'Activated Carbon By Treatment of Lignites With Potassium and/or Sodium Hydroxide Or Salts And Adsorption Therewith"

FIELD OF THE INVENTION

The present invention relates to improved activated carbon for adsorption of organic vapors, gases or vapors, using potassium and sodium hydroxide or potassium and sodium salts so as to achieve an activated carbon with increased working capacity, lower apparent density and increased capacity for the removal of gases, vapors and liquids.

BACKGROUND OF THE INVENTION

The present invention _^ involves the manufacture of activated carbons from inexpensive lignite (brown coal) to achieve superior adsorptive working capacity and other desirable attributes in the improved adsorptive material.

These activated carbons are useful in removing, adsorbing or storing organic gases, vapors and liquids; specific uses include, but are not limited to, evaporative loss control canisters on automobiles, in-line filters, solvent recovery filters and other applications. For example, when activated carbon is used in evaporative loss canisters on automobiles, the activated carbon should have the highest possible hydrocarbon working capacity and the lowest possible apparent density. Activated carbon

SUBSTITUTE SHEET

formulations that include the use of potassium impregnation before activation for these purposes are known; however, known formulations lack desirable (working) adsorptive capacity, may require the use of more adsorptive material than is desirable in many applications, and/or may be expensive to produce. In the case of carbon filters or beds to be used in evaporative control devices in vehicular fuel systems, the drawbacks associated with such known fuel vapor control carbons can be significant.

The activated carbons of the present invention may also involve the further use of special impregnants to remove gases, vapors and liquids that would not otherwise be removed through the use of unimpregnated activated carbons.

Attempts have been made to enhance the properties of activated carbons using potassium ions. For example, in German Publication No. 3,834,745 to Karl, moist brown coals are impregnated with potassium. The activated carbons in Karl are produced by heating or steam activating moist brown (unprocessed) coals that are impregnated with KOH before activation. Karl discloses the use of carbons with moisture levels of about 46-59% and KOH levels up to about 2%. Activated carbons with iodine adsorptivity of 600-1100 mg/g were produced with brown coal at a final activation yield of about 14% (dry basis). Also, U.S. Patent No. 4,039,473 to Schafer describes a carbon (lignite) that is treated with potassium, charred and leached (to remove the potassium), and optionally steam-activated to achieve additional activity.

The shortcomings of these and other known carbons are easily understood; they may require more space, in that they have less adsorptive working capacity

SUBSTITUTESHEET

per unit volume. As minimum fill volumes must be used to meet the required size and weight restrictions in fuel evaporative loss control canisters and other applications, the activated carbon must have very high working capacities per unit volume.

Using lignites (brown coal), the present invention results in activated carbons having an improved and unique pore structure which yields superior working capacity resulting in a more effective carbon that results in cost, weight and space savings. By using a lower cost feed stock (lignite), the present invention results in an immediate cost savings.

Accordingly, the present invention is intended to provide a more effective, more compact, and less expensive activated carbon that is effective in removing and/or storing gases and/or vapors, and liquids for use in automotive and industrial applications. The potassium and/or sodium hydroxides and salts thereof used for the treatment of the lignites (brown coals) of the present invention are applied prior to charring and activation in an efficient, reliable and cost-effective manner.

SUMMARY OF THE INVENTION

The present invention comprises an activated carbon manufactured from lignite (brown coal) by impregnating a lignite with a solution of potassium and/or sodium hydroxide or salt thereof followed by charring and thereafter activation. For the purpose of the present invention, the terms "lignite" and "brown coal" are low-rank coals having an oxygen content greater than from about 15 to 17% up to about 30%. The present invention solves the problem of the required use of more expensive carbons, as it achieves improved performance by using

_S UBSTITUTESHEET

potassium and/or sodium hydroxide and/or salts thereof on relatively inexpensive feed stock. Further, the carbons of the present invention attain greater hydrocarbon working capacities, lower apparent densities and increased hydrocarbon capacities. The activated carbons of the present invention have improved and unique pore structures, which yield superior working capacities; these carbons yield cost savings, environmental benefits and superior performance.

The carbons of the present invention can be significantly less expensive to produce, and have greater working capacities than known carbons produced from known KOH impregnated carbonaceous materials, to include known steam activated wood-based pellet carbons, or brown coals and pit-wet lignites impregnated with KOH before activation.

One commercial wood-base carbon was found to have a gasoline working capacity of 34 g/1. A brown coal carbon was found to have a gasoline working capacity of 29.4 g/1. The carbon present invention can achieve gasoline working capacities greater than 57.0 g/1. The activated carbons of present invention can therefore achieve an increase in working capacity of at least 91% over the aforementioned ^' brown coal carbon and at least 67% over the aforementioned wood-base carbon. The wood-based carbon is a more expensive material and is less effective than the lignite (brown coal) based carbons of the present invention. The known brown coal or pit-wet lignite substitute, on the other hand, is "heavy" to ship due to its high water content and produces less effective activated carbon.

Due to the increased working capacities of the activated carbons of the present invention, less carbon filter material is required to achieve the same working

SUBSTITUTE SHEET

capacities of less adsorptive carbons. Significant cost savings can be achieved using smaller canisters filled with activated carbons of the present invention.

The activated carbons of the present invention also avoid unnecessary excess weight of pit-wet material, and provides for processing equipment advantages. The product of the present invention may be granulated, pelletized or "shaped" into other forms without a binder via (most desirably) pressure formation, or prepared with a binder.

The process of the present invention is effective in an expanded range of carbonaceous material compositions, treated prior to charring and activation with potassium or sodium. The amount (by weight) of potassium or sodium hydroxide or salts thereof present on the carbon after charring and activation can increase, for example double, due to carbon shrinkage following activation. The formulations may be varied, depending on the particular application and/or performance characteristics desired.

However, preferred embodiments of the present invention comprise a low-rank coal, preferably a lignite, treated prior to charring and activation to provide a composition containing, by weight, up to about 30% potassium (as KOH or an equivalent potassium salt). Up tc about 120% water may also be used. This potassium or sodium hydroxide or salt thereof can be effectively dispersed over agglomerated lignite granules (8x30, 6x20, 12x30, and other mesh sizes), or any similar carbonaceous media. The apparent density of the carbons of the present invention are desirably less than 370 g/1, and have a high hexane working capacity (defined hereinafter) of at least 5.4%.

SUBSTITUTE SHEET

A specific preferred embodiment of the present invention comprises lignite (brown coal) treated with about 5% potassium (KOH) and 25% water prior to charring and activation.

The activated carbons of the present invention have a wide-range of utility and provide balanced performance. For certain applications, such as in automotive applications, the activated carbons provide high working capacities which can be regenerated by passage of ambient air over the carbon. Other advantages of the activated carbons of the present invention will become apparent from a perusal of the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Figure 1 is a flow diagram of the process of the present invention for manufacturing pelletized activated carbon;

Figure 2 is a flow diagram of the process of the present invention for manufacturing a nonagglomerated granular activated carbon; and

Figure 3 is a flow diagram of the process of the present invention for manufacturing an agglomerated granular activated carbon.

PRESENTLY PREFERRED EMBODIMENTS

The present invention is directed to the manufacture of activated carbons from lignites (brown

SUBSTITUTE SHEET

coals) having increased surface area and superior working capacities for the adsorption of hydrocarbons and other gases, vapors and liquids. Activated carbons prepared according to the present invention are superior adsorbents for evaporative loss control canisters, solvent recovery and other applications.

The process of the present invention is effective in using a broad range of potassium or sodium hydroxides and/or salts thereof treatment compositions. These formulations may be varied, depending on the particular application and/or performance characteristics desired for the activated carbon.

A number of process parameters have been found to affect the performance of the carbon product. These parameters include: (1) the particular potassium or sodium hydroxide or salt used, the level of these hydroxides and salts, and the source from which these hydroxides or salts are obtained; (2) the method of hydroxide and salt treatment of the lignite (brown coal); (3) the process used to char the lignite (brown coal); (4) the process used to activate the carbon; (5) the procedure used to dry and/or wash the activated carbon; (6) the amount of moisture present or added at each step; and (7) the activity of the starting lignite (brown coal). Other parameters also affect the final characteristics of the activated carbon of the present invention, but to a lesser extent.

Tests were performed to provide a description of the unique qualities of activated carbons of the present invention. Also, preferred methods of preparing the activated carbons are set forth. It has been found, for example, that Australian lignite (brown coal) produced carbons with good hexane adsorption and working capacities. It has also been found that other lignites

SUBSTITUTE SHEET

produce exceptionally good working capacities. In these tests, the lignite (brown coal) was treated with a solution of KOH, charred in an inert atmosphere and activated with steam.

Initial preparation comprises pulverizing the dry lignite (brown coal) and then treating it with a solution of KOH and, thereafter, forming it into two-millimeter pellets. This procedure maximizes the advantages of the potential interaction between KOH and oxygenated surface groups of lignite (brown coal) for the present invention. These pellets were charred in an inert atmosphere and activated in steam. This preferred method of treating the lignite with KOH prior to charring yielded surprisingly desirable results.

In preliminary tests, two-millimeter pellets were formed from lignite (brown coal) with a 2.5% KOK loading. The pellets were air-dried for three days, charred in nitrogen at 700°C, and activated in steam at 870-950°C. About 20 to 25 mg of activated carbon were p in a micro balance and were kept at constant temperature of 30°C. A stream of 100 ml/minute of pure nitrogen saturated with n-hexane at 0°C was passed through the carbon for 20 minutes. Then a stream of 100 ml/minute of pure nitrogen was passed through the carbon saturated with hexane for 20 minutes. The difference in weights of the carbon is defined as the hexane working capacity.

Hexane capacity and working capacity of pellets that were activated in the range of 870-900°C reached approximately 42-45% and 13-14%, respectively. Butane capacity and working capacity reached approximately 36-46% and 22-24%, respectively. This butane capacity range iε higher than the highest capacity of the known commercial

SUBSTITUTE SHEET

carbons ( WVA1100™ with 34%). Therefore, the treatment of dry lignite (brown coal) with KOH prior to charring resulted in an activated carbon having a large adsorption capacity and a high working capacity.

Different levels of potassium hydroxide or salt treatment and charring temperatures were studied to produce optimal working capacity. Tests were run on Australian lignite (brown coal) pellets with up to 30% KOH (by weight). A hexane working capacity of 20% was achieved using this level of KOH impregnation.

These test led to products made with 5% KOH having a variety of highly desirable characteristics, particularly for use in gas phase applications such as evaporative loss control canisters, solvent recovery, and others. Although lignites (brown coal) impregnated with 2.5% and 1.25% KOH resulted in activated carbons useful for some applications, the 5.0% level of KOH resulted in carbons with higher activity. Carbons at the 2.5% KOH level show greater butane working capacities than 1.25% KOH carbons. Potassium hydroxide catalyzed activation of lignite (brown coal), both in granular agglomerated and sized nonagglomerated forms, resulted in activated carbons with good hexane capacities and working capacities.

Further samples of the lignite (brown coal) were treated with a solution of KOH, and thereafter agglomerated into 6x20 mesh granular form or sized into 8x30 mesh directly then charred in a nitrogen atmosphere at 700°C for 10 minutes. The char was then activated in steam. Flow diagrams for the preparation of three forms of activated carbon of the present invention are shown in Figures 1, 2 and 3.

Westvaco Corporation,

SUBSTITUTE SHEET

To achieve an enhanced pore structure, KOH solution was sprayed into the pulverized lignite to a KOH loading of 5%. The wet mixture of coal and KOH was pelletized without further addition of any binder to form 2mm pellets. The wet pellets were charred in a nitrogen atmosphere at 700°C for 10 minutes. It was discovered that by reducing the steam delivery rate and by diluting the activation gas with nitrogen, hexane working capacity was increased dramatically and increased the yield of the carbon. Hexane working capacity was 20.5%.

In further tests, KOH loading of 5.2% in the char feedstock produced the activated product with hexane capacity reaching approximately 45% at activation temperatures of 870-900°C. The hexane working capacity of these products reached approximately 13-14%. (The hexane capacity and hexane working capacity are approximately 32% and 17%, respectively for a first commercially used carbon and 44% and 20%, respectively for a second commercially used carbon) .

Further tests were performed using pulverized dry lignite (brown coal) having a water content of 14.4% mixed with a solution consisting of 25% (by weight of lignite) water and 5% (by weight of lignite) potassium hydroxide. The mixture was agglomerated, crushed, and screened to 8x30 mesh. This granular material was charred under an inert atmosphere at 800°C for 20 minutes. The char was activated in a steam atmosphere at 870°C to produce activated carbons of the present invention. Superior activated carbons were produced by this process; a first sample activated for 230 minutes had a hexane working capacity of 17.7%, and a second example activated for 250 minutes had a hexane working capacity of 22.1%.

The activated carbon of the present invention using lignites (brown coal) has an apparent density as low

SUBSTITUTE SHEET

-li¬

as 250 g/1 or lower. These surprising low densities are partly responsible for the superior performance. These attributes of surprisingly low apparent density and superior hexane working capacity provide an activated carbon which is remarkably effective as an adsorbent for a large range and number of applications.

Of the various treatment methodologies available, the most reliable method is treatment of the lignite (brown coal) by spraying with a solution of the hydroxide or salt. Methods of preparation may include one, two or more treatment cycles with the selected hydroxide or salt(s) by spraying, wetting or other process.

In producing the carbons of the present invention, the constituents should preferably be applied to the lignite (brown coal) using a single application.

The resulting mixture may thereafter be, as desired or necessary, thermally processed by charring and steam activation into the desired activated carbon product. In addition to this single application method, a method involving a second application (by solution or spray operation) can also be used to achieve the desired hydroxide or salt loadings. This two-step application can also be used to achieve uniform high hydroxide or salt loadings, or application of different liquids than those applied during the first loading.

Formulations containing various loadings of potassium or sodium hydroxide or salts thereof on granular, pelletized, and other shaped carbonaceous particles of differing mesh sizes will likewise desirably perform. Low-moisture content (e.g. , less than 20% by weight) of the lignite (brown coal) prior to charring and was discovered to be a factor in enhancing the performance of the carbons after activation.

SUBSTITUTE SHEET

-12-

Numerous examples of lignites (brown coals) were prepared in accordance with the present invention and tested. The results of these tests are described in Table I.

TABLE I

About 20 to 25 mg of activated carbon were put in a micro balance and were kept at constant temperature of 30 ^β C. A stream of 100 ml/minute of pure butane was passed through the carbon for 15 minutes. Then a stream of 100 ml/minute of pure nitrogen was passed through the carbon saturated with butane for 20 minutes. The difference in weights of the carbon is defined as the butane working capacity.

SUBSTITUTE SHEET

TABLE I (continued)

SUBSTITUTE SHEET

TABLE I ( continued )

Comparative Carbon Tests

Comparative performance studies were conducted using several known products- and the improved KOH treated product of the present invention. Table II below sets forth a comparison of two carbons against the carbons of the present invention.

SUBSTITUTE SHEET

Table I I

COMPARATIVE WORKING CAPACITY TESTS

Product (substrate) Gasoline Working Capacity (g/1)

Prior Art Sample 1 34.0 g/1

(wood pellet)

Prior Art Sample 2 29.4 g/1

(lignite)

Present invention 49.7 g/1 Sample 1 (pellet)

Present Invention 57.0 g/1

Sample 2 (granular )

Present Invention 65.0 g/1

Sample 3 (granular )

Prior art sample 1 (a wood-char-based pellet impregnated with KOH, steam activated) has a 34 g/1 working capacity. The prior art sample 2 (Karl

Higher gasoline working capacities are generally associated with higher hexane working capacities. Test for determining gasoline working capacity is set forth in SAE Technical Paper Series 890621, 1989.

SUBSTITUTESHEET

publication) lignite (brown coal) yields a 29.4 g/1 capacity. The present invention sample 1 activated carbon has a gasoline working capacity of 49.7 g/1. As established above, present invention sample 1 achieves an increase in gasoline working capacity of at least 69% over prior art sample 1 and at least 46% over prior art sample 2. The favorable results achieved with the present invention sample 2 activated carbon are even more remarkable, demonstrating an increase in working capacity of at least 91% over prior art sample 2 and at least 67% over prior art sample 1.

Due to the increased gasoline working capacities of the carbons of the present invention, less activated carbon filter material is required to achieve the working capacities of less adsorptive carbons. Significant savings can be achieved using smaller canisters filled with activated carbons of the present invention.

Although the carbons offered by the present invention have been described in detail in the foregoing for purposes of illustration, it is to be understood that such details are solely for that purpose and that variations may be made therein by those skilled in the art without departing from the spirit and scope of the invention as described in the following claims.

SUBSTITUTESHEET