1
EQUIPMENT
INTRODUCTION
The Fluid Catalytic Cracking unit is exposed to severe temperature, erosion and corrosion effects. The equipment has been designed to withstand these conditions for an acceptable mechanical life, but the life can be drastically shortened by abuse or poor operations. Proper control and operation will avoid the unnecessary problems which lead to premature failure.
The FCC unit should be inspected every turnaround. The inspection can be done by a UOP inspector, the refinery inspection department, or both. Adequate record keeping of these inspections is necessary to develop the unit history which will aid the refiner to judge equipment life, determine potential problems and evaluate the effect of different metallurgy and process conditions related to the equipment.
A good inspection will include the following:
1) An evaluation of the equipment which has experienced erosion and/or corrosion
2) A list recommending minor repair work
3) An assessment of mechanical problems caused by the operating conditions of the previous run
4) A list of spare parts required for the next turnaround
Erosion and corrosion are not always obvious. A thorough inspection of the equipment will reveal the extent of any damage and help determine the cause and effect relationship. Also, because no equipment lasts forever, the inspection will help the refiner determine when equipment must be replaced so advance orders can be made.
Minor repair work can prevent small problems from expanding to larger problems. It is difficult to predict the amount of minor work which will be required. The desired length of the next run will determine the extent of the repairs.
PREPARATION for INSPECTION
Preparation for inspection should begin well before the shutdown. Proper scheduling of inspection and maintenance will avoid delays and wasted time. As soon as an inspector finds a problem, parts can be ordered and work can be scheduled so the completion of the turnaround will not be delayed.
The normal shutdown procedure will remove most of the catalyst from the unit and cool the vessels to 200-250°F (90°-120°C). The manways should be opened on both the reactor and regenerator to air cool the vessels. Vacuum connections are provided to remove any remaining catalyst. The vacuuming operation can begin while the manways are being opened, if there is sufficient manpower. When the vacuuming is complete, water washing can be started. This will remove the dust and fines which would hinder a complete inspection. A simple water spray is usually sufficient, with the water draining out of the reactor and regenerator at the bottom of each vessel. The water will not cause any problems with the vessel internals, even stainless steel. A high-pressure blast could obviously damage the refractory;
common sense is required. Clean, potable water with less than 50 ppm chlorides should be used. Excess water should not be allowed to stand on the equipment for long periods of time. An air hose can be used to blow away these puddles. There are a few areas that are difficult to drain, such as the regenerator plenum chamber.
These can be cleaned with a heavy-duty vacuum cleaner. Because there are different reactor and regenerator designs, the exact cleaning method should be determined by the refiner.
The main column and gas concentration section should also be cleaned for inspection after the normal shutdown procedures have been completed.
Hydrocarbon and sour water should be pumped or pressured out of the unit. Gas and vapors are removed by steaming out the vessels. Any remaining material can
be removed as necessary by water washing the equipment. Do not allow any water in the wet gas compressor. Normal refinery safety practices should be followed for toxic vapors, explosivity, oxygen content of vessels, etc. The refinery safety engineer should follow the turnaround carefully.
AIR BLOWER
The main blower of an FCC unit supplies large quantities of air to the regenerator.
Advances in rotating machinery technology have led to the replacement of the old positive displacement reciprocating air compressor with centrifugal and axial machines. Blowers can be driven by steam or gas turbines, electric motors, or flue gas turbines, usually referred to as power recovery expanders. Depending on the mode of operation and other factors such as feed quality, the FCC unit needs 10-14.5 pounds of air per pound of coke, which is approximately 2000-3000 SCF/bbl (330-500 Nm3air/m3FF). Air is filtered through a screened suction housing that should be designed for noise abatement. Compressed air leaves the blower at about 300-450°F (150-230°C) and 30-60 psia (2.1-4.2 kg/cm2(a)).
CENTRIFUGAL MACHINE
Air flow rates on a centrifugal machine are controlled by varying the speed of rotation, throttling the suction, or venting off excess air. Four to six stages are common, with labyrinth seals used to prevent leakage between stages. These machines normally use forced lubrication systems for the bearings and may be equipped with temperature and vibration probes for early detection of mechanical problems.
AXIAL MACHINE
An axial blower, shown in Figure 1, uses rotating blades to move the air. The air flows through the machine in a straight line, each successive stage adding pressure energy much like a propeller blade. The air flow rate is most often controlled by varying the shaft speed. This can be accomplished either through a turbine driver, or by including a Variable Speed Drive (VSD) unit on a motor. If the machine is designed for constant speed, other means of flow control must be provided. One option is to snort (vent) excess air to atmosphere. However, for normal control this would require power to compress air that is vented back to atmosphere, and is simply not energy efficient. Practical options include the use of small variable pitch blades known as “stators” on the blower housing. The variable pitch stators redirect the air flow into the path of the rotating elements. When this redirection is at a steeper angle, more air is transferred. These machines use forced lubrication systems and are normally equipped with temperature and vibration probes.
Another option to control air flow from a fixed speed blower is to include a suction throttle valve. This mode of operation is very common in older machines, but as with a snort valve, it is not an energy efficient system. The suction throttle valves on motor driven air blowers can be eliminated through the installation of a VSD unit onto the motor.
Axial compressors are generally more efficient at larger capacities than centrifugal machines. They are smaller and lighter than an equivalent size centrifugal unit.
Choice of machine depends on the individual refiner, but axial blowers are more common for larger units.
Specific operation of these large machines is too complex to describe in this manual. Individual manufacturer's instructions should be followed for each unit.