US2893956A - Process for the recovery of high-boiling ingredients from coal tar by distillation - Google Patents

Description

PROCESS FCR THE RECQJVERY F lHGH-BOILING ggREDENTS FROM COAL TAR BY DISTHJLA- Julius Geller and Richard Schumacher, Bad i-lomburg vor der Hohe, and Karl Friedrich Lang, Frankfurt am Main, Germany; said Geller and said Schumacher assiguors to 'Riitgerswerke-Alrtiengesellschaft, Frankfurt am Main, Germany Applicafion August 27, 1956, Serial No. 606,240

Claims priority, application Germany August 29, 1955 I 7 Claims. (Cl. 208-366) This invention relates to the continuous production by distillation of high-boiling ingredients of coal tar, which become highly concentrated in highly-boiling coal tar fractions, in order to facilitate further processing, for example, the recovery of pure products such as anthracene, carbozol or pyrene, and to increase the yields of these ingredients.

In the processes known from the prior art, the unlimited application of rectification for the separation and recovery by distillation of certain coal tar ingredients, some of which are very valuable, was prevented by the lack of thermal stability of said coal tar ingredients.

In view of the above reason, processing by distillation of the coal tar by evaporation processes in one or more steps, has retained its economic importance for a long time, although in the application of these evaporation processes, the yield and concentration of certain iugredi ents which are of economic importance, are relatively low so that further processing is rather expensive. In this connection the following evaporation processes known from the art can be mentioned.

One-step evaporation (1) The coal tar charge of a single evaporator is subjected to evaporation without reflux and the condensed products are recovered in different receivers (discontinuous distillation).

(2) The coal tar is continuously introduced into an evaporator and is subjected to evaporation without refiux up to about one-half of the charge in the evaporator. The vapors are introduced in succession into condensers which are cooled to varying degrees and are thus subdivided by fractional condensation.

Evaporation in several steps (3) The coal tar is continuously passed successively through several evaporators which are heated to increasing temperatures. The fractions of the individual evaporation steps are condensed and collected after condensation in separate receivers.

(4) The coal tar is continuously passed at a constant temperature successively through evaporators which are kept under increasing vacuum. The vapors of the individual vacuum steps are thus separated according to their boiling ranges. (5) It has also been known from the art to subject individual coal tar fractions resulting from one of the above mentioned evaporation processes, or a combination thereof, to further processing by rectifying redistillation.

In the case of the above mentioned evaporation process (1), this can be done discontinuously only, while in the case of processes (2)(4), it can be done discontinuously or continuously. A process corresponding to the above mentioned procedure (4) is, for example, described in German Patent No. 260,060. A process 2,03,50 Patented July i, 1059 corresponding to a combination of the above sections (4) and (5) has also been known from the art.

A known process based on the combination of the procedures according to the above sections (2), (3) and (4) is described in US. Patent No. 2,029,883. The process described in German Patent No. 767,001 utilizes the basic process mentioned above in section (2) and combines it with redistillation according to the above section (5), whereby the surface evaporator according to German Patent No. 260,060 is replaced by a tube furnace, like in US. Patent No. 2,020,883.

In all of the above mentioned processes for the treatment of coal tar by distillation, the crude tar is supplied only with the amount of heat which is sufficient for a single evaporation of the tar fractions to be separated therefrom. The yields and concentrations of the economically most important tar ingredients in the thus separated fractions are unsatisfactory and cause, therefore, relatively high expenses resulting from the necessary subsequent treatment.

The yields and concentrations can be increased by discontinuous rectification of the coal tar with heat supply to the coal tar still, but the discontinuous character of the procedure and the thermal instability of the pitch remaining in the still, limit said increase of yields and concentrations.

French Patent No. 1,018,333 describes a process for the recovery of carbozol and an anthracene-phenanthrene mixture by continuous distillation of coal tar. In this process, heated coal tar which has been freed from the ingredients boiling below phenanthrene, is subjected to distillation in an evaporator provided with a column in order to distill ofii-in addition to higher boiling ingredientsfirst of all, a crude carbazol fraction. The latter is separated in a fractionating column, having a stripping portion and a side column, into a head fraction containing mainly anthracene and phenanthracene, into one or more intermediate fractions and a sump product which contains mainly carbazol.

In this last mentioned process, too, the object of which is the recovery of high-boiling ingredients of coal tar, it has still been avoided to supply to the sump product of the respective column the amount of rectification heat, which is necessary for a complete sharp separation and high yield in separating the carbazol fraction. It is for this reason that the pitch discharged from the column sump still contains carbazol and pyrene in this process.

It has now been found that the difficulties and disadvantages of the known processes can be avoided by proceeding according to the process of this invention as follows. v

In this process, all coal tar ingredients boiling below 300 C. are separated under vacuum. Subsequently the heavy fractions of coal tar having a boiling range above 300 C. up to about 400 C. are recovered in a fractionating column (pitch column) from the residual coal tar by transmitting the rectification heat to the sump product of this column, in a fraction denoted herein collective fraction, as the head product of said column and said head product is subsequently separated in 3 or more fractionating columns connected in series, into the desired heavy fractions.

The new process of this invention avoids the disadvantages of the prior art processes and yields highly concentrated fractions with high yields, which can be further processed to pure products in a simple manner.

According to the invention, in the fractionating columns arranged in succession and connected in series, first a phenanthrene-anthracene fraction, subsequently an acridine fraction, and finally a carbazol fraction, is separated, whereby a fluoranthene-pyrene fraction is recovered as the sump product of the last fractionating column.

The fluoranthene-pyrene fraction, which is the highest boiling and largest heavy fraction obtained from the head product of the pitch column, is used as the carrier of rectification heat in all distillation steps after the pitch column.

The present invention is based on novel and fundamental considerations and tests, the results of which could not be expected and are surprising. This invention is the result of a combination of inventive concepts which are not in harmony with the conventional ideas and experiences in connection with the processing by distillation of coal tar ingredients boiling in the range of about 300400 C.

It has been unexpectedly found that it is entirely possible to supply considerable amounts of heat to a briquetting pitch having a solidification point of about 75 -85 C., without adversely affecting the quality of the pitch in the manner assumed in the hitherto prevailing views of the art, if by suitable selection ofthe individual fractions and sharpness of separation in the pitch column in the manner described hereinafter, care is taken to avoid the presence in the sump of the pitch column of ingredients which are liable to decomposition and of coal tar ingredients which cause the occurrence of resinification and coke formation, and boil below approximately 385 C.

Comparative tests have indicated that in the thermal treatment of coal tar ingredients boiling above about 300 C. and up to 400 C., first of all, the linear B-nuclear substances of the coal tar, for example: anthracene, acridine and carbazol, are considerably liable to decomposi tion, particularly in the vapor phase. Thereby, on the one hand, the carbon content of the pitch is increased, and, on the other hand, the yield of valuable ingredients of the coal tar is reduced in an undesirable manner.

Such undesirable decomposition is reduced, or even extensively prevented, by the following basic process steps:

(a) In the first distillation step, in contrast to the French Patent No. 1,018,333, the charged material which still contains anthracene and carbazol, is not heated under a predetermined pressure in a tube furnace, but an additional amount of rectification heat is transmitted only to the circulating sump product of the pitch column, which is free from anthracene and carbazol.

(b) By simultaneous separation of the fluoranthenepyrene fraction from the residual coal tar, the anthracene and carbazol concentration of the collective fraction is reduced and thereby decomposition of the linear 3-nuclear substances in the vapor phase is prevented.

The fact that the collective fraction contains the fluoranthene-pyrene fraction as a diluting medium has an advantageous effect, because said diluent reduces the solidify ng point of this fraction to a considerable extent, so that the first column can be operated under a considerably higher vacuum than the three subsequent columns connected therewith. Although the pressure in the first column could be reduced up toabout 1 Torr. in order to avoid decomposition, it has been found that it is economically suificient to adjust said pressure to about 20 to 50 Torr., While the pressure in the subsequent columns must normally amount to about 100 to 300 Torr.

It has been found that it is of extraordinary advantage to have in the subsequent distillation steps for the separation of the anthracene fraction, recovery of an acridinecontaining intermediate fraction and first of all for the distillation of the carbazol fraction, a thermally more stable butter available dueto the presence of the fluoranthene-pyrenc fraction, which is passed through the sumps of these columns and renders it possible to supply the rectification heat necessary for a satisfactory separation of the most important compounds, to these distillation systems arranged after the pitch column.

In order to further reduce the carbazol concentrations and thus restrain the tendency of decomposition in the sumps of the subsequent columns following the pitch column, it has been further found to be of advantage to '4 t admix part of the fiuoranthene-pyrene fraction in a cycle to the head product of the first fractionating column (2).

In order to restrain the tendency of decomposition of the 3-nuclear substances, particularly carbazol, which are present in increased concentrations in the evaporating parts of said subsequent columns, steam can be introduced, if desired, according to the invention into the columns or into the heaters for the sump product; This steam, like the fluoranthene-pyrene fraction present in the first step of the process, has an effect of reducing decomposition in the respective steps, particularly in the recovery of carbazol.

By the above mentioned steps of the invention, it is attained that anthracene, acridine as well as carbazol, appear in concentrated form practically only as distillates and need not be taken up as sump products by correspoudingly higher temperatures, so that, for example in the case of carbazol, the yield is increased by more than 60% in comparison with the procedure in which carbazol is obtained as a sump product.

The process of the invention is diagrammatically illustrated in the appended drawing.

A particular feature of the process consists in that part of the fluoranthene-pyrene fraction is used, after cooling to about 120 C., as an effective washing agent for the waste gases of the columns which are operated under vacuum. This fraction is particularly suitable as a washing agent for high-boiling ingredients of coal tar, because this fraction does not solidify when cooled even up to 20 C. and is not volatile at the temperatures used in the process, in contrast to normal washing oil, which is conventionally used for washing in vacuum processes.

The portion of the fluoranthene-pyrene fraction, which has been used as a washing agent, is then preferably added in a cycle to the distillate of the first fractionating column and introduced with this product as feed of the column (2). In this manner, losses of material in the waste gases of the subsequent column are avoided and the yield is still further increased.

Example In carrying out the invention, first the ingredients boiling under 300 C., which amount to about 25% of the coal tar, are separated from the coal tar. The residual coal tar is introduced into column 1, from which about of the charge introduced into the column are obtained as pitch having a solidification point of about -85 C., preferably about C., as the sump product, while about /3 of the charge are discharged at head product of said column 1. To a portion of the sump product, rectification heat is supplied=-by means of a tube furnace 5 operated under adjustable pressure-for the sharp separation of pyrene from the pitch, in such an amount that the vapor condensed in condenser 9 approximately corre sponds to the amount ofthe introduced product. At a pressure of, for example, 30 Torr. (measured at the head of the column), the head temperature will amount to about 240 C.

The collective fraction obtained as the head product in column 1 (said fraction corresponding to about 25% of the coal tar) contains three heavy fractions comprising as the most important components phenanthreneanthracene, acridine, carbazol, methylcarbazol, fluoranthene and pyrene. This fraction contains the linear 3-ni1- clear substances (which are relatively easier decomposed than the non-linear 4nuclear ingredients of the fraction), for example, anthracene, in an amount of about 7% and carbazol in an amount of about 8%. This corresponds to about 1.75% of anthracene and 2.07% carbazol based on the amount of coal tar.

Anthracene is recovered, together with phenanthrene, in column 2 as the head product in an amount corresponding to about 6.3% of the coal tar. Acridine and carbazol are discharged together with an about 3.5 times higher amount of a fluoranthene-pyrene fraction which acts as a stabilitybuffer, as the sump productof this 001- umn, which is supplied to a middle plate of the successiveadjacent column 3. Under a pressure of about 200 Torr, the temperature amounts, for example, at the head to about 270 C.

The acridine fraction (which is an intermediate fraction and corresponds to about 1.7% of the coal tar) is obtained as the head product in column 3, While the carbazol compounds-together with the fluoranthenepyrene fraction which is then present in an about 6.5 times higher amount and acts as a stability bufleris recovered as the sump product of this column and serves as the material to be supplied to column 4. The temperatures amount, for example, at the head of the column up to about 280 C. at a pressure of about 200 Torr.

In column 4, the carbazol fraction (which corresponds to about 2.4% of the coal tar) is obtained as the head product and the fluoranthene-pyrene fraction (which corresponds to about 14.6% of the coal tar) is discharged preferably together with the methyl carbazols as the sump product. The temperatures at the head amount up to about 290 C. under a pressure of about 200 Torr.

As in the stripping portions and the sumps of columns 2-4 (which are connected in succession With each other and with column 1), the thermally more stable fluoranthene-pyrene fraction is present in each case as a stability buifer, it is possible to supply to the sump product of columns 2-4 by means of tube furnaces 6, 7 and 8, such amounts of rectification heat which are sufiicient to cause in condensers 10, 11 and 12 reflux liquid in amounts which are 2-3 times higher than the amount charged in each case to said columns. A portion of the fluoranthene-pyrene fraction can be used in scrubber 13 for washing the gases before they enter the vacuum pump and said portion can be then added to the head product of column 1, whereby any loss of valuable distillates is eliminated.

By carrying out the process of the invention in conformity with the above example, the following results are obtained.

The carbazol obtained as the head product of column 4 shows the following analysis by fractional distillation:

Start 40% 50% 60% 70% 80% 90% This carbazol is practically free from methylcarbazols.

The head product of column 4 has a carbazol content of 36%, which corresponds to a yield of at least 73% based on the total amount of carbazol present in the coal tar.

The head product of column 2 has an anthracene content of about 25%, which corresponds to a yield of at least 88% based on the total amount of anthracene present in the coal tar. This fraction can be further processed in a separate step.

The head product of column 3 contains as an intermediate fraction-in addition to acridine which is characteristic of this fraction--by far most of the residual amount of anthracene and carbazol, while the sump product of column 4 contains the methylcarbazols, together with fluoranthene and pyrene.

The above example illustrates the unexpectedly high yields of anthracene (about 88%) and carbazol (about 7 3%) which are obtainable by the process of this invention (based on the total amount of said ingredients present in the coal tar used as starting material).

It should be emphasized that the ratio of anthracene/ carbazol in the anthracene fraction amounts to more than 15/ 1 and in the carbazol fraction to about l/ 36. These yields and degrees of purity ofier conditions for further processing which are economically extremely favorable.

It will be understood from the above that the present invention 'is' based on a'novel and original combination of steps and that this combination yields unexpected and surprising results. In carrying out this invention, first all ingredients which boil below 300 C. under atmospheric pressure, are removed from the coal tar by distillation. The resulting residual coal tar, which contains the ingredients boiling above 300 C. under atmospheric pressure, is introduced into a first fractionating column, in which the ingredients which boil above 300 C. and up to about 400 C. and include phenanthrene, an thracene, acridine, carbazol, fluoranthene and pyrene, are vaporized to form a fraction-which is denoted collective fraction herein and is recovered as the head product in said first fractionating column, which is operated under vacuum of, e.g. 30 Torr. (measured at the head of the column), whereby the temperature at the column head is about 240 C. The sump of this first fractionating column contains pitch which is free from anthracene and carbazol, and to which rectification heat is supplied by causing part of it to circulate through a tube furnace. Theamount of said rectification heat is selected in such a manner that the tar ingredients which boil in the range of 300400 C. and amount to about 25% by weight of the coal tar starting material, are vaporized and escape as the head product of the column. Thereby, the temperature in the tube furnace 5 of fractionating column 1 amounts to about 370-410 C. and the temperature of the pitch in the sump of column 1 amounts to about 350380 C. under the above mentioned conditions.

The head product of fractionating column 1 is condensed and subjected to further fractionation in subsequent columns in the above described manner, whereby thermal decomposition is reduced or eliminated to a satisfactory extent by the presence of the fluoranthene-pyrene fraction during said further fractionation.

It will also be understood that this invention is not limited to the specific steps, temperatures, pressures and other details specifically described above and illustrated in the drawings and can be carried outwith various modifications without departing from the scope of the invention as defined in the appended claims.

The term Torr. used herein denotes a unit of pressure equal to of the normal atmospheric pressure.

As mentioned in the first paragraph of the above example, the amount of vapor condensed in condenser 9 approximately corresponds to the amount of the introduced product. Thus, for example 21 tons per hour of the starting material are introduced into the middle of column 1. About /3 of this amount, i.e., 7 tons per hour, are vaporized and then condensed in condenser 9 and passed as distillate to column 2. However, in tube furnace 5 additionally an amount of heat is produced, which is sufiicient for vaporizing 14 tons per hour. These 14 tons represent the inner circulation of vaporized liquid of the column, which is additionally condensed in condenser 9 and flows from there to the head of the column 1, in which it is vaporized again.

What is claimed is:

1. A process for the recovery of high-boiling ingredients from coal tar by distillation, comprising (at) removing by distillation from coal tar the ingredients having under atmospheric pressure boiling points below 300 C.; subjecting the residual tar resulting from step (a) to a first fractional distillation under vacuum in order to separate said residual tar into a vaporized collective fraction containing ingredients having under atmospheric pressure boiling points in the range of 300-400 C. and a pitch residue which has a solidification point of 75 -85 C. and is substantially free from anthracene and carbazol, heat being supplied to said pitch residue only by causing part of it to circulate through an elongated heating zone of restricted cross-section at a temperature in the range of 3704l0 C., whereby the pitch residue in the sump is heated to about 350 380 C.; condensing the collective fraction and separating the ingredients thereof by subsequent fractional vacuum distillations connected with each other in series.

2. A process as claimed in claim 1, in which the collective fraction is separated in succession into: (a) a phenanthrene-anthracene fraction; (b) an acridine fraction; (c) a carbazol fraction, and (d) a fluoranthrene-pyrene fraction, said fractions (a), (b), and (c), being obtained as head products, and said fraction (d) being obtained as the sump product of the last fractional distillation; the necessary fractionation heat being supplied to these fractional distillations by causing parts of the sump products to circulate through a tube furnace. H

3. A process as claimed in claim 2, in which part of the fluoranthrene-pyrene fraction is added in a cycle to the head product of the first fractionation of the collective fraction. Y

4. A process as claimed in claim 2, in which steam is introduced into sump products resulting from the fractionation steps. V

5. A process as claimed in claim 2, in which part of the fluoranthene-pyrene fraction is cooled to ahoi1t120 C. and used as a washing fluid for the Waste gases of vacuum fractionations;

6. A process as claimed in claim 5, in which used washing fluid is added in acycle to the head product of the first fractionation of the collective fraction.

7.- A process as claimed in claim 1, in which vaporization ofthe ingredients having boiling points in the range of 300-400 C. is carried out under a higher vacuum than the subsequent fractional vacuum distillations of the collective fraction.

References Cited in the file of this patent UNITED STATES PATENTS 2,029,833 MacC'ubBin er a1. Feb. 4, 1936 2,099,434 Culbertson Nov. 16, 1937 2,203,930 Smith J ne 11, 1940 2,250,072 Wilton Oct. 21, 1941 2,594,352 Schmalenbach Apr. 29, 1952 FOREIGN PATENTS 303,038 Great Britain Dec. 27, 1928

Claims (1)

1. A PROCESS FOR THE RECOVERY OF HIGH-BOILING INGREDIENTS FROM COAL TAR BY DISTILLATION, COMPRISING (A) REMOVING BY DISTILLATION FROM COAL TAR THE INGREDIENTS HAVING UNDER ATMOSPHERIC PRESSURE BOILING POINTS BELOW 300*C., SUBJECTING THE RESIDUAL TAR RESULTING FROM STEP (A) TO A FIRST FRACTIONAL DISTILLATION UNDER VACUUM IN ORDER TO SEPARATE SAID RESIDUAL TAR INTO A VAPORIZED COLLECTIVE FRACTION CONTAINING INGREDIENTS HAVING UNDER ATMOSPHERIC PRESSURE BOILING POINTS IN THE RANGE OF 300*-400*C. AND A PITCH RESIDUE WHICH HAS A SOLIDIFICATION POINT OF 75*-85*C. AND IS SUBSTANTIALLY FREE FROM ANTHRACENE AND CARBAZOL, HEAT BEING SUPPLIED TO SAID PITCH RESIDUE ONLY BY CAUSING PART OF IT TO CIRCULATE THROUGH AN ELONGATED HEATING ZONE OF RESTRICTED CROSS-SECTION AT A TEMPERATURE IN THE RANGE OF 370*-410*C., WHEREBY THE PITCH RESIDUE IN THE SUMP IS HEATED TO ABOUT 350*-380*C., CONDENSING THE COLLECTIVE FRACTION AND SEPARATING THE INGREDIENTS THEREOF BY SUBSEQUENT FRACTIONAL VACUUM DISTILLATIONS CONNCETED WITH EACH OTHER IN SERIES.

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