A new combination packing, which consists of bundles of structured packing and spacers, and which itself is conjoined to the elements of packed film and film-drop assemblies, is examined. The heat-and mass-exchange efficiency, and the dependence of the hydraulic resistance of the new packing on the velocity of the air flow are cited for various irrigation densities.Efficiency of heat and mass exchange with direct contact between the liquid and gaseous phases is ensured, above all, by the developed surface of their contact, which can be created by a packed assembly. Interaction between the phases may be film, film-drop, drop, or jet-drop in nature depending on the type of packed assembly [1][2][3]. A multitude of new types of packings, among which structured packings classed with the family of packed film assemblies, have come into the most widespread use in the past 15 years.Structured packings (SP) are regular packings represented as bundles assembled from flat or crimped sheets, which form a three-dimensional multichannel structure. The sheet components of the packing, which are fashioned from metallic foil, meshes, and polymeric, ceramic, and other materials, may be arranged in a vessel as a set of plates, spirals, cylinders, and may also be assembled into a honeycomb or cellular structure. Coaxial channels, which are formed by the packing elements, may assume different configurations -from simple (a circular or polygonal section) to complex three-dimensional [4].The term structured packings has also been expanded to include film heat-exchange contact assemblies for cooling towers (sprinklers) [5].Despite their high heat-and mass-exchange efficiency, SP have a number of deficiencies, primary among which is nonuniform distribution of the gaseous and liquid flows throughout the cross section of the vessel [1], since the geometric structure of the packing excludes communication between the free channels formed by neighboring sheets. To improve the operating efficiency of SP, it is necessary to ensure uniformity of gas and liquid throughout all channels, even at the inlet to a layer of packing. It is possible to organize sufficiently uniform distribution of flows, however, only on relatively small mock-up installations with a column vessel and in the presence of a liquid distributor, which ensures uniform irrigation over its entire cross section. In practice, it is virtually impossible to provide ideal irrigation of the entire surface of SP in industrial vessels, especially such large-scale vessels as cooling towers.Packings that organize the drop surface of phase contact are distinguished by an appreciably smaller specific surface as compared with packed film assemblies, and, consequently, also by low efficiency [1], although they provide free commu-

Various types of structured packing, such as Pyrapak F, Mellapak 250.Y, Pyrapak G, DAO TsKBN, etc., are examined. Relationships of hydraulic resistance of dry and irrigated structured packings with F-factor of gas flow rate in the column are given for these packings.Structured packings were developed in early 1960s, and in the 1970s they came into wide use in various separation processes as contact devices of industrial columnar apparatuses for implementing heat and mass exchange processes, such as absorption, fractionation, extraction, purification and drying of gas, etc.We propose to define structured packings as regular packings that are comprised of packets assembled from flat or corrugated sheets forming a spatial multichannel structure. Sheet packing components made of metal foil, network, and polymer, ceramic and other materials may lie in the apparatus as a pack of sheets, may be twisted into spirals and cylinders, and assembled into honeycombed or cellular structures. Coaxial channels thus formed, depending on the shape and relative dispositions of individual sheets, have various configurations ranging from simple (round or polygonal cross section) to complex spatial packing periodically changing with height. Thus, the proposed term "structured packings" encompasses a whole multitude of film-type heat-exchanging contact devices (for instance, sprinklers) for cooling towers [1].Most popular in the industry are structured packings of the type that are well adaptable to process conditions and geometric dimensions of the apparatuses. Such packings are distinguished by equally high and stable indices of separation of the components of mixtures in a wide range of diameters of the mass-exchange columns (80 mm-20 m). They are usable in almost all process conditions regardless of pressure in the apparatus, gas flow velocity, and liquid load per unit cross sectional area of the apparatus.In this packing family, the most known are Mellapak 250.X/250.Y pressed into service in the industry in 1987 by the Swiss company Sulzer Chemtech. Almost all major packing companies manufacture modified versions of this packing [2][3][4].A notable feature of the referred structured packings is large specific surface, which ensures developed phase contact surface with a high free volume (0.9-0.98 m 3 /m 3 ). This made them widely applicable in processes occurring with high gas and low liquid loads. Low differential pressure across the packing height and high mass exchange efficiency are among the advantages of the structured packings. For instance, for getting equal separating power using Pall rings, at least four times larger column volume is required than when Sulzer BX cellular structured packings are used [5]. Under the same conditions 1 Moscow State University of Engineering Ecology (MGUIÉ). 2 TsNTU RINVO ZAO.

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

Hydrodynamic parameters are investigated for a structured packing (analogy of the Mellapak 250.X packing) in a water-air system under atmospheric pressure. Relationships between the hydraulic resistance of the irrigated packing on the hypothetical gas velocity, and also its holding capacity on the Re number are presented for the liquid phase.Regular structured packings (RSP) assembled from corrugated sheets are widely used as contact devices in industrial columns for heat-and mass-exchange processes, for example, absorption, fractionation, cleaning, and drying of gas. The most widely known RSP carries the Mellapak trademark 250.X/250.Y, and can be used over a broad range of conditions under which these processes take place, and was introduced to industry in 1987 by the Sulzer Chemtech Company (Switzerland) [1,2].A characteristic feature of RSP is a large specific surface, which provides for a developed area of contact for the phases (with a free space of 0.9-0.98 m 3 /m 3 ); this makes it possible to use them for processes with heavy gas and light liquid loads. The low pressure gradient over the height of the packing, and high gas-exchange efficiency should be referred to as advantages of RSP. Owing to its ordered structure, the RSP can be used in columns with diameters of up to 20 m, ensuring uniformity of liquid-phase distribution over the entire volume of the vessel.In studying the performance of packings, major interest is focused on the hydrodynamic pattern of the process, and the character of the gas and liquid in the packed layer.The most critical hydrodynamic characteristics of RSP are the amount h t of liquid retained, the pressure gradient ∆p/∆z over the height of the packing, and the range of stable operation, i.e., maximum and minimum allowable gas and liquid loads.Familiar advertizing information on the hydrodynamic characteristics of RSP is intended only for comparative evaluation, and is not recommended for engineering calculations performed by manufacturing companies. Because these calculations are required in practice, we investigated the hydrodynamic parameters of RSP in an air-water system on an experimental plant (Fig. 1) under atmospheric pressure.The packing under investigation (analogy of the Mellapak 250.X packing) was placed in an absorption column formed from acrylic plastic with an effective section of 250 × 250 mm.