Regarding the selection of pre-lift dry gas for catalytic cracking units [

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The pre-lift dry gas currently used in our catalytic cracking unit is the dry gas from the desorption stage; this means that this dry gas circulates within the system. However, due to insufficient upgrades to the hardware of the absorption and desorption systems during previous renovations, the unit is now operating at its maximum capacity, which results in high levels of carbon-3 in the dry gas and high levels of carbon-2 in the liquefied gas. This limits the operational flexibility of the unit and constitutes a bottleneck for increasing its processing capacity. To reduce the load on the absorption and separation system, I have an idea of changing the pre-heated dry gas to rich gas. However, the rich gas contains a certain amount of liquefied gas components, and their entry into the reactor will lead to a decrease in the yield of liquefied gas, particularly affecting the yield of the valuable product propylene. I would appreciate any suggestions regarding this idea. Dry gas can passivate heavy metals and improve the distribution of products. Using dry gas for pre-heating can reduce the yield of coke and dry gas while increasing the yield of the desired product. At our plant, dry gas and pre-heating steam are used together. In fact, the impact of pre-lifting dry gas on the product distribution is limited! Although there is industrial test data supporting the claim that dry gas pre-lifting technology can reduce dry gas and coke yields, this is conditional. ASHLAND Company states that for dry gas pre-raising to reduce coking, hydrogen must be activated at a temperature of 704 degrees ； When the carbon content in the regenerant is below 0.15%, passivation is completed within one second. American Universal Petroleum Company believes that for plants with a regenerant temperature of 716 degrees and heavy metals at 3000 PPM, the dry gas pre-boosting process offers good benefits. However, using dry gas can at least save some steam and reduce wastewater discharge, which is an aspect worth promoting. However, dry gas alone seems insufficient as well; both UOP and ASHLAND use a mixture of dry gas and steam for pre-vaporization. We have also tried using dry gas alone in our device; on the one hand, the low temperature of the dry gas, at 40 degrees, causes the catalyst temperature to drop and the circulation rate to increase. On the other hand, the flow pattern at the bottom of the riser deteriorates, resulting in poor vaporization of the catalyst when it comes into contact with the feedstock ； The amount of raw coke increased significantly, and later some steam was added back into use. Currently, all the units are operating at full capacity; the dry gas cycle inevitably consumes part of the compressors’ capacity, which affects the processing volume. As a result, the dry gas circulation is stopped. 　　 Dry gas lift is not intended to save steam, and it is not entirely accurate to say that it serves to prevent catalyst thermal degradation; its main function is to allow the dry gas to come into contact first with the highly active regenerated catalyst located in the lift section and regeneration inclined tubes. Firstly, dry gas contains a large amount of hydrogen, and according to expert research, this hydrogen has a passivating effect on metals such as nickel adsorbed on the catalyst. Secondly, the hydrocarbons in the dry gas react on the highly active molecular sieves of the catalyst, which helps to reduce the dry gas yield and increase the liquid yield. Pre-raising of dry gas has the following advantages: 1. It can reduce the aging and thermal degradation of high-temperature catalysts caused by steam, which helps to preserve the catalyst’s activity and minimize catalyst loss. 2. It increases the partial pressure of dry gas inside the reactor, which facilitates the reaction to proceed in a direction that suppresses the formation of dry gas, improves the product distribution, and enhances the yield of the desired product. 3. Low-molecular-weight hydrocarbons have the effect of deactivating heavy metals, thereby reducing the hydrogen content in the dry gas. 4. It can reduce wastewater discharge, offering good environmental benefits. With the adoption of this technology, the total air inflation volume of the air compressor increases, and its rotational speed must also increase. 1. The molecular weight of dry gas is slightly lower than that of water vapor; by injecting the same amount of each, it is possible to maintain a constant flow rate within the riser reactor. Furthermore, due to the low specific heat capacity of dry gas, less heat is carried to the distillation tower, which helps to reduce the load on the overhead condenser. 2. Dry gas mainly contains hydrogen and light hydrocarbons; using dry gas as the lift medium increases the hydrogen partial pressure in the lift tube reactor, which is beneficial for suppressing dehydrogenation reactions. 3. It can reduce the hydrothermal deactivation of the catalyst caused by water vapor, stabilize the onset temperature of the reaction, effectively protect the acidic sites of the catalyst, and help maintain its activity. 4. Injecting hydrogen-containing dry gas is equivalent to creating a passivation zone between the two reactors; under appropriate conditions, the hydrogen activates the NiO or Ni2O2 on the catalyst, reducing them to metallic nickel. This metallic nickel tends to accumulate together, reducing the number of active metal centers and thus serving a passivation function. Furthermore, after V2O5 is reduced to V2O3 or metavanadic acid, it also loses its destructive effect, reducing catalyst poisoning and heavy metal contamination. 5. Increased gas partial pressure. When dry gas is injected into the lift pipe, reactions that occur in the gaseous product are suppressed, the yield of dry gas is reduced, and the product distribution is slightly improved. 6. Dry gas lift is used, requiring a certain margin in the capacity of the plant’s rich gas compressor.