Scale-up and testing

The heat transfer coefficient combines the surface coefficient for the condensing steam, the resistance of the metal wall and the surface coefficient on the working side. Because conditions vary with the type of material involved, the amount of moisture it contains, the thickness of the layer in contact with the surface, the structure of this layer, and many other factors, it is impossible to construct an overall heat transfer coefficient without experimental data. 

Scaleup of laboratory data is a critical step and requires considerable experience. Since scaleup is subject to many factors that are not quantifiable, it is based primarily on experience and is a function of the specific dryer. When the heating surface is known, it is easy to calculate the working volume and the dryer s geometrical volume (Fig. 7). 

For a pan dryer, the percent of the total volume occupied by the batch is called the working volume, which is another critical consideration. As the working volume approaches 100% of the total volume, there is less void space available for material movement and contacting of the heated surface. 

In the vacuum batch dryer, approximately 60% total volume is required for optimum drying result. A very simple and approximate equation can be used for the scaleup of vertical pan and horizontal paddle dryers  

Drying time Heat transfer surface, Vessel working volume, m Pilot plant Industrial size plant 

Laboratory filtration investigations are of three main kinds  

with small vacuum or pressure leaf filters  

with pilot plant equipment of the types expected to be suitable for the plant. 

Before any SLS equipment of substantial size is finally selected, it is essential to use the results of pilot plant tests for guidance. Although many vendors are in a position to do such work, pilot equipment should be used at the plant site where the slurry is made. Because slurries often are unstable, tests on shipments of slurry to the vendors pilot plant may give misleading results. It may be possible to condition a test slurry to have a maximum possible resistivity, but a plant design based on such data will have an unknown safety factor and may prove uneconomical. 

There is much evidence, however, that the resistivity behavior of a cake under filtration conditions may be different from that measured in a CP cell. The literature is reviewed by Wakeman (1978). CP cell data are easily obtained and may be of value in a qualitative sense as an indication of the sensitivity of resistivity to pressure, but apparently are not of acceptable engineering accuracy for the design of filtration equipment. The deduction of resistivities from filtration tests is illustrated in Example 11.1. 

TABLE 11.7. Specific Resistances of Some Filter Cakes 

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As with the design of the other columns described above, the design of a Scheibel column must be based on pilot plant tests and scale-up, The following procedure is recommended ... 

Since testing and scale-up are different for batch and continuous filtration, discussion in this section will be limited to continuous filtration. 

Laboratory Extractors. Pilot-Scale Testing, and Scale-Up. Several laboratory units arc useful in analysis, process control, and process studies. The AKUFVE contactor incorporates a separate mixer and centrifugal separator. It is an efficient instrument for rapid and accurate measurement of partition coefficients, as well as for obtaining reaction kinetic data. Miniature mixer-settler assemblies set up as continuous, bench-scale, multistage, countercurrent, liquid-liquid contactors are particularly useful Tor the preliminary laboratory work associated with flow-sheet development and optimization because these give a known number of theoretical stages. 

Pratt, H. R. C., and Stevens, G. W. (1992). Selection, design, pilot-testing, and scale-up of extraction equipment. In Science and Practice of Liquid-Liquid Extraction (J. D. Thornton, ed.), Vol. 1, pp. 492-589. Oxford University Press, New York. 

Vacuum and pressure laboratory filtration assemblies are shown in Figure 11.7. Mild agitation with air sometimes may be preferable to the mechanical stirrer shown, but it is important that any agglomerates of particles be kept merely in suspension and not broken up. The test record sheet of Figure 11.8 shows the kind of data that normally are of interest. Besides measurements of filtrate and cake amounts as functions of time and pressure, it is desirable to test washing rates and efficiencies and rates of moisture removal with air blowing. Typical data of these kinds are shown in Figure 11.3. Detailed laboratory procedures are explained by Bosley (1977) and Dahlstrom and Silverblatt (1977). Test and scale-up procedures for all kinds of SLS equipment are treated in the book edited by Purchas (1977). 

Laboratory Testing and Scale-Up 317 Compression-Permeability Cell 317 The SCFT Concept 317... 

It is not within the scope of this report to give a full account of the basic tests and scale-up theory but there are some fundamental concepts in particle characterization that have to be stated here. 

H. P. C. Prati and C. Hanson, Selection, Pilot Testing, and Scale-Up of Commercial Extractors,... 

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