Evaporators Special types

Floating Roof A special type of storage tank roof which floats on the surface of the oil in storage. This helps to eliminate tank venting and reduces evaporative loss. 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

Many special types of equipment have been developed for particular industries, possibly extreme examples being the simple open ponds for solar evaporation of brines and recovery of salt, and the specialized vacuum pans of the sugar industry that operate with syrup on the tubeside of calandrias and elaborate internals to eliminate entrainment. Some modifications of basic types of crystallizers often carry the inventor s or manufacturer s name. For their identification, the book of Bamforth (1965) may be consulted. 

The wiped-film evaporator is a special type of evaporator that permits evaporation to a much higher concentration of solids than do other evaporators. Liquid is fed into a heated cylinder that contains rotating blades or wipers to reduce the liquid to a film, thereby improving the heat-transfer efficiency. Wiped-film evaporators can also be operated as dryers. Other equipment that can be used for drying and calcining non-high-level waste is the same as for HLW, e.g., spray calciners, fiuidized-bed calciners, and rotary kilns. 

Selection of equipment, elimination of obviously unsuitable equipment, or proof of applicability of standard or special types of equipment or machines such as pumps, bottles, evaporators... 

A special type of organic solvent-based adhesive is the so-called contact adhesive. This makes use of the fact that certain elastomeric or rubbery solids (e.g., polychloroprene) have the property of autohesion, i.e., they can stick readily to themselves, especially if compounded with resins and containing small amounts of solvents. The bonding takes place by a diffusion process, the adhesive being applied to both surfaces to be bonded. Thus substrates may be coated with a contact adhesive, the adhesives can be allowed to dry till most of the solvent has evaporated (the dry adhesive film at this point will contain... 

Distillation processes may be operated continuously or batchwise. The operating pressure may be low (10 Pa-100 Pa), medium (10 Pa-0.1 Pa) or even high vacuum (0.1 Pa-10 Pa) (molecular distillation) to avoid thermal damage to the mixture, or to increase efficiency. Special types of distillation are equilibrium or flash distillation (flash evaporation) and carrier distillation processes. 

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. 

An evaporator is a special type of heat exchanger in which steam is used to heat a solution to partially boil off some of the solvent. In the evaporator shown in Figure P2.24, a brine containing 1% (weight) salt in water is fed at... 

Several studies have been published on the use of special stationary phases (using packed GC columns) or special types of deactivated retention gaps [11,12]. Among these, a series of papers by Goosens et al. [13-15] on the use of a carbo-wax-deactivated retention gap to transfer acetonitrile-water eluents from the LC to the GC part of the system, merit attention. When using an on-column interface and a solvent vapor exit (SVE), up to 200 Xl of aqueous-organic eluent could be introduced, provided that the water content of the eluent did not exceed that of the azeotropic mixture (16 vol%). Otherwise, water will be left in the retention gap after evaporation of the azeotropic mixture and will mar the analysis. In order... 

Evaporate sediments are a special type of sedimentary rock that is formed from the concentration of dissolved salts through evaporation (for example rock salt/haHte). 

The basic assumption is that the Langmuir equation applies to each layer, with the added postulate that for the first layer the heat of adsorption Q may have some special value, whereas for all succeeding layers, it is equal to Qu, the heat of condensation of the liquid adsorbate. A furfter assumption is that evaporation and condensation can occur only from or on exposed surfaces. As illustrated in Fig. XVII-9, the picture is one of portions of uncovered surface 5o, of surface covered by a single layer 5, by a double-layer 52. and so on.f The condition for equilibrium is taken to be that the amount of each type of surface reaches a steady-state value with respect to the next-deeper one. Thus for 5o... 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

Product Quality Considerations of product quahty may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer charac teristics at low temperature. Product quality may also dic tate special materials of construction to avoid met hc contamination or a catalytic effect on decomposition of the product. Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high hquid and vapor velocities used, the frequent presence of sohds in suspension, and the necessary concentration differences. 

Whether a vessel is called an evaporator or a crystallizer depends primarily on the criteria used in arriving at its sizing. In an evaporator of the salting-out type, sizing is done on the basis of vapor release. In a ciystaUizer, sizing is normally done on the basis of the volume required for crystallization or for special features required to obtain the proper produc t size. In external appearance, the vessels could be identical. Evaporators are discussed in Sec. 11. 

Pipe coils within a water tank (see Figures 7.1c, 7.5 and 7.6). Both flooded and dry expansion evaporators are in use. Water is circulated by pumps and/ or special agitators. This type of water chiller maybe operated without formation of ice, or ice maybe allowed to accumulate intentionally (see below). 

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