Subcooled condensate, cooling

The vapor is drawn into a steam jet (discussed in Chap. 16). The steam condensate flows into the boot or hot well. The water in the boot is slightly subcooled. This is accomplished by a pair of baffles that create a small zone of condensate backup. The subcooled condensate, cooled to perhaps 10°F below its boiling or bubbling point, is easier to pump. As the pressure in the hot well is subatmospheric, the hot-well pump typically develops a AP of at least 30 to 50 psi. 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

Coolant flow is generally not throttled for pressure control, but it is occasionally adjusted for temperature control of the subcooled condensate. Unless there is significant sub-cooling, this latter temperature loop is often ineffectual. At best, it will require loose tuning, or often it will be placed into manual for seasonal adjustment only. 

The condenser is the stage where overhead vapors are condensed and liquid is returned to the top of the column as reflux. The condenser is partial if only part of the vapor is condensed and refluxed and the remainder leaves the condenser as vapor distillate. This type of condenser adds one equilibrium stage to the column trays because it holds a vapor phase and a liquid phase at equilibrium with each other. A total condenser is one where the entire overhead vapor is condensed (cooled to the bubble point temperature or subcooled to a lower temperature), part of the condensate is returned as reflux, and the remaining part is taken as liquid distillate. This type of condenser does not count as an equilibrium stage because no vapor-liquid separation takes place in it. The liquid distillate composition is identical to the composition of the vapor leaving the column top tray. 

For energy exchange equipment Supply sufficient excess of heat transfer area in reboilers, condensers, cooling jackets, and heat removal systems for reactors to be able to handle the anticipated upsets and dynamic changes. Sometimes extra area is needed in overhead condensers to subcool the condensate to prevent flashing in the downstream control valves. Too frequently, overzealous engineers size the optimum heat exchangers based on an economic minimum based on steady-state conditions and produce uncontrollable systems. 

If water is used as the cooling medium in the condenser and the condensate is not subcooled, the cooling-water requirement is... 

Another, completely different approach is to run the column at a slight pressure, say 3—5 psig, or under vacuum. Then the condenser cooling water may be manipulated by the pressure controller while subcooling is controlled by manipulating the vent (see Figure 3.10). This is discussed more fully in the next section. There is, however, a limitation to this technique for protection... 

The cooling water (or other medium) must absorb enough heat to balanee the heat of vaporization and eondensate subcooling. Piping and hot wells must be sized based upon the maximum condenser requirement. The following example illustrates the method of calculating the quantity of eooling water for a specific service. 

Note that the method described assumes that the high-pressure condensate has not been sub-cooled. If any subcooling has taken place, then the figure taken for the enthalpy of water at the higher pressure is reduced by the amount of sub-cooling. The chart or table can still be used if the upstream pressure is taken as that corresponding to saturated steam at the same temperature as the sub-cooled condensate. 

A condenser is required to condense a flowrate of 7 kg-s-1 of isopropanol. The condensation takes place isothermally at 83°C without subcooling of the condensate. The cooling is provided by cooling water between 25 and 35°C. The condenser can be assumed to be steel with 20 mm tubes... 

Meanwhile, the tubes covered with stagnant water would begin to cool. The steam condensate itself around these tubes would cool. This cooled water would be colder than the saturation temperature of the condensing steam. The tubes would then be said to be submerged in subcooled water. 

Mechanics of subcooling. As the condensed steam flow out of the radiator is restricted, the surface area of the radiator, available to cool the hot water, increases. Hence, the water temperature leaving the radiator decreases. To summarize, the effect of restricting the condensate flow from a radiator or condenser is to... 

The reason is condensate backup. The condensate backup causes subcooling that is, the liquid is cooled below its bubble point, or saturated liquid temperature. Perhaps a rat has lodged in the condensate outlet pipe. The rat restricts condensate drainage from the shell side. To force its way past the dead rat, the propane backs up in the condenser. The cold tubes in the bottom of the shell are submerged in liquid propane. The liquid propane is cooled below its bubble-point temperature. 

In the aforementioned process, the heat for the reboiler is usually available as waste heat from the steam cracker, for example, and is essentially cost-free. If this heat is not available, a heat pump can be used. The heat pump can upgrade the heat, at an exergetic cost, to the desired temperature level. If the separation is viewed in isolation, this means that the heat rejected by the condenser at relatively low temperature, can be upgraded to be the higher temperature heat input for the reboiler. A schematic of the heat pump process is given in Figure 10.2. The overhead vapors are heated slightly in the reflux subcooler, which enables these vapors to be compressed and cooled in the condenser-reboiler. 

The reactor in Problem 13.14 is to be cooled by autorefrigeration. Determine the boilup rate in the reactor assuming that the condensate is returned to the reactor without subcooling. 

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

Vapor can condense on a cooled surface in two ways. Attention has mainly been given in this chapter to one of these modes of condensation, i.e.. to him condensation. The classical Nusselt-type analysis for film condensation with laminar film flow has been presented hnd the extension of this analysis to account for effects such as subcooling in the film and vapor shear stress at the outer edge of the film has been discussed. The conditions under which the flow in the film becomes turbulent have also been discussed. More advanced analysis of laminar film condensation based on the use of the boundary layer-type equations have been reviewed. 

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