Liquid subcooling experiments

Actually, the PV behavior predicted in this region by proper cubic equations of state is not wholly fictitious. When the pressure is decreased on saturated liquid devoid of vapor-nucleation sites in a carefully controlled experiment, vaporization does not occur, and the liquid phase persists alone to pressures well below its vapor pressure. Similarly, raising the pressure on a saturated vapor in a suitable experiment does not cause condensation, and the vapor persists alone to pressures well above the vapor pressure. These nonequilibrium or metastable states of superheated liquid and subcooled vapor are approximated by those portions of the PV isotherm which lie in the two-phase region adjacent to the saturated-liquid and saturated-vapor states. 

A distillation process has been set up to separate an ethylene-ethane mixture as shown in Fig. P4.94. The product stream will consist of 98% ethylene and it is desired to recover 97% of the ethylene in the feed. The feed, 1000 lb per hour of 35% ethylene, enters the preheater as a subcooled liquid (temperature = — 100 F, pressure = 250 psia). The feed experiences a 20 F temperature rise before it enters the still. The heat capacity of liquid ethane may be considered to be constant and equal to 0.65 Btu/(lb)(°F) and the heat capacity of ethylene may be considered to be constant and equal to 0.55 Btu/(lb)("F). Heat capacities and saturation temperatures of mixtures may be determined on a weight fraction basis. An optimum reflux ratio of 6.1 lb reflux/lb product has been previously determined and will be used. Operating pressure in the still will be 250 psia. Additional data are as follows ... 

The need for maintaining subcooled liquid in the supply tank is based upon the need for net positive suction head (NPSH), which is the total head of fluid in excess of vapor pressure, at the pump inlet. Years of experience in designing and testing pumps for fluids other than hydrogen have demonstrated the need for NPSH at the pump inlet to ensure proper operation. However, test data for hydrogen suggest that unusual suction performance can be expected. This expectation is especially valid if the available thermodynamic head at the pump inlet is considered. In this case, thermodynamic head is defined as the reduction in enthalpy of the fluid when expanded isentropically. This process involves vaporization, i.e., cavitation of the fluid. 

Fig. 8.7 Experimental apparatus at Kyushu University for a condensation experiment with high temperature vapor in a subcooled liquid pool (takcm from ref. [38] and used with permission from Atomic Energy Society of Japan)...

The pressure suppression type containment like that of BWRs is a candidate for the containment of the Super LWR and Super FR. The steam temperature and pressure discharged into the suppression pool through the vent pipes or safety relief valve (SRV) pipes will be higher than those in BWRs. Especially from the SRV pipes, superheated steam or supercritical steam may be discharged. The characteristics of its condensation in the subcooled water at low pressure (containment pressure) need to be well understood. Kyushu University researchers carried out a unique experiment to observe condensation behavior with a high speed camera and to measure pressure oscillation of vapor discharge into a subcooled liquid pool [38]. HCFC123 was used in this experiment and the experimental apparatus is shown in Fig. 8.7 [38]. 

---