Separations Technology: Plants Polish Towers

Revamping of distillation columns often can provide important benefits

By Seán Ottewell, Editor at Large

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A vast number of distillation columns operate at sites worldwide. Many of these towers offer significant opportunities for upgrading, experts say. For example, Henry Kister, senior fellow and director of fractionation technology for Fluor, Aliso Viejo, Calif., points out that preheating is valuable when a tower bottleneck occurs below the feed in a tower that has spare capacity above.

Preheating increases the total heat energy requirement compared to adding heat in the reboiler, so it is a first-law-of-thermodynamics energy guzzler. However, because the feed is colder than the tower bottoms, an economizing medium such as waste heat or the bottoms can provide a large portion of the heat, turning preheaters into a second-law-of-thermodynamics energy savers, he notes.

“Benefits from preheating with an economizing heating medium usually far outweigh the first law losses. Generally, the effectiveness of preheating for either hydraulically unloading the bottom section or for saving energy is maximized when the fraction vaporized is of similar order to the fraction of lights in the feed, as detailed by the classic paper of Patterson and Wells (Chem. Eng., Sept. 1977),” Kister adds.

Inter-reboilers can save more energy and gain more capacity than the preheater, he says, but at the expense of greater complexity.

It’s a similar story with precoolers and inter-condensers: they both can save energy when an economizing medium can pick up some of the overhead condenser heat duty but lead to a higher condensing energy requirement in the tower.

Another issue for optimizing post-revamp efficiency he highlights is manipulation of column operating pressure/pressure drop.

Raising the pressure boosts vapor density and reduces vapor velocity, thus increasing vapor handling capacity. In most distillations, a higher pressure lowers relative volatility, pushing up reflux and reboil requirements for achieving the same separation. This counters the vapor density effect and tends to lower capacity.

“At lower pressures (<50 psia), usually the vapor density effect dominates, so raising pressure enhances capacity. At higher pressure (>150 psia), and especially when a tower is limited by liquid handling capacity, lower distillation pressure tends to maximize capacity. Lower pressure usually leads to lower energy consumption due to the lower reflux and reboil requirements. See Kister and Doig, CEP, August 1981, for more,” he explains.

Kister cautions that while trays come in high-capacity versions there are no high-efficiency ones, despite what sometimes is claimed.

He notes that except for the flow path length, once a tray design is hydraulically sound, changing the tray geometry can’t greatly improve efficiency, as can be inferred from the O’Connell correlation (see “Perry’s Chemical Engineers’ Handbook”) that has been the standard of the industry for decades, as well as from Kister’s own relatively recent detailed article (CEP, June 2008).

 

“Lowering tray spacing can increase the number of trays in an existing tower, thus improve separation, at the price of reduced capacity. Features like forward push devices, high ratios of top-to-bottom downcomer areas, using multi-pass trays, are all invaluable means of enhancing capacity, but do little to improve efficiency,” he emphasizes.

One Company’s Experience

AkzoNobel, Amsterdam, is carrying out many revamps worldwide. “As well as increasing capacity, [these revamps] are also important when different raw materials, different process set-ups and different product compositions are needed,” says Maurits Romme, lead process engineer, projects & engineering, who is based in Arnhem, The Netherlands.

Preheaters loom large in many of his revamp considerations, particularly when the opportunity exists to expand column capacity by using a lower-cost source of heat.

In one recent project, the original design already used the vapor from the top of the column to preheat the feed, notes Romme. However, the majority of the heat required for evaporation was delivered by the vapor of the high boiling component generated in the reboiler. The reboiler was heated with a hot oil system.

“To reduce the heat load on this hot oil system — c. 3–5 MW, saving around €1 million/y [$1.1 million/y] in operational costs — we installed an extra pre-evaporator using low-pressure steam as the heating medium. In this way. we could prevent the installation of a new hot oil furnace, saving an estimated €3.5 million [$3.9 million],” he states.

One slightly unexpected challenge came in overcoming operators’ perceptions of how the new system works. Erich Stuy, Arnhem-based discipline specialist process technology, projects & engineering, explains: “They started up the pre-evaporator with some or no reduction of the reboiler duty. This resulted in a large amount of extra vapor over the top of the column, resulting in even more duty in the existing preheaters. It took some time before they could notice this effect in the column. Because of this leadtime, they thought that the complete effect was already there and increased the steam flow. This always resulted in suddenly a much larger vapor flow.” A brief training session settled the issue.

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  • Dear Mr. Ottewell: I read with a great interest your article and its proper time when we are talking about energy efficiency in plants. I would like to raise some points related to important and valuable insights in the article, if I may: 1- The troubles of the hotspots in the packing and its spikes from time to time when it replaced the trays, I was very glad to mention at the end of the article that installing Liquid Re-distributers throughout the column ( Structured Packing) has brought significant improvements to the overall performance of the column which is great. An enquiry! Are there any side effects that I should be aware of besides the low capacity? 2- Another point that I was expecting to be raised in the article; although I think it was mentioned indirectly in the statements of that "Mass Transfer and Column Hydrodynamics" and " Advance in ...."; is the: The Advancement in "Split Columns or Wall Columns". My question is: Would you please shed some light and insight for me about such column designs, issues such as the internal design of the column, simulation and ....etc.? It was reported on many occasions that "Split Columns or Wall Columns" are having also great effects when it comes to Equipment Energy Efficiencies and consequently overall plant energy efficiency. Thank you for a very informative article and I look forward to hearing back from you. Rabih Zayed rzayed@hotmail.com

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