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Fuel Processing Technology 70Ž. 2001 115–134 www.elsevier.comrlocaterfuproc A review of air separation technologies and their integration with energy conversion processes A.R. Smith), J. Klosek Air Products and Chemicals, Inc., Allentown, PA 18195, USA Received 1 April 2000; received in revised form 1 October 2000; accepted 30 November 2000 Abstract Cryogenic air separation technology has been successfully employed for many years to supply oxygen for the gasification of a wide range of hydrocarbon feedstocks to generate synthesis gas for the production of fuels, chemicals and other valuable products. Examples include the conversion of liquid and solid refinery waste streams into hydrogen for use within the refinery along with the coproduction of electricity, and the growing interest in gas-to-liquidsŽ. GTL processes which convert natural gas into synthetic crude oil, waxes and fuels. Recently, increased attention has been focused on methods of integrating the oxygen production process with the downstream hydrocarbon processing units to reduce facility cost or increase efficiency. A review of traditional and developing processes to generate oxygen is presented, along with integration schemes to improve the economics of these facilities. q 2001 Published by Elsevier Science B.V. Keywords: Air separation; Gas-to-liquids; Gasification; Oxygen; Partial oxidation; Power; Synthesis gas 1. Introduction Two questions frequently asked by industrial gasŽ. oxygen, nitrogen, argon con- sumers are: v which technology is optimum for a specific rate, purity, pressure and use pattern? v what are the integration opportunities between the air separation unit and other processes? ) Corresponding author. Tel.: q1-610-481-6628; fax: q1-610-481-6489. E-mail address: [email protected]Ž. A.R. Smith . 0378-3820r01r$ - see front matter q2001 Published by Elsevier Science B.V. PII: S0378-3820Ž. 01 00131-X 116 A.R. Smith, J. KlosekrFuel Processing Technology 70() 2001 115–134 This paper describes the processes for separating industrial gases from air and notes economic or other limits for each process. Integration opportunities for cryogenic and non-cryogenic industrial gas processes are presented for facilities incorporating gas turbines or having large amounts of excess heat or energy available as a byproduct. The processes associated with these characteristics are generally the oxygen-blown, partial oxidation, Fischer–TropschŽ. F–T plants producing chemicals, fuels, power, synthesis gasŽ. syngas or synthetic crude oil products. A brief review of integration solutions for cryogenic industrial gas plants and integrated gasification combined cycleŽ. IGCC power production facilities is presented as background information. A few conceptual integra- tion opportunities for cryogenic and non-cryogenic industrial gas plants and energy conversion processes are described. 2. Non-cryogenic industrial gas processes 2.1. Adsorption Adsorption processes are based on the ability of some natural and synthetic materials to preferentially adsorb nitrogen. In the case of zeolites, non-uniform electric fields exist in the void spaces of the material, causing preferential adsorption of molecules, which are more polarizable as those that have greater electrostatic quadrapolar moments. Thus, in air separation, nitrogen molecules are more strongly adsorbed than oxygen or argon molecules. As air is passed through a bed of zeolitic material, nitrogen is retained and an oxygen-rich stream exits the bed. Carbon molecular sieves have pore sizes on the same order of magnitude as the size of air molecules. Since oxygen molecules are slightly smaller than nitrogen molecules, they diffuse more quickly into the cavities of the adsorbent. Thus, carbon molecular sieves are selective for oxygen and zeolites are selective for nitrogen. Zeolites are typically used in adsorption-based processes for oxygen production. A typical flowsheet is shown in Fig. 1. Pressurized air enters a vessel containing the adsorbent. Nitrogen is adsorbed and an oxygen-rich effluent stream is produced until the bed has been saturated with nitrogen. At this point, the feed air is switched to a fresh vessel and regeneration of the first bed can begin. Regeneration can be accomplished by heating the bed or by reducing the pressure in the bed, which reduces the equilibrium nitrogen holding capacity of the adsorbent. Heat addition is commonly referred to as temperature swing adsorptionŽ. TSA , and pressure reduction as pressure or vacuum swing adsorptionŽ. PSA or VSA . The faster cycle time and simplified operation associated with pressure reduction usually makes it the process of choice for air separation. Variations in the process that effect operating efficiency include separate pretreatment of the air to remove water and carbon dioxide, multiple beds to permit pressure energy recovery during bed switching, and vacuum operation during depressurization. Opti- mization of the system is based on product flow, purity and pressure, energy cost and expected operating life. Oxygen purity is typically 93–95 vol.%. Due to the cyclic nature of the adsorption process, bed size is the controlling factor in capital cost. Since A.R. Smith, J. KlosekrFuel Processing Technology 70() 2001 115–134 117 Fig. 1. Adsorption-based air separation process. production is proportional to bed volume, capital costs increase more rapidly as a function of production rate compared to cryogenic plants. 2.2. Chemical processes A number of materials have the ability to absorb oxygen at one set of pressure and temperature conditions, and desorb the oxygen at a different set of conditions. One such process that Air Products and ChemicalsŽ. Allentown, PA investigated in the early 1990s was MOLTOXe, a molten salt chemical process depicted in Fig. 2. The process variation shown is based on absorption of oxygen by a circulating molten salt stream, Fig. 2. Chemical air separation process. 118 A.R. Smith, J. KlosekrFuel Processing Technology 70() 2001 115–134 followed by desorption through a combination of heat and pressure reduction of the salt stream. Air is compressed from 20 to 185 psia and treated to remove water and carbon dioxide in an adsorbent-based system. Water and carbon dioxide would both degrade the salt if not removed at this stage. Air flows through an adsorbent bed until bed saturation is reached. The beds are switched and the saturated bed is regenerated by dry nitrogen from the process. The clean, dry air is heated against returning product streams to between 9008F and 12008F in the main heat exchangers. The hot air flows to the bottom of the absorber where it contacts molten liquid salt. The oxygen in the air reacts chemically with the salt and is removed with the liquid salt leaving the bottom of the absorber. The oxygen-bearing salt is heat interchanged with oxygen-free salt and further heated before being reduced in pressure and flowing to the desorber. Gaseous oxygen leaves the top of the desorber, while oxygen-lean salt is removed from the bottom of the desorber, heat interchanged and sent to the top of the absorber vessel to close the loop. The hot oxygen and hot nitrogen streams enter the main heat exchanger and are cooled against feed air. The oxygen is compressed to delivery pressure, while a portion of the nitrogen is used to regenerate the air pretreatment system. The major process advantage of the TSA-based system is that air has only to be compressed to a pressure that overcomes pressure drop through the air pretreatment and heat exchanger, thus reducing the amount of air compression power compared to a cryogenic plant. A source of thermal energy must be available to liberate the salt via heating. Air Products and Chemicals operated a small-scale pilot unit that verified process conditionsŽ 99.9% oxygen purity at expected salt loading. , however, corrosion of the saltroxygen two-phase areas of the facility was determined to be an economic problem. 2.3. Polymeric membranes Membrane processes using polymeric materials are based on the difference in rates of diffusion of oxygen and nitrogen through a membrane which separates high-pressure and low-pressure process streams. Flux and selectivity are the two properties that determine the economics of membrane systems, and both are functions of the specific membrane material. Flux determines the membrane surface area, and is a function of the pressure difference divided by the membrane thickness. A constant of proportionality that varies with the type of membrane is called the permeability. Selectivity is the ratio of the permeabilities of the gases to be separated. Due to the smaller size of the oxygen molecule, most membrane materials are more permeable to oxygen than to nitrogen. Membrane systems are usually limited to the production of oxygen enriched air Ž.25–50% oxygen . Active or facilitated transport membranes, which incorporate an oxygen-complexing agent to increase oxygen selectivity, are a potential means to increase the oxygen purity from membrane systems, assuming oxygen compatible membrane materials are also available. A typical membrane system is shown in Fig. 3. A major benefit of membrane separation is the simple, continuous nature of the process and operation at near ambient conditions. An air blower supplies enough head pressure to overcome pressure drop through the filters, membrane tubes and piping. Membrane materials are usually assembled into cylindrical modules that are manifolded together to provide the required A.R. Smith, J. KlosekrFuel Processing Technology 70() 2001 115–134 119 Fig. 3. Polymeric membrane air separation process. production capacity. Oxygen permeates through a fiberŽ. hollow fiber type or through sheetsŽ. spiral wound type and is withdrawn as product. A vacuum pump typically maintains the pressure difference across the membrane and delivers oxygen at the required pressure. Carbon dioxide and water usually appear in the oxygen enriched air product, since they are more permeable than oxygen for most membrane materials. As with adsorption systems, capital is essentially a linear function of production rate and product backup is typically not available without a separate liquid oxygen storage tank and delivery support system.