Why is the one-pass conversion rate generally controlled at 60–70% in hydrocracking processes with 100% conversion?

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The results regarding the distribution of VGO hydrocracking products are as follows: Although different types of catalysts and reaction conditions were used in the two sets of experiments, the trends in conversion depth and product distribution indicate that increased secondary cracking led to higher yields of gases and light components, thereby reducing the yield of intermediate distillates. The overall liquid yield also decreased; thus, pursuing excessively high one-pass conversion rates is uneconomical. When the conversion rate is above 60%, not only does the yield of the target product decrease, but the chemical hydrogen consumption in the process also increases. This explains why, in hydrocracking processes with 100% conversion, the single-pass conversion rate is generally controlled at 60–70%, and the unconverted residue oil is then subjected to cyclic cracking in order to improve the selectivity of the process.