Reboiler sensible heating

Rgure 17.2 Reboiler sensible heating medium control, (a) Reboiler bjrpass, poor control (b) reboiler bypass, good control (c) reboiler bypass with PDC control (d) direct flow control (e) Btu control. 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. 

The large fraction of tube length used for sensible heating in vacuum reboilers leaves little density difference for thermal circulation. This fact, plus the frequent... 

Inputs reboiler heat input Qb + feed sensible heat HF. 

The condensing temperature of the steam is 300°F. The process into which the heat is transferred is at a constant temperature of 20O°F. The overall heat transfer coefficient is 300 Btu/h °F ft. The reboiler has S09 tubes that arc 10 feet long and 1 inch inside diameter. The steam and condensate are inside the tubes. The density of the condensate is 62,4 Ib ft and the latent heat of condensation of the steam is 900 Btu/lb . Neglect any sensible heat transfer. 

This calculation is typical, in that 94% of the heat is liberated at the 320°F condensing temperature of the saturated steam. Another way of stating the same idea is that a steam reboiler depends on latent-heat transfer, and not on sensible-heat transfer. 

When a vapor condenses to a liquid, we say that the latent heat of condensation of the vapor is liberated. In a steam reboiler, this liberated heat is used to reboil the distillation tower. When a vapor, or more commonly a liquid, cools, we say that its sensible heat is reduced. For a small or slight temperature change, the change in latent heat might be large, while the change in sensible heat will be very small. 

There is a condition on this rule of thirds—it is to be limited to areas of latent heat and not to the tube area dedicated to sensible heat. Most reboilers of this type comply to the requirement for very little sensible heat because the large flywheel of circulating liquid is near or exactly equal to the fractionator column bottoms temperature. Therefore, only latent heat applies, and this rule of thirds also applies. 

For vertical tubes, the superficial vapor velocity (based on the total heat-transfer surface) can be obtained by multiplying the value calculated from the preceding equation by 0.22. This assumes that there is adequate liquid circulating past the surface to satisfy the mass balance. For thermosiphon reboilers, a detailed analysis must be made to establish circulation rate, boiling pressure, sensible heat-transfer zone, boiling heat-transfer zone, and mean temperature difference. If hquid circulation rates are not adequate, ah hquid will be vaporized and superheating of the vapor wih occur with a resultant decrease in heat-transfer rates. 

SubcooUng heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturation temperature in the mean temperature difference. For horizontal and vertical thermosiphons, a separate calculation is required for the sensible heat transfer area, using appropriate sensible heat transfer coefficients and the liquid temperature- profile for the mean temperature difference. 

HEATING AND COOLING REQUIREMENTS. Heat loss from a large insulated column is relatively small, and the column itself is essentially adiabatic. The heat effects of the entire unit are confined to the condenser and the reboiler. If the average molal latent heat is X and the total sensible heat change in the liquid streams is small, the heat added in the reboiler is VX, either in watts or Btu per hour. When the feed is liquid at the bubble point (q 1), the heat supplied in the reboiler is approximately equal to that removed in the condenser, but for other values of q this is not true. (See page 554.)... 

A two-stage evaporator is used to concentrate a brine solution of NaCl in water. Assume that the NaCl is completely nonvolatile. Steam is fed into the reboiler in stage 1 at a rate Vq (kg/hr). If we neglect sensible heat effects, this amount of steam will produce about the same amount of vapor in stage I V, kg/hr), and condensing V in a reboiler in stage 2 will produce about the same amount of vapor in stage 2 Vi, kg/hr). Thus, for a simplified model we will assume that 0=121= 2= V. Since NaCl is nonvolatile, these vapors are pure water. 

When the liquid arrives at the reboiler base, it is usually subcooled because of the effect of static pressure and heat losses from the line. When the liquid enters the tubes, heat is applied to the liquid. Initially, the subcooled liquid is heated to its boiling point by sensible heat transfer only. After the boiling point is reached, vaporization begins and two-phase flow regimes are established. 

Forced-circulation reboilers are similar to vertical thermosiphon reboilers, but do not depend on the natural thermosiphon action, and commonly operate at high circulation rates. The pump replaces the level in the reboiler sump as the driving force that sets rehoiler circulation. Heat transfer is mainly by sensible heat, often followed hy nucleate boiling. Additional boiling occurs when the heated mixture is flashed at the reboiler outlet. Forced-circulation reboilers may be installed horizontally or vertically horizontal units are often easier to clean, but consume more plot space. 

The above considerations generally apply regardless of the control scheme used. A discussion of the common control schemes used for reboiling with sensible heat follows. 

Sensible-heat reboilers are most commonly controlled by the b rpass scheme in Fig. 17.2a. This scheme is inexpensive and can almost always be implemented, but it suffers from sluggishness. An increase in reboiler duty closes the bypass valve, which initially raises the reboiler flow. This is accompanied by a rise in reboiler pressure drop, which routes some of the added flow back through the bypass. Eventually, the system will reach equilibrium, but only after some back-and-fbrth flow fluctuations. This flow sluggishness may compound the temperature sluggishness described earlier. 

A common problem with sensible heat reboilers is variation in heating medium inlet temperature. A heat input (Btu) controller (Fig. 7.2e) can alleviate this problem. Heat input control can be implemented either with a direct (Fig. 7.2e) or a bypass control system. The Btu controller compensates for changes in heating medium temperature. Heat input controllers are psuticularly useful in services where the temperature difference across the exchanger is small (AT effects dominate), and small variations in heating medium temperature significantly affect boilup. 

Additional separation can be obtained by operating without a liquid water phase in the column. Reducing the number of phases increases the degrees of freedom by one. Operation must be at a temperature higher than that predicted by Eq. f8-15L or a liquid water layer will form in the column. Thus, the column must be heated with a conventional reboiler and/or the sensible heat available in superheated steam. The latent heat available in the steam cannot be used, because it would produce a layer of liquid water. Operation without liquid water in the column reduces the energy requirements but makes the system more complex. 

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