Tower spray

Since mass transfer in packed or spray towers occurs differentially rather than stagewise, their performance should be expressed in terms of the number of transfer units (NTU) rather than the number of theoretical stages (NTS). For dilute systems, the number of transfer units is given in terms of the terminal concentrations and the equilibrium relation by 

In order to permit sizing a tower, data must be available of the height of a transfer unit (HTU). This term often is used interchangeably with the height equivalent to a theoretical stage (HETS), but strictly they are equal only for dilute solutions when the ratio of the extract and raffinate flow rates, E/R, equals the distribution coefficient, K = xE/xR (Treybal, 1963, p. 350). Extractor performance also is expressible in terms of mass transfer coefficients, for instance, KEa, which is related to the number and height of transfer units by 

Packed towers may be employed when 5-10 stages suffice. They are not satisfactory at interfacial tensions above lOdyn/cm. Even at this condition, sieve trays have greater efficiency, and at much higher interfacial tensions some form of agitated tower is required. 

Metal and ceramic packings tend to remain wetted with the 

Intalox saddles and pall rings of 1-1.5-in. size are the most commonly used packings. Smaller sizes tend to be less effective since their voids are of the same order of magnitude as drop 

Flgnre 14.12. Tower extractors without agitation, (a) Spray tower with both phases dispersed, (b) Two-section packed tower with light phase dispersed, (c) Sieve tray tower with light phase dispersed, (d) Sieve tray construction for light phase dispersed (left) and heavy phase dispersed (right), (e) Redistributor for packed tower with light phase dispersed Treybal, 1963), 

Occasionally gasAiquid processes are carried out in spray towers. The liquid is sprayed at the top, the drops (that should not be too fine) fall downwards and a gas flow moves either upward or downward through the column. One of the main advantages of a spray column is that viscous liquids can be handled without great difficulty. 

At first glance one would expect that it would not make much practical difference whether the gas or the liquid phase is dispersed, as long as a sufficiently large contact area is created. However, there are practical situations where the differences are important. Spray towers may be advantageous when a relatively small liquid holdup is required, e.g., when the effects of undesired side reactions or consecutive reactions, taking place in the bulk of the liquid phase, have to be minimized (see. section 54.2.2). Spray towers are also used with liquids of high viscosity, since it is not practical to disperse gas in viscous liquids. Spray towers should not be used when there is a risk of superheating of the liquid phase. The most important chacteristic of the spray column is the size of the droplets. 

Usually nozzles can be selected that give a certain average drop size. When this is known, the mass transfer rate can be calculated relatively easily. With eqs. (4.40) and (4.41) the terminal velocity can be calculated (assuming the drops are rigid spheres), so that the contact time can be estimated. The mass transfer within the drops can be described in terms of non-steady state diffusion. When contact times are relatively short, which they normally are, the effect of non-steady state diffusion can be expressed as a mass transfer coefficient, that is time dependent  

For a contact time of r the time averaged value is then 

This is also known as the penetration theory. Eq. (4.63b) is identical with eq. (4.24) combined with eq. (4.30), when is replaced by d v. 

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Fig. 2. Types of spiay towers (a) horizontal spray chamber (b) simple vertical spray tower (c) cyclonic spray tower, Pease-Anthony type (d) cyclonic spray...

Fig. 18. Target efficiency of a single water droplet in a gravitational spray tower (278,279). From Ref. 280,...

Fig. 19. Predicted performance cut diameter for typical spray towers (271) (a) vertical countercurrent spray tower (b) horizontal cross-current spray chamber. Liquid—gas ratio is 1 m of Hquid/1000 m of gas. Drop diameter curve 1, 200 p.m curve 2, 500 p.m curve 3, 1000 lm. Uq = 0.6 m/s.

Fig. 20. Spray droplet target efficiency in a centrifugal spray tower with a centrifugal field of 100 g (140,281).

N2, and traces of PH, CO2, E, and S large furnaces generate off-gas at a rate of about 120—180 m /min. In most installations the off-gas is passed through a series of Cottrell electrostatic precipitators which remove 80—95% of the dust particles. The precipitators ate operated at temperatures above the 180°C dew point of the phosphoms. The collected dust is either handled as a water slurry or treated dry. Einal disposal is to a landfill or the dust is partially recycled back to the process. The phosphoms is typically condensed in closed spray towers that maintain spray water temperatures between 20 and 60°C. The condensed product along with the accompanying spray water is processed in sumps where the water is separated and recycled to the spray condenser, and the phosphoms and impurities ate settled for subsequent purification. 

Burning Pyrites. The burning of pyrite is considerably more difficult to control than the burning of sulfur, although many of the difficulties have been overcome ia mechanical pyrite burners. The pyrite is burned on multiple trays which are subject to mechanical raking. The theoretical maximum SO2 content is 16.2 wt %, and levels of 10—14 wt % are generally attained. As much as 13 wt % of the sulfur content of the pyrite can be converted to sulfur trioxide ia these burners. In most appHcations, the separation of dust is necessary when sulfur dioxide is made from pyrite. Several methods can be employed for this, but for many purposes the use of water-spray towers is the most satisfactory. The latter method also removes some of the sulfur... 

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... 

Spray Drying and Agglomeration. Most instant coffee products are spray-dried. Stainless steel towers with a concurrent flow of hot ak and atomized extract droplets are utilized for this purpose. Atomization, through pressure nozzles, is controUed based on selection of the nozzles, properties of the extract, pressures used, bulk density, and capacity requkements. Low inlet ak temperatures (200—280°C) are preferred for best flavor quaHty. The spray towers must be provided with adequate dust coUection systems such as cyclones or bag filters. The dried particles are coUected from the conical bottom of the spray drier through a rotary valve and conveyed to bulk storage bins or packaging lines. Processors may screen the dry product to... 

FIG. 14-126 Predicted spray-tower cut diameter as a function of sprayed length and spray droplet size for (a) vertical-countercurrent towers and (b) horizontal-cross-flow towers per Calvert [ J. Air Polliit. Control Assoc., 24, 929 (1974)]. Curve 1 is for 200-pm spray droplets, curve 2 for 500-pm spray, and curve 3 for 1000-pm spray. QHQc is the volumetric hqiiid-to-gas ratio, L liqiiid/nri gas, andt/cis the siiperBcial gas velocity in the tower. To convert liters per ciihic meter to cubic feet per cubic foot, multiply by 10 . ... 

In these expressions, B = ZJd, Nps = dVp/EE, Np r = dVn/Eii, where d = some characteristic length such as dp for packed towers or T for spray towers. Ep and Er are the longitudinal dispersion coefficients, which must ultimately be deter-... 

Spray Towers These are simple gravity extractors, consisting of empty towers with provisions for introducing and removing liquids at the ends (see Fig. 15-32). The interface can be run above the top distributor, below the bottom distributor, or in the middle, depending on where the best performance is achieved. Because of severe axial back mixing, it is difficult to achieve the equivalent of more than one or two theoretical stages or transfer units on one side of the interface. For this reason they have only rarely been applied in extraction applications. 

FIG. 15-32 Spray tower with both phases dispersed,... 

FIG. 23-25 Typ es of industrial gas/Hqiiid reactors, (a) Tray tower, (h) Packed, counter current, (c) Packed, parallel current, (d) Falling liquid film, (e) Spray tower, if) Bubble tower, (g) Venturi mixer, h) Static in line mixer, ( ) Tubular flow, (j) Stirred tank, (A,) Centrifugal pump, (/) Two-phase flow in horizontal tubes. 

By far the greatest number of instaUations is for the removal or recoveiy of mostly small concentrations of acidic and other components from air, hydrocarbons, and hydrogen. Hundreds of such plants are in operation, many of them of great size. They mostly employ either packed or tray towers. Power requirements for such equipment are small. When the presence of solid impurities could clog the equipment or when the pressure drop must be low, spray towers are used in spite of their much larger size for a given capacity and scrubbing efficiency. 

Sulfur Dioxide, Spray Towers Flue gases and offgases from sulfuric acid plants contain less than 0.5 percent SO9 smelter gases like those from ore processing plants may contain 8 percent. The high-concentration streams are suitable for the manufacture of sulfuric acid. The low concentrations usually are regarded as contaminants to be destroyed or recovered as elemental siilfur by, for example, the Claus process. 

Of the removal processes that have attained commercial status, the current favorite employs a shiny of lime or limestone. The activity of the reagent is promoted by the addition of small amounts of carboxylic acids such as adipic acid. The gas and the shiny are contacted in a spray tower. The calcium salt is discarded. A process that employs aqueous sodium citrate, however, is suited for the recoveiy of elemental sulfur. The citrate solution is regenerated and recycled. (Kohl and Riesenfeld, Gas Purification, Gulf, 1985, p. 356.)... 

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

Not many operating data of large-scale hquid/hquid reactions are published. One study was made of the hydrolysis of fats with water at 230 to 260°C (446 to 500°F) and 41 to 48 atm (600 to 705 psi) in a continuous commercial spray tower. A small amount of water dissolved in the fat and reacted to form an acid and glycerine. Then most of the glycerine migrated to the water phase. Tlie tower was operated at about 18 percent of flooding, at which condition the HETS was found to be about 9 m (30 ft) compared with an expec ted 6 m (20 ft) for purely physical extrac tion (Jeffreys, Jenson, and Miles, Trans. In.st. Chem. Eng., 39, 389-396 [1961]). A similar mathematical treatment of a batch hydrolysis is made by Jenson and Jeffreys (In.st. Chem. Engrs. Symp. Ser, No. 23 [1967]). 

Pot charging Particulates (dust), CO, HF, SO, CF4, and hydrocarbons High-efficiency cyclone, baghouse, spray towers, floating-bed scrubber, electrostatic precipitators, chemisorption, wet electrostatic precipitators... 

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