Heat-integrating a petroleum refinery fluid catalytic cracking (FCC) unit main fractionator and its unsaturate gas plant (gas concentration unit) is common. This paper expands upon the fundamental principles in the analysis of a modern FCC unit. Too often, high-technology analytical tools seduce designers away from consideration of fundamentals such as operability, flexibility, maintenance costs, and investment minimization. The design of an actual unit is compared to a configuration reengineered with some practical considerations in mind. The reengineered (and simpler) system improved energy recovery by 5.54 Mkgcal/HR (22 MBTU/HR); reduced the investment cost by eliminating two large pumps and miscellaneous process control equipment; eliminated long-term pump maintenance; reduced horsepower consumption; and simplified the unit operation.
Details presented include review of the design of the FCC unit main fractionator and gas concentration unit deethanizer reboiler system developed to recover low-temperature main fractionator heat. The increased low-temperature heat recovery was due to a main fractionator heat balance shift to optimize gasoline yields. An optimum main fractionator heat removal system utilizes five pumparound loops. Process issues of this design are discussed. Additionally, emphasis is placed on the interaction between process and equipment design details. The design of the system implemented and an alternate design with its benefits of better operability, lower maintenance, and lower capital investment are presented.
In the example, the heat integration between the main fractionator and the deethanizer column reboiler system was unnecessarily complicated and the column process control system was fundamentally inadequate. The objective of the revamp was to recover heat shifted from the FCC unit main fractionator heavy cycle oil (HCO) pumparound (PA) to the top pumparound (TPA). The new gasoline regulations and economic incentives during gasoline season associated with maximum gasoline recovery always shifts the main fractionator heat removal to the top section of the column. The heat removal shift from high-temperature HCO (329°C, 625°F) pumparound to the top pumparound (177°C, 350°F) makes the overall energy recovery more challenging and the unit heat-integration more complex.
With more complex heat-integration, the unit operability becomes more difficult. The overall operability of this unit could have been improved (in comparison to the revamp design) while reducing capital investment and long-term maintenance costs. The deethanizer control and operability could have been improved by analyzing some basic process issues of the process heat-integration scheme. Fundamental chemical engineering principles are the foundation of a good process design, not high-technology tools. Refinery distillation troubleshooter Norm Lieberman stated that he has "no unique knowledge, only a fundamental grasp of basic chemical engineering tools." The general rule of "The simpler, the better" is always true because the realities of day-to-day operation override the complexity of "high technology" solutions.
The following points will be covered in detail:
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