- Reactive distillation
Reactive distillation is a process where the chemical
reactoris also the still. Separation of the product from the reaction mixture does not need a separate distillationstep, which saves energy (for heating) and materials.
This technique is especially useful for equilibrium-limited reactions such as
esterificationand ester hydrolysisreactions. 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, columnhardware 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.
*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
The conditions in the reactive column are suboptimal both as a
chemical reactorand 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.
Typically, reactive distillation equipment comprises a reactive column, with the feed input, with directly attached stripping and enriching columns with the outputs.
Reactive distillation can be used with a wide variety of chemistries, including the following:
The esterification of
acetic acidwith alcohols like n-butanol, ethanol, isobutanoland amyl alcoholfall in a typical class of reacting systems. n-Butyl acetateis 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
azeotropeof 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, being the least volatile component in the system, is realized as a bottom product.
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