US2002704A - Recovery of tar acids, etc. - Google Patents

Description

2 Sheets-Sheet l S. P. MILLER RECOVERY 0F TAR ACIDS, ETC

Filed sept. 2,6, 1951 May 28, 1935.

INVENTOR ATTORNEYS INVENTOR 5PM BY 7; ATTORNEYS May 28, 1935. s. P. MILLER RECOVERY OF TAR ACIDS, ETC

Filed Sept. 26, 1931 Patented May 28, 1935 UNITED STATES RECOVERY F TAR ACIDS, ETC.

Stuart vParmelee Miller, Englewood, N. J., as-

signor to The Barrett Company, New York, N. Y., a corporation of New Jersey Application September 26, 1931, Serial N0.'565,270

6 Claims.

This invention relates to improvements in the recovery of tar acid oils and other products at coal distillation plants, particularly at coke oven plants.

In the ordinary operation of by-product coke ovens the gases produced by the coking operation pass from the individual ovens through uptake pipes and goose-necks to a collector main common to the ovens of the battery. The gases commonly known as foul gases leave the ovens at a high temperature, for example 600'700 C. or higher, and are ordinarily cooled as rapidly as possible by the application of sprays of ammonia liquor or ammonia liquor and tar in the goosenecks and collector main. The rapid cooling causes separation of a. heavy tar in the collector main. Further cooling is effected in the crossover main which connects the collector to the condensing system where further cooling takes place and where tarry oils are separated from the gases. The heavy tar and tarry oils are commonly admixed with each other and subjected to distillation and the resulting distillate oils or selected fractions thereof are treated for the separation of tar acids or phenols therefrom.

The tars and tarry oils commonly recovered at by-product coke oven plants cannot be directly extracted to separate tar acids therefrom largely because of the tarry impurities which interfere with the action of the caustic solutions employed for the extraction and prevent satisfactory extraction from being effected. It is customary accordingly to ship the coal tar from a coke oven plant to a tar distillation plant there to be sub- Jected to distillation and to treat the resulting distillate oils for the extraction of the tar acids. Handling losses, freight charges and distillation costs add materially to the cost of production of tar acid oils,l tar acids and other tar distillation products, while the distillation in ordinary tar stills is accompanied with more or less decomposition and loss or change in tar acid constituents.

The improved process of the present invention eliminates the necessity of transportation of tar to a tar distillation plant and eliminates the ne-` cessity of distillation of the tar. It provides an improved method wherebyvtar acid oil can be recovered directly from the coke oven gases and tar acids can then be extracted from the oils and recovered at the coke oven plant.

According to the present invention the coal distillation gases from the coke ovens are subjected to a cleaning treatment while still at such a high temperature that the tar acids in the gases leaving the collector main are for the mostV part or entirely maintained in a vapor state thereby removing from the hot gases all or substantially all suspended pitch constituents and leaving clean gases containing clean vapors. The cleaning of the gases at a high temperature is effected by passing the gases through an electrical precipitator at a regulated speed and subjecting the gases to an electrical discharge of sufllcient intensity to effect substantially complete cleaning of the gases. Pitch constituents carried in the gases are re- 1o moved in the precipitator and drawn off as a separate pitch product and the tar acid vapors remain admixed with the clean gases and other clean vapors issuing from the precipitator. The clean gases are then cooled to separate therefrom a clean oil rich in tar acids from which the tar acids are extracted directly. The gases are subsequently treated for the recovery of ammonia, benzol and other light oils therefrom. No tar is produced according to this process, the pitch constituents entrained in the gases being removed as pitch, while the gases are still hot.

Archibald F. Meston in U. S. Patent No. 1,444,627, issued February 6, 1923, describes a method of detarring coke oven or retort gases in an electrical precipitator and then removing ammonia and benzol from the gases. He provides two precipitators and a cooling chamber for cleaning the gases. Part of the tar is removed from the gases in one precipitator and then, after cooling, all such tar particles as may have escaped the action of the first precipitator and have been subsequently condensed by cooling are removed from the gases in the second precipitator. The cleaning process in the Meston process is not completed until after the gases have been cooled below the dewpoint of the gases for water. The clean gases are treated for the removal of ammonia and benzol. The present invention may be considered in the nature of an improvement over the Meston process in so far as it provides for cleaning the gases, while hot, to such an extent that on cooling there is obtained a tar acid oil of sufficient purity to permit extraction of the tar acids directly with anV alkaline reagent.

In ordlnarycoke oven systems the gases are cooled in the collector main by the introduction of ammonia liquor or ammonia liquor and tar therein, and heavy tar is thrown out of the gases in this main; Some of the tar'acids may be 50 thrown down from the gases with the heavy tar separated in the collector main. The present invention contemplates regulated cooling of the gases in the collector main. By keeping the temperature of the gases escaping from the main 55 sufficiently high the greater part of the tar acids will be carried over in vapor form from the collector main to the electrical precipitator.

'I'he temperature at which the gases are subjected to cleaning may be varied depending Aupon whether only lower tar acids are desired or whether higher tar acids are also desired. Even at temperatures around 90 C. a considerable amount of the lower boiling tar acids are retained in the vapor state in normal coke oven gases and if the gases are cleaned at this temperature and then cooled, the oil which is obtained will contain substantial amounts of tar acids, particularly phenol and the cresols. A higher yield of tar acids including most of the lower boiling tar acids is obtained by cleaning the gases at a somewhat higher temperature, around 100 or 110 C.,.for example. Cleaning at 120 C. gives a high yield of tar acid oils. By cleaning the gases at a temperature around 150 C. most, if not all, of the tar acids in the gases will be retained as vapors and on subsequent cooling a tar acid oil containing substantially all of the tar acids will be obtained in admixture with a large percentage of non-acid or neutral oils. The temperature at which the gases are cleaned can be regulated and varied to obtain a larger or smaller yield of the tar acids as tar acid oils. By fractional cooling, two or more separate tar acid oils may be obtained suitable for the extraction of separate tar acid fractions. For example, by cooling the cleaned gases first to a temperature above the dew point of the gases for water, collecting the condensed oils, and then cooling to a temperature below, say 35 C., two tar-acid oils may be condensed, the lower one yielding tar acids rich in the commercially valuable cresols and phenols.

After the gases have been subjected to cleaning in the 'electrical precipitator they will contain the tar acids in the form of vapors together with nonacid or neutral oils also in the form of vapors and the gases will be substantially free from suspended pitch impurities. 'Ihe substantially complete removal of suspended particles from the gases at a temperature above the dew point of the tar acids which are to be recovered and production of a commercially clean oil are essential features of the invention because the presence of any` substantial amount of pitch constituents in the tar-acid oils presents diihculties during the extraction with an alkaline reagent and makes the direct recovery of tar acids from such oils diiiicult and expensive on a commercial scale. Free carbon is an easily determinable pitch constituent and is commonly used as a measure of the suitability of a tar acid oilfor extraction. Since the free carbon content is not the sole factor governing emulsion formation during extraction, there will be some variation under different conditions and with different oils of the permissible free-carbon content of the oil. Ordinarily afree-carbon content of not over 0.2% is desirable, but as much as 0.5% may be permissible under special circumstances.

By free carbon is meant benzol-insoluble material determined as described by Weiss for tar in the Journal of Industrial and Engineering Chemistry, Vol. (1918), p. 736, with such modification as is necessary for applying to oil, ibid. p'. 911. l

In recovering the tar acids from the cleaned gases, the gases should, in general, be cooled to a temperature around 25-30 C. in order to separate as much of the oils containing tar acids as possible. This cooling can be effected by indirect cooling or by direct cooling and is advantageously effected by introducing a spray of water or ammonia liquor into the gases so that they are rapidly cooled to around 25-30" C. The gases at this lower temperature maybe again subjected to the action of an electrical precipitator to effect substantially complete removal therefrom of all suspended liquid constituents, thereby insuring that the maximum amount of condensed oils will be obtained, and leaving the gases substantially free from such suspended -oil.

'I'he gases from which the tar acid oils have been removed will still contain large amounts of ammonia, although some of the ammonia will have been removed therefrom in the condensers, in which the tar acid oils have been removed, particularly where a direct water or ammonia liquor spray is y'employed for cooling the gases to separate the tar acid oils. The gases after cooling to separate the tar acid oils can be passed directly to a saturator or absorber for the absorption of ammonia in sulfuric acid but they may advantageously be reheated somewhat to a temperature around 55 C., for example, before being passed to the absorber. The gases leaving the absorber are cooled and are then subjected to scrubbing with a suitable solvent for the recovery of light oil, such as benzol, etc., therefrom. The benzol recovered in this manner will be free from tarry constituents present in gases which have not been subjected to a prior cleaning operation.

The improved process 0f the present invention can be carried out with existing equipment by the provision of an electrical precipitator at the exit of the gases from the collector main and by regulation of the temperature of the gases entering and leaving the collector main. The electrical precipitator will be constructed so as to be capable oi' withstanding the high temperatures to which it is subjected in the operation as. described and it will be so constructed and operated as to effect the removalof substantially all suspended particles from the gases while they are still at a high temperature and before the condensation of the tar acid oils therefrom. The precipitator may advantageously be insulated to prevent loss of heat during the cleaning operation.

The invention will be further illustrated by the following more detailed description in connection with the accompanying drawings, which show apparatus embodying the invention.

Fig. 1 is a plan view of a portion of a coke oven plant illustrating the application of the invention thereto;

Fig. 2 is an enlarged sectionalview of the electrical `precipitator and collector main; 5

Fig. 2A is a sectional elevation of the precipitator; and

Fig. 3 is an enlarged sectional view of the tar acid extraction apparatus,

Referring to the drawings, 5 indicates a battery of coke ovens the individual ovens of which are connected by uptake pipes and goose-necks 6 to a collector main I having a center box I. Ammonia liquor or other aqueous cooling agent, or tar and ammonia liquor, can be supplied to the goose-neck and collector main through spray nozzles 9 and 9' therein which are supplied by a pipe I from a suitable source of the cooling liquid. As hereinbefore indicated, the amount of ammonia liquor thus supplied maybe regulated so that the temperature of the gases leaving the collector main is, for example, not lower than 100 C. and in many cases higher so that the gases will carry from the collector main a considerable proportion of the oil constituents con taining tar acids. 'I'he tar or pitch and ammonia liquor which accumulate in the center box can be withdrawn through a pipe II to a decanter I2 wherein the tar or pitch is separated from the ammonia liquor. The tar and ammonia liquor can be utilized for any desired purpose. For example, the tar or pitch can be distilled to separate additional quantities of oil therefrom and to produce pitch adapted for various commercial purposes.

The gases and vapor leaving the collector main are conducted directly through a pipev It to an electrical precipitator I5 which is arranged preferably closely adjacent to or upon the collector main in order that the temperature of the gases may not be reduced to any considerable extent before the separation of suspended pitch impurities from the gases. The electrical precipitator consists of a shell enclosing a number oi .tubes I5' supported in heads I6 and Il within the shell. An inlet I8 near the bottom of the shell communicates with a chamber is which is partially separated from the tube section by a Abaille 20. An outlet 2i permits the escape of the gases from the separator after the latter have passed through the tubes.

Extending through each tube is a metal rod 22 which is supported from a bus-bar 23! located near the upper end of the tubes. The bus-bar 23 extends at Aboth ends into casing 2i which enclose insulators 25 upon which the bus-bar is supported. The high tension current line extends into one of the casings 24 and connects with the bus-bar 23. The current is ordinarily obtained by the stepping-up and rectification of an alternating current oi-lower voltage.

The casing of the precipitator is grounded or otherwise connected to the source of current to complete the circuit. The casing and tubes form the positive electrode, the rods 22 connected to the bus-bar being the negative electrode. The form and arrangement of the conductors in the circuit can be varied.

The size and number of tubes in the precipitator is such that the gases in passing through the tubes remain in the tubes and are subjected to the electrical discharge therein for a period of about cne second or more in order to effect the substantially complete removal of suspended particles required for the preparation of tar acid such longer tubes. Tubes of shorter length may i beemployedandinthiscasethespeedofthe gases should be decreased proportionately. lOwing to the slowing up of the process by the use of such shorter tubes, longer tubes are generally preferred. Tubes around nine feet long will generally be employed.

To maintain the cleaning eiliciency of the preeipitator at the uniformly high standard required for producing the clean tar acid oils of this invention it is very necessary to keep the secondary voltage of the precipitator, i. e. the voltage between the discharge and collecting electrodes at very nearly the break down voltage, i. e. the voltage which produces sparking between the electrodes. Using collecting electrodes six inches in diameter this break down voltage is approximately 50,000 volts but will vary somewhat depending upon several factors, among which are the amount of suspended matter in the gases being cleaned and the condition of the inside walls ofthe collecting electrodes.

' The use of tubes of maximum diameter is desirable to obtain maximum cleaning capacity with the greatest simplicity of construction. 'I'he practical limit at present is largely determined by the maximum voltage that can be used without undue losses or other disadvantages. Tubes of 6 inches internal diameter have been found in practice to meet operating conditions satisfactorily in this respect, and are the preferred size in this process.

If deposits oi pitch build up on the inside walls of the tubes so as to permit a concentration of surface charge at any one point, a spark discharge will take place between the discharge electrode and the tube. When this occurs the cleaning eillciency drops to practically zero. A voltage somewhat under the break down voltage is lmaintained between the electrodes and in cleaning gases in six inch tubes a voltage of about 45,000 to 4?',000 volts will ordinarily be maintained between the electrodes. Voltages as low as 40,000 volts may be employed in some cases but when the break down voltage falls `below about 40,000 volts the precipitator must be cleaned.

larger tubes require higher voltages, for example about 75,000 volts for ten inch tubes and A 325,000 volts for thirty-six inch tubes.

With tubes of the preferred length, i. e. nine feet, the rate of flow of thegases through the tubes is preferably not greater than nine feet per second, the gases thus being subjected to cleaning 'for approximately one second. This rate of ilow should be maintained regardless of the diameter of the tubes. The velocity of the gases may be somewhat less than nine feet per second, but in order to operate most eiciently the precipitator will be designed so that a velocity of eight or nine feet per second will be maintained in treating ordinary coke oven gases in tubes nine feet long. In any case a maximum velocity of ten or even eleven feet per second should usually not be exceeded. The content of suspended matter in the gases isat least to some extent dependent upon the temperature of the gases, so that the time of treatment may vary somewhat with the temperature at which the gases are cleaned.

Since the rate of ilow of gases from the coke oven plant is subject to, variation due to changes in the coking cycle and changes in the temperature of the gases^treated, a precipitator should be designed to maintain the desired emciency of clexning under peak conditions.

The hot gases from the collector main, carrying pitch in the form of globules or tar fog,

enter the electrical precipitator and pass through the tubes in the precipitator, being subjected therein to the electrical discharge which causes the suspended particles to separate from the gases and condensable vapors. The pitch is thus thrown out of the gases and runs down the inner walls of the tubes into the chamber i9 and is withdrawn from the bottom of the precipitator through the outlet 25.

The cleaned gases carrying the condensable vapors escape through the outlet 2| to a crossover main 26 and are delivered thereby to condensers 21. The condensers may be of any suitable form, for example, the ordinary direct or wet type which is 'employed frequently in byproduct recovery systems. The gases and vapors are cooled therein by contact with grid surfaces wet by sprays of ammonia liquor, for example, to a temperature of 25 C. or 35 C., and the resulting condensate is withdrawn through pipes 21' to a decanter 28 wherein the oil is separated from the ammonia liquor. The gases pass from the condensers through a pipe 29 to an exhauster 30. 'I'he gases are then conducted, with or without reheating, to an ammonia absorber or saturator 3| to recover ammonia therefrom and thereafter to suitable apparatus (not shown) for the recovery of light oils, etc.

'Ihe oil separated in the decanter 28 can be conducted to tanks 32' and thence through a pipe 32 to extraction apparatus 33 which may comprise a tank having suitable agitating means 34 therein. 'I'he extraction agent, for example, caustic alkali, can be added in suitable proportions to the tank through an inlet 35 and mingled with the oil by agitation until the extraction is accomplished. The agitation canY then be discontinued. The carbolate will settle to the bottom of the tank, providing caustic soda solution of 10% or higher strength is used. After suicient settling the carbolate is drawn oil? through pipe 36. I'he line of demarcation between the carbolate and the oil is suiiiciently distinct to allow a close separation to be made. When the carbolate has been completely removed, the extracted oil is removed to storage. The separation is easily conducted if a sight box 31 is used. The carbolate can be treated in the ordinary manner for removal of dissolved oils and liberation and recovery of tar acids.

The removal of pitch from the gases in the prey cipitator must be complete or so nearly complete that the oils obtained by cooling the resulting gases can be extracted directly for tar acids. 'I'he presence of any considerable amount of pitch or tarry constituents inthe oils will cause the formation of emulsions when treated with caustic.' 'Ihis invention contemplates the production of clean oils which can be extracted directly.

The relation of cleaning eiilciency to time of treatment of the gases in the precipitator for a given set of operating conditions is shown in the following table:

Percentage oi Time oi treatment in gtglrdrg seconds moved .as es. 4 l. 95. 2 1. 40 98. l

Unless the time of treatment is approximately one second, the cleaning effected in the precipitator is not adequate to produce a tar acid oil which can be extracted directly for tar acids without formation of troublesome emulsions.

For example, the gases from a battery of twenty coke ovens, each charged with thirteen tons of coal, operating on a coking cycle of nineteen hours and therefore coking 329 tons of coal each twentyfour hours, can be cleaned at 100 C. in a precipitator containing 63 tubes, six inches in diameter and nine feet long, by maintaining a voltage of 45,000 to 47,000 volts between the electrodes. If the gases are treated at an appreciably higher temperature a larger number of tubes is required owing to the larger Volume of hotter gases.

An installation for twenty ovens using nine feet tubes six inches in diameter should include somewhat more than 63 tubes to allow for variations in the length of the coking cycle, the diiIerent temperatures at which the gases may be treated and other changes in operating conditions, and those tubes which are not needed may be blocked oi by suitable means vknown in the art.

According to a preferred method of carrying out the process, the gases are cleaned at a temperature of 110 C. `At this temperature pitch melting at about 110 F. (cube infwater method) will be thrown down in the precipitator and by cooling the cleaned gases to 30 C. an oil yield of approximately 20% of the tar will be obtained. If the gases are cleaned at 100 a pitch with a oat test of approximately 50-160 seconds at 50 C. (A. S. T. M. 139-27) will be obtained and a somewhat lower oil yield will result, but the phenol content of the oil will be relatively somewhat higher and the content'of lhigher' boiling tar acids will be relatively somewhat lower than if the gases are cleaned at 110 C.

After cooling the gases to throw down the tar acid oil, it is desirable to reheat the gases to 50- 55 C. before recovering the ammonia by the direct method of absorption in sulfuric acid.

While various methods for the recovery of ammonia may be employed, the removal of ammonia by direct contact with sulfuric acid will ordinarily be preferred.

-The extraction of the clean, directly recovered tar acid oil for the separation of tar acids therefrom can be effected in apparatus of the kind commonly used for extracting tar acids from distillate oils, using an aqueous solution of caustic soda or sodium sulfide or other phenol absorbing reagent and bringing about intimate contact of the oil with the caustic alkali solution to cause the tar acids to combine with the caustic alkali. etc. A solution containing about ten percent of caustic soda can advantageously be employed for the extraction of the tar acids, although a,

stronger or weaker solution may be employed. The solution, when brought into intimate contact with the oil extracts the tar acids therefrom and forms a phenolate or carbolate solution containing the alkali salts of the tar acids.' This solution can be separated from the extracted `neutral oils which are thus obtained substantially ree from tar acids and the solution can then be treated to recover the tar acids therefrom.

The treatment of the phenolate solution may be preceded by evaporation or distillation to drive oir volatile oil constituents such as pyridine bases, etc. therefrom. It can then be treated with an acid, such as carbon dioxide, to set free the phenols or'tar acids ima separate layer, which can then be separated and subjected to-further treat ual phenols, o1' commercial mixturesy of phenols, can be obtained, e. g., cresols, cresylic acid, etc.

'I'he neutral oils remaining after the extraction of the tar may be further fractionated, if desired, to recover therefrom oils having diierent boiling points and adapted for various uses. 'Ihe neutral oil is a valuable oil foruse in lampblack manufacture. It may be converted to lampblack by incomplete combustion. The oil, particularly heavier fractions thereof, can also be advantageously employed in creosoting compositions.

This application is in part a continuation of my application Serial No. 202.736, filed June 30, 1927.

I claim:

l. The method of recovering from hot coal distillation gases a tar acid oil, which comprises cooling the gases to a regulated temperature at which tar acids are retained in vapor phase but below that at which the oils remaining in the gases in vapor phase constitute a creosote oil, subjecting the gases to cleaning to remove substantially all suspended matter therefrom, and cooling the resultant clean gases to separate therefrom a tar acid oil substantially free from tarry constituents.

2. 'I'he method of recovering a tar acid oil from hot coal distillation gases, which comprises cooling the gases to a temperature above their dew point for water and below about 150 C., cleaning the gases at this temperature to remove suspended matter therefrom, and cooling the re- -sultant clean gases to separate therefrom a tar acid oil substantially free from pitch constituents. V3. The method of recovering a tar acid oil from hot coal distillation gases, which comprises cooling the gases to a temperature above their dew point for Water and below about 150 C., cleaning the gases at this temperature to remove suspended matter therefrom, and cooling the resultant clean gases to separate therefrom a tar acid oil contain-I ing not more than about .5 free carbon.

4. The method of recovering from hot coal dis-- tillation gases a tar acid oil, which comprises cleaning the gases by electrical precipitation while yet at a temperature above their dew point for water and below about 150 C. to remove suspended matter therefrom, and cooling the resultant clean gases to separate therefrom a tar acid oil substantially free from pitch constituents.

5. The method of recovering from hot coal distillation gases a tar acid oil readily extractable with aqueous caustic soda for removal of tar acids therefrom, which comprises cleaning the gases by electrical precipitation while yet at a temperature above their dew point for water and below about 150 C. to remove suspended matter from the gases and' leave in the gases not more than .5% of free carbon basedvon the weight of oil vapors retained in the gases, and cooling the resultant clean gases to separate therefrom a caustic extractable tar acid oil containing not more than about .5% of free carbon.

6. The method of recovering from hot coal distillation gases a tar acid oil readily extractable with aqueous caustic soda for removal of tar acids therefrom, which comprises cleaning the gases by electrical precipitation while yet at a temperature above their dew point for water and below about 150 C. to remove suspended matter from the gases and leave in the gases not more than .5% of free carbon based on the weight of oil vapors retained in the gases, and cooling the resultant clean gases to a temperature below about 35 C. to separate therefrom a caustic extractable tar acid oil containing not more than about .5% of

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