Cryogenic air separation is the preferred technology for producing very high purity oxygen and nitrogen. It is the most cost effective technology for high production rate plants.  All plants producing liquefied industrial gas products utilize cryogenic technology. 

The complexity of the cryogenic air separation process, the physical sizes of equipment, and the energy required to operate the process all vary with the number of gaseous and liquid products to be produced, their required product purities and required delivery pressures. 

Nitrogen-only production plants are less complex and require less power to operate than an oxygen-only plant making the same amount of product.  Co-production of both products, when both are needed, is cost effective, in terms of both capital and energy utilization.  Producing liquid oxygen and liquid nitrogen, as opposed to directly-usable gaseous products, requires additional equipment and more than doubles the amount of power required per unit of delivered product. 

Argon production is economical only as a co-product with oxygen.  Making it at high purity adds to the physical size and complexity of the plant. 

Air - The Raw Material for Making Nitrogen, Oxygen and Argon:

General Process Description - Cryogenic Air Separation Units: 

All cryogenic air separation processes consist of a similar series of steps.  Variations in selected process configuration and pressure levels reflect the desired product mix (or mixes) and the priorities/ evaluation criteria of the user. Some process cycles minimize capital cost, some minimize energy usage, some maximize product recovery, and some allow maximum operating flexibility. 

This illustrative cryogenic air separation flow diagram illustrates (in a generic fashion) many of the important steps in separating air and purifying its primary constituents into nitrogen, oxygen and argon as gas and liquid products.  It does not represent any particular plant.

Steps in Cryogenic Air Separation and Process and Physical Configuration Options:

The first process step in any air separation plant is filtering, compressing, and cooling the incoming air.

In most cases the air is compressed to somewhere between 5 and 8 bar (about 75 to 115 psig), depending upon the intended product mix and desired product pressures. The compressed air is cooled, and much of the water vapor in the incoming air is condensed and removed, as the air passes through a series of interstage coolers plus an aftercooler following the final stage of compression.  

Because the final temperature of the air leaving the compression system is limited by the temperature of the available cooling medium (which in almost all cases is limited by the wet or dry bulb temperature of the ambient air) the temperature of the compressed air is often well above optimum temperature for maximum efficiency of downstream unit operations. Consequently, the air is usually cooled further by a mechanical refrigeration system.

This step provides several benefits. It allows removal of additional water vapor by condensation.  It also lowers and  stabilizes the inlet temperature to downstream systems, which enhances the efficiency and stability of the overall air separation process. In particular, it reduces the water-removal load in the molecular sieve pre-purification system.

In some cases, compressed air cooling may be accomplished, totally or in part, by a direct contact aftercooler system (DCAC). DCAC systems utilize cool, dry, nitrogen-rich waste gas to chill a a circulating cooling water stream in a "chill tower", and then use the chilled water stream to cool the compressed air in a second tower.  

Following compression and cooling, he next major step in the air separation process is removal of impurities, in particular, residual water vapor plus carbon dioxide. 

These components of the incoming air feed must be removed to meet product quality specifications. The water vapor and carbon dioxide must be removed prior the air entering the cryogenic distillation portion of the plant; because at very low temperatures they would freeze and deposit on the surfaces within the process equipment. 

There are two basic approaches to removing the water vapor and carbon dioxide -  "molecular sieve units" and "reversing exchangers".  

• Almost all new air separation plants employ a “molecular sieve” "pre-purification unit" (PPU) to remove carbon dioxide and water from the incoming air by adsorbing these molecules onto the surface of "molecular sieve" materials at near-ambient temperature. The mix of adsorbent materials in these units can also be easily adjusted to remove other contaminants, such as hydrocarbons, which may be found in an industrial environment. The adsorbent materials are typically contained in two identical vessels; one of which is used to purify the incoming air while the other is being regenerated using clean waste gas.  The two beds switch service at frequent intervals.  Molecular sieve pre-purification is the natural choice when a high ratio of nitrogen recovery is desired.

• The other approach is to use “reversing” heat exchangers to remove water and CO2.  While often thought of as "old" technology, reversing exchangers can be more cost effective for smaller production rate nitrogen or oxygen plants.  In plants which utilize reversing heat exchangers, the cool-down of the compressed air feed is done in two sets of brazed aluminum heat exchangers.

The incoming air is cooled in "warm end" heat exchangers to a low enough temperature that the water vapor and carbon dioxide freeze out onto the walls of the  heat exchanger.  At frequent intervals, a set of valves reverse the duty of the the air and waste gas passages. After switching, very dry,partially-warmed waste gas evaporates the water and sublimes the carbon dioxide ices that were deposited during the air cooling period.  These gases are returned to the atmosphere, and after they have been fully removed, the reversing heat exchanger is ready for another reversal of passage duty.  

When reversing heat exchangers are used, cold absorption units are installed to remove any hydrocarbons which make their way into the distillation system. (When a molecular sieve "front end" is used, hydrocarbon impurities are removed along with water vapor and carbon dioxide, in the PPU.) 

The next step in cryogenic air separation is additional heat transfer, between product and waste gas streams and the incoming air, which bring the air feed to cryogenic temperature (approximately -300 degrees Fahrenheit or -185 degrees Celsius). 

This cooling occurs in brazed aluminum heat exchangers which exchange heat between the incoming air feed and cold product and waste gas streams exiting the cryogenic distillation process.  The exiting gas streams are warmed to close-to-ambient air temperature.  Recovering refrigeration from the gaseous product streams and waste stream minimizes the amount of refrigeration that must be produced by the plant. 

The very cold temperatures needed for cryogenic distillation are created by a refrigeration process that includes expansion of one or more elevated pressure process streams.   

The next step in the cryogenic air separation / product purification process is distillation, which separates the air into desired products. 

To make oxygen as a product, the distillation system uses two distillation columns in series. These are commonly called the “high” and “low” pressure columns (or, alternatively, the "lower' and "upper" columns).  Nitrogen plants may have only one column, although some very high purity plants may have two.  Nitrogen leaves the top of each distillation column; oxygen leaves from the bottom.  Impure oxygen produced in the initial (higher pressure) column is further purified in the second, lower pressure column when it is a desire product.  If ultra-pure nitrogen is desired, the upper or low pressure column is used to eliminate essentially all of the oxygen not removed in the first stage of distillation. 

Argon has a boiling point which is similar to that of oxygen and it will preferentially stay with the oxygen product if only oxygen and nitrogen are desired as products. This limits the oxygen purity to a maximum of about 97% in a simple two-column system. If low purity oxygen is acceptable (e.g. for combustion enrichment) the oxygen purity can be as low as 95%.  Howerver, If high purity oxygen is required, argon must be removed from the distillation system. 

Argon removal, when needed or desired, takes place at a point in the low pressure column where the concentration of argon is highest.  The argon which is removed is processed in an additional "side-draw" crude argon distillation column which is integrated with the low pressure column. The removed impure (or "crude") argon stream may be vented, further processed on site to remove both oxygen and nitrogen to become "pure" argon, or collected as liquid and shipped to a remote "argon refinery". The choice depends primarily upon the quantity of argon available and economic analysis of the various alternatives. As a general rule, argon purification is most economically viable when at least 100 tons per day of oxygen is being produced.       

Pure argon is produced from crude argon by a multi-step process. The traditional approach is removal of  the two to three percent oxygen present in the crude argon in a “de-oxo” unit; which is a small multi-step set of processes, which chemically combine the oxygen with hydrogen in a catalyst-containing vessel and then removes the resultant water (after cooling) in a molecular sieve drier. The resulting oxygen-free argon stream is further processed in a "pure argon" distillation column to remove residual nitrogen and unreacted hydrogen.

Advances in packed-column distillation technology have created a second argon production option, totally cryogenic argon recovery that uses a very tall (but small diameter) distillation column to make the difficult argon/ oxygen separation. Argon by distillation requires many stages of distillation because of the relatively small difference in boiling points between oxygen and argon.

The amount of argon that can be produced by a plant is limited by the amount of oxygen processed in the distillation system; plus a number of other variables that affect the recovery percentage. These include the amount of oxygen produced as liquid and the steadiness of plant operating conditions. Due to the naturally-occurring ratio of gases in air, argon production cannot exceed 4.4% of the oxygen feed rate (by volume) or 5.5% by weight.  

The cold gaseous products and waste streams that emerge from the air separation columns are routed back through the front end heat exchangers.  As they are warmed to near-ambient temperature, they chill the incoming air.  As noted previously, the heat exchange between feed and product streams minimizes the net refrigeration load on the plant and, therefore, energy consumption. 

Refrigeration is produced at cryogenic temperature levels to compensate for heat leak into the cold equipment and for imperfect heat exchange between incoming and outgoing gaseous streams. 

Cryogenic air separation plants use a refrigeration cycle that is similar, in principle, to that used in home and automobile air conditioning systems.  One or more elevated pressure streams (which may be nitrogen, waste gas, feed gas, or product gas, depending upon the type of plant) are reduced in pressure, which chills the stream. To  maximize chilling and plant energy efficiency, the pressure reduction (or expansion) takes place inside an expander (a form of turbine).  Removing energy from the gas stream during expansion reduces its temperature more than would be the case with simple expansion across a valve.  The energy produced by the expander may be used to drive a process compressor, an electrical generator, or other energy-consuming device such as an oil pump or air blower. 

Gaseous products from a cryogenic oxygen plant / air separation unit typically exit the cold box (the insulated vessel containing the distillation columns and other equipment operating at very low temperatures) at close to atmospheric temperature, but at relatively low pressure; often just over one atmosphere (absolute).  In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process. 

While lower pressure favors lower separation power requirements, if the products must be delivered at higher pressure, either product compressors will be needed or one of various cycle options can be used to supply nitrogen or oxygen at higher delivery pressure directly from the cold box. By eliminating a product compressor and its power, these higher delivery pressure processes can be, on an overall basis, more cost effective than separation followed by compression. 

UIG will assess available process options to ensure that the most cost effective option is offered for each client's unique situation. 

If gaseous oxygen is required at moderate pressure, a process option is to use a "LOX boil" or "pumped LOX" cycle.  These process cycles vaporize liquid oxygen, drawn from the bottom of the upper column, The withdrawn liquid is pumped to just above delivery pressure, and then vaporized against incoming air which has been boosted to an elevated pressure which allows it to partially condense against the vaporizing liquid oxygen. 

These cycles have appeal because they effectively substitute additional stages of air compression and a cryogenic pump for an oxygen compressor; which can result in a more compact and less expensive plant.   They can easily coproduce a limited amount of LOX to be used for plant backup.

The portions of the cryogenic air separation process that operate at very low temperatures, i.e., the distillation columns, heat exchangers and cold interconnecting piping, must be well insulated.  These items are  located inside sealed (and nitrogen purged) “cold boxes”, which are relatively tall structures that may be either rectangular or round in cross section. Depending upon the complexity and capacity of the plant, one or several cold boxes may be installed.  Cold boxes are "packed" with rock wool or perlite to provide insulation and minimize convection currents. Depending on plant type and capacity, cold boxes may measure 2 to 4 meters on a side and have a height of 15 to 60 meters.  They may be totally shop fabricated for rapid field erection, or the distillation columns, heat exchangers, and their interconnecting manifolds may shop fabricated for field assembly and erection.  This is done when a shop fabricated box would be too large or heavy to ship to the site.  

LIN assist plants are a special kind of cryogenic plant that can cost-effectively produce gaseous nitrogen at relatively low production rates.  They differ from "normal" cryogenic plants in that they do not have their own mechanical refrigeration system.  They effectively "import" the refrigeration required for on-site nitrogen production from a remote high-volume, high efficiency merchant liquid plant. They accomplish this by continuously injecting a small amount of liquid nitrogen into the distillation process, where the "imported" LIN provides reflux for distillation, then vaporizes and mixes with the locally-produced gaseous nitrogen, becoming part of the final product stream.  Use of LIN-assist instead of a mechanical refrigeration system simplifies the plant design, makes the system somewhat more compact, reduces capital cost and can, under the right conditions, provide better overall economics than either an all-bulk-liquid supply or a new cryogenic nitrogen plant with a standard internal refrigeration cycle. 

Liquefiers

These units are called liquefiers and they use nitrogen as the primary working fluid. The required liquefier capacity is determined by considering the anticipated daily demand for bulk liquid products to be sold into the local merchant liquid market, and the need to produce additional liquid for back up to any on-site gas customers served out of the same air separation plant.  Liquefier capacity may range from a small fraction of the air separation plant capacity up to the plant's maximum production capacity for oxygen plus nitrogen and argon. 

The basic process cycle used in liquefiers has been unchanged for decades.  The basic difference between newer and older liquefiers is that the maximum operating pressure rating of cryogenic heat exchangers has increased as cryogenic heat exchanger manufacturing technology has improved. A typical new liquefier can be more energy efficient than one built thirty years ago if it employs higher peak cycle pressures and higher efficiency expanders. 

A classic "stand alone" liquefier takes in near-ambient-temperature-and-pressure nitrogen, compresses it, cools it, then expands the high pressure stream to produce refrigeration.  In some liquefier systems a second refrigeration system using an environmentally-friendly form of refrigerant provides some of the higher temperature duty. 

A stand-alone liquefier cycle produces only liquid nitrogen.  If it is desired to produce liquid oxygen, and both the ASU and liquefier will be new units, a portion of the liquid nitrogen production will be sent to the ASU to provide the refrigeration which is needed to allow withdraw the desired amount of liquid oxygen from the cold box. 

If the liquefier is being added to an existing ASU, the ASU may not have been designed to allow high rates of liquid oxygen withdrawal. In that case, one solution is to add an extra heat exchanger circuit to liquefy gaseous oxygen by concurrently vaporizing liquid nitrogen. 

In highly integrated air separation and liquefaction plants, most if not all of the refrigeration for both air separation and product liquefaction is produced in the liquefier section.  Refrigeration is transferred to the air separation section of the plant through heat exchangers and injection of liquid nitrogen as distillation column reflux.  Highly integrated merchant liquid production plants are less expensive to build and more thermodynamically efficient.  They can be very flexible in the sense of allowing production of varying mixes of liquid nitrogen and liquid oxygen.

When a totally new air separation plant is designed, an important question to address is whether the ASU and NLU (Nitrogen Liquefier Unit) will typically operate in tandem, or whether independent operation may be desirable. Bulk liquid only plants are good candidates for close integration with the air separation process cycle. "Piggyback" plants with substantial pipelined gas demand may want the ability to operate independently of the liquefier. 

Being able to operate the ASU without also also operating the liquefier can be advantageous:

• When liquid inventories are at high levels but a pipeline-supplied gaseous oxygen customer continues to require a large amount of product, or

• When total liquid demand is consistently less than the full plant capacity.  In this case, plants with independent liquefiers may be operated in what is commonly called a "campaign" mode - where periods of full capacity operation of the liquefier are alternated with periods when the liquefier is idled.

Campaign operations take advantage of the facts that liquefiers are most energy efficient when operating near full capacity and that shutdown and startup of an independent liquefier system can be done relatively easily and with little adverse impact on air separation plant operation.  When the efficiency savings available with campaign operation are coupled with production run timing that takes advantage of lower-cost power periods (nights, weekends, etc.), significant operating cost savings can be achieved versus constant operation at reduced liquid production rates.

More on Air Separation and Supply System Optimization:

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