Heavy desorbent

Zinnen, H.A. (1991) Zeolitic para-ethyltoluene separation with tetralin heavy desorbent. U.S. Patent 4,956,522. 

Unlike the gasoline Molex process that employs a iso-butane and n-butane desorbent mixture, the MaxEne process employs a heavy desorbent system. A heavy desorbent system means that the bottom product from both the Sorbex extract and raffinate frachonation columns is desorbent while the feed components are recovered as overhead products. In the MaxEne process case, heavy normal paraffin such as n-dodecane is employed as the desorbent though desorbents as light as n-decane and as heavy as n-tetradecane are possible candidates too. 

The C4 Olex process is designed with the full allotment of Sorbex beds in addition to the four basic Sorbex zones. The C4 Olex process employs sufficient operating temperature to overcome diffusion limitations with a corresponding operating pressure to maintain liquid-phase operation. The C4 Olex process utilizes a mixed paraffin/olefin heavy desorbent. In this case it is an olefin/paraffin mix consisting of n-hexene isomers and -hexane. A rerun column is needed to remove heavy feed components such as Cs/C because they would contaminate or dilute the hexene/hexane desorbent. Table 8.5 contains the typical feed and product distributions. 

The normal regeneration temperature for siUca gel is 175°C. In hydrocarbon service, higher temperatures (225—275°C) are recommended to desorb heavy hydrocarbons, which tend to foul the adsorbent during prolonged use (see Silicon compounds). 

Fig. 3. Schematics of a matrix-assisted laser desorption time-of-flight mass spectrometer. Ions are desorbed from the target, accelerated into the machine, and left to drift freely in the flight tube. Heavy ions are slower than light ions and reach the detector later. The flight time is converted into mass using an appropriate calibration.

Thermotech Systems Corporation (Thermotech) has patented a thermal desorption, two-stage tandem soil remediation unit (TDU) that treats and desorbs light and heavy hydrocarbons from contaminated soils, clays, and drilling muds. Thermotech s TDU does not incinerate soil, but rather cleans and recycles it. The technology has been commercially available since 1991. 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

No products of disproportionation have ever been observed in the studies of Katzer s and Satterfield s groups, probably because either these products would be too heavy to desorb and thus to be detected, or steric hindrance due to the second ring, absent in the pyridine-piperidine reactions, prevents the alkyl transfer. However Schultz et al.33 found alkyl addition products, but did not specify whether these were A-alkyl or C-alkyl molecules. The formation of C-alkyl products will be shown later with the low pressure reaction. 

For all samples pretreated under vacuum at temperatures in the 298 - 573 K range, optimal H/D exchange was observed if D20 adsorption was performed for lh at room temperature, followed by evacuation at the same temperature as pretreatment. Deuteration may be considered as a rehydration of the surface, with heavy water, followed by H/D exchange. Product HDO and H20 molecules are thermally desorbed in the evacuation step. In this way, H/D exchange occurs without rehydroxylation, the deuterated silica has a deuterated surface silanol distribution identical to the initial pretreated silica. 

As mentioned above, in each section of the SMBR, a specific task must be fulfilled and certain binary selectivities can be chosen as key adsorption parameters to explain the separation behavior in each section. In section I, the heavy product (water) must be desorbed by the eluent (methanol) from the ion-exchange resin, and therefore a low selectivity between water and methanol is desirable in order to reduce the methanol flow rate in this section as much as possible. In section IV, the fight component (methyl acetate) is removed from the eluent stream, and consequently a low-selectivity methanol/methyl acetate is advantageous. In sections II and III, the products water and methyl acetate are separated and a high-selectivity water/ methyl acetate is therefore beneficial. The selectivity of acetic acid to methyl acetate and the water/acetic acid selectivity should also be high in order to permit an easy separation of acetic acid from the products - which in turn allows the residence time of acetic acid to be increased and the conversion to be raised. 

A light desorbent (toluene) can be used with the hrst type of feed. With the others, heavy eluants are preferable (diethylbenzene and more spedfically p-diethylbenzcse or a 70/30 mixture of p< ethylbenzene and to C 3 paraffins, etc.). Sieve life is about 5 years. 

Cadmium leached much more readily than the other metals and may have been bound by a different mechanism, possibly by particle surface adsorption rather than by tight complexation. Cadmium was the most soluble cationic heavy metal used in this experiment. It desorbs at higher pH and is less tightly adsorbed than chromium or lead. It was first detected in the fourth... 

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