By Donald F. Shaw and Richard E. Walter
The previous article in this series discussed catalyst losses in Fluid Catalytic Cracking Units (FCCUs) and how trickle valves are used to minimize these. This article describes the commonly used trickle valve designs, the pros and cons of each, the forces that act on trickle valves, and measures that may be taken to improve their performance.
There are at least five different trickle valve designs widely used in FCCUs. Typical considerations for selecting trickle valve design are as follows:
The following summarizes some of the pros and cons for the various trickle valve designs.
U.S. Patent 4,996,028
The forces acting on a counterweighted trickle valve are similar for both the horizontal and angled valve, and these will be discussed first since it is relatively straightforward. Looking at Figures 4 and 5, we can see that the closure force that is built into the design of the counterweighted design is established by hanging a temporary weight on the center of the flapper plate and moving the counterweight along a moment arm to balance that weight (including that of the flapper plate). It is relatively easy to calculate the equivalent hydrostatic pressure that is required to just open the flapper (i.e., offset the counterweight force). It should be noted that these calculations ignore any accumulation of catalyst or coke on the horizontal flapper or any dynamic forces associated with the momentum of the catalyst. Some allowance based on experience is typically included in the settings for these considerations.
The forces that act on and open a traditional miter body trickle valve are also quite simple but are more complicated to calculate. The following addresses the forces and moments that control the opening and operation of a typical cyclone trickle valve installed in the dilute phase of a fluidized bed reactor or regenerator using negative pressure primary or secondary cyclones. For diplegs that operate in a bed, there are additional hydrostatic forces due to the turbulent nature of the bed that are not considered here. Also for diplegs submerged in a bed, the hydrostatic head of the catalyst bed height above the trickle valve discharge must be accounted for.
The calculations are based primarily on the forces acting on the trickle valve flapper plate as it begins to open. The calculation model is based on the fact that the flapper plate is supported at the top with two hinge rings, and that the plate tends to rotate around the hinge rings as it opens. As opening forces act on the flapper plate, the rotation around the support hinges causes the opening at the bottom of the seat to increase more than the opening at the top of the plate. Of course the hinge ring is not an absolute pivot point and there can be some small lateral translation of the plate. This small translation, however, does not significantly affect the calculations. Also, these calculations do not include any dynamic forces, which will tend to further open the trickle valve once it has opened and catalyst is flowing. See Figure 6 for the terminology used for a miter trickle valve.
The following forces act upon a trickle valve to seat and unseat the flapper plate against the opening in the trickle valve body.
An opening force arises from the pressure acting on the flapper plate (area at the valve opening) due to the hydrostatic head of catalyst in the dipleg. This pressure acts horizontally on the surface of the flapper plate. For simplification, we take the calculated pressure at the middle of the trickle valve opening and spread this over the entire area of the elliptical trickle valve opening. This assumption is valid as long as there is a reasonable catalyst level in the dipleg.
A net closing force occurs due to the differential gas pressure that acts across the flapper plate (i.e., the external pressure that exists inside the vessel, Pv, minus the pressure inside the cyclone body, Pc). This net differential pressure only acts on the area of the valve discharge opening, and tends to close the valve in a negative pressure cyclone system since the vessel pressure is higher than the pressure inside the cyclone.
Since the trickle valve flapper plate hangs from its support ring at a small angle (approximately 3° from vertical), the weight of the flapper plate creates a seating force against the valve opening. This seating force is determined by the moment created by the horizontal offset (y) of the flapper plate mass from the vertical centerline through the hinge ring support.
Dynamic forces are caused by the momentum of the falling gas/catalyst stream in the dipleg impinging on the flapper plate, but are not included in the calculations. Instead, the calculations determine the condition where the trickle valve just opens and any such dynamic forces will tend to further open the valve. As the valve opens further, the offset (y) increases, thus increasing the closure forces until the opening forces/moments (catalyst level) are in balance with the closing forces/moments.
To calculate the effect of the trickle valve flapper weight, catalyst head in the dipleg, and the differential pressure between the vessel and inside the cyclone, one takes moments around the flapper plate vertical support point (point C in Figure 6). For static equilibrium, the summation of Moments (Mc) around point C must equal zero. At the point of the trickle valve opening, the moment due to pressure on the inside of the flapper (Pc + static catalyst head in the dipleg) must equal the closure moment due to the weight of the trickle valve flapper and the pressure in the vessel.
Calculations demonstrate that flapper weight does not have a significant effect on the opening of a typical FCCU trickle valve when compared to cyclone Delta P where the normal hanging angle is 3° from vertical. It should be noted, however, that increasing the weight does have a positive effect on the stability of the flapper and the dipleg level since small pressure fluctuations are dampened out by the weight of the flapper or changes in the level of the catalyst in the dipleg.
The weight stability effect has also been reported in cold flow studies of light plastic flappers where the catalyst level in the dipleg can fluctuate significantly because small pressure fluctuations cause erratic movement of the flapper. Cold flow studies with steel flapper plates simulating an actual trickle valve also report that increasing the weight of the trickle valve flapper can stabilize the catalyst level in the dipleg, especially as the flapper plate weight is increased (due to the addition of refractory).
Figure 3 shows a type of trickle valve used on FCCU diplegs in dilute phase service which employs a patented elbow configuration (U.S. Patent 4,996,028) rather than a miter to provide the seating surface. The principal benefit of the elbow is that the vertical opening (O in Figure 6) is reduced by approximately 25%. Also, this design increases the closure force since it adds weight to the flapper with refractory on both sides. Typically the vertical opening for a 35° miter valve is approximately 2D whereas the vertical opening for an elbow valve is typically 1.5D.
Based on observing erosion patterns on trickle valve flapper plates it has been determined that as the flapper opens, gases from the vessel tend to enter the dipleg at the top of the flapper. A principal advantage of the elbow valve and trickle valve with blank out is that they reduce the quantity of gas that enters the dipleg. Typical erosion patterns on flappers have been observed as inverted horseshoe in shape (See Figure 7). As these gases enter the top, they tend to cause flow disruption in the dipleg and erratic dipleg levels.
The following summarizes possible causes of trickle valve malfunction.
A variety of trickle valve designs have been used in FCCUs, each with advantages and disadvantages. Experience is an important consideration in selecting the appropriate valve type for a specific application. Counterweighted valves have primarily been used in dilute phase service. Trickle valves with flappers hanging at a small angle have been used in both bed and dilute phase service.
The horizontal counterweighted trickle valves are more complex and may be more vulnerable to binding at the internal bearings or hanging up due to coking or chunks of refractory. The angled counterweight valve alleviates some of these concerns. The conventional miter trickle valve has the advantage of being much simpler but has the disadvantage for certain services of providing a long vertical opening that can allow gas to enter at the top while discharging catalyst at the bottom. There are several enhancements that help alleviate this limitation.
The perception that the weight of the flapper plate is a significant factor in the maloperation of a trickle valve is not supported by the calculations for a negative pressure cyclone. The major forces controlling the opening of the trickle valves are the differential pressure forces across the flapper. These forces are balanced by the catalyst head in the dipleg. A “heavy” flapper is an unlikely cause of high catalyst losses.
Submerging the bottom of the dipleg in the bed offers some advantages; however, this is not always desirable from the process viewpoint. The secondary trickle valve in the dilute phase is often considered by many as the most difficult application since the catalyst flux can be quite small. Also, gas leakage into the dipleg during periods with low flux can cause problems if the gap between the flapper and the seat allows too much gas to leak into the dipleg. For these cases, the fabrication tolerances can be critical.
A properly designed trickle valve should not be the first item on the list of potential reasons for high catalyst losses. Many other items, such as mechanical damage, high attrition rate catalyst, potential coking or coke spalling from the cyclones, and unexpected high velocity attrition source, should be heading the list of potential causes for high catalyst losses.
The authors wish to thank EMTROL, LLC and Fisher-Klosterman, Inc. for providing some sketches on typical valve types.