PASC

Remi caraballo, Carole Arnal-Herault, Andreea Pasc-Banu, Andre AYRAL, Arie van der Lee, Eddy Petit, Anca Meffre and Yves Marie Legrand. 

Cazacu, A., Pasc-Banu, A. and Barboiu, M. (2006) Molecular and supramolecular dynamics — a versatile tool for self organization of polymeric membranes systems. Macromolecular Symposia, 245—246, 435—438. 

Fig. 2. Separation and analysis of polyaromatic sulfur-containing compounds (PASC). Oils 3-1, 3-2, and 3-3 were analyzed by gas chromatography-mass spectroscopy (GC-MS) and gas chromatography-atomic emission detection (GC-AED). Reproduced from Ref. 12, with permission.

In seeking new and improved ways for achieving the ultralow levels of sulfur in the fuels of the future, it is important to understand the nature of the sulfur compounds that are to be converted (especially PASCs), as described in Section III. It is equally important to understand how these transformations occur through interactions with catalytic surface species, the pathways involved during these transformations, and the associated kinetic and thermodynamic limitations. These considerations dictate the process conditions and reactor process configurations that must be used to promote such transformations. In this section, we describe the reactor configurations and process conditions being used today what is known about the catalyst compositions, structure, and chemistry and what is known about the chemistry and reaction pathways for conversion of PASCs in conventional HDS processes. 

Most of today s distillate HDS processes consist of fixed-bed, down-flow reactors configured in a manner similar to that shown in Fig. 8 (7). It should be noted that hydrogen is used in excess and is recirculated after scrubbing out the H2S byproduct. Care must be used in the scrubbing operation as it is necessary to maintain a low but optimum level of H2S in the recycle stream to maintain catalyst stability and activity. The consequence of this H2S requirement when hydrotreating PASCs to extinction is discussed in more detail in later sections, but at this point it should be mentioned that H2S is a strong inhibitor of HDS for PASCs. 

To put these problems into perspective, as already discussed, the PASCs remaining at the 0.20% S level are lower in reactivity, by a factor of 10-50, than the sulfur compounds that are now removed in lowering the sulfur level from 1.2 to 0.20%, Even with catalysts 2 times as active, the reactor volume may have to be doubled to convert the required 75% of the least-reactive PASCs to achieve the 0.05% S target. Based on the composition of a typical gas oil and using the first-order rate constants for the different classes of sulfur compounds (14), a theoretical HDS severity plot is presented in Fig. 9. It can be seen that reactor volumes will have to be increased by about a factor of 4 to meet the new specifications unless much more active catalysts can be developed than are presently available. 

It is generally agreed that there is a common sequence of mechanistic pathways for all PASCs as shown earlier in Scheme 1. Though the partially hydrogenated thiophene intermediate is illustrated in that scheme as a tetrahydro derivative, it has been shown that this intermediate is, in reality, an equilibrium mixture of the tetra- and hexahydrodibenzothiophenes. The rate of equilibration is at least 10 times greater than the other associated rates. Thus, the pair can be treated as a single kinetic species (26). 

The results presented in Figs. 12a and 12b provide one other piece of information. As discussed in more detail later, NiMoS /A Oj has a relatively high activity for aromatic ring hydrogenation. This indicates that the catalyst of choice for high-conversion HDS may change as PASCs become more refractory due to steric hindrance imposed by alkyl substituents. This is discussed in more detail in the next section. 

In the previous sections, we have discussed the pathways by which PASCs are desulfurized and what is presently known about the structure of the active species in supported Co(Ni)-Mo-S catalysts. In this section, we discuss the chemical reactions and intermediates involved in the catalytic sequence that results in desulfurization. This sequence is often called the catalytic mechanism. 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Low reactivity of alkyl-substituted dibenzothiophenes (PASCs) that are substituted in positions adjacent to the sulfur atom... 

Insufficient activity of present catalysts to overcome the low-reactivity PASCs using existing moderate-pressure (<5 MPa) hydrotreating reactors... 

Harada, M., Hashimoto, K., and Fujiwara, K., 1994, Immunohistochemical distribution of CD44 and desmoplakin I II in Hailey-Hailey s disease and Darier s disease. J. Dermatol. 21, 389—393 Harper, C., Wootton, L., Michelangeli, F., Lefievre, L., Barratt, C., and Publicover, S., 2005, Secretory pathway ( a2 A l Pasc (SPCA1) Ca2+ pumps, not SERCAs, regulate complex [Ca2+]j signals in human spermatozoa. J. Cell Sci. 118, 1673—1685... 

Amal-Herault C, Barboiu M, Pasc A, Michau M, Perriat P, van der Lee A (2007) Constitutional self-organization of adenine-uracil-derived hybrid materials. Chem Eur J 13 6792-6800... 

Amal-Herault C, Pasc A, Michau M, Cot D, Petit E, Barboiu M (2007) Functional G-quartet macroscopic membrane films. Angew Chem Int Ed Engl 46 8409-8413... 

Michau M, Barboiu M, Caraballo R, Amal-Herault C, Perriat P, Van Der Lee A, Pasc A (2008) Ion-conduction pathways in self-organised ureidoarene-heteropolysiloxane hybrid membranes. Chem Eur J 14 1776—1783... 

Cazacu A, Legrand YM, Pasc A, Nasr G, Van der Lee A, Mahon E, Barboiu M (2009) Dynamic hybrid materials for constitutional self-instructed membranes. Proc Natl Acad Sci USA 106 8117-8122... 

Amal-Herault, Mathieu Michau, Andreea Pasc, Arie van der... 

Amal-Herault C, Pasc-Banu A, Barboiu M, Michau M, van der Lee A Amplification and transcription of the dynamic supramolecular chirality of the G-quadruplex. Angew. Chem. 2007 119 4346-4350 Angew. Chem. Int. Ed. 2007 46 4268-4272. 

Cazacu A, Michau M, Arnal-Herault C, Pasc-Banu A, Meffre A, Caraballo R, Pasc A, Barboiu M. Hybrid supramolecular dynamic membranes as selective information transfer devices. Desalination 2006 199 521-522. 

Consequently, Friedrich Pasc hen (1804-1873) included the culminations of the moon in his water level analyses at the Wismar gauge station (Paschen, 1856) in order to prove the existence of Baltic Sea tides. His primary goal was to clearly disprove the widespread notion that the ebb and flood tides are not noticeable in the Baltic Sea. This was also supported by the observation that, with constant wind conditions, the mouths of rivers discharging into the Baltic Sea could be observed to alternate several times a day between inflow and outflow. 

Pasc-Banu, A., Sugisaki, C., Gharsa, T., Marty, J.-D., Gascon,... 

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