Hydrotreat reaction

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

Ramirez de Agudelo, M. M. Galarraga, C., and Pimentel, M., The Competitive Inhibition in Hydrotreatment Reactions. inDiv. Pet. Chem. ACS Meeting, Symp. Mechanism ofHDS-HDN Reactions, 1993., August 22-27 Preprint 12pp. 

The arguments proving tiiis mechanism and the irrelevance of the 18 or so dtemative explanations proposed in literature are presented elsewhere (9,10). These include several proofs that the popular so-called "CoMoS" phase does not exist. If some unstable association characterized by a Mossbauer signal of Co detected both when Co is alone or in the presence of Mo, this fades away during activation or when catalysts are in the real conditions of hydrotreatment reactions. 

Catalysts for hydrotreatment reactions such as hydrodesulfurization (HDS), based on alumina-supported Mo or W (promoters Co or Ni) are active in the sulfided state. Thus, the oxidie eatalyst precursors have to be activated by treating with a mixture of H2S and H2. This sulfidation process can be studied by TPS. 

First order kinetic plots for hydrotreatment reactions (after Paraskos et al., I975). 

Noble metals are strongly inhibited by sulfur compounds and unable to promote any hydrotreatment reactions consequently they must be associated with a hydrotreatment catalyst. 

Thermochemical Liquefaction. Most of the research done since 1970 on the direct thermochemical Hquefaction of biomass has been concentrated on the use of various pyrolytic techniques for the production of Hquid fuels and fuel components (96,112,125,166,167). Some of the techniques investigated are entrained-flow pyrolysis, vacuum pyrolysis, rapid and flash pyrolysis, ultrafast pyrolysis in vortex reactors, fluid-bed pyrolysis, low temperature pyrolysis at long reaction times, and updraft fixed-bed pyrolysis. Other research has been done to develop low cost, upgrading methods to convert the complex mixtures formed on pyrolysis of biomass to high quaHty transportation fuels, and to study Hquefaction at high pressures via solvolysis, steam—water treatment, catalytic hydrotreatment, and noncatalytic and catalytic treatment in aqueous systems. 

Reactions occurring in hydrotreatment units are mainly hydrodesulfurization and hydrodenitrogenation of sulfur and nitrogen compounds. In... 

Reactions occurring in hydrotreatment include both hydrogenation and hydrogenolysis reactions. 

The selectivity of a catalyst is typically optimized towards a reaction type, but some operations required a high level of removal for more than one contaminant. In fact, the treatment of a VGO, for instance, involves the removal of metals, S and N. Depending on the quality of the feed and on the specifications of the desired product, the hydrotreatment may require more than one catalyst. The catalyst can be stacked in a single reactor or disposed in sequential stages, when more than one reactor is available. Stacked-bed reactors with more than one catalyst type are a common practice in HDT. 

Some other processes are based on a severe hydrotreatment followed by a stage for octane recovery. Octgain from ExxonMobil [57] and ISAL from UOP-Intevep [58], Deep desulfurization is achieved by an increase in severity, causing lost in octane by olefins saturation. In the first case, in a second reactor octane number is recovered by a combination of cracking and isomerization reactions. In the latter case, the catalyst employed during desulfurization possess isomerization capabilities inhibiting an excessive octane lost. Other mentioned functionalities of the catalyst include dealkylation and conversion. 

Kinetics studies of the hydrotreatment (and hydrocracking) of VR has led to the conclusion that most of the metals, sulfur and nitrogen removal takes place during the first 50% of the whole VR conversion [119-123], More than one reactor was needed for HDM and HDS of a Maya VR, when HDT is used as feed pretreatment [119,120], Although vanadium removal appears easier and faster than nickel removal, their kinetics results showed very similar values of the activation energy for the demetallization reactions [122],... 

The process involves reacting butenes and mixtures of propenes and butenes with either a phosphoric acid type catalyst (UOP Process) or a nickel complex-alkyl aluminum type catalyst (IFP Dimersol Process) to produce primarily hexene, heptene, and octene olefins. The reaction first proceeds through the formation of a carbocation which then combines with an olefin to form a new carbocation species. The acid proton donated to the olefin initially is then released and the new olefin forms. Hydrotreatment of the newly formed olefin species results in stable, high-octane blending components. 

The products were solvent fractionated into hexane soluble (HS), hexane insoluble-benzene soluble (HI-BS), and benzene insoluble (Bl) fractions. The yields of these solvent-fractionated products after hydrotreatment of SRC are plotted against the reaction time in Fig. 13. The overall activities of the catalysts were very similar to those of the commercial catalyst in spite of their lower surface areas. Both exploratory catalysts (Cat-A and Cat-B) showed similar reaction profiles, which were markedly different from those of the commercial catalyst. The BI fraction decreased over the exploratory catalysts equally as well as the over the commercial catalyst. However, the HS fraction hardly increased as long as the BI fraction was present. As the result, the HI-BS fraction increased to a maximum just before the BI fraction disappeared and then rapidly decreased to complete conversion after about 9 hr. The rate of HS formation increased correspondingly during this time. Thus, the exploratory catalysts were found to exhibit a preferential selectivity for conversion of heavier components of SRC, compared to the commercial catalyst. These results emphasize that the chemical and physical natures of the support are important in catalyst design (49). 

Fig. 13. The yields of solvent-fractionated products in the hydrotreatment of SRC vs reaction time. Catalyst (a) commercial catalyst (b) Cat.-A (c) Cat.-B. Reaction conditions SRC/ Cat. ratio (weight), (a) and (c) 10/1 (b) 5/1 reaction temp, 380°C H2 initial pressure, 100 kg/cnr, B1 (O) HI-BS ( ) HS (A) open first run closed second run.

For Runs 2, 3, 1, and 5 the feed entered the hydrotreater directly from the supercritical extraction unit. The extract contained about k% shale oil in toluene. Runs 2 and 3 were carried out at 81+2F, and Runs k and 5, at T50F. At the lower reaction temperature (750F), the yield of gases dropped to less than 2, and the yield of heavy oil fraction increased by about 10. The extent of nitrogen removal was reduced significantly at the lower temperature. However, the sulfur removal seemed to be unaffected by the lowering of reaction temperature from 8 2F... 

Hydrotreat To contact a hydrocarbon with hydrogen at moderate to high temperatures and pressures in order to perform hydrogenation reactions. 

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

The following developments will be restricted to laminar liquid flow with weak gas-liquid interactions. However, this is not a limitation of the proposed methodology which could be easily applied to any other flow regime. Applications will be presented for the modelling of the irrigation rate, the dynamic liquid holdup and the apparent reaction rate in the absence of external mass transfer limitations and in the case of non volatile liquid reactants (i.e. approximatively the operating conditions of petroleum hydrotreatment). 

---