Sometimes innovative ideas take extraordinary amounts of time to germinate. Such is the case with dividing wall columns — an innovation that is introducing unprecedented cost efficiency, energy savings and emissions reductions to the natural gas liquids processing industry.
Distillation has been the central element in processing natural gas liquids (NGL) for about a century. It has traditionally relied on a four-tower design, but dividing wall columns (DWCs) now enable this process to be scaled down to three towers.
The DWC concept has been explored for at least 40 years, but a variety of factors have slowed development, particularly after early research and development failed to demonstrate significant benefits under a few limited operating conditions. The concept began to be proved as technological advancements allowed thousands of simulations to be run under dozens of operating scenarios.
The result is a DWC design that has been patented by Burns & McDonnell and is already being used in commercial operation. Results in the field are confirming early evaluations that showed up to a 40% reduction in energy requirements, up to 30% less capital investment and up to 50% less plot required when compared to a traditional system.
DWCs offer midstream NGL processors, refineries and chemical plants the ability to reduce operating and capital costs by reconceptualizing traditional distillation systems, which are capital-intensive to build and require significant energy to operate.
A DWC sounds like a simple concept, but is surprisingly complex. It is not only a physical wall, but also a varied process configuration that exploits the wall to improve thermal efficiency and reduce capital cost. These dividing sections combine what previously required multiple distillation systems into a single system with multiple products.
A traditional configuration would be composed of two distillation systems — a depropanizer and debutanizer. The first step in integrating the systems would be using a single column; however, this configuration has a significant drawback — it is impossible to produce high-purity butane (the intermediate product).
A second, smaller column is necessary to maintain purity. This column is fitted with either an independent reboiler and/or condenser using a side stripper and/or absorber. The two columns are partially thermally coupled, resulting in removal of one thermal system and a modest reduction in heat and capital cost. If the two columns are integrated into either a closed top or closed bottom DWC, capital costs may be reduced further. Even more savings may be realized under a final level of integration to remove another thermal system by opening the closed section and allowing vapors and liquids to move in and out of the bottom of the divided section.
The composition of the liquid stream coming into an NGL facility can vary. Depending on market conditions, it could have a lot of ethane or very little. In other instances, it may have a lot of gasoline materials, or may be mostly propane. The patented DWC application is flexible enough to handle all those variations and meet specifications under unique conditions for each of the five purity products: ethane, propane, isobutane, normal butane and natural gasoline.
That flexibility — in combination with lower carbon emissions, easier permitting processes and big capital cost savings — translates into a big industry win that will only grow in importance as pressure mounts to meet environmental, social and governance (ESG) goals.