US3513194A - Process for producing adipic acid - Google Patents

Description

United States Patent US. Cl. 260-531 4 Claims ABSTRACT OF THE DISCLOSURE A process is provided for preparing adipic acid by oxidation in the liquid phase of cyclohexanol and cyclohexanone with an oxygen containing gas in an acetic acid solvent in the presence of maganese catalyst; the procedure is characterized in that the oxidation is effected in the presence of cyclohexane, whereby adipic acid is produced in markedly high yields not only from cyclohexanone and cyclohexanol, but also from the cyclohexane.

This invention relates to a process for producing adipic acid. More particularly, the invention pertains to a method for preparing adipic acid by oxidizing in the liquid phase cyclohexanol, cyclohexanone and cyclohexane with molecular oxygen.

For the production of adipic acid, a variety of processes have heretofore been proposed, such as for example, in old times, a process carried out by oxidizing with nitric acid cyclohexanol and cyclohexanone obtained by airoxidation of cyclohexane, and, more recently, a process conducted by oxidizing a starting oil comprising cyclohexanol and cyclohexanone with an oxygen-containing gas at an elevated temperature in the presence of a metalcontaining catalyst (e.g. British Pat. No. 941,662, and US. Pat. No. 3,234,271).

However, the above conventional processes are defective in the following points. That is, the former process is high in yield of adipic acid based on cyclohexane but suffers from such a drawback that the amount of nonrecoverable nitric acid becomes substantially equal to that of adipic acid, While the latter process, which has been considered as one of the most advanced processes for the production of adipic acid, is poor in yield of adipic acid based on the starting material.

An object of the present invention is to provide a process for producing adipic acid at a cost lower than in said conventional processes.

Another object is to provide a process for preparing adipic acid from cyclohexanol, cyclohexanone and cyclohexane in yields higher than in the conventional processes.

A further object is to provide a process for preparing adipic acid by oxidizing in the liquid phase cyclohexanol and cyclohexanone with an oxygen-containing gas in an acetic acid solvent in the presence of a manganese catalyst, characterized in that the oxidation is effected in the presence of cyclohexane, whereby adipic acid can be produced in markedly high yields not only from cyclohexanol and cyclohexanone but also from cyclohexane.

A still further object is to provide a process for obtaining adipic acid in high yield from a starting oil comprising cyclohexanol, cyclohexanone and cyclohexane in which the cyclohexane is used as a solvent for the cyclohexanol and cyclohexanone.

The process of the present invention is far more advantageous than the conventional process in, for example, that since a part of cyclohexane in the starting oil is directly oxidized to adipic acid in high yield, a portion of the starting material can be replaced by cyclohexane and hence is more inexpensive than in said conventional processes (e.g. British Pat. No. 941,662). The process of the present invention is further advantageous in that a mixture formed by separating a part of unreacted cyclohexane from the oxidation product of cyclohexane with an oxygen-containing gas can be directly used as starting oil. Therefore, no complete separation of unreacted cyclohexane is required and therefore the separation means to be used becomes simple. Moreover, the separation of cyclohexane can be effected at a relatively low temperature, so that the secondary change of cyclohexanol and cyclohexanone can be minimized.

The above objects and advantages of the present invention can be attained by oxidizing in the liquid phase a starting oil comprising cyclohexanol, cyclohexanone and cyclohexane with molecular oxygen in an acetic acid solvent in the presence of a manganese salt soluble in the reaction system. The present invention may also be practiced by use of a catalyst comprising a manganese salt soluble in the starting oil, and at least one member selected from the group consisting of cobalt salts, copper salts and mixtures thereof which are soluble in the starting oil.

The most marked difference between the present invention and aforesaid British Pat. No. 941,662 lies in that in the former, cyclohexane is made present in oxidizing cyclohexanol and cyclohexanone with an oxygen-containing gas in an acetic acid solvent in the presence of a manganese catalyst. In the above oxidation reaction, cyclohexane is not only one component of the starting oil employed for the production of adipic acid but is an excellent solvent for cyclohexanol and cyclohexanone, and results in smoothly progressing the oxidation of cyclohexanol and cyclohexanone to adipic acid. Cyclohexane further effectively removes, by azeotropic action, water formed in the oxidation reaction which lowers the reaction rate of adipic acid formation and the yield of adipic acid. Another action of cyclohexane is that it lowers the viscosity of oxidation reaction liquid with the result that the diffusion constant of liquid phase in the reaction zone becomes greater to bring about such advantage that local super-oxidation is inhibited to reduce the amounts of by-products due to the oxidation reaction. A further action of cyclohexane is that it prevents the corrosion of apparatus, and in oxidation reaction of cyclohexanol and cyclohexanone with an oxygen-containing gas in an acetic acid solvent in the presence of a manganese salt catalyst, it reduces the corrosion of equipment to about /3 the corrosion brought about in the oxidation reaction carried out in the absence of cyclohexane.

The amount of cyclohexane to be used is desirably from 10 to 400% by weight based on the sum of cyclohexanol and cyclohexanone in the starting oil. In case the amount is less than 10% by weight the yield of adipic acid from cyclohexanol and cyclohexanone becomes low, while in case the amount is more than 400% by weight, the yield of adipic acid based on consumed cyclohexane 3 is lowered, though the yield of adipic acid from cyclohexanol and cyclohexanone is improved, with the result that the adipic acid yield becomes poor as a whole.

In the present invention, the cyclohexanol, cyclohexanone and cyclohexane to be used as components of the starting oil for adipic acid may be those prepared according to any processes. As to the cyclohexanol and cyclohexanone, there may be used a cyclohexanol-cyclohexanone mixture which is obtained by the air-oxidation of cyclohexane in the liquid phase in the presence of a cobalt naphthenate catalyst, or by the hydrolysis of boric acid ester or the like formed by the liquid phase air oxidation of cyclohexane in the presence of boric acid. Alternatively, there may be used cyclohexanol pre' pared by the hydrogenation of phenol, in admixture of cyclohexanone obtained by the dehydrogenation thereof. The ratio of cyclohexanol to cyclohexanone in the starting oil, regardless of the preparation process thereof, is preferably from 2:1 to 1:4.

As the catalyst, a manganese salt soluble in the reaction system, such as manganese acetate, is used either alone or in combination with other metal salt soluble in the reaction system. The manganese salt employed is desirably present, calculated as manganese metal, in an amount of more than 100 p.p.m. based on the total amount of the liquid fed to the reactor. In case the manganese concentration is markedly high, combustion reaction and carbon-carbon scission reactions tend to be enhanced to increase the formation of acids lower than adipic acid, such as glutaric, succinic, valeric and propionic acids. For this, the addition of a copper salt soluble in the reaction system serves to inhibit the siderea tions due to the increase of manganese concentration. As the copper compound soluble in the reaction system copper acetate, copper naphthenate or the like are usable. The amount of such copper compound is preferably more than 200 p.p.m., calculated as copper metal, based on the total amount of the liquid fed to the reactor and is, particularly preferably, several times the amount of fed manganese. However, in case the amount of copper added is excessive, the reaction rate is inhibited and not only the conversion of cyclohexanol and cyclohexanone to adipic acid but also the conversion of a part of cyclohexane to adipic acid become low. In order to overcome the above drawbacks, the addition of a suitable amount of a cobalt compound soluble in the reaction system is markedly effective. As such as soluble cobalt compound, cobalt acetate or cobalt naphthenate is preferred. The amount of the cobalt compound to be added is more than 25 p.p.m. calculated as cobalt metal based on the total amount of the liquid fed to the reactor, and is preferably from one-seventh to one-half the concentration of manganese. Both yield and selectivity of crystalline adipic acid increase with increasing cobalt concentration and gradually decrease via a peak of a relatively narrow optimum concentration range.

The optimum catalyst concentration more or less varies depending on the composition of reaction system, partial pressure of oxygen and reaction temperature. In any case, however, the amount of metal-calculated manganese is desirably more than 200 p.p.m. based on the total amount of feed liquid, the amount of copper is more than 250 p.p.m., and the amount of manganese is more than 40 p.p.m. More preferably, the amounts of manganese, copper and cobalt are within the ranges of from 250 to 1000 ppm, from 500 to 3000 p.p.m. and from 50 to 500 p.p.m., respectively. Generally, more excellent effects can be attained by effecting the reaction at a higher copper concentration and a lower cobalt concentration.

The catalyst may be fed in any of such manners that soluble salts of respective components, such as acetates or naphthenates, are fed to the reactor after dissolving in the feed liquid, or solid compounds of manganese,

copper and cobalt are first fed to the reactor and are then dissolved in the reactor.

The total amount of feed liquid referred to in the above signifies, in case the reaction is effected in a batch manner, the total amount per batch of feed liquid containing starting materials for adipic acid production and a solvent, and in case the reaction is effected in a continuous manner, it means the total amount per unit time of feed liquid containing starting materials for adipic acid production and a solvent.

-In the present invention, the solvent to be used is acetic acid or an organic liquid composed mainly of acetic acid. The amount of acetic acid employed is at least 100% by weight' preferably from 250 to 500% by weight based on the sum of cyclohexanol and cyclohexanone. In the case where acetic acid is to be used in combination with other polar solvent, it is desirable that those two are to be used at such a proportion that the resulting solvent is composed mainly of acetic acid.

In accordance with the process of the present invention, the oxidation reaction initiates at a temperature of from to 80 C., without any substantial induction period. In order to improve the yield of adipic acid, however, it is necessary that a substantial portion of the reaction be eflfected at a temperature of below 85 C. and that the whole reaction be conducted under such condition that according to the progress of the reaction, the temperature is suc essively elevated within the range of from 60 to 120 C. The reaction temperature to be employed in each stage more or less varies depending on the conditions adopted. However, in case the reaction is effected at 3 stages, for example, it is preferable to conduct the first stage reaction at to 85 C., the second stage reaction at 81 to 85 C., and the third stage at 89 to 110 C., while in case the reaction is effected at 2 stages, it is desirable to carry out the first stage reaction at to 88 C., and the second stage at 89 to 105 C.

Generally, the oxidation reaction is conducted by use of a low oxygen concentration gas, e.g. air, under a pressure of from 10 to 30 atm. In this case, it is preferable that the amount of oxygen, per kg. of cyclohexanol and cyclohexanone, to be fed to the reactor is about 0.8 kg. when the weight ratio of cyclohexane to cyclohexanol-t-cycloexanone is 0.1 to 1, about 1.2 kg. when said ratio is 1:1, and about 1.8 kg. when said ratio is 4:1. It is further desirable that the space time reaction amount of oxygen be successively reduced according the increase of reaction temperature and the ratio of the amount of oxygen reacted at below 88 C. to that at above 88 C. be within the range of from 2:1 to 50:1, preferably from 5:1 to 20:1.

The oxidation reaction is carried out by use of oxygen in the above manners, and is desirably ceased when the conversion of cyclohexanol has reached more than 70%; the conversion of cyclohexanone has reached more than preferably more than 98%; and the conversion of cyclohexane has reached 30 to 70% by Weight based on the sum of fed cyclohexanol and cyclohexanone.

The process of the present invention is effected according to the procedures as detailed above and the whole reaction is complete in a reaction time of from 4 to 10 hours, particularly from 6 to 8 hours. In case the ratio of the reaction time at below 88 C. to that at above 88 C. is made from 2:1 to 20:1, preferably from 4:1 to 10:1, adipic acid can be obtained in higher yields.

The above-mentioned specific conditions concerning the amount of oxygen and the reaction time are based on such knowledges found by the present inventors that in accordance with the process of the present invention, high purity adipic acid can be obtained in high yields by effecting at relatively low temperatures of below 88 C. a ma jor portion of the co-oxidation reaction of cyclohexanol, cyclohexanone and cyclohexane, and completing at temperatures higher than said temperature and in a short time, the oxidation of the resulting precursor for adipic acid formation such as, for example, e-caprolactone, hydroxycaproic acid, adipoaldehyde or semi-adipoaldehyde.

In the oxidation reaction, the presence of water undesirably slows the reaction rate and lowers the yield of adipic acid. It is therefore essential that the water content of the liquid to be fed to the reactor be less than preferably less than 2%.

The oxidation reaction liquid obtained in the manners detailed above deposits and separates crystals of adipic acid, when allowed to cool as such. However, in order to effect the separation of adipic acid more efficiently, it is desirable that the liquid be cooled, prior to crystallization, to below 50 C., preferably to a temperature of from to C., after separation of cyclohexane, oxygen-containing compounds close in boiling point to cyclohexane, Water and a part of acetic acid. Because, the former two compounds adhere to the crystals of adipic acid to bring about a cause for lowering the quality of the crystals, and water increases the solubility of adipic acid to lower the yields of crystals thereof. For the crystallization of adipic acid, any of known crystallizers may be used.

30 to 90% by weight of the liquid which has separated the crystals is suitably dehydrated and is recycled to the reactor, whereby the oxidation of cyclohexanol and cyclohexanone progresses quite smoothly.

The following examples fully illustrate the present invention in which all percents are by Weight unless otherwise defined.

Example 1 A starting oil comprising 2.5 kg. of cyclohexanol, 2.5 kg. of cyclohexanone and 5.0 kg. of cyclohexane, and 16.0 kg. of acetic acid containing, based on the total amount of the feed liquid, 600 p.p.mpof manganese, 2000 p.p.m. of copper, and 200 p.p.m. of cobalt each in the form of a soluble acetic acid salt, were charged in a reactor provided with an exhaust gas condenser and a cooling jacket. The mixture was reacted by elevating the temperature to 65 C., while stirring With 3 turbine blades, injecting air at a flow rate of 2.5 N m. /hr., while maintaining the pressure at 15 atm. and elevating the reaction temperature from 65 C. to 90 C. over a period of 12 hours. After completion of the reaction, the reaction liquid was taken out and was cooled to 25 C. in a tank-type crystallizer to obtain 6.38 kg. of adipic acid crystals. In addition thereto, 0.99 kg. of adipic acid was found in the mother liquor. In this example, the conversion of cyclohexanol was 91.8%, the conversion of cyclohexanone was 99.5%, the adipic acid selectivity based on the total amount of consumed cyclohexanol, cyclohexanone and cyclohexane was 84 mol percent, and the yield of adipic acid crystals based on the fed cyclohexanol and cyclohexanone was 127.6%.

The adipic acid selectivity and adipic acid crystal yield in each of the examples shown below are values calculated in the same manners as in this example.

Example 2 Example 1 was repeated, except that the amount of fed cyclohexane was varied, to obtain the results as set out in Table 1.

Example 2 Example 1 was repeated, except that the concentration of manganese, copper and cobalt of the catalyst were varied. The results are shown in Table 2.

TABLE 2 Catalyst concentration (based on total amount of feed liquid) Adipic acid Adipic acid crystal yield Manganese Copper Cobalt selectivity (percent (p.p.m.) (p.p.m.) (p.p.m.) (mol percent) by weight) Example 4 Example 1 was repeated, except that the feed amount of cyclohexanol and cyclohexanone was made 5.0 kg. and the feed ratio of cyclohexanol to cyclohexanone was varied. The results are shown in Table 3.

50 kg. of cyclohexane in which had been dissolved cobalt naphthenate (20 p.p.m., calculated as cobalt) was charged in a reactor provided with a stirrer and an exhaust gas condenser. To the reactor, air was fed at a flow rate of 3,4 N mfi/hr. for 1.5 hours at C. under a total pressure of 25 atm. During this reaction, 1.53 kg. of oxy gen was reacted. After ceasing the reaction, a part of unreacted cyclohexane and water formed were separated by distillation to obtain 7.29 kg. of a reaction liquid containing 45.4% of cyclohexane, 21.2% of cyclohexanol and 16.1% of cyclohexanone. (In this reaction liquid were contained 1.26 kg. of high boilings having an adipic acid content of 0.24 kg.) The amount of cyclohexane consumed in the above reaction was 3.30 kg, and the total selectivity of cyclohexanol, cyclohexanone and adipic acid based on the consumed cyclohexane was 73.4 mol percent. To 7.29 kg. of the reaction product obtained by the above oxidation of cyclohexane was added 8.5 kg. of acetic acid containing 1000 p.p.m., 3000 p.p.m. and 300 p.p.m., based on the total amount of feed liquid, of manganese, copper and cobalt, respectively, each in the form of acetate. The mixture was charged in a reactor provided with an exhaust gas condenser and a jacket cooler and was elevated in temperature to 65 C., while stirring with 3 turbine blades. Into the reactor, air was injected at a flow rate of 3.0 N mfi/hr. under a pressure of 25 atm. The temperature was gradually elevated from 65 C. to 90 C. in a period of 13 hours, and the reaction was completed at 90 C. The reaction liquid was taken out and was cooled to 25 C. in a tank-type crystallizer to obtain 3.40 kg. of adipic acid crystals. In the filtrate was present 0.92 kg. of dissolved adipic acid.

In the second stage oxidation step, the conversions of cyclohexanol, cyclohexanone and cyclohexane were 92.4%, 98.9% and 27.2%, respectively, and the adipic acid selectivity based on the cyclohexanol, cyclohexanone and cyclohexane was 76.0 mol percent, excluding the adipic acid produced in the first stage oxidation. The amount of cyclohexane consumed throughout the first and second 8 a stirrer, a cooler and an exhaust gas condenser, respectively. On the other hand, air was fed to each reactor and the pressure of each reactor was maintained at 25 atm.

The reaction conditions of respective reactors were as stage oxidation steps was 4.20 kg., the adipic acid selec- 5 follows:

TAB LE 5 Space time Oxygen Amount of reaction Reactor Reaction Flow rate conversion reacted amount of volume Residence tem erafeed air (volume oxygen oxygen (1.) time (hr.) ture C.) (N mfl/hr.) percent) (kg/hr.) (mol/L/hr.)

Reactor N o.:

tivity based on the consumed cyclohexane was 59.2 mol percent, and the adipic acid crystal yield based on the consumed cyclohexane was 81% by weight.

Example 6 3.0 kg./hr. of a starting oil comprising 20% of cyclohexanol, 20% of cyclohexanone. and 60% of cyclohexane, and 4.0 kg./hr. of acetic acid containing, based on the total amount of the feed liquid, 800 p.p.m. of manganese, 2000 p.p.m. of copper and 300 p.p.m. of cobalt, each in the form of an acetate were charged in the first reactor of 3 reactors connected in series which had been provided with a stirrer, a cooler and an exhaust gas condenser, respectively. On the other hand, air was fed to each reactor, and the pressure of each reactor was maintained at 25 atm.

Reaction conditions of respective reactors were as fo1- lows:

TABLE 4 Reactor Temperature Rcsi- Flow 01 volume Maximum dcnce 1 feed 0.) time (hr.) (N m5) Reactor No.:

of reacted oxygen, per kg. of the fed cyclohexanol and I cyclohexanone, in the first and second reactors to that in the reactor 3 was 12:1.)

The reaction liquid was subjected to distillation to separate unreacted cyclohexane and formed water, and was then cooled to 25 C. in a classification-type (DTB-type) crystallizer to obtain adipic acid crystals at a rate of 1.59 kg./hr.

In the filtrate was present dissolved adipic acid, and crude adipic acid was obtained from said liquid at a rate of 0.33 kg./hr.

In this example, the conversions of cyclohexanol, cyclohexanone and cyclohexane were 92.5%, 99.1% and 31.5%, respectively. The selectivity of produced adipic acid based on said consumed components consumed was 71.6 mol percent and the yield of adipic acid crystals based on the fed cyclohexanol and cyclohexanone was 132.5%.

Example 7 3.0 kg./hr. of a starting oil comprising 16.7% of cyclohexanol, 33.3% of cyclohexanone and 50% of cyclohexane, and 4.5 kg./hr. of acetic acid containing, based on the total amount of feed liquid, 1000 p.p.m. of manganese, 3000 p.p.m. of copper and 600 p.p.m. of cobalt, each in the form of acetate, were fed to the first reactor of 4 reactors connected in series which had been provided with In the above reaction, the total residence time was 6.7 hours (the ratio of the reaction time in the first to third reactors to that in the fourth reactor was 4:1). Further, the amount of reacted oxygen was 1.75 kg./hr., which corresponded to 1.17 kg. per kg. of the fed cyclohexanol and cyclohexanone. (The ratio of the amount of reacted oxygen, per kg. of the fed cyclohexanol and cyclohexanone, in the first to third reactors to that in the fourth reactor was 11.5:1.)

The reaction liquid was subjected to distillation to separate unreacted cyclohexane and formed water, and was then cooled to 25 C. ina classification-type (D.T.B.-type) crystallizer to obtain adipic acid at a rate of 1.92 kg./hr.

In the filtrate was present dissolved adipic acid, and from said liquid, adipic acid was obtained at a rate of 0.37 kg./hr.

In this example, the conversions of cyclohexanol, cyclohexanone and cyclohexane were 93.0%, 99.0% and 40.6%, respectively. The selectivity of adipic acid based on said consumed components was 71.3 mol percent, and the yield of adipic acid crystals based on the fed cyclohexanol and cyclohexanone was 128% by weight.

Example 8 0.30 kg./hr. of cyclohexane, 0.10 kg./hr. of cyclohexanol, 0.15 kg./hr. of cyclohexanone, 0.85 kg./hr. of a crystal-separated filtrate obtained in the same manner as in this example (the composition of said filtrate will be mentioned later), and 0.20 kg./hr. of an acetic acid solvent containing, based on the total amount of the feed liquid, 2000 p.p.m. of manganese, 3000 p.p.m. of copper and 5000 p.p.m. of cobalt, each in the form of acetate, were fed to the first reactor of 5 reactors connected in series which had been provided with a stirrer, a cooler and an exhaust gas condenser, to effect oxidation reaction. Each reactor was maintained under a pressure of 25 atm., and in the first reactor, the temperature was 63 C., the air-injection rate was 0.06 N m. /hr., and the average contact time was 3.0 hours; in the second reactor, 70 C., 0.06 N m. /hr. and 3.5 hours; in the third reactor, 76 C., 0.06 N m. /hr. and 1.5 hours; in the fourth reactor, 80 C., 0.03 N m. /hr. and 1.5 hours; and in the fifth reactor, 88 C., 0.02 N m. /hr. and 1.0 hour.

The total amount of reacted oxygen was 0.26 kg./hr., which corresponded to 1.15 kg. per kg. of fed cyclohexanol and cyclohexanone. After completion of reaction, unreacted cyclohexane and formed water were separated from the reaction liquid by distillation. Subsequently, the reaction liquid, which had been reduced in water content to less than 0.2%, was cooled to 25 C. in a tank-type crystallizer to obtain adipic acid crystals at a rate of 0.35 kg./hr. On the other hand, the crystal-separated filtrate contains 4.7% of adipic acid, 0.2% of cyclohexanol, about 30% of glutaric acid and other reaction precursors, and acetic acid. 0.85 kg./ hr. of the filtrate, which corresponded to 71% of the total crystal-separated filtrate, was recycled in the oxidation system.

The process of this example was continuously effected for 200 hours, but no lowering in yield and purity of adipic acid crystals was observed, and the selectivity of produced adipic acid based on the consumed cyclohexanol, cyclohexanone and cyclohexane was 84.2 mol percent, and the yield of adipic acid crystals based on the fed cyclohexanol and cyclohexanone was 140% by weight.

When the recycle of the crystal-separated liquid in this example had been discontinued, said adipic acid selectivity and adipic acid crystal yield become 79.5 mol percent and 130% by weight; respectively.

While the above has been described in connection with preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as falling within the true spirit and scope of the invention.

What We claim is:

1. A process for producing adipic acid which comprises oxidizing a mixture of cyclohexanol, cyclohexanone and cyclohexane, the weight ratio of cyclohexanol to the cyclohexanone being 2:1 to 1:4, and the cyclohexane comprising 10400% by weight based on the total of cyclohexanol and cyclohexanone, in a solvent of acetic acid with molecular oxygen containing gas in the presence of a catalyst of cobalt acetate, copper acetate and manganese acetate at a temperature range of 60-120 C., the acetates being present at a concentration of 50500 ppm, 50- 3000 ppm. and 2504000 p.p.m., respectively, calculated as the metals, and the acetic acid being present in quantity greater than the quantity of cyclohexanol and cyclohexanone, and the temperature being increased during the process within said range of 60120 C.

2. A process according to claim 1 wherein an amount of oxygen per hour per volume of the mixture is reduced as the reaction temperature increases.

3. A process in accordance with claim 1 wherein the mother liquor obtained after separation of the product adipic acid is recycled.

4. A process in accordance wit-h claim 1 wherein the amount of acetic acid employed is from 250500% by weight based on the sum of cyclohexanol and cyclo-i hexanone.

References Cited UNITED STATES PATENTS 2,831,024 4/1958 Brown et al. 260--531 3,361,806 1/1968 Lidov 260-531 2,223,493 12/1940 Loder 260533 3,236,897 2/1966 Hornig et al. 260--533 2,439,513 4/1948 Hamblet et a1. 260533 3,390,174 6/1968 Schulz et al.

LORRAINE A. WEINBERGER, Primary Examiner D. E. STENZEL, Assistant Examiner U.S. Cl. X.R, 260-533, 537