(17a) Cracked Gas Compressor / Turbine

The cracked gas compressor (CGC) is still the most critical unit in the entire ethylene production facility. Its energy demand and potential plant run length, linked to its volumetric flow, molecular weight, pressure ratio and associated contaminant removal, is a major design element within the entire ethylene unit.

As plants become larger, ~1500 KTA ethylene production, various CGC situations are pushed to their limited capacities. Naphtha crackers convert the feedstock molecule to ~4 molecular products; hence to maximize ethylene production and recovery, the CGC suction pressure must be low (~ 3 psig) and the discharge pressure must be high enough to condense methane (~475 psig). This inevitably requires five CGC stages. Ethane crackers are less demanding; feedstock product is ~ 2 molecules per single ethane molecule, and recovery scheme design is easier. Hence the suction / discharge pressure ratio is lower and the CGC can often employ only four stages.

But all cracked gas delivered to the CGC often contains a combination of key contaminants, several of which can foul the compressor. This involves various internal special metallurgies and coatings with regular cleaning injections.

The mega-plant CGC capacities and contaminants impact the optimum fin tip velocities and the number of impellers per each stage. Stage 1 inevitably requires a double suction design.

Many plants, particularly recent ones, specify a wide range of potential feedstocks in order to optimize periodic feedstock costs and ethylene / propylene ratios. As a consequence, the CGC must be designed not only for its standard API-617 margins, but also for a relatively wide range of feedstock MW and flowrates.

The API design requirement for the CGC sets the respective power margins for both the CGC and its turbine driver. Several plants also request operational capacity margins; this directly impacts the CGC design. A related ideal requirement is the turndown flowrate without recycle re-compression. These respective higher / lower operational ratios will be discussed. Another key element in the successful CGC is the anti-surge instrumentation. That will be mentioned also.

The capital cost of our CGC compressor / turbine system is directly linked to the equipment capacities and powers. This will be determined principally by the number of CGC casings and turbine drivers. We have designed units with 2or 3 or 4 CGC casings and 1 or 2 steam turbine drivers.