US 3488278 A
Description (OCR text may contain errors)
jo N7@ E. F. NELSON PROCESS FOR TREAT'ING COAL Filed Jan. 25, l968 United States Patent O 3,488,278 PROCESS FOR TREATING COAL Edwin F. Nelson, Arlington Heights, Ill., assignor to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Jan. 25, 1968, Ser. No. 700,535 Int. Cl. Cg 1/08, 1/04 U.S. Cl. 208-10 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to a coal treating process. It also relates to a process for liquefying coal using a selective solvent. It particularly relates to a process for obtaining valuable liquid hydrocarbons from solid particulate coal utilizing counter-current solvent extraction and, preferably, simultaneous hydrogenation within the extraction zone.
It has long been known that hydrocarbon gases, liquids, pitch and chemicals may be obtained in some form from coal which is mined from the earth. Usually, the prior art has employed destructive distillation or other gasification processes for the conversion of coal into these more valuable and useful products. Recently, the prior art has developed a high pressure hydrogenation of coal technique to effectuate such conversion. Still more recently, methods involving solvent extraction techniques have been developed for obtaining useful fuels and chemicals from coal whereby the coal is contacted with a selective solvent which acts as a hydrogen donor for supplying sufficient hydrogen to the coal to aid in converting it into a liquid state.
Following the solvent extraction step, the prior art schemes have utilized various recovery procedures, such as hydrogenation of the liquid coal extract, for increasing its value and utility together with retorting or coking of the residual materials obtained from the solvent extraction step to still further convert these coal derived products into more valuable products. However, none of the aforementioned prior art procedures have been commercially efficient or feasible to warrant widespread cornmercial exploitation of converting coal into liquid products. Generally, the deficiencies in the prior art schemes have not only involved capital investment problems, and disposal problems of the residue or waste frequently having high metals content, but also involved liquid product quantity and quality problems which have yet to be solved in an economical and facile manner.
Since it is clear to those skilled in the art that the vast mineral reserves of butuminous coal represent an extremely important supply of energy, it would be desirable to improve upon the prior art techniques, particularly the solvent extraction technique in order to reduce the cost of obtaining high quality petroleum-type products from coal. Accordingly, the present invention provides an improved process for the conversion of solid coal into liquefied products utilizing an improved solvent extraction technique.
3,488,278 Patented Jan. 6, 1970 SUMMARY OF THE INVENTION Therefore, it is an object of this invention to provide a process for treating coal.
It is also an object to provide a process for the liquefaction of coal whereby valuable liquid hydrocarbons are obtained therefrom.
It is a specific object of this invention to provide an improved process for treating coal which involves the counter-current extraction of solid particulate coal with a selective solvent.
It is another specific object of this invention to provide an improved method for the liquefaction of particulate coal by a counter-current solvent extraction in a. facile and economical manner.
Thus, in accordance with the practice of the present invention, there is provided a process for treating coal which comprises passing crushed coal into the upper end of a vertically disposed liquefying zone maintained under coal liquefying conditions, introducing coal solvent into said zone at a plurality of spaced points from the lower end of said zone, removing from the upper end of said zone rich solvent containing dissolved coal, and withdrawing from the lower end of said zone a residue stream containing ash and undissolved material.
Another embodiment of this invention provides a process comprising contacting crushed coal in a contacting zone with solvent in continuous counter-current fashion, said coal being introduced into said zone at the upper end thereof and said solvent being introduced at a plurality of spaced points from the lower end thereof, under conditions sufficient to dissolve more than 50% M.A.F. coal into said solvent and thereafter separating the solvent containing dissolved coal from the ash and undissolved material.
It can be seen from the description of the invention presented thus far that the essence of the present invention embodies the use of counter-current solvent extraction whereby the lean solvent is introduced into the lower end of the extraction zone at a plurality (e.g. from 2 to 5) of spaced points within the zone. It is believed that the introduction of the lean solvent in this unique fashion results in improved contact between the solvent and the coal while providing a continuous manner of converting solid coal into liquefied products in a more facile and economical manner.
In brief, the operation of the present invention involves maintaining the coal liquefier or extraction zone under conditions of elevated temperature and pressure sufiicient to maintain the lean solvent in substantially liquid phase through the extraction zone while providing the necessary dissolving conditions to transform the solid coal into liquefied products. A suitable residence time is maintained in the extraction Zone by utilizing the technique of hindered settling whereby the upflow velocity of the liquid solvent provides hindrance to the gravity pull of the solid coal particles down the zone. By introducing the solvent into a plurality of points within the zone, the hindered settling technique can be advantageously exploited by providing varying degrees of solvent purity and solvent velocity throughout the extraction zone.
Additionally, as will become evident from the description presented hereinbelow, the efficiency of the coal treating process can be increased by the introduction of hydrogen or a hydrogen-containing gas into the extraction zone, preferably, at a lower end thereof. The use of the hydrogen-containing gas can be further enhanced by introducing into the extraction zone'a suitable solid hydrogenation catalyst which, preferably, would be introduced into the upper end of the solvent extraction zone.
It is believed that one of the reasons the practice of the preferred embodiment of the invention produces such desirable results is the manner in which hydrogen balance is maintained in the system. For example, during the extraction operation it is also believed that hydrogen is transferred from the solvent to the coal thereby aiding in the conversion of the coal into a liquid product. Thus, the solvent, in effect, becomes reduced in hydrogen content. Preferably, therefore, hydrogen gas is introduced into the extraction zone in order to restore the hydrogen content of the solvent to substantially its original hydrogen content. In other words, in the broad embodiment of the invention, as the solvent passes upwardly through the solvent extraction zone, it becomes more and more depleted in hydrogen content as it contacts the crushed coal passing downwardly through the zone. While unique advantages were found in this manner of operation over the prior art schemes, it was also discovered that the inventive method could be further improved by having the extraction take place in the presence of a hydrogen gas in such a manner that the solvent substantially throughout the extraction zone is maintained, in effect, in its highest hydrogen content thereby significantly increasing the efficiency of the coal conversion reaction.
By operating in this preferred manner and in some cases utilizing hydrogenation catalyst in the extraction zone, one of the ultimately recovered hydrocarbon products is now eminently suitable for recycle to the pulverization zone since its hydrogen content has been restored to a high level.
The coal preferred for use in the practice of the present inventive process is of the bituminous type, such as Pittsburgh Seam Coal. More preferably, however, the bituminous coal is a high volatile content coal having a solids content greater than about 20 weight percent of M.A.F. coal (moisture and ash-free coal). Although the inventive method will be described with reference to the treating of bituminous coal, it is within the concept of the present invention to apply the inventive process to a sub-bituminous coal, lignite, and other solid carbonaceous materials of natural origin.
Apparatus for use in pulverizing coarse coal to prepare feed to the present process may be of any type known to those skilled in the art. Conventional ball mills or rod mills may be used with satisfactory results. Preferably, the apparatus must be able to pulverize lump or coarse coal in the presence of significant quantities of liquids, such as the coal solvent, without difiiculty. Those skilled in the art are familiar with the kinds of apparatus for processing wet solids and the crushing yand grinding thereof so that no detailed discussion of the apparatus need be presented herein. The primary requirement for crushing and grinding of the lump coal is that coarse coal usually having an average particle diameter in excess of 0.08 inch and, typically, 0.25 to 2.0 inches must be processed therein and reduced in size to an average particle diameter which would be of at least a -8 Tyler screen size and, preferably, would be reduced to an average particle size of -14 Tyler screen size. As used herein, the term Tyler screen refers in all instances to the commercial Tyler standard screens. A correlation between Tyler screen mesh and average particle diameter is as follows:
Tyler screen Average diameter of The coal feed utilized in the present invention may be introduced into the solvent extraction zone in its substantially dry state through lock hopper means, screw conveyor means, or other means available to those skilled in the art. In some cases, however, it may be advantageous to introduce the coal into the extraction zone as a slurry, i.e., a mixture of coal and solvent. The slurry method may advantageously be used if the solvent has been present during the crushing and grinding operation of the coarse coal. In many cases, it is advantageous to have the solvent present during the crushing and grinding operation.
With respect to the benefit gained from having the solvent present during the pulverization step, it is believed that at the point of shear for the crushing and grinding of the coal, the shear site is extremely reactive and hydrogen, therefore, can be transferred into that site more easily than if the coal is pulverized prior to contact with the solvent. In addition, the smaller particles of coal whichare sheared away from a relatively large lump irnmediately exposes not only the highly reactive shear site to the solvent, but also exposes an extremely large surface area to the solvent, thereby enabling the resulting small particles of coal to almost immediately dissolve in the solvent and become a part of the liquid coal extract.
Additionally, there has been some discussion in the prior art that the presence of oxygen or oxygen compounds on the surface of the coal makes it diflicult for the coal to react properly with the suitable solvents for the conversion thereof into liquid coal extract. It is further believed therefore, that by pulverizing or by crushing the coal in the presence -of :a liquid solvent, oxygen compounds or the presence of oxygen have been excluded from the highly reactive shear sites along the coal, thereby further enabling the transfer of the hydrogen from the solvent to the coal to become of significantly increased efficiency.
Extraction of coal by means of a selective solvent is, by definition, a partial conversion of the coal since not only is the coal reacted with the hydrogen which is transferred from the solvent, but is, in the preferred embodiment, also reacted with the hydrogen which is added during the extraction operation. In addition, there is a solution phenomenon which actually dissolves the coal which has accepted the hydrogen into the solvent. Therefore, as used herein, the terms liquid coal extract and liquefied coal fraction or dissolved coal or other words of similar import are intended to include the liquid product which is obtained by treating the coal with a selective solvent and, generally, will be described on the basis of being solvent-free even though a portion of the liquid coal extract comprises hydrocarbons suitable for use as the selective solvent.
The practice of the present invention is preferably performed under conditions which increase the kinetics of the reaction while maintaining the components therein (except hydrogen) primarily or substantially in liquid phase; although, in some cases it may be desirable to practice this invention in the presence of a vaporized solvent by using a vapor-iiuid extraction technique.
Suitable solvents for use in the practice of this invention are those which are of the hydrogen-donor type and are at least partially hydrogenated and include naphthalenic hydrocarbons. Preferably, the solvent is one which is in liquid phase at the recommended temperature and pressure for the extraction and/or pulverization step. Mixtures of hydrocarbons are generally employed and preferablly are derived from intermediate or final products obtained from subsequent processing following the practice of this invention. Typically, these solvent hydrocarbons or mixtures of hydrocarbons boil between about 260 C. and 425 C. Examples of suitable solvents are tetrahydronaphthalene (Tetralin), Decaln, biphenyl,
methylnaphthalene, dimethylnaphthalene, etc. Other types of solvents which may -be added to the preferred solvents of this invention for special reasons include phenolic compounds such as phenols, cresols, and xylenols. It is also to be recognized that in some cases it may be desirable during a subsequent separation step prior to the removal of the solvent from the liquid coal extract to add an anti-solvent, such as saturated paraffinic hydrocarbon like hexane, to aid in the precipitation of tarry and solid residue from the coal extract of the invention.
However, in the selection of a suitable solvent it must be recognized that the solvent must have the ability to transfer hydrogen to the pulverized coal during the extraction step. In other words, it is a requirement of the broad embodiment of this invention (Le. no added hydrogen) that the rich solvent leavingthe extraction zone having coal dissolved therein has a reduced hydrogen content compared to the hydrogen content of the lean solvent which is added to the extraction zone. It has also been explained that another critical feature of the preferred embodiment of this invention is the selective hydrogenation of the solvent during extraction in order to increase its hydrogen content so that hydrogen may be more easily transferred from the solvent to the coal.
`One of the convenient ways of optimizing the specific hydrotreating operation during extraction is to use the I-factor analysis for determining the degree to which hydrogen has been added to the solvent. This analytical technique permits the characterization of various types of aromatics in a hydrocarbon mixture by means of the I-factor analysis. The technique utilizes mass spectrometer analysis employing a low ionizing voltage. The ionizing voltage is chosen such that only those hydrocarbons to be characterized are ionized while other hydrocarbon types are not ionized under the potential chosen. For example, since compounds more saturated than aromatic hydrocarbons, such as the paraffin hydrocarbons, have an ionization level above v volts the ionization chamber is thus maintained at a potential of about 7 volts so that only the aromatic hydrocarbons are ionized and the saturated compounds will not be observed on the mass spectrum. As those skilled in the analytical art know, the mass spectrum reveals molecular ion peaks which correspond to the molecular weight of the aromatic compound. Thus, the technique permits characterization of the aromatic hydrocarbons 'by means of the general formula CnH2n J where I is the herein referred to I- factor for the practice of the present invention. The following table shows the relationship between the J -factor and the type of aromatic.
Type of aromatic hydrocarbon I-factor number Alkyl benzenes and benzene 6 Indanes, Tetralins 8 Indenes 10 Alkyl naphthalenes and naphthalene 12 Acenaphthenes, tetrahydroanthracene 14 Acenaphthalenes, dihydroanthracenes 16 Anthracenes, phenanthrenes 18 Using this J-factor analysis in characterizing the degree of hydrogenation during extraction allows for the optimum treatment of said solvent to produce a high quality hydrogen enriched solvent for use in converting coal into liquid coal extract. The I -factor analysis is simply a convenient means for optimizing the hydrogenation technique in the practice of the preferred embodiment of the present invention.
The operating conditions maintained in the extraction zone may be Varied widely. The temperature, for example, may be varied essentially from atmospheric temperat'ure to a relatively high temperature. It is distinctly preferred in the practice of this invention that the temperature of the coal and the solvent be maintained at a relatively high level, say, from 300 C. to 500 C. The pressure in similar manner may be varied over an extremely wide range; for example, from atmospheric pressure, to, say, 10,000 p.s.i.g. with a preferred pressure being about 500 p.s.i.g. In all cases it is distinctly preferred that the operating conditions be chosen so as to maintain the solvent and dissolved coal in substantially liquid phase. These conditions should also be chosen so as to maintain the extraction zone substantially liquid-full; at least in liquid phase over the volume of the extraction zone where the major portion of the dissolving action takes place.
The amount of solvent which is used in the present invention should be from 0.2 to 10 pounds of solvent per pound of solid coal entering the extraction zone. As previously mentioned, it may be desirable to use the slurry technique for feeding the coal into the extraction zone. In such case, if solvent is used as the carrying medium in the slurry, then, of course, solvent lwould enter the extraction zone along with the solid coal feed. It is to be noted that in the slurry technique of feeding coal to the extraction zone there has already been at least partial liquefaction of the coal at that point. 'Satisfactory results may be obtained in the extraction zone in utilizing approximately equal amounts of solvent to coal on a weight basis. The amount of solvent entering the plurality of spaced points (excluding the coal feed point in the case of slurry feed) may be varied in any .manner desired by those skilled in the art. It is preferred, however, that from 50% to 90% by volume of the total lean solvent entering the extraction zone be introduced through the uppermost point. Therefore, for example, if three introduction points are used the amount of solvent may be varied as :15 :5, 60:30:10, 70:15:15, 70:20:10, etc. In the preferred embodiment of his invention, three introduction points are used and the amount of solvent introduced should be in the 70:20:10 ratio. The 10% amount of solvent which enters the lowermost introduction point passes into the extraction zone and in most cases ows in essentially a downward fashion thereby washing the lower end of the extraction zone free of ash and residue.
As used herein, the term spaced or words of similar import is intended to include spaced arrangements whereby the inlet points are in a straight line, i.e. one totally above the other; or in an upwardly staggered arrangement, or in an upwardly spiral arrangement equally spaced around the extraction shell; or in any other arrangement desired `by those skilled in the art as long as the plurality of inlet points are arranged in a vertically disposed direction. It is also distinctly preferred that the spaced points be concentrated in the lower end of the extraction zone, ie., below the said point of a vertically disposed extraction column.
As previously mentioned, the upwardly flowing solvent in the extraction zone and, preferably, the upwardly owing hydrogen gas provide a residence time for the solid coal to be in contact with the solvent through the hindered settling technique. Generally, a residence time from 30 seconds to 5 hours is sufficient and, preferably, the amount of hydrogen gas introduced into the system is sucient to dissolve the coal and to substantially maintain the hydrogen content of the solvent at substantially the level of the lean solvent. In all cases, the combination of operating conditions should be suicient so that a total in excess of 50% by weight and, typically, from 70 to by weight of the M.A.F. coal has been liquefied. The amount of hydrogen gas necessary to perform this pressured function may range from 1,000 to 100,000 standard cubic feet per barrel of lean solvent entering the system. Typically, however, the amount of hydrogen added to the extraction zone in the preferred embodiment will be in the range from 2,000 to 10,000 standard cubic feet per barrel. However, the amount of hydrogen entering the extraction zone should not be in excess of that which would cause foaming or carryover of solid coal out of the upper end of the extraction zone. Those skilled in the art are familiar with the teachings presented herein and general knowledge will understand how to control the amount of hydrogen in conformity with the J-factor analysis on the solvent leaving the extraction zone and on other design parameters.
While the purpose of the extraction zone, including the preferred embodiment of adding hydrogen to the extraction zone, is to substantially complete the conversion of the coal into a liquid coal extract, it may also be desirable to add to the extraction zone a hydrogenation catalyst. The catalyst used may be conventional, may be homogenous `or heterogenous and may be introduced in the pulverization zone and/or extraction zone in admixture with the liquid solvent or with the solid coal or may be introduced as a separate stream into the upper end of the extraction zone. Those skilled in the art, from a knowledge of the characteristics of the coal, solvent and the properties desired for the end product will know whether or not it may be desirable to use any or all of these desirable features in the extraction zone. Conventional solid, particulate hydrogenation catalyst (e.g. finely divided) may be desirable, such as palladium on an alumina support or a cobalt-molybdate catalyst or any other hydrogenation catalyst known to those skilled in the art and applicable to the solvent-coal system environment maintained in the extraction zone including the use of a slurry-catalyst system.
Hydrogenation in the extraction zone generally accomplished the following functions: transfer of hydrogen directly to coal molecules; transfer of hydrogen to hydrogen donor molecules; transfer of hydrogen from hydrogen donor molecules to coal molecules; and combinations of the above. Homogenous catalysts may be introduced with the coal, or hydrogen donor compounds, in the pulverization step prior to the extraction zone. Examples of catalysts suitable include compounds containing tin, nickel, molybdenum, tungsten, and cobalt.
Following the solvent extraction zone the solvent containing dissolved oil is passed into a separation zone for the recovery therefrom of valuable hydrocarbon products. Typically, these products are normal gasoline boiling range products and/or chemicals, aromatic hydrocarboncontaining fractions, heavy fuel oil fractions, and the like, the utility of which is well known to those skilled -in the art. As previously mentioned, at least a portion of the coal extract is suitable for use as a coal solvent and may, therefore, be recycled at least in part to the solvent extraction zone as lean solvent therein.
The invention may be more fully understood with reference to the appended drawing which is a schematic representation of apparatus for practicing the preferred embodiment of the invention.
DESCRIPTION DRAWING Referring now to the drawing, crushed coal having an average particle diameter of at least 8 Tyler screen size is introduced from hopper 10 into coal liqueiier 11 which is maintained under coal liquefying conditions. Lean solvent is introduced into the system via line 12 and is split into spaced introduction points via lines 13, 14, and 15 in an amount suflicient to provide an upward flowing velocity which hinders the settling of the coal particles passing downwardly through liqueer 11. The amount of lean solvent entering the spaced introduction points is in a ratio of 80:15:5, respectively, by volume.
Suicient hydrogen gas is introduced into l-iqueer 11 via line 19 and passes in an upwardly flowing direction through the extraction zone. A typical cobalt-molybdate hydrogenation catalyst in solid particulate form -is introduced into the upper end of liqueiier 11 via line 19. Thus, the lean solvent and hydrogen pass in counter-current fashion with the coal and catalyst particles.
Rich solvent containing dissolved coal together with other gases including hydrogen is removed from liqueer 11 via line 16 and passed into recovery facilities. Conventionally, the hydrogen gas may be separated (by means not shown) from the other materials and recycled to liqueiier 11. Similarly, a portion of the liquid coal extract may also be recovered (by means not shown) and returned to liqueer 11 as lean solvent therein. Ash and residue including solid catalyst is removed from the system via line 17 and passed into recovery facilities not shown for the reclaiming of the catalyst and reuse in the extraction zone.
In a manner of operating, the materials in contact at point 23 include. solvent which is relatively lean having been introduced via line 13, hydrogen gas, downward passing coal particles and catalyst particles. In many respects the condition of the materials at point 23 may be termed semiplastic. As the material passes further down the column, the condition at point 22 may be termed a condition of relatively high ash content zone since at that point substantially all of the coal to be extracted has been converted into liquid phase. In addition, the velocity of the materials owing upwardly at point 23 is greater than the upward flow velocity of materials at point 22. One of the reasons for controlling the conditions in this manner of relative velocity is that at point 23 there are significantly greater numbers of relatively tine coal particles than at point 22.
By similar analogy, the condition at point 21 is one of extremely high ash content with essentially no coal particles to be dissolved. The introduction of the hydrogen into the zone at point 21 creates a condition of relatively high turbulence which aids in the further separation of any remaining coal particles from the undissolved ash, catalyst, and residue which continue down the column. Point 20 defines a settling zone where low Reynolds number liquid iiows are maintained so that the ash, residue, and catalyst will have a chance to settle out of the liquid phase and be washed by at least a portion of the lean solvent which enters liqueer 11 vial line 15 and passes downward as a wash through the settling zone.
It was discovered in the practice of the present invention that the utilization of hydrogen gas in the coal liquetier permitted the use of less solvent than would otherwise be required. Even though substantial benefits over the prior art may be obtained through the use of the plurality of inlet points for the lean solvent, without the use of either hydrogen or a catalyst, the benefits to be obtained through the hydrogenation technique permit significant economies to be obtained over the prior art scheme.
PREFERRED EMBODIMENT From the discussion presented hereinabove, the preferred embodiment of the present invention includes a process for liquefying solid particulate coal which comprises introducing crushed coal having an average particle diameter of at least -8 Tyler screen into the upper end of a vertically disposed liquefying zone maintained under coal liquefying conditions, introducing solid particulate coal, hydrogenation catalyst into the upper end of said zone; passing coal solvent into said first zone at from 2 to 5 spaced points from their lower end of said zone; introducing hydrogen into the lower end of said zone; removing from the upper end of said zone rich solvent containing dissolved coal; and withdrawing from the lower end of said zone a slurry comprising solvent, ash, and said catalyst.
In a distinctly preferred embodiment of the present invention there is provided the preferred process hereinabove wherein said conditions are sufficient to maintain said solvent substantially in liquid phase.
The invention claimed:
1. Process for treating coal which comprises passing crushed coal into the upper end of a vertically disposed liquefying zone maintained under coal liquefying conditions, introducing hydrogen-donor type coal solvent into said zone at a plurality of vertically spaced points in the lower end portion of said zone, passing said crushed coal downwardly through said zone in a state of hindered settling, removing from the upper end of said zone rich solvent containing dissolved coal, and withdrawing from the lower end of said zone a residue stream containing ash and undissolved material.
2. A process according to claim 1 wherein said coal liquefying conditions include the presence of hydrogenation catalyst and hydrogen.
3. Process according to claim 1 wherein said plurality of solvent introduction points comprises from 2 to 3 points.
4. Process comprising contacting crushed coal in a contacting zone with hydrogen-donor type solvent in continuous countercurrent fashion, said coal being introduced into said zone at the upper end thereof and said solvent being introduced at a plurality of vertically spaced points in the lower end portion thereof, passing said crushed coal downwardly through said zone in a state of hindered settling, under conditions suiicient t dissolve more than 50% M.A.F. coal into said solvent, and thereafter separating the solvent containing dissolved coal from the ash and undissolved material.
5. Process according to claim 4 wherein said plurality of spaced points comprises from 2 to 3 points and said conditions include the presence of hydrogen gas.
6. Process for liquefying solid particulate coal which comprises introducing crushed coal having an average particle diameter of at least -8 Tyler screen into the upper end of a vertically disposed liquefying zone maintained under coal liquefying conditions, introducing solid particulate coal hydrogenation catalyst into the upper end of said zone, passing hydrogen-donor type coal solvent into said zone at from 2 to 5 vertically spaced points in the lower end portion of said zone, removing from the upper end of said Zone rich solvent containing dissolved coal, passing said crushed coal downwardly through said zone in a state of hindered settling, and withdrawing from the lower end of said zone a slurry comprising solvent, ash and said catalyst.
7. Process according to claim 6 wherein said conditions are sufficient to maintain said solvent substantially in liquid phase.
8. Process according to claim 7 wherein said coal solvent is passed through in spaced points in a manner such that from t0 90% by volume is passed through the uppermost point.
References Cited UNITED STATES PATENTS Re. 25,770 4/1965 Johanson 208-10 2,707,163 4/1955 Thibaut 208-10 3,184,401 5/1965 GOrIl 208-8 DELBERT E. GANTZ, Primary Examiner V. OKEEFE, Assistant Examiner