BACKGROUND OF THE INVENTION There is a plethora of references (both patents and literature articles) dealing with the formation of acids, one of the most important being adipic acid, by oxidation of hydrocarbons. Adipic acid is used to produce Nylon 66 fibers and resins, polyesters, polyurethanes, and miscellaneous other compounds.

There are different processes of manufacturing adipic acid. The conventional process involves a first step of oxidizing cyclohexane with oxygen to a mixture of cyclohexanone and cyclohexanol (KA mixture), and then oxidation of the KA mixture with nitric acid to adipic acid. Other processes include, among others, the "Hydroperoxide Process", the"Boric Acid Process", and the"Direct Synthesis Process", which involves direct oxidation of cyclohexane to adipic acid with oxygen in the presence of solvents, catalysts, and promoters.

The Direct Synthesis Process has been given attention for a long time.

However, to this date it has found little commercial success. One of the reasons is that although it looks very simple at first glance, it is extremely complex in reality. Due to this complexity, one can find strikingly conflicting results, comments, and views in different references.

It is well known that after a reaction has taken place according to the Direct Synthesis, a mixture of two liquid phases is present at ambient temperature, along with a solid phase mainly consisting of adipic acid. The two liquid phases have been called the"Polar Phase"and the"Non-Polar Phase. "However, no attention has been paid so far to the importance of the two phases, except for separating the adipic

acid from the"Polar Phase"and recycling these phases to the reactor partially or totally with or without further treatment.

It is also important to note that most studies on the Direct Synthesis have been conducted in a batch mode, literally or for all practical purposes.

The following references, among others, may be considered as representative of oxidation processes relative to the preparation of diacids and other intermediate oxidation products.

U. S. Patent 5,463,119 (Kollar) discloses a process for the oxidative preparation of C5-C8 aliphatic dibasic acids by (1) reacting, (a) at least one saturated cycloaliphatic hydrocarbon having from 5 to 8 ring carbon atoms in the liquid phase and (b) an excess of oxygen gas or an oxygen-containing gas in the presence of (c) a solvent comprising an organic acid containing only primary and/or secondary hydrogen atoms and (d) at least about 0.002 mole per 1000 grams of reaction mixture of a polyvalent heavy metal catalyst; (2) removing the aliphatic dibasic acid; and (3) recycling intermediates, post oxidation components, and derivatives thereof remaining after removal of the aliphatic dibasic acid into the oxidation reaction.

U. S. Patent 5,374,767 (Drinkard et al.) discloses formation of cyclohexyladipates in a staged reactor, e. g., a reactive distillation column. A mixture containing a major amount of benzene and a minor amount of cyclohexene is fed to the lower portion of the reaction zone and adipic acid is fed to the upper portion of the reaction zone, cyclohexyladipates are formed and removed from the lower portion of the reaction zone and benzene is removed from the upper portion of the reaction zone.

The reaction zone also contains an acid catalyst.

U. S. Patent 5,321,157 (Kollar) discloses a process for the preparation of C5-C8 aliphatic dibasic acids through oxidation of corresponding saturated cycloaliphatic hydrocarbons by (1) reacting, at a cycloaliphatic hydrocarbon conversion level of between about 7% and about 30%, (a) at least one saturated cycloaliphatic hydrocarbon having from 5 to 8 ring carbon atoms in the liquid phase and (b) an excess of oxygen gas or an oxygen containing gas mixture in the presence of (c) less than 1.5 moles of a solvent per mole of cycloaliphatic hydrocarbon (a), wherein said solvent comprises an organic acid containing only primary and/or secondary hydrogen atoms and (d) at least about 0.002 mole per 1000 grams of reaction mixture of a polyvalent heavy metal catalyst; and (2) isolating the C5-C8 aliphatic dibasic acid.

U. S. Patent 3,987,100 (Barnette et al.) describes a process of oxidizing cyclohexane to produce cyclohexanone and cyclohexanol, said process comprising contacting a stream of liquid cyclohexane with oxygen in each of at least three <BR> <BR> <BR> <BR> <BR> successive oxidation stages by introducing into each stage a mixture of gases comprising molecular oxygen and an inert gas.

U. S. Patent 3,957,876 (Rapoport et al.) describes a process for the preparation of cyclohexyl hydroperoxide substantially free of other peroxides by oxidation of cyclohexane containing a cyclohexane soluble cobalt salt in a zoned oxidation process in which an oxygen containing gas is fed to each zone in the oxidation section in an amount in excess of that which will react under the conditions of that zone.

U. S. Patent 3,932,513 (Russell) discloses the oxidation of cyclohexane with molecular oxygen in a series of reaction zones, with vaporization of cyclohexane

from the last reactor effluent and parallel distribution of this cyclohexane vapor among the series of reaction zones.

U. S. Patent 3,530,185 (Pugi) discloses a process for manufacturing precursors of adipic acid by oxidation with an oxygen-containing inert gas which process is conducted in at least three successive oxidation stages by passing a stream of liquid cyclohexane maintained at a temperature in the range of 140"to 200°C and a pressure in the range of 50 to 350 p. s. i. g. through each successive oxidation stage and by introducing a mixture of gases containing oxygen in each oxidation stage in an amount such that substantially all of the oxygen introduced into each stage is consumed in that stage thereafter causing the residual inert gases to pass countercurrent into the stream of liquid during the passage of the stream through said stages.

U. S. Patent 3,515,751 (Oberster et al.) discloses a process for the production of epsilon-hydroxycaproic acid in which cyclohexane is oxidized by liquid phase air oxidation in the presence of a catalytic amount of a lower aliphatic carboxylic acid and a catalytic amount of a peroxide under certain reaction conditions so that most of the oxidation products are found in a second, heavy liquid layer and are directed to the production of epsilon-hydroxycaproic acid.

U. S. Patent 3,361,806 (Lidov et al.) discloses a process for the production of adipic acid by the further oxidation of the products of oxidation of cyclohexane after separation of cyclohexane from the oxidation mixture, and more particularly to stage wise oxidation of the cyclohexane to give high yields of adipic acid precursors and also to provide a low enough concentration of oxygen in the vent gas so that the latter is not a combustible mixture.

U. S. Patent 3,234,271 (Barker et al.) discloses a process for the production of adipic acid by the two-step oxidation of cyclohexane with oxygen. In a preferred embodiment, mixtures comprising cyclohexanone and cyclohexanol are oxidized. In another embodiment, the process involves the production of adipic acid from cyclohexane by oxidation thereof, separation of cyclohexane from the oxidation mixture and recycle thereof, and further oxidation of the other products of oxidation.

U. S. Patent 3,231,608 (Kollar) discloses a process for the preparation of aliphatic dibasic acids from saturated cyclic hydrocarbons having from 4 to 8 cyclic carbon atoms per molecule in the presence of a solvent which comprises an aliphatic monobasic acid which contains only primary and secondary hydrogen atoms and a catalyst comprising a cobalt salt of an organic acid, and in which process the molar ratio of said solvent to said saturated cyclic hydrocarbon is between 1.5: 1 and 7: 1, and in which process the molar ratio of said catalyst to said saturated cyclic hydrocarbon is at least 5 millimoles per mole.

U. S. Patent 3,161,603 (Leyshon et al.) discloses a process for recovering the copper-vanadium catalyst from the waste liquors obtained in the manufacture of adipic acid by the nitric acid oxidation of cyclohexanol and/or cyclohexanone.

U. S. Patent 2,565,087 (Porter et al.) discloses the oxidation of cycloaliphatic hydrocarbons in the liquid phase with a gas containing molecular oxygen and in the presence of about 10% water to produce two phases and avoid formation of esters.

U. S. Patent 2,557,282 (Hamblet et al.) discloses production of adipic acid and related aliphatic dibasic acids; more particularly to the production of adipic acid by the direct oxidation of cyclohexane.

U. S. Patent 2,439,513 (Hamblet et al.) discloses the production of adipic acid and related aliphatic dibasic acids and more particularly to the production of adipic acid by the oxidation of cyclohexane.

U. S. Patent 2,223,494 (Loder et al.) discloses the oxidation of cyclic saturated hydrocarbons and more particularly to the production of cyclic alcohols and cyclic ketones by oxidation of cyclic saturated hydrocarbons with an oxygen-containing gas.

U. S. Patent 2,223,493 (Loder et al.) discloses the production of aliphatic dibasic acids and more particularly to the production of aliphatic dibasic acids by oxidation of cyclic saturated hydrocarbons with an oxygen-containing gas.

German Patent DE 44 26 132 A1 (Kysela et al.) discloses a method of dehydration of process acetic acid from liquid-phase oxidation of cyclohexane with air,

in the presence of cobalt salts as a catalyst after separation of the adipic acid after filtration, while simultaneously avoiding cobalt salt precipitates in the dehydration column, characterized in that the acetic acid phase to be returned to the beginning of the process is subjected to azeotropic distillation by the use of added cyclohexane, under distillative removal of the water down to a residual content of less than [sic] 0.3-0.7%.

PCT International Publication WO 96/03365 (Constantini et al.) discloses a process for recycling a cobalt-containing catalyst in a direct reaction of oxidation of cyclohexane into adipic acid, characterized by including a step in which the reaction mixture obtained by oxidation into adipic acid is treated by extraction of at least a portion of the glutaric acid and the succinic acid formed during the reaction.

U. S. Patent 5,221,800 (Park et al.) discloses a process for the manufacture of adipic acid. In this process, cyclohexane is oxidized in an aliphatic monobasic acid solvent in the presence of a soluble cobalt salt wherein water is continuously or intermittently added to the reaction system after the initiation of oxidation of cyclohexane as indicated by a suitable means of detection, and wherein the reaction is conducted at a temperature of about 50°C to about 150°C at an oxygen partial pressure of about 50 to 420 pounds per square inch absolute.

IJ. S. Patent 4,263,453 (Schultz et al.) discloses a process claiming improved yields by the addition of water at the beginning of the reaction, generally of the order of 0.5 to 15% relative to monobasic aliphatic acid solvent, and preferably 1 to 10% relative to the solvent.

U. S. Patent 3,390,174 (Schultz et al.) discloses a process claiming improved yields of aliphatic dibasic acids when oxidizing the respective cyclic hydrocarbons at temperatures between 130° and 160°C, while removing the water of reaction substantially as quickly as it is formed.

Lee, Glasgow, Erickson and Patel, Department of Chemical Engineering, Kansas State University, Manhattan, Kansas, 66506, Biotechnology Processes, pages 50-59, published by American Institute of Chemical Engineers, New York, NY, describe a mathematical model regarding airlift fermentation as compared to stirred tank fermentation.

None of the above references, or any other references known to the inventors disclose, suggest or imply, singly or in combination, the intricate and critical controls and requirements of the instant invention as described and claimed.

Our U. S. Patents 5,580,531,5,558,842,5,502,245,5,654,475, and our copending U. S. applications 08/477,195 (filed 06/07/95). 08/587,967 (filed 01/17/96), and published PCT patent application WO 96/07056, all of which are incorporated herein by reference, describe methods and apparatuses relative to controlling reactions in atomized liquids. Our copending U. S. applications 08/812,847, filed on March 6, 1997; 08/824,992, filed on March 27,1997; 08/859,985, filed on May 21 1997; 08/861,281, filed on May 21 1997.08/861,180, filed on May 21 1997; 08/861,176, filed on May 21 1997; 08/859,890, filed on May 21 1997; 08/861,210, filed on May 21 1997; 08/876,692, filed on June 16 1997; 08/900,323, filed on July 25 1997; 08/931,035, filed on September 08/932,875, filed on September 08/934,253, filed on September 19,1997; 08/989,910, filed on December 12,1997 ; 60/074,068 filed on February 9,1998; and 60/075,257, filed on February 19,1998, are all also incorporated herein by reference.

All of our following PCT patent applications, are also incorporated herein by reference: PCT/US97/10830, filed on June 23,1997 of Mark W. Dassel, David C.

DeCoster, Ader M. Rostami, Eustathios Vassiliou, and Sharon M. Aldrich, titled "Methods and Devices for Oxidizing a Hydrocarbon to Form an Acid" ; and PCT/US97/12944, filed on July 23,1997 of David C. DeCoster, Eustathios Vassiliou, Mark W. Dassel, Sharon M. Aldrich, and Ader M. Rostami, titled "Methods and Devices for Controlling the Reaction Rate and/or Reactivity of Hydrocarbon to an Intermediate Oxidation Product by Adjusting the Oxidant Consumption Rate."

SUMMARY OF THE INVENTION As aforementioned, this invention relates to methods of oxidizing hydrocarbons, such as cyclohexane for example, to respective intermediate oxidation products, such as adipic acid for example.

More particularly, in one aspect, this invention pertains a method of controlling the oxidation of a hydrocarbon to a respective dibasic acid in a reaction zone at a bulk temperature, the bulk temperature being within a desired bulk temperature range, the method comprising steps of : (a) removing reaction heat from the reaction zone by a cooling element having a cooling element temperature; and (b) controlling the cooling element temperature to be within a range of a low cooling element temperature under which solids are deposited on the cooling element and/or a second liquid phase forms; and a high cooling element temperature, lower than the bulk temperature and higher than the low cooling element temperature.

Preferably, the bulk temperature is maintained within the desired bulk temperature range, at least partially by controlling the cooling element temperature.

The cooling element may be at least partially within the reaction zone, or outside the reaction zone, or both.

The heat removal from the reaction zone is preferably controlled by flow rate of a liquid being used to control the temperature of the cooling element.

In a preferred embodiment, the hydrocarbon is cyclohexane and the dibasic acid is adipic acid. In this case, the bulk desired temperature is preferably in the range of 70°-130°C, and the low cooling element temperature is selected from the range of 50°-70°C In a related aspect, this invention pertains a method of controlling the oxidation of a hydrocarbon with oxygen to form an intermediate oxidation product in a reaction zone connected to a condenser, in which condenser, hydrocarbon vapors are at least partially condensed leaving behind condenser off-gases, the method comprising steps of :

(a) monitoring the concentration of oxygen in the condenser off- gases; and (b) controlling feed of oxygen into the reaction zone so that the concentration of oxygen in the condenser off-gases is lower than flammable oxygen concentration.

Preferably, the concentration of oxygen in the condenser off-gases is maintained lower than 90% by volume of the flammable oxygen concentration, and more preferably at a level of about 70% by volume of the flammable oxygen concentration.

Preferably, the hydrocarbon comprises a compound selected from a group consisting of cyclohexane, cyclohexanone, cyclohexanol, o-xylene, m-xylene, p- xylene, a mixture of at least two of cyclohexane, cyclohexanone, cyclohexanol, and cyclohexylhydroperoxide, and a mixture of at least two of o-xylene, m-xylene, p-xylene and a major portion of the intermediate oxidation product comprises a compound selected from a group consisting of adipic acid, cyclohexanol, cyclohexanone, cyclohexylhydroperoxide, phthalic acid, isophthalic acid, terephthalic acid, a mixture of at least two of cyclohexane, cyclohexanone, cyclohexanol, and cyclohexylhydroperoxide, and a mixture of at least two of phthalic acid, isophthalic acid, and terephthalic acid.

More preferably, the hydrocarbon comprises cyclohexane and the intermediate oxidation product comprises adipic acid. In this case the concentration of oxygen in the condenser off-gases preferably should not be allowed to exceed 10% by volume.

In a further related aspect, this invention pertains a method of controlling the oxidation of a hydrocarbon to form an intermediate oxidation product in a reaction zone, the method comprising a step of feeding oxygen and inert gas (preferably nitrogen) in the reaction zone, the inert gas and the oxygen having preferably a ratio of inert gas to oxygen in the range of 1: 5 to 1: 200, and more preferably in the range of 1: 10 to 1: 100.

The method may further comprise a step of recycling off-gases to the reaction zone at a recycle rate falling within a desired recycle rate range. The desired recycle rate range is preferably such as to result in a buoyant energy release in the reaction zone of preferably 2-40 hp/m gal, more preferably 5-20 hp/m gal, and even more preferably about 10 hp/m gal. In general it is preferable that the desired recycle rate range is such as to eliminate need for mechanical stirring in the reaction zone.

Preferably, the hydrocarbon comprises a compound selected from a group consisting of cyclohexane, cyclohexanone, cyclohexanol, o-xylene, m-xylene, p-xylene, a mixture of at least two of cyclohexane, cyclohexanone, cyclohexanol, and cyclohexylhydroperoxide, and a mixture of at least two of o-xylene, m-xylene, p-xylene and a major portion of the intermediate oxidation product comprises a compound selected from a group consisting of adipic acid, cyclohexanol, cyclohexanone, cyclohexylhydroperoxide, phthalic acid, isophthalic acid, terephthalic acid, a mixture of at least two of cyclohexane, cyclohexanone, cyclohexanol, and cyclohexylhydroperoxide, and a mixture of at least two of phthalic acid, isophthalic acid, and terephthalic acid.

Even more preferably, the hydrocarbon comprises cyclohexane and the intermediate oxidation product comprises adipic acid.

The above methods and aspects may be practiced exclusively or <BR> <BR> <BR> <BR> <BR> inclusively, i. e., any two or more of the above methods may be practiced in a single process for oxidizing a hydrocarbon to an intermediate oxidation product, although no two methods are necessarily combined in a single oxidation product.

The above methods may further comprise a step of reacting the intermediate oxidation product, which preferably is a dibasic acid, with a reactant selected from a group consisting of a polyol, a polyamine, and a polyamide in a manner to form a polymer of a polyester, or a polyamide, or a (polyimide and/or polyamideimide), respectively. The method may further comprise a step of spinning the polymer into fibers. Also, the polymer may be mixed with fillers and/or other additives and/or processed by methods well known to the art to form composites.

BRIEF DESCRIPTION OF THE DRAWINGS The reader's understanding of this invention will be enhanced by reference to the following detailed description taken in combination with the drawing figure, wherein: Figure 1 illustrates a block diagram of a preferred embodiment of the present invention, wherein the contents of the reactor are at least partially cooled by an internal cooling element (for example cooling coil).

Figure 2 illustrated another preferred embodiment of the present invention, wherein the contents of the reactor are at least partially cooled by an external cooling element (for example a heat exchanger).

DETAILED DESCRIPTION OF THE INVENTION As aforementioned, this invention relates to methods and devices for oxidizing hydrocarbons, such as cyclohexane for example, to respective intermediate oxidation products, such as adipic acid for example.

Therefore, this invention is exemplified by the case of oxidizing cyclohexane to adipic acid, but it also applies to the oxidation of other hydrocarbons to intermediate oxidation products.

In the practice of this invention, two independent variables (reactor operating pressure and nitrogen diluent feed rate) are very important, along with reactor system dependent variables, which include % oxygen conversion, reactor and condenser heat duties, oxygen concentration in the reactor off-gas, reactor size, off-gas recycle rate, and off-gas compressor energy requirement.

In general, it is believed by the inventors, that higher operating pressures (>1000 K Pascals) are favorable because they results in an about lOx smaller reactor size (due to attendant higher oxygen partial reaction pressure) and in an about lOx smaller off-gas purge stream (assuming zero nitrogen feed), also taking into account carbon dioxide dissolution effects. Very low nitrogen feed rates relative to oxygen feed (e. g., ratios of less than 0.1) are favorable because purge gas rates can be drastically reduced (by 100x), again taking into account carbon dioxide dissolution effects, and

oxygen requirement (for constant cyclohexane conversion) can be cut in about half- compared to ratios >3/1 (e. g., air). In the case of high pressure operation coupled with low nitrogen addition, oxygen conversions greater than 99% are possible. The entire system can be operated safely outside the flammable envelope (oxygen concentration preferably <90%, more preferably <80% and even more preferably <70% of flammable concentration by volume). Low pressure operation requires excessive reactor hold-up- time and size. Higher pressure operation requires that most energy of reaction is removed from the reactor (instead of the condenser). Off-gas recycle is highly preferable because it adds sufficient energy to the reactor, thereby eliminating the need for a reactor agitator. Among other disadvantages of an agitator or stirrer, there is the potential of sparking, and hence introduction of an ignition source. For safety purposes, high pressure operation also require the installation of a reactor with as large as feasible over-pressure design rating (e. g., at least a 1,000 psi to 2,000 psi design rating), the installation of a rupture disc on the reactor to protect against the failure scenarios most likely to lead to pressure excursions, design redundancies to insure diminimous potential (e. g., less than once per 20,000 years) for reactor rupture, and perhaps the installation of the reactor within a barricade to prevent loss of life in the event of such an unlikely rupture.

Further, in general, considering reactor size, compressor energy, oxygen conversion, purge gas rate, purge gas composition, purge gas scrubbing requirements, reactor liquid degassing requirements, and vessel pressure design requirements to prevent vessel rupture in the event of a deflagration, it would be preferable to maintain a pressure around 200 psia, but design the reaction chamber for a pressure rating of 1,000 psia to 2,000 psia. Assuming 0.3 hours hold-up-time, 15% vapor space and a 2/1 L/D ratio, the vessel would be 9 feet in diameter (225 million Ibs. per year adipic plant basis). However, in a case of 100 psia and 0.66 hours hold-up-time, the reaction chamber would be 11 feet in diameter. Both reactors when built for a design rating of 1,000 psia would require less than 4 inches wall thickness.

According to an example of this invention, oxygen concentration in the condenser off-gas is continuously monitored. It is controlled at a certain level

(preferably under 10% by volume, and more preferably around 8% by volume) by making minor adjustments to oxygen makeup feed to the reactor (alternately, adjustments to nitrogen feed rate). The off-gas recycle rate is adjusted to insure an adequate buoyant and jetting energy release to the reactor (for good mixing). System pressure is maintained at a constant level by venting purge gas through a pressure letdown valve. Reactor liquid feed (containing cyclohexane, recycled intermediates, acetic acid, optionally initiator and/or promoter, some water, and dissolved catalyst) is maintained at constant feed rate and composition. Reactor temperature is maintained constant (preferably in the range of 90° to 110°C) by means of heat removal from the reactor (e. g., internal coils) by controlling tempered cooling water temperature at constant flow (or by adjusting flow rate). Condenser outlet temperature is maintained constant by tempered cooling water temperature.

Reactor heat removal can be accomplished by means of internal coils or an external heat exchanger with liquid recirculation. Internal coils may perform this function. If internal coils are used, they offer several avantages: (1) Elimination of recirculation pumps.

(2) Easier heat transfer because--while in both alternatives the tempered cooling water must be fed at >70 degrees to prevent"frosting"--in the case of internal cooling the heat is transferred from a well-agitated liquid at a constant temperature of 100 degrees, for example, whereas in the external case the hot liquid comes in at one temperature, 100 degrees for example, and exits the exchanger at a lower temperature, 75 degrees for example, based on a 5 degree approach. Assuming that the cooling water exits at 80 degrees in both cases (hence same cooling water rate) and a 5 degree approach in the external case, the log mean delta temperature is 2.3x higher in the internal cooling case.

(3) No recirculation liquid de-gassing issues exist.

The use of an external exchanger instead of internal coils eliminate liquid recirculation patterns associated with the coil (these may be either good or bad), and may enable the use of a smaller sized reactor (minimum essential reactor volumes sufficient for higher pressure operation may be too small to accommodate requisite coil

areas). Cleaning issues of an external heat exchanger are generally less costly (downtime, etc.) as compared to the use of coils inside the reaction chamber.

Regarding flammability, the controlling factor (substantially independent of pressure and temperature) in the case that the hydrocarbon is cyclohexane, is oxygen concentration. Specifically, in the region of interest for the production of adipic acid by direct oxidation of cyclohexane the flammable oxygen concentration is-12 v%.

Therefore, it is highly preferable that the condenser off-gas concentration in all cases is controlled at 8% oxygen by volume.

The oxygen concentration in the reactor off-gas is always less than that in the condenser off-gas. This is believed by the inventors to occur principally due to cyclohexane condensation across the condenser, but more inclusively due to the condensation of volatiles in the reactor off-gas. Oxygen concentration in the reactor off-gas approaches that in the condenser off-gas when the system pressure is increased (due to lower cyclohexane concentration in the reactor off-gas), or when the condenser off-gas temperature is increased (less cyclohexane condensation). This belief of the inventors is an assumption, and should not limit the scope of the invention.

Therefore, by controlling the oxygen concentration in the condenser off- gas such that that gas is non-flammable, the reactor off-gas is necessarily also non- flammable.

Preferably, the oxygen concentrations in both the reactor off-gas and the condenser off-gas should be continuously monitored. The analyzer response should preferably be on the order of milliseconds.

The preferable reactor start-up, according to this invention, is conducted by first be pressurizing the reaction chamber with an inert gas to the desired operating pressure. The system pressure controller is on-line and set at nominal. The reactor is then inventoried with a certain amount of reaction liquid containing promoter and catalyst. Reactor temperature is increased to nominal by using heating water on the internal coils, for example. The recycle gas compressor is started and off-gas recycle rate is established at nominal. Nitrogen feed is established at some low nominal level.

Oxygen feed is established at a low rate and slowly ramped up to nominal condenser

off-gas concentration (but not allowed to exceed the safe non-flammable level). When reaction kicks off, oxygen feed rate is again increased so as to maintain a safe nominal concentration in the condenser off-gas.

A preferable way of shutdown or emergency shutdown is conducted by terminating oxygen feed. Oxygen feed and the recycle gas compressor are preferably interlocked"off'on either an"high-high"condenser off-gas analyzer result or an"high high"continuous reaction chamber off-gas analyzer result. Loss of off-gas recycle results in immediate cessation of reaction. Therefore, in order to maintain nominal reactor operating temperature, it is preferable to provide for fast acting reduction of tempered cooling water flow to the reactor cooling system. It should be taken into account that a sudden reduction of reactor temperature results in higher oxygen concentration in the reactor off-gas due to reduced cyclohexane volatilization. Activate of nitrogen"dump"on an interlock signal to the reactor vapor space, may also be utilized.

In a reactor system as described above (about 225 million lbs. per year adipic plant basis), the off-gas recycle rate should be preferably controlled to be of the order of 10 hp/m gal in the reaction chamber. It is beiieved by the inventors that this level of release is sufficient to eliminate the need for a reactor agitator with its attendant mechanical utility problems. However, energy releases of as high as 20 hp/m gal (or higher) may be required, or as low as 5 hp/m gal (or lower) may be adequate.

Carbon dioxide, produced as reaction by-product, can be effectively used as an inerting agent to help keep off-gases outside the flammable envelope. At higher operating pressures, however, a substantial amount of this by-product carbon dioxide may be purged from the system as a dissolved gas in the reaction liquid product stream.

The product stream must subsequently be de-gassed, and the de-gassed carbon dioxide carries cyclohexane with it, which has to be removed prior to venting to atmosphere.

These effects have been accounted for as aforementioned. It is worth noting that the addition of a small amount of diluent nitrogen is believed to substantially reduce the carbon dioxide dissolution effect.

Preferable conditions for the operation of a reactor system (assuming production of about 225 MM lbs. per year adipic acid), according to the instant invention, are the following, among others: (1) Oxygen is not to exceed 8 v% in the condenser off-gas.

(2) Buoyant energy release in the reactor is to be of the order of 10 hp per 1,000 gallons of reactor volume.

(3) Cyclohexane feed conversion is to be about 30%.

(4) Reactor H. U. T. is to be 1 hr if the partial pressure of oxygen in the reactor gas-phase is 4 psi, for example.

(5) Reactor H. U. T. is to equal to: (1 hour) times (actuai oxygen partial pressure in psi in the reactor gas-phase) divided by (4 psi), for example.

(6) Carbon dioxide generated to equal that purged in the purge gas stream plus that dissolved in the liquid product stream exiting the reactor.

General comments regarding this invention are: (1) Reactor Volume. While the reaction rate is directly and proportionally related to oxygen partial pressure in the reactor gas phase, it is also related to gas/liquid interfacial area in the same way, and it should be taken into account.

(2) Pressure Increase Effects. Increasing the pressure from about 250 to about 1100 KPascals (in cases with no nitrogen addition) has the following major effects: (a) The partial pressure of oxygen in the reactor off-gas is increased by about 1 Ox.

(b) Required reactor holdup time is reduced by a factor of about lOx, assuming constant interfacial area for constant buoyant energy release.

(c) Condenser duty as a percent of total heat of reaction energy removal is reduced by about 6x.

(d) Compressor energy is reduced by about 4x.

(e) Cyclohexane in the purge gas is reduced by about 4x.

(3) Minor Nitrogen Rate Increase Effects. Increasing the nitrogen feed rate to the reactor from zero to about 30 kg moles per hour (in cases where system pressure is about 1000 KPascals) has the following major effects: (a) The oxygen rate in the purge gas is increased by about 2x.

(b) The cyclohexane rate in the purge gas is increased by about 2x.

(c) The purge gas rate is increased by about 2x.

(d) Carbon dioxide purged in reactor liquid product as a percent of the total purge is decreased by about 3x.

(4) Major Nitrogen Rate Increase Effects. Increasing the nitrogen rate from zero to about 1500 kg moles per hour (i. e., equivalent to a molar ratio of nitrogen to oxygen of 3/1) has the following major effects (in cases where system pressure is about 280 KPascals): (a) Carbon dioxide purged in reactor liquid product as a percent of the total purge is decreased by about 50x to diminimous levels.

(b) The recycle gas rate (based on that required to supplement the feed gas to yield 10 hp per 1,000 gallons) is reduced to about zero.

(c) The purge gas rate is increased by about 60x to about 2,000 kg moles per hour.

(d) The oxygen rate in the purge gas is increased by about 60x.

(e) The cyclohexane rate in the purge gas is increased by about 50x.

Referring now to Figure 1, there is depicted a reactor device or system 10, comprising an oxidation or reaction chamber or reactor 12 containing an oxidation or reaction zone 14. The reactor device 10 is only partially shown for demonstrating the components necessary to exemplify the present invention. Miscellaneous unnecessary elements for illustration of the invention are not shown for purposes of brevity and clarity. The oxidation or reaction chamber or reactor 12 may be any type of reactor,

such as for example stirred tank reactor, atomization reactor, recirculation reactor, etc.

The oxidation chamber may also be supplied by a stirrer or agitator (not shown).

In this example, the reaction or oxidation chamber or reactor 12 contains a cooling element 16, such as a coil, for example. The cooling element, in other examples, may be partially or totally inside or outside the reaction chamber 12. The part being outside the reaction chamber 12, is preferably in the form of a heat exchanger 18, as shown for example in Figure 2. The coil 16 is connected to a cooler/heater 20 through inlet line 22, pump 24, and outlet line 26.

The reactor 12 is connected to a reactor off-gas line 28, which in turn is connected to a first oxygen analyzer 30, and a condenser/decanter 32.

The condenser/decanter 32 is connected to an off-gas splitter 34 and to a second oxygen analyzer 38 through a condenser off-gas line 36. The decanter part of the condenser/decanter 32 is connected to a polar phase line 42, and to the reactor 12 through a non-polar phase return line 40.

The splitter 34 is connected to the reactor 12 through off-gas return line 44, and to a chiller 46 through line 48. The splitter is further connected to a purge line 50 and to a condensate line 52.

The reactor 12 is connected to a liquid feed line 11, to an oxygen line 13, and to an inert gas line 15. The oxygen gas line provides a gas containing predominantly oxygen, while the inert gas line provides a gas containing predominantly an inert gas, such as nitrogen for example. Preferably lines 13 and 15 merge with line 44 to merge line 45. A temperature monitor 17 is connected to the reactor 12, and monitors the temperature of the reaction or oxidation zone 14 (bulk temperature). The reactor 12 is also connected to a liquid outlet line 19, which transfers the reaction products to other stations (not shown) of the reactor device 10 for further treatment.

As aforementioned, the cooling element may be partially or totally outside the reactor 12. One such example is illustrated in Figure 2. Liquid matter from the reaction zone 14'of the reactor 12'is circulated through line 56 and a heat exchanger 18 by pump 54. At the same time a pump 24'circulates an appropriately

cooled liquid by cooler/heater 20'through line 22', through the cooling part of the heat exchanger 18, and through line 26'.

The operation of the embodiment illustrated in Figure 1, will be better understood by using a specific example of raw materials and products. This approach, however, should not limit the scope of the instant invention to said materials and products. The scope of this invention should only be interpreted by the language of the claims and its equivalents.

A hydrocarbon. such as cyclohexane for example, catalyst, such as a cobalt salt for example, solvent, such as acetic acid for example, and an optional initiator, such as cyclohexanone or acetaldehyde, for example are fed to the reaction zone 14 of the reactor 12, through liquid feed line 11. Although the liquid feed line 11 is shown as a single feed line, it could be more than one lines, and it could include lines for recycled matter or other matter. The miscellaneous predominantly liquid moieties entering the reaction or oxidation zone 14 may be pre-mixed totally or partially in any desired combination before entering the reaction zone 14, or some of them or all of them may be introduced individually. In the reaction zone 14 of the reactor 12, all matter is mixed together either by a mixer or agitator (not shown), or by gases entering the reaction zone 14 through merge line 45, or both. It is preferable that an adequately large amount of gases enter the reaction zone 14 to provide all the mixing required, as earlier indicated. Mixers or agitators are preferably avoided because of mechanical problems or failures that may introduce.

The gases entering the reaction zone 14 of the reactor 12 are preferably introduced by the single merge line 45. However, they may be introduced individually or in any combination. In the example shown in Figure 1, recycled gases from line 44, a gas containing predominantly oxygen from line 13, and a gas containing predominantly an inert gas, such as nitrogen for example, are merged into line 45, and enter the reaction zone 14 of the reactor 12, preferably in the form of bubbles passing through the liquid mixture contained in the reactor 12. It is preferable that the gas entering through line 13 is oxygen, and the gas entering through line 15 is nitrogen, and more preferably air. The total oxygen to the total inert gas entering the reaction zone 14

through merge line 45 is preferably in the range of 1: 5 to 1: 200, and more preferably in the range of 1: 10 to 1: 100. This low ratio is beneficial and critical in reducing substantially the carbon dioxide dissolution effect at a later stage, especially at higher pressures, as aforementioned. Larger ratios introduce too much inert gas, such as nitrogen for example, thus causing an excess of gas which finally has to be purged from the system.

The reactor off-gases are directed to the condenser/decanter 32 through reactor off-gas line 28. The oxygen concentration is monitored in line 28 by the first oxygen analyzer 30, and if it exceeds a certain limit, it preferably cuts off the oxygen supply to the reaction zone. It may also cause other actions, such as for example, introduction of an abundance of nitrogen through a blanketing line (not shown) or through line 15, ceasing the liquid feed through line 11, ceasing recycling through line 44, completely shutting the system off, etc. Thus, the above mentioned limit should be lower (preferably in the vicinity of 20-40%. lower than the flammable or explosion oxygen concentration. In the case of production of adipic acid from cyclohexane, the concentration of oxygen at or over which an explosion may occur is about 12% by volume. Thus the limit in this case should be set preferably between 7.2% and 9.6% oxygen concentration by volume After condensation takes place in the condenser/decanter 32, the condenser off-gases are transferred to the splitter 34 through condenser off-gas line 36, in which the concentration of oxygen is monitored by the second oxygen analyzer 38. It is preferable that the analysis data from this oxygen analyzer are utilized to ensure that the system is outside the explosion region. Preferably if the oxygen concentration detected by analyzer 38 exceeds 90%, more preferably 80% and even more preferably 70% (by volume) of the flammable or explosion oxygen concentration, the oxygen feed through line 15 is reduced in a manner to ensure that the oxygen concentration in the condenser off-gases remains under but in the vicinity of the desired limit.

The splitter 34 splits the condenser off-gas to two streams. One stream passes through line 48, a chiller 46 in which further condensation of non-volatiles occurs, and then it is purged out of the system through line 50. The condensed non-

volatiles, such as for example cyclohexane, acetic acid, other hydrocarbons, etc., are preferably returned to the reactor 12. The second stream enters line 44 and it is recycled back to the reactor 12, as explained earlier.

During operation, the rate at which the condenser off-gases are recycled through line 44 is such as to produce preferably a buoyant energy release in the reaction zone 14 of 2-40 hp/m gal, more preferably 5-20 hp/m gal, and even more preferably about 10 hp/m gal. In general, it is preferable that the buoyant energy release in the reaction zone 14 is adequately high to eliminate the need for mechanical stirring or agitation.

The condenser/decanter 32 actually represents a condenser and a decanter in cooperation. The non-volatiles condensed in the condenser/decanter 32, are separated by the decanter part to an upper non-polar and to a lower polar phase. The upper non-polar phase is recycled to the reactor 12 through line 40, while the polar phase enters line 42 part or all of it may be returned to the reactor 12. If the design is such that it is desirable to return the totality of both polar and non-polar phases back to the reactor 12, the decanter part of the condenser/decanter 32 is not necessary.

However, in most occasions, it is preferable to control the amount of polar phase to be returned to the reactor 12, mainly for controlling the amount of water present in the reaction zone 14.

The bulk temperature of the liquids in the reaction zone 14 is monitored by the temperature monitor 17, and maintained within a desired bulk temperature range, which in the case of direct oxidation of cyclohexane to form adipic acid is preferably about 70°-130°C, and more preferably about 90°-110°C. The bulk temperature is maintained within the desired range by the cooling effect of the condenser 32 (if present), and by the cooling element 16. The cooler/heater 20 recirculates a fluid through the cooling element, such as a coil for example, and through lines 22 and 26, by using pump 24. It is critical that the temperature of the cooling element 16 is maintained within a range of a low cooling element temperature under which solids are deposited on the cooling element and/or a second liquid phase forms; and a high cooling element temperature, lower than the bulk temperature and higher than the low

cooling element temperature. In the case of direct oxidation of cyclohexane to form adipic acid the low cooling element temperature is preferably about 50°C, and more preferably about 70°C. Although the main purpose of the cooling element is to remove heat of reaction from the liquid mixture inside the reaction zone 14 during the oxidation, it is also used to heat up the liquid mixture at the start up of the reactor in a manner to bring the mixture to reaction temperature. The heat removal from the liquid mixture in the reaction zone 14 is controlled not only by the temperature of the cooling element 16 (and the temperature of the fluid inside the coil) but also by the flow rate of the fluid controlled by pump 24. The higher the flow rate and the lower the temperature of he fluid, the faster the heat removal from the liquid mixture inside the reaction zone 14, and therefore the lower the bulk temperature, if all other conditions remain constant.

The cooling element may be in the reactor as described above, or it may be outside the reactor, or both. Figure 2 illustrates one case that the cooling element is outside the reactor. In this case, liquid mixture from the reaction zone 14'of the reactor 12'is recirculated through the heat exchanger 18 by pump 54, and through lines 56.

The heat exchanger is driven by the cooler/heater 20', which pumps fluid through the heat driving portion of the heat exchanger 18 by pump 24'. and through lines 22'and 26'. The operation in this case is substantially the same as in the case that the cooling element is inside the reactor 12. The higher the flow rate and the lower the temperature of he fluid pumped by pump 24', the faster the heat removal from the liquid mixture of the reaction zone 14, and therefore the lower the bulk temperature, if all other conditions remain constant.

According to this invention, when 0, is the limiting reagent, the mole <BR> <BR> <BR> <BR> fraction °2 in the reactor off-gas is fairly small (e. g, less than say 10%), the reaction stoichiometry remains constant over the region of interest, cyclohexane feed rate and liquid concentration is kept constant, the cyclohexane conversion is not too great (e. g., less than 40%), the nitrogen addition rate is kept constant, and an increase in Cyclohexane conversion does not substantially effect a significant change in CO, contribution to the non-condensible reactor purge, and assuming that: --Hold up time is constant;

Cyclohexane conversion is proportional (i. e., a direct linear function of) to mole fraction O2 in reactor off-gas; -- The mole fraction of O2 in reactor off-gas is a direct linear function of the moles per unit time ofunreacted 0, -- The mole fraction of O2 in reactor off-gas is a direct linear function of the moles per unit time of unreacted 0,.

-- The O2 reacted is a direct linear function of the Cyclohexane reacted; and -- The non-condensible purge rate from the reactor (i. e., CO, by-product formation plus N, addition) is constant; then the surprising results are that: -0, conversion will be constant; Cyclohexane conversion will be a direct linear function of O2 feed rate.

-- Mole fraction O, in the reactor off-gas will be a direct linear function of °2 feed rate.

-- Moles per unit time of O, in reactor off-gas will be a direct linear function of °2 feed rate.

The practical implications bearing on continuous reactor operation are that: -- Mole fraction O, in reactor off-gas can be easily controlled by adjusting 02 feed rate; -- A 10% increase in 0., feed rate will increase mole fraction O2 in reactor off- gas by 10%, and will increase cyclohexane conversion by the same amount; For constant cyclohexane feed rate, the molar ratio of O2/cyclohexane is directly proportional to mole fraction °2 in reactor off-gas; -- A 10% increase in O2/Cyclohexane molar ratio will increase mole fraction O, in reactor offgas by 10%, and will increase Cyclohexane conversion by the same amount; -- The O2 conversion will be relatively constant regardless of changes in O2 feed rate (or 0,/Cyclohexane molar ratio); -- O2 conversion will be a inverse linear function of non-condensible (i. e., CO2 by-product formation plus N2 addition) reactor purge rate;

To increase 02 conversion, decrease nitrogen feed rate. The percentage increase will be proportional to the increase in nitrogen rate (best when the molarN/O, ratio is <0.1); and --If mole function O, in reaction off-gas is 10% below standard operating conditions (SOC) and Cyclohexane conversion is at SOC, increase mole fraction O, to SOC by increasing 0,, feed rates by 10% and maintain CHX conversion at SOC by increasing Cyclohexane feed rate by 10% (i. e., increase 0,, feed rate by 10% and keep 0,/Cyclohexane molar ratio constant).

It should be understood that according to the present invention, any liquids or gases or off-gases may be recycled totally or partially from any section to any other section, if so desired. Further, any combinations of the exemplifying matter, in part or in total, or any equivalent arrangements or any combinations of equivalent arrangements may be utilized. and are within the scope of the present invention.

Although miscellaneous functions are preferably controlled by a computerized controller, it is possible, according to this invention, to utilize any other type of controller or even manual controls and/or labor for controlling one or more functions. Preferred computerized controllers are artificially intelligent systems (expert systems, neural networks, and fuzzy logic systems, well known to the art). Of the three types of the artificially intelligent systems, the neural network, which is a learning system, collects information from different places of the device (for example pressure, temperature, chemical or other analysis, etc.), stores this information along with the result (pressure drop rate, reaction rate, reactivity, and the like, for example), and is programmed to use this information in the future, along with other data if applicable, to make decisions regarding the action to be taken at each instance. The expert systems are programmed based on the expertise of experienced human beings. The fuzzy logic systems are based on intuition rules in addition to expertise rules.

Oxidations according to this invention, are non-destructive oxidations, wherein the oxidation product is different than carbon monoxide, carbon dioxide, and a mixture thereof, such as adipic acid for example. Of course, small amounts of these

compounds may be formed along with the oxidation product, which may be one product or a mixture of products.

Examples include, but of course, are not limited to preparation of C5-C8 aliphatic dibasic acids from the corresponding saturated cycloaliphatic hydrocarbons, such as for example preparation of adipic acid from cyclohexane. Other examples include preparation of aromatic carboxylic acids such as benzoic acid, phthalic acid, isophthalic acid, and terephthalic acid, among others. Cyclohexanone, cyclohexanol, cyclohexylhydroperoxide, and mixtures thereof, are addition example of intermediate oxidation products.

Regarding adipic acid, the preparation of which is especially suited to the methods and apparatuses of this invention, general information may be found in a plethora of U. S. Patents, among other references. These include, but are not limited to: U. S. Patents 3,234,271; 3,36i, 806; 3,530,185 ; 3,987JCO; 4,032,569; 4,105,856; 4,158,739 (glutaric acid); 4,902,827: 5,221,800; and 5,321,157.

Dibasic acids (for example adipic acid, phthalic acid, isophthalic acid, terephthalic acid, and the like) or other suitable compounds may be reacted, according to well known to the art techniques, with a reactant selected from a group consisting of a polyol, a polyamine, and a polyamide in a manner to form a polymer of a polyester, or a polyamide, or a (polyimide and/or polyamideimide), respectively. Preferably the polyol, the polyamine, and the polyamide are mainly a diol, a diamine, and a diamide, respectively, in order to avoid excessive cross-linking. The polymer resulting from this reaction may be spun by techniques well known to the art to form fibers. Also, the polymer may be mixed with fillers and/or other additives and/or processed by methods well known to the art to form composites.

Examples demonstrating the operation of the instant invention have been given for illustration purposes only, and should not be construed as limiting the scope of this invention in any way. In addition it should be stressed that the preferred examples discussed in detail hereinabove, as well as any other examples encompassed within the

limits of the instant invention, may be practiced individually, or in any combination thereof, according to common sense and/or expert opinion. Individual sections of the examples may also be practiced individually or in combination with other individual sections of examples or examples in their totality, according to the present invention.

These combinations also lie within the realm of the present invention. Furthermore, any attempted explanations in the discussion are only speculative and are not intended to narrow the limits of this invention.