Rod-Based Baffle Heat Exchangers Minimize Pressure Drop at an Aromatics Facility

Phillips Petroleum Co. today produces aromatics at its Puerto Rico Core facility (Guayama, PR), thanks to two RODbaffle heat exchangers—including the world's longest. RODbaffle heat exchangers, which replace baffle plates with an arrangement of rods that support heat exchanger tubes, have been successfully used in a variety of refining, chemical, and water-treatment applications around the world.

Phillips saved money, energy and possibly the Puerto Rico Core plant itself by replacing two trains of five horizontal plate-baffle heat exchangers each, with only two vertical RODbaffle exchangers. The two heat exchangers, huge size and vertical placement gained the nickname "Texas Towers." The RODbaffle heat exchangers have an excellent performance record after four years of service, with pressure drop across the exchangers less than originally calculated, even at increased charge rates. Feedrates are about 110% of design.

In 1993, Phillips Petroleum faced the possibility of having to shut down Puerto Rico Core's aromatics production due to rising feedstock cost and inability to recycle the process stream to convert the last portion to benzene, toluene and xylene (BTX)—the bulk of the plant's aromatic products. Phillips engineers found a licensed technology to help solve this problem, but discovered that existing plant heat exchangers couldn't support the process without a major upgrade. The advanced process added a new reactor between the existing plate-baffle heat exchanger trains.

"The extra reactor produced a higher pressure drop than the plant's original design, and created a challenge that the existing heat-exchanger trains couldn't handle," said Maria Scharre, P.E., a Phillips senior process engineer. The solution to keeping the plant upgrade cost-effective was to design and build two large RODbaffle heat exchangers to replace existing plate-baffle exchangers.

The new RODbaffle heat exchangers also saved Phillips money. The estimated cost to upgrade existing plate-baffle heat exchangers was $2.3 million, but cost for the RODbaffle exchangers was about $1 million less, due to RODbaffle technology design advantages.

The world's largest—82-in. (2,083-mm)-dia. by 80-ft (24.4-m)-long—RODbaffle exchanger was fabricated by Fabsco, Inc., Sapulpa, OK., while the smaller, 72-in. (1,829-mm)-dia. by 22-ft (6.7-m)-long exchanger was fabricated by Krueger Engineering and Manufacturing Co., Houston. The larger unit's tube bundle was inserted into its shell on a rail car, since the final. assembly was too large for Fabsco to lift and move once it was completed.

Design considerations

Phillips Petroleum selected the RODbaffle heat exchanger design for its superior pressure drop performance and its ability to operate without tube vibration failures. The RODbaffle's tube-support rods create very little shellside form drag, absorbing very little energy and minimizing the shellside pressure drop. Pressure drop through a RODbaffle heat exchanger can be 50% less than a typical plate-baffle exchanger, and the energy savings can significantly reduce online costs.

The RODbaffle tube-bundle design provides maximum tube vibration protection. The positive four-point tube containment, and minimum, convex-point contact between rods and tubes virtually eliminate tube vibration failures, providing a high-integrity bundle for longer onstream performance.

The decision to use RODbaffle exchangers for the BTX plant came after stringent operational and economic studies of several heat exchanger designs. "We examined operational and reliability claims of plate baffle and welded plate exchangers, and found they wouldn't work as well as the RODbaffle design in this application," Scharre said. "We wanted the best heat-exchanger design possible at the plant, so we specified RODbaffle.

"When we held both RODbaffle and plate-baffle exchanger designs to 2.5-psi (0.172-bar) pressure drop, the RODbaffle design cost about $1 million less than an equivalent plate-baffle exchanger," she said. "The plate baffle exchanger might have been price-competitive at 10-psi (0.69-bar) pressure drop, but at those levels we couldn't afford to keep the Puerto Rico Core facility operating."

Mel Atkins, P.E., Phillips senior engineering specialist who designed the heat exchanger systems, added, "With the more stringent pressure loss criteria, we could achieve higher heat-transfer rates with the RODbaffle design. This resulted in less heat-transfer surface area needed and smaller, less costly, exchangers.

"The RODbaffle technology reduced total heat transfer surface area by almost 40%, compared with plate-baffle heat exchangers. Each heat exchanger train was reduced from two parallel sections to one section each," Atkins added.

The RODbaffle exchangers vaporize incoming feed on the tube side, using heat from the reactor effluent on the shellside. Feed to the tallest RODbaffle feed/effluent heat exchanger enters at about 100°F (38°C) and leaves at 600°F (316°C). The feedstream then flows through the added process reactor before entering the second RODbaffle exchanger, which increases the stream temperature from 650°F (343°C) to 760°F (404°C) for feed to the charge heater.

RODbaffle design and fabrication

The RODbaffle design not only offers superior pressure-drop characteristics, but its four-point containment system virtually eliminates the vibration encountered in plate-baffle heat exchangers, which must provide clearance between the tube and plate-baffle hole for fabrication.

The Texas Tower RODbaffle heat exchangers shell and tube were fabricated with carbon steel. The larger exchanger has 157 rod-baffle rings supporting 7,238 plain tubes that are 80-ft (24.4 m) long, 0.625-in. (15.875 mm) O.D.. The smaller RODbaffle exchanger has 42 rod-baffle rings supporting 4,615 plain tubes that are 22-ft (6.7 m) long, 0.75-in. (19.05-mm) O.D.

All baffles are on 6-in. (152-mm) centers and fabricated of 0.1875-in. (4.7-mm) rods. The Phillips-designed RODbaffle exchangers alternate between vertical and horizontal baffle placement to lock in the tubes. The zero-clearance, convex point contact between rods and tubes, with alternating rod-layout, completely locks each tube into a positive four-point containment system.

Due to the unusually large 82-in. (2.083-m)-dia. tubesheet and the high weight fraction of liquid on the tubeside feed inlet (0.714 wt% liquid), Dr. C.C. Gentry, Phillips corporate engineering branch manager, and R.W. Johnson, Fluor Daniel Williams Brothers, Tulsa, designed a novel spray injection system. The advanced injection system ensures uniform, two-phase flow distribution to all 7,238 tubes, and is now the standard for larger RODbaffle units with high liquid-inlet weight fractions.

Moving a giant

The larger RODbaffle exchanger was so heavy (219 tons) that Fabsco used railcars to insert the tube bundle into the shell. Once completed, the exchanger was moved by rail to the Port of Catoosa, OK, and loaded on a barge for transport to Houston, where it was transferred to a larger, ocean-going barge for shipment to Puerto Rico.

Once the exchanger arrived in Las Mareas, it was moved onto a special multi-wheeled trailer and towed over 2.6 miles of dirt road to the Puerto Rico Core facility, where it was lifted into place using several cranes which were shipped with it from Houston.

For more information on the Phillips RODbaffle Heat Exchanger design, contact G.E. (Jeff) Scanlon Licensing Division, Phillips Petroleum Company, 242 Patent & Library Building Bartlesville, OK 74004. Tel: 918-661-1244, fax: 918-662-2007.

By Nick Basta