Hydrotreatment

In naphtha reforming, hydrotreatment is always applied to protect the platinum-containing catalyst against sulphur poisoning. The specification of sulphur content for the feed for the reforming unit is less than 1 ppm. 

The hydrogen used in naphtha hydrotreatment is a by-product of the catalytic reforming. When hydrotreatment of heavy residue is performed, separate H2 production units are often required. 

Unstable byproducts are formed in naphtha cracking, and these are hydrotreated in order to increase their stability. One of the important reactions occurring is the saturation of diolefins. 

The severity of the processing conditions depends on the feed for light petroleum fractions it will be milder than for heavy residues. Moreover, it is common practice to compensate for deactivation of the catalyst by increasing the temperature of the reactor. A simplified flow scheme involving a trickle flow reactor is shown in Fig. 2.5. 

Catalysts are mixed metal sulphides on a carrier. The major examples are supported mixed phases of CoS and M0S2, of NiS and M0S2 and NiS and WS2. There is as yet no full agreement on the structure. For the first catalyst the best description is small MoS2 clusters with CoS units at the edges. Typical dimensions of these clusters are 1 nm and a stacking between 1 and 3. 

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Table 3.7 gives results obtained by this method for a gas oil before and after hydrotreatment. 

The gas oils from visbreaking and coking have better cetane numbers than LCO but they are unstable and need hydrotreatment before they can be used. 

Numerous works have been directed towards studying the influence of diesel fuel hydrotreatment on emissions. 

Table 5.28 gives the modifications in physical/chemical characteristics resulting from deeper and deeper hydrotreatment (Martin et al., 1992). The sulfur contents could thus be reduced to first as low as a few hundred ppm, then to a few ppm. The level of aromatics in the selected example drops from 39% to 7% while the cetane number increases from 49 to 60. Note here that such a treatment, possible through experimental means, does not correspond to current industrial practice because of its high cost and its very high hydrogen consumption. 

Influence of deep hydrotreatment on the emissions of a diesel vehicld passenger car - ECE and EUDC cycle. ... 

A refinery lubricant base stock is obtained having an viscosity index around 100, certain hydrotreatments result in Vi s of 130, and paraffin hydroisomerization provides oils with a VI close to 150. 

Upstream of the refornjiing unit, the feedstock undergoes hydrotreatment so as to eliminate impliritles such as S, N, olefins, and metals which are all catalyst poisons. 

The treated water containing sodium chloride, cyanides, phenols and traces of H2S and NH3 is recycled to the crude desalting unit and used as wash water for the hydrotreaters and FCC units. 

Martin, B. and P.-H. Bigeard (1992), Hydrotreatment of diesel fuels -its impact on light- duty diesel engine pollutants . SAE paper No. 92-2268, International fuels and lubricants meeting, San Francisco, CA. 

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. 

Fig. 1. Distillation—hydrotreat process, where CW = cooling water.

Fig. 3. Simplified process flow diagram of a naphtha hydrotreater and rheniformer.

The cat products become feed to other units, such as alkylation and polymerization plants. High boiling liquid products are used to make lubes, and the gas goes into the refinery fuel systems. Cat cracking feed stocks come from atmospherie and vacuum stills, phenol extraction plants, hydrotreaters, deasphalters and cokers. 

Most sulfur compounds can be removed from petroleum streams through hydrotreatment processes, where hydrogen sulfide is produced and the corresponding hydrocarbon released. Hydrogen sulfide is then absorbed in a suitable absorbent and recovered as sulfur (Chapter 4). 

Reduce harmful impurities in petroleum fractions and residues to control pollution and to avoid poisoning certain processing catalysts. For example, hydrotreatment of naphtha feeds to catalytic reformers is essential because sulfur and nitrogen impurities poison the catalyst. 

Products from coking processes vary considerably with feed type and process conditions. These products are hydrocarbon gases, cracked naphtha, middle distillates, and coke. The gas and liquid products are characterized by a high percentage of unsaturation. Hydrotreatment is usually required to saturate olefinic compounds and to desulfurize products from coking units. 

Other important catalytic processes are those directed toward improving the product quality through hydrotreatment. These processes use heterogeneous hydrogenation catalysts. 

Feeds to hydrotreatment units vary widely they could he any petroleum fraction, from naphtha to crude residues. The process is relatively simple choosing the desulfurization process depends largely on the feed type, the level of impurities present, and the extent of treatment needed to suit the market requirement. Table 3-12 shows the feed and product properties from a hydro treatment unit. ... 

Vacuum gas oil hydrotreater Vacuum residuum hydrotreater Atmospheric residuum desulfurization hydrotreating... 

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant. The cobalt-molybdenum system supported on alumina was found to be an effective catalyst. 

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

For nonhydrotreated feed, the gasoline sulfur level is about 10% of the feed sulfur level. For hydrotreated feed, it is about 5%. For example, if the sulfur content of a nonhydrotreated feed is 1.0 r (10,000 ppm), the sulfur in FCC gasoline will be 1,000 ppm. Assuming 80% desulfurization, feed to the FCC unit will contain 0.2% (2000 ppm) sulfur, resulting in FCC gasoline containing 100 ppm sulfur. Sulfur compounds that survive the hydrotreater are in the heav fraction and tend to end up in the LCO, decanted oil, and coke. 

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