Moisture Contamination in High Purity Gas Separation Products
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MOISTURE CONTAMINATION IN HIGH PURITY GAS SEPARATION PRODUCTS T. G. Ustyugova and M. Yu. Kupriyanov
UDC 621.565
Research results of impurity water accumulation and removal processes in cryogenic separation units for high-purity inert gas production (with a purity of more than 99.999 %) are presented. Dewpoint temperature calculations for gases at various pressures are made. Main sources of moisture contamination in the product gaseous flows of cryogenic separation units are determined. Recommendations regarding both the pre-purification technology for cryogenic separation unit feed gas and the cylinder filling process are provided, which make it possible to reduce water vapor concentration in gas to the level of ppb –9 (parts per billion, 1 ppb = 10 mol/mol). Keywords: сryogenic separation unit, inert gas, dew point, impurity, water.
Impurities in inert gas concentrates are divided into several groups [1–3]. The control of the concentration of impurities leading to clogging of cryogenic equipment (H2O, CO2, N2O) is necessary for the reliable and safe operation of a cryogenic technical system. However, the patterns of accumulation of impurities and their removal from the system have not been sufficiently studied [4–6]. Water is a common impurity in any technical system. When assessing the moisture content of gases, the following concepts are used: water vapor concentration, dewpoint temperature (the temperature to which a gas can be cooled at a fixed pressure without formation of the liquid phase). In cryogenic units at low temperatures, condensed water from the liquid state goes into the solid state. In gas separation plants, the accumulation of ice can significantly affect the quality of the product gas, as well as completely or partially block flow through pipeline cross-sections, affect the increase in hydraulic resistance in heat exchangers and the deterioration of the heat exchange characteristics of devices. In this paper, the reasons for the accumulation of impurity water (moisture contamination) in cryogenic gas separation units are considered. Recommendations are formulated for cleaning the initial gas mixture (entering the cryogenic unit) and for filling the cylinders with gas, which make it possible to ensure ppb (parts per billion) concentration of water vapor in gases. Moisture Contamination in Gas Separation Products Data on the relationship between the volumetric molar concentration of water vapor and the dewpoint temperature according to the ideal gas equation of state (Table 1) are presented on the websites of industrial gas producers. The dewpoint temperature can be determined by calculating the vapor-liquid equilibrium of a binary gaswater mixture according to the equations of state of real gases. Calculations are carried out using cubic equations of state: Peng–Robinson (PR), Soave–Redlich–Kwong (SRK), Cubic–Plus–Association (CPA) [7]. The concentration of water vapor in the gas phase is determined at fixed values of temperature and pressure. Bauman Moscow State Technical University, Russia; e-
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