Thermodynamics is applied to HCI to predict the conditions which lead to condensation of moisture and liquid HCI. A Mollier diagram is used to identify the offending temperatures, pressures, and operating conditions. Practices that reduce corrosion in HCI delivery systems are recommended.
Corrosion in hydrogen chloride (HCl) gas handling systems has long been a formidable problem in the semiconductor industry . Although anhydrous HCI gas is noncorrosive to austenitic stainless steels, moist HCI can severely corrode the type 316L stainless steel process equipment widely used in the industry. HCI gas also happens to be extremely hygroscopic. Besides the moisture inherent in HCI gas cylinders, HCl picks up moisture as it travels through the process piping network. Moisture tends to coat the tube walls (as well as other components) and is very difficult to remove even with the best purging techniques. Consequently, delivery of dry HCI through a piping network is exceedingly difficult.
To understand and prevent corrosion in HCi gas handling systems, the size and morphology of corrosion products as well as the surface chemistry taking place during the corrosion process have been studied [2,3]. The condensation of moisture and liquid HCI intensifies these corrosion processes at the regulator orifice and diaphragm.
Since HCI cools as it passes through a restricted orifice, moisture and even liquid HCi may condense and cause "regulator creep" or "regulator hunting," wherein the HCl regulator creeps or oscillates around the set pressure. In the present study, Joule-Thomson expansion is examined as being the cause of these deleterious condensation effects.
A temperature change occurs when a gas expands without doing work, e.g., in passing through the orifice of a needle valve or pressure regulator. A drop in temperature, for example, can occur due to the energy absorbed in overcoming the cohesion of the molecules of the gas. The almost instantaneous expansion allows little time for the gas to reequilibrate with its surroundings, so such processes tend to be adiabatic or isenthalpic (i.e. constant enthalpy, H = E + PV, where E is the internal energy, P is the pressure, and V is the volume).
The Joule,Thomson coefficient (micro) is the ratio of change in temperature to change in pressure (dT/dP) during an adiabatic expansion (constant H) . Gas temperature may rise or fall on expansion, depending on its inversion temperature. J-T cooling, i.e., a positive value for (micro), occurs below the inversion temperature; p. is negative above the inversion temperature, in which case temperature increases as the gas expands at constant enthalpy.
J-T coefficients have been reported in literature for a very limited number of compounds such as air, argon, nitrogen, methane, and ethane as well as commonly used refrigerants such as ammonia, carbon dioxide, and chlorofluorocompounds [4-7]. Unfortunately, no such detailed data is available for HCI.
A Mollier diagram is a set of Pressure-Enthalpy (P-H) curves, convenient for determining the Joule-Thomson effect, Although enthalpy departure functions (departure from ideal gas behavior) have been reported for HCI, most of the data...
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