Method for production of hydroxylammonium phosphate in the synthesis of caprolactam

A method for production of caprolactam. The method involves: PA1 (a) reacting air with ammonia gas in an ammonia conversion zone to produce nitric oxide; PA1 (b) oxidizing at least a portion of the nitric oxide to nitrogen dioxide to produce an NO.sub.x -rich process gas stream; PA1 (c) reactively absorbing the NO.sub.x -rich gas stream with phosphoric acid containing solution in an absorption zone to form nitrate ions; PA1 (d) contacting the nitrate ions with air in a degassing zone to produce a nitrate-rich aqueous process stream; PA1 (e) reducing the nitrate-rich aqueous stream with hydrogen in the presence of phosphoric acid to produce hydroxylammonium phosphate; PA1 (f) oximating the hydroxylammonium phosphate with cyclohexanone to produce cyclohexanone oxime; and PA1 (g) converting the cyclohexanone oxime to caprolactam. According to the invention, supplemental oxygen is added downstream of the ammonia conversion zone to increase the quantity and rate of formation of nitrogen dioxide in the NO.sub.x -rich process gas stream.

BACKGROUND OF THE INVENTION
 Caprolactam can be produced from three hydrocarbon feedstocks: cyclohexane,
 phenol, and toluene. Approximately 68% of the world's caprolactam capacity
 is produced from cyclohexane, 31% from phenol, and 1% from toluene. All of
 the cyclohexane and phenol-based production proceeds via the formation of
 cyclohexanone oxime. In 94% of the cyclohexane and phenol-based
 caprolactam capacity, the formation of this oxime requires an ammonia
 oxidation step.
 In the processes involving ammonia oxidation, caprolactam production from
 cyclohexane or phenol can be broken down into the following steps:
 Oxidation of cyclohexane or hydrogenation of phenol, to synthesize
 cyclohexanone;
 Oxidation of ammonia to form nitric oxide, followed by various reaction
 steps to form a hydroxylamine salt;
 Synthesis of cyclohexanone oxime by reaction of cyclohexanone and the
 hydroxylamine salt; and
 Treatment of the cyclohexanone oxime with sulfuric acid followed by
 neutralization with aqueous ammonia to form caprolactam.
 One such method for producing caprolactam is the DSM-HPO (Dutch State
 Mines-Hydroxylammonium Phosphate-Oxime) process, also known as the
 Stamicarbon process. Such process is disclosed, for example, in Weissermel
 and Arp, Industrial Organic Chemistry (VCH Verlagsgesellschaft mbH 1993),
 pp. 249-258. In the DSM-HPO process, hydroxylammonium phosphate (NH.sub.3
 OH.H.sub.2 PO.sub.4) is reacted with cyclohexanone in toluene solvent to
 synthesize the oxime.
 The hydroxylammonium phosphate is synthesized in the DSM-HPO process in the
 following manner:
 Catalytic air oxidation of ammonia to form nitric oxide:
EQU 4 NH.sub.3+5 O.sub.2.fwdarw.4 NO+6 H.sub.2 O (I)
 Continued oxidation of nitric oxide to form nitrogen dioxide, among other
 nitrogen oxides:
EQU NO+1/2 O.sub.2.fwdarw.NO.sub.2 (II)
 Reactive absorption of nitrogen dioxide in a buffered aqueous phosphoric
 acid solution to form nitrate ions:
EQU 3 NO.sub.2 +H.sub.2 O.fwdarw.2 HNO.sub.3 +NO (III)
EQU HNO.sub.3 +H.sub.2 PO.sub.4.fwdarw.NO.sub.3 +H.sub.3 PO.sub.4 (IV)
 Catalytic hydrogenation of nitrate ions to form hydroxylammonium phosphate:
EQU NO.sub.3.sup.- +2 H.sub.3 PO.sub.4 +3 H.sub.2.fwdarw.NH.sub.3 OH.H.sub.2
 PO.sub.4 +H.sub.2 PO.sub.4.sup.- +2 H.sub.2 O (V)
 Oximating the cyclohexanone with hydroxylammonium phosphate to produce
 cyclohexanone oxime:
EQU C.sub.6 H.sub.10 O+NH.sub.3 OH.H.sub.2 PO.sub.4.fwdarw.C.sub.6 H.sub.11
 NO+H.sub.3 PO.sub.4 +H.sub.2 O (VI)
 The process for forming hydroxylammonium phosphate in the DSM-HPO process
 is shown in the flow sheet depicted in FIG. 1 of the attached drawing. As
 shown therein, an air stream 3 is initially compressed in a compressor 10,
 introduced as a "primary" air stream through feed line 12 into admixture
 with a gaseous ammonia stream 1, and thereafter fed to a catalytic ammonia
 converter 20. Typically, 100% ammonia conversion and 95% selectivity to NO
 are achieved in that reaction. Upon exiting the converter, some of the NO
 is further oxidized to NO.sub.2 to form an NO.sub.x -rich process gas
 stream 15. Some of the NO.sub.2 in the NO.sub.x -rich process stream 15
 dimerizes to form N.sub.2 O.sub.4.
 The NO.sub.x -rich process gas stream 15 is contacted countercurrently with
 an aqueous inorganic acid stream 37 in a trayed absorption tower 40. In
 the conventional DSM-HPO process, a "secondary" air stream 11 is added
 into a degasser 50 in amounts of from 5 to 20 volume % of the total air
 flow to the system. The secondary air stream 11 becomes laden with nitric
 oxide and the resulting nitric oxide laden air stream 17 is added to the
 base of the absorption tower 40. A nitrate-rich liquid stream 13 exiting
 the absorption tower 40 is routed to the degasser 50, and an NO,
 containing vent gas 5 exits the absorption tower.
 The vent gas 5 exiting the absorption tower 40 must normally be properly
 regulated to minimize the emission of NO,. An increase in production of
 hydroxylammonium phosphate typically results in a corresponding increase
 in NO.sub.x emission in the vent gas 5.
 The aqueous inorganic acid stream 37 added to the top of the absorption
 tower 40 contains a mixture of water, phosphoric acid (H.sub.3 PO.sub.4),
 ammonium nitrate (NH.sub.4 NO.sub.3), and monoammonium phosphoric acid
 (NH.sub.4 H.sub.2 PO.sub.4). The acid stream 37 is continuously cycled
 from the oximator train (consisting of an oximator 70, oxime extractor 80,
 and a hydrocarbon stripper 90) to the hydroxylamine train (consisting of
 the absorption tower 40, degasser 50, and a nitrate hydrogenator 60).
 Nitric oxides in the NO.sub.x -rich process gas stream 15 reactively
 absorb in the phosphoric acid solution in the absorption tower 40 to form
 nitrate ions.
 The nitrate-rich liquid stream 13 exiting the absorption tower 40 is passed
 through the degasser 50, where it is contacted countercurrently with
 secondary air 11 entering the degasser 50. The secondary air 11 removes
 unreacted nitric oxides from the nitrate-rich liquid stream 13. The nitric
 oxide-containing air stream 17 exiting the degasser 50 is routed to the
 absorption tower 40.
 A nitrate-rich liquid stream 19 exiting the degasser 50 is combined with an
 aqueous inorganic acid stream 21 from the oximator train, and the
 combination 31 fed to the nitrate hydrogenator 60. A hydrogen stream 7 is
 also added to the nitrate hydrogenator 60. Nitrate ions are reduced with
 hydrogen in the nitrate hydrogenator 60 over a palladium catalyst to form
 hydroxylammonium phosphate. An aqueous stream of hydroxylammonium
 phosphate, phosphoric acid, ammonium nitrate, and monoammonium phosphoric
 acid 23 exits the nitrate hydrogenator 60.
 The hydroxylammonium phosphate containing aqueous stream 23 then reacts
 with a stream of cyclohexanone in toluene solvent 25 in the oximator 70 to
 produce cyclohexanone oxime. An oxime-toluene stream 9 exits the oximator
 70 and is processed into caprolactam. An aqueous stream 27 also exits the
 oximator 70, and is routed to a oxime extractor 80 which removes entrained
 oxime 39, and adds it to the stream of cyclohexanone in toluene solvent
 25. An aqueous stream 29 exiting the oxime extractor 80 is routed to a
 hydrocarbon stripper 90 where entrained cyclohexanone and toluene 33 are
 removed and added to the stream of cyclohexanone in toluene solvent 25,
 which is routed to the oximator 70. Thus, the entrained oxime 39 obtained
 in the oxime extractor 80 and the cyclohexanone-toluene 33 obtained in the
 hydrocarbon stripper 90 are returned to the oximator 70. The aqueous
 stream 35 leaving the hydrocarbon stripper 90 is routed back to the
 hydroxylamine train, where a portion 21 is distributed to the nitrate
 hydrogenator 60 and a portion 37 is distributed to the absorption tower
 40. Typically, about 90% of aqueous stream 35 is routed to stream 21, and
 about 10% routed to stream 37.
 In view of the strict environmental regulation of NO.sub.x emissions, the
 quantity of NO.sub.x in the vent gas 5 cannot be increased. Accordingly,
 any increased hydroxylammonium phosphate production (and subsequent
 caprolactam production) must be obtained without any increase in NO.sub.x
 emissions. This can be accomplished by increasing the amount of air and
 ammonia fed to the process while increasing the plant size, e.g., the size
 of the absorption tower 40 and air compressor 10. However, such an
 increase in equipment capacity requires substantial capital investment.
 There is therefore a need for the development of improved techniques in the
 DSM-HPO process for producing caprolactam, by which increased amounts of
 hydroxylammonium phosphate and, consequently, caprolactam can be produced
 without large capital investment, and without increasing NO.sub.x
 emissions.
 SUMMARY OF THE INVENTION
 The present invention provides an improvement in the DSM-HPO process for
 production of caprolactam involving:
 (a) reacting air with ammonia gas in an ammonia conversion zone to produce
 nitric oxide;
 (b) oxidizing at least a portion of the nitric oxide to nitrogen dioxide to
 produce an NO.sub.x -rich process gas stream;
 (c) reactively absorbing the NO.sub.x -rich gas stream with phosphoric acid
 containing solution in an absorption zone to form nitrate ions;
 (d) contacting the nitrate ions with air in a degassing zone to produce a
 nitrate-rich aqueous process stream;
 (e) reducing the nitrate-rich aqueous stream with hydrogen in the presence
 of phosphoric acid to produce hydroxylammonium phosphate;
 (f) oximating the hydroxylammonium phosphate with cyclohexanone to produce
 cyclohexanone oxime; and
 (g) converting the cyclohexanone oxime to caprolactam.
 In accordance with the invention, the foregoing process is improved by
 adding supplemental oxygen downstream of the ammonia conversion zone to
 increase the quantity and rate of formation of nitrogen dioxide in the
 NO.sub.x -rich process gas stream.
 Desirably, a portion of secondary air, normally introduced into the
 degassing zone is rerouted to the ammonia conversion zone to increase the
 production of nitric oxide formed in the ammonia conversion zone without
 increasing the level of NO.sub.x contained in the gas vented from the
 absorption zone.
 Utilizing the improved technique of the invention, desirably by rerouting a
 portion of the secondary air to the ammonia conversion zone and
 maintaining the volumetric percentage of ammonia fed to the conversion
 zone at a constant or increased level, the production of NO in the ammonia
 conversion zone is increased. By adding supplemental oxygen according to
 the invention, both the amount and rate of conversion of NO to NO.sub.2
 are increased, thereby promoting formation of nitrate in the absorption
 zone, without any adverse effect on the NO.sub.x content of gases vented
 from the absorption zone. Alternatively, the addition of supplemental
 oxygen may be used to lower NO.sub.x emissions, with or without rerouting
 of secondary air to the ammonia conversion zone, and with or without
 increases in nitrate (and consequently hydroxylammonium phosphate and
 caprolactam) production. The invention also encompasses adding
 supplemental oxygen according to the invention without rerouting a portion
 of secondary air to the ammonia converter, but increasing the volumetric
 percentage of ammonia fed to the conversion zone to increase production of
 NO. This ultimately results in an increase in formation of
 hydroxylammonium phosphate and caprolactam without an increase in NO.sub.x
 emissions.
 The method of the present invention thus facilitates an increase in
 hydroxylammonium phosphate production in the DSM-HPO process for
 synthesizing caprolactam, while maintaining NO.sub.x emissions at
 constant, or decreased, levels. It is estimated that use of the method of
 the invention normally results in an increase of between about 5 and 15%
 in the production of hydroxylammonium phosphate without increasing
 NO.sub.x emissions. Furthermore, this is accomplished without substantial
 capital investment, such as would otherwise be required to increase plant
 capacity. Moreover, by substituting oxygen for inert nitrogen present in
 the secondary air conventionally fed to the absorption zone, the oxygen
 partial pressure in the system may be increased and residence times for
 the intermediates formed in the various stages of the process may be
 lowered.
 In the production of nitric acid, it is known that direct injection of
 supplemental oxygen can boost nitric acid synthesis while controlling
 NO.sub.x emissions.
 Such addition of oxygen is described, for example, in U.S. Pat. Nos.
 4,183,906; 4,183,906; 4,235,858; and 5,167,935; UK Patent No. 803211; and
 EP published Patent Applications Nos. 799794 and 808797. Oxygen addition
 is also described in Kongshaug, Extension of Nitric Acid Plant Capacity by
 Use of Oxygen, Nitric Acid Symposium (1981); and by Faried et al.,
 Boosting Existing Nitric Acid Production, The Fertiliser Society (1986).
 For example, EP 808797 describes an improved process for nitric acid
 production in which supplemental oxygen is added to the cooler/condenser,
 the absorption tower, the ammonia converter, and/or the bleacher, to cause
 an increase in nitric acid production without increasing NO, emissions. No
 supplemental oxygen addition of this type is believed to have been
 previously disclosed in connection with the synthesis of caprolactam.
 Feeding oxygen to the ammonia converter has been employed in the BASF and
 Inventa processes for the synthesis of caprolactam. (See Kirk Othmer
 Encyclopedia of Chemical Technology, 4th Edition, 4: 831 (1992); U.S. Pat.
 No. 5,777,163.) In these processes, however, no supplemental oxygen is
 added downstream of the converter. Also, the BASF and Inventa processes
 differ substantially from the DSM-HPO process for producing caprolactam in
 that they do not add air to the ammonia converter, and do not involve the
 formation of intermediates analogous to those produced in the DSM-HPO
 process.

DETAILED DESCRIPTION OF THE INVENTION
 All patent applications, patents, and literature references cited in this
 specification are hereby incorporated by reference in their entirety.
 In accordance with the present invention, a supplemental oxygen stream 43,
 (see FIG. 2) is injected downstream of the ammonia converter 20 of the
 hydroxylammonium phosphate reaction train (FIG. 2) in the DSM-HPO process
 for the synthesis of caprolactam.
 As used herein, the term "supplemental oxygen" refers to pure oxygen or any
 oxygen-enriched gaseous stream containing more than about 50%, and
 preferably more than about 90%, oxygen by volume. Suitable supplemental
 oxygen sources include pipeline oxygen, independent cryogenic oxygen
 plants or PSA/VPSA oxygen plants, liquid oxygen tanks or oxygen-enriched
 air streams.
 The supplemental oxygen is desirably injected in place of at least a
 portion of the secondary air introduced to the degasser in the DSM-HPO
 process through line 11 (FIG. 1). In accordance with a preferred
 embodiment of the present invention, air that would otherwise have been
 employed as "secondary" air is instead fed through feed line 12 for
 introduction as primary air into the ammonia converter 20. Gaseous
 mixtures containing about 8 to 12 mole % ammonia and about 18 to 20 mole %
 oxygen are thus introduced into the ammonia converter, and converted
 therein under the reaction conditions, e.g., temperature, pressure and
 catalyst, utilized in the DSM-HPO process to produce gaseous reaction
 mixtures containing in mole %, about:
 7 to 12% NO
 11 to 18% H.sub.2 O
 67 to 72% N.sub.2
 3 to 10% O.sub.2
 By thus increasing the flow of primary air introduced into the ammonia
 converter, the amount of NO formed therein is increased by about 5 to 15%
 as compared with the ammonia oxidation step in the absence of the addition
 of supplemental oxygen according to the invention.
 The supplemental oxygen is preferably added to the absorption zone in the
 proportion of about 1.8 to 4.0 moles of O.sub.2 per mole of incremental NO
 produced in the ammonia converter (i.e., per mole of additional NO
 produced as a result of oxygen addition according to the invention). By
 thus increasing the amount of oxygen introduced into the absorption zone,
 both the quantity and rate of formation of NO.sub.2 and nitrate ions are
 enhanced.
 FIG. 2 shows the portion of the process for producing caprolactam by the
 DSM-HPO method to which the present invention relates. According to the
 invention, oxygen is injected in the process for hydroxylammonium
 phosphate synthesis, downstream of the reaction of air with ammonia gas.
 The invention encompasses injection of oxygen in any manner that increases
 the formation of NO.sub.2, thereby decreasing the amount of NO.sub.x that
 would otherwise be emitted.
 The supplemental oxygen may be added through any of the alternative streams
 shown in FIG. 2. For example, in one embodiment of the invention the
 supplemental oxygen stream 43 is injected via line 43b into the process
 gas line 15 entering the absorption tower 40. Alternatively, the
 supplemental oxygen 43 may be injected through line 43a directly into the
 absorption tower 40. It is also feasible to inject the supplemental oxygen
 43 into the degasser via line 43c into process gas line 11 supplying
 secondary air to the degasser, or inject the supplemental oxygen 43 via
 line 43d directly into the degasser. The invention also encompasses the
 direct addition of the supplemental oxygen at several locations in the
 absorption tower and the degasser. The supplemental oxygen is preferably
 introduced under positive pressures of between about 2 and 20 psig,
 typically about 5 psig.
 As noted above, practice of the improved method of this invention does not
 involve any capital investment of the order of that which would be
 required to, e.g., expand the capacity of the absorption tower.
 Furthermore, retrofitting of existing plants to practice the improved
 technique of the invention can be easily carried out by providing the
 necessary supplemental oxygen supply lines and connecting them by
 conventional means to the relevant process line or process unit as
 outlined above.
 The invention is further illustrated by the following example, which is
 intended to exemplify practice of the invention, and not to be construed
 as limiting its scope.
 Example
 The method of the invention was employed to modify an existing caprolactam
 production plant using the DSM-HPO process wherein the upper section of
 the absorber had been damaged. This, in turn, reduced the ability of the
 absorber to reoxidize NO to NO.sub.2, and to absorb nitrate ions. Thus,
 production was limited, and NO.sub.x emissions increased.
 The method of the invention was employed in this process to increase
 hydroxylammonium phosphate production and to lower NO.sub.x emissions.
 Specifically, oxygen was injected via a sparger into the nitrate-rich
 liquid in the degassing tower. A portion of the secondary air that would
 otherwise have been fed to the degasser was rerouted to the converter, and
 the amount of ammonia fed to the converter increased The table below shows
 the amounts of oxygen and ammonia added in seven tests that were run. The
 "secondary air flow", "NH.sub.3 Flow", and "NO.sub.x in vent gas" values
 shown represent the percent changes relative to the DSM-HPO process as
 operated without oxygen injection according to the invention. The percent
 increase in hydroxylammonium phosphate achieved using the method of the
 invention was the same as the percent increase in NH.sub.3 flow shown. The
 "O.sub.2 /NH.sub.3 added" values shown indicate the molar ratio of oxygen
 injected to NH.sub.3 flow.

Test 1 2 3 4 5 6 7
 Sec- -49% -50% -51% -100% -100% -100% -94%
 ond
 Air
 Flow
 NH.sub.3 +5.7% +5.6% +5.6% +9.5% +9.8% +9.3% +8.2%
 Flow
 O.sub.2 / 3.2 3.2 3.2 3.3 3.2 3.3 3.8
 added
 NH.sub.3
 NO.sub.x NA -24% -43% NA -2% NA NA
 in
 vent
 gas
 Thus, in these tests from about 50% to 100% of the secondary air was
 diverted from the degasser to the converter. Oxygen was added according to
 the present invention at a rate of from 3.2 to 3.8 moles of oxygen per
 mole of added ammonia. The oxygen addition removed dissolved NO.sub.2 from
 the nitrate-rich stream, thereby promoting increased oxidation of NO to
 NO.sub.2, and an increase in hydroxylammonium phosphate production of up
 to 9.8%. Furthermore, the increase in hydroxylammonium phosphate
 production was achieved without an increase in NO.sub.x content in the
 vent gas.
 While preferred embodiments of the process hereof are described
 hereinabove, it will be apparent to those skilled in the art that various
 changes may be made therein without departing from the scope of the
 invention as defined in the claims appended hereto.