Spray cyclones

A spray cyclone is based on the same idea as tiie bubble cyclone. A gas is introduced with a high velocity (say, 40 m/s) tangentionally into the cyclone, and liquid is sprayed in the axis (see figure 4.14). The liquid drops travel with great speed towards the wall, and flow downward as a thin film. Because of the greatly enhanced gas phase mass transfer coefficients, gas phase components can be separated on the basis of their effective liquid phase mass transfer coefficients (Schrauwen, 1986). These effects are discussed further in section 5.42.2 see eq. (5.45). 

Wetted wall colums, or falling film reactors 

A falling film reactor is essentially a vertical cylinder, where liquid flows downward in a thin film along the wall, and gas flows in the core. Relatively high mass transfer coefficients are obtained both in the liquid and in the gas phases. The liquid phase can be cooled effectively via the wall. Therefore this type of reactor is preferred for very rapid exothermic gas liquid reactions. There are two variations, one consists of a tube bundle, the other consists of one cylinder with a rotor. 

In the first type, the tubes are mounted in tube sheets in a shell, very much as in 

In the second type a coaxial rotor is used, that grazes the wall with a clearance of the order of 1 mm. This is a very effective way to increase both the liquid/gas mass transfer and the liquid to wall heat transfer, especially for very viscous liquids. These reactors are relatively expensive per unit of heat transfer area. 

---

The experimental determination of the feed consumed, the char yield, and the noncondensible gas yields are relatively straightforward. However, the primary vapor and water yields have proven difficult to measure directly due to the formation of aerosols. These aerosols escape high-pressure sprays, cyclonic separators, and impingement or inertial collection techniques. The use of condensible steam as the carrier gas makes the water yield very... 

A typical device, known as spray cyclonic scrubber, consists on a cylindrical charnber with a conical bottom as illustrated in Figure 10.14. The scrubber... 

Figure 4.14, Spray cyclone, used for obtaining very high mass transfer coefficients in the gas phase (Schrauwen, 1986).

For given conditions, this selectivity can be increased by raising the ratio of gas- and liquid side mass transfer coefficients. It has been demonstrated that a considerable increase in this selectivity can be achieved when the flow rate of the gas relative to the liquid is raised, e.g., in wetted wall columns (Blauwhoff et al. 1983) and in spray cyclones (Schrauwen and Thoenes, 1988 figure 4.14). 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

Includes cyclonic, dynamic, filtration, inertial impaction (wetted targets, packed towers, turbulent targets), spray chambers, and venturi. 

Fig. 2. Types of spiay towers (a) horizontal spray chamber (b) simple vertical spray tower (c) cyclonic spray tower, Pease-Anthony type (d) cyclonic spray...

A filter cake from the wringer is washed to remove absorbed acid, transferred to a slurry tank of water, and quickly submerged, after which the nitrocellulose is pumped to the stabilization operation as a diluted water slurry. Exhaust systems are installed to protect personnel and equipment from acid fumes, and water sprays and cyclone separators are used for acid fume recovery before venting to the air. 

The reaction mixture can either be crystallised, centrifuged, and dried, or spray-dried and cyclone-separated to produce a fine crystalline powder having a particle size of 50 p.m. Metal analysis of the AP produced by this method is reported to be less than 0.02 p.g/g. 

Fig. 13. Spray-dryer system designed for production of agglomerated food powders with instant properties (82) A, Hquid-feed system B, spray-dryer chamber C, drying air heater D, cyclones for fines recovery E, vibrofluidizer as afterdryer F, vibrofluidizer as aftercooler and G, fines return to drying...

Spray Drying and Agglomeration. Most instant coffee products are spray-dried. Stainless steel towers with a concurrent flow of hot ak and atomized extract droplets are utilized for this purpose. Atomization, through pressure nozzles, is controUed based on selection of the nozzles, properties of the extract, pressures used, bulk density, and capacity requkements. Low inlet ak temperatures (200—280°C) are preferred for best flavor quaHty. The spray towers must be provided with adequate dust coUection systems such as cyclones or bag filters. The dried particles are coUected from the conical bottom of the spray drier through a rotary valve and conveyed to bulk storage bins or packaging lines. Processors may screen the dry product to... 

Fig. 7. Plant for spray-drying of detergents and soaps. 1, Burner 2, air combustion 3, cold air fan 4, hot air generator 5, spray-drying tower 6, cleaning ring 7, no22les circuit 8, tower suction cyclones group 9, tower suction fan 10, double-flap dischargers 11, recovery powder cyclone 12, recovery powder fan 13, belt conveyor 14, air lift 15, air lift suction cyclones 16, air lift suction fan 17, double-flip dischargers 18, vibrating sieve.

Spray dryers may lose dried egg products out of the stack. Cyclone-type coUectors, usually used as the secondary coUector, are not properly designed to coUect products efficiently. A weU-designed cyclone coUector recovers only 85—90% of spray-dried egg white, with 10—15% lost out of the stack, and 94—95% of egg products co-dried with carbohydrates. Bag-type coUectors are needed for 100% recovery of both these products. Plain whole egg and plain egg yolk products are capable of being coUected almost 100% with a cyclone coUector, but a bag coUector is advised (see Airpollution control methods). 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

If the product in no way adheres to the dryer parts and simple cyclone collectors are sufficient for gas-sohds separation, batch operation of a spray dryer may be considered. Otherwise, the time and costs for cleaning the large equipment parts make them rarely economical for other than continuous processing of a single material. 

Pot charging Particulates (dust), CO, HF, SO, CF4, and hydrocarbons High-efficiency cyclone, baghouse, spray towers, floating-bed scrubber, electrostatic precipitators, chemisorption, wet electrostatic precipitators... 

---