Distillation is bubbling up

High capacity utilization is prompting significant efforts to optimize column performance

By Seán Ottewell

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Properly install column internals
Hopes that fallout from the subprime lending fiasco would mainly impact the U.S. and Western European economies seem largely misplaced now as its malaise spreads ever wider around the globe. The ongoing tribulations of the world economy are making even the modest growth predictions made late last year by the American Chemistry Council (ACC), Arlington, Va., seem quite optimistic. ACC did warn, though, that ongoing financial problems and an oil price shock could take a toll.

Other problems are emerging, too. Even before subprime problem surfaced, China was always going to be the economic driving force for global development over the next few years. Now, however, that country is suffering its worst winter in decades. Food costs have climbed 18% as a result of snow-related supply problems. Inflation has hit an 11-year high in January, an annualized 7.1% — up by more than ½% compared to the previous month.

Source: Invensys Process Systems.

On the other hand, however, a low dollar is good for U.S. exports, particularly if overseas growth remains robust. If this is the case, last year’s $154 billion export figure could rise to $169 billion this year and $180 billion in 2009, says ACC.

Latest data from ACC show shipments of chemicals rising and capacity utilization hovering just below 80%. So, continuing strong demand for exports should edge U.S. capacity utilization to 80% in 2008.

It’s not just the mainstream chemical producers that are enjoying high capacity utilization. In August, the American Petroleum Institute (API), Washington, D.C., reported 90.8% refinery utilization rates, the highest level since September 2006. Production of gasoline hit 9.325 million bbl/d, another new record. By November, refinery activity was the highest in three years.

API’s annual statistical report published in late January noted that overall capacity utilization averaged 88.1% for all of 2007, up slightly from a year before.

Meanwhile, the International Energy Agency (IAE), Vienna, Austria, predicts that global refinery crude runs will average 74.3 million bbl/d for the first quarter of 2008, a year-on-year increase of 1 million bbl/d.

While it remains to be seen how much further damage the subprime fiasco can do to the world economy — there are rumors of major investments in the Far East being postponed by many years — high capacity utilization is spurring companies to place even more emphasis on asset optimization. And, because at many plants, distillation plays a crucial role, demand for mass transfer products and services is booming. Operating companies are looking to gain improvements through everything from better packings and trays to improved control systems and simulators.

“The biggest trend at the moment is the massive number of projects. There’s a huge uptake after the lull in the late 1990s,” says Gary Sturtevant, industry segment leader in UOP’s Refining Business Unit, Des Plaines, Ill. UOP also is building up a healthy stream of business for its new Ecofining technology. This converts raw materials such as vegetable oils into biodiesel. “The challenge here is that we are not dealing with traditional hydrocarbon feedstocks, so we are having to do a lot of development work associated with their [the biofeedstocks] handling and processing,” he notes.

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.

For instance, in early January Emerson Process Management, Austin, Texas, announced the acquisition of The Automation Group (TAG), Houston. The deal will help Emerson expand its technical and management services for the design, engineering and implementation of automation systems, especially for distillation-based processes.

Yokogawa, Tokyo, which will supply an integrated production control system to the new Shell Eastern Petrochemicals complex in Singapore, was awarded the contact “…because we have the ability to improve the operability and safety of large-scale plants by integrating their production control and safety instrumented systems,” notes Teruyoshi Minaki, Yokogawa’s executive vice president.

Safety of distillation operations is receiving increasing emphasis in response to major incidents in the last couple of years.

Column internals that offer better performance such as this MD tray remain in high demand. Source: UOP

“Safety is now the number one consideration, so much so that it has become easier to get budget to improve safety than to improve efficiency,” says Harpreet Gulati, a product director for SimSci-Esscor, South Lake Forest, Calif., a unit of Invensys Process Systems (IPS), London. SimSci-Esscor’s heritage is in distillation simulation, with its PRO/II well established in that area. “And, importantly, we partner with various third-party companies to provide enhanced product capabilities using it,” he adds.

Among IPS’s partners is Koch-Glitsch, which enhances PRO/II with a number of its own modules. For example, Ratefrac performs rigorous rate-based distillation modelling while Batchfrac handles design and analysis of batch reactors and distillation columns.

This partnership also extends to Koch-Glitsch’s own KG-Tower tray and packing hydraulics software. Here, PRO/II has the option of converting tower hydraulic information it calculates to a file that can be read directly by KG-Tower, eliminating the need for manual transfer of data. PRO/II also supports hydraulic sizing and rating for a wide array of structured packings from Koch-Glitsch.

It’s a similar story with Sulzer Chemtech, as PRO/II supports hydraulic sizing and rating for a wide range of structured packings from that company. 

“At our latest user meeting, both companies were showing their own in-house design packages for equipment such as high efficiency packings. So they start with PRO/II, which can design the overall column and take account of factors such as fluid types, flows and viscosities. Then they can use their own packages to find, for example, how many meters of packing are required in a particular column,” notes Gulati.

Other SimSci-Essscor partnerships involve software supplied by organizations as diverse as FRI, Det Norske Veritas, Innotec and the Institut Français du Pétrole.

The number two consideration for the distillation process is now energy management in an around columns, according to Gulati. “We’re talking here about real-time heat and material balances,” he says.
Years ago, if there was a problem in the column, operations or technical support staff assigned to that particular unit would have to start by taking readings, calculating heat and mass balances, plugging this information into some sort of model and then deciding on a way forward, he says. “It’s all online today, so every few minutes the data can be reconciled. So we can calculate the overall yield, for example, or how much energy is being used. It can also be used for predictive analysis. For example, what increase in efficiency would I get if I clean the re-boiler now?”

IPS is taking what Gulati describes as a holistic approach to distillation optimization. “By combining maintenance, visualisation, data and design/modeling together, we can immediately identify any potential problems and alert maintenance staff. In this way, the cost of problems can be quantified and they get sorted quickly.”

Overall, while business is good for those involved in distillation, a major constraint is emerging: a lack of skilled labor.

“Companies like ours — contractors — are very short of people to do project development,” says UOP’s Sturtevant. “Lots of training simulator operators will be retiring in the next ten years, so we need to train the next generation. But there is a real problem finding people to do this training now. The engineering companies are getting most of the talent,” notes Gulati.

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