Low pressure amine reactor

A method for the production of an amine from a nitrile utilizing hydrogenation by; feeding hydrogen and nitrile into a reactor including catalyst, water and inorganic base to form a reaction medium; mixing the reaction medium to provide a uniform bulk concentration of the nitrile in at least one direction across the reactor to minimize the reactor volume where local bulk nitrile concentration exceeds that stoichiometrically required to completely deplete the local bulk hydrogen concentration; and hydrogenating the nitrile to form the amine.

FIELD OF THE INVENTION
 The invention relates to a process for the production of an amine from a
 nitrile, where the reaction is conducted in the presence of a catalyst.
 The invention also relates to a reactor for producing an amine from a
 nitrile.
 BACKGROUND OF THE INVENTION
 It is well known that amines such as hexamethylene diamine, propyl amines,
 butyl amines, benzyl amines, tallow amines, ethyl amines, etc., may be
 produced by the catalytic hydrogenation of nitriles such as
 proprionitrile, butyronitriles, tallow nitriles, acetonitriles, etc., in
 the presence of catalysts and other substances such as ammonia and/or
 caustic alkali. As set forth in U.S. Pat. No. 3,821,305, the entire
 subject matter of which is incorporated herein by reference, one such
 process is described in which hydrogenation is conducted in liquid phase
 at pressures of from 20-50 atmospheres and temperatures of
 60.degree.-100.degree. C. in the presence of finely divided Raney catalyst
 and an inorganic base. Hydrogen and adiponitrile are fed into a liquid
 reaction medium consisting of hexamethylenediamine, water, the inorganic
 base, and the catalyst, in which medium the content of base is maintained
 in the range of 0.2-12 moles per kilogram of catalyst, while the content
 of water is maintained in the range of 2-130 moles per mole of the base.
 In typical continuous processes utilizing a Raney nickel or Raney cobalt
 hydrogenation catalyst, the rate at which the catalyst is fed into the
 reaction medium must be carefully controlled. Active catalysts of that
 type are pyrophoric, however, and are therefore normally kept out of
 contact with air by transporting and storing the catalyst in a relatively
 inert liquid. Hence, in some of the aforementioned processes, the rate at
 which the catalyst is fed into the reaction medium is desirably controlled
 by suspending the catalyst in such a liquid so as to disperse the catalyst
 substantially uniformly through the liquid in a known concentration of
 catalyst per unit volume of the suspension, and then controlling the
 volumetric flow rate of the suspension into the reaction mixture. Examples
 of processes in which the catalyst feed rate may be conveniently
 controlled in this way are described in U.S. Pat. No. 3,821,305, the
 disclosure of which is incorporated herein by reference, and in U.S. Pat.
 No. 3,056,837, the disclosure of which also is incorporated herein by
 reference.
 However, Raney nickel and cobalt catalysts in such processes have been
 plagued by high deactivation rates under certain conditions when utilized
 in the hydrogenation of nitrites. For example, an article in Chemical
 Engineering Science, Vol. 47, No. 9-11, 2289-94 (1992), indicates that
 nitrites deactivate nickel or cobalt catalysts, such as Raney nickel
 catalysts. More recently, efforts have been made to reduce such catalyst
 deactivation rates. For example, it is also known in such low pressure
 hydrogenation systems to utilize high liquid recirculation velocities in
 an attempt to provide good mixing conditions found in turbulent flow so as
 to enhance catalyst stability and increase mass transfer coefficients as
 set forth in Chemical Engineering Science, Vol. 35, 135-141 (1980).
 Additionally, efforts have been made to study reactors to determine the
 effect of operating conditions on catalyst deactivation rates. For
 example, in Catalysis Today, 24, 103-109 (1995) catalyst deactivation
 effects under various operating conditions for hydrogenation of
 adiponitrile, in a continuous bench scale slurry bubble column reactor
 were investigated. The reactor was considered to be mixed perfectly
 because the temperatures at the top and bottom of the column were
 identical and the differences in concentration between the samples taken
 at the top and the bottom of the column were less than 15%.
 Efforts have also been made to reduce catalyst deactivation by physically
 blocking the active catalyst sites or access to the sites and equipment
 fouling in hydrogenation reactions by increasing mass transfer rates in
 the reactor system, i.e., see "Pumped-up Mixer Improves Hydrogenation,"
 Chemical Engineering, June 1998, p. 19, in which increased mass transfer
 rates reduced catalyst physical deactivation and equipment fouling in
 hydrogenation reactions for the production of edible oils. However, the
 local bulk concentrations of reactants in such reactors varies
 considerably and will not inhibit chemical catalyst deactivation (i.e.,
 catalyst deactivation by irreversibly depleting the catalyst of elements
 (e.g., interstitial hydrogen)) necessary for adequate catalyst activity in
 nitrile hydrogenation reactions.
 However, it has now been discovered that contrary to the assumptions and
 inferences made in the above-mentioned prior reactor configurations for
 the hydrogenation of nitrites, the reactants in such reactors are not
 perfectly mixed across the diameter of the reactor. According to the
 present invention, studies have been made that indicate the local bulk
 nitrile concentration in such reactors is not uniform and through most of
 the reactor the local bulk nitrile concentration exceeds that
 stoichiometrically required to completely deplete the local bulk hydrogen
 concentration, which leads to an increased catalyst chemical deactivation
 rate. Accordingly, there is a need to provide certain reactor conditions
 that would provide reduced chemical catalyst deactivation rates in nitrile
 hydrogenation systems.
 SUMMARY OF THE INVENTION
 The present invention relates to a method for the production of an amine
 from a nitrile by hydrogenation comprising; feeding hydrogen and nitrile
 into a reactor comprising catalyst, water and inorganic base to form a
 reaction medium; mixing the reaction medium to provide a uniform local
 bulk concentration of the nitrile in the reactor; and hydrogenating the
 nitrile to form the amine. Moreover, the present invention relates to a
 method for the production of an amine from a nitrile by hydrogenation
 while minimizing the reactor volume where local bulk nitrile
 concentrations exceed that stoichiometrically required to completely
 deplete the local bulk hydrogen concentration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 While the invention is applicable to the process for the production of any
 amine including aliphatic and aromatic amines and their derivatives, such
 as hexamethylene diamine, propyl amines, butyl amines, benzyl amines,
 tallow amines, ethyl amines, etc., produced from a nitrile including
 aliphatic and aromatic nitrites and their derivatives such as
 proprionitrile, butyronitriles, tallow nitrites, acetonitriles, benzyl
 nitrites, etc., in which finely divided catalyst is suspended in the
 liquid reaction medium, the invention will be described in the context of
 a preferred process for such production.
 For example, a process for an amine may be carried out at pressures of
 20-50 atmospheres and at temperatures of 60.degree. to 120.degree. C., by
 feeding hydrogen and nitrile into a liquid reaction medium containing,
 along with the amine produced, water, inorganic base and a finely divided
 nickel or cobalt catalyst dispersed in the liquid components of the
 reaction medium. The catalyst, which preferably is Raney nickel, with or
 without promotor metals such as chromium and/or iron, loses all or most of
 its activity during hydrogenation.
 To maintain a given level of catalytic activity within the catalytic mass,
 it is necessary for the catalyst in the reaction medium to be gradually
 replaced. This replacement is effected by feeding fresh catalyst to the
 reaction vessel and removing a quantity of reaction medium which contains
 an amount of catalyst equal to that supplied. The feed catalyst may
 consist of a mixture of fresh catalyst and of recycled catalyst. Recycled
 catalyst is catalyst that has been washed prior to re-use.
 The reaction medium preferably contains:
 (1) a quantity of catalyst in excess of 1 part, by weight, per 100 parts of
 liquid reaction medium (amine, water and inorganic base), the upper limit
 depending solely on the fluidity of the reaction medium; the preferred
 range is from 3 to 35 parts per 100 parts by weight of the liquid reaction
 medium;
 (2) a quantity of inorganic base in the range of 0.2 to 12 moles per
 kilogram of catalyst and preferably between 1 and 3 moles per kilogram of
 catalyst;
 (3) a quantity of water in the range of 2 to 130 moles per mole of
 inorganic base and preferably between 7 and 70 moles per mole of inorganic
 base.
 Preferably, the inorganic base comprises alkali metal hydroxide, such as
 sodium, potassium, lithium, rubidium, or cesium. More preferably, the
 inorganic base comprises a mixture of two or more alkali metal hydroxides.
 For example, synergistic results (e.g., improved catalyst stability and
 improved selectivity for the primary amine) have been obtained using a
 mixture of sodium hydroxide and potassium hydroxide.
 The liquid part of the reaction medium, under the starting conditions
 already specified, and within the preferred range of ratio of water to
 inorganic base, consists of an aqueous solution of inorganic base whose
 concentration is in the range of 25 to 70%, preferably 30 to 60%, and,
 more preferably 40 to 50% by weight of the aqueous solution. The other
 phase consists of amine containing water and small amounts of inorganic
 base. The aqueous solution of inorganic base, which is the heavier phase,
 contains most of the catalyst.
 In accordance with the present invention, so as to reduce the chemical
 catalyst deactivation rate, it has been discovered that the local bulk
 concentration of nitrile be uniform in at least one direction across the
 reactor with substantially the same extent of reaction. Moreover, in
 accordance with the present invention, chemical catalyst deactivation may
 be minimized by maintaining conditions such that the local bulk nitrile
 concentration is less than that stoichiometrically required to completely
 deplete the local bulk hydrogen concentration in the reactor; for example,
 one mole per liter of adiponitrile is stoichiometrically required to
 completely deplete 4 moles per liter of hydrogen (H2) to produce to one
 mole per liter of hexamethylenediamine. As defined herein, "chemical"
 catalyst deactivation refers to the reduction in activity of the catalyst
 by altering the chemical composition of the catalyst, "physical" catalyst
 deactivation refers to the reduction in activity of the catalyst by
 limiting the accessible number of active sites of the catalyst, such as by
 blocking the pores of the catalyst (e.g., coking), and local bulk
 concentration refers to the average concentration of a chemical species in
 a sample volume centered on a catalyst particle with the sample volume
 having a diameter between about 100 times the catalyst particle diameter
 and about 0.1 times the characteristic length scale of the reactor, e.g.
 the reactor diameter in a tubular reactor. In the present invention,
 `local bulk` nitrile concentration gradients refers to gradients in
 nitrile concentration over length scales of the order of magnitude of the
 dimensions of the reactor rather than gradients in nitrile concentration
 over length scales with dimensions of the order of magnitude of the
 catalyst particles.
 For example, in a tubular reactor, the extent of reaction is substantially
 constant in a plane perpendicular to the axis of the tube, while in a
 stirred tank reactor the extent of reaction is substantially the same at
 all points in the reactor. As above-mentioned, chemical catalyst
 deactivation rates are higher in reactor zones where the local bulk
 nitrile concentration exceeds that stoichiometrically required to
 completely deplete the local bulk hydrogen concentration and where such
 local bulk nitrile concentration comes into contact with the catalyst.
 Accordingly, the present invention reduces the rate of chemical catalyst
 deactivation by substantially eliminating local bulk concentration
 gradients through zones in the reactor with substantially the same extent
 of reaction while also minimizing the volume of the reactor where local
 bulk nitrile concentration exceeds that stoichiometrically required to
 completely deplete the local bulk hydrogen concentration.
 Generally, within a zone of constant extent of reaction, the coefficient of
 variation of bulk nitrile concentration (100 times standard deviation
 divided by mean) is less than 250%, preferably less than 150%, and even
 more preferably less than 100%. Generally, the conditions in the reactor
 are maintained such that the local bulk concentration of nitrile is less
 than the local bulk concentration of hydrogen throughout greater than
 about 30% of the volume of the reactor, preferably greater than about 40%,
 and more preferably greater than about 50%.
 In an embodiment of the present invention, the nitrile hydrogenation
 process may be performed in tubular reactors, such as gas lift reactors.
 An example of such a reactor, which is not limitive of the invention, is
 shown in the accompanying drawing (FIG. 1).
 The equipment for continuous operation of the process is of conventional
 type. An example of this, which is not limitive of the invention, is shown
 in the accompanying drawing. The equipment consists essentially of a
 vertical tubular reaction vessel 1 provided inside with an injection
 device 2, such as to promote the agitation of the reaction medium
 resulting from the hydrogen flow 9, mixing device 30, and at the top with
 containers 3 and 4, which enable the separation of the gas from the liquid
 and the drawing off from the reaction vessel of a hydrogenated product
 having a low content of catalyst thus making it possible to maintain in
 the reaction vessel relatively high concentration of catalyst--for
 example, 10 to 30 parts of catalyst per 100 parts by weight of liquid
 reaction medium.
 The equipment also includes a gas re-cycling pump 5 and pipes for feeding
 the reaction vessel with adiponitrile 8, aqueous caustic solution 7, and
 hydrogen 9. The hydrogen consumed is replaced by feeding fresh hydrogen
 through pipe 10.
 Part of the gas is vented through pipe 11, the purpose of this release
 being to maintain the hydrogen content in the re-cycled gas above a given
 value.
 Product stream 12 from the reactor is discharged into decanter 14 where the
 upper layer containing crude hexamethylene diamine is discharged through
 pipe 15 and on to settling tank 16, thence through pipe 24 to further
 purification measures including distillation. The lower layer of the
 decanter, 14, is separated into two portions, the first going to pipe 6
 which is returned to the reactor and the second going to pipe 25 which
 discharges into wash tank 17. Wash tank 17 is fed by pipe 26 containing
 water, and the washed catalyst is returned to the reactor via catalyst
 tank 19 and pipe 20. The catalyst wash water is discharged from tank 17
 into hold tank 22 via pipe 21, thence through pipe 23 to pipe 16.
 In tubular reactors, the flow rate of the reaction medium for hydrogenation
 of nitrites is quite high (i.e., turbulent flow with Reynolds numbers
 above 2000). Even with turbulent flow, which would have been expected to
 provide sufficient mixing of the reaction medium, nonuniform local bulk
 concentration in these reactors exists. Additional mixing mechanisms must
 be utilized to provide the uniform local bulk nitrile concentration of the
 present invention. Such additional mixing may be provided by static
 mixers, mechanical mixers, jet mixers or by reactor design. In tubular
 reactors, the mixing is provided preferably by static mixers.
 For example, the mixing device 30 may be a static mixer, such as a low
 pressure drop vortex mixer, an orifice mixer, a mixing nozzle, valves, a
 pump, an agitated line mixer, packed tubes, or a long pipe line.
 Additionally, the mixing device 30 may be a mechanical mixer, such as an
 impeller, a pump, or the like; or the mixing device 30 may be a jet mixer.
 Preferably, the mixing device is a static mixer, more preferably, a low
 pressure drop vortex mixer. The mixing device may be placed at various
 locations in the reactor. However, in order to be more effective in
 providing uniform local bulk nitrile concentration, the mixer is placed in
 the reactor vessel in close proximity to the nitrile feed stream.
 The uniform bulk nitrile concentration may be implemented with other
 reactor configurations, such as stirred tank reactors, bubble column
 reactors, or the like. Such mixing conditions may be implemented as
 mentioned herein.
 EXAMPLES
 The mixing may be characterized by flow visualization experiments
 involving, for example, injection of a dye into a scaled or full-scale
 transparent mock-up of the reactor, or may be caluclated using
 computational fluid dynamics. In the present examples, a gas lift reactor,
 as illustrated in the FIGURE, is utilized to prepare hexamethylene diamine
 from adiponitrile and hydrogen with a Raney nickel catalyst.
 Flow condition are maintained such that the Reynolds number of the reaction
 mass was about 1.6 million.
 The following table indicates the coefficient of variation in total nitrile
 concentration as a function of position in the reactor, with and without a
 mixer. The mixer utilized in these examples is a static mixer,
 particularly a low pressure drop vortex mixer. The mixer is placed
 directly above the adiponitrile feed stream in the reactor.