Thermodynamics: Saturation Pressure

Saturation pressure and temperature in a physicochemical process are essential active factors in the thermodynamic system to identify deviations and aid in the calculation of liquid holdups, levels, and flow rate in the chemical reactor. The first indicator that is essential to process plant operation is pressure, which gives information about the range of phase flow from one place to another. Each pipe flow has a pressure gauge or transducer placed to properly calibrate and navigate pressure ranges. Manometers, piezoresistive, piezoelectric, piezocapacitive, and Bourdon pressure gauges are among the several types of pressure gauges. Installing precise pressure gauges with a particular unit is necessary to determine the saturation pressure measurement. 

This graph is a phase diagram that illustrates the relationship between a substance's temperature (in Kelvin) and pressure (on a logarithmic scale, in MPa), as well as its distinct states of matter (solid, liquid, and vapour). The black curves represent phase boundaries, where two phases exist in equilibrium. The liquid–vapour equilibrium curve, which terminates at the critical point (represented as a filled circle at high temperature and pressure), is represented by the curved line extending to the right. The solid and vapour areas are separated by the sublimation curve on the left, while the solid and liquid areas are separated by the melting curve on the right. The triple point, a critical location where the solid, liquid, and vapour phases are in equilibrium, is denoted by the grey circle. The relative quantities of each phase in an entirely liquid system are illustrated in the inset bar graph. This graphic elucidates the manner in which the state of a substance is influenced by temperature and pressure.

The temperature-entropy (T-s) diagram for carbon dioxide is a thermodynamic property chart that is often used in the study of refrigeration systems and heat pump cycles. The temperature (in °C) is shown on the vertical axis (ordinate) of this diagram, while the entropy (measured in kJ·kg⁻¹·K⁻¹) is shown on the horizontal axis (abscissa). The saturated zone is defined as a specific region/point that is bordered on the right by the saturated vapour line and on the left by the saturated liquid line. This region includes the two-phase mixing region, where the refrigerant exists in a vapour-liquid equilibrium. Within this region, the vapour percentage is indicated by the constant-quality lines (dashed curves). The performance assessment and energy balance of the cycle rely on both isobars (constant-vaporisation lines) and isenthalpies (constant-enthalpy lines), which are illustrated by the blue and green curves, respectively. 

At (~31 °C), where critically lower, which denotes the apex of the saturation region, is a crucial observation for CO₂. After this, the refrigerant converts into the supercritical region, where heat rejection occurs at supercritical pressures instead of through typical condensation, and phase change does not take place. Diagram differs from traditional refrigerants due to its thermophysical behaviour, which requires the usage of trans-critical cycles in CO₂-based refrigeration and heat pump systems. So that, engineers may accurately evaluate and optimise CO₂ thermodynamic cycles by using the T–s diagram to illustrate compression (an almost isentropic process), heat rejection, throttling (isenthalpic expansion), and heat absorption. The Temperature–Entropy (T–s) graph demonstrates that thermodynamic processes of a CO₂-based vapour compression refrigeration cycle are going quite well. The cycle starts with isentropic compression of low-pressure vapour (state 1 → 2). During this process, the temperature and pressure rise quickly and move up the T–s plane, while the refrigerant entropy stays virtually the same. Because of its lower critical existence temperature (~31°C), the supercritical region is where the high-pressure vapour often occurs after entering the gas cooler/heat rejection process (states 2–3). As heat is rejected to the environment at nearly constant pressure during this operation, entropy falls, moving the state to the left on the diagram. 

The refrigerant is then processed through the isenthalpic throttling/expansion (state 3 → 4), which is observed on the T–s chart as a vertical line pointing downward since enthalpy remains constant while entropy rises. In subcritical operation, this mechanism moves the refrigerant into the two-phase region; in trans-critical operation, it moves into a lower enthalpy superheater/supercritical state. The refrigerant moves rightward across the two-phase region (quality increases from liquid-rich to vapour-rich mixture) during the last stage, heat absorption in the evaporator (state 4 → 1), where it evaporates at constant pressure until it reaches saturated or slightly super-heated vapour conditions at the compressor inlet. This diagram highlights the unique thermodynamic properties of CO₂, particularly its inability to condense under typical ambient conditions. This characteristic necessitates the use of a trans-critical cycle arrangement. For chemical and refrigeration engineers, understanding the T-s diagram is essential for improving the design and operational efficiency of these applications. 

Note: - The diagram serves as a vital tool for analysing work input, heat transfer, and overall cycle efficiency.