Reboiler refrigerant vapor

The partially vaporized refrigerant flows into the evaporator. In Fig. 22.1, the evaporator shown is similar to a kettle-type reboiler (see Chap. 5). The process fluid flows through the tube side of the kettle evaporator. The refrigerant liquid level is maintained by the letdown valve. The refrigerant vapor flows from the top of the kettle, to the compressor suction. 

Figure 3.11. Vaporizers (reboilers), (a) Vaporizer with flow-rate of HTM controlled by temperature of the PF vapor. HTM may be liquid or vapor to start, (b) Thermosiphon reboiler. A constant rate of heat input is assured by flow control of the HTM which may be either liquid or vapor to start, (c) Cascade control of vaporizer. The flow control on the HTM supply responds rapidly to changes in the heat supply system. The more sluggish TC on the PF vapor resets the FC if need be to maintain temperature, (d) Vaporization of refrigerant and cooling of process fluid. Flow rate of the PF is the primary control. The flow rate of refrigerant vapor is controlled by the level in the drum to ensure constant condensation when the incoming PF is in vapor form.

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

Reboiler Film-transfer Rates. Heat is transferred by a boiling film in such equipment as reboilers, refrigerator evaporators, and waste-heat boilers. As the rate of heat transfer is increased, vapor tends to blanket the surface and the rate is sharply retarded. This usually occurs at a temperature difference of 120 to 150 F, and the mimmum heat density or heat that can be transferred per square foot-hour is about as indicated in Table 17-5. 

The kettle unit used in the reboiling service usually has an internal weir to maintain a fixed liquid level and tube coverage. The bottoms draw-off is from the weir section. The reboiling handled in horizontal thermosiphon units omits the disengaging space because the liquid-vapor mixture should enter the distillation tower where disengaging takes place. The chiller often keeps the kettle design but does not use the weir because no liquid bottoms draw off when a refrigerant is vaporized. 

If the feed is partially vaporized, the vapor flow below the feed will be lower than the top of the column. For above ambient temperature separations, the cost of operating the distillation will be dominated by the heat load in the reboiler and the vapor flow in the bottom of the column. For below ambient temperature separations, the cost of operating the column will be dominated by the cost of operating the refrigerated condenser and hence the vapor flow in the top of the column. If constant molar overflow is assumed, the vapor flow in the bottom of the column V is related to the vapor flow in the top of the column by... 

Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280psig and —75°F. Freon refrigerant at — 90°F is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification. 

The most widely used is ammonia absorption in water. A flowsketch of the process is in Figure 8.27. Liquid ammonia at a high pressure is obtained overhead in a stripper, and then is expanded through a valve and becomes the low temperature vapor-liquid mixture that functions as the refrigerant. The low pressure vapor is absorbed in weak liquor from the bottom of the stripper. Energy input to the refrigeration system is primarily that of the steam to the stripper reboiler and a minor amount of power to the pump and the cooling water circulation. 

Most of the water at the bottom of IB, now enriched to 0.117 a/o deuterium, is converted to vapor in the reboiler and returned to IB, but around 12 percent is pumped ahead to the top of 2A. Vapor from the top of 2A is condensed in a condenser refrigerated with ammonia, to prevent loss of the now valuable water. This condenser is also vented to a steam qector, which maintains a pressure of 130 Ton. 

Conventional, (Fig. 17.8) Heat Pump with External Refrigerant, (Hg. 17.11) Heat Pump with Compression of Overhead Vapor, (Fig. 17.12) Heat Pump with Reboiler Liquid Rashing, (Fig. 17.13)... 

Figure 17.10. Use of heat pumps in distillation, (a) Heat pump with external refrigerant, (h) Heat pump with compression of overhead vapor, (c) Heat pump with reboiler liquid flashing. [H. R. Null, Chem. Eng, Progr., 72 (7), 58-64 (1976).]...

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