Spray Distributors

It is difficult to compare the performance of various spray towers since the type of spray distributor used influences the results. Data from Hixson and Scott 33 and others show that KGa varies as G70-8, and is also affected by the liquid rate. More reliable data with spray columns might be expected if the liquid were introduced in the form of individual drops through a single jet into a tube full of gas. Unfortunately the drops tend to alter in size and shape and it is not possible to get the true interfacial area very accurately. This has been investigated by Whitman et a/. 34 , who found that kG for the absorption of ammonia in water was about 0.035 kmol/s m2 (N/m2), compared with 0.00025 for the absorption of carbon dioxide in water. 

Spray distributors (Fig. 14-68b) are pipe headers with spray nozzles fitted on the underside. The spray nozzles are typically wide-angle (often 120°) full-cone. Spray distributors are unpopular in distillation but are common in heat transfer, washing and scrubbing services... 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

Ladder and spray distributors rely on pressure for their action. They provide a large gas flow area but a somewhat limited liquid flow area they are light and cheap but are sensitive to corrosion, erosion, and to a certain extent plugging. They are most suitable for high gas/liquid ratio applications. 

Liquid distributors function to spread an even distribution of liquid over the packing bed. Liquid distributors are commonly broken down by motive force into pressurized or gravity distributors. Pressurized distributors include pipe-oriflce distributors and spray distributors. Gravity distributors include pan-oriflce distributors and trough distributors. 

Pressurized liquid distributors use pressure drop across an orifice to deliver a smooth liquid distribution to a packed bed. Two main types exist, pipe-orifice distributors and spray distributors. Of the two, spray distributors are the most common. In specifying either type, the main criteria to be set are the pressure drop available for use and the minimum orifice size. These... 

The operating conditions for solid-state fermentation for cellulase production are dependent on the strain to be used, the reactor type and the medium composition, but the basic operating procedure remains the same as shown in Fig. 2. The final product can be obtained as crude solid cellulase, liquid cellulase or powder cellulase according to the application. Figure 3 shows a process flowsheet for cellulase production [25]. In the process, wheat bran is used as substrate. Seeds are prepared in a stirred-tank fermentor and then sprayed into the medium by a spray distributor. The fermentation is performed in a shallow-tray fermentor. The temperature and humidity in the fermentor are automatically regulated. After fermentation, cellulase is recovered by water extraction and purified by salt precipitation and ion exchange. The final product is concentrat-... 

The spray nozzle is not widely used. If more than one nozzle is used, it is difficult to obtain a uniform spray pattern because of overlap and underlap of the patterns. Also, liquid entrainment from the sprays is a problem. Spray distributors are sometimes used in petroleum refinery vacuum columns. The full cone nozzle is normally used, singly or in banks. 

Figure 3.4 Common types of liquid distributors, (a) Ladder pipe distributor (6) perforated ring distributor (c) spray distributor id) orifice pan distributor (e) tunnel orifice distributor (f) notched-trough distributor (g) weir-riser distributor. (Parts a and c to f reprinted courtesy of Norton Company part b, reprinted courtesy of Koch Engineering Company, Inc. part g, reprinted courtesy ofGlitsch, Inc.)...

Other performance characteristics of spray distributors are generally similar to perforated-pipe distributors. Like perforated-pipe distributors, spray distributors are of simple construction, are easy to support, and are inexpensive. Compared to perforated-pipe distributors, spray distributors offer an even larger open area, a greater liquid flow rate, easier replacement of corroded or eroded sections, and more extensive irrigation. On the debit side, spray distributors require higher pumping horsepower than other distributors, and an overhead mist eliminator is mandatory in order to control entrainment. Compared to perforated-pipe distributors, spray distributors also consume much more vertical space. 

Guidelines for selection, design, and operation of spray distributors are listed below ... 

Typically, spray distributors use wide-angle (120°) sprays, and are located 18 to 36 in above the bed, providing irrigation area of the order of 5 to 10 ft per spray nozzle. Typical pressure drops are 5 to 30 psi. 

Spray distributors are not suitable for vapor-containing streams. 

Spray distributors should never be embedded in the packing, because this is likely to cause premature flooding (237). For good distribution, a spray distributor containing more than a single spray nozzle should be placed at least 18 in, and preferably more than 24 in, above the bed. 

Like perforated-pipe distributors, the spray distributor turndown ratio is about 2 1 (111). Excessive liquid flow rates may create a mist problem, while low liquid flow rates reduce the cone diameter and may create poorly irrigated areas. For this reason, oversized spray nozzles must be avoided (237). As with perforated pipe distributors, a dual liquid distributor can be used to enhance turndown (Sec. 3.8). 

The distributor should be located at least 6 to 12 in above the packing to permit vapor disengagement from the bed before passing through the distributor (305). Greater distances (at least 18 to 24 in) above the bed are recommended for spray distributors. 

Plugging and erosion in spray distributor nozzles that have been in service often occur at the nozzle spirals (109) and may be invisible from outside. It is best to water-test these distributors during the shutdown such a test will also reveal internal leakage. However, this is not always practical. Alternatively, a sample of nozzles (about one in five) should be dismantled and inspected. Even better, these nozzles can be rigged and water-tested outside the column. 

If the liquid flow to the packing is low and a spray distributor is used, the sprays can collapse so that the liquid contact with the vapor degrades causing low viscosity and poor volatility of the stream below. If the pumparound rate is too high, liquid may be atomized at the spray distributor and the small liquid droplets may be entrained upwards. If the liquid stream is from the pumparound, the product above the pumparound will be contaminated with heavy components. If there is leakage or overflow Ifom the sidestream draw off pan, the falling liquid does not contact the vapor and it will reduce the viscosity of the stream below and also result in poor volatility. 

Spray distributors, wetting the whole cross-section of the colunm. 

A usual construction of spray distributor (type DP-S) product by Raschig [1] is presmted in Figs. 49 and 50. 

In order to achieve perfect function, a spray distributor with several nozzles must be arranged so that the spray cones overlap, when they fall on the packings. It is only in this way that it can be ensured that liquid reach each sections of the column. 

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