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with packing than in trays, and it takes more trials “to get it right” than with trays. This makes trays more robust. Complex towers. Interreboilers, intercondensers, cooling coils, and side drawoffs are more easily incorporated in trays than in packed towers. In packed towers, every complexity requires additional distribution and/or liquid collection equipment. Feed composition variation. One way of allowing for design uncertainties and feedstock variation is by installing alternate feed points. In packed towers, every alternate feed point requires expensive distribution equipment. Performance prediction. Due to their sensitivity to maldistribution there is greater uncertainty in predicting packed column performance. Chemical reaction, absorption. Here the much higher liquid holdup on trays provides greater residence time for absorption or chemical reaction than does packing. Turndown. Moving valve and bubble-cap trays normally give better turndown than packings. Unless very expensive distributors are used, packed tower turndown is usually limited by distributor turndown. Weight. Tray towers usually weigh less than packed towers, saving on the cost of foundations, supports, and column shell. Trays vs. Random Packings The following factors generally favor trays compared to random packings, but not compared to structured packings. Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. Process surges. Random packings are usually more troublesome than trays in services prone to process surges (e.g., those caused by slugs of water entering a hot oil tower, relief valve lifting, compressor surges, or instability of liquid seal loops). Structured packings are usually less troublesome than trays in such services. Trays vs. Structured Packings The following factors generally favor trays compared to structured packings, but not compared to random packings. Packing fires. The thin sheets of structured packing (typically 0.1 mm) poorly dissipate heat away from hot spots. Also, cleaning, cooling, and washing can be difficult, especially when distributors or packing plug up. Many incidents of packing fires during turnarounds (while towers with structured packings were open to atmosphere) have been reported. Most of these fires were initiated by pyrophoric deposits, hot work (e.g., welding) above the packing, opening the tower while hot organics were still present, and packing metallurgy that was not fire-resistant. Detailed discussion can be found in Fractionation Research Inc. (FRI) Design Practices Committee, “Causes and Prevention of Packing Fires,” Chem. Eng., July 2007. Materials of construction. Due to the thin sheets of structured packings, their materials of construction need to have better resistance to oxidation or corrosion. For a service in which carbon steel is usually satisfactory with trays, stainless steel is usually required with structured packings. Column wall inspection. Due to their snug fit, structured packings are easily damaged during removal. This makes it difficult to inspect the column wall (e.g., for corrosion). Washing and purging. Thorough removal of residual liquid, wash water, air, or process gas trapped in structured packings at startup and shutdown is more difficult than with trays. Inadequate removal of these fluids may be hazardous. High liquid rates. Multipass trays effectively lower the liquid load “seen” by each part of the tray. A similar trick cannot be applied with packings. The capacity of structured packings tends to rapidly fall off at high liquid rates. Capacity and Efficiency Comparison Kister et al. [Chem. Eng. Progr., 90(2), 23 (1994)] reported a study of the relative capacity and efficiency of conventional trays, modern random packings, and conventional structured packings. They found that, for each device optimally designed for the design requirements, a rough guide could be developed on the basis of flow parameter L/G (ρ G/ρ L) 0.5 (abcissa in OTHER TOPICS FOR DISTILLATION AND GAS ABSORPTION EQUIPMENT 14-81 Figs. 14-31, 14-55, and 14-56) and the following tentative conclusions could be drawn: Flow Parameter 0.02–0.1 1. Trays and random packings have much the same efficiency and capacity. 2. Structured packing efficiency is about 1.5 times that of trays or random packing. 3. At a parameter of 0.02, the structured packing has a 1.3–1.4 capacity advantage over random packing and trays. This advantage disappears as the parameter approaches 0.1. Flow Parameter 0.1–0.3 1. Trays and random packings have about the same efficiency and capacity. 2. Structured packing has about the same capacity as trays and random packings. 3. The efficiency advantage of structured packing over random packings and trays decreases from 1.5 to 1.2 as the parameter increases from 0.1 to 0.3. Flow Parameter 0.3–0.5 1. The loss of capacity of structured packing is greatest in this range. 2. The random packing appears to have the highest capacity and efficiency with conventional trays just slightly behind. Structured packing has the least capacity and efficiency. Experience indicates that use of structured packings has capacity/ efficiency disadvantages in the higher-pressure (higher-flow-parameter) region. Zuiderweg and Nutter [IChemE Symp. Ser. 128, A481 (1992)] explain the loss of capacity/efficiency by a large degree of backmixing and vapor recycle at high flow parameters, promoted by the solid walls of the corrugated packing layers. SYSTEM LIMIT: THE ULTIMATE CAPACITY OF FRACTIONATORS Liquid drops of various sizes form in the gas-liquid contact zones of tray or packed towers. Small drops are easily entrained upward, but their volume is usually too small to initiate excessive liquid accumulation (flooding). When the gas velocity is high enough to initiate a massive carryover of the larger drops to the tray above, or upward in a packed bed, liquid accumulation (entrainment flooding) takes place. This flood can be alleviated by increasing the tray spacing or using more hole areas on trays or by using larger, more open packings. Upon further increase of gas velocity, a limit is reached when the superficial gas velocity in the gas-liquid contact zone exceeds the settling velocity of large liquid drops. At gas velocities higher than this, ascending gas lifts and carries over much of the tray or packing liquid, causing the tower to flood. This flood is termed system limit or ultimate capacity. This flood cannot be debottlenecked by improving packing size or shape, tray hole area, or tray spacing. The system limit gas velocity is a function only of physical properties and liquid flow rate. Once this limit is reached, the liquid will be blown upward. This is analogous to spraying water against a strong wind and getting drenched (Yangai, Chem. Eng., p. 120, November 1990). The system limit represents the ultimate capacity of the vast majority of existing trays and packings. In some applications, where very open packings (or trays) are used, such as in refinery vacuum towers, the system limit is the actual capacity limit. The original work of Souders and Brown [Ind. Eng. Chem. 26(1), 98 (1934), Eq. (14-80)] related the capacity of fractionators due to entrainment flooding to the settling velocity of drops. The concept of system limit was advanced by Fractionation Research Inc. (FRI), whose measurements and model have recently been published (Fitz and Kunesh, Distillation 2001: Proceedings of Topical Conference, AIChE Spring National Meeting, Houston, Tex., 2001; Stupin, FRI Topical Report 34, 1965 available through Special Collection Section, Oklahoma State University Library, Stillwater, Okla.). Figure 14-75 is a plot of FRI system limit data (most derived from tests with dual-flow trays with 29 percent hole area and 1.2- to 2.4-m tray spacing) against liquid superficial velocity for a variety of systems (Stupin, loc. cit.,
14-82 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION FIG. 14-75 Effect of liquid rate on ultimate capacity at higher liquid rates. (From Stupin, W. J., and H. Z. Kister, Trans. IChemE, vol. 81, Part A, p. 136, January 2003. Reprinted courtesy of IChemE.) 1965). The data show a constant-slope linear dependence of the system limit C-factor on the liquid load. There was a shortage of data at low liquid loads. Later data (Fig. 14-76) showed that as the liquid load was reduced, the system limit Cs,ult stopped increasing and reached a limiting value. Based on this observation, Stupin and Kister [Trans. IChemE 81, Part A, p. 136 (January 2003)] empirically revised the earlier Stupin/FRI correlation to give σ � ∆ρ C1 = 0.445(1 − F) � � 0.25 σ � ∆ρ C 2 = 0.356(1 − F) � � 0.25 − 1.4LS (14-167) (14-168) FIG. 14-76 Comparison of original ultimate capacity correlation to test data, C 6/C7, 1.66 bar. (From Stupin, W. J., and H. Z. Kister, Trans. IChemE, vol. 81, Part A, p. 136, January 2003. Reprinted courtesy of IChemE.) where Cs,ult = smaller of C1 and C2 (14-169) 1 F = �� (14-170) 1 + 1.4(∆ρ/ρG) 1/2 In Eqs. (14-167) through (14-170), Cs,ult is the system limit C-factor based on the tower superficial area [see Eq. (14-77) for C-factor definition]; LS is the liquid superficial velocity, m/s; σ is the surface tension, mN/m; ∆ρ is the difference between the liquid and gas densities, kg/m 3 ; and ρG is the gas density, kg/m 3 . Stupin and Kister (loc. cit.) relate the flattening of the curve in Fig. 14-76 at low liquid loads to the formation of more, smaller, easier-toentrain liquid drops when the liquid load is lowered beyond the limiting liquid load. It follows that devices that can restrict the formation of smaller drops may be able to approach the system limit capacity predicted by Stupin’s original equation [Eq. (14-167)] even at low liquid loads. The only devices capable of debottlenecking a tray system-limit device are those that introduce a new force that helps disentrain the vapor space. Devices that use centrifugal force (see “Centrifugal Force Deentrainment”) are beginning to make inroads into commercial distillation and have achieved capacities as high as 25 percent above the system limit. Even the horizontal vapor push (see “Truncated Downcomers/Forward-Push Trays”) can help settle the entrained drops, but to a much lesser extent. It is unknown whether the horizontal push alone can achieve capacities exceeding the system limit. WETTED-WALL COLUMNS Wetted-wall or falling-film columns have found application in masstransfer problems when high-heat-transfer-rate requirements are concomitant with the absorption process. Large areas of open surface
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