Cryogenic Process of Air Separation
Cryogenic Process of Air Separation
Air has a composition of various gasses, of which nitrogen (N2) and oxygen (O2) collectively account for around 99.03 % of the total sample volume. Dry air contains by volume around 78.08 % of nitrogen, around 20.95 % of oxygen, and around 0.93 % of argon along with traces of a number of other gases like hydrogen, neon, helium, krypton, xenon, and carbon dioxide. Ambient air can contain varying amount of water vapour (depending upon humidity) and other gases produced by natural processes and human activities. Oxygen and nitrogen are produced by means of an air separation process, which entails the separation of air into its constituents. The rare gasses like, for example, argon, krypton can be recovered as byproducts of the air separation process.
The separation of air into its constituent gases is done through the implementation of a specific air separation technology. There different air separation technologies which are available at present, each one aimed at exploiting different attributes with regard to the difference in physical properties between the constituent gases of the air. In other words, an air separation technology is based on the fact that each of the constituent gases of air has different physical properties and hence, air separation is realized through exploiting a physical property such as (i) distinguishing between molecule sizes of the constituent gases, (ii) distinguishing between difference in diffusion rates through certain materials, (iii) adsorption preference which special materials have towards certain gases, and (iv) difference in boiling temperatures etc.
Some of the technologies being used today include cryogenic, adsorption, chemical processes, polymeric membranes, and ion transport membrane (ITM). Out of these technologies, cryogenic air separation technology is in a mature stage of its life cycle, thus making it the only feasible means from the presently available technologies for the mass production of air products such as oxygen, nitrogen, and argon.
Air separation technologies are used for the production of oxygen and / or nitrogen as gases and sometimes as liquid products. Some plants also produce argon either as a gas, or a liquid, or both. All air separation processes start with compression of air. All air separation plants employ either non cryogenic based technologies or cryogenic based technologies. Air separation plants employing non cryogenic air separation technologies produce gaseous oxygen or nitrogen products using near ambient temperature separation processes. These plant produce oxygen which is typically 90 % to 95.5 % pure or nitrogen which is typically 95.5 % to 99.5 % oxygen free. Air separation plants can produce more than three times more nitrogen than oxygen, but a nitrogen-to-oxygen product ratio of 1:1 to 1.5:1 is normally maintained.
The cryogenic process was first developed by Carl Von Linde in 1895 and improved by George Claude in the 1900s to produce oxygen on a small scale for meeting the requirements of various industrial processes such as welding, and cutting and as a medical gas.
Cryogenic air separation on industrial scale started in the beginning of 20th century fostering the development of metallurgy and other branches of industry highly dependent on the availability of oxygen, nitrogen, and finally argon. Cryogenic air separation plants (ASP) are characterized by very good quality of the products, big capacities, and high reliabilities. In spite of other emerging technologies of air separation, cryogenics air separation technology remains the basic technology for oxygen production. Cryogenic air separation plants are most commonly used to produce high purity gaseous products. However, the use of this technology is restricted for the applications needing the gases in high quantities normally above several hundred tons of the separated gases per day. They can produce products as gases or liquids.
The cryogenic air separation technology utilizes difference in boiling points of gases for their separation. It is based on the fact that the different constituent gasses of air have different boiling points and by manipulating the immediate environment in terms of temperature and pressure, the air can be separated into its components. The boiling point of oxygen at a 1 atmosphere pressure and 0 deg C is minus 182.9 deg C and that at 6 atmosphere pressure and 0 deg C is minus 160.7 deg C. The corresponding boiling points of nitrogen are minus 195.8 deg C and minus 176.6 deg C, and those for argon are minus 185.8 deg C and minus 164.6 deg C respectively.
Cryogenic separation is most effective process when any of the three criteria need to be met namely (i) high purity oxygen is needed (higher than 99.5 %), (ii) high volumes of oxygen are needed (greater than 100 tons of oxygen / day), or (iii) high pressure oxygen is needed. Cryogenic air separators take more than an hour to start up. Additionally, since cryogenics can produce such a high purity of oxygen, the waste nitrogen stream is of a usable quality. This can add considerable financial benefits to a process integrated with a cryogenic air separation plant.
Cryogenic separation of air into its constituent gases involves various processes. Combination of these processes are needed in a cryogenic air separation plant, of which the fundamental ones are (i) air compression, (ii) air purification, (iii) heat exchanging, (iv) distillation, and (v) product compression. Fig 1 shows these processes.
Fig 1 Fundamental processes involved in the cryogenic separation of air
Cryogenic air separation plants are based on cryogenic air separation processes. The basic process since its commercialization early in the 20th century, has been under continuous development as an industrial process. A large number of process configuration variations have emerged, driven by the desire to produce particular gas products and product mixes as efficiently as possible at various required levels of purity and pressure. 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 at the heart of the overall process since it performs the actual separation of air into its constituents. The air products are produced with a certain purity, which is defined as the ratio of the quantity of 100 % pure air product to the quantity of total of air product at the output.
In the distillation process, trays are used. The basic function of the trays is to enable efficient contact of the descending liquid and the rising gas. Hence, the tray sets the stage for (i) cooling and partial condensation of the rising gas, and (ii) heating and partial vapourization of the descending liquid. Fig 2 shows a typical distillation column with fractional distillation tray. This distillation column has only one vapourizer and one condenser. Distillation is made possible by efficient liquid-gas contact and this is enabled through proper contact between the descending liquid and the rising gas. The respective purities of the most volatile and less volatile elements differ at each tray, with the lower and upper sides of the distillation column being the two extremes, which is also where the pure elements are obtained.
Fig 2 Typical distillation column with fractional distillation trays for oxygen and nitrogen production
Fig 2 shows that the tray provides the rising gas with a certain resistance, and thus creates a pressure drop. The pressure drop is to be as small as possible since it has a significant impact on the energy consumption of the air compressor and is also an important parameter in development of the tray’s technology. Distillation packing is another technology that is being used and, as opposed to fractional distillation trays, ensures a much smaller total pressure drop as well as improved liquid-gas contact.
For producing oxygen, a liquid mixture of oxygen and nitrogen and a column, equipped with a vapourizer at the bottom, is needed, while for producing nitrogen, a gaseous mixture of oxygen and nitrogen as well as a column, equipped with a condenser at the top, is needed and in this process, a byproduct, rich in oxygen, is also produced. By stacking theses two types of columns on top of one another and by routing the oxygen rich liquid, which is obtained at the bottom of the nitrogen column, to the top of the oxygen column it is possible to produce oxygen and nitrogen by using only a condenser. This is shown in Fig 2.
An oxygen rich liquid enters the top of the upper distillation column, and through distillation, results in liquid oxygen (LOX) at the bottom of the same column. Vapourization of the LOX into gaseous oxygen (GOX) is realized by means of the heat exchanging which occurs between the gaseous nitrogen (GAN) at the top of the lower column and the LOX at the bottom of the upper column. At the top of the upper column a waste product, consisting of a nitrogen and oxygen gas mixture, is also produced.
In practice, the function of the condenser is fulfilled by a heat exchanger which ensures that proper heat is carried over from the GAN to the LOX and vice versa, in order to enable vapourization of the LOX and condensation of the GAN, which is required for the continuous operation of the distillation columns. In this model the columns are stacked on top of each other, but it is also possible to place them alongside one another, as occasionally being done in practice.
Cryogenic air separation process is an energy intensive, low-temperature process which separates air into its component gases. Energy consumption of oxygen separation is an increasing function of oxygen purity. The cost of electric energy is the largest single operating cost incurred in air separation plants. It is normally in the range of one third or two thirds of the operating costs associated with producing gas and liquid products. Since the steel industry uses extensively oxygen, nitrogen, and argon gases, the price of these gases affects the production cost of steel and steel products. The energy efficiency of ASP is considerably influenced by the production ratio of oxygen and nitrogen, which can be varied depending upon the requirement.
The thermodynamic minimal work of oxygen separation from air is equal to 53.1 kWh / ton of oxygen. Presently, the best constructed cryogenic ASPs are characterized by energy consumption which exceeds the thermodynamic minimum by around three times.
The complexity of the cryogenic air separation process, the physical sizes of equipment, and the energy needed to operate the process vary with the number of gaseous and liquid products, required product purities, and required delivery pressures. Plants with production of only nitrogen gas are less complex and require less power to operate than plants with production of only oxygen gas. Co-production of both gases increases capital cost and energy efficiency. Making these gases in liquid form needs additional equipment and more than double the amount of power needed per unit of delivered gas.
Argon production is economical only as a co-product with oxygen. Producing it at high purity adds to the physical size and complexity of the air separation plant. Flow diagram of a typical cryogenic air separation plant is shown in Fig 3. The flow diagram shows typical inter-relationships between the various components of the plant. However, the actual relationship is dependent on the design of the air separation plant which can vary for meeting the requirements.
Fig 3 Flow diagram 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 filtering, compressing and cooling of the incoming air. In most of the cases air is compressed between 5 MPa and 8 MPa depending on the product mix and the needed product pressures. In this step the compressed air is cooled and majority of the water vapour in the incoming air is condensed and removed as the air passes through a series of inter stage coolers plus an after cooler following the final stage of compression.
The second step consists of removal of impurities, in particular, but not limited to, residual water vapour plus carbon dioxide (CO2). These components are removed to meet the product quality specifications and prior to air entering the distillation portion of the plant. There are two basic approaches for removal of water vapour and CO2. They are (i) molecular sieve units (ii) reversing exchangers. Most of the new air separation plants employ a molecular sieve pre purification unit to remove water vapour and CO2 from the incoming air. Reversing exchangers for removal of water vapour and CO2 are more cost effective for smaller plants. In plants utilizing reversing heat exchangers, the cool down of the compressed air feed is done in two sets of brazed aluminum heat exchangers. When reversing heat exchangers are used, cold absorption units are installed to remove any hydrocarbons.
The third step is additional heat transfer against product and waste gas streams to bring the air stream to cryogenic temperature (- 185 deg C). 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 leaving the separation process. During the heat exchange, the leaving gas streams are warmed to close to the ambient air temperature. Recovering refrigeration from the gaseous product streams and waste stream minimizes the amount of refrigeration which is to be produced by the plant. The very cold temperatures needed for cryogenic distillation are created by a refrigeration process which includes expansion of one or more elevated pressure process streams.
The fourth step is the process of distillation which separates the air into the desired products. To make oxygen, the distillation system uses two distillation columns in series, which are normally called the high and low pressure columns. Nitrogen plants can have only one column, although many have two. Nitrogen leaves the top of each distillation column while 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 preferentially stays with the oxygen. If high pure oxygen is needed then argon is to be removed. Argon removal takes place at a point in the low pressure column where the concentration of argon is at its highest level. The argon, which is removed, is normally processed in an additional ‘draw’ crude argon distillation column that is integrated with the low pressure column argon refining facilities. Cold gaseous products and crude argon can be vented, further processed on site, or collected as liquid, or vapourized to produce gaseous argon.
Waste streams which 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. The heat exchange between feed and product streams minimizes the net refrigeration load on the plant and hence the 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. In the refrigeration cycle of air separation plants, one or more elevated pressure streams (which can be intake air, 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 in 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 any other energy-consuming device.
Gaseous products typically leave the cold box (the insulated vessel containing the distillation columns and other equipment operating at very low temperatures) at relatively low pressures, frequently just over one atmosphere (absolute). In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process. The product gas is then compressed in compressors to the pressure needed by the product gas for its use.
Portions of the cryogenic air separation process which operate at very low temperatures (for example distillation columns, heat exchangers, and cold interconnecting piping) are to be well insulated. These items are located inside sealed (and nitrogen purged) ‘cold boxes’ which are relatively tall structures which are either rectangular or round in cross section. Cold boxes are packed with rock wool to provide insulation and minimize convection currents. Depending on plant type and capacity, cold boxes can measure 2 meters to 4 meters on a side and have a height of 15 meters to 60 meters.
Production of argon
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-ox’ 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 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 argon production option, totally cryogenic argon recovery which uses a very tall (but small diameter) distillation column to make the difficult argon / oxygen separation. The amount of argon which can be produced by a plant is limited by the amount of oxygen processed in the distillation system plus a number of other variables which 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.
Production of liquid products
When liquid products are produced in a cryogenic air separation plant then normally a supplemental refrigeration unit is added to (or integrated into) the basic air separation plant. This unit is called liquefier and use nitrogen as the primary working fluid. Liquefier capacity can range from a small fraction of the air separation plant capacity upto maximum production capacity for oxygen plus nitrogen and argon of the air separation plant.
The basic process cycle used in liquefiers has been unchanged for decades. A typical liquefier takes in near ambient temperature and pressure nitrogen, compresses it, cools it and then expands the high pressure stream to produce refrigeration. 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.