The cracked gas compressor (CGC) is still the most critical unit in the entireethylene production facility. Its energy demand and potential plant run length, linkedto its volumetric flow, molecular weight, pressure ratio and associated contaminantremoval, is a major design element within the entire ethylene unit.As plants become larger, ~1500 KTA ethylene production, various CGC situations arepushed to their limited capacities. Naphtha crackers convert the feedstock molecule to~4 molecular products; hence to maximize ethylene production and recovery, the CGCsuction pressure must be low (~ 3 psig) and the discharge pressure must be highenough to condense methane (~475 psig). This inevitably requires five CGC stages.Ethane crackers are less demanding; feedstock product is ~ 2 molecules per singleethane molecule, and recovery scheme design is easier. Hence the suction / dischargepressure 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 keycontaminants, several of which can foul the compressor. This involves various internalspecial metallurgies and coatings with regular cleaning injections.The mega-plant CGC capacities and contaminants impact the optimum fin tip velocitiesand the number of impellers per each stage. Stage 1 inevitably requires a doublesuction design.Many plants, particularly recent ones, specify a wide range of potential feedstocks inorder to optimize periodic feedstock costs and ethylene / propylene ratios. As aconsequence, the CGC must be designed not only for its standard API-617 margins, butalso for a relatively wide range of feedstock MW and flowrates.The API design requirement for the CGC sets the respective power margins for both theCGC and its turbine driver. Several plants also request operational capacity margins;this directly impacts the CGC design. A related ideal requirement is the turndownflowrate without recycle re-compression. These respective higher / lower operationalratios will be discussed. Another key element in the successful CGC is the anti-surgeinstrumentation. That will be mentioned also.The capital cost of our CGC compressor / turbine system is directly linked to theequipment capacities and powers. This will be determined principally by the number ofCGC casings and turbine drivers. We have designed units with 2or 3 or 4 CGC casingsand 1 or 2 steam turbine drivers.
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