Troubleshooting FCC Circulation Problems-Practical Considerations
At one time or another, almost every operator of a fluid catalytic cracking unit has experienced problems with catalyst circulation. Usually, the problem is solved through adjustment of standpipe operating parameters, or change of catalyst physical properties. Sometimes, the problem just goes away as mysteriously as it began.
As plant operations engineers advance to other positions, they take their experience and solutions to these problems with them, leaving the new engineer and his operators with the same problems, but not necessarily the solutions.
This paper is intended to help solve this problem, provide a roadmap in effect, for troubleshooting circulation difficulties on fluid catalytic cracking units. The emphasis will be on smooth flow in standpipes, but the basic principles will also carry over to other parts of the unit.
In order to properly discuss troubleshooting of circulation problems, the pressure balance and dynamics which control the fluid catalytic cracking (FCC) unit must be understood. Figure 1 is an outline drawing of a typical FCC unit. Figure 2 is a pressure balance worksheet which may be followed while troubleshooting pressure balance.
The regenerated catalyst flows from the regenerator in a catalyst transport line, usually a standpipe, to the feed injection point via a control valve. The control valve is adjusted to maintain a constant reactor temperature. This regenerated catalyst must provide sufficient pressure above the control valve to overcome the valve pressure drop and pressure drop associated with transporting the catalyst and feed to the reactor vessel. The catalyst provides heat for the cracking reaction and catalytic sites for selective cracking of feed to desired products.
From the reactor, the catalyst flows through a stripper vessel where hydrocarbon vapors are stripped with a counter current flow of steam. Spent catalyst is then transported to the regenerator where coke is burned off. The catalyst must provide the pressure to overcome pressure losses in the transport system and control valve pressure drop as it flows between the stripper and the regenerator.
The catalyst in the FCCU must remain in a fluidized state at all times as it circulates through the reactor/regenerator system. The reactor and regenerator are operated at different pressures in most cat crackers and the "static head" of the fluidized catalyst provides the motive force to overcome the pressure differential and the pressure drop across control valves in the catalyst transport lines.
Circulation problems are usually caused by the catalyst becoming defluidized as a result of lack of fluidization media, or the catalyst being held up in a standpipe due to too much fluidization media being injected at one point.
There are three major controls on the FCCU itself, which maintain correct catalyst circulation rates. They are shown in Figure 1.
The reactor temperature is controlled by a valve, which allows the hot, regenerated catalyst to flow into the reaction zone. The reactor temperature is controlled at the desired level at the reactor outlet, thereby including the heat of feed vaporization, heat losses, and heat of cracking in the reactor heat demand.
The stripper level is the second major control. The stripper level is controlled by a valve at the base of the spent catalyst standpipe. Stripper level control is important in order to provide sufficient residence time in the stripper for the stripping steam to displace strippable hydrocarbons for recovery downstream. The stripper level is also important to provide sufficient pressure to keep the air in the regenerator from reverse flow into the reaction system, thereby causing a hazard.
The reactor/regenerator differential pressure controller is the final major control on the FCCU. This controller operates the flue gas slide valve to maintain a safe differential pressure between the reactor and regenerator. When the pressure balance is proper, the differential pressure maintains about equal pressure drop across both the regenerated and spent catalyst control valves. The differential pressure may be adjusted to overcome small circulation difficulties from time to time. It may also be used to balance pressure drops across the two catalyst flow control valves.
Now come the big questions. How can you tell when you have a circulation problem? What happens to the unit, and what do you watch for?
The most obvious indication of a problem with catalyst circulation is a dramatic drop or fluctuation in the pressure drop across one or both of the catalyst control valves. Often, these fluctuations are only momentary and the problem corrects itself before other process parameters are affected. However, this momentary drop in differential pressure across the catalyst control valve is a signal that more drastic upsets may be in the offing if steps are not taken to remedy the situation.
These momentary pressure fluctuations which seem to correct themselves often recur over extended time periods. In these cases, they tend to worsen with each recurrence and cause ragged reactor temperature or stripper level control. When the reactor temperature or stripper level begin to swing, this may also begin to cause pressure swings in other parts of the unit such as the main fractionator and wet gas compressor suction due to the change in volume of the reacted products with the reactor temperature change. Similarly, the regenerator pressure may begin to fluctuate due to temperature changes caused by the irregular flow of spent catalyst and/or changes in the coke load due to conversion swings (riser temperature). These pressure changes only serve to aggravate an already bad situation.
In most cases, problems with circulation can be narrowed down to incorrect standpipe aeration, standpipe obstructions, changes in equipment performance, (i.e. cyclones) causing the bulk catalyst fluidization properties to change, or changes in the catalyst particle properties. Regardless of the cause of the circulation problem, it must be corrected before the unit can return to stable, safe operation.
Procedure for Troubleshooting Circulation Problems
Following is a step-by-step procedure, which may be followed in trouble shooting a circulation problem in a fluid catalytic cracking unit.
Many FCC units have been in operation since the early 1940's. Most of these units have been revamped to take advantage of modern catalysts. However, in many instances, the revamps have not included a thorough look at the effect of catalyst changes and process operating condition changes on the aeration system.
The entire system of standpipe aeration needs to be inspected, paying particular attention to the following general guidelines.
Restriction orifices which are used to distribute aeration flow should be checked for proper size and installation. Pressure drop across the orifices should be sufficient to maintain a constant flow of aeration media, regardless of changes in downstream pressure due to normal process variations. This usually means that the orifices need to operate in the critical flow regime along the entire length of the standpipe. In some instances, operators have installed flow indicators and control valves on each individual aeration tap.
Particular attention must be paid to units with long vertical standpipes where compression effects are important. Many modern FCCU's rely on a maximum aeration tap spacing of about 8 ft. to reduce the chance of a single tap introducing so much gas that a bubble is formed in the standpipe causing flow restrictions. On the other hand, the spacing should also be close enough to prevent the catalyst from defluidizing in the standpipe due to aeration gas compression from static head. In low pressure units which contain long standpipes, compression effects are more important and tap spacing may be reduced to 4 - 6 ft.
There are two catalyst "densities" needed to calculate the standpipe aeration requirement. They are (1) the catalyst flowing density in the standpipe (usually about 35 - 40 Lb/Ft3), and (2) the catalyst bulk density. The now compacted bulk density is used to approximate the void space in the standpipe at incipient fluidization. Typical rules of thumb assume the density at flowing conditions in the standpipe are 90 - 95% of the catalyst bulk density. This density should be used for calculating the amount of excess gas above minimum fluidization that is entrained with the catalyst into the standpipe. At a minimum, sufficient aeration must be added to maintain a constant value of excess gas above minimum fluidization along the length of the standpipe. The catalyst flowing density should be used for calculating the compression of the fluidizing gas caused from the static head in the standpipe. An example calculation of fluidization requirements is shown in Figure 3.
Properties of the aeration gas should also be checked. Condensate is one of the most common problems where steam is used for fluidization of standpipes. Condensate may cause difficulties in several ways. Condensate flowing in internal pipe runs may cause stress corrosion cracking. The liquid may also cause the catalyst at the nozzle tip to turn to "mud" which can plug the nozzle. Finally, as the condensate is introduced into the hot catalyst in the standpipe, it vaporizes causing an interruption in catalyst flow due to momentary over-aeration. This vaporization problem can occur whether the aeration media is steam or refinery fuel gas.
Studies have shown that due to its viscosity, air and/or nitrogen can be much better media than steam for aerating standpipes. Of course, the media must be safe for the location and air would not normally be used for standpipe aeration where the possibility of a hazardous situation could exist when oxygen could be introduced into the reactor system. Nitrogen is usually too expensive to be considered for standpipe aeration. Other factors which make nitrogen an expensive choice include increased wet gas compressor horsepower and lower C3 recoveries in the gas plant.
Changes in catalyst properties are also often the cause of circulation problems. Mechanical failures may cause a loss of fines, or catalyst density may change, causing difficulty if other variables, such as aeration are not manipulated. Several publications (including Catalyst Report T. I. 800) have given methods for determining the fluidization quality of solids so I will not go into detail here. However, the method of Geldart and Radtke will be used to illustrate how changes in fluidization media, particle size distribution, and particle density may affect fluidization in a standpipe. Figure 4 gives the results of a study comparing fluidization characteristics of different catalysts and aeration media. Generally, the larger the ratio of Umb/Umf, the better the fluidization properties of the solid.
Historical data for both fresh and equilibrium catalyst will often help determine if changes in fluidization stem from increased cyclone losses, or from a change in fresh catalyst properties.
Quite often there are other things happening to the unit which manifest themselves as catalyst circulation problems. These may include times of feed rate instability; feedstock vaporization upstream of the feed distribution nozzles causing slug flow; varying aeration media pressure; and broken aeration taps. However, the most common problem is over-aeration of the standpipe.
A complete pressure profile is often a valuable tool in establishing the existence of mechanical problems. A pressure profile is also very helpful in isolating the cause and location of aeration problems.
Over-aeration of the standpipe is usually the result of process or catalyst changes not being recognized as significant enough to warrant changes in operations procedures. The tendency to adjust standpipe aeration during periods of rough circulation should be resisted until or unless the investigation of other factors such as catalyst particle size or density changes warrant such a change. It should be noted that low pressure units are more sensitive to changes in aeration rate than higher pressure units.
The above guidelines will allow refinery process engineers to troubleshoot circulation difficulties, train operations in the proper use of aeration in standpipes, and assist in understanding the dynamics of the FCCU and its dependence on maintaining a proper pressure balance for smooth operation.
This paper has given the new FCC engineer an overview of the FCC pressure balance dynamics and the effect of major controls on operations. In addition, a procedure for troubleshooting catalyst circulation problems has been outlined, and examples of the calculations necessary to carry out successful aeration system checkouts have been given.
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