Cryogenic Air Separation and Liquefier Systems
Cryogenic air separation is the most cost effective technology for larger plants and for producing very high purity oxygen and nitrogen.
Cryogenic air separation processes are routinely used in large- or medium-scale plants to produce nitrogen, oxygen and argon as gases and/or liquid products. The energy required to operate cryogenic plants depends on the product mix and required product purities. Gas-producing plants use less power than those producing some or all of the product as liquid. More than twice as much power is required to produce a unit of product in liquid form than as a gas.
Cryogenic Air Separation. There are many variations in the air separation cycles used to make industrial gas products. Design choices are made depending upon:
How many products are desired: nitrogen; oxygen; both oxygen and nitrogen; or nitrogen, oxygen and argon.
Required product purities.
Gaseous product delivery pressures.
Whether one or more product must be produced in liquid form.
All cryogenic air separations begin with a similar series of steps. Subsequent variations reflect the desired product mix (or mixes) and the priorities or evaluation criteria of the user. Some designs minimize capital cost, some minimize energy usage, some maximize product recovery and some allow greater operating flexibility. The cryogenic air separation flow diagram (figure 1) has been drawn to illustrate many of the important steps in producing nitrogen, oxygen and argon, and how they relate to one another. It does not represent any particular plant.
Dry air, the raw material used to make oxygen, nitrogen and argon, is relatively uniform in composition.
The first process step in any air separation plant is filtering and compressing air (most commonly, to about 90 psig, or 6 bar). The compressed air then is cooled to close-to-ambient temperature by passing through water- or air-cooled heat exchangers. Sometimes, it is cooled a bit more in a mechanical refrigeration system. Condensed water is removed from the air as it is compressed and then cooled.
The next step is removing the remaining water vapor and carbon dioxide. These components of air, if not removed, would freeze and plug the very cold portions of the plant. Other contaminants such as hydrocarbons are removed as well. Many, but not all, plants that have been designed since the mid-'80s use a "molecular sieve" pre-purification unit (PPU). Earlier practice employed reversing heat exchangers to remove the water and CO2, plus cold absorbers to remove hydrocarbons. Reversing plants are quite cost-effective for small plants, but the trend has been to use molecular sieve cleanup in most new plant designs, in particular when it is desired to make argon or relatively high ratios of nitrogen to oxygen.
Additional heat transfer, in brazed aluminum heat exchangers, cools the air to cryogenic temperature, or approximately -300oF (-185oC). The cold comes from product and waste gases exiting the separation process as they are being warmed to close-to-ambient air temperature. The very cold temperatures required for cryogenic distillation are created within the system by a refrigeration process that includes expansion of one or more internal process streams.
Distillation columns separate the air into desired products. Nitrogen plants may have only one column. Oxygen plants will have both "high" and "low" pressure columns where impure oxygen from the high pressure column receives further purification in the low pressure column. Because the boiling points for argon and oxygen are similar, plants producing very high purity oxygen will have an additional distillation column to remove argon.
Pure argon is made by additional purification of the "crude argon" produced by the argon removal column. The final argon purification process includes additional distillation in a "pure argon" column, which is preceded by a "de-oxo" step that removes the bulk of the oxygen present in the crude argon. This traditionally is done by chemically combining the trace amounts of oxygen with hydrogen to make water, which is then removed (after cooling) relatively easily in a molecular sieve dryer. Relatively recent advances in packed-column distillation technology have allowed the option of totally cryogenic argon recovery.
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. This heat exchange between feed and product streams minimizes the net refrigeration load on the plant and, therefore, energy consumption.
Refrigeration sections that produce cryogenic temperatures compensate for heat leak into the cold equipment and for imperfect heat exchange between incoming and outgoing gaseous streams. 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, which is a form of turbine. The expander drives a compressor or electrical generator, removing energy from the gas and reducing its temperature more than would be the case with simple expansion across a valve.
Gaseous products normally emerge from the plant at relatively low pressures, often just over one atmosphere (absolute). In general, the lower the delivery pressure, the higher the plant efficiency. When product will be used at relatively low gauge pressure (up to several atmospheres) the plant can be designed to produce product at that level. In most cases, it is most cost-effective to produce product at low pressure and use a blower or compressor to achieve required delivery and gaseous storage pressures. If gaseous oxygen is required at several atmospheres pressure, it may be produced by a "LOX boil" process or "pumped LOX" process that vaporizes liquid at elevated pressure, eliminating the need for product oxygen compression.
The portions of the cryogenic air separation process that operate at very low temperatures -- that is, the distillation columns, heat exchangers and cold interconnecting piping -- must be well insulated. These items are located inside insulated, sealed and nitrogen purged "cold boxes." Cold boxes are relatively tall structures that may be rectangular or round in cross section. Depending on plant type and capacity, they may measure 6.6 to 13' (2 to 4 m) on a side and have a height of 49 to 196' (15 to 60 m).
Liquefiers. When a large percentage of plant production must be produced as a liquid, a supplemental refrigeration unit must be added to, or integrated into, the 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 an analysis that considers local plant backup needs and anticipated demand for merchant liquid products to be supplied out of the plant. As a consequence, the ability to produce liquefied product may range from a small fraction of the air-separation plant capacity up to the 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 30 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 classic stand-alone liquefier cycle produces only liquid nitrogen. If it is desired to produce liquid oxygen, an extra heat exchanger circuit may be provided to revaporize some of the liquid nitrogen while liquefying the oxygen product. Alternatively, in some cases, it is possible to return some of the liquid nitrogen to the air separation system and use the contained refrigeration to permit withdrawal of a similar amount of liquid oxygen directly from the plant.
When a totally new air separation plant is designed, the liquefier cycle may be closely integrated with the air-separation process cycle. This is most advantageous if the plant will make a large amount of liquid product, as in a merchant liquid plant.
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, and they can be flexible in the sense of allowing production of varying mixes of liquid nitrogen and liquid oxygen. However, one potential disadvantage is that the liquefier cannot be shut down independently of the air separation unit.
What about LIN?
LIN assist plants are a special kind of cryogenic plant. They differ from "normal" cryogenic plants in that they do not have their own mechanical refrigeration system. They effectively import the required refrigeration from a high-volume, high efficiency merchant liquid plant by continuously injecting a small amount of liquid nitrogen into the distillation process. The LIN provides reflux for distillation, then vaporizes and mixes with the locally produced gaseous nitrogen, becoming part of the final product stream.
This arrangement simplifies the plant, reduces capital cost vs. a "normal" cryogenic plant with its own refrigeration cycle and can provide better overall economics than either an all-bulk-liquid supply or a new cryogenic nitrogen plant with a standard internal refrigeration cycle.