1961-07-26

Abstract of GB873607 The permeability of a carbon or graphite article, e.g. a tube, is decreased by impregnating with furfural and successively polymerizing and carbonising the furfural under pressure and heat in the absence of a catalyst. The furfural may be admixed with furfuryl alcohol, or may contain cellulose acetate, ethyl cellulose, or pitch. The article may be first evacuated and then impregnated in a nitrogen atmsphere. The nitrogen pressure may then be raised to 800-1100 p.s.i. and the temperature to 305-310 DEG C. to polymerize and carbonise the furfural. The treated article may be calcined at a temperature of 1000 DEG C. at atmospheric pressure in air surrounded by graphite powder or nitrogen, or in a vacuum. The treated article may be graphitised by heating to 2800 DEG C. The successive steps of impregnation, polymerization, and carbonisation may be repeated a number of times. The permeability of the calcined article may be 9.3 X 10-7 darcys and its bulk density 1.93 gm. Examples are given.

0

1961-07-26

C04B35/52 ; C04B35/52

C04B35/52C

GB19580020006 19580623

GB19580020006 19580623

1	Improvements in or relating to processes for production of low permeability carbon
	Inventor: RIVINGTON HERBERT LAWRENCE	Applicant: GEN ELECTRIC CO LTD
	EC:C04B35/52C	IPC: C04B35/52;C04B35/52
	Publication info: GB873607 A - 1961-07-26

2 A process as claimed in Claim 1,

wherein the pores of the carbon are evacuated prior to impregnating the carbon.

3 A process as claimed in Claim 1 or Claim 2, wherein the impregnant is a mixture of not less than 95 % furfural and not more than 5 % furfuryl alcohol.

4 A process as claimed in Claim 3, wherein the impregnant contains from 3 % to 5 %,' furfuryl alcohol.

A process as claimed in any preceding claim, wherein the impregnant contains a cellulose plastic in solution.

6 A process as claimed in Claim 5, wherein the cellulose plastic is cellulose acetate or ethyl cellulose.

7 A process as claimed in any preceding claim, wherein the carbon whose permeability has been reduced is purified by a calcining treatment.

8 A process for the production of low permeability graphite from graphite of substantially higher permeability, substantially as hereinbefore described in any one of the specific examples.

9 Graphite of reduced permeability produced in accordance with any one of the preceding claims.

For the Applicants, F S PEACHEY, Chartered Patent Agent.

to five successive treatments each as described in the first example The specimen was not, however, heated to graphitise the deposited carbon The bulk density of the specimen after treatment had increased from 1.72 g/cc to 1 93 g/cc, the tensile strength of the specimen being increased correspondingly, and the permeability was reduced by the order of 1,000,000 times.

In the fourth example, the process according to the first example was carried out but the furfural impregnant contained a small amount, 10 %o by weight, of ethyl cellulose in solution Because of the skin formed round the specimen by the ethyl cellulose during the early part of the first heating stage it was not necessary to carry out this stage with the specimen immersed in furfural.

In the fifth example, the process according to the fourth example was carried out using cellulose acetate instead of ethyl cellulose.

If furfural, furfuryl alcohol, and mixtures thereof, are simply heated under pressure, without a carbon body to be impregnated, so that they polymerise and decompose to produce a carbon body, the bulk density of the body after subsequent heating to 1000 ' C ranges from about 0 7 g/cc, for furfural, to 1 4 g/cc, for furfuryl alcohol.

WHAT WE CLAIM IS: 1 A process for the production of low permeability carbon from carbon of substantially higher permeability, comprising impregnating the carbon whose permeability PROVISIONAL SPECIFICATION.

Improvements in or relating to Processes for Production of Low Permeability Carbon.

We, THE GENERAL ELECTRIC COMPANY LIMITED, of Magnet House, Kingsway, London, W C 2, a British Company, do hereby declare this invention to be described in the following statement:The present invention relates to processes for the production of low permeability carbon from carbon of substantially higher permeability.

The permeability to liquids and gases of carbon in the form, for example, of standard commercial graphite constitutes a serious disadvantage to the use of this material for certain applications One such application, of increasing importance at the present time, lies in the field of nuclear reactors Taking this field by way of example, graphite is used as a moderator or reflector material in thermal and intermediate nuclear reactors and it is desirable from several points of 90 view that this material should have as low a permeability as possible Thus, such a low permeability graphite presents fewer problems when fluid cooling medium is passed through channels in a moderator 95 made of this material so as to cool uranium or plutonium fuel elements contained within the channels, more particularly if liquid cooling medium, such as liquid metal, is employed in unlined channels in the graphite 100 Even with a gaseous cooling medium, where the effects of absorption possibly combined with chemical interaction are not in general so serious, the mass transfer of graphite from one part of a channel to another is appreci 105 ably diminished with a low permeability graphite Again, if the low permeability is associated with a higher bulk density so thfat 873,607 the effect is not confined to the surface of the graphite, the efficiency of the material as a moderator or reflector is correspondingly increased.

It is an object of the present invention to provide an improved process for the production of low permeability carbon suitable for such applications.

According to the present invention, a process for the production of low permeability carbon from carbon of substantially higher permeability comprises impregnating the carbon whose permeability is to be reduced with furfural, polymerising the furfural under pressure and heat so as to cause the furfural to become plastic, this being done without a catalyst, and heating the impregnated carbon to a temperature sufficient to carbonise the furfural impregnant.

By avoiding the use of a catalyst in this process, the introduction of elements having a relatively high capture cross section for neutrons is avoided.

It may be an advantage to evacuate the pores in the carbon prior to impregnating the carbon with the furfural.

If a very low degree of permeability is required, a carbon specimen may be subjected repeatedly to the process according to the invention.

In carrying the invention into effect according to one example of the process, a specimen of relatively high permeability carbon such as commercial graphite is first treated so as to evacuate the air from the pores of the carbon The specimen is then immersed in furfural, which readily wets the carbon, and pressure is applied to force the furfural into the pores of the specimen.

The impregnated graphite specimen is placed in a pressurised vessel or bomb and the pressure of air or nitrogen within the bomb is raised to approximately 1200 p s i, and the temperature is raised to about 300 ' C Under these conditions the furfural first polymerises, becoming plastic, this being achieved without the presence of a catalyst, and upon further heating the furfural decomposes into carbon which is deposited within the pores of the specimen, and steam The heating at this stage is continued for a time long enough to ensure that substantially all the plasticised furfural has decomposed, after which the bomb is cooled, the pressure released and the specimen removed from the 55 bomb.

if the graphite specimen is to be of high purity after treatment it is now subjected to a further heating stage This further heating may take place in atmospheric pressure 60 either in air surrounded by graphite powder or nitrogen Alternatively, the specimen may be heated in a good vacuum The rate of heating can be fast, the final temperature of approximately 10000 C being reached by 65 a gradual increase in temperature at a rate of the order of 3000 C an hour This heating rate enables the residual volatile products decomposition to diffuse gradually out through the small remaining pores in the 70 graphite specimen without the formation of pockets of high pressure gas which might finally force a rapid passage to the surface of the specimen When this further stage of heating is carried out, it is found that the 75 carbonaceous material left after the decomposition of the furfural under high pressure at the preceding stage shrinks gradually to form a particularly dense, non-porous form of carbon, and graphite impregnated in this 80 way has the minimum content of unwanted volatile products while its permeability is improved by a factor of the order of 100 times after being subjected only once to the complete process 85 If a further low permeability is required, the graphite specimen is subsequently again subjected to the whole process, and this repeated application of the process may be continued any desired number of times 90 Alternatively, the final stage of heating to 1000 ' C which is necessary in order to obtain a very pure product may be reserved and purified only once after several successive impregnations and subsequent heating 95 to temperatures between 275 C and 3000 C.

It has been found advantageous in certain applications to use, instead of pure furfural, mixtures of furfural with other furfural derivatives For example, with a mixture 100 of 95 % furfural and 5 % furfuryl alcohol, the method described above has produced a greatly improved graphite in which the deposited carbon can be readily graphitised.

For the Applicants, F S PEACHEY, Chartered Patent Agent.

Abingdon: Printed for Her Majesty's Stationery Office, by Burgess & Son (Abingdon), Ltd -1961 Published at The Patent Office, 25, Southampton Buildings, London, W C 2, from which copies may be obtained.

873,607

PATENT SPECIFICATION

NO DRAWINGS.

Inventor:-HERBERT LAWRENCE RIVINGTON.

Date of filing Complete Specification: June 15, 1959.

Application Date: June 23, 1958 No 20006 i 58.

Complete Specfication Published: July 26,1961.

Index at Acceptance:-Class 90, K 4.

International Ulassification:-O O lb.

COMPLETE SPECIFICATION.

Improvements in or relating to Processes for Production of Low Permeability Carbon.

We, THE GENERAL ELECTRIC COMPANY LIMITED, of Magnet House, Kingsway, London, W C 2, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -

The present invention relates to processes for the production of low permeability carbon from carbon of substantially higher permeability.

The permeability to liquids and gases of carbon in the form, for example, of standard commercial graphite constitutes a serious disadvantage to the use of this material for certain applications One such application, of increasing importance at the present time, lies in the field of nuclear reactors Taking this field by way of example, graphite is used as a moderator or reflector material in thermal and intermediate nuclear reactors and it is desirable from several points of view that this material should have as low a permeability as possible Thus, such a low permeability graphite presents fewer problems when fluid cooling medium is passed through channels in a moderator made of this material so as to cool uranium or plutonium fuel elements contained within the channels, more particularly if liquid cooling medium, such as liquid metal, is employed in unlined channels in the graphite Even with a gaseous cooling medium, where the effects of absorption possibly combined with chemical interaction are not in general so serious, the mass transfer of graphite from one part of a channel to another is appreciably diminished with a low permeability graphite Again, if the low permeability is associated with a higher bulk density so that lPrice 3 s 6 d l the effect is not confined to the surface of the graphite, the efficiency of the material as a moderator or reflector is correspondingly increased.

It is an object of the present invention to provide tan improved process for the production of low permeability carbon suitable for such applications.

According to the present invention, a process for the production of low permeability carbon from carbon of substantially higher permeability comprises impregnating the carbon whose permeability is to be reduced with furfural alone or in admixture, polymerising the furfural under pressure and heat, this being done without a catalyst, and heating the impregnated carbon to a temperature sufficient to carbonise the impregnant.

In the Specification, "furfural" means "furfuraldehyde" and is to be distinguished from furfuryl alcohol, which is sometimes called "furfural" in the plastics art.

By avoiding the use of a catalyst in this process, the introduction of elements having a relatively high capture cross section for neutrons is avoided.

It may be an advantage to evacuate the pores in the carbon prior to impregnating the carbon with the furfural.

If a graphite specimen treated according to the present invention is to have a low volatile matter content, it may be subjected to a further heating stage, for example to 10000 C, in which the specimen is calcined.

This further heating may take place at atmospheric pressure either in air surrounded by graphite powder or nitrogen Alternatively, the specimen may be heated in a good vacuum The rate of heating in this further L 8739607 heating stage can be fast, the final temperature being reached by a gradual increase in temperature at a rate of the order of 3000 C.

an hour This heating enables the residual volatile products of decomposition to diffuse gradually out through the small remaining pores in the graphite specimen without the formation of pockets of high pressure gas which might finally force a rapid passage to the surface of the specimen When this further stage of heating is carried out, it is found that the carbonaceous material left after the decomposition of the furfural at the preceding stage shrinks gradually to form a particularly dense, non-porous form of carbon, and graphite impregnated in this way has the minimum content of unwanted volatile products while its permeability is improved by a factor of the order of 100 times after being subjected only once to the complete process.

If a further low permeability is required, the graphite specimen may be again subjected to the whole process, and this repeated application of the process may be continued any desired number of times Alternatively, the stage of calcining which is necessary in order to obtain a very pure product may be omitted during successive repetitions, the specimen being purified only once by calcining after several successive impregnations and subsequently heating under pressure to polymerise and carbonise the furfural impregnant.

It has been found advantageous in certain applications to use, instead of pure furfural, mixtures of furfural with other furan derivatives For example, with a mixture of 95 % furfural and 5 % furfuryl alcohol, the method has produced an improved graphite in which the deposited carbon can be readily graphitised Amounts of furfuryl alcohol in the range 0-5 % may be used, but the best results are obtained with from 3-5 % of the alcohol.

Because the viscosity of the furfural decreases during the first heating it is an advantage to dissolve in it a quantity of cellulose acetate, ethyl cellulose or other o cellulose plastic; if this is done the dissolved plastic increases the viscosity of the furfural and also forms a skin around the specimen which counters the tendency of the furfural to drain away from the graphite.

Furfural is a solvent for pitch, and mixtures of pitch and furfural may be used for impregnating high permeability carbon in accordance with the invention.

In order that the invention may be more readily understood, a number of specific examples of processes in accordance with the invention, and which we have carried out, will now be described.

In the first example, a specimen of nuclear grade graphite in the form of a tube having an external diameter of 5 inch and an internal diameter of 8 inch, and having an initial permeability of 1 16 x 103 darcys (viscosity in poises) was immersed in furfural in a pressure vessel or bomb The pressure 70 O in the bomb was reduced to evacuate the air from the pores in the graphite, and nitrogen was admitted under pressure so that the graphite was impregnated with the furfural.

The pressure of the nitrogen was next raised 75 to 1100 p s i and the temperature within the bomb was raised to 305 C, the temperature reaching 2500 C after one hour's heating and 3050 C after a further three hours heating Under these conditions the 80 furfural first polymerised and then decomposed into carbon, which was deposited within the pores of the specimen, and steam.

At the end of this heating stage the bomb was cooled, the pressure was released, and 85 the specimen was removed from the bomb.

The specimen was next subjected to a further heating stage in an atmosphere of nitrogen, the temperature being raised to 10000 C at a rate of 3000 C per hour, and 90 at the end of this further heating the permeability of the specimen was found to be 9.3 x 10-7 darcys The specimen was heated further to a temperature of 28000 C, to graphitise the carbon deposited in the pores, 95 and the permeability was found to be 2.32 x 110 darcys.

In the second example, a specimen of nuclear grade graphite in the form of a tube having an external diameter of 5 inches and 100 an internal diameter of 4 inches, and having an initial permeability of 1 2 x 10 darcys was immersed in a mixture containing 97 % furfural and 3 % furfuryl alcohol The pressure was reduced to 26 inches of mercury 105 to evacuate the air from the pores of the graphite, and was raised again by admitting nitrogen to the bomb so that the mixture was caused to penetrate into the pores The pressure was raised further to 800 p s i and 10 the specimen was heated to 310 C at a gradually decreasing rate over a period of four hours, the temperature reaching 2500 C.

-after one hour's-heating At the end of this heating stage, the mixture having polymer 115 ised and decomposed, the bomb was cooled, the pressure was released, and the impregnated specimen was removed.

The specimen was next subjected to a further heat treatment in an atmosphere of 120 nitrogen at atmospheric pressure, the temperature being raised to 10000 C in three hours At the end of this further heat treatment the permeability of the specimen was found to be 1 14 x 10-5 darcys, and its weight 125 had increased by 4 50.

In the third example a specimen of nuclear grade graphite in the form of a tube having an external diameter of g inch and an internal diameter of 3 inch, was subjected 130 -2 873,607 is to be reduced with furfural alone or in admixture, polymerising the furfural under pressure and heat, this being done without a catalyst, and heating the impregnated carbon to a temperature sufficient to carbonise the impregnant.