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Drying is a complex operation involving transient transfer of heat and mass along with several rate processes, such as physical or chemical transformation, in which may cause changes in product quality as well as the mechanism of heat and mass transfer.
In paper making drying of paper is by dryer cylinder. New drying technologies, better operational strategies and control of industrial dryer as well as improved methodologies can contribute to energy efficient and better quality dried product. In this paper approach towards energy saving in drying of paper during manufacturing.
Keywords: Drying, Energy efficient, Paper drying, Quality of product, Drying technologies.
Drying is perhaps the oldest, most common and most diverse of chemical engineering unit operations. It competes with distillation as the most energy-intensive unit operation due to the high latent heat of vaporization and the inherent inefficiency of using hot air as the (most common) drying medium. Various studies report national energy consumption for the industrial drying operations ranging from 10 to 20% all the developing countries based on the mandatory energy audit data supplied by industry and hence are more reliable.
Drying occurs by effective vaporization of the liquid by supplying heat to the wet feedstock. Heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers), radiation or volumetrically by placing the wet material in a microwave or radio frequency electromagnetic field. Over 85 percent of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium. Over 99 percent of the applications involve removal of water. All modes except the dielectric (microwave and radio frequency)supply heat at the boundaries of the drying object so that the heat must diffuse into the solid primarily by conduction. The liquid must travel to the boundary of the material before it is transported away by the carrier gas (or by application of vacuum for non-convective dryers).
Drying of wet web of paper is among the largest steam users at any mill. Drying starts by heating the wet web of 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 including overall heat losses, using less air, and increasing the heat extraction from each unit of steam used for drying. This is the traditional drying technique. With escalating energy costs and need to mitigate environmental pollution due to emissions due to combustion of fossil fuels, it is increasingly important to develop innovative drying technologies. Furthermore, drying is also affects quality of the dried product. The conventional method with a cylinder dryer, this new technique resulted in higher drying rates and produced a paper of better quality. However, none of them was capable of substituting the multi-cylinder dryer. (Mujumdar, 1996).
Drying occurs in three different periods, or phases, which can be clearly defined.
The first phase, or initial period, is where sensible heat is transferred to the product and the contained moisture. This is the heating up of the product from the inlet condition to the process condition, which enables the subsequent processes to take place. The rate of evaporation increases dramatically during this period with mostly free moisture being removed.
The second phase, or constant rate period, is when the free moisture persists on the surfaces and the rate of evaporation alters very little as the moisture content reduces. During this period, drying rates are high, and higher inlet air temperatures than in subsequent drying stages can be used without detrimental effect to the product. There is a gradual and relatively small increase in the product temperature during this period.
The third phase, or falling rate period, is the phase during which migration of moisture from the inner interstices of each particle to the outer surface becomes the limiting factor that reduces the drying rate.
Measuring moisture content allows control of the drying process such that drying is carried out until a specific level of moisture content is achieved rather than for a fixed time period. Electrical resistance type meters operate on the principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Most of these types of instruments are suitable for measuring moisture content in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee, jute, tobacco, rice, copra, and concrete. Resistance meters have an average accuracy of + 1% MC over their operating range.
Calculation of the quantity of water to be evaporated is explained below with a sample calculation.
If the throughput of the dryer is 60 kg of wet product per hour, drying it from 55% moisture to 10% moisture, the heat requirement is:
60 kg of wet product contains 60 x 0.55 kg water = 33 kg moisture and 60 x (1 - 0.55) = 27 kg bone-dry product.
As the final product contains 10% moisture, the moisture in the product is 27/9 = 3 kg and so moisture removed = (33 - 3) = 30 kg
Latent heat of evaporation = 2257 kJ kg-1(at 100 °C so heat necessary to supply = 30 x 2257 = 6.8 x l04 kJ
REVIEW OF MAJOR DRYERS
Rotary dryers potentially represent the oldest continuous and undoubtedly the most common high volume dryer used in industry, and it has evolved more adaptations of the technology than any other dryer classification. All rotary dryers have the feed materials passing through a rotating cylinder termed a drum. It is a cylindrical shell usually constructed from steel plates, slightly inclined, typically 0.3-5 m in diameter, 5-90 m in length and rotating at 1-5 rpm. It is operated in some cases with a negative internal pressure (vacuum) to prevent dust escape. Solids introduced at the upper end move towards the lower or discharge end
The pneumatic or 'flash' dryer is used with products that dry rapidly owing to the easy removal of free moisture or where any required diffusion to the surface occurs readily. Drying takes place in a matter of seconds. Wet material is mixed with a stream of heated air (or other gas), which conveys it through a drying duct where high heat and mass transfer rates rapidly dry the product. Applications include the drying of filter cakes, crystals, granules, pastes, sludges and slurries; in fact almost any material where a powdered product is required.
Spray drying has been one of the most energy-consuming drying processes, yet it remains one that is essential to the production of dairy and food product powders. Basically, spray drying is accomplished by atomizing feed liquid into a drying chamber, where the small droplets are subjected to a stream of hot air and converted to powder particles. As the powder is discharged from the drying chamber, it is passed through a powder/air separator and collected for packaging. Most spray dryers are equipped for primary powder collection at efficiency of about 99.5%, and most can be supplied with secondary collection equipment if necessary.
Fluidized Bed Dryers
Fluid bed dryers are found throughout all industries, from heavy mining through food, fine chemicals and pharmaceuticals. They provide an effective method of drying relatively free flowing particles with a reasonably narrow particle size distribution. In general, fluid bed dryers operate on a through-the-bed flow pattern with the gas passing through the product perpendicular to the direction of travel. The dry product is discharged from the same section.
This is the simplest and cheapest mode of drying woven fabrics. It is mainly used for intermediate drying rather than final drying (since there is no means of controlling fabric width) and for predrying prior to stentering. Fabric is passed around a series of steam heated cylinders using steam at pressures varying from 35 psi to 65 psi. Cylinders can be used to dry down a wide range of fabrics, but it does give a finish similar to an iron and is therefore unsuitable where a surface effect is present or required. In stenters, the fabric is width wise stretched for width fixation by a series of holding clips or pins mounted on a pair of endless chains.
Paper & Pulp 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 I dried in Cylinder/Can dryers. Paper pulp takes many shapes as molded materials, boards, light and heavy weight paper, 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.
Paper Making Process
The energy and material flow diagram of an integrated paper mill is shown below,
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 20 comes off the machine ready for reeling up into large reels, each of which may contain up to 20 tones 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.
Theoretical steam requirement in paper drying cylinders
Equation for Evaporation Rate, Lbs/hr/sq.ft
Where T = Temperature of saturated steam, degree F.
The surface area refers to the contact surface of the paper with the cylinder.
Approach to energy saving
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
Make sure that water can be efficiently drained away from the forming section in the most effective manner.
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.
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.
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.
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.
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.
Radio frequency (R-F) drying
In a radio frequency drying system, the RF generator creates an alternating electric field
between two electrodes. The material to be dried is conveyed between the electrodes, where the alternating energy causes polar molecules in the water to continuously re-orient themselves to face opposite poles-much in the same way magnets move in an alternating magnetic field. The friction of this movement causes the water in the material to rapidly heat throughout the material's entire mass.
RF drying offers numerous benefits to ceramic and glass manufacturers, including moisture control and uniformity; reduction in surface cracking; and savings in energy, drying time and plant space.
Precise Control of Moisture Content and Uniformity. Heating in an RF dryer occurs
selectively in those areas where heat is needed because water is much more responsive to RF energy than most other dielectric materials. Since wetter areas absorb more RF power than dryer areas, more water is automatically removed from wet areas, resulting in a more uniform moisture distribution.
Energy Savings. The efficiency of convection dryer drops significantly as lower moisture levels are reached and the dried product surface becomes a greater thermal insulator. At this point, the RF dryer provides an energy-efficient means of achieving the desired moisture objectives. Typically, 1 kW of RF energy will evaporate 1 kg of water per hour. Additionally, because RF is a "direct" form of applying heat, no heat is wasted in the drying process.
Optimal design for drying individual or in combination of different drying techniques which will be energy efficient, cost effective, improved product quality and quantity, reducing environmental influence, safe operation, and easy control.
Better operational strategies and control of industrial dryers, improved and more reliable scale-up methodology can contribute to better cost effectiveness and better quality dried product. Water is removed from a wet paper web by the combined action of mechanical pressure and intense heat. This result in increased dewatering rates, increased smoothness and increased density.