Zeolites normal paraffin separation adsorbents

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. 

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. 

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 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 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. 

---