GB2086867A

GB2086867A - Process for preparing activated carbon

Info

Publication number: GB2086867A
Application number: GB8032594A
Authority: GB
Original assignee: PERTANIAN MALAYSIA, University of
Current assignee: PERTANIAN MALAYSIA, University of
Priority date: 1980-10-09
Filing date: 1980-10-09
Publication date: 1982-05-19
Also published as: GB2086867B; MY8700322A

Abstract

In a process for preparing activated carbon, subatmospheric pressures are employed. Carbonaceous material, for example coconut shell or palm-oil shell, is heated in a vacuum which is preferably lower than 1 mm. Hg. Decompostion products may be prevented from reaching the vacuum pump by a cold trap which may be cooled by liquid nitrogen.

Description

SPECIFICATION Process for preparing activated carbon This invention relates to activated carbon, and especially to a process for the production thereof.

Activated carbon is conventionally prepared from carbonaceous materials, e.g. from vegetable or animal sources by treatment with activating chemicals, such as calcium chloride or zinc chloride, at high temperature and pressure and with limited oxygen supply. The kiln used in this process must be of extremely robust construction in order to withstand the high internal pressures, which are usually maintained by the use of super-heated steam.

It has now been found that activated carbon may be formed from carbonaceous material by heating under reduced pressure.

According to the present invention, there is provided a process for preparing activated carbon comprising heating carbonaceous material under an operating pressure less than atmospheric pressure.

Preferably, the operating pressure is less than 5 mm-Hg and, even more preferably, less than 2 mm-Hg, for example 1 mm-Hg.

In a particularly preferred embodiment, activation takes place in a vessel which is in communication with a cold trap, which may be cooled with liquid nitrogen.

Preparation of activated carbon by the process of the present invention is now described, by way of example, with reference to the accompanying drawing, which is a schematic representation of a vertical section through apparatus suitable for carrying out the process.

Carbonaceous material 1, which may be wood chips, sawdust, bones, coconut shell, palm-oil shell, bamboo, seeds etc., is suitably fragmented, and then packed into thick-walled glass vessel 2. If the apparatus used is on a sufficiently large scale, it may not be necessary to fragment the carbonaceous material before packing into the vessel. It is preferred, however, that the carbonaceous material is present as pieces which are of the same order of thickness as coconut shell i.e. approx. 0.6 mm.

A loose plug of glass wool 3 is inserted in order to retain the raw material, but at the same time to allow passage of decomposition products such as gaseous material and tar. The vessel 2 is then connected to a cold trap 4, which has two limbs 7 and 8. The limbs 7 and 8 are immersed in liquid nitrogen 5 to provide cooling. Alternative forms of cooling, for example liquid air, may be used provided that the resultant temperature of the trap is sufficiently low to achieve and maintain the required degree of vacuum. The interior 9 of the cold trap 4 is in communication via a passage 10 with a vacuum pump (not shown).

In use, heat is applied to the vessel 2 from a heater 6, and the vacuum pump is used to evacuate the vessel 2 and the cold trap 4. A pressure of 1 mm-Hg or preferably less has been found to be particularly advantageous, and the apparatus must be capable of sustaining such a vacuum. Liquid decomposition products collect directly in the first limb 7 of the cold trap 4, while more volatile components condense in the second limb 8. The presence of the cold trap thus helps to prevent decomposition products reaching the vacuum pump. After heating, the product is allowed to cool and the vacuum then released.

It is found that the effectiveness of the process is dependent both on the temperature and on the duration of heating. A parameter which may be used to measure the effectiveness of the process is the absorptive surface area of the resulting activated carbon. When coconut shell was used as the raw material, optimal efficiency was obtained by heating at 5260 for 3 hours. The product in this case was found to have a surface area of 1 90 m2/g. The effect of variation in temperature and duration of heating at a pressure of 1 mm-Hg is illustrated by the following Table.

TABLE 1 Heating Temperature Duration Surface Area ( C) (hours) (m2/g) % of optimum 458 3 136 71.6 458 2 123 64.7 520 1 142 74.7 520 2 157 82.6 520 3 185 97.4 520 4 134 70.5 572 2 151 79.5 572 3 168 88.4 572 4 122 64.2 Thus, heating the material for about 3 hours at a temperature of 5000--5700C yields a product with approximately 90% of the adsorptive efficiency of that obtained under optimal conditions.

When palm-oil shell was used as the raw material, optimal efficiency was obtained by heating at 41 50C for 2 2 hours, the product having a surface area of 470 m2/g. The results obtained by the application of other heating conditions and a pressure of 1 mm-Hg are illustrated in Table 2.

TABLE 2 Heating Duration Surface Area ( C) (hours) (m2/g) % of optimum 321 2 416 88.5 321 3 402 85.5 395 2 467 99.4 395 3 437 93.0 395 4 430 91.5 470 2 447 95.1 470 3 420 89.4 530 3 390 83.0 Heating for 2 to 3 hours at a temperature of 3200--4700C is seen to produce a product having an efficiency of approximately 90% or more.

The present invention provides an improved method of preparing activated charcoal with greatly reduced capital and running costs.

Many of the disadvantages of the previous known methods are eliminated by the present technique. It is found that the preparation of activated carbon at reduced pressure eliminates the need for an activating agent, such as zinc chloride or calcium chloride. Furthermore, the time required for the preparation is reduced to only a few hours and the operating temperature is also reduced with concomitant reductions in cost. A further effect of the reduced temperature and the absence of activating agent is that corrosion of the plant is much reduced, and the life-time of the plant thereby increased: The problems of production of super-heated steam and washing are eliminated, and the single step process results in a product which is clean, dry and ready for use.

Claims

1. A process for preparing activated carbon comprising heating a carbonaceous material at a pressure less than atmospheric pressure.

2. A process according to claim 1 wherein the operating pressure is less than 2 mm-Hg.

3. A process according to claim 2 wherein the operating pressure is less than 1 mm-Hg.

4. A process according to claim 1, 2 or 3, wherein the material is heated in a vessel which is in communication with a cold trap.

5. A process according to claim 4 wherein the cold trap is cooled with liquid nitrogen.

6. A process according to any of claims 1 to 5, wherein the carbonaceous material is coconut shell.

7. A process according to claim 6 wherein said shell is heated at a temperature of about 5000 to 5700C for about 3 hours.

8. A process according to any of claims 1 to 5 wherein the carbonaceous material is palm-oil shell.

9. A process according to claim 8 wherein said shell is heated at a temperature of about 3200 to 4700C for about 22 hours.

10. A process for preparing adtivated carbon substantially as hereinbefore described with reference to the accompanying drawing.

11. Activated carbon when prepared by a process according to any of the preceding claims.