Multiple-effect falling film evaporators

1. EVAPORATION
IN THE
SUGAR INDUSTRY
Chandrima Shrivastava
13FET1001

2.  Water has to be evaporated from juice to allow its crystallization
in the next stage-will occur only if water content is low (Brix 70°)
 Multiple-Effect Evaporator
◦ commonly 4 or 5 effects
◦ forward feed
 Falling Film evaporator - most common
 Highly energy efficient
FALLING FILM

3. Evaporator Auxiliaries
1. Condensers - condense the vapour from
evaporator
2. Vacuum Pumps - maintain vacuum within
evaporator and remove the non-condensing gases
(accumulation will eventually stop process)
3. Entrainment Separators - remove droplets of
liquid carried away by vapour-prevent loss of liquid
from system-cyclone separator may be used
Falling Film
Evaporators

4.  Liquid pre-heated to BP, enters from top of calandria
through distribution plates
 Steam used as heating medium
 A thin film of product flows downwards in tubes due
to gravity, product and vapour in co-current flow
 Product velocity accelerated due to drag of vapour
generated by boiling
 Vapour condensate removed at base
 Vapour separated from concentrate at bottom of
calandria - entrainment separator
Working

5. Advantages
 Suitable for high viscous liquids -
flow of film assisted by gravity
 High heat transfer coefficents even
at low boiling temperature 2000-
5000 W/m2K for water
 Short residence times on the
heated surface, 5-10 s without
recirculation
 Suitability for vacuum operation
 High evaporation ratios, 70%
without and 95% with recirculation
 Low susceptibility to fouling
 Minimum cost operation
 Suitable for highly heat sensitive
products such as juices
Disadvantage
s
 Distribution of feed to tubes may
use perforated plates above the
tubes or spray nozzles - not suitable
for the suspensions as solids clog
the perforated plate
 High head-room requirements
 Recirculation may be required
Applications
Concentration of dairy products
(whey, skim milk, milk protein),
sugar solutions, urea, black liquor,
yeast extract, gelatin manufacture,
tea and coffee extracts

6. Multiple-Effect
Evaporator

7. Multiple-Effect Evaporator
 Clarified juice passed through heat exchangers to preheat, arrives
evaporation station
 Steam from steam boilers, flows to turbine for power generation
(cogeneration), exhaust steam sent to evaporator
 Steam used to indirectly heat 1st effect, and steam from water
evaporated in 1st effect used to heat 2nd effect
 Partially concentrated juice leaving 1st effect introduced into 2nd effect,
vapour produced from 1st effect used as a heating medium in 2nd effect
 Product from 2nd effect fed to 3rd effect, and so on
 After last effect, steam recovered in condensers, along with
condensates from other effects as hot water, attends to other heat
requirements of process - efficient multiple utilisation of steam

8. Principles
 DRIVING FORCE: Temperature difference between heating medium
(steam), and feed - to provide the latent heat of vaporisation
 PRESSURE-BP RELATION: Water boils at 100°C under atmospheric
conditions but with reduction of pressure (application of vacuum), BP
is lowered
 BOILING POINT ELEVATION: Juice - a solution of sugar in water, BP
higher than that of pure water under same pressure
BPR depends on concentration of sucrose and impurities in solution
 EFFECT OF HYDROSTATIC HEAD: In a column of boiling liquid, the
pressure at a depth is more than that at the surface - hydrostatic head
-important for liquids boiling in tubular calandrias

9. 4-stage Vacuum Evaporation
In Sugar Cane Industry

10.  Steam temperature is higher in 1st than in 2nd effect – for steam exiting
1st effect to boil liquid in 2nd effect, BP in 2nd effect must be lower
 Each proceeding effect at lower pressure than the previous - to get
temperature difference between steam and juice
 Later effects under vacuum - concentrate will flow in forward direction
without pumping - forward feed
 Vacuum necessary for heat-sensitive materials - sucrose inversion,
decomposition of reducing sugars, discoloration at high temperatures
 As concentration increases, salts (limesalts, Na
compounds, SiO2) thrown out of solution as
their solubility is exceeded – scale deposition
on inside of tubes - formidable barrier to heat
transfer
 Scale removal - time consuming - every 2-3
weeks

11. INLET OUTLET
Stage Thin Juice Thick Juice (syrup)
State Pale yellow liquid Yellow-brown syrup
Brix 15.5° 67-72°
Water content 83-85% 28-33%
Invert Sugar 0.1% 3.5-4.5%
Flow rate 133 kg/sec 28 kg/sec
Viscosity 1 cP 10 cP
pH 9-9.5 8.7-9.2
Colour/100 parts
of sugar
34.3 41.6

12. Vapour per
unit steam
(kg)
Evaporation
Coefficient
(wt of vapour
released/m2/hr)
Steam Supplied 1 --
1st effect 0.923 37
2nd effect 0.972 35
3rd effect 1.047 30
4th effect 1.162 30
Parameter
Optimal Dimensions
1st vessel 2nd vessel 3rd vessel 4th vessel
Tube Length (m) 4 3.5 3 2.5
Tube Diameter (mm) 35 38 44.43 50
Surface Area (m2) 0.4398 0.4178 0.4187 0.3927
Number of tubes 415 135 118 126
Vessel Length (m) 12 10.5 9 7.5
Construction
Material:
304/316
stainless steel