Reactive distillation

Reactive distillation is a process where the chemical reactor is also the still. Separation of the product from the reaction mixture does not need a separate distillation step, which saves energy (for heating) and materials.

This technique is especially useful for equilibrium-limited reactions such as esterification and ester hydrolysis reactions. Conversion can be increased far beyond what is expected by the equilibrium due to the continuous removal of reaction products from the reactive zone. This helps reduce capital and investment costs and may be important for sustainable development due to a lower consumption of resources.

Being a relatively new field, research on various aspects such as modeling and simulation, process synthesis, column hardware design, non-linear dynamics and control is in progress. The suitability of RD for a particular reaction depends on various factors such as volatilities of reactants and products along with the feasible reaction and distillation temperature. Hence, the use of RD for every reaction may not be feasible. Exploring the candidate reactions for RD, itself is an area that needs considerable attention to expand the domain of RD processes. Although invented in 1921, the industrial application of reactive distillation did not take place before the 1980s.

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Benefits

• Increased speed and improved efficiency
• Lower costs – reduced equipment use, energy use and handling
• Less waste and fewer byproducts
• Improved product quality– reducing opportunity for degradation because of less heat

Difficulties

The conditions in the reactive column are suboptimal both as a chemical reactor and as a distillation column, since the reactive column combines these.

The introduction of an in situ separation process in the reaction zone or vice versa leads to complex interactions between vapor-liquid equilibrium, mass transfer rates, diffusion and chemical kinetics, which poses a great challenge for design and synthesis of these systems.

Side reactors, where a separate column feeds a reactor and vice versa, are better for some reactions, if the optimal conditions of distillation and reaction differ too much.

Setup

Typically, reactive distillation equipment comprises a reactive column, with the feed input, with directly attached stripping and enriching columns with the outputs.

Applicable Processes

Reactive distillation can be used with a wide variety of chemistries, including the following:

• Acetylation
• Aldol condensation
• Alkylation
• Amination
• Dehydration
• Esterification
• Etherification
• Hydrolysis
• Isomerization
• Oligomerization
• Transesterification

Example

The esterification of acetic acid with alcohols like n-butanol, ethanol, isobutanol and amyl alcohol fall in a typical class of reacting systems.

n-Butyl acetate is an industrially important chemical with wide applications as a versatile solvent. It is manufactured by the esterification of acetic acid with n-butanol in the presence of suitable acid catalyst. The alcohol is sparingly soluble in water and the ester is almost insoluble.

Another interesting feature of this system is that it is associated with the formation a minimum boiling ternary azeotrope of ester, alcohol and water, which is heterogeneous in nature. Hence, in a typical reactive distillation column that consists of both reactive and non-reactive zones, the heterogeneous azeotrope or a composition close to the azeotrope can be obtained as the distillate product. Moreover, the aqueous phase that forms after the condensation of the vapor is almost pure water.

Depending on the requirement either of the phases can be withdrawn as a product and the other phase can be recycled back as reflux. The pure ester i.e. butyl acetate is the least volatile component in the system is realized as a bottom product.