A consistent supply of ultrapure liquefied gas is critical for fabrication of semiconductors and other products that have no tolerance for contamination. Although liquefied gasas are typically used in the vapor phase, they are stored in cylinders as liquids under their own vapor pressure. The trend, however, is toward bulk gas supply. Driving this trend is the reduction in potential contamination, improved safety and handling, and lower costs. Many specialty gases are stored in a liquefied state. The typical container for liquefied gases is a cylinder in sizes of 7, 16, 30 or 44 L. Materials of construction include aluminum, carbon steel and nickel. For worker protection, cylinders are mounted in a gas cabinet, and the components are controlled by a programmable logic controller (PLC). Bulk supply is centralized, with redundancy of key components in the event of a disruption in service. Compared with cylinder cabinets, bulk supply involves fewer source changes, reducing the risk of operator exposure and system contamination. Delivery equipmentcan be classified into four basic categories: 1) Standard gas cabinet. A typical cabinet contains three cylinders; two supply liquefied gas, and the third contains nitrogen for purging the control panels. 2) Gas cabinet with integrated heating control. The all-vapor-phasr (AVP) delivery system is identical in appearance to the standard cabinet. For greater heat efficiency, energy input is through heaters located under the cylinders. 3) Bulk evaporation purification by gas-phase (BEPS-G). This system can be fitted with a heating element to increase the heat available for vaporization and the rate of gas withdrawal. 4) Bulk evaporation purification by liquid-phase (BEPS-L). This system permits direct vaporization of the liquid. Increasing the size or number of vaporizers accommodates high flowrates. Performance of the four different delivery systems is evaluated for HCl, NH3, Cl2 and HF. The economics of delivery systems for ultrapure liquefied gas can be estimated on the flowrate and delivery-pressure requirements. A software modeling program is used to determine the pressure-drop performance data for equipment with gases at selected flowrates. The model can also be used to predict the capabilities of equipment for a given flowrate with a required delivery pressure. Most bulk source vessels are horizontal in orientation. Given the operating parameters of the equipment, the cost of materials for various scenarios can be estimated. At higher flowrates, the cost advantage shifts in favor of bulk supply. Selection of delivery systems for ultrapure specialty gas is highly dependent on gas type, flow, pressure and temperature requirements.
Special delivery: Ultrapure liquefied gas
Liefersysteme für ultrareines Flüssiggas
Chemical Engineering, New York ; 107 , 11 ; 115-118
2000
3 Seiten, 8 Bilder, 1 Tabelle, 6 Quellen
Article (Journal)
English
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