Non Cryogenic Processes of Air Separation

Non Cryogenic Processes of Air Separation 

Dry atmospheric air contains by volume 78.08 % of nitrogen, 20.95 % of oxygen, and 0.93 % of argon along with traces of a number of other gases (Fig 1). Atmospheric air can also contain varying amount of water vapour (depending upon humidity) and other gases produced by natural processes and human activities. There are two primary technologies to separate the gases of the air such as (i) cryogenic distillation, and (ii) non cryogenic separation processes. The non cryogenic processes are typically used to separate a single component from the atmospheric air.

Fig 1 Composition of atmospheric air

Non cryogenic air separation processes are near ambient temperature separation processes and are used for the production of either nitrogen or oxygen as gases. These processes are cost effective choices when demand of gases are relatively small and when very high purity of the gases is not needed. Non cryogenic air separation plants are compact and produce gaseous nitrogen which is typically 95.5 % to 99.5 % oxygen free or gaseous oxygen which is 90 % to 95.5 % pure.

Non-cryogenic plants are less energy efficient than cryogenic plants (for comparable product purity) but at the same time cost less to build. The physical size of the plant can be reduced as required purity is reduced, and the power needed to operate the unit is reduced as well.  Non-cryogenic plants are relatively quick and easy to start up and can be brought on line in less than half an hour. This is useful when product is not needed full time. Like cryogenic plants, non cryogenic air separation processes also start with compression of air.

Unlike cryogenic plants which use the difference between the boiling points of nitrogen and oxygen to separate and purify these products, non cryogenic air separation plants use physical property differences such as molecular structure, size, and mass to produce nitrogen and oxygen. Non cryogenic processes are based on either selective adsorption or permutation through membranes.

The most common technologies used for non cryogenic air separation plants are adsorption process technology, (ii) chemical process technologies, (iii) membrane separation technology, and (iv) ion transport membrane technology.

Adsorption process technology

Adsorption process technology is based on the ability of some natural and synthetic materials to preferentially adsorb either nitrogen or oxygen. This technology is used to produce either nitrogen or oxygen by passing compressed air at several atmospheric pressures through a vessel containing adsorbent materials. Adsorbents are chosen on the basis of their adsorption characteristics. Special adsorptive materials are used as a molecular sieve, preferentially adsorbing the target gas species. A desirable adsorbent has much greater affinity for non product molecules than for the product gas (nitrogen or oxygen).  This characteristic results in most of the molecules of the product gas passing through the bed and into the product stream, while other components of the air are captured by the adsorbent.

Oxygen production plants using adsorption technology normally use zeolite molecular sieves to adsorb nitrogen, carbon dioxide, residual water vapour and other gases.  Typical oxygen delivery pressures leaving the plant are 1 atmosphere to 3 atmospheres.  Oxygen purity normally is typically in the range of 93 % to 95%, and is limited primarily by the argon content, which is normally in the range of 4.5 % to 5 %.

Nitrogen production plants using adsorption technology use an activated carbon molecular sieve material which removes oxygen and other undesired components by adsorption. Alternatively, a ‘de-oxo’ unit is added which catalytically combines hydrogen with the oxygen in the nitrogen product leaving the adsorption process, producing water. This water is removed by cooling and additional adsorption. Nitrogen is typically delivered from the production unit at pressures of 6 atmospheres to 8 atmospheres and at a purity of 95 % to 99.5 %.  If a higher purity is needed then both the equipment size and the ratio of air feed to product make is to go up.  The waste stream from a nitrogen production plant is enriched in oxygen which is frequently to around 40 % oxygen.  This stream is sometimes used for combustion enhancement or waste treatment equipment operation at the plant.

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 which have higher 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 leaves the bed. This is because zeolites are selective for nitrogen.

In case of carbon molecular sieves, they have pore sizes of 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.

The adsorbent process is basically a batch process, as the adsorbent bed needs periodic desorption. Hence production plants based on this technology normally have at least two adsorbent vessels to provide operational continuity.  At any time, one of the vessels is making product by adsorbing undesired components of the air, while the other vessel is undergoing regeneration by depressurization to atmospheric pressure. When the adsorbing vessel approaches saturation, a set of valves quickly switches the streams to other vessel.  A surge vessel (buffer vessel) downstream of the absorbers makes sure that delivery of the product gas is continuous.  While the two vessel system is most common, mono vessel or three vessel configurations are also sometimes used. The mono vessel system provides capital savings while the three vessel system provides for greater continuity of production.  A typical flowsheet for the air separation process based on adsorption technology is shown in Fig 2.

Fig 2 Air separation process based on adsorption technology

Pressurized air enters a vessel containing the adsorbent bed. Nitrogen / oxygen are adsorbed and an oxygen / nitrogen rich effluent stream is produced until the adsorbent bed has been saturated with nitrogen / oxygen. At this point, the feed air is switched to a fresh vessel and regeneration of the adsorbent bed in the first vessel can begin. Regeneration can be accomplished by heating the adsorbent bed or by reducing the pressure in the adsorbent bed, which reduces the equilibrium nitrogen / oxygen holding capacity of the adsorbent.

Heat addition is commonly referred to as temperature swing adsorption (TSA), and pressure reduction as pressure or vacuum swing adsorption (PSA or VSA). Air separation plants using vacuum is referred to as VPSA (vacuum pressure swing adsorption), VSA (vacuum swing adsorption) or PVSA (pressure vacuum swing adsorption) plants. It is used for the production of oxygen. The process cycle is similar to that of PSA plants except that vacuum pumps are used to reduce the desorption pressure. The lower desorption pressure reduces the inlet pressure. The faster cycle time and simplified operation associated with pressure reduction normally makes it the process of choice for the air separation.

A VPSA plant produces oxygen at around 0.2 atmospheres (gauge).  When higher oxygen delivery pressures are required, an oxygen booster compressor is added to the plant. Overall, VPSA plants are more costly but more energy efficient than PSA plants for the same product flow, pressure and purity conditions.

VPSA plants regenerate the sieve material under vacuum conditions. It results into more fully regenerated molecular sieve material. This sieve material is more selective than material subjected to the regeneration process in a PSA plant. As a result, a higher percentage of available oxygen is recovered which means that less air is to be processed.  Air compressor power is greatly reduced compared to a PSA plant because of lower air flow and lower compressor discharge pressure which is normally less than half an atmosphere (gauge). However there is an off-set to the air compression power savings due to power needed to operate the vacuum pump.

VPSA units are normally more cost effective than PSA units when the desired production rate is more than 20 tons per day. They are normally the most cost-effective oxygen production choice up to 60 tons per day provided high purity oxygen is not needed.  Above 60 tons per day, cryogenic plants are normally the oxygen production technology of choice, although in some cases, two VPSA plants allow for better matching of large step-changes in demand.

Variations in the process which have effect on the operating efficiency include separate pretreatment of the air to remove water and carbon dioxide, multiple vessels to permit pressure energy recovery during adsorbent bed switching, and vacuum operation during depressurization. Optimization of the system is based on product flow, purity and pressure, energy cost and expected operating life. Due to the cyclic nature of the adsorption process, adsorbent bed size is the controlling factor in capital cost. Since production is proportional to the volume of the adsorbent bed, capital costs increase more rapidly as a function of production rate compared to cryogenic plants.

Chemical process technologies

A number of materials have the ability to absorb oxygen at one set of pressure and temperature conditions, and to desorb the oxygen at a different set of conditions. One such process which was investigated in the early 1990s was MOLTOXe process, a molten salt chemical process. The process is shown in Fig 3.

Fig 3 Chemical process for air separation

The process variation shown is based on absorption of oxygen by a circulating molten salt stream, followed by desorption through a combination of heat and pressure reduction of the salt stream. Air is compressed from 1.4 atmospheres to 12.5 atmospheres and treated to remove water and carbon dioxide in an adsorbent-based system. Water and carbon dioxide both degrade the salt if not removed at this stage.

Air flows through an adsorbent bed until the bed saturation is reached. The adsorbent beds are switched and the saturated adsorbent bed is regenerated by dry nitrogen from the process. The clean, dry air is heated against returning product streams to between 480 deg C and 650 deg C 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 main process advantage of the TSA based system is that air has only to be compressed to a pressure which 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 is to be available to liberate the salt through heating. A small-scale pilot unit was operated which verified process conditions (99.9 % oxygen purity at expected salt loading), however, corrosion of the salt / oxygen two-phase areas of the facility was determined to be an economic problem.

Membrane separation technology

The process based on the membrane separation technology makes use of the different rates at which air gases diffuse through a polymer membrane. 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. Membrane separation technology uses tube bundles made of special polymers, frequently configured in a manner similar to a shell and tube heat exchanger.  The air separation principle is that different gases have different permeation rates through the polymer film.  A schematic of polymeric membrane process for air separation is shown in Fig 4.

Fig 4 Polymeric process for air separation

Flux and selectivity are the two properties which 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 which 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.

The air separation principle is that different gases have different permeation rates through the polymer film.  Oxygen along with water vapour and carbon dioxide are considered ‘fast gases’ that diffuse more rapidly through the tube walls than the ‘slow gases’ argon and nitrogen.  This allows dry air to be converted to a product which is an inert mix of mostly nitrogen gas and argon, and a low pressure ‘permeate’ or waste gas which is rich in oxygen, water vapour and carbon dioxide that is vented from the shell.

Atmospheric air is filtered, compressed to the required pressure, dried and then passed through a membrane module. The air components with the higher rate of diffusion (O2 and CO2) penetrate the polymer membrane fibres more quickly, resulting in a nitrogen-rich flow as the primary product. The purity of the N2 gas flow depends on the flow rate through the membrane module, reaching 93 % to 99.5% and more if operated efficiently.

Nitrogen product emerges from membrane units at close to the compressed air feed pressure. Since there are no moving parts in the separation process, membrane units can be rapidly activated when needed and shut down when they are not.

Membrane separation plants are normally made in standard size modules with nitrogen production ratings which depend upon the desired nitrogen purity.  For a given standard module nitrogen production rate increases with higher inlet air flow rates but at the same time the purity of nitrogen decreases. When the needed production capacity (at a specified purity level) is more than the largest standard module size, a number of smaller units are usually combined in a manifold to allow them to operate in parallel.

Membrane plants are cost effective for relatively low demand applications.  Since larger capacity plants are normally made up of multiple smaller capacity modules, membrane plants have a close to constant cost per plant of production capacity over a wide range of production rates. This is in contrast to the declining cost for marginal capacity that is typical with PSA nitrogen plants and cryogenic air separation nitrogen plants.

In case desired product is oxygen then the membrane systems are normally limited to the production of oxygen enriched air (25 % to 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.

Oxygen permeates through a fibre (hollow fibre type) or through sheets (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 normally appear in the oxygen enriched air product, since they are more permeable than oxygen for most membrane materials.

A major benefit of membrane separation is that it is a simple, continuous nature of the process which is operating 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 normally assembled into cylindrical modules which are manifolded together to provide the needed production capacity.

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. Membrane systems readily fit applications upto 20 tons per day, where air enrichment purities with water and carbon dioxide contaminants can be tolerated. This technology is newer than adsorption or cryogenics and improvements in materials could make membranes attractive for somewhat larger oxygen requirements. The fast start-up time, due to the near ambient operation, is especially attractive for oxygen-use systems than exhibit discontinuous usage patterns. The passive nature of the process is also appealing.

Membrane plants are cost effective for relatively low demand applications.  Since larger capacity plants are normally made up of multiple smaller capacity modules, membrane plants have a close to constant cost per plant of production capacity over a wide range of production rates. .

Ion transport membrane (ITM) technology

ITMs are solid inorganic oxide ceramic materials which produce oxygen by the passage of oxygen ions through the ceramic crystal structure. These systems operate at high temperatures, normally above 600 deg C. Oxygen molecules are converted to oxygen ions at the surface of the membrane and transported through the membrane by an applied electric voltage or oxygen partial pressure difference, then reform oxygen molecules after passing through the membrane material. Membrane materials can be fabricated into flat sheets or tubes. Fig 5 shows a simple schematic of an ion transport membrane air separation process.

Fig 5 Ion transport membrane air separation process

For large energy conversion processes the pressure difference transport driving force is the method of choice. Membranes, which operate by a pressure difference, are referred to as mixed conducting membranes since they conduct both oxygen ions and electrons. The oxygen ions travel through the ITM at very high flow rates and produce nearly pure oxygen on the permeate side of the membrane. The oxygen can be separated as a pure product, or another gas can be used to sweep on the permeate side of the membrane to produce a lower purity product. If a reactive sweep gas is used, an oxidative product can be produced directly, e.g. natural gas methane sweeps to make synthesis gas for gas-to-liquid (GTL) conversion.

Air is compressed and then heated to operating temperature by exchange against the hot process streams (non-permeate and oxygen product) and then auxiliary heat addition. In general, the heating of air can be done by either indirect heat exchange and / or direct firing of fuel. The oxygen stream is compressed to delivery pressure. The pressurized nitrogen enriched non-permeate stream is used elsewhere in balance of the energy conversion process, for example, expanded in an integrated gas turbine cycle to generate electric power.

The ITM oxygen process is suited to integration with power generation and energy conversion processes which need oxygen as a feedstock for combustion or gasification, or in any oxygen-based application with a need for power.

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