US2440234A - Process for oxidation of benzene to phenol promoted by added ethers - Google Patents

Description

APril 20 1948- R. H. KRIEBLE Er AL 2,440,234 f PROCESS FOR OXIDATION OF BENZENE TO PHENOL PROMOTEDBY ADDED ETHERS Fiied June 5, 1945 4 ww @ww Patented Apr. I20, 1948 PROCESS FOR OXIDATION OF BENZENE TO PHENOL PROMOTED BY ADDED ETHmS Robert H. Krieble, Schenectady, N. Y., and William I. Denton, Woodbury, N. J., assignors to Socony-Vacuum Oil Company, Incorporated, a corporation of New York Application June 5, 1945, sei-isi No. 551,658

This invention has to do with the production of 8 Claims. (Cl. Zitti-621) phenol by the oxidation of benzene and is more particularly concerned with that typev of process wherein aureaction mixture comprising benzene and oxygen or molecular oxygen-containing gas such as air is passed through a reaction zone void of solid `catalyst, under pressure and at elevated temperature.

'I'his application is a4 .continuation-impart of our copending application Serial No. 565,928, illed November 30, 1944, which, in turn, is a continuation-in-part of our application Serial No. 395,016, iled May 24, 1941, now abandoned.

We are 'aware of the fact that processes of this general character have heretofore been proposed, and in this regard, reference is made to the Bone et al. Patent No. 2,199,585 and the Moyer et al. Patent No. 2,223,383. Each of these patents discloses a process of the general character above' referred to, and each mentions that the process therein disclosed can be carried out by recycling. We have discovered, however, that although the process of Bone, forexample. if used with a relatively impure grade of commercial benzene, can be operated to obtain a. fair yield of phenol on a small number of passes in a recycling operation, the conversion to phenol rapidly decreases with successive cycles, thus materially restricting the scope of the process and rendering it impractical as a recycle operation. We have also observed that if a chemically pure (reagent grade) of benzene is subjected to the process of the Bone patent,

no substantial conversion to phenol is obtained.

Since the percentage conversion of benzene to phenol in an oxidationtprocess of the class described is relatively small, it is obviously essential 'from the standpoint of practicability that the operation be one in which the unconverted benzene can be continuously recycled with a substantially constant conversion and yield of phenol under a given set of operating conditions. It is the primary object oi this invention to provide such a process.

Ethers typified by diethyl ether are phenol and other oxidation products. The ethers, which form the subject matter of this invention, may be aliphatic, aromatic, cyclic or terpenic, or their substituted analogs. Representative ethers l which have Ibeen found to be effective herein are diethyl ether, Chlorex (Ay-dichiaro ethyl ether), dioxane and ascaridole.

This invention is predicated upon the discovery that a minor proportion of an ether added to benzene substantially improves the conversion to. and yield of, phenol from benzene under the conditions of reaction in the process contemplated herein, over the conversion and yield obtained without said ether. The present invention is also predicated upon the related discovery that if a minor proportion of an ether is continuously in..

` troducecl into the stream of benzene entering the reaction mixture, the conversion to, and yield of, phenol from benzene is appreciably increased over the conversion and yield obtained without said zene and air, or other oxygen-containing gas, is

thoroughly mixed and pre-heated to the desired temperature and passed through a reactor tube which has beenpre-heated and which for the purpose of pre-heating and dissipating the heat of the exothermic reaction is immersed in a suitable heat-transfer bath, such as salt. The reaction mixture, as will hereinafter be explained, Ais preferably maintained under pressure. With properly regulated conditions a part of the benzene is thus oxidized to phenol. This phenol and the gaseous and high-boiling products of oxidation are then separated from the benzene. which is subsequently recycled through the mixer-preheater and reactor, Additional air is of course added prior to the admission of the :benzene to the pre-heater-mixer, and in a continuous operation makeup benzene is also added to replace the benzene converted. K

In such an operation we have found that where a commercial grade of benzene, such'as 90% Benzol," is used as the charging stock and is recycled through the reactor with no additions of make-11D view that the mechanism 3 make-up the benzene converted, the reactivity and conversion to phenol is not markedly lncreased. However, in an operation of the recycle type just described, if a minor proportion oi' an ether,l such as diethyl ether. is continuously added to the charge of benzene, a steady conversion performance may be obtained and the reactivity, as indicated by the per cent conversion and yield of phenol at a given temperature, is greatly increased. We have found that the reactivity in a process of the class described under a given set of conditions may be greatly increased by the incorporation in the reaction mixture of a minor proportion of an ether.

As indicated above, no substantial conversion of a. chemically pure (reagent grade) benzene to phenol takes place in the process contemplated herein. The results provided below in Table 1 serve to illustrate the improvement realized by the use of an ether in the oxidation of benzene to phenol. The single-pass runs were made with' reagent grade benzene in a stainless steel tube 15' long and 0.12" inside diameter, at 1000 pounds per square inch and at a reaction time of about 35 seconds with the approximate mixture BCeHs-l-Oz-l-lNz High boiling products obtained in the runs are designated as H. Bs." in the table.

It is readily seen that small, and almost negligible, conversions of benzene to phenol areefiected when reagent grade benzene is used alone; and, where typical ethers are used, appreciable conversions are effected. Although the conversions with' these preferred inductors are only of the order of about 2%, such conversions may be maintained in the continuous operation by maintaining the proper Inductor concentration in each pass.

Although we do not wish to be bound by any theory as to the mechanism of this reaction or the part whichth ether plays in increasing the reactivity and conversion to phenol, it is our as represented by the following equations will explain most of the observed facts:

CQHIOH (b) Reaction I is slow and determines the rate. Reactions II and III, since they involve radicals, should be extremely rapid. Reaction III is shown as a competition of phenyl and hydroxyl radicals for phenyl and is probably over simplified. The essential fact is that in the presence of a material which can be more readily dehydrogenated by O2 than benzene.- H0z is produced independently yof reaction I and at a lower temperature. It is then capable oi' reacting rapidly with bell- 4 zene by Reactions II and III to produce the usualproducts.'

Under these conditions, far more phenol survives, less diphenyl is formed, and the yield is greatly improved.

According tothe foregoing mechanism, then. an inductor or promoter for use in a process of th'e type contemplated herein is any compound which is a betterhydrogen donor than benzene under 4the conditions of the reaction'zone; or, stated in another way, a promoter or Inductor may be denned as a compound containing hydrogen and possessing 'the properties of being in the gaseous phase under the conditions of the reaction zone, and under said conditions, reacting with oxygen to form water at a lower temperature than the temperature at which benzene so reacts with oxygen.

'Of the ethers contemplated herein, diethyl ether and chlorex are particularly preferred.

As stated hereinabove, the promoters contemplated by the present invention are better hydrogen donors than benzene under the conditions of the' reaction zone. In other words, the promoters in our process are hydrogen-containing materials which release hydrogen more readily than benzene under the conditions of reaction. 'I'he rates of hydrogen release from numerous materials are available in the chemical literature and may be resorted to in selecting effective promoting materials. From such data we have derived potential hydrogen donor ability (P. H. D. A.). values for various materials and have found those-materials havingvalues above a certain wherein K is the speciic velocity constant; C is minimum are effective in our process. The P, H.

AD. A. value relationship thus provides a means whereby a chemist skilled in the art may readily determine, with a high degree of certainty. whether or not a material will be an eiective promoter. A

'I'he P. H.`D. A. values obtained for various materials are derived in the following manner. Hydrogen atoms are classied here into four categories: primary, secondary, tertiary and hydrogens attached to an oleilnic bond or an aromatic ring. No attempt li'as been made to evaluate bonds other than the carbon-hydrogen bond. With this as a starting point and taking activation energy values from the chemical literature, it is possible to evaluate relative ratios of release of the different hydrogen atoms from hydrogencontaining materials. Average activation energy values taken from 'I'he Aliph'atic Free Radicals (F. O. Rice and O. K. Rice,- Johns Hopkins Press;

1935) are as follows:

Aterge Difference in Type of Hydrogen Atom Enragon Activation Kilocalories Energy Oleilnlc (Aromatic) 103 0 Primary 97 6 Relative rates of hydrogen release under conditions typical of our process, as at 427 C., are

calculated from the well known relationship:

E K=0e RT a constant; E represents activation energy (in calories); R is a constant (1.987 calories); and T is absolute temperature (in degrees Kelvin). The relative rate of a primary hydrogen atom as vconstant value has 5 i Y compared with an oleiinie hydrogen atom is calculated as follows:

am Kaw?" im. im)

log Kay-F4364 In this manner it ii found that the several hydrogen atoms have the following K values at -Olennlc (aromatic). 1

Secondary 215.

Tertiary 12st The foregoing Karo. values indicate that a primary hydrogen atom is released 'I3 times faster than an oleflnic or aromatic hydrogen atom. Simplifying this (0l value to an oleiinic or aromatic hydrogen drogen atoms. Hydrogen atoms attached to carbon atoms which are in the alpha position -to an olennic bond or some electronegativ'e group such at keto (C=O) ether (-0-) or ester ('-COOR) are released from the molecule at an increased rate. It is dimcultto evaluate this eiIect since it depends largely upon the structure oi. the molecule. It is recognized that diilerent substituent groups such as olefin, ketone. etc., groups may activate hydrogen atoms to different degrees. For convenience, however. a

been assigned to such activation. vAccordingly,'whenever a primary hydrogen is in an activated position, it is assigned a value equivalent to a normal secondary hydrogen. Similarly, atom is assigned a value equivalent to a normal tertiary hydrogen atom; and for an activated tertiary hydrogen atom, the normal rate is doubled. The P. H. D. A. of a compound, on a weight basis. is the sum of all the evaluated hydrogen atoms multiplied by a factor of 'ten and divided by the molecular weight of the compound.

From the Km o. values of 0. 1, 3 and 15 shown above. the P. H. D. A. values of hydrogen-containing materials are easily calculated. By way of illustration, diethyl ether has six primary and four activated secondary (alpha to an olefinv bond) hydrogen atoms: therefore, diethyl ether has a P. H. D. A. value, on a weight basis of y 9.0

.process as promoters are in substantial agreelment with the' experimental results` obtained.

However. in the case of first members of the various chemical classes which often behave differently than all other members of their classes.

relationship by assigning a zero ester, ether,

an activated secondary hydrogen 6 somediscrepancies obtain between the RED. A. In general.

values, on a weight basis.' of are ineffective. and those greater than about 2.5 are effective: however. those having values greater than! are generally oharactcrld by a high degree of effectiveness. 'Ihis relationship is shown in the following tabu- 10 lation:

l0 As will be readily apparent benzene and lnimma. 0mm t (weight) tetra ll-ethyl. 8propyl wolein turpentias taken as pinene) ath lnnth diauliide the art. the apparatus used in carrying out a lprocess of the type contemplated herein may take various forms. In the accompanying drawings. however, we have shown diagrammatieally one form of apparatus which may be satisfactorily used in carrying out an operation for the continuous conversion of benzene to phenol. As will hereinafter appear, the conditions of operation embody a number of variables which may be changed with respect to one another over relatively wide limits, and no attempt will be made in describing the apparatus to take account of these possible chanses in variables.

Referring now to the drawing, reference numerals IlV and il indicate conduits which carry are connected through suitable valves I2 and I2' with the inlet I3 of a pump Il. vThe pump Il delivers the benzene through a conduit li to a T-connection Ii, where it is introduced into a mixing conduit i1 leading to the coils I U of a mixer and pre-heater mounted in an insulated case I! which is lled with a suitable heat-exchange medium such as Dowthermf Reference numeral pressor which discharges into pipe 2| connected through the connection 22 to a compressed-air storage reservoir 23. The pipe 2l discharges through a pressure-reducing valve 24 and an orifice now-control 2B into the conduit I1through which the airand benzene-reaction mixture is conducted to the mixing and pre-heating coil il. The inductor or promoter is introduced into the reaction mixture at any suitable point, preferably in the benzene stream, through a conduit as indicated by nunerall 28, such conduit being provided with a metering valve 29 to control the quantity of an inductor. such as diethyl ether, which is introduced.

The pre-heating and mixing coll II, wherein the benzene-promoter-alr mixture is initimately mixed and pre-heated to a. temperature below 20 indicates an air-comr trated. the liquid products transfer bath, such as a fused salt bath. capable of maintaining a close temperature control, such bath being contained in the insulated case 42, which is provided with an inlet conduit 43 having a pump 44, the inlet 45 of which connects with a heat-exchanger 48 which can be used to raise the temperature of the salt bath for initiating the reaction and, after the exothermic reaction y has started, can be used to dissipate the heat of reaction and maintain a constant temperature. The heat exchanger 46 receives the heat-cxchange medium from the tank 42 through a discharge conduit 41.

In the form of the apparatus shownvin the drawing. the reactor tubes are illustrated as being U`shaped. and the discharge portion 4I' of the respective reactor tubes connect with a header conduit 50 which discharges into the coil 5 I of a heat-exchanger 52. For the thermal balance of the process this heat-exchanger 52 is shown as being connected through conduits 53 and 54 with the mixer and pre-heater I9 so that the heat-exchange medium is circulated by means of pump 55 from the bottom of the mixer and preheater. I9 to the bottom of the heat-exchanger 52 and back to the mixer and pre-heater through the pipe 53. y

The cooled reaction mixture containing the phenol and other products of the reaction discharges from the heat-exchanger 52 through pipe 56-58' and filter 51 into a high-pressure mist-breaking tower 58 having a high-pressure gas-discharge valve 59 which may lead to a turbine. Discharge valve 59 is controlled by orifice ow-control 25 to maintain a constant flow of air in pipe I 1. The liquid product accumulating in the bottom of the high-pressure tower 58 is conducted through a pressure-reducing valve 60 into a lowpressure-packed tower 6I provided 62 to release gaseous prod- This liquid product, which is a mixture of phenol, benzene and hig -bilers," is delivered to the benzene-recovery still 62; In the still, as illuspass through a preheater 63 and are discharged through the discharge pipe 63' intothe bottom of thestill 62, where the benzene is distilled oil' by a steam coil reboiler 64. The still 62 is shown as being equipped With bubble plates 65 and a water coil reflux-condenser 85'.V` The benzene vapors are discharged through conduit 58 into a benzene condenser 66', from which the liquid product discharged through conduit 61 is pumped by means zene conduit Il'.

The bottom of the benzene-recovery still `62 is provided with a discharge conduit 1 0 through which the mixture of phenol and high-boilers is pumped into a vacuum still 1I. wherein the phenol is distilled oil by means of the steam reboiler coil 12 into the phenol-discharge conduit pump 8|. l been previously pointed out, the process contemplated herein embodies a number of as to the effect of these variables, each of them will now be discussed individually.

'I'he proportion of air that can be used t Preferred operating actor tubes, for example, include a benzene to oxygen ratio of from 2.5:1 to 8:1, and particularly a ratio of 4:1. In terms of air then, the most preferred ratio is 4CaHa:1Oz:4N2.

Reaction time value, there is a high-boilers and a corresponding decline in As preferred, the reaction time should fall within the range of 2 seconds to about 100 seconds.

The optimum reaction time is about 8.5 seconds under the following preferred operating conditions:

charge Benzene havihg added thereto about 1% (by volume) of diethyl ether.

Reactor 0.36 inch internal diameter v stainless steel tube 60 feet long. l

MlXture 4CsH-f--l-4Nz Pressure 750pounds per square inch. Temperature--- '175 F.

Pressure We have vobtained phenol in good conversion perature, other conditions'being the same. Thus,

pounds per square-inch temperatures of the order of 1200 F. are required, at 2000 pounds pressure good 'results have been obtained at 675 F. Since ferrous metal surfaces strongly catalyze total combustion at temperatures above 1000 F., it is" necessary to use inert surfaces such as glass, enamel, silica. etc..

conditions with 0.36 inch4 in the reactor when operating at low pressures and the concomitant high temperatures. In this connection, good results have been obtained in nickel tubes at a pressure of 350 pounds per square inch where the optimum reaction temperature was about 1030* F., and probably still lower pressures and higher temperatures could have been successfully used. With iron tubes, sufficient pressure to keep the reaction temperature below 1000 F. is required, usually at least 50o-1000 pounds per square inch. However, at 1000 pounds pressure,- the use of nickel tubes instead of iron or stainless steel tubes presents no advantage.

The use of high pressure has certain fundamental advantages, however. At low pressures, even though a glass-lined reactor be used, the loss to oxides of carbon is materially higher than at high pressures in stainless steel equipment. Furthermore, the minimum critical reaction time` apparently increases with decreasing pressure; and, since for a constant reaction time the rate of feed varies directly with the pressure (by deflnition), the maximum permissible rate of throughput falls oil! very rapidly with decreasing pIBSSule.

There is also an upper useful limit of pressure. As the pressure is raised, the temperature required to initiate reaction decreases, as explained above. Furthermore, the boiling point of benzene is raised until the critical temperature of liquid benzene (550 F.) is reached. where it remains unchanged as the pressure is further increased. Since itis advantageous to mix thoroughly the benzene vapor and air before initiating reaction, in order that local regions of undesirably high oxygen-concentration be eliminated. it is not desirable to indenitely lower the reaction temperature by the use of extremely high pressures. There is a relationship between the maximum useful pressure and the internal diameter of the reactor tube, since we found that the smaller the internal diameter of the reactor tube, the higher the temperature necessary to initiate reaction. In a 0.36 inch internal diameter reactor tube, 1500 pounds per square inch appears to'be about the upper useful limit, `and we prefer a pressure of about '150 pounds per square inch. In general', however. pressures within the range of about 350 pounds to about 1500 pounds per square inch serve the purposes of this invention.

` Temperature T-heV optimum reaction temperature can be varied from 600 F. to 950 F. by changing the operating conditions, among the most effective of which are pressure, promoter concentration and internal diameter of the reaction tube. As indicated above in the discussion of reaction time. a temperature of about 775 F., is particularly preferred under certain well-defined conditions.

Reactor tube enamel, glazed porcelain. or silica, is imperative. At moderate pressures (several hundred pounds per square inch), nickel is satisfactory. At pressures in the neighborhood of 1000 pounds per v square inch, low-carbon steel and stainless 'steel are satisfactory, and nickel prei' its no advantage.

As to the internal diametr if the reactor tube (4l-4V) successful operation has been obtained in tubes varying from 0.086 inch to 0.875 inch in internal diameter. For iron tubes, the smaller the internal diameter the higher the required pressure. We prefer to use a tube having an internal diameter in the neighborhood of 0.36 inch and to operate with pressures in the range of from 350 to 1500 pounds per square inch. Smaller tubes are equally satisfactory, but they require higher operating pressures. For a tube having an internal diameter of 01.086 inch, pressures of from 2000 pounds to 3000 pounds per square inch are recommended. Y

. The length of the reactor tube (4i-4 i apparently determines the rate of throughput, and the existence of a critical minimum reaction time for a tube of fixed length and internal diameter has already been discussed. Satisfactory results in the operation of a process of the class described have been obtained with tube lengths of feet. and' tubes of at least this length are recommended.

Mier-pre-heater tube As to the mixer-pre-heater tube (i8) any material having suitable mechanicaly properties such as stainless steel is satisfactory.` The internal diameter of this tube should be sufficiently small to prevent reaction taking place therein, and the tube should be sumciently long to provide the heating surface necessary for a heat-exchange that will maintain the-feed temperature within F. to 50 E. of the temperature of the preheater bath. The temperature of the pre-heater bath should be such that the reaction mixture discharged from the pre-heater coil i8 is at a temperature of about -150" F. below the temperature maintained in the reactor bath in the case 42.

Inductor The required properties oi the inductors and the theory of their action. together with certain preferences therein to diethyl ether and chlorex and the like,y having already been discussed.

The concentration of the inductor may be varied depending upon the character of the make-upstock, the conditions of operations, etc. With diethyl ether. which is a preferred inductor, we have found that this concentration should be maintained in the neighborhood of from about 0.2 per cent to about 3.0 per cent, by volume, based upon the quantity of benzene in the fresh i'eed. We prefer, however, to use a concentration of 0.5 per cent to about 1.5 per cent, by volume, in the total feed. and have found that increasing the concentration of the inductor greatly lowers the optimum reaction temperature.

It will be seen from the foregoing discussion that a process of the type contemplated herein vis susceptible of numerous operating conditions through manipulation of the variables discussed;

and the present invention is not concerned with any particular set of operating conditions, but as aforesaid, is predicated upon the discovery that greatly improved results and a continuous recycling operation may be obtained by introducing surfaces of `the reactor tubes, such as glass, into the reaction mixture an inductor or promoiing compound of the type hereinabove discussed. It is to be understood, therefore, that although we have described and illustrated a specific form of apparatus and have discussed in considerable detail various reaction conditions which may be employed in the operation of such apparatus, the invention is not limited to this apparatus or to any particular set of operating conditions, but includes within its scope such changes and modifications as fairly come within the spirit of the appended claims.

We claim:

l. In a method for the continuous manufacture of phenol from benzene wherein a reaction mixture ofbenzene vapor and oxygen-containing gas is passed under pressure through a reaction zone void of solid catalyst and maintained at a temperature of between about 600 F. and about 950 F., to convert a part of the benzene to phenol and other oxidation products, and wherein unconverted benzene is separated from the phenolv and other oxidation products and is returned for recycling in the reaction mixture; the improvement which comprises: continuously incorporating in the benzene so rezone, a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume, of diethyl ether.

2. In a method for the continuous manufac- .25 cycled, prior to its admission to the reaction" ture of phenol from benzene wherein a reaction mixture of benzene vapor and oxygen-containiing gas is passed under pressure through a reaction zone void of solid catalyst and maintained at a temperature of between about 600 F. and about 950 F., to convert a. part of the benzene to phenol and other oxidation products, and whereinunconverted benzene is separated from the phenol and other oxidation products and is returned for recycling" in the reaction mixture;

the improvement which comprises: continuously incorporating in the benzene so recycled, prior to its admission to the reaction zone, a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume, of dioxane.

3. In a method for the continuous manufacture of phenol from benzene wherein a reaction mixture of benzene vapor and oxygen-containing gas is passed under pressure through a reaction zone void of solid catalyst and maintained at4 a temperature of between about 600 F. and about 950 F., to convert a part of the benzene to phenol and other oxidation products, and wherein unconverted benzene is separated from the phenol and other oxidation products and is returned for recycling in the reaction mixture;

the improvement which comprises: continuously incorporating in the benzene so recycled, prior to its admission to the reaction zone, a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume, of beta, beta'dichloro diethyl ether.

4. In the method of making phenol `wherein ether, beta, betr-ammore diethyl ether, and di` oxane, in an amount falling within the range of from about 0.2 per cent to about 3.0 per cent by volume based on the quantity of benzene in said reaction mixture.

5. In a method for the continuous manufacture of phenol from benzene wherein a reaction mixture of benzene vapor and oxygen-containing gas is passed under pressure through a reaction zone void of solid catalyst and maintained at an elevated temperature to convert a part of the benzene to phenol and other oxidation products, and wherein unconverted benzene is separated from the phenol and other oxidation products and is returned for recycling in the reaction mixture; the improvement which comprises continuously incorporating in the benzene so recycled, prior to its admission to the reaction zone, an ether selected from the group consisting of diethyl ether, beta, betadichloro diethyl ether, and dioxane, in an amount falling within the range of from about 0.2 per cent to about 3.0 per cent by volume.

6. In the method of making phenol wherein a reaction mixture of benzene vapor and oxygencontaining gas is passed under pressure through a reaction zone void of solid catalyst and maintained at a temperature of between about 600 F. and about 950 F.; y the step of incorporating in said reaction mixture diethyl ether in an amount falling within the range lof from'about 0.2 per cent to about 3.0 per cent'by volume based on the quantity of benzene in said reaction mixture.

7. In the methodfof making phenol wherein a reaction mixture of benzene vapor and oxygencontaining gas is passed under pressure through a reaction zone void of solid catalyst and maintained at a temperature of between about 600 F. and about 950 F.; the step of incorporating in said reaction mixture beta, beta'-dichloro diethyl ether in an amount falling within the range of from about 0.2 per cent to about 3.0 per cent by volume based on the quantity of benzene in said reaction mixture.

8. In the method of making phenol wherein a reaction mixture4 of benzene vapor and oxygen-l containing gas is passed under pressure through `a reaction zone void of solid catalyst and main-

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