Meanwhile, he offers a warning about plugging. “Approximately half of the world’s distillation columns are packed. It is impossible to climb through such towers. The key to their performance is the distributors, which are easily plugged with dirt, polymer, gums and other materials. During annual turnarounds, all packed towers should be opened and the distributors should be inspected for hole plugging. Better to do these inspections than restart such towers and have them malperform six months later because the half-plugged distributors became fully plugged. Strainers are good, here, too.”
Azeotropes often can pose challenges for distillation — and modeling can provide important insights, says Vikas Dhole, vice president of engineering and product management at AspenTech, Burlington, Mass. He points to a recent project on a Reliance Industries’ plant in Mumbai, India. The challenge was to troubleshoot and debottleneck the acetic-acid heterogeneous azeotropic dehydration column. AspenPlus software simulated para-xylene removal.
“Users often have entrainer losses between columns. In this case, simulation helped to reduce losses by optimizing the packing in the column bed to increase capacity,” adds senior product manager Dave Tremblay. The result is a 0.4 kg/metric ton/yr reduction in entrainer consumption, equivalent to a $550,000/yr savings.
In distillation, capacity and energy consumption are intertwined. “As you squeeze pump-around, vapor flows within the column are reduced, as is energy consumption… There is not only an energy benefit, but a capital benefit, too,” says Dhole.
A project at the China National Offshore Oil Corp.’s refinery in Guangdong, China, illustrates this. Here, the refiner wanted to optimize energy consumption during the design phase to achieve a best-in-class energy intensity index (EII). AspenTech used a combination of pinch and column analysis, together with a total site analysis that considered inter-unit integration. The result was a final design EII of 65.3 compared to the general contractor’s 71. This improvement equals $16 million/yr less in energy costs.
At the Samsung-Total aromatic complex in Chungnam, Korea, process modeling, coupled with pinch, column, distillation-sequence and thermal-integration analyses, improved product purity as well as slashed 20% off the complex’s energy costs, saving $12 million/yr.
“Overall, distillation is a key element for us and we are investing heavily in it,” notes Dhole. In February, the company launched release 7.1 of its apenONE engineering software.
AspenTech certainly isn’t alone in updating its software tools. For instance, Koch-Glitsch, Wichita, Kan., just introduced version 4.0 of its KG-Tower equipment rating program. This assists in specifying mass-transfer equipment, including conventional and high-performance trays and random and structured packings. The latest release provides, e.g., new sieve-tray design/rating capability, an updated tray model and the ability to rate additional packing products.
Hardware, not software, is Koch-Glitsch’s main focus, though — and the company just debuted Intalox Ultra random packing. It boasts low pressure drop/high capacity, high efficiency and high strength-to-weight ratio. For a new installation, this translates into smaller column diameter or less column height. On revamps, benefits depend on the application and include more capacity at current product purity, less energy consumption per unit of product, higher purity at current product rates and lower pressure drop.
Cutting the size of columns assumes more importance — both in terms of cost and fabrication — as the definition of “worldscale” rises to 1,500 metric ton/yr and beyond, notes Greg Wisniewski, product line manager for UOP Process, Des Plaines, Ill.