Nitration reactor and method

Abstract

A nitration reactor (10) incorporating sections of downward flow for use in preparing nitrated organic compounds. It comprises a first vertically-oriented reactor section (12), a second vertically-oriented reactor section (14), a connecting section (16) between the two reactor sections, one or more inlets (20, 22) for introducing nitration reactants into the reactor, an outlet (24) for the removal of nitration reaction products, a vertically-downward flowpath (26) for the nitration reactants in one of the reactor sections or the connecting section, and operating conditions that produce a flow regime in the vertically-downward flowpath that is a dispersed flow regime or a bubbly flow regime. The invention overcomes the limitations of prior art nitration reactors of the type in which fluids flow largely in a vertically upward direction, with respect to hydrostatic demands and plant layout considerations.

Claims

1. A nitration reactor for use in nitrating aromatic organic compounds, comprising: (a) a first vertically-oriented reactor section; (b) a second vertically-oriented reactor section; (c) a connecting section between the first reactor section and the second reactor section for the flow of nitration reactants from the first reactor section to the second reactor section; (d) one or more inlets for introduction of the nitration reactants into the reactor; (e) an outlet for the removal of nitration reaction products from the reactor; (f) a vertically-downward flowpath for the nitration reactants in at least one of the first reactor section, the connecting section and the second reactor section; and (g) wherein operating conditions in the section of the reactor having the vertically-downward flowpath are such that a stability parameter Φ is in the interval of 0<Φ≤1, where $\varphi =\frac{\beta }{a.Math.\sqrt{Ri}+b.Math.\mathrm{Eo}+c}$ a=−1.1836×10.sup.−1 b=2.2873×10.sup.−5 c=1.1904×10.sup.−1 Ri=Richardson Number $\mathrm{Ri}=\frac{gD\left({\rho }_c-{\rho }_d\right)}{{\rho }_c{U}^2}$ β=volumetric fraction of a dispersed, organic phase, $\beta =\frac{Q_d}{Q_d+Q_c}$ Eo=Eötvös Number $Eo=\frac{\left({\rho }_c-{\rho }_d\right)g{D}^2}{\sigma }$ U=bulk fluid velocity $U=\frac{Q_d+Q_c}{A}$ D=downflow section hydraulic diameter $D=\frac{4A}{P}$ A=downflow section cross-sectional area, P=downflow section cross-sectional perimeter, g=gravitational acceleration constant, ρ.sub.c=density of continuous phase, ρ.sub.d=density of dispersed phase, Q.sub.c=volumetric flow of continuous phase, Q.sub.d=volumetric flow of dispersed phase, and σ=interfacial tension.

2. The nitration reactor according to claim 1, wherein the section having the vertically-downward flowpath is the connecting section.

3. The nitration reactor according to claim 1, wherein the section having the vertically-downward flowpath is the first reactor section.

4. The nitration reactor according to claim 1, wherein the section having the vertically-downward flowpath is the second reactor section.

5. The nitration reactor according to claim 1, wherein the first reactor section and the second reactor section have the same configuration.

6. The nitration reactor according to claim 1, wherein the first reactor section and the second reactor section have a different configuration.

7. The nitration reactor according to claim 1, further comprising a plurality of additional vertically-oriented reactor sections and connecting sections connected in series downstream of the second reactor section, at least one of said additional reactor sections and connecting sections having an additional vertically-downward flowpath for the nitration reactants, and the operating conditions produce a flow regime in at least one of the additional vertically-downward flowpaths that is a dispersed flow regime or a bubbly flow regime.

8. The nitration reactor according to claim 1, wherein the section having the vertically-downward flowpath has a cross-section that is circular.

9. The nitration reactor according to claim 1, wherein the section having the vertically-downward flowpath has a cross-section that is non-circular.

10. The nitration reactor according to claim 1, further comprising one or more mixing devices for mixing the nitration reactants.

11. The nitration reactor according to claim 10, wherein at least one said mixing device is positioned within a reactor section having a vertically-downward flowpath.

12. The nitration reactor according to claim 1, further comprising a venting conduit arranged to vent a gas from the reactor.

13. The nitration reactor according to claim 1, further comprising a draining conduit arranged to drain a liquid from the reactor.

14. A method of nitrating an aromatic organic compound using a nitration reactor comprising a first vertically-oriented reactor section, a second vertically-oriented reactor section, a connecting section between the first reactor section and the second reactor section for the flow of nitration reactants from the first reactor section to the second reactor section, and a vertically-downward flowpath for the nitration reactants in at least one of the first reactor section, the connecting section and the second reactor section, the method comprising the steps of: (a) introducing the nitration reactants into the first reactor section; (b) flowing the nitration reactants through the first reactor section, the connecting section and the second reactor section under nitrating conditions to produce nitration products; (c) selecting operating conditions in the section of the reactor having the vertically-downward flowpath such that a stability parameter Φ is in the interval of 0<Φ≤1, where $\varphi =\frac{\beta }{a.Math.\sqrt{Ri}+b.Math.\mathrm{Eo}+c}$ a=−1.1836×10.sup.−1 b=2.2873×10.sup.−5 c=1.1904×10.sup.−1 Ri=Richardson Number $\mathrm{Ri}=\frac{gD\left({\rho }_c-{\rho }_d\right)}{{\rho }_c{U}^2}$ β=volumetric fraction of a dispersed, organic phase, $\beta =\frac{Q_d}{Q_d+Q_c}$ Eo=Eötvös Number $Eo=\frac{\left({\rho }_c-{\rho }_d\right)g{D}^2}{\sigma }$ U=bulk fluid velocity $U=\frac{Q_d+Q_c}{A}$ D=downflow section hydraulic diameter $D=\frac{4A}{P}$ A=downflow section cross-sectional area, P=downflow section cross-sectional perimeter, g=gravitational acceleration constant, ρ.sub.c=density of continuous phase, ρ.sub.d=density of dispersed phase, Q.sub.c=volumetric flow of continuous phase, Q.sub.d=volumetric flow of dispersed phase, and σ=interfacial tension; and (d) removing the nitration products from the nitration reactor.

15. The method according to claim 14, wherein the nitration reactor further comprises a plurality of additional vertically-oriented reactor sections and connecting sections connected in series downstream of the second reactor section, at least one of said additional reactor sections and connecting sections having an additional vertically-downward flowpath for the nitration reactants, and the method further comprises the step of selecting operating conditions in the at least one section of the reactor having the additional vertically-downward flowpath such that a flow regime in said at least one section is dispersed flow or bubbly flow.

16. The method according to claim 14, wherein the section having the vertically-downward flowpath is the connecting section.

17. The method according to claim 14, wherein the section having the vertically-downward flowpath is the first reactor section.

18. The method according to claim 14, wherein the section having the vertically-downward flowpath is the second reactor section.

19. The method according to claim 14, wherein the section having the vertically-downward flowpath has a cross-section that is circular.

20. The method according to claim 14, wherein the section having the vertically-downward flowpath has a cross-section that is non-circular.

21. The method according to claim 14, further comprising one or more mixing devices for mixing the nitration reactants.

22. The method according to claim 21, wherein at least one said mixing device is positioned within a reactor section having a vertically-downward flowpath.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a nitration reactor according to one embodiment on the invention.

(2) FIGS. 2(a) to (h) are schematic diagrams of nitration reactors according to other embodiments of the invention.

(3) FIG. 3 is a graph showing flow regimes in a downward flow in the nitration reactor as related to the parameters Φ and Ri.

DETAILED DESCRIPTION

(4) In adiabatic nitration reactors having a section with vertically downward flow of the two phase reactants, four distinct flow regimes may be observed in the down-flow section. These are Dispersed flow, Bubbly flow, Churn flow and Annular flow. The applicant has experimentally determined that these observed flow regimes are relatively well characterized by three classic dimensionless parameters: Richardson Number (Ri), Void Fraction (β), and Eötvös Number (Eo). These parameters are defined as follows:

(5) $\begin{array}{c}\mathrm{Ri}=\frac{gD\left({\rho }_c-{\rho }_d\right)}{{\rho }_c{U}^2}\\ \beta =\frac{Q_d}{Q_d+Q_c}\\ \mathrm{Eo}=\frac{\left({\rho }_c-{\rho }_d\right)g{D}^2}{\sigma }\\ D=\frac{4A}{P}\\ U=\frac{Q_d+Q_c}{A}\end{array}$
where: Ri=Richardson Number β=dispersed phase volumetric fraction Eo=Eötvös Number U=bulk fluid velocity D=hydraulic diameter A=downflow section cross-sectional area P=downflow section cross-sectional perimeter g=gravitational acceleration constant ρ.sub.c=density of continuous phase ρ.sub.d=density of dispersed phase Q.sub.c=volumetric flow of continuous phase Q.sub.d=volumetric flow of dispersed phase, and σ=interfacial tension.

(6) However, it is difficult to reliably predict transition from stable ‘Dispersed’ and ‘Bubbly’ flow regimes to unstable ‘Churn’ and ‘Annular’ flow regimes using these three parameters.

(7) A support vector machine (SVM) algorithm was used to separate desirable ‘Dispersed’ and ‘Bubbly’ flow regimes from unstable or unsafe ‘Churn’ and ‘Annular’ flow regimes. A new dimensionless parameter (Φ) was discovered based on the output of the SVM algorithm that allows the transition from unstable to stable flow regimes to be reliably predicted in a nitrator including extended regions of downward flow.

(8) The parameter Φ is defined as:

(9) $\varphi =\frac{\beta }{a.Math.\sqrt{Ri}+b.Math.\mathrm{Eo}+c}$
where: Φ=Stability Parameter a=−1.1836×10.sup.−1 b=2.2873×10.sup.−5 c=1.1904×10.sup.−1 Ri, Eo and β are as defined above.

(10) As shown in FIG. 3, transition from stable to unstable flow regimes can be predicted at approximately Φ=1, with Bubbly and Dispersed flow regimes occurring in the interval of 0<Φ≤1.

(11) The four flow regimes observed in downward flowing sections of an experimental apparatus were found to be reliably predicted by two parameters: Richardson Number (Ri) and the stability parameter (Φ). The two stable flow regimes (‘Dispersed’ and ‘Bubbly’) are reliably predicted by the stability parameter alone.

(12) As seen in FIG. 3, the dispersed flow regime exists at smaller values of the stability parameter (Φ). Indeed, the stability of two phase downward flow was observed to increase as the stability parameter was reduced towards zero. While stable two phase downward flow exists in the interval of 0<Φ≤1, it is preferred to design a nitration reactor incorporating sections of downward flow with smaller values of Φ. A more preferred embodiment of such a nitration reactor would operate with a stability parameter in the interval of 0<Φ≤0.75, and an even more preferred embodiment would operate with the stability parameter in the interval of 0<Φ≤0.5.

(13) Surprisingly, the stability of the vertical down-flow sections is independent of the selection or location of the mixing devices installed in the up-flow section. It is clear that a reactor according to the present invention can be operated with any type of mixing device designed to create fine dispersions of light phase, without initiating unstable flow.

(14) References in this disclosure to “vertically-oriented” reactor sections and “vertically-downward” flowpaths and the like means sections and flows that are at an angle of greater than 45 degrees. In practice, the sections and flows are substantially vertical. Likewise, references to “horizontal” sections and flows means sections and flows that are at an angle of less than 45 degrees. References to “reactor sections” includes ones with cross-sections that may be circular or non-circular or of arbitrary, regular cross-section, e.g., square, rectangular or polygonal.

(15) Referring to FIG. 1, which shows one embodiment of the invention, the nitration reactor 10 has a first vertically-oriented reactor section 12, a second vertically-oriented reactor section 14 and a connecting section 16 between the first reactor section and the second reactor section for the flow of nitration reactants from the first reactor section to the second reactor section. The connecting section 16 has a region 26 with a vertically-downward flowpath. The cross-section of the connecting section is preferably circular, i.e., the connecting section is a cylindrical conduit, but can be any regular cross-sectional shape, for example ellipsoidal, square, rectangular or polygonal. The two reactor sections 12, 14 are mounted at the same elevation. Mixing devices 18 are located in the sections 12, 14. Reactant inlets 20, 22 are located at the lower end of the first reactor section 12. For example, inlet 20 is for the introduction into the reactor of the organic phase and inlet 22 for the introduction of the aqueous, mixed acid phase. It will be apparent that the reactants may be introduced individually, which would require three separate inlets, or they may be previously mixed, requiring only one inlet, or, as in FIG. 1, the sulphuric acid and the nitric acid may be previously mixed, requiring two inlets. The second reactor section 14 has an outlet 24 for the removal of the nitration products, e.g., MNB and MNT.

(16) The individual reactor sections 12, 14 are preferably supported at a common elevation. This reduces differences in thermal expansion during operation and avoids the need for flexible piping elements such as expansion joints. However, it may be necessary in some applications to support the individual reactor sections at different elevations.

(17) The reactor 10 can be site assembled. It can be skid mounted to facilitate remote construction and shipment of the completed sections to the site.

(18) The reactor 10 is operated according to the following method. The nitration reactants, for example benzene, sulphuric acid and nitric acid, are fed into the first reactor section 12 and are flowed through the first reactor section 10, the connecting section 16 and the second reactor section 14, under nitrating conditions. Operating conditions within the connecting section, and in particular its down-flow region 26, are selected such that the flow regime in that section is Dispersed flow or Bubbly flow, and/or that the stability parameter is in the interval 0<Φ≤1. The nitration products, e.g. MNB, are withdrawn from the reactor through the outlet 24.

(19) FIGS. 2(a) to (h) illustrate several alternative embodiments of the nitration reactor 10.

(20) FIGS. 2(a) and (g) illustrate embodiments in which the reactor 10 includes a third reactor section 30 connected in series to the second reactor section 14 by a second connecting section 32 having a region 26 with a vertically-downward flow. The reactor 10 can comprise any suitable number of reactor sections, connected in series, for example five or eight or twenty reactor sections.

(21) A reactor section can have downward flow, instead of only upward flow as in FIG. 1. For example, in FIG. 2(b), the second reactor section 14 has downward flow, and in FIG. 2(e), the first reactor section 12 has downward flow.

(22) As explained above, the reactants can be introduced individually in three inlets, as shown in FIG. 2(b), with an inlet 20 for the organic stream, an inlet 21 for the sulphuric acid stream and an inlet 23 for the nitric acid stream; or all reactants may be previously mixed, requiring only one inlet 25, as shown in FIGS. 2(a), (e), (f), (g) and (h). Alternatively, the sulphuric acid and organic stream could be previously mixed in inlet 29 and then nitric acid could be added in a separate inlet 27, as shown in FIG. 2(c).

(23) As shown in FIG. 2(a), a reactor section may have no mixing devices or it may have one or more mixing devices 18. Under certain process conditions, the reactor residence time limits the reaction rate and no mixing devices are required in a section of a reactor.

(24) Mixing devices 18 are generally installed in vertical sections 12, 14 of the reactor, but they may be installed in horizontal sections, as illustrated in FIG. 2(c).

(25) As depicted in FIG. 2(c), adjacent reactor sections 12 and 14 with reactants flowing upwards and downwards may be combined by a full-sized connecting section 34, such that the connecting section 34 functions as a reactor section.

(26) The reactor sections may be installed all at the same elevation, as in FIG. 1, or they may be installed at higher or lower elevations. For example, as illustrated in FIG. 2(d), the second reactor section 14 is at a higher elevation than the first reactor section 12. Here, a portion of the second reactor section 14 has an elevation in common with the first reactor section 12.

(27) The reactor sections 12, 14 may have substantially identical configurations and volumes, as shown in FIG. 1, as this design simplifies manufacturing and assembly. Alternatively, the reactor may be designed with differing reactor sections so as to meet certain process requirements such as minimizing the formation of a specific byproduct. For example, FIG. 2(b) depicts a reactor 10 in which the second reactor section 14 is smaller than the first reactor section 12.

(28) The reactor inlet or inlets may be located on the bottom of the first reactor section as shown in FIG. 1 or may be located on the top of the first reactor section 12 as depicted in FIG. 2(e).

(29) The reactors 10 may include some sections or portions of sections having horizontal flow. For example, FIG. 2(b) shows a portion of the connecting section 16 that is horizontal. Likewise the reactors can include reactor sections that are horizontal. However, it is preferred that the use of horizontal sections is limited, so that more than 50% of the reactor volume is flowing vertically upwards or downwards.

(30) To improve safety during a loss in circulation and to improve stability during startup, vent and drain lines may be installed connecting to one or more reactor sections. These allow gas to be quickly vented during startup and reactants to be quickly drained during shutdown. In the event of a normal shutdown in which the reactor remains flooded the vent and drain lines allow organic material to be purged out of the reactor. FIGS. 2(f) and (h) depict reactors 10 having a draining conduit 36 at the bottom of the reactor. FIGS. 2(g) and (h) depict reactors 10 having a venting conduit 38 at the top of the reactor.

(31) Valves may be installed in the venting and draining conduits to prevent flow through these conduits during operation. Orifices or another flow restriction device may be installed in these conduits to limit bypassing flow during operation and possibly act as a small parallel reactor. Flow in these conduits is generally forward by design, but fluid may occasionally flow against the direction of normal operation during, for instance, reactor draining.

Examples

(32) In a first example, the first downflow section in a nitration reactor conveys a continuous phase consisting of a mixture of sulphuric and nitric acid with Q.sub.c=0.771 ft.sup.3/s and ρ.sub.c=95.51b/ft.sup.3, and a dispersed phase of mostly benzene with Q.sub.d=0.056 ft.sup.3/s and ρ.sub.d=50.91b/ft.sup.3 between which exists an interfacial tension of σ=0.055 lb/s.sup.2. The downflow section has perimeter P=1.57 ft, and area A=0.196 ft.sup.2. On evaluation, it is found that the downflow section has a hydraulic diameter of D=0.5 ft, and bulk fluid velocity of 4.212 ft/s. It can then be seen that the void fraction, 13=0.0677, Eötvös Number, Eo=6,505, and Richardson Number, Ri=0.423. Using these values, the stability parameter, Φ, is found to be 0.355. Using the flow map in FIG. 3, it is predicted that this section of downflow will operate in the ‘Bubbly’ flow regime, close to the ‘Dispersed’ flow regime. Note that the units of the physical parameters are canceled such that parameters β, Eo, Ri and Φ are dimensionless.

(33) In a second example, the seventh downflow section in a larger nitration reactor conveys mostly sulphuric acid as the nitric acid has been converted to product with Q.sub.c=280.9 m.sup.3/hr and ρ.sub.c=1497 kg/m.sup.3, and a dispersed phase of mostly mononitrobenzene (MNB) with Q.sub.d=25.3 m.sup.3/hr and ρ.sub.d=1087 kg/m.sup.3, between which exists an interfacial tension of σ=0.016 N/m. In this reactor, the downflow section has a square cross-section with perimeter P=1.4 m, area A=0.122 m.sup.2, and hydraulic diameter D=0.35 m. Bulk fluid velocity U=0.694 m/s, β=0.0826, Eo=30,784, and Ri=1.95. In this downflow section of the larger reactor, the stability parameter, Φ, evaluates to 0.126. Using FIG. 3, it can be predicted that this downflow section of the reactor will operate in the ‘Bubbly’ flow regime.

(34) Throughout the foregoing description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details have been set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

(35) As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the following claims.