Case Studies on the Applications of Cyclic Distillation #ipumusings #appliedchemistry #biochemistry #cyclicdistillation
Case Studies on the Applications of Cyclic Distillation
Abstract: More often than not, the introduction of Process Intensification methods complicates the control of the process. In the case of cyclic distillation, however, the modifications are simpler and more convenient to implement concerning the conventional process and these adaptations are significant to efficient and effective process control of distillation from an industrial point of view. This article aims to discuss the industrial scenarios in which cyclic distillation has directly enhanced the otherwise traditional, and therefore less effective, conventional distillation process.
The two industrial case studies encompassed in the following passages establish the concept of cyclic distillation as a proven technology. Process intensification methods typically demand more challenging and complicated control techniques. Conversely, when considered from a practical standpoint of the control of a process, cyclic distillation is simpler and more robust when compared to conventional distillation. Model-based control systems can be employed to solve the problems of additional optimal control (dynamic optimization). One such technique is Model Predictive Control, which is beyond the scope of this study.
Modelling of an Ethanol–Water Stripping Column
The Lipnitsky Alcohol Plant situated in Ukraine implements a cyclic distillation column, the modelling of which is the subject of this case study. The cyclic distillation column serves primarily as a stripper to intensify the concentration of alcohol to a higher extent. In the process of beer production, an ethanol-water mixture is procured by fermentation. The beer stripping column in this case is modelled for the actual plant production of 30,000 litres per day of ethanol food grade. There are approximately 30 additional impurities as components that contribute to less than 0.2 mol% in the feed stream.
The alcohol concentration is 3.29 mol%—equivalent to 10 vol% in the feed stream. In the top tray of the stripping column, the feed stream is introduced while steam is injected directly at the bottom of the column. The ethanol concentration typically has a value of about 18.25 mol% in the top distillate and it varies between 13 and 24 mol% while the ethanol concentration in the bottom product is never more than 0.004mol%. For this system, simple modelling of the cyclic distillation column was carried out, based on the previously discussed theoretical model in [see article].
In table 1 the effect of the diffusion potential factor (λ= mG/L), local point efficiency (EOG) and the liquid transfer delay factor (F) on the Murphree efficiency (EMV) and the number of trays (N) is illustrated in a cyclic distillation column for ethanol concentration. Consequently, when the perfect displacement mode of operation is attained a higher efficiency with a lower number of trays is possible.
Column for the concentration of impurities
The Lipnitsky Alcohol Plant situated in Ukraine also employs the column for concentrating impurities and therefore this column is essential in industrial plants producing food-grade alcohol. The wastes generated are 5% of the plant capacity and this waste is recycled back into the plant by using the column concentrating the impurities. Consequently, the total waste generation is reduced to 0.5% of the capacity of the plant and an additional output of alcohol is produced which is 4.5% of the total plant capacity.
The column for concentrating impurities is similar to the hydro-selection column as they perform the same task of removing the volatile components and also share identical impurities and the same working conditions. The major difference between the cyclic distillation column and the hydro-selection column is that the latter has a higher concentration of impurities which increases the yield of the desired product.
The efficiency of both columns can be compared using the Fenske equation under the same conditions. The reflux ratio is close to 50 mol/mol for both columns.
Table 2 below represents the concentration of impurities (ppm) for the column concentrating the impurities and also for the hydro-selection column in the liquid stream.
The industrial data report of the Lipnitsky Alcohol Plant in Ukraine suggests it can be concluded that the separation efficiency of the cyclic distillation column of 15 trays is twice or thrice as much as that in the conventional bubble cap trays column of 56 trays which translates to an increase of efficiency to about 200–300%.
Cyclic distillation has also been carried out at an experimental scale for the separation of the mixtures of benzene and toluene, methanol and water, acetone and water, and methyl- cyclohexane and n-heptane in columns equipped with conventional internals:
• Benzene and toluene, using brass and mesh-screen plates
• methyl-cyclohexane and n-heptane, using packed-plates
• acetone and water, using sieve trays
• methanol and water, using conventional stages.
From the two case studies, it can be inferred that a cyclic distillation column can be utilised in the food industry, to increase alcohol concentration from about 8 wt% to 27–45 wt%. In the beer manufacturing process, the ethanol-water mixture contains about 30 impurities when leaving the fermenter which amounts to less than 0.2 mol% of the stream.
At the industrial level, cyclic distillation is already being used for kerosene fractionation in the petrochemicals industry.
However certain challenges need to be addressed for cyclic distillation to reach its potential:
• improve the features in the latest industrial applications that involve complex separations, e.g., close-boiling mixtures, high-purity standards, considering larger column capacities
• develop process control procedures to address high purity requirements or feed inconsistencies
• further develop and implement precise and meticulous design and simulation techniques (for use in process simulators) which incorporate hydrodynamic models, for instance, by utilising modern computing power and contemporary advancements in computational fluid dynamics (CFD)
• develop enhanced tray designs that permit absolute separation of phase movements, and expand the worldwide availability of that apparatus by increasing its manufacture.
Conclusion:
These case studies highlight how the cyclic operation mode is better than conventional distillation in terms of higher capacity and tray efficiency which allows higher feed rates to be processed and assists the necessary separation to be attained by the means of lower energy consumption without compromising the number of trays or vice versa.
References:
1. Revive Your Columns with Cyclic Distillation. (2021). Retrieved 5 July 2021, from pdf.
2. 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
About the Author:
Esha Chatterjee, a graduate student of University School of Chemical Technology, GGSIP University, Delhi. She is pursuing her degree in chemical engineering. She wants to build her career in Industrial Chemistry and biotechnology.
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