## Breaking News

Author: Esha Chatterjee

Abstract

Process Intensification (PI) in distillation columns, in the case of the cyclic mode of operation, has significant advantages, in terms of increased mass transfer efficiency and reduced costs of operations, over the classical process along with a few limitations and challenges. This article aims to scrutinize the characteristics of cyclic distillation and examine its range of application in modern mass transfer technologies.

The mathematical models which employ theoretical stages while reflecting upon the hydrodynamic flow regime in a cyclic distillation column facilitate the discernment of process intensification in the distillation operation and make accessible the suitable equipment to design the distillation columns for controlled periodic operation. [¹]

During liquid overflow, the mixing of the liquid from one tray to another enhances the separation efficiency. The process operating line which is continuous, concave and discrete in nature, is a reflection of the equilibrium line. A perfect mixing, the operating line at its limit can verge on the operating line L/G.  The local point efficiency (EOG) which represents the mass transfer kinetics impacts the position of the kinetic curve and the operating line as well as the shape of the operating line. At EOG→0, the operating line L/G yields the ultimate position of both lines. The physical differences between the efficiencies of the cyclic and the conventional processes can be observed at the values F→1 and EOG →1. The Murphree efficiency of a tray increases continuously with an increase in the local point efficiency and is subject to the changes in the stripping factor and hydrodynamic conditions of the liquid flow regime. [²]

Fig. 1: Depiction of how model parameters influence the position and shape of the operating line: (1) equilibrium line, (2) operating line (L/G), and (3) perfect displacement operating line (F = 1; EOG = 1).

The mathematical model discussed in the article [introduce the link for the previously submitted article “Mathematical models in cyclic distillation”] has several advantages over the conventional steady-state mode of operation. To name a few, the separation efficiency is independent of the diameter of the column. Since the amount of liquid holdup in a tray remains constant while only cyclic frequency is changed, the load of the column does not increase the resistance of the hydraulic operations. The consequence of secondary self-evaporation in the liquid can be noted. The separate movement of phases (SPM) in the mass transfer technology encompasses more degrees of freedom which interject a better process control. [³]

The major economic advantages of cyclic distillation, observed both theoretically and experimentally are: [⁴]

• Decrease in investment cost as much as 20-50%, due to cost conservation in column parameters such as reduced column height and column diameter, less space requirement, less steel construction and smaller area of heat exchangers.
• Reduced hot utility usage of up to 20-35% consistent with an increased mass transfer efficiency and a reduced reflux rate.
• Reduced cold utility usage of up to 20-35% consistent with a reduced reflux rate.
• Enhanced product quality subject to increased mass transfer efficiency and highly concentrated key product.
• Augmented product yield corresponds to increased mass transfer efficiency and highly concentrated impurities.
• Improved process sustainability is analogous to reduced GHG emissions and energy usage.

The major technological benefits of the cyclic mode of operation include: [5]

• Thrice the mass transfer efficiency when compared to that in a classic arrangement.
• The reduced residence time of liquid overflow in column and a regular arrangement of liquid in trays.
• In the case of reactive distillation, enhanced control opportunities with ease of regulation of the amount of liquid in the tray and reaction time.
• Dynamic geometric configuration facilitates the building of dividing wall columns with a tray.
• Less complicated industrial scale up as a result of separation efficiency of liquid being independent of column diameter.
• Flexible packing is available between the trays which further enhances the mass transfer efficiency.
• The amount of liquid holdup in a tray remains constant while only cyclic frequency is changed (range: 1-30 m³/m² hr liq. load) and therefore there is no effect of pressure drop on liquid holdup.
• The vapour velocity depends on the pressure in the column and has a range of 0.2-20 m/s.
• Even if the concentrations of the key components are changed, the operation maintains its stability and efficiency. [⁶]

Limitations of Cyclic Distillation:

The cyclic mode of operation has certain limitations most of which account for problematic applications to vacuum systems and separation of the vapour liquid periods which critically affect the performance enhancement of the process. However, the introduction of sluice chambers has eased the use of simple trays for vacuum applications. [⁷]

Fig. 2 Design for Cyclic Distillation: Trays with valves and sluice chambers below them.

A noteworthy breakthrough was achieved in recent times in the design of trays precisely for the operation of cyclic distillation. These trays engage the use of valves and sluice chambers positioned under the trays. When the valves are closed during the vapour flow period, the liquid stays on the tray after which the valves are opened in the following liquid flow period and the liquid in the trays flows down to the sluice chamber in absence of vapour. When the next cycle begins the sluice chambers open and the liquid from trays flow to the ones below them. This arrangement eradicates the complexity of using a large number of trays. [⁸]

The earliest design and simulation models of cyclic distillation came up long before the computing powers were efficient and inexpensive which in turn affected the accuracy of the outcomes of the model. These models only took into account binary mixtures and included major assumptions to simplify the operation such as a linear vapour-liquid equilibrium, insignificant condenser holdup and infinite reboiler. In the later studies, these assumptions were replaced with a more realistic approach. Moreover, recent developments were made in the model to incorporate noniterative design algorithms which could be applied to non-binary mixtures with non-linear valour-liquid equilibrium. [⁹]

These advancements equipped more reliable design and control methods, highly efficient computing power and introduced specialized hardware to allow separate phase movement and therefore invoked attention to cyclic distillation. [10]

Industrial Applications:

Up until now, cyclic distillation has been majorly used in the food industry especially for concentrating alcohol. However, the application range is broadening to other industries, such as petrochemicals, biofuels, oil refining, chemistry, pharmaceuticals, etc. Moreover, cyclic distillation offers new prospects by applying fundamentals similar to those of the cyclic operation mode to other intensified processes, such as dividing-wall column (DWC) or catalytic distillation (CD). Nowadays, cyclic distillation technology is employed in stripping columns, tray dividing wall columns and rectification columns, with diameters ranging from as low as 400 to 1700 mm. [¹¹]

Cyclic distillation technology is also utilised in the recovery of methanol from water and/or acetone, in the production of ethanol food-grade, biofuels, in the distillation of various chemicals (formalin, aniline, cyclohexane, butyl acetate, methyl acetate, BTX, ethers, propylene, propanol, hexane, ethylbenzene), in the dehydration of isopropyl alcohol, distillation industrial solvents (white spirit), raw coal benzole cleaning, fractional distillation of kerosene (with implementations in Republic of Belarus, Ukraine and Saudi Arabia). [¹²]

Maleta CD is an Estonia-based company dedicated to providing cyclic distillation products and services such as distillation columns and trays. Absorption columns, Azeotropic distillation columns, Dividing-Wall Column (DWC), Extraction distillation columns, Fractional distillation columns, Reactive distillation columns, Rectification distillation columns, Stripping distillation columns, Vacuum distillation columns are the different types of distillation columns manufactured by Maleta CD. These columns have diameters of up to 4m and can work in pressures of up to 40 atm and also in a vacuum. [¹³]

Maleta CD has been closely involved in the industrial implementation of cyclic distillation technology by setting up plants producing ethanol in the food industry, as well as installing the earliest application of cyclic distillation dividing-wall columns for fractionating kerosene. [¹⁴]

In petroleum refining, the cyclic distillation technology is primarily used in high-quality narrow fractions during vacuum / atmospheric distillation of petroleum; high-quality fractionation of gas; combining reaction and distillation processes (catalytic distillation); second refining of all types of petroleum cracking; obtaining isomers of high added value: mesitylene (1,3,5-trimethyl benzene); 1,2,3-trimethyl benzene; iso-durol (1,2,3,5-tetramethylbenzol); durene (1,2,4,5- tetra-methylbenzene) and others. However, there are some conceivable limitations of the process such as in crude oil processing the liquid overflow on the tray is limited to 30 m³/m²hr. [¹⁵]

EndNote Citations and References:

[1] [2] [3] Maleta, V., Kiss, A., Taran, V., & Maleta, B. (2011). Understanding process intensification in cyclic distillation systems. Chemical Engineering And Processing: Process Intensification, 50(7), 655-664. doi: 10.1016/j.cep.2011.04.002

[4] [5] [11] [12] [14] [15] BÃ®ldea, C., PÄƒtruÅ£, C., JÃ¸rgensen, S., Abildskov, J., & Kiss, A. (2016). Cyclic distillation technology - a mini-review. Journal Of Chemical Technology & Biotechnology, 91(5), 1215-1223. doi: 10.1002/jctb.4887

[6] [7] [8] [9] [10] Revive Your Columns with Cyclic Distillation. (2021). Retrieved 5 July 2021, from weblink

[13] Distillation column. Retrieved 20 July 2021, from  weblink