WO1996017665A1

WO1996017665A1 - Separation process

Info

Publication number: WO1996017665A1
Application number: PCT/GB1995/002685
Authority: WO
Inventors: William David Parten
Original assignee: Imperial Chemical Industries Plc
Priority date: 1994-12-06
Filing date: 1995-11-16
Publication date: 1996-06-13
Also published as: GB9424557D0; AU3874995A

Abstract

A feedstream containing components which form a homogeneous binary azeotrope is subjected to distillation but, prior to the distillation, it is contacted with a third component with respect to which the azeotrope forming components of the feed have substantially different solubilites, the resulting mixture being allowed to separate in a decanting vessel (52) to form two liquid phases (58, 60) in which the concentrations of the azeotroping components are significantly different from the azeotropic composition. The resulting liquid phases are supplied to respective distillation columns (54, 56) to produce bottoms products essentially consisting of the components to be separated, each substantially free of the other component.

Description

SEPARATION PROCESS

This invention relates to a process for separating components from a liquid mixture. In the oil and chemical processing mdustπes. the process of distillation is widely used for effecting the separation of components with different vapour pressures from a liquid phase mixture containing the components. In some instances, the components in the mixture may be capable of forming a homogeneous binary azeotrope such that the boiling point of the mixture is less than the two pure components and the composition of the single liquid phase is the same as the vapour phase. Such mixtures are impossible to separate into the constituent pure components in a single simple distillation step. One method of breaking a mixture containing components which form homogeneous binary azeotropes is to employ a third component which, when added to the mixture, forms with the azeotroping components to be separated a heterogeneous ternary azeotrope (ie one which exhibits two liquid phases). The third component is selected with reference to the solubilities therein of the components of the binary azeotrope. For example, water and isoproply alcohol (IPA) form a binary homogeneous azeotrope and such a mixture has traditionally been broken by the addition of benzene as the third component. IPA having a significantly greater solubility in benzene than water. More recently, because of the toxological properties of benzene, cyclohexane has been used as the third component.

Thus, in a typical flowsheet for the dewateπng of IPA, a feed containing IPA and water is initially concentrated using a distillation column to remove water as bottoms product and obtain the binary azeotrope or a composition very close thereto as the overheads product. The resulting azeotropic composition is then contacted with the third component, for instance cyclohexane. in a second distillation column to produce an overheads stream with a composition close to that of the heterogeneous azeotrope. the overheads stream being condensed and allowed to split into the two liquid phases in a decanter while substantially water and eπtrainer tree IPA is recovered as the bottoms product. The separate liquid phases in the decanter compnse an organic phase and an aqueous phase, the organic phase being refluxed back to the second column and the aqueous phase being subjected to processing in a third distillation column in which water is obtained as bottoms product and a mixture of IPA and cyclohexane is obtained as overheads, the latter mixture being recycled for instance to the decanter.

Variations of this flowsheet are known but each is characteπsed by the need to supply to the second column a feed having a concentration close to that of the homogeneous azeotropic composition and by the use of the third azeotrope forming agent to draw the lower solubility component of the mixture (ie water in the case of a water/IPA mixture) overhead. In addition to the requirement for the feed to the second column to be close to the binary azeotrope in order to minimise the amount of low solubility component to be dragged overhead in the ternary azeotrope, the third component generally needs to have a relatively high boiling point so as to maximise the amount of lower solubility component in the ternary azeotrope. According to the present invention there is provided a process for the separation of a feed containing components which form a homogeneous binary azeotrope, comprising contacting the feed with a third component with respect to which the azeotrope forming components of the feed have substantially different solubilities, allowing the resulting mixture to separate in a decanting vessel to form two liquid phases in which the concentrations of the azeotropmg components are significantly different from the azeotropic composition and separately treating each liquid phase so formed.

The separate treatment of each liquid phase so formed may for example be effected by supplying each of the liquid phases obtained to respective distillation apparatuses from each of which a respective one of the azeotrope forming components is recovered.

The third component is preferably selected so as to magnify the difference in solubilities of the azeotrope forming components. In contrast with the previously discussed state of the art flowsheet, it is not necessary that the third component should be capable of forming a heterogeneous ternary azeotrope of a particular composition with the feed. Often the third component will have a lower boiling point than the azeotrope forming components to be separated, although this need not always be the case.

By appropriate selection of the third component, the liquid phases forming in the decanter may be such that, in one of the two liquid phases the more soluble azeotrope forming component is substantially in excess of its concentration in the binary azeotrope, whilst in the second liquid phase the less soluble azeotrope forming component is substantially in excess of its concentration in the binary azeotrope.

Typically the feed contains the azeotrope forming components in proportions which are substantially removed from the azeotropic composition. For instance, one component may, relative to the other component, be present in the feed in an amount which is at least 20% (eg at least 50%) greater than the amount corresponding to the azeotropic proportion. For instance, in the case of water/IPA where the azeotrope compnses in parts by weight 12.6 water to 87.4 IPA, the feedstream may have a water content which is at least 50% greater than 12.6, ie greater than 18.9. Often the excess will be such that one component, relative to the other component, is present in the feed in an amount which is at least 100% (eg at least 200%) greater than the amount corresponding to the azeotropic proportion. Thus, for example, in applying the process of the invention to water/IPA the feedstream may vary widely in its water/IPA content, eg from 10wt/% water-90wt % IPA through to 90wt% water-10wt% IPA.

The invention will now be descπbed by way of example only with reference to the accompanying drawings, in which: Figure 1 illustrates an existing approach for effecting separation of a feedstream containing components which form a homogeneous binary azeotrope; and Figure 2 is a flowsheet illustrating the process of the present invention. Referring to Figure 1 , an existing plant design for breaking homogeneous binary azeotropes compnses a first column 10 to which a feedstream containing the azeotrope forming components is supplied via line 12. The column 10 serves to concentrate the feedstream to form the binary homogeneous azeotrope or a composition very close thereto, the azeotropic proportions for a water/IPA system being about 12.6 : 87.4 parts by weight. In a typical example which will be referred to below to illustrate the process, the feedstream may comprise 50% water and 50% IPA by weight and to achieve the azeotropic composition water must be removed. Thus, in column 10, the feedstream undergoes distillation to produce a bottoms product comprising one of the components to be recovered from the feedstream, eg water, and an overheads stream in proportions very close to azeotropic proportions for the two components.

The overheads stream is supplied to a second column 14 via line 16 and in the upper part of column 14 is contacted with entrainer supplied via lines 18. It is important to supply the column 14 with a water/IPA composition close to the azeotropic proportions so that the amount of water that has to be dragged overhead in the distillation column 14 can be minimised. The entrainer is selected for its ability to form a heterogeneous ternary azeotrope with the azeotrope forming components of the oπginal feedstream and also to drag water into ovemeads of column 14. In this manner, a substantial proportion of the IPA orgmally present in the feedstream can be recovered as the bottoms product of column 14. A suitable entrainer for water/IPA mixtures is cyclohexane. For efficient operation, heavier rather than light entrainers are suitable, eg for the water/IPA separation cyclohexane rather than cyclopentane is to be preferred; if a lighter entrainer such as cyclopentane is used, larger amounts of entrainer are needed and/or greater energy expenditure in the form of heating is required to drag the same amount of water up the column for a given volume of entrainer. The ternary azeotrope containing ovemeads stream from column 14 is condensed in a condenser system (not shown) and supplied to a decanter 20 so as to separate into two liquid phases, a heavy aqueous phase and a lighter organic phase. The lighter phase is refluxed back to the column 14 via lines 18 while the heavier phase is passed via line 22 to a third distillation column 24 which is operated so that water is obtained at the base of the column and an organics containing overheads stream is obtained, the latter being returned to the decanter. From the foregoing, it will be seen that the existing approach requires: treatment of the feedstream to secure the azeotropic composition, or one close thereto, which implies the use of the first distillation column to concentrate the feedstream to the desired concentration; and a relatively heavy entrainer having a relatively high boiling point and which must form a heterogeneous ternary azeotrope with the binary azeotrope forming components of the feedstream.

Referπng now to Figure 2. in a process according to the present invention, the need to adjust the concentration of the incoming feedstream is removed, thereby eliminating a distillation column for effecting partial removal of one of the components (eg water in the case of a 50/50, by weight, mixture of water and IPA). Thus, instead of supplying the feedstream to a column for adjusting the component concentrations in accordance with the binary azeotrope composition, the feedstream is supplied via line 50 directly to a decanting vessel 52 along with a third component which is preferably selected using the following cπtena. 1. The components to be separated should have markedly different solubilities with respect to the third component.

2. It should be effective to split the feedstream into two separate liquid phases in one of which the the more soluble component is far in excess of the amount corresponding to the amount that would be present in the binary azeotrope and in the other of which the less soluble component is similarly far in excess.

3. It should be relatively easily separable from the respective azeotropmg components and sufficiently volatile to ensure that it remains in the column overheads and does not slip into the column bottoms product.

It is not essential for the third component to be one which is capable of forming a heterogeneous temary azeotrope with the binary azeotrope forming components of the oπginal feedstream. A suitable third component for the water/IPA separation is cyclopentane as it enables the separation to be earned out at atmospheπc pressure. Other possible compounds for use as the third component include diethyl ether, pentane and butane. Butane for instance would require operation at pressure. The third component is deπved as a recycle from distillation columns 54 and 56. Of the two liquid phases 58 and 60 forming in the decanter 52, the heavier phase 58 (the aqueous phase in the case of the water/IPA system) is fed via line 53 to column 54 which is operated to separate a substantial proportion of water from the heavy phase, the water exiting as the bottoms product (line 55) The organic ovemeads stream (line 57), which will contain IPA and the third component (eg cyclopentane), is condensed in a condenser system (not shown) and returned to the decanter 52. The lighter phase 60 forming in the decanter 52 is supplied, via line 59. to column 56 which is operated to separate the other binary azeotrope forming component (eg IPA) as the bottoms product (line 61) for recovery. The overheads organic stream (line 63) from column 56 which will contain pπmaπly the third component (eg cyclopentane) is condensed and recycled to the decanter 52. The overheads streams from the columns 54 and 56 may be condensed in a common condenser (not shown). Conveniently at least one, and preferably, both are mixed with the feedstream pπor to entry into the decanter, such mixing being effected by any suitable mixing device such as a static mixer.

In an implementation of a process as desenbed with reference to Figure 2 earned out using the software programme Aspen version 8 5 (using default UNIFAC parameters), contact of a feed containing water and isopropyl alcohol with cyclopentane (CP) in the decanting vessel, involved the following feed, ovemeads and bottoms product compositions (all in parts by weight): COLUMN 54

Aqueous phase feed 415 IPA 2011 water 15 CP

Overheads 403 IPA 124 water 15 CP

Bottoms 12 IPA 1887 water O CP

COLUMN 56

Organic phase feed 2515 IPA 157 water 8306 CP

Overheads 649 IPA 153 water 8306 CP

Bottoms 1866 IPA 4 water O CP

In the above example, it will be seen that the bottoms products contain very little of the unwanted component. These amounts can be readily reduced further by for example increasing the number of plates in the respective columns.

A number of advantages accrue from a process as described with reference to Figure 2:

1. Where the feedstream contains an excess of one of the components to be separated (ie excess in terms of the amount present in the azeotrope) which will invariably be the case in practice, concentration of the feedstream to achieve a composition close to the azeotropic composition is unnecessary thereby reducing the number of distillation columns needed with consequent reduction in capital costs. Thus, for example, where the feedstream is constituted by an effluent from a process, it may be treated in accordance with the present invention without any upstream treatment involving adjustment of the relative proportions of the azeotroping components.

2. The distillations carried out do not need to be operated close to the binary homogeneous azeotrope composition and are therefore easier to achieve with less energy consumption which again minimises energy consumption.

3. Because the third component is not required to form a ternary azeotrope, it is feasible to use a lower boiling point third component.

Although the invention is described above by way of example in the context of a water/IPA system, the invention may be applied to a variety of other homogeneous binary azeotrope forming components such as water and normal propyl alcohol, and water and methyl acetate.

Claims

1. A process for the separation of a feed containing components which form a homogeneous binary azeotrope, comprising contacting the feed with a third component with respect to which the azeotrope forming components of the feed have substantially different solubilities, allowing the resulting mixture to separate in a decanting vessel to form two liquid phases in which the concentrations of the azeotroping components are significantly different from the azeotropic composition and separately treating each liquid phase so formed.

2. A process as claimed in Claim 1 in which at least one of the two liquid phases formed in the decanting vessel is subjected to distillation in order to obtain one of said components in substantially pure form.

3. A process as claimed in Claim 1 in which each of the two liquid phases formed in the decanting vessel is subjected to separate distillation stages to recover each component in a form in which is it is substantially free of the other component and also said third component.

4. A process as claimed in Claim 3 in which each component is recovered as the bottoms product from a respective distillation column, the ovemeads stream from at least one of the distillation columns being recvcled to the decanting vessel.

5. A process as claimed in any one of Claims 1 to 4 in which the feed is contacted with the third component pπor to introduction of the mixture into the decanting vessel.

6. A process as claimed in any one of Claims 1 to 4 in which the feed and the third component are introduced separately into the decanting vessel.

7. A process as claimed in any one of the preceding claims in which the feedstream compnses an effluent from an upstream process and is contacted with the third component without any intervening treatment of the effluent stream to effect adjustment of the relative concentrations of the components to be separated.

8. A process as claimed in any one of Claims 1 to 7 in which, relative to the other component, one of said components of the feed is present in an amount which is at least 20% greater than the proportion in the azeotropic composition.

9. A process as claimed in any one of Claims 1 to 7 in which, relative to the other component, one of said components of the feed is present in an amount which is at least 50% greater than the proportion in the azeotropic composition.

10. A process as claimed in any one of the preceding claims in which the feed compnses isopropyl alcohol and water.

11. A process as claimed in Claim 10 in which the water component relative to the isopropyl alcohol is in excess of the azeotropic composition.

12. A process as claimed in any one of the preceding claims in which the separation is carried out at atmospheπc pressure.

13. A process as claimed in any one of the preceding claims in which the third component is a C5 or lower hydrocarbon.

14. Apparatus for effecting separation of a feed containing components which form a homogeneous binary azeotrope, compnsing first and second distillation stages and a decanting vessel, characteπsed by means for supplying the feed to the decanting vessel together with a third component which is effective to split the feed into two separate liquid phases each having a composition which differs significantly from that of the binary azeotrope and means for supplying the liquid phases each to a respective distillation stage in which each of said components of the feed is recovered in a form substantially free of the other feed component and said third component.

15. Apparatus as claimed in Claim 14 in which the said components of the feed are each recovered as a bottoms product from the respective distillation stages.