Nitrogen is used extensively throughout a number of high-technology industries, including those concerned with the manufacture of ceramics, carbon fibers and silicon wafers. Nitrogen is a major chemical for the electronics industry and is by far the largest used gas in the production of semiconductor devices. Because the fabrication of silicon wafers requires extremely low-contaminant atmospheres, it is imperative that nitrogen for the electronics industry be supplied at high purity specifications.
The major source of nitrogen is air, from which it is typically produced by cryogenic separation. One of the contaminants in air is carbon monoxide; the carbon monoxide concentration in air is typically 0.1 to 2 vppm, but may be as high as 5 vppm. Due to the reactive nature of carbon monoxide, it extremely critical that the nitrogen delivered to the electronics industry be free of this impurity. The concentration of carbon monoxide in carbon monoxide-free nitrogen should be less than 0.1 vppm and preferably below 10 vppb. Thus, efficient processes for the production of carbon monoxide-free nitrogen are essential for the cost-effective manufacture of semiconductor devices.
The most common method for the production of nitrogen is by the cryogenic distillation of air. The distillation system typically consists of either a single distillation column or a double-column arrangement. Details of the single-column process can, be found in the "Background of the Invention" section of U.S. Pat. Nos. 4,867,773 and 4,927,441 assigned to Air Products. Details of the double-column nitrogen generator can be found in U.S. Pat. Nos. 4,994,098 and 5,006,137, also assigned to Air Products. However, in these known processes, a significant fraction of the carbon monoxide in the feed air shows up in the final nitrogen product. To overcome this inability of conventional air separation units in reducing the concentration of carbon monoxide in the nitrogen product, a number of schemes have been proposed. These previous solutions ,can all be classified into two major categories.
Processes in the first group remove the carbon monoxide up-front from the feed air, which is then sent to the distillation system for the production of the desired carbon monoxide-free nitrogen. In these processes, the carbon monoxide is usually removed using a noble metal catalyst such as the ones based on palladium or platinum. Compressed warm air is sent over a catalyst bed to react the carbon monoxide These catalysts are usually expensive.
Processes in the second class remove the carbon monoxide by further purifying the nitrogen that is produced by the distillation system. Usually some form of chemisorption operation is carried out to reduce the concentration of the carbon monoxide to the desired level. U.S. Pat. No. 4,869,883, assigned to Air Products, describes in detail a typical process that employs a catalytic purifier for the removal of carbon monoxide.
Both of the above-described solutions for the reduction of the carbon monoxide, concentration in the nitrogen product possess a number of inherent drawbacks. First, both schemes require the application of an additional unit operation on either the feed air or the standard nitrogen product from the distillation system to produce the desired product. This extra processing step adds complexity and cost to the overall process. In the case of the catalytic purifier, the additional operation can be very expensive since the catalyst used is often a noble metal such as platinum or palladium. Second, when the nitrogen gas is treated in a catalytic unit, particulates are introduced into the gas which subsequently need to be filtered out. Requiring a filtration system adds yet another processing step to the schemes, adding even further to the cost and complexity of the overall system.
Clearly then, it can be seen that there exists a need for a process for the production of carbon monoxide-free nitrogen that does not suffer from the weaknesses of the above-mentioned schemes. A desirable process would be one in which the concentration of the carbon monoxide in the nitrogen product is reduced directly within the distillation system, thus removing the need for additional processing steps.