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Green design practices: focus on efficiency

Green design is more than recycling scrap materials or calculating carbon footprints. It minimizes negative environmental impact through skillful design and operating practices to produce efficient, better-functioning processes. Because green practices reduce resource requirements, in many cases, they can also improve the profitability of the entire process.

One of the most effective ways to make a process greener is to focus on column efficiency. Distillation is an energy intensive process. Even though most columns are thoroughly heat integrated, their energy consumption is high. Historically, many column designs have focused on throughput at the expense of efficiency. Now with increasing energy costs and the emphasis on greener designs, efficiency is becoming an important topic.

Efficiency in Packed Columns
Increasing the efficiency of column internals typically lowers energy usage, yielding a greener design. More specifically, by increasing the number of theoretical stages within a given column section, the required reflux rates or stripping steam can be reduced, saving large amounts of utilities. An excellent way to increase stages is by using high performance packing. Depending on the packing type selected, high performance packing allows the designer to increase capacity or efficiency. When emphasizing a green design, we would select the higher efficiency option.

As an example, Sulzer MellapakPlus™ 352.Y high performance structured packing can be used in place of a standard M250.Y packing to obtain 25% higher efficiency with 20% lower pressure drop. The added efficiency and lower pressure drop combine to substantially reduce the overall process energy consumption, especially in vacuum applications where pressure drop is critical.

Efficiency in Trays
Optimizing efficiency in a tray design requires that the vapor and liquid contact each other uniformly to maximize the generation of interfacial area. From the vapor side, this means selecting properly sized orifices or valves and locating them uniformly across the entire tray deck. Valves are typically more efficient because they discharge vapor laterally into the liquid on the tray deck for maximum contacting. As for optimum size, testing at Fractionation Research, Inc. (FRISM) has shown that ¾” (19 mm) orifices give an ideal combination of capacity and efficiency. Sulzer MVG™ valves provide a ¾” effective orifice size and their small size allows for uniform distribution across the tray deck to maximize efficiency.

As for the liquid phase, ideally it should travel uniformly across the deck in plug flow with no backmixing. Tray designs with long flow paths and/or smaller downcomers can create maldistribution or stagnant regions that can lower efficiency by as much as 25%.

In these cases, strategically placed push valves can be used to maintain a uniform liquid flow across the tray deck, thereby maximizing tray efficiency. This proven technology can increase efficiency by 15 - 25%, depending upon the tray design. Since push valves are integral to the tray deck, they don’t require any additional material to be used. Higher efficiency with no additional material usage makes this another effective, green design option.

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