Liquid/gas ratio

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.

Other Considerations For organic vapor HAP control applications, low outlet concentrations will typically be required, leading to impractically tall absorption towers, long contact times, and high liquid-gas ratios that may not be cost-effective. Wet scrubbers will generally be effective for HAP control when they are used in combination with other control devices such as incinerators or carbon adsorbers. 

The collection efficiency of wet scrubbers is dependent on parameters such as the size and quantity of liquid droplets, the liquid/gas ratio, high wa-ter-to-particle relative velocity, wettability of dust, particle density, gas viscosity, etc. For any specific application, the design procedure is to review operating data available from the technical literature or from manufacturers for similar applications. If data are not available, it may be necessary to per form pilot scale tests, which can be used for scale-up purposes. 

Monsanto Enviro-Chem under the Dyna-Wave trademark case histories can be downloaded from a web site (http //www.enviro-chem. com/airpol/common/rjstop.html). Nonatomizing froth scrubbing is described in those patents as occuring within defining boundaries on a new dimensionless velocity versus dimensionless liquid/gas ratio two-phase flow regime map, shown here as Fig. 17-51. 

Mechanical draft towers are always used for cooling systems requiring a low approach and typically have an L/G (liquid/gas) ratio within the range of 1 0.75 to 1 1. 

Venturi scrubbers. In a venturi scmbber, a liquid jet issues from a nozzle. The jet induces cocurrent gas flow into the throat of the jet. Mass transfer takes place between the gas and the atomized liquid downstream of the nozzle. Mass transfer is usually poor and depends on the throat velocity or pressure drop, the liquid/gas ratio, and the liquid atomization pattern. Because of the cocurrent nature of contacting, the maximum solute removal does not exceed a single theoretical stage. Venturi scmbbers are used primarily for separation of fine particulate matter or... 

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). 

Rotary wheel atomizers require 0.8 to 1.0 kWh/1000 L. The lateral throw of a spray wheel requires a large diameter to prevent accumulation on the wall the ratio of length to diameter of 0.5 to 1.0 is in use in such cases. The downward throw of spray nozzles permits smaller diameters but greater depths LID ratios of 4 to 5 or more are used. Spray vessel diameters of 15 m (50 ft) or more are known. Liquid/gas ratios are 0.2 to 0.3 gal/MSCF. Flue gas enters at 149°C (300°F) at a velocity of 2.44 m/s (8 ft/s). Utilization of 80 percent of the solid reagent may be approached. Residence times are 10 to 12 s. At the outlet the particles are made just dry enough to keep from sticking to the wall, and the gas is within 11 to 28°C (20 to 50°F) of saturation. The fine powder is recovered with fabric filters. In one test facility, a gas with 4000 ppm S02 had 95 percent removal with lime and 75 percent removal with limestone. 

The basic mechanism of foam degradation in porous medium is film coalescence. It depends on film thickness and capillary pressure. In the process of advancement the film thickness changes considerably thickens in the narrow parts (pore throats) and thins in the wider parts (pore bodies). Visual observations of such a stretching-squeezing mechanism are reported by Huh et al. [178]. Therefore, the film thickness would depend on the liquid/gas ratio, the rate of movement and the ratio of pore-body to pore-throat. When the critical capillary (disjoining) pressure is reached, the film will rupture. 

Experimental values of the total liquid holdup were obtained by simultaneous closure of the inlet and exit solenoid valves, followed by drainage over a 15-min period. In some experiments, these values were checked by actual weighing of the section of the column in between the solenoid valves. The liquid holdup data were obtained by correlating the conductivity to the liquid/gas ratio. The details of the liquid holdup measurement technique are given in the Pittsburgh Energy Research Center s quarterly reports.22... 

Determined from average of gas-phase values from p. 151 of Reference 10, multiplied by average of liquid/gas ratios on p. 162. 

The importance of the minimum liquid-to-gas ratio lies in the fact that column operation is frequently specified as some factor of the minimum liquid-gas ratio. For example, a typical situation frequently encountered is that the slope of the actual operating line, is 1.5 times the minimum,... 

Assume a 180 MW boiler burning coal with 2.5% sulfur by weight, and a heating value of 12,767 BTU/lb. An appropriate limestone scrubber with forced oxidation would operate with a liquid/gas ratio of 130 gal. liquid per 1000 ft. of flue gas, and a pressure drop of Sin. water. Such a scrubber would consume 2.549 MW to operate, with the breakdown as shown in Table 3. This corresponds to 1.42% of the total power output of the plant. Such a scrubber would remove approximately 93% of the sulfur, while consuming approximately 13,000 Ib/hr of limestone being added at 35% solids. ... 

Equation (13.8) can be used for multiple solute cases, since it can accommodate absorption factors Ai, Aj, A, . .., each giving a different value of E. The liquid/gas ratio does not change, since it is based on overall flows. Thus, Equation (13.9) can give different values of N (theoretical stages). It is here that the key component enters into the picture. The required stages for that component are calculated, and then the same value of N is substituted back in Equation (13.8) to determine the relative absorption effectiveness of the other components. 

The gas flows horizontally, contacting by downflowing liquid. The effective driving force for mass transfer is between that for counter- and cocurrent contactors. Crossflow scrubbers have low pressure drop and usually require a lower liquid/gas ratio than either counter- or cocurrent scrubbers. The time of contact between gas and liquid is relatively low, and crossflow units are not reconunended for most chemical absorptions. Design procedures follow a finite-element approach the scrubber volume is divided into cubes, each of which is assumed to reach equilibrium. 

The power required to move flue gas through the scrubber, from the absorber inlet to the mist eliminator outlet, was calculated from pressure drop and gas flow-rate data reported in the literature for a range of superficial velocities, liquid/gas ratios, and internal scrubber packings. A fan of the wet induced-draft type was assumed for each case, operating on saturated flue gas at 125°F. The gas-side power input was added to the power delivered through the slurry recirculation pumps which was calculated from the volumetric flow rates and the minimum discharge pressures required for the given scrubbers. The total power input for SO2 absorption was thus determined as ... 

Work is delivered to the spray tower primarily through the slurry pumps. The TCA scrubber delivers most of the work through the fans due to its lower liquid/gas ratio and higher pressure drop. If the mechanical efficiencies of the pumps and fans were... 

For GC analysis, HS is a preconcentration technique particularly suitable for the sampling of volatile organic compounds in air, water, and solids. Few reports have been published on the use of static headspace in the analysis of free amines in aqueous samples because of the high polarity and solubility in water of these compounds." In one experiment," static headspace preconcentration was developed for the gas chromatographic analysis of aliphatic amines in aqueous samples. A liquid-gas ratio of 1, an incubation temperature of 80°C (15 min), a pH of 13.7, and a mixture of salts (NaCl and K2SO4) at saturation concentration gave a maximal headspace amine concentration (Table 11.4). 

Absorption and stripping are usually conducted in packed columns or in trayed towers. Packed columns are preferred when (1) the required column diameter is less than 60 cm (2) the pressure drop must be low, as for a vacuum service (3) corrosion considerations favor the use of ceramic or polymeric materials and/or (4) low liquid holdup is desirable. Trayed towers are preferred when (1) the liquid/gas ratio is very low, and (2) frequent cleaning is required. If there is no overriding consideration, cost is the major factor to be taken into account when choosing between packed columns and trayed towers for absorption or stripping. 

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