Crystallization process

Transcription

Crystallization process:
·
·
1) nucleation (formation of nuclei)
2) crystal growth
need conditions for nucleation and crystal growth is existence
of supersaturated sugar solution
driving force of nucleation and crystal growth is difference
between actual concentration in solution and
concentration of saturated solution (supersaturation)
Supersaturation of sugar solution is expressed as supersaturation
coefficient Kp, which is defined as
Kp = H/H1,
H - weight ratio P/W in solution
H1 - weight ratio P/W in saturated solution
© P.Kadlec - Food Technology
and Biotechnology I - Sugar2
1
Metastable zone
 no creation of new nucleus, only
growth crystals
 zone of supersaturation suitable
for evaporation and cooling
crystallization
 limits of metastable zone are
influenced chiefly by temperature,
purity and presence of crystals
 the lower limit correspond to
saturated solution
2
Theory of growth crystals from solution
Phases of crystallization process:
a) transport of sucrose molecules from solution to diffusion layer
b) diffusion of sucrose molecules cross diffusion layer
c) surface diffusion of sucrose molecules on crystal surface (in reaction
layer) and deposition into crystal lattice
surface
reaction diffusion supersaturated
of crystal layer
layer
solution
distance from
crystal
3
Diffusion of sucrose molecules - Fick law
dm/d = A . D/d . (c - cr) = A . kd . (c - cr)
m

A
D
d
c
cr
kd
weight of sucrose transported cross diffusion layer
time
crystal surface area
diffusion coefficient
thickness of diffusion layer
mean solution concentration
concentration of solution on boundary line between
diffusion and reaction layers
constant
4
Deposition of sucrose molecules into crystal lattice
(surface reaction)
dm/d = kr . A . (cr - c0)r
kr
c0
r
growth constant of surface reaction
concentration on crystal surface – concentration
of saturated solution
order of surface reaction (r = 1)
growth rate of crystallization in steady state
kd . kr
v = dm/(d . A) = ----------- . (c - c0) = K . (Kp - 1)
k d + kr
K
constant of crystallization
5
Scheme of sugar boiling house
evaporation
crystallization
in vacuum pan
massecuite
cooling
crystallization
centrifugation
1-reservoirs,
2-vacuum pan
3-barometric
condenser
4-crystallizer
5-distributing
trough
6-centrifuge
7-sirup
8-sugar
6
Massecuite boiling
1) preparation
- evaporation by steam, test of tightness
- adjustment of sirups - 85 oC, 70 - 75 %, alkalinity
2) concentration
- sirup on base of boiling
- boiling temperature increses with increasing
concentration of sirup,
- boiling temperature is 78 - 82 °C,
pressure 67 až 80 kPa
3) seeding
- Kp 1,10 - 1,25
- microseed - suspension of sugar particles in ethanol
or propanol, diameter 1 - 10 m
4) stabilization of nuclei
- transit from phase seeding to phase of crystal
growth
- metastable area, decreasing of Kp to 1,08 - 1,12
7
5) crystal growth
Kp 1,10, sucrose crystallizes from mother sirup,
mother sirup is exhausted, new sirup is feeded and
crystallizes and these processes are repeated till
above level of massecuite in vacuum pan is reached
6) concentration
concentration of massecuite to dry substances
93 - 95 %
7) end of boiling
massecuite is set off to crystallizer where is added
heated mixing sirup to decrease Kp of mother sirup,
during set off temperature of massecuite decreases
8
Boiling of massecuite with inoculation magma
Advantages:
Improving of grain size analysis of boiled crystal
Energy saving (steam) during shortened time of boiling
Simplification of boiling scheme
Course of massecuite boiling
1) Preparation to boiling
2) Feeding of inoculation magma – low grade sugar+thick juice
3) Crystal growth – massecuite boiling
4) Concentration of massecuite
5) End of boiling
9
Automatization of boiling
main control value – supersaturation coefficient
subsidiary values – content of crystalls,
level of massecuite in pan
For undirect measurement of Kp is used:
electric conductivity
viscosity (consistency)
10
vacuum pans
crystallizers
control room
centrifuges
11
A-massecuite
White
crystal syrup
Magma
seed
B-sugar
Thick juice
A-sugar
Green
crystal syrup
B-massecuite
Magma
seed
Green
B-syrup
C-massecuite
C-sugar
B-magma
White
B-aff.syrup
C-magma
Green
B-aff.syrup
1st aff.sugar
Molasses
White
C-aff.syrup
Thin juice
or water
Green
C-aff.syrup
2nd aff.sugar
Liquor
12
Molasses
Composition:
S = 80-85 %, Q = 60 - 65 %, A = 10 %
pH molasses should be higher than 8,3, alkalinity on phenolphtalein
For storage and expedition is need to adjust S = 80 %
Use of molasses
1) By direct feeding or preparation of molasses fodder
2) Raw material for biotechnological production
- traditional fermentation production (spirit, yeast, fodder yeast, organic acid
- citric, lactic, vinegar, organic solvent, aminoacids)
- modern biotechnological production
3) Isolation of nitrogen compounds (betain, aminoacids, …)
4) Sugar exhaustion
- Separation of sugar in form calcium saccharate - Steffen way
- Separation of nonsugar by means of ion exchangers – demineralizaton or
ionic exclusion
- sugar fraction is treated to liquid sugar
- nonsugar fraction as fodder or fertilizer
13
Scheme of refinery
Raw sugar
Magma preparation
Affination
Liquor preparation
Liquid sugar
Filtration
Decolorization
Boiling
Centrifugation
Drying
Classification
Milling
Packaging
Pressed cube
14
Drying and cooling of crystals
Sugar from centrifuge – 1 – 1,5 % H20
Transport of moisture from crystal – surface layer of
saturation sugar solution – to surrounding air
Surface layer of saturation sugar
solution
Layer of amorphous sucrose
Saturation sugar solution
15
Turbine cooler
Course of drying and
cooling of sugar
16
Fluidization drying
Wet sugar
Dry sugar
Air
granulity
moisture
temperature
moisture
temperature
temperature
relat. humidity
0,6 - 1,0 mm
0,8 %
50 - 70 °C
max. 0,08 %
35 - 45 °C
20 - 30 °C
max. 30 %
17
Crystal classification
Plate classifiers
Vibrating classifiers
Minimal content of the main fraction of crystals and powder sugar (%)
Dimension of screen aperture (mm)
Crystal
Caster
sugar
Powdered
sugar
18
Conditions of sugar storage in silos
Sugar crystal
Perfect dust off
Uniform quality
Moisture 0,03-0,05 %
Temperature 20 – 25 %
Ash 0,02
Reducing compounds 0,01 %
Air
Relative humidity 50-60 %
Temperature 20-22 °C
Small overpressure inside silo
19
Equilibrium
moisture of
sugar (%)
Relative moisture of air (%)
Course of sorptive isotherms for crystals of various quality
1 – the lowest quality, 2 – the medium quality, 3 – the highest quality
20
Weibull sugar silo
Defects:
• sugar moisten
• drying-out of sugar
• increasing of RL
• hardening of sugar
• sugar contains much fine
dust
21
Explosibility of sugar dust
Explosive concentration
settled sugar dust
difficult inflamable
min. ignition temperature 400 - 500 oC
turbid sugar dust
inflammable
min. ignition temperature 300 - 450 oC
very explosive mixture
lower explosive limit
4 - 50 g/m3
dangerousness of sugar dust explosion
shock waves
Diameter of sugar particles
22
1 – crystal conveyer
Sugar cube production
2 – crystal container
3 – scale
1
4 – water container
2
4
5 – homogeniser
6 – container before press
3
5
moistening
to 1-2 %
7 – rotary press
8 – cubes conveyer
9 – dryer
6
9
10 – cooler
11 – packaging
7
8
10
11
23
Chambon cube process
24
Types of sugar according Czech Food Law
Group
Sugar extra
Subgroup
crystals
caster sugar
mixture of crystals, bulk material
mixture of smaller crystals or pulverized crystals,
bulk material
mixture of fine pulverized crystals, bulk material
powder
crystals
Sugar white
caster sugar
powder
crystals
Sugar semi-white caster sugar
powder
Powdered sugar can include (max. 3 %) anti caking agent
Pressed cube sugar (cube, bridge, loaf)
Sugar with additives
Natural sugar
bulk, granular, light yellow crystal, slightly sticky
Candys
mixture of big crystals, yellow – brown color
Liquid products
invert syrup
caramel
25
Physical and chemical requierements on sugar quality
according Czech Food Law
Sucrose
Ash
Invert sugar Moisture
Color in
solution
Extra-white
White
Semi-white
26
Biological purification of waste water
Using of function mixture of heterotrof aerobic and partly
anaerobic bacteria, fungi, yeasts, protozoons, …
Organic compounds from waste water are used by means of
these microorganisms as substrate. This substrate is partly
oxidiced to CO2 and water, partly is changed to new biomasse.
Function microorganisms are cultivated in suspension - forms
activated sludge. This sludge is circulated to anaerobic step of
purification.
27
1st step – anaerobic fermentation
combination of acid and methane fermentation
(acido- and methanogenesis)
initial compounds: polysaccharides, proteins, lipids, …
process
hydrolysis
interproducts:
monosaccharides, aminoacids,
alifatic acids, glycerol, …
process
acidogenesis
interproducts:
low alifatic acids and alcohols, …
process
methanogenesis
End product:
stabilized sludge, sludge water,
biogas (65-75 % methan, 25-35 % CO2,
heating value 20-30 MJ/kg)
2nd step – aerobic fermentation
28
Biological purification of waste water
1-waste water inlet
2-heating of waste water
3-anaerobic reactor
4-degassing tank
5-plate clarifier
6-activation
7-clarifier
8-air systém
9-purified water
10-excess sludge outlet
11-burner of rest biogas
12-biogas
29
30

Crystallization process

Transcription

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