Cyclone Basics-Problem Solving

Buell

Division of

Fisher-Klosterman, Inc.®

FCC CYCLONE BASICS AND PROBLEM SOLVING

BY EDWIN D. TENNEY

BUELL DIVISION OF FISHER-KLOSTERMAN, INC. LEBANON, PENNSYLVANIA, U.S.A.

FCC CYCLONE BASICS AND PROBLEM SOLVING

CYCLONE BASICS Cyclone Types Two types of cyclones have been used in fluid catalytic crackers. In some early units 230 to 305 mm (9" to 12") I.D. Axial Inlet Cyclones like the one pictured on the left side of Figure 1 were used. Gases entered through the top of each cyclone and were induced to spin by the turning vanes in the top of the cyclone. However, it was soon found that the high loading of catalyst in the gases resulted in frequent plugging of the turning vanes, which meant that the units had to be frequent shut down. To alleviate this problem some refiners decided to try larger cyclones with tangential inlets like the one pictured on the right side of Figure 1. The diameter of these initial cyclones was about 1000 mm (40"). It was found that these cyclones did not plug, but the losses from a single stage of cyclones in a Regenerator in many cases were high so external collectors were added to some units to reduce catalyst losses. As a result of these initial successes most Axial Inlet Cyclones were replaced with Tangential Inlet Cyclones. In the few units where they were not immediately replaced, they became second stage cyclones proceeded by a stage of Tangential Inlet Cyclones. For the past thirty years all new or replacement cyclone systems in (or attached to) Reactor and Regenerator Vessels have been Tangential Inlet Cyclones. Axial Inlet Cyclones have only been used in some separators external to Regenerator Vessels. In these separators, usually referred to as Tertiary Separators, some of the catalyst particles remaining in the gases leaving regenerators are collected and the rest of the particles are ground to a fineness of less than 10 microns before the gases enter expander turbines in which some of the pressure and heat energy in the gases is converted into electricity. However, even in this application, when an operational upset results in high catalyst losses from the Regenerator, the cyclone vanes may become plugged. For this reason most new Tertiary Separators utilize Tangential Inlet Cyclones, either housed in a vessel or located externally as individual pressure vessels. In the remainder of this paper all references to cyclones will refer to Tangential Inlet Cyclones.

Cyclone Nomenclature 1

Figure 2 shows the names most frequently used for the parts of a cyclone. Some may wonder about the term "radish" for the cyclone hopper. A radish is a ball shaped root, red on the outside and white on the inside, with leaves flaring out in a cone shape from the top center and a thin section of root extending out the bottom center. On most cyclones over fifteen years old the hopper cylinder does not extend up to the cyclone cone, but has a conical roof between the hopper cylinder and the cyclone cone. To some construction people eating radishes during their lunch break the cyclone hopper with a conical top and bottom, the cyclone cone coming out of the top and the dipleg coming out of the bottom looked like a radish. They started saying "the radish" when referring to a cyclone hopper. These workers traveled from job to job and the name "radish" became widely used in the industry.

Gas Flows in Cyclones In a cyclone there are the three gas flows. These are shown on Figure 3. The entering gases spiral down the walls of the cyclone cylinder and cone. The exiting gases, rising in the center of the cyclone, form a cone with the apex at the bottom and the base at the entrance to the gas outlet tube. All along the interface between these two gas streams, gases flow from the descending stream into the ascending stream. Thus, while the amount of downward flowing gases is constantly decreasing, the constantly decreasing cone diameter keeps the gas velocity nearly constant. The final transfer of gases occurs in the hopper.

Cyclone Inlet Scrolls The purpose of the inlet scroll on a cyclone is to keep the inlet gas stream and the entrained particles away from the entrance to the gas outlet tube. This is particularly important when the inlet stream initially passes the entrance because the particles are still randomly distributed throughout the gas stream. In most cyclones the scroll also prevents the impingement of entering particles on the gas outlet tube. Some will tell you that is not necessary to have inlet scrolls on second stage regenerator cyclones and single stage cyclones when the distance between the gas outlet tube and the cyclone wall is slightly greater than the width of the inlet. The omission of inlet scrolls reduces the initial cost of a regenerator cyclone system about 2 percent and the initial cost of a single stage system about 4 percent. However, this omission results in reduced cyclone efficiency and in most cases, because gases expand when they exit a duct, impingement of particles on the outside of the gas outlet tube. The bottom picture on Figure 4 shows some particles hitting the outside of the outlet tube and others traveling toward the entrance to the outlet tube.

Figure 4 also shows the error in the statement "Gases exiting a first stage regenerator cyclone are concentrated along the outside wall of the second stage cyclone so an inlet scroll is not necessary." First, while the coarse particles may be concentrated along the outside wall of the inlet, the finer particles (the ones most easily lost) remain distributed 2

through the gas stream. This is the result of turbulences induced in the gas stream when it leaves the round first stage cyclone outlet tube and enters the horizontal duct to the second stage cyclone. Second, the elimination of the scroll moves all of the entering particles closer to the gas outlet tube entrance when they pass by the entrance. Thus, the chances of a particle being carried by the gas stream going from the descending stream to the ascending stream and the chances of the particle bouncing off the inlet wall into the ascending stream are significantly increased. In an earlier paper entitled "Cyclones, Facts and Fiction" it was shown that the use of second stage cyclones without scrolls is not even the most cost effective way to reduce initial costs. Cyclones with inlet scrolls that are slightly smaller in diameter than the cyclones without scrolls, are more efficient and have a lower cost than the cyclones without scrolls. As can be seen in Figure 5 (taken from the above paper), even the argument that limited space in the vessel makes it is necessary to use second stage cyclones without scrolls is not justified. The smaller, more efficient cyclones with scrolls will fit in the same space.

Cyclone Cones and Hoppers Some will say that it is not necessary to have a hopper on a cyclone. This is acceptable when the inlet catalyst loading to the cyclone is very high and the total length of the cylinder and cone is 4 or more times the inside diameter of the cyclone cylinder. An example of a cyclone with a heavy inlet loading would be one directly connected to a reactor riser. However, for most first stage cyclones and all single stage and second stage cyclones, the hopper is an essential part of the cyclone. As shown in Figure 6 the hopper allows the collected catalyst to separate from the gas streams and to move away from the apex of the vortex formed by the gas streams. In operation the gas steam vortex moves around randomly, similar to the tail of a dog. In a cyclone without a hopper, the moving vortex intermittently contacts catalyst on the wall of the dipleg. Some catalyst is reentrained, the dipleg wall is eroded and the eroding catalyst is attrited, producing very fine particles which become future catalyst losses. In Figure 6 one should also observe that both gases and catalyst must pass through the cyclone cone outlet into the hopper. If the flow rate of collected catalyst is too great, there will be insufficient space for passage of the two gas streams through the cone outlet. When this occurs, some of the collected particles are re-entrained in the rising gas stream and carried out of the cyclone. We have found that when the weight rate of catalyst particles entering a cyclone divided by the cross sectional area of the cone outlet exceeds 390 kg/m2s (80 lb/ft2-s), re-entrainment of particles starts to occur. This flow rate is about one half the 735 kg/m2-s (150 lb/ft2-s) we recommended as the maximum flow rate in a dipleg. There are published articles in which the authors report measuring mass flows in excess of 975 kg/m2-s (200 lb/ft2-s) through large diameter diplegs during laboratory testing. We agree that under controlled conditions such flow rates are possible. However, in an operating catalytic cracker the catalyst loading entering a cyclone is not constant. Based on 3

data from many operating units, we have found that our maximum recommended mass flow rate compensates for these fluctuations.

Cyclone Diplegs Cyclone diplegs are the means used to return catalyst collected in cyclones to the bottom of the vessel. Each dipleg also provides a barometric type seal to prevent or minimize gas leakage from the vessel into the cyclone hopper outlet. Gas leakage through a hopper outlet would re-entrain some of the collected catalyst particles and carry them out of the cyclone with the exiting gases. When designing a cyclone system, one must calculate the catalyst level in each stage of diplegs. The maximum catalyst level in a dipleg should be a minimum of 600 mm (2 ft) below the hopper-dipleg weld line. As noted above one must also determine the required pipe size for first stage diplegs based on the amount of catalyst collected in the first stage cyclones. Since the efficiency of the first stage cyclones is over 99.9 percent, one normally uses the entire amount of catalyst entering a first stage cyclone to calculate the pipe size for the first stage diplegs. Because the mass flow in the second stage cyclone diplegs is normally very low, second stage diplegs normally have a cross sectional area between 1/4 and 1/2 of the cross sectional area of the first stage diplegs. Some add to this criteria the additional restriction that the second stage diplegs should not be larger than 324mm (12 3/4") O.D. - between 299 and 305mm (11 3/4" and 12") I.D.

CYCLONE PROBLEMS Erosion in Second Stage Cyclones Among the most commonly heard statements describing problems with cyclones are reports of holes or extensive erosion in the conical transition portion of the second stage hoppers just above the diplegs and in the second stage diplegs just below the hoppers cones. These areas are shown on Figure 7. Erosion in these areas result from one or more of the conditions listed here and shown on Figure 8: High second stage cyclone inlet velocities High second stage cyclone gas outlet tube velocities Excessive gas leakage into and up the second stage diplegs High catalyst carryover to the second stage cyclones

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High Second Stage Cyclone Inlet Velocities To understand this, one must remember that the hardness of refractory linings is equal to or greater than the hardness of the catalyst. Therefore, at the same time catalyst is eroding a refractory lining, the refractory lining is attriting the catalyst. Each eroding catalyst particle is itself broken into many tiny particles which are returned to the catalyst stream circulating in the unit. Shortly after re-entering the catalyst stream, these tiny particles are again entrained in the vessel gases and carried back into the cyclones. But these tiny particles, which are too small to be collected by any cyclone, pass through the cyclone system and add to the cyclone losses.

High Second Stage Cyclone Gas Outlet Tube Velocities High gas outlet tube velocities are generally found in second stage regenerator cyclones, where the resulting higher cyclone pressure drops have little or no effect on downstream operations. High second stage cyclone gas outlet tube velocities, like high second stage cyclone inlet velocities, are frequently the result of unit operation at conditions higher than those specified for the design of the cyclone system. However, high outlet tube velocities also frequently occur when the second stage cyclones are supplied without inlet scrolls, a means used to lower initial cost. This is done when a Purchaser is known to look mainly at initial cost with little or no consideration given to operating reliability and future maintenance requirements. This design has been accepted by some process licensers who have not recognized the long term disadvantage!

Excessive Gas Leakage Into And Up Second Stage Diplegs Excessive gas leakage into and up second stage diplegs primarily occurred in reactors where the riser discharged into the vessel and the diplegs discharged above the catalyst bed or stripper backup. A significant portion of the catalyst separated from the gases at the riser outlet before the gases entered the first stage cyclones. Most of the remaining catalyst was collected in the first stage cyclones. Very little catalyst entered the second stage cyclones. Even when each second stage dipleg had a horizontally closing counterweighted valve, there was seldom enough collected catalyst in each dipleg to cover the perimeter of the valve seat. The differential pressure across each valve and dipleg, normally about 0.14 kg/cm2 (2.0 lb/in2), can suck a significant amount of gas into the dipleg. Not only does this entering gas carry catalyst into the dipleg, but it also re-entrains some of the catalyst in the dipleg. When the rising gas leakage meets the spinning vortex of cyclone gases at the top of the dipleg, the catalyst particles in the rising gas stream are accelerated by the spinning vortex. These accelerated particles erode the upper portion of the dipleg and the lower portion of the cyclone hopper. Because of this, process licensers now specify single stage cyclones in 5

reactors where the riser discharge provides the primary catalyst separation. These single stage cyclones are designed for higher efficiency and higher pressure drop than first stage cyclones in a two stage reactor cyclone system. In most cases the number of single stage cyclone required is greater than the number of first stage cyclones required.

High Catalyst Carryover to the Second Stage Cyclones High catalyst mass flows in first stage cyclone cones and diplegs are the most common cause of high catalyst carryover to second stage cyclones. The design basis for mass flow through first stage cyclone cone outlets was discussed above under "Cyclone Cones and Hoppers". However, an obvious question is, if the mass flow through one’s first stage cyclone cones is excessive because of operation at higher than design conditions, can any modifications be made to the cyclones that will reduce the problem? In most cases the answer to this question is yes. Normally, the diameter of the cyclone cone opening is 4/10 of the cyclone diameter and the diameter of the cylindrical portion of the cyclone hopper is 6/10 of the cyclone diameter. On most cyclones supplied in the last fifteen (15) years the cylindrical portion of the cyclone hopper extends up to the cyclone cone. This means that the portion of the cyclone cone with a diameter between 4/10 and 6/10 of the cyclone diameter is inside the cyclone hopper. As shown on Figure 9, one can cut off part or all of the portion of the cone that is inside the hopper, as required to reduce the catalyst mass flow through the cone opening to less than 390 kg/m2-s (80 lb/ft2-s). While the efficiency of a cyclone with a cone opening larger than 4/10 of the cyclone diameter will be a little less than the efficiency of a cyclone with a cone opening that is 4/10 the cyclone diameter, the reductions in both catalyst attrition and catalyst re-entrainment will result in significantly reduced catalyst losses.

Some Reasons For High Catalyst Losses High catalyst losses usually occur as the result of one of the following situations: Mechanical Failures Catalyst Attrition Excessive Mass Flows in Cyclones or Diplegs Insufficient Dipleg Length

Mechanical Failures The first thing most people think of when catalyst losses suddenly or gradually increase is a mechanical failure. Since these have been frequently discussed in the literature, only some of the more common mechanical failures are listed here: Leaks at broken welds or high stress tears Holes formed by erosion in cyclones or diplegs Blockage in diplegs 6

Dipleg valves which do not operate Dipleg valves which do not close because of bent or lost closure plates Anyone who has been involved with fluid catalytic cracker cyclones will have his own tale of a mechanical failure which he believes to be unique.

Catalyst Attrition Some reasons for catalyst attrition in cyclones were discussed earlier. Catalyst attrition also occurs in other areas of the fluid catalytic cracker. Some of the sources or causes of this attrition are: Improperly designed, eroded or missing orifices in steam lines Excessive velocities through the air grid High catalyst velocities through slide valves High turbulence caused by a broken air grid

Excessive Mass Flows in Cyclones and Diplegs The way excessive mass flow rates through the cyclone cone outlets increase cyclone losses has been described above. Similarly, when the quantity of catalyst entering first stage cyclones becomes greater than the maximum amount that will flow down the diplegs, usually resulting from increased gas rates to the cyclones, the catalyst level backs up into the cyclone hoppers until it reaches a level where it is re-entrained by the cyclone gases and carried to the second stage cyclones. Significant catalyst attrition also occurs during the reentrainment process.

Insufficient Dipleg Length While most cyclone systems, when designed, have diplegs which are long enough so that the catalyst level in the diplegs is 600mm (24") or more below the cyclone hopper-dipleg weld line, increases in throughput can raise the required dipleg level to the point where it reaches the cyclone vortex. When this occurs, the results will be both attrition of the catalyst and erosion of the cyclone cones and hoppers. In addition, some of the catalyst will be re-entrained in the exiting gas stream and carried out of the cyclones. Since the highest catalyst level is normally in the diplegs of the last stage of cyclones, the re-entrained catalyst becomes additional losses. High catalyst losses for any reason except catalyst attrition will result in a reduction in the amount of 0 to 40 micron particles in the equilibrium catalyst. In some units the effect of this loss of fines on catalyst circulation is more critical than the increased catalyst losses. Current Cyclone Designs 7

Cyclone design concepts that have changed in the past few years are the following: The number of cyclone stages used in a reactor The length of the cyclone compared to the cyclone diameter

The Number of Cyclone Stages in a Reactor When the riser in a reactor vessel has a discharge device other than a cyclone, the gases will then pass through a single stage of cyclones. These single stage cyclones are more efficient than the first and second stage cyclones previously used in two stage reactor cyclone systems and currently used in two stage regenerator cyclone systems. The easiest way to describe these more efficient cyclones is by listing how they are different from normal first stage cyclones. Start with the inlet area. As shown in Figure 10, the inlet areas are the same in both cyclones. The gas outlet tube diameters are either the same or, in the more efficient cyclone, the gas outlet tube has a smaller diameter. However, the cylinder (or barrel) diameter of the more efficient cyclone is about 20% greater than the first stage cyclone diameter and all other dimensions of the more efficient cyclone are 20% or more greater than the corresponding first stage cyclone dimensions. Since only a single stage of cyclones is used, it is possible to fit these larger cyclones in the vessel. Most reactors have several sets of cyclones. By using a greater number of single stage cyclones than the number of two stage sets, the diameter of each more efficient cyclone can be nearly the same as the diameter of the first stage cyclones. The primary reason for this design change is to eliminate leakage in to the second stage diplegs. The results have been reduced catalyst carry-over to the fractionator and reduced capital equipment costs. More recently there has been a trend back to two stage reactor cyclone systems, but in these systems the first stage cyclones are directly connected to the riser. The first stage cyclone gas outlets are located in close proximity to or directly connected to the inlets to the second stage cyclones. These systems are designed to obtain a more rapid separation of the catalyst from the gases and to reduce or eliminate the time the gases spend in the reactor vessel, thereby reducing "over-cracking" of the products.

The Length of a Cyclone Compared to the Cyclone Diameter If one measures the inside length of a cyclone that is five or more years old and divides this length by the inside cyclone diameter, this ratio will probably be between 3.5 and 3.7. However, it is possible that this ratio may be as much as 5.0. Figure 11 shows a side-byside comparison of two cyclones which are the same except for the lengths of their respective cones and hoppers. A comparison of the erosion found in cones, hoppers and diplegs of longer second stage cyclones with that found in the corresponding areas of shorter second stage cyclones has shown that there is significantly less erosion in the longer cyclones. However, longer second stage cyclones have one disadvantage that must be 8

considered. In order to make the cyclones longer, it is necessary to reduce the length of the second stage diplegs. In many vessels the length of dipleg required for proper operation of the cyclones is such that it is not possible to use longer second stage cyclones. However, when considering a cyclone replacement, the use of longer cyclones should be discussed with your process licenser and your cyclone supplier. There is one other advantage to be gained by using longer cyclones and that is the efficiency of longer cyclones is greater than the efficiency of shorter cyclones. For this reason most single stage reactor cyclones and many first stage cyclones now being installed have a greater than traditional length-to-diameter ratio. When it is not possible to have a length-todiameter ratio of 5, it may be possible to have a ratio between 4 and 5. More efficient first stage cyclones reduce the catalyst loading to the second stage cyclones. When it is possible to submerge the second stage diplegs in the catalyst bed, the reduced loading to the second stage cyclones reduces catalyst losses, catalyst attrition and cyclone erosion.

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