Production of Butyl Acrylate

Case:

Reaction: Direct esterification
Raw materials: Acrylic acid , butanol, sulfuric acid (catalyst)
Product: Butyl acrylate , water
CH₃(CH₂) ₃OH + CH₂CHCOOH <===> CH₂CHCOOC₄H₉ + H₂O
Butyl acrylate is prepared by the equilibrium-limited esterification reaction of acrylic acid-and n-butanol. The reaction is conducted at elevated temperature in the presence of a catalyst. Typically, the coproduct, water, is removed during the course of the reaction to drive the reaction more toward the butyl acrylate product. In conducting this reaction, impurities in the raw materials and side reactions generate by-products that must be removed from the butyl acrylate.

This system includes two homogeneous azeotropes (AA–BuOH and BA–AA) and two two-component heterogeneous azeotropes (BA–H2O and BuOH–H2O) and one three-component heterogeneous azeotrope (BA–BuOH–H2O).

Comments:

Since the mixture exhibit azeotropic behavior, a conventional distillation column cannot be used. Basically, there are, at least, two options of using distillation column in dealing with this problem: 1) Pressure swing distillation (PSD), or 2) Entrainer.

In the use of PSD, two columns are operated with different pressure. First column is operated to reach the maximum product purity for the region below the azeotropic point, while pressure is changed for the second column in order to further increase the purity. To do this, you need to identify the appropriate pressure of each column according to the vapor-liquid equilibrium of the binary mixture.

On the other hand, the use of entrainer will involve third chemicals as an “extracting agent”. The third chemicals (called as entrainer) is able to break the azeotrope so that there is azeotropic behavior anymore, hence conventional distillation can be used. However, since there is another component in the mixture, column configuration need to be considered.

However, it will be better to avoid the azeotropic behavior. Start thinking of the reaction part.

In your case,

The fractionating column should be one that can be cleaned readily if a polymer is formed in it. A large number of plates is not required, though the column should be capable of separating the methanol-methyl acrylate azeotrope (b.p. 62–63°) from methyl acrylate(b.p. 80°), and butanol (b.p. 117°) from butyl acrylate (b.p. 145°). The necessity of effecting the latter separation can be practically eliminated by allowing the reaction to go virtually to completion, all the butanol thus being consumed. This can be done by extending the reaction period as long as reaction occurs and by adding a considerable excess of methyl acrylate. Instead of the twofold excess specified, three or four times the theoretical amount may be used with benefit. The larger amount is especially desirable when the acrylate of a relatively unreactive alcohol is being prepared.

Ref: Organic Syntheses, Coll. Vol. 3, p.146 (1955); Vol. 26, p.18 (1946).

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Multi-vessel batch distillation

Introduction

It is obvious that separating azeotropic mixture or close boiling components into pure component is not an easy way. The use of pressure swing operation or introducing entrainer (as additional solvent) has been intensively investigated.

For dealing with fine or specialty chemicals (that are commonly found in pharmaceutical industries), which is usually operated in batch mode, the use of batch distillation system is much more advantageous. Its flexibility offers an attractively cheaper investment, by which for separating mixture with a number of components to be pure component, only single column is required. The concern is that the operating cost will highly depend upon the operation time. Time means energy, so that the longer the process runs, the more expensive the operating cost.

In the case where a mixture contains two close boiling components, the difficulty of separation will be much higher. Introducing extractive distillation by adding an entrainer (solvent) is common, but it will need further separation processes as this mode produces a two-phase liquid.

This research will investigate the use of multi-vessel batch distillation, in which there is no additional chemicals introduced. This is carried out for ‘virginity’ reason, since we process essential oils for food and pharmaceuticals.

Basically, this type of batch distillation consists of several (at least two) columns with only one reboiler (still pot). Between two columns, a (middle) vessel is placed. In addition, a vessel is also installed as a condenser holdup but larger size. The equipment is operated in total reflux policy, so that there is no product drawn during the process.

Within a certain time of operation, the products can be collected from top vessel (condenser holdup), middle vessel, and still pot. The questions that we would like to investigate are:

1. Is this equipment able to tackle close boiling mixture?
2. If so, how long should the process be run to obtain the desired products?

Case study

Clove oil will be used as the mixture to be separated into its pure component. The major components of clove oil are eugenol (70-80%), caryophyllene (20-24%), alpha-copaene (2-4%), and many trace components. The objective is to obtain high purity eugenol from clove oil by utilizing multi-vessel batch distillation. In this case, eugenol and caryophyllene as the most major components have close boiling points at around 254-256 deg C, hence it is difficult to separate them.

The proposed method is expected to handle this problem so that the separation of those two components becomes technically easier, with respect to no additional solvent.

Lecture notes

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Chemical Process Development

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