Countercurrent direct condensation

Fig. 2-89. Chemical reactor with countercurrent direct condensation column.

CC Countercurrent direct condensation column R Chemical reactor C Condenser CF Cold raw material feed PF Preheated raw material V Vapor, generated by chemical reaction and/ or by evaporative cooling RC Remaining condensate... 

In a distillation column, vapor and liquid flow in countercurrent directions to each other. Liquid is vaporized at the bottom, and vapor is condensed from the top product and withdrawn from the column. A number of trays are placed in the column, or the column is packed with open material, so that the vapor phase contacts the liquid phase, and components are transferred from one phase to the other. As you proceed up the column the temperature decreases, and the net effect is an increase in the more volatile component(s) in the vapor and a decrease in the less volatile components in the liquid. Vapor is withdrawn from the top of the column and liquid from the bottom. Feed to the column usually enters part way up the column. 

Dehumidification of air can be effected by bringing it into contact with a cold surface, either liquid or solid. If the temperature of the surface is lower than the dew point of the gas, condensation takes place and the temperature of the gas falls. The temperature of the surface tends to rise because of the transfer of latent and sensible heat from the air. It would be expected that the air would cool at constant humidity until the dew point was reached, and that subsequent cooling would be accompanied by condensation. It is found, in practice, that this occurs only when the air is well mixed. Normally the temperature and humidity are reduced simultaneously throughout the whole of the process. The air in contact with the surface is cooled below its dew point, and condensation of vapour therefore occurs before the more distant air has time to cool. Where the gas stream is cooled by cold water, countercurrent flow should be employed because the temperature of the water and air are changing in opposite directions. 

Under a countercurrent of nitrogen, a flask containing FeCl2n (2 g, 15.8 mmole) is substituted for the flask at B. The system is evacuated and after the flask is cooled at position B to -78°, 40 mL of a tetrahydrofuran solution of P(CH3)3 (2.9 g, 38.2 mmole) is condensed into it. [This solution is made by condensing 3.8 mL of P(CH3)3 from a flask, fitted with a stopcock, through a PVC tube into the tetrahydrofuran-containing flask, which is attached directly... 

The material exits the predesolventizing trays of the DT and falls onto the top countercurrent tray. This is perhaps the most critical tray of the DT. As most of the heat is transferred into the meal layer by condensation of direct steam, a deep layer of 700-to 1000-mm meal depth is held above the tray. The solvent-laden meal is stirred above the top countercurrent tray by the rotating sweeps. The direct steam passes from below up through apertures in the countercurrent tray. As the direct steam penetrates the upper meal layer, it reaches the solvent-laden meal and condenses, providing direct latent heat to evaporate solvent. The solvent evaporates and exits the meal layer as vapor. The condensation of steam causes the meal exiting the tray to be wet, typically in the range of 17-21% moisture. After the majority of the solvent evaporates, the meal temperature increases by direct and indirect steam... 

Simple extraction is often a rather inefficient process and may be modified to magnify the effect of one extraction. This may be done in either of two ways by using continuous extraction, in which the solvent is recycled through the solution by the use of a distillation-condensation cycle or by countercurrent extraction, in which the original solution and the extracting solvent are permitted to flow in opposite directions through a tube designed to promote contact between the phases. These methods will be described in turn. 

Consider the condensation of a vapor mixture inside the vertical tube as shown in Figure 15.1. Vapor enters the top of the tube and flows downwards. The condensate flows cocurrently with the remaining vapor down the tube. The coolant may flow cocurrently with or countercurrently to the vapor and liquid streams. Condensation on a horizontal tube was illustrated in Figure 15.2. In this case the condensed liquid drips off the tube and eventually collects at the bottom of the heat exchanger. The vapor flow is directed along or, more likely, across bundle of tubes. The design equations are developed in the same way for both types of condenser. 

Billet [176 c] shows that deviations from optimum column lay-out may lead to a drastic increase in cost. The connexion of main and side columns and the use of partial condensation in direct and countercurrent operation, where appropriate are also discussed. These principles should be considered in laboratory and semitechnical work. Comparative studies of the applicability of heuristic rules and approximate methods in the selection of appropriate column connexions for nmlticomponenl countercurrent distillation were made by Jobat et al. [176d]. 

Typical dryer exhaust emissions are sulfur compounds such as hydrogen sulfide, carbon disulfide, carbonyl sulfide, and methyl and w-propyl mercaptans. In addition to ammonia, the only amine present is trimethyl amine. Since the emissions from the dryers contain considerable moisture at temperature of about 95°C, necessary means should be provided to remove most of this moisture and to cool the air before further odor treatment. Also, there may be dust particles in the cyclone exhaust that should be removed before effective odor measures can be applied. This is normally accomplished by either direct or indirect contact (e.g., shell and tube) water-cooled condensers. The direct-contact type includes cocurrent flow venturi scrubbers and countercurrent... 

The solution to Part (a) is shown in Figure E17.7faT The horizontal line at 250°C is for the steam condensing at constant temperature. The sloped line is for heating of the process stream. The arrow on the sloped line indicates the direction of flow of the process stream, because it is being heated. The arrow on the condensing steam line is opposite because of countercurrent flow. 

Vertical bundle with coolant on shell side. This design comes in two variations reflux design, where the vapor goes up the tubes and is countercurrent to the condensate falling down, and a design with a chimney in the center such that the vapor rises in it, reverses direction, and comes down the tubes with the condensate. 

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