Reactive Distillation: Will a Sea Change Occur?

Reactive distillation has some strong proponents and is finding growing use in niche applications.

By Seán Ottewell, Editor at Large

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Reactive distillation, which involves performing reaction and separation in the same vessel, long has intrigued chemical makers. However, the technique poses drawbacks as well as benefits, and so has achieved only a limited industrial role. Companies such as Dow, BASF, Eastman, Sulzer Chemtech and Chemoxy do see its value.

“Reactive distillation holds promise for process simplification, particularly with applications that are limited by chemical equilibrium. We have developed a number of commercial processes using the technology, particularly for the manufacture of specialty chemicals,” explains Timothy Frank, fellow, Core R&D, Dow Chemical, Midland, Mich. “I would add that reactive distillation is really one part of the broader category of reactive separation — along with reactive extraction and reactive crystallization. So, in that sense, it’s one of several tools.”


At the same time, the technology does have constraints. “Essentially the reaction and separation have to occur at the same or very similar temperatures, for example. So that limits the window of operation for the technology. However, some of our big successes with reactive distillation have seen as much as a 50%–70% reduction in capital and a 70% reduction in energy used per product manufactured,” adds John Pendergast, research fellow, Performance Materials, Dow Texas Operations, Freeport, Texas.

Alongside thermodynamic constraints is the challenge of sunk capital. Pendergast explains: “The existing plants we work with already have reactors and distillation columns. Even if the technology fits, the plant might already be producing and the net capital already sunk. So, we really have to be in the situation where we need a new process to replace an existing one, or a new process to manufacture a new product. We need to develop these processes out in front of the needs so that we can be prepared to respond when the need arises. This is a generally true statement about all of our process synthesis efforts in the sense that we have to know what is possible before the urgent need for a new process arises.”

To develop new reactive distillation processes — several more will be onstream in the next five years — Dow carries out a lot of mini-plant (i.e., scaled down) and bench-scale work. “Also, one of the most important developments is in our ability to swiftly simulate potential new processes. The power of modern simulation software continues to grow and make rapid and accurate simulation easier,” notes Pendergast.
    
Reactive distillation gets criticized for failing to live up to its early potential, comments Pendergast. “Some companies have regarded reactive distillation as if it is a hammer looking for a nail. It is much better to find a new place where it brings value rather than putting it up against the economics and sunk costs, etc., that you already have. We are most successful when there is a need and a solid value proposition, not when we are pushing the technology,” he says.

“Dow is a very large company with lots of different products and processes, so there are many opportunities to apply reactive distillation in different situations. But, in the great majority of cases, what you do need is an equilibrium-limited reaction. Remember, too, that reactive distillation is the most common form of reactive separation, but there are other ways we can practice reactive separation as well. However, when it is successfully applied, it is one of the most powerful game-changers out there. The manufacture of methyl acetate, for example, is the poster child for reactive distillation,” notes Frank.

PROCESS OPTIMIZATION TOOL
For BASF, reactive distillation is an important option when it comes to process optimization. “BASF has therefore spent considerable efforts on the development of this technology, for instance in public joint ventures such as the EU [European Union] INSERT (INtegrating SEparation and Reactive Technology) project,” says Regina Benfer of the company’s Department of Chemical and Process Engineering, Ludwigshafen, Germany.

INSERT focuses on developing new ways to integrate reaction and separation processes in such a way that they outperform existing technologies.

“Today, there are numerous commercial processes in BASF using reactive distillation. Typical reactions involved are esterifications, transesterifications or hydrolysis of esters, acetalizations and hydrolysis of acetals as well as etherifications. In most cases, homogenous or autocatalysis is applied; in few cases we use heterogenous catalysis too,” she adds.

BASF — sometimes in cooperation with manufacturers of column internals — has developed and patented special packings and reaction trays for both types of catalyzed reactions. For example, one is designed specifically to ensure a long residence time combined with good mixing performance for the liquid phase. Another is used with heterogenous reactive distillations.

Any new or improved process always has to meet economic or sustainability thresholds (Figure 1). “That is also true for the application of reactive distillation. Savings based on this technology are mostly a consequence of minimized energy and raw material costs, but also as a result of improvements in waste water or offgas management,” says Carsten Knösche, also with the Department of Chemical and Process Engineering in Ludwigshafen.

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  • <p>There are patents, more than forty years old, where reactive distillation technology was used to produce product. Patents never mentioned reactive distillation but one versed in chemistry and chemical engineering could easily see that. </p> <p>I recall one batch process where about 40 kg product was produced in eight hours. However, the reactive process had to be continuous and produced at about 25 kg per hour. </p> <p>One has to be well versed in marrying physical properties of chemicals (reactants and products) and chemical engineering principles to exploit. </p>

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  • <p>The Reactive distillation concept and combining two unit operation in one, it looks on the paper good and also to academics as they have not looked at the operations and economics of reactive distillation. Worked on Reactive Distillation and licensed for 20 years and find following drawbacks which are easily measureable. Reactive distillation might be competitive when the WHSV are in the range of 10 to 15 after that it is a stretch and one can sell snake oil also if one is clever in marketing. But in general the Reactive distillation has major problems unless one can find a method to have bulk catalyst in the column where you can do not simultaneous distillation and reaction but in stages alternately. I have been involved with over 20 years and was part of one of major companies which licenses these application if applicable/economical or not and have been able to sell to some big companies in USA but not to the parent company. Following drawback is ignored by people who want to use the Reactive distillation in economics. 1. The catalyst cost is 3 to 4 times the bulk catalyst used as bulk catalyst in the fixed bed or any other type of reactor. This price difference and saving in some equipment by Reactive distillation is paid off by the catalyst cost differences in 3 to 6 months depending on catalyst requirement based on WHSV. The more catalyst one requires the earlier it is paid off. 2. The energy and utility requirements are higher due heat of reaction has got to be taken care of by higher reflux, which increases the column diameter and height to install the catalyst and bigger reflux pump, reflux drum and reflux condenser. In some cases the pressure has to be increased which requires either higher level heat or furnace. So careful consideration is to be given to these issues in evaluating Reactive distillation against conventional and Smart configuration. In some cases one might have the high level heat medium but that heat can be used somewhere else with better benefit than using LP steam if pressure is not raised. This is case the saturating the benzene in reformate if used this application. 3. Installing the catalyst in structure which can be installed through the Man way is cumbersome operation of loading and unloading and twice the time compared to conventional equipment and additional resources to take these modules install/and take out when replacing. 4. Smart configuration will provide with simple couple of equipment the objective of going to maximum conversion to extinction of feed/reactants, which are paid off in 3 to 6 months as mentioned above for the cost of these pump/drum and other small modification to column. 5. Reactive distillation where the cost are higher has been used in gasoline HDS, alpha to Beta isomerization of butenes and also as a packing for mixing in Sulfuric acid alkylation and the cost in first two applications are 25 % as in the first application it needs an extra stage of HDS compared to competing technologies and in butenes it takes too much catalyst. In the case of Sulfuric acid alkylation the packing is prone to crushed and its integrity is questionable. Additionally there are new innovation which provide mixing with unique equipment and also the auto refrigeration vapor can be condensed by raising the pressure by small amount and at the temperature it will condense and one can remove the compressor saving Opex and Capex by about 40 to 50 %. So researchers and licensees should look smart operation to overcome the equilibrium reactions. 6. The complexity and cost of packaged catalyst makes Reactive distillation a very uneconomical option against Smart configuration and researchers/Academics and licensees should look at it with caution. 7. The present operation with reactive distillation can easily be revamped to conventional and Smart configuration with minimal cost as in all these like HDS, isomerization, hydrotreating, mercaptan removal as sulfides can be achieved with conventional operation at low cost and saving in energy in HDS are more than 30 percent and CAPEX 25 % additional HDS stage ( payout is 3 to 6 months) so please look into the economics with care as it looks to academics/and layman a process which is alchemy and hype it has lot of issues which makes the economics lopsided and also cumbersome to install and uninstall the catalyst and needs longer to do the job. 8. Smart configuration provides the benefits of removing equilibrium constraints and conventional operation. 9. Even in special application one can provide Smart configuration which will remove the equilibrium constraints and also will be in any application will be cheaper in CAPEX and OPEX. </p>

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