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11Contact devices for mass-exchange processes in packed and plate columns are investigated. A comparative analysis is made of contact devices (sieve plate, irregular packed, and regular structured packings) for the purpose of selecting equipment optimal for a blow-through column used to produce weak nitric acid.Selection of optimal contact devices is a critical problem in the development of new, and modernization of existing vessels for mass-exchange processes. Many mass-exchange processes (absorption, desorption, distillation, etc.) are carried out in column vessels with plate, packed, or film contact devices (regular structured packings).The hydrodynamic situation in packed columns (PC) with irregular packings (IP) is characterized by disordered movement of flows over time and in cross sections over the height of the vessel, and longitudinal displacement, as well as displacement as liquid flows move toward the walls of the column. Owing to the indicated characteristic features of a PC, its operating efficiency decreases with increasing diameter. Figure 1 shows packed elements of one type of IP for PC. In PC fitted with structured packings (SP), the flow of liquid through the SP is predominately film in nature, and the movement of both liquid, and also gas flows is ordered over the height of the column and in the cross section of the vessel (in providing uniformity of the flow introduced to the packed layers). In PC with SP, displacement of liquid toward the walls of the column is eliminated by creating discontinuities in the liquid film and its redistribution at the joints between packing bundles, which are turned 90°relative to one another. PC with SP (for example, the Sulzer Chemtech Co. [1], Fig. 2) have low hydraulic resistance and high mass-exchange characteristics.The movement of flows in plate columns (PLC) is clearly organized both within the limits of a single plate, and also on the plates over the height of the column. Since channel formation, by-passing, and longitudinal displacement do not occur on the plates, the separation efficiency in PLC, as calculated per unit height of the effective zone of the vessel, is higher than in PC, and is less dependent on scale factor, since sectioned plates are normally used in large-diameter columns. Figure 3 shows a schematic diagram of a sectional sieve (zero-gradient [2]) plate 2600 mm in diameter. For a uniform distribution of flows, this design of plate ensures a scale-transition factor close to unity with increasing diameter of the PLC [2].Analysis of PC and PLC indicated the following: 1. As compared with PLC, PC have a low hydraulic resistance, as calculated per unit height of transfer, since the plates operate in a submerged regime, and the gas passing through the column must overcome a resistance equal to the total pressure of the liquid columns on all plates. The hydraulic resistances of PLC and PC with SP differ particularly significantly. The low

An actual problem relative to selection of the optimal type (shape) and size of a packing for tower equipment used in chemical engineering is examined. Appropriate criteria are proposed for comparative evaluation of different designs of packings. Practical examples are cited for solution of this important problem.A packing with maximum geometric surface per unit volume, i.e., fine packing, is usually chosen for column equipment in the chemical industry. This is not always correct, since the active surface will depend little on the size of the packing under an irregular loading (for example, a ring packing).When a coarser packing is used, operation of the tower at high gas velocities is possible. It is precisely for this reason that a coarser packing is found more effective than a fine packing in practice, especially for the absorption of readily soluble gases. When slightly soluble gases are absorbed, it may be found more preferable as compared with a fine packing [1].Loss of pressure (head) ∆p rm in the layer of packing as calculated over 1 m (running meter) is the most critical indicator of the packing. When contact equipment is used under a pressure close to atmospheric, the pressure loss in the unit should be minimal with respect to energy and economic considerations. In that case, it is possible to use regular structured packings with a highly developed specific surface and low head loss [2, 3].Let us examine comparative characteristics of the most widely used irregular packings. Problems confronting equipment designers who must select packings may differ:• selection of a packing for the packed unit being developed, or replacement of a packing that has served its time, or is insufficiently effective; • provision for minimum consumption factors with respect to energy outlays, which would not require capital expenditures; this is important for regions with limited energy resources; • capital expenditures for equipment should be minimal for unlimited consumption factors with respect to energy outlays; and • provision for minimum combined energy and financial expenditures in the calculation of a unit of production throughout the scheduled period of work production. It is convenient to present the results of hydrodynamic investigations of each of the competitive packings as a bundle of broken lines in ∆p rm -F-factor coordinates (Fig. 1), and also in L-w 0 (or F-factor) coordinates (Fig. 2). Data with respect to effectiveness in β lv (or K)-F-factor coordinates are presented in Fig. 3. Here, F-factor = w 0 γ g 1/2 in m/sec·(kg/m 3 ) 1/2 , L is the water concentration in m 3 /(m 2 ·h), β lv is the mass efficiency in the liquid phase in 1/h, K is the coefficient of mass transfer in 1/h, w 0 is the velocity of the air flow in calculating the total section of the empty unit in m/sec, and γ g is the specific gravity of the gas in N/m 3 .

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