Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
2002-06-20
2004-11-30
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S659000
Reexamination Certificate
active
06824752
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to a system for protecting a gas purification system from damage. In particular, this invention is directed to a process and system for operating an ultra high purity gas purifier using a getter while minimizing the chance for damage to the gas purification system.
BACKGROUND OF THE INVENTION
Ultra-high purity (UHP) gas purification systems are used to supply customers with UHP nitrogen. The initial source of nitrogen (the distillation plant, or the liquid nitrogen supply) typically contains impurities including about 1 ppm oxygen by volume. The oxygen level is checked by an oxygen analyzer before passing into the gas purification vessel, which contains material that reacts with impurities in the nitrogen to produce purified gas.
Ultra-high purity inert gas purification systems generally employ a chemically-reactive getter metal that is comminuted by some means, and then dispersed in a matrix of a comparatively inert substrate material (usually alumina or similar). Once activated by a reduction process of some kind, this high surface area metal can then react extremely rapidly with various impurity gases (oxygen, hydrogen and others, depending on the getter material and temperature) to chemically bond with such gases, and so remove them from the gas stream: a process known as chemisorption.
The speed of the reaction, plus its highly exothermic (heat-evolving) nature means that processing inert gases containing high levels of a reactive impurity may cause significant damage and personal danger. For example, it is known that exposing activated nickel-based getters to oxygen concentrations greater than 1% will generally cause heat-damage to the catalyst, and possibly also to the reactor vessel and downstream customer equipment. The cost of the damage in such an instance ranges from tens of thousands to millions of dollars, when accounting for the impact on downstream processing.
One safety scheme is to measure the temperature of the catalytic bed using judiciously placed temperature measuring devices, such as thermocouples. If the bed temperature rises due to exothermic reactions, action is taken to safeguard the bed. This action will typically consist of diverting the feed gas and venting the purifier to rid it of remaining reactive gases and preventing additional reactive gases from entering. This will typically be performed automatically using a control unit that recognizes that a temperature setpoint has been reached and actuates valves to the shutdown condition. A major problem with this approach is that it requires that the bed be exposed to high levels of reactive impurities before action is taken, since this is necessary to raise the temperature of the bed.
Another safety scheme is to sample the gas stream prior to entering the bed. When a predetermined level of contaminants is reached, typically orders of magnitude above that normally present in the feed gas, action is taken to safeguard the bed. There are several types of impurity-level monitoring device. Typically, commercially available gas analyzers can be used. The approach of measuring the gas stream prior to entering the bed has the advantage that it can potentially allow more rapid reaction than an approach using thermocouples placed inside the purifier that is being protected, i.e., it can take action to safeguard the bed before the bed is exposed to high levels of reactive species. Further, it is possible to detect levels of reactive species that are higher than normal operation but are below that required to raise the temperature of the bed significantly. Thus, it is possible to design a system that is more sensitive to reactive species than one that simply embeds thermocouples inside the purifier bed and waits for these to register an increase in temperature.
One drawback to this approach is the cost of the unit, both in terms of the initial purchase cost and the maintenance required to keep the analyzer in good working order. For example, oxygen is commonly the species from which the purifier must be protected. Two standard varieties of oxygen-detection cell provide an electrical current output from either (i) a cell that operates at ambient temperature, containing salt solution that needs to be maintained at a fairly constant level by continuous replenishment with deionized water or new salt solution, or (ii) a zirconia (ZrO
2
) cell maintained at high temperature (greater than 600° C.), that has a typical lifetime of two years or less.
The cost of using analysis external to the bed is compounded by the need to protect the gas from a backflow condition. Backflow may occur due to errors in operation or piping hook-up, or as a result of upset conditions that cause the pressure within the purifier to be lower than the pressure in what is usually the downstream direction and thus cause gas to enter the bed in a direction counter to that during normal operation. Protection against backflow requires that both the stream entering and leaving the purifier must be sampled. This increases cost and reduces system reliability by requiring two measuring devices as opposed to one.
It is therefore a priority to ensure (by monitoring) that such instances are avoided. An ideal system to achieve (or monitor) this will have three main features. It must 1) respond rapidly to increases in impurity level; 2) be easily maintained and operated, and 3) be cost effective enough to allow several redundant analyzers to be employed to monitor reactive impurities in the gas stream. This redundancy guarantees superior reliability, which is necessary to minimize the risks, both personal and financial, as discussed above.
Some prior art references have attempted to provide a system that attempts to safeguard the getter. U.S. Pat. No. 6,168,645B1 discloses a safety device located both up and downstream of the getter. The safety device contains getter material, along with thermocouples. This patent discloses a method of detecting impurities in gases by their reaction with a purification material that exhibits an exothermic reaction when an impure gas is passed over it. The resultant change in temperature is then detected by various means, including measuring the temperature of the gas using a thermocouple, or melting of the purification material. Once a certain temperature has been reached, a control system is then triggered, to cause it to carry out remedial action.
However, the '645 patent has a significant disadvantage in that a continual low level of impurity will eventually consume the purification material, and will cause a gradual reduction in both the speed and the level of response (smaller change of temperature) to increases in the impurity concentration. For example, it is well known in the industry that a container filled with activated nickel getter has a limited capacity to react with oxygen, hydrogen and other impurities. The oxidation reaction is:
Ni+½ O
2
-->NiO+heat
However, once all the nickel has reacted in this way, there will be no further heat output by the purification material, regardless of how high the oxygen concentration is in the gas or other fluid passing over it.
It is believed, therefore, that the '645 patent may be practiced only if there is the capability for either regeneration or replacement of the purification material. The condition of the purification material must itself be monitored to ensure that the safety device will give the appropriate response when exposed to excessive amounts of impurity.
U.S. Pat. Nos. 6,068,685 and 6,156,105 disclose protecting the purifier both upstream and downstream. A first temperature sensor is disposed in a top portion of the getter material that constitutes the purifier bed. The first temperature sensor is located in a melt zone to detect rapidly the onset of an exothermic reaction which indicates the presence of excess impurities in the incoming gas to be purified. A second temperature sensor is disposed in a bottom portion of the getter material. The second temperature sensor is
Mackie Andrew Christopher
Terbot Charles Edward
Praxair Technology Inc.
Schwartz Iurie A.
Silverman Stanley S.
Strickland Jonas N.
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