Patent Application
20130327067
The invention relates to the application of Liquid Argon for inerting the atmosphere above a metal body in a furnace.
Many metals react to water and/or oxygen in air which intensifies when the metal is melted. There exist a variety of techniques to reduce the level or these air constituents sufficiently. A widely used technique is commercially named SPAL™. The SPAL™ process involves pouring liquid cryogens over the metal to create a continuous covering. As this liquid vaporizes, the surface of the metal is protected from oxygen and water in the air. One continuing issue with SPAL™ is the loss of liquid cryogen prior to pouring on the metal surface. Delivery systems have been optimized with e.g. vacuum jacketed insulation to minimized vaporization in the piping. Most SPAL™ systems still have enough vapor formation within the delivery system to require terminal phase separators. The vapor from such phase separators is generally vented to the atmosphere. In certain advanced SPAL™ systems, the vapor is directed onto the metal surface to augment the inerting by the liquid covering. While these advanced SPAL™ systems make use of the loss vapor from the liquid cryogen, the inerting value of this vapor is not as high that derived by an equal amount of liquid cryogen poured onto the metal. Consequently, it would be useful in many instances if vaporization losses in SPAL™ systems could be further reduced. From a cost analysis perspective, reduction of losses to vaporization will have the most impact when liquid Argon is the inerting liquid cryogen.
The invention primarily addresses the losses of liquid Argon in a foundry or other metallurgy facilities utilizing a SPAL™ system to provide protection of metals in furnaces from atmospheric exposure. The basic technique to improve liquid Argon utilization efficiency (or possibly use of other cryogenic liquids or mixture) is the sub-cooling of Argon from a liquid Argon bulk source tank. The Argon is ideally sub-cooled as close to the point of dispensation onto the metals as is practical. The sub-cooling should be sufficient to either a) compensate for subsequent in transit heating to reduce the amount of liquid Argon that becomes vaporized prior to dispensation onto the metal or b) condense a portion of Argon vapor that evolves from the liquid Argon due to prior in transit heating and/or pressure reduction between the tank and the SPAL™ piping system, or both (“target temperature”).
Bulk storage tanks are often pressurized while most SPAL™ piping and delivery systems are not pressurized. The depressurization of bulk tank liquid Argon causes a significant amount of liquid Argon from the bulk tank to vaporize upon depressurization. An intervening sub-cooling step can be adapted to condense some or even most of the gaseous Argon back to liquid Argon while also sub-cooling the liquid Argon to the target temperature.
Finally, the sub-cooling of liquid Argon will reduce the amount of Argon gas in the SPAL™ piping and delivery systems. This will provide an added benefit of reduced flow rate variation and sputtering of liquid Argon from a dispensing lance due to gas build up in the pipes.
Target Temperature
The target temperature will vary depending on the specific facility SPAL™ system. For example, foam insulated pipes will generate more gaseous Argon than vacuum insulated pipes. The piping distance from the point of sub-cooling to the dispensing lance will affect the degree of transit associated heat gain and thus the quantity of liquid Argon that is vaporized en route. Other facility specific factors will impact the target temperature.
In addition to facility specific factors, the target temperature for liquid Argon sub-cooling is governed in part by physical limitations. Argon freezes at −189 degrees C. Thus, −189 degrees C. constitutes a lowest end target temperature for making a liquid/solid slush. A liquid/solid slush would need to be sufficiently composed of liquid Argon to flow in the SPAL™ piping. The solid Argon mixed in with the liquid would contribute more heat absorption capacity for the mixture due to the heat required to melt the solid. Forming Argon slush is not required for the invention to operate. For example, this maximum level of sub-cooling will not be of sufficient benefit in terms of Argon vaporization mitigation to justify the energy consumption required. In addition, from a process control perspective, forming consistently flowing liquid/solid slush will be quite difficult. Over-freezing will block the piping and stop flow. Thus, highly preferably the target temperature will be sufficiently above the freezing point to avoid formation of any solid Argon.
The upper end of the target temperature range will be governed in part by the applicable boiling point which in turn depends in part on the pressure. Liquid Argon in bulk storage tanks is generally maintained under pressure (for delivery of liquid Argon from the bulk tank) and at a temperature below the boiling point at the bulk tank pressure. An example from current commercial systems, Liquid Argon may be maintained in bulk tanks at 45±2 psig (310.26 kPa) and −176 degrees C. The pressure in the SPAL™ system will generally be atmospheric to e.g. 22±2 psig (253±115.11 kPa). This means liquid Argon will equilibrate by vaporization-cooling until the temperature of the remaining liquid reaches the boiling point temperature at the lower pressure (at atmospheric pressure, roughly −185.7 degrees C.). Thus, for example, the target temperature for sub-cooling in a pressurized system component could be different than in an atmospheric pressure component of the same system.
Sub-Cooling Location
In principle, the liquid Argon in the bulk tank may be sub-cooled as the sole sub-cooling step, or in combination with a downstream sub-cooling step or series of sub-cooling steps. Preferably however, a single sub-cooling step is integrated into the SPAL™ system as close to the dispensing lance as is practical.
If a particular SPAL™ system has Argon losses primarily due to depressurization from the bulk tank to the SPAL™ system piping, the sub-cooling step may be carried out as close as possible to the bulk tank to also improve flow rate and flow consistency through the piping system which is negatively affected by the presence of large gas volumes.
Multiple sub-cooling steps may be used such as both close to the bulk tank and as close to the SPAL™ lance as possible.
Sub-Cooling Step Equipment
The liquid Argon sub-cooling and/or gaseous Argon condensation to liquid may be implemented by any suitable equipment. For example, liquid Argon in a bulk storage tank may be sub-cooled by the same refrigeration process and similar equipment as used in cryogenic distillation. Alternatively, liquid and gaseous Argon may be passed through a sub-cooling heat exchanger close to the dispensing lance. The refrigerant in the heat exchanger may for example be pressurized Argon gas from the headspace of the bulk storage tank. Alternatively, a separate source of another liquid cryogen such as liquid Nitrogen may be used. Heat from the Argon condensing and sub-cooling will be transferred to the liquid Nitrogen, resulting in Nitrogen vapor generation. The Nitrogen vapor may be vented to the atmosphere.
The invention is described in part by the following numbered sentences:
Liquid Argon source 10 source is generally a bulk tank supplied with liquid Argon 40. The liquid Argon is transported by pipe 20 into phase separator 30 then out to a SPAL process generally by diffuser lance 50 with an optional auxiliary phase separator. Liquid Nitrogen source 60 is also generally a bulk tank supplied with liquid Nitrogen. Liquid Nitrogen is delivered by pipe 70 to condensing coil 80 and the liquid and vaporous Nitrogen returns via pipe 90 to liquid Nitrogen source 60. Venting phase separator 100 removes and expels vaporized Nitrogen from the pipe 90 prior to return of the recycled liquid Nitrogen. The liquid Nitrogen should be sufficiently cold to recondense vaporized Argon when passed through the condensation coil. Argon boils at −185.85° C. under standard atmospheric pressure whereas liquid Nitrogen boils at −195.79° C. Nitrogen also has a greater specific heat capacity than Argon. Thus liquid Nitrogen will under normal circumstances be able to recondense the vapor phase Argon in a liquid-vapor Argon phase separator.
The liquid Nitrogen temperature (and pressure) in coil 80 should be selected to provide sufficient cooling under operating condition to condense Argon vapor 45 without freezing it or the liquid Argon 40. The precise operating conditions will depend on the pressure and temperature of the Argon. An optimally balanced system will preferably cool the liquid Argon 40 (which may be in direct contact with cooling coil 80) to a target temperature half way in-between the boiling point and freezing point of the Argon. For example, at 31 psig (315.06 kPa) the boiling point of Argon is −173 degrees C. and the freezing point is −189 degrees C. The preferred target temperature for sub-cooling would thus be −181 degrees C. Because Argon has a narrow temperature range between boiling and freezing, target temperatures at e.g. −188 degrees C. run the risk of excessive Argon freezing due to variations in liquid Nitrogen temperature. By targeting a median temperature in the liquid phase range, the system will tolerate some downward temperature fluctuations in the liquid Nitrogen cooling system without overly sacrificing Argon gas condensation efficiency.
Prophetic example 1 relates to the mode for carrying out the invention shown in
H
sat'd. liq
190 psig=(1−x)*Hsat'd. liq0 psig+xHsat'd. vap0 psig=Hsat'd. liq0 psig+sΔHvap0 psig
Based on this equation, 26.6% of the liquid Argon would vaporize upon depressurization to decrease the temperature of the remaining Argon. By sub-cooling all of the Argon to 110.2 K (−261.4° F.; −163° C.) by heat exchange with 200 psig (1480.27 kPa) liquid Nitrogen, the fraction of Argon vapor will decrease to 17.6%. If the pressure of the liquid Nitrogen is decreased from 200 to 60 psig (515 kPa) to decrease the liquid Nitrogen temperature prior to sub-cooling the liquid Argon, the temperature of the sub-cooled Argon will be decreased by heat exchange to 94.4 K (−289.8° F.; −178.78° C.). At this temperature, only 6.4% of the Argon will be in the gas phase.
The present invention is at least industrially applicable to the protection of metals in foundry furnaces from air.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
