Drying operation

The drying of solids is a topic of considerable proportions that merits an entire monograph for its proper treatment. Our purpose here is to give tiie reader a brief survey of the operation and to provide practice in carrying out simple calculations. 

Drying can be carried out in a variety of physical configurations in which the solids can be stationary, conveyed on a moving belt, or allowed to tumble through an inclined rotary kiln. What these operations have in common is the use of heated air to assist in the drying process. When air passes through a stationary mass of solids, which may be contained in a set of perforated 

The drying of solids in general is a highly complex process involving both heat and mass transfer. If the solid is porous, moistme content and the temperatme will vary internally, as well as externally in the direction of airflow. Thus we could be dealing with at least two coupled PDEs (mass and energy balance) in time and two dimensions. 

In the following illustration, which deals with the drying of a steamed activated carbon bed, we assume that the operation takes place entirely in the constant-rate period. This is based on the fact that carbon is a hydrophobic substance that allows little penetration of its porous structure by the condensate produced dming the steaming process. In other words, the condensate is assmned to be present entirely as surface moisture, which is removed by a constant-rate drying mechanism. 

Mass Transfer and Separation Processes Principles and Applications 

we could be dealing with at least two coupled PDEs (mass and energy balance) in time and two dimensions. 

The focus here will be on the dominant constant-rate period that is well described by conventional design equations for coimtercurrent gas-liquid operations. We can, in fact, adopt the gas scrubber equation (Equation 8.2) with only a slight change in form and write 

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Cakes are dried in low temperature, high humidity conditions for a long time in order to minimize strain and absorbency variabiUty. The continuing usage of the cake system almost a century after its invention owes much to the desirable strain-free yam arising from its washing and drying operations. 

Solution Casting. The production of unsupported film and sheet by solution casting has generally passed from favor and is used only for special polymers not amenable to melt processes. The use of solvents was generally very hazardous because of their flammabiUty or toxic nature. The cost of recovery and disposal of solvents became prohibitive for many lower price film appHcations. The nature of the drying operations leads to problems with solvent migration and retention that are not problems with melt-processed polymers. 

Dryers. A drying operation (see Drying agents) needs to be viewed as both a separation and a heat-exchange step. When it is seen as a separation, the obvious perspective is to cut down the required work. This is accompHshed by mechanically squeezing out the water. The objective is to cut the moisture in the feed to the thermal operation to less than 10%. In terms of hardware, this requires centrifuges and filters, and may involve mechanical expression or a compressed air blow. In terms of process, it means big crystals. 

Batch Saponification. Batch saponification, the oldest PVA manufacturing method (252), is mainly used in the 1990s for the production of specialty products. The process uses a kneader in which the hydrolysis, washing, and drying operations are performed. This is the simplest method of saponification, but the production rates are low, and producing the product quaUty needed by many end uses is difficult. 

In order to eliminate the expensive naphtholating and drying operations some further important developments were introduced by Chemische Fabrik Griesheim-Elektron under the names of Rapid Fast, Rapidazol, and Rapidogen combinations (mixtures of substantive coupling components with antidiazotates, diazosulfonates, and diazoamino compounds, respectively). 

A hard, mst-resistant shaft of at least 0.25 micrometer finish is usually required. Common shaft surfaces are hardened tool steel, chrome plate, high strength bronze, and carbide and ceramic overlays. Test results over a broad speed range from 0.05 to 47 m/s (10 to 9200 fpm) iadicate that a coefficient of friction of 0.16—0.20 and a wear factor of 14 X 10 m /N(70x 10 ° in. min/ft-lb-h) are typical for dry operation of weU appHed grades of carbon—graphite (29). 

The principal appHcations of pneumatic conveyors are for materials that are nonsticky and readily dispersible in the gas stream as drying must be entirely constant rate. Many are employed as predryers ahead of longer residence time fluid-bed and rotary dryers in polymer drying operations. 

Efforts have also been made to overcome compHcated processes. Methods to reduce the number of steps or to use new starting materials have been studied extensively. l-Amino-2-chloro-4-hydroxyanthraquinone (the intermediate for disperse red dyes) conventionally requires four steps from anthraquinone and four separation (filtration and drying) operations. In recent years an improved process has been proposed that involves three reactions and only two separation operations starting from chloroben2ene (Fig. 2). 

A pseudo-convective heat-transfer operation is one in which the heating gas (generally air) is passed over a bed of solids. Its nse is almost exchisively limited to drying operations (see Sec. 12, tray and shelf dryers). The operation, sometimes termed direct, is more aldu to the coudnctive mechanism. For this operation, Tsao and Wheelock [Chem. Eng., 74(13), 201 (1967)] predict the heat-transfer coefficient when radiative and conductive effects are absent by... 

Another deep-bed spiral-activated solids-transport device is shown by Fig. ll-60e. The flights cany a heat-transfer medium as well as the jacket. A unique feature of this device which is purported to increase heat-transfer capability in a given equipment space and cost is the dense-phase fluidization of the deep bed that promotes agitation and moisture removal on drying operations. 

Definitions Drying generally refers to the removal of a liquid from a solid by evaporation. Mechanical methods for separating a liquid from a solid are not generally considered drying, although they often precede a drying operation, since it is less expensive and frequently easier to use mechanical methods than to use thermal metnods. 

The quantity of material to be processed does not justify investment in more expensive, continuous equipment. This case would apply in many pharmaceutical-drying operations. 

Vapors (from drying) are removed at the feed end of the dtyer to the atmosphere through a natural-draft stack and settling chamber or wet scrubber. When employed in simple drying operations with 3.5 X 10 to 10 X 10 Pa steam, draft is controlled by a damper to admit only sufficient outside air to sweep moisture from the cylinder, discharging the air at 340 to 365 K and 80 to 90 percent saturation. In this way, shell gas velocities and dusting are minimized. When used for solvent recovery or other processes requiring a sealed system, sweep gas is recirculated throu a scrubber-gas cooler and blower. 

Moisture content is another common and controllable flow factor. Most materials can safely absorb moisture up to a certain point further addition of moisture can cause significant flow problems. Specifications can control the amount of moisture content present in purchased raw materials. Moisture content can be lowered in the plant by including a drying operation in the process line. The costs incurred in drying may be offset by more efficient flow, lower shipping cost, and control of deterioration losses. 

Before nitration the moisture content of the purified linters is reduced to well below 5% since the presence of water will modify the progress of the reaction and tends to produce undesirable products. The drying operation is carried out by breaking open the cotton linters and passing along a hot air drier. 

The casein must be dried with considerable care. If dried too rapidly an impervious layer is formed on the outside of the particles which prevents the inside of the particles from drying out. Too slow a drying operation will lead to souring. 

Figure 16. Process flow scheme for a batch vacuum drying operation in the processing of rubber wastes.

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