Packed towers

Packed towers are used for further scmbbing after the venturi scmbber. 

It shall be possible to introduce liquid injection spray at centre through pipes 

This is able to create a draught for the incoming gases due to the spray of scmbbing liquor. This will be useful for flow of gases. 

However, liquid-gas contact is not as good as venturi because the mixing is not as intense. It is useful where concentration of pollutants and particulate matter is less. 

The unit is mounted on a liquor circulation tank and thus saves separate liquid exit piping (with U-seal). The construction can be of fibreglass-reinforced plastic or rubber-lined steel tank. 

Packed towers, used for continuous contact of liquid and gas in both counter-current and cocurrent flow, are vertical columns which have been filled with packing or devices of large surface, as in Fig. 6.27. The liquid is distributed over, and trickles down through, the packed bed, exposing a large surface to contact the gas. 

The tower packing, or fill, should offer the following characteristics  

Have structural strength to permit easy handling and installation. 

These may be of wood, metal, chemical stoneware, acidproof brick, glass, plastic, plastic- or glass-lined metal, or other material depending upon the corrosion conditions. For ease of construction and strength they are usually circular in cross section. 

In the case of random packings, the packing density, i.e., the number of packing pieces per unit volume, is ordinarily less in the immediate vicinity of the tower 

The diameter of a packed tower is calculated from an estimated flooding velocity with a continuity equation similar to Eq. (14.11) for tray towers  

Depth of plastic random packings may be limited by the deformability of the packing elements to 10-15 ft. For metal random packings, this height can be 20-25 ft. For both random and structured packings, the height between redistributors is limited to 20-25 ft. because of the tendency of the phases to maldistribute. 

The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with large surface per unit volumes that are usually randomly dumped into the tower. Structured packings are normally layers of elements fabricated form expanded metal, woven wire screen, or sheet metal, and are stacked into the tower very carefully. 

The first of the widely used random packings were Raschig rings which are hollow cylinders of ceramics, plastics, or metal. They were an economical replacement for the crushed rock often used then. Because of their simplicity and their early introduction, Raschig rings have been investigated thoroughly and many data of their performance have been obtained which are still useful, for 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

Ultimately, the choice of packing is based on pressure drop, mass transfer efficiency, and cost. Structured packings made from sheet metal now cost less than 100 per cubic foot. The hydraulics of packed beds will be discussed later in this section. 

A factor that is of concern with bubblecap trays is the development of a liquid gradient from inlet to outlet which results in corresponding variation in vapor flow across the cross section and usually to degradation of the efficiency. With other kinds of trays this effect rarely is serious. Data and procedures for analysis of this behavior are summarized by Bolles (in Smith, 1963, Chap. 14). There also are formulas and a numerical example of the design of all features of bubblecap trays. Although, as mentioned, new installations of such trays are infrequent, many older ones still are in operation and may need to be studied for changed conditions. 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Depth of packing without intermediate supports is limited by its deformability metal construction is limited to depths of 20-25 ft, and plastic to 10-15 ft. Intermediate supports and liquid redistributors are supplied for deeper beds and at sidestream withdrawal or feed points. Liquid redistributors usually are needed every 2 -3 tower diameters for Raschig rings and every 5-10 diameters for pall rings, but at least every 20 ft. 

Used for further scrubbing after the venturi scrubber. Pressure drop is more than 

The vendor should be asked to supply a spray nozzle with a 120-135 ° spray angle so that the cross section is properly covered. The scrubbing liquor pump should 

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Industrially an aqueous solution of chlorine dioxide can be prepared by passing nitrogen dioxide up a packed tower down which sodium chlorate(V) flows ... 

Sometimes the height equivalent to a theoretical plate (HETP) is employed rather than and to characterize the performance of packed towers. The number of heights equivalent to one theoretical plate required for a specified absorption job is equal to the number of theoretical plates,... 

Includes cyclonic, dynamic, filtration, inertial impaction (wetted targets, packed towers, turbulent targets), spray chambers, and venturi. 

When two phases are present the situation is quite complex, especially in beds of fine soHds where interfacial forces can be significant. In coarse beds, eg, packed towers, the effects are often correlated empirically in terms of pressure drops for the single phases taken individually. 

Decomposition by Caustic Scrubbing. The waste gas stream is led through packed towers where a sodium hydroxide solution is introduced at the top of the towers. Venturi scmbbers can also be used. Makeup sodium hydroxide is added under pH control (32). 

Environmental Protection. Fumes resulting from exposure of anhydrous aluminum chloride to moisture are corrosive and acidic. Collection systems should be provided to conduct aluminum chloride dusts or gases to a scmbbing device. The choice of equipment, usually one of economics, ranges from simple packed-tower scmbbers to sophisticated high energy devices such as those of a Venturi design (11). 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Plants producing oleum or Hquid SO typically have one or two additional packed towers irrigated with oleum ahead of the normal SO absorption towers. Partial absorption of SO occurs in these towers, and sulfuric acid is added to maintain desired oleum concentrations. Normally, oleum up to about 35 wt % free SO content can be made in a single tower two towers are used for 40 wt % SO. Liquid SO is produced by heating oleum in a boder to generate SO gas, which is then condensed. Oleums containing SO >40 wt % are usually produced by mixing SO with low concentration oleum. 

Process air in sulfur-burning plants is dried by contacting it with 93—98 wt % sulfuric acid in a countercurrent packed tower. Dry process air is used to minimise sulfuric acid mist formation in downstream equipment, thus reducing corrosion problems and stack mist emissions. 

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Types of air strippers include packed towers, tray towers, and spray towers. Packed towers are packed or filled with small forms made of polyethylene [9002-88-4] stainless steel, poly(vinyl chloride) (PVC) [9002-86-2] or ceramic that provide large surface area to volume ratios which increase transfer rates into the air stream. Packed towers operate in countercurrent mode, that is, the aqueous stream enters at the top of the tower while air is blown in from the bottom. An example of this type of unit is shown in Figure 1. Channeling or short circuiting of the aqueous stream is minimized by... 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

In a steam stripper, steam is introduced into a packed tower, which causes volatiles to be removed in the vapor phase. An a2eotropic mixture is formed, resulting in a separation of the volatiles from the water. An effluent recycle is usually employed to reduce volatiles in the Hquid effluent. 

Copper(II) oxychloride [1332-65-6], Cu2Cl(OH)2, is found in nature as the green hexagonal paratacamite [12186-OOA] or rhombic atacamite [1306-85-0]. It is usually precipitated by air oxidation of a concentrated sodium chloride solution of copper(I) chloride (13—15). Often the solution is circulated through a packed tower of copper metal, heated to 60—90°C, and aerated. 

Packed vs Plate Columns. Relative to plate towers, packed towers are more useful for multipurpose distillations, usually in small (under 0.5 m) towers or for the following specific appHcations severe corrosion environment where some corrosion-resistant materials, such as plastics, ceramics, and certain metaUics, can easily be fabricated into packing but may be difficult to fabricate into plates vacuum operation where a low pressure drop per theoretical plate is a critical requirement high (eg, above 49,000 kg/(hm ) (- 10, 000 lb/(hft )) Hquid rates foaming systems or debottlenecking plate towers having plate spacings that are relatively close, under 0.3 m. 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

The stagnant-film model discussed previously assumes a steady state in which the local flux across each element of area is constant i.e., there is no accumulation of the diffusing species within the film. Higbie [Trans. Am. Jn.st. Chem. Eng., 31,365 (1935)] pointed out that industrial contactors often operate with repeated brief contacts between phases in which the contact times are too short for the steady state to be achieved. For example, Higbie advanced the theory that in a packed tower the liquid flows across each packing piece in laminar flow and is remixed at the points of discontinuity between the packing elements. Thus, a fresh liquid surface is formed at the top of each piece, and as it moves downward, it absorbs gas at a decreasing rate until it is mixed at the next discontinuity. This is the basis of penetration theoiy. 

The predictions of correlations based on the film model often are nearly identical to predictions based on the penetration and surface-renewal models. Thus, in view of its relative simphcity, the film model normally is preferred for purposes of discussion or calculation. It should be noted that none of these theoretical models has proved adequate for maldug a priori predictions of mass-transfer rates in packed towers, and therefore empirical correlations such as those outlined later in Table 5-28. must be employed. 

Investigators of tower packings normally report kcCi values measured at very low inlet-gas concentrations, so that yBM = 1, and at total pressures close to 100 kPa (1 atm). Thus, the correct rate coefficient For use in packed-tower designs involving the use of the driving force y — y /yBM is obtained by multiplying the reported k co values oy the value of pf employed in the actual test unit (e.g., 100 kPa) and not the total pressure of the system to be designed. 

Effects of System Physical Properties on Ug and Ui When designing packed towers for nonreacting gas-absorption systems for... 

Effective Interfacial Mass-Transfer Area a In a packed tower of constant cross-sectional area S the differential change in solute flow per unit time is given by... 

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

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