Dry gas and steam are mixed into the riser
Thread Content
In a heavy oil catalytic cracking unit of our company, the reaction riser was modified to mix dry gas and steam into the pre-lift section. During current operation, no abnormal phenomena have been seen. Although the load of the air compressor has increased, the impact is not significant. It's just that the effect is not very obvious.
Both dry gas and steam can be used as pre-lift media. The difference is that the temperature of dry gas is generally lower and some heat is required to heat it up. Although water vapor has a higher temperature, it also has a large heat capacity and absorbs a large amount of heat, which may cause thermal collapse of the catalyst. Therefore, generally speaking, it is better to use dry gas as the pre-lifting medium. There are reports that using dry gas pre-boost can reduce steam consumption. This is inappropriate because dry gas cycle compression requires the air compressor to consume some steam.
Dry gas pre-lifting is not to save steam, and it is not entirely correct to prevent thermal collapse of the catalyst. Its main function is that the dry gas is the first to contact and react with the high-temperature and high-activity regenerated catalyst coming from the regeneration inclined tube under the lifting jurisdiction. First, the catalytic dry gas contains a large amount of hydrogen. According to expert research, the hydrogen in the dry gas has a passivation effect on nickel and other metals adsorbed on the catalyst. Second, the hydrocarbons in the dry gas react on the highly active molecular sieve of the catalyst, which can reduce the dry gas yield and increase the liquid recovery.
The reasons are as follows: 1. The molecular weight of dry gas is slightly smaller than that of water vapor. After the same amount of medium is injected, the flow rate in the riser reactor can be guaranteed to remain unchanged. In addition, due to the low specific heat capacity of dry gas, less heat is brought to the fractionation tower, which is beneficial to reducing the load of the overhead condenser. 2. Dry gas mainly contains hydrogen and light hydrocarbons. Using dry gas as the lifting medium will increase the hydrogen partial pressure in the riser reactor, which is beneficial to inhibiting the dehydrogenation reaction. 3． It can reduce the hydrothermal deactivation of the catalyst caused by water vapor, stabilize the starting temperature of the reaction, effectively protect the acidic center of the catalyst, and help ensure the activity of the catalyst. 4． Injecting dry gas containing hydrogen is equivalent to setting up a passivation area between the two devices. Under suitable conditions, hydrogen is activated, and NiO or Ni2O2 on the catalyst is reduced to metallic nickel by hydrogen. It is easy to accumulate together and reduce the number of metal activity centers, which plays a passivation effect. In addition, after V2O5 is reduced to V2O3 or metavanadate, it will also lose its destructive effect, reducing catalyst poisoning and heavy metal pollution. 5. Increased gas partial pressure. When dry gas is injected into the riser, the reaction to the gas product is suppressed, the yield of dry gas is reduced, and the product distribution is slightly improved. 6. The use of dry gas liters requires a certain margin in the capacity of the rich gas compressor.
Things to note: When the air compressor is shut down, the dry gas pre-lift valve must be closed immediately and replaced with steam pre-lift.
study* Well, I originally only understood that dry gas pre-lifting is to prevent thermal collapse of the catalyst, but it turns out that it also has a passivation effect. Thanks!
It is definitely better to use dry gas than steam, as long as the margin of the rich gas compressor is enough. :) :)