Tower heat removal

From the viewpoint of the tower heat balance, a low estimate of the exit bottoms temperature will cause the various tower heat removal quantities to be calculated as lower than actual. This applies especially to the cooling requirements for the vacuum residuum. 

Theoretical possible heat removal per pound of air circulated in a cooling tower depends on the temperature and moisture content of air. An indication of the moisture content of the air is its wet-bulb temperature. Ideally, then, the wet-bulb temperature is the lowest theoretical temperature to which the water can be cooled. Practically, the cold-water temperature approaches but does not equal the air wet-bulb temperature in a coohng tower this is so because it is impossible to contact all the water with fresh air as the water drops through the wetted fill surface to the basin. The magnitude of approach to the wet-bulb temperature is dependent on tower design. Important factors are air-to-water contact time, amount of fill surface, and breakup of water into droplets. In actual practice, cooling towers are seldom designed for approaches closer than 2.8°C (5°F). 

When the solute has a large heat of solution or when the feed gas contains high percentages of the solute, one should consider the use of internal coohng coils or intermediate external heat exchangers in a plate-type tower to remove the heat of absorption. In a packed tower, one could consider the use of multiple packed sections with intermediate hquid-withdrawal points so that me hquid coiild be cooled by external heat exchange. 

Equipment Constraints These are the physical constraints for individual pieces of eqiiipment within a unit. Examples of these are flooding and weeping limits in distillation towers, specific pump curves, neat exchanger areas and configurations, and reactor volume limits. Equipment constraints may be imposed when the operation of two pieces of equipment within the unit work together to maintain safety, efficiency, or quahty. An example of this is the temperature constraint imposed on reactors beyond which heat removal is less than heat generation, leading to the potential of a runaway. While this temperature could be interpreted as a process constraint, it is due to the equipment limitations that the temperature is set. 

A temporary extra heat load was placed on an olefin plant cooling tower one winter. The operations people asked the question, Will the cooling tower make it next summer with the extra load Of course the tower would deliver the heat removal, so the real question was, What will the cold water temperature be next summer ... 

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

If the tower is sufficiently tall, the interface temperature can fall below the dry bulb temperature of the air (but not below its wet bulb temperature), and sensible heat will then be transferred from both the air and the water to the interface. The corresponding temperature and humidity profiles are given in Figure 13.18ft. In this part of the tower, therefore, the sensible heat removed from the water will be that transferred as latent heat less the sensible heat transferred from the air. 

Ammonia is to be recovered from a 5 per cent by volume ammonia-air mixture by scrubbing with water in a packed tower. The gas rate is 1.25 m3/m2s measured at 273 K and 101.3 kN/m2 and the liquid rate is 1.95 kg/m2s. The temperature of the inlet gas is 298 K and the temperature of the inlet water 293 K. The mass transfer coefficient is Kaa = 0.113 kmol/m3s (mole ratio difference) and the total pressure is 101.3 kN/m2. What is the required height of the tower to remove 95 per cent of the ammonia. The equilibrium data and the heats of solutions are ... 

There are a few other kinds of industrial plants. An air-lift tower fermenter was developed by ICI for the production of single-cell protein from methanol which employs an external loop for heat removal. Trickle beds for the treatment of waste waters employ packing structures or packing elements of 4-6 cm dia and porosities above 50%. Microbial films are formed on the packing that react with the organic substrate and the air as the liquid flows down and the air up. 

Often, we remove heat from a tower, at an intermediate point, by use of apumparound or circulating reflux. Figure 7.11 is a sketch of such a pumparound. In many towers, the liquid flows in the pumparound section are greater than in the other sections, which are used for fractionation. That is why we are often short of capacity and initiate flooding in the pumparound or heat-removal section of a column. 

We could reduce the amount of diesel product from the tower. That could wash the heavier gas oil out of the diesel. But it would also increase the amount of diesel in the gas oil. Increasing the heat removed in the pumparound would have a similar effect less gas oil in diesel, but more diesel in gas oil. 

If the air entering the tower is saturated, as much as one-third of the heat removed from the water may go into heating the air, while the balance will go into evaporating the water. Thus the water consumption will be only about two-thirds of what would be required if the entire heat load went into evaporation of the water. On the other hand, under unusual conditions at light loads, with a low temperature range and very dry air, evaporation of the water may actually reduce the air dry-bulb temperature so that heat is removed from, rather than added to, the air, and the amount of heat going to evaporate the water actually exceeds the heat load on the tower. 

Asphalt is withdrawn from the bottom of the extractor. Since this asphalt contains a small amount of solvent, it is heated through a furnace and fed to the flash tower to remove most of the solvent. Asphalt is then sent to the asphalt stripper, where the remaining portion of solvent is completely removed. 

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