Hydrotreating Process Flow

A modem petroleum refinery may have four or more hydrotreating units. Strictly speaking, hydrotreaters are not conversion imits because the breaking of carbon-to-carbon bonds is minimal. However, it is convenient to discuss hydrotreating together with hydrocracking and mild hydrocracking because they employ similar catalysts and process flow schemes. 

The key differences are presented in Table 14. Hydrocrackers tend to operate at higher pressure, using different catalysts, and with lower linear hourly space velocity (LHSV). LHSV is equal to the volume of feed per hour divided by the catalyst volume. A lower required LHSV means that a given volume of feed requires more catalyst. In terms of process conditions and conversion, mild hydrocracking lies somewhere between hydrotreating and full-conversion hydrocracking. 

To one extent or another, all of the chemical reactions listed in Table 15 occur in hydrotreaters and hydrocrackers. The reactions are discussed in greater detail in Chapters 7-9. 

Hydrotreatiiig is exothemiic (heat-releasing), so many commercial units comprise several catalyst beds separated by quench zones. In a quench zone, hot process fluids from the preceding bed are mixed with relatively cold, hydrogen-rich quench gas before passing to the next bed. 

HDS and HDN reactions produce H2S and NH3, respectively. Wash water is injected into the effluent from the last reactor to remove ammonia, which goes into the aqueous phase as ammonium bisulfide, NHiHSCaq). The NHjHSCaq) is rejected from the imit as sour water in downstream flash drums. 

---

Trickle-bed reactors are widely used in hydrotreating processes, i.e., hydrodesulfurization of gasoline and diesel fuel, in petroleum refining, chemical, petrochemical, and biochemical processes. The knowledge of hydrodynamic parameters is vital in the design of a TBR because the conversion of reactants, reaction yield, and selectivity depend not only on reaction kinetics, operating pressure, and temperature, but also on the hydrodynamics of the reactor. Special care is also required to prevent flow maldistribution, which can cause incomplete catalyst wetting in some parts... 

E.g. in the so-called "pseudo-equilibrium model, developed by Sylvester [53-56], the same design procedure is used as in a single phase catalytic gas phase reaction, where the mass transfer resistance is replaced by a suitable overall term. Bulk flow and dispersion of the liquid phase are neglected and the whole transport mechanisms are lumped into the equilibrium of the reactant concentrations between gas-, liquid- and particle phase. It is an application of the same principle used successfully in fluid/fluid reactions [57], But the necessary precondition is that the rate of reaction is slow compared to the transfer rate across the phase boundaries, so that equilibrium can really by assured. This might be justified in some of the hydrotreating processes, but certainly not in case of an aqueous liquid phase, existing in waste water treating. Earlier models used in petroleum industry have taken in-... 

In both the liquid- and vapor-phase extraction processes, the kerosene feed is typically prefractionated to narrow the feed to the desired four-carbon number range (either Cjo-Cu or C11-C14). This heartcut is hydrotreated to remove the majority of kerosene contaminants that may compromise the performance of life of the adsorbent or subsequent quality of the LAB or LAS properties. In some process flow schemes, the fractionation into the discrete n-paraffin cuts may be deferred until after the extraction process. 

Although there are about thirty hydrotreating processes available for licensing, most of them have essentially the same process flow for a given application. Figure 1 illustrates a typical... 

A typical flow diagram for hydrocracking is shown in Fig. 15.17. Process flow is similar to hydrotreating in that feed is pumped to operating pressure, mixed with a hydrogen-... 

Air emissions from hydrotreating may arise from process heater flue gas, vents, and fugitive emissions. Figure 6 provides a simplified flow diagram. 

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. 

In the Energy Research and Development Administration s SYNTHOIL process, slurries of coal in recycle oil are hydrotreated on Co-Mo/Si02 Al203 catalyst in turbulent flow, packed-bed reactors. The reaction is conducted at 2,000 to 4,000 psi and about 450° C under which conditions coal is converted to low-sulfur liquid hydrocarbons and sulfur is eliminated as E2S. 

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation). 

In many large-scale reactors, such as those used for hydrotreating, and reaction systems where deactivation by poisoning occurs, the catalyst decay is relatively slow. In these continuous-flow systems, constant conversion is usually necessary in order that subsequent processing steps (e.g., separation) are not upset. One w to maintain a constant conversion with a decaying catalyst in a packed or fluidized bed is to increase the reaction rate by steadily increasing the feed temperature to the reactor. (See Figme 10-26.)... 

In a trickle bed reactor the gas and liquid flow (trickle) concurrently downward over a packed bed of catalyst particles. Industrial trickle beds are typically 3 to 6 m deep and up to 3 m in diameter and are filled with catalyst particles ranging irom to in. in diameter. The pores of the catalyst are filled with liquid. In petroleum refining, pressures of 34 to 100 atm and temperatures of 350 to 425°C are not uncommon. A pilot-plant trickle bed reactor might be about 1 m deep and 4 cm in diameter. Trickle beds are used in such processes as the hydrodesulfurization of heavy oil stocks, the hydrotreating of lubricating oils, and reactions such as the production of butynediol from acetylene and aqueous formaldehyde over a copper acetylide catalyst. It is on this latter type of reaction,... 

---