Reducing the circulation ratio to increase the yield of liquid products in coking units

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The circulation ratio is an important operating parameter in coking units; by adjusting it, it is possible to change the product distribution and control the quality of the products. Currently, the recycle ratio of these units can be classified into the following four categories: 1. High recycle ratio of 25% – this is primarily aimed at ensuring proper material compatibility, so as to provide wax oil as a raw material for downstream units; the purpose of a high recycle ratio is to control the quality of the wax oil (in terms of residue, impurities, and final boiling point). 2. Low circulation ratio: 10% 3. Ultra-low circulation ratio: 5% 4. Zero circulation ratio. Comparison of yields for low circulation ratio, ultra-low circulation ratio, and zero circulation ratio: Low circulation ratio, Ultra-low circulation ratio, Zero circulation ratio – Dry gas LV%: 5.59, 5.24, 5.22; Liquid gas LV%: 8.79, 8.34, 8.32; Liquid product yield LV%: 72.20, 73.79, 74.50; Coke weight percentage: 30.04, 28.69, 27.50. The comparison shows that reducing the circulation ratio can increase the yield of liquid products in coking units. However, reducing the cycle ratio also presents problems: 1. The quality of coker wax oil deteriorates, with higher levels of residue and metals, which affects subsequent processing units. 2. It will promote the formation of pellet coking. 3. It is likely to cause coking on the bottom trays of the distillation column. In summary, the circulation ratio should be determined based on the operation of the equipment and the overall material balance of the plant, with the goal of achieving optimal overall efficiency. Adjusting the cycle ratio also allows for adjusting the product distribution; production schemes that yield more diesel or more steam cracking feedstock can be adopted based on market considerations and process objectives.