Reactions and Separations Producing Nitrogen via Pressure Swing Adsorption Svetlana Ivanova Pressure swing adsorption (PSA) can be a Robert Lewis Air Products cost-effective method of onsite nitrogen generation for a wide range of purity and flow requirements. itrogen gas is a staple of the chemical industry. effective, and convenient for chemical processors. Multiple Because it is an inert gas, nitrogen is suitable for a nitrogen technologies and supply modes now exist to meet a Nwide range of applications covering various aspects range of specifications, including purity, usage pattern, por- of chemical manufacturing, processing, handling, and tability, footprint, and power consumption. Choosing among shipping. Due to its low reactivity, nitrogen is an excellent supply options can be a challenge. Onsite nitrogen genera- blanketing and purging gas that can be used to protect valu- tors, such as pressure swing adsorption (PSA) or membrane able products from harmful contaminants. It also enables the systems, can be more cost-effective than traditional cryo- safe storage and use of flammable compounds, and can help genic distillation or stored liquid nitrogen, particularly if an prevent combustible dust explosions. Nitrogen gas can be extremely high purity (e.g., 99.9999%) is not required. used to remove contaminants from process streams through methods such as stripping and sparging. Generating nitrogen gas Because of the widespread and growing use of nitrogen Industrial nitrogen gas can be produced by either in the chemical process industries (CPI), industrial gas com- cryogenic fractional distillation of liquefied air, or separa- panies have been continually improving methods of nitrogen tion of gaseous air using adsorption or permeation. German production and supply to make them more efficient, cost- engineer Carl von Linde developed cryogenic distillation of Carbon Mist Filter After Eliminator Air Filter Gas Product Adsorption Buffer Compressor Nitrogen to Towers Vessel Air Customer Condensate Air Condensate Gaseous Nitrogen Air Buffer Tank Vent p Figure 1. PSA systems can provide a reliable, low-cost nitrogen supply to meet a wide variety of process requirements. 38 www.aiche.org/cep June 2012 CEP air, the oldest method of nitrogen production, in 1895 (1). The two most important factors Cryogenic distillation is still used today in large commercial to consider when choosing air separation plants, and accounts for nearly 65–70% of the total nitrogen production (2). among nitrogen supply options Leonard Pool (the founder of Air Products) introduced are the required nitrogen purity the concept of generating industrial gases onsite in the and the required nitrogen flowrate. early 1940s. Onsite cryogenic plants were built on or near the user’s site, and the product was delivered by pipe- line. This method provided a low-cost, reliable supply for is depressurized to remove the adsorbed oxygen, which is large-volume users of industrial gases. However, due to the then vented to the atmosphere. The automatic cycling of relatively high capital and power costs associated with onsite adsorption and desorption between the two beds enables the cryogenic plants, smaller-volume users were typically lim- continuous production of nitrogen. ited to liquid nitrogen supply delivered by vacuum-insulated A large range of flow and purity combinations can be trucks. The stored liquid nitrogen was vaporized and piped met by adjusting the size of the air compressor and adsorp- as needed. tion vessels containing the CMS. PSAs can economically In the 1980s, alternative methods of onsite gaseous produce nitrogen gas at flowrates from less than 5,000 scfh nitrogen generation, such as PSA and membrane separation, to greater than 60,000 scfh, and at purities ranging from came into practice. Initially, these techniques were suit- 95% to 99.9995%. able only for small-volume, low-purity applications. Today, Membrane separation. Membrane systems operate on however, PSA and membrane systems are an efficient supply the principle of selective gas permeation. A typical mem- mode for a variety of volumes, purity requirements, and brane process (Figure 2) uses several membrane modules, usage patterns. each containing thousands of hollow fibers. Every molecule PSA systems operate on the principle of adsorption, passing through the fibers has a characteristic permeation whereas membrane systems separate based on selective rate that is a function of its ability to dissolve in, diffuse permeation. through, and dissolve out of the hollow-fiber membrane. Pressure swing adsorption. In the PSA process The permeation rate is the product of the solubility and dif- (Figure 1), compressed air first passes through a combina- fusivity rates of the gas in the membrane. When compressed tion of filters to remove entrained oil and water. The purified air passes through the fibers, oxygen, water vapor, and air is then directed to one of two adsorption vessels that are carbon dioxide are selectively removed, creating a nitrogen- packed with carbon molecular sieves (CMS). The remaining rich product stream. impurities, such as carbon dioxide and residual moisture, Membrane systems typically produce nitrogen with a are adsorbed by the CMS at the entrance of the adsorbent purity of 95–99.5%, and, in some cases, greater than bed. At high pressures, the CMS selectively adsorbs oxygen, 99.9% nitrogen purity. Product purity depends on the feed allowing nitrogen to pass through at the desired purity level. purity, available differential partial pressure, and desired While one vessel is producing nitrogen, the second vessel recovery level. Article continues on next page Air Permeate Compressor Electric Nitrogen Heater Production Filters Membranes Nitrogen to Customer Condensate Nitrogen Oxygen Buffer Analyzer Vessel p Figure 2. Membrane systems use selective gas permeation to generate nearly pure nitrogen. CEP June 2012 www.aiche.org/cep 39 Reactions and Separations ments must be determined, as well as the plant’s day-to-day nitrogen flow require- ments. These two factors will help deter- mine the best system for nitrogen supply (Figure 3). 100% Nitrogen purity. Nitrogen provides safety and quality for chemical manufac- 99% turing processes (3). Because nitrogen is an inert gas, it is used to protect sensitive Purity materials and prevent fires and explo- 98% sions. It can be a challenge to determine the most suitable nitrogen purity. How- 97% ever, nitrogen costs can be reduced if a low purity is acceptable. 10 100 1,000 10,000 Nm³/h PSA can produce nitrogen at a range 350 3,500 35,000 350,000 scf/h of purities. The lower the purity, the Flowrate lower the unit cost of the nitrogen (Figure 4). For example, the quality of Cryogenic Liquid Delivered Adsorption Onsite Generation some vegetable oils can be maintained by Permeation Onsite Generation Cryogenic Onsite Generation blanketing and/or sparging with 99.5% nitrogen purity. This can be achieved p Figure 3. Choosing between nitrogen supply options depends on flowrate and purity requirements. easily by PSA. The nitrogen purity required to blanket a flammable material can be determined based on the material’s limiting oxygen concentration (LOC) or lower flammability limit (LFL). LOC values for many chemicals 99.999+% N2 can be found in chemical engineering and chemistry hand- books, as well as in the National Fire Protection Associa- tion’s NFPA 69: Standard on Explosion Prevention Systems 99.9% N2 (4). Table 1 lists the LOC for a few common chemicals. A substance’s LFL can be found on the safety data sheet (SDS) ogen Cost 99% N2 provided by the manufacturer. Nitr NFPA 69 requires hazardous processes to operate well 98% N2 below the LOC and LFL, typically at around 60% of these values. For example, a flammable material with an LOC of 10% would require an atmosphere of 94% nitrogen to meet Flowrate NFPA guidelines. However, a more-conservative 25% of the LOC, or 97.5% nitrogen, adds a larger safety buffer. A purity p Figure 4. Nitrogen purity and flowrate requirements can affect nitrogen of 94–97.5% can be supplied by a PSA system. cost. Higher purity comes at a price, but higher volume generally reduces the unit cost of nitrogen. For maximum savings, use a nitrogen purity no higher than the application requires. Table 1. LOC for some common materials at ambient temperature and pressure. A table of LOC for many more materials can be found in NFPA 69 (4). When to select PSA Material LOC, vol.% O With multiple nitrogen supply options and technologies 2 available, selecting the right system for a specific application Propylene Oxide 5.8 can seem complicated. However, the two most important Methanol 8.0 factors to consider when choosing among PSA onsite gen- Ethanol 8.5 eration, permeation membrane systems, cryogenic distilla- Acetone 9.5 tion, and liquid delivery are the required nitrogen purity and Benzene 10.1 the required nitrogen flowrate. The nitrogen purity necessary to meet the application’s safety and product quality require- Vinyl Chloride 13.4 40 www.aiche.org/cep June 2012 CEP Steady Periodic Erratic ogen Flow Nitr Time p Figure 5. Plotting nitrogen flowrate versus time reveals the application’s flow pattern. Nitrogen demand patterns. PSA nitrogen generators However, if the duration of the valleys is short, a PSA com- operate most economically at their full design capacity. Size bined with a large product buffer tank may be sufficient. optimization is critical for maximizing the economic benefit An erratic flow pattern represents the most common sce- of a PSA system. For this reason, it is important to under- nario in chemical plants. This flow pattern has a substantial stand both the utilization rate (i.e., hours of operation per continuous flow with some short irregularities. A PSA system month) as well as the nitrogen flow pattern. Identifying the can be sized to handle most of the nitrogen requirements, flow pattern is crucial if instantaneous flowrates vary widely.