Compression processes, thermodynamics

To evaluate the integral in Equation B.l requires the pressure to be known at each point along the compression path. In principle, compression could be carried out either at constant temperature or adiabatically. Most compression processes are carried out close to adiabatic conditions. Adiabatic compression of an ideal gas along a thermodynamically reversible (isentropic) path can be expressed as ... 

These assumptions have been expanded upon (Shah and Capps, 1968 Lucassen-Reynders, 1973 Rakshit and Zografi, 1980), especially in regard to the application of the ideal mixing relationship in gaseous films (Pagano and Gershfeld, 1972). It has been pointed out that water may contribute to the energetics of film compression if the molecular structures of the surfactants are sufficiently different (Lucassen-Reynders, 1973). It must be noted that this treatment assumes that the compression process is reversible and the monolayer is truly stable thermodynamically. It must therefore be applied with considerable reservation in view of the hysteresis that is often found for II j A isotherms. 

Energy is needed to eompress gases. The compression work depends on the thermodynamic compression process. The ideal isothermal compression cannot be realized. Even more energy is needed to compact hydrogen by liquefaction. Low density and extremely low boiling point of hydrogen increases the energy cost of compression or hquefaction. 

Example 163 What is the thermodynamic efficiency of the compression process of Example 7.8 if T0 = 300 K ... 

FIG. 4-3 Adiabatic compression process. [Smith, Van Ness, and Abbott, Introduction to Chemical Engineering Thermodynamics, 7th ed., p. 274, McGraw-Hill, New York (2005).]... 

Real compression processes operate between adiabatic and isothermal compression. Actual compression processes are polytropic processes. This is because the gas being compressed is not at constant entropy as in the adiabatic process, or at constant temperature as in the isothermal processes. Generally, compressors have performance characteristics that are analogous to those of pumps. Their performance curves relate flow capacity to head. The head developed by a fluid between states 1 and 2 can be derived from the general thermodynamic equation. 

Work of Compression. Section 10.4.3.1 discusses the thermodynamics of the compression process and derived the energy consumed in certain limiting cases. When operation is isothermal, for example. 

Adiabatic process Thermodynamic process in which there is no exchange of heat or mass between a metaphorical parcel of air and its surroundings thus, responding to the decrease in atmospheric density with height, rising air cools adiabatically due to expansion and sinking air warms due to compression. 

Consider a Claude liquefier with parahydrogen as the working fluid. The compressor operates isothermally at 295 K from 0.101 to 3.03 MPa. The expander flow-rate ratio is 0.7 and the expander work is utilized in the compression process. The condition of the gas at the inlet to the expander is 279 K and 3.03 MPa. What is the quantitative effect on the liquid yield, work per unit mass of parahydrogen compressed, and the work per unit mass of parahydrogen liquefied if the thermodynamic efficiency of the expander is 70% rather than 100% Assume that the compressor has a thermodynamic efficiency of 70% for both situations and the mechanical efficiencies of both compressor and expander can be neglected. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

Surely, it is now time to reformulate the questions considered to be fundamental to shock-compression science. The questions must consider shock-compressed matter as it exists as a highly defective solid, heterogeneous in character, with significant anisotropic components and heterogeneous processes that are not in thermodynamic equilibrium. 

The thermodynamic processes that may occur during a compression operation are ... 

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