Condensers temperature

Separation of low-molecular-weight materials. Low-molecular-weight materials are distilled at high pressure to increase their condensing temperature and to allow, if possible, the use of cooling water or air cooling in the column condenser. Very low... 

A widening of the temperature differences across columns, since light nonkey components cause a decrease in condenser temperature and heavy nonkey components cause an increase in the reboiler temperature. 

This wetness fraction is possibly too high, since high levels of wetness can cause damage to the turbine. To allow a lower wetness fraction of, say, a = 0.15, then the outlet pressure of the turbine must be raised 0.2 bar, corresponding to a condensing temperature of 60°C. However, in so doing, the shaftwork generation decreases to 4.2 MW. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Table 3. Threshold Condensation Temperatures for Substituted />Xylylene Monomers...

The process operates at 1 kPa (10 mbars) and 450 kW of power. When the condenser temperature reaches 580°C, the power is reduced to 350 kW. Cooling water is appHed to the condenser, throughout distillation, by means of sprays. Normally distillation takes 10—12 hours and the end point is signified by an increase in furnace temperature and a decrease in vapor temperature to 500—520°C. At this point the power is turned off and the vacuum pump is shut down. Nitrogen is then bled into the system to prevent oxidation of 2inc. 

Thermodynamic efficiency is hurt by the large ATbetween the temperatures of melting and freezing. In an analogy to distillation, the high a comes at the expense of a big spread in reboiler and condenser temperature. Erom a theoretical standpoint, this penalty is smallest when freezing a high concentration (ca 90%) material. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

Evaporative condensers (Fig. 11-88) are widely used due to lower condensing temperatures than in the air-cooled condensers and also lower than the water-cooled condenser combined with the cooling tower. Water demands are far lower than for water-cooled condensers. The chemical industry uses shell-and-tube condensers widely, although the use of air-cooled condensing equipment and evaporative condensers is on the increase. 

In this analysis F will be constant but it could be described more accurately as a function of parameters influencing heat transfer in the condenser (temperature, pressure, flow rate, fluid thermodynamical, and thermophysical characteristics. . . ). 

FIG. 11-106 Effect of steam pressure on steam demand at 38 C condenser temperature (ASHRAE 1983 Equipment Handbook). 

Chilled-water temperature. As the chilled-water outlet temperature decreases, the ratio of steam/refrigeration effect decreases, thus increasing condensing temperatures and/or increasing the con-densing-water requirements. 

FIG. 13-12 Liq iiid boiling points and vapor condensation temperatures for maximum-boiling azeotrope mixtures of chloroform and acetone at 101.3 kPa (1 atm) total pressure. 

It is advantageous to have the expander generate its refrigeration at the lowest possible temperature in the process (see Fig. 29-45), and this frequently encounters the condensation temperature of the pi ocess stream. Steam-turbine pracrice advises against operating on... 

The real atmosphere is more than a dry mixture of permanent gases. It has other constituents—vapor of both water and organic liquids, and particulate matter held in suspension. Above their temperature of condensation, vapor molecules act just like permanent gas molecules in the air. The predominant vapor in the air is water vapor. Below its condensation temperature, if the air is saturated, water changes from vapor to liquid. We are all familiar with this phenomenon because it appears as fog or mist in the air and as condensed liquid water on windows and other cold surfaces exposed to air. The quantity of water vapor in the air varies greatly from almost complete dryness to supersaturation, i.e., between 0% and 4% by weight. If Table 2-1 is compiled on a wet air basis at a time when the water vapor concentration is 31,200 parts by volume per million parts by volume of wet air (Table 2-2), the concentration of condensable organic vapors is seen to be so low compared to that of water vapor that for all practical purposes the difference between wet air and dry air is its water vapor content. 

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