|Cryogenic air separation processes are routinely used in medium to large scale plants to produce nitrogen, oxygen, and argon as gases and/ or liquid products.
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, 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, increases capital and energy efficiency. Making these products in liquid form 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:
| Dry air is relatively uniform in composition, with primary constituents as shown below. Ambient air, may have up to about 5% (by volume) water content and may contain a number of other gases (usually in trace amounts) that are removed at one or more points in the air separation and product purification system.
General Process Description – Cryogenic Air Separation:
| There are numerous variations in air separation cycles used to make industrial gas products. Design variations arise from differences in user requirements. Process cycles are somewhat different depending upon how many products are desired (either nitrogen or oxygen; both oxygen and nitrogen; or nitrogen, oxygen and argon); the required product purities; the gaseous product delivery pressures desired; and whether one or more products will be produced and stored in liquid form.
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.
The cryogenic air separation flow diagram shown below illustrates (in a generic fashion) many of the important steps in producing nitrogen, oxygen and argon as both gas and liquid products. It does not represent any particular plant.
Steps in Cryogenic Air Separation:
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, 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 compressed air 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 air, the temperature of the compressed air is sometimes well above optimum for maximizing the efficiency of downstream unit operations.
Consequently, the compressed air is often cooled to a somewhat lower temperature in a mechanical refrigeration system. In addition to lowering and stabilizing the inlet temperature to downstream compression and heat exchange systems, which enhances the efficiency and stability of the overall air separation process, reducing the compressed air temperature allows removal of additional water vapor by condensation, reducing the water-removal load in molecular sieve pre-purification equipment. with a mechanical refrigeration system or, In some cases, cooling may be accomplished with a direct contact aftercooler system (DCAC) instead of mechanical refrigeration. DCAC systems utilize cool, dry 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.
The next major step is removal of impurities, in particular, but not limited to, residual water vapor plus carbon dioxide.
These components of air must be removed to meet product quality specifications. In addition, they must be removed prior the air entering the distillation portion of the plant; because very low temperatures would cause the water and carbon dioxide to 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”.
In the “warm end” heat exchangers, the incoming air is cooled to a low enough temperature that the water vapor and carbon dioxide freeze out onto the walls of the heat exchanger air passages. At frequent intervals, a set of valves reverse the duty of the the air and waste gas passages. After a passage in the heat exchanger is switched from incoming air cooling to waste gas warming service, the very dry, partially-warmed waste gas evaporates the water and sublimes the carbon dioxide ices that were deposited during the last air cooling period. These gases return to the atmosphere, and after they have been fully removed, the passage is return to incoming air cooling service.
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, hydrocarbons are removed along with water vapor and carbon dioxide in the PPU.)
The next step is additional heat transfer against product and waste gas streams to bring the air feed to cryogenic temperature (approximately -300 degrees Fahrenheit or -185 degrees Celsius).
This cooling is done in brazed aluminum heat exchangers which allow the exchange of heat between the incoming air feed and cold product and waste gas streams exiting the separation 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 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, which are commonly called the “high” and “low” pressure columns. Nitrogen plants may have only one column, although many 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.
Argon has a boiling point similar to that of oxygen and will preferentially stay with the oxygen product. If high purity oxygen is required, argon must be removed from the distillation system.
Argon removal takes place at a point in the low pressure column where the concentration of argon is its highest level. The argon which is removed is usually processed in an additional “side-draw” crude argon distillation column that is integrated with the low pressure column. Crude argon may be vented, further processed on site, or collected as liquid and shipped to a remote “argon refinery”. The choice depends upon the quantity of argon available and economic analysis of the various alternatives.
Pure argon is typically 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. These small units chemically combine the oxygen with hydrogen in a catalyst-containing vessel. The resultant water is easily removed (after cooling) in a molecular sieve drier. The 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. 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.
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 reduces its temperature more than would be the case with simple expansion across a valve. The energy produced by the expander is put to use to drive a process compressor, an electrical generator, or other energy-consuming device such as an oil pump or air blower.
Gaseous products typically exit the cold box (the insulated vessel containing the distillation columns and other equipment operating at very low temperatures) at relatively low pressures, often just over one atmosphere (absolute). In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process.
When products will be used at relatively low gauge pressure (up to several atmospheres) plants can be designed and operated to produce product at the required pressure. In many cases, however, it is more cost effective to produce the product at low pressure and compress the product gas to the required delivery pressure(s).
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 at just above delivery pressure, against incoming air which has been boosted in pressure to allow 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.
“Pumped LOX” systems are most applicable when there is fairly constant product demand. The heat for vaporizing and warming the vaporized LOX is drawn from the air feed, which is partially condensed and sent to the distillation system, Rapid changes in oxygen demand will negatively affect plant performance, as each sudden change will tend to “bounce” the distillation columns.
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. 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.
| When a large percentage of plant production must be produced as liquid product(s), a supplemental refrigeration unit must be added to (or integrated into) a basic air separation plant.
These units are called liquefiers and most use nitrogen as the primary working fluid. The required liquefier capacity is determined by considering the anticipated average daily demand for bulk liquid products and the need to produce some additional liquid to back up 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 typically 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 extra heat exchanger circuit to liquefy gaseous oxygen while 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. On the other hand, they have a potential disadvantage – the liquefier cannot be shut down independently of the air separation unit
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 operating the liquefier can be advantageous:
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.