HDS reactor

Another consequence of increased reaction temperatures is the shift in the equilibrium concentrations of polyaromatic hydrocarbons and their hydrogenated derivatives. At high temperatures, the fully aromatic hydrocarbons are thermodynamically favored, especially at the pressures of today s HDS reactors. As the concentration of polyaromatics increases, the fluorescence of the finished fuel increases. This may cause problems with other specifications, as discussed shortly. 

The catalyst pretreatment process for both the clay-supported and the reference catalysts consists of loading into the HDS reactor under N2, purging in N2 at 20°C for 30 min at 1000 cmVmin., drying in N2 at 150°C for 60 min and at 400°C for 60 min, and finally sulfiding in a 5% H2S/H2 mixture at 400°C for two hr prior to use as catalysts. The laboratory scale liquid-phase continuous-flow HDS reactor consists of a thick-walled 0.375" ID 316 SS tube, with 1 g catalyst diluted with 5 g tabular alumina (LaRoche T-1061, 10 m2/g) sitting between plugs of quartz wool. Beneath the lower plug is a 0.125" ID, 0.375" OD deadman used to minimize volume between the reactor and the liquid receiver. The liquid test feed consisted of 0.75 wt % sulfur as dibenzothiophene (DBT), dissolved in hexadecane and is representative of a middle distillate oil. All liquid-filled lines were heated to 50°C. The reaction was carried out at 400°C LHSV = 10-40/hr. 

The Neutralization Module accepts the ton container contents from the TCC module and destroys the agent batchwise through hydrolysis with water followed by caustic addition. The Neutralization Module consists of three units, each located inside a Containment Level A toxic cubicle. There are two HD Reactors and one TCC Effluent Tank in each of the three neutralization units. In each neutralization unit, drained agent held in the Agent Holding Tank is processed in batch neutralization reactions. The rinse and spray water from the TCC Module and spent decontamination solution are stored in the TCC Effluent Tank and process in separate batch reactions. In the neutralization reaction HD reacts with water to yield the principal hydrolysis products of thiodiglycol and hydrochloric acid. After the hydrolysis is complete and sample analysis results confirm the destruction of HD, 18 percent sodium hydroxide is added to the reactor to raise pH in order to increase the hydrolysate biodegradability. The hydrolysate is then pumped to the Hydrolysate Tank in the VOC Treatment Module. 

The vent streams from the HD reactors and the TCC Effluent Tank are combined, scrubbed with a sodium hydroxide scrubber, condensed and passed through dedicated carbon filters before entering the site cascade ventilation system. 

Figure 9-35 Liquid distribution system in the Unicracking/HDS reactor. 

The H2S produced is removed downstream from the HDS reactor by adsorption on ZnO particles at about 400-500°C. 

The commercial trickle-bed reactors, such as hydrodesulfurization and hydrocracking reactors, are often operated adiabatically. The temperature rise in such reactors is often controlled by the additions of a quench fluid (normally hydrogen) at one or more positions along the length of the reactor. A schematic of an adiabatic trickle-bed HDS reactor with a single quench is shown in Fig. 4-7. 

As time progresses, the catalyst in the HDS reactor decays because of metal (vanadium and nickel) and coke depositions. The deposition of these metals occurs nonuniformly along the length of the reactor (more deposits occur near the reactor inlet than at the reactor outlet). In normal plant operations, the catalyst activity decline is counterbalanced by a rise in feed temperature, a reduction in the amount of quench fluids fed to the reactor or both, so as to achieve the same quality product. The process is terminated upon the attainment of a maximum allowable temperature (MAT) anywhere in the reactor. The catalyst bed is then regenerated. The time required to achieve the MAT is often called the reactor cycle life. 

A model for an adiabatic HDS reactor (see Fig. 4-7) with a single quench is given by Shah et al.46 Under plug-flow conditions and assuming that there are no external mass-transfer resistances, the governing material and energy-balance equations are... 

Figure 4-7 Schematic diagram of an adiabatic fixed-bed HDS reactor with a single quench (after Shah et alft6).

Shah et al.46 also carried out a dynamic analysis of the commercial HDS reactor similar to the one described above, assuming the activity functions to be dependent upon the coke content of the catalyst. Exponentially decaying catalyst activity functions similar to the ones described by Szepe55 were used. This analysis gave results qualitatively similar to the ones described above. 

Normally, commercial HDS reactors are operated under adiabatic conditions. The heat removal is achieved by the addition of quench fluids. Mhaskar and Shah27 carried out a similar analysis for reactors which are operated non-isothermally under the conditions of either constant wall temperature or constant wall heat flux. For simplicity, they assumed that the catalyst desulfurization activity function

rate equation proposed by Szepe,55 namely,... 

The pressure-drop correlations outlined above assume a constant value of e, the bed void fraction. In industrial-hydroprocessing trickle-bed operations, such as in a hydrodcsulfurization reactor, the pressure drop has been found to increase with time. A typical behavior of the pressure drop across an industrial HDS reactor as a function of time is shown in Fig. 6-4. The pressure drop remains essentially constant over a long initial period, where the correlations given above should be useful. After a while, however, as shown in Fig. 6-4, the pressure drop increases very rapidly with time until the operation requires termination due to an excessive pressure drop across the bed. 

It is clear that a behavior such as the one shown in Fig. 6-4 is caused by plugging of the catalyst bed by solids deposition. The plugging causes the bed void fraction to decrease. Some speculations on the modes of deposition can be made from analysis of the plugging material. This material consists primarily of oxides of iron in the form of loose particles and is densely deposited in the inlet portion of the reactor. The material is probably carried by the fluid while it is flowing through rusted pipelines. As the reaction proceeds, bed plugging in an HDS reactor also occurs as a result of metal deposits, both nickel and vanadium, and coking of the catalyst. 

A mathematical formulation of the transient behavior of pressure in an HDS reactor is, of course, extremely complex and. as yet, has not been analyzed. In the limiting case, when the catalyst bed is heavily plugged with solid deposits, the problem of pressure-drop calculations is very similar to the pressure drop through an oil reservoir.72 The well-known Darcy type of equation should be applicable in this case. 

Mears,53 Paraskos et al.,66 Montagna and Shah,38 and Montagna et al.59 have recently shown that ineffective catalyst wetting can cause the reactor performance to be dependent on the liquid velocity. The y used a correlation of Puranik and Vogelpohl69 for the effectively wetted surface area of the packing to explain the effects ofliquid hourly space velocity and the length of the catalyst bed on the performance of bench-scale HDS reactors. 

Figure 9. Topical T versus t curve for various kinetic models and a cctnmercial HDS reactor. Ref [9]. Reproduced by permission Academic Press, he.

Another approach to reach 500 ppm sulphur in product is to adjust the plant capacity working at lower liquid hourly space velocity [LHSV]. If such a reduction is not acceptable, the volume of catalyst will have to be increased by addition a new HDS reactor. 

Figure 1,14 . Expanded view of a multi-bed HDS reactor showing the gas distribution tray at the top and the catalyst support tray in the middle these reactors can range in diameter from 13-5 m. Courtesy of UOP LlC.

Figure 1,13 Cross-sectional view of a commercial HDS reactor containing four fixed-bed reactors with interstage cooling/quenching quenching is necessary to limit the temperature increase for this exothermic reaction. Adapted from UOP LlC.

Your R D department has done substantial pilot plant work on this new process and has determined the following correlations to assist you in designing the HDS reactor. You also have processing data available from an older HDS unit within your company to use as a baseline. 

Another need for deep desulfurization is for potential application in fuel cell. Gasoline is the ideal fuel for fuel cell because of its high-energy density, ready availability, safety, and ease in storage. However, to avoid poisoning of the catalyst for the water-gas shift reaction and that in the electrode of the fuel cell, the sulfur concentration should be preferably below 0.1 ppmw. To reduce the sulfur content of diesel from 500 ppmw to this level, an estimate showed that the HDS reactor size needed to be increased by a factor of 7 (Whitehurst et al., 1998). Another estimate showed that in order to reduce the sulfur level in diesel from 300 to less than 10 ppm, the HDS reactor volume needed to be increased by a factor of about 15 at 600 psi, or by a factor of 5 at 1,000 psi (Parkinson, 2001 Avidan and Cullen, 2001). 

With respect to the increasing demand for practically S-free fuels (<10ppm), S-species with a very low reactivity like dibenzothiophene derivatives have to be converted (Figure 6.8.6), typically in a second deep HDS step, which leads to additional high investment and operating costs. The enormous expenses needed today for such a deep HDS of already hydrotreated (pre-desulfurized) diesel oil can be illustrated by typical industrial data [MiRO-refinery 2004 (oral communication) Wache et al. (2006)] Deep HDS reactors have a volume of up to 500 m to treat about 400 of oil per h. The feed rate of fresh H2 is about 40 m (NTP) per m oil, the recycle rate is about three, and for a typical pressure of 6 MPa the pressure drop of about 1 MPa has to be compensated by a huge recycle compressor (1 MW). 

For these reasons the design of a desulphurisation system must be undertaken with care. If the fuel cell plant has a source of hydrogen-rich gas (usually from the reformer exit), it is common practice to recycle a small amount of this back to a hydrodesulphurisation (HDS) reactor. In this reactor, any organic sulphur-containing compounds are converted, over a supported nickel-molybdenum oxide or cobalt-molybdenum oxide catalyst, into hydrogen sulphide via hydrogenolysis reactions of the type... 

One key issue to automate the hazard evaluation process is to find a systematic way to represent the fault propagation in the plant process. This section describes a new way of representing the fault propagation in process plant using examples from HDS reactor CGU. 

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