US2774716A - Process for removing finely divided solids from raw low temperature carbonization coal tars - Google Patents

Description

.. q 1956 M. D. KULIK 2,774,716

PROCESS FOR REMOVING FINEILY DIVIDED souns FROM RAM LOW TEMPERATURE CARBQNIZATION COAL TARS Filed Jan. 29, 1954 LT.O|L

Rw A TANK 24 2e 34 @IB 33 LHQUOR STRIPPING COLUMN 3o commuous INVENTOR METRO D. KULIK ATTORNEY United States Patent O" Metro D. Kulik, Pittsburgh, Pa., assignor to Pittsburgh Consolidation Coal Company, Pittsburgh, Pa., a corporation of Pennsylvania Application January 29, 1954, Serial No. 406,926

11 Claims. (Cl. 196 14.1)

The present invention relates to a process for separating finely divided solids from low temperature carbonization coal 'tars, and more particularly to a process for effecting this separation by agglomerating the solids with a portion of the tar itself.

The present application is a continuation-in-part of copending U. S. application S. N. 260,265, filed by me on December 6, 1951, now abandoned, and entitled Process for Removing Finely Divided Solids from Raw Low Temperature Carbonization Coal Tars.

In continuous processes for carbonizing coal at low temperature, the tar product often contains a high concentration of finely divided particles of coal or partially devolatilized solid fuel. The presence of these particles in the product tar is occasioned by entrainment of'the particles in the efiluent gas and tar vapors from the carbonization process. Some low temperature tars will contain 15 to 25 percent by weight of solids. Frequently more than 50 percent by weight of these solids will pass through a 325 mesh U. S. standard screen. Because of the very fine consistency of thesesolids, they will not settle out of the raw tar onjstanding. fMoreover, their removal from the tar by filtration is impractical since the particles instantly clog the interstices of any conventional filtering equipment. The high viscosity of the tar and the hard sediment makes continuous centrifuging 'inefiective for the removalof'these solids.

The presence of these solids in the tar seriously reduces the value of the tar. Conventional processing of the solids laden raw tar for recovery of valuable tar acids and oils by such methods as distillation or extraction is difficult. Moreover if the solids laden tar is refined, the solids become even more'concentrated infthe heavy pitch remaining after the lighter, morevaluable constitutents have' been removed. Hence the' ash content of the residual pitch increases and "its value correspondingly decreases,

The primary object of this invention is to recover a 7 By the term coal I comprehend all ranks of coail,in--

eluding bituminous, sub-bituminous, brown coal,lign ite', 'andthelike. s i V. According to the present process, raw tar containing finely divided solids is agitated vwith .a mixture: of a predominantly non-aromatic low boiling distillate tar oil containing a'controlled amount of low boilingv tar'acids.

Forthe'purposeof this specification, this rnigitureiof 2,774,716 Patented Dec. 18, 1956 "ice boiling tar oils and a small amount of low boiling tar acids shall be'termed solvent.

The solvent dissolves a major portion of the normally liquid tar and rejects from solution a small amount of the heavy pitch, Droplets of this rejected pitch become coated with the powdery solidsfrom the tar and settle out from the mixture in discrete agglomerated particles which are recovered separately from the liquid tar. The solvent is stripped from the liquid tar and recycled. A significant feature of the present invention is the autogenous production of small amounts of solvent from the raw tar being treated. This feature permits continuous operation of the solids separation process without requiring the addition of extrinsic solvent.

The low boiling tar oils which form the solvent are those low boiling hydrocarbons normally present in low temperature carbonization tars. Those low boiling tar oils represent less than about 5 percent of the weight of low temperature carbonization tars, and usually less than about 2 percent, depending upon the distillation techniques selected. The composition of the low boiling tar oil from a typical low temperature carbonization tar is presented in The Journal of the Institute of Fuel, vol. XX, No. 113, April 1947, at page 111. As set forth in Table IV of the reference, the low boiling oil represents about 2 percent of the total condensed liquid products from low temperature carbonization. Its composition is as follows (in percent):

Crude phenols 12' Saturated hydrocarbons (parafirns and naphthenes)- 21.5 Olefinic hydrocarbons 34.5 Aromatic hydrocarbons 30 The exact composition of low boiling tar oils may vary somewhat from this typical analysis, but in all cases, low temperature carbonization tars are predomiv nantly non-aromatic in contrast to the tars resulting from high temperature carbonization. v

For the purpose of this invention I prefer to use as solventthe entire low boiling tar distillate fraction having a boiling end point temperature below 200 C. but above 155 C. The initial boiling point of this fraction is generally about C., but may vary somewhat with'the nature of the tar from which the low boiling tar oil is derived, While the entire fraction is preferred for ease of operation, nevertheless any narrower fraction thereof may be employed; The low boiling tar oil must in addition contain at least 2 percent by volume of tar acid in order to comprise the solvent of this invention. Insome instances the solvent may be a distillate fraction of the low temperature carbonization tar being an end point in excess of-ZOO" C. In other instances the solvent may be ablendof two or more distillate fractions ,of the tar.

I have discoveredthat a solvent fulfilling the specifications and suited to the purposes of this invention can be made without the deliberateuaddition of tar acids by recovering the.,e ntire distillate fraction from the low temperature carbonization tar which inherently contains the requisite quantity of tar acids. Thedis'tillation end temperature may beselectedgso as to control the tar acids content quite. closely. Those fractionshaving-a distillation end point below about C. will contain virtually no tar acids whereas fractions having-a distillation end point of about 200 C. might contain 15 to, 30 percent tar acids. .;The solvent from low temperature carbonization tar, by itself, is not wholly miscible with a batch of raw tar; instead, the solvent effects-a separation of a tacky nonfi-lterable pitch phase together with the particles of solid fuel-contained in the raw tar The liquid portion of raw low temperature carbonization tars is wholly' miscible with tar acids by themselves; thus tar acids alone will not reject pitch from low temperature carbonization tars.

Nevertheless, if solids-laden tars are treated with low boiling tar oils containing the proper amount of tar acids for the particular raw tar, an optimum quantity of pitch can be rejected from the raw tar to bind virtually all the raw tar solids into readily filterable largeragglomerated particles. Insufficient taracids in the treating solvent re.- sult in excessive pitch rejection and accompanying nonfilterability. Excessive tar acids in the treating solvent result in insufiicient pitch rejection and very little improvement in the filterability of the raw tar.

The treating solvent should contain at least 2 and preferably less than percent by volume :of tar acids, with the balance comprising low boiling tar oils. The exact optimum concentration depends upon the raw tar being treated. Factors controlling the raw tar composition include the type of coal used .in the carbonization process, the nature of the carbonization process, the technique of tar recovery and the fraction of solid fuel particles found in the raw tar.

For a better understanding of my invention and its objects, reference should be had to the drawing which is a schematic representation of the preferred embodiment of my invention.

In the drawing, 19 is a mixing vessel, 12 is a continuous filter, 14 is a solvent stripping column, 16 is a cooler, 18 is a condenser, and 20 is a solvent storage vessel.

Raw tar from a low temperature carbonization process is introduced continuously into the mixing vessel 19 through conduit 24. This raw tar contains all the normally condensible overhead products from a low temperature carbonization process. In addition, the tar contains particles of solid fuel which have been entrained in the vapors leaving the carbonizing apparatus. These particles of solid fuel are coal or partially devolatilized coal or a mixture of both, depending upon the carbonizing apparatus and the starting coal. In the Disco carbonization process, for example, tars are recovered containing up to weight percent of solid fuel particles, which are coal and partially devolatilized fuel, with the coal particles predominating. In other carbonization processes, the solid fuel particles in the tar are almost exclusively partially devolatilized fuel. These fines, of course, possess a high ash composition.

Solvent from vessel 20 is introduced into the mixing vessel 10 through conduit 26. In general from 0.5 to 3.0 volumes of solvent are employed for-each volume of raw tar. Preferably the solvent to raw tar ratio is about 1:1.

Low solvent to tar ratios (i. e., less than 0.521) produce tacky, non-filterable pitch precipitates. There appears to be little advantage in increasing the solvent to tar ratio above 3:1.

in the mixing vessel 10, the intermixing of the solvent and the raw tar rejects a portion of the heavy pitch from the liquid phase. Droplets of this rejected, sticky pitch become coated with the powdery solids from the raw tar and settle out as discrete, non-tacky, agglomerated particles.

The mixture of agglomerated particles, tar and solvent passes from the mixing vessel 10 through conduit 28 to a continuous filter 12. The precipitated pitch and agglomerated solids are collected as a filter cake which can be recovered and briquetted along with additional coal or char to produce a solid fuel briquet. Alternatively, since the process of this invention is integrated with a low temperature carbonization operation, the filter cake can be returned to the carbonization vessel as a feed material. Since the filter cake contains some solvent and tar components, it can be treated to permit recovery of the valuable liquids.

The filtrate from the continuous filter '12 is pumped Development of the Disco iProcess :of Low Temperature Carbonizationw'by C. E. Lesher, Mining Engineering 4, 287-99 (March 1952).

through conduit 30 to a stripping column where the solvent is distilled overhead. Since the raw tar from the carbonization plant contains low boiling tar oils, as well as low boiling tar acids, there are no solvent make-up problems associated with the present process. Each unit of raw tar being treated supplies a small amount of the low boiling tar oils and tar acids which constitute the solvent. This autogenous solvent production is sufiicient to compensate for any reasonable operating solvent losses in the system.

it is significant to note that the solvent and the filtered tar need not be separated immediately. After the initial pitch rejection in the agitation stage, no further separation Will occur through the action of the solvent. The solvent (as the term is defined in this application) of this invention has no tendency to react continuously with the heavy components of the pitch. Not all the pitch contained in the raw tar is rejected by the solvent treatment of this invention. Much of the pitch remains in solution after mixing with the solvent and is recovered as a valuable ash-free product.

The composition of the solvent is controlled by regulating the operating conditions of the stripping column which provides an overhead low boiling tar oil product containing the desired tar acids concentration. The lowest boiling tar acid, phenol, has a normal boiling temperature of l C. in its pure form. However, small amounts of phenols and other low boiling tar acids form azeotropes and boil along with the low boiling tar oils at temperatures below the normal boiling point of the pure tar acids. In fact I have found that filtrate fractions having a boiling end point temperature of about C. normally contain the proper amount of tar acids required to treat some tars in the agglomerating step of the process. Thus the tar oil distillate fraction boiling below about 160 C. usually contains about 2 to 10 percent tar acids by volume resulting from the azeotropic character of the low boiling tar acids. For most raw tars, this 160 C..boiling end point fraction will be a suitable treating solvent. The discovery that this low boiling tar oil possesses tar acid azeotropes and can be used as treated solvent greatly simplifies the solids removal process.

Solvents with high tar acids concentration precipitate less pitch from the raw tar and hence permit the recovery of slightly greater quantities of liquid product tar. However, when the tar acids concentration in the solvent is too high, no pitch precipitation occurs and the objectionable solids cannot be readily removed from the liquid tar.

The stripped solvent passes overhead from the stripping column 14 through conduit 32 and condenser 18 to the solvent storage vessel 20. A portion of the overhead solvent is returned .as reflux to the column 14 through conduit 33. Any water in the solvent stream settles in vessel 20 and is withdrawn through a conduit 34. The Water is combined with carbonization plant liquor for treatment.

The bulk of the tar and pitch contained in the initial raw tar is recoveredas a bottom product from the stripping column 14 through conduit 36 and cooler 16. This tar is substantially free of any solid material and can be processed easily in conventional distillation and extraction operations. The pitch contained in the bottom product from the stripping column, because it is essentially ash-free, finds many valuable uses, e. g., as a raw material .for the production of electrode carbon.

'In order to commence commercial operation of a solids removal system for raw low temperature carbonization tars it is frequently impractical to attempt to collect a supply of low boiling tar oils to be used as solvent. Accordingly for the initial operation of a rawtar solids removal system, commercial low boilingmaterials may be obtained which will serve the same purpose .as the solvent .described herein. For example, .benzene alone is an excellent material for treating .raw tar to effect the desired agglomeration of pitch together with dry agglomerated solid particles. The benzene would circulate through the system until replaced by the autogenous solvent of this invention. Foreign solvents such as benzene migh't be selected which would efiect the proper amount of pitch rejection to bind the solids in the raw tar. Benzene, of course, is an unsatisfactory solvent for continued solids removal use because of its expense and because excessive solvent and tar are retained in the filter cakes resulting from operation with benzene.

The pitch rejection step of the present invention is quite effective in causing a particle size accretion of the solids contained in raw tar. In fact it is possible to recover solids-free tar simply by decantation of the liquid remaining above the rejected pitch and solids. Such a system, of course, could be employed in combination with a vacuum drying of the settled solids to recover the liquids associated with the solids. A centrifuging step can be substituted for the filtration step described in connection with the drawing. The presence of the solvent tends to reduce the viscosity of the tar and the increased size of the solids readily permits their centrifugal separation. 7

Example The present process was employed to remove solids from a raw tar derived from the low temperature carbonization of Pittsburgh seam coal.

The. raw tar contained about 16 percent by weight of finely divided solids which had a differentialscreen analysis as follows:

Sieve size: Weight percent on 48 0.0 100 0.6

325 48.6 through -325 50.2

This tar was unfilterable in its raw state. However, after agitating this raw tar at 24 C. with an equal volume of solvent composed of low boiling tar oils having a boiling end point of 160 C. and containing 6 percent by volume of low boiling tar acids, the mixture was filtered without difiiculty.

The liquid filtrate contained virtually no ash and the recovered solvent was a greater volume than that employed in the agitation stage. The solvent stripped tar was about 78 percent by weight of the initial raw tar. The differential screen analysis of the filter cake solids was as follows:

1 If desired, filtration rates for the tar can be increased even further by adding small quantities of coarse char to the raw tar before the solvent treatment.

As seen from the example, the present process can be operated at ordinary room temperatures. Slightly higher temperatures permit operation of the process with a reduced tar acids concentration in the solvent.

According to the provisions of the Patent Statutes, I have explained the principle, preferred construction, and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim: e 1 -f 1.A process for'removing solids consisting essentially of finely divided solid particles from the class of coal and partially devolatilized coal from raw low tempera ture carbonization tar. which comprises mixing the raw tar with 0.5 to 3.0 volumespf a predominantly nonaromatic low temperature carbonization tar distillate fraction containing at least 2 percent by volume of tar acids, separating rejected pitch and agglomerated solids, and recovering the liquids.

2. In the removal from raw low temperature carbonization tar those particles of solid fuel which become entrained in the tar during the carbonization and tar recovery processes, the improvement which comprises mixing said raw tar with 0.5 to 3.0 volumes of a solvent comprising a predominantly nonaromatic distillate fraction from low temperature carbonization tar containing at least 2 percent tar acids by volume, separating from the resulting mixture rejected pitch and agglomerated solids, and recovering the liquids.

. 3. A process for removing solids consisting essentially of finely divided particles from the class of coal and partially devolatilized coal from raw low temperature carbonization tar which comprises mixing the raw tar with 0.5 to 3.0 volumes of a predominantly nonaromatic low temperature carbonization tar distillate fraction having a boiling end point below 200? C. and containing 2 to 20 percent byvoilume of tar acids, separating rejected pitch and agglomerated solids and recovering the liquids.

4. A process for removing solids consisting essentially of finely divided particles from the class of coal and partially devolatilized coal from raw low temperature carbonization tar which comprises mixing raw low temperature carbonization tar with 0.5 to 3.0 volumes of a predominantly nonaromatic low temperature carbonization tar distillate fraction having a boiling end point below 200 C. and containing 2 to 20 percent by volume of tar acids, filtering the resulting mixture and recovering the filtrate;

5. A process for removing solids consisting essentially of finely divided particles from the class of coal and partially devolatilized coal from raw low temperature carbonization tar which comprises adding to the raw tar 0.5 to 3.0 volumes of a predominantly nonaromatic low temperature tar distillate fraction having a boiling end point below 200 C. and containing 2 to 20 percent by volume of tar acids, separating rejected pitch and agglomerated solids from the mixture, recovering the liquids, separately recovering from said-liquids a distillate fraction having a boiling end point below 200 C. and containing 2 to 20 percent by volume of tar acids, and separately recovering the remaining liquids.

6. A process for recovering ash-free tar and pitch from raw low temperature carbonization tar which comprises continuously agitating each volume of tar along with 0.5 to 3.0 volumes of solvent comprising a predominantly nonaromatic low temperature carbonization tar distillate fraction having a boiling end point below 200 C. and containing 2 to 20 percent tar acids by volume, continuously filtering the mixture of tar and solvent, recovering the filtrate, recovering from said filtrate a distillate frac- -tion having a boiling end point below 200 C. and containing 2 to 20 percent by weight of tar acids, separately recovering the remaining filtrate, and recovering said distillate fraction as solvent.

7. A continuous process for removing solids consisting of finely divided particles from the class of coal and partially devolatilized coal from raw low temperature carbonization tar which comprises continuously mixing each volume of said tar with 0.5 to 3.0 volumes of solvent comprising a predominantly nonaromatic low temperature carbonization tar distillate fraction having a boiling end point below 200 C. and containing 2 to 20 percent tar acids by volume, continuously filtering the mixture of tar and solvent, recovering the filtrate, recovering from said filtrate a distillate fraction having a boiling end point temperature below 200 C., said temperature being selected so that the distillate fraction will contain 2 to 20 percent by volume of tar acids, separately recovering the remaining filtrate and recovering said distillate fraction to be used as solvent in a cyclic manner.

8. A process for removing solids consisting essentially of finely divided particles from the class of coal and partially devolatilized coal from the raw tar produced by the low temperature carbonization of high volatile bituminous coal which comprises mixing each volume of said tar with 0.5 to 30 volumes of solvent comprising a predominantly nonaromatic distillate fraction of low temperature carbonization tar having a boiling end point below 200 C. and containing 2 to 20 percent tar acids by volume, filtering the mixture of tar and solvent, recovering the filtrate, recovering as solvent from said filtrate the distillate fraction having a boiling end point below 200 C. and separately recovering the remaining filtrate.

9. A continuous process for removing solids consisting essentially of finely divided particles from the class of coal and partially devolatilized coal from the raw tar produced by the low temperature carbonization of high volatile bituminous coal which comprises mixing each volume of said tar with 0.5 to 3.0 volumes of solvent comprising a predominantly nonaromatic distillate fraction of low temperature carbonization tar having a boiling end point below 200 C. and containing 2 to 20 percent tar acids by volume, continuous-1y filtering the mixture of tar and solvent, recovering the filtrate, recovering as solvent from said filtrate a distillate fraction having a boiling end point below 200 C., separately recovering the remaining filtrate, and recycling said distillate fraction to be used as solvent in a cyclic manner.

10. The process of claim 9 in which the high volatile bituminous coal is Pittsburgh seam coal.

11. In the removal from raw low temperature carbonization tar of those particles of solid fuel which become entrained in the tar during the carbonization and tar recovery processes, the improvement which comprises mixing said raw tar with 0.5 to 3.0 volumes of a solvent comprising a predominantly nonaromatic distillate fraction from low temperature carbonization tar having an end boiling point below 200 C. and containing at least 2 percent by volume of tar acids, separating from the resulting mixture rejected pitch and agglomerated solids and recovering the liquids.

References Cited in the file of this patent UNITED STATES PATENTS 1,960,679 Parkhurst May 29, 1934 2,631,982 Donegan Mar. 17, 1953 FOREIGN PATENTS 590,748 France Mar. 25, 1925

Claims (1)

1. A PROCESS FOR REMOVING SOLIDS CONSISTING ESSENTIALLY OF FINELY DIVIDED SOLID PARTICLES FROM THE CLASS OF COAL AND PARTIALLY DEVOLATILIZED COAL FROM A RAW LOW TEMPERATURE CARBONIZATION TAR WHICH COMPRISES MIXING THE RAW TAR WITH 0.5 TO 3.0 VOLUMES OF A PREDOMINANTLY NONAROMATIC LOW TEMPERATURE CARBONIZATION TAR DISTILLATE FRACTION CONTAINING AT LEAST 2 PERCENT BY VOLUME OF TAR ACIDS, SEPARATING REJECTED PITCH AND AGGLOMERATED SOLIDS, AND RECOVERING THE LIQUIDS.

Images