Jul-2000

Low-cost methods to improve FCCU energy efficiency

The first step to finding low-cost ways of improving FCC unit efficiency under new environmental regulations is to determine the true limits of the existing process equipment and flow scheme

Scott W Golden and Scott Fulton, Process Consulting Services

Article Summary

Improving FCC unit energy efficiency is becoming more important as refiners try to comply with new government environmental regulations. European and US government mandates are dramatically reducing FCC gasoline sulphur, refinery total NOx and CO2 emissions. Without improved energy utilisation within the FCC, reducing gasoline sulphur will actually increase energy use in the refinery. This will increase NOx and CO2 emissions by increasing the consumption of boiler or new fired heater fuel gas.

Most FCC units were built before the 1970s, when energy costs were considerably lower than today. Low capital cost was the primary investment objective, not efficient energy utilisation. With few exceptions, the capital cost of rejecting heat to air or water is lower than recovering it.

While refinery energy consumption is a significant operating cost and is closely monitored, investment in improved energy utilisation often cannot be justified by economic factors alone. In today’s competitive market, many refiners require revamp payouts of one year or less and the cost of energy does not often meet investment hurdle rates. Increasingly, environmental issues are driving energy efficiency improvements.

Improving energy efficiency in a cost effective manner requires a thorough review of the existing unit process and major equipment design [Martin G R, “Keeping down the cost of revamp investment”, Petroleum Technology Quarterly, Summer 1999]. Pinch studies, while addressing important theoretical issues, often do not consider constraints imposed by the existing equipment and process flow schemes and fail to determine all the equipment changes that must be made to improve energy efficiency.

A more practical revamp approach is to consider energy efficiency improvement options, process flow scheme changes, and equipment system limits concurrently to find a minimum cost solution [Barletta T, “Practical considerations for crude unit revamps”, Petroleum Technology Quarterly, Autumn 1998].

This requires a thorough understanding of the existing unit bottlenecks, integrating specific process unit knowledge, and general process engineering skills of hydraulics, heat transfer, compression, distillation and process control. Also, a rudimentary understanding of cost estimating is important to separate high and low cost solutions.

Evaluating alternative process flow schemes should be part of this practical design approach. An FCC unit will have a finite number of potential pumparound and product stream heat sources and certain available heat sinks such as riser feed, gas plant reboilers, or utility streams such as BFW, condensate, and waste heat steam generation.

Matching the heat sources and heat sinks while minimising major equipment changes or additions is the key to practical energy efficiency improvements. For instance, using HCO pumparound heat to reboil the FCC gas plant deethaniser is not an effective use of high temperature heat. Also, energy users (heat sink) temperatures should not all be considered fixed.

Any potential process flow scheme change must be tempered by the reality of the existing process and major equipment design. Most revamps, including energy efficiency improvements, are done in the comfort of a design office with little or no details on the current unit and equipment performance available.

Understanding real unit limitations requires a comprehensive benchmarking that starts with a field test run. Test runs include field-measured pressure, temperature, and composition profiles throughout the unit. The engineer responsible for the revamp should be in the field installing pressure gauges and portable thermocouples to get a feel for the existing plant operation. Ultimately, computer modelling must include the realities of the existing plant; otherwise the model results may not match reality and the investment cost may be much higher than required, or the revamp might not work.

Existing major equipment constraints must be circumvented or the equipment modified to minimise capital costs. This requires evaluating alternative energy efficiency options, understanding the existing unit bottlenecks in detail, and having sufficient equipment knowledge to understand the difference between high and low cost equipment changes.
Equipment modifications must be identified. Identifying and designing low cost equipment changes can make the difference between revamp success and failure. Some of the practical energy efficiency improvement considerations involve: Distillation: heat source and sink temperatures
- Number of pumparounds
- Product draw location
- Gas plant side and once-through reboilers.

Exchanger LMTD: increasing pumparound circulations rates
Process flow changes: adding an exchanger service or changing an existing exchanger service
Low cost equipment changes: pump impellers/motors and heat exchanger tube bundles

FCC energy consumption
FCC units consume a considerable amount of energy. The FCC reactor uses regenerator heat to vaporise the feed and provide the endothermic heat of reaction. Figure 1 (on previous page) is a simple block diagram of the FCC reactor/regenerator.

The reactor feed is atmospheric/vacuum gas oils, coker gas oil, and other heavy hydrocarbons. The reactor products are a mix of hydrocarbons, hydrogen, hydrogen sulphide and coke. The reactor product stream feeds the main column and contains a large quantity of heat. Coke leaves the reactor on the catalyst and is burned in the regenerator.

Catalyst flows back and forth between reactor and regenerator to exchange heat, burn the coke, and provide energy for the reactor. The FCC reactor effluent stream enters the main fractionator at temperatures between 950-1050°F (510-565°C).