¡Descarga Secadores ejercicios y más Ejercicios en PDF de Calor y Transferencia de Masa solo en Docsity!
Module # 4
DESIGN OF DRYERS: INTRODUCTION, TYPES OF DRIERS, DESIGN
CONSIDERATION OF DRIERS
1. INTRODUCTION
2. PHYSICAL MECHANISM OF DRYING
3. CLASSIFICATION OF DRYERS
4. DRYING EQUIPMENT
4.1 Batch Type Dryers 4.1.1 Tray Dryer 4.1.2 Pan Dryer 4.1.3 Agitated Vacuum Dryer 4.2 Continuous Dryer 4.2.1 Rotary Dryer 4.2.2 Drum Dryer 4.2.3 Flash Dryer 4.2.4 Fluidised Bed Dryer 4.2.5 Screen Conveyor Dryers
5. NOVEL DRYING TECHNOLOGIES 5.2 Microwave Drying **5.3 Supercritical Fluid Extraction and its application to Drying
- SELECTION OF DRYING EQUIPMENT
- DESIGN OF DRYER**
Lecture 1: INTRODUCTION AND TYPES OF
DRIERS
1. INTRODUCTION
The term drying refers generally to the removal of moisture from a substance. It is one of the oldest, most commonly used and most energy consuming unit operation in the process industries. Drying is often necessary in various industrial operations particularly in chemical process industries to remove moisture from a wet solid, a solution or a gas to make it dry and choice of drying medium is depends on the chemical nature of the materials. Three basic methods of drying are used today 1) sun drying, a traditional method in which materials dry naturally in the sun, 2) hot air drying in which matrials are exposed to a blast of hot air and 3) freeze drying, in which frozen materials are placed in a vacuum chamber to draw out the water. The fundamental nautre of all drying porcess is the removal of volatile substances (mainly moisture) from mixture to yield a solid product. In general drying is accomplished by thermal techniques and thus involves the application of heat, most commonly by convection from current of air. Throughout the convective drying of solid materials, two processes occur simultaneously namely, transfer of energy from the local environemnt in the dryer and transfer of moisture from within the solid. Therefore this unit operation may be considered as simultaneous heat and mass transfer operation. Drying processes and equipment may be categorised according to several criteria, incuding the nature of material and the method of heat supply and the method of operation. For example In the sugar industry washed and centrifuged sugar crystals are dried to get finisehd product for packing. Drying is an important operation in food processing. Milk is dried in a spray chamber to produce milk powder. All the above examples indicates that wet material loses moisture in direct contact with hot air/gas. The hot air/gas supplies the energy required for drying and also carries away the moisture released by the solid. For heat sensitive materials much of the resistance to drying resides within the material. Unduly high heat and mass transfer rates applied at the surface only result in overheating or over drying of the surface layer resulting in quality problems without major increase in the drying kinetics. The rate of migration of the moisture from within the solid to the evaporation front often controls the overall drying rate. Therefore, drying may be defined as an operation in which the liquid, generally water, present in a wet solid is removed by vaporization to get a relatively liquid free solid product. Drying of a solid does not demand or ensure complete removal of the moisture. Sometimes it is desirable to retain a little mositure in the solid after drying. Dryer and drying process selection for a specific operation is a complex problem, and many factors have to be taken into account. Though, the overall selection and design of a drying system for a perticular material is dictated by the desire to achieve a favourable combination of a product quality and process
The moisture content of solid in excess of the equilibrium moisture content is refered as free moisture. During drying, only free moisture can be evporated. The free moisture content of a solid depends upon the vapour concentration in the gas.
The moisture contents of solid when it is in equilibrium with given partial pressure of vapour in gas phase is called as equilibrium moisture content. Similalry, the moisture content at which the constant rate drying peroid ends and the falling rate drying period starts is called critical moisture content. During the constant rate drying period , the moisture evporated per unit time per unit area of drying surface remains constant and in falling rate drying period the amount of moisture evporated per unit time per unit area of drying surface continuously decreases.
3. CLASSIFICATION OF DRYERS
Drying equipment is classified in different ways, according to following design and operating features. It can be classified based on mode of operation such as batch or continuous, In case of batch dryer the material is loaded in the drying equipment and drying proceeds for a given period of time, whereas, in case of continuous mode the material is continuously added to the dryer and dried material continuously removed. In some cases vacuum may be used to reduce the drying temperature. Some dryers can handle almost any kind of material, whereas others are severely limited in the style of feed they can accept. Drying processes can also be categorized according to the physical state of the feed such as wet solid, liquid, and slurry. Type of heating system i.e. conduction, convection, radiation is another way of categorizing the drying process. Heat may be supplied by direct contact with hot air at atmospheric pressure, and the water vaporized is removed by the air flowing. Heat may also be supplied indirectly through the wall of the dryer from a hot gas flowing outside the wall or by radiation. Dryers exposing the solids to a hot surface with which the solid is in contact are called adiabatic or direct dryers, while when heat is transferred from an external medium it is known as non-adiabatic or indirect dryers. Dryers heated by dielectric, radiant or microwave energy are also non adiabatic. Some units combine adiabatic and non adiabatic drying; they are known as direct-indirect dryers.
To reduce heat losses most of the commercial dryers are insulated and hot air is recirculated to save energy. Now many designs have energy-saving devices, which recover heat from the exhaust air or automatically control the air humidity. Computer control of dryers in sophisticated driers also results in important savings in energy.
4. DRYING EQUIPMENT
4.1 Batch Type Dryers
4.1.1 Tray Dryer
Schematic of a typical batch dryer is shown in figure 2.1. Tray dryers usually operate in batch mode, use racks to hold product and circulate air over the material. It consists of a rectangular chamber of sheet metal containing trucks that support racks_._ Each rack carries a number of trays that are loaded with the material to be dried. Hot air flows through the tunnel over the racks. Sometimes fans are used to on the tunnel wall to blow hot air across the trays_. Even b_ affles are used to distribute the air uniformly over the stack of trays. Some moist air is continuously vented through exhaust duct; makeup fresh air enters through the inlet_._ The racks with the dried product are taken to a tray-dumping station.
Figure 2.1: Tray dryer
Inlet air Exhaust
Hot side
Temperatures
Cold side
Temperatures
designing includes fatigue consideration. Designing the impeller needs consideration of characteristics of the material before and after drying.
4.2Continuous Dryer
4.2.1 Rotary Dryer
The rotary drier is basically a cylinder, inclined slightly to the horizontal, which may be rotated, or the shell may be stationary, and an agitator inside may revolve slowly. In either case, the wet material is fed in at the upper end, and the rotation, or agitation, advances the material progressively to the lower end, where it is discharged. Figure (2.2) shows a direct heat rotary drier. Typical dimensions for a unit like this are 9 ft diameter and 45 ft length. In direct-heat revolving rotary driers, hot air or a mixture of flue gases and air travels through the cylinder. The feed rate, the speed of rotation or agitation, the volume of heated air or gases, and their temperature are so regulated that the solid is dried just before discharge.
Figure 2.2: Counter current direct heat rotary dryer The shell fits loosely into a stationary housing at each end. The material is brought to a chute that runs through the housing; the latter also carries the exhaust pipe. The revolving shell runs on two circular tracks and is turned by a girth gear that meshes with a driven pinion. The inclination is one in sixteen for high capacities and one in thirty for low ones. As the shell revolves, the solid is carried upward one-fourth of the circumference; it then rolls back to a lower level, exposing fresh surfaces to the action of the heat as it does so. Simple rotary driers serve well enough when fuel is cheap. The efficiency is greatly improved by placing longitudinal plates 3 or 4 in. wide on the inside of the cylinder. These are called lifting flights. These carry part of the solid half-way around the circumference and drop it through the whole of a diameter in the central part of the cylinder where the air is hottest and least laden with moisture. By bending the edge of the lifter slightly inward, some of the material is delivered only in
the third quarter of the circle, producing a nearly uniform fall of the material throughout the cross section of the cylinder. The heated air streams through a rain of particles. This is the most common form of revolving rotary cylinder. It has high capacity, is simple in operation, and is continuous. Table 2.1: Rotary dryers practical ranges of dimension and operating parameters
Shell i.d. : D = 1 to 10 ft Length, L = 4 D to 15 D Radial flight height: D/12 to D/8; shell rpm: 4 to 5
Pripheral shell speed: 50 – 100 ft/min
The flight count per circle: 2.4D to 3 D Inclination of the shell to the horizontal: up to 8cm/m
Avg. solid retention time: 5 min to 2h
Mass flow rate of the drying gas: 300 to 5000 lb/h.ft^2
Drying capacity: 0.4 to 2.5 lb moisture/(h) (ft^3 dryer volume) Number of heat transfer units in the dryer (NT): 1.5 to 2
Solid hold up m(i.e. fraction of the shell volume occupied by the solid at any time): 5-15% Courtesy: Principle of Mass Transfer and Separation Processes, B.K. Dutta, 2007.
Figure 2.3a: Single drum dryer
Figure 2.3b: Double drum dryer
4.2.3 Flash Dryer
The flash driers (figure 2.4), also called pneumatic dryers, are similar in their operating principle to spray dryer. The materials that are to be dried (i.e. solid or semisolid) are dispersed in finely divided form in an upward flowing stream of heated air. These types of dryer are mainly used for drying of heat sensitive or easily oxidizable materials. The wet materials that are to dried can be passed into a high- temperature air stream that carries it to a hammer mill or high-speed agitator where the exposed surface is increased. The drying rate is very high for these dryers (hence the term flash dryers ), but the solid temperature does not rise much because of the short residence time. A flash dryer is not suitable for particles which are large in size or heavy particles. The special advantage of this type of dryer is that no separate arrangement is required for transporting the dried product. The fine particles leave the mill through a small duct to maintain the carrying velocities (drying gas) and reach a cyclone separator. A solid particle takes few seconds to pass from the point of entry into the air stream to the collector. The inlet gas temperature is high and varies from 650 oC to 315oC, for example, in 2 seconds, or from 650oC to 175oC in 4 seconds. The thermal efficiency this type of dryer is generally low. A material having an initial moisture content of 80 % may be reduced to 5 or 6 % in the dried product.
Figure 2.4: Flash dryer
4.2.5 Screen Conveyor Dryers
Screen conveyor dryer is also called a direct heat continuous type dryer. The solid to be dried are fed on to endless, perforated, conveyor belt through which hot air is forced. The belt is housed in a long rectangular drying chamber or tunnel (figure 2.6). The chamber is divided into series of separate sections, each with its own fan and air heater. Air may be recirculated through, and vented from each section separately or passed from one section to another counter current to the solid movement. The solid is carried through the tunnel and discharged at the opposite end. In order to prevent the higher flow rate of hot air through thinner regions of the bed a uniform feeding rate and distribution of the material over the conveyor is necessary. Coarse granular, flakey, or fibers materials can be dried by through circulation without any pretreatment and without loss of material through the screen. High drying rate can be achieved with good product quality control. Thermal efficiency of this type of dryer is high and with steam heating, the steam consumption for heating the drying gas can be as low as 1.5 kg per kg of water evaporated. Only disadvantage of this type of dryer are high initial cost and high maintenance cost due to the mechanical belt.
Figure 2.6: Screen conveyor dryer
6. NOVEL DRYING TECHNOLOGIES
Newer technologies focus on saving in energy consumption that result in considerable overall improvement in energy efficiency. In addition, the final quality of the product is greatly influenced by the drying technique and strategy. A brief overview of some novel drying techniques is given below:
5.1 Microwave Drying
Microwave heating is a direct drying method. High-frequency radio waves are utilized in microwave drying. A high-frequency generates the waves and wave channel guides them in to an oven that is designed to prevent the waves from leaving the chamber. In microwave drying, heat is generated by directly transforming the electromagnetic energy in to kinetic molecular energy, thus the heat is generated deep within the material to be dried. Selection of proper wavelength is necessary to ensure thorough penetration into the material. Apart from these, other parameters such as material type and depth of material being exposed also affect the penetration. Therefore, selection of proper wavelengths and dehydration condition for each product is selected individually. This type of heating is instantaneous, uniform and penetrating throughout the material, which is a great advantage for the processing of pharmaceutical compounds. In case of microwave drying the waves bounce from wall to wall, until the product absorbs eventually all of the energy, generating heat within the material, resulting in dehydration. Vapour from the liquid evaporating inside the product is emitted through the pore structure of the solid material‟s macro-capillary system, resulting in a high drying rate. This type of dryer is highly efficient and power utilization efficiencies are generally greater than 70 %. Important commercial aspects of this dryer includes the ability to maintain colour, moisture and quality of the natural food.
6.2Supercritical Fluid Extraction and its application to Drying
The supercritical fluid (SCF) is a substance at a temperature and pressure above its critical point. It can effuse through solids like a gas, and dissolve materials like a liquid. Supercritical fluids possess unique properties that enable them to extract components selectively from a mixture. This ability has been investigated as an alternative to currently used separation processes such as distillation or liquid extractions. In addition, close to the critical point, small changes in pressure or
There are some general guidelines which need to be followed to select a dryer, but it should be recognized that the rules are far from rigid and exceptions not uncommon. Often batch dryers are used when the production rate of dried product is less than 150 to 200 kg/h, while continuous dryers are suitable for production rates greater than 1 or 2 tons/h. To handle intermediate production rates other factors must be considered. The dryer must also operate reliably, safely, and economically. Operation and maintenance costs must not be excessive; pollution must be controlled; energy consumption must be minimized. As with other equipment these requirements may be conflict with one another and a compromise needs to be reached in finding the optimum dryer for a given service. As far as the drying operation itself is concerned, adiabatic dryers are generally less expensive than non-adiabatic dryers, in spite of the lower thermal efficiency of adiabatic units. Unfortunately there is usually a lot of dust carry over from adiabatic dryers, and these entrained particles must be removed from the drying gas. Elaborate particle-removal equipment may be needed, equipment that may cost as much as the dryer itself. This often makes adiabatic dryers less commercially attractive than a “buttoned-up” non-adiabatic system in which little or no gas is used.
Lecture 3: DESIGN CONSIDERATION OF DRIERS
8. DESIGN OF DRYER
Design of a rotary dryer only on the basis of fundamental principle is very difficult. Few of correlations that are available for design may not prove to be satisfactory for many systems. The design of a rotary dryer is better done by using pilot plant test data and the full scale operating data of dryer of similar type if available, together with the available design equations. A fairly large number of variables are involved such as solid to be dried per hour, the inlet and exit moisture contents of the solid, the critical and equilibrium moisture contents, temperature and humidity of the drying gas. The design procedure based on the basic principles and available correlations is discussed below. In this case we assume that the solid has only unbound moisture and as shown in fig 2.7 in stage II the solid is at the wet bulb temperature of the gas.
Figure 2.7: Temperature profile for solid and gas in a counter current rotary dryer
- Heat losses from dryer surfaces are neglected.
- Once the capacity of the dryer is known, the drying gas flow rate, its temperature and humidity are decided considering a number of factors. And the following moisture & enthalpy balances need to be satisfied. Gs (Y 1 - Y 2 ) = Ms (X 1 – X 2 ) Gs (Hg 2 – Hg 1 ) = Ms (HS2 – Hs 1 ) Here, Gs = flow rate of air (dry basis, kg/h), Ms = flow rate of solid (kg/h, dry basis), Hs = humidity of air (kg/H 2 O/kg dry air)
Example 2.1 : Size of the rotary dryer can be estimated for the following case. A moist non hygroscopic granular solid at 26^0 C is to be dried from 20% initial moisture to 0.3% final moisture in a rotary dryer at a rate of 1500 kg/h. The hot air enters the dryer at 135^0 C with a humidity of 0.015. With condition that the temperature of the solid leaving the dryer must not exceed 110^0 C and the air velocity must not exceed 1.5 m/s in order to avoid dust carry over. Cps = 0.85 kJ/kg.K. Recommend the diameter, length and other parameters of the dryer. Solution: Basis of calculation is 1 hr operation Solid contains 20% initial moisture Mass of dry solid = MS = 1500 (1-0.2) = 1200 kg/hr Moisture in the wet solid = X 1 = 20/80 = 0. Moisture in the dry solid = X 2 = 0.3/99.7 = 0. Water evaporated, mS, evaporated = MS (X 1 – X 2 ) = 1200 (0.25 – 0.00301) = 296.4 Kg Given data: TS1 = 26^0 C; TG2 = 135^0 C; Y 2 = 0. Let us assume that the exit temperature of the gas is TG1 = 60 oC and for solid TS2 = 100 oC Now enthalpy of different streams (suppose ref temp = 0oC) HS1 = [CPS + (4.187) X 1 ] [TS1 – 0] = [0.85 + (4.187) 0.25] [26 – 0] = 49.31 KJ/kg dry air HS2 = [CPS + (4.187) X 1 ] [TS1 – 0] = [0.85 + (4.187) 0.0.00301] [100 – 0] = 86.2 KJ/kg dry solid Hg2 = [1.005 + (1.88) 0.015] [135 – 0] + (0.015) (2500) = 177 KJ/kg Hg1 = [1.005 + (1.88) Y 1 ] [60 – 0] + Y 1 (2500) = 60.3 + 2613 Y 1 Overall mass balance GS (Y 1 – Y 2 ) = MS (X 1 – X 2 ) GS (Y 1 – 0.015) = 296.
GS = 296.4/(Y 1 – 0.015) MS [HS2 – HS1] = GS [Hg2 – Hg1]
1200 [86.2 – 49.31] = 296.4/(Y 1 – 0.015 ) × (177 – 60.3 -2613Y 1 )
Y 1 = 0.04306 and Gs = 296.4/(Y 1 – 0.015) = 10560 Kg/h Shell Diameter Volume of humid inlet gas (135^0 C and Y 2 = 0.015) VH2 = 1.183 m^3 /Kg dry air Volume of humid exit gas (60^0 C and Y 1 = 0.04306) VH1 = 1.008 m^3 /Kg dry air The max. volumetric gas flow rate = Gs.VH = 10560 × 1.183 = 12490 m^3 /h The working velocity i.e. superficial velocity = 1.5 – 0.2 × 1. = 1.2 m/s / 4 × d^2 (1.2) = d = 1.98 m, say 2.0 m Heat Transfer Unit Dryer is divided into three zones and therefore, the stage wise calculation of temperature and humidity of the stream can be obtained by material and energy balance. Stage III Very less water left for vaporization in stage III. Consider solid is at TSB, the wet bulb temperature of the air at location between III & II. assume TSB = TSA = 41^0 C Enthalpy of solid at the inlet to stage III HSB = [0.85 + (0.00301) (4.187)] (41-0) = 35.37 KJ/kg dry solid Humid heat of gas entering stage III CHB = [1.005 + (1.88) (0.015)] = 1.003 KJ/kg.K Heat balance over stage III MS [HS2 - HSB ] = GS (CHB)III (135 – TGB) TGB = 129^0 C Adiabatic saturation temperature of air entering stage II (129^0 C & humidity of 0.015) is 41.3^0 C. At the boundary B, ∆TB = 129 -41 = 88^0 C At end 2, ∆T 2 = 135 -100 = 35^0 C
