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  Air Separation Technology

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 is then cooled to close-to-ambient temperature by passing through water-cooled or air-cooled heat exchangers. Sometimes it is cooled a bit more in a mechanical refrigeration system to improve impurity removal efficiency and minimize power consumption. 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 must be removed to meet product quality specifications, and most importantly, they must be removed prior to chilling the air feed to low temperatures in a cryogenic plant, where the water and carbon dioxide would freeze and deposit in process lines and heat exchanger passages. Many, but not all, plants that have been designed since the mid-eighties use a "molecular sieve" "pre-purification unit" (PPU). These "front end cleanup" systems also remove other contaminants such as hydrocarbons. The earlier standard practice was to use "reversing" heat exchangers to remove water and CO2 and cold absorbers to remove hydrocarbons. Reversing heat exchanger plants are still built, and can be the most cost-effective technology choice for small plants; however, molecular sieve cleanup is used in most new plants, in particular when it is desired to produce argon or relatively high ratios of nitrogen to oxygen.

Further heat transfer, in brazed aluminum heat exchangers, cools the air to cryogenic temperature (approximately -300 degrees Fahrenheit or -185 degrees Celsius). The cooling is accomplished with cold product and waste gas streams exiting the separation process. These exiting gas streams are warmed to close-to-ambient air temperature, which reduces the amount of refrigeration that must be produced in the process. 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.

Distillation columns separate the air into desired products. 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. Nitrogen plants may have only one column, although many also have two.

Because the boiling points for oxygen and argon are similar, plants producing very high purity oxygen require more distillation stages and remove argon from a near the mid-point of the low pressure column where its concentration is highest. The removed argon is usually processed in an additional "side-draw" distillation column that is integrated with the low pressure column. The argon-rich stream removed by this column is called "crude argon". 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 uncombined hydrogen.

Relatively recent 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 will be less than 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. This heat exchange between feed and product streams minimizes the net refrigeration load on the plant and, therefore, energy consumption.

Refrigeration must be 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 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 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 most cases, however, it is more cost effective to produce the product at low pressure and use a blower or compressor to achieve required delivery and gaseous storage pressures.

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, after withdrawal it from the distillation column and pumping to the desired pressure (if necessary). These cycles have appeal because oxygen compressors are expensive to purchase and install. To achieve energy-efficient vaporization of the product oxygen, a portion of the air feed, which is almost 80% nitrogen, must be compressed to higher than normal pressure.

The required booster compressor is often integrated with the main air compressor. Because 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, "Pumped LOX" systems are most applicable when there is fairly constant product demand. Rapid and repeated changes in 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. The "imported" 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 (versus a "normal" cryogenic plant with its own refrigeration cycle) 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.

Important

The information set out in this document constitutes a set of general guidelines and should not be construed or relied upon as specialist advice. Independent legal advice should always be sought. Therefore Risktechnik accepts no responsibility towards any person relying upon these Risk Management Guides nor any liability whatsoever for the accuracy of data supplied by another party or the consequences of reliance upon it.

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