PTQ Q4 2025 Issue

Shut the tap when not in use Shutting the tap when not in use can significantly reduce CO₂ emissions (while saving the company considerable money), as shown in the following examples: dP instrument bypass: The final step in ethylene plant dis - tillation, the C₂ splitter (see Figure 2 ) separates the ethyl- ene product from the ethane recycle. This close separation under pressure (typically 200-280 psig) produces high- purity ethylene (typically a few hundred ppm ethane impu- rity). It requires a large number of trays, with about 100 trays being common. One C₂ splitter in an 800 kilometric tons per year steam cracker was retrofitted with high-capacity trays. Upon restart, the ethylene purity specification could not be met. Tray efficiency was less than half the design. Eventually, the plant got the ethylene ‘on spec,’ but at the price of over- refluxing, losing tons of ethylene in the bottoms, and operating at less than half of its design throughput. For an ethylene plant, this is a performance disaster, costing tens of thousands of dollars, maybe more, per day. The plant recruited experts and troubleshooters and conducted gamma scans. There were meetings, discussions, brain- storming, theories, and ideas. The gamma scans surpris- ingly showed no apparent problems, but they were not believed. Two weeks later, an operator climbed up the tower. The C₂ splitter had a differential pressure transmitter (Figure) mounted right at the top of the tower. This location mini- mises issues with liquid accumulation in the impulse lines, but is inconvenient to inspect and maintain. The operator noticed that around the transmitter, there was a short bypass line with a ¾in valve. To his surprise, the valve was open, probably to isolate the transmitter and protect it during some commissioning operation. It should have been closed when the transmitter was reconnected, but this did not happen. The open valve passed a large quan- tity of ethane from the tower bottom to the top product. A typical differential pressure in this tower was about 8-10 psi, which would push a lot of ethane into the ethylene via this ¾in line. When the operator closed the bypass valve, the column returned to proper functioning, and the trays operated efficiently. One of the authors had a similar experience with an analyser cross connection on another C₂ splitter that was left open. This tower also had problems making on-spec ethylene upon restart. Steam cracking produces 1.5-3 metric tons of CO₂ per ton of ethylene produced. Over two weeks, an 800-kilo - tons-per-year plant would normally produce 30,000 metric tons of ethylene, emitting roughly 60,000 tons of CO₂. With the dP bypass valve left open, nearly the same quantity of CO₂ was emitted, but half of it was wasted as only about 15,000 metric tons of ethylene were produced. So, 30,000 tons of CO₂ were unnecessarily emitted because of the open valve. With an average car emitting3 4.29 metric tons of CO 2 per year, leaving the valve open for just two weeks had a carbon footprint equivalent to that of 7,000 cars over a year. Caustic leak: In an unrelated incident at a major refinery, a

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Lieberman measured a pressure drop of 0.1 psi across Trays 21-24 of the atmospheric tower, where the pres- sure drop should have been at least 0.4 psi. He concluded that Trays 21-24 were mechanically deficient and needed repair. However, his advice was not accepted, and the new heater project went ahead. The following is an estimate of the CO 2 emissions in this case history:  For four years, the vacuum heater duty was 40 MM Btu/h higher than it should have been. Based on a heating value of 1,020² Btu/scf fuel gas (methane) and an 80% heater efficiency, generating 40 MM Btu/h takes 49,020 scfh of methane. The combustion of this will generate 5,683 lb/h of CO 2 or 22,581 metric tons of CO 2 per year (based on 8,760 hours per year, no outages). An average car emits3 4.29 metric tons of CO2 per year. So not reinstalling four manways resulted in a carbon footprint equivalent to that of 5,264 cars over four years. v A 200 MM Btu/h vacuum heater consumes tons of steel, nickel, and chrome. The mining and fabrication of each of these tons has an immense carbon footprint that is difficult to assess. w The concrete for the foundation of the new heater weighs tons and has a high carbon footprint. x Transportation of the heater to the site adds to the car- bon footprint. Takeaway: Inadequate manway installation inspection carries with it a high carbon footprint. Proper diagnostics greatly reduce the carbon footprint – and costs. Figure 2 C 2 splitter tower, showing the dP bypass Source: H.Z. Kister, Practical Distillation Technology , Course Manual . Copyright © 2013; reprinted with permission

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PTQ Q4 2025