Cryogenic process of air separation

 Air consists of various gases， of which nitrogen (N2) and oxygen (O2) together account for about 99.03% of the total volume of the sample. Dry air contains about 78.08% nitrogen， about 20.95% oxygen and about 0.93% argon， as well as traces of some other gases， such as hydrogen， neon， helium， krypton， xenon and carbon dioxide. Ambient air may contain varying amounts of water vapor (depending on the humidity) and other gases produced by natural processes and human activities. Oxygen and nitrogen are produced by air separation processes， which require the separation of air into its components. Rare gases such as argon and krypton can be recovered as by-products of the air separation process.

The separation of air into its constituent gases is accomplished by implementing specific air separation technologies. There are different air separation technologies available， each of which is designed to exploit the different properties of the physical properties between the constituent gases of air. In other words， air separation techniques are based on the fact that each of the constituent gases of air has different physical properties and， therefore， air separation is accomplished by exploiting certain physical properties such as (i) differentiation of molecular sizes of constituent gases， (ii) differentiation of diffusion rate differences through certain materials， (iii) adsorption preferences of particular materials for certain gases， and (iv) boiling temperature differences， etc.

Some of the techniques used today include cryogenic， adsorption， chemical processes， polymeric membranes， and ion transport membranes (ITM). Among these technologies， cryogenic air separation technology is at a mature stage of its life cycle and， as such， it is the only viable method currently available for the large-scale production of air products such as oxygen， nitrogen and argon.

Air separation technology is used to produce oxygen and/or nitrogen， sometimes as a liquid product. Some equipment also produces argon， either as a gas， a liquid， or both. All air separation processes start with compressed air. All air separation plants use either non-cryogenic or cryogenic technology. Air separation plants using non-cryogenic air separation technology produce gaseous oxygen or nitrogen products using a separation process that is close to ambient temperature. These plants typically produce oxygen that is 90 to 95.5 percent pure or nitrogen that is typically 95.5 to 99.5 percent oxygen-free. Air separation equipment can produce more than three times as much nitrogen as oxygen， but typically maintains a 1:1 to 1.5:1 ratio of nitrogen to oxygen product.

The cryogenic process was first developed by Carl von Linde in 1895 and refined by Georg Claude in the 1900s for small-scale production of oxygen for various industrial processes such as welding and cutting， and as a medical gas.

Industrial-scale cryogenic air separation began in the early 1900s， facilitating the development of metallurgy and other industrial sectors that are highly dependent on oxygen， nitrogen and argon. Cryogenic air separation plants (ASP) are characterized by good product quality， high capacity and reliability. Despite the availability of other emerging air separation technologies， cryogenic air separation remains the basic technology for oxygen production. Cryogenic air separation equipment is most commonly used to produce high purity gas products. However， the use of this technology is limited for applications that require large volumes of gas， typically more than a few hundred tons of separated gas per day. They can produce either gaseous or liquid products.

Cryogenic air separation technology uses the difference in boiling points of gases for separation. It is based on the fact that the different component gases of air have different boiling points and by manipulating the direct environment in terms of temperature and pressure， air can be separated into its component parts. Oxygen has a boiling point of minus 182.9 degrees Celsius at 1 atmosphere and 0 degrees Celsius， and minus 160.7 degrees Celsius at 6 atmospheres and 0 degrees Celsius. The corresponding boiling points for nitrogen are minus 195.8 degrees Celsius and minus 176.6 degrees Celsius， and for argon are minus 185.8 degrees Celsius and minus 164.6 degrees Celsius， respectively.

Cryogenic separation is the most efficient process when any of the three criteria need to be met， i.e. (i) high purity oxygen (above 99.5%) is required， (ii) large quantities of oxygen (>100 tons of oxygen/day) are required， or (iii) high pressure oxygen is required. Cryogenic air separators require more than one hour to start up. In addition， since cryogenic technology can produce such high purity oxygen， the waste nitrogen stream also has a usable mass. This can add considerable economic benefit to a process combined with a cryogenic air separation plant.

The cryogenic separation of air into its constituent gases involves a variety of processes. A combination of these processes is required in a cryogenic air separation plant， the most basic of which are (i) air compression， (ii) air purification， (iii) heat exchange， (iv) distillation， and (v) product compression. Figure 1 shows these processes.

Figure 1 Basic process involving low-temperature air separation

Cryogenic air separation equipment is based on the cryogenic air separation process. This basic process has been evolving as an industrial process since its commercialization in the early 20th century. A large number of process configuration variations have occurred due to the desire to produce specific gas products and product combinations as efficiently as possible at a variety of desired purity and pressure levels. These air separation process cycles have evolved in parallel with advances in compression machinery， heat exchangers， distillation technology and gas expander technology.

The distillation process is the heart of the process as it physically separates the air into its components. The air product produced has a certain degree of purity， which is defined as the ratio of the amount of 100% pure air product to the total amount of air product output.

In the distillation process， trays are used. The basic function of the trays is to bring the falling liquid and the rising gas into effective contact. Thus， the tray provides the stage for (i) cooling and partially condensing the rising gas， and (ii) heating and partially vaporizing the falling liquid. Figure 2 shows a typical distillation column with a fractionation tray. This distillation column has only one distiller and one condenser. Distillation is achieved by effective liquid-gas contact， which is achieved by proper contact between the falling liquid and the rising gas. The respective purity of the most volatile and less volatile elements differs on each tray， and the lower and upper sides of the distillation column are the two extremes， which is where the pure elements are obtained.

Figure 2 Typical distillation column with fractionation trays for the production of oxygen and nitrogen

Figure 2 shows that the tray provides a certain resistance to the rising gas and therefore creates a pressure drop. The pressure drop has to be as small as possible because it has a significant impact on the energy consumption of the air compressor and is an important parameter in the development of tray technology. Distillation packing is another technology that is being used to ensure a much smaller total pressure drop and improved liquid-gas contact compared to partial distillation trays.

For the production of oxygen， a liquid mixture of oxygen and nitrogen and a column with a vaporizer at the bottom are required， while for the production of nitrogen， a gas mixture of oxygen and nitrogen and a column with a condenser at the top are required， in which an oxygen-rich by-product is also produced. By stacking these two types of columns together and transporting the oxygen-rich liquid obtained at the bottom of the nitrogen column to the top of the oxygen column， it is possible to produce oxygen and nitrogen with only one condenser. This is shown in Figure 2.

The oxygen-rich liquid enters the top of the upper distillation column and， by distillation， produces liquid oxygen (LOX) at the bottom of the same column. The liquid oxygen vaporizes to gaseous oxygen (GOX) through heat exchange between gaseous nitrogen (GAN) at the top of the lower distillation tower and liquid oxygen at the bottom of the upper distillation tower. A waste product is also produced at the top of the upper column， consisting of a mixture of nitrogen and oxygen.

In practice， the function of the condenser is performed by a heat exchanger that ensures the proper heat transfer from the GAN to the LOX and vice versa in order to vaporize the LOX and condense the GAN， which is required for the continuous operation of the distillation column. In this model， the distillation columns are stacked on top of each other， but it is possible to place them side by side， as is occasionally done in practice.

The cryogenic air separation process is an energy-intensive， low-temperature process that separates air into its constituent gases. The energy consumption for oxygen separation is an increasing function of oxygen purity. The cost of electrical energy is the single largest operating cost incurred by air separation plants. It is typically in the range of one-third or two-thirds of the operating costs associated with the production of gas and liquid products. Due to the extensive use of oxygen， nitrogen and argon in the steel industry， the price of these gases affects the cost of producing steel and steel products. the energy efficiency of an ASP is largely influenced by the ratio of oxygen to nitrogen production and can be changed as required.

Figure 3 Flow chart of a typical cryogenic air separation plant

Steps in the cryogenic process of air separation

There are several steps in the cryogenic process of air separation. The first step is the filtration， compression and cooling of the incoming air. In most cases， the air is compressed between 5 MPa and 8 MPa， depending on the product structure and the desired product pressure. In this step， the compressed air is cooled and most of the water vapor entering the air is condensed and removed as the air passes through a series of interstage coolers and an aftercooler after the final stage of compression.

The second step includes the removal of impurities， specifically， but not exclusively， residual water vapor and carbon dioxide (CO2). These components are removed to meet product quality specifications and before the air enters the distillation section of the equipment. There are two basic methods of water vapor and CO2 removal. They are (i) molecular sieve devices (ii) reversing exchangers. Most new air separation plants use molecular sieve pre-purification units to remove water vapor and carbon dioxide from the incoming air. Heat exchangers for water vapor and carbon dioxide removal are more cost effective for smaller plants. In plants utilizing heat exchangers， cooling of the compressed air feed is accomplished in two sets of brazed aluminum heat exchangers. When a reversing heat exchanger is used， a cold absorption unit is installed to remove any hydrocarbons.

The third step is an additional heat transfer for the product and exhaust air streams to bring the air streams to a low temperature (-185 degrees Celsius). This cooling is performed in a brazed aluminum heat exchanger， which allows heat exchange between the incoming air feed and the cold product and waste gas streams leaving the separation process. During the heat exchange process， the leaving gas stream is heated to a temperature close to that of the ambient air. Recovery of refrigeration from the gaseous product stream and the waste gas stream minimizes the amount of refrigeration produced by the plant. The very low temperatures required for cryogenic distillation are produced by a refrigeration process that includes the expansion of one or more high-pressure process streams.

The fourth step is the distillation process， which separates the air into the desired product. To create oxygen， the distillation system uses two distillation columns in series， often called high and low pressure columns. Nitrogen generators can have only one column， although many have two. Nitrogen leaves from the top of each distillation column， while oxygen leaves from the bottom. The impure oxygen produced in the initial (high pressure) column is further purified in a second low pressure column. Argon has a similar boiling point to that of oxygen and stays preferentially with oxygen. If high purity oxygen is required， then argon is removed. The removal of argon is done at the highest concentration of argon in the low-pressure column. The removed argon is typically processed in an additional "pull" crude argon distillation column that is integrated with the low pressure column argon refining facility. The cold gaseous product and crude argon can be vented， further processed on site， collected as a liquid， or vaporized to produce gaseous argon.

Waste streams from the air separation column are sent back through a heat exchanger at the front end. As they are heated to near ambient temperature， they are cooled against the incoming air. The heat exchange between the feed and product streams minimizes the net refrigeration load on the plant and thus minimizes energy consumption.

Refrigeration is generated at low temperature levels to compensate for heat leakage into the cold plant and incomplete heat exchange between the incoming and outgoing gas streams. In the refrigeration cycle of an air separation plant， one or more high-pressure gas streams (which can be inlet， nitrogen， exhaust， feed gas or product gas， depending on the type of equipment) are reduced in pressure， thereby cooling the gas stream. To maximize cooling and the energy efficiency of the equipment， the pressure reduction (or expansion) is performed in an expander (a form of turbine). Energy is removed from the gas stream to make it cooler than the simple expansion through the valve. The energy produced by the expander is used to drive a process compressor， generator， or any other energy consuming device.

The gaseous product typically leaves the cold box (the insulated vessel containing distillation towers and other equipment operating at very low temperatures) at relatively low pressure， often just over one atmosphere (absolute). In general， the lower the delivery pressure， the more efficient the separation and purification process will be. The product gas is then compressed in a compressor to the required pressure of the product gas for its use.

Parts of the cryogenic air separation process that operate at very low temperatures (e.g. distillation columns， heat exchangers and cold interconnect piping) are well insulated. These items are located in sealed (and nitrogen purged) "cold boxes"， which are relatively high structures with rectangular or circular cross sections. The cold box is filled with rock wool to provide insulation and minimize convection. Depending on the type and capacity of the equipment， the cold box can have sides of 2 m to 4 m and heights of 15 m to 60 m.

Argon production

Pure argon is usually produced from crude argon by a multi-step process. The traditional method is to remove the 2% to 3% oxygen present in crude argon in a "deoxygenation" unit. These small devices chemically combine oxygen and hydrogen in a vessel containing a catalyst. The resulting water is easily removed (after cooling) in a molecular sieve dryer. The oxygen-free argon gas stream is then processed in a "pure argon" distillation column to remove residual nitrogen and unreacted hydrogen.

Advances in packed column distillation technology have created a second method of argon production， full cryogenic argon recovery， which uses very tall (but small diameter) distillation columns to perform difficult argon/oxygen separations. The amount of argon that can be produced by a facility is limited by the amount of oxygen handled in the distillation system， plus a number of other variables that affect the recovery rate. These variables include the amount of oxygen produced as a liquid and the stability of the operating conditions of the equipment. Due to the percentage of gas naturally occurring in the air， argon production cannot exceed 4.4% of the oxygen feed rate by volume， or 5.5% by weight.

Production of liquid products 

When producing liquid products in a cryogenic air separation plant， a supplementary refrigeration unit is usually added (or integrated) to the basic air separation plant. This unit is called a liquefier and uses nitrogen as the primary working fluid. The capacity of the liquefier can range from a small fraction of the capacity of the air separation plant to the maximum capacity of the air separation plant for oxygen plus nitrogen and argon.

The basic process cycle used in liquefiers has remained unchanged for decades. A typical liquefier draws in nitrogen at near ambient temperature and pressure， compresses it， cools it， and then expands the high-pressure gas stream to produce refrigeration. The basic difference between newer and older liquefiers is that the maximum operating pressure rating of a cryogenic heat exchanger has increased with improvements in cryogenic heat exchanger manufacturing technology. If a typical new liquefier has a higher peak cycle pressure and a more efficient expander， it may be more energy efficient than a thirty year old liquefier.