The separation of acetic aid acid and water by simple rectification very difficult, requiring a column with many stages and a high reflux ratio, thus incurring high running costs.

In practice other processes are used depending on the concentration of acetic acid present in the feed. Between 50 and 70% w/w acetic acid, extractive distillation is used. By adding a third component, the volatility of water is increased and the separation can be achieved with less energy. Below 40% acetic acid, liquid-liquid extraction is most appropriate. Acetic acid is extracted from water by a suitable solvent in order to obtain substantially pure acetic acid. Liquid-liquid extraction is also useful, independent of concentration, when other contaminants such as salts interfere with direct distillation. QVF is able to offer systems using either technique.

This case study describes the recovery of acetic acid by liquid-liquid extraction.

Choice of Solvent in Liquid-Liquid Extraction

In order to minimize energy costs in the distillation stage, a lower boiling point solvent is usually chosen, In practice, ethyl acetate or methyl isobutyl ketone are usually preferred.

Figure 1 shows the equilibrium diagram for the ternary system ethyl acetate/acetic acid/water. It can be seen that the distribution coefficient is approximately 1. This is true for most solvents for which the extraction mechanism is based on physical solubility. Moreover, one can see that when ethyl acetate is used a reciprocal solubility with water exists. Furthermore, the two phase region is quite small, limiting the maximum feed concentration for the liquid-liquid extraction system to less than 30%. However, the two phase region for the ternary system MIBK/acetic acid/water is larger and therefore can be used for higher feed concentrations. Furthermore, the reciprocal solubility is less. Mixtures containing up to 50% acetic acid can thus be treated by liquid-liquid extraction using MIBK as a solvent.

The Extraction Process

Figure 2 shows a flowsheet of an extraction plant for the recovery of acetic acid. It comprises, essentially, the extraction column, the solvent recovery column and the aqueous phase stripping column. Since the feed mixture has a higher density than the solvent it is introduced at the top of the extraction column. It flows to the bottom of the column transferring acetic acid to the solvent. The usual concentration in the bottom is 0.1-0.5% but it is possible to improve this if required. At the same time, the aqueous phase is saturated with solvent which is removed in the stripping column suing live steam.

In the solvent rectification column, solvent and water leave the column at the top. After condensation the two liquid phases are separated. A part of the solvent phase is refluxed to the column. The remainder is returned to the extraction column.

The aqueous phase is stripped of any solvent in the stripping column. Under certain conditions, it is also important to reflux a part of the aqueous phase to the column. The bottoms product from the solvent recovery column contains acetic acid, where a concentration of between 95% and 100% can be obtained. If there is a possibility that in the extraction process higher boiling components will go into the organic phase, it is recommended that the acetic acid is removed as a vapor stream.

Solvent Rectification

The economics of the whole process are strongly dependent on the costs for the separation of the acetic acid from the solvent by distillation. The scale up and the control of this column should e done very carefully. The actual process design depends on the solvent used.

In the system ethyl acetate/acetic acid/water, a binary azeotrope exists between water and ethyl acetate (Boiling point 70.4 Deg. C). The separation by distillation of water and acetic acid in the ethyl acetate can be done without too many problems. As far as running costs are concerned. one must take card that the ration of ethyl acetate/water used in the column corresponds to the maximum volatility for acetic acid. Consequently the feeds to the extraction column must be held within closely controlled limits, necessitating a carefully designed control system.

The separation of acetic acid from the system MIBK/acetic acid/water can be more of a problem because the boiling point of MIBK is very close to that of acetic acid. This separation is thus achieved by azeotropic distillation in which water is used to bread the azeotrope, see Figure 3.

The binary azeotrope MIBK/water contains about 25% w/w of water. Recovery of the highest concentration acetic acid is only possible if the feed to the rectification stage lies on the line between the azeotrope point AZ and pure acetic acid E. On the other hand, since the point should also lie on the ternary equilibrium curve, only the point SA corresponds to these conditions.

The concentration the extraction column (Point M) should correspond to mixture of the feed and, at the bottom of the column, solvent saturated with water corresponding to the pint SE. The ration of the flow rate of solvent and mixture is being given by the lever rule.

When this ratio is chose, however, the extraction equilibrium must be considered. Generally in the system it is necessary to operate with a high proportion of solvent to mixture. The point M shift to M’ ad the extraction phase reaches the state SA’. Hence, because the feed to be rectified must bon on the AZ-E line, it is necessary to add water to Reach point SA'.

Choice of a Suitable Extraction Column

The problem can be solved in general with a pulsed column for which the operation conditions must be determined by pilot plant trials. QVF Process Systems has a suitable pilot unit and is able to carry out pilot plant trials on the customer’s behalf.

If corrosive substances are present in the mixture a column fabricated from corrosion resistant materials must be chosen. Borosilicate glass is and ideas material for extraction columns - allowing the drops and interfaces to be readily observed.

QVF also has the capability to design such columns in other corrosion resistant materials - stainless steels, titanium, hastelloys, lined steel etc. and to scale up data obtained from pilot plant trials.

This Process Profile supersedes all previous issues.

QVF Process Systems pursues a policy of continuous product improvement. We therefore reserve the right to alter any product or process as described and illustrated.