Patent Application
20080000239
The objects and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figures wherein like numbers denote same features throughout and wherein:
The manufacture of semiconductor devices, LEDs, LCDs and solar/photovoltaic cells requires the delivery of vapor phase, low vapor pressure gases to a point-of-use. These fluids must meet customer purity and flow requirements. The present invention provides a means to transport a compressed, liquefied low vapor pressure gas from the gas manufacturer, and process this non-air fluid so as to deliver a low vapor pressure vapor stream which is lean in low volatility contaminants to the point-of-use. As utilized herein, the term “lean” shall mean a vapor stream having a lower level of low volatility contaminants therein than the liquid or two-phase fluid provided by the gas manufacturer. The system provides the requisite purity on a consistent basis. Further, the transport/storage vessel (referred below, as the transport vessel) is preferably designed to carry more than about 2,000 lbs. and preferably between 20,000 and 50,000 lbs. of low vapor pressure fluid. Additionally, it is preferable that the vessel be capable of being shipped, and is compliant with International Standards Organization (ISO) requirements (e.g., ISO container standards).
Typically, low vapor pressure non-air fluids are stored in a transport vessel under their own vapor pressure. While the fluid contained in the transport vessel delivered to the point-of-use is process dependent, for ease of reference ammonia is utilized as the fluid of choice, but it will be understood that any number of low vapor pressure non-air fluids may be utilized. The transport vessel can be constructed from a material such as carbon steel, type 304 and 316 stainless steel, Hastelloy, nickel or a coated metal (e.g., a zirconium-coated carbon) which is strictly non-reactive with the fluids utilized and can withstand both a vacuum and high pressures.
The transport vessel, such as an ISO container, is installed “on-site,” that is in close proximity to the manufacturing facility and may be installed outdoor, where the temperature can be as low as −30° C., or indoor. The manufacturing facility is preferably equipped with automatic gas sensors and an emergency abatement system in case of an accidental leakage or other malfunctions of the system.
The transport vessel is not typically insulated. As a result, the temperature of the transport vessel contents during transport and storage at the facility is similar to ambient temperature. With reference to
Most manufacturing facilities require ammonia delivery pressures in excess of 100 psig. To meet these pressure requirements, the temperature of the transport vessel contents must be elevated by applying heat from a heat source.
In one exemplary embodiment of the invention, and as illustrated in
While initially a liquid-vapor phase equilibrium is maintained in ISO container 210, this equilibrium is upset when the manufacturing facility begins to withdraw vapor phase ammonia. In operation, ammonia fluid in vapor phase is withdrawn from either ISO container 210 or 220 at flow rates ranging from about 0 to 10,000 standard liters per minute (slpm), preferably from about 0 to 7,500 slpm, and most preferably from about 0 to 3,500 slpm. As the manufacturing facility draws vapor phase ammonia, the amount of vapor phase ammonia in the ISO container decreases. This causes the vessel pressure to fall. To return the ISO container pressure to its initial level, some of the liquid phase ammonia must be vaporized to replace the vapor mass that was withdrawn.
Ammonia in the ISO container typically has some contaminant level. Some of these contaminants, for example water, are less volatile than ammonia. Therefore, their concentration in the liquid phase is higher than their concentration in the vapor phase. For example, and with reference to
As vapor is withdrawn from ISO container 210, it passes through containment device 230 which is typically purged with nitrogen. The containment device contains valves, fittings, etc., that have the potential to leak. Vapor phase ammonia is conveyed from containment device 230 to a source gas panel 240, which regulates the flow rate of ammonia to the point-of-use.
As demonstrated previously, the pressure within the ISO container 210 falls as vapor ammonia is withdrawn. This causes the temperature of the remaining fluid in the container to likewise fall, as shown in
In order to maintain the ISO container temperature and pressure, energy in the form of heat must be transferred to the ISO container contents. The amount of energy required to sustain the ISO container pressure and temperature at given flow rate must be considered, as well as potential heat losses. For example, to sustain a vapor flow rate of 3,500 slpm at 70° F., the heat transfer to ISO container 210 is on the order of 50 to 60 kW, assuming no heat losses. As explained in U.S. Pat. No. 6,363,728 which is incorporated herein by reference in its entirety, the rate of heat transfer between the heating means and ISO container 210 is governed by: (1) the overall heat transfer coefficient; (2) the surface area available for heat transfer; and (3) the temperature difference between the heaters and the contents of ISO container 210.
The source of energy is one or more energy delivery devices disposed on the lower portion of the ISO container. The energy delivery devices are typically electrical resistance type heating means/elements typically selected from blanket heaters, heating bars, cables and coils, band heaters, heater tape and heating wires. Alternative heating elements include intermediate fluid based heaters and inductive heaters.
Intermediate fluid based heaters transfer heat to an intermediate fluid, such as water, which then transfers heat to the transport vessel and ultimately to the low vapor pressure fluid. The intermediate fluid may transfer heat to the transport vessel by a number of mechanisms, such as by passing the intermediate fluid through heating coils. Inductive heaters generate a magnetic field, which is then used to generate heat. This heat could then be passed to a device such as a band or coil which is in contact with the transport vessel.
In the exemplified embodiment, vapor phase ammonia is withdrawn from ISO container 210 at a variable rate. To maintain the ISO container pressure in response to this variable rate, a pressure controller is used, which regulates the energy input to ISO container 210. Delivery system 200 includes a closed-loop control means to monitor the pressure at which the ammonia vapor withdrawn and to compensate for the energy of vaporization utilized to deliver the ammonia vapor at a desired flow rate. Suitable control means 260 are known in the art, and include, for example, a programmable logic controller (PLC) or microprocessor (not shown).
In the exemplified embodiment, a pressure sensor (not shown) sends a measurement signal to the PLC thereby indicating the pressure of the vapor phase ammonia delivered to the source gas panel 240. The measured pressure is compared to a pressure set point. Should the pressure decrease below the expected pressure, a signal is transmitted from the PLC to the energy delivery device to deliver energy to ISO container 210. Thus, the thermal energy is employed to restore the pressure necessary to maintain demanded flow rate of vapor delivered to the point-of-use. In the event the level of ammonia fluid in ISO container 210 should drop to below the level at which the desired flow rate can be sustained as determined by the PLC, system 200 would switch to ISO container 220 so as to deliver the vapor to containment device 250, and in turn to source gas panel 250, which regulates the flow rate of ammonia to the point-of-use, as discussed with respect to ISO container 210. It will be understood that heater controls need to include a mechanism to prevent the heating means from overheating if the pressure loss becomes excessive.
Alternatively, an algorithm could be employed to determine the transport vessel 210 surface temperature required to sustain the set point pressure in conjunction with a pressure vs. temperature curve for the ammonia system employed. Upon deriving the required transport vessel surface temperature, its value is compared with a surface temperature set point range. In the event that the temperature falls below the lower limit of the range, energy in the form of heat is applied. Conversely, if the temperature is above the range, energy is removed.
Returning to the energy delivery device, these devices are not only positioned at the lower portion of the vessel, but are configured to the contour of the vessel to efficiently transfer energy/heat to the vessel. Although the heating means/elements discussed above are adequate means for providing energy to the system, in some instances they do not conform well to the contour of the vessel or are otherwise difficult to hold in close proximity or contact with the wall of the transport vessel. As a result, at the contact points between the transport vessel and the heating means/elements can become very hot and exceed the transport vessel's design temperature. Liquid ammonia contained near these “hot spots” can boil vigorously, causing liquid droplets containing high-low volatility contaminant levels to be carried over into the vapor stream. As a result, the low-volatility contaminant level may exceed acceptable limits.
Away from the contact points, energy will not transfer efficiently from the heating means/elements to the vessel surface, resulting in increased heat losses and excessive power consumption. Further, the heating means are susceptible to overheating and burn out at those locations for which contact between the heating means/elements and the transport vessel is poor.
To ensure uniform, intimate contact between the energy delivery devices and the transport vessel, an efficient energy delivery system 400 was developed, as depicted in
The insulation 430 is placed between the cradle and the heating elements such that heat is directed from the heating elements 420 to the transport vessel, thereby minimizing heat losses. The insulation is preferably pliable and conforms well to the shape of the transport vessel. One such type of insulation is silicone rubber sheet insulation.
The cradle can be made of any substantially rigid material, including but not limited to stainless steel, such that it supports and maintains the heating means in close proximity with the lower portion of vessel which cradle 410 encompasses, so that it does not sag, bulge, wrinkle or otherwise lose contact with the wall of the vessel.
Insulation 430 is preferably attached to the cradle using an adhesive, which is not depicted. Further, the heater elements are preferably attached to the insulation using a second adhesive layer, which is also not depicted. Because the heating elements and insulation are adhered to the cradle, the opportunity for heater warping or bulging is eliminated.
With reference to
Each of the substantially rigid support devices (i.e., crescent-shaped cradles) is attached to the ISO container, preferably using straps or springs attached to both ends of the support devices and which wrap around the top of the container where they are connected by buckles. Alternatively, the straps or springs may attach to fixed objects located on the upper portion of the ISO container, such as the sun shield support brackets. By attaching the cradle to the transport vessel in this manner, the heating elements are compressed between the cradle and the transport vessel, ensuring intimate contact. This eliminates the possibility of heater buckling or sagging.
Using this attachment method, the support devices are easily removed and employed with other transport vessels. Therefore, a specific transport vessel does not need to be dedicated to each manufacturing facility, nor does a transport vessel have to be purchased for use at a given manufacturing facility (transport vessels may be leased from any supplier and remain compatible with the heating equipment).
Because it is large, it is likely that the ISO container will be located outdoor at the manufacturing facility. Typically, it is desirable to maintain the pressure within the ISO container at a level of at least 100 psig, implying that the temperature within the ISO container is approximately 70° F. If ambient temperature is less than this value, heat losses will be experienced from the ISO container itself to ambient. To minimize these losses, it may be desirable to surround the ISO container with a second insulation means. The second insulation means is preferably easily transferred from vessel to vessel. For example, the second insulation means may be an insulating tarp that is draped over the ISO container or the ISO container frame. This insulating tarp may be constructed of one of many insulating materials, such as foam insulations.
While the invention has been described in detail with reference to exemplary embodiments thereof, it will become apparent to one skilled in the art that various changes and modifications can be made, and equivalents employed, without departing from the scope of the appended claims.
