Nitrogen gas is commonly used in a broad range of pharmaceutical manufacturing processes, and it is a critical component in the manufacturing of modern drug formulations. The inert gas protects the reactants and products from oxygen and/or water vapor to avoid degradation, and it is frequently employed to move a reaction mixture from one vessel to another. In many facilities, nitrogen is obtained from an outside supplier that generates it via the fractional distillation of air. Many fractional-distillation facilities are designed to generate oxygen for a broad range of basic industrial processes such as the smelting of iron ore, metal cutting, welding, and chemical reactions with nitrogen is obtained as a secondary product.
Nitrogen boils at –196 °C (–320 °F) and must be pressurized and/or liquefied to transport it to the desired location using specially designed containers. Refrigerated trucks are used to deliver the gas to the end-user's facility on a routine basis, and the gas is then stored on site. If the fraction-distillation facility is relatively close to the end-user's site, the transportation costs are relatively low. In many cases, the end-user's facility is distant from the fractional-distillation site, and transportation costs can dramatically increase the overall cost of the gas. Fractional-distillation facilities are typically located near major industrial facilities in urban areas. Because pharmaceutical manufacturing is labor intensive, many facilities are located in lower labor-cost areas that may not be in close proximity to a fractional-distillation site. Thus, the cost of the providing nitrogen to the facility may be extremely high (e.g., nitrogen provided to islands located in the Caribbean basin is generated on the mainland and shipped to desired location). Because fractional distillation and the transport of nitrogen may not be a cost-effective approach for supplying the gas to some facilities, other approaches may well create dramatic cost savings. This paper describes the use of in-house generation of nitrogen via pressure swing absorption methodology for providing industrial-scale quantities of nitrogen and explains how it can provide a cost-effective approach. In addition to the cost savings, in-house generation of nitrogen provides significant environmental benefits.
Basics of pressure-swing adsorbtionA pressure-swing adsorption (PSA) generator is used to separate nitrogen from oxygen under pressure on the basis of the preferential adsorption and desorption of oxygen on a solid surface. Pressurized air is passed through a vessel packed with a molecular sieve such as activated carbon or zeolite that absorbs oxygen while the nitrogen is allowed to pass out of the vessel. Once the molecular sieve is saturated with oxygen, the pressure is lowered and the gas that has been trapped is released to the atmosphere. Because very small particles are employed, the surface area is quite large and the molecular sieve has a large capacity.
A molecular sieve such as activated carbon (e.g., charcoal) has a very large surface area available for adsorption and is extremely porous. Due to its high degree of microporosity, 1 g of activated carbon may have a surface area in excess of 500 m2 (equivalent to the area of about 2 tennis courts) as determined by nitrogen gas adsorption.
To obtain a continuous flow of nitrogen and maximize system utility, two vessels are connected in parallel so that one vessel is providing nitrogen to the system while the other vessel is being regenerated. In addition to producing essentially continuous nitrogen, the use of two adsorbent vessels allows for pressure equalization, in which the gas from one vessel is used to pressurize the other vessel, reducing the overall cost of operation.