US3468792A - Simultaneous solvent extraction of a light and heavy fraction - Google Patents

Description

Sept. 23, 1969 K. D. UlTTl SIMULTANEOUS SOLVENT EXTRACTION OF A LIGHT AND HEAVY FRACTION Filed Aug. 17. 1967 Heavy Raff/note Heavy Feed Light Raff/note L Light Feed N VE/V T0 R= Kenneth 0. 01m

A TTORNEYS United States Patent US. Cl. 208-312 7 Claims ABSTRACT OF THE DISCLOSURE Process for separating and recovering aromatic hydrocarbons, such as benzene, from two different feedstock mixtures via a dual zone solvent extraction technique. The preferred solvent comprises sulfolane and the extraction zones are refluxed individually with specific nonaromatic hydrocarbon streams.

Background of the invention This invention relates to a separation process. It also relates to a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures using a selective solvent. It particularly relates to a solvent extraction process whereby aromatic hydrocarbons are recovered simultaneously from two different feed stocks utilizing a dual extraction system.

The technique of aromatic hydrocarbon separation by solvent extraction is well-known in the prior art. Generally, according to a well-known procedure, a mixture of hydrocarbons is introduced into an extraction zone at an intermediate point thereof and is contacted therein with a solvent which is selective for aromatic hydrocarbons. A raflinate phase comprising substantially all of the non-aro matic hydrocarbons in the feed stock is removed from one end of the extraction zone at which the solvent is introduced and an extract phase comprising the aromatic components of the feed stock dissolved in the selective solvent is removed from the other end of the extraction zone. The non-aromatic raflinate phase is generally utilized in gasoline blending. The extract phase is subsequently passed through various separation means for the recovery therefrom of aromatic hydrocarbons in relatively high concentration and high purity with a residue fraction comprising lean solvent which is generally suitable for reuse in the extraction zone. One feature in the prior art process is the substantial freeing of the extract phase of substantially all of the non-aromatic components dissolved by the solvent from the feedstocks. This is generally accomplished by introducing a relatively light hydrocarbon reflux stream, typically comprising naphthenes and paraflins, into the extraction zone at a point intermediate between .the feedintroduction point and the extract phase withdrawal point. This reflux phase is more volatile in the presence of the solvent than either of the aromatic or raflinate components of the feedstock and in effect acts as a displacing agent for the non-aromatic hydrocarbons into the raffinate stream.

Normally in a solvent extraction process of the type described hereinabove, the raflinate stream is recovered as a separate product stream and the aromatic hydrocarbons are recovered as a second product stream. Both of these product streams have well-known uses. Accordingly, if petroleum refiners have various hydrocarbon feedstocks possessing different characteristics, it is common practice to pass these feedstocks through the solvent extraction system in blocked out operation so as to avoid contamination of the product streams with undesirable components. If the volume of different feedstocks is sufficient, if is also 3,468,792 Patented Sept. 23 1969 common practice for the prior art to build two separate and distinct solvent extraction systems; again, in order to avoid cross contamination between the various products. It would be desirable therefore to provide a process whereby different hydrocarbon feedstocks could be processed through the solvent extraction system with a minimum of excessive equipment and with a technique which would avoid cross contamination of the products.

Summary of the invention Therefore, it is an object of this invention to provide a separation process.

It is another object of this invention to provide a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures.

It is still another object of this invention to provide a process for the recovery of aromatic hydrocarbons from two different feedstocks via a dual solvent extraction technique in a facile and economic manner.

According to the practice of the present invention there is provided a process for separating aromatic hydrocarbons from a relatively heavy hydrocarbon mixture and a relatively light hydrocarbon mixture which comprises the steps of: (a) introducing said light mixture into a first solvent extraction zone in contact with a hereinafter specified lean solvent stream and a hereinafter specified light reflux stream under conditions sufficient to produce a light raffinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein; (b) removing from said first zone said light ratfinate as a first product stream; (c) introducing said heavy mixture into a second solvent extraction zone in contact with hereinafter specified lean solvent stream and hereinafter specified heavy reflux stream under conditions sufficient to produce a heavy raffinate stream and an extract stream comprising solvent having aromatic hydrocarbon dissolved therein; (d) removing from said second zone said heavy raifinate as a second product stream; (e) passing the extract stream from said first zone and from said second zone to a separation zone maintained under separation conditions sufficient to produce a relatively light non-aromatic fraction, a relatively heavy non-aromatic fraction, and a solvent fraction containing a concentrate of aromatic hydrocarbons; (f) introducing light non-aromatic fraction into said first extraction zone as said specified reflux therein, and introducing heavy non-aromatic fraction into said second extraction zone as said specified reflux stream therein; (g) separating a concentrate of aromatic hydrocarbons from said solvent fraction of step (e) thereby producing a lean solvent stream; (h) returning a portion of said produced lean solvent to said first extraction zone and another portion of said produced lean solvent stream to said second extraction zone as specified therein; and, (i) recovering aromatic hydrocarbons in high concentration as a third product stream.

Another embodiment of the present invention includes the process hereinabove wherein said lean solvent comprises aqueous sulfolane.

Still another embodiment of the present invention includes the process hereinabove wherein said concentrateof aromatic hydrocarbons comprises a combined mixture of aromatic hydrocarbons which were originally present in said light mixture and originally present in said heavy mixture.

As a broad general class, suitable feedstocks for the practice of this invention include fluid hydrocarbon mixtures having a sufficiently high concentration of aromatic hydrocarbons to economically justify recovery of the aromatics. The present invention is particularly applicable to hydrocarbon feed mixtures which are different in some characteristic. The most applicable characteristic for the practice of this invention includes hydrocarbon mixtures which are different in terms of boiling point. Therefore, feedstocks applicable for use in the present invention include a relatively heavy hydrocarbon mixture and a relatively light hydrocarbon mixture. In other words, these different feedstocks include a relatively high boiling aromatic hydrocarbon containing feedstock and a relatively low boiling aromatic hydrocarbon containing feedstock. These different feedstocks preferably should contain at least 25% by weight aromatic hydrocarbons and should have at least one aromatic hydrocarbon type which is common between the two different feedstocks although this common expedient is not a requirement thereof, only a preferred embodiment thereof. The suitable carbon number range of the feedstocks is from about six carbon atoms per molecule to about twenty carbon atoms per molecule and preferably from six to ten. One source of feedstock is the debutanized reactor eflluent from a reforming processing unit. Another source is the liquid by product from a pyrolysis processing unit which has been hydrotreated to saturate diolefins and olefins and to remove other contaminants thereby producing an aromatic hydrocarbon concentrate suitable for solvent extraction techniques. An example of a relatively light hydrocarbon feedstock includes a mixture of hexane, cyclohexane, and benzene. An example of a relatively heavy hydrocarbon feedstock includes a Wide boiling benzene, toluene, xylene stream mixed with the corresponding parafiins and naphthenes which has been obtained from a conventional catalytic reforming unit.

The preferred solvent which may be utilized in the aromatics extraction process of the present invention is a solvent of the sulfolane type. This solvent is well-known and typically possesses five-membered ring containing one atom of sulfur and four atoms of carbon with two oxygen atoms bonded to the sulfur atom of the ring. Those skilled in the aromatics extraction art are well-versed in the characteristics of this type of solvent and more detail thereof need not be presented at this time.

Other solvents which may be included and may be satisfactorily used in the practice of the present invention are the sulfolenes such as 2-sulfolene and 3-sulfolene. Still other typical solvents which have a high selectivity for separating aromatic hydrocarbons from non-aromatic hydrocarbons, and which may be processed within the scope of the present invention, are Z-methyl-sulfolane, 2-4, dimethyl-sulfolane, methyl-Z-sulfonylether, n-aryl-3-sulfonylacetate, diethyleneglycol, various polyethyleneglycols, dimethylsulfoxide, n-methylpyrrolidone, and the like. The specifically preferred solvent for use in the practice of the present invention is sulfolane.

The aromatic selectivity of the solvents can usually be further enhanced by the addition of water to the solvent. Preferably, the solvents utilized in the practice of this invention contain small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbons over non-aromatic hydrocarbons.

without reducing substantially the solubility of the solvent phase for aromatic hydrocarbons. The presence of water in the solvent composition furthermore provides a relatively volatile material therein which is distilled from the solvent in the extractive stripper, more fully discussed hereinafter, to vaporize the last traces of non-aromatic hydrocarbons from the solvent stream by steam distillation. The solvent composition of the present invention preferably contains from about 0.5% to about 20% by weight water and preferably from about 5% to about by weight depending upon the particular solvent utilized and the process conditions at which the extraction zones and extractive stripper are operated.

The dual extraction zones of the present invention are operated at elevated temperature and at a sufiiciently elevated pressure to maintain the feedstock, sol-vent, and reflux streams in liquid phase. The operating condition in each extraction zone may be the same or may be difierent depending upon the characteristics of the particular feedstock charged to the particular extraction zone. Typically suitable temperatures when utilizing sulfolane as the solvent are within the range from F. to about 400 F., and preferably from about 175 F. to about 300 F. Suitable pressures are generally Within the range of from about atmospheric pressure up to about 400 p.s.i.g. and preferably from 50 to 150 p.s.i.g.

It is noted from the description of the present invention thus far, that one of the critical problems to which this invention is directed is the displacement of nonaromatic hydrocarbons from the extract phase in the lower end of the extraction zone by utilizing the technique of non-aromatic reflux at that point. Therefore, it is a feature of the present invention to use a relatively light reflux material on the extraction zone which is charging the relatively light or low boiling hydrocarbon feedstock. Conversely, it is a feature of this invention to utilize a relatively heavy reflux on the lower end of the extraction zone which charges these relatively heavy or high boiling hydrocarbon feedsto-cks to the extraction zone. The source of the specific reflux streams utilized herein will be more fully developed hereinbelow. In any event, the volume of reflux material introduced into the lower end of the extraction zone should be at least 10% by volume of the extract phase leaving the extraction zone in question in order to effectively displace the nonaromatic hydrocarbons from the extract. By operating in this manner, cross-contamination of the respective rafiinate streams products is avoided.

The extractive stripper is operated at moderate pressures and sufiiciently high reboiler temperatures to drive all of the non-aromatic material and some of the aromatics, water, and solvent overhead. Typical stripper pressures are from atmospheric pressure to about p.s.i.g., although generally the top of the stripper is preferably maintained at from about 1 p.s.i.g. to about 20 p.s.i.g. The reboiler temperature, of course, is dependent upon the composition of the feedstock and solvent. Those skilled in the art from a knowledge of the prior art are well versed in the operation of the stripper column and additional details thereof need not be presented here.

The solvent recovery column is operated at generally low pressures and sufiiciently high temperatures to distillate the aromatic hydrocarbons overhead as a distillate fraction and thereby producing a solvent bottoms fraction which is suitable for reuse in both of the solvent extraction zones previously mentioned. Again, the choice of operating conditions depends on the feedstock characteristics and the composition of the solvent. Preferably, the top of the solvent recovery column is operated at from 100 to 400 millimeters of mercury absolute. These subatmospheric pressures must be employed in order to maintain a sufliciently low reboiler temperature to avoid thermal decomposition of the solvent; particularly when the solvent is of the sulfolane type. Preferably, the reboiler temperature should be maintained below about 360 F. when using saturated sulfolane as the solvent.

Thus, from the description presented thus far, the present invention involves a solvent extraction process utilizing a dual extraction zone and a combined stripperaromatic recovery system whereby solvent is recovered for reuse within the process in both extraction zones and an aromatic hydrocarbon product stream is obtained which contains aromatic hydrocarbons which were originally present in both of the feedstocks.

The overhead stream from the extractive stripper column is in vapor state and comprises primarily nonaromatic hydrocarbons of the naphthene and/or parafiin type. In a preferred embodiment of this invention the vapor stream from the extractive stripper column is partially condensed in a heat exchanger condenser thereby producing a liquid fraction which is enriched in relatively heavy non-aromatic hydrocarbons. This enriched liquid stream is then returned at least in part to the extraction zone as reflux thereon which originally had charged to it the relatively heavy feed mixture. The vapor phase remaining from the partial condenser is next passed into a final condensing zone thereby producing a liquid comprising relatively light or low boiling non-aromatic hydrocarbons. This latter liquid stream is similarly utilized at least in part as reflux on the extraction zone which charged the relatively light or low boiling feed mixture.

The bottoms material from the extractive stripper is passed into a solvent recovery column which is operated at low pressures as previously mentioned in order to separate and recover the aromatic hydrocarbons from the solvent phase and to recover a lean solvent stream suitable for reuse in both of the extraction zones.

Thus, it is seen that the present invention utilizes a common extractive stripper column, a common solvent recovery column which is related to the dual extraction zone in an inter-dependent manner which eifects considerable economies of operation and in capital expense.

The invention may be further understood with reference to the appended drawing which is a schematic representation of apparatus for practicing one embodiment of the invention.

Description of the drawing Referring now to the drawing, a relatively heavy hydrocarbon feedstock containing aromatic hydrocarbons such as a conventional mixed product from a conventional catalytic reforming unit comprising benzene, toluene, and xylene mixed with corresponding naphthenes and paraffins enters the system via line 10. Simultaneously therewith, a relatively light hydrocarbon feedstock such as one comprising hexane, cyclohexane, and benzene enters the system via line 11.

Referring now to the relatively heavy feedstock operation: the feed enters extractor column 12 via line at an intermediate point. Aqueous sulfolane as the lean solvent enters the upper portion of extractor 12 via line 17 and a relatively heavy reflux stream enters extractor 12 at a lower end thereof via line 29, the source of which is more fully developed hereinafter. A relatively heavy raifinate stream is withdrawn from extractor 12 via line 19 as a product stream. An extract phase comprising solvent having dissolved therein aromatic hydrocarbons and also which is contaminated with small amounts of relatively heavy-non-aromatic hydrocarbons, such as naphthenes and paraflins, is withdrawn from extractor 12 via line 18.

Referring now to the extraction step of the relatively light feed: the relatively light hydrocarbon feed enters extractor 13 via line 11. Aqueous sulfolane as the lean solvent enters the extractor at a upper portion thereof via line 16 and a relatively light non-aromatic reflux stream enters extractor 13 at the lower end thereof via line 33, the source of which is now fully developed hereinbelow. A relatively light raffinate stream is withdrawn from extractor 13 via line 14 as a'product stream. An extract phase comprising solvent having aromatic hydrocarbons dissolved therein and which is contaminated with minor amounts of realtively light non-aromatic hydrocarbons such as hexane is withdrawn from extractor 13 via line 15.

As a preferred embodiment of this invention, the extract phase in line 18 and the extract phase in line are admixed and passed via line 20 into extractor stripper column 21. Additional solvent may be added to extractor stripper column 21 as needed, from a source not shown, in order to enhance the separation of the nonaromatic hydrocarbons from the solvent containing the aromatic hydrocarbons. Suilicient separation conditions are maintained in extractor stripper 21 to produce a bottoms fraction comprising solvent having dissolved therein the desired aromatic hydrocarbons which is removed from stripper 21 via line 22.

A distillate fraction comprising non-aromatic hydrocarbons and minor amounts of solvent aromatic hydrocarbons is withdrawn from stripper 21 via line 26 and passed into partial condenser 27 which is maintained under conditions suflicient to produce a liquid phase and a vapor phase in separation zone 28. Normally the conditions maintained in condenser 27 will be sutficient to produce from 5% to liquid in separator 28 depending upon the relative amount of non-aromatic hydrocarbons present in both feedstocks. The liquid which is condensed in separator 28 comprises relatively heavy nonaromatic hydrocarbons of the bulk of which typically, was originally present in the relatively heavy feed which entered the system via line 10. On the other hand, the relatively heavy non-aromatic hydrocarbon stream has a higher volatility than the rich solvent phase in the lower end of extractor 12 to which ultimately this material is utilized as reflux, as previously mentioned. Thus, the liquid phase in separator 28 is withdrawn via line 29 and passed into the lower end of extractor 12. The utilization of this specific stream in extractor 12 permits the displacement from the extract phase of additional non-aromatic hydrocarbons thereby enhancing the removal of non-aromatic hydrocarbons into the heavy rafiinate phase which is withdrawn via line 19.

The remaining vapor from separator 28 is withdrawn via line 30 and passed into final condenser 31 for the production of a liquid phase containing relatively light non-aromatic hydrocarbons which is accumulated in accumulator 32. This liquid phase in accumulator 32 is passed via line 33 into the lower end of extractor 13 thereby displacing additional non-aromatic hydrocarbons from the extract phase at the lower end of extractor 13.

Referring now to the solvent and aromatic hydrocarbon stream in line 22: this material is passed into recovery column 23 which is maintained under conditions suflicient to separate the aromatic hydrocarbons from the solvent phase. The aromatic hydrocarbons are concentrated as a distillate fraction and removed from recovery column 23 via line 24. It is to be noted that the material in line 24 contains, for example, benzene which was originally present in the heavy feed in line 10 and the light feed in line 11 and is removed from the system as a combined product stream. The operation of recovery column 23 includes the use of steam stripping in the column in order to remove the final traces of aromatic hydrocarbons from the solvent. Therefore, the lower end of column 23 contains the desired solvent plus water in sufficient amounts such that the material Withdrawn from column 23 via line 25 comprises lean solvent generally suitable for reuse in both extraction zones. It is noted that this lean solvent stream in line 25 is split and a portion thereof is passed via line 17 into extractor 12 and another portion passed v1a line 16 into extractor 13 as previously described.

Thus, it is seen that the practice of the present invention enables the simultaneous handling of two different feedstocks which avoids the contamination of the light raffinate phase with non-aromatic hydrocarbons from the heavy feed system and enables the production of a combrned aromatic stream in high concentration and high purity without the necessity of either blocked-out operation or absolute duplicity of equipment.

Preferred embodiment Therefore, from the presentation presented hereinabove, the preferred embodiment of the invention provides a process for separating and recovering a combined aromatic hydrocarbon stream from two different feedstocks which comprises the steps of: (a) introducing a relatively low boiling, aromatic hydrocarbon-containing feedstock into a first solvent extraction zone and introducing a hereinafter specified low boiling reflux stream into said first zone, said first solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, suflicient to separate said low boiling feedstock into a light raflinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing light non-aromatic hydrocarbons as contaminants therein; (b) removing from said first zone said light raflinate as a first product stream; introducing a relatively high boiling, aromatic hydrocarbon-containing feedstock into a second solvent extraction zone and introducing a hereinafter specified high boiling reflux stream into said second zone, said second solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, sufiicient to separate said high boiling feedstock into a heavy rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing heavy non-aromatic hydrocarbons as contaminants therein; (d) removing from said second zone said heavy rafiinate as a second product stream; (e) admixing the extract stream from the first extraction zone with the extract stream from the second extraction zone and passing said admixture into a first distillation zone maintained under distillation conditions suflicient to produce a first vapor fraction comprising nonaromatic hydrocarbons and a liquid fraction comprising solvent having aromatic hydrocarbons dissolved therein; (f) partially condensing said first vapor fraction to produce a relatively high boiling liquid fraction comprising non-aromatic hydrocarbons and a second vapor fraction; (g) passing at least a portion of said high boiling liquid fraction into said second solvent extraction zone as the specified reflux stream; (h) condensing said second vapor fraction, thereby producing a relatively low boiling liquid fraction comprising nonaromatic hydrocarbons; (i) passing at least a portion of said low boiling fraction into said first solve'nt extraction zone as the specified reflux stream; (3) introducing the liquid solvent fraction from step (e) into a second distillation zone maintained under distillation conditions sufficient to produce a second distillate fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent suitable for reuse in said extration zones; and, (k) recovering said second distillate fraction.

A still further preferred embodiment of this invention includes the process hereinabove wherein said solvent comprises sulfolane.

The invention claimed:

1. Process for separating aromatic hydrocarbons from a relatively heavy hydrocarbon mixture and a relatively light hydrocarbon mixture which comprises the steps of:

(a) introducing said light mixture into a first solvent extraction zone in contact with a hereinafter specified lean solvent stream and a hereinafter specified light reflux stream under conditions sufficient to produce a light raflinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein;

(b) removing from said first zone said light raffinate as a first product stream;

(c) introducing said heavy mixture into a second solvent extraction zone in contact with a hereinafter specified lean solvent stream and hereinafter specified heavy reflux stream under conditions suflicient to produce a heavy raffinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein;

(d) removing from said second zone said heavy raflinate as a second product stream;

(e) passing the extract stream from said first zone and from said second zone into a distillation zone maintained under conditions suflicient to produce a distillate stream comprising a concentrate of non-aromatic hydrocarbons and a bottoms stream comprising solvent having aromatic hydrocarbons dissolved therein;

(f) separating said distillate stream into a relatively light non-aromatic hydrocarbon fraction and a relatively heavy non-aromatic hydrocarbon fraction;

(g) introducing said relatively light non-aromatic fraction into said first extraction zone as said specified reflux stream therein and introducing said relatively heavy non-aromatic fraction into said second extraction zone as said specified reflux stream therein;

(h) separating a concentrate of aromatic hydrocarbons from said solvent fraction of step (e) thereby producing a lean solvent stream;

(i) returning a portion of said produced lean solvent to said first extraction zone and another portion of said produced lean solvent stream to said second extraction zone as specified therein; and,

(j) recovering the aromatic hydrocarbons produced in step (f) in high concentration as a third product stream.

2. Process according to claim 1 wherein said lean solvent comprises aqueous sulfolane.

3. Process according to claim 1 wherein said concentrate of aromatic hydrocarbons comprises a combined mixture of aromatic hydrocarbons which were originally present in said light mixture and originally present in said heavy mixture.

4. Process for separating and recovering a combined aromatic hydrocarbon stream from two different feedstocks which comprises the steps of:

(a) introducing a relatively low boiling aromatic hydrocarbon containing feedstock into a first solvent extraction zone and introducing a hereinafter specified low boiling reflux stream into said first zone, said first solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, suflicient to separate said low boiling feedstock into a light rafiinate stream and an extract stream comprising solvent aromatic hydrocarbons dissolved therein and containing light non-aromatic hydrocarbons as contaminants therein;

(b) removing from said first zone said light raflinate as a first product stream;

(c) introducing a relatively high boiling, aromatic hydrocarbon containing feedstock into a second solvent extraction zone and introducing a hereinafter specified high boiling reflux stream into said second zone, said second solvent extraction zone being maintained under extraction conditions, including the presence of lean solvent, suflicient to separate said high boiling feedstock into a heavy rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing heavy non-aromatic hydrocarbons as contaminants therein;

(d) removing from said second zone said heavy raflinate as a second product stream;

(e) admixing the extract stream from the first extraction zone with the extract stream from the second extraction zone and passing said admixture in a first distillation zone maintained under distillation conditions suflicient to produce a first vapor fraction comprising non-aromatic hydrocarbons and a liquid fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(f) partially condensing said first vapor fraction to produce a relatively high boiling liquid fraction comprising non-aromatic hydrocarbons, and a second vapor fraction;

(g) passing at least a. portion of said high boiling liquid fraction into said second solvent extraction zone as the specified reflux stream;

(h) condensing said second vapor fraction thereby producing a relatively low boiling liquid fraction comprising non-aromatic hydrocarbons;

(i) passing at least a portion of said low boiling fraction into said first solvent extraction zone as the specified reflux stream;

(j) introducing the liquid solvent fraction from step (e) into a second distillation zone maintained under distillation conditions sufficient to produce a second distillate fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent suitable for reuse in said extraction zones; and,

(k) recovering said second distillate fraction.

5. Process according to claim 4 wherein said solvent comprises sulfolane.

6. Process according to claim 5 wherein said aromatic hydrocarbons comprise single ring type aromatic hydrocarbons.

7. Process according to claim 6 wherein said single ring hydrocarbons comprise benzene.

References Cited 5 UNITED STATES PATENTS 2,492,787 12/1949 Davis 208-321 2,921,015 1/1960 Shiras 208-312 3,262,875 7/1966 Girroti et al. 208-321 10 3,361,664 1/1968 Broughton et a1. 208-321 HERBERT LEVINE, Primary Examiner US. Cl. X.R.

Images