Molecular sieve paraffin separation adsorbent

Displacement-Purge and Inert-Purge Cycles By far the most widespread embodiment of these cycles Is the separation of normal and lso-parafflns In a variety of petroleum fractions. These fractions In general contain several carbon numbers and can Include molecules from about C5 to about C g All of these separations use 5A molecular sieve as the adsorbent Its 0.5 nm pore diameter Is such that normal paraffins can enter but lso-parafflns are excluded this constitutes the basis for separation. 

However, ia some cases, the answer is not clear. A variety of factors need to be taken iato consideration before a clear choice emerges. Eor example, UOP s Molex and IsoSiv processes are used to separate normal paraffins from non-normals and aromatics ia feedstocks containing C —C2Q hydrocarbons, and both processes use molecular sieve adsorbents. However, Molex operates ia simulated moving-bed mode ia Hquid phase, and IsoSiv operates ia gas phase, with temperature swiag desorption by a displacement fluid. The foUowiag comparison of UOP s Molex and IsoSiv processes iadicates some of the primary factors that are often used ia decision making ... 

Separation of Norma/ and Isoparaffins. The recovery of normal paraffins from mixed refinery streams was one of the first commercial appHcations of molecular sieves. Using Type 5A molecular sieve, the / -paraffins can be adsorbed and the branched and cycHc hydrocarbons rejected. During the adsorption step, the effluent contains isoparaffins. During the desorption step, the / -paraffins are recovered. Isothermal operation is typical. 

Even if 5A zeolite is widely used in iso-paraffin separation from an n/iso paraffin mixture, the adsorbent is affected by a slow deactivation mainly due to coke formation inside the molecular sieve porosity. Its aging phenomenon decreases its sorption properties. According to previous studies, 5A zeolite deactivation results essentially from heavy carbonaceous compound formation in a-cages blocking the 5A zeolite microporosity [1-2]. 

Union Carbide s OlefinSiv Process. Union Carbide s OlefinSiv process is used mainly to separate n-butylenes from isobutylene 31). The basic hardware is the same as for the IsoSiv process for n-paraffin separation, and the process uses a rapid cycle, fixed-bed adsorption. Since this process separates straight-chain olefins from branched-chain olefins, it is reasonable to assume that a 5A molecular sieve is used as the adsorbent. Product purities are claimed to be above 99% for both n-butylene and isobutylene streams. 

The most comrson use for these cycles is the separation of normal and isoparaffins in a variety of petroleum fractions. Distillation is not practical for these separations because the boiling ranges of the two products overlap. The feedstock most often contains several caibon numbers and can range from about C3 to CIS The adsorbent used is 5A molecular sieve, whose 0.5 nm pones ndmit normal paraffins and exclude isoparaffins. 

Adsorption processes with molecular sieves as adsorbents are a good tool to separate mixtures of -paraffins, isoparaffins, olefines, and cyclic hydrocarbons according to the PSA mode. 

Several different adsorption processes for the separation of linear paraffins have been developed including Ensorb (Exxon), IsoSiv (Union Carbide), T. S. F. (Texaco), the Shell process, and the Leuna Werke process. The latter has been called Parex (paraffin extraction) but the choice of name is unfortunate because of possible confusion with the UOP Parex process for separation of xylene isomers. All these processes use a 5A molecular sieve, generally in binderless form to minimize nonselective adsorption. The C,o-C,g linear paraffins are strongly adsorbed even at temperatures as high as 350°C. Thermal swing desorption is not feasible since the temperature required for desorption is so high that coking would occur. The alternatives are therefore vacuum desorption, which is used in some versions of the IsoSiv process, or displacement desorption which is used in most if not all of the other processes. 

Type 5A (five angstroms). Molecular sieve is the calcium form of the zeolite. Type 5A adsorbs molecules having a critical diameter of less than five angstroms (e.g., methanol, ethane, propane). Type 5A sieves can be used to separate normal paraffins from branched-chain and/cyclic hydrocarbons through a selective adsorption process. 

Molecular-sieving effects based on size/shape exclusion are common in rigid zeolites and molecular sieves. One famous example is the separation of normal paraffins from branched-chain and cyclic hydrocarbons by using a 5-A molecular sieve. Similar selective adsorption effects have been observed in several porous MOFs. Kim and coworkers reported that Mn(HCOO)2 has a robust 3D framework structure with ID channels interconnected by small win-do ws/apertures. This material can selectively adsorb H2 over N2 and Ar at 78 K, and CO2 over CH4 at 195 K, as indicated by the gas adsorption isotherms. In both cases, the uptake of the excluded gases N2, Ar, and CH4 was negligible. Thus, the selectivity was attributed to the small aperture of the channels. An interpenetrated MOF, PCN-17, contains nanoscopic cages with a window size of 3.5A and displays selective adsorption of H2 and O2 over N2 and CO. ° MIL-96 " and Zn2(cnc)2(dpt) were also found to selectively adsorb CO2 over CH4 based on size/shape... 

Al-Damkhi, A.M. Al-Ameeri, R.S. Jeffreys, G.V., and Mumford, C.J., Optimal separation of n-paraffins from Kuwait kerosene using a molecular sieve adsorbent, J. Chem. Technol. Biotechnol., 37(4), 215-228 (1987). Aracil, J., Use of factorial design of experiments in the determination of adsorption equilibrium constants (methyl iodide on charcoals), J. Chem. Technol. Biotechnol.. 38(3), 143-152 (1987). 

The main applications of PSA are to be found in the production of oxygen from air, dehumidification of gases and purification of hydrogen. Other applications include removal of carbon dioxide, recovery of radioactive waste gas, enrichment recovery of rare gases, purification of helium, purification of natural gases, separation of isomers and separation of carbon monoxide. Separation of iso-parafllns from normal paraffins is accomplished by using a shape-selective adsorbent such as a molecular sieve. Separation of carbon monoxide involves chemical adsorption on complex adsorbents. 

Due to these properties zeolites are applied to numerous industrial processes as catalysts, selective adsorbents, ion exchangers as well as molecular sieves for the separation of n-paraffins, air constituents, the sweetening of natural gases, as ion exchanger in detergents and for the extraction of metal ions from waste water including those of radioactive isotopes from nuclear plants. 

A mixture of hydrocarbons can be separated by three selective adsorbents molecular sieves which retain the n-paraffins, mercuric perchlorate which adsorbs the unsaturated compounds, and a stationary liquid phase which retains the aromatic hydrocarbons. Boric acid on a stationary phase such as Chromosorb P is used for the removal of alcohols from a mixture of organic compounds, the products are nonvolatile esters. A similar technique was used for removal of terpene alcohols from a mixture of terpenoids. Acids can be adsorbed on potassium hydroxide deposited on quartz powder. This technique was found to be suitable for the analysis of compounds having active hydrogens such as fluorene, indene, carbazole, indole, and pyrrole as well as steroids such as estrogens and ketosteroids. o-Dianisidine quantitatively subtracts aldehydes, ketones, and epoxides, and phosphoric acid subtracts epoxides. 

Because of its electrical charge, the aluminosilicate network is hydrophilic and is used to dry non-aqueous solvents. The aluminate ion may be removed froni the structure without significant modification of the framework, giving the network a hydrophobic character. The resulting material is used as a molecular sieve for the separation of paraffins linear molecules are adsorbed in the network whereas branched molecules are excluded from it. 

The most important class of inorganic adsorbents is the zeolites, a subclass of molecular sieves. These are crystalline aluminosilicates with specific pore sizes located within small crystals. Two common classes have simple cubic crystals (type A) or body-centered cubic crystals (type X). Sometimes, the type is assigned a number equal to a nominal pore size in the crystals. For example, zeolite 5A with a nominal 5 A pore size is used to separate normal from branched paraffins. 

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