US3227525A - Apparatus for pyrolyzing vapors - Google Patents

Description

Jan. 4, 1966 M. E. DEGEORGES ET AL 3,227,525

APPARATUS FOR PYROLYZING VAPORS Filed Aug. 4, 1964 2 Sheets-Sheet 1 ATTORNEY 51 I I 4 i 1 Y /27 25 4/ 43 5 Y I I i i 58 59 i i i i MARGEL E. DEG'EORG'ES Y P MAURICE JA won 0 2/ i :26 i "24 23 INVENTORS ll i F i r i LEW i l 7' L) BY J "a ll i I" 1 Jan. 4

, 1965 M. E. DEGEORGES ET AL 3,227,525

APPARATUS FOR PYROLYZING VAPORS 2 heets-Sheet 2 Filed Aug. 4, 1964 FIG. 2.

MA R65 L E. DE 65 0/765 .5

MA URIOE JA YMO/VD INVENTORS United States Patent 3,227,525 APIARATUS FOR PYRGLYZING VAPORS Marcel E. Dgeorges and Maurice Jaymond, Lyon, France, assignors to Societe Progil, a corporation of France Filed Aug. 4, 1964, Ser. No. 388,346 Claims priority, application France, Dec. 6, 1960, 846,600 1 Claim. (Cl. 23--277) This is a continuation-in-part of my co-pending patent application, Serial Number 125,197, filed July 19, 1961, and now abandoned.

This invention relates to apparatus for pyrolyzing vapors and, more particularly, to apparatus arranged to cause a chemical reaction in chemical vapors by passing the vapors through a tube to the exterior of which heat is applied.

Some chemical transformations are best carried on by a process known as pyrolyzation. An example of such a conversion is the transformation of benzene to diphenyl in which benzene vapors are passed through tubes to the exterior of which heat is applied, In this and similar processes, the eflectiveness of the process depends to a great extent upon the efficiency with which the heat is transferred through the tube wall into the vapor within. In the past, pyrolyzation has been carried on in an apparatus having heat transfer tubes of very small diameter because it was believed that heat transfer would take place less effectively with tubes of larger diameter. Furthermore, the amount of turbulence of the vapor within the tube has been limited considerably since it has always been believed that very high turbulence would atfect the reaction adversely. For that reason, pyrolyzation apparatus in the past have been capable of only very inefiicient operation. For instance, in the pyrolysis of benzene vapors to obtain diphenyl, only 5% of the benzene vapors in a given pass through the tubing have been converted; furthermore, the conversion has taken place in such a manner that only approximately 90% of the conversion was to diphenyl, the remainder being undesired elements, such as polyphenyls, tar, and carbon. The biggest problem in pyrolyzation apparatus in the past, however, has been the accumulation of carbon and tar on the inside surface of the tubes so that the process could be carried on for only a short time before it was necessary to shut down the apparatus and clean the tubes. Because it was felt necessary to use relatively small pyrolysis tubes, the eltect was that, in order to obtain industrial production, a very large number of tubes was used in the pyrolysis apparatus with resultant complications and difiiculties of maintenance. These and other difficulties experienced with the prior art apparatus have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide an apparatus for the efiective pyrolyzation of vapors of hydrocarbon.

Another object of this invention is the provision of a pyrolysis apparatus making use of tubes of relatively large diameter whereby the heat exchange portions are very simple and free of maintenance.

A further object of the present invention is the provision of a high production pyrolysis apparatus which is relatively free of the problem of coating of the inside surfaces of the tube by carbon or tars.

It is another object of the instant invention to provide an apparatus for the pyrolysis of hydrocarbon vapors which is capable of continuous operation for long periods of time without shutdown for maintenance.

It is a further object of the invention to provide apparatus for the pyrolytic reaction of hydrocarbon vapors 3,227,525 Patented Jan. 4, 1966 'ice adapted to operate with flow of high turbulence whereby carbon accumulation difiiculties are obviated.

A still further object of this invention is the provision of pyrolyzation apparatus for hydrocarbon vapors making use of a small number of relatively large diameter tubes and operating with vapor flow of extremely high turbulence whereby extremely high conversion rates take place.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claim appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a somewhat schematic view of apparatus embodying the principles of the present invention; and

FIG. 2 is an isometric view with portions broken away of a portion of the apparatus.

For the purposes of description, the apparatus and its operation will be described in connection with the pyrolyzation of benzene vapors to produce diphenyl. Referring first to FIG. 1, which best shows the general features of an apparatus embodying the principles of the invention, a pyrolysis apparatus, indicated generally by the reference numeral 10, is shown as having an entrance pipe 11 leading into a storage tank 12. The bottom of the storage tank is connected by a pipe 13 to one side of a valve 14, the other side of which is connected to one end of a pipe 15 extending through a vaporizer 16. The vaporizer is provided with a heating coil 17 connected to a source of heating steam (not shown). It is also provided with another coil 18 whose use will be described further hereinafter. A pipe 19 connects the pipe 15 of the vaporizer 16 to a tubular heat exchanger 21, the other end of which is connected to a pyrolysis furnace 23 having a tube 24 which will be described in greater detail later. The entrance end of the tube 24 is connected to the pipe 22 and the other end is connected through a pipe 25 to a tube 26 lying within the heat exchanger 21.

The other end of the tube 26 is connected by a pipe 27 to one end of the coil 18 which lies in the vaporizer 16. The other end of the coil 18 is connected by a pipe 28 to the lower end of a fractionating column 29. The central portion of the fractionating column 14 is connected to a reflux regulator valve 31. The upper part of the fractionating column 29 is connected to the upper part of a condenser 32, the bottom of which is connected to a cylinder- 33, the bottom ofwhich is connected by a pipe 34 to the storage tank 12. The top of the cylinder 33 is connected to a cooler 35 having a top eXit pipe 36 from which hydrogen or other generated gas can be removed. The bottom of the fractionating column 29'passes into the top of a second column 37 having a heating coil 38. Any benzene remaining is distilled off through a pipe 39 lead. ing back to the first column 29. The bottom of the colrnun 37 is connected by a pipe 41 to a third colmun 42 having a heating coil 43 at its lower portion. A reflux coil 44 is located at the upper part of the column 42 and is connected through a valve 45 to a water storage container 46. The water which may be evaporated in the coil 44 passes through a pipe 47 to a condenser 48, the bottom of which is connected by a pipe 49 to the top of the water storage tank 46. The top of the column 42 is connected by a pipe 51 to a coil 52 lying in a vessel 53. The coil 52 is completely immersed in a body 54 of boiling water the vapor from which is condensed by a cooling coil 55 located in the upper part of the vessel 53. Finally, pure product, such as diphenyl, leaving the bottom of the coil 52 through a pipe 56 which is connected to a storage drum 57. Similarly, the bottom of the column 42 is connected by a pipe 58 to a drum 59 in which the undesired higher molecular weight products, such as polyphenyls are stored.

The pyrolysis furnace 23 is shown in detail in FIG. 2. The furnace is provided with a vertical tubular steel wall 61 which is lined with insulating material 62 in which are embedded electrical heating elements 63. Lying within the housing defined by the wall 61 and the insulating material 62 is the coil 24 which is arranged in a series of convolutions lying in an imaginary cylinder coaxial with the wall 61. The entrance pipe 22 is connected to the coil as is the exit pipe 25. Each convolution consists of a U-shaped bend 64 which merge into vertical straight portions 65.

The inside diameter of the tube 24 may be selected in the range between 55 and 205 mm. One of the factors which must be taken into account in selecting the diameter is the desired hourly production of the product, such as diphenyl. However, suitable diversification of production may be obtained with a pyrolysis tube having a diameter in the range between 80 mm. and 140 mm. with best results in the range of from 100 to 120 mm. With tubes having too great a diameter, heat exchange becomes difficult, while, when the tubes are selected of too small a diameter, the difficulties described above in connection with the prior art processes are encountered. The length of the tube is, of course, determined by the exact process and the diameter of the tube. However, experience shows that industrial production is greatly facilitated when the tubes have a length of from 30 to 100 meters and, more particularly, in the range of from 45 to 75 meters. Preferably, the tubes should be made of refractory steel.

As is evident in FIG. 2, the tube is bent in a plurality of portions of its length; however, it is advisable that the tube have a bend every length in the range of from 2 to meters with preference being given to the range of from 3 to 5 meters. In contrast to the conventional helical coils which are often used in the prior art, the bends of the tube 24 have their radius of curvature as little as possible compatible with the diameter of the tube. It is preferable that the radius of the axis of the tube in the bend be of the same order as the external diameter of the tube; good results are obtained when the radii equal 1 or 2 times the outside diameter. At the same time, it is important that the straight parts 65 form an angle to one another as close as possible to 0, and in any case, an angle of less than 30. With the radius of curvature selected in the manner shown, the distance between two successive straight portions 65 will be in the same order as the outside diameter of the tube.

The operation of the apparatus will now be readily understood in the light of the above description, making use of the pyrolysis of benzene vapors as'an example. Ben zene is fed through the pipe 11 into the storage tank 12 and from the tank the benzene passes into the vaporizer 4 through the pipe 13, the flow being controlled by the valve 14. The interior of the vaporizer is heated by steam passing through the heating coil 17 and the benzene, then passing through the pipe 15 is converted into a vapor which leaves the vaporizer 16 through the pipe 19. The vapor reaches the tubular heat exchanger 21 and there its temperature is raised by the absorption of heat from the vapors arriving at the heat exchanger through the pipe 25 and residing within the tube 26. Benzene vapor then passes through the pipe 22 into the pyrolysis furnace 23. After passing through the coil 24 in which the reaction takes place, the pyrolyzed vapors when leaving the furnace by the pipe 25 now contain benzene, diphenyl, polyphenyls, and hydrogen. These then pass through the heat exchange tube 26 in the heat exchanger 21 and then through the pipe 27 into the coil 18 within the vaporizer 16. There heat passes into the fresh benzene in the tube 15 as it comes from the storage tank 12. These vapors are conducted by the pipe 28 to the bottom of the first column 29 in which the benzene is separated from the pyrolysate. Benzene distilled in the column 29 is liquified by the condenser 32. The central region of the column 29 is kept at constant temperature by means of the reflux regulator valve 31. From the condenser 32 liquid benzene flows into the intermediate cylinder 33 from which it returns to the storage tank 12. So far as the hydrogen evolved is concerned, it is cooled to a low temperature within the cooler 35 in order to recover any benzene it carries before it is taken away at the top through the pipe 36.

Crude diphenyl (which still contains polyphenyls and also a little benzene) is the liquid fraction recovered at the bottom of the column 29, and this is permitted to run into the second column 37 having the heating coil 38 in its lower portion. From the column 37 the remainder of the benzene distills off to the column 29, while a mixture of diphenyl and polyphenyls enter the third distillation column 42. These two elements are then separated. The heat required for the separation is provided by the heating coil 42 at the bottom of the column. Adequate reflux is obtained at the top of the column 42 by circulating water through the coil 44, the inlet of which is provided with the valve 45. Water vaporized within the coil 44 liquifies in the condenser 48 from which it returns through the pipe 49 to the storage container 46, the lower part of which is connected with the coil 44 through the valve 24. Purified diphenyl vapor, which leaves the top of the column 42, is condensed to the liquid state within the coil 52 in the vessel 53. This coil is situated in the bottom part of the vessel 53 and the vessel contains the body 54 of boiling water suflicient for the coil 52 to be completely immersed. As the water boils, its vapor is condensed in the upper coil 55 within which cold water is circulated. In this way, the condensation of pure diphenyl is formed without any crystallization of this compound within the coil 52. Finally, the pure liquid diphenyl which flows down from the coil 52 through the pipe 56 is cooled and crystallized in the drum 57, while the polyphenyls separated at the bottom of the column 42 pass through the pipe 58 into the drum 59 where they are solidified.

Now, returning to the passage of the benzene vapors through the coil 24 of the furnace 23, it will be understood that heating of the substance takes place because of the electrical heating element 63. As has been stated above the diameter of the tube has been related to the temperatures and so on of the benzene vapor according to the formula for the Reynolds number in such a way that the Reynolds number of the flow through the tube is in the range of from 100,000 to 700,000. These coils are, of course, connected to an electrical source (not shown). The following are several examples illustrating the effectiveness of the process of the present invention:

Example I In an installation similar to that shown in FIG. 1, 3,000 kilograms of diphenyl were produced continuously each 24 hour period. The furnace in reaction tube was similar to those shown in FIG. 2. The tube was 50 meters long and had 11 successive bends, so that there were 12 straight portions of about 4.08 meters each. The internal diameter of the tube was mm. and the radius of curvature of the bends at the axis of the tube was mm. The distance between the axes of two adjacent straight portions was about 235 mm. The temperature of the henzene vapor entering the apparatus through the pipe 22 was 560 C. and the heating by means of coil 63 was so controlled that the vapor left the apparatus through the pipe 25 at a temperature of 800 C. The tube was continuously fed with 1390 kilograms per hour of benzene vapor. The flowing vapor remained within the tube 24 for 1.32 seconds, which means that the vapor flowed with a linear velocity of 38.4 meters per second. The average Reynolds number of the vapor, therefore, was 180,000. With each passage of the vapor through the pyrolysis tube 9% of the benzene was converted into diphenyl with a yield of 92%. There was practically no formation of tar or carbon within the reaction tube and the tube worked formation.

In a process carried on in the manner similar to that :of Example I, the pyrolysis tube, however, was 67 meters long, instead of 50 meters. The benzene vapor entered through the pipe 22 at 600 C. and left the tube 24 through the pipe 25 at a temperature of 750 C. The time of its passage through the pyrolysis zone was 1 second and the Reynolds number was 320,000. During each passage through the pyrolysis zone 10% of the benzene was transformed into diphenyl with a yield of 87%. 4.5 tons of diphenyl was thus produced every 24 hours and it was possible to vary the daily production between 1.5 and 5 tons with the same reaction tube. The above results should be compared with tests carried out according to the prior art at the same average temperature of 675 C. with the time pyrolysis of 1 second within a tube having internal diameter of 25 mm. and a length of 8 meters. In this case, the Reynolds number was 4,000 and only 5.5% of the benzene was converted into diphenyl in each passage through the pyrolysis tube.

Example 111 A production of 6 tons of diphenyl per 24 hours was carried'on with a single pyrolysis tube 67 meters long in an apparatus otherwise similar to that used in Example II.

360,000 and during each passage through the pyrolysis tube 10% of the benzene was transformed into diphenyl with a yield of 85%. The continuous process was disturbed neither by carbon deposits in the tube nor by tar It is worthy of note that the reaction tube used produced 360 grams of diphenyl per hour per liter of its capacity compared with the prior art processes in which'never more than 85 grams per hour per liter were produced.

Example IV With the apparatus used in Example I equipped, however, with a reaction tube having an internal diameter of Benzene vapor remained within the tube was for 0.3 second at a mean temperature of 765 C. (inlet temperature 700 C. and outlet temperature 830 C.) and its turbulence corresponded to a Reynolds number of 460,000. Under these conditions 3.3 tons of diphenyl were prepared every 24 hours with a conversion rate of benzene of 3.5% at each passage and a yield of 96.6%.

Example VI The production of diphenyl was effected within a pyrolysis tube 96 meters long having an internal diameter of 200 mm. The benzene vapor entered the tube at 406 C. and left it at 710 C., the mean temperature being 585 C. The vapor passed through the tube in 2.7 secends with what corresponded to a Reynolds number of 352,000. The conversion of benzene in the diphenyl was 4.5% with a yield of 96% and there was no indication of carbon deposit.

Example VII The same tube used in Example VI was used at a constant temperature of 700 C. with the flow through the tube taking place in 1.17 seconds and a Reynolds number of 680,000. With each passage of the vapor through the tube 3.4% of the benzene was converted into diphenyl giving a yield of 96.5%. The daily production of diphenyl was 7.5 tons, which meant that 105 grams of diphenyl per hour per liter of capacity of the reaction tube takes place. There was no indication of carbon deposit.

Example VIII In this case, the pyrolysis tube was meters long and 77 mm. diameter. It was used with a temperature of 550 C. at the inlet and 800 C. at the outlet, this being a mean temperature of 680 C. The benzene vapor remained within the tube for 1.32 seconds (as in Example I) and the Reynolds number was 120,000. The conversion of benzene into diphenyl was 8.8% with each pass and a yield of 92%. Daily production was 1.33 tons which meant that 236 grams per hour, per liter of the capacity of the tube took place. No carbon was deposited on the inside surfaces of the tube.

The following is a chart comparing the operation and results of the prior art processes with the results obtained with the process carried on according to the present invention:

Prior Art Examples of Application A l B C 1 2 3 4 Inlet temp. C 700 675 700 560 600 630 675 Outlet temp. C 700 675 700 800 750 770 800 Mean temperature, degrees 675 700 680 675 700 737 Internal diameter 2r of reaction tube 2. 5 2. 5 5. 3 11.5 11. 5 11. 5 11. 5 Linear velocity of vapor in cmJsec 21. 5 92. 7 225 3, 840 6, 700 7, 900 11, 100 Length of reaction tube in meters 7. 5 7. 5 4. 00 5 67 67 50 Capacity of reaction tube in liters 3. 7 3. 7 8.8 518 694 694 518 VLII Percent of benzene converted into diphenyl per 23.8 6.6 3.3 9 10 10 5 passage. IX Yield on benzene, percent 66. 5 92 94 92 87 95 X Time 05 passage of vapor through reaction tube, 34.8 8. 1 1. 76 1. 32 l 0.85 0.45

secon s. XI Reynolds number 1, 020 1, 050 4, 580 180, 000 320, 000 360, 000 470, 000 XII Diphenyl produced in grams per hour per liter of 85. 5 33 6. 246 313 360 375 reaction tube capacity.

1 Pressure, 3.55/kg./em.

perature was 675 C. at the inlet of the tube and 800 C. at its outlet. Vapor remained within the pyrolysis zone for 4.45 second and its turbulence amounted to a Reynolds number of 470,000 At each passage 5% of the benzene' was transformed into diphenyl with a yield of 95%.

The hourly production of diphenyl per liter of capacity of the pyrolysis tube amounted to 375 grams. There is no indication of carbon deposit.

Example V Diphenyl was produced in a reaction tube, diameter of which was mm. and the length 35 the internal meters. 75 to a pressure of 1.2 kgs. per centimeter squared. As a practical matter, in general, a pressure is used of from 0.4 to 1.4 kgs. per centimeter squared and more often than not 0.9 to 1.2 kgs. per centimeter squared are used in addition to the atmospheric pressure at the inlet in the pipe 22 of the pyrolysis tube 2e. Of course, there is a certain loss of head within the tube, depending on the form and size of the latter. For example, when two tons of diphenyl are produced at 770 C. within a tube 76 meters long and 115 mm. in diameter with 18 bends, the loss of head is from 0.4 to 0.5 kg. per centimeter squared.

The figures in the tables show that the peeentage conversion of benzene into diphenyl per second of pyrolysis time is much greater in the apparatus of the invention than in the prior art (compare the horizontal lines VIII and X). In fact, when one divides the figures in line VIII by those of line X it is seen that the percent of benzene converted per second within the pyrolysis tube are, according to the prior art, (a) 0.685, (b) 0.815 and (c) 1.865 while, according to the invention, Example I gives 6.8, Example II gives 10.0, Example 111 gives 11.8 and Example IV gives 11.1. The table also shows that the production of diphenyl per hour per liter of capacity of the reaction tube is considerably increased (see Line XII).

The advantages of the invention with regard to ciliciency and good yield appear very clearly when we compare the present apparatus with one of the best devices previously known, as described in US. Patent No. 2,702,- 307. When the patented apparatus is used, there is a yield of about 93% diphenyl with respect to benzene for each passage of the benzene vapor through the pyrolyzing zone, while the rate of conversion of benzene is 5%. Now, with the present invention, the conversion rate of benzene is from 9 to 10% which is about twice the conversion rate. Moreover, the production of diphenyl per liter of the capacity of the reactor used per second is 10 times that which can be obtained according to the above-mentioned patent. Another comparison that can be made would be comparing the present apparatus with that described in US. Patent No. 2,099,350. When that apparatus is used, the tube within which the pyrolysis reaction is carried out must be replaced at least every 8 to 10 days because of the carbon deposits formed on the inner wall of the tube. In comparison, when the present apparatus is used with the pyrolysis tube described above, the process can work uninterruptedly as long as several months with each single tube daily producing 5 to 10 tons of diphenyl.

The benefits of the present invention result from the fact that the production of diphenyl is improved when the benzene vapor passing through the reaction zone is subjected to intense turbulence, much stronger than that which generally prevailed in the prior art reaction tubes. The usefulness of a certain degree of turbulence is well known. For example, in US. Patent No. 1,907,498, which issued in 1933, the application of this knowledge led to passing the vapor rapidly through the pyrolyzing tube, but the velocity of the vapor was limited by the fact that the conversion rate in each passage decreased as the velocity increased. At that time, a turbulence corresponding to a Reynolds number of 100,000 was considered as the upper admissible limit, but in practice, operations were carried out at much lower turbulences, say in the order of 600 to 5,000 Reynolds number.

It will be seen that it is possible, by use of the present invention, to simplify the equipment required for a pyrolysis unit by substituting a single tube for a considerable number of tubes in the prior art apparatus. The apparatus of the invention is particulary useful for such pyrolyzing operations as the non-catalytic cracking of aliphatic hydrocarbons, alcohols, or other organic compounds and especially that of the arylic hydrocarbons; for example, in the production of diphenyl, terphenyls and other polyphenyls. It will be understood that, although the special apparatus as suggested by the present invention preferably uses the tube of inner diameter between 55 and 205 mm., the size will be determined, to a certain extent, by the desired hourly production. Of course, with diameters which are too large, heat exchange becomes difiicult, while with tubes of small cross-section the problems of the prior art are encountered. Furthermore, the length of the tube can be determined according to the specific process, but experience shows that industrial production demands the use of tubes having a length of from 30 to 100 meters and, particularly, those in the range of from 45 to meters.

It should be noted that in contrast to the conventional helical coils used in the prior art, the bends of the tube of the apparatus of the invention have their radii of curvature as little as possible compatible with the diameter of the tube. It is preferable, as has been stated, that the radius measured along the axis of the tube is of the same order as the external diameter of the tube, good results being obtained with radii equal to 1 to 2 times the said diameter. At the same time, it is important that the straight parts of the tube adjacent to the same bend make with respect to each other an angle as close as possible to zero degrees, and in any case, an angle of less than 30. However, angles of less than 17 are most advisable.

Although the examples set forth in the preceding description have to do with the preparation of diphenyl, it will be understood that similar processes may be carried out in the apparatus of the invention with extremely high percentages of conversion and with high yields.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

Invention having been thus described, what is claimed as new and desire to secure by Letters Patent is:

Apparatus for pyrolyzing benzene vapors or the like, comprising a housing of cylindrical conformation, a heatexchange tube mounted within the housing having an internal diameter in the range from 55 to 205 mm. and a length in the range from 30 to meters, the tube being arranged in a sinuous conformation along the surface of an imaginary cylinder concentric with the housing, the tube consisting of U-shaped bends joined by straight portions lying along the generatrices of the imaginary cylinder, the radius of each bend being in the order of the outside diameter or" the tube so that the straight portions are spaced a distance apart approximately equal to the outside diameter of the tube, means for heating the interior of the housing, including an insulated wall surrounding the housing and carrying heating elements, and a pump bringing about a flow of vapor through the tube with a turbulence having a Reynolds number in the range from 120,000 to 500,000 with a residence time of the vapor in the tube in the range from 0.4 to 1.5 seconds at a temperature at the entrance in the range from 550 C. to 650 C. and a temperature at the exit in the range from 750 C. to 800 C.

References Cited by the Examiner UNITED STATES PATENTS 2,925,377 2/1960 Mayer 208-138 MORRES O. \VOLK, Primary Examiner,

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