US3135681A - Method of dewatering tar - Google Patents

Description

June 2, 1954 J. 1 PFLAsTi-:RER 3,135,631

METHOD OF' DEWATERING TAR Filed May l1, 1962 gf?, s :s L

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Q) L Q l@ N l` United States Patent O 3,135,631 METHOD GF DEWA'I'ERING TAR James L. Pflasterer, Portage, Ind., assigner to United States Steel Corporation, a corporation of New .ersey Filed May 11, 1962, Ser. No. 194,133 8 Claims. (Cl. 20S- 39) This invention relates generally to the recovery of crude coal tar resulting from the destructive distillation of bituminous coal, and more particularly to methods for dewatering the crude coal tar thus produced, improving the subsequent tar distillation and products obtained therefrom.

ln conventional coke-plant practice, the foul gas from the coke ovens is cooled by spraying with flushing liquor which, along with the condensible tarsand other compounds, ows to a decanter for separation of the liquor from the condensed tar. The flushing liquor is recycled for further gas cooling while the condensed tar is pumped to a wet-tar storage tank. The gas and remaining vapors are further cooled in primary coolers, the condensate comprising tar and liquor being decanted and separated. The tar is pumped to a wet-tar storage tank. Primary-cooler tar is less viscous and has a lower speciic-gravity range than flushing-liquor tar. These tars may be combined or separately stored for diierent end uses. In the method to be describedthey are combined in a Wet-tar storage tank for subsequent dewatering and distillation.

Production of crude coal tar as hereinabove described is effected by condensation of a vapor to a liquid. The condensation results in entrainment of liquor with the tar which does not separate completely in the described decantations, resulting in so-called wet tar. The liquor contains a number of dissolved salts, including ammonium chloride which, in any subsequent tar distillation are deleterious to steel equipment, especially at the higher temperatures attained therein. Prior to any distillation itis preferable to dewater the tar to decrease equipment size and heat requirements. Dewatering also vreduces the entrainedliquor content and the salts dissolved therein.

ln conventional practice various methods of separating the water from high moisture tars have been used. These methods include the use of centrifuges, additives from various outside sources, decanters, and flash-dehydration units. While some of these methods are reasonably successful, they may in turn be expensive and time consuming or the additive may cause ditiiculties later in the operation, due to disposal or adulteration problems.

I have discovered that numerous advantages over conventional practices are obtained by adding a coal-tar distillate fraction to the heavy, water-containing tar, and agitating the contents of the tank by recirculating the admixture until it is fairly homogeneous. The tank and its contents are then held in a relatively quiescent state, between about 90 and 98 C. for a period of at least about 24 hours, after which time, dewatered tar, containing between about l and 3% moisture may be drawn ofI the bottom. Thev water or liquor layer, which separates and rises to the top of the tank, may be disposed of by conventional means such as siphoning and removal through cocks on the side of the tank.

Itis the generally accepted belief of the chemical industry that the greater the difference between the specific gravities of two liquids, the better will be the resultant phase separation upon gravity settling. While this is generally true, my invention is an exception to accepted principle. This is so because of the relationship I have found between surface tension, viscosity and specific gravity. I have discovered that, with a reduction in the specic-gravity difference between water and tar by the admixture of a coal-tar distillate fraction with wet coal tar, there is substantial improvement bothv in the speed 3,135,681 Patented June 2, 1964 and the degree of water separation therefrom. l believe the improved separation occurs because the distillate fraction reduces the surface tension of the tar, allowing the occluded water droplets to ascend to the surface. To increase the mobility of the water droplets an ascending temperature gradient should be maintained from the top of the tar-storage tank to the bottom, between about 90 and 98 C. in order to reduce further the restrictive tar viscosity. Tar recovered from most coke-ovens has a surface tension in a range of about 40 and 42 dynes/cm. Since the procedure for determination of surface tension is delicate and not susceptible to rapid performance for plant control, a correlation has been established between the surface-tension effects and rapidly determinable specitic gravity for this purpose, in practicing my invention.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawing illustrating the present preferred practice. The single ligure of the drawing is a diagrammatic representation of a system for carrying out the method of my invention.

Referring now in detail to the drawing, wet coal tar is introduced by a pipe 10 from a decanter (not shown) to a wet-tar storage tank 12, which may be one of a number of such tanks found in a tar plant. The entrained-liquor or water content of the tar may vary widely. It is usually between about 5% and 12% by volume of tar, but may be as high as 20%. The specific gravity thereof may vary between about 1.20 and 1.28 at Y C., having an and a surface tension of about 33" dynes/cm. The light;

creosote-oil fraction is injected through pipe 14 into pipe 10. Control valve 16 supplies suicient oil to reduce the speciiic gravity of the admixture to between about'LlO v and 1.18 at between about 5% and 20% The amount of oil achnxed with'the wet tar may vary (by volume) depending upon the water content of the tar, its initial viscosity, surface tension and speciiic gravity. Steam coil lserves to maintain the etiluent-tar temperature between about and 95 C.

The surface tension of the homogeneous admixture should be sufficiently low to release the water droplets occluded in the tar mixture flowing from the bottom of tank 12 is transferred by pump 20 through pipe 22 into residence-time storage tank 24. Steam-heating coil 26 serves to maintain the tar-temperature gradient from top to bottom in this tank between about C. and 98 C. This temperature gradient further reduces the viscosity of the wet tar-oil admixture. The mobility of the water droplets is increased and the water-tar separation is thereby facilitated.. When storage tank 24 is suicientlyfull, the contents are` thoroughly mixed by recirculating them through pump 30, valve 32, pipe 34, and valve 36 into When the admixture is fairly homogeneous, as evidenced by a specic gravity variation in the contents of not more than about .02, pump 30 is stopped, valves 28, 32 and 35 valve 28, tank 24.

C.) and preferably is a light creosote oil boiling overV and to permit them to rise. The ad-YV are closed and the contents of the tank are then held in a relatively quiescent state for a period of at least about 24 hours at the aforesaid temperature gradient, during Vwhich time the water separates from the tar and rises to the top of tank 24. When the WaterA separation is completed, water is drawn olfr the top of tankY 24 through test cocks 24A and pipe 24B, whence it returns to the aforementioned decanters or is recycledwas ushing liquor for further gas cooling. Tar containing between about 1.8% and 2.2% moisture, or less, ows from the bottom of tank 24 and is movedjby pump 30 and pipe 4@ into dewateredtar tank 42.V The ,dewateredl tar flows from the bottom Vof tank 42 and is delivered by a pump 46 through a pipe48 to tar a,135,es1

adrnixture in the Wet-tar storage tank will vary in temperature between about 75 and 95 C. according to the season and liquor content. The temperature will normal- Y ly be maintained higher in colderV weather'and as the liquor content increases, to aid in the separation of the liquor. It is preferable for good mixing to add to the wet tar a coal-tar distillate fraction through pipe 14, although the total fraction or any additional fraction required to attain a preferred admixture speciic gravity may be made through pipe 50.

Y lt should not be assumed from the above, that it is mandatory tohave a quiescent periodV of kat least about '24 f hours before. more lwet-tar-oil admixture is pumped to the stills (not shown) for fractionation. and the recovery of Y pitch. Y

The above-described, relatively. quiescent state continuing for a periodrof at least about 24 hours at a temperature gradient from top to bottom ofthe tank between about 90 andr98r C., is termed the residencetime. Once established for Vany ,tank,rthe advantages of. my invention are obtained by substantiallyl balancing admixture admission and dewatered-tar withdrawal at a rate such as Y to permit any oneY gallon of tar to remain in a tank for atleast about 24 hours, as its residence time.V The longer VtheV Vresidence time over 'about 24 hours, theV better the Y water-tar separation. However, a practical limit for resi-` dence time is vabout 48 hours, since VVbeyond this time theV moisture content of the dewatered tar will not be substantially below about 1%.Y Y l A speciic example of the practice of the invention giving quantities` involved is set forth below.

tank and dewatered4 tar is withdrawn therefrom for distilation. After a fairly homogeneous admixturey is astablished in tank 24 and the quiescent Vperiod begins, it.

has been found that an 8-12% moisture tar willhave hadA Balanced mixture in the top of the tank is Ywithdrawn as dewatered tar from the bottom thereof. During this period, the

'In a typical coke lant producing about V150,000gallonsy f of tar per day, the combined iiushing liquor and primary cooler tars contain between about 8% and 12% of entrained liquor, have a specific gravity between about 1.22 and 1.24 and a surface tension betweenabout 40 and 42 dynes/cm. The wet tar is pumped throughpipe 10 to,V

tank 12, which may have'a capacity of 200,000 to k500,000 gallons.V A creosote-oil fraction is pumped through pipe 14 into pipe 10.` This fraction Vhas a boiling Vrange',bej tween about 211 and 305 C., anaverage specific gravity` of 1.060 at an averagesurface tension of 33 dynes/ cm. The fraction is admixed with thev wet tar in a quantityv suicient to produce an admixture having a specicgravity between about 1.14 and 1.16 at l The quantity of the fraction admixedis between about 8% and.10% by volume of the wet tar dependent upon Vthe. source of the original tar. Coil 18 heats the admixture in tank 12 to between about 85 and 95 C. The admixture Yis pumped to a1,000,000 gallon dwell-time tank 24 and recirculatedby pump 30 until the admixture isfairly homogeneous, that is, until the specific gravityY thereof does not vary much more than .02 rbetween successive pump samples. Should the specific gravity of the admixture be' greater than about 1.16, additional light creosote oil may be added to the admixture in tank 24 through pipe 50, vvalve 52, pump and pipe 22. After thorough mixing, the contents of tank 24 are kept relatively quiescent forabout 2 4 hours. The temperature gradient from top to bottom of tank 24 is maintained between about 90 and 95 C. Thereafter, dewatered tar having a moisture content between about 17.8 and-2.2% is Withdrawn from tank 24 and pumped to tank 42 or direct- .ly to a still (not shown) at a rate of about 100`gallons per minute. The separated liquor is removed through cocks 24A and the input and output ofthe tank are balanced.

optimum reduction in the moisture content willV be pro-v gressively attained. Y I

Any fraction having a specific gravity between `about 1.04 and 1.12 at Y f may be used. While I prefer an admixture having a Below about 11.10 specific gravity, improved surface tension Yelfects will be overshadowed by the danger of creating troublesome emulsions. Above( about 1.18 speciic gravity, water" lseparation will become progressively more diicult' since the surface tension of the admixture approaches that of undiluted' tar. Ordin-v arily, the fraction lis admixed -to between about 8% and 10% by volume of the wet tar, the maximum being about` 20%. The fraction, whatever its boiling( range and grav,-

ity, is totally miscible in all proportions with the tar.

The maximum amount of a fraction admixed is usually determined by the size of distillation and handling equipment in an existing tar plant. Ordinarily, beyond athree-r fold increase in'an'yl boiling-range fraction over that naturally occurring in undiluted tar will require vthe uneconomid reduction in overall admixfure throughput in the tar still. The so-called wide-boiling range creosote oil fractionn (B.P.` 200 C.-350 C.) comprisingV lightandv heavy-Jl creosote oils may be admixed.V The light'creosote oilV is preferred for its more pronounced, surface-tension ei'ects.

VNarrow-boiling fractions, for example, a tar-acid fraction or a methyl-naphthalene fraction may be-used toladvantage,`if the economics permit. It must be understood, however, that the cheapest widefboiling fraction is effective. i

' cost of obtaining a narrow-boiling fraction or product.

QIt has been found that the viscosity of an adi-mixtureV with flushing-liquor and primary-cooler tars will be `re, duced suiiiciently to permit ecient separation of liquorV with bottom heating to between about 95 fand 98 C.

Within the practical limits of a temperature gradientV of on such factors as the type and quantity of the contents,`

this will produce a temperature at the-top of the/tankbetween about Y and 95 C.

The invention is characterized by severalA distinct advantages. In the irst place, the separation' of liquorfand tar is a relatively simple, inexpensive and eicient process,VV

Any further'fractionation thereof adds to theV` as compared to former practices. Suitable wide-boiling or narrow-boiling fractions are readily available in any coal-tar plant. A fraction may be recovered through distillation in unchanged form and may be reused indefinitely. Wet tar with a liquor content of 5% to 12%, and higher, can be reduced consistently to between about 1% and 2.5% liquor content.

Secondly, the addition of a coal-tar fraction improves tank cleaning and heating. The fraction is a solvent for the binders holding coke and coal dust into a sludge-form requiring frequent tank cleaning. Additionally, binder removal permits the coke and coal dust to move with the tar and increases the quinolineand benzene-insoluble content of pitch distilled from the tar. In the production of special electrode pitches this is desirable and reduces the need for addition of pulverized material.

Thirdly, my process can be easily varied to handle separately, flushing-liquor tar and primary-cooler tar, the former having both a higher viscosity and speciiic-gravity range. Utilizing the principles of this invention, tars with viscosities outside these limits may be processed.

Finally, an operator may be faced with a sudden and even prolonged period of excessively high liquor content in the incoming wet tar. In such an emergency one or more of the principles taught by my invention may be utilized. In the wet-tar tank and/or the residence-time tank there may be admixed a greater volume of a coaltar fraction. There may be a change to a fraction that will produce a lower surface tension in the admixture. The temperature gradient and residence time may be increased. Where there is a greater than preferred variation in specic gravity, the residence-time tank may be recirculated. Were there assurance that such an emergency would never occur, it would be possible to eliminate the wet-tar tank, the dewatered-tar tank and operate solely with a residence-time tank.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

l. A method of dewatering wet coal tar comprising admixing therewith a coal-tar creosote-oil fraction, conducting the admixture to the top of a storage tank, maintaining an ascending temperature gradient in said admixture from the top of said tank to the bottom while allowing the admiXture to separate, conducting separated liquor from the top of said tank and dewatered tar from the bottom thereof.

2. A method as defined in claim 1, characterized by admixing sufficient of said creosote-oil fraction to produce 6 an admixture having a specific gravity between about 1.10 and 1.18 at 3. A method as defined in claim 1, characterized by said creosote-oil fraction having a boiling range between about 200 and 350 C.

4. A method as defined in claim 1 characterized by recirculating the admixture in said storage tank, prior to allowing the admixture to separate, until the contents thereof have a specific gravity variation of no more than about .02.

5. A method as defined in claim 1, characterized by maintaining said temperature gradient between about and 98 C.

6. A method as defined in claim 1, characterized by substantially balancing dewatered tar withdrawal and admixture admission to said tank and at a rate such as to permit a residence time for the admixture of at least about 24 hours.

7. A method of dewatering wet coal tar containing between about 5% and 12% entrained liquor by volume, comprising admixing therewith suicient coal-tar creosoteoil fraction having a boiling range between about 200 and 310 C. to produce an admixture having a specific gravity of between about 1.14 and 1.16 at conducting the admixture to the top of a storage tank, recirculating the admixture in the tank to mix it until substantially all of it is in said range of speciiic gravity, maintaining an ascending temperature gradient in said admiXture from the top of said tank to the bottom of between about 90 and 98 C., substantially balancing liquor withdrawal from the top of said tank plus dewatered tar withdrawal from the bottom thereof with admixture admission to said tank and at a rate such as to permit a dwell time for the admixture of at least about 24 hours.

8. A method as delined in claim l, characterized by distilling dewatered tar and recovering pitch, thereby removing binders for sludge-forming, quinolineand benzene-insolubles in the tar and substantially reducing sludge formation while dewatering tar and recovering pitch with an increased content of said quinolineand benzene-insolubles.

Kunk Dec. 1s, 1956

Claims (1)

1. A METHOD OF DEWATERING WET COAL TAR COMPRISING ADMIXING THEREWITH A COAL-TAR CREOSOTE-OIL FRACTION, CONDUCTING THE ADMIXTURE TO THE TOP OF A STORAGE TANK, MAINTAINING AN ASCENDING TEMPERATURE GRADIENT IN SAID ADMIXTURE FROM THE TOP OF SAID TANK TO THE BOTTOM WHILE ALLOWING THE ADMIXTURE TO SEPARATE, CONDUCTING SEPARATED LIQUOR FROM THE TOP OF SAID TANK AND DEWATERED TAR FROM THE BOTTOM THEREOF.

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