Distillation is bubbling up

High capacity utilization is prompting significant efforts to optimize column performance

By Seán Ottewell

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Improvements in internals

UOP is seeing continuing strong demand for its high performance distillation trays, notes Sturtevant, particularly for its high capacity MD ones (Figure 1).

They typically are used for large liquid loads, especially when the volumetric ratio between vapor and liquid rates is low — common conditions in medium- to high-pressure services. Because MD trays can be used at close spacings, they can reduce both the height and diameter of a new column compared to one fitted with conventional multipass trays, says the company, thus significantly cutting vessel shell costs. The closer spacing means that retrofitted towers can contain far more trays, increasing product purity and recovery while reducing reflux ratio and therefore energy consumption.

“The point here is that we are able to get more out of existing columns and make new columns smaller to achieve the same throughput,” Sturtevant says.

It’s a similar story with Koch-Glitsch, Wichita, Kan., another major player in the mass transfer equipment and services market. Like UOP, the company has a long tradition of manufacturing trays and in pioneering specialty high capacity designs.

For instance, Superfrac trays, which are the culmination of 10 years of development work, are designed to produce the maximum capacity and maximum vapor/liquid contact efficiency achievable with crossflow distillation trays. They have provided the highest combined capacity and efficiency of any crossflow tray tested so far at Fractionation Research Inc. (FRI), Bartlesville, Okla., boasts Koch-Glitsch. 

When designing Superfrac trays, the company targeted three major areas to give them enhanced performance over conventional trays. First, a variety of valve styles and technologies are available to enhance the vapor/liquid contacting that takes place on a tray deck. Second, the downcomer is precisely sized and shaped to maximize the active area available for vapor/liquid contact. Finally, inlet area improvements provide more capacity and better froth initiation/bubbling activity on the tray — increasing vapor contact efficiency.

Together, these enhancements eliminate the vapor and liquid maldistribution and stagnant zones that can occur on conventional trays, claims the company.

They promote uniform flow distribution at the tray inlet and the perimeter areas, a great benefit to the tray’s hydraulic performance and contact efficiency.

Sulzer Chemtech, Winterthur, Switzerland, another major player in the mass transfer field, offers a variety of trays, including high performance chordal downcomer, multi-downcomer and ultra-system-limit trays. In addition, it’s the authorized supplier of Shell’s high-end trays and other equipment.

Many distillation columns rely on structured or random packings. And, here too, developments are pushing up performance. For instance, Koch-Glitsch has just introduced the Intalox Ultra random packings (Figure 2). These boast industry-leading strength-to-weight ratios, efficiencies and capacities, according to the company. This translates to reduced column diameter or height in new columns. On a revamp, the new packings offer a range of benefits, including more capacity at current purity, less energy consumption per unit of product, higher purity at current product rates, and lower pressure drop.

In early February, Sulzer Chemtech signed an agreement with Kuehni, Allschwil, Switzerland, to work together on packed liquid/liquid extraction columns. Sulzer is providing the structured packing know-how and Kuehni the stirred exraction column knowledge so that customers can develop optimized designs for their extractors.

Besides hardware, most major vendors of mass transfer hardware offer services to optimize the operation of columns; demand for such help is strong.

Better and safer operation

Optimizing distillation assets involves more than the mass transfer hardware. Enhanced column control can play an essential role. Major automation vendors such as Emerson, Yokogawa and Invensys are developing new technologies and alliances designed to improve distillation efficiency.

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