Air, separation

The fractionation of air into nitrogen and oxygen is a classical process with a great industrial importance, even after a history of over 100 years (Baldus et al. 1983). The special feature of the process is the very low temperature level required for the operation of the distillation colunm, approximately -180 to -190°C. Hence, it is not possible to heat and cool the colurrms by steam and water, respectively. 

The system nitrogen/oxygen is a nearly ideal and very wide boiling mixture (relative volatility a 4) that can, in principle, very easily be fractionated in a single distillation colunm The industrial process, however, uses two distillation colurrms, see Fig. 11.1-7. 

In column C-1 the feed mixture (20% oxygen) is prefractionated into pure nitrogen (overhead fraction) and an oxygen-rich mixture with approximately 40% oxygen (bottom fraction). As the feed is a two-phase mixtote with high vapor corrtent, no 

The technology associated with pressure swing adsorption is well developed. Air at a pressure of several bar is brought into contact with large beds of LiX 

A membrane process that has grown rapidly in the last few decades is the separation of the air into nitrogen- and oxygen-enriched streams. The main part of the membranes in use are oxygen selective, therefore the nitrogen-rich stream is recovered in the high pressure side, whereas an Oa-enriched stream is obtained as permeate at a low pressure. 

Significant efforts have been made for increasing O2/N2 selectivity of the polymeric membranes these two molecules have a very close kinetic diameter (nitrogen, 3.64 A oxygen, 3.46 A), thus the use of a membrane whose selective properties are related only on the molecular size selection was very difficult. As reported by Baker [7], the first membranes used for this separation showed an O2/N2 selectivity of ca. 4. Significant improvement in membrane selectivity allowed values ranging from 8 to 12 to be reached, implying relevant reduction in compressor size and equipment costs. 

nitrogen separation by membrane systems is the largest GS process in use. Membrane selectivity does not need to be high in order to produce a relatively pure nitrogen stream, thus they became the dominant technology instead of pressure swing adsorption (PSA) or cryogenic distillation. 

Promising results have been obtained with the facilitated transport membranes in which an oxygen-complexing carrier compound acts like a shuttle to transport the oxygen selectively throngh the membrane [44], 

---

Air separation plants produce about 99% of the gas, while electrolysis plants produce about 1%. 

Chemical Conversion. In both on-site and merchant air separation plants, special provisions must be made to remove certain impurities. The main impurity of this type is carbon monoxide, CO, which is difficult to separate from nitrogen using distiHation alone. The most common approach for CO removal is chemical conversion to CO2 using an oxidation catalyst in the feed air to the air separation unit. The additional CO2 which results, along with the CO2 from the atmosphere, is then removed by a prepuritication unit in the air separation unit. 

A Na+ Naj2[(A102)j2(Si02)j2] obstmcted 8-ring 0.38 desiccant CO2 removal air separation (N2)... 

A Ca + Ca5Na2[(A102)j2(Si02)j2] free 8-ring 0.44 linear paraffin separation air separation (O2)... 

A, 5A, and 13X zeoHtes are the predorninant adsorbents for CO2 removal by temperature-swing processes. The air fed to an air separation plant must be H2O- and C02-ftee to prevent fouling of heat exchangers at cryogenic temperatures 13X is typically used here. Another appHcation for 4A-type zeoHte is for CO2 removal from baseload and peak-shaving natural gas Hquefaction faciHties. 

Bulk Separations. Air separation, methane enrichment, and iso-/normal separations are the principal bulk separations for PSA. Others are the recovery of CO and CO2. 

The Eastman Chemicals from Coal faciUty is a series of nine complex interrelated plants. These plants include air separation, slurry preparation, gasification, acid gas removal, sulfur recovery, CO /H2 separation, methanol, methyl acetate, and acetic anhydride. A block flow diagram of the process is shown in Eigure 3. The faciUty covers an area of 2.2 x 10 (55 acres) at Eastman s main plant site in Kingsport, Teimessee. The air separation plant is... 

The latest of three ethylene recovery plants was started in 1991. Sasol sold almost 300,000 t of ethylene in 1992. Sasol also produces polypropylene at Secunda from propylene produced at Sasol Two. In 1992 Sasol started constmction of a linear alpha olefin plant at Secunda to be completed in 1994 (40). Initial production is expected to be 100,000 t/yr pentene and hexene. Sasol also has a project under constmction to extract and purify krypton and xenon from the air separation plants at Sasol Two. Other potential new products under consideration at Sasol are acrylonitrile, acetic acid, acetates, and alkylamines. 

Medium Heat- Value Gas. Medium heat-value (medium Btu) gas (6,7) has a heating value between 9 and 26 MJ/m (250 and 700 Btu/fT). At the lower end of this range, the gas is produced like low heat-value gas, with the notable exception that an air separation plant is added and relatively pure oxygen (qv) is used instead of air to partially oxidize the coal. This eliminates the potential for nitrogen in the product and increases the heating value of the product to 10.6 MJ /m (285 Btu/fT). Medium heat-value gas consists of a mixture of methane, carbon monoxide, hydrogen, and various other gases and is suitable as a fuel for industrial consumers. 

Fig. 5. Production of helium-group gases in a classical air-separation plant.

X 10 m (l.9x lO " /t ) of heHum in each cubic meter (35 ft ) of air entering the air separation process, the small quantities of cmde coUected in even a large air separation unit may be easily appreciated. It is sometimes desirable, therefore, to combine cmdes coUected from several air plants and to process them at a centralized location in specialized equipment. 

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

The U.S. production of argon is summarized in Table 5. Because argon is a by-product of air separation, its production is ca 1% that of air feed. Total 1988 United States consumption of neon, krypton, and xenon was 36,400, 6,800, and 1,200 m, respectively (88). 

Russia, nitrogen (qv) from the adjacent air-separation plant, and reformed gas with the purified fuel gas stream from the plant. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

---