Troubleshooting FCC Unit Circulation and Fluidization Problems
The FCC unit is one of the more profitable processes in a modern refinery. It is a versatile unit which can be operated at a variety of conditions in order to maximize profits, in spite of changing feed quality, feed rate or fluctuations in product prices. Occasionally, the flexibility and productivity of the unit are negatively affected by catalyst circulation problems.
It is often easy to confuse the terms "fluidization" and "circulation" when referring to FCC unit operation. "Fluidization" refers specifically to the ease with which the catalyst bed can become and be maintained in a fluid state. "Circulation" refers to the overall rate at which catalyst can be circulated around the reactor-regenerator circuit. Circulation can be influenced by fluidization but is more broadly influenced by a number of both catalyst and unit parameters, which can at times lead to circulation problems. These circulation problems could be related to mal-operation of the unit, the mechanical condition of the unit, or the fluidization properties of the catalyst.
The observations noted from the experiments described in this article suggest that increasing the fines content and reducing the equilibrium catalyst density would alleviate circulation problems in those units experiencing rapid de-aeration in the standpipes. Making changes in the equilibrium catalyst's physical properties should be tried after the refinery has attempted to correct the difficulty by readjusting the aeration rates. In the great majority of cases of poor circulation, the problem is solved by air rate readjustment.
It should also be noted that units which encounter circulation limitations due to "low" pressure differentials across the slide valves most likely are not encountering a fluidization problem. In such situations the first thing to check is the aeration to assure that this is not the problem. Once this has been determined, switching to a higher density catalyst should be considered since this will increase the P across the valve and provide additional flexibility to the unit.
Problems Caused By Improper Aeration
The fluid cracking unit requires a large catalyst circulation rate, in order to keep the unit in heat balance. The type of fluidization encountered in the circuit ranges from dilute phase transport in the riser to dense phase transport in the spent and regenerated catalyst stand pipes. If a unit encounters circulation difficulties it is most likely that the problem will occur in the dense phase transport areas. These areas are sensitive to changes in aeration rates. Improper aeration, too low or too high, may cause low or erratic differential pressures across the slide valves and result in reduced operating flexibility. An aeration rate that is too high tends to form large bubbles that interfere with the catalyst flow, while the aeration rate that is too low permits catalyst to de-aerate or slump and cause problems. BASF has studied the de-aeration rate phenomenon in the laboratory on a small fluidization test unit illustrated in Figure 1. The data generated illustrates de-aeration rate differences of various equilibrium catalysts as a function of apparent bulk densities and particle size distribution.
The fluidization test consists of a Pyrex glass tube. 48 inches long and 4 inches in diameter, with a 5mm fine porous frit at the bottom. An adjustable gauge (10 inches of water) is used to measure the pressure generated inside the tube. Dry air regulated at 10 psi is used as the fluidization agent. A rotometer tube is used to monitor and control the velocity of the air.
The procedure requires one gallon of catalyst sample. The sample is loaded into the glass tube to a height of about 18 inches. The exact height is then measured. A 1/4 inch stainless steel tube, used as a pressure transfer line to the gauge, is inserted down the center of the glass tube until the tip is exactly 12 inches above the frit.
The fluidization velocities used for the test range between 0.04 ft /sec and 0.10 ft/sec. Three velocities are required for evaluation. The catalyst is fluidized for about 5 minutes, and the fluidization air is then halted. As the catalyst settles, a dense zone forms at the base of the bed, and the dense/dilute interface moves upward with time.
The time for the catalyst bed to settle is determined by measuring the time for the dense/dilute interface to move one foot from the base of the bed to the outlet of the pressure transfer line (i.e. for the pressure gauge to return to zero). The velocity for settling, (i.e. de-aeration rate), is then calculated as 1 ft/settling time. This measurement is carried out at each of the fluidization velocities chosen for evaluation.
The data from the tests are used to calculate the true de-aeration rate by generating a linear plot of the three fluidization velocities versus the measured de-aeration rates. The true de-aeration rate is defined as the intersection of the experimental curve and an equivalency line of a slope equal to 1. The de-aeration rate for the example shown (Figure 2), is 0.073 ft/sec.
De-aeration rates determined in this manner indicate the minimum aeration rate that can maintain fluidization for that catalyst. The lower the value, the better are the fluidization properties of the catalyst.
Effects of Catalyst Particle Size, Shape and Density
Aside from determining relative de-aeration rates for equilibrium catalyst samples, the test unit can be used to determine the effects of other physical properties of the catalyst on fluidization. The key catalyst properties affecting fluidization are the particle size distribution, particle density and particle shape. Leva et, al developed a relationship which shows that the minimum fluidization is proportional to the particle density and the particle diameter squared.
Vmp = minimum fluidization velocity, ft/sec
(Pp) = particle density, Ibs/ft3
(Dp) = particle diameter, ft
(1) LEVA. M
McGraw-Hill Book Co. NY (1959)
Similar types of relationships can be observed using our test method and studying the affects of particle size distribution. The presence of particles <40 microns and those >80 microns and their effects on fluidization were studied as well as the affects of density and shape factor. The results can be summarized as follows:
- As shown in Figure 3, an absolute increase of 10% wt. of <40 micron particles size fraction of the catalyst reduced de-aeration rate by 40%.
- An absolute reduction of 10% wt. in the >80 micron particle size fraction of the catalyst reduced de-aeration rate 6%.
- A reduction in density of 33% resulted in de-aeration rate reduction of 20%.
- A change in catalyst shape from spherical to oblong gives a 19% reduction in de-aeration rate.
In summary, the data indicated that the variable which affects catalyst fluidization the most is the quantity of <40 micron particles in the catalyst. Improvements can also be made by a reduction in equilibrium catalyst density and a change of particle shape. A reduction in the >80 micron fraction has an influence but it is not a major factor.
The test we use for screening fluidization properties of equilibrium catalyst, is only one of many that the private industry and academic world use for this purpose. It provides a guide to potential problems in fluidization relating to the catalyst properties. The majority of circulation problems in the fluid crackers are associated with mal-operation or mechanical problems. The former includes lack of proper aeration due to plugged nozzles or changes in total aeration rate. Mechanical problems could be related to refractory partially blocking entrances to the stand pipes or stand pipe design.
The combination of all the above contributing factors make catalyst circulation problems unit specific. A solution to the problem for one refiner may be totally inappropriate for another. In general, if a catalytic cracker experiences circulation problems, the refiner should try those changes which have minimum effect on his overall unit operation, such as changing aeration rate, checking for plugged aeration taps, changing the differential pressure between the Reactor-Regenerator vessels and increasing the fines content in the circulating catalyst inventory.