By Nick Basta
Few operations can be considered more basic to chemical processing than distillation, whose roots can be traced back millennia. Many consider distillation a mature technology; however, new developments in both hardware and software are boosting its performance. In addition, radical departures from conventional distillation continue to attract attention.The high energy consumption of distillation, which some observers say accounts for 40% of the production cost of petrochemicals, certainly is a tantalizing target area for improvement. Notwithstanding current energy concerns, however, the latest developments in distillation hardware tend to focus on achieving higher capacity, not higher efficiency. Trends in tray designBoth of the leading suppliers, Koch-Glitsch, Wichita, Kan., and Sulzer Chemtech USA, Pasadena, Texas, tout the benefits of their valve trays compared to conventional sieve trays. Sulzer Chemtech introduced its mini-V grid (MVG) trays in the mid-1990s and has continually refined designs, says Mark Pilling, the company’s Tulsa, Okla.-based manager of technology. Most recently, the firm introduced an MVG tray with a truncated downcomer. Tests at Fractionation Research Inc. (FRI), Stillwater, Okla., showed a capacity improvement from the new downcomer. This tray, being marketed as the MVGPlus, is said to have the highest combination of capacity and efficiency of any tray FRI has tested.The MVG tray includes slots and stationary valves that allow liquid to swirl around the surface of the tray. It can provide up to 70% more capacity than conventional sieve trays. Depending upon the application, however, 10%-20% of this added capacity may be needed to handle the necessary higher reflux, cutting the overall capacity increase to 50% or less. “Having that capacity improvement gives the operator several options,” Pilling says. “Increasing a refinery column’s output from, say, 10,000 bbl/day to 15,000 bbl/day is one. Another is to use fewer trays in the column. A third is to have shorter columns.” During the past decade the company also has reduced valve size from openings of several inches in diameter to less than 1 in. The truncated downcomer is a channel on one side of the tray that allows liquid to cascade onto the tray below. By shortening it — allowing a few more inches of space between the downcomer and the deck of the tray underneath — a more efficient bubbling zone is created above the bottom tray, further enhancing capacity, Pilling says.Sulzer Chemtech may be on the verge of the next jump in performance with new technology licensed from Shell Global Solutions, Houston. The design, called ConSep, divides a tray into two sections: one where contacting occurs between vapor and liquid, and another where the separation of the two phases is enhanced by the use of tubes that accelerate movement. José Bravo, business development manager for Shell, says this tray could increase capacity by another 50%-60% above that of current high-capacity tray designs. Koch-Glitsch offers a cross-flow tray, of which it has steadily improved the design, mostly to gain extra capacity. The tray first saw service in refining, but now is used in gas processing and petrochemicals, says Izak Nieuwoudt, the company’s director of R&D. “Conventional sieve-type distillation trays are fundamentally limited by gravity. When a droplet is entrained by the upward flow of vapor, you are reaching a flooding state, and mass-transfer efficiency goes down,” he explains. Koch-Glitsch offers several variations of this type of tray, including MaxFrac, SuperFrac and UltraFrac (Figure 1). The designs differ in the types of valves on the tray deck, downcomer designs and fouling resistance.Another vendor, Jaeger Products Inc., Houston, which is best known for its random packings, is developing a cross-flow tray with support from the U.S. Department of Energy (DOE). A company representative says the tray might be available for commercial use by the end of the year. Sieve trays also have their proponents, one of which is UOP, Des Plaines, Ill. UOP provides slotted sieve trays for styrene, alcohols and other types of service. The company says the design provides even vapor distribution across the deck, lower pressure drop than high-capacity trays, and more predictable scale-up. Another design that competes against valve trays is the firm’s multiple downcomer (MD) tray, which is recommended for fouling service.One of the main obstacles to such designs is tradition. Many plant operators prefer to replace tower internals with the same type of hardware because they want the column performance to keep to its familiar history, vendors say. That certainly suits Amistco, a Houston fabricator that provides standard, nonproprietary designs for smaller- to medium-size projects. The company says its column-internals business has doubled during the past three years. “Industry prodded us to do this,” says Bernard Kalis, technical director, “because as the larger companies gobbled up their competitors and the business has consolidated, getting smaller-sized supply contracts has become more difficult.” (During the past several years, Koch-Glitsch has absorbed Otto York and Norton, whereas Sulzer has acquired Nutter Engineering and rights to technology from Julius Montz GmbH.) Kalis says that especially for petroleum refiners, which are running their plants flat-out these days to meet rising fuel demand, “Plant operators don’t want any hiccups, such as might occur when one type of tray is replaced with another during a turnaround.” Even so, Amistco now is working with researchers at Texas A&M, College Station, Texas, on some alternative tray designs.Figure 1. The Ultra-Frac is one of several cross-flow trays that promises increased capacity but differs in the type of valve and downcomer design.Progress in packingA similar evolution is taking place in structured packings, which find use in vacuum- to low-pressure applications and come in a variety of shapes and materials.The biggest innovations relate to the discovery that the transition from one layer or block of structured packing to the next is an area of significant liquid holdup and pressure drop. Sulzer Chemtech’s solution was to bend the outer layers of structured packing to give a smooth transition from one layer to the next, reducing the holdup. This is now a standard feature in the company’s Metallpak-Plus packing and was extended to the company’s BX-Plus (Figure 2) gauze-structured packing last year. The capacity of a column with the “Plus” versions of structured packing can be 20%-40% higher, with no change in efficiency, technology manager Pilling says.At Koch-Glitsch, the response to the liquid holdup problem was a joint development with Praxair, Tonawanda, N.Y., to adjust the interfaces between layers of packing. This innovation, now called Flexipac HC by Koch-Glitsch, led to a 25%-40% increase in capacity, R&D director Nieuwoudt says.However, the world of random packings remains a “jungle,” Neiuwoudt says. “There is a considerable array of shapes, materials of construction and performance characteristics, and much of it is not well understood,” he says. So, in many cases, the best option when making a choice is to experiment with different designs and column internals, which may not be optimal. Random packings are preferred for higher-pressure applications, vapor/liquid stripping and certain other applications, vendors say.
Figure 2. BX-Plus structured packing provides a smooth transition between layers, which reduces holdup.Source: Sulzer ChemtechSoftware improves designColumn design benefits from the application of new software tools, especially computational fluid dynamics (CFD). Sulzer Chemtech performed its own studies, which resulted in the redesign of its truncated downcomers. Koch-Glitsch makes use of the CFD capabilities at a sister organization, John Zink Co., Tulsa, Okla. It employed CFD for new types of vapor distributors and liquid collectors. Both of these vendors, as well as others, have ready-made software downloads on their Web sites for doing basic distillation-column design and equipment specification.Vendors are developing a “flooding predictor” — a program that predicts when flooding conditions are about to occur in a column and recommends operating changes to prevent it. One effort, just commercialized, is Tray Zone Valve (TZV), a program developed by The Distillation Group and marketed by 3rdPhaseEngineering, Ful-shear, Texas. Brad Fleming, one of the partners of The Distillation Group, says that the Windows-based program has both a predictive flooding function as well as a design function, and incorporates tray design elements from most major tray suppliers. “There are numerous public-domain flooding-prediction programs, but they tend to be overly conservative at low pressures and overly optimistic at high pressures,” Fleming says, adding that the transition tends to occur around 2 lb/ft3 densities. The result, of course, is low-pressure columns that are too large and high-pressure systems that are too small.With a similar goal in mind, DOE, together with the Separations Research Program at the University of Texas at Austin, is supporting the development of a program from 2ndpoint LLC, St. John, Ind. The software uses pattern-recognition technology (rather than a first-principles calculation) to predict flooding, says George Dzyacky, a refinery engineer at the BP refinery in Whiting, Ind., which developed the technology. “One of the advantages of this system is that it doesn’t depend on the equipment specifications of any one vendor,” says Chris Lewis, research engineer at the University of Texas, who has worked on the technology. Roughly a year ago, Koch-Glitsch sold the marketing rights to RateFrac and BatchFrac to SimSci-Esscor, a unit of Invensys, Lake Forest, Calif. SimSci-Esscor has said it would incorporate the programs into its Pro/II simulation program.Unconventional column designProcess intensification, reactive distillation and divided-wall columns aim to take distillation in new directions.Process intensification has been bouncing around industry and academic circles for many years. The idea is either to combine a standard unit operation like distillation with something else (such as the separation effects of centrifugation, a combination that ICI experimented with in the 1970s and came to be known as HiGee separation) or to employ aspects of evolving nanotechnology R&D, such as the production of ceramic sheets with nanoporous channels for heat or mass transfer. “We are looking at these [nanoporous channels], as are others,” says Koch-Glitsch’s Nieuwoudt. “But don’t hold your breath waiting for their implementation.”On the reactive-distillation front, the leading player is CD-Tech, a Houston-based joint venture between ABB Lummus and Chemical Research and Licensing (which is owned by Shell). Numerous CD-Tech systems were installed during the 1980s to produce MTBE for gasoline octane enhancement. In recent years, though, MTBE production has decreased after the material’s use was banned by several states. Mitchell Loescher, process development manager, says the company has several proposals out to refiners for producing ethyl tert-butyl ether (ETBE) as an alternative. “ETBE has the advantages of using bioethanol as a feedstock, which has significant governmental support, and it is a less soluble material than MTBE, which should limit its environmental impact,” he says. The process schematics for ETBE would be similar to that of MTBE: A CD-Tech-supplied structured packing, incorporating porous catalyst containers, is placed in the column at a position where reactivity is maximized, while separation of product ETBE and unreacted feed is easier to accomplish. CD-Tech technology also is in use in limited applications for converting benzene to ethylbenzene, hydrogenating methyl and vinyl acetylene byproducts during butadiene production, and desulfurizing alkylation streams that are high in olefins (as a way to minimize saturation of the olefins). “Applications tend to be fairly specialized,” Loescher says.Meanwhile, Julius Montz GmbH, Hilden, Germany, has been pursuing both reactive distillation and an old-but-new-again technology, divided wall distillation columns, in conjunction with research partners at Delft University of Technology, Delft, Netherlands; BASF, Ludwigshafen, Germany; and others. Aside from the specific structured packing sold to Sulzer Chemtech, Montz’s U.S. representative is ACS Industries, Houston.A divided wall column (DWC) features a vertical wall inside part of the column. In essence, two columns are contained within a single shell. The technique, which has been used for many years by BASF, requires less energy than two or more separate columns. According to a paper presented at this year’s AchemAsia meeting (Beijing, May 11-15), Montz’s technology, now called a “dividing wall column,” has been enhanced by enabling the wall to be moved within the column to adjust to process conditions. Although the company maintains that “the dividing wall column is now considered to be an accepted technology,” it has not announced any major new implementations.Nick Basta is editor at large for Chemical Processing magazine. E-mail him at [email protected].