Producing Nitrogen via Pressure Swing Adsorption Article

Pressure swing adsorption (PSA) can be a cost-effective method of onsite nitrogen generation for a wide range of purity and flow requirements. - American Institute of Chemical Engineers article from June 2012
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  Reactions and Separations Producing Nitrogen via Pressure Swing dsorption SVETLANA IVANOVA ROBERT LEWIS IR PRODUCTS Pressure sw ing adsorption P SA can be a cost-e ff ec tive meth od of ons it e nitrogen ge nera ti on fo r a wide ran ge of purity and flow requirements. N itrogen gas is a staple oflhe chemical industry. Because it is an iuet1 gas, nitrogen is suitable for a wide range of applications covering various aspects of chemical manufacnrring, processing, handling, and shipping. Due to its low reactivity, nitrogen is an excellent blanketing and purging gas Ihal can be used 10 protect val u- able products from hannful contaminants. It also enables the safe storage and use offialIlluable compOlmd s, and can help prevent combu stible du st explosions. Nitrogen gas can be used to remove contaminants from process streams through methods s uch as stripping and spargillg. Because of the widespread and growing use of nitrog en illihe chemical process industries CPI  , industrial gas com panies have been continually improv ing methods of nitrogen production and supply to make them more efficient, co st-effective, and convenient for chemical processors. Multiple nitrog en tecl molog ies and supply mode s now exi st to m ee t a range of specifications, including purity, usage pattem , portability, footprint, and power con sumption. Choo sing among supply options can be a challenge. On site nitrogen generators, such as pr essure swing ad sOlption PSA ) or mem brane system s, can be more co st-effective than traditional CIY O- genic di stillation or stor ed liquid nitrogen, particularly if an extremely high purity e.g., 99.99 99 ) is not required. Generating nitrogen gas hldu s trialnitro g en gas can be produced by either cryogenic fractional distillation of liquefied air, or separation of gaseous air using ad sOlption or penneation . Gemlatl en gi nee r Carl v on Linde developed clyogenic di stillation of f----D Aftec £ .f Adsorption Towers Buffer Vessel Filter Gas Product Nitrogen to Customer le Condensate Condensate Ai r Buffer Tank Vent Air Gaseous Nitrogen ... Figure 1. PSA systems can provide a re liab le, ow-cost nitrogen supp ly o meet a wide variety of process requ ir ements. 8 www .aiche.orgfcep June2012 EP Copy light @2012Amelican Institute of Chemical Engineers AIChE)  air, the oldest method of nitrogen production, in 1895 Cryogenic distillation is still used today in large conullerc ia l air separation plants, and acc OlUl ts for nearly 65-70  of the total nitrogen production (2). Leonard Pool (the fOlUlder of Air Products) introduced the concept of generating industrial gases OIlSite in the early 1940s. Onsite cryogenic plants were built on or near the user  s site, and the product was delivered by pipe- line. TIlls method provided a low-cost, reliable supply for large-volume users of industrial gases. However, due to the relatively high capital and power costs associated wi th OIlSi te cryogenic plants, smaller-voltmle users were typically lim ited to liquid nitrogen supply delivered by vacuum-insulated trucks. TIle stored liquid nitrogen was vaporized and piped as needed. In the 1980 s, altemative methods of onsite gaseous nitrogen ge ne ration, such as PS and membra ne separation, came into practice. Initially, these teclmiques we re suit- able on ly for small-volume, l ow -purity applications. Today, however, PSA a nd membrane systems are an efficient supply mode for a variety of volumes, purity requirements, and usage pa tt ems. PSA systems operate on the principle of adsOlption, whereas membrane systen lS separate based on selec ti ve penneation. Pressllre swing adsorption hI the PSA process (Figure 1), compressed air first passes through a combina ti on of filters to remove entrained oil a nd water. The purified air is then directed to one of two adsOlption vessels that are packed with carbon molecular sieves ( eMS ). The itnpurities, such as carbon dioxide and residual mo is ture, are adsorbed by the CMS at the entrance of the adsorbent bed. At l i gh pressures, the CMS selec ti vely adsorbs oxygen, allowing nitrogen to pass through at the desired purity leve l. WlJ.i.le one vessel is producitlg nitrogen, the second vessel T he two most im portant factors to consider when choosing among nitrogen supp ly options are the requi red nitrogen purity and the required nitrogen flowrate. is depressurized to remove the adsorbed oxygen, which is then vented to the atmosphere. TIle automatic Cyclitlg of adsoiption and desOlption between the two beds enables the contitmous production of IJ.i.tro gen. A large range of B ow a nd purity combina tiOi lS can be met by adjusting the size of the air compressor and adsOlp tion vess el s containing the CMS. PSAs can eco nonJ.i. cally produce nitrogen gas at fiowrates from le ss than 5,000 scfh to greater th an 60,000 SCtll, a nd at purities rangitlg from 95 to 99.9995 . emb rane separation  Membrane systems operate on the principle of selective gas pennea ti on. A typical mem brane process (Figure 2) uses several membrane module s, each con taitJ.i.n g thousa nd s of hollow fi bers. Every molecule passitlg through the fibers has a characteris ti c penlleation rate that is a function of its ability to dissolve itl, diffilse through, a nd d is solve out of the hollow- fi ber membrane. The pel.111eation rate is the product of the solubility a nd dif - fusivity rates of the gas itlthe membrane. Wilen compressed air passes through the fiber s, oxygen, wat er vapor, and carbon dioxide are selectively removed, creating a IJ.i.trogen- rich product stream. Membrane systems typically produce nitrogen with a purity of 95-99.5 , a nd , itl some cases, greater than 99.9 nitrogen purity. Product purity depends on the feed purity, available differential partial pressme, and desired recovery le vel. Article continues on nerr page Permeate Compressor Fitters Condensate Electric Heater Nitrogen Production Membranes ig u re 2. Membrane sys ems use se l ective gas permeation to generate nearly pure nitroge n. Cop yright 2 12 American Institute of Chem ical Engi nee rs AIChE) Oxygen Analyzer Nitrogen to Custome r Nitrogen Buffer V. . ~ EP J...oe 2012 \o .aiche.orgIcep 9  Reactions and Separations 100% 99% ~ , - 98% 97% I 10 350 100 3,500 FJowrate 1,000 35,000 I 10,000 NlT1 th 350,000 setth ment s mu st be detenlli.ned, as well as the plant's day- t o-day nitrogen flow require ment s. The se two factors wi ll help deter mine the best syst em for nitrogen s upply (Figure 3). itrogen u r ty Nitroge n provides s afety and quality for c hemi cal mallufac hiring processes (3). Because nitrogen is an inert gas, it is used to protect s en sitive materials and pre ve nt fires and explos ion s. It can be a challenge 10 detemune the m ost suitable nitrogen purity. H ow ever, nitro ge n co sts can be reduced if a low purity is acceptable. PSA can produce nitrogen at a range of purities. The l ower the purity, the lo we r th e lULi co st of the nitrog en ã Cryogenic Uquid Delivered D Pecmeation Onsile Generation ã Adsorption Onsile Generation (Figure 4). For example , the quality of s ome veg etable oils can be maintained by blanketing and/or s parging with 99.5 nitrogen puri ty. TItis can be achiev ed easily by PSA. ã Cryogenic Onsite Generation ... Figure 3 Choosing between nitrogen supply options depends on fIowrate and purity requ i rements. 3 } z Fl owrate ... Figlre 4. Nitrogen p urity and fIowrate req uirements can affect n troge n cos t. Higher purity comes at a price , but high er vo l ume genera l ly reduces th e unit cost of n itrog en. For maxi m um sav i ngs, use a nitrogen purity no h gher tha n he application req u res . When to select PSA Wi th multiple nitrog en supply optio ns and tec hn ol ogies available, selecting the right sys tem for a specific application can see m complicated . H oweve r, the two most impol1ant factors to con s ider w hen choosing anlOng PSA onsite gen eration, penneation membrane sys tem s, cryogenic distilla tion, and liquid delivery are the required nitrogen purity and the required nitrogen flowrate. TIle nitrogen purity nece ssary to meet the application's safety and product quality require- 4 www .aiche.orgfcep June2012 C EP TIle nitrogen purity required to blanket a flallunable material can be detennined based on the material's limiting oxygen co ncentration (LOC ) or lower flanurutbility limit (LFL). LOC va lues for lrutny che micals can be found in chemical engineering and che mi s try handbooks, as well as in the Na tional Fire Protection Association 's NFPA 69: Standard on Explo s ion Prevention Sys t ems (4). Table I lists the LOC for a few conunon chemical s. A s ub stan ce's LFL can be fmmd on the safety data s heet (S DS ) pro v ided by the lrutnufacttrrer. NFPA 69 requires hazardous proce sses to ope rate well below the LO C and LFL , typically at around 60 of th ese v alue s. For example , a fialillnable material with an LO C of 10 wo uld require an atm os phere of 94% nitro gen to meet NFPA guidelines. However, a more-conservative 25 of the LOC , or 97. 5 nitro ge n, adds a larger safety buffer. A purity of94-97.5 can be supplied by a PSA system . Table 1 LOe fo r some common materials at ambient temperature and pressure A table of LOe for many more malenals can be found in NFPA 69 (4). LOC , vol. 4 O 2 Prop yl ene Oxide 5.8 Methanol 8.0 Ethanol 8.5 Acetone 9.5 e   10.1 Vi n yl Chloride 13.4 Copyright@201 2American In stitute of Chemical En gineers (AIChE)  Steady Periodic Erratic ~ 0 ã z TI me R gu re 5. Plotting n itrog en flowrale versus lime reveals the application 's flo w pattern. Nitrogen demand p t t  m s PSA nitrogen generators operate mo st economically at their full design capacity. Size optimization is critical for maximizing the economic benefit ofa PSA system. For this reason, it is important to lUlder stand both the utilization rate i. e , hours of ope ration per month) as well as the nitrogen flow pattem. Identifying the fl ow pa tt em is cmcial if instantaneous flowrates vary widely. It can be di ffi cult to predict the exact nitrogen usage pattem f or a n ew process. Often, a new process is started up using liquid nitrogen and operated with a fl ow recorder installed on the main nitrogen supply line for an extended peri od of time e.g., 2 -4 weeks). TIus will provide an accu rate picnu'e of the lutrogen fl ow pattem. Nitrogen fl ow pattem s at chellucal plants typically fit into one of thr ee ca tegories: steady, pe riodic, or erratic (Fig ure 5). A PSA nitrogen system is an excellent fit for a steady flow pa tt em , where the usage rate as a function of time is essentially constant. TIle PSA lIlut size can easily be matched to the me asured or estimated usage rate. Fur thel111 0re, the unit will operate continuously at or near its full capacity, wluch m ake s nitrogen production the most economical. A PSA system is not a good fit for a process with a peri odic fl ow pattem, where the flow is characterized by peaks and valleys. An onsite generat or feeding tlus fl ow denland, particularly if it is sized for a peak fl ow , will operate at partial capacity or idle for a significant alllOllllt of time. lhi s results in operational inefficiency and lugh operating costs. However, if the duration of the valleys is short, a PSA c om bllled with a large product buffer tank may be sufficient. An erratic fl ow pattern represents the most COi lUll on sce nario chenucal plants. lhi s flow pattern has a substantial continu ou s fl ow with some s h0l1 irregularities. A PSA system can be sized to handle mo st of the nitrogen requirement s, supplemented with liquid nitrogen during peak-demand pe riods (Figure 6). A properly designed sy stenl ca n aclueve a utilization rate of90 or better for tlus demand pattelll. Using nitrogen safely Ni trogen is often nu stakenly considered harnlless because it is nontoxic and largely lllert. However, lutrogen can act as an as ph yxiant by displaclllg oxygen in air to levels below those required f or sruviva l. TIlerefore, use caution when handllllg lutrogen. Reference 5 provides more llUor mation about the safe handllllg of lutrogen. ypical applications Because lutrogen generated by PSA generally contallls small alllOllllts of oxygen, some types of chenucal processes lend themselves better than others to tlus type of supply. Manufacturing pressure-sensitive adhesives. The manufacture of pre ssure-sensi ti ve adhesives lllVol ves flammable solvents and po wders that present some safety ha zar ds. In tlus process, finely ground mbber and fillers are added to flammable or galuc solvents in agitated tanks. Because vapors from the solvents are typically flanunable III air, and the powders ca n generate significant sparks fi Oln the buildup of static electricity, .... Li quid Backup reacti on s involving these lnaterials ca n be dangerous III the presence of air i.e., 20.9  oxy gen ). For these reasons, an adhesives man uf acnll'er wa s com pelled t r ---- ~  _ _ - r_-- f__\   BufferV~   :. - --,1-   --\ ,-1 --- .- +   -- PSA Nitrogen Generator gure 6. The use of a PS generator with l iquid n itrogen backup for peak shaving can provide the optimal nitrogen supply system for e rra tic usage patterns. Copyright ~ 2012 American Institute of Chemical Engineers (AIChE) to take steps to reduce the risk of explosions and fires. First, the co mp any installed static electricity elllninators. Howe v er , as these devices do not provide complete protection, occasional sparklllg was still a risk. Co nsequentl y, the mallufac-  EP June2012 www.aiche.orgIcep 4
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