Multiple Dividing Wall Column Distillation Doesn’t Get any Better Than This
The multiple dividing wall column separates multi-substance mixtures in a single equipment item, thus reducing investment and energy costs. So far, this complex device has remained a pipe dream, but that is about to change: The first plant worldwide has been put into operation and should come as close as possible to the thermodynamic optimum.
Fred Flintstone, process engineer? In fact, prehistoric people of the Neolithic age were already busy making carbon products as soon as they had learned to tame fire. Pitch and tar, and later essential oils, were extracted early on with simple distillation vessels made of clay. Later, the ancient Arabs discovered how to distil crude oil, which they called naft, and developed the typical distilling vessel called an alembic, which allowed the distillation of alcohol.
From then on, things went from strength to strength: There was hardly a medieval alchemist’s room without a distilling flask simmering away somewhere in a dark corner. In due course, with the extraction of the countless hydrocarbons used to generate plastics, polymers, paints and colors, separation by evaporation became the basis of complete value chains in the chemical, pharmaceutical and energy industries.
At the Heart of the Industry
Distillation is one of the oldest thermal separation processes known to man. Moving forward to today, fractional distillation is the most common form of separation process technology used in petroleum refineries, petrochemical and chemical plants, and natural gas processing units. Columns as tall as houses and giant crackers form the core of large chemical sites and shape the image of the industry.
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And even if in the course of decarbonization the industry is now planning ways to shift from fossil resources to alternative raw materials and electric heating, the basic necessity to separate substances thermally will remain virtually unchanged.
About Ten Percent of the Global Energy Production for Distillation
But “thermally” also conventionally means energy-intensive: About ten per cent of the power produced worldwide is needed for different distillation processes, and there is very little that can be done about that.
Or is there? In southern Germany, a group of researchers begs to differ: Here, a team led by Professor Thomas Grützner, from the Institute of Chemical Engineering at the University of Ulm, is working on a small revolution in process technology: a multiple dividing wall column.
So far, separating multi-substance mixtures by means of distillation in a single process step has not been possible, at least with conventional distillation columns. Instead, typically several columns are connected in series, each separating another substance from the mixture in a single step.
The Dream of the Separation Wall
In the Ulm University project, the separation of multi-substance mixtures is the number one priority, Grützner explains. But in order to reliably isolate up to four different fractions from each other, the researchers claim to need only a single column.
The multiple separation column itself reaches almost ten meters in height, an astonishing feat for a pilot plant. And even if the actual apparatus can only be glimpsed from the outside through the surrounding scaffolding and thermal jackets, the plant is nothing short of a small revolution. For the first time, a distillation plant with several partitions separated by dividing walls has been realized here, Grützner explains.
Multiple Separation Wall Column
The idea of dividing up the interior of a distillation column by adding an internal partition wall, and thus being able to separate several fractions in a single step, is nothing particularly new. The first patents were filed as early as the 1940s, and the first actual plants were tested about a decade later. Further studies showed that the additional equipment costs more than paid for themselves, thanks to the ability to eliminate a second column and its peripherals, as Grützner points out.
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The thermodynamics also ensure that the dividing wall column consumes less energy than two or three separate units that do the same job. However, the design of these plants, just like their instrumentation and operation, is a highly complex task. Even today, only a few plant specialists or global companies with large engineering departments even dare to work on relatively simple single dividing wall columns.
A Vision is to Become Reality
But if ternary systems can be partitioned so successfully and efficiently, why shouldn’t such an approach also be feasible for four-component mixtures? As so often, the devil is in the details. “The complexity literally explodes,” explains Grützner. “That is why such a plant has often been discussed, but never built.” Grützner, on the other hand, was already considering how to distil several fractions in a single column when he was still working in research and development at a big chemical company.
“After I transitioned to the University of Ulm, I immediately started with the development of this new type of distillation apparatus,” the chemical engineer recalls. But apart from planning and building the pilot plant, he says, designing the column proved to be a Herculean task that could only be solved with years of modeling and simulation.
2021 was the year: the plant was started up for the first time in September. It cost around 600,000 euros and was built by distillation specialist Iludest in Waldbüttelbrunn, near Würzburg. Set up in an explosion-proof environment, the plant has a heating capacity of 10 kW and is equipped with over 70 sensors. It can operate under either pressure or vacuum, depending on which mixture is to be separated.
From Simulation to Distillation
“The system is primarily designed not to separate practical mixtures into their highly pure components, but to help us understand the process,” Grützner says. For example, the liquid feed flowing into the head of the column from above, and the vapor streaming up from below, are distributed to the separate partitions by means of splitters, which also have an influence on one other. “Understanding these interrelationships is now the goal for the next few years,” Grützner explains.
Also on the agenda is the study of so-called multiple steady states, in which a particular mixture can stabilize at several operating points. This behavior has been described many times in theory and simulation, but it unsettles potential investors and operators in the industry. If the separation process were to stabilize outside the desired parameters, production to the required specifications would be jeopardized. It’s therefore necessary to try to recreate these conditions in the real plant — after all, Grützner points out, these could also be merely mathematical artefacts.
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In any case, the aim is to understand and safely control the column, to achieve different degrees of purity and to compare the behavior of the real plant with the expectations from the simulation: How can such a plant be started up? How can the desired states of equilibrium be achieved? And what should the control of such a process look like?
Three Channels Make One Column
After all, the Ulm researchers are entering completely new territory. “We have the first plant in the world here in the laboratory and we want to use our research to help understand how these plants work and to identify and ensure robust operation,” explains the distillation expert. “Our goal is to pave the way and take away the fear of this complex apparatus from users in the industry.”
In the construction of the plant, however, the developers used a little trick: Instead of a single column body with two partitions, the Ulm-based company used three parallel channels, each designed as a separate glass body but with a common condenser and evaporator. It doesn’t matter for the actual distillation process whether the individual channels are designed as single glass tubes or areas in a common steel column, separated by partition walls, Grützner explains.
In this way, the behavior of a three-partition column can be simulated even with the small diameters of typical pilot plants, without having to consider heat transfer through the column walls. This effect, which is quite significant for small plants, can increasingly be neglected at the large diameters of typical industrial systems. For the same reason, Iludest enclosed the individual channels in a single heating jacket.
Up to 50 Percent Energy Savings Possible
Since distillation in a dividing wall column can be carried out with almost no backmixing, the new process requires not only much less equipment compared to three columns connected in series, but also much less energy. In fact, the energy input is up to 50 per cent less than for a typical distillation cascade — an enormous effect that should also convince skeptical industrial users, Grützner is sure: “Dividing wall columns work very close to the thermodynamic optimum. You simply cannot use even less energy.”
Close to the Thermodynamic Limit
“A mixture of substances will not separate on its own, so you have to add energy,” Grützner says. “In the multiple dividing wall column, we realize exactly the thermodynamically necessary minimum amount of energy — that’s pretty cool!” In practice, the only limits are set by heat losses and the efficiency of auxiliary equipment, so this efficiency monster comes as close as technically possible to the minimum energy input.
The team succeeded in reducing to just a few hours the long start-up period needed for a typical column — the process of charging and heating until the plant reaches a stable operating state. This considerably simplifies trials in the everyday life of a university that lacks the round-the-clock shift operation of a large chemical company. Here, too, the solution was impressively simple, Grützner underlines: the desired steady-state operating condition is quickly achieved by placing the heavy reboiler inside a sump at the bottom of the column.
One for All, All for One?
Grützner is sure that almost all doors are open to the dividing wall column — only with regard to azeotropic mixtures is the researcher from Ulm reluctant to commit himself. Distillation at different pressure levels, which is by no means unusual in industrial practice, also does not work in a single apparatus, of course.
“If the multiple dividing wall column finds application in industry, our research could make a massive contribution to climate protection,” explains the German process scientist. “Even in the future, companies will not be able to do without chemical separation processes, which are responsible for their own share of global energy demand. The multiple dividing wall column points the way to how we can make this fundamental process more energy-efficient.” n