Space hydrocracking

Hydrocracking reaction conditions vary widely, depending on the feed and the required products. Temperature and pressure range from 400 to 480°C and 35 to 170 atmospheres. Space velocities in the range of 0.5 to 2.0 hr" are applied. Figure 3-8 shows the Chevron two-stage hydrocracking process. 

VGO hydrocracking is typically carried out at temperatures of 260-480°C, total pressure between 500-3000 psig, and liquid hourly space velocity ranging form 0.5 to 4h 1, leading to hydrogen consumption of 170-340 Nm3/m. ... 

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

In most applications, the reaction occurs between a dissolved gas and a liquid-phase reactant in the presence of a solid catalyst. In some cases, the liquid is an inert medium and the reaction takes place between the dissolved gases at the solid surface. These reactors have many diverse applications in catalytic processes and are used extensively in the chemical industry. Trickle-bed reactors have been developed by the petroleum industry for hydrodesulfurization, hydrocracking, and hydrotreating of various petroleum fractions of relatively high boiling point. Under reaction conditions, the hydrocarbon feed is frequently a vapor-liquid mixture that reacts at liquid hourly space velocities (LHSV in volume of fresh feed, as liquid/volume of bed, hr) in the... 

The selectivity of the hydrocracking process is dependent upon the severity of operation. The experimental design was such that conversion was largely dependent upon temperature. It was not practical to achieve equivalent conversions by changing liquid hourly space velocity (LHSV). A typical selectivity change with temperature is illustrated in Table V. 

Accelerated deactivation tests in hydrocracking have been reported (38), where a constant conversion mode was run at much higher space velocity (and hence temperature) than under actual operation conditions. Differences in deactivation were measured that were later substantiated in commercial operation (38). Although all these approaches aim at accelerating the catalyst deactivation reaction in Equation 7, such tests should obviously not be applied to catalyst systems that — at the high space velocity — operate at such high temperatures that very high polyaromatics concentrations prevail. 

Catalytic tar elimination over nickel-based catalysts mainly proceeds by steam and dry (CO2) reforming reactions, although there can be simultaneous thermal reactions of cracking and, perhaps, of hydrocracking. Therefore, the steam and COj contents in the flue gas have an important role in the overall tar elimination. Tar conversion (elimination) depends on the properties of the catalyst used, on the space-time (or space-velocity), bed temperature, H2O to carbon to be reformed ratio and on the operation variables of the upstream gasifier such as equivalence ratio and temperatures in the bed and in the freeboard. 

The initial reaction rates for hydrodealkylation, hydrocracking and condensation were determined as slope of the dependence of the conversion on reciprocal space velocity at 1/WHSV=0. The obtained rates as a function of the amount of strong acid sites are given in Figure 1. 

Whatever the exact explanation, on the process side, the mild solvent extraction step of the heavier feed fractions resulted in no less than doubling the hydrocracker space velocity, a spectacular benefit. Needless to say, feedstock solvent extraction became a popular process step. Lube yields based on feed also improved by 5 vol. % although based on crude, lube yields actually declined (Table 7.7). As IFP had found previously, a decrease in feed dewaxed VI reduced yields at a constant product VI, in Sun s case by about 1.3 volume percent per VI change (Table 7.8). 

FIGURE 7.20 Hydrocracking distillates for lubes bottoms composition—isoparaffins and mononaphthene levels versus conversion and space velocity. 

The assessment first looked into the opportunities associated with design margin. By optimizing process conditions and pushing the equipment to the absolute limits, the plant could make majority of the spare capacity and push plant capacity by 10% with some minor modifications. However, beyond a 10% expansion, several major limitations were identified from detailed simulations, which included the feed heater flux limit before the main fractionator column, jet flooding limit in the main fractionation column, and reactor space velocity limit in the hydrocracking unit. 

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. 

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