THERMODYNAMICS AND HYSYS

Thermodynamics and HYSYS



Thermodynamics and HYSYS

The word cryogenics means production of icy cold, but today it is synonym for low temperatures. On temperature scale there is no clear distinction where ordinary refrigeration range ends and cryogenics begins. However the National Bureau of Standards at Boulder, Colorado consider temperature below 123K as cryogenic operating temperature. This can be accepted because boiling temperature of permanent gases like helium (He), hydrogen (H), neon (Ne) etc are below 123K.

Cryogenic engineering deals with development and improvement of low temperature techniques, processes and equipment. It deals with utilization of low temperature phenomena. In general cryogenic system refers to interacting group of components involving low temperature. Examples are: Air liquefaction plant, helium refrigerators etc.

There are many textbook available regarding cryogenic systems. I have referred to cryogenic systems by Barrons (2).

Principle of Liquefaction:

Liquefaction of gases is always accomplished by refrigerating the gas to some temperature below its critical temperature so that liquid can be formed at some suitable pressure below the critical pressure. Thus gas liquefaction is a special case of gas refrigeration and cannot be separated from it. In both cases, the gas is first compressed to an elevated pressure in an ambient temperature compressor. This high-pressure gas is passed through a countercurrent recuperative heat exchanger to a throttling valve or expansion engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may be formed. The cool, low-pressure gas returns to the compressor inlet to repeat the cycle. The purpose of the countercurrent heat exchanger is to warm the low-pressure gas prior to recompression and simultaneously to cool the high-pressure gas to the lowest temperature possible prior to expansion. Both refrigerators and liquefiers operate on this basic principle. In a continuous refrigeration process, there is no accumulation of refrigerant in any part of the system. This contrasts with a gas liquefying system, where liquid accumulates and is withdrawn. Thus, in a liquefying system, the total mass of gas that is warmed in the countercurrent heat exchanger is less than that of the gas to be cooled by the amount liquefied, creating an imbalance mass flow in the heat exchanger. In a refrigerator the warm and cool gas flows are equal in the heat exchanger. This results in what is usually referred to as a "balanced flow condition" in a refrigerator heat exchanger. The thermodynamic principles of refrigeration and liquefaction are identical. However the analysis and design of the two systems are quite different because of the condition of balanced flow in the refrigerator and unbalanced flow in liquefier systems.

The Joule-Thomson coefficient is a property of each specific gas. It is a function of temperature and pressure, and may he positive, negative, or zero. For instance, hydrogen, helium, and neon have negative J-T coefficients at ambient temperature. Consequently, to be used as refrigerants in a throttling process they must first be cooled either by a separate pre coolant liquid. Only then will throttling cause a further cooling rather than a heating of these ...