Cooling with Entrainment

So far we have treated a rising air parcel as a closed system. This is rarely the case in real clouds, where air from the rising parcel is mixed with the surrounding air. If m is the mass of the air parcel one defines the entrainment rate e as 

The entrainment rate is often written as e 1 //, where l is the lengthscale characteristic of the entrainment process. If the water vapor mixing ratio of the environment around the rising parcel is w v and its temperature 7, one can show that the lapse rate in the cloud Fc is given by (Pruppacher and Klett 1997) 

Let us assume that the air parcel has mass m and air density p (without including the liquid water). The velocity of the air parcel will be the result of buoyancy forces and the gravitational force due to liquid water. The buoyancy force is proportional to the volume of the air parcel, m/p, and the density difference between the air parcel and its surroundings, p — p. The liquid water mass is mwi and the corresponding gravitational force gmwL-The equation for conservation of momentum is 

FIGURE 15.12 Schematic description of the cloud formation mathematical framework. 

The rate of change of temperature can be calculated using (15.55), noting that dTfdt = WdTfdz, and also that w s should be replaced by Wv to allow the creation of supersaturations. The final result is 

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The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

Manipulation. Since thionyl chloride attacks rubber, and since the usual reaction products are HCl and SO2, the reagent should be manipulated in all-glass apparatus with silicon grease on the glass joints, and provision should be made for entrainment of the gases. The best practice is to carry out the reaction in a round-bottomed flask fitted with a reflux condenser and drying tube and mounted in a hood the flask later can be fitted with an adapter for removal of excess thionyl chloride at the pressure of an aspirator on the steam bath and then for vacuum distillation of the product. Many workers employ thionyl chloride in large excess, but the evidence seems to show that no excess is needed in case the reaction is run at room temperature or if the mixture is refluxed under an efficient condenser cooled with water at 20° or below. 

Particulates in glass furnaces come from two sources (1) physical entrainment of the batch into the flue system and (2) volatilization of glass and batch constituents that recombine and condense upon cooling. Physical entrainment of particles is reduced when an air/fuel furnace is enhanced with oxygen. The flue gas volume decreases, thereby lowering the gas velocity across the furnace and entering the flue system. 

The thermal structure reveals the existence of a barrier layer in the SCS. The barrier layer usually weakens the cooling effect entrained at the bottom of the mixed layer. There are barrier layers in both the NSCS and SSCS, but they are thinner than that in the western equatorial Pacific. A barrier layer in the SSCS has a seasonal variation, and its depth has a positive correlation with temperature in the mixed layer. In addition, the barrier layer often exists in summer and autumn. The structure of the barrier layer in the SSCS is significantly modulated by the wind field, as well as by development of the mixed layer. In summer, relatively fresh water in the upper layer in the SSCS piles up in the southeast SCS because of the combined action of southeastward Ekman transport and downwelling in the eastern SCS. The high temperature water at the bottom of the mixed layer remains in a thermally uniform layer after separating from the mixed layer. The deepest barrier layer lies in the southeastern SCS, at about 30 m depth. The location of the thickest barrier layer almost overlaps the SCS Warm Water, which suggests that the heat barrier effect may stimulate the development of the SCS Warm Water. 

Scrubbers. Scrubbers are designed to contact a liquid with the particle-laden gas and entrain the particles with the liquid. They offer the obvious advantage that they can be used to remove gaseous as well as particulate pollutants. The gas stream may need to be cooled before entering the scrubber. Some of the more common types of scrubbers are shown in Fig. 11.2. 

Having assisted desolvation in this way, the carrier gas then carries solvent vapor produced in the initial nebulization with more produced in the desolvation chamber. The relatively large amounts of solvent may be too much for the plasma flame, causing instability in its performance and, sometimes, putting out the flame completely. Therefore, the desolvation chamber usually contains a second section placed after the heating section. In this second part of the desolvation chamber, the carrier gas and entrained vapor are strongly cooled to temperatures of about 0 to -10 C. Much of the vapor condenses out onto the walls of the cooled section and is allowed to drain away. Since this drainage consists only of solvent and not analyte solution, it is normally directed to waste. 

Hot product char carries heat into the entrained bed to obtain the high heat-transfer rates required. Feed coal must be dried and pulverized. A portion of the char recovered from the reactor product stream is cooled and discharged as product. The remainder is reheated to 650—870°C in a char heater blown with air. Gases from the reactor are cooled and scmbbed free of product tar. Hydrogen sulfide is removed from the gas, and a portion is recycled to serve as the entrainment medium. 

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. 

Many organisms are exposed to some of the thermal, chemical, and physical stresses of entrainment by being mixed at the discharge with the heated water this is plume entrainment. The exact number exposed depends on the percentage of temperature decline at the discharge that is attributed to turbulent mixing rather than to radiative or evaporative cooling to the atmosphere. 

The second Hquefaction process is carried out at temperatures from 261 to 296 K, with Hquefaction pressures of about 1600—2400 kPa (16—24 atm). The compressed gas is precooled to 277 to 300 K, water and entrained oil are separated, and the gas is then dehydrated ia an activated alumina, bauxite, or siHca gel drier, and flows to a refrigerant-cooled condenser (see Drying agents). The Hquid is then distilled ia a stripper column to remove noncombustible impurities. Liquid carbon dioxide is stored and transported at ambient temperature ia cylinders containing up to 22.7 kg. Larger quantities are stored ia refrigerated iasulated tanks maintained at 255 K and 2070 kPa (20 atm), and transported ia iasulated tank tmcks and tank rail cars. 

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