Refrigeration – Cryogenic treatment of gas or gas mixture – Automatic control
Reexamination Certificate
2002-08-13
2003-06-24
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Automatic control
C062S050200
Reexamination Certificate
active
06581412
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the delivery of gases at high flow rates, and more specifically, this invention is related to a method and system for delivering high purity semiconductor gases at high flow rates.
BACKGROUND OF THE INVENTION
There is a growing need in semiconductor manufacturing to deliver specialty gases to the point of use at high flow rates. Conventional compressed gas storage vessels (i.e., cylinders and ton containers) have liquefied gas under its own vapor pressure at ambient temperature. As the vapor is withdrawn from the vessel, the liquid evaporates at an equivalent rate to account for the decrease in pressure. This consumes energy from the remaining liquid in the tank. In the absence of heat transfer to the vessel, the liquid temperature drops, leading to a corresponding drop in the vapor pressure. Further vapor withdrawal eventually subcools the liquid and the flow of vapor reduced.
Along with liquid subcooling, rapid vapor withdrawal and uncontrolled heat transfer to the storage vessel also induces violent boiling at the vessel walls. This results in carryover of metastable liquid droplets into the vapor phase. In addition, the conventional sources of compressed gas storage deliver saturated vapor. A decrease in its temperature or a flow restriction in the process line leads to condensation. The presence of liquid droplets in the vapor stream is detrimental to most instruments and therefore needs to be minimized.
The problem, therefore, was to deliver high vapor flowrates from conventional sources, with minimal liquid carryover and without liquid subcooling.
The prior art has disclosed some ways of delivering high vapor flow rates from conventional sources, but none of the prior art teaches or suggest a method and system for such delivery at high flow rates using an external source with optimal heat transfer from the storage vessel walls to the liquid as well as minimizing the liquid droplet formation n the process lines.
U.S. Pat. No. 6,122,931 discloses a system that transports liquefied gas from a storage vessel to a distillation column and uses the distillate to deliver ultra-high purity vapor to the point of use. Additional processing steps are involved using liquefied gas.
U.S. Pat. No. 6,076,359 discloses increasing the heat transfer between the ambient and the gas cylinder placed in a gas cabinet. The increase is achieved by altering air flow rate in the cabinet and adding fins internal to the cabinet. This enhances the heat transfer from the ambient to the cylinder. The resulting flow rate is comparatively low. However, the increase in delivery flowrate is still not significant enough to meet the current demands.
U.S. Pat. No. 5,894,742 discloses a liquefied compressed gas pumped into evaporators, which convert the liquid into vapor phase before delivering the gas to the point of use. Using a number of such evaporators, each corresponding to a use point allows for high throughput through the delivery system.
U.S. Pat. No. 5,673,562 discloses the use of a storage vessel fitted with an internal heat exchanger, which maintains the temperature of the liquid-gas interface. The heat is transferred to the interface either by radiation or conduction through the gas phase.
U.S. Pat. No. 5,644,921 discloses superheating the vapor withdrawn from a storage vessel containing liquefied compressed gas heated using an external heat exchanger. This superheated vapor is then used to exchange heat with the liquid phase by passing the vapor through heating tubes immersed in the liquid phase. This cools the vapor and induces liquid boiling to maintain a minimum vapor pressure in the vessel. The cooled vapor is then delivered to the point of use.
All the methods presented in the patents discussed above provide means of supplying additional energy to the liquid through external sources. However, these methods are not adaptable to existing sources of compressed gas storage and require additional equipment. This makes those inventions capital intensive. Further, these inventions only address the issue of supplying additional energy to the system. There is no teaching or suggestion on methods of decreasing the various heat transfer resistances, which allows for optimal operation of the delivery system.
Udischas R. et al., “Performance and Cost Comparison for Various Bulk Electronic Specialty Gas Delivery Solutions”, presented in Workshop on Gas Distribution Systems, SEMICON West 2000 compared the economic advantage of various delivery systems for compressed gases. The maximum delivery flow rate used for the comparison was 400 standard liters per min (slpm) ammonia flowing for two hours and 1000 slpm HCl flowing for one hour.
Yucelen B. et al., “High Flow Delivery Systems for Bulk Specialty Gases”, presented in Workshop on Gas Distribution Systems, SEMICON West 2000 disclosed that externally heating the ton containers can deliver high flow rates (up to 1500 slpm). The focus of the paper is to analyze the moisture carryover in the vapor at high flow rates.
In view of the prior art, there is a need for a method and system which 1) facilitates the withdrawal of vapors from the existing sources of compressed gas storage (cylinders and ton containers) at high flow rates using an external heat source; 2) proposes a control strategy which allows for optimal heat transfer from the storage vessel walls to the liquid, and 3) develops a method to deliver high vapor flow rates while minimizing liquid droplet formation in the process lines.
SUMMARY OF THE INVENTION
One aspect of this invention is directed to a method for controlling the temperature of a liquefied compressed gas in a storage vessel comprising passing a liquefied compressed gas into a storage vessel; positioning a temperature measuring means onto the wall of the compressed gas storage vessel; positioning at least one heating means proximate to the storage vessel; monitoring the temperature of the compressed gas within the storage vessel with the temperature measuring means; and adjusting the output of the heating means to heat the liquefied compressed gas in the storage vessel.
In another embodiment, this invention is directed to a method for maintaining the evaporation of a liquefied compressed gas in a storage vessel during vapor delivery comprising passing a liquefied compressed high-purity semiconductor gas into a storage vessel; positioning a temperature measuring means onto the wall of the storage vessel; positioning at least one heating means proximate to the storage vessel; monitoring the temperature of the compressed gas within the storage vessel with the temperature measuring means; positioning a pressure measuring means at an outlet of the storage vessel; monitoring the pressure of the compressed gas within the storage vessel with the pressure measuring means; passing a portion of a gas out of the storage vessel; and adjusting the heat output of the heating means to maintain a desired pressure.
In yet another embodiment, this invention is directed to a method for delivering a liquefied compressed gas with a high rate of flow comprising passing a liquefied compressed high-purity semiconductor gas into a storage vessel; positioning a temperature measuring means onto the wall of the compressed gas storage vessel; positioning at least one heating means proximate to the storage vessel; monitoring the resulting temperature with the temperature measuring means; positioning a pressure measuring means at the outlet of the storage vessel and monitoring the vessel pressure; adjusting the heat output of the heating means to heat the liquefied compressed gas in the storage vessel to control the evaporation of the liquefied compressed gas in the storage vessel; and controlling the flow of the gas from the storage vessel.
In yet another embodiment, this invention is directed to a method for delivering ammonia with a high rate of flow comprising passing a high-purity liquefied compressed ammonia gas into a ton container; positioning a thermocouple onto the wall of the ton container; positioning at l
Leshner Mike
Pant Atul
Richardson Melvyn
Capossela Ronald
Follett Robert J.
Praxair Technology Inc.
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