Why is pressurized air supplied to the external heat exchanger in catalytic cracking?

A pressure difference is created to ensure proper fluidization.

The pressure of the pressurizing air in the external heat exchanger is 100 KPa higher than that of the main air, thereby overcoming the gravitational force acting on the catalyst inside the external heat exchanger and ensuring that the catalyst can return to the regenerator.

The use of fluidized beds in the two main units of a catalytic cracking unit determines its characteristics. When working with catalytic cracking, it is essential to maintain pressure balance in these two units, and this applies equally to the external heat exchanger as well. The external heat exchanger is usually located below the regenerator; therefore, its pressure is higher than that of the regenerator. If primary air is used, the pressure of this primary air needs to be determined based on the pressure at the bottom of the external heat exchanger. However, the low flow rates of the air used for fluidization and lifting in the external heat exchanger result in unnecessary energy waste. In some units, a small high-pressure fan is used for these purposes, with the primary air being drawn directly from the atmosphere and supplied exclusively to the external heat exchanger~~~

To maintain the pressure difference with the regenerator for external heat extraction, a blower is typically used to supply the fluidization air; it is also feasible to use unpurified air as the fluidization air for external heat extraction, which can reduce energy consumption to some extent (although this is to some degree affected by utility services)

This post was last edited by JINGBOREN on 2011-11-2 at 18:01. The fluidization air is pressurized air, at a pressure higher than that of the regenerator. The externally heated pressurizing air is divided into two types: 1. Externally heated fluidization air: which keeps the catalyst in the external heat exchanger in a good fluidized state; 2. Externally heated lift air: which provides the force necessary for the catalyst to return to the regenerator, as the external heat exchanger is located below the regenerator

It depends on the type of external heat exchanger used. If it is of the bottom-valve-controlled type, then pressurized air is not necessary, as the main air used in this external heat exchanger serves only to facilitate fluidization. The catalyst in the external heat exchanger returns to the bottom of the regenerator due to the force of gravity

If the device is small, main air can also be used in place of pressurized air

Overcome the weight of the catalyst itself to ensure normal fluidization for external heat removal; pressurized air is used for external heat removal

It mainly breaks the internal pressure balance between the regenerator and the external heat exchanger, ensuring proper fluidization of the catalyst in order to remove excess heat from the regenerator. If the device is small and there is an ample supply of unpurified air, unpurified air can also be used as a substitute.