Cryogenic Air Separation Plant
Cryogenic Air Separation Unit
Cryogenic Air Separation Plant is routinely used in medium to large scale industries to generate nitrogen, oxygen, and argon as gases and or liquid products. Cryogenic air separation is the adopted technology for making very distinguished virtue of oxygen and nitrogen gases. It is the most cost-effective technology for high production standard plant. All plants delivering liquefied industrial gas produce utilize cryogenic technology. The complexity of the cryogenic air separation process, the substantial volumes, and the power needed to run the process all vary with the number of gaseous and liquid products to be produced, their required product purities and required delivery pressures.
Cryogenic Air Separation Unit mainly consists of the following equipment.
- Plate type heat exchanger
- Medium pressure column
- Low-pressure column
- Expansion turbine
- Air liquefier
- Rich liquid sub-cooler
- Liquid Nitrogen separator
- Liquid Oxygen separator
Cryogenic Air Separation Process
The initial step in any air separation unit is separating, compressing, and cooling the incoming air.
In most circumstances the air is reduced to around within 5 and 8 bar (about 75 to 115 psi), depending upon the expected product mix and desired product requirements. The compressed air is cooled, and much of the water vapour 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 air leaving the concentration operation is bounded by the temperature of the available cooling mechanism (which in almost all situations is limited by the wet or dry bulb temperature of the ambient air) the temperature of the compressed air is often well above optimum temperature for maximum efficiency of downstream unit operations. Consequently, the air is normally cooled further by an automatic cooling system.
This level gives numerous advantages. It enables the removal of additional water vapour by condensation. It also reduces and stabilizes the channel temperature to downstream operations, which improves the productivity and stability of the overall air separation process. In particular, it decreases the water-removal load in the molecular sieve pre-purification system.
In some cases, compressed air cooling may be achieved, totally or in part, by a direct contact after cooler system (DCAC). DCAC systems utilize cool, dry, nitrogen-rich waste gas to chill 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.
Following concentration and cooling, the subsequent major action in the air separation process is the removal of impurities, inappropriate, residual water vapour plus carbon dioxide.
These elements of the incoming air feed must be removed to meet product quality specifications. The water vapour and carbon dioxide must be separated earlier to the air entering the cryogenic distillation part of the plant because at very low temperatures they would freeze and deposit on the surfaces within the process equipment.
There are two basic methods for separating the liquid vapour and carbon dioxide - "molecular sieve units" and "reversing exchangers".
Essentially all modern air separation plants operate a “molecular sieve” "pre-purification unit" (PPU) to separate carbon dioxide and liquid from the incoming air by adsorbent these particles onto the surface of "molecular sieve" materials at near-ambient temperature. The mix of adsorbent materials in these units can also be easily adjusted to remove other contaminants, such as hydrocarbons, which may be found in an industrial environment. The adsorbent materials are typically contained in two identical vessels; one of which is used to purify the incoming air while the other is being regenerated using clean waste gas. The two beds change service at regular intervals. Molecular sieve pre-purification is the natural choice when a high ratio of nitrogen recovery is desired.
The other procedure is to use “reversing” heat exchangers to remove liquid and Carbon Di-Oxide. While often thought of as "primitive" technology, reversing exchangers can be more cost-effective for smaller production rate nitrogen or oxygen plants. In plants which utilize reversing heat exchangers, the cool-down of the compressed air feed is done in two sets of brazed aluminium heat exchangers.
The incoming air is cooled in "warm end" heat exchangers to a low sufficient temperature that the liquid vapour and carbon dioxide freeze out onto the walls of the heat exchanger. At regular intervals, a set of valves reverse the duty of the air and waste gas passages. After switching, very dry, partially-warmed waste gas evaporates the water and sublimes the carbon dioxide ices that were collected during the air cooling period. These gases are returned to the atmosphere, and after they have been fully removed, the reversing heat exchanger is ready for another reversal of passage duty.
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, hydrocarbon impurities are removed along with water vapour and carbon dioxide, in the PPU.)
The next step in cryogenic air separation is further heat transfer, among product and waste gas streams and the incoming air, which bring the air feed to cryogenic temperature (approximately -300 degrees Fahrenheit or -185 degrees Celsius).
This cooling happens in brazed aluminium heat exchangers which transfer heat between the incoming air feed and cold product and waste gas streams exiting the cryogenic distillation 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 cryogenic air separation/product purification process is a distillation, which separates the air into desired products.
To make oxygen as a product, the distillation system uses two distillation columns in order. These are commonly called the “high” and “low” pressure columns (or, alternatively, the "lower' and "upper" columns). Nitrogen plants may have only one column, although some very high purity plants may 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 when it is the desired product. If ultra-pure nitrogen is desired, the upper or low-pressure column is used to eliminate essentially all of the oxygen not removed in the first stage of distillation.
Argon has a boiling point which is comparable to that of oxygen and it will preferentially stay with the oxygen product if only oxygen and nitrogen are craved as products. This restricts the oxygen purity to a maximum of about 97% in a simple two-column system. If low purity oxygen is adequate (e.g. for combustion enrichment) the oxygen purity can be as low as 95%. However, If high purity oxygen is required, argon must be separated from the distillation system.
Argon removal, when needed or desired, takes place at a point in the low-pressure column where the concentration of argon is highest. The argon which is removed is processed in an additional "side-draw" crude argon distillation column which is integrated with the low-pressure column. The separate impure (or "crude") argon stream may be vented, moreover processed on site to remove both oxygen and nitrogen to become "pure" argon, or collected as liquid and shipped to a remote "argon refinery". The choice depends primarily upon the quantity of argon available and economic analysis of the different options. As a general rule, argon purification is most economically viable while at least 100 tons per day of oxygen is being produced.
Pure argon is produced from crude argon by a multi-step process. The traditional approach is a removal of the two to three per cent oxygen present in the crude argon in a “de-oxo” unit; which is a small multi-step set of processes, which chemically combine the oxygen with hydrogen in a catalyst-containing vessel and then removes the resultant water (after cooling) in a molecular sieve drier. The resulting oxygen-free argon stream is further processed in a "pure argon" distillation column to remove residual nitrogen and unreacted hydrogen.
Progress in packed-column distillation technology has produced 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. Argon by distillation requires many stages of distillation because of the relatively small difference in boiling points between oxygen and argon.
The quantity of argon that can be created by a plant is limited by the quantity 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 the 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.
Cooling is performed at cryogenic temperature levels to compensate for heat leak into the cold equipment and for imperfect heat exchange between incoming and outgoing gaseous streams.
Cryogenic air separation plants use a cooling 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 a turbine). Eliminating heat from the gas stream during expansion reduces its temperature more than would be the case with simple expansion across a valve. The energy produced by the expander may be used to drive a process compressor, an electrical generator, or another energy-consuming device such as an oil pump or air blower.
Gaseous produce from a cryogenic oxygen plant / air separation unit typically exit the cold box (the insulated vessel containing the distillation columns and other equipment operating at very low temperatures) at close to atmospheric temperature, but at relatively low pressure; often just over one atmosphere (absolute). In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process.
While lowering pressure support lower separation power requirements, if the products must be delivered at higher pressure, either product compressors will be needed or one of the various cycle options can be used to supply nitrogen or oxygen at higher delivery pressure directly from the cold box.
By reducing a product compressor and its power, these higher delivery pressure processes can be, on an overall basis, more cost-effective than separation accompanied by compression.