Considerations for specifying structured packings

Solids. Wire mesh packings easily plug and are best avoided altogether in solid-containing services. Corrugated-sheet packings, especially those with wider spacings between adjacent layers, are more tolerant to solids, but their distribution equipment may not be. If a potential for fouling exists, accurate data must be gathered and com-

Sulzer Trays
Figure 8.13 Flow of liquid over a vertical surface of Koch-Sulzer® wire mesh (left) and metal sheet (right). (Reprinted courtesy of Koch Engineering Company, Inc.)

municated to the manufacturer. Any economic analysis must account for the costs of plugging prevention. Several measures are described elsewhere (40).

Corrosion and oxidation. Thickness of the corrugated sheets of structured packings is typically around 30 gage (Woo in), and seldom exceeds 20 gage (V32 in). A 30-gage sheet is eight times thinner than a typical stainless steel tray, and several times thinner than the corrosion allowance for carbon steel trays. The thinness of the sheets makes them extremely vulnerable for attack by corrosive chemicals and oxidants. Further, the large surface area per unit volume, which is conducive to mass transfer, is also conducive for corrosion and oxidation. Thorough removal of residual liquid, wash water, air, or process gas trapped in structured packings at startup and shutdown is difficult, and the leftovers promote corrosion, oxidation, or other undesirable reactions.

It is not uncommon to find bits of corroded structured packings at the bottom of the column. There have also been several instances where a bed of structured packings caught fire when air entered the column at shutdown. These fires were initiated by small amounts of flammable or pyrophoric materials adhered to packing surfaces.

It is imperative that the packing manufacturer be supplied with accurate data on the corrosiveness of the system. Readily oxidizable metals such as carbon steel should be avoided. Generally, the metallurgy of a structured packing should be more corrosion- and oxidation-resistant than the metallurgy normally used to handle a service. Special consideration should be given to startup, shutdown, commissioning, air leakage, and vacuum relief (e.g., air entering a hot column); rapid oxidation of the packing is hazardous.

Sensitivity to upsets. Structured packings due to their lower pressure drop and bricklike structures, can weather pressure surges (such a* those due to introduction of a pocket of water into a hot hydrocarbon tower) much better than random packings.

Maintenance and troubleshooting. Detecting fabrication or installation defects, and inspection inside a "brick" of structured packing are extremely difficult and may damage the packings. Inspection of the column walls (e.g., for corrosion) can also be difficult and require damaging many packing elements. Inspecting and maintaining randos packings is far easier.

Cost. Structured packings typically cost 3 to 10 times more per unit volume than 2-in random packings. However, structured packings are more efficient and will afford a shorter column. Savings due to the reduced height usually outweigh the additional packing cost. Further, pressure drop per theoretical stage is far lower for structured packing than for random packings (Sec. 8.1.10). In vacuum, this leads to lower reflux, lower operating costs, and smaller column and auxiliaries. Finally, the price per unit volume depends on the quantity ordered, the economic climate, and marketing considerations, and in some cases may be low. Therefore, the only meaningful cost comparison is of the complete designs.

8.1.12 Types of grids

Wooden and plastic grids, similar to those used in cooling towers, have been used in distillation and absorption for decades. The first modern grid was the Glitsch C-grid®, developed in the early 1960s as a means of utilizing sheet metal from which circular valve units (for valve trays) were punched out. Modern grid geometries are designed to promote desirable features, i.e., a high open area, a high capacity, a high resistance to fouling and plugging, and a low pressure drop. The efficiencies of grids are considerably lower than those of both random and structured packings. Grids are primarily used in direct-contact heat transfer, scrubbing, and deentraining services.

Glitsch C-Grfd® (Fig. 8.14«). This grid is constructed from metal sheets in which 1.5-in holes are punched. The sheet is bent into a multiple V shape, with the holes on the sides of each V. The resulting layer is about IV2 in thick. Each layer is placed horizontally, and with the long sides at 45° to the layer below. C-Grid® has a higher capacity, but a lower efficiency than the Glitsch EF-25A Grid® (below). It is relatively infrequently used, mainly in applications where more capacity is required than EF-25A can offer.

Glitsch EF-25A Grid® (Fig. 8.14to,c). This is a seamless structure manufactured in 60 in long x 15 in wide x 2Vs in high open area panels composed of horizontal, vertical, and slanted members. Adjacent layers of the grid are rotated 45° to produce a crisscross effect, giving it a lattice structure. The Glitsch EF-25A Grid® was introduced in the 1960s, has been popular, and is available in metals and plastics. The Norton Company's Intalox® Grid (in metal) is often considered equivalent to the Glitsch EF-25A Grid®.

Koch Flextgrid® #2 (or Koch High-Capacity Flexigrid®) (Fig. 8.140). This grid is constructed of 60 in long x 16 in wide x 23A in high open area panels. The vertical, parallel blades of the grid are held in a fixed rela-

Flexigrid Refining

Figur» 8.14 Common grid packings, (a) Glitsch C-Grid*; (6) Glitsch EF-25A Grid*. (Parts aandb courtesy of Glitsch Inc.)

25a Grid Packing

Figur* 8.14 (Continued) Common grid packings, (c) An element of Glitsch EF-25A Grid*; (d) an element of Koch Flexigrid® #2 (Part c, courtesy of Glitsch Inc. Part d, courtesy of Koch Engineering Company, Inc.)

Nutter Snap Grid Packing

Figure 8.14 (Continued) Common grid packings, (e) An element of Koch Flexigrid* #1. if) Three layers of Nutter #3 Snap-Grid® (Port e, courtesy of Koch Engineering Company, Inc.; part f, courtesy of Nutter Engineering Corp.)

Figure 8.14 (Continued) Common grid packings, (e) An element of Koch Flexigrid* #1. if) Three layers of Nutter #3 Snap-Grid® (Port e, courtesy of Koch Engineering Company, Inc.; part f, courtesy of Nutter Engineering Corp.)

Orientation direction of the orifice in the expanded metal

Additional orifices

Orientation direction of the orifice in the expanded metal

Additional orifices

Figur* 8.14 (Continued) Common grid packings, (g) Perform® Grid. (Port g from K. Hoppe, J. Keller and L. Krell, Chem, Engnr., February, 1978, p. 110; reprinted courtesy of the Institution of Chemical Engineers, UK.)

tionship to one another by welded cross members. Each successive layer of grid is rotated by 45° to the previous one. Compared to the EF-25A Grid®, the projections are smaller and angled (Fig. 8.14ci). This reduces interference to vapor flow but also the area for vapor-liquid contact, giving the grid its higher capacity and lower efficiency. Koch Flexi-grid® #2 was introduced in the early 1980s and is available in metals.

Koch Flexigrid® #3 (or Koch High-Efficiency Flexigrid®) (Fig. 8.14«). This grid is constructed from panels identical to those of Koch Flexigrid® #2, but the projections are larger and rectangular. This not only makes more area available for vapor-liquid contact and improves the spread of this area, but also increases the resistance to vapor flow. Compared to Flexigrid® #2, Flexigrid® #3 gives higher efficiency at the expense of lower capacity. Koch Flexigrid® #3 was introduced in the 1970s and is available in metals.

Nutter #3 Snap-Grid® (Fig. 8.14f). This grid is constructed from high open area panels composed of spacing bars, I-beam-shaped elements, slotted baffles, and curved tabs. Each panel is at 45" to the panel below. The objective of this intricate design is to maximize vapor-liquid contact and mechanical strength while maintaining a high capacity. The Nutter Snap-Grid® was introduced in the early 1980s and is available in metals.

Perform® grid (Fig. 8.14s). This grid is made of expanded metal components. The direction of orientation of small form-punched orifices changes both horizontally from one element to the next and vertically from one layer to another. Every layer is rotated at an acute angle (3 to the layers below. To provide an additional open area, windows are cut in the material, with the flaps projecting out of the material at an acute angle y. The Perform® grid was introduced in the 1970s and is available in metals from VEB Chemieanlagenbaukombinat Leipzig-Grimma, Germany.

8.1.13 Grid versus other packings

Capacity and efficiency. Capacity data by Glitsch (42) suggest that the Glitsch EF-25A Grid® has a capacity practically identical to CMR* #4. The same source also states that EF-25A has an efficiency two-thirds of that of 2-in Pall® rings in distillation services. Since CMR* #4 has about half the specific surface area as 2-in Pall® rings, it appears comparable to EF-25A Grid® in efficiency as well. Roughly, grids have high capacity and low efficiency, similar to the larger second- and third-generation random packings.

Pressure drop. Due^to its high open área and high capacity, grid is one of the lowest-pressure-dropdevices. Compared to 2-in Pall® rings, grids typically have a pressure drop three to five times lower; some high-capacity grids have a pressure drop 10 times lower (43).

Wetting. The minimum liquid rate recommended for adequate wetting is about 0.2 to 0.3 gpm/ft2 (44), although rates down to 0.1 gpm/ft2 have been successfully used (45,46). These rates are low and are comparable to the minimum wetting rates for corrugated-sheet structured packings (Sec. 8.1.10). Grids therefore can achieve high turndown and perform well at low liquid rates.

Solids. Grids are suitable for solid-containing streams and fouling services. Grid surfaces are designed to be self-draining, avoiding areas where sediment may trap or hot liquid may coke or polymerize. The low pressure drop keeps liquid holdup at a minimum, which reduces the residence time of liquid in the heated zone. The ability of grids to handle solids and minimize fouling is one of their greatest advantages.

Liquid to grids is usually supplied by spray nozzles, and these may plug. As with other distributors, adequate measures are required to prevent plugging, and the grid manufacturer must be supplied with good data on the fouling potential of the service.

Mechanical strength. The lattice structure of grids gives them a mechanical strength advantage compared to random and structured packings. Close attention is required to the metal thickness of the grid. This usually ranges from 18 to 14 gage; the mechanical strength depends on this thickness. A thinner gage gives a cheaper grid, at the expense of lower strength.

Sensitivity to upsets. The high open area and rugged construction give grids a strong resistance to pressure surges, even greater than that of structured packings. Strong mechanical design and good fastening practices (42,46) must be followed to achieve this resistance.

Corrosion and oxidation. Due to the relatively thick metal, low specific surface area, and rugged construction of grids, their tendency to oxidize is low. Grids tend to degrade to a much lesser degree than either random or structured packings.

Maintenance and troubleshooting. Due to their layered structure, grids are easy to install, remove, inspect, and maintain. Grid cleaning (especially if coked up) may be difficult, often impractical, but even in such cases, it is easier than that of random or structured packings.

Cost. Grids are far less expensive than structured packings, and their volumetric costs are of the same order as those of random packings.

Grid-Ring Combination9 (GRC®). In many condensing heat transfer services (such as refinery fractionator pumparound services), the vapor and liquid loads taper as one ascends the bed, due to condensation of vapor. To achieve the high capacity required at the bottom of the bed, the entire bed can be packed with grid alone. A patented improvement (42) that permits shortening of the bed is to use grid near the bottom and to dump random packings on top of the grid. The grid height is designed to condense enough of the vapor so that the remaining vapor and liquid loads do not exceed the allowable capacity of the random packing. The random packings complement the condensation at a higher efficiency than the grid, thus permitting an overall shorter bed to be used.

One alternative and analogous technique is to use structured packings instead of random packings to complete the condensation. Another (47) is to use a "fine" (i.e., high-surface-area, low-capacity) grid instead of the random packings.

8.2 Packing Hydraulics 8.2.1 Pressure drop flow regimes

At low liquid flow rates (region A-B in Fig. 8.15), the open cross-sectional area of the packing is about the same as in a dry bed. The

Figure 8.15 Typical presfiure drop characteristics of packed tower?. (From. H. Z. Kister, Distillation Operation. Copyright <& by McGraw-Hill, Inc., reprinted by permission.)

Figure 8.15 Typical presfiure drop characteristics of packed tower?. (From. H. Z. Kister, Distillation Operation. Copyright <& by McGraw-Hill, Inc., reprinted by permission.)

pressure drop is entirely by Mctional losses through a series of openings, and therefore is proportional to the square of the gas flow rate. In random packings, the openings are randomly sized and located, and pressure drop is due to expansion, contraction, and changes of direction. In structured packings, the openings are regular and of uniform size, and pressure drop is due to changes in direction.

As liquid rates are raised, the liquid occupies some of the cross-section area, making the openings for gas flow smaller. The pressure drop curve will parallel A-B but will be somewhat above it. At high liquid flow rates, the packing voids fill up with frothy liquid. A portion of the energy of the gas is used to support the liquid in the column and pressure drop becomes proportional to the gas rate raised to a power different (usually lower) than 2 (region A'-B'). The point where the packing voids fill up, i.e., when tower operation switches from vapor-continuous (normal) to liquid continuous is termed phase inversion.

For all liquid flow rates, as gas flow rate is raised, a point is reached when the gas velocity begins to interfere with the free drainage of liquid. Liquid will start to accumulate or "load" the bed, giving this region the name "the loading region." The accumulation of liquid reduces the cross-section area available for gas flow and therefore accelerates the pressure drop rise. In this region (B-C and B'-C' in Fig. 8.15) the slope of the curve increases to a power distinctly above 2.

Upon further increases in gas rate, more liquid accumulates, until the liquid surface becomes continuous across the top of the packing. The slope of the curve ^creases, until it becomes very steep. When this occurs (points C and C' in Fig. 8.15) the column is flooded.

A stable operating region beyond flooding (above points D and D') was discovered (48,49). In this region, the packed column is essentially inverted to a "bubble column." This region is of little significance in industrial practice.

8.2.2 Efficiency flow regimes

Figure 8.16c illustrates typical variation in column efficiency as gas rate is raised at a constant L/V ratio (i.e., liquid rate increases with the gas rate). To the left of point A on Fig. 8.16« is the turndown maldistribution region. Upon turndown from normal operating rates, a point is reached where efficiency drops because either the distributor or the packing reaches a turndown limit. Most distributors are designed for turndown ratios of 2 to 4 from their normal design liquid rates. At lower liquid rates, irrigation to the bed is poor, giving poor efficiency. When liquid remains well-distributed in the column upon turndown, point A represents the minimum wetting rate of the packing. Below minimum wetting the falling liquid film breaks up, some of the packing surface unwets, and the efficiency drops.

Generally, the minimum wetting rate is at 0.5 to 2 gpm/ft2 for random packings, and 0.1 to 0.2 gpm/ft2 for structured packings (Sec. 8.2.15). It follows that point A is usually a distributor turndown limit. Regardless of which limit point A represents, it is extremely sensitive to maldistribution (Fig. 8.166). When liquid distribution is poor, it will take more liquid to wet the entire bed, and point A will shift to the right. If distribution is veiy poor, point A may never be observed, and the curve will have no flat region at all. A V-shaped curve is not uncommon, and is indicative of poor distribution.

Region A-B (Fig. 8.16a) has turbulent liquid film, good wetting of the packing, good mass transfer, and essentially constant efficiency. This region is ideal for packed column design and operation.

Raising gas velocity past point B moves column operation into the loading region. Initially, efficiency improves because of the greater liquid holdup (region B-E), but this improvement is short-lived. As the flood point is approached, the efficiency passes through a maximum (point E), and then drops (region E-C) because of excessive entrapment.

Packed towers are usually designed for region A-B. Although region

Gosrote -*

Gosrote -*

Liquid rate-

Figure 8.16 Typical efficiency characteristics of packed towers. (a) Typical efficiency characteristics for random packings and for most corrugated-sheet structured packings; (£>) effect of liquid distribution on the efficiency characteristics of part a. (Part b from H. Z. Kister, Distillation Operation. Copyright €> by McGraw-Hill, Inc. Reprinted by permission.)

Liquid rate-

Figure 8.16 Typical efficiency characteristics of packed towers. (a) Typical efficiency characteristics for random packings and for most corrugated-sheet structured packings; (£>) effect of liquid distribution on the efficiency characteristics of part a. (Part b from H. Z. Kister, Distillation Operation. Copyright €> by McGraw-Hill, Inc. Reprinted by permission.)

B-F gives the highest efficiency, it is usually avoided in design because of the proximity of the flood point. In practice, operation is normally stable and design efficiency or better is achieved throughout region A-F.

Most wire-mesh and some corrugated-sheet structured packings t LJ

Gosrote Liquid rote -

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