Sugar Dryer

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Latest SysCAD Version: 19 March 2024 - SysCAD 9.3 Build 139.35102

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General Description

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

The sugar dryer is a counterflow dryer used to cool and dry sugar crystals in the final stage of processing. The dryer receives hot sugar crystal feed into one end. Air enters the other end of the dryer and flows in the opposite direction to the sugar.

The hot sugar crystal feed is coated with a film of molasses and as the sugar moves through the dryer heat is transferred from the sugar to the air and water evaporates from the sugar into the air. As water evaporates the concentration of sugar in the molasses film increases, the solubility of the sugar decreases and some of the aqueous sucrose in the film precipitates onto the existing sugar crystals. The change in the molasses film properties with increasing concentration due to evaporation and decreasing purity as sucrose precipitates affects the partial pressure of water and thus the evaporation rate of water from the film to the air and also the precipitation rate of sucrose from the film to the crystal. This is all taken into account in the model.

The model also has the option to add a water spray to the feed to allow adjustment of the final the moisture content of the sugar.

NB The model iteratively solves a finite element problem to find a solution. This is computationally expensive and can slow down the overall project so it is recommended that the dryer unit operation is only switched on after the rest of the circuit is solved.

Input and Output Connections

A sugar stream and an air stream are required connections. The sugar stream must contain sugar crystal and have the specific surface area (SSA) quality present or an error is displayed. The sugar feed stream should not contain any gases. There are no specific checks built in for the air stream although it should contain only gases. The air stream may defined with water vapour and the pseudo-species Air or it may be defined in terms of the individual gas species. There may be up to five sugar feed streams. There is only one air stream connection allowed.

There is only one air flow and one sugar flow out of the dryer.

There is an option for a water addition stream to the sugar feed. Although it would normally be expected to be pure water, there are no restrictions on the added water composition other than it may not contain any vapours. It could contain aqueous sugar or impurities and can contain some solids. The added water is assumed to mix perfectly with the molasses film and any solutes or solids are included in the calculation of the solution.

Label	Required / Optional	Input /Output	Number of Connections		Description
			Min	Max
Feed	Required	In	1	5	Sugar Crystal Feed to the Dryer.
Air	Required	In	1	1	Air Flow to the Dryer.
Water Addition	Optional	In	0	1	Added Water Feed to the Dryer.
Dried Sugar Out	Required	Out	1	1	Sugar out of the Dryer.
Exhaust air	Required	Out	1	1	Air Flow out of the Dryer.

Behavior when Model is OFF

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

• All Feed material and any added water will flow through to the Product Sugar Stream with no changes or heat exchange.
• The air stream in will flow through unchanged with no heat exchange or mass transfer from the sugar stream.
• There is no environmental heat transfer.

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

Model Theory

Operation

The dryer is an inclined cylindrical kiln type dryer which rotates and generally include lifts which pick up sugar and drop it through the air stream to increase contact area with the gas stream. Air flow is normally ambient air which is drawn or forced through the dryer.

The sugar feed is high purity sugar crystals covered with a film of molasses. The mass fraction of molasses is in the order of several percent of the total mass and the moisture fraction in the order of one percent of the feed. The sugar particles are typically in the range of 0.5 to 1 mm. Although the sugar has a relatively high surface area per unit mass, only approximately 10% to 20% of the surface area is in active contact with the gas stream at any one time. The surface area of the sugar is calculated from diameter and coefficient of variation, the active area fraction is an input into the model.

Sugar in active contact with the air loses heat to the air through convective heat transfer and water via convective mass transfer (evaporation). The sugar crystal is pure (or almost pure) sucrose. The molasses film is an aqueous mixture of water, sucrose and impurities. As the molasses loses water, the sucrose concentration in the film increases and as the molasses loses heat, the sucrose solubility decreases. The molasses is generally close to saturation at the start of the drying process and with evaporation and cooling the molasses film becomes supersaturated in sucrose near the inlet of the dryer. Sucrose precipitation begins there and generally continues through the rest of the dryer process. As sucrose precipitates, the purity of the molasses decreases and as evaporation occurs the concentration of solutes increases which increases the boiling point elevation of the molasses film and slows evaporation.

Depending on the temperature and moisture of the feed sugar and the temperature and humidity of the air, water may be added to the feed sugar to control the final moisture content of the sugar to prevent over-drying.

The model also includes options for environmental heat loss from the dryer which may have a significant effect on the dryer operation. When the total heat loss rate is specified, the heat loss is divided uniformly between all of the segments and taken 50/50 from the sugar and the air in each segment. When heat loss to ambient method is selected, the heat losses from the air and sugar are based on the heat loss coefficient and the temperature difference from ambient at each segment.

Numerical Solution

The dryer model is solved by breaking it into a series of segments and solving the combined heat and mass transfer problem in each of the segments. A relaxation technique is used to solve the counter flow problem. Convergence is based on relative change between iterations. The number of segments is user selectable and up to 500 segments are allowed. However, there is generally no need for more than 50 segments and typically 30 to 50 is sufficient for accuracy.

There is an option to add damping. In general damping is not required and will slow down convergence. However, when temperature differences are large or heat transfer coefficients are very high then some damping may help (suggested values for damping are in the range of 0.1 to 0.25).

The dryer model solves a complete numerical finite element model at each iteration step. The model saves the last converged solution and uses that as its starting point so when there is no change from the last step, it will take only one step to converge and when there is only little change from the last step, convergence is usually fairly quick.

It is important to note that this model may consume a significant amount of computation time to solve at each step. Using a large number of segments may take some ten's of seconds or more to solve. There are some controls which can be used to optimize the solution speed. However, when the dryer unit is part of a larger process model it is possible that it can slow down the overall model. Given that this unit is generally at the end of the process and there is no recycle through it, a better approach may be to either use a small number of segments or to turn the unit off until the rest of the model is solved and then turn it back on - that way it will only need to solve once.

Environmental Heat Exchange Options

The sugar dryer may lose heat to the environment. The heat loss is taken from both the air and sugar streams. How the loss is accounted for depends on the method chosen.

The heat loss options are set from a drop down list and include;

1. None - No Heat Exchange occurs.
2. Fixed_Loss - A Heat Flow Rate to the Environment is specified by the user. The heat loss is assumed to be taken evenly over all the segments the

loss is assumed to come evenly from both the sugar and the air streams. For example, if there are 50 segments, then the loss per segment is 1/50 of the total heat loss specified with 1/100 coming from each stream.

1. Ambient - An overall 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 loss is calculated individually for each segment and individually for the air and sugar streams. For example if there are 50 segments, then the heat loss from the sugar stream in segment(i) is; Qloss(i) = (0.5) * (1/50) * Constant * (Tsugar(i) - Tamb)

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

The model follows the flow of sugar and air through dryer stages.

Feed - All feed streams are assumed to mix perfectly. Any added water is assumed to mix perfectly with the molasses film on the sugar crystals. Sugar crystals are required to be present in the feed stream and the molasses film must be a Sugar type stream.

Air - The air stream in is characterized by its mass flow, temperature and water content.

Area - Each segment in the dryer has an active surface area for heat and mass transfer that is calculated from the solids mass flow rate, retention time in the dryer, the characteristic particle diameter, the coefficient of variation (CV) of size distribution of the feed crystal and the active surface area fraction. The active area fraction accounts for the fact that only a fraction of the sugar is exposed to the air stream at any one time and the rest is buried below the surface of the sugar. A typical active area for a dryer is in the order of 1,000 to 20,000 m^2. The total area is estimated as;

• [math]\displaystyle{ Total Area= \cfrac{\left(RetentionTime \times Qm_{solids} \times 6.0 \times \left(1.0 + CV^2\right) \right)} {\left(Diameter \times Rho \times (1.0 + 3.0CV^2)\right)} }[/math]

And the active area per segment is;

• [math]\displaystyle{ Area= \left(Total Area\right) \times \cfrac{\left(Active Area Fraction \right)} {\left(Number of Segments \right)} }[/math]

Heat Transfer

Convective heat transfer is calculated as;

• [math]\displaystyle{ Q_{hex} = h \times Area \times \left( T_{slurry} - T_{gas} \right) }[/math]

The heat transfer coefficient, h, is a user input. Typical values of h for a sugar particles falling in a gas stream or in the order of 1 W/m^2.K to 10 W/m^2.K. The literature suggests a value of about 4 W/m^2.K for a typical application.

Evaporation

Mass transfer from the surface of the molasses film is a function of the saturation pressure of the water in the film, the partial pressure of the water in the air stream and the mass transfer coefficient, h_m. The mass transfer coefficient may be estimated from the film coefficeint, h, using heat and mass transfer analogies.

• [math]\displaystyle{ \cfrac{h}{h_m} = \cfrac{k}{D_{AB}Le^n} }[/math]

Where h is the heat transfer convection coefficient, h_m is the mass transfer coefficient [m/s], k is thermal conductivity, D_AB is the binary diffusivity and Le is the Lewis number - all evaluated at the film temperature. The mass transfer is then;

• [math]\displaystyle{ Q_{m} = h_m \times Area \times \left( \rho_{slurry} - \rho_{gas} \right) }[/math]

where ρ_slurry is the saturation density of water vapor in the slurry and ρ_gas is the density of water vapor in the gas stream. The saturation pressure for water vapour over the high concentration, low purity molasses is given by;

• [math]\displaystyle{ Z_1 = \cfrac{100\times(W/I)\times \left(100-Purity(\%)\right)}{50} }[/math]

• [math]\displaystyle{ Z_2 = 0.01\times \left(51.2\times log_{10}\left( Z_1\right) - 25.0 \right) }[/math]

• [math]\displaystyle{ P_{sat} = P_{sat,pure H_2O} \times Z_2 }[/math]

If evaporation rates are high compared to the diffusion rate of water through the molasses, then the surface of the molasses coating the crystals may have less water water and a higher sugar content which will reduce the vapor pressure of water and slow the evaporation rate. To account for this there is an optional correction term in the evaporation rate calculation. The correction for segment i is of the form;

• [math]\displaystyle{ C_1* \exp \left(\cfrac{-C_2 \times t_{res} \times i }{segments}\right) }[/math]

Where C₁ is a constant (generally about 1), C₂ is the diffusion rate of sucrose in water (~0.0035), t_res is the residence time in the dryer and i is the segment number and segments is the total number of segments. This correction makes the evaporation rate taper off towards the end of the dryer. NB Setting C₁ to 1 and C₂ to zero will make the correction term equal to 1 and take it out of the calculations.

Crystallization

As the sugar cools and water evaporates, the molasses coating the sugar crystals will become supersaturated in sucrose and sucrose will precipitate onto the sugar crystals. The mass rate of precipitation is the growth rate times the area available for precipitation. The growth rates is given by;

• [math]\displaystyle{ E_a = 15 - 0.2\times\left(T(K)-333.15\right) }[/math]

• [math]\displaystyle{ FT = \cfrac{-E_a}{0.001987} \times\left(1/T(K)-1/333.15\right) }[/math]

• [math]\displaystyle{ \text{Growth Rate (m/s)} = 0.00000206\times\left(SSN - 1.0046\right)\times\exp\left(FT - 1.75*I/W\right)\times GrowthFactor }[/math]

Where I/W is the impurity to water mass ratio, GrowthFactor is a tuning parameter (with a default value of 0.4) and the supersaturation, SSN, is given by;

• [math]\displaystyle{ SSN = \cfrac{M_{sucrose}\times \left(100-PureSol(\%)\right)}{M_{water} \times PureSol(\%)\times\left(1.0-0.088I/W\right)} }[/math]

and PureSol(%) is the solubility of pure sucrose in water.

The area for precipitation in a segment is;

• [math]\displaystyle{ Area=\cfrac{Qm_{sugar}\times t_{res} \times6\times\left(1+CV^2\right)}{\rho_{crystal}\times Dia_{crystal}} }[/math]

The precipitation rate is;

• [math]\displaystyle{ Qm_{precip}= Area\times GrowthRate \times ResidenceTime }[/math]

Data Section

SugarDryer

Unit Type: SugarDryer - 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.
ShowQFeed	Tickbox	Option to Massecuite Feed Stream.
ShoqQProd	Tickbox	Option to display Dried Sugar Stream.
Numerical Solution
MaxIterations	Input
Iterations	Display	Maximum number of iterations (suggest ~50)
Damping	Input	Damping factor (suggest ~ 0.2)
Tolerance	Input	Relative Tolerance for convergence (suggested range 5e-05 to 1e-06)
Segments	Input	Number of segments to use for numerical solution (suggest 30 to 50)
Display Segment Data
ShowSegmentData	Tickbox	Option to display a matrix of data for the solution
Segment.Total	Display	total number of segments.
Segment.display	Input	Number of segments to display (max = Segment.Total+2 or 256)
Residence Time
ResTime	Input	Total residence time in dryer
SegResTime	Display	Residence time in individual segments
Surface Area
TotArea	Display	Total calculated surface area of crystals
ActiveFrac	Input	Active fraction of surface area (exposed to gas stream)
ActiveArea	Display	Active surface area
Environmental Heat Loss
Method	None	No heat exchange with the environment.
	Fixed_Loss	User specified heat loss rate.
	Ambient	Heat loss determined as a constant times temperature difference from ambient. Qloss = ThermalLossAmbient * (Tprod - Tamb)
EnvLossRqd	Input	The Required Heat Loss Rate - This is only visible if the Fixed_Loss Method is selected.
EnvLossAmbient	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 Dryerto Environment.).
Air Stream In
AirInT	Input	Inlet Air Temperature.
AirInQm	Input	Inlet Air Mass Flow.
DryAirQm	Input	Inlet Air Dry Air Mass Flow.
MoistureQm	Input	Inlet Air Water Vapor Mass Flow.
AirInHumidity	Input	Inlet Air Relative Humidity.

Crystallization

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

Tag / Symbol	Input / Calc
Feed Crystal Diameter
DiamMethod	Steam_DIA	Crystal Diameter from Input Stream.
	User_DIA	User Specified Crystal Diameter.
InputDiameter	Input	User input crystal diameter (only displayed if User_DIA is selected).
StreamDiameterIn	Display	Feed stream crystal diameter.
StreamDiameterOut	Display	Product stream crystal diameter.
Feed Size Distribution CV
CrystalCVin	Input	User input crystal CV.
CrystalCVout	Display	Product stream crystal CV.
Crystal Growth Factor
GrwthFactor	Input	Scaling factor for crystal growth rate (nominal value 0.4).
Crystallization
AvgGrowthRate	Display	Average crystal linear growth rate through the dryer.
AvgPrecipRate	Display	Average sucrose precipitation rate through the dryer.
Evaporation
DiffusionLimit	TickBox	Use diffusion limited model for evaporation rate.
Evap_Diff_Factor1	Input	Diffusion limited evaporation factor 1 (default = 1.0).
Evap_Diff_Factor2	Input	Diffusion limited evaporation factor 2 (default = 3.5e-03)
AvgEvapRate	Display	Average water evaporation rate through the dryer.
Water Addition
WaterT	Display	Added water temperature.
WaterQm	Display	Added water mass flow rate.
Feed
FeedT	Display	Feed Temperature.
FeedQm	Display	Feed total mass flow.
FeedCrystal	Display	Feed sucrose crystal mass flow.
FeedMoisture	Display	Feed moisture (not including water addition).
Feed Molasses
FeedMolasses	Display	Feed Molasses mass flow.
FeedWs	Display	Feed Molasses sucrose mass fraction.
FeedBrix	Display	Feed Molasses Brix.
FeedPurity	Display	Feed Molasses Purity.
FeedItoWratio	Display	Feed Molasses Impurities/Water mass ratio.
FeedSSN	Display	Feed Molasses supersaturation.
Total Moisture
TotalMoisture	Display	Total feed moisture into dryer including water addition.
Product
ProdT	Display	Prod Sugar Temperature.
ProdQm	Display	Prod total mass flow.
ProdCrystal	Display	Prod sucrose crystal mass flow.
ProdMoisture	Display	Prod moisture.
ProductMolasses
ProdMolasses	Display	Product Molasses mass flow.
ProdWs	Display	Product Molasses sucrose mass fraction.
ProdBrix	Display	Product Molasses Brix.
ProdPurity	Display	Product Molasses Purity.
ProdItoWratio	Display	Product Molasses Impurities/Water mass ratio.
ProdSSN	Display	Product Molasses supersaturation.

Data Access Page

Data for various points through the drier may be displayed on this page. The data display is turned on or off in the first tab (see display segment data above). The data is displayed in a matrix format with the headings below arrayed across the top of the page.

It is possible to display up to the number of segments plus 2 data points through the dryer. Fewer points may be selected. The data displayed for any segment are the exit conditions for the sugar and air streams from that segment.

Segment 0 is is the inlet condition for the sugar feed and exit condition for the air stream.

Segment n+1 is the inlet condition for the air stream and exit condition for the product sugar.

Segments 1 through n are data for the segments. The points displayed depend in the specified number of points to display. These would normally be the inlet and exit points plus evenly spaced segments in between.

NB if the number of data points to display is changed, SysCAD must be run for at least one iteration to refresh the display values.

Tag / Symbol	Input / Calc	Description
Product Crystal
Display	Display	Display point number.
Segment	Display	Segment number.
Time	Display	Feed time in dryer from start.
Tslurry	Display	Feed slurry temperature.
QmXtal	Display	Sucrose crystal mass flow rate.
QmFilm	Display	Molasses film mass flow rate.
Moist	Display	Sugar slurry moisture.
Wds	Display	Molasses film sucrose mass fraction.
Purity	Display	Molasses film purity.
SSN	Display	Molasses film supersaturation.
QmPrcp	Display	Molasses to crystal sucrose precipitation rate.
XtalDia	Display	Sugar crystal diameter.
QmEvap	Display	Evaporation rate.
BPE	Display	Molasses film boiling point elevation.
PPsat	Display	Molasses film water saturation partial pressure.
Qhex	Display	Heat transfer rate.
Tgas	Display	Air Stream temperature.
hm	Display	Mass transfer coefficient.
QmVapor	Display	Air Stream water vapor mass flow.
Humidity	Display	Air Stream relative humidity.

Warnings

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

Warning Message	Comments
Invalid Feed - check streams	Missing Sugar Feed or Drying Air.
Feed has no SSA - turn on SSA	Sugar Feed must have SSA property, may need to turn on SSA.
No Crystal in Feed - Check Stream Compositions	There must be sucrose crystal in the Dryer feed stream.
Feed is not a Sugar Species model	The feed stream must be a sugar stream.
Model not Converged - mismatched mass flows? increase damping, more/less iteration steps	Model has not converged, check operating conditions, number of segments and damping.

Adding this Model to a Project

Add to Configuration File

Sort either by DLL or Group:

	DLL:	SugarUnits.dll	→	Units/Links	→	Sugar: Sugar Dryer
or	Group:	Sugar	→	Units/Links	→	Sugar: Sugar Dryer

See Model Selection for more information on adding models to the configuration file.

Insert into Project Flowsheet

Insert Unit

→

Sugar

→

Sugar Dryer

See Insert Unit for general information on inserting units.