Packed-Tower Scale-up

According to this method, it is not necessaiy to investigate the kinetics of the chemical reactions in detail, nor is it necessary to determine the solubihties or the diffusivities of the various reactants in their unreacted forms. To use the method for scaling up, it is necessaiy independently to obtain data on the values of the interfacial area per unit volume a and the physical mass-transfer coefficient /c for the commercial packed tower. Once these data have been measured and tabulated, they can be used directly for scahng up the experimental laboratory data for any new chemic ly reac ting system. 

It would be desirable to reinterpret existing data for commercial tower packings to extract the individual values of the interfacial area a and the mass-transfer coefficients fcc and /c in order to facilitate a more general usage of methods for scaling up from laboratory experiments. Some progress in this direction has afready been made, as discussed later in this section. In the absence of such data, it is necessary to operate a pilot plant or a commercial absorber to obtain kc, /c , and a as described by Ouwerkerk (op. cit.). 

References 77 through 90 provide additional information on packed tower wet scrubbers, design and scale-up principles, as well as operational guidance. 

The height of the transfer unit has not been satisfactorily correlated for application to a wide variety of systems. If pilot plant or other acceptable data are available to represent the system, then the height of packing can be safely scaled-up to commercial units. If such data are not available, rough approximations may be made by determining Hg and Hl as for absorption and combining to obtain an Hqg (Ref. 74, pg. 330). This is only very approximate. In fact it is because of the lack of any volume of data on commercial units that many potential applications of packed towers are designed as tray towers. 

The scale-up and design configurations of fluid-bed chemical reactors have evolved rapidly and empirically. An example is fluid catalytic cracking (FCC) [13]. The general fluid-bed concepts developed early. However, the correlations describing the various rate processes and other operational phenomena developed slowly because they could not easily be related back to already established data bases developed for other systems in the case of trickle-bed reactors, data developed for packed-bed absorption towers were utilized. 

Danckwerts and Gillham did not investigate the influence of the gas-phase resistance in their study (for some processes gas-phase resistance may be neglected). However, in 1975 Danckwerts and Alper [Tram. Imt. Chem. Eng., 53, 34 (1975)] showed that by placing a stirrer in the gas space of the stirred-cell laboratory absorber, the gas-phase mass-transfer coefficient kc in the laboratory unit could be made identical to that in a packed-tower absorber. When this was done, laboratory data obtained for chemically reacting systems having a significant gas-side resistance could successfully be scaled up to predict the performance of a commercial packed-tower absorber. 

A pilot study was performed by Bilello and Singley (3) using a 15 in. (38.1 cm) diameter PVC column, scaled up from a 6 in. (15.2 cm) column used in earlier studies. The effects of varying the air-to-water ratio, tower height, packing material, and temperature were studied. Good correlation between data obtained by each column was observed. 

Packing for towers consists of rings, berl saddles, Fiberglas pads, and helices (see Perry s Chemical Engineers Handbook, 3d ed., p. 685). Because of uncertainties of scale-up and ease of flooding, packed towers are seldom used for large-scale operations, being limited to diameters less than 2 ft. 

Pay attention to differences in distribution equipment when scaling up packed tower efficiency. 

The above experiments, if done under conditions equivalent to full scale ones with a well-mixed stirred tank reactor at steady state, give the basic rate of overall reaction plus information on what influences it. These can be used for scale-up calculations, either keeping to a stirred tank, or where appropriate, scaling up a different type of reactor, e.g. a bubble column for Regime I, a cascade of stirred tanks if plug flow is required in Regime II, or a packed tower or gas-liquid annular flow tubular reactor for Regime III or for gas-fllm controlled mass transfer. 

The evolution of chemical processes and process equipment is closely related to the methods and apparatus used in the chemistry laboratory. At the early stage of evolution of chemical industries, process steps in the manufacture of a chemical mimicked the steps used in the chemistry lab in its preparation. Most of these processes were batch processes. Some of these evolved into continuous processes as the production volumes increased. Batch processes occupy the preeminent position, even today, in the pharmaceutical and fine-chemical industries. Some of the process equipment - stirred vessels, packed towers, filters, and so on - are the up-sealed versions of the apparatus used in the chemistry laboratory of yesteryear. Process intensification (PI), which represents a paradigm shift in equipment as well as in process design, takes advantage of advances in reaction engineering and transport phenomena in the design of equipment and processes (as opposed to the mere scale-up of the apparatus of the chemistry lab and mimicking the step in the laboratory preparation). 

One commercial method to obtain nitric acid concentrations above 68% uses concentrated sulfuric acid to dehydrate the azeotropic composition. Hot nitric acid vapor is passed upward against concentrated sulfuric acid, which moved downward (countercurrent) in a tower packed with chemical stoneware to obtain 90+%HNO3 and a diluted sulfuric acid stream (Fig. 11.6). If this process is practiced on only a small scale, the sulfuric acid may be reconcentrated by addition of oleum, and a portion of the buildup of sulfuric acid in this circuit may be used to make up a commercial nitrating mixture with some of the fuming nitric acid made. Larger scale operation requires the use of a sulfuric acid boiler and a large heat input to reconcentrate the dehydration acid. There is also a noticeable sulfate contamination of the nitric acid product from this process. 

---