(71c) Prediction of Pressure Drop in Gas Cylinders with Vapor Extraction at High and Low Flow Rates

Electronic specialty gases (ESGs) are used in the manufacturing processes for semiconductors, fiber optics and flat panel displays. Some ESGs are stored in cylinders in the liquid phase under their own vapor pressure. In order to estimate the amount of gas a cylinder can supply at a given flow rate and required pressure, models are needed for the prediction of the pressure drop in cylinders and the downstream distribution lines as vapor is extracted. The approach for modeling the liquid and gas phase contained in the gas cylinder is similar to the one used for modeling storage tanks and propellant tanks, which assumes uniform liquid and gas temperatures. We have recently improved our model to allow non-uniform liquid temperature.

This improvement has been motivated by the observation that modeling results do not always agree with experiments for low flow rates. We attribute these discrepancies to the boiling phenomenon that is associated with the extraction of vapor from the cylinder. It has been demonstrated experimentally that vapor extraction, at high flow rates, is associated with violent boiling. Therefore, it is expected that for high flow rate for extraction, boiling and natural convection will fully mix the liquid leading to a uniform liquid temperature. For low flow rates, the mixing will not be as pronounced, resulting in non-uniformity of the temperature in the liquid phase. The non-uniformity of temperature is characterized by a subcooled liquid/gas interface in the cylinder, resulting in a lower delivery pressure than predicted by assuming a uniform temperature.

In practice, low flow extraction is usually associated with low pressure gases such as tungsten hexafluoride and dichlorosilane. The temperature non-uniformity can be corrected by the use of All Vapor Phase (AVP) gas cabinets [1]. The AVP gas cabinet incorporates a pressure controlled cylinder-bottom heater to more completely mix the liquid in the cylinder, thus improving the extraction flow rate.

This paper presents an improved model that takes into account the non-uniformity of temperature in the liquid phase as well as boiling and natural convection within the liquid. The simulation results with the improved model, which compare well with the experimental results, are presented and discussed.

[1] H-C Wang, R. Udischas and B. Jurcik, ?All Vapor Phase delivery of electronic specialty gases?. Proceedings of SEMICON West '97 Gas Distribution Workshop, San Francisco, CA.