US2868713A - Continuous distillation of coal tar - Google Patents

Description

Jan. 13, 1959 H. R41-TE E'rAl., 2,858,713

coNTIINUous nIsTILLATIoN 0F com. TAR

Filed Aug. 2, 1954 k FIRST l MAIN coLuMN wvelvroks: HEM/mcy RATTE JUL/U5 GELLER United, States Patent VC) CONTINUOUS DISTILLATION OF COAL TAR f Heinrich Ratte, Frankfurt arn'Main, and Julius-Geller, BadA Homburg vor` der Hohe, Germany, assignors to RtgersWerke'-Aktiengesellschaft, Frankfurt am Main, Germany Application August 2, -1954,Serial No. 447,182I

Claims `priority,lapplication Germany'May `14, 1949 4 Claims.v (Cl. 208-42) For separating coal tar by continuous distillation into several fractions various methods have hitherto been employed.v It has been known to separate coal tar by distillation-in'successive stages, the water and the light oil vapors first formed being distilled off under atmospheric pressure and condensed in a cooler, while lthe constituentsA of .highery .boiling points evaporated in' ,the next .following steps are drawn .off under vacuum. and treated in fractionating columns. In. this procedure only a few fractions with relatively wide .boilingranges are obtained.. Moreover, the Vheatconsumption is very vconsiderable, as .in .this lprocedure recovery of heat from the fractions .is possible only to a limited extent. Finally, in this known method the tar is superheated, so thatrthe yields ofthe oil fractions to be recovered decrease to ka correspondi-ng extent.

In. another .known process the crudeV coal tar isltrst continuously heated and dehydrated in a tubev furnace and then y'heated in a .second tube furnace until all .volatile constituents `with the exception of the pitch-like residue are evaporated. The vapors thus formed are-subjected to'fractional condensation inv one large column orrin several .columns arranged onebehindy the other. In the latter case, pitch will accumulate in the sump vof the first column,';while at thel head, through partial condensation; high boiling oils, e. g. anthracene oils, are recovered in the treatment,y of coke-.oven tar;k The constituents .not condensed in said first column are conveyed to the second column, iinithe sumpv .of which oils of lower boiling point are collected,.1andso:on, while the vaporsleavingfthehead of the. last column .consistxof light oils.

In .order to -avoid .undue overheating of the charge, n this last mentioned process, it has alreadyzbeen :proposed to operate* the fractionatingcolumns undenvacuum. However, Aas' the.. entire .charge including `the low-'boiling constituents .must be, heated/in :the tubefurnaces to .the required 'maximum 'r distillation temperature', a notxinf considerable. part Vof the volatile constituents will vbe sub-i jected to unnecessary. `thermal stress, .so -that thefoil yield. is:l correspondingly reduced; Furthermore: .then-.partial superheating of the charge results in anincreasednheat consumption. The fact that the low-boiling constituents accompany the higher boiling ingredients on'- their' way through the column and are thelast to be condensed, renders fractionationlmore ditiicult.I Furthermore,fo`r the vsame reason, :the condensation heat of .thevapors cannot Vbe sufficiently utilized for preheating 4the crude. tar.y

The kknown process has the further;V disadvantage,V that the-columns, in which `the low-boiling fractionsare ,rer

covered, are under the :highest vacuum, while. qthe. preceding columns, in .which fractions .having higher b oil. ing points .are treated,y operate -undena lowerfvacuum,l corresponding to the pressu-re .loss within the-columns.- Consequently, just in .thefractionation of higher-boiling constituents, where -a specially: lowpressure lis desirable,- the..distillation temperatures. .cannot Abe .reduced-:to .the full desired extent. Moreover, the great quantities of w 2,868,713 Patented Jan.` 13, 1959 vapors t'o be handled render the production of a high vacuum ditlcult. y y

Finally, during heating of the crude tar in the tube furnace to high temperature corrosions will occur and deposits will .be formed, which will result in the necessity of interruptions of the process for overhauling the tube furnace after it has been in operation for a certain time.

The disadvantages of the known methods are avoided in the vmethod according to the present invention, which in addition, makes possible a 'particularly economical separation of coal'tar into .a relatively great lnumber of rather sharply cut individual fractions. The new method isv carried out in. lseveral fractionating columns connected with each other in series, whereby the temperatures are increased from column to column or step to step, while the pressure is decreased from column to column, to a high vacuum of about 10-'100 mm. Hg in the last column. For further fractionation of material branched off from main columns, the latter are provided with one or more side columns. The vacuum in the individual steps can be adapted or adjusted within wide limits, and the displacement of boiling points with respect to one another in'vacuum, can be extensively utilized. The coal tar can be preheated and is then fed to a middle section of the rst main. column (I) at the head Of which the lowest boiling fraction -is taken off. This column is operated'undera pressure .of about 760 to'llOO mm. Hg. The temperature at the head of this column is about to 1409 C..and the temperature in the sump of this column is :kept'by regulation of the reflux and by a reheater at about 280 to 320 C. Infone or more side columns of the first main column, azcorrespondingnumber of fractions can `be recovered. The residue from the sump of the firstcolumnisintroduced into a middle section of the second main column (Il). Here again the lowest boiling fraction is discharged at the column head. The procedure in the third or any further main column will be similar. The second column and those following it are kept undera vacuum which isy increased from column to columnvor .step by step depending on the nature of the desired fractions. In order to increase yields as much as' possible and/or to keep decomposition of pitch-like residuek at a minimum, the lower part of the last main column can be operated under particularly high vacuum. Incontrast tothe knownmethods, in the method according to the present invention, a very good vacuum can be easily produced even with high boiling fractions, so that the fractional-distillation proceeds very gently and produces ahigh yield of oil with a relatively low amount of pitchl residue. As the amounts of vapor to be drawn off are relatively low, the vacuum can be produced at relatively low cost.

In .the procedure according to the presentinvention the condensation heat of all fractions, including high-boiling amount of heat to the tar and this great amount of heatV is applied to the tar and its fractions according to the invention .in such manner that sharply cut fractions are obtained xwith. a minimum of decomposition and maximum'savings of heat. According tothe invention, the

distillation heat is `supplied to the main columns substan`r tially vbyrindirect'heating with circulation and l.partial evaporation ofthe residues accumulating Yin the column sump. In order, to avoid oVer-heatingof the pitch-like.

residue lof the highest boiling point, according to the ini vention the residue of he penultimate main columnwhich .is to be introduced into the last mainl column, is passed,

before entering the latter, through a heater, Where it isk heated to provide it with the necessary amount of heat.

In order to carry out heating of the residue accumulating in the column sump as gently as possible, said residue is circulated through a tube furnace. In thisway the temperature drop in the heat transfer in the tube furnace `can be reduced and harmful vsuper-heating of the tar avoided. As in separating the tar into several fractions the main columns are preferably provided with side columns, heat must be supplied also to these side columns.` It has been found to be of advantage to heat the sump of these side columns by a portion of distillation residue reheated in the tube furnace. Por this purpose the side columns are provided with a suitable reboiler or reheater, through which the branched-off portion of the distillation residue is passed as the heating medium.

An apparatus for carrying out the invention is illustrated by way of example in theaccompanying drawing which is a vertical sectional diagrammatical view of an apparatus for carrying out the invention.

It is intended to separate coal tar a by distillation into the fractions: light oil b, water c, carbolic oil d, naphtha lene oil I e, naphthalene oil II f, Washing oil g, anthracene oil I h, anthracene oil II i, heavy oils j and pitch k.

The crude coal tar a passes rst into the heat exchangers 1, 2 and 3, where the head vapors of the column, with or without those of the side columns, give olf their condensation heat to the crude c'oal tar. From the heat exchanger 3 the crude tar passes to the heat exchanger 4 for the intermediate reux of main column III and finally to the heat exchanger 5, where thepitch gives off a portion of its sensible heat. Finally, the extensively preheated crude coal tar is introduced into a middle section of column I. From the head of this column, the water and the lowest boiling tar fraction b are taken off and passed through heat exchanger 1 and cooler 6 to the separating vessel 7. The light oil b passes into the receiver 8, from where the pump 9 conveys a portion of the light oil as reux to the head of column I. The quantity of the return flow is so adjusted that the temperature in the column head amounts to about 80 to 140o C. From an upper section of the column I a portion is branched otf and introduced into the sidev column 10 in which it is rectified to separate it from the lower boiling consttutents. The residue accumulating in the sump of the column I is circulated by the pump 11 through the tube furnace 12 and kept continuously in circulation. It is returned from the tube furnace into the column under expansion and partial evaporation and the evaporated constituents rise in the column.

A portion'of the residue in the sump of column I, corresponding to the difference in the inflow of coal tar and the fractions b, c, d of column I is conveyed through pipe 13 to a middle section of the column Il. As in the 4column I an absolute pressure of about 760 to 1100 mm. Hg and in the column II a pressure of about 40 to 400 mm. Hg is maintained, this pressure drop is sufficient for passing liquid from the sump of column I into ycolumn II.

From the head of column II the fraction e is taken off, which passes throught heat exchanger 2 and cooler 14 into receiver 1S. This receiver is connected to vacuum pump 16. A portion of the fraction e is pumped by pump 17 as reflux to the head of the column II, and is adjusted in such manner that a temperature of about 120 to 190 C. is maintained there.

The higher boiling fractions and g are drawn off from a middle section of the column II and rectified in side columns 18 and 19. The residue accumulating in the sump of column Il is forced by the pump 20 through 'the tube furnace 21, the pump keeping said residue in constant circulation. Here again a portion of the residue is evaporated, Whilst a portion corresponding to the inow to column II is forced by the pump 22 through the tube furnace 23 to a middle section of column III.

From the head of this column the fraction. h, i. e.

' l'anthracene oil I, is taken off, which by Way of the heat Y. exchanger 3 and cooler 24 passes into receiver 25. This receiver is connected to vacuum pump 26. As the coolers 14 and 24 are exposed to relatively high temperatures, salts are deposited from cooling water in the coolers on the water side and thereby hinder heat exchange in the cooler. Therefore, it is advisable to use in these coolers a suitable tar oil as cooling medium. i

The pump 27 conveys a portion of fraction h into the head of the column III, the inflow being so vregulated that the temperature at the head of this column is in the range of to 160 C.

In order to obtain the highest possible yield of oil while using the lowest possible distillation temperature, the absolute pressure in the column III is reduced to about 10-100, preferably 10-40 mm. Hg. In order to assist this expedient, liquid is drawn o from an upper tray of column III and cooled in the heat exchanger 4. From there the pump 28 conveys the cooled liquid back to the adjacent section of column III. This withdrawal of heat reduces the amount of vapors in the upper part of the column and on the way to the head after the heat exchanger, causing considerable reduction of the pressure loss at these places.

The fraction i. e. anthracene oil II, is conveyed from a middle section of column III into side column 29 and is rectified there. This subsidiary column is provided, like the side columns of the preceding main columns with reheaters which can be heated with superheatedwater vapor. A particularly economical heating can be obtained, if according to the invention a portion' of the residues circulating through the tube furnaces is branched off behind the tube surfaces and if this branched otf material gives off its condensation heat and some of its sensible heat to the respective reheater of a side column. The fractionation in the side columns leads to a particularly sharp separation, if the temperature in the sump of the side columns is kept unchanged by corresponding regulation of the inow.

The portions of the charge, which do not evaporate on the supply tray of column III, are subjected in the lower part 30 of the column to a particularly high vacuum produced by the vacuum pump 31. By suitable construction of the lower column part, the pressure loss between evaporation and condensation can be kept so small that an absolute pressure of about 10 mm. Hg can be maintained. The condensate thus obtained is discharged separately, as heavy oils j by the condenser 32 while the pitch, which has a temperature of about 320 to 380 C. ows first into heat exchanger 5 and from there to the vacuum receivers.

Depending on the number of the desired fractions, the individual main columns can be provided with one or more side columns. Furthermore, the number of main columns can be increased beyond those shown in the above example and the drawing. By connecting or disconnecting side columns, the number and nature of the fractions can be Widely varied, so that the method of the invention can be easily adjusted to the market conditions.

It will be understood from the above that the process of the present invention results in the recovery of a considerable number of sharply cut fractions and products from coal tar, by the application of a high amount of heat in such manner that no undue thermal decomposition occurs and, due to the re-use of heat, the consumption of heat in the process is kept at a very low level. These results are attained by carrying out continuous fractionation of coal tar in several steps while subjecting it to heating at temperatures which increase from step to step but at pressures which decrease from step to step up to a high vacuum in the last step, in the above described novel manner, including the use of main columns, each of which is provided with at least one side column and causing the distillation residue of each main column, except the last column, to be reheated by circulation through a reheater and then to be returned to the column under expansion and partial evaporation, whereupon the thus reheated sump liquid, including the pitch ingredients, is passed into a rectifying stripping zone of the adjacent main column.

It will be also understood that the invention is not limited to the specic steps, conditions, devices and other details described above and illustrated in the drawings and can be carried out with various modifications. For example, instead of the 3 main columns shown in the drawings, more main columns, e. g. 4 or 6 main columns can be used and the number of side columns can also be varied.

In carrying out the present invention branched of portions of the material treated in the main columns are introduced into an upper section of a side column for further rectification, after which the sump liquid of the side column is discharged, whilehead products of the side column are reintroduced into the respective main column. The main columns are conventlonal fractionating columns containing e. g. 40 to 60 trays, and the Column Head Tem- Sump Tem- Head Pressure,

perature, C. perature, C. mm. Hg

. 80 to 140-.- abt. 280 to 320.. abt. 760 to 1,100. 110 to 180-- abt. 150 to 220 abt. 800 to 1,260.

120 to 190.. abt. 300 to 350 abt. 40 to 400. 130 to 200.. abt. 140 to 220" abt. 50 to 425. 140 to 210-- abt. 160 to 280-. abt. 60 to 450. 150 to 260.- abt. 320 to 380-. abt. 10 to 100. 210 to 340.. abt. 240 to 340.. abt. to 125. 240 to 380.- abt. 10 to 40.

The side columns may be provided with reboilers as shown in the drawing, or the sump products of the side columns may be reheated by circulation through tube furnaces, as the sump products of the main columns.

The lower part of the main column III is provided with evaporating plates which will cause no substantial pressure loss. Thereby it is possible to supply heat eventually also to the sump of the main column lII, so that the major part of the volatile constituents of the pitch can be distilled 01T.

The terms naphthalene oil I and naphthalene oil II are used to denote fractions containing mainly naphthalene as crystallizable solid hydrocarbon and having a boiling range of 215 to 221 C. and 237 to 241 C., respectively. The term anthracene oil I and anthracene oil II, are used to denote fractions containing anthracene in mixture with other crystallizable ingredients and having boiling ranges of 332 to 344. C. and 344 to 366 C., respectively.

Reference is made to` our co-pendng application Ser. No. 192,702, filed on October 28, 1950, for Continuous Distillation of Tars and Hydrocarbon Oils of All Kinds, now abandoned, of which this is a continuation-impart.

What is claimed is:

1. A process for the continuous rectication of coal tar to sharply cut fractions and to enriched individual components of the coal tar, said fractions and com ponents having, under a pressure of 40 mm. Hg, boiling points up to 350 C., which comprises a combination of the following steps: (a) introducing the coal tar for stepwise fractionation into a plurality of main fractionat- 6 ing steps having a head zone, a middle rectifying stripping zone and a sump zone and following each other in series, whereby the crude coal tar is introduced into said middle zone of the first main fractionating step, in which water and low boiling oilsare driven oif, and which is operated under an absolute pressure of about 760 to 1100 mm., at a head temperature of about to 140 C., and a sump temperature of about 280 to 320 C., and the temperature is gradually increased and the pressure is gradually decreased in the subsequent main lfractionating steps in the direction of the last main fractionating step which is operated under an absolute pressure of 10 to 100 mm. Hg, a head temperature of about 150 to 260 C. and a sump temperature of about 320-3 80 C., and each main fractionating step has branched oi therefrom at least one side fractionating step for further fractionating parts of the tar branched olf from said main steps; (b) subjecting sump liquid accumulating from each main fractionation step, with the exception of the last main fractionating step, to non-destructive indirect heating in a step consisting of continuous forced circulation through a tube furnace and continuous separation into vaporous and liquid ingredients upon its return to the sump zone of the same main fractionating step under expansion and partial evaporation, in order to mainly supply to the process the heat required for said sharp fractionation; (c) passing sump liquid from the sump zone `of each main fractionation step, with the exception of the last but one main fractionation step, into the rectifying stripping zone of the succeeding main fractionating step and (d) continuously discharging the products thus separated by fractionation.

2. A process as claimed in claim 1, which comprises a rst, second and last main fractionating step, the second fractionating step being operated under an absolute pressure of 40-400 mm. Hg, at a head temperature of about to 190 C. and a sump temperature of about 300 to 350 C.

3. A process as claimed in claim 2, in which the residue of material from the second main fractionating step is subjected to indirect heating prior to its introduction into the last main fractionating step and subjected to expansion and rectification under a pressure of 10 to 40 mm. Hg for the rectiiication of the residual volatile tar components.

4. A process as claimed in claim 2, in which the sump product in the last main fractionating step is subjected to a higher vacuum at the lower level of said last main fractionating step than the vacuum at an upper level of this main fractionating step and the vapors formed at said lower level are separately discharged from said last main fractionating step.

References Cited in the le of this patent UNITED STATES PATENTS Radasch Mar. 30, 1954 OTHER REFERENCESA Petroleum Renery Engineering, by Nelson, 3rd edition, McGraw-Hill Book Co. (1949), pages 208-9 relied f On.

Claims (1)

1. A PROCESS FOR THE CONTINUOUS RECTIFICATION OF COAL TAR TO SHARPLY CUT FRACTIONS AND TO ENRICHED INDIVIDUAL COMPONENTS OF THE COAL TAR, SAID FRACTIONS AND COMPONENTS HAVING, UNDER A PRESSURE OF 40 MM. HG, BOILING POINTS UP TO 350* C., WHICH COMPRISES A COMBINATION OF THE FOLLOWING STEPS: (A) INTRODUCING THE COAL TAR FOR STEPWISE FRACTIONATION INTO A PLURALITY OF MAIN FRACTIONATING STEPS HAVING A HEAD ZONE, A MIDDLE RECTIFYING STRIPPING ZONE AND A SUMP ZONE AND FOLLOWING EACH OTHER IN SERIES, WHEREBY THE CRUDE COAL TAR IS INTRODUCED INTO SAID MIDDLE ZONE OF THE FIRST MAIN FRACTIONATING STEP, IN WHICH WATER AND LOW BOILING OILS ARE DRIVEN OFF, AND WHICH IS OPERATED UNDER AN ABSOLUTE PRESSURE OF ABOUT 760 TO 1100 MM., AT A HEAD TEMPERATURE OF ABOUT 80 TO 140* C., AND A SUMP TEMPERATURE OF ABOUT 280 TO 320* C., AND THE TEMPERATURE IS GRADUALLY INCREASED AND THE PRESSURE IS GRADUALLY DECREASED IN THE SUBSEQUENT MAIN FRACTIONATING STEP WHICH THE DIRECTION OF THE LAST MAIN FRACTIONATING STEP WHICH IS OPERATED UNDER AN ABSOLUTE PRESSURE OF 10 TO 100 MM. HG, A HEAD TEMPERATURE OF ABOUT 150 TO 260* C. AND A SUMP TEMPERATURE OF ABOUT 320-380* C., AND EACH MAIN FRACTIONATING STEP HAS BRANCHED OFF THEREFROM AT LEAST ONE SIDE FRACTIONATING STEP FOR FURTHER FRACTIONATING PARTS OF THE TAR BRANCHED OFF FROM SAID MAIN STEPS; (B) SUBJECTING SUMP LIQUID ACCUMULATING FROM EACH MAIN FRACTIONATION STEP, WITH THE EXCEPTION OF THE LAST MAIN FRACTIONATING STEP, TO NON-DESTRUCTIVE INDIRECT HEATING IN A STEP CONSISTING OF CONTINUOUS FORCED CIRCULATION THROUGH A TUBE FURNACE AND CONTINUOUS SEPARATION INTO VAPOROUS AND LIQUID INGREDIENTS UPON ITS RETURN TO THE SUMP ZONE OF THE SAME MAIN FRACTIONATING STEP UNDER EXPANSION AND PARTIAL EVAPORATION, IN ORDER TO MAINLY SUPPLY TO THE PROCESS THE HEAT REQUIRED FOR SAID SHARP FRACTIONATION; (C) PASSING SUMP LIQUID FROM THE SUMP ZONE OF EACH MAIN FRACTIONATION STEP, WITH THE EXCEPTION OF THE LAST BUT ONE MAIN FRACTIONATION STEP, INTO THE RECTIFYING STRIPPING ZONE OF THE SUCCEEDING MAIN FRACTIONATING STEP AND (D) CONTINUOUSLY DISCHARGING THE PRODUCTS THUS SEPARATED BY FRACTIONATION.

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