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Mass transfer is the net movement of mass from one location to another. Mass transfer is used by different scientific disciplines for different processes and mechanisms. The phrase is commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems. Some common examples of mass transfer processes are the evaporation of water from a pond to the atmosphere; the diffusion of chemical impurities in lakes, rivers, and oceans from natural or artificial point sources; separation of chemical components in distillation columns. In Cooling towers, hot water flows down over the fill material as air flows up and contact between water and air evaporates some of the water. Evaporation requires heat; the heat is removed from the remaining water lowering its temperature.
Mass Transfer Mass Transfer Table of Contents 2 Short Details: Channeling Loading Flooding Weeping Entrainment Detailed Description Dryers Types of Dryers Draying Curves Procedure of Drying Cooling Tower Types of Cooling Tower Also Included Packing , its Types Why we use Packing Packing Materials Evaporation Function of Evaporators Types of Evaporation Dry bulb Temperature Wet bulb Temperature Mass Transfer E vaporation is the removal of solvent as vapor from a solution or slurry. For the overwhelming majority of evaporation systems the solvent is water. The objective is usually to concentrate a solution; hence, the vapor is not the desired product and may or may not be recovered depending on its value. Therefore, evaporation usually is achieved by vaporizing a portion of the solvent producing a concentrated 3 solution, thick liquor, or slurry. Evaporation often encroaches upon the operations known as distillation, drying, and crystallization. In evaporation, no attempt is made to separate components of the vapor. This distinguishes evaporation from distillation. Evaporation is distinguished from drying in that the residue is always a liquid. The desired product may be a solid, but the heat must be transferred in the evaporator to a solution or a suspension of the solid in a liquid. The liquid may be highly viscous or a slurry. Evaporation differs from crystallization in that evaporation is concerned with concentrating a solution rather than producing or building crystals. Function of Evaporators: As stated above, the object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy. In most cases the solvent is water, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces. For evaporators to be efficient, the equipment selected and used must be able to accomplish several things: (1) Transfer large amounts of heat to the solution with a minimum amount of metallic surface area. This requirement, more than all other factors, determines the type, size, and cost of the evaporator system. (2)Achieve the specified separation of liquid and vapor and do it with the simplest devices available. Separation may be important for several reasons: value of the product otherwise lost; pollution; fouling of the equipment downstream with which the vapor is Contacted; corrosion of this same downstream equipment. Inadequate separation may also result in pumping problems or inefficient operation due to unwanted recirculation. Mass Transfer (3)Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-pounds of solvent 4 evaporated per pound of steam used. Heat is required to raise the feed temperature from its initial value to that of the boiling liquid, to provide the energy required to Separate liquid solvent from the feed, and to vaporize the solvent. The greatest increase in energy economy is achieved by reusing the vaporized solvent as a heating medium. This can be accomplished in several ways to be discussed later. Energy efficiency may be Increased by exchanging heat between the entering feed and the leaving residue or condensate. (4) Meet the conditions imposed by the liquid being evaporated or by the solution being concentrated. Factors that must be considered include product quality, salting and scaling, corrosion, foaming, product degradation, holdup, and the need for special types of construction. Evaporator Classification & Types Evaporators are often classified as follows: Heating medium separated from evaporating liquid by tubular heating surfaces, Heating medium confined by coils, jackets, double walls, flat plates, etc., Heating medium brought into direct contact with evaporating liquid, and Heating with solar radiation. Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface. Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered. Evaporators may be operated batch wise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small Mass Transfer amounts must be evaporated. Batch operation generally requires more energy than continuous operation. Batch evaporators, strictly speaking, are operated such that 5 filling, evaporating, Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation. Evaporators may be operated either as once-through units or the liquid may be re circulated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials. Types Jacketed Vessels Coils Horizontal Tube Evaporators Long Tube Vertical Evaporators Short Tube Vertical Evaporators Forced Circular Evaporators Plate Evaporators Mechanically Aided Evaporators Submerged Combustion Evaporators Flash Evaporators Mass Transfer Packing and its Types 6 Packed columns are used for distillation, gas absorption, and liquid-liquid extraction; only distillation and absorption will be considered in this section. Stripping (desorption) is the reverse of absorption and the same design methods will apply. The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapour, counter-currently, up the column. In some gas-absorption columns co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. A schematic diagram, showing the main features of a packed absorption column, is Given in Figure 11.36. Choice of plates or packing The choice between a plate or packed column for a particular application can only be made with complete assurance by costing each design. However, this will not always be worthwhile, or necessary, and the choice can usually be made, on the Mass Transfer 7 Basis of experience by considering main advantages and disadvantages of each type; which are listed below: Plate columns can be designed to handle a wider range of liquid and gas flow- rates than packed columns. Packed columns are not suitable for very low liquid rates. The efficiency of a plate can be predicted with more certainty than the equivalent Term for packing (HETP or HTU). Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns. It is easier to make provision for cooling in a plate column; coils can be installed on the plates. It is easier to make provision for the withdrawal of side-streams from plate columns. If the liquid causes fouling, or contains solids, it is easier to make provision for cleaning in a plate column; manways can be installed on the plates. With small diameter columns it may be cheaper to use packing and replace the packing when it becomes fouled. For corrosive liquids a packed column will usually be cheaper than the equivalent plate column. The liquid hold-up is appreciably lower in a packed column than a plate column. This can be important when the inventory of toxic or flammable liquids needs to be kept as small as possible for safety reasons. Packed columns are more suitable for handling foaming systems. Mass Transfer The pressure drop per equilibrium stage (HETP) can be lower for packing than plates; and packing should be considered for vacuum columns. 8 Packing should always be considered for small diameter columns, say less than 0.6 m, where plates would be difficult to install, and expensive. Types of Packing The principal requirements of a packing are that it should: Provide a large surface area: a high interfacial area between the gas and liquid. Have an open structure: low resistance to gas flow. Promote uniform liquid distribution on the packing surface. Promote uniform vapour gas flow across the column cross-section. Many diverse types and shapes of packing have been developed to satisfy these requirements. They can be divided into two broad classes: 1. Packings with a regular geometry: such as stacked rings, grids and proprietary structured Packings. 2. Random packings: rings, saddles and proprietary shapes, which are dumped into the Column and take up a random arrangement. Mass Transfer 9 Why We Use Packing :( Applications of Packing) The term structured packing refers to packing elements made up from wire mesh or perforated metal sheets. The material is folded and arranged with a regular geometry, to give a high surface area with a high void fraction. Structured packings are produced by a number of manufacturers. The basic construction and performance of the various proprietary types available are similar. They are available in metal, plastics and stoneware. The advantage of structured packings over random packing is their low HETP (typically less than 0.5 m) and low pressure drop (around 100 Pa/m). They are being increasingly used in the following applications: For difficult separations, requiring many stages: such as the separation of isotopes High vacuum distillation For column revamps: to increase capacity and reduce reflux ratio requirements Figure 11.38 Make-up of structured Packing The applications have mainly been in distillation, but structured packings can also be used in absorption; in applications where high efficiency and low pressure drop are needed. Mass Transfer 10 The cost of structured packings per cubic meter will be significantly higher than that of random packings, but this is offset by their higher efficiency. The manufacturers’ technical literature should be consulted for design data. A review of the types available is given by Butcher (1988). Generalised methods for predicting the capacity and pressure drop of structured packings are given by Fair and Bravo (1990) and Kister and Gill (1992). The use of structured packings in distillation is discussed in detail in the book by Kister (1992). SHORT Detailed Topics Flooding Flooding The consequence of excessive column liquid loading where, in effect, the liquid on trays becomes too deep for the vapour to pass through or where the vapour flow rate is too high, creating an excessive differential pressure or a decrease in the differential temperature across the column. Flooding in Packed Towers Mass Transfer 11 To understand what is meant by flooding in a bed of packing, one ought to first understand what is meant by the idea of holdup. Let’s imagine we are operating the air- water scrubber in the Unit- Ops Lab at Cooper Union School of Engineering (Fig. 10.5). The water is circulating from the bottom to the top of the tower. The water level in the bottom is 2 ft. The height of the packed bed is 20 ft. At 3:30 P.M., Professor Liebskind shuts off our air compressor and the water circulation pump. As the water, which had been held up on the packing by the flowing air, drains down, the water level in the bottom of the tower rises from 2 to 5 ft. We say, then, that the holdup of the packing was 15 percent [(5 ft _ 2 ft) 20 ft_15 percent]. If the liquid holdup is too low, fractionation efficiency will be bad. We say that the height equivalent to a theoretical plate (HETP) will be high. If the liquid holdup is too high, fractionation efficiency will also be poor. We again say that the HETP will be high. This idea is expressed in Fig. 10.6. When the holdup rises above the point that corresponds to the minimum HETP, we can say that the packing is Beginning to flood. The minimum HETP point on Fig. 10.6 corresponds to the point of incipient flood, discussed in Chap. 3, for trayed towers. For structured-type packing, a liquid holdup of 4 to 5 percent corresponds to this optimum packing fractionation efficiency. For 1-in Raschig rings, this optimum holdup would be roughly 10 to 12 percent. Mass Transfer 12 Dry Bulb & Wet Bulb Temperature Dry bulb temperature the temperature of the air indicated by thermometer not affected by the water vapour content of the air Wet bulb temperature the lowest temperature which a water wetted body will attain when exposed to an air current. This is the temperature of adiabatic saturation. Wet bulb thermometer A thermometer whose bulb is covered with a piece of fabric such as muslin or cambric that is saturated with water; it is most often used as an element in a psychrometer. Entrainment The conveying of particles of water or solids from the boiler water by the steam Entrainment can be estimated from the correlation given by Fair (1961), Figure 11.29, Which gives the fractional entrainment (kg/kg gross liquid flow) as a function of the liquid-vapour factor FLV, with the percentage approach to flooding as a parameter. The percentage flooding is given by As a rough guide the upper limit of can be taken as 0.1; below this figure the effect on efficiency will be small. The optimum design value may be above this figure, see Fair (1963) Weep point (Weeping) The lower limit of the operating range occurs when liquid leakage through the plate holes becomes excessive. This is known as the weep point. The vapour velocity at the weep point is the minimum value for stable operation. The hole area must be chosen so that at the lowest operating rate the vapour flow velocity is still well above the weep point. Several correlations have been proposed for predicting the vapour velocity at the Mass Transfer weep point; see Chase (1967). That given by Eduljee (1959) is one of the simplest to use, and has been shown to be reliable. 13 Mass Transfer 14 Drying The drying of materials is often the final operation in a manufacturing process, carried out immediately prior to packaging or dispatch. Drying refers to the final removal of water, or another solute, and the operation often follows evaporation, filtration, or crystallization. In some cases, drying is an essential part of the manufacturing process, as for instance in paper making or in the seasoning of timber, although, in the majority of processing industries, drying is carried out for one or more of the following reasons: To reduce the cost of transport. To make a material more suitable for handling as, for example, with soap powders, dyestuffs and fertilizers. To provide definite properties, such as, for example, maintaining the free-flowing nature of salt. To remove moisture, this may otherwise lead to corrosion. One example is the drying of gaseous fuels or benzene prior to chlorination. With a crystalline product, it is essential that the crystals are not damaged during the drying process, and, in the case of pharmaceutical products, care must be taken to avoid contamination. Shrinkage, as with paper, cracking, as with wood, or loss of flavor, as with fruit, must also be prevented. With the exception of the partial drying of a material by squeezing in a press or the removal of water by adsorption, almost all drying processes involve the removal of water by vaporization, which requires the addition of heat. In assessing the efficiency of a drying process, the effective utilization of the heat supplied is the major consideration. RATE OF DRYING Drying periods In drying, it is necessary to remove free moisture from the surface and also moisture from the interior of the material. If the change in moisture content for a material is determined as a function of time, a smooth curve is obtained from which the rate of drying at any given moisture content may be evaluated. The form of the drying rate curve varies with the structure and type of material, and two typical curves are shown in Figure 16.2. In curve 1, there are two well-defined zones: AB, where the rate of drying is constant and BC, where there is a steady fall in the rate of drying as the moisture Mass Transfer 15 Content is reduced. The moisture content at the end of the constant rate period is represented by point B, and this is known as the critical moisture content. Curve 2 shows three stages, DE, EF and FC. The stage DE represents a constant rate period, and EF and FC are falling rate periods. In this case, the Section EF is a straight line, however, and only the portion FC is curved. Section EF is known as the first falling rate period and the final stage, shown as FC, as the second falling rate period. The drying of soap gives rise to a curve of type 1, and sand to a curve of type 2. A number of workers, including SHERWOOD (1) and NEWITT and co-workers (2–7), have contributed various theories on the rate of drying at these various stages. Time for drying If a material is dried by passing hot air over a surface which is initially wet, the rate of drying curve in its simplest form is represented by BCE, shown in Figure 16.3 Mass Transfer 16 DRYING EQUIPMENT Classification and selection of dryers Because of the very wide range of dryer designs available, classification is a virtually impossible task. PARKER (18) takes into account, however, the means by which material is transferred through the dryer as a basis of his classification, with a view to presenting a guide to the selection of dryers. Probably the most thorough classification of dryer types has been made by KRO¨ LL (19) who has presented a decimalized system based on the following factors: (a) Temperature and pressure in the dryer, (b) The method of heating, (c) The means by which moist material is transported through the dryer, (d) Any mechanical aids aimed at improving drying, (e) The method by which the air is circulated, (f) The way in which the moist material is supported, (g) The heating medium, and (h) The nature of the wet feed and the way it is introduced into the dryer. Mass Transfer 17 Types of Dryers Spray Dryers: Water may be evaporated from a solution or a suspension of solid particles by spraying the mixture into a vessel through which a current of hot gases is passed. In this way, a Large interfacial area is produced and consequently a high rate of evaporation is obtained. Drop temperatures remain below the wet bulb temperature of the drying gas until drying is almost complete, and the process thus affords a convenient means of drying substances which may deteriorate if their temperatures rise too high, such as milk, coffee, and plasma. Furthermore, because of the fine state of subdivision of the liquid, the dried material is obtained in a finely divided state. In spray drying, it is necessary to atomize and distribute, under controlled conditions, a wide variety of liquids, the properties of which range from those of solutions, emulsions, and dispersions, to slurries and even gels. Most of the atomizers commonly employed are designed for simple liquids that are mobile Newtonian liquids. When atomizers are employed for slurries, pastes, and liquids having anomalous properties, there is a great deterioration in performance and, in many cases, atomizers may be rapidly eroded and damaged so as to become useless. There is therefore much to be gained by considering various types and designs of atomizer so that a suitable selection can be made for the given duty. The performance of a spray dryer or reaction system is critically dependent on the drop size produced by the atomizer and the manner in which the gaseous medium mixes with the drops. In this context an atomizer is defined as a device which causes liquid to be disintegrated into drops lying within a specified size range, and which controls their spatial distribution. Rotary Dryers For the continuous drying of materials on a large scale, 0.3 kg/s (1 tone/h) or greater, a rotary dryer, which consists of a relatively long cylindrical shell mounted on rollers and driven at a low speed, up to 0.4 Hz is suitable. The shell is supported at a small angle to the horizontal so that material fed in at the higher end will travel through the dryer under gravity, and hot gases or air used as the drying medium are fed in either at the upper end of the dryer to give co-current flow or at the discharge end of the machine to give countercurrent flow. One of two methods of heating is used: Mass Transfer 18 (a) Direct heating, where the hot gases or air pass through the material in the dryer. (b) Indirect heating, where the material is in an inner shell, heated externally by hot gases. Alternatively, steam may be fed to a series of tubes inside the shell of the dryer. The shell of a rotary dryer is usually constructed by welding rolled plate, thick enough for the transmission of the torque required to cause rotation, and to support its own weight and the weight of material in the dryer. The shell is usually supported on large tyres which run on wide rollers, as shown in Figure 16.10, and although mild steel is the usual material of construction, alloy steels are used, and if necessary the shell may be coated with a plastics material to avoid contamination of the product.
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