METHOD FOR CONTROLLING THE PROCESS FOR MAKING ISOCYANATES

Abstract

The invention relates to a method for preparing an aromatic polyisocyanate in an isocyanate plant comprising a reaction section for a phosgenation reaction, wherein a primary aromatic amine is reacted with phosgene compounds in a reaction section to obtain an isocyanate comprising reaction product and wherein CO.sub.2 concentration in the gases coming from the reaction section is measured and analyzed, and wherein the conditions in the phosgenation reaction are adjusted in case the CO.sub.2 concentration in the gases coming from the reaction section is higher than a background CO.sub.2 concentration.

Claims

1. A method for preparing an aromatic polyisocyanate in an isocyanate plant comprising a reaction section for a phosgenation reaction, the method comprising the steps of: a) providing a primary aromatic amine stream and a phosgene stream via an inlet in the reaction section; b) reacting the primary aromatic amines with the phosgene compounds in the reaction section to obtain an isocyanate comprising reaction product; c1) measuring CO.sub.2 concentration in the gases coming from the reaction section; d1) analyzing the CO.sub.2 concentration coming from the reaction section by comparing with a background CO.sub.2 concentration, wherein the background CO.sub.2 concentration comprises the CO.sub.2 concentration in the phosgene stream; e) adjusting conditions in the phosgenation reaction in case the CO.sub.2 concentration in the gases coming from the reaction section is higher than a background CO.sub.2 concentration and/or deviates with more than 2 vol % in view of the total volume of the gases coming from the reaction section.

2. A method for preparing an aromatic polyisocyanate in an isocyanate plant comprising a reaction section for a phosgenation reaction and a work-up section which is downstream of the reaction section and treats an isocyanate comprising stream, the method comprising the steps of: a) providing a primary aromatic amine stream and phosgene stream via an inlet in the reaction section; b) reacting the primary aromatic amine compounds with the phosgene compounds in the reaction section to obtain an isocyanate comprising reaction product; c2) measuring CO.sub.2 concentration in the gases coming from the reaction section and the work-up section; d2) analyzing the CO.sub.2 concentration coming from the reaction section and the work-up section by comparing with a background CO.sub.2 concentration range; e) adjusting conditions in the phosgenation reaction in case the CO.sub.2 concentration in the gases coming from the reaction section and the work-up section is higher than a background CO.sub.2 concentration and/or deviates with more than 2 vol % in view of the total volume of the gases coming from the reaction section.

3. The method according to claim 1, wherein the inlet comprises a mixing nozzle having at least two conduits one providing the primary aromatic amine stream and one providing the phosgene stream and of which the conduits have a conduit end opening where the primary aromatic amine stream and phosgene stream are discharged in the reaction section.

4. The method according to claim 3, wherein the adjustment of the conditions of the phosgenation reaction is the adjustment of the size of at least one conduit end opening in the nozzle.

5. The method according to claim 4, wherein the adjustment of the size is decreasing the size of at least one conduit end opening.

6. The method according to claim 3, wherein the adjustment of the conditions of the phosgenation reaction is the adjustment of the angle of at least one of the conduits.

7. The method according to claim 3, wherein the adjustment of the conditions of the phosgenation reaction is the adjustment of the distance between at least two of the conduits.

8. The method according to claim 1, wherein the inlet comprises a mixing device which is a rotor-stator mixer having a rotor and the adjustment of the conditions of the phosgenation reaction is the adjustment of the speed of the rotor.

9. The method according to claim 1, wherein the adjustment of the conditions of the phosgenation reaction is adjusting the flow ratio of the primary aromatic amine compounds stream and the phosgene stream at the inlet of the reaction section, by decreasing or increasing the flow ratio.

10. The method according to claim 1, wherein the adjustment of the conditions of the phosgenation reaction is adjusting the pressure in the reaction section.

11. The method according to claim 1, wherein the adjustment of the conditions of the phosgenation reaction is adjusting the temperature in the reaction section.

12. The method according to claim 1, wherein the adjustment of the conditions of the phosgenation reaction is adjusting the residence time of the reaction mixture of the primary aromatic amine compounds with the phosgene compounds in the reaction section.

13. The method according to claim 1, wherein the adjustment of the conditions of the phosgenation reaction is adjusting the concentration of the primary aromatic amine in the primary aromatic amine stream and/or the phosgene compounds in the phosgene stream.

14. The method according to claim 1, wherein the CO.sub.2 concentration is measured via infrared, gas chromatography, titration, near infrared, and/or UV.

15. The method according to claim 1, wherein the background CO.sub.2 concentration further comprises the CO.sub.2 concentration in the primary aromatic amine stream.

Description

[0036] The invention is further explained by the following figures and example that are non-limiting for the purpose of the invention.

[0037] FIG. 1 is a flow diagram of a process of making isocyanates.

[0038] FIG. 2 is a flow diagram representing an embodiment of the invention.

[0039] FIG. 3 represents a cross-sectional view of a possible inlet according to the invention.

[0040] FIG. 4 is a curve showing on the Y axis the amount of CO.sub.2 that has been found in the gasses coming from the work-up section and the reaction section and the X axis shows the amount of the back pressure of the primary aromatic amine stream, which is a measure of the inlet opening.

[0041] FIG. 1 shows a diagram where it is represented that the primary aromatic amine stream and the phosgene stream enter via an inlet into the reaction section. Gases formed in the reaction are taken off the reaction section (represented as off gases). The reactants enter the work-up section, also here gases that are formed are taken off. During work-up, the stream of crude isocyanates is formed. The crude isocyanates can be split further. All these steps are conventionally used and known by the skilled person. The crude isocyanates can be analysed via on-line or off-line analysis methods, to find out whether and how much by-products have been formed in the reaction section. When by-products are formed, the process controller can adjust the process conditions so that less by-products are formed.

[0042] FIG. 2 represents an embodiment of the invention. Here the same steps occur as in FIG. 1, but besides that the off gases coming from the reaction section can also be analysed to see whether CO.sub.2 is formed in the reaction section, which CO.sub.2 derives from the formation of by-products. Also the off gases coming from the work-up section can be analysed, e.g. either by analysing a combined gas stream coming from the reaction section and the work-up section (la) or via analysing the gas streams separately. Also the phosgene stream can be analysed to see whether and how much CO.sub.2 can be found in here so that the background CO.sub.2 can be set. The process represented in FIG. 2 allows that the process control can adjust faster in the sequence the process conditions and prevents sooner in the process that further by-products are formed.

[0043] Referring now to FIG. 3, there is shown an example of an inlet comprising a mixing nozzle for providing the primary aromatic amine stream and the phosgene stream in the reactor. The mixing nozzle is an impinging coaxial assembly 100 comprising an inner conduit 101 having an inner conduit end opening 102 disposed coaxially inside an outer conduit 103 having an outer conduit end opening 104. Flow chamber 105 is defined as the rectangular space inside inner conduit 101 and inner conduit end opening 102. The inner end conduit opening is the place where the phosgene stream or the primary aromatic amine stream is discharged. Flow chamber 106 begins as the rectangular space between outer conduit 101 and inner in conduit end 102. Flow chamber 106 continues as the rectangular space between outer conduit end 104 and inner conduit 101. Flow chamber 106 continues further as the rectangular space between outer conduit end opening 104 and inner conduit end opening 102. The outer end conduit opening 104 is the place where the phosgene stream or the primary aromatic amine stream is discharged, which stream is different than the stream discharged at the inner end conduit opening 102.

EXAMPLE 1

[0044] In an industrial plant for making MDI, an MDA stream and a stream of phosgene which is dissolved in MCB are fed via a mixing nozzle in a phosgenation reactor. The gases formed in the reactor are taken off and the reactants are fed to the work-up section. Gases that are formed in the work-up section are combined with the gases coming from the reactor and analysed via infrared spectroscopy to see the amount of CO.sub.2. The background CO.sub.2 was around 0.10 vol %. The amount of CO.sub.2 was considered too high (higher than 0.6 vol %) and the nozzle opening was decreased. This has as a consequence that the back-pressure [p], which is the pressure of the primary amine stream entering the nozzle, increases. This back-pressure can be measured and reflects the size of the nozzle opening.

[0045] FIG. 4 shows that when the back pressure is higher, and thus the nozzle opening is decreased, less CO.sub.2 is measured and thus less by-products have been formed.