By Donald F. Shaw and Richard E. Walter
High or abnormal catalyst losses from Fluid Catalytic Cracking Unit (FCCU) cyclone systems are sometimes attributed to plugged diplegs, often believed to be caused by one or more malfunctioning trickle valves. Units are sometimes shut down and catalyst unloaded to “clear” a plugged dipleg. Applied forces affect the opening and closing of a trickle valve for both positive and negative pressure cyclones. These forces are caused by the trickle valve design, weight of the flapper or counterweight, and catalyst level in the dipleg.
This article discusses catalyst losses in FCCUs and how trickle valves are used to minimize these. A subsequent article will describe the trickle valve designs commonly used in FCCUs, the pros and cons of each, the opening and closing forces exerted on these trickle valves, and measures that may be taken to improve their performance.
The FCCU process includes the circulation of huge quantities of solids to convert feed stocks to desired products (e.g., vacuum gas oils to gasoline). Catalyst circulation rates can range from 10 to over 100 tons/minute between the reactor and the regenerator.
Cyclone systems typically consist of primary and secondary stage cyclones in both the reactor and regenerator to recover the solid catalyst from the reactor products and the regenerator flue gases. As more catalyst is collected with the cyclone systems, fewer processing problems occur in the product recovery circuit, and there will be lower solids emissions in the regenerator flue gas stream.
The role of the trickle valve has become increasingly more important as FCC technology has advanced from primarily bed cracking to Short Contact Time (SCT) Riser reactors, and environmental concerns have driven refiners to minimize catalyst losses. FCCUs initially did not use trickle valves on the cyclone diplegs. This worked fine once a bed level was established above the level of the dipleg discharges. The bed acts to seal the diplegs, and low catalyst losses can be achieved. However, high catalyst losses were observed during unit startups before a bed was established, or during upset situations where the bed levels were reduced below the level of the dipleg.
Adding a trickle valve to the end of the dipleg was successful in maintaining low catalyst losses during the unit startup phase before bed levels are established, and also during upset situations. The trickle valves also help maintain the dipleg seal when negative pressure reactor cyclones are utilized in SCT or other operations which discharge the catalyst into a dilute phase.
While overall experience with trickle valves has been good, now there was a mechanical device which adds to the possible list of reasons for increased catalyst losses from either the reactor or regenerator. In addition to the catalyst and other hardware, there is now the concern that the trickle valves could be directly involved in causing increased catalyst losses. While it is possible for trickle valves to cause high catalyst losses, a properly designed trickle valve should not be the primary cause.
The purpose of the trickle valve on the discharge end of the dipleg is similar to that of a check valve in a process line in that the trickle valve flapper prevents catalyst or gas flow up the dipleg. A cyclone will not work properly with gas flow up the dipleg. Upflow is prevented either by pressure differential, a mechanical device, or by massive down flow of solids. Figure 1 (see page 5) shows how a typical cyclone dipleg works. Cyclones can operate at a positive or negative pressure relative to the containment vessel. Also the diplegs can discharge into a bed or into a dilute phase. All these factors affect how the trickle valve functions and what it is expected to do.
The purpose of trickle valves that are inserted into the bed is primarily to minimize catalyst losses during start up and during other periods when the bed level is below the valve. Typically trickle valves that operate in the bed are provided with a shroud arrangement to prevent a gas bubble from entering the dipleg. Few problems are typically experienced with these trickle valves if they are immersed in a well-fluidized bed.
Positive pressure cyclones are not expected to experience flow up the diplegs except during start up or other low-rate periods. Typically the diplegs are inserted in a bed with positive pressure cyclones in an attempt to create a backpressure and to minimize gas flow down the dipleg. Except for certain coupled cyclone systems in reactors, cyclones in FCCUs typically operate at a negative pressure relative to the containment vessel.
Figure 1 shows a typical negative pressure cyclone system with pressure in the vessel, Pv, and pressure in the cyclone, Pc, with Pc less than Pv typically by the amount of the pressure drop through the cyclones. This differential pressure tends to push the flapper closed. To balance this closure force, a level of catalyst must build up in the dipleg and create a “hydrostatic” force to open the flapper. The weight of the flapper (or counterweights) creates an additional closure force that must also be overcome by the catalyst level.
Several trickle valve designs are widely used, each with advantages and disadvantages. The next article in this series will discuss these, along with measures that may be taken to improve their performance.