Change-of-phase operations

Sometimes, you will see a change-of-phase operation illustrated in a form that you may associate with chemical reactions, such as the equation for melting ice that you see following this paragraph. Don t be confused by the format, it is still showing a physical change. 

You are probably familiar with most change-of-phase operations. In Lesson 1-2 we discussed the process of sublimation, which you may not have been familiar with. Recall that sublimation is the process of a substance changing from a solid phase directly into a vapor phase. Even if you don t know the term, you have probably seen sublimation taking place. When dry ice, which is frozen carbon dioxide, appears to give off a white smoke, you see evidence of the solid carbon dioxide turning directly into vapor. Carbon dioxide is actually an invisible gas. The white smoke is actually water vapor condensing because of the cold gas coming off the solid block of dry ice. 

In any operation in which a material undergoes a change of phase, provision must be made for the addition or removal of heat to provide For the latent heat of the change of phase plus any other sensible heating or cooling that occurs in the process. Heat may be transferred by any one or a combination of the three modes—conduction, convection, and radiation. The process involving change of phase involves mass transfer simultaneous with heat transfer. 

According to Eqs. (8) and (9), the effective RF fields are scaled unsymmet-rically (in respect to the sideband number n) by the scaling factor Xn, in response to the unsymmetrical excitation profile. When K<0, the nth effective RF field becomes negative, which corresponds to a sign change of the operator Ix from positive to negative, or equivalently, a 180° phase shift is introduced to the nth effective RF field. Consequently, a phase inversion occurs in the transverse magnetization of the nth excitation band. 

The fact that the order parameter vanishes above does not mean that Nature does not have an inkling of things to come well below (or above) T. Such indicators are indeed found in many instances in terms of the behaviour of certain vibrational modes. As early as 1940, Raman and Nedungadi discovered that the a-) transition of quartz was accompanied by a decrease in the frequency of a totally symmetric optic mode as the temperature approached the phase transition temperature from below. Historically, this is the first observation of a soft mode. Operationally, a soft mode is a collective excitation whose frequency decreases anomalously as the transition point is reached. In Fig. 4.4, we show the temperature dependence of the soft-mode frequency. While in a second-order transition the soft-mode frequency goes to zero at T, in a first-order transition the change of phase occurs before the mode frequency is able to go to zero. 

Two feasible methods for removal of as much water as desired from the azeotrope are depicted on Figure 13.27. The dual pressure process takes advantage of the fact that the azeotropic composition is shifted by change of pressure operations at 100 and 760Torr result in the desired concentration of the mixture. In the other method, trichlorethylene serves as an entrainer for the water. A ternary azeotrope is formed that separates into two phases upon condensation. The aqueous layer is rejected, and the solvent layer is recycled to the tower. For economic reasons, some processing beyond that shown will be necessary since the aqueous layer contains some acetonitrile that is worth recovering or may be regarded as a pollutant. 

Catalyst performance has of course been a permanent theme in industry. For example, the catalytic activity of oxo catalysts (in hydroformylation) has improved in the past 50 years by a factor of 10 000 change from diadic and triadic process technology to continuous plant operation, replacement of cobalt by rhodium, tailoring of the ligand sphere (phosphines), change of phase application (from mono- to two-phase processes). At the same time, an improvement of selectivity has been achieved, apart from the ease of product/catalyst separation [132]. A similar development seems to occur in the Monsanto acetic acid process [49]. 

The condition of interest is illustrated in Figure 16.6. Here the open arrows represent effects discussed in Chapter 13 these by themselves would lead to change of phase or migration of the interface through the material. But the solid arrows show a compensating effect, so that the set of processes all operating together keep the interface stationary. We consider the open arrows and then the solid arrows in turn. 

The actual PFD may change with time due to system changes and changes of the operational and environmental conditions. The actual PFD should therefore be assessed at several stages in the operational phase. The... 

There are two types of heat transfer operation sensible heat and change of phase. Sensible heat operations involve heating or cooling of a fluid in which the heat transfer results only in a temperature change of the fluid. Change-of-phase... 

Membrane separation offers important advantages over other separation methods. There is usually no change of phase in membrane separation. Other important advantages are the simple operation, compact equipment, low energy consumption, and high selectivity. 

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

---