porous walled basket in the fugal which is then spun at high speeds. The crystalline sugar is retained in the basket and centrifugal force pulls the molasses out of the sugar crystals and through the basket walls where it is collected and removed. Water sprays may be used to wash further molasses from the sugar crystals during the process. Some sugar crystal may be dissolved by the wash water during this stage of the operation. The sugar is then plowed out of (removed from) the basket leaving a small fraction of the sugar on the walls. A basket wash spray is used to dissolve this remaining sugar and the the cycle is then repeated. The purged molasses, washings and basket washings may be collected into a single stream leaving the fugal or they may be split into a high and low purity stream leaving the fugal.

In reality a sugar fugal is a batch device. However, the cycle times are generally short (typically about 200 seconds) so the fugal may be approximated as a continuous operation for steady state modeling purposes.

The incoming feed to a fugal is a slurry of sugar crystals and mother molasses. The sugar crystal is pure or almost pure sucrose. The molasses is an aqueous mixture of water, sucrose and impurities. Virtually all of the impurities in the slurry are in the molasses. As molasses is removed, the massecuite purity of the cake increases. Thus the purity of the cake is a measure of how much of the mother molasses has been retained with the cake. The purity of the molasses may also increase as sugar crystal is dissolved when wash water is added. Thus molasses purity is a measure of the extent of dissolution of sugar. In this model it is assumed that there is perfect mixing of the wash water and the molasses during the wash phase. In practice there may be other effects occurring which affect the action of the wash water - wash water may bypass through the cake, wash water may displace some of the molasses from the cake and remain in the cake or there may be inhomogeneities in the molasses remaining with the crystal. These effects have not been included in this model.

The Sugar Fugal process is described by the following steps;

Purge Molasses is removed by centrifugal force in a purge step. In this stage only molasses is removed and all crystal is assumed to remain in the basket along with some fraction of the mother molasses. The purity of the sugar crystals (cake) rises as impurities are removed with with molasses. The amount of molasses retained with the cake determines the massecuite purity (NB the measure of cake purity is always the massecuite purity).

Wash 1 An Optional Wash Stage is used to wash out more of the molasses. In this stage it is assumed the wash water mixes perfectly with the molasses and that some sucrose crystal may dissolve. As this happens, the purity of the cake increases as molasses is removed and the purity of the molasses increases with sugar going into solution. The wash process is described by the amount of molasses removed from the cake and the extent of dissolution of sugar crystal.

The amount of impurities remaining in the cake may be controlled by (1) specifying the cake purity rise, (2) specifying the target cake purity, (3) specifying the fraction of impurities retained or (4) entering constants for an equation which calculates the fraction of impurities retained.

The dissolution of sugar crystals may be specified by (1) the purity rise in the molasses or (2) by specifying the level of sucrose supersaturation in the molasses. The saturation measure used here is the supersaturation coefficient which is the actual sucrose to water ratio divided by the equilibrium sucrose to water ratio (at the same temperature and impurities to water ratio). An energy balance is solved simultaneously for the final temperature taking into account the incoming temperatures of the sugar and wash streams, the enthalpy of dissolution for any dissolving sucrose and any heat loss to the environment. After mixing and dissolution of wash water, molasses is removed until either a user specified cake purity or user specified cake purity rise is achieved. When no wash is connected, there is no wash action and this step is skipped.

Wash 2 A second Optional Wash Stage - when a second wash is connected, the process works the same as the first wash stage described above.

Basket Wash A user specified fraction of the sucrose crystal mass is assumed to remain in the basket after plow out. A fraction of the washed crystal and molasses mixture from the last wash stage is retained in the basket to give the required crystal mass. The rest of the washed crystal/molasses mixture leaves the unit as product sugar. Basket wash water is sprayed onto the basket to dissolve the remaining sugar and the wash water sugar solution leaves as basket washings. The solubility of sucrose in the wash water is calculated. Generally the extent of dissolution should be 100% and all sucrose dissolved. However, if there is insufficient wash water or if the wash water is too cold, sugar will only dissolve to the limit of solubility. If this happens the basket washings will contain some sucrose crystal and a warning will be displayed.

Sugar Outlet Connection A sugar crystal product stream is always required for the washed sugar crystals.

Molasses Outlet Connections There are two outlet connection configurations for the molasses and washings from the fugal. The molasses, washings and basket wash streams may be sent out as a single combined stream low purity molasses stream or they may be split between a low purity and a high purity molasses outlet stream (for example purge molasses goes to a low purity outlet stream and washings and basket washings to a high purity outlet stream). In the model, a low purity connection is always required and the high purity stream is optional.

Molasses Split When there are two molasses outlet connections, there is also the option to control the sharpness of the cut between the outlet streams. This accounts for the fact that there is always some material retained in the trough and some mixing between the streams from the various stages of the process. In the model this is specified by the separation efficiency. For example, if the separation efficiency for the purge stream is 90%, then 90% of the purge flow will go to the low purity outlet connection and 10% will go to the high purity outlet connection.

Vapors Any vapours in the input streams are sent to the vent stream - a warning is displayed if there are vapours in any of the input streams or if there is evaporation and there is no vent connected.

Pressure The exit pressure of all streams is atmospheric pressure (NB the atmospheric pressure that is specified in the project - see Plant_Model#Environment_Tab).

Heat Loss The sugar fugal model includes optional heat loss to the environment and there are several different heat loss models available. The wash water is typically hotter than the massecuite and is sprayed onto the sugar. The environmental heat loss is taken from the wash water before it mixes with the massecuite. The total heat loss is calculated and then taken proportionally from the wash water streams based on their relative mass flows.

Evaporation The sugar fugal also includes the option to specify an evaporation rate from the wash water. The overall evaporation rate is taken from the individual wash water streams in proportion to relative mass flow. Heat loss and evaporation are included in the energy balance.

Input and Output Connections

A number of the inputs and outputs for the sugar fugal are optional. You connect the ones that you want to use depending on how you want the unit to operate. NB it is possible to have some of the output connections in place and direct the flows elsewhere so that they are effectively unused.

Label Required / Optional Input /Output Number of Connections Description
      Min Max.  
FugalFeed Required In 1 5 Massecuite Feed to the Fugal.
WashWater1 Optional In 0 5 Wash Water for Wash Stage 1.
WashWater2 Optional In 0 5 Wash Water for Wash Stage 2.
BasketWash Required In 1 5 Wash Water for Basket Wash Stage.
LowPurityMol Required Out 1 1 Lower Purity Molasses from Fugal.
HiPurityMol Optional Out 0 1 Higher Purity Molasses from the Fugal.
Sugar Required Out 1 1 Purged and Washed Sucrose Crystal.
Vent Optional Out 0 1 Vent outlet containing all Vapors - NB required if there are vapors present.

Behavior when Model is OFF

The sugar fugal may be turned Off by deselecting the On tick box in the access window. When the unit is off the following behavior occurs:

  • If the unit has a valid feed stream (some mass flow and sucrose crystals present), then all material will flow through to the Product Sugar Stream with no changes or heat exchange.
  • If the feed stream is not valid (no mass flow or no sugar crystals), then all material will flow through to the Purge Molasses stream with no changes or heat exchange.

NB any gases in either feed or wash water streams will remain with those streams when the unit is off.


Environmental Heat Exchange Options

The sugar Fugal may lose heat to the environment. The heat loss is taken from the wash water streams as they enter the unit. The heat loss options are set from a drop down list and include;

  1. None - No Heat Exchange occurs.
  2. FixedLoss - A Heat Flow Rate to the Environment is specified by the user.
  3. Ambient - A Heat Loss Constant is specified and heat loss is calculated as heat loss constant times the temperature difference between the product stream and ambient temperatures, Qloss = Constant * (Tprod - Tamb).

The Heat Flow is displayed for all options and heat flow from the fugal to the environment is positive in sign.

NB If a negative heat flow is reported, it indicates heat flow to the fugal from the environment.

Evaporation Loss Options

There may be significant evaporation from a sugar fugal when hot water is used for washing. The model has an option to allow evaporation. An evaporation loss rate, EvapLossQm, may be entered and this quantity of water will be split from the wash water streams coming into the unit and flashed to vapor. The vapor will flow out through the vent (along with any other vapors that enter the unit). Although the evaporation mass rate will generally be small, the main effect of evaporation will be to cool the wash water.

Evaporative losses are assumed to occur from the wash water sprays. The specified amount of vapor is made up from each of the wash streams in proportion to its mass flow. The phase change energy is taken into account along with any environmental heat loss to recalculate the wash water temperature before it enters the unit. An estimate of the energy loss due to evaporation is displayed when evaporation is occurring.

Model Theory

The model follows the flow of massecuite through the different fugal stages. The process stages are;

Feed - All feed streams to the basket mix perfectly.

Purging - The massecuite in the basket is spun and molasses is removed from the sugar crystal. There is assumed to be no dissolution of crystal sugar during this stage. As molasses is removed, the massecuite purity increases. The extent of molasses removal is determined by specifying either the final massecuite purity or the purity rise.

Washing Stage 1 - This stage is optional. When in use, wash water is sprayed onto the sugar in the basket to displace molasses remaining on the sugar after the purging stage. The wash water stream is adjusted for any heat loss or evaporation before it enters the sugar bed. The molasses in the cake and wash water are assumed to mix perfectly. During this stage some of the sugar crystal may dissolve. As sugar dissolves the purity of the molasses increases (sugar is added to the solution but no impurities). The extent of dissolution may be defined so that molasses stream leaving has a user specified supersaturation coefficient or the user may specify an molasses purity rise and sugar crystal will dissolve to achieve this target. All solids are assumed to remain with in the basket and only liquids leave in the molasses stream. Molasses is removed from the sugar bed until the target massecuite purity is achieved. The massecuite in the basket and the molasses stream are in thermal equilibrium at the end of the stage. NB Wash water coming in may actually have any composition, for example it could be a dilute molasses stream.

Then the equation impurities retained equation option is selected, the equation used is;


[math] \mathrm{ImpurityRetention(\%}) = 100 - k_1 \times \left(1-e^{-k_2 X}\right) [/math]

[math] \text{where } [/math]

[math] X = \text{is wash water mass percent on Massecuite} [/math]

[math] k_1 = \text{defines limits for impurities removal by washing} [/math]

[math] k_2 = \text{defines sensitivity to initial application of water}[/math]


Some typical values for the constants are [ref: Wright, Broadfoot and Miller, "A model of Batch Sugar Centrifugal Productivity", Proceedings of Australian Society of sugar Cane Technologists, 1995]:

[math]\begin{matrix} \text{Massecuite} &k_1 &k_22\\ \text{A Massecuite} &83\text{ to }91 &1.8\text{ to }2.2\\ \text{B Massecuite} &62\text{ to }76 &1.0\text{ to }1.8 \end{matrix}[/math]



Washing Stage 2 - This stage is optional. This stage works the same as stage 1 washing above with massecuite purity of the remaining sugar and sucrose saturation of the molasses specifying the performance of the stage.

Basket Wash - During the basket wash stage most of the sugar is plowed out of the basket and goes off as the product stream. Basket wash water is then sprayed in to dissolve any remaining sugar. The mass percentage of sugar crystal remaining after plow out is an input parameter. The wash water and remaining sugar are assumed to mix perfectly and the sugar crystal dissolves until it is all in solution or until it reaches the sucrose solubility limit. This stream then leaves as Basket Washings. If there is not enough water to dissolve all of the sugar a warning message is displayed.

Outlet Connections - The sugar output stream is the sugar Crystal that is plowed out of the basket after the centrifuging. The vent connection is optional and only required when there are gases present (either int he feed streams or due to evaporation). There are two possible molasses connections - a low purity molasses stream which is a required connection and a high purity molasses steam which is an optional connection. The default case is to have only one molasses exit flow with all purge, wash and basket wash molasses flows going to it. When the high purity connection is present, the fraction of each of the internal stage flows to it may be specified. This allows the split of molasses to be modelled.

Outlet flow Splits - When there are two molasses outlet connections, there is not a sharp cut-over between the flows. For example some of the purge molasses may end up in the high purity stream. The access page for the fugal shows the stage flows and the outlet connection flows. The stage molasses flows are the flows from each of the processes in the fugal cycle (purge, wash 1, wash 2 and basket wash) and the outlet molasses flows are the flows after the separation efficiency has been taken into account. All of the mixing processes take energy balancing into account.

Gases - Any gases present in any of the input streams are split off and go to the vent stream. If there is no vent attached, the gases are sent out with the purge molasses stream and a warning is displayed that there are gases present, but no vent.

NB the temperature of the different outlet streams may be different depending on wash water temperatures.

Control inputs are defined in terms of parameters that are normally measured in sugar factories.

Flow Chart

SugarFugal.jpg

Sugar Fugal Access Pages

SugarFugal

Unit Type: SugarFugal - The first tab page in the access window will have this name. User inputs and results are displayed on this page.

In general, the control inputs are defined in terms of parameters that are normally measured in sugar factories.

Tag / Symbol
Input / Calc
Description
Common Data on First Tab Page

Requirements

On Tickbox Tickbox used to turn the unit ON or OFF (off behavior is described above).
TrackStatus Tickbox Option to display warnings.

Environmental Heat Loss

Method None No heat exchange with the environment.
FixedLoss User specified heat loss rate.
Ambient Heat loss determined as a constant times temperature difference from ambient. Qloss = ThermalLossAmbient * (Tprod - Tamb)
ThermalLossRqd Input The Required Heat Loss Rate - This is only visible if the FixedLoss Method is selected.
ThermalLossAmbient Input The Heat Loss Rate Constant (kW/K) - This is only visible if the Ambient Method is selected.
EnvHeatLoss Display This field displays heat flow to the environment (positive value is heat flow from Fugalto Environment.).

Evaporation Loss

EvapLossQm Input User specified evaporation rate (set to zero for no evaporation).
EvapHeatLoss Display Heat loss due to evaporation (latent heat of vaporization).

Molasses Outlet Connections

Set Stage Fractions to High Purity Outlet

PurgeHPmol Input Fraction of Purge Molasses going to high purity outlet connection, balance to low purity molasses connection.
Wash01HPMol Input Fraction of Wash 1 Molasses going to high purity outlet connection, balance to low purity molasses connection.
Wash02HPMol Input Fraction of Wash 2 Molasses going to high purity outlet connection, balance to low purity molasses connection.
BsktWashHPMol Input Fraction of Basket Wash Molasses going to high purity outlet connection, balance to low purity molasses connection.

Purity

The second tab page in the access window. Input Parameters, results and calculated data are displayed on this page.

Tag / Symbol
Input / Calc

Purge

CakePurityControl Purity Rise User Specified Cake Purity Rise.
Target Purity User Specified Cake Final Purity.
FeedMassPurity Display Massecuite purity of the feed stream.
PurgePurityReq Input Required purity after purging (this controls the amount of molasses that remains in the sugar).
PurgeMassPurity Display Actual Prewash purity.
PurgePurityRiseReq Input Required Purity Rise of the Purge molasses stream.
PurgePurityRise Display Purity Rise of the Purge molasses stream.
PurgeImpurityFrac Display Impurity mass fraction of the Cake.
PurgeImpRetained Display Mass percentage of impurities retained in the basket with the sugar crystal after Purging (NB calculation includes any solid impurities present).
PurgeSolFraction Display Mass percentage of sugar crystal in the basket after purging.
FeedMolPurity Display Purity of Feed Molasses (NB molasses not massecuite).
PurgeMolPurity Display Purity of Purged Molasses.
PurgeMolSSNCoeff Display Supersaturation Coefficient of Purged Molasses.

Wash Stage 1 and 2

Inputs and outputs for wash stage 2 are the same for both wash stage 1 (substitute 02 for 01).

NB Wash 1 and Wash 2 are only displayed if there is wash water connection and there is a flow of wash water.

Wash control Options

WashCakeControl Purity Rise User Specified Cake Purity Rise.
Target Purity User Specified Cake Final Purity.
Impurities Retained User specified Impurities Fraction Retained.
Imp Retained Eqn User input Constants for Impurities Retained equation.
ImpRetK1 Input Impurities retained parameter k1 (only displayed with Imp retained Eqn option).
ImpRetK2 Input Impurities retained parameter k2 (only displayed with Imp retained Eqn option).
MolassesControl Purity Rise User Specified Molasses Purity Rise.
Target SSN User Specified Molasses Final Sucrose Supersaturation.
WashWaterRatio01 Display The Mass Ratio of Wash Water to feed massecuite (expressed as percent).
WashWater01Brix Display Brix of the Wash Water.
WashWater01Purity Display Purity of the Wash Water.
Wash01MassPurityIn Display Massecuite Purity into Wash Stage 1.
Wash01MassPurityReq Input Target Massecuite Purity Out of Wash Stage 1 (must be > Purity in).
Wash01MassPurity Display Actual Massecuite Purity out of Wash stage 1.
Wash01MassPurRiseReq Input Required Massecuite purity out of Wash stage 1.
Wash01MassPurityRise Display Massecuite Purity out of Wash stage 1.
Wash01ImpurityRet Display Mass fraction of Impurities retained in the basket through Wash Stage 1 (NB calculation includes any solid phases impurities).
Wash01SolidsFrac Display Mass percentage of sugar crystal in the basket after Wash 1.
Wash01MolPurity Display Molasses Purity into Wash 1.
Wash01MolPurRiseReq Input Molasses Purity Rise Required through Wash Stage 1.
Wash01MolPurRise Display Molasses Purity rise through Wash Stage 1.
Wash01SSN_Req Input Sucrose SuperSaturation Coefficient required in Wash Stage 1.
Wash01SSN Display Molasses Sucrose Supersaturation out of Wash Stage 1.
Wash01XtDissolved Display Mass Percentage of Sugar Crystal dissolved in Wash Stage 1.

Basket Wash

BsktWWtoMassRatio Display Mass ratio of Basket Wash Water to Feed Mass (expressed as percent).
BsktWashWtrBrix Display Basket Wash Water Brix.
BsktWashWtrPurity Display Basket Wash Water Purity.
BasketSugarFrac Input Mass Fraction of Solids remaining in Basket after plow out (expressed as percent).
BasketSugarXt Display Mass of Sugar remaining in Basket after plow out.
BsktXtDissolved Display Mass of sugar dissolved into basket washings NB should be the same as mass of sugar remaining when there is sufficient wash water.
BsktWashingsXt Display Sugar Crystal remaining in basket washings NB a warning will be displayed if there is any crystal in the basket washings.
BsktWashingsPurity Display Purity of the basket washings stream.
BsktWashingsSSN Display Sucrose Supersaturation of basket washings stream.

Data Access Page

Class: Data - The second tab page in the access window will have this name. Results and calculated data are displayed on this page.


Tag / Symbol
Input / Calc
Description

Product Crystal

FeedCrystal Display Mass Flow Rate of Sugar Crystal in Feed.
ProductCrystal Display Mass Flow Rate of Product Crystal.
CrystalDissolved Display Mass Fraction of Crystal Dissolved through Fugal.
ProductPurity Display Product Massecuite Purity.

Wash Water Mass Flows

WashWater01 Display Mass Flow of Wash Water to Stage 1 (before any evaporation).
WashWater02 Display Mass Flow of Wash Water to Stage 2 (before any evaporation).
BasketWash Display Mass Flow of Wash Water to Basket Wash (before any evaporation).

Stage Molasses Mass Flows

These are the properties of molasses from the different internal process stages - some streams may be combined before exiting the Fugal.

QmFeedMol Display Mass Flow of Molasses in Feed.
QmPurgeMol Display Mass Flow of Molasses from Purging.
QmWash01Mol Display Mass Flow of Molasses from Wash Stage 1.
QmWash02Mol Display Mass Flow of Molasses from Wash Stage 2.
QmBsktWash Display Mass Flow of Basket Washings.

Stage Molasses Purities

These are the properties of molasses from the different stages - some streams may be combined before exiting the Fugal.

PurityFeedMol Display Purity of Feed Molasses.
PurityPurgeMol Display Purity of Molasses from Purging.
PurityWash01Mol Display Purity of Molasses from Wash Stage 1.
PurityWash02Mol Display Purity of Molasses from Wash Stage 2.
PurityBsktWashMol Display Purity of Basket Washings.

Stage Molasses Brix

These are the properties of molasses from the different stages - some streams may be combined before exiting the Fugal.

BrixFeedMol Display Brix of Feed Molasses.
BrixPurgeMol Display Brix of Molasses from Purging.
BrixWash01Mol Display Brix of Molasses from Wash Stage 1.
BrixWash02Mol Display Brix of Molasses from Wash Stage 2.
BrixBsktWashMol Display Brix of Basket Washings.

Outlet Connection Molasses Flows

These are the actual outlet streams that are connected.

QmLPmolasses Display Mass Flow to Low Purity Molasses Connection.
QmHPmolasses Display Mass Flow to High Purity Molasses Connection.

Outlet Connection Molasses Purities

These are the actual outlet streams that are connected.

PurityLPmolasses Display Purity of Low Purity Molasses.
PurityHPmolasses Display Purity of High Purity Molasses.

Outlet Connection Molasses Brix

These are the actual outlet streams that are connected.

BrixLPmolasses Display Brix of Low Purity Molasses.
BrixHPmolasses Display Brix of High Purity Molasses.

Vent Mass Flow

VentQm Display Mass flow of Gases to Vent connection.

Warnings

The model will report errors and warnings for the following conditions.

Warning Message Comments
Vapour in Feed or Wash Streams and NO Vent Connected When vapors are present in any input streams or evaporation is occurring a vent stream must be connected.
No Crystal in FugalFeed - Check Stream Compositions There must be sucrose crystal in the fugal feed stream.
FugalFeed is not a Sugar Species model The feed stream must be a sugar stream.
Purge Purity Target < Feed Purity Purge Purity Target must be greater than the Feed Purity.
Crystal in Basket Washings - insufficient Wash Water There is insufficient wash water available in the basket wash stage to dissolve all the sugar crystal remaining in the basket or the water is too cold.
Purge Target Purity NOT Achieved Purge purity target not achieved.
Wash01 Target Purity NOT Achieved The purity target is set lower than the feed to this stage.
Wash02 Target Purity NOT Achieved The purity target is set lower than the feed to this stage.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

 

DLL:

SugarUnits.dll

Units/Links

Sugar: Sugar Fugal

or

Group:

Sugar

Units/Links

Sugar: Sugar Fugal|}

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


Insert into Project

 

Insert Unit

Sugar

Sugar Fugal

See Insert Unit for general information on inserting units.