January 2021

Special Focus: Sustainability

Cyclic distillation: A novel enhanced technology for processing hydrocarbons and derivatives

Distillation is the most widespread separation method in history.

Distillation is the most widespread separation method in history.¹ Despite its simplicity and flexibility, distillation remains a very energy-inefficient process. Novel distillation concepts based on process intensification (PI) principles can deliver major advantages in terms of significantly lower energy use and reduced capital costs while improving ecological efficiency.

The PI concept has been around since the mid-20th century, but has received more attention in the last two decades. The objective of PI is to design substantially smaller plants while improving their operational safety, environmental performance and energy efficiency. The Rapid Advancement in Process Intensification Deployment (RAPID) Institute, supported by the U.S. Department of Energy, has placed a spotlight on PI in the U.S. RAPID aims to target and promote major advances in energy efficiency and productivity in the process industries.²

With respect to PI, distillation has enjoyed a number of major enhancements based on process intensification—e.g., HiGee distillation, dividing-wall column, heat-integrated distillation, membrane distillation and reactive distillation.³ Among these advanced technologies, cyclic distillation stands out as a new challenger in fluid separations due to a different way of contacting the liquid and vapor phases.⁴ In contrast to classic operation, cyclic distillation uses separate phase movement (SPM) that can be achieved with specific internals and a periodic operation mode.⁵ Cyclic distillation can also bring new life to old distillation columns by replacing classic internals with new ones and using a cyclic operation mode. One operating cycle consists of two key parts: a vapor flow period (when the thrust of rising vapor prevents liquid down flow) followed by a liquid flow period (when the liquid flows down the column, dropping by gravity, first to a lock chamber and then moving to the tray below). FIG. 1 illustrates this operating cycle.⁶

From the outside, a cyclic distillation column looks like a regular tower. However, the cross-section view inside a cyclic distillation column shows the absence of downcomers and the presence of different internals (FIG. 1, left) that allow an efficient separate-phase movement in practical operation. Recent articles and a mini-review of cyclic distillation provide more details about the history of cyclic distillation, working principle, design and control methods, benefits and limitations.^7–10

Theoretical background

The theory of the cyclic distillation processes has been developed over several decades of engineering. The best established design algorithms for cyclic distillation columns include analytical and numerical methods of increased complexity and usability, developed from 1977–2014.¹¹ FIG. 2 illustrates the modeling of a classic theoretical stage and the operating lines in case of classic vs. cyclic distillation—where L and G are the liquid and gas (vapor) flowrates, H is the liquid holdup on the tray (mol), τ is the vapor period duration (sec), and x and y are the liquid and vapor composition, respectively.⁴

FIG. 3 shows an example of the operating lines for binary separations in the case of classic distillation vs. cyclic distillation. The operating lines are at the basis of designing distillation columns, and a distinction stems from the different theoretical-stage concepts. This difference is demonstrated by the effectiveness of the minimum number of theoretical plates required for the separation at total/infinite reflux. The operating lines provide the number of stages, which is much lower in the case of cyclic distillation, due to the larger driving force of the process. A visual clue of this advantage is the operating line that is actually a mirror image of the equilibrium line relative to the diagonal.

Recent studies proposed iterative numerical solutions of the cyclic distillation simulation model with simultaneous tray drainage. An iterative procedure has been proposed⁸ to calculate the tray holdups and compositions at the end of vapor flow and liquid flow periods, respectively.

A method was developed¹² for organizing cyclic modes in a sectioned apparatus, with the operation of one of the sections in the liquid flow mode and the other sections in the vapor flow mode. A driving force was proposed¹¹ based on a design adapted from classic continuous distillation to cyclic distillation. Recently, both mass and energy transfer were considered in a new model¹⁰ that is able to describe binary and multiple-component systems with both ideal and nonideal liquid phases, as well as account for multiple feed locations and side draws.

An integrated process design and control approach was proposed⁹ based on the application of simple graphic design methods known from conventional distillation columns. This approach has shown that operating a cyclic distillation column at the largest driving force results in an optimal design in terms of controllability and operability, which is less sensitive to disturbances in the feed and has the inherent ability to efficiently reject disturbances.

Techno-economic advantages

The cyclic mode of operation leads to major economic advantages, reported both theoretically and experimentally:

• 20%–50% lower investment cost (due to lower column height, smaller column diameter, smaller area of the heat exchangers, less steel construction and less space used)
• 20%–35% lower hot utility usage (due to high mass transfer efficiency and reduction of the reflux rate)
• 20%–35% lower cold utility usage (due to lower reflux rate)
• Improvement of product quality (due to high mass transfer efficiency and higher concentration of key product)
• Increase of product yield (due to high mass transfer efficiency and more concentrated impurities or other fractions)
• Enhanced process sustainability (due to less GHG emissions due to lower energy usage).

The technological advantages of cyclic distillation include the following:

• Mass transfer efficiency of one cyclic distillation tray is equal to three classic distillation trays
• Reduction of the residence time of liquid in the column, and the uniform arrangement of liquid on the tray
• Ability to control the amount of liquid on the tray and the reaction time (in the case of reactive distillation)
• Any geometric configuration of the trays, which allows the possibility to build dividing-wall columns with trays
• The separation efficiency does not depend on the column diameter, which allows easy industrial scale-up
• Placement of any type of packing between the trays further increases the mass transfer efficiency
• The pressure drop in the column does not depend on the liquid load in the column since the amount of liquid on the trays is constant and only the frequency of cycles is changed (range: 1 m³/m²hr–30 m³/m²hr liquid load)
• The vapor velocity in the column typically ranges from 0.2 m/sec–20 m/sec, as it depends on the pressure in the column
• The operation remains stable and efficient in case of changed concentrations of the key components.

However, cyclic distillation is also characterized by important limitations when considering the technology for various applications. It is difficult to apply to vacuum systems due to the pressure drop on the trays, and the performance enhancement critically depends on the separation of the vapor and liquid periods.

Industrial equipment

Industrial interest in cyclic distillation has been renewed due to the availability of design and control methods and the introductions of special trays that allow better control of phase movement. Due to these developments, during the past decade, cyclic distillation has become accepted as a proven technology and emerged as a challenger of the status quo in distillation. Along with the scientific research carried out at several universities in the EU, cyclic distillation has sparked the interest of large companies,¹³ software vendors and technology providers.¹⁴

Although all simulation studies predicted enhanced separation performance, when applying conventional internals in practice, the fluid dynamic limitations restrained the industrial breakthrough of cyclic distillation technology. However, this barrier has been removed with the development of new internals.⁴ Also, the cyclic operation uses reliable stop valves (butterfly valves with pneumatic drive) from leading manufacturers, which can withstand up to 20 MM cycles (e.g., the first industrial columns operated well for more than 10 yr).

The present leader offering cyclic distillation technology has developed novel engineering solutions for more efficient contact devices (e.g. trays with lock chambers allowing the liquid to follow the path of tray to lock chamber to the tray below), as shown in FIG. 4, and a full column in FIG. 5.⁶ Practical implementation at pilot and large scale has showed the following benefits achieved in operation:

• High tray efficiencies (140%–200% Murphree efficiency) translates into reduced equipment cost
• Higher throughput and equipment productivity than conventional distillation
• Reduced energy requirements (20%–35% savings) translate into lower operating costs
• Increased product quality due to higher separation efficiency.

Industrial applications

So far, cyclic distillation has been used mostly in the food industry, but the application range is expanding to other areas with potential use, such as oil refining, chemicals, petrochemicals, pharmaceutics, biofuels, etc.

Ethanol separation and purification. Ethanol is an essential chemical that can be produced by fermentation of sugars by yeasts, or via petrochemical processes. It is used in the food industry, for medical applications, as a chemical solvent, in manufacturing processes and also as a renewable fuel (bioethanol). FIG. 6 shows the flowsheet of a typical ethanol production plant.¹⁵ An ethanol production plant consists of a beer column pre-concentrating ethanol (C1), a hydro-selection column (C2), a rectification column (C3), a column for end-cleaning, which produces high-purity ethanol (C4), a column for concentrating impurities (C5), a fusel column (C6) and a methanol column (C7). The light (head), intermediate and heavy (end) impurities are removed by distillation columns C2, C5 and C7.

The main goals in the purification of ethanol are to obtain a product with a minimum impurities content and to maximize product yield. In addition, energy savings can be achieved by operating the columns at different pressures; for example, the vapor from the beer column is used to heat the hydro-selection and the methanol columns. All these distillation columns can be replaced with cyclic distillation units. However, for the production of food-grade ethanol, column C3 remains due to insufficient knowledge about the actual distribution of impurities in this multi-feed, multi-product column in cyclic operation mode.

FIG. 7 illustrates the difference in the shape of the XY diagrams for the ethanol-water mixture for classic distillation and cyclic distillation. The operating lines for cyclic distillation allow a much lower number of theoretical stages due to the larger driving force of the process.

FIG. 8 (left) illustrates the separation efficiency in a cyclic distillation column (perfect displacement model) vs. classic distillation (perfect mixing mode) for an ethanol-water mixture.¹⁴ At minimum reflux ratio operation, fewer trays are required for cyclic distillation as compared to classic distillation. Similarly, a lower reflux ratio is needed for a cyclic distillation operation as compared to classic distillation, for the same number of trays used. FIG. 8 (right) shows the difference between the cyclic operation in a pilot column working in parallel to an existing industrial column, thereby performing the same separation task.¹⁴ Note the large steam usage (per liter of ethanol product), which translates into 30% energy savings in the case of cyclic distillation.

FIG. 9 illustrates the quantitative effect of applying the cyclic operation mode to ethanol-water mixtures. The optimal reflux ratio (corresponding to the minimum annualized cost) shifts to the left (lower values), thereby allowing 25% lower energy use as compared to classic distillation. The reflux ratio and the number of trays can be used to achieve various targets—e.g., at reflux ratio 4, the energy savings are 0% but 2.5 times fewer trays are required; at reflux ratio 3.5, the energy savings are 10% and 2.3 times fewer trays are required; and at reflux ratio 2.6, the steam savings are 30% and require 1.5 times fewer trays as compared to classic distillation.

Other industrial applications. Cyclic distillation also offers new opportunities by applying the same principles of the cyclic operation mode to other intensified processes, such as catalytic distillation (CD)¹⁶ or even dividing-wall column (DWC). FIG. 10 illustrates cyclic distillation DWCs with a coaxial partition for multi-component separations.⁶

An interesting phenomenon that also occurs in a DWC is the evaporation of superheated liquid when the steam supply to the column is shut off. This results in added process efficiency, especially when obtaining highly pure substances, as volatiles are the first to evaporate. During the vapor period, the liquid gathers much energy; however, when the vapor flow is stopped, the pressure drops and the superheated liquid boils throughout the DWC.

Modern cyclic distillation technology works in stripping columns, rectification columns and tray DWCs with a diameter range of 400 mm–1,700 mm. Cyclic distillation technology has been used in the production of ethanol for biofuels and food, in the recovery of methanol from water and/or acetone, in the distillation of various chemicals (ethers, propylene, propanol, hexane, formalin, aniline, cyclohexane, butyl acetate, ethylbenzene, methyl acetate, BTX), in isopropyl alcohol dehydration, in industrial solvents distillation (white spirit), in the cleaning of raw coal benzole, and in the fractionation of kerosene, with several implementations in Ukraine, Belarus and Saudi Arabia.

For example, since 2006, one company has built and installed commercial-scale plants with cyclic distillation columns of 5–42 trays and column diameters of 0.4 m–1.7 m. In 2014, the same company built an industrial-scale DWC using cyclic operation (42 trays, 1.5 m/1.7 m diameter, and a capacity of 25 m³/hr) for a plant processing kerosene and white spirit.

Promising industrial use. Some promising areas for using cyclic distillation include the production of high-purity substances, such as:

1. Organic synthesis (e.g., ethyl acetate, methyl acetate) in reactive distillation systems, which offers the ability to control the reaction on a tray, along with the added bonus of avoiding erosion of the solid catalyst due to fluid overflow.
2. Ecological applications (e.g., recovery of solvents and refrigerants and CO₂ absorption processes). A pilot industrial unit was built in Finland by Eco Scandic Oy. An additional pilot unit is planned to conduct research on the development of new technologies. Typically, refrigerants are collected for regeneration by their type and not as mixtures; however, in practice there are no ideal conditions. Mixtures of different types of refrigerants may be present in one tank; fractionation is required to separate and purify them for further reuse. Eco Scandic Oy invested in building a distillation unit for the fractionation of refrigerants (as illustrated in FIG. 11). This is a great prospect for countries with hot climates and heavy use of refrigerants, as it can significantly reduce the environmental burden. In addition, this unit can be used for research purposes, since the unit was designed for pressures up to 30 bar and deep vacuum.
   
   

   FIG. 11. Cyclic distillation column for refrigerants fractionation.
3. Separations of close boiling components (such as isomers of hydrocarbons or isotopes of water).

FIG. 12 illustrates the first use of cyclic distillation DWCs for fractionating kerosene.⁶

The main areas of use for cyclic distillation technology in petroleum refining include high-quality fractionation of gas, high-quality narrow fractions during vacuum/atmospheric distillation of petroleum, second refining of all types of petroleum cracking, combining reaction and distillation processes (catalytic distillation), obtaining isomers of high added value 1,2,3-trimethylbenzene, mesitylene, durene, isodurol and others. However, a potential limitation of the process is crude oil processing, as liquid load on the tray does not exceed 30 m³/m²hr.

Takeaway

Cyclic distillation can significantly improve fluid separations for processing hydrocarbons and their derivatives, providing key benefits such as lower energy requirements (by 20%–35%), reduced capital costs (by 20%–50%), increased column throughput and better separation performance (with up to three times higher efficiency than classic distillation trays). The separate phase movement also provides more degrees of freedom that contribute to good process control and simple operation.

Yet, similar to the history of other process intensification technologies (e.g., DWC), the chemical industry and hydrocarbon processing industry seem to be reluctant in adopting new technologies due to various perceived issues: difficult process control, unavailability of adequate models in process simulators or reliability of moving parts to sustain the cyclic operation. For this reason, it is absolutely crucial for industrial champions to lead by example and implement cyclic distillation while addressing the perceived and real issues associated with the technology. HP

ACKNOWLEDGMENTS

1. A. Kiss gratefully acknowledges the Royal Society Wolfson Research Merit Award.

LITERATURE CITED

1. Kiss, A. A., “Distillation technology—Still young and full of breakthrough opportunities,” Journal of Chemical Technology and Biotechnology, Vol. 89, 2014.
2. Baldea, M., T. F. Edgar, B. L. Stanley and A. A. Kiss, “Modularization in chemical processing,” Chemical Engineering Progress, Vol. 114, Iss. 3, 2018.
3. Kiss, A. A., Advanced Distillation Technologies—Design, Control and Applications, Wiley-Blackwell, Chichester, UK, 2013.
4. Maleta, V. N., A. A. Kiss, V. M. Taran and B. V. Maleta, 2011, “Understanding process intensification in cyclic distillation systems,” Chemical Engineering and Processing, Vol. 50.
5. Kiss, A. A. and C. S. Bildea, “Revive your columns with cyclic distillation,” Chemical Engineering Progress, Vol. 111. Iss. 12, 2015.
6. Kiss, A. A. and V. N. Maleta, “Cyclic distillation technology: A new challenger in fluid separations,” Chemical Engineering Transactions, Vol. 69, 2018.
7. Patrut, C., C. S. Bildea, I. Lita and A. A. Kiss, “Cyclic distillation—Design, control and applications,” Separation and Purification Technology, Vol. 125, 2014.
8. Bildea, C. S., C. Patrut, S. B. Jorgensen, J. Abildskov and A. A. Kiss, “Cyclic distillation technology—A mini-review,” Journal of Chemical Technology and Biotechnology, Vol. 91, 2016.
9. Andersen, B. A., R. F. Nielsen, I. A. Udugama, E. Papadakis, K. V. Gernaey, J. K. Huusom, S. S. Mansouri and J. Abildskov, “Integrated process design and control of cyclic distillation columns,” IFAC—PapersOnLine, Vol. 51, Iss. 18, 2018.
10. Rasmussen, J. B., S. S. Mansouri, X. Zhang, J. Abildskov and J. K. Huusom, “A mass and energy balance stage model for cyclic distillation,” AIChE Journal, 2020.
11. Nielsen, R. F., J. K., Huusom and J. Abildskov, “Driving force based design of cyclic distillation,” Industrial & Engineering Chemistry Research, Vol. 56, 2017.
12. Krivosheev, V. P. and A. V. Anufriev, “Mathematical modeling of the cyclic distillation of binary mixtures with a continuous supply of streams to the column,” Theoretical Foundations of Chemical Engineering, Vol. 52, 2018.
13. Buetehorn, S., J. Paschold, T. Andres, A. Shilkin and C. Knoesche, “Impact of the duration of the vapor flow period on the performance of a cyclic distillation,” Chemie Ingenieur Technik, Vol. 87, 2015.
14. Maleta, B. V., A. Shevchenko, O. Bedryk and A. A. Kiss, “Pilot-scale studies of process intensification by cyclic distillation,” AIChE Journal, Vol. 61, 2015.
15. Maleta, B. V., O. Bedryk, A. Shevchenko and A. A. Kiss, “Pilot-scale experimental studies on ethanol purification by cyclic stripping,” AIChE Journal, Vol. 65, 2019.
16. Patrut, C., C. S. Bildea and A. A. Kiss, “Catalytic cyclic distillation—A novel process intensification approach in reactive separations,” Chemical Engineering and Processing, Vol. 81, 2014.

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