US2740753A - Continuous fractionation of tar - Google Patents

Description

April 3, 1956 A. SCHMALENBACH 2,740,753

CONTINUOUS FRACTIONATION OF TAR Filed Oct. 7, 1950 2 Sheets-Sheet l Jnventor:

Adolf Schmalenbaah BY 5W, 5W

ATTORNEYS April 1956 A. SCHMALENBACH 2,740,753

CONTINUOUS FRACTIONATION OF TAR Filed Oct. 7, 1950 2 Sheets-Sheet 2 6o 67 59 H as m it; 62 53 1 l 64 III II I 1 ATTORN E Y8 rectification column or columns.

United States Patent O CONTINUOUS FRACTIONATION F TAR Adolf Schmalenbach, Essen-Ruhr, North-Rhine-Westphalia, Germany, assignor to Gesellschaft fiir Teerverwertung mit beschrankter Haftung, Duisburg-Meiderich, Germany, and Heinrich Koppers, Geselischaft mit beschrankter Haftimg, Essen, Germany Application October 7, 1950, Serial No. 188,912

Claims priority, application Belgium October 7, 1949 4 Claims. (Cl. 196-76) This invention relates to a process for the continuous fragtionation of tars containing naphthalene, particularly coal coking tar, by flash vaporization and rectification of the vapours formed or freed thereby, that is, from the liquid, as well as to devices for carrying out this process. Flash vaporization is to be understood as implying processes in which the tar is continuously heated and then decomposed in a vaporizing chamber into gaseous and liquid fractions corresponding to the temperature and pressure used in the vaporizing chamber, under phase equilibrium of both fractions, by spontaneously forming a vapor mixture with separation from the fractions remaining liquid in the vaporizing chamber or with the formation of vapours during the heating and the subsequent freeing, that is, separation of the vapours from the liquid residue in the vaporizing chamber or under conditions lying between these two extremes, whilst the heating may be carried out under increased pressure, for example in a tube furnace, with the avoidance of a formation of substantial quantities of vapours.

It is known to carry out the fractionation of tar with the employment of flash vaporization, in which the tar, for example, raw tar, is heated in a tube furnace, for example under increased pressure and without substantial vaporization and is then expanded into a vaporizing chamber whereafter the vapours resulting from the expansion or the vapours set free thereby are treated in a This process has given extraordinarily good results in comparison with the discontinuous treatment of tars by batch distillation.

Raw tar, or even, for example, dehydrated or partially dehydrated tar, or topped tar, in which the water or other constituents have been removed in any suitable manner (such as is known, for example, in a flash vaporization process), can be subjected to such a process.

It has now been found that considerable advantages can be obtained and a considerable proportion of the cost of the installation can be saved by subdividing such a process into two or more stages each of which consists of heating, flash vaporization and rectification of the vapours formed or freed thereby, this being done in such a manner that in the first flash vaporization the constituents including at least naphthalene oil are obtained in the vapour fraction and are separated and recovered therefrom, whereafter the residue is subjected to a further,

second or a further stage is carried out at reduced pressure, for example, below 100 mm. mercury. This second or further flash vaporization is applied with advantage to the residue of the vaporization of the first stage together with the bottom product or a part thereof resulting from the rectification of the vapours of the first stage.

Raw tar, as received, may be used as the starting material for the process according to the invention, or even a tar dehydrated, or partly dehydrated in any suitable way, for example advantageously a tar containing less than about 3% or preferably 2% of water, and more particularly less than 1% of water. Removal of this water can advantageously be effected in the known manner by decanting the tar, heated under increased pressure, with the avoidance of vaporization, in a pressure vessel. In the preliminary removal of water, or before this step, water for washing out salts may be added.

If raw tar or a tar with for example, less than about 3 or 2%, and preferably with about 1% water is heated in a tube furnace to a comparatively low temperature lying as a rule not above 320 C., for example with the avoidance of a substantial degree of vaporization, and is then reduced to atmospheric pressure, there takes place in a vaporizing chamber a division into a residue containing on the one hand the pitch and a substantial part of the anthracene oil together with possibly the lower boiling fraction, and on the other hand a mixture of vapours containing the light oil fractions as well as at least the naphthalene oil fraction of the tar employed as the start ing material and possibly higher boiling constituents, for example, the washing oil fraction or a part of the washing oil fraction. This vapour mixture is according to the invention rectified with the recovery of the desired fractions whilst the light constituents at least up to the naphthalene oil are separated and recovered.

The residue of the first flash vaporization is then, preferably together with the base product of the rectification of the vapours of the first flash vaporization or a part of the base product, subjected to a second, for example final, flash vaporization. Before this second vaporization, under practical conditions a reheating of this residue of the first vaporization or of the mixture of residue and bottom product is necessary, preferably again to comparatively low temperatures lying in particular not above 350 C. During this heating which, for example, like wise takes place continuously in a tube furnace, the mixture may be subjected to pressure, preferably with the avoidance of the formation of substantial proportions of vapours, and subsequently expanded into a vaporization chamber kept under reduced pressure, preferably to a pressure of below mm. mercury. In this second vaporization pitch is formed as a residue, for example, and according to the quantity of the oil fraction remaining in the pitch, there will be formed soft pitch, normal pitch or hard pitch on the one hand and on the other hand a vapour mixture which contains the volatile fractions up 'to including the anthracene oil and is rectified with the recovery of the desired high boiling fractions of the tar.

It may also be arranged that there is recovered first of all a residue which still has substantial quantities of fractions capable of distillation, this residue then being subjected to a final flash vaporization in which, for example, hard pitch remains as the residue.

As explained above, the process according to the invention is distinguished in that the costs of the plant Ci which contains low boiling constituents and is not therefore liable to create blockages.

A further advantage of the present invention is the easy applicability of fractionation columns provided with bubble caps of the usual type of construction. A particularly small sized plant is thus obtained if according to the aforesaid preferred form of execution a comparatively Water-free tar is used as the starting material. If the above indicated preferred form of execution is carried out in such a way that the total quantity of naphthalene oil is present in the vapours of the first stage of the process of the invention, and is removed and recovered therefrom, there is obtained not only the aforesaid advantage that in the second or a further stage, the vacuum stage, a blockage by condensation of solid naphthalene is avoided, and a washing with oil to remove such difliculties before the vacuum stage, can be dispensed with, but that in this way very good washing oil, practically free from naphthalene or very poor in naphthalene, can be obtained. This is particularly the case if the conditions, particularly the temperature to which the first stage is heated, are so chosen that not only is the naphthalene oil found in the vapours of the flash vaporization of the first stage, but the washing oil also, or at least a part of the washing oil, and if the washing oil or a part of the washing oil is recovered as a separate fraction in the rectification of the vapours of the first stage. With such a procedure a part of the washing oil remains in the residue of the first flash vaporization and this part is very poor in naphthalene or practically free from naphthalene, and is then recovered in the second stage.

According to a further preferred form of execution of the invention the procedure is such that the washing oil fraction occurring in the rectification device, which operates at substantially atmospheric pressure is heated outside the rectification device in such a way, for example in a side column with evaporator, that the vapours containing naphthalene escaping from the washing oil fraction in this treatment are led back into the rectification device. In this way it is possible, even in the rectification of the first stage, to obtain a washing oil particularly poor in naphthalene and with favourable properties. If the combination of the washing oil fraction of the tar is desired this washing oil fraction can be combined with the washing oil fraction which is obtained in the rectification of the vapours of the subsequent stage according to this preferred form of execution, for example as a head fraction.

As is suggested in another place, there is a particular advantage in the process according to the invention, in employing the residue of the flash vaporization for the vaporization of a part of the bottom product of a subsequent rectification column or even a side column, by indirect heat exchange, whilst increasing the return flow thereof or of several columns.

The invention further relates to particular and advantageous forms of construction for carrying out the process according to the invention.

A preferred form of construction of a device for carrying out the flash vaporization of the first stage consists in a rectification column with a flash vaporizing chamber arranged in its lower part. Preferably, there is employed according to the invention a flash vaporization chamber in which the tar or residue coming from the tube furnace enters substantially tangentially, and from which the resulting vapours are withdrawn through a central pipe which extends into the flash vaporization chamber. Below this flash vaporization chamber there may be provided according to the invention a further chamber (vapour liberation chamber) in which the liquid residue of the flash vaporization is distributed over a larger surface so that it can give up the volatile materials practically completely.

According to a further important characteristic of the invention the tube furnace employed for carrying out the process is provided with two tube systems heated independently of each other, of which one serves for the flash vaporization at reduced pressure and the other for that under atmospheric pressure.

Further important characteristics of the invention will be evident from the following description of a preferred form of construction of the invention which is illustrated on the drawing.

Fig. 1 of the drawing shows diagrammatically a plant for carrying out the process according to the invention whilst,

Fig. 2 shows a section through a fractionating column together with a flash vaporization chamber according to the invention.

The tar to be distilled, for example raw tar, with about 6% water comes from a supply tank or the like, not illustrated, through the pipe 1 and is led through the pipe 3 to a heat exchanger 4 by means of the tar pump 2 and passes from there through the pipe 5 to the heat exchangers 6 and 7. In the heat exchangers 4, 6 and 7 the raw tar is under increased pressure and is preheated in a turbulent flow to a temperature of ab. l40-l50 C. whilst the pressure is maintained so that vaporization of water and other constituents is inhibited. The raw tar thus preheated under pressure then passes through the pipe 8 without reduction of pressure into a separator container 9 in which a substantially laminar flow takes place so that the washing water contained in the raw tar or which may still be added thereto separates with the removal of ammonium chloride and other salts from the tar whose water content is thereby decreased to below 2%. The separated water is withdrawn from the container 9 through the pipe 10 and is released through a cooler 43 into the waste water pipe 42.

The tar thus previously freed from water flows through the pipe 11 directly to the pipe system 12 of a tube furnace 13. The pressure of the raw tar can be higher in the separator container 9 than the entry pressure of the tar in the pipe system 12; in certain circumstances the pressure in the pipe 11 may be correspondingly decreased.

The pipe system 12 lies in the so-called convection zone of the tube furnace 13 which is limited by the intermediate walls 14, 15. The heating of the tube furnace is effected by burners 16, 17 whilst the waste gases, as indicated at 18, are withdrawn at the lower end of the convention zone space to the chimney, not illustrated.

The tar preheated in the tube system 12 passes into the pipe system 19, subjected to the direct steam of the heating gases, where it is heated to the temperature, about 320 0, required for the flash vaporization under atmospheric pressure, the pressure amounting to about 2 to 3 atmospheres so that the vaporization in the tube system 19 is comparatively small. In this connection substantially atmospheric pressure with this flash vaporization is to be understood as the pressure which corresponds with the surrounding atmosphere or which differs therefrom to a certain extent. In particular, the pressure may in some circumstances be less than atmospheric pressure but it should not however, correspond to the pressure of the second flash vaporization or approximate so closely thereto that the specific advantages of rectification under normal pressure are lost. The resulting mixture of raw tar and vapours then passes through the pipe 20 into the flash vaporization chamber 21 of the fractionating column 22 which operates a substantially atmospheric pressure. In the chamber 21 the hot tar is completely reduced in pressure whereby vaporization occurs to a point at which a residue is obtained consisting substantially of pitch, the anthracene oil fraction and a certain quantity of washing oil. The resulting vapours rise in the fractionating part of the column 22 (provided with a dephlegmator for the production of the reflux), from which a naphthalene oil fraction can be withdrawn at 23, a phenol oil fraction at 24 and as a head product at a light oil fraction which also contains the residual water.

The residue of the flash vaporization in the chamber 21 together with the base product of the column 22 introduced into this chamber flows through the vapour liberation chamber 26 and then passes into the heat exchanger 6. In certain circumstances the hot residue or a part thereof may be supplied through the pipe 27 to the cooker 28 of a side column 29 which is provided in order to produce a particularly pure naphthalene oil fraction. The hot residue in the boiler 28 serves in the known manner for reheating the separated naphthalene oil for the purpose of forming a reflux in the side column 29.

The residue of the first flash vaporization after passing through the heat exchanger 6 flows through the pipe 30 into a second flow pipe system 31 of the tube furnace 13. The pipe system 31, as can be seen from the drawings, is provided in a particular part of the tube furnace which is only connected through the opening 32 with the chamber of the convection zone and of the flue gas pipe 18. In this Way it is possible, if necessary after closing the opening 32, to heat and operate the pipe system 12 and 19 separately. The burner device 17 serves for heating the pipe system 31.

In the pipe system 31 the residue is heated to the temperature required for the subsequent vacuum vaporization, approximately 350 C., whilst it is provided that in the subsequent flash vaporization the pressure is decreased to about 70 mm. Hg.

The residue thus heated then flows through the pipe 33 into the flash vaporization chamber 34 of the vacuum column 35 constructed in a similar manner to the column 22.

In the chamber 34 the hot residue is vaporized under reduction of pressure as far as a pitch fraction, which after passing a vapour liberation chamber, goes through the pipe 36 through the heat exchanger 7 into the pitch receiving vessel 41.

The vapours pass upwardly from the chimney 34 and are fractionated in the part of the column 35 provided with plates or trays so that an anthracene oil fraction can be withdrawn at 37, whilst the washing oil fraction is withdrawn at 38 at the head of the column 35. This is discharged into the coolers 39 behind which is connected the vacuum pump 40.

The fractionating column used in the device according to the invention is shown in detail in Fig. 2. It consists of an upper series of compartments 50 of a suitable number arranged one above the other, a charging body part 51, a vaporization chamber part 52 and a vapour liberation part 53.

The fractionating compartments 50 are provided in the usual way with trays 54 having openings through which project pipe legs 55 which are covered by bell-shaped hoods 56. The vapours rising from below through the openings of the trays 54 are compelled by the bellshaped hoods 56 to come into intimate contact with the liquid accumulating on the trays. The liquid flows from the trays 54 downwardly through the overflow pipe 57. On passing through the fractionating compartment 50 a material exchange takes place in the known manner between the vapour and liquid phase. The so-called head product is removed from the top of the column at 58.

The heated tar to be fractionated or other liquid comes through the pipe 59 into the tangentially opening entry 60 of the vaporization chamber 52. On the cover 61 of the vaporization chamber 52 there is provided a central downwardly directed pipe branch 62 through which the vapours arising in the chamber 52 can be withdrawn upwardly. The liquid residue occurring accumulates on the trays 63 from whence it flows over the upwardly-curved edge 64 of the opening 65 into the vapour liberation part 53 of the column. In this there is arranged a series of intermediate walls 66 provided with non-aligned openings 67. The liquid residue is distributed over the intermediate trays 66 and is thereby spread out over a large surface so that the volatile constituent still remaining therein can be practically completely removed. The residue freed from vapour is finally withdrawn through the opening 68.

In order to improve the separation of liquid entrained by the vapours there is provided above the vaporization chamber 52 a further filling body compartment 51 which is closed underneath by a grate-like intermediate base 69 on which suitable filling bodies, for example, the so-called Doctor Raschigfs rings are arranged.

The repeated flash vaporization of the pitch residue and subsequent rectification of the vapour mixture produced with preferably still smaller pressure also provides the possibility of recovering a valuable chrysene fraction, and in some cases other valuable materials, from the pitch.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

l. A process for the continuous fractionation of tars containing naphthalene comprising heating the tar in a pipe coil still at a pressure that no substantial vaporization of any tar constituents occurs; feeding said heated tar into a first flash vaporization chamber at such conditions of pressure and temperature that the vapors resulting from such flash vaporization contain besides other lower boiling constituents practically all of the naphthalene contained in the tar; drawing-off said vapors from said flash vaporization chamber into a rectification column in order to separate sharp cutted tar oil fractions from said vapors; feeding the liquid residue free from naphthalene resulting as a bottom product in the first flash vaporization chamber into a pipe coil still for re-heating at superatmospheric pressure so that no substantial vaporization of any tar constituents occurs to a temperature higher than that to which the tar was heated before entering the first flash vaporization chamber; feeding said heated residue into a second flash vaporization chamber maintained at a reduced pressure below mm. of mercury, partially vaporizing the residue solely by the sensible heat contained in said heated residue; drawing-off the resulting vapors from said second vaporization chamber for rectification and recovering the unvaporized residue in the form of a high melting pitch.

2. A process as recited in claim 1 in which the maximum temperature to which the tar, after naphthalene has been removed therefrom, is heated is about 350 C.

3. The process of claim 1 wherein the tar employed as starting material is a dehydrated tar containing less than 3% water.

4. The process of claim 1 wherein removal of the naphthalene fraction is effected at atmospheric pressure.

References Cited in the file of this patent UNITED STATES PATENTS 1,466,411 Peters, Jr Aug. 28, 1923 1,759,816 MacCubbin et a1 May 20, 1930 1,831,887 Sieck, Ir. Nov. 17, 1931 2,029,883 MacCubbin et a1. Feb. 4, 1936 2,260,072 Wilton Oct. 21, 1941 2,366,900 Weir Jan. 9, 1945 2,594,352 Schmalenbach Apr. 29, 1952

Claims (1)

1. A PROCESS FOR THE CONTINUOUSL FRACTIONATION OF TARS CONTAINING NAPHTHALENE COMPRISING HEATING THE TAR IN A PIPE COIL STILL AT A PRESSURE THAT NO SUBSTANTIAL VAPORIZATION OF ANY TAR CONSTITUENTS OCCURS; FEEDING SAID HEATED TAR INTO A FIRST FLASH VAPORIZATION CHAMBER AT SUCH CONDITIONS OF PRESSURE AND TEMPERATURE THAT THE VAPORS RESULTING FROM SUCH FLASH VAPORIZATION CONTAIN BESIDES OTHER LOWER BOILING CONSTITUENTS PRACTICALLY ALL OF THE NAPHTHALENE CONTAINED IN THE TAR; DRAWING-OFF SAID VAPORS AND SAID FLASH VAPORIZATION CHAMBER INTO A RECTIFICATION COLUMN IN ORDER TO SEPARATE SHARP CUTTED TAR OIL FRACTIONS FROM SAID VAPORS; FEEDING THE LIQUID RESIDUE FREE FROM NAPHTHALENE RESULTING AS A BOTTOM PRODUCT IN THE FIRST FLASH VAPORIZATION CHAMBER INTO A PIPE COIL STILL FOR RE-HEATING AT SUPERATMOSPHERIC PRESSURE SO THAT NO SUBSTANTIAL VAPORIZATION OF ANY TAR CONSTITUENTS OCCURS TO A TEMPERATURE HIGHER THAN THAT TO WHICH THE TAR WAS

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