Hydrocracking Reactor Operation

Kinetics is the study of the rates of reaction. The rates of reaction determine the key properties of a hydrocracking catalyst initial activity, selectivity, stability and product quality. The temperature required to obtain the desired product at the start of the run measures the initial activity. In general, the catalyst activity is a measure of the relative rate of feedstock conversion. In hydrocracking, activity is defined as the temperature required obtaining fixed conversion under certain process conditions. Hydrocracking conversion is usually defined in terms of change of endpoint  

Catalyst selectivity is a measure of the rate of formation of a desired product relative to the rate of conversion of the feed (or formation of other products). Hydrocracking selectivity is expressed as the yield of desired product at a specific conversion. Yield is determined by the rate of formation of the desired product relative to the feed rate. At 100% conversion, catalyst yield equals catalyst selectivity. Hydrocracking selectivity is affected by operating conditions. In general, more severe operating conditions cause higher selectivity for secondary products. 

The product quality is a measure of the ability of the process to yield products with the desired use specification such as pour point, smoke point or octane. Table 8.2 shows some of the important product quality measurements and the chemical basis for these measurements. 

High Octane High Raio of i/n Paraffins High Concentration of Aromatics High Concentration of Naphthenes 

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Pressure Vessels. Refineries have many pressure vessels, e.g., hydrocracker reactors, cokers, and catalytic cracking regenerators, that operate within the creep range, i.e., above 650°F. However, the phenomenon of creep does not become an important factor until temperatures are over 800°F. Below this temperature, the design stresses are usually based on the short-time, elevated temperature, tensile test. 

In the two-stage operation, the feed is hydrodesulfurized in the first reactor with partial hydrocracking. Reactor effluent goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The liquid portion from the separator is fractionated, and the bottoms of the fractionator are sent to the second stage reactor. 

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. 

The LC-Fining process is a hydrocracking process capable of desulfurizing, demetallizing, and upgrading a wide spectrum of heavy feedstocks by means of an expanded bed reactor. Operating with the expanded bed allows the processing of heavy feedstocks, such as atmospheric residua, vacuum residua, and oil sand bitumen. 

A diagram of a five-bed hydrocracker is shown in Fig. 15. The dimensions of this particular commercial reactor were 10 ft diameter with 8in.-thick walls. The design of hydrocracking reactors must be carefully considered on account of the severe operating conditions to which they are subjected, namely ... 

Most commercial trickle-bed reactors operate adiabatically at high temperatures and high pressures and generally involve hydrogenation, oxidation, desulfurization, hydrocracking, etc. The most important hydrodynamic properties for trickle-bed reactors are i) the liquid holdup that controls the liquid-to-gas reactant... 

During design, limits on temperature rise (Tise = Tom - T ) set the size of catalyst beds and determine the number and location of quench zones. During operation, when feeds (and maybe catalysts) are different, the Tnse is also different - sometimes dangerously different. A sudden spike in Tnse can lead to a temperature runaway or temperature excursion. These are dangerous. The rates of cracking reactions increase exponentially with temperature - the hotter they get, the faster they get hot. In a few cases, temperature mnaways have melted holes in the stainless steel walls of hydrocracking reactors. This is remarkable, because the walls were more than 8 inches (20 cm) thick. 

When a hydrocracker is operated with recycle of the unconverted feed, PNA with more than seven benzene rings are created. These are called HPNA (heavy polynuclear aromatics). The consequences of PNA formation are shown in Figure 13. The HPNA produced on the catalyst may exit the reactor and cause downstream fouling or they may deposit on the catalyst and form coke, which deactivates the catalyst. Their presence results in plugging... 

Slurry-phase hydrocracking systems convert heavy vacuum residues however, these processes are not yet fully commercialized. The feed to this type of reactor is the petroleum residue plus a solid carrier (commonly known as additive). The purpose of the additive is to provide a surface for the deposition of converted asphaltenes and metals, as the residue is hydrocracked. Slurry reactors operate at high temperature and pressure, and residue conversions higher than 90% (Kressmann et al., 1998). Unfortunately, these units produce poor-quality, hydrogen-deficient distillate and vacuum products that cannot be used as fuel, unless blended with something else, for example, coal or heavy fuel oil, due to their high content of sulfur and metals (Ancheyta and Speight, 2007). 

Galiasso, R. 2007. Effect of recycling the unconverted residue on a hydrocracking catalyst operating in an ebullated bed reactor. Fuel Process. Technol. 88 779-785. 

Galiasso, R., Caprioli, L. 2005. Catalyst pore plugging effects on hydrocracking reactions in an ebullated bed reactor operation. Catal. Today 109 185-194. 

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