Spray dryer nozzles used

Drying of liquid and pasty materials on the surface of heated inert particles is one of the newer drying techniques. The inert material serves not only as a carrier for the liquid film but also as a heat transfer medium. The size of the inert particles may be from 20 to 40 times larger than that of the dispersed material dried, which allows for higher gas velocity and thus increased dryer productivity. Moreover, intensive motion of the inert particles (e.g., in fluid or spouted beds) results in good dispersion of the liquid feed, so coarse spray nozzles can be used. As compared to the standard spray dryer, the use of a fluid bed of inert particles to dry slurries results in 15- to 17-fold higher volumetric ev oration rates under identical thermal conditions. Table... 

Historically, spray dryers were used because of their abihty to produce a constant quality product under full operational control. Normally, emulsion PVC (E-PVQ is water wet in a slurry and dried to a powder in one single-pass operation with high capacities. The slurry is atomized using a rotary wheel or nozzle. Evaporation takes place under constant and falling rate conditions. Rapid evaporation maintains a low temperature of the spray droplets so that high dry gas temperature can be applied without affecting polymer quahty. Conical spray-dryer chambers are commonly employed. 

Drum Dyers. Indirect-heat dmm dryers, like spray dryers, are usable only for materials that are fluid initially and pumpable. Drying is effected by applying a thin film of material onto the outer surface of a rotating heated dmm using appHcator roUs, spray nozzles, or by dipping the dmm into a reservoir. Usually the dmm is cast iron or steel and chrome-plated to provide a smooth surface for ease of product release by doctoring. Dmm rotational speed is such that... 

Thus, spray-dried xylan/ESlOO microparticles were produced at different polymer weight ratios dissolved in alkaline and neutral solutions, separately. More precisely, xylan and ESIOO were dissolved in 1 1 and 1 3 weight ratios in 0.6 N NaOH and phosphate buffer (pH 7.4). Then, the suspensions were spray-diied at the feed rate of 1.2 mL/min (inlet temperature of 120°C) using a Biichi Model 191 laboratory spray-dryer with a 0.7 mm nozzle, separately. Cross-linked xylan microcapsules were also coated by ESIOO after spraydrying at the same conditions. 

Spray dryers are normally used for liquid and dilute slurry feeds, but can be designed to handle any material that can be pumped. The material to be dried is atomised in a nozzle, or by a disc-type atomiser, positioned at the top of a vertical cylindrical vessel. Hot air flows up the vessel (in some designs downward) and conveys and dries the droplets. The liquid vaporises rapidly from the droplet surface and open, porous particles are formed. The dried particles are removed in a cyclone separator or bag filter. 

In the jet spray dryer, cold feed is introduced(42) into preheated primary air which is blown through a nozzle at velocities up to 400 m/s. Very fine droplets are obtained with residence times of around 0.01 s, and an air temperature of 620 K. This equipment has been used for evaporating milk without adverse effect on flavour and, although operating costs are likely to be high, the system is well suited to the handling of heat-sensitive materials. 

A spray-dryer eonsists of a feed tank, a rotary or nozzle atomizer, an air heater, a drying ehamber, and a eyelone to separate the powder from the air. A rotary atomizer uses eentrifugal energy to form the droplet. Pressure-nozzle atomizers feed solution to a nozzle under pressure, whieh forms the droplet. Two-fluid nozzles feed solutions separately into a nozzle head, whieh produces high-speed atomizing air that breaks the solution into tiny droplets. Both the feed solution and the drying air are fed into the drying ehamber in a standard eoeurrent flow [27]. 

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

As in spray dryers, a variety of devices have been used or suggested for producing droplets from the melt. Centrifugal devices, such as spinning discs and rotating perforated baskets impart an initial radial velocity to the droplets. Such devices require larger tower cross-sections and may lead to inefficient air/droplet contact due to non-uniform prill distribution across the tower [6]. These devices are best-suited to prill tubes of circular cross-section. Atomizing nozzles produce small droplets which are only suitable when fine prills are required. 

Two-fluid nozzles do not operate efficiently at high capacities and consequently are not used widely on plant-size spray dryers. Their chief advantage is that they operate at relatively low pressure, the liquid being 0 to 400 kPa/m pressure, while the atomizing fluid is usually no more than 700 kPa/m pressure. The atomizing fluid may be steam or air. Two-fluid nozzles have been employed for the dispersion of thick pastes and filter cakes not previously capable of being handled in ordinary atomizers [Baran, Ind. Eng. Chem., 56(10), 34-36 (1964) and Turba, Brit. Chem. Eng., 9(7), 457-460 (1964)]. 

Fig. 7 Buchi/Brinkmann Lab Scale Spray-Dryer. A stock solution or suspension is pumped from the beaker through a nozzle which sits above the large glass particle formation vessel on the left-hand side. The nozzle atomizes the feedstock at temperatures ranging from ambient to 250° C into the particle formation vessel, using air or an inert gas such as nitrogen to dry and move particles into the cyclone and collection chamber on the right-hand side. Product temperature is monitored by a temperature probe mounted between the particle formation vessel and the cyclone. Solvent or water are exhausted through a fine particle filter bag in series with the cyclone, which also collects fines.

Although it lacks the flexibility of the rotary atomizer, the pressure nozzle is nevertheless widely used in spray drying applications. For many products the requirement for nondusty appearance calls for large mean particle size and lack of a fines fraction that cannot be met with a rotary atomizer. In the other end of the particle size range, some products require finer particles than are practically achievable with a rotary atomizer. This is the range where two-fluid nozzles are applied. The following guidelines may be used as an indication of the particle sizes obtainable in spray dryers ... 

Essentially, three major types of atomizers are used in practice (1) rotary wheel (or disk) atomizers, (2) pressnre nozzle, and (3) two-fluid nozzle. Figure 23.20 shows some typical atomizer designs. Ultrasonic and electrostatic atomizers can also be used for special applications to produce monodisperse sprays, but they are very expensive and low capacity. Most spray dryers operate at slight negative pressnre. New designs with low-pressure chambers enhance drying rates at lower temperatnres to dry heat-sensitive products. 

Average drop diameters in a spray dryer range from 20 /rm when a disk atomizer is used to 180 /im with a coarse spray nozzle. Residence times vary from 3 to 6 s in cocurrent dryers to as much as 25 to 30 s in countercurrent dryers. 

Dispersion of the wet feed is typically accomplished by specially designed nozzles. Although different designs of nozzles (see also Section 7.4.2) are offered and used by competing manufacturers of spray dryers, below, as examples, only two often applied nozzle designs will be described. 

Spray dryers with nozzle atomizers use two different types of dispersers singlephase, hydraulic pressure nozzles and two-phase, pressurized gas assisted nozzles (see also Section 7.4.2). The latter has a relatively limited application in spray drying because of the relatively large flow of atomizing gas, which influences the flow pattern of the drying gas in the tower, and the broader particle size distribution that is produced by this type of nozzle. 

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