DE896652C - Process for the production of phenol - Google Patents

Description

Process for the preparation of phenol The invention relates to a process for the production of phenol by the oxidation of benzene with oxygen or Oxygen-containing gases such as air. The procedure is done without the use of solid Catalysts carried out under pressure and at elevated temperatures.

Such processes have already been proposed, which in the cycle, but work with low yields and their yield in the course of the process decreases.

The process also works in a cycle in contrast to the previous one practically applied procedures; however, the yields remain constant achieved on phenol.

The method of the invention is based on the knowledge that a small Addition of a substance that gives off hydrogen more easily under the reaction conditions as benzene, for example cyclohexane or cyclohexanol, to the reaction mixture Substantially improved phenol yield. It consists in a mixture of Benzene vapor, an oxygen-containing gas and a small proportion of a hydrogen-containing gas organic compound that is in the gas phase under the reaction conditions and under these conditions gives off hydrogen more easily than benzene a pressure greater than about 35 kg / cm2 and a temperature between about 343 and about 663 ° is passed continuously through a reaction zone.

The phenol obtained and the high-boiling by-products of the oxidation are separated from the unconverted benzene. The benzene returns with the addition of more oxygen back into the circuit. Technical benzene, d. H. A large amount of benzene quickly loses its ability to oxidize in the circulation. if on the other hand, if small amounts of cyclohexane are continuously added, one obtains a significantly increased yield of phenol. For example, results in a cycle with 6% freshly added benzene a conversion into phenol of 0.1% of the total batch, while with the addition of 100% cyclohexane a conversion of 2 percent by weight phenol is obtained.

Instead of cyclohexane, any other compound can be used, which gives off hydrogen more easily than benzene, gaseous under the reaction conditions and forms water with oxygen at a temperature lower than the temperature at which benzene reacts with oxygen.

Cyclohexane is particularly useful because it reacts in benzene converted, thereby avoiding any contamination of the reaction mixture. Furthermore, the substances can serve as reaction accelerators, which are partially Let hydrogenation of benzene win. Other accelerators are: cyclohexanol, Cyclohexene, tetrahydronaphthalene, 2-ethyl-3-propyl-acrolein, octaldehyde, n-hexane, Diethyl ether, aniline, turpentine, n-hexes and cracked gasoline. All of these substances contain carbon and hydrogen, some of them also contain oxygen and nitrogen. Cyclohexanol has the same advantages as cyclohexane, but is considerably more expensive. The hydrogenated aromatic compounds, such as cyclohexane, cyclohexene, are preferred Tetrahydronaphthalene and the olefins, their typical representatives n-hexene and cracked Are gasoline.

The amounts of hydrogen given off by these substances can be found in the chemical literature. In literature, the ability the release of hydrogen (potential hydrogen donar ability) with P.H.D.A. designated, and it has been found that substances with values above a certain minimum are useful are. The P.H.DA. value is therefore a means of predicting with some certainty to determine whether a substance is effective as an accelerator in the present process is or not.

In general, substances with a PHDA on a weight basis of o to about 2.5 are ineffective and substances greater than 2.5 are effective. Substances with a value greater than 4 are generally characterized by a high degree of effectiveness. The PHDA values of various compounds are shown in the table below. PHDA Connection value (on weight Base) Carbon disulfide. . . . . . . . . . . . . . O Thiophene ....................... o Pyridine ........................ o Ethyl alcohol .................... 2.0 Acetone ......................... 2.0 Toluene .... .................... 1, o Xylene .......................... 1.7 Isopentane ...................... 4.2 PHDA: Connection value (on weight Base) n-hexane ....................... 3.5 n-Hexen-i ...................... 5.4 Cyclohexane ..................... 4.4 Cyclohexene ..................... 5.8 Cyclohexanol ................... 5.4 Cyclohexanone ................... 8, o Tetrahydronaphthalene ........... 5.5 Ethyl ether .......... ............ g, o 2-ethyl-3-propyl-acrolein. . . . . . . . . 8, o Turpentine (as a-pinene) .......... 8.9 The drawing shows a system for carrying out the method.

Through the lines ii and ii 'through the valves 12 and 12' with are connected to the inlet 13 of a pump 14, benzene is supplied. The pump 14 promotes the benzene through a line 15 to a T-piece 16 and further into one Mixing line 17, which leads to the coils 18 of a mixer and preheater ig, which is filled with a heat exchange fluid.

An air compressor 2o conveys compressed air through a line 21 or 22 into a storage container 23. Via a pressure reducing valve 24 and a control nozzle 25, the compressed air flows into line 17, through which the air-benzene mixture to Preheater ig got. The accelerator is placed in a suitable place, preferably into the pure benzene stream through a line 28 which is provided with a valve 2g is provided for quantity control.

The accelerator can be obtained by hydrogenating part of the recycled benzene. For this purpose, a valve 30 is provided in the benzene line 15, and through a discharge line 31 with a valve 31 'upstream of the valve 30 , part or the entire amount of the benzene stream is fed into a hydrogenation chamber 32, from which the partially hydrogenated benzene is carried through the return line 33 Valve 33 'flows back into line 15 behind valve 30.

The above-mentioned hydrogenation chamber contains a suitable catalyst, z. B. nickel, and is kept at an elevated temperature of about 121 to 2o4 °. Hydrogen is introduced through a pipe 37 via a measuring apparatus 38.

From the preheater ig a distribution line 40 leads to a number of Reaction tubes 41. These are located in a bath of a molten salt, whereby a good temperature control is possible. A line goes from the bath tank 42 43 with pump 44 to the outlet 45 of a heat exchanger 46, which at the start of the reaction to increase the temperature of the salt bath and during the exothermic reaction to Cooling, d. H. Maintaining a constant temperature, is used. To the Heat exchanger 46 is supplied with the salt from container 42 through line 47.

The reaction tubes 41 are U-shaped and their return lines 41 'are connected to a manifold 50 which is in the coil 51 of a heat exchanger 52 flows out. This heat exchanger 52 is through for the heat balance of the process the lines 53 and 54 connected to the preheater ig so that the heat exchange fluid with the aid of the pump 55 from the preheater ig to the heat exchanger 52 and back.

The cooled reaction mixture, which contains the phenol and other products of the reaction, passes from the heat exchanger 52 through the line 56, 56 'with filter 57 into a high pressure tower 58 for removal of the gaseous components via a gas valve 59, from which the gases are fed into a turbine can flow. The gas valve 59 is regulated by the control nozzle 25 so that a constant air flow in the line 17 is maintained. The liquid products collect at the bottom of the high pressure tower 58 and are guided through a pressure reducing valve 6o into a low pressure tower 61 which is provided with an outlet 62 containing a valve for discharging the remaining gaseous products. The liquid products that collect at the bottom of this tower 61 are discharged through line 61 '. These liquid products, consisting of phenol, benzene and high-boiling substances, go via a preheater 63 and the line 63 'into the benzene collecting tank 62. In the tank 62, the benzene is distilled off by the steam coil 64. The container 62 has baffles 65 and a water coil serving as a reflux condenser 65 '. The benzene vapors pass through line 66 into a benzene condenser 66 ', from which the liquid products pass through line 67 with the aid of the pump 68 into a benzene washing tower 69. From this, the unconverted benzene is returned to the benzene line ii '.

The bottom of the tank 62 serving for benzene recovery is with a line 7o provided through which the mixture of phenol and high-boiling substances is pumped into a vacuum tower 71. There the phenol is extracted with the help of the steam coil 72 distilled off into line 73. This stands with the phenol capacitor 74 in Compound from which the condensed phenol through line 75 into a collecting container 76 flows, one vacuum line 77 provided with a valve and one for drainage of the phenol certain valved conduit 78 having a pump 79 is connected to peel off the phenol. The high-boiling products of oxidation are withdrawn from the container 71 through the line 8o with the aid of a pump 81.

The method has a number of variables that relate to each other can be adjusted to achieve optimal working conditions. These variables are: the amount of oxygen and benzene in the reaction mixture, the reaction time, the pressure under which the process is carried out, the temperature of the reaction, the material, the inner diameter and the length of the reaction tube, the inner The diameter and length of the preheater coil and the temperature of the preheater bath, the hydrogen-containing organic compound used as an accelerator, and the amount of that connection. The individual variables and explains their relationships to one another.

Reaction mixture The amount of air used has a practical upper limit Limit that varies with other conditions, especially with the inner one Diameter of the reaction tubes 41.

For pipes with an inner diameter of 5 to 9 mm, those with a Working pressure of 70 kg / cm2, good yields of phenol are obtained with mixtures, which are not richer in oxygen than a mixture containing ¢ moles of benzene to 1 mole Contains oxygen and 4 moles of nitrogen. Mixtures with a higher oxygen content rather give rise to the formation of high-boiling substances, which affects the yield of phenol is greatly reduced even if the conversion to phenol as such is somewhat greater. Good yields of phenol are also obtained with reaction tubes of this size with mixtures in the ratio of 2 moles of benzene to i llol of oxygen and 4 moles of nitrogen thereby been obtained by keeping the temperature of the reaction bath so low that only half of the oxygen is consumed; however, this requires a great deal precise temperature control and offers no advantage either in terms of conversion nor in relation to the yield of phenol compared to the 4: 1: 4 mixture. Under the conditions described is the use of mixtures with 8 to 4 parts Benzene to some extent preferable to oxygen.

With a narrower reaction tube with an inner diameter of about 2.2 mm the proportions are a little different. The response here is much less temperature sensitive, so that the bath temperature can be regulated very easily so that only part of the oxygen in the mixture is consumed. With this pipe are the best results with a mixture of 2 moles of benzene to 1 mole of oxygen and 4 moles of nitrogen have been obtained at temperatures at which 1 / to 1/2 des available oxygen is consumed. Under these conditions it becomes higher Yield of phenol obtained as with a mixture of 8 moles of benzene to 1 mole of oxygen and 4 moles of nitrogen, although the conversion to phenol does not change significantly.

With a narrower pipe, higher pressures of about 14o to about 2io kg / cm2 is desirable. Response time The response time is considered to be the time which one molecule of the reaction mixture needs to under the reaction conditions to pass through the reaction tube. The response time is not an important variable, provided that it exceeds a certain minimum value. Above this so-called critical minimum value can increase the response time by a Change in the reaction temperature can be compensated. Meanwhile, response times are increasing below the minimum value the formation of high-boiling substances increases sharply and the phenol yield accordingly, although the actual conversion into phenol does not have much influence will. The value of this so-called critical minimum reaction time depends on the type of installation and how it is operated, and it is likely that the influencing factor is the ability of the system to dissipate heat.

The critical minimum reaction time is about ten seconds under the following conditions: Batch ....... pure benzene plus z% cyclohexane. Reaction chamber tube made of corrosion-resistant Steel of 3.8 m in length and 9 mm clear space. Mixture ..... 8 moles of benzene plus z moles of 02 plus 4 N2. Pressure . . . . . . . . 7o kg / cm2. If the reaction time is between 13 and 140 seconds, the optimum reaction temperature is between 371 and 398 °, the conversion to phenol is between 2 and 3% and the yield of phenol is 55 to 65%.

The minimum critical reaction time is around 2 seconds under the following conditions: Batch ....... thiophene-free benzene plus z% cyclo- 'hexane. Reaction chamber steel tube of 3.8 m length and 2.2 mm clear width. Mixture. . . . . 2 moles of benzene plus z moles of 02 plus 4 moles of N2. Pressure . . . . . . . . 7o kg / cm2. With a reaction time of 2.3 seconds, phenol was obtained with 3% conversion and 50% yield at 50 °. A better yield is achieved if the pressure is increased to 140 kg / cm- 'under the above conditions. Phenol is then obtained with a conversion of 3 to 4% and a yield of 60 to 65% with a reaction time of 5 to 35 seconds. The optimal reaction temperatures are then between 421 and 449 °. Pressure Phenol was obtained with good conversion at pressures of 42-211 kg / cm '. The higher the pressure, the lower the optimal reaction times under otherwise identical conditions. At 40 kg / cm2 pressure, good results were obtained at 357 °. Since steel surfaces catalytically favor total combustion at temperatures above 538 °, it is necessary to use inert surfaces such as glass, enamel, silica, etc. in the reaction chamber when working at relatively low pressure and the associated high temperatures. In the case of steel pipes, the pressure required is so high that the temperature remains below 538 ° - usually at least 35 to 70 kg / cm2. At 70 kg / cm 'there is no advantage in using a nickel tube in place of steel or corrosion-resistant steel.

However, the use of high pressure has certain fundamental advantages. At low pressures there is even a glass-lined reaction space is used, the loss due to carbon oxide formation is considerably higher than with high Pressing with equipment made of corrosion-resistant steel. Furthermore, the critical increases Minimum reaction time evidently with the decrease in pressure on, and there on with a constant response time, the conveying speed is directly related to the Pressure changes, the maximum possible throughput falls very quickly with the Decrease in pressure.

There is also an upper one. practical limit of pressure. If the Pressure rises and, as indicated, the temperature required to start the reaction falls is required. The boiling point of benzene continues to rise until the critical temperature of liquid benzene (287 °) is reached. As it is beneficial to use the benzene vapor and to mix the air well before starting the reaction so that local areas are undesirable high oxygen concentration are avoided, it is not desirable to lower the reaction temperature to be reduced without restriction by using extremely high pressures.

There is also a relationship between the practical maximum pressure and the inner diameter of the reaction tube. The smaller the inner diameter of the reaction tube, the higher the temperature necessary for the reaction to get going. So can pressures of at least 211 kg / cm2 and probably used even more advantageously in a reaction tube with 2.2 mm internal width while in a tube with 9 mm inner diameter 140 kg / cm2 pressure above the upper usable limit seem to lie. A pressure of 70 kg / cm2 is preferred. The minimum pressure is around 35 kg / cm2.

As a result of this relationship between the inner diameter and the Reaction temperature and the catalytic effect of the walls of the steel pipe requires a pipe with an inner diameter of 2.2 mm has a pressure of at least 7o kg / cm ', while a tube with an inner diameter of 9 mm gives good yields 35 kg / cm2 pressure.

Temperature The optimal reaction temperature can be from 343 to 663 ° vary by changing the reaction conditions, the most important of which are: the pressure, the concentration of the accelerator and the inner diameter of the reaction tube. This is discussed in the Pressure and Accelerator sections.

Reaction tube As mentioned, a reaction tube without solid catalyst and made of a material that is not essentially catalytic Has an effect on the oxidation of benzene. At the relatively high temperatures, those present in an oxidation at relatively low pressure are inert internal ones Surfaces of the reaction tubes, such as glass, enamel, glazed porcelain or silica, necessary.

At pressures of about 70 kg / cm 'it is low carbon steel and corrosion-resistant steel works well.

An advantageous mode of action is obtained in pipes 2.2 to 22 mm clear width. at Steel pipe is the required one The smaller the clearance, the higher the pressure. A tube with about g mm clearances Width has proven itself at pressures of around 35 to 140 kg / cm2; tighter pipes are also satisfactory, but require higher pressures. For a pipe with 2.2 mm pressures of 140 to 211 kg / cm 'are recommended for inner widths.

The length of the reaction tube obviously determines the size of the throughput, and the existence of a critical minimum reaction time for a particular dimensioned tube has already been mentioned. Satisfactory results have been obtained with a pipe length of 4.6 m and pipes of at least this length are therefore recommended. Mixer Preheater Tube Any material having suitable mechanical properties can be used for the preheater coil 18; corrosion-resistant steel has proven itself. The clear width of this pipe coil should be small enough to prevent a reaction in it. The tube should be of sufficient length and the temperature of the preheater bath such that the reaction mixture coming from the preheater coil 18 has a temperature which is about 28 to 84 ° below the temperature in the reaction bath in the container 42. Accelerator The required amounts of the hydrogen-containing organic compound acting as an accelerator have already been mentioned. The concentration of the accelerator can vary depending on the nature of the starting material, the conditions of the process, etc. The favorable concentration is from about 0.2 to about 3 percent by volume based on the amount of benzoin in the reaction mixture. A concentration of 0.5 to z percent by volume is preferred. As the concentration of the accelerator increases, the optimum reaction temperature is greatly reduced.

Claims (3)

PATENT CLAIMS: x. Process for the production of phenol by oxidation of benzene, characterized in that a mixture of benzene vapor, an oxygen-containing Gas and a small amount of a hydrogen-containing organic compound, which is in the gas phase under the reaction conditions and under these Conditions give off hydrogen more easily than benzene, at a pressure above about 35 kg / cm2 and a temperature between about 343 and about 663 ° continuously a reaction zone is passed. 2. The method according to claim r, characterized in that that the amount of hydrogen-containing organic compound is about 0.2 to about 3 Volume percent of the amount of benzene. 3. The method according to claim z or 2, characterized in that the hydrogen-containing organic compound is hydrogenated part of the recirculated benzene before it re-enters the reaction zone is produced.