Adsorbents normal paraffin separation

The fifth and final adsorbent characteristic is zeolite type. The adsorbent used in the Molex process is a proprietary and is a particularly effective adsorbent for normal paraffin separation [4, 5] and has achieved purity and recovery targets for the Molex processes. A sampUng of various molecules (and their corresponding dimensions) that Molex can easily separate is listed in Table 8.1. As discussed in Chapter 6, a zeoUtes s pore structure is dependent on its silica aluminum ratio and the proprietary Molex adsorbent possess a uniform repeating three-dimensional porous structure with pores running perpendicular to each other in the x. 

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. 

Examples of rate-selective adsorption are demonstrated using silicalite adsorbent for separation of Ciq-Cm n-paraffins from non- -paraffins [40, 41] and Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45]. Silicalite is a ten-ringed zeolite with a pore opening of 5.4A x 5.7 A [22]. In the case of -paraffins/non-n-paraffins separation [40, 41], n-paraffins enter the pores of silicalite freely, but non-n-paraffins such as aromatics, naphthenes and iso-paraffins diffuse into the pores more slowly. However, the diffusion rates of both normal -paraffins and non-n-paraffins increase with temperature. So, one would expect to see minimal separation of n-paraffins from non-n-paraffins at high temperatures but high separation at lower temperature. 

The third desorbent characteristic is that the desorbent material must be easily separated from the two Sorbex process products extract and raffinate. The adsorbent chamber s composition profile produces extract and raffinate streams comin-gled with desorbent. In order for the process to be economical, the separation of the feed components from the desorbent (achieved through fractionation) is set by the boiling point differences between the species. Depending on the selectivity possessed by the desorbent over that of the feed normals, the subsequent desorbent rates needed to flush feed normal paraffins from the adsorbent s selective volume and the resulting extract or raffinate streams from the Sorbex chambers could contain in some cases more than 50% desorbent. High concentration of desorbent demonstrates the importance of the desorbent characteristics when selecting a desorbent. 

As promised, this chapter outhnes numerous liquid-phase non-aromatic adsorption processes that enable one to economically separate a commercially desirable component from a mixture when the separation is impossible (given the closeness of their relative volatilities) by conventional means such as distillation. We review process that can separate a wide range of normal paraffins from a mixture of their corresponding feedstocks. In addition to this, we also review how to separate mono branched paraffins and olefins from similar feedstocks. Finally we review liquid-phase adsorption processes to isolate desired carbohydrates, fatty acids and citric acid from their feed source and for each separation we reveal insight on the corresponding operating conditions, process configuration and adsorbent necessary to achieve the separation. 

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. 

In the TIP process the Hysomer process is combined with the ISOSIV process which separates normal paraffins from branched ones by selectively adsorbing the normal fraction into zeolite CaA (pressure swing adsorption). Ajfter desorption (by applying vacuum) the normal paraffins are recycled. A schematic view... 

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. 

The last example to be discussed was mentioned near the introduction of the chapter, namely the selective separation of normal and iso-paraffins over zeolite 5A (Ca-LTA). While normal paraffins can readily adsorb into the LTA material, the iso-paraffins cannot. This is one of the few examples in gas separations wherein a true sieving effect is utilized. What is observed in practice is that upon exposing the zeolite bed to the feed stream, iso-paraffins are quickly observed at the exit of the bed, as they do not diffuse through the particles but rather around them. At a later time, once the zeohte has been saturated with normal paraffins they are then observed at the bed exit. 

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... 

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. 

Before 1925, there were a few compounded oils made for special purposes, such as lubrication of marine engines and steam cylinders, but additives were not used in automotive crankcase oils. In the 1930 s, chemical compounds made by condensation of chlorinated paraffin wax with naphthalene were found to lower the pour points of oils. Pour depressants (9) apparently are adsorbed on small wax crystals which separate from oils when they are chilled. The protective adsorbed layer of additive prevents the normal interlacing of larger wax crystals which forms a gel. In 1934 polymerized unsaturated hydrocarbons first came into large scale commercial use to lower the temperature coefficient of viscosity of oils. Other compounds for increasing the viscosity index of oils have since become common. 

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