Spray dryer requirements

In spite of the higher energy requirements, the spray dryer has gained ia popularity because of the reduced heat effect oa the product as compared to the dmm dryer. Modifications such as foam sprayiag ate being developed to reduce the heat effect further. 

Spray dryers are shown in Figure 8.13d. Here, a liquid or slurry solution is sprayed as fine droplets into a hot gas stream. The feed to the dryer must be pumpable to obtain the high pressures required by the atomizer. The product tends to be light, porous particles. An important advantage of the spray dryer is that the product is exposed to the hot gas for a short period. Also, the evaporation of the liquid from the spray keeps the product temperature low, even in the presence of hot gases. Spray dryers are thus particularly suited to products that are sensitive to thermal decomposition, such as food products. 

Figure 2.13 Influence of moisture content on the temperature of powder in a spray dryer (tp), dryer outlet temperature (f0) and sticking temperature (rs). The minimum product tempera-tured required to avoid problems with sticking is at TPC with the corresponding dryer outlet temperature TOC. (Modified from Hynd, 1980.)...

In as much as the spray dryer operates by drying a finely divided droplet, the feed to the dryer must be capable of being atomized sufficiently to ensure that the largest droplet produced will be. dried within the. retention time provided. There are different requirements on the degree of... 

Capital Equipment Costs The initial investment made by a utility to control pollutant emissions is in the equipment comprising the flue gas treatment system. The amount of the investment in this equipment purchase is directly related to the size and complexity of the equipment itself. The vessels required in this application must be large because of the volume of flue gas that must be treated. Some equipment, such as spray dryers or absorbers, can be 20 to 50 feet in diameter, and spray towers can be over 100 feet in height. Considerable preparation must go into sizing these vessels properly in order to maximize gas-sorbent contact time and minimize scaling and other operational problems. Much of this equipment, especially in wet systems, must be constructed of costly corrosion-resistant materials. Elaborate valving systems must be set up to control flows to the system. 

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. 

Spray dryers are operated continuously and commonly used for foods, enzymes, and pharmaceutical intermediates and products. Most if not all of the solvent phase can be removed provided that the feed slurry is converted to a fine spray to maximize the surface area for heat and mass transfer. Spray dryers have a relatively large space requirement in a plant and have a high energy consumption. They can be difficult to clean effectively but are well suited to single products where continuous operation is desirable. Again dust can be a problem and contained pack off systems are recommended to maintain a clean environment around the spray dryer unit. 

An aqueous gel (1 liter per min) is spray dried at 25°C in a counter-current spray dryer. The droplets produced by atomization are 50 pm in diameter and remain that size throughout drying. If the drying rate is limited by mass transfer in the boimdary layer, what height of the spray dryer of cross-sectional area 1 m is required if the drying air (100 liter per min) enters at 150°C and = 0% relative humidity. See problem 3 for the data. 

A safe estimate of power requirement for double drum dryers is approx 0.67 HP/(rpm)(100sqft of surface). Maintenance can be as high as 10%/yr of the installed cost. Knives last from 1 to 6 months depending on abrasiveness of the slurry. Competitors for drum dryers are solid belt conveyors that can can handle greater thicknesses of pasty materials, and primarily spray dryers that have largely taken over the field. 

Hydraulic pressure nozzles cannot combine the capability for fine atomization with high feed capacity in one single unit. Many spray dryer applications, where pressure nozzles are applied, therefore require multinozzle systems with the consequence that start-up, operational control, and shutdown procedures become more complicated. 

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 ... 

A quick scoping estimate of the size of an industrial spray dryer can be made on this basis. The required evaporation rate or product rate can be multiplied by the relevant ratio from the table to give the mass flow rate of the drying gas. The next step would be to calculate the size of a spray drying chamber to allow the drying gas at outlet conditions approximately 25 s of residence time. A cylindrical chamber with diameter D and height H equal to D and a 60° conical bottom has a nominal volume of... 

Accordingly a zinc sulfate spray dryer with a drying capacity of 2 t/h would require a drying gas flow rate of approximately 8.45 kg/s. With an outlet gas density of 0.89 kg/m and the above-mentioned gas residence time, this results in a required chamber volume of... 

Estimate the water requirement of a spray dryer (dry SO2 scrubber) at a coal-fired incineration facility that treats 150,000 Ib/h of a flue gas at 2180°F. Assume an approach temperature to the adiabatic saturation temperature (AST) of 40°E The AST can be assumed to be 180°R... 

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