FCC unit revamps at minimum cost
Selective modifications for FCC unit revamps can deliver the best return on investment
GARY R MARTIN
ReCon Management Services, Inc.
Viewed : 3600
To develop a minimum cost revamp of a fluid catalytic cracking (FCC) unit requires a detailed and rigorous analysis analysing alternate flow schemes and changes in operation to overcome existing equipment limitations by taking advantage of under-utilised equipment. A common mistake is assuming a revamp project can be broken down into evaluating individual pieces of equipment or circuits to develop the modifications necessary in meeting revamp objectives without considering solutions to the limitations from a ‘big picture’ perspective. While this would determine the equipment that is limiting in meeting the design objectives it does not provide the process evaluation necessary to develop the minimum cost modifications. It leads to replacing the limiting equipment with larger equipment instead of employing other under-utilised equipment to circumvent equipment limitations, or modifying other lower cost equipment to overcome the high cost equipment limitation.
The lead process engineer must not only have a very good understanding of the process but also of the equipment. The lead process engineer needs to be able to see the big picture. Knowing the interaction of the process with good knowledge of the individual pieces of equipment enables one to develop changes to the process flow configuration or implement changes to lower cost equipment to debottleneck other high capital cost equipment. This approach to revamp design is required to develop a process design solution that provides the best return on investment (ROI). Examples provided illustrate how this methodology is implemented in the revamping of FCC units.
Air blower through wet gas compressor
The regenerator air blower and the main fractionator overhead wet gas compressor are linked as shown in Figure 1. In comparison to designing a new unit, understanding the ultimate limitations and operation of the integrated equipment is more important in the revamp of an existing unit. To minimise investment, existing equipment performance must be pushed to maximum realistic capacities. To determine maximum realistic capacities requires that the existing equipment performance be thoroughly understood.
The reactor, regenerator, and main fractionator vessels, as well as the air blower and wet gas compressor, are major cost items to replace. A revamp design based on replacing any of these items will be expensive and will often prove not financially viable. Understanding how to change the unit operation and selectively modify existing equipment, instead of replacement, yields a minimum cost revamp.
The first example considers the revamp of an FCC unit to expand the capacity from 33 500 b/d to 40000 b/d (5.33 m3/h to 6.36 m3/h) and increase the conversion from 78.5 vol% to 81.8 vol%. The regenerator operating pressure currently limits the charge rate to the FCC unit. The regenerator’s maximum allowable working pressure is 30 psig (2.07 bar). The critical operating limit is set at 90% of the PRV set point, which is 27 psig (1.86 bar). Operations must reduce the charge rate when the regenerator pressure reaches this pressure. The current operating pressure profile is shown in Figure 1.
The current and revamp design reactor yields are shown in Table 1. It should be noted that the dry gas yields are increasing from 2.25 wt% to 2.71 wt% in addition to the 19.4% increase in unit throughput; this has a significant effect on the wet gas compressor load. The wet gas compressor is already driver limited.
Prorating the system pressure drop based on the increase in throughput determines a system pressure drop increase from 4.9 psi to 7.0 psi (0.34 bar to 0.48 bar) between the reactor and the wet gas compressor suction. This yields a corresponding regenerator operating pressure that exceeds the critical operating pressure of the regenerator. This operation is not feasible without replacing the regenerator vessel.
Major equipment constraints at current throughput and conversion include:
• The operating pressure of the regenerator is at the vessel’s critical operating pressure limit
• The wet gas compressor motor is at current limitation.
Replacing the regenerator vessel is not economically feasible and requires a solution that involves modifications to other equipment to resolve the problem with the regenerator’s critical pressure limitation. The solution requires either lowering the system pressure drop between the regenerator and the wet gas compressor inlet, lowering the compressor suction pressure, or some combination of the two.
Wet gas compressor
Two-stage centrifugal machines are normally selected for wet gas compressors. However, there are older FCC units with reciprocating wet gas compressors still in use. These reciprocating wet gas compressors are less reliable. The characteristics of a centrifugal compressor are determined by the impeller, diffuser, and return channel (see Figure 2). A typical centrifugal compressor designed for FCC wet gas compressor service is designed with backward-leaning bladed impellers due to its higher overall stage efficiency. The designer can select a tip width and blade angle that best fits the desired head and efficiency for the application.
The general shape of the performance curves, as a function of compressor speed, for a centrifugal wet gas compressor is shown in Figure 3. Parameters such as diffusor width, blade angle, gas density, compressor speed, and Mach number play a role in the development of the compressor’s characteristic curve shape. For a typical backward-leaning bladed impeller, as the flow decreases at constant speed, the gas velocity relative to the blade decreases. This makes the tangential velocity increase, which increases the head output. This head increase with decreasing flow is what causes the basic slope of the performance curve. For a given compressor speed, as the flow rate is increased at some point the increase in flow rate results in an excessive decrease in head, which is known as stonewall or choke. This occurs because the Mach number is approaching 1.0. This is why the right hand side of the performance curve rapidly drops off. As the compressor flow rate is decreased at a given speed there is a point where the head decreases with a decrease in flow. At this point of peak head surge flow occurs. Surge flow should be avoided because it can be damaging to a compressor. Flow reversal occurs, resulting in reverse bending of nearly all compressor components.
For this revamp case it is desired to decrease the compressor suction pressure while achieving an increase in throughput. A centrifugal compressor develops a certain head for any given inlet flow rate. The operation of the compressor can be changed to provide a lower suction pressure. However, for a given inlet volumetric flow rate the compressor will produce a head according to its polytropic head performance curve.
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