When Imperial Sugar Co., Sugar Land, TX, decided to rebuild the sugar refinery at Port Wentworth, GA, following a February 2008 explosion and fire, it sought out the best available technologies to reduce hazards inherent in sugar production. According to Brian Harrison, vice president, technology, who was charged with overseeing the $200 million-plus reconstruction project that began production in mid-2009, one of its priorities was to determine how to best convey the billions of pounds of sugar produced, and this led it to investigate the advantages offered by pneumatic conveying systems.
Prior to the incident, the company used mechanical systems such as bucket elevators and screw and belt conveyors to transport sugar to the packaging plant. Historically, these systems have been used in sugar refineries because they require less power; however, these systems also have limited flexibility, and if a plant expands or modifies processes, it can result in inefficient layouts. And while nearly any maintenance staff could maintain these mechanical systems, they can regularly require repairs because of their numerous moving parts and pieces.
On the other hand, pneumatic conveying requires more power but has fewer moving parts and greater flexibility for plants with changing needs. Also, the enclosed pipelines protect the material from contamination and virtually eliminate the breakdown of the product into smaller particles that can present a dust hazard.
Imperial Sugar engineers visited various plants, not necessarily sugar refineries, to look at conveying systems and to get an understanding and feedback from people who had worked with pneumatic conveying over a period of years. The company also talked to different equipment manufacturers during its investigation to confirm the advantages of pneumatic conveying and to assess how each company would approach its process challenges, according to Mr. Harrison.
Ultimately, Imperial Sugar selected MAC Equipment, Kansas City, MO, to construct continuous dense-phase pneumatic conveying systems for the reconstructed plant. Dense-phase conveying uses air pulses ranging from 400 to 1,600 ft per minute. Although dense-phase is typically a batch process, it can be made approximate continuous by adding a second vessel; the two vessels alternating between filling and conveying cycles.
The systems installed at Imperial Sugar use extremely reliable Spheri Valves, which are available exclusively from MAC Equipment and its parent company Clyde Process Solutions. Spheri Valves feature dome-shaped closures mounted on half shafts that are moved through 90 degrees by means of a pneumatic cylinder. The rubber seal-mounted valves are inflated by compressed air when closed to obtain a gas-tight closure. When the valve opens, the seal is first deflated, causing it to retract away from the dome. Because no direct contact between the dome and seal occur when the dome is moving, abrasive wear of the seal is avoided. The valves are rated for up to 1 million cycles, according to Kent Carolan, senior project manager, MAC Equipment.
Imperial Sugar chose MAC as a supplier for its pneumatic conveying system because it demonstrated a complete understanding of its systems, according to Mr. Harrison. “MAC was very enthusiastic about the opportunity to work on our project and developing solutions for our unique challenges,” he added. “We were betting 100% of our material handling on MAC’s system. Because it would handle all of the production of this facility, we wanted somebody who would be comprehensive in their approach to providing engineering solutions, and we thought MAC brought that to the table from the beginning.”
When Imperial Sugar chose its pneumatic conveying vendor, it had outlined the scope of the project, but that was all. “We were moving fast with this project,” Mr. Harrison said. “Time was of the essence for us, and we were looking to our vendor partners to seamlessly collaborate with us to compete final designs.”
MAC committed the resources to move the project forward as quickly as possible, according to Mr. Harrison. “MAC had people on-site for a year and a half, and it had the resources to commit to that kind of application,” he noted. “At times during startup phases, MAC people were required on-site 24 hours a day.”
Overall, MAC installed 16 continuous dense-phase systems at the refinery with rates at 150 tons per hour in dual pipelines, according to Doug Carroll, strategic account executive, MAC Equipment. These were complete systems with modularized dense-phase vessels, Spheri valves, airlocks and dust collectors. In addition, MAC designed the safety systems using best practice technology. Although it is highly unlikely an event could occur due to the nonabrasive conveying, out of an abundance of caution, pressure vessels were built for explosion containment with pressure ratings of 650 psi, according to Mr. Carolan. Conveying lines and ancillary equipment is fitted with either explosion venting or explosion suppression to minimize the chance of an event being propagated through the system.
Dense phase conveying enable separation and isolation of unit operations without the complexity that historical approaches would have presented and reduced the risk of mechanical failure substantially.
“Continuous dense-phase conveying presents several advantages for the sugar processor,” Mr. Harrison explained. “First, it does not degrade the size and shape of sugar crystals while moving the product at high rates of speed over considerable distances. We want to minimize the breakage of sugar crystals during the material handling and we want to safely contain any entrained dust. Second, pneumatic conveying requires less maintenance and use few parts that could contaminate the product when they wear or fail.
Mr. Carroll added, “Dense-phase conveying assists processors in meeting Occupational Safety and Health Administration requirements and the National Fire Protection Association guidelines. Our pneumatic conveying system is an ideal way of conveying friable solids.”
Imperial Sugar has been extremely pleased with MAC’s support throughout the project, according to Mr. Harrison. “As we’ve gone through the different phases of the startup, its checkout procedures were first class,” he said. “Its systems started with very few faults. Any issues we did have were minor, and MAC always had the right people on-site to address those.”