PART-II PACKED BED COLUMN
TYPES OF PACKING
Tower packings can be divided into two distinct categories: random (or dumped) and structured (or ordered) packing. Random packing consists of individual pieces which are dumped into the tower where they are oriented randomly. Structured packing is usually in modular form (blocks or sections), and the modules are designed and placed in a tower with specific orientations
RANDOM, STRUCTURED RANDOM PACKING RASCHIG RING Shaped cylindrically with solid walls and open ends Manufactured in metal, carbon, plastic, and ceramic. Carbon Raschig rings are still used in extremely corrosive, nonoxidizing environments.
PALL RING Commonly used in refinery and petrochemical applications. Available in metal and plastic under various trade names. "Pall Ring" , "Ballast Ring" and "Flexiring". Modifed metal Pall rings which provide slightly better performance are available under the trade names "HyPak" , "Ballast-Plus Ring" , and "K-Pac" .
Plastic Pall rings are primarily used in low temperature, low pressure systems.
SADDLES Saddles are available in plastic and ceramic. Trade names "Intalox Saddles" and "Flexisaddles". Ceramic saddle is thick, providing a low void fraction. This causes a higher pressure drop and lower capacity relative to other types of packings. Properly designed tower bottoms strainers are needed to protect pumps from damage. For these reasons, the ceramic saddles are not preferred for most refinery and petrochemical applications despite their relatively low cost.
PROPRIETARY Designed to compete against the Pall ring in efficiency, pressure drop, and capacity. Examples of these packings are the Miniring , IMTP etc. In certain applications, some proprietary packings can offer a significant advantage over the Pall ring. Particularly in retrofit situations where modest incremental improvement in a tower's efficiency, pressure drop, or capacity can make or break justification for a project. The main disadvantage of the proprietary packings compared to Pall rings can be the cost.
Raschig Rings Random Packing
Pall Rings Random Packing.
Saddles Random Packing.
Proprietary Random Packing.
STRUCTURED PACKING
Types are the corrugated and grid types. Structured packings can be used in any service random packings are used. They generally provide higher mass transfer efficiencies, lower pressure drops, and higher capacities than random packings. Major drawback is cost. CORRUGATED SHEETS Vertically oriented corrugated layers. The angled corrugations of adjacent layers are reversed relative to one another. Most commonly manufactured in metal. Available under the trade names Flexipac , Gempac , Intalox and Max-Pak.
GRID Lowest pressure drop and highest capcity of any packing. Very high resistance to fouling because of its open design. This open design also provides a very low specific surface area, which in turn reduces the mass transfer provided by the vaporliquid interface on the packing surface. Primarily used for heat transfer services, where it is quite effective.
Corrugated Structured Packing.
Intalox Grid Structured Packing.
INTERNALS : Packing Support , Liquid Distributors, Bed Limiters/Hold Downs, Flashing Feed Galleries
Packing may perform poorly if the internals aren't designed for good liquid and vapor distribution.
PACKING SUPPORTS Bar type, grid type and the gas injection type. Packing support, support the weight of the packed bed while providing little resistance to the flow of vapor and liquid. Maintaining a high open area, yet not so open that the packing is not retained.
Gas injection support Most commonly used in random packing applications. “Hump" design can provide an open area that exceeds 100% of the tower CSA when metal and approximately 80% for other materials.
Grid type support Most commonly used to support structured packing Commonly consists of parallel bars braced with crosspieces. The modular or block form of the structured packing allows the bars in the grid to be spaced far apart, which provides a very high open area. Bar type supports Consisting of thick parallel bars, are primarily made of and used for ceramics, glass, and graphite. Lowest open area of all supports In some very corrosive environments, they may be the only economically viable option.
LIQUID DISTRIBUTORS Weir type and orifice type. Weir types are designed for gravity flow of liquid over a weir. Orifice types are designed for gravity or forced flow of liquid. From a mechanical standpoint, the trough, tubular, and plate distributors are the most common. Orifice Flow A gravity-flow orifice distributor normally provides accurate and uniform flow control. The flow is controlled by the liquid head above the orifice. Maintain a uniform flow over the cross-section of a tower even when slightly out-of-level. Main drawback is plugging.
The forced-flow orifice distributor's effectiveness depends much on the distribution header and orifice pressure drops. Perforated pipe type distributors are primarily used to distribute a tower liquid inlet stream onto multipass trays or large packed bed liquid distributors. Used where the distribution requirements are not stringent. Weir Flow Used in high liquid flow and dirty or fouling systems Modelled by using the Francis formula. A slightly out-of-level distributor can result in substantial maldistribution. Designs with narrow rectangular weirs and high liquid levels over the weirs can provide good distribution. Weir flow distributors are less prone to plugging.
Plate Type Control liquid flow using weirs, orifices, or a combination of both. Consists of a plate with risers for vapor flow. Attached to a support ring that is welded to the tower wall. Often preferred for flashing feeds because it can save tower height and is relatively inexpensive. Trough Type Control liquid flow using weirs, orifices, or a combination of both. Consist of a header trough that distributes into lateral troughs. Supported either by a support ring or rest on a distributor support grid which rests directly on top of the packing. Provide a high open area, and the vapor pressure drop is negligible. Reliable in dirty or fouling services.
Tubular Type Employ both gravity and forced orifice flow. The gravity flow tubular distributor consists of a vertical feed box, horizontal main header (pipe or closed channel), and lateral pipes. Feed enters at the top of the feed box in which the liquid level is maintained. Designed to provide high turndown and low liquid residence time relative to trough types Highly susceptible to plugging. The forced flow tubular distributors have no feed box, and the liquid feed enters through a main pipe header, and flows through orifices in lateral pipes. The two main types are ladder and spray.
BED LIMITERS/HOLDOWNS The function of bed limiters and holddown plates is to prevent the packing from being displaced from the bed. In extreme cases, random packing can be blown into the liquid distributor and even into the tower overhead line. These plates are an open type of grid placed above the packed bed which retains the packing and allows passage of vapor and liquid. A bed limiter is attached to the support ring or clips, whereas a holddown plate rests on the packing. Sometimes the bed limiter is integral with the liquid distributor. They are rarely used with structured packing, which is less likely to become displaced, unless there are serious uplift concerns (resulting from process upsets). Bed limiters and holdown plates are available in metal, FRP, plastic, ceramic, and graphite materials.
Bed Limiter for Random Packing
Bed Limiter for Structured Packing
FLASHING FEED GALLERIES Flashing Feed Gallery is used to disengage the vapor phase from a two-phase feed. It consists of two plates: an upper gallery, which is 50% open for vapor disengagement, and a lower plate for liquid distribution. Each plate requires a separate support ring. The two-phase feed is fed to the upper gallery where the vapor disengages from the liquid. Inlet Deflector Baffle is typically used in front of the feed nozzle to deflect the feed around the tower wall. Holes in the bottom of the upper gallery transfer the liquid to the lower plate where the liquid is distributed over the packing. Flashing Feed Distributor is fabricated in sections for passage through column manways.
Flashing Feed Gallery
Manufacturers and catalogues
Major Manufacturers in the field of Column internals and their web address (Refer website for their products, catalogues and design softwares ) are as follows : Sulzer Chemtech : (also taken over “Nutter”) http://www.sulzerchemtech.com Montz : http://www.montz.de Koch-Glitsch : http://www.koch-glitsch.com Saint-Gobain Norpro : (earlier known as Norton) http://www.nortoncppc.com
PACKING TERMINOLOGY AND CRITERIA
Flooding Occurs when the vapor flow is high enough to entrain the countercurrent flowing liquid and cause a liquid "stack-up". The pressure drop through the tower substantially increases to overcome the liquid head. The operation becomes very unstable and is normally characterized by very high pressure drop and very low mass transfer efficiency. Flooding can also occur if the liquid rate is simply too great for it to flow down without "stacking up". Liquid Holdup Volume of liquid in a packed bed, normally expressed as a percentage of the total volume. A higher holdup reduces the volume available for vapor flow. Liquid holdup is a strong function of the packing type, liquid rate and properties, and the vapor rate.
Packing Factor Determined empirically from experimental data for each packing. Using vapor and liquid flowrates and densities, liquid viscosity, and the packing factor, packing pressure drops can be calculated over a wide range of operating conditions. Packing Size The size convention of random packings was first derived from the raschig ring which has a height equal to its diameter. Specifying a 1" pall ring, for example, meant that it would be 1" high and 1" in diameter. Unfortunately, all random packings can't be specified in the same way. The style numbers for a particular type of packing provide a relative size comparison, with the higher style numbers usually corresponding to larger sizes.
Specific Surface Area The surface area of a packing per unit volume, e.g., on a ft2 per ft3 basis. Depends on the piece count per unit volume, and important to note that a standardized method for establishing piece counts and specific surface area has not been established. Reported values provide a relative indication of packing surface available for vapor-liquid contacting. A given surface area for one type of packing may be considerably more effective than the same area for another type. Void Volume Volume of a packing container not occupied by the packing material divided by the total volume of the container. Expressed as a fraction or percentage and provide a relative indication of open space available for vapor and liquid traffic.
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