Ultra-high purity (UHP) gas purification systems are generally used to supply customers with UHP nitrogen. The initial source of nitrogen (the distillation plant, or the liquid nitrogen supply) typically contains about 1 ppm oxygen by volume. The oxygen level is monitored 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 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 reactive 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% by volume (10,000 ppm O2) will generally cause heat-damage to the getter bed, 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 the impact on downstream processing is accounted for.
It is therefore a priority to ensure that such instances are avoided. As such, safety schemes have been devised to protect the bed from excessive levels of contaminant, and thus excessively high temperatures.
One safety scheme is to measure the temperature of the bed using judiciously placed temperature measuring devices, such as thermocouples. If the bed temperature rises due to the 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 measuring 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 react more rapidly than 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 that the purifier must be protected from. 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 (ZrO2) cell maintained at high temperature (typically 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 downstream and thus cause gas to enter the bed in a direction counter to that during normal operation. Protection against backflow requires that both the streams entering and leaving the purifier must be sampled. This increases cost and reduces system reliability by requiring two measuring devices as opposed to one. The cost and reliability implication of monitoring the purifier both upstream and downstream is the problem that this current invention addresses.
U.S. Pat. Nos. 6,068,685 and 6,156,105 disclose the need to protect 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 located in a melt zone to detect rapidly the onset of an exothermic reaction which indicates that excess impurities are being backfed into the getter column. In these patents, a purifier vessel contains getter material. A gas stream enters the purifier vessel and a separate stream exits from the vessel. A thermocouple is placed at the inlet side of the bed. Another thermocouple is located near the exit of the bed. The temperature at both locations is sent to a control unit. The registered temperatures are compared with setpoint values. Action is taken to protect the bed if the setpoint is exceeded in either thermocouple.
U.S. Pat. No. 6,168,645B1 also discloses the need to protect the purifier both upstream and downstream. A first safety device is located upstream of a purifier and a second safety device is located downstream of the purifier. A sample stream is drawn from the feed stream, upstream of the purifier, and sent to an upstream safety device. Similarly, a sample stream is drawn downstream of the purifier and sent to the downstream safety device. The level of reactive species is sent from both safety devices to the control unit. If either safety device registers levels of reactive species is in excess of some defined setpoint, action is taken to safeguard the bed. On passing through the safety devices, the sample streams are typically sent to vent. Since this represents loss of product gas, there is a motivation to minimize the quantity of sample streams withdrawn.
Also disclosed in the '645 patent are low cost means of measuring the quantity of reactive species in the stream downstream of the purifier. In one embodiment, the safety device is simply a small sample of the getter material inside the purifier. The temperature rise in this guard bed is used as an indicator of the level of reactive species. Such an approach is not suitable for monitoring the upstream case because the level of reactive species is such that the bed material would react with the oxygen over time and thus become inert to the presence of additional reactive species. For example, activated nickel getter, commonly used to remove oxygen, hydrogen and carbon monoxide from bulk inert gases, has a limited capacity to react with oxygen, hydrogen and other impurities. The oxidation reaction is:Ni+½O2-->NiO+heatOnce all the nickel has reacted in this way, there will be no further heat output by the purification material, no matter how high the oxygen concentration in gas or other fluid passing over it.
It is believed that the number of patents describing safety schemes for getter-based purifiers is limited. A series of patents disclose determining the end-of-life of a purifier, meaning the point at which the purifier allows unacceptable concentrations of impurities to break through. U.S. Pat. No. 5,150,604 discloses the use of pressure drop across the bed to determine the end of its useful life. The '604 patent discloses pressure transmitters that are located up and downstream of the purifier. The signals from these transmitters are sent to a control device to see if the pressure drop is above some setpoint.
There is therefore a need in the art to provide for a low cost and reliable process or system for monitoring the purifier both upstream and downstream of the getter.