4.1
Textile Industry
Materials
like wool or viscose are more hygroscopic and those like Nylon or polyester are
hydrophobic. The drying proceeds in 2 phases of moisture content. After initial
heating, the rate of evaporation is constant from 1 kg moisture/kg of bone-dry
material up to say, 0.2 kg/kg of bone-dry material (critical moisture content).
Then the drying recedes inside and drying rate is reduced as diffusion and
capillary forces control it. If this material is over dried, (say up to 2% moisture),
it absorbs the moisture from atmosphere and stabilises at a level called
equilibrium moisture content (about 7%).
The
productivity of drying operation is reduced if the critical moisture content is
higher. That is, transition from constant rate of drying to falling rate of
drying starts quickly. The critical and equilibrium moisture content of typical
textile materials is given below in table 4.1.
Table 4-1: Equilibrium moisture content of textiles
|
Material
|
Critical
moisture content
|
Equilibrium
moisture content
|
|
Cotton
|
17.5
to 26
|
7
|
|
Wool
|
39
|
16
|
|
Viscose
rayon
|
38
|
12.5
|
|
Silk
|
30
|
-
|
|
Nylon
|
-
|
4
|
|
Polyester
|
-
|
0.5
|
|
67:33
Polyester -cotton
|
-
|
2.5
|
|
67:33
Polyester-wool
|
-
|
5.5
|
The
fabric in sheet form is dried in cylinders or in hot air chambers with or
without tension. The hot air dryers are called stenters, hot flue, float
dryers, loop dryers etc
4.1.1
Approach to energy saving in Cylinder Dryers:
1.
Increase drying rate by:
Squeezing out incoming moisture
Avoid over drying
Use maximum permissible steam pressure
Provide efficient condensate and air removal
systems
Clean heating surfaces
2.
Increase thermal efficiency by
Stop all live steam leaks
Provide insulation on piping and cylinder ends
Use as much of drying surface as possible
Typical
drying speeds for drying cotton poplin fabric 0f 0.1 kg/m from 75% moisture
content to 7% are as follows:
Table 4-2: Steam pressure and drying speed
|
Steam pressure, kg/cm2
|
Speed
per cylinder, m/min
|
|
|
570 mm dia cylinder
|
760 mm dia cylinder
|
|
|
1
|
4
|
5
|
|
2
|
5
|
6.5
|
|
4
|
6
|
8
|
4.1.2
Approach to energy saving in hot air dryers/stenters
High
temperature air at temperatures varying from 80 to 200° C is used in stenters.
The heat requirement is similar to that of a cylinder dryer, except that there
is an additional consumption towards heating the fresh air, which has to be
drawn in matching quantities with the exhaust.
In
hot air dryers, the drying rate is increased by:
high temperature of air jets with high steam
pressures in heaters ( about 7 bar) or high temperature thermic fluid in the
heaters
adequate heater capacity and cleanliness of
heaters and fins
proper removal of condensate and air in case
of steam heaters and proper circulation of non-deteriorated thermic fluid in
case of thermic fluid heating
Operating at designed air jet velocity of 30
to 40 m/s and avoiding drop in air velocity due to choking of filters, damaged
fan blades or belt slippage in fan drives, opening or leaks in air ducts
Maintaining optimum air humidity and avoiding
high humidity.
Avoiding stoppages and steam leaks
In
modern design of stenters, the following features are incorporated.
Heating medium is circulating thermic fluid so
that steam leakage loss and condensate losses are avoided. Where possible,
direct gas fired burners are used to avoid heat transmission losses and
heaters.
Air to air or air to water heat exchanger is
used. Any lubricating oil vapours in exhaust are recondensed and pollution due
to fumes is avoided.
Blowers and exhaust moors are interlocked with
the main drive so that when machine stops they also stop.
Control systems are provided to monitor
productivity and also to measure and control the moisture on the fabric leaving
the stenter. Systems are also available to adjust the speed as per the pre-set
dwell time required in drying chamber.
Recirculating fan and exhaust are provided
with variable speed drives to regulate air circulation rates and pressures.
Exhaust is minimised by adopting super heated
steam drying in some of the latest designs.
4.2
Paper & Allied Products Industry
Drying
of pulp or paper is among the largest steam users at any mill. Drying starts by
heating the pulp or paper sheet from the temperature at which it leaves the
press section. Important ways of improving the efficiency of paper drying, in
addition to higher solids from the press section, include reducing overall heat
losses, using less air, and increasing the heat extraction from each unit of
steam used for drying. Several technologies to increase solids from the press
section
and alternatives to the conventional cylinder drying that would impact energy
use are being developed or are already in use. More revolutionary drying
concepts include the Condebelt process and impulse drying. Bulk of the paper in
sheet form is dried in Cylinder/Can dryers. Paper pulp takes many shapes as
molded materials, boards, light and heavy weight paper, and resin
impregnated/coated paper as laminates/wall papers. While molded articles are
dried in truck tray tunnels or continuous conveyor sheet dryers, special coated
paper is handled in continuous festoon
dryers.
4.2.1
Paper Making Process
The
energy and material flow diagram of an integrated paper mill is shown below.
Figure 4-1: Energy flow diagram
The
first section of the machine is called the 'Wet End'. This is where the diluted
stock first comes into contact with the paper machine. It is poured onto the
machine by the flow box, which is a collecting box for the dilute paper stock.
A narrow aperture running across the width of the box allows the stock to flow
onto the wire with the fibers distributed evenly over the whole width of the
paper machine.
Press
section consists of a number of heavy rollers. The paper is conveyed through
these rollers on thick felts of synthetic fiber. More moisture is squeezed out
of the paper like a mangle, and drawn away by suction. At this stage of the
process the paper is still very moist. In drying section, the paper passes
through a large number of steam-heated drying cylinders. The sheet enters the
dryer with a moisture content of 60–75% depending upon the product and
the
effectiveness of the presses. The paper leaving the dryer has a moisture
content of 2–10%, but typically has a final moisture content of between 5–7%.
Paper mill steam consumption with cylinder drying is about 4GJ/tonne of
product. The ratio of energy use between the dryer and press sections is
typically 15:1.
Steam
of 6 to 12 bar is brought into the cylinders where it condenses. Water in the
sheet is removed by evaporation. The temperature at the cylinder surface varies
from 100oC to 165oC. There can be up to 50 or 60 cylinders on a fast running
paper machine. Synthetic dryer fabrics carry the web of paper round the
cylinders until the paper is completely dry. Part way down the bank of drying
cylinders is the size press. It is here that a solution of water and starch can
be added to the sheet in order to improve the surface for printing purposes.
The paper then continues through the drying section. The calendar consists of a
stack of polished iron rollers mounted one above the other. The calendar
'irons' the paper. The surface of the paper is smoothed and polished. The paper
now comes off the machine ready for reeling up into large reels, each of which
may contain up to 20 tonnes of paper. These large reels are either cut into
sheets or slit into smaller reels according to the customer's requirements.
The
theoretical steam requirement in Cylinder drying, as indicated by TAPPI studies
are given below.
Table 4-3: Theoretical steam requirement in paper drying
cylinders
|
Paper type
|
Equation for Evaporation Rate,
Lbs/hr/sq.ft
|
|
Kraft
|
0.300T-5.26
|
|
Tissue
|
0.0205T-3.15
|
|
Glassine
|
0.0340T-6.26
|
|
Writing
|
0.0820T-17.8
|
|
Paper Board
|
0.0147T-1.51
|
|
Newsprint
|
0.0300T-4.82
|
|
Pulp
|
0.0147T-2.13
|
Where
T = Temperature of saturated steam, degree F.
The
surface area refers to the contact surface of the paper with the cylinder.
4.2.2
Approach to energy saving
1.
Various designs of stationary and rotary syphons were used to remove condensate
from the dryers. Poor designs and inadequate control resulted in thick
condensate layers and dryer flooding that reduced drying capacity. Syphon
designs were upgraded to achieve reliable condensate removal, to minimize
cross-machine temperature variation, and to maximize dryer surface
temperatures. Modern syphons have predefined clearance to dryer shells to
control thickness of the condensate layer and improve heat transfer and at the
same time provide high mechanical stability.
2.
Differential pressures of 2 to 4 psig can be run with stationary syphons. Lower
differential pressure and lower blowthrough steam flows with stationary syphons
have led to growing use of cascade steam and condensate systems in recent
years. Cascade systems have been the standard in Scandinavia for many years to
minimize energy cost and eliminate the need for high-pressure motive steam
headers to the machine.
3.
Turbulator bars increase heat transfer efficiency and improve cross-machine
temperature profiles by creating turbulence in the condensate layer when the
dryer is operating above rimming speed. A stagnant film of water in dryers
creates high resistance to heat transfer in rimming dryers.
4.
Rimming speed depends on a variety of factors including the amount of
condensate in a dryer, dryer diameter, and dryer speed. Rimming usually occurs
at dryer surface speeds over 900 fpm. Manufacturers
recommend the installation of Turbulator bars on dryinglimited paper machines
operating at speeds as low as 900 fpm and machines with poor CD moisture
profiles.
5. Bar design and configuration can have a significant impact on
performance. Syphon clearance is also closely related to the optimal
performance of dryer bars. With the correct condensate depth, the condensate
resonates "in tune" with dryer rotation. This greatly increases the
rate of heat transfer from the dryer cylinder.
6. For some applications, improvement in uniformity of heat
transfer is needed without any increase in the rate of heat transfer.
"De-tuned" Turbulator bars were developed for such applications.
These have been installed on some newsprint machines to reduce heat transfer
efficiency while maintaining uniform CD profiles. Many newsprint machines have excess
drying capacity and must operate at very low dryer steam pressures, due to improved
press section water removal and lower basis weights. "De-tuned" bars
permit running higher steam pressures with improved steam and condensate system
control.
When the paper sheet enters the paper machine Dryer Section, it is
about 50% water. It must be dried to less than 10% water for a finished
product. The drying section of the process consumes around 90% of the steam
demand of a typical paper mill. Less energy is used in removing water from the
web by mechanical means than by evaporation.
Monitor product dryness leaving the press section; a 1% increase
in dryness leaving the press results in a 4% decrease in steam consumption of
the drying section. There is a balance between removing water at the wet end
and in presses through increased electrical power for presses and vacuum
against the value of the lower cost steam saved. Dewatering in the papermaking
machine is achieved by increasing the nip pressure and by applying it uniformly
in the cross direction.
_
Examine compliance of final product
dryness and overall evenness of quality. Poor moisture profile is usually
corrected by over drying
_
Cylinder wall finish and
cleanliness and close contact between the feedstock and the cylinder external
surface will affect drying rates.
_
Characteristics of both the paper
and the type of felt used will affect operational efficiencies.
_
Make sure that water can be
efficiently drained away from the forming section in the most effective manner.
Check collection points, weirs, pipe-work and sumps for downstream blockages.
_
Ensure proper maintenance of the
vacuum system removing water through the suction boxes. Check seals for condition
and leakage. Power is wasted if too high a vacuum is maintained, so ensure
adequate levels are maintained and that controls are operable and accurate. For
overall drying operations. Develop a figure for energy input per kg water
evaporated, (theoretical minimum is 0.63 kWh/kg water).
_
Examine suitability and efficacy of
drying mechanism controls. Check whether the end point temperature and humidity
controls installed and working correctly. Less energy is used in removing water
from the web by mechanical means than by evaporation; check on moisture levels
at the interface.
_
Examine compliance of final product
dryness and overall evenness of quality. Poor moisture profile is usually
corrected by over drying.
_ Monitor dryer inlet and outlet air temperatures and flows over
daily/weekly operations. Link to product throughput and moisture levels to
establish a heat and mass balance for overall drying operations. Develop a
figure for energy input per kg water evaporated, (theoretical minimum is 0.63
kWh/kg water).
_ Ensure adequate removal of condensate and uncondensed gases from
within drying cylinders. Uneven distribution of the steam supply over the
internal surface could affect paper condition.
The
concepts for saving energy in cylinder dryers for textiles discussed in
previous section apply to paper drying as well.
4.2.3
New Technologies for efficient drying
Impulse
drying is a technology that improves the mechanical dehydration of paper and consequently
reduces the amount of water that has to be removed in the drying section. The press
cylinder is heated by steam or electro-techniques (infrared, induction
heating). Very high temperatures (200-500oC) are used and contact time is very
short.
In
the Condebelt drying concept a wet web (sheet of paper) is carried between two
steel bands, one hot band and one cold band, and subjected to high pressure
(max. 10 bar) and temperature (max. 180oC). Heat is transferred from the hot
band to the sheet; moisture evaporates and traverses through two wire screens
to the cold band, where it condenses. The condensate is carried away by the
thickest of the two wire screens. The sheet is dried in absence of air. In
contrast with conventional pressing technologies and impulse drying the pressure
is maintained for several seconds, resulting in good paper qualities. Drying
rates are 5-15 times as high as in conventional drying. Condensing belt drying
can dry paper from 44% (exit conventional pressing section) to 94%. The
technical life of paper machines is approximately 20 years and investment costs
are extremely high. Demonstration of new pressing and drying technologies will
be difficult. The first Condebelt dryer is delivered to Finnish paper mill
(Pankakoski) and would start production in the 1996. Condensing belt will be available
for all types of paper, except tissue.
4.3
Chemical/Pharmaceutical/Food/Dairy Industry
In
Chemical Industry, Inorganic salts and insoluble organic dyes require drying.
Many of the materials are heat resistive and require time temperature control
to prevent degradation and exact get exact shades. These require tray/vacuum
dryers in batch process and semi continuous truck and tray tunnel dryers,
direct and indirect rotary dryers, continuous through circulation dryers and
spray dryers for large productions.
In
Pharmaceutical industry, the material in powder, granular or crystalline form
having moisture/solvents needs drying. These are generally heat sensitive.
These require all kinds of tray dryers, fluidised bed dryers and vibratory
conveyor dryers for small productions and rotary dryers, flash dryers,
Continuous through circulation and fluidised bed for large production. Very sensitive
materials have to be dried in Spray dryers, High vacuum tray dryers and freeze dryers.
Dryers
for liquids
Simple
and colloidal solutions, emulsions such as salt solutions, extracts, milk,
blood, waste liquors, rubber latex etc. are examples. For large production,
spray dryers of direct contact and continuous operation can be used. It permits
use of high temperatures with heat sensitive materials. The product usually is
powdery, free flowing, spherical and has low bulk density.
Another
method for continuous drying is Film drum dryers at atmospheric pressure and vacuum.
The product is usually flaky and dusty and maintenance costs may be high. For
small batches, jacketed pan types dryers are used. These can be cleaned and
amenable to solvent recovery. For heat sensitive and readily oxidised
pharmaceutical materials like Penicillin and blood, freeze dryers are useful.
Dryers
for Slurries:
Pumpable
suspensions such as pigment slurries, soap and detergents, calcium carbonate, bentonite,
clay slip lead concentrates etc. are examples of slurries require drying in
chemical industries. Spray dryers could be used with pressure nozzle atomisers.
Film dryers with twin are widely used. For small batches, vacuum shelf dryers
can be used. Tray/compartment dryers are used for very small –laboratory type
production.
Dryers
for pastes and sludges
Filter
press cakes, sedimentation sludges, centrifuged solids, starch etc. require
drying in chemical/food industry. Continuous Tray tunnels are suitable for
small and large productions. For small batches, tray-compartment dryer is used.
These have very long drying times and for larger production, investment and
operating costs are high. If the material can be preformed, then batch type or
continuous through circulation is possible. For heat sensitive, readily
oxidisable material, indirectly heated vacuum shelf dryer can be used. Spray
dryers would need very special pumping equipment to feed the atomiser.
Dryers
for free flowing powders
100
mesh or less free flowing when wet but dusty when dry such as cement, clay,
pigments, precipitates etc. are examples. Screw conveyors and indirectly heated
rotary dryers suit a large range of materials and capabilities and have
continuous dust free operation. Drying with steam is possible. Rotary vacuum
dryers are considered for large batches of heat sensitive material where
solvent recovery is also desired. For large capacities, pneumatic conveying
type direct contact dryers are suitable if the material can be suspended and
looses moisture easily. If dusting is not too severe, direct rotary dryers of
continuous type can suit many materials. Fluidised bed batch type dryers can be
used in case of non-dusty materials.
Dryers
for granular/Crystalline or fibrous materials
Larger
than 100 mesh such as sand ores, salt crystals, rayon staples, potato strips,
synthetic rubber etc. are the typical materials. For most materials and
capacities, continuous rotary dryers are suitable. The limitation comes only in
the form of dust and abrasion. For large batches of heat sensitive materials,
or where solvent is to be recovered, batch type indirect vacuum rotary dryers
can be used. Product is subjected to some grinding action and dust collection
may be required. Screw conveyor and indirect rotary dryer with continuous
operation have low dust loss. Continuous pneumatic conveying direct type dryers
have high capacities and can handle materials that are easily suspended.
Fluidised bed dryers are suitable for crystals, granules and short fibers.
Tray/vacuum tray dryers may be selected for small batches, keeping in mind that
drying times are long. Where primarily surface moisture only is to be
removed,
infra red dryers can be considered.
Approach
to energy saving:
Heat recovery from exhaust air to preheat
incoming air
Proper mechanical dewatering of feed before
entering the dryer
Online instrumentation and automatic feed
forward controls
Energy saving by optimising auxiliary
equipment operation.
4.4
Tea Industry
The
main objectives of tea drying are to arrest enzyme reaction as well as
oxidation to remove moisture from the leaf particles and to produce a stable
product with good keeping quality. On an average 100 kg of fresh leaf produces
22.5 kg of dried tea containing residual 3% moisture. The difference of 77.5 kg
between the figures represents the moisture evaporated during the process. Of
the 77.5 kg, about 20-25 kg are evaporated during withering and around 20-50 kg
are evaporated during drying.
Common
fuel consumption figures per 1 Kg tea are given below.
Table 4-4: Tea Dryer
Conventional
Dryer
|
Coal (Kg) Hand stoked
|
Oil (1)
|
Natural gas (m3)
|
||
|
Indirect
|
Direct
|
Indirect
|
Direct
|
|
|
Drying only
|
1-1.10
|
0.3-0.4
|
0.5-0.6
|
-
|
|
Including wither
|
1.15-1.25
|
0.4-0.5
|
0.6-0-7
|
0.50-0.85
|
Fluid
Bed Dryer
|
Coal (Kg)
|
Oil (1)
|
||
|
Drying only
|
0.39-0.70
|
0.17-0.20
|
0.17 Kg
|
Energy
saving approach in tea drying includes:
•
Heater insulation
•
Excess air control
•
Adoption of gasifiers
•
Recirculation of exhaust air/ Waste heat recovery
•
Use of Solar heaters
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