WO1993010082A1

WO1993010082A1 - Dimerisation of acrylonitrile

Info

Publication number: WO1993010082A1
Application number: PCT/GB1992/002076
Authority: WO
Inventors: Gordon Shaw; Jose Merono Lopez
Original assignee: Imperial Chemical Industries Plc
Priority date: 1991-11-15
Filing date: 1992-11-10
Publication date: 1993-05-27
Also published as: GB9124303D0

Abstract

After dimerisation of acrylonitrile to a linear C6 dinitrile in a homogenous liquid phase comprising a phosphonite or phosphinite catalyst, an inert proton donating solvent and a non-hydroxylic solvent, the reactor effluent is mixed with a sufficient volume of a further solvent, preferably pentane to quench the reaction, is then fed to means for separating the mixture into a first phase consisting of the major part of the solvents, catalyst and acrylonitrile and a second phase consisting of the major part of the dinitrile and polymeric by-products.

Description

DIMERISATION OF ACRYLONITRILE

This invention relates to the dimerisation of acrvlonitrile (ACN) to a linear C . dinitrile. o

It is known for the dimerisation to be effected (preferably in a continuous flow reactor) in a liquid phase consisting of a phosphonite or phosphinite catalyst (CAT) and an inert proton donating solvent (PDS) as disclosed in UK Patent Specification No 1546807. The main product of the dimerisation is 1,4 dicyanobutene (DCB) although a minor proportion of methylene glutaronitrile (MGN) is also formed.

In addition to the DCB/MGN, unwanted polymeric by-product (PBP) consisting of various oligomers, in particular the hexamer, is also formed and the reference suggests the addition of a non-hydroxylic solvent (NHS) to the reaction mixture to reduce such formation.

The reaction conditions (ie temperature and the ratio of the components) may be selected to maximise DCB/MGN formation and minimise PBP formation. However, the reaction continues during removal of the mixture from the reactor for further processing (ie recovery of the product) and under these circumstances where conditions are no longer optimum, there is a tendency for increased PBP formation.

Hence the further processing must prevent the unwanted reaction whilst effecting efficient separation of the components that can be re-cycled (ACN, CAT, PDS and NHS) from DCB/MGN and PBP.

Recovery of CAT is of particular importance as the cost of CAT is extremely relevant to the overall cost of the process. European Patent Application No 314383 suggests that these objects can be achieved by including in the reaction mixture a further solvent having a higher boiling point than ACN, PDS and NHS. The lower boiling components can be distilled off leaving a mixture which separates into an upper phase containing CAT and the high boiling point solvent and a lower phase containing DCB/MGN and PBP. In this process, the preferred PDS is isopropanol (IPA) and the preferred NHS is benzene as it forms a low boiling point azeotrope with ACN and IPA.

Unfortunately, it has been found that when this process is carried out on a larger scale, the hexamer content of the PBP (which unlike the other components is a solid) can be deposited in the distillation column, causing reduced efficiency and eventual blockage.

In practice, therefore, it is necessary to filter the reaction mixture to remove PBP before the mixture passes to the distillation column.

Whilst the overall process is preferably continuous, clearly the filtration must be carried out in a batch mode. Hence, the process must be stopped whilst the filter is changed or at least two (but preferably three) filters and appropriate valves and pipework must be provided.

Additionally, as the catalyst system is susceptible to oxygen and moisture, the filter must be purged with nitrogen before it is brought back 'on-line'. According to one aspect of the invention, there is provided a process for the dimerisation of acrylonitrile to a linear C_fi dinitrile in a homogenous liquid phase comprising a phosphonite or phosphinite catalyst, an inert proton donating solvent and a non-hydrox lic solvent, characterised in that the reactor effluent is mixed with a sufficient volume of a further solvent to quench the reaction and fed to a means for separating the mixture into a first phase consisting of the major part of the solvents, catalyst and acrylonitrile and a second phase consisting of the major part of the dinitrile and polymeric by-products.

According to a preferred aspect of the invention, the second phase passes to an extraction column fed with said further solvent to remove residual solvents, catalyst and acrylonitrile. The thus extracted components may be added to the reactor effluent along with the quenching volume of further solvent. Preferably the further solvent is pentane. The separating means may be either a simple settling vessel or may be a centrifuge.

The proton donating solvent is preferably isopropanol, although any of the solvents listed in EPA 314383 may be used. Likewise any of the non-hydroxylic solvents listed in

EPA 314383 may be used. However, the process of the invention has the advantage that because no distillation step is involved, toluene may be used instead of the lower boiling point benzene preferred in EPA 314383. The catalyst and reaction conditions may be as specified in EPA 314383.

According to a further preferred aspect of the invention, the first phase from the separating means is fed to a distillation column to separate the further solvent from the recyclable components.

The invention will now be described in the following exampl .

EXAMPLE A first feed solution consisting of pentane 3.98% w/w isopropanol 5.18% w/w acrylonitrile 0.14% w/w toluene 80.5 % w/w dicyano- butene 3.95% w/w isopropyl bis

-p-tolylphos- phinite 5.43% w/w and a second feed solution consisting of isopropanol 16.8% w/w acrylonitrile 83.2% w/w at respective feed rates of 0.87 ml/min and 0.13 ml/min, were mixed and passed into six continuous flow, stirred reactors having a total volume of 300 mis. The reaction temperature was maintained at 40 C and the residence time of 5 hours was sufficient to achieve an acrylonitrile conversion in the range 70-80%.

Analysis of the feed to the reactors immediately after mixing and of the reactor effluent was carried out and the results (the average of 5 runs) are shown in the Table (Sample Points 1 and 2).

Acrylonitrile conversions obtained over a two day period were in the range 72.3 to 75.6%, giving an average of 74.4%. The average selectivity to the linear dinitriles at this stage was 93.9%.

The reactor effluent at a rate of 1 ml/min was mixed with an equal volume of pentane to quench any further reaction. The resulting mixture was passed to a settler. The bottom layer, consisting of the major part of the

DCB/MGN and the PBP together with residual components was fed by gravity to a 60cm long by 1.3cm internal diameter pulsed extraction column. Pentane was introduced at the bottom of the column and after rising up the column, together with the entrained residual components, was mixed with the reactor effluent.

The top layer from the settler which, as shown in the Table, (Sample Point 3) consisted mainly of pentane together with smaller quantities of isopropanol, acrylonitrile, toluene and the extracted catalyst, was fed to the middle of a sealed, jacketed, distillation column 60cm long by 3cm internal diameter packed with 3mm diameter glass beads. The base of the column was attached to a falling film re-boiler having a wall temperature of 70 C. The distillation action in the column was provided by a stream of dry, deoxygenated nitrogen at a rate of 40 litres/hour. The overhead stream from the column was condensed and collected for recycle to the base of the extraction column and to the mixer. Analysis (Sample Point 4) showed that the stream consisted mainly of pentane with minor amounts of isopropanol and acrylonitrile and only trace amounts of toluene and products. The tails stream from the column, which analysis (Sample Point 5) showed to consist mainly of toluene together with catalyst and small amounts of pentane, isopropanol, acrylonitrile and products, was recycled back to the reactors. The raffinate product from the base of the extraction column (Sample Point 6) consisting mainly of the products and by-products together with some pentane, isopropanol and acrylonitrile, was passed to a falling film reboiler at 60 C and 100mm pressure to remove the low boiling components. Analysis of the tails stream (Sample Point 7) indicated an average selectivity to DCB of circa 90%.

CRM/AAL

Claims

A process for the dimerisation of acrylonitrile to a linear C dinitrile in a homogenous liquid phase comprising a phosphonite or phosphinite catalsyt, an inert proton donating solvent and a non-hydroxylic solvent, characterised in that the reactor effluent is mixed with a sufficient volume of a further solvent to quench the reaction and fed to a means for separating the mixture into a first phase consisting of the major part of the solvents, catalyst and acrylonitrile and a second phase consisting of the major part of the dinitrile and polymeric by-products.

A process according to claim 1, characterised in that the further solvent is pentane.

A process according to claim 1 or 2, characterised in that the second phase passes to an extraction column fed with said further solvent to remove residual solvents, catalyst and acrylonitrile.

A process according to claim 3, characterised in that the extracted components are mixed with the quenching volume of further solvent and added to the reactor effluent.

A process according to claim 1,2,3 or 4 characterised in that the first phase is fed to a distillation column to separate off the further solvent.

A process according to claim 1,2,3,4 or 5 characterised in that the separating means is a settling vessel or a centrifuge.