US3647907A

US3647907A - Process for quenching a gas obtained by thermal cracking of hydrocarbons

Info

Publication number: US3647907A
Application number: US852275A
Authority: US
Inventors: Takehiko Sato; Hiroyuki Sagami
Original assignee: Mitsubishi Petrochemical Co Ltd
Current assignee: Mitsubishi Petrochemical Co Ltd
Priority date: 1968-09-06
Filing date: 1969-08-22
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07
Also published as: DE1945139B2; GB1249559A; DE1945139A1; NL6913589A; JPS4624681B1; NL155302B

Abstract

PROCESS FOR QUENCHING A GAS OBTAINED BY THERMAL CRACKING OF HEAVIER HYDROCARBONS THAN KEROSENE WHICH COMPRISES QUENCHING INDIRECTLY THE HIGH TEMPERATURE CRACKED GAS IN A QUENCHING HEAT EXCHANGER TO A TEMPERATURE CRACKED REPRESENTED BY THE FOLLOWING EQUATION:

T=0.56X+A

WHEREIN T IS A VOLUMETRIC AVERAGE BOILING POINT OF THE FEED HYROCARBONS AND T= T10+T30+T50+T70+T90/5 IN WHICH T10, T30, T50, T70 AND T90 IS A TEMPERATURE OF (*C.) AT WHICH 10 VOLUME PERCENT, 30 VOLUME PERCENT, 50 VOLUME PERCENT, 70 VOLUME PERCENT AND 90 VOLUME PERCENT OF FEED HYDROCARBONS IS DISTILLED IN ASTM DISTILLATION, RESPECTIVELY, AND A IS A TEMPERATURE IN THE RANGE OF 340-240* C., AND T IS A TEMPERATURE ABOUVE 450* C. BUT NOT EXCEEDING 600*C., SAID TEMPERATURE (T) BEING A TEMPERATURE OF THE CRACKED GAS AT AN OUTLET OF THE QUENCHING HEAT EXCHANGER, AND FURTHER QUENCHING DIRECTLY THE CRACKED GAS TO A TEMPERATURE OF 150-250*C.BY SPRAYING HEAVIER OIL.

Description

March 7, 1912 TAKEHIKO SATQ EI'AL 3,647,907

PROCESS FOR QUENCHING A GAS OBTAINED BY THERMAL CRACKING 0F HYDROCARBONS 2 Sheets-Sheet 1 Filed Aug. 22, 1969 FIG. I

" foo O O o O O O 5 O VOLUMETRIC AVERAGE BOILING POINT (C) INVENTORS TAKEHIKO SATO HIROYUKI SAGAMI avg/ M ATTORNEYS- March 7, 1972 TAKEHIKQ SATO E.TAL' 3,647,907

PROCESS FOR QUENCHING A GAS OBTAINED BY THERMAL CRACKING OF HYDROCARBONS 7 Filed Aug. 22, 1969 2 Sheets-Sheet 2 INVENTORS TAKEHIKO SATO HIROYUKI SAGAMI ATTORNEYS United States Patent US. Cl. 260-683 4 Claims ABSTRACT OF THE DISCLOSURE Process for quenching a gas obtained by thermal cracking of heavier hydrocarbons than kerosene which comprises quenching indirectly the high temperature cracked gas in a quenching heat exchanger to a temperature (T) represented by the following equation:

wherein t is a volumetric average boiling point of the feed hydrocarbons and in which 1' r 1 I and 1 is a temperature of C.) at which 10 volume percent, 30 volume percent, 50 volume percent, 70 volume percent and 90 volume percent of feed hydrocarbons is distilled in ASTM distillation, respectively, and a is a temperature in the range of 340-420 C., and T is a temperature above 450 C. but not exceeding 600 C., said temperature -(T) being a temperature of the cracked gas at an outlet of the quenching heat exchanger, and further quenching directly the cracked gas to a temperature of 150250 C. by spraying heavier oil.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to a process for quenching a high temperature cracked gas obtained by thermal cracking of hydrocarbons efiiciently and economically.

(2) Description of the prior art The high temperature cracked gas obtained by thermal cracking of hydrocarbons contains polymeric tarry matter and cokes and is still reactive enough to produce these undesirable by-products. This tendency becomes more remarkable as the volumetric average boiling point of feed hydrocarbons to be cracked becomes higher. Therefore, there has not been found a satisfactory process for quenching the high temperature gas obtained by thermal cracking of kerosene, light gas oil or heavy gas oil etc.

The cracked gas obtained by thermal cracking of hydrocarbons has generally temperature of 700900 C. Therefore, it is required to quench the high temperature cracked gas to send the gas to a successive separating zone in which the cracked gas is separated into many useful fractions. And at the same time it is required to recover the heat energy as high pressure steam from economical standpoint. The quenching has to be done very quickly for preventing the polycondensation or polymerization of ice olefin, or aromatic hydrocarbons contained in the cracked gas. When the residence time of the high temperature cracked gas in quenching apparatus is longer, undesirable secondary reactions of the cracked gas occur remarkably, and the cracked gas is converted to products having lower value, for instance, gases such as hydrogen and methane, heavier cracked oil and cokes which deposit on the surface of tubes through which the gas passes.

In general, the quenching of the cracked gas of lighter hydrocarbons than naphtha is carried out in an indirect quenching apparatus which is connected with the cracking furnace.

If the cracked gas obtained by thermal cracking of the heavier hydrocarbons than kerosene is quenched in the conventional indirect quenching apparatus used for light hydrocarbons than naphtha, tarry matter and cokes contained in the cracked gas deposit on the surfaces of the tubes of quenching heat exchanger, and as a result, they are completely plugged in a short period of time.

As described above, the most troublesome drawback of the conventional quenching process, in which the thermal cracked gas at 750-850 C. is quenched to a temperature below 400 C. by conventional heat exchanger, is that severe coking occurs in the surfaces of tubes of quenching heat exchanger, particularly the surfaces of tubes of the outlet portion thereof.

When the heavier hydrocarbons than kerosene, such as kerosene, light gas oil or heavy gas oil are cracked and when the cracking condition becomes severe, above mentioned coking occurs more violently. Because of this coking, the pressure of the thermal cracking furnace is increased by increase of the pressure drop across the quenching heat exchanger, which leads to a lower yield of olefin.

Accordingly, the operation of the thermal cracking furnace and the quenching heat exchanger is compelled to interrupt in a short period of time to remove the cokes deposited on the surfaces of the tubes of heat exchanger. These are large obstacles and disadvantageous to the continuous operation of the thermal cracking furnace.

From above mentioned troublesome drawback, the conventional quenching process of cracked gas was practically impossible to apply for heavier hydrocarbons than kerosene.

There have been proposed many oil direct quenching processes, in which liquid hydrocarbons are sprayed on the cracked gas directly. But these processes are uneconomical for commercial application because of difiiculty of keeping steady and safety operations.

In case of quenching of cracked gas obtained by severe cracking in which useful gas components are increased, much tarry matter and cokes are violently formed in a quenching process by polycondensation and polymerizations of quenching oil, because the quenching oil is in contact with hot cracked gas, and is kept at too high temperature to recover heat energy as high pressure steam.

These tarry matter and cokes interrupt the steady operation and recovering of high pressure steam.

On the other hand, when the feed hydrocarbons are thermally cracked at a medium temperature, the yields of the useful components contained in the cracked gas are lowered, and it is diflicult to recover a high pressure steam as an energy source. Furthermore it is required in these processes to handle a large quantity of a quenching oil in the vicinity of the cracking furnace, and it is not desirable from the standpoint of safety. In order to separate and remove the cokes which are deposited on the oil quenching apparatus, mechanical means might be used, but it brings undesirable results arising from the complication of the apparatus. Thus, the oil direct quenching process is not economical to the quenching of cracked gas obtained by high temperature thermal cracking.

SUMMARY OF THE INVENTION An object of the present invention is to provide a quenching process of thermal cracked gas of heavier hydrocarbons than kerosene for producing olefin hydrocarbons in high yield under stable operating conditions. According to the present invention, the object described above is achieved by a process which comprises quenching indirectly the high temperature cracked gas containing lower hydrocarbons such as ethylene and propylene which is obtained by thermal cracking of a heavier hydrocarbon oil than kerosene such as kerosene, light gas oil and heavy oil in a quenching heat exchanger at a mass velocity 50-l20 kg./m. -second, preferably 60-110 kg./m. -second, and in a period of time not more than 0.05 second, preferably not more than 0.04 second and to a temperature (T) represented by the following equation:

so so 70 90 in which r r r r and t is a temperature C.) at which 10 volume percent, 30 volume percent, 50 volume percent, 70 volume percent and volume percent of feed hydrocarbons is distilled in ASTM distillation, respectively, and or is a temperature in the range of 340-420" C., and T is a temperature above 450 C. but not exceeding 600 C., said temperature (T) being a temperature of the cracked gas at an outlet of the quenching heat exchanger, and further, quenching directly the cracked gas to a temperature of ISO-250 C. by spraying the heavier oil of which 50% recovery temperature of ASTM distillation is in the range of 200-400" C. and preferably in the range of 250350 C.

The present invention is based on our discovery that the deposition of the coke on the surfaces of the tubes of the quenching heat exchanger in a thermal cracking of heavy hydrocarbons such as kerosene, light gas oil or heavy gas oil, which is a dominant factor causing the troubles on the continuous operation, can be prevented by limiting the mass velocity of the cracked gas passing therethrough to 50-120 kg./m. -sec0nd and the cooling time therein to not more than 0.05 second and regulating a temperature of the cracked gas at the outlet of the quenching heat exchanger to a specific temperature, depending upon cracking conditions and the boiling point of the feed hydrocarbons, and the tar and coke contained in the cracked gas can be discharged out of the system without deposition by succeeding oil direct quenching.

If the mass velocity of more than kg./m. 'second is employed, the pressure loss in the quenching heat exchanger increases thereby bringing about the elevation of pressure in the thermal cracking furnace, which leads to the undesirable reduction of the yield of olefin and an increase of coking.

In the mass velocity of less than 50 kg./m. -second, heavy fractions contained in the cracked gas tend to ad here to the surface of the tubes of the quenching heat exchanger and results in the undesirable deposition of cokes. The quenching in the quenching heat exchanger has an eifect of reducing a polymerization or polycondensation reaction of the quenching oil in the succeeding oil spraying quenching apparatus.

According to the present invention, it is possible to suppress the deposition of cokes on the surfaces of the tubes in the quenching heat exchanger, which are brought about by the polycondensation, polymerization and car bonization of olefin, diolefin and polycyclic aromatic compounds having high boiling points contained in the cracked gas, by adjustments of the mass velocity of the cracked gas through the quenching heat exchanger, and the resi- .4 dence time of the cracked gas in the quenching heat eX- changer to specific ranges and the temperature of the cracked gas at an outlet of the quenching heat exchanger, depending upon the nature of the starting hydrocarbon oil and the thermal cracking conditions used. The high mass velocity used in the present invention has an effect of removing physically the cokes adhered to the surfaces of the tubes.

The temperature T of the cracked gas at an outlet of quenching heat exchanger is a temperature in the range of 450600 C. depending upon a volumetric average boiling point t of the feed hydrocarbons and is represented by the following equation:

wherein at is a temperature in the range of 340420 C. depending upon the thermal cracking conditions used and the feed hydrocarbon properties. FIG. 1 shows the relations of the volumetric average boiling point of the feed hydrocarbons and the temperature of the cracked gas at an outlet of quenching heat exchanger. With the ranges of the temperatures of the cracked gas at an outlet of quenching heat exchanger shown in the FIG. 1, at a given feed hydrocarbons, the value of or becomes larger as the thermal cracking condition becomes severe, and vice versa.

For example, the temperature of cracked gas at an outlet of quenching heat exchanger T is approximately in the range of 460540 C. for kerosene having boiling point 230 C., and 500-575 C. for light gas oil having boiling point 210-350 C., and 520-590 C. for heavy gas oil having boiling point 230-420 C. It was heretofore impossible to operate continuously the quenching heat exchanger due to the coking on the surfaces of the tubes when the cracked gas was quenched to 300380 C. at the outlet thereof, but if the temperature of the cracked gas at the outlet of the quenching heat exchanger is adjusted according to the equation described above, the continuous operation is possible for a long period of time because of the lack of the formation of the cokes and the deposit thereof. The upper limit of the temperature of the cracked gas at an outlet of the quenching heat exchanger is limited from various view points including the stop of thermal cracking reactions, the prevention of undesirable secondary reactions of the highly reactive cracked gas mixture, the improvement of heat recovery and the prevention of polymerization or condensation reaction of the quenching oil used in the succeeding quenching oil injection apparatus, and it is desirable below 600 C. The lower limit depends upon the formation of cokes on the surfaces of the tubes of the quenching heat exchanger. At a temperature lower than 450 C., the heavy fraction contained in the cracked gas is condensed on the surfaces of tubes of the quenching heat exchanger and form undesirable cokes. Therefore, a temperature lower than 45 0 C. is not desirable.

The heat transfer area of the quenching heat exchanger used in the present invention can be calculated on the basis of the temperature of the cracked gas at an outlet of quenching heat exchanger, which is determined by the above mentioned equation. When wide flexibility is desired for the feed hydrocarbons and the thermal cracking conditions, it is one of effective method to make a quenching heat exchanger having a maximum heat transfer area satisfying all of the desired requirements and to vary the temperature of the cracked gas at an outlet of the quenching heat exchanger by changing the pressure and level of the coolant. It is necessary to increase a temperature of cracked gas at the outlet of the quenching heat exchanger as the feed hydrocarbon become heavier and the thermal cracking conditions become more severe.

The present invention can be effectively practised in the vertical type quenching heat exchanger which is composed of a cooling tube consisting of a plurality of curved tubes, and a cylindrical vessel capable of accommodating the whole of the coolant which is in contact with the outer wall of the cooling tube. For instance, the high temperature cracked gas is passed from the lower portion of the cylindrical type vertical quenching heat exchanger to the upper direction thereof, and thus it is possible to adjust the temperature of the cracked gas at an outlet of the quenching heat exchanger by varying the heat transfer area of the cooling tube or the level of the coolant depending upon the properties of the feed hydrocarbon and the thermal cracking conditions.

Various types of conventional quenching heat exchangers and methods may be conveniently used in addition to the vertical quenching heat exchanger.

Now referring to FIG. 2, a process according to the present invention will be explained wherein the high temperature cracked gas is quenched according to the process of the present invention and the resulting gas is fed to a separating zone in which it is separated into useful components. The steam for diluting a feed hydrocarbons is introduced from an inlet (a) to a thermal cracking furnace 1 and the feed hydrocarbons are from inlet (b) introduced to the heating furnace 1. The thermal cracking heat furnace 1 is composed of a convection portion 2 and a radiation portion 3. In the convection portion 2 of the heating furnace 1, a mixture of the feed hydrocarbons and steam is preheated, and in the radiation portion 3, the hydrocarbon oil is thermally cracked. The thermally cracked gas mixture from the heating furnace 1 is quenched in the quenching heat exchanger 4. The line 5 which connects the heating furnace 1 with the quenching heat exchanger 4 is made of a pipe as short as possbile. The cracked gas mixture is rapidly introduced to the quenching heat exchanger wherein it is possible to cool the mixture to a temperature at which the thermal cracking is stopped. The thermally cracked gas mixture from the quenching heat exchanger 4 reaches the quenching oil injecting apparatus 7 through the line 6, and is cooled by the heavier oil which is injected through the line 10, and the thus cooled gas is forwarded to the separating zone 9 through the line 8. The mixture is separated in to the thermally cracked gas and the heavier oil in the separating zone 9, and the cracked gas is fed through the line 11 to the distillation system (not shown in the drawing) for recovering ethylene or other resulting hydrocarbons. The separated heavier oil is passed through the heat exchanger 12 via the line to recover steam, and is circulated in the quenching oil injection apparatus 7, and a portion thereof is removed out of the system via line 13.

In the present invention, the temperature of the cracked gas at an outlet of the quenching heat exchanger 4 is regulated depending upon the properties of the feed hydrocarbon and the thermal cracking conditions. That is, as the volumetric average boiling point of the feed hydrocarbon or the temperature of the thermal cracking become higher, the temperature of the cracked gas at an outlet of quenching heat exchange 4 has to be elevated. By this method, the coking in the quenching heat exchanger 4 which makes the continuous operation impossible may be prevented and the tarry matter contained in the cracked gas is liquefied in the quenching oil injection apparatus 7, and discharged out of the system continuously via line 13. In the quenching oil injection apparatus 7, the quenching oil of 2-8 times as much as the cracked gas is injected. The temperature of the quenching oil to be circulated through line 10 is maintained at an optimum temperature depending upon the properties of the feed hydrocarbons to be cracked and the cracking conditions, because the properties of the quenching oil are varied depending upon the properties of the feed hydrocarbons and the cracking condition, and the deterioration of the quenching oil and the coking at the quenching portion occur at a too high temperature of quenching oil.

A lower temperature of quenching oil is advantageous from a standpoint of the prevention of coking in the quenching oil injection apparatus 7 and the prevention of the deterioration of the quenching oil, it is accompanied by disadvantages in that the steam pressure which is recovered at the heat exchanger 12 becomes lower or the cost of installation for heat recovery becomes increased. In the circulating system of the quenching oil, it is a common practice to employ the highest temperature at which the properties of the quenching oil is stable, and in general, the temperature of the quenching oil is preferably in the range of ISO-250 C.

PREFERRED EMBODIMENT OF THE INVENTION The present invention will be described in detail in the following examples, but the present invention is not limited thereto.

COMPARATIVE EXAMPLE 1 The kerosene diluted with steam was thermally cracked in an external heating type tubular thermal cracking furnace. The properties of the feed kerosene are shown in Table 1.

Table 1 Properties of starting kerosene:

Specific gravity: 0.792 ASTM distillation:

10 volume percent recovered temperature:

C. 30 volume percent recovered temperature:

The thermal cracking was conducted at a flow rate of 3000 kg./hr., of kerosene, 1950 kg./hr. of steam, and at a temperature of 780 C. at the outlet of the thermal crackmg furnace. The composition of a gas at the outlet of the furnace was as shown in Table 2.

Table 2 Composition of cracked gas of kerosene (weight percent):

Above 180 C 16.2

The quenching heat exchanger was cooled with water having a pressure of kg./cm. g. When the cracked gas was passed through the quenching heat exchanger continuously under the conditions of mass velocity of 48 kg./m. -second, initial outlet temperature of 360 C. increased to 430 C. after a continuous operation of 15 days. While the pressure loss was initially 0.31 kg./cm. 1t mcreased to 0.5 kg./cm. after 15 days. There was a further tendency of increasing the pressure loss. As a result, the yield of olefin was lowered, and it was compelled to decoke the apparatus.

The gas leaving the quenching heat exchanger was subjected to the spraying of the heavier oil in the quenching oil injection apparatus to quench the gas to 200 C.

The heavier oil used as quenching oil was a heavy fraction which was separated from the cracked gas and having the properties shown in the following Table 3.

Table 3 Properties of quenching oil:

Specific gravity: 1.037 ASTM distillation:

Initial point: 185.0 C. 50%: 286.5 70%: 325.5 Total distillate: 75%

The quenching oil contained 10.8 weight percent as insoluble substance in n-heptane, but no coking occurred in the process line or the portion to which the quenching oil is injected. In the following examples, the quenching oil injection apparatus was an identical apparatus used in the Comparative Example 1.

EXAMPLE 1 As described above, coking occurred and pressure loss was large in the quenching exchanger of the Comparative Example 1. In this example, the Comparative Example 1 was repeated except that the mass velocity of the cracked gas was 70 kg./cm. -second and a quenching heat exchanger having a heat transfer surface of 70% of that of the quenching heat exchanger used in the Comparative Example 1.

While the temperature of a cracked gas at the outlet of the quenching heat exchanger was initially 450 C., it increased to 510 C. after days. While pressure loss was initially 0.22 kg./cm. it increased to 0.31 kg./cm. after 25 days. There was no necessity of decoking. After 90 days, the pressure loss and the outlet temperature remained nearly the same and there was no necessity of decoking.

COMPARATIVE EXAMPLE 2 The thermal cracking of the light gas oil having the properties as shown in Table 4 was conducted using the cracking furnace and quenching heat exchanger as used in the Comparative Example 1.

Table 4 Properties of starting light gas oil:

Specific gravity: 0.837 ASTM distillation:

10 volume percent recovered temperature:

C. volume percent recovered temperature:

Naphthens: 13.1 volume Aromatics: 20.1 volume The thermal cracking was conducted at flow rates of 3000 kg./hr. of light gas oil, 2250 kg./hr. of steam, and at a temperature of 790 C. at the outlet of the thermal cracking furnace.

8 Table 5 Compositions of cracked gas obtained by cracking light gas oil at 790 C.

Weight percent H 0.48 CH 10.5 (3 H 23.0 C H 3.4 0 14.2 c 8.0 C 180 C. 18.5 Over 180 C. 21.92

In the quenching heat exchanger of the Comparative Example 2, the severe coking was encountered and the pressure loss was large. Therefore, Comparative Example 2 was repeated except that the quenching heat exchanger having heat transfer surface of 56% of that of the quenching heat exchanger of the Comparative Example 1 was used and the mass velocity of the cracked gas was 75 kg./m. The temperature of the cracked gas at an outlet of the quenching heat exchanger was initially 480 C. and reached 550 C. after 20 days. The pressure loss was initially 0.25 kg./cm. and reached 0.38 kg./cm. after 20 days. Thereafter, the pressure loss and the temperature of the cracked gas an outlet of the quenching heat exchanger remained nearly the same and there was no neces sity of decoking of the quenching heat exchanger after 60 days.

As it is clear from the foregoing examples, when the mass velocity of the cracked gas passing through the quenching heat exchanger is low and the temperature of the cracked gas at an outlet of the quenching heat exchanger is low, the severe coking occurs in the quenching heat exchanger. On the contrary, according to the present invention, it is possible to conduct the continuous operation without forming cokes 'on the surface of the tubes by adjusting the temperature of the cracked gas at an outlet of the quenching heat exchanger in the range defined in the claim by controlling the heat transfer area of the quenching heat exchanger and increasing the mass velocity of the gas passing through the quenching heat exchanger. There is a tendency of causing remarkable cok ing as the properties of the feed hydrocarbons become heavier and thermal cracking conditions become more severe, which results in an increase of the pressure loss. According to the present invention the progress of the coking can be prevented and the stable continuous operation period can be greatly extended by adjusting the mass velocity and the temperature of the cracked gas at an outlet of the quenching heat exchanger within a specific range defined in the claim.

Quantity of heavier polycyclic aromatics which is susceptible to coking is increased as the feed hydrocarbon be comes heavier or the cracking conditions become more severe. The features of the present invention reside in that the condensation, adhesion and carbonization of the heavier substances on the surface of the cooling tube of the quenching heat exchanger can be prevented, and the heavier substance are maintained in fluid condition by the succeeding injection of the quenching oil, and easily discharged out of the system as cracking residue oil. According to the present invention, the coking in the quenching 1. Process for quenching hot gaseous products obtained by thermal cracking of kerosene or heavier hydrocarbons than kerosene having a volumetric average boiling point not exceeding 460 C. which comprises quenching indirectly the high temperature cracked gas in a quenching heat exchanger to a temperature (T) represented by the following equation:

wherein t is a volumetric average boiling point of the feed hydrocarbons and 10+ 30+ so-ivo-l- 90 in which r r I 2 and 1 is a temperature C.) at which 10 volume percent, 30 volume percent, 50 volume percent, 70 volume percent and 90 volume percent of feed hydrocarbons is distilled in ASTM distillation, respectively, and a is a temperature in the range of 340420 C., and T is a temperature above 450 C. but not exceeding 600 C., said temperature (T) being a temperature of the cracked gas at an outlet of the quenching heat exchanger, and further quenching directly the cracked gas to a temperature of 150-250 C. by spraying heavier oil 10 which is the heavier component of cracked gas of which recovery temperature of ASTM distillation is in the range of 200-400 C.

2. Process according to claim 1 wherein mass velocity is -120 kg./m. 'second in the quenching heat exchanger.

3. Process according to claim 1 wherein the residence time of cracked gas in the quenching heat exchanger is not more than 0.05 second.

4. Process according to claim 1 wherein said heavier feed hydrocarbons are selected from kerosene, light gas oil and heavy gas oil.

References Cited UNITED STATES PATENTS 3,242,225 3/1966 Danz et al. 260679 3,347,949 10/1967 Dollinger et al. 2606-79 2,188,982 2/19'40 Nagel 260683 3,103,485 9/1963 Cahn 208-130 2,951,029 8/1960 Johnston et al. 208-- 2,172,228 9/1939 Johannes van Peski 260-683 2,366,521 1/1945 Guichet 196-69 2,789,149 4/1957 Bogart et al. 260679 3,236,905 2/ 1966 Otsuka et al 260679 2,443,210 6/ 1948 Upham 260679 3,392,211 7/1968 Buschmann et al. 260683 3,065,165 11/1962 Amis et al. 208--48 3,580,838 5/1971 Lutz 208-48 DELBERT E. GANTZ, Primary Examiner I. M. NELSON, Assistant Examiner US. Cl. X.R. 208-48; 260679