Modos de enseñanza y aprendizaje de la ciudadanía

SPRAY NOZZLES

Spray nozzles are used to inject water through an atomizing nozzle to cool off the flue gas in the plenum chamber if the unit has power recovery. The spray water should be clean, such as steam condensate. Contaminants such as sodium will cause problems by deactivating the catalyst or contributing to its breakup.

Mechanically, the spray nozzles are similar to the torch oil nozzles.

CATALYST COOLER

The ability to control and vary the amount of heat removed from the regenerator creates an additional degree of freedom by moderating the regenerator temperature as a limiting constraint. The catalyst cooler provides a variable heat sink, which

allows the refiner to vary the catalyst/oil ratio, reactor temperature, and feed temperature independently of one another.

The catalyst cooler tube bundle is inserted into a refractory lined shell off the side or bottom head of the regenerator. The tubes of this exchanger are the bayonet type.

The boiler feed water enters the cooler through the inner tubes and the mixture of water and steam exits the cooler through the annulus between the inner and outer tubes. The outer tubes are 3 inch (75mm) O.D. made from 1¼ Cr, ½ Mo seamless tube material. The inner tubes are 1-3/8 inch (35mm) O.D. made from carbon steel seamless tubing.

The stainless steel fluidizing air lances distribute air into the cooler near the bottom of the tubes. The air creates turbulence and increases heat transfer coefficient as the bubbles travel upward. The backmixing created by the bubbles also brings hot catalyst into the cooler from the regenerator. The air is delivered to a common manifold supplying all the lances through a flow controller. The lances contain a restriction orifice, located near the piping header at the top of each lance, to help distribute the air uniformly over the cross sectional area of the cooler. The countercurrent fluidizing air improves heat transfer by creating turbulence and mixing in the region of contact between the hot catalyst and the tubes. A differential pressure transmitter, with taps located above and below the cooler, gives a direct indication of the density of the fluidized catalyst at various conditions of catalyst flow and air injection.

Mechanical reliability is achieved by locating the cooler in the dense phase of the regenerator. In the dense phase, the heat transfer coefficient is higher which permits lower catalyst and fluidization air velocities. Lower velocities minimize erosion within the cooler. In addition, the cooler tubes are located in the vertical plane. This feature generates a uniform heat transfer coefficient over the entire tube surface thereby preventing uneven surface temperatures which cause localized stress.

Catalyst coolers have been designed and built to fit virtually every regenerator configuration, including single-stage bubbling beds, high-efficiency combustors, and two-stage regenerators. Three basic styles of UOP catalyst coolers are currently available:

Flow-through catalyst cooler – The catalyst flows downward into the cooler shell and exits into the cooled catalyst standpipe near the bottom of the tube bundle. The standpipe transports the cooled catalyst through a slide valve and expansion joint into the combustor on a high efficiency regenerator or to the second stage regenerator on an RFCC. In a single stage bubbling bed regenerator the catalyst can be lifted back into the regenerator with air through a lift riser. Both the catalyst flow through the cooler and the fluffing air rate are used to control the cooler duty.

Backmix catalyst cooler – This style contains no catalyst exit standpipe. Hot catalyst enters the cooler by backmixing as a result of fluidization air injected near the bottom of the tube bundle. The major advantage of this cooler design is that no slide valve, expansion joint, or standpipe is required. This configuration also permits the cooler to be lower to the ground if elevation is a limiting constraint. The duty of a back mix cooler is ~60% of an equal sized flow through cooler and is controlled only with the fluffing air.

Hybrid catalyst cooler – The combination of flow-through and backmix operation constitutes the hybrid catalyst cooler. In a hybrid, the catalyst exits into a standpipe located at the midsection of the tube bundle (instead of at the bottom as in flow-through coolers). In the hybrid cooler, the upper portion of the bundle operates in the flow-through mode, and the bundle length below the catalyst outlet operates in the backmix mode. This configuration achieves somewhat less heat-removal capacity than a full flow-through cooler but still transfers cooled catalyst down to the lower portion of the regenerator.

The catalyst cooler steam generation circuit includes the cooler, steam drum, and circulation pumps. Boiler feedwater is pumped to the bottom head, enters the inner tubes, then flows down through the annulus between the inner and outer tubes where it absorbs heat to generate steam. The steam-water mixture leaves the catalyst cooler to be separated in the steam drum. Makeup boiler feed water is delivered to the steam drum through a flow controller which is cascaded to signals from the drum level and steam generation flow transmitters. Steam flows from the drum through a stop check non-return valve and a superheater (either part of the flue gas cooler or a fired heater) before entering the refinery steam header.

Figures 31-35 show some examples of catalyst coolers and catalyst cooler-regenerator configurations that have been constructed.

Figure 31