Compression of a gas

It suffices to carry out one such experiment, such as the expansion or compression of a gas, to establish that there are states inaccessible by adiabatic reversible paths, indeed even by any adiabatic irreversible path. For example, if one takes one mole of N2 gas in a volume of 24 litres at a pressure of 1.00 atm (i.e. at 25 °C), there is no combination of adiabatic reversible paths that can bring the system to a final state with the same volume and a different temperature. A higher temperature (on the ideal-gas scale Oj ) can be reached by an adiabatic irreversible path, e.g. by doing electrical work on the system, but a state with the same volume and a lower temperature Oj is inaccessible by any adiabatic path. 

Virial equation represents the experimental compressibility of a gas by an empirical equation of state ... 

The flow of air through the compressor may be regarded as the compression of a gas with properties (Cpa)i2 and (ya)i2 (the double subscript indicates that a mean is taken over the relevant temperature range). The work required to compress the unit mass of air in the compressor is then represented as... 

Space-Filling Models. For most of this century, chemists have tried to answer the size question by using a special set of molecular models known as space-filling or CPK models. The space-filling model of an atom is simply a sphere of fixed radius. A different radius is used for each element, and the radii are chosen to reproduce certain experimental observations, such as the compressibility of a gas, or the spacing between atoms in a crystal. 

Thus, from an investigation of the compressibility of a gas we can deduce the values of its critical constants. We observe that, according to van der Waals theory, liquid and gas are really two distant states on the same isotherm, and having therefore the same characteristic equation. Another theory supposes that each state has its own characteristic equation, with definite constants, which however vary with the temperature, so that both equations continuously coalesce at the critical point. The correlation of the liquid and gaseous states effected by van der Waals theory is, however, rightly regarded as one of the greatest achievements of molecular theory. 

For an irreversible process it may not be possible to express the relation between pressure and volume as a continuous mathematical function though, by choosing a suitable value for the constant k, an equation of the form Pv = constant may be used over a limited range of conditions. Equation 2.73 may then be used for the evaluation of / 2 v dP. It may be noted that, for an irreversible process, k will have different values for compression and expansion under otherwise similar conditions. Thus, for the irreversible adiabatic compression of a gas, k will be greater than y, and for the corresponding expansion k will be less than y. This means that more energy has to be put into an irreversible compression than will be received back when the gas expands to its original condition. 

Compressibility of a gas flowing in a pipe can have significant effect on the relation between flowrate and the pressures at the two ends. Changes in fluid density can arise as a result of changes in either temperature or pressure, or in both, and the flow will be affected by the rate of heat transfer between the pipe and the surroundings. Two limiting cases of particular interest are for isothermal and adiabatic conditions. 

Equation 6.30 shows that, in general, expansion or compression of a gas is accompanied by a change of temperature. 

Protein biomolecules consisting of polypeptide chain with large molecular mass Protostar early stage in the formation of a star when gases and dust start to contract due to gravitational forces P-V Work work associated with the expansion or compression of a gas Pyrethroids synthetic forms of pyrethrins, insecticides based on extracts from chrysanthemums... 

Fig. 6-2. Typical shape of the coefficient of isothermal compressibility of a gas as a function of pressure at constant reservoir temperature.

We recognize that the equation of state for an ideal gas does not describe adequately tire behavior of gases at temperatures and pressures normally encountered in petroleum reservoirs. However, Equation 6-6 does illustrate that we can expect the coefficient of isothermal compressibility of a gas to be inversely proportional to pressure. Equation 6-6 can be used to determine the expected order of magnitude of gas compressibility. 

At pressures above the bubble point, the coefficient of isothermal compressibility of oil is defined exactly as the coefficient of isothermal compressibility of a gas. At pressures below the bubble point an additional term must be added to the definition to account for the volume of gas which evolves. 

Work in polytropic compression of a gas with equation of state PV = zRT is entirely analogous to Eq. (7.26). The hydrodynamic work or the work absorbed by the gas during the compression is... 

COMPRESSION (Gas). The compressibility of a gas is delined as Ihe rate of volume decrease with increasing pressure, per unit volume of the gas. The compressibility depends not only on the stale of the gas. but also on the conditions under which the compression is uchicved. Thus, if the temperature is kepi constant during compression, the compressibility so defined is called Ihe isothermal compressihilily ft ... 

In order to make magnetic refrigeration practical one must use a continuous cyclic process in which heat must be rejected when the ferromagnet experiences a magnetic field increase (just as the heat is rejected during the compression of a gas in a conventional gas cycle... 

Compression of a gas generates heat. The compression of bubbles during cavitation is more rapid than thermal transport, which generates a shortlived, localized hot-spot. 

Thus, compression of a gas by a shock wave proves to be a necessary, possible, and indeed the only means of igniting a gas and causing a chemical reaction in a propagating detonation wave. 

These methods provide an accurate means of investigating translation-vibration and translation-rotation transfer. The passage of a sound wave through a gas involves rapidly alternating adiabatic compression and rarefaction. The adiabatic compressibility of a gas is a function of y, the ratio of the specific heats, and the classical expression for the velocity, V, of sound in a perfect gas is... 

In a compression process, the isentropic work, as given by Eq. (7.27), is the minimum shaft work required for compression of a gas from a given initial state to a given discharge pressure. Thus we define a compressor efficiency as... 

Our discussion has centered on a single nonflow process, the expansion of a gas in a cylinder. The opposite process, compression of a gas in a cylinder, is described in exactly the same way. There are, however, many processes which are driven by other-than-mechanical forces. For example, heat flow occurs when a temperature difference exists, electricity flows under the influence of an electromotive force, and chemical reactions occur because a chemical potential exists. In general, a process is reversible when the net force driving it is only differential in size. Thus heat is transferred reversibly when it flows from a finite object at temperature T to another such object at temperature T - dT. 

From the first law of thermodynamics, the internal work W needed for the compression of a gas can be calculated from the enthalpies and before and after the... 

First and second laws of therniodynaniics applied to compression of a gas ... 

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